Overview
The nature and control of individual metal atoms on insulators are of great importance in emerging atomic-scale technologies. Individual gold atoms on an ultrathin insulating sodium chloride film supported by a copper surface exhibit two different charge states. By applying a voltage pulse within a scanning tunneling microscope (STM), the individual gold atoms can be reversibly switched between a neutral and a negatively charged state [1].
Gold atoms adsorbed onto a sodium chloride film consisting of only two atomic layers in thickness are imaged as large protrusions in the STM (see right atom in the STM image). Applying a positive voltage larger than 0.6 V to the sample, while the tip is positioned directly above a gold atom, causes the transformation of this atom into a negative ion. Most importantly, both states are stable, that is, the additional charge remains on the negative ion until it is removed by a voltage pulse of reversed sign. In the STM image, the negative gold ion appears as a smaller protrusion with a circular trough around it.
The experimental findings are interpreted with the help of first-principles density functional theory calculations. In agreement with the experiments, the theoretical investigation also finds two different stable states for Au atoms: One is nearly neutral, the other is negatively charged by one electron. Simulated STM images are in very good agreement with the experimental ones, corroborating the assignment of the two states. Moreover, the calculations reveal that the large ionic polarizability of the NaCl film is the reason for the stability of the two different charge states. An additional electron on the gold atom forces the Cl- ion underneath the gold to move downward, whereas the surrounding Na+ ions move upward. This relaxation pattern creates an attractive potential for the additional charge on the Au adatom.
The electrical charge of an atom determines the way the atom reacts with the rest of the world. Therefore, the control of the charge state of gold atoms is associated with the control of other physical and chemical properties as well. This is documented in diffusion experiments, in which the diffusion for the negative gold ion sets in at lower temperatures than for the neutral gold atom. Moreover, in the neutral state the 6s electron is unpaired, resulting in a net (spin) paramagnetic moment, whereas in the charged state it is paired and the adatom is nonmagnetic.
Whereas single Au atoms show only charge bistability in the case of Ag atoms, even charge tristability could be observed. Here neutral as well as Ag anion and Ag cations can be formed [2].
The charge state can also be determined using non-contact atomic force microscopy (AFM). AFM reveals a higher attractive force above a negatively charged Au atom than it is abovce a neutral Au atom. Moreover, the local contact potential difference (LCPD) measured with Kelvin probe force microscopy (KPFM) allows the discrimination of positively charged, neutral, and negatively charged atoms [3].
References
Images
Figure 1. Scanning tunneling microscope
(STM) false-color image of two gold atoms on an insulating NaCl
film surface. The atom on the left-hand side has been intentionally
transferred from its neutral state into a negatively charged
ion by means of STM manipulation.
Figure 2. STM image of two gold atoms
on an insulating NaCl film surface. The atom on the left-hand
side has been intentionally transferred from its neutral state
into a negatively charged ion by means of STM manipulation.
The change in charge state is revealed by the dark trough around
the gold atom.
