Thermoelectric devices that utilize the Seebeck effect convert heat flow into electrical energy and are highly desirable for the development of portable, solid state, passively powered electronic systems. The conversion efficiencies of such devices are quantified by the dimensionless thermoelectric figure of merit (ZT), which is proportional to the ratio of a device's electrical conductance to its thermal conductance. In this paper, a recently fabricated two-dimensional (2D) semiconductor called phosphorene (monolayer black phosphorus) is assessed for its thermoelectric capabilities. First-principles and model calculations reveal not only that phosphorene possesses a spatially anisotropic electrical conductance, but that its lattice thermal conductance exhibits a pronounced spatial-anisotropy as well. The prominent electrical and thermal conducting directions are orthogonal to one another, enhancing the ratio of these conductances. As a result, ZT may reach the criterion for commercial deployment along the armchair direction of phosphorene at T = 500 K and is close to 1 even at room temperature given moderate doping (∼2 × 10(16) m(-2) or 2 × 10(12) cm(-2)). Ultimately, phosphorene hopefully stands out as an environmentally sound thermoelectric material with unprecedented qualities. Intrinsically, it is a mechanically flexible material that converts heat energy with high efficiency at low temperatures (∼300 K), one whose performance does not require any sophisticated engineering techniques.

We have systematically investigated the effect of oxidation on the structural and electronic properties of graphene based on first-principles calculations. Energetically favorable atomic configurations and building blocks are identified, which contain epoxide and hydroxyl groups in close proximity with each other. Different arrangements of these units yield a local-density approximation band gap over a range of a few eV. These results suggest the possibility of creating and tuning the band gap in graphene by varying the oxidation level and the relative amount of epoxide and hydroxyl functional groups on the surface.

Recent observation of intrinsic ferromagnetism in two-dimensional (2D) CrI3 is associated with the large magnetic anisotropy due to strong spin-orbit coupling (SOC) of I. Magnetic anisotropy energy (MAE) defines the stability of magnetization in a specific direction with respect to the crystal lattice and is an important parameter for nanoscale applications. In this work we apply the density functional theory to study the strain dependence of MAE in 2D monolayer chromium trihalides CrX3 (with X = Cl, Br, and I). Detailed calculations of their energetics, atomic structures and electronic structures under the influence of a biaxial strain ε have been carried out. It is found that all three compounds exhibit ferromagnetic ordering at the ground state (with ε=0) and upon applying a compressive strain, phase transition to antiferromagnetic state occurs. Unlike in CrCl3 and CrBr3, the electronic band gap in CrI3 increases when a tensile strain is applied. The MAE also exhibits a strain dependence in the chromium trihalides: it increases when a compressive strain is applied in CrI3, while an opposite trend is observed in the other two compounds. In particular, the MAE of CrI3 can be increased by 47% with a compressive strain of ε = 5%.

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The phonon dispersions of monolayer and few-layer graphene (AB bilayer, ABA and ABC trilayers) are investigated using the density-functional perturbation theory (DFPT). Compared with the monolayer, the optical phonon E2g mode at Γ splits into two and three doubly degenerate branches for bilayer and trilayer graphene, respectively, due to the weak interlayer coupling. These modes are of various symmetry and exhibit different sensitivity to either Raman or infrared (IR) measurements (or both). The splitting is found to be 5 cm −1 for bilayer and 2 to 5 cm −1 for trilayer graphene. The interlayer coupling is estimated to be about 2 cm −1 . We found that the highest optical modes at K move up by about 12 cm −1 for bilayer and 18 cm −1 for trilayer relative to monolayer graphene. The atomic displacements of these optical eigenmodes are analyzed.