Vacancy-dependent electrical transport in vertical transition metal dichalcogenide memristors: first principles study*

 Nonvolatile resistive switching devices have garnered considerable attention owing to their demonstrations in the ongoing development of next-generation flexible memory and neuromorphic computing systems. Recently, such memristive devices based on two-dimensional materials have been experimented on the benefit of size scaling^1-3.  However, the underlying mechanisms in these promising experiments are yet to be elucidated.  Here, we study changes in the electronic band structures and transport properties of hybrid heterostructures formed with monolayer MoSe₂ and Au electrodes using the first-principles calculations. Specifically, we analyze changes introduced in pristine systems due to the presence of different types of defects including vacancies (e.g. V_Se, V_Mo, and V_2Se) and substitutions (e.g. Au_Se and Au_Mo) using density functional theory in combination with quantum ballistic transport calculations.  The presence of vacancies significantly reduces van der Waals gap between the electrode and the monolayer MoSe₂. Additionally, through the analysis of their band structures, we find that chalcogen vacancies result in the formation of midgap states near the Fermi level of the electrode.   These changes in the geometric and electronic structures also translate into an increase of electrical conductivity with respect to the pristine heterostructures, offering a plausible explanation to the mechanism governing transition metal dichalcogenide (TMD)-based memristors at the atomic scale. Furthermore, we compare the results of hybrid heterostructures formed with other noble metals and TMDs. Outcomes of this work may shed light on the design of versatile memristors scaled based on these few-atom-thick crystalline layers.

[1] Ge et al., Nano Lett. 18, 434–441 (2017).

[2] Kim et al., Nat. Commun. 9, 2524 (2018).

[3] Wu et al., Adv. Mater. 31, 1806790 (2019).