Table of contents

In this topical review, we outline the construction of a reflection high-energy electron diffraction (RHEED) surface pole figure from a polycrystalline film by recording multiple RHEED patterns as the substrate is rotated around the surface normal. Due to the short penetration depth of electrons, the constructed pole figure is a surface pole figure. It is in contrast to the conventional x-ray pole figure which gives the average texture information of the entire polycrystalline film. Examples of the surface pole figure construction processes of a fibre texture and a biaxial texture are illustrated using Ru vertical nanorods and Mg nanoblades, respectively. For a biaxially textured film, there often exists an in-plane morphological anisotropy. Then additional intensity normalization must be applied to compensate for the effects of anisotropic morphology on RHEED surface pole figure construction. Rich information on the texture evolution, such as the change in the tilt angle of the texture axis, has been obtained from the in situ study of oblique angle vapour deposition of Mg nanoblades using RHEED surface pole figures. Finally we make a comparison between the RHEED surface pole figure and the conventional x-ray pole figure techniques.

Two series of epitaxial Pr–Co thin films with a nominal composition of Pr₂Co₁₇ were grown on Cr buffered MgO(1 1 0) single crystal substrates by pulsed laser deposition, in which the deposition temperature T_D and the post annealing temperature T_ann were varied. Pr–Co films prepared at different deposition temperatures crystallize in the metastable PrCo₇ structure (TbCu₇ type) for 450 °C ≤ T_D ≤ 650 °C. For T_D ≤ 600 °C these films exhibit a (h 0 0) texture, with the c-axis of the Pr–Co grain aligned in the substrate plane along MgO[0 0 1]. However, raising T_D to 650 °C, an additional out-of-plane c-axis texture is observed. Post annealing experiments on films, which are deposited at 450 °C to obtain the unique in-plane texture, reveal that crystal structure, texture and also the uniaxial magnetocrystalline anisotropy of the metastable PrCo₇ structure is maintained for annealing temperatures up to 800 °C. The influence of deposition temperature and post annealing temperature on phase stability and extrinsic magnetic properties are discussed.

One of the main concerns in the preparation of alloy nanowires is the ability to synthesize compositionally uniform nanowires along the axis. Since most of the conventional mild acidic permalloy (Ni₈₀Fe₂₀) electroplating baths consist of an extremely low concentration of Fe ions compared with Ni ions, the electrodeposition of iron is controlled by mass transfer, which leads to a significant change in the composition along the axis of the nanowire. To overcome this obstacle, we developed a new acidic chloride electrolyte with a high concentration of Fe and Ni ions to electrodeposit homogeneous nanowires. After synthesizing nanowires, the temperature dependent magneto- and electro-transport properties of individual nanowires were investigated. The temperature coefficient of resistance of a nanowire is much lower than the bulk counterpart, which might be attributed to a higher residual resistivity. The magnetoresistance shows a typical anisotropic magnetoresistance behaviour where the maximum anisotropic magnetoresistance ratio decreased with increasing temperature. The angular dependence of the magnetization switching field indicated that curling is the magnetization reversal mode at all temperatures.

We present findings on the synthesis and processing of various FePt nanostructures using pulsed laser deposition. FePt was deposited as (a) continuous thin films, (b) single layered nanodots and (c) multi-layered nanodots. Integration of these FePt nanostructures with Si (1 0 0) was achieved by using epitaxial TiN as a template buffer layer. Epitaxy and L1₀ order in the FePt structures were investigated in detail by x-ray diffraction and transmission electron microscopy techniques. Remarkably, we achieved ⟨0 0 1⟩ c-axis growth orientation and L1₀ ordering at a low processing temperature of 500 °C. The following epitaxial relationship was obtained in the FePt structures: FePt (0 0 1) ⟨0 0 1⟩ ∥ TiN (1 0 0) ⟨0 0 1⟩ ∥ Si (1 0 0) ⟨0 0 1⟩. The 9.5% misfit strain between FePt and TiN is relaxed by domain matching epitaxy where 10/19 and 11/10 domains alternate across the interface. The composition of the nanostructures was determined by Rutherford backscattering and was found to be Fe₄₁Pt₅₉. The magnetic properties of each of the three nanostructured systems were studied individually and then compared to evaluate them as potential information storage media. Room temperature perpendicular coercivity of 3200 Oe for the single layered FePt nanodot sample was much improved when compared with the continuous epitaxial FePt film sample showing a coercivity of 2250 Oe. In the FePt nanodot system, each nanodot acts as a single domain and assuming one bit of information is stored in each nanodot, magnetic storage density can exceed one terabit per square inch, corresponding to 250 million pages of information per chip.

Magnetic levitation technology, having the characteristics of low cost and high quality, has been considered a preferable option for the next generation of launcher systems. A world-wide research design on the conceptual level has been carried out on the highly reusable space transportation systems by applying magnetic levitation to the launch assistance. Recently, a research plan has been implemented in our laboratory by constructing a scale-model suspension system with high temperature superconductor (HTS henceforth) bulks over a 7 m Nd–Fe–B permanent-magnet (PM henceforth) track for the launch assistance. An experimental platform was built to investigate the dynamic responses of the PM–HTS interaction at different field-cooled positions. The critical frequencies and amplitudes which lead to the instability of levitation drift were investigated. The stiffness and the vibration damping were also discussed at the zero-field-cooled position.

A series of Ni₄₅Mn_44−xCr_xSn₁₁ (x = 0, 1, 2) ferromagnetic shape memory alloys were prepared. With slight doping of Cr, the martensitic transition temperatures decrease rapidly. The magnetic entropy changes at a low magnetic field were investigated in these alloys. The maximum value of ΔS_M is 23.4 J kg⁻¹ K⁻¹, which was observed in Ni₄₅Mn₄₃CrSn₁₁ alloys. The origin of the magnetic entropy changes in these alloys has been discussed. The giant low-field magnetic entropy changes and low cost make Ni₄₅Mn_44−xCr_xSn₁₁ alloys a promising candidate for magnetic refrigeration.

Nanocrystalline n-GaN in thin film form was deposited onto fused silica substrates by high pressure dc sputtering of Si(1 at%) doped GaN target. An Al/n-GaN/Pd Schottky diode was realized by depositing n-GaN nanocrystalline layer onto Al-coated fused silica substrate under identical deposition conditions. Top Pd contacts were obtained by evaporating Pd using an appropriate mask. Corresponding current–voltage characteristics of the Schottky diodes were recorded before and after hydrogen gas exposure. I–V variations were analysed in the light of the existing theories. The ideality factor for the Schottky diodes varied between 3.4 and 4.5. The series resistance of the diode before gas and after gas exposure were found to be ∼500 kΩ and ∼300 kΩ, respectively. The barrier height could be seen to decrease significantly with increased hydrogen concentration while it increased with temperature.

A Mg_xZn_1−xO p–n homojunction light-emitting diode (LED) was fabricated on the GaAs substrate by metal–organic chemical vapour deposition (MOCVD). The I–V curve of the device exhibited typical rectifying behaviour of the p–n diode. The forward turn-on voltage was about 3.5 V. The photoluminescence spectra of the n-type and p-type Mg_xZn_1−xO layers exhibited obvious near-band-edge emission peaks and weak deep-level emission peaks. In the electroluminescence (EL) spectra of the diode, a broad emission band from 2.3 to 2.8 eV can be observed. The emission band near 3.31 eV did not appear until the injection current reached 180 mA. The differences in the EL spectra between ZnO and Mg_xZn_1−xO LEDs are also discussed.

Elastic strain fields at the interface of the epilayer and buffer layer of the InGaAsP/InP heterostructure were characterized by electron backscatter diffraction (EBSD) technology based on scanning electron microscopy. The InGaAsP/InP heterostructure which contained lattice misfit was under a dislocation-free condition. Image quality (IQ) was used as the strain sensitive parameter. From the image quality map and image quality curve, we observed directly the distribution of the elastic strain fields at the interface along the direction perpendicular to the interface as well as the interface structure between the epilayer and buffer layer by transmission electron microscopy and high resolution transmission microscopy.

The scattering of a light pulse by a sphere has been explored. The pulse has a trigonometric envelope. Equations have been developed for the scattering and extinction efficiencies, both for real-time and for time-averaged situations. In the latter case the time averaging is carried out analytically and only trigonometric functions are involved. The results are largely in agreement with expectation from previous studies, though there is no indication of a negative extinction efficiency as predicted by Shifrin and Zolotov (1995, Appl. Opt.34 552–8). A detector with a time constant greater than approximately 2 ps would effectively measure the same total scattering as would be predicted for a continuous plane wave, apart from a small difference that may be attributed to whispering gallery modes.

The study of electron mobility of bis(2-methyl 8-hydroxyquinoline) (triphenyl siloxy) aluminium (SAlq) by transient electroluminescence (EL) is presented. An EL device is fabricated in bilayer, ITO/N,N'-diphenyl-N, N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD)/SAlq/LiF/Al configuration. The temporal evaluation of the EL with respect to the step voltage pulse is characterized by a delay time followed by a fast initial rise, which is followed by a slower rise. The delay time between the applied electrical pulse and the onset of EL is correlated with the carrier mobility (electron in our case). Transient EL studies for SAlq have been carried out at different temperatures and different applied electric fields. The electron mobility in SAlq is found to be field and temperature dependent and calculated to be 6.9 × 10⁻⁷ cm² V⁻¹ s⁻¹ at 2.5 × 10⁶ V cm⁻¹ and 308 K. The EL decays immediately as the voltage is turned off and does not depend on the amplitude of the applied voltage pulse or dc offset.

Nitrogen doped SrTiO₃ (N-STO) was fabricated by the ion implantation technique. The microstructure and photoluminescence (PL) of virgin N-STO and HF acid etched N-STO were studied in detail. X-ray diffraction measurements indicate that the nitrogen dopants can enhance the crystal structure of STO, and that there are no other impurity phases in N-STO except that of cubic STO. However, there is nano-crystal phase of Sr₂Ti₆O₁₃ in the HF-acid-etched STO and N-STO. The room temperature PL intensity of the original STO crystal is quenched by nitrogen dopants, which might be related to the absence of trapped holes and electrons due to the enhanced crystal structure. In contrast, the room temperature PL intensity is greatly increased by HF acid etching for both STO crystal and N-STO. This increase in PL intensity may be related to the order–disorder effect and the stabilization of self-trapped excitons at room temperature due to the nano-crystal phase of Sr₂Ti₆O₁₃.

In the laser direct metal deposition process, interaction between the laser beam and powder from a coaxial powder delivery nozzle alters the temperature of powder and the amount and spatial distribution of laser intensity reaching the deposition melt pool. These factors significantly affect the process and are also important input parameters for any finite element or analytical models of the melt pool and deposition tracks. The analytical model in this paper presents a method to calculate laser attenuation and powder temperatures at every point below such a nozzle. It is applicable to laser beams that are approximately parallel over the beam–powder interaction distance of any initial intensity distribution (Top Hat, Gaussian, TEM₀₁^* or other). The volume below the nozzle is divided into the region above the powder consolidation plane, where the powder stream is annular, and below it, where it is a single Gaussian stream, and expressions derived for each region. Modelled and measured results are reasonably matched. Results indicate that attenuation is more severe once the annular powder stream has consolidated into a single stream but is not zero before that point. The temperature of powder reaching any point is not constant but the mean value is a maximum at the centre of the stream.

The performances of In_0.65Ga_0.35N single-junction solar cells with different structures, including various doping densities and thicknesses of each layer, have been simulated. It is found that the optimum efficiency of a In_0.65Ga_0.35N solar cell is 20.284% with 5 × 10¹⁷ cm⁻³ carrier concentration of the front and basic regions, a 130 nm thick p-layer and a 270 nm thick n-layer.

Cu-doped nanoporous silica glass was prepared and broadband upconversion luminescence was observed when excited with a 800 nm femtosecond laser. The optical properties of the Cu-doped nanoporous silica glass were investigated by absorption, steady state excitation and emission, luminescence decay and time-resolved spectroscopy. The analysis of the upconversion mechanism indicates that three-photon absorption is responsible for the visible luminescence. A three-dimensional solid state display was also demonstrated and the full colorization of a volumetric three-dimensional solid state display could be made possible by designing and controlling the combination of doping species in the nanoporous silica glass.

A collisional–radiative model is proposed to describe the behaviour of a helium plasma sustained in a resonant cavity at atmospheric pressure. A set of rate constants is proposed at 2450 K. Indeed, most of the available data in the literature are reliable below 500 K. It is shown that the time post-discharge is mainly controlled by ambipolar diffusion during the first ten µs, next by electron-assisted recombination of helium ions and finally by chemionization from the excited state. This post-discharge lasts for hundreds of µs. It is mainly due to the slow recombination of electrons together with chemionization. Then, at steady state in CW mode, the electron temperature is found to be lower than 1 eV since low reduced fields are needed to sustain the discharge.

Arc voltage fluctuations are studied for two different plasma torches. One of them is home-made, the other is a Sultzer Metco plasma torch. Both work under similar operating conditions, but they show very different voltage waveforms and spectra, depending on their electrode configurations. The first torch shows a rather intermittent behaviour and works under the restrike mode with a rather low fluctuation amplitude and with non-reproducible spectral components. The second torch also shows characteristic features related to the restrike mode, but they are superimposed on more regular oscillations which are due to pressure variations in the cold gas chamber, located upstream the arc region. This part of the torch together with the nozzle channel appears to be a Helmholtz resonator, whose resonance frequency is theoretically evaluated as a function of the torch geometry and of the operating conditions. The theoretical predictions are in very good agreement with the measured frequency of the main peak in the voltage spectrum of the second torch. A discussion about the coupling between the pressure and the voltage is proposed to explain how the torch design could influence the Helmholtz resonance.

Direct current plasma jet source operated in a glow regime is studied by applying nitrogen as a feed gas. This plasma jet generates fluxes of atoms, which are produced in the small region near the cathode tip characterized by high luminosity and a high dissociation rate of molecules. The nitrogen atoms carried away from this region by the feed gas flow form the narrow effluent of the plasma jet source. The gas temperature in the effluent flow is lower than 400 K despite the relatively high temperature of up to 950 K in the active plasma zone near the cathode tip.

The fluxes of chemical active particles in the effluent of plasma jets are very important parameters for different applications. Herein, we present a method of measurement and an algorithm to obtain plasma parameters and particle fluxes from the atmospheric pressure direct current (dc) plasma jet operated in nitrogen/oxygen mixtures. We determine the atom and radical steady state densities and particles fluxes by means of emission spectroscopy and numerical modelling. The plasma parameters, electron density n_e and electron velocity distribution function, are obtained by means of emission spectroscopy, microphotography and solutions of the Boltzmann equation. The steady state densities of particles in the effluent region are calculated by solving transport equations (thermal conductivity and diffusion) and chemical kinetics equations. The calculated densities of nitrogen atoms are compared with emission spectroscopy measurements at various positions outside the nozzle of the plasma source. The measurements confirm model results. The fluxes of chemically active particles are determined by means of steady state particle densities and known gas flow rate.

Multi-wall carbon nanotubes (CNTs) were modified by an oxygen radio frequency plasma treatment, the appropriate duration of which was determined from post-treatment high-resolution transmission electron microscopy (HRTEM) images. A comparison of HRTEM results from gold-decorated pristine and plasma treated CNTs showed that the plasma treatment improves the uniformity of the distribution of surface defects. X-ray photoelectron spectroscopy confirms that oxygen atoms attach to the surface of CNTs. Changes in the C 1s photoemission signal point towards a loss of delocalization of valence electrons, confirmed by the modifications of valence band spectra, and consistent with currently accepted theory.

Fluorocarbon plasmas are widely used in applications and as model systems for fundamental investigations of complex plasmas. In recent years pulsing of the rf discharge has been used as an additional parameter for process control, because many plasma parameters, e.g. densities and temperatures, become time dependent when the rf power is modulated. In this work tunable diode laser absorption spectroscopy in the mid-IR (IR-TDLAS) was applied to measure time-resolved densities of the transient species CF and CF₂ and that of the stable product C₂F₄ in pulsed CF₄/H₂ asymmetrical capacitively coupled radio-frequency plasmas at 13.56 MHz. Simultaneously, the thickness of amorphous thin fluorocarbon films (a-C:F) on the powered electrode was determined by means of in situ ellipsometry. Therefore, it was possible to study the correlation between gas phase species and thin film formation. The decay curves of the CF and CF₂ densities in the off-phase of the pulsed rf plasma were fitted with a combination of first and second order processes involving the loss processes of these radicals in the gas phase and at the surfaces. Particularly, in the plasma off-phase, the loss of CF₂ radicals forming C₂F₄ was found to be dominant in the CF₂ kinetics, but of minor importance for C₂F₄ production. Plasma process parameters such as total pressure, gas composition, power and power modulation were varied to investigate the interaction between gas phase species and surfaces.

Currently, there is a strong tendency to replace rigid electronic assemblies by mechanically flexible and stretchable equivalents. This emerging technology can be applied for biomedical electronics, such as implantable devices and electronics on skin. In the first step of the production process of stretchable electronics, electronic interconnections and components are encapsulated into a thin layer of polydimethylsiloxane (PDMS). Afterwards, the electronic structures are completely embedded by placing another PDMS layer on top. It is very important that the metals inside the electronic circuit do not leak out in order to obtain a highly biocompatible system. Therefore, an excellent adhesion between the 2 PDMS layers is of great importance. However, PDMS has a very low surface energy, resulting in poor adhesion properties. Therefore, in this paper, PDMS films are plasma treated with a dielectric barrier discharge (DBD) operating in air at medium pressure (5.0 kPa). Contact angle and XPS measurements reveal that plasma treatment increases the hydrophilicity of the PDMS films due to the incorporation of silanol groups at the expense of methyl groups. T-peel tests show that plasma treatment rapidly imparts adhesion enhancement, but only when both PDMS layers are plasma treated. Results also reveal that it is very important to bond the plasma-treated PDMS films immediately after treatment. In this case, an excellent adhesion is maintained several days after treatment. The ageing behaviour of the plasma-treated PDMS films is also studied in detail: contact angle measurements show that the contact angle increases during storage in air and angle-resolved XPS reveals that this hydrophobic recovery is due to the migration of low molar mass PDMS species to the surface.

The temperature determination by spectroscopic measurements in high-current high-pressure arcs in a polytetrafluoroethylene (PTFE) nozzle under the assumption of an optically thin plasma has been investigated. Assuming local thermodynamic equilibrium the radial temperature distributions as well as the plasma pressures have been determined by fitting a model to measured spectral radiances considering line and continuum absorption. It is shown that absorption has to be included in the error estimate of the experimental results. The different effects, which cause deviations from the optically thin case, have been analysed numerically and by using a simplified analytical model. The theoretically estimated pressures sensitively depend on the Stark broadening. In the studied plasmas the calculated large electron densities indicate a marked reduction of the Stark widths by nonideality effects. The applicability of the experimental method has been proved for suitably chosen lines.

Based on the hydrodynamic equations, the characteristics of nonlinear dust-ion-acoustic shock propagation in an inhomogeneous dusty plasma are numerically studied, by means of the fifth-order weighted essentially non-oscillatory (WENO) finite difference scheme. The inhomogeneous dust charging process for different plasma parameters has been incorporated and its effect on the shock propagation has been investigated. Comparisons of the shocks in the cases of different ion density distributions and different electron-to-ion density ratios are presented. The results show that the dust charging process results in wave damping and dissipation, and the decrease of the electron-to-ion density ratio leads to the increase of the speed, amplitude and width of the shock front. Moreover, the dispersion effect is dominant in the case of sharp shocks whereas the smooth shocks are easily dissipated.

We report the flashover development phenomenon across a polished silicon wafer and its damage effect on the silicon surface under pulsed excitation of ∼0.4/2.5 µs with a peak value of >6 kV in atmospheric air. A visible light channel connecting two electrodes is observed before the complete flashover occurs. The filament and pit–jut damages induced by the flashover are found on the silicon surface. A comprehensive flashover development hypothesis is proposed according to which first the filamentary current process occurs in the surface layer of a semiconductor and then the ionization process of air ambient above the surface occurs. The observed surface damages are considered to be closely correlated with the two processes.

The effect of Nb doping on the electrochromic properties of WO₃ thin films is investigated. The systematics of different properties such as the structural phase change, the colour-bleach kinetics, colouration efficiency, reversibility, stability, ion insertion/extraction capacity and change in charge states from W⁺⁶ to W⁺⁵ are found to depend on the Nb doping concentration. The x-ray diffraction results reveal the solid solution WNb₂O₈ phase with mixed WO₃–Nb₂O₅ (monoclinic ε-WO₃ and orthorhombic Nb₂O₅) phases at lower Nb concentration (2 at.%) and mixed WO₃–Nb₂O₅ phases at higher Nb concentrations (6 and 10 at.%). The scanning electron microscopy results show a drastic change in surface morphology as the doping levels increase. The surface morphology changes from the fibrous-reticulate present in undoped films to a rough and granular structure for doped films. The cycle stability, charge storage capacity and reversibility of the films are improved upon doping; however, colouration efficiency decreases for doped films and is minimum for 6 at.% Nb doped film. The x-ray photoelectron spectroscopy measurements show that the valance state of Nb is not changed after colouration, indicating no direct contribution of Nb in the colouration process. These results are discussed in terms of the effects of doping induced compositional inhomogeneity and disorder structure on the film properties.

A three-dimensional elastic–perfectly-plastic finite element model of asperity interaction has been developed. This model consists of two connected neighbouring deformable hemispheres which are brought into contact with a flat rigid surface. For two asperities of the same height and the same radius, as the approach of the rigid surface is increased, the contact area changes from two isolated nearly circular contacts to a wasp-waist area, and finally to a single oval shaped contact region. For asperities of different heights the results can be qualitatively similar to those for equal heights, but can also exhibit a shielding effect in which the first contact grows to eventually encompass the second lower peak. Results are given using dimensionless quantities for the force versus interference, and the contact area versus interference. These relationships are plotted for different asperity height differences and asperity separations. Also provided are graphs of the contact area boundary for different values of the approach. Curve-fit equations are presented as an aid in using this work as a building block in a statistical multi-asperity contact model which directly includes the effect of asperity interactions.

Two experimental techniques of nanoindentation and tensile testing were used at room temperature to investigate the strain rate sensitivity of an electrodeposited Ni with a mean grain size (d) of 20 nm, respectively. It was found that the nanocrystalline (nc) Ni possessed a higher strain rate sensitivity exponent (m) during nanoindentation than during tensile testing. Furthermore, a higher m was accompanied by a smaller activation volume (V). It is believed that the higher stress concentration could activate a shorter dislocation line length (L), which should be responsible for the higher m value during the nanoindentation. Based on a model of dislocation nucleation or bowing-out mechanism, the relationship between m and d for Ni and its alloys was investigated. In the end, a simple and straightforward equation relating m to d was proposed in aid of a simple assumption associating L with d, which implied that the enhanced m in nc Ni and its alloys with d > ∼6 nm should be due to the reduction of the dislocation line length.

(Pb, La)(Zr, Sn, Ti)O₃ (PLZST) antiferroelectric (AFE) thin films with different compositions were deposited on Pt-buffered silicon wafers by the sol–gel process. The phase structure and the surface morphology of the PLZST AFE thin films were analysed by XRD and SEM, respectively. The electric field induced AFE-to-ferroelectric (AFE–FE) phase transformation behaviour of the PLZST thin films was examined by polarization versus field (P–E) and relative permittivity versus field (ε_r–E) measurements, with emphasis placed on composition-dependent phase switching field. The phase switching current was investigated as a function of a gradually changed dc electric field. Furthermore, the effect of the composition of the PLZST thin films on the Curie temperature (T_c) was also studied in detail.

The pressure-induced structural transition in amorphous TiO₂ nanoparticles has been studied in a spherical model of different diameters of 2, 3 and 4 nm under non-periodic boundary conditions. We use the pairwise interatomic potentials proposed by Matsui and Akaogi. Models have been compressed from 3.8 g cm⁻³ up to very high density (i.e. high pressure) in order to investigate the pressure-induced structural changes. We found the change from the low-density amorphous (lda) form with Z_Ti–O ≈ 6.0 to the high-density amorphous (hda) one with Z_Ti–O ≈ 7.0 to be like those observed in practice. We found that the transition pressure is nanoparticle size dependent due to the surface effects. In order to compare and highlight the features of such transition in nanoparticles, we also present the results for the same transition in the amorphous TiO₂ models containing 3000 atoms under periodic boundary conditions, which could be considered as bulk counterparts. Structural properties of nanoparticles at 700 K have been analysed in detail through the partial radial distribution functions, interatomic distances, coordination number and bond-angle distributions. Moreover, we also show the radial density profile in nanoparticles.

On the metalorganic chemical vapour deposition growth of AlN, by adjusting H₂+N₂ mixture gas components, we can gradually control island dimension. During the Volmer–Weber growth, 2-dimensional coalescent islands induces an intrinsic tensile stress. Then, this process can control the in-plane stress: with the N₂ content increasing from 0 to 3 slm, the in-plane stress gradually changes from 1.5 GPa tensile stress to −1.2 GPa compressive stress. Especially, with the 0.5 slm N₂ + 2.5 slm H₂ mixture gas, the in-plane stress is only 0.1 GPa, which is close to the complete relaxation state. Under this condition, this sample has good crystal and optical qualities.

Electrical conductivity of a 99.999% aluminium wire with a 400 nm width and a 350 nm thickness was measured using the four-point atomic force microscope (AFM) technique. This technique is a combination of the principles of the four-point probe method and standard AFM. The equipment is capable of simultaneously measuring both surface topography and local electrical conductivity. Experiments show the microprobe to be mechanically flexible and robust. The repeatability of conductivity measurements indicates that this four-point AFM probe could be used for fast in situ characterization of local electrical properties of nanocircuits and nanodevices.

We have carried out a detailed temperature-dependent Raman scattering investigation on coupled longitudinal–optical phonon–plasmon modes in N–In codoped p-type ZnO thin films with different hole densities. In combination with the theoretical analysis for the temperature-dependent frequencies, linewidths, and lifetimes, we have revealed an asymmetrical decay process with frequencies near 450 and 130 cm⁻¹, an increasing anharmonic effect with hole density, as well as a decreasing lifetime with enhanced anharmonicity. The variation in the relative contribution from three- and four-phonon processes can be further attributed to the diversification of the local phonon density of states due to N–In codoping.

The reduction in switchable polarization during fatigue largely limits the application of PZT thin films in ferroelectric nonvolatile memories. So, it is very important to understand the fatigue mechanism in PZT films, especially at a nanoscale level. In this paper, nanoscale fatigue properties in PZT thin films have been studied by piezoresponse force microscopy and local piezoloops. It has been found that a piezoloop obtained on a fatigued point exhibits a much more pinched shape and a local imprint phenomenon is observed after severe fatigue. Furthermore, the domain structure evolves from a simple single-peak profile to a complex fluctuant one. However, there is only some shift of the piezoloop when a unipolar field with the same amplitude is applied on the film. The available experimental data show that there exist obvious domain wall pinning and injection of electrons into the film during fatigue. Finally, a schematic illustration is suggested to explain the possible fatigue mechanism.

Single-crystal LiNbO₃ films were deposited by liquid phase epitaxy (LPE) onto inductively coupled plasma-assisted reactive ion etched (RIE) LiTaO₃ substrates. The RIE-etched channels were 6 µm wide by 0.9 µm deep. Surface roughness at the bottom of trenches was 100 nm or better with sidewall roughness below detection limits. This study also compares the suitability of the LiBO₂–LiNbO₃ and the LiVO₃–LiNbO₃ flux systems. From high-resolution x-ray diffraction and scanning electron microscopy (SEM) data, LiBO₂–LiNbO₃ was determined to be the superior flux system with respect to diffraction profile widths of the 0 0 12 reflection. Films grown from boron-based flux were observed to have narrower x-ray rocking curve widths than films grown from vanadium-based flux, with typical values of the FWHM of 0.018° and 0.041°, respectively. Boron-based LPE was then used to deposit LiNbO₃ film onto (RIE) LiTaO₃ substrates. SEM imaging and energy dispersive x-ray analysis showed a sharp composition change between film and substrate indicative of a step index change suitable for waveguiding.

We present friction characteristics of sliding textured silicon surfaces at the submicrometre scale. A two-dimensional submicrometre dimple array on the Si surface is fabricated by femtosecond laser processing. Direct femtosecond laser nano-structuring of the Si (1 0 0) substrate by polystyrene particle-assisted near-field enhancement is used. In the investigated hole diameter domain from 229 to 548 nm, an increase in the friction coefficient with the decrease in the hole size is found experimentally. The fabricated submicrometre dimples act evidently as lubricant reservoirs to supply lubricants and traps to capture wear debris. The fluctuation of the friction coefficient is also increased by reducing the dimple size. The lowest friction coefficient of 1.41 × 10⁻² is achieved with the dimple array having a diameter of about 550 nm. This value is 2.6 times lower than that of non-structured substrates.

Topographic and optical contrasts formed by Ga⁺ ion irradiation of thin films of amorphous silicon carbide have been investigated with scanning near-field optical microscopy. The influence of ion-irradiation dose has been studied in a pattern of sub-micrometre stripes. While the film thickness decreases monotonically with ion dose, the optical contrast rapidly increases to a maximum value and then decreases gradually. The results are discussed in terms of the competition between the effects of ion implantation and surface milling by the ion beam. The observed effects are important for uses of amorphous silicon carbide thin films as permanent archives in optical data storage applications.

The influence of Poisson noise on the accuracy of x-ray reflectivity analysis is studied with an aluminium oxide (AlO) layer on silicon. A null hypothesis which argues that other than the exact solution gives the best fitness is examined with a statistical p-value test using a significance level of α = 0.01. Simulations are performed for a fit instead of a measurement since the exact error caused by noise cannot be determined from the measurement. The p-value is studied by comparing trial curves to 1000 'measurements', each of them including synthetic Poisson noise. Confidence limits for the parameters of Parratt's formalism and the Nevot–Croce approximation are determined in (mass density, surface roughness), (thickness, surface roughness) and (thickness, mass density) planes. The most significant result is that the thickness determination accuracy of AlO is approximately ±0.09 nm but the accuracy is better for materials having higher mass density. It is also shown that the accuracy of mass density determination can be significantly improved using a suitably designed fitness measure. Although the power of the presented method is demonstrated only in one case, it can be used in any parameter region for a plethora of single layer systems to find the lower limit of the error made in x-ray reflectivity analysis.

The microstructural properties of heteroepitaxial ZnO thin films prepared by laser molecular beam epitaxy (L-MBE) were investigated on SrTiO₃ substrates and BaTiO₃/SrTiO₃ pseudo substrates with different orientations. The interface characteristics were in situ monitored by reflection high-energy electron diffraction (RHEED), and the epitaxial orientation relations were reconfirmed by ex situ x-ray diffraction (XRD) measurements. ZnO films grown on SrTiO₃(0 0 1) and BaTiO₃/SrTiO₃(0 0 1) contained a poly-domain structure. For the former, the lattice mismatch was about −1.7% by four types of domain growth with the epitaxial relation of ZnO(1 1 0)||SrTiO₃(0 0 1) and ZnO[−1 1 1]|| SrTiO₃⟨100⟩. For the latter, twin domains would result in a smaller mismatch of −0.8% by the epitaxial relation of ZnO(0 0 1)||BaTiO₃(0 0 1) and ZnO[1 1 0]|| BaTiO₃⟨1 1 0⟩. On SrTiO₃(1 1 1) and BaTiO₃/SrTiO₃(1 1 1), single-domain films following the c-axial direction were observed with in-plane orientation ZnO[1 1 0]||SrTiO₃[1 1 0] and ZnO[1 0 0]||BaTiO₃[1 1 0], respectively. This 30° rotation in the in-plane direction of the ZnO epilayer with respect to the perovskite surfaces increased the lattice mismatch from about −2% to −14.5% after inserting BaTiO₃ layers. The orientation of ZnO films could be attributed to the characteristic difference of the interface energy. It is determined entirely by interface stress and crystallographic symmetry for the growth on nonpolar (0 0 1)-orientated perovskite surfaces while the competition between elastic energy and chemical energy plays an important role for that on polar (1 1 1)-surfaces.

TiN–Fe films with various iron concentrations were deposited on Si and NaCl single-crystal substrates by direct current reactive magnetron sputtering. The structure and chemical composition of the films were examined by x-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), energy-dispersive x-ray and x-ray photoelectron spectroscopy (XPS). The effects of Fe addition on the structural, mechanical and magnetic properties of TiN films were studied. XRD and HRTEM revealed for TiN–Fe a cubic B-1 structure while no sign of an iron phase was observed. It was found that the lattice parameter and the grain size decreased with an increase in the Fe/Ti atomic ratio. At Fe/Ti = 0.2, the XRD revealed a change in the preferential orientation from (1 1 1) to (2 0 0) with a tendency of line broadening as the iron concentration increased. The Fe 2p core level peak of XPS indicates that the greater part of Fe atoms in TiN–Fe films exists as free pure metallic iron while the lesser part was in the form of iron oxide. Film annealing at 500 and 600 °C led to iron precipitation of the α-Fe phase indicating that below 500 °C Fe–TiN, films can be considered as nanocomposites materials, as confirmed by nanoindentation measurements and HRTEM observations.

Ge₁₅Te_85−xSi_x (2 ≤ x ≤ 12) glasses of a wide range of compositions have been found to exhibit electrical switching at threshold voltages in the range 100–600 V, for a sample thickness of 0.3 mm. The samples become latched to the ON state (memory behaviour) at higher ON state currents (>1 mA). However, the switching is found to be reversible (threshold behaviour) if the ON state current is limited to lower values (≤0.7 mA). While Ge₁₅Te_85−xSi_x glasses with x ≤ 5 exhibit a normal electrical switching, an unstable behaviour is seen in the I–V characteristics of Ge₁₅Te_85−xSi_x glasses with x > 5 during the transition to the ON state. Further, a sparking in the electrode region and the splashing of the active material is observed in Ge₁₅Te_85−xSi_x glasses with x > 5. It is also interesting to note that the switching voltage (V_T) and initial resistance (R) of Ge₁₅Te_85−xSi_x glasses increase with addition of Si, exhibiting a change in slope at a composition x = 5 (⟨r⟩ = 2.4).

The observed electrical switching behaviour of Ge₁₅Te_85−xSi_x glasses has been understood on the basis that the composition x = 5 is the rigidity percolation threshold of the Ge₁₅Te_85−xSi_x system. It is also proposed that the Ge₁₅Te_85−xSi_x glasses with x < 5 are likely to be more suitable for phase change memory applications.

We report on a novel luminescent phenomenon in Gd₂O₂S : Er,Yb prepared by a solid-state reaction. After irradiation at 980 nm, the afterglow emission of Er³⁺ is observed. X-ray diffraction, photoluminescent emission, long-lasting phosphorescence emission and decay curves were used to characterize the phosphor. It is found that the afterglow is improved with the co-doping of Ti⁴⁺ and Mg²⁺. Also the upconversion luminescent spectrum of Er³⁺ is changed. We ascribe this phenomenon to the change in the environment of activator ions. The phosphorescence properties of the sample are researched in detail. Due to the dependence of afterglow intensities on different irradiative powers, a possible two-step-absorption upconversion long-lasting phosphorescence mechanism is proposed. This kind of mechanism may overcome the shortcomings of the long persistent phosphors which are used for UV irradiation.

Lead-free ceramics Bi_0.5(Na_{0.925−x−y}Li_0.075K_xAg_y)_0.5TiO₃ have been prepared by an ordinary sintering technique, and their ferroelectric and piezoelectric properties have been studied. From the results of x-ray diffraction, Li⁺, K⁺ and Ag⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) exists at 0.15 < x < 0.25. Compared with pure Bi_0.5Na_0.5TiO₃, the substitutions of K⁺ and Ag⁺ lower the coercive field E_c greatly and increase the remanent polarization P_r of the ceramics. Because of the MPB and low E_c, the piezoelectricity is significantly enhanced. For the ceramics with compositions near the MPB, the piezoelectric coefficient d₃₃ = 178–219 pC N⁻¹, the planar electromechanical coupling factors k_P = 35–39% and k_t = 44–51%. Also, the depolarization temperature T_d reaches a minimum value near the MPB. The results of the ferroelectric and dielectric properties at high temperatures suggest that the ceramics may contain both the polar and non-polar regions at temperatures above T_d.

We have grown BiFeO₃ (BFO) and 5% Ti doped BiFeO₃ (BFTO) thin films on LaNiO₃/Si structures by simple sol–gel deposition. X-ray diffraction analysis confirmed that BFO has an intense (1 1 0) peak and the BFTO film is randomly oriented and adopts a rhombohedrally distorted perovskite structure. No impure phase was identified in the two films. A cross section scanning image revealed that the BFMO film has homogeneous thickness. Surface scanning electron microscopy images indicated that the BFTO film has a more compact structure. The BFTO film showed larger remanent polarization (Pr) and a small coercive field. The Pr values are 16.0 µC cm⁻² and 8.0 µC cm⁻² and coercive fields are 189 kV cm⁻¹ and 416 kV cm⁻¹ for the BFTO and BFO films at the same applied electric field, respectively. Through the Ti substitution, the dielectric property is also enhanced and leakage conduction is reduced.

The transmission coefficient and effective permittivity of frequency selective composites with embedded long conductive fibres have been investigated experimentally and numerically at microwave frequencies. A cluster effect due to the overlapping of fibres is proposed to explain the dispersive microwave properties of composites with randomly distributed fibres. It is observed that the fibre clusters have multiple resonance frequencies that are related to the lengths of the cluster or those of the individual fibres. Interaction between overlapping fibres in a cluster cannot be eliminated by electrical isolation. Cluster effect can explain the appearance of broad resonance peak found in randomly distributed fibres, though the resonance frequency remains close to that of single fibre. This behaviour can be used in the design of composites with broadband frequency selective properties.

Fe modified Pb_0.92La_0.08(Zr_0.65Ti_0.35)O₃ (PLZT) was prepared by the conventional method based on the solid state reaction of mixed oxides. The x-ray analysis clearly shows the formation of a PLZT single rhombohedral phase. The crystallite size calculated from Sherrer's equation and the grain size determined from microstructures were found to decrease with Fe doping up to 4%. Calculations carried out for the peak intensity of the x-ray diffraction analysis suggest that Fe occupied the A site up to 4% doping and beyond that it occupied both A and B sites in the ABO₃ perovskite structure. Dielectric and pyroelectric measurements show a peak at around the Curie transition temperature. The width of the peaks was found to increase with Fe doping along with decreasing peak dielectric and pyroelectric maxima up to 4%. The P–E loop measurements of PLZT samples with Fe doping up to 6% have shown saturated single loops. P_r and E_C from the P–E loop were found to increase with increasing applied field and reached saturation at higher applied fields (25 kV cm⁻¹). However, 8% and 10% Fe doped PLZT have shown double unsaturated hysteresis loops. This anti-ferroelectric phase in Fe doped PLZT samples beyond 8% doping is expected to be due to the occupation of Fe in the B site replacing the Ti atom in the ABO₃ perovskite structure.

Electrical, magnetotransport, magnetic and thermoelectric power (TEP) studies carried out on the (1 − x)Pr_2/3Ba_1/3MnO₃ + xPdO composite system are reported here. The electrical resistivity of the pristine sample Pr_2/3Ba_1/3MnO₃ (PBMO) exhibits two insulator–metal (I–M) transitions (T_P1 and T_P2). The higher temperature transition (T_P1) becomes sharper and the lower temperature transition (T_P2) disappears with higher content of PdO (>10 mole%). The intrinsic magnetoresistance (MR) becomes enhanced whereas the extrinsic MR decreases monotonically. The observed MR behaviour of the composite system is attributed to the decrease in the electrical resistivity (due to PdO decomposition to metallic Pd), the opening of new conducting channels and the decrease of spin dependent scattering at the grain boundaries due to the disappearance of the barrier formed at the grain boundary from the canting of spins, defects, etc. Electrical resistivity above T_P1 follows the non-adiabatic small polaron hopping model whereas the ferromagnetic metallic region is governed by the electron–magnon scattering process. The Curie temperature (T_C) of the composite system remains unchanged; however, magnetization at low temperatures decreases with PdO addition. TEP exhibits an I–M transition near T_P1 in all the manganites. The TEP crossover observed at temperature T^* suggests that the dominating carriers in these compounds change from electrons to holes below T^*.

Aiming for polarization transfer enhancement of ¹⁴N nuclear quadrupole resonance (NQR) for the detection of explosives with low NQR frequencies, we examine the potential and limitations of this method. As illustrative sample materials two non-explosive compounds, urotropine (C₆H₁₂N₄) and urea (CON₂H₄) with NQR frequencies of 3.3 MHz and 2.8 MHz, respectively, have been chosen. In both substances the NQR signal can be easily seen. In urotropine no signal enhancement has been detected. The reason is a ¹⁴N spin-lattice relaxation time being much shorter than the ¹H–¹⁴N polarization transfer time. Although in urea the signal enhancement is significant there is, because of the long ¹H polarization time, still no effective gain as compared with the pure NQR signal accumulated during the same time interval. To estimate the expected NQR signal enhancement, a polarization enhancement factor has been derived in terms of a simplified theoretical treatment, neglecting spin-lattice relaxation. The substantial influence of relaxation effects on the signal enhancement has been discussed in a qualitative manner in connection with the experiments performed for urea and urotropine.

It has been observed that low-temperature (950 °C) fast-fired Pb(Zr_0.52Ti_0.48)O₃ (LTFF-PZT) shows strong low-frequency dispersion (LFD). Considering the presence of inhomogeneity (clusters) in LTFF-PZT, such LFD can be explained by the universal dielectric response (UDR) formalism. In the presence of moisture, LFD is exaggerated in conformity with the principle of UDR, where the long-range, cooperative interactions are enhanced due to screening of the surface field of the ferroelectric by polar water molecules. Preliminary study indicates that LTFF-PZT can be exploited to make novel pore-free moisture sensors.

Multiferroic particulate composites with composition xNi_0.93Co_0.02Mn_0.5Fe_1.95O_4−δ + (1 − x)PbZr_0.52Ti_0.48O₃ where the molar fraction x varies as 0, 0.1, 0.2, 0.3, 0.4 and 0.5 were prepared by the conventional ceramic method. The presence of two phases was confirmed by x-ray diffraction and scanning electron microscopy. The temperature variation of the longitudinal modulus (L) and the internal friction (Q⁻¹) of these particulate composites at 104.387 kHz was studied in the wide temperature range 30–420 °C. The temperature variation of the longitudinal modulus (L) in each composition of these particulate composites showed two abrupt minima. One minimum coincided with the ferroelectric–paraelectric Curie transition temperature (θ_E) and the other with the ferrimagnetic–paramagnetic Curie transition (θ_M) temperature. The internal friction (Q⁻¹) measurements also showed two sharp peaks in each composition corresponding to those temperatures where the minima were noticed in the temperature variation of the longitudinal modulus behaviour. The Curie transition temperature of pure ferrite was found to be 560 °C. Addition of 10% of ferrite to ferroelectric in a magnetoelectric (ME) composite resulted in a 360 °C fall in θ_M and with a further increase in ferrite content the θ_M variation was found to be very nominal. However, no significant ferroelectric Curie transition temperature shift could be noticed. This behaviour is explained in the light of structural phase transitions in these multiferroic particulate composites. These ME composites were prepared with a view to using them as ME sensors and transducers.

Sn_xSb₂₀Se_80−x chalcogenide alloys (with x = 8, 10, 12, 13.5, 15, 16.5 and 18 at.%) were prepared by the melt quenching technique. The average coordination number and the overall mean bonding energy were calculated. The x-ray diffraction studies revealed that the alloys with x = 8 and 10 at.% were amorphous and the alloy with x = 12 at.% was partially crystalline whereas the alloys with x = 13.5, 15, 16.5 and 18 at.% were crystalline. The glass transition temperature, the crystallization temperature, the melting temperature and the glass-forming tendency of the amorphous samples were determined from the differential scanning calorimetry measurements. The glass transition activation energies and the crystallization activation energies were determined using Kissinger as well as Augis and Bennett methods. Upon annealing the bulk amorphous samples at 573 K, two different crystalline phases, SnSe₂ and Sb₂Se₃, were observed. X-ray reflectometry analysis revealed that the surface roughness increased with increasing annealing temperature. The density of the as-prepared films increased with increasing Sn content. Upon annealing at 523 K the density increased, whereas the density of most of the films decreased upon annealing at 573 K. Thermal gravimetric analysis showed a steeper weight loss upon heating after 533 K.

Dispersion characteristics of the two-dimensional sonic crystals consisting of elliptic rods are investigated. Due to the geometric anisotropy of the elliptic rods, we can obtain different structure factors for varying the equifrequency surfaces, (EFSs) and the refractive direction can be tuned. The plane wave expansion method is used to calculate the EFSs for the sonic crystals with a triangular lattice. The pressure distribution in the sonic crystal is also obtained by the finite element method. The refraction angles are found to be dependent on the frequency of the incident wave and the orientation angle of the elliptic rods. By employing the dispersion characteristics of the elliptic rod sonic crystal, we can design and develop new acoustic devices, such as acoustic switches and splitters.

The temperature dependence of elastic constants and thermodynamic properties of light rare earth (RE) Mg–Pr, heavy RE Mg–Dy and Mg-Y intermetallics are studied using the modified analytic embedded atom method. The present calculations are generally in agreement with the experimental data and other theoretical results. The bulk moduli of Mg–Pr alloys are smaller than that of Mg–Dy and Mg–Y alloys, which explains why the mechanical properties of Mg-based alloys can be further improved by the addition of heavy RE metals experimentally. Furthermore, heat capacity, vibrational free energy and vibrational entropy of Mg–Pr, Mg–Dy and Mg–Y alloys are calculated in the harmonic approximation, using the vibrational density of states (VDOS) obtained from Born–von Karman theory. The partial VDOS indicates that the VDOS originates from RE atoms at low frequency and from Mg atoms at high frequency and the contribution of RE atoms is dominant in frequency since they are heavier than Mg atoms.

Recently, there has been a great deal of interest in extending the lattice Boltzmann method (LBM) to model transport phenomena in the non-continuum regime. Most of these studies have focused on single-component flows through simple geometries. This work examines an ad hoc extension of a recently developed LBM model for multi-component mass diffusion (Joshi et al2007 J. Phys. D: Appl. Phys.40 2961) to model mass diffusion in the non-continuum regime. In order to validate the method, LBM results for ternary diffusion in a two-dimensional channel are compared with predictions of the dusty gas model (DGM) over a range of Knudsen numbers. A calibration factor based on the DGM is used in the LBM to correlate Knudsen diffusivity to pore size. Results indicate that the LBM can be a useful tool for predicting non-continuum mass diffusion (Kn > 0.001), but additional research is needed to extend the range of applicability of the algorithm for a larger parameter space. Guidelines are given on using the methodology described in this work to model non-continuum mass transport in more complex geometries where the DGM is not easily applicable. In addition, the non-continuum LBM methodology can be extended to three-dimensions. An envisioned application of this technique is to model non-continuum mass transport in porous solid oxide fuel cell electrodes.

This paper presents a concise, although admittedly non-exhaustive, didactic review of some of the main concepts and approaches related to the use of molecularly modified metal nanoparticles in or as chemical sensors. This paper attempts to pull together different views and terminologies used in sensors based on molecularly modified metal nanoparticles, including those established upon electrochemical, optical, surface Plasmon resonance, piezoelectric and electrical transduction approaches. Finally, this paper discusses briefly the main advantages and disadvantages of each of the presented class of sensors.

Immunosensors based on surface plasmon resonance (SPR) have become a promising tool in sensor technology for biomedical, food, environmental, industrial and homeland security applications. SPR is a surface sensitive optical technique, suitable for real-time and label-free analysis of biorecognition events at functional transducer surfaces. Fabrication of highly active and robust sensing surfaces is an important part in immunoassays because the quality, quantity, chemistry and topography of the interfacial biomembranes play a major role in immunosensor performance. Eventually, a variety of immobilization methods such as physical adsorption, covalent coupling, Langmuir–Blodgett film, polymer thin film, self-assembly, sol–gel, etc, have been introduced over the years for the immobilization of biomolecules (antibody or antigen) on the transducer surfaces. The selection of an immobilization method for an immunoassay is governed by several factors such as nature and stability of the biomolecules, target analyte, application, detection principle, mode of signal transduction, matrix complexity, etc. This paper provides an overview of the various surface modification methods for SPR based immunosensor fabrication. The preparation, structure and application of different functional interfacial surfaces have been discussed along with a brief introduction to the SPR technology, biomolecules and detection principles.

Porous silicon samples were obtained by the electrochemical method and were impregnated with Pd by the electroless process. X-ray photoelectron spectroscopy illustrated that the surface of the samples is oxidized during the palladium deposition. Scanning electron microscopy showed how factors such as morphology and pre-oxidations of porous samples and the plating parameters including Pd-salt concentration, HCl concentration, temperature of the electroless solution and the time of process affect the growth and nucleation of Pd particles. Observations demonstrated that the illumination-assisted process on p-type samples has a drastic effect on the growth of palladium. Hydrogen sensing of these samples, which works on the basis of change in the Schottky barrier between the silicon and the palladium interface, was tested. Variations of the electrical resistance in the presence of diluted hydrogen at room temperature revealed that the best samples can sense hydrogen at levels down to several thousand ppm. This value is far below the flammability limit of hydrogen gas. In addition, the best samples have a very good selectivity to hydrogen.

A new class of low temperature proton-conducting-type hydrogen gas sensor was developed using Dion–Jacobson type layered perovskite oxides. A laminated structure with a junction of charge carriers at the interface between a predominantly ionically and predominately electronically conducting material was prepared by using the multistep-impregnation-reduction method for the deposition of Pt on top of a perovskite oxide. The proton conductivity of the layered perovskite materials was studied between room temperature and 250 °C. The sensing characteristic was studied by using H₂ concentrations between 1% and 7%. The optimum operating temperature of the sensor was found to be at 45 °C. The formation of the galvanic cell voltage is described in terms of reactions at the interfaces and the surface of the electrodes. The experimental results indicate the motion of electrons within the Pt and of protons within the perovskite oxide along the interface. Hence, modelling the system response upon a change in gas concentrations can be beneficial for understanding the individual processes and optimizing the overall performance.

The paper provides a critical assessment of the progress made in the field of gas sensing metal oxide semiconductors (MOX) based on the Hall effect measurement as the main experimental data source. At the beginning, the focus is on the morphologic and the functional peculiarities of this type of material that differentiate them among the larger class of polycrystalline, granular and porous semiconductors. On this basis the complications in defining and extracting the electrical parameters of MOX, especially under operating conditions, are discussed. Different experimental solutions addressing the Hall effect measurements on gas sensing MOX reported in the literature are then briefly presented as the foundation of the discussions about the most utilized modelling procedures. Two main trends are identified: the solid-state and the chemo-physical ones. The solid-state methods focus on the crystallite energy structure, introducing the external influences as model parameters describing the interface trap distribution. The chemo-physical models firstly give a physical picture of the chemical interaction of the ambient with the solid-state surface by connecting the target gases partial pressures with the surface trap concentration and energy and, afterwards, follow the formalism used by the solid-state models. For each type of approach, representative examples and applicative illustrations are considered. An extended overview of the accessible data is included at the end of the paper in a tabulated form.

In this paper we discuss the challenges and opportunities afforded by surface-based photoswitchable chemical sensors. We focus on spiropyrans as it is a well-studied system that can be photonically switched between two states, only one of which exhibits ion-binding behaviour. Surface immobilization and protection within a polymer matrix is identified as a route that can successfully address the need for a localized hydrophobic environment in which a user can maintain control over the spiropyran-merocyanine equilibrium and at the same time improve photo-fatigue resistance. Furthermore, we discuss the excellent potential of light emitting diodes as light sources and detectors for photoswitching between the states of spiropyran and measurement of bound species. A simple, low-cost, low-power experimental setup provides spatial and temporal control of surface illumination and surface binding. This, coupled with low irradiance, is shown to generate significant improvement in fatigue resistance of spiropyrans-modified films, and may prove to be an important step towards the realization of chemical sensors that can be deployed in large-scale wireless chemical sensor networks.

This paper describes a high-speed adsorption sensor based on thin-film interference at the interfaces. The sensor can be used as a stand-alone instrument or in combination with a direct surface force measurement, which yields a wide range of additional information on molecular interactions on adsorbed films. The achieved mass resolution of the presented method (1–10 ng cm⁻² Hz^−1/2) is comparable to or better than other modern bio-sensors. The dependence of mass resolution on various factors is presented and demonstrated in a number of relevant examples. The described method is suitable for the implementation of a low-cost bio-sensor with a minimal number of optical elements.

The measurement spot size is one micrometre or more and sampling rates >10 Hz are readily possible. In contrast to other bio-sensors, the signal baseline has a remarkable long-term stability since the measured signal is virtually independent of refractive index changes in the fluid medium above the sensor surface.

In combination with an optical spectral correlation method, the classical computer calculations are substituted by an optical calculator and a label-free real-time imaging adsorption sensor is realized. We demonstrate sensor operation both inside the extended surface forces apparatus as well as in a stand-alone bio-sensor configuration. As a final point, we illustrate the imaging capability of this new sensor technology on a patterned bio-functionalized surface.

ZnO nanowires were deposited using the vapour phase technique. The nanowires cover the substrate and appear uniform in morphology with lengths of up to several micrometres and uniform lateral size of the order of tenths of a nanometre. The electrical and optical properties of ZnO nanowires have been characterized in the presence of nitrogen oxide. Electrical measurements highlight a remarkable response even at low operating temperatures, with detection limits lower than 200 ppb and an optimal operating temperature of 100°C, while the ZnO nanowire photoluminescence is reversibly quenched by the introduction of a few ppm of NO₂ even at room temperature.

Sensors are specific analog devices that convert a physical quantity, like the temperature or external pressure or concentration of carbon monoxide in a confined atmosphere, into an electrical signal. Considered in this way, every sensor is then a part of the artificial interface, which connects the human world to the world of machines. The other side of the interface is represented by actuators. Most often, after processing the data they are used to convert the out-coming electrical power into counteracting physical action. In the last few years, thanks to inexpensive silicon technology, enormous capability for data processing has been developed and the world of machines has become increasingly invasive. The world of sensors has become increasingly complex too. Applications range from classical measurements of the temperature, vibrations, shocks and acceleration to more recent chemical and bio-sensing technologies. Chemical sensors are used to detect the presence of specific, generally toxic, chemical species. To measure their concentration, one uses some specific property, generally a physical one, like the intensity of infrared absorption bands. Bio-sensors are new, more complex, devices that combine a bio-receptor with a physical transducer. The bio-receptor is a molecule (for instance, an enzyme like glucose oxidase) that can recognize a specific target (glucose molecules in the case of glucose oxidase). The enzyme must be fixed on the transducer and, as a consequence of recognition, the transducer must convert the event into a measurable analytical signal. A common feature of many chemical and bio-sensors is that they require a large surface of interaction with the outside world. For that reason and in order to increase efficiency, either nanoparticles or pores or a combination of both, made from various materials including (but not limited to) porous silicon, are often used as the functional transducer interface.

The reviews in this Cluster Issue of Journal of Physics D: Applied Physics describe some recent advances in this field and the very different approaches and/or techniques that can be used for the sensors' implementation. They include the use of molecularly modified metal nanoparticles in or as chemical sensors, especially for high sensitivity hydrogen sensors. Hydrogen sensing can also be achieved by performing galvanic measurements on a thin layer of perovskite oxide covered with platinum. In this case, one mixes an ionic (proton) transport in the oxide with an electronic one in the metal. Another focus is on optical and electrical read-out techniques, like surface–plasmon resonance (SPR), such as for immuno-sensor applications or piezo-electrical and electro-chemical detection. Toward this end, the preparation, structure and application of functional interfacial surfaces are described and discussed. A totally different approach based on the use of Hall effect measurements performed on a granular metal-oxide-semiconductor layer and different experimental solutions is also presented. Finally, optical sensors are addressed through the photonic modulation of surface properties or transmission interferometric absorption sensors. Mixed electrical and optical chemical sensors are also examined.