Ken Ostrikov

Plasma-controlled metal catalyst saturation and the initial stage of carbon nanostructure array growth

The kinetics of the nucleation and growth of carbon nanotube and nanocone arrays on Ni catalyst nanoparticles on a silicon surface exposed to a low-temperature plasma are investigated numerically, using a complex model that includes surface diffusion and ion motion equations. It is found that the degree of ionization of the carbon flux strongly affects the kinetics of nanotube and nanocone nucleation on partially saturated catalyst patterns. The use of highly ionized carbon flux allows formation of a nanotube array with a very narrow height distribution of half-width 7 nm. Similar results are obtained for carbon nanocone arrays, with an even narrower height distribution, using a highly ionized carbon flux. As the deposition time increases, nanostructure arrays develop without widening the height distribution when the flux ionization degree is high, in contrast to the fairly broad nanostructure height distributions obtained when the degree of ionization is low.

Contribution of radicals and ions in catalyzed growth of single-walled carbon nanotubes from low-temperature plasmas

The growth kinetics of single-walled carbon nanotubes (SWCNTs) in a low-temperature, low-pressure reactive plasma is investigated using a multiscale numerical simulation, including the plasma sheath and surface diffusion modules. The plasma-related effects on the characteristics of SWCNT growth are studied. It is found that in the presence of reactive radicals in addition to energetic ions inside the plasma sheath area, the effective carbon flux, and the growth rate of SWCNT increase. It is shown that the concentration of atomic hydrogen and hydrocarbon radicals in the plasma plays an important role in the SWCNT growth. The effect of the effective carbon flux on the SWCNT growth rate is quantified. The dependence of the growth parameters on the substrate temperature is also investigated. The effects of the plasma sheath parameters on the growth parameters are different in low- and high-substrate temperature regimes. The optimum substrate temperature and applied DC bias are estimated to maximize the growth rate of the single-walled carbon nanotubes.

Enhanced ultraviolet photocatalytic activity of Ag/ZnO nanoparticles synthesized by modified polymer-network gel method

Ag/ZnO nanoparticle (NP) heterostructures are synthesized through a modified polymer-network gel method in which glucose is added to the precursor solution to prevent the gel from drastically shrinking during drying of the aqueous solution. Structural and optical properties of the samples are characterized by a range of techniques including XRD, SEM, TEM, XPS, UV–Vis, and PL. The high-quality Ag-ZnO heterostructure is evidenced clearly by high-resolution TEM. The Ag/ZnO heterostructure nanocomposites exhibit a higher photocatalytic activity in the degradation of methyl orange than pure ZnO. Especially, Ag/ZnO NP heterostructures with the Ag/Zn molar ratio of 5:95 (sample ZA-5) show the highest degradation efficiency, which is 11 times higher compared with pure ZnO. The photoluminescence properties of the heterostructures and O defect states are studied to well explain the observed photocatalytic effects. ZA-5 also exhibits competitive photocatalytic activity for the degradation of other pollutant dyes such as Methylene blue and Rhodamine B compared with the recently reported techniques, while showing excellent catalyst photostability as well as offering simplicity and reliability.

H2O/air plasma-functionalized carbon nanotubes decorated with MnO2 for glucose sensing

Multi-walled carbon nanotubes (MWCNTs) were functionalized using dielectric barrier discharge plasma in water vapor-saturated air at 70 °C. MnO2 was deposited on the MWCNTs by chronoamperometry, followed by glucose oxidase (GOx) immobilization, and the resulting GOx/MnO2/MWCNTs electrode was used for electrochemical detection of glucose. Structural, morphological, and elemental microanalysis was performed. Plasma-induced oxygen-based functional groups were confirmed on the MWCNT surfaces and improved their dispersion in aqueous solutions. The maximum amount of these groups was created at the optimum exposure time of 4 min. The GOx immobilized on the MnO2/MWCNTs hybrid showed a well-defined, reversible and surface-controlled redox wave around −0.45 V and a peak to peak separation of 0.04 V. The coefficient and rate constant for electron transfer of GOx were calculated as 0.41 and 1.08 s−1, respectively. The GOx/MnO2/MWCNT-modified electrode exhibited a linear behavior in the range of 0.1–3.2 mM glucose concentration with the competitive detection limit of 3.0 μM and a sensitivity of 24.2 μA mM−1 cm−2. This highly-stable glucose sensing electrode retained more than 76% of its initial faradic current value after 71 days. These results are relevant to the development of next-generation glucose sensors for diverse health- and food-related applications.

Visible photoluminescence from plasma-synthesized SiO2-buffered SiNx films: Effect of film thickness and annealing temperature

The effect of the film thickness and postannealing temperature on visible photoluminescence (PL) from SiNx films synthesized by plasma-assisted radio frequency magnetron sputtering on SiO2 buffer layers is investigated. It is shown that strong visible PL is achieved at annealing temperatures above 650°C. The optimum annealing temperature for the maximum PL yield strongly depends on the film thickness and varies from 800to1200°C. A comparative composition-structure-property analysis reveals that the PL intensity is directly related to the content of the Si–O and Si–N bonds in the SiNx films. Therefore, sufficient oxidation and moderate nitridation of SiNx∕SiO2 films during the plasma-based growth process are crucial for a strong PL yield. Excessively high annealing temperatures lead to weakened Si–N bonds in thinner SiNx films, which eventually results in a lower PL intensity.

Control of radial propagation and polarity in a plasma jet in surrounding Ar

In recent years, the use of shielding gas to prevent the diffusion of the ambient air, particularly oxygen and nitrogen species, into the effluent of the atmospheric pressure plasma jet, and thus control the nature of chemical species used in the plasma treatment has increased. In this paper, the radial propagation of a plasma jet in ambient Ar is examined to find the key determinants of the polarity of plasma jets. The dynamics of the discharge reveal that the radial diffusion discharge is a special phenomenon observed only at the falling edge of the pulses. The radial transport of electrons, which is driven by the radial component of the applied electric field at the falling edge of the pulse, is shown to play an important role in increasing the seed electron density in the surrounding Ar. This result suggests a method to provide seed electrons at atmospheric pressure with a negative discharge. The polarity of the plasma jet is found to be determined by the pulse width rather than the polarity of the ap...

Electron energy probability function in the temporal afterglow of a dusty plasma

The kinetic description of the electron energy probability function (EEPF) in a dusty afterglow plasma is considered for two typical cases: when the rate of electron-neutral momentum-transfer collisions is independent of the electron energy and when it is a power function of the electron energy. The electron Boltzmann equation is solved using the method of characteristics and analytical expressions for the EEPF are obtained for different initial EEPFs (including both Maxwellian and Druyvesteyn distributions) at electron energies larger than the dust-surface potential. The analytical EEPF functions are then used to analyze several experimental parameter regimes of the dust radius and density, the dust-charge decay time, the afterglow duration, etc. It is also found that absorption of electrons by the dust particles plays an important role in determining the EEPF in a dusty afterglow.

Radial constraints and the polarity mechanism of plasma plume

Free to read on publisher website Plasma plumes have found a wide range of applications over the recent decade, stimulating studies of characteristics of plasma plumes generated under different conditions. Regardless of whether they propagate within a dielectric tube or different shielding gases, the behavior of these plumes will be affected by the boundary condition of the plume. Yet, at present, little is known about the behavior of plasma plumes of different polarities, especially negative plasma plumes, when propagating under different boundary conditions. To bridge this gap, in this paper, the characteristics of positive and negative plasma plumes propagating within a quartz tube, ambient Ar, and air are studied. The results reveal that the behavior of the positive plasma plume is similar under three different boundary conditions. However, this is not the case for the negative plasma plume, the behavior of which differs significantly between the three cases. Numerical simulation suggests that electron loss due to the drift in the radial direction impacts significantly the characteristics of the negative plasma plume.

Orientation-controlled, low-temperature plasma growth and applications of h-BN nanosheets

Dimensionality and orientation of hexagonal boron nitride (h-BN) nanosheets are promising to create and control their unique properties for diverse applications. However, low-temperature deposition of vertically oriented h-BN nanosheets is a significant challenge. Here we report on the low-temperature plasma synthesis of maze-like h-BN nanowalls (BNNWs) from a mixture of triethylamine borane (TEAB) and ammonia at temperatures as low as 400 °C. The maze-like BNNWs contained vertically aligned stacks of h-BN nanosheets. Wavy h-BN nanowalls with randomly oriented nanocrystalline structure are also fabricated. Simple and effective control of morphological type of BNNWs by the deposition temperature is demonstrated. Despite the lower synthesis temperature, thermal stability and oxidation resistivity of the maze-like BNNWs are higher than for the wavy nanowalls. The structure and oxidation of the nanowalls was found to be the critical factor for their thermal stability and controlled luminescence properties. Cytotoxic study demonstrated significant antibacterial effect of both maze-like and wavy h-BN nanowalls against E. coli. The reported results reveal a significant potential of h-BN nanowalls for a broad range of applications from electronics to biomedicine.