Abstract
Optical properties of aged WS2 monolayers grown by CVD method on Si/SiO2 substrates are studied using temperature dependent photoluminescence and reflectance contrast spectroscopy. Aged WS2 monolayers have a typical surface roughness about 0.5 nm and, in addition, a high density of nanoparticles (nanocaps) with the base diameter about 30 nm and average height of 7 nm. The A-exciton of aged monolayer has a peak position at 1.951 eV while in as-grown monolayer the peak is at about 24 meV higher energy at room temperature. This red-shift is explained using local tensile strain concept, where strain value of 2.1% was calculated for these nanocap regions. Strained nanocaps have lower band gap energy and excitons will funnel into these regions. At T=10K a double exciton and trion peaks were revealed. The separation between double peaks is about 20 meV and the origin of higher energy peaks is related to the optical band gap energy fluctuations caused by random distribution of local tensile strain due to increased surface roughness. In addition, a wide defect related exciton band XD was found at about 1.93 eV in all aged monolayers. It is shown that the theory of localized excitons describes well the temperature dependence of peak position and halfwidth of the A-exciton band. The possible origin of nanocaps is also discussed.
Highlights
Layered semiconductor compounds involving transition metals from group VI and chalcogens (TMDs) are promising candidates for studying atomically thin structures and have attracted considerable attention because of their unique properties and their potential applications in various opto-electronic devices.[1]
We show that aged WS2 monolayers exhibit different local strains due to surface topography leading to excitons localization
The thickness of as-grown WS2 monolayers was measured with atomic force microscope (AFM) and was about 0.8 nm and the sample surface was quite smooth.[12]
Summary
Layered semiconductor compounds involving transition metals from group VI and chalcogens (TMDs) are promising candidates for studying atomically thin structures and have attracted considerable attention because of their unique properties and their potential applications in various opto-electronic devices.[1]. Even in WS2 monolayers prepared by mechanical exfoliation there is a remarkable inhomogeneous strain across the monolayer’s area and spatial non-uniformity of the electron density across the monolayer surface due to oxygen chemisorption.[23] Besides a strain many structural defects exist in CVD-grown WS2 on Si/SiO2 These defects introduce localized states in the bandgap, leading a dramatic decrease in the carrier mobility. In our recent paper[29] optical properties of aged MoSe2 monolayers were studied These monolayers showed quite high surface roughness leading to a random distribution of local tensile strain and, spatial optical band gap energy fluctuations.
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