TWENTY YEARS OF LOW-DIMENSIONAL ORGANIC CONDUCTORS

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THOMAS J. KISTENMACHER, DWAINE O. COW AN, and THEODORE O. POEHLER
TWENTY YEARS OF LOW-DIMENSIONAL
ORGANIC CONDUCTORS
One of the most vigorous and successful of the collaborative programs between the Applied Physics
Laboratory and the Homewood campus of The Johns Hopkins University is in the synthesis and study of
the crystal structure and physical properties of low-dimensional (nearly one-dimensional or nearly twodimensional) organic charge-transfer salts exhibiting metallic or semiconducting properties. This article
surveys the more interesting and important contributions of the Johns Hopkins community to the
experimental and theoretical chemical physics of organic conductors during the past two decades and
reveals some of the reasons for the continuing vitality and import of this field of research. The cited results
have been culled to give an overview of the hard-won experimental data and resulting theoretical
interpretations as well as a sense of the considerations that have determined the direction of the research.
BACKGROUND
In late 1972, a group of experimentalists and theoreticians from the Department of Chemistry at Homewood
and the Applied Physics Laboratory began what was to
become an intensive research effort to synthesize electrically conducting organic charge-transfer salts, which
prompted an equally spirited preoccupation with the systematic exploration of their crystalline structures and
the measurement and theoretical interpretation of their
physical properties. At the beginning, two of the present
authors (D.O.C. and TJ.K.), along with Jerome H. Perlstein (now a research scientist at Eastman Kodak) and
Aaron N. Bloch (now Associate Provost for Research at
Columbia University), were members of the Department
of Chemistry at Homewood, and Theodore O. Poehler
resided in the Milton S. Ei senhower Research Center of
the Applied Physics Laboratory. In addition, numerous
undergraduate and graduate students (including Terry E.
Phillips, Richard S. Potember, and Wayne A. Bryden,
who are all presently members of the Milton S. Eisenhower Research Center), postdoctoral fellow s, and
visiting scientists contributed enormously to the initial
advances and to the continuing strength of the organic
conductors program.
INTRODUCTION
The electrical conductivity of most organic solids is
generally expected to be limited (largely to insulating
behavior) by the covalent contribution to the binding
energy that stabilizes such materials. About twenty-five
years ago, however, several groups of researchers discovered that a class of organic materials exhibiting substantial room-temperature electrical conductivity could be
synthesized. The electrical conductivities of some of
these materials proved to be nearly as high as those
observed for such common metals as lead and copper.
256
Specifically, researchers demonstrated that planar, conjugated molecular radical (possessing an unpaired electron) anions- such as those derived from tetracyanoethylene (TCNE) and tetracyano-p-quinodimethane (TCNQ)or planar conjugated organic radical cations-such as
those derived from N-methylphenazene ( MP) or tetramethylparaphenylenediamine (TMPD)-form charge-transfer salts in which donors or acceptors can be stacked
along a common direction with their molecular planes
parallel to each other. Such a crystalline arrangement
maximizes the intermolecular 7r-orbital overlap along the
chain propagation direction. In these materials, moreover,
as in those to be described, the interchain 7r-orbital overlaps are very small. As a result, the electronic structure
and dependent electrical properties are markedly anisotropic, with the ratio of the conductivities in the intrachain
to the interchain directions often exceeding 103 . In the
jargon of condensed matter physics, such materials are
said to be quasi-one-dimensional.
The charge-transfer salts synthesized were generally of
three kinds: (1 ) those invol ving organic electron donors
with simple inorganic anions, such as MP- X, where x is
a halide anion; I (2) those composed of organic electron
acceptors with simple inorganjc cations, such as
Cs2(TC Qh; and, (3) those where both donor and acceptor
were drawn from conjugated organic molecules, such as
MP-TC Q, TMPD-TC Q, and TMPD(TCNQ)z. An elegant,
comprehensive, and masterful review of the chemistry,
spectroscopy, and crystallography of these early materials
has been presented by Herbstein. I
The potential for fundamental discoveries from continued studies of conductive organic systems was certainly
a sufficient motivation for further research into the synthesis of new donors and acceptors and the preparation
of new charge-transfer salts. Although these early matef ohns Hopkins APL Technical Digest, Vo lume 13, Number I (1992)
rium (lTeF, see Fig. 1) analogues, the chalcogen atoms
have been shown to bear the most prominent fraction of
the positive charge.
Also shown in Figure 1 are two particularly important
fulvalene donors containing selenium: TMTSF (tetramethyltetraselenafulvalene), initially synthesized at Johns
Hopkins and a key element in the first family of super-
rials were limited in many of their properties (generally
to a temperature dependence typical of a semiconductor),
the enormous power of synthetic organic chemistry and
the equally broad range of potential markets for useful
devices based on inexpensive organic materials (see the
boxed insert) provided more than sufficient additional
impetus for continued exploratory research at many university, industrial, and government laboratories both here
and abroad.
The Johns Hopkins program in synthetic chemistry has
had an early and continuing focus on organic donors,
particularly those based on a class of molecules derived
from chalcogen-substituted (s ulfur [S] , seleni um [Se] ,
and tellurium [TeD fulvalenes. The molecular structure
of the simplest of these donors , TIF (tetrathiafulvalene),
is illustrated in Figure 1. This symmetrical donor has a
relatively simple molecular geometry with two chalcogen-substituted five-membered rings joined by a carboncarbon double bond, which results in a near overall planarity. (It is important to recognize and retain the point
that the donor TIF and its derivatives shown in Figure 1
have a structure suggestive of a playing card.) The comparatively low first ionization potential (see Fig. 1) of TIF
makes its radical cation accessible through charge
transfer to a sufficiently strong inorganic or organic
acceptor or via simple electrochemical techniques. Finally, in the cation of TIF (TIP) and its selenium and tellu-
H
H
\ C..----- \
II
/
c--.... /
C=C
\ __ C
\
H
H
TTF (6.9)
~Te
\
/ --------(
/
H
C
\
I
/ C=C
C--....Te
/
\
Te......... /
C
H
II
.--C
Te
\
H
H
TTeF (7. 2)
TMTSF (6.6)
HMTSF (6 .5)
Figure 1. The electron donor TIF and several of its derivatives .
The numbers in parentheses are first ion ization potentials in
electron vo lts (a measu re of th e ease of formati on of the rad ical
cati ons).
OPPORTUNITIES IN, AND APPLICATIONS OF, CONDUCTIVE ORGANIC
CHARGE- TRANSFER SALTS
,
Transduce rs
(electret microphones)
Photocopier
solar cells
Piezoelectric
phenomena,
ferroelectric
phenomena
O rgan ic
photoconductors,
organic
semiconductors
Electronic
com ponents,
Metals ~
plastic ~
batteri es
/
Josephson junction
computer logic gates,
high-field magnets,
generators, motors,
power transm ission
Johns Hopkins APL Technical Digest, Volum e 13, Number J (1 992)
/
Solid-state
photochemical
reactions
t
0
.
h '
rganlc synt eSls
/t
Supercond uctors
Optical
information
storage
Electro-optics ,
nonlinear optical
phenomena
t
Frequency doublers,
modulators,
integrated optics ,
optical compute rs
~ Solid-state
Chem ical
reactions ~ sensors
Ferromagnetism
Magnetic recording ,
magneto-optic
recording
257
T. 1. Kistenma cher, D. O. Cowan , and T. O. Poehler
conducting organic salts, (TMTSF)2X?,3 and HMTSF (hex amethylenetetraselenafulvalene), the electron donor in a
number of unusual salts, some of which are described
below. These donors have been synthesized by substituting selenium for sulfur in the fulvalene rings and replacing various functional groups (such as methyl) of the
exocyclic protons of TIE One major consequence of these
chemical changes has been a substantial decrease in the
first ionization potentials of these donors relative to that
of TIF (see Fig. 1), making their cations even more
accessible.
A prominent family of electron acceptors employed in
the Johns Hopkins program has its antecedents in the
acceptor TCNQ and its fluorinated derivatives depicted in
Figure 2. Like TIF, TCNQ is an essentially planar molecule
with a relatively high first electron affinity, which makes
its anion (TCNQ-) available through charge transfer from
a sufficiently dative inorganic or organic donor or via
electrochemical methods. In this context, note that the
acceptor TCNQF4 has a significantly higher electron affinity than TCNQ (see Fig. 2). Like the TIF-based donors
shown in Figure 1, the TCNQ-based acceptors of Figure
2 also resemble a playing card in shape. Finally, in the
anions of TCNQ and its fluorinated derivatives, the cyano
nitrogen atoms bear the most prominent fraction of the
negative charge.
In the simplest of experiments, charge-transfer salts are
synthesized in a glass H-tube apparatus by allowing one
of the electron donors of Figure 1 and one of the electron
acceptors of Figure 2 to interact as shown in Figure 3.
The electron donor is dissolved in a suitable solvent in
one half of the H-tube apparatus, and the other half of
the apparatus contains a solution of the electron acceptor
in an appropriate solvent. (Best results are usually obtained if the donor and acceptor solvents are at least
miscible.) These solutions are separated by a glass frit
(with pores on the order of a few microns) to ensure a
mixing rate sufficiently slow (usually days to weeks to
months) to allow an equilibrium growth of the chargetransfer salt. Nucleation of crystals (usually black acic-
N
/
J
Figure 2. The electron acceptor TCNQ
and several of its fluorinated derivatives.
The numbers in parentheses are first
electron affinities in electron volts (a
measure of the ease of formation of the
radical anions).
N
In the two decades since the achievement at Johns
Hopkins of true metallic conductivity in the purely organic charge-transfer salt TIF-TCNQ, probably no other molecular conductor has engendered as much excitement in
the chemistry and physics communities. It seems appropriate, then, to begin this brief review by describing
the crystalline structure and some of the physical properties of this still novel charge-transfer salt.
TTF-TCNQ: An Auspicious Beginning
The charge-transfer salt TIF-TCNQ is composed of the
now familiar electron donor TIF (see Fig. 1) and the
equally familiar electron acceptor TC Q (see Fig. 2). The
crystal structure4 ,5 of TIF-TCNQ is illustrated in Figures 4
and 5, and some of its crystallographic properties are
given in Table 1. Only a few-but very importantaspects of the crystal structure of TIF-TCNQ need to be
comprehended: (1) The relatively low symmetry (monoclinic) unit cell describing the crystal structure has a very
acicular shape reminiscent of its molecular components
(see the previous section) with unit cell axis lengths of
\
I
c=c
\
/
H
I
\
/
\c
\N
H
/
C
!!
N
c
!!
N
H
I
CI
\ c=cI
I
\
c
c=c
\ c=c
/
H
N
\
H
,
N
TCNQF (2 .95)
;
IC
C===C
\
I
C===C
/
\
F
F\
\ e=cI
c=c
F\
!
IC===C\
e=c
N
\C
CI
TCNQ (2.85)
C\
\
C
,
F
TCNQF 4 (3.20)
258
N
I
\
c=c
l
\
\ e=c
c
PRESENTATION OF RESULTS
AND DISCUSSION
H
H
\C
ular prisms for conductive salts and brightly colored red
or yellow blocks Jor insulating salts) occurs at the rough
surface of the glass frit as is visible in Figure 3.
Depending largely on the relative ionization potential
of the electron donor and the relative electron affinity of
the electron acceptor, the charge states and crystal structures of organic donor-acceptor salts may be classified as
(1) mixed (insulating) stacks of essentially neutral donors
and acceptors, (2) mixed or segregated (insulating to
semiconducting) stacks of fully ionized donors and
acceptors, or (3) mixed (semiconducting) stack or segregated (metallic) stack arrays of fractionally charged
donors and acceptors. Although fractional charge transfer
is not restricted to organic salts, its clear role in the
conductivity of organic charge-transfer salts cannot be
overemphasized.
N
2, 5-TCNQF 2 (3.02)
Johns Hopkins APL Technical Digest, Vo lume 13, Number J (1992)
Twenty Years of Low-Dimensional Organic Conductors
Table 1. Some crystallographic properties of selected organic
charge-transfer salts exhibiting a segregated stack motif.
Crystal
system
Salt
TTF- TC Q
monoclinic
TSF- TC Q
monoclinic
TTeF- TC Q
triclinic
TMTTF- TC Q
monoclinic
TMTSF-TC Q
triclinic
HMTTF- TC Q
orthorhombic
HMTSF- TCNQ
monoclinic
HMTSF- TC QF4 monoclinic
az i
Figure 3. H-tube growth of an organic charge-transfer salt. The
electron donor is dissolved in a su itable solvent in one half of the
apparatus , and the other half contains a solution of the electron
acceptor in an appropriate solvent. The black, acicular crystals of
the charge-transfer salt, which are typically a few millimeters long
and a few hundredths of a square millimeter in cross section , have
nucleated at the fine glass frit separating the solutions .
x
Johns Hopkins APL Technical Digest, Vo lume 13, Number 1 (1992)
Unit cell
Stacking volume
za axis (A) (A 3)
2
3.819
840
2
3.872
869
475
3.947
2
3.850
1062
544
3.883
2
3.901
1050
2
3.890
1076
2
4.018
1137
the number of formula units per cell.
approximately 4, 12, and 18 A. (2) Segregated, onedimensional stacks of donors and acceptors run parallel
to the crystallographic b axis (the ~4- A cell constant)
with their cross sections packed in a herringbone motif
(the dihedral angle between contiguous donor and acceptor planes is 58.5°) as best seen in Figure 5. (3) The
segregated stacks of donors and acceptors are arranged
in a checkerboard fashion as is more readily seen in
Figure 4, and the interchain coupling is dominated by
S ... N interactions that are significantly shorter than the
x
x
x
Space
group
P2/c
P2/c
PI
P2/c
PI
Pmna
C2/m
C2/m
x
Figure 4. The crystal structure of the
charge-transfer (0.59e-) salt TIF-TCNQ
projected onto the ac crystallographic
plane . The shortest S ... N intermolecu lar contacts are indicated with dashed
lines.
x
259
T. 1. Kistenmacher, D. O. COl>l an, and T. O. Poehler
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
Figure 5. The crystal structure of TIF-TCNO projected onto the be
crystallographic plane . The TCNO anions (blue lines) are in one
plane , and the TIF cations (yellow lines) are in a separate plane .
The dihedral angle between alternate stacks of donors and acceptors is 58.5°.
sum of their van der Waals radii, which suggests the
influence of factors other than dispersive forces.
The degree of charge transfer in ITF-TC Q is a major
consideration. Several methods of determining the degree
of charge transfer have been utilized and include assessing the geometrical dependence of the molecular components on charge transfer; 6 using infrared spectroscopy to
measure the linear frequency shift of the molecular vibration, which is largely confined to the stretching of the
cyano bonds of the TC Q acceptor with increased charge
transfer;7 and measuring the variation in the diffuse Xray scattering wavevector with charge transfer,8 which is
the most precise method. Although each of these approaches yields a consistent measure of the degree of
charge transfer, the value of 0.5ge- from the X-ray measurements is the most accurate determination.
Two aspects of the degree of charge transfer in ITFTC Q are worthy of special comment: (1) The significant
charge transfer suggests a meaningful electrostatic component to the S .. .N intermolecular contacts shown in
Figure 4 and an important Madelung energy contribution
to the crystal cohesive energy.9 (2) The charge transfer
is fractional and is due largely to the magnitude of the
difference between the ionization potential of ITF and the
electron affinity of TC Q (although other factors are
known to be imp0l1ant).l o
The room-temperature electrical conductivity of ITFTCNQ is given in Table 2 in comparison with other chargetransfer salts, and the dependence of its electrical conductivity (normalized to the value at 300 K) on temperature 11 ,12 is illustrated in Figure 6. A strong increase in conductivity, rising to a maximum of 2 X 104 (ohm·cmt l
(a value near that of lead), is evident with decreasing
temperature, which is behavior typical of a conventional
metal. A sharp drop in conductivity occurs near 60 K, and
an insulating state ultimately arises near 30 K. The physics of ITF-TCNQ at temperatures near 60 K has been examined with great energy and insight. It is beyond the
scope of this article to cite all of the pertinent results,
discussions, and theories that have evolved to explain the
observed experimental behavior. (For example, one can
perceive a sequence of three closely spaced transitions
near 60 K, two of which are easily distinguished in the
inset to Figure 6.) Suffice it to say that low-dimensional
260
Table 2. Charge-transfer and conductivity data for selected
conducting heterofulvalene-TCNO charge-transfer salts.
Temperature
Fractional Conductivity
Maximum of maximum
charge
at room
transfer temperature conductivity conductivity
(e-)
(K )
Salt
(ohm·cmt' (ohm·cmt'
2 X 104
TIF-TC Q
59
0.59
500
1 X IQ4
40
TSF-TC Q
0.63
800
1 X 104
2200
100
TIeF-TC Q
0.71
5 X 103
60
TMTIF-TC Q 0.65
350
7 X 103
61
TMTSF-TC Q 0.57
1200
2 X 103
75
500
HMTIF- TC Q 0.72
7 X 103
32
HMTSF-TC Q 0.74
2000
15
~
.!z
10
t>
~
J'"
:~
"'):..
b
-g
10
1
0
()
;i-
'0
S
.~
:g
Cii
§ 0.1
:::J
'0
c
z
0
30
50
40
Temperature (K)
u
'0
60
Q)
.!:::!
(U
5
E
(5
z
O L---~------~----------~----------~
o
100
200
300
Temperature (K)
Figure 6. Temperature dependence of the normalized electrical
conductivity for TIF-TCNO (0" = conductivity). The inset shows two
of the three phase transitions occurring at about 53, 47, and 38 K.
systems with fractional charge transfer are electronically
unstable and that at the critical temperature the crystal
structure becomes distorted, a gap opens at the Fermi
energy level (see Fig. 7), and a metal-insulator transition
occurs. This phenomenon is commonly termed a Peierls
transition l3 and is known to involve the condensation of
a charge density wave.
Because of the inherent instability of quasi-one-dimensional systems, much effort has been devoted in trying to design and synthesize new donors and acceptors
that will form charge-transfer salts of greater dimensionality (i.e. , with a more two-dimensional set of intermolecular interactions in the crystalline structure) in order
to suppress the Peierls transition. In the following section,
HMTSF-TCNQ, a successful end product of the quest for
higher dimensionality is discussed, and the very prominent role the degree of charge transfer plays in electrical
activity is highlighted with reference to a companion salt,
HMTSF- TC QF4 .
f ohns Hopkins APL Technical Digest, Vo lume 13, Number 1 (1992 )
Twenty Years of Low-Dimensional Organic Conductors
A ~______________, -____________~~
B
p
~--------------~--------------~
E
Empty
states
Energy {
gap
-
-
-
EF
Filled
states
-hl2b
-hl4b
o
hl4b
hl2b P
Figure 7. Energy versus momentum for electrons in a onedimensional periodic potential (E = energy , EF = Fermi level ,
p = momentum , b = b-cell constant, h = Planck's constant). A.
Original undistorted lattice. B. Distorted lattice caused by electronic instability at the Fermi level (Peierls transition).
HMTSF-TCNQ and HMTSF-TCNQF4:
Higher Dimensionality, Isomorphism, and
the Importance of Charge Transfer
The crystal structure 14 of the charge-transfer salt
is presented in Figures 8 and 9, and some
of its crystallographic properties are given in Table 1.
Once again, the prominent features of the crystal structure are a low (monoclinic) crystal symmetry and the
presence of segregated columns of donors and acceptors
running parallel to a short (-4 A) cell axis. One very
noticeable structural difference between TTF-TC Q and
HMTSF-TCNQ (see Figs. 5 and 9) is that the herringbone
motif (the dihedral angle between contiguous donor and
acceptor planes is 58.5°) in TTF-TC Q has been replaced
by a nearly parallel arrangement (the dihedral angle between contiguous HMTSF donor planes and TC Q acceptor
planes is 10.9°) of donors and acceptors in HMTSF- TC Q.
This altered motif and an increased charge transfer
(0.74e-) are stabilized by very short Se... N interaction
distances and an even stronger interaction between columns of donors and acceptors. In fact, the increased
coupling between donor and acceptor columns (see Fig.
8) leads to quasi-two-dimensional planes running approximately normal to the crystallographic a axis. Because of the poorer coupling between these two-dimensional planes, stacking faults 14 are inherent in the crystal
physics of HMTSF- TC Q.
The effect of the increased dimensionality in HMTSFTCNQ, moreover, is most notably expressed in the temperature dependence of its relative electrical conductivityl Sas shown in Figure 10. Here, in contrast to TTF- TCNQ,
the electrical properties are only modestly dependent on
temperature. Near 50 K, the conductivity has increased
by about a factor of five compared with the room temperature value, corresponding to a decrease in resistivity
by the same magnitude. At about 10K, however, the
HMTSF-TC Q
Johns Hopkins APL Technical Digest, Volume 13. Number J (1992)
Figure 8. The crystal structure of the charge-transfer salt (-0.74e-)
HMTSF-TCNQ projected onto the ab crystallograph ic plane. The
dominant Se ... N intermolecular contacts are indicated with dashed
lines.
conductivity has returned to its room temperature value
and is subsequently nearly independent of temperature
down to 0.1 K. The absence of a clear indication of a
metal- insulator transition in HMTSF-TC Q has primarily
been attributed 14, 15 to the increased structural and electronic dimensionality, but the effects of disorder owing
to the presence of the stacking faults may also playa role
in suppressing the Peierls transition.
In considering the salt formed between HMTSF (see Fig.
1) and the modified acceptor TC QF4 (see Fig. 2), note that
TC QF4 has a considerably higher electron affinity than
TC Q. The HMTSF-TC QF4 salt has a crystal structure, as
shown in Figure 11 ,16 that is isomorphous with that of
HMTSF- TCNQ. That is, the structures of these two salts
display the same symmetry and have unit cells of about
the same size (see Table 1). Pictorially, the parallels in
symmetry and unit cell size of these salts can be seen by
comparing Figures 8 and 9 with Figure 11.
If this were the end of the story of HMTSF- TCNQF4 , it
would be a simple one indeed and relatively devoid of
interest. The electrical properties 17 of this salt, however,
(see Fig. 12) are dramatically different from those of
HMTSF-TCNQ (see Fig. 10) and kindled-and continue to
generate--considerable experimental and theoretical interest. In particular, the activated behavior of the temperature dependence of the electrical resistivity of HMTSF261
T. 1. Kistenmacher, D. O. Cowan, and T. O. Poehler
Figure 9. The crystal structure of HMTSFprojected onto the ac crystallographic plane . The TCNQ anions (blue
lines) are in one stack, and the HMTSF
cations (orange lines) are in a separate
stack. The dihedral angle between alternate stacks of donor cations and acceptor
anions is 10.9°.
TCNQ
10
'"
0
5
b
F..
b
~
2
S
U
::::J
'0
C
0
()
'0
Q)
0.5
.!::!
co
E
(5 0.2
z
0.1
1
2
5
10
20
50
100 200
500 1000
Temperature (K)
Figure 10. Temperature dependence of the normalized electrical
conductivity for HMTSF-TCNQ (a = conductivity).
TCNQF4 (see Fig. 12A) is typical of a semiconductor, with
an experimental activation energy of about O.leY. Briefly
stated, the salt HMTSF-TC QF~ behaves as a semiconductor
because (I) the electron charge transfer from donor to
acceptor is unity; (2) each HMTSF cation and TC QF4 anion
has one unpaired electron in its outermost molecular
orbital; and (3) in view of the unpaired electron availability, the excitation of an electron along a donor or acceptor
stack is activated with an activation energy on the order
of the onsite electron-electron repulsion parameter.
The electrical behavior of HMTSF-TCNQF.. is not just that
of a simple semiconductor, however, for a transition can
be seen from one semiconducting state to another in
Figure 12B. This change is a spin-Peierls transition because it involves the correlation of the free electron spins
of donor and acceptor electrons as against the correlation
of their spatial wavefunctions, which was the case for the
conventional Peierls transition active in TTF-TCNQ (see
the previous section) .
The isomorphism of the organic conductor HMTSFTCNQ and the organic semiconductor HMTSF- TCNQF4 of262
fers the possibility of preparing alloys of the form (HMTSF)
(TC Q)x (TCNQF~)I .x whose constitution spans the full available range. The study of the structural properties (with
the experimental data taken largely from the Ph.D. thesis
of Thomas J. Emge with the help of Louis K. Frevel of
Dow Chemical) , 18 electrical properties (extracted largely
from the Ph.D. thesis of James P. Stokes), 19 and magnetic
properties (taken from the Ph.D. thesis of Wayne A.
Bryden)2o of these alloys covers a truly vast sweep of
theory and experimentation.
Salts near one or the other of the extremes, to summarize the research results, show properties that can be
considered as clearly deriving from either those of
HMTSF-TC Q or HMTSF-TC QF4. Near the equimolar alloy
composition, however, phase separation into a mixture of
the original salts develops instead of alloy formation.
Such a result is consistent with a first-order transition
separating the two phases and culminating in a miscibility
gap. All of these aspects of the properties of HMTSF-TCNQ
and HMTSF-TC QF4 and their alloys, including the detailed
nature of the first-order phase transition, the miscibility
gap and phase separation, and the general features of the
important contributors to crystal cohesion in these lowdimensional materials have been described by Bloch,IO·21
and the interested reader is referred to the primary literature for a more elaborate exposition of the theory of
cohesion and its applications.
TMTSF-TCNQ: Polymorphism and
the Red and Black Phases
The donor TMTSF (see Fig. 1) was first synthesized22
in 1974 and subsequently coupled in the same crystallization experiment with the acceptor TCNQ to yield two
types of charge-transfer salts-one a black conductor and
the other a red insulator. The first of these is considered
a typical low-dimensional organic conductor whose crystal structure (see Figs. 13 and 14)23 is dominated by
segregated stacks of TMTSF cations and TCNQ anions. The
crystalline structure ofTMTsF-TcNQ is one of the simplest
known for an organic charge-transfer salt and features
triclinic symmetry (see Table 1), strong donor-acceptor
John s Hopkin s APL Technical Digest, Volume 13. Numb er I (1992)
Twenty Years of Low-Dimensional Organic Conductors
Figure 11 . Crystal structure of the unit charge-transfer salt HMTSF-TCNOF 4 projected onto the ab (top) and ac (bottom) crystallographic
planes (the blue lines denote the TCNOF4 anions, and the orange lines signify the HMTSF cations). The dominant Se . .. N and Se ... F
intermolecular contacts are indicated with dashed lines.
coupling in the form of short Se . .. N interactions, and
strong interchain coupling via short Se .. .Se interactions,
which were not observed in the charge-transfer salts considered thus far. The experimental charge transfer is fractional for this black, conductive phase ofTMTsF-Tc Q with
a magnitude (O.57e-, see Table 2) very similar to that found
for TIF- TCNQ.
Our discussion of the properties of HMTSF- TCNQ and
HMTSF- TCNQF4 having just been completed, one might be
tempted to speculate that the degree of charge transfer is
unity in the red phase of TMTSF-TCNQ and that on-site
electron-electron correlations limit this salt, like HMTSFTCNQF4, to semiconducting behavior. Such a notion would
be wrong, of course, as the acceptor in each phase of the
TMTSF salt is the same, and therefore a difference in
charge transfer is not expected as was the case for the
different acceptors in the HMTSF salts. Thus, the source
of the difference in the electrical properties of the two
polymorphs of TMTSF-TCNQ must lie in some other fundamental property. It is in the crystal structure of the red
phase of TMTSF-TCNQ24 that the solution to this dilemma
is found. As shown in Figures 15 and 16, the red polyJohns Hopkins APL Technical Digest, Vo lume 13, Number 1 (1992)
morph of TMTSF-TCNQ adopts a mixed-stack array of
donors and acceptors characterized by columns of alternating, nearly parallel TMTSF donors and TCNQ acceptors
that propagate along the short crystallographic direction
(see Table 2). Additionally, short interchain Se ... N interactions are seen between these mixed stacks.
This excursus demonstrates rather emphatically that a
very delicate balance of forces contributes to the adopted
crystalline motif in these charge-transfer salts . Quite obviously, the crystalline structures of the two polymorphs
of TMTSF- TCNQ are similar in that one-dimensional columns are an essential element in each of their crystalline
motifs. It is equally obvious, however, that the crystalline
structures of the two polymorphs of TMTSF- TCNQ are quite
dissimilar, for the one-dimensional columns are composed of segregated stacks of donors and acceptors in the
black, conducting phase and mixed stacks of donors and
acceptors in the red, semiconducting phase. The latter
motif is manifestly supported by electrostatic considerations, whereas the former motif is clearly favored by a
covalent-type bonding factor. Thus, near equality of these
competing forces is suggested in the TMTSF- TCNQ salts.
263
T. 1. Kistenmacher, D. O. Cowan, and T. O. Poehler
Temperature (K)
A
300
250
200
150
8 ~~------~--------~----------~--~
0.
Ol
6
.2
4
B 5 ~~----~--------~------------~
3 L-~~----~--------~--------~-----
3
4
5
6
103fT (K-1)
Figure 12. Inverse temperature dependence for the semicon ducting salt HMTSF-TCNOF4 • A. Log (resistivity) where p is in a·cm.
B. Resistance (R) derivative d(lnR)/d(1/T)(1 03 K-l).
TTeF-TCNQ: Final Third of a Matched Set
Finally, we will consider the properties of a quite recent charge-tran fer salt derived from a tellurium-based
donor and the acceptor TC Q. Early on after the excitement generated by the synthesis and physical properties
of TIF-TC Q, a common (but very difficult) synthetic
approach was "simply" to replace the sulfur atoms of TIF
with selenium atoms to yield the donor tetraselenafulvalene (TSeF). In fact, the synthesis of TSeF and its metallic
charge-transfer salt with TC Q was reported as far back as
1974 by Engler and Pate1. 25 Significantly, the isomorphism of TSeF-TC Q and TIF-TC Q permitted the preparation of alloys in the form (TSeFMTIF)'_xTC Q, which
enabled important deductions to be made about the separate contributions of the segregated donor and acceptor
stacks to the electrical conductivity, magnetism, and lattice dynamics of the phase transitions in TIF-TCNQ and
TSeF-TC Q.26
Figure 13. The crystal structure of the black, conducting chargetransfer salt TMTSF-TCNO projected onto the crystallographic be
plane . The dominant Se ... N and Se . .. Se contacts are indicated
with dashed lines
Expectations immediately intensified for the synthesis
of the third member of this family of donors , tetratellurafulvalene (TIeF, see Fig. 1). Given the properties ofTIFTCNQ and TSeF-TC Q, researchers envisioned that the preparation of the charge-transfer salt TIeF-TC Q would likely lead to (1) an increase in the intrastack conduction
bandwidth of the donor stack because of the increased 7roverlap resulting from the more diffuse p orbitals of
tellurium; (2) an increase in structural and electronic
dimensionality owing again to the relative spatial extent
of the tellurium-based orbitals now used in the interstack
interactions; (3) a reduction in the on-site electron-elec-
Figure 14. The crystal structure of the
black polymorph of TMTSF-TCNO projected
onto the ae crystallographic plane. The
TCNO anions (blue lines) are in one plane ,
and the TMTSF cations (orange lines) are
in a separate plane . The dihedral angle
between alternate planes of donor cations
and acceptor anions is 54.9°.
264
Johns Hopkins APL Technical Digest, Vo lum e 13, Number J (1992)
Twenty Years of Low-Dimensional Organ ic Conductors
o
/
/
/
/
/
/
/
cf
o
I
I
o
I
o
»-«
/
d
Figure 15. Two layers of the crystal structure of the red , semiconducting polymorph of TMTSF-TCNQ projected onto the ab crystallographic plane. The TMTSF cations (filled circles) are in one plane ,
and the TCNQ anions (open circles) are in a separate plane. The
important intermolecular Se . .. N and Se ... Se contacts are indicated with dashed lines. The dihedral angle between alternate
sheets of donors and acceptors is about 1°.
Figure 17. Projections of the segregated-stack crystal structure
of TTeF-TCNQ onto the ae (top) and be (bottom) crystallographic
planes. The principal Te . . .Te and Te ... N intermolecular interactions are indicated with dashed lines.
Figure 16. Projection of the crystal structure of the red polymorph
of TMTSF-TCNQ onto the crystallographic ae plane . The TCNQ
anions are denoted by blue lines, and the TMTSF cations are
indicated by orange lines. Note the mixed-stack arrays of donors
and acceptors .
tron repulsion because of the increased polarizability
added by the tellurium atoms; and, (4) a difference in
charge transfer as a consequence of a change in the ionization potential of the donor.
In 1987, the synthesis of TTeF was finally reported at
Johns Hopkins,27 and initial and detailed studies of the
crystal structure and physical properties of the chargetransfer salt TTeF_TCNQ28-30 (see Table 2), which has a
fractional charge-transfer of 0.71e-, soon followed. Surprisingly, although the crystal structure of TTeF-TCNQ
(see Fig. 17) consists of segregated stacks of TTeF cations
and TCNQ anions, the adopted motif is not isomorphous
with that of TTF- TCNQ and TSeF-TCNQ. It seems reasonable, then, to conjecture that it is the interstack interactions that lead to the structural pattern alteration. As
expected, strong Te ... N interactions are present in the
Johns Hopkins APL Technical Digest, Vo lume 13, Number J (/992)
crystal structure of TTeF-TCNQ that are analogous to the
S ... N interactions of TTF-TCNQ and the Se ... N interactions of TSeF-TCNQ. In addition, however, rather short
Te ... Te contacts exist (recall the absence as shown in
Figure 4 of this type of interaction in the crystal structures
ofTTF-TcNQ and TSeF-TCNQ) that are apparently sufficient
to sway the cohesive energetics in favor of the modified
crystal structure.
In fact, the crystalline motif of TTeF-TCNQ is akin in
symmetry (see Table 1) and in interchain coupling to that
of the black, conducting polymorph of TMTSF-TCNQ (see
Figs. 13 and 14) with its combination of Se ... Se and
Se ... N inters tack interactions. The electrical properties
of TTeF-TCNQ (see Fig. 18), however, are reminiscent of
those of HMTSF-TCNQ (see Fig. 10), which suggests that
the Te . .. Te interactions are of sufficient strength to give
a two-dimensional electronic band structure and to suppress the Peierls transition effectively.
SUMMARY
Some of the more compelling contributions of the
Johns Hopkins community to the synthesis and study of
organic charge-transfer salts have been presented, and a
chronological outline of the accomplishments of the synthetic organic conductors program is given in the boxed
insert. Review articles by Johns Hopkins authors are cited
265
T. 1. Kisrenmacher, D. O. Cowan , and T. O. Poehler
1.0 ,-------------~--------------,-------------~
0.9
'"g
t5' 0.8
-."..
t::>
.~
>
U
0.7
~
"0
C
0
u
0.6
"0
in the reference list (see Refs. 31 through 42) for further
and more detailed reading.
The synthetic organic conductors program remains an
active and vital collaborative link between Homewood
and the Applied Physics Laboratory on a number of scientific fronts given the continued interest in the diversity
of crystalline organic materials exhibiting superconductivity. Some of the noteworthy ventures in progress include the syn.thesis and study of novel donors and acceptors and real-space imaging of crystalline motifs
through scanning tunneling microscopy.
Q.)
.!::!
Cii
E 0.5
REFERENCES
z
He rbstein , F. H., " Crystalline p-Molecu lar Compounds: Chemistry, Spectroscopy, and Crystallograph y," in Perspecti ves in Structural Chemistry, Vol. 4 ,
Dun itz, 1. D. , and Ibers, 1. A. (eds.), John Wiley and Sons, New York, pp.
166-395 ( 1971 ).
2Jerome, D., Mazaud, A. , Ribault, M., and Bechgaard, K. , "Superconductivity
in a Synthetic Organi c Conductor (TMTSF)2PF6'" 1. Phys. (Paris) Lell. 41 ,
L95-L98 ( 198 1).
3Bechgaard , K., Carne iro , K., Ras mussen, F. B., Ol sen, M. , Rindorf, G ., et aI. ,
"Supe rconducti vity in an Organic Solid. Synthesis, Structure and Conductivity
of Bi (tetrameth yltetrase lenaful valenium Perchlorate, (TMTSFh CI04," 1.
Amer. Chem. Soc. 102, 2440-2442 (1984).
· Ph illips, T. E. , Kiste nm acher, T. J ., Ferraris, J. P., and Cowan, D. 0 ., " Crystal
Structure of the Radi cal-Cati on Radi cal-Anion Salt from 2,2'-Bi- I,3 -Dithiole
and 7,7,8 ,8-Tetracya no-p -quinodimeth ane," 1. C . S. Chem. Commun ., 471-472
(1973).
5Ki stenmacher, T. J. , Phillips, T. E. , an d Cowan, D. 0 ., "The Crystal Structure
of the I: I Radi cal Cation-Radical Ani on Salt of 2,2'-Bi-I ,3-Dithiofulv alene
(TIF) and 7,7 ,8,8-Tetracyano-p-q uinodimethane (TC Q)," Acta Crystallogr.
B 30, 763-768 ( 1974).
6Ki stenmacher, T . J., Emge, T. J., Wiygul , F. M. , Bryden, W . A., Chappell,
J . S., et aI. , " DBTIF- TCNQ, A Fracti onall y-Charged Organic Salt with a
Mi xed-Stack Crystalline Motif," Solid State Commun. 39, 415-417 (1981 ).
7Chappell , J. S., Bl och, A. ., Bryden, W. A., Max fi eld , M. , Poehler, T . 0. , et
aI. , "Degree of Charge Transfer in Organi c Cond uctors by Infrared Absorpti on
Spectroscopy," 1. Amer. Chem . Soc. 103, 2442- 2445 ( 198 1).
8Comes, R., and Shirane, G .. " X-Ray and eutron Scanering fro m OneDimensional Condu ctors," in Highly Conducting Olle-Dimensional Solids ,
Dev reese, J. T. , Evrard. R. P., and van Doren, V. E. (eds.), Plenum Press, ew
York, pp. 17-fJ7 (1979).
9Metzger, R. M., and Bloch, A. ., " Crystal Co ul omb Energies. VII. The
Electrostati c Binding Energy Defect in Tetrath iaful valinium 7,7,8,8-Tetracyanoquinodimethanide," J. Ch ern . Phys. 63, 5098-5107 ( 1975).
IOMazu mdar , S., and Bloch, A. ., "Systemat ic Trends in Short-Range Coulomb
Effects Among earl y One- Dimensional Organic Conductors," Phys. Rev.
Lell . 50, 207-2 10 ( 1983).
I IFerraris, J., Cowan, D. 0. , Walatka, V. , and Perl tei n, 1. H., " Electron Transfer
in a New Highl y Condu ctin g Donor-Acceptor Complex ," 1. Amer. Chem . Soc.
95 , 948-951 (1973).
12Coleman, L. B. , Cohen, M. J., Sandm an, D. J., Ya magishim , F. G ., Garito,
A. F. , et aI. , "Superconductin g Fluctuati ons and the Pe ierls Instability in an
Organi c So lid," Solid State Commlln. 12, 11 25- 1128 ( 1973).
13Pe ierls, R. E., Quantum Theory of Solids, Oxford Uni versity Press ew York,
p. 108 (1955).
14Phillips, T. E. , Kistenmacher, T. J. , Bloch, A. ., and Cowan, D. 0 ., " X-Ray
Crystal Structure of the Organi c Condu ctor from 2,2 '-Bi-(2 ,4-di selenabicyclo[3.3.0]octyJidene) and 7,7,8,8-Tetracyanoparaquinodimethane (HMTSFTC Q)," 1. C. S. Ch em. Commull . 334-335 ( 1976).
15Bl och, A. ., Cowan, D. 0. , Bec hgaard, K., Pyle, R. E., Banks, R. H., et aI. ,
" Low-Te mperature Metal lic Behavior and Resistance Minimum in a ew
Quasi One- Dimensional O rgani c Conductor," Phys. Rev. Lell . 34, 156 1- 1564
( 1975).
16Emge, T. 1. , Cowan . D. 0 ., Bl och, A. ., and Ki stenmacher, T. J., " On the
C rysta l Structure of the Organic Charge-Transfer Salt Deri ved from
Hexameth ylenetetrase lenaful valene (HMTSF) and Tetrafluoro -7,7,8,8-tetracyano-p-quinodimeth ane (TC QF4 ), HMTSF-TC QF4'" Mol. Cryst. Liq.
Cryst. 95 , 19 1-207 ( 1983).
17Hawley, M. E. , Poehle r, T. 0 ., Carruthers, T. F., Bloch, A. ., Cowan, D. 0 .,
et aI. , " Preparati on, Structure, and Electrical Properties of an Organi c
Semiconductor, HMTSF-TCNQF4 ," Bull. Amer. Phys. Soc. 23, 424-425
( 1978).
18Emge, T. J., "Structural Characteristi cs of Organic Charge-Transfer Salts:
From e utral Components to Unit Charge-Tran fer Salts," Doctoral di ssertati on, The Johns Hopki ns Uni versity ( 198 1).
19Stokes, J. P., " I. Mon Transition in Organi c Charge Transfer Complexes; II.
Resistance Minima in Amorphous (FexMn l.xhs PI6B6AI3'" Doctoral dissertati on, The Johns Hopkins Uni versity (1981 ).
(;
I
0.4
0.3
0
100
200
300
Temperature (K)
Figure 18. Temperature dependence of the normalized electrical
conductivity of TIeF-TCNQ (0" = conductivity) .
HIGHLIGHTS OF THE JOHNS HOPKINS
UNIVERSITY (JHU) LOW-DIMENSIONAL
ORGANIC CONDUCTORS PROGRAM
1972
1974
1975
1978
1979
1980
1982
1983
1987
1990
266
TTF-TCNQ: First organic solid with a true metallic temperature profIle synthesized; crystal struc ture
and electrical properties studied; electronic instabilty found to cause a metal- insulator transition at
low temperatures.
TMTSF-TCNQ: Replacement of S by Se resulted
in wider electronic bands and higher conductivity;
metallic and in sulating polymorphs discovered.
HMTSF-TCNQ: Increased two-dimensional crystal structure found to suppress electronic instability;
salt observed to remain metallic to 0.1 K; no evidence of superconductivity di scovered.
HMTSF-TCNQF4: electrical semiconductor observed to be isomorphous with the metallic TCNQ
salt; HMTSF-(TCNQMTCNQF4)I .x alloys emphasized
the importance of fractional charge transfer and
stimulated Aaron Block 's theory of cohesion.
Cu-TCNQ: Field-induced electrical switching
and memory di scovered in simple copper salt of
TC Q acceptor.
(TMTSFhX: Donor synthesized at JHU served as
basis for first organic salts exhibiting superconductivity.
HMTTeF, DBTTeF: First tellurium donors synthesized.
(BEDT-TTFhX: Donor synthesized at JHU
served as basis for second class of organic salts
di splaying superconductivity; critical temperature
raised to 12 K.
TTeF: Tellurium analogue ofTTF synthesized ; simple salts prepared and studied.
TTeF-TCNQ: Tellurium congener of TIF- TCNQ
crystallized; structure and physical properties characterized .
Johns Hopkins APL Technical Digest, Volum e 13, Nu mber I (1992 )
Twenty Years of Low-Dimensional Organic Conductors
20Bryden, W. A., " Magnetism and the Mott Transition : Studies on Solid
Solutions of Organic Charge-Transfer Salts," Doctoral dissertation, The Johns
Hopkins University ( 1982).
2l Mazumdar, S. , Dixit, S. N., and Bloch, A. N. , "Correlation Effects on ChargeDensity Waves in arrow-Band One- Dimensional Conductors," Phys. ReI'. B
30,4842-4848 ( 1984).
22Bechgaard , K., Cowan, D.O. , and Bloch , A. ., '"Synthesis of the Organic
Conductor Tetramethyltetraselenafulvalenium 7,7 ,8,8-Tetracyano-p-quinodimethanide (TMTSF-TCNQ) [4,4',5,5'-Tetrameth yl-t>-2,2'- bis-1 ,3-diselenolium 3,6-bis-(dicyanomethylene)cyclohexadienide] ," J . C. S. Chem . Commun .
937- 938 (1974).
23Bechgaard, K. , Kistenmacher, T. J. , Bloch, A. ., and Cowan , D. O., "The
Crystal and Molecular Structure of an Organic Conductor from 4,4',5,5'Tetramethyl-t>-2,2'-bi- I,3-diselenole and 7,7,8,8-Tetracyanoparaquinodimethane [TMTSF-TC Q] ," Acta Crystallogr. B 33, 417-422 (1977).
24Kistenmacher, T. J., Emge, T. 1., Bloch, A. ., and Cowan , D. O. , "S tructure
of the Red, Semiconducting Foml of 4,4',5,5'-Tetramethyl-t>-2,2'-bis-l,3diselenole-7 ,7 ,8,8-Tetracyano-p-quinodimethane, TMTSF- TC Q," Acta Crystallogr. B 38, 11 93-1199 (1982).
25Engler, E. M., and Patel , Y. Y., "Structure Control in Organic Metal s.
Synthesis of Tetraselenafulvalene and Its Charge Transfer Salt wi th
Tetracyano-p-quinodimethane," 1. Amer. Chem. Soc. 96, 7376--7378 (1974).
26Schultz, T. D. , and Craven, R. A., "The Organic Metals (TSeF).(TTF)x-TC Q
-A Systematic Study ," in Highly Conducting One-Dimensional Solids,
Devreese, J. T. , Evrard, R. P. , and van Doren, Y. E. (eds.), Plenum Press, New
York , pp. 147-225 (1979) .
27McCullough, R. D., Kok, G. B., Lerstrup, K. A. , and Cowan, D.O. ,
"Tetratellurafulvalene (TTeF)," J. Amer Chem. Soc. 109, 4115-4116 ( 1987).
28Mays, M. D., McCullough , R. D. , Cowan , D.O., Poeh ler, T. 0., and
Kistenmacher, T. 1., " Initial Studies on a ew Tellurium Containing Organic
Metal: Tetratellurafulvalene-Tetracyanoquinodimethane (TTeF-TC Q),"
Solid State Commun. 65, 1089- 1092 (1988).
29Mays, M. D., McCullough, R. D. , Bailey, A. B., Cowan, D.O. , Bryden, W. A. ,
et aI., "Electrical and Magnetic Studies on Some New Organic Conductors
Made with Tetratellurafulvalene (TTeF)," Synthetic Metals 27, B493-B499
( 1988).
30Cowan , D.O. , Mays, M. D. , Kistenmacher, T. J. , Poehler, T. 0. , Beno, M. A. ,
et aI., "Structural and Electronic Properties of TXF- TCNQ (X = S, Se, Te),"
Mol. Cryst. Liq. Cryst. 181, 43-58 (1990).
31 Bloch, A. N. , "Design and Study of One-Dimensional Organic Conductors 1.
The Role of Structural Di sorder," in Energy and Charge Tran sfer in Organic
Semiconductors, Mas uda, K.. and Silver, M. (eds.), Plenum Press, New York,
pp. 159-166 ( 1975).
32Bloch , A. N. , Cowan, D. O. , and Poehler, T. 0 ., "Design and Study of OneDimensional Organic Conductor II . TTF-TCNQ and Other Organic Semimetals," in Energy and Charge Transfer in Organic Semiconductors, Masuda, K. ,
and Silver, M. (eds.), Plenum Press, ew York , pp. 167- 173 (1975).
33Poehler, T. 0., "Organic Conductors," Johns Hopkins APL Tech. Dig. 15(4),
13-21 (1976).
34Cowan , D. O., Shu , P., Hu, c., Kru g, W., Carruthers, T. F., et aI. , "The
Organic Metallic State: Some Chemical Aspects," in Chemistry and Physics of
One-Dimensional Metals , Keller, H. J. (ed.), Plenum Press, ew York, pp. 2546 (1977).
35Bloch, A. N., Carruthers, T. F. , Poehler, T. 0 ., and Cowan, D. O. , "The
Organic Metallic State: Some Physical Aspects and Chemical Trends," in
Chemistry and Physics of One-Dimensional Metals , Keller, H. J. (ed.), Plenum
Press, New York, pp. 46--86 (1977).
36Peristein, 1. H. , "Organic Metal s-The Inteml0lecular Migration of Aromaticity," Angew. Chem. 16, 519-532 (1977).
37Kistenmacher, T. J. , "Structural Relationships in the Heterofulvalene- TC Q
Family of Organic Conductors," Annals N. Y. Acad. Sci. 313, 333-342 ( 1978).
38Kistenmacher, T. J., " Partial Charge Transfer and Charge Density Wave
Modulation in the TTF- TC Q Famil y of Quasi One-Dimensional Organic
Materials," in Modulated Structures 1979 (Kailua Kona, Ha waii), Cowley,
1. M. , Cohen, J. B. , Salamon, M. D., and Weunsch, B. J. (eds.), American
Institute of Physics Conference Proceedings, No. 53 , pp. 193-204 ( 1979).
39Wiygul, F. M. , Metzger, R. M., and Kistenmacher, T. J., "Madelung Energy
Systematics in the Heterofulvalene-TCNQ Charge-Transfer Salts," Mol. Cryst.
Liq . Cryst. 107, 115-131 (1984).
40Cowan, D.O., and Wiygul, F. M. , "The Organic Solid State," Chem. Eng.
News 64, 28-45 (1986).
4l Potember, R. S., Hoffman, R. c., and Poehler, T. 0., "Molecular Electronics,"
Johns Hopkins APL Tech. Dig. 7(2), 129-141 (1986).
42Kistenmacher, T. J., "Structural Aspects of Organic Superconductors," Johns
Hopkins APL Tech. Dig . 7(2), 142-151 (1986).
ACKNOWLEDGMENTS: We are deeply indebted to the many scientific collaborators, especially Aaron N. Bloch, who have contributed so much of their time
and talent to ensure the success of thi s program . Finally, the National Science
Foundation has provided continuous and generous patronage of this work after
initial support from the Defense Advanced Research Projects Agency and the IR&D
Program at the Applied Physics Laboratory.
Johns Hopkins APL Technical Digest, Volume 13, Number I (1992 )
THE AUTHORS
THOMAS J. KISTENMACHER is
a Principal Profess ional Staff
chemist in APL'S Milton S. Eisenhower Research Center. He obtained a B.S. degree in chemistry
from Iowa State Univers ity and
M.S. and Ph.D. degrees in chem istry from the University of Illinois.
During 1969-71 , he was a Junior
Fellow in chemical physics at the
California Institute of Technology.
During 1971-82, he served on the
faculties of The Johns Hopkins
University and the California institute of Technology. Dr. Ki stenmacher joined APL in 1982 as a
member of the Microwave Physics
Group. In 1984, he became a member of the Materials Science Group,
where his current research interests include crystalline structure and
structure- physical property relationships in highly conductive organic
solids, local structure and magnetic properties of amorphous thin films
and multilayers, the elucidation of structural model s for icosahedral
materials, the structural basis for high- Tc oxide ceramics, and large
bandgap metal-nitride semiconductors.
DW AINE O. COW AN is professor
of chemistry at The Johns Hopkins
University. He received a B.S. in
chemistry from Fresno State College in 1958 and a Ph.D. in chemistry from Stanford University in
1962. After a postdoctoral fellowship at the California Institute of
Technology, he joined the John s
Hopkins faculty in 1963. Dr.
Cowan has been a fellow of the
Sloan Foundation and spent a year
at the University of Basel in Switzerland as a Guggenheim fellow.
His current research program in the
organic solid state, pursued in collaboration with scienti sts at APL and
the Homewood campus, combines synthesis, physical measurements,
and theory aimed at the systematic development of new materials and
the exploration of physical phenomena. His other research interests
include organometallic chemistry, organic photochemistry, mixed
valence compounds, structural property correlations, smart materials,
and the synthesis of heterocyclic compounds containing sulfur,
selenium, and tellurium.
THEODORE O. POEHLER is currently the Associate Dean for Research of the G.W.c. Whiting
School of Engineering and Research Professor in the Department
of Electrical Engineering. He also
holds a joint appointment as a
member of the Principal Professional Staff at APL. Dr. Poehler
previously served as Director of
the Milton S. Eisenhower Research
Center, where he developed and
managed the research programs,
evaluated their effectiveness, and
participated in the overall institution management policies. He also
served as supervi sor of the Quantum Electronics Group, for which he led research and development
efforts in optical lasers, sensing and detection, and optical information
processing. Dr. Poehler has over twenty-five years' experience in
research and development, solid-state physics, semiconductors, optical
lasers, and optical information processing and storage.
267
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