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Intervalley collisions, which scatter electrons from one valley or band to another, can be detrimental to thermoelectric performance in materials with multiple valleys/bands. In this study, density functional theory is used to investigate the electron-phonon scattering characteristics of three lead chalcogenides (PbS, PbSe, PbTe) and three half-Heuslers (ScNiBi, ScPdSb, ZrNiSn), which all possess multiple equivalent conduction valleys, in order to characterize and analyze their intravalley/intervalley components. To elucidate what controls the degree of intravalley and intervalley transitions, the scattering rates are decomposed into the product of the phase space (a measure of how much scattering is possible) and the average electron-phonon coupling. To help guide the search for improved thermoelectric and high-conductivity materials, simple and approximate approaches are demonstrated that can be adopted to identify materials with reduced intervalley scattering, which circumvent the need for computationally-demanding electron-phonon scattering calculations. In addition, the benefits of selecting materials with large-energy zone-edge phonons are explored in the limit $\hbar\omega \gg k_BT$, and found to potentially suppress intervalley processes by up to an order of magnitude, leading to a 70% and 100% increase in conductivity and power factor, respectively., Comment: 13 pages, 9 figures

High drive current is a critical performance parameter in semiconductor devices for high-speed, low-power logic applications or high-efficiency, high-power, high-speed radio frequency (RF) analog applications. In this work, we demonstrate an In2O3 transistor grown by atomic layer deposition (ALD) at back-end-of-line (BEOL) compatible temperatures with a record high drain current exceeding 10 A/mm, the performance of which is 2-3 times better than all known transistors with semiconductor channels. A record high transconductance of 4 S/mm is also achieved among all transistors with a planar structure. It is found that a high carrier density and high electron velocity both contribute to this remarkably high on-state performance in ALD In2O3 transistors, which is made possible by the high-quality oxide/oxide interface, the metal-like charge-neutrality-level (CNL) alignment, and the high band velocities induced by the low density-of-state (DOS). Experimental Hall, I-V and split C-V measurements at room temperature confirm a high carrier density up to 6-7*10^13 /cm2 and a high velocity of about 10^7 cm/s. Ultra-thin oxide semiconductors, with a CNL located deep inside the conduction band, represent a promising new direction for the search of alternative channel materials for high-performance semiconductor devices.

With the goal of maximizing the thermoelectric (TE) figure of merit $ZT$, Mahan and Sofo [Proc. Natl. Acad. Sci. U.S.A. 93, 7436 (1996)] found that the optimal transport distribution (TD) is a delta function. Materials, however, have TDs that appear to always be finite and non-diverging. Motivated by this observation, this study focuses on deriving what is the optimal bounded TD, which is determined to be a boxcar function for $ZT$ and a Heaviside function for power factor. From these optimal TDs upper limits on $ZT$ and power factor are obtained; the maximum $ZT$ scales with $\Sigma_{\rm max} T /\kappa_l$, where $\Sigma_{\rm max}$ is the TD magnitude and $\kappa_l$ is the lattice thermal conductivity. These results help establish practical upper limits on the performance of TE materials and provide target TDs to guide band/scattering engineering strategies., Comment: 7 pages, 3 figures

The temperature fields and heat fluxes created by a circular heat source (CHS) are derived for the case of a CHS embedded between two identical and isotropic semi-infinite media, when different thermal contact resistances (CRs) are present on each surface of the CHS. Three different temperature fields are derived: on the CHS surface and in each one of the media surrounding the CHS. The derivation of the three-dimensional heat flow solution uses first principles with no prior assumptions, and employs the Hankel and Laplace transforms. The analytical solution presented here is exact with no approximations, but is given in integral form, which requires a facile numerical evaluation. Finite element simulations performed with COMSOL Multiphysics are provided and validate the analytical solution. The application of the solution to thin film thermal conductivity virtual measurements is demonstrated. Regressions of the numerical results yield thin film resistances in excellent agreement with the actual values (all within 1.5% error), which can readily be translated into thin film thermal conductivities.

To increase the performance of thermoelectric materials, the electronic parameters in the figure of merit must be improved. In this paper, we use full, numerical band structures and solve the Boltzmann equation in the relaxation time approximation using energy-dependent scattering times informed by first principles simulations. By varying the strength of the electron-phonon coupling or the lattice thermal conductivity, we compute the thermoelectric figure of merit, zT, vs. a generalized thermoelectric quality factor. More than a dozen different complex electronic structures are examined. Surprisingly, we find that at a given quality factor, none provides a better figure of merit than that of a material with a simple, parabolic band and acoustic deformation potential scattering. A qualitative argument for this unexpected finding is presented. This apparent universal behavior suggests that even for complex electronic band structures, the thermoelectric figure of merit depends solely on the ratio of electrical to thermal conductivity; the Seebeck coefficient and Lorenz number need not be considered. This observation should simplify the search for promising new materials, but if exceptions to this behavior can be identified, new paths for increasing thermoelectric material performance will open up., Version 5: The discussion section and conclusion were rewritten and supplementary material was added

The McKelvey–Shockley (McK–S) flux method is a semi-classical transport theory that captures ballistic and non-equilibrium effects and can treat carrier flow from the nano-scale to the macro-scale. This work introduces a revised formulation of the McK–S flux equations for electron transport, in order to resolve the energy dependence of the fluxes, capture the effect of electric field, and include acoustic/optical phonon scattering. This updated McK–S formalism is validated by simulating electron transport across a finite-length semiconductor under the influence of a constant electric field under varying conditions, from ballistic to diffusive and from near-equilibrium to non-equilibrium, and benchmarked against solutions of the Boltzmann transport equation (BTE). The McK–S results display good agreement with those of the BTE, including the directed fluxes and heating profiles, with the electron density showing larger differences when far from equilibrium. Compared to other more rigorous techniques, the McK–S flux method is physically intuitive and computationally efficient and, thus, well suited to treat systems that are complex and/or span multiple length scales.

Materials with ring-shaped electronic bands are promising thermoelectric (TE) candidates since their unusual dispersion shape is predicted to give large power factors. In this paper, we use density functional theory to investigate single and double quintuple-layer ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3},\phantom{\rule{0.28em}{0ex}}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, and ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ and to compare the TE properties using three scattering approximations: constant relaxation time, constant mean free path, and scattering rates proportional to the density of states (the so-called DOS model). Focus is placed on elucidating how these particular dispersion shapes influence the TE characteristics and on understanding how each scattering model impacts TE transport. The single quintuple-layer materials possess two ring-shaped valence-band maxima that provide an abrupt increase in conducting channels, which benefits the power factor. Below the band edge a ring-shaped minimum is found to further enhance TE performance within the DOS model due to a sharp drop in the DOS and, thus, scattering. An analytic ``octic'' dispersion model, designed to capture the observed characteristics of the band structure, is introduced and shown to qualitatively reproduce the first-principles results. The double quintuple-layer materials display notably worse TE properties since their dispersions are significantly modified compared to the single quintuple-layer case and lose much of the ring-shaped character. Our analysis shows that the benefits of ring-shaped bands are sensitive to the alignment of the two ring maxima and to the degree of ring anisotropy. Moreover, the predicted TE performance can vary significantly depending on the choice of scattering approximation, which could benefit from further study to assess the accuracy of these simple scattering models.

Thermoelectric devices are widely used as solid-state refrigerators and have potential energy generation appli-cations. Their characterization is key to develop more efficient devices and monitor their performance. Electrical impedance spectroscopy has been proved to be a useful method for the characterization of thermoelectric modules. However, deviations from current impedance models still exist in experimental results, especially in the high frequency part of the impedance spectrum, which limits its use. Here, we present a new comprehensive impedance model (equivalent circuit) which covers all the key phenomena that affects the module performance, and it is able to explain the observed deviations. The new equivalent circuit includes, as new additions, the thermal influence of the metallic strips (electrodes), combined with the thermal contact resistance between the metallic strips and the outer ceramic layer. Moreover, a new more accurate spreading-constriction impedance element, which considers the variation of the heat flow in the radial direction at the outer ceramic surfaces, is also developed. The comprehensive equivalent circuit was used to perform fittings to impedance spectroscopy measurements of modules fabricated by different manufacturers. From the fittings, it was possible to identify, among other key properties, the internal thermal contact resistances, whose direct determination is very chal-lenging. Thermal contact resistivities at the metallic strips/thermoelectric elements interface in the range 2.20 ×10-6-1.26 ×10-5 m2KW 1 were found. An excellent thermal contact was identified at the metallic strips/ceramic layers. This opens up the possibility of using impedance spectroscopy as a powerful tool to evaluate, monitor, and identify issues in thermoelectric devices. Funding for open access charge: CRUE-Universitat Jaume I

Using first-principles modeling, we investigate how phonon transport evolves in layered/van der Waals materials when going from 3D to 2D, or vice versa, by gradually pulling apart the atomic layers in graphite to form graphene. Focus is placed on identifying the features impacting thermal conductivity that are likely shared with other layered materials. The thermal conductivity $\kappa$ of graphite is found to be lower than that of graphene mainly due to changes in the phonon dispersion driven by van der Waals coupling. Specifically, as the atomic layers are brought closer together, the acoustic flexural phonons in graphene form low-energy optical flexural phonons in graphite that possess lower in-plane velocities, density-of-states and phonon occupation, thus reducing $\kappa$. Similar dispersion changes, and impact on thermal conductivity, can be expected in other van der Waals materials when transitioning from 2D to 3D. Our findings also indicate that the selection rules in graphene, which reduce phonon-phonon scattering and contribute to its large $\kappa$, effectively hold as the atomic layers are brought together to form graphite. While the selection rules do not strictly apply to graphite, in practice similar scattering behavior is displayed due in part to the weak inter-layer coupling. This suggests that van der Waals materials, in bulk 3D form, may have lower phonon-phonon scattering rates than other non-layered bulk materials., Comment: 14 pages, 10 figures

This paper is the first in a series of papers describing the temperature fields created by a circular area heat source (CHS) in various situations. In this paper, the derivation of the solution of the 3D transient heat flow from an embedded finite CHS supplying heat to an infinite medium is shown by employing different methods. The different adopted techniques include Laplace, Hankel and Fourier Cosine transforms, and solutions are based on first principles with minimum assumptions. Both isotropic and orthotropic infinite media are considered. Finite element simulations performed in COMSOL Multiphysics are provided and compared with the analytical solutions, with deviations between analytical solution and simulation of

Topological insulators (TI) have attracted extensive research effort due to their insulating bulk states but conducting surface states. However, investigation and understanding of thermal transport in topological insulators, particularly the effect of surface states, are lacking. In this work, we studied thickness-dependent in-plane thermal and electrical conductivity of Bi2Te2Se TI thin films. A large enhancement in both thermal and electrical conductivity was observed for films with thicknesses below 20 nm, which is attributed to the surface states and bulk-insulating nature of these films. Moreover, a surface Lorenz number much larger than the Sommerfeld value was found. Systematic transport measurements indicated that the Fermi surface is located near the charge neutrality point (CNP) when the film thickness is below 20 nm. Possible reasons for the large Lorenz number include electrical and thermal current decoupling in the surface state Dirac fluid, and bipolar diffusion transport. A simple computati...

In this study, we calculate the $T=300\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ scattering and thermoelectric transport properties of rhombohedral GeTe using first-principles modeling. The room-temperature phase of GeTe has a layered structure, with cross-plane and in-plane directions oriented parallel and perpendicular to [111], respectively. Based on rigorous electron-phonon scattering, our transport calculations reveal unusual anisotropic properties; $n$-type GeTe has a cross-plane electrical conductivity that is roughly $3\ifmmode\times\else\texttimes\fi{}$ larger than in plane. $p$-type GeTe, however, displays opposite anisotropy with in-plane conducting roughly $2\ifmmode\times\else\texttimes\fi{}$ more than cross plane, as is expected in quasi-two-dimensional materials. The power factor shows the same anisotropy as the electrical conductivity, since the Seebeck coefficient is relatively isotropic. Interestingly, cross-plane $n$-GeTe shows the largest mobility and power factor approaching $500\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{2}/\mathrm{V}$-s and $32\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{W}/\mathrm{cm}\text{\ensuremath{-}}{\mathrm{K}}^{2}$, respectively. The thermoelectric figure of merit, $zT$, is enhanced as a result of this unusual anisotropy in $n$-GeTe since the lattice thermal conductivity is minimized along cross plane. This decouples the preferred transport directions of electrons and phonons, leading to a threefold increase in $zT$ along cross plane compared to in plane. The $n$-type anisotropy results from high-velocity electron states formed by Ge $p$ orbitals that span across the interstitial region. This surprising behavior, that would allow the preferential conduction direction to be controlled by doping, could be observed in other quasi-two-dimensional materials and exploited to achieve higher-performance thermoelectrics.

Recent first-principles electron-phonon scattering calculations of heavily-doped semiconductors suggest that a simple DOS scattering model, wherein the electronic scattering rates are assumed to be proportional to the density-of-states, better approximates the rigorous scattering characteristics compared to the commonly used constant relaxation-time and constant mean-free-path approximations. This work investigates how the thermoelectric properties predicted with the DOS model compare to the other two scattering models, using three analytical electron dispersions (parabolic band in 3D/2D/1D, Kane band in 3D/2D/1D, and ring-shaped quartic band in 2D). Our findings show that the scattering models can lead to significant differences, and can disagree about whether certain band structures can provide benefits. A constant relaxation-time is found to always be optimistic compared to a constant mean-free-path, while the DOS scattering model shows no such clear trend. Notably, the 1D parabolic band and 2D quartic band exhibit the highest power factors with the DOS model, resulting from a rapid decrease in density-of-states, and thus scattering - suggesting a possible strategy for improved thermoelectrics based on engineering band structures with sharp/discontinuous drops in density-of-states. The DOS scattering approximation also suggests that searches for materials with a delta function-like DOS (as a proxy to the transport distribution) or converged bands may yield limited benefits, due to the increase in scattering. This work highlights the importance of simple and accurate scattering models when rigorous ab-initio scattering calculations are not feasible., Comment: 14 pages, 10 figures

Semiclassical electron and phonon transport is typically simulated by solving the electron or phonon Boltzmann transport equation. More recently, quantum transport of electrons and phonons has been simulated by the non-equilibrium Green’s function (NEGF) approach. The Landauer approach is a simple, physical approach to analyze electron and phonon transport. It is widely used to treat ballistic electron transport at the nanoscale. Less well known is that under the right conditions, it works all the way from the ballistic to diffusive limits in nanoscale to macroscale structures. The Landauer approach can also be used for phonon transport – and also from the ballistic to diffusive limits and from the nano- to macro-scales. Because the Landauer approach can be derived from the Boltzmann equation or from the NEGF equations, it provides a direct connection to more rigorous and fine-grained treatments. We believe that it should be more widely-used for three reasons: 1) it provides a smooth bridge from the ballistic to diffusive limits and from small to large structures, 2) it provides a common “language” for electron and phonon transport, and 3) it provides a clear connection to more fundamental semiclassical and quantum approaches to transport. Our goals in this talk are: 1) to review the Landauer approach to electron and phonon transport, 2) to illustrate how it compares to more rigorous treatments with some new benchmarking calculations, and 3) to illustrate how this approach helps identify good thermoelectric materials. The electrical conductance can be written as the quantum of electrical conductance, times the number of thermally active channels for electrical conduction, times the transmission. The quantum of conductance is a set of fundamental constants. The number of thermally active channels depends only on electronic structure and can be rigorously computed from first principles. The transmission is one for a ballistic conductor and small for a diffusive conductor. For large structures, this approach reduces to the traditional (Boltzmann equation) expressions of near-equilibrium electron transport. For small structures, it describes quantized, ballistic conduction. It is common to use the Landauer approach for nanostructures and the Boltzmann equation for large structures, but there is no need for two separate approaches – the Landauer approach covers both regimes of transport. The Landauer approach also applies to phonon transport using the same physical concepts described above. For example, the thermal conductance can be written as the quantum of thermal conductance, times the number of thermally active channels for phonon conduction, times the phonon transmission. This talk will present a brief overview of this technique applied to electron and phonon transport, and will discuss some new examples that benchmark the Landauer approach and illustrate the benefits and limitations of the method. It is sometimes felt that electrons and phonons are different because electrons are fermions and phonons are bosons. It turns out that this makes little difference. It does affect the thermal window functions that determine the number of thermally active channels, but the window functions turn out to be quite similar in shape. There are two significant differences – in practice, but not in principle. The first is that the energy bandwidth of the electron dispersion is typically much, much larger than the width of the thermal window function for electrons, so that simplified dispersions often work very well (e.g. parabolic energy bands). For phonons, the width of the thermal window function is comparable to the width of the phonon dispersion, so simplified phonon dispersions usually lead to large errors. The second major difference has to do with the electron and phonon mean-free-paths. Both mean-free-paths are energy-dependent, but since electrons typically occupy only a small range of energies near the bottom or top of a band, the use of a constant mean-free-path is often sufficient. In contrast, there is a very strong energy dependence of the phonon mean-free-path, and phonons with a very wide range of mean-free-paths contribute to the thermal conductivity. In spite of these technical differences, the formal equations and computational approaches are essentially identical for electrons and phonons. For most electronic devices, both electron and phonon transport play a role, and for thermoelectric devices, the intimate coupling of the two flows is critical. For problems like thermoelectrics, the use of a common approach to analyze electron and phonon transport is beneficial and can provide new insights. In this talk, we will illustrate what makes a good thermoelectric material from a Landauer perspective considering both traditional bulk and novel 2D materials as examples. Figure 1

Thin-film transistors (TFTs) based on multilayer molybdenum diselenide (MoSe

We review the basic optical, electronic, optoelectronic, thermoelectric and mechanical properties of few-layer black phosphorus (BP), a layered semiconductor that can be exfoliated from bulk BP, the most stable allotrope of phosphorus. The distinguishing trait of BP is its highly anisotropic crystal structure, which leads to strong optical linear dichroism, elliptical plasmonics energy surfaces, anisotropic electronic and thermal conductivities and elasticity. We provide tutorial-like discussion of these phenomena and their theoretical models.

Complex electronic band structures, with multiple valleys or bands at the same or similar energies can be beneficial for thermoelectric performance, but the advantages can be offset by inter-valley and inter-band scattering. In this paper, we demonstrate how first-principles band structures coupled with recently developed techniques for rigorous simulation of electron-phonon scattering provide the capabilities to realistically assess the benefits and trade-offs associated with these materials. We illustrate the approach using n-type silicon as a model material and show that intervalley scattering is strong. This example shows that the convergence of valleys and bands can improve thermoelectric performance, but the magnitude of the improvement depends sensitively on the relative strengths of intra- and inter-valley electron scattering. Because anisotropy of the band structure also plays an important role, a measure of the benefit of band anisotropy in the presence of strong intervalley scattering is presented.

Steady-state thermal transport in nanostructures with dimensions comparable to the phonon mean-free-path is examined. Both the case of contacts at different temperatures with no internal heat generation and contacts at the same temperature with internal heat generation are considered. Fourier's Law results are compared to finite volume method solutions of the phonon Boltzmann equation in the gray approximation. When the boundary conditions are properly specified, results obtained using Fourier's Law without modifying the bulk thermal conductivity are in essentially exact quantitative agreement with the phonon Boltzmann equation in the ballistic and diffusive limits. The errors between these two limits are examined in this paper. For the four cases examined, the error in the apparent thermal conductivity as deduced from a correct application of Fourier's Law is less than 6%. We also find that the Fourier's Law results presented here are nearly identical to those obtained from a widely-used ballistic-diffusive approach, but analytically much simpler. Although limited to steady-state conditions with spatial variations in one dimension and to a gray model of phonon transport, the results show that Fourier's Law can be used for linear transport from the diffusive to the ballistic limit. The results also contribute to an understanding of how heat transport at the nanoscale can be understood in terms of the conceptual framework that has been established for electron transport at the nanoscale., 23 pages, 11 figures

A first-principles informed approach to the calculation of Lorenz numbers for complex thermoelectric materials is presented and discussed. Example calculations illustrate the importance of using accurate band structures and energy-dependent scattering times. Results obtained by assuming that the scattering rate follows the density-of-states show that in the non-degenerate limit, Lorenz numbers below the commonly assumed lower limit of 2(kB/q)^2 can occur. The physical cause of low Lorenz numbers is explained by the shape of the transport distribution. The numerical and physical issues that need to be addressed in order to produce accurate calculations of the Lorenz number are identified. The results of this study provide a general method that should contribute to the interpretation of measurements of total thermal conductivity and to the search for materials with low Lorenz numbers, which may provide improved thermoelectric figures of merit, zT., Comment: 21 pages, 8 figures. Supplementary information is available from the first author

Understanding phonon transport across heterojunctions is important to achieve a wide range of thermal transport properties. Using the McKelvey-Shockley flux method with first-principles modeling, we theoretically investigate the phonon transport properties of a Si–Ge interface with a focus on the role of inelastic bulk phonon processes. We observe significant inelastic scattering near the interface that redistributes the heat among the phonons as a result of non-equilibrium effects driven by the junction. These effects are most pronounced when the length of the junction is comparable to the average phonon mean-free-path. What controls these inelastic processes is elucidated.

In the past two decades, significant progress has been achieved in the large-scale fabrication of nanostructures where quantum transport properties of charge and spin are closely coupled to the discreteness of the device material. Multitudes of emerging device concepts and new materials with interesting application potential have been discovered. In order to understand the experimental data and device physics of nanoelectronics, an important task is to develop appropriate theoretical formalisms and associated modeling tools which are capable of making quantitative and material specific predictions of device characteristics without any phenomenological parameters. Here we review the atomistic modeling method based on carrying out density functional theory (DFT) within the nonequilibrium Green's function (NEGF) formalism. Since its original implementation ten years ago, the NEGF-DFT technique has emerged as a very powerful and practically very useful method for predicting nonlinear and nonequilibrium quantum transport properties of nanoelectronics. Recent new developments concerning nonequilibrium disorder scattering will also be presented. Large-scale and scalable computations have allowed NEGF-DFT to model Si structures reaching the present day realistic channel sizes.

Molybdenum disulfide (MoS 2 ) is a promising 2D material since it has a finite band gap, and its electronic band structure depends on the layer thickness. The tunability of the gate voltage on band alignment of different MoS 2 layers is analyzed. For this purpose, the multipurpose nanodevice simulation tool NEMO5 was altered by several new features: electronic bandstructure calculations in maximally localized Wannier function (MLWF) representation and self-consistent charge calculations with subatomic electrostatic resolution.

The Poisson's ratio of a material characterizes its response to uniaxial strain. Materials normally possess a positive Poisson's ratio - they contract laterally when stretched, and expand laterally when compressed. A negative Poisson's ratio is theoretically permissible but has not, with few exceptions of man-made bulk structures, been experimentally observed in any natural materials. Here, we show that the negative Poisson's ratio exists in the low-dimensional natural material black phosphorus and that our experimental observations are consistent with first-principles simulations. Through applying uniaxial strain along armchair direction, we have succeeded in demonstrating a cross-plane interlayer negative Poisson's ratio on black phosphorus for the first time. Meanwhile, our results support the existence of a cross-plane intralayer negative Poisson's ratio in the constituent phosphorene layers under uniaxial deformation along the zigzag axis, which is in line with a previous theoretical prediction. The phenomenon originates from the puckered structure of its in-plane lattice, together with coupled hinge-like bonding configurations.

A mechanically formed electrical nanocontact between gold and tungsten is a prototypical junction between metals with dissimilar electronic structure. Through atomically characterized nanoindentation experiments and first-principles quantum transport calculations, we find that the ballistic conduction across this intermetallic interface is drastically reduced because of the fundamental mismatch between s wave-like modes of electron conduction in the gold and d wave-like modes in the tungsten. The mechanical formation of the junction introduces defects and disorder, which act as an additional source of conduction losses and increase junction resistance by up to an order of magnitude. These findings apply to nanoelectronics and semiconductor device design. The technique that we use is very broadly applicable to molecular electronics, nanoscale contact mechanics, and scanning tunneling microscopy.

Thin-film transistors (TFTs) based on multilayer molybdenum diselenide (MoSe2 ) synthesized by modified atmospheric pressure chemical vapor deposition (APCVD) exhibit outstanding photoresponsivity (103.1 A W-1 ), while it is generally believed that optical response of multilayer transition metal dichalcogenides (TMDs) is significantly limited due to their indirect bandgap and inefficient photoexcitation process. Here, the fundamental origin of such a high photoresponsivity in the synthesized multilayer MoSe2 TFTs is sought. A unique structural characteristic of the APCVD-grown MoSe2 is observed, in which interstitial Mo atoms exist between basal planes, unlike usual 2H phase TMDs. Density functional theory calculations and photoinduced transfer characteristics reveal that such interstitial Mo atoms form photoreactive electronic states in the bandgap. Models indicate that huge photoamplification is attributed to trapped holes in subgap states, resulting in a significant photovoltaic effect. In this study, the fundamental origin of high responsivity with synthetic MoSe2 phototransistors is identified, suggesting a novel route to high-performance, multifunctional 2D material devices for future wearable sensor applications.

It is assessed in detail both experimentally and theoretically how the interlayer coupling of transition metal dichalcogenides controls the electronic properties of the respective devices. Gated transition metal dichalcogenide structures show electrons and holes to either localize in individual monolayers, or delocalize beyond multiple layers - depending on the balance between spin-orbit inter- action and interlayer hopping. This balance depends on layer thickness, momentum space symmetry points and applied gate fields. A good quantitative agreement of predictions and measurements of the quantum confined Stark effect in gated MoS2 systems unveils intralayer excitons as major source for the observed photoluminesence.

Using a model, developed earlier, of trap controlled conduction, which includes both the Meyer–Neldel rule and field assisted detrapping, we have simulated picosecond timescale drift mobility and drift velocity measurements in a-Si:H. This model is able to duplicate both the picosecond and nanosecond data, for all values of temperature and field, using one set of fixed parameters. We observe that under certain conditions, the drift mobility is field independent. Coherence between the picosecond and nanosecond results is shown.

Developing simplified, but accurate, theoretical approaches to treat heat transport on all length and time scales is needed to further enable scientific insight and technology innovation. Using a simplified form of the Boltzmann transport equation (BTE), originally developed for electron transport, we demonstrate how ballistic phonon effects and finite-velocity propagation are easily and naturally captured. We show how this approach compares well to the phonon BTE, and readily handles a full phonon dispersion and energy-dependent mean-free-path. This study of transient heat transport shows i) how fundamental temperature jumps at the contacts depend simply on the ballistic thermal resistance, ii) that phonon transport at early times approach the ballistic limit in samples of any length, and iii) perceived reductions in heat conduction, when ballistic effects are present, originate from reductions in temperature gradient. Importantly, this framework can be recast exactly as the Cattaneo and hyperbolic heat equations, and we discuss how the key to capturing ballistic heat effects is to use the correct physical boundary conditions., 9 pages, 5 figures

Black phosphorus has been revisited recently as a new two-dimensional material showing potential applications in electronics and optoelectronics. Here we report the anisotropic in-plane thermal conductivity of suspended few-layer black phosphorus measured by micro-Raman spectroscopy. The armchair and zigzag thermal conductivities are ~20 and ~40 W m$^{-1}$ K$^{-1}$ for black phosphorus films thicker than 15 nm, respectively, and decrease to ~10 and ~20 W m$^{-1}$ K$^{-1}$ as the film thickness is reduced, exhibiting significant anisotropy. The thermal conductivity anisotropic ratio is found to be ~2 for thick black phosphorus films and drops to ~1.5 for the thinnest 9.5-nm-thick film. Theoretical modeling reveals that the observed anisotropy is primarily related to the anisotropic phonon dispersion, whereas the intrinsic phonon scattering rates are found to be similar along the armchair and zigzag directions. Surface scattering in the black phosphorus films is shown to strongly suppress the contribution of long-mean-free-path acoustic phonons., Comment: Accepted by Nature Communications

An important challenge facing society involves reducing our energy consumption and shifting away from fossil fuels and towards sustainable, abundant sources of power that do not harm the environment. Waste heat is a tremendous unutilized energy source, accounting for roughly 60% of the energy humans produce, that if we could only partially recover would have a major impact. Thermoelectrics (TE) provide a solution as materials than can convert heat into electrical power. A major goal is to enhance the TE conversion efficiency, characterized by the figure-of-merit ZT, which depends on the material TE properties: Seebeck coefficient, electrical conductivity and electronic/lattice thermal conductivities. Increasing ZT is critical for the advancement of TE technologies, but difficult since the TE properties are interrelated. To discover more high-ZT materials, it is important to explore and design new materials with unique properties tailored for TE conversion. In the past decade, there have been several breakthroughs in this direction, including nanostructured alloys, resonant impurity states, superlattice structures, all-scale defect engineering, and ultra-low thermal conductivity materials. These, like most advances, were experimentally led and based on trial-and-error, which can be expensive and time-consuming. To help guide experimental efforts and expedite discoveries, there is an opportunity to explore emerging materials using predictive theoretical modeling. In this talk, I will present recent work focused on the development and application of a first principles approach for TE parameter predictions. I will describe our adopted framework, which combines density functional theory (DFT) and the Boltzmann transport equation (BTE). DFT is employed to extract the full electron and phonon dispersions, and to compute the detailed electron-phonon and phonon-phonon lifetimes, the dominant scattering mechanisms for electrons and phonons, respectively. These material properties serve as input for the BTE, which is solved for the TE transport coefficients. With this DFT-based approach, all TE parameters are obtained predictively, including Seebeck coefficient, electrical conductivity and electronic/lattice thermal conductivities, as a function of temperature and carrier concentration. Recent results on select materials, for example SnSe, will be presented to demonstrate the approach and to illustrate how we can gain physical insight. This work will help guide experimental breakthroughs by theoretically exploring and identifying new TE materials with enhanced properties.

It is generally understood that Fourier's law does not describe ballistic phonon transport, which is important when the length of a material is similar to the phonon mean-free-path. Using an approach adapted from electron transport, we demonstrate that Fourier's law and the heat equation do capture ballistic effects, including temperature jumps at ideal contacts, and are thus applicable on all length scales. Local thermal equilibrium is not assumed, because allowing the phonon distribution to be out-of-equilibrium is important for ballistic and quasi-ballistic transport. The key to including the non-equilibrium nature of the phonon population is to apply the proper boundary conditions to the heat equation. Simple analytical solutions are derived, showing that i) the magnitude of the temperature jumps is simply related to the material properties and ii) the observation of reduced apparent thermal conductivity physically stems from a reduction in the temperature gradient and not from a reduction in actual thermal conductivity. We demonstrate how our approach, equivalent to Fourier's law, easily reproduces results of the Boltzmann transport equation, in all transport regimes, even when using a full phonon dispersion and mean-free-path distribution., Comment: 9 pages, 5 figures

We use molecular dynamics simulations to investigate how the energy input and distribution in contacts affect the thermal transport in silicon as described by the Stillinger-Webber potential. We create a temperature difference across a Si specimen by maintaining the temperature of two contacts (also made of Si) using widely used thermostats: the deterministic Nose-Hoover approach and a stochastic Langevin bath. Quite surprisingly, the phonon thermal conductivity of the channel obtained using the two thermostats but under otherwise identical conditions can differ by a factor of up to three. The discrepancy between the two methods vanishes as the coupling strength between the thermostat and material is reduced and for long channels. A spectral analysis of the contacts and channel shows that increasing the coupling of the stochastic Langevin thermostat affects the spectral energy distribution in the contacts away from that based on the vibrational density of states, broadening peaks and smoothening the distr...

A microdevice was used to measure the in-plane thermoelectric properties of suspended bismuth telluride nanoplates from 9 to 25 nm thick. The results reveal a suppressed Seebeck coefficient together with a general trend of decreasing electrical conductivity and thermal conductivity with decreasing thickness. While the electrical conductivity of the nanoplates is still within the range reported for bulk Bi2Te3, the total thermal conductivity for nanoplates less than 20 nm thick is well below the reported bulk range. These results are explained by the presence of surface band bending and diffuse surface scattering of electrons and phonons in the nanoplates, where pronounced n-type surface band bending can yield suppressed and even negative Seebeck coefficient in unintentionally p-type doped nanoplates.

Epitaxial ScN(001) thin films were grown on MgO(001) substrates by dc reactive magnetron sputtering. The deposition was performed in an Ar/N-2 atmosphere at 2 x 10(-3) Torr at a substrate temperature of 850 degrees C in a high vacuum chamber with a base pressure of 10(-8) Torr. In spite of oxygen contamination of 1.6 +/- 1 at. %, the electrical resistivity, electron mobility, and carrier concentration obtained from a typical film grown under these conditions by room temperature Hall measurements are 0.22 m Omega cm, 106 cm(2) V-1 s(-1), and 2.5 x 10(20) cm(-3), respectively. These films exhibit remarkable thermoelectric power factors of 3.3-3.5 x 10(-3) W/mK(2) in the temperature range of 600 K to 840 K. The cross-plane thermal conductivity is 8.3 W/mK at 800 K yielding an estimated ZT of 0.3. Theoretical modeling of the thermoelectric properties of ScN calculated using a mean-free-path of 23 nm at 300 K is in very good agreement with the experiment. These results also demonstrate that further optimization of the power factor of ScN is possible. First-principles density functional theory combined with the site occupancy disorder technique was used to investigate the effect of oxygen contamination on the electronic structure and thermoelectric properties of ScN. The computational results suggest that oxygen atoms in ScN mix uniformly on the N site forming a homogeneous solid solution alloy. Behaving as an n-type donor, oxygen causes a shift of the Fermi level in ScN into the conduction band without altering the band structure and the density of states. (C) 2013 AIP Publishing LLC [http://dx.doi.org/10.1063/1.4801886]

Over the past few years, there has been a growing interest in layered transition metal dichalcogenides (TMD) such as molybdenum disulfide (MoS2). Most studies so far have focused on the electronic and optoelectronic properties of single-layer MoS2, whose band structure features a direct bandgap, in sharp contrast to the indirect bandgap of thicker MoS2. In this paper, we present a systematic study of the thickness-dependent electrical and thermoelectric properties of few-layer MoS2. We observe that the electrical conductivity () increases as we reduce the thickness of MoS2 and peaks at about two layers, with six-time larger conductivity than our thickest sample (23-layer MoS2). Using a back-gate voltage, we modulate the Fermi energy () of the sample where an increase in the Seebeck coefficient () is observed with decreasing gate voltage () towards the subthreshold (OFF state) of the device, reaching as large as in a four-layer MoS2. While previous reports have focused on a single-layer MoS2 and measured Seebeck coefficient in the OFF state, which has vanishing electrical conductivity and thermoelectric power factor (), we show that MoS2-based devices in their ON state can have as large as in the two-layer sample. The increases with decreasing thickness then drops abruptly from double-layer to single-layer MoS2, a feature we suggest as due to a change in the energy dependence of the electron mean-free-path according to our theoretical calculation. Moreover, we show that care must be taken in thermoelectric measurements in the OFF state to avoid obtaining erroneously large Seebeck coefficients when the channel resistance is very high. Our study paves the way towards a more comprehensive examination of the thermoelectric performance of two-dimensional (2D) semiconductors., Comment: 35 pages, 20 figures

Strain can be used as an alternate way to tune the electronic properties of graphene. Here we demonstrate that it is possible to tune the uniform strain of graphene simply by changing the chemical vapor deposition growth temperature of graphene on copper. Due to the cooling of the graphene on copper system, we can induce a uniform compressive strain on graphene. The strain is analyzed by Raman spectroscopy, where a shift in the 2D peak is observed and compared to our ab initio calculations of the graphene on copper system as a function of strain.

We present theoretical results for the backaction force noise and damping of a mechanical oscillator whose position is measured by a mesoscopic conductor. Our scattering approach is applicable to a wide class of systems; in particular, it may be used to describe point contact position detectors far from the weak tunneling limit. We find that the backaction depends not only on the mechanical modulation of transmission probabilities but also on the modulation of scattering phases, even in the absence of a magnetic field. We illustrate our general approach with several simple examples, and use it to calculate the backaction for a movable, Au atomic point contact modeled by ab initio density functional theory., Published version; erratum added

We report a first principles study of spin-transport under finite bias through a graphene-ferromagnet (FM) interface, where FM=Co(111), Ni(111). The use of Co and Ni electrodes achieves spin efficiencies reaching 80% and 60%, respectively. This large spin filtering results from the materials specific interaction between graphene and the FM which destroys the linear dispersion relation of the graphene bands and leads to an opening of spin-dependent energy gaps of roughly 0.4-0.5 eV at the K points. The minority spin band gap resides higher in energy than the majority spin band gap located near the Fermi level, a feature that results in large minority spin dominated currents., Comment: 5 pages, 4 figures

We report a theoretical investigation of dephasing effects to quantum transport properties of molecular junctions. The quantum transport analysis is done by density functional theory carried out within the nonequilibrium Green's function framework, and the dephasing effect is modeled within the B\"uttiker-probe approach. We observe two distinct behaviors in the three systems we studied: either an increase or a decrease in electronic conduction with dephasing. For a 1,4-benzenedithiol molecule and an atomic gold chain, where the conducting molecular levels are located away from the Fermi level, conduction is seen to increase due to reduced destructive interference resulting from the B\"uttiker probe. On the other hand, for a very thin Al nanowire we find that backscattering dominates over the phase-randomization and the current decreases with dephasing. The resistance follows Ohm's law while the resistivity scales linearly with the scattering rate. Finally, a comparison between the B\"uttiker-probe model and a more microscopic dephasing model shows nearly identical transport characteristics. From a computational point of view, the B\"uttiker-probe model has an order of magnitude speed up.

The ballistic thermoelectric performance of ultrathin films of Bi2Te3, ranging in thickness from 1 to 6 quintuple layers, is analyzed using density functional theory combined with the Landauer approach. Our results show that the thinnest film, corresponding to a single quintuple layer, has an intrinsic advantage originating from the particular shape of its valence band, leading to a large power factor and figure-of-merit exceeding bulk Bi2Te3. The interaction between the top and bottom topological surface states is key. The thinnest film yields a six-fold increase in power factor compared to bulk.