Absorption and Raman spectra of Se8-ring clusters in zeolite 5A

Solid State Communications,

Vol. 100, No. 12, pp. 841-843, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/96 $12.00+.00

Pergamon

PII: SOO38-1098(96)00526-l

ABSORPTION

AND RAMAN SPECTRA

OF Se8-RING CLUSTERS

IN ZEOLITE

5A

Z. Lin,” Z. Wang,” W. Chen,= L. Lir,” G. Li,b Z. Liub H. Hanb and Z. Wangb “Laboratory of Semiconductor Ma’ trials Sciences, Institute of Semiconductors, Chinese Academy of Science+ P.O. Box 912, Beijing 100083, China bNational Laboratory for Superlattices and Microstructures Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China (Received and accepted 13 August 1996 by 2. Gun)

Samples with different weight ratio of Se to zeolite 5A (Se composition) have been prepared by loading Se into the cages of zeolite 5A and the measurements of the absorption and Raman spectra have been carried out for the prepared samples. The measured absorption edges of the samples are close and blue shifted to the value for monoclinic Se containing Sea-ring, suggesting the formation of Sea-ring clusters? in the cages. The continuous and broadening features of the absorption spectra are interpreted by the strong electron-phonon coupling in Ses-ring clusters. The sample with high Se composition has a red shift of the absorption band edge relative to the samples with less Se composition. It is tentatively attributed to the reason that with different Se composition, single Ses-ring clusters and double &s-ring clusters are formed in the cages of zeolite 5A. A single broad band at about 262 cm-’ is observed in the Raman spectra, that gives the further support of the formation of &s-ring clusters. Copyright 0 1996 Elsevier Science Ltd Keywords: A. nanostructures, B. nanofabrications, D. electron-phonon interactions.

1. INTRODUCTION DURING the past decade, the research of semiconductor clusters has been quite popular because of their potential applications in opto-electronic devices and technology. Many methods have been developed to prepare semiconductor clusters. In these methods, an attractive one is to form clusters by loading semiconductor materials into zeolite cages, in which zeolites are used as templates. Clusters prepared by the method are almost same in size, and their distribution are even. These characteristics provide a great convenience for the study of the physical properties of clusters. Loading Se into zeolite A, the work has been reported [l-4], and it is shown that Sesring clusters are formed in zeolite A. In order to study the different Se composition in the samples how to affect the properties of prepared See-ring clusters. We used zeolite

t Ses-ring clusters include double Ses-ring clusters.

single Ses-ring

clusters and 841

D. optical properties,

5A as a template loading Se into the zeolite cages to form See-ring clusters and prepared Ses-ring cluster samples with different Se composition. We found that with different Se composition, single &s-ring ClUSterS and double Ses-ring clusters can be formed in the cages of zeolite 5A and in Ses-ring clusters, there is strong coupling between electrons and phonons, the optical absorption is the absorption of the strong electronphonon coupling quantum system. 2. EXPERIMENTAL

PROCEDURES

We used the powders of zeolite 5A and Se (2 99.95% purity), the size of the powder particles was about 1 pm. Zeolite 5A was dehydrated by heating gradually in a glass ampoule up to 400°C on a vacuum line and kept in vacuum for’ 2 h. The dehydrated zeolite 5A and Se powders were mixed with a certain weight ratio of Se to zeolite 5A and the mixture was ground for 2 h. Then, the ground mixed powders were sealed in the ampoule without exposing them to air. The ampoule was heated

842

ABSORPTION AND RAMAN SPECTRA OF Ses-RING CLUSTERS

uniformly at 400°C for 8 h and cooled to room temperature gradually. All the optical measurements were made at room temperature and within several days after sample preparation. Because water does not react upon Se and zeolite 5A, a certain amount of Se-loaded zeolite 5A powder was put into deionized water to form a solution with a certain sample concentration. The absorption spectra of solutions were measured by using a Hitachi 340 spectrophotometer. Pure zeolite 5A has no absorption within the wavelength range (300nm-800nm). The obtained spectra contains only the absorption of the Se clusters and the scattering effect of the solution. The absorption coefficient of the Se clusters was obtained from the following analysis. Because of the less sample density in solutions, we neglect the light energy flow toward the reverse direction of incident light. The intensity of light through the solution, I, can be presented as Z =Zeexp[-(a,+S)L],

(I)

where Za is the intensity of the incident light, CYis the absorption coefficient of the powder sample (Se clusters), L is the thickness of the solution, S is the scatter rate for light passing the solution. Since the absorption of the samples can be neglected in infrared range (a = 0), S can be obtained by S=tlnF, 1

(2)

where Zr is the measured light intensity in infrared range, in fact, S is the measured absorption coefficient in infrared range. The absorption coefficient of the Se clusters in the whole measured range (300 rmr-800 run) is then obtained as o=tln+-S.

Vol. 100, No. 12

spectra of sample solutions are shown in Fig. 1. The absorption edges in Fig. 1 are close and blue shifted to the value for monoclinic Se which contains Ses-ring [2]. These data suggest that in zeolite 5A, Ses-ring clusters are formed. Furthermore, it is shown [3] that in the wavelength range (300 nm-800 nm), there is little optical absorption for those Se atoms which are not filled into zeolite A cages. So we can conclude that the optical absorption in Fig. 1 is caused by Ses-ring clusters in zeolite 5A cages. The inner diameters of the cage and the window in zeolite 5A are 1.14 nm and 0.5 nm, respectively [5]. It is shown [l] that the cluster-cluster interaction in zeolite A is little, they are isolated from each other. For a lonely cluster within 1.14nm size range, its electronic energy states should be discrete due to quantum confinement effects, but the absorption spectra in Fig. 1 is continuous and broad. It is tentatively attributed to two reasons. One is electron-phonon coupling in Ses-ring clusters, which leads to the transition of different electronic states along with absorbing and emitting phonons. The other is that the energy levels of excited states in Ses-ring clusters are broadened due to their short lifetime which can be caused by surface states and electron-phonon relaxation. Another important feature of the absorption spectra (Fig. 1) is that the sample with 35 wt % Se composition has a red shift of the absorption band edge relative to the samples with less Se composition. Because the clustercluster interaction in zeolite A is little, it can be inferred that the red shift is caused by the ring-ring interaction in the cages. Simple calculations based on the cage density and volume of zeolite 5A [5] reveal that assuming all Se atoms being filled into the cages and corresponding to 35 wt % Se, 8 wt % Se and 2 wt % Se samples, the figure

(3)

Thus, the absorption spectra removed scattering effect can be obtained by subtracting the absorption in infrared range from measured absorption spectra. For the Raman measurement, the sample powder was pressed into a thin piece. We use the back scattering configuration. The samples were excited by the 488.0 nm line of the SP-165-09 Ar+ laser with the exciting power about 8mW. The scattered light was collected by the JY-HRD-2 double grating monochromator equipped with a RCA ~31034 photomultiplier. 3. RESULTS AND DISCUSSION We prepared three kinds of Se composition (2wt % Se, 8 wt % Se, 35 wt % Se) Se-loaded zeolite 5A samples and made them up into solutions in which the concentration of zeolite 5A was 0.001 (mall-‘). The absorption

0 300

350

400

450

500

550

600

Wavelength

Fig. 1. Absorption spectrum of prepared samples: (a) 2 wt % Se sample, (b) 8 wt % Se sample, (c) 35 wt % Se sample.

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ABSORPTION AND RAMAN SPECTRA OF Se,-RING CLUSTERS

843

not again restricted by the law of conservation of momentum in Raman scatter, which brings about that the band in Raman spectra (Fig. 2) is broad. It is shown in Fig. 2 that with the increase of Se composition, the intensity of band peak is increased. This can be explained by the increase of Ses-ring density, because the increase of Ses-ring density will result in the increase of scattered light intensity. 4. CONCLUSIONS

100

150

200

RAMAN SHIFT

250

300

(cm-‘)

Fig. 2. Raman spectra of prepared samples: (a) 2 wt % Se sample, (b) 8 wt % Se sample, (c) 35 wt % Se sample.

of Se atoms in each cage on an average is -13, -3 and -0.7, respectively. While Se atoms are mainly filled into the cages which are at the surface of zeolite 5A particles. Thus, it is possible that there are double Ses-ring clusters in 35 wt % Se sample and in 8 wt % Se and 2 wt % Se samples, only single Ses-ring clusters are existent. Because there is interaction between two single Sesring clusters, the energy bands of double Ses-ring clusters are broadened comparing with that of single Ses-ring clusters. Figure 2 shows the Raman spectra of three kinds of Se composition samples. There is only a single broad band at about 262cm-‘. This band results from the symmetric extension of the bonds and the spectra are basically consistent with the Raman spectrum of Seloaded zeolite NaA [4]. Because there is not a certain momentum for the phonons in Ses-ring clusters, they are

Three kinds of Se composition Se-loaded zeolite 5A samples have been prepared and their absorption and Raman spectra have been measured at room temperature. The results show that with different Se composition, single Ses-ring clusters and double Ses-ring clusters can be formed in the cages of zeolite 5A. The optical absorption of Ses-ring clusters is accomplished by the strong electron-phonon coupling system. Acknowledgements-The authors would like to thank the financial support from the President Foundation of Chinese Academy of Sciences and partly support from the National Nature Science Foundation of China. REFERENCES 1. 2. 3. 4. 5.

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