J . Phys. Chem. 1988, 92, 4988-4994
4988
Photoluminescence and Relaxation Dynamics of CdS Superclusters in Zeolitest Ying Wang* and Norman Herron* Central Research and Development Department, E . I . DuPont de Nemours & Co., Wilmington, Delaware 19898 (Received: January 14, 1988)
Photoluminescence properties of CdS superclusters in zeolites have been studied. Three different emission bands, all attributed to defects rather than carrier recombination, are observed. The yellow-green emission, which can be observed in zeolites X, Y, and A, is attributed to Cd atoms. The red emission is attributed to sulfur vacancies, following previous works on colloidal CdS. The blue emission is due to shallow donors, the nature of which is not yet clear but we suggest it is sulfur related. Both red and blue emissions can be observed only in zeolite A. The temperature dependences of the emission intensities and lifetimes are quite different for different emission bands. For the blue emission, the temperature dependence can be described by an Arrhenius equation with an activation energy of 4.6 meV. We attribute the relaxation process to thermal ionization of donor defects or bound excitons. Both red and yellow-green emissions show temperature dependence characteristic of multiphonon-induced radiationless processes. The relaxation processes belong to the intermediate-coupling regime from theoretical analyses. The effective mediating phonon frequency deduced, 500-600 cm-', is much higher than the highest phonon frequency of CdS. These results indicate that interface and host (zeolite) phonons are mainly responsible for the radiationless relaxation processes of CdS superclusters in zeolites. Combining with the previous results on colloidal CdS, we conclude that as the semiconductor cluster size decreases, although the optical absorption is still characteristic of the semiconductor itself, the excited-state relaxation dynamics is gradually dominated by the interface and host.
Introduction
Semiconductors with reduced dimensions such as superlattice and multiple quantum well structures can now be routinely prepared with modern epitaxial growth techniques.] In these materials, the excitons are confined along the growth direction and are free to move in the other two dimensions. Many interesting optical properties have been observed for these materiah2 There is also much current effort to fabricate materials where excitons are confined in two dimensions and three dimensions, often referred to as quantum wires and quantum dots, respectively. Small colloidal semiconductor particles belong to the latter category and can be conveniently prepared with synthetic chemical techn i q u e ~ . ~ - It ' ~appears that, at least in principle, a wide variety of semiconductor particles can be prepared in solution^,^-'^ g l a ~ s e s , ' and ~ ~ ' ~polymer^,'^^'^ although the identification is often not unambiguous." We have recently16-'* prepared a novel class of semiconductor/zeolite composite materials where well-defined cubic (CdS)4 clusters are stabilized inside the 5-A sodalite cages of the zeolite and are interconnected to form an extended structure that we refer to as a supercluster.ls These materials can be viewed as threedimensional arrays of mutually interacting quantum dots with geometric structures imposed by the zeolite internal pore structure. Different spatial arrangements of these quantum dots can be achieved by using different zeolites as the template. This is in essence a three-dimensional superlattice structure, different from the traditional superlattice in the sense that the spatial pattern of the latter can be controlled only along one dimension (the growth direction). In the previous paper1* we reported the preparation, the structure, and the optical properties of CdS superclusters in zeolites. We now report their photoluminescence properties and excited-state dynamics. The nature of the defects and the effects of the host will be discussed. Experimental Section
Synthesis. The syntheses of CdS superclusters in zeolites and their structure determinations have been described in detail in a separate paper.Is The samples studied here have the following unit cell compositions: Cd1s,3S6,1A156Si136 (CdS in zeolite Y), (CdS Cd21S14A190Silo2 (CdS in zeolite X), and Cd2,52S3,1Al12Si12 in zeolite A). All samples are Cd-rich except the CdS/zeolite A sample, which is slightly S rich by 0.87 wt %. Photoluminescence Spectra and Lifetime Measurements. Photoluminescence spectra were taken on a Spex Fluorolog Contribution no. 4632.
0022-3654/88/2092-4988$01.50/0
fluorimeter. All the reported spectra have been corrected for the photomultiplier response. Samples were cooled in either a nitrogen cryostat (Oxford Instruments DN1704) or a continuous-flow helium cryostat (Oxford Instruments CF1204). Temperature was read from the inner wall of the vacuum chamber, rather than directly from the sample. To ensure that the sample achieves an equilibrium temperature with the surrounding helium exchange gas, we monitor the emission intensity of the sample until it reaches a plateau. This usually takes about 1-2 h after the Dewar flask is cooled to 4 K. Fluorescence lifetimes were measured by a time-correlated single-photon-counting technique with an apparatus from Photochemical Research Associates. A hydrogen lamp provides the excitation pulse with a half-width of about 2 ns. The data were (1) (a) Chang, L. L.; Esaki, L.; Howard, W. E.; Ludeke, R. J . Vac. Sci. Technol. 1973,10, 11. (b) Cho, A. Y.; Arthur, J. R. Prog. Solid State Chem. 1975, 10, 157. (c) Dingle, R.; Gossard, A. C.; Wiegmann, W. Phys. Reo. Lett. 1975, 34, 1327. (2) Chemla, D. S.; Miller, D. A. B. J . Opt. SOC.Am. B Opt. Phys. 1985, 2, 1155. (3) Berry, C. R. Phys. Rev. 1967, 161, 848. (4) (a) Brus, L. E. J . Phys. Chem. 1986,90,2555, and references therein. (b) Brus, L. E. J . Chem. Phys. 1984, 80, 4403. (c) Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J . Chem. Phys. 1985, 83, 1406. (5) (a) Weller, H.; Schmidt, H. M.; Koch, U.; Fojtik, A,; Baral, S.; Henglein, A.; Kunath, W.; Weiss, K.; Dieman, E. Chem. Phys. Letr. 1986, 124, 557, and references therein. (b) Weller, H.; Koch, U.; Gutierrez, M.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 649. (6) (a) Nozik, A. J.; Williams, F.; Nenadovic, M. T.; Rajh, T.; Micic, 0. I. J . Phys. Chem. 1985, 89, 397. (b) Nedeljkovic, J. M.; Nenadovic, M. T.; Micic, 0. I.; Nozik, A. J. J . Phys. Chem. 1986, 90, 12. (7) Ramsden, J. J.; Webber, S. E.; Gratzel, M. J . Phys. Chem. 1985,89, 2740. (8) Tricot, Y.-M.; Fendler, J. H. J . Phys. Chem. 1986, 90, 3369. (9) Variano, B. F.; Hwang, D. M.; Sandroff, C. J.; Wiltzins, P.; Jing, T. W.; Ong, N . P.J . Phys. Chem. 1987, 91, 6455. (10) Dannhauser, T.; ONeil, M.; Johansson, K.; Whitten, D.; McLendon, G. J . Phys. Chem. 1986, 90, 6074. (11) Wang, Y.; Herron, N. J . Phys. Chem. 1987, 91, 5005. (12) (a) Ekimov, A. I.; Onushchenko, A. A. JETP Lett. 1984, 40, 1136. (b) Ekimov, A. I.; Efros, Al. L.; Onushchenko, A. A. Solid State Commun. 1985, 56, 921. (c) Efros, Al. L.; Efros, A. L. Sou. Phys.-Semicond. (Engl. Transl.) 1982. 16. 772. (13)'Borreilii N. FY; Hall, D. W.; Holland, H. J.; Smith, D. W. J . Appl. Phys. 1987, 61, 5399. (14) Wang, Y.; Mahler, W. Opt. Commun. 1987, 61, 233. (1 5) Wang, Y . ;Suna, A,; Mahler, W.; Kasowski, R. J . Chem. Phys. 1987, 87.. 7315. - (16) Wang, Y.; Herron, N. J . Phys. Chem. 1987, 91, 257. (17) Parise, J. B.; MacDougall, J.; Herron, N.; Farlee, R.; Sleight, A. W.; Wang, Y.; Bein, T.; Moller, K.; Moroney, L. M. Inorg. Chem. 1988, 27, 221. (18) Herron, N.; Wang, Y.; Eddy, M.; Stucky, G.; Cox, D. E.; Bein, T. J . A m . Chem SOC.,in press.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 4989
Dynamics of CdS Superclusters in Zeolites
CdS/zeolite Y 0.901
' 3
/
/
/
/
J
CdS/zeolite X
I \
\ \
\
\ \
\
\ \ \ \ \
0.00
200
- . . -_ _ , . 300
400 500 600 w a v e l e n g t h , nm
' l ) , this effective phonon frequency approaches the highest phonon frequency in the medium. In the intermediate-coupling regime (S l ) , lower frequency phonons can also contribute, and this effective phonon frequency represents an “average” of all contributing phonons. In the strong-coupling limit (S < l), when the relaxation involves a relatively large displacement in the nuclear configuration (e.g., isomerization, diffusion, ionic conductivity, etc.), we have an activated rate process. In this case, the nonradiative transition rate can be expressed in the familiar Arrhenius form?’
-
k,, =
exp(-E,/kT)
(11)
where A is the preexponential factor, k is the Boltzmann constant, and E, is the activation energy. Comparison between Theory and Experiments. To compare with the experimental data, we may rewrite eq 1 in terms of the relative emission intensity:
I O / [ = ( 1 + knr/kr)/(l + kn,O/kr)
(12)
where Io and kmorepresent the emission intensity and nonradiative decay rate a t 4 K, respectively, I and kn,represent the emission intensity and nonradiative decay rate at temperature T , respectively, and k, represents the radiative rate. K , and kmoare defined by eq 1 . Fitting of eq 12 to experimental data involves the optimization of three parameters: C / k , , G, and ha. The first parameter has no physical meaning unless the radiative rate constant is known. If we assume that nonradiative decay dominates over radiative decay, then eq 12 reduces to
I o / I = knr/kn,O
(13)
Fitting of eq 13 to experimental data still involves three parameters: C,G, and ha. The difference is that the first parameter, C, now has physical meaning. Its magnitude reflects the strength of electronic coupling. We have fitted the data using a nonlinear least-square fitting program containing IMSL subroutine ZXSSQ, LINV3F, UGETIO, and USPLO. The program minimizes the sum of squares using a finite difference Levenberg-Marquardt algorithm. To make sure that the solution is unique, we also determine the lower and upper limit of a parameter with acceptable standard deviation by fixing one parameter and optimizing the other two. Figure 4 shows the intensities of the yellow-green emission band of CdS/zeolite Y , relative to the emission intensity at 4 K, as a
function of temperature. Also plotted are the relative decay rates for comparison purpose. These are taken to be the first exponential of the biexponential fit to the decay curves (Figure 5 ) . The decay rate data and the intensity data agree with each other semiquantitatively. Here we use the relative emission intensity data for theoretical analysis. From eq 13 and 1, the following parameters give the best fit to the observed data in Figure 4: h w = 510 cm-’ (450-650),G = 32.8 (29-36), C = 1.6e-32(1.2-2.6). The standard deviation of the fit is 0.36. With these parameters, we calculate the coupling parameter S 1 , which corresponds to the intermediate-coupling regime of the radiationless transition theory. The values within parentheses represent the lower and upper limit of the parameter. At these limits, the fit is already bad even visually. We have also fitted the data with eq 12 and 1 and obtained similar results. At present, it is not clear whether the low-temperature decay is mostly radiative or n ~ n r a d i a t i v e . ~ ~ We therefore do not attach too much significance to the value of C. However, the values of G and h w are significant. We have also analyzed the red emission from CdS/zeolite A and similar results are obtained (Figure 9 ) . Parameters giving the best fit are h w = 630 cm-I, G = 23.9, C = 2.2e-31,and S 1 . Again, this belongs to the intermediate coupling regime. The most interesting result coming out from these analyses is the very high effective mediating phonon frequency, around 500-600 cm-’, that is responsible for the nonradiative relaxation. This frequency is considerably higher than the highest phonon frequency in bulk CdS,34300 cm-’. For CdS superclusters in zeolite Y, the Cd-S bond distance, 2.47 &Is is slightly shorter than that of bulk CdS, 2.52 A. We therefore expect the phonon frequency of CdS superclusters to be slightly higher than that of bulk CdS, but it still cannot be as high as 500 cm-1.35 On the other hand, the vibrational frequency of Cd-0 is around 500 cm-I, and the zeolite framework has a vibrational frequency around 1000 cm-I due to Si-0 and A1-0 bonds. It seems that it is these interface and host phonons that are responsible for the radiationless relaxation processes. These results also suggest that CdS superclusters have to be strongly coupled to the zeolite lattice. This is supported by the synchrotron X-ray results,l* which show the distance between Cd atoms and oxygen atoms of the sodalite cages is only 2.48 A, slightly longer than the bulk CdO bond distance, 2.35 A.36 In a recent study22on the photoluminescence of 22- and 38-A CdS colloidal particles, a similar multiphonon-induced radiationless process was also observed. The emission lifetimes in that case stayed about constant up to 40-50 K, much lower than the 100 K observed in the present case, and then suddenly quenched. The emission was attributed to trapped donor-acceptor recombination. A different version of the radiationless transition theory, but in the same spirit as the present one, was used to analyze the data. The effective mediating phonon frequency was found to be about 140-160 cm-’ and was attributed to the CdS phonons. The combination of these two studies indicate that for CdS in the size range above 20 A, the defect relaxation processes are still predominantly controlled by the CdS lattice phonons. As the cluster size becomes smaller, the interface and host phonons start to dominate the dynamics. We note that although the dynamics of CdS superclusters are dominated by the interface and host, the optical absorption is still dominated by CdS itself. This is reasonable since CdS is such a strong chromophore. An appropriate
-
-
(33) We estimate the emission quantum yield to be
