Plastic Transistors’ Problems Solved by Insulating Layer

Plastic Transistors’ Problems Solved by Insulating Layer

by John Emsley

WHAT'S HOT IN CHEMISTRY
Rank	Paper	Citations This Period (Jan-Feb 06)	Rank Last Period (Nov-Dec 05)
1	X.Y. Kong, et al., "Single-crystal nanorings formed by epitaxial self-coiling of polar nanobelts," Science, 303(5662): 1348-51, 27 February 2004. [Georgia Inst. Tech., Atlanta] *778JN	22	10
2	L.C. Storoni, A.J. McCoy, R.J. Read, "Likelihood-enhanced fast rotation functions," Acta Cryst. D, 60: 432-8, March 2004. [U. Cambridge, U.K.] *778CQ 22	22	1
3	J.L.C. Rowsell, et al., "Hydrogen sorption in functionalized metal-organic frameworks," J. Am. Chem. Soc., 126(18): 5666-7, 12 May 2004. [U. Michigan, Ann Arbor] *818TW	18	†
4	L.-L. Chua, et al., "General observation of n-type field-effect behaviour in organic semiconductors," Nature, 434(7030): 194-9, 10 March 2005. [U. Cambridge, U.K.; Natl. U. Singapore; Inst. Materials Res. Eng., Singapore] *904JU	17	†
5	P. Wernet, et al., "The structure of the first coordination shell in liquid water," Science, 304(5673): 995-9, 14 May 2004. [6 institutions worldwide] *820IV	16	†
6	X.D. Wang, C.J. Summers, Z.L. Wang, "Large-scale hexagonal-patterned growth of aligned ZnO nanorods for nano-optoelectronics and nanosensor arrays," Nano Lett., 4(3): 423-6, March 2004. [Georgia Inst. Tech., Atlanta] *802NQ	14	†
7	M.S. Chen, D.W. Goodman, "The structure of catalytically active gold on titania," Science, 306(5694): 252-5, 8 October 2004. [Texas A&M U., College Station] *861MC	13	†
8	M. Law, et al., "Nanoribbon waveguides for subwavelength photonics integration," Science, 305(5688): 1269-73, 27 August 2004. [U. Calif., Berkeley; L. Berkeley Natl. Lab., CA] *850PJ	13	†
9	X.Y. Zhang, W.J. Goux, S.K. Manohar, "Synthesis of polyaniline nanofibers by ‘nanofiber seeding’," J. Am. Chem. Soc., 126(14): 4502-3, 14 April 2004. [U. Texas Dallas, Richardson] *811DJ	13	†
10	S. Helveg, et al., "Atomic-scale imaging of carbon nanofibre growth," Nature, 427(6973): 426-9, 29 January 2004. [Tech. U. Denmark, Lyngby] *767UF	12	†
SOURCE: Thomson Scientific's Hot Papers Database. Read the Legend.

rganic polymers can be conducting, but to make useful transistors they need to move both electrons and electron "holes"—in other words, to provide both n-type and p-type conductors. While the latter kinds have been known for some ten years or so, the former types have proved elusive. In theory, electrons are just as able to move along the backbones of conducting polymer chains as are holes, but they seem to prefer to go somewhere else—but where? The answer is to be found in paper #4, reporting the groundbreaking work of Lay-Lay Chua et al., of the Cavendish Laboratory, University of Cambridge, U.K., and the National University of Singapore.

They showed that OH groups at the interface between the semiconductor and the commonly used silicon dioxide insulator layer were trapping the electrons, and when contact with these groups was prevented, n-type behavior was observed. The researchers used multiple-reflection attenuated-total-reflection FTIR spectrometry to focus on the O-H stretching vibration at 3,630 cm^-1 which revealed the presence of the trapped electrons. Standard silicon dioxide crystals grown at 1,000DEGREES C can have around 10^-1³ SiOH groups per square cm on its surface, and while this may seem a lot they are in fact relatively rare in terms of the atomic scale, but they can nevertheless negate the electron-carrying ability of the conducting polymer.

The answer to the problem was to insert a layer of insulating material between the silicon dioxide and the semiconductor polymer, and devices which incorporated this feature then displayed uninhibited electron flow. The insulator they preferred was BCB (short for benzocyclobutene) but other materials such as poly(ethylene) and poly(methyl methacrylate) were also effective. The work reported in #4 has made possible the demonstration of light-emitting transistors which work by electron-hole capture within the transistor active channel. (Jana Zaumseil and colleagues in Cambridge recently reported on this in Nature Materials, 5[1]:69-74, 2006.)

Also in the current list is another remarkable paper, #9, which will surely have implications for the synthesis of all organic semiconducting polymers in the future. It comes from Sanjeev Manohar’s group at the University of Texas at Dallas and reports "nanofiber seeding" as an excellent method for growing bulk quantities of the conducting polymer poly(aniline) and without the need for conventional templates, surfactants, polymers, or organic solvents. The group’s technique is to add very small amounts of nano-sized fibers to act as seeds for the growing polymer. Manohar believes that trace amounts of alien substances, such as fabric lint and dust, may account for the puzzling behavior that has been reported for poly(aniline) in the literature—indeed, samples prepared by different researchers never appeared to be the same material.

Manohar’s work is based on the theory that polymer nanostructures formed at the start of the polymerization act to orchestrate the bulk formation of the rest of the polymer. His group tested nanofibers from a variety of organic, inorganic, and biological systems. When they added around a milligram of these as seeds, then the poly(aniline) product consisted entirely of nanofibers, as opposed to non-seeded polymer which was shown to consist of particles or granules of polymer. The group were able to synthesize bulk quantities of nanofibers of the conducting polymer, poly(aniline), at room temperature and in a single step free of the need for other chemicals, in effect being a "green chemistry" approach. They have also extended their seeding method to synthesizing nanofibers of all major classes of conducting polymers. (See X.Y. Zhang, et al., Chemical Communications, 42: 5328-30, 2005; and X.Y. Zhang and S.K. Manohar, Journal of the American Chemical Society, 126[40]: 12714-5, 2004.)

Currently Manohar is working on synthesizing aligned nanofibers of conducting polymers and biodegradable polymers for use in tissue engineering as cellular scaffolds. Speaking to Science Watch, he predicts a new era for organic materials: "I venture to say that we might have stumbled onto the first organic analogues of inorganic crystallizations where added seed crystals dramatically alter the crystal shape and growth. So far we have only been able to obtain fibers or spheres of polymers. I would like to see if we can go beyond these elemental shapes to other shapes with pre-selected properties."

Dr. John Emsley is based at the Department of Chemistry,
University of Cambridge, U.K.


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Science Watch^®, July/August 2006, Vol. 17, No. 4
Citing URL: http://www.sciencewatch.com/july-aug2006/sw_july-aug2006_page7.htm