Tunable quantum phase transitions and novel emergent fermions in solid state materials are fascinating subjects of research. Here, we propose a new stable two-dimensional (2D) material, the blue phosphorene oxide (BPO), which exhibits both. Based on first-principles calculations, we show that its equilibrium state is a narrow-bandgap semiconductor with three bands at low energy. Remarkably, a moderate strain can drive a semiconductor-to-semimetal quantum phase transition in BPO. At the critical transition point, the three bands cross at a single point at Fermi level, around which the quasiparticles are a novel type of 2D pseudospin-1 fermions. Going beyond the transition, the system becomes a symmetry-protected semimetal, for which the conduction and valence bands touch quadratically at a single Fermi point that is protected by symmetry, and the low-energy quasiparticles become another novel type of 2D double Weyl fermions. We construct effective models characterizing the phase transition and these novel emergent fermions, and we point out several exotic effects, including super Klein tunneling, supercollimation, and universal optical absorbance. Our result reveals BPO as an intriguing platform for the exploration of fundamental properties of quantum phase transitions

Since surface functionalization can profoundly tune the physical and chemical properties of materials, we performed a comparative study on the thermal conductivities of fluorinated diamane (FD) and compared them with the hydrogenated diamane (HD) to examine the influence of functional groups on the thermal transport properties of diamane. Our results reveal a significant impact of a functional group on the thermal conductivity of diamane. The FD shows an 82% reduced thermal conductivity as compared with the HD. Most strikingly, the dominant phonon modes in thermal transport switches from out-of-plane acoustic (ZA) modes in HD to optical modes in FD. Those results can be understood by the heavy atomic mass of fluorine as opposed to the light hydrogen, which leads to remarkably softened phonon dispersion and the entanglement of optical modes with the acoustic modes. These two factors result in reduced group velocities and enhanced phonon scattering in FD, both of which account for the significantly dropped thermal conductivity of FD. Hence, the mass of functional groups could be employed to tune the thermal transport behavior of 2D materials effectively.

The effect of biaxial tensile strain on the thermal transport properties of MoS(2) is investigated by combining first-principles calculations and the Boltzmann transport equation. The thermal conductivities of single layer MoS(2) are found to be heavily suppressed by the applied strains; even a moderate biaxial tensile strain, 2 ∼ 4%, could result in a 10 ∼ 20% reduction in the thermal conductivity. Most interestingly, the reduction rate of thermal conductivity is size dependent,which is due to different dominant phonon scattering mechanisms at different sizes of MoS(2) samples. The sensitive strain dependence of thermal conductivity indicates that strain engineering could be an effective method to enhance the figure of merit for thermoelectric applications of MoS(2).

Thermal conductivities of monolayer holey CN nanosheets are investigated via equilibrium molecular dynamics simulations. As compared with graphene, the lattice thermal conductivities of CN decrease by two orders in magnitude, which are around 40 W m K at 300 K along both zigzag and armchair directions. The lattice dynamics calculations reveal that the reduced group velocities and shortened phonon lifetimes, due to the incorporation of nitrogen atoms and the holey structure, account for such a giant reduction in thermal conductivity. Our study also indicates that pyridinic-like nitrogen doping would be a more efficient way than graphite-like nitrogen doping to suppress the thermal conductivity of graphene.