J. Phys. Chem. 1992, 96, 1428-1430
1428
iconductor surface. However, when there is no interaction between the semiconductor and the sensitizer, one can expect the triplet excited state to participate in a diffusionantrolled charge injection process. The charge injection from the triplet excited thionine into ZnO colloids ( k = 6.7 X lo4 s-I) and a quantum yield for the net charge transfer (4 = 0.1) were found to be maximum at monolayer werage of thionine. Reverse electron transfer between
the injected charge and the dye cation radical is still a limiting factor in controlling the efficiency of net charge transfer.
Acknowledgment. The work described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Contribution No. NDRL-3403 from the Notre Dame Radiation Laboratory.
Evolution of Polypyrrole Band Structure: A Scannlng Tunneling Spectroscopy Study R. Yang, W. H. Smyrl,D.F.Evans,* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455-01 32
and W. A. Hendrickson 3M Company, Corporate Research Laboratory, 3M Center, Bldg. 208-1 -01, St. Paul, Minnesota 55144 (Received: August 26, 1991; In Final Form: October 1, 1991)
Scanning tunneling spectroscopy (STS) has been used to determine the band structure of polypyrrole. The polypyrrole films were electropolymeridonto a highly oriented pyrolytic graphite surface using a rotating disk electrode. Differential conductance spectra measurements allowed us to follow the evolution of the band gap as a function of doping level. Before and after the STS measurements, the film structure was imaged by scanning tunneling microscopy. The STS results agree with data obtained by inverse photoemission spectroscopy.
Introduction Scanning tunneling spectroscopy (STS) is a new technique which permits characterization of electron local surface states and has been used to measure the surface state band gap of semiconductor~.’-~When STS and scanning tunneling microscopy (STM) are combined, both structural and electronic surface states can be determined with nanometer resolution. In a previous series of papers$” we presented STM images of polypyrrole and polythiophene conducting polymers which provide evidence for highly ordered structures near surfaces which transform to amorphous structures with increasing f i i thickness. In this paper, we report STS measurements on polypyrrole as a function of doping level. These measurements allow us to follow the development of the polaron and bipolaron energy levels within the band gap. The STS measurements provide electronis state data that agree with inverse photoemission spectra on the same polymer fibers.
Experimental Section The polypyrrole films were electrochemically deposited onto highly oriented pyrolytic graphite (HOPG,grade A, Union Carbide) using a rotating disk electrode (RDE). The RDE apparatus used in this study has been fully described elsewhere.6 The HOPG surfaces (-0.35 cm2)were cleaved in air prior to use. The aqueous electrolytic medium contained 0.1 M pyrrole monomer and 0. l M tetraethylammonium tetrafluoroborate. All chemicals were reagent grade (Aldrich) and were used without further purification. The solution was purged with nitrogen gas before starting the electropolymerization. (1) Henson, T. D.; Saris, D.; Bell, L. S.J. Micros. (Oxford) 1988, 152, 467. (2) Shih, C. K.;Feenstra, R. M.; Martensson, P. J. Vuc. Sci. Technol. A 1990.8 (4), 3379. ( 3 ) Fan, F. R.; Bard, A. J. J. Phys. Chem. 1990, 94, 3761. (4) Yang, R.; Evans, D. F.; Christensen, L.; Hendrickson, W. A. J. Phys. Chem. 1990, 94, 6117. (5) Yang, R.; Naoi, K.;Evans, D. F.; Smyrl, W. H.; Hendrickson, W. A. Langmuir 1991, 7, 556. (6) Smyrl, W. H. J. Electrochem. Soc. 1985, 132, 1555.
The electrochemical cell employed a saturated calomel reference electrode (SCE), a platinum mesh counter electrode, and a rotating electrode (2000 rpm). The polymerization reaction was carried out at a constant current density of 2.5 mA/cm2. The film thickness was monitored by measuring the charge that passed during the electropolymerization. The electrolysis time was typically 5-20 s, and bilayers ranging from 20 to 200 nm were produced. The samples with low doping densities in the neutral state were prepared by electrochemical reduction of conducting polymer samples. After the electrochemical deposition, the samples were rinsed with purified water, dried in a nitrogen gas stream, and stored under nitrogen gas atmosphere. The following procedure was used in determining the scanning tunneling spectra. First the area to be analyzed was imaged by STM in air, since the polymer is stable under ambient conditions. Second, with the feedback circuit inactivated, the tunneling current was recorded as a function of the voltage. In this measurement, the voltage between sample and tip was swept from a preset negative sample voltage, which corresponds to tunneling from the filled metal tip states to empty sample states, to an equal positive sample voltage, which corresponds to tunneling from fded sample states to empty metal tip states. After the STS measurements, the area analyzed was reimaged by STM and compared to the original image to ascertain that the STS process did not alter the polymer morphology. Mechanically formed Pt-Ir tips were used.
Results and Discussion Figure 1 shows a STM image of a highly doped polypyrrole film in which individual polymer fibers are aligned to form long-range ordered structures. The STM images at different locations reveal that the orientation of the fibers varies across the sample surface in a manner determined by the rotating streamlines of the RDE. This becomes more obvious when the fiber film grow thicker, forming a whorled spiral pattern. The electrodeposition of metal follows the streamline on a RDE as well, under certain conditions. The morphology of the polymer film depends on the RDE processing conditions, and more precise measurements to establish the relationship between operating parameters and film structure are underway in our laboratories. Imaga similar to those
0022-365419212096-1428$03.00/0 0 1992 American Chemical Society
Evolution of Polypyrrole Band Structure
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1429
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Voltage (V) Figure 3. The characteristics of the tunneling current, I, differential conductance, dl/d V, and normalized differential conductance, [dl/ dVJ/[I/V], as a function of scanning voltage for polypyrrole film with an intermediate doping level.
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correspond to the actual DOS features of the sample surface at different energy levels. The current-voltage behavior of the neutral polymer (Figure 2a) establishes the breakdown voltages while the differential conductance spectrum (Figure 2b) defines the energy band gap between the conduction and valence bands. From the normalized differential conductance spectra (Figure 2c), the band gap for undoped polypyrrole is determined to be 3.2 f 0.1 eV (n = 17 measurements on three samples). The small peaks at Ep, Ed = f(1.5 f 0.1) V (n = 9), are associated with localized polaron states remaining in the dedoped polypyrrole film. STM images and IPES studies, discussed below, confirmed that the doping density was not uniform in the polypyrrole film. That some polaron defects remained in the dedoped polymer film can be ascertained from the spectra which were taken at single point positions. In Figure 3a, the current-voltage behavior of the intermediate doping polymer shows three distinct regions: I, zero current; 11, small current; and 111, sharply increased current. The slope of the I - Vcurve in region I1 increases with doping density, as can be seen by comparing Figures 3a and 4a. Inspection of Figures 2c and 3c shows that the band gap decreases as the level of dopant increases, because new energy levels, bipolaron states, located at E,, E,, = f(1.3 f 0.1) V, arise ( n = 21 measurements on three samples). The two small peaks located at =-1.0 (n = 8) and ==+0.7 V (n = 8) reflect nonuniform doping density and occur only at higher doping density as the bipolaron energy states start to overlap. The measurements also show some detectable shift of the conduction and valence band edges after the new bipolaron energy states overlap. The spectrum of the highest doped polypyrrole (Figure 4c) shows the overlapping of bipolaron energy levels, which lead to the formation of two new energy bands within the gap. The two major peaks at EbEv = f(1.1 f 0.1) V, in Figure 4c correspond to the bipolaron band positions. The full width of the two bipolaron bands is approximately 0.5 eV ( n = l l measurements on three samples). The total energy band gap between the conduction and valence bands increases from 3.2 f 0.1 to 3.5 f 0.1 eV (n = 12). There were some small peaks in the spectrum; whether there are actual DOS features or background noise signals remain unresolved. We found that there are two important factors in acquiring reliable STS spectra. We initially observed that the conductance spectra sometimes changed noticeably after making a series of STS measurements at a fixed position. Subsequent STM images
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Voltage (V) Figure 2. The characteristics of the tunneling current, I, differential conductance, dI/d V, and normalized differential conductance, [ d l / dVJ/[I/V], as a function of scanning voltage for the almost neutral polypyrrole film.
shown in Figure 1 were obtained on all of the highly and partially doped samples employed in the STS measurements. Because of the low conductivity, no STM images were acquired for the completely dedoped samples. Figures 2-4 show the characteristics of the tunneling current, Z, differential conductance, dZ/d V,and normalized differential conductance, [dl/d v] / [ I / v], as a function of sample voltage, V, for the almost neutral, intermediate, and highly doped polypyrrole films, respectively. The tunneling current, I, and differential conductance,dZ/d V,vary exponentially with tip sample separation, s, according to exp(-2rcs), where K is the decay constant. We normalized dZ/dV to the conductance of the junction by computing the ratio of differential to total conductance, (dI/d V ) / ( I / V) = d(ln I)/d(ln V), thereby cancelling out the exponential dependence of voltage, V,on separation, s. Thus, the surface electronic density of states (DOS),p(E), of the sample varies as p(E) 2 [d(ln I)/d(ln V)] = [(dZ/dV)/(I/V)].' Using this relationship, the peaks measured in the normalized differential conductance spectra (7) Feenstra, R. M.;Stroscio, J. A.; Fein, A.
P.S u r - Sci. 1987,181,295.
Yang et al.
1430 The Journal of Physical Chemistry, Vol. 96. No. 3, 1992
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revealed that a hole had been created in the surface. The size and depth of the hole depended on the voltages used and the number of times scanning at that position. The tip was examined after use and showed no evidence of the damage associated with a crashed tip. No debris and broken pieces were observed on the surrounding area, indicating that the polymer film was ejected by the high electrical field rather than by mechanical contact. In fact, several groups have used this method to create surface features.* We routinely compared STM images taken before and after the STS measurements in order to ascertain that the polymer morphology had not been altered during the STS measurements. Another factor influencing the accuracy of the measured spectra is the STM tip structure. Multiple or nonuniform tips change the peak positions in the conductance spectra. Multiple tips combine information from different sample areas and shift peak positions in the conductance spectra. Nonuniform tips (which means here that the DOS of a tip varies a lot within its Fermi energy) will alter the sample local DOS features because some of the features will come from the tip DOS contribution. This can sometimes can be minimized by applying a higher sample voltage. All STS measurements were checked by using several tips, and the data reported here have none of the artifacts discussed above. Figure 5 shows the inverse photoemission spectrum (IPES) of the highly doped polypyrrole film employing the sample used in the STS measurements (Figure 4). In the spectrum, the peaks at 1.0 and 2.6 eV are attributed to the bonding and antibonding bipolaron bands. The band width of 0.4 eV was determined by measuring the full width at half-maximum of the two bipolaron bands. The peak at 3.6 eV is attributed to the conduction band minima. The change in photon intensity of the polypyrrole film as a function of doping density agrees with the STS data.9 Figure 6 provides a summary of the energy level diagram of polypyrrole as a function of the level of BFL doping. In the neutral state, polypyrrole has a 3.2-eV band gap (Figure 2c) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). At low doping densities, new polaron energy states arise at about 0.2 eV (Figure 2c, E, and Ep')away from either side of the gap. At still higher doping (8) Albrecht, T.R.; Dovek, M.;Kirk, M.D.; Lang, C. A.; Quae, C.; Smith, D. P. E. Appl. Phys. Lett. 1989, 55, 1727. (9) Hu,Y.; Yang, R.; Evans, D. F.; Weaver, J. H. Phys. Rev.B, in press.
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Figure 6. Energy level diagram of polypyrrole upon BF4- doping: (A) neutral state shows a large band gap; (B) polaron states arise within the gap at very low doping levels; (C) bipolaron states are created at increased doping levels; (D) two new bonding and antibonding bipolaron bands form at the highly doped level.
levels, the polaron states combine with each other to form bipolaron states, which are located at about 0.5 eV (Figure 3c, Em and Em#) away from either edge of the gap. The bipolaron states start overlapping, and the gap edges begin shifting away from each other. Finally, at the highest doping states, two completely new energy interbipolaron bands form that are approximately 0.5 eV wide and are located about 0.7 eV away from either side of the gap. Our data are in good agreement with theoretical calculations of Bredas et al.,1° which predict that the bonding and antibonding bipolaron bands appear at 1.0 and 2.7 eV above the valence band maximum.
Acknowledgment. U.S.Army Grant DA-DAAL-03-89-K-058, The Center for Interfacial Engineering (CIE), a National Science Foundation Engineering Research Center, and The 3M Company, a CIE sponsor company, are gratefully acknowledged. Registry No. Et4N+BF4-,429-06- 1; polypyrrole (homopolymer), 30604-81-0. (10) Bredas, J. L.;Scott, J. C.; Yakushi, K.;Street, G.B. Phys. Rev. B 1984, 30 (2), 1023.
