Formation and Interlayer Decoupling of Colloidal MoSe₂ Nanoflowers

We report the colloidal synthesis of substrate-free MoSe₂ nanostructures with a uniform flower-like morphology and tunable average diameters that range from approximately 50–250 nm. The MoSe₂ nanoflowers contain a large population of highly crystalline few-layer nanosheets that protrude from a central core. Aliquot studies and control experiments indicate that the nanoflowers are generated through a two-step process that involves the formation of a core in the early stages of the reaction followed by outward nanosheet growth that can be controlled based on the concentrations of reagents. The effects of laser-induced local heating, bulk-scale heating using a temperature stage, and nanostructuring on the ability to trigger and tune interlayer decoupling were also investigated. Notably, laser-induced local heating results in dynamic and reversible interlayer decoupling. Such capabilities provide a pathway for achieving quasi-two-dimensional behavior in three-dimensionally structured and colloidally synthesized transition metal dichalcogenide nanostructures

Fast and High-Performance Self-Powered Photodetector Based on the ZnO/Metal–Organic Framework Heterojunction

Electrical conductive metal–organic frameworks (EC-MOFs) are emerging as an appealing class of highly tailorable electrically conducting materials with potential applications in optoelectronics. Here, we in situ grew nickel hexahydroxytriphenylene (Ni-CAT) on the surface of ZnO nanorods (NRs). The self-powered photodetectors (PDs) were fabricated with heterojunctions formed at the interface of ZnO NRs and Ni-CAT. With this, the built-in electric field (BEF) can effectively separate the photogenerated electron–hole pairs and enhance the photoresponse. We observe that the PDs based on hybrid ZnO/Ni-CAT with 3 h of growth time (ZnO/Ni-CAT-3) show good photoresponse (137 μA/W) with the fast rise (3 ms) and decay time (50 ms) under 450 nm light illumination without biased voltage. This work provides a facile and controllable method for the growth of the ZnO/Ni-CAT heterojunction with an effective BEF zone, which will benefit their optoelectronic applications

Effects of Stigmasterol and β‑Sitosterol on Nonalcoholic Fatty Liver Disease in a Mouse Model: A Lipidomic Analysis

To study the effects of stigmasterol and β-sitosterol on high-fat Western diet (HFWD)-induced nonalcoholic fatty liver disease (NAFLD), lipidomic analyses were conducted in liver samples collected after 33 weeks of the treatment. Principal component analysis showed these phytosterols were effective in protecting against HFWD-induced NAFLD. Orthogonal projections to latent structures–discriminate analysis (OPLS-DA) and S-plots showed that triacylglycerols (TGs), phosphatidylcholines, cholesteryl esters, diacylglycerols, and free fatty acids (FFAs) were the major lipid species contributing to these discriminations. The alleviation of NAFLD is mainly associated with decreases in hepatic cholesterol, TGs with polyunsaturated fatty acids, and alterations of free hepatic FFA. In conclusion, phytosterols, at a dose comparable to that suggested for humans by the FDA for the reduction of plasma cholesterol levels, are shown to protect against NAFLD in this long-term (33-week) study

Field-Effect Transistors Based on Few-Layered α‑MoTe₂

Here we report the properties of field-effect transistors based on a few layers of chemical vapor transport grown α-MoTe₂ crystals mechanically exfoliated onto SiO₂. We performed field-effect and Hall mobility measurements, as well as Raman scattering and transmission electron microscopy. In contrast to both MoS₂ and MoSe₂, our MoTe₂ field-effect transistors are observed to be hole-doped, displaying on/off ratios surpassing 10⁶ and typical subthreshold swings of ∼140 mV per decade. Both field-effect and Hall mobilities indicate maximum values approaching or surpassing 10 cm²/(V s), which are comparable to figures previously reported for single or bilayered MoS₂ and/or for MoSe₂ exfoliated onto SiO₂ at room temperature and without the use of dielectric engineering. Raman scattering reveals sharp modes in agreement with previous reports, whose frequencies are found to display little or no dependence on the number of layers. Given that MoS₂ is electron-doped, the stacking of MoTe₂ onto MoS₂ could produce ambipolar field-effect transistors and a gap modulation. Although the overall electronic performance of MoTe₂ is comparable to those of MoS₂ and MoSe₂, the heavier element Te leads to a stronger spin–orbit coupling and possibly to concomitantly longer decoherence times for exciton valley and spin indexes

Excited Excitonic States in 1L, 2L, 3L, and Bulk WSe₂ Observed by Resonant Raman Spectroscopy

Resonant Raman spectroscopy (RRS) is a very useful tool to study physical properties of materials since it provides information about excitons and their coupling with phonons. We present in this work a RRS study of samples of WSe₂ with one, two, and three layers (1L, 2L, and 3L), as well as bulk 2H-WSe₂, using up to 20 different laser lines covering the visible range. The first- and second-order Raman features exhibit different resonant behavior, in agreement with the double (and triple) resonance mechanism(s). From the laser energy dependence of the Raman intensities (Raman excitation profile, or REP), we obtained the energies of the excited excitonic states and their dependence with the number of atomic layers. Our results show that Raman enhancement is much stronger for the excited A′ and B′ states, and this result is ascribed to the different exciton–phonon coupling with fundamental and excited excitonic states

Tellurium-Assisted Low-Temperature Synthesis of MoS₂ and WS₂ Monolayers

Chemical vapor deposition (CVD) is a scalable method able to synthesize MoS₂ and WS₂ monolayers. In this work, we reduced the synthesis temperature by 200 °C only by introducing tellurium (Te) into the CVD process. The as-synthesized MoS₂ and WS₂ monolayers show high phase purity and crystallinity. The optical and electrical performance of these materials is comparable to those synthesized at higher temperatures. We believe this work will accelerate the industrial synthesis of these semiconducting monolayers

Electric-Field-Assisted Directed Assembly of Transition Metal Dichalcogenide Monolayer Sheets

Directed assembly of two-dimensional (2D) layered materials, such as transition metal dichalcogenides, holds great promise for large-scale electronic and optoelectronic applications. Here, we demonstrate controlled placement of solution-suspended monolayer tungsten disulfide (WS₂) sheets on a substrate using electric-field-assisted assembly. Micrometer-sized triangular WS₂ monolayers are selectively positioned on a lithographically defined interdigitated guiding electrode structure using the dielectrophoretic force induced on the sheets in a nonuniform field. Triangular sheets with sizes comparable to the interelectrode gap assemble with an observed preferential orientation where one side of the triangle spans across the electrode gap. This orientation of the sheets relative to the guiding electrode is confirmed to be the lowest energy configuration using semianalytical calculations. Nearly all sheets assemble without observable physical deformation, and postassembly photoluminescence and Raman spectroscopy characterization of the monolayers reveal that they retain their as-grown crystalline quality. These results show that the field-assisted assembly process may be used for large-area bottom-up integration of 2D monolayer materials for nanodevice applications

Metal to Insulator Quantum-Phase Transition in Few-Layered ReS₂

In ReS₂, a layer-independent direct band gap of 1.5 eV implies a potential for its use in optoelectronic applications. ReS₂ crystallizes in the 1T′-structure, which leads to anisotropic physical properties and whose concomitant electronic structure might host a nontrivial topology. Here, we report an overall evaluation of the anisotropic Raman response and the transport properties of few-layered ReS₂ field-effect transistors. We find that ReS₂ exfoliated on SiO₂ behaves as an <i>n</i>-type semiconductor with an intrinsic carrier mobility surpassing μ_i ∼ 30 cm²/(V s) at <i>T</i> = 300 K, which increases up to ∼350 cm²/(V s) at 2 K. Semiconducting behavior is observed at low electron densities <i>n</i>, but at high values of <i>n</i> the resistivity decreases by a factor of >7 upon cooling to 2 K and displays a metallic <i>T</i>²-dependence. This suggests that the band structure of 1T′-ReS₂ is quite susceptible to an electric field applied perpendicularly to the layers. The electric-field induced metallic state observed in transition metal dichalcogenides was recently claimed to result from a percolation type of transition. Instead, through a scaling analysis of the conductivity as a function of <i>T</i> and <i>n</i>, we find that the metallic state of ReS₂ results from a second-order metal-to-insulator transition driven by electronic correlations. This gate-induced metallic state offers an alternative to phase engineering for producing ohmic contacts and metallic interconnects in devices based on transition metal dichalcogenides

Manganese Doping of Monolayer MoS₂: The Substrate Is Critical

Substitutional doping of transition metal dichalcogenides (TMDs) may provide routes to achieving tunable p–n junctions, bandgaps, chemical sensitivity, and magnetism in these materials. In this study, we demonstrate in situ doping of monolayer molybdenum disulfide (MoS₂) with manganese (Mn) via vapor phase deposition techniques. Successful incorporation of Mn in MoS₂ leads to modifications of the band structure as evidenced by photoluminescence and X-ray photoelectron spectroscopy, but this is heavily dependent on the choice of substrate. We show that inert substrates (i.e., graphene) permit the incorporation of several percent Mn in MoS₂, while substrates with reactive surface terminations (i.e., SiO₂ and sapphire) preclude Mn incorporation and merely lead to defective MoS₂. The results presented here demonstrate that tailoring the substrate surface could be the most significant factor in substitutional doping of TMDs with non-TMD elements