Borophene shows highly anisotropic metallic behaviour (a) due to anisotropic structure (b). The optical properties of borophene are orientation-dependent (c). This provides opportunities for applications in display technologies, photovoltaics and flexible electronics.

Our work shows that the thermal conductivityκof MoS₂is largest among the three materials (Fig. a and b) due to largest Debye temperature. WS₂has stronger bonding (Fig. e) and lower anharmonicity (Fig. h), leading to a much largerκcompared to MoSe₂.

It has been argued that stanene has lowest lattice thermal conductivity among two-dimensional (2D) group-IV materials because of its 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 $\kappa$ for graphene, silicene, and germanene decreases monotonically with decreasing Debye temperature, unexpected higher $\kappa$ 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 $\kappa$ 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 new 2D material, borophene, has been grown successfully recently on single crystal Ag substrates. Three main structures have been proposed ( , 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 , and striped borophene. Free-standing and 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. Our work shows a deep connection between stability and strength, and helps researchers to estimate accurately the mechanical performance of 2D materials. GRAPHICAL ABSTRACT IMPACT STATEMENT

The Ba₉Lu₂Si₆O₂₄:Ce³⁺cyan phosphor has excellent luminescence properties and displays promising application in NUV-based white LEDs with high performance.

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.