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Nobel Lecture: Semiconducting and Metallic Polymers: the Fourth Generation of Polymeric Materials*

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Nobel Lecture: Semiconducting and Metallic Polymers: the Fourth Generation of Polymeric Materials* REVIEWS OF MODERN PHYSICS, VOLUME 73, JULY 2001 Nobel Lecture: Semiconducting and metallic polymers: The fourth generation of polymeric materials* Alan J. Heeger Department of Physics, Materials Department, Institute for Polymers and Organic Solids, University of California, Santa Barbara, Santa Barbara, California 93106 (Published 20 September 2001) I. INTRODUCTION Conducting polymers are the most recent generation of polymers (Ra˚nby, 1993). Polymeric materials in the In 1976, Alan MacDiarmid, Hideki Shirakawa, and I, form of wood, bone, skin, and fibers have been used by together with a talented group of graduate students and man since prehistoric time. Although organic chemistry postdoctoral researchers, discovered conducting poly- as a science dates back to the eighteenth century, poly- mers and the ability to dope these polymers over the full mer science on a molecular basis is a development of the range from insulator to metal (Chiang et al., 1977; twentieth century. Hermann Staudinger developed the Shirakawa et al., 1977). This was particularly exciting be- concept of macromolecules during the 1920s. cause it created a new field of research on the boundary Staudinger’s proposal was openly opposed by leading between chemistry and condensed-matter physics, and scientists, but the data eventually confirmed the exis- because it created a number of opportunities: tence of macromolecules. Staudinger was awarded the • Conducting polymers opened the way to progress in Nobel Prize in Chemistry in 1953 ‘‘for his discoveries in understanding the fundamental chemistry and physics of the field of macromolecular chemistry.’’ While carrying ␲-bonded macromolecules; out basic research on polymerization reactions at the • Conducting polymers provided an opportunity to DuPont Company, Wallace Carothers invented nylon in address questions that had been of fundamental interest 1935. Carothers’s research showed the great industrial to quantum chemistry for decades: potential of synthetic polymers, a potential that became Is there bond alternation in long-chain polyenes? reality in a remarkably short time. Today, synthetic poly- What is the relative importance of the electron- mers are used in larger quantities than any other class of ␲ elecron and the electron-lattice interactions in -bonded materials (larger by volume and larger by weight, even macromolecules? though such polymers have densities close to unity). • Conducting polymers provided an opportunity to Polymer synthesis in the 1950s was dominated by Karl address fundamental issues of importance to condensed- Ziegler and Giulio Natta, whose discoveries of polymer- matter physics as well, including, for example, the metal- ization catalysts were of great importance for the devel- insulator transition as envisioned by Neville Mott and opment of the modern ‘‘plastics’’ industry. Ziegler and Philip Anderson and the instability of one-dimensional Natta were awarded the Nobel Prize in Chemistry in metals discovered by Rudolph Peierls (the ‘‘Peierls In- 1963 ‘‘for their discoveries in the field of the chemistry stability’’). and technology of high polymers.’’ Paul Flory was the • Finally—and perhaps most important—conducting next ‘‘giant’’ of polymer chemistry; he created modern polymers offered the promise of achieving a new gen- polymer science through his experimental and theoreti- eration of polymers: Materials which exhibit the electri- cal studies of macromolecules. His insights and deep un- cal and optical properties of metals or semiconductors derstanding of macromolecular phenomena are summa- and which retain the attractive mechanical properties rized in his book, Principles of Polymer Chemistry, and processing advantages of polymers. published in 1953 and still useful (and used) today. Flory Thus, when asked to explain the importance of the dis- was awarded the Nobel Prize in Chemistry in 1974 ‘‘for covery of conducting polymers, I offer two basic an- his fundamental achievements, both theoretical and ex- swers: perimental, in the physical chemistry of macromol- (i) They did not (could not?) exist. ecules.’’ (ii) They offer a unique combination of properties not Because the saturated polymers studied by available from any other known materials. Staudinger, Flory, Ziegler, and Natta are insulators, they The first expresses an intellectual challenge; the second were viewed as uninteresting from the point of view of expresses a promise for utiltity in a wide variety of ap- electronic materials. Although this is true for saturated plications. polymers (in which all of the four valence electrons of carbon are used up in covalent bonds), in conjugated polymers the electronic configuration is fundamentally *The 2000 Nobel Prize in Chemistry was shared by Alan J. different. In conjugated polymers, the chemical bonding Heeger, Alan G. MacDiarmid, and Hideki Shirakawa. This lec- leads to one unpaired electron (the ␲ electron) per car- ture is the text of Professor Heeger’s address on the occasion bon atom. Moreover, ␲ bonding, in which the carbon 2 of the award. orbitals are in the sp pz configuration and in which the 0034-6861/2001/73(3)/681(20)/$24.00 681 © The Nobel Foundation 2000 682 Alan J. Heeger: Semiconducting and metallic polymers FIG. 1. Molecular structures of examples of conjugated polymers. Note the bond- alternated structures. orbitals of successive carbon atoms along the backbone doping (Chiang et al., 1977; Shirakawa et al., 1977) to- overlap, leads to electron delocalization along the back- gether with a variety of studies of the neutral polymer bone of the polymer. This electronic delocalization pro- have eliminated the antiferromagnetic Mott insulator as vides the ‘‘highway’’ for charge mobility along the back- a possibility. bone of the polymer chain. In real polyacetylene, the structure is dimerized as a As a result, therefore, the electronic structure in con- result of the Peierls instability with two carbon atoms in Ϫ ␲ ducting polymers is determined by the chain symmetry the repeat unit, ( CHvCH)n . Thus the band is di- (i.e., the number and kind of atoms within the repeat vided into ␲ and ␲* bands. Since each band can hold unit), with the result that such polymers can exhibit two electrons per atom (spin up and spin down), the ␲ semiconducting or even metallic properties. In his lec- band is filled and the ␲* band is empty. The energy ture at the Nobel Symposium (NS-81) in 1991, Professor difference between the highest occupied state in the ␲ Bengt Ra˚nby designated electrically conducting poly- band and the lowest unoccupied state in the ␲* band is ␲ ␲ mers as the ‘‘fourth generation of polymeric materials’’ the - * energy gap Eg . The bond-alternated structure (Ra˚nby, 1993). of polyacetylene is characteristic of conjugated polymers Ϫ The classic example is polyacetylene, ( CH)n , in (see Fig. 1). Consequently, since there are no partially which each carbon is ␴ bonded to only two neighboring filled bands, conjugated polymers are typically semicon- ␲ carbons and one hydrogen atom with one electron on ductors. Because Eg depends upon the molecular struc- each carbon. If the carbon-carbon bond lengths were ture of the repeat unit, synthetic chemists are provided Ϫ equal, the chemical formula, ( CH)n with one unpaired with the opportunity and the challenge to control the electron per formula unit, would imply a metallic state. energy gap by design at the molecular level. Alternatively, if the electron-electron interactions were Although initially built upon the foundations of quan- Ϫ too strong, ( CH)n would be an antiferromagnetic Mott tum chemistry and condensed-matter physics, it soon be- insulator. The easy conversion to the metallic state on came clear that entirely new concepts were involved in Rev. Mod. Phys., Vol. 73, No. 3, July 2001 Alan J. Heeger: Semiconducting and metallic polymers 683 the science of conducting polymers. The discovery of nonlinear excitations in this class of polymers, solitons in systems in which the ground state is degenerate and con- fined soliton pairs (polarons and bipolarons) in systems in which the ground-state degeneracy has been lifted by the molecular structure, opened entirely new directions for the study of the interconnection of chemical and electronic structure. The spin-charge separation charac- teristic of solitons and the reversal of the spin-charge relationship (relative to that expected for electrons as fermions) in polyacetylene challenged the foundations of quantum physics. The study of solitons in polyacety- lene (Heeger et al., 1988), stimulated by the Su- Schrieffer-Heeger (SSH) papers (Su et al., 1979, 1980), dominated the first half of the 1980s. The opportunity to synthesize new conducting poly- mers with improved/desired properties began to attract the attention of synthetic chemists in the 1980s. Al- though it would be an overstatement to claim that chem- ists can now control the energy gap of semiconducting polymers through molecular design, we certainly have come a long way toward that goal. Reversible ‘‘doping’’ of conducting polymers, with as- sociated control of the electrical conductivity over the full range from insulator to metal, can be accomplished either by chemical doping or by electrochemical doping. Concurrent with the doping, the electrochemical poten- tial (the Fermi level) is moved either by a redox reaction FIG. 2. Electrical conductivity of trans-(CH), as a function of or an acid-base reaction into a region of energy where (AsF5) dopant concentration. The trans and cis polymer struc- there
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