Melissa A. Hines, Chemistry and Chemical Biology, has long been captivated by nanoscale surfaces. Ever since her years as an undergraduate at Massachusetts Institute of Technology, Hines has dedicated her research to understanding various chemical reactions that take place at the surface of materials.
“It’s so different from the kinds of reactions that chemists are used to thinking about in a beaker or in a bulk material,” says Hines. “The fact that it’s at an interface adds not only a level of complexity, but a level of fascination.”
From Silicon to Metal Oxide Surfaces
Hines has spent several years studying the surface of silicon. In collaboration with Alan Zehnder, Mechanical and Aerospace Engineering, Hines showed how changing silicon’s surface alters its mechanical properties. By changing a single layer of molecules on a silicon surface, Hines and Zehnder found dramatic changes both in the way silicon vibrates and its strength against breakage.
In 2012, her lab developed a technique to make atomically flat silicon using a chemical agent that etched away the rough and bumpy parts. It was a major development, given silicon’s ubiquitous role in the semiconductor and electronics industries.
The ideal silicon wafer is perfectly flat, improving the performance of transistors that are made from the wafer. But as computers and circuits became smaller and smaller, the problems caused by atomic-scale defects increased. Hines and her lab were able to flatten industry-standard silicon and potentially improve transistor performance.
“It was a very fundamental question, good for academic research but fueled by a problem in industry,” says Hines.
Today, Hines’ lab has shown that it’s possible to do similar chemical etching on surfaces of metal oxides. In particular, Hines is interested in better understanding surface reactions of titanium oxide (TiO2).
If you’ve ever used white paint or paper or toothpaste, you’ve come in contact with TiO2, the most commonly used white pigment worldwide. But TiO2 isn’t limited to its pigmentation abilities; it catalyzes many more interesting chemical reactions under specific circumstances. Apply nanocrystalline TiO2 to fluorescent lights and those lights become self-cleaning. Add it to a glass of water under sunlight, and it will split H2O to make hydrogen. It has wide-ranging application potential, from solar cells to data storage.
Despite its prevalence, researchers don’t yet fully understand the chemistry of TiO2 reactions. Hines’ lab is working to explain them. What makes it the ideal challenge for Hines is that changing the shape of the nanocrystalline TiO2 alters its reactivity.
“That tells me that there’s a very specific site on the surface that is chemically reactive,” says Hines. “I want to know which site on the surface is chemically reactive and once I know that, I want to make the material so that it has a lot of that site.”
Hines employs scanning tunneling microscopy to capture images of individual atoms on a surface, before and after chemical reactions, to see what the patterns can tell her about the reactions that took place. The surface will literally change its atomic pattern between reactions. The challenge is translating the images into chemistry. To aid the process, Hines’ lab uses computer simulations of tens of thousands of different reactions to find the one that best matches the experiment.
Growing Titanium Oxide Nanocrystals
Understanding the fundamentals of TiO2’s surface chemistry could have vast implications for its use in the energy landscape.
“It has some really exciting reactions that, if we could make them faster, have the potential to solve a lot of our energy problems,” says Hines. That said, she adds, “I don’t want to oversell it because people have been trying to do this for 40 years. So, it’s not clear that TiO2 will be fast enough to be commercialized, but that’s the potential of these kinds of materials.”
It’s this potential and the well-known applications that have drawn Hines to TiO2. But instead of using the etching method used in silicon, Hines’ lab is growing TiO2 in varying shapes and structures in order to study how shape affects chemical reactivity.
“In principle, if you make the right shape, you increase the reactivity. Could that do something really interesting like split water into hydrogen very fast?” says Hines. “We don’t know how big this effect is going to be, but are trying to see how much we can enhance reactivity.”
Hines’ lab grows TiO2 crystals in different structures using a process called "epitaxy." Much like the way rock candy is grown, the researchers grow TiO2 crystals using a crystalline substrate as the “string.” They carefully choose the type of material to make the string out of so that the crystals grow in a certain manner that mimics the substrate orientation, and then study how chemistry affects the shape of the growing crystals.
“We’re trying to show that if you choose the right string and the right chemistry, you get a specific shape,” says Hines. “What we want to be able to do is to learn to make crystals in the most reactive shape.” The problem is that the most reactive crystals are tiny, far too small to study in the scanning tunneling microscope. The lab has recently used this technique to grow crystals large enough for their microscope.
Collaborative Research at the Cornell Center for Materials Research
Since 2005, Hines has acted as the Director of the Cornell Center for Materials Research (CCMR). But Hines’ relationship with the CCMR dates back to her earliest days at Cornell.
“The CCMR was very important to me when I was starting out, and still is,” says Hines. “It’s had a profound impact on the campus, and it has been a force that brings people from different departments together to do interdisciplinary research.”
Despite its prevalence, researchers don’t yet fully understand the chemistry of titanium oxide reactions; Hines’ lab is working to explain them.
The CCMR funded Hines’ initial research on making flat silicon, as well as her work with Zehnder on silicon’s mechanical properties.
Today, the CCMR funds several projects, including three large Interdisciplinary Research Groups (IRGs) on atomic membranes, complex electronic materials, and spin manipulation. The Center also invests in exploratory seed projects, or as Hines puts it, pie in the sky ideas that need money for a short amount of time to obtain initial results.
“Groups at the CCMR have finite lifetimes, but then they dissolve and re-nucleate around a new topic, with new funding,” says Hines. The CCMR makes it possible to collect those early results and for Cornell researchers to be successful in the future.
The Center runs shared research facilities that are available to researchers across campus. Facilities include electron and optical microscopy, x-ray diffraction analysis, surface analysis and characterization, and more.
“When the directorship changed hands some people twisted my arm and said that I should step up to the plate and be director,” Hines jokes. In seriousness, however, Hines says, “we need to make sure that these institutions and facilities stay at Cornell. It’s what makes Cornell unique. It may not be obvious to outsiders that this is a special place, but Cornell has a completely different level of interdisciplinary research. Centers like the CCMR have really changed the culture of research across campus.”