Scanning Quantum Dot Microscopy (SQDM)

Our single-molecule manipulation experiments have led us to the discovery of a new scanning probe technique that is able to image nanoscale variations of the electrostatic potential of surfaces with very high sensitivity and spatial resolution.

Scanning Quantum Dot Microscopy (SQDM) utilises the fact that a nanometer-sized molecule fixed with atomic precision to the tip apex of a low-temperature non-contact atomic force microscope (NC-AFM) [1, 2] acts as a quantum dot. Tunnelling of single electrons in and out of the quantum dot occurs when a sufficient electric bias voltage V is applied to the NC-AFM tip. The tunnelling changes the charge state of one of the molecular orbitals of the quantum dot between -e, 0 and +e, where e is the elementary charge [3]. The charging leads to abrupt jumps in the attractive electrostatic force acting from the surface on the tip. A sensitive mechanical resonator which continuously oscillates the quantum dot probe with sub-Ångstrom amplitudes responds to the abruptly changing force by sharp spikes in its oscillation frequency. Scanning the quantum dot probe at a constant height over the surface and adjusting V such that a particular frequency spike (0 → +e or 0 → -e) occurs, SQDM records maps of corresponding voltages V+ and V-, which finally yield the distribution of the electrostatic potential Φ over the scanned surface [4, 5]. We have developed a fast controller which tracks the spikes while scanning the surface, making SQDM an efficient und easy-to-use scanning probe technique [6].