Criteria for Assessing the Interlayer Coupling of van der Waals Heterostructures Using Ultrafast Pump–Probe Photoluminescence Spectroscopy

Abstract

van der Waals (vdW) heterostructures of transition metal dichalcogenides (TMDCs) provide an excellent paradigm for next-generation electronic and optoelectronic applications. However, the reproducible fabrications of vdW heterostructure devices and the boosting of practical applications are severely hindered by their unstable performance, due to the lack of criteria to assess the interlayer coupling in heterostructures. Here we propose a physical model involving ultrafast electron transfer in the heterostructures and provide two criteria, η (the ratio of the transferred electrons to the total excited electrons) and ζ (the relative photoluminescence variation), to evaluate the interlayer coupling by considering the electron transfer in TMDC heterostructures and numerically simulating the corresponding rate equations. We have proved the effectiveness and robustness of two criteria by measuring the pump–probe photoluminescence intensity of monolayer WS2 in the WS2/WSe2 heterostructures. During thermal annealing of WS2/WSe2, ζ varies from negative to positive values and η changes between 0 and 4.5 × 10–3 as the coupling strength enhanced; both of them can well characterize the tuning of interlayer coupling. We also design a scheme to image the interlayer coupling by performing PL imaging at two time delays. Our scheme offers powerful criteria to assess the interlayer coupling in TMDC heterostructures, offering opportunities for the implementation of vdW heterostructures for broadband and high-performance electronic and optoelectronic applications.

Structure modulation of two-dimensional transition metal chalcogenides: recent advances in methodology, mechanism and applications

“…The six heterostructures, i.e., MoS 2 / MoSe 2 , MoS 2 /WS 2 , MoS 2 /WSe 2 , MoSe 2 /WS 2 , MoSe 2 /WSe 2 and WS 2 /WSe 2 , all possess type-II band alignments. 271,[309][310][311][312][313] In general, intralayer excitons and trions are generated from the intralayer electrons and holes at the K valleys for each layer, and interlayer excitons often come from electrons at K valley of one layer and holes at G/K valley of the other (an example of MoS 2 / MoSe 2 shown in Fig. 23(a)).…”

“…As an essential factor to realize interlayer coupling and exciton formation, the interlayer tunnelling of electrons and holes can be quantitatively evaluated by the charge transfer ratio. 313 This offers a quantitate method for evaluating the interlayer coupling strength, which can provide secure evidence for optoelectronic performances. For example, the MoS 2 /MoSe 2 interface with weak interlayer coupling tends to induce the formation of intralayer trions through charge transfer.…”

Approaching the Intrinsic Properties of Moiré Structures Using Atomic Force Microscopy Ironing

“…In this perspective, the quality of the interfaces between the layers in the stack is of utmost importance to unveil their intrinsic properties, which are highly sensitive to their surroundings . Moreover, the mechanical exfoliation of bulk crystals unavoidably entails some form of interface disorder, which include charged impurities, traps, wrinkles, or bubbles. − These localized strain gradients impact profoundly the electronic properties of layered materials − and are considered to be one of the main sources of disorder in high-quality samples . Providing long-range homogeneity of the interface represents an ongoing challenge, and the lack thereof explains the considerable variations of the optoelectronic properties of nominally identical heterostructures. ,, Reports on the interlayer exciton in TMD heterobilayers show significant variation in the number of peaks and their energy, with a range of up to 110 meV. − Even in high-quality monolayers, the energy of the exciton transition can vary as much as 10 meV …”

“…The area we selected straddles the edge of the top hBN flake, highlighted by the blue dashed line visible in Figure (a–c). A large number of bubbles and wrinkles can be noted, resulting from the stamping process of the exfoliated flakes. − We then scan an area of 10 μm × 10 μm in contact mode, by applying subsequently increasing forces ranging from 5 nN to 120 nN. After each scan, the initial area (12 μm × 12 μm) was imaged in dynamic mode, to verify the effects of the scans in contact mode.…”