ITHACA, N.Y. Cornell University researchers are now able to observe atomic bonds by combining scanning tunneling microscopes (STMs) with vibrational spectroscopy.
Beginning in the 1980s as a new observational technique, STMs have become an important research tool for nanoscale devices and a means of individually moving atoms to create very small structures.
"Other labs mostly use STM for imaging or to break up molecules and study them. But we are making molecules bond together and studying them with vibrational spectroscopy, something that is much more difficult but potentially much more rewarding," said Wilson Ho, who invented the technique a year ago.
Vibrational spectroscopy has allowed the Cornell team to study how chemical bonds form by cataloging the individual bond's vibrational signatures and comparing them to those of newly bonded molecules. By modulating the frequency of the electrical field at the STM tip, it is possible to identify the bond directly below the tip with increased conductance-each atomic type responds with a current surge only at a "signature" frequency. Falsely colored STM images, after analysis by vibrational spectroscopy, reveal answers to age-old questions about how chemical bonds are oriented.
"Our technique supplies answers to questions about how to mimic living organisms. For instance, researchers have wondered how carbon monoxide bonds to iron in hemoglobin," said Ho. "We have shown for the first time that it is at an angle, even when there is only a single atom."
Medical concerns over the iron/carbon-monoxide bond stem from hemoglobin's role as a carrier for oxygen in the blood. When the bond usually used to transport oxygen is used to transport carbon monoxide (CO), suffocation results before the victim even feels distress. The reason for the preference for bonding to carbon-monoxide has remained a mystery.
Though the medical implications have yet to be sorted out, Ho has discovered some details about the critical FeCO bonds. When two CO atoms bond to an iron atom, they form a rabbit-ears configuration, where each CO molecule sticks out at about a 45 angle from the Fe atom, forming a 90 angle between the two COs. But even when only one CO molecule bonds to Fe, it does so at an angle-like rabbit ears with one ear broken off.
"Such a detailed understanding of how atoms go together to form bonds was not possible before the STM and vibrational spectroscopy," said Ho. Encouraged by that discovery, the researchers continue to study CO bonding with other metals to determine the uniqueness of iron bonding.
Ho said that he has just discovered how CO and copper bond. Unlike iron, copper forms a stable linear bond with CO. Instead of sticking out at an odd angle that can easily break off, it lines up straight with the end of the iron atom.
"Maybe the angle that iron forms made it the logical choice for hemoglobin instead of copper; it's not for me to say," Ho said. "But any researcher trying to mimic nature will benefit from a more detailed understanding of how the chemical bonds involved are formed."
Scanning probe microscopes have resulted from the ability to control sample positions within nanometer tolerances and from microfabrication techniques that allow a probe tip to be sharpened to similar dimensions.
Ho's main tool, the STM, has a tip so sharp that it narrows to a single atom and is held just 6 angstroms above the sample to be studied. A voltage potential is then induced and the tip is raised and lowered adaptively as it scans the sample, maintaining a constant current.
The tip is so narrow that only single electrons can "tunnel" across the gap, making the distance proportional to the shape of the sample. A computer program converts the pits and valleys into the topography of the sample atom and converts it into a falsely colored photograph.
For this project, Ho began with atoms of iron and molecules of carbon monoxide on a silver surface in a vacuum cooled to 13 Kelvin (--260 Celsius or 13 degrees above absolute zero).
Ho took advantage of the STMs' ability to "stick" individual atoms to its tip and set them atop other selected atoms, bypassing the cumbersome chemical mechanisms for making elements bond together. Current flow into the instrument picks up a molecule, and a reversed current flow into the sample releases it.
Ho also recently discovered a way to dramatically increase the resolution of the STM to see inside the bonds between atoms. The technique first attaches a tiny CO molecule to the tip of the STM, and then leaves it in place while scanning a sample.
The tiny tip formed by the CO molecule permits the STM to see the lattice of silver atoms on the working surface and the bonding sites for Fe and CO molecules.
Specific molecular bonds are formed where the molecule is dropped by the STM. But typically, the molecule slides to either side of the host atom at a spontaneous, repeated angle. After putting the STM in its vibrational spectroscopy mode, it is then possible to verify that the chemical bond truly formed as planned. The procedure then analyzes the bonded molecule to produce a photograph of its orientation.
Here, Ho created an iron carbonyl Fe (CO) molecule, but theoretically, any set of "designer" molecules could be created from simple ingredients. For instance, the technique can be used to create exotic semiconductor materials that cannot be created by any other known means today. Unfortunately, only minute quantities of any substance can be formed. The approach is suited only for research, unless some sort of self-reproduction can be achieved.
"There are many uses of our techniques for building up nanoscale structures, even self-replicating ones, but for now my lab is more interested in charting the known universe of chemical bonds," said Ho.
In the future, Ho expects to bond other metals and clusters of metals not only to CO, but also to hydrogen, which could be important for the petroleum industry. "There is a lot of interest in how hydrocarbon bonds are formed there are so many surfaces covered with them from burning fossil fuels," said Ho.
For the electronics industry, Ho is also targeting magnetic molecules for the nanoscale magnetic memories of the future. By coming to understand the specific orientation of bonds formed by magnetic molecules used in memory media, he hopes to find the "holy grail" of magnetic memories one bit per molecule.
The technology behind scanning probe microscopes, the microscopic cantilevers and tips used to sense physical features, are also being proposed as a route to a new type of memory device. By building arrays of the tips using micromachine technology, it becomes possible to use magnetic, or other physical properties such as surface pits to represent bits. The arrays can then scan over a surface encoding nanometer scale data sets to read data in parallel. The array methods might also result in practical methods for nanoscale device fabrication.