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

Abstract

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.

Figure 1

Side view of the supercell with the sulfur divacancy V_S2 point defect and the corresponding energy of formation, E_form: (a) two configurations for MoS₂ and (b) three configurations for PtS₂.

Figure 2

Top view of a 6 × 6 supercell with the single sulfur vacancy V_s forming the extended line and localized cluster for (a) MoS₂ and (b) PtS₂. The variation in formation energy (E_form) per V_s for line defect vs cluster for (c) MoS₂ and (d) PtS₂.

Figure 3

Side view of a 3 × 3 supercell for bilayer MoS₂ (top row) and PtS₂ (bottom row) with single vacancy (V_S) or divacancy (V_S2) sulfur defects. Given the vacancy placement in a layer (circled in black), there are two different possibilities for the alignment of the vacancy in the adjacent layer: vertically aligned (circled red) and vertically skewed (circled blue).

Figure 4

Electron localization function (ELF) for MoS₂ (top row) and PtS₂ (bottom row) for pristine, single sulfur point defects (V_S) and sulfur divacancy defects (V_S2).

Figure 5

Electronic band structure and Brillouin zone path for V_s (inline), V_S2 (inline), and V_S (line defect) for MoS₂ (top row) and PtS₂ (bottom row) in solid blue and pristine MoS₂ and PtS₂ in semitransparent red. The shaded region in the band diagram corresponds to the out-of-plane directions.

Figure 6

Side view of a 3 × 3 supercell for bilayer MoS₂ (top row) and PtS₂ (bottom row) with gold(Au)/copper(Cu) in the vdW gap at (a) position A (left column) and (b) position B (right column).

Figure 7

(a) Electron localization function (ELF) and (b) electronic band structure (shown in the solid blue line) for MoS₂ (top row) and PtS₂ (bottom row) and for Au (Position A) and Cu (Position B). The band structures for pristine MoS₂ and PtS₂ are also shown in semitransparent red. The shaded region in the band diagram corresponds to the out-of-plane directions.

Figure 8

Zero-bias NEGF transmission spectrum comparing V_S (inline, red), V_S2 (inline, green), V_S (line defect, blue), Au (Position A), and Cu (Position B) for (a) MoS₂ and (b) PtS₂.