This CAREER project aims to substantially enhance research and education in the area of nanometer-scale electronic materials. The research emphasis is on studying, identifying and understanding fundamental mechanisms of synthesis and electronic properties of new functional materials - Y-shaped carbon nanotube junctions (Y-junctions). It is anticipated that Y-junctions can be formed by fusing together three pieces of nanotubes with different structure and electronic properties through judicious introduction of topological defects, such as pentagons, heptagons and octagons, into the hexagonal carbon network. In this way metal-metal, metal-semiconductor and semiconductor-semiconductor junctions within individual nanotube molecules are sought. A goal is to elucidate the synthesis mechanism via answering a series of critical questions, including: Are the Yjunctions grown by splitting one nanotube to two or by merging two individual nanotubes to one? What is the growth thermodynamics? What is the relation between the type of topological defects and the chiralities (helicities) of the three branches? How does the nanostructure of Y-junctions affect the electronic/electrical properties? The approach includes: 1) Adapting a chemical vapor deposition technique to synthesize Y-junctions; 2) Detailed studies of nanostructure by a combination of scanning probe microscopy and transmission electron microscopy to reveal the growth mechanism; 3) Investigation of electronic/electrical properties by scanning probe microscopy and electrical transport measurement to relate the electronic/electrical properties to the nanostructures. The methodology established for synthesis, testing and analysis is expected to contribute to rational design of functional carbon nanotube heterostructures and the development of carbon nanotube based devices. Non-Technical Broader Impact: The project will provide training for students at FIU (Florida International University) in electronic material synthesis, characterization and design. Graduate and undergraduate students, especially women and minority students, will be recruited and involved in the proposed research projects. Students will be exposed to forefront research and will participate in scientific discovery in the rapidly developing nanoscience field. Advanced materials physics curricula with focus on nanoscience and nanotechnology will be developed for both undergraduate and graduate students to improve science achievement by enriching science teaching in both classroom and laboratories. It is anticipated that this research and education program will provide FIU students with fundamental knowledge and unique technological skills. It will also have substantial impact on the current Ph.D. program in Physics, and on the Ph.D. program in Materials Science and Engineering, currently in the final stages of approval. As an important part of the proposed outreach program, a "Summer Camp of Nanomaterials Science" for local high school students will be organized through collaboration with the on-going FIU "Physics Learning Center" (PLC) and "Upward Bound program" (UBP) programs serving high school students in South Florida. During the five-week "Summer Camp", students will be involved with research topics through presentations, interactive demonstrations, and hands-on-laboratory experiences. The PI will also participate in outreach to the local community through lectures with the aim of enhancing public understanding of nanoscience and nanotechnology.
(branched carbon nanotubes). It is found that cobalt nitrate and magnesium nitrate are very effective catalyst precursors when thiophene (C4H4S) is used as carbon source for synthesizing branched nanotubes. Catalyst 20 wt% Co/Mg results in high yield branched carbon nanotubes at temperature of 1000 °C and pressure of 200 Torr. Sulfur element in C4H4S is also playing a very important role in the formation of branched nanotubes. A concentration of 0.76-1.5 vol.% of C4H4S in Argon buffer gas will be beneficial to the formation of branched nanotubes. At high C4H4S concentration carbon fibers will form while at low concentration Co9S8 nanowire-filled carbon nanotube will form. High concentration C4H4S vapor deactivates the catalyst and results in the formation of carbon fibers or short nanotubes with poor graphitization while low concentration of C4H4S is not sufficient for functionalizing the catalyst to grow branched nanotubes. The final form of catalyst responsible for the growth of branched carbon nanotubes is Co9S8 and the presence of MgO and MgS as substrates for the Co9S8 catalyst nanoparticles is also indispensable. Initially the catalyst is Co and then it is converted to Co9S8 after the nanotube growth. The nanotube growth and catalyst sulfidation take place simultaneously. The local sulfidation of the catalyst nanoparticle will change its active site for carbon precipitation, which will result in branched nanotubes. The branched nanotube is catalyzed by and grown on a single catalyst particle, rather than through the split or mergence of catalyst nanoparticles. The branched carbon nanotubes have herringbone-like structures. The straight herringbone-like carbon nanotubes exhibit much higher resistivity (5460 μW×cm) than the regular concentric carbon nanotubes. The high resistivity is a contribution of both the resistivity parallel to the basal plane and the resistivity perpendicular to the basal plane of the graphite layers. The resistivity of a bended herringbone-type carbon nanotube is even higher than that of a straight counterpart. Besides the straight carbon nanotubes, the herringbone-type carbon nanotube junctions such as Y-junctions, V-junctions, and T-junctions have also been measured. The measured junctions show metallic conduction behaviors at room temperature. No gating effect has been observed in these junctions. In this project period, four postdoc, five graduate students, eighteen undergraduate students, and seven high school students have participated in the proposed research. Six of the thirty-four participants are women. Among the eighteen undergraduate students, one was from Miami-Dade College, one was from Auburn University, one was from Carnegie Melon University, and the rest are from the Department of Physics and Department of Engineering at Florida International University (FIU). Undergraduate students have performed guided research in the PI’s laboratory and the Advanced Materials Engineering Research Institute at FIU. They have learned to synthesize nanomaterials by using techniques such as self-assembly, chemical vapor deposition technique, electron beam evaporation, and hydrothermal/solvothermal method. They are exposed to basic nanomaterials characterization techniques, such as X-ray diffractometry, scanning electron microscopy, and atomic force microscopy techniques. High school students are mentored by the PI, postdocs and graduate students during their research training. Students are trained to present their research results to the group and write research reports at the end of each semester. Graduate students have been involved in intensive research activities. They are trained with critical experimental skills and assigned with specific research projects. Two students have graduated with Master Degree and each of them has published two research papers in peer reviewed journals. Three current students have published their first papers on their research results and are making progress towards their graduation. The graduate students have been supported to attend conferences such as the American Physical Society March meeting and Materials Research Society meeting to disseminate their research results and listen to the talks of renowned scientists in the field. The PI has offered course entitled "Widely Applied Physics" to undergraduate students from physics and engineering majors. Each year, about 20 undergraduate students attend this class to gain knowledge of nanosciene and technology. The course provides a broad view of the applications of physics principles to nanoscience and nanotechnology. In addition, the PI has given lectures or seminars to undergraduate and graduate students in Department of Physics and Department of Engineering at FIU. During the project period, the PI participated in the annual Physics Open House during which he gave lab tours and research demonstrations, and presented research findings on nanomaterials to high school students, teachers, and families. Each year, about 50-100 high school students, teachers, and families visited the PI’s lab and enjoyed the research demonstration and presentation.
