Intentionally created localized bridges for electron transport through graphene monolayer between two metals

Abstract

Monolayer graphene (1LG) is frequently unpredictably modified by supporting material so that it limits development of devices. Van der Waals interaction is dominant in the models describing the in-plane processes, including the electrical charge transport. However, the current flow perpendicular to the plane of the graphene is still less understood. This report analysed specific aspect of the perpendicular current and disclosed an original way to create transport bridges perpendicular to the plane across the 1LG. The most extraordinary finding is that the electron transport between two parallel metal surfaces can be shut down and opened if the metals are separated by the 1LG. The electron transmission can be intentionally varied in this metal – 1LG – metal (M-G-M) system by pressure. In the experimental study the AFM force curve and tunnelling current measurements were combined when the external load force (0 – 1200 nN) and electrical potential (-1.5 V – +1.5 V) were used. It is proved that for low voltages (< ±9 mV) a bridge is opened perpendicular to the graphene across the M-G-M systems by the external force, if the compression dependent Fermi level crosses electronic states in the interfaces and graphene. The localised bridges with diameter about 10 – 40 nm can be opened and kept continuously by the stabilised force in separated points of the system. However, the predictable changes can be produced in the system if the voltage and the force exceeded critical magnitudes. A combined model was proposed acceptable to explain the bridging and predictably modify the characteristics.

Relationship between changes in interface characteristics and external voltage under compressing force in metal–graphene–metal stacks

“…There was also another model limitation on the energy interval defined by ±1 eV from the Dirac point, within which the Fermi level shift can be digitally analyzed assuming linear dependence of the density of the electronic states. This restriction was acceptable here in the force model analysis because the compression force produced a lower shift in the Fermi level as it was proved for the flat construction of the M-G-M system [22]. The Fermi level shift ∆E F-eq-MGM important for the present study is illustrated in figure 2(a the model from previous work and including V app in the relationship:…”

“…More details on the methodology of the investigation of the mechanical and electrical properties in an M-G-M vertical structure were presented in a previous work [22].…”

“…The transfer method was described in detail in a previous study [24]. The substrate with the graphene was annealed in Ar atmosphere at 300 • C for 124 min before the investigation [22]. In this work, the complete samples consisted of the SiO 2 /Si substrate, supporting Au film, a graphene layer and a conductive probe as the top electrode.…”

“…It follows from equations ( 2) and ( 4) that the force terms F C and F tQ are related to the parameters d MG1 , d MG2 and N gQ that are the characteristics of the interface properties in the M-G-M system. These parameters can be calculated for the M-G-M system in the equilibrium state by the model proposed initially in earlier work and successively adapted to M-G-M systems in a recent work [16,22]. The phenomenological version of the model was used to perform a quantitative comparison between the model and the experiments.…”

“…Aiming to explain the highly specific results, the layout of the atoms in the adjacent layers was suggested as the primary cause for the variations in the interlayer distances [21]. The extraordinary relationship between the applied compressing force and the electrical current was explained by the Fermi level dependence with the force leading to specific changes in the tunneling current across the interface barrier [22]. The changes in the electrical charge distribution in such M-G-M systems change the electrostatic forces, which are part of an experimentally measured adhesion force [23].…”