Table of contents

Colloidal quantum dots are semiconductor nanocrystals well dispersed in a solvent. The optical properties of quantum dots, in particular the wavelength of their fluorescence, depend strongly on their size. Because of their reduced tendency to photobleach, colloidal quantum dots are interesting fluorescence probes for all types of labelling studies. In this review we will give an overview on how quantum dots have been used so far in cell biology. In particular we will discuss the biologically relevant properties of quantum dots and focus on four topics: labelling of cellular structures and receptors with quantum dots, incorporation of quantum dots by living cells, tracking the path and the fate of individual cells using quantum dot labels, and quantum dots as contrast agents.

PbS nanocrystals are synthesized using colloidal techniques and have their surfaces capped with oleic acid. The absorption band edge of the PbS nanocrystals is tuned between 900 and 580 nm. The PbS nanocrystals exhibit tuneable photoluminescence with large non-resonant Stokes shifts of up to 500 meV. The magnitude of the Stokes shift is found to be dependent upon the size of PbS nanocrystals. Time-resolved photoluminescence spectroscopy of the PbS nanocrystals reveals that the photoluminescence has an extraordinarily long lifetime of 1 µs. This long fluorescence lifetime is attributed to the effect of dielectric screening similar to that observed in other IV–VI semiconductor nanocrystals.

In this paper we describe the preparation of homogeneously needle-shaped cobalt ferrite (CoFe₂O₄) nanocrystals on a large scale through the smooth decomposition of urea and the resulting co-precipitation of Co²⁺ and Fe³⁺ in oleic acid micelles. Furthermore, we found that other ferrite nanocrystals with a needle-like shape, such as zinc ferrite (ZnFe₂O₄) and nickel ferrite (NiFe₂O₄), can be prepared by the same process. Needle-shaped CoFe₂O₄ nanocrystals dispersed in an aqueous solution containing oleic acid exhibit excellent stability and the formed colloid does not produce any precipitations after two months, which is of prime importance if these materials are applied in magnetic fluids. X-ray diffraction (XRD) measurements were used to characterize the phase and component of the co-precipitation products, and demonstrate that they are spinel ferrite with a cubic symmetry. Transmission electron microscopy (TEM) observation showed that all the nanocrystals present a needle-like shape with a 22 nm short axis and an aspect ratio of around 6. Varying the concentration of oleic acid did not bring about any obvious influence on the size distribution and shapes of CoFe₂O₄. The magnetic properties of the needle-shaped CoFe₂O₄ nanocrystals were evaluated by using a vibrating sample magnetometer (VSM), electron paramagnetic resonance (EPR), and a Mössbauer spectrometer, and the results all demonstrated that CoFe₂O₄ nanocrystals were superparamagnetic at room temperature.

The mechanical properties of sliding carbon nanotubes have been investigated by classical molecular dynamics simulations in the canonical ensemble. In particular we have studied damped oscillations in the separation between the centres of mass of the inner and outer tubes of double-walled carbon nanotubes (DWCN). Incommensurate DWCNs forming (7, 0)@(9, 9) structures were simulated for systems at 298.15 K with axial lengths from 12.21 to 98.24 nm. The oscillations exhibited frequencies in the range of gigahertz with the frequency decreasing as the length of the system increases. The time until oscillations become negligible exhibited a nearly linear dependence on the length of the system. Two macroscopic models were developed in order to understand the forces involved in terms of macroscopic properties like friction and shear. The first model considered constant restoring forces during the whole event, while in the second the value of these constant restoring forces depended on the initial conditions of each oscillation. Both models reproduced the oscillations quite well, while the second model allows us to predict the dynamic shear strength in terms of the axial length of the system for tubes with the same diameters. The calculated dynamic shear strength exhibited monotonic behaviour with an inverse dependence on the length of the system. For systems with unequal axial lengths, the restoring force, which drives the oscillation, is reduced compared to the system with equal lengths, regardless of whether the outer nanotube is longer or shorter.

We propose a step-by-step experimental procedure for characterization of the nonlinear contact stiffness on surfaces using contact-mode atomic force microscopy. Our approach directly estimates the first-, second-, and third-order coefficients of the contact stiffness. It neither uses nor requires the underlying assumptions of the Hertzian contact theory. We use a primary resonance excitation of the probe to estimate the linear coefficient of the contact stiffness. We use the method of multiple scales to obtain closed-form expressions approximating the response of the probe to a subharmonic resonance excitation of order one-half. We utilize these expressions and higher-order spectral measurements to independently estimate the quadratic and cubic coefficients of the contact stiffness.

An electrospinning method was used to spin semi-crystalline poly(L-lactide) (PLLA) nanofibres. Processing parameter effects on the internal molecular structure of electrospun PLLA fibres were investigated by x-ray diffraction (XRD) and differential scanning calorimetry (DSC). Take-up velocity was found as a dominant parameter to induce a highly ordered molecular structure in the electrospun PLLA fibres compared to solution conductivity and polymer concentration, although these two parameters played an important role in controlling the fibre diameter. A collecting method of a single nanofibre by an electrospinning process was developed for the tensile tests to investigate structure–property relationships of the polymer nanofibres. The tensile test results indicated that higher take-up velocity caused higher tensile modulus and strength due to the ordered structure developed through the process.

The magnetic properties of the system MnZnO prepared by conventional ceramic procedures using ZnO and MnO₂ starting powders are studied and related to the nanostructure. Thermal treatment at 500 °C produces a ferromagnetic phase, although this temperature is not high enough to promote proper sintering; thus the thermally treated compact shows brittle characteristics of unreacted and poorly densified ceramic samples. Scanning electron microscopy and x-ray analysis reveal the appearance of a new phase, most probably related to the diffusion of Zn into MnO₂ oxide nanocrystals. The magnetic properties deviate considerably from what would be expected of an unreacted mixture of ZnO (diamagnetic) and MnO₂ particles (paramagnetic above 100 K and anti-ferromagnetic below that temperature), exhibiting a ferromagnetic like behaviour from 5 to 300 K and beyond mixed with a paramagnetic component. The ferromagnetic phase seems to be originated by diffusion at the nanoscale of Zn into MnO₂ grains. The Curie temperature of the ferromagnetic phase, once the paramagnetic component has been subtracted from the hysteresis loops, is measured to be 450 K. EPR resonance experiments from 100 to 600 K confirm a ferromagnetic to paramagnetic like transition above room temperature for these materials.

A great step forward in science and technology was made when it was discovered that lattice mismatch can be used to grow highly ordered, artificial atom-like structures called self-assembled quantum dots. Several groups have in the meantime successfully demonstrated useful infrared photodetection devices which are based on this technology. The new physics is fascinating, and there is no doubt that many new applications will be found when we have developed a better understanding of the underlying physical processes, and in particular when we have learned how to integrate the exciting new developments made in nanoscopic addressing and molecular self-assembly methods with semiconducting dots. In this paper we examine the scientific and technical questions encountered in current state of the art infrared detector technology and suggest ways of overcoming these difficulties. Promoting simple physical pictures, we focus in particular on the problem of high temperature detector operation and discuss the origin of dark current, noise, and photoresponse.

The electronic structure and the conductance of a carbon nanotube based metal/semiconductor/metal intramolecular junction is investigated numerically. The nature of electronic states at the interfaces and in the semiconductor section is analysed. The quantum conductance of the system is calculated in the coherent regime and its variations with energy and length are shown to be related to contributions from different kinds of electronic state.

We present a comparison of three different methods to calibrate the spring constant of two different types of silicon beam shaped atomic force microscope (AFM) cantilevers to determine each method's accuracy, ease of use and potential destructiveness. The majority of research in calibrating AFM cantilevers has been concerned with contact mode levers. The two types of levers we have studied are used in force modulation and tapping mode in air. Not only can these types of cantilevers have spring constants an order of magnitude greater than contact mode levers, but also their geometries can be quite different from the standard V-shape contact lever. In this work we experimentally determine the correction factors for two of the calibration methods when applied to the tapping mode cantilevers and also demonstrate that the force modulation levers can be calibrated easily and accurately using these same techniques.

The conductance of a single 4,4 bipyridine (44BPD) molecule connected to two gold electrodes is calculated using a density functional theory based Green function method. The atomic geometry of such a molecular junction is constructed from the optimized structure of a gold trimer–44BPD–gold trimer complex. Resonant conduction is the main feature of its transport properties. The magnitude of the transmission coefficient at the Fermi level is determined to be T = 1.01 × 10⁻², which is in excellent agreement with the experimental value. The dependence of the transmission on the Au–N bond length and the torsion angle is also discussed.

α- and β-MnO₂ single-crystal nanowires have been successfully synthesized through a soft chemical process, which involves no catalysts or templates. The diameters of the nanowires are about 10–40 nm and the lengths up to several tens of micrometres. Experimental results showed that MnO₂ polymorphic forms can be synthesized by selected-control reaction temperatures and their crystal phases in the nanowires were confirmed by XRD and TEM measurement. Further, the formation process of nanowires is discussed.

Using the Stillinger–Weber (SW) potential model, we have performed molecular dynamics (MD) simulations to investigate the melting of silicon nanoclusters comprising a maximum of 9041 atoms. This study investigates the size, surface energy and root mean square displacement (RMSD) characteristics of the silicon nanoclusters as they undergo a heating process. The numerical results reveal that an intermediate nanocrystal regime exists for clusters with more than 357 atoms. Within this regime, a linear relationship exists between the cluster size and its melting temperature. It is found that melting of the silicon nanoclusters commences at the surface and that T_m,N = T_m,Bulk−αN^−1/3. Therefore, the extrapolated melting temperature of the bulk with a surface decreases from T_m,Bulk = 1821 K to a value of T_m,357 = 1380 K at the lower limit of the intermediate nanocrystal regime.

Uniform arrays of nano-scale electrolyte–molecule–silicon capacitors have been successfully fabricated. This was done by a combination of reactive ion etch and a selective wet etch through an anodic aluminium oxide mask to form nano-holes in silicon oxide/silicon nitride insulator layers on silicon. Self-assembled monolayers of 4-ferrocenylbenzyl alcohol were then attached to the exposed silicon surfaces at the bottom of the nano-holes. Characterization by conventional capacitance and conductance techniques showed very high capacitance and conductance peaks near −0.6 V, that were attributed to the charging and discharging of electrons into and from discrete levels in the monolayer owing to the presence of the redox-active ferrocenes.

Scanning probe microscope (SPM) oxidation was used to form zirconium oxide features on 200 nm thick ZrN films. The features exhibit rapid yet controlled growth kinetics, even in contact mode with 70 V dc applied between the probe tip and substrate. The features grown for times longer than 10 s are higher than 200 nm, and reach more than 1000 nm in height after 300 s. Long-time oxidation experiments and selective etching of the oxides and nitrides lead us to propose that as the oxidation reaches the silicon substrate, delamination occurs with the simultaneous formation of a thin layer of new material at the ZrN/Si interface. High-voltage oxide growth on ZrN is fast and sustainable, and the robust oxide features are promising candidates for multiscale (nanometre-to-micrometre) applications.

Hollow polyhedra and cubes of nanostructured Cu₂O particles have been synthesized by reduction of CuSO₄ with ascorbate acid in the solution phase. The nanostructures were obtained when the cetyltrimethylammonium (CTAB) concentration ranged from 0 to 0.03 M in the presence of NaOH. Structural characterizations, by means of x-ray photoelectron spectroscopy (XPS) for measuring Cu valence states and by electron microscopy for microstructure and chemical analyses, suggest that most Cu₂O nanoparticles are covered with a thin CuO shell arising possibly from reaction of the adsorbed oxygen on the Cu₂O particle surface. The blue shift is observed as microstructures of Cu₂O nanoparticles changed from cubic to hollow in ultraviolet and visible (UV–visible) absorption spectra. Both the Cu₂O hollow and cubic nanostructures show certain quantum-confined effects. A cationic CTAB template mechanism is proposed to interpret the formation of the Cu₂O nanoparticles.

The thermal annealing conditions in nitrogen ambient for the self-synthesis of tungsten carbide nanowires from sputter-deposited WC_x films were investigated. Experimental results show that the temperature window for the growth of nanowires lies in the range of 500–750 °C with the corresponding annealing time interval ranging from 2.5 to 0.25 h. The diameter, length, and density of the grown nanowires are in the range of 10–15 nm, 0.1–0.3 µm, and 210–410 µm⁻², respectively. The degree of carbon depletion in the annealed WC_x films plays a crucial role in determining both the shape and density of the self-synthesized nanowires. Nanowires synthesized at lower temperatures were seen to be smaller in dimension but higher in density. Material analysis reveals that the phase transition from WC to W₂C arising from decarburization of the WC_x film during thermal annealing should be responsible for the self-synthesis of nanowires.

We present Raman scattering and scanning tunnelling microscopy (STM) measurements on hydrogen plasma etched single-wall carbon nanotubes (SWNTs). Interestingly, both the STM and Raman spectroscopy show that the metallic SWNTs are dramatically altered and highly defected by the plasma treatment. In addition, structural characterizations show that metal catalysts are detached from the ends of the SWNT bundles. For semiconducting SWNTs we observe no feature of defects or etching along the nanotubes. Raman spectra in the radial breathing mode region of plasma-treated SWNT material show that most of the tubes are semiconducting. These results show that hydrogen plasma treatment favours etching of metallic nanotubes over semiconducting ones and therefore could be used to tailor the electronic properties of SWNT raw materials.

LaPO₄ and CePO₄ nanorods/nanowires with controlled aspect ratios have been successfully synthesized using a hydrothermal microemulsion method under mild conditions. It has been shown that the obtained LaPO₄ has a monoclinic structure, while CePO₄ exists in the hexagonal structure. Uniform nanorods/nanowires with diameters of 20–60 nm and lengths ranging from several hundreds of nanometres to several micrometres were obtained. The aspect ratios of the obtained 1D nanostructures can be fine-tuned by simply changing the [H₂O ]/[surfactant] molar ratios. The possible growth mechanism of LaPO₄ and CePO₄ nanorods/nanowires was explored in detail.

CoPt nanoparticles with an average size of 3 nm and narrow distribution were synthesized by chemical reduction of Co(CH₃COO)₂ and Pt(acac)₂ by polyethyleneglycol-200. The as-prepared nanoparticles have a disordered fcc structure which transformed after thermal treatment to an ordered fct structure, which results in coercivity up to 6 kOe at room temperature and 9 kOe at 5 K because of the high magnetocrystalline anisotropy of the tetragonal structure .

We demonstrate selective growth of vertically aligned ZnO nanowires on a (100) Si substrate using a patterned thin film ZnO seed layer. Metal catalysts, which can be a source of contamination, were not used. A single-crystalline structure with c-axis preferred orientation and a strong intrinsic near band edge photoluminescence peak at 380 nm with no detectable visible photoluminescence indicate a lack of defects and the high quality of the ZnO nanowires.

ZnO nanowires have been synthesized on porous silicon substrates with different porosities via the vapour–liquid–solid method. The texture coefficient analysed from the XRD spectra indicates that the nanowires are more highly orientated on the appropriate porosity of porous silicon substrate than on the smooth surface of silicon. The Raman spectrum reveals the high quality of the ZnO nanowires. From the temperature-dependent photoluminescence spectra, we deduced the activation energies of free and bound excitons.

Different configurations for BN nanocones with 240° disclination are analysed through first-principles calculations based on the density-functional theory. The studied cone tips involve either four pentagons with two homonuclear bonds or two squares and two hexagons with no homonuclear bonds. The structures are both terminated by two three-coordinated atoms. Cohesive energy results show that the cone terminated in pentagons is slightly energetically preferred. The influence of a 1.7 V Å⁻¹ external electric field, applied along the axis, on the structural changes and charge distributions is analysed. The responses of the charge distributions to the external electric field demonstrate the main role played by the B atoms for the applications of BN cones as probes in electronic microscopy as well as field emitters.

Modulation-doped GaAs v-groove quantum wires (QWRs) have been fabricated with novel electrical contacts made to two-dimensional electron-gas (2DEG) reservoirs. Here, we present longitudinal photocurrent (photoconductivity/PC) spectroscopy measurements of a single QWR. We clearly observe conductance in the ground-state one-dimensional subbands; in addition, a highly temperature-dependent response is seen from other structures within the v-groove. The latter phenomenon is attributed to the effects of structural topography and localization on carrier relaxation. The results of power-dependent PC measurements suggest that the QWR behaves as a series of weakly interacting localized states, at low temperatures.

Electro-orientation of rod-like particles in liquids, under the application of an external AC field, is analysed. A rod shape is suitable for particle light valve (PLV) applications. When they are aligned with their long axes parallel to the electric field (and the direction of light is assumed to be parallel to the applied electric field), then it can lead to good transmission of light. Various criteria to arrive at appropriate parameters for PLV applications are proposed. It is found that good electric conductors are excellent rod materials for PLV applications. They lead to an appropriate orientation of the rods and at the same time result in maximum orientational torque. Water-like liquids with higher values of permittivity are appropriate choices as suspending liquids since the Brownian dispersion in the presence of the electric field is minimized. The time it takes the rods to fully diffuse in the orientational space, once the electric field is turned off, decreases with decreasing liquid viscosity.

ZnO films were grown on polycrystalline Zn foil by cathodic electrodeposition in an aqueous zinc chloride/calcium chloride solution at 80 °C. Variation in the electrochemical parameters resulted in a variation in growth morphology from 1D (nanorods), 2D ('nanoplates') to 3D crystal growth. An as-received or mechanically polished substrate proved the most suitable substrate finish and allowed more highly aligned, dense structures to be grown; in contrast, electropolished substrates formed inhomogeneous deposits. Substrate annealing gave rise to large homogenous areas of nanorod deposition. Two-dimensional sheet growth was found to occur in conjunction with nanorods under specific electrochemical conditions. Hexagonal 'plates' approximately 50 nm in thickness and several microns in diameter were formed normal to the substrate.

A phenomenological study of the phase transitions occurring in the adsorption systems Pb/Ge(111) and Sn/Ge(111) is presented. The starting point of such a study is the Landau theory. The critical behaviour expected theoretically for the two interfaces, and the corresponding influence of defects, are discussed in detail. Symmetry arguments show that, contrary to general belief, the critical behaviours of Pb/Ge(111) and Sn/Ge(111) are essentially different. The Landau-like approach employed to study the influence of defects provides a consistent and general manner to interpret the existing experimental data. Special attention is paid to the influence of hopping defects in Sn/Ge(111).

The optical and electrical properties of semiconductor nanoparticles are strongly dependent on their size. A flexible control of the size of the nanoparticles is of interest for tuning their properties for different applications. Here we use a coupled method to control the size of CdS nanoparticles. The method involves the photochemical growth of CdS nanoparticles together with the use of a capping agent as an inhibiting factor. CdS nanoparticles were formed through a photoinduced reaction of CdSO₄ and Na₂S₂O₃ in an aqueous solution. Mercaptoethanol (C₂H₆OS) was used as the capping agent, and we investigated the effect of illumination time, illumination intensity and the concentration of capping agent on the nanoparticle size. Transmission electron microscopy (TEM) shows crystalline nanoparticles with relatively low dispersion. Optical absorption spectroscopy was mainly used to measure the band gap and size of the nanoparticles. Increasing the illumination time or illumination intensity increases the nanoparticle size, while higher capping agent concentration leads to smaller nanoparticle size. A band gap range of 2.75–3.4 eV was possible with our experimental conditions, corresponding to a 3.2–6.0 nm size range.