Surface properties of 1T-TaS2 and contrasting its electron-phonon coupling with TlBiTe2 from helium atom scattering

We present a detailed helium atom scattering study of the charge-density wave (CDW) system and transition metal dichalcogenide 1T-TaS2. In terms of energy dissipation, we determine the electron-phonon (e-ph) coupling, a quantity that is at the heart of conventional superconductivity and may even "drive" phase transitions such as CDWs. The e-ph coupling of TaS2 in the commensurate CDW phase (λ = 0.59 ± 0.12) is compared with measurements of the topo-logical insulator TlBiTe2 (λ = 0.09 ± 0.01). Furthermore, by means of elastic He diffraction and resonance/interference effects in He scattering, the thermal expansion of the surface lattice, the surface step height, and the three-dimensional atom-surface interaction potential are determined including the electronic corrugation of 1T-TaS2. The linear thermal expansion coefficient is similar to that of other transition-metal dichalcogenides. The He-TaS2 interaction is best described by a corrugated Morse potential with a relatively large well depth and supports a large number of bound states, comparable to the surface of Bi2Se3, and the surface electronic corrugation of 1T-TaS2 is similar to the ones found for semimetal surfaces.

High-Fidelity Transfer of Chemical Vapor Deposition Grown 2D Transition Metal Dichalcogenides via Substrate Decoupling and Polymer/Small Molecule Composite

Current polymeric transfer methods of 2D materials often bring about the presence of wrinkles, cracks, and polymer residue, limiting the quality of the transferred materials and performance of devices. Herein, we report a transfer approach combining pretreatment by liquid nitrogen and lithium ion intercalation with polymer composite of small molecules and polystyrene to achieve high-fidelity transfer of 2D transition metal dichalcogenides (TMDs) grown by chemical vapor deposition. In this method, the as-grown samples were pretreated by liquid nitrogen and lithium ion intercalation to weaken the bonding between the TMD and the substrate. A polymer composite incorporating small molecules, namely camphor or naphthalene, was used to increase the dissolution of the polymer film. These two processes work synergistically to enable nearly 100% transfer of monolayer TMDs virtually free of wrinkles, cracks, or organic residue with retained optical properties. Our technique can be generalized for the efficient and high quality transfer of other 2D materials.

Interface-Driven Partial Dislocation Formation in 2D Heterostructures

Van der Waals (vdW) epitaxy allows the fabrication of various heterostructures with dramatically released lattice matching conditions. This study demonstrates interface-driven stacking boundaries in WS₂ using epitaxially grown tungsten disulfide (WS₂ ) on wrinkled graphene. Graphene wrinkles function as highly reactive nucleation sites on WS₂ epilayers; however, they impede lateral growth and induce additional stress in the epilayer due to anisotropic friction. Moreover, partial dislocation-driven in-plane strain facilitates out-of-plane buckling with a height of 1 nm to release in-plane strain. Remarkably, in-plane strain relaxation at partial dislocations restores the bandgap to that of monolayer WS₂ due to reduced interlayer interaction. These findings clarify significant substrate morphology effects even in vdW epitaxy and are potentially useful for various applications involving modifying optical and electronic properties by manipulating extended 1D defects via substrate morphology control.

Raman scattering studies on very thin layers of gallium sulfide (GaS) as a function of sample thickness and temperature

Gallium sulfide is a semiconducting material with a layered structure and a characteristic low interlayer interaction. Because of weak van der Waals forces, GaS crystals are relatively easy to exfoliate to very thin layers. In this work nanometric-GaS layers were obtained by a micro-mechanical exfoliation process and were transferred to Si/SiO₂ substrate. The thickness of these layers was estimated from AFM measurements. Raman spectra were collected for different layer thicknesses ranging from one layer to bulk crystal. An analytical function fitted to experimental data is proposed to determine layer thickness from Raman measurements. For the first time, the Raman position and the FWHM of the main Raman peaks were measured on very thin GaS layers as a function of temperature in the range from 80 to 470 K. The first order temperature coefficients of the A _1g Raman peaks were determined. Phonon decay due to anharmonic processes at temperatures above 300 K in layers of thickness below 4 nm was observed. Contribution of optical phonon scattering processes to thermal properties of very thin GaS layers is discussed.

Oxidation of 2D-WS2 nanosheets for generation of 2D-WS2/WO3 heterostructure and 2D and nanospherical WO3

Evolution of 2D-WS₂/WO₃ heterostructures as well as 2D and nanospherical WO₃ during the oxidation of WS₂ nanosheets in air.

Surface enhanced resonant Raman scattering in hybrid MoSe2@Au nanostructures

We report on the surface enhanced resonant Raman scattering (SERRS) in hybrid MoSe₂@Au plasmonic-excitonic nanostructures, focusing on the situation where the localized surface plasmon resonance of Au nanodisks is finely tuned to the exciton absorption of monolayer MoSe₂. Using a resonant excitation, we investigate the SERRS in MoSe₂@Au and the resonant Raman scattering (RRS) in a MoSe₂@SiO₂ reference. Both optical responses are compared to the non-resonant Raman scattering signal, thus providing an estimate of the relative contributions from the localized surface plasmons and the confined excitons to the Raman scattering enhancement. We determine a SERRS/RRS enhancement factor exceeding one order of magnitude. Furthermore, using numerical simulations, we explore the optical near-field properties of the hybrid MoSe₂@Au nanostructure and investigate the SERRS efficiency dependence on the nanodisk surface morphology and on the excitation wavelength. We demonstrate that a photothermal effect, due to the resonant plasmonic pumping of electron-hole pairs into the MoSe₂ layer, and the surface roughness of the metallic nanostructures are the main limiting factors of the SERRS efficiency.

Temperature induced crossing in the optical bandgap of mono and bilayer MoS2 on SiO2

Photoluminescence measurements in mono- and bilayer-MoS₂ on SiO₂ were undertaken to determine the thermal effect of the MoS₂/SiO₂ interface on the optical bandgap. The energy and intensity of the photoluminescence from monolayer MoS₂ were lower and weaker than those from bilayer MoS₂ at low temperatures, whilst the opposite was true at high temperatures above 200 K. Density functional theory calculations suggest that the observed optical bandgap crossover is caused by a weaker substrate coupling to the bilayer than to the monolayer.

Edge Delamination of Monolayer Transition Metal Dichalcogenides

Delamination of thin films from the supportive substrates is a critical issue within the thin film industry. The emergent two-dimensional, atomic layered materials, including transition metal dichalcogenides, are highly flexible; thus buckles and wrinkles can be easily generated and play vital roles in the corresponding physical properties. Here we introduce one kind of patterned buckling behavior caused by the delamination from a substrate initiated at the edges of the chemical vapor deposition synthesized monolayer transition metal dichalcogenides, led by thermal expansion mismatch. The atomic force microscopy and optical characterizations clearly showed the puckered structures associated with the strain, whereas the transmission electron microscopy revealed the special sawtooth-shaped edges, which break the geometrical symmetry for the buckling behavior of hexagonal samples. The condition of the edge delamination is in accordance with the fracture behavior of thin film interfaces. This edge delamination and buckling process is universal for most ultrathin two-dimensional materials, which requires more attention in various future applications.

A sensitive, uniform, reproducible and stable SERS substrate has been presented based on MoS2@Ag nanoparticles@pyramidal silicon

By combine the Ag nanoparticles, pyramidal silicon and molybdenum disulfide, the MoS₂@AgNPs@PSi substrate shows high performance in terms of sensitivity, uniformity, reproducibility and stability.