Selective control of molecule charge state on graphene using tip-induced electric field and nitrogen doping

Abstract

The combination of graphene with molecules offers promising opportunities to achieve new functionalities. In these hybrid structures, interfacial charge transfer plays a key role in the electronic properties and thus has to be understood and mastered. Using scanning tunneling microscopy and ab initio density functional theory calculations, we show that combining nitrogen doping of graphene with an electric field allows for a selective control of the charge state in a molecular layer on graphene. On pristine graphene, the local gating applied by the tip induces a shift of the molecular levels of adsorbed molecules and can be used to control their charge state. Ab initio calculations show that under the application of an electric field, the hybrid molecule/graphene system behaves like an electrostatic dipole with opposite charges in the molecule and graphene sub-units that are found to be proportional to the electric field amplitude, which thereby controls the charge transfer. When local gating is combined with nitrogen doping of graphene, the charging voltage of molecules on nitrogen is greatly lowered. Consequently, applying the proper electric field allows one to obtain a molecular layer with a mixed charge state, where a selective reduction is performed on single molecules at nitrogen sites.

Monolayers of MoS2 on Ag(111) as decoupling layers for organic molecules: resolution of electronic and vibronic states of TCNQ

“…This structure is stabilized by dipole-dipole interactions between the cyano endgroups and the quinone center of neighboring molecules. This assembly is very similar to typical self-assembled TCNQ islands on weakly interacting substrates [5,23,[47][48][49]. When measured at a lower bias voltage (e.g., at V = 0.2 V in Figure 4a), the molecules appear with featureless elliptical shape, reflecting only the topographic extent of the molecules.…”

“…For the identification of molecular orbitals, it is often sufficient to compare the d I /d V maps with the shape of the gas-phase molecular orbitals. Using this method, the U-shaped features have previously been associated to the LUMO of TCNQ [ 5 , 23 , 49 ]. Here, we corroborate this assignment by simulating constant-height d I /d V maps of a free, flat-lying molecule.…”

“…For the identification of molecular orbitals, it is often sufficient to compare the dI/dV maps with the shape of the gas-phase molecular orbitals. Using this method, the U-shaped features have previously been associated to the LUMO of TCNQ [5,23,49].…”

Electronic properties of chemically doped graphene

“…DFT calculations revealed that a local charge transfer occurs at the nitrogen site, so that the molecule gains 0.33 electron, as compared to 0.20 electron at carbon sites ( Fig. 10c and d) [74]. A combination of this effect with local gating allows to obtain a molecular layer with a mixed charge state.…”

“…This effect has been exploited to perform catalysis, or electrochemical sensing with nitrogen-doped graphene [2,71]. The electronic properties of molecules interacting with nitrogen sites has been investigated with scanning probe microscopy [61,[72][73][74]. STM experiments on free base tetraphenylporphyrin molecules (H 2 TPP) have shown that at nitrogen sites, the molecules undergo a downshift of their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states, evidencing a local charge transfer at these sites ( Fig.…”