Excitation-Dependent High-Lying Excitonic Exchange <i>via</i> Interlayer Energy Transfer from <i>Lower</i>-<i>to</i>-<i>Higher</i> Bandgap 2D Material

High light absorption (∼15%) and strong photoluminescence (PL) emission in monolayer (1L) transition metal dichalcogenides (TMDs) make them ideal candidates for optoelectronic device applications. Competing interlayer charge transfer (CT) and energy transfer (ET) processes control the photocarrier relaxation pathways in TMD heterostructures (HSs). In TMDs, long-distance ET can survive up to several tens of nm, unlike the CT process. Our experiment shows that an efficient ET occurs from the 1Ls WSe₂-to-MoS₂ with an interlayer hexagonal boron nitride (hBN), due to the resonant overlapping of the high-lying excitonic states between the two TMDs, resulting in enhanced HS MoS₂ PL emission. This type of unconventional ET from the <i>lower-to-higher</i> optical bandgap material is not typical in the TMD HSs. With increasing temperature, the ET process becomes weaker due to the increased electron-phonon scattering, destroying the enhanced MoS₂ emission. Our work provides new insight into the long-distance ET process and its effect on the photocarrier relaxation pathways.