IBM opens new possibilities in nanoelectronics with measurement of
single atom charge
16 June 2009
IBM (NYSE:IBM) scientists in collaboration with the University of
Regensburg, Germany, and Utrecht University, Netherlands, for the first
time demonstrated the ability to measure the charge state of individual
atoms using noncontact atomic force microscopy.
Measuring with the precision of a single electron charge and
nanometer lateral resolution, researchers succeeded in distinguishing
neutral atoms from positively or negatively charged ones. This
represents a milestone in nanoscale science and opens up new
possibilities in the exploration of nanoscale structures and devices at
the ultimate atomic and molecular limits. These results hold potential
to impact a variety of fields such as molecular electronics, catalysis
As reported in the June 12 issue of the journal Science ,
Leo Gross, Fabian Mohn and Gerhard Meyer of IBM's Zurich Research
Laboratory in collaboration with colleagues at the University of
Regensburg and Utrecht University imaged and identified differently
charged individual gold and silver atoms by measuring the tiny
differences in the forces between the tip of an atomic force microscope
and a charged or uncharged atom located in close proximity below it.
To conduct these experiments, researchers used a combined scanning
tunneling microscope (STM) and atomic force microscope (AFM) operated in
vacuum at very low temperature (5 Kelvin) to achieve the high stability
necessary for these measurements.
The AFM in principle uses a sharp tip to measure the attractive
forces between the tip and the atoms on a substrate. In the setup of the
present work, the AFM uses a qPlus force sensor consisting of a tip
mounted on one prong of a tuning fork, the other prong being fixed.
The tuning fork, which is like those found in ordinary wristwatches,
is actuated mechanically and oscillates with amplitudes as small as 0.02
nanometer — which is about one-tenth of an atom's diameter.
As the AFM tip approaches the sample, the resonance frequency of the
tuning fork is shifted due to the forces acting between sample and tip.
By scanning the tip over a surface and measuring the differences in the
frequency shift, a precise force map of the surface can be derived.
The extremely stable measurement conditions were crucial for sensing
the minute differences in the force caused by the charge state switching
of single atoms. The difference between the force of a neutral gold atom
and that of a gold atom charged with an additional electron, for
example, was found to be only about 11 piconewton, measured at the
minimum distance to the tip of about half a nanometer above the atom.
The measurement accuracy of these experiments is better than 1
piconewton — which is equal to the gravitational force that two adults
exert on each other over a distance of more than half a kilometer.
Moreover, by measuring the variation of the force with the voltage
applied between tip and sample, the scientists were able to distinguish
positively from negatively charged single atoms.
This breakthrough is yet another crucial advance in the field of
atomic-scale science. In contrast to the STM, which can be used only on
conducting materials, the AFM is independent of conductivity and can be
used for investigating materials of all kinds, most importantly
In the field of molecular electronics, which aims at using molecules
as functional building blocks for future computing devices, as well as
for single-electron devices, an insulating substrate is needed in order
to avoid the leakage of electrons. This makes noncontact atomic force
microscopy the investigation method of choice.
"The AFM with single-electron-charge sensitivity is a powerful tool
to explore the charge transfer in molecule complexes, providing us with
crucial insights and new physics to what might one day lead to
revolutionary computing devices and concepts," explains Gerhard Meyer,
who leads the STM and AFM-related research efforts at IBM's Zurich
To study the charge transfer in molecule complexes, scientists
envision that, in future experiments, single atoms could be connected
with molecules to form metal-molecular networks (see figure 1 below).
Using the tip for charging these atoms, scientists could then inject
electrons into the system and measure their distribution directly with
the non-contact AFM .
|Figure 1. Diagram of an
AFM manipulating atoms (in orange). Understanding the charge
distribution in molecules and molecular networks is a crucial
step in the exploration of future computing elements on the
IBM researcher Leo Gross points out other areas of impact beyond
nanoscale computing: "The charge state and charge distribution are
critical in catalysis and photoconversion. Mapping the charge
distribution on the atomic scale might deliver insight into fundamental
processes in these fields."
This achievement follows a string of remarkable scientific advances
achieved by IBM scientists in recent years and represents a fundamental
step towards building computing elements at the molecular scale —
computing elements that are expected to be vastly smaller, faster and
more energy-efficient than today's processors and memory devices.
Using the qPlus AFM, a team at the IBM Almaden Research Center was
the first to measure in 2008 the force necessary to move an atom over a
surface, paving the way for the present experiment.
In 2007, Gerhard Meyer's team at IBM's Zurich Lab demonstrated a
single-molecule switch that can operate flawlessly without disrupting
the molecule's outer frame or shape.
In 2004, the same group controllably manipulated the charge state of
individual atoms using an STM. By inducing voltage pulses through the
STM tip, they succeeded in charging an individual atom on a thin
insulating film with an additional electron.
Importantly, the negatively charged atom remained stable until a
voltage pulse with the opposite bias was applied via the STM tip. This
is the method used by scientists in the present experiments to charge
the individual atoms.
1. L Gross, F Mohn, P Liljeroth, J Repp, FJ Giessibl, and G Meyer.
Measuring the Charge State of an Adatom with Noncontact Atomic Force
Microscopy. Science, Volume 324, Issue 5933, pp. 1428 - 1431
(12 June 2009).
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