In Situ XPS Study of the Interactions of Evaporated Copper Atoms with

Department of Physics, National University of Singapore, Kent Ridge, Singapore 119260. E. T. Kang. Department of Chemical Engineering, National Univer...
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Langmuir 1998, 14, 5305-5313

5305

In Situ XPS Study of the Interactions of Evaporated Copper Atoms with Neutral and Protonated Polyaniline Films S. L. Lim and K. L. Tan* Department of Physics, National University of Singapore, Kent Ridge, Singapore 119260

E. T. Kang Department of Chemical Engineering, National University of Singapore, Kent Ridge, Singapore 119260 Received February 20, 1998. In Final Form: June 12, 1998 X-ray photoelectron spectroscopy was employed for the in situ study of the interactions between thermally evaporated copper atoms and neutral polyaniline (PANI) films of various intrinsic oxidation states ([d N-]/[-N-] ratios), as well as with the HClO4-protonated PANI film. Quantitative changes in the N 1s core-level spectra, evolution of the Cu 2p core-level signals and the Cu LMM Auger line shapes, and angle-resolved changes in surface stoichiometry of these films with the progressive deposition of Cu atoms were carefully monitored. For the neutral films, the incoming Cu atoms appear to affect predominantly the nitrogen sites, resulting in a decrease in the intrinsic oxidation state of the polymer. Nevertheless, the effect was observed to be rather weak, possibly through a slight disruption of the π-electron conjugation of the polymer backbone. However, for the protonated film, interactions with incoming copper atoms resulted in the decomposition of the perchlorate dopant and the formation of copper chloride species. Diffusion of Cu atoms into the bulk of the polymer was observed for all types of films studied. The interactions of the Cu atoms with the PANI surfaces were also compared with those of the Al atoms.

Introduction During past two decades, the synthesis and characterization of electroactive polymers have become one of the most important research areas in polymer science. The earlier works on conjugated conductive polymers were summarized in the fine review of Billingham.1 A handbook drawing on two decades of pioneering research on the subject has also been compiled recently.2 V arious surface analysis techniques have been employed in the studies of the intrinsic structure of conjugated polymers, especially the X-ray photoelectron spectroscopic (XPS) and the secondary ion mass spectroscopic (SIMS) techniques.3,4 Among the various electroactive polymers, polyaniline (PANI) has been of particular interest because of the controllable electrical conductivity,5 environmental stability,6 and interesting redox properties associated with the chain heteroatoms.7 In particular, the electrical conductivity of the polymer can be increased from ∼10-10 S/cm to over 10 S/cm through protonation in aqueous acid solution.8 The aniline polymers have the general formula [(-B-NH-B-NH-)y(-B-NdQdN-)1-y]x, in which B * Corresponding author: [email protected].

(fax) (65) 772-6126; (e-mail)

(1) Billingham, N. C.; Calvert, P. D. Adv. Polym. Sci. 1989, 90, 2. (2) Handbook of Organic Conducting Molecules and Polymers; Nalwa, H. S., Ed.; John Wiley: Chichester, 1997; Vols. I-IV. (3) Abel, M. L.; Leadley, S. R.; Brown, A. M.; Petitjean, J.; Chehimi, M. M.; Watts, J. F. Synth. Met. 1994, 66, 85. (4) Huan, C. H. A.; Wee, A. T. S.; Gopalakrishnan, R.; Tan, K. L.; Kang, E. T.; Neoh, K. G.; Liaw, D. J. Synth. Met. 1993, 53, 193. (5) Ray, A.; Asturias, G. E.; Kershner, D. L.; Richter, A. F.; MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1989, 29, E141. (6) Neoh, K. G.; Kang, E. T.; Khor, S. H.; Tan, K. L. Polym. Degrad. Stab. 1990, 27, 107. (7) Tan, K. L.; Tan, B. T. G.; Kang, E. T.; Neoh, K. G. J. Chem. Phys. 1991, 94, 5382. (8) MacDiarmid, A. G.; Chiang, J. C.; Richter, A. F.; Epstein, A. J. Synth. Met. 1987, 18, 285.

and Q denote the C6H4 rings in the benzenoid and quinonoid forms, respectively.5 Thus, the aniline polymers are basically poly(p-phenyleneamineimine)s, in which the neutral intrinsic redox states can vary from that of the fully oxidized pernigraniline (PNA, y ) 0), through the 50% intrinsically oxidized emeraldine (EM, y ) 0.5), to that of the fully reduced leucoemeraldine (LM, y ) 1).8 The 75% oxidized neutral polymer has been termed nigraniline (NA, y ) 0.25). The chemical structures for the various intrinsic oxidation states of PANI are shown in Figure 1. Recent XPS studies on chemically9,10 and electrochemically11,12 synthesized PANIs have demonstrated that the quinonoid imine (dN- structure), benzenoid amine (-NH- structure), and positively charged nitrogen, corresponding to any particular intrinsic oxidation state and protonation level of PANI, can be quantitatively differentiated in the properly curve-fitted N 1s core-level spectrum. They correspond respectively to peak components with BEs at ∼398.4, 399.6, and >400.0 eV. XPS has thus provided a unique surface analytical tool for the investigation of the intrinsic structure of the aniline polymers. The studies of metal-polymer interfaces have received considerable attention in the past decade, due to the related technological applications in molecular semiconductor devices, corrosion protection coatings, and microelectronics packagings. Conjugated polymers, in particular, have attracted ever growing interest in relation (9) Tan, K. L.; Tan, B. T. G.; Kang, E. T.; Neoh, K. G. Phys. Rev. B 1989, 39, 8070. (10) Kang, E. T.; Neoh, K. G.; Tan, K. L. In Handbook of Organic Conducting Molecules and Polymers; Nalwa, H. S., Ed.; John Wiley: Chichester, 1997; Vol. 3, Chapter 3. (11) snauwaert, P.; Lazzaroni, R.; Riga, J.; Verbist, J. J.; Gonbeau, D. J. Chem. Phys. 1990, 92, 2187. (12) Kumar, S. N.; Gaillard, F.; Bouyssoux, G.; Sartre, A. Synth. Met. 1990, 36, 111.

S0743-7463(98)00205-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/07/1998

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Figure 1. Chemical structures of the various intrinsic oxidation states of PANI.

to their outstanding electronic properties.13,14 The potential of conjugated polymers as promising new materials for semiconductor devices was clearly demonstrated by the fabrication of the organic light-emitting diode15 and field-effect transistor.16 A common feature of these devices is the presence of interfaces between the polymeric semiconductor and the metal contacts. The performance of these devices depends predominantly on the chemical, electronic, and mechanical properties of these interfaces. Hence, the understanding and characterization of the various types of metal-conjugated polymer interfaces are of paramount importance. In view of the potential applications of PANI in lightemitting diodes17,18 and in corrosion protection of metals,19,20 a good understanding of the charge-transfer interaction at the metal-polymer interface will facilitate the application of the aniline polymers in the fabrication of molecular-based electronics devices and in the corrosion prevention of metals. The present work supplements the earlier fine works21 on the metallization of PANI films by providing a comparison of the interactions of thermally deposited Cu with the neutral PANI films, as well as with the protonated EM films. Comparison is also made, whenever appropriate, with our earlier studies of Al deposition onto the various neutral and protonated PANI films.22,23 (13) Conjugated Polymers: The Novel Science and Technology of Highly Conducting and Nonlinear Optically Active Materials; Bre´das, J. L., Silbey, R., Eds.; Kluwer: Dordrecht, 1991. (14) Organic Materials for Electronics, Proceedings of Symposium D on Polymer Interfaces with Metals and Semiconductors of the 1994 E-MRS Spring Conference; Bre´das, J. L., Salaneck, W. R., Wegner, G., Eds.; North-Holland: Elsevier: Amsterdam, 1994. (15) Burroughes, J. H. Jones, C. A.; Friend, R. H. Nature 1988, 335, 137. (16) Garnier, F.; Horowitz, G.; Peng, X.; Fichou, D. Adv. Mater. 1990, 2, 592. (17) Wang, H. L.; MacDiarmid, A. G.; Wang, Y. Z.; Geblor, D. D.; Epstein, A. J. Synth. Met. 1996, 78, 33. (18) Chen, S. A.; Chuang, K. R.; Chao, C. I.; Lee, H. T. Synth. Met. 1996, 82, 207. (19) Lu, W. K.; Elsenbaumer, R. L.; Wessling, B. Synth. Met. 1995, 71, 2163. (20) Ahmad, N.; MacDiarmid, A. G. Synth. Met. 1996, 78, 103. (21) Lazzaroni, R.; Gre´goire, C.; Chtaı¨b, M.; Pireaux, J. J. In PolymerSolid Interfaces; Pireaux, J. J., Bertrand, P., Bre´das, J. L., Eds.; IOP: Bristol, U.K., 1992; p 213. (22) Lim, S. L. Tan, K. L.; Kang, E. T. J. Vac. Sci. Technol. A 1998, 16, 13. (23) Lim, S. L.; Tan, K. L.; Kang, E. T. Synth. Met. 1998, 92/3, 213.

Experimental Section The chemical oxidative polymerization of aniline monomer was carried out at ∼5 °C using (NH4)2S2O8 as the oxidant, similar to the method described in the literature,8 except that 1 M H2SO4 was used instead of 1 M HCl. The EM base was obtained in powder form by treating the PANI salt with excess 0.5 M NaOH. The base polymer was subsequently cast into free-standing films of ∼20 µm in thickness by heating the concentrated N-methylpyrrolidinone (NMP) gel solution (containing ∼8% EM base by weight) at 150 °C in an air-circulated oven for 6 h and dried under reduced pressure by dynamic pumping. The intrinsic oxidation state of the as-cast EM base film can be substantially enhanced by an acid-base treatment cycle to give rise to the 75% intrinsically oxidized NAlike state.24 The fully reduced LM film was obtained by treating the EM base film with phenylhydrazine, according to the method of Green and Woodhead.25 The protonation of the as-cast EM base film was carried out by equilibrating the film in 1 M HClO4 aqueous solution for 1 h and subsequent rinsing with an excess amount of 0.1 M HClO4. The protonated film was dried by exhaustive pumping. The deposition of Cu was performed using a thermal evaporator installed inside the preparation chamber which is connected to the XPS analysis chamber via a gate valve. The evaporator filaments were made of tantalum (99.9% purity, obtained from Goodfellow Inc., Cambridge, U.K.). High-purity Cu wire segments (99.999% purity, Goodfellow Inc.) were attached to these filaments for evaporation by resistive heating. Each evaporation was carefully controlled to minimize overexposure. The pressure in the preparation chamber during the deposition remained below 8 × 10-8 Torr. In the present work, the amount of Cu deposited onto the PANI films is expressed in terms of the molar ratio of Cu to nitrogen, i.e., [Cu]/[N]. This is believed to be a more direct and quantitative parameter than the thickness of the deposited metal for monitoring the interaction of Cu atoms with the conjugated polymer. It should be pointed out that the “thickness” of the deposited Cu is not well defined in such study, especially at the initial stages (24) Kang, E. T.; Neoh, K. G.; Tan, K. L. Surf. Interface Anal. 1993, 20, 833. (25) Green, A. G.; Woodhead, A. E. J. Chem. Soc. 1912, 101, 1117.

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of the metal deposition process. The problem is further aggravated by the fact that Cu atoms readily diffuse into the bulk of the polymer films before forming a continuous film at the surface (see Results and Discussion section below). The XPS measurements were made on a VG Escalab MKII spectrometer with a nonmonochromatic Mg KR X-ray source (photon energy of 1253.6 eV), at a constant retard ratio of 40. The X-ray source was operated at a reduced power of 120 W (12 kV with emission current of 10 mA) to minimize possible degradation of the polymer films. All reported spectra were for the photoelectron takeoff angle (with respect to the film surface) of 75°, unless stated otherwise. The pressure in the analysis chamber was maintained at 8 × 10-9 Torr or lower during each measurement. In peak synthesis, the line width (full width at half-maximum, fwhm) of the Gaussian-shaped peaks was kept practically constant for all components in a particular spectrum. To compensate for surface charging effect, all binding energies (BEs) were referenced to the neutral C 1s peak at 284.6 eV. Surface elemental stoichiometry was determined from the peak-area ratios, after correcting with the experimentally determined sensitivity factors, and is reliable to within (10%. The sensitivity factors were determined using stable binary compounds of well-defined stoichiometries. Results and Discussion Cu Deposition on PANI Films of Various Intrinsic Oxidation States. The N 1s core-level spectra of the pristine EM film, the once acid-base cycled EM film (NAlike film), and the LM film before Cu deposition, are shown in Figures 2a, 3a, and 4a, respectively. The N 1s envelopes of the pristine and acid-base cycled EM films can be resolved into four components. The two major components with BEs at about 398.4 and 399.7 eV are attributable to the nitrogen atoms of the quinonoid imine (dN-) and benzenoid amine (-NH-), respectively.9,11,12 The ratio of the dN- and -NH- components provides a direct measure of the intrinsic oxidation state of the aniline polymers.26,27 Hence, the approximately equal intensity of the dN- and -NH- components for the pristine EM film (Figure 2a) is consistent with the corresponding chemical structure of a 50% intrinsically oxidized PANI. Furthermore, it can also be seen that a simple acid-base cyclic treatment has significantly enhanced the intrinsic oxidation state of the EM film surface (Figure 3a), as reported earlier.27 In the case of the fully reduced LM film, the peak assignment is also consistent with the fact that only one major N 1s component with a BE of ∼399.7 eV, attributable to the benzenoid amine, was observed (Figure 4a). For each of these N 1s spectra, although the residual high BE tail has been curve-fitted with up to two components on the basis of the fixed fwhm approach, it is likely that it may have resulted from surface states or species with a continuous distribution of BE. The surface species are probably associated with the surface oxidation products or with the weakly charge-transfer complexed oxygen species. These assignments are consistent with the presence of a fairly strong O 1s core-level signal for most aniline polymers. Contribution from the imine nitrogen shake-up satellite to the high BE tail has also been suggested.11 Finally, contribution from interchain (26) MacDiarmid, A. G.; Chiang, J. C.; Richter, A. F.; Epstein, A. J. Synth. Met. 1987, 18, 285. (27) Kang, E. T.; Neoh, K. G.; Tan, K. L. Surf. Interface Anal. 1993, 20, 833.

Figure 2. N 1s core-level spectra of the pristine EM film (a) before Cu deposition and (b-d) at different stages of the Cu deposition process.

hydrogen bonding in thermally induced PANI aggregates may also have to be taken into account.28 Upon Cu deposition, the N 1s line shapes of the pristine and acid-base cycled films were progressively modified. Figure 2b-2d, and Figure 3b-3d show the changes in the N 1s core-level line shapes, together with the curvefitted peak components, for the EM and NA films, respectively, at different stages of the metal deposition process. The intrinsic oxidation states of these films are observed to decrease with Cu coverage, by approximately equal extents. The calculated ratios of the [dN-] and [-N-] components, together with the surface compositions, of these films are tabulated in Table 1. The changes in [dN-]/[-N-] ratios as a function of Cu coverage are also plotted in Figure 5. To compare the effects of metals of different work functions deposited on PANI films of various intrinsic oxidation states, the results of an earlier study of in situ Al deposition22 are also shown in Figure 5. It can be seen that changes induced in these films by deposited Cu atoms are less drastic in comparison with those induced by deposited Al atoms. The weaker effect of deposited Cu atoms on PANI base films is probably consistent with the increased work function of Cu. A recent study on the deposition and reduction of very high work function metals, such as gold and palladium, on EM films has shown that the deposition of these high work function metals has virtually no effect on the [dN-]/[(28) Angelopoulos, M.; Liao, Y.; MacDiarmid, A. G.; Zheng, W. G.; Epstein, A. J.; Long, S.; Arbuckle, G. Proc. ICSM ‘96, July 28-August 2, 1996, Snowbird, UT, 1996; p 150.

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Figure 3. N 1s core-level spectra of the acid-base cycled EM film (a) before Cu deposition and (b-d) at different stages of the Cu deposition process.

Figure 4. N 1s core-level spectra of the fully reduced LM film (a) before Cu deposition and (b-d) at different stages of the Cu deposition process.

N-] ratios of the polymer.29 On the other hand, other investigators have reported that no significant changes were observed for XPS C 1s and N 1s signals upon Cu deposition.21 The observation of decreasing intrinsic oxidation state in the pristine and acid-base cycled films seems to imply that the incoming Cu atoms affect predominantly the imine sites along the conjugated backbone of the polymer, similar to the observation reported earlier for Al deposition on such polymer films.22 Theoretical calculations have also indicated that the imine-quinonoid-imine groups of the PANI polymer are relatively more reactive.30 On the other hand, the deposition of Cu does not seem to induce any discernible changes in the N 1s core-level signal of the fully reduced LM film, as shown in Figure 4b-4d. Thus, the observation implies that there is no direct interaction between the deposited Cu atoms and the amine sites of the aniline polymers, as in the case of Al deposition.22 In addition, it should also be noted that the deposition of Cu does not give rise to any significant shift in the BEs of the dN- and -NH- components and neither does the emergence of any new component. The absence of such features seems to suggest that there is no direct charge transfer between the deposited Cu atoms and the dN- sites. A probable mechanism of indirect interactions between the Cu atoms and the conjugated polymer is through the disruption of π-electron conjugation to give rise to a decrease in the [dN-]/[-N-] ratio.

The intrinsic oxidation states of the pristine and acidbase cycled films seem to approach rather different asymptotic limits with increasing Cu coverage, as seen in Figure 5. The respective values of these limits are higher than those in the corresponding cases of Al deposition. This observation, together with the fact that Cu atoms can diffuse into the film, would readily testify to a lower reactivity for Cu than Al. Diffusion of Cu into various polymer substrates has been widely reported.21,31,32 More direct evidence for the diffusion of Cu atoms into the PANI films can be obtained from the XPS measurements taken at different photoelectron takeoff angles with respect to the film surface. Figure 6 shows the stoichiometric concentrations of Cu measured at takeoff angles of 20 and 75°, respectively, at each stage of the metal deposition process. Results from the earlier study of Al deposition are also shown for comparison. It can be seen clearly that, for Cu deposition, the concentrations of Cu measured at the more surface glancing angle of 20° (i.e., nearer to the surface) are only slightly higher than those measured at 75°, indicating that the incoming Cu atoms can readily diffuse into the polymer films. On the other hand, for Al deposition, the concentrations of Al near the surfaces are much higher than those deeper into the films. Thus, the deposited Al atoms are practically trapped at the topmost region of the film surface. Diffusion of Cu atoms into the bulk of PANI films has also been reported by Lazzaroni

(29) Kang, E. T.; Neoh, K. G.; Huang, S. W.; Lim, S. L.; Tan, K. L. J. Phys. Chem. 1997, 101, 10744. (30) dos Santos, M. C.; Bre´das, J. L. Phys. Rev. B 1989, 40, 11997.

(31) Haight, R.; White, R. C.; Silverman, B. D.; Ho, P. S. J. Vac. Sci. Technol. A 1988, 6, 2188. (32) Ringenbach, A.; Jugnet, Y.; Tran Minh Duc, Y. J. Adhes. Sci. Technol. 1995, 9, 1209.

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Table 1. Intrinsic Oxidation States, Surface Elemental Stoichiometries, and Modified Auger Parameters of the Three Types of Neutral PANI Films, before and at the Different Stages of Cu Deposition [Cu]/[N]

[dN-]/[-N-]

0.000 0.065 0.118 0.165 0.216 0.284 0.428 0.591

1.16 1.15 0.952 0.806 0.811 0.729 0.637 0.561

0.000 0.016 0.073 0.130 0.219 0.399 0.887 1.928 0.000 0.013 0.044 0.062 0.087 0.138 0.235 0.340 0.490 a

C:N:O:Cu

Pristine EM Film 7.22:1:0.621:0.000 7.10:1:0.606:0.066 7.10:1:0.595:0.118 6.94:1:0.612:0.165 6.81:1:0.608:0.216 6.98:1:0.616:0.284 7.09:1:0.607:0.428 7.18:1:0.634:0.591

modified Auger param (eV)

1847.5 1847.9 1847.7 1847.6 1847.7 1847.6 1847.7

Acid-Base Cycled EM Film (NA-like State) 1.60 6.90:1:0.728:0.000 1.58 6.68:1:0.681:0.016 naa 1.46 6.89:1:0.714:0.073 1847.4 1.20 6.88:1:0.706:0.130 1848.1 1.14 6.85:1:0.690:0.219 1848.2 1.19 7.05:1:0.695:0.399 1847.9 0.943 7.15:1:0.725:0.887 1848.7 1.02 8.36:1:0.812:1.93 1850.1 0 0 0 0 0 0 0 0 0

Fully Reduced LM Film 6.19:1:0.340:0.000 6.23:1:0.324:0.013 6.28:1:0.328:0.044 6.50:1:0.328:0.062 6.22:1:0.307:0.087 6.44:1:0.321:0.138 6.02:1:0.310:0.235 6.46:1:0.358:0.340 6.54:1:0.360:0.490

Figure 6. Stoichiometric concentration of deposited metals, at different stages of the deposition process, measured at photoelectron takeoff angles of 20 and 75°, respectively.

1847.1 1847.5 1847.2 1847.6 1847.8 1848.4 1849.2 1849.6

Not available.

Figure 7. Changes in surface oxygen stoichiometry of PANI films with Cu and Al deposition.

Figure 5. Changes in intrinsic oxidation states of PANI films with Cu and Al deposition.

et al.,21 based on the nonproportionality between the intensity of XPS Cu 2p core-level signal and the deposited amount detected by quartz microbalance. The fact that Cu atoms can diffuse readily into the bulk of the PANI films is also consistent with the earlier deduction that the interaction between Cu atoms and the conjugated backbone of the polymer is weaker, in comparison with that between the Al atoms and the polymer. The surface stoichiometries of the three types of films, both before and at different stages of Cu deposition, are deduced from the corresponding XPS core-level spectral area based on carefully calibrated instrumental sensitivity factors of the elements involved and are tabulated in Table 1. The stoichiometry of carbon in each of the as-prepared

films deviates slightly from the ideal value expected from the chemical structure of PANI. This is probably due to the inaccuracy of the experimentally determined sensitivity factors, as well as surface hydrocarbon contaminants. Cross-linking induced during the film-casting process could also contribute to the higher concentration of carbon observed. It should also be noted from Table 1 that oxygen has been detected in all the three types of as-cast films. The presence of oxygen in PANI films has been widely reported.12,21,22 Given the reactive nature of the conjugated polymers, the bulk of a PANI film probably can be perceived as an oxygen reservoir. It should be emphasized that, for all the types of films studied, the surface concentration of oxygen remains practically constant throughout the various stages of Cu deposition process, as shown in Figure 7. This is in sharp contrast to the case of Al deposition on these PANI films, also plotted in Figure 7 for comparison. The deposition of Al atoms had led to the surface migration of the adsorbed oxygen in response to the incoming Al atoms.22 The corresponding data for Al deposition on poly(tetrafluoroethylene) (PTFE) film is also shown in Figure 7, which rules out any significant contribution of oxygen from the Al evaporation process. The data in Figure 7 clearly indicate that there is no migration of bulk adsorbed oxygen toward the surface of the polymer in response to the deposition of the Cu atoms. Hence, it seems that oxygen does not play a direct role in the interactions between Cu atoms and the conjugated backbone of the aniline polymers.

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Figure 8. Cu 2p core-level spectra, and the corresponding Auger Cu LMM signals, of the pristine EM film at different stages of the Cu deposition process.

Figure 9. Cu 2p core-level spectra, and the corresponding Auger Cu LMM signals, of the acid-base cycled EM film at different stages of the Cu deposition process.

The XPS Cu 2p3/2 core-level spectra, and the corresponding Auger LMM signals, acquired at the different stages of metal deposition process for the pristine EM, acid-base cycled EM, and fully reduced LM films are shown, respectively, in Figures 8-10. Each Cu 2p3/2 spectrum for the three types of PANI films can be curvefitted with one major peak component (>85%) with BE at ∼932.8 eV throughout the various stages of Cu coverage. On the other hand, the corresponding Auger LMM signal shifts continuously toward the higher kinetic energy (KE) with progressive coverage of Cu. The modified Auger parameter, R′, defined as the summation of the BE of Cu 2p3/2 core-level signal and the KE of the corresponding Auger LMM signal, increases from ∼1847 eV to ∼1850 eV for the acid-base cycled EM and the fully reduced LM films, but remains essentially unchanged for the pristine EM film, with Cu deposition. At the initial stages of Cu deposition, the observation of the Cu 2p3/2 major peak component at BE of 932.8 eV and the R′ parameter at ∼1847.3 eV seems to imply that the interactions between the incoming Cu atoms and the polymer occur predominantly within the close proximity of the nitrogen sites of the conjugated backbone, since this result closely resembles the observation for the CuCN compound (in which the Cu 2p3/2 component has BE at 933.1 eV, with the

Lim et al.

Figure 10. Cu 2p core-level spectra, and the corresponding Auger Cu LMM signals, of the fully reduced LM film at different stages of the Cu deposition process.

corresponding R′ at 1847.6 eV).33 With increasing Cu coverage, the upward shift in the R′ parameter is probably due to the increasing contribution from metallic Cu (for which Cu 2p3/2 has BE at ∼932.6 eV, with corresponding R′ at ∼1851.2 eV).33 On the basis of the above observations, i.e., (a) the decrease in the intrinsic oxidation states of pristine and acid-base cycled EM films with Cu deposition, (b) the absence of features associated with direct charge-transfer interaction, (c) the diffusion of Cu atoms into the bulk of the films, (d) the absence of oxygen migration toward the surface in response to the incoming Cu atoms, and (e) the persistence of the main Cu 2p3/2 signal at BE of ∼932.8 eV for all films throughout the different stages of Cu deposition, as well as the upward shift of the modified Auger parameter R′ with increasing Cu coverage, it can be proposed that the deposited Cu atoms interact preferentially, but weakly, with the imine sites of the polymer. Such interactions could have manifested through the disruption of the π-electron conjugation along the conjugated backbone. The observation of the enhanced shakeup satellite structure in the C 1s signal (Figure 11), attributable to phenyl rings of the EM backbone, with increasing Cu coverage is also consistent with the disruption of π-conjugation and, hence, the localization of π-electrons within the benzene rings. The fact that the interactions must be weak in nature is supported by the high mobility of Cu atoms within the polymer films. It should also be pointed out that, unlike the case of Al deposition, such interactions most probably do not involve oxygen. Cu Deposition on HClO4-Protonated EM Film. The surface elemental stoichiometry of the HClO4-protonated EM film as revealed by the XPS technique is tabulated in Table 2. The stoichiometric ratio of carbon to nitrogen of the as-prepared film was again observed to be slightly higher than the ideal value of 6, similar to that observed for neutral PANI films. On the other hand, the ratio of oxygen to chlorine was determined to be approximately 4.1, which is consistent with the chemical structure of the perchlorate ClO4- dopant. Figure 12a shows the curve-fitted N 1s core-level spectrum for the HClO4-protonated EM film prior to Cu (33) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Chastian, J., Ed.; PerkinElmer: Eden Prairie, MN, 1992; p 203.

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Figure 11. Changes in the high BE tail of C 1s core-level spectra (C 1s satellite structure) of the acid-base cycled EM film with Cu deposition. Table 2. Intrinsic Oxidation States, Elemental Stoichiometries, and Modified Auger Parameters of HClO4-Protonated PANI Film, before and after the Different Stages of Cu Deposition [Cu]/ [dN-]/ [N] [-N-] 0 0.032 0.069 0.120 0.166 0.225 0.326 a

0.000 0.038 0.069 0.093 0.113 0.126 0.145

C:N:O:Cl:Cu 6.77:1:1.78:0.435:0.000 6.73:1:1.79:0.443:0.032 6.89:1:1.83:0.464:0.069 6.80:1:1.72:0.481:0.120 6.90:1:1.68:0.530:0.166 6.95:1:1.75:0.529:0.225 6.85:1:1.69:0.553:0.326

modified {[ClO4*] + [ClO4-]}/ Auger param (eV) [N+] naa 1847.9 1847.9 1847.9 1847.7 1847.6

1.12 1.17 1.15 1.08 1.04 1.20 1.10

Figure 12. N 1s core-level spectra of the HClO4-protonated EM film (a) before Cu deposition and (b-d) at different stages of the Cu deposition process.

Not available.

deposition. The protonated film has a doping level, as measured by the [Cl]/[N] ratio, of 0.44. The N 1s spectrum exhibits a major component at BE of ∼399.6 eV, which is characteristic of amine -NH- nitrogen, and two other components, based on the fixed fwhm approach in peak synthesis, with BEs above 400 eV that correspond to the positively charged N+ nitrogen. The corresponding curvefitted Cl 2p core-level signal before Cu evaporation is shown in Figure 13a. The major feature at the BE of ∼208 eV is known to be due to the anionic perchlorate dopant34 and can be well fitted with two spin-orbit split doublets (Cl 2p3/2 and Cl 2p1/2) with the BEs of the corresponding Cl 2p3/2 peaks lying at about 207.7 and 209.1 eV, respectively. The lower BE doublet can be attributed to the ClO4- anion, whereas the higher BE doublet can be assigned to the perchlorate species (ClO4*) resulting from the charge-transfer interactions between the dopant anion and the metal-like conducting state of the polymer.35 The observation of two perchlorate and two chloride anionic species has been widely reported in PANI-HClO4 and PANI-HCl complexes,36 respectively, as well as in complexes of other conjugated polymers.37,38 It is worth (34) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Chastian, J., Ed.; PerkinElmer: Eden Prairie, MN, 1992; p 63. (35) Kang, E. T.; Neoh, K. G.; Tan, K. L. Adv. Polym. Sci. 1993, 106, 135.

Figure 13. Cl 2p core-level spectra of the HClO4-protonated EM film (a) before Cu deposition and (b-d) at different stages of the Cu deposition process.

pointing out that the ratio of the concentration of the perchlorate species (taken to be the sum of the two major fitted components of the Cl 2p signal at ∼208 eV) to that of the N+ sites is ∼1.1 before Cu deposition, and it remains

5312 Langmuir, Vol. 14, No. 18, 1998

closely within the range of 1.1 ( 0.1 throughout all stages of the metal deposition process. The slight deviation of the ratio from unity could be due to the inclusion of a small contribution from the adsorbed perchlorate species. Based on the requirement of charge neutrality, the observed ratios therefore indicate that both the anionic ClO4- and charge-transfer ClO4* species are involved in the charge-transfer interactions with the polymer, consistent with an earlier study of Al deposition onto protonated PANI films.23 The much weaker feature of the Cl 2p signal at the BE of ∼200 eV, before Cu deposition (Figure 13a), is attributed to the KR3,R4 contributions of the nonmonochromatic Mg X-ray source. This assignment is based on the observation that the ratio of the signal intensities at ∼200 and ∼208 eV has a value of 0.12, for the as-prepared protonated film before Cu deposition, in excellent agreement with that reported by Moulder et al.39 Since the BE range of the KR3,R4 contributions overlaps with that of chlorine in most chloride states, this Cl 2p spectrum obtained before Cu evaporation thus provides a reference (baseline) for extracting the net contributions of the chloride species which emerge as a result of Cu deposition (see below). As shown in Figure 13, it can be seen that the lower BE feature (at BE of ∼200 eV) of the Cl 2p spectra was strongly modified by Cu deposition, whereas the line shape of the higher BE one (at BE of ∼208 eV) appeared unaffected. Analysis of these features with careful curve fitting indicates that the concentrations of both the anionic ClO4species and the charge-transfer ClO4* complex, or the [ClO4-]/[N] and [ClO4*]/[N] ratios, decrease with progressive evaporation of Cu. The relative proportion of these two components increases from about 4.6:1 before metal deposition to about 6.6:1 at high Cu coverage, probably indicating that the incoming Cu atoms react more readily with the charge-transfer complexed species. The decrease in the total concentration of the perchlorate species clearly implies that the deposition of Cu has resulted in the decomposition of the perchlorate species and, hence, the deprotonation of the protonated film. Another evidence of the deprotonation of the film is the emergence of a minor but discernible peak component in the N 1s core-level spectra, with BE at ∼398.4 eV, which is characteristics of the imine dN- nitrogen, as well as a corresponding decrease in the proportion of the positively charged nitrogen, with increasing Cu coverage (Figure 12). It can be noted (Figure 13) that the lower BE feature of the Cl 2p signal at ∼200 eV increases rapidly with Cu coverage. This feature can be resolved into two spinorbit split doublets with the BEs of the corresponding Cl 2p3/2 peaks lying at about 198.3 and 199.8 eV, respectively. These doublets are attributed, respectively, to the formation of CuCl and CuCl2.40 This assignment is consistent with the observed decomposition of the two perchlorate species with increasing Cu coverage. The reaction between the perchlorate species and the Cu atoms that resulted mainly in the formation of CuCl species (with corresponding Cl 2p3/2 BE at 198.3 eV) is (36) Neoh, K. G.; Kang, E. T.; Tan, K. L. J. Phys. Chem. 1991, 95, 10151. (37) Kang, E. T.; Neoh, K. G.; Tan, K. L. Phys. Rev. B 1991, 44, 10461. (38) Tan, K. L.; Tan, B. T. G.; Kang, E. T.; Neoh, K. G. J. Mater. Sci. 1992, 27, 4056. (39) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Chastian, J., Ed.; PerkinElmer: Eden Prairie, MN, 1992; p 18. (40) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Chastian, J., Ed.; PerkinElmer: Eden Prairie, MN, 1992; p 218.

Lim et al.

Figure 14. Cu 2p core-level spectra, and the corresponding Auger Cu LMM signals, of the HClO4-protonated EM film at different stages of the Cu deposition process.

further supported by the analysis of the corresponding Cu 2p core-level signal, as well as the Cu LMM Auger line shape, at the different stages of Cu coverage (Figure 14 and Table 2). Each Cu 2p3/2 line shape can be fitted consistently with a major peak component at BE of ∼932.8 eV, with the corresponding modified Auger parameter R′ remaining practically constant at ∼1847.8 eV, throughout the various stages of Cu coverage. These results clearly imply that, in the final products, the Cu atoms are mainly in the Cu(I) state. It should be pointed out that diffusion of Cu atoms into the bulk of the HClO4-protonated PANI film is also observed (Figure 6), as evidenced by the approximately identical stoichiometric concentrations of Cu at both takeoff angles of 20 and 75° at each stage of the metal deposition process. On the other hand, no migration of adsorbed oxygen from the bulk of the protonated film toward the surface in response to Cu deposition is observed, as the slight increase in surface oxygen stoichiometry of the protonated film (Figure 7) can be accounted for by oxygen from the decomposition of dopant anions. These observations are in contrast to that observed during the study of Al deposition onto HClO4protonated film.23 On the basis of the above observations, it appears reasonable to suggest that the incoming Cu atoms reacted strongly with the perchlorate species and resulted in the decomposition of these anionic species and, hence, the formation of the copper chlorides. To compensate for the decomposed ones, there appears to be a continuous migration of the perchlorate dopants to the film surface. However, due to the high diffusion rate of Cu atoms into the bulk of the polymer film, the supply of the dopant anions by migration was impaired. In view of the requirement of charge neutrality, the deficiency of dopant anions thus resulted in the deprotonation of the N+H sites to the imine dN- state. In addition, the absence of migration of oxygen suggests that, unlike the case of Al deposition, the role of oxygen in such polymer-metal interactions is insignificant, if any. Conclusion The interactions of thermally evaporated Cu atoms with neutral PANI films of three different intrinsic oxidation states (viz., the pristine EM film, the NA-like state from the acid-base cycled EM film, and the fully reduced LM film), as well as with the HClO4-protonated EM film, were

Cu Interactions with Polyaniline Films

investigated in situ by XPS. In the cases of neutral films, the gradual decrease in the intrinsic oxidation states of the pristine and acid-base cycled neutral EM films seems to imply that the incoming Cu atoms affect predominantly the imine (dN-) sites along the conjugated backbone. However, such interaction is believed to be rather weak and possibly through the disruption of π-electron conjugation. From the observation that the BE of the major component of the Cu 2p core-level spectra remains consistently at ∼932.8 eV for all neutral films, as well as the gradual increase in the modified Auger parameter R′, it appears that during the initial stages of metal deposition, the Cu atoms are within the close proximity of the nitrogen sites and are in the Cu(I) state. With higher coverage of Cu, the increasing contribution of Cu(0) from the formation of metallic clusters is believed to have resulted in the upward shift of the R′ parameter. On the other hand, for

Langmuir, Vol. 14, No. 18, 1998 5313

the HClO4-protonated film, the incoming Cu atoms interact strongly with the perchlorate dopants. This interaction resulted in the decomposition of the dopant anions and subsequently the formation of copper chloride species. Migration of the perchlorate dopant is observed to occur with progressive Cu coverage, presumably to replace the decomposed ones and maintain charge neutrality on the film surface. However, deprotonation of the doped film was observed at high Cu coverage, probably due to the impairment of the migrating mechanism of the anionic dopant by the high mobility of Cu atoms into the bulk of the films. Finally, diffusion of the deposited Cu atoms into the bulk of the aniline polymers was observed for all types of films studied. LA980205+