Yes, our paper describes two different approaches to the synthesis of metal nanostructures with well-controlled shapes and facets. In the first approach, poly(vinyl pyrrolidone) (PVP) was used as a capping reagent to control both nucleation and growth of silver atoms, and thus to obtain silver nanocubes of controllable sizes as monodispersed samples and in large quantities. These nanocubes were single crystals and were characterized by slightly truncated corners and edges. They were mainly enclosed by {100} facets. The key to the success of this synthesis was PVP, a polymer capable of selectively capping the {100} rather than {111} or {110} facets. In a related demonstration, we also showed that uniform silver nanowires (with five-fold twined, pentagonal cross-sections) could be synthesized by controlling the amount of PVP added to the reaction medium in the nucleation step (see, for example, Y. Sun, B. Mayers, T. Herricks and Y. Xia, "Polyol synthesis of uniform silver nanowires: A plausible mechanism and the supporting evidence," Nano Letters 3: 955, 2003). In the second approach, the silver nanocubes served as sacrificial templates for a galvanic replacement reaction between silver and chloroauric acid to generate single crystalline nanoboxes—truncated, hollow cubes bounded by six {100} and eight {111} facets—of gold (as well as its alloys with silver). More recently, we further demonstrated that the second approach could be readily extended to fabricate metal (e.g., gold, palladium, and platinum) nanostructures characterized by hollow interiors and a wide variety of morphologies (e.g., nanoscale boxes or tubes with highly porous walls). For those interested in this work, please refer to the following publications: Y. Sun, B. Mayers and Y. Xia, "Metal nanostructures with hollow interiors," Advanced Materials 15: 641, 2003; Y. Sun and Y. Xia, "Alloying and dealloying processes involved in the preparation of metal nanoshells through a galvanic replacement reaction," Nano Letters 3: 1569, 2003; Y. Sun and Y. Xia, "Mechanistic study on the replacement reaction between silver nanostructures and chloroauric acid in the aqueous medium," Journal of the American Chemical Society, 2004, in press; Y. Sun and Y. Xia, "Multiple-walled nanotubes made of metals," Advanced Materials, 2004, in press. These studies fully demonstrated the capability and feasibility of the synthetic strategies as outlined in our Science paper. More importantly, these metal nanostructures are immediately useful in applications such as surface plasmonics, SERS detection, optoelectronics, biomedical imaging, catalysis, and hydrogen storage.

I worked with Professor George M. Whitesides as a Ph.D. student and then a postdoctoral fellow at Harvard University. My thesis work involved the development of soft lithography (see, for example, Y. Xia and G. M. Whitesides, Angewandte Chemie International Edition in English 37: 551, 1998), which represents a top-down approach to the fabrication of nanostructures. When I launched my own research at the University of Washington, I wanted to explore different routes to nanostructures, in particular, the bottom-up approach that involves the formation of nanostructures from building blocks with smaller dimensions. I asked my student (Byron Gates) to begin with trigonal selenium and tellurium, two solids well-known for their highly anisotropic crystallographic structures (both of them are constructed from helical chains of atoms) and interesting photoconductive properties. By taking advantage of the anisotropy in structure, we were able to grow uniform nanowires with diameters in the range of 10-100 nm and lengths up to hundreds of micrometers (a recent review: B. Mayers, B. Gates and Y. Xia, "One-dimensional nanostructures of chalcogens and chalcogenides," International Journal of Nanotechnology 1/2: 86, 2004). While working on this system, I also asked myself if a similar chemical approach could be applied to the synthesis of metal nanowires. It was soon realized that most of metals crystallize in the face-centered cubic lattice (which is a highly symmetric structure), and one has to introduce a capping reagent to help control the shape and structure of seeds formed in the nucleation process. I asked Dr. Yugang Sun, a postdoc who just joined my group at that time, to conduct some survey experiments with silver by using the well-known polyol process (see, for example, F. Fievet, J. P. Lagier and M. Figlarz, Materials Research Society Bulletin 14[12]: 29, 1989). In a typical polyol synthesis, PVP is often added as a surfactant to stabilize the silver colloids. After we had completed a number of syntheses, we realized that the silver nanostructures obtained using the polyol method were extremely rich in terms of morphological variation. The product of each synthesis usually contained a mixture of many types of structures such as cubes, tetrahedrons, rods, quasi-spheres, and irregular particles. After analyzing the possible function of each component in this system, we concluded that PVP not only served as a stabilizer for the product but also played a critical role in dictating the nucleation and growth of silver atoms. By controlling how the PVP was added and how much of it was added (through the use of syringe pumps), we finally succeeded in directing the product of each synthesis solely to one particular morphology or shape.