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.

石墨烯与分子的结合提供了实现新功能的有希望的机会。在这些混合结构中，界面电荷转移在电子性能中起关键作用，因此必须被理解和掌握。使用扫描隧道显微镜和从头算密度函数理论计算，我们显示出将石墨烯的氮掺杂与电场结合可以选择性控制石墨烯分子层中的电荷状态。在原始石墨烯上，尖端施加的局部门控会引起吸附分子的分子水平移动，并可用于控制其电荷状态。从头算计算表明，在电场的作用下，杂化分子/石墨烯系统的行为就像静电偶极子，分子和石墨烯亚单元中的电荷相反，而电荷与电场幅度成正比，从而控制电荷转移。当局部门控与石墨烯的氮掺杂相结合时，氮上分子的充电电压会大大降低。因此，施加适当的电场可使人们获得具有混合电荷状态的分子层，其中在氮位点对单个分子进行选择性还原。