Ultramicroscopy 42-44 (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. Hoge rvo r s t b, P .M.L.O. Scholte a and F. Tu ins t ra 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 Schottky barrier between the metallic tip and the semiconducting polymer.

1. Introduction

The discovery that doped polyacetylene is elec- trically conducting [1] has generated considerable research effort on the properties of doped or- ganic conducting polymers. They constitute a new class of materials combining the processability, light weight and durability of plastics with the electrical conductivity of metals. Many polymers have been reported to have good electrical con- ductivities, including conjugated systems such as polythiophene (PTP) and polypyrrole (PPY). The doping process involves the incorporation of dopant ions into the polymer by an oxidation (p-doped) or reduction (n-doped) reaction. Dur- ing the oxidation reaction of e.g. PPY an electron is removed from the n-electron system of a pyr- role ring. Removal of a second electron in the same polymer chain leads to the formation of a doubly charged and spinless quasiparticle, a bipo- 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 electronic p-type conductor. The doping (oxida- tion) level normally reaches values of one elec- tron per three monomer units.

The physical properties of the polymers de- pend on the dopant ion and on the conditions during the polymerization reaction. Considerable effort has been invested to determine the struc- ture of the polymer films. Recently several poly- mer films have been investigated with scanning tunneling microscopy (STM). It was shown that thin films of polypyrrole show semicrystalline or- der, while thick films have an amorphous struc- ture. Also helical strands were observed with pitches of 7-10 A or 20-30 A [2-5]. In this paper we present the results of scanning tunneling ex- periments on very thin films of p-doped polypyr- role (PPY) and poly(3-hexylthiophene) (PHT). In the next section some experimental details are summarized. Subsequently, we discuss the struc- ture of thin PHT and PPY films. Finally, we show that the apparent height of a polymer strand is affected by the formation of a Schottky-like bar- rier between the tunnel tip and the polymer.

2. Experiment

Poly(3-hexylthiophene) (PHT) and polypyrrole (PPY) films were deposited by electropolymeriza-

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F.G.C. Hoogenraad et al. / Schottky-barrier formation in conducting polymers 1005

tion on highly oriented pyrolytic graphite (HOPG) substrates: PHT 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 PHT 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 PHT 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.

The samples were investigated with a commer- cially available scanning tunneling microscope of the "Beet le" type [6]. Pictures were taken in air, in constant-current mode at a tunnel current of 1-4 nA and with the tip voltage varying between - 3 0 0 and +600 mV. Structural studies were done with the tip voltage set at typically - 1 0 0 m g .

Fig. 1. 1220 A by 1220 ,& scan of a PHT micro-island and polymer strands. The width of the strands is typically 20 ,&

and the diameter of the micro-island 600 .&.

lieve that the images presented in this study are of molecular origin. The H O P G substrates were thoroughly examined on several occasions. We

3. Results and discussion

We have investigated the samples in the transi- tion area between continuous film and bare sub- strate.

For the PHT 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

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 H O P G can be easily mistaken for macromolecu- iar strands in STM images [7]. However, we be-

? = i ~ i ~

~i~ ~ ~

Fig. 2. PHT strands connected to each other. The connecting angle is 30 °. Scan size of the picture is 1220 ,~ by 1220 ,~.

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- plies with the simple helixes observed by Yang et 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

.........

Fig. 3. 1220 ,& by 1220 A scan of PPY superhelixes on HOPG. The length of the parallel strands in the picture is 1200 A.

Fig. 4. Enlarged view of a PPY superhelix. The width is 25 A. The average pitch is 24 ,~. Scan size of the image is 380 A by

380 ,~.

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 PHT 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 PPY-to luenesulphonate 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 observe superhelixes (figs. 3 and 4). The width of the superhelix shown in fig. 4 is 28 ,~. It has been

F.G.C. Hoogenraad et al. / Schottky-barrier formation in conducting polymers 1007

analysed with a one-dimensional Fourier trans- form. The main Fourier components indicate a

o pitch of 28 and 74 A, which can also be clearly observed in the picture. Two less intense peaks at 42 and 156/~ are observed as well.

This superhelix differs markedly from the PPY superhe!ix reported by Yang et al. [3] that was 50-60 A wide and had a pitch of 26 A. Yang observed the superhelixes on the strands that connect micro-islands. The superhelix shown in fig. 4 is "free standing", i.e. it does not connect two micro-islands. The difference in width could 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.

Presently it is difficult for us to explain this pitch, since our STM is not equipped with a spectroscopic mode. The effect may be due to a topological distortion of the superhelix. An analy- sis of fig. 4 shows that the distance between two low-intensity coils is somewhat smaller than the distance between a high- and a low-intensity coil: 12-16 A versus 19-23 A. This may be caused by buckling of the helix, but also by species being shifted in between the coils. However, one cannot exclude that the cause of the three-fold pitch is chemical, e.g. an uneven distribution of the dopant anions and bipolarons along the helix. The number of counter-ions in the individual helix is unknown. Normally, electropolymeriza- tion of pyrrole results in polypyrrole films with an oxidation level of one elementary charge per 3 to 4 pyrrole rings.

During the experiments it appeared that the best pictures were obtained at a negative tip voltage. At a tunnel current of 9 nA, it was not possible to image the polymer without crashing at positive tip voltages. However, at low tunnel cur- rents it is possible to image a polymer strand both at positive and at negative tip voltages. In fig. 5 we have plotted the apparent height of the super- helix of fig. 4 as a function of tip voltage. The tunnel current used in these experiments was 3 nA. The height has been determined by measur- ing the difference in grey level of the STM image of the bare HOP G substrate and a coil of the superhelix. We did not try to convert this to

70-

~. 60

=. 50

"~ 4 0 ~

3 0

20 -400 -200 0 200 400 600 800

Vtip (mV)

Fig. 5. A p p a r e n t he igh t o f a supe rhe l i c a l s t r a n d in a r b i t r a r y

uni t s ve rsus t ip vo l tage . T h e t u n n e l c u r r e n t was set a t 3 nA.

T h e sol id l ine serves to gu ide the eye.

absolute height differences to avoid calibration ambiguities. It can be seen that the apparent height is highly asymmetric. For negative tip volt- ages it rises steeply and saturates at - 7 0 mV; for positive tip voltages a slow increase can be ob- served.

It is known that the elastic deformation of a contamination layer, such as water, influences the observed corrugation in a STM [8]. But such an elastic artefact cannot explain the asymmetry of the apparent height and the saturation at - 7 0 mV.

The electronic structure of doped conducting polymers is well understood [9]. The undoped polymers have a band-gap between the valence and conduction band of several eV (2.2 eV for PTP). Upon doping bipolaron states are formed in this gap. If the doping level is high enough, these bipolaron states start to overlap and form a band. The width of the two bipolaron bands depends on the doping level [9]. In the case of p-doping the bipolaron bands are empty and we may consider the conducting polymer as a p-type semiconductor with a band-gap of the order of 0.5 eV and a conduction band with a finite width (typically 100 meV).

When the tunnel tip is brought in the proxim- ity of the polymer the electron states of tip an polymer start to overlap. This gives rise to band bending of the polymer bands, similar to the band bending that occurs in a Schottky diode. Conse- quently, the holes that tunnel between polymer and tip have to cross a Schottky barrier as well as

1008 F.G.C. Hoogenraad et al. / Schottky-barrier formation in conducting polymers

the vacuum barrier. The tunnel current I t can be expressed as a function of the tip voltage V t

I t = I 0 exp( - 2kd)[1 - exp( - V t / k s T ) ] ,

where d is the width of the vacuum barr ier and k equals approximately 1 .~-1. i0 is the maximum tunnel current that can be used without the tip crashing for positive tip voltages (i.e. for d be- coming less than 0). F rom our experiments it can be deduced that I 0 must be less than 9 nA. It is also clear that the apparen t height of the polymer will be highly asymmetric for positive and nega- tive tip voltages, as we have observed. Therefore , we propose that a Schottky-like barr ier is formed between tip and polymer.

The saturat ion of the apparen t height for tip voltages less than - 7 0 m e V is due to the finite width of the lowest b ipolaron band. A fur ther lowering of the tip voltage does not result in more states becoming accessible to tunnel into. Consequently, the apparen t height saturates.

4. Conclusion

Thin films of P H T - C I O 4 and P P Y - t o l u e n e - sulphonate show microcrystalline order that is similar to the order in o ther conduct ing polymer films. One observes micro-islands connected by parallel strands and single strands. We did not observe the simple helix pitch, as repor ted by Yang et al. [3]. In PPY we did observe the super- helical structure. The Four ier t ransform showed a strong componen t at 74 .& besides the compo- nent at 28 A. We proposed the format ion of a

Schottky-like barr ier between polymer and metal- lic tip, to explain the asymmetry in the apparen t height of a single (superhelical) strand.

Acknowledgements

The authors want to thank Mr. A. v.d. Waal and Mr. Th. Kock for the prepara t ion of the samples. Mr. G. van Kempen and Mr. J. Mullikin are acknowledged for providing the Four ier anal- ysis of the superhelix. Par t of this work is finan- cially suppor ted by the Dutch Ministry of Eco- nomic Affairs, Innovat ion Or ien ted Research Program on Polymer Composi tes and Special Polymers ( I O P - P C B P project BP202).

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