Nanoscale charge transfer and diffusion at the MoS₂/SiO₂ interface by atomic force microscopy: contact injection versus triboelectrification

Abstract

Understanding the process of charge generation, transfer, and diffusion between two-dimensional (2D) materials and their supporting substrates is very important for potential applications of 2D materials. Compared with the systematic studies of triboelectric charging in a bulk sample, a fundamental understanding of the triboelectrification of the 2D material/insulator system is rather limited. Here, the charge transfer and diffusion of both the SiO surface and MoS/SiO interface through contact electrification and frictional electrification are investigated systematically in situ by scanning Kelvin probe microscopy and dual-harmonic electrostatic force microscopy. Different from the simple static charge transfer between SiO and the PtSi alloy atomic force microscope (AFM) tip, the charge transfer between the tip and the MoS/SiO system is complicated. Triboelectric charges, generated by contact or frictional electrification with the AFM tip, are trapped at the MoS/SiO interface and act as floating gates. The local charge discharge processes can be obtained by monitoring the surface potential. The charge decay time (τ) of the MoS/SiO interface is one (or two) orders of magnitude larger than the decay time τ of the SiO surface. This work facilitates an understanding of the triboelectric and de-electrification of the interface between 2D materials and substrates. In addition to the charge transfer and diffusion, we demonstrate the nanopatterns of surface and interfacial charges, which have great potential for the application of self-assembly of charged nanostructures.

Atomically Asymmetric Inversion Scales up to Mesoscopic Single-Crystal Monolayer Flakes

“…Figure 3h demonstrates the SPM image of the MCD of the WS 2 sample, which was obtained by recording the secondorder amplitudes of the cantilever vibration (A 2ω ). 34 Given the relation between the MCD and the bandgap (E gap ), we deduced that the local bandgap in the ZR E gap (Z) is larger than that in the KR E gap (K) (see details in the Supporting Information); this is, again, consistent with our theoretically predicted always larger local bandgap in the ZR. This local bandgap difference was also confirmed by Raman mapping, which was previously used to depict local bandgap distributions and thus to show strain distribution.…”

“…Figure d,e depicts typical topographies of the WS 2 monolayer showing a uniform hexagonal shape. Local surface potential (LSP) and surface mobile charge carrier density (MCD) were measured using scanning Kelvin probe microscopy (SKPM) − and dual-harmonic electrostatic force microscopy (DH-EFM), − respectively, as presented in Figure f and h. The LSP of the WS 2 flake shows six domains, which are categorized into two LSP levels with an appreciable difference of 8 meV (see Figure g), consistent with the theoretical work-function difference of 4–6 meV. Figure h demonstrates the SPM image of the MCD of the WS 2 sample, which was obtained by recording the second-order amplitudes of the cantilever vibration ( A 2ω ) .…”

“…Local surface potential (LSP) and surface mobile charge carrier density (MCD) were measured using scanning Kelvin probe microscopy (SKPM) − and dual-harmonic electrostatic force microscopy (DH-EFM), − respectively, as presented in Figure f and h. The LSP of the WS 2 flake shows six domains, which are categorized into two LSP levels with an appreciable difference of 8 meV (see Figure g), consistent with the theoretical work-function difference of 4–6 meV. Figure h demonstrates the SPM image of the MCD of the WS 2 sample, which was obtained by recording the second-order amplitudes of the cantilever vibration ( A 2ω ) . Given the relation between the MCD and the bandgap ( E gap ), we deduced that the local bandgap in the ZR E gap (Z) is larger than that in the KR E gap (K) (see details in the Supporting Information); this is, again, consistent with our theoretically predicted always larger local bandgap in the ZR.…”

Tailoring Domain Morphology in Monolayer NbSe2 and WxNb1–xSe2 Heterostructure

“…The bright contrast observed for the needle-like NbSe 2 domain in the phase image is attributable to strong electrostatic interaction of the metallic NbSe 2 compared with the semiconducting WSe 2 domain and SiO 2 region (Figure S8). 46 The NbSe 2 /W x Nb 1−x Se 2 heterostructure is characterized by confocal Raman intensity mapping in the 245−250 cm −1 range, which includes the mixed E 1 2g +A 1g mode of WSe 2 (∼249 cm −1 ) and the E 1 2g mode of NbSe 2 (∼248 cm −1 ) (Figure 4d). 47 The Raman intensity gradually decreases from the central to the outer region in the W x Nb 1−x Se 2 domain (white dotted line), while no distinguishable signal appears in the NbSe 2 region (yellow dotted line) due to the relatively low intensities of the Raman modes of NbSe 2 (Figure S9).…”

Switching friction at a manganite surface using electric fields

“…Under the friction measurement conditions used here, where the specimen is only electrically connected through the tip, the electrostatic forces come from the contact potential difference between the tip and specimen and from any trapped charges or charges generated by triboelectrification (which may be quite different from those generated by contact electrification [32]). However, since spontaneous resistive switching was not observed during the friction measurements, the maximum electric potential generated during friction must be less than the threshold voltage for switching, leading to a maximum change in contact area of 28%.…”