Low Temperature, High Visibility
Physicists and chemists dream of studying progressively more complicated objects built atom-by-atom on a variety of surfaces. To continue developing this technology, they need to learn more about the forces between an atom and a probe tip. In the 20 March PRL a Swiss team reports the first direct measurements of those forces. By cooling their system with liquid helium, they held a microscopic force probe steady enough to measure the force of a single silicon atom bonding with the probe tip at several different distances. This force- vs.-distance data is the most fundamental information on the interaction between two atoms. The deep freeze also allowed the highest resolution images yet made with a force-based microscope, revealing atoms one layer below the surface of a silicon crystal.
Scanning tunneling microscopes (STMs) give beautiful images of atoms at the surfaces of metals and semiconductors based on the electron current between the probe tip and the orbital clouds of the atoms. The STM is excellent for studying electronic properties, but it gives only indirect information on the forces those electrons create in response to other atoms, including those in the probe. What’s more, an STM is useless on a nonconducting surface like sodium chloride or sapphire, which may one day serve as the field upon which metal atoms will be assembled into nanoscopic structures for physics and chemistry research. “You would like to play with the atoms,” says Hans Hug of the University of Basel in Switzerland, “and feel them” as you move them–that is, sense the forces.
A scanning force microscope (SFM)–also called an atomic force microscope (AFM)–measures the mechanical deflection of the probe as it’s moved across the surface, rather than measuring the current, so it works on any material. Researchers have achieved atomic resolution images with the SFM only within the past five years or so, using the so-called dynamic mode: vibrating the probe and measuring the change in its resonant frequency as surface atoms attract or repel the tip. Or alternatively, they keep the frequency shift constant by varying the probe-to-sample distance as it sweeps across the surface. But at room temperature slow drifts in position and random thermal motions of the tip make it too unstable to be reliably positioned at specific distances from a single atom.
By cooling their apparatus to 7 K, Hug and his colleagues could measure the attraction between a single silicon atom in a crystal and another atom at the tip of a silicon probe at five distances in the range of 5 to 6 Å. In this range, the force curve was steepest, even steeper than a theoretical prediction with which they compared their data. The team says the theory may not fully account for the atoms moving slightly toward one another as the probe approached the surface.
The combination of low temperature and unusually sharp probe tips allowed the Swiss team to make extremely clean images, showing the so-called rest atoms, which are one layer below the top-most atoms and visible in spaces between them. Previously only STM images captured these elusive atoms on film.
“The possibility of doing spectroscopy on an individual atomic site is wonderful,” says Franz Giessibl, of the University of Augsburg in Germany. He says the direct measure of the bond strength of a single atom is important for understanding the basic properties of materials–such as tensile strength–at an atomic level.