Besides the conventional forms of carbon, graphite and diamond, new forms of carbon such as fullerenes, carbon nanotubes and carbon onions have been discovered. A majority of current research focuses on the potential applications of carbon nanotubes (CNT). Carbon nanotubes are considered as ultra-fine unique devices, which can offer significant advantages over many existing materials due to their remarkable mechanical, electronic and chemical properties. With strong covalent bonding they possess unique one-dimensional structures. Nanotubes can be utilised as electronics devices, super-capacitors, lithium ion batteries, field emission displays, fuel cells, actuators, chemical and biological sensors and electron sources. Some of the R&D areas include: nanoscale phenomena, atomic and mesoscale modeling, carbon nano structures and devices, nano composites, biomaterials and systems, bio nano computational methods, fluidics and nano medicine. CNTs are typically longer in length, usually measuring from about a few tens of nanometers to several micrometers, with a diameter up to 30 nanometers and as small as 2.5 nanometers. Apparently, they are hollow cylinders extremely thin with a diameter about 10,000 times smaller than a human hair. Each nanotube is a single molecule made up of a hexagonal network of covalently bonded carbon atoms. Freestanding carbon nanotubes can be grown by chemical vapor deposition (CVD) across the predefined trenches. The trenches can be fabricated lithographically in SiO2 and then by depositing Pt over the sample to serve as the conducting substrate. Explicitly, they adhere to metallic semiconducting properties along with good thermal conductivity. Other essential properties they possess are:
• High tensile strength and high resilience
• High current densities
The ongoing research that is being carried out all over the world is based on the study of conductive and high-strength composites, energy storage and energy conversion devices, sensors for field emission displays and radiation sources, hydrogen storage media and nanometer-sized semiconductor devices such as probes and interfacings. Nanotube-based design scenarios anticipate the design of gears and bearings and hence the development of machines at the molecular level.
The team at the Paul Pascal Research Center has been working on overcoming the obstacle of the formation the carbon nanotubes and has developed a process for aligning them in the form of fibers and strips. Variorum structural constructions can be formed through appropriate methods. Continuous sputtering of carbon atoms from the nanotubes lead to a dimensional change, which facilitates surface reconstruction with annealing. An X-like junction with diverse angles between the branches can be formed. Under careful irradiation one of the branches of the X-junction can be removed, thereby creating Y- and T-like junctions. This new class of carbon junctions exhibits an intrinsic non-linear transport behavior, depending mainly on its pure geometrical configuration and on the kind of topological defects (Terrones, 2000; Andriotis, 2002; Bir'o, 2002). Calculation and measurement of characteristic curves, like current versus voltage of different sets of Y junctions, show robust rectification properties, giving rise to the possibility of using these junctions as nanoscale three-point transistors (Latg'e et al. 2004). Advanced computational techniques, including large-scale parallelisable molecular dynamic simulations of the growth mechanism and first-principles calculations of the electronic structure, are being applied to model the self-assembly and the electronic properties of nanostructures (Lambin 1995). Based on computational results, Han et al. at NASA Ames Research Center have suggested that nanotube-based gear (Fig.1.1(b)) can be made and operated and the gears can work well if the temperature is lower than 600-1000 K and the rotational energy is less than the teeth tilting energy at 20°.
1.14 Molecular Logic Gates
Logic gates are the fundamental components of digital circuits, which process binary data encoded, in electrical signals. The increasing need for the miniaturisation of logical circuits is reaching the physical limit of the metal oxide semiconductor field effect transistors (FET). There has been tremendous advancement as far as a search for alternatives is concerted. For example, a molecule-based scheme for logical operations, with different in its features, has been proposed. Various other approaches are (Prasanna and Nathan 2004):
• Chemically-controlled fluorescent and transmittance-based switches
• DNA oligonucleotides with fluorescence readout
• Oligonucleotide reactions with DNA-based catalysts
• Chemically-gated photochromics and reversibly denaturable proteins
• Molecular machines with optical and electronic signals
• Two-photon fluorophores
Because of their discrete orbital levels, molecules at room temperature possess large energy level separations. At the nanometer size, the levels make themselves independent of broadband properties. Electronically transduced photochemical switching is possible in organic monolayers and thin films, enzyme monolayers, redox-enzymes tethered with photoisomerisable groups, enzymes reconstituted onto photoisomerisable FAD-cofactors among others. It is found that the photoinduced electron transfer process (PET) occurs in many organic compounds, leading to an on/off mechanism. For instance, pyrazole derivative emits electrons only when the concentration of H+ is low. Photo-induced electron transfer from the central pyrazoline unit to the pendant benzoic acid quenches the fluorescence of the protonated form. Therefore, the emission intensity switches from a high to low value when the corresponding concentration of H+ changes from a low to high value. A detailed description is found in this book.
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