Size-dependent phase stability in transition metal dichalcogenide nanoparticles controlled by metal substrates

Nanomaterials based on MoS₂ and related transition metal dichalcogenides (TMDCs) are remarkably versatile; MoS₂ nanoparticles are proven catalysts for processes such as hydrodesulphurization and the hydrogen evolution reaction, and transition metal dichalcogenides in general have recently emerged as novel 2D components for nanoscale electronics and optoelectronics. The properties of such materials are intimately related to their structure and dimensionality. For example, only the edges exposed by MoS₂ nanoparticles (NPs) are catalytically active, and extended MoS₂ systems show different character (direct or indirect gap semiconducting, or metallic) depending on their thickness and crystallographic phase. In this work, we show how particle size and interaction with a metal surface affect the stability and properties of different MoS₂ NPs and the resulting phase diagrams. By means of calculations based on the Density Functional Theory (DFT), we address how support interactions affect MoS₂ nanoparticles of varying size, composition, and structure. We demonstrate that interaction with Au modifies the relative stability of the different nanoparticle types so that edge terminations and crystallographic phases that are metastable for free-standing nanoparticles and monolayers are expressed in the supported system. These support-effects are strongly size-dependent due to the mismatch between Au and MoS₂ lattices, which explains experimentally observed transitions in the structural phases for supported MoS₂ NPs. Accounting for vdW interactions and the contraction of the Au(111) surface underneath the MoS₂ is further found to be necessary for quantitatively reproducing experimental results. We finally find that support-induced effects on the stability of nanoparticle structures are general to TMDC nanoparticles on metal surfaces, which we demonstrate also for MoS₂ on Au(111), WS₂ on Au(111), and WS₂ on Ag(111). This work demonstrates how the properties of nanostructured transition metal dichalcogenides and similar layered systems can be modified by the choice of supporting metal.