Ultra fine powders with particle diameters less than 100 nm are becoming increasingly important for various electronic, magnetic and chemical applications as well as in nanostructured materials with extremely fine grained components. Nowadays there are a lot of methods available to synthesize nanoscaled powders. One of the most versatile production methods delivering clean particle surfaces is laser evaporation. Interaction of high intensity laser beams with materials leads to evaporation and subsequent condensation of very small mostly spherical particles with diameters in the order of 10 nm. This means specific surfaces of the powders in the order of 100 m2/g. Depending on the lasers intensity, the surrounding gas, its pressure, and the evaporated material the particles have mean diameters ranging from 4 to 20 nm. Their distribution is generally lognormal with a geometrical standard deviation in the narrow range between 1.5 and 1.7. These properties of the powders can be explained by their originating mechanism, nucleation and particle growth in the surrounding gas. With this method nanosized powders of various materials can be produced like pure metals, alloys, metal oxides, other ceramics and new phases which are probably stabilized by the small particle size. Using a 1 kW - Nd:YAG - laser powder production rates of up to 100 g/h can be achieved for alumina. Contrary to other synthesis methods e.g. the sol-gel method the produced nanoparticles are in particular free of solvents or other residuals. This makes laser synthesised powders extremely usefiil for catalytic or gas sensor applications where defined surface qualities are necessary.

High quality nanoscale, phase-pure hexagonal gallium nitride (GaN) crystallites have been synthesized using the thermally induced explosion of the molecular precursor (Et3N)Ga(N3)3. The method allows the control of the particle size from 2 to 24 nm. X-ray diffraction and Rietveld simulations revealed a platelet-like shape of the larger particles in contrast to the rather spherical smaller crystallites. An increasing blue-shift of the high energy luminescence with decreasing crystal size was found (up to 4.4 eV at 5 K). A preliminary analysis of the surface chemistry of has been carried out based on silylation and deuteration experiments and IR spectroscopy, indicating hydrophilic, O-H and N-H terminated particles.

Microstructural analysis was performed on several crystalline SiC samples previously prepared by three separate processing methods and subsequently sintered under high pressure and high temperature conditions using the cubic anvil cell MAX80 at Hasylab. Microcrystalline SiC was prepared using SHS conditions, while nanocrystalline SiC was prepared using both combustion synthesis methods and polymer precursors. High purity, highly disordered nanocrystalline SiC powders, with average particle diameters below 100 nm, were synthesized via combustion methods from precise mixtures of silane and acetylene. The properties of the silicon carbide powders prepared in this manner were dependent on the initial stoichiometry and pressure of the combustion mixture. Pyrolysis of polymer precursors to SiC was also used to fabricate ceramic powders containing uniformly-sized, highly disordered nanocrystalline SiC grains. The grain sizes ranged from approximately 3 nm to greater than 50 nm, and depended on the initial composition of the polymer, the pyrolysis conditions, as well as the annealing atmosphere, temperature and time. This paper describes the preparation methods for each of the SiC powders, the densification procedure, and preliminary results obtained primarily from transmission electron microscopy and X-ray diffraction analysis.

Nanosized silicon carbide powders are synthesized by the Chemical Vapor Synthesis (CVS). Grain and particle sizes are measured by XRD and BET. Agglomeration, crystallinity and the chemical composition of the SiC samples are investigated as a function of storage conditions and the CVS process parameters: precursor material, synthesis temperature and reactor length. Grain and particle sizes below 10 nm are observed in all samples.

Sensors and Transducers, and in the specific context of this paper gas sensors, are currently amongst the largest growth areas in the modem electronics industry and this seems likely to continue for the foreseeable future. Nanocrystalline materials posses many properties that could make them ideal as potential gas sensing elements with many advantages over their microcrystalline counterparts. Most importantly these include increased surface area coupled with reduced sintering temperatures and times. However, it should also be noted that there are several disadvantages including the comparatively high cost of materials and increased electrical resistance.

This paper reviews the operating mechanisms of semiconductor gas sensors and the possible advantages of using nano sized powders to produce gas sensitive devices. Results are presented which have been obtained from several materials produced by laser evaporation including alumina (Al2O3), zirconia (ZrO2), and tin dioxide (SnO2) in contaminated atmospheres incorporating carbon monoxide, hydrogen and methane.

SnO2 nanoparticles are of interest for gas sensor applications because the surface area is much larger compared to conventional powders. Thus, interactions between the material and the gases, which occur on the surface sites of the particles, are increased considerably. The preparation of SnO2 powders has been investigated following two forced precipitation systems: the hydrolysis reaction of SnC14 in an emulsion media and the hydrolysis reaction of Sn2+ in the presence of a complexing ligand (CH3COO−). Spherical nanoparticles in the 10 to 100 nm range and with a narrow size distribution were synthesized by both precipitating routes. In both cases, it has been demonstrated that the most important parameter which controlled the particle size was the nature of the associated anion. When this associated anion or ligand is able to form a strong complex with the colloidal subunits, a barrier against Van der Waals attraction is created which results in little growth. This greatly influences the agglomeration/growth kinetics during the precipitation. The effect of acetate chelating ligands which resulted in the SnO2 nano-powders formed of 5–10 nm crystallites will be presented and discussed. Preliminary results on the gas (N2, NO) adsorption studies on pellets formed from these powders are also presented.

Antimony-doped tin oxide (ATO) nanophase materials containing up to 43 mole% antimony have been synthesized by a wet-chemistry method. These ATO materials exhibit enhanced electrochromic properties for display devices. The performance of the display devices is related to the nanostructure of the synthesized ATO materials. The average sizes of the ATO nanocrystallites depend on dopant level, annealing conditions, and synthesis processes. Antimony inhibits the growth of tin dioxide nanocrystallites during annealing at moderate temperatures. At higher annealing temperatures, however, antimony segregates to form separate oxide phases and ATO nanocrystallites grow significantly. A systematic study of the structural evolution of the synthesized ATO materials is presented and the relationship between the nanostructure of the ATO materials and the enhanced performance of the corresponding electrochromic devices is discussed.

Starting with a hydrated tin dioxide powder of 3nm size obtained by the sol-gel method, the effects of thermal treatments and grinding on the final structural properties of the nanoparticles have been investigated. It is observed that two regions with different structural properties can be distinguished in the range of temperatures between 250 and 1000°C. In spite of their large grain size (>15nm), above 350–450°C, these nanocrystals can be used as precursor powders for gas sensors because of their higher stability. Grinding of the starting material before calcination will be proved as a suitable method to obtain the sarne type of stability with lower grain size.

It is well established that the dispersion of nanosized metal particles in a polymer matrix can induce non linear optical properties, yet very little is known about the effect of semiconducting transition metal oxide nanoparticles on both electrical and luminescence properties of conjugated polymers. In this paper, we report the synthesis of a nanostructured TiO2/poly(pphenylenevinylene) system and show by diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) that a stable conjugated nanocomposite is obtained. Investigation of the photoluminescence (PL) properties reveals both a broadening and a blue shift of the emission spectra. Adsorption of oxygen is found to be stronger on the nanocomposite than on PPV and to reversibly quench the PL emission, thus suggesting enhanced gas sensing properties. A tentative mechanism explaining the role of n-TiO2 is briefly discussed.

The oxygen adsorption on a polycrystalline titanium surface at room temperature has been studied by MDS in conjunction with UPS and AES. MDS and UPS spectra were measured using a pulsed-dischaxge-type metastable helium atom source which we have newly developed. FRom the analysis of the measured spectra, we found the following about the local density of states (LDOS) of the oxygen-adsorbed titanium surface: (1) With increasing oxygen exposure at 0-2 L, the LDOS on the first atomic layer (SDOS) at 0-1 eV below the Fermi level (EF) decreases more steeply than that of deeper layers. (2) The SDOS at 5–8 eV below EF remains small at 0-2 L, and begins to increase at ca. 2 L while the LDOS of subsurface layer increases first. We discuss the positions of adsorbed oxygen atoms based on these results.

Silver nanoparticles which are embedded in a plasma polymer matrix were obtained by transmission electron microscopy. Their sizes and shapes were analyzed with optical image processing. As most of the particles appeared as elongated rotational ellipsoids, the major and minor half axes were determined for each particle. The model of Gans in the Rayleigh approximation was used to calculate the extinction spectra for each investigated particle using the data from the image processing. Various spectra for 1049 silver particles were added to get a total extinction spectrum which was in good agreement with the experimental spectrum.

There is strong interest in the research & development of the particle/polymer composite technology for functional applications. Current activities are directed towards polymer assisted synthesis of ultrafine metal or semiconductor particles in solution, with an ultimate goal to arrange the particles in a matrix in well-controlled fashion, for possible electro-optical applications. In the present work the optical properties of poly(acrylic acid) polymers containing gold chloride have been investigated. This material system permits synthesis of gold nanoparticles by the reduction of gold chloride. The polyelectrolyte initially forming upon mixture of the reagents shows a gelating behavior. In-situ reduction into gold clusters can be achieved by several ways. Rheological properties, optical absorption/extinction spectra as well as photoluminescence and photoluminescence excitation spectra will be presented and discussed. Different possible combinations of gold ligands and co-ordinations states will be considered in the discussion. Gold may be present in the form of complexes, to the polyelectrolytes, or in form of nanoparticles or larger aggregates depending on the relative concentrations of the reagents and polymer molecular weight.

We have studied precipitation of noble metals with a view to extending these synthetic processes to other metals. In this paper we discuss the synthesis of gold clusters on the surface of oxide fine particles (in the range 60–100nm). The influence of pH of the solution, the concentration of metal and the reducing agent are discussed. The size distribution of the precipitated gold clusters, determined by transmission electron microscopy, are around 2–10 nm. The interface between the gold clusters and oxide has been investigated by HRTEM. Optical measurements of the zirconia supported gold clusters shows a significant shift in the plasmon resonance. A model based on effective medium theory is proposed taking into account the high refractive index of zirconia.

The surface modification of titania and tin dioxide nanopowders by hexamethyldisilazane and hexamethyldisiloxane grafting has been followed in situ by FT-IR spectroscopy. A grafting mechanism is proposed for both compounds and the formation of new surface species is discussed. Since TiO2 and SnO2 are widely used in chemical gas sensors due to their electrical properties, the respective behaviors of the non-grafted and grafted samples in reducing (CO) environment as well as the humidity effects are compared. Because the transmitted IR energy depends on the concentration of the free carriers, a correlation between the electrical conductivity variation and the perturbation of the IR spectra is attempted.

The surfaces of two tin oxide nanosized powders synthesized by laser evaporation and presenting different average grain sizes were studied by Fourier transform infrared spectrometry. The oxygen adsorption onto activated and oxidized surfaces appeared to be dependent upon the grain size. Moreover, the nature of surface acidic sites probed by pyridine was shown to be dependent on the oxidizing treatment. A simultaneous study of the infrared energy variations and the spectrum perturbations when CO and mixtures of CO and O2 were in contact with the SnO2 samples, showed both the formation of CO2 and the CO reducing effect. The need of an oxygen environment for the surface recovery was clearly brought in evidence as well as the importance of the temperature in the CO sensing mechanism.

A new method for dispersing magnetic nanoparticles in a polymer matrix is described. SmCo5 nanoparticle powders are first prepared by ball milling, and then embedded in thermoplastic powder by further milling. The nanoparticle-embedded thermoplastic powder was compacted to form solid magnetic composites which were characterized by transmission electron microscopy (TEM) and by SQUID magnetometry. The conventional TEM measurements on microtomed sections of the composites showed spherical nanoparticles clearly separated from their nearest neighbors, thereby eliminating exchange interactions and greatly reducing magnetostatic interactions. Lorentz microscopy illustrated the potential for imaging single particles and determining the magnetization directions of isolated and interacting particles. Magnetic measurements on dilute samples show that the processing method does not degrade the magnetic properties of the particles.

A plasma oxidation-reduction process has been found to be suitable to produce silver surfaces showing Surface Enhanced Raman (SER) activity. SER spectra of monodisperse polystyrene films and of other smaller molecules like alkanethiole, have been obtained in order to study the correlation between enhancement and nanomorphology as probed by Atomic Force Microscopy. It was found that the enhancement is strongly correlated with a coalescence of the initially present silver grains (10–50 nm in size). This coalescence leads to the formation of 100–300 nm silver hills in which each silver grain is still clearly distinguishable. The increase of the effective surface area due to the particle coalescence has been correlated to the SERS enhancement in straightforward fashion while self assembled alkanethiole monolayers have been used as spacer for electromagnetic SERS enhancement of polystyrene.

A study by X-ray photoelectron spectroscopy (XPS) of the local atomic environment in laser-synthetized Si/C/N nanopowders is presented. The Si 2p, C Is, and N 1s core levels as well as the valence band distribution are investigated. The attention is focused on samples with C/N atomic ratio around 0.6 which exhibit interesting properties concerning annealing treatments. The presence of C-N bonds after 1500°C annealed samples is evidenced.

FeCo nanoparticles are synthesized in a radio frequency (RF) plasma torch from metal powder precursors and simple gases. The first precursor consists of pre-alloyed FeCo powder; the second precursor is a mixture of elemental Fe and Co powders. A protective carbon coating on the particles is achieved by injection of acetylene gas into the plasma. X-ray diffraction reveals phase purity, low carbon concentration, and minimal oxidation. Analytical electron microscopy is used to examine the nanocomposite morphology and composition of individual nanoparticles. Both synthesis routes produce alloy product, but nanoparticles produced from pre-alloyed precursors exhibit smaller variations in Fe/Co ratio than particles produced from elemental powders.

An overview of our work on the synthesis of nanostructured powders and coatings using the polyol process is presented.