We studied various gas molecules (NO2, O-2, NH3, N-2, CO2, CH4, H2O, H-2, Ar) on single-walled carbon nanotubes (SWNTs) and bundles using first principles methods. The equilibrium position, adsorption energy, charge transfer, and electronic band structures are obtained for different kinds of SWNTs. Most molecules adsorb weakly on SWNTs and can be either charge donors or acceptors to the nanotubes. We find that the gas adsorption on the bundle interstitial and groove sites is stronger than that on individual nanotubes. The electronic properties of SWNTs are sensitive to the adsorption of certain gases such as NO2 and O-2. Charge transfer and gas-induced charge fluctuation might significantly affect the tran,;port properties of SWNTs. Our theoretical results are consistent with recent experiments.
In this report, exploitation of the unique properties of single-walled carbon nanotubes (SWNT) leads to the achievement of direct electron transfer with the redox active centres of adsorbed oxidoreductase enzymes. Flavin adenine dinucleotide (FAD), the redox active prosthetic group of flavoenzymes that catalyses important biological redox reactions and the flavoenzyme glucose oxidase (GOx), were both found to spontaneously adsorb onto carbon nanotube bundles. Both FAD and GOx were found to spontaneously adsorb to unannealed carbon nanotubes that were cast onto glassy carbon electrodes and to display quasi-reversible one-electron transfer. Similarly, GOx was found to spontaneously adsorb to annealed, single-walled carbon nanotube paper and to display quasi-reversible one-electron transfer. In particular, GOx immobilized in this way was shown, in the presence of glucose, to maintain its substrate-specific enzyme activity. It is believed that the tubular fibrils become positioned within tunnelling distance of the cofactors with little consequence to denaturation. The combination of SWNT with redox active enzymes would appear to offer an excellent and convenient platform for a fundamental understanding of biological redox reactions as well as the development of reagentless biosensors and nanobiosensors.
We demonstrate continuous label-free detection of two cardiac biomarker proteins (creatin kinase and myoglobin) using an array of microfabricated cantilevers functionalized with covalently anchored anti-creatin kinase and anti-myoglobin antibodies. This method allows biomarker proteins to be detected via measurement of surface stress generated by antigen-antibody molecular recognition. Reference cantilevers are used to eliminate thermal drifts, undesired chemical reactions and turbulences from injections of liquids by calculating differential deflection signals with respect to sensor cantilevers. The sensitivity achieved for myoglobin detection is below 20 mug ml(-1). Both myoglobin and creatin kinase could be detected independently using cantilevers functionalized with the corresponding antibodies, in unspecitic protein background. This approach permits the use of up to seven different antigen-antibody reactions simultaneously, including an additional thermomechanical and chemical in situ reference. Applications lie in the field of early and rapid diagnosis of acute myocardial infarction.
We study the pull-in voltage characteristics of several nanotube electromechanical switches, such as double-wall carbon nanotubes suspended over a graphitic ground electrode. We propose parametrized continuum models for three coupled energy domains: the elastostatic energy domain. the electrostatic energy domain and the van der Waals energy domain. We compare the accuracy of the continuum models with atomistic simulations. Numerical simulations based on continuum models closely match the experimental data reported for carbon-nanotube-based nanotweezers. An analytical expression, based on a lumped model, is derived to compute the pull-in voltage of cantilever and fixed-fixed switches. We investigate the significance of van der Waals interactions in the design of nanoelectromechanical switches.
Electrical contacts between a metal probe and molecular monolayers have been characterized using conducting, atomic force microscopy in an inert environment and in a voltage range that yields reversible current-voltage data. The current through alkanethiol monolayers depends on the contact force in a way that is accounted for by the change of chain-to-chain tunnelling with film thickness. The electronic decay constant, beta(N), was obtained from measurements as a function of chain length at constant force and bias, yielding beta(N) = 0.8 +/- 0.2 per methylene over a +/-3 V range. Current-voltage curves are difficult to reconcile with this almost constant value. Very different results are obtained when a gold tip contacts a 1,8-octanedithiol film. Notably, the current-voltage curves are often independent of contact force. Thus the contact may play a critical role both in the nature of charge transport and the shape of the current-voltage curve.
Knowledge of the interaction forces between surfaces gained using an atomic force microscope (AFM) is crucial in a variety of industrial and scientific applications and necessitates a precise knowledge of the cantilever spring constant. Many methods have been devised to experimentally determine the spring constants of AFM cantilevers. The thermal fluctuation method is elegant but requires a theoretical model of the bending modes. For a rectangular cantilever, this model is available (Butt and Jaschke). Detailed thermal fluctuation measurements of a series of AFM cantilever beams have been performed in order to test the validity and accuracy of the recent theoretical models. The spring constant of rectangular cantilevers can also be determined easily with the method of Sader and White. We found very good agreement between the two methods. In the case of the V-shaped cantilever, we have shown that the thermal fluctuation method is a valid and accurate approach to the evaluation of the spring constant. A comparison between this method and those of Sader-Neumeister and of Ducker has been established. In some cases, we found disagreement between these two methods; the effect of non-conservation of material properties over all cantilevers from a single chip is qualitatively invoked.
We present here the use of amino-terminated DNA strands in functionalizing the open ends and defect sites of oxidatively prepared single-walled carbon nanotubes, an important first step in realizing a DNA-guided self-assembly process for carbon nanotubes.
A novel chemical functionalization method for multiwalled carbon nanotubes (MWNTs), through an oxidation and silanization process, is presented. The method allows us to have different organo-functional groups attached to the MWNTs, which improves their chemical compatibility with specific polymers for producing new nanotube-based composites. The corresponding moieties were characterized by infrared, Raman and energy dispersion spectroscopies.
Conducting nanotubes of polyaniline (PANI) about 80-180 nm in diameter were synthesized by a chemical template-free method in the presence of D-10-camphorsulfonic acid (D-CSA) as the dopant, and ammonium persulfate ((NH4)(2)S2O8) as the oxidant. The effect of synthetic conditions, such as the molar ratio of D-CSA to aniline (An), the concentration of D-CSA in the polymerization media, the reaction temperature and time, on the morphology and size as well as the electrical properties of the PANI-(D-CSA) was investigated. It was found that the above synthetic conditions, especially the molar ratio of D-CSA to An, strongly affected the morphology and formation probability of the resulting PANI. The micelles formed by D-CSA and anilinium cations act as the templates in the formation of PANI-(D-CSA) nanotubes.
Alumina-borate/PVA composite fibres were prepared using sol-gel processing and an electrospinning technique. After calcination of the thin fibres, ultra-fine fibres of alumina-borate oxide with a diameter of about 550 nm could be prepared. The fibres were characterized by SEM, XRD and FT-IR. The results showed that the crystalline phase and morphology of alumina-borate fibres were largely influenced by the calcination temperature.
We report on the fabrication of field emission microcathodes which use carbon nanotubes as the field emission source. The devices incorporated an integrated gate electrode in order to achieve truly low-voltage field emission. A single-mask, self-aligned technique was used to pattern the gate, insulator and catalyst for nanotube growth. Vertically-aligned carbon nanotubes were then grown inside the gated structure by plasma-enhanced chemical vapour deposition. Our self-aligned fabrication process ensured that the nanotubes were always centred with respect to the gate apertures (2 mum diameter) over the entire device. In order to obtain reproducible emission characteristics and to avoid degradation of the device, it was necessary to operate the gate in a pulsed voltage mode with a low duty cycle. The field emission device exhibited an initial turn-on voltage of 9 V. After the first measurements, the turn-on voltage shifted to 15 V, and a peak current density of 0.6 mA cm(-2) at 40 V was achieved, using a duty cycle of 0.5%.
The transmission of light through an aperture in a metal film is extremely small when the aperture diameter is much smaller than the optical wavelength. But when the metal surface surrounding the subwavelength hole is corrugated, the incident light can couple to surface plasmons (SP), excitation modes on the metal surface. A resonant interaction leads to an enhanced transmission at wavelengths determined by the corrugation pitch. We discuss applications of the SP enhanced transmission in near-field scanning optical microscopy and in high-density optical data storage.
Two polymer-montmorillonite (MMT) nanocomposites have been synthesized by in situ intercalative polymerization. The styrene monomer is intercalated into the interlayer space of organically modified MMT, a layered clay mineral. Upon the intercalation, the complex is subsequently polymerized in the confinement environment of the interlayer space with a free radical initiator, 2,2-azobis isobutyronitrile. The aniline monomer is also intercalated and then polymerized within the interlayer space of sodium- and copper-MMT initiated by ammonium peroxodisulphate and interlayer copper cations respectively. X-ray diffraction indicates that the MMT layers are completely dispersed in the polystyrene matrix and an exfoliated structure has been obtained. The resulting polyaniline-MMT nanocomposites show a highly ordered structure of a single polyaniline layer stacked with the MMT layers. Fourier transform infrared spectra further confirm the intercalation and formation of both polymer-MMT nanocomposites.
The idea of nature as engineer is an old one, but the realization that this metaphor can be extended (should we say retracted?) to the molecular scale has become common currency only over the past two decades or so. Two reasons for this are perhaps paramount. First, the picture of the cell has been transformed from that of a 'wet chemical' melange-'a vessel, filled with a homogeneous solution, in which all chemical processes take place', as Franz Hofmeister put it in 1901-into an image of a sort of fluid factory, a production plant in which molecular machinery works in near-fantastic orchestration to generate complex products from raw materials. This mechanism is self-assembling, self-repairing and self-replicating. The concept of proteins and nucleic acids as 'molecular machines' is now a mainstream one in cell biology. Second, technological advances have made us accustomed to the idea that engineering can be conducted at scales too small to see with the naked eye, yet employing principles-mechanical, electrical. hydraulic, optical, tribological-familiar from the macroscopic world. Molecular electronics and computing, microelectromechanical devices and nanotechnology, are now mainstream concepts, and are validated by at least some degree of physical realization. In this article I shall briefly review some of nature's principles and practices at the molecular, supramolecular and submicrometre scales, and attempt to illustrate how these can be adapted for developing, synthetic chemical and materials systems sharing the kind of superior properties and special functions that natural systems exhibit.
Techniques are presented for making ohmic contacts to nanowires with a thick oxide coating. Although experiments were carried out on Bi nanowires, the techniques described in this paper are generally applicable to other nanowire systems. Metal electrodes are patterned to individual Bi nanowires using, electron beam lithography. Imaging the chemical reaction on the atomic scale with in situ high-resolution transmission electron microscopy shows that annealing in H-2 or NH3 can reduce the nanowires' oxide coating completely. The high temperatures required for this annealing, however, are not compatible with the lithographic techniques. Low-resistance ohmic contacts to individual bismuth nanowires are achieved using a focused ion beam (FIB) to first sputter away the oxide layer and then deposit Pt contacts. By combining electron beam lithography and FIB techniques, ohmic contacts stable from 2 to 400 K are successfully made to the nanowires. A method for preventing the burnout of nanowires from electrostatic discharge is also developed.
The proposed nanometre-sized electronic devices are generally expected to show an increased probability of errors both in manufacturing and in service. Hence, there is a need to use fault-tolerant techniques in order to make reliable information processing systems out of those devices. Here we examine and compare four fault-tolerant techniques: R-fold multiple redundancy; cascaded triple modular redundancy; von Neumann's multiplexing method; and a reconfigurable computer technique. It is shown that the reconfiguration technique is the most effective technique, able to cope with manufacturing defect rates of the order of 0.01-0.1, but the technique requires enormous amounts of redundancy, of the order of 10(3)-10(5). However, in the case of transient errors, multiple modular redundancy and multiplexing are the only feasible options.
For the first time, silica nanofibres with diameters of 200-400 nm were prepared by using electrospun fibres of polyvinylalcohol/silica composite as precursor. The products were characterized by the scanning electron microscopy (SEM), Fourier transform-infrared spectroscopy (FT-IR), and x-ray diffractometry (XRD) methods. The results showed that the crystalline phase and morphology of silica fibres were substantially influenced by the calcination temperature. The amorphous silica nanofibres could be obtained at 550degreesC.
Atomic force microscopy (AFM) is a technique to image surfaces with unprecedented vertical and lateral resolution. Many related techniques have been derived front AFM, taking advantage of local interactions between a tip on a cantilever and a surface. However, cantilevers can also be used for sensing applications. These so-called nanosensors feature extreme sensitivity for the detection of chemical vapours or adsorption of molecules. Upon adsorption to the cantilever surface, the molecules cause the cantilever to bend. Thus physical, chemical or biochemical processes are directly transduced into nanomechanical motion. We show that measurement of the deflection of a single cantilever might be misleading. Reliable information can only be obtained by using a sensor cantilever and at least one reference cantilever integrated into an array. We have built an electronic nose using polymer layers as partially selective cantilever coatings to recognize chemical vapours and odours by evaluating the cantilevers' bending pattern. Major applications lie in the fields of process and quality control, biosensing, medical diagnostics, molecular recognition and proteomics.
We report on the development of a nanoplotter that consists of an array of microfabricated probes for parallel dip-pen nanolithography. Two types of device have been developed by using microelectromechanical systems micromachining technology. The first consists of 32 Silicon nitride cantilevers separated by 100 mum, while the second consists of eight boron-doped silicon tips separated by 310 mum. The former offers writing and imaging capabilities, but is challenged with respect to tip sharpness. The latter offers smaller linewidths and increased imaging capabilities at the expense of probe density. Parallel generation of nanoscopic monolayer patterns with a minimum linewidth of 60 nm has been demonstrated using an eight-pen microfabricated probe array.