Thin-film deposition with high pressure capillary micro-discharges under different supersonic flow and shock regimes

Hollow cathode microplasma jets operating under different supersonic flow regimes were investigated for spray deposition of nanostructured CuO thin films. A variety of supersonic flow phenomena (e. g., shock fronts, stagnation regions, flow instabilities, and turbulent mixing) were studied using fluorescence imaging of the jet afterglow, and were seen to have a dramatic effect on film deposition. Material growth with under-expanded flows was dominated by flux inhomogeneities related to standoff shocks and stagnation regions due to complex fluid interactions between the jet and substrate, resulting in dense, polycrystalline films with variable thickness. For a pressure-matched jet, film thickness was more uniform and morphology evolved from dense polycrystalline films to crystalline nanowires oriented perpendicularly to the substrate as source-substrate distance was increased. Overall, this work demonstrates that microplasma jet-based deposition is a scientifically rich and promising approach for nanostructured thin-film growth, which involves a unique combination of supersonic flow phenomena, plasma physics at high pressure, and mass transport processes.