Atmospheric Pressure Nonequilibrium Plasma Jets for Synthesis, Deposition, and Modification of Various Oxides
Authors: Paxton William F , Ravipati Srikanth , Ajayi Babajide , Sunkara Mahendra Kumar 
Published: February 2020

Atmospheric pressure nonequilibrium plasma jets (APNP-Js) have garnered increasing interest over the last decade due to their simplicity, high excitation selectivity, and relatively low temperature operation.1 The utility of such plasmas has already been realized in areas ranging from medical applications (sterilization, intracellular treatments, and surface functionalization, among others) to industrial material processing (etching, cleaning, and surface treatments). However, not until recently have these microjet plasmas been explored as an alternative to costly and complex plasma sources (i.e. microwave, glow discharge, and electron cyclotron resonance, among others) for the deposition and synthesis of materials.2-4

Current efforts at the University of Louisville’s Conn Center for Renewable Energy Research are being directed toward employing various APNP-J configurations for this purpose. One promising application we are presently investigating entails the use of dielectric barrier discharge-like APNP-Js to deposit ceramic nanoparticles on a desired substrate. Here, a low frequency (<100kHz) RF plasma driver is used to ignite a helium plasma inside a quartz tube. An aerosol of the ceramic nanoparticles is then generated in a custom-built fluidization system and directed through the plasma via a second inlet. It has been found that successful deposition typically occurs at total flow rates less than 3 SLM. The plasma deposition of particles into thin films seems to offer an interesting way to form uniform, pinhole-free layers with thicknesses ranging from 50nm to 2µm. It is envisioned that this technique can serve as an intermediary between atomic layer deposition and spin coating. The sintering of particles during plasma deposition of particles is an interesting phenomenon, which needs to be understood further.
Another promising application extends upon our group’s recently published plasma oxidation-based synthesis technique for the production of mixed metal oxide solid solutions.5 Favorable results in this study were obtained by exposing nickel, iron, and manganese nitrate solutions to an atmospheric microwave plasma flame reactor. As an alternative to this microwave driven plasma, we have recently observed that single electrode plasma jets can achieve similar results on comparable time scales. The plasma apparatus utilized for this application uses the same RF plasma driver described above to generate a helium plasma. However, a different configuration was designed to permit the introduction of oxygen to the plasma without extinguishing it. The resulting plasma jet is much more concentrated with characteristics that border on the arc regime of operation. Upon exposure of metal-nitrate solutions to this plasma, their colors were observed to change rapidly (<2 seconds) to black and dark grey. Results were verified by x-ray diffraction and electron microscopy. Work is currently on-going to create a wide ranging metastable mixed metal oxide alloys for future material development.

Fig. 1 Scanning electron micrograph of metal oxide nanopowder deposited on silicon substrates via APNP. (a) Top view and (b) Cross-sectional view.

Acknowledgements:Authors gratefully acknowledge the Conn Center for Renewable Energy Research for facilities and access to characterization equipment
In addition to these two promising applications, we are currently exploring APNP-Js for use in a wide range of other applications including greenhouse gas reduction, nanowire deposition, and surface functionalization. It is our goal to further extend the capabilities of this technology in an effort to simplify and reduce the cost of plasma processing in mass-production.

References:

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2Gandhiraman, R. P. et al. Plasma Jet Printing of Electronic Materials on Flexible and Nonconformal Objects. ACS Applied Materials & Interfaces
6, 20860-20867, doi:10.1021/am505325y (2014).

3Gandhiraman, R. P. et al. Plasma jet printing for flexible substrates. Applied Physics Letters
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4Kona, S., Kim, J. H., Harnett, C. K. & Sunkara, M. K. Carbon Nanotube Growth Studies Using an Atmospheric, Microplasma Reactor. IEEE Transactions on Nanotechnology
8, 286-290, doi:10.1109/TNANO.2008.2010544 (2009).

5Ajayi, B. P. et al. A rapid and scalable method for making mixed metal oxide alloys for enabling accelerated materials discovery. Journal of Materials Research
FirstView, 1-12, doi:doi:10.1557/jmr.2016.92 (2016).

Figure 1