Written by Aditi Risbud
On Wednesday evening, Hongjie Dai of Stanford University discussed his body of work on carbon nanotubes from the last two decades, highlighting the efforts of his students and colleagues along the way.
As a material that can be made with high atomic precision, carbon nanotubes are unique one-dimensional systems that present opportunities to study basic chemistry, physics, and materials science. Dai recognized the influence of his mentors Charles Lieber and Richard Smalley, both pioneers in the field of nanomaterials.
One key aspect of Dai’s work was producing high-quality carbon nanomaterials through synthetic routes for nanotubes and nanoribbons. By controlling the growth of these materials and their direction, structural order can emerge. From this start, two decades of research by Dai, his colleagues and students have resulted in dense arrays of ordered nanotubes arranged vertically turned into a thin, transparent film. Today, Foxconn uses this technology for cell phone touch screens.
Dai also discussed his work with Calvin Quate, inventor of the atomic force microscope (AFM), who told Dai “it might be wise to do something different every ten years or so.”
Their collaboration focused on “islands” of nanotubes grown on a wafer. This led to patterning single iron nanoparticles on these islands and using them to grow carbon nanotubes with precise locations and size, and using electrical fields in a chemical vapor deposition technique to control their orientation.
In addition, Dai and his colleagues showed that silicon posts on a wafer could serve as power lines, with nanotubes grown from one post to another. These suspended nanotubes can be used to study fundamental properties of the nanotube: for example, by probing, or stretching, the nanotube with an AFM tip, the electrical properties can be gleaned. This system can be modified to grow a nanotube across a trench with electrical contacts underneath, an “electrically wired, extremely clean one-dimensional system in its native state” that can be used to investigate the fundamental physical properties of nanotubes.
Dai discussed carbon nanotube-based transistors and other devices with remarkable properties (“nearly ballistic”) with low resistances and high currents that could outperform traditional silicon-based devices. Based on these findings, many researchers are now pursuing the idea of making computer chips from carbon nanotubes.
In keeping with Quate’s suggestion, Dai has moved on to studying graphene nanoribbons and, more recently, using proteins and antibodies as sensors on nanotubes to study biological systems. His group has developed sensors for detecting antibodies for autoimmune disease and used carbon nanotubes as a mechanism for drug delivery and targeting cancer cells. Now, he is using the fluorescence of carbon nanotubes to image the brain with less light scattering and without damaging the nerves. This imaging technique uses 1,000-1,700 nm wavelength light (the so-called second near-infrared window) and can be used to image the flow of blood in mouse blood vessels. He also discussed work on electrocatalysis and batteries using carbon-based materials.
Dai reflected on his twenty years as a researcher thus far, saying the award “is really quite special to me, a recognition that is beyond my expectation.” He also joked that a mid-career award suggests he has “twenty more years to go.”
The Mid-Career Research Award is endowed by Aldrich Materials Science.