ARTICLE pubs.acs.org/JPCC
Ionic Vibration Spectrum of Nanocrystalline MEL Pure Silica Zeolite Film Yasuhisa Kayaba,*,† Tadashi Sato,‡ Yutaka Seino,§ Takafumi Yamamoto,† and Takamaro Kikkawa† †
Research Institute for Nanodevice and Bio Systems (RNBS), Hiroshima University, 1-4-2 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan ‡ Hiroshima Prefectural Institute of Industrial Science and Technology, 3-10-32 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-0046, Japan § Nanodevice Innovation Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan ABSTRACT: Skeletal siloxane bonding structure of nanocrystalline MEL pure silica zeolite film was investigated. Nanocrystalline zeolite (nc-zeolite) suspension was prepared by the twostage hydrothermal treatment, and thin film was obtained by spin-coating. After a quantitative analysis of Fourier-transform infrared spectra with the variable incidence angles of light probe, skeletal complex dielectric function concerning the siloxane bond vibration was derived in the 1400400 cm1 range. Then, the widening of the AS1-(LO-TO) frequency splitting width and the enhancement of the normalized integrated oscillator strength of AS vibration mode were observed in the nc-zeolite film in comparison with those of solgel derived amorphous silica film. Those improvements must be attributed to the increased siloxane network connectivity and the crystal structure of zeolite. The effect of the UV assist anneal was also discussed.
1. INTRODUCTION Zeolite crystal films are a widely studied material for the applications in molecular sieve membrane,13 sensing devices,4 and chemical syntheses5 because of their unique micropore structures.6 Pure silica MFI (silicalite-1) and MEL (silicalite-2) zeolite films79 have attracted much attention as a low dielectric constant (low-k) material for large-scale integrated (LSI) circuit due to these excellent features such as high thermal conductivity (≈1.0 W/(m K)),10 high Young’s modulus (>10 GPa),11 and low dielectric constant (as low as 1.5).9 For such low-density materials, the investigation of bonding structure is quite important because the mechanical strength is strongly affected by the skeletal siloxane bonding structures (the short-range order (SiOSi bonding angle), the middle-range order (siloxane ring structure),12 and the long-range order (siloxane network connectivity,13,14 as well as amorphous or crystal structure)) with keeping the low density as investigated for mesoporous silica film. Our aim of study is to elucidate the molecular bonding structure of zeolite film. In previous studies,79 the structure of zeolite film was characterized in crystallinity by X-ray diffraction (XRD) spectrometry, pore size distribution by gas adsorptiondesorption isotherm measurement, size and yield of zeolite particle in the suspension by dynamic light scattering, and so on. Measurement and discussion about the siloxane bonding structure were not performed. Fourier transform infrared (FTIR) spectroscopy is a powerful technique to investigate the siloxane bonding structures r 2011 American Chemical Society
as studied extensively in the R-quartz,15 amorphous silicon dioxide,1619 solgel derived silica,20,21 chemical vapor deposition (CVD) silicon oxides,22,23 and others.24 On the basis of these important works, it has become possible to discuss the local bonding structure and siloxane network connectivity. For a quantitative discussion, one of the authors has been developed a nondestructive evaluation technique of ionic vibration spectrum.25,26 In this work, the FTIR spectrum of the spin-on thin film derived from the suspension of nanocrystalline MEL pure silica zeolite is measured, and the complex dielectric function concerning the ionic vibration of siloxane bonding is calculated. On the basis of the data, the siloxane bonding structures are discussed. For the optical measurement of thin film, low roughness at the surface and interface, which is sufficiently smaller than the wavelength of prove light, is demanded. The nanocrystalline zeolite (nc-zeolite) film9 has homogeneous film morphology and low surface/interfacial roughness, therefore, which is adequate for the optical characterization.
2. EXPERIMENTAL PROCEDURE A precursor solution of nanocrystalline MEL pure silica zeolite film was prepared by the two-stage hydrothermal synthesis Received: February 12, 2011 Revised: May 12, 2011 Published: May 12, 2011 11569
dx.doi.org/10.1021/jp201400b | J. Phys. Chem. C 2011, 115, 11569–11574
The Journal of Physical Chemistry C
ARTICLE
Table 1. Refractive Index (n) at 633 nm and Porosity (P) of the Spin-Coated Film and Those of UV-Assisted Annealed Films n at
second stage label
time duration
anneal condition
633 nm
P
gel-silica
no synthesis
in air
1.40
0.12
MEL-F
first stage only
in air
1.37
0.18
MEL-S11h
11 h
in air
1.33
0.27
MEL-S21h MEL-S41h
21 h 41 h
in air in air
1.20 1.19
0.55 0.57
gel-silica-UV
no synthesis
UV assisted
1.38
0.16
MEL-S41h-UV
41 h
UV assisted
1.15
0.65
method.9 At first, a solution of silica sol was synthesized. 32.1 mL of tetraethoxysilane was added dropwise to 27.5 mL of tetrabutylammonium hydroxide in PFA bottle with vigorous stirring at 400 rpm. Then, 14 mL of secondary deionized water was added. At the first stage of the hydrothermal treatment, to promote the nucleation of zeolite, the solution in sealed PFA bottle was heated at 80 C for 48 h in an oil bath with stirring at 250 rpm. At the second stage, the solution was transferred to Teflon lined autoclave bottle and heated at 114 C for various time durations between 11 and 41 h without stirring. After sonicated for 30 min, the zeolite suspension was spin-coated on a single crystalline silicon substrate (110 ohm 3 cm resistivity, 340 μm thick) in a controlled humid environment (20 C, RH 55%). Typically, the spinning condition was 2000 rpm for 30 s. The film was baked at 80 C in air for 10 h to promote the gelation and was annealed in air at 400 C for 2 h after heated to 400 at 1 C/min. In this work, further post-treatment such as silylation was not performed. In Table 1, each entry of the samples is shown with each preparation condition. Gel-silica is the film obtained from the sol solution without any hydrothermal treatment. MEL-F is the film obtained from the solution after the first stage hydrothermal treatment. MEL-S11h, MEL-S21h, and MEL-S41h mean the films obtained from the sol solution after the second stage hydrothermal treatment for 11, 21, and 41 h, respectively. Film thickness and refractive index were measured by a UVvis spectroscopic ellipsometer (J.A. Woollam. Co., M-2000). In this analysis, the fitting range was taken at 5001000 nm. The XRD pattern of the nc-zeolite film was measured by a XRD spectrometer (Rigaku Co., RINT 2100). For this measurement, spin deposition condition was controlled to 1000 rpm with the acceleration rate of 20 rpm/s and held for 20 s in order to increase the film thickness. The FTIR spectrum was collected by a spectrometer (JASCO Co., FTIR 660-plus). The ambient was purged with nitrogen. To avoid the diffusive scattering of light at the backside silicon/air interface, two-side polished silicon wafer was used. Reference silicon wafer was carefully selected to have the same optical adsorptions to the silicon substrate of the samples.
3. RESULTS AND DISCUSSION 3.1. Film Characteristics. Figure 1 shows the XRD patterns of the annealed films. In the MEL-S21h and MEL-S41h films, diffraction peaks are found at 7.9, 8.9, and around 23.5. These correspond to those of MEL type zeolite.6 According to ref 9, at the first stage and the second stage with short time duration (
