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Chemical vapor deposition (CVD) is widely used in the semiconductor industry for deposition of material on various substrates. A gas or vapor precursor is transformed into solids such as thin films, powders, or various structured materials inside a reactor. Recently, CVD has been used to synthesize a variety of nanostructured materials, including carbon nanowires and nanotubes.
CVD techniques include:
3.b Carbon Nanotubes
Carbon nanotubes are one-dimensional graphitized tubular forms of carbon in a concentric multi-shell form (MWNTs). The original nanotubes were produced by arc-discharge of graphite and had outer diameters of ~ 5 nm to tens of nanometers. Later, single-walled tubes (SWNTs) were synthesized by the same method with diameters as low as ~ 1 nm. Another method for producing carbon nanotubes is laser vaporization of graphite with embedded metal particles, which results in SWNTs with narrow diameters in bundles or ropes, as the nanotubes are held tightly together.
CVD has been effectively used to synthesize high quality carbon nanotubes. Typically, the process consists of dissociation of a carbon feed, interaction with catalytic particles (or centers), and growth of the nanotubes.
Carbon Nanotubes Properties:
Carbon nanotubes have many unique properties. Some of them are
Nanotubes are rolled graphite layers and can be assumed to be infinite along their axis but have atomic dimensions along the circumference. The electronic structure of the carbon nanotube is obtained based on the two-dimensional structure of graphite. As a consequence of the Heisenberg’s Uncertainty Principle, the nano-size of the CNTs causes quantization of the wave vector in the circumferential direction.
Magnetic measurements on nanotubes confirmed that nanotubes are diamagnetic and show a pronounced anisotropy of susceptibility. Magnetic susceptibility of tubes aligned parallel to the field was found to be much greater than that of tubes perpendicular to the field. Theoretical predictions indicate the contrary and the reason for this discrepancy is not clear.
Carbon nanotubes have non-linear optical properties, which depend strongly on the diameter and symmetry of the tubes. Also the perpendicular nanotube films were found to be optically isotropic while for parallel films the optical properties were strongly dependent on whether light was polarized parallel/perpendicular to the tubes.
Carbon Nanotubes possess many desirable properties with regard to their flexibility, high strength, ability to withstand cross-sectional and twisting distortions, their extensibility, and their ability to withstand compression without fracture. It can be bent around small circles or about sharp bends without breaking. Early work was done by David Tomanek and his colleagues and confirmed theoretically that nanotubes should have exceptional mechanical strength and rigidity than that of any known material.
Carbon Nanotube Sensors:
Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM) have been used to measure the surface topography and perform direct manipulation and fabrication at the atomic scale. Nanotube has been considered as a better alternative to the conventional Silicon and Silicon nitride tips. For good results, the tip needs to be precisely defined, should maintain its integrity after repeated use not only in vacuum but also in air and water should survive tip crash with the surface, should be flexible and mechanically robust. As CNTs offer above advantages, they have been used as AFM and STM probe tips. These were found useful to investigate biological structures in the area of amyloid-beta protein aggregation and chromatin, spatial arrangement of chemical functional groups as a chemical force microscopy (CFM).
Scanning force microscopy (SFM) probe tips are prepared by bonding the MWNT (CNT) to the side of the tip of a conventional silicon cantilever. It is operated in tapping mode. A high quality-oscillating cantilever is driven at its resonant frequency and the amplitude is monitored as the tip taps the surface. The CNT tip is stiff, gentle and does not crash on the surface. Once the force on the nanotube exceeds the Euler Buckling Force, nanotube will bend to larger amplitude and ensure that maximum force that can be transmitted to the sample is FEULER, hence preventing damage to the surface. The other advantage of MWNT probe tip lies in its ability to reach into deep trenches previously inaccessible to high-resolution scanning probes.
Apart from sensing applications, CNTs are used in other purposes mainly in electronics like:
Nanowires are one-dimensional wires with nanometric widths that can grow up to several microns in length. Nanowires can be produced from materials like silicon, germanium, gallium nitride, metals, oxides, and others. The structures can range from crystalline to polycrystalline to nearly amorphous. The grown nanowires are often easy to modify through processing and functionalization. Relative to carbon nanotubes, they are generally more compatible with post-processing techniques.
Carbon nanotubes and nanowires have enormous potential for experimentation and applications. Electrical applications include transistors, chemical and bio sensors, and integrated circuits. Mechanical applications include composites, scanning probe microscopy tips, and materials. NEMS, or nanoelectromechanical systems are also possible.
Nanowires can be grown to hundreds of microns or even millimeters in length. However, at this high length to diameter ratio, they become highly susceptible breaking. Growth of nanowires of over 9µm long, and arrays of Nanowires of size 10µm×10µm have been reported.
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