Technical. This project addresses locally catalyzed nucleation and growth of semiconductor nanowires (NWs); the approach combines simulation and modeling with in situ and ex situ ex-periments. The closely coupled computational and experimental approach should help to gain fundamental understanding of mechanisms of NW structure evolution and how they can be con-trolled. Atomistic modeling and controlled experiments on NW nucleation and growth kinetics will be carried out so as to provide direct comparisons. The following questions related to metal-catalyzed NW synthesis will be addressed: Q1. What is the mechanism of NW nucleation on dif-ferent substrates and how does it relate to the observed deep sub-eutectic nucleation reported in some catalyst-NW systems? Q2. What mechanisms control normal NW growth and result in growth abnormalities (e.g. kinking)? Q3. What is the mechanism of the termination of NW growth (catalyst nanoparticle solidification)? Using the Si/Ge NW system as a test bed, in situ and ex situ measurements during CVD growth at controlled temperature and pressure conditions will be compared with numerical predictions, validating and guiding the development of compu-tational methods to overcome time scale challenges in the modeling of nucleation events. The latter is a well known major limit to the range of applicability of atomistic simulations to date. The new understanding gained is expected to benefit the broader field of atomistic modeling due to the common occurrence of nucleation events in physical, chemical and biological processes. Non-Technical. The project addresses fundamental research issues in a topical area of elec-tronic/photonic materials science having technological relevance. This basic research, being con-ducted within the context of worldwide efforts among academic and industrial labs to use semi-conductor NWs in new devices, is expected to have strong technological impact. Results ob-tained through this project on NW synthesis may have impact on the development of these and other emerging technologies. The prospect of new science and technology breakthroughs enabled by semiconductor NWs attracts strong interest in such research among undergrads, graduate stu-dents, and faculty. The PIs plan to leverage this 'excitement' and build on it in their education and outreach activities. Specific initiatives will be pursued in: 1) science education outreach to under-represented minorities through a collaboration with a high school science teacher; 2) un-dergraduate experiences in both computational and experimental components of the proposed re-search; and 3) undergraduate and graduate course module development based on research find-ings.
This project lead to a successful investigation of the fundamental growth mechanisms of semiconductor nanowires. The intellectual merit of this project lies in the closely coupled computational and experimental approaches to gain a fundamental understanding of nanowire structure evolution and growth anomalies. This project has had a broader impact in the entire field of atomistic modelling by showing how experiments and modelling complement each other to solve challenging problems. It also impacts the worldwide efforts among academic and industrial researchers to synthesize semiconductor nanowires controllably, as required to develop emerging nanowire-based technologies. The following are some of the major accomplishments in this project. 1. We developed an empirical interatomic potential model for the Au-Si binary system that is fitted to the experimental phase diagram. 2. Using the new atomistic model, we performed the first molecular dynamics (MD) simulations of Si crystal growth in contact with a Au-Si alloy liquid. Several growth mechanisms are identified for growth on different solid surface orientations. 3. We have observed early stage kinking of Ge nanowires in controlled growth environments in the absence extrinsic factors such as temperature and pressure changes. The side wall faceting of nanowires is found to have a significant effect on the onset of kinking. 4. The solidification of Au nano-particles into a recently-discovered hexagonl close packed (HCP) phase during the termination of vapor-liquid-solid Ge nanowire growth was systematically studied by X-ray diffraction. The volume fraction of the HCP phase, among all the Au nanoparticles present after Ge nanowire growth, was measured. 5. Photoluminescence (PL) of Ge nanowires have been studied. This is a useful tool to measure the crystalline quality of the synthesized Ge nanowires and the presence of defects on their surfaces.
