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Probing interlayer excitons in a vertical van der Waals p-n junction using scanning probe microscopy technique
摘要： Two dimensional (2D) semiconductors feature exceptional optoelectronic properties controlled by strong confinement in one dimension. In this contribution, we studied interlayer excitons in a vertical p-n junction made of bilayer n-type MoS2 and few layers of p-type GaSe using current sensing atomic force microscopy (CSAFM). The p-n interface is prepared by mechanical exfoliation onto highly ordered pyrolytic graphite (HOPG). Thus the heterostructure creates an ideal layered system with HOPG serving as the bottom contact for the electrical characterization. Home-built Au tips are used as the top contact in CSAFM mode. During the basic diode characterization, the p-n interface shows strong rectification behavior with a rectification ratio of 104 at ±1 V. The I-V characteristics reveal pronounced photovoltaic effects with a fill factor of 0.55 by an excitation below the band gap. This phenomenon can be explained by the dissociation of interlayer excitons at the interface. The possibility of the interlayer exciton formation is indicated by density functional theory (DFT) calculations on this heterostructure: the valence band of GaSe and the conduction band of MoS2 contribute to an interface-specific state at an energy of about 1.5 eV. The proof of excitonic transitions to that state is provided by photoluminescence measurements at the p-n interface. Finally, photocurrent mapping at the interface under an excitation wavelength of 785 nm provides evidence of efficient extraction of such excitons. Our results demonstrate a pathway towards a two dimensional device for future optoelectronics and light harvesting assisted by interlayer excitons in a van der Waals heterostructure.
关键词： optoelectronics,van der Waals heterojunction,GaSe,density functional theory,MoS2,interlayer exciton,p-n junction
更新于2025-09-23 15:23:52
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Ultrafast formation and dynamics of interlayer exciton in a large-area CVD-grown WS <sub/>2</sub> /WSe <sub/>2</sub> heterostructure
摘要： A WS2/WSe2 heterostructure is constructed by stacking a WS2 monolayer on the top of WSe2 monolayer fabricated with chemical vapor deposition (CVD) method. Ultrafast transient spectroscopy is used to demonstrate the ultrafast charge transfer and interlayer exciton dynamics in the heterostructure. When the WS2/WSe2 heterostructure was photoexcitated at 617 nm (2.01 eV) to excite the A-exciton transition of WS2, an ultrafast photobleaching was observed around the WSe2 A-exciton transition at 749 nm. The bleaching signal lasts several nanoseconds, which is much longer than the A-exciton lifetime in both the WS2 and WSe2 monolayer film. Moreover, by selectively photoexciting the A-exciton of WSe2 at 749 nm in the heterostructure, an ultrafast photobleaching occurs around the WS2 A-exciton transition, the recovery of the bleaching shows a single exponential relaxation with typical time constant of ~1.8 ps. The very fast relaxation in the heterostructure probing around 620 nm is indicative that rich defect states exist below the conduction band in WS2, which can efficiently trap these electrons transferred from the WSe2 upon photoexcitation. Our spectroscopic results reveal that our CVD-grown WS2/WSe2 bilayer film has a type II heterostructure in nature at room temperature. With photoexcitation, electrons and holes can be separately confined in the WS2 and WSe2 layer, respectively; as a result, interlayer excitons are formed.
关键词： charge transfer,interlayer exciton,type-II heterostructure,transition metal dichalcogenides,transient spectroscopy
更新于2025-09-10 09:29:36