In the semiconductor industry, plasma-based surface processes such as reactive ion etching and chemical vapor deposition of thin films are vitally important to the manufacturing of integrated circuits. Under certain operating conditions, the formation of small particles in the gas phase has been observed known as a dusty plasma. While plasma synthesis of nanoparticles is attractive for nanotechnological applications, in these ?batch? processes, it is difficult to control particle nucleation and growth in order to obtain narrow dispersions of nanometer-sized particles. In addition, the low operating pressures (<10 Torr) and large infrastructure normally associated with these processes makes it difficult to scale-up for commercial applications. This research focuses on the synthesis of narrowly dispersed, surface-functionalized Group IV semiconductor nanoparticles including silicon and diamond. The Group IV semiconductors are important materials because of their excellent electrical and chemical properties and compatibility with microelectronics processing. However, fundamental understanding of nanoscale properties in these materials remains unknown because of synthetic challenges. Here, a synthesis methodology is proposed based on a continuous-flow, atmospheric-pressure microplasma. Microplasmas are miniaturized versions of direct-current (dc) glow discharges characterized by large concentrations of energetic electrons (1-10 eV) which stabilize the plasma at high pressures. Because of the small volume defined by the microplasma (less than 1 nL), particle growth is restricted, allowing the production of nanometer-sized, non-agglomerated particles. Continuous production of nanoparticles at atmospheric pressure will be monitored by aerosol size classification to obtain particle size distributions in real time. In addition, the particles will be collected and characterized by high-resolution transmission electron microscopy (HRTEM), micro-Raman spectroscopy, and photoluminescence (PL). The availability of precisely tuned silicon and diamond nanoparticles will lead to a fundamental understanding of electronic confinement in these materials. Broader impacts: The research is closely integrated with educational activities that include the establishment of a new program for young women in K-12 schools. The program is comprised of two components: 1) expose middle school and high school students to research in current technological areas including nanotechnology through a new research-based elective class and 2) provide high school students an opportunity to conduct independent research at the university. The outreach activities are expected to enhance the education of students in math and science by exposing them to the practice of science and engineering.
Nanoparticle formation in plasma chemical processes by homogenous nucleation has been recognized for several decades. The formation of "dust" particles during chemical vapor deposition of thin films is undesired in microelectronics device fabrication where the particles can deposit during film growth and cause deleterious effects on device performance. Recently, the advent of nanotechnology and enormous interest in nanomaterials has led to studies of plasma processes to purposefully synthesize nanoparticles. Plasma-based processes offer several potential advantages for nanoparticle synthesis including a non-equilibrium chemical environment, high-purity, and scalability. Unfortunately, plasma systems can be complex, making it difficult to control particle nucleation, often resulting in broad size distributions and agglomeration. In this project, we have developed a new class of plasmas termed microplasmas that offer a unique and potentially transformative platform for nanoparticle synthesis. Microplasmas are plasmas formed in confined electrode geometries of dimensions less than approximately 1 mm. The small spatial scales allow gaseous precursors to be dissociated to nucleate particles by homogenous nucleation and rapidly quenched to limit their size to the nanoscale. In addition, the microreactor geometry focuses the residence time distribution for particle nucleation and growth, leading to narrow size distributions of particles without any further size selection. The process is continuous and carried out at atmospheric pressure, both of which are attractive for scale up. In this project, we have focused our investigations on two classes of materials. First, we studied the synthesis of metal nanoparticles from organometallic precursors. Mono- and multimetallic nanoparticles were synthesized by mixing different precursors in the microplasma. We have shown that the size and composition of the nanoparticles can be tuned by controlling the precursor vapor concentrations in the microplasma which has important implications for catalytic studies. The availability of well-defined catalyst nanoparticles enables fundamental studies of chemical catalysis. We have carried out catalytic growth of carbon nanotubes and found a link between catalyst size and composition and nanotube nucleation and growth. Alloys of nickel and iron were found to lower the minimum growth temperature and activation energy for nanotube nucleation as compared to either pure nickel or iron. Significantly, we also found that the catalyst helped determine the diameter and chirality of nanotubes. The chirality distribution of single-walled carbon nanotubes was found to shift with the catalyst composition, with a narrow distribution of predominantly semiconducting tubes obtained for a Ni0.27Fe0.73 catalyst. This finding has important implications in future electronics where homogeneous nanotubes are required. A second area of study was the synthesis of carbon nanoparticles and, specifically, the nucleation of diamond-phase carbon (see Fig. 1). We showed that carbon nanoparticles can be homogeneously nucleated from ethanol vapor, resulting in particles 2-5 nm diameter that exhibit crystal structures corresponding to cubic diamond and lonsdaleite. The purity of the diamond phase in the aerosol product could be enhanced by adding hydrogen gas to the microplasma. While graphite is the stable form of bulk carbon at normal conditions, diamond has been predicted to be stable at the nanoscale for hydrogen terminated structures with dimensions less than approximately 3 nm. Our results support these predictions and open up the possibility of producing diamond at conditions far from their thermodynamic equilibrium for applications such as polymer coating. Moreover, the possibility of homogenous nucleation of diamond brings into question how diamond is formed in CVD processes where substrate nucleation is often assumed. In addition to the scientific and technological contributions, broader impacts of the project include the involvement of more than 20 undergraduate and 7 high school students in the research. Three of the undergraduate students have won NSF Graduate Fellowships and 7 of the undergraduate students have gone on to Ph.D. programs in engineering. Prof. Sankaran has also developed and taught an elective course on nanotechnology at a local girl’s high school (Hathaway Brown Upper School, Shaker Heights, OH). Exposing young women to science and current research topics has the potential to recruit and train women for careers where they are currently a minority. As a result of the research, Prof. Sankaran has also edited a new book entitled "Plasma Processing of Nanomaterials." The research has led to many interactions with other universities and companies, which promises to increase the impact of this topic.
