Highly Efficient and Anomalous Charge Transfer in van der Waals Trilayer Semiconductors.

Two-dimensional materials, such as graphene and monolayer transition metal dichalcogenides, allow the fabrication of multilayer structures without lattice matching restriction. A central issue in developing such artificial materials is to understand and control the interlayer electron transfer process, which plays a key role in harnessing their emergent properties. Recent photoluminescence and transient absorption measurements revealed that the electron transfer in heterobilayers occurs on ultrafast time scales. However, there is still a lack of fundamental understanding on how this process can be so efficient at van der Waals interfaces. Here we show evidence suggesting the coherent nature of such interlayer electron transfer. In a trilayer of MoS-WS-MoSe, electrons excited in MoSe transfer to MoS in about one picosecond. Surprisingly, these electrons do not populate the middle WS layer during this process. Calculations showed the coherent nature of the charge transfer and reproduced the measured electron transfer time. The hole transfer from MoS to MoSe is also found to be efficient and ultrafast. The separation of electrons and holes extends their lifetimes to more than one nanosecond, suggesting potential applications of such multilayer structures in optoelectronics.