Technical: This project is to study growth mechanism of nanostructures that are grown using encapsulation and controlled release of metal catalyst. The controlled catalyst release allows the exposed size/area of the catalyst be tailored, which leads to growth of unusual nanostructures, such as nanocones made of silicon carbide. The goal of the research project is to gain new mechanistic knowledge of all stages of the metal-catalyzed nano- and hetero-structure formation including the migration of the metal nanoparticle from the carbon shell and agglomeration behavior, the formation of the SiC at the metal/nanocone interface, and the nucleation and initial growth of the heterostructures. The primary research tool is transmission electron microscopy (TEM) and both in-situ and ex-situ TEM experiments will be carried out. Electrical and property measurements will be used to establish the synthesis-structure-property relationships. The potential outcome of the project will be a predictive tool for the manufacturing of specific nanowires with a precise distribution and geometry, as well as branched structures, and how processing conditions, such as temperature, size, and type of catalyst material, will alter the nanowire morphology, structure, and properties.
The project addresses fundamental research issues in a topical area of importance to materials science. It aims for a quantitative and mechanistic understanding of the catalytic reaction governing one-dimensional nanostructure formations. Such knowledge is necessary for the design-based production of nanodevices with specific architectures and properties. The project constitutes an effective integration of research and education through training of postdoctoral, graduate, and undergraduate students in nanoscience and nanotechnology. The in-situ TEM nanocharacterization represents a unique opportunity for the students. The research program will also contribute to a new suite of graduate-level courses in nanotechnology, particularly nanofabrication and nanocharacterization. Undergraduate education will be enhanced via undergraduate summer research and senior projects.
Intellectual Merit: Silicon carbide has many applications ranging from abrasive cutting tools to high impact armor and diode to LEDs. Silicon carbide can act as either an n-type or p-type semiconductor and is a strong material able to withstand harsh environments. It is a promising candidate to replace silicon in electronic applications especially in harsh enviornments. Nanowires of silicon carbide are especially promising as wires or transistor circuit elements due to its high electron transport and low thermal expansion properties. A remarkably elegant approach to produce 1-dimensional (1D) nanostructures is by metal-catalyzed nanowire formation via the vapor-liquid-solid (VLS) mechanism. By encapsulation and controlled release of the metal catalyst, the exposed size/area of the catalyst could be tailored, leading to unusual 1D nanostructures, such as nanocones. The research program in Judith Yang’s group at University of Pittsburgh demonstrated the successful synthesis of unusual silicon carbide (SiC) nanocones and Y, T branched heterostructures as catalyzed by an iron nanoparticle originally enclosed within a graphite-like carbon shell. Silicon carbide nanocone and heterostructures structures were formed by heating the iron nanoparticles encapsulated in carbon with a mixture of silicon monoxide and silicon powder at elevated temperatures of 1200-1400 C in a tube furnace. These nanoscale particles and structures were analyzed by using an electron microscope to visualize these minute features in by taking real-time video (in situ) and pictures (ex situ). Yang’s group hypothesized that the unusual nanostructures were a result of the iron nanoparticle escaping from the carbon shell and then agglomerating during growth of the SiC. The in situ heating electron microscopy studies of the iron nanoparticles enclosed in a carbon shell revealed that the hypothesis is reasonable and that the iron nanoparticles could either diffuse as atoms or clusters on the surface of the carbon shell or in through the interior. Electron microscopy, especially dynamic environmental electron microscopy, provided mechanistic knowledge of this novel "released" metal-catalyzed nano- and heterostructure formation. Broader impact: This body of work contributes to our understanding on the manufacturing of nanocones, as well as branched structures. Such knowledge is necessary for the design-based production of nanodevices using 1D nanostructures with specific architectures and properties. The impact could be a manufacturing toolbox that determines and predicts a specific nanowire structure, composition and length, as well as its distribution. This program contributed to the education and training of one post-doc, one PhD student and two undergraduates, who learned about nanoscience and technology, synthesis, and nanocharacterization, especially in situ. Collaborations with universities abroad (e.g., China and Spain) as well as with companies (e.g., RJ Lee Group and Protochips) enriched the research experiences of the post-doc and graduate student. The results from this research program also contributed to a graduate level Electron Microscopy course that Yang teaches. This program also supported efforts in the Pennsylvania Junior of Academy of Science (PJAS) "Nano-day", where electron microscopy of everyday materials were shown and explained to high school students to excite and explain materials science.
