Scanning Quantum Dot Microscopy

Abstract

We introduce a scanning probe technique that enables three-dimensional imaging of local electrostatic potential fields with subnanometer resolution. Registering single electron charging events of a molecular quantum dot attached to the tip of an atomic force microscope operated at 5 K, equipped with a qPlus tuning fork, we image the quadrupole field of a single molecule. To demonstrate quantitative measurements, we investigate the dipole field of a single metal adatom adsorbed on a metal surface. We show that because of its high sensitivity the technique can probe electrostatic potentials at large distances from their sources, which should allow for the imaging of samples with increased surface roughness.

Forces from periodic charging of adsorbed molecules

“…We just note that two or more charging levels have been observed for molecules before. 65,67 Irrespective of the nature of the two levels that are being shifted across the Fermi level, we can investigate whether the observed distance dependence fits to a simple model according to the picture behind the charging. As the fraction of the total bias voltage that drops between molecule and substrate is relatively small, the electronic level responsible for charging has to be close to the Fermi level also without any field in the junction.…”

“…This might be particularly interesting, if the charging occurs at the tip instead of the sample side as was reported recently. 67 Next we reconsider the scaling of ∆E with bias voltage and distance. Irrespective of the shape of the junction and the detailed spatial dependence of the potential, it is reasonable to assume that the change in potential ∆V ( r) scales linearly with V − V CPD 79 in good approximation.…”

“…This enables the use of a suitably functionalized tip as a sensitive probe to map out the potential in front of the surface. 67 After introducing the methods in Sec. II, previous results 66 will be recapped in Section III.…”

Local tunneling decay length and Kelvin probe force spectroscopy

“…Kelvin probe force spectroscopy (KPFS) [21] is an established technique to reliably extract variations of the contact potential on the very local scale [22][23][24][25][26][27][28][29][30]-at least as long as the tip is far enough from the sample and not entering the regime of Pauli repulsion [31,32]. Further improvements of the interpretation and data acquisition of KPFS and related techniques are active fields of research, progressing fast at the moment [33][34][35][36][37]. KPFS is based on atomic force microscopy, where the minimization of the electrostatic interaction as a function of bias voltage yields the voltage of compensated contact potential difference between the tip and the sample.…”

Electric-Field Sensing with a Scanning Fiber-Coupled Quantum Dot

“…In 2015, Wagner et al used a noncontact atomic force and scanning tunneling microscope functionalized with a single molecule to image the dipole field of an adatom on a surface [10]. This first demonstration of scanning quantumdot microscopy (SQDM) registered single-electron charging events of a molecular quantum dot (QD) to produce three-dimensional images of the local electrostatic potential with subnanometer resolution.…”