Borophene (two-dimensional boron sheet) is a new type of two-dimensional material, which was recently grown successfully on single crystal Ag substrates. In this paper, we investigate the electronic structure and bonding characteristics of borophene by first-principle calculations. The band structure of borophene shows highly anisotropic metallic behaviour.The obtained optical properties of borophene exhibit strong anisotropy as well. The combination of high optical transparency and high electrical conductivity in borophene makes it a promising candidate for future design of transparent conductors used in photovoltaics.Finally, the thermodynamic properties are investigated based on the phonon properties.arXiv:1601.00140v3 [cond-mat.mes-hall] 3 Apr 2016 2

Phonons are essential for understanding the thermal properties in monolayer transition metal dichalcogenides, which limit their thermal performance for potential applications. We investigate the lattice dynamics and thermodynamic properties of MoS 2 , MoSe 2 , and WS 2 by first principles calculations. The obtained phonon frequencies and thermal conductivities agree well with the measurements. Our results show that the thermal conductivity of MoS 2 is highest among the three materials due to its much lower average atomic mass. We also discuss the competition between mass effect, interatomic bonding and anharmonic vibrations in determining the thermal conductivity of WS 2 . Strong covalent W-S bonding and low anharmonicity in WS 2 are found to be crucial in understanding its much higher thermal conductivity compared to MoSe 2 .

It has been argued that stanene has lowest lattice thermal conductivity among 2D group-IV materials because of largest atomic mass, weakest interatomic bonding, and enhanced ZA phonon scattering due to the breaking of an out-of-plane symmetry selection rule. However, we show that although the lattice thermal conductivity κ for graphene, silicene and germanene decreases monotonically with decreasing Debye temperature, unexpected higher κ is observed in stanene. By enforcing all the invariance conditions in 2D materials and including Ge 3d and Sn 4d electrons as valence electrons for germanene and stanene respectively, the lattice dynamics in these materials are accurately described. A large acoustic-optical gap and the bunching of the acoustic phonon branches significantly reduce phonon scattering in stanene, leading to higher thermal conductivity than germanene. The vibrational origin of the acoustic-optical gap can be attributed to the buckled structure. Interestingly, a buckled system has two competing influences on phonon transport: the breaking of the symmetry selection rule leads to reduced thermal conductivity, and the enlarging of the acoustic-optical gap results in enhanced thermal conductivity. The size dependence of thermal conductivity is investigated as well. In nanoribbons, the κ of silicene, germanene and stanene is much less sensitive to size effect due to their short intrinsic phonon mean free paths. This work sheds light on the nature of phonon transport in buckled 2D materials.

A fundamental understanding of phonon transport in stanene is crucial to predict the thermal performance in potential stanene-based devices. By combining first-principle calculation and phonon Boltzmann transport equation, we obtain the lattice thermal conductivity of stanene. A much lower thermal conductivity (11.6 W/mK) is observed in stanene, which indicates higher thermoelectric efficiency over other 2D materials. The contributions of acoustic and optical phonons to the lattice thermal conductivity are evaluated. Detailed analysis of phase space for three-phonon processes shows that phonon scattering channels LA + LA/TA/ZA ↔ TA/ZA are restricted, leading to the dominant contributions of high-group-velocity LA phonons to the thermal conductivity. The size dependence of thermal conductivity is investigated as well for the purpose of the design of thermoelectric nanostructures.

A new two-dimensional (2D) material, borophene (2D boron sheet), has been grown successfully recently on single crystal Ag substrates by two parallel experiments [Mannix et al., Science, 2015, 350, 1513] [Feng et al., Nature Chemistry, 2016, advance online publication]. Three main structures have been proposed (β 12 , χ 3 and striped borophene). However, the stability of three structures is still in debate. Using first principles calculations, we examine the dynamical, thermodynamical and mechanical stability of β 12 , χ 3 and striped borophene. Free-standing β 12 and χ 3 borophene is dynamically, thermodynamically, and mechanically stable, while striped borophene is dynamically and thermodynamically unstable due to high stiffness along a direction. The origin of high stiffness and high instability in striped borophene along a direction can both be attributed to strong directional bonding.This work provides a benchmark for examining the relative stability of different structures of borophene.

Phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS) has been established as a powerful technique for surface characterization, but for it to generate a reliable spectrum, accurate phase measurement with a well-defined phase reference is most important. Incorrect phase measurement can lead to significant distortion of a spectrum, as recently seen in the case for the air/water interface. In this work, we show theoretically and experimentally that a transparent, highly nonlinear crystal, such as quartz and barium borate, can be a good phase reference if the surface is clean and unstrained and the crystal is properly oriented to yield a strong SF output. In such cases, the reflected SF signal is dominated by the bulk electric dipole contribution and its phase is either +90° or -90°. On the other hand, materials with inversion symmetry, such as water, fused quartz, and CaF2 are not good phase references due to the quadrupole contribution and phase dispersion at the interface. Using a proper phase reference in PS-SFVS, we have found the most reliable OH stretching spectrum for the air/water interface. The positive band at low frequencies in the imaginary component of the spectrum, which has garnered much interest and been interpreted by many to be due to strongly hydrogen-bonded water species, is no longer present. A weak positive feature however still exists. Its magnitude approximately equals to that of air/D2O away from resonances, suggesting that this positive feature is unrelated to surface resonance of water.

Controlling heat transport through material design is one important step toward thermal management in 2D materials. To control heat transport, a comprehensive understanding of how structure influences heat transport is required. It has been argued that a buckled structure is able to suppress heat transport by increasing the flexural phonon scattering. Using a first principles approach, we calculate the lattice thermal conductivity of 2D mono-elemental materials with a buckled structure. Somewhat counterintuitively, we find that although 2D group-V materials have a larger mass and higher buckling height than their group-IV counterparts, the calculated κ of blue phosphorene (106.6 W mK) is nearly four times higher than that of silicene (28.3 W mK), while arsenene (37.8 W mK) is more than fifteen times higher than germanene (2.4 W mK). We report for the first time that a buckled structure has three conflicting effects: (i) increasing the Debye temperature by increasing the overlap of the p orbitals, (ii) suppressing the acoustic-optical scattering by forming an acoustic-optical gap, and (iii) increasing the flexural phonon scattering. The former two, corresponding to the harmonic phonon part, tend to enhance κ, while the last one, corresponding to the anharmonic part, suppresses it. This relationship between the buckled structure and phonon behaviour provides insight into how to control heat transport in 2D materials.

Recently a stable monolayer of antimony in buckled honeycomb structure called antimonene was successfully grown on 3D topological insulator Bi 2 Te 3 and Sb 2 Te 3 , which displays semiconducting properties. By first principle calculations, we systematically investigate the phononic, electronic and optical properties of α− and β− allotropes of monolayer arsenene/antimonene. We investigate the dynamical stabilities of these four materials by considering the phonon dispersions. The obtained electronic structures reveal the direct band gap of monolayer α−As/Sb and indirect band gap of β−As/Sb. Significant absorption is observed in α−Sb, which can be used as a broad saturable absorber.