This collaborative grant between the University of Texas at Arlington (UTA) and University of Minnesota (UMN) funds a combined experimental and theoretical/computational research program that aims to gain fundamental understanding into the mechanics of hierarchically structured gold nanospheres. Multiple length scales such as grain size, shell thickness, and diameter of sphere exist within an individual structure. Specifically, our gold nanospheres are about 100 nm in diameter, and feature a polycrystalline shell with a thickness ranging from 20 to 50 nm, and grain size around 5 nm. Such combination provides a ?golden? opportunity to explore the effects of structural hierarchy on the deformation mechanisms of nanoscale materials. Due to their hollow nature, the gold nanospheres can be easily observed and tested with in-situ TEM nanoindentation without elaborate sample preparation. They can be positioned precisely into an addressable array using the ?electrostatic funneling method?. This greatly facilitates nanoindentation experiments, enabling a statistical examination. Model polycrystalline hollow nanoparticles with similar characteristics will be simulated with state-of-the-art multiscale theoretical/computational techniques that combine Molecular Dynamics and Discrete Element Method. Experimental input from the in-situ TEM nanoindentation will greatly facilitate the multiscale modeling, specifically the critical issues of time-acceleration and coarse-graining. The obtained simulation results will help comprehend experimental data and identify deformation mechanisms with atomistic resolution.
The general mechanical behavior of nanoscale metals is of great interest for technological applications such as new ultrastrong structural materials, microelectronics and micro- and nano-electromechanical devices and systems. If successful, this research will help establish important design principles targeting such applications. The proposed combination of Molecular Dynamics with Distinct Element Method to tackle problems at different length scales is expected to find broad applications in the modeling community. The proposed research program is integrated with a multi-layered education and outreach program that includes development of new courses designed to educate students in an interdisciplinary field, summer camps for K-12 students, and outreach activities through MRSEC-UMN, existing REU, cyber module development, and demonstrations at high schools and the Science Museum of Minnesota. A coordinated outreach plan in partnership with the Society for Hispanic Professional Engineers is proposed to provide opportunities to underrepresented groups by targeting and involving the large Hispanic population in the State of Texas, which includes a minority recruitment program, a SciTech Latino Summer Camp for K-12 Hispanic students, and a public awareness component.
Gold nanoparticles have been used in many different areas. For example, they can be used as nanomedicine for photothermal therapy of cancers; they can be used as imaging contrast agents for diagnostics; they can be used as catalysts for many chemical reactions. We developed a process to produce a special type of gold nanoparticles, polycrystalline hollow gold nanoparticles (HGNPs). Such hollow nanoparticles (<100 nm) have a polycrystalline gold shell with tunable thickness in the range of 20 to 50 nm and a grain size around 5 nm. HGNPs can potentially be used as nano-containers to store and deliver gaseous/liquid materials. Stored materials can also be released by either photothermal heating through their fascinating plasmonic properties or just diffusion process. This unique property opens up many new possibilities, such as gaseous drug deliveries, new hydrogen storage schemes and catalysts using gaseous molecules. HGNPs can be easily produced in large amounts with a narrow size distribution using bubble template synthesis method. In this process, electrochemically evolved hydrogen nanobubbles serve as templates and reducing agent to reduce Au complex ions into metal Au. Metal Au covers the bubbles to form hollow Au nanoparticles. Anodic aluminum oxide (AAO) membrane which has 300 nm diameter through channels were used inside the solution to collect nanoparticles. In this project, we conducted a systematic investigation on the effect of key process parameters on the structural properties of hollow gold nanoparticles (HGNPs), and obtained synthesis conditions for tuning the number, size, morphology and microstructures of hollow gold nanoparticles. Nowadays, nanostructured materials including nanocrystalline materials, nanoparticles, and ultrathin films have been used in many fields such as new ultrastrong structural materials, microelectronics and micro- and nanoeletromechanical devices and systems. The mechanical properties of nanoscale metals is of great interest for their technological applications. HGNPs are nanostructures made of nanocrystalline materials, belonging to the growing class of nanostructures with inherent structural hierarchy (multiple length scales existing within an individual structure: grain size, shell thickness and diameter of sphere). HGNPs provides the unique opportunity to complement previous efforts on measuring/probing mechanical behavior of small volumes and add a new perspective that can elevate our understanding of nanoscale mechanics. We conducted a series of mechanical testing on individual HGNPs. We found that the Au shell in HGNPs behave mechanically in between solid thin film and simple agglomerate of nanoparticles; softer and more flexible than solid thin films but much stronger than an agglomerate of nanoparticles. This is the first time that mechanical behavior of polycrystalline materials with not-fully developed grains has been observed. Other polycrystalline materials fabricated from chemical synthesis could have similar behaviors. In addition, during this project, we developed a unique process to coat HGNPs with other precious metals. In this method, a Cu layer is first coated to Au nanoparticles by a simple but robust electroless deposition process. Then, the Cu layer can be readily replaced by less reactive metals through simple replacement reactions using aqueous solution containing metal ions without any other additives. This Cu-mediated process provides a facile way to incorporate isotopes into Au nanoparticles from solutions with a trace level concentration to form brachytherapy nanoseeds for the treatment of cancers and other diseases. This method can also be used for producing more effective catalysts. Total 9 papers have been published in peer-reviewed journals, and 11 presentations have been given in international conferences. As a major educational component, we developed an experiment module, 'colloidal gold', for the experiment-oriented course, 'MSE 4320/5320 Nanoscale Materials', which is offered in every spring semester at University of Texas at Arlington. Every year; about 15 students enrolled in this class, usually including 20% undergraduate students. We also participated in annual Summer Camp in Materials Science for Middle and High School students funded by ASM International and the North Texas Chapter of ASM. Every year, about 25 students got into our labs, and exposed to the research activities related to this project. Total two PhD students, two MS students, and three undergraduate students have participated in this project. They got trained on the whole scientific research process, from experiment design, implementation to data analysis. Graduate students also worked as Teaching Assistant for the course of Nanoscale Materials, conducting lab lectures and demonstrations. They also conducted all summer camp activities. These help them to obtain critical teaching skills.
