Observation of Strong -Aggregate Light Emission in Monolayer Molecular Crystal on Hexagonal Boron Nitride.

aggregates are widely used in studies of light-matter interaction and organic optoelectronic devices. Although -aggregate films can be fabricated on salt by epitaxial growth method, the size is limited to hundreds of nanometer. In this work, with hexagonal boron nitride (h-BN) as a substrate, highly crystalline -aggregate ultrathin films of ,'-ditridecylperylene 3,4,9,10-tetracarboxylic diimide (PTCDI-C) are achieved by physical vapor transport (PVT) method. Significant bathochromically shifted absorption band and narrowed 0-0 transition are observed in the monolayer PTCDI-C crystal on h-BN. The exciton coherence number of monolayer aggregate film extracted from the photoluminescence (PL) spectrum is up to 15 at = 140 K, which is higher than that of the epitaxially grown layer on salt. Beyond the first molecular layer, the multilayer crystal on h-BN is dominated by -aggregates. Further study shows that that the first molecular layer on h-BN adopts the highly ordered face-on configuration, while the overlayers adopt the edge-on motif. As a comparison, only aggregate PTCDI-C ultrathin films are found on SiO substrates, but no -aggregates. The results suggest that high-quality -aggregates can be prepared by utilizing appropriate substrates via physical vapor transport.