Since the discovery of carbon nanotubes in 1991 , researchers have envisaged potential applications such as nanoscale electronic circuits and the construction of complex carbon-based nano-machines. Thus, the assembly of basic building blocks of complex nano-architectures, such as conjugated polymers and nanotubes, has been a driving goal of much of the nano-science community. A first step towards realizing this goal may be the attachment to, or modification by carbon nanotubes of structures such as polymers. This leads to the possibility of assembling individual polymer molecules onto carbon nanotubes with the net effect being the modification of the polymer's electronic properties and structure in a predictable way. To accomplish this, clearly, a more detailed understanding of the interactions between conjugated polymers and carbon nanotubes must be sought. In this paper, we describe the assembly of the polymer, poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) (PmPV), into a coating around single-walled carbon nanotubes. Using scanning tunnelling microscopy, and scanning tunnelling spectroscopy, we demonstrate that the low-energy electronic structure of the assembled material is dominated by the one-dimensional nature of the nanotube as reflected in van Hove singularities. Further, we examine the modifications to electronic structure at higher energies using spectroscopy, which suggests that the polymer's electronic structure is altered by the introduction of nanotubes. (Some figures in this article are in colour only in the electronic version).
Zn2SiO4:Mn2+ nanocrystals were grown in an oxidized porous silicon layer using a chemical impregnation method. Apparently two classes of samples have been obtained. One is characterized by the formation of alpha-phase zinc silicate crystalline particles, which show green luminescence, and the other one is characterized by beta-phase particles, showing yellow luminescence. It was found that in general prolonged annealing, as well as a high degree of impregnation leads to the formation of green-luminescent samples. The decay time of both yellow and green luminescence decreases with the concentration of Mn activator, while generally the decay time of yellow luminescence is considerably larger than that of green luminescence.
Molecular dynamics studies are performed to investigate the evolution of the deformed region during nanoindentation of silicon. A new approach based on a local strain diagnostic to identify and characterize the plastic rearrangements occurring during indentation is presented. During indentation, the response of the substrate changes from elastic to plastic to relieve the accumulated stress. The plastic rearrangements involve the displacement of atoms from the lattice sites to interstitial sites. The formation of interstitials results in the transformation of the deformed region to a denser amorphous phase. During retraction of the tip, the deformed region undergoes an incomplete elastic recovery signifying the plastic nature of rearrangements.
A simple linear electromechanical model for an electrostatically driven resonating cantilever is derived. The model has been developed in order to determine dynamic quantities such as the capacitive current flowing through the cantilever-driver system at the resonance frequency, and it allows us to calculate static magnitudes such as position and voltage of collapse or the voltage versus deflection characteristic. The model is used to demonstrate the theoretical sensitivity on the attogram scale of a mass sensor based on a nanometre-scale cantilever, and to analyse the effect of an extra feedback loop in the control circuit to increase the Q factor.
By combining conventional silicon microfabrication and direct three-dimensional growth using electron-beam induced carbon contamination, we have developed a scheme for fabricating nanotweezers with a gap of 25 nm. Four silicon oxide cantilevers with a spacing of 1.5 mum extending over an edge of a silicon support chip, were covered with a thin layer of metal. By focusing an electron beam at the ends of the cantilevers, narrow supertips grew from the substrate. Careful alignment of the substrate made the supertips converge to form a nanoscale gap. We demonstrate customization of the shape and size of the tweezer arms, using a simple scheme that allows conveniently fine-tuning of the tip features and the gap to within 5 nm. The supertips can be metallized subsequently, to be made conducting, without significantly affecting the shape of the tweezers. By applying a voltage on the outer electrodes with respect to the inner two electrodes, the gap can be opened and closed. This enables the device to grab and manipulate small particles, with the option of direct electrical measurement on the particle. The advantage of our approach is that no voltage difference is applied between the tweezer arms, making the device ideal for application with such fragile structures as organic objects.
Chemically capped CdS nanoparticles are embedded in porous silicon (PS) by a dip coating method. Atomic force microscopy measurements reveal that the PS surface is covered with CdS nanoparticles forming well-defined rectangular blocks of nearly uniform size (200 x 200 nm(2)). Photoelectron spectroscopy and energy dispersive x-ray analysis confirm the presence of CdS in PS. Optical and electrical properties of the heterojunctions so-formed are investigated. Junction characteristics show that the composite so-formed exhibits very high forward current density (145 mA cm(-2)) and high reverse breakdown voltage (15 V).
The use of metal nanoparticles as seed layers for controlling the microstructures of tin oxide (SnO2) films on temperature controllable micromachined platforms has been investigated. The study is focused on SnO2 due to its importance in the field of chemical microsensors. Nanoparticle seeds of iron, cobalt, nickel, copper and silver were formed by vapour deposition on the microhotplates followed by annealing at 500 degreesC prior to self-aligned SnO2 deposition. Significant control of SnO2 grain sizes, ranging between 20 and 121 nm, was achieved depending on the seed-layer type. A correlation was found between decreasing the SnO2 grain size and increasing the melting temperature of the seed-layer metals, suggesting the use of high temperature metals as being appropriate choices as seed layers for obtaining a smaller SnO2 grain structure. Smaller grain diameters resulted in high sensitivity in 90 ppm ethanol illustrating the benefits of nanoparticle seeding for chemical sensing. The initial morphology, particle size and distribution of the seed layers was found to dictate the final SnO2 morphology and grain size. This paper not only demonstrates the possibility of depositing nanostructured oxide materials for chemical microsensor applications, but also demonstrates the feasibility of conducting combinatorial research into nanoparticle growth using temperature controllable microhotplate platforms. This paper also demonstrates the possibility of using multi-element arrays to form a range of different types of devices that could be used with suitable olfactory signal processing techniques in order to identify a variety of gases.
Experimental and theoretical investigations of nanoindentation into fcc silver and bcc iron were performed, including an investigation of the effect near the grain boundaries. Experimentally, micrograph images of the surfaces and force-depth curves were obtained which were used to determine the hardness and Young's modulus of the materials. Molecular dynamics simulations, on smaller systems than those investigated experimentally, exhibit the main experimental attributes, showing the plastic deformation of the substrates with piling-up of the work material along the indenter sides. The simulations also show how defects in the substrates form and these are contrasted with the various materials under investigation.
The controlled assembly of metal nanoparticles into macroscopic materials using DNA oligonucleotides has opened new directions of research in nanoscience and nanotechnology. Here, we describe recent ab initio calculations on structural and electronic properties of the subsystems forming these materials: bare and thiol-passivated gold nanoclusters, gold nanowires and fragments of DNA chains. Our results indicate that gold nanoclusters are distorted dramatically by a passivating methylthiol monolayer, that monatomic gold chains are stable in zigzag geometries and that dry acidic gimel -DNA is a good insulator These results provide useful insights towards the complete understanding, design and proper utilization of hybrid DNA-gold nanostructured materials.
Initial stages of epitaxial growth and formation of CaF2 nanostructures on Si(001) were studied. A variety of nanostructures were grown including ultrathin two-dimensional layers at 750 degreesC, quasi-one-dimensional stripes at 650 degreesC and well-ordered dots at lower growth temperatures. Atomic force microscopy and reflection high-energy electron diffraction were used to measure the nanostructure shape and lattice orientation. The evolution of the surface electronic structure under different growth conditions was studied by ultraviolet photoelectron spectroscopy and metastable de-excitation spectroscopy. The leading role of the wetting layer in high-temperature formation of the fluorite-silicon interface was established.
Building upon traditional nb initio quantum chemistry calculations, we present a theoretical study of the transport properties of C-60 molecules connected in realistic ways to Al metallic electrodes. A Green function technique that combines standard density functional calculations with an effective tight-binding model allows us to calculate the ab initio electrical transport properties of the fullerenes in contact with the electrodes. Our results are relevant for the correct interpretation of scanning tunnelling microscope visualization of these and related molecules adsorbed on different substrates as well as for predicting the electrical conduction properties of molecular devices currently under study.
The peculiarities of the local oxidation process of ultrathin amorphous titanium films by scanning probe microscope are discussed. It is shown that the tip-induced oxidation process can be considered as electrochemical anodic oxidation. A model of the tip-induced oxidation kinetics is proposed. It is shown that film resistance, relative humidity, applied voltage and duration of oxidation are effects on the rate and resolution of the process. The possibility of formation of 8 mn oxide patterns by tip-induced oxidation is demonstrated.
Angle resolved emission spectra of microcavities with quantum wells embedded in the active layer have been investigated under the resonant and quasi-resonant excitation into the lower polariton (LP) branch. The conditions have been found at which macroscopic filling of polariton modes is reached in the strong coupling regime. The strong nonlinear effects in the intensity and the degree of polarization of polariton emission have been observed and investigated. Both experimentally observed renormalization of the dispersion and strong narrowing of polariton mode have been explained using an interacting polariton model predicting that the coupling between the LP mode E-LP(k) and the mode of composite polaritons E-CP(k) = 2homega(k(0)) - E-LP(2k(excitation) - k) is qualitatively different from that of the exciton and photon.
The equivalent electrical circuit of a single C-60 electromechanical transistor in a planar lay-out is presented using its experimental STM characteristics. This circuit is used to demonstrate that such a hybrid molecular electronic device can be used as a class A amplifier, a NOT or NOR gate and to implement an SRAM memory point. All the devices are simulated using the SPICE routine to find their optimum load resistance and cantilever grid size. The class A amplifier can operate with a cut-off frequency of a few gigahertz while the logic gate and memory are limited to a few tens of megahertz, but for a very small power design in the picowatt range.
A new NMR quantum computer scheme based on semiconductor nanostructures is discussed theoretically. We also propose using an infrared laser as a new independent instrument for manipulating qubits. Methods of single- and many-qubit gate realization are discussed. Resonant transfer of the electron to several quantum dots helps in implementing complex many-qubit gates. The energy spectrum of the spin-qubit system is obtained. This variant of a quantum computer is simpler to control and more coherent. The distance between qubits and hence the structural requirements are more achievable using modem nanotechnology.
Modulation-doped N-AlGaAs/GaAs/InAs/GaAs/InAs/GaAs heterostructures with InAs quantum dots (QDs) in the device channel have been grown and investigated. Their photoluminescence spectra and electron transport properties in both low and high electric fields were studied. Using these structures. modulation-doped field effect transistors have been fabricated and analysed. It was demonstrated that the QD field effect transistors present a new type of hot-electron device. promising for high-speed applications.
Electronic devices have decreased in size to the extent that they are now in the nanometric range, and this requires quantum mechanics which understands their operation and optimization. Many features associated with quantum effects are not desirable from an engineering point of view: the charging of a nanocapacitor runs into a Coulomb blockade; the dielectric constants of nanoparticles is much reduced; the binding energy of the shallow dopants for nanoscale quantum dots is a multiple of k(B) T resulting in intrinsic behaviour regardless of the doping density; etc. There are other serious problems preventing the implementation of redundancy and robustness which are so essential to electronic devices, for example, inadvertent defects cannot be avoided, contacts and input/output have to be sufficiently small resulting in pushing beyond current lithographic technology. This paper discusses some of the fundamental points which still require more understanding, and what lies ahead in nanoelectronics involving augmented cavity interactions.
The first successful results of the application of multifractal analysis to a quantitative description of mosaic-structure peculiarities, that are typical of a GaN epitaxial layer with hexagonal modification grown on (0001) sapphire substrates, have been obtained. The characteristic size of the mosaic structure has been measured to be 200-800 nm. The direct dependence of mobility on the multifractal parameters (the Renyi dimension or the self-organization degree and the order-degree index) of the surface topology of the mosaic structure has been observed for all GaN layers investigated.
Different methods have been applied for the stretching of DNA molecules on chemically functionalized surfaces by various modified reagents, i.e. 3-aminopropyltriethanoxysilane or polylysine on mica and 2-mercaptoethylamine on Au(111)/mica by a moving interface technique, magnesium cation (Mg2+) on mica by a spin-stretching method and DNA on an atomic-level flat mica by a free-flowing method. The long lambda -DNA molecule is well elongated using the moving interface technique. The DNA molecule array density can be controlled by the change of surface charge density and the DNA concentration. On the other hand, the novel free-flowing method is very useful for the alignment of short polynucleotide molecules. Shadow-mask evaporation has been used to fabricate a gold electrode contacted electrically to the oriented DNA molecules. The intrinsic electrical properties of individual DNA molecules are directly measured using a conducting probe atomic force microscope equipped with a gold-coated conductive tip. The DNA molecule is considered as a promising molecular wire.