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Electrochemical Quartz Crystal Microbalance Study of the Growth Characteristics of N-Alkylpyrrole: Organic Monolayers as Nucleation Sites for Ordered Polymer Growth† D. Brad Wurm* and Yeon-Taik Kim Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35401 Received November 2, 1999. In Final Form: February 28, 2000 The electrochemical polymerization of the n-alkylpyrrole, N-hexadecylpyrrole (HDPy) to form poly(Nhexadecylpyrrole) (PHDPy) was monitored at both a bare gold electrode surface and a gold surface modified with bis(ω-(N-pyrrolyl)-n-undecyl) sulfide (BPUS). The film growth process was monitored by chronoamperometry and electrochemical quartz crystal microbalance (EQCM). Both techniques revealed that there is a fundamental difference in the nucleation and growth of the polymer films on the modified versus the unmodified electrode surface. EQCM data revealed that the PHDPy film formed on the BPUS-modified electrode was much denser and grew in a more ordered manner than that formed on the bare gold surface. The film formed on the modified electrode was also found to be more resilient than that formed on the bare gold surface, withstanding repeated cycling in an electrolyte solution. These experiments demonstrate that the surface-confined pyrrole units on the modified electrode serve as specific nucleation sites to induce long-range order in the polymer film.
Introduction The ease of fabrication and of varying the physical properties of conducting polymer films make them attractive candidates for real world applications. Potential applications include rechargeable polymer batteries,1 electrochromic displays,2 antistatic materials,3 polymerbased diodes and transistors,4 chemical sensors,5 and memory devices.6 However, making these applications become a reality has been a slow process due in part to a lack of understanding of the fundamental processes involved in polymer formation.7 One of the most studied of these heterocyclic, π-conjugated conducting polymers has been polypyrrole.7-10,12-15,17-19,23 However, the low polymer oxidation potential of polypyrrole makes it highly susceptible to air degradation.8,9 For this reason, we have been interested in poly(N-alkylpyrrole)s as potential electronic materials due to their better air stability. It * To whom correspondence may be addressed. Current address: Department of Chemistry and Physics, Southeastern Louisiana University, Hammond, LA 70402. † This paper is dedicated to the late Yeon-Taik Kim. (1) (a) Nigrey, P. J.; MacInnes, D.; MacDiarmid, A. G.; Heeger, A. J. J. Electrochem. Soc. 1981, 128, 1651. (b) Mermilliod, M.; Tanguy, J.; Petiot, F. J. Electrochem. Soc. 1986, 133, 1073. (c) Kaufman, J. H.; Chun, T. C.; Heeger, A. J.; Wudl, F. J. Electrochem. Soc. 1984, 131, 2092. (d) Biserni, M.; Marinangeli, A.; Mastragostino, M. J. Electrochem. Soc. 1986, 132, 1597. (e) Kitani, A.; Kaya, M.; Sasaki, K. J. Electrochem. Soc. 1986, 133, 1069. (f) Genies, E. M.; Lapkowski, M.; Santier, C.; Vieil, E. Synth. Met. 1987, 18, 393. (g) MacDiarmid, A. G.; Yang, L. S.; Huang, W.-W.; Humphrey, B. D. Synth. Met. 1987, 18, 393. (2) (a) Inganas, O.; Lundstrom, I. Synth. Met. 1987, 21, 13. (b) Kuwahata, S.; Yoneyama, K.; Tamura, H. Bull. Chem. Soc. Jpn. 1984, 57, 2247. (c) Yoneyama, H.; Wakomoto, K.; Tamura, H. J. Electrochem. Soc. 1985, 132, 2414. (d) Kitani, A.; Yano, J.; Sasaki, K. J. Electroanal. Chem. 1986, 209, 227. (3) Heinze, J.; Dietrich, M. Synth. Met. 1991, 41, 503. (4) White, H. S.; Kittlesen, G. P.; Wrighton, M. S. J. Am. Chem. Soc. 1984, 106, 5375. (5) (a) Thackeray, J. W.; Wrighton, M. S. J. Phys. Chem. 1986, 90, 6674. (b) Josowicz, M.; Janata, J. Anal. Chem. 1986, 58, 514. (6) Meyer, H. W., Kiess, K.; Bingelli, B.; Meier, E.; Harbeke, G. Synth. Met. 1985, 10, 255. (7) Heinze, J. Synth. Met. 1991, 41-43, 2805. (8) Skotheim, T. A., Ed. Handbook of Conducting Polymers; Marcel Dekker: New York, 1986; p 95.
has been reported that the quality of these films is highly dependent on the substituent on the nitrogen, with a rough and wrinkled film resulting when the substituent is propyl or larger, presumably due to steric effects distorting the conjugated π system from coplanarity.9a In contrast to this, we recently reported the formation of reflective, high-quality films of poly(N-hexadecylpyrrole) (PHDPy).10 The films were obtained by electropolymerization of n-hexadecylpyrrole on modified gold electrodes. Alkane and ω-substituent alkanethiols spontaneously form ordered and densely packed monolayers upon immersion of Au substrates into thiol-containing solutions.11 Since the importance of the nucleation process is recognized as influencing the growth process of conducting polymers,12-14 we used the self-assembly method to create nucleation sites for the electrochemical growth of PHDPy. There is precedent for this type of rationale. Wrighton and co-workers self-assembled N-(3-(trimethoxysilyl)propyl)pyrrole on n-type silicon electrodes. The pendant pyrrole was proposed to covalently anchor polypyrrole to the semiconductor service, thereby guarding against photocorrosion.15 Rubenstein and co-workers preadsorbed (9) (a) Diaz, A. F.; Castillo, J. I.; Kanazawa, K. K.; Logan, J. A.; Salmon, M.; Fajardo, O. J. Electroanal. Chem. 1982, 133, 233. (b) Diaz, A. F.; Castillo, J. I.; Logan, J. A.; Lee, W.-Y. J. Electroanal. Chem. 1981, 129, 115. (10) Wurm, D. B.; Brittain, S. T.; Kim, Y.-T. Langmuir 1996, 12, 3756. (11) (a) Ulman, A. An Introduction to Ultrathin Organic Films: from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (b) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (c) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (12) (a) Asavapiriyanont, S.; Chandler, G. K.; Gunawardena, G. A.; Pletcher, D. J. Electroanal. Chem. 1984, 177, 229. (b) Asavapiriyanont, S.; Chandler, G. K.; Gunawardena, G. A.; Pletcher, D. J. Electroanal. Chem. 1984, 177, 245. (c) Noftle, R. E.; Pletcher, D. J. Electroanal. Chem. 1987, 277, 229. (13) Genies, E. M.; Bidan, G.; Diaz, A. F. J. Electroanal. Chem. 1983, 149, 101. (14) Kim, Y.-T.; Allara, D. L.; Collins, R. W.; Vedam, K. Thin Solid Films 1990, 193/194, 350. (15) Simon, R. A.; Ricco, A. J.; Wrighton, M. S. J. Am. Chem. Soc. 1982, 104, 2031.
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p-aminothiophenol onto a gold electrode and found the density of galvanostatically grown polyaniline on the modified electrode to be greatly increased over that formed at the unmodified electrode surface.16 Willicut and McCarley and Sayre and Collard electropolymerized pyrrole and 3-ethylpyrrole on a polycrystalline gold surface modified with surface-confined pyrroles in an aqueous and a nonaqueous solvent.17,18 A great improvement in adhesion was achieved for the polymers grown on the modified electrode surfaces. The electrochemical quartz crystal microbalance (EQCM) has proven useful in monitoring the growth of conducting polymers. Reynolds and co-workers used the EQCM to study the growth of polypyrrole on a gold electrode surface in nonaqueous solvents. The simultaneous measurement of charge and mass proved useful in elucidating the effect of various experimental conditions on physical parameters such as reaction kinetics and efficiency of polymerization.19 The EQCM has also been used to monitor ion and solvent movement in polypyrrole,20 polyaniline,21 and composite polymers.22 However, there has been very little study of the effects of preassembled nucleation sites on the subsequent polymer growth by EQCM.23 Here we used self-assembled monolayers of bis(ω-(Npyrrolyl)-n-undecyl) sulfide (BPUS) to serve as nucleation sites for the electrochemical growth of PHDPy. That the terminal pyrrole units of BPUS function as nucleation sites is further tested by chronoamperometry. Growth of PHDPy by cyclic voltammetry on both a BPUS modified and a bare gold electrode is monitored by EQCM. The BPUS self-assembled monolayer is shown to dramatically influence the growth mechanism and quality of the resulting polymer film. Experimental Section Chemicals. Pyrrole, acetonitrile, and tetrabutylammoniumhexafluorophosphate were purchased from Aldrich. Bu4N(PF6), which had been recrystallized and stored in a glovebox, was used to prepare a 0.1 M electrolyte solution. Acetonitrile was purified by distillation over CaH2. HDPy was synthesized according to the procedure reported in the literature.24 BPUS was synthesized as reported elsewhere.25 Self-Assembly. Thin film gold working electrodes were prepared by vapor deposition of 100 Å of Cr undercoat followed by 1500 Å of Au onto glass microscope slides at a pressure of 2 × 10-6 Torr. After backfilling the deposition chamber with nitrogen, the gold electrodes were immediately removed and placed in solutions for self-assembly modification or promptly used for electrochemistry experiments. Self-assembly modification of the electrodes involved placing them in 1 mM solutions of BPUS in hexane for 12-24 h. EQCM Measurements. A homemade electrochemical quartz crystal microbalance and 10-MHz AT cut quartz crystals (16) Rubenstein, I.; Rishpon, J.; Sabatani, E.; Redondo, A.; Gottesfeld, S. J. Am. Chem. Soc. 1990, 112, 6135. (17) (a) Willicut, R. J.; McCarley, R. L. J. Am. Chem. Soc. 1994, 116, 10824. (b) Willicut, R. J.; McCarley, R. L. Langmuir 1995, 11, 296. (18) Sayre, C. N.; Collard, D. M. Langmuir 1995, 11, 302. (19) Baker, C. R.; Reynolds, J. R. J. Electroanal. Chem. 1987, 251, 307. (20) (a) Kaufman, J. H.; Kanazawa, K. K.; Street, G. B. Phys. Rev. Lett. 1984, 53, 2461. (b) Bilger, R.; Heinze, J. Synth. Met. 1991, 43, 2893. (21) (a) Orata, D.; Buttry, D. A. J. Am. Chem. Soc. 1987, 109, 3574. (b) Miras, M. C.; Barbero, C.; Kotz, R.; Haas, O. J. Electroanal. Chem. 1994, 369, 193. (22) Basak, S.; Bose, C. S. C.; Rajeshwar, K. Anal. Chem. 1992, 64, 1813. (23) Wurm, D. B., Zong, K.; Kim, Y.-T.; Shin, M.; Jeon, I.-C. J. Electrochem. Soc. 1998, 145, 1483. (24) Josey, A. D. Org. Synth. 1967, 47, 81. (25) Zong, K.; Brittain, S. T.; Wurm, D. B.; Kim, Y.-T. Synth. Commun. 1997, 27, 157.
Wurm and Kim (International Crystal Manufacturing Co., OK) were used. The sensitivity of the EQCM is 0.9 ng/Hz. Each electrode was precoated with approximately 100 Å of Cr and was overcoated with 1500 Å of Au. All the measurements were carried out in situ at room temperature. The Au-coated crystals were cleaned with UV light before the experiment. A platinum wire was used as the counter electrode, and a freshly cleaned silver wire was used as a quasi-reference electrode. The Ag quasi-reference electrode was calibrated by determining the oxidation potential of ferrocene in 0.1 M Bu4N(PF)6 in acetonitrile solution. The Ag quasireference potential was found to be approximately 150 mV lower than a saturated calomel reference electrode. HDPy was polymerized by cycling between 0.4 and 1.4 V relative to the silver quasi-reference electrode. Details of the cell construction and its configuration as well as EQCM design can be found elsewhere.26 Chronoamperometry. A three-electrode, single-compartment glass electrochemical cell was used with a silver wire quasireference electrode, platinum gauze counter electrode, and gold thin film on glass serving as the working electrode. All the chronoamperograms were measured in air with a PAR 273 potentiostat equipped with software provided by the company.
Results and Discussion Chronoamperometry. HDPy was electropolymerized by cyclic voltammetry as well as potentiostatic means. We have noted the effect of BPUS modification on the resulting film morphology grown by cyclic voltammetry in a previous publication.10 The films were grown potentiostatically in hopes of gaining more insight into the role of the nucleation process during film growth. Figure 1a is a typical chronoamperogram corresponding to the potentiostatic growth of HDPy on the BPUS-modified electrode surface where the potential was jumped from 0.3 to 1.25 V vs Ag for growth of the polymer film. A steady, constant current following a rising transient is observed as has been reported for polypyrrole.12a The resulting film is continuous, shiny, and reflective. Figure 1b is a typical chronoamperogram for potentiostatic growth of HDPy on the bare gold surface under the same conditions. There is a noticeable hump in the current response after the initial rising portion of the curve and an unsteady current flow after this point. For typical three-dimensional growth one expects the current to increase with time.12a The fact that this is not observed may imply that there is a break in the polymer film during the growth process, resulting in an abrupt change in the electroactive area of electrode. We have proposed that the terminal pyrrole functional groups of the BPUS monolayer serve as nucleation sites for HDPy monomer species in solution by forming radical cations upon oxidation.10 At oxidative potentials positive enough to oxidize the HDPy in solution, the surfaceconfined cation radicals can then undergo a radical coupling reaction with the HDPy radical cations in solution, thereby forming a covalent bond and anchoring the monomer units to the electrode surface as seen in Figure 2. This mechanism is supported by the observation that the film morphology is very sensitive to the applied potential during the film growth, with the best films resulting when the pyrroles of the BPUS monolayer are fully oxidized. A poor brownish film resulted on the BPUSmodified surface if the applied potential was not sufficient to oxidize the surface-confined pyrroles. Even though the peak potential of the broad oxidation peak characteristic of surface-confined pyrroles10,17,18,23 was 1.18 V vs Ag under these experimental conditions, the best quality films were obtained with the potential fixed at 1.25 V during the film growth. At this potential the pyrroles of the monolayer are fully oxidized, thereby providing the maximum number (26) Shin, M.; Kim, E.-Y.; Kwak, J.; Jeon, I.-C. J. Electroanal. Chem. 1995, 34, 87.
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Figure 1. (a) Chronoamperogram of PHDPy growth at the BPUS-modified electrode surface. Potential was jumped from 0.3 to 1.25 V vs Ag quasireference. (b) Chronoamperogram of PHDPy growth at bare gold under the same conditions.
Figure 2. Schematic representation of chemical interaction between the surface-confined pyrroles of BPUS and the HDPy monomers in solution.
of nucleation sites for the HDPy species in solution. At potentials intermediate between 1.18 and 1.25 V, an intermediate quality of film resulted, presumably because only a portion of the surface-confined pyrroles are oxidized
resulting in limited nucleation sites. Also, it was not possible to form a smooth, shiny film on a BPUS-modified electrode surface if it had been electrochemically oxidized prior to potentiostastic electrodeposition of HDPy as has been noted for cyclic voltammetry.10 The resulting film morphology is the same as that formed on an unmodified gold electrode. EQCM. While it is true that viscoelastic effects do play a role in frequency changes when characterizing films by EQCM, it has been shown that EQCM is an effective tool when characterizing sufficiently thin films.19 There may be some ambiguity in interpreting the results where solvent and ion movement is concerned, but it suffices to say that there is a fundamental difference in the solvent/ ion transport for the films formed on the modified and unmodified electrodes as well as in the growth process of the films. Figure 3 shows the cyclic voltammogram and corresponding frequency change during the growth of poly(hexadecylpyrrole) on the modified gold electrode surface. There are immediately apparent differences in both the cyclic voltammogram (CV) and the frequency response when compared to the growth on the bare electrode (see
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Figure 4. Mass attributed to anion movement, mass attributed to solvent movement, and total mass change during film reduction for each successive scan.
Figure 3. (a) Cyclic voltammogram of PHDPy grown on a BPUS-modified gold electrode by cycling from 0.4 to 1.4 V vs Ag at 50 mV/s. (b) Corresponding frequency change during growth at the BPUS-modified electrode surface.
below). Both the polymer oxidation and reduction waves are considerably broader than those observed for the film grown on the unmodified electrode. In fact there are two peaks in the polymer oxidation region as discussed below. The frequency response consists of decreasing frequency in roughly even increments for each successive cycle during the polymer film growth, in stark contrast to that formed on the bare electrode surface. An increase in mass is first noticed at about 1.3 V on the first scan as was noted for the bare case. The mass continues to increase past the switching potential of 1.4 V until ∼0.96 V in the reverse scan. The mass then begins to decrease due to anions departing from the film to the solution upon reduction. In subsequent scans, the mass increases slowly starting at ∼0.52 V and continuing to ∼1.2 V in the forward scan because of the injection of anions upon oxidation. A rapid increase due to monomer oxidation and further film precipitation is observed from 1.2 V in the forward scan to ∼0.96 V in the reverse scan. The process repeats itself with reduction of the film resulting in a slow mass decrease from ∼0.96 V in the reverse scan to ∼0.52 V in the forward scan. For the film growth on the BPUS-modified surface, these trends are unambiguous because of the stability of the film. It is interesting to note that the mass loss in the region corresponding to anion and solvent departure from the film upon reduction decreases in almost identical
amounts for each scan, indicating growth of the film in even increments on the electrode surface with each successive cycle. The charge consumed during reduction of the film (0.960.4 V) was compared to the frequency change in this region. The mass change in this region arises from an anion leaving the film to maintain charge neutrality as well as the solvent responsible for solvation of the PF6anion.21b,23,27 In the first cycle 62.4 µC of charge was passed upon reduction. This corresponds to 9.4 × 10-8 g or 3.9 × 1014 molecules of PF6- ions. The frequency increase of ∼250 Hz represents a loss in mass of 2.15 × 10-7 g. Therefore, 1.21 × 10-7 g of mass change is due to the solvent. A solvation number of 4.56 is arrived at, which is in close agreement with the solvation number of PF6in acetonitrile as determined by the same means during the growth of polypyrrole.23 This is significant in that it implies a film with much less void volume in the initial stages of growth than for the film grown at the unmodified substrate. An interesting trend is observed during reduction of the films in the following scans. The amount of charge due to polymer reduction increases in identical increments of 120 µC (1.8 × 10-7 g) for scans 2-4. This illustrates that the film is growing in an ordered manner, with similar masses being deposited each cycle. However, the mass loss attributable to solvent leaving the film increases in increments of only approximately 2.2 × 10-8 g with each additional scan, as shown in Figure 4. This is due to the dense film structure of the PHDPy film on the modified electrode, prohibiting the flow of solvent into the film during the oxidation process. A picture emerges in which the anion is able to flow into the polymer backbone to maintain charge neutrality, while the solvent is confined primarily to the outermost region of the polymer film. The polymer film is so dense that the anion must travel into the polymer backbone unsolvated. In the fifth cycle only approximately 100 additional µC are due to anion leaving during the reduction process, indicating a slowing of the film growth process as will be discussed below. (27) (a) Naoki, K.; Lien, M.; Smryl, W. H. J. Electrochem. Soc. 1991, 138, 440.
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Figure 6. Frequency change during cycling of the film at a BPUS-modified electrode surface.
Figure 5. (a) Cyclic voltammogram of PHDPy grown on a bare gold electrode by cycling from 0.4 to 1.4 V vs Ag at 50 mV/sec. (b) Corresponding frequency change during growth at the bare gold surface.
Results of the analogous experiments for the film formed on the bare gold electrode surface are shown in Figure 5. As can be seen, the film does not grow in ordered, even amounts with each successive cycle as it does on the BPUSmodified electrode. In fact, after the first two cycles, the film is observed to deteriorate, with apparently little or no additional mass being deposited on the electrode. This is probably due to the fact that the nonadherent film is being detached from the surface simultaneously with film deposition, but there is a large degree of ambiguity associated with interpretation of any results for the film growth at the bare gold surface. As mentioned above, the polymer oxidation region for the film formed on the BPUS-modified substrate is much broader than that of the same region for the film grown on the unmodified substrate, and there are actually two oxidation peaks. The first peak current is at approximately 0.7 V. This first region ranges from approximately 0.4 to 0.8 V as marked in Figure 3a. The second polymer oxidation region starts at approximately 0.8 V and continues to the onset of monomer oxidation at ∼1.1 V, with an approximate peak current position of 1.0 V. Notice that this second region corresponds to approximately the same polymer oxidation region as for the HDPy electropolymerized at the bare gold surface. The two peaks
observed in the modified case correspond to two distinctly different types of polymer formation. The lower oxidation potential of polypyrrole compared to n-alkyl analogues has been attributed to better ring-ring planarity along the polymer chain.9a Therefore, we propose that the polymer giving rise to the oxidative current flow in region 1 has better ring-ring planarity than the polymer in region 2. Since there is no analogous oxidative current flow from 0.4 to 0.8 V for the film produced at the bare gold surface, this more coplanar film formation is attributed to the presence of the BPUS monolayer serving as nucleation sites for growth of the polymer. The controlled nucleation provides an ordered means by which the monomers can nucleate, thereby allowing them to maintain coplanarity, whereas the random nucleation process results in a polymer film that is distorted from coplanarity due to steric interactions. This is of practical consequence since the more coplanar film is known to have higher electrical conductivity.9 The amount of disorder and resulting decrease in the chain order of the polymer film is marked by a shift in the polymer oxidation potential of the first region upon continued cycling to higher oxidation potentials. This is noticed in Figure 3a as the “beginning potential” of region 1 is 0.65 V for the fifth cycle. The amount of additional polymer resulting from the fifth cycle is 20 µC less than that added in the 2-4 cycles as noted above. This is due to a 20 µC decrease in the amount of polymer formation in region 1. As we have noted many times, with additional cycling there is a successive shift to higher polymer oxidation potentials for the film on the modified surface. Films formed on the BPUS-modified substrate are observed to turn brownish in as few as eight cycles. From this we can infer that disorder in the film increases as the film is grown out from the controlled nucleation sites, with film quality eventually resembling that of the film grown at the unmodified surface. Long-term stability of electrically conducting polymer films is particularly important if practical applications of the films are to be realized. In light of this, the stability of the film on the electrode surfaces was then tested by cycling from 0.4 to 1.1 V in 0.1 M blank electrolyte solution. Figure 6 shows the frequency response for the film formed on the BPUS-modified substrate. There is no mass loss
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can be seen in Figure 7. Here, the film is almost completely removed from the electrode surface after only one cycle, with subsequent cycles resulting in even more film loss. Conclusions The electrochemical growth process of poly(N-hexadecylpyrrole) on both a gold electrode modified with a BPUS monolayer and a bare gold electrode was studied by EQCM. The surface-confined pyrrole units serve as specific nucleation sites for subsequent polymer growth as demonstrated by chronoamperometry. Mass changes at the electrode surface showed that the film formed at the bare gold surface was not stable, with dissolution of the film occurring even during the growth process. The film grown at the modified electrode surface was stable even upon repeated cycling. This behavior illustrated that a controlled nucleation process can improve the long-range order of electrochemically grown polymer films. In situ real time spectroscopic ellipsometry is currently being used to monitor the film growth and further elucidate the role of the nucleation process. Figure 7. Frequency change during cycling of film on the bare gold surface.
when cycling the film even after dozens of cycles. Reversible intake and expulsion of anion and solvent are observed with a mass change for both processes of 1.2 × 10-6 g, further indicating the stability of the film. This is in stark contrast to what is observed for the analogous experiment performed on the film formed on the bare gold surface, as
Acknowledgment. This work was supported from the School of Mines and Energy Development Grant and Research Grant Committee Fund at the University of Alabama. D.B.W. thanks the Alabama Space Grant Consortium, NASA Training Grant NGT-40010, for support. This paper is gratefully dedicated to the memory my coauthor and mentor, Yeon-Taik Kim. LA991439B
