Blood, sweat and serendipity
Probing the nanoscale for unexpected insights
Discovering how electrons move through materials tells scientists a lot about their physical qualities. Are they good insulators? Good conductors? Can they be polarized? The nuances of "electron transport" account for phenomena as exotic as the northern lights and as commonplace as the information stored on the memory chip in your phone.
Physicists normally handle measurements of this process with kid gloves, taking care not to change the materials through the act of measuring them. By this standard, the analytical methods used by ORNL materials scientist Peter Maksymovych and his colleagues-blasting materials with high-energy electrons and subjecting them to intense electric fields-seem a bit iconoclastic.
Experience and perspective
"Most people think that if you stimulate a material with a strong electric field or inject high-energy electrons into it, that will significantly alter the material, or simply destroy it, and you will not learn anything," says ORNL materials scientist Peter Maksymovych. "Our research team disagrees profoundly."
Maksymovych's skepticism is based both on experience and perspective. While many scientists in the field work with relatively large material samples, at the laboratory's Center for Nanophase Materials Sciences, Maksymovych and his team probe mate- rials at the nanoscale-at the level of single molecules.
"Because we use scanning probe microscopy to examine materials," he says, "we have a unique perspective. We can see phenomena at the nanoscale that aren't apparent in larger samples."
Maksymovych and his team are currently trying to find evidence of new behaviors, new phenomena and new materials by exposing ferroelectric materials, such as those used to make memory chips and other electronic devices, to very large electric fields and high electron currents.
One of the issues they have been studying is related to the production of ferroelectric-based memory chips, or FE-RAM-a specialized type of memory chip that is faster and requires less energy than many other forms of digital memory. Data is recorded on FE-RAM by creating very small polarized and non-polarized regions on the material. This is read as 1s and 0s by the computer, just like any other binary memory medium. Unfortunately, polarization-based FE-RAM memory requires a lot of space- relatively speaking-and can't be scaled down beyond a certain point.
"The challenge with FE-RAM has been scalability," Maksymovych explains. "The FE-RAM uses a layer of ferroelectric film to store information. Making a bit (the smallest unit of information) smaller than 100-200 nanometers across (about as much as a fingernail grows in two minutes) and still read it has proven to be fundamentally difficult. That's because reading FE-RAM depends on detecting a charge on the surface of the material. There's a limit to our ability to measure that. As a result, while we could use conventional techniques to write data to a smaller area of ferroelectric film, we couldn't read it."
Serendipity steps in
Just as the research team was coming up empty in its search for a way around this "reading" problem, Maksymovych says serendipity took over. They found that, contrary to conventional wisdom, when they applied a large electric field to a tiny area of the ferro-electric film, which is normally an insulator, the area became electrically conductive.
This behavior not only opens up the potential for writing, reading and erasing much smaller "bits" of information (down to at least 10 nanometers), suggesting a way around the scalability problem, but it also hints at possibilities for new electronic applications for ferroelectric materials.
The discovery is also of interest to researchers because of the type of conductivity it uncovered. The team found that applying an electric field to the otherwise insulating material had turned on metallic conductance. This is significant because, unlike many other conducting materials, metallic conductors work at all temperatures-opening them up to a much broader range of potential applications.
"If you take the material down to very low temperatures where the ability of semiconducting materials to conduct electricity would nearly die out," Maksymovych explains, "the difference in conductance is seven to eight orders of magnitude. In addition, the amount of current being conducted is easily two to four orders of magnitude higher when ferroelectric materials become metallic. This phenomenon is particularly attractive because it has these colossal magnitudes tied to it."
Also attractive to scientists is the prospect of being able to create conductive "nanodomains" within an otherwise insulating material. Maksymovych suggests that this behavior alone opens up a new realm of nanotechnology which he refers to as "topological nanostructure."
"Instead of depositing a material here or changing the composition of a material there," he says, "all we have to do is apply electric fields to the ferroelectric material and we can control conductance. By doing this we were able to observe phenomena that had been hypothesized, but certainly not observed."
A beautiful playground
Shrink the size of the probed region by another order of magnitude, and you enter the world of molecules. Maksymovych suggests that the ability to control molecules using electric fields is a compelling platform for understanding physics in general.
"Today there are still many mysteries surrounding molecular physics," he says. "Even though we can develop complex simulations, we still don't know everything we want to know. Ideally, we would like to apply what we have learned so far to developing a better understanding of organic molecules."
Maksymovych envisions devising a way to inject high-energy electrons into molecules and then applying the knowledge his team has accumulated and the analytical techniques they have devised to the task of understanding how electrons interact within organic molecules and how molecules interact with each other "on a whole new level." One specific area of interest is the role electron transport plays in conductivity at the molecular level, including materials like "Mott insulators" that can be either insulators or conductorsódepending on the conditionsó and inorganic superconductors.
"Some people will say that we already understand electron behavior in molecular conductors pretty well," he observes, "but the simplicity of a lattice of organic molecules (like the molecules in a sugar crystal) has the potential to give us more insight, particularly at the nanoscale. We can count the number of molecules, count the number of defects, create new defects, and sometimes even count the number of electrons. We cannot do that in inorganic materials yet. This kind of research would move us in that direction."
Scientists normally look for the simplest possible setting in which to study a phenomenon, and Maksymovych and his research team believe molecular assemblies are an elegantly simple platform for studying the nature of materials.
"Often these kinds of studies are done using individual atoms at extremely low temperaturesóthe so called cold atom lattices," he says, "but there are lots of things a single atom can't tell you. Molecules, or lattices of molecules, give us the opportunity to study a much wider range of phenomena."
Maksymovych is betting that observing how electrons enable molecules to "communicate" with one another will have multiple benefits: providing us with a better understanding of their structure, affording insight into elusive phenomena such as superconductivity and uncovering more unanticipated properties of everyday materials.
"If we use molecules in this way, I believe we can build a new and beautiful playground for the study of electron physics," he says. óJim Pearce