Ultrathin silica films with a nanoporous monolayer

Article abstract of DOI:10.1246/cl.2004.1408

Ultrathin silica films with a monolayer of uniform nanopores were fabricated on a silicon substrate by contacting triblock copolymer films with tetraethoxysilane vapor followed by calcination to remove the copolymer.

Full text of DOI:10.1246/cl.2004.1408

1408
Chemistry Letters Vol.33, No.11 (2004)
Ultrathin Silica Films with a Nanoporous Monolayer
Shunsuke Tanaka, Norikazu Nishiyama,ꢀ Yasushi Hayashi,y Yasuyuki Egashira, and Korekazu Ueyama
Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University,
1-3 Machikaneyama, Toyonaka, Osaka 560-8531
yResearch Laboratories, DENSO CORPORATION, 500-1 Minamiyama, Komenoki-cho, Nisshin, Aichi 470-0111
(Received July 13, 2004; CL-040828)
Ultrathin silica films with a monolayer of uniform nano-
the nanostructure remained uniform and did not shrink. The lat-
tice spacing d corresponds to the thickness of a monolayer nano-
porous silica film. The FE-SEM image shown as an inset in
Figure 2 is a portion of the box in Figure 1, revealing that the
pore diameter was about 7.5 nm. The pillar size in the middle
and the repeated distance from pillar to pillar were estimated
as about 6.0 and 15 nm, respectively.
pores were fabricated on a silicon substrate by contacting tri-
block copolymer films with tetraethoxysilane vapor followed
by calcination to remove the copolymer.
Techniques for producing thin films are very important in
core industry fields, such as information-communication, ener-
gy, and environmental technologies. Silica thin films have been
attracting much attention because of their central roles in silicon
technology and many other applications. The various methods
used for preparing these films include physical vapor deposition
(e.g., evaporation, pulsed laser ablation, sputtering),1 chemical
vapor deposition,2,3 oxidation, and sol–gel synthesis.4–6
More recently, investigators have succeeded in replicating
an ordered mesophase of surfactants into silica substances via
the co-assembly of silica precursors and surfactants.7–11 Such
nanostructured silica thin films are usually prepared by spin
coating, dip coating, or epitaxial growth.12–16 The removal of
the surfactants can be performed through pyrolysis or solvent ex-
traction, converting the silica into a porous form, while retaining
the nanostructural features of the surfactant. Although many pre-
vious studies have reported on the preparation of these porous
silica thin films, few have controlled the number of layers in
an ordered structure. It seems to be difficult to fabricate ultrathin
silica films with monolayer of nanopores by liquid deposition
methods.
We have previously prepared nanoporous silica films by a
vapor phase method.17,18 In the present study, we have applied
this technique to the fabrication of ultrathin silica films with a
monolayer of nanopores, and have characterized the products
by field emission scanning electron microscope (FE-SEM) and
X-ray diffraction (XRD). The detailed synthesis and characteri-
zation methods are shown in the references and notes sec-
tion.19,20
Figure 1 shows an FE-SEM image of the cross-section of an
ultrathin silica film with a monolayer of nanopores prepared by
this vapor phase synthesis. This film was about 15-nm thick,
continuous and homogeneous on the silicon substrate. The nano-
pores were observable from different angles (see Supplemental
Information, Figures S1) and seemed to be connected in a planar
matrix.
Surface of the silica film
Section of the silicon substrate
100 nm
Figure 1. FE-SEM image of the cross-section of an ultrathin
silica film with monolayer of uniform nanopores. The film was
prepared by vapor phase synthesis.
6000
Film
5000
4000
7.5
nm
15
nm
Silicon substrate
3000
2000
1000
0
(C)
(B)
(A)
0
1
2
3
4
5
6
7
8
2 η / deg.
Figure 2. XRD patterns of the monolayered silica film. Trace A
is a copolymer film; Trace B shows the copolymer film after
treatment with the TEOS vapor (containing the copolymer);
Trace C shows the film after calcination (copolymer-free). The
inset is a portion of the box in Figure 1.
The triblock copolymer film does not have an ordered peri-
odic structure (Figure 2A). A X-ray reflection peak appeared af-
ter treating it with tetraethoxysilane (TEOS) vapor (Figure 2B),
corresponding to a lattice spacing of d ¼ 15:4 nm. The nano-
phase transition may occur dynamically via contact between
the copolymer film and TEOS vapor. The reflection peak hardly
shifted after calcination (Figure 2C). Because the full-width at
half-maximum (FWHM) of the peak did not change, we believe
The number of layers and the film thickness were very sen-
sitive to the triblock copolymer concentration in the precursor
solution. Figure 3 shows the FE-SEM image of the cross-section
of a double-layer nanoporous silica film. These films were ob-
tained using a precursor solution with double the concentration
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.11 (2004)
1409
of the copolymer. The film thickness was about 30 nm, which
was double of the monolayer films. On the basis of the FE-
SEM observations (see Supplemental Information, Figures S2),
we suggest that the porous structure of this film is the same as
that of the monolayer nanoporous films. In the nanoporous films
with five and eight layers of nanopores, the nanopores at the
film-substrate interface are hemispheric in shape and the silica
pillars adhere to the substrate.18 In contrast, in the mono- and
double-layered films, the silica layer adheres to the substrate.
The thin copolymer layer may allow rapid permeation of the
TEOS and a high mobility of the silicate-copolymer composite
film formed at the film-substrate interface. The thermal conduc-
tivity of the double-layer nanoporous silica film (containing the
copolymer) was 0:174 ꢁ 0:007 W mꢂ1 Kꢂ1, a level which was
lower than that of the bulk silica. The measurements were car-
ried out at 25 ꢃC in vacuum (<0:02 Pa). After calcination, the
thermal conductivity was decreased by 70% (0:125 ꢁ 0:005
W mꢂ1 Kꢂ1).
pores should perform very well as nanodevices and nanoflasks.
We gratefully acknowledge the assistance of the GHAS
laboratory and Mr. M. Kawashima of Osaka University for the
FE-SEM measurements.
References and Notes
1
2
3
4
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Schlenker, D. H. Olson, and E. W. Sheppard, Chem. Mater., 6,
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Figure 4 shows the ultrathin silica films obtained by spin
coating synthesis. The periodicity of the nanopores was poor al-
though three or four layers of nanopores were observed. It was
difficult to fabricate silica films with less than three layers by
the spin coating synthesis method.
10 Q. Huo, R. Leon, P. M. Petroff, and G. D. Stucky, Science, 268,
1324 (1995).
11 D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F.
Chmelka, and G. D. Stucky, Science, 279, 548 (1998).
12 M. Ogawa, J. Am. Chem. Soc., 116, 7941 (1994).
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(1996).
Surface of the silica film
Surface of the substrate
14 Y. Lu, R. Ganguli, C. A. Drewien, M. T. Anderson, C. J. Brinker,
W. Gong, Y. Guo, H. Soyez, B. Dunn, M. H. Huang, and J. I. Zink,
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16 D. A. Doshi, A. Gibaud, V. Goletto, M. Lu, H. Gerung, B. Ocko,
S. M. Han, and C. J. Brinker, J. Am. Chem. Soc., 125, 11646 (2003).
17 N. Nishiyama, S. Tanaka, Y. Egashira, Y. Oku, and K. Ueyama,
Chem. Mater., 15, 1006 (2003).
Section of the silicon substrate
60 nm
Figure 3. FE-SEM image of the cross-section of an ultrathin
silica film with double-layer of nanopores. The film was pre-
pared by vapor phase synthesis.
18 S. Tanaka, N. Nishiyama, Y. Egashira, Y. Oku, and K. Ueyama,
J. Am. Chem. Soc., 126, 4854 (2004).
19 We fabricated ultrathin nanoporous silica films by vapor phase and
spin coating synthesis. Vapor phase synthesis: Nonionic amphiphilic
triblock copolymer Pluronic F127 (EO106PO70EO106) was used as a
templating agent. A precursor solution of 0.05 g of Pluronic F127,
9.2 g of ethanol, and 3.6 g of deionized water was dropped onto a sil-
icon substrate (5 ꢄ 5-cm piece), and then the substrate was spun at a
rate of up to 4000 rpm for 1 min using a SPINCOATER 1H-DX2
(Mikasa Co.). Next, the triblock copolymer film was placed vertical-
ly in a closed vessel (200 cm3) along with a separate, small amount of
TEOS and HCl (5 N). The vessel was then placed in an oven at 90 ꢃC.
Thus, the copolymer film was exposed to a saturated TEOS vapor
under autogenous pressure. Removal of the copolymer template from
the composite was subsequently carried out by calcination at 400 ꢃC
in air for 5 h with a heating rate of 1 ꢃC/min. Spin coating synthesis:
A precursor solution of TEOS, Pluronic F127, ethanol, and deionized
water was spin-coated on a silicon substrate. The film thickness was
controlled by adjustment of the spinning rate and the concentrations
in the precursor solution.
Surface of the silica film
Section of the silicon substrate
60 nm
Figure 4. FE-SEM image of the cross-section of an ultrathin
silica film with about four layers of nanopores. The film was ob-
tained by spin coating synthesis.
20 FE-SEM images were recorded on a Hitachi S-5000L microscope at
an acceleration voltage of 21 kV. The samples were not coated before
the FE-SEM measurements. The ordered structure of the films was
investigated with a Philips X’ Pert-MPD diffractometer using Cu Kꢀ
In conclusion, we have successfully fabricated, by vapor
phase synthesis, ultrathin silica films with a monolayer of uni-
form nanopores. We were able to control the number of layers
via changes in the triblock copolymer concentration in the pre-
cursor solution. Such ultrathin films with a monolayer of nano-
ꢀ
radiation with ꢁ ¼ 1:5418 A. The copper anode was operated at 40
kV and 30 mA. The peak evolution was followed in the ꢂ/2ꢂ
Bragg–Brentano scattering geometry, 2ꢂ varying from 0.2 to 8
degrees, with a step size of 0.01 degree and a time per step of 1.0 s.
Published on the web (Advance View) September 29, 2004; DOI 10.1246/cl.2004.1408