Qu Yue

Adsorption of gas molecules on monolayer MoS2 and effect of applied electric field

AbstractUsing first-principles calculations, we investigate the adsorption of various gas molecules (H2, O2, H2O, NH3, NO, NO2, and CO) on monolayer MoS2. The most stable adsorption configuration, adsorption energy, and charge transfer are obtained. It is shown that all the molecules are weakly adsorbed on the monolayer MoS2 surface and act as charge acceptors for the monolayer, except NH3 which is found to be a charge donor. Furthermore, we show that charge transfer between the adsorbed molecule and MoS2 can be significantly modulated by a perpendicular electric field. Our theoretical results are consistent with the recent experiments and suggest MoS2 as a potential material for gas sensing application.

Bandgap tuning in armchair MoS2 nanoribbon.

We report on the first-principles calculations of bandgap modulation in armchair MoS(2) nanoribbon (AMoS(2)NR) by transverse and perpendicular electric fields respectively. In the monolayer AMoS(2)NR case, it is shown that the bandgap can be significantly reduced and be closed by transverse field, whereas the bandgap modulation is absent under perpendicular field. The critical strength of transverse field for gap closure decreases as ribbon width increases. In the multilayer AMoS(2)NR case, in contrast, it is shown that the bandgap can be effectively reduced by both transverse and perpendicular fields. Nevertheless, it seems that the two fields exhibit different modulation effects on the gap. The critical strength of perpendicular field for gap closure decreases with increasing number of layers, while the critical strength of transverse field is almost independent of it.

High-Performance Few-layer Mo-doped ReSe2 Nanosheet Photodetectors

Transition metal dichalcogenides (TMDCs) have recently been the focus of extensive research activity owing to their fascinating physical properties. As a new member of TMDCs, Mo doped ReSe2 (Mo:ReSe2) is an octahedral structure semiconductor being optically biaxial and highly anisotropic, different from most of hexagonal layered TMDCs with optically uniaxial and relatively high crystal symmetry. We investigated the effects of physisorption of gas molecule on the few-layer Mo:ReSe2 nanosheet based photodetectors. We compared the photoresponse of the as-exfoliated device with annealed device both in air or ammonia (NH3) environment. After annealing at sub-decomposition temperatures, the Mo:ReSe2 photodetectors show a better photoresponsivity (~55.5 A/W) and higher EQE (10893%) in NH3 than in air. By theoretical investigation, we conclude that the physisorption of NH3 molecule on Mo:ReSe2 monolayer can cause the charge transfer between NH3 molecule and Mo:ReSe2 monolayer, increasing the n-type carrier density of Mo:ReSe2 monolayer. The prompt photoswitching, high photoresponsivity and different sensitivity to surrounding environment from the few-layer anisotropic Mo:ReSe2 can be used to design multifunctional optoelectronic and sensing devices.

Magnetic and electronic properties of α-graphyne nanoribbons.

Based on the first-principles calculations, we investigate the magnetic and electronic properties of α-graphyne nanoribbons (NRs). We show that all the armchair α-graphyne NRs are nonmagnetic semiconductors with band gaps as a function of ribbon widths. The zigzag α-graphyne NRs are found to have magnetic semiconducting ground state with ferromagnetic ordering at each edge and opposite spin orientation between the two edges. Under the application of transverse electric field, we further predict the existence of half-metallicity in the zigzag NRs which strongly depends on the width of the ribbon.

Highly efficient gas molecule-tunable few-layer GaSe phototransistors

Herein we present a systematic study on the effects of different gas molecules on the photoelectric response of few-layer GaSe phototransistors before and after introducing defects. After introducing defects by thermal annealing, the phototransistors become largely photo-responsive (18.75 A W−1) with high external quantum efficiency (EQE) (∼91.53%), high photocurrent on–off ratio, fast photo-response, and good stability in an O2 rich environment when illuminated by 254 nm ultraviolet light.

Metal to semiconductor transition in metallic transition metal dichalcogenides

We report on tuning the electronic and magnetic properties of metallic transition metal dichalcogenides (mTMDCs) by 2D to 1D size confinement. The stability of the mTMDC monolayers and nanoribbons is demonstrated by the larger binding energy compared to the experimentally available semiconducting TMDCs. The 2D MX2 (M = Nb, Ta; X = S, Se) monolayers are non-ferromagnetic metals and mechanically softer compared to their semiconducting TMDCs counterparts. Interestingly, mTMDCs undergo metal-to-semiconductor transition when the ribbon width approaches to ∼13 A and ∼7 A for zigzag and armchair edge terminations, respectively; then these ribbons convert back to metal when the ribbon widths further decrease. Zigzag terminated nanoribbons are ferromagnetic semiconductors, and their magnetic properties can also be tuned by hydrogen edge passivation, whereas the armchair nanoribbons are non-ferromagnetic semiconductors. Our results display that the mTMDCs offer a broad range of physical properties spanning from met...

Abnormal low-temperature behavior of a continuous photocurrent in Bi2S3 nanowires

High-quality Bi2S3 nanowires are synthesized and their photoresponses are investigated in detail. Our results show that the photoresponsive curves have distinctly different characteristics at low-temperature (50 K) compared to those at room temperature (290 K). The transferred-electron effect is believed to cause this difference. A first principle calculation shows that Bi2S3 has many energy valleys, which agree with our experimental analysis. At low temperature, due to the lack of sufficient phonon energy, the photoexcited electrons in Bi2S3 mainly aggregate at the bottom of the conduction band. When this electron concentration increased to a high enough level after illumination, an electron transfer between the energy valleys happened and the photocurrent began to decrease slowly after the rapid increase in the first stage. After the transfer process reaches equilibrium, the photocurrent reaches a minimum, thus the trap states play a dominant role and the photocurrent rises slowly again. Furthermore, photocurrent curves at different temperatures were recorded to estimate the phonon energy value needed to assist the electron transitions. The required phonon energy is calculated to be about 16.3 meV (corresponding to 190 K), which fits well with previous results.

Low temperature electrical and photo-responsive properties of MoSe2

MoSe2 was fabricated by a facile hydrothermal method, and a simple device based on it was prepared to investigate the low temperature electrical and photo-responsive (PR) properties. PR current of MoSe2 under 650 nm red illumination is 2.55 × 10−5 A and remains approximately at low temperatures, which demonstrates its fine PR property. As the temperature became lower, electrical conductivity of MoSe2 first decreased from 300 to 43 K and then increased at temperatures from 43 to 13 K. Mechanisms of such electrical and PR phenomenon were proposed. Our findings revealed potential method to adjust band gap of transition metal dichalcogenides and demonstrated their potential applications under special environment.

Tunable band gaps in graphene/GaN van der Waals heterostructures

Van der Waals (vdW) heterostructures consisting of graphene and other two-dimensional materials provide good opportunities for achieving desired electronic and optoelectronic properties. Here, we focus on vdW heterostructures composed of graphene and gallium nitride (GaN). Using density functional theory, we perform a systematic study on the structural and electronic properties of heterostructures consisting of graphene and GaN. Small band gaps are opened up at or near the Γ point of the Brillouin zone for all of the heterostructures. We also investigate the effect of the stacking sequence and electric fields on their electronic properties. Our results show that the tunability of the band gap is sensitive to the stacking sequence in bilayer-graphene-based heterostructures. In particular, in the case of graphene/graphene/GaN, a band gap of up to 334 meV is obtained under a perpendicular electric field. The band gap of bilayer graphene between GaN sheets (GaN/graphene/graphene/GaN) shows similar tunability, and increases to 217 meV with the perpendicular electric field reaching 0.8 V Å(-1).