Direct Formation of Interlayer Exciton in MoSe₂/WSe₂ Heterostructure Measured via Broadband Transient Absorption Spectroscopy

In Transition Metal Dichalcogenide Heterostructures (TMD HS) with Type II band alignment, such as MoSe₂/WSe₂, photoexcited carriers can form interlayer excitons (ILX) with electrons/holes residing in different layers. Spatially-indirect ILXs exhibit longer recombination times than intralayer excitons (10 – 100x longer) making these HS a promising platform for observing possible Bose-Einstein condensation of excitons and for applications in spin/valley optoelectronics.

Our understanding of the ILX kinetics is limited to recombination resolved via photoluminescence spectroscopy. We directly study the early formation process of the ILX in a MoSe₂/WSe₂ HS on a sub-ps timescale using transient absorption spectroscopy. Upon selective photoexcitation of the MoSe₂ layer, we resolve interlayer hole scattering to WSe₂ and the signature of the ILX around ~1.4 eV, which grows in on a 400 fs timescale (i.e. slower than the interlayer charge transfer process). This suggests photoexcited carriers are not instantaneously converted into the ILX following interlayer scattering, but rather intermediate scattering processes take place. Moreover, valley-contrasting physics of intra/inter-layer excitons are explored.