The Properties of Silicon Clusters in Zeolite
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methods are applied, however, it is still difficult to make homogeneous materials. Now we direct our attention to zeolite as the substrate. Zeolite is a kind of aluminosilicate and its rigid structure constitutes nanometer-sized cages whose size and shape depend on the kind of the zeolite. To utilize this characteristic, many researchers have attempted to embed various materials in zeolite cages and have reported several interesting properties [6-8] . Some researchers have also used zeolite as the substrate of semiconductor clusters such as CdS [8], and blue shift of the PL peak was confirmed. Furthermore, zeolite is transparent in the visible region, as is quartz, and this property also seems to be suitable for use in optical devices. In this paper, we calculate the thermodynamic stability and electronic structure of silicon clusters in zeolite. CALCULATION METHOD We chose hydrogenated spherical silicon clusters, Si10 Hl 6 and'Si 20 H36 , and dehydrated zeolite-A whose structural formula for the unit cell is expressed as M,/,(A102)n(SiO2)24-, where M denotes the cation of valence x. Although synthetic zeolite-A generally contains the same number of Si and Al atoms, which corresponds to n=12, we adopted n=O as a calculation model. Figure 1 shows the unit cell of all-silica zeolite-A. There are two types of cages, the a-cage and the f-cage, whose diameters are about 11 A and 6 A, respectively. 361 Mat. Res. Soc. Symp. Proc. Vol. 486 01998 Materials Research Society
Figure 1: The unit cell of dehydrated all-silica zeolite-A. Its framework is illustrated with sticks which indicate Si-O-Si bonds. Si atoms are located at the vertices, and 0 atoms are located in the middle of the sticks. The spheres represent the cages constituted by the framework; the cages located at the center and the apex of the unit cell are called the a-cage and the f-cage, respectively.
We embedded one cluster in one of these cages, and calculated the properties for such a structure, employing periodic boundary conditions. First we performed molecular mechanics (MM) simulations to obtain optimized structures of each cluster, zeolite and their compound, and their potential energies, Vduster, Vzeolite and Vcompoun.. From these potential energies, we calculated the stabilization en-
ergy,
EsSZhii,,E5o.n,
using
Estabilti..o,= VKompund - (VMius.t. + Vzeoiite). This value indicates the thermodynamic stability of the compound. Then a molecular dynamics (MD) simulation was carried out to observe the motion of the atoms at finite temperature. For these MM and MD calculations, we used the Discover [9] program with the consistent-valence forcefield (CVFF). After we had verified the stability of the compound, its electronic structure and optical properties were calculated by density functional method (DFT). For these calculations we used two programs, DSolid [10] and ESOCS [11], and compared their results. The main difference between the algorithms used in these programs is in the basis functions used to expand the wave functions. During these electron
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