 transition 
 metal dichacogenides represent a unique class of two - dimensional layered 
 materials that can be exfoliated into single or few atomic layers . 
 
 tungsten diselenide ( wse2 ) is one typical example with 
 p - type semiconductor characteristics . 
 bulk wse2 has an 
 indirect band gap ( 1.2 ev ) , which transits into a direct band 
 gap ( 1.65 ev ) in monolayers . 
 monolayer wse2 , therefore , 
 is of considerable interest as a new electronic material for functional 
 electronics and optoelectronics . 
 however , the controllable synthesis 
 of large - area wse2 atomic layers remains a challenge . 
 the 
 studies on wse2 are largely limited by relatively small 
 lateral size of exfoliated flakes and poor yield , which has significantly 
 restricted the large - scale applications of the wse2 atomic 
 layers . here , we report a systematic study of chemical vapor deposition 
 approach for large area growth of atomically thin wse2 film 
 with the lateral dimensions up to 1 cm2 . 
 microphotoluminescence 
 mapping indicates distinct layer dependent efficiency . 
 the monolayer 
 area exhibits much stronger light emission than bilayer or multilayers , 
 consistent with the expected transition to direct band gap in the 
 monolayer limit . 
 the transmission electron microscopy studies demonstrate 
 excellent crystalline quality of the atomically thin wse2 . 
 electrical transport studies further show that the p - type wse2 field - effect transistors exhibit excellent electronic characteristics 
 with effective hole carrier mobility up to 100 cm2 v1 s1 for monolayer and up to 350 
 cm2 v1 s1 for few - layer 
 materials at room temperature , comparable or well above that of previously 
 reported mobility values for the synthetic wse2 and comparable 
 to the best exfoliated materials . 
Supplementary Material