July 14, 2008 — Just about every assumption we make about the materials of everyday life start to collapse when objects become very small. At the micron scale — or one-millionth of a meter — they retain their familiar properties, but as they move to the nanoscale — with dimensions measured in billionths of a meter — strange things start to happen. Notoriously inert elements like platinum or gold become potent chemical catalysts, and gold turns into a liquid at room temperatures. Normally stable, aluminum becomes combustible, and copper becomes transparent.
It is a fascinating and, in many respects, an unknown world, but it is one we must master if we are to continue our pursuit of ever smaller and more powerful electronic and mechanical devices. For materials scientists like Petra Reinke, an associate professor in the University of Virginia's Department of Materials Science and Engineering, the goal is to be able to engineer materials and construct structures that perform in predictable ways at the nanoscale.
Reinke's approach to building nanoscale structures is to use a surface as a template. By varying this template, she is learning how to control the size and the organization in space of different nanomaterials layered on it, determining their properties and their interactivity. "My aim is to unravel the relationship between nanoscale structure and properties at a surface," she says.
Currently, silicon is the material of choice for semiconductors, but conventional silicon-based electronics are reaching their limits in terms of feature size, performance and other characteristics. Spintronics, which uses the spin of electrons rather than their charge, will be the basis for a new generation of devices that will be much smaller, more versatile and more robust than those currently making up silicon chips and circuit elements. Realizing the promise, spintronics will require the ability to fabricate and manipulate ferromagnetic semiconductors, which combine silicon with materials that have magnetic properties at the nanoscale.
One of these materials is manganese, but creating manganese structures on silicon has proven difficult. As a step in this direction, Reinke has placed a layer of manganese under a layer of silicon, applied heat, and, using a variety of high-tech tools like scanning probe microscopy and photoelectron spectroscopy, characterized the structures that form at different temperatures.
Reinke's work also has applications for organic solar cells, which offer an alternative to conventional silicon-based technology that is expensive, brittle and difficult to fabricate. Improving the efficiency of organic solar cells so that they approach that of silicon cells will require a more precise arrangement of photoabsorber molecules and fullerenes, in order to use their interaction to produce a current and thus electricity. Reinke creates defects in a graphite substrate and deposits fullerenes on it as a first step, noting how islands of fullerenes form around damaged areas. "I am learning how to generate structures at a surface in predictable ways, which provides insight into the forces that operate at this scale," she said.
Given what remains to be discovered at the nanoscale, Reinke is very much an explorer. One of the advantages of her position is that all her observations, even those that do not suit her immediate purpose, provide important clues to the phenomena that shape this tiny world.
— By Charlie Feigenoff