Fundamental Studies of Photo-Assisted Chemical Vapor Deposition on Aerosol Nanoparticles

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National Science Foundation - Division of Chemical &Transport Systems ? Particulate & Multiphase Processes Program (1415)Proposal Number: 0730184 Principal Investigators: Steven L. Girshick Affiliation: University of Minnesota Proposal Title: Fundamental Studies of Photo-Assisted Chemical Vapor Deposition on Aerosol Nanoparticles For nanoparticles to be useful in a wide variety of applications, methods must be developed to control their surface properties. This can be accomplished either by coating the nanoparticle with a thin film, producing a 'core-shell' structure, or by attaching chemical functional groups to the nanoparticle surface. In some cases the goal is to stabilize or passivate the nanoparticle surface, in other cases to impart some desired functionality.A new method was recently demonstrated by the PIs, in which aerosol nanoparticles are coated by photo-assisted chemical vapor deposition (photo-CVD), driven by vacuum ultraviolet radiation from excimer lamps. Photo-CVD has several potential advantages as a method for coating nanoparticles. It can be achieved in a low-temperature, atmospheric-pressure gas, features that may have important advantages over alternative methods. Excimer lamps are relatively economical and easy to use, and are increasingly being used in industry for a variety of applications, including thin film deposition on large-area substrates?but not, to this point, on nanoparticles.While the feasibility of this new technology has been demonstrated, fundamental scientific questions have yet to be addressed. What are the relative roles of photodissociation in the gas phase versus chemistry driven by UV radiation incident on the particle surface? What is the relationship between coating growth rate and particle size? What is the effect of temperature on coating growth? Under what conditions can nanoparticles be coated while avoiding photoinduced homogeneous nucleation of particles from the reactant gas? Does UV-induced particle charging affect coating growth? Can UV-induced particle charging be used to suppress coagulation? Specific experiments are proposed that are designed to test hypotheses associated with each of these questions. These hypotheses will be tested in the context of three model chemical systems, involving metallic, semiconductor, and oxide nanoparticles: amorphous organic films on aluminum nanoparticles, dense organic monolayers on silicon nanoparticles, and SiO2 films on magnetic iron oxide nanoparticles. Potential applications of the resulting core-shell nanoparticles range from solid fuel propulsion to photovoltaics and photonics, and from biological imaging to tumor destruction. For each of these systems, coating formation will be studied using online diagnostics including tandem differential mobility analysis and Fourier transform infrared spectroscopy, and off-line, by high-resolution transmission electron microscopy and related diagnostics such as energy dispersive X-ray spectroscopy.The proposed research has a wide range of broader impacts. On the technical side, it will lead to the development of a new and widely applicable method for coating nanoparticles. Relatively few methods exist for creating such coatings, especially in a room-temperature, atmospheric-pressure gas-phase environment. Photo-CVD has the additional advantages of being scalable and capable of high throughput processing of nanoparticles. It is therefore expected that this process will attract the attention of both the academic and industrial science and engineering communities.The proposed research will also serve as a springboard for a number of education and outreach activities. These include K-12 outreach through local public school districts, involvement of undergraduates in research, training of at least two graduate research assistants in a highly interdisciplinary research environment, teaching of interdisciplinary graduate courses, and fostering greater involvement of women and underrepresented groups in our graduate research programs.