Van der Waals heterostructures of two-dimensional (2D) materials are a promising platform for understanding the nanoscale energy transport of room temperature excitons. Since the orientation of exciton dipole in type-II heterostructures of 2D transition metal dichalcogenides (TMDCs) is fixed due to electron and hole existence in different monolayers , it offers a controlled platform to study excitonic transport.
In this work, we experimentally demonstrate a spatial, temporal, and spectral visualization of exciton transport in monolayer molybdenum disulfide (MoS2), tungsten diselenide (WSe2) and a MoS2/WSe2 heterostructure. The sample was made by stacking two mechanically exfoliated MoS2 and WSe2 monolayers using a dry transfer technique followed by high temperature annealing [2,3]. The photoluminescence (PL) measurement of the heterostructure revealed a dominant spectral peak at 1.59 eV that was red shifted from the PL emission peak of monolayer MoS2 (1.90 eV) and WSe2 (1.66 eV) confirming the existence of the interlayer exciton in the heterostructure. By performing diffusion experiments using an avalanche photodiode (APD), we were able to estimate the lifetime, diffusivity and diffusion length of each excitons . At room temperature, interlayer excitons in the MoS2/WSe2 heterostructure showed a longer lifetime (0.955 ns) compared to the intralayer excitons in MoS2 monolayer (0.236 ns) and WSe2 monolayer (0.705 ns). Furthermore, the diffusivity and diffusion length of interlayer excitons (2.25 cm2/s, 654 nm) was found to be higher than the intralayer excitons in the MoS2 monolayer (0.44 cm2/s, 56 nm) and WSe2 monolayer (1.4 cm2/s, 444 nm) reconfirming the electronic coupling in the heterostructure.
In conclusion, our results quantify and confirm that interlayer excitons live longer and diffuse faster and further than intralayer excitons. Controlling the transport of such intralayer excitons can potentially enable a platform to fabricate room temperature, energy efficient, exciton-based information processing devices.
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2. Andres Castellanos-Gomez et al. Deterministic transfer of two-dimensional materials by all-dry viscoelastic stamping. 2D Mater. 1 011002(2014).
3. Zhixing Lu et al. Universal Transfer and Stacking of Chemical Vaper Deposition Grown Two-Dimensional Atomic Layers with Water-Soluble Polymer Mediator. ACS Nano. 10, 5237-5242(2016)
4. Gleb M. Akselrod et al. Visualization of exciton transport in ordered and disordered molecular solids. Nat. Commum. 5, 3646(2014)