Nanostructures for phononic applications

Resume : The thermal conductivity of a silica-gallium nitride nanocomposite is investigated by means of Scanning Thermal Microscopy and predicted using Molecular Dynamics Simulations. In this work we report on the effect of several geometric parameters, as the volume fraction of the nanoinclusions, the total interfacial area and the surface/volume ratio of the nanoinclusions on the thermal properties of the nanocomposite. Upon increasing the size and the volume fraction of the crystalline nanoinclusions, the thermal conductivity increases rapidly even if there are no interconnections among the crystalline islands and it can reach values up to 40% of the thermal conductivity of bulk crystalline GaN. On the contrary, increasing the number of nanoinclusions does not lead to enhancement of the thermal transport. These results are discussed and physical insights are presented.

Resume : Nanostructuring is a promising approach for next generation thermoelectric materials yielding ultra-low thermal conductivities and enhanced thermoelectric performance. More specifically, some of the lower thermal conductivities in nanocrystalline materials have been achieved in materials that include hierarchically sized structures, at the atomic size, the nanoscale, and mesoscale, which can scatter phonons of various wavelengths and reduce phonon transport throughout the spectrum. In this work, we describe the development of a large scale, comprehensive Monte Carlo simulator to model thermal transport in nanostructured materials with a large and arbitrary degree of hierarchical disorder. Geometry induced scattering of phonons on grain boundaries, surfaces, several defects, voids, and dislocations as in realistic nanocomposite which all contribute to reducing thermal conductivity, are investigated. Although this study focuses on Si-based materials, we discuss extensions of our simulator capabilities to other type of promising thermoelectric materials with various types of embedded nanoinclusions. For this, we discuss how we couple useful transport properties from molecular dynamics and ab initio calculations. We believe that this multi-physics/multi-scale approach could play a very useful part in optimizing thermal conductivity reductions not only in advanced new-generation thermoelectric materials.

Resume : Dopants play an important role in engineering the electronic properties of semiconductor materials. At the same time they can strongly influence the phonon scattering processes and thereby the thermal conductivity. We have recently shown how Boron, when substituted in place of Carbon in 3C-SiC, acts as a “super-scatterer” and exhibits resonant phonon scattering which is one to two orders of magnitude higher than other defects.[1] The increased scattering leads to a thermal conductivity that is suppressed by one to two orders of magnitude. The resonance is caused by a large perturbation in the 2nd order inter-atomic force constants (IFCs). This large IFC perturbation is the result of a small lattice distortion accompanied by a change from Td to C3v symmetry around the Boron atom. In order to understand the factors causing resonance in semiconductors, we have studied the influence of charge on the defect structure and phonon scattering.[2] We explain the results with the help of a simple 1D mono-atomic linear chain.[2] We show that small lattice distortions emanating from two or more close energy minima in potential energy surface lead to a very large IFC perturbation. Such a behavior is characterized by a peak in the trace of imaginary part of the T matrix (which is closely related to the scattering rates) and a reflection coefficient approaching unity. The strong influence of the potential energy surface surrounding the defect atom on the thermal conductivity opens a new path to tailor thermal conductivities where required values range from very low in thermoelectric materials to very high in power electronics applications. All the calculations presented were obtained with almaBTE (www.almabte.eu) [3], and supported by the H2020 programme through project ALMA. [1] A. Katre, J. Carrete, B. Dongre, G. K. H. Madsen and N. Mingo, Physical Review Letters 119, 075902 (2017). [2] B. Dongre, J. Carrete, A. Katre, N. Mingo and G. K. H. Madsen submitted [3] J. Carrete, B. Vermeersch, A. Katre, A. van Roekeghem, T. Wang, G. K. H. Madsen, N. Mingo, Comput. Phys. Commun., 2017, 220, 351

Resume : Since 2014 the layered semiconductor SnSe is known to be the most efficient thermoelectric material. At room temperature it crystalizes in the Pnma space group changing its phase at around 800 K to Cmcm. The character of the transition is not fully understood and the thermal conductivity of the high temperature phase has only been studied perturbatively. Using the Stochastic Self-Consistent harmonic Approximation (SSCHA) we show that it is a second-order structural phase transition driven by the anharmonic renormalization of the phonon spectrum. With the anharmonic force-constants calculated using SSCHA we show that non-perturbative effects are inherent of the system and yield a thermal conductivity in good agreement with experiments.

Resume : GeTe, a well-known ferroelectric and thermoelectric material, undergoes a structural phase transition from a rhombohedral to the rocksalt structure at ~600-700 K. We model this phase transition using density functional theory by minimizing the Helmholtz free energy using the elastic and quasi-harmonic approximations and Gruneisen theory. By accounting for up to the fourth order elastic constants and their temperature dependence, we obtain the temperature variation of the structural parameters of rhombohedral GeTe (the lattice constant, the angle between the primitive lattice vectors and the internal atomic displacement) in good agreement with experiment [1]. From the calculated temperature dependence of the transverse optical (TO) mode, we extracted the critical temperature of 701 K and the critical exponent of 0.17, which are in good agreement with experiment [2]. We find that the divergence of the thermal expansion coefficients near the phase transition in GeTe is induced by acoustic phonon coupling to soft TO modes. [1] T. Chattopadhyay et al, J. Phys. C 20, 1431 (1987) [2] E. F. Steigmeier and G. Harbeke, Solid State Commun. 8, 1275 (1970) This work is supported by Science Foundation Ireland PI Award 15/1A/3160.

Resume : We present a comprehensive study of the optical, vibrational, and thermal properties of Ga2O3 in the alpha-, beta-, gamma-, and epsilon-modification. The studied materials include beta-Ga2O3 in different crystal cut orientations grown by the Czochralski method, alpha-Ga2O3 and epsilon-Ga2O3 grown on sapphire, and epsilon-Ga2O3 grown on MgAl2O4 substrates. In addition, we also explore the influence of alloying Ga2O3 with Al in the whole compositional range. We will give a broad overview of the phononic structure of Ga2O3 polymorphs and alloys. The optical vibrational modes (Raman) of these structures will be discussed for different symmetries with special focus on addressing the components of the Raman tensor through polarized Raman measurements. I will discuss the temperature dependence of the thermal conductivity of several oxide polymorphs. The samples were investigated using the 3-omega method, although other state-of-the-art contactless experimental approaches based on Raman spectroscopy will be considered to address thermal anisotropy. The influence of size effects, composition, and temperature on the temperature conductivity reduction will be addressed for most cases.

Resume : The study of nanoscale heat transport is of growing interest following the industry’s trend of downsizing chip components. It is of great interest for both the industrial and scientific community to understand heat transfer at low dimensions in order to improve devices performances as well as understanding physical phenomena emerging from this downsizing. One important challenge to overcome is to accurately measure the thermal properties of low-dimensional materials. Indeed, creating both a heat source and a thermometer in size-constrained materials is challenging from a technological point of view, and the interpretation of experimental data is as well more complex when the size of the probed material becomes comparable to the heat source, thermometer or even to the heat carriers characteristic lengths. The so-called 3ω method is an electrical-based technique allowing the thermal conductivity measurement of substrates and thin films. A thin metallic layer deposited on top of the studied material acts simultaneously as a heat source and as a thermometer, resistance thermometry then allows to extract the underlying material thermal properties accurately. The thermal conductivity measurement of very thin layers is challenging due to the presence of interfacial thermal resistances between the studied film and the neighboring materials. The contribution of these unknown thermal resistances to the total heat balance impedes a sensitive estimation of the thermal conductivity of the thin films. The constraints on the geometries and thermal properties of the system (transducer, stacking of thin films and substrate) often lead to use approximate models to facilitate data treatment, or to use comparative technique such as the so-called differential 3ω in order to extract the thin film contribution from the total measured thermal resistance. Here we will present experimental results comparing the thermal conductivity of SiN very thin films (30-120nm) extracted from the complete analytical solution given by Borca et al. and the thermal conductivity obtained from a differential setup (requiring two samples) [1]. The results obtained with two different techniques are in perfect agreement and allow an estimation of the interfacial resistance of the whole stacking being measured. We also present interfacial thermal resistance measurements of a multilayer system composed of Ge/Al2O3/Pt as a function of temperature. The interfacial thermal resistance measurements are carried out on several stacking with varying thicknesses of the Al2O3 layer to isolate the effect of the interfacial thermal resistances. This allows as well the extraction of the Al2O3 thermal conductivity as a function of temperature with the two techniques (differential and analytical solution). Although each technique has its benefits and limitations, the present results show a great viability and reproducibility between the two methods. [1] Borca-Tasciuc, T., Kumar, A. R., & Chen, G. (2001). Data reduction in 3ω method for thin-film thermal conductivity determination. Review of Scientific Instruments, 72(4), 2139–2147. https://doi.org/10.1063/1.1353189

Resume : Progress in the fields of electronics and photonics has brought a wide range of technological developments in the last decades. In comparison, the control of phonons is still falling short. Logical operations using phonons to switch, amplify and route signals can be realized by nanoscaling. An important building block of a phononic integrated circuit would be a thermal diode. A thermal diode is a device where the heat flow is suppressed when a heat gradient is applied in one direction, as compared to the heat flow when the gradient is applied in the opposite direction. In this work the thermally rectifying behavior is explored in semiconductor nanowires with a sudden change in diameter, referred to as “telescopic nanowires”. Increased boundary scattering in the thin part reduces the temperature dependence, causing a dependence on the direction of the heat flow. Theoretical work on telescopic nanowires showed a potential rectification of more than 50%[1]. In this work the thermal transport behavior of telescopic silicon nanowires is studied experimentally. These wires are created by partly embedding a silicon nanowire in a polymer and subsequent etching of the exposed part of the nanowire. The thermal transport properties of the wires are studied using micro-Raman thermometry, combined with electronic heaters to control the thermal gradient. A heat transfer model developed to describe telescopic nanowires will also be discussed. [1] X. Cartoixà, et. al., Nano Letters, 15 (12), 8255-8259 (2015)

Resume : According to our current understanding, thermal rectification in heterojunctions derives from the different temperature dependence of the thermal conductivity of the constituent materials. This view, however, neglects thermal boundary resistance, TBR, that plays a central role in nanoscale systems. Here we show that in nanoscale systems the physical mechanisms that result in heat rectification are more complex and include an active role of the interface through the temperature dependence of its TBR. In these conditions a suitably designed resistance mismatch, at odds with the matching requirement for bulk systems, can lead to a large heat rectification, exceeding the typical values obtained in bulk systems. Besides it can be much larger, it can even change sign with respect to the cases in which it can be neglected. We show, with theoretical calculations of a Si/GaP and a graphene/bilayer graphene heterojunction, that whenever the TBR of a heterojunction is large compared to the overall thermal resistance, a common situation in nanoscale systems, its temperature dependence can be exploited to engineer an interface-driven heat rectification. This effect is amplified in a suitable resistance mismatching condition, which results in very different interface temperatures, and thus TBR, under forward and reverse thermal bias. This is at odds with the design strategy of a bulk thermal diode, where the resistance matching condition maximizes the heat rectification. These findings reveal a new physical mechanism uderlying thermal rectification in nanoscale systems and open the way to a more flexible design of efficient thermal diodes.

Resume : When the characteristic length is shorter than the phonon mean free paths in nanostructured materials, the interface thermal resistance (ITR) will play a dominant component of the total thermal resistance. There are lots of factors affect the ITR and make the ITR prediction become a mathematical high dimensional problem. Here, we demonstrated by both experiments and data science approach an alternative strategy for achieving thermal insulating nanostructure. The BiSi system was selected from the machine learning prediction model and manufactured with various parameters by laboratory-built combinatorial sputtering system. The BiSi nanostructure is composed of crystallized Bi and amorphous Si without any additional phase. The Bi grain sizes, Bi/Si interface and Si/Bi volumetric ratio in the BiSi nanostructure have major effects on the thermal resistance. The ideal parameter range of Bi/Si interface design for the thermal insulating nanostructure is defined and an extremely low thermal conductivity, 0.21 W/mK, measured by frequency-domain thermoreflectance (FDTR) in a dense material is achieved. The power of data science in materials innovation is limited by data availability and reliability. “Data science + physics and chemistry “is a solution to obtain maximum effect with minimum data. By means of the machine learning prediction model, the potential systems for specific thermal management and application such as thermoelectric materials can be discovered and realized.

Resume : Extensive research is ongoing into the sources of dielectric loss in ferroelectric materials at low frequencies, and several mechanisms were identified. Recent scanning impedance microscopy measurements have pinpointed the domain walls in multiferroic hexagonal manganites as a source of loss in the GHz frequency range. The impedance peaks at ferroelectric domain walls have been attributed to the excitation of the domain wall translation mode, that leads to the domain wall oscillations in the periodic potential of the lattice. These domain wall-localized vibrations can be easily excited and guided along domain walls. Modeling of the slow dynamics of domain walls in ferroelectrics and the implications for possible devices will be discussed. [1] Xiaoyu Wu et al., Science Advances, 3, e1602371 (2017) DOI: 10.1126/sciadv.1602371

Resume : Elastic strain engineering utilizes stress to realize unusual material properties. For instance, strain is used to enhance the electron mobility in semiconductor thin films. In the context of nanomechanics, the pursuit of ultra-coherent resonators has led to intense study of ?dissipation dilution?, a technique where the stiffness of a material is effectively increased without added loss. Dissipation dilution causes the anomalously high quality factor of thin-film Si3N4 nanomechanical resonators; however, the paradigm has so far relied only on the strain produced during material deposition. Geometric strain engineering techniques?capable of producing local stresses near the material yield strength?remain largely unexplored. Here, we will present a spatially non-uniform phononic crystal pattern, used to co-localize the strain and flexural motion of a stoichiometric silicon nitride nanobeam, while increasing the former to near the yield strength. This combined approach produces string-like modes with Qf products approaching 10^15 Hz, exceeding previous values for a room-temperature mechanical oscillator of any size. The devices we have realized can have force sensitivities of aN/rtHz perform hundreds of quantum coherent cycles at room temperature, and attain Q>400 million at megahertz frequencies.

Resume : The capability to tune the acoustic phonon dynamics in technologically relevant group IV nanostructures provides a promising prospect to control the propagation of acoustic and thermal phonons with great implications on nanoscale hypersound and thermal transport. Despite their fundamental importance, accurate measurements of acoustic phonon lifetimes are challenging and their values are still unknown for most materials. Even in the case of the extensively studied group IV semiconductors, measurements of phonon lifetimes are scarce and the impact of strain engineering is not well understood. In this work, we address the influence of tensile stain on the thermo-mechanical properties of suspended Ge nano-bridges with thicknesses below 100 nm. It is shown that the acoustic phonon lifetimes can be tuned both by strain engineering of the suspended structures and strain modification by temperature variation in addition to a strong dependence on the thickness of the suspended structures [1]. The combination of temperature dependent micro-Raman and femtosecond reflectivity measurements allows for a complete decoupling of the effects of temperature, geometry, and strain on the acoustic phonon dynamics [2, 3]. [1] J. Cuffe et al., Phys. Rev. Lett. 110, 095503 (2013). [2] S. Neogi et al., ACS Nano 9, 3820 (2015). [3] M. R. Wagner et al., Nano Letters 16, 5661 (2016).

Resume : Progress in the last few decades in nano-scale thermal transport has enabled a significant degree of control over heat and sound propagation by lattice vibrations - phonons. The latest investigations on the thermal properties of silicon, the most common material in electronics, micro- and nano-electro-mechanical systems (MEMS and NEMS) and photonics, have pointed to nanostructuring as a highly efficient approach to acoustic phonon engineering [1-3]. Heat conduction in silicon can be effectively engineered by means of sub-micrometer porous thin free-standing membranes [1,4]. Tunable thermal properties make these structures good candidates for integrated heat management units such as waste heat recovery, rectification or efficient heat dissipation. However, possible applications require detailed thermal characterization at high temperatures which, up to now, has been an experimental challenge. In this work we use the contactless two-laser Raman thermometry to study heat dissipation in periodic porous membranes at high temperatures via lattice conduction and air-mediated losses [4,5]. We find the reduction of the thermal conductivity and its temperature dependence closely correlated with the structure feature size. Based on two-phonon Raman spectra, we attribute this behavior to diffuse (incoherent) phonon-boundary scattering. Furthermore, we investigate and quantify the heat dissipation via natural air-mediated cooling, which can be tuned by engineering the porosity. References: 1. M. Maldovan, Nature 503, 209 (2013). 2. M.R. Wagner, B. Graczykowski, J.R. Reparaz et al., Nanoletters 16(9), 5661–5668 (2016). 3. B. Graczykowski, M. Sledzinska, F. Alzina et al., Physical Review B 91, 075414 (2015). 4. B. Graczykowski, A. El Sachat, J.R. Reparaz et al. Nature Communications 8, 415 (2017). 5. J. Reparaz, E. Chavez-Angel, M. Wagner et al., Review of Scientific Instruments 85, 034901 (2014).

Resume : Nanoscale phononic crystals can be used in many areas including high-frequency signal processing and the control of thermal phonons. Technological applications of phononic crystals require in many cases a precise control of the vibrational band structure. Particularly important is the ability to control the frequency and width of vibrational band gaps. In this work, molecular dynamics simulations and finite element method calculations are used to study the vibrational band structures of nanoscale two-dimensional phononic crystals made from silicon. A comparison of results obtained with both methods shows that a simple two-dimensional linear-elasticity model is able to reproduce quantitatively the low-frequency part of the band structure obtained from computationally much more demanding molecular dynamics simulations of a three-dimensional atomistic model. The linear elasticity model is then used to study the effects of lattice symmetry, mass density distribution and grain boundaries on the vibrational band structures. The results show that these mechanisms affect the two lowest phononic band gaps in different manners. This makes it possible to control the frequency and width of the two band gaps independently.

Resume : We characterize nanometer-thin suspended silicon membranes in the pristine state or perforated in phononic crystals with periods below 100 nm, by means of scanning thermal microscopy (SThM) and Raman thermometry (RTh). Spatial resolution of SThM is shown to be similar to the one of RTh on these samples, once the air contribution is eliminated. Temperature distributions along the membranes are obtained, but require precise determination of the optical absorption for RTh as the membranes are also photonic objects. We find that the techniques are sensitive to cross-plane and in-plane heat conduction, and that the native oxide on both sides of the membranes cannot be neglected. The perforation depth of the holes is also a key quantity. For instance, the reduction of thermal conductivity in comparison to bulk silicon is 2.5 for a pristine membrane of 60 nm thickness, and limited to 6 for moderately-perforated arrays with pitch equal to thickness. The results are analyzed in light of phonon mean free path computations in complex geometries.

Resume : Doubly-clamped pre-stressed silicon nitride string resonators excel as high Q nanomechanical systems enabling room temperature quality factors of several 100,000 in the 10 MHz eigenfrequency range. Dielectric transduction is implemented via electrically induced gradient fields, providing an ideal platform for actuation, displacement detection and frequency tuning. The two orthogonal fundamental flexural modes of a single string vibrating in- and out-of-plane with respect to the sample surface can be engineered to tune reversely. This allows bringing both modes into resonance where a pronounced avoided crossing is observed, indicating strong mechanical coupling and the formation of a mechanical two-mode system. Here we focus on the nonlinear dynamics of the string subject to a strong resonant or parametric actuation, and discuss the string’s response both far from the avoided crossing where the individual eigenmodes can be considered, and in the coupling region where the normal modes of the two-mode system have to be taken into account. Depending on the actuation amplitude and frequency, satellite resonances arise which enable deep insights into fundamental properties of the system.

Resume : Acoustic phonons in the sub-THz range have emerged as a suitable platform to study complex wave physics phenomena. This has spurred the development of a large bandwidth of versatile nanophononic devices for full control and manipulation of phonons on the few-nm length scale. Furthermore, the strong interactions between acoustic phonons and other excitations in solids extend the range of applications for nanophononic devices into other areas of research such as electronics and optomechanics. For example, recent advances in material science and fabrication techniques enabled the fabrication of nanometric devices in which photons (VIS-NIR) and phonons (GHz-THz frequencies) are simultaneously confined in a single resonant cavity giving rise to unprecedented large optomechanical coupling factors. In addition, the engineering of acoustic waves with GHz-THz frequencies is also at the base of the study of mechanical quantum phenomena and non-classical states of mechanical motion. In this work I will first introduce and compare strategies to generate, manipulate and detect ultra-high frequency acoustic phonons using either ultrashort laser pulses or high resolution Raman scattering. Second, I will describe the acoustic behavior of standard nanophononic Fabry-Perot resonators and finally present experimental and theoretical results on a series of novel nanomechanical devices able to control the interactions between light, sound and charge at the nanoscale.

Resume : In nano-phononics experimental studies and applications, features related to the phonon transport are entangled with other phenomena also due to the strongly confined heat sources which are needed to activate phonon transport. In order to correctly design devices and control experiments accurate modelling is needed, especially when the reference systems are complex 3D structures composite by ~nm wide objects with different shapes and made of different materials. We present a computation tool, which we have developed for the simulation of phononic systems' features. The code is designed to efficiently simulate 3D structures with TCAD capabilities for the design of the system. Coupling between system's regions ruled by advanced transport models (derived by the Boltzmann transport equation) and regions ruled by the conventional Fourier law is considered. Heating can be also evaluated by means of self consistent solutions of the Maxwell equations where optical constants are functions of the local average phonons' energy (temperature in quasi-equilibrium conditions). A preliminary calibration is provided for given materials/phases. Applications of the simulations will be discussed in order to demonstrate the potentiality of the method.

Resume : The direct conversion between thermal and electrical energies by thermoelectric devices is becoming an important alternative for the clean energy generation. Nanowires seem to be promising candidates, whose efficiency is determined by the dimensionless thermoelectric figure-of-merit (ZT) that can be calculated by using the Boltzmann formalism [1]. The inherent correlation between the thermoelectric quantities, such as electrical and thermal conductivities, makes difficult to improve the value of ZT. In this work, we study the thermoelectric properties of nanowires with periodic and quasiperiodically placed Fano defects [2] by means of a real-space renormalization plus convolution method developed for the Kubo-Greenwood formula [3], in which tight-binding and Born models are respectively used for the study of electric and lattice thermal conductivities [4]. The results reveal an enhanced ZT when the quasiperiodicity is introduced, because it diminishes the thermal conduction of long wavelength acoustic phonons that are not easy to block their transmission since they do not feel local defects neither impurities. This work has been supported by CONACyT-252943 and UNAM-DGAPA-IN106317. Computations were performed at Miztli of DGTIC-UNAM. [1] J. E. Gonzalez, V. Sanchez, C. Wang, J. Electron. Mater. 46, 2724 (2017). [2] V. Sanchez, C. Wang, Phil. Mag. 95, 326 (2015). [3] V. Sanchez, C. Wang, Phys. Rev. B 70, 144207 (2004). [4] C. Wang, F. Salazar, V. Sanchez, Nano Lett. 8, 4205 (2008).

Resume : One intriguing feature of van der Waals materials is the layer thickness and misorientation angle dependence that involve stark optical gain and electrical transport modulation. Yet, the phase transformation modulated by the misorientation angle has never been accessible to date. Here, we report misorientation angle-dependent phase transformation of multilayer MoS2 via in situ electron beam irradiation. AA’ stacked-bilayer MoS2 undergoes structural transformation from 2H semiconducting to 1T’ metallic phase similar to monolayer MoS2, which is confirmed via in situ transmission electron microscopy. Meanwhile, non-AA’ stacking which has no local AA’ stacking order in Moire pattern does not reveal such a phase transformation. While collective sliding motion of chalcogen atoms easily occurs during transformation in AA’ stacking, such a collective motion in non-AA’ stacking is suppressed by weak van der Waals strength and furthermore by the interlocked chalcogen atoms at different orientations, which unfavor their kinetics by the increased entropy of mixing.

Resume : Silicon nanowires have attracted high attention for their possible application in nanoelectronics and thermoelectricity. Because of phonon boundary scattering, thermal transport in such structures strongly depends on the dimensions and the surface state (roughness) and is still not fully understood. With means of Non Equilibrium Molecular Dynamics and Monte Carlo resolution of Boltzmann Transport Equation, we studied the radial evolution of the heat flux inside silicon nanowires. It is found with both methods that the heat flux is maximum at the center of the nanowire, while it decreases when reaching the exterior surfaces of nanowires. Surprisingly, this phenomenon does not depend on the diameter of the nanowire as long as d < 500nm. For macroscopic diameters, the heat flux becomes constant as boundary scattering weakly affect thermal transport. Phonon Density of States (DOS) were computed by Molecular Dynamics for different regions in the structure. DOS close to the free surface is found to differ from the one at the center of the nanowire, revealing more populated modes at lower frequencies.

Resume : The high cost and complex production techniques restrict the use of the conventional thermoelectric (TE) generators. Providing a low-cost TE device with a respectable performance is even more important than providing a very high-performance TE device which would cost an unexpected price. Silicon meets the criteria of abundance and non-toxicity, but it is characterized by a poor TE efficiency with a low figure of merit, ZT. We have established a versatile, cost-effective, reproducible and scalable synthesis route for the fabrication of a thermoelectric metamaterial based on nanostructured fibres. The architecture developed is self-supported and can easily be moulded to any shape. For p-silicon gives very low thermal conductivity due to the influence of phonon boundary scattering at the surface which leads to a figure of merit as high as 0.34 at T=823K. The extension to other materials is also possible and will be shown. We finally propose cost-efficient and environment-friendly TE devices for middle and high operating temperatures by harnessing silicon and silicon technology. By using the silicon-based thermoelectric metamaterial, we greatly reduce the cost production of the thermoelectric generator module.

Resume : Silicon nanowires are promising candidates for thermoelectric harvesting since they combine an easy integration in electronic devices with an enhancement of thermoelectric performance conferred by nanostructuring. The measurement of the thermoelectric figures of these nanostructures is a complex task, requiring sophisticated strategies to obtain rigorous results. In this work, Si nanowires were integrated in different planar micromachined structures. Vapour-Liquid-Solid synthesis in Chemical-Vapour-Deposition reactors (CVD-VLS) was used to grow the nanowires, using gold particles as seeds. This bottom-up approach allows fabricating doped epitaxial nanowires with controlled properties (diameter, length, doping level and alloy composition) fully integrated in silicon microdevices, i.e. monolithically grown with low contact resistances. Different thermoelectric characterization silicon micromachined structures were used for the assessment of the thermal and electrical properties of the single nanowires and arrays. As a result, it was possible to determine ZT of a series of individual suspended nanowires from room temperature up to 350°C. The epitaxial nature of the growth allows overcoming the possible artefacts coming from contact resistances. The results arisen from this study demonstrate the feasibility of a practical implementation of this technology in integrated thermoelectric power generators.

Resume : Thermoelectric performance of Ag nanoparticle dispersed Indium selenide bulk materials Pallavi Dhama, Aparabal Kumar and P. Banerji Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India Direct conversion of thermal energy to electrical energy can be realised through the transmission and interaction of charge carriers and phonons in the solid by using thermoelectric effect. To tailor the thermoelectric properties of In4Se3, concept of energy filtering of the charge carriers applied in the present work. The energy barriers at the grain interfaces of the Ag and In4Se3 are supposed to control carrier scattering. Polycrystalline In4Se3:Ag samples were prepared by solid state reaction, ball milling and spark plasma sintering at 693 K for 5 min with a pressure of 70 MPa under vacuum condition. Structural characterizations were performed to study the phase purity, crystal structure and microstructure. To study thermoelectric properties of all the samples, temperature dependent electrical resistivity, Seebeck coefficient, Hall and thermal conductivity measurements were performed in the temperature range of 300 K – 700 K. All samples had negative thermopower which indicated that electrons transported the electric charge. In4Se3:Ag samples showed increased thermopower and decreased thermal conductivity which was attributed to the Ag nanoparticles as an effective phonon scattering centres.

Resume : Optimisation of thermoelectric materials requires a process of structural tuning in the nano/meso-scale regime to reduce thermal conductivity with minimal effect on electrical properties. Here we use in-plane thermal conductivity measurements which are particularly sensitive to thin film morphology, and apply this to several materials including conductive polymers and halide perovskites. Halide perovskite films of various thicknesses and morphologies were formed by thermal co-evaporation of methyl ammonium iodide with lead chloride. Despite an intrinsically low thermal conductivity in this material, we measure a sizeable Kapitza resistance (thermal boundary resistance) and find a further reduction in thermal conductivity when grain sizes are in the range 10-100nm. I will also present our results on anisotropic thermal conductivity in highly aligned polymer films, using doped polymers of interest for thermoelectric applications. An asymmetry in thermal conductivity of ~2 is observed between the directions parallel and perpendicular to the chain alignment, with implications for using highly oriented polymer films for thermoelectric applications.

Resume : Thermoelectricity is one of the best alternative sustainable energy sources to fulfill the global demand for energy with a rapidly increasing population as it converts waste heat into electricity. In the present work, Ge doped Cu3SbSe4 samples (Cu3Sb1-xGexSe4, x = 0 to 0.05) were made by melt growth, ball milling and spark plasma sintering. Structural and microscopic characterizations such as XRD, SEM and TEM were carried out to analyze the phase purity, crystal structure, crystallite size, morphology and microstructure of the samples. Electrical conductivity, thermal conductivity, and Seebeck coefficient measurements were performed in the temperature range 300 - 650 K. Enhanced power factor in Ge doped samples is attributed to the increased electrical conductivity via the increase in carrier concentrations while low thermal conductivity is due to the increased phonon-phonon interactions. Moreover, presence of nanograins in Cu3SbSe4 enhance the scattering of phonon by grain boundaries and defects. Transport mechanism involved in all the samples are explained well within the limit of acoustic phonon scattering approximations with the single parabolic band model. Our results show that Ge doping significantly increases the figure of merit (ZT) of Cu3SbSe4 due to low lattice thermal conductivity and high power factor with maximum ZT (~0.78) in Cu3Sb0.98Ge0.02Se4 at 625 K. Thus, the Ge doped p-type Cu3SbSe4 is a suitable material for green energy application.