The performance of devices and systems based on two-dimensional material systems depends critically on the quality of the contacts between 2D material and metal. A low contact resistance is an imperative requirement to consider graphene as a candidate material for electronic and optoelectronic devices. Unfortunately, measurements of contact resistance in the literature do not provide a consistent picture, due to limitations of current graphene technology, and to incomplete understanding of influencing factors. Here we show that the contact resistance is intrinsically dependent on graphene sheet resistance and on the chemistry of the graphene-metal interface. We present a physical model of the contacts based on ab-initio simulations and extensive experiments carried out on a large variety of samples with different graphene-metal contacts. Our model explains the spread in experimental results as due to uncontrolled graphene doping and suggests ways to engineer contact resistance. We also predict an achievable contact resistance of 30 Ω·μm for nickel electrodes, extremely promising for applications.

Platinum Diselenide (PtSe 2 ) is an exciting new member of the two-dimensional (2D) transition metal dichalcogenide (TMD) family. It has a semimetal to semiconductor transition when approaching monolayer thickness and has already shown significant potential for use in device applications. Notably, PtSe 2 can be grown at low temperature making it potentially suitable for industrial usage. Here, we address thickness dependent transport properties and investigate electrical contacts to PtSe 2 , a crucial and universal element of TMD-based electronic devices. PtSe 2 films have been synthesized at various thicknesses and structured to allow contact engineering and the accurate extraction of electrical properties. Contact resistivity and sheet resistance extracted from transmission line method (TLM) measurements are compared for different contact metals and different PtSe 2 film thicknesses. Furthermore, the transition from semimetal to semiconductor in PtSe 2 has been indirectly verified by electrical characterization in field-effect devices. Finally, the influence of edge contacts at the metal -PtSe 2 interface has been studied by nanostructuring the contact area using electron beam lithography. By increasing the edge contact length, the contact resistivity was improved by up to 70 % compared to devices with conventional top contacts. The results presented here represent crucial steps towards realizing high-performance nanoelectronic devices based on group-10 TMDs.

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