When Mildred S. Dresselhaus, professor of physics and electrical engineering at Massachusetts Institute of Technology, recently spoke at the European Materials Research Society Spring Meeting, delegates crowded into a hot conference room and spilled out into the corridor. She herself admits, “I’m one of those people that if you have a manuscript of something new you send it to me and say, ‘What do you think of it?’” When Materials Today caught up with her shortly after, not only was Dresselhaus keen to recount her recent research, but to consider what has driven interest in nanotechnology and the challenges and opportunities that might be in store for the field. “Lots of new physics that was totally unexpected propelled the field,” she explains. “That’s what nanoscience and technology is all about: finding all these differences from three-dimensional systems and exploiting them, perhaps, for devices. That’s the ultimate objective that society wants from research in the final analysis.”
Dresselhaus has been working on carbon since 1961. “At that time physics equaled high energy physics,” she recalls, “but I didn’t much like the lifestyle. I saw that physics was becoming ‘living out of a suitcase’, and that was not really what I had in mind. I went against the tide, but over the years the kind of work that I do has become more like that done in physics departments worldwide.”
Her work on graphite naturally led to research on the relation of energy levels in carbon nanotubes to their structure. “Many things that we know about nanotubes come from graphite, but they are very different from graphite at the same time.” By applying Raman spectroscopy to single nanotubes, Dresselhaus was able to determine their structure from the energy spectra. “What this does, from the point of view of materials science, is to provide a methodology for structural determination — you have a new tool for determining structures of nanosystems.” Her work has also demonstrated the possibility of using other techniques, in conjunction with Raman spectroscopy, on the same nanotube, allowing many properties to be measured in terms of the nanotube geometry. “That’s interesting because, for example, whether a nanotube is semiconducting or metallic depends on structure,” she explains. “With this experiment we can, among other things, easily identify that property.”
Dresselhaus is also interested in nanowires. “Nanowires are much more recent in a sense,” she says. “With nanowires, we use quantum mechanics to make phase transitions to go from one kind of system like a semi-metal to a semiconductor. That’s something that’s only possible at these very small sizes. There again we have new properties.” It is this theme of understanding new properties that arise from confinement that is constantly reinforced by Dresselhaus. She sees this as bringing all the excitement and interest to the field. “The biggest thing is the opportunity opened up by the ability to work on much smaller objects. We know now that there are many new physical principles that will be found. At this scale, quantum mechanics plays a much bigger role than at any other scale, so there will be many new phenomena.”
So opportunity knocks for nanoscale science, but new properties and new phenomena also bring new technological challenges if devices are to be made. The main challenge in working with carbon nanotubes is their synthesis. Devices will require monodisperse nanotubes with identical, controlled properties. “When you have some synthesis process, you make semiconducting and metallic ones, all diameters, all chiralities, and they all have different properties,” says Dresselhaus. “So if you want to make a reliable device you have some challenges ahead. One of the hopes is that we will soon figure out some way to have a dial you can turn and get a particular nanotube. Then it will be possible to produce nanotubes in large quantities, to order, all the same size.”
Nanotechnology, then, has novelty and challenge, but what about those potential devices and applications? Dresselhaus believes there will be applications, “But it’s very early in the field to know exactly where that will be.” She gives the development of the laser as an analogy, where it was many years before practical applications became apparent. “No one would have guessed that the laser would have something to do with the checkout counter at the grocery store. That’s kind of where nanoscience and technology is right now. We shouldn’t promise too much. I think everybody believes that something will come of this, but it’s not at all clear what it will be.”
One of the great attributes of nanoscience is its excitement. At a time when there is a decline in student numbers in the physical sciences, and industry is worried about finding a trained workforce in five to ten years’ time, that attribute could prove essential. “What nanoscience and technology has done is sparked interest,” says Dresselhaus. “It’s one thing that excites students because they see it’s not just pushing the next decimal point. There are totally new phenomena available and new opportunities.”