Engineers Provide Insight Into the Dynamics of Molecular Self-Assembly

July 5, 2007

By studying how a layer of molecules grows into an ordered layer from the edge of a rectangular silicon wafer, engineers at North Carolina State University, working with researchers from the National Institute of Standards and Technology (NIST), have established the time evolution of self-propagating self-assembly fronts. The team is the first to confirm the phenomenon in a real physical system.

The NC State researchers, Dr. Jan Genzer, professor of chemical and biomolecular engineering, and Dr. Kirill Efimenko, research assistant professor of chemical and biomolecular engineering, and NIST researchers, Dr. Jack Douglas, Dr. Daniel Fischer and Dr. Frederick Phelan, examined the spontaneous assembly of organosilane molecules into a monolayer film formed on an oxidized silicon surface.

They found that if a supply of the carbon-silicon-based molecule is placed along one edge of a treated silicon wafer, under controlled conditions, the organosilane molecules spontaneously organize themselves into a well-ordered layer, creating a carpet-like layer on the silicon that advances from the edge of the wafer at a constant velocity where the ordering initiates, ultimately covering the surface at long times. By following this process using a high resolution synchrotron X-ray technique and computer simulations, the NC State/NIST team established that the propagating wavefronts did not follow the constant width predicted by the classical mean-field theory that is widely believed to govern reaction-diffusion and self-assembly processes. (A wavefront is the leading edge of a wave or line of points that have the same phase or stage in a process.) What actually occurred is described as a “power-law broadening in time” when an autocatalyst is present.

A paper describing the research appears in the June 19, 2007, issue of the Proceedings of the National Academy of Sciences; the paper is also mentioned as a highlight on the journal cover.

“We found that simple diffusion is not going to explain our data; it will not govern the molecular processes involved in our experiment,” Genzer said. “So we began looking for the connection between diffusion and wavefront propagation, something the NIST team of theorists has been looking for some time.”

Propagating fronts are fueled by catalysts, which typically involve small chemical compounds that participate in the reaction and recover themselves after it. In the present case, the process may be governed by a very different type of autocatalyst, namely the system’s confinement. One of the benefits of the present set up is that the “broadening” of the front can be adjusted (and thus studied systematically) by varying the characteristics of the “building blocks,” the organosilane molecules, and the conditions under which the gradients are formed.

In addition to providing validation for simulation and theoretical predictions for precisely how these fronts should broaden, the researchers say that these results should be important to understanding self-organization in diverse other material processing and biological systems where similar fronts arise. Examples include, but are not limited to, material sciences, human health, social sciences, anthropology, and many others.

“Processes in nature follow just a few principles that apply to nanoscale, microscale and macroscale,” said Genzer. “Since pattern formation processes exist everywhere in the natural world, we believe the model can be used to verify dynamics in other (seemingly rather unrelated) phenomena such as the spread of disease, tumor growth, wound healing, the spread of epidemics, and the spread of languages across Europe.”

Source: North Carolina State University

Explore further: Atom-by-atom growth chart for shells helps decode past climate

Related Stories

Electrics touted at Paris car show, but await their moment

September 29, 2016

When will electric-powered cars become a practical choice for ordinary people? The question hangs in the air at the Paris auto show, where Volkswagen, Mercedes-Benz and General Motors are showing off electric cars they hope ...

Recommended for you

133 million-year-old dinosaur brain fossil found in England

October 28, 2016

Soft tissues such as hearts and muscles are very rarely preserved in the fossil record. For that reason, nearly all study of dinosaur soft tissue has to be reconstructed from fossil bones. However, researchers in the United ...

Making energy-harvesting computers reliable

October 28, 2016

A revolutionary and emerging class of energy-harvesting computer systems require neither a battery nor a power outlet to operate, instead operating by harvesting energy from their environment. While radio waves, solar energy, ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.