Centre d’Élaboration de Matériaux et d’Etudes Structurales

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Metallic nano-islands on bulk insulators

Flat metallic islands on an insulating substrate can be used as electrons reservoir to contact a molecule or a graphene nanoribbon in a planar geometry for molecular applications. The challenge is then to control and to stabilize the charge on a metallic nanocrystal for a time long enough to perform in-plane operations. This challenge was take up using the tip of a NC-AFM microscope to control the charging of 2D Au nano-islands synthesized ex-situ and deposited on a SiO2 insulating substrate. We image the platelets in the nc-AFM mode [1] and characterize their charge state by Kelvin Probe Force Microscopy (KPFM) (see the figure) [2,3]. Our results demonstrate that the charge of a metallic island on an insulating layer can be controlled by electron field emission to or from the tip of a nc-AFM by monitoring ∆f(V) spectroscopy curves, as shown in figure (c). The procedure works for both polarities, electrons being emitted by the tip to the nano-island or reversely. As shown by an analytical model and complementary numerical simulations, the rise of the island’s potential upon charging leads to a constant charging current and tip-island electric field [4]. Our measurements suggest that this method can be used to set the island potential with 10 mV precision, corresponding to the transfer of a single electron. This degree of control is achieved thanks to the increased stability and sensitivity provided by the UHV environment. The procedure is robust and opens the way to original experiments, such as establishing a bias at the extremities of a molecule connected between two islands or exploring locally the charge leaking mechanisms across an insulating layer.