Self-organized patterns appear in many biological, chemical and physical systems, including electric discharges. Under certain conditions, self-organized patterns also form in plasmas generated below room temperature. These so-called cryoplasmas have also shown promise for low-damage materials processing; however, the underlying mechanisms and experimental conditions that lead to either uniform discharges or those containing self-organized patterns are still not understood completely. Here, we investigated the formation and dynamics of self-organized patterns in dielectric barrier cryoplasmas generated at plasma gas temperatures ranging from 264 down to 7 K at a constant gas density ρ = 5 × 10 19 cm −3 . The electrode gap was 0.15 mm and the cryoplasmas were generated at voltages between 0.8 and 1.5 kV, at frequencies ranging from 20 to 30 kHz. The discharges were characterized by time-resolved imaging, optical emission spectroscopy and current-voltage measurements. For temperatures down to 250 K, the discharges are uniform, whereas between 250 and about 140 K, self-organized, bright filamentary patterns form. Below that temperature, the discharge regime changes again to a uniform glow and for temperatures below 20 K, different types of discharges-uniform, but also self-organized dark solitons and bright stripe patterns-are observed. The cryoplasmas show current-voltage characteristics that are similar to atmospheric pressure glow discharges and the different types of uniform or self-organized discharges are suggested to be caused by the disappearance of impurities in the plasma as the temperature is lowered, and changes in the mobilities of ion species and surface charges.

The generation of plasmas in a microreactor is demonstrated in xenon from atmospheric pressure up to supercritical conditions. Ac high voltage at a frequency of 15 kHz was applied across a 25-µm discharge gap between a tungsten wire and a fused silica micro-capillary tube in a coaxial configuration. Using this continuous flow supercritical fluid microreactor, it was possible to synthesize diamantane and other diamondoids up to nonamantane, using adamantane as a precursor and seed. It is anticipated that plasmas generated in supercritical fluid microreactors may not only allow faster fabrication of diamondoids, but also offer opportunities for the fabrication of other nanomaterials.

We have synthesized diamondoids using dielectric barrier discharge microplasmas generated inside a microcapillary reactor in supercritical xenon. The plasmas were generated near the critical temperature ( ) and pressure ( ) of xenon in the ranges of and under both batch-type and continuous flow conditions with gas flow rates of 0.01–0.5 mL min−1. Micro-Raman spectra of the synthesized particles showed features characteristic of diamondoids, while gas chromatography–mass spectrometry measurements revealed that diamondoids up to undecamantane were possibly synthesized. Further, the amount of obtained diamantane was greater than those obtained using previously reported diamondoid synthesis processes that involve plasmas in supercritical fluids. This increase is attributed to the higher solubility of the supercritical medium, i.e., xenon, and the higher efficiency of the microreactor. A detailed gas chromatography–mass spectrometry analysis showed that higher diamondoids grow in a stepwise manner via the alternate removal of hydrogen atoms and the addition of methyl groups.

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