1. Ultramicroscopy42-44 (1992) 1004-1008
North-Holland
Schottky-barrier formation in conducting polymers
F . G . C . H o o g e n r a a d a, A . C . R . H o g e r v o r s t b, P . M . L . O . Scholte a a n d F. T u i n s t r a a
Department of Applied Physics, Solid State Physics, Delft University of Technology, Lorentzweg 1,
2628 CJ Delft, The Netherlands
b TNO Plastics and Rubber Research Institute, P.O. Box 6031, 2600 JA Delft, The Netherlands
Received 12 August 1991
Thin films of poly(3-hexylthiophene) and polypyrrole on highly oriented pyrolytic graphite have been studied with
scanning tunneling microscopy. We observed semicrystalline order in the form of micro-islands and parallel strands of
polymer. The orientation of these strands is determined by the substrate. Also, a new and until now unreported effect has
been observed. The corrugation measured perpendicular to the strands in constant-current mode is different for positive
and negative bias voltages. This difference can be attributed to the formation ofa Schottkybarrier between the metallic tip
and the semiconductingpolymer.
1. Introduction The physical properties of the polymers de-
pend on the dopant ion and on the conditions
The discovery that doped polyacetylene is elec- during the polymerization reaction. Considerable
trically conducting [1] has generated considerable effort has been invested to determine the struc-
research effort on the properties of doped or- ture of the polymer films. Recently several poly-
ganic conducting polymers. They constitute a new mer films have been investigated with scanning
class of materials combining the processability, tunneling microscopy (STM). It was shown that
light weight and durability of plastics with the thin films of polypyrrole show semicrystalline or-
electrical conductivity of metals. Many polymers der, while thick films have an amorphous struc-
have been reported to have good electrical con- ture. Also helical strands were observed with
ductivities, including conjugated systems such as pitches of 7-10 A or 20-30 A [2-5]. In this paper
polythiophene (PTP) and polypyrrole (PPY). The we present the results of scanning tunneling ex-
doping process involves the incorporation of periments on very thin films of p-doped polypyr-
dopant ions into the polymer by an oxidation role (PPY) and poly(3-hexylthiophene) (PHT). In
(p-doped) or reduction (n-doped) reaction. Dur- the next section some experimental details are
ing the oxidation reaction of e.g. PPY an electron summarized. Subsequently, we discuss the struc-
is removed from the n-electron system of a pyr- ture of thin PHT and PPY films. Finally, we show
role ring. Removal of a second electron in the that the apparent height of a polymer strand is
same polymer chain leads to the formation of a affected by the formation of a Schottky-like bar-
doubly charged and spinless quasiparticle, a bipo- rier between the tunnel tip and the polymer.
laron. These bipolarons are responsible for the
conduction properties of the polymer, because
the dopant anions are not very mobile. Therefore
a p-doped polymer can be regarded as a truly 2. Experiment
electronic p-type conductor. The doping (oxida-
tion) level normally reaches values of one elec- Poly(3-hexylthiophene) (PHT) and polypyrrole
tron per three monomer units. (PPY) films were deposited by electropolymeriza-
0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

2. F.G.C. Hoogenraad et al. / Schottky-barrier formation in conducting polymers 1005
tion on highly oriented pyrolytic graphite ( H O P G )
substrates: P H T from a solution of 1 vol% 3-
hexylthiophene and 0.1M LiC10 4 in acenotrile;
PPY from a solution of 1 vol% pyrrole and 0.1M
toluenesulphonic acid in acenotrile. The polymer-
ization reaction was carried out at a constant
current density (20 m A / c m 2 for PHT, 2 m A / c m z
for PPY). In this way p-doped P H T with C10 4
counter ions, and p-doped PPY films with tolue-
nesulphonate counter ions, respectively, were ob-
tained.
The film thickness was controlled by monitor-
ing the charge that passed during the reaction.
This charge was 40 m C / c m 2 for P H T and 2
m C / c m 2 for PPY. During the sample prepara-
tion the H O P G substrate was only partially sub-
merged in the solution, which resulted in a film
with graded thickness. After deposition the films
were dried and stored in air. Fig. 1. 1220 A by 1220 ,& scan of a PHT micro-island and
The samples were investigated with a commer- polymer strands. The width of the strands is typically 20 ,&
cially available scanning tunneling microscope of and the diameter of the micro-island 600 .&.
the "Beetle" type [6]. Pictures were taken in air,
in constant-current mode at a tunnel current of lieve that the images presented in this study are
1-4 nA and with the tip voltage varying between of molecular origin. The H O P G substrates were
- 3 0 0 and +600 mV. Structural studies were thoroughly examined on several occasions. We
done with the tip voltage set at typically - 1 0 0
mg.
3. Results and discussion
We have investigated the samples in the transi-
tion area between continuous film and bare sub-
strate.
For the P H T films semicrystalline order was
observed in the form of micro-islands connected
by parallel polymer strands and individual poly-
mer strands. In fig. 1 a micro-island is shown of
approximately 600 ,~ diameter, to which parallel
strands connect. The width of these strands is
o
? = i~i ~
typically 20 A; the length of these strands varies
from 100 up to 1000 A. Also strands connected to
each other were observed, see fig. 2. The angle
between the two strands is 30 °. This angle proba-
bly is determined by the H O P G substrate.
Recently, it has been shown that artefacts in ~i~~ ~
H O P G can be easily mistaken for macromolecu- Fig. 2. PHT strands connected to each other. The connecting
iar strands in STM images [7]. However, we be- angle is 30 °. Scan size of the picture is 1220 ,~ by 1220 ,~.

3. 1006 F.G.C. Hoogenraad et al. / Schottky-barrier formation in conducting polymers
never found similar features as shown in this
paper. Also on several occasions we were able to
move the observed molecular strands with the
STM tip. The observed strands were either con-
nected to micro-islands of polymeric material, or
were lying in the direct neighborhood of such
islands.
Yang et al. observed helical PPY and PTP
strands with a width of 15-18 A and a pitch of
5 - 8 ,&. They also observed a superhelical strand
50-60 A wide and with a pitch of 26 A. This
superhelix was proposed to be a helical confor-
mation of the simple helix with pitch 5 - 8 ,~ [2,3].
Micro-islands were also observed of the same size
as we present here for PHT.
One may conclude that the overall semicrys-
talline order in P H T is similar to the order re-
ported for PPY and PTP. However, we did not
observe a significant helical structure of the
strands, although the width of the strands com- Fig. 4. Enlarged view of a PPY superhelix. The width is 25 A.
plies with the simple helixes observed by Yang et The average pitch is 24 ,~. Scan size of the image is 380 A by
380 ,~.
al. We did observe periodic structures on the
strands, but they did not reproduce from strand
to strand. During the measurements the quality
of the tip was regularly checked by imaging the bare H O P G substrate with atomic resolution.
Caple et al. observed strands both with and with-
out periodic structure in PTP films on platinum
[4,5]. The width of the rod-like strands was 30 A,
while that of the helical strands was only 10 ~,.
However, PTP and P H T differ by the large hexyl
chains that are connected to the heterocycle.
From simple steric hindrance arguments one
would expect the P H T strands to have a greater
tendency to adapt to a helical conformation than
......... the PTP strands. The absence of helicity could be
due to counter-ion specificity. Yang et al. ob-
served a slight counter-ion specificity in the struc-
ture of P P Y - t o l u e n e s u l p h o n a t e and P P Y - B F 4
helixes [3].
For PPY films, we observed the same
semicrystalline order in the transition region be-
tween continuous film and bare substrate as in
the case of PHT. The strands observed were
15-25 ,~ wide and extended over distances up to
1000 A. Again we did not observe the periodicity
of 5 - 8 A of the simple helix. However, we did
Fig. 3. 1220 ,& by 1220 A scan of PPY superhelixes on HOPG.
observe superhelixes (figs. 3 and 4). The width of
The length of the parallel strands in the picture is 1200 A. the superhelix shown in fig. 4 is 28 ,~. It has been

4. F.G.C. Hoogenraad et al. / Schottky-barrier formation in conducting polymers 1007
analysed with a one-dimensional Fourier trans- 70-
form. The main Fourier components indicate a
o ~. 60
pitch of 28 and 74 A, which can also be clearly
observed in the picture. Two less intense peaks at =. 50
42 and 156/~ are observed as well. "~ 40 ~
This superhelix differs markedly from the PPY
30
superhe!ix reported by Yang et al. [3] that was
50-60 A wide and had a pitch of 26 A. Yang 20
observed the superhelixes on the strands that -400 -200 0 200 400 600 800
Vtip (mV)
connect micro-islands. The superhelix shown in Fig. 5. A p p a r e n t h e i g h t o f a s u p e r h e l i c a l s t r a n d in a r b i t r a r y
fig. 4 is "free standing", i.e. it does not connect u n i t s v e r s u s tip v o l t a g e . T h e t u n n e l c u r r e n t w a s set at 3 n A .
two micro-islands. The difference in width could T h e solid line serves to g u i d e t h e eye.
be explained if the superhelix of Yang consists of
two parallel (superhelical) strands. However, the
three-fold pitch seen in figs. 3 and 4 was not
observed by Yang. absolute height differences to avoid calibration
Presently it is difficult for us to explain this ambiguities. It can be seen that the apparent
pitch, since our STM is not equipped with a height is highly asymmetric. For negative tip volt-
spectroscopic mode. The effect may be due to a ages it rises steeply and saturates at - 7 0 mV; for
topological distortion of the superhelix. An analy- positive tip voltages a slow increase can be ob-
sis of fig. 4 shows that the distance between two served.
low-intensity coils is somewhat smaller than the It is known that the elastic deformation of a
distance between a high- and a low-intensity coil: contamination layer, such as water, influences the
12-16 A versus 19-23 A. This may be caused by observed corrugation in a STM [8]. But such an
buckling of the helix, but also by species being elastic artefact cannot explain the asymmetry of
shifted in between the coils. However, one cannot the apparent height and the saturation at - 7 0
exclude that the cause of the three-fold pitch is mV.
chemical, e.g. an uneven distribution of the The electronic structure of doped conducting
dopant anions and bipolarons along the helix. polymers is well understood [9]. The undoped
The number of counter-ions in the individual polymers have a band-gap between the valence
helix is unknown. Normally, electropolymeriza- and conduction band of several eV (2.2 eV for
tion of pyrrole results in polypyrrole films with an PTP). Upon doping bipolaron states are formed
oxidation level of one elementary charge per 3 to in this gap. If the doping level is high enough,
4 pyrrole rings. these bipolaron states start to overlap and form a
During the experiments it appeared that the band. The width of the two bipolaron bands
best pictures were obtained at a negative tip depends on the doping level [9]. In the case of
voltage. At a tunnel current of 9 nA, it was not p-doping the bipolaron bands are empty and we
possible to image the polymer without crashing at may consider the conducting polymer as a p-type
positive tip voltages. However, at low tunnel cur- semiconductor with a band-gap of the order of
rents it is possible to image a polymer strand both 0.5 eV and a conduction band with a finite width
at positive and at negative tip voltages. In fig. 5 (typically 100 meV).
we have plotted the apparent height of the super- When the tunnel tip is brought in the proxim-
helix of fig. 4 as a function of tip voltage. The ity of the polymer the electron states of tip an
tunnel current used in these experiments was 3 polymer start to overlap. This gives rise to band
nA. The height has been determined by measur- bending of the polymer bands, similar to the band
ing the difference in grey level of the STM image bending that occurs in a Schottky diode. Conse-
of the bare H O P G substrate and a coil of the quently, the holes that tunnel between polymer
superhelix. We did not try to convert this to and tip have to cross a Schottky barrier as well as

5. 1008 F.G.C. Hoogenraadet al. / Schottky-barrierformation in conductingpolymers
the v a c u u m barrier. T h e tunnel current I t can be Schottky-like barrier between polymer and metal-
expressed as a function of the tip voltage Vt lic tip, to explain the asymmetry in the a p p a r e n t
height of a single (superhelical) strand.
I t = I 0 exp( - 2kd)[1 - exp( - V t / k s T ) ] ,
where d is the width of the v a c u u m barrier and k
equals approximately 1 .~-1. i0 is the m a x i m u m Acknowledgements
tunnel current that can be used without the tip
crashing for positive tip voltages (i.e. for d be- T h e authors want to thank Mr. A. v.d. Waal
coming less than 0). F r o m our experiments it can a n d Mr. Th. Kock for the p r e p a r a t i o n of the
be d e d u c e d that I 0 must be less than 9 nA. It is samples. Mr. G. van K e m p e n and Mr. J. Mullikin
also clear that the a p p a r e n t height of the polymer are acknowledged for providing the Fourier anal-
will be highly asymmetric for positive and nega- ysis of the superhelix. Part of this work is finan-
tive tip voltages, as we have observed. Therefore, cially supported by the D u t c h Ministry of Eco-
we p r o p o s e that a Schottky-like barrier is f o r m e d nomic Affairs, Innovation O r i e n t e d R e s e a r c h
between tip and polymer. P r o g r a m on Polymer Composites and Special
T h e saturation of the a p p a r e n t height for tip Polymers ( I O P - P C B P project BP202).
voltages less than - 7 0 m e V is due to the finite
width of the lowest bipolaron band. A further
lowering of the tip voltage does not result in
m o r e states b e c o m i n g accessible to tunnel into. References
Consequently, the a p p a r e n t height saturates.
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