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Solids facing a plasma are a common situation in many astrophysical systems and laboratory setups. Moreover, many plasma technology applications rely on the control of the plasma-surface interaction, i.e., of the particle, momentum and energy fluxes across the plasma-solid interface. However, presently often a fundamental understanding of them is missing, so most technological applications are being developed via trial and error. The reason is that the physical processes at the interface of a low-temperature plasma and a solid are extremely complex, involving a large number of elementary processes in the plasma, in the solid as well as fluxes across the interface. An accurate theoretical treatment of these processes is very difficult due to the vastly different system properties on both sides of the interface: Quantum versus classical behavior of electrons in the solid and plasma, respectively; as well as the dramatically differing electron densities, length and time scales. Moreover, often the system is far from equilibrium. In the majority of plasma simulations surface processes are either neglected or treated via phenomenological parameters such as sticking coefficients, sputter rates or secondary electron emission coefficients. However, those parameters are known only in some cases and with very limited accuracy. Similarly, while surface physics simulations have often studied the impact of single ions or neutrals, so far, the influence of a plasma medium and correlations between successive impacts have not been taken into account. Such an approach, necessarily neglects the mutual influences between plasma and solid surface and cannot have predictive power. In this paper we discuss in some detail the physical processes of the plasma-solid interface which brings us to the necessity of coupled plasma-solid simulations. We briefly summarize relevant theoretical methods from solid state and surface physics that are suitable to contribute to such an approach and identify four methods. The first are mesoscopic simulations such as kinetic Monte Carlo and molecular dynamics that are able to treat complex processes on large scales but neglect electronic effects. The second are quantum kinetic methods based on the quantum Boltzmann equation that give access to a more accurate treatment of surface processes using simplifying models for the solid. The third approach are simulations of surface process that are based on density functional theory (DFT) and time-dependent DFT. The fourths are nonequilibrium Green functions that able to treat correlation effects in the material and at the interface. The price for the increased quality is a dramatic increase of computational effort and a restriction to short time and length scales. We conclude that, presently, none of the four methods is capable of providing a complete picture of the processes at the interface. Instead, each of them provides complementary information, and we discuss possible combinations.

Reactive oxygen and nitrogen species (RONS) are among the key factors in plasma medicine. They are generated by atmospheric plasmas in biological fluids, living tissues and in a variety of liquids. This ability of plasmas to create a delicate mix of RONS in liquids has been used to design remote or indirect treatments for oncological therapy by treating biological fluids by plasmas and putting them in contact with the tumour. Documented effects include selective cancer cell toxicity, even though the exact mechanisms involved are still under investigation. However, the “right” dose for suitable therapeutical activity is crucial and still under debate. The wide variety of plasma sources hampers comparisons. This review focuses on atmospheric pressure plasma jets as the most studied plasma devices in plasma medicine and compiles the conditions employed to generate RONS in relevant liquids and the concentration ranges obtained. The concentrations of H O , NO , NO and short-lived oxygen species are compared critically to provide a useful overview for the reader.

The synthesis of CdO, Ag O (5 nm) and Ag (~20‒30 nm) nano-objects is achieved simultaneously by nanosecond-pulsed discharges in liquid nitrogen between one cadmium electrode and one silver electrode. Oxidation occurs when liquid nitrogen is fully evaporated and nanoparticles are in contact with the air. No alloy is formed, whatever the conditions, even though both elements are present simultaneously, as showed by time-resolved optical emission spectroscopy. This lack of reactivity between elements is attributed to the high pressure within the discharge that keeps each metallic vapor around the electrode it comes from. Each element exhibits a specific behavior. Cubic Cd particles, formed at 4 kV, get elongated with filamentary tips when the applied voltage reaches 7 and 10 kV. Cd wires are formed by assembly in liquid nitrogen of Cd nanoparticles driven by dipole assembly, and not by dielectrophoresis. On the contrary, silver spherical particles get assembled into 2D dendritic structures. The anisotropic growth of these structures is assumed to be due to the existence of pressure gradients.

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