IBM scientists directly measure charge states of atoms

Tony Austin
Monday, 15 June 2009 07:38

Opinion and Analysis

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 insulators.

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,” explained Gerhard Meyer, who leads the STM and AFM-related research efforts at IBM’s Zurich Research Laboratory.

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. 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 (see figure 2).

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.