Tracking Ultrafast Charge Transfer Processes in Heterostructures of 2D Materials

Layered materials are solids consisting of crystalline sheets with strong in-plane covalent bonds and weak van der Waals out-of-plane interactions. These materials can be easily exfoliated to a single layer (1L), obtaining two-dimensional (2D) materials with radically novel physico-chemical characteristics compared to their bulk counterparts. The field of 2D materials began with graphene and quickly expanded to include semiconducting transition metal dichalcogenides (TMDs). 2D materials exhibit very strong light-matter interaction and exceptionally intense nonlinear optical response, enabling a variety of novel applications in optoelectronics and photonics. They also offer the unique opportunity of creating vertical stacks of 1L materials, referred to as heterostructures (HS). Weak interlayer van der Waals forces between individual layers bypass the lattice parameter matching constraints of conventional semiconductor HS and preserve the electronic structure of each constituent layer, enabling the creation of new materials with tailored electronic and optical properties which differ from those of the isolated components, depending on the layers type, stacking sequence and relative angle.<br/>In this presentation we use high time resolution ultrafast transient absorption (TA) and two-dimensional (2D) spectroscopy to resolve the interlayer charge scattering processes in HS of 2D materials. We first study a WSe₂/MoSe₂ HS, which displays type II band alignment with a staggered gap, where the valence band maximum and the conduction band minimum reside in different layers. By two-colour TA spectroscopy, we selectively photogenerate intralayer excitons in MoSe₂ and observe hole injection in WSe₂ on the sub-picosecond timescale, leading to the formation of interlayer excitons, which is detected by the weak photobleaching/stimulated emission signal formed with a further delay with respect to the hole injection. The temperature dependence of the build-up and decay of interlayer excitons provide insights into the layer coupling mechanisms. We also employ 2D spectroscopy, which guarantees simultaneously high temporal and spectral resolution, to study the interlayer charge transfer in a MoS₂/WS₂ HS. We unambiguously time resolve both interlayer hole and electron transfer processes with very fast 34 ± 14 and 69 ± 9 fs time constants, respectively. Finally, we investigate a graphene/WS₂ HS where, for excitation well below the bandgap of WS₂, we observe the characteristic signal of the A and B excitons of WS₂, indicating ultrafast charge transfer from graphene to the semiconductor. The nonlinear excitation fluence dependence of the TA signal reveals that the underlying mechanism is hot electron/hole transfer, whereby a tail the hot Fermi-Dirac carrier distribution in graphene tunnels through the Schottky barrier. Hot electron transfer is promising for the development of broadband and efficient low-dimensional photodetectors.