Controlling plasma composition during carbon film deposition in microwave discharges by means of optical-emission spectroscopy

Optical emission spectra from the microwave discharge plasma that is used to activate gas-phase deposition of carbon films are systematically investigated under various deposition conditions. The line emission intensities from CH and C₂ radicals, which are responsible for the growth of the diamond and graphite phases, respectively, are studied as functions of the main macroparameters of the process. To find the relation between the features of the emission spectra and the composition of the films obtained, the films were examined using Raman spectroscopy and electron microscopy. It is shown that monitoring the relative intensities of the spectral lines can be used to obtain the desired type of film, in which case the state of the substrate surface and the presence of a catalyst on it also play an important role. Experiments on the deposition of carbon films in the pulsed regime of plasma excitation show the possibility of changing the phase composition of the film by varying both the pulse repetition rate and the off-duty factor. At the same average microwave power, the rate of film deposition in the pulsed regime of plasma excitation is lower than that in a continuous discharge; however, the growth rate of the graphite phase decreases insignificantly.     

Polycrystal diamond growth in a microwave plasma torch

Diamond films of different structures were deposited on quartz, WC-Co, and molybdenum substrates in a microwave plasma torch discharge in an argon-hydrogen-methane gas mixture in a sealed chamber at pressures close to atmospheric by using the chemical vapor deposition technique. Images of diamond polycrystal films and separate crystals, as well as results of Raman spectroscopy, are presented. The spectra of optical plasma radiation recorded during film deposition demonstrate the presence of intense H_α hydrogen and C₂ radical bands known as Swan bands.     

The Mirror Image Symmetry in the Optical Absorption/Luminescence Spectra of Silver Nanoparticles in Ag/Ag2O Composites Synthesized by Oxygen Plasma Treatment of Silver Thin Films

The aim of this paper is to compare the spectral features of the response plasmon peak in both the optical absorption and photoluminescence spectra of the Ag/Ag₂O composites synthesized by treating silver thin films manufactured by thermal evaporation method with oxygen plasma afterglow. Results show that close values of the relative area and spectral width of the plasmon response peaks in the optical absorption/photoluminescence spectra were obtained in the case of samples with high oxygen content. In addition, the intensity ratio (I_absorption/I_luminescence) of these samples decreases linearly with increasing silver oxide grain size.

     

Linear dichroism and the orientation of reaction centers of Rhodopseudomonas sphaeroides in dried gelatin films

Aqueous mixtures of reaction centers of Rhodopseudomonas sphaeroides and gelatin were dried to form thin films. Following hydration, these films were stretched as much as two to three times their original length. Polarized absorption spectra showing linear dichroism were obtained for both unstretched and stretched films, with the planes and stretching axes of the films mounted in various geometries relative to the electric vector of the measuring beam. These data were analyzed in terms of the following model: Reaction centers possess an axis of symmetry that is fixed in relation to the reaction center structure. In unstretched films this axis is confined to the film plane and oriented at random within the plane. In stretched films the symmetry axis is aligned with the direction of stretching. In both preparations reaction centers are distributed randomly with respect to rotation about the axis of symmetry. The data are consistent with this model when the analysis acknowledges less than perfect orientation. For perfect orientation in a stretched film the model predicts uniaxial symmetry about the axis of stretching. The approach to this condition was examined with films stretched to different extents. Extrapolation yielded dichroic ratios for the ideal case of perfect orientation, and allowed calculation of the angles between the axis of symmetry and the various optical transition dipoles in the reaction center. This treatment included the two absorption bands of the bacteriochlorophyll ‘special pair’ (photochemical electron donor) in the Q_x region, at 600 and 630 nm, which we were able to resolve in light minus dark difference spectra.     

Production of singlet oxygen in a non-self-sustained discharge

The production of O₂(a¹Δ_g) singlet oxygen in non-self-sustained discharges in pure oxygen and mixtures of oxygen with noble gases (Ar or He) was studied experimentally. It is shown that the energy efficiency of O₂(a¹Δ_g production can be optimized with respect to the reduced electric field E/N. It is shown that the optimal E/N values correspond to electron temperatures of 1.2–1.4 eV. At these E/N values, a decrease in the oxygen percentage in the mixture leads to an increase in the excitation rate of singlet oxygen because of the increase in the specific energy deposition per O₂ molecule. The onset of discharge instabilities not only greatly reduces the energy efficiency of singlet oxygen production but also makes it impossible to achieve high energy deposition in a non-self-sustained discharge. A model of a non-self-sustained discharge in pure oxygen is developed. It is shown that good agreement between the experimental and computed results for a discharge in oxygen over a wide range of reduced electric fields can be achieved only by taking into account the ion component of the discharge current. The cross section for the electron-impact excitation of O₂(a¹Δ_g and the kinetic scheme of the discharge processes with the participation of singlet oxygen are verified by comparing the experimental and computed data on the energy efficiency of the production of O₂(a¹Δ_g and the dynamics of its concentration. It is shown that, in the dynamics of O₂(a¹Δ_g molecules in the discharge afterglow, an important role is played by their deexcitation in a three-body reaction with the participation of O(³P) atoms. At high energy depositions in a non-self-sustained discharge, this reaction can reduce the maximal attainable concentration of singlet oxygen. The effect of a hydrogen additive to an Ar: O₂ mixture is analyzed based on the results obtained using the model developed. It is shown that, for actual electron beam current densities, a significant energy deposition in a non-self-sustained discharge in the mixtures under study can be achieved due to the high rate of electron detachment from negative ions. In this case, however, significant heating of the mixture can lead to a rapid quenching of O₂(a¹Δ_g molecules by atomic hydrogen.     

Absorption spectra of mixed monomolecular films of chlorophyll and photosynthetic electron carriers at a gas-water interface

Absorption spectra of mixed monomolecular films containing chlorophyll and endogenous redox reagents are studied at a gas-water interface. Overlapping absorption spectra are resolved by difference spectroscopy and fourth derivative analysis. Monomolecular films are formed in a Langmuir trough using a Wilhelmy film balance. A reaction between vitamin K₁ and chlorophyll is observed both in the dark and after illumination. A smaller interaction occurs between α-tocopherolquinone and chlorophyll. A light-driven reaction occurs between oxidized plastocyanin and chlorophyll, but not between reduced plastocyanin and chlorophyll. An interaction is observed between cytochrome c and chlorophyll both in the dark and after illumination. Evidence is obtained indicating that the presence and amount of aggregated species of chlorophyll are dependent on the presence of specific reagents. We suggest that redox reagents of Photosystem II and Photosystem I of photosynthesis also serve to “induce” the formation of distinct chlorophyll species.     

A mass spectrometric analysis of sensitizer solution used for dye-sensitized solar cell

We report a mass spectrometric study of ruthenium(II) sensitizer ((C₄H₉)₄N)₂[Ru(4-carboxy-4′-carboxylate-2,2′bipyridine)₂(NCS)₂] (N719) in various organic and aqueous solvents. At the equilibrium different species were found, depending on solution properties. After stoichiometry confirmation through high-resolution mass spectra and isotopic pattern analysis, the formation of these complexes was found to strongly depend from the polarity and the ionic strength of solvent medium. Mass spectra obtained in acetonitrile/tert-butanol (1:1) mixture, used for the absorption of N719 on TiO₂, suggests that the main species involved in the adsorption process is the dicarboxylate form of N719 with one (C₄H₉)₄N as counter ion. The photovoltaic data of the N719 sensitizer adsorbed on TiO₂ films exhibited a remarkable power conversion efficiency, 11.20% at 1 sun.     

Multinuclear NMR of cis-dicarbonylbis(dithiocarbamato)iron(II) complexes in chloroform solution

The ¹³C and ¹⁵SN NMR spectra of eleven cis-Fe(S₂CNRR′)₂(CO)₂ complexes, where R and R′ are organic substituents, have been measured at ambient temperature in CDCl₃ (0.08–0.16 M). The ¹³C absorptions for the carbonyl ligands correlate well with the force constants for the CO stretching vibrations in CHCl₃ solution. Each of the parameters (¹³CO absorption and k_cis for CO) correlate well with the aqueous solution pK_a for⁺H₂NRR′, corrected for the phenyl-containing substituents, high pK_a values corresponding to high ¹³CO absorptions and low k_cis CO force constants. [p ]Evidence was found in the ¹³C NMR spectra for hindered rotation about the CN bond in S₂CNC₂ in complexes with higher pK_a(corr) values and in the ¹³C spectra of the corresponding thiuram disulfides. [p ]The ¹⁵N (natural abundance) NMR spectra for each of the complexes was determined. Each revealed a single sharp absorption in a region of the ¹⁵N NMR spectrum which indicates substantial CN double bond character, as one would expect for coordinated dithiocarbamate ligands.     

Atomic Layer Deposition of Tin Monosulfide Thin Films

Thin film solar cells made from earth‐abundant, non‐toxic materials are needed to replace the current technology that uses Cu(In,Ga)(S,Se)₂ and CdTe, which contain scarce and toxic elements. One promising candidate absorber material is tin monosulfide (SnS). In this report, pure, stoichiometric, single‐phase SnS films were obtained by atomic layer deposition (ALD) using the reaction of bis(N,N′‐diisopropylacetamidinato)tin(II) [Sn(MeC(N‐ⁱPr)₂)₂] and hydrogen sulfide (H₂S) at low temperatures (100 to 200 °C). The direct optical band gap of SnS is around 1.3 eV and strong optical absorption (α > 10⁴ cm^?1) is observed throughout the visible and near‐infrared spectral regions. The films are p‐type semiconductors with carrier concentration on the order of 10¹⁶ cm^?3 and hole mobility 0.82–15.3 cm²V^?1s^?1 in the plane of the films. The electrical properties are anisotropic, with three times higher mobility in the direction through the film, compared to the in‐plane direction.     

Effect of thermal annealing on the structural and optical properties of tris‐(8‐hydroxyquinoline)aluminum(III) (Alq3) films

Tris‐(8‐hydroxyquionoline)aluminum (Alq₃) was synthesized and coated on to a glass substrate using the dip coating method. The structural and optical properties of the Alq₃ film after thermal annealing from 50°C to 300°C in 50° steps was studied. The films have been prepared with 2 to 16 layers (42–324 nm). The thickness and thermal annealing of Alq₃ films were optimized for maximum luminescence yield. The Fourier transform infrared spectrum confirms the formation of quinoline with absorption in the region 700 ? 500/cm. Partial sublimation and decomposition of quinoline ion was observed with the Alq₃ films annealed at 300°C. The X‐ray diffraction pattern of the Alq₃ film annealed at 50°C to 150°C reveals the amorphous nature of the films. The Alq₃ film annealed above 150°C were crystalline nature. Film annealed at 150°C exhibits a photoluminescence intensity maximum at 512 nm when excited at 390 nm. The Alq₃ thin film deposited with 10 layers (220 nm) at 150°C exhibited maximum luminescence yield. Copyright © 2014 John Wiley & Sons, Ltd.     

V2O5 Nano‐Electrodes with High Power and Energy Densities for Thin Film Li‐Ion Batteries

Nanostructured V₂O₅ thin films have been prepared by means of cathodic deposition from an aqueous solution made from V₂O₅ and H₂O₂ directly on fluorine‐doped tin oxide coated (FTO) glasses followed by annealing at 500°C in air, and studied as film electrodes for lithium ion batteries. XPS results show that the as‐deposited films contained 15% V⁴⁺, however after annealing all the vanadium is oxidized to V⁵⁺. The crystallinity, surface morphology, and microstructures of the films have been investigated by means of XRD, SEM, and AFM. The V₂O₅ thin film electrodes show excellent electrochemical properties as cathodes for lithium ion intercalation: a high initial discharge capacity of 402 mA h g^?1 and 240 mA h g^?1 is retained after over 200 cycles with a discharging rate of 200 mA g^?1 (1.3 C). The specific energy density is calculated as 900 W h kg^?1 for the 1^st cycle and 723 W h kg^?1 for the 180^th cycle when the films are tested at 200 mA g^?1 (1.3 C). When discharge/charge is carried out at a high current density of 10.5 A g^?1 (70 C), the thin film electrodes retain a good discharge capacity of 120 mA h g^?1, and the specific power density is over 28 kW kg^?1.     

Structural differences among phosphatidylcholine, phosphatidylethanolamine, and mixed phosphatidylcholine/phosphatidylethanolamine multilayers: an infrared absorption study

We have examined the infrared absorption spectra from 4000 to 250 cm^?1 of multilayers of phosphatidylcholine, phosphatidylethanolamine, and phosphatidylcholine/phosphatidylethanolamine (1:1 m/m) as a function of hydration, pH, and fatty acid composition. Characteristic splittings of the CH₂ bending and rocking modes and the position of the phosphoryl absorption at ca. 1240 cm^?1 reveal differences in acyl chain packing and head group conformation in the various films. Spectra demonstrate the importance of NH → O hydrogen bonding of the ethanolamine head group and the prerequisite head group conformation (tangent to the multilayer plane) in establishing these structural differences. The general appearance of the P-O-C stretching region (~1050 cm^?1) in the pure and mixed films further supports these conclusions and shows that the spectra clearly distinguish among the different head group orientations. Self-association of phosphatidylethanolamine is sometimes sufficient to prevent formation of mixed phases with phosphatidylcholine at neutral pH. The amount of fine structure, particularly in the low-frequency (800?200 cm^?1) region, in spectra of films of anhydrous, saturated-chain phospholipids decreases considerably when the films are monohydrated, when mixed phases exist, or when there are unsaturations in the acyl chains. These changes likely result from decreased crystal field effects in the spectra as the phosphatide packing density is decreased by any of the above procedures. Furthermore, the absence of other changes upon complete hydration of phosphatidylcholine films suggests that only the initial water is tightly bound to the lipid.     

Formation of CuInSe2 and CuGaSe2 Thin‐Films Deposited by Three‐Stage Thermal Co‐Evaporation: A Real‐Time X‐Ray Diffraction and Fluorescence Study

Thin film solar cells based on co‐evaporated Cu(In,Ga)Se₂ absorber films present the highest efficiencies among current polycrystalline thin‐film technologies. Thanks to the development of a novel experimental setup for in situ growth studies, it was possible to follow the formation of the crystalline phases during such deposition processes for the first time. This synchrotron‐based energy‐dispersive X‐ray diffraction and fluorescence setup is suited for real‐time studies of thin film vapor deposition processes. Focusing on the growth of CuInSe₂ and CuGaSe₂ fabricated by three‐stage processing, we find that the phase transitions in the Cu‐In‐Se system follow the reported pseudo‐binary In₂Se₃‐Cu₂Se phase diagram. This requires a transformation of the Se sublattice during the incorporation of Cu‐Se into the In₂Se₃ precursor film from the first process stage. In the Cu‐Ga‐Se system, besides an increase in the lattice spacings, we observe no transformation of the Se sublattice. Furthermore, the structural defects of the Ga‐Se precursor film are preserved until the CuGaSe₂ stoichiometry is reached. By means of model calculations of the fluorescence signals, we confirm in both systems the segregation of Cu₂Se at the surface near a concentration of 25 at.% Cu shortly after the recrystallization of the films. The modeling also reveals that Cu₂Se penetrates into the CuInSe₂ film, whereas it remains at the surface of the CuGaSe₂ film.     

Controlling the characteristics of a repetitive volume discharge in CFC-12 and its mixtures with argon

The parameters of a repetitive volume discharge in CF₂Cl₂ (CFC-12) and its mixtures with argon at pressures of P(CF₂Cl₂)≤0.4 kPa and P(Ar)≤1.2 kPa are studied. The discharge was ignited in an electrode system consisting of a spherical anode and a plane cathode by applying a dc voltage U_ch≤1 kV to the anode. The electrical and optical characteristics of a volume discharge (such as the current-voltage characteristics; the plasma emission spectra; and the waveforms of the discharge voltage, the discharge current, and the total intensity of plasma emission) are investigated. It is found that, by shunting the discharge gap with a pulsed capacitor with a capacitance of C₀≤3.5 nF, it is possible to control the amplitude and duration of the discharge current pulses, as well as the characteristics of the pulsed plasma emission. The increase in the capacitance C₀ from 20 to 3500 pF leads to a significant increase in the amplitude and duration of the discharge current pulses, whereas the pulse repetition rate decreases from 70 to 3 kHz. The glow discharge exists in the form of a domain with a height of up to 3 cm and diameter of 0.5–3.0 cm. The results obtained can be used to design an untriggered repetitive germicidal lamp emitting in the Cl₂(257/200 nm) and ArCl (175 nm) molecular bands and to develop plasmachemical methods for depositing amorphous fluorocarbon and chlorocarbon films.     

Probing the FQR and NDH activities involved in cyclic electron transport around Photosystem I by the ‘afterglow’ luminescence

Far-red illumination of plant leaves for a few seconds induces a delayed luminescence rise, or afterglow, that can be measured with the thermoluminescence technique as a sharp band peaking at around 40-45 °C. The afterglow band is attributable to a heat-induced electron flow from the stroma to the plastoquinone pool and the PSII centers. Using various Arabidopsis and tobacco mutants, we show here that the electron fluxes reflected by the afterglow luminescence follow the pathways of cyclic electron transport around PSI. In tobacco, the afterglow signal relied mainly on the ferredoxin-quinone oxidoreductase (FQR) activity while the predominant pathway responsible for the afterglow in Arabidopsis involved the NAD(P)H dehydrogenase (NDH) complex. The peak temperature T_m of the afterglow band varied markedly with the light conditions prevailing before the TL measurements, from around 30 °C to 45 °C in Arabidopsis. These photoinduced changes in T_m followed the same kinetics and responded to the same light stimuli as the state 1-state 2 transitions. PSII-exciting light (leading to state 2) induced a downward shift while preillumination with far-red light (inducing state 1) caused an upward shift. However, the light-induced downshift was strongly inhibited in NDH-deficient Arabidopsis mutants and the upward shift was cancelled in plants durably acclimated to high light, which can perform normal state transitions. Taken together, our results suggest that the peak temperature of the afterglow band is indicative of regulatory processes affecting electron donation to the PQ pool which could involve phosphorylation of NDH. The afterglow thermoluminescence band provides a new and simple tool to investigate the cyclic electron transfer pathways and to study their regulation in vivo.     

The long rod-shaped Sr2MgSi2O7:Eu2+,Dy3+ with ultrahigh afterglow performance

The afterglow properties of long afterglow luminescent materials are greatly affected by their defects, which are distributed on the grain surface. Increasing the exposed surface area is an important method to improve the afterglow performance. In this research, long rod-shaped long afterglow materials Sr₂MgSi₂O₇:Eu²⁺,Dy³⁺ were prepared using the hydrothermal-coprecipitation method. When the reaction time reached 96 h, the length of the afterglow materials could grow to 2 mm, and the sintering temperature was just 1150°C. The emission spectra of all obtained samples upon excitation at 397 nm had a maximum of 465 nm, which belonged to the representative transition of Eu²⁺. The initial brightness was 1.35 cd/m². The afterglow time could reach 19 h, giving a good afterglow performance. The research on this kind of material has essential significance in the exploration of luminescence mechanisms and their applications.     

Diagnostics of a nonequilibrium nitrogen plasma from the emission spectra of the second positive system of N2

A method is proposed for determining the electron density N _e and the electric field E in the non-equilibrium nitrogen plasma of a low-pressure discharge from the spectra of the second positive system of N₂. The method is based on measuring the specific energy deposition in the plasma and the distribution of nitrogen molecules over the vibrational levels of the C ³Π _u state, as well as on modeling this distribution for a given energy deposition. The fitting parameters of the model are the values of N _e and E. A kinetic model of the processes governing the steady-state density of the C ³Π _u nitrogen molecules is developed. The testing of this method showed it to be quite reliable. The method is of particular interest for diagnosing electrodeless discharges and provides detailed information on the processes occurring in the discharge plasma. Preliminary data are obtained on the plasma parameters in a cavity microwave discharge and an electrode microwave discharge. In particular, it is found that the electric field in an electrode microwave discharge in nitrogen is lower than that in a hydrogen discharge. This effect is shown to be produced by stepwise and associative processes with the participation of excited particles in nitrogen.     

Low-pressure glow discharge in a Xenon/Chlorine mixture

The spatial, electrical, and optical characteristics of a transverse glow discharge and a volume discharge with a spherical anode and plane cathode in low-pressure Xe/Cl₂ mixtures are studied. It is shown that the transverse glow discharge in mixtures with a low chlorine content occupies most of the interelectrode gap and exists in the form of strata. As the total pressure (P≥300 Pa) and the partial chlorine pressure (P(Cl₂)≥80 Pa) increase, a solitary plasma domain with a volume of 1–2 cm³ forms in the discharge gap. It acts as a selective source of UV radiation in the XeCl(D-X) 236-nm, Cl₂ (D′-A′) 257-nm, and XeCl(B-X) 308-nm bands. In certain Xe/Cl₂ mixtures, plasma self-oscillations in the frequency range 1–100 kHz are observed. The current of a low-pressure volume discharge with a spherical anode and plane cathode and the emission from it have both a dc and an ac component. The pressure and composition of the working mixture, as well as the average current of the volume discharge are optimized to attain the maximum emission intensity of the XeCl(D,B-X) bands. Low-pressure volume discharges in xenon/chlorine mixtures can be used as active media in low-pressure large-aperture planar or cylindrical excimer-halogen lamps emitting modulated or repetitive pulsed UV radiation.     

Simulation of the Plasma Afterglow in the Discharge Gap of a Subnanosecond Switch Based on an Open Discharge in Helium

The phenomenon of subnanosecond electrical breakdown in a strong electric field observed in an open discharge in helium at pressures of 6–20 Torr can be used to create ultrafast plasma switches triggering into a conducting state for a time shorter than 1 ns. To evaluate the possible repetition rate of such a subnanosecond switch, it is interesting to study the decay dynamics of the plasma remaining in the discharge gap after ultrafast breakdown. In this paper, a kinetic model based on the particle-in-cell Monte Carlo collision method is used to study the dynamics of the plasma afterglow in the discharge gap of a subnanosecond switch operating with helium at a pressure of 6 Torr. The simulation results show that the radiative, collisional-radiative, and three-body collision recombination mechanisms significantly contribute to the afterglow decay only while the plasma density remains higher than 10¹² cm^?3; the main mechanism of the further plasma decay is diffusion of plasma particles onto the wall. Therefore, the effect of recombination in the plasma bulk is observed only during the first 10–20 μs of the afterglow. Over nearly the same time, plasma electrons become thermalized. The afterglow time can be substantially reduced by applying a positive voltage U_c to the cathode. Since diffusive losses are limited by the ion mobility, the additional ion drift toward the wall significantly accelerates plasma decay. As U_c increases from 0 to +500 V, the characteristic time of plasma decay is reduced from 35 to 10 μs.