Presentation on theme: "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."— Presentation transcript:
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
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. )
http://terra.cm.kyushuu.ac.jp/lab/ research/nano/Quantum.html 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.
The simulate model image of MCM -41and MCM-48 www.ill.fr/AR-99/page/ 34liquids.htm (a) MCM-41 has hexagonal arrangement of unidirectional pores (b) MCM-48 has a cubic structure, gyroid minimal surface.
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.
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 N 2 gas atmosphere for 30 sec, respectively.
Experiment Photoluminescence measurement Laser line filter Notch filter Raman measurement 396 nm pulse laser Monochromator Polarizer Polarization of photoluminescence
The adsorption mechanism is controlled by the characterization of microporous and mesoporous materials. Six characteristic shapes of the physisoption isotherms. [K. S. W. Sing et al. Pure. Appl. Chem.57 (1985) 603] Microporous (<2nm) Non-porous Mesoporous Macoporous (>50nm) Macoporous
The Profile of MCM-41 X-ray diffraction pattern of siliceous MCM-41 nanotubes. Isotherms of N 2 adsorption on siliceous MCM-41 nanotubes. The inset shows the pore-size distribution curve.
Result and Discussion PL spectrum of as-synthesized MCM-41 and MCM-48 at room temperature. The dashed lines are fitted Gaussian components 1 1 2 2
SiO 2 surface Si Si Si Si Si OOO O HHHHH O OOO Hydrogen bonded silano groups Single silanol group
NBOHC Si OOO OH hν Si OOO O ‧ + H ‧ Single silanol group
Result and Discussion Photoluminescence spectra of MCM-41 and MCM-48 after RTA at room temperature.
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.
As T RTA increases further (T RTA ＞ 400 o C), the single silanol groups with longer distance can then be dehydroxylated and give rise to the formation of the strained siloxane bridges. 1. Strained siloxane bridge has been demonstrated to create NBOHCs and surface E’ centers (i.e.,≡Si) 2.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 ] (strain siloxane bridge)
Result and Discussion PL degradation 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).
Result and Discussion PL degradation of MCM-48 as a function of irradiation time
Result and Discussion The Red-PL degradation of MCM-41 and MCM-48 as a function of irradiation time in air and O 2 ambient gases.
Evolution of PL intensity of MCM-48 as a function of irradiation time in O 2 gas. Result and Discussion 2.25 eV we suggest that O 2 － molecules can recombine with NBOHC on the surface, leading to the quenching of NBOHCs
Result and Discussion PL spectrum of MCM-41 at room temperature.
Result and Discussion Photoluminescence spectra of MCM-41 after RTA at room temperature.
(strain siloxane bridge) Both surface E’ centers and NBOHCs increase after the RTA treatment with T RTA ＞ 400 o C E’ centersNBOHCs 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
Result and Discussion B. L. Zhang et al. The first-principles calculations. The T 1 → S 0 is about 2.5 eV, is in agreement with our experimental result. B. L. Zhang and K. Raghavachari, Phys. Rev. B 55, R15993 (1997)]
Results and Discussion PLE spectrum of the 2.5-eV emission band from MCM-41.
Result and Discussion Polarized PL spectra of MCM-41 nanotubes
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)]
Result and Discussion The photoluminescence decay profile of MCM-41 at different temperatures.
Result and Discussion Temperature dependence of the recombination time constant
Result and Discussion Raman spectra of MCM-41 nanotubes (nonbridge oxygen atom)
Result and Discussion Y. Kanemitsu attributed the active energy E a to the phonon-related processes 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)
Result and Discussion The variation of the luminescence intensity with temperature of the MCM-41.
Result and Discussion 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).
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 O 2 － and NBOHC. Published in Solid State Comm. 122, 65 (2002) Micrpor. Mespor. Mater. 64, 135 (2003)
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)