Encapsulation of silicon nanoclusters in zeolite Y

Luminescent silicon clusters have been synthesized by the chemical vapor deposition of Si2H6 into the alpha-cages of H32Na24Y zeolite. The synthetic process was monitored by FTIR, TGA-MS, and Si-29 and H-1 solid-state NMR spectroscopies. In the initial step at 100 degrees C, Si2H6 reacts with the Bronsted acid sites to produce anchored ZO-Si2H5. Si2H6 is also chemisorbed at Na+ cation sites to give Si2H6/NaHY and is possibly physically trapped within the alpha-cage by the anchored disilyl groups. Multiple quantum H-1 NMR spin counting shows that each alpha-cage contains 38 H atoms. This is equivalent to 14 Si atoms present as a combination of disilyl and disilane. Subsequent thermal treatment of the entrapped disilane precursors leads, via H-2 and SiH4 elimination, to the formation of Si clusters. The formation of Si clusters is complete at 550 degrees C. These clusters are capped by up to 5 H atoms (determined by H-1 NMR spin counting) and attached to the zeolite framework through SiOx linkages (determined by Si K-edge XANES). The average size of the resulting silicon clusters is 12 +/- 2 Si atoms (determined by XPS and Si K-edge XANES). The encapsulated Si clusters are air-stable and exhibit a room-temperature photoluminescence in the green-yellow region with a peak energy at similar to 2.2 eV. The HOMO-LUMO energy gap in the Si cluster is estimated to be 2.2 eV, from a comparison of the band edges of the Si clusters and bulk Si (c-Si) (determined by synchrotron photoabsorption (Si K-edge XANES) and photoemission spectroscopies). The close correspondence of the HOMO-LUMO energy gap and the photoluminescence peak energy confirms the origin of luminescence from the Si cluster as a predominantly electron-hole radiative recombination process.