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Optical studies of meso-porous siliceous

東南技術學院 九十二學年第二學期 資訊工程系第二次論文發表會. Optical studies of meso-porous siliceous. Y. J. Lee a,c J. L. Shen b,c a Department of Computer Science and Information Engineering, Tung Nan Institute of Technology, Taipei, Taiwan, R.O. C. b Department of Chemistry, Chung Yuan Christian University,

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Optical studies of meso-porous siliceous

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  1. 東南技術學院 九十二學年第二學期 資訊工程系第二次論文發表會 Optical studies of meso-porous siliceous Y. J. Lee a,c J. L. Shen b,c a Department of Computer Science and Information Engineering, Tung Nan Institute of Technology, Taipei, Taiwan, R.O. C. b Department of Chemistry, Chung Yuan Christian University, Chung-Li, Taiwan, R.O.C c Center for Nanotechnology, CYCU, Chung-Li, Taiwan, R.O.C

  2. Introduction • The scientists of the Mobil Oil company firstly synthesized M41S-type meso-porous materials, such as MCM-41and MCM-48 in 1992. (MCM:Mobil Composition of Matter n.)

  3. The simulate synthesis process of MCM-41 MCM-41 has hexagonal arrangement of unidirectional pores with very narrow pore size distribution, which can be systematically varied in size from approximately ~20 to 200Å. http://terra.cm.kyushuu.ac.jp/lab/ research/nano/Quantum.html

  4. The simulate model image of MCM -41and MCM-48 (a) MCM-41 has hexagonal arrangement of unidirectional pores (b) MCM-48 has a cubic structure, gyroid minimal surface. www.ill.fr/AR-99/page/ 34liquids.htm

  5. Introduction • There have been few reports on the optical properties of MCM-41 and MCM-48. • The optical properties are not only offer a convenient way to clarify the structural defects, but also provide useful information for extending their applications to optical devices.

  6. Experiment • The photoluminescence (PL) spectra were taken by using a focused Ar+ laser (488nm) and He-Cd laser (325nm) at room temperature. • The Time-resolved Photoluminescence (TRPL) spectra were measured with temperature dependence and using a solid-state laser (396 nm) with a pulse duration 50 ps as the excitation source. • The MCM-41 and MCM-48 samples were subjected to rapid thermal annealing (RTA) at 200 ℃,400℃,600℃,800℃ in N2 gas atmosphere for 30 sec, respectively.

  7. Notch filter Laser line filter Polarizer 396 nm pulse laser Raman measurement Polarization of photoluminescence Experiment Monochromator Photoluminescence measurement

  8. Experiment IRTA

  9. Macoporous (>50nm) Macoporous Non-porous Microporous (<2nm) Mesoporous Six characteristic shapes of the physisoption isotherms.[K. S. W. Sing et al. Pure. Appl. Chem .57 (1985) 603] The adsorption mechanism is controlled by the characterization of microporous and mesoporous materials.

  10. The Profile of MCM-41 Isotherms of N2 adsorption on siliceous MCM-41 nanotubes. The inset shows the pore-size distribution curve. X-ray diffraction pattern of siliceous MCM-41 nanotubes.

  11. Result and Discussion 2 1 1 2 PL spectrum of as-synthesized MCM-41 and MCM-48 at room temperature. The dashed lines are fitted Gaussian components

  12. H O O O O Hydrogen bonded silano groups H H H H O O O O SiO2 surface Si Si Si Si Si Single silanol group

  13. NBOHC H ‧ O hν O ‧ + H Si Si O O O O O O Single silanol group

  14. Result and Discussion Photoluminescence spectra of MCM-41 and MCM-48 after RTA at room temperature.

  15. The hydrogen-bonded silanol groups are dehydroxylated due to water removing and form siloxane bonds and single silanol groups. The dehydroxylation of hydrogen-bonded silanol groups take place to form single silanol groups, leading to the generation of NBOHCs and the increase of the PL intensity of MCM-41 and MCM-48 simultaneously.

  16. (strain siloxane bridge) As TRTA increases further (TRTA＞ 400 oC), the single silanol groups with longer distance can then be dehydroxylated and give rise to the formation of the strained siloxane bridges. • Strained siloxane bridge has been demonstrated to create NBOHCs and surface E’ centers (i.e.,≡Si•) • We suggest that the 2.16-eV PL origins from the NBOHCs associated with the strained siloxane bridges. • [ D. L. Griscom and M. Mizuguchi, J. Non-Cryst. Solids 239 (1998) 66 ]

  17. Result and Discussion PLdegradation of MCM-41 and MCM-48 as a function of irradiation time. The inset plots MCM-48 PL degradation as a function of irradiation time, including a dark period (without laser irradiation).

  18. Result and Discussion PL degradation of MCM-48 as a function of irradiation time

  19. Result and Discussion The Red-PL degradation of MCM-41 and MCM-48 as a function of irradiation time in air and O2 ambient gases.

  20. 2.25 eV Result and Discussion Evolution of PL intensity of MCM-48 as a function of irradiation time in O2 gas. we suggest that O2－molecules can recombine with NBOHC on the surface, leading to the quenching of NBOHCs

  21. Result and Discussion PL spectrum of MCM-41 at room temperature.

  22. Result and Discussion Photoluminescence spectra of MCM-41 after RTA at room temperature.

  23. The surface E’ centers can combine and produce the twofold-coordinated silicon centers, which emits the blue-green luminescence in the triplet-to-singlet transition (strain siloxane bridge) E’ centers NBOHCs Both surface E’ centers and NBOHCs increase after the RTA treatment with TRTA＞ 400 oC

  24. Result and Discussion B. L. Zhang et al. The first-principles calculations. The T1→ S0 is about 2.5 eV, is in agreement with our experimental result. B. L. Zhang and K. Raghavachari, Phys. Rev. B 55, R15993 (1997)]

  25. Results and Discussion PLE spectrum of the 2.5-eV emission band from MCM-41.

  26. Result and Discussion Polarized PL spectra of MCM-41 nanotubes

  27. Result and Discussion • The PLE measurement: The value for the direct singlet-to-triplet excitation transition in two-coordinated Si is around 3.3 eV [L. Skuja J. Non-Cryst. Solids 149, 77 (1992)] [ G. Pacchioni and G. Ierano, J. Non-Cryst. Solids 216, 1 (1997) ] • The Polarized PL spectra: The degree of polarization P of 2.5 eV calculated was found to be 0.25, which agrees well with the P value (0.22) obtained from the reported triplet-to-singlet transition in twofold-coordinated silicon [L. Skuja, A. N. Streletsky, and A. B. Pakovich Solid State Commun. 50, 1069 (1984)]

  28. Time-resolved Photoluminescence (TRPL) Detector Sample Lens Lens Lens Pulse Laser Mirror

  29. Result and Discussion The photoluminescence decay profile of MCM-41 at different temperatures.

  30. Result and Discussion Temperature dependence of the recombination time constant

  31. Result and Discussion Raman spectra of MCM-41 nanotubes (nonbridge oxygen atom)

  32. Result and Discussion Y. Kanemitsuattributed the active energy Ea to the phonon-relatedprocesses in the inhomogeneous surface of the oxidize Si nanocrystals. For nonradiative recombination process, they suggested that the carriers undergo the phonon-assisted tunneling from the radiative recombination centers to the nonradiative centers Y. Kanemitsu, Phys. Rev. B 53, 13515 (1996)

  33. Result and Discussion The variation of the luminescence intensity with temperature of the MCM-41.

  34. At low temperatures: only phonon emission At high temperatures: phonon absorption become dominant The phonon-assisted transition dominates the recombination process at high temperatures, and the time constant of PL decay and the PL intensity decreases. The PL intensity reaches the maximum value at 40 K, implies that the radiative transition is pronounced and fast enough to overcome the nonradiative escape due to the small activation energy in radiative transition (Δ). Result and Discussion

  35. Conclusion Two PL bands were observed at around 1.9 eV and 2.15 eV ,which can be explained by the surface chemistry in MCM-41 and MCM-48. The around 1.9 eV is assigned to the NBOHCs and the around 2.15 eV is related to the NBOHCs associated with the strained siloxance bridges. The PL intensity can be enhanced by the RTA treatment. We suggest the PL degradation origins from the recombination of O2－and NBOHC. Published in Solid State Comm. 122, 65 (2002) Micrpor. Mespor. Mater. 64, 135 (2003)

  36. The blue-green PL in MCM-41 and MCM-48 were attributed to the twofold-coordinated silicon centers, which emit luminescence by the triplet-to-singlet transition. The PL intensity can be enhanced by the RTA treatment with increased the concentration of the surface E’ center. We consider the PL decay dynamics with temperature dependence by TRPL measurement and depict that the nonradiative process, which is associated with the phonon-assisted transition, dominates the recombination mechanism at high temperatures. Published in J. Phys-condens. Mater. 15, L297 (2003)

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