Moiré potential impedes interlayer exciton diffusion in van der Waals heterostructures

Abstract

The properties of van der Waals heterostructures are drastically altered by a tunable moiré superlattice arising from periodically varying atomic alignment between the layers. Exciton diffusion represents an important channel of energy transport in transition metal dichalcogenides (TMDs). While early studies performed on TMD heterobilayers suggested that carriers and excitons exhibit long diffusion, a rich variety of scenarios can exist. In a moiré crystal with a large supercell and deep potential, interlayer excitons may be completely localized. As the moiré period reduces at a larger twist angle, excitons can tunnel between supercells and diffuse over a longer lifetime. The diffusion should be the longest in commensurate heterostructures where the moiré superlattice is completely absent. Here, we experimentally demonstrate the rich phenomena of interlayer exciton diffusion in WSe2/MoSe2 heterostructures by comparing several samples prepared with chemical vapor deposition and mechanical stacking with accurately controlled twist angles.

Interlayer exciton formation, relaxation, and transport in TMD van der Waals heterostructures

“…The influence of the moiré potential on interlayer exciton diffusion was also studied recently via spatially and spectrally resolved PL imaging in MoSe 2 /WSe 2 heterostructures with different twist angles (Fig. 10a-f) 162 . In this study, three types of heterostructures were used, with the first prepared by the CVD method with a twist angle of 0°( no moiré pattern) and the second and third prepared by the mechanical exfoliation and transfer (MET) method with twist angles of~1.1°(moiré period of~17 nm) and 3.5°(moiré period of~5 nm), respectively.…”

“…5d ). The moiré effect on the interlayer exciton lifetime was also studied in MoSe 2 /WSe 2 heterostructures prepared by chemical vapor deposition (CVD, twist angle of 0°) and mechanical stacking with twist angles of ~1.1° and ~3.5° 162 . The sample at a twist angle of 0° without a moiré pattern possessed an averaged interlayer exciton lifetime of ~1.7 ns, which was similar to that at a twist angle of ~1.1° with a moiré period of ~17 nm.…”

“…d – f PL line profiles extracted from a – c for the three samples at a few selected energies. The PL line profiles for sample A-1 are truncated by the boundaries of the heterostructure region (the white dashed lines in a ) 162 . The 660 nm excitation laser profile and the 900 nm laser profile are represented by the bottom gray lines and the top gray shaded area, respectively.…”

“…a – f Reprinted with permission from ref. 162 [American Association for the Advancement of Science]. g , h Reprinted with permission from ref.…”

A roadmap for interlayer excitons

“…When the moiré period is sufficiently large, a well-defined periodic potential can trap the interlayer excitons and induce correlations. The feature of such a potential modulation appears in not only excitonic transition energies 11 – 13 but also exciton diffusion dynamics 14 , 15 .…”