Extraordinary Room-Temperature Photoluminescence in Triangular WS 2 Monolayers (original) (raw)
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Two-dimensional (2D) transition metal dichalcogenides (TMDs), especially MoS 2 and WS 2 recently attract extensive attentions due to their rich physics and great potential applications. Superior to graphene, MS 2 (M = Mo/W) monolayers have a native direct energy gap in visible frequency range. This promises great future of MS 2 for optoelectronics. To exploit properties and further develop more applications, producing large-scale single crystals of MS 2 by a facile method is highly demanded. Here, we report the synthesis of large-scale triangular single crystals of WS 2 monolayer from a chemical vapor deposition process and systematic optical studies of such WS 2 monolayers. The observations of high yield of light emission and valley-selective circular dichroism experimentally evidence the high optical quality of the WS 2 monolayers. This work paves the road to fabricate large-scale single crystalline 2D semiconductors and study their fundamentals. It must be very meaningful for exploiting great potentials of WS 2 for future optoelectronics. Being entangled in controlling the electronic properties of graphene for next-generation electronics 1-2 , monolayer transition metal dichalcogenides such as MS 2 (M = Mo, W) are intriguing great interest as two-dimensional (2D) semiconductors with a native direct energy gap in visible frequency
The Effect of Preparation Conditions on Raman and Photoluminescence of Monolayer WS2
Scientific reports, 2016
We report on preparation dependent properties observed in monolayer WS2 samples synthesized via chemical vapor deposition (CVD) on a variety of common substrates (Si/SiO2, sapphire, fused silica) as well as samples that were transferred from the growth substrate onto a new substrate. The as-grown CVD materials (as-WS2) exhibit distinctly different optical properties than transferred WS2 (x-WS2). In the case of CVD growth on Si/SiO2, following transfer to fresh Si/SiO2 there is a ~50 meV shift of the ground state exciton to higher emission energy in both photoluminescence emission and optical reflection. This shift is indicative of a reduction in tensile strain by ~0.25%. Additionally, the excitonic state in x-WS2 is easily modulated between neutral and charged exciton by exposure to moderate laser power, while such optical control is absent in as-WS2 for all growth substrates investigated. Finally, we observe dramatically different laser power-dependent behavior for as-grown and tra...
Temperature dependent Raman and photoluminescence of vertical WS2/MoS2 monolayer heterostructures
Science Bulletin, 2017
Heterostructures from two-dimensional transition-metal dichalcogenides MX 2 have emerged as a hot topic in recent years due to their various fascinating properties. Here, we investigated the temperature dependent Raman and photoluminescence (PL) spectra in vertical stacked WS 2 /MoS 2 monolayer heterostructures. Our result shows that both E 1 2g and A 1g modes of WS 2 and MoS 2 vary linearly with temperature increasing from 300 to 642 K. The PL measurement also reveals strong temperature dependencies of the PL intensity and peak position. The activation energy of the thermal quenching of the PL emission has been found to be equal to 69.6 meV. The temperature dependence of the peak energy well follows the bandgap shrinkage of bulk semiconductor.
Vertical and in-plane heterostructures from WS2/MoS2 monolayers
Nature Materials, 2014
Layer-by-layer stacking or lateral interfacing of atomic monolayers has opened up unprecedented opportunities to engineer two-dimensional heteromaterials. Fabrication of such artificial heterostructures with atomically clean and sharp interfaces, however, is challenging. Here, we report a one-step growth strategy for the creation of high-quality vertically stacked as well as in-plane interconnected heterostructures of WS 2 /MoS 2 via control of the growth temperature. Vertically stacked bilayers with WS 2 epitaxially grown on top of the MoS 2 monolayer are formed with preferred stacking order at high temperature. A strong interlayer excitonic transition is observed due to the type II band alignment and to the clean interface of these bilayers. Vapour growth at low temperature, on the other hand, leads to lateral epitaxy of WS 2 on MoS 2 edges, creating seamless and atomically sharp in-plane heterostructures that generate strong localized photoluminescence enhancement and intrinsic p-n junctions. The fabrication of heterostructures from monolayers, using simple and scalable growth, paves the way for the creation of unprecedented two-dimensional materials with exciting properties.
Local Optical Properties in CVD-Grown Monolayer WS2 Flakes
The Journal of Physical Chemistry. C, Nanomaterials and Interfaces, 2021
Excitons dominate the light absorption and re-emission spectra of monolayer transition-metal dichalcogenides (TMD). Microscopic investigations of the excitonic response in TMD almost invariably extract information from the radiative recombination step, which only constitutes one part of the picture. Here, by exploiting imaging spectroscopic ellipsometry (ISE), we investigate the spatial dependence of the dielectric function of chemical vapor deposition (CVD)-grown WS2 flakes with a microscopic lateral resolution, thus providing information about the spatially varying, exciton-induced light absorption in the monolayer WS2. Comparing the ISE results with imaging photoluminescence spectroscopy data, the presence of several correlated features was observed, along with the unexpected existence of a few uncorrelated characteristics. The latter demonstrates that the exciton-induced absorption and emission features are not always proportional at the microscopic scale. Microstructural modula...
Investigation of spatially localized defects in synthetic WS2 monolayers
Physical Review B
While the spatially nonhomogeneous light emission from synthetic WS 2 monolayers is frequently reported in the literature, the nature of this phenomenon still requires thoughtful investigation. Here, we combine several characterization techniques (optical imaging, scanning probe and electron microscopy) along with density functional theory to investigate the presence of substitutional doping localized at narrow regions along the S zigzag edge of WS 2 monolayers. We verified that photoluminescence quenching along narrow regions is not related to grain boundaries but to substitutional impurities of lighter metals at the W sites, which modify the radiative and nonradiative decay channels. We also found potential candidates for occupying the W site through ADF-STEM analysis and discussed their impact on photoluminescence quenching by performing density functional theory calculations. Our findings shed light on how atomic defects introduced during WS 2 monolayer's synthesis impact the crystalline quality and, therefore, the development of high-performance optoelectronic devices based on semiconducting 2D materials.