Zengqi Yu

Optical and RF electrical characteristics of atmospheric pressure open-air hollow slot microplasmas and application to bacterial inactivation

We report electrical properties of radio frequency (RF)-driven hollow slot microplasmas operating in open air but with uniform luminous discharges at RF current densities of the order of A cm −2 . We employ interelectrode separations of 100–600 µm to achieve this open-air operation but because the linear slot dimension of our electrode designs are of extended length, we can achieve, for example, open-air slot shaped plasmas 30 cm in length. This creates a linear plasma source for wide area plasma driven surface treatment applications. RF voltages at frequencies of 4–60 MHz are applied to an interior electrode to both ignite and sustain the plasma between electrodes. The outer slotted electrode is grounded. Illustrative absolute emission of optical spectra from this source is presented in the region from 100 to 400 nm as well as total oxygen radical fluxes from the source. We present both RF breakdown and sustaining voltage measurements as well as impedance values measured for the microplasmas, which use flowing rare gas in the interelectrode region exiting into open air. The requirement for rare gas flow is necessary to get uniform plasmas of dimensions over 30 cm, but is a practical disadvantage. In one mode of operation we create an out-flowing afterglow plasma plume, which extends 1–3 mm from the grounded open slot allowing for treatment of work pieces placed millimetres away from the grounded electrode. This afterglow configuration also allows for lower gas temperatures impinging on substrates, than the use of active plasmas. Work pieces are not required to be part of any electrical circuit, bringing additional practical advantages. We present a crude lumped parameter equivalent circuit model to analyse the effects of changing RF sheaths with frequency of excitation and applied RF current to better understand the relative roles of sheath and bulk plasma behaviour observed in electrical characteristics. Estimates of the bulk plasma densities are also provided. Finally, we present results of afterglow plasma based bacteria inactivation studies (Escherichia coli, Bacillus atrophaeus and B. atrophaeus spores) in which we employ the flowing afterglow plume from a hollow slot microplasma device rather than the active plasma itself, which is fully contained between electrodes. (Some figures in this article are in colour only in the electronic version)

Disk hydrogen plasma assisted chemical vapor deposition of aluminum nitride

We employ a well‐confined hydrogen plasma of disk shape both as a vacuum ultraviolet (VUV) lamp operating primarily at 121.5 nm and as a source of atomic hydrogen radicals. Both VUV photons and atomic hydrogen act to dissociate feedstock gases used in low‐temperature (<400 °C) metalorganic chemical vapor deposition (MOCVD). Thin films have been deposited both with the confined hydrogen plasma and with an excimer laser operating at 193 nm in order to compare the two methods. Preliminary chemical and electrical properties of the films deposited via the two methods indicate the superiority of the atomic hydrogen assisted MOCVD technique.

Radio-frequency-driven near atmospheric pressure microplasma in a hollow slot electrode configuration

We report hollow electrode geometries that support near atmospheric (89 kPa) microdischarges in Ar and Ar–air mixtures when excited at 13.56 MHz with rf voltages of 50–230 V. The slot dimensions are 200 μm wide by 400–600 μm deep and 3–35 cm in length. Optical emission along the slot length is uniform in intensity at applied rf power levels up to 10 W/cm of slot length under the experimental conditions described herein. When metal electrodes are employed, we can draw 0.18 A/cm of self-bias dc current into the rf-driven plasma. Measurements of the electrode self-bias dc voltage level indicate operation in both negative as well as positive self-bias discharge modes. The latter mode, to our knowledge, has not been reported previously in rf microdischarges at near atmospheric pressures.

Absolute UV and VUV emission in the 110-400 nm region from 13.56 MHz driven hollow slot microplasmas operating in open air

We present absolute optical emission spectra in the 110–400 nm regions from radio-frequency-driven (13.56 MHz) hollow slot microplasmas operating in open air at atmospheric pressure. The term microplasma in our research refers to inter-electrode separation (100–600 µm) only, as electrode lengths are scalable from 1 to 30 cm. This creates an extended slot plasma and an associated afterglow plume as described herein. Spectra are presented for gas flows through the microelectrodes of argon and helium with small admixtures of hydrogen and nitrogen into open air. The spectra are discussed in terms of species origin and magnitude of the dominant emission lines. Atomic O and N lines dominate the 110–200 nm region, whereas, in the 200–400 nm region, NO, N2, and NH molecular lines are strongest. The role of the state in the open air microplasmas is discussed and the second positive system of molecular nitrogen (N2(C 3Πg–B 3Πg)), is used to measure the rotational (gas) temperature. Finally, we compare the efficiency and magnitude of light emission from the open air microplasmas with values attainable from commercial sealed mercury lamps in the UVB and UVC regions.

Electrical and optical emission characteristics of radio-frequency-driven hollow slot microplasmas operating in open air

We employ hollow slot electrodes, with pd values of ∼10 Torr cm and average E/N values of ∼70 Td, to create plasmas in open air. We measure the 13.56 MHz Irf–Vrf electrical characteristics of the plasma. Stable discharges, with sinusoidal currents, are obtained up to power densities of 14 kW/cm3, and root-mean-square radio-frequency (rf) currents of 1.5 A/cm of slot length, before nonsinusoidal currents and rf glow-to-arc transitions occur. We report the absolute optical emission in the vacuum ultraviolet region located between 110 and 155 nm, with a focus on the 149 nm atomic nitrogen line. For this atomic N line alone, we find an emitter efficiency of 0.0024.

Bacterial Inactivation of Wound Infection in a Human Skin Model by Liquid-Phase Discharge Plasma

Background We investigate disinfection of a reconstructed human skin model contaminated with biofilm-formative Staphylococcus aureus employing plasma discharge in liquid. Principal Findings We observed statistically significant 3.83-log10 (p<0.001) and 1.59-log10 (p<0.05) decreases in colony forming units of adherent S. aureus bacteria and 24 h S. aureus biofilm culture with plasma treatment. Plasma treatment was associated with minimal changes in histological morphology and tissue viability determined by means of MTT assay. Spectral analysis of the plasma discharge indicated the presence of highly reactive atomic oxygen radicals (777 nm and 844 nm) and OH bands in the UV region. The contribution of these and other plasma-generated agents and physical conditions to the reduction in bacterial load are discussed. Conclusions These findings demonstrate the potential of liquid plasma treatment as a potential adjunct therapy for chronic wounds.

The energy of thermal electrons in electron beam created helium discharges

Abstract We have measured the electron energy of the thermal group of electrons in both longitudinal and transverse electron beam created helium glow discharges. The measurement technique employs the ratio of intensities of spectral lines in the 2s3S−np3P He I series. Values of kTe between 0.07 and 0.11 eV were obtained. These energies are typical of the beam-generated electric field free plasmas. The competitive loss of helium ions by recombination and by charge transfer in a He−Hg electron beam created plasma is calculated. The results are applied to the Hg+ laser pumping scheme using a electron beam created He−Hg plasma.

Transverse electron guns for plasma excitation

We report a plasma gun, which generates on a continuous basis, kilowatt electron beam discharges (e.g., 0.4 A at 4 kV) in an ambient pressure 0.1–1 Torr without differential pumping. Gun design characteristics, operating parameters, and measured beam profiles are given. Electron beam generation on a pulsed basis has also been studied. More than 100 A of beam current has been measured with a Faraday cup biased to −100V. A trapped electron beam scheme for achieving efficient deposition of the electron beam energy in a gas medium is described.

Decomposition of trimethylgallium in the downstream region of a near afterglow plasma

The downstream region of a near afterglow plasma (Δt=100 μs) has been used to decompose trimethylgallium (TMGa) and the products analyzed using a mass spectrometer. The major TMGa dissociation products with a hydrogen plasma were identified as methane and ethane while with a helium plasma ethane, ethylene, and acetylene were the main dissociation species. Time‐resolved optical emission originating from the downstream reaction zone shows that decomposition of TMGa occurs primarily via photodissociation with the hydrogen plasma while both photon and metastable species (He 21 S and 23 S) contributed to the TMGa decomposition with the helium plasma. VUV transmitting and VUV absorbing windows placed between the upstream hydrogen plasma and the downstream TMGa source provided further direct evidence of the role of VUV photodissociation in the TMGa decomposition.

Wide area disk‐shaped vacuum ultraviolet lamp

A wide area vacuum ultraviolet (VUV) lamp employs a ring‐shaped cold cathode to produce an electron beam excited plasma of disk geometry. When excited by the soft vacuum electron beam, molecular hydrogen, nitrogen, and atomic helium emit strong atomic resonance radiation at 121.6, 120, and 58 nm, respectively. The VUV optical power emitted on the atomic resonance line is typically 6–10% of the total applied discharge power. The spatial uniformity of the VUV emission intensity at a discharge input power level of 30 W approaches 6% across the entire disk diameter, up to a maximum of 20 cm.