Liyuan Wu

Structural Properties and Phase Transition of Na Adsorption on Monolayer MoS2.

First-principles calculations are performed to investigate the structural stability of Na adsorption on 1H and 1T phases of monolayer MoS2. Our results demonstrate that it is likely to make the stability of distorted 1T phase of MoS2 over the 1H phase through adsorption of Na atoms. The type of distortion depends on the concentration of adsorbed Na atoms and changes from zigzag-like to diamond-like with the increasing of adsorbed Na atom concentrations. Our calculations show that the phase transition from 1H-MoS2 to 1T-MoS2 can be obtained by Na adsorption. We also calculate the electrochemical properties of Na adsorption on MoS2 monolayer. These results indicate that MoS2 is one of potential negative electrodes for Na-ion batteries.

Quasiparticle and optical properties of strained stanene and stanane

Quasiparticle band structures and optical properties of two dimensional stanene and stanane (fully hydrogenated stanene) are studied by the GW and GW plus Bethe–Salpeter equation (GW-BSE) approaches, with inclusion of the spin-orbit coupling (SOC). The SOC effect is significant for the electronic and optical properties in both stanene and stanane, compared with their group IV-enes and IV-anes counterparts. Stanene is a semiconductor with a quasiparticle band gap of 0.10 eV. Stanane has a sizable band gap of 1.63 eV and strongly binding exciton with binding energy of 0.10 eV. Under strain, the quasiparticle band gap and optical spectrum of both stanene and stanane are tunable.

Tension-induced mechanical properties of stanene

In this paper, elastic properties of stanene under equiaxial or uniaxial tensions along armchair and zigzag directions are investigated by first-principles calculations. The stress strain relation is calculated and the relaxation of the internal atom positions is analyzed. The high-order elastic constants are calculated by fitting the polynomial expressions. The Youngs modulus and Poisson ratio of the stanene is calculated to be 24.14 N/m and 0.39 N/m, respectively. The stanene exhibits lower Youngs modulus than those of the proceeding group IV elements, which is attributed to the smaller sp(2)-sp(3) bond energy in stanene than those of silicene and germanene. Calculated values of ultimate stresses and strains, second-order elastic constants (SOCEs) and the in-plane Youngs modulus are all positive. It proves that stanene is mechanically stable.

Electronic and excitonic properties of two-dimensional and bulk InN crystals

Motivated by potential extensive applications in nanoelectronics devices of III-Vmaterials, we calculate the structural and optoelectronic properties of two-dimensional (2D) InN as well as its three-dimensional (3D) counterparts by using density functional theory (DFT). Compared with the 3D form, the In-N bonding in the 2D InN layer is stronger in terms of the shorter bond length, and the formation of the 2D one is higher in terms of the lower cohesive energy. The bandgap of monolayer InN is 0.31 eV at PBE level and 2.02 eV at GW(0) level. By many-body GW(0) and BSE within RPA calculations, monolayer InN presents an exciton binding energy of 0.12 eV. The fundamental bandgap increases along with layer reduction and is converted from direct (0.7-0.9 eV) in bulk InN to indirect (2.02 eV) in monolayer InN. Under biaxial compressive strain, the bandgap of 2D-InN can be further tuned from indirect to direct.

Electronic and Optical Properties of Arsenene Under Uniaxial Strain

The electronic and optical properties of strained monolayer arsenene were calculated based on first-principle density functional theory. Our theoretical calculations demonstrated that monolayer arsenene was transformed from indirect to direct band gap semiconductor by inducing uniaxial tensile strain along armchair and zigzag directions. Compared to the biaxial tensile strain of 0.04, this transformation occurred at the strain of 0.06 and 0.10 along armchair and zigzag direction, respectively. Spin-orbital coupling is available to tune the bandgaps. The spin-orbit interaction opens a 0.2 eV bandgap in the Γ-point on the unstrained monolayer arsenene. The absorption properties were calculated and a clear red shift was observed with the increasing strain.

Raman scattering studies of dilute InP1−xBix alloys reveal unusually strong oscillator strength for Bi-induced modes

Room-temperature Raman scattering studies of new InP1-xBix alloys grown by molecular beam epitaxy are reported. Two new Bi-induced vibrations observed at 149 and 171 cm-1 are assigned to InBi-like TO and LO phonon modes, respectively, and exhibit an unusually strong intensity for the dilute regime. Two additional modes at 311 and 337 cm-1 are resolved as well with unknown origins. The Raman intensities of the InBi-like TO and LO bands, as well as the new mode at 337 cm-1, exhibit strong and linear dependence on the Bi concentration for the composition range studied, 0.003 ≤ x ≤ 0.023. This correlation may serve as a fast and convenient means of characterizing bismuth composition not only in the ternary alloy InP1-xBix but also in the quaternaries such as In1-yGayP1-xBix and In1-yAlyP1-xBix.

High n-type and p-type thermoelectric performance of two-dimensional SiTe at high temperature

From a device perspective, achieving great merits for both n- and p-type thermoelectric systems is particularly desirable. By first-principles calculations, electronic, phonon, and thermoelectric transport properties of 2D SiTe with three different structural phases are investigated, which are quadruple layer (QL), black-phosphorene-like (α-SiTe) and blue-phosphorene-like (β-SiTe), respectively. Of these three structure phases, β-SiTe possesses the best thermoelectric properties. This is because the DOS peak near the valence band results in a high Seebeck coefficient, further leading to a high power factor. We also demonstrate that strong phonon scattering heavily influences the lattice thermal conductivity Kl of β-SiTe. With the combination of high power factor and low Kl, the ZTmax value of β-SiTe reaches 0.95 at T = 1300 K for both n- and p-type doped systems. Therefore, 2D β-SiTe is a promising candidate for future high-temperature solid-state thermoelectric generators with a balanced performance of the n- and p-legs.

Phase-selective synthesis of 1T′ MoS 2 monolayers and heterophase bilayers

Two-dimensional (2D) MoS2, which has great potential for optoelectronic and other applications, is thermodynamically stable and hence easily synthesized in its semiconducting 2H phase. In contrast, growth of its metastable 1T and 1T′ phases is hampered by their higher formation energy. Here we use theoretical calculations to design a potassium (K)-assisted chemical vapour deposition method for the phase-selective growth of 1T′ MoS2 monolayers and 1T′/2H heterophase bilayers. This is realized by tuning the concentration of K in the growth products to invert the stability of the 1T′ and 2H phases. The synthesis of 1T′ MoS2 monolayers with high phase purity allows us to characterize their intrinsic optical and electrical properties, revealing a characteristic in-plane anisotropy. This phase-controlled bottom-up synthesis offers a simple and efficient way of manipulating the relevant device structures, and provides a general approach for producing other metastable-phase 2D materials with unique properties.The 1T′ phase of MoS2 monolayers, as well as the 2H phase and their heterophase bilayers, can be grown directly by tuning the potassium concentration in the reaction atmosphere. The pure 1T′ phase demonstrates in-plane anisotropic properties.

Thermoelectric properties of two-dimensional hexagonal indium-VA

The electrical properties and thermoelectric (TE) properties of monolayer In-VA are investigated theoretically by combining first-principles method with Boltzmann transport theory. The ultralow intrinsic thermal conductivities of 2.64 Wm−1K−1 (InP), 1.31 Wm−1K−1 (InAs), 0.87 Wm−1K−1 (InSb), and 0.62 Wm−1K−1 (InBi) evaluated at room temperature are close to typical thermal conductivity values of good TE materials (κ < 2 Wm−1K−1). The maximal ZT values of 0.779, 0.583, 0.696, 0.727, and 0.373 for InN, InP, InAs, InSb, and InBi at p-type level are calculated at 900 K, which makes In-VA potential TE material working at medium-high temperature.