On the chemically-assisted excitonic enhancement in environmentally-friendly solution dispersions of two-dimensional MoS2 and WS2

Current rectifying p-n junction devices are demonstrated from solution exfoliated two-dimensional (2D) molybdenum disulfide (MoS2), and excitonic effects are elucidated for solution dispersions of not only MoS2, but also another refractory 2D sulfide, tungsten disulfide (WS2). The excitonic enhancement effects are correlated to new solution chemistries using environmentally friendly terpineol. The role of sonication time and centrifugation are analyzed in the presence of terpineol with isopropyl alcohol and surfactant ethyl cellulose, where the red-shift in the excitonic peaks is correlated to the size distribution of the nanoparticles using optical interferometry. This was correlated to the data obtained using photoluminescence, Raman spectroscopy, scanning electron microscopy and particle size analysis which also yielded results that were consistent with this finding. The terpineol dispersion exhibits the least red-shift of 5 meV from the top to the bottom of the vial, in contrast to non-terpineol dispersions where the red-shift is calculated to be as high as 90 meV, indicating terpineol's effectiveness in exfoliating a larger population of mono-to few-layer nanomembranes. Analysis of the optical absorption spectra allows for the extraction of the energy band gap for MoS2 and WS2. These results clearly show evidence of quantum confinement effects in solution dispersions of chemically exfoliated 2D MoS2 and WS2 which can be harnessed for a wide variety of optoelectronic devices that are amenable to scalable and high-throughput synthesis routes, using environmentally friendly solution chemistries.