Alain Karma - US grants

TECHNICAL SUMMARY

This award supports integrated research and education in computational approaches aimed at understanding and quantifying the growth physics of nanocrystals, in particular semiconducting nanowires. The combination of quantum confinement, superior transport and diverse surface structures have led to the emergence of semiconducting nanowires as materials of choice for next generation nanoelectronic devices and nanoelectromechanical systems. The realization of almost all of the envisioned applications relies on high-yield nanowire synthesis with controlled morphology and composition. The research focuses on classical vapor-liquid-solid growth, wherein a low melting nanoparticle catalyses vapour phase reduction of the gas precursor and also serves as a conduit for essentially one-dimensional mass transfer to the growing nanowire. Fundamental questions remain on aspects related to nanowire growth rate, steady-state diameter and growth orientation selection, in particular for small nanowires with diameters less than a few tens of nanometers. The underlying size dependent effects related to structure, energetics and dynamics of constituent interfaces necessitate a detailed understanding of the growth process that bridges atomic and continuum scales.

The PIs will combine computational approaches on atomic and continuum scales to study both equilibrium and non-equilibrium aspects of nanocrystalline growth, focusing on the well-characterized Au-catalyzed silicon nanowire growth system. The atomistically informed multi-physics approach is centered around classical molecular dynamics and suitably tailored Monte-Carlo based techniques, integrated within appropriately designed kinetic Monte-Carlo and phase field simulations. The atomistic computations are aimed at quantifying the energetics and kinetics of the consitutent interfaces, and the equilibrium as well as near-equilibrium structure, composition and morphology of the nanowire/catalyst particle system. The atomistic understanding is transferred to i) a tailored kinetic Monte-Carlo approach, and ii) phase-field models that allow the PIs to address the growth aspects in their full complexity at the meso- and continuum scales. It is expected that the insights gained from this research will be applicable to a broad set of technologically relevant nanowire systems.

The research component will be integrated into educational and outreach activities that include i) the summer research discovery program and research internships made available through an NSF-funded interdisciplinary program to promote interest in Mathematics, Physics, Biology, and the sciences among college and high-school students, ii) participation and mentorship within the Materials Research Society chapter at Northeastern University, iii) the development of nanoscale-relevant curricula, iv) design of two capstone projects on nanowire growth and mechanics, v) participation in outreach at local schools and museums in the Boston area and through the Society for Women Engineers at Northeastern University, and vi) integration of related computational efforts within the region via the formation of a New England Network on Computational Sciences.

NONTECHNICAL SUMMARY

This award supports theoretical and computational research and educational activities centered on improving our fundamental understanding of the synthesis of technologically relevant materials that have some of their spatial dimensions confined to very small length scales. A primary focus will be semiconducting "nanowires", which are extended along one direction and have cross-sectional diameters of the order of up to several "nanometers", where a nanometer is one billionth the size of a meter. Such nanowires are of significant technological interest for next-generation electronic devices, energy systems, as well as systems that integrate electronic and mechanical functionality at the nanometer length scale. In spite of detailed experimental observations, the mechanisms that govern the formation of nanowires including size, shape, growth orientation and composition remain poorly understood. Since the realization of almost all of the envisioned applications of nanowires relies on high-yield nanowire synthesis with controlled structure and composition, a detailed theoretical understanding of the growth process at a fundamental level is urgently needed. In this research program, the PIs will combine various state-of-the-art computational approaches on the atomistic and continuum scales to elucidate basic mechanisms of nanowire formation. The insight gained from this multi-physics approach is expected to apply to a broad set of technologically relevant elemental and compound materials at the nanoscale.

The research component will be integrated into educational and outreach activities that include i) the summer research discovery program and research internships made available through an NSF-funded interdisciplinary program to promote interest in Mathematics, Physics, Biology, and the sciences among college and high-school students, ii) participation and mentorship within the Materials Research Society chapter at Northeastern University, iii) the development of nanoscale-relevant curricula, iv) design of two capstone projects on nanowire growth and mechanics, v) participation in outreach at local schools and museums in the Boston area and through the Society for Women Engineers at Northeastern University, and vi) integration of related computational efforts within the region via the formation of a New England Network on Computational Sciences.

DESCRIPTION (provided by applicant): Unraveling the genetic basis of common polygenic diseases, such as hypertension, diabetes and heart failure, will require fresh approaches to view how genes work together in groups rather than singly. In this proposal, we investigate gene network analysis as a promising new approach. Our goal is to identify specific expression patterns of gene modules, rather than single genes, which predict susceptibility to heart failure (HF). A network analysis of DNA microarray data typically groups 20,000 genes into 20-30 modules, each containing 10's to 100's of gene, drastically reducing number of possible candidates required to perform a gene network- based Gene Module Association Study (GMAS), which will be complementary to GWAS. To test the GMAS concept, we will use a systems genetics approach integrating DNA microarray analysis with physiological studies and computational modeling, to examine whether gene module expression patterns predict susceptibility to heart failure (HF) induced by cardiac stress. For this purpose, we will utilize a novel resource developed at UCLA, the Hybrid Mouse Diversity Panel (HMDP), consisting of 102 strains of inbred mice from which a common mouse cardiac modular gene network comprised of 20 gene modules has been constructed. Our preliminary findings reveal that different HMDP strains show considerable variability in both gene module expression patterns and phenotypic response to chronic cardiac stress (isoproterenol). Using biological and computational experiments, we will test the hypothesis that gene module expression patterns among HMDP strains represent different good enough solutions, all of which are adequate for normal excitation-contraction- metabolism coupling, but have different abilities to adapt to chronic cardiac stress. Three Specific Aims integrating experimental and computational biology and combining discovery-driven, hypothesis-driven, and translational elements are proposed, towards the goal of relating HMDP results directly to human heart failure.

Toughness is a material's ability to withstand fracture. Understanding and predicting this key property remains a major challenge for most structural materials. In biological systems high toughness is commonly associated with composite microstructures. Often, soft flexible proteins are found in combination with a hard mineral crystal, organized with specific orientations. This project will utilize novel methods for constructing synthetic composite materials in which the material components can be arranged in a controlled way to achieve a large array of different microstructures. The materials will be tested mechanically to determine their strength and toughness. Computer models of the materials will be generated in order to predict crack propagation, giving insight into the critical physical principles governing tough materials. This project will determine critical characteristics of tough materials. Thereby, the anticipated research outcomes will improve the ability to construct materials with optimal mechanical properties. Undergraduate students and high school summer interns will be involved in the construction and mechanical testing of the materials. A K-8 module entitled 'Being tough' will be developed to teach students underlying principles of mechanics of materials, including composite structures, orientation of components, and material properties.

This project combines computational and experimental studies of crack propagation to determine the relative importance of material anisotropy and heterogeneities in crack path selection and fracture toughness. Novel synthetic discontinuous fiber composites will be produced whereby inhomogeneity and anisotropy of the composite can be tuned with a magnetic field. Numerical simulations will employ the phase field method to predict complex crack paths in materials with defined anisotropy and heterogeneities. Crack propagation will be experimentally measured and computationally predicted in various loading configurations. The interaction of cracks with macroscopic heterogeneities, and crack growth in anisotropic materials will be investigated. With this research we can determine what type and amount of anisotropy (elastic moduli versus fracture energy) lead to crack destabilization, how these instabilities manifest for different modes of fracture in two and three dimensions, and what relative importance anisotropy and heterogeneity have in promoting crack deflection and increased toughness.