Materials Chemistry, Inorganic chemistry — Low-dimensional nanoscopic building blocks are used to assemble complex architectures with novel electronic and photonic properties.
Semiconductor nanowires have witnessed an explosion of interest in the last decade due to advances in synthesis and the unique thermal, optoelectronic, chemical, and mechanical properties of these materials. The potential applications of single-crystalline nanowires are truly impressive, including computational technology, communications, spectroscopic sensing, alternative energy, and the biological sciences. In the context of global energy needs, low-cost solution-phase nanowire synthesis has also sparked interest in novel solar cell architectures which may play a significant role in the renewable energy sector. Additionally, the use of compact, integrated optical sensors can be envisioned for the detection of pathogenic molecules in the arena of national security or for the diagnosis and study of human disease. This breadth of application naturally requires a multidisciplinary community, including but not limited to materials scientists, chemists, engineers, physicists, and microbiologists, all converging to solve challenging problems at nanometer length scales. However, sine qua non, the materials must be synthesized and characterized before the exploration of their properties and applications can take place.
Semiconductor systems with photon, phonon and/or electron confinement in two dimensions offer a distinct way to study electrical, thermal, mechanical, and optical phenomena as a function of dimensionality and size reduction. These structures have cross-sectional dimensions that can be tuned from 5 to 500 nm, with lengths spanning hundreds of nanometers to millimeters. The vapor-liquid-solid crystal growth mechanism has been utilized for the general synthesis of nanowires of different compositions, sizes, and orientation. Precise size control of the nanowires can be readily achieved using metal nanocrystals as the catalysts. Epitaxial growth plays a significant role in making such nanowire heterostructures and their arrays. To this end, we have successfully synthesized superlattice nanowires and core-sheath nanostructures. Achieving such high level of synthetic control over nanowire growth allows us to explore some of their very unique physical properties. For example, semiconductor nanowires can function as self-contained nanoscale lasers, sub-wavelength optical waveguides, photodetector and efficient nonlinear optical mixer. It was also discovered that the thermoconductivity of the silicon nanowires can be significantly reduced when the nanowire size in the 20 nm region, pointing to a very promising approach to design better thermoelectrical materials. In addition, semiconductor nanowire arrays can be used as potential substrates to achieve high energy conversion efficiency in photovoltaics.
The rapid pace of research in the field of one-dimensional nanostructures is driven by the very exciting scientific challenges and technological potential of mesoscopic systems. Our synthetic capabilities continue to expand quickly, while progress with the difficult tasks of precision property control and assembly inches forward. There are several outstanding scientific challenges in the field that need to be addressed urgently, the most significant of which is the integration and interfacing problem. The ability to create high-density arrays is not enough: how to address individual elements in a high-density array and how to achieve precise layer-to-layer registration for vertical integration are just two of the many challenges still ahead. To achieve reproducible nanostructural interfaces, semiconductor-semiconductor and metal-semiconductor alike, requires careful examination and understanding of the chemistry and physics occurring at the interface. Equally important is the very precise control of the size uniformity, dimensionality, growth direction, and dopant distribution within semiconductor nanostructures, as these structural parameters will ultimately dictate the functionality of the nanostructures. In particular, the physical significance of the dopant distribution and the interfacial junction, and their implications in device operation and performance, will likely require careful re-examination and/or re-definition at the nanometer length scale. Lastly, accurate theoretical simulations appropriate to the above-mentioned mesoscopic regime are becoming feasible with the enhanced computing power available, and should assist our understanding of many of these size- and dimensionality-controlled phenomena.
Professor, B. A. Chemistry, University of Science and Technology in China (1993); Ph. D. Chemistry, Harvard University (1997); Postdoctoral Fellow, University of California, Santa Barbara (1997-1999); Camille and Henry Dreyfus New Faculty Award (1999); 3M Untenured Faculty Award (2000). Research Innovation Award (2001); Alfred P. Sloan Fellow (2001); NSF CAREER Award (2001); Hellman Family Faculty Award (2001); ACS ExxonMobil Solid State Chemistry Award (2001); Beckman Young Investigator Award (2002). MIT Tech. Review TR 100 (2003); ChevronTexaco Chair in Chemistry, Berkeley (2003); First Chairperson for American Chemical Society, Nanoscience subdivision (2003); Camille Dreyfus Teacher-Scholar Award (2004); Dupont Young Professor Award (2004), Julius Springer Prize for Applied Physics (2004), MRS Outstanding Young Investigator Award (2004), ACS Pure Chemistry Award (2005), University of Wisconsin McElvain Lectureship (2006), Chinese Academy of Science Molecular Science Forum Lectureship (2006), NSF A. T. Waterman Award (2007), Scientific American 50 Award (2008).