2D materials for energy applications

Resume : Rabi splitting is important for many optical and non-linear optical applications, and has potential use in the development of solar cells. It occurs when the rate of energy exchange between a plasmon and quantum emitter is faster than their intrinsic dissipation rates. This causes the coupled plasmon and exciton energy to split into two new eigenstates of higher and lower energy than the original plasmon and exciton (anti-crossing). Rabi splitting was detected between the longitudinal plasmons of gold bipyramids and the A exciton of monolayer MoS2. This was investigated numerically using FDTD simulations and experimentally at the single particle level at room temperature. Bipyramids were shown to have the ideal shape to couple with a 2D material. This is because only one of a deposited bipyramid's ten sides touches the substrate's surface, tilting the plasmon resonance towards the MoS2. Therefore, the high electric field strength confined at the bipyramid's tip has a stronger overlap with the dipole moment of the transition metal dichalcogenide (TMDC) than other similar nanostructures such as rods or prisms. Further, the electric field enhancement is confined at the very sharp tip, touching the surface of the 2D material, keeping the mode volume of the resonator low, and hence increasing the coupling strength. It was found that larger bipyramids exhibit a stronger Rabi splitting effect than smaller ones, contrary to what has previously been reported for different nanoparticle shapes such as prisms and spheres. This is because a decreased aspect ratio of the bipyramid is necessary to keep the plasmon energy the same as that of the A exciton for larger bipyramids, but has the further effect of increasing the angle of the bipyramid's tilt towards the substrate. It is also because the electric field is stronger at the tips of larger bipyramids. Bipyramids of a large variety of lengths, and with plasmons of a large variety of energies were coupled with the A exciton in MoS2 to reveal clear anti-crossing behaviour. For plasmons directly overlapping with the exciton, a splitting as large as 80 meV was found, demonstrating a very large coupling strength for a single bipyramid on MoS2 at room temperature.

Resume : The advancements in graphene based functional materials has raised tremendous interest in fields of science, engineering and technology. Graphene supported inorganic hetero-structures, hybrid nanocomposites of metal oxides/hydroxides and graphene doped polymer hybrids etc are widely explored in materials research. The high surface area and high conductivity of graphene makes it an ideal template for energy storage application. In this pursuit, we have used surface functionalized spherical reduced graphene oxide (rGO) shells as nanotemplates for the growth of layered double hydroxide (LDH) nanostructures. The chemical moieties present on the graphene backbone enhances the electrochemical properties and also increases the effective surface area of the nanostructures. The synthesized coronal architectured hybrid material has a well aligned nanostructured morphology with ultrathin LDH nanosheets radially woven over the rGO nano-networks. The rGO nano-networks encapsulated inside eases the electrolytic access and enhances the overall specific charge storage capacity. The synthesized material has also 3 times higher charge storage capacity than the pristine material. This surface modification technique can also be extended to various transition metal based hydroxides.

Resume : Although gallium chalcogenides are among the most promising 2D semiconducting materials, there is currently no clear and reliable understanding of the properties of defects in these materials. In order to provide such critical knowledge we present a systematic theoretical study, based on state-of-the-art first-principles calculations, of a large set of native defects in monolayer GaS and GaSe. Specifically, using carefully determined chemical potentials, we obtained their formation energies and thermodynamic charge transition levels for different realistic growth conditions. We then evaluate their expected concentrations at thermal equilibrium applying a self-consistent approach. Based on these results we provide valuable insights to understand the physical mechanism behind the growth-method-dependent doping behaviours observed for monolayers of GaS and GaSe and we discuss the possible growth conditions to minimise the presence of harmful native defects in these materials.

Resume : Portable batteries are a reliable source of mobile energy to power smart wearable electronics, medical devices, communications, and others internet of thing (IoT) devices. There is a continuous increase in demand for thinner, more flexible battery with high energy density and reliability to meet the requirement. For a flexible battery, factors that affect these properties are the stability of current collectors, electrode materials and their interfaces with the corrosive electrolytes. State-of-the-art conventional and flexible batteries utilise carbon as an electrode and current collector, which cause high internal resistance (~100 ohms) and limit the peak current to ~1mA. This makes them inept for a wide range of applications. Replacing the carbon parts with metallic current collector would reduce the internal resistance (and hence reduce parasitic loss), but significantly increases the risk of corrosion due to galvanic interactions within the battery. To overcome these challenges, low cost electroplated metals (such Ni, Sn, and Ag) on copper (Cu) were studied as a potential anode current collector for a zinc-manganese oxide primary battery with an aqueous acidic electrolyte. Open circuit potential (OCP) of electroplated metals were monitored using electrical impedance spectroscopy (EIS) for different concentration of electrolytes to optimise the thickness of the plated coatings. Our results show that the electroplated coatings suffer excessive corrosion in the acidic electrolyte. Corrosion rate of different coatings were calculated with Tafel analysis. The results demonstrate that channelling and/or open porosity provide an easy pathway for electrolyte to penetrate thorough the electroplated coatings to corrode the Cu/coating interface. We further investigated the incorporation of a printed carbon/graphene corrosion protection layer and their effects on the shelf-life, internal resistance and the overall capacity of the printed flexible battery system.

Resume : The method of electrodeposition was used for the one-step fabrication of films based on graphene, graphene oxide and multi-wall carbon nanotubes. Since the dispersion of carbon nanomaterials is critical for the development of good composites, the surfactant sodium dodecylbenzene sulfonate was used to produce stable dispersions. The electrodeposited films on copper were later irradiated with a xenon lamp. Structural and chemical characterisation of the electrode material was carried out. Αll-solid-state symmetric supercapacitors were successfully fabricated with the use of a gel electrolyte of polyvinyl alcohol – phosphoric acid and the respective films on the electrodes. The electrochemical performance of the supercapacitors was evaluated by different electrochemical methods including cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. The all-solid-state supercapacitor with the best electrochemical properties was the symmetric supercapacitor from irradiated films of multi-wall carbon nanotubes with areal capacitance 78.8 μF/cm2, energy density 0.04 μW·h/cm2 and power density 63.03 μW/cm2.

Resume : Layered double hydroxides (LDHs) are a class of cationic layers with exchangeable anions in the interlayer space that exhibit a hydrotalcite-like structure. The high tunability of these phases from the point of view of metallic composition, stoichiometry, and the interlayer anion gives LDHs a wide versatility; resulting in different applications such as catalysis, sensing, magnetism or energy storage, to name a few.[1] Indeed, these anionic 2D materials exhibit a great potential for the design of hybrid heterostructures. With this goal in mind, the covalent functionalization of LDHs represents a very promising strategy that remains almost unexplored.[2] Herein, we report for the first time the reversible covalent functionalization with (3-Aminopropyl)triethoxysilane (APTES) of the NiFe–LDH.[3] The NiFe–APTES materials were fully characterized using X-ray diffraction, infrared spectroscopy, electron microscopy, thermogravimetric analysis coupled with mass spectrometry and 29Si solid-state nuclear magnetic resonance, among others. Moreover, we investigated the electrochemical performance of the NiFe-APTES for the oxygen evolution reaction (OER) in basic media, as well as its application in environmental remediation (adsorption of Cr(VI)). Finally, the pH-dependent retro-functionalization was demonstrated, opening the door to the synthesis of novel architectures. [1] Layered Double Hydroxides; Duan, X., Evans, D. G., Eds.; Structure and bonding; Springer: Berlin ; New York, 2005. [2] Park, A.-Y.; Kwon, H.; Woo, A. J.; Kim, S.-J. Layered Double Hydroxide Surface Modified with (3-Aminopropyl)Triethoxysilane by Covalent Bonding. Advanced Materials 2005, 17 (1), 106–109. [3] Carrasco J. A., Seijas‐Da Silva A., Oestreicher V., Romero J., Márkus B. G., Simon F., Vieira B. J. C., Waerenborgh J. C., Abellán G., Coronado E., Chemistry – A European Journal 2020, 26, 6504 – 6517.

Resume : The extraordinary properties of carbon-based nanomaterials have attracted great interest in material science, and graphene (Gr) and Carbon Dots (CDs) are two remarkable examples of that. In particular, the extraordinary transport properties of Gr make this material appealing for the production of nanosized microelectronic devices. On the other hand, CDs –a peculiar type of carbon nanoparticles– have variously shown their excellent optical (high emission efficiency, emission tunability, environment sensitivity) properties which make them promising candidates to replace other optically active nanoparticles lacking in biocompatibility and production affordability. The use of such materials in nanocomposite systems is usually studied in liquid phase and exploited in photocatalysis showing a powerful combination of their transport and optical properties. Besides, many other applications based on light conversion are of interest but need solid phase systems: light harvesting, optoelectronic devices. Herein, we report a μ-Raman/PL spectroscopy study at different wavelengths of solid CDs/Gr composite. PL-mapping reveals that Gr strongly affects the emission efficiency of CDs deposited onto it. In addition, the analysis of correlation between G and 2D bands evidences a modification of charge carrier population (doping) of Gr due to the presence of additional electrons from photexcited CDs. The reported results suggest a photoinduced electron transfer between Gr and CDs in solid phase.

Resume : In optics the measurable properties of absorbing monolayers become dictated by a thin-film limit, which is greatly simplified compared to typical Fresnel optics. Distinct properties of thickness, refractive index and absorption coefficient are effectively replaced by only the mathematical product of their values. However new implications appear, such as a strong dependence on ambient refractive index [1]. Motivated by this we investigate how the thickness also affects the electrical performance of a layer in solar cell. As the object of the study we take modern solar cell architecture and the role of the thickness of a selective contact [2]. First, we look for how a reduced thickness affects the sensitivity of device efficiency to light intensity for which the device is designed and we found logarithmic dependence of required thickness on light intensity. Second, we found another logarithmic relation between thickness and required effective work function. The conclusion from this study is that when going to thin absorbers requires not only high absorption coefficient, but also high refractive index and when going to thin contact layers high work function or restriction to low illuminations is necessary. [1] J. Holovský, et al., Effect of the thin-film limit on the measurable optical properties of graphene, Sci. Rep. 5 (2015) 15684. [2] B. Conrad, et al. Illumination-Dependent Requirements for Heterojunctions and Carrier-Selective Contacts on Silicon, IEEE J. Photovolt. 10 (2020) 1214–1225.

Resume : We present the first example of a working bulk heterojunction solar cell incorporating metal-organic nanosheets (MONs) - an emerging class of two-dimensional materials which allow the creation of diverse, well-defined architectures with tailored properties. MONs are the two-dimensional analogues of metal-organic frameworks (MOFs), consist of self-supporting, sheet-like materials approaching monolayer thickness and can be readily processed as suspensions and spin-coated to form thin films. MONs provide within a single material the highly ordered structure of inorganic materials with the chemically tailorable properties and low cost of organics. As with other two-dimensional materials such as graphene, boron nitride and molybdenum disulfide, MONs have been found to outperform their bulk counterparts in a range of applications thanks to their high external surface area, aspect ratios, nanoscopic dimensions and novel magnetic, electronic and optical properties. In this work we synthesized ultra-thin porphyrin based MONs with tunable bandgaps to match the choice of donor and acceptor semiconductors to form a bulk heterojunction type solar cell. This work introduces for the first time the concept of including metal-organic framework as nanosheets into bulk heterojunctions in order to control the morphology of the active layer and improve absorption and charge mobility in order to enhance the performance of BHJ solar cells. We anticipate that our approach and the insights gained through this study along with the ease with which the electronic, optical and surface properties of MONs can be modified will enable new generations of high efficiency organic solar cells and other electronic devices.

Resume : New low power technologies, as the one largely developed for the internet of Things (IoT), have fostered the development of sustainable energy sources that can harvest the available environmental green energy and that can be portable. The operation of many wearable and portable electronics and sensors requires only few tens of microwatts up to few mW of power supply, making it possible their integration with energy harvesters such as mechanical energy harvesters. Triboelectric nanogenerators (TENGs) are a relatively new technology that has gained more attention, since it can provide a green power supply by harvesting the almost ubiquitous environmental mechanical energy employing low cost and sustainable materials. Recent developments in TENGs technology show that they can be used in different applications going from wearable and self-powered sensors to wind and sea wave energy harvesting (blue energy).1 Differently from piezoelectric generators, which rely on the use of rare earth metals and often toxic element such as lead, TENGs can be fabricated form non-toxic, biodegradable and recyclable materials. Importantly, TENGs have shown to provide high power output also in the low frequency range overcoming a main limitation of more conventional mechanical harvesters as piezoelectric and electromagnetic generators.2 However, despite the wide number of TENGs developed so far, an in depth understanding of their working mechanism operating at the sub-microscale is still missing. 2D materials have been largely employed in TENGs, both as triboelectric material and as electroactive additives embedded into an insulating triboelectric material, showing their beneficial effect in rising the TENGs’ power output. However, it is not often clear which specific intrinsic properties of 2D materials are really playing a role in favoring the TENGs performance, therefore hampering the establishment of new design principles. Here, we highlight the fundamental role played by the interface between the triboelectric material and the electrode collector in contributing to the TENG’s power generation. We will show the importance of the electrodes work function and capacitance, a main aspect which has been often overlooked in previous work. The specific role of few-layers graphene electrodes, doped graphene and other 2D materials embedded at the interface between the triboelectric materials and the electrodes will be highlighted and clarified.3 The outcomes of our studies provide essential insights for the proper design of new device structures, materials selection and optimization, paving the way to the further enhancement of TENGs performances. 1 C. Wu, A. C. Wang, W. Ding, H. Guo and Z. L. Wang, Adv. Energy Mater., 2019, 9, 1–25. 2 Y. Zi, H. Guo, Z. Wen, M. H. Yeh, C. Hu and Z. L. Wang, ACS Nano, 2016, 10, 4797–4805. 3 G. *Pace, A. Ansaldo, M. Serri, S. Lauciello and F. Bonaccorso, Nano Energy, 2020, 76, 104989.

Resume : Two-dimensional layered transition metal dichalcogenides (TMDCs) have attracted interest recently for their promising optoelectronic properties resulting from their rich excitonic structure with exciting possibilities for spin-valley physics. Moreover, their interlayer Van der Waals bonding enables the stacking of many different low-dimensional materials into heterostructures, allowing rich variation of their behaviour. Promising candidates for stacking are inorganic lead-halide perovskite quantum dots, as their excellent optical properties and tunability hold great promise for future optoelectronic applications. In this work, we combine these two classes of materials and investigate their resulting photo physics. We study the interaction and carrier transfer of heterostructures of MoS2 layers and CsPbBr3 quantum dots by various spectroscopic techniques and investigate the role of the quantum-dot ligands in the charge and energy transfer process. As expected, the interface between the two materials plays a decisive role in determining energy- and charge transfer efficiencies and we identify ligand treatments to improve the interface quality. These results open up new directions for versatile, highly flexible future optoelectronic devices with tailor-made properties.

Resume : Boron-Carbon-Nitrogen (B-C-N) ultrathin layers have been grown by plasma enhanced chemical vapour deposition by using methylamine borane as a single source molecular precursor. This easy and inexpensive method allows controlled and reproducible growth of B-C-N layers onto different substrates, such as Cu [1] and Ti [2]. Their morphological, structural and chemical properties have been characterized by scanning electron microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy and transmission electron microscopy coupled with electron energy loss spectroscopy. The ultrathin B-C-N layers segregate into C-rich and h-BN-rich domains with very high mutual doping levels, despite some disorder has been observed. The obtained layers present enhanced electrocatalytic activity for the oxygen evolution reaction in alkaline aqueous solutions. Moreover, owing to their ultrathin nature, the B-C-N layers grown on Ti substrates preserve the photocurrents of the underlying native titanium oxide layer, acting as transparent electrodes with high conductivity for the photogenerated charge carriers and improved electrocatalytic activity for the oxidation of water to oxygen gas. The present results may have important applications in the development of metal-free photochemical water splitting devices. [1] Leardini et al. 2D Mater. 2019, 6, 035015 [2] Jiménez Arévalo et al. ACS Appl. Energy Mater. (in press, 2020), https://dx.doi.org/10.1021/acsaem.9b02339

Resume : Transition metal dichalcogenides (TMDCs) are layered semiconductors that can be assembled into ultra-thin solar cells. Although such structures have shown remarkable light absorption for their thickness, device performance has so-far been limited by low open-circuit voltages (VOC). Here we report on a MoS2 homojunction with a VOC above 1 V under an illumination level equivalent to 40 suns. This has been achieved by substitutional doping with Nb (p-type) and Fe (n-type) which results in a favourable band alignment compared to heterostructures. In our devices we find that blocking the light on the metal contacts increases the VOC by ~0.4 V, which can be understood by an equivalent circuit model which considers photoactive Schottky barriers at the TMDC/metal interface. The VOC of 1.04 V at an optical band gap EG of 1.29 eV represents a remarkably small band gap‐voltage offset (WOC) of 0.25 V which can be compared to WOC values in transition metal dichalcogenide cells which to-date are above 0.8 V. Our results demonstrate that substitutional doping is a viable approach for achieving unprecedented VOCs in TMDC based devices and our simple device structure is a feasible path to develop ultra-thin low-WOC solar cells based on van der Waals structures.

Resume : Amongst 2D materials, the family of transition metal chalcogenides (TMCs) is attracting great attention due to the outstanding properties that they exhibit in fields such as magnetism, catalysis and/or optoelectronics.[1] Concretely, the group of metal phosphorous trichalcogenides has attracted great attention not only because of their magnetic and ferroelectric properties but, more interestingly, for their catalytic activity in hydrogen and oxygen evolution reactions (HER and OER), application that can be improved by going from the bulk to the exfoliated material.[2] Among the family of trichalcogenides, we have focused our work on MnPS3. Until now, this TMC has been mostly exfoliated by dry methods producing only a few layers over a substrate,[3] but regarding the exfoliation in liquid phase, published works are scarce and incomplete.[4][5] Our work has focused on the optimization of a solution approach to obtain high quality ultrathin MnPS3 layers by means of weak functionalization with organic compounds that can be removed in a final step. The resulting layers have been deeply characterized and their magnetic and electrochemical properties studied.[6] [1] Wang, L., et al. J. Mater. Chem. A 5(44) (2017) 22855-22876. [2] Liu, et al. J.Chem Phys. 140 (2014) 054707 [3] Long, G., et al. ACS Nano. 11(11) (2017) 11330-11336 [4] Frindt, R. F., et al. J. Mat. Res. 20(5) (2005) 1107-1112. [5] Bai, W., et al J. Am. Chem. Soc. 142 (2020) 10849-10855 [6] R. Gusmão, et al. Adv. Funct. Mater. 29 (2) (2019) 1805975.

Resume : Layered two-dimensional transition-metal dichalcogenides (TMDCs) have garnered intense attention due to the different properties achieved by the modification of their composition and number of layers. One of the most studied members of this family is MoS2. This compound is an indirect band-gap semiconductor in bulk, that turns into direct-bandgap when it is exfoliated from bulk down to a monolayer, which makes it very promising for its integration in electronic, optoelectronic and photovoltaic devices.[1] Recently, many studies have targeted the tuning of MoS2 optical and electrical properties by means of bandgap modulation with an applied external strain. In this direction, the recovery of the indirect-bandgap behavior has been demonstrated by applying a ~2% of strain.[2] This transition can dramatically alter the electrical behavior of the 2D material, as well as the photoluminescence (PL) displayed by the direct-bandgap MoS2. In this work, we propose an original approach to induce a reversible strain on MoS2 ultrathin layers. It consists on the surface functionalization of the 2D layers with switchable molecular systems that undergo a reversible volume change upon the application of an external stimulus. In this context, the use of spin-crossover (SCO) systems as mechanical actuators is presented as a promising alternative. These SCO molecular materials can change their ground spin state between low spin (LS) and high spin (HS) upon the exertion of various external physical or chemical stimuli, resulting in a variation of volume that presses the flake. We have achieved the decoration of MoS2 flakes with SCO nanoparticles of the well-known core-shell compound [Fe(Htrz)2(trz)](BF4)@SiO2.[3] This material presents a transition above room temperature with a broad hysteresis, 40 K, and a change in volume of ca. 10 %. The hybrid composite exhibits an abrupt change in conductivity, near the NPs transition temperature, that can be associated with a decrease in the bandgap. Finally, a direct probe of the bandgap narrowing has been obtained by monitoring the PL of the composite in both spin states, observing the expected redshift upon the spin transition from 1.88 eV (LS) to 1.82 eV (HS). [1] D. Jariwala, V.K. Sangwan, L.J. Lauhon, T.J. Marks, M.C. Hersam, ACS nano, 2014, 8 (2), 1102. [2] J. Pető, G. Dobrik, G. Kukucska, P. Vancsó, A.A. Koós, J. Koltai, L. Tapasztó, npj 2D Materials and Applications, 2019, 3 (1), 1. [3] R. Torres-Cavanillas, L. Lima-Moya, F.D. Tichelaar, H.W. Zandbergen, M. Giménez-Marqués, E. Coronado, Dalton Trans. 2019.

Resume : Semiconductor titanium dioxide (TiO2) photocatalyst has ability of photocatalytic water splitting and can inactivate pathogenic microorganisms (viruses and bacteria) effectively. However, the large bandgap of TiO2 (ca. 3.0 eV) restricts its utilization of the whole solar spectrum. Recently, graphite-like carbon nitride g-C3N4, as a metal-free polymeric semiconductor with photoactivity in visible light, and a moderate band gap of 2.7 eV, has generated a lot of interest as photocatalytic material. In this work, we introduce a facile procedure to obtaining of g-C3N4/TiO2 binary composite films on titanium foil substrate by one-step CVD approach using melamine as precursor under air atmosphere. To produce partially oxidized product (TiO2) on the Ti surface we carried out the pyrolysis of precursor at the presence of a fixed volume of air. Deposited onto Ti surface the composite g-C3N4/TiO2 films were characterized by XRD, SEM, XPS and IR spectroscopy. It was found that the visible-light-induced photodegradation of methylene blue was remarkably increased upon coupling TiO2 with g-C3N4, possibly due to heterojunctions which enhanced electron-hole separation efficiency as a result of effective interfacial electron transfer between TiO2 and g-C3N4. The facile deposition method can be promising for the fabrication of efficient and low-cost photocatalyst based on g-C3N4/TiO2 composite films for microorganisms inactivation and H2-evolution by photocatalytic water splitting.

Resume : Colloidal lead halide perovskites (LHPs) are an emerging class of semiconductors for inexpensive, solution-processable, and ultra-color pure, quantum dot light-emitting diodes (LEDs) for next-generation displays and illumination devices. However, the efficiencies of quantum dot LEDs are adversely suffered due to intrinsic characteristics of LHPs that include low photoluminescence (PL) quantum efficiency and poor light-out coupling efficiency because of the random distribution of emission dipoles and mismatch in refractive indices. Here, we demonstrate efficient, ultra-pure green EL based on the colloidal two-dimensional (2D) superlattices by introducing the formamidinium cations into methylammonium lead bromide perovskites. For our champion perovskite LED devices, comprising asymmetric 2D superlattices thin-films with near-unity PL quantum efficiency, show a maximum external quantum efficiency of >20% and luminous efficacy of >120 lm/W. Thus, very high performance in the LEDs is due to over 95% internal quantum efficiency coupled with balanced hole and electron injection, effective charge carriers, and exciton confinement within the emissive layer. Moreover, the color gamut covers over 99% of the Rec. 2020 standard in the CIE 1931 color space, respectively, representing the “greenest” LEDs ever reported. By using the 2D superlattices, our champion device also demonstrates an operational lifetime (LT50) of 15 hours at 100 nits. These are among the highest-performance colloidal perovskite quantum dots LEDs ever demonstrated by far.

Resume : The global need for energy, combined with a desire to be more environmentally friendly, has led to a real increase in electricity consumption, with an average annual increase of 3.4%, 1.2 percentage points higher than average annual growth of energy consumption.[1] As not all the electricity produced can be used immediately, the utilisation of batteries to store unconsumed electricity becomes a key point in these energy issues. In this optic, lithium-oxygen batteries could be the best candidate, due to their high theoretical energy density which could allow to increase by 15 times the actual storage of lithium-ion batteries (11,680 W.h.kg-1).[2,3] Nevertheless, performances of current lithium-oxygen batteries are still quite far from the theoretical values. Indeed, improving electrochemical performances such as rate capability or cycle life are still challenging.[4] In this talk, I will present the different structures and materials obtained in our laboratory in order to evaluate their performance as cathodes in lithium oxygen batteries. These innovative structures will be multiple, ranging from carbon-based structures with hierarchically ordered porosity following Murray's law allowing optimal diffusion of the electrolyte within the cathode, to carbon slurry containing catalysts based on various metal oxides to enhance OER/ORR processes, or finally, spinel-based nanograss allowing optimized deposition of lithium peroxide during discharge. [1] Liu.Z, Global Energy Interconnection,1, 91-100 (2015) [2] K. M. Abraham and Z. Jiang, J. Electrochem. Soc, 143, 1, 1-5 (1996). [3] P. G. Bruce, S. A. Freunberger, L. J. Hardwick and J.-M. Tarascon, Nat. Mater, 11, 19–29 (2012). [4] P. Tan, H. R. Jiang, X. B. Zhu, L. An, C. Y. Jung, L. Wu, L. Shi, W. Shyy, and T. S. Zhao, Applied Energy, 204, 780–806 (2017).

Resume : Over recent years, thermoelectricity has become a forefront, green technology in capturing the waste heat to generate electrical power. The binary compound InTe has recently emerged as an interesting thermoelectric material with a high dimensionless thermoelectric figure of merit ZT of 0.9 at 600 K in polycrystalline samples [1]. One of its remarkable key properties is the extremely low lattice thermal conductivity reaching ~ 0.3 – 0.4 W m-1 K-1 at 600 K [1,2]. Despite several reports on the electronic properties measured on single-crystalline InTe, no detailed investigation of its thermal properties have been undertaken so far. In this work, a detailed study of the thermoelectric properties at high temperatures (300 – 800 K) was performed on single-crystalline InTe. The successful growth of a large single crystal of InTe was realized using the Bridgman technique. As observed in prior studies, InTe exhibits very low lattice thermal conductivity, explained by the presence of low-energy, optical phonon modes evidenced by inelastic neutron scattering experiments [3]. Our results also confirmed the cleavage plane of single-crystalline InTe to correspond to the (110) plane, confirming that InTe has a layered crystal structured with weak interchain bonds. A high ZT of 0.71 was obtained along the [110] direction at 780 K. References 1 M.K. Jana, K. Pal, U.V. Waghmare, and K. Biswas, Angewandte Chemie 128, 7923 (2016). 2 S.Y. Back, H. Cho, Y.-K. Kim, S. Byeon, H. Jin, K. Koumoto, and J.-S. Rhyee, AIP Advances 8, 115227 (2018). 3 S. Misra, C. Barreteau, J.-C. Crivello, V.M. Giordano, J.-P. Castellan, Y. Sidis, P. Levinský, J. Hejtmánek, B. Malaman, A. Dauscher, B. Lenoir, C. Candolfi, and S. Pailhès, Physical Review Research 2, 043371 (2020).

Resume : carbon nanostructure functionalized with porphyrin derivative were prepared and characterized with spectroscopic techniques. Porphyrins are photoactive molecules generally known to be photoconductors,optoelectronics and capable of light induced charging and facilitate charge transfer to acceptors under the visible light irradiation. In this work, the carbon nanostructure nanocomposites based were obtained using porphyrin derivative and their optical properties were investigated by means of Photoluminecence in steady state. The Photoluminecence spectrum of porphyrin precursor and carbon nanostructure based composite indicates an important interaction suggesting charge transfer from porphyrin precursor to carbon nanostructure in steady state.

Resume : Transition metal dichalcogenide semiconductors, particularly MoS2, are known as promising alternative non-precious hydrogen evolution reaction (HER) electrocatalysts to high-cost Pt. However, their performance is strongly limited by the poor conductivity and lack of active sites in the basal plane. Therefore, it is desirable to find alternatives with active basal plane sites or develop facile strategies to optimize the inert basal plane. In this work, we study the HER over topological semimetal Nb2S2C based on its basal plane. We report the first successful activation and optimization of the basal plane of Nb2S2C by synergistic using trivial surface states (SSs) and nontrivial topological surface states (TSSs). We find that the binding strength towards hydrogen adsorption of the easily cleaved sulfur(S)-terminated Nb2S2C surface can be stronger than that of 2H-MoS2, attributing to the presence of trivial SSs and nontrivial TSSs in Nb2S2C. By creating S vacancy on the basal plane, the binding strength towards hydrogen adsorption can be greatly optimized. The TSSs together with dangling-bonds reduce the Gibbs free energy to 0.31 eV, close to the peak of the volcano plot. This study provides a promising strategy for the joint utilization of the basal plane trivial SSs and TSSs for the HER.

Resume : Exotic electronic states are realized in various topological phases, from topological insulators to recently reported Weyl/Dirac semimetals, nodal line semimetals, and magnetic semimetals. They strongly influence the surface electronic structures of the investigated materials and could serve as a good platform to gain insight into the catalytic mechanism of surface reactions. Topological Semimetals such as PtSn4 and Co3Sn2S2 adopt quasi-two-dimensional structures and could expose the crystal surfaces constructing by transition metal atoms.1,2 Topological non-trivial surface states are observed at the crystal surfaces, which are located near the Fermi level. These topological surface states can act as both electron acceptors or donators for small adsorbed molecules, consequently tailoring the adsorption energy and Gibbs free energy in the electrochemical catalytic reactions. References: (1) Li, G., et al. Dirac Nodal Arc Semimetal PtSn4 : An Ideal Platform for Understanding Surface Properties and Catalysis for Hydrogen Evolution. Angewandte Chemie 2019, 58, (2) Li, G., et al. Surface states in bulk single crystal of topological semimetal Co3Sn2S2 toward water oxidation. Sci. Adv. 2019, 5, eaaw9867.

Resume : All-solid-state Li-ion micro-batteries are promising candidates to power miniaturized sensors for Internet of Things (IoT) applications [1]. Such applications have created a high demand for battery systems to provide larger power and energy densities. To fulfill the performance requirements, positive electrodes with both high storage capacity and high operating potential should be developed. A promising candidate is the spinel LiMn1.5Ni0.5O4 (LMNO) which exhibits a mean operating potential of 4.75 V vs. Li/Li+, a theoretical specific capacity of 147 mAh•g−1, with appealing properties such as low cost and environmental friendliness due to the use of the high amount of manganese [2]. In this work, the structural evolution during Li+ extraction/insertion in sputtered disordered spinel LMNO thin films deposited on Si/ Al2O3/ Cr-Pt substrate is investigated for the first time by Raman spectroscopy. A selected area of 0.38 cm2 on the 400 nm thick LMNO layer is cycled in a homemade cell using 1 M LiClO4 EC: DMC electrolyte in the 4.4 - 4.8 V potential range. A discharge capacity of 60 µAh cm-2 µm-1 (~0.95 Li/mole) is obtained on the second cycle at a 1C rate with 90% capacity still retained even after 50 cycles. The XRD study shows a 2-phase mechanism consisting of a solid solution region followed by a diphasic region, in good agreement with a previous study on bulk disordered phase powder [3]. The Raman spectral variations displayed during the first charge-discharge cycle are shown to be strongly associated with the change in the transition metals valence states. A careful electrochemical and spectroscopic analysis allows identifying pertinent descriptors of the Ni2+/Ni3+/Ni4+ species in the Raman spectra and providing their relative ratio in the LNMO thin film at different oxidation-reduction states. Spectral variation during the discharge highlight the good reversibility of the structural changes. This work demonstrates the efficiency of Raman spectroscopy to determine the state of charge (SOC) of the LMNO thin film cathode. [1] Maxime Hallot et al., Energy Storage Materials, 15, 396-406, (2018) [2] Arumugam Manthiram et al., Energy Environmental Science 7, 1339, (2014) [3] J.H. Kim et al., Chemistry of Materials, 16, 906-914, (2004)

Resume : Chalcogenides of group VIB transition metals are of interest as absorber materials for thin film solar cells. To investigate fundamental properties such as conductivity, doping behaviour and opto-electronic properties single crystals are in demand. We have successfully grown MoSe2, MoTe2 as well as WSe2 by chemical vapor transport (CVT) using halogens as transport agent. While the crystals of the selenides and tellurides have 1 mm thickness and areas of the 00.1 face in cm2 range, single crystal growth of the sulphide counterparts MoS2, WS2 still remains challenging to delimitate the growth conditions concerning temperature, gas phase composition and concentration of transport agent. From series of CVT experiments in which we varied the growth conditions, we observed that the transport of the powder proceeded very fast. However, the grain size of the transported sulphides remained unchanged with extension of the 00.1 face smaller <1 mm. To improve the yield of the transport reaction and to verify the influence of oxygen-containing species (WO2/WO3) on the transport of tungsten the gas phase composition was varied systematically. Using bromine as transport agent, red transparent crystals were obtained as alternative product beside transported MoS2 / WS2 powders in some ampoules. The crystals grown could be assigned to the oxybromides WO2Br2 and MoO2Br2 crystallizing in the space group Cc. It was found that significant amounts of oxygen lead to a rapid increase of pressure within the ampoule. The occurrence of oxybromide crystals inhibits the transport of tungsten dichalocogenides, no single crystal growth of WS2/ MoS2 could be observed. On the other hand, the occurrence of oxybromide crystals shows that residual oxygen is present in the system, even if none was added. We were able to show that the gas phase composition plays an important role in the chemical vapor transport. Furthermore, work needs to be done on the growth conditions in order to achieve larger MoS2, WS2 single crystals.

Resume : Unlike localized excitons in organic molecules, emissive states in semiconducting nanocrystals are extended band edge states of complex symmetry characterized by degenerate transition dipole moments (TDMs), usually leading to isotropic emission. The strength and spatial distribution of TDMs play a key role in light-matter interactions and its understanding and engineering is central in outcoupling efficiency of light-emitting diodes, fluorescence resonant energy transfer (FRET) and on-chip photonic applications. Engineering the shape and dimensionality of semiconducting nanostructures has been showed to be a promising route to overcome the isotropic distribution of TDMs typical of spherical quantum dot solids. Mixed dimensionality systems are especially interesting platforms to explore the role of the TDM in determining the optoelectronic properties of semiconductor colloidal nanocrystals. Here, we first examine binary mixtures of few-monolayer-thick lead halide perovskite nanoplatelet (NPL) solids and we characterize the FRET to highlight discrepancies showed in 2D extended systems with respect to classical Förster theory. By mixing NPLs with diverse aspect ratios we achieve a bimodal distribution of TDMs where thinner platelets have more pronounced in-plane character than their thicker partners. Since different TDM orientations exhibit variations in the angular emission pattern, we observe that the binary mixtures display an emission color gradient as the angle of view changes, which is proportional to the difference in NPL aspect ratio. The tunability of the angular color gradient is then enhanced by the integration of the NPL thin film with a fully solution processed polymeric 1D photonic crystal. The distributed Bragg reflector (DBR) acts as a tunable and stretchable color filter which increments the portion of color space spanned by the bare film and can be easily lifted-off from the substrate. Hence, we report the fabrication of a flexible and transferable film exhibiting angular variable fluorescence by TDM engineering in lead halide perovskite NPL solids aimed at cryptographic and anticounterfeit applications.

Resume : By means of the hybrid exchange-correlation functionals, ab initio calculations for energy applications were performed for LaScO3 as well as main ABO3 perovskite (A=Sr, Ba, Pb, Ca and B=Ti) surfaces, namely SrTiO3, BaTiO3, PbTiO3 and CaTiO3 [1-6]. For ABO3 perovskite (001) surfaces, with a few exceptions, all atoms of the upper surface layer relax inwards, all atoms of the second surface layer relax outwards, and all third layer atoms, again, inwards. The relaxation of (001) surface metal atoms for ABO3 perovskite upper two surface layers for both AO and BO2-terminations, in most cases, are considerably larger than that of oxygen atoms, what leads to a considerable rumpling of the outermost plane. The ABO3 perovskite (001) surface energies always are smaller than the (011) and especially (111) surface energies. The ABO3 perovskite AO and BO2-terminated (001) surface band gaps always are reduced with respect to the bulk values. The B-O chemical bond population in ABO3 perovskite bulk always are smaller than near the (001) and especially (011) surfaces. References: 1. R.I. Eglitis and A.I. Popov, J. Saudi Chem. Soc. 22, 459-468 (2018) 2. R.I. Eglitis and D. Vanderbilt, Phys. Rev. B 76, 155439 (2007) 3. R.I. Eglitis and D. Vanderbilt, Phys. Rev. B 77, 195408 (2008) 4. R.I. Eglitis and D. Vanderbilt, Phys. Rev. B 78, 155420 (2008) 5. R.I. Eglitis, Applied Surface Science 358, 556-562 (2015) 6. R.I. Eglitis, J. Kleperis, J. Purans, A.I. Popov, Ran Jia, J. Mater. Sci. 55, 203-217 (2020)

Resume : Transition metal dichalcogenides (TMDs) offer huge flexibility in tuning electronic properties. Their electronic structure is found to change dramatically from bulk to few monolayer samples. Moreover, some bulk TMDs show potential for high thermoelectric (TE) performance (large TE power factor (PF)) . Guiding ideas [1] have shown that electronic states generated by very directional atomic orbitals (d and d-p) which allow the presence of very anisotropic flat-and-dispersive electronic bands can maximize the PF and at the same time create low-dimensional electronic transport. Using first-principles calculations, we studied the electronic and TE properties of bulk TMDs, which form in 2H (MoS2, MoSe2, MoTe2, WS2, WSe2, WTe2) and 1T (SnS2, SnSe2, HfS2, HfSe2, HfTe2, ZrS2, ZrSe2) layered structures, evaluating the anisotropy of electronic bands and their multiplicity, and identifying the high performance TE materials. The first-principles calculations have been performed within density functional theory (DFT) using the hybrid functional B1-WC [2]. The dependence of electronic and TE properties on layer thickness and applied epitaxial strain will be discussed in order to further improve TE performance of these 2D materials. 1. D. I. Bilc, G. Hautier, D. Waroquiers, G. M. Rignanese, and Ph. Ghosez, Phys. Rev. Lett. 114, 136601 (2015). 2. D. I. Bilc, R. Orlando, R. Shaltaf, G.-M. Rignanese, Jorge Iniguez and Ph. Ghosez, Phys. Rev. B 77, 165107 (2008).

Resume : Currently employed materials in Thermoelectric Generators (TEGs) such as bismuth telluride or lead telluride are scarce, expensive, toxic, and environmentally harmful. In recent years, the thermoelectrics paradigm has changed mainly due to the introduction of low-dimensional materials able to reduce the thermal conductivity by phonon scattering. Semiconductor nanowires have demonstrated fascinating properties with application in various fields, including energy and information technologies. In particular, increasing attention has been focused on Si and SiGe nanowires for application in thermoelectric generation after recent successful implementation in miniaturized devices. Despite this interest, a proper evaluation of such nanostructures' thermal conductivity still represents a great challenge, especially when the characterization of the device-integrated nanowire is desired. Scanning Thermal Microscopy (SThM) offers an alternative for spatially-resolved, non-destructive, and flexible evaluation of individual nanostructures' thermal conductivity. However, a big challenge remains for micro and nanostructures presenting high aspect ratios, such as the here studied suspended nanowires integrated into micro platforms. In those cases, the background change along the abrupt topography of the nanostructures can be of the same order of magnitude as the signal of the nano/microstructure of interest, and thus the signal extraction is often challenging to perform. In this work, we present an alternative approach to currently existing intricate techniques for precisely measuring the thermal properties of integrated 1D nanostructures (NSs). The procedure consists in performing a series of z scan approaches over the studied sample using the conductance contrast mode of the SThM. By correlating the force-z and the probe current-z curves, the heat change produced in the contact can be accounted for and used to compute the thermal conductivity. This approach eliminates the need for background subtraction. As a case study, the thermal conductivities of Si and SiGe NWs integrated in silicon microdevices were measured. Thermal conductivity values of 15.8 ± 0.8 W/m·K and 4.2 ± 0.3 W/m·K were measured for Si and SiGe nanowires, respectively. Indeed, up to our knowledge, this is the first-time reported use of SThM where the thermal conductivity of a device integrated 1D nanostructure is assessed using a conventional SThM. Finally, the range of applicability according to the sample thermal conductance and associated errors are discussed to establish the potential of the novel technique. The results presented here show the remarkable utility of scanning thermal microscopy for the challenging thermal characterization of integrated nanostructures and the development of multiple devices such as thermoelectric generators or photovoltaic cells. References: [1] G. Gadea, Á. Morata, A. Tarancón, J. D. Santos, and M. Pacios Pujadó, “Integration of Si/Si-Ge nanostructures in micro-thermoelectric generators,” Universidad de Barcelona, 2017.

Resume : Two-dimensional oxide materials are a well-studied, interesting class of materials, enabled by the fact that their bulk layered metal oxides, such as titanates and niobates, can be easily exfoliated within minutes into 2D nanosheets. However, some promising oxide materials, such tantalum oxide, are much more difficult to delaminate, taking several weeks, due to the higher charge density resulting in stronger Coulombic interactions between the layers. This intrinsic constraint has limited detailed studies for exploiting the promising properties of tantalum oxide 2D nanosheets towards enhanced catalysis and energy storage. Here, we have studied in detail the exfoliation mechanism of high charge density 2D materials, specifically tantalum oxide (TaO3) nanosheets. Optimization of tetrabutylphosphonium hydroxide (TBPOH) as the exfoliation agent in a 2:1 ratio to HTaO3 has resulted in a dramatic reduction of the exfoliation time down to only 36 hours at 80 oC. Furthermore, single monolayers of TaO3 nanosheets with >95% coverage have been achieved by Langmuir-Blodgett deposition, while thicker layers (ranging from several tens of nanometers up to microns) exhibiting long-range ordering of the present nanosheets have been realized through ink-jet printing. Interestingly, scanning tunneling microscopy analysis indicated a wide bandgap of ~5 eV for the single TaO3 nanosheets. This value is significantly higher than the reported values between 3.5 and 4.3 eV for the layered RbTaO3 parent compound, and opens up new opportunities for 2D oxide materials.

Resume : In this study, we demonstrate the concept of preferential leaching of cerium oxide on ternary Pt-C-CeO2 compound in order to develop a cost-effective catalyst for the cathode in Proton exchange membrane fuel cells(PEMFCs). The Pt-C-CeO2 thin film catalyst was prepared by simultaneous magnetron co-sputtering of Pt, C and CeO2. The morphology, structure and composition of Pt-C-CeO2 layer was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Both half-cell and single-cell tests were performed to determine its activity and durability. During an activation electrochemical cycling procedure, CeO2 was leached from the compound leaving behind a porous Pt-C matrix which exhibited almost three times higher electrochemically active surface area in comparison with pure Pt with identical loading before and even after accelerated degradation tests in the half-cell. When used as the cathode in a single-cell membrane electrode assembly decomposed Pt-C-CeO2 also showed greater power density than pure Pt.

Resume : Recently, two-dimensional layered transition metal dichalcogenides (TMDCs) have attracted much interest due to their excitonic structure and their exciting possibilities in solid state electronics, energy harvesting, energy storage and spin- and valleytronics. Furthermore, the interlayer van der Waals-bonding of 2D TMDCs facilitates the fabrication of heterostructures, but also enables the creation of different stacking orders in multilayers of the same material. Of these TMDC materials, the group-VII TMDC ReS2 has unique optoelectronic properties due to its distorted triclinic structure. This leads to anisotropic optical characteristics like polarized photoluminescence and quasi-1D hole dynamics that persist from bulk material to the monolayer limit. However, the co-existence of two distinct stacking orders in mechanically exfoliated samples has presented challenges in unambiguously characterizing optical and electronic behaviour of ReS2 multilayers. Adequate characterization of multilayer samples must therefore include stacking order determination. In this work, we elucidate the distinct excitonic properties of both stacking orders by means of temperature dependent photoluminescence (TDPL) spectroscopy and low temperature differential reflection spectroscopy. By fitting the Arrhenius equation to TDPL data and fitting Rydberg series to differential reflection data, we conclusively determine distinct binding energies for both excitons in both stacking orders. Furthermore, we compare the level of optical anisotropy of both stacking orders through polarization resolved PL.

Resume : There is an increasing interest in two-dimensional (2D) Ruddlesden-Popper perovskites for solar harvesting and light-emitting applications due to their superior chemical stability as compared to bulk perovskites.[1,2] Both, purely 2D and blends of 2D/3D phases have been successfully employed in solar cells with efficiencies of >18% and >21%, respectively.[3,4] As with earlier advances in the field of perovskites, these technological improvements are advancing at a pace that far exceeds our understanding of the physical mechanisms underlying their performance. Particularly, the reduced dimensionality in 2D perovskites results in excitonic excited states which dramatically modify the dynamics of charge collection. While the carrier dynamics in bulk systems is increasingly well understood, a detailed understanding of the spatial dynamics of the excitons in 2D perovskites is lacking.[5] Here, we present direct measurements of exciton transport in single-crystalline layered perovskites. Using transient photoluminescence microscopy, we can follow the temporal evolution of a near-diffraction-limited exciton population with sub-nanosecond resolution revealing the spatial and temporal exciton dynamics. We observe two distinct temporal regimes: For early times excitons undergo unobstructed normal diffusion, while at later times exciton transport becomes subdiffusive as excitons get trapped. Interestingly, the diffusivity at early times depends sensitively on the choice of the organic spacer which can yield diffusion lengths that differ by up to an order of magnitude. We find a clear correlation between lattice stiffness and diffusivity, suggesting exciton–phonon interactions to be dominant in the spatial dynamics of the excitons in perovskites, consistent with the formation of exciton–polarons. Our findings provide a clear design strategy to optimize exciton transport in these systems.[6] In addition, we show that the complex exciton dynamics observed at later times can be leveraged to provide a detailed map of the trap-state landscape in 2D perovskites, in particular when used in combination with transient photoluminescence spectroscopy and a rigorous diffusion model that accounts for a distribution of radiative shallow trapping sites.[7] 1. Krishna, A., Gottis, S., Nazeeruddin, M. K. & Sauvage, F. Mixed Dimensional 2D/3D Hybrid Perovskite Absorbers: The Future of Perovskite Solar Cells? Adv. Funct. Mater. 29, 1806482 (2019). 2. Ortiz‐Cervantes, C., Carmona‐Monroy, P. & Solis‐Ibarra, D. Two‐Dimensional Halide Perovskites in Solar Cells: 2D or not 2D? ChemSusChem 12, 1560–1575 (2019). 3. Li, P. et al. Phase Pure 2D Perovskite for High-Performance 2D-3D Heterostructured Perovskite Solar Cells. Adv. Mater. 30, 1805323 (2018). 4. Yang, R. et al. Oriented Quasi-2D Perovskites for High Performance Optoelectronic Devices. Adv. Mater. 30, 1804771 (2018). 5. Straus, D. B. & Kagan, C. R. Electrons, Excitons, and Phonons in Two-Dimensional Hybrid Perovskites: Connecting Structural, Optical, and Electronic Properties. J. Phys. Chem. Lett. 9, 1434–1447 (2018). 6. Seitz, M. et al. Exciton diffusion in two-dimensional metal-halide perovskites. Nat. Commun. 11, 2035 (2020). 7. Seitz, M. et al. Mapping the Trap-State Landscape in 2D Metal-Halide Perovskites using Transient Photoluminescence Microscopy. Adv. Opt. Mat. (accepted 2021).

Resume : Two-dimensional materials such as, SnSe2, MoS2 and reduced graphene oxide (RGO) have attracted immense attention for various opto-electronic applications. Particularly, SnSe2 having promising photodetection properties like high absorption coefficient, tunable direct band gap, and high charge carrier concentration have low charge carrier mobility (~ 1-10 cm2/Vs). A heterostructure based on SnSe2/PEDOT: PSS has recently shown self-driven photodetection with low responsivity of (1.4 – 2.6) × 10-6 A/W [1]. This is mainly attributed due to low charge carrier mobility. To improve the result, here SnSe2 incorporated with RGO which has high charge carrier mobility (>1000 cm2/Vs). The device was fabricated by drop-casting solvothermal synthesized SnSe2-RGO composite on pulsed laser deposited MoS2. SnSe2-RGO/MoS2 heterostructure device shows excellent photoresponse with responsivity 12.5 A/W and detectivity 5.22 x 1012 Jones in self driven mode under IR light irradiation. A built-in voltage generated at SnSe2-RGO/MoS2 drives the device to operate in self driven mode. Ideality factor was estimated by power law I α Pθ, with θ = 0.64. It shows that effective charge carrier separation and transport is occurring at the bulk heterojunction interface. The rise and fall rates (10% - 90 %) at zero bias estimated as 0.10 s. The device also shows excellent photoresponse under visible light illumination. These results suggest that hybrid bulk heterojunction could be the new possible alternative for highly responsive self-powered photodetector. [1] Mukhokosi, E. P.; Krupanidhi, S. B.; Nanda, K. K. An Extrinsic Approach Toward Achieving Fast Response and Self-Powered Photodetector. Phys. status solidi 2018, 215 (21), 1800470.

Resume : Waste heat is one of the largest available sources of energy worldwide; therefore, harvesting this vast source helps alleviate the expanding energy demand foreseen for the coming years, especially in the rapidly growing field of Internet of Things (IoT) and Wireless Sensor Networks (WSN) for Smart and Green cities. Nanostructured thermoelectric (TE) materials have been a novel way to improve the Figure-of-Merit, ZT= S2σT/κ, due to their high lattice scattering. Hence, silicon nanowires (SiNWs) with diameters in the order of ~100 nm present a significantly reduced thermal conductivity κ and can be obtained with controlled morphology, crystallinity, and doping. Nonetheless, implementing NWs in devices that can benefit from their thermoelectric properties has been challenging, as they are highly susceptible to the troublesome effects of electrical and thermal contact resistances. In this work, we demonstrate how electrical and thermal contact resistances were suppressed due to the monolithic integration of the NWs into the microdevice, leading to efficient, scalable, and free from contact/parallel resistances of TE Si-based nanostructures that have not been demonstrated before. For this, we studied boron-doped <111> Si NWs with rough surfaces grown and monolithically integrated into micro-fabricated test devices through the bottom-up CVD-VLS approach. The electrical (σ) and thermal (κ) conductivities were measured in the very same single nanowire using the self-heating method. Thermal conductivities, as low as 10 W/K·m, were obtained thanks to the nanostructuration. Likewise, different precursors concentrations were used during CVD-VLS growth to optimize the relation between doping concentration and electrical conductivity. Finally, complementary Seebeck measurements of similar integrated NWs into specific devices that mimic the final application were used to compute the Seebeck coefficient. This way, full characterization (ZT) of the nanostructure is achieved in a wide range of temperatures, with values ranging from 0.02 at 300 K to 0.2 at 620 K.

Resume : 3D printing is gaining importance for the sustainable manufacturing of energy devices with superior efficiency and customizable design. The high mass loading of active material enabled by 3D structures ensures the storage of a large amount of charge per unit area, whereas the complex architectures enabled by the 3D printing process improve the charge transport kinetics, resulting in superior electrochemical performance. Among extrusion-based printing processes, Direct Ink Writing (DIW) presents the widest portfolio of printable materials, allowing the fabrication of multi-material devices in a single manufacturing step. However, the fabrication of 3D devices via DIW is bound to the formulation of printable inks. DIW inks must satisfy stringent rheological requirement to be printable; hence the formulation of DIW inks from 2D energy materials, such as graphene and TMDs, is often challenging. For this reason, the DIW of supercapacitors and batteries has so far largely centered on reduced graphene oxide (rGO), presenting hydrophilic moieties which ensure a suitable rheology in concentrated aqueous dispersions. However, devices based on rGO require complicated post-print processes involving high temperatures and hazardous chemicals. Additionally, the structure of GO remains highly defective even after reduction, affecting its electrical properties. Here, the fabrication of 3D printed supercapacitors based on pristine graphene (PG) platelets is demonstrated. PG inks have a superior electrical conductivity to rGO inks and do not require harsh post-print processing, allowing the sustainable fabrication of printed supercapacitors with ultra-high areal capacitance. The pristine graphene structures are also promising as scaffolds for the growth of pseudocapacitive and battery-like materials, which would further extend the application of PG inks to pseudocapacitors and batteries.

Resume : Catalysts undergo poisoning and degradation during their utilisation in many different fields. One energy technology that features prominent catalyst degradation is the hydrogen bromine redox flow battery (H2-Br2 RFB). H2-Br2 RFBs are a cheap and efficient solution to large scale energy storage. The main hinderance of the technology is the poisoning of the hydrogen evolution reaction/hydrogen oxidation reaction (HER/HOR) catalyst by bromine species which have crossed over the proton exchange membrane. Without a revolution in membrane development, this crossover appears unpreventable. Therefore, we sought to protect the catalyst locally. Single-walled carbon nanotubes (SWCNTs) have previously been shown to block diffusion of larger anions into their internal cavities, whilst still allowing the diffusion of water and protons. This work sought to harness this effect to protect an encapsulated catalyst from bromide and tribromide, which would usually poison and corrode the catalyst. Platinum nanoparticles were synthesised within the internal cavities of small diameter SWCNTs, through a simple impregnation and drying procedure. High resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM) and scanning tunnelling electron microscopy (STEM) were used to characterise the particles and demonstrate that they were confined within the SWCNTs. The typical hydrogen under potential deposition peaks were observed on a cyclic voltammogram of the sample, whereas the oxide region was heavily suppressed compared to Pt/C. Some diffusion limitation was observed during the HOR, indicating that the electrolyte diffusion pathway is through the SWCNTs, but the same mass transport limited current was attained. The oxygen reduction reaction (ORR) mass transport limited current was much lower than expected for Pt (2 mA cm-2), indicating a selectivity for hydrogen over oxygen. The stability of these platinum nanoparticles in the presence of Br-/Br3-solution was vastly increased compared to the standard 50% Pt/C catalyst, shown by x-ray photoelectron spectroscopy (XPS) and electrochemistry. It is proposed that this effect is caused by steric and electrostatic repulsion of the large tribromide ion by the SWCNT cavity (internal diameter of 2 nm). Density functional theory (DFT) was used to study the stability of H , H2, Br- and Br3- depending on their position relative to a SWCNT. The calculations revealed that H and H2 were not affected by the SWCNT, whereas Br- and Br3- suffered a severe energy penalty upon entrance into the SWCNT. The encapsulated platinum also featured a vastly higher mass activity when cycled in a cell, indicating better Pt utilisation due to the small particle size. This opens a new route for imparting selective access to active sites of a catalyst, hence increasing the stability of the catalyst, a potential solution to many problems faced by technologies that rely on catalysts.

Resume : Recent research has shown the the stabilization of a hexagonal phase known as ω in steel samples, demonstrated to be aided by the presence of C in TEM measurements. The presence of large amounts of C, therefore can lead to the degradation of the steel samples containing ω-phase, through surface adsorption. We investigated the adsorption mechanism of CO2 on ω-Fe(0001) surface, considering various adsorption sites and found that it strongly adsorbs horizontally with a bent configuration. The adsorption is assisted by significant charge transfer from the surface Fe atoms to the CO2 molecule, also evidenced by structural modification of the molecule. Electronic structure analysis showed that hybridization and subsequent charge transfer is probable between the d orbitals of Fe and p orbitals of CO2, resulting in strong chemisorption, that further leads to spontaneous dissociation of the molecule.

Resume : The application of graphene for energy conversion applications often requires functionalization via covalent binding or adsorption to tune the material’s electronic properties. To achieve a uniform functionalization, the local surface properties of the pristine graphene can be decisive. Therefore, real-space imaging of the graphene topography as well as mapping the electronic properties with a nanometer resolution is essential. Atomic force microscopy (AFM) not only visualizes surface topography, but can additionally resolve functional properties, such as the surface potential and work function, adhesion and elasticity. Here, we investigated electronic and mechanical properties of a CVD grown wafer scale graphene film on sapphire via functional AFM techniques. In particular, using sideband Kelvin probe force microscopy (KPFM) we were able to resolve a heterogeneous surface potential distribution with distinct contrast between the bulk film, graphene wrinkles as well as on the underlying sapphire terraces and step edges. The surface potential distribution correlated with the mechanical behavior, including deformation and adhesion, which gave a distinct contrast on the same features, resolved via PinPoint nanomechanical AFM. The correlation of the surface potential and the sample’s nanomechanical response likely originates from delamination. Especially the local change of the surface potential has to be considered when functionalizing the graphene surface for energy applications.

Resume : The discovery of graphene has caught the attention of the scientific community and driven substantial research efforts for the investigation of other two-dimensional (2D) layered atomic crystals with a wide range of electronic and optical properties ranging from metallic to semiconducting structure Unlike graphene, 2D transition metal dichalcogenides (TMDs) possess tunable band gaps ranging from visible to near-infrared (between 1-2 eV). These appealing properties are crucial for building optoelectronic devices, such as single-layer transistors or photodetectors. Moreover, beyond structures consisting of atomically thin TMDs, heterostructures based on 2D TMDs with various bandgaps can serve as building blocks for future lightweight multi-junctional solar cells, light-emitting diodes, double heterojunction lasers, photodetectors, and other optoelectronic applications. Tungsten disulfide (WS2) is an intriguing representative of the semiconducting TMDs. Unlike its bulk counterpart, monolayer (1L) WS2 exhibits a direct bandgap of ~2 eV, as demonstrated by both theoretical and experimental studies. 1L-WS2 provides a convenient platform to design light emission from 2D confined excitonic systems due to the strong light absorption and emission at visible wavelengths that opens great opportunities for its integration in ultrathin optoelectronic devices. Here, we report on the reproducible and highly controllable two-step synthesis process of 1L-WS2 via high-temperature sulfurization in a sulfur-rich atmosphere of non-stoichiometric tungsten oxide films obtained by pulsed laser deposition (PLD) on atomically smooth sapphire. This approach yields 1L-WS2 with roughness and photoluminescence properties comparable to monolayers grown by chemical vapor deposition (CVD). Our results reveal that by changing the growth conditions in PLD, the stoichiometry of the epitaxial WOx films can be significantly varied. Moreover, we will show that the composition of the tungsten oxide films can have a significant impact on the quality of the 1L-WS2, and this was carefully examined by XPS, Raman spectroscopy, PL quantum yield measurement, SEM, AFM, and TEM analysis.

Resume : Layered van der Waals (vdW) materials, consisting of atomically thin layers, are of paramount importance in physics, chemistry, and materials science owing to their unique properties and various promising applications. However, their fast and large‐scale growth via a general approach is still a big challenge, severely limiting their practical implementations. Here, we report a universal method for rapid (~60 min) and large‐scale (gram scale) growth of phase‐pure, high‐crystalline layered vdW materials from their elementary powders via microwave plasma heating in sealed ampoules. This method can be used for growth of 30 compounds with different components (binary, ternary, and quaternary) and properties. The ferroelectric and transport properties of mechanically exfoliated flakes validate the high crystal quality of the grown materials. Our study provides a general strategy for the fast and large‐scale growth of layered vdW materials with appealing physiochemical properties, which could be used for various promising applications.

Resume : Two-dimensional transition metal dichalcogenides (2D-TMDCs) offer a new generation of active material for a variety of electrochemical applications, such as hydrogen evolution.[1-3] However, the 2D configuration of the material distorts the electronic properties compared to bulk counterpart, as well as being highly heterogeneous with terraces, edges, and different layer-thickness all contributing to the electrochemical performance.[4] In this presentation the influence of the layer thickness (monolayer, bilayer and trilayer) of 2D MoS2, MoSe2, WS2, WSe2 on the reduction of a simple outer sphere redox couple will be reported. We use a scanning electrochemical cell microscopy (SECCM) approach to conduct localized electrochemical measurements on pristine surface of 2D TMDCs. Coupling electrochemical kinetics using a Gerischer formularization and diffusional mass transfer by finite element simulation, we determine the rate of electron transport for each layer thickness. A tunneling barrier model, which includes subtrate/TMDCs and TMDCs/electrolyte interface effects, is suggested to explain the kinetic variations we observe. The simplicity of the redox probe chosen allow to differ fundamental insight of electron transfer processes in 2D-TMDCs, which play a very relevant role in its electrochemical use for energy storage applications.

Resume : The synthesis of two-dimensional graphene-like gallium nitride (g-GaN) allows extensive applications in nanoelectronics devices as added quantum confinement permits further modulation of its optoelectronic properties. In this work, we examine the stability, electronic, and optical properties, as well as the stacking, biaxial strain, and external electric field effects on g-GaN multilayers using van der Waals-corrected density functional theory calculations. Monolayer g-GaN is predicted to be thermodynamically stable with an indirect wide bandgap, which makes it interesting, especially for optoelectronic applications. The particular stacking order of g-GaN monolayer can construct bilayer, trilayer, and multilayers van der Waals crystals. They can present unique characteristics that can be regulated by g-GaN layers number. Mainly, their bandgap diminishes with an increased g-GaN layers number. Application of both strain and the electric field effectively modify the bandgap and carrier effective mass of the studied g-GaN multilayers. Furthermore, the biaxial strain application can notably redshift the g-GaN monolayer' optical spectra into the visible-light region, thereby expanding the light-harvesting range. Such a stacking-dependent tunability of the optoelectronic properties in the presence of an applied electric field and strain engineering uncovers opportunities for new technological applications at the nanoscale.

Resume : Over the last several decades, there has been a sustained research activity in the area of anomalous heat conduction, which has resulted in the introduction of number of models for the description of this phenomena. One of the prospective approaches is the covariational analysis of lattice equations of motion in the harmonic approximation with the nearest-neighbor interaction. This method allows to derive analytically the equation which describe dynamics of the kinetic temperature perturbations caused by the ultrafast heating. The process described by this equation corresponds to purely ballistic heat conduction. In an experiment it can be achieved, for example, with a femtosecond laser excitation at cryo-temperatures. Limitations of aforementioned analytical approach are associated with anharmonicity of interaction, which may not influence the process at low but manifests itself at higher temperatures. This study aims to perform numerical simulation and to determine how background temperature influence the process of heat conduction, analyze the influence of interatomic potential, test the limitations of analytical approach and to compare the results of simulation with available experimental data. Graphene was chosen as a material of investigation, because it has an interesting for analytical analysis crystal lattice geometry, a number of empirical potentials have been developed and fitted for carbon, the technological achievements made it possible to obtain defect-free samples which were used in available experimental investigations. The classical molecular dynamics method is used for simulation. Two problems are considered. Ultrafast heating of a circular region on a graphene surface and relaxation of a sinusoidal profile. Four nonlinear potentials are used for modeling, Tersoff 1989, Tersoff 2010, LCBOP and AIREBO. Results of simulation show that all anharmonic potentials lead to temperature profiles which qualitatively correspond to what the analytical harmonic model predicts, which is also an indication of ballistic heat transport. There is a quantitative difference between potentials. Each of potential also quantitatively differ from the analytical harmonic model. With increase of the temperature the qualitative correspondence with harmonic model vanishing. Molecular dynamic simulation results with nonlinear potentials suggests that the harmonic model is a promising candidate for the description of thermal processes at very low temperatures in real materials. For higher temperatures it is necessary to take into account the nonlinearity. The results obtained in low temperature range both from simulation and analytics are both in qualitative agreement with the available experimental data.

Resume : We will present the thermoelectric properties of free-standing films of electrochemically exfoliated graphene oxide (EGO). The synthesis of our films consisted of electrochemical intercalation of perchlorate anions and acetonitrile in graphite, the expansion of the graphite using microwaves to evaporate the trapped acetonitrile, followed by electrochemical exfoliation in DMF and deposition on Nylon filters which were subsequently removed to obtain free standing films. EGO films of different thicknesses and different levels of oxidation were characterised. Raman spectra revealed similar spectra for all samples before and after reduction treatment with an I_D/I_G= 1.2. XPS spectra showed clear indications of reduction from the deconvolution of the C1s peak showing an increase in sp2 carbon during the reduction process. The electrical conductivity at room temperature was 15 S/cm before reduction and 240 S/cm after reduction. The Seebeck coefficient was measured between 100K-400K allowing us to identify the material as p-type with room temperature Seebeck coefficient of ~31μV/K. The room temperature power factors were 1 μW/mK^2 and 23 μW/mK^2, prior and after reduction treatment. Lower density films yielded a larger power factor of 48 μW/mK^2. These power factors are the highest seen in the literature for p-type EGO films. In addition, this performance was achieved for film thicknesses >10 microns demonstrating a potential for scalability for TE applications

Resume : The past decade witnessed several developments on 2D graphene analog, also known as Xenes, and significant milestones have been achieved on material synthesis and their application prototypes[1]. Contrarily, piling up Xene layers on top of each other to realize a Xene heterostructure remains currently an unresolved issue. Indeed, even if two different Xenes can be grown on the same substrate, their stacking remains quite puzzling as matching between the two materials is requested in order to avoid clustering and amorphization. Making Xene heterostructures in this framework therefore will provide substantial technological throughputs for various applications, for instance nanoelectronics, optoelectronics, plasmonic, and energy. We present two Xene heterostructures based on two well-established configurations: silicene-on-Ag(111)[2] and stanene-on-Ag(111)[3]. We demonstrate that one Xene layer can act as a suitable template for the other reciprocal Xene layer. The insights of deposition mechanisms, in addition to the influence of epitaxial parameters on two heterostructures will be discussed and detailed with various in situ and ex situ probes. In addition Density Functional Theory calculations, carried out taking experimental data as starting input models, will be presented in order to give a deeper insight on structural stability and electronic properties of the two synthesized Xene heterostructures. 1. Tao, L. et al. Silicene field-effect transistors operating at room temperature. Nature Nanotechnology 10, 227–231 (2015). 2. Vogt, P. et al. Silicene: Compelling experimental evidence for graphenelike two-dimensional silicon. Phys. Rev. Lett. 108, 1–5 (2012). 3. Yuhara, J. et al. Large area planar stanene epitaxially grown on Ag(1 1 1). 2D Mater. 5, (2018).

Resume : The cathode microporous layer (MPL), as one of the key components of the proton exchange membrane fuel cell (PEM-FC), requires specialized carbon materials to realize the two-phase flow and inter-facial effects. In this respect, we designed a novel MPL based on super-hydrophobic nitrogen doped graphene nano-walls (GNW). Employing radio-frequency plasma deposition techniques directly on carbon paper we produced high quality microporous layers at competitive yield-to cost ratio with distinctive MPL properties: high porosity, good stability, considerably durability, superhydrophobic and substantial conductivity. Platinum-ink, serving as fuel cell (FC) catalyst, was directly sprayed on the MPLs and incorporated in the FC assembly by hot-pressing against a polymeric membrane. The integrated PEM-FCs were tested in a single cell PEM-FC on a BT-112 Single Cell Test System, showing power performance comparable to industrial quality membrane assemblies (330 mW·cm-2), with elevated working potential (0.95 V) and impeccable fuel crossover for a low-cost system resulting from a highly scalable, inexpensive, and rapid manufacturing method. Key-words: graphene nano-walls, plasma deposition, microporous layer. Acknowledgements: This work was financially supported by UEFISCDI Romania, in the frame of the projects: PN-III-P1-1.1-PD-2019-0684 contract 106PD/2020, PN-III-P1-1.1-PD-2019-0540 contract 103PD/2020.

Resume : Improve energy conversion is a major societal concern, particularly in nano-electronics. In this domain, the discovery of 2D materials has opened new routes of investigation. The thermoelectric efficiency at a given temperature, ZT, is the relevant parameter for applications that researchers struggle to increase. It depends on the material Seebeck coefficient (S) and on its electrical (sigma) and thermal (k) conductivity. In this context, transition metal dichalcogenides (TMDs) represent emergent materials to explore. First measurements on isolated TMDs have revealed very high S and low k values, particularly interesting for increasing ZT. Our recent research activity aims to unveil the potential of TMD inserted in van der Waals (vdW) heterostructures for energy conversion. Here I will present the fabrication of hBN-supported WSe2 (4-6L) for thermoelectric investigations. The choice of the metallic contact plays a fundamental role in determining the charge injection by the introduction of localized energy states at the conduction and valence band edges. By using a local gate underneath the hBN flake, we have measured the Seebeck coefficient of WSe2 with Ti, Ag, Co and Pd contacts in the hole and electron transport regime. We reveal particularly high value of S, up to 900 µV/K. Combining the Seebeck coefficient with charge transport measurements, we extract power factors (PF=S^2sigma) as high as 30 µW/cm K^2. Combined with the low in-plane WSe2 thermal conductivity (1-2 W/mK), such high values allow to foreseen maximum ZT higher than 1, revealing the great potential of TMD assembled in van der Waals heterostructures for energy conversion at the nanoscale.

Resume : 2-dimentional (2D) nano sized graphite carbon nitride (g-C3N4) has become an excellent material for energy strorage and optoelectronics application in recent years. Graphite carbon nitride contains high C/N ratio. It is gaining considerable attaention for electrochemical energy storage and biosensing due to metal free characteristic, high surface area, nitrogen rich frame work and low cost. Graphite carbon nitride accomodates high number of nitrogen fuctional group which is enhanced the electrical conductivity as well as specific capacitance. Graphite carbon nitride have been recently proposed as an alternative to carbon based graphene structures in supercapacitor electrode applications. In this paper, we report a low cost pyrolysis synthesis protocol forming melanin to graphite carbon nitrid , which shows high surface area and tunable pore structure. The detailed characterization of graphite carbon nitride is performed using XRD, SEM, TEM, Raman, FTIR and BET charaterization techniques. The electrochemical properties were investigated by cyclic voltammetry and charge-discharge measurements in 3 M KOH, clearly indicating improved cyclic stabilities upto 2500 cycles, with specific capacitance of 84 F g-1 at 1 A g-1 current density. A symmetric supercapacitor, has also been fabricated, using this material as active electrode, which shows ~ 11Wh Kg-1 energy density at a power density of 500 W Kg-1.These kind of electrodes can be utilized in biosensing application. Graphite carbon nitride electrode materials were also used as electrochemical sensor for the detection of hydrogen peroxide. The result of cyclic voltammetry and chronoamperometric experiments are discussed which showed that the linear response ranges are 0-80 nM, with a detection limit of 0.42 nM.

Resume : The nonlinear heat devices are the potential game-changer in excess heat removal or energy harvesting using the waste heat produced by electronics and industrial processes. One of the examples is the thermal rectifier that enables preferential heat transport in one direction. Despite the new materials the mature technologies such as silicon or silicon on insulator (SOI) remain the first choice in robust and miniaturized devices production, mostly used in high-temperature environments. In this work, we report on single-material thermal diode operating at high temperatures. We used silicon membrane to fabricate the structure with spatially asymmetric and temperature-dependent thermal conductivity. We achieved the rectification of about 14% in vacuum. Furthermore, we demonstrated air atmosphere could enable additional features of the device, i.e., thermal switching and passive cooling. Reference: M. Kasprzak et al, High-temperature silicon thermal diode and switch, Nano energy 78, 105261 (2020) The authors acknowledge the support from: Polish National Science Centre (Sonata UMO-2018/31/D/ST3/03882 and Preludium UMO-2019/33/N/ST5/02902)

Resume : Recently, the use of graphene layer, as an efficient absorber substrate on active metallic conductors like silver, gold, has been intensively considered for designing optoelectronic sensors with high sensitivity. It was demonstrated both numerically and experimentally that a significant electromagnetic induced transparency (EIT) effect can be realized due to the variation of graphene layer. In the aim of increasing the sensitivity of the previous phenomenon, graphene on MoS2 were already been reported in several papers. In this contribution, we focus on the use of other materials to enhance the surface sensitivity of a designed biosensor that offers the ability of sustaining electromagnetic field generated during the coupling concept. To solve the sensitivity issue in preferential conditions, generally required in biosensing, various arrangements between a polar material, metamaterial layer and a dielectric gap (SiO2) were investigated. The optical response from asymmetric nanostructures was simulated by the transfer matrix approach. Thus, to estimate the overall limits of the sensitivity, deduced from the analysis of angular-spectra probed through the sensing structure, an operating wavelength of 815nm of TM-polarization light is considered. In this theoretical analysis, the whole of thicknesses of the media are turned in the subwavelength scale and the sensitivity was determined on both the change of refractive index of the sensing medium, I (surrounded by active materials) and its layer thickness. Finally, according to the obtained results described on the response of resonant phenomena, the multilayer sensor based on the property of polar material, metamaterial layer, and the dielectric gap, can be used as a sensing platform to probe interface phenomena due to its highest sensitivity and detection accuracy.

Resume : Graphitic-carbon nitride (gCN) has attracted much attention in the past few years as an interesting platform to host different catalytic processes such as CO2 reduction or water splitting, just to mention two of the most important ones in the field of environmental or energy applications. Indeed, the main reason for such an interest is represented by its layered structure, where each sheet is composed of regularly polymerized monomers and, consequently, regularly spaced holes: such voids expose N-rich borders, which are the driving motif of the great reactivity of the material. Moreover, gCN can be synthesized through many different pathways, possibly benefiting from large-scale or industrial level techniques. However, as several experimental works have already pointed out, different configurations can be obtained, which correspond to similar stoichiometries of C/N atoms, but whose actual structure can largely influence the reactivity of the material. In this context, a thorough characterization of the phases contained in a sample is paramount if one aims at an accurate description of the catalytic processes. In our work, we have theoretically investigated the atomic and the electronic structure of different gCN allotropes from a DFT standpoint and compared our results with the experimental findings from a very common pathway: the condensation of melamine (C3N3(NH2)3) precursor into melem (C6N7(NH2)3) and the subsequent polymerization of the monomers controlled by the temperature. We take into account not only pristine phases, but also modified systems containing H and O adatoms or NH2 radicals, which could be present during the synthesis. We have analyzed the optimized structures for the 2-dimensional gCN allotropes by constructing their XRD spectra and explored their electronic configurations through both simulated XPS and NEXAFS spectroscopies. Through this characterization study we could identify the fingerprints of each phase considered. Finally, together with our experimental collaborators, we investigated the incorporation of Cu adatoms in the gCN network and found again that the presence of different phases plays a crucial role.

Resume : Semiconducting transition metal dichalcogenides (TMDs), such as MoS2, MoSe2, WS2 and WSe2, have recently gathered a lot of interest for applications in electronics (ultrathin body transistors), optoelectronics (photodetectors) and energy (photovoltaics, water splitting,..). As a matter of fact, methods for the production of monolayer or few layer TMDs on large area are needed for the development of devices . However, TMD films obtained by the state-of-the-art deposition methods, such as chemical vapor deposition, still suffer from a lower electronic quality as compared to flakes obtained by mechanical exfoliation from bulk crystals. To overcome the main limitation of the exfoliation method, i.e. the small (micrometer) lateral size of the flakes and their thickness inhomogeneity, in the last years several groups developed the “gold-assisted” exfoliation to separate extended (cm^2) monolayer films of MoS2 from molydbenite stamps, by exploiting the strong affinity between Au substrates or deposited layers and the topmost S atoms of MoS2 [1,2]. The thickness uniformity of the exfoliated MoS2 was found to be crucially dependent on the cleanness and planarity of the gold surface [2]. Furthermore, the strong Van der Waals (VdW) interaction with Au strongly influences the doping, strain and photoluminescence (PL) yield of exfoliated monolayer MoS2 [2]. In this work, we report a systematic characterization of large area MoS2 films exfoliated on Au/Ni/SiO2 samples with optimized Au surface roughness (Root Mean Square=0.2 nm). By combined application of optical microscopy, micro-Raman (uR) spectroscopy and atomic force microscopy (AFM), a calibration curve correlating the number of MoS2 layers with the positions and separation of characteristic Raman vibrational peaks (A1g and E1g) was obtained for the specific case of MoS2/Au, and it has been used for rapid identification of the exfoliated layer thickness. Furthermore, uR and microPL mapping of monolayer regions with high statistics allowed to evaluate the doping and strain distribution (by correlative plots of the A1g and E1g positions), as well as their impact on local PL spectra. Finally, nanoscale resolution current mapping and local I-V measurements by conductive-AFM permitted to get insight in the current injection mechanisms at monolayer MoS2/Au interface and to evaluate key electrical parameters, such as the Schottky barrier height [3]. All these information will be crucial for perspective energy applications of the MoS2/Au system. [1] S. B. Desai, et al., “Gold-Mediated Exfoliation of Ultralarge Optoelectronically-Perfect Monolayers”, Adv. Mater. 28, 4053−4058 (2016). [2] M. Velický, et al., “Mechanism of Gold-Assisted Exfoliation of Centimeter-Sized Transition-Metal Dichalcogenide Monolayers ” ACS Nano 12, 10463−10472 (2018). [3] F. Giannazzo, et al., “Conductive atomic force microscopy of semiconducting transition metal dichalcogenides and heterostructures”, Nanomaterials 10, 803 (2020).

Resume : Layered TMDCs, particularly molybdenum disulfide (MoS2),is envisioned as future material owing to their remarkable properties such as indirect bandgap, sustainability to high power, and wide spectral response, thus reveal the possibilities for next-generation electronic and optoelectronic applications. Since multilayer MoS2 consists of strong in-plane bonding (covalent/ ionic) and weak out of plane bonding (Van der Waals), the cleaved surface is expected to have no dangling bond, thereby raising the prospects for devices of cleaner interface due to low defect density. Recently, MoS2|ZnO heterostructure received a special interest in photocatalysis application. However, the electrical properties of such interface on the basis of electrical characterization have not been understood. In this work, an attempt has been made to understand the effect of band bending and interface processes in MoS2|ZnO heterostructure.A specifically designed Au|multilayer MoS2| ZnO (n-n ) isotype device structure was fabricated with mechanically exfoliated MoS2 flake of thickness ~10um. We have systematically studied the electrical properties of fabricated n-n heterostructure device by temperature-dependent current-voltage characteristics. Measured temperature-dependent current-voltage characteristics show a sudden rise in current density at a particular bias for every temperature and reflectinstability in flat band voltage. This signifiesthat, with decreasing temperature, the upward movement of Fermilevel leads to deionization of shallow levels at the heterostructure and controls the flat band instability. Once flat band is achieved, the band bending at the heterostructure controls accumulation or depletion at the heterostructure. Here at every temperature, a threshold voltage (forboth positive and negative applied bias) is required to make heterostructure ohmic.These results reveal that the band bending and impurity present in the MoS2 flake strongly controls the electrical properties of theMoS2| ZnO n-n isotypeheterostructure device. These results will further pave the way for engineering and design strategies for multilayer MoS2|ZnO based electronic and optoelectronic devices.

Resume : Two-dimensional transition metal chalcogenides (TMDCs) exhibit excellent optical properties. This has encouraged the solar cell community to combine TMDCs with organic semiconductors to form heterostructures with tailorable properties that possess advantages of both materials [1, 2, 3]. Recently, reasonable photovoltaic performance has been demonstrated for hybrid TMDC/Organic semiconductor devices [4]. Here, we study the excitation dynamics of bilayer samples of monolayer MoS2 and α-NPD upon excitation of the TMDC. From the photoluminescence studies, we conclude efficient exciton dissociation due to. In parallel, ultrafast transient absorption spectroscopy shows significant exciton-exciton interactions in the neat MoS2, which is fully suppressed in the bilayer. These findings suggest rapid charge transfer between the MoS2 monolayer and α-NPD occurs. Interestingly, the decay of the charge-induced signal in the TAS is independent of the excitation fluence. This indicates that the interfacial electron-hole pair remains bound and that it undergoes geminate recombination. These preliminary results serve a nice platform for optimization of the bilayer towards realization of efficient solar cells. References: [1] Z, Chengmei, et al., journal of physical chemistry letters 9, 10 (2018). [2] K, Tika R., et al. Journal of the American Chemical Society 141, 28 (2019). [3] T. R. Kae, et al., ACS nano 17, 164 (2017). [4] T. A. Shastry, et al., ACS nano 10, 10573 (2016).

Resume : Topological insulators are materials that are insulators in bulk however, conductors on its surface. The electric current circulating in a topological insulator does not suffer any loss of energy. This property opens great possibilities of application in electronics, since it would enable the fabrication of more efficient, faster and low-energy consumption devices. For technological applications one of the challenges has been the creation of a magnetic topological insulator. Magnetic topological insulators usually are created by the so-called extrinsic route, which consists of doping nonmagnetic topological insulators with magnetic atoms. Recently the first intrinsic magnetic topological insulator, with chemical formula MnBi2Te4 was predicted theoretically (1). In this study MnBi2Te4 crystals are prepared by using High Temperature Solution (HTS) method. Millimeter-sized MnBi2Te4 single crystals are successfully grown and characterized using EDS, XRD, Raman spectroscopy and SEM. The elemental composition close to the stoichiometric one was obtained by EDS. The as-grown MnBi2Te4 single crystal exhibits layered structure, which is composed of a septuple Te-Bi-Te-Mn-Te-Bi-Te sequences as determined by powder X-ray diffraction and SEM. Eg and A1g Raman active modes typical for MnBiTe compounds are observed. Magnetic and transport properties are measured on crystals and exfoliated film samples. Acknowledgement: This research was supported by the European Union’s Horizon 2020 research and innovation programme FETPROACT, under grant agreement No. 824140 Reference: 1. M.M. Otrokov et al. Prediction and observation of an antiferromagnetic topological insulator. Nature 576, 416–422 (2019)

Resume : Atomically thin two-dimensional (2D) materials are promising candidates for electronic, optoelectronic, and sensing applications. Due to quantum confinement, 2D layered materials exhibit several novel optical and electronic properties compared to bulk semiconductors. They show the absence of surface dangling bonds and defect states that lead to intrinsically weak charge carrier scattering as well as bandgap that is controlled by the number of atomic layers. 2D materials are used in easy-to-fabricate van der Waals heterojunctions with enhanced photodetection and photovoltaic properties and in performant (photo)transistors immune to short channel effects due to their ultrathin nature. The large surface to volume ratio enhances the interaction with light and chemical molecules favoring their use in sensors. In this talk, several applications of 2D materials in transistors, sensors, and field emitting devices are reported. The focus is on the wide family of transition-metal dichalcogenides (TMDs), such as MoS2, WSe2, PdSe2, and PtSe2, as well as on GeAs and BP. Nanosheets of TMDs, obtained by either mechanical exfoliation or chemical vapor deposition on SiO2/Si substrates, will be used to discuss electric transport, modulation of the conductivity by a back-gate, photoresponse, effect of electron irradiation, and the role of surface adsorbates. It will be shown that light and electron irradiation cause photoconductive and photogating effects, which might result in both positive and negative photoconductivity. Electron irradiation will be exploited to reduce the Schottky barrier at the contacts and improve the TMD/metal contacts. It will be highlighted how adsorbates can change the polarity of the charge-carriers and enhance the hysteresis in the transfer characteristics of TMD-based field-effect transistors. The dominant n-type behavior in a high vacuum and the sharp-edge geometry, as well as the presence of defects, facilitate the extraction of electrons (field emission) from 2D materials upon application of an electric field. It will be shown that TMDs are effective field emitters and that their emission current can be modulated by a back-gate. The concept of a new transistor based on field emission will be proposed and demonstrated. Finally, the temperature dependence of the anisotropic conduction of GeAs nanosheets and the origin of an unexpected peak at 75 K in the carrier density per area will be discussed. References 1. F. Urban et al. Isotropic conduction and negative photoconduction in ultrathin PtSe2 films, Applied Physics Letters 117 (2020) 193102 2. A Pelella et al. Electron irradiation of metal contacts in monolayer MoS2 Field-Effect Transistors ACS Applied Materials and Interfaces, 12 (2020) 40532 3. A Di Bartolomeo, et al. Electron irradiation on multilayer PdSe2 field effect transistors Nanotechnology 31 (2020) 375204 4. A Di Bartolomeo et al. Field emission in ultrathin PdSe2 back-gated transistors Advanced Electronic Materials, 6 (2020) 2000094 5. A. Di Bartolomeo Emerging 2D Materials and Their Van Der Waals Heterostructures Nanomaterials 2020 (2020) 579 6. F. Urban et al. Gas dependent hysteresis in MoS2 field effect transistors 2D Materials 6 (2019) 045049 7. A. Di Bartolomeo et al. Pressure?Tunable Ambipolar Conduction and Hysteresis in Thin Palladium Diselenide Field Effect Transistors Advanced Functional Materials 29 (2019) 1902483 8. F. Giubileo et al. Effect of Electron Irradiation on the Transport and Field Emission Properties of Few-Layer MoS 2 Field Effect Transistors The Journal of Physical Chemistry C 123 (2019) 1454 9. A. Di Bartolomeo et al. A WSe2 vertical field emission transistor Nanoscale 11 (2019) 1538 10. A. Di Bartolomeo et al. Asymmetric Schottky Contacts in Bilayer MoS2 Field Effect Transistors Advanced Functional Materials 28 (2018) 1800657 11. A. Di Bartolomeo et al. Hysteresis in the transfer characteristics of MoS2 transistors 2D Materials 5 (2018) 015014 12. A. Di Bartolomeo et al. Electrical transport and persistent photoconductivity in monolayer MoS2 phototransistors Nanotechnology 28 (2017) 214002

Resume : The solar-driven conversion of carbon dioxide (CO2) with water into fuels has been regarded as an artificial photosynthesis process for the development of the renewable energy and addressing the global warming. In this work, we report a novel approach to convert CO2 into renewable fuels by atomically tuning graphene/3C-SiC Schottky junction as a photoelectrode [1]. High-quality and uniform graphene layers were epitaxially grown on cubic silicon carbide (3C-SiC) surface. 3C-SiC has a relatively small bandgap of 2.36 eV, which is favorable for visible sunlight absorption and is close to the hypothetical ideal bandgap (2.03 eV) of a single material for a maximum of the solar water splitting efficiency [2]. Most importantly, the conduction and valence band positions of 3C-SiC ideally straddle the water redox potentials, indicating that the photogenerated carriers have enough energy to overcome the energetic barrier of water splitting. The Schottky junction between graphene layers and 3C-SiC with a tunable barrier and the built-in electric field can be precisely controlled by atomically tuning the number of graphene layers [1]. The tuned graphene/3C-SiC Schottky junction was demonstrated to promote charge separation and transport in a photoelectrochemical CO2 conversion system for solar-to-fuel conversion under a low bias. Moreover, we demonstrate that the monolayer graphene with an extremely high conductivity facilitates the charge transfer towards the loaded co-catalyst and protects the surface of 3C-SiC from photo-corrosion, thus achieving a synergetic enhancement of the efficiency and stability [1]. References: [1]. Hao Li, Yuchen Shi, Huan Shang, Weimin Wang, Jun Lu, Alexei A. Zakharov, Lars Hultman, Roger I. G. Uhrberg, Mikael Syväjärvi, Rositsa Yakimova, Lizhi Zhang, and Jianwu Sun*, ?Atomic-Scale Tuning of Graphene/Cubic SiC Schottky Junction for Stable Low-Bias Photoelectrochemical Solar-to-Fuel Conversion?,ACS Nano, 14, 4905?4915, (2020). [2]. Murphy, A. B.; Barnes, P. R. F.; Randeniya, L. K.; Plumb, I. C.; Grey, I. E.; Horne, M. D.; Glasscock, J. A., ?Efficiency of solar water splitting using semiconductor electrodes.? Int. J. Hydrog. Energy, 31, 1999-2017, (2006).

Resume : Transition metal dichalcogenides (TMDCs) are large family of layered materials that show unique properties (different from their bulk counterparts) when thinned down to nanoscale thicknesses. TMDCs with stable metallic and semi-metallic phases (e.g., the 1T or 2H phase of TaS2, 1T-TiSe2, 2H-NbSe2, and Td-WTe2) are attractive for investigations of charge density waves (CDW), superconductivity, magnetoresistance, and other topological phases of mater [1]. The preparation of WTe2 as monolayer or few layers by MOCVD and CVD is found to be challenging, therefore a bulk single crystal method is sought in order to obtain crystal samples with good quality. In this study WTe2 single-crystals were synthesized through Chemical Vapor Transport (CVT) method using Br2 as transport agent. WTe2 orthorhombic Td structure is confirmed by single crystal and powder X-ray diffraction. The structure is further verified by Raman spectroscopy analyses. Layered morphology and elemental composition stoichiometry are observed by SEM and EDS respectively. Acknowledgement: This research was supported by the European Union?s Horizon 2020 research and innovation programme FETPROACT, under grant agreement No. 824140 Reference: 1. H. Li et al. Epitaxial Growth of Two-Dimensional Layered Transition-Metal Dichalcogenides: Growth Mechanism, Controllability, and Scalability. Chem. Rev. 118, 6134-6150 (2018)

Resume : Transition metal carbides have attracted significant attention in electrocatalysis applications, especially as electrodes in hydrogen evolution reaction (HER). In this talk, we will present results considering the controlled synthesis of ultrathin Mo2C films and graphene/Mo2C vertical heterostructures, on liquid catalysts via chemical vapor deposition. We demonstrate experimental results on the role of graphene as a diffusion barrier, resulting in the growth of Mo2C crystals with a smaller size and higher nucleation density [1]. We use Cu-based alloys, like Cu-Sn, as growth substrates, which enable the growth in reduced temperatures, with respect to pure Cu. This alloy has a lower melting point, which is an obligatory condition to enable Mo diffusion. Moreover, we present results on the enhanced HER activity of the graphene/Mo2C compounds, thanks to graphene’s contribution to faster charge kinetics [2]. Then, we discuss on the increased electrocatalytic performance of the heterostructure when we apply a ´´flipped´´ transfer method which permits to reverse the vertical order of the heterostructure, allowing Mo2C to stand on the graphene and preventing the function of the latest as an electrochemical barrier [3]. Finally, we demonstrate the CVD synthesis of Mo2C films through chemical conversion of Mo foil surface, in a Cu vapor-enriched atmosphere [4]. References [1] S. Chaitoglou et al. Journal of Crystal Growth 495 (2018) 46–53 [2] S. Chaitoglou et al. Nanotechnology 30 (2019) 415404 [3] S. Chaitoglou et al. Nanotechnology 30 (2019) 125401 [4] S. Chaitoglou et al. Appl. Surf. Sci. (2019) Under revision

Resume : Since many years, a lot of research have been focused on two-dimensional (2D) chalcogenide-based nanomaterials. Among the transition metal dichalcogenides that can be isolated in 2D form, MoS2 based materials are the most used in catalysis, tribology and since a few years have been considered as materials of high potential for batteries electrodes. Given their widen applicability, further improvements of these materials are highly desirable, which could be achieved by a better control of the design of nanoparticles and their assembly at a mesoscopic scale. This study aims at synthesizing bimetallic NiMoS nanoparticles with controlled textural and self-assembly properties by means of « one-pot » tetrathiomolybdate soft chemistry routes in aqueous solutions. To do so, two synthesis routes have been explored. The first route, called metathesis (M), is a straightforward and fast reaction while the other route, called nucleation growth (NG), consists of a reduction of ammonium tetrathiomolybdate in presence of H2S. The properties of the materials prepared by these two methods have been compared and revealed that the NG method leads to lower stacking and size of MoS2 layers and to an increase of the specific surface area compared to M method. Furthermore, the addition of polymers as structuring agent has also been studied for a better control of the physico-chemical properties of the MoS2 phase. The specific surface area of these materials increases when the amphiphilic Pluronic® P123 copolymer is used during the synthesis (e.g. four-fold increase following M method in presence of P123) and a reduction in stacking (down to two layers) and size (down to two nm) of MoS2 slabs is observed. Further analyses such as XPS and catalytic tests have been performed in order to better understand the relationships between textural / active phases properties and reactivity of these materials. All those results will be described and discussed.

Resume : Exploration of precious-metal-free catalysts for water splitting is of great importance in developing renewable energy conversion and storage technologies. In this study, we investigate the link between the oxygen evolution reaction (OER) activities and the electronic properties of pure and first-row transition-metal-doped AlN and GaN monolayers. We find that Ni-doped layers are singularly appealing because they lead to a low overpotential (0.4 V). Early transition-metal dopants are not suited for OER because they bind the intermediate species OH or O too strongly, which leads to very large overpotentials or to no OER activity at all. The late transition-metal dopants Cu and Zn show little or no OER activity as they bind intermediate species too weakly. Whereas in many cases the overpotential can be traced back to an OOH intermediate species being adsorbed too weakly compared to an OH species. Ni dopant breaks this trend and stabilizes instead the OOH adsorbant which can be correlated with a switch from a high- to a low-spin state of the dopant atom. This ability to change spin states offers an exciting ingredient for the design of OER catalysts. Liang, Brocks, Zhang, Bieberle-Hütter, J. Phys. Chem. C 123 (43) (2019) 26289. DOI: 10.1021/acs.jpcc.9b06704

Resume : Unlike atomically-thin graphene, monolayers of the transition metal dichalcogenides (TMDs) are three-atom thick and exist in numerous polymorphs, where transition metal coordination changes from trigonal-prismatic (2H phase) to octahedral and distorted octahedral (1T and 1T’ phases). Crystal phase control in layered TMDs provides an additional tool to exploit their electronic properties ranging from semiconducting and metallic to quantum spin Hall insulators and two-dimensional superconductors.1 The metastable phases, such as the 1T’ phase of the group VI TMDs, are commonly produced via the distortion of the thermodynamically stable (2H) phase, while their direct synthesis remains challenging. In this work, we report on the direct, solution phase synthesis of the metastable distorted octahedrally coordinated structure (1T’ phase) of the WSe2 nanosheets.2 We design a kinetically-controlled bottom-up synthesis from molecular precursors to enable the formation of the metastable phase. 1T’ WSe2 branched few-layered nanosheets are produced in high yield and in a reproducible manner. We further demonstrate that the 1T’ phase is fully convertible into the semiconducting 2H phase upon thermal annealing at 400 oC, while the material retains the branched morphology. The 1T’ WSe2 nanosheets exhibit a metallic nature evidenced by an enhanced electrocatalytic activity for hydrogen evolution reaction as compared to the 2H WSe2. References: 1 H. Yang, S. W. Kim, M. Chhowalla and Y. H. Lee, Nat. Phys., 2017, 13, 931–937. 2 M. S. Sokolikova, P. C. Sherrell, P. Palczynski, V. L. Bemmer and C. Mattevi, Nat. Commun., 2019, 10, 712.

Resume : MXenes are a large family of 2D materials [1] with excellent potential for energy storage applications [2]. Ti3C2Tx (where Tx refers to surface terminations -O, -OH, -F, etc, present in varying amounts depending on synthesis conditions) is one of the most studied MXenes to date. Due to hydrophilic surfaces and weak attractive forces between the negatively charged layers, MXenes can retain significant amounts of water between the layers, and they can be intercalated with a variety of cations and molecules [3]. Studying the structure and dynamics of confined water between MXene sheets is important for understanding the behaviour of confined water in MXene electrodes for practical energy storage applications. Infrared spectroscopy is sensitive to the hydrogen bonding environment of water molecules and thus well suited for the study of water structures [4]. MXenes, on the other hand, exhibit broadband absorption and a lack of mid-infrared active features, making them a less suitable sample for infrared spectroscopic studies. Indeed, to the best of our knowledge, there are so far no reports on in situ infrared spectroscopy studies of MXenes. Here, for the first time, we use infrared spectroscopy to probe the vibrational dynamics of water confined between Ti3C2Tx MXene sheets during electrochemical charging and discharging. In addition to potential-dependent, reversible changes in the O-H stretching modes of confined water, we observe a clear signature of hydrated protons in sulfuric acid electrolyte. References [1] Naguib et al. Two-dimensional nanocrystals produced by exfoliation of Ti3AlC2. Adv. Mater. 2011, 23, 4248−4253 [2] Jun et al. Review of MXenes as new nanomaterials for energy storage/delivery and selected environmental applications. Nano Res. 2019, 12, 471–487 [3] Osti et al. Effect of metal ion intercalation on the structure of MXene and water dynamics on its internal surfaces. ACS Appl. Mater. Interfaces 2016, 8, 8859−8863 [4] Ohno et al. The effect of cooperative hydrogen bonding on the OH stretching-band shift for water clusters studied by matrix-isolation infrared spectroscopy and density functional theory. Phys . Chem. Chem. Phys. 2005, 7, 3005–3014

Resume : Electrochemical conversion of abundant feedstocks to fuels and value-added chemicals is rapidly gaining significance as a promising method to harness renewable electricity. Specific reactions within this context that my research group is focused on are the reduction of CO2 and oxidation of biomass platforms to fuels and value-added chemicals. Because the design of new catalytic systems is inherently linked to a precise understanding of how these reactions proceed on heterogeneous surfaces, we put considerable efforts in developing methodology for operando probing with vibrational spectroscopy. In all, I show how using these experiments provide the key mechanistic information on surface reaction mechanisms that enhance our understanding of functional hybrid interfaces and how the research provide avenues for future materials design within the context of electrosynthesis of fuels and chemicals.

Resume : 2D nanomaterial graphene is at the top of the considerable interest due to the giant electron and hole mobility, charge carrier multiplication, flexibility, optical transparency, chemical inertness. Graphene is already considered as a new transparent conductor, monolayer alternative to the Schottky contact metals and even as an active layer of the semiconductor devices. Particularly graphene is intensively explored as a new photovoltaic material for the fabrication of the various solar cells. The list is pretty long: monocrystalline silicon, inorganic chalcogenide thin film, organic, perovskite, dye sensitized solar cells can be mentioned. One of the main limitations stopping the wider application of the graphene in semiconductor device technology is a complex graphene transfer procedure. In this case, graphene is synthesized on the catalytic Cu or Ni foils. Afterward, follows the long process of the graphene transfer onto the targeted semiconductor or dielectric substrates. During that process, graphene can be contaminated by different adsorbents. Transfer causes wrinkles or ripples to form on graphene. In such a case control of the graphene layer or graphene-semiconductor contact properties is complicated. Recently there were shown that direct synthesis of the graphene on semiconducting or dielectric substrates is possible. However, the development of this technology is the very beginning. In the present research graphene layers were directly synthesized by microwave plasma enhanced chemical vapor deposition on the monocrystalline Si(100) substrates. The structure of the films was investigated by multiwavelength Raman scattering spectroscopy and atomic force microscopy. A number of the graphene layers was evaluated by using Raman scattering spectroscopy and optical reflectance spectra. Graphene/Si(100) heterojunction based diodes were fabricated. The effects of the deposition conditions on the structure of the graphene layers were studied. The influence of the nitrogen and fluorene doping was considered. There were revealed that both vertical graphene flakes and planar graphene layers can be synthesized by setting appropriate deposition conditions. Graphene grown on textured silicon surface was studied. Current-voltage characteristics, as well as photovoltaic and photoelectric properties of the different graphene/Si(100) diodes and solar cells, were investigated. Acknowledgements. The research project No. 09.3.3-LMT-K-712-01-0183 is funded under the European Social Fund measure „Strengthening the Skills and Capacities of Public Sector Researchers for Engaging in High Level R&D Activities“ administered by the Research Council of Lithuania.

Resume : Given their unique optical and electrical properties, including strong photoluminescence and high carrier mobility, 2D transition-metal dichalcogenides (TMDCs) can become key components in optoelectronic devices with truly atomic-scale thicknesses. Despite tremendous progress, the fundamental challenge in this rapidly growing field is the fabrication of high quality 2D crystals with a well-controlled structure. Pulsed Laser Deposition (PLD) has emerged as a catalyst-free method for the synthesis of 2D materials. However, the main figure of merit, i.e. photoluminescence (PL) yield, is rather low. In this paper, we will present our advances on the synthesis of MoS2 monolayers (MLs) by a hybrid approach based on PLD combined with a sulfur evaporation beam. This approach allows a good control over the number of layers and Mo:S ratio, as indicated by Raman and X-ray photoelectron (XPS) spectroscopy, respectively. We report on the PL emission from an ultimately thin MoS2 monolayer (0.7 nm) grown by PLD. The atomic resolution images of the MoS2 MLs reveal a rich variety of defects, including single or double sulfur vacancies, antisite MoS defects, adatoms, etc., as well as 1800 rotated mirror-like grain boundaries (GBs). No triangle-shaped single crystals, specific to the bottom-up synthesis approach by Chemical Vapor Deposition, were identified. Instead, the 2D films grown by the PLD method are continuous, and exhibit nanometer-size domains connected by multiple GBs. Finally, we will compare the atomic resolution images with first-principles calculations in order to explore the electronic structures of the experimentally observed intrinsic point defects and GBs.

Resume : Inorganic perovskite materials research has known an incredible growth over the past decade. Recent record of 18% PCE with CsPbI3 perovskite underpin the expectation for these materials to be a great candidate for solar cells. However, as for the majority of perovskite materials, stability against time and moisture is an issue to be addressed for industrialization. The use of 2D materials for perovskite phase stabilization has been largely studied in the literature, generally using big organic spacers such to induce the separation of large perovskite blocks, which avoid moisture penetration and gives a longer life-time to photovoltaic device. However, the use of organic spacers can increase the bandgap of perovskite active film, and induce thermodynamic barriers that modifies the energetic landscape and that might be detrimental to charge carrier mobility. Our main focus is the use of the quite recently discovered Ruddlesden Popper structure Cs2PbCl2I2 for the stabilization of CsPbIxBr1-x for an all-inorganic 3D/2D structure. We show the friendly synthesis of such 2D structure by conventional spin-coating route and the photovoltaic properties of the pure phase. This phase is then added to the so-called 3D all-inorganic perovskite solution for spin-coated thin film deposition. In-situ XRD analysis is operated on freshly spin-coated films on ITO substrate. The 3D/2D mix shows an interesting preferential orientation along 14° and 28° peaks that is not observed for pure all-inorganic 3D perovskite. This effect is reproducible and can be observed on different Bromine/Iodine stoichiometry. We meticulously studied stability of such mixture upon heating, and showed that the texture brought by 2D phase allows to stabilize the CsPbI3 black phase at room temperature under nitrogen, while the reference sample shows the reappearance of the undesired yellow non-perovskite structure. The effect of annealing temperature over thin film morphology is observed as well, and shows increased compacity and grains size as 2D phase is added. Such all-inorganic 3D/2D phase is quite new in the literature, and offers a remarkable possibility to stabilize perovskite structure without the use of organic molecules.

Resume : Previous theoretical studies have shown that the tin carbide (SnC) nanosheet with honeycomb lattice structure is stable. Moreover, together with other carbide monolayers, the SnC monolayer has been proposed as a potential photocatalyst for water splitting, even though its indirect band gap would hinder its catalytic performance. The addition of hydrogen to its surface could lead to an indirect-direct band gap transition. In this work, we comparatively study the elastic constants and the phonon and electronic band structures of pristine and hydrogenated SnC nanosheets. For the latter, we propose two hydrogenation schemes. In one of them, all the hydrogen atoms are on the same surface face, whereas in the other scheme, the hydrogen atoms alternatively lie on each of the two surface faces. Finally, the relative stability and the nature of the band gaps of the three studied systems are discussed. Acknowledgment: This work was supported by UNAM-PAPIIT IN109320. Computations were performed at the supercomputer Miztli of DGTIC-UNAM (Project LANCAD-UNAM-DGTIC-180). A.L.M.V. would like to thank CONACYT and BEIFI-IPN for her scholarship.

Resume : Molybdenum disulfide (MoS2) is a promising candidate for both nanoscale and flexible electronics due to its exotic electrical and optical properties. While MoS2 has been synthesized by various techniques like CVD, PLD, etc. But for the potential applications like 2D/3D or 3D/2D heterostructures, transparent and flexible electronic devices, sensors, and storage devices, layer transfer of MoS2 is essential. In this work, we report an efficient and damage-free approach to transfer single and few-layer MoS2 onto arbitrary substrates. First, MoS2 was grown on single crystalline silicon dioxide (SiO2) substrate with the assistance of NaCl by a single-temperature zone chemical vapor deposition method then transferred onto desired substrates. The proposed transfer route is based on the dissolution of the water-soluble layer (Na2S/Na2SO4) formed underneath MoS2 flakes/film simultaneously during the growth process. Optical, morphological and transmission electron microscopy results show the slight increase in bandgap (20¬¬-30 meV) and improved crystalline quality of MoS2 after the transfer which makes it a better transfer process than the conventional method. Furthermore, our transfer process overcomes the limitation of choosing an appropriate substrate for the MoS2 film to be transferred. Also, it allows the reuse of growth substrate, which makes it an inexpensive and scalable process that can be used for the transfer of other 2D materials. A successful transfer of MoS2 on flexible substrate (mica) makes it suitable for high-performance flexible devices.

Resume : Carbon based materials, such as, carbon quantum dots (CQD) are prominent material for vast variety of functions, though, the mobility of such materials should be considered as free ranging nanomaterials is not a sustainable model of use. Thus, a stable carrier is necessary. Titanium dioxide is a well-known photocatalyst with high stability and abundance, but functionalization of the material is still under development as it has low photo generated charge separation, high recombination rate and mostly low selectivity in CO2 reformation and water splitting to hydrogen production efficiency. One of clear ways to improve catalytic properties is introduction of additional materials. CQD could provide necessary conduction and valence band edge shift. In addition, CQD/TiO2 can increase charge separation as well as increase photocatalytic properties and reduction selectivity. This work focuses on synthesis of nitrogen doped CQD (N-CQD) and it’s influence on anodic TiO2 properties and photocatalytic activity. N-CQD deposition on TiO2 is performed during anodization to lower the possibility of N-CQD separation and diffusion into environment. Initial carbon material is synthesized in electrochemical exfoliation and microwave synthesis. Thence, gained nanoparticles are added into titania during anodization process. Morphology is investigated with microscopy, structure and composition using Raman spectroscopy, XPS, XRD, FTIR. Photocatalytic activity is determination from electrochemical properties and potentials, i.e. flat band potential and organic pollutant degradation and CO2 reformation to value-added chemicals. Comparison of the N-CQD properties and loading influence on TiO2 material with commercial QD in various sizes is done. Acknowledgement. Authors acknowledge financial support from Latvian Science Council project No LZP-2018/1-0194.

Resume : To increase the energy density of lithium-ion batteries and to ensure stability at the electrode/electrolyte interface (EEI), for this work we studied the compatibility among Si monolayer (Si-ML) electrodes and LaTiO3 (LTO) or LixLa1-xTiO3 (LLTO) perovskite type slab electrolytes. The study was performed in the Density Functional Theory (DFT) scheme. To avoid any artificial dipole-dipole interaction between periodic images, the LTO and LLTO slabs were modeled using symmetric (001) TiO2-terminated free surfaces. The Si-ML was placed at high symmetry absorption sites on the LTO or LLTO slabs: Ti-top, O-top and hollow sites. Along the structural relaxation, the atomic coordinates of the central layers of the LTO and LLTO slabs were kept fixed, because the main interest is to study the structural and physical properties at the EEI. The results shown there are electronic charge concentrations along the Si-Ti bonds at the EEI, when the Si-ML was placed at the Ti-top site. In contrast, there are Si atoms’ electrons’ loss when the Si-ML was placed at the O-top and hollow sites, leading to a negative charge gain by the O atoms around the Si-O bonds. The most favorable interface, energetically speaking, occurs when the Si-ML is placed at the O-top sites. Acknowledgments. This work was partially supported by the IPN-SIP-2020-1114 project. J. E. Antonio and H. Muñóz acknowledge the scholarship from CONACYT and support from the IPN-BEIFI program.

Resume : Germanane (GeH), a germanium analogue of graphane, has recently attracted considerable interest because its remarkable combination of properties makes it an extremely suitable candidate to be used as 2D material for field effect devices, photovoltaics, and photocatalysis. Up to now, the synthesis of GeH has been conducted by substituting Ca by H in a β-CaGe2 layered Zintl phase through topochemical deintercalation in aqueous HCl. This reaction is generally slow and takes place over 6 to 14 days. The new and facile protocol presented here allows to synthesize GeH at room temperature in a significantly shorter time (a few minutes), which renders this method highly attractive for technological applications. The GeH produced with this method is highly pure and has a band gap (Eg) close to 1.4 eV, a lower value than that reported for germanane synthesized using HCl, which is promising for incorporation of GeH in solar cells. In addition, a facile and controllable approach based on the Langmuir-Schaefer deposition to produce homogeneous and dense germanane monolayer films on various substrates is reported.

Resume : The work is devoted to ascertainment of the regularities for thermostimu-lated formation of the phase composition and structure of CoSb3-scutterudite-based films deposited by the vacuum condensation method, as well as the effect of the nanoscale factor on their thermoelectric properties. It was determined that the presence of the nanoscale factor (the single-phase crystalline structure of CoSb3 scutterudite with an extended area of existence in the film with increased structural defect due to the sublimation of antimony and reduction in the grain size) causes an increase in the thermoelectric efficiency coefficient of Co-Sb films in 8 times as compared to the bulk material. In studies, to increase the ZT, we plan to dope of nanosize Co–Sb films with different chemical elements Fe, Yb, Li, Eu, La, Се, Ba to form a structure that can better conduct electric current (as a crystalline conductor) and poorly conduct a heat (like a glass). This will make possible to reduce a phonon component of the heat conductivity and much more increase thermoelectric efficiency coefficient ZT. Doping elements occupy voids in the crystal lattice – atomic polyhedrons of large sizes. This provides an effective phonon scattering, which in turn results to decreasing of the heat conductivity without a substantial impact on electrical conductivity due to mainly ionic character of interaction between phonons and atoms of scutterudite carcass and covalently bonded carcass with a small probability of chemical bonds.

Resume : Thin-film solid-state Li-ion batteries (TFB) are promising candidates to power miniaturized sensors for Internet of Things (IoT) applications [1]. Such applications have created a high demand for battery systems to provide larger power and energy densities. TiO2 (anatase) is of great interest as a negative electrode for TFB because of its interesting capacity of 168 mAh g-1 corresponding to lithium inserted into half the vacant octahedral holes, giving a composition of Li0.5TiO2 [2]. Other advantages of TiO2 are its rapid discharge and charge properties, low cost, and non-toxicity. Lithium insertion into TiO2 (anatase) attracted considerable attention in the past. However, the mechanism of the Li+ storage in TiO2 thin films has not been studied yet. Because of the sensitive probing of short-range structures, Raman spectroscopy is preferable over diffraction methods for acquiring structural data on the Li/TiO2 system for thin film nanosized materials. A wealth of information about the local disorder and variations in bond lengths and coordination environments can be provided [3]. Furthermore, because of the sensitive spatial resolution of Raman microscopy, the homogeneity of the electrode reaction can be investigated. In this work, anatase type TiO2 thin films were deposited by atomic layer deposition on Si/ Al2O3/ Pt substrate. A selected area of 0.38 cm2 on the 150 nm thick TiO2 layer was cycled in a homemade cell using 1 M LiClO4 EC: DMC electrolyte in the potential range of 1-3 V vs. Li/Li+. A discharge capacity of 10 µAh cm-2 is obtained on the second cycle at C/10 rate with a good efficiency of the lithium insertion/extraction reaction. The retained capacity observed is approximately 95% even after 130 cycles. Raman spectroscopy experiments were performed on the anatase LixTiO2 system (0 ≤ x ≤ 0.5) produced from the electrochemical discharge of a TiO2 thin film at room temperature. The biphasic transition from tetragonal TiO2 to orthorhombic titanate LixTiO2 is clearly evidenced. The formation of orthorhombic LixTiO2 manifests itself from x = 0.1, through several changes in the Raman spectra, indicating an extension of the solid solution domain in the nanosized thin film electrode compared to previous data reported for micrometric TiO2 powders [2,4]. For the first time in the thin film configuration, the rich Raman fingerprint of the pure orthorhombic Li0.5TiO2 phase, made of 20 components, is fully observed for x = 0.5. The high quality of obtained Raman spectra allows quantifying the amount of orthorhombic phase at different oxidation-reduction states, showing the efficiency of Raman spectroscopy as a powerful probe to evaluate the state of charge of the TiO2 thin film electrode material. [1] M. Letiche et al., Adv. Energy Mater. 7 (2017) 1601402 [2] L. J. Hardwick et al., Electrochim. Acta 52 (2017) 5357-5367 [3] R. Baddour-Hadjean et al., Chem. Rev 110 (2010) 1278-1319 [4] R. Baddour-Hadjean et al., J. Raman Spectrosc.35 (2004) 577-585

Resume : Zinc ferrite (ZnFe2O4) has a high theoretical capacity of ~1000 mAh/g, low-toxicity, and made from abundant elements which make it a primary material for synthesizing "green" and low-cost supercapacitor electrodes. However, it has low conductivity and stability which limit its practical application. To overcome this drawback, ZnFe2O4 is often hybridized with conductive carbon such as CNT and graphene. Though this approach is successful at mitigating some of the aforementioned problems, the performance of the composites still lags way behind the theoretical capacity; hence, a new approach is needed. In this study, we demonstrate that ZnO has the unique ability to boost the energy storage performance of ZnFe2O4 nanoparticles embedded in graphitic carbon (g-C). ZnO does this by tuning the electronic structure of ZnFe2O4 at their heterojunction which promotes the adsorption of oxygenated species and fast electron transfer. Consequently, the specific capacitance, energy density and capacity of ZnO-ZnFe2O4/g-C is comparable to that of reported state-of-the-art supercapacitor electrodes which are based on much expensive and toxic materials.

Resume : The electrochemical reduction of carbon dioxide (CO2) to selective production of methanol (MeOH) is currently a hot topic. MeOH is used as fuel additive and as solvent. There are distinct types of homo and heterogeneous catalytic systems. However, the main issues for electrochemical reduction of CO2 are formation of many byproducts, less stability of catalyst, and low faradic efficiency (FE). These key issues are not solved. In current century several types of metal oxide nanoparticle and metal organic framework (MOF) were synthesized to address the issues such as MOF-8 and MOF-67. Same idea was adopted to produce MeOH from CO2 using engineer optimization self-assembled monolayer of pyridine@Cu-MOF. The synthesized catalyst worked at -1.3 V to form MeOH with high current and FE. Moreover, given catalyst could be active around 120 min without changing FE.

Resume : Self-powered sensors running on small temperature differences are considered promising candidates to cover the increasing demand for sustainable and maintenance-free wireless sensor networks for the Internet of Things (IoT). Under this context, a cost-effective and self-powered hydrogen thermoelectric sensor is presented here as a proof of concept for battery-less IoT nodes. The device is based on low-density paper-like fabrics made of functionalized thermoelectric silicon nanotubes able to harvest energy from the heat released by exothermic reactions, such as hydrogen catalytic oxidation. This sensor gives an accurate value of the reacting gas concentration without any external power requirement. Experimental results confirm that this self-powered sensor can autonomously measure concentrations as low as 250 ppm of hydrogen in air while generating power densities up to 0.5 μW/cm2 for 3% H2 at room temperature that can be eventually used to store or send the reading. Moreover, an analytical study of the device's controllable parameters predicts that there is still a wide margin of improvement in the performance. Varying parameters such as the fabric TE and catalyst layer thickness, doping level, device area, and fabric density allow tuning the sensor output accordingly to the point of operation. This way, an optimization of the performance can be achieved at a given final scenario. In addition to the aforementioned features, the cost-effective, reproducible, and scalable synthesis; its adaptability to any desirable shape; the abundance, non-toxicity, and integrability of silicon make the proposed device very attractive for future implementation of self-powered wireless sensors nodes with countless direct potential applications. References: [1] M. P. Pujadó, J. M. S. Gordillo, H. Avireddy, A. Cabot, A. Morata, and A. Tarancón, "Highly Sensitive Self‐Powered H 2 Sensor Based on Nanostructured Thermoelectric Silicon Fabrics," Adv. Mater. Technol., vol. 2000870, p. 2000870, 2020. [2] A. Morata et al., "Large-area and adaptable electrospun silicon-based thermoelectric nanomaterials with high energy conversion efficiencies," Nat. Commun., vol. 9, no. 1, p. 4759, Dec. 2018.

Resume : Upconversion (UC) emission is an important non-linear optical process of trivalent lanthanide ions (Ln3 ). Ln3 -activated materials absorb two or more low energy near infrared (NIR) photons to generate a higher energy photon in the range of NIR to visible radiation. Transparent thin films with UC properties have received considerable attention in the last years for application in solar cells technology and solid-state lighting. For example, UC of low-energy photons to high-energy ones has recently gained great interest among the photovoltaics research community as it allows spectral conversion of the light wavelengths that are not absorbed by the conventional solar cells, resulting in a higher overall efficiency (ACS Energy Lett. 2017, 2, 1346). The different emission colors of the UC Ln3 ions (red for Er3 , blue for Tm3 and green for Ho3 ), also allow for the design of color solid state lighting devices or even for the development of white light solid state lighting through the rational combination of their red, blue and green emissions. Matrices doped with Er3 , Tm3 or Ho3 show, in general, very low UC efficiency. The co-doping with Yb3 (sensitizer) can remarkably enhance the UC efficiency because of the efficient energy transfer from Yb3 to the three mentioned Ln3 ions. Due to their low phonon energy, which minimizes the non-radiative processes, rare earth trifluorides (REF3) co-doped with Yb:Er, Yb,Ho or Yb,Tm pairs have aroused extensive interest as UC materials (CrystEngComm, 2013, 15, 7142). However, REF3 show a relatively low chemical and thermal stability. On the contrary, rare earth oxifluorides (REOF) show higher chemical and thermal stability than REF3 and still benefit from a low phonon energy compared to the other end-member of the REF3 – RE2O3 system, the rare earth oxides (J. Mater. Chem. C, 2016,4, 331). Up to now, just one method for fabrication of upconverting YOF films by pulsed liquid injection metal–organic chemical vapor deposition has been reported, which is tedious due to the use of a reactor with controled oxygen partial pressure and high temperature (Dalton Trans., 2018, 47, 2655). In this work, we report on the first successful deposition of YOF thin films co-doped with Yb:Er, Yb,Ho or Yb,Tm on silica substrates using a simple deposition method. The method consisted on the preparation of colloidal suspensions of YF3 nanoparticles co-doped with Yb:Er, Yb,Ho or Yb,Tm that are then spin coated on silica substrates at room temperature. The films were then annealed, for 2 hours in air, at increasing temperature. The crystalline phases resulting from the annealing process were monitored through small angle XRD. The YF3 – YOF phase transformation was observed after annealing at 400ºC for all three films. The transparency of the films, analysed from ballistic transmittance spectra, showed values well above 80% for any wavelength analyzed, which indicates a high optical quality of the layers. Finally, the emission spectra of the three thin films were recorded under 980 nm excitation. The Er,Yb-YOF film showed almost exclusively an emission band located at 650 nm, which is responsible for the red color of the film. The emission spectrum of the Tm,Yb-YOF film presented an emission band centered on the blue region while that of the Ho-Yb film showed two bands located in the green and red regions. These results demonstrate that it is possible to prepare transparent thin films of YOF phase with different emission colors using spin coating, a simple and inexpensive method. The prepared films are potentially useful for color solid state lighting and for white light solid state lighting. The latter can be achieved by combination of Er-, Ho- and Tm-based films in the adequate proportions depending on their emission intensity. These films could also be implemented in solar cell technology to gain a higher overall cell efficiency.

Resume : Thin sheets of layered transition metal dichalcogenide (TMD) crystals have attractive interest for its practically applications as high capacity electrode materials for electrochemical energy storage devices/ The Resonance Raman scattering in TMD crystals has been experimentally inestigated. For spatial symmetry groups of crystals and two-periodical monolayers of transition metal dichalcogenides the projective classes and characters of the projective representations for the high symmetry points of Brillouin zone have been defined. The origin of spin-dependent splitting of energy spectra of elementary excitations of electronic pi-bands in these structures have been established. The interpretation of the obtained experimental results has been done.

Resume : SiO2 – based materials have attracted considerable interest as anode candidates for lithium-ion batteries due to their environmental friendliness, low cost, high theoretical capacity and low electrochemical potential. Despite its low electronic conductivity and volume modification during electrochemical cycling, silica remains challenging as next generation anode material for lithium-ion batteries (LIBs). Thus, nanostructured SiO2-based carbon composites seem to bee an effective solution to improve the cycling performance and accommodate the volume changes. Herein, we propose a simply and eco-friendly preparation route of SiO2 – carbon composites as anode materials for lithium-ion batteries. This consists in simple mixture of nano-SiO2 and sucrose solution followed by thermal annealing under nitrogen atmosphere. Scanning Electron Microscopy (SEM-EDAX), Transmission Electron Microscopy (TEM) and X-Ray Diffraction (XRD) investigations confirmed the structure of the SiO2-carbon composite. Also, the electrochemical performance has been evaluated. The SiO2-carbon composites with different sucrose concentration exhibits enhanced electrochemical performance, good reversible capacity and cycling stability.

Resume : Lithium-ion batteries (LIBs) have received great interest for portable electronics and electrical vehicles because of their merits of high energy density and cycle performance. Great efforts have been focused on optimizing the overall performance of LIBs, of which anode materials play a critical role. However, graphite, the currently commercial anode, exhibits low operating voltage (<0.25 V vs lithium) and thus easily leads to an unstable cycling process. In recent decades, mixed titanium-niobium oxides are considered to be promising candidates because the presence of multiple redox couples (Nb5+/Nb4+, Nb4+/Nb3+ and Ti4+/Ti3+) above 1.0 V suppress the formation of lithium dendrites. Among various titanium-niobium oxides, layered HTiNbO5 exfoliated from KTiNbO5 is able to deliver a theoretical capacity of 242 mAh g-1 based on two redox couples of Ti4+/Ti3+ and Nb5+/Nb4+. Nevertheless, its application is hindered by the intrinsic low electronic conductivity of titanium-niobium oxides. Ti3C2Tx Mxene, one of the most widely studied two-dimensional (2D) materials, possesses good electronic conductivity and thus becomes the potential conductivity framework for the as-mentioned titanium-niobium oxides. Given the fact that both HTiNbO5 nanosheets and Ti3C2Tx can be obtained through acid-induced flocculation, we propose in this work a co-flocculation strategy to fabricate 2D HTiNbO5/H-Ti3C2Tx nanohybrid. The addition of HCl into the mixed [TiNbO5]-/ Ti3C2Tx colloidal suspension eliminate negative-charged surface, and thus lead to random restacked between HTiNbO5 and H-T3C2Tx. Such 2D nanohybrid with 2D plane-to-plane contact area is expected to enhance the ionic and electronic conductivity of the composite and lead to the high-rate performance by boosting the pseudocapacitively dominated Li+ intercalation process. The electrochemical performance of HTiNbO5/H-Ti3C2Tx nanohybrid with different mass feeding rates is investigated. When served as anode materials for LIBs, HTiNbO5/H-Ti3C2Tx nanohybrid with mass feeding rate of 3:1 shows the best lithium storage performance, delivering superior rate performance and exhibiting excellent cycling stability. Our exfoliation and co-flocculation strategy efficiently improve the pseudocapacitive properties for lithium storage, paving the way to designing layered titaniumniobate with high-rate performance.

Resume : Water splitting is an environmentally benign way to store the energy harvested from sustainable but intermittent sources (e.g. Wind, solar etc.) in the form of hydrogen. However, as commercial value of water splitting is impeded by the sluggish kinetics of cathodic and anodic half reactions involved during water splitting, only 4% of the total global H2 production is done using water electrolysis. In theory both half reactions of water electrolysis contribute equally to overall efficiency of the process however, being 4 electron transfer process that suffers from indolent kinetics and unfavourable thermodynamics Oxygen evolution reaction (OER) is considered more challenging of the two. Thus, if OER is substituted by other less energy intensive anodic reactions like urea oxidation reaction (UOR), the overall cost of hydrogen production from water electrolysis can be significantly reduced. In this regard, Herein, we have prepared a highly active composite electrode by depositing polyhedral iron oxide on the surface of nickel foam. As prepared Fe3O4 modified nickel foam (Fe3O4-NF) demonstrated excellent catalytic activity for both UOR and hydrogen evolution reaction (HER). Notably this catalyst was able to achieve benchmark current density of 10 mA cm-2 at low potential of 1.33 V and 0.187 V for UOR and HER respectively without any visible loss in activity during 24 h operation. A two-electrode symmetrical electrolyser established by using Fe3O4-NF as both cathode and anode delivers the current density of 10 mA cm-2 at applied potential of 1.51 V and 1.72 V for urea and water electrolysis respectively. The proposed Fe3O4-NF || Fe3O4-NF electrolyser system can be driven by an AA battery in an alkaline solution containing urea, which emphasizes the practical utility of this system for hydrogen production.

Resume : Lithium ion batteries with high energy density cathodes, as nickel-rich layered oxide LiNi0.8Mn0.1Co0.1O2 (NMC811) materials, have attracted great interest due to the urgent demand of energy storage technologies for electric vehicles, hybrid electric vehicles and smart grids. In order to reduce fading capacity of NMC811 cathode, new electrolyte formulations including additives are investigated, being one of the most economical approach to reduce flammability and gas swelling. In this case, boron compounds as tris(trimethylsilyl) borate (TMSB) are used into the electrolyte EC/DMC with 1M LiPF6 in NMC811/graphite full cells. Electrochemical characterization, conductivity and morphological analysis have been conducted in order to reveal the improvements beyond state of art via safe electrolyte systems.

Resume : Among various energy sources, hydrogen energy gained increasing attention because of its highest energy density and carbon-free emission. Electrochemical water splitting into molecular hydrogen and oxygen is a most clean, convenient and sustainable technique to produce energy with low emission of greenhouse gases. The electrochemical water splitting comprises two half-cell reactions; one is hydrogen evolution reaction and second is oxygen evolution reaction (OER). Till date, Ir and Ru-based electrocatalysts showed the superior catalytic performance for OER. However, its high cost and scarcity restricts their large-scale practical application. Discovering of efficient and cost-effective electrocatalyst for electrochemical water splitting is one of the major scientific challenge, becuase of sluggish kinetics during oxygen evolution reactions (OER) and hydrogen evolution reaction (HER). In addition we have focused on synthesis of low-cost catalysts with high surface area, high conductivity, good catalytic activity, and minimization of gas bubble adhesion. Until now, several catalyst candidates based on earth abundant elements including transition-metal oxides(TMOs), hydroxides, sulfides, selenides, phosphides, nitrides, and carbides exhibited great potential because of their catalytic activities can be easily tuned by controlling their composition and structure. Herein, we have synthesized several TMOs based layered double hydroxides, oxide, sulfide and phosphide materials for water splitting applications. All synthesized electrocatalysts exhibits superior catalytic activity and long-term stability. The enhanced electrochemical performance mainly attributed to its hierarchical structure, enhanced charge-transport and high electrochemical surface area with plenty active sites.

Resume : Previous research has reported that the performance of perovskite solar cells can be influenced by their contact layers. The power conversion efficiency can be improved by designing well-matched hole transport layers or electron transport layers, own to the reduced charge recombination and enhanced charge extraction. Moreover, the morphology and crystallinity of perovskite layer also can be controlled by adjusting substrates that they grow on. The improved morphology and enhanced crystallinity not only can improve the photovoltaic performance of perovskite solar cells, but also have potential to promote their long-term stability. Our group are interested in two dimensional materials, such as graphene and its derived nanomaterials, as charge transport layers. They are promoting candidates for this purpose, due to their unique electronic, optical and mechanical properties. A well-matched energy level can be obtained by controlling the work function of graphene according to the energy level of perovskite layers. In addition, the hydrophobic nature of the graphene is beneficial for the crystallisation of perovskite layer and improving their long-term stability by preventing moisture from atmosphere.

Resume : One of the most intensively studied group of 2D materials are transition metal dichalcogenides (TMDCs). Due to unique properties of TMDC materials, they are potential candidates for 2D electronics and optoelectronics. As atomically thin-layered semiconductors they give possibility to create p–n junction at the ultimate thickness limit. Van der Waals junctions composed of p- and n-type semiconductors are predicted to exhibit completely different charge transport characteristics than bulk heterojunctions. The atomically thin MoS2 p–n and n-p junctions present a significant potential of the two-dimensional crystals for flexible, transparent and high-efficiency optoelectronic applications. To investigate this phenomenon, junctions in p-n and n-p configurations based on MoS2 doped on n- and p-type monolayers were fabricated on gold and aluminum substrates. This allowed comparing the influence of the substrate material on the value of work function. In this work, the results of the Kelvin Probe Force Microscopy (KPFM) measurements and contact potential difference (CPD) calculations of the MoS2 monolayers of the p- and n-type were presented. Using CPD calculations it was possible to determine the work function of n-p and p-n junction MoS2. In addition, a change in the work function was observed depending on the layer thickness. Preliminary measurements and CPD calculations showed that the work function for n-p and p-n structures on gold substrate equals 5,18 eV and 6,44 eV, respectively. For the aluminum substrate, work function equals 4,70 eV for n-p structure and 4,78 eV for p-n structure. In both cases, the work function value of the junction structures is greater than the width of the bandgap. Acknowledgements: The research leading to these results was funded by the NCN OPUS 13 Grant - “Electrical and mechanical investigations of the membranes based on transition metal dichalcogenides” (Grant No. 2017/25/B/ST7/01203).

Resume : One of the most common methods used on a laboratory scale to obtain monolayers of 2D materials such as transition metal dichalcogenides is mechanical exfoliation, also known as the scotch tape method. The process requires only adhesive tape, tweezers, and a substrate on which we want to place the material. The greatest advantage of this procedure is that the surface of the flakes is not chemically modified and allows the best characterization of the material. It is well known that in order to obtain monolayer or few-layered material by mechanical exfoliation, this process should be repeated about 10 – 20 times, but the mathematical principle behind this phenomenon remains unknown. We will show how the thickness of the flake changes with subsequent separations. The tests were conducted on two widely used dicing tapes, and Scotch removable tape. At the outset, we assumed that with each subsequent cleavage of the tape, the thickness of the material will decrease by half. Additionally, the exfoliated flakes are observed to break during the process and the fragmentation factor is also discussed. The experiment was carried out as follows: a piece of MoS2 (SPI Supplies) was placed on the adhesive tape, all stiffened by mounting the tape to the glass slide. In the next step, another piece of tape was placed on the top of the material and torn off to remove the excess crystal. Then prepared material was placed under an optical microscope and the photo of the sample was taken, each time AFM measurement of the height of the remaining material was taken. After each new piece of tape was torn off, the measurements were repeated. Additionally, after reducing the material to monolayers, the flakes were transferred to silicon substrates and optical properties of the obtained flakes, such as photoluminescence and Raman spectroscopy, were measured. In this experiment, mechanical exfoliation was performed using three different adhesive tapes: the commonly used blue Nitto tape, Scotch magic removable tape, and Clear Low Tack SPV214R dicing tape. By the measurements with the use of the AFM microscope, changes in the thickness of the material were observed and an equation describing this process was proposed. The efficiency of the process was the highest with the Clear Low Tack SPV214R dicing tape, the Nitto tape was slightly less effective, both of which resulted in obtaining 10 nm flakes after 10 repetitions. While Scotch exfoliation was the least effective, even after 30 breaks, the material thickness did not drop below 10 nm. Acknowledgments: The research leading to these results was funded by the NCN OPUS 13 Grant - “Electrical and mechanical investigations of the membranes based on transition metal dichalcogenides” (Grant No. 2017/25/B/ST7/01203).

Resume : In electrochemical storage, the current advantages (high volumetric and gravimetric capacitances) shown by Lithium-ion batteries (LIB) collide with the limited natural abundance and not-uniform distribution at global level of certain elements involved in this technology (e.g.: Li and Co); Rechargeable Na-ion batteries (NIB) are regarded as complementary energy storage technology to LIB. The electrochemical performance of NIBs are limited by the low stability and capacity of the best material for anode known like “Hard Carbon”. Recently, MXenes, a new class of 2D materials obtained by the chemical exfoliation of carbides, known like MAX phase, have been proposed for hybrid NIB devices. Their lamellar structure facilitates the intercalation of many alkaline metal ions, on an extended range of charge-recharge rates for thousands number of cycles. In this work, the preliminary results about the development of an integrated «Lab-scale» process from MAX Phase synthesis to NIB half-cell testing, are shown and commented. The electrochemical behaviour of MXenes for storage applications in NIBs have been studied as a function of some critical parameters in the different productive steps. In our approach, Spark Plasma sintering (SPS) has been tested like a high-throughput method to produce pure TI-Al-C MAX-phases; the effects of the SPS parameters used for the synthesis of Ti2AlC2 MAXPhase samples been correlated with the electrochemical performance of the related MXenes. Exfoliation conditions and post process treatments have been also tested in order to modify the functionalization of MXene lamellas in order to maximize storage performance and extend cyclability in half-cell Vs metallic Na. Preliminary results of full-cell based on Ti3C2Tx MXenes (Vs Na0.44MnO2) will be also commented in order to validate the potentiality of this class of materials as anodes for NIBs.

Resume : The n-β-Ga2O3/p-GaSe heterojunctions were fabricated by heat treatment of GaSe lamella of micrometric thickness in an air ambiance containing water vapors. Depending on heat treatment regime two type of structures were obtained: I – planar structures with β-Ga2O3 thin layer on GaSe substrate; II – structures with β-Ga2O3 layer formed along to GaSe lamella edge perimeter. The nanometric thickness GaSe lamella were obtained by mechanical and ultrasonic cleaving from single crystal plates. The β-Ga2O3 layer chemical composition and surface morphology were studied by X-rays diffraction XRD, EDXS, SEM and light combined (Raman) scattering. The β-Ga2O3 layer onto GaSe substrate and these formed on the GaSe lamella edge represents a dense set of nanoformations such as β-Ga2O3 nanofibers and nanostrips. The β-Ga2O3 layer direct bandgap was determined from the analysis of the reflection spectrum by using the Kubelka-Munk function and equals to 4,75 eV. The investigated structures photoresponse is determined by the mechanism of nonequilibrium charge carriers generation in β-Ga2O3 layer (in the spectral range of 200 – 300nm) as well as in GaSe substrate (in the visible region of spectrum).

Resume : Magnesium hydride (MgH2) is a light hydride able to accumulate high amounts of hydrogen (7.6 % wt H2). However, thermodynamic restrictions as well as poorly H-kinetics preclude its utilization for mobile applications due to the high temperatures (300ºC) requited to drive the H2-absorption/desorption reactions. To overcome these drawbacks, MgH2 nanostructures are being widely investigated which promote the hydrogenation process by reducing the activation energies [1] related to bulk processes such as H-diffusion, hydride nucleation, etc. However, the key role of the surface on the hydrogenation process has hardly been investigated. Theoretical calculations [2] have shown the poor ability of clean magnesium surface to dissociate the H2-molecule contribution but barely experimental works have been performed. Moreover, the high reactivity of the magnesium promotes the formation of several phases such as oxides, hydroxides and carbonates which could drastically affect to H2-dissociation and H-diffusion into few atomic layers. Therefore, the main purpose of this work is to shed light about the influence of the surface (and subsurface) composition on hydrogen-related process. To this aim, nanocrystalline magnesium films (thickness80 nm) were deposited by e-beam evaporation. Oxides and hydroxides were formed onto magnesium surface. Pd-capped Mg films were also synthesised for comparison purposes. Hydrogenation mechanism was investigated by “in situ” optical measurements under different temperatures and H2-pressures. All films were characterized by profilometry, XRD, FEG-SEM and Raman and FTIR spectroscopy. A drastic influence of the oxide/hydroxide layer on hydrogenation process was observed and it will be discussed in a detailed way at this work. [1] K.F.Aguey-Zinsou, J.R. Ares-Fernandez. Energy Environ. Sci. 2010, 3, 526–543. [2] NB Arboleda, H.Kasai, K. Nobuhara ,W.A. Dino, H.Nakanishi. J Phys Soc Jpn 2004, 73, 745-8.

Resume : The electronic and mechanical properties of TiSe2 monolayers were studied, and the effects of Li atoms on the exposed surface. Li atoms were placed at different adsorption sites at the TiSe2 monolayer, to study the consequences on the electronic and mechanical properties, and to identify the most favourable adsorption site for Li in the TiSe2 systems; that could be helpful to diminish degradation of the monolayer if it is used as a cathode in Li-ion batteries. Calculations were carried out in the Density Functional Theory scheme with the Generalized Gradient Approximation. The monolayer without the Li atom shows a semiconductor behaviour with a bandgap of 0.152 eV; however, when Li atoms are placed at the surface, the behaviour is metallic. That behaviour could be favourable to facilitate the electronic transport by the monolayer. Furthermore, the G/B ratio indicates that all systems are brittle, while the Poisson´s ratio changes from ionic-covalent to strongly covalent when Li atoms are placed at the surface. Finally, the mechanical properties’ analysis supported that the better adsorption sites are those labelled as Top and Hollow. These results would help to understand the degradation process of the cathodes in Li-ion batteries and to make devices that fulfil more demanding functions. Acknowledgments This work was partially supported by the multidisciplinary project SIP-IPN 20201114. J.E. Antonio and H. Muñóz want to acknowledge support from CONACyT and BEIFI-IPN.

Resume : The sol-gel process is a generic route for the liquid to solid transformation of materials. We concentrate in this study in the processing of WO3 to achieve MoTe2-xSex films. Thin amorphous films of WO3 have been grown on Si by spin-coating of a sol-gel precursor and consolidated by ten minutes annealing treatments at 200, 400, 600 and 800ºC. Subsequently, a reduction process was carried out for 2 hours at the same reference temperature in a partial atmosphere of H2. After this process, the films processed at temperatures of 600 and 800ºC have transitioned to metallic tungsten with bcc and cubic crystalline structure, respectively. To achieve MoTe2-xSex films, the samples reduced at 600ºC were exposed to the chalcogenide’s vapors by isothermal closed space vapor transport [1]. This process was done at 600ºC for 15 minutes in a partial atmosphere of H2. To characterize the samples, field emission scanning electron microscopy (FESEM) has been used together with EDAX, X-ray diffraction (XRD), ellipsometry, Rutherford backscattering spectroscopy (RBS) and Raman. On the one hand, FESEM micrographs show that the sol-gel nucleated films are porous and amorphous. After annealing, XRD, ellipsometry and RBS confirm the presence of WO3, with increasing W (metal) clusters after the reduction treatment. On the other hand, FESEM micrographs of tellurized / selenized samples show, for processing conditions at 600ºC, crystalline microstructures in the form of nanoflakes. Stoichiometry has been studied using XRD, EDAX, RBS and Raman, allowing to determine an excesses of Te, which can be useful in the protection of the structures against oxydation. [1] O. de Melo. Journal of Materials Chemistry C, (6), 6799-6807, 2018.

Resume : Producing flexible supercapacitors via inkjet printing is particularly important for future development of electronics. The composition of the ink and the nanoparticles (size, shape and distribution) influence the electrochemical properties of the electrodes. The main problem of inkjet printing is that the conductive inks must possess remarkably high purity in solution form, in order to avoid clogging of the nozzles. The efficiency of the method is directly dependent on the viscosity and the surface tension of the ink. Nevertheless, the preparation of a stable, homogeneous, conductive nanocarbon ink is a huge challenge. Therefore, before manufacturing of printed electrodes, close examination of the ink should be performed. In this project, aqueous dispersions of graphene and multi-wall carbon nanotubes (MWCNTs) with the addition of the surfactant lithium dodecyl sulfate (LDS) are used as the ink for inkjet printing. Tip sonication is applied to obtain individual sheets of graphene and MWCNTs. The formulation is further enhanced by altering the type and the concentration; and by incorporating conductive additives, such as metal nanoparticles. Gold nanoparticles are synthesised using a simple approach of citrate reduction. After examination with AFM and DLS, two populations of nanoparticles are found, one with diameter 2.2 nm and a bigger one with 40 nm. These gold nanoparticles are added into the nanocarbon ink, aiming to produce a 3D network that will present high electrochemical properties when printed as supercapacitor electrodes. They are expected to improve the overall electronic conductivity of the electrode and produce a porous carbon network, thus strongly increase the overall exhibited capacitance of the electrode and the cycling stability. To verify the uniformity and assess the quality of the dispersions, techniques such as XRD, SEM, Raman and ATR-FTIR spectroscopies and electrical characterisation are applied. A general and scalable inkjet-printing technique for construction of supercapacitors, utilizing inks of 0D gold nanoparticles, 1D CNTs and 2D graphene-derivatives is proposed. Here, nanoparticles enhance the areal capacitance of the printed electrode, a continuous network of nanowires forms highly conductive electron paths that promote charge transfer and graphene provides good rate capability and high cyclability. Electrodes with different patterns and layers are printed onto flexible paper and PET substrates. Using a gel electrolyte between them, these electrodes are then assembled in high-performance all-solid-state supercapacitors.

Resume : Thermoelectric (TE) materials exploiting the Seebeck effect are attracting growing attention,1 as TE-based devices allow the direct conversion of waste heat into electricity,2 so improving the overall energy production efficiency. Moreover, TE devices fed by the heat produced during metabolic processes are today considered as promising energy sources for wearable electronics, such as fitness trackers, smart watches, and medical sensors. This latter class of devices should join good TE efficiency and suitable fitting to the human body, as allowed by flexible TE materials. These properties can be achieved by hybrid TE materials combining inorganic electrical conductivity with typically organic flexibility and thermal resistivity. A strategy to achieve these characteristics is that of intercalating organic molecules within the matrix of 2D inorganic compounds.2-3 The design of this class of materials can be rationally carried out by first principles calculation, allowing to forecast the relevant TE properties as concerns in particular the dependence of electronic features, such as density of states, band gap and electron mobility, on the intercalated organic species. In this paper we present the results relative to the intercalation of various linear amines within the van der Waals gap of TiS2, which is a reference compound for 2D TE hybrid materials.4 The calculations are carried out using the QUANTUM ESPRESSO suite.5 The simulations are validated making reference to the structural features of synthesized compounds. References (1) C. Gayner, K. K. Kar, Recent advances in thermoelectric materials, Prog. Mater. Sci. 83 (2016) 330–382. (2) Y. Liu, W. Wang, J. Yang, S. Li., Recent Advances of Layered Thermoelectric Materials, Adv. Sustainable Syst. 2 (2018) 1800046. (3) H. Jin, J. Li, J. Iocozzia, X. Zeng, P. C. Wei, C. Yang, N. Li, Z. Liu, J. H. He, T. Zhu, T. Zhu, J. Wang, Z. Lin, S. Wang, Hybrid Organic-Inorganic Thermoelectric Materials and Devices, Angew. Chem. Int. Ed. 58 (2019) 15206 – 15226. (4) C. Wan, R. Tian, M. Kondou, R. Yang, P. Zong, K. Koumoto, Ultrahigh thermoelectric power factor in flexible hybrid inorganic-organic superlattice, Nat. Commun. 8 (2017) 1024. (5) P. Giannozzi, S. Baroni, N. Bonini, M. Calandra, R. Car et al., QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials, J. Phys.: Condens. Matter 21 (2009) 395502.

Resume : A considerable obstacle to cost-efficient catalysis of the Hydrogen Evolution Reaction (HER) is that inexpensive alternatives to state-of-the-art platinum group metal catalysts are typically unstable in acidic media. Furthermore, these materials suffer very poor efficiencies in alkaline solutions. As such, the development of inexpensive electrocatalysts optimised for alkaline water electrolysis is receiving considerable attention in the research community. Here, we present heterostructured N-carbon/2D electrodes with different catalyst components tailored to enhance specific processes within the multistep alkaline HER. Inexpensive materials such as nitrogen-doped graphitic carbon and 2D transition metal dichalcogenides are employed to this end. Highly graphitised graphitic carbons with controlled distribution of nitrogen functional groups are produced via physical vapour deposition and decorated with molybdenum disulfide flakes to enhance the proton adsorption during the Volmer step of the HER. Materials characterisation via spectroscopic and microscopic techniques probes the structural and chemical environment of these heterostructures. Electrochemical investigation of the catalyst performance is carried out using voltammetry, Tafel analysis, chronopotentiometry etc. Catalysts are evaluated using key performance metrics such as determining onset potential, rate determining step, stability etc. We demonstrate that tailoring the chemical environment of the heterostructured electrocatalyst surface enhances the overall activity of alkaline HER by promoting the efficiency of rate determining steps of the reaction. Thus, inexpensive electrocatalyst systems can be developed to promote alkaline HER and drive down costs associated with electrochemical hydrogen production.

Resume : The effects of relative positions of Se atoms in a real monolayer alloy MoS2(1-x)Se2x have been studied. It is demonstrated that the distribution of Se atoms between top and bottom chalcogen planes is most energetically favorable. For a more probable distribution of Se atoms MoS2(1-x)Se2x monolayer alloy is a direct semiconductor with the fundamental band gap equal 2.35 eV (x = 0.25). We have also evaluated the optical band gap of alloy at 77 K (1.86 eV, x = 0.25) and room temperature (1.80 eV, x = 0.25), which is in good agreement with the experimentally measured band gap of 1.79 eV.