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.

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

(210)

(200)(110)

(100)

2£c(deg.)

Inte

nsi

ty (

a. u

.)The Profile of MCM-41

0.0 0.2 0.4 0.6 0.8 1.0100

200

300

400

500

600

700

800

2 3 4 5

3.3 nm

Pore diameter (nm)

absorption deabsorption

Vol

um

e A

dso

rbed

(cm

3 /g)

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

PL

In

ten

sity

(a.

u.)

Energy (eV)

MCM-41

1

12

2

SiO2 surface

Si Si Si Si Si

OOOO

H H H H H

O

OO O

Hydrogen bonded silano groups

Single silanol group

NBOHC

Si

OO O

O

H

hν

Si

OO O

O‧

+ H‧

Single silanol group

Result and Discussion

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

UnRta

MCM-41

Energy (eV)Energy (eV)

PL

In

ten

sity

(a.

u.)

200¢J

800¢J

600¢J

400¢J

UnRta

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)

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).

0 1000 2000 3000

0.2

0.4

0.6

0.8

1.0

0 200 400 600PL

Int

ensi

ty (

a. u

.)

Time (sec)

MCM-48

PL

Int

ensi

ty (

Nor

mal

ized

)

Time (sec)

Red

MCM-41

Result and Discussion

0 500 1000 1500 2000 2500 3000 35000.0

0.2

0.4

0.6

0.8

1.0

O2

Air

H2

Vacuum

PL

Int

ensi

ty (

Nor

mal

ized

)

Time (sec)

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

Result and Discussion

0 1000 2000 3000

0.2

0.4

0.6

0.8

1.0

O2

Air

PL

Int

ensi

ty (

Nor

mal

ized

)

Time (sec)

MCM-41

0 1000 2000 30000.0

0.2

0.4

0.6

0.8

1.0

O2

Air

PL

Int

ensi

ty (

Nor

mal

ized

)

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.

Result and Discussion

0 2000 4000 6000 8000 10000 120000.0

0.2

0.4

0.6

0.8

1.0

PL

In

ten

sity

(N

orm

aliz

ed)

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 －

Result and Discussion

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

PL

In

ten

sity

(a.

u.)

Energy (eV)

Result and Discussion

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

UnRTA

PL

In

ten

sity

(a.

u.)

Energy (eV)

(strain siloxane bridge)

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

C

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

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)]

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

Inte

nsi

ty (

a.u

.)

Energy (eV)

1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

PL

In

ten

sity

(a.

u.)

Energy (eV)

Result and Discussion

Polarized PL spectra of MCM-41 nanotubes

I I

PI I

I

I

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)]

Pulse LaserMirror

Lens

Lens Lens

Sample

Detector

Time-resolved Photoluminescence (TRPL)

0 4 8 12 16 0 4 8 12 16

0 4 8 12 16 0 4 8 12 16

(a) 15K

(b) 40K

2.5

eV P

L I

nten

sity

(a.

u.)

Time (nsec)

(c) 100K

Time (nsec)

(d) 300K

Result and Discussion

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

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

Result and Discussion

0.00 0.02 0.04 0.062.0

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

Tim

e C

onst

ant

(nse

c)

1/T(K-1)

Time Constant E

a=28 meV

Temperature dependence of the recombination time constant

1 1 1

1 exp( )

r nr

anr

E

kT

: radiative recombination time

: nonradiative recombination timer

nr

Result and Discussion

0 200 400 600 800 1000

£G=30meV

Inte

nsi

ty (

a.u

.)

Wavenumber (cm-1)

MCM-41 Raman

396 nm

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

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)

Result and Discussion

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

100K

200K

PL

In

ten

sity

(a.

u.)

Energy (eV)

300K

12K

40K

0 50 100 150 200 250 300 350

PL

Int

ensi

ty (

a. u

.)

Temperature (K)

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).

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)