Moneesh Upmanyu - 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.

In this NSF-Designing Materials to Revolutionize and Engineer our Future (DMREF) project funded by the Division of Chemistry and the Civil, Mechanical, Manufacturing and Innovation Division, Professors Yung Joon Jung, Carol Livermore-Clifford, Moneesh Upmanyu, and David Kaeli of Northeastern University are studying how to create high-performance carbon fibers that could be used for applications in aerospace, high power density energy storage, and lightweight cabling/wiring. The main challenge of creating such fibers is that they need to be mechanically strong while also being exceptional conductors of heat and electricity. To accomplish this goal, the researchers are studying how to fuse networks that contain many nanometer-sized carbon tubes ("carbon nanotubes") into a larger, more seamless structure. The variables being studied include ways to organize the carbon nanotubes into the network and to use electric voltage to fuse the carbon nanotubes together. Experimental studies, computational simulations, and data mining techniques are being applied to understand the complex relationships between the structure of the fused networks and how they perform.

The combination of superior electronic, thermal, and mechanical properties makes carbon nanotube networks an ideal building block for high-performance multifunctional materials, but these advantages are eroded in van der Waals connected networks. If van der Waals interactions were replaced with covalent bonds to create macroscopic seamless carbon nanostructures, performance should increase significantly. This research project is focusing on a novel carbon nanostructure engineering process called nanotube fusion. The method controls input voltages across the network to create covalently bonded molecular junctions (cross-links) between carbon nanotubes, transforming them into larger diameter single-walled carbon nanotubes, multi-walled carbon nanotubes, or multi-layered graphene nanoribbons with measurable property improvement. The research is engaging interdependent experimental, simulation, and data mining efforts to enable scalable multifunctional fibers. The experimental parameters include fusion polarity, frequency, voltage, source-on-time, and external temperature, as well as carbon nanotube structure, assembly process and initial fiber architecture. Characterization data and coarse-grained atomistic simulation of fused fibers relate physical properties to structure and structure to processing. These efforts are complemented by statistical data mining efforts to extract the complex relationship between fiber processing and their properties. The team is working to involve high school students directly in the research; to enable research opportunities for undergraduates from a diversity of backgrounds; and to create symposia on materials science and on multifunctional nanostructured networks.