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.


Section 2: Estimation of diffusivity, diffusion length and activation energy for diffusion
We estimated the values of diffusivity, diffusion length, and activation energy using mean field diffusion and thermodynamic nucleation theories. 1We estimate nucleation density (N, grains per µm 2 ), from the number of individual grains per unit area using AFM images (Gwyyidion 2.60 software).The diffusivity (D) of In considering 2D growth is related to nucleation density as: Where,  is the cluster size 1 .For  = 1, and N = 64.9± 2.46 µm -2 ( for 1L MoS2), diffusivity (D) was calculated as: (2) -6 µm -2 s -1 .The nucleation density is related to activation energy for diffusion (E d)   and can be expressed as: For  = 1, following equation is used to calculate activation energy for diffusion (E d) : ). (4) We have also provided,      (5)

Section 3 :
Figure S2.Morphology of In metal films deposited on different thicknesses of graphene.AFM (2×2 μm 2 ) images of 5 nm In deposited on (a) 2L, (b) 3L, and (c)Bulk graphene.Scale bar = 500 nm.(d) Mean grain size and R rms (before metal deposition) of graphene flakes for different thicknesses of graphene.(e) In nucleation density on graphene for different thicknesses of graphene.Error bars in (d) and (e) represent the standard deviation across 5 AFM images (2×2 μm 2 ).

Figure S5 .
Figure S5.(a) Optical microscope image of a graphene flake with regions having different thicknesses.AFM image (5×5 µm 2 ) of region in (a) marked inside red box (b) before metal deposition and (c) after 15 nm thick In metal deposition.The image shows difference in grain size on different thicknesses of graphene, 1L graphene and 5L graphene, and SiO2 substrate.Scale bar is 1 µm.

Figure S7 .
Figure S7.SEM image of surface morphology 5 nm Au film on (a) pristine and (b) defective MoS2/SiO2.Scale bar is 200 nm.

Table S4 . Calculated Values for Diffusivity (D), and Activation energy for Diffusion (E d ) for Three Different Thicknesses of Graphene (𝒊= 2)
In coated substrate) is the Raman peak intensity measured at 5 nm In coated MoS2.We take A(hotspot) using the average radius (30 nm) of the In grains for 5 nm In coated MoS2.I(bare substrate) is the MoS2 Raman peak intensity measured at pristine flake without any metal deposition.The A(excitation spot) is the area of the laser excitation spot.We used 100× lens with 514 nm laser and used A(excitation spot) with radius of 450 nm..;Xie, Y. H. Spectroscopic Signatures of AA' and AB Stacking of Chemical Vapor Deposited Bilayer MoS2.ACS Nano 2015, 9 (12), 12246-12254.