Tailored Preparation of Quantum-Sized ZnS Nanoparticles by the Gas

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Tailored Preparation of Quantum-Sized ZnS Nanoparticles by the Gas-Phase Condensation Method J. C. Sa´nchez-Lo´pez, A. Justo, and A. Ferna´ndez* Instituto de Ciencia de Materiales de Sevilla and Departamento Quı´mica Inorga´ nica, Centro de Investigaciones Cientı´ficas Isla de la Cartuja, Avda. Ame´ rico Vespucio s/n, 41092-Sevilla, Spain Received December 15, 1998. In Final Form: June 21, 1999 The effects of collection geometry, vapor cooling, gas pressure, and the nature of the inert gas have been investigated with respect to particle size, morphology, and microstructure of ZnS nanocrystalline powders prepared by the gas-phase condensation method. The materials have been characterized by transmission electron microscopy, X-ray diffraction, and UV-vis absorption spectroscopy. It has been found that adequate control of the experimental conditions can produce powders of controlled mean particle size and morphology. This tailored nanocrystalline microstructure also determines the electronic characteristics of the materials, in particular, band gap absorption values.

Introduction Research on semiconductor nanostructured materials has become widely spread in recent years. In particular, ZnS, CdS, and also telluride compounds are some of the materials extensively investigated for the fabrication of optolelectronic devices.1-3 In addition, zinc and cadmium sulfides have been investigated as photocatalysts and for solar cells devices.4-6 The preparation of nanosized particles of ZnS and CdS has been approached by a large variety of wet chemical methods.7-10 Recently, in a previous paper11 we have described that the gas-phase condensation method (also called inert gas evaporation)12 can be used for the preparation of both cadmium and zinc sulfides as ultrafine powders constituted by aggregates of spherical nanoparticles. The gas-phase condensation method for the preparation of nanostructured materials12 consists of the evaporation of the material into an inert gas atmosphere so that through interatomic collisions with the inert gas atoms, the evaporated atoms lose kinetic energy and condense in the form of small crystals of a few nanometers in size. From previous studies, some important conclusions with respect to the mechanism of the inert gas evaporation method can be summarized: The formation of the particles proceeds by condensation of the vapor material, nucleation, and growth.13 Two main regions can be distinguished in the smoke formed during the inert-gas evaporation procedure:14 one (1) Tsai, C. T.; Chuu, D. S.; Chen, G. L.; Yang, S. L. J. Appl. Phys. 1996, 79, 9105. (2) Britt, J.; Ferekides, C. Appl. Phys. Lett. 1993, 62, 2851. (3) Rack, P. D.; Holloway, P. H. Mater. Sci. Eng., R 1998, 21, 171. (4) Hetterich, W.; Kisch, H. Chem Ber. 1989, 122, 621. (5) Chopra, K. L.; Das, S. R. Thin Film Solar Cells; Plenum Press: New York, 1983. (6) Gao, G. Q.; Deng. Y.; Kispert, L. D. J. Phys. Chem. B 1998, 102, 3897. (7) Kamat, P. V.; Patrick, B. J. Phys. Chem. 1992, 96, 6829. (8) Korgel, B. A.; Monbouquette, H. G. J. Phys. Chem. 1996, 100, 346. (9) Parvathy, N. N.; Pajonk, G. M.; Venkateswa, R. Nanostruct. Mater. 1997, 8, 929. (10) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (11) Sa´nchez-Lo´pez, J. C.; Reddy, E. P.; Rojas, T. C.; Sayague´s, M. J.; Justo, A.; Ferna´ndez, A. Nanostruct. Mater. 1999, 12, 459. (12) Gleiter, H. Adv. Mater. 1992, 4, 474. (13) Bassett, G. A. Proc. Eur. Reg. Conf. Electron Microsc. 1960, 270. (14) Yatsuya, S.; Kasukabe, S.; Uyeda, R. Jpn. J. Appl. Phys. 1973, 12, 1675.

is the so-called vapor zone, in the proximity of the evaporation source, showing a higher concentration of vapor material; the second region is around this first one and contains condensed particles in a higher concentration. However, it has been shown that inside the vapor zone, nucleation and growth can take place.15,16 Two mechanisms have been described to be responsible for the growth of the particles,17 absorption of vapor species (atoms or molecules), or coalescence of small particles. In the present paper we will take profit from these previous conclusions, to achieve a tailored synthesis with respect to the structural and morphological characteristics of ZnS ultrafine powders. In particular we have studied the influence of different experimental parameters (collection point, gas pressure, type of gas, etc) on the final properties of the materials. Quantum-sized particles of ZnS showing different band-gap values have been obtained. Experimental Section The experimental device used for the synthesis of the ZnS ultrafine powders is shown in Figure 1 (left). It consists of a small high vacuum chamber pumped to a residual vacuum better than 5 × 10-7 Torr. A tantalum boat containing a pellet of commercial ZnS (from Aldrich, ref no. 33,327-1) was heated resistively to a temperature of ca. 1373 K in an inert gas atmosphere. After evaporation the obtained powder was passivated by oxygen (3 Torr for 10 min) before exposure to air. The loose material was collected (smoothly stripped off) from two surfaces: one in direct contact with a liquid nitrogen reservoir situated above the evaporation source (see Figure 1, 2a) and a second one, which consists of a Cu foil cylinder surrounding the evaporation device (see Figure 1, 2b). In Figure 1 (right) we have also depicted the direction of the convection lines within our preparation chamber during evaporation in an inert gas atmosphere. The two regions, A and B, from which we collected samples are also marked in Figure 1 (right). Transmission electron microscopy (TEM) examination of the samples was carried out with a Philips CM200 microscope working at 200 kV. The samples were dispersed in ethanol by sonication and dropped on a copper grid coated with carbon film. Particle size distribution was evaluated from several micrographs (15) Oda, M. Dissertation (in Japanese), Nagoya University; H.U.T., Mita, 1986; p 115. (16) Saito, Y. Jpn. J. Appl. Phys. 1989, 28, 2024. (17) Shiojiri, M.; Kaito, C.; Fujita, K. J. Cryst. Growth 1981, 52, 168.

10.1021/la981718j CCC: $18.00 © 1999 American Chemical Society Published on Web 09/18/1999

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Figure 1. (Left) Experimental setup for the preparation of nanostructured materials by gas-phase condensation method: (1) thermal evaporator; (2a) cold trap; (2b) cold copper substrate; (3) valves; (4) pressure gauge. (Right) Scheme of the preparation chamber during an evaporation experiment; A and B indicate the zones from which samples were collected for investigation. using an automatic image analyzer. The number of particles selected for consideration was around 400, which resulted in stable size distribution statistic. The particle size is defined as twice the average radius measured from the center of the area to the perimeter of the particle. X-ray diffraction (XRD) analysis was carried out by using Cu KR radiation in a Siemens D5000 diffractometer. UV-vis absorption spectra were recorded for the powdered samples in the reflectance mode (R∞) and transformed to a magnitude proportional to the extinction coefficient through the Kubelka-Munk function (F(R∞)).18 Measurements were carried out in a Shimadzu UV-2102 PC spectrometer equipped with an integrating sphere accessory for diffuse reflectance, and BaSO4 powder was used as standard.

Results and Discussions A series of preliminary experiments determined the best geometry and temperature of evaporation for the systematic study of the effect of the experimental parameters. An evaporation temperature of 1373 K was selected because of good yields of nanocrystalline materials but without decomposition of the sulfide. By changing the distance between the evaporation boat and the liquid nitrogen reservoir, we found the best condition, attending to homogeneity and small particle sizes, at a distance of around 12 cm. This position and the chosen temperature were fixed for the following experiments. (a) Influence of the Collection Point. In a first experiment we studied the influence of the collection point on the microstructure of the obtained powders. The liquid nitrogen reservoir was filled, and the samples were collected directly on its surface (position A) and from several positions on the copper substrate (position B) at different distances h from position A (see Figure 1, right). Figure 2 shows the micrographs corresponding to samples collected at different places and obtained by evaporation in 3 Torr helium. The particle size of the materials increases when going from position A to the positions nearer the evaporation source. A statistical analysis was carried out, and the mean particle size and standard deviation values are summarized in Table 1. In addition, Figure 3 illustrates the particle size distribution histograms for the mentioned samples. It is found that together (18) Klier, K. Catal. Rev. 1967, 1, 207.

with an increase in particle size also a wider particle size distribution is obtained by going from position A to positions on the Cu substrate farther from the cold source and nearer the evaporation source. It should be also emphasized from the images in Figure 2 that a smaller particle size is accompanied by a higher degree of coalescence, while the bigger particles appear more isolated. The effect of increasing particle size depending on the collection position can be also concluded from the X-ray diffraction pattern of the powders. As shown in Figure 4, a broadening in the peak width is observed when the position of collection is located nearer the cold trap. For positions h ) 12.5 and h ) 16 cm the diffraction peaks are very similar in width as determined by adequate programs. Other evidence from the XRD analysis is the formation of a certain amount of ZnS (wurtzite), although the raw material was ZnS in the sphalerite phase. This phase is the stable form for bulk ZnS at room temperature and atmospheric pressure. It can be concluded that during the inert gas evaporation procedure, the conditions were achieved for stabilization of the wurtzite phase and some particles could be frozen with that structure (see Figure 4). In fact, the wurtzite phase has been reported in the literature19 as the high-temperature polymorph of sphalerite, forming at temperatures of around 1296 K. In Figure 5 are depicted the X-ray diffraction patterns for the asprepared ZnS nanocrystalline sample collected on the liquid N2 cooled substrate and after heating in a vacuum to 773 and 1023 K, respectively. The heating treatment produces the crystallization of the powder and allows us to distinguish between the sphalerite and wurtzite components of the sample. The content of wurtzite present was small and very similar in all the samples collected along the Cu substrate. Only a small increase in the amount of wurtzite could be detected for samples collected very near the evaporation source. (b) Influence of the Vapor Cooling Rate. The effect of vapor cooling rate can be studied by repeating the previous experience in the same conditions but without liquid nitrogen in the reservoir. The first important result was the absence of powder deposition on point A. Figure (19) Skinner, Barton Am. Mineral. 1960, 45, 612.

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Figure 3. Particle size distribution histograms for the three samples depicted in Figure 2.

Figure 2. TEM micrographs for ZnS samples as obtained by evaporation in 3 Torr He: (a) sample collected on the liquid nitrogen reservoir (zone A), (b) and (c) samples collected on the Cu substrate (zone B) at distances of 3 and 12.5 cm from position A, respectively. Container filled up with liquid nitrogen.

6 shows two representative micrographs of samples collected in the copper substrate (position B) at two different distances (h ) 10 and 13 cm) from point A and their corresponding particle size distributions. The morphology of particles collected at h ) 10 cm appears quite homogeneous and consists of small interconnected particles of ca. 12 nm in diameter. However, the sample collected at h ) 13 cm seems to be more heterogeneous in size distribution as compared to the previous one, showing also a high degree of aggregation. As can be seen in Table 2, these features are in good agreement with the

Figure 4. X-ray diffraction patterns for ZnS samples as obtained by evaporation in 3 Torr He and collected at different distances (h) on the copper substrate (see Figure 1, position B). For h ) 0 the sample was collected in position A (see Figure 1).

data obtained by statistical calculus on particle size and standard deviation for this experiment. At this point we can consider the three-step mechanism for the formation of the nanoparticles: condensation of the vapor material, nucleation, and growth. By comparison of the results obtained in sections a and b, the following points can be concluded with respect to the overall mechanism: (i) Nucleation occurs mainly at the edges of the evaporation boat where the vapor produced by the heating is cooled very quickly by the upward convection flow of the inert gas. The subsequent growth takes place by absorption of vapor species supplied through diffusion from

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Table 1. Mean Particle Size (D) and Standard Deviation (σ), As Determined by TEM, for the ZnS Samples Obtained by Evaporation in 3 Torr He and Collected at Different Points According to Figure 1a

a

h (cm)

D (nm)

σ

powder color

0 3 12.5

8 11 18

1.1 1.6 3.5

white white white

Liquid nitrogen filled reservoir during the experiments.

Table 2. Mean Particle Size (D) and Standard Deviation (σ), As Determined by TEM, for the ZnS Samples Obtained by Evaporation in 3 Torr He and Collected at Different Points According to Figure 1a

a

h (cm)

D (nm)

σ

powder color

0 10 13

no sample 12 13

2.0 3.8

white gray

No liquid nitrogen was used in this experiment.

Table 3. Mean Particle Size (D) and Standard Deviation (σ), As Determined by TEM, for the ZnS Samples Obtained by Evaporation under Different Helium Gas Pressuresa A P (Torr) D (nm) 1 3 45

5 8 15

σ 1.1 1.1 2.3

B

powder color D (nm) light yellow white white

7 11 27

σ

powder color

2.1 1.6 6.0

gray white black

a Samples were collected on position A and position B (h ) 11.512.5 cm) according to Figure 1. Trap cooled with liquid nitrogen during this experiment.

the vapor zone. Thus, the region situated above it (inner zone, see Figure 1 right), becomes deficient of the vapor atoms for the reason that most of them go outward due to the diffusion. Also the trajectory determines the particle size. Those particles that are not rapidly removed from the evaporation source can continue growing by absorption of new vapor material and later, along their flight up to get the substrate, by coalescence of colliding particles. The particles collected in the copper foil (position B) have undergone further growing, and their sizes are bigger than those collected on the position A and much bigger nearer to the hot evaporation source. As the temperature drops, the coalescence rate decreases leading to agglomerate formation. A more detailed description of nucleation and growth theory as well as population dynamics theory applied to the gas-phase condensation method is included in ref 20. (ii) The lack of liquid nitrogen in the corresponding container produces on one hand the absence of product in position A. This fact confirms the importance of the gas convection flows to remove the powder from the vapor zone increasing the deposition efficiency. Second, the vanishing of the cooling rate produces an increase in the particle size and agglomeration of the sample. The larger heterogeneity points out the importance of the convection flows to separate the particles according to their different sizes. (iii) We can also conclude that the smallest particles show a higher degree of coalescence through neck formation (see Figure 2, sample collected in point A). We explain this point by considering that the very small particles have a high surface energy and tend to coalesce forming chains of particles while the temperature is not elevated enough to lead to a complete coalescence.

Figure 5. XRD diffraction pattern from a ZnS sample obtained by evaporation in 3 Torr helium and collected in position A. Evolution of the diffraction pattern upon heating in a vacuum.

(iv) The log-normal distribution function describes a general growth phenomenon where any initial distribution which grows by accretion of smaller particles into larger ones will result in a broadened distribution with a tail extending into the larger particle region. This type of particle size distribution has been normally observed in these types of materials.21,22 (c) Influence of the Inert Gas Pressure. Another experiment consisted of the evaporation of zinc sulfide at T ) 1373 K but choosing different helium pressures in the gas phase (1, 3, 10 and 45 Torr). Table 3 includes the mean particle size and standard deviation data as obtained by TEM for the synthesis at 1, 3, and 45 Torr. Measurements were carried out for samples collected at point A and also for samples collected in region B at a distance (h ) 11.5-12.5 cm) from the liquid nitrogen reservoir. From the study of these results it is concluded that an increase in gas pressure produces an increase in particle size and heterogeneity of the samples. The X-ray diffraction analysis of the samples obtained under different He pressures shows an increase in the amount of wurtzite for the samples obtained under higher pressures. It has been also observed that at the higher pressures, especially in region B, the material can be partially decomposed to form metallic zinc. This produces a black color in the samples, indicative of the presence of zinc particles. In Figure 7 we have included the pattern obtained for two samples prepared at 1 and 10 Torr He after heating in a vacuum at 1023 K to get well-defined diffraction peaks. It is clear that the sample obtained at 10 Torr He contains a higher amount of wurtzite although the major phase continues to be sphalerite in all cases. Some conclusions can be outlined from the previously described experiment: (i) An increase in the gas pressure favors a confinement of the particles in the proximity of the evaporation source; whereas an increase in particle size can be achieved by absorption of atomic vapor or total coalescence between particles. However, once the particles grow to bigger sizes, later phenomena of coalescence are less likely because of the reduction of excess of kinetic and surface free energies. Hence, at high gas pressures the formation of strongly bound particles as a result of coalescence through neck (20) Flagan, R. C.; Lunden, M. M. Mater. Sci. Eng. A 1995, 204, 113. (21) Kaatz, F. H.; Chow, G. M.; Edelstein, A. S. J. Mater. Res. 1993, 8, 995. (22) Hass, V.; Birringer, R. Nanostruct. Mater. 1992, 1, 491.

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Figure 6. TEM micrographs for ZnS samples as obtained by evaporation in 3 Torr He. Samples were collected on the Cu substrate (zone B) at distances of 10 and 13 cm from position A. The container was not filled with liquid nitrogen. Table 4. Mean Particle Size (D) and Standard Deviation (σ), As Determined by TEM, for the ZnS Samples Obtained by Evaporation under Different Inert Gas Pressuresa He

A (0 cm) B (11.5-12.5 cm)

N2

Ar

D (nm)

σ (nm)

F

D (nm)

σ (nm)

F

D (nm)

σ (nm)

F

8 18

1.1 3.5

0.91 0.93

18 24

3.2 5.2

0.93 0.91

13 15

2.2 4.1

0.88 0.87

a Samples were collected on region A and region B (h ) 11.5-12.5 cm) according to Figure 1. Trap cooled with liquid nitrogen during this experiment.

formation was not observed and the particles appear more isolated. (ii) The conversion to the wurtzite phase and the decomposition of the zinc sulfide to zinc metal are favored

at the highest pressures. These effects are related to the temperature distribution in the gas phase. At constant volume, higher inert gas pressures lead to higher temperatures in the vapor phase due to increasing collision

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Figure 7. XRD diffraction pattern of ZnS samples obtained in 1 and 10 Torr helium after heating in a vacuum at 1023 K. Samples were collected on the Cu substrate (position B). Table 5. Relative Band Gap Values, E (eV), As Obtained by UV-vis Absorption Spectroscopy, for Different ZnS Samples Obtained in 3 Torr He and Collected at Different Distances (h, see Figure 1) Measured from the Surface of the Filled Liquid Nitrogen Reservoir h)0 cm

h)3 cm

h)8 cm

h ) 12.5 cm

h ) 16 cm

commercial

3.70

3.71

3.69

3.67

3.61

3.55

events, and these severe conditions help both processes to happen. In addition to that, it has been shown previously23 that sulfur deficiency is favoring the sphalerite to wurtzite transformation. (d) Influence of the Nature of the Inert Gas. During the last series of experiments samples were prepared by evaporation in 3 Torr of nitrogen and argon. The TEM images of the samples collected in position A are represented in Figure 8 and may be compared to the sample obtained under the same conditions in the presence of helium in the gas phase (Figure 2a). Table 4 summarizes the morphological characteristics of the particles obtained under 3 Torr of the three gases (He, N2, and Ar) and deposited on both substrates (regions A and B). The largest particles and standard deviations are produced in a nitrogen atmosphere while He and Ar lead to smaller particle sizes but with a higher degree of coalescence. Some conclusions can be obtained from these results: (i) The nature of the gas employed during inert gas evaporation influences the morphological characteristics of the obtained materials. However, there is not a direct relationship between mean particle size and the molecular weight of the inert gas such has been proposed in previous studies.24,25 Its effect must be a balance of several parameters such as diffusion coefficient, thermal conductivity, molecular weight, molecular diameter, density, etc. (ii) The empirical study presented here reveals that helium is the most adequate gas to obtain the smallest particle size together with a higher interconnection between particles while nitrogen is the gas that produces smaller coalescence between particles together with the highest mean particle size. Attending to their thermodynamics properties, helium is the inert gas with highest thermal conductivity, diffusion coefficient, and mean free (23) Barnes, Scott Geochim. Cosmochim. Acta 1972, 36, 1275. (24) Kato, M. Jpn. J. Appl. Phys. 1976, 15, 757. (25) Hahn, H. Nanostruct. Mater. 1997, 9, 3.

Figure 8. TEM micrographs for ZnS samples obtained by evaporation in 3 Torr N2 (a) and Ar (b). Both samples were collected on the liquid nitrogen reservoir (position A). The experiments were carried out with a cooled liquid nitrogen trap.

path values while nitrogen and argon are rather similar but the former has a lower mean free path because of its greater molecular diameter. (e) Effect of the Microstructure on the UV-vis Absorption Behavior of the Samples. The abovedescribed morphological characteristics of the ZnS ultrafine powders may also be reflected in the optical properties of the materials. In particular, band gap absorption from nanometric particles may show quantumsize effects. A blue shift has been described to appear in small semiconductor particles.26-27 In Figure 9 we have plotted [F(R∞) hν]2 (where hν is the photon energy) against hν for different ZnS samples obtained in 3 Torr He at different distances from the cooled trap as compared to that of a commercial microcrystalline ZnS powder. For direct interband transitions the linear portions of this curve can be extrapolated to absorption equal to zero28 leading to relative band gap values. The obtained values calculated by extrapolation of curves in Figure 9 are summarized in Table 5. For the microcrystalline commercial sample a value of 3.55 eV has been measured. The highest band gap shifts have been obtained for the samples collected closer to the cold trap that correspond to those with smaller particle size as we have measured by TEM (see Figure 2). There are two main contributions to the band-gap shift: the first term corresponds to the location energy of the electron-hole pair spatially confined in a three-dimensional spherical potential well while the second term corresponds to a shielded Coulomb interaction between the electron and the hole. The location energy (26) Weller, H. Adv. Mater. 1993, 5, 88. (27) Brus, L. J. Phys. Chem. 1986, 90, 2555. (28) Zhang, D. H.; Brodie, D. E. Thin Solid Films 1992, 213, 109.

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Figure 9. UV-vis absorption spectra in the band-gap region for ZnS nanocrystalline powders prepared in 3 Torr He. Samples were collected directly on the cooled trap (position A) or along the Cu substrate (position B) at different distances (h) from the cooled trap. Also a commercial ZnS has been included as reference.

increases the value of the shift inversely to the radius squared while the Coulomb interaction decreases it by R-1. Therefore, as the particle size decreases the quantum-confinement term is getting more significant and leads to an increment in the energy of the band-gap transition as really observed in Figure 9. It should be emphasized here that our preparation method produces materials in which each individual nanoparticle is a crystalline single domain. This effect can be observed in the high-resolution TEM image depicted in Figure 10 for the sample obtained in 3 Torr N2 atmosphere and collected in region B. It should be mentioned here that the presence of the wurtzite phase might produce an increase in the band gap value in comparison to pure sphalerite ZnS phase. However, we have found a continuous blue shift by decreasing particle size, and this effect is not related to an increase in the amount of the wurtzite phase. This phase is always minor and only reaches significant content for samples with higher particle sizes. This would produce an opposite trend to the observed one due to particle size effects. Finally, we have included in Table 6 the relative band gap values obtained for samples collected in positions A and B at different He pressures. It is obvious from this table how the increase in mean particle size is also accompanied by a decrease in the band-gap absorption energy for samples A. Samples B appear however always more heterogeneous in size and more agglomerated so that the band gap values show almost no changes up to 45 Torr He where the presence of a small amount of metallic zinc phase in the sample would produce a decrease

Figure 10. High-resolution TEM micrograph for ZnS nanoparticles obtained by evaporation in 3 Torr N2 and collected in region B. Table 6. Relative Band Gap Values, As Obtained by UV-vis Absorption Spectroscopy, for Different ZnS Samples Obtained in Different Pressures of Helium and Collected from Positions A and B (h ) 11.5-12.5 cm)a E (eV) 1 Torr

3 Torr

10 Torr

45 Torr

3.73 3.68

3.70 3.69

3.69 3.68

3.66 3.63b

A B

a Trap cooled with liquid nitrogen during this experiment. sample contains metallic zinc.

b

This

in the band-gap absorption energy that does not correspond to a particle size effect. Conclusions In the present work we have shown how the inert gas evaporation method can be useful to prepare ultrafine powders of nanocrystalline zinc sulfide. According to the preparation conditions, samples can be obtained showing different mean particle sizes and degrees of aggregation. The tailored microstructure also gives a variation in bandgap values for the different samples according mainly to quantum-size effects. Acknowledgment. The authors thank the DGES (Projects PB96-0863-C02-02 and HA1997-0055) for financial support. LA981718J