1 Scientific RepoRts | 7: 5109 | DOI:10.1038/s41598-017-05069-7 www.nature.com/scientificreports electrical properties of graphene- metal contacts teresa Cusati 1 , Gianluca Fiori 1 , Amit Gahoi 2,3 , Vikram passi 2,3 , Max C. Lemme 2,3 , Alessandro Fortunelli 4 & Giuseppe Iannaccone 1 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 infuencing 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 diferent 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. Low and reproducible metal-graphene contact resistance R C (i.e., smaller than 100 Ω × μm) is an imper- ative requirement for the industrial adoption of graphene in electronics 1–3 and for the adoption of other two-dimensional materials, which ofen rely on the use of graphene-metal interfaces 4 . However, graphene contact fabrication technology is not yet mature and fully reproducible, and therefore a broad range of experimental values of R C is found in the literature for the same metal 5–12 . Measurements of graphene-metal contact resistance for diferent metals (Cr, Ti, Cu, Au, Ni, Pd and Pt) via transfer-length and four-probe methods are strongly dependent on factors such as deposition temperature and process conditions, in addition to intrinsic factors such as metal work function, number of graphene layers, back-gate voltage 5 . In addition, photoresist residues are generally an issue that leads to high contact resistance in experimental devices. A reduced contact resistance has been reported in the case of contacts to graphene edges or defects, and has been attributed to stronger covalent bonding of graphene and metal or to a reduction of the bonding distance, which would entail a larger orbital overlap compared to van der Waals contacts 8, 10, 13–16 . Teoretical work has provided insightful contributions into the physics of graphene-metal contacts 15–24 . In the model proposed by Xia et al. 17 , electrons frst tunnel through the graphene-metal interface and then transfer from the graphene region under the metal to the graphene channel. Ji et al. 18 used this concept in a systematic study of the contact resistance, including both single-sided and double-sided contacts for diferent graphene-metal sys- tems. However, they compute graphene-metal tunnelling with the Wentzel–Kramers–Brillouin approximation, that is inadequate for high transmission, and their study is limited to a single geometry. Similar ab-initio studies only investigate lateral in-plane transmission, and therefore completely neglect the critical issue of vertical trans- port through the heterointerface 19–22 . Te contact geometry afecting the formation of covalent bonds at the interface certainly plays a role. Stokbro et al. 23 showed that the nickel-graphene contact resistance is independent of the orientation of graphene and of the contact area, in agreement with experimental observations, but the actual resistance calculation is roughly approximated. Liu et al. 5 further analysed the impact of molecular orbitals involved in the contact on trans- mission, fnding that the conductance of the metal-graphene-metal junction is afected not only by the interfa- cial binding, but also by which molecular orbitals are involved and their symmetry, and that contact resistance decreases with the increase of the contact area at low bias voltage. 1 Dipartimento di Ingegneria dell’Informazione, Università di Pisa Via G. Caruso 16, 56122, Pisa, Italy. 2 University of Siegen, Hölderlinstrasse 3, 57076, Siegen, Germany. 3 RWTH Aachen University, Chair for Electronic Devices, Aachen, Germany. 4 CNR-ICCOM, Istituto di Chimica dei Composti Organometallici, Via G. Moruzzi 1, 56124, Pisa, Italy. Correspondence and requests for materials should be addressed to G.I. (email: g.iannaccone@unipi.it) Received: 23 February 2017 Accepted: 5 May 2017 Published: xx xx xxxx OPEN