Finding the art behind the science of nanocrystal synthesis

Enter the office of Argonne physicist Zhili Xiao, and you might feel as though you've set foot in the studio of an accomplished artist.

Numerous images of tubes, wires, and dots arranged in perfectly symmetrical patterns hang over his computer like paintings. On his bookshelf rests a set of large-ringed binders, which, like photo albums, contain dozens of pictures of tripods, brushes and other three-dimensional crystals on a scale 100,000 times smaller than the period at the end of this sentence.

"These binders represent many years of hard work, explained Xiao, flipping through pages of exquisitely shaped structures. "Each type of crystal structure was painstakingly crafted and imaged by me and my colleagues.

Xiao, who is also an associate professor of physics at Northern Illinois University, works with materials at the nanoscale — a level so small that its basic unit, the nanometer, is equivalent to a billionth of a meter. To give some perspective, a nanometer is roughly 10,000 times smaller than the thinnest human hair. Dealing with materials at this scale can be tricky, since they can behave unpredictably at such miniscule dimensions. This unpredictability has made nanocrystal synthesis more of an art form than anything else — one that Xiao seems to have mastered, judging by the wide variety of structures in his image collection.

However, researchers like Xiao are now beginning to understand the science behind the art of making nanocrystals. His scientific team, a joint effort in the Material Sciences Division at Argonne and the Physics Department of Northern, has figured out the basics of using electricity to control the shape of nanostructures. Their findings provide a practical method of generating large quantities of these particles for use in electronic, optic and superconducting applications.

These efforts are part of a larger nanoscale research initiative sponsored by the U.S. Department of Energy's Office of Science — one that seeks to develop "the ability to create materials atom by atom, and to precisely control chemical reactions, according to the Office of Science Strategic Plan. As Xiao and other Argonne scientists make further breakthroughs in synthesizing and characterizing new nanomaterials, the vision as set forth in this initiative is quickly starting to become reality.

What's in a shape?

Ever since the 1980s, when the emergence of a variety of tools made it possible to study materials at the nanoscale, scientists have been fascinated by unique properties occurring at the level of atoms and molecules. Carbon, for example, is an element renowned for its ability to form a diverse array of compounds ranging from methane gas to DNA — the molecular backbone of organic life. When grown as tube-like structures one to 100 nanometers wide, the bonds between carbon atoms become so robust that, weight-for-weight, the structures are at least 100 times as strong as steel. Indeed, a twisted carbon-nanotube string half the width of a pencil can support more than 40,000 kilograms, the weight of about 40 small cars piled one on top of another.

Yet, size isn't the only thing that can determine the properties of a given material. In recent years, scientists have also discovered that a material's characteristics can be changed simply by altering its shape.

"When you alter the shape of a nanocrystal, you're basically setting new boundaries to the space in which its electrons can move, said Wai-Kwong Kwok, leader of the Superconductivity and Magnetism group in the Materials Science Division. "This, in turn, affects its physical properties, which explains why a triangle and a sphere made of lead can have completely different physical and chemical properties.

Further investigation into the nature of these size- and shape-dependent properties revealed that they primarily occur in structures with dimensions ranging from a few nanometers to 10 microns (millionths of a meter) — a finding that has inspired researchers to develop more reliable ways of synthesizing these miniscule structures for further scientific study.

The search for a nanocrystal recipe

Xiao's initial fascination with nanocrystals mainly revolved around their potential for unique electronic and magnetic properties. As a physicist specializing in superconductivity, he was particularly interested in studying the electron behavior in nanocrystals in hopes of gaining better control over the physical properties of materials.

"Throughout my life, I've always had a love of making and studying interesting materials, Xiao explained, with a twinkle in his eye. "You could say I'm something of a cross between an engineer and a physicist.

Xiao, a wiry young man with an animated personality, had drawn upon this passion many times throughout his career as a researcher. From his college years in China to his Ph.D. experience in Germany, he spent many hours poring over scientific journals, searching for the most efficient methods to make fascinating materials.

"I found, early on, that a lot of work had already been done on nanostructure synthesis. My task then became that of finding out what contribution I could make as a physicist and as a materials scientist, said Xiao.

Indeed, each scientific discipline seems to have its own method of synthesizing nanomaterials. In general, chemists tend to be molecular choreographers — using hot plates and beakers to guide the interaction of chemical compounds in solution. Physicists, with their love for precision, typically use techniques such as electron-beam lithography and focused-ion-beam milling for an atom-by-atom approach to constructing nanostructures.

However, as Xiao soon discovered, things can get much more complicated when trying to produce a batch of nanocrystals that have the exact same size and shape. Traditionally, scientists fabricated these structures through a chemical approach — rapidly injecting compounds into a solution heated to high temperatures. The downside to this method, however, is the difficulty of controlling the solution concentration, which changes as the reaction proceeds. This change in concentration leads to changes in the electrochemical potential — the measure of a compound's ability to react in solution. Since a stable electrochemical potential is crucial for forming well-shaped nanocrystals, scientists using this method often found themselves struggling to control solution concentrations and to time the right moment to stop the reaction.

Electrical Magic

In the end, Xiao's contribution to the field of nanocrystal synthesis would come as a fusion of chemistry and physics.

"When I first came to Argonne, my expertise was on the physics of electron behavior in superconducting materials, he recalled, "I didn't know much about the various methods that chemists use to make materials — until my colleagues taught me about electrochemistry.

It didn't take Xiao long to latch onto the importance of electrochemistry, the study of the relationship of electricity to chemical changes. In the late 1700s, scientists first observed this relationship when they discovered that electricity could be generated through chemical reactions — a discovery that paved the way for batteries and fuel cells. More recently, it was found that the reverse process, adding electrical voltage to chemical solutions, could also produce interesting effects.

"Because the behavior of chemicals depends on the activity of their electrons, running an electrical current through a given solution can be an effective way to control chemical reactions. Thus, we thought that we could use electrochemistry as a way to control the architecture, or shape, of our nanocrystals, explained Xiao.

The thinking of Xiao and his colleagues proved to be correct. In contrast to traditional methods, they found that it was easy to control the electrochemical potential, and thus the architecture, of the nanocrystals by using electrical voltage. The scientists used a technique called electrodeposition, which uses electricity passing through an electrode to reduce ions from solution on a given surface. By changing the applied voltage value and the type of chemicals in the solution, the Argonne researchers were able to synthesize large quantities of nearly 30 different nanostructures, including nanoparticles of various shapes, nanowires, nanobrushes and nanoscale tripods.

"We found, for example, that shaped nanoparticles tend to form at lower voltages, while higher voltages tend to produce structures such as nanowires and nanobrushes, said Xiao.

The electrodeposition technique developed by Xiao and other Argonne scientists has caught the eye of key researchers involved with nanocrystal synthesis. Reginald Penner, a University of California-Irvine professor and a leading scientist in the field, recently praised the team for gaining a "new and deeper understanding of the mechanism behind metal nanostructure growth.

"Dr. Xiao's team was among the first to demonstrate that complex shapes, and shape control, can be achieved for the growth of metal nanoparticles, said Penner. "The sensitivity of nanoparticle geometry to applied voltage has never been observed in such a systematic fashion.

In addition, the paper that Xiao and his colleagues had published early in 2004 in the Journal of the American Chemical Society on this work was recognized by Science magazine as one of the highlights of recent literature.

Climbing the Mt. Everest of nanoscience

With large quantities of these nanocrystals in hand, scientists are now concentrating on exploring their unique physical and chemical properties. These structures can lead to discoveries of new phenomena and applications, such as the use of ferromagnetic nanocrystals as components in ultra-high-density storage media and the use of certain metal nanocrystals as catalysts for hydrogen production and sensing. Xiao also envisions the possibility of constructing a single wire with segments that function as magnets and others that function as superconductors — a capability that could speed up the development of high-tech nanoscale devices.

Still, the size- and shape-dependent properties of nanocrystals remain largely unexplored. Xiao, for his part, is excited and optimistic about the field's infancy — a feeling he frequently sums up by quoting the words of Charles Lieber, a chemist and nanoscience pioneer from Harvard University.

"If nano research is Mt. Everest, we have barely reached the base camp, quoted Xiao, savoring each word. "It's a good feeling to have so many directions left to explore.

The research work by scientists in Argonne's Materials Science Division was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, and the University of Chicago-Argonne National Laboratory Consortium for Nanoscience Research.