Temperature dependent dc and microwave conductivity data together with EPR and optical reflectance measurements on the "mixed" system (pe)2(ASF6)0,75(PF6)0,35 times 0,85 CH2Cl2 are described. The data prove metallic behaviour of this organic solid down to 200 K

Geserich, Hans Peter

Harms, Ralf H.

Helberg, Hans W.

Keller, Heimo J.

Koch, P.

Ruppel, Wolfgang

Schweitzer, Dieter

Schäfer, Herbert

Wilckens, Rudolf

Online Publikationen der Universität Stuttgart

Mol. Cryst. Liq. Cryst .• 1982, Vol. 86, pp. 87-101 
0026-8941/82/8601-0087$06.50/0 
Cl 1982 Gordon and Breach, Science Publishers, Inc. 
Printed in the United States of America 
(Proceedings of the International Conference on Low-Dimensional Conductors, Boulder, 
Colorado, August 1981) 
HIGHLY CONDUCTING PERYLENE RADICAL SALTS+ 
P. KOCH, and D. SCHWEI7ZER 
Abt. far Molekulare Physik 
Max Planck-Institut far Med. Forschung 
Heidelberg 
R. H. HARMS, and H. J. KELLER 
Anorganisch-Chemisches Institut 
University of Heidelberg 
H. SCHAFER, and H. W. HELBERG 
Drittes Physikalisches Institut 
University of Gottingen 
R. WILCKENS, H. P. GESERICH, and W. RUPPEL 
Institut far angewandte Physik 
University of Karlsruhe 
Received for publication October 20, 1981 
Temperature dependent dc and microwave con-
juctivity data together with EPR and optical 
reflectance measurements on the~mixedHsystem 
(pe)2(AsF 6 )0,75(PF 6 )0,35 x 0,85 CH 2C1 2 are 
described. The data prove metallic behaviour 
of this organic solid down to 200 K. 
+This work was supported by Stiftung Volks-
wagenwerk 
[1827]/87 
88/[1828] p, KOCH et ai, 
INTRODUCTION 
Recently electrochemically prepared single crys-
tals of quasi one-dimensional organic conductors 
have caused considerable interest because of a 
number of exciting electronic properties. Speci-
ally crystals of the series of tetramethyltetra-
selenafulvalene (TMTSF) salts [(TMTSF) 2X with ty-
pical anions X = PF~, ASF~, CIO ~ etc.J show a va-
riety of fascinating properties and are the first 
, l'd h h b d ' 1 organ~c so ~ s w ic ecame supercon uct~ng . On 
the other hand also simple hydrocarbons such as 
2 
naphthalene , perylene (pe), coronene, anthracene, 
pyrene, triphenylene 3 and fluoranthene 4 were used 
to generate quasi one-dimensional crystals by 
electrochemical methods. It was found that pery-
lene [(pe)2(PF6 )1,1 XO ,8 CH 2 Cl 2 and (pe)2 
(ASF 6 )1,1 x 0,7 CH 2Cl 2] is specially well suited 
to form highly conducting organic metals with up 
12 (n ) -1 d " 3 to 00 cm room temperature con uct~v~ty . 
Here we report electronic properties of elec-
trochemically prepared single crystals of (pe)2· 
(ASF 6 )0,75(PF 6 )O,35 x 0,85 CH 2Cl 2 . Some of these 
crystals show strongly increasing conductivities 
with decreasing temperature. In addition to the 
temperature dependent conductivities EPR- and op-
tical reflectance data confirm a metallic beha-
viour of these crystals at temperatures above 200K. 
HIGHLY CONDUCTING PERYLENE RADICAL SALTS [1829]189 
EXPERIMENTAL 
Crystals were prepared in a 100 ml electrochemi-
cal cell of 3 compartments separated by two glass 
wires with filters from each other. Platinum 
2 platinum plates (1,5 x 1,5 cm ) at the end were 
+ 
used as electrodes. 0,1 g perylene,0,39 g BU4NPF6 (Bu4N 
= tetrakis(n-butyl)ammonium ion) and 0,43 9 
BU 4NASF 6 were dissolved in 100 ml CH 2C1 2 and sa-
turated with dry nitrogen. Equimolar mixtures of 
the electrolyte salts were used. The crystals 
were grown using a constant current of 10 pA re-
sulting in a potential of about 1,9 V, at a tem-
perature of 300 K and under exclusion of light. 
Crystals of the following composition were ob-
tained. 
Analysis: (pe)2(AsF 6 )0,75(PF 6 )0,35 xO ,85 CH 2C1 2 
(C40,85H25,7ASOJ5PO.35Cl1,7F6,6) 
Calc.: C 63,78%; H 3,36%; As 7,31%; P 1,41%; Cl 7,83%; 
F 16,30% 
Found: C 63,98%; H 3,44%; As 6,9 %; P 1,41%; Cl 7,82%; 
F 16,6 % 
C 64,13%; H 3,41%; As 7,1 %; P 1,33%; Cl 7,85%; 
F 16,4 % 
The crystals have typical dimensions as des-
cribed earlier 3 but in addition very "thin" crys-
tals .(cross sections <lOp x lOp) of a few milli-
meter length were obtained. Conduction measure-
90/[1830) f. KOCH " al. 
ments were performed on "thick" and "thin" crys-
tals. 
The dc-conductivity was measured by the 
standard four-probe method 5 along the needle 
axis, while for the conductivity perpendicular 
6 to the perylene stacks the method of Montgomery 
was used. The samples were mounted on gold wires 
in a teflon block and measured with the same ap-
3 paratus as described recently 
In addition to the dc-conductivity the mi-
crowave conductivity was studied. The temperature 
dependent conductivity 0'= 2 Jr ff. E." (f frequen-
o 
cy, E = 8,854 x 10- 12 As/Vm dielectric constant 
o 
of free space, £" = imaginary part of the complex 
dielectric constant e: = E:.' -i€.") was measured in 
the range from 80 to 300 K at both 9,3 and 23,5 
GHz using cavity perturbation technique. The fre-
quency generator was controlled by the resonance 
frequency of the cavity in a feedback loop (fre-
quency accuracy ~10-7). The qualities of the ca-
vities at room temperature are 6000 and 4700 at 
9,3 and 23,5 GHz, respectively. The single crys-
tals were brought into the maximum of the elec-
tric field inside a rectangular TE 103 transmis-
sion cavity with the needle axis parallel to the 
field. The relative change in halfwidth and the 
normalized frequency shift were used to calcu-
late ~ wi th the perturbation formula 7 in the qua-
sistatic regime 1v'€·k·d/21«1 (k wave vector, d 
sample diameter) and with the surface impedance 
formula 8 in the skin effect regime l(£. ·k·d/2!»1. 
9 A general formula valid in both cases can be 
HIGHLY CONDUCTING PERYLENE RADICAL SALTS [1831J/91 
used to calculate ~ in the difficult 
region between both approximations. 
Temperature dependent EPR spectra were ta-
ken on a Varian spectrometer E-101 equipped with 
a Varian NMR-Gauss-meter for field calibration. 
Spectral reflectance measurements were performed 
and analyzed by a Drude model similar as descri-
10 bed for (pe)2(PF6 )1,1 x 0,8 CH 2Cl 2 . 
RESULTS AND DISCUSSION 
"Thick" crystals 
The temperature dependence of the dc-conductivity 
of "thick" specimens of the "mixed system" (pe) 2· 
(ASF 6 )0,75(PF 6 )0,35 x 0,85 CH2C~ is very similar 
to the characteristics of (pe)2(PF 6 )1 1 x 0,8 
, 3 
CH 2Cl2 and of (pe)2(AsF6 )1,1 x 0,7 CH 2Cl 2 . The 
d .. ,.....dc d . room temperature con uct~v~ty v t range typ~-
r -1 
cally between 200 and 1200 (Ohm cm) along the 
needle axis but is reduced by a factor of at 
least 400 perpendicular to this axis. 
The room temperature microwave conductivity 
O"mw was in the average smaller than the O""dC_ 
rt rt 
values. Nevertheless, a very similar temperature 
dependence was observed. Typical microwave con-
ductivity data on (pe)2(AsF 6 )0,7S(PF6b ,75 x 
0,85 CH 2Cl 2 single crystals are shown in fig. 1. 
With decreaSing temperature the mw-conductivity 
reaches (similar to the dc-conductivity) a flat 
maximum ~ at temperature T and passes 
max max 
921[1832] 
1000 
({,2cm)-1 
500 
t 200 
> 
~ 100 
> 
~ 
u 
~ 
o 
z 
o 
u 
50 
20 
10 
2 
P. KOCH et al. 
-T 
300 200 150 100 
'Q 
'Q 
QPDUIbooo 
o 
o 
o 
o 
1000 
-T--
o 
80 K 
v 
o 'Q 
o 
~O 
o 
o 
~ 
FIGURE 1 Microwave conductivity of (pe)2· 
(AsF 6) 0,75 (PF 6) 0, 3SxO, 85 CH2C12 single crystals vs. re-
ciprocal temperature (three different crystals at (<» 9.3 
and (~,[J) 23,5 GHz, one crystal at (~) 9,3 and (0) 23,5 
GHz) . 
HIGHLY CONDUCTING PERYLENE RADICAL SALTS [1833]193 
through a smooth transition into a semiconducting 
region at lower temperatures. Values for ~ / 
max 
~ range between 1,0 and 1,1 at a T between 
rt max 
200 and 250 K. From sample to sample there are 
considerable variations of ~mw (20 750 (Ohm 
-1 rt 
em) ) and of the activation energy Emw in the se-
a 
mw 
miconducting range. Crystals with a larger ~rt 
have larger E:
w 
too. (pe)2(ASF6)0,7S(PF6)0,3~wX 
0,85 CH 2C1 2 crystals usually have a larger Ea 
(14 to 27 me V) than (pe)2(PF 6 )1,1 x 0,8 CH 2 C1 2 
or (pe)2(AsF6 )1,1 x 0,7 CH 2C1 2 samples (E a = 8 to 
17 me V). The crystal with the highest conducti-
vity in Figure 1 [O""':~ = 750 (Ohm cm) -1J belonC]ed 
to the class of "thin" crystals (cross section 14 
x 8 p) and was not cooled below 200 K in order to 
avoid its destruction by thermal tension and to 
preserve it for other measurements. The same crys-
tal was used earlier for temperature dependent 
EPR-measurements. 
The frequency dependent mw-conductivity was 
measured on the same samples at both frequencies 
thus avoiding quality effects of different crys-
tals. All crystals (see fig. 1) do not exhibit a 
significant frequency dependence of the conducti-
vity between 9,3 and 23,5 GHz. 
The spectral reflectance data confirm a me-
tallic like behaviour of the "mixed" (pe)2(AsF6 )0,7S· 
(PF6 )0,3S x 0,85 CH 2 C1 2 system. Figure 2 
shows the reflectance spectra with the direction 
of the polarization of the incident light paral-
lel (R,,) and perpendicular (R.l.) to the (a-)axis 
of the ~erylene stacks. The R,,-spectrum is typi-
94/[1834] 
2000 
P. KOCH el al. 
Frequency (cm-1) 
5000 10 00 20000 50000 
100~~~----~----~----~------~ 
80 
60 
40 
20 R. 
10 
.... 
8 
~ ~ 6 
III 
U 4 
c 
" 
U 
III 2 
.... 
III 
a: 
Q2 6 
Photon Energy (eV) 
FIGURE 2 Polarized reflectance spectra of a "thick" 
(pe)2(AsF6)0,75(PF6 )0,35 x 0,85 CH 2C12 single crystal. R" 
and Rk refer to the polarization direction with respect to 
the axis of the molecular stacks, full line: experimental 
curve, dotted line: Drude fit. 
HIGHLY CONDUCTING PERYLENE RADICAL SALTS (1835]/95 
cal for a solid with free charge carriers. The 
values of the reflectance reach almost 80 % at 
low photon energies and exhibit a steep plasma 
edge with a pronounced minimum. At higher photon 
energies the spectral distribution of the reflec-
tivity is dominated by the electronic transition 
in the organic molecules. If the direction of po-
larization is perpendicular to the stack axis 
((R~), the crystal behaves optically as a non-
conducting organic compound. Probably due to lar-
ger scattering losses at the surface above 1,3 eV 
the reflectivity here is smaller than for (pe)2· 
10 (PF 6 )1,1 x 0,8 CH 2C1 2 crystals . The RH-spec-
trum was fitted by a Drude model (dashed curve) 
Therefore in this case a somewhat larger error 
must be assumed than in the case of the (pe)2. 
(PF 6 ) 1 1 x 0,8 CH 2C1 2 crystals, where a Lorentz-
, 10 
Drude model could be used 
With the Drude parameter h(4) = 0,95 eV, 't""'= 
-15 P 3,3 x 10 sec and e~= 2,7 an optical value for 
the electrical dc-conductivity of cr = 1700 
-1 0 (Ohm cm) was calculated. This value is about 
40 % larger as the largest value obtained from 
dc-measurements. Such differences are common 10 . 
From the charge carrier concentration N = 
+21 -3 1,40 x 10 em and the plasma frequency ~p 
the effective mass of the free charge carriers 
• can be calculated to m = 0,77 m . On the basis 
o 
of a tight binding model for the band structure 
these effective mass value can be translated in-
to a band-width 10 of t = 1,22 eV. A repeating 
distance of ~29 R was used for this calculations. 
96/[1836] P. KOCH et al. 
A Fermi vector kF = 0,275 'j(/a was calculated. This 
value of the bandwidth is about 10 % larger as 
10 
the one for (pe)2(PF 6 )1,1 x 0,8 CH 2C1 2 crystals. 
"Thin" crystals 
The dc-conductivity of "thin" (pe)2(AsF 6 )0,75 
(PF 6 )0,35 x 0,85 CH 2C1 2 crystals shows a somewhat 
different temperature characteristic. With respect 
to their conductivities at least two types of 
"thin" crystals can be discerned. 
One type whose conductivity characteristic 
is shown for a typical example in figure 3, 
[0-' ~~ = 1240 (Ohm cm) -1] shows an increase in con-
ductivity by a factor of 5 to 10 and a maximum 
- 1 
value of about 10000 (Ohm cm) around 200 K. 
There is a phase transition between 200 and 180 K 
to a semiconducting state. 
A second, more seldom type of "thin" crys-
tals has a sharp increase in dc-conductivity in 
a small temperature range with a sudden decrease 
at lower temperatures. After the sharp decrease 
in conductivity a weak increase before the tran-
sition to the semiconducting state is observed by 
lowering the temperature. Figure 4 shows the tem-
perature dependence of an extreme example of such 
a second type" thin" crystal (~dC = 1400 (Ohm 
rt 
cm)-l]. Within a temperature range of 15 K the 
conductivity increases by a factor of 50 having a 
-1 
maximum value of about 70000 (Ohm cm) • After 
the sharp decrease in conductivity the crystal 
has a further plateau and a weak increase in con-
HIGHL Y CONDUCTING PERYLENE RADICAL SALTS 
300 200 150 
4-·T(K) 
100 "TO 60 
100-,~~~~~------~--------~--~ 
10 
,.V~ .. 
\ 
\ 
• 
10.2 
0.5 
Perylene [PF6 , AsF6 ] 
• 
n-1 -1 
a[300] ~~ em 
• 650 
_1240 
-" 
-
"" -~ 
.~ . 
• \. 
\ 
1.5 
100/T (K-1) -+ 
(1837]197 
FIGURE 3 Normalized dc-conductivity of "thin" (pe) 2 . 
(ASF6)O,75(PF6)O,35xO,85 CH 2Cl 2 crystals vs. recprocal 
temperature (see text) 
98/[1838] P. KOCH et al. 
300 200 150 
+-T(K) 
100 70 60 
101G-~~~~~------L-------~----; 
..QTIl 
U(3OO1 Perylene (PF6 • AsF6 ) 
£")-1 _1 
0'(300) = 1400 u em 
10 
FIGURE 4 Normalized dc-conductivity of an extreme ex-
ample of a "thin" (peI2(AsF6lo,7S(PF6)O,3SxO,8S CH2C12 crys-
tal vs. reciprocal temperatur (see text) 
HIGHLY CONDUCTING PERYLENE RADICAL SALTS [1839)/99 
ductivity at around 300 -1 (Ohm-cm) . In a second 
temperature cycle with 
ductivity of ~dc = 350 
(J' rt 
a reduced "starting" con-
-1 (Ohm cm) the sharp con-
de dc ---
ductivity increase disappeared (0'" t/(J" only 3). 
r max 
There is experimental evidence that the dis-
appearance of the huge conductivity peak after 
temperature cycling is connected with the 200 K 
transition. The technical problems with dc-con-
ductivity measurements on such "thin" crystals 
with cross sections of the order of 10 P x 7jU 
may explain the so far limited experimental data. 
Comparison of "thick" and "thin" crystals 
The question arises: 
What is the difference between the "thin" and 
the "thick" crystals? An elementary analysis 
could not be done because of the small amount of 
"thin" crystals available. Furthermore, up to 
now it was not possible to investigate the crys-
tal structure of the "thin" specimens by x-ray 
methods, because of their small cross section. 
Nevertheless, EPR-data show a clear difference 
between both types which indicate that the "thin" 
crystals might be more "one-dimensional" than the 
"thick" ones. For all crystals, "thick" and "thin" 
ones, only one EPR-line was observed with an iso-
tropic g-value of 2.0022. All crystals show the 
same small anisotropy range with a maximum value 
gmax 2.0025 and a minimum value of gmin 
2.0019 by turning the crystals around the needle 
(a-)axis and with the magnetic field perpendicular 
100/[1840] P. KOCH et al. 
to this axis. But the EPR-lines of "thick" and 
"thin" 
crystals differ appreciably in their linewidth. 
The EPR linewidth of the "thick" crystals ranges 
between 0,5 and 1,2 Gauss depending on the indi-
vidual crystal, while the "thin" crystals have a 
linewidth of only 95 mGauss. In all cases the EP~ 
linewidth and the intensity of the signal is tem-
perature independent in the metallic range down 
to 200 K. This is also true for those "thin" 
crystals which have the sharp dc-conductivity 
peak. 
To clarify the difference between "thin" and 
"thick" (pe)2(As~6)0,75(PF6)0,35 x 0,85 CH 2C1 2 
crystals further experiments are necessary. Ad-
ditional information might be obtained by vary-
ing the concentrations of the ASF 6 and PP6 an-
ions in the preparation. 
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Highly conducting perylene radical salts

Temperature dependent dc and microwave conductivity data together with EPR and optical reflectance measurements on the “mixed” system (pe)2(ASF6)0,75(PF6)0,35 × 0,85 CH2Cl2 are described. The data prove metallic behaviour of this organic solid down to 200 K

Highly conducting perylene radical salts

Abstract

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