Ultrafast Interlayer Charge Transfer Outcompeting Intralayer Valley Relaxation in Few-Layer 2D Heterostructures

Abstract

While 2D transition metal dichalcogenides (TMDs) feature interesting layer-tunable multivalley band structures, their preeminent role in determining the photoexcitation charge transfer dynamics in 2D heterostructures (HSs) is yet to be unraveled, as previous charge transfer studies on TMD HSs have been mostly focused on monolayers with a direct bandgap at the K valley. By ultrafast transient absorption spectroscopy and deliberately designed few-layer WSe 2 /WS 2 HSs, we have observed an ultrafast interlayer electron transfer from photoexcited few-layer WSe 2 to WS 2 , prior to intralayer relaxation to lower lying dark valleys. More interestingly, we have identified an unconventional ∼0.5 ps electron back-transfer process after the initial interlayer electron transfer in HSs with WSe 2 layers ≥ 3, regenerating indirect intralayer excitons. The result reveals an ielectron and valley relaxation pathway mediated by interlayer charge transfer in 2D HSs, faster than intralayer relaxation. It also sheds light on the origin of generally observed robust ultrafast interlayer charge transfer in TMD HSs and provides guidance toward optoelectronic and valleytronic devices using few-layer TMDs.

Harnessing Hot Carriers in Two-Dimensional Materials

“…Afterward, we explored the competitive relationship between the hot electron transfer and the intervalley hot electron relaxation within the layer of 2D semiconductors with an indirect bandgap (e.g., few-layer WSe 2 ). In deliberately designed WSe 2 /WS 2 heterostructures, the results of transient absorption spectra after selectively exciting the WSe 2 layer exhibit the same ultrafast rising process (<50 fs) of WS 2 kinetics in 1L_- and 3L_WSe 2 /WS 2 heterostructures, and a 0.54 ps fast decay in 3L_WSe 2 /WS 2 heterostructures but not in 1L_WSe 2 /WS 2 heterostructures (Figure e), which indicate an ultrafast interlayer electron transfer from photoexcited few-layer WSe 2 to WS 2 , occurring prior to intralayer hot carrier relaxation to lower-lying dark valleys (Figure f) . These results highlight the robust interlayer charge transfer in TMD heterostructures and the advantages of 2D monolayer semiconductors as extremely thin light absorbers for efficient optoelectronic devices by harnessing hot carriers.…”

“…The efficiency of optoelectronic conversion can experience a significant enhancement through the efficient extraction of hot carriers from semiconductors prior to their rapid cooling process. ,, Given extremely fast hot carrier cooling in semiconductors via electron–phonon scattering (tens to hundreds of femtoseconds), − ,− an ultrafast interfacial charge transfer process is essential for extracting photogenerated hot carriers. While this is challenging in conventional 3D semiconductors where time-consuming carrier diffusion is a prerequisite, it can be achieved in low-dimensional semiconductors.…”

“…(f) Scheme of electron transfer and back-transfer processes in the 3L_WSe 2 /WS 2 heterostructure after selectively exciting the WSe 2 layer. Adapted from refs and with permission. Copyright 2020 for (a), (b), (c), and (d), AIP Publishing.…”

“…Recently, atomically thin 2D semiconductor materials have shown promising potential in hot carrier physics and in devices. Compared to their 3D counterparts, 2D semiconductors exhibit enhanced many-body interactions due to reduced dielectric screening and quantum confinement, resulting in significant exciton binding energy (e.g., up to 500 meV for monolayer semiconductors) and ultrafast electron–electron scattering within femtoseconds. − Additionally, the 2D geometry and atomic flatness without dangling bonds guarantee high density of states and allow readily interfacing 2D semiconductors with charge extraction components to achieve ultrafast interfacial charge transfer within 100 fs. ,− The combination of ultrafast electron–electron scattering and interfacial charge transfer in 2D semiconductors suggests exciting potential to explore and realize efficient hot carrier harvesting. Indeed, with the help of powerful transient spectra, efficient hot carrier transfer from graphene and other 2D semiconductors before interband electron–electron scattering has been observed, and multiple exciton generation and transfer with high yield and low threshold have been reported in MoTe 2 and black phosphorus (BP). − In this Perspective, we highlight three primary directions: (1) hot electron harnessing from graphene; (2) hot electron transfer from 2D semiconductors; and (3) multiexciton generation in 2D semiconductors.…”