Atom-Moving Force Measured

The force it takes to move individual atoms on a surface has been measured for the first time, an achievement that could mean future nanoelectronic devices for information technology, medicine and data storage will contain integrated circuits thousands of times smaller than those made today.

Atom manipulation, first demonstrated 20 years ago, is used widely in research today to build, probe and manipulate objects at the scale of individual atoms. However, an answer to the fundamental question -- "How much force does it take to move an atom on a surface?" -- has eluded scientists until now.

An illustration of an atomic force microscope's (AFM) oscillating tip measuring the force it takes to move a cobalt atom on a crystalline surface. The ability to measure the exact force it takes to move individual atoms is one of the keys to designing and constructing the small structures that will enable future nanotechnologies. (Images courtesy IBM)

In answering the question, scientists from IBM’s Almaden Research Center in San Jose and their collaborators at the University of Regensburg in Germany have gained access to important information for designing future atomic-scale devices.

Understanding the force necessary to move specific atoms on specific surfaces is one of the keys to designing and constructing next-generation nanotechnology structures, and is relatable to building vastly larger ones. Just as civil engineers need to know the strength of individual materials before they use them to build a bridge, nanotechnology engineers need to ensure that they use the right atoms for the right job, such as using strongly bonded ("sticky") atoms for structures that need to remain rigidly in place or weak chemical bonds for groups of atoms that need to be able to move.

Forces acting on the AFM tip are shown in this illustration of a cobalt atom on a copper surface. It shows the measured energy landscape when a cobalt atom is dragged over a copper surface. The arrows show the forces that are acting on the AFM tip as it manipulates the molecules.

"This result provides fundamental information about atomic-scale fabrication and could pave the way for new data storage and memory devices," said Andreas Heinrich, lead scientist in the scanning tunneling microscopy lab at the Almaden Research Center.

In the paper "The Force Needed to Move an Atom on a Surface," published in today's issue of Science, the scientists show that the force required to move a cobalt atom over a smooth platinum surface is 210 piconewtons, while moving a cobalt atom over a copper surface takes only 17 piconewtons. To put this in perspective, the force required to lift a copper penny that weighs just three grams is nearly 30 billion piconewtons -- 2 billion times greater than the force to move a single cobalt atom over a copper surface. (A video of the research can be seen here)

The heart of the specialized AFM used by IBM researchers to measure the force needed to move individual atoms.

This knowledge will enable a deeper understanding of the atomic-scale processes at the heart of future nanotechnology endeavors, furthering progress toward ever-smaller computing and medical devices.

More than 40 years ago, Intel co-founder Gordon Moore predicted that the number of transistors on a computer chip would double about every two years, an observation known as Moore's Law. Shrinking the transistors allows integrated circuits to use less power while operating faster and more cheaply. One of the industry's most pressing challenges is finding designs and manufacturing methods that will allow that process to continue.

Miniaturizing these devices to the ultimate limit -- the scale of just a few atoms -- requires radically new designs and manufacturing methods. The ability to measure the force it takes to move an atom provides a new window into the workings of atom-by-atom construction and operation for future nanodevices.

In the paper, the researchers describe their use of a sensitive atomic force microscope (AFM) to measure both the strength and direction of the force exerted on an atom or molecule on a surface using a sharp metal tip to move the atom. The team discovered that the force varies dramatically depending on the material used for the surface. The amount of force also changes greatly when a small molecule is used instead of a single atom.

An close-up look at the miniature "tuning fork" inside the AFM. The tuning fork measures the interaction between the tip of the microscope and the atoms on a surface; when the tip is positioned close to an atom on the surface, the frequency of the tuning fork changes slightly. The frequency change can be analyzed to determine the force exerted on the atom.

The AFM uses a sharp tip mounted on a flexible beam -- akin to a tiny diving board -- to measure the interaction between the tip and the atoms on a surface. In the present work, the flexible beam was actually a miniature quartz tuning fork of the type commonly found in clocks and wrist watches. When the tip is positioned close to an atom on the surface, the frequency of the tuning fork changes slightly. The frequency change can be analyzed to determine the force exerted on the atom.

"It is amazing to see how this tool, which at its heart uses the tuning fork of an everyday wrist watch, can be used to measure forces between individual atoms," said professor Franz Giessibl of the University of Regensburg.

For more information, visit: www.research.ibm.com