Mapping Image Potential States on Graphene Quantum Dots

Fabian Craes, Sven Runte, Jürgen Klinkhammer, Marko Kralj, Thomas Michely, i Carsten Busse

Physical Review Letters 111, 056804 (2013)

Electrons trapped in image potential states (IPS’s) at surfaces are an ideal example for the free electron in a two-dimensional electron gas. Moreover, two-dimensional nanostructures offer a possibility for lateral localization and modification of these electrons, depending on the shape and size of the nanostructure. However, experimental realization of such system is very demanding and burdened by the issues of structural stability, depending on temperature and interactions that are present between the nanostructure and scanning probe tips which measures the local properties.

Graphene quantum dots on iridium explored in this work, turn to be very stable and resistant against intrinsic experimental problems described above. Moreover, the size of synthesized quantum dots can be varied in a broad range, from few tens of nm2 and up. Finally, by intercalation, i.e. by chemical doping, it is easy to modify the energies of IPS’s.

By the application of scanning tunneling microscopy and spectroscopy (STM and STS) it was possible to visualize IPS wave function localization above the quantum dots, and to determine spectroscopically their energy precisely. By such an approach, the quantity of information otherwise accessible by other spectroscopic techniques such as two-photon photoemission, is significantly refined.

In the STM experiment, for the first time the dependence of lateral localization was clearly related to the size of quantum dots, as well as on the parameters of the localization potential through modification of the quantum dot work function realized by oxygen intercalation. Figure 1 shows a set of graphene quantum dots of different sizes (Fig. 1a), which, depending on their size have specific spectroscopic signature (Fig. 1b). The pronounced peak splitting in spectra from smallest islands is a sign of lateral localization of IPS, which is visualized in Fig. 2 showing dI/dV STM maps.

Figure 2.

Figure 1.