J. Phys. Chem. 1993,97, 13767-13772
13767
Growth of CdS Particles in Cadmium Arachidate Films: Monitoring by Surface Plasmon Resonance, UV-Visible Absorption Spectroscopy, and Quartz Crystal Microgravimetry Norman J. Ceddes,Robert S. Urquhart, D. Neil Furlong,' Christopher R. Lawrence,+ Kentaro Tanaka,t and Yoshio Okahatas CSIRO, Division of Chemical and Polymers, Private Bag 10, Rosebank MDC, Clayton, Victoria 31 68,Australia Received: July 7 , 1993"
The growth of Q-state CdS particles during exposure of Langmuir-Blodgett films of cadmium arachidate to hydrogen sulfide has been monitored by surface plasmon resonance (SPR), UV-visible absorption, and quartz crystal microbalance (QCM) measurements. The growth kinetic profile, expressed in terms of increasing refractive index (SPR), absorption or mass increase (QCM), was the same when films were gassed under similar conditions. Microgravimetry showed the quantitative conversion of Cd2+to CdS in such systems; UV-visible absorption exhibited the blue shift typical of Q-state particles of 2-3-nm diameter. SPR reflectance curves obtained after H2S treatment were initially fitted assuming that either the film thickness or the real component of the relative permittivity (ET) was constant during Q-state particle formation. Film thickness changes (assuming cr was constant) were similar to those previously reported. Relative permittivity changes, determined assuming the film thickness was constant, indicated that for thinner films (Le., fewer than about 10 layers) effects such as film breakup during particle formation and H2S exposure to the gold surface of the SPR crystal were important. The change in e, for a 20-layer cadmium arachidate film after extensive gassing was calculated using Maxwell-Garnett theory. Use of this value in the fitting routine indicated that the film thickness change during particle formatior. was 0.13 nm/layer or 4.9%.
Introduction In recent years, considerable interest has been shown in synthesizing ultrasmall or so called Q-state semiconductor particles (which are typically less than 10 nm in diameter) due to their interesting optical and redox properties. These particles have potential application in new technologies such as nonlinear optics.' To date they have been produced in various media, including homogeneous solution (with the particles stabilized by organic or inorganic polymers),2~eolites,~ gla~ses,~ and surfactant systems such as reversed micelles,5 vesicles: air-water monol a y e r ~ ,and ~ - ~Langmuir-Blodgett (LB) filmsS9-l2A common feature of all such systems is the "control" of particle growth needed to ensure that particle size does not exceed the Q-state dimension. An added advantage of LB films is that particles can be coated onto a substrate for incorporation into a device. Q-state semiconductor particles in LB films have been studied by a number of techniques such as UV-visible and infrared spectrometry,10J2JX-ray photoelectronspectroscopy,I electron microscopy,'O and ellipsometry.I2 Experimental studies during particle formation have not been common, and hence descriptions of the mechanisms of particle growth in the film, or of the control of particle size in the film,I0are not well developed. This paper describes results from surface plasmon resonance (SPR) measurements obtained during the formation of cadmium sulfide Q-state particles formed by gassing cadmium arachidate (CdAr) LB films with hydrogen sulfide. SPR measurements provide information on film refractive index and thickness. The technique of surface plasmon resonance (SPR) has become widely accepted for the evaluation of the optical properties of thin films, where such films have first been deposited on an appropriate metal surface.IsJ6 Due to SPR's inherent high sensitivity,it is currently being utilized in areas like gas adsorption (to measure changes To whom correspondence should be addressed. t Permanent address: Thin Film and Interface Group, University of Exeter,
Stocker Road, Exeter, Devon, EX4 445, UK. 8 Permanent address: Dept. of Biomolecular Engineering, Tokyo Institute of Technology, Tokyo 152, Japan. *Abstract published in Advance ACS Abstracts, November 15, 1993.
in the bulk properties of thin films) and biosensing (to monitor processes such as the binding of analytes to receptor layers).17J8 In the present study, the SPR measurements are complemented by UV-visible absorption spectroscopy and quartz crystal microbalance (QCM) gravimetry.
Experimental Section Materials. Cadmium chlorideand potassium bicarbonate were analytical grade reagents from BDH and were used as supplied. Arachidic acid was puriss grade (Fluka) and was used without further purification. Milli-Q-filtered water (which had a conductivity of less than 1 rS cm-I and surface tension of greater than 71.7 mN m-1 at 25 "C) was used to prepare monolayer subphases. Chloroform (Ajax, Spectrol grade) was used to prepare monolayer spreading solutions and was used to wash glass prisms and QCM electrodes before use. The Langmuir trough was cleaned with methanol, dichloromethane, and hexane (BDH or Ajax, HPLC grade) before use. Quartz plates, made ofSupraci1glass (H.A. Grois Ltd.), andQCM electrodes (Kyushu Dentsu Co.) were used as the substrates for UV-visible and mass uptake measurements, respectively. Before LB coating, quartz plates were hydrophobed in a 10% v/v solution of dichlorodimethylsilane (Aldrich, 99%) in hexane according to the method of Trau et al.I9 Hydrogen sulfide, obtained from Matheson Gas Products Inc., was used as supplied. Equilateral glass prisms (H.A. Grois Ltd.) of refractive index 1.65 were used as the substrate for SPR measurements. A thin gold film was deposited on one face by sputtering in a 2-Pa atmosphereof argon at a rate of 0.8 nm/s to yield a final thickness of about 38 nm (as measured by a quartz crystal microbalance). Prior to the LB deposition of cadmium arachidate, the real and imaginary components of the relative permittivity ( t r and ti), as well as the exact thickness (d) of the gold film, were determined by SPR. Methods. Surface Plasmon Resonance (SPR). The surface plasmon, a coupled electromagnetic field charge density oscillation, can exist at a metal-dielectric interface. It is resonantly
0022-3654/93/2097-13767$04.00/0 @ 1993 American Chemical Society
Geddes et al.
13768 The Journal of Physical Chemistry, Vol. 97, No. 51, 1993 I20
. 3 #J
intensity I
3
100
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60 . DIELECTRIC (high refractive indea: glass prism)
- METAL (air or aqueous subphase)
-DIELECTRIC EO
F i p e 1. Schematic diagram of the attenuated total reflection configuration used in the SPR measurements where Bi and 8, are the internal and external angles of incidence, respectively.
excited by p-polarized light once conditions for momentum conservation are satisfied, i.e., when the surface wave vectors of the incident radiation and the surface plasmon waves are matched. The matching configuration used in the present study is the Kretschmann20technique of attenuated total reflection. A metal film is sandwiched between two dielectric media of permittivities (2 and €0 where co < €2, as shown in Figure 1. The medium of high permittivity ( 4 consists of a high refractive index glass prism, while the medium of lower permittivity (€0) is gas (air) or an aqueous solution. In a SPR measurement,the system is excited by monochromatic ppolarized light of wavelength A. The intensity of the reflected light (ZR)from the system is monitored as a function of the internal angle Bi as shown in Figure 1. The reflected light intensity from the system reaches a minimum at internal angle OsPp, where Os, is given approximately by the expression
where el, is the real part of the permittivity of the metal film. The presence of a Langmuir-Blodgett film on the metal surface or changes in the optical properties of such a film will modify the value of co and therefore cause a change in the angle Bspp at which the reflected light intensity is at a minimum. A full analysis of the system, using Fresnel's equations applied to a multilayer system, shows that the value of Bspp and the variation of reflected light intensity with internal angle (particularly around the angle Bspp) are dependent on the optical properties of film. When the experimental angular dependence of reflected light intensity is fitted to that predicted by Fresnel's equations, the relative permittivity and thickness of the film can be obtained. LB films of cadmium arachidate were deposited directly onto the gold film of the SPR prism. The prism was placed on a rotating table geared so that the detector moved at twice the angular speed of the prism (to ensure that the reflected beam from the gold/prism interface could be monitored as the angle Be wasvaried). The prism was illuminated with p-polarized light from a He-Ne laser (A = 632.8 nm, mechanically chopped at 1.7 kHz). A beam splitter reflected a small percentage of the incident beam onto a reference detector to allow for fluctuations in laser intensity. The signals from the prism and reference were monitored by phase-sensitive detectors which used the chopper signal as their phase input. The DC outputs from the phasesensitive detectors were then stored on computer. Reflectivity values were calculated by dividing the signal from the prism by the reference. External angles of incidence (e,) were recorded and converted to internal angles within the prism (ei), and corrections were made for reflection at the entrance and exit faces of the prism. Reflectance curves were then fitted to Fresnel theory assuming an idealized "layer model", Le., perfectly uniform isotropiclayers on a substrate with perfectly flat interfaces. These
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total gassing time I minutes Figure 2. QCM mass uptake versus total H2S gassingtime for a 20-layer cadmiumarachidatefilm. The line (a) correspondsto the value expected for complete conversion of cadmium ions in the film to CdS by cq 1. fits provided estimates of the real (e,) and imaginary (ci) components of the film relative permittivity as well as its thickness
(4. Quartz Crystal Microgravimetry (QCM). The experimental system is described in detail elsewhere?' A 9-MHz signal is used to *drive" gold QCM electrodes that sandwich an AT cut quartz crystal. The resulting oscillation frequency is measured on a Iwatsu frequency counter (Model SC-7201) and recorded on a Epson computer. Changes in the frequency of the QCM electrodes (AF in Hz) were converted to mass changes (Am in ng) by use of the equation Am = -0.8 AFSz2 UV-Visible Absorption. Measurements were recorded using a Hewlett-Packard 845 1A diode array spectrophotometer. LBcoated quartz plates were secured in a specially designed Teflon holder which was fitted into a standard UV-visible absorbance cell. This experimental arrangement ensured that the quartz plate was at right angles to the beam of the diode array and that the area of the plate illuminated by the beam was the same during a given experiment. The absorbance contributions due to the quartz plate and the LB film have been subtracted out in the spectra shown in the figures. All the spectra shown are the results of the subtraction of single-wavelength scans of the LB-coated plates before and after gassing. Film Fabrication/Generationof CdS Particles. LangmuirBlodgett (LB) filmsof cadmium arachidate (CdAr) were prepared from a precursor monolayer formed by spreading a 2.5 X le3 M arachidic acid solution in chloroform on a subphase containing 4 X 10-4 M CdCl2 and 4 X 10-4 M KHCO3 (pH = 7.1 i 0.1 at 21 "C). The horizontal dipping technique was used to coat CdAr films ontoequilateral glass prisms (for SPR measurements),while vertical dipping was used for the gold QCM electrodesand quartz plates (for microgravimetryand UV-visible absorbance spectra). Transfers were performed at a monolayer pressure of 21 mN m-1 with a dipping rate of 0.5 cm min-I. There was a 15-min delay between dips. Films were stored for 24 h under desiccated conditions before further characterization. LB-coated substrates were placed in a 250-cm3sealed cell and repeatedly exposed to a flowing stream of H S for various periods of time. After each exposure, the properties of the gassed CdAr layers were investigated. The time periods shown in the figures correspond to the total time the LB films were in contact with HIS gas. Results and Discussion
Quartz CrystalMicrogravimetryand UV-Visible Spectrcwcopy. Figure 2 shows the mass uptake on a QCM crystal coated with 20 layers of cadmium arachidate on repeated gassing with H2S. There is an abrupt increase in mass during the first 10 min or so; thereafter the total mass approaches constancy. The limiting mass at 90-100 ng corresponds very closely to the 93.4 ng (as indicated by line a in Figure 2) that is predicted if all the Cd2+
The Journal of Physical Chemistry, Vol. 97, No. 51, 1993 13769
Growth of CdS Particles in Cd Arachidate Films
0.08
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wavelength / nm Figure 3. UV-visible absorbance s p t r a of Q-state CdS particles in a 20-layer CdAr film recorded during repeated H2S gassing treatments. Contributions to the absorbance due to the quartz substrate and the LB film have been subtracted. Total gassing times for lower to upper spectra are 15 s, 30 s, 1 min, 2 min, 5 min, 10 min, and 20 min, respectively. The inset shows the absorbance of the Q-state particles (after subtraction of the absorbancedue to the substrate and the LB film) versus total gassing time for selective wavelengths.
30
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Internal Angle (degrees)
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-
[CH,(CH2)l,CO~]2Cd*+ + H2S 2CH,(CH2)l,C02H
+ CdS
(1)
The number of Cd2+ions in the film was calculated using the film mass of 2016 ng determined by QCM measurements. This mass corresponds to an LB transfer ratio of 0.96 (calculated using the area of the QCM gold electrode (0.318 cm2) and the area of 0.185 nmz per Cd0.5Armolecule at 21 mN m-I, as determined by surface pressure-area measurements). Figure 3 shows the UV-visible absorbance spectrum recorded during the repeated gassing of a quartz plate coated with a 20layer CdAr film. The QCM crystal and quartz plate were gassed simultaneously, and so both LB films were subjected to the same gassing conditions. Absorbance from the substrate and the LB film have been subtracted, so only the absorbance changes due to particle formation are shown in Figure 3. The onset of CdS absorption (determined as the wavelength at which linear extrapolations of the initially steeply rising portion of the CdS absorption spectrum and the background absorbance/scatter intersected) does not shift appreciably during repeated gas exposuresand for all measurements was in the range 449 f 6 nm. This agrees well with the value of about 450 nm determined in other LB studies of this system.l2 The absorption spectra were compared to the CdS absorption property-particle size relationships of Henglein2and Wang.z3 The inflection points of the CdS absorption spectra (determined by finding the minimum in the first derivative of the absorption spectra) were in the range 422 & 5 nm. The exciton energies throughout H2S gassing (determined by finding the minimum in the second derivative of the absorptionspectra) were in the range 3.08 0.05 eV. Comparison of these results with the exciton energy-particle size data of Wangz3and the absorption inflection point-particle size data of Hengleinz indicated that the CdS particles (if it is assumed they are spherical) are between 2 and 3 nm in diameter. The inset to Figure 3 shows the absorbance of the Q-state CdS particles at three chosen wavelengths versus total H2S gassing time. The absorbancebehavior mirrors the mass uptake behavior shown in Figure 2 in that there is a marked change in absorbance atshort totalgassing times whileafterabout 10mintheabsorbance is fairly constant. Thesimilarity between theabsorbance behavior and the mass uptake data suggests that the reaction of HzS with the CdAr film causes the direct conversion of cadmium ions in the film to Q-state CdS particles. This process is consistent with
*
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Number of layers Figures. Film thicknessversus number of cadmium arachidate LB layers prior to (crossed points) and after 70 min (square points) of exposure to H2S. Film thicknesses were calculated assuming the value of e, for the LB film was equal to 2.402 for all measurements. The linear fits to the data beforeandaftergassingareshownin the figureand yieldedequivalent monolayer thicknesses of 2.67 and 2.95 nm, respectively,before and after gassing.
eq 1 and continues during repeated gassings until all the available cadmium ions in the film are converted to CdS. Surface Plasmon Resonance (SPR) Measurements. Growth of CdS Particles. Before characterization of the CdAr layers it is necessary to determine the optical constants of the gold film, as these are required in the fitting routine for the CdAr film. For the gold film, it is possible to independently determine the real (e,) and imaginary (ti) components of the relative permittivity, as well as the film thickness (d), by taking a full plasmon curve which includes the critical angle.24 Figure 4 shows the experimental reflectivity data and theoretical fits for plain gold. In the case of a LB-coated substrate it is not possible to obtain a unique solution to the three parameters, as the presence of the surfactant film on the surface merely perturbs the resonance observed for the bare gold substrate. (Similar difficulties are encountered when very thin films such as these are measured using ellipsometry.19) Therefore, for the initial fit of the reflectivity data a value of 2.402 was assumed for e, of the CdAr film (chosen from previous work for p-polarized lightz5),and d and ei were allowed to vary to obtain the best fit. Figure 4 shows the experimental and fitted curves for the gold with four CdAr layers and the shift after 70 min of exposure to HIS. The inset in Figure 4 gives the calculated values for e,, ti, and d, obtained from a minimizationof a least-squares fit between the experimental data and theory. Figure 5 shows the values of d obtained, using this method to fit the SPR data, for CdAr LB films of various thickness
1
Geddes et al.
13770 The Journal of Physical Chemistry, Vol. 97, No. 51, 1993
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Internal Angle (degrees) Figure 6. Comparison of theoretical fits (solid lines) to the reflectivity data (crossed points) for the case when (a, top) e, and ti are fixed and d is allowed to freely fit and when (b, bottom) only e, is fixed with both ti and d freely fitting. The inset in each figure shows the theoretical fit parameters describing the solid line in each figure for a four-layer film after 5 5 min of exposure to H2S. (before gassing with H2S). An average thickness of 2.67 nm per CdAr layer is obtained from Figure 5 . This value is in good agreement with the accepted value of 2.68 nm.25 There are various approaches that can be adopted to fit reflectivity data obtained after gassing of the CdAr layers. In a previous ellipsometricstudy,I2the value of ei was set to zero and a value chosen for e, that was invariant during the fitting of the reflectivity data. The experimental/theoreticalfit was then used to estimate thickness changes in the filmcaused by gassing. Figure 6 shows such a fit for our four-layer CdAr film exposed to H2S for a total of 55 min (the value of e, has been fixed at 2.402). The fitted parameters are given in the insets in Figure 6. Visually, a very poor fit to the experimental reflectivity data is obtained. A better fit, with a tenfold reduction in the least-squares error, is obtained when ci is allowed to freely fit. A slight reduction is obtained for the value of d. It will be shown later that ei changes dramatically upon gassing of the CdAr films, probably due to scattering effects. It seems then that the restriction of ti to a zero value is of questionable validity. Two alternate approaches wereevaluated. First, e, was fixed (at the value of 2.402 in our case), and d and ci were allowed to vary due to gassing (that is, interaction of the LB film with H2S). Second, d was fixed at the value calculated prior to gassing (using e, = 2.402 for the ungassed LB film), and then e, and ei were allowed to vary due to gassing. Figure 5 shows the values of d obtained from the first option. The data were obtained after gassing for a total of 60-70 min, by which time there appeared to be no further change in the reflectivity data with gassing. A linear fit indicates that the average thickness change per layer after CdS particle formation (assuming e, does not change) is approximately 0.3 nm. At first sight this value
A , A E
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40 60 80 100 Time (minutes) Figure 7. The change in (a, top) cr and (b, bottom) ti versus time during CdS particleformationfor different thickness of CdAr layersfor successive exposures to H2S.
0
20
seems low, given the growth of 2-3-nm particles in a film starting at 2.67 nm per layer. A similar change in thickness was obtained from previous ellipsometricmeasurements.12 In this study,12the small changes in film thickness during H2S gassing were rationalized by proposing that the CdS particles were disk shaped, with the larger dimension in the plane of the film layers. More discussion on this point is given below. Parts a and b of Figure 7 show the changes in e, and ei, respectively, for the second fitting option described above, that is, when d is held as a constant during the fitting of the reflectivity curves after each successive exposure to H2S. The changes in either €,/ti or d (depending on which parameters are assumed to be constant in the fit) are essentially complete after about 5 min of gassing. In fact, the majority of thechangeoccursvery quickly, within the first 10sofexposure. Permittivityvaluesareessentially constant for cumulative exposure times of 10-70 min. These kinetics are in qualitative agreement with the CdS particle growth behavior observed by UV-visible and QCM measurements. The largest changes in permittivity occur for the thinnest films. For a two-layer film, e, increases by about 0.4 compared to 0.17 for 20 layers. The difference in ei is even more dramatic, 0.3 compared with only 0.02. It should be noted that changes in ei can originate from both absorption and scatter in the film. The large increases in e, for the thinner layer films could be associated with film disruption during particle growth. For the two-layer CdAr film, the size of the Q-state CdS particles (assuming they are spherical) is about half the thickness of the original surfactant film. Hence, disruption would not be unexpected and would probably give increased optical scatter. The disruption induced by particle formation may generate sufficient film heterogeneity to invalidate the modeling assumption used to fit SPR data (that is, parallel interfaces for each component of the SPR system). It
The Journal of Physical Chemistry, Vol. 97, No. 51, 1993 13771
Growth of CdS Particles in Cd Arachidate Films
R
The value of t, for the CdAr films after gassing can be estimated by an “effectivemedium model”,in which the dielectric constants of CdS and arachidic acid (ArH) are used to calculate an effective dielectric constant ecff for the composite film. For the particular system under study, the Maxwell-Garnett (MG) m0del26.2~is applicable since it deals specifically with composites in which particles (of CdS) are dispersed through a host medium (ArH). This model predicts that
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Internal Angle (degrees) Figure 8. Comparison of thearetical fits (solid lines) to the reflectivity data (crosses) for ‘before” and ‘after” exposureof plain gold to H2S. The inset shows the values obtained for the gold parameters, for two separate gold films, (1) for 5 min and for (2) 5 and 55 min of exposure. is likely that disruption would cause the greatest relative heterogeneity for the thinnest films. It is also possible that the larger changes in relative permittivity for the thinner films may result from diffusion of H2S through the CdAr film, leading to direct exposure of the gold. If this effect is not taken into account prior to fitting (i.e., by adjusting the gold parameters appropriately), then the fitted values of e, and ei would show much larger changes as the LB films were subjected to H2S gas. Figure 8 shows the plasmon curves for fresh gold and gold after 5 min of H2S gassing. A small shift in the plasmon minimum is observed, which alters the gold parameters slightly (see Figure 8, inset). Gassing of the gold films had little effect on the fitted values of ei and d but did change the value of e, slightly. In the case of a gold film exposed to H2S gas for a total of 55 min, the value of e, increased 0.44 from its original value. The reflectivity data for 2 and 20 CdAr layers after 55 min of exposure to H2S were refitted with the e, of gold increased 0.44 from the value used in the original fit. The value of er for the 20-layer film was only 0.04 less than the value obtained when it was assumed that H2S did not interact with the gold substrate. The thinner film showed a marked change as the fitted value of e, was slightly below that of the initial ungassed film value of 2.402 when gold/HzS interactions were considered. The presence of a LB film on the gold surface would be expected to reduce such H2S/gold interactions since the LB film provides a barrier which the gas must penetrate before interaction with the gold surface can occur. However, it may still be the case that for the thinner films, where breakup of the CdAr layers may occur due to particle growth, gold exposure to H2S may occur. It is difficult to take account of this in the modeling of the layers, but it may indicate that the greater changes in the fitted dielectric properties of the thinner films are caused in part by this change to the gold. Due to this effect, as well as the smaller apparent contribution of film breakup to the SPR curves of the thicker films studied, it appears that the SPR data for the thickest films (particularly those of 20 layers) may be considered to give a truer reflection of the changes in the properties of the CdAr layers due to CdS particle formation. Effective Medium Model. As mentioned above, SPR measurements as described so far cannot independently elucidate all three of the e,, ti, and d parameters for the LB films of interest. For ungassed CdAr films it is reasonable to assume that e, is constant with film thickness, since there is no chemical change in the film as layers are added. However, the reaction of H S with CdAr during gassing clearly produces other chemical species which have different optical constants. Thus, values of the optical parameters would vary during the gassing process. Moreover, the film thickness may also change during gassing.
wheref w represents thevolume fraction occupied by the particles. The model assumes that fCds is relatively small (such that the optical properties of the CdS merely perturb those of the host, rather than dominating them) and that the particles are distributed isotropically throughout the medium. Values of the permittivities and the volume fraction of CdS in the film were estimated so that a value of tefffor the mixed CdS/ ArH system after gassing could be evaluated. Martin and Sambles2*found that the optical properties of arachidic acid layers are best described with a biaxial model, with exx = 2.31 f 0.01, eyy = 2.32 f 0.01, and ezr = 2.44 f 0.01 representing the three components of the dielectric tensor (the r-direction lying normal to the substrate). As the dielectric properties of Q-state CdS particles are not known, the bulk dielectric properties of CdS were used. Bulk CdS is a uniaxial crystal with the components of the dielectric tensor being exx = eyy = 6.28 and e,, = 6.40.29 The Maxwell-Garnett model assumes an isotropic medium, and therefore average values of the ArH and CdS dielectric tensors were used, giving values of CA,H = 2.36 and CCdS = 6.32. UV-visible absorption spectra (section 1) indicate that the CdS particles are between 2 and 3 nm in diameter. If it is assumed that they are spherical with the same density as bulk CdS and have a radius of 1.25 nm, then the mass of one particle can be calculated. The cadmium can only originate from the CdAr molecules in the film. Quantitative conversion, as shown by quartz crystal gravimetry, determines the ratio of CdS particles to arachidic acid molecules as 1:329. The thickness of an arachidic acid layer in a Langmuir-Blodgett film has been determined by wave guide studies to be 2.42 f 0.02 nm while the molecular area (from surface pressure-area isotherms at 21 mN m-I) is 0.200 nm2.30 With these dimensions, the volume fraction of a CdS particle in 329 molecules of arachidic acid is 0.0489 (Le., 4.89% by volume of the film was CdS). This value was found to be independent of particle size and shape. With €CdS = 6.32, CA,H = 2.36, and fcds = 0.0489, the effective dielectric constant of the CdS/arachidic acid composite was calculated to be 2.486. This value showed very little variation when the values of €CdS, ~ A ~ Hand , fCds were changed within reasonable limits. The results in Figure 7a show that the value of er obtained for a 20-layer CdAr film after extensive gassing (assuming Ad was constant) was 2.572. This value corresponds to an e, change of +0.170 after gassing. As the fitted value of e, (assuming Ad = 0) is quite different from that estimated by MG theory, it is likely that there is some change in film thickness during Q-state particle formation. SPR curve for the 20-layer CdAr film after 55 min of exposure to H2S (which showed minimal effects due to processes such as exposure of the gold surface to H2S and LB film disruption) was refitted, allowing d and ei to vary and fixing the value of Cr at 2.486 (as predicted by MG theory). The value of d for the 20layer film after extensive gassing using this fitting procedure was found to be 55.99 nm. This correspondsto a film thicknessincrease after extensive gassing of about 0.13 nm/layer or 4.9%. This thickness change is about half the 0.3 nm/layer found in a previous ellipsometric study of this system12where the ellipsometric data were fitted assuming that e, after extensive gassing was the same as CdAr and ci was set to zero. This shows that the fitted value
13772 The Journal of Physical Chemistry, Vol. 97, No. 51, 1993
of film thickness change is dependent, when fitting these types of experiments, on the choice of cr for the particular system. The choice of the value of c, calculated by Maxwell-Garnett theory seems to be more appropriate than assuming that the value of c, is constant during the gassing process. On first investigation, the growth of Q-state particles with diameters of the order of a single LB layer would be expected to cause much larger thickness changes than those reported in this paper. Smotkin er a1.l2,in an ellipsometric study of this system, rationalized small changes in film thickness after H2S exposure by p r o p i n g that the particleswerediskshaped. Theapproximate volume change in the film due to spherical particle formation, however, can be estimated by use of a simple model which utilizes the molecular sizes of the components in the LB film before and after gassing. If it is assumed that a monolayer of CdAr is 2.67 nm thick (as determined in this study) and that each CdAr molecule has a cross sectional area of 0.185 nmz (as determined by surface pressure-area measurements at 21 mN m-l), then the volume of 329 CdAr molecules can be calculated. When this volume is compared to the same number of arachidic acid molecules plus their corresponding CdS particle (i.e., after conversion has occurred), it is found that the film volume would increase by about 3%. Even though this is a very simple model, it does show that large increases in film volume would not be expected if spherical CdS particles were formed upon gassing the LB film with H2S.
Conclusions UV-visible and QCM measurements have shown that the reaction of HzS with the CdAr film is consistent with reaction 1 and that the reaction proceeds until all the available cadmium ions in the film are depleted. Measurements by these techniques as well as SPR have shown that the kinetics of Q-state CdS production are similar when samples are subjected to similar gassing conditions. In the case of very thin films, of the order of a few layers, effects such as film disruption and interaction between the gold surface and H2S seem to be important, particularly in the analysis of SPR curves. These seem far less important for the thicker films studied. The film thickness change of a 20-layer CdAr film on extensive gassing, using the value of cr predicted by Maxwell-Garnett theory, was 0.13 nm/layer or 4.9%. Acknowledgment. We thank Dr. Scott Martin and Prof. Roy Sambles (University of Exeter, UK) for their considerable help
Geddes et al. in setting up the plasmon equipment and access to the software for the fitting routines. R.S.U. is grateful to the CSIRO for a postdoctoral research award. We acknowledge support for our Australia-Japan collaboration from the Department of Industry, Trade and Commerce, Commonwealth Government of Australia.
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