Table of contents

Since its invention in 1981, scanning tunnelling microscopy (STM) is well-known for its supreme imaging resolution enabling one to observe atomic-scale structures, which has led to the flourishing of nanoscience. As successful as it is, there still remain phenomena which are observed using STM but are beyond our understanding. Graphite is one of the surfaces which have been most extensively studied using STM. However, there are a number of unusual properties of graphite surfaces. First reported in the 1980s, superlattices on graphite have since been observed many times and by many groups, but as yet our understanding of this phenomenon is quite limited. Most of the observed superlattice phenomena are widely believed to be the result of a Moiré rotation pattern, arising from the misorientation between two graphite layers, as verified experimentally. A Moiré pattern is a lattice with larger periodicity resulting from the overlap of two lattices with smaller periodicities. As graphite layers are composed of hexagonal lattices with a periodicity of 0.246 nm, as observed using STM, when there are misoriented graphite layers overlapping each other, a Moiré pattern with larger periodicity, depending on the misorientation angle, will be produced and appear as a superperiodic hexagonal structure on top of the graphite atomic lattice of the topmost surface layer. It is important to study graphite superlattices because, firstly, knowledge of this phenomenon will enable us to properly interpret STM images; secondly, it helps us to understand the correlation between electronic structures and atomic-structure rearrangement of graphite which is of tremendous aid for engineering material properties; thirdly, and perhaps most importantly, the observation of the phenomenon exhibits the capability of STM to produce images indicating the nature of internal defects which are below the surface. Over recent years, experimental and modelling techniques have been developed to study this anomalous regime of STM; however, there is a lack of a systematic classification of this scattered information. This review article thus serves the purpose of organizing all these results so as to enable a more comprehensive understanding of this phenomenon. We review the discovery of graphite superlattices, the observation of the associated properties, and the research efforts on this subject. An effort is made to envision the future experimental and theoretical research possibilities to unveil the mystery of this anomaly of STM. Applications of graphite superlattices are also proposed.

In this paper we introduce a new method for producing nanoscale spin valve arrays. The deposition of NiFe/Cu/NiFe/MnNi spin valve structures was carried out in self-organized nanoporous anodized aluminium templates by using an electrodeposition technique involving three baths of different solutions. Field emission scanning electron microscopy observation reveals that the nanometric spin valve arrays, of uniform size, are well separated and exhibit a very perfect two-dimensional array with a hexagonal pattern. Post-thermal treatment yields highly ⟨111⟩ oriented spin valve films with a giant magnetoresistance ratio of above 4%. The pinning field is about 700 Oe for the nanoscale spin valves film and it stays constant up to 200 °C. These nanoscale spin valve arrays are highly thermally stable and, thus, suitable for the application of high density recording heads.

We describe a Monte Carlo algorithm that we have developed to simulate the electron beam induced current (EBIC) contrast of a surface perpendicular dislocation. The contrast was obtained by simulating the random diffusion and collection of the carriers that are generated at point-like sources S_i randomly distributed within the generation volume. The dislocation is described as a cylinder with a radius r_D where the minority carrier lifetime (τ_D) is lower than that in the bulk (τ_B).

The dependence on the electron range R_e of the simulated EBIC contrast profiles, their full width at half maximum (FWHM) and their maximum (C_max) is analysed for a germanium sample. It is shown that with increasing R_e, the FWHM of the contrast profile increases steadily in agreement with experiment while the maximum contrast goes through a maximum and converges to zero for R_e → 0. The variation of C_max upon the diffusion length L_D within the dislocation cylinder is analysed too. The results of our simulations show that the values of the contrast obtained by the analytical approach using the first order approximation are underestimated for dislocations of strength exceeding 5.

We have calculated the photonic band structures of two-dimensional periodic arrays of well aligned ZnO nanorods with a frequency dependent dielectric constant. The ZnO nanorod photonic crystal can exhibit a photonic band gap in the visible range. The dependence of the band gap on the period and radii of the nanorods was shown. From the finite difference time domain simulations, we also showed that the luminescence of a ZnO nanorod could be altered by the photonic band gap effect. Controlling the luminescence of ZnO nanorods can be useful in the implementation of efficient light emitting devices.

The thermoluminescence (TL) and optically stimulated luminescence (OSL) characterization of Er³⁺ and Yb³⁺ doped Y₃Al₅O₁₂ nanocrystalline samples prepared by the precipitation process and exposed to β-rays are discussed. The TL as well as the OSL were two orders of magnitude higher in Er³⁺ doped than in Yb³⁺ specimens. The charge trapping and the radiative thermally stimulated recombination processes in Y₃Al₅O₁₂ : Er³⁺ involve four trapping states at 166, 243, 342 and 424 °C, but just two trapping levels at 219 and 413 °C for Y₃Al₅O₁₂ : Yb³⁺ at a heating rate of 10 °C s⁻¹. The photostimulation with 470 nm light causes in both phosphors a radiative recombination of the optically free charge carriers belonging to the same trapping states. The TL and the OSL as a function of radiation dose behaviour were linear in the 10–100 Gy dose range. The results provide evidence of the potential uses of these materials in radiation storage and dosimeter devices.

In the current study, the influence of heating rates between 1 and 50 °C s⁻¹ on the response of TL glow peaks of CaF₂ : Dy (TLD-200) has been investigated using a computer glow curve deconvolution (CGCD) method before and after application of the thermal cleaning method at 90 and 170 °C. The trapping parameters of the main glow peaks were also obtained using the CGCD method as a function of heating rate. The results showed that the peak temperatures of all glow peaks increase with increasing heating rates. The peak height method showed that the peak value (I_m) in the glow curve below 230 °C is approximately constant for low heating rates up to 10 °C s⁻¹, but slightly increased after that by about 10% with increasing heating rate. The integrated total area of low temperature peaks below 230 °C is more stable. It remains constant for heating rates less than 25 °C s⁻¹, then it slowly increases by about 10% with increasing heating rate up to 50 °C s⁻¹. The increase in the total area of low temperature peaks is reduced after the application of the thermal cleaning method. The peak intensities and total area of high temperature peaks above 260 °C decrease continuously with increasing heating rate in the investigated region of heating rates.

In this work we investigated the electron emission properties of high-pressure discharge lamp cathode tips. The work function (Φ) of the cathode tip was measured by using the Kelvin probe method and by work function spectroscopy (WFS). The Kelvin probe method was used to measure the average work function of tips under atmospheric pressure in air. By WFS we could measure the local work function value of tips in the selected spots under ultra high vacuum conditions. The chemical composition analysis was carried out in the same chamber by Auger electron spectroscopy.

The focus of this study is to investigate the influence of sintering temperature of cathodes (1500–1700 °C) and lamp operation time (0–12 000 h) on the work function. The comparison of the work function of both cathodes as a function of operation time originating from the two different ends of the ceramic tube is also considered. In order to understand the structure of the layers on the cathode tips we also give results obtained on a flat tungsten foil covered with Ba-containing emission material. The flat samples were measured using x-ray photoelectron spectroscopy and WFS.

A preliminary investigation of tomographic reconstruction of arc plasma in three dimensions has been carried out. The main goal of this work was to define both the optimal experimental scheme for tomographic measurements and the most appropriate tomographic method with minimum constraints to obtain images of good quality in real situations. Numerical calculations were developed and performed to define a test case corresponding to an experimental device. The multiplicative algebraic reconstruction technique (MART) was applied for reconstruction of the emission profile from the acquired projections. Numerical reconstruction from two, three, four and seven projections are presented and discussed in a theoretical three-dimensional (3D) transferred arc configuration. The dependence of the reconstructed image quality on both the projection directions and the noise level was studied. Numerical simulation demonstrated that MART was perfectly suitable for reconstructing satisfactory 3D emission and temperature profiles of the arc plasma with a four-view configuration, proving the feasibility and the utility of tomography to characterize a 3D plasma medium.

The emission spectra of N₂ hollow cathode and pulsed discharges were investigated and computational simulations have been made for the determination of the rotational and vibrational temperatures. A calibration curve was obtained by rotational simulation and was applied to study the space-resolved temperature in the pulsed discharge plasma. The rotational temperature was found to decrease rapidly along the radial axis of the plasma. The vibrational temperature in the pulsed discharge plasma was estimated by spectral simulation and was found to be more than three times higher than the rotational temperature.

Streamer-to-spark transition in short non-uniform air gaps is simulated. Both kinetic and gas dynamic effects are accounted for. The dependence of the streamer-to-spark transition time τ_br on the applied voltage is obtained. Calculated values of τ_br agree with the available experimental data.

The ratios of intensities of the spectral bands of molecular nitrogen corresponding to transitions ), N₂(C ³Π_u, v = 0) → N₂(B ³Π_g, v = 0) and N₂(C ³Π_u, v = 2) → N₂(B ³Π_g, v = 5) as a function of the applied electric field strength were measured for air in the pressure range of 300 to 10⁵ Pa. The non-self-sustaining dc discharge in a parallel-plane gap was used for excitation of gas molecules. The reduced field strength was varied in the range of (150–5000) × 10⁻²¹ V m². The measured ratio of intensities as a function of electric field strength is compared with the theoretical estimates made by other authors. The obtained intensity ratio versus field strength curves can be used for field strength estimation in plasmas if the nitrogen molecules are excited dominantly from the ground state directly by the electron impact.

Current pulses induced by pulsed laser radiation at a metal target were studied at radiation intensities below thresholds for laser ablation. Measurements were made in air in weak dc fields where ionization by electron impact does not occur. Current pulses as a function of radiation intensity were recorded. Experimental conditions allowed a distinction between charged particles injected into the gap from the target electrode and those created in the gas medium. With the intensity growth, the increase in the number of target-injected charge carriers stops and gas-created particles make a large contribution to the total current. Modelled current pulses correspond to those recorded in the experiment. Furthermore, the modelling verifies that the space charge of negative ions controls the current of target-injected charge carriers.

A model that allows the quantitative analysis of heterogeneous strain fields in epitaxial thin films using x-ray diffraction (XRD) is presented. Particular emphasis is laid on the modelling of the two-component XRD profiles (i.e. profiles made of the superposition of a narrow coherent Bragg peak and a broad diffuse scattering profile) encountered in the XRD investigation of epitaxial thin films containing localized strain fields. The spatial properties of the strain field are included in a correlation function based on phenomenological parameters such as the defect correlation length ξ and the level of disorder σ_∞. No assumption regarding the nature of the defect is hence required. The statistical properties of the strain field are described by means of Lévy-stable distributions which allow us to account for profile shapes ranging between the Gaussian and profiles exhibiting pronounced power law-type tails, as well as for asymmetrical profiles. The effects of finite size of the domains (crystallites) over which diffraction is coherent are rigorously taken into account by calculating the auto-correlation function of the crystallite shape including the size distribution effects. The effects of each parameter are presented and discussed in detail and the applicability of the model is illustrated with two examples.

A theoretical model is suggested that describes emission of partial Shockley dislocations from triple junctions of grain boundaries (GBs) in deformed nanocrystalline materials. In the framework of the model, triple junctions accumulate dislocations due to GB sliding along adjacent GBs. The dislocation accumulation at triple junctions causes partial Shockley dislocations to be emitted from the dislocated triple junctions and thus accommodates GB sliding. Ranges of parameters (applied stress, grain size, etc) are calculated in which the emission events are energetically favourable in nanocrystalline Al, Cu and Ni. The model accounts for the corresponding experimental data reported in the literature.

We present an analytical method for diagnosing the rapid thermal annealing (RTA) condition of low energy ion-implanted silicon by reflective second harmonic generation (RSHG). The phenomena of recrystallization and implant diffusion of arsenic and phosphorous atoms implanted Si are easily observed by comparing a series of RSHG pattern. The implant atoms enter the Si site when the activation energy is sufficient to reach during the RTA process, and the residual electrical dipoles are presented accompanying the formation of Si–As and Si–P dipoles which are estimated by the bond orbital approach. The RSHG intensity is strongly dependent on the recrystallization degree of implanted silicon and the implantation density near the surface region which is a function of RTA temperature. With the assistance of the secondary ion mass spectrometry analysis, the actual distribution of the implantation density could be obtained and considered in the final analysis model. The anisotropic term of polarized RSHG patterns is an index to find the optimum RTA conditions; they indicate the recrystallization condition of the implanted silicon and the distribution of the implantation density near the surface.

We report here a low-temperature approach which is simple and catalyst free to synthesize aligned ZnO nanorods on flat substrates, i.e. decomposing zinc acetate (C₄H₆O₄Zn) in air at ∼250 °C. The ZnO nanorods prepared by this method exhibited strong photoluminescence at ∼377 nm, ∼387 nm and ∼401 nm. Thermal annealing experiments suggested that the emission at ∼401 nm might be due to the defects in the ZnO nanorods which can be removed by low-temperature annealing, and that the near band-edge emissions at ∼377 and ∼387 nm might be caused by the difference in the diameter of the nanorods.

The surface evolution of an elastic conducting material subject to an infinitesimal surface perturbation and uniform loading in an electric field was evaluated with respect to lattice diffusion. A dispersion relation describing morphological evolution of the elastic material as a function of the electric field intensity was derived, and the time evolution of the surface perturbation was obtained. The critical spatial frequency of the infinitesimal surface perturbation, at which the growth rate of the perturbation is zero, increases with the increase of the electric field intensity and is independent of Young's modulus of the elastic halfspace. An electrical field enhances the surface growth of elastic conducting solids for atomic migration controlled by lattice diffusion, while tensile stress tends to smooth surface perturbations.

Compaction characteristics of granular materials subjected to axial loading are investigated for both sphere and non-sphere granular assemblies. The computational study is based on the discrete element method (DEM). The compressive stress–strain relation obtained from three-dimensional DEM simulations is compared with that of an idealized two-dimensional plane-strain compression test and physical experiments using a bronze sphere assembly. We observed good agreement between the experimental and three-dimensional DEM simulation results, while two-dimensional simulations significantly underestimate the stiffness of particulate bed, particularly at large strains. This demonstrates that two-dimensional analysis is generally inadequate to model the compaction characteristics of granular systems. We performed a detailed analysis on the force–transmission characteristics of granular materials at microscopic level and present a connection between the directional orientation of force-networks and the invariants of the macroscopic stress tensor: the non-sphere systems were able to build up a strongly anisotropic network of heavily loaded contacts. Several complex phenomena, both geometric and kinematic, that are operative in sphere and non-sphere assemblies due to inter-particle interactions during compression are presented here. It is often assumed that the ratio of invariants of the stress tensor is uniform and constant in uni-axial compression tests. Our results show that the ratio of invariants of the stress tensor is non-uniform and non-constant even when the granular assemblies are subjected to the so-called uni-axial compressive loading, which is in agreement with other recent studies (e.g. Gu et al 2001 Int. J. Plasticity17 147) performed using the finite element method. The non-homogeneous characteristics that are reported at the particulate scale need to be accounted in considering possible continuum models for the granular systems.

The compensation effect (also known as the Meyer–Neldel (MN) rule) is observed in many activated phenomena, including solid-state diffusion in crystals and polymers, dielectric relaxation, conduction and thermally stimulated processes in polymers and electronic conduction in amorphous semiconductors. In the present paper, we have investigated the compensation effect for isothermal crystallization in glassy Se_80−xTe₂₀M_x (M = Cd, Ge, Sb) alloys. The extent of crystallization was estimated from the dc conductivity measurements for the present chalcogenide glasses. We have observed the MN rule relating the pre-exponential factor K₀ and the activation energy of crystallization E_c for all three glassy systems.

A theoretical model which includes considerations of the effects of an interfacial nanolayer formed by liquid molecule layering on the particle/liquid interface and of micro-convection caused by thermal motion of nanoparticles has been proposed to calculate the effective thermal conductivity of nanofluids. This model accounts for the enhancement in effective thermal conductivity of a nanofluid with respect to the suspended nanoparticle size, volume fraction, temperature and thermal conductivities of the nanoparticle and base fluid. The predicted results are in good agreement with some recently available experimental data.

Carbon nanotubes have generated considerable excitement in the scientific and engineering communities because of their exceptional mechanical and physical properties observed at the nanoscale. Carbon nanotubes possess exceptionally high stiffness and strength combined with high electrical and thermal conductivities. These novel material properties have stimulated considerable research in the development of nanotube-reinforced composites (Thostenson et al 2001 Compos. Sci. Technol.61 1899, Thostenson et al 2005 Compos. Sci. Technol.65 491). In this research, novel reaction bonded silicon carbide nanocomposites were fabricated using melt infiltration of silicon. A series of multi-walled carbon nanotube-reinforced ceramic matrix composites (NT-CMCs) were fabricated and the structure and properties were characterized. Here we show that carbon nanotubes are present in the as-fabricated NT-CMCs after reaction bonding at temperatures above 1400 °C. Characterization results reveal that a very small volume content of carbon nanotubes, as low as 0.3 volume %, results in a 75% reduction in electrical resistivity of the ceramic composites. A 96% decrease in electrical resistivity was observed for the ceramics with the highest nanotube volume fraction of 2.1%.

In this work, La_0.4Ni_0.4Co_3.6Sb₁₂ filled skutterudite was prepared successfully by mechanical alloying (MA) and subsequent hot pressing. With increasing hot press (HP) temperature, the content of the filled skutterudite phase increases and its composition tends to the nominal composition; a single phase filled skutterudite compound can be obtained when the HP temperature is higher than 550 °C. The Seebeck coefficient and electrical resistivity of the hot pressed sample increased while its thermal conductivity decreased and the resultant figure of merit increased with the increase in the HP temperature. A fractograph observation shows that there are two nanocrystalline microstructures in the hot pressed filled skutterudite sample—one is the prime skutterudite phase formed by MA with a relatively large size, about 300 nm, and the other is the skutterudite produced during the HP process which has an average grain size about 80 nm.

A wide range glass system based on the TeO₂–WO₃–ZnF₂ was prepared. A number of physical properties, namely, the glass transition temperature, optical energy gap, Urbach energy, optical basicity, ultrasonic velocity (longitudinal and shear), elastic moduli, Poisson's ratio and Debye temperature were investigated by using several techniques such as: differential thermal analysis, dilatometric, UV–NIR–IR spectra and pulse-echo. These glasses have a high transmission in the visible, the near infrared and infrared regions, very low OH-vibration absorption bands and high thermal stability beyond 169 °C. The results indicated that most properties are observed to be dependent on the ZnF₂ content. These physical properties are reported with a view to drawing fluorotellurite optical fibres.

A novel type of liquid crystal device is described, based on a four-electrode unit, arranged in a hexagonal array. Full three-dimensional simulations were performed using a finite elements algorithm demonstrating a 2π rotation of the director in the plane parallel to the substrate surface. Applications for the device are situated in the field of multistable wave plates, spatial light modulators and electrically controllable anchoring.

The thermoelectric method for non-destructive evaluation of inhomogeneous samples is analysed using the finite element technique. As a case study, the magnetic field generated by thermoelectric currents induced in niobium plates with tantalum inclusions is considered. It is shown that this technique can be effectively used to evaluate niobium plates used for superconducting radio frequency resonators. It is further shown that this technique could also be used for non-contact weld inspection and is superior to weld characterization by direct measurement of the thermopower using a reference electrode in contact.

A curious property of the electrostatic image theory for the perfectly conducting prolate spheroid, introduced in 2001, is pointed out: the potential outside the conducting spheroid remains unchanged if the conductor is replaced by a dielectric spheroid with an embedded charge which coincides with the image charge for the conducting spheroid case. The present note is a generalization of the corresponding observation about the classical Kelvin image theory for the conducting sphere, made in 1988.

Highly sensitive K₂Ca₂(SO₄)₃ : Eu thermoluminescence detector phosphor was irradiated at room temperature by ⁷Li ion beams at 24 and 48 MeV for different ion fluences in the range 10⁹–10¹² ions/cm² using a 16 MV Tandem Van de Graff Type Electrostatic Pelletron Accelerator at the Nuclear Science Centre, New Delhi, India. The samples from the same batch were also irradiated with γ-rays from a Cs¹³⁷ source for comparative studies. Glow curves of the ion beam irradiated samples mainly consist of two prominent peaks at around 392 and 411 K while the γ-rays irradiated samples show only one peak at around 411 K. The appearance of the new peak (392 K peak) may be attributed to the defects/trapping centres due to ⁷Li ions that have been implanted deep inside during irradiation and act as a source of emission of thermoluminescence (TL). This was confirmed from the glow curve structure of Eu, Li ion co-doped samples.