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.

Mechanical exfoliation of 2D materials has triggered an explosive interest in low dimensional material research. We extend this idea to 1D van der-Waals materials. Three 1D semiconductors (SbSeI, SbSI, and SbSBr) with high stability and novel electronic properties are discovered using first principles calculations. Both the dynamical and the thermal stability of these 1D materials are examined. We demonstrate that their nanowire thinner than 7Å can be easily obtained by mechanical exfoliation, hydrothermal method, or sonochemical method. The bulk-to-1D transition results in dramatic changes in band gap, effective mass, and static dielectric constant due to quantum confinement, making 1D SbSeI a highly promising channel material for transistors with gate length shorter than 1 nm. Under small uniaxial strain, these materials are transformed from indirect into direct band gap semiconductors, paving the way for optoelectronic devices and mechanical sensors. Moreover, the thermoelectric performance of these materials is significantly improved over their bulk counterparts. These highly desirable properties render SbSeI, SbSI, and SbSBr promising 1D materials for applications in future microelectronics, optoelectronics, mechanical sensors, and thermoelectrics.

Sulfur vacancies in monolayer MoS2 can provide unexpected opportunities for tailoring the properties and device applications via defect engineering. However, determining the effect of vacancies in thermal transport remains a big challenge. Using a first-principles supercell approach, we reveal the dominant role of defect-induced quasi-localized phonon states in reducing thermal conductivity of MoS2. These states are related to flattened dispersions in phonon spectrum, which comes from perturbations in atomic mass and interatomic bonding. Although the scattering strength of each modes remains similar, the phonon group velocities are much lower near the quasi-localized modes, while the Umklapp scattering are significantly enhanced. Thus, the thermal conductivity of defective MoS2 is severely reduced. Our results contribute to fundamental understanding of the effect of vacancies on thermal transport, and can be used to assess the defect concentrations in semiconductors quantitatively.

At room temperature, the uniaxial strain (εx = −8%) can enhance the hole mobility of monolayer penta-SiC2 along the b-direction by almost three orders of magnitude up to 1.14 × 106 cm2 V−1 s−1, which is much larger than that of graphene.

In article number https://doi.org/10.1002/adts.201700005, Hao Zhang and co‐workers discover 1D SbSeI, SbSI and SbSBr with high stability and novel properties based on first principles calculations. Bulk V‐VI‐VII compounds are earth‐abundant, and isolating one chain from them can be experimentally feasible. The electronic, optoelectronic, mechanical and thermoelectric performances are significantly improved over their bulk counterparts. A new pathway to obtain 1D nanostructures with highly desired properties is demonstrated.