The first demonstration of catalysis on a nanometer scale has been reported by scientists at LBL's new Molecular Design Institute. Using an Atomic Force Microscope, modified to function like an ultrafine-point pen for catalytic calligraphy, the scientists were able to create a reaction that changed the chemical composition of the surface of a material one molecule at a time.
This demonstration of molecular synthesis on a nanometer scale represents a promising step towards the development of nano-fabrication. According to a growing legion of experts, the future belongs to a new technology in which constructs are measured in terms of nanometers or billionths of a meter. From gigabit memory chips stuffed into hand-held supercomputers, to molecular-sized machines that can repair damaged cells inside the human body, the world will enter the nanotechnology age once scientists master a technique for fabricating devices with complex nano-sized features.
The research team that carried out the proof-of-principle experiment was led by Peter Schultz, a chemist with LBL's Materials Sciences Division. Other MSD team members were physicists Paul McEuen, David Klein, John Clarke, and Thomas Lee, and chemist Wolfgang Muller. Schultz and Muller are also members of UCB's Chemistry Department. The other team members are affiliated with UCB's physics department. The team published its results in a recent issue of the journal Science.
The key to the success of this experiment was the combination of atomic force microscopy with a technique from organic chemistry called molecular self-assembly.
"Atomic force microscopes have become important analytic tools in materials science and have been used to directly modify surfaces," Schultz says. "However, this application has been limited by the complexity of the structures that can be fabricated. Discussions with the physicists convinced us that the potential for constructing novel nanostructures would be enhanced significantly if the resolution of the AFM could be combined with the wide array of catalytic transformations available in chemistry."
For their experiment, the researchers first created a self-assembled monolayer (SAM) of alkylazide molecules, organic molecules that are capped with a crown of three nitrogen atoms collectively known as an azide. Next they deposited chromium onto the silicon tip of an AFM to make it adhesive and coated it with a layer of platinum.
The tip of an AFM converges to a point only a few atoms wide. When an AFM tip is touched to a sample, forces (such as attraction and repulsion) interact between atoms in the tip and on the surface of the sample. These forces can either be recorded and translated into three-dimensional images of the sample's atoms and molecules, or used to reposition those atoms and molecules.
In the LBL-UCB experiment, coating the AFM tip with platinum transformed it into an instrument for nanoscale catalysis. Samples of the alkylazide SAM were soaked with a hydrogen-containing solvent, then scanned with the AFM over an area measuring 10 by 10 microns (a micron is a millionth of a meter). The idea was for the platinum-coating on the tip to catalyze a reaction in which hydrogen atoms would be added to the azides to transform them into amines (a molecule of one nitrogen and two hydrogen atoms).
To prove that their attempts at catalysis had been successful, the researchers added a fluorescent tag that binds to amines but not to azides. Under fluorescent microscopy, the scanned area became a glowing green square exactly where the tip had passed, demonstrating that a catalytic-tipped AFM can be used to precisely control where on a surface a chemical reaction occurs.
With the right choice of reactants and catalysts, and adding on other molecules in the same manner that they added the fluorescent tags, Schultz, McEuen and the other team members believe they could assemble a wide variety of complex nanostructures.
"Given the large number of heterogeneous catalytic reactions involving the transition metal catalysts, our approach may provide a general strategy for performing chemistry on a nanometer scale," Schultz says.
Even if the use of a catalytic AFM proves too slow to be a useful manufacturing process, Schultz says it should still be a valuable research tool for investigating different aspects of the catalytic process; for example, helping scientists to identify how long a given catalyst must be in contact with specific reactants to produce a desired reaction.