Atomic-layer triangular WSe2 sheets: synthesis and layer-dependent photoluminescence property.

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 WSe2 two-dimensional sheets, for the first time we demonstrate a straightforward catalyst-free vapor-solid (VS) growth method to synthesize ultrathin, even monolayer, WSe2 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 WSe2 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 WSe2 sheets, further prove the high optical and crystal quality of our WSe2 nanosheets via the VS growth approach. Our efforts may open up new exciting opportunities in future valley-based electronics, optoelectronics and photonics.