Metal Films on Two-Dimensional Materials: van der Waals Contacts and Raman Enhancement

Electronic devices based on two-dimensional (2D) materials will need ultraclean and defect-free van der Waals (vdW) contacts with three-dimensional (3D) metals. It is therefore important to understand how vdW metal films deposit on 2D surfaces. Here, we study the growth and nucleation of vdW metal films of indium (In) and non-vdW metal films of gold (Au), deposited on 2D MoS₂ and graphene. In follows a 2D growth mode in contrast to Au that follows a 3D growth mode. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to image the morphology of metal clusters during growth and quantify the nucleation density. As compared to Au, In atoms exhibit nearly 50 times higher diffusivity (3.65 × 10^-6 μm^-2 s^-1) and half the nucleation density (64.9 ± 2.46 μm^-2), leading to larger grain sizes (∼60 nm for 5 nm In on monolayer MoS₂). The grain size of In can be further increased by reducing the 2D surface roughness, while the grain size for Au is limited by its high nucleation density due to the creation of interface defects during deposition. The vdW gap between In and MoS₂ and graphene leads to strong enhancement (>10³) in their Raman signal intensity due to localized surface plasmon resonance. In the absence of a vdW gap, the plasmon-mediated enhancement in Raman does not occur.