Table of contents

A simple method for the fabrication of highly photoactive nanocrystalline two-layer TiO₂ electrodes for solar cell applications is presented. Diluted titanium acetylacetonate has been used as a precursor for covering SnO₂:F (FTO) films with dense packed TiO₂ nanocrystallites. The nanoporous thick TiO₂ film follows the dense packed thin TiO₂ film as a second layer. For the latter, amorphous TiO₂ nanoparticles have been successfully synthesized by a sol–gel technique in an acidic environment with pH<1 and hydrothermal growth at a temperature of 200 °C. The acidic nanoparticle gel was neutralized by basic ammonia and a TiO₂ gel of pH 5 was obtained; this pH value is higher than the recently reported value of 3.1 (Park et al 2005 Adv. Mater.17 2349–53). Highly interconnected, nanoporous, transparent and active TiO₂ films have been fabricated from the pH 5 gel. SEM, AFM and XRD analyses have been carried out for investigation of the crystal structure and the size of nanoparticles as well as the surface morphology of the films. Investigation of the photocurrent–voltage characteristics has shown improvement in cell performance along with the modification of the surface morphology, depending on pH of the TiO₂ gel. Increasing the pH of the gel from 2.1 to 5 enhanced the overall conversion efficiency of the dye-sensitized solar cells by approximately 30%. An energy conversion efficiency of 8.83% has been achieved for the cell (AM1.5, 100  mWcm⁻² simulated sunlight) compared to 6.61% efficiency in the absence of ammonia in the TiO₂ gel.

MgZnO/ZnO quantum wells on top of ZnO nanowires were grown by pulsed laser deposition. Ensembles of spatially fluctuating and narrow cathodoluminescence peaks with single widths down to 1 meV were found at the spectral position of the quantum well emission at 4 K. In addition, the number of these narrow QW peaks increases with increasing excitation power in micro-photoluminescence, thus pointing to quantum-dot-like emission centers. Indeed, laterally strained areas of about 5 nm diameter were identified at the quantum well positions on top of the nanowires by high-resolution transmission electron microscopy.

The electron transport properties of semiconducting carbon nanotube (SCNT) Schottky diodes are investigated with atomic models using density functional theory and the non-equilibrium Green's function method. We model the SCNT Schottky diode as a SCNT embedded in the metal electrode, which resembles the experimental set-up. Our study reveals that the rectification behaviour of the diode is mainly due to the asymmetric electron transmission function distribution in the conduction and valence bands and can be improved by changing metal–SCNT contact geometries. The threshold voltage of the diode depends on the electron Schottky barrier height which can be tuned by altering the diameter of the SCNT. Contrary to the traditional perception, the metal–SCNT contact region exhibits better conductivity than the other parts of the diode.

Here we demonstrate that an astigmatic detection system (ADS), constructed with a modified digital-versatile-disk (DVD) optical head, can achieve real-time measurement of a linear displacement and two-dimensional (2D) tilt angles with a high sensitivity. An atomic force microscope (AFM), using our detection system to sense the deflection of microfabricated cantilevers, can resolve single atomic steps on graphite surfaces with a noise level less than 0.04 nm in topographic images. This astigmatic detection system can even detect mechanical resonances due to thermal vibrations of microfabricated cantilevers. The high sensitivity, small detecting size and high bandwidth of this detection system is suitable for dynamic characterization of elements in micromachined components. Further optimization of the system will promise many other applications in diverse technological fields.

There is currently a need for superior stand-off detection schemes for protection against explosive weapons of mass destruction. Fluorescence detection at small distances from the target has proven to be attractive. A novel unexplored route in fluorescence chemical sensing that utilizes the exceptional spectroscopic capabilities of nonlinear optical methods is two-photon excited fluorescence. This approach utilizes infra-red light for excitation of remote sensors. Infra-red light suffers less scattering in porous materials which is beneficial for vapor sensing and has greater depth of penetration through the atmosphere, and there are fewer concerns regarding eye safety in remote detection schemes. We demonstrate this method using a novel dendritic system which possesses both excellent fluorescence sensitivity to the presence of TNT with infra-red pulses of light and high two-photon absorption (TPA) response. This illustrates the use of TPA for potential stand-off detection of energetic materials in the infra-red spectral regions in a highly two-photon responsive dendrimer.

Although research on the use of single walled carbon nanotubes (SWNTs) as the acceptor in polymer photovoltaic cells is currently making great progress, their poor dispersion in a polymer matrix has greatly hindered the overall performance of the devices. Here a novel bulk heterojunction structure based on a poly(phenyleneethynylene)/SWNT composite was designed to improve the dispersion of SWNTs in the composite based on their structural similarity and strong interaction. Better dispersion and higher performance are achieved compared with a common control device based on a poly(3-octylthiophene)/SWNT composite layer.

The magnetic-field-assisted approach has been used in the shape-controlled synthesis of single Bi nanocrystals. By tuning the magnetic field strength in the solvothermal process, Bi nanowires with dimension of 40–200 nm and lengths up to tens of micrometers were synthesized. Various techniques such as x-ray diffraction, scanning electron microscopy, transmission electron microscopy and Fourier transform infrared spectrometry have been used to characterize the products obtained. The results show that the magnetic field plays a key role in the crystal growth of the Bi nanowires. All nanowires were highly oriented single crystals with the growth direction along the c-axis.

Nanostructured nickel matrix composites reinforced with nanosized, undoped, tetragonal zirconia has been synthesized by cathodic pulsed electrodeposition. The reinforcement is synthesized by the aqueous combustion synthesis route with glycine as the fuel and zirconyl nitrate as the oxidizer. The reinforcement and composite have been characterized by XRD, TEM and SEM coupled with EDS. The microhardness and thermal stability (Kissinger method) of the composite are evaluated. These values are compared with those of pure nickel deposited under the same conditions. The results show that the microhardness of the nickel matrix is enhanced by the presence of the reinforcement from 450 to 575 VHN. Also the strengthening due to grain size effects and dispersion strengthening effect are evaluated individually and the interparticle separation is estimated to be around 85 nm. The volume fraction of the reinforcement is estimated to be 12–15% and the particles are uniformly distributed and monodispersed in the nickel matrix. The thermal stability of the composite is better than that of pure nickel in contrast to some of the reported literature.

Silver nanoparticles (Ag NPs) are one of the active substrates that are employed extensively in surface-enhanced Raman scattering (SERS), and aggregations of Ag NPs play an important role in enhancing the Raman signals. In this paper, we fabricated two kinds of SERS-active substrates utilizing the electrostatic adsorption and superior assembly properties of type I collagen. These were collagen-Ag NP aggregation films and nanoporous Ag films. Two probe molecules, 4-aminothiophenol (4-ATP) and methylene blue (MB), were studied on these substrates. These substrates showed reproducible SERS intensities with relative standard deviations (RSDs) of 8–10% and 11–14%, respectively, while the RSDs of the traditional thick Ag films were 12–28%. Also, the intensities for the 4-ATP spectrum on the collagen-templated nanoporous Ag film were approximately one order higher than those on the DNA-templated Ag film.

Water-soluble multi-walled carbon nanotubes (MWNTs) with a high solubility of 29.2 mg ml⁻¹ were obtained by polymer dispersant hydrolyzed poly(styrene-co-maleic anhydride) (HSMA) assisted exfoliation and centrifugation. The MWNTs were exfoliated and dispersed in aqueous solution by non-covalent modification with polymer dispersant of HSMA. Characterizations of HSMA-coated MWNTs were conducted via transmission electron microscopy (TEM), UV–vis and fluorescence spectroscopy, and thermal gravimetric analysis (TGA). The as-prepared HSMA-coated MWNTs showed good dispersibility and stability in water.

DNA interactions with multivalent cations, leading to wrapping around the cations and thermodynamically stable structure formation, followed by electrodeposition, have yielded a narrow size distributed single-crystalline HgTe–DNA quantum dot (QD) hybrid system. The mechanisms of the DNA interactions resulting in self-assembled HgTe QDs through phosphate–cation linkages and superstructure formation by nitrogen base interactions have been established by their respective binding energy shifts as evidenced from x-ray photoelectron spectroscopic studies. The photoluminescence peak position associated with HgTe QD single stranded DNA is red shifted in the presence of its conjugate and suggests the system as a potential optical probe for biomolecular recognition applications.

Phosphatidylcholine (PC) is a versatile ligand for synthesizing gold nanoparticles that are soluble in either organic or aqueous media. Here we report a novel route to organic-soluble, PC-stabilized gold nanoparticles that can be re-suspended in water after removal of the organic solvent. Similarly, we show that PC-stabilized gold nanoparticles synthesized in water can be re-suspended in organic solvents after complete removal of water. Without complete removal of the solvent, the nanoparticles retain their original solubility and do not phase transfer. This change in solvent preference from organic to aqueous and vice versa without the use of an additional phase transfer reagent is novel, visually striking, and of utility for synthetic modification of nanoparticles. This approach allows chemical reactions to be performed on nanoparticles in organic solvents followed by conversion of the products to water-soluble materials. A narrow distribution of PC-stabilized gold nanoparticles was obtained after phase transfer to water as characterized by UV–visible (UV–vis) spectroscopy and transmission electron microscopy (TEM), demonstrating that the narrow distribution obtained from the organic synthesis is retained after transfer to water. This method produces water-soluble nanoparticles with a narrower dispersity than is possible with direct aqueous synthesis.

A reducing system involving M13 virus-mediated FCC Fe nanoparticles was employed to achieve uranium reduction and synthesize uranium dioxide nanocrystals. Here we show that metastable face-centered cubic (FCC) Fe nanoparticles were fabricated around the surface of the M13 virus during the specific adsorption of the virus towards Fe ions under a reduced environment. The FCC phase of these Fe nanoparticles was confirmed by careful TEM characterization. Moreover, this virus-mediated FCC Fe nanoparticle system successfully reduced contaminable U(VI) into UO₂ crystals with diameters of 2–5 nm by a green and convenient route.

Hexagonal AlN nanorod and nanoneedle arrays were synthesized through the direct reaction of AlCl₃ and NH₃ by chemical vapor deposition at about 750 °C. Both the AlN nanoneedle and nanorod samples were of wurtzite structure and grew preferentially along the c-axis. With an increase in the ratio of NH₃ to Ar, an evolution from nanorods to nanoneedles was observed. A growth model was proposed to explain the possible growth mechanism. Measurements in field emission show that AlN nanoneedle arrays have a much lower turn-on field (3.1 V µm⁻¹) compared to nanorod arrays (15.3 V µm⁻¹), due to their large curvature geometry. The AlN nanoneedle arrays have potential applications in many fields, such as electron-emitting nanodevices and field-emission-based flat-panel displays.

This study uses molecular dynamics to simulate the nanoscale cutting of a Cu single crystal by a conical diamond tool. Actual nanoscale straight-line cutting experiments were performed, and the experimental results are compared with the simulation results. The heaping of copper atoms is qualitatively quite consistent with the simulation result. This paper also proposes a nanoscale contact pressure factor (NCP factor) that is applicable to the probes at different tip radii. An estimation model of the cutting force for nanoscale cutting is established. This model can estimate the cutting force during actual nanoscale cutting. Actual nanoscale cutting experiments were performed for verification, and the difference between the cutting force estimated by this model and the actual force is very small.

Recombination dynamics in CdTe/CdSe core–shell type-II quantum dots (QDs) has been investigated by time-resolved photoluminescence (PL) spectroscopy. A very long PL decay time of several hundred nanoseconds has been found at low temperature, which can be rationalized by the spatially separated electrons and holes occurring in a type-II heterostructure. For the temperature dependence of the radiative lifetime, the linewidth and the peak energy of PL spectra show that the recombination of carriers is dominated by delocalized excitons at temperatures below 150 K, while the mixture of delocalized excitons, electrons and holes overwhelms the process at higher temperature. The binding energy of delocalized excitons obtained from the temperature dependence of the non-radiative lifetime is consistent with the theoretical value. The energy dependence of lifetime measurements reveals a third power relationship between the radiative lifetime and the radius of QDs, the light of which can be shed by the quantum confinement effect. In addition, the radiative decay rate is found to be proportional to the square root of excitation power, arising from the change of wavefunction overlap of electrons and holes due to the band bending effect, which is an inherent character of a type-II band alignment.

We report Raman spectroscopic studies of the nanosized rare earth sesquioxides, namely yttrium sesquioxide (Y₂O₃), gadolinium sesquioxide (Gd₂O₃) and samarium sesquioxide (Sm₂O₃), under high pressure. The samples were characterized using x-ray diffraction, Raman spectroscopy and atomic force microscopy at atmospheric pressures. Y₂O₃ and Gd₂O₃ were found to be cubic at ambient, while Sm₂O₃ was found to be predominantly cubic with a small fraction of monoclinic phase. The strongest Raman peaks are observed at 379, 344 and 363 cm⁻¹, respectively, for Y₂O₃, Sm₂O₃ and Gd₂O₃. All the samples were found to be nanosized with 50–90 nm particle sizes. The high pressures were generated using a Mao–Bell type diamond anvil cell and a conventional laser Raman spectrometer is used to monitor the pressure-induced changes. Y₂O₃ seems to undergo a crystalline to partial amorphous transition when pressurized up to about 19 GPa, with traces of hexagonal phase. However, on release of pressure, the hexagonal phase develops into the dominant phase. Gd₂O₃ is also seen to develop into a mixture of amorphous and hexagonal phases on pressurizing. However, on release of pressure Gd₂O₃ does not show any change and the transformation is found to be irreversible. On the other hand, Sm₂O₃ shows a weakening of cubic phase peaks while monoclinic phase peaks gain intensity up to about a pressure of 6.79 GPa. However, thereafter the monoclinic phase peaks also reduce in intensity and mostly disordering sets in which does not show significant reversal as the pressure is released. The results obtained are discussed in detail.

The hexagonal quantum well (QW) is studied as a model for hexagonal nanowires, and the effects of donor impurities and geometrical deformations of the well are treated. By use of the Poisson equation the donor potential is calculated and the eigenspectrum of the hexagonal QW is shown to converge to that of a paraboloid quantum well with increasing donor density. Small deformations of the hexagon are shown to change the eigenspectrum significantly and give strong splittings of degenerate eigenvalues. Analytical approximations for the potential and eigenfunctions on the deformed hexagons are given.

A constrained three-dimensional atomistic model of a cracked aluminum single crystal has been employed to investigate the growth behavior of a nanoscale crack in a single crystal using molecular dynamics simulations with the EAM potential. This study is focused on the stress field around the crack tip and its evolution during fast crack growth. Simulation results of the observed nanoscale fracture behavior are presented in terms of atomistic stresses. Major findings from the simulation results are the following: (a) crack growth is in the form of void nucleation, growth and coalescence ahead of the crack tip, thus resembling that of ductile fracture at the continuum scale; (b) void nucleation occurs at a certain distance ahead of the current crack tip or the forward edge of the leading void ahead of the crack tip; (c) just before void nucleation the mean atomic stress (or equivalently its ratio to the von Mises effective stress, which is called the stress constraint or triaxiality) has a high concentration at the site of void nucleation; and (d) the stress field ahead of the current crack tip or the forward edge of the leading void is more or less self-similar (so that the forward edge of the leading void can be viewed as the effective crack tip).

Onion-like carbons (OLC) obtained by thermal transformation of nanodiamonds are agglomerates of multi-shell fullerenes, often covered by an external graphitic mantle. For the present work, elemental OLC units were constructed on the computer by coalescence of several two-layer fullerenes, in a structure similar to carbon peapods with a corrugated external wall. The electrical polarizability of such pod-of-peas fullerenes has been computed by a classical monopole–dipole atomistic theory. The description of pod-of-peas fullerenes was further simplified by representing them as linear arrays of point-like objects, whose polarizability matches that of the starting molecules. Calculations demonstrated that the static polarizability of spherically shaped assemblies of these arrays, modeling real OLC materials, is weakly dependent on the geometry of its constituent molecules and is chiefly proportional to the volume of the whole cluster. It increases with increasing filling fraction of the pod-of-peas fullerenes in the OLC aggregate. The polarizability so obtained can be used in Maxwell–Garnett theory to predict the permittivity of OLC-based composites, at least for static excitations. Experimental results obtained at GHz frequencies reveal a weak attenuation for OLC- and nanodiamond-based polydimethylsiloxane composites. In these silicone composites, we did not find long chains of coupled OLCs. Quite separated clusters were found instead, which contribute little to the polarizability and to the dielectric properties, in good agreement with our theoretical predictions.

Silicon nanocrystals (Si-nc) and amorphous silicon (α-Si) produced by silicon implantation in fused silica have been studied by micro-Raman spectroscopy. Information regarding the Raman signature of the α-Si phonon excitation was extracted from Raman depth-probing measurements using the phenomenological phonon confinement model. The spectral deconvolution of the Raman measurements recorded at different laser focusing depths takes into account both the Si-nc size variation and the Si-nc spatial distribution within the sample. The phonon peak associated with α-Si around 470 cm⁻¹ is greatest for in-sample laser focusing, indicating that the formation of amorphous silicon is more important in the region containing a high concentration of silicon excess, where large Si-nc are located. As also observed for Si-nc systems prepared by SiO_x layer deposition, this result demonstrates the presence of α-Si in high excess Si implanted Si-nc systems.

Shrinking feature sizes and energy levels coupled with high clock rates and decreasing node capacitance lead us into a regime where transient errors in logic cannot be ignored. Consequently, several recent studies have focused on feed-forward spatial redundancy techniques to combat these high transient fault rates. To complement these studies, we analyze fine-grained rollback techniques and show that they can offer lower spatial redundancy factors with no significant impact on system performance for fault rates up to one fault per device per ten million cycles of operation (P_f = 10⁻⁷) in systems with 10¹² susceptible devices. Further, we concretely demonstrate these claims on nanowire-based programmable logic arrays. Despite expensive rollback buffers and general-purpose, conservative analysis, we show the area overhead factor of our technique is roughly an order of magnitude lower than a gate level feed-forward redundancy scheme.

Cadmium sulfide (CdS) nanoparticles dotted on the surface of multiwalled carbon nanotubes (MWCNTs) have been synthesized by the polyol method. The as-prepared materials were characterized by x-ray powder diffraction, transmission electron microscopy, scanning electron microscopy, and Brunauer–Emmett–Teller adsorption analysis. The results indicate that CdS nanoparticles with diameter of 5–8 nm are thickly and uniformly coated on the surface of the MWCNTs. The photodegradation of azo dye using these materials was evaluated by the degradation of Brilliant Red X-3B under visible light. The coated nanotubes show higher photocatalytic activity than both CdS alone and a CdS/activated carbon sample; in addition, there is an optimum content of MWCNTs. The presence of MWCNTs can also hamper the photocorrosion of CdS. The mechanism for the enhancement of MWCNTs on the adsorption and photocatalytic property of CdS is investigated for the first time.

The value of the surface anisotropy constant as obtained from equation (5) in line 29 of the first column of page four should be K_s=0.07 x 10^-4 Jm^-2 in place of 0.07 x 10⁵ Jm^-2.