Materials Science and Technology

In this paper metal vapour deposition and metal organic chemical vapour deposition have been used to produce nanoscale metal particles on various TiO₂(110) surfaces. The nanoparticles (Pd and Rh) are imaged directly by scanning tunnelling microscopy at ambient and elevated temperature in ultra high vacuum. The surface reconstruction of the TiO₂ to a cross-linked (1 × 2) is used to form highly corrugated rectangular arrays that direct the growth of the Pd clusters. Thermal annealing at first induces a flattening and then a sintering of the nanoparticles. Ultimately the particles form a wide distribution of shapes and sizes. A crystal displaying crystallographic shear planes, composed of a self assembled array of alternating up -down step edges on the terrace, is used to direct the growth of Rh nanoparticles from [Rh(CO)₂Cl]₂. The particles are shown to grow mainly at the top edge of each step that runs at an angle to the principal directions of the surface.

Single sized PbS nanocrystals are selectively deposited from the gas phase onto photoresist patterned substrates at atmospheric pressure, forming patterned nanocrystal thin films. Selective deposition is obtained by means of electrostatic control of the charged nanocrystals in the gas phase. The nanocrystal deposition process is characterised by low cost and high parallelism. A lift off process is introduced which removes the photoresist without removing the nanocrystal film in the interested area. Thus, further processing can follow the nanocrystal film deposition. During the deposition of nanocrystals out of the gas phase a preferred deposition into resist openings, i.e. onto the substrate surface is observed. An electrostatic model is developed to explain this focusing effect yielding smaller line width compared with the resist pattern.

It is well known that boron (B) forms surface phases if deposited on Si surfaces. One interesting phase is the so called √3 × √3-R30° boron surface phase (BSP) on Si(111). This surface phase is different from all other √3 × √3-R30° surface reconstructions formed by group III atoms because B alone is able to induce this reconstruction in two different configurations. Few facts are known about the step by step formation of this √3 × √3-R30° BSP and its Si overgrowth, although this is particularly important for the incorporation of BSPs into novel electronic devices. Therefore, the formation of the surface phase by in situ scanning tunnelling microscopy (STM) has been investigated. Examination showed that deposition of B led to the breakdown of the 7 × 7 reconstruction and to the formation of a Si surface with a BSP located on T₄ lattice sites (B-T₄). Thermal annealing of the structure causes the B atoms to migrate from T₄ to S₅ positions. The two phases exhibit completely different properties as a result of their different chemical binding. A technique has also been developed to distinguish both surface phases and to visualise the phase transition from B-T₄ to B-S₅. This has been achieved using work function measurements in a hydrogen atmosphere. The Si overgrowth of B-T₄ surface phases has been studied by STM. In comparison with the overgrowth of a B-S₅ surface phase, epitaxial growth occurs at lower temperatures. As a result of the increased surface mobility on the B-T₄ BSP, the density of epitaxial islands formed during overgrowth is much lower compared with the overgrowth of a B-S₅ phase or of Si(111)-7 × 7.

Using frequency dependent capacitance spectroscopy the dynamics of tunnelling is investigated from a three-dimensional back contact into ensembles of self assembled InAs quantum dots. An equivalent RC circuit is derived from the balance of charge which allows the charging time for each state of the dots to be determined. This method is applied to investigate the charging dynamics of samples with double layers of vertically aligned InAs quantum dots. In these structures the interplay between Coulomb blockade effects and sequential tunnelling can be seen: The tunnelling probability for the dots in the second layer is either increased by sequential tunnelling through states of the first dot layer or reduced by Coulomb blockade effects.

A novel hybrid organic/inorganic heterostructure is described, comprising a highly ordered discotic liquid crystal layer, self-organised on an ultrathin Au film. The electronic properties of the hybrid structure have been investigated using a range of theoretical techniques. The strong bonding of the molecules onto the metal surface, combined with the high degree of periodicity across the molecular layer, results in the formation of a hybrid metal/molecular band structure which has been calculated using a periodic Newns-Anderson model. The in-plane conductivity was calculated using the Kubo-Greenwood formula, and electron trapping in the molecular layer was considered by use of a diffusion model. The effect of trapping is apparent in the frequency domain conductivity. The sensitivity of the frequency domain response to variations in the trapping parameters was studied, and interpreted in terms of potential gas sensing functionality.

A code has been developed for the simulation of the electronic properties of an array of semiconductor nanocrystals embedded in a thin dielectric layer. Such nanocrystal layers, stacked between the gate and the channel of a field effect transistor, can be effectively used as a storage medium for non-volatile memory applications, with perspectives of improved performance in terms of power consumption, ease of programming, and shorter write-erase times with respect to conventional non-volatile memories. Results are given from a detailed simulation based on the self-consistent solution of the Poisson-Schrödinger equation on a three-dimensional grid, focusing on the charging process and on the effect of charge stored in the nanocrystals at transistor threshold voltage.

A detailed model is proposed of the leakage current due to tunnelling through defect traps in thin insulating layers. Such leakage currents are a major concern for the reliability of semiconductor devices based on the insulating properties of dielectric layers, such as metal-oxide-semiconductor transistors and memories, and of nanoscale devices whose operation is based on controlled tunnelling through thin dielectrics. The proposed model allows the addressing of both dc and noise properties of trap assisted currents. Results from a numerical implementation of the model are also presented.

Monte Carlo simulation of deposition of non-stoichiometric amorphous SiO_x nanolayers from vapour phase onto a polymer surface is reported. The model developed is based on the network properties of silica and takes into account dangling bonds arising during the real process of deposition. The model is validated via comparison of the radial and bond angle distribution functions for the simulated Si and SiO₂ structures with those obtained from experiment for bulk materials. Porosity of the simulated amorphous layer is characterised by the relative volume of pores and the ratio of the pore surfaces to the pore volume. It was found that porosity strongly depends on nucleation sites density (NSD) on the polymer substrate. At NSD lower than 1 nm^-2 the porosity may reach as much as 30% of the layer volume, while at NSD higher than 4 nm^-2 it decreases down to 3/7%. Possible implications of the obtained results are discussed.

Focused ion beam systems are similar to scanning electron microscopes except that a beam of focused ions rather than electrons is rastered over the sample. Images are formed by either the secondary ions or electrons that are emitted as the ion beam interacts with the sample and the typical image resolution using the electrons is of the order of 10 nm. A system can be used to implant/sputter away material, deposit or preferentially etch metals and insulators in selected areas. Up to about 10 years ago, focused ion beam systems were mainly used in the semiconductor industry for circuit modifications; by the cutting and re-routing of interconnects. However, they are now being used for a range of novel micro and nanoengineering applications. In this paper, the application and potential use of focused ion beam systems to micro and nanoengineering is discussed. Examples of applications, such as the milling of two-dimensional and three-dimensional shapes, ion lithography, and the preparation of site specific specimens for materials characterisation, are outlined.

The morphology of thin films of conjugated polymers and block copolymers (containing one conjugated segment and one (or two) non-conjugated segments) is analysed with a combination of atomic force microscopy data and molecular mechanics/dynamics calculations. The rigid character of the conjugated segment induces specific morphological characteristics, with respect to the situation usually found in coil-coil copolymers. The copolymers organise as very long, thin objects with constant width and height in the nanometre range (nanoribbons). Decreasing the amount of polymer on the substrate allows the observation of isolated nanoribbons. The comparison of the experimental data with the theoretical modelling indicates that the ribbons are made of highly regular stacks of conjugated chains surrounded by coiled non-conjugated segments.

Nanostructures on TiO₂ and CeO₂ have been created by treatment under ultrahigh vacuum conditions. Crystallographic shear in the bulk of rutile TiO₂ leads to line structures on the (110) surface. Regular trenches have been observed after calcium segregation to the (110) surface of TiO₂. Network structures formed by self-organisation are formed after high temperature anneal and rapid cooling of TiO₂(001). On CeO₂(111) regularly arranged defect types of various shapes have been observed. The examples illustrate that a scanning tunnelling microscopy is a suitable tool for studies of surface structures on the nanometre scale.

Fluorescence scanning near field optical microscopy and tapping mode atomic force microscopy have been used to investigate a blend of the polyfluorene based polymers F8BT and PFB. Images acquired from the film, which contains 10 wt-% F8BT, show the presence of micrometre size regions protruding from the surface of the film, unambiguously identified from their fluorescence as F8BT rich. However, comparatively strong and locally varying fluorescence from F8BT is also detected in the PFB rich phase, which is topographically flat apart from small ∼30 nm diameter protrusions identified as PFB from their presence in a pure PFB film. It is demonstrated that the combined application of scanning near field optical microscopy and atomic force microscopy to the study of such blends is able to assist our understanding of the nature of phase separation and photoinduced charge transfer.

Most state of the art adhesives fail to bond under wet conditions. Therefore, knowledge of the intrinsic properties of natural adhesives might give valuable information for future engineering approaches. This work investigates the adhesive that Eunotia sudetica, a species of benthic freshwater diatoms, produces to attach itself to a substrate. Atomic force spectroscopy under aqueous solution reveals the modular, self-healing properties of this natural adhesive.

The response of two different types of nanostructured gas sensor to oxygen has been investigated. The first (optical) is based on the photoluminescence quenching effect of a porous silicon sample, the second on the changes of the electrical conductance v. environment of a porous silicon free standing membrane on an insulating neutral substrate. The response of both the devices to oxygen have been measured and compared. The optical based gas sensor exhibits a quenching following the Stern-Volmer model. The corresponding reactivity rate constant is found to depend on a characteristic nanodimension of the wire. The electrically operated sensor is more sensitive to oxygen and shows an opposite behavior if exposed to a reducing environment.

Solid materials based on gold nanoparticles cross-linked with a dithiol spacer molecule were prepared and their dielectric properties examined. The gold nanoparticles were synthesised via a standard toluene/water two phase system using sodium borohydride as the reducing agent. Two different sized nanoparticle colloids were produced, 4.8 and 1.2 nm diameter. These were cross-linked with two dithiol compounds, 1,5-pentanedithiol and 1,9-nonanedithiol. The combination of two different size colloids and two different spacer molecules led to four final materials. The dielectric properties of the four materials were investigated using a broadband dielectric spectrometer in the frequency range of 1 Hz to 10 MHz and the temperature range 123-373 K. The imaginary component of the dielectric loss factor is presented for each material over the frequency and temperature range quoted. The 1 Hz room temperature conductivity for each material was between 0.1 and 7.2 μS cm^-1. The temperature dependence of the conductivity followed that predicted for a non-metallic activated electron hopping mechanism with the electrostatic activation energy for each material being between 0.034 and 0.120 eV. These were calculated by applying the simple Arrhenius based equation first described by Neugebauer.

The hot deformation behaviour of the discontinuously reinforced metal matrix composite aluminium alloy 2009(Al-3.8 wt-%Cu-1.3 wt-%Mg) - 15 vol.-%SiC_w, produced via powder metallurgy, was studied by compression testing in the strain rate range 10^-4 - 10¹ s^-1 at temperatures ranging from 450 to 525 ^°C. It was shown that the composite exhibits superplasticlike behaviour, which can be explained in terms of a threshold stress. This approach allowed calculation of the true activation energy and true values of the stress exponent. A strong temperature dependence of the threshold stresses was determined. Optical metallographic studies and TEM investigations were carried out to examine microstructural evolution. It was shown that superplastic deformation does not lead to the formation of a new macrostructure. Under superplastic conditions the SiC whiskers retain their orientation, while at both lower and higher strain rates the reinforcements rotate during deformation. As a result, superplastic deformation yields increased service properties. The origin of this phenomenon is discussed. MST/4998

An ultra high strength low alloy steel having the composition Fe - 0.3C - 1.35Cr - 0.9Mo - 0.25V - 0.1Nb (wt-%), has been developed using the electroslag refining process. In order to understand the failure behaviour of the material, tensile coupons, surface crack tension (SCT) specimens, compact tension (CT) specimens, and pressure chambers were fabricated and tested. The fracture parameters of the material were evaluated from the test data. Using these parameters a failure assessment diagram was generated to quantify the effects of the presence of cracks on material strength. Fracture strength estimates on SCT and CT specimens compare well with test results. The effects on fracture properties owing to weld/heat treatment processes have also been examined. To select a suitable failure theory for this material, failure assessment has been carried out on pressure chambers and verified with test results. Fracture analysis has also been carried out on the cylindrical vessels containing axial surface cracks and it was found that failure pressure decreases with increasing in crack size.

Hot compression tests were conducted on an austenitic stainless steel, BS 304S31, to clarify the origin of the grain coarsening phenomenon that is observed for carbon steels deformed slightly at a high temperature. The grain size was measured after hot compression testing, and the distribution was compared with that of the equivalent plastic strain ϵ_eq calculated using a rigid plastic finite element analysis technique. Similarly to carbon steels, a clear band zone was observed, although in a different region, in the plane difined by working temperature and ϵ_eq. The adoption of a material without a phase transformation has provided strong evidence supporting the hypothesis presented for carbon steels that the grain coarsening phenomenon is strain induced grain growth in the austenite region. A peculiar, staggered grain boundary was observed as the equivalent plastic strain approached the threshold value for the initiation of grain coarsening.

A one-dimensional predictive model is proposed to estimate the interfacial heat transfer coefficients during unidirectional solidification of a cast iron alloy, vertically upwards, against a sand block. The model is based on the surface roughness characteristics of the casting and sand surfaces and the concave deformation of the initial solidified casting skin towards the sand surface. The modelled interfacial heat transfer coefficients and predicted temperatures inside the casting and the sand block showed an approximate agreement with experimentally determined values. The model showed that radiation was a significant mode of casting/sand interfacial heat transfer with the predicted contribution of radiation to the overall heat transfer being nearly 50%. The evaluation of the model in comparison to the interfacial heat transfer models proposed by Zeng and Pehlke suggested that the interfacial conditions considered in this model, namely, the mean peak to valley heights of the casting/sand mould surfaces and the gap width calculated from the deformation of the initial solid skin, gave a more accurate prediction. This predictive heat transfer model has an advantage over the inverse modelling technique as the matching of experimentally measured temperatures to determine the boundary conditions is avoided and the heat transfer coefficients can be estimated as an integral part of the casting simulation.

In the present work, the evolution of microstructure in the nickel based superalloy Inconel 713C was investigated for the vertical continuous casting of small cross-section rods (10 mm dia.), using several microstructural characterisation techniques. Microstructural evolution was greatly affected by the mould design, average casting speed, and distance from the rod surface, but the strongest influence was from the alternating drawing mode consisting of the drawing stroke, the resting period, and the reverse stroke, which caused periodic orientation changes of columnar γ crystal grains. Combined with other casting parameters this determined the local solidification conditions, influencing the formation and growth of γ dendritic grains and primary and eutectic MC carbide, as well as the stress distribution in the solidified shell, which caused shell deformation and the appearance of the inverse segregation and, occasionally, hot tears. A physical model is presented to explain the influence of casting parameters on microstructural evolution on the continuous casting of this alloy.

In the present work, an ingot of a commercial aluminium alloy, A356 (Al-7Si-0.3Mg), was deformed by rolling; a second ingot was extruded; and a third was cast in such a way as to obtain fine sized dendrites. All specimens were partially remelted in a salt bath, and water quenched. Microstructural examination indicated that the particle growth rate was reasonably similar for all specimens; however, while rolling gave an almost ideal microstructure of equiaxed particles, extrusion produced microstructural heterogeneities that led to abnormal grain growth and liquid segregation. Texture studies indicated that the rolled specimen recrystallised during heating to the semisolid state, while the extruded specimen recrystallised immediately after deformation. Simple compression experiments (parallel plates) showed that for high solid fractions (0.80) the extruded material exhibited the highest deformation response among specimens, but when the solid fraction was lower (0.60) the deformation behaviour of as cast, extruded, and rolled specimens was similar. Skeleton formation and liquid segregation were found to be associated with both non-spherical morphologies and high solid fractions.