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Showing papers in "Nanotechnology in 2008"


Journal Article
TL;DR: In this article, a surface acoustic wave (SAW) atomizer was used to generate monodisperse aerosols and particles for drug delivery applications, including insulin liquid aerosols for pulmonary delivery and solid protein nanoparticles.
Abstract: We describe the fabrication of a surface acoustic wave (SAW) atomizer and show its ability to generate monodisperse aerosols and particles for drug delivery applications. In particular, we demonstrate the generation of insulin liquid aerosols for pulmonary delivery and solid protein nanoparticles for transdermal and gastrointestinal delivery routes using 20 MHz SAW devices. Insulin droplets around 3 mu m were obtained, matching the optimum range for maximizing absorption in the alveolar region. A new approach is provided to explain these atomized droplet diameters by returning to fundamental physical analysis and considering viscous-capillary and inertial-capillary force balance rather than employing modifications to the Kelvin equation under the assumption of parametric forcing that has been extended to these frequencies in past investigations. In addition, we consider possible mechanisms by which the droplet ejections take place with the aid of high-speed flow visualization. Finally, we show that nanoscale protein particles (50-100 nm in diameter) were obtained through an evaporative process of the initial aerosol, the final size of which could be controlled merely by modifying the initial protein concentration. These results illustrate the feasibility of using SAW as a novel method for rapidly producing particles and droplets with a controlled and narrow size distribution

1,306 citations


Journal ArticleDOI
TL;DR: It is proposed that anatase formation is dominated by surface energy effects, and that rutile and brookite formation follows a dissolution-precipitation mechanism, where chains of sixfold-coordinated titanium complexes arrange into different crystal structures depending on the reactant chemistry.
Abstract: We report on the synthesis of phase-pure TiO2 nanoparticles in anatase, rutile and brookite structures, using amorphous titania as a common starting material. Phase formation was achieved by hydrothermal treatment at elevated temperatures with the appropriate reactants. Anatase nanoparticles were obtained using acetic acid, while phase-pure rutile and brookite nanoparticles were obtained with hydrochloric acid at a different concentration. The nanomaterials were characterized using x-ray diffraction, UV–visible reflectance spectroscopy, dynamic light scattering, and transmission electron microscopy. We propose that anatase formation is dominated by surface energy effects, and that rutile and brookite formation follows a dissolution–precipitation mechanism, where chains of sixfold-coordinated titanium complexes arrange into different crystal structures depending on the reactant chemistry. The particle growth kinetics under hydrothermal conditions are determined by coarsening and aggregation–recrystallization processes, allowing control over the average nanoparticle size.

1,010 citations


Journal ArticleDOI
TL;DR: The results suggest that silver nanoparticles induce a dose-dependent toxicity in embryos, which hinders normal development.
Abstract: This study was initiated to enhance our insight on the health and environmental impact of silver nanoparticles (Ag-np). Using starch and bovine serum albumin (BSA) as capping agents, silver nanoparticles were synthesized to study their deleterious effects and distribution pattern in zebrafish embryos (Danio rerio). Toxicological endpoints like mortality, hatching, pericardial edema and heart rate were recorded. A concentration-dependent increase in mortality and hatching delay was observed in Ag-np treated embryos. Additionally, nanoparticle treatments resulted in concentration-dependent toxicity, typified by phenotypes that had abnormal body axes, twisted notochord, slow blood flow, pericardial edema and cardiac arrhythmia. Ag+ ions and stabilizing agents showed no significant defects in developing embryos. Transmission electron microscopy (TEM) of the embryos demonstrated that nanoparticles were distributed in the brain, heart, yolk and blood of embryos as evident from the electron-dispersive x-ray analysis (EDS). Furthermore, the acridine orange staining showed an increased apoptosis in Ag-np treated embryos. These results suggest that silver nanoparticles induce a dose-dependent toxicity in embryos, which hinders normal development.

976 citations


Journal ArticleDOI
TL;DR: An efficient and user-friendly method was developed for the synthesis of uniform hexagonal-phase NaYF(4):Yb, Er/Tm nanocrystals with upconversion fluorescence, by forming small solid-state crystal nuclei and further growth and ripening of the nuclei.
Abstract: Hexagonal-phase NaYF4:Yb, Er/Tm nanocrystals are the best IR-to-visible upconverting materials to date, but user-friendly methods for making pure hexagonal-phase NaYF4:Yb, Er/Tm nanocrystals with upconversion fluorescence are still lacking. Most of the methods reported so far require excess fluoride reactants in a high-temperature reaction which are very unfriendly to users and raise safety concerns. In this work, an efficient and user-friendly method was developed for the synthesis of uniform hexagonal-phase NaYF4:Yb, Er/Tm nanocrystals with upconversion fluorescence, by forming small solid-state crystal nuclei and further growth and ripening of the nuclei. NaYF4:Yb, Er/Tm nanoplates, nanospheres and nanoellipses were also selectively produced by varying the concentration of the surfactant. All the nanocrystals showed strong upconversion fluorescence, and fluorescence from the nanoplates was observed even when the laser power density was reduced to about 50 mW cm−2. These nanocrystals have great potential for use in biology and medicine as fluorescent labels or imaging probes.

693 citations


Journal ArticleDOI
TL;DR: Nanostar dispersions display a well-defined optical response, which was found to comprise a main mode confined within the tips and a secondary mode confined in the central body.
Abstract: Multipod Au nanoparticles (nanostars) with single crystalline tips were synthesized in extremely high yield through the reduction of HAuCl4 in a concentrated solution of poly(vinylpyrrolidone) (PVP) in N,N-dimethylformamide (DMF), in the presence of preformed Au nanoparticle seeds, but with no need for external energy sources. Nanostar dispersions display a well-defined optical response, which was found (through theoretical modeling) to comprise a main mode confined within the tips and a secondary mode confined in the central body. Calculations of the surface enhanced Raman scattering (SERS) response additionally show that this morphology will be relevant for sensing applications.

668 citations


Journal ArticleDOI
TL;DR: In this review, recent progress on the development of different types of CNT-based nanosensors is summarized and the focus was placed on the means used by various researchers to improve the sensing performance (sensitivity, selectivity and response time) through the rational functionalization of C NTs with different methods (covalent and non-covalents and with different materials).
Abstract: The development of carbon nanotube-(CNTs-)based gas sensors and sensor arrays has attracted intensive research interest in the last several years because of their potential for the selective and rapid detection of various gaseous species by novel nanostructures integrated in miniature and low-power consuming electronics. Chemiresistors and chemical field effect transistors are probably the most promising types of gas nanosensors. In these sensors, the electrical properties of nanostructures are dramatically changed when exposed to the target gas analytes. In this review, recent progress on the development of different types of CNT-based nanosensors is summarized. The focus was placed on the means used by various researchers to improve the sensing performance (sensitivity, selectivity and response time) through the rational functionalization of CNTs with different methods (covalent and non-covalent) and with different materials (polymers and metals).

607 citations


Journal ArticleDOI
TL;DR: In this study, copper nanoparticles were synthesized through a relatively large-scale, high-throughput (0.2 M) process through the chemical reduction of copper sulfate with sodium hypophosphite in ethylene glycol within the presence of a polymer surfactant (PVP), which was included to prevent aggregation and give dispersion stability to the resulting colloidal nanoparticles.
Abstract: Copper nanoparticles are being given considerable attention as of late due to their interesting properties and potential applications in many areas of industry. One such exploitable use is as the major constituent of conductive inks and pastes used for printing various electronic components. In this study, copper nanoparticles were synthesized through a relatively large-scale (5 l), high-throughput (0.2 M) process. This facile method occurs through the chemical reduction of copper sulfate with sodium hypophosphite in ethylene glycol within the presence of a polymer surfactant (PVP), which was included to prevent aggregation and give dispersion stability to the resulting colloidal nanoparticles. Reaction yields were determined to be quantitative while particle dispersion yields were between 68 and 73%. The size of the copper nanoparticles could be controlled between 30 and 65 nm by varying the reaction time, reaction temperature, and relative ratio of copper sulfate to the surfactant. Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) images of the particles revealed a spherical shape within the reported size regime, and x-ray analysis confirmed the formation of face-centered cubic (FCC) metallic copper. Furthermore, inkjet printing nanocopper inks prepared from the polymer-stabilized copper nanoparticles onto polyimide substrates resulted in metallic copper traces with low electrical resistivities (≥3.6 µΩ cm, or ≥2.2 times the resistivity of bulk copper) after a relatively low-temperature sintering process (200 °C for up to 60 min).

511 citations


Journal ArticleDOI
TL;DR: A controlled and up-scalable biosynthetic route to nanocrystalline silver particles with well-defined morphology using cell-free aqueous filtrate of a non-pathogenic and commercially viable biocontrol agent Trichoderma asperellum is being reported for the first time.
Abstract: A controlled and up-scalable biosynthetic route to nanocrystalline silver particles with well-defined morphology using cell-free aqueous filtrate of a non-pathogenic and commercially viable biocontrol agent Trichoderma asperellum is being reported for the first time. A transparent solution of the cell-free filtrate of Trichoderma asperellum containing 1 mM AgNO3 turns progressively dark brown within 5 d of incubation at 25 °C. The kinetics of the reaction was studied using UV–vis spectroscopy. An intense surface plasmon resonance band at ~410 nm in the UV–vis spectrum clearly reveals the formation of silver nanoparticles. The size of the silver particles using TEM and XRD studies is found to be in the range 13–18 nm. These nanoparticles are found to be highly stable and even after prolonged storage for over 6 months they do not show significant aggregation. A plausible mechanism behind the formation of silver nanoparticles and their stabilization via capping has been investigated using FTIR and surface-enhanced resonance Raman spectroscopy.

502 citations


PatentDOI
TL;DR: In this article, the authors disclose the response of normal human cells to ZnO nanoparticles under different signaling environments and compare it to response of cancerous cells, showing that ZnOs exhibit a strong preferential ability to kill cancerous T cells (˜28-35X) compared to normal cells.
Abstract: Here we disclose the response of normal human cells to ZnO nanoparticles under different signaling environments and compare it to the response of cancerous cells. ZnO nanoparticles exhibit a strong preferential ability to kill cancerous T cells (˜28-35X) compared to normal cells. Interestingly, the activation state of the cell contributes toward nanoparticle toxicity as resting T cells display a relative resistance while cells stimulated through the T cell receptor and CD28 costimulatory pathway show greater toxicity in direct relation to the level of activation. The novel findings of cell selective toxicity towards potential disease causing cells indicate a potential utility of ZnO nanoparticle in the treatment of cancer and/or autoimmunity.

492 citations


Journal ArticleDOI
TL;DR: A way to extend the capabilities of plasma-enhanced chemical vapour deposition to the synthesis of freestanding few-layer graphene is presented and the resulting graphene flakes are aligned vertically to the substrate surface and grow according to a three-step process.
Abstract: If graphene is ever going to live up to the promises of future nanoelectronic devices, an easy and cheap route for mass production is an essential requirement. A way to extend the capabilities of plasma-enhanced chemical vapour deposition to the synthesis of freestanding few-layer graphene is presented. Micrometre-wide flakes consisting of four to six atomic layers of stacked graphene sheets have been synthesized by controlled recombination of carbon radicals in a microwave plasma. A simple and highly reproducible technique is essential, since the resulting flakes can be synthesized without the need for a catalyst on the surface of any substrate that withstands elevated temperatures up to 700 °C. A thorough structural analysis of the flakes is performed with electron microscopy, x-ray diffraction, Raman spectroscopy and scanning tunnelling microscopy. The resulting graphene flakes are aligned vertically to the substrate surface and grow according to a three-step process, as revealed by the combined analysis of electron microscopy and x-ray photoelectron spectroscopy.

483 citations


Journal ArticleDOI
TL;DR: This paper presents some simplified non-local elastic beam models, for the bending analyses of small scale rods, and shows that this paradox may be overcome with a gradient elastic model as well as an integral non-Local elastic model that is based on combining the local and the non- local curvatures in the constitutive elastic relation.
Abstract: Non-local continuum mechanics allows one to account for the small length scale effect that becomes significant when dealing with microstructures or nanostructures. This paper presents some simplified non-local elastic beam models, for the bending analyses of small scale rods. Integral-type or gradient non-local models abandon the classical assumption of locality, and admit that stress depends not only on the strain value at that point but also on the strain values of all points on the body. There is a paradox still unresolved at this stage: some bending solutions of integral-based non-local elastic beams have been found to be identical to the classical (local) solution, i.e. the small scale effect is not present at all. One example is the Euler-Bernoulli cantilever nanobeam model with a point load which has application in microelectromechanical systems and nanoelectromechanical systems as an actuator. In this paper, it will be shown that this paradox may be overcome with a gradient elastic model as well as an integral non-local elastic model that is based on combining the local and the non-local curvatures in the constitutive elastic relation. The latter model comprises the classical gradient model and Eringen's integral model, and its application produces small length scale terms in the non-local elastic cantilever beam solution.

Journal ArticleDOI
TL;DR: The fabrication of silicon nanowire-based solar cells on silicon wafers and on multicrystalline silicon thin films on glass is described, which shows a strong broadband optical absorption, which makes them an interesting candidate to serve as an absorber in solar cells.
Abstract: The fabrication of silicon nanowire-based solar cells on silicon wafers and on multicrystalline silicon thin films on glass is described. The nanowires show a strong broadband optical absorption, which makes them an interesting candidate to serve as an absorber in solar cells. The operation of a solar cell is demonstrated with n-doped nanowires grown on a p-doped silicon wafer. From a partially illuminated area of 0.6 cm2 open-circuit voltages in the range of 230–280 mV and a short-circuit current density of 2 mA cm−2 were obtained.

Journal ArticleDOI
TL;DR: The excellent antibacterial activity of the Ag-fabric composite against Escherichia coli and Staphylococcus aureus cultures was demonstrated.
Abstract: Silver nanoparticles were synthesized and deposited on different types of fabrics using ultrasound irradiation. The structure of silver-fabric composites was studied by physico-chemical methods. The mechanism of the strong adhesion of silver nanoparticles to the fibers is discussed. The excellent antibacterial activity of the Ag-fabric composite against Escherichia coli (gram-negative) and Staphylococcus aureus (gram-positive) cultures was demonstrated.

Journal ArticleDOI
TL;DR: Results show it is unlikely that morphology alone is sufficient to explain the difference in elastic moduli for metastatic cancer cells and benign mesothelial cells, and indicate that biomechanical-based functional analysis may provide an additional platform for cytological evaluation and diagnosis of cancer in the future.
Abstract: Recently biomechanics of cancer cells, in particular stiffness or elasticity, has been identified as an important factor relating to cancer cell function, adherence, motility, transformation and invasion. We report on the nanomechanical responses of metastatic cancer cells and benign mesothelial cells taken from human body cavity fluids using atomic force microscopy. Following our initial study (Cross et al 2007 Nat. Nanotechnol. 2 780-3), we report on the biophysical properties of patient-derived effusion cells and address the influence of cell morphology on measured cell stiffness. Using a cytocentrifugation method, which yields morphologically indistinguishable cells that can be prepared in 1 min and avoids any possible artifacts due to 12 h ex vivo culture, we find that metastatic tumor cells are more than 80% softer than benign cells with a distribution over six times narrower than that of normal cells. Consistent with our previous study, which yielded distinguishable cell populations based on ex vivo growth and morphological characteristics, our results show it is unlikely that morphology alone is sufficient to explain the difference in elastic moduli for these two cell types. Moreover, analysis of non-specific cell adhesion inherent to tumor and normal cells collected from patients show surface adhesion of tumor cells is ∼33% less adhesive compared to that of normal cells. Our findings indicate that biomechanical-based functional analysis may provide an additional platform for cytological evaluation and diagnosis of cancer in the future.

Journal ArticleDOI
Ding Wang1, Pengcheng Song, Changhong Liu1, Wei Wu, Shoushan Fan 
TL;DR: This work presents a simple and effective macroscopic manipulation of aligned CNT arrays called 'domino pushing' in the preparation of the aligned thick buckypaper with large areas, and indicates that these buckypapers with controllable structure have better performance on thermal and electrical conductance.
Abstract: Paper-like carbon nanotube (CNT) materials have many important applications such as in catalysts, in filtration, actuators, capacitor or battery electrodes, and so on. Up to now, the most popular way of preparing buckypapers has involved the procedures of dispersion and filtration of a suspension of CNTs. In this work, we present a simple and effective macroscopic manipulation of aligned CNT arrays called 'domino pushing' in the preparation of the aligned thick buckypapers with large areas. This simple method can efficiently ensure that most of the CNTs are well aligned tightly in the buckypaper. The initial measurements indicate that these buckypapers have better performance on thermal and electrical conductance. These buckypapers with controllable structure also have many potential applications, including supercapacitor electrodes.

Journal ArticleDOI
TL;DR: A nanowire-based gas sensor is fabricated using a simple method of growing SnO(2) nanowires bridging the gap between two pre-patterned Au catalysts, in which the electrical contacts to the Nanowires are self-assembled during the synthesis of the nanOWires.
Abstract: We fabricated a nanowire-based gas sensor using a simple method of growing SnO2 nanowires bridging the gap between two pre-patterned Au catalysts, in which the electrical contacts to the nanowires are self-assembled during the synthesis of the nanowires. The gas sensing capability of this network-structured gas sensor was demonstrated using a diluted NO2. The sensitivity, as a function of temperature, was highest at 200 °C and was determined to be 18 and 180 when the NO2 concentration was 0.5 and 5 ppm, respectively. Our sensor showed higher sensitivity compared to different types of sensors including SnO2 powder-based thin films, SnO2 coating on carbon nanotubes or single/multiple SnO2 nanobelts. The enhanced sensitivity was attributed to the additional modulation of the sensor resistance due to the potential barrier at nanowire/nanowire junctions as well as the surface depletion region of each nanowire.

Journal ArticleDOI
TL;DR: The unique characteristics in electrical resistance change for different volume fractions imply that MWCNT/PEO composite films can be used as tunable strain sensors and for application into embedded sensor systems in structures.
Abstract: The strain-dependent electrical resistance characteristics of multi-walled carbon nanotube (MWCNT)/polymer composite films were investigated. In this research, polyethylene oxide (PEO) is used as the polymer matrix. Two representative volume fractions of MWCNT/PEO composite films were selected: 0.56 vol% (near the percolation threshold) and 1.44 vol% (away from the percolation threshold) of MWCNT. An experimental setup which can measure electrical resistance and strain simultaneously and continuously has been developed. Unique and repeatable relationships in resistance versus strain were obtained for multiple specimens with different volume fractions of MWCNT. The overall pattern of electrical resistance change versus strain for the specimens tested consists of linear and nonlinear regions. A resistance change model to describe the combination of linear and nonlinear modes of electrical resistance change as a function of strain is suggested. The unique characteristics in electrical resistance change for different volume fractions imply that MWCNT/PEO composite films can be used as tunable strain sensors and for application into embedded sensor systems in structures. (Some figures in this article are in colour only in the electronic version)

Journal ArticleDOI
TL;DR: The method offers a generic tool for electrical functionalization of nanoparticle structures and has the feasibility for patterning, systematic control of the final conductivity and in situ process monitoring.
Abstract: A method for sintering nanoparticles by applying voltage is presented. This electrical sintering method is demonstrated using silver nanoparticle structures ink-jet-printed onto temperature-sensitive photopaper. The conductivity of the printed nanoparticle layer increases by more than five orders of magnitude during the sintering process, with the final conductivity reaching 3.7 × 10 7 Sm −1 at best. Due to a strong positive feedback induced by the voltage boundary condition, the process is very rapid—the major transition occurs within 2 μs. The best obtained conductivity is two orders of magnitude better than for the equivalent structures oven-sintered at the maximum tolerable temperature of the substrate. Additional key advantages of the method include the feasibility for patterning, systematic control of the final conductivity and in situ process monitoring. The method offers a generic tool for electrical functionalization of nanoparticle structures. (Some figures in this article are in colour only in the electronic version)

Journal ArticleDOI
TL;DR: The electrical properties of polymer nanocomposites containing a small amount of carbon nanotube (CNT) are remarkably superior to those of conventional electronic composites, based on three-dimensional (3D) statistical percolation and 3D resistor network modeling.
Abstract: The electrical properties of polymer nanocomposites containing a small amount of carbon nanotube (CNT) are remarkably superior to those of conventional electronic composites. Based on three-dimensional (3D) statistical percolation and 3D resistor network modeling, the electrical properties of CNT nanocomposites, at and after percolation, were successfully predicted in this work. The numerical analysis was also extended to investigate the effects of the aspect ratio, the electrical conductivity, the aggregation and the shape of CNTs on the electrical properties of the nanocomposites. A simple empirical model was also established based on present numerical simulations to predict the electrical conductivity in several electronic composites with various fillers. This investigation further highlighted the importance of theoretical and numerical analyses in the exploration of basic physical phenomena, such as percolation and conductivity in novel nanocomposites.

Journal ArticleDOI
TL;DR: The open-circuit voltage was found to increase proportionally with reductions in QD size, which may relate to a bandgap widening effect in Si QDs or an improved heterojunction field allowing a greater split of the Fermi levels in the Si substrate.
Abstract: Silicon (Si) quantum dot (QD) materials have been proposed for 'all-silicon' tandem solar cells. In this study, solar cells consisting of phosphorus-doped Si QDs in a SiO2 matrix deposited on p-type crystalline Si substrates (c-Si) were fabricated. The Si QDs were formed by alternate deposition of SiO2 and silicon-rich SiOx with magnetron co-sputtering, followed by high-temperature annealing. Current tunnelling through the QD layer was observed from the solar cells with a dot spacing of 2 nm or less. To get the required current densities through the devices, the dot spacing in the SiO2 matrix had to be 2 nm or less. The open-circuit voltage was found to increase proportionally with reductions in QD size, which may relate to a bandgap widening effect in Si QDs or an improved heterojunction field allowing a greater split of the Fermi levels in the Si substrate. Successful fabrication of (n-type) Si QD/(p-type) c-Si photovoltaic devices is an encouraging step towards the realization of all-silicon tandem solar cells based on Si QD materials.

Journal ArticleDOI
TL;DR: The main strategies for improving the power conversion efficiency of organic solar cells, namely raising the open circuit voltage V(oc) and increasing the short circuit current density J(sc), are discussed.
Abstract: In this paper we focus on the current status of organic solar cells based on small molecules. Since their discovery, much progress has been made, and the main steps are highlighted that led to the current state-of-the-art devices. However, organic solar cells still need to be improved further, and the main strategies for improving the power conversion efficiency, namely raising the open circuit voltage V(oc) and increasing the short circuit current density J(sc), are discussed. In theory, power conversion efficiencies of around 15% should be possible with a single heterojunction; for higher efficiencies, stacked solar cell concepts have to be employed.

Journal ArticleDOI
TL;DR: This research utilizes electrically conductive networks of carbon nanotubes as in situ sensors for detecting damage accumulation during cyclic loading of advanced fiber composites and identifies a parameter that may be utilized as a quantitative measure of damage.
Abstract: Developments in producing nanostructured materials with novel properties have opened up new opportunities in which unique functionality can be added to existing material systems. As advanced fiber composites are utilized more frequently in primary structural applications there is a key challenge to enhance the performance and reliability while reducing maintenance. As a consequence there is tremendous scientific and technical interest in the development of techniques for monitoring the health of composite structures where real-time sensing can provide information on the state of microstructural damage. In this research we utilize electrically conductive networks of carbon nanotubes as in situ sensors for detecting damage accumulation during cyclic loading of advanced fiber composites. Here we show that, by combining load and strain measurements in real-time with direct current electrical resistance measurements of the carbon nanotube network, insight can be gained toward the evolution and accumulation of damage. The resistance/strain relations show substantial hysteresis due to the formation and opening/closing of cracks during cyclic loading. Through interpreting the resistance response curves we identify a parameter that may be utilized as a quantitative measure of damage.

Journal ArticleDOI
TL;DR: Large-area slantingly-aligned silicon nanowire arrays (SA-SiNW arrays) on Si(111) substrate have been fabricated by wet chemical etching with dry metal deposition method and employed in the fabrication of solar cells for the first time.
Abstract: Large-area slantingly-aligned silicon nanowire arrays (SA-SiNW arrays) on Si(111) substrate have been fabricated by wet chemical etching with dry metal deposition method and employed in the fabrication of solar cells for the first time. The formation of SA-SiNW arrays possibly results from the anisotropic etching of silicon by silver catalysts. Superior to the previous cells fabricated with vertically-aligned silicon nanowire arrays (VA-SiNW arrays), the SA-SiNW array solar cells exhibit a highest power conversion efficiency of 11.37%. The improved device performance is attributed to the integration of the excellent anti-reflection property of the arrays and the better electrical contact of the cell as a result of the special slantingly-aligned structure. The high surface recombination velocity of minority carriers in SiNW arrays is still the main limitation on cell performance.

Journal ArticleDOI
TL;DR: The excellent agreement of the results with available experimental data indicates that the approach of this work could be used as an efficient predictive tool to help the optimization of dye-sensitized solar cells.
Abstract: We present a theoretical study of the lineup of the LUMO of Ru(II)-polypyridyl (N3 and N719) molecular dyes with the conduction band edge of a TiO2 anatase nanoparticle. We use density functional theory (DFT) and the Car?Parrinello scheme for efficient optimization of the dye?nanoparticle systems, followed by hybrid B3LYP functional calculations of the electronic structure and time-dependent DFT (TDDFT) determination of the lowest vertical excitation energies. The electronic structure and TDDFT calculations are performed in water solution, using a continuum model. Various approximate procedures to compute the excited state oxidation potential of dye sensitizers are discussed. Our calculations show that the level alignment for the interacting nanoparticle?sensitizer system is very similar, within about 0.1?eV, to that for the separated TiO2 and dye. The excellent agreement of our results with available experimental data indicates that the approach of this work could be used as an efficient predictive tool to help the optimization of dye-sensitized solar cells.

Journal ArticleDOI
TL;DR: The high stability of graphene-coated copper nanoparticles makes them economically a most attractive alternative to silver or gold nanocolloids, and will strongly facilitate the industrial use of metal nanOColloids in consumer goods.
Abstract: Metallic copper nanoparticles were synthesized by a bottom-up approach, and in situ coated with protective shells of graphene in order to get a metal nanopowder of high air stability and chemical inertness. Using an amphiphilic surfactant, a water-based copper nanocolloid could be prepared and successfully printed onto a polymer substrate by conventional ink-jet printing using household printers. The dried printed patterns exhibited strong metallic gloss and an electrical conductivity of >1 S cm−1 without the need for a sintering or densification step. This conductivity currently limits use in electronics to low current application or shielding and decorative effects. The high stability of graphene-coated copper nanoparticles makes them economically a most attractive alternative to silver or gold nanocolloids, and will strongly facilitate the industrial use of metal nanocolloids in consumer goods.

Journal ArticleDOI
TL;DR: The composite represents an ideal case of an environmentally friendly and stable catalyst, which works under heterogeneous as well as micro-heterogeneous conditions with the advantage of nanoscopic particles as the catalyst.
Abstract: In this paper, we report on the catalytic activity of a new metal nanoparticle–polymer composite consisting of Ag nanoparticles (NPs) and environmentally friendly ('green') chitosan. The polymer (chitosan) not only acted as the reducing agent for the metal ions, but also stabilized the product NPs by anchoring them. The majority of the particles produced in this way had sizes less than 5 nm. The catalytic activity of the composite was investigated photometrically by monitoring the reduction of 4-nitrophenol (4NP) in the presence of excess NaBH4 in water, under both heterogeneous and micro-heterogeneous conditions. The reaction was first order with respect to the concentration of 4NP. We also observed that the apparent rate constant, kapp, for the reaction was linearly dependent on the amount of Ag NPs present in the composite. Moreover, the turn-over frequency (TOF) of the catalyst was found to be (1.5 ± 0.3) × 10−3 s−1, when the reaction was carried out under heterogeneous conditions. The Ag NPs in the composite retained their catalytic activities even after using them for ten cycles. Our observations also suggest that the catalytic efficiency under micro-heterogeneous conditions is much higher than under heterogeneous conditions. Thus the composite we have represents an ideal case of an environmentally friendly and stable catalyst, which works under heterogeneous as well as micro-heterogeneous conditions with the advantage of nanoscopic particles as the catalyst.

Journal ArticleDOI
TL;DR: The induction of reactive oxygen species (ROS), degradation of mitochondrial membrane integrity, disruption of the actin cytoskeleton, and reduction in proliferation after stimulation with nerve growth factor were found after incubation with Ag nanoparticles at concentrations of 25 µg ml(-1) or greater, with a more pronounced effect produced by the hydrocarbon-based Ag nanoparticle in most cases.
Abstract: Silver (Ag) nanoparticles have unique plasmon-resonant optical scattering properties that are finding use in nanomedical applications such as signal enhancers, optical sensors, and biomarkers. In this study, we examined the chemical and biological properties of Ag nanoparticles of similar sizes, but that differed primarily in their surface chemistry (hydrocarbon versus polysaccharide), in neuroblastoma cells for their potential use as biological labels. We observed strong optical labeling of the cells in a high illumination light microscopy system after 24 h of incubation due to the excitation of plasmon resonance by both types of Ag nanoparticle. Surface binding of both types of Ag nanoparticle to the plasma membrane of the cells was verified with scanning electron microscopy as well as the internalization and localization of the Ag nanoparticles into intracellular vacuoles in thin cell sections with transmission electron microscopy. However, the induction of reactive oxygen species (ROS), degradation of mitochondrial membrane integrity, disruption of the actin cytoskeleton, and reduction in proliferation after stimulation with nerve growth factor were found after incubation with Ag nanoparticles at concentrations of 25 μ gm l −1 or greater, with a more pronounced effect produced by the hydrocarbon-based Ag nanoparticles in most cases. Therefore, the use of Ag nanoparticles as potential biological labels, even if the surface is chemically modified with a biocompatible material, should be approached with caution. (Some figures in this article are in colour only in the electronic version)

Journal ArticleDOI
TL;DR: A broad experimental study to elucidate the limiting factors to the solar cell performance and investigates the process of TiO(2) mesopore infiltration with spiro-MeOTAD, which aims to improve the physical composite formation.
Abstract: The solar to electrical power conversion efficiency for dye-sensitized solar cells (DSCs) incorporating a solid-state organic hole-transporter can be over 5%. However, this is for devices significantly thinner than the optical depth of the active composites and by comparison to the liquid electrolyte based DSCs, which exhibit efficiencies in excess of 10%, more than doubling of this efficiency is clearly attainable if all the steps in the photovoltaic process can be optimized. Two issues are currently being addressed by the field. The first aims at enhancing the electron diffusion length by either reducing the charge recombination or enhancing the charge transport rates. This should enable a larger fraction of photogenerated charges to be collected. The second, though less actively investigated, aims to improve the physical composite formation, which in this instance is the infiltration of mesoporous TiO2 with the organic hole-transporter 2,2',7,7'-tetrakis(N,N-di-p-methoxypheny-amine)-9,9'-spirobifluorene (spiro-MeOTAD). Here, we perform a broad experimental study to elucidate the limiting factors to the solar cell performance. We first investigate the charge transport and recombination in the solid-state dye-sensitized solar cell under realistic working conditions via small perturbation photovoltage and photocurrent decay measurements. From these measurements we deduce that the electron diffusion length near short-circuit is as long as 20 µm. However, at applied biases approaching open-circuit potential under realistic solar conditions, the diffusion length becomes comparable with the film thickness, 2 µm, illustrating that real losses to open-circuit voltage, fill factor and hence efficiency are occurring due to ineffective charge collection. The long diffusion length near short-circuit, on the other hand, illustrates that another process, separate from ineffective charge collection, is rendering the solar cell less than ideal. We investigate the process of TiO2 mesopore infiltration with spiro-MeOTAD by examining the cross-sectional images of and performing photo-induced absorption spectroscopy on devices with a range of thickness, infiltrated with spiro-MeOTAD with a range of concentrations. We present our interpretation of the mechanism for material infiltration, and by improving the casting conditions demonstrate efficient charge collection through devices of over 7 µm in thickness. This investigation represents an improvement in our understanding of the limiting factors to the dye-sensitized solar cell. However, much work, focused on composite formation and improved kinetic competition, is required to realize the true potential of this concept.

Journal ArticleDOI
TL;DR: Vertical light emitting diodes based on GaAs/InGaP core/shell nanowires, epitaxially grown on GaP and Si substrates, have been fabricated, enabling applications such as on-chip optical communication.
Abstract: Vertical light emitting diodes (LEDs) based on GaAs/InGaP core/shell nanowires, epitaxially grown on GaP and Si substrates, have been fabricated. The devices can be fabricated over large areas and can be precisely positioned on the substrates, by the use of standard lithography techniques, enabling applications such as on-chip optical communication. LED functionality was established on both kinds of substrate, and the devices were evaluated in terms of temperature-dependent photoluminescence and electroluminescence.

Journal ArticleDOI
TL;DR: A novel method was developed for preparing high specific surface area one-dimensional TiO(2) nanostructures co-doped with C, N and S by the nano-confinement effect, and results indicated that the anatase nanowires grew along the [101] direction.
Abstract: A novel method was developed for preparing high specific surface area (156.2 m2 g−1) one-dimensional TiO2 nanostructures co-doped with C, N and S by the nano-confinement effect. A nonmetal doping source (thiourea) was first intercalated into the inner space of H-titanate nanotubes prepared by the hydrothermal method, and then calcined at 450 °C for 2 h in air. The as-prepared C, N and S co-doped TiO2 nanowires exhibited high visible light and enhanced UV–vis activities in photocatalytic degradation of toluene in the gas phase. The samples were characterized by x-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, fast Fourier transform analysis, x-ray photoelectron spectroscopy, UV–vis diffuse reflectance spectra and photoluminescence. The results indicated that the anatase nanowires grew along the [101] direction. Doping TiO2 nanowires with C, N and S could not only broaden the light adsorption spectra into the visible region (400–600 nm), but also inhibit the recombination of photo-induced carriers. A mechanism is proposed to elucidate the nano-confinement effect of H-titanate nanotubes in the formation of C, N and S co-doping. Based on this mechanism, the effect of C, N and S co-doping on the band structure of TiO2 nanowires is also discussed.