Table of contents

We present a new method of realizing single nanocavities in individual colloidal particles on the surface of silicon dioxide artificial opals using a focused ion beam milling technique. We show that both the radius and the position of the nanocavity can be controlled with nanometre precision, to radii as small as 40 nm. The relation between the defect size and the milling time has been established. We confirmed that milling not only occurs on the surface of the spheres, but into and through them as well. We also show that an array of nanocavities can be fashioned. Structurally modified colloids have interesting potential applications in nanolithography, as well as in chemical sensing and solar cells, and as photonic crystal cavities.

The temperature dependence of the time-resolved photoluminescence (PL) of self-assembled InAs quantum dots (QDs) with InGaAs covering layers was investigated. The PL decay time increases with temperature from 50 to 170 K, and then decreases as the temperature increases further above 170 K. A model based on the phonon-assisted transition between the QD ground state and the continuum state is used to explain the temperature dependence of the PL decay time. This result suggests that the continuum states are important in the carrier capture in self-assembled InAs QDs.

The crosslinking of polymers in a polymeric material will alter the mechanical properties of the material. Control over the mechanical properties of polyelectrolyte multilayer films (PEMs) could be useful for applications of the technology in medicine and other areas. Disulfide bonds are 'natural' polypeptide crosslinks found widely in wild-type proteins. Here, we have designed and synthesized three pairs of oppositely charged 32mer polypeptide to have 0, 4, or 8 cysteine (Cys) residues per molecule, and we have characterized physical properties of the peptides in a PEM context. The average linear density of free thiol in the designed peptides was 0, 0.125, or 0.25 per amino acid residue. The peptides were used to make 10-bilayer PEMs by electrostatic layer-by-layer self-assembly (LBL). Cys was included in the peptides to study specific effects of disulfide bond formation on PEM properties. Features of film assembly have been found to depend on the amino acid sequence, as in protein folding. Following polypeptide self-assembly into multilayer films, Cys residues were disulfide-crosslinked under mild oxidizing conditions. The stability of the crosslinked films at acidic pH has been found to depend on the number of Cys residues per peptide for a given crosslinking procedure. Crosslinked and non-crosslinked films have been analysed by ultraviolet spectroscopy (UVS), ellipsometry, and atomic force microscopy (AFM) to characterize film assembly, surface morphology, and disassembly. A selective etching model of the disassembly process at acidic pH is proposed on the basis of the experimental data. In this model, regions of film in which the disulfide bond density is low are etched at a higher rate than regions where the density is high.

We present a comparison between the structural, chemical, and electrical properties of Mg-doped GaN nanowires grown by hot-wall chemical vapour deposition using two different Mg sources, namely, metallo-organic bis(methylcyclopentadienyl) magnesium and magnesium nitride powder. We find that Mg from the solid nitride source is more effectively incorporated into the nanowires while better maintaining the nanowire integrity. After Mg activation, the nanowires are partially or fully compensated. In comparison, vapour phase doping results in an obvious degradation of the nanowire morphology in spite of lower Mg incorporation levels.

We present the synthesis and dielectric characterization of core–shell LaMn_0.5Co_0.5O₃-based composites that we have prepared with the aim of improving the dielectric properties of the pure mixed oxide, especially reducing its high dielectric losses. For this purpose we have coated the LaMn_0.5Co_0.5O₃ particles with SiO₂ nanoshells ∼20 nm thick. From the practical point of view, the composite material obtained shows at room temperature a still rather high dielectric constant (ε'_r∼200 at ν = 10⁶ Hz) while its dielectric losses are at least one order of magnitude smaller than those of the base oxide (tanδ = 0.8 at ν = 10⁶ Hz).

Arrays of nickel and gold nanorods have been grown on glass and silicon substrates using porous alumina templates of less than 500 nm thickness. A method is demonstrated for varying the diameter of the nanorods whilst keeping the spacing constant. Optical extinction spectra for the gold nanorods show two distinct maxima associated with the transverse and longitudinal axes of the rods. Adding small quantities of oxygen to the aluminium before anodization is found to improve the sharpness of the extinction peaks. The spectral position of the longitudinal peak is shown to be sensitive to the nanorod diameter for constant length and spacing. For the nickel nanorods it is shown that the magnetic properties are governed by both interactions between the wires and shape anisotropy.

By using two-photon excitation spectroscopy we take advantage of the increased spatial resolution to perform time-integrated and time-resolved photoluminescence measurements on several discrete stacks of GaN quantum discs. The stack structure consisted of coupled asymmetric GaN quantum discs with embedded AlGaN barriers, which were grown at the tip of a GaN nanocolumn. We observed that with increasing optical excitation power the carrier lifetime decreased due to free-carrier screening, with an enhancement of the screening effect in the larger quantum disc due to carrier tunnelling from the smaller quantum disc.

The effects of interfacial bonding on mechanical properties of single-walled carbon nanotube reinforced copper matrix nanocomposites were investigated. The nanocomposites were fabricated by means of a powder metallurgy process, which consists of mixing carbon nanotubes with matrix powder followed by hot-pressing. The mixing process was carried out by ultrasonicating the nanotubes and copper powder in ethanol. The interfacial strength between the nanotubes and the copper matrix was improved by coating the nanotubes with nickel. The displacement rate of the nanotube reinforced nanocomposites was found to increase at 200 °C, whereas that of the nickel-coated nanotube reinforced nanocomposites significantly decreased. The incorporation of carbon nanotubes and nickel-coated carbon nanotubes in the copper matrix composites improved tribological properties compared with those of pure copper specimens.

We report a structure of (In, Ga)As/GaAs quantum dots which are vertically correlated and laterally aligned in a hexagonal way thus forming three-dimensionally ordered arrays. The growth pathway is based on a mechanism of self-assembly by strain-mediated multilayer vertical stacking on a planar GaAs(100) substrate, rather than molecular-beam epitaxy on a prepatterned substrate. The strain energy of lateral island–island interaction is minimum for the arrangement of hexagonal ordering. However, realization of hexagonal ordering not only depends on a complicated trade-off between lateral and vertical island–island interaction but is also related to a delicate and narrow growth kinetics window.

A convenient nanotechnique is used to place analyte molecules between closely spaced silver-capped Si nanowires for investigating surface-enhanced Raman scattering (SERS). It is revealed that the SERS intensity (or sensitivity) is closely related to the etching time used to prepare the Si nanowires from wafer. As the etching leaves the nominal spacing between the nanowires unaffected, the observed effect can be explained based on different gaps between the Ag particles due to the different lengths of the Si nanowires. Large SERS intensity for short etching times can be elucidated in terms of the rigidity of the nanowires and the smaller SERS intensities for longer etching times can be explained by considering the bending of nanowires and the agglomeration of the Ag caps due to gravity and van der Waals forces.

We report the growth of exceptionally well aligned and vertically oriented GaN nanowires on  r-plane sapphire wafers via metal–organic chemical vapour deposition. The nanowires were grown without the use of either a template or patterning. Transmission electron microscopy indicates the nanowires are single crystalline, free of threading dislocations, and have triangular cross-sections. The high degree of vertical alignment is explained by the crystallographic match between the  oriented nanowires and the r-plane sapphire surface. We find that the degree of alignment and size uniformity of the nanowires are highly dependent on the nickel nitrate catalyst concentration used, with the highest degree of uniformity and alignment occurring at concentrations much more dilute than typically employed for vapour–liquid–solid-based nanowire growth. Additionally, we report here a strong dependence of the optical and electrical properties of the nanowires on the growth temperature, which we hypothesize is due to increased carbon incorporation at lower growth temperatures.

The mechanisms of photoinduced charge transfer in composites of TiO₂ nanorods with a conjugated polymer (poly(2-methoxy-5-(2'-ethyl)(hexyloxy) 1,4-phenylenevinylene) (MEH-PPV) have been investigated by steady-state, time-resolved photoluminescence (PL) spectroscopy and photoluminescence excitation (PLE) spectroscopy. Efficient charge separation takes place at the TiO₂-nanorod/polymer interfaces when the polymer is excited, leading to quenching of the photoluminescence efficiency η and shortening of the measured lifetime τ_PL. In addition, the low-temperature absorption and photoluminescence spectra show that the inclusion of TiO₂ nanorods in polymer can reduce disorder in conformation and enhance conjugation in the polymer chain. A photovoltaic solar cell device based on the MEH-PPV/TiO₂-nanorod composite material is also presented, which shows a two order increase in short-circuit current J_SC compared to that based on the pristine MEH-PPV.

Upconverting materials, which can be efficiently excited by near infrared light and emit strong visible light through a process termed 'upconversion fluorescence', have shown great potential for use in biological labelling and imaging. Some upconverting nanoparticles such as NaYF₄ doped with lanthanide ions have been synthesized; however, these nanoparticles are not soluble in water, not biocompatible and do not have functional chemical groups for conjugation of biomolecules, and as a result their bioapplications are very limited unless some surface modifications are performed. Here we report a method for one-pot synthesis of polyethylenimine/NaYF₄ nanoparticles doped with lanthanide ions, which are water soluble and biocompatible. The amino groups of polyethylenimine existing on the nanoparticles can be used for attachment of biomolecules. The nanoparticles showed a spherical shape with an average size of about 50 nm. Different lanthanide ions (Yb³⁺, Er³⁺ and Tm³⁺) were doped into the nanoparticles, which showed strong upconversion fluorescence of different colours in aqueous solutions under excitation at 980 nm.

We report a method of fabricating nanotextured Ag surfaces using a template of self-assembled inorganic-containing block copolymer, polystyrene-b-polyferrocenylsilane. The Ag surfaces with periodically ordered nanoscale features created by the self-organized block copolymer are capable of producing enhanced Raman signals. Using benzenethiol as a probe molecule, an enhancement factor of up to 10⁶ has been observed. More importantly, the enhancement is very uniform; less than 10% Raman signal variation has been obtained. Furthermore, since the size and spacing of the nanostructures can be adjusted by tailoring the polymer chain length, the electromagnetic field can potentially be tuned to achieve even higher surface-enhanced Raman scattering (SERS) activity. This inorganic-containing block copolymer template approach not only provides a simple and straightforward method to fabricate SERS active substrates but also offers a means to experimentally examine the SERS mechanism.

Carbon nitride nanostructures have been created using the novel technique of liquid phase pulsed laser ablation of a graphite target in ammonia solution. For ablation times between 1 and 7 h, 'leaf-shaped' nanostructures are formed by self-assembly of carbon nitride nanorods. With increasing ablation time, the crystallinity of the nanorod building blocks improves, and the length of the nanoleaf structures increases while the width remains constant. X-ray photoelectron spectroscopy results confirmed that nitrogen is chemically incorporated into the carbon network. The N–C (sp²) bonding increases with increasing ablation time, whereas the N–C (sp) and N–C (sp³) bonding behaviour is more complex. The optical properties for these leaf-like structures as a function of ablation time are also discussed.

A multi-scale model is presented that captures the experimentally observed behaviour of electroluminescence (EL) in carbon nanotube field-effect transistors (CNFETs) under ambipolar bias conditions, namely variations in mobile EL intensity, localized EL at a contact, and localized EL at a charge defect. A full, quantum mechanical approach is used to describe tunnelling and thermionic emission at the contacts, and the drift-diffusion equations, with a field-dependent mobility, are used for transport in the long devices (CN length ≥10 µm). We find that contact-localized EL is only present when the height of the Schottky barrier at the ends of the CN favours the injection of one type of carrier. Charge defects on the CN surface also lead to localized EL, which is present only under certain bias conditions.

A new γ-Fe₂O₃ MION ferrofluid has been developed with a salt-assisted solid-state reaction. Characterizations show that the ferrofluid is composed of maghemite nanoparticles with a mean diameter of 2.7 nm. Though the nanoparticles are ultrafine, they are well crystallized, with a saturation magnetization value of 34.7 emu g⁻¹, making them suitable for MRI applications. In spite of the absence of any surfactant, the ferrofluid can be stable for more than 6 months. An in vitro cytotoxicity test revealed good biocompatibility of the maghemite nanoparticles, suggesting that they may be further explored for biomedical applications. NMR measurements revealed significantly reduced water proton relaxation times T₁ and T₂. The MR images of the nanoparticles in aqueous dispersion were investigated using a 3 T clinical MR imager. These preliminary experiments have demonstrated the potential of the as-synthesized ultrafine, cap-free maghemite MIONs in functional molecular imaging for biomedical research and clinical diagnosis.

Oridonin, a lipophilic Chinese medicine, has very low oral bioavailability due to its poor solubility. Solid lipid nanoparticle (SLN) delivery systems of oridonin have been formed using stearic acid, soybean lecithin and pluronic F₆₈ in our studies to overcome this problem. Emulsion evaporation–solidification at low temperature was used to prepare SLN dispersions. The particle size and morphology were examined by transmission electron microscopy (TEM), and the zeta potential was measured by a television micro-electrophoresis apparatus. Process and formulation variables have been studied and optimized on the basis of entrapment efficiency. Differential scanning calorimetry (DSC) and powder x-ray diffraction (PXRD) studies were performed to characterize the state of the drug. In vitro release studies were performed in phosphate-buffer solution (PBS) (pH 7.4). The tissue distribution in mice and the pharmacokinetics in rabbits were studied to evaluate the tissue targeted property of SLNs. Stable SLN formulations of oridonin having a mean size range of 15–35 nm and mean zeta potential −45.07 mV were developed. More than 40% oridonin was entrapped in SLNs. DSC and PXRD analysis showed that oridonin is dispersed in SLNs in an amorphous state. The release pattern of the drug was analysed and found to follow the Higuchi equations. In vivo studies demonstrated that oridonin-loaded SLNs obviously increased the concentration of oridonin in liver, lung and spleen, while its distribution in heart and kidney decreased.

Electrospinning is a relatively simple and versatile method to produce polymer nanofibres and their composites. In this work, functionalized multiwalled carbon nanotubes (f-MWNTs) were used for the fabrication of conducting nanocomposite fibres, in comparison with the composite nanofibres made of unfunctionalized MWNTs (u-MWNTs). Our results showed that the addition of f-MWNTs could improve the dispersion of carbon nanotubes in the polymer solution and therefore result in composite nanofibres with uniform diameters by electrospinning. Alignment of the composite nanofibres was achieved by using a rotating drum as the collector. F-MWNTs were found to align parallel to the axis direction of the nanofibres. DC electrical properties of a single composite fibre were investigated at room temperature as well as cryogenic states (100–300 K). An electrical percolation phenomenon was observed for nanofibres with different mass fractions of MWNTs. It was shown that the conductivity of the material could be significantly improved above the percolation threshold. The conductivity could be of several orders of magnitude higher than the pure PVAc.

A method for synthesizing multiwalled carbon nanotubes (MWNTs) beaded with Cu₂O nanospheres has been developed. In this work Cu(CH₃COO)₂·H₂O served as the precursor and NaBH₄ as the reducing agent, and the produced Cu₂O nanoparticles were deposited on the MWNTs with the help of poly(vinyl pyrrolidone) (PVP). After increasing the ageing time, grapelike Cu₂O nanospheres adhering to the MWNTs gradually came into being. The synthesized products were systematically studied by x-ray powder diffraction, scanning electron microscopy and transmission electron microscopy. UV–vis absorption spectra were used to estimate the band gap energies of the hybrid system. The synthesis process was facile, efficient and not time-consuming. This method may shed light on integrating MWNTs with other oxide nanospheres and/or nanoparticles using a PVP medium, and lead to an exploration of the potential properties of nanocomposite hybrid materials.

In this paper, through a simple and fast electroless metal deposition route, gold dendritic nanostructures are synthesized in aqueous conditions. The gold dendrites with a threefold symmetric characteristic were built up of numerous nanoparticles roughly 5–10 nm in size. The aggregated nanoparticles spontaneously experience a self-assembly process along crystallographic orientations and finally form a monocrystalline dendrite. An oriented attachment mechanism can be used to explain the nanoparticle-aggregated self-assembly process.

Periodical alignment of the InAs dots along the and directions was observed on an elastically relaxed InGaAs buffer layer grown at 500 and 450 °C, respectively, on the vicinal GaAs(001) substrate. Due to alignment along these directions, the InAs dots were arranged into a quasi-two-dimensional hexagonal lattice. Such a periodical arrangement of InAs dots may be explained in terms of modulation in strain as well as composition along [110] as observed by using cross-sectional transmission electron microscopy.

We report a simple method for producing magnetic nanoparticles with enhanced maghemite (γ-Fe₂O₃) to haematite (α-Fe₂O₃) phase transition temperature. By controlling the properties of the solvent media, we have been able to tune the particle size from 2.3 to 6.5 nm. Nanoparticles with higher transition temperatures have been achieved by using NaOH as an alkali during the co-precipitation. The maghemite to haematite phase transition was complete at a temperature below 600 °C for nanoparticles prepared using ammonia, whereas the phase transition was not complete until 750 °C for the samples prepared with NaOH. The increase in average particle size after heat treatment at 600 °C is attributed to coalescence of particles by solid state diffusion, where the system reduces its free energy by reducing the surface area. The final particle diameters of the haematite after the heat treatment were 35.4, 31.38 and 26.85 nm respectively for nanoparticles of initial diameters 5, 7 and 9.8 nm. Our studies on the effect of the initial particle size on the transition temperature show that the transition temperature decreases with decreasing particle size due to the reduced activation energy of the system.

Crystalline β-Ga₂O₃ nanorods have been prepared from GaCl₃ and humid SBA-15 microparticles (SMP) at low temperature (280–300 °C) using the solvothermal method. The as-synthesized product was characterized by x-ray powder diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and room-temperature photoluminescence (PL). Formation mechanisms related to the β-Ga₂O₃ nanorods on SMP are suggested. The room-temperature PL spectrum of the β-Ga₂O₃ nanorods shows a strong blue luminescence (BL) band centred at 400 nm, which is ascribed to be the recombination of surface states.

We report from ab initio calculations that the band-gap sensitive side-wall functionalization of a carbon nanotube is feasible with the fluorine molecule (F₂), which can provide a route to the extraction of semiconducting nanotubes by etching away metallic ones. In the small diameter cases like (11, 0) and (12, 0), the nanotubes are easily functionalized with F₂ regardless of their electronic properties. As the diameter becomes larger, however, the fluorination is favoured on metallic CNTs with smaller activation barriers than those of semiconducting ones. Our results suggest that low-temperature exposure to F₂ molecules in the gas phase can make a dominant portion of fluorinated metallic nanotubes and unfluorinated semiconducting ones. This is consistent with recent experimental reports.

We present the results of calculations performed to model the process of lateral manipulation of an oxygen vacancy in the MgO(001) surface using the non-contact atomic force microscope (NC-AFM). The potential energy surfaces for the manipulation as a function of tip position are determined from atomistic modelling of the MgO(001) surface interacting with a Mg terminated MgO tip. These energies are then used to model the dynamical evolution of the system as the tip oscillates and at a finite temperature using a kinetic Monte Carlo method. The manipulation process is strongly dependent on the lateral position of the tip and the system temperature. It is also found that the expectation value of the point at which the vacancy jumps depends on the trajectory of the oscillating cantilever as the surface is approached. The effect of the manipulation on the operation of the NC-AFM is modelled with a virtual dynamic AFM, which explicitly simulates the entire experimental instrumentation and control loops. We show how measurable experimental signals can result from a single controlled atomic scale event and suggest the most favourable conditions for achieving successful atomic scale manipulation experimentally.

The surface treatment of thiol-capped CdTe quantum dots (QDs) was carried out with a small amount of sodium borohydride (NaBH₄) in aqueous solution at room temperature. The treatment effectively enhanced the photostability of QDs and increased the photoluminescence (PL) efficiency by a factor of two as against the original ones. By measuring the PL trajectories of single QDs with a total internal reflection fluorescence microscope under the same irradiation density of 0.72 µW µm⁻² at 532 nm, the photostable lifetimes were determined to be 15.2 ± 5.9 s for surface-treated QDs, and 5.8 ± 1.9 s for the original ones, respectively. Both the original and treated QDs could be ingested into human hepatocellular carcinoma cells (QGY) by incubation. The photobleaching of cellular QDs was measured with a confocal microscope. Remarkable enhancement was found for the photostability of the surface-treated QDs in QGY cells, demonstrating its advantage for cellular labelling applications.

Ammonium tetrafluoroborate, a cheap and commonly used chemical, was successfully utilized in this study to synthesize BN-coated MgF₂ nanowires and collapsed BN nanotubes, although previous investigations have indicated that the compound cannot be used as a chemical vapour deposition (CVD) reaction precursor of amorphous or crystalline BN. Our study reveals that when MgCl₂ is used as a promoter a crystalline BN phase can be obtained on a large scale. The MgCl₂ also controls the product morphology, resulting in collapsed BN-coated MgF₂ nanowires at the first stage of CVD. The detailed morphology of the composite nanowires also depends on the reaction temperature. Increase in temperature stimulates the nanowire formation. The MgF₂ inclusions can be fully removed via a simple high-temperature evaporation procedure, forming collapsed BN nanotubes of high purity and yield.

Ga⁺ focused ion beam (FIB) patterning was used to structure highly oriented pyrolytic graphite surfaces with square, periodic arrays of amorphous carbon defects (mesh sizes: 300 nm–2 µm). Controlled oxygen etching of these arrays leads to matrices of uniform, orientationally aligned, nm-sized, hexagonal holes. The properties of the resulting hole assembly (hole depths and lateral hole dimensions) have been investigated by means of atomic force microscopy, scanning electron microscopy and FIB sectioning. The hole dimensions and uniformity both depend on the FIB parameters and etching conditions. Etching temperatures from 500 to 700 °C were applied. Initial etch rates of up to 10⁶ C s⁻¹ per individual hole were observed when using oxygen pressures of 200 mbar. For an etch temperature of 590 °C the rate of etching of individual holes was found to depend measurably on the inter-hole separation. This confirms that the associated reaction kinetics is mediated by the finite diffusion length of reactive oxygen species along the graphite basal plane. Prolonged etching results in hole–hole contact and generation of mesa arrays of controllable size and shape.

The effect of a focusing electric field on the formation of carbon nanotubes in a direct current arc-plasma is investigated. The hard deposits on the surface of the cathode are the main products, rich in multi-walled carbon nanotubes. It is seen that the focusing electric field has a distinct influence on the yield, purity and morphology of the nanotubes. The yield of the carbon nanotubes under the 'focused field condition' has been found to be higher than that derived from the normal electrode configuration. It has been observed that the deposition of carbonaceous soot on the reactor wall is considerably reduced on application of the focusing electric field. Transmission electron microscopy has been used to determine the morphology of the nanotubes. In addition, Raman spectroscopy has helped in distinguishing the graphene-like structures from the disordered carbon networks and helped in analysing the morphology of the tubes. Thermal analysis gave a qualitative estimation of the relative yield of carbon nanotubes within the cathode deposits and their thermal stabilities. The crystalline nature of the samples has been confirmed by x-ray diffraction analysis. The results clearly indicate that the focusing electric field confines the positively charged carbon precursors within the cathode–anode space causing high relative yield and purity and has a distinct effect on controlling the inner diameter of the as-synthesized carbon nanotubes.