Local probing techniques & in-situ measurements of energy storage & conversion materials

Resume : Single crystals of Topological Insulators (TI) such as Bi2Te3 and Sb2Te3 as well as thin films and nanowires of Bi2Te3 alloys are investigated under 488 nm optical excitation using micro-Raman and photoluminescence spectroscopy in as-grown state or after irradiation. The thin films and nanowires are obtained by electrodeposition in various conditions and are generally amorphous in the as-deposited state. The films are characterised using transmission electron spectroscopy (TEM) and scanning electron microscopy (SEM) equipped with electron dispersive x-ray (EDX) for chemical analysis. It is shown that the single crystals and the thin films irreversibly evolve under the 488 nm excitation both in the as-grown and irradiated state. The ageing under 488 nm illumination depends on the history of the single crystals and films.

Resume : Since its discovery in 1960, metallic glasses have focused a tremendous interest as candidate in engineering, electronics and bio-compatible materials[1-2]. The high Young modulus of this material which can sustain a large elastic stress fundamentally rely on the nanoscale structural characteristics. However, the local atomic structure of these materials still remains poorly understood. Here, we systematically characterize the surface of the Ni40Ta60 metallic glass by means of spectroscopic techniques and scanning tunneling microscopy (STM). High-resolution STM images reveal the coexistence of short atomically ordered chains and disordered domains of icosahedral–like clusters. Time-resolved STM imaging further demonstrates the dynamic of surface clusters at amorphous areas rather than on top of the small crystalline chains. This observation suggests that the amorphous areas correspond to shear transition zones whereas ultra-small crystallites strengthen the material at the nanometer scale. We conclude that concomitant amorphous and crystalline domains are the key to foresee the struggling macroscale peculiarities of metallic glasses.

Resume : The photovoltaic modules (PV) are currently characterized and qualified for moderate climates according to international standard described in IEC 61215 for crystalline silicon PV modules. Unfortunately, there is currently no consensus in the scientific community of PV on a physical standard representing the exact behavior of the PV modules from various technologies in the natural environment and particularly in specific climates. The environmental parameters induce a heat stress in PV module components and particularly encapsulant based on plastic materials such as Ethylene Vinyl Acetate (EVA) after aging. The EVA encapsulant exhibits mechanical properties loss followed by PV module delamination leading to the degradation its electrical performances. The aim of this work is to contribute in this area by using new thermal analysis methods for the characterization of the EVA encapsulant before and after aging. The correlation of PV modules performances at standard testing conditions after aging with the physical and chemical properties modification of the EVA is required. This helps to understand and elucidate the influence of the aging over the life time of the PV module in the real operating conditions. Due to various aspects of the subject, in the first part we have examined the physical and chemical variation of EVA encapsulant characteristics induced by crosslinking reaction during the thermal PV module encapsulation process. In the second part, various analysis techniques were used to quantify EVA degradation within PV module performances loss. The use of thermally stimulated current technique (TSC) in the field of PV encapsulant materials is the most relevant point of this scientific contribution. Keywords: EVA Material; Analysis techniques; PV module.

Resume : EVA as a cell encapsulation material Ethylene-vinyl acetate (EVA) is still the market leader and standard encapsulation material for solar cells. EVA’s widespread use is explained by its favorable processing properties as a cross-linking rubber-elastic material in vacuum Laminators, its adapted technical product features and, by no means least, its low price. For a long time there have been no alternatives on the horizon, as finished photovoltaic (PV) modules produced since the Nineties have a service life of at least 20 years and all series products have to be certified. It is therefore difficult for other plastics to gain a foothold on the market if the points in their favor are rarely measurable. To ensure PV module durability and long-term power generation, encapsulant materials have to display certain important features such as: Mechanical protection of the cell; Electrical insulation; Barrier to oxygen and water vapor; Prevention of cell corrosion. Polyvinyl Butyral (PVB) encapsulant material is being evaluated as a candidate for use in photovoltaic solar cells encapsulation process due to high stability against UV radiation and the high adhesive force to glass. This material is used for a long time in various industrial applications. The long experience in this sector can direct be carried over to the photovoltaic industry. The purpose of this experimental investigation is to better understand the electrical properties and thermal stability of PVB based encapsulant material and their dependence on temperature will be presented. An overview of some main electrical and thermal properties of PVB is compared to EVA. Keywords: Encapsulant materials, Photovoltaic Module, Encapsulation, Electrical properties, Thermal Analysis.

Resume : Controlling the doping within semiconductor nanowires (NWs) is a key issue in order to obtain high performance electronic devices with well-defined characteristics. For instance promising Tunnel Field-Effect Transistors with subthreshold slope less than 60 meV/dec based on the band to band tunnelling of carriers and Esaki tunnel diodes require abrupt junctions and high doping levels. In this work, different axial doping junctions in Au-catalyzed Si NWs were characterized using local probed techniques, i.e., scanning capacitance microscopy (SCM) and Kelvin probed force microscopy (KPFM). Particularly, we will discuss here the observed doping contrast on an axial p-n junction in Si NWs. SCM reveals an abrupt junction thanks to the beneficial use of HCl during the growth which prevents the conformal growth and therefore avoids the formation of a superdoped shell. The homogeneous distribution of dopants on both part is demonstrated and the relative doping is consistent with that expected from the dopants (B2H6 or PH3) over SiH4 flux ratios. On the other hand, a low doping contrast is measured by KPFM due to the presence of surface states. The resulting band bending was exactly quantified by additional photoelectron emission microscopy measurements.

Resume : Assembly of the nanoparticles can be treated in different ways. Some properties can be assigned for separated particles and some for the array. Regardless of the point of view in any case it become extremely complex superstructure. So many parameters can heavily modify nanostructures characteristic due to the large surface-to-volume ratio. Each boundary brings an extraordinary modification of the structure or the properties. Phase transformation of the inorganic materials, can be observed as a result of impact of the high and low temperatures, sonication, pressure, humidity of the environment or physical treatment of the material. Investigation of such transition can be performed by many various methods, where among others XRD and Mössbauer spectroscopy can be applied. In the paper we would like to present phase transition from magnetite to hematite through maghemite of the various type of magnetite nanoparticles. The transition temperature and width of the intermediate state depends strongly of the crystallinity of the separate particles. Discussion of the results will be done on base of XRD, TEM, and Mössbauer spectroscopy.

Resume : SSRM is an AFM-based technique used to probe the 2D-carrier distribution in semiconductor devices. In SSRM, a bias voltage is applied between a conductive tip and a back-contact (BC), and one is recording the current flowing through the sample at every position while scanning. In most cases the current is limited by the current spreading through the tip-sample nanocontact and the so-called spreading resistance (SR) dominating the total resistance is a measure for the local resistivity and thus carrier concentration. However in some circumstances (i.e. devices presenting confined volumes and/or potential barriers) the measured resistance may be dominated by the BC and/or bulk series resistance preventing the use of SSRM. We developed a novel mode, named Fast Fourier Transform-SSRM to overcome this limitation realizing that only the SR depends on the applied force. Hence applying a sinusoidal modulation of the force to the tip, generates a modulation of the contact size and in turn of the SR, whereas the series resistance components (bulk and BC) remain unaffected. Using a FFT analysis, we can isolate the SR amplitude at the applied modulation frequency. Within this work, we present its application on poly-Si solar-cells. We have identified the impact of the grain boundaries on the current conduction. We have also been able to identify variations in active dopant concentration inside the frontside implant from grain to grain (while SSRM was exhibiting saturated profiles).

Resume : Scanning confocal micro-Raman spectroscopy in combination with Kelvin probe force microscopy (KPFM) was applied to study the structure, morphology and electrostatic properties of the graphene flakes on Ni film. Graphene flakes with varied number of layers were obtained on the Ni surface by vacuum thermal treatment of the “sandwich” a-SiC/Ni multilayer structure deposited on silicon wafer by magnetron sputtering. The lateral size of graphene flakes was estimated to be about hundreds of micrometers while the thickness estimated using Raman scattering varied from one to few layers in case of vacuum annealing. Atom force microscopy (AFM) is not able to detect graphene flakes in regime of surface topology examination due to large roughness of Ni surface. Employment of KPFM allowed to demonstrate occurrence of the layers with potential different from potential of the nickel surface, which corresponds to graphene layers, and to perform correlation between number of graphene layers obtained from micro-Raman measurements and potential of the layers. It was shown that KPFM possesses a high sensitivity to graphene layers and graphite nanoclusters and allows us to observe them even in absence of optical contrast from these structures. Additionally confocal micro-Raman mapping of the individual graphene flake make it possible to reveal local microscale variations in number of graphene layers and defects, and correlates them with surface topology and distribution of surface potential.

Resume : This work demonstrates application of confocal micro-Raman spectroscopy to study spatial distribution of charge carriers in bulk GaN crystals grown by hydride vapor phase epitaxy (HVPE) and containing V-shaped defects with high carrier concentration level, and periodic HVPE GaN structures grown at controllably varied composition of the carrier gas (H2 and N2+10%H2). Raman spectra were measured at room temperature using T64000 spectrometer equipped with confocal microscope and piezo-driven XYZ stage with scanning step of 100 nm. Concentration of charge carriers is estimated from the analysis of plasmon-LO-phonon coupled modes (LOPC) in the Raman spectra. Studied HVPE GaN crystals are shown to contain heavily doped (n ~ 2.0÷4.0•10^19 cm^-3) regions inside the V-shaped defects, which is caused by the preferential incorporation of oxygen on the non-polar (semi-polar) planes inside the defects. The interface between the defects and the undoped matrix (n ≤ 10^17 cm^-3) is shown to have sharp step-like carrier profile in micrometer scale. Annealing at high temperatures and high pressures results in diffusion blurring of the carrier profiles at a distance less than 10 um from the interface allowing for the evaluation of oxygen diffusion coefficient in GaN as D ≤ 3.1*10^-12 m^2/s (at T = 3375 K and P = 9 GPa). Obtained results are compared with the results of selective photo-etching and SIMS measurements, and advantages and limitation of the micro-Raman spectroscopy are discussed.

Resume : Magnesium hydride is one of the most promising materials for hydrogen storage applications, having an elevated H-storage capacity (7.6 wt%) and low cost. However, its high thermodynamic stability implies too slow hydrogen sorption kinetics at low temperature. Up to now, different strategies have been investigated to properly tune the thermodynamic properties of magnesium hydride and one of these consists in adding in the Mg matrix transition metal dopants (Nb, Pd, V among others) that often aggregate in nanoclusters. We will show new results of an ad-hoc in situ experiment on the hydrogen desorption kinetics of Nb-doped Mg hydride. In particular, combining for x-ray absorption spectroscopy (at Nb K-edge) with synchrotron x-ray diffraction (at the Italian beamline of the European Synchrotron Radiation Facility) on Nb-doped Mg hydride heated to induce hydrogen desorption, the evolution of the crystalline phases of both the matrix and the dopant is monitored and correlated with the local site of Nb, as probed by x-ray absorption spectroscopy. These results allow to link the hydrogen desorption from Nb hydride nanostructures (about 10-nm sized) with the one from Mg hydride. Finally, the x-ray absorption spectroscopy analysis on the very early stages of Nb cluster formation allows to elucidate the fundamental role of small Nb clusters in triggering the H2 desorption kinetics.

Resume : Half-Heusler intermetallic compounds based on XNiSn (X = Ti/Zr/Hf) and their solid solutions are one of the most promising materials for medium temperature range thermoelectric energy conversion, which is a way to directly convert heat to electricity. A basic requirement for a practical application of a material is to resist real world operating conditions: high temperatures, high temperature gradients and oxidizing atmosphere. The presented work focuses on Ti0.33Zr0.33Hf0.33NiSn Half-Heuslercompound exposed to air at temperatures as high as 873 K. Several experimental methods were applied to study the oxidation behaviour and the phase stability of Ti0.33Zr0.33Hf0.33NiSn: thermogravimetric analysis, ex-situ optical and electron microscopy with energy dispersive X-ray spectroscopyand in-situ atomic force microscopy with scanning Kelvin probe force microscopy. The results show that the examined compound is a two-phase system with dendritic-shaped grains of (Zr,Hf)-rich and intergranular Ti-rich half-Heusler phases, the latter being thermody-namically less stable. The oxidation starts at 545 K. Moreover, tracking of the initial stages of the phase decomposition (temperatures up to 423 K) reveals the important role of grain boundaries and voids, causing local thermodynamic instabilities.Concluding the experimental results, an improvement ofthe stability ofthe XNiSn half-Heusler system can be achieved by limiting the Ti content.

Resume : Thin layers and multi-layered thermoelectric structures are potential candidates for variety of thermoelectric applications such as solid-state coolers, and generators, thermoelectric transducers or thermal sensors. Knowledge of material thermoelectric properties with lateral resolution of tens of nanometres and the problem of accurate characterization is extremely important for these nanodevices. We have been developing a new method for a relative thermal conductivity characterization of thin thermoelectric structures in nanometer scale. The method uses a scanning thermal microscope working in an active constant current mode. For our experiments high quality thermoelectric layers with thermal conductivity in the range from about one-tenth to few tens of Wm-1K-1 with a very smooth surface are needed. The increased surface roughness might cause inaccuracy and bring some difficulties such as artefacts, which typically occur when using any scanning probe microscope measurement technique, including thermal microscope. To fulfil these requirements the study of an influence of the main deposition conditions that include the substrate temperature and the laser beam density on the quality of surface of the Bi2Te3 layers prepared by pulsed laser deposition was done and its results are presented. Crystallinity, composition and morphology of the layers are presented. Nanocrystallites observed on the layer’s surface are studied by X-ray Diffraction and by Atomic Force Microscope. Keywords: thermoelectric materials, thin layers, pulsed laser deposition

Resume : At third generation synchrotrons focused X-ray beams have been used for about two decades adding spatial resolution to diffraction experiments. Whilst the early days the typical beam size was at the 10 micron scale, today nano-beams ranging from few tens of nanometers to a few hundred nanometers are available to investigate micro-/nano-crystal arrangments. We will discuss instrumental aspects of nano-beam scanning diffraction, exemplified on the ID13 nano-probe. This comprises the production of nano-beams, requirements of positional stability, the selection of different types of area detectors, the layout of the nano-goniometer, and sample manipulation. Many scientific topics that can in principle be addressed with nano-diffraction can be enhanced by the use of complementary methods or sample environments in order to perform the experiment under in-situ/in-operando conditions such as working at elevated temperatures [1] and mechanical deformation. Examples from a wide range of scientific applications from material science to polymer research will be presented in the second part of this contribution. Nano-diffraction can be used, for instance, to investigate texture, strain, particle size, and phase composition of thin films and other materials. Mapping the orientation of its crystalline component, the longitudinal morphology of high performance polymer fibres can be imaged in detail at the 100 nm scale. We will conclude with an outlook on future opportunities of nano-diffraction in view of the availability of high performance pixel array detectors and substantial upgrades foreseen of the 3rd generation synchrotrons at the ESRF and elsewhere. [1] Rosenthal, M., et al (2014) J. Synchrotron Radiat. 21 223-228

Resume : Investigation of photovoltaic and photo-catalytic materials by surface photovoltage techniques Abstract: Charge separation in space is crucial for the application of photovoltaic and photo-catalytic materials. Surface photovoltage (SPV) measurements in the Kelvin-probe and fixed capacitor arrangements give information about spectral, time and temperature dependent charge separation in space in semiconducting materials and in ultra-thin charge-selective layers or layer systems. SPV signals contain information about mechanisms of charge separation, electronic transitions from which charge separation is possible, charge transport and recombination. Advantages of SPV techniques are (i) the local sensitivity in one dimension related to regions of charge separation, (ii) the high sensitivity up to charge separation in molecular monolayers, (iii) the ability to bridge the pressure gap between ultra-high vacuum, gas atmospheres and electrolytes and (iv) the special access to materials specific properties. The complex dependence of SPV signals on photo-generation, carrier dynamics and photo-chemical processes makes a straight forward interpretation of SPV signals often difficult so that the investigation of model systems is required. A brief introduction into SPV techniques will be given. Examples of mechanisms of charge separation will be demonstrated for the characterization of charge separation in surface treated semiconductors and across organic and quantum dot monolayers and nano-composites.