Ultramicroscopy42-44 (1992) 1004-1008

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

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1006                      F.G.C. Hoogenraad et al. / Schottky-barrier formation in conducting polymers

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

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1992 schottky barrier formation in conducting polymers

  1. 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. 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. 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. 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. 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. [1] H. Shirakawa, E.J. Louis, A.G. MacDiarmid, C.K. Chiang and A.J. Heeger, J. Chem. Soc. Chem. Commun. (1977) 578. 4. Conclusion [2] R. Yang, K.M. Dalsin, D.F. Evans, L. Christensen and W.A. Hendrickson, J. Phys. Chem. 93 (1989) 511. Thin films of P H T - C I O 4 and P P Y - t o l u e n e - [3] R. Yang, D.F. Evans, L. Christensen and W.A. Hendrick- sulphonate show microcrystalline order that is son, J. Phys. Chem. 94 (1990) 6117. [4] G. Caple, B.L. Wheeler, R. Swift, T.L. Porter and S. similar to the o r d e r in other conducting polymer Jeffers, J. Phys. Chem. 94 (1990) 5639. films. O n e observes micro-islands connected by [5] T.L. Porter, S. Jeffers, G. Caple, B.L. Wheeler and R. parallel strands and single strands. W e did not Swift, Surf. Sci. Lett. 238 (1990) L433. observe the simple helix pitch, as r e p o r t e d by [6] K. Besocke, Surf. Sci. 181 (1987) 145. Y a n g et al. [3]. In P P Y we did observe the super- [7] C.R. Clemmer and Th.P. Beebe, Science 251 (1991) 640. [8] H.J. Mamin, E. Ganz, D.W. Abraham, R.E. Thomson and helical structure. T h e Fourier transform showed J. Clarke, Phys. Rev. B 34 (1986) 9015. a strong c o m p o n e n t at 74 .& besides the c o m p o - [9] J.L. Br6das, E. Th6mans, J.G. Fripiat, J.M. Andr6 and nent at 28 A. W e p r o p o s e d the formation of a R.R. Chance, Phys. Rev. B 29 (1984) 6761.