Monolayer tungsten disulfide (WS) has emerged as an active material for optoelectronic devices due to its quantum yield of photoluminescence. Despite the enormous research about physical characteristics of monolayer WS, the defect-related Raman scattering has been rarely studied. Here, we report the correlation of topography and Raman scattering in monolayer WS by using tip-enhanced resonance Raman spectroscopy and reveal defect-related Raman modes denoted as D and D' modes. We found that the sulfur vacancies introduce not only the red-shifted A mode but also the D and D' modes by the density functional theory calculations. The observed defect-related Raman modes can be utilized to evaluate the quality of monolayer WS and will be helpful to improve the performance of WS optoelectronic devices.
We successfully achieve the tip-enhanced nano Raman scattering images of a tungsten disulfide monolayer with optimizing a fabrication method of gold nanotip by controlling the concentration of etchant in an electrochemical etching process. By applying a square-wave voltage supplied from an arbitrary waveform generator to a gold wire, which is immersed in a hydrochloric acid solution diluted with ethanol at various ratios, we find that both the conical angle and radius of curvature of the tip apex can be varied by changing the ratio of hydrochloric acid and ethanol. We also suggest a model to explain the origin of these variations in the tip shape. From the systematic study, we find an optimal condition for achieving the yield of ~60% with the radius of ~34 nm and the cone angle of ~35°. Using representative tips fabricated under the optimal etching condition, we demonstrate the tip-enhanced Raman scattering experiment of tungsten disulfide monolayer grown by a chemical vapor deposition method with a spatial resolution of ~40 nm and a Raman enhancement factor of ~4,760.
Defect
investigation in two-dimensional transition-metal dichalcogenides
(TMDs) is required because structural defects significantly affect
the optical and electrical properties of TMD. Raman scattering can
be an essential tool to study the defects in TMD, but defect-related
Raman modes have been rarely studied. Here, we investigated the influence
of sulfur vacancies and oxygen substitution on the optical properties
of WS2 using the laser irradiation technique. The defect-induced
photoluminescence (PL) exhibits distinct features depending on the
type of defects, which shows different changes in the intensities
and peak positions of the excitons, biexcitons, and defect-bound excitons.
Defect-activated Raman modes revealed information about the defects
and demonstrated the origin of the alteration in PL. The defect analysis
of TMDs based on the correlation between PL and Raman scattering provides
a clear understanding of the variations in their optical properties.
We elucidated chain-extension effects of a benzodithiophene (BDT) and thienopyrroledione-based push−pull conjugated polymer (CP) on its exciton-dissociation mechanism within aggregate systems using transient absoption spectroscopy. The side-group extension CP with benzothiophene on the BDT unit induced H-type excitons with excess energy owing to decreased chain stiffness. This led to interchain polaron-pair (PP)-mediated exciton dissociation. The stiff side-group extended with thienothiophene on the BDT unit also induced H-type excitons, but the decreased energy and breadth of the density of states suppressed the interchain PP-mediated exciton dissociation. The main-chain-extension CP with two thiophenes on either side of the BDT unit has a curved structure disturbing the interchain packing. Thus, the driving force of exciton dissociation between the chains decreased, leading to intrachain PP-mediated exciton dissociation. Our findings can facilitate the development of novel CPs to further increase the efficiencies of polymer solar cells.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.