Structure and stability of copper nanoclusters on monolayer tungsten dichalcogenides

Layered materials, such as tungsten dichalcogenides (TMDs), are being studied for a wide range of applications, due to their unique and varied properties. Specifically, their use as either a support for low dimensional catalysts or as an ultrathin diffusion barrier in semiconductor devices interconnect structures are particularly relevant. In order to fully realise these possible applications for TMDs, understanding the interaction between metals and the monolayer they are deposited on is of utmost importance. The morphology that arises due to given metal-substrate combinations determines their possible applications and thus is a central characteristic. Previous theoretical studies typically focus on the effects which single metal adatoms, or dopants, have on a TMDs' electronic and optical properties, thereby leaving a knowledge gap in terms of thin film nucleation on TMD monolayers. To address this, we present a density functional theory (DFT) study of the adsorption of small Cu clusters on a range of TMD monolayers, namely WS₂, WSe₂, and WTe₂. We explore how metal-substrate and metal-metal interactions contribute to both the stability of these Cu clusters and their morphology, and investigate the role of the chalcogen in these interactions. We find that single Cu atoms adsorb most strongly to the adsorption site above the W atom, however as nanocluster size increases, Cu tends to be adsorbed atop the chalcogen atoms in the monolayer to facilitate Cu-Cu bond formation. We show that Cu-Cu interactions drive the stability of the adsorbed Cu nanoclusters, with a clear preference for 3D structures on all 3 monolayers studied. Furthermore, significant Cu migration occurs during 0 K relaxation. This, combined with the small activation barriers found for Cu migration suggest facile and dynamic cluster behaviour at finite temperature on all three monolayers. Finally, we find that Cu clusters are generally most stable on WTe₂ and least stable on WSe₂. This difference however is typically only in the range of 0.1 eV.