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22nd International Symposium on Plasma Chemistry July 5-10, 2015; Antwerp, Belgium Microplasmas as short length and time scale reactors for controlled fabrication of nanomaterials R.M. Sankaran Department of Chemical Engineering, Case Western Reserve University, Cleveland, Ohio, U.S.A. Abstract: This invited talk will present recent progress on the development of microplasma-based schemes for the synthesis of nanomaterials, primarily nanoparticles. In the first scheme, the microplasma dissociates vapour precursors to nucleate aerosol nanoparticles in the gas phase. In the second scheme, the microplasma is employed as an electrochemical cathode and reduces metal ions in water or dispersed in polymer films. Spatial and temporal scales are critical in both schemes to controlling particle nucleation and growth and ultimately producing well-defined nanomaterials. Keywords: microplasma, nanomaterials, nanoparticle, plasma-liquid, nanodiamond 1. Introduction Plasma processing is a well-established top-down process to fabricate microscale patterns in thin films. There has been growing interest in implementing plasmas in bottom up schemes to produce nanomaterials that are smaller in size than possible by lithography. However, nucleation and growth must be precisely controlled to produce well-defined nanomaterials. Here, two schemes are presented based on microplasmas to synthesize nanomaterials. Microplasmas are confined to sub-millimetre dimensions, leading to unique spatiotemporal effects that help control precursor dissociation, particle nucleation and growth, and agglomeration. 2. Experimental Details Fig. 1 schematically illustrates the two schemes that have been developed to synthesize nanomaterials based on microplasmas. The schemes bear similarities to chemical vapour deposition (CVD) and electrodeposition, two well-known techniques for thin film fabrication. In the first scheme, vapour precursors are dissociated in the microplasma, similar to CVD, but the dissociated precursor species homogeneously nucleate particles, free of a substrate. In the second scheme, the microplasma is formed at the surface of an aqueous solution and metal cations either generated from oxidation of the metal anode, or dissolved from a metal salt precursor, are reduced at the cathode, similar to electrodeposition, but nucleate and colloidally disperse in solution, free of a substrate. 3. Results and Discussion 3.1. Gas-phase synthesis of nanodiamonds We have recently applied the aerosol synthesis of nanoparticles by microplasma dissociation of vapour precursors to the formation of nanodiamonds. Nanodiamonds are carbon nanoparticles that exhibit diamond phase and properties. Theory has shown than IN-01 Fig. 1. Two general schemes for synthesizing nanomaterials (nanoparticles) using microplasmas: (a) dissociation of vapour precursors to nucleate aerosol nanoparticles, and (b) cathodic reduction of aqueous metal cations to nucleate colloidal nanoparticles. while graphite is the thermodynamically stable form of bulk carbon at normal conditions, at the nanoscale, tetrahedral forms of carbon which are the precursor to diamond are more stable than hexagonal forms of carbon which are the precursor to graphite. In support of these predictions, we have carried out experiments with a carbon precursor, ethanol, and produced nanometre-sized particles by the microplasma process [1]. Extensive materials characterization of the carbon nanoparticles confirms that diamond-phase carbon is produced with sizes less than 5 nm and structures corresponding to cubic diamond, lonsdaleite, and n-diamond. The importance of the non-equilibrium chemistry on the synthesized material will also be discussed. 3.2. Microplasma reduction of Ag and Au precursors in baths, water jets, and polymeric films Using a microplasma as a cathodic electrochemical electrode, we have studied the reduction of various metal salts in aqueous solutions. In a static bath without any 1 stirring, nanoparticles are formed, but the nucleation and growth depends on the bath volume since the microplasma is formed at the surface and the metal cations must diffuse to the interfacial region. To introduce forced convection and control the reaction zone, we have recently formed a microplasma at the surface of a liquid water jet [2]. Metal cations are transported in the water jet at a controlled flow rate, through the plasmaliquid interface, to nucleate and grow nanoparticles. This approach allows metal nanoparticles to be continuously produced, and the residence time to be controlled so that it is possible to synthesize well-defined nanoparticles. Alternatively, we have dispersed metal salts with polymers, cast films, and exposed the films to a scanning microplasma to reduce metal cations on a substrate. This direct-write method leads to the fabrication of flexible, microscale patterns of metal nanoparticles. Under appropriate conditions, the patterns exhibit high electrical conductivity, enabling applications as flexible electrodes [3]. Characterization of the reduced films suggests that the reduction process involves a diffusion/reaction mechanism whereby the metal cations in the film bulk are electrophoretically drawn to the film surface to be reduced by the microplasma. 4. Conclusions Microplasmas offer a unique platform for synthesizing well-defined nanomaterials by spatiotemporally controlling nucleation and growth. In a continuous flow geometry, particle nucleation and growth are controlled by the confined plasma volume and the flow rate of the gas or liquid which carries the reactant into and out of the reactor. Alternatively, microplasmas can be scanned across a film surface to control nucleation and growth of supported nanomaterials. In addition to the unique nonequilibrium chemistry, these approaches open up possibilities to produce novel materials for a broad range of applications. 5. References [1] A. Kumar, P.A. Lin, A. Xue, B. Hao, Y.K. Yap and R.M. Sankaran. “Formation of nanodiamonds at near ambient conditions via microplasma dissociation of ethanol vapor”. Nature Comm., 4, 2618 (2013) [2] S. Ghosh, B. Bishop, I. Morrison, R. Akolkar, D. Scherson and R. M. Sankaran. “Generation of a direct-current, atmospheric-pressure microplasma at the surface of a liquid water microjet for continuous plasma-liquid processing”. J. Vac. Sci. Technol. A, in press (2015) [3] S. Ghosh, R. Yang, M. Kaumeyer, C.A. Zorman, S.J. Rowan, P.X.-L. Feng and R.M. Sankaran. “Fabrication of electrically-conductive metal patterns at the surface of polymer films by microplasma-based direct writing”. ACS Appl. Mater. Interfaces, 6, 3099-3104 (2014) 2 IN-01