Table of contents

Due to their interesting properties, research on colloidal nanocrystals has moved in the last few years from fundamental research to first applications in materials science and life sciences. In this review some recent biological applications of colloidal nanocrystals are discussed, without going into biological or chemical details. First, the properties of colloidal nanocrystals and how they can be synthesized are described. Second, the conjugation of nanocrystals with biological molecules is discussed. And third, three different biological applications are introduced: (i) the arrangement of nanocrystal–oligonucleotide conjugates using molecular scaffolds such as single-stranded DNA, (ii) the use of nanocrystal–protein conjugates as fluorescent probes for cellular imaging, and (iii) a motility assay based on the uptake of nanocrystals by living cells.

A systematic characterization of single-wall carbon nanotube (SWCNT) material after successive purification steps, including reflux treatment with nitric acid, air oxidation, and annealing, has been performed. Inductively coupled plasma–optical emission spectroscopy shows that a considerable reduction of the metal impurities by up to 95% can be obtained by the nitric acid reflux treatment. During this process, Raman spectroscopy clearly proves that HNO₃ molecules are intercalated into the bundles of SWCNTs. At the same time, SWCNTs have suffered a high degree of degradation and defects are being introduced. The subsequent thermal processes lead to the removal of further defect carbon materials and to the almost complete de-intercalation of the HNO₃ molecules. Transmission electron microscopy reveals that the remaining SWCNT bundles tend to form thick bundles. Thus the applied purification process results in a high-purity SWCNT material with a drastically reduced content of metal nanoparticles and composed of large bundles of SWCNTs.

The phase stability of nanograined Fe–12Ni powders is studied by means of both experiment and theoretical calculations. By means of a magnetic field accompanied by an ultrasonic vibration dispersal, the nanograined Fe–12Ni powders synthesized by the gas condensation were successfully separated into a face-centred-cubic (fcc, γ-) phase with a grain size distribution ranging from 20 to 50 nm and a body-centred-cubic (bcc, α-) phase with a grain size less than 30 nm. The experimental results show that the most significant factor affecting the phase stability of the nanograined powders is the grain size. The critical grain size (d^∗) for the fcc phase to exist at 300 K was quantitatively calculated on the basis of a dilated crystal model and the effect of magnetism energy was considered in the calculations. The calculated d^∗ is 27 nm when the excess volume for nanograined Fe–Ni (ΔV) is taken as 0.1, so the calculated d^∗ is comparable to the experimental result. The calculation results for the nucleation barrier to fcc-to-bcc phase transformation indicate that the grain size plays an important role in the fcc phase stability of nanograined Fe–12Ni powders.

The Ni/Cu(001) metallic interface shows interesting magnetic properties due to the lattice misfit. Its components exhibit a misfit of 2.6% in their lattice parameters. Hence, the growing thin film–substrate interface is strained. We are interested exclusively in the solid phase formation effects; therefore, the growth kinetics effects will be avoided. This work is focused on the analysis of atomic distances and deformations in this system and on the calculation of the interface energy. It is shown how the stabilization on an atomic scale of different Ni nanocrystals set down on top of a large enough Cu(001) crystal is achieved. The adjustment between the lattice parameters of the Ni clusters on the Cu substrate is analysed. Specially, changes in the atomic distances at the interface are quantified. The main result is that the anisotropy of the structural matching causes a cubic lattice to become a tetragonal one. In addition, we carry out the energetic analysis of this interface.

A self-forming nanostructure—a wave-ordered structure with a controllable period (20–180 nm)—results from the off-normal bombardment of amorphous silicon layers by low-energy (∼ 1–10 keV) nitrogen ions. The nanostructure has been modified by reactive-ion etching in plasma to form a periodic nanomask on the surface of the channel region of a metal–oxide–semiconductor field-effect transistor (MOSFET). Implantation of arsenic ions through the nanomask followed by the technological steps completing the fabrication of the MOSFET resulted in a periodically doped channel field-effect transistor (PDCFET), which can be considered as a chain of short-channel MOSFETs with a common gate. Having worse subthreshold characteristics, PDCFETs show greater drain current and transconductance than to MOSFETs without a periodically doped channel. This improvement in device performance is attributed to the fact that the channel length is cut by the length of high-conductivity doped areas in the channel and that the voltage is distributed between the areas, depressing the scaling rules for short-channel MOSFETs and allowing the channel to be less doped between the areas, thus keeping drift mobility high.

We report the observation of anomalously high currents of up to 500µA during direct oxide nanolithography on the surface of n-type silicon {100}. Conventional nanolithography on silicon with an atomic force microscope (AFM) normally involves currents of the order of 10⁻¹⁰ –10⁻⁷ A and is associated with ionic conduction within a water meniscus surrounding the tip. The anomalous current we observe is related to an electrical breakdown resulting in conduction dominated by electrons rather than ions. We discuss the electron source during the AFM-assisted nanolithography process, and the possibility of using this breakdown current for nanoscale parallel writing.

The design of a 1-bit half-adder diode logic circuit inside a single molecule is investigated, with the chemical groups for diodes and wires bonded together to form the molecular circuit. With a circuit working in the ballistic transport regime, interference effects between the different electron paths in the circuit make the optimization of the circuit's logic function very delicate. In the tunnelling regime, these effects are partly suppressed. But the exponential decay of the current with the wire length imposes additional constraints for circuit design. A programmable gate logic array-like architecture would be expected be more useful for the design of a 1-bit adder in the ballistic regime due to the regularity of the circuit lattice, which might reduce interferences. On the other hand, a dedicated design which minimizes the amount of wiring might be the better choice for the tunnelling regime. However, we find that the logic output of classical diode logic circuits cannot be reproduced in either regime because Kirchhoff-like circuit rules do not apply. Furthermore, the geometry dependence of electron transmission in both regimes would make it impractical to build up logical functions like the SUM of an adder from simple OR- and AND-gates, even if the output pattern of these gates could be perfectly reproduced.

High-yield production of quasi-aligned carbon nanotube (CNT) bundles is presented. Benzene was used as the precursor for this catalytic process at a moderate temperature of 650°C over γ-Al₂O₃-supported Fe–Ni alloy catalysts with different mole ratios. The maximum yield of high-quality CNTs per hour is even greater than 350% for the optimized catalyst. The products were characterized by thermal gravimetry/differential scanning calorimetry and high-resolution transmission electron microscopy. The results indicated that the products are well graphitized with tube diameters of about 20 nm. Three typical morphologies of the end parts of the CNTs were identified as the open end capped with an Fe–Ni catalyst, the open end without catalyst and the closed end, respectively. A hexagonal carbon-cluster-based growth for this synthesis is suggested.

ZnO, MgO, and GeO₂ nanowires were successfully synthesized by simply heating the desired metal powder to a temperature above its melting point in a flow of mixed gases (20% O₂, 80% Ar, with the total flow rate of 120 sccm). Transmission electron microscopy observations show that as-synthesized products are exclusively nanowires, structurally uniform and single crystalline. The same technique was used to fabricate arrays of ZnO nanowires on silicon substrates, which would be of particular interest for direct integration in the current silicon-technology-based optoelectronic devices. Based on our experimental results, a metal self-catalytic growth mechanism was proposed and described conceptually. Because of the absence of impurities such as transition metal or noble metal throughout the whole growth process, the intrinsic properties of the resulting metal oxide nanowires could be expressed and utilized. And with in-depth understanding of the growth mechanism, our method could be efficient and controllable in extension to many other low-melting-point metals, such as Al, In, and Sn, for the synthesis of corresponding metal oxide nanostructures.

A finite-element analysis was carried out to study the effect of the vertical stacking period on the strain field in stacked InAs quantum dots embedded in a GaAs matrix. A comparison between stacked and unstacked structures was established. The hydrostatic strain was reinforced slightly as stacking occurred and was roughly constant regardless of the studied stacking period. On the other hand, the biaxial, radial and vertical strain components were relieved with stacking and the relief was significantly enhanced with a decrease in the stacking period. The drastic changes in the strain field with the stacking period due to strain field interaction are associated with the modification of electronic states and photoluminescence properties.

Semimetallic antimony nanotubes were synthesized via a hydrothermal reduction method at 120°C using zinc powder as the reducing agent. X-ray powder diffraction and energy dispersive x-ray analysis showed that the as-prepared sample was elemental antimony. High-resolution transmission electron microscopy characterization indicated that the walls of the antimony nanotubes were constructed with (012) crystal planes of rhombohedral antimony. A possible mechanism for the formation of antimony nanotubes is proposed.

The vertical and lateral ordering of quantum dot superlattices is dependent on the elastic stress/strain fields induced by the buried quantum dots within the previous superlattice layers. It has been found that quantum dots may be vertically correlated or may exhibit anti-correlation for different material systems. These differences are caused by the elastic anisotropy as well as the crystallographic orientation of the materials. The finite element method is used in this work to investigate the effects of anisotropy of the elastic property and crystal orientation on the elastostatic fields of quantum dot superlattices. The strain energy density distribution obtained from the calculations can be used to deduce the locations whereby subsequent quantum dot islands will nucleate by locating the minima positions. A tetragonal body centred arrangement is observed for the CdSe/ZnSe(001) system with a sufficiently thick spacer layer. Through comparison with a less anisotropic system (InAs/GaAs), it is shown that an increase in the degree of anisotropy favours an anti-correlated island growth. Spatial correlation in different crystal orientations is also investigated and discussed.

Single-walled nanotubes (SWNTs) covalently attached to phthalocyanine molecules through amide bonds have been prepared. A survey on the functionalization of SWNTs is also given.

We describe a simple process for preparing sub-3 nm gaps by means of controllable breaking of gold wires lithographed on a SiO₂/Si substrate at low temperature (4.2 K). We show that the mechanism involved is thermally assisted electromigration. We investigate the effect of the high electric field developed at the final stage of the breaking of the nanowire and observe that the current–voltage characteristics (I–V) of the resulting electrodes are stable up to ∼5 V. This high-electric-field stability gives access to the well-known Fowler–Nordheim regime (eV > Φ₀) in the I–V characteristic, thus allowing an accurate characterization of the gap size. The size of the gap is found to be between 1 and 2 nm. We validate this characterization by fabricating single-electron tunnelling devices based on alkylthiol capped gold nanoparticles.

Colloidal Ti has been synthesized following a method described in the literature. Extended x-ray absorption fine structure measurements indicate the presence of small colloidal entities, consisting of only a few core Ti atoms which are coordinated by O atoms from tetrahydrofuran (THF). The results can be explained by the proposed structure of Ti₁₃ · 6THF, in which the Ti cluster has the shape of a distorted icosahedron. The Ti colloid was used to prepare a functional nanocomposite by ball-milling the clusters with NaAlH₄. The nanocomposite showed superior hydrogen exchange kinetics when compared to the state of the art in the literature.

In this paper we report a technique that allows a fast replication of sub-100 nm scale patterns in a thin polymer film on a substrate from a patterned mask. Using the new pattern transfer technique, we fabricated 100 nm period polymer gratings with a 50 nm linewidth above a Si substrate as an example to demonstrate its capability of producing sub-100 nm nanostructures with direct industrial applications. In our technique, a mask with protruding patterns is used to induce similar pattern formation in the molten polymer film through an electrohydrodynamic instability process. A solid positive replica of the mask is obtained by cooling the polymer below its glass transition temperature. The mask is removed afterwards for the next fabrication procedure. The polymer structures formed can be used either directly as functional devices or as etching masks for further lithography processes. The mechanism that leads to the instability and subsequent pattern formation in the polymer layer is explained. Several important physical parameters that control the whole instability process are also identified. Our theory and experiments show that the pattern transfer technique developed here is well suited for the fabrication of sub-100 nm surface patterns in thin polymer films.

In this study, different weight fractions of multiwalled carbon nanotubes were dispersed in epoxy to produce toughened adhesives. The reinforced adhesives were used to bond the graphite fibre/epoxy composite adherends. Single lap joint samples were prepared and the average shear strengths were experimentally measured. Significant enhancement of the bonding performance was observed as the weight fraction of carbon nanotubes was increased.

The effect of mechanical milling on the luminescence properties of ZnO microcrystalline samples has been studied by means of cathodoluminescence in a scanning electron microscope. The samples consisted of pressed pellets of commercially available ZnO powder which were ball milled to investigate the possibility of nanocrystalline ZnO formation. Changes observed in the relative intensities of the characteristic ultraviolet and green band of ZnO are discussed in terms of defects generated during milling. The effect of nano- and picosecond pulsed laser irradiation on the particle size and luminescence of the milled samples has been also investigated.

We present measurements on nanomechanical resonators operating in the radio frequency range. We apply a set-up which allows the comparison of two schemes of displacement detection for mechanical resonators, namely conventional power reflection measurements of a probing signal and direct detection by capacitive coupling via a gate electrode. For capacitive detection, we employ a preamplifier, mounted close to the sample and connected to it via bond wires, which enables direct measurements of the resonator's displacement. We observe that the response of the mechanical resonator depends on the detection technique applied, which is verified in model calculations. We show results for the detection of subharmonics.

Nanoindentation techniques provide a unique opportunity to obtain mechanical properties of materials of very small volumes. The load–displacement and load–area curves are the basis for nanoindentation tests, and their interpretation is usually based on the main assumptions of the Hertz contact theory and formulae obtained for ideally shaped indenters. However, real indenters have some deviation from their nominal shapes leading researchers to develop empirical 'area functions' to relate the apparent contact area to depth. We argue that for both axisymmetric and three-dimensional cases, the indenter shape near the tip can be well approximated by monomial functions of radius. In this case problems obey the self-similar laws. Using Borodich's similarity considerations of three-dimensional contact problems and the corresponding formulae, fundamental relations are derived for depth of indentation, size of the contact region, load, hardness, and contact area, which are valid for both elastic and non-elastic, isotropic and anisotropic materials. For loading the formulae depend on the material hardening exponent and the degree of the monomial function of the shape. These formulae are especially important for shallow indentation (usually less than 100 nm) where the tip bluntness is of the same order as the indentation depth. Uncertainties in nanoindentation measurements that arise from geometric deviation of the indenter tip from its nominal geometry are explained and quantitatively described.

The optical band gap (E_g) of Pb_1−xFe_xS solution-grown nanoparticle films was varied from 2.65 to 2.22 eV with an increase in iron concentration 0.25 ≤ x ≤ 0.75 in films grown at fixed pH and temperature by the chemical bath deposition method. The presence of excitonic structure in the ternary Pb_1−xFe_xS for x ≥ 0.50 suggests increasing binding energy with increase in iron concentration in the films. A shift of excitonic peak towards higher energy with an increase in iron concentration is also observed.

Chemical functionalization on nano-scaled silica particles was performed through the reaction between the silanol group of silica and the oxirane ring of epoxy compounds. The occurrence of the surface functionalization and the chemical structures of the resulting products were characterized with FTIR and ¹H NMR. Energy dispersive x-ray spectrometry and transmission electronic micrography observation on the silica particles also confirmed the surface functionalization. This approach provides another method to organically functionalize nano-scaled silica particles and can be applied to the preparation of organic–inorganic nanocomposites.

The finite element method is used for modelling of self-positioning microstructures and nanostructures. The geometrically nonlinear problem with large rotations and large displacements is solved using a step procedure with coordinate updating after each step. It is shown that the real shape of the self-positioning structures is rather complicated and the analytical formulae have limited applicability in the estimation of such parameters as the curvature radius and/or angle of elevation.

The development of reliable, eco-friendly processes for the synthesis of nanoscale materials is an important aspect of nanotechnology. In this paper, we report on the use of an alkalotolerant actinomycete (Rhodococcus sp.) in the intracellular synthesis of gold nanoparticles of the dimension 5–15 nm. Electron microscopy analysis of thin sections of the gold actinomycete cells indicated that gold particles with good monodispersity were formed on the cell wall as well as on the cytospasmic membrane. The particles are more concentrated on the cytoplasmic membrane than on the cell wall, possibly due to reduction of the metal ions by enzymes present in the cell wall and on the cytoplasmic membrane. The metal ions were not toxic to the cells and the cells continued to multiply after biosynthesis of the gold nanoparticles.