Fully Optical in Operando Investigation of Ambient Condition Electrical Switching in MoS₂ Nanodevices

Abstract

processes are different for microscale and nanoscale electrodes. [27] To our knowledge, only two works investigate MoS 2 biased vertically with nanosized electrodes. [18,20] Both report volatile conductive filaments formed by metallic atoms diffusing from electrodes into sulfur vacancies (V S ). [18,20] However, those measurements are performed in vacuum conditions, again leading to different switching dynamics and device performances of the ones observed in the air. [13,18,20,27] Therefore, due to the lack of adequate imaging technologies capable of peering into commercially relevant devices, i.e., at the nanoscale and in ambient conditions, the origin and the volatility of electrical switching in MoS 2 remains unclear. [16,27] We have recently developed the first non-invasive technique able to track material's morphology in situ, at the nanoscale, and in ambient conditions, via plasmon-enhanced dark field (DF) nanospectroscopy. [28][29][30] Here, we expand it with additional capabilities offered by nano-Raman and nano-photoluminescence to study the switching mechanism in MoS 2 . The method's principle is presented in Figure 1a. An 80 nm gold nanoparticle (AuNP) placed in the vicinity of a gold substrate is illuminated with white light (λ ≈ 400-900 nm) to produce plasmonic resonances within the AuNP (single mode) and in a spacer between AuNP and the substrate (gap mode). [31,32] The resonances are detected using DF scattering microscope configuration and the gap mode's wavelength and intensity depend on the spacer's refractive index, thickness, and geometry. [31,32] Using a AuNP as a nanosized (≈700 nm 2 ) top contact of an electrical switch [29,33] results in a strong field enhancement localized within the nanoscale switching channel. This greatly enhances both Raman and photo vluminescence (PL) signals, [34] conveniently highlighting the otherwise undetectable nanoscale switching dynamics.Many switching mechanisms are proposed in the literature for MoS 2 , as summarized in Table 1. Among these are migrations of sulfur vacancies (V S ), [3,9,10] movements of oxygen in oxidized MoS 2 , [6,12] charge trapping and detrapping, [2] phase change from semiconducting (2H) to metallic (1T'), [4,7] and metal ions intercalating from electrodes [5,13,17,18,20] We note that all the above mechanisms would trigger changes in optical signals (Raman, PL) which are detectable with our experimental capabilities. In particular, the density of V S , which are observed in all MoS 2 nanosheets studied by transmission electron microscopy (TEM), [36][37][38] is correlated to the PL peak at ≈750 nm, [39,40] the intensity ratio of MoS 2 's A/B excitons [39,41] MoS 2 nanoswitches have shown superb ultralow switching energies without excessive leakage currents. However, the debate about the origin and volatility of electrical switching is unresolved due to the lack of adequate nanoimaging of devices in operando. Here, three optical techniques are combined to perform the first noninvasive in situ characterization of nanosized MoS 2 devices....

Controllable Epitaxial-like Growth of Bi2S3/MoS2 Hybrid Aerogel Nanostructures for Sensitive NO2 Detection at Room Temperature

“…The intensity ratio of E 1 2g / A 1g in the composite material (0.47) is greater than that of MoS 2 (0.56), revealing that the Bi 2 S 3 /MoS 2 -2 composite exposes numerous edge-terminated structures . We present a spatial map of the frequency difference of the A 1g and E 1 2g Raman peaks (Δ) 24 cm –1 of Bi 2 S 3 /MoS 2 -2, corresponding to few-layer MoS 2 . For comparison, the Raman frequency difference of the pristine MoS 2 aerogel is measured to be about 28 cm –1 , corresponding to bulk MoS 2 flakes.…”

“…30 We present a spatial map of the frequency difference of the A 1g and E 1 2g Raman peaks (Δ) 24 cm −1 of Bi 2 S 3 /MoS 2 -2, corresponding to few-layer MoS 2 . 31 For comparison, the Raman frequency difference of the pristine MoS 2 aerogel is measured to be about 28 cm −1 , corresponding to bulk MoS 2 flakes. In this regard, Raman analysis is highly consistent with SEM and TEM observations.…”

Scanning Plasmon-Enhanced Microscopy for Simultaneous Optoelectrical Characterization

“…Pristine Interface. Thanks to the lack of adhesion layer (e.g., Ti or Cr) and ligands (removed from the top facet by etching, leaving AuNP stabilized by the parylene coating 55 ), SPEM delivers an immaculate van der Waals (vdW) interface. This is confirmed by the current−voltage (I−V) characteristics for different thicknesses of MoS 2 (see Figure 1c).…”

“…As shown in Figure 2d, the plasmonics of SPEM and standard NPoM are comparable. 55 Notably, in standard NPoMs, the dipolar gap mode l 10 is damped, most likely by ligands at the AuNP/material interface 55 (red-shifted modes due to parylene coating�see Figure S2a). This is not observed for SPEM, indicating again a pristine junction that guarantees reliable optical mapping.…”

“…Finally, SPEM represents a pivotal achievement in advancing the NPoM geometry. 50,55 In the traditional NPoM method, AuNPs are randomly drop-cast on a test material. Unavoidable variability in AuNP geometries induces gap modes with differing wavelengths and intensities.…”