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Register a new userHigh-Throughput Computational Screening of Two-Dimensional Semiconductors
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By performing high-throughput calculations using density functional theory combined with a semiempirical van der Waals dispersion correction, we screen 97 direct- and 253 indirect-gap two dimensional nonmagnetic semiconductors from near 1000 monolayers according to the energetic, thermodynamic, mechanical and dynamic stability criterions. We present the calculated results including lattice constants, formation energy, Youngs modulus, Poissons ratio, shear modulus, band gap, band structure, ionization energy and electron affinity for all the candidates satisfying our criteria.
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We search for novel two-dimensional materials that can be easily exfoliated from their parent compounds. Starting from 108423 unique, experimentally known three-dimensional compounds we identify a subset of 5619 that appear layered according to robust geometric and bonding criteria. High-throughput calculations using van-der-Waals density-functional theory, validated against experimental structural data and calculated random-phase-approximation binding energies, allow to identify 1825 compounds that are either easily or potentially exfoliable, including all that are commonly exfoliated experimentally. In particular, the subset of 1036 easily exfoliable cases---layered materials held together mostly by dispersion interactions and with binding energies up to $30-35$ meV$cdottext{AA}^{-2}$---provides a wealth of novel structural prototypes and simple ternary compounds, and a large portfolio to search materials for optimal properties. For the 258 compounds with up to 6 atoms per primitive cell we comprehensively explore vibrational, electronic, magnetic, and topological properties, identifying in particular 56 ferromagnetic and antiferromagnetic systems, including half-metals and half-semiconductors.
Based on high throughput density functional theory calculations, we performed systematic screening for spin-gapless semiconductors (SGSs) in quaternary Heusler alloys XX 0 YZ (X, X 0 , and Y are transition metal elements without Tc, and Z is one of B, Al, Ga, In, Si, Ge, Sn, Pb, P, As, Sb, and Bi). Following the empirical rule, we focused on compounds with 21, 26, or 28 valence electrons, resulting in 12, 000 possible chemical compositions. After systematically evaluating the thermodynamic, mechanical, and dynamical stabilities, we successfully identified 70 stable SGSs, confirmed by explicit electronic structure calculations with proper magnetic ground states. It is demonstrated that all four types of SGSs can be realized, defined based on the spin characters of the bands around the Fermi energy, and the type-II SGSs show promising transport properties for spintronic applications. The effect of spin-orbit coupling is investigated, resulting in large anisotropic magnetoresistance and anomalous Nernst effects.
We analyze the low-energy properties of two-dimensional direct-gap semiconductors, such as for example the transition-metal dichalcogenides MoS$_2$, WS$_2$, and their diselenide analogues MoSe$_2$, WSe$_2$, etc., which are currently intensively investigated. In general, their electrons have a mixed character -- they can be massive Dirac fermions as well as simple Schrodinger particles. We propose a measure (Diracness) for the degree of mixing between the two characters and discuss how this quantity can in principle be extracted experimentally, within magneto-transport measurements, and numerically via ab initio calculations.
We have performed a computational screening of topological two-dimensional (2D) materials from the Computational 2D Materials Database (C2DB) employing density functional theory. A full textit{ab initio} scheme for calculating hybrid Wannier functions directly from the Kohn-Sham orbitals has been implemented and the method was used to extract $mathbb{Z}_2$ indices, Chern numbers and Mirror Chern numbers of 3331 2D systems including both experimentally known and hypothetical 2D materials. We have found a total of 46 quantum spin Hall insulators, 7 quantum anomalous Hall insulators and 9 crystalline topological insulators that are all predicted to be dynamically stable. Roughly one third of these were known prior to the screening. The most interesting of the novel topological insulators are investigated in more detail. We show that the calculated topological indices of the quantum anomalous Hall insulators are highly sensitive to the approximation used for the exchange-correlation functional and reliable predictions of the topological properties of these materials thus require methods beyond density functional theory. We also performed $GW$ calculations, which yield a gap of 0.65 eV for the quantum spin Hall insulator PdSe$_2$ in the MoS$_2$ crystal structure. This is significantly higher than any known 2D topological insulator and three times larger than the Kohn-Sham gap.
Electrical contact resistance to two-dimensional (2D) semiconductors such as monolayer MoS_{2} is a key bottleneck in scaling the 2D field effect transistors (FETs). The 2D semiconductor in contact with three-dimensional metal creates unique current crowding that leads to increased contact resistance. We developed a model to separate the contribution of the current crowding from the intrinsic contact resistivity. We show that current crowding can be alleviated by doping and contact patterning. Using Landauer-Buttiker formalism, we show that van der Waals (vdW) gap at the interface will ultimately limit the electrical contact resistance. We compare our models with experimental data for doped and undoped MoS_{2} FETs. Even with heavy charge-transfer doping of > 2x10^{13} cm^{-2}, we show that the state-of-the-art contact resistance is 100 times larger than the ballistic limit. Our study highlights the need to develop efficient interface to achieve contact resistance of < 10 {Omega}.{mu}m, which will be ideal for extremely scaled devices.
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