Probing Chemical Vapor Deposition Growth Mechanism of Polycrystalline MoSe2 by Near-Field Photoluminescence

Chemical vapor deposition (CVD) growth of atomically thin 2D materials, such as transition metal dichalcogenides (TMDs), is a complex process that has not been completely understood. Large-scale growth of monolayer TMDs often leads to polycrystalline films with heterogeneous morphological and optical properties. Previous optical studies of 2D materials by far-field photoluminescence (PL) provided insights into CVD growth mechanisms of simple crystals, which were, however, limited in spatial resolution due to the diffraction limit. Here we performed correlated morphological and tip-enhanced PL (TEPL) imaging of CVD-grown polycrystalline monolayer molybdenum diselenide (MoSe2) flakes with heterogeneous optical response. We observed nanoscale spatial variations of the optical band gap due to growth-induced thermal strain, revealing the different roles of aligned particles (APs) at crystal edges and grain boundaries (GBs) in thermal strain relaxation. TEPL imaging showed the strain-free near-field PL at GBs, revealing the direct connection between MoSe2 and APs. These results may be used to improve nanoscale bandgap engineering techniques and CVD growth scalability, leading to optimized crystal growth and high performance optoelectronic nanodevices.