Thin Layered Transition Metal Dichalcogenides Research Articles

PVD growth of spiral pyramid-shaped WS2 on SiO2/Si driven by screw dislocations.

Atomically thin layered transition metal dichalcogenides (TMDs), such as MoS2 and WS2, have been getting much attention recently due to their interesting electronic and optoelectronic properties. Especially, spiral TMDs provide a variety of candidates for examining the light-matter interaction resulting from the broken inversion symmetry, as well as the potential new utilization in functional optoelectronic, electromagnetic and nanoelectronics devices. To realize their potential device applications, it is desirable to achieve controlled growth of these layered nanomaterials with a tunable stacking. Here, we demonstrate the Physical Vapor Deposition (PVD) growth of spiral pyramid-shaped WS2 with ∼200 in size and the interesting optical properties via AFM and Raman spectroscopy. By controlling the precursors concentration and changing the initial nucleation rates in PVD growth, WS2 in different nanoarchitectures can be obtained. We discuss the growth mechanism for these spiral-patterned WS2 nanostructures based on the screw dislocations. This study provides a simple, scalable approach of screw dislocation-driven (SDD) growth of distinct TMD nanostructures with varying morphologies, and stacking.

Quantitatively Deciphering Electronic Properties of Defects at Atomically Thin Transition-Metal Dichalcogenides.

Defects can locally tailor the electronic properties of 2D materials, including the band gap and electron density, and possess the merit for optical and electronic applications. However, it is still a great challenge to realize rational defect engineering, which requires quantitative study of the effect of defects on electronic properties under ambient conditions. In this work, we employed tip-enhanced photoluminescence (TEPL) spectroscopy to obtain the PL spectra of different defects (wrinkle and edge) in mechanically exfoliated thin-layer transition metal dichalcogenides (TMDCs) with nanometer spatial resolution. We quantitatively obtained the band gap and electron density at defects by analyzing the wavelength and intensity ratio of excitons and trions. We further visualized the strain distribution across a wrinkle and the edge-induced reconstructive regions of the band gap and electron density by TEPL line scans. The doping effect on the Fermi level and optical performance was unveiled through comparative studies of edges on TMDC monolayers of different doping types. These quantitative results are vital to guide defect engineering and design and fabrication of TMDC-based optoelectronics devices.

High-Temperature-Induced Intervalley Carrier Transfer in Two-Dimensional Semiconductors: WSe2 versus WS2

Thin layer transition metal dichalcogenides (TMDs) have shown great potential in the field of electronics and optoelectronics devices due to their unique electronic and optical properties. Multilayer WSe2 is an indirect bandgap semiconductor and generally optically inactive. To improve the optical properties of multilayer TMDs, heating as a simple and effective method was widely chosen. Herein, we analyze high-temperature photoluminescence (PL) enhancement results on excitons in three-layer and four-layer WS2 and WSe2 with a theoretical analysis of their spectral behavior. Both direct and indirect exciton emissions in WSe2 are relatively enhanced, which is different from the high-temperature exciton emission behavior of WS2 in the same layer. The PL enhancement in WSe2 could be attributed to the transfer of thermally activated electrons from the Λ valley to K valley involving K → K and K → Γ transition at high temperature, where the conduction band extreme is located at the Λ valley. We quantitatively describe this enhancement phenomenon and demonstrate that the observed spectral behavior reflects the competition between intervalley carrier transfer and high-temperature PL quenching. An excellent agreement between calculated and measured intensities is obtained. This research provides a deeper understanding of the thermally induced intervalley carrier transfer model in multilayer TMDs.

TFT Channel Materials for Display Applications: From Amorphous Silicon to Transition Metal Dichalcogenides.

As the need for super-high-resolution displays with various form factors has increased, it has become necessary to produce high-performance thin-film transistors (TFTs) that enable faster switching and higher current driving of each pixel in the display. Over the past few decades, hydrogenated amorphous silicon (a-Si:H) has been widely utilized as a TFT channel material. More recently, to meet the requirement of new types of displays such as organic light-emitting diode displays, and also to overcome the performance and reliability issues of a-Si:H, low-temperature polycrystalline silicon and amorphous oxide semiconductors have partly replaced a-Si:H channel materials. Basic material properties and device structures of TFTs in commercial displays are explored, and then the potential of atomically thin layered transition metal dichalcogenides as next-generation channel materials is discussed.

Incoherent phonon population and exciton-exciton annihilation dynamics in monolayer WS2 revealed by time-resolved Resonance Raman scattering.

Atomically thin layer transition metal dichalcogenides have been intensively investigated for their rich optical properties and potential applications on nano-electronics. In this work, we study the incoherent phonon and exciton population dynamics in monolayer WS2 by time-resolved Resonance Raman scattering spectroscopy. Upon excitation of the exciton transition, both Stokes and anti-Stokes scattering strength of the optical and the longitudinal acoustic two phonon modes exhibit large reduction. Based on the assumption of quasi-equilibrium distribution, the hidden phonon population dynamics is retrieved, which shows an instant build-up and a relaxation lifetime of ∼4 ps at the exciton density ∼1012cm-2. A phonon temperature rises of ∼20 K was identified due to the exciton excitation and relaxation. The exciton relaxation dynamics extracted from the transient vibrational Raman response shows strong excitation density dependence, signaling an important bi-molecular contribution to the decay. These results provide significant knowledge on the thermal dynamics after optical excitation, enhance the understanding of the fundamental exciton dynamics in two-dimensional transition metal materials, and demonstrate that time-resolved Resonance Raman scattering spectroscopy is a powerful method for exploring quasi-particle dynamics in optical materials.

Membraneless reproducible MoS2 field-effect transistor biosensor for high sensitive and selective detection of FGF21

Fibroblast growth factor 21 (FGF21) serves as an essential biomarker for early detection and diagnosis of nonalcoholic fatty liver disease (NAFLD). It has received a great deal of attention recently in efforts to develop an accurate and effective method for detecting low levels of FGF21 in complex biological settings. Herein, we demonstrate a label-free, simple and high-sensitive field-effect transistor (FET) biosensor for FGF21 detection in a nonaqueous environment, directly utilizing two-dimensional molybdenum disulfide (MoS2) without additional absorption layers. By immobilizing anti-FGF21 on MoS2 surface, this biosensor can achieve the detection of trace FGF21 at less than 10 fg mL−1. High consistency and satisfactory reproducibility were demonstrated through multiple sets of parallel experiments for the MoS2 FETs. Furthermore, the biosensor has great sensitivity to detect the target FGF21 in complex serum samples, which demonstrates its great potential application in disease diagnosis of NAFLD. Overall, this study shows that thin-layered transition-metal dichalcogenides (TMDs) can be used as a potential alternative platform for developing novel electrical biosensors with high sensitivity and selectivity.

Enhanced Exfoliation of Biocompatible MoS2 Nanosheets by Wool Keratin

The unique properties of thin-layer transition metal dichalcogenides (TMDs) materials have been extensively investigated for both fundamental studies and practical applications in recent years. To ...

Sub-20 nm patterning of thin layer WSe2 by scanning probe lithography

The electronic properties of thin layer transition metal dichalcogenides have raised considerable interest in the fabrication of advanced field-effect transistors and ultrasensitive sensors. Downscaling those devices to the nanoscale depends on the development of cost-effective and robust alternative nanolithographies. Here we demonstrate the direct, resist-less and reproducible nanopatterning of tungsten diselenide thin layers. By using oxidation scanning probe lithography (o-SPL) we have generated arrays of dots with a width of 13 nm and periodicity of 40 nm. We have also patterned a point contact of 35 nm and a nanoscale field-effect transistor. The direct and resistless fabrication of WSe2 nanoscale devices by oxidation scanning probe lithography opens a straightforward and reliable method for processing transition metal dichalcogenides materials.

Photoluminescence Architectures for Disease Diagnosis: From Graphene to Thin-Layer Transition Metal Dichalcogenides and Oxides.

Ever since the discovery of graphene, increasing efforts have been devoted to the use of this stellar material as well as the development of other graphene-like materials such as thin-layer transition metal dichalcogenides and oxides (TMD/Os) for a variety of applications. Because of their large surface area and unique optical properties, these two-dimensional materials with a size ranging from the micro- to the nanoscale have been employed as the substrate to construct photoluminescence architectures for disease diagnosis as well as theranostics. These architectures are built through the simple self-assembly of labeled biomolecular probes with the substrate material, leading to signal quenching. Upon the specific interaction of the architecture with a target biomarker, the signal can be spontaneously restored in a reversible manner. Meanwhile, by co-loading therapeutic agents and employing the inherent photo-thermal properties of the material substrates, a combined disease imaging and therapy (theranostics) can be achieved. This review highlights the latest advances in the construction and application of graphene and TMD/O based thin-layer material composites for single-target and multiplexed detection of a variety of biomarkers and theranostics. These versatile material architectures, owing to their ease in preparation, low cost and flexibility in functionalization, provide promising tools for both basic biochemical research and clinical applications.

Anisotropic Electron–Phonon Coupling in Colloidal Layered TiS2 Nanodiscs Observed via Coherent Acoustic Phonons

Atomically thin layered transition metal dichalcogenides with highly anisotropic structure exhibit strong anisotropy in various material properties. Here, we report the anisotropic coupling between the interband optical transition and coherent acoustic phonon excited by ultrashort optical excitation in a colloidal solution of multilayered TiS2 nanodiscs. The transient absorption signal from the diameter- and thickness-controlled TiS2 nanodiscs dispersed in solution exhibited an oscillatory feature, which is attributed to the modulation of the interband absorption peak by the intralayer breathing mode. However, the signature of the interlayer acoustic phonon was not observed, while it has been previously observed in noncolloidal exfoliated sheets of MoS2. The dominance of the intralayer mode in modulating the interband optical transition was supported by the density functional theory (DFT) calculations of the optical absorption spectra of TiS2, which showed the stronger sensitivity of the interband absorption peak in the visible region to the in-plane strain than to the out-of-plane strain.

Nonlinear photoluminescence in atomically thin layeredWSe2arising from diffusion-assisted exciton-exciton annihilation

We studied multi-exciton dynamics in monolayer WSe2 using nonlinear photoluminescence (PL) spectroscopy and Monte Carlo simulations. We observed strong nonlinear saturation behavior of exciton PL with increasing excitation power density, and long-distance exciton diffusion reaching several micrometers. We demonstrated that the diffusion-assisted exciton-exciton annihilation model accounts for the observed nonlinear PL behavior. The long-distance exciton diffusion and subsequent efficient exciton-exciton annihilation process determined the unusual multi-exciton dynamics in atomically thin layered transition metal dichalcogenides.