Optical studies of meso-porous siliceous

1
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.
• (MCMMobil 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/Q
uantum.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
Experiment
Monochromator
Photoluminescence measurement
8
Experiment
IRTA
9
Non-porous
Microporous (lt2nm)
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
PL spectrum of as-synthesized MCM-41 and MCM-48
at room temperature. The dashed lines are fitted
Gaussian components
12
Hydrogen bonded silano groups
H
H
H
H
O
O
O
O
SiO2 surface
Si Si Si Si Si
Single silanol group
13
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 (TRTAgt 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
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).
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
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
(strain siloxane bridge)
E centers
NBOHCs
Both surface E centers and NBOHCs increase after
the RTA treatment with TRTAgt 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. 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)
33
Result and Discussion
The variation of the luminescence intensity with
temperature of the MCM-41.
34
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)