Defect-induced photoluminescence of tungsten disulfide monolayers

Aswin Asaithambi, Universität Duisburg-Essen, Duisburg, Germany
Roland Kozubek, Universität Duisburg-Essen, Duisburg, Germany
Guenther Prinz, Universität Duisburg-Essen, Duisburg, Germany
Francesco Reale, Imperial College London, London, UK
Cecilia Mattevi, Imperial College London, London, UK
Marika Schleberger, Universität Duisburg-Essen, Duisburg, Germany
Axel Lorke, Universität Duisburg-Essen, Duisburg, Germany

The intriguing electronic properties of graphene have triggered the interest in other 2D materials, such as transition metal dichalcogenides (TMDCs) monolayers. TMDCs, similar to graphite, have a layered structure with an indirect band gap, which becomes direct at the K point in momentum space if only a monolayer is present. This leads to strongly enhanced photoluminescence (PL), compared to the bulk. However, these monolayers are never defect free and the defects present in the material affect their opto-electronic properties drastically, which makes it necessary to study and characterize their optical properties. In this contribution, we present highly sensitive, non-destructive, temperature- and power- dependent PL measurements to study the defects present in tungsten disulfide (WS2) monolayers. For this purpose, monolayers of WS2 were irradiated with different fluences of Xe30+ ions to create defects with controllable density. Low temperature spectra of pristine (unirradiated) samples show a single peak, associated with neutral exciton recombination, and no signature of defect-bound exciton recombination. For higher temperature or power, again, only the neutral exciton recombination is observed in the spectrum. For the different defect densities created in the monolayers, we observed changes in the PL spectrum concerning PL intensity and FWHM at room temperature. Low temperature spectra of irradiated samples show two to three different peaks depending on the fluence of Xe30+ ions. These peaks are associated with neutral and defect-bound exciton recombination. Interestingly, in the excitation power-dependent studies, where the sample was gradually subjected to higher laser powers from 0.5µW until 1500µW and gradually back to 0.5µW, every sample displays laser-induced changes of the PL emission properties (“laser annealing”). Defect-bound emissions exhibit a gradual shift to higher energies when first increasing the laser power. Furthermore, the samples only show the neutral exciton emission after laser annealing. These results will be discussed and compared to data published in the literature. Our study is expected to contribute to optical defect engineering in WS2 and related materials.

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