Atomic-layer triangular WSe_2 sheets: Synthesis and layer-dependent photoluminescence property

摘要

The exotic band structures and distinctive physical properties of two-dimensional materials have exhibited great potential for fundamental research and technical applications in spintronics, electronics, photonics, optoelectronics, and so forth. Facing the challenge of effective synthesis of WSe_2 two-dimensional sheets, for the first time we demonstrate a straightforward catalyst-free vapor-solid (VS) growth method to synthesize ultrathin, even monolayer, WSe_2 sheets with high yield, regular shapes and high quality optical properties on sapphire substrates. By detailed layer-number-dependent photoluminescence measurements at a low temperature of 40 K, we find the spin-orbit splitting at the K point of the WSe_2 valence band with a fixed energy difference of 0.36 eV independent of layer number and the transition of indirect-to-direct gap when the thickness decreases to monolayer. These results, comparable to those of mechanically peeled WSe_2 sheets, further prove the high optical and crystal quality of our WSe_2 nanosheets via the VS growth approach. Our efforts may open up new exciting opportunities in future valley-based electronics, optoelectronics and photonics.