Polarity-Reversed Robust Carrier Mobility in Monolayer MoS₂ Nanoribbons

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) cm² V^–1s^–1 in sheet, and 49.72 (h) and 190.89 (e) cm² 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

Modulating Carrier Density and Transport Properties of MoS₂ by Organic Molecular Doping and Defect Engineering

Using first-principles calculations, we investigate the effect of molecular doping and sulfur vacancy on the electronic properties and charge modulation of monolayer MoS₂. It is found that tetrathiafulvalene and dimethyl-<i>p</i>-phenylenediamine molecules are effective donors, whereas tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) are effective acceptors, and all these molecules are able to shift the work function of MoS₂. For MoS₂ containing sulfur vacancies, these molecules are able to change the position of the defect levels within the band gap and modulate the carrier density around the defect center. Charge transfer analysis shows that TCNE and TCNQ induce a free-carrier depletion of the defect states, which is beneficial for the suppression of the nonradiative trionic decay and a higher excitonic efficiency due to a decrease in the screening of excitons. Furthermore, the effects of molecular adsorption on Seebeck coefficient of MoS₂ are also explored. Our work suggests that an enhanced excitonic efficiency of MoS₂ may be achieved via proper defect engineering and molecular doping arising from the charge density modulation and charge screening

An Anomalous Formation Pathway for Dislocation-Sulfur Vacancy Complexes in Polycrystalline Monolayer MoS₂

Two-dimensional (2D) molybdenum disulfide (MoS₂) has attracted significant attention recently due to its direct bandgap semiconducting characteristics. Experimental studies on monolayer MoS₂ show that S vacancy concentration varies greatly; while recent theoretical studies show that the formation energy of S vacancy is high and thus its concentration should be low. We perform density functional theory calculations to study the structures and energetics of vacancy and interstitial in both grain boundary (GB) and grain interior (GI) in monolayer MoS₂ and uncover an anomalous formation pathway for dislocation-double S vacancy (V_2S) complexes in MoS₂. In this pathway, a (5|7) defect in an S-polar GB energetically favorably converts to a (4|6) defect, which possesses a duality: dislocation and double S vacancy. Its dislocation character allows it to glide into GI through thermal activation at high temperatures, bringing the double vacancy with it. Our findings here not only explain why V_S is predominant in exfoliated 2D MoS₂ and V_2S is predominant in chemical vapor deposition (CVD)-grown 2D MoS₂ but also reproduce GB patterns in CVD-grown MoS₂. The new pathway for sulfur vacancy formation revealed here provides important insights and guidelines for controlling the quality of monolayer MoS₂

Strain-Robust and Electric Field Tunable Band Alignments in van der Waals WSe₂–Graphene Heterojunctions

We study the band alignments and band structures of van der Waals WSe₂–graphene heterojunctions by varying out-of-plane external electric field and in-plane mechanical strain using density-functional calculations. We find that the electronic properties of WSe₂–graphene heterojunctions are insensitive to the change of the mechanical strain, showing strong robustness. However, the external electrical field intensity is able to significantly change the band alignments of WSe₂–graphene heterojunctions, while a constant band gap value of WSe₂ in the heterojunctions is nearly maintained. We further show that the highest hole concentration injected by the external electric field is estimated as high as 6.40 × 10¹² cm^–2, while the highest electron density is about 3.00 × 10¹² cm^–2. These findings suggest that the WSe₂–graphene heterojunctions are a promising structure instrumental for electronic device applications

Realizing Indirect-to-Direct Band Gap Transition in Few-Layer Two-Dimensional MX₂ (M = Mo, W; X = S, Se)

In the applications of two-dimensional (2D) transition metal dichalcogenides (TMDs) for solar cell and optoelectronic devices, two challenging issues remain: (1) the direct-to-indirect band gap transition from single layer to a few layers and (2) the absence of an effective and robust doping procedure. In this study, we explore the feasibility to realize indirect-to-direct band gap transition and control the Fermi level by intercalating few-layer TMDs with embedded metals. Specifically, utilizing density functional theory calculations, we examine the electronic properties of few-layer MX₂ (M = Mo, W; X = S, Se) intercalated with metals (Zn, Sn, Mg and Ga). Our calculation results reveal that (1) Ga intercalation can realize an indirect-to-direct band gap transition in few-layer TMDs, and as a result, the absorption efficiency is increased by two orders compared with that of pristine MX₂; and (2) intercalated Ga acts as an n-type shallow donor, which markedly increases the charge density and electrical conductivity. Therefore, Ga intercalation may provide a potential practical route for manipulating few-layer TMDs for high performance solar and optoelectronic devices

Anisotropic Wetting Characteristics of Water Droplets on Phosphorene: Roles of Layer and Defect Engineering

We study the wetting behavior of water droplets on pristine and defective phosphorene using molecular dynamics simulations. It is found that unlike prototypical two-dimensional materials such as graphene and MoS₂, phosphorene exhibits an anisotropic contact angle along armchair and zigzag directions. This anisotropy is tunable with increasing the number of layers and vacancy concentration. More specifically, the water contact angles decrease with increasing the number of layers, indicating the importance of water–substrate interactions. The contact angles along both armchair and zigzag directions increase with the increasing vacancy concentration, and the anisotropy disappears when the defect concentration is high. For an in-plane pristine-defective phosphorene heterostructure, when the junction is zigzag-oriented, a spontaneous diffusion of water droplets from the defective region to the pristine region occurs; when the junction is armchair-oriented, however, the spontaneous motion is suppressed. The energetic factor plays a role for the difference in the motion of water droplets along zigzag and armchair directions. Our work highlights the unique and fascinating directional wetting behavior of water droplets on phosphorene

Anisotropic Wetting Characteristics of Water Droplets on Phosphorene: Roles of Layer and Defect Engineering

We study the wetting behavior of water droplets on pristine and defective phosphorene using molecular dynamics simulations. It is found that unlike prototypical two-dimensional materials such as graphene and MoS₂, phosphorene exhibits an anisotropic contact angle along armchair and zigzag directions. This anisotropy is tunable with increasing the number of layers and vacancy concentration. More specifically, the water contact angles decrease with increasing the number of layers, indicating the importance of water–substrate interactions. The contact angles along both armchair and zigzag directions increase with the increasing vacancy concentration, and the anisotropy disappears when the defect concentration is high. For an in-plane pristine-defective phosphorene heterostructure, when the junction is zigzag-oriented, a spontaneous diffusion of water droplets from the defective region to the pristine region occurs; when the junction is armchair-oriented, however, the spontaneous motion is suppressed. The energetic factor plays a role for the difference in the motion of water droplets along zigzag and armchair directions. Our work highlights the unique and fascinating directional wetting behavior of water droplets on phosphorene

Ende der Talfahrt fuer die ostdeutsche Wirtschaft in Sicht?

SIGLEIAB-90-0DD0-101200 AU 533 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Anisotropic Wetting Characteristics of Water Droplets on Phosphorene: Roles of Layer and Defect Engineering

We study the wetting behavior of water droplets on pristine and defective phosphorene using molecular dynamics simulations. It is found that unlike prototypical two-dimensional materials such as graphene and MoS₂, phosphorene exhibits an anisotropic contact angle along armchair and zigzag directions. This anisotropy is tunable with increasing the number of layers and vacancy concentration. More specifically, the water contact angles decrease with increasing the number of layers, indicating the importance of water–substrate interactions. The contact angles along both armchair and zigzag directions increase with the increasing vacancy concentration, and the anisotropy disappears when the defect concentration is high. For an in-plane pristine-defective phosphorene heterostructure, when the junction is zigzag-oriented, a spontaneous diffusion of water droplets from the defective region to the pristine region occurs; when the junction is armchair-oriented, however, the spontaneous motion is suppressed. The energetic factor plays a role for the difference in the motion of water droplets along zigzag and armchair directions. Our work highlights the unique and fascinating directional wetting behavior of water droplets on phosphorene

Anisotropic Wetting Characteristics of Water Droplets on Phosphorene: Roles of Layer and Defect Engineering

We study the wetting behavior of water droplets on pristine and defective phosphorene using molecular dynamics simulations. It is found that unlike prototypical two-dimensional materials such as graphene and MoS₂, phosphorene exhibits an anisotropic contact angle along armchair and zigzag directions. This anisotropy is tunable with increasing the number of layers and vacancy concentration. More specifically, the water contact angles decrease with increasing the number of layers, indicating the importance of water–substrate interactions. The contact angles along both armchair and zigzag directions increase with the increasing vacancy concentration, and the anisotropy disappears when the defect concentration is high. For an in-plane pristine-defective phosphorene heterostructure, when the junction is zigzag-oriented, a spontaneous diffusion of water droplets from the defective region to the pristine region occurs; when the junction is armchair-oriented, however, the spontaneous motion is suppressed. The energetic factor plays a role for the difference in the motion of water droplets along zigzag and armchair directions. Our work highlights the unique and fascinating directional wetting behavior of water droplets on phosphorene