Spectroscopic analysis demonstrates strong photoluminescence in monolayer transition metal dichalcogenides, indicating a shift from indirect to direct band gaps in 2D forms.
We mechanically exfoliate mono- and few-layers of the transition metal dichalcogenides molybdenum disulfide, molybdenum diselenide, and tungsten diselenide. The exact number of layers is unambiguously determined by atomic force microscopy and high-resolution Raman spectroscopy. Strong photoluminescence emission is caused by the transition from an indirect band gap semiconductor of bulk material to a direct band gap semiconductor in atomically thin form.
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Tonndorf et al. (2013) studied this question.
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