Role of Point Defects and Ion Intercalation in Two-Dimensional Multilayer Transition Metal Dichalcogenide Memristors

Two-dimensional materials, in particular transition metal dichalcogenides (TMDs), have attracted a nascent interest in the implementation of memristive architectures. In addition to being functionally similar to synapses, their nanoscale footprint promises to achieve the high density of a biological neural network in the context of neuromorphic computing. However, in order to advance from the current exploratory phase and reach reliable and sound memristive performances, an understanding of the underlying physical mechanisms in TMD memristors seems imperative. Despite the distinctive transport medium inherent to multilayer TMDs, the memristance is routinely attributed to defects or metal atoms present in the system, with their precise contribution remaining elusive. Specifically, the role of intrinsic point defects in the formation of conductive channels, although shown for monolayer TMDs, is not conclusively studied for multilayer samples. In this work, using density functional theory (DFT) and nonequilibrium Green's function (NEGF) formalism, a systematic study is carried out to analyze the impact that defects and metal atoms produce on the out-of-plane conductivity of multilayer TMDs. MoS2, a representative of the 2H structural configuration, and PtS2, a representative of the 1T structure, the most common crystal arrangements among TMDs, are used for this analysis. It is found that the intrinsic sulfur vacancies, which are the dominant defects present in both TMDs, appear to be insufficient in causing resistive switching on the application of an external bias. The claim that the intrinsic point defects on their own can realize a valence change memory-type device by providing a controllable conductive channel through the van der Waals structure seems, according to our study, improbable. The presence of metallic atoms is demonstrated to be essential to trigger the memristive mechanism, emphasizing the proper choice of a metal electrode as being critical in the fabrication and optimization of memristors using TMDs.