Wednesday, October 14, 2009

Functionalized Carbon Nanotubes-A Thousand and One Uses

Carbon nanotubes (CNTs) are a wonder material that is stronger than steel, lighter than aluminum and more conductive than copper. They can be conducting or semiconducting depending on how the tube is formed. They are found in computers, cars, cosmetics and many other products. While
 carbon nanotubes are pure carbon, functionalized carbon nanotubes have other molecules attached somewhere along the tube's circumference, inside the nanotube, attached at the ends or in any number of combinations for an equally diverse number of applications. Depending on what molecules are added, the carbon nanotube will perform a variety of different functions such as delivering various drugs to specific cells in the body. They can also serve as additives to make stronger cement, polymer and epoxy composites with special properties. They can act as sensors for deadly chemicals or diseases. CNTs can serve as components of integrated circuits or photonic devices used in computers and fiber optic communication.

It is estimated that more than 1000 types of functionalized carbon nanotubes have been developed. These newly designed molecules are patentable materials and big business. Functionalized carbon nanotubes command high prices. Functionalized CNT prices range for $1800 per kilogram to $80,000 per kilogram. Production of carbon nanotubes is expected to more than triple over the next five years. Sony Corp, Rice University and du Pont have all received recent patents for functionalized carbon nanotube, each serving a different purpose.

Sony Corporation (Tokyo, JP) researcher Houjin Huang has developed a simpler method for making field-effect transistors and semiconductor devices using functionalized carbon nanotubes.  A bare carbon nanotube is joined at one end with a functional group that includes electrode material to make a field-effect transistor.  The electrodes may be composed of elements from Groups III to XIII in the periodic table, according to U.S. Patent 7,601,322.  Field-effect transistors are used in liquid crystal displays (LCDs) in phones and TVs

Sony’s process does not involve complicated procedures such as using an atomic force microscope (AFM) or a high-temperature process such as chemical vapor deposition (CVD) but can position carbon nanotubes on predetermined electrodes with high accuracy.  A chemical substance is used to cover the conductive material of the electrodes so that the connection between the functional groups at the ends of the carbon nanotubes and electrodes can be strengthened.

William Marsh Rice University (Houston, TX) researchers reveal in a better way of integrating carbon nanotubes into epoxy polymer composites via chemical functionalization of carbon nanotubes as well as new carbon nanotube-epoxy polymer composites in U. S. Patent 7,601,421.  Rice researchers Valery N. Khabashesku, Jiang Zhu, Haiqing Peng, Enrique V. Barrera and John L. Margrave maintain that integration is enhanced through improved dispersion and/or covalent bonding with the epoxy matrix during the curing process. In general, such methods involve the attachment of chemical moieties (i.e., functional groups) to the sidewall and/or end-cap of carbon nanotubes such that the chemical moieties react with either the epoxy precursor(s) or the curing agent(s) (or both) during the curing process. Additionally, the additional chemical moieties can function to facilitate dispersion of the carbon nanotubes by decreasing the van der Waals attractive forces between the nanotubes.

Depending on their atomic structure CNT's have either metallic or semiconductor properties, and these properties, in combination with their small dimensions makes them particularly attractive for use in fabrication of nano-devices. A major obstacle to such efforts has been the diversity of tube diameters, chiral angles, and aggregation states in nanotube samples obtained from the various preparation methods. Aggregation is particularly problematic because the highly polarizable, smooth-sided fullerene tubes readily form parallel bundles or ropes with a large van der Waals binding energy. This bundling perturbs the electronic structure of the tubes, and it confounds all attempts to separate the tubes by size or type or to use them as individual macromolecular species.

E.I. du Pont de Nemours and Company (Wilmington, DE) achieved good dispersion of carbon nanotubes by functionalizing them with nucleic acids.  In United States Patent 7,588,941 reveals  how du Pont inventors Ming Zheng and Bruce A. Diner using nucleic acid molecules in a stabilized solution of single stranded DNA and RNA were able to disperse a high concentration of bundled carbon nanotubes into an aqueous solution. They found that nucleic acids are very effective in dispersing the nanotubes, forming nanotube-nucleic acid complexes based on non-covalent interactions between the nanotube and the nucleic acid molecule.

du Pont’s method is particularly useful since a major obstacle to the manipulation and use of carbon nanotubes (CNT's) as structural materials has been their poor solubility and their tendency to aggregate in bundles or clusters.  Zheng and Diner also reveal a novel separation method that involves either gel electrophoresis chromatography or a phase separation method.  Bothe separation methods are said to be “rapid and facile and permit the separation of nanotube-nucleic acid complexes into discreet fractions based on size or charge.”  Gel electrophoresis chromatography and phase separation methods have been applied to the separation and the recovery of other coated nanoparticles

Once formed the dispersed nanotube-nucleic acid complexes containing ligands or binding pairs may be either immobilized on a solid substrate or rationally associated with other complexes in a process of nano-device fabrication. Preferred binding pairs for immobilization of the du Pont’s complexes are biotin/streptavidin or biotin/avidin.  du Pont has also developed peptide-based carbon nanotube hair colorants and cosmetic compositions.

Interuniversitair Microelektronica Centrum vzw (IMEC) (Leuven, BE) used functionalized carbon nanotubes to make photoelectrochemical cells.   The functionalization of semiconductor surfaces is such that its semiconducting and light generating properties are maintained and the surface becomes stable in wet environments. U.S. Patent Application 2009021527 details unstable semiconductor materials which have photocurrent generating properties, and methods for the functionalization of surfaces with metallic carbon nanotubes (CNTs).

IMEC’s CNT functionalized semiconductor electrode structure still has the photo-generating properties of the semiconductor layer, as the electrochemical reaction occurs at the CNT. The combination enables the use of unstable semiconductor materials such as Si, Ge and III-V semiconductors in wet electrochemical cells. The semiconductor-CNT electrodes can be used as photo-anodes (n-type semiconductor) or photo-cathodes (p-type semiconductor) in photo-electrochemical cells such as photovoltaic cells or as photosensing electrodes in electrochemical sensors. The electrodes can also be used without light where opportune. In the dark, semiconductor-CNT electrodes can be used as electrochemical Schottky diodes

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