Table of contents

The field of semiconductor nanowires has attracted much attention in recent years, from the areas of basic materials science, advanced characterization and technology, as well as from the perspective of the applications of nanowires. Research on large-sized whiskers and wires had already begun in the 1960s with the pioneering work of Wagner, as well as by other researchers. It was, however, in the early 1990s that Kenji Hiruma at Hitachi Central Research Laboratories in Japan first succeeded in developing methods for the growth of nanowires with dimensions on the scale of 10–100 nm, thereby initiating the field of growth and applications of nanowires, with a strong emphasis on epitaxial nucleation of nanowires on a single-crystalline substrate. Starting from the mid-1990s, the field developed very rapidly with the number of papers on the subject growing from ten per year to several thousand papers on the subject published annually today, although with a rather generous definition of the concept of nanowires. With this rapid development we have seen many new and different approaches to the growth of nanowires, technological advances leading to a more well-controlled formation of nanowires, new innovative methods for the characterization of structures, as well as a wealth of approaches towards the use of nanowires in electronics, photonics and sensor applications.

This issue contains contributions from many different laboratories, each adding significant detail to the development of the field of research. The contributions cover issues such as basic growth, advanced characterization and technology, and application of nanowires.

I would like to acknowledge the shared responsibilities for this special issue of Nanotechnology on the synthesis and integration of nanowires with my co-Editors, S Tong Lee and M Sunkara, as well as the highly professional support from Dr Nina Couzin, Dr Ian Forbes and the Nanotechnology team from the Institute of Physics Publishing.

We present a process for fabricating a field-effect transistor based on vertically standing InAs nanowires and demonstrate initial device characteristics. The wires are grown by chemical beam epitaxy at lithographically defined locations. Wrap gates are formed around the base of the wires through a number of deposition and etch steps. The fabrication is based on standard III–V processing and includes no random elements or single nanowire manipulation.

ZnO nanowires and nanobelts are two representatives of one-dimensional semiconductor nanomaterials possessing potential applications as optoelectronic and sensor devices. In this study, we applied a vapour-transport-deposition method to synthesize both types of nanostructures using relatively low temperatures (860 °C) by controlling the source materials. We found that the resulting product under similar growth conditions can be switched between [0001]-axial nanowires and -axial nanobelts simply by adding indium to the source. The former appear as ordered vertical arrays of pure ZnO while the latter are belts without spatial ordering. Both represent defect-free single crystals grown via the vapour–liquid–solid mechanism using nanosphere lithography-fabricated catalyst Au templates. Examination of the early growth stage suggests that the dissolution of In into Au influences the nucleation of ZnO at the solid–liquid interface, and subsequently defines the structure and crystallographic orientation of the nanobelts. The optical properties of both nanostructures are studied by photoluminescence and resonant Raman scattering, which indicate consistently that the doped nanobelts have a higher carrier concentration than the nanowires.

An accurate evaluation of the radial dopant profile in a nanowire is crucial for designing future nanoscale devices synthesized using bottom-up techniques. We developed a very slow wet chemical etchant for gradually reducing the diameters of metal-catalysed, boron-doped silicon nanowires with varying diameters and lengths. Particular care has been taken to perform the experiment at room temperature to prevent dopant segregation, which is common in high temperature processes. By ensuring identical surface conditions subsequent to diameter reduction, the resistance of the nanowires was measured and, as anticipated, was found to increase with decreasing diameter. As the diameters were shrunk using wet-chemical etching, nanowires exhibited a non-linear increase of the resistance when the diameter was reduced to ∼50 nm. This is an indication of near-complete depletion in the nanowires caused by nanowire surface charges. The dopant concentration of the nanowires was found to be 2.1 × 10¹⁸ cm⁻³ and the corresponding surface charge density was around 2.6 × 10¹² cm⁻².

A comparison of the transport properties of populations of single-crystal, In₂O₃ nanowires (NWs) grown by unassisted hot-wall chemical vapour deposition (CVD) versus NWs grown by laser-ablation-assisted chemical vapour deposition (LA-CVD) is presented. For nominally identical growth conditions across the two systems, NWs fabricated at 850 °C with laser-ablation had significantly higher average mobilities at the 99.9% confidence level, 53.3 ± 5.8 cm² V⁻¹ s⁻¹ versus 10.2 ± 1.9 cm² V⁻¹ s⁻¹. It is also observed that increasing growth temperature decreases mobility for LA-CVD NWs. Transmission electron microscopy studies of CVD-fabricated samples indicate the presence of an amorphous In₂O₃ region surrounding the single-crystal core. Further, low-temperature measurements verify the presence of ionized impurity scattering in low-mobility CVD-grown NWs.

This account briefly summarizes our research on the synthesis of nanocrystals of a host of materials from metal chloride precursors via atmospheric pressure chemical vapour deposition (APCVD). Successful synthesis of nanocrystals of various sulfides, oxides, silicides, and metals, demonstrates metal chlorides as useful precursors for nanoscience and nanotechnology. Oxide-assisted growth (OAG) is found to dominate the formation of a lot of metal sulfide nanowire/nanosheets. Our hypotheses of geometrically kinetic competition for the interaction between the silica sheath and the inner core materials (hexagonal Fe₇S₈ and face-centred cubic Cu_7.2S₄) have fairly well explained the phenomena of silica vapour pressure-dependent growth of the sulfide crystals. But experimental results for some sulfides call for ab initio analysis to determine its effective region associated with torsions.

Four types of one-dimensional (1D) heterojunctions, including Ag/Si, Pt₆Si₅/Si, Ni/CNT (carbon nanotube) and Ag/a-CNT (amorphous CNT), have been synthesized by combining electrochemical deposition and chemical vapour deposition, with porous membranes of anodic aluminium oxide (AAO) as templates. Each of these 1D heterojunctions consists of two different inorganic nanowires (or nanotubes) connected end to end. Moreover, after an important improvement has been made on the AAO templates, ordered arrays of these 1D heterojunctions are obtained in the AAO templates, where the filling ratios of the 1D heterojunctions in the pores of the AAO templates are 100%. Thus, the electrical properties of individual 1D heterojunctions in the corresponding arrays can be accurately measured by conductive atomic force microscopy. As a result, it is found that, in the arrays of Ag/a-CNT 1D heterojunctions, the contact between the Ag nanowire and the a-CNT of every heterojunction is ohmic. This finding has universal significance, and so offers a wide opportunity for designing large-scale interconnections among nanowires and nanotubes.

We show the epitaxial integration of III–V semiconductor nanowires with silicon technology. The wires are grown by the VLS mechanism with laser ablation as well as metal–organic vapour phase epitaxy. The hetero-epitaxial growth of the III–V nanowires on silicon was confirmed with x-ray diffraction pole figures and cross-sectional transmission electron microscopy. We show preliminary results of two-terminal electrical measurements of III–V nanowires grown on silicon. E-beam lithography was used to predefine the position of the nanowires.

A biosensor based on modified silicon nanowires has been fabricated to detect glutathione in solution via the cyclic voltammetry (CV) method. Owing to the strong sorption ability and high electrical conductivity of the modified silicon nanowires, the biosensor shows a fast response with good linear concentration dependence on glutathione in the range 0.33–2.97 µM. This facile modification approach could be extended to fabricate similar biosensors for the detection of other biomolecules.

Although dielectrophoresis has been used previously to manipulate a variety of nanoscale materials, manipulation in ionic solutions is more difficult due to the high dielectric constant of water and the formation of electrical double-layers. Here, we report experiments aimed at the manipulation of nanowires in aqueous media and real-time detection of nanowire bridging events. Real-time video images demonstrate the ability to manipulate individual nanowires in aqueous media by capturing them along the edges of electrodes, and using a slow fluid flow to transport them until they bridge across micron-sized electrode gaps. By using special cancellation schemes, we demonstrate that it is possible to eliminate the effects of background currents through the electrolyte, and to electrically detect the bridging of electrodes by individual nanowires and nanowire bundles. These results have been obtained using gold nanowires with diameters ranging from ∼50 to 250 nm, ∼50 nm diameter silicon nanowires, and ∼70 nm diameter carbon nanofibres.

We demonstrate how light force, irrespective of the polarization of the light, can be used to run a simple nanorotor. While the gradient force of a single beam optical trap is used to hold an asymmetric nanorod, we utilize the scattering force to generate a torque on the nanorod, making it rotate about the optic axis. The inherent textural irregularities or morphological asymmetries of the nanorods give rise to the torque under the radiation pressure. Even a small surface irregularity with non-zero chirality is sufficient to produce enough torque for moderate rotational speed. Different sized rotors can be used to set the speed of rotation over a wide range with fine tuning possible through the variation of the laser power. We present a simple dimensional analysis to qualitatively explain the observed trend of the rotational motion of the nanorods.

Metal-catalysed silicon nanowires were grown between silicon electrodes and exposed to vapours containing HCl or NH₃ at reduced pressure. Charge from adsorbed vapour modulated the conductance of the nanowires by changing the number of mobile carriers. Exposing the nanowires to HCl vapour increased the conductance while exposure to NH₃ vapour decreased the conductance. The observed results suggest the use of an array of nanowire sensors integrated with silicon electronics. Preliminary area estimates indicate that integrated amplification and signal processing is feasible for an array of 1000 sensors.

In this study we report a novel technology for synthesizing silica-based nanosprings with a yield higher than 90%, and with 100% repeatability. The nanospring mats are grown via the liquid–vapour–solid mechanism using a gold catalyst, where the deposition temperature can be as low as 350 °C. XPS analysis shows that the as-grown nanosprings have components of silicon and oxygen with an atomic ratio close to silica. Both SEM and TEM images illustrate that the helical structure of the nanosprings is extremely uniform. Two types of nanosprings are observed using TEM. The first type of silica nanospring is formed from a single nanowire, whereas the second type consists of multiple intertwined nanowires. Patterned deposition of nanosprings has been achieved using this technology.

ZnO nanowires, grown on transparent conducting oxide substrates from aqueous solutions of methenamine and Zn(NO₃)₂, were integrated as the wide band gap semiconductor into dye-sensitized solar cells. ZnO nanowires and their growth mechanisms were studied using electron microscopy, x-ray diffraction and photoluminescence measurements. The solution growth method forms dense arrays of long nanowires oriented normal to the substrate surface because nanowires growing at off-normal angles are prevented from growing further when they run into neighbouring wires. Dye-sensitized solar cells with ZnO nanowires were assembled and characterized using optical and electrical measurements. Short circuit current densities of 1.3 mA cm⁻², and overall power conversion efficiencies of 0.3% were achieved with 8 µm long nanowires. Photocurrent and efficiency increase with increasing nanowire length and improved light harvesting. Low surface area and a shunt that appears under light illumination limit the solar cell performance. Internal quantum efficiencies were similar for nanowires of all lengths, indicating that electron transport is not limited by the nanowire dimensions for aspect ratios less than 70.

The solvothermal/hydrothermal method is an important technology for producing semiconductor nanowires at low temperature. This paper presents an integrated discussion of nanowire growth, mainly from four aspects in the solvothermal/hydrothermal process. These aspects, including materials with highly anisotropic crystal structures, coordination directing/mixed solvents, surfactants/capping reagents, and reaction at relatively high temperature, have a key effect on nanowire formation in the solvothermal/hydrothermal process. These factors have instructional significance for nanowire synthesis and research of their growth process in the future.

Large-area and high-density arrays of AlN nanorods were synthesized at low temperature via a template-free and catalyst-free chemical vapour deposition. The quasi-aligned AlN nanorods were identified to grow along the c-axis and preferentially orient with their growth direction perpendicular to the substrate. Further studies showed that the AlN nanorods were grown on a buffer layer formed at the beginning of the reaction. By changing the flow rate of the carrier gas at the beginning of the reaction, we successfully obtained nanorods with different orientations on the substrate. The Raman spectrum and cathodoluminescence spectrum of the AlN nanorods at room temperature reveal the existence of oxygen-related defects in the nanorods.

We fabricated dual-gate ZnO nanorod metal–oxide semiconductor field-effect transistors (MOSFETs) where a Si substrate with a 200 nm thick SiO₂ layer was used as a bottom-gate and a Au electrode with a 100 nm thick SiO₂ layer was used as a top-gate. From current–voltage characteristic curves of the nanorod MOSFETs, the top-gate mode operation exhibited significantly enhanced device characteristics compared with the bottom-gate case. A switch current ON/OFF ratio of the top-gate mode (10⁵–10⁷) was at least one order of magnitude larger than that of the bottom-gate mode (10⁴–10⁶). Normalized transconductance, one of the key transistor parameters, was also drastically increased from 0.34 µS µm⁻¹ for the bottom-gate to 2.4 µS µm⁻¹ for the top-gate mode. The enhanced device performance can be explained in terms of geometric field enhancement and the resulting efficient gating effect for the top-gate mode geometry.

We report diameter-selective growth of GaN nanowires (NWs) by using mono-dispersed Au nanoparticles (NPs) on a ligand-modified Si substrate. The thiol-terminal silane was found to be effective in producing well-dispersed Au NPs in low density on Si substrates so that the agglomeration of Au NPs during growth could be avoided. The resultant GaN NWs exhibited a narrow diameter distribution and their mean diameter was always larger than, while keeping a deterministic relation with, the size of the Au NPs from which they were grown. A self-regulating steady growth model is proposed to account for the size-control process.

Materials capable of highly efficient, direct thermal-to-electric energy conversion would have substantial economic potential. Theory predicts that thermoelectric efficiencies approaching the Carnot limit can be achieved at low temperatures in one-dimensional conductors that contain an energy filter such as a double-barrier resonant tunnelling structure. The recent advances in growth techniques suggest that such devices can now be realized in heterostructured, semiconductor nanowires. Here we propose specific structural parameters for InAs/InP nanowires that may allow the experimental observation of near-Carnot efficient thermoelectric energy conversion in a single nanowire at low temperature.

The mechanical properties of poly(vinyl alcohol) matrix composites incorporating SiC and Al₂O₃ nanowires (NWs) have been investigated. A marked increase in the elastic modulus (up to 90%) has been observed even with the addition of a small quantity (0.8 vol%) of nanowires. This observation cannot be explained by iso-stress analysis, which is appropriate for describing the variation of properties with the reinforcement volume fraction in discontinuously reinforced composites. Crystallization of the polymer induced by the NWs, the high aspect ratio and the surface-to-volume ratio of the NWs as well as the possible in-plane alignment of the NWs during processing are considered to be responsible for the increase in the stiffness. A significant increase in the strength of the composite with the addition of NWs is also observed. This is due to the significant pull-out of the NWs and the corresponding stretching of the matrix due to the complete wetting of the NW surface by the polymer. The increase in tensile strength is found to saturate at higher vol% of NW addition due to the reduced propensity for shear-band induced plastic deformation.

The effects of thermal annealing on the field emission properties of AlN nanorods treated in different ways were investigated. A strong correlation was observed between the nitrogen vacancy concentration and the field emission properties of the samples. The sample annealed in ammonia had a higher emission current than the as-grown and vacuum-annealed samples. Raman and cathodoluminescence analyses revealed that the sample treated with ammonia had the smallest nitrogen vacancy density and the lowest work function, leading to the best field emission properties. These results indicate that post-treatment is an effective way to improve the field emission properties of AlN nanostructures.

We use metal–organic vapour phase epitaxy for growth investigations of epitaxial nanowires in III–V materials, such as GaAs, GaP, InAs, and InP. In this paper we focus on gold assisted growth of nanowires. The nature of the metal particle—whether it is in the solid or liquid state—is discussed. For InAs and InP we have demonstrated that gold assisted wires can only grow at temperatures where the particle is solid. We continue with a discussion concerning the kinetic aspects of nanowire growth. Under common growth conditions one observes that thinner wires grow faster than thicker wires, contrary to what was described in the early days of whisker growth. We address and resolve this discrepancy by discussing a simple transport model and comparing the supersaturations of different systems. Finally, we describe the morphology of epitaxial III–V nanowires with emphasis on the crystal structure.

Using a crystalline embedding scheme it has recently become possible to study free-standing III–V nanowires with cross-sectional scanning tunnelling microscopy (XSTM). In the present paper we discuss how this novel method can be used for direct atomically resolved imaging of the interior of nanowires. We will focus in particular on two areas where this method provides unique possibilities. First we discuss the growth of the nanowire at the substrate as studied by XSTM and determine the facets of the nanowire growth on the surface and at the onset of free-standing nanowire growth. Second, we demonstrate how individual defects can be studied inside the wires, indicating a unique way for investigating dopant structures and concentrations in nanowires. We identify a carbon defect/dopant in the nanowire positioned on arsenic sites and establish quantitative limits on the defect density in the nanowires.

We grew GaAs wires as thin as 20 nm on a GaAs(111)B substrate using organo-metallic vapour-phase epitaxy (OMVPE), with Au as a growth catalyst. To investigate the growth characteristics, we compared two methods of depositing Au. In the first, Au was deposited by vacuum evaporation, and the deposition thickness was varied to form a planar Au layer. We found that an Au layer thickness of 1 nm was best for forming cylindrical shaped wires. Next, a new method of injecting Au onto an area of a few micrometres was tested using a focused ion beam (FIB), and this method was found to be effective for growing wires as thin as 30–80 nm. However, the wire width did not depend on the injected density of Au. We based our analysis of the results on an ion implantation model. GaAs wires with a p–n junction along the direction were formed by changing dopants from silicon to carbon during growth. We observed an optical emission with a peak intensity at the wavelength of 910–920 nm during continuous current injection into the wires at 300 K. A spectral blue-shift in the light emission and a polarization along the wire growth direction were also revealed at 77 K.

Nanowires of Ag and Cu have been produced from metal-loaded zeolites and micropore-containing mesoporous silica SBA-15 in situ inside the electron microscope. The structures of the nanowires have been characterized by using high-resolution electron microscopy and the compositions have been examined by energy dispersive x-ray spectroscopy. The growth of single-crystal nanowires was controlled by the electron beam density. The growth mechanism of the metal nanowires is discussed.

Nanocrystalline hydroxyapatite (Nano HA), a prototype of minerals of bones and teeth, attracts increasing interest in medicine and dentistry. Different parameters for synthesis and post-treatment were investigated to determine their effects on crystallinity of nano HA, and in vitro cell responses to nano HA were studied. XRD and TEM analyses indicate that the crystallinity of nano HA synthesized by a chemical method was within the range of 15–50 nm, which is adapted to natural minerals of hard tissues. Increasing the ageing temperature significantly increased the crystallinity of nano HA, while lengthening the ageing time or varying the post-ageing drying process did not have any influence on its crystallinity. Nano HA annealed between 300 and 900 °C showed a small increase in crystallinity with increasing annealing temperature due to the long-range ordering effect. Cell attachment and spreading on nano HA were lower than those on pure titanium, and decreased as the crystallinity of nano HA increased. However, cells on nano HA demonstrated well-developed filopodia and lamelliopodia, which facilitate migration of the cells on it. This may benefit osteogenesis at the interface between bone and nano HA in vivo.

The fabrication of interdigitated titanium nanoelectrode arrays of 50 nm in width and spacing is described in this work. The nanoarrays have been realized using a Ga⁺ focused ion beam (FIB). FIB milling is typically accompanied by redeposition of removed material, which represents an important hindrance for milling closely spaced nanostructures. Redeposition effects have been reduced by means of XeF₂ gas assistance, which increases the etch yield by a factor of seven compared with pure ion milling. Furthermore, we used a simple adsorption model, which led to the conclusion that dwell time and refresh time should be <500 ns and >30 ms, respectively, for optimized XeF₂ assisted Ti milling. The measured resistance R of the electrodes is higher than 1 GΩ.

A study of the field emission characteristics of novel structures of ZnO, namely marigolds, multipods and microbelts, has been carried out in both the close proximity configuration and the conventional field emission microscope. The use of a conventional field emission microscope overcomes the drawback of arc formation at high field values. The nonlinearity in the Fowler–Nordheim (F–N) plot, a characteristic feature of semiconductors has been observed and explained on the basis of electron emission from both the conduction and the valence bands. The current stability exhibited by these structures is also promising for future device applications.

We systematically investigated the correlation between morphological and optical properties of InGaAs self-assembled quantum dots (QDs) grown by solid-source molecular beam epitaxy on GaAs (n 11)B (n = 9, 8, 7, 5, 3, 2) substrates. Remarkably, all InGaAs QDs on GaAs(n 11)B under investigation show optical properties superior to those for ones on GaAs(100) as regards the photoluminescence (PL) linewidth and intensity. The morphology for growth of InGaAs QDs on GaAs (n 11)B, where n = 9, 8, 7, 5, is observed to have a rounded shape with a higher degree of lateral ordering than that on GaAs(100). The optical property and the lateral ordering are best for QDs grown on a (511)B substrate surface, giving a strong correlation between lateral ordering and PL optical quality. Our results demonstrate the potential for high quality InGaAs QDs on GaAs(n 11)B for optoelectronic applications.

We have observed the formation of nanoscale Si spikes atop mesoscopic Si pillars during laser irradiation at intensities sufficiently high to melt silicon. The spikes have a radius on the order of 100 nm (<50 nm at the tip) and up to several micrometres in length. Nanoscale clusters formed during laser ablation deposit on the surface of the melt and nucleate the solidification of a mantle. The spikes are extruded through a solid mantle that applies pressure to a molten core as the liquid fuses. The growth mechanism is shown to be the same as the mechanism for formation of centimetre-scale ice spikes formed from freezing pure water. This mechanism of spike formation scales from the macroscopic to the nanoscopic regime and should be completely general and applicable to any material that expands upon freezing.

Brush-type amphiphilic block copolymers of 3-(trimethoxysilyl)propyl methacrylate (TMSPMA) and stearyl methacrylate (SMA) were prepared through the initiation of chloroacetylferrocene using atom transfer radical polymerization (ATRP) in anisole solution. The resultant copolymers were characterized by ¹H NMR and GPC. We investigated the morphologies and sizes of organic/inorganic hybrid nanoballs using aggregates of these multi-block copolymers as precursors after a sol–gel process. The effect of the copolymer–solvent interaction on these aggregations was discussed.

We examine the morphological and electrical characteristics of nanowires fabricated on DNA templates via palladium (Pd) reduction. λ-DNA molecules were stretched and aligned on a mica surface using a molecular combing technique, followed by an electroless deposition of palladium, resulting in formation of nanowires with nominal width of 7 nm. We investigated the size distribution of nanowires with atomic force microscopy and made electrical connections to the wires by metal evaporation through multiple shadow masks. Electrical characterization of the nanowires under various bias conditions, variable temperature, and with different contact metal work functions revealed a conduction mechanism resembling that of granular metals.

Superhydrophobic surfaces as well as low adhesion and friction are desirable for various industrial applications. Certain plant leaves are known to be hydrophobic in nature due to their roughness and the presence of a thin wax film on the surface of the leaf. The purpose of this study is to fully characterize the leaf surfaces on the micro- and nanoscale while separating out the effects of the micro- and the nanobumps of hydrophobic leaves on the hydrophobicity. Hydrophilic leaves were also studied to better understand the role of wax and roughness. Furthermore, the adhesion and friction properties of hydrophobic and hydrophilic leaves were studied. Using an optical profiler and an atomic/friction force microscope (AFM/FFM), measurements were made to fully characterize the leaf surfaces. It is shown that the nanobumps play a more important role than the microbumps in the hydrophobic nature as well as friction of the leaf. This study will be useful in developing superhydrophobic surfaces.

This paper reports the results of an investigation on the effect of shear deformations on wave propagation in carbon nanotubes embedded in an elastic matrix. A multi-walled carbon nanotube is considered as a multiple shell coupled together through van der Waals forces between two adjacent tubes. The surrounding matrix is considered as a spring element defined by the Winkler model. Using the variational calculus of Hamilton's principle, dynamic governing equations considering the shear deformation and rotary inertia terms are derived. Numerical examples describe the effects of shear deformation, rotary inertia and elastic matrix on the velocity, the critical frequency, the cut-off frequency and the amplitude ratio of wave propagation in multi-walled carbon nanotubes embedded in an elastic matrix, respectively. The results obtained show that wave propagation in carbon nanotubes appears in a critical frequency or a cut-off frequency for different wave modes; the effect of shear deformation decreases the value of critical frequency; the critical frequency increases as the matrix stiffness increases; the inertia rotary has an obvious influence on the wave velocity for some wave modes in the higher frequency region.

Magnetite nanoparticles (Fe₃O₄) of three different sizes below the limit for single domain magnetic behaviour have been obtained by thermal decomposition of an iron precursor in an organic medium in the presence of a surfactant. Good agreement between mean particle size obtained by TEM, crystal size calculated from x-ray diffraction and magnetic diameter calculated from magnetization curves measured at room temperature shows that the samples consist of uniform, crystalline and isolated magnetite nanoparticles with sizes between 5 and 11 nm. High saturation magnetization and high initial susceptibility values have been found, the latter decreasing as the particle size decreases. The main contribution to the anisotropy is magnetocrystalline and shape anisotropy, since surface anisotropy is suppressed by the oleic acid molecules which are covalently bonded to the nanoparticle surface.

A significant enhancement of ultraviolet (UV) emission has been observed in chemically grown ZnO samples using a thermal treatment at 200 °C. The intensity of UV emission can reach up to fifty times its initial value, while that of the visible emission decreases to a negligible value. Based on the thermal desorption spectroscopy results, the origin of this effect was attributed to the reduction of non-irradiative centres and hydrogen passivation through desorption of adsorbed water and hydroxyl groups. By precisely controlling the local desorption in ZnO with an electron beam, we have not only created optical nanotags on individual ZnO nanorods, but have also written sub-micrometre UV-emission patterns on ZnO films. It is believed that this patterning technique will extend the applications of ZnO to many other fields, such as high-density optical data storage and high-resolution UV-emission displays.

Molecular dynamics (MD) simulations are conducted for water flow through carbon nanotube (CNT) junctions as molecular nozzles. The fluidized piston model (FPM) is employed to drive the inlet flow at streaming velocities of 25 and 50 m s⁻¹. Water flow through the CNT junctions is found to undergo an increase in streaming velocity, a decrease in pressure, and an increase in temperature. Although the difference of the upstream velocities does not generally lead to an appreciable density difference in the downstream CNT, the higher streaming velocity causes the upstream density to increase. The streaming velocity remains almost constant in the upstream CNT, but increases dramatically in the junction region. The ratio of downstream to upstream streaming velocities increases with the ratio of upstream to downstream cross section. A higher inlet velocity results in larger acceleration, which is generally more noticeable at larger cross-sectional ratios, and less prominent in junctions with smaller cross-sectional ratios. The cross-sectional ratio calculated from the internal radii of the CNTs based on the oxygen atomic density profile of water is closer to the ratio of downstream to upstream streaming velocities than the cross-sectional ratio calculated from the radii given by the carbon atomic centres.

Nanocrystalline powders of undoped and lanthanide-doped scandium oxide were prepared by propellant synthesis and characterized by x-ray powder diffraction, electron microscopy, EDX spectroscopy and luminescence spectroscopy. The obtained material has the Sc₂O₃ cubic structure (space group ) with unit cell parameter increasing with the size of the dopant. The crystallite size is in the range 20–40 nm. The lanthanide-doped samples form Sc_2−xLn_xO₃ solid solutions with x≈0.2 (Ln = Eu or Er). No inhomogeneity was found by microanalysis on the micron scale. The emission spectrum of the Eu³⁺ doped Sc₂O₃ sample shows strong bands in the visible region assigned to 4f–4f transitions of the lanthanide ions.

In this paper we describe a new configuration for producing narrow extinction lineshapes for light scattering from one-dimensional arrays of silver nanoparticles. In this configuration, which is specifically concerned with an array with a finite number of relatively large (radius greater than around 30 nm) nanoparticles, the wavevector of the light is chosen to be parallel to the array axis, while the polarization direction is perpendicular to the array axis. This leads to narrow plasmon/photonic lineshapes when the particle spacing is half the incident wavelength. This effect stands in contrast to the narrow lines previously found for wavevector and polarization vector perpendicular to the array axis, where the optimum spacing is close to the wavelength. The results are rationalized using a semi-analytical evaluation of the coupled dipole interaction, and it is demonstrated that the parallel and perpendicular chains have very different dependence on the number of particles in the chain. Results as a function of chain orientation relative to the wavevector are also considered, as is the possibility of sensing using an array configuration that combines the parallel and perpendicular chain directions.

The localized surface plasmon resonance (LSPR) is a collective oscillation of the nanoparticle conduction electrons. LSPR excitation in silver and gold nanoparticles produces strong extinction and scattering spectra that in recent years have been used for important sensing and spectroscopy applications. Tuning the optoelectronic properties by controlling coupled SP modes in metals is one of the major challenges in the area of metal nanomaterials. Here we develop a simple method to fabricate linear-chainlike aggregates of gold nanoparticles (so-called nanochains), tuning the linear optical properties in a wide wavelength range from visible to the near infrared. The aggregation behaviour and linear self-assembly mechanism of citrate-stabilized gold colloids as provoked by the addition of cetyltrimethylammonium bromide (CTAB) are also analysed. The CTAB with appropriate concentration serves as the 'glue' that can link the {100} facets of two neighbour Au NPs, which leads to an anisotropic distribution of the residual surface charge, and this extrinsic electric dipole formation is responsible for the linear organization of the gold NPs into short chains.

Work on the synthesis and characterization of well-ordered mesostructured SiO_xN_y thin films (MSONTFs) on silicon wafers is described in this paper. The MSONTFs, characterized by high surface area, good order and uniform mesopores, were prepared by thermally treating the calcined mesoporous silica thin films (MSTFs) on silicon wafers in flowing ammonia at high temperatures. The nitrogen content of the MSONTFs was controlled by simply changing the nitridation temperature. At the same time, the specific surface area and wall thickness of the MSTFs all decreased with increasing nitridation temperature. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy confirmed that nitrogen was incorporated into the framework of the MSTFs. High-resolution transmission electron microscopy (HRTEM), small-angle x-ray diffraction (XRD) and N₂ sorption evidenced good structural ordering of the MSONTFs obtained. Scanning electron microscopy (SEM) images of cross sections and the surfaces of the samples showed that the MSONTFs on the Si substrates were continuous, homogeneous and did not crack, though there was evident shrinkage after the MSTFs were thermally treated in flowing ammonia at high temperatures.