Research
The soaring global demand for housing, transportation and infrastructure in the face of diminishing natural resources requires the development of lightweight, durable and multifunctional structural materials with performance far beyond that achievable today. Nanomaterials will be critical for meeting this challenge because of their unprecedented high strength, toughness and ductility in conjunction with their novel electronic, optical and transport behavior. Technological breakthroughs using structural nanomaterials require an improved understanding of the origin of mechanical size effects and nanoscale phenomena, as well as advances in the manufacture and assembly of nanostructures. The Gu research group studies the mechanical behavior of architected nanomaterials, in order to design nanocomposites with extraordinary mechanical performance, and sensors that turn mechanical energy into optical or electronic signals.
The Gu group uses the tools of mechanical testing and design, bottom-up colloidal synthesis and self-assembly, 3D printing, in-situ electron microscopy, and materials characterization in the following research areas:
1) Two-photon Lithography
Two-photon lithography offers a precise means of creating architected structures at the microscale. However, typically available materials are limited to polymers. Here, we introduce metallic nanoclusters in a PETA photo resin to act as a two-photon photo-initiator. These clusters enable the high-precision printing of metal-polymer composite materials in a single step. These metal nanoclusters take part in the polymerization process and simultaneously act as a stiff filler material which improves the mechanical performance of the printed structures. For instance, these novel composites have been shown to be highly impact-resistant and recoverable. The two-photon lithography process then enables the production of microlattice metamaterials which take advantage of the superior mechanical properties of the photoresist. Research in this area is branched in these two directions: advancing the material capabilities of two-photon lithography and metamaterial lattice design.
2) Nanomaterials at High Pressure
The diamond anvil cell (DAC) is a high-pressure technique that can be used to measure structural changes in nanocrystals, which overcomes many issues of contact-based nano-mechanical testing techniques. The optical and X-ray transparency of the diamond anvils allows the precise measurement of structural and chemical changes using diffraction and spectroscopic techniques, which has been widely used to study high-pressure phenomena in bulk solids and fluids. In our group, monodisperse ensembles of nanoclusters and nanocrystals are placed within the DAC as colloidal suspensions for absorbance and photoluminescence measurements to understand the photo-physics of deformed nanomaterials. High-pressure X-ray measurements are used to understand the corresponding structural changes.
3) Self Assembly of Lithographed Microparticles
Self-assembly is the process of taking a disordered system of components and ordering them in a specific structure through local interactions. The structure of the assembly dictates its chemical, electrical, and mechanical properties. In colloidal self-assembly, the shape of particles controls the final structure of the assembly. In our group, we use two-photon lithography to 3D print micron-sized particles of different shapes and study the effect of particle shape on the final assembled state. Structural self-assembly has been experimentally observed for simple structures and shapes but has yet to rival the complexity or tunability of biological self-assembly. We are working on bridging this gap by controlling the microscale forces that guide this process.
4) Mechanical Behavior of Nanoparticles and Nanoparticle Assemblies
Metallic nanoparticles can be used as model systems to understand the mechanical behavior of bulk and architected materials. Through in situ SEM/TEM compressions coupled with computational/analytical modeling, we link atomistic deformation mechanisms to mechanical properties such as strength. By first understanding the properties of single particles, we can rationally design more complex systems that use nanoparticles as building blocks. Bulk metallic glasses compacted from nanoparticles are studied for their mechanical properties such as elastic modulus, hardness, yield strength, and ductility. The microstructure and interfaces within and between the nanoparticles are tuned to increase the yield strength and ductility of bulk structures. Recent research efforts include:
1. Colloidal synthesis of multi-metal or high entropy metallic glass boride nanoparticles
2. Crystallization effects in multi-metal boride nanoparticles
3. Assembly of core shell metallic glass structures
4. Interface studies in bulk metallic glasses
5) In-situ Study of Hydrogen Embrittlement Mechanisms
Hydrogen releases three times more energy per mass than gasoline with zero greenhouse gas emissions. Implementing a hydrogen economy requires distribution infrastructure resistant to hydrogen embrittlement (the mechanical degradation of material in the presence of hydrogen). Our group develops in situ techniques to probe fundamental mechanisms of hydrogen embrittlement in high-strength metal. By correlating mechanical behavior in the presence of hydrogen with material microstructure, we build a holistic understanding of hydrogen’s complex interactions with structural materials. This knowledge can help predict the viability of existing infrastructure for hydrogen transport and inform future design of embrittlement-resistant material.
