Lecture V Carbon Nanotubes Carbon nanotubes are a tubular form of carbon with diameter as small as 1 nm and length ranging from a few nm to microns. Its configuration is similar to 2D graphene sheets rolled in a tube. There are two types of carbon nanotubes- 1. Single wall nanotubes(SWNT) They are made up of a single rolled graphene layer. It is difficult to synthesize in pur, defect-free form. It accumulates less in the body, hence it's used for biomedical purposes. 2. Multi wall Nanotubes(MWNT) They are made of a bunch of graphene layers rolled together. Bulk synthesis is easy and high purity is attained. It has more accumulation in the body. CNTs have unique properties which make it useful for a spectrum of applications. Its strength to weight ratio is ~500 times that of Aluminium, steel and titatnium. It is as hard as diamond and its electrical conductivity is ~1000 times that of copper. Besides, its thermally stable up to 4000K. Nanocomposites can be used for dam and bridge construction in place of steel. Its light weightiness can be exploited for making aircrafts and satellites. It can even be used as interconnect transistors (CMOS industry, Nanowires) Certain limitations exist to its usage. 1. Its difficult to produce CNTs in bulk, maintaining homogeneity and purity. 2. Its scarcely soluble in water. To deal with the solubility issue, fictionalization i.e. modification of surface of CNT by absorption, electrostatic interaction or covalent bonding of a functional group. These functional groups (like amides, fluorine) render the CNTs more hydrophilic. Fictionalization enhances its biocompatibility and prevents aggregation of CNTs. Application- Its small size can be exploited to make biosensors. Probe molecule can be attached to the tip of CNTs. Target molecules will bind to the probe and produce a unique signal. CNTs are used because they are scale close to molecules. This increases the signal to noise ratio and renders high sensitivity to even small amount of target molecules. Fabrication of gene chip can be done wherein ~30 sensor dyes are immobilized on a 4" silicon wafer. This arrangement can be used for cancer detection, environmental monitoring and pathogen detection. Quantum Dots A crystalline nanoparticle made of semiconductor material, small enough to show quantum mechanical properties. The excitons (excited electrons and electron holes) of a quantum dot are confined in all three spatial dimensions. Also, the electronic properties are intermediary of bulk semiconductor and discrete molecules. They are closely related to their shape and size, the size and band gap being inversely proportional. They are made from a range of materials, most common being zinc sulphide, lead sulphide, cadmium selenide and indium phosphide. Since, many applications of quantum dots see them

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