Preparation and Applications of Mechanically Exfoliated Single-Layer and Multilayer MoS₂and WSe₂Nanosheets

Abstract

Although great progress has been achieved in the study of graphene, the small current ON/OFF ratio in graphene-based field-effect transistors (FETs) limits its application in the fields of conventional transistors or logic circuits for low-power electronic switching. Recently, layered transition metal dichalcogenide (TMD) materials, especially MoS2, have attracted increasing attention. In contrast to its bulk material with an indirect band gap, a single-layer (1L) MoS2 nanosheet is a semiconductor with a direct band gap of ~1.8 eV, which makes it a promising candidate for optoelectronic applications due to the enhancement of photoluminescence and high current ON/OFF ratio. Compared with TMD nanosheets prepared by chemical vapor deposition and liquid exfoliation, mechanically exfoliated ones possess pristine, clean, and high-quality structures, which are suitable for the fundamental study and potential applications based on their intrinsic thickness-dependent properties. In this Account, we summarize our recent research on the preparation, characterization, and applications of 1L and multilayer MoS2 and WSe2 nanosheets produced by mechanical exfoliation. During the preparation of nanosheets, we proposed a simple optical identification method to distinguish 1L and multilayer MoS2 and WSe2 nanosheets on a Si substrate coated with 90 and 300 nm SiO2. In addition, we used Raman spectroscopy to characterize mechanically exfoliated 1L and multilayer WSe2 nanosheets. For the first time, a new Raman peak at 308 cm(-1) was observed in the spectra of WSe2 nanosheets except for the 1L WSe2 nanosheet. Importantly, we found that the 1L WSe2 nanosheet is very sensitive to the laser power during characterization. The high power laser-induced local oxidation of WSe2 nanosheets and single crystals was monitored by Raman spectroscopy and atomic force microscopy (AFM). Hexagonal and monoclinic structured WO3 thin films were obtained from the local oxidization of single- to triple-layer (1L-3L) and quadruple- to quintuple-layer (4L-5L) WSe2 nanosheets, respectively. Then, we present Raman characterization of shear and breathing modes of 1L and multilayer MoS2 and WSe2 nanosheets in the low frequency range (<50 cm(-1)), which can be used to accurately identify the layer number of nanosheets. Magnetic force microscopy was used to characterize 1L and multilayer MoS2 nanosheets, and thickness-dependent magnetic response was found. In the last part, we briefly introduce the applications of 1L and multilayer MoS2 nanosheets in the fields of gas sensors and phototransistors.

Unconventional solution-phase epitaxial growth of organic-inorganic hybrid perovskite nanocrystals on metal sulfide nanosheets

“…Based on TEM and atomic force microscopy (AFM) measurements (Fig. S1), the as-prepared MoS 2 nanosheets were 200-500 nm in lateral size and 1.3-3.0 nm in thickness, suggesting that they are single-to few-layer thick [41]. The subsequent preparation of MAPbBr 3 /MoS 2 heterostructures is schematically illustrated in Fig.…”

Recent Advances on Anilato-Based Molecular Materials with Magnetic and/or Conducting Properties

“…The weak natures of the intermolecular interactions between the magnetic neutral layers play a crucial role for the exfoliation of the layers. In fact this new magnetic network is very peculiar since it can be easily exfoliated by using the socalled Scotch tape method which is a micromechanical method, capable to produce in a very efficient way, highly crystalline thin microsheets of a layered material [173][174][175]. To the best of our knowledge this method has never been applied to such layered materials.…”

Two-dimensional inorganic analogues of graphene: transition metal dichalcogenides

“…Transistors made of mechanically exfoliated MoS 2 nanosheets with one-to four-layer thicknesses have been used to detect up to 2 ppm levels of NO with 80% decrease in the channel conductance [105]. He et al [106] have fabricated flexible transistors with MoS 2 as the thin-film channel and reduced graphene oxide (RGO) as the electrode material for sensing NO 2 .…”