Controllable Synthesis of Band-Gap-Tunable and Monolayer Transition-Metal Dichalcogenide Alloys

Abstract

The electronic and optical properties of transition-metal dichalcogenide (TMD) materials are directly governed by their energy gap; thus, band-gap engineering has become an important topic recently. Theoretical and some experimental results have indicated that these monolayerTMD alloys exhibit direct-gap properties and remain stable at room temperature, making them attractive for optoelectronic applications. Here, we systematically compared the two approaches of forming MoS 2x Se 2(1-x) monolayer alloys: selenization of MoS 2 and sulfurization of MoSe 2 .The optical energy gap of as-grown chemical vapor deposition MoS 2 can be continuously modulated from 1.86 eV (667 nm) to 1.57 eV (790 nm) controllable by the reaction temperature. Spectroscopic and microscopic evidences show that the Mo-S bonds can be replaced by the Mo-Se bonds in a random and homogeneous manner. By contrast, the replacement of Mo-Se by Mo-S does not randomly occur in the MoSe 2 lattice, where the reaction preferentially occurs along the crystalline orientation of MoSe 2 and thus the MoSe 2 /MoS 2 biphases are easily observed in the alloys, which makes the optical band gap of these alloys distinctly different. Therefore, the selenization of metal disulfide is preferred and the proposed synthetic strategy opens up a simple route to control the atomic structure as well as optical properties of monolayer TMD alloys.

Tailoring molybdenum nanostructure evolution by low-energy He+ ion irradiation

“…For all high-resolution XPS spectra, the major fitting parameters, binding energy (BE) and full width at half maxima (FWHM) are summarized in Table 1. As expected, the in-situ XPS spectra for 3d doublets of the sputter-cleaned pristine Mo sample shows the presence of Mo3d 3/2 (228.5 eV) and Mo3d 5/2 (231.6 eV) XPS peaks [53][54][55] that confine the metallic Mo in the samples; no impurity, including that of oxidized Mo (MoO 3 ), was seen. However, ex-situ XPS for the pristine sample shows the presence of metallic Mo 3d doublet XPS peaks Mo3d 3/2 (228.6 eV) and Mo3d 5/2 (231.2 eV) XPS peaks [53][54][55], in addition to oxidized Mo 3d doublet XPS peaks MoO 3 3d 3/2 (236.5 eV) and MoO 3 3d 5/2 (233.3 eV) XPS peaks [53][54][55].…”

Spectroscopic Signatures for Interlayer Coupling in MoS2–WSe2 van der Waals Stacking

“…Based on these, we can determine the exciton binding energy of the MoS2/WSe2 inter-layer exciton, as marked by solid double arrow line with gray background. [49][50][51][52] We find that the MoS2/WSe2 inter-layer exciton has a lower exciton binding energy 0.26 eV when compared to MoS2 or WSe2. The band alignment reveals the important role of the interlayer coupling, which is crucial for band engineering.…”

Strain relaxation via formation of cracks in compositionally modulated two-dimensional semiconductor alloys

“…The ternary alloys of 2D TMDs can be synthesized using two general approaches: (i) a direct chemical vapor deposition (CVD)based technique in which the simultaneous reaction of three precursors yields ternary alloys 7-10 and (ii) an indirect two-step technique in which a MX 2 binary compound forms in the first step and later serves as a host that accommodates the atoms of the third element (M′ or X′) in the second step (alloying step). 16,18,19 Although the direct approach yields high quality 2D alloys, it is not an optimal method for realizing lateral heterostructures as it provides minimal spatial control over the chemical composition. In contrast, the indirect two-step approach can be combined with standard lithography processes to deterministically form lateral heterostructures with intended dimensions in predefined locations.…”