Highlights of 2009

Heisenberg exchange parameters for various Heusler compounds with L2₁ structure were calculated using the Korringa–Kohn–Rostoker method and by employing the magnetic-force theorem to calculate the total energy changes associated with a local rotation of magnetization directions. Random occupation was treated within the coherent potential approximation. Further, the Curie temperatures were calculated in the mean-field approximation and have been found to be in good agreement with the experiment. A procedure for evaluating the spin-stiffness constants for the case of multiple magnetic sublattices is given and the results were compared with measured values. Magnon dispersion curves were obtained by Fourier transforming the calculated exchange parameters.

Bottom pinned IrMn/[Co/Pt] multilayer films with relatively thin Pt layers (∼10 Å) grown on a Ta/Pt buffer have been found to exhibit favourable perpendicular exchange bias (PEB) and well-defined perpendicular anisotropy at room temperature. However, even the optimum film suffers from the same problem as those reported previously, i.e. the exchange bias field is just slightly larger than the coercivity. By replacing the Co layer in contact with IrMn by a 6–8 Å Co₆₀Fe₄₀ layer, the exchange bias is drastically enhanced while the large perpendicular anisotropy is sustained and the coercivity changes little. The exchange bias field for the IrMn/CoFe/[Pt/Co] films with sufficient high perpendicular anisotropy can reach a value as high as 950 Oe, which is almost three times the coercivity, and the corresponding unidirectional anisotropy, 0.22 erg cm⁻², considerably exceeds the best result at room temperature for various PEB systems ever reported. The present result suggests that PEB strongly depends on the composition of the ferromagnetic layer across the interface, which opens an effective avenue to boost PEB.

We have found that the electromechanical resonance frequency of Terfenol-D/Pb(Zr_xTi_1−x)O₃ laminated composites can be tuned by an applied dc magnetic (H_dc) bias. With increasing H_dc from 0 to 5000 Oe, the resonance frequency (f_r) can be shifted over the range of 40 ⩽ f_r ⩽ 55 kHz. This finding offers a means by which to enhance the effective bandwidth of resonance enhanced magnetoelectric devices.

Epitaxially grown magnetic tunnel junctions (MTJs) with a stacking structure of Co₂MnSi/MgO/CoFe were fabricated. Their tunnel magnetoresistance (TMR) effects were investigated. The TMR ratio and tunnelling conductance characteristics of MTJs were considerably different between those with an MgO barrier prepared using sputtering (SP-MTJ) and those prepared using EB evaporation (EB-MTJ). The EB-MTJ exhibited a very large TMR ratio of 217% at room temperature and 753% at 2 K. The bias voltage dependence of the tunnelling conductance in the parallel magnetic configuration for the EB-MTJ suggests that the observed large TMR ratio at RT results from the coherent tunnelling process through the crystalline MgO barrier. The tunnelling conductance in the anti-parallel magnetic configuration suggests that the large temperature dependence of the TMR ratio results from the inelastic spin–flip tunnelling process.

High magnetization nanoparticles coated with a biocompatible polymer have attracted considerable interest in recent times as potential materials for biomedical applications associated with targeted drug delivery, detection and the treatment of cancer. This paper considers the use of sodium borohydride reduction of metal salts to form Fe based nanoparticles coated with carboxyl terminated polyethylene glycol (cPEG). By mixing the reactants in a Y-junction, the synthesis produces uniform nanoparticles in the size range 10–20 nm with a core–shell structure. The particles are subsequently coated with a 1–3 nm thick layer of cPEG. These nanoparticles are soft ferromagnets with H_c = 400 Oe. Exciting these nanoparticles with a 4 Oe, 500 kHz alternating magnetic field leads to particle heating with a maximal increase in the saturation temperature as the particle size is decreased. For the largest particles considered here, the temperature reaches 35 °C with a 10 mg sample mass whilst for the smallest nanoparticles considered the temperature exceeds 40 °C.

We report on an experimental study of the charge transfer dynamics in a P3HT : PCBM blend by means of a femtosecond fluorescence up-conversion technique. Using two-photon excitation we probe the exciton dynamics in P3HT and a P3HT : PCBM blend with a weight ratio of 1 : 1 at excitation densities of up to 6 × 10¹⁸ cm⁻³. In both samples we find strongly nonexponential decay traces compatible with (i) diffusion-limited exciton–exciton annihilation and (ii) diffusion-limited donor–acceptor charge transfer in the polymer blend. Additionally, our results indicate that in the P3HT : PCBM blend about 50% of the photogenerated excitons undergo a prompt charge transfer process on a time scale of about 150 fs. Our study shows that fluorescence spectroscopy with femtosecond time resolution is a powerful technique for probing ultrafast charge transfer processes in solar cell materials.

A new class of integrative radial crystals (Krause et al 2006 Phys. Rev. Lett. 96 165502) and radial superlattices (RSLs) (Deneke et al 2004 Appl. Phys. Lett. 84 4475) has been realized by the roll-up of semiconductor-based hybrid layer systems on a substrate surface (Prinz et al 2000 Physica E 6 828, Schmidt and Eberl 2001 Nature 410 168). After a brief overview of different approaches to realize hybrid superlattices, we review the structure of semiconductor-based radial crystals and RSLs, and provide in-depth insight into the formation of these hybrid multilayer systems. Besides various transmission electron microscopy investigations, x-ray techniques to determine the lattice parameters, strain and mismatch are discussed. The overview is meant to provide a comprehensive structural summary of semiconductor/oxide, semiconductor/organic as well as semiconductor/metal hybrid RSLs at the current state of research. Furthermore, we focus on different interface types found in RSLs. Results from laterally resolved chemical analysis techniques are discussed to complete this review paper.

Here we present a simple and novel approach of fabricating three dimensional (3D) n-Si nanowires (NWs) and poly(3-octylthiophene) hybrid solar cells incorporating carbon nanotubes (CNTs). Vertically aligned n-Si NWs arrays were fabricated by electroless chemical etching of a n-Si [1 1 1] wafer. n-Si NWs/poly(3-octylthiophene) hybrid solar cells were fabricated with and without functionalized CNTs incorporation. Fabricated solar cells incorporating CNTs show open circuit voltage (V_oc), short circuit current density (J_sc) fill factor (FF) and conversion efficiency as 0.353, 7.85 mA cm⁻², 22% and 0.61%, respectively. In fabricated devices n-Si NWs arrays form multiple heterojunctions with the polymer and provide efficient electron collection and transportation, whereas CNTs provide efficient hole transportation.

We compare the characteristics of photonic crystal (PC) light-emitting diodes (LEDs) with the same hole pattern density of 12 ea µm⁻². The PC LED with periodic hole structure demonstrated an increased output power, improved external efficiency at high current operation and uniform radiation owing to the periodic nanoscale features generating a photonic band gap (PBG), when compared with those of the random hole (RH) LED. The electroluminescence images obtained by confocal scanning electroluminescence microscopy (CSEM) show the difference of light emission propagation from the random and periodic structures.

As infrared imaging systems have evolved from the first generation of linear devices to the second generation of small format staring arrays to the present 'third-gen' systems, there is an increased emphasis on large area focal plane arrays (FPAs) with multicolour operation and higher operating temperature. In this paper, we discuss how one needs to develop an increased functionality at the pixel level for these next generation FPAs. This functionality could manifest itself as spectral, polarization, phase or dynamic range signatures that could extract more information from a given scene. This leads to the concept of an infrared retina, which is an array that works similarly to the human eye that has a 'single' FPA but multiple cones, which are photoreceptor cells in the retina of the eye that enable the perception of colour. These cones are then coupled with powerful signal processing techniques that allow us to process colour information from a scene, even with a limited basis of colour cones. Unlike present day multi or hyperspectral systems, which are bulky and expensive, the idea would be to build a poor man's 'infrared colour' camera. We use examples such as plasmonic tailoring of the resonance or bias dependent dynamic tuning based on quantum confined Stark effect or incorporation of avalanche gain to achieve embodiments of the infrared retina.

Ca₃Sc₂Si₃O₁₂ : Ce³⁺ phosphors with a single phase and fine size were successfully obtained at a lower temperature (1100 °C) using the gel-combustion method compared with the conventional solid-state reaction method (about 1500 °C). The crystal phase and the microstructure of the phosphors and their photoluminescence were investigated. The particle size is about 1 µm, which is much less than that obtained by the solid-state reaction. Smaller particle size can reduce internal scattering when particles are mixed with silicon and coated onto a blue light-emitting diode (LED). A bright green emission is observed, which is attributed to the characteristic emissions from 5d–²F_5/2 and 5d–²F_7/2 transitions of Ce³⁺. The excitation spectra show a broad and strong absorption at about 460 nm, suggesting that it is very suitable for use as a colour converter in white LEDs. The decay time of the gel-combustion phosphor is 54.65 ns.

During the last two decades atmospheric (or high) pressure non-thermal plasmas in and in contact with liquids have received a lot of attention in view of their considerable environmental and medical applications. The simultaneous generation of intense UV radiation, shock waves and active radicals makes these discharges particularly suitable for decontamination, sterilization and purification purposes. This paper reviews the current status of research on atmospheric pressure non-thermal discharges in and in contact with liquids. The emphasis is on their generation mechanisms and their physical characteristics.

Recently several investigators reported on various means of generating cold plasma jets at atmospheric pressure. More interestingly, these jets turned out to be not continuous plasmas but trains of small high velocity plasma packets/bullets. However, until now little is known of the nature of these 'bullets'. Here we present experimental insights into the physical and chemical characteristics of bullets. We show that their time of initiation, their velocity and the distance they travel are directly dependent on the value of the applied voltage. We also show that these bullets can be controlled by the application of an external electric field. Using an intensified charge coupled device camera we report on their geometrical shape, which was revealed to be 'donut' shaped, therefore giving an indication that solitary surface ionization waves may be responsible for the creation of these bullets. In addition, using emission spectroscopy, we follow the evolution of various species along the trajectory of the bullets, in this way correlating the bullet propagation with the evolution of their chemical activity.

Semiconductor nanocrystals have attracted considerable interest for a wide range of applications including light-emitting devices and displays, photovoltaic cells, nanoelectronic circuit elements, thermoelectric energy generation and luminescent markers in biomedicine. A particular advantage of semiconductor nanocrystals compared with bulk materials rests in their size-tunable optical, mechanical and thermal properties. While nanocrystals of ionically bonded semiconductors can conveniently be synthesized with liquid phase chemistry, covalently bonded semiconductors require higher synthesis temperatures. Over the past decade, nonthermal plasmas have emerged as capable synthetic approaches for the covalently bonded semiconductor nanocrystals. Among the main advantages of nanocrystal synthesis in plasmas is the unipolar electrical charging of nanocrystals that helps avoid or reduce particle agglomeration and the selective heating of nanoparticles immersed in low-pressure plasmas. This paper discusses the important fundamental mechanisms of nanocrystal formation in plasmas, reviews the range of synthesis approaches reported in the literature and discusses some of the potential applications of plasma-synthesized semiconductor nanocrystals.

Antimicrobial effectiveness of a nanosecond-pulsed dielectric barrier discharge (DBD) was investigated and compared with that of a microsecond-pulsed DBD. Experiments were conducted on the Escherichia coli bacteria covering a topographically non-uniform agar surface acting as one of the DBD electrodes. They reveal that the nanosecond-pulsed DBD can inactivate bacteria in recessed areas whereas the microsecond-pulsed and conventional DBDs fail to do so. Charged species (electrons and ions) appear to play the major role in the bacteria inactivation with the nanosecond-pulsed DBD. Moreover, the nanosecond-pulsed DBD kills bacteria significantly faster than its microsecond-pulsed counterpart.

A new collisional–radiative model for atmospheric-pressure low-temperature argon discharges is proposed, which illustrates the significant effect of electron density on the excited atom population distribution. This makes it possible to determine the electron density from the intensity ratio of emission lines of excited atoms. Results of this new method in several types of atmospheric-pressure discharges are found to be in agreement with those of the Stark broadening method and the electric model over a wide electron density range 10¹¹–10¹⁶ cm⁻³.

This paper presents results from a study into the generation of ozone by a stable atmospheric glow discharge, using dry air as the feeding gas for ozone generation. The power supply is 50 Hz ac, with the use of a perforated aluminium sheet for the electrodes and soda lime glass as a dielectric layer in a parallel-plate configuration, stabilizing the generation process and enabling ozone to be produced. The stable glow discharge spreads uniformly at a gas breakdown voltage below 4.8 kV and requires only 330 mW discharge power, with a limitation of 3 mm on the maximum gap spacing for the dry air. With the technique providing a high collision rate between the electrons and gas molecules during the discharge process, a high ozone yield is obtained. An analysis of the effect on the production rate of parameters such as the input voltage, gas flow rate and reaction chamber dimensions resulted in a highest efficiency of production of almost 350 g kWh⁻¹ and confirms its potential as an important ozone generation technology.

For about 10 years, surface dielectric barrier discharges (DBDs) have been widely used as plasma actuators in subsonic airflow control applications. However, the extension length of a single surface DBD is limited to about 2 cm, which could restrict its use to small-scale applications. One way to extend the plasma actuation surface consists of using several single surface DBDs in series, energized by zero phase delayed or phase shifted high voltages. However, the mutual interaction between successive discharges affects the benefits of such standard multi-DBD actuators. This paper deals with a new design electrode for large-scale flow control applications. It consists of replacing each single two-electrode DBD by a three-electrode DBD where the third electrode acts as a shield between two successive DBDs. Experimental measurements by laser doppler velocimetry, pressure probe and time-resolved particle image velocimetry show that the mutual interactions can be strongly reduced, resulting in a constant electric wind velocity above the multi-DBD actuator.

One-dimensional metal/semiconductor heterojunction nanomaterials have opened many new opportunities for future nanodevices because of their novel structures and unique electrical and optical properties. In this work, sulfhydryl-containing multi-walled carbon nanotube/gold nanoparticle (MWCNT/Au) heterojunctions were synthesized in high yield by a sulfhydryl- functionalized self-assembly strategy. The component, size, structure, morphology and bond mode of the MWCNT/Au heterojunctions thus prepared were investigated and demonstrated by transmission electron microscopy, scanning electron microscopy, x-ray diffraction, energy-dispersive x-ray spectroscopy, Fourier-transform infrared and UV–visible measurements. Cyclic voltammogram and electrochemical impedance spectroscopy studies indicate that the MWCNT/Au heterojunctions have a novel electron transfer property, which retards electron transfer of the horseradish peroxidase or the ferricyanide in the underlying electrodes. We believe that MWCNT/Au heterojunctions with high stability and a unique electrical property are expected to find potential applications for nanodevices.

The authors report on hydrophobic self-cleaning surfaces of transparent zinc oxide (ZnO) thin films tuned by controlling uniformity of the film surfaces. ZnO films were synthesized by a simple, low-cost sol–gel technique. The degree of hydrophobicity of the films was strongly dependent on the number of coatings with ZnO. The surface of the polycrystalline ZnO thin film with 8-coatings was maximum hydrophobic with a static contact angle of 114° ± 3°. The films were highly transparent with average transmission exceeding 80% in the visible region. Thus, new hydrophobic ZnO thin films can be of great importance in commercial application as transparent self-cleaning surfaces.

Two different configurations of photoanodes based on anodic iron oxide were investigated for photoelectrochemical water oxidation. A self-ordered and vertically oriented array of iron oxide nanotubes was obtained by anodization of pure iron substrate in an ethylene glycol based electrolyte containing 0.1M NH₄F + 3 vol% water (EGWF solution) at 50 V for 15 min. Annealing of the oxide nanotubes in a hydrogen environment at 500 °C for 1 h resulted in a predominantly hematite phase. The second type of photoanode was obtained by a two-step anodization procedure. This process resulted in a two-layered oxide structure, a top layer of nano-dendrite morphology and a bottom layer of nanoporous morphology. This electrode configuration combined the better photocatalytic properties of the nano-dendritic iron oxide and better electron transportation behaviour of vertically oriented nano-channels. Annealing of these double anodized samples in an acetylene environment at 550 °C for 10 min resulted in a mixture of maghemite and hematite phases. Photocurrent densities of 0.74 mA cm⁻² at 0.2 V_Ag/AgCl and 1.8 mA cm⁻² at 0.5 V_Ag/AgCl were obtained under AM 1.5 illumination in 1M KOH solution. The double anodized samples showed high photoconductivity and more negative flat band potential (−0.8 V_Ag/AgCl), which are the properties required for promising photoanode materials.

The ability to synthesize carbon nanofibres (CNFs) with a high degree of control over their geometry, location and structure via catalytic plasma-enhanced chemical vapour deposition has expanded the possibility of new applications. The nanoscale dimensions and high aspect ratio of vertically aligned carbon nanofibres (VACNFs), along with favourable physical and chemical characteristics, has provided a nanostructured material with properties that are well-suited for interfacing with live cells and tissues. This review surveys the aspects of synthesis, integration and functionalization of VACNFs, followed by examples of how VACNFs have been used to interface with live systems for a variety of advanced nanoscale biological applications.

Digital image correlation is a measurement technique that allows one to retrieve displacement fields 'separating' two digital images of the same sample at different stages of loading. Because of its remarkable sensitivity, it is possible to not only detect cracks with sub-pixel opening, which would not be visible but also to provide accurate estimates of stress intensity factors. For this purpose suitable tools have been devised to minimize the sensitivity to noise. Working with digital images allows the experimentalist to deal with a wide range of scales from the atomistic to the geophysical one with the same tools. Various examples are shown at different scales, as well as some recent extensions to three-dimensional cracks based on x-ray Computed micro-tomographic images.

Graphite oxide was synthesized using various oxidation times and characterized by its physical and chemical properties. The degree of oxidation of the graphite oxide was systematically controlled via oxidation time up to 24 h. Three phases of interlayer distances were identified by x-ray diffraction: pristine graphite (3.4 Å), intermediate (4 Å) and fully expanded graphite oxide (6 Å) phases. These phases were distinguished by an atomic ratio of O/C, which occurred from the different compositions of epoxide, carboxyl and hydroxyl groups. The band gap of the graphite oxides was also tuned via the oxidation time, resulting in direct band gap engineering from 1.7 to 2.4 eV and strong correlation with the atomic ratio of O/C.

Nanocomposite films were fabricated by supersonic cluster beam deposition (SCBD) of palladium clusters on poly(methyl methacrylate) (PMMA) surfaces. The evolution of the electrical conductance with cluster coverage and microscopy analysis show that Pd clusters are implanted in the polymer and form a continuous layer extending for several tens of nanometres beneath the polymer surface. This allows the deposition, using stencil masks, of cluster-assembled Pd microstructures on PMMA showing a remarkably high adhesion compared with metallic films obtained by thermal evaporation. These results suggest that SCBD is a promising tool for the fabrication of metallic microstructures on flexible polymeric substrates.

An analytic formula is derived for the elastic bending modulus of monolayer graphene based on an empirical potential for solid-state carbon atoms. Two physical origins are identified for the non-vanishing bending stiffness of the atomically thin graphene sheet, one due to the bond-angle effect and the other resulting from the bond-order term associated with the dihedral angles. The analytical prediction compares closely with ab initio energy calculations. Pure bending of graphene monolayers into cylindrical tubes is simulated by a molecular mechanics approach, showing slight nonlinearity and anisotropy in the tangent bending modulus as the bending curvature increases. An intrinsic coupling between bending and in-plane strain is noted for graphene monolayers rolled into carbon nanotubes.

Composite metamaterials (CMMs) combining a subwavelength metallic hole array (i.e. one-layer fishnet structure) and an array of split-ring resonators (SRRs) on the same board are fabricated with gold films on a silicon wafer. Transmission measurements of the CMMs in the terahertz range have been performed. Dual-band magnetic resonances, namely, an LC resonance at 4.40 THz and an additional magnetic resonance at 8.64 THz originating from the antiparallel current in wire pairs in the CMMs, are observed when the electrical field polarization of the incident light is parallel to the gap of the component SRR. The numerical simulations agree well with the experimental results and further clarify the nature of the dual-band magnetic resonances.

Zinc oxide (ZnO) is a wide band gap semiconductor material with attractive features for light emitting devices, photovoltaics, chemical sensors and spintronics. In the past 10 yr ZnO has attracted tremendous interest from the materials science and semiconductor physics research communities, and in this review recent progress in (i) bulk growth, (ii) understanding of the role of hydrogen and (iii) formation of high-quality Schottky barrier (SB) diodes, are discussed for single crystalline ZnO. In (i), the emphasis is put on hydrothermally grown material and how the concentration of intentional and unintentional impurities, such as In and Li, can be controlled and modified by high temperature treatment and defect engineering involving vacancy clusters. In (ii), different possible configurations of hydrogen as a shallow donor are evaluated based on results from calculations employing the density-functional-theory as well as from experimental studies of local vibrational modes using Fourier transform infrared spectroscopy. Further, hydrogen is demonstrated to be very reactive and the interaction with zinc vacancies, group I and group V elements, and transition metals are elucidated. Moreover, the diffusion of hydrogen is found to be rapid and limited by the concentration of traps in hydrothermal samples, and it is argued that isolated (free) hydrogen is not very likely to exist in ZnO at room temperature. In (iii), a compilation of the literature data illustrates that the SB heights for metals deposited on n-type samples have no correlation with the metal work function, violating the fundamental Schottky–Mott model. The role of surface preparation cannot be overestimated and in several cases an oxidation of the surface prior to metal deposition is shown to be beneficial for the formation of high barrier SB diodes. The effects of near-surface defects, such as oxygen vacancies, and contact inhomogeneity are also addressed. However, in spite of the significant progress made in the past 5–7 years, a thorough understanding of the SB formation to ZnO is still lacking. Finally, results from characterization of electrically active point defects employing the SB contacts and junction spectroscopic techniques are reviewed and the identification of some prominent bandgap states is critically evaluated.

Oxygen and zinc vacancies are unambiguously shown to be formed in as-grown ZnO bulk crystals grown from melt without being subjected to irradiation, from electron paramagnetic resonance (EPR) and optically detected magnetic resonance (ODMR) studies. Concentrations of the defects in their paramagnetic charge states and are estimated to be ∼2 × 10¹⁴ cm⁻³ and ∼10¹⁵ cm⁻³, respectively. The defect is concluded to act as a deep acceptor and to exhibit large Jahn–Teller distortion by 0.8 eV. The energy level of the defect corresponding to the (2–/–) transition is E_v + 1.0 eV. The isolated Zn vacancy is found to be an important recombination centre and is concluded to be responsible for the red luminescence centred at around 1.6 eV. On the other hand, the oxygen vacancy seems to be less important in carrier recombination as it could be detected only in EPR but not in ODMR measurements. Neither isolated nor centres participate in the so-called 'green' emission. It is also shown that whereas the concentrations of both defects can be reduced by post-growth annealing, the Zn vacancy exhibits higher thermal stability. The important role of residual contaminants such as Li in the annealing process is underlined.

Dielectrophoresis of non-spherical particles is attracting attention, particularly for the aligned deposition of nanowires and nanotubes. The orientation and translation of ellipsoids are studied theoretically, and it is found that the orientation plays a significant role in determining the particle trajectory. A new equation for dielectrophoresis is presented which allows for orientation to be taken into account. Numerical simulation of particle motion is performed to demonstrate these effects.

Visualization of Nd : YAG laser ablation of aluminium targets was performed by a shadowgraph apparatus capable of imaging the dynamics of ablation with nanosecond time resolution. Direct observations of vaporization, explosive phase change and shock waves were obtained. The influence of vaporization and phase explosion on shock wave velocity was directly measured. A significant increase in the shock wave velocity was observed at the onset of phase explosion. However, the shock wave behaviour followed the form of a Taylor–Sedov spherical shock below and above the explosive phase change threshold. The jump in the shock wave velocity above phase explosion threshold is attributed to the release of stored enthalpy in the superheated liquid surface. The energy released during phase explosion was estimated by fitting the transient shock wave position to the Taylor scaling rules. Results of temperature calculations indicate that the vapour temperature at the phase explosion threshold is slightly higher than the critical temperature at the early stages of the shock wave formation. The shock wave pressure nearly doubled when transitioning from normal vaporization to phase explosion.