Fast Interlayer Energy Transfer from the Lower Bandgap MoS2 to the Higher Bandgap WS2

Abstract

Energy transfer (ET) is a dipole-dipole interaction, mediated by the virtual photon. Traditionally, ET happens from the higher (donor) to lower bandgap (acceptor) material. However, in some rare instances, ET can happen from the lower-to-higher bandgap material, depending on the strong overlap between the acceptor photoluminescence (PL) and the donor absorption spectra. In this work, we report an ET process from the lower bandgap MoS2 to the higher bandgap WS2, due to a near 'resonant' overlap between the MoS2 B and WS2 A excitonic levels. Changing the MoS2 bandgap from direct-to-indirect by increasing the layer number results in a reduced ET rate, evidenced by the quenching of the WS2 PL emission. Our work shows at 300 K, the ET timescale of ~33 fs is faster than the reported thermalization of the MoS2 excitonic intervalley scattering (K to K') time and competing with the ultrafast charge transfer timescale. Thus, allowing us to open a new direction in understanding the competing inter/intralayer processes.