I received a Ph.D. in physics from Cornell University working with Wilson Ho. In my Ph.D. research, I used scanning tunneling microscopy to study vibrational and electronic excitations of single molecules on clean metal surfaces in an ultrahigh vacuum. The ability to "see" and actually manipulate single atoms and molecules motivated me to pursue research exploring the limits of our ability to control the organization of matter on the nanoscale.

In 2000, I took a postdoctoral position with Charles Lieber [see also, see also] in the Department of Chemistry and Chemical Biology at Harvard University. At the time, Lieber was well known for work on carbon nanotubes and had recently begun working on semiconductor nanowire synthesis and characterization. I was eager to get involved in the nanowire effort because I saw the potential to make semiconductor nanostructures that could not be made any other way. As a physicist by training, I felt that getting involved in the synthesis of new materials would ensure that I would have interesting subjects to study in the future.

My research group, in the Department of Materials Science and Engineering at Northwestern University, is exploring the extent to which the properties of one-dimensional semiconductor nanowires can be rationally controlled by doping and composition modulation on the nanoscale. Beyond just the synthesis of new materials, this objective compels us to develop new characterization techniques that are capable of establishing quantitative connections between nanoscale structure and the resulting electrical, optical, and magnetic properties. To that end, we recently demonstrated a technique to map the composition of individual nanowires atom by atom. We have also been combining electrical transport measurements of nanowire devices with scanning probe microscopy to map the electronic and magnetic properties of nanowires on relevant length scales. Ultimately, we aim to establish reliable processing-structure-property relationships to provide a basis for engineering new or improved technologies based on one-dimensional materials.

This paper was a group effort arising from several strands of research within the Lieber group, where I did my postdoctoral studies. I think the paper received the attention it did because it effectively demonstrated the Lieber group’s vision for nanowire-based nanotechnology: it showed that these nanowires could act as functional building blocks within larger hierarchically-assembled structures to produce (relatively) complex functionality. Assembly is still a major challenge, of course.

In two subsequent papers in Nature, my colleague Mark Gudiksen and I showed that the vapor-liquid-solid (VLS) growth method could be exploited to build functionality into nanowires during synthesis, providing useful new capabilities to nanowire devices and creating new opportunities and flexibility in the assembly of hierarchical device structures. I continue to be interested in pushing the limits of the VLS growth process in my own research here at Northwestern.

I am not aware of any nanowire-based technologies on the market, but there are promising near-term applications in areas including sensing and energy conversion. Working nanowire-based technologies have been demonstrated in these and other areas—whether or not a particular approach is "practical" will ultimately be dictated by cost. If nanowires can be synthesized on a large scale, processed in solution, and "printed" on a variety of substrates, one can envision many potential applications for nanowire composite materials. In contrast, much of the basic research to date has focused on the interesting properties of single nanowires and nanowire devices, but this could be changing.

The pace of development has been sufficiently rapid that it is difficult to pick just a few advances. Generally speaking, the breadth of applications being considered for nanowires represents a great advance in our understanding of their potential. More specifically, important advances in synthesis have been those which demonstrate some of the unique possibilities, including directed growth from selected sites and the low-temperature growth of heterostructures of dissimilar materials. With regards to electrical characterization, several groups have demonstrated that nanowires can behave close to the ideal one-dimensional electron boxes that physicists imagine, which is exciting from a fundamental perspective and from the perspective of nanotechnology development.

Looking to the near future, there are manifold opportunities for advances in nanowire electromechanics and heat transport, and in the synthesis of nanowires with magnetic functionality or multiple functionalities, like multiferroics. Materials which exhibit large changes in properties of interest near phase transitions are good targets for new phenomena or enhanced properties in nanowire form. In five years, I think we might view this "field" of nanowires as part of a much larger exploration of multi-functional nanostructured materials. The idea is that you can design the response of a material from the bottom up by selecting appropriate building blocks and then assembling a material with the appropriate hierarchical structure. Nanowires are somewhat unique in that they bridge length scales by having nanoscale diameters but microscopic lengths. We are already seeing a good deal of work on carbon nanotubes in composites, and I expect the work in nanowires to follow. Ironically, semiconductor nanowires were discovered long before carbon nanotubes, and they were being considered for composites with high mechanical strength based on their degree of crystalline perfection. I’m not sure where the field will be in 10 years —if the funding agencies were giving 10-year grants I’d have a better answer.