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.

First-principles calculations are performed to investigate the interaction of physisorbed small molecules, including CO, H 2 , H 2 O, NH 3 , NO, NO 2 , and O 2 , with phosphorene, and their energetics, charge transfer, and magnetic moment are evaluated on the basis of dispersion corrected density functional theory. Our calculations reveal that CO, H 2 , H 2 O and NH 3 molecules act as a weak donor, whereas O 2 and NO 2 act as a strong acceptor. While NO molecule donates electrons to graphene, it receives electrons from phosphorene. Among all the investigated molecules, NO 2 has the strongest interaction through hybridizing its frontier orbitals with the 3p orbital of phosphorene. The nontrivial and distinct charge transfer occurring between phosphorene and these physisorbed molecules not only renders phosphorene promising for application as a gas sensor, but also provides an effective route to modulating the polarity and density of carriers in phosphorene. In addition, the binding energy of H 2 on phosphorene is found to be 0.13 eV/H 2 , indicating that phosphorene is suitable for both stable room-temperature hydrogen storage and its subsequent facile release.

Vertical integration of two-dimensional materials has recently emerged as an exciting method for the design of novel electronic and optoelectronic devices. Using density functional theory, we investigate the structural and electronic properties of two heterostructures, graphene/phosphorene (G/BP) and hexagonal boron nitride/phosphorene (BN/BP). We found that the interlayer distance, binding energy, and charge transfer in G/BP and BN/BP are similar. Interlayer noncovalent bonding is predicted due to the weak coupling between the p z orbital of BP and the π orbital of graphene and BN. A small amount of electron transfer from graphene and BN, scaling with the vertical strain, renders BP slightly n-doped for both heterostructures. Several attractive characteristics of BP, including direct band gap and linear dichroism, are preserved. However, a large redistribution of electrostatic potential across the interface is observed, which may significantly renormalize the carrier dynamics and affect the excitonic behavior of BP. Our work suggests that graphene and BN can be used not only as an effective capping layer to protect BP from its structural and chemical degradation while still maintain its major electronic characteristics, but also as an active layer to tune the carrier dynamics and optical properties of BP. Supporting Information Placeholder

Using first-principles calculations and non-equilibrium Green's function method, we investigate the ballistic thermal transport in single-layer phosphorene. A significant crystallographic orientation dependence of thermal conductance is observed, with room temperature thermal conductance along zigzag direction being 40% higher than that along armchair direction. Furthermore, we find that the thermal conductance anisotropy with the orientation can be tuned by applying strain. In particular, the zigzag-oriented thermal conductance is enhanced when a zigzag-oriented strain is applied but decreases when an armchair-oriented strain is applied; whereas the armchair-oriented thermal conductance always decreases when either a zigzag-or an armchair-oriented strain is applied. The present work suggests that the remarkable thermal transport anisotropy and its strain-modulated effect in single-layer phosphorene may be used for thermal management in phosphorene-based electronics and optoelectronic devices.

We have synthesized high-quality, micrometer-sized, single-crystal GeSe nanosheets using vapor transport and deposition techniques. Photoresponse is investigated based on mechanically exfoliated GeSe nanosheet combined with Au contacts under a global laser irradiation scheme. The nonlinearship, asymmetric, and unsaturated characteristics of the I-V curves reveal that two uneven back-to-back Schottky contacts are formed. First-principles calculations indicate that the occurrence of defects-induced in-gap defective states, which are responsible for the slow decay of the current in the OFF state and for the weak light intensity dependence of photocurrent. The Schottky photodetector exhibits a marked photoresponse to NIR light illumination (maximum photoconductive gain ∼5.3 × 10(2) % at 4 V) at a wavelength of 808 nm. The significant photoresponse and good responsitivity (∼3.5 A W(-1)) suggests its potential applications as photodetectors.