Table of contents

This review covers the results obtained in the period 2006–2014 in the field of plasma-assisted combustion, and in particular the results on ignition and combustion triggered or sustained by pulsed nanosecond discharges in different geometries. Some benefits of pulsed high voltage discharges for kinetic study and for applications are demonstrated. The necessity of and the possibility of building a particular kinetic mechanism of plasma-assisted ignition and combustion are discussed. The most sensitive regions of parameters for plasma–combustion kinetic mechanisms are selected. A map of the pressure and temperature parameters (P–T diagram) is suggested, to unify the available data on ignition delay times, ignition lengths and densities of intermediate species reported by different authors.

We have conducted systematic investigations of magnetic circular dichroism (MCD) spectra and optical absorption spectra of the III–V diluted magnetic semiconductor (DMS) Ga_1−xMn_xAs. With an increase in Mn concentration x, the MCD structure originating from the L-critical points of the zinc-blend type band structure shifted towards lower energy while the corresponding absorption spectrum did not show the shift although they are expected to show the same dependence. This indicates that the s, p–d exchange interaction of Ga_1−xMn_xAs has a very localized nature, because MCD is only active in a region where band structure is affected by Mn spins, and optical absorption results from the overall sample response. The MCD spectrum of a ferromagnetic Ga_0.97Mn_0.03As sample was decomposed into contributions from Γ-critical points (E₀ and E₀ + Δ₀), L-critical points (E₁ and E₁ + Δ₁), and impurity band-related optical transitions. Using the intensities of MCD and optical absorption, the Zeeman splitting energy at E₁ was estimated to be larger than +8 meV at 6 K. By assuming the same ratio between the Zeeman splitting energies at E₀ and E₁, which is experimentally reported in a II–VI paramagnetic DMS Cd_1−xMn_xTe, the Zeeman splitting at E₀ of Ga_0.97Mn_0.03As was estimated to be larger than +120 meV at 6 K.

FePt alloy systems in the L1₀ phase are characterized by high anisotropy values that lead to exceptionally high coercive fields, making them a primary candidate for high density data storage. In this paper, we report the synthesis of L1₀ FePt nanoparticles by thermal annealing of chemically disordered FePt nanoparticles. The high degree of ordering and high coercivity achieved in these systems is further studied using Monte Carlo simulations by incorporating the effects of anisotropy, interactions, and disorder, with a view to understand the role of these interactions. A comparison of the simulation results with the experiment results also helps us to estimate an anisotropy value of ∼2.25–2.5 MJ m⁻³.

This work focused on the development of a new single bath technique to fabricate compositionally modulated Co/Cu, CoFe/Cu and Fe/Cu multilayer nanowires in nanoporous alumina templates prepared by the hard and mild anodization methods. The approach was based on ac pulse electrodeposition, employing successive cycles of alternating continuous and pulsed sine waves with designated off-time between pulses and reduction/oxidation voltages. The substantial control over the composition of each segment was achieved by simultaneous change in the off-time between pulses and the ac deposition voltage. The multilayered nature of the nanostructures was substantiated by transmission electron microscopy. Each layer thickness was also nearly uniform, and could be readily adjusted by the number of pulses. The proposed method facilitates the fabrication of various multilayer nanowires in a single bath, which speeds up the fabrication process and is desirable for their application in nanodevices and nanoelectronics. The effect of magnetic layer thickness on the magnetic behaviour was also studied. Decreasing the magnetic layer thickness caused the parallel coercivity and squareness values to approach those measured in the perpendicular direction. The magnetic easy axis changed from parallel to nearly perpendicular to the nanowire axis, depending on the magnetic layers' aspect ratio and shape anisotropies.

In this work we report on the growth of Fe_1−xGa_x films by means of the electrodeposition technique in baths containing sodium citrate as an antioxidant agent. We have investigated the effect of the electrolyte composition in terms of its Ga content on the structural and magnetic properties. Our experimental results indicate that for the optimized bath composition, O-free Fe_1−xGa_x alloys can be achieved for a potential of E = −1.4 V and magnetic stirring at 500 rpm. Both the composition and the coercivity of the samples deposited from this electrolyte can be tuned by means of the duration of the rest pulse at E = −1.12 V. However, when the composition of the bath is not optimized the films exhibit a large amount of O that cannot be dissolved during the rest pulse. Our experimental results show that O-free Fe_1−xGa_x alloys with a Ga content of around 19 at% and optimum magnetic properties can be deposited on Au substrates when using an optimized electrolyte and long rest pulses.

We report on the performance enhancement of n-type organic field-effect transistors (OFETs) through the use of gold source and drain electrodes that are both modified with rubidium carbonate (Rb₂CO₃) reducing the electron injection barrier. Devices are fabricated using n-channel N, N'-ditridecyl-3,4,9,10-perylenetetracarboxylicdiimide (PTCDI-C₁₃) and a polymeric gate dielectric with various thicknesses of Rb₂CO₃, and the dependence of device's electrical performance on Rb₂CO₃ thickness is investigated. The device with 10 Å Rb₂CO₃ exhibits the best performance, and its mobility is five times higher than that of the device without Rb₂CO₃. UV–visible, x-ray and ultraviolet photoemission spectroscopy are used to investigate the interface between Rb₂CO₃ and PTCDI-C₁₃, and we find that charge transfer from Rb₂CO₃ to PTCDI-C₁₃ occurs, resulting in the reduction of the electron charge injection barrier from the gold electrode. The charge injection mechanism and OFET performance enhancement with Rb₂CO₃ are discussed in detail.

The origin of the high leakage current measured in several vertical-type diamond Schottky devices is conjointly investigated by conducting probe atomic force microscopy and confocal micro-Raman/photoluminescence imaging analysis. Local areas characterized by a strong decrease of the local resistance (5–6 orders of magnitude drop) with respect to their close surrounding have been identified in several different regions of the sample surface. The same local areas, also referenced as electrical hot-spots, reveal a slightly constrained diamond lattice and three dominant Raman bands in the low-wavenumber region (590, 914 and 1040 cm⁻¹). These latter bands are usually assigned to the vibrational modes involving boron impurities and its possible complexes that can electrically act as traps for charge carriers. Local current–voltage measurements performed at the hot-spots point out a trap-filled-limited current as the main conduction mechanism favouring the leakage current in the Schottky devices.

In this work we present a two-dimensional (2D) model for an organic thin film photo-conductive sensor containing a planar heterojunction and in-plane electrodes. The model simulates the flow of charge carriers based on the standard one-dimensional semiconductor transport and continuity equations, and combines this with a 2D model for the electric field. This procedure results in a hybrid differential/integral equation formulation. We present and analyse simulation results that resemble very well measured current–voltage characteristics of a real sensor under different illumination levels. We find that for currents below a critical value the sensor behaves like a resistor. Above this critical current the current increases much more slowly due to space charge accumulation close to the cathode. We explain the critical current as the maximum reverse current of the solar cell formed by the heterojunction covering the cathodic electrode.

Na-substituted BiFeO₃ films were prepared on FTO/glass substrates using the sol–gel method. XRD results showed that all films adopted random orientation and an R3m perovskite structure. UV–Vis absorption results indicated that the films exhibited intense absorbance around 450 nm. The band gaps were 2.59 eV, 2.63 eV, 2.62 eV and 2.52 eV for the films substituted with 0%, 5%, 10% and 20% Na, respectively. A substantially enhanced photovoltaic effect was observed in the 20% Na substituted BiFeO₃ film; its short-circuit current density was 1.26 µA cm⁻² and the open circuit voltage was −0.66 V. Meanwhile, polarization-modulated J_sc and V_oc were observed in the 20% Na substituted BiFeO₃ film and the mechanism was studied using the Schottky barrier combined with the ferroelectric polarization model.

Strong and stable room-temperature photoluminescence (PL) emission is achieved in MoO₃ nanoplates and lamellar crystals doped with Er and Eu by ion implantation and subsequent annealing. Micro-Raman and PL spectroscopy reveal that optical activation of the rare earth ions and recovery of the original MoO₃ structure are achieved for shorter annealing treatments and for lower temperatures in nanoplates, as compared with lamellar crystals. Er seems to be more readily incorporated into optically active sites in the oxide lattice than Eu. The influence of the dimensionality of the host sample on the characteristics of the PL emission of both rare earth dopants is addressed.

The structural parameters of AlInN/AlN/GaN high mobility field effect transistors (FETs) determine their electrical properties. The AlN-interlayer (spacer) thickness especially plays an important role to enhance the mobility and the density of the two dimensional electron gas (2DEG). However, structural characterization of this ultra-thin AlN-interlayer is ambiguous when only high resolution x-ray diffraction (HRXRD) and x-ray reflectometry (XRR) are taken into account. Here a combined layer analysis was performed using HRXRD, XRR and grazing incidence x-ray fluorescence (GIXRF) for the determination of the AlN-interlayer thickness. A sample series of AlInN/AlN/GaN FETs on Si(1 1 1) has been grown and analysed. The growth time of the AlN-interlayer was changed from 0 to 12 s and the AlInN barrier was grown nearly lattice matched to GaN with a nominal thickness of 5 nm. By the combination of HRXRD, XRR, GIXRF and simultaneous simulation of the data the determination of the spacer thickness was successfully performed.

Photoreflectance (PR) spectroscopy was applied to study the band gap in GaSb_1−xBi_x alloys with Bi < 5%. Obtained results have been interpreted in the context of ab initio electronic band structure calculations in which the supercell (SC) based calculations are joined with the alchemical mixing (AM) approximation applied to a single atom in the cell. This approach, which we call SC-AM, allows on the one hand to study alloys with a very small Bi content, and on the other hand to avoid limitations characteristic of a pure AM approximation. It has been shown that the pure AM does not reproduce the GaSb_1−xBi_x band gap determined from PR while the agreement between experimental data and the ab initio calculations of the band gap obtained within the SC-AM approach is excellent. These calculations show that the incorporation of Bi atoms into the GaSb host modifies both the conduction and the valence band. The shift rates found in this work are respectively −26.0 meV per % Bi for the conduction band and 9.6 meV per % Bi for the valence band that consequently leads to a reduction in the band gap by 35.6 meV per % Bi. The shifts found for the conduction and valence band give a ∼27% (73%) valence (conduction) band offset between GaSb_1−xBi_x and GaSb. The rate of the Bi-related shift for the split-off band is −7.0 meV per % Bi and the respective increase in the spin–orbit split-off is 16.6 meV per % Bi.

A model of the isolated streamer head based on Meek's criterion of the avalanche to streamer transition is applied for description of the plasma bullet propagation in a helium/air admixture. According to the model previously proposed by Kulikovsky for streamers in air, along with the knowledge of one of three parameters: electric field, ionization integral or the width of the space charge layer, the other two parameters could be determined. Furthermore, using the streamer current or radius, it is possible to determine the electric field-streamer velocity functional dependence. Obtained results showed satisfactory agreement with both the results of the fluid model from the literature and the experimental results of plasma jet studies. Finally, for the sake of comparison, streamer velocity dependence on the electric field strength range of 10–250 kV cm⁻¹ is determined for helium, argon and air.

Pulsed corona-induced partial oxidation of methane in humid oxygen or carbon dioxide atmospheres has been investigated for future fuel synthesis applications. The obtained product spectrum is wide, i.e. saturated, unsaturated and oxygen-functional hydrocarbons. The generally observed methane conversion levels are 6–20% at a conversion efficiency of about 100–250 nmol J⁻¹. The main products are ethane, ethylene and acetylene. Higher saturated hydrocarbons up to C6 have been detected. The observed oxygen-functional hydrocarbons are methanol, ethanol and lower concentrations of aldehydes, ketones, dimethylether and methylformate. Methanol seems to be exclusively produced with CH₄/O₂ mixtures at a maximum production efficiency of 0.35 nmol J⁻¹. CH₄/CO₂ mixtures appear to yield higher hydrocarbons. Carboxylic acids appear to be mainly present in the aqueous reactor phase, possibly together with higher molecular weight species.

A microscale plasma and a spherical microscale bubble were generated by the application of a single pulsed discharge in water with no pre-existing bubble. The microscale corona discharges were created at the tip of a microelectrode by applying a voltage at around −11 kV with a rise time of around 20 ns. The energy inputs for microplasma generation were controlled by varying the durations of the discharges from nanoseconds to microseconds. Two different energy inputs of 103 and 0.5 mJ were studied in detail and the differences in the microplasma-generated microbubbles, such as the maximum radii, numbers of oscillations and durations of the bubble were observed. These microbubbles were visualized using a microscope based optical system with two different high speed cameras. Images of the discharges were captured by a nanosecond gated intensified charge-coupled device (CCD) camera, and the microbubbles' dynamics were recorded by a million-frame-per-second CCD video camera. A Rayleigh–Plesset (RP) model considering both condensable (water vapour) and incondensable (H₂ and O₂) gases in the microbubble predicts the bubbles' dynamics accurately. Comparisons of the experimental results and the RP models allow estimation of the thermodynamic states of the microplasmas and microbubbles. The energies in the microbubbles are analysed quantitatively from the model and rough approximations for energy dissipation and the energy of the microplasma are made. The microplasma energy can be significantly less than the applied energy input. Such low initiation energy is the reason that the size of microplasmas is at the micron scale and all microplasmas are confined in a spherical microbubble. All the microbubbles reported in this paper are spherical. The low energy also provides conditions for non-equilibrium plasmas in liquid.

This work considers the ignition process of mercury-free high-intensity discharge lamps used for car headlights. These lamps have to run-up fast. This is achieved with a high xenon pressure of about 15 bar (cold) in the inner bulb. The high filling-gas pressure causes an increased ignition voltage compared with lower-pressure lamps used in general-lighting applications.

In this paper the possibility is investigated to reduce the ignition voltage by optimizing a dielectric-barrier discharge (DBD) in the outer bulb working as ignition aid. A special outer bulb was built up allowing gas exchange and adjustment of the gas pressure. For diagnostic purposes different electrical and optical methods are used, namely the recording of ignition voltage, ignition current and light emission by a photo-diode signal on nanosecond time scale as well as short-time photography by a intensified charge-coupled device camera.

It was found that the DBD mainly generates a potential distribution within the lamp which supports ignition by an increase in the E-field in front of the electrodes and the wall. It is shown that this effect is distinctly more effective than UV radiation potentially emitted by the DBD.

This paper focuses on the investigation of CF₄ decomposition in a low-pressure inductively coupled plasma by means of a global model. The influence of O₂ on the CF₄ decomposition process is studied for conditions used in semiconductor manufacturing processes. The model is applied for different powers and O₂ contents ranging between 2% and 98% in the CF₄/O₂ gas mixture. The model includes the reaction mechanisms in the gas phase coupled with the surface reactions and sticking probabilities of the species at the walls. The calculation results are first compared with experimental results from the literature (for the electron density, temperature and F atom density) at a specific power, in the entire range of CF₄/O₂ gas mixture ratios, and the obtained agreements indicate the validity of the model. The main products of the gas mixture, obtained from this model, include CO, CO₂ and COF₂ together with a low fraction of F₂. The most effective reactions for the formation and loss of the various species in this process are also determined in detail. Decomposition of CF₄ produces mostly CF₃ and F radicals. These radicals also contribute to the backward reactions, forming again CF₄. This study reveals that the maximum decomposition efficiency of CF₄ is achieved at a CF₄/O₂ ratio equal to 1, at the applied power of 300 W.

CuO nanostructures doped with Ce at different concentration levels have been synthesized via a simple co-precipitation technique. The prepared samples have been characterized by x-ray diffraction, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and UV–visible absorption spectroscopy. Structural studies exhibit the presence of a monoclinic structure of CuO for undoped and Ce doped samples without any additional impurity phases. SEM images have revealed the rod-like morphology with an average diameter of 30 nm for undoped CuO. FTIR results of undoped and Ce doped CuO nanostructures have further confirmed the formation of monoclinic CuO. The optical band gap calculated from the Tauc relation has been observed to be 2.48 eV for undoped CuO nanostructures, which is found to decrease down to 2.2 eV with the increase in the Ce doping level. This tuning in the optical band gap may be attributed to the merging of the impurity band with the conduction band of CuO. The Ce doping induced effects on the antibacterial activity of the CuO nanostructures have been examined by recording the growth curves of bacteria in the presence of prepared nanostructures. It has been observed that S. aureus bacterium may be completely eradicated by the application of Ce doped CuO nanostructures. Finally, the cytotoxicity analysis has shown that the synthesized undoped and Ce doped CuO nanostructures are biocompatible and non-toxic towards the human cell line SH-SY5Y cells.

Carbon nanotubes (CNT) generate smooth-spectra sound over a wide frequency range (1–10⁵ Hz) by means of thermoacoustics (TA). The protective encapsulation of CNT sheets in inert gases between rigid vibrating plates provides resonant features in the TA sound projector. The vibrational modes of plates and the compliance of the soft sealing spacers between those plates are studied with the aim of creating efficient, tunable underwater sound generation at relatively low frequencies, 10 Hz–10 kHz.

We report on thermal conductivity measurements of aluminum nitride (AlN) films using the fast pulsed photo-thermal technique. The films were deposited by high-power impulse magnetron sputtering with different thicknesses ranging from 1000 to 8000 nm on (1 0 0) oriented silicon substrates. The films were characterized by x-ray diffraction (XRD), Raman spectroscopy, profilometry, scanning electron microscopy and atomic force microscopy. The XRD measurements showed that AlN films were textured along the (0 0 2) direction. Moreover, x-ray rocking curve measurements indicated that the crystalline quality of AlN was improved with the increase in film thickness. The thermal conductivities of the samples were found to rapidly increase when the film thickness increased up to 3300 nm and then showed a tendency to remain constant. A thermal boundary resistance as low as 8 × 10⁻⁹ W⁻¹ K m² and a thermal conductivity as high as 250 ± 50 W K⁻¹ m⁻¹ were obtained for the AlN films, at room temperature. This high thermal conductivity value is close to that of an AlN single crystal and highlights the potential of these films as a dielectric material for thermal management.

Polycrystalline and highly preferred $(1\,0\,\bar{{2}})$ orientated Bi_0.5Sr_0.5FeO_3−δ thin films were grown by pulsed laser deposition (PLD) on n-Si (2 0 0) and MgO (2 0 0) single crystalline substrates respectively. The thin films were inspected using x-ray diffraction, scanning electron microscopy, energy dispersive x-ray spectroscopy and atomic force microscopy techniques. The electrical surface-resistivity, dielectric resonance, electric polarization, and magnetic properties of the thin films were studied. At room temperature, depending on deposition conditions, the polycrystalline thin films grown on n-Si substrates were found to exhibit an electrical surface-resistivity of the order of 10³–10⁶ Ω, a piezoelectric resonance in the frequency range of about 25–26 MHz, a relaxor-type ferroelectric hysteresis with a maximum polarization of 0.015–0.055 µC cm⁻² and magnetic hysteresis. Similarly, the thin films grown on MgO substrates exhibited an electrical surface-resistivity of the order of 10⁹ Ω, multiple piezoelectric resonances in the frequency range of about 8–45 MHz, a linear variation of polarization with applied electric field and either a linearly varying magnetization or magnetic hysteresis which depends on the deposition conditions.

Carbon–tungsten layers deposited on graphite by thermionic vacuum arc (TVA) were directly irradiated with a femtosecond terawatt laser. The morphological and structural changes produced in the irradiated area by different numbers of pulses were systematically explored, both along the spots and in their depths. Although micro-Raman and Synchrotron-x-ray diffraction investigations have shown no carbide formation, they have shown the unexpected presence of embedded nano-diamonds in the areas irradiated with high fluencies. Scanning electron microscopy images show a cumulative effect of the laser pulses on the morphology through the ablation process. The micro-Raman spatial mapping signalled an increased percentage of sp³ carbon bonding in the areas irradiated with laser fluencies around the ablation threshold. In-depth x-ray photoelectron spectroscopy investigations suggested a weak cumulative effect on the percentage increase of the sp²-sp³ transitions with the number of laser pulses just for nanometric layer thicknesses.

This paper presents an accurate yet straightforward methodology for characterizing time-dependent anelastic mechanics of thin metal films employed in metalic microelectromechanical systems (MEMS). The deflection of microbeams is controlled with a mechanical micro-clamp, measured with digital holographic microscopy and processed with global digital image correlation (GDIC). The GDIC processing directly incorporates kinematics into the three-dimensional correlation problem, describing drift-induced rigid body motion and the beam deflection. This yields beam curvature measurements with a resolution of <1.5 × 10⁻⁶ µm⁻¹, or for films thinner than 5 µm, a strain resolution of <4 με. Using a simple experimental sequence, these curvature measurements are then combined with a linear multi-mode time-dependent anelastic model and a priori knowledge of the Young's modulus. This allows the characterization of the material behaviour in the absence of an additional explicit force measurement, which simplifies the experimental setup. Using this methodology we characterize the anelasticity of 5 µm-thick Al(1 wt%)-Cu microbeams of varying microstructures over relevant timescales of 1 to 1 × 10⁵ s and adequately predict the time and amplitude response of experiments performed for various loading conditions. This demonstrates the validity of the methodology and the suitability for thin film mechanics research for MEMS development.