Optical studies of meso-porous siliceous

transcript

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.

IntroductionThere 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.

ExperimentThe 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.

Experiment

Photoluminescence measurement

Laser line filter

Notch filter

Raman measurement

396 nm pulse laser

Monochromator

Polarizer

Polarization of photoluminescence

Experiment

IRTAIRTA

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

1 2 3 4 5 6

(200)(110)

2£c(deg.)

.)The Profile of MCM-41

0.0 0.2 0.4 0.6 0.8 1.0100

2 3 4 5

3.3 nm

Pore diameter (nm)

absorption deabsorption

Relative Pressure (P/P0)

X-ray diffraction pattern of sili

ceous MCM-41 nanotubes.

Isotherms of N2 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

1.4 1.6 1.8 2.0 2.2 2.4 2.6

MCM-48

Energy (eV)

MCM-41

SiO2 surface

Si Si Si Si Si

H H H H H

Hydrogen bonded silano groups

Single silanol group

+ H‧

Single silanol group

Photoluminescence spectra of MCM-41 and MCM-48

after RTA at room temperature.

1.6 2.0 2.4 1.6 2.0 2.4

200¢J

800¢J

600¢J

400¢J

MCM-41

Energy (eV)Energy (eV)

200¢J

800¢J

600¢J

400¢J

MCM-48

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 TRTA increases further (TRTA ＞ 400 oC), the single silanol groups wit

h 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)

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).

0 1000 2000 3000

0 200 400 600PL

Time (sec)

MCM-48

Time (sec)

MCM-41

0 500 1000 1500 2000 2500 3000 35000.0

Vacuum

Time (sec)

PL degradation of MCM-48 as a function of irradiation time

0 1000 2000 3000

Time (sec)

MCM-41

0 1000 2000 30000.0

Time (sec)

MCM-48

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

Evolution of PL intensity of MCM-48 as a function of irradiation time in O2 gas.

0 2000 4000 6000 8000 10000 120000.0

Degassing on

Time (sec)

2.25 eV

we suggest that O2－ molecules can recombine with NBOHC

on the surface, leading to the quenching of NBOHCs

2 2O Oe －

PL spectrum of MCM-41 at room temperature.

1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

Energy (eV)

Photoluminescence spectra of MCM-41 after RTA at room temperature.

1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

800¢J

600¢J

400¢J

200¢J

Energy (eV)

(strain siloxane bridge)

Both surface E’ centers and NBOHCs increase after the RTA treatment with TRTA ＞ 400 o

E’ centers NBOHCs

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

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)]

Results and Discussion

PLE spectrum of the 2.5-eV emission band from MCM-41.

2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0

Energy (eV)

1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

Energy (eV)

Polarized PL spectra of MCM-41 nanotubes

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)]

Pulse LaserMirror

Lens Lens

Sample

Detector

Time-resolved Photoluminescence (TRPL)

0 4 8 12 16 0 4 8 12 16

(a) 15K

(b) 40K

Time (nsec)

(c) 100K

Time (nsec)

(d) 300K

The photoluminescence decay profile of MCM-41 at different temperatures.

0: ( ) exp - Theory fit I t I t

0.00 0.02 0.04 0.062.0

1/T(K-1)

Time Constant E

a=28 meV

Temperature dependence of the recombination time constant

1 exp( )

: radiative recombination time

: nonradiative recombination timer

0 200 400 600 800 1000

£G=30meV

Wavenumber (cm-1)

MCM-41 Raman

396 nm

Raman spectra of MCM-41 nanotubes(nonbridge oxygen atom)

Y. Kanemitsu attributed the active energy Ea 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)

The variation of the luminescence intensity with temperature of the MCM-41.

1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

Energy (eV)

0 50 100 150 200 250 300 350

Temperature (K)

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).

ConclusionTwo 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)

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)

Optical studies of meso-porous siliceous

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