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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
J. Repp, G. Meyer, F.E. Olsson, and M. Persson, Science 305, 493
(2004).
F. E. Olsson, S. Paavilainen, M. Persson, J. Repp , G. Meyer ,
Phys. Rev. Lett. 98, 176803 (2007).
L. Gross, F. Mohn, P. Liljeroth, J. Repp, F. J. Giessibl, G. Meyer, Science 324, 1428 (2009). Abstract | Full text
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IBM Research
