Physicists from Université de Strasbourg and CNRS (France) have disentangled the contributions from interlayer charge and energy transfer from atomically thin semiconductors to graphene.
Research into two-dimensional materials is one of the hottest topics in physics and material science. These thin, flexible structures exhibit many unique material, optical, and electrical properties that make them well suited to applications as diverse as photodetectors, solar cells, light-emitting screens, and biosensors. Taking this a step further, innovative devices could be built by tightly stacking atomically thin layers to create designer materials. These structures, known as van der Waals heterostructures (vdWHs), can exchange charge carriers and energy across the interfaces between layers on short timescales following optical excitation. These competing processes govern the optoelectronic response of vdWHs but remain poorly understood. Here, we experimentally study a model vdWH to better understand these interlayer charge and energy transfers.
Specifically, we investigate a metal-semiconductor junction made of a graphene monolayer transferred onto a transition metal dichalcogenide (TMD, here molybdenum diselenide) monolayer. By measuring light emission from the TMD, we demonstrate that interlayer coupling to graphene drastically reduces the photoluminescence yield and exciton lifetime. Additionally, using Raman spectroscopy, we finely probe the frequency and linewidth of the optical phonon modes in the monolayers. These parameters are highly sensitive to the charge carrier density and show a net photo-induced electron transfer from TMD to graphene. Remarkably, exciton dynamics in a TMD-graphene vdWH is largely independent of the existence of a net charge transfer.
This key result strongly suggests that picosecond interlayer energy transfer from TMD to graphene dominates the photoresponse of these heterostructures. Highly efficient energy transfer now has to be carefully considered when designing devices based on 2D materials.
Charge versus energy transfer in atomically-thin graphene-transition metal dichalcogenide van der Waals heterostructures
Guillaume Froehlicher, Etienne Lorchat, Stéphane Berciaud
Physical Review X 8, 011007 (2018) doi: 10.1103/PhysRevX.8.011007
Preprint at: https://arxiv.org/abs/1703.05396
Contact: Stéphane Berciaud, Professor at Université de Strasbourg
Figure1: Illustration of photoinduced charge and energy transfer from a monolayer of transition metal dichalcogenide (here MoSe2) to graphene (Gr) (b) Optical image of a MoSe2/graphene (Gr) heterostructure. The map of the Gr Raman G-mode frequency (c) and of the MoSe2 photoluminescence intensity (d) reveals clear signatures of interlayer coupling on the heterostructure (dashed contour in b-d).