Optical Properties of Ultrathin Electrodeposited CdS Films Probed by

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J. Phys. Chem. 1996, 100, 9480-9484

Optical Properties of Ultrathin Electrodeposited CdS Films Probed by Resonance Raman Spectroscopy and Photoluminescence B. Edward Boone and Curtis Shannon* Department of Chemistry, Auburn UniVersity, Auburn, Alabama 36849-5312 ReceiVed: January 11, 1996; In Final Form: March 19, 1996X

We report resonance Raman and photoluminescence data from ultrathin films of CdS grown on Au substrates using electrochemical atomic layer epitaxy (ECALE). Samples ranged in coverage from 25 to 200 monolayers. Quantitative analysis of resonance Raman intensities leads to several important conclusions. First, ECALE does not involve growth by random precipitation of CdS onto the Au surface; contiguous thin layers of material are deposited. Second, the electronic structure of the films in this coverage regime corresponds to that of bulk CdS. Finally, charge carriers are rapidly trapped at the surface of the semiconductor on the time scale of the Raman scattering event; the trapping rate decreases linearly as a function of CdS coverage.

Introduction Electrochemical deposition is emerging as a new and powerful tool for the growth of high-quality semiconductor ultrathin films and nanostructures. Much recent effort has focused on achieving atomic level control of the growth process, leading to the formation of well-ordered, epitaxial deposits of a desired material.1 Other experiments have been directed toward achieving local, nanometer-scale deposition of materials or their growth from extremely small domains.2 These recent developments have increased the need for characterization techniques that can provide molecular level information about the electronic and optical properties of electrodeposited materials. Resonance Raman spectroscopy is an excellent probe of molecular or solid-state vibrations that are coupled to electronic transitions. Quite detailed information about both the ground and the excited states of the system can be obtained from steady-state resonance Raman measurements.3 Raman scattering in polar semiconductors depends on the interaction of lattice vibrations (phonons) and electrons; since phonons are very sensitive to their local environment, resonance Raman spectroscopy can, in principle, provide information about materials on the scale of a few lattice constants. Resonance Raman spectroscopy has been used extensively to study the electronic states of small semiconductor particles and the evolution of these states to those of the bulk material.4 There is one report of using resonance Raman spectroscopy to monitor the growth of CdS thin films on InP(110) substrates.5 In addition, there is a quantitative relationship between resonance Raman intensities and the dynamics of the excited state.6 Therefore, resonance Raman spectroscopy can also provide insight into photochemical processes taking place within an electrodeposited material. This aspect of resonance Raman spectroscopy, which has been widely exploited in studies of condensed phase molecules, has not been as extensively used to study surface dynamic processes. In the present study, resonance Raman spectroscopy and photoluminescence were used to probe the nature of ultrathin films of CdS grown onto Au substrates using electrochemical atomic layer epitaxy (ECALE). This growth technique exploits the phenomenon of underpotential deposition (upd, the electrochemical deposition of an element onto a dissimilar material * To whom correspondence should be addressed. E-mail: shanncg@ mail.auburn.edu; Tel: 334.844.6964; Fax: 334.844.6959. X Abstract published in AdVance ACS Abstracts, May 1, 1996.

S0022-3654(96)00222-5 CCC: $12.00

at potentials less oxidizing or reducing than the Nernst potential) for the production of well-ordered atomic layers.7 Sequential upd of multiple atomic layers is a viable route to thin films of simple inorganic compounds and has been used to grow binary materials possessing a one-to-one stoichiometry, including CdTe, CdSe, and CdS.8 In this paper, we present resonance Raman and room temperature photoluminescence data for ultrathin films of CdS on Au substrates ranging in coverage from 25 to 200 monolayers. Our resonance Raman data demonstrate that ECALE does not involve growth by random precipitation of CdS onto the Au surface. Rather, contiguous thin layers of material are formed. From an analysis of resonance and off-resonance Raman scattering, and from the position of the band-edge photoluminescence, we show that the electronic structure of the films in this coverage regime corresponds to that of bulk CdS. Changes in the relative intensity of the overtone bands in resonance Raman spectra as a function of film coverage are therefore due to dephasing effects. These effects arise from trapping of one of the charge carriers at the semiconductorambient interface. Finally, resonance Raman spectroscopy can be used to obtain extensive information on semiconductor thin films as long as effects having to do with electron-phonon coupling and dephasing can be separated. We suggest that one way to accomplish this is through controlling nanostructure morphology. In particular, films of mesoscopic thickness possess the bulk electronic structure and a large enough surfaceto-volume ratio that the interfacial phenomena that give rise to dephasing can be independently observed on the resonance Raman time scale. Experimental Section Sample Preparation. Several of the CdS specimens were generously provided by Prof. J. L. Stickney (University of Georgia). Samples produced in our laboratory were grown using published materials and methods.8d,9 Deposition of sulfide was carried out from a solution that was 2 mM Na2S (as Na2S‚ 9H2O) in 0.1 M NaOOCCH3/0.01 M KOH. Cadmium depositions were performed using a 2 mM CdSO4 (as Cd(CH3CO2)2‚2H2O) in 1 mM KOH/2 mM CH3CO2H solution. All electrochemical experiments were performed using a Pine AFRDE-4 bipotentiostat, an HP-7015B X-Y recorder, and a single-compartment, three-electrode Teflon cell. The bipotentiostat electronics were modified to minimize noise at high © 1996 American Chemical Society

Optical Properties of Ultrathin Electrodeposited CdS Films sensitivities. In all cases, the reference electrode was Ag/AgCl, and a platinum wire served as the counter electrode. The electrochemical cell was connected to a solution handling manifold that allowed solutions to be changed without exposing the electrode to the laboratory ambient and which allowed all depositions to be carried out under ultrahigh-purity (UHP) Ar. All cell and manifold parts that came into contact with the electrolyte were made of Teflon or Kel-F. Resonance Raman Spectroscopy. Resonance Raman scattering was excited using the 457.9, 488.0, and 514.5 nm lines of a Coherent Innova-70 Ar+ laser. CW power levels at the sample were maintained at ca. 80 mW to minimize laser-induced damage to the CdS thin films. Spectra obtained of samples run in air and under nitrogen at these power levels were found to be identical; thereafter, all spectra were obtained in air. The incident light was p-polarized (p-, in the plane of incidence) and was focused to a ca. 100 µm spot on the surface using a spherical lens. The angle of incidence and the collection angle (also in the plane of incidence) were approximately 50° and 40° with respect to the surface normal, respectively. The scattered radiation was collected and focused onto the slits of a SPEX 14018 double monochromator using an f-matched f/1.4 camera lens (Canon). The scattered light was detected using a RCA photomultiplier, Pacific Precision Instruments photon counting electronics, and a Phillips PM6680 frequency counter. The spectrometer drive unit and photon counting electronics were controlled using LabView. The instrumental slit width was approximately 6 cm-1 in all experiments, and the acquisition time per spectrum was typically 10 min. Ambient temperature photoluminescence spectra were collected using the Raman apparatus and were excited with 80 mW of 457.9 nm radiation. Results and Discussion Resonance enhancement of Raman scattering from lattice vibrations in polar semiconductors depends in a complicated way on several variables but can be understood qualitatively by considering a simplified form of the KHD equation derived by Albrecht.10

r ) (Me)2

2π

∑ h ν (ν

〈j|ν〉〈ν|i〉

νi

- ν0) + iΓ

(1)

In this equation, R is the Raman transition polarizability tensor between ground state vibrational levels i and j; the intensity of a given transition is proportional to the square of R. Me is the pure electronic transition moment between the ground and excited states, and the remaining symbols have their conventional meanings. The numerator, which is a product of FranckCondon factors, is proportional to the magnitude of the coupling between the lattice vibration and the resonant electronic transition. In semiconductors such as CdS, electrons are coupled to lattice vibrations through the Fro¨hlich mechanism.11 When light is absorbed by a polar material, electron-hole pairs are formed which induce a deformation in the crystal lattice. The magnitude of this distortion depends on the ionicity of the lattice and on the spatial overlap and symmetry of the electron and the hole wave functions. In the denominator, the difference term sets the resonance condition, while the imaginary term is related to the Lorenzian line width of the resonant electronic absorption. Γ is the total dephasing rate and contains both lifetime and pure dephasing contributions.12 Together, these factors determine the relative intensities of the Raman bands and their overtones in the scattered spectra. In principle, therefore, physical changes in an electrodeposited material that affect the electron-phonon coupling, the resonance condition,

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Figure 1. Resonance Raman spectrum of 200 monolayers of CdS grown on Au by electrochemical atomic layer epitaxy showing the LO phonon band at 305 cm-1 and the progression of overtones at higher frequencies. The excitation wavelength was 488.0 nm.

or dephasing can be studied by observing the corresponding changes in the resonance Raman spectrum. In practice, however, the relative contributions of each term can be difficult to separate. The resonance Raman spectrum of 200 monolayers of CdS on Au from 200 to 1700 cm-1 is shown in Figure 1. This spectrum is typical of what we observed from approximately 20 points across the surface of the film and is characterized by the CdS longitudinal optical (LO) phonon mode at 305 cm-1 and a progression of overtone bands at higher frequencies. An analysis of the relative intensities of the fundamental and overtone bands provides detailed information about the CdS exciton and can be used to study how the nature of this state evolves as a function of film thickness. As the first few monolayers are deposited, an increase in the relative intensity of the overtone bands is expected, corresponding to the anticipated increase in the electron-phonon coupling which accompanies the development of the bulk band structure of CdS.13 Once the film thickness exceeds the bulk exciton diameter, however, there should be no significant further change in the electron-phonon coupling. For films whose thickness exceeds this critical dimension, the relative intensities of bands in the scattered spectra will be determined solely by dephasing effects. The critical film thickness at which the electronphonon coupling reaches its bulk value can be estimated from the known exciton diameter in bulk CdS of approximately 6.0 nm. Given an interlayer spacing of about 0.35 nm in bulk CdS,14 we expect this transition to occur at a CdS coverage of approximately 20 monolayers. Although multiple overtone bands are clearly seen in the thin film spectra, only the integrated intensities of the fundamental and its first overtone are analytically useful due to the weak signal levels observed from the lowest coverage films. Representative resonance Raman spectra of CdS thin films as a function of the number of monolayers deposited are given in Figure 2. The fundamental of the LO band and its first overtone are shown. In addition, the spectral intensities have been scaled so that the height of the fundamental band is approximately the same in all cases. Clearly, the overtone intensity is changing significantly in a coverage regime where the electron-phonon coupling is expected to be constant. A complete data set that illustrates this trend more clearly is shown in Figure 3. The integrated overtone intensity ratios (Iovertone/Ifundamental) are plotted as a function of the number of monolayers of CdS deposited for Raman data obtained under the resonance condition expected

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Boone and Shannon

Figure 4. Room temperature photoluminescence spectra of CdS thin films as a function of coverage. The excitation wavelength was 457.9 nm. The sharp bands superimposed on the luminescence is CdS Raman scattering.

Figure 2. Resonance Raman spectra of bulk CdS thin films electrodeposited on Au as a function of coverage. There is a 3-fold change in the relative intensity of the overtone band in this thickness regime. The number of monolayers of CdS is indicated on each spectrum.

Figure 3. Integrated overtone ratio (Iovertone/Ifundamental) as a function of the number of layers of CdS. Filled squares correspond to the on-resonance and open triangles to off-resonance experiments.

for bulk CdS (488.0 nm). The overtone intensity ratio increases linearly by a factor of 2 as the CdS coverage is increased from 25 to 200 monolayers. The open triangles show off-resonance data obtained at 514.5 nm for comparison. This data, in which the overtone ratio is observed to be independent of coverage, proves that we are indeed observing a resonance phenomenon. That is, in all cases the films have the electronic structure of bulk CdS. Room temperature photoluminescence spectra independently support this conclusion (Figure 4). These spectra are characterized by two features: a relatively narrow highenergy peak corresponding to the band-edge luminescence at approximately 505 nm and a broad red-shifted luminescence peak near 700 nm that originates from midgap states (trap sites). In the case of the two thicker films, the band-edge luminescence λmax is unambiguously indicative of bulk CdS. In the case of the 100-monolayer film, however, no band-edge luminescence

can be discerned. The background luminescence observed in this spectrum, which peaks at ca. 520 nm, originates from the Au substrate itself.15 There are several explanations for the absence of CdS luminescence in this spectrum. First, the integrated intensity of the photoluminescence peaks does not scale linearly with CdS coverage due to the quenching of this relatively long-lived luminescence by the conductive metal surface.16 Because of the d-3 dependence of this quenching, photoluminescence from low-coverage films will be quenched more efficiently than that originating from thicker films. As will be discussed in more detail in the next section, it is also possible that, due to extremely rapid trapping of the electron or hole at the semiconductor-ambient interface, band-edge luminescence is entirely absent from the 100-monolayer spectrum. This is analogous to what is observed in the luminescence of small CdS particles, for example, where band-edge luminescence is not seen due to the short exciton lifetime.17 More importantly, there is no evidence of blue-shifted emission from this sample which would be characteristic of quantum confinement effects.17 In sum, the dramatic coverage dependence in the resonance Raman overtone intensities indicates that there are significant dephasing effects taking place in these thin films. Before discussing the nature of these dynamic processes, however, we first consider the effects of structural nonidealities on the resonance Raman spectra and quantify the relationship between the experimentally measured overtone intensities and the damping term, Γ, appearing in eq 1. Scanning probe microscopy results from our laboratory8d,e and preliminary X-ray scattering data18 both indicate that the degree of crystallinity and order in these films is high; nevertheless, they are polycrystalline, and the effect of structural nonidealities (i.e., inhomogeneous broadening) on the resonance Raman spectra must be considered. Although inhomogeneous broadening may influence the intensity of the overtone band, it cannot explain the observed coverage dependence. For instance, if structural defects were such that the distributions of crystallites were not all being excited at their resonance maximum, the overtone intensity measured for the ensemble would be reduced compared to a sample in which there were no inhomogeneous broadening. This is because the intensity of overtone bands decreases off-resonance (i.e., overtones are generally not observed in normal Raman scattering). Regardless of the magnitude of the overtone band, however, the fact that structural

Optical Properties of Ultrathin Electrodeposited CdS Films defects propagate as the film grows means that inhomogeneous broadening should scale with the number of monolayers deposited. In other words, the intensity of the overtone band would decrease as the film thickness increases if the films were becoming more inhomogeneous. However, this stands in contrast to the observed increase in overtone intensity with CdS coverage. Our results also rule out the possibility that the ECALE growth mechanism involves random precipitation of CdS from solution during growth.19 In the case of random precipitation, which would produce a film composed of small crystallites, the overtone intensity would be a function of the size distribution of crystallites deposited but would be independent of the total coverage of CdS. In fact, our results suggest that contiguous thin films of CdS are formed in the deposition process. The changes we observe in the overtone intensity therefore can only be due to lifetime or pure dephasing effects and not to inhomogeneous broadening or other structural factors. If the electron-phonon coupling constant is independently known, it is possible to calculate Γ directly from the KHD equation or by using equivalent time-domain methods.20 In particular, for materials in which the coupling constant is large, as is the case for bulk CdS, Heller has shown that there is a simple linear relationship between the overtone ratio and Γ-1.21 Calculations using the known bulk electron-phonon coupling constant of CdS bear this out.22 Thus, the change in overtone intensity in our films reflects the fact that the total electronic dephasing rate decreases linearly as the CdS coverage increases. The last remaining question concerns the relative importance of lifetime and pure dephasing effects, since Γ contains contributions from both. In thin films such as these, pure dephasing would presumably involve coupling of the exciton transition to a bath of acoustic phonons and would be analogous to internal conversion in a molecular system.23 If such a mechanism were operative, Γ (the dephasing rate) might be expected to increase slightly as the density of phonon states increased, that is, as the film became thicker.24 On the other hand, in quantum confined systems, damping to acoustic modes has been shown to follow a r -5 dependence (r, particle radius).25 In the 25-200-monolayer coverage regime, we would anticipate at most a slight increase in Γ with coverage, but not the linear decrease that is observed experimentally. We therefore conclude that the overtone intensities are changing due to coveragedependent changes in the exciton lifetime and that our results indicate the presence of a nonradiative decay pathway which depopulates the excited state on the time scale of the resonance Raman process.26 Furthermore, this nonradiative decay process shows a linear dependence on the CdS coverage. This result immediately suggests the existence of surface states which rapidly trap charge carriers. A large body of experimental evidence indicates that extremely rapid trapping of carriers at surface states occurs in small particles of CdS and other II-VI nanocrystals.27 Furthermore, the photoluminescence data in Figure 4 suggest that the carriers are being trapped at the semiconductor-ambient interface as opposed to the metalsemiconductor junction. If metal-derived interface states were involved in the trapping, we would expect to observe significant differences between the photoluminescence of adsorbed and free CdS particles. However, the thin film photoluminescence spectra in Figure 4 are strikingly similar to what has been observed from colloidal particles in solution.17 Therefore, it appears that the metal surface plays only a minor role in determining the optical properties of these systems. Simple energy transfer from the CdS thin film to the conductive Au surface cannot account for the coverage dependence of Γ because both the observed thickness dependence and the relative

J. Phys. Chem., Vol. 100, No. 22, 1996 9483 time scales are inconsistent with the energy transfer mechanism.16 Therefore, although the substrate modulates the intensity of long-lived luminescence, as reflected in the photoluminescence spectra, thin film electronic structure and optical properties, as manifested in the resonance Raman data, are essentially a function only of the electrodeposited material. Conclusions Resonance Raman spectroscopy and photoluminescence have been used to study ultrathin films of CdS grown using the ECALE method. Resonance Raman data rule out the possibility of growth by random precipitation and, taken together with recent scanning probe results,8d,e strongly suggests that a layerby-layer growth mechanism is operative in ECALE. Resonance Raman spectroscopy provides a wealth of information about semiconductor thin film properties as long as dephasing and electron-phonon coupling phenomena can be controlled independently. One method to accomplish this is to couple relative and absolute intensity measurements.3b We suggest that an alternative approach is to control nanostructure morphology. In particular, precise control of film thickness using ECALE allows dephasing effects to be studied in bulk CdS films. Temperaturedependent Raman measurements may help to elucidate the nonradiative decay mechanism in more detail. This work is currently underway and will be reported in the near future. Acknowledgment. The financial support of the Society of Analytical Chemists of Pittsburgh and the Auburn University Grant-In-Aid program is gratefully acknowledged. We thank Ms. Lisa Colletti (University of Georgia) for growing some of the samples used in this study and Mr. Umit Demir (Auburn University) for his help in sample preparation. We thank Prof. John L. Stickney (University of Georgia) for helpful discussions. References and Notes (1) (a) Switzer, J. A.; Shane, M. J.; Phillips, R. J. Science 1990, 247, 444. (b) Switzer, J. A.; Hung, C.-J.; Breyfogle, B. E. Science 1994, 264, 1573. (c) Golden, T. D.; Rafaelle, R. D.; Switzer, J. A. Appl. Phys. Lett. 1993, 63, 1501. (d) Gregory, B. W.; Suggs, D. W.; Stickney, J. L. J. Electrochem. Soc. 1991, 138, 1279. (e) Hodes, G. Isr. J. Chem. 1983, 33, 95. (f) Golan, Y.; Margulis, L.; Rubenstein, I.; Hodes, G. Langmuir 1992, 8, 749. (g) Klein, J. D.; Herrick, II, R. D.; Palmer, D.; Sailor, M. J.; Brumlick, C. J.; Martin, C. R. Chem. Mater. 1993, 5, 902. (h) Li, W.; Virtanen, J. A.; Penner, R. M. J. Phys. Chem. 1992, 96, 6529. (i) Wei, C.; Rajeshwar, K. J. Electrochem. Soc. 1992, 139, L40. (j) de Tacconi, N. R.; Rajeshwar, K. J. Phys. Chem. 1993, 97, 6504. (2) (a) Li, W.; Virtanen, J. A.; Penner, R. M. J. Phys. Chem. 1994, 98, 11751. (b) Li, W.; Virtanen, J. A.; Penner, R. M. Appl. Phys. Lett. 1992, 60, 1181. (3) (a) Heller, E. J.; Sundberg, R. L.; Tannor, D. J. Phys. Chem. 1982, 86, 1822. (b) Myers, A. B.; Matthies, R. A In Biological Applications of Raman Spectrometry: Resonance Raman Spectra of Polyenes and Aromatics; Spiro, T. G., Ed.; John Wiley & Sons: New York, 1987; Vol. 2, p 1. (4) (a) Alivisatos, A. P.; Harris, T. D.; Carroll, P. J.; Steigerwald, M. L.; Brus, L. E. J. Chem. Phys. 1989, 90, 3463. (b) Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1985, 82, 552. (c) Brus, L. E. J. Chem. Phys. 1986, 90, 2555. (5) Zahn, D. R. T.; Maierhofer, CH.; Winter, A.; Reckzu¨gel, M.; Srama, R.; Thomas, A.; Horn, K.; Richter, W. J. Vac. Sci. Technol. 1991, B9, 2206. (6) Reid, P. J.; Lawless, M. J.; Wickham, S. D.; Mathies, R. A. J. Phys. Chem. 1994, 98, 5597. (7) (a) Kolb, D. M. In AdVances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley-Interscience: New York, 1978; Vol. 11, p 125. (b) Hubbard, A. T. Crit. ReV. Anal. Chem. 1973, 3, 201. (8) (a) Colletti, L. P.; Teklay, D.; Stickney, J. L. J. Electroanal. Chem. 1994, 369, 145. (b) Suggs, D. W.; Stickney, J. L. Surf. Sci. 1993, 290, 375. (c) Stickney, J. L.; Villegas, I.; Gregory, B. W.; Suggs, D. W. J. Vac. Sci. Technol. 1992, A10, 886. (d) Demir, U.; Shannon, C. Langmuir 1994, 10, 2794. (e) Demir, U.; Shannon, C. Langmuir, in press. (9) Huang, B. M.; Colletti, L. P.; Gregory, B. W.; Anderson, J. L.; Stickney, J. L. J. Electrochem. Soc. 1995, 142, 3007.

9484 J. Phys. Chem., Vol. 100, No. 22, 1996 (10) Tang, J.; Albrecht, A. C. In Raman Spectroscopy, Szymanski, H. A., Ed.; Plenum: New York, 1970; Chapter 2, p 33. (11) Pinczuk, A.; Burstein, E. In Light Scattering in Solids; Cardona, M., Ed.; Springer: Berlin, 1983. (12) Ziegler, L. D. Acc. Chem. Res. 1994, 27, 1. (13) Shiang, J. J.; Risbud, S. H.; Alivisatos, A. P. J. Chem. Phys. 1993, 98, 8432. (14) Landolt-Bornstein Numerical Data and Functional Relationships in Science and Technology; Helwege, K. H., Ed.; Springer: Berlin; New Series III, Vols. 22a and 23a. (15) Mooradian, A. Phys. ReV. Lett. 1969, 22, 185. (16) Chance, R. R.; Prock, A.; Silbey, R. AdV. Chem. Phys. 1978, 37, 1. (17) Fendler, J. H.; Meldrum, F. C. AdV. Mater. 1995, 7, 607. (18) Prof. J. L. Stickney, University of Georgia, personal communication. (19) Boone, B. E.; Gichuhi, A.; Shannon, C. J. Electrochem. Soc., submitted. (20) Myers, A. B. J. Opt. Soc. Am. B: Opt. Phys. 1990, 7, 1665.

Boone and Shannon (21) Lee, S.-Y.; Heller, E. J. J. Chem. Phys. 1979, 71, 4777. (22) Shannon, C. Langmuir, Manuscript in preparation. (23) (a) Friedman, H.; Wilson-Gordon, A. D. Chem. Phys. Lett. 1979, 64, 337. (b) Alivisatos, A. P.; Harris, A. L.; Levinos, N. J.; Steigerwald, M. L.; Brus, L. E. J. Chem. Phys. 1988, 89, 4001. (24) On the basis of a simple golden rule argument. (25) Mittelman, D. M.; Schoenlein, R. W.; Shiang, J. J.; Colvin, V. L.; Alivisatos, A. P.; Shank, C. V. Phys. ReV. B 1994, 49, 14435. (26) We note that dephasing of the exciton through elastic collisions from structural defects almost certainly contributes significantly to the total dephasing rate. However, since we expect the defect density to be roughly constant (or to increase slightly) as a function of coverage, this mechanism cannot account for the observed trend. (27) (a) Bawendi, M. G.; Carroll, P. J.; Wilson, W. L.; Brus, L. E. J. Chem. Phys. 1992, 96, 946. (b) Colvin, V. L.; Alivisatos, A. P. J. Chem. Phys. 1992, 97, 707.

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