Photocurrent Imaging of Multi-Memristive Charge Density Wave Switching in Two-Dimensional 1T-TaS₂

Abstract

Transport studies of atomically thin 1T-TaS 2 have demonstrated the presence of intermediate resistance states across the nearly commensurate (NC) to commensurate (C) charge density wave (CDW) transition, which can be further switched electrically. While this presents exciting opportunities for memristor applications, the switching mechanism could be potentially attributed to the formation of inhomogeneous C and NC domains. Here, we present combined electrical driving and photocurrent imaging of ultrathin 1T-TaS 2 in a heterostructure geometry. While micron-sized CDW domains are seen upon cooling, electrically driven transitions are largely uniform, indicating that the latter likely induces true metastable CDW states, which we then explain by a free energy analysis. Additionally, we are able to perform repeatable and bidirectional switching across the intermediate states without changing sample temperature, demonstrating that atomically thin 1T-TaS 2 can be further used as a robust and reversible multimemristor material for the first time.

Evidence for a thermally driven charge-density-wave transition in 1T-TaS2 thin-film devices: Prospects for GHz switching speed

“…Charge-density wave (CDW) phenomena have recently witnessed renewed interest, particularly in the context of two-dimensional (2D) van der Waals materials. [1][2][3][4][5][6][7][8] This interest is driven by both exciting physics, and possible practical applications, that can be derived from such materials.…”

Observation and manipulation of a phase separated state in a charge density wave material

“…22 In the FFT of the C-like region only the fundamental, three-fold symmetric CDW modes are observed. It is important to note that the observed state is not simply an NC state with asymmetrically arranged domain walls nor is it an NC state with a domain wall period different from the equilibrium NC value as has been previously suggested, 14,17 but rather the state is specifically composed of defined, irregularly shaped NC-like and C-like domains.…”

“…Only recently have attempts been made to specifically study the spatial characteristics of the phases present in flakes within the NC-C hysteresis region, but these experiments have been limited to characterizing the phases at the micron length scale. 14,18,19 In this work, the nanoscopic nature of the CDW states in the hysteresis region of the NC-C phase transition of ultrathin 1T-TaS 2 is elucidated. We image exfoliated flakes using scanning tunneling microscopy (STM) and observe the existence of an inhomogeneous electronic state.…”

“…Previous results in ultrathin flakes of 1T-TaS 2 have indicated that within the hysteresis region of the NC-C transition, the resistivity of the material can be bidirectionally switched, driven reversibly through intermediate states whose resistivities lie between the extrema values associated with the NC and C states. [14][15][16][17] While this property suggests the use of this material as a memory element in future technologies, the nanoscopic nature of these non-equilibrium intermediate phases remains an open question. Efforts to characterize the CDW phases in 1T-TaS 2 have typically focussed on bulk samples, within regions of the phase diagram where the CDW states are well-defined.…”

“…This result was previously seen in the literature where the authors were unable to drive the sample back to the low resistance value. 14 However, Figure 4a demonstrates that we are able to fully toggle between the saturation resistance level and the resistance minimum by using a DC bias when driving from the high to the low resistance state, and an AC signal when driving the sample in the opposite direction.…”