Excitation-wavelength dependent upconverting surfactant free MoS2 nanoflakes grown by hydrothermal method

Two-dimensional (2D) materials such as graphene and layered transition metal dichalcogenides (LTMDs) have attracted a great deal of attention by the scientific community because of its exceptional electronic, mechanical and optoelectronic properties. Unlike graphene, monolayer MoS2 is a non-centrosymmetric material featured with a direct band gap energy of 1.9 eV that identifies the presence of strong luminescence. Here we report the growth of quantum confined MoS2 nanoflakes with desired optical properties such as pronounced excitation wavelength dependent luminescence at room temperature. The emission luminescence was observed to shift from 560 to 704 nm with increasing excitation wavelength. In addition to the down-conversion photoluminescence, interestingly, MoS2 nanoflakes also exhibited unusual upconversion photoluminescence. These enhanced optical properties of MoS2 provide various exciting technological applications and thus opening up new possibilities in the field of bio-imaging, optical displays, photovoltaic light harvesting cells and other optical devices.