A type-I van der Waals heterobilayer of WSe₂/MoTe₂

We present a joint theoretical/experimental study of a van der Waals heterobilayer with type-I band alignment formed by monolayers of WSe₂ and MoTe₂. Our first-principles computation suggests that both the valence band maximum and the conduction band minimum of the WSe₂/MoTe₂ heterobilayer reside in the MoTe₂ layer. The type-I band alignment allows efficient transfer of excitons from WSe₂ to MoTe₂. Since monolayer MoTe₂ is a direct semiconductor with a bandgap in the infrared range, this heterobilayer is attractive for infrared light emission applications. Time-resolved measurements of photocarrier dynamics were conducted to provide experimental evidence of the type-I nature of this heterobilayer. In these measurements, we found that excitation energy can transfer from WSe₂ to MoTe₂ efficiently, but not along the opposite direction. The efficient energy transfer can serve as an optical gain or wavelength conversion mechanism for efficient emission from MoTe₂, which can be utilized in ultrathin and efficient infrared light sources.