Computational guide to optimize electric conductance in MoS$_2$ films

TL;DR

This paper uses first-principles simulations to analyze how edge terminations and flake overlaps affect charge transport in MoS$_2$ films, providing insights for optimizing their electronic conductance.

Contribution

It offers a detailed computational study of edge states and overlap effects on conductance, guiding experimental optimization of MoS$_2$ thin films.

Findings

Edge states resemble donor and acceptor levels, enabling control over charge carriers.

Overlapping flakes reduce overall conductance, with specific reductions depending on flake shape and environment.

Optimal interflake overlap of 6.5 nm maximizes conductance.

Abstract

Molybdenum disulfide (MoS$_2$) is a high-potential material for nanoelectronic applications, especially when thinned to a few layers. Liquid phase exfoliation enables large-scale fabrication of thin films comprising single- and few-layer flakes of MoS$_2$ or other transition-metal dichalcogenides (TMDCs), exhibiting variations in flake size, geometry, edge terminations, and overlapping areas. Electronic conductivity of such films is thus determined by two contributions: the intraflake conductivity, reflecting the value of each single layer, and charge transport across these overlapping flakes. Employing first-principles simulations, we investigate the influence of various edge terminations and of the overlap between flakes on the charge transport in MoS$_2$ film models. We identify characteristic electronic edge states originating from the edge atoms and their chemical environment, which resemble donor and acceptor states of doped semiconductors. This makes either electrons or holes to majority carriers and enables selective control over the dominant charge carrier type (n-type or p-type). Compared to pristine nanosheets, overlapping flakes exhibit lower overall conductance. In the best performing hexagonal flakes occurring in Mo-rich environments, the conductance is reduced by 20% compared to the pristine layer, while the drop by 40%, and 50% is predicted for truncated triangular, and triangular flakes, respectively in S-rich environments. An overlap of 6.5 nm is sufficient to achieve the highest possible interflake conductance. These findings allow for a rational optimization of experimental conditions for the preparation of MoS$_2$ and other TMDC semiconducting thin films.