Band gap engineering of atomically thin two-dimensional (2D) materials is the key to their applications in nanoelectronics, optoelectronics, and photonics. Here, for the first time, we demonstrate that in the 2D system, by alloying two materials with different band gaps (MoS2 and WS2), tunable band gap can be obtained in the 2D alloys (Mo(1-x)W(x)S(2) monolayers, x = 0-1). Atomic-resolution scanning transmission electron microscopy has revealed random arrangement of Mo and W atoms in the Mo(1-x)W(x)S(2) monolayer alloys. Photoluminescence characterization has shown tunable band gap emission continuously tuned from 1.82 eV (reached at x = 0.20) to 1.99 eV (reached at x = 1). Further, density functional theory calculations have been carried out to understand the composition-dependent electronic structures of Mo(1-x)W(x)S(2) monolayer alloys.

We summarize our recent progresses in developing first-principles methods for predicting the intrinsic charge mobility in carbon and organic nanomaterials, within the framework of Boltzmann transport theory and relaxation time approximation. The electron-phonon couplings are described by Bardeen and Shockley's deformation potential theory, namely delocalized electrons scattered by longitudinal acoustic phonons as modeled by uniform lattice dilation. We have applied such methodology to calculating the charge carrier mobilities of graphene and graphdiyne, both sheets and nanoribbons, as well as closely packed organic crystals. The intrinsic charge carrier mobilities for graphene sheet and naphthalene are calculated to be 3 × 10(5) and ∼60 cm(2) V(-1) s(-1) respectively at room temperature, in reasonable agreement with previous studies. We also present some new theoretical results for the recently discovered organic electronic materials, diacene-fused thienothiophenes, for which the charge carrier mobilities are predicted to be around 100 cm(2) V(-1) s(-1).

We show here that the carrier mobility in the novel sp-sp(2) hybridization planar 6,6,12-graphyne sheet should be even larger than that in the graphene sheet. Both graphyne and graphene exhibit a Dirac cone structure near the Fermi surface. However, due to the sp-sp(2) hybridization forming the triple bonds in graphyne, the electron-phonon scattering is reduced compared with that of graphene. The carrier mobility is calculated at the first-principles level by using the Boltzmann transport equation coupled with the deformation potential theory. The intrinsic mobility of the 6,6,12-graphyne is 4.29 × 10(5) cm(2) V(-1) s(-1) for holes and 5.41 × 10(5) cm(2) V(-1) s(-1) for electrons at room temperature, which is found to be larger than that of graphene (∼ 3 × 10(5) cm(2) V(-1) s(-1)).

Tuning carrier concentration via chemical doping is the most successful strategy to optimize the thermoelectric figure of merit. Nevertheless, how the dopants affect charge transport is not completely understood. Here we unravel the doping effects by explicitly including the scattering of charge carriers with dopants on thermoelectric properties of poly(3,4-ethylenedioxythiophene), PEDOT, which is a p-type thermoelectric material with the highest figure of merit reported. We corroborate that the PEDOT exhibits a distinct transition from the aromatic to quinoid-like structure of backbone, and a semiconductor-to-metal transition with an increase in the level of doping. We identify a close-to-unity charge transfer from PEDOT to the dopant, and find that the ionized impurity scattering dominates over the acoustic phonon scattering in the doped PEDOT. By incorporating both scattering mechanisms, the doped PEDOT exhibits mobility, Seebeck coefficient and power factors in very good agreement with the experimental data, and the lightly doped PEDOT exhibits thermoelectric properties superior to the heavily doped one. We reveal that the thermoelectric transport is highly anisotropic in ordered crystals, and suggest to utilize large power factors in the direction of polymer backbone and low lattice thermal conductivity in the stacking and lamellar directions, which is viable in chain-oriented amorphous nanofibers.

High-throughput (HTP) material design is an emerging field and has been proved to be powerful in the prediction of novel functional materials. In this work, an HTP effort has been carried out for thermoelectric chalcogenides with diamond-like structures on the newly established Materials Informatics Platform (MIP). Specifically, the relaxation time is evaluated by a reliable yet efficient method, which greatly improves the accuracy of HTP electrical transport calculations. The results show that all the compounds may have power factors over 10 μW/cm·K if fully optimized. A new series of diamond-like chalcogenides with an atomic ratio of 1:2:4 possess relatively higher electrical transport properties among all the compounds investigated. One particular compound, CdInTe, and its variations have been verified experimentally with a peak ZT over 1.0. Further analysis reveals the existence of general conductive networks and the similar Pisarenko relations under the same anion sublattice, and the transport distribution function is found to be a good indicator for the power factors for the compounds investigated. This work demonstrates a successful case study in HTP material screening.

Both intrinsic and extrinsic charge transport properties of methylammonium lead triiodide perovskites are investigated from first-principles. The weak electron-phonon couplings are revealed, with the largest deformation potential (~ 5 eV) comparable to that of single layer graphene. The intrinsic mobility limited by the acoustic phonon scattering is as high as a few thousands cm2 V−1 s−1 with the hole mobility larger than the electron mobility. At the impurity density of 1018 cm−3, the charged impurity scattering starts to dominate and lowers the electron mobility to 101 cm2 V−1 s−1 and the hole mobility to 72.2 cm2 V−1 s−1. The high intrinsic mobility warrants the long and balanced diffusion length of charge carriers. With the control of impurities or defects as well as charge traps in these perovskites, enhanced efficiencies of solar cells with simplified device structures are promised.

We investigated the composition-dependent electronic properties of two-dimensional transition-metal dichalcogenide alloys (WxMo1-xS2) based on first-principles calculations by applying the supercell method and effective band structure approximation. It was found that hole effective mass decreases linearly with increasing W composition, and electron effective mass of alloys is always larger than that of their binary constituents. The different behaviors of electrons and holes in alloys are attributed to the fact that metal d-orbitals have different contributions to conduction bands of MoS2 and WS2 but almost identical contributions to valence bands. We examined the conduction polarity of WxMo1-xS2 monolayer alloys with four metal electrode materials (Au, Ag, Cu, and Pd). It suggests the main carrier type for transport in transistors could change from electrons to holes as W composition increases if high work function metal contacts were used. The tunable electronic properties of two-dimensional transition-metal dichalcogenide alloys make them attractive for electronic and optoelectronic applications.

Control of doping is crucial for enhancing the thermoelectric efficiency of a material. However, doping of organic semiconductors often reduces their mobilities, making it challenging to improve the thermoelectric performance. Targeting on this problem, we propose a simple model to quantitatively obtain the optimal doping level and the peak value of thermoelectric figure of merit (zT) from the intrinsic carrier mobility, the lattice thermal conductivity, and the effective density of states. The model reveals that high intrinsic mobility and low lattice thermal conductivity give rise to a low optimal doping level and a high maximum zT. To demonstrate how the model works, we investigate, from first-principles calculations, the thermoelectric properties of a novel class of excellent hole transport organic materials, 2,7-dialkyl[1]benzothieno[3,2-b][1]benzothiophene derivatives (C n -BTBTs). The first-principles calculations show that BTBTs exhibit high mobilities, extremely low thermal conductivities (∼0.2 W m–1 K–1), and large Seebeck coefficients (∼0.3 mV K–1), making them ideal candidates for thermoelectric applications. Moreover, the maximum zT predicted from the simple model agrees with that observed from the first-principles calculations. This study has provided new insights to guide the search for organic thermoelectric materials and their optimization.