Composites of conjugated poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) demonstrate an efficient photogeneration of mobile charge carriers. Thermal annealing of P3HT:PCBM based devices gives rise to a significant increase of the photovoltaic efficiency, as follows from measurements of the external quantum efficiency and the current-voltage characteristics. Upon annealing, the absorption spectrum of the P3HT:PCBM composite undergoes a strong modification, whereas in the pure components it remains unchanged. The absorption of the annealed blends becomes stronger and red shifted in the wavelength region ascribed to P3HT, while the absorption due to the PCBM contribution does not change. Atomic force microscope measurements on P3HT:PCBM disclose some variation in morphology due to the crystallization of PCBM. The concentration of the PCBM clusters and their size (up to 500 nm) were found to be correlated with the amount of PCBM in the blend. We have studied the performance of photovoltaic devices with different weight ratios of P3HT:PCBM, namely, 1:3, 1:2, 1:1.5, 1:1, 1:0.9, 1:0.8, and 1:0.7. The photocurrent and the power conversion efficiency showed a maximum between 1:1 and 1:0.9. We conclude the variation in the absorption spectrum and the red shift to result from molecular diffusion of PCBM out of the polymer matrix upon annealing. The growth of the PCBM clusters leads to formation of percolation paths and, therefore, improves the photocurrent. Above a certain concentration, the PCBM crystals provide mechanical stress on the metal electrode, therefore possibly damaging the interface. Optimization of the composite weight ratio reveals the important role played by morphology for the transport properties of bulk heterojunction P3HT:PCBM based solar cells.
Carbon nanotubes change their electronic properties when subjected to strains. In this study, the strain sensing characteristic of carbon nanotubes is used to develop a carbon nanotube film sensor that can be used for strain sensing on the macro scale. The carbon nanotube film is isotropic due to randomly oriented bundles of single-wall carbon nanotubes (SWCNTs). Using experimental results it is shown that there is a nearly linear change in voltage across the film when it is subjected to tensile and compressive stresses. The change in voltage is measured by a movable four-point probe in contact with the film. Multidirectional and multiple location strains can be measured by the isotropic carbon nanotube film.
The incorporation of 23 nm titanium dioxide nanoparticles into an epoxy matrix to form a nanocomposite structure is described. It is shown that the use of nanometric particles results in a substantial change in the behaviour of the composite, which can be traced to the mitigation of internal charge when a comparison is made with conventional TiO2 fillers. A variety of diagnostic techniques (including dielectric spectroscopy, electroluminescence, thermally stimulated current and photoluminescence) have been used to augment pulsed electro-acoustic space charge measurement to provide a basis for understanding the underlying physics of the phenomenon. It would appear that, when the size of the inclusions becomes small enough, they act cooperatively with the host structure and cease to exhibit interfacial properties, leading to Maxwell-Wagner polarization. It is postulated that the particles are surrounded by high charge concentrations in the Gouy-Chapman-Stern layer. Since nanoparticles have very high specific areas, these regions allow limited charge percolation through nano-filled dielectrics. The practical consequences of this have also been explored in terms of the electric strength exhibited. It would appear that there was a window in which real advantages accrue from the nano-formulated material. An optimum loading of about 10% (by weight) is indicated.
We report the new application of electrospun TiO2 fibres as an electrode for dye-sensitized solar cells (DSSCs). TiO2 fibre electrode was electrospun directly onto a conducting glass substrate from a mixture of titanium(IV) propoxide and poly(vinyl acetate) (PVAc) in dimethyl formamide. The TiO2 fibres are composed of one-dimensionally aligned nanofibrils about 20 nm thick with an islands-in-a-sea morphology, which was obtained from the phase separation of TiO2 gel and PVAc during the solidification process. The porous structure of the electrospun TiO2 electrode was found to be efficiently penetrated by a viscous polymer gel electrolyte. In order to improve the photocurrent generation, we treated the electrospun TiO2 electrode with TiCl4 aqueous solution. The rutile crystal was grown on the surface of anatase TiO2 fibres. An additional TiO2 layer increased the volume fraction of active materials, resulting in an increase of sensitizer adsorption. The energy conversion efficiency obtained from electrospun TiO2 electrodes with a PVDF-HFP gel electrolyte was over 90% of that from a liquid electrolyte system.
For the continuous monitoring, diagnosis, and treatment of neural tissue, implantable probes are required. However, sometimes such neural probes (usually composed of silicon) become encapsulated with non-conductive, undesirable glial scar tissue. Similarly for orthopaedic implants, biomaterials (usually titanium and/or titanium alloys) often become encapsulated with undesirable soft fibrous, not hard bony, tissue. Although possessing intriguing electrical and mechanical properties for neural and orthopaedic applications, carbon nanofibres/nanotubes have not been widely considered for these applications to date. The present work developed a carbon nanofibre reinforced polycarbonate urethane (PU) composite in an attempt to determine the possibility of using carbon nanofibres (CNs) as either neural or orthopaedic prosthetic devices. Electrical and mechanical characterization studies determined that such composites have properties suitable for neural and orthopaedic applications. More importantly, cell adhesion experiments revealed for the first time the promise these materials have to increase neural (nerve cell) and osteoblast (bone-forming cell) functions. In contrast, functions of cells that contribute to glial scar-tissue formation for neural prostheses (astrocytes) and fibrous-tissue encapsulation events for bone implants (fibroblasts) decreased on PU composites containing increasing amounts of CNs. In this manner, this study provided the first evidence of the future that CN formulations may have towards interacting with neural and bone cells which is important for the design of successful neural probes and orthopaedic implants, respectively.
Polystyrene nanofibres were electrospun with the inclusion of cationic surfactants, dodecyltrimethylammonium bromide (DTAB) or tetrabutylammonium chloride (TBAC), in the polymer solution. A small amount of cationic surfactant effectively stopped the formation of beaded fibres during the electrospinning. The cationic surfactants were also found to improve the solution conductivity, but had no effect on the viscosity. Only DTAB had an effect on the surface tension of the polymer solution, the surface tension decreasing slightly with an increase in the concentration of DTAB. The formation of beaded fibres was attributed to an insufficient stretch of the filaments during the whipping of the jet, due to a low charge density. Adding the cationic surfactants improved the net charge density that enhanced the whipping instability. The jet was stretched under stronger charge repulsion and at a higher speed, resulting in an exhaustion of the bead structure. In addition, a polymer/surfactant interaction was found in the polystyrene-DTAB solution system, while this interaction was not found in the poly styrene-TBAC system. The polymer/surfactant interaction led to the formation of thinner fibres than those formed in the absence of the interaction. The effects of a non-ionic surfactant, Triton X-405, on the electrospun fibres were also studied. The addition of Triton X-405 did not eliminate the fibre beads, but reduced the bead numbers and changed the morphology. Triton X-405 slightly improved the solution conductivity, and had a minor effect on the surface tension, but no effect on the viscosity.
Flower-like ZnO nanostructures, which consisted of sword-like ZnO nanorods, have been prepared by an organic-free hydrothermal process. The XRD pattern indicated that the flower-like ZnO nanostructures were hexagonal. The SAED and HRTEM experiments implied that the sword-like ZnO nanorods were single crystal in nature and preferentially grew up along the  direction. The effects of temperature, pH value and mineralizer on the morphology have been also investigated. It is considered that pH value is the main factor to influence the morphology because of its effect on the initial nuclei and growth environment of ZnO. Finally, the mechanism for organic-free hydrothermal synthesis of the flower-like ZnO nanostructure is discussed.
Nanocomposites of polyaniline (PANI)-titanium dioxide (PANI-TiO2) are prepared from a colloidal sol of TiO2 nanoparticles. The dc and ac conductivities of samples with different concentrations of PANI have been investigated as a function of frequency and temperature. The dc conductivity follows three-dimensional variable range hopping. The ac conductivity has been interpreted as a power law of frequency. The temperature variation of the frequency exponent suggests a correlated barrier hopping conduction process in the nanocomposites. A very large dielectric constant of about 3700 at room temperature has been observed. An electric modulus presentation is used to interpret the dielectric spectra. The interface between polyaniline and TiO2 plays an important role in yielding a large dielectric constant in the nanocomposite.
Technological growth in the electronics industry has historically been measured by the number of transistors that can be crammed onto a single microchip. Unfortunately, all good things must come to an end; spectacular growth in the number of transistors on a chip requires spectacular reduction of the transistor size. For electrons in semiconductors, the laws of quantum mechanics take over at the nanometre scale, and the conventional wisdom for progress (transistor cramming) must be abandoned. This realization has stimulated extensive research on ways to exploit the spin (in addition to the orbital) degree of freedom of the electron, giving birth to the field of spintronics. Perhaps the most ambitious goal of spintronics is to realize complete control over the quantum mechanical nature of the relevant spins. This prospect has motivated a race to design and build a spintronic device capable of complete control over its quantum mechanical state, and ultimately, performing computations: a quantum computer. In this tutorial we summarize past and very recent developments which point the way to spin-based quantum computing in the solid state. After introducing a set of basic requirements for any quantum computer proposal, we offer a brief summary of some of the many theoretical proposals for solid-state quantum computers. We then focus on the Loss-DiVincenzo proposal for quantum computing with the spins of electrons confined to quantum dots. There are many obstacles to building such a quantum device. We address these, and survey recent theoretical, and then experimental progress in the field. To conclude the tutorial, we list some as-yet unrealized experiments, which would be crucial for the development of a quantum dot quantum computer.
This tutorial article presents a 'bottom-up' view of electrical resistance starting from something really small, like a molecule, and then discussing the issues that arise as we move to bigger conductors. Remarkably, no serious quantum mechanics is needed to understand electrical conduction through something really small, except for unusual things like the Kondo effect that are seen only for a special range of parameters. This article starts with energy level diagrams (section 2), shows that the broadening that accompanies coupling limits the conductance to a maximum of q(2) / h per level (sections 3, 4), describes how a change in the shape of the self-consistent potential profile can turn a symmetric current-voltage characteristic into a rectifying one (sections 5, 6), shows that many interesting effects in molecular electronics can be understood in terms of a simple model (section 7), introduces the non-equilibrium Green function (NEGF) formalism as a sophisticated version of this simple model with ordinary numbers replaced by appropriate matrices (section 8) and ends with a personal view of unsolved problems in the field of nanoscale electron transport (section 9). Appendix A discusses the Coulomb blockade regime of transport, while appendix B presents a formal derivation of the NEGF equations. MATLAB codes for numerical examples are listed in appendix C.
The properties of organic/inorganic poly(3-hexylthiophene) (P3HT):TiO2 nanocomposite films and nanocomposite based solar cells as a function of TiO2 concentration and the solvent used for the film fabrication were studied. For low nanoparticle concentration (20-30%) the device performance was worse compared to pure P3HT, while for nanoparticle concentration of 50% and 60% significant improvements were obtained. P3HT photoluminescence quenching in 600-800 nm spectral region changes by a factor of two for the increase in TiO2, concentration from 20% to 60%, while the AM1 power conversion efficiency increases similar to35 times. Photoluminescence quenching and solar cell efficiency were found to be strongly dependent not only on nanoparticle concentration but also on the solvent used for spin-coating. The changes in the film and device properties were explained by the change in the film morphology. For optimal fabrication conditions, external quantum efficiency up to 15% and AM1 power conversion efficiency of 0.42% were obtained.
Nanomechanical properties of single-walled carbon nanotube (SWNT) reinforced epoxy composites with varying weight percentage (0, 1, 3, and 5 wt%) of nanotubes were measured by nanoindentation and nanoscratch techniques. Hardness and elastic modulus were measured using a nanoindenter. Scratch resistance and scratch damage were studied using the AFM tip sliding against the SWNT reinforced sample surfaces. Nanoindentation/nanoscratch deformation and fracture behaviour was studied by in situ imaging of the indentation impressions/scratch tracks. Viscoelastic properties of the nanocomposites were measured using nanoindentation dynamic mechanical analysis tests. The reinforcing mechanisms are discussed with reference to the nanotube dispersion, interfacial bonding, and load transfer in the SWNT reinforced polymer composites.
Copper-doped zinc oxide nanowires were fabricated on copper-coated silicon substrate by sintering a mixture of zinc oxide and graphite powders at high temperature. Copper functioned as a catalyst in the zinc oxide nanowire growth and was incorporated during the growth as a dopant. The size of copper-doped zinc oxide nanowires ranges from 30 to 100 nm in diameter and tens to hundreds of microns in length. The photoluminescent excitation spectra showed multiple absorption peaks in the ultraviolet and blue/green region. Correspondingly, broad and continuous photoluminescence spectra were observed extending from the ultraviolet to the red region with shoulder peaks at room temperature, which is different from that of the bulk. The x-ray photoelectron spectroscopy and low temperature photoluminescence were employed to analyse the luminescent mechanism.
One of the most popular methods for calibrating the spring constant of an atomic force microscope cantilever is the thermal noise method. The usual implementation of this method has been to position the focused optical spot on or near the end of the cantilever, acquire a force curve on a hard surface to characterize the optical lever sensitivity and to then measure the thermal motion of the cantilever. The equipartition theorem then allows the spring constant to be calculated. In this work, we measured the spring constant as a function of the spot along the length of the cantilever. The observed systematic variation in the spring constant as a function of this position ranged from approximate to15% for a short 60 mum cantilever up to approximate to50% for a 225 mum cantilever we examined. In addition, the thermally calibrated spring constants systematically disagreed with spring constants calibrated using the Sader and Cleveland methods: by approximate to50% for the short 60 mum cantilever and by approximate to25% for the longest, 225 mum cantilever. By using a model that accounts for the spot diameter and position on the cantilever, the thermally measured spring constants were brought into better than 10% agreement with the other methods.
Single-walled carbon nanotubes are candidates for a number of electronics and sensing applications, provided nanotubes with semiconducting and metallic band structure can be separated. Dielectrophoresis has recently been demonstrated as a route towards the separation of metallic nanotubes from semiconducting nanotubes, and is moreover a method for controlled assembly of the nanotubes on microstructures that has the possibility to be scaled to wafer-level manufacturing. In this paper we will present numerical calculations of carbon nanotubes subjected to dielectrophoresis, drag force and Brownian motion induced by application of an ac voltage to a set of microelectrodes in a microliquid channel. We calculate the probability of capturing various types of carbon nanotubes, the time frame for the assembly and the efficiency of separation, for different experimental parameters. Our results suggest that relatively low frequencies, where both semiconducting and metallic nanotubes are subject to positive dielectrophoresis, may be optimal for separation, due to large differences in the magnitude of the dielectrophoretic force.
The controlled deposition of functional layers is the key to converting nanomechanical cantilevers into chemical or biochemical sensors. Here, we introduce inkjet printing as a rapid and general method to coat cantilever arrays efficiently with various sensor layers. Self-assembled monolayers of alkanethiols were deposited on selected Au-coated cantilevers and rendered them sensitive to ion concentrations or pH in liquids. The detection of gene fragments was achieved with cantilever sensors coated with thiol-linked single-stranded DNA oligomers on An. A selective etch protocol proved the uniformity of the monolayer coatings at a microscopic level. A chemical gas sensor was fabricated by printing thin layers of different polymers from dilute solutions onto cantilevers. The inkjet method is easy to use, faster and more versatile than coating via microcapillaries or the use of pipettes. In addition, it is scalable to large arrays and can coat arbitrary structures in non-contact.
Nanotechnology is an area receiving increasing attention as progress is made towards tailoring the morphology of polymeric biomaterial for a variety of applications. In the present study an attempt was made to electrospin poly(L-lactide-co-glycolide) biodegradable polymer nanofibres. In this process, polymer fibres with diameters down to the nanometre range are formed by subjecting a fluid jet to a high electric field. The nanofibres were collected on to a rotating Teflon mandrel and fabricated to tubes or conduits, to function as nerve guidance channels. The feasibility of in vivo nerve regeneration was investigated through several of these conduits. The biological performance of the conduits were examined in the rat sciatic nerve model with a 10 mm gap length. After implantation of the nanofibre nerve guidance conduit to the right sciatic nerve of the rat, there was no inflammatory response. One month after implantation five out of eleven rats showed successful nerve regeneration. None of the implanted tubes showed tube breakage. The nanofibre nerve guidance conduits were flexible, permeable and showed no swelling. Thus, these new poly(L-lactide-co-glycolide) nanofibre conduits can be effective aids for nerve regeneration and repair. Improvements could be done by impregnating nerve growth factors or Schwann cells and may lead to clinical applications.
We report simultaneous lateral growth of a high density of highly oriented, metal-catalyzed silicon nanowires on a patterned silicon substrate and bridging of nanowires between two vertical silicon sidewalls, which can be developed into electrodes of an electronic device. After angled deposition of catalytic metal nanoparticles on one of two opposing vertical silicon surfaces, we used a metal-catalyzed chemical vapour deposition process to grow nanowires and eventually form mechanically robust 'nanobridges'. The growth and bridging of these nanowire arrays can be integrated with existing silicon processes. This method of connecting multiple nanowires between two electrodes offers the high surface-to-volume ratio needed for nanosensor applications.
Gold nanoparticles immobilized on a silicon wafer were used to semi-quantitatively study the Raman enhancement from the individual particles and the interparticle coupling, with p-aminothiophenol (p-ATP) molecules as Raman probes. At low coverage of gold nanoparticles on the surface, the gross intensity of the p-ATP Raman peaks was found to be proportional to the number of particles per unit area, suggesting that the spectra could be a sum of contributions from the isolated particles while the interparticle coupling could be neglected. However, a sharp nonlinear increase in the intensity of the p-ATP Raman peaks versus the number of particles per unit area was observed at higher coverages, indicating that the contribution from the interparticle coupling became remarkable at reduced interparticle distances. By comparing the apparent enhancement factors of the isolated particles with that of particles at saturated surface coverage, a 10-20 times extra-enhancement can be derived which could be attributed to the interparticle electromagnetic coupling. The critical centre-to-centre interparticle distance for significant electromagnetic coupling is found to be about twice the particle diameter. Further efforts will be made in quantitatively correlating the present results with the possible alterations in plasmon resonance of the assembled gold nanoparticles by the substrate and the interparticle coupling.
Scale dependence of micro/nanotribological properties is studied for various materials, coatings and lubricants used in micro/nanoelectromechanical systems (MEMS/NEMS). The adhesive force and friction force dependence on rest time and sliding velocity and the effect of relative humidity and temperature on the scale dependence of these properties is studied. The scale dependence of the coefficient of friction is attributable to the sample surface roughness and the scan size. For larger scan sizes the sliding interface encounters larger asperities and so friction force is higher. The adhesive force is higher on the microscale although on the nanoscale surface forces such as electrostatic attraction that are generally negligible on the microscale can become dominant. The difference in the adhesive force on the micro- and nanoscale for different rest times, relative humidities and temperatures is due to the meniscus force dependence on the sample surface roughness. The velocity dependence of the friction force shows significant scale dependence due to the scale dependent roughness and the higher contact pressures that are encountered on the nanoscale.