Table of contents

Germanium (Ge) nanostructures embedded in Al₂O₃ dielectric are synthesized by the vacuum electron-beam co-evaporation method. A clear blue-shift of the absorption edge and a large third-order nonlinear optical susceptibility are observed due to the quantum confinement effects. Current–voltage and capacitance–voltage characteristics of metal–insulator–semiconductor structures containing Ge nanocrystals are studied in the dark and under illumination. Negative photoconductivity due to the screening effects of the negatively charged Ge nanocrystals is experimentally demonstrated.

This paper gives a brief review of our recent work on a new method of nanoscale pattern formation of thin oxide film on Si substrate by using a scanning tunnelling microscope (STM): a field-emitted electron beam (e-beam) extracted from the STM tip is used for selective removal of the oxide film by e-beam-induced reduction and thermal annealing at moderately high temperatures (300–700 °C). The process is dependent on electron dose and the patterning is controllable by adjusting the emission current and exposure time. One can draw nanoscale open-window patterns directly on oxide-covered Si substrates, e.g. lines and concentric circles of a few tens of nanometres in line width and spacing. Such patterning on the Si oxide layer shows good reproducibility and flexibility of the nanofabrication method, which suggests a further development and application of this method in nanotechnology. The beam profile of the extracted e-beam is measured and the beam-energy dependence of the quantum yield of the process is derived. Based on the excitation function, we consider that the decomposition is activated by core-level excitations as is the Knotek–Feibelman mechanism. One can also make use of this technique to diagnose the Si/SiO₂ interface topography on a sub-nanometre scale.

The development of simulation techniques that can elucidate the function of biomolecular nanodevices is still in its infancy. In this paper we summarize our approach to the investigation of structural properties of biomolecular systems with stochastic optimization methods. We briefly review the stochastic tunnelling method and summarize applications in two important areas of biomolecular structure prediction: protein folding and protein–ligand association.

We have produced nanostructured titanium dioxide thin films by supersonic cluster beam deposition. The as-deposited films have a nanocrystalline or amorphous structure depending on the mass distribution of the precursor clusters. This can be controlled by aerodynamic separation effects typical of supersonic expansions. On thermal annealing at temperatures from 400 to 800 °C in ambient atmosphere, amorphous-to-anatase and anatase-to-rutile phase transitions have been observed. The nanostructure and microstructure evolution of the film upon annealing has been characterized by atomic force microscopy and transmission electron microscopy. The influence of the precursor clusters in the evolution of the film nanostructure at high temperatures has been demonstrated. This observation opens up new perspectives for batch fabrication of devices based on cluster-assembled materials.

Metal oxide semiconductor structures with silicon nanocrystals (Si-nc), embedded within SiO₂ were investigated. These structures were fabricated by low temperature chemical vapour deposition of silicon on a thin tunnel oxide (3 nm thick) followed by oxidation and annealing steps. The high temperature annealing is critical since it determines the structural and consequently the charging properties of the samples. Two annealing temperatures were used, 900 and 1100 °C. Samples annealed at 900 °C showed strong charging phenomena due to electron and hole trapping. The density of these trapping sites was estimated to be around 2 × 10¹² cm⁻². Hydrogen annealing showed that the majority of these trapping sites was due to unsaturated bonds, located in the Si-nc plane, probably at the interface of Si-nc with SiO₂. In contrast, samples annealed at 1100 °C showed a symmetrical 'memory window' of 0.6 V due to electron and hole trapping, which was not significantly affected by hydrogen annealing. The density of the trapping sites was in this case estimated to be 2.5 × 10¹¹ cm⁻².

A method is presented for calculating the bulk optical properties of aligned carbon nanotubes. The method is based on an improved equivalent dielectric function of carbon nanotubes and an analytical series expression for the effective dielectric function, ε_eff, of periodic composites. The optical properties of aligned carbon nanotubes are studied using the effective-medium approximation (EMA), the Maxwell–Garnett theory (MGT) and the method presented. It shows that the calculation results using the method presented fit experimental data excellently and are better than those of EMA and MGT.

Arrays of nanoscale depressions were observed during sputtering of Si/SiGe heterostructures using 500 eV O₂⁺ ions at angles of incidence between 46° and 56° with simultaneous sample rotation in a secondary ion mass spectrometry system. Both the atomic force microscope and transmission electron microscope results suggest that these depressions were produced as a result of preferential sputtering of Ge and only occur when there are undulations present at the Si/SiGe interface. Formation of the interface undulation appears to occur in tandem with the increase in the Ge surface segregation at the Si/SiGe interface for layers grown at elevated temperatures.

CaF₂/Si/CaF₂ double-barrier resonant-tunnelling diodes have been fabricated by various processes with molecular beam epitaxy in SiO₂ windows patterned on Si(111) substrates. Two types of I–V characteristics with negative differential resistance (NDR) were observed in the temperature range from 75 K up to about 200 K. The one is very stable in forward and backward voltage sweeps, and, in contrast to earlier reports, it does not show any hysteresis or trapping effects. Typical diode parameters were: peak voltage V_p = 0.7 V, peak-to-valley current ratio P/V = 1.7 and peak current density I_p = 2 mA cm⁻² at 105 K. The second is less stable and the NDR appeared only during forward bias sweeps, possibly due to local trapping effects. But it has larger P/V ratios, the typical parameters being: V_p = 1.2 V,P/V = 5 and I_p = 11 mA cm⁻² at 77 K. The observed peak current densities of both NDR types are about five orders of magnitude higher than those reported previously for diodes with the similar structure. We assume that an inhomogeneous Si quantum-well thickness with the current flow localized in small regions of the diode area causes the observed variations in peak voltages, multiple NDR regions, peak spreading and exponentially rising background currents.

At present molecular dynamics (MD) simulations are the major tools used to investigate the mechanical behaviour of fullerenes. In this study, the energy changes of three fullerene balls, C₆₀, C₈₀ and C₁₈₀, under uniform inflation/deflation and axial tension/compression, are investigated using the second-generation Brenner potential. The energy changes can be precisely described by a cubic function of the inflation/deflation strain or tension/compression deflection. A spherical shell, the continuum model of the fullerene balls, is established and analysed by means of non-linear elasticity theory. The results of calculations using this model are in good agreement with those of MD simulations. A regular non-linear pattern of energy change of fullerene balls subject to external loads is elucidated, and the possibility of applying continuum elasticity theories for studying shell-type nanostructures is demonstrated.

The washing and elution behaviour of carbon nanotubes (CNTs), following soaking in radioactive (¹²⁵I)NaI and (^110mAg)AgNO₃ solutions, was investigated using the radioactive-trace technique. High-resolution electron microscopy and x-ray energy dispersive spectroscopy observations showed the validity of filling CNTs with soluble inorganic salts using this simple method. A great difference in the release of (¹²⁵I)NaI and (^110mAg)AgNO₃ from CNTs was found and discussed in terms of the chemical transformation occurring in CNT cavities. The amount of the filled materials was also estimated using the radioactive-trace technique.

Nanoindentation induces versatile deformation mechanisms. The situation is examined here for the case of nanocrystalline Cu by means of parallel molecular dynamics simulations. The indenter is applied to the grain boundary (GB) or to the grain interior. The inter-atomic interaction of Cu is modelled by both the Lennard-Jones potential and the embedded atom method potential. The burst and arrest of stacking faults hold the key for the plastic deformation of nanocrystalline Cu under nanoindentation, in clear contrast to the case of nanoindentation on single-crystal Cu. The following descending order of indentation resistance is found: single crystal (or polycrystal of large grain size), nanocrystal when indented on the grain interior, and nanocrystal indented on the GB.

The focus of this research was to use the solid state glass devitrification transformation to develop nanocomposite microstructures in iron-based materials. Mechanical deformation studies on the nanocomposite structures at room temperature revealed that although they exhibited high yield strength (975–1200 MPa), they were brittle and exhibited no tensile elongation. A specialized two-step annealing process was employed which involved first holding for extended times below the crystallization onset temperature to devitrify extremely fine matrix grains followed by heating to temperatures above the crystallization onset temperature for short times. Using this approach, nanocomposite microstructures could be formed consisting of α-Fe, Fe₂₃C₆ and Fe₃B type triplex matrix phases of the order of 75 nm in size which were stabilized by dispersions of 2–10 nm α-Fe precipitates along the grain boundaries and embedded in the Fe₂₃C₆ type phase. At 750 °C and a strain rate of 10⁻³ s⁻¹ this stabilized nanoscale microstructure was found to exhibit superplasticity with an ultimate tensile strength of 1800 MPa and a tensile elongation of 230%. Subsequent testing using 'jump tests' revealed that the strain rate sensitivity factor was equal to 0.51, which, consistent with the microstructural observations, revealed that the main mechanism for elevated temperature deformation was grain boundary sliding/rotational processes.

The growth and evolution of nanometre-sized Ni metal islands deposited under low-temperature non-ultra high vacuum conditions as a function of metal layer thickness, growth temperature and time is reported. The temperature of formation of the islands has been intentionally kept low for possible applications in flat panel display technology and also to act as a catalyst for carbon nanotube growth. It is shown that the size and distribution of the islands depends critically on the annealing temperature and the initial thickness of the metal layer. The mechanism of formation of the islands is described in terms of an Ostwald ripening mechanism of mass transport of either weakly bound individual Ni atoms or smaller clusters into larger more dispersed clusters.

The focus of this research was on gaining a fundamental understanding of the structure/property relationships that exist in an Fe-based multicomponent glass-forming alloy. While composition is one key parameter in determining the final microstructure, the specific transformation conditions (temperature and time) related to how a glass devitrifies were found to have significant effects on the development of the devitrified microstructure. Microstructural studies have revealed that recovery, relaxation, crystallization, and recrystallization phenomena are important effects which lead to large differences in the ensuing microstructure resulting in significant differences in properties such as devitrified hardness. The information gained in this study, combined with further investigations into the mechanistic processes governing these transformations, may allow for the further development of nanostructured materials with specific and targeted sets of properties.

Silicon nanoparticles have been fabricated in both oxide and nitride matrices by using plasma-enhanced chemical vapour deposition, for which a low substrate temperature down to 50 °C turns out to be most favourable. High-rate deposition onto such a cold substrate results in the formation of nanoscaled silicon particles, which have revealed an amorphous nature under transmission electron microscope (TEM) examination. The particle size can be readily controlled below 3.0 nm, and the number density amounts to over 10¹² cm⁻², as calculated from the TEM micrographs. Strong photoluminescence in the whole visible light range has been observed in the as-deposited Si-in-SiO_x and Si-in-SiN_x thin films. Without altering the size or structure of the particles, a post-annealing at 300 °C for 2 min raised the photoluminescence efficiency to a level comparable to the achievements with nanocrystalline Si-in-SiO₂ samples prepared at high temperature. This low-temperature procedure for fabricating light-emitting silicon structures opens up the possibility of manufacturing integrated silicon-based optoelectronics.