Printer Friendly

Imaging technique reveals hidden atoms.

One of today's celebrity scientific instruments, the atomic force microscope (AFM), is valued despite some quirks. Famous for rendering atoms visible, it can also be blind.

That shortfall has been particularly glaring when it comes to graphite. AFM images reveal only three of the six carbon atoms in each of the material's basic hexagonal units. In an upcoming Proceedings of the National Academy of Sciences, a team of German physicists describes how it solved that problem. The advance may lead to techniques to image biological materials, the physicists say.

In graphite, the hexagonal units fuse into sheets resembling miniaturized chicken wire. Loose connections between these sheets make graphite soft; it's these sheets that a pencil leaves behind on paper. When intact, the sheets stack such that every other carbon in each ring rests directly above a carbon in the sheet below. These are known as alpha atoms. The other carbons, called beta atoms, have nothing directly underneath.

When the AFM's cantilever tip passes over the graphite, it gently tugs on each carbon atom but can detect the attractive forces only between the tip and the beta atoms. That's because electrons in the alpha atoms overlap with those of the atoms below, restricting interactions between the electrons and the AFM tip. In contrast, the less-fettered electrons of the beta atoms show up in AFM images.

Jochen Mannhart and his colleagues at the University of Augsburg in Germany modified their AFM to measure repulsive forces instead of attractive ones. The tip pushes down on each atom "like an atomic braille system," explains Yip-Wah Chung at Northwestern University in Evanston, Ill.

The researchers needed to make sure the AFM tip wasn't pushing down on the graphite surface too forcefully. "Otherwise, the carbon [atom] will disappear inside the material" says Mannhart.

As the tip approaches a carbon atom, the electron clouds and the tip repel each other, changing the cantilever's vibration frequency. In this mode, both alpha and beta atoms become visible.

The procedure is slow. To prevent subtle motions in the sample and instrument that would blur the images, the measurements must be carried out at just a few degrees above absolute zero.

Other types of microscopes can image hard, electrically conducting materials with atomic resolution, but soft, nonconducting materials such as graphite and biological molecules have been difficult to image.

To probe DNA and proteins, says North-westerns Mark Hersam, the German technique needs to be modified so that it works at much warmer temperatures that preserve the samples' biologically relevant structures.
COPYRIGHT 2003 Science Service, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2003, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:atomic force microscope; A Soft Touch
Author:Goho, A.
Publication:Science News
Geographic Code:4EUGE
Date:Sep 27, 2003
Words:419
Previous Article:Poodle DNA compared with that of mice, people.
Next Article:Lake yields core of pre-Inca silver making.
Topics:


Related Articles
Surface maps of organic molecules.
High-tech microscope makes molecules move.
Getting a feel for atoms: 'magic wrist' takes scientists into a new sensory realm.
Microscope's misleading tips.
Using light to guide atomic deposition.
Teeny-weeny transistors.
Atom tinkerer's paradise: innovations to atom-imaging microscopes create labs on tips.
Calibration of High-Resolution X-Ray Tomography With Atomic Force Microscopy.
Images of tiny ion may help battery designers. (Lithium Sees the Light).
Microscope goes mini.

Terms of use | Privacy policy | Copyright © 2020 Farlex, Inc. | Feedback | For webmasters