ABSTRACT
Spontaneous flexoelectricity in transition metal dichalcogenide (TMD) nanotubes is critical to the design of new energy devices. However, the electronic properties adjusted by the flexoelectric effect in TMD nanotubes remain vague. In this work, we investigate the effect of flexoelectricity on band engineering in single- and double-wall MS2 (M = Mo, W) nanotubes with different diameters based on first-principles calculations and an atomic-bond-relaxation method. We find that the energy bandgap reduces and the polarization and flexoelectric voltage increase with decreasing diameter of single-wall MS2 nanotubes. The polarization charges promoted by the flexoelectric effect can lead to a straddling-to-staggered bandgap transition in the double-wall MS2 nanotubes. The critical diameters for bandgap transition are about 3.1 and 3.6 nm for double-wall MoS2 and WS2 nanotubes, respectively, which is independent of chirality. Our results provide guidance for the design of new energy devices based on spontaneous flexoelectricity.
ABSTRACT
Current achievements show that inserting an insulator at the interface in van der Waals (vdW) heterostructures can improve the photoelectric conversion efficiency. However, the underlying mechanism of the intercalated insulator effect on photocarrier collection and recombination, etc., remains unclear at the atomic level. Herein, we investigate the influence of intercalated hexagonal boron nitride (h-BN) on the optoelectronic properties of MoS2/WSe2 vdW heterostructures based on the interface bond relaxation and detailed balance principles. It was found that the band offsets (barrier height) at the heterointerface decrease with increasing h-BN size and decreasing transition metal dichalcogenide (TMD) thickness. Moreover, the power conversion efficiency (PCE) in monolayer MoS2/11-layers h-BN/monolayer WSe2 can reach 3.23% and enhances with increasing thickness of TMDs. Also, the optimal thickness of h-BN in MoS2/h-BN/WSe2 decreases from 3.64 to 2.86 nm as the thickness of TMDs increases. Our results show that the PCE and open-circuit voltage of the MoS2/h-BN/WSe2 vdW heterostructure are obviously improved compared with the bilayer heterostructure without h-BN, and this further proves the feasibility of the intercalated insulator as a booster for highly efficient photovoltaic and optoelectronic nanodevices.
ABSTRACT
Layered two-dimensional transition-metal dichalcogenide (TMD) alloys with strong intralayer ionic-covalent bonds and weak interlayer van der Waals bonds have been extensively studied in recent years owing to their tunable electronic and optoelectronic properties. However, the relationship among atomic bond identities, band offset, and related semiconductor-to-metal transition in ternary alloys of TMDs with different thicknesses under hydrostatic pressure at the atomic level remains largely unexplored, despite the fact that it plays an important role in the functionality of TMD-based devices. In this work, we investigate the thickness-dependent band offset and semiconductor-to-metal transition in Mo(1-x)W x S2 with different thicknesses under hydrostatic pressure based on the atomic-bond-relaxation correlation mechanism. It was found that the compression ratio in the out-of-plane direction is significantly higher than that of in-plane, and the band shift and semiconductor-to-metal transition are significantly modulated by the hydrostatic pressure, number of layers, and composition. The theoretical predictions are consistent with the experimental observations and calculations, suggesting that our approach can be suitable for other layered TMDs.
ABSTRACT
Based on field survey, information collection and experts consultation, the quantitative grading index system and assessment standards for preference conservation of rare and endangered plant species of forest-meadow ecotone in Sanjiangyuan Nature Reserve were established by using the methods and principles of systematical analysis. The quantitative grading index system included endangered coefficient, genetic coefficient, and useful value coefficient. In addition, 10 indicators used to evaluate endangered grading and conservation priorities sequenc, were also included in 3 subsystems respectively. Furthermore, the weights of 3 subsystem and 10 indicators were given through experts consultation and analytic hierarchy process. Endangered coefficient and conservation priorities coefficient, which respectively described the endangered grading, and preferential conservation of plant species were calculated by mathematic models and computer program. Contrasting to the standards of endangered grading and conservation priorities for plant species, we quantitatively evaluated the status of endangered and conservation priorities of plant species. The results showed that the number of endangered species was 4, vulnerable species 68, lower risk species 179, safety species 695; the number of the first class species was 8, the second class species 78, the third class species 164, and the delayed conservation species 696. Finally, we discussed the problems of indicator system and its weight, the relationship between endangered grading and conservation priorities sequence, and the spatial scale problem of plant species assessment.
