J. Phys. Chem. C 2008, 112, 20149–20153
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Inorganic/Organic Semiconductor Heterostructures: Optical Properties of Quaterthiophene Intercalated in Cadmium Phosphorus Trisulfide Massimiliano Labate,*,† Peter J. S. Foot,‡ Hugh O’Malley,‡ Luciano Miozzo,† Antonio Papagni,† Leonardo Silvestri,† P. Spearman,†,‡ and Silvia Tavazzi† Dipartimento di Scienza dei Materiali, UniVersita` degli Studi di Milano-Bicocca Via Cozzi 53, 20125 Milano, Italy, and School of Chemical and Pharmaceutical Sciences, Kingston UniVersity, Kingston upon Thames KT1 2EE, U.K. ReceiVed: July 16, 2008; ReVised Manuscript ReceiVed: October 9, 2008
The transmittance, reflectance, and photoluminescence optical spectra of lamellar cadmium phosphorus trisulfide (CdPS3) crystals are reported, as measured before and after intercalation with quaterthiophene (4T) molecules. The optical properties of pristine CdPS3 and 4T intercalates have been characterized in the 2-6 eV spectral region by the full complex refractive index. This latter has been deduced from transmittance and reflectance measurements and by Kramers-Kronig relations. The comparison of optical properties of CdPS3 and its 4T intercalate provide information on the arrangement of the molecules inside the inorganic layers and on the interactions between the organic molecules and between the organic and inorganic components. I. Introduction Cadmium phosphorus trisulfide (CdPS3) belongs to the family of the MPS3 compounds, where M represents a divalent transition metal. It crystallizes in a layered structure with P2 pairs and Cd ions occupying the octahedral holes between alternate planes of sulfur atoms. The forces between adjacent layers are weaker than the forces in a single layer, so that twodimensional interstitial spaces are present, which are accessible for the intercalation of atoms, ions, and molecules to obtain heterostructures with specific designed properties.1 From the optical point of view, CdPS3 is considered a wide gap semiconductor.2,3 A few empirical schemes of the energy levels and some electronic structures calculated with different methods have been reported for some members of the metal phosphorus trisulfide family.4–11 For example, Calareso et al.3 reported the reflectivity spectra of CdPS3 up to 5.5 eV and the optical absorption for energies lower than 3.5 eV. For interpreting the experimental results, they used the so-called transition-metal weakly interacting model.5 In particular, they studied the featureless long tail of the absorption below 3.5 eV, which has been interpreted in terms of an indirect interband transition with energy gap at 3.06 eV and involving a phonon of 74 meV. The possibility to intercalate organic molecules in the interstitial spaces between adjacent layers explains the recent interest toward the MPS3 class of materials. The interplane distance in CdPS3 is about 6.5 Å12 which can expand upon accommodation of the intercalant. It is transparent in the visible spectral region which makes it suitable for optical studies of the intercalated organic compounds with a strong transition in the optical region. For example, photoinduced charge transfer states have been reported between cointercalated ruthenium tris(bipyridyl) cation and dimethylviologen cation.13,14 Moreover, CdPS3 has also been found to react with cobaltocene, alkylamines,15 dimethylamino-N-methylstilbazolium cations, and * Corresponding author. Telephone +39 02 64 48 52 36. Fax: +39 02 64 48 54 00. E-mail:
[email protected]. † Dipartimento di Scienza dei Materiali, Universita` degli Studi di MilanoBicocca. ‡ School of Chemical and Pharmaceutical Sciences, Kingston University.
several organic radical cations.16,17 The encapsulation of biological molecules cytidine monophosphate monohydrate and adenosine monophosphate into Cd0.8PS3 has been recently reported by Westreich et al.18 Other authors report studies on the orientation and the motion of both potassium and water in the galleries of layered Cd0.75PS3K0.5(H2O)1.2, prepared by the ionexchange intercalation of hydrated potassium ions into CdPS3.19 Water molecules have been found to form monolayer-thick twodimensional islands. Moreover, two types of interlamellar arrangement are reported, namely a loosely bound isotropically tumbling water and a more tightly bound water with restricted degrees of rotational freedom. The magnetic properties of different intercalated compounds have also been recently studied. For example, cationic iron(III) complexes have been inserted in layered MnPS3 host lattices20 The magnetic properties of the resulting intercalation compounds and the occurrence of a spin transition have been discussed. Le´austic et al. report the photoinduced modifications of the magnetization of a stilbazolium-MnPS3 layered intercalate, where the molecules have been inserted by ion exchange.21 More recently, the paramagnetism from 120 to 300 K and the antiferromagnetic phase transition at 100 K of an hybrid compound of Fe0.76PS3 and bis(ethylenedithio)tetrathiafulvalene have been reported.22 Several preliminary studies on the optical properties of CdPS3 intercalated with different organic molecules have been reported. Jakubiak et al.14 have discussed the emission properties of ruthenium tris(bipyridyl) cations and dimethylviologen cations intercalated in CdPS3. They attributed the emission to states of the intercalated molecules, with a slight shift depending on the environment and on interactions with lattice vacancies or between different cointercalated species. However, we underline that a complete description of the optical properties based on the full determination of the complex refractive index of CdPS3 up to 6 eV, where the material is strongly absorbing, is still lacking. Knowledge of both the real and the imaginary parts of the refractive index of CdPS3 for energies much higher than the energy gap is fundamental for the description of the interband transitions, for the design of devices, for photoinducing chemical reactions in the two-dimensional interstitial spaces,
10.1021/jp806274w CCC: $40.75 2008 American Chemical Society Published on Web 12/04/2008
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and also for a comparison with the optical properties of intercalated compounds based on CdPS3. It enables the study of any modification of the electronic band structure as a consequence of intercalating organic molecules and the possible mechanisms of energy- or charge-transfer between the inorganic matrix and the intercalant by analysis of both the absorption spectra and the excitation profiles of the photoluminescence over a wide spectral range. In this paper, we report the optical spectra and the full complex refractive index of CdPS3 crystals in the spectral region from 2 to 6 eV, deduced from transmittance and reflectance spectra and from the Kramers-Kronig relations. The results provide the required information on the absorption and emission properties of CdPS3 and on the penetration depth of the light inside these crystals, thus allowing the comparison and the interpretation of the optical spectra of CdPS3 after the intercalation of quaterthiophene molecules. Information is deduced on the arrangement of the molecules inside the layers and on the interactions between the molecules and on the synergistic effect of the organic and inorganic components. II. Experimental Methods Crystals of CdPS3 were grown by heating a mixture of stoichiometric amounts of elemental Cd, P and S powder under vacuum sealed in 20 cm quartz tube under a temperature gradient of 600-680 °C for ten days.12,23 The resulting materials contain lamellar crystals up to 1 cm2 in area and with thickness ranging from 5 to 20 µm. They were confirmed by X-ray diffraction to be monoclinic CdPS3.24 4T25 was intercalated by refluxing CdPS3 crystals in a saturated solution of 4T in pyridine. This latter opens the layer spacing and facilitates 4T intercalation.23 X-ray diffraction showed that homogeneous intercalation had occurred, and the interlayer spacing is expanded by 3.2 Å, indicating that 4T molecules were lying with their long axes parallel to the host layers.26 Absorbance spectra were taken at normal incidence and at room temperature using a Perkin-Elmer Lambda 900 spectrometer equipped with Glan-Taylor calcite polarizers. Reflectance measurements were performed at near-normal incidence (20°) using a VASE ellipsometer from J.A. Woollam Co. Inc. Photoluminescence emission spectra (PL) and photoluminescence excitation profiles (PLE) were taken at room temperature, using a homemade apparatus equipped with a Xe lamp and a cooled CCD detector.
Figure 1. Reflectance spectra R(E) of a CdPS3 sample taken at nearnormal incidence (20°) for different azimuthal polarizations of the incident light (a) with parallel polarizer and analyzer, (b) with orthogonal polarizer and analyzer, and (c) without analyzer. In panel c, the absorbance spectrum taken at normal incidence with unpolarized light is also reported, where the absorbance is defined as -log(T(E)) and T(E) is the measured transmittance.
III. Results and Discussion III.a. Full Determination of the Complex Optical Functions of CdPS3 Crystals in the Spectral Region from 2 to 6 eV. Figure 1 shows the measured reflectance spectra R(E) of a CdPS3 sample as a function of the energy E taken at near-normal incidence for different polarizations of the incident light with respect to the plane of incidence (a) with parallel polarizer and analyzer, (b) with orthogonal polarizer and analyzer, and (c) without the analyzer. In panel c, the absorbance spectrum taken at normal incidence with unpolarized light is also reported for energies lower than 3.6 eV since at higher energies the spectrum is affected by saturation. Absorbance is defined as -log(T(E)), where T(E) is the measured transmittance. For energies lower than about 3.0 eV, the crystal is transparent and the measured reflectance is affected by multiple reflections at the front and rear edges of the sample. In principle, an infinite number of multiple reflections can be considered and the measured value is higher than the reflectivity of the single crystal/air interface. For increasing energies up to about 3.5 eV,
the absorption of the material is relatively weak, but it progressively increases, leading to a decrease in the number of multiple reflections, so that the difference between measured reflectance and reflectivity of a single interface progressively vanishes. Finally, above about 3.5 eV the sample is strongly absorbing and reflectance can be assumed to coincide with the reflectivity of one single interface between air and the crystal. This effect explains the presence of a pronounced minimum in the measured spectra centered at about 3.5 eV, which is therefore affected by multiple reflections, as also reported for other members of the chalcogenophosphate family.3,27 For this reason, we start by focusing our attention on the highenergy portion of the spectra. First of all, we notice that the spectra taken without analyzer shown in panel c do not depend on the polarization of the incident light. These findings suggest that the sample is optically isotropic in the plane of its most developed face. The isotropy is confirmed by the similarities between the interference fringes observed in the low-energy portion of the reflectance spectra taken with different polariza-
Optical Properties of 4T Intercalated in CdPS3
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Figure 2. Reflectance spectrum R(E) measured at near-normal incidence and transmittance spectrum T(E) measured at normal incidence (dotted lines), together with the reflectivity R(E) and transmission T(E) spectra (continuous lines) deduced below 3.6 eV from eq 2aand eq 2b. The high-energy tail for the reflectivity extrapolated above 6 eV is also shown (dashed line).
tions, as shown in the inset of Figure 2 for 0° and 90°. A further confirmation of the isotropy in the plane comes from the detailed analysis of the spectra based on the well-known Fresnel optics.28 Indeed, by comparing the different polarizations, we notice that for 0° and 90° only, the polarization of the incident light is maintained after reflection (Figure 1b). For all the other polarization directions, some reflected light is detected with crossed polarizer and analyzer, corresponding to a decrease of the intensity measured with parallel polarizer and analyzer (Figure 1a). This effect is maximum at 45°, when the polarization direction of the reflected light is 90° rotated with respect to the incident one. To explain this effect, we consider that during the measurements the sample was randomly oriented and the spectra were taken at an angle of incidence of 20°. The polarizations 0° and 90° in Figure 1 correspond to purely pand purely s-polarized incident light, respectively. For these two polarizations, no reflected light is expected with crossed polarizer and analyzer if the sample is isotropic in the plane (i.e the principal axes of the system can be arbitrarily defined in the plane of the sample), in contrast to the case of anisotropic samples, thus confirming our hypothesis. To explain the experimental observations at the other angles of polarizations, we consider that, in these cases, the incident electric field has both s and p components. For angles of incidence lower than the Brewster angle (as in our case), a 180° dephasing is expected between the p and s components after reflection. This modification is responsible for the rotation of the reflected electric field with respect to the incident one for all the polarization angles different from 0° and 90°, the rotation corresponding to 90° when the incident light is polarized at 45° to the plane of incidence. Considering that CdPS3 is optically isotropic in the plane, a possible approach to deduce its complex refractive index nˆ(E) as a function of the energy E is based on the knowledge of both reflectivity R(E) and transmission T(E) in the same spectral range. R(E) is the reflectivity of one single air/sample interface and it is related to the complex refractive index by
R(E) )
|
|
nˆ(E) - 1 2 nˆ(E) + 1
(1a)
T(E) is the transmission spectrum defined as
(
T(E) ) exp -
2E Im(nˆ(E))d pc
)
(1b)
where d is the thickness of the sample, c is the velocity of light in vacuum, pis the Planck constant.
Figure 3. Real and imaginary parts of the in-plane refractive index of CdPS3 as deduced below 3.6 eV from the R(E) and T(E) spectra (continuous lines) and above 3.5 eV from R(E) and the KK relations.
In our case, below 3.6 eV both the reflectance R(E) and transmittance T(E) have been measured. Assuming an infinite number of multiple internal reflections, these quantities are related to R(E) and T(E) by:29
R(E) ) R(E) +
R(E)(1 - R(E))2T(E)2 1 - R(E)2T(E)2
(2a)
(1 - R(E))2T(E)2 1 - R(E)2T(E)2
(2b)
T(E) ) R(E) +
Figure 2 shows the measured R(E) and T(E) spectra, together with R(E) and T(E) below 3.6 eV deduced using eqs 2a and 2b. At 2 eV, the sample can be considered transparent and the refractive index n is a real quantity which is found to be 2.67 by using eq 1a. From the energy separation ∆ between consecutive interference fringes shown in the inset of Figure 2, we also deduced the thickness of the sample as d ) (pcπ)/ (n∆), equal to 10.9 µm. Finally, from R(E), T(E), and d, the complex refractive index has been deduced by using eqs 1a and 1b in the range below 3.6 eV. Its real and imaginary parts are shown in Figure 3 (continuous line). For energies higher than 3.6 eV, R(E) can be directly measured (it can be confused with R(E), but T(E) and, in turn, T(E) are not measurable due to the strong absorption). In this range, a general experimental method to determine the complex refractive index of a solid is based on the Kramers-Kronig (KK) relations. Indeed, nˆ(E) is related to the Fresnel coefficient rˆ(E) by nˆ(E) ) (1 + rˆ(E))/(1 - rˆ(E)). The coefficient rˆ(E) can be written in the form rˆ(E) ) R(E)eiθ(E), where θ(E) is the phase shift between the reflected and the incident light. Since we only know R(E), the KK relations are required to calculate the phase shift as30
θ(E) )
E π
⁄ R(E)) dE′ ∫0∞ ln(R(E′) 2 (E - E′2)
(3)
In our case, R(E) below 3.5 eV is taken from Figure 2 as deduced from eqs 2a and 2b; between 3.5 and 6 eV it is assumed equal to R(E) and taken from the measured curve in Figure 2, while at higher energies it has been extrapolated as R(E) ) R(6 eV)(6 eV/E)p, with p adjusted so that the obtained nˆ(E) from the KK relations agrees with the already known real and imaginary parts below 3.6 eV. This high-energy tail (p ) 1.2) is shown in Figure 2. This allowed the deduction of the complex Fresnel coefficient and thus nˆ(E), whose real and imaginary parts
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Figure 4. Penetration depth of the incident light at normal incidence on CdPS3 crystals as deduced from the imaginary part of the refractive index in Figure 3 in the corresponding energy range. Inset: penetration depth on an enlarged sale.
are depicted by dotted lines in Figure 3. Above 6 eV, the refractive index deduced from the KK relations is omitted since it is strongly dependent on the line shape of the high-energy tail. The obtained complex refractive index in Figure 3 shows a strong absorption in the high-energy portion of the spectrum, with a maximum at about 4.8 eV. This strong absorption is attributed to transitions between the valence and conduction bands of the material. Considering that the maximum value obtained for Im(nˆ) is about 1.9 and the typical upper limit of the commercial spectrophotometers is about 6-8 absorbance units, we note that absorbance could be measured without saturation effects only for CdPS3 samples whose thickness is less than 150-200 nm, but such thin samples cannot easily be obtained. The structure which is clearly seen in Re(nˆ) at about 3.3 eV derives from the structure at about the same energy in the R(E) spectrum in Figure 2. It has also been observed by Calareso et al.3 and attributed to internal photon scattering caused by the layer structure of the samples whose cleavage planes are parallel to their surfaces. An alternative explanation for this structure could be the assignment to a phonon-assisted indirect interband transition. Consistently, a corresponding band should be observed in Im(nˆ) in Figure 3, probably with a maximum slightly shifted to higher energy with respect to the maximum in Re(nˆ), as typically deduced in the framework of the Lorentz model describing the optical properties of materials.30 In Figure 3, this structure is not observed in Im(nˆ) since it is expected in the intermediate spectral region between the curves deduced by the two different approaches, where the sensitivity of both the methods is poor. Finally, from the imaginary part of the refractive index, the penetration depth for incident light impinging at normal incidence on CdPS3 can also be deduced as (pc)/(2E Im(nˆ(E))) and it is shown in Figure 4. It represents the depth inside the sample at which the intensity of the light is reduced by a factor 1/e. The knowledge of this parameter is fundamental for a comparison with the optical properties of intercalated samples and for those chemical processes and reactions photoinduced inside the layers by the incident light. III.b. Optical Properties of CdPS3 Crystals Intercalated with 4T. Figure 5a shows the absorbance and reflectance spectra taken at normal incidence of a sample of CdPS3 intercalated with 4T. The optical measurements performed with polarized light (not shown here) exhibit that the samples are isotropic in the plane similarly as the nonintercalated ones The reflectance
Figure 5. (a) Absorbance and reflectance spectra of CdPS3 intercalated with 4T and absorption spectrum of the 4T isolated molecule.33 (b) PL emission spectra of a CdPS3 crystal, of a CdPS3 intercalated with 4T taken with different excitation energies, and PLE spectra of the CdPS3 sample intercalated with 4T taken at 1.7 and 2.2 eV emission energies.
spectrum shows a pronounced minimum at about 3.3 eV, similarly as the nonintercalated CdPS3 crystal (see Figures 1 and 2). The absorbance spectrum also shows a strong increase at about 3.2 eV, attributed to the strong absorption of the inorganic CdPS3 matrix above its absorption edge. Weak structures are detected at 2.0 and 2.2 eV in the reflectance spectrum and at about 1.9 and 2.1 eV in the absorbance spectrum whose origin is unknown. We tentatively attribute them to charge-transfer states of the intercalated system.31 At higher energy, an absorption band with replicas at about 2.72 and 2.91 eV is observed and structures centered at about 2.64 and 2.86 eV are detected in the reflectance curve. These structures are in the same spectral region as the absorption of the 4T isolated molecule,32,33 which is also shown in Figure 5a for comparison. We conclude that the excitonic interaction between the 4T molecules intercalated in the CdPS3 matrix is negligible.31 No effect of molecular aggregation is observed in the spectra, probably as a consequence of the reduced space for 4T between the layers (the length of the single 4T molecule is about 15 Å to be compared with the space between the layers of the order of 6-7 Å). As a consequence of the reduced surface quality, the scattering of light on the interface between air and the intercalated samples has prevented the absolute values of reflectivity to be measured, thus preventing the complete characterization of the complex refractive index of the crystals intercalated with 4T. Figure 5b shows the emission spectra of CdPS3 intercalated with 4T taken at different excitation energies, together with the emission spectrum of the nonintercalated CdPS3 matrix for comparison. The latter shows a typical band centered at about 2.6 eV, while a broad band at lower energy is detected in the spectra of the intercalated compounds. Its maximum shifts from
Optical Properties of 4T Intercalated in CdPS3 1.80 to 1.92 eV as a function of the excitation energy. We attribute this effect to the contributions of two different emissive species with different relative intensity depending on the excitation energy. For relatively high and relatively low excitation energies (as indicated by the spectra taken at 4.43 and 2.38 eV in Figure 5b), the emission is centered at about 1.8 eV. This emission is attributed to lattice defect states induced during the intercalation process. For intermediate excitation energies, such as at 2.82 eV in Figure 5b, the band is broader and shows a contribution at higher energy attributed to the emission from the 4T molecule.32 Indeed, this contribution has been observed for excitation energies between about 2.5 and 3.2 eV, approximately the range of absorption of the isolated molecule indicated in Figure 5a. Separating the two emissions is not straightforward due to their large overlap and to the relatively weak 4T contribution, whose emission is not favored, being at higher energy. Indeed, as reported in Figure 5b, the excitation profiles taken at 1.7 eV and at 2.2 eV show the same line shape. This indicates that, when the 4T is not directly excited, the emission comes from low-lying defect states; when the excitation energy corresponds to the absorption of the 4T molecules, a weak 4T emission is observed, almost masked by the strong emission from the lower defect states, which are favored. For excitation energies higher than 3.5 eV, the emission is strongly suppressed. This cutoff in the PLE profile corresponds to the absorption edge of the CdPS3, thus indicating that the emission is not intrinsic of the inorganic matrix, but it comes from a localized region in the interplane space where both defects and 4T are present and where light can enter only for energies lower than the absorption edge of the matrix. Moreover, the strong decrease of the emitted intensity for excitation energies higher than the absorption edge indicates that no mechanisms of energy transfer occur between the inorganic component and the organic intercalated molecules. IV. Conclusions The reflectance and transmittance optical spectra of CdPS3 crystals are reported as taken in different experimental configurations. From polarized measurements, the samples have been found isotropic in the plane of their most developed face. The real and imaginary parts of the corresponding refractive index have been deduced in the whole range between 2 and 6 eV either by combining transmittance and reflectance spectra in the low portion of the spectrum or by using the KK relations in the high energy region, where transmittance cannot be measured as a consequence of the strong absorption. The determination of the complete optical functions is fundamental to study the interband transitions, to design devices where the knowledge of the refractive index is required, and to deduce the penetration depth of light in the spectral range which is expected to be useful for photoinduced chemical processes and reactions inside the two-dimensional interstitial spaces. In our case, the results have provided the required information for the comparison and the interpretation of the optical spectra of CdPS3 intercalated with 4T molecules. The excitonic interactions between these molecules inside the layers has been found negligible, the optical spectra showing the typical bands of the isolated molecule. This result is in agreement with the results of the structural analysis, which indicate that the space among the inorganic planes is of the order of 6-7 Å, to be compared with 15 Å, the length of the 4T molecule. A contribution from
J. Phys. Chem. C, Vol. 112, No. 51, 2008 20153 the 4T molecules has also been observed in the emission spectra after directly exciting the 4T itself in its typical absorption spectral range, but the emission is strongly masked by the emission attributed to lattice defect states at lower energy. Both the 4T and the defect emissions are strongly suppressed for excitation energies higher than the absorption edge of the inorganic matrix, as a consequence of the reduced penetration depth of light. This indicates that the emitting species are localized inside the interplane space and that no energy-transfer mechanisms in the UV-vis region occur between the inorganic and organic components. Acknowledgment. Fondazione Cariplo is acknowledged for financial support. References and Notes (1) Lacroix, P. G.; Cle´ment, R.; Nakatani, K.; Zyss, J.; Ledoux, I. Science 1994, 263, 658. (2) Brec, R.; Schleich, D. M.; Ouvrard, G.; Louisy, A.; Rouxel, J. Inorg. Chem. 1979, 18, 1814. (3) Calareso, C.; Grasso, V.; Silipigni, L J. Appl. Phys. 1997, 82, 6228. (4) Khumalo, F. S.; Hughes, H. P. Phys. ReV. B 1981, 23, 5375. (5) Piacentini, M.; Khumalo, F. S.; Olson, G. C.; Anderegg, J. W.; Lynch, D. W. Chem. Phys. 1982, 65, 289. (6) Piacentini, M.; Khumalo, F. S.; Leveque, G.; Olson, G. C.; Lynch, D. W. Chem. Phys. 1982, 72, 61. (7) Whangbo, M. H.; Brec, R.; Ouvrard, G.; Rouxel, J. Inorg. Chem. 1985, 24, 2459. (8) Mercier, H.; Mathey, Y.; Canadell, E. Inorg. Chem. 1987, 26, 963. (9) Kurita, N.; Nakao, K. J. Phys. Soc. Jpn. 1989, 58, 232. (10) Ouvrard, G.; Brec, R. Eur. J. Solid State Inorg. Chem 1990, 27, 477. (11) Zhukov, V.; Boucher, F.; Alemany, P.; Evain, M.; Alvarez, S. Inorg. Chem. 1995, 34, 1159. (12) Klingen, W.; Ott, R.; Hahn, H. Z. Anorg. Allg. Chem. 1973, 396, 271. (13) Lifshitz, E.; Clement, R.; Yu-Hallada, L. C.; Francis, A. H. J. Phys. Chem. Solids 1991, 52, 1081. (14) Jakubiak, R.; Francis, A. H. J. Phys. Chem. 1996, 100, 362. (15) Foot, P. J. S.; Shaker, N. G. Mater. Res. Bull. 1983, 18, 173. (16) Clement R. In ACS Symposium Series on Hybrid-Organic-Inorganic Composites; Mark, J. E., Lee, C. Y. C., Bianconi, P. A., Eds.; Americal Chemical Society: Washington, DC, 1995. (17) Audiere, J. P.; Cle´ment, R.; Mathey, Y.; Mazieres, C. Physica B 1980, 99, 133. (18) Westreich, P.; Yang, D.; Frindt, R. F. J. Phys. Chem. Sol. 2004, 65, 583. (19) Arun, N.; Vasudevan, S.; Ramanathan, K. V. J. Am. Chem. Soc. 2000, 122, 6028. (20) Floquet, S.; Salunke, S.; Boillot, M.-L.; Cle´ment, R.; Varret, F.; Boukheddaden, K.; Rivie`re, E. Chem. Mater. 2002, 14, 4164. (21) Le´austic, A.; Rivie`re, E.; Cle´ment, R. Chem. Mater. 2003, 15, 4784. (22) Chen, X.; Zhou, H.; Zou, L.; Yang, C.; Qin, J.; Inokuchi, M. J. Incl. Phen. Macr. Chem. 2005, 53, 205. (23) Hill, P. G.; Foot, P. J. S.; Davis, R. Synth. Met. 1996, 76, 289. (24) Boucher, F.; Evain, M.; Brec, R. J. Alloys Compd. 1994, 215, 63. (25) Trabattoni, S.; Laera, S.; Mena, R.; Papagni, A.; Sassella, A. J. Mater. Chem. 2003, 14, 171. (26) Foot, Spearman et al, manuscript in preparation. (27) Calareso, C.; Grasso, V.; Neri, F.; Silipigni, L. J. Phys.: Condens. Matter 1997, 9, 4791. (28) Born, M.; Wolf, E. Principles of Optics; Pergamon Press: Oxford, U.K., 1965. (29) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: Amsterdam, 1997. (30) Wooten, F. Optical Properties of Solids; Academic Press: New York, 1972. (31) Davydov, A. S. Theory of Excitons; Plenum Press: New York, 1971. (32) Trabattoni, S.; Borghesi, A.; Laera, S.; Moret, M.; Papagni, A. Synth. Met. 2004, 145, 7. (33) Laicini, M.; Spearman, P.; Tavazzi, S.; Borghesi, A. Phys. ReV. B 2005, 71, 045212.
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