Excitation-dependent photoluminescence from WS2 nanostructures synthesized via top-down approach

Crystalline and hetero-dimensional nanostructures of WS2 quantum dots and a few layered sheets have been successfully synthesized in a single-step process using liquid exfoliation in deionized water. The X-rays diffraction analysis confirmed the presence of hexagonal as well as tetragonal phase of WS2 in quantum dots. The nanostructures so obtained exhibit strong photoluminescence, which is otherwise difficult to detect (due to low quantum yield) when WS2 is in bulk form. The UV-Vis spectra from these nanostructures display strong excitonic absorptions together with band edge absorption at approximate to 5.1 eV, sufficiently larger than the indirect band gap (approximate to 1.3 eV) of bulk WS2. The average statistical size distribution of quantum dots was found to be 3-4.5 nm and corresponds to a band gap of 6 and 3.4 eV, respectively, under effective mass approximation. Further, the photoluminescence spectra shifts with the excitation wavelength, a signature of size-dependent photoluminescence emission from these nanostructures. These results suggest that quantum confinement effects together with band gap transformation from indirect to direct, not only shift the band edge in the ultraviolet region but also give rise to enhancement in the photoluminescence emission.