Using first-principles calculations and deformation potential theory, we investigate the intrinsic carrier mobility (μ) of monolayer MoS2 sheet and nanoribbons. In contrast to the dramatic deterioration of μ in graphene upon forming nanoribbons, the magnitude of μ in armchair MoS2 nanoribbons is comparable to its sheet counterpart, albeit oscillating with ribbon width. Surprisingly, a room-temperature transport polarity reversal is observed with μ of hole (h) and electron (e) being 200.52 (h) and 72.16 (e) cm(2) V(-1) s(-1) in sheet, and 49.72 (h) and 190.89 (e) cm(2) V(-1) s(-1) in 4 nm nanoribbon. The high and robust μ and its polarity reversal are attributable to the different characteristics of edge states inherent in MoS2 nanoribbons. Our study suggests that width reduction together with edge engineering provide a promising route for improving the transport properties of MoS2 nanostructures.

Using first-principles calculations, we study the electronic properties of few-layer phosphorene focusing on layer-dependent behavior of band gap, work function band alignment and carrier effective mass. It is found that few-layer phosphorene shows a robust direct band gap character, and its band gap decreases with the number of layers following a power law. The work function decreases rapidly from monolayer (5.16 eV) to trilayer (4.56 eV), and then slowly upon further increasing the layer number. Compared to monolayer phosphorene, there is a drastic decrease of hole effective mass along the ridge (zigzag) direction for bilayer phosphorene, indicating a strong interlayer coupling and screening effect. Our study suggests that 1). Few-layer phosphorene with a layer-dependent band gap and a robust direct band gap character is promising for efficient solar energy harvest. 2). Few-layer phosphorene outperforms monolayer counterpart in terms of a lighter carrier effective mass, a higher carrier density and a weaker scattering due to enhanced screening. 3). The layer-dependent band edges and work functions of few-layer phosphorene allow for modification of Schottky barrier with enhanced carrier injection efficiency. It is expected that few-layer phosphorene will present abundant opportunities for a plethora of new electronic applications.

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The anharmonic behavior of phonons and intrinsic thermal conductivity associated with the Umklapp scattering in monolayer MoS 2 sheet are investigated via first-principles calculations within the framework of density functional perturbation theory. In contrast to the negative Grüneissen parameter ( ) occurring in low frequency modes in graphene, positive in the whole Brillouin zone is demonstrated in monolayer MoS 2 with much larger for acoustic modes than that for the optical modes, suggesting that monolayer MoS 2 sheet possesses a positive coefficient of thermal-expansion. The calculated phonon lifetimes of the infrared active modes are 5.50 and 5.72 ps for E ′ and A 2 ′′ respectively, in good agreement with experimental result obtained by fitting the dielectric oscillators with the infrared reflectivity spectrum. The lifetime of Raman A 1 ′ mode (38.36 ps) is about 7 times longer than those of the infrared modes. The dominated phonon mean free path of monolayer MoS 2 is less than 20 nm, about 30-fold smaller than that of graphene. Combined with the nonequilibrium Green's function calculations, the room temperature thermal conductivity of monolayer MoS 2 is found to be around 23.2 Wm -1 K -1 , two orders of magnitude lower than that of graphene.