Session D12: Focus Session: Low Dimensional Thermoelectric Systems and Theory I

Molecular Dynamics Study of Heat Transport in Silicon--Germanium Nanoscale Metamaterials

We have studied the thermal properties of Si--Ge metamaterials with lattice constants of up to tens of nanometers using molecular dynamics simulations and the Green--Kubo method. Validation of this approach is provided by comparing computed thermal conductivities to experimental data for bulk Si--Ge alloy systems. Close agreement with experiment in a large temperature range is found when isotopic effects are taken into account and interatomic potentials are directly parameterized against higher-level semilocal density-functional theory calculations. These simulations highlight the importance of surface morphology, isotopic substitution, alloy fraction, and superlattice periodicity in determining the thermal properties of these metamaterials, suggesting design strategies to control heat transport in nanostructures.   

First principles study of thermoelectric properties of IV-VI semiconductor superlattices

Thermoelectric materials (TE) have attracted great attention due to their ability to convert heat directly into electricity. However, to be commercially competitive with existing technology, TE devices must have a higher value of figure of merit ZT. It has been proposed to improve ZT by using multilayered systems or superlattices (SLs) resulting in 1D or 2D carrier confinement, reduction of the phonon thermal conductivity, and introduction of anisotropy effects. Here we study the TE properties of IV-VI derived semiconductor SLs. By using the Boltzmann transport theory, within the constant scattering time approximation, in conjunction with first principles calculations, we study the Seebeck coefficient (S) and ZT of PbTe/SnTe SLs. The calculated S shows good agreement with recent experimental data. An anisotropic behavior is observed for low carrier concentrations less than 10\textasciicircum 18cm\textasciicircum -3. For T $=$ 900 K, a large value of ZT$_{\, }$parallel to the SL axis equal to 2.6 is predicted for n$=$1.2x10\textasciicircum 18cm\textasciicircum -3, whereas ZT perpendicular to the SL axis peaks at the value 1.4 for n$=$5.5x10\textasciicircum 17 cm\textasciicircum -3. Both electrical conductivity enhancement and reduction of thermal conductivity are analyzed, and a comparison with other multilayered systems such as planar-doped PbTe is done.   

How bilayer excitons can greatly enhance thermoelectric efficiency

Presently, a major nanotechnological challenge is to design thermoelectric devices that have a high figure of merit. To that end, we propose to use bilayer excitons in two-dimensional nanostructures. Bilayer exciton systems are shown to have an improved thermopower and an enhanced electric counterflow and thermal conductivity, with respect to regular semiconductor-based thermoelectrics. We suggest an experimental realization of a bilayer exciton thermocouple. Based on current experimental parameters, a bilayer exciton heterostructures of $p$- and $n$-doped Bi$_2$Te$_3$ can enhance the figure of merit an order of magnitude compared to bulk Bi$_2$Te$_3$. Another material suggestion is to make a bilayer out of electron-doped SrTiO$_3$ and hole-doped Ca$_3$Co$_4$O$_9$.   

Effect of phonon-blocking at sintered interfaces

With an aim to develop high figure-of-merit silicon nanocrystalline thermoelectrics, controllability of thermal conductivity is demonstrated by combining computation, measurement, and material synthesis. Direct measurements of interfacial thermal conductance at sintered interfaces using a 2D model interface reveal that the interfacial thermal conductance can be greatly reduced by precipitating silicon oxide crystalline nano-dots at the interface. Furthermore the impact of the reduction in interfacial thermal conductance on the overall thermal conductivity of the bulk nanocrystalline material is identified by multiscale phonon transport calculation using intrinsic phonon properties obtained from first principles. These analyses help us identify the required interfacial structure and grain size (mean value and distribution) for a target thermal conductivity. Attempts to implement this in the actual material development will be also introduced.   

Thermal boundary resistance in Si/Ge interfaces determined by approach-to-equilibrium simulations

Nanostructured materials hold great promises as efficient thermoelectrics. In such materials, the propagation of phonons is hindered by the internal interfaces (grain boundaries), leading to a reduced overall thermal conductivity and, therefore, to a larger figure of merit. Any further improvement in this field does, however, require a better fundamental understanding of the specific interface effects on thermal transport. In the present work we use approach-to-equilibrium molecular dynamics simulations (AEMD) [1] to investigate the interfacial thermal resistance (ITR) of Si/Ge interfaces, occurring in very promising nanostructured SiGe alloys [2]. We discuss how ITR depends on the thickness of the interface layer, as well as on its composition. Furthermore, the effect of the heat flux direction has been investigated at ambient temperature showing lower ITR for thermal transport from Si to Ge than vice versa. This feature is discussed in connection to possible rectification effects.\\ Reference: [1] C. Melis, R. Dettori, S. Vandermeulen, L. Colombo, Eur. Phys. J. B 87, 96 (2014) [2] C. Melis and L. Colombo, Phys. Rev. Lett. 112, 065901 (2014)   

Enhancing the thermoelectric performance and bridging the $p$- and $n$-type carrier asymmetry of Bi$_{2}$Te$_{3}$ thin films \textit{via} topological surface states

It has been recognized that some of the best thermoelectric materials are also topological insulators (TIs), yet whether these two classes of materials are inherently connected remains mysterious and conceptually perplexing. Here we combine first-principles calculations and Boltzmann theory to study the thermoelectric properties of Bi$_{2}$Te$_{3}$ thin films in the few quintuple layer regime, and demonstrate how the \textit{ZT} values of such strong three-dimensional TIs can be tuned by both the film thickness and relaxation time of the topological surface states relative to the bulk states. We first show that when the surface and bulk states have comparable relaxation times, such films could actually have higher \textit{ZT} values in the non-TI regime than those in the TI regime. Nevertheless, the very existence and robustness of the topological surface states in the TI regime offers unique new design strategies to not only significantly enhance their \textit{ZT} values, but also potentially bridge the long-standing challenge of $p$- and $n$-type carrier asymmetry faced by the broad thermoelectric research and industrial communities.   

Specific Heat and Thermoelectric Power of Germanane

Germanane(GeH) is a new two-dimensional hydrogen-terminated germanium graphane analogue semiconductor that has been successfully synthesized only recently [1]. We will report on the temperature dependence of the specific heat Cp of GeH from 2K to 300K. The specific heat differs considerably from the Debye model for the parent three-dimensional solid Ge. At low temperature, Cp follows a power law that approaches a T$^{3}$ law, but no saturation to a Dulong-Petit value is observed up to 300 K. Errors of this experiment mainly come from mass uncertainty. Theoretical calculation of the phonon spectra will be shown, and the calculated specific heat compared to the experimental one. The calculated Debye temperatures for the different modes are higher than 400 K, which is above the temperature where the material becomes amorphous. The thermopower of p-type doped material will also be reported. \\[4pt] [1] E. Bianco \& al., ACS Nano 7 4414-4421 (2013).   

Length scale dependent of thermal conductivity of Si-Ge alloys

A crucial aspect of the optimization of the thermoelectric figure of merit involves manipulation of the lattice thermal conductivity without significantly effecting electronic mobility. In order to fully understand the contributions to the lattice thermal conductivity, we present a calculations based on a phonon frequency-dependent model. This model, derived using the effective medium method, predicts the lattice thermal conductivity reduction due to the presence of nanoinclusions in a matrix. We further extend our work to study fully nanostructured materials. By using this method, the dependence of lattice thermal conductivity on various length scale is determined. We validate these models with experiment results obtained via time-domain thermoreflectance. By varying the modulation frequency of this pump-probe technique, we are able to measure the thermal conductivity of Si and Si-Ge systems over a variety of thermal penetration depths. We use this combination of modeling and experimental findings to gain insight into the relationship between phonon mean free path and the lattice thermal conductivity.   

Calculations of the thermopower in materials with nano-inclusions using quantum mechanical simulations

Inclusion of nano-crystalline structures in thermoelectric materials has shown the potential to greatly enhance their thermopower. In this work we present a fully quantum mechanical simulation study of thermoelectric transport through 2D channels in the presence of 0D nano-inclusions, which effectively act as barriers for energy filtering. For this, we use the non-equilibrium Green's function (NEGF) method. We show that improved thermopower can be achieved in such structures compared to pristine geometries. We find that these materials disturb the flow of low energy electrons more compared to the flow of high energy electrons, which enhanced anisotropy and benefits the thermopower. Furthermore, we show that the largest improvements in the Seebeck coefficient can be achieved when the channels become very narrow, on the order of the nano-inclusions' feature sizes. Under such large confinement, the channel provides only a few paths for electrons to flow, and allows only high energy electrons to propagate easily, which improves filtering and, thus, increases the Seebeck coefficient.   

Electronic structure and thermoelectric properties of (PbSe)$_{\mathrm{m}}$/(SnSe)$_{\mathrm{n}}$ superlattice: A first principles study

Figure of merit (ZT) of thermoelectric materials can be enhanced by lowering thermal conductivity or/and increasing electrical conductivity. The extremely high ZT of layered structure SnSe\footnote{Li-Dong Zhao \textit{et al}., Nature \textbf{508}, 373 (2014).} opened up a new direction in study of thermoelectricity due to its low thermal conductivity, which, however, is limited to high temperature. Here, we performed first principles density functional calculations to explore room-temperature thermoelectricity. We consider (PbSe)$_{\mathrm{m}}$/(SnSe)$_{\mathrm{n}}$ superlattices with different period, whose quantum well structure is expected to increase electrical conductivity by modulation of charge doping at interface. Calculations of Seebeck coefficients for the superlattices are presented.   

Tuning thermal transport ultra-thin silicon membranes: Influence of surface nanostructures

A detailed understanding of the behaviour of phonons in low-dimensional and nanostructured systems provides opportunities for thermal management at the nanoscale, efficient conversion of waste heat into electricity, and exploration of new paradigms in information and communication technologies. We elucidate the interplay between nanoscale surface structures and thermal transport properties in free-standing silicon membranes with thicknesses down to 4 nm. We demonstrate that whereas dimensional reduction affects the phonon dispersion, the surface nanostructures provide the main channel for phonon scattering leading to the dramatic reduction of thermal conductivity in ultra-thin silicon membranes. The presence of surface nanostructures, by means of pattern formation and surface oxidation, leads to a 40-fold reduction in the in-plane thermal conductivity of the thinnest membrane. We also investigate the effect of chemical substitution and the geometry of the nanostructures in the thermal transport properties of the membranes. We show that local strain induced by nanostructuring enables tuning of the thermal conductivity of these nanophononic metamaterials[1]. [1] B. L. Davis and M. I. Hussein, Phys. Rev. Lett., 112, 055505 (2014).   

Thermal Conductivity of Nanocrystalline Silicon Prepared by Chemical-Vapor Deposition

Thin film nanocrystalline silicon prepared by chemical-vapor deposition is an established material used in multijunction amorphous silicon solar cells. Its potential in low cost and scalable thermoelectric applications depends on the reducing grain sizes to nanometers while simultaneously maintaining a high crystalline to amorphous ratio. In this work, we show that by varying the hydrogen dilution of silane gas flow during deposition, we can reduce average grain sizes to a few nanometers while still maintaining $\sim$ 90{\%} crystallinity of the material. Annealing at 600 $^{\circ}$C improves crystalline content with only a small increase of the grain sizes. The values of thermal conductivity, measured from 85 K to room temperature as function of hydrogen dilution ratio from full amorphous to nanocrystalline silicon, remain at a level that is typical for amorphous silicon.   

Intrinsic low thermal conductivity and optimization of the thermoelectric figure of merit in epitaxial thin films of CrN

Thermoelectric properties have been measured in single crystal epitaxial thin films of CrN. Combined with ab-initio calculations, we demonstrate that the rock-salt structure of this system has an intrinsic lattice instability similar to the resonant bond state in classic thermoelectric and phase change materials. The optimized figure of merit of CrN reaches zT$\approx$ 0.12 at 300 K, increasing rapidly with temperature. This results are promising for high temperature applications.