Monolayer transition metal dichalcogenides in the distorted octahedral 1T$^\prime$ phase exhibit a large bulk bandgap and gapless boundary states, which is an asset in the ongoing quest for topological electronics. In single-layer tungsten diselenide (WSe$_2$), the boundary states have been observed at well ordered interfaces between 1T$^\prime$ and semiconducting (1H) phases. This paper proposes an effective 4-band theory for the boundary states in single-layer WSe$_2$,describing a Kramers pair of in-gap states as well as the behaviour at the spectrum termination points on the conduction and valence bands of the 1T$^\prime$ phase. The spectrum termination points determine the temperature and chemical potential dependences of the ballistic conductance and thermopower at the phase boundary. Notably, the thermopower shows an ambipolar behaviour, changing the sign in the bandgap of the 1T$^\prime$ - WSe$_2$ and reflecting its particle-hole asymmetry. The theory establishes a link between the bulk band structure and ballistic boundary transport in single-layer WSe$_2$ and is applicable to a range of related topological materials.
The thermoelectric transport coefficients of electrons in two recently emerged transition metal dichalcogenides(TMD), MoS2 and WSe2, are calculated by solving Boltzmann Transport equation and coupled electrical and thermal current equations using Rode iterative technique. Scattering from localized donor impurities, acoustic deformation potential, longitudinal optical (LO) phonons, and substrate induced remote phonon modes are taken into account. Hybridization of TMD plasmon with remote phonon modes is investigated. Dynamic screening under linear polarization response is explored in TMDs sittin…
The electronic and thermoelectric properties of one to four monolayers of MoS$_{2}$, MoSe$_{2}$, WS$_{2}$, and WSe$_{2}$ are calculated. For few layer thicknesses,the near degeneracies of the conduction band $K$ and $Σ$ valleys and the valence band $Γ$ and $K$ valleys enhance the n-type and p-type thermoelectric performance. The interlayer hybridization and energy level splitting determine how the number of modes within $k_BT$ of a valley minimum changes with layer thickness. In all cases, the maximum ZT coincides with the greatest near-degeneracy within $k_BT$ of the band edge that results in…
The electronic Seebeck response in a conductor involves the energy-dependent mean free path of the charge carriers and is affected by crystal structure, scattering from boundaries and defects, and strain. Previous photothermoelectric (PTE) studies have suggested that the thermoelectric properties of polycrystalline metal nanowires are related to grain structure, though direct evidence linking crystal microstructure to the PTE response is difficult to elucidate. Here, we show that room temperature scanning PTE measurements are sensitive probes that can detect subtle changes in the local Seebeck…
Grain boundaries are commonly observed in carbon nanostructures, but their influence on thermal and electric properties are still not completely understood. Using a combined approach of density functional tight-binding theory and non-equilibrium Green functions we investigate electron and phonon transport in carbon based systems. In this work, quantum transport and thermoelectric properties are summarized for graphene sheets, graphene nanoribbons and carbon nanotubes with a variety of grain boundary types in a wide temperature range. Motivated by previous findings that disorder scatters phonon…
Introducing hierarchical disorder from multiple defects into materials through nanostructuring is one of the most promising directions to achieve extremely low thermal conductivities and thus improve thermoelectric performance. The success of nanostructuring relies on charge carriers having shorter mean-free-paths than phonons so that the latter can be selectively scattered. Nevertheless, introducing disorder into a material often comes at the expense of scattering charge carriers as well as phonons. In order to determine the tradeoff between the degradation of the lattice thermal conductivity…
The resonant tunneling model is the simplest model for describing electronic transport through nanoscale objects like individual molecules. A complete understanding includes not only charge transport but also thermal transport and their intricate interplay. Key linear response observables are the electrical conductance G and the Seebeck coefficient S. Here we present experiments on unspecified resonant tunnel junctions and molecular junctions that uncover correlations between $G$ and $S$, in particular rigid boundaries for $S(G)$. We find that these correlations can be consistently understood …
We analyze thermally induced spin and charge transport in HgTe/CdTe quantum wells on the basis of the numerical non-equilibrium Green's function technique in the linear response regime. In the topologically non-trivial regime, we find a clear signature of the gap of the edge states due to their finite overlap from opposite sample boundaries -- both in the charge Seebeck and spin Nernst signal. We are able to fully understand the physical origin of the thermoelectric transport signatures of edge and bulk states based on simple analytical models. Interestingly, we derive that the spin Nernst sig…
Nanocrystalline silicon and silicon-germanium alloys are promising thermoelectric materials that have achieved substantially improved figure of merits compared to their bulk counterparts. This enhancement is typically attributed to a reduction in lattice thermal conductivity by phonon scattering at grain boundaries. However, further improvements are difficult to achieve because grain boundary scattering is poorly understood, with recent experimental observations suggesting that the phonon transmissivity may depend on phonon frequency rather than being constant as in the commonly used gray mode…
We study hydrodynamic electron transport in Corbino graphene devices. Due to the irrotational character of the flow, the forces exerted on the electron liquid are expelled from the bulk. We show that in the absence of Galilean invariance, force expulsion produces qualitatively new features in thermoelectric transport: (i) it results in drops of both voltage and temperature at the system boundaries and (ii) in conductance measurements in pristine systems, the electric field is not expelled from the bulk. We obtain thermoelectric coefficients of the system in the entire crossover region between …
Nanocomposites are promising candidates for the next generation of thermoelectric materials since they exhibit extremely low thermal conductivities as a result of phonon scattering on the boundaries of the various material phases. The nanoinclusions, however, should not degrade the thermoelectric power factor, and ideally should increase it, so that benefits to the ZT figure of merit can be achieved. In this work we employ the Non-Equilibrium Greens Function (NEGF) quantum transport method to calculate the electronic and thermoelectric coefficients of materials embedded with nanoinclusions. Fo…
Debye temperature decrease, residual resistivity increase and electron-phonon coupling constant increase as crystallite size decreases have been found from electrical resistivity in temperature range 5 K to 300 K of well-characterized Ag nanoparticles synthesized with oleylamine, trioctylphosphine and/ polyvinylpyrrolidone with Scherrer sizes ranging from 15.1 nm to 33.4 nm. Notably, about 36 % reduction in Debye temperature in 15.1 nm compared to bulk Ag is found. Remarkably, usual phonon drag peak found in Seebeck coefficient for bulk Ag turned into dips or phonon drag minima in these NPs th…
In this review, we discuss the impact of interfaces and heterojuctions on the electronic and thermoelectric transport properties of materials. We review recent progress in understanding electronic transport in two-dimensional (2D) materials ranging from graphene to transition metal dichalcogenides (TMDs), their homojunctions (grain boundaries), lateral heterojunctions (such as graphene/MoS$_2$ lateral interfaces), and vertical van der Waals (vdW) heterostructures. We also review work in thermoelectric properties of 2D heterojunctions, as well as their applications in creating devices such as r…
The dominance of Coulomb interactions over kinetic energy of electrons in narrow, non-trivial moiré bands of magic-angle twisted bilayer graphene (TBG) gives rise to a variety of correlated phases such as correlated insulators, superconductivity, orbital ferromagnetism, Chern insulators and nematicity. Most of these phases occur at or near an integer number of carriers per moiré unit cell. Experimental demonstration of ordered states at fractional moiré band-fillings at zero applied magnetic field $B$, is a challenging pursuit. In this letter, we report the observation of states near half-inte…
