SPECIAL REPORT
Dwalne 0. Cowan and Frank M. Wlygul, Johns Hopkins University
The Organic Solid State Organic materials show interesting and useful electrical, magnetic, and optical properties in the solid state
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July 21, 1986 C&EN
This article, we hope, will encourage more chemists— particularly organic chemists—to think about and start research on organic materials that may exhibit interesting electrical, optical, or magnetic properties in the solid state. We now have an almost bewildering battery of techniques to study the solid state. Given this degree of readiness, opportunities are nearly unlimited for physical organic chemists to correlate structure with solid-state physical properties, as well as for organic and organometallic chemists to synthesize new and sometimes exotic compounds. The scientist who acts on these exciting challenges has the added opportunity and—usually—pleasure of a multidisciplinary effort involving almost all areas of theoretical and experimental chemistry, applied physics, materials science, and electrical engineering. In our work at Johns Hopkins University, supported by the materials science division of the National Science Foundation and the Air Force Office of Scientific Research, we have encountered substantial cross-pollination of ideas, as well as encouragement, from colleagues in adjunct disciplines. It is important for the experimentalist, the industrial or academic group leader, and the funding company or agency to understand that the reward of a detailed understanding of the chemistry and physics will go to those who make relatively long-term commitments to the study of the organic solid state. For example, it may be far more glamorous to study only organic metals (organic compounds that conduct electricity as metals do) and organic superconductors (materials that conduct an electrical current without resistance). However, we are convinced it is equally important to study the semiconductors and insulators so often encountered along the path to new organic metals. Only with a detailed study of the entire range of materials, using a number of techniques, can we gain a better appreciation of how to structurally design the molecules of interest. The potential for applying the knowledge derived from these studies to the next generation of electronic devices and energy conversion systems is enormous. Although it is tempting, it is too early to suggest that organic-based materials will play an even more important role in such devices than will more traditional inorganic compounds like silicon and germanium. Already, however, the study of organic radical cation salts like those derived from the electron donor tetrathiafulvalene (TTF) and its selenium and tellurium analogs has given us a wealth of information about a new area of chemistry and physics. Most organic solids are electrical insulators. However, several salts formed from the organic electron acceptor tetracyanoquinodimethane (TCNQ) and donor ions wiere discovered to be semiconductors in the 1960s, primarily as a result of work at Du Pont. It was not until 1972 that Dwaine O. Cowan, John P. Ferraris,
Opportunities abound in the study of the organic solid state Transducers (electret microphones)
Photocopiers, solar cells
t
\ Organic photoconductors organic semiconductors ;tors Electronic components, plastic batteries
Optical information storage
Piezoelectric phenomena, ferroelectric phenomena
/ Solid-state photochemical rreactions e
A
Metals Organic and organometallic synthesis
7T
Superconductors
Electro-optics, nonlinear optical phenomena Josephson junction computer logic gates, high-field magnets, generators, motors, power transmission
Ferromagnetism
Liquid crystals Magnetic recording, magneto-optic recording
/
Frequency doublers, modulators, integrated optics, optical computers
and their group at Johns Hopkins found that single crystals of the salt TTF-TCNQ showed metal-like electrical conductivity. Both the magnitude and the temperature dependence of the electrical conductivity indicated that the first organic metal had been found. Specifically, single crystals of TTF-TCNQ have a conductivity of about 500 (ohm-cm) -1 at room temperature. [For comparison, copper has a room-temperature conductivity of about 106 (ohm-cm) -1 .] Also, the conductivity of TTF-TCNQ increases as temperature decreases, as is true of metals, until a maximum conductivity of about 104 (ohm-cm) -1 is reached at 59 K. Below 59 K, conductivity drops off and behaves like a semiconductor—that is, the conductivity decreases with decreasing temperature. The electrical conductivity in TTF-TCNQ is also highly anisotropic, varying along different crystal axes. The ratio of the conductivity in the three principal directions is approximately 500:5:1. The synthesis of TTF-TCNQ was a conscious effort on the part of the Hopkins group to optimize those
\ Electronic displays
factors that are likely to produce an organic metal. Soon afterward, Alan J. Heeger, Anthony F. Garito, and coworkers at the University of Pennsylvania confirmed the high conductivity of single crystals of TTFTCNQ and noted that a small number of samples showed an even larger conductivity maximum. They suggested these samples could be showing superconducting precursor effects. Although most workers now believe that these observations were experimental artifacts, that work helped stimulate a search that has, in fact, led to organic superconductors. Such electrical properties, unique among organic compounds, drew the attention of many researchers in subsequent years. The basis for the electrical properties of TTF-TCNQ lies in the structure of the crystalline material. Since both TTF and TCNQ are planar molecules, the individual molecules can approach quite closely in the direction perpendicular to their molecular plane. In the salt, the molecular species exist in two kinds of July 21, 1986 C&EN
29
I I Organic donors and acceptors are planar molecules with extended IT networks
XKX
00-00
Tetrathiafuivalene(TTF): R = H , X - S Tetraselenafulvalene(TSF): R = H,X=Se Tetramethyltetraihiafuivalene (TMTTF) : R=CH3tX-S Tetramethyltetraselenafulvalene (TMTSF) : R-CH 3 ,X--Se
Hexamethylenetetrathiafulvalene (HMTTF): X-,S Hexamethylenetetraselenafulvalene (HMTSF): X - S e Hexamethyienetetrateilurafulvalene (HMTTeF): X - T e
ακο
Dibenzotetrateliurafulvaiene(DBTTeF)
QXJQ Bis(ethylenedithiolo)tetrathiafuiva!ene (BEDT-TTF)
X —X
χ—
Perylene
χ
Tetrathiatetracene (TTT) : X = S Tetraseienatetracene (TST) : X - Se
:N
N-
\ Ν >—Ν Tetracyanoquinodimethane (TCNQ): R — Η Dirnethyltetracyanoquinodirnethane (DMTCNQ): R - C H 3
Tetracyanonaphthoquinodimethane(TNAP)
/ Μ
Ν Ν-
A^N Metal phihalocyanine [M(Pc)3
stacks: One contains solely TTF molecules; the other is made up of TCNQ molecules. This gives rise to the term segregated stacking. The direction of high con ductivity is along these stacks. The short interplanar distances between adjacent molecules (3.17 A for TCNQ and 3.47 Â for TTF) allow significant interaction between π-molecular orbitals of neighbors, leading to formation of a band (that is, a restricted range of al lowed energies for the electrons) in the solid. The basic ideas of band theory in solids are impor tant for understanding the unique properties of organ ic charge-transfer salts like TTF-TCNQ. When a great number of atoms (as in elemental metals or semicon ductors) or molecules (as in organic metals) are brought together into a crystalline solid, the electronic states mix so as to form bands, each band consisting of elec tronic states whose energies form a continuous range. This is analogous to the splitting of atomic energy levels as two atoms are brought together to form a molecule with higher and lower energy levels. With the combination of 1020 or so atoms and translational periodicity, energy levels are continuous over a range called the bandwidth. In fact, usually more than one band is available. The bands may overlap in energy or there may be an energy gap, that is, a range of forbidden energies. In band theory the appropriate 30
July 21, 1986 C&EN
quantum number is called the wave vector; it repre sents the momentum of the electron. Band filling can be thought of as analogous to the Aufbau principle for atoms: Electrons are placed in the lowest energy states and then succeeding higher ener gy states are filled. The highest occupied state is called the Fermi level. Only the states that are near in energy to the Fermi level are readily accessible and influence physical properties. It simply requires too much ener gy to promote an electron into empty states far above the Fermi level. If the highest filled band (often called the valence band) is only partially full, empty states will exist infinitesimally close to the Fermi level, and those elec trons nearby in energy can take part, for example, in electrical conduction. On the other hand, if the highest filled band is entirely full, and there is some band gap between it and the next lower band (conduction band), then a relatively large energy (compared to the avail able thermal energy) will be needed to put electrons into states where they will be available for conduction and where they may influence other properties. Metallic conduction is associated with incompletely filled bands, which have a large number of electrons available for conduction. The dominant influence on the electrical conductivity is scattering of these elec-
trons by lattice vibrations (called phonons). Because there are fewer Conductivity of materials extends over wide range vibrations at lower temperatures, Conductivity (ohm-cm) 1 the conductivity of a metal in-10 8 creases as temperature falls. A completely filled band leads to |— Copper semiconducting properties. In this Silver 10 e Gold case, there are fewer electrons TinConductive around to serve as charge carriers, Lead' organic Bismuth —I—10" and the number of available carriMetalscrystals _(TMTSF)2-TCNQ ers is strongly dependent on tem(TMTSF) 2 CI0 4 TTF-TCNQ perature. At higher temperatures, 10< Polyphenylene, polypyrrole more electrons can be promoted into the conduction band, and T Poly(phenylene sulfide) hence conductivity increases with increasing temperature. Recent research has led to a numGermanium -j SemiMO conductors ber of new conducting chargeDoped transfer salts. In general, the donor polyacetylenes Doped Silicon -j polymers and acceptor components are platrans10 Polyacetylene -| nar molecules with an extended ir network. These planar molecules Boron -\ are stacked in a close array, with a -10 relatively small distance between Soda-lime glass -\ molecular planes, typically 3 to 4 A. 10 These donor-acceptor salts can c/s-Polyacetylene -j form a segregated stacking pattern, Fused quartz in which there are two kinds of 10 Polyvinyl chloride stacks. Or they can form mixedWhite phosphorus -j stack structures, which are limited Insulators —\ to being semiconductors or insula10 tors. Although so limited, mixedstack salts are still of interest. Jerry -10 B. Torrance of IBM in San Jose, Calif., has observed a fascinating pressure-induced neutral-to-ionic Polyethylene " M O phase transition in some such materials. However, at the present time, Sulfur -J Note: Vertical scale shows conductivity at room little is known about the factors temperature in (ohm-cm) . Teflon -L-10 controlling whether the stacking pattern will be mixed or segregated The electrical conductivity of materials varies by more than 20 orders of magnitude and more experimental and theobetween a good insulator and a metal. The range of conductivity at room temperature retical work is needed in this area. found for some organic materials is depicted on the right-hand side of the diagram. At When the molecules are in segrea temperature of about 1 K, (TMTSF) 2 CI0 4 becomes a superconductor (zero resisgated stacks, a band of electronic tance). The classifications of insulators, semiconductors, and metals on the left-hand states is formed from the ^-molecuside of the drawing are only approximate. For example, a material with a roomlar orbitals of individual molecules. temperature conductivity of 100 (ohm-cm) -1 could be a metal, but it could also be a The tight-binding theory usually semiconductor. A decision can be made only after its electrical transport properties employed to describe the bands is have been studied as a function of temperature and structural studies performed analogous to simple Hiickel molecular orbital theory for an infinite linear polyene. The 7r-highest occupied molecular orbital (7T-HOMO), for a donor, or irBand filling in segregated-stack salts is determined by the extent of charge transfer from donor to acceptor lowest unoccupied molecular orbital (7T-LUMO), for an molecules. A band based on single orbitals from each of acceptor, play the role of 7r-atomic orbitals in Hiickel theory. This approach assumes that the energetic cost n molecules in a uniform stack will contain n states into which can be placed 2n electrons. Transfer of a unit of putting two electrons on the same site (molecule) is charge for salts like TTF-TCNQ implies n electrons in relatively small. Using the tight-binding theory, a transfer integral, t, can be calculated which describes the band. This produces a half-filled band, which should be able to support conduction. the extent of interaction between adjacent molecules in the stack. This transfer integral corresponds to the However, with exactly one electron per molecule, Hiickel beta. electrical conduction (moving electrons down the
T
1
July 21, 1986 C&EN
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Special Report chain) must involve placing two electrons on the same molecule with a consequent energetic penalty because of the coulomb repulsion (the repulsive force between like charges). Therefore, single electrons become associated with each molecule, insulating properties result, and the material behaves as a type of insulator known as a Mott-Hubbard insulator. If, however, charge transfer (Z) is less than one, there will be some "empty" sites, onto which electrons can move. This makes the conduction process energetically more favorable. A theoretical explanation advanced by Aaron N. Bloch of Exxon's research labs in Annandale, N.J., implies that the effective on-site coulomb potential is well screened at intermediate charge transfer (0.5 < Z < 0.8), which allows formation of a metallic band. Because in TTF-TCNQ charge transfer is only partial (on the average, only 59 electrons are transferred to a TCNQ stack for each 100 molecules in the stack), the band is only partially occupied and conduction can take place. The anisotropy that is observed in the electrical conductivity and other properties of TTF-TCNQ is a general characteristic of organic charge-transfer salts with segregated stacks. In fact, the anisotropy is usually so pronounced that the materials are often treated as onedimensional, that dimension being the high-conductivity direction. The anisotropy arises from the much greater value of the transfer integral (7r-overlap) in the stacking direction. The nearest neighbors in the stacking direction are 3 to 4 A away, whereas nonstacking neighbors often may be as much as 10 to 15 A distant. In addition, the 7r-orbitals are better oriented for interaction along the stacking direction. Consequently, the term quasi-one-dimensional is commonly used to describe the nearly uniaxial nature of the physical properties of such organic charge-transfer salts. The temperature dependence of the electrical conductivity of TTF-TCNQ is also prototypical for the class of organic metals. Conductivity reaches a maximum at some temperature, usually less than room temperature. Above this point, the temperature dependence is metallic, that is, conductivity increases as temperature decreases. Below it, the temperature dependence is that of a semiconductor. In other words, organic metals often undergo a metal-semiconductor transition. The physical reason for this behavior is intimately related to the quasi-one-dimensional nature of the segregated stack salts. Quasi-one-dimensional electron systems are subject to instabilities in which there is a distortion of the translational periodicity of the crystal lattice. The distortion leads to a band gap at the Fermi level so the material has insulating properties at low temperature. This is known as the Peierls instability or transition. It is analogous to the familiar Jahn-Teller distortion in molecules. As temperature is increased, electrons are thermally excited across the Peierls gap into states that are higher in energy than the empty states resulting from the uniform (undistorted) crystal lattice. Consequently, at higher temperatures the electronic energy advantage resulting from the distorted lattice is reduced, whereas elastic forces favor the uniform lattice. As a result, a 34
July 21, 1986 C&EN
transition from the distorted lattice structure (semiconducting) to the uniform lattice structure (metallic) is expected at some finite temperature. In contrast, in a two- or three-dimensional system, it is difficult, and in some cases impossible, to rearrange the lattice so that a gap of forbidden energy levels is opened. One of the chemical aspects of research into organic charge-transfer salts has been the synthesis of new donors and acceptors. Two basic paths to new donors have been used: Addition of substituents onto the TTF skeleton in place of its hydrogen atoms and substitution of other heteroatoms (selenium and, most recently, tellurium) in place of its sulfur atoms. A number of different donors have been synthesized by these approaches, alone or in combination. New acceptors based on TCNQ as a prototype also have been synthesized, many by substitution onto the quinone ring. Some contain a more extensive 7r network in place of the six-membered ring.
Current and voltage measurements indicate conductivity Constant current source
The electrical conductivity of single crystals usually is determined by a technique using four probes or electrodes. Because little current flows in the voltage-measuring circuit, any voltage drop due to contact resistance is minimized. The conductivity (a) is then calculated from the measured current (I), the voltage drop (V), and the sample dimensions (A and L). a = (l/V) (L/A) (ohm-cm)" 1 The temperature dependences of the conductivity or resistivity (p) of a semiconductor and metal frequently are characterized by the following expressions where a0, p0, Eg, A, and 7 are material-dependent constants. semiconductor: a(T) = a0 e
9
= a0 e~ A / T
metal: p(T) = 1/
