Tunable Direct Bandgap Optical Transitions in MoS₂ Nanocrystals for Photonic Devices

Highly luminescent MoS2 nanocrystals (NCs) with controlled size distribution have been achieved using a simple yet inexpensive and impurity free sono-chemical exfoliation method followed by gradient centrifugation. The size of nanocrystals could be varied within the diameter range of ∼4 to 70 nm. Typical MoS2 nanocrystal has exhibited high crystalline quality with 0.25 nm lattice fringe spacing for (002) planes for 2-H phase of MoS2. Raman spectra has revealed that both out-of-plane and in-plane vibrational modes are stiffen due to the edge effect of MoS2 NCs. The size tunable optical properties of MoS2 NCs have been investigated by optical absorption and photoluminescence spectroscopy. The coexistence of direct band gap emission from 2D MoS2 nanosheets and quantum confined nanocrystals has been achieved. A strong and tunable photoluminescence (560–518 nm) emission due to the quantum size effect of tiny NCs below a critical dimension is reported for the first time. The photocurrent measurement of the Au/MoS2–NCs/Au junction has been performed at room temperature to investigate the optical responsivity and switching characteristics, demonstrating the potential of MoS2 nanocrystals for next generation photonic devices.