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Posted: Aug 10, 2006
A novel approach to making organic thin film transistors
(Nanowerk Spotlight) Organic thin film transistors (OTFTs) based on π-conjugated molecules have attracted a great deal of attention as they are the critical components to fabricate low cost and large area flexible displays and sensors for future application in organic electronics technology. However, the major problem to use organic thin film transistor in logic circuits is the high operating voltage required. Researchers in India believe this problem can be solved by using organic materials with high dielectric constant as gate dielectrics.
Prior to a recent paper by researchers in India there was no organic compound with high permittivity known in the literature. Dr. Shyamal Saha, together with colleague Dipankar Chakravorty, both from the Unit on Nanoscience at the Indian Association for the Cultivation of Science published their findings, titled "One-dimensional organic giant dielectrics" in the July 28, 2006 online edition of Applied Physics Letters.
The dielectric properties of nanomaterials has generated strong interests in the scientific community and nanomaterials having such properties are increasingly being referred to as "nanodielectrics". In general, a nanodielectric would consist of a single or multi-component dielectric possessing nanostructures, the presence of which results in the change of one or several of its dielectric properties.
"Surprisingly" Saha tells Nanowerk, "the area of nanodielectrics is relatively unexplored, despite the enormous scope of potential applications in power electronics and as gate electrodes in thin film transistors. Perhaps the greatest opportunity in nanodielectrics is in nanoparticulate materials." The area of organic materials for use in nanodielectrics has been completely unexplored.
The major finding in Saha's latest research is that a long, well aligned, conjugated polymer chain shows giant dielectric permittivity as a result of quantum confinement of the electronic wave functions.
In conjugated polymers, the polymer chains are intrinsically metallic in nature but in the bulk phase the chemical defects, cross-linking, entanglement of the chains and sufficient disorder cause the chain to behave like a semiconductor. Their electronic properties can be tuned by tailoring the morphology, defects and cross-linking of the polymer chains.
For his research, Saha assumed that a long conjugated polymer chain with metallic behavior interrupted by chemical defects or structural disorder is the most ideal system to consider as a disordered metal filament to achieve giant dielectric response.
Saha explains the novel nature of his work: " So far, most of the high dielectric materials which are being used are ceramic materials. Very recently, some research has been carried out on high dielectric permittivity in one dimensional metal strands ("Observation of giant dielectric constant in an assembly of ultra fine Ag particles") as a result of quantum confinement of the electrons. But, there is no report on organic compounds which show high dielectric constant. This is the first, in which we report a one dimensional aligned conjugated polymer chain that shows giant dielectric permittivity."
The potential application of this material is to fabricate a molecular capacitor which can be used in molecular electronic circuits. The most important application in the semiconductor industry would be to use a segment of this conjugated polymer chain as high-κ dielectric gate in organic thin film transistor to reduce the operating voltage.
The future direction of Saha's research is to minimize the loss factor by attaching insulating molecule at the ends of the chain segment and tailoring the structure at the nanoscale level.