Laminar and turbulent flow modes of a cold atmospheric pressure argon plasma jet are investigated in this work. The effects of the gas flow rate, applied voltage, and frequency on each plasma mode and on intermodal transitions are characterized using photographic, electrical, and spectroscopic techniques. Increasing the gas flow rate increases the plasma jet length in the laminar mode. Upon transition to the turbulent mode, increasing the gas flow rate leads to a decrease in the plasma jet length. The flow rate at which the jet transitions from laminar to turbulent increases with the applied voltage. The presence of nitric oxide (NO) radicals is indicated by the emission spectra of the turbulent plasmas only, while excited Ar, N₂, OH, and O excited species are produced in both laminar and turbulent modes. With no distinctive behavior observed upon transition between the two operating modes, the power consumption was found to be insensitive to gas flow rate variation, while the energy density was found to decrease exponentially with the gas flow rate. Rotational and vibrational temperature measurements of the two plasma modes indicated that they are of the non-thermal equilibrium plasma type. Since they offer NO radicals while maintaining the benefits of the laminar plasma jet, the turbulent plasma jet is more useful than its laminar counterpart in biomedical applications.

Thermal atomic layer etching (ALE) of nickel (Ni) may be performed with a step of thin-layer oxidation of its surface and another step of its removal by gas-phase hexafluoroacetylacetone (hfacH) as an etchant. In this study, adsorption of hfacH and possible formation of volatile nickel hexafluoroacetylacetonate Ni(hfac)2 on a NiO surface were investigated based on the density functional theory (DFT) with more realistic surface material models than those used in the previous study [A. H. Basher et al., J. Vac. Sci. Technol. A 38, 022610 (2020)]. It has been confirmed that an hfacH molecule approaching a NiO surface deprotonates without a potential barrier and adsorbs on the surface exothermically. In addition, stable adsorption of two deprotonated hfacH molecules on a NiO (100) surface was found to occur not on a single Ni atom but over a few Ni atoms instead, which makes the formation of a Ni(hfac)2 complex on the flat surface very unlikely even at elevated temperature. However, if the surface is rough and a Ni atom protrudes from the surrounding atoms, two hexafluoroacetylacetonate anions (hfac−) can bond to the Ni atom stably, which suggests a possibility of desorption of a Ni(hfac)2 complex from the surface at elevated temperature. Given the experimentally observed fact that desorption of Ni(hfac)2 complexes typically takes place on a NiO surface at a temperature of ∼300–400 °C, our DFT calculations indicate that the surface roughness of an oxidized Ni surface facilitates the formation and desorption of organometallic complexes Ni(hfac)2, and therefore, the resulting Ni surface after ALE can be smoother than the initial surface.

Cold atmospheric pressure sterilization is one of the nominated and efficient techniques to prevent the spread of diseases. Reactive species such as O and OH and other radicals play a major role in the mechanism of plasma sterilization. Therefore, in this work, oxygen was mixed with different parentage from (0.2 to 1.2%) to argon to enhance the generation of the reactive species and increase the argon atmospheric pressure plasma sterilization efficacy. The emission spectra from the jet increase the radicle line intensities by increasing the percentage admixture of O₂ with the argon gas to reach a maximum power at 0.8; then, it gradually decreases with a higher O² percentage. The OH band intensity decreases with increasing the admixture of O₂. The jet with different O₂ percentages was tested against Gram-positive S. epidermidis, which is the causal agent of nosocomial infections. The maximum reduction in colony-forming units (CFU) was observed at 0.2% O₂. No bacterial growth was observed at the later concentration applied for 8 min and the same case was detected at 0.4% O₂ applied to 16 min.

Adsorption of enol hexafluoroacetylacetone (hfacH) on nickel oxide (NiO) fcc (100) and metallic Ni fcc (100) surfaces and the stability of the adsorbate was examined using first-principles quantum mechanical simulations. It was shown that an hfacH molecule can be unstable and dissociate on an Ni metal surface. On an NiO surface; however, an hfacH molecule can be deprotonated and form a hexafluoroacetylacetonate anion (hfac−) bonded stably with positively charged Ni atoms of the surface. The results are consistent with observations of the interaction of hfacH with NiO and Ni surfaces in earlier experiments. The results also explain the mechanisms of the adsorption steps in the thermal atomic layer etching of Ni based on the cyclic processes of surface oxidation and formation of volatile organo-nickel complexes.

In thermal atomic layer etching (ALE) of Ni, a thin oxidized Ni layer is removed by a hexafluoroacetylacetone (hfacH) etchant gas at an elevated surface temperature, and etching ceases when a metallic Ni surface appears (self-limiting step). However, atomistic details of the self-limiting step was not well understood. With periodic density-functional-theory calculations, it is found that hfacH molecules barrierlessly adsorb and tend to decompose on a metallic Ni surface, in contrast to the case of a NiO surface, where they can form volatile Ni(hfac)₂. Our results clarify the origin of the self-limiting process in the thermal ALE.