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TiO2 thin films with reflection-enhancing function
Optical Materials 28 (2006) 1381–1384 www.elsevier.com/locate/optmat
Layer-by-layer assembly of polyelectrolyte/TiO2 thin films with reflection-enhancing function H.H. Yu *, D.S. Jiang, X.F. Li, D.S. Yu, L.D. Zhou Fiber Optic Sensing Technology Research Center, Wuhan University of Technology, Wuhan 430070, China Received 24 December 2004; accepted 18 August 2005 Available online 27 September 2005
Abstract Colloidal TiO2 was prepared by hydrolyzing tetra-n-butyl titanate. Composite multilayer films of poly(sodium 4-styrenesulfonate) (PSS) and colloidal TiO2 particles were layer-by-layer assembled onto optic fibers and microscope glass slides. As the PSS/TiO2 film was deposited onto the end face of a glass fiber, the reflected optic intensity periodically oscillated as the bilayer number of the film increased. After a 24-bilayer film was coated onto the both sides of a glass slide, the transmittance at 850 nm decreased more than 20%, which means that the film could serve the function as a reflection-enhancing coating. X-ray diffraction analysis and data of TEM electron diffraction analysis show that the colloidal TiO2 particles are mainly brookite nanocrystals and that the PSS/TiO2 films are polycrystalline films. Scratching experiments indicate that the composite films are of relatively high hardness. Ó 2005 Elsevier B.V. All rights reserved. PACS: 42.79.W; 68.55; 78.66.Q; 82.70.D Keywords: Polyelectrolyte; TiO2; Layer-by-layer assembly; Thin film; Reflection-enhancing
1. Introduction Optic thin films, a sort of functional thin films based on optic interference, can be used versatilely as thin film devices, such as anti-reflecting films, high reflecting films, optic filters and so on. TiO2 possesses excellent chemical stability, and is often used to prepare high refractive index films and wave-guides by different methods, such as vapor deposition method and sol–gel process. With the development of optic information technology, the application area of optic thin films has constantly spread out, and, as a result, investigation on the preparation methodology of optic thin films on substrates with special shapes like optic fibers has become more significant. The optic thin films can be prepared on optic fibers via different approaches. However, there are some disadvantages
*
Corresponding author. Fax: +86 27 87665287. E-mail address: [email protected] (H.H. Yu).
0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.08.015
by using those methods to prepare optic thin films on optic fibers. For example, the sol–gel process is convenient for use, but it is difficult to control the thickness of the films. Decher and his coworker reported a novel thin filmmaking method in 1991 [1], so-called layer-by-layer assembly multilayer technique (LbL). The LbL technique, possessing many advantages including no restriction on the shape and size of substrates, can be used to incorporate polyelectrolyte and nanoparticles or clusters into thin films, and easily build up optic films on optic fibers. Claus and his coworkers once developed TiO2/polymer nanocomposite films [2], and formed gold cluster/polyelectrolyte-based nanometer-scale Fabry–Perot interferometers on optic fibers using this technique [3]. In the work we described that colloidal TiO2 nanoparticles and anionic polyelectrolyte were incorporated into films, and assembled onto the end faces of optic glass fibers and microscope glass slides. The results demonstrated that the films could serve the function as reflection-enhancing coatings.
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2. Experimental LED
PIN optic fiber
assembled multilayer thin film
Fig. 1. Schematic of fiber optic system.
The assembling process of a composite film on the end face of an optic fiber was monitored using an optic fiber system shown in Fig. 1. As the bilayer number of a PSS/TiO2 thin film assembled onto the end face of the optic fiber reached a given number, the film would be dried with N2 flow, and the light intensity reflected by the thin film was measured after the system reached equilibrium. The assembling process of a composite film on a microscope slide was monitored using the UV–vis spectrophotometer, and the transmittance of the film-coated glass slide at given wavelength was measured using the same spectrophotometer after the composite film was dried with N2 flow. The composite films were peeled off the glass slides and observed with JEOL JEM-100CXII transmission electron microscope (TEM) under an acceleration voltage of 100 kV. 3. Results and discussion Fig. 2 is the UV–vis absorption spectrum of colloidal TiO2 solution. The onset of absorption appears at 372 nm. At wavelengths longer than 372 nm, almost no absorption can be observed on the spectrum, which implies that the colloidal TiO2 is good for making optic thin films transparent beyond UV range. Fig. 3 is the X-ray diffraction curve, on which the Bragg diffraction peaks corresponding to brookite phase can be observed. The XRD curve indicates that the main crystalline phase is brookite. However, the widening of the Bragg diffraction peaks implies that the TiO2 particles are very small and their crystallization is incomplete. 1.0
0.8
Absorbance/a.u.
Colloidal TiO2 was prepared using tetra-n-butyl titanate, isopropanol and nitric acid as raw materials in the process similar to that reported in Ref. [4]. The preparation process is briefly described here. Thirteen milliliter tetra-nbutyl titanate and 4 mL isopropanol were mixed in a glass beaker. Two milliliter HNO3 was added into 150 mL ultrapure water and stirred for 15 min. The mixed tetra-n-butyl titanate and isopropanol solution was slowly dropped into the aqueous HNO3 and stirred for 1 h at 75–80 °C. Then, the obtained colloid was diluted with ultrapure water to 900 mL, stirred overnight and ready for use. The pH value of the final TiO2 colloid was about 1.0. The absorption spectrum of the colloidal TiO2 was measured using a Pgeneral TU-1901 UV–vis spectrophotometer (made in Beijing, China), and the dried TiO2 particle powder was characterized using a Rigaku D/MAX-YB X-ray diffractometer. Poly(sodium 4-styrenesulfonate) (PSS) was purchased from Aldrich. The powder was dissolved in ultrapure water, and the solution was stirred overnight. The concentration of the solution was 2.0 mg/mL. PDDA [poly(diallyldimethylammonium) chloride] was also purchased from Aldrich. The original solution was diluted to 0.01 mol/L with ultrapure water. Other reagents included 98% H2SO4, 30% H2O2 and 25% ammonia. The substrates used to assemble thin films were optic glass fibers and microscope glass slides. To clean and have the optic glass fibers and microscope slides negatively charged, we treated them with ‘‘piranha’’ solution, cleaned with ultrapure water, treated with ammonia/H2O2/H2O solution, and thoroughly cleaned with ultrapure water again. The assembling process of the composite films was similar to that reported in Ref. [2] and briefly described here. The cleaned and negatively charged optic glass fibers and microscope slides were dipped into the aqueous PDDA solution for 20 min, then took them out and cleaned with ultrapure water. The PDDA monolayer would have the substrate surfaces positively charged because of the existence of quaternary ammonium groups of PDDA. PSS with –SO3 groups is negatively charged due to its ionization in water. When the PDDA-monolayer-surfaced optic glass fibers and microscope slides were dipped into the aqueous PSS solution for 5 min, the PSS monolayer would adsorb onto the positively charged PDDA monolayer due to the electrostatic attraction. The isoelectric point of colloidal TiO2 is about 5.8 [5]. So the TiO2 particles are positively charged in the prepared colloid. After the PSSmonolayer-surfaced samples were cleaned with ultrapure water and dipped into colloidal TiO2 for 5 min, colloidal TiO2 would adsorb onto the anionic PSS monolayer to form a TiO2 layer. After the dipping, the sample was cleaned with ultrapure water again. By repeating this dipping–washing process circularly, PSS/TiO2 composite films could be build up on the substrates until desired bilayer numbers or thickness was reached.
optic fiber 2 x 2 coupler
0.6
0.4
0.2
0.0 200
250
300
350
400
450
Wavelength/nm Fig. 2. UV–vis absorption spectrum of colloidal TiO2.
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H.H. Yu et al. / Optical Materials 28 (2006) 1381–1384
brookite anatase
Intensity/a.u.
300
200
100
0 10
20
30
40
50
60
70
2θ/º Fig. 3. X-ray diffraction curve of dried TiO2 nanoparticle powder.
The working wavelength used in the fiber optic system illustrated in Fig. 1 is 850 nm. Fig. 4 shows the relationship between the light intensity reflected by the composite film and the bilayer number of the film. As it can be observed from the curve, with the bilayer number increasing, the reflected light intensity oscillates periodically and almost smoothly. It is well known that, if a thin film is a homogeneous medium without light absorption, the reflectance of the light incident into a film will periodically oscillate with the increasing optic thickness of the film. Therefore, we can consider that the assembled PSS/TiO2 film is a homogeneous medium although it is composed of PSS and TiO2. If the assembling experimental conditions are properly controlled, after every bilayer is assembled onto the substrate, the film will increase the same geometrical thickness. Thus the optic thickness of the film is proportional to the bilayer number of the thin film, and the reflectance of the sample exhibits periodical change with the increment of the bilayer number. A homogeneous optic coating can serve the function as reflection-enhancing coating when its refractive index is
greater than that of the substrate. The first peak on the curve of Fig. 4 is a maximum, which indicates that the refractive index of the assembled thin film is greater than that of the optic glass fiber, and that the PSS/TiO2 thin film can serve the function as a reflection-enhancing coating. Fig. 5 shows the transmittance spectra of 7-, 15-, 24-, 30and 40-bilayer PSS/TiO2 thin films assembled onto the both sides of glass slides. The transmittance at 500 nm of blank glass slides used in this experiment is about 90.8%. As it can be observed from Fig. 3, after a 7-bilayer thin film is assembled, the transmittance of the sample decreases; after a 15-bilayer thin film is assembled, the transmittance decreases further, to 75.0% at 500 nm and to 70.5% at 800 nm. When the bilayer number of the film reaches 24, the transmittance at 850 nm decreases to 68.2%. In other words, the transmittance at 850 nm can decrease more than 20% after a 24-bilayer thin film was assembled onto the both sides of a slide. The above results show that PSS/ TiO2 thin films possess a pronounced reflection-enhancing function. When the bilayer number reaches 30, the transmittance at 580 nm of the sample is up to 90.8%, i.e. the transmittance of a blank substrate, which means that the 96 90
Transmittance/%
400
1383
d
84
a
78
c
e b
72 66 60 300
400
500
600
700
800
900
Wavelength/nm Fig. 5. Transmittance curves of PSS/TiO2 multilayer films assembled on both sides of glass slides (a-7, b-15, c-24, d-30, and e-40 bilayers).
180
Reflected intensity/a.u.
λ = 850 nm
150
120
90
60
0
10
20
30
40
50
60
70
Bilayer number Fig. 4. Light intensity reflected by a PSS/TiO2 film on the end face of an optic fiber against the bilayer number of the film.
Fig. 6. TEM photograph of the composite PSS/TiO2 thin film with a inset of electron diffraction pattern.
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thin films have no effect to the transmittance and reflection of the incident light. Fig. 6 is a TEM photograph of the composite PSS/TiO2 thin film. On the photograph, TiO2 particles are visible at the edges of the film. Distinct diffraction rings can be observed on the TEM electron diffraction pattern of the film, and the analysis data show that colloidal TiO2 particles are mainly brookite nanocrystals, which is in agreement in the XRD experimental result. Thus PSS/TiO2 thin films are polycrystalline thin films composed of brookite nanocrystals. In addition, scratching experiments indicate that the composite films are of relatively high hardness. It is rather difficult to scrape the films from the substrate with a blade, suggesting that the thin films be promising for practical applications. 4. Conclusion In this work, it is demonstrated that, after a PSS/TiO2 thin film was layer-by-layer assembled onto the end face of an optic fiber, the reflected optic intensity periodically
oscillated as the bilayer number increased; the film could serve the function as a reflection-enhancing coating; the transmittance at 850 nm decreased more than 20% after a 24-bilayer thin film was assembled onto both sides of a glass slide. Furthermore, if the film-making process parameter is given, the thickness of the film can be controlled through controlling the bilayer number of the film, and subsequently, the optic properties of the films can be regulated. Colloidal TiO2 particles are mainly brookite nanocrystals, and the PSS/TiO2 thin films are polycrystalline thin films with relatively high hardness. The experimental results suggest that the LbL technique is promising to develop functional optic films and fiber optic devices. References [1] G. Decher, J.D. Hong, Ber. Bunsen Phys. Chem. 95 (1991) 1430. [2] Y. Liu, A. Wang, R.O. Claus, J. Phys. Chem. B 101 (1997) 1385. [3] F.J. Arregui, I.R. Matias, Y. Liu, K.M. Lenahan, R.O. Claus, Opt. Lett. 24 (1999) 596. [4] Y. Shen, L. Wang, Z. Lu, Y. Wei, Chin. J. Mater. Res. 9 (1995) 81 (in Chinese). [5] Z. Shen, G. Wang, Colloidal and Surface Chemistry, second ed., Chemical Industry Press, Beijing, China, 1997 (in Chinese).
Layer-by-layer assembly of polyelectrolyte/TiO2 thin films with reflection-enhancing function H.H. Yu *, D.S. Jiang, X.F. Li, D.S. Yu, L.D. Zhou Fiber Optic Sensing Technology Research Center, Wuhan University of Technology, Wuhan 430070, China Received 24 December 2004; accepted 18 August 2005 Available online 27 September 2005
Abstract Colloidal TiO2 was prepared by hydrolyzing tetra-n-butyl titanate. Composite multilayer films of poly(sodium 4-styrenesulfonate) (PSS) and colloidal TiO2 particles were layer-by-layer assembled onto optic fibers and microscope glass slides. As the PSS/TiO2 film was deposited onto the end face of a glass fiber, the reflected optic intensity periodically oscillated as the bilayer number of the film increased. After a 24-bilayer film was coated onto the both sides of a glass slide, the transmittance at 850 nm decreased more than 20%, which means that the film could serve the function as a reflection-enhancing coating. X-ray diffraction analysis and data of TEM electron diffraction analysis show that the colloidal TiO2 particles are mainly brookite nanocrystals and that the PSS/TiO2 films are polycrystalline films. Scratching experiments indicate that the composite films are of relatively high hardness. Ó 2005 Elsevier B.V. All rights reserved. PACS: 42.79.W; 68.55; 78.66.Q; 82.70.D Keywords: Polyelectrolyte; TiO2; Layer-by-layer assembly; Thin film; Reflection-enhancing
1. Introduction Optic thin films, a sort of functional thin films based on optic interference, can be used versatilely as thin film devices, such as anti-reflecting films, high reflecting films, optic filters and so on. TiO2 possesses excellent chemical stability, and is often used to prepare high refractive index films and wave-guides by different methods, such as vapor deposition method and sol–gel process. With the development of optic information technology, the application area of optic thin films has constantly spread out, and, as a result, investigation on the preparation methodology of optic thin films on substrates with special shapes like optic fibers has become more significant. The optic thin films can be prepared on optic fibers via different approaches. However, there are some disadvantages
*
Corresponding author. Fax: +86 27 87665287. E-mail address: [email protected] (H.H. Yu).
0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.08.015
by using those methods to prepare optic thin films on optic fibers. For example, the sol–gel process is convenient for use, but it is difficult to control the thickness of the films. Decher and his coworker reported a novel thin filmmaking method in 1991 [1], so-called layer-by-layer assembly multilayer technique (LbL). The LbL technique, possessing many advantages including no restriction on the shape and size of substrates, can be used to incorporate polyelectrolyte and nanoparticles or clusters into thin films, and easily build up optic films on optic fibers. Claus and his coworkers once developed TiO2/polymer nanocomposite films [2], and formed gold cluster/polyelectrolyte-based nanometer-scale Fabry–Perot interferometers on optic fibers using this technique [3]. In the work we described that colloidal TiO2 nanoparticles and anionic polyelectrolyte were incorporated into films, and assembled onto the end faces of optic glass fibers and microscope glass slides. The results demonstrated that the films could serve the function as reflection-enhancing coatings.
1382
H.H. Yu et al. / Optical Materials 28 (2006) 1381–1384
2. Experimental LED
PIN optic fiber
assembled multilayer thin film
Fig. 1. Schematic of fiber optic system.
The assembling process of a composite film on the end face of an optic fiber was monitored using an optic fiber system shown in Fig. 1. As the bilayer number of a PSS/TiO2 thin film assembled onto the end face of the optic fiber reached a given number, the film would be dried with N2 flow, and the light intensity reflected by the thin film was measured after the system reached equilibrium. The assembling process of a composite film on a microscope slide was monitored using the UV–vis spectrophotometer, and the transmittance of the film-coated glass slide at given wavelength was measured using the same spectrophotometer after the composite film was dried with N2 flow. The composite films were peeled off the glass slides and observed with JEOL JEM-100CXII transmission electron microscope (TEM) under an acceleration voltage of 100 kV. 3. Results and discussion Fig. 2 is the UV–vis absorption spectrum of colloidal TiO2 solution. The onset of absorption appears at 372 nm. At wavelengths longer than 372 nm, almost no absorption can be observed on the spectrum, which implies that the colloidal TiO2 is good for making optic thin films transparent beyond UV range. Fig. 3 is the X-ray diffraction curve, on which the Bragg diffraction peaks corresponding to brookite phase can be observed. The XRD curve indicates that the main crystalline phase is brookite. However, the widening of the Bragg diffraction peaks implies that the TiO2 particles are very small and their crystallization is incomplete. 1.0
0.8
Absorbance/a.u.
Colloidal TiO2 was prepared using tetra-n-butyl titanate, isopropanol and nitric acid as raw materials in the process similar to that reported in Ref. [4]. The preparation process is briefly described here. Thirteen milliliter tetra-nbutyl titanate and 4 mL isopropanol were mixed in a glass beaker. Two milliliter HNO3 was added into 150 mL ultrapure water and stirred for 15 min. The mixed tetra-n-butyl titanate and isopropanol solution was slowly dropped into the aqueous HNO3 and stirred for 1 h at 75–80 °C. Then, the obtained colloid was diluted with ultrapure water to 900 mL, stirred overnight and ready for use. The pH value of the final TiO2 colloid was about 1.0. The absorption spectrum of the colloidal TiO2 was measured using a Pgeneral TU-1901 UV–vis spectrophotometer (made in Beijing, China), and the dried TiO2 particle powder was characterized using a Rigaku D/MAX-YB X-ray diffractometer. Poly(sodium 4-styrenesulfonate) (PSS) was purchased from Aldrich. The powder was dissolved in ultrapure water, and the solution was stirred overnight. The concentration of the solution was 2.0 mg/mL. PDDA [poly(diallyldimethylammonium) chloride] was also purchased from Aldrich. The original solution was diluted to 0.01 mol/L with ultrapure water. Other reagents included 98% H2SO4, 30% H2O2 and 25% ammonia. The substrates used to assemble thin films were optic glass fibers and microscope glass slides. To clean and have the optic glass fibers and microscope slides negatively charged, we treated them with ‘‘piranha’’ solution, cleaned with ultrapure water, treated with ammonia/H2O2/H2O solution, and thoroughly cleaned with ultrapure water again. The assembling process of the composite films was similar to that reported in Ref. [2] and briefly described here. The cleaned and negatively charged optic glass fibers and microscope slides were dipped into the aqueous PDDA solution for 20 min, then took them out and cleaned with ultrapure water. The PDDA monolayer would have the substrate surfaces positively charged because of the existence of quaternary ammonium groups of PDDA. PSS with –SO3 groups is negatively charged due to its ionization in water. When the PDDA-monolayer-surfaced optic glass fibers and microscope slides were dipped into the aqueous PSS solution for 5 min, the PSS monolayer would adsorb onto the positively charged PDDA monolayer due to the electrostatic attraction. The isoelectric point of colloidal TiO2 is about 5.8 [5]. So the TiO2 particles are positively charged in the prepared colloid. After the PSSmonolayer-surfaced samples were cleaned with ultrapure water and dipped into colloidal TiO2 for 5 min, colloidal TiO2 would adsorb onto the anionic PSS monolayer to form a TiO2 layer. After the dipping, the sample was cleaned with ultrapure water again. By repeating this dipping–washing process circularly, PSS/TiO2 composite films could be build up on the substrates until desired bilayer numbers or thickness was reached.
optic fiber 2 x 2 coupler
0.6
0.4
0.2
0.0 200
250
300
350
400
450
Wavelength/nm Fig. 2. UV–vis absorption spectrum of colloidal TiO2.
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H.H. Yu et al. / Optical Materials 28 (2006) 1381–1384
brookite anatase
Intensity/a.u.
300
200
100
0 10
20
30
40
50
60
70
2θ/º Fig. 3. X-ray diffraction curve of dried TiO2 nanoparticle powder.
The working wavelength used in the fiber optic system illustrated in Fig. 1 is 850 nm. Fig. 4 shows the relationship between the light intensity reflected by the composite film and the bilayer number of the film. As it can be observed from the curve, with the bilayer number increasing, the reflected light intensity oscillates periodically and almost smoothly. It is well known that, if a thin film is a homogeneous medium without light absorption, the reflectance of the light incident into a film will periodically oscillate with the increasing optic thickness of the film. Therefore, we can consider that the assembled PSS/TiO2 film is a homogeneous medium although it is composed of PSS and TiO2. If the assembling experimental conditions are properly controlled, after every bilayer is assembled onto the substrate, the film will increase the same geometrical thickness. Thus the optic thickness of the film is proportional to the bilayer number of the thin film, and the reflectance of the sample exhibits periodical change with the increment of the bilayer number. A homogeneous optic coating can serve the function as reflection-enhancing coating when its refractive index is
greater than that of the substrate. The first peak on the curve of Fig. 4 is a maximum, which indicates that the refractive index of the assembled thin film is greater than that of the optic glass fiber, and that the PSS/TiO2 thin film can serve the function as a reflection-enhancing coating. Fig. 5 shows the transmittance spectra of 7-, 15-, 24-, 30and 40-bilayer PSS/TiO2 thin films assembled onto the both sides of glass slides. The transmittance at 500 nm of blank glass slides used in this experiment is about 90.8%. As it can be observed from Fig. 3, after a 7-bilayer thin film is assembled, the transmittance of the sample decreases; after a 15-bilayer thin film is assembled, the transmittance decreases further, to 75.0% at 500 nm and to 70.5% at 800 nm. When the bilayer number of the film reaches 24, the transmittance at 850 nm decreases to 68.2%. In other words, the transmittance at 850 nm can decrease more than 20% after a 24-bilayer thin film was assembled onto the both sides of a slide. The above results show that PSS/ TiO2 thin films possess a pronounced reflection-enhancing function. When the bilayer number reaches 30, the transmittance at 580 nm of the sample is up to 90.8%, i.e. the transmittance of a blank substrate, which means that the 96 90
Transmittance/%
400
1383
d
84
a
78
c
e b
72 66 60 300
400
500
600
700
800
900
Wavelength/nm Fig. 5. Transmittance curves of PSS/TiO2 multilayer films assembled on both sides of glass slides (a-7, b-15, c-24, d-30, and e-40 bilayers).
180
Reflected intensity/a.u.
λ = 850 nm
150
120
90
60
0
10
20
30
40
50
60
70
Bilayer number Fig. 4. Light intensity reflected by a PSS/TiO2 film on the end face of an optic fiber against the bilayer number of the film.
Fig. 6. TEM photograph of the composite PSS/TiO2 thin film with a inset of electron diffraction pattern.
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thin films have no effect to the transmittance and reflection of the incident light. Fig. 6 is a TEM photograph of the composite PSS/TiO2 thin film. On the photograph, TiO2 particles are visible at the edges of the film. Distinct diffraction rings can be observed on the TEM electron diffraction pattern of the film, and the analysis data show that colloidal TiO2 particles are mainly brookite nanocrystals, which is in agreement in the XRD experimental result. Thus PSS/TiO2 thin films are polycrystalline thin films composed of brookite nanocrystals. In addition, scratching experiments indicate that the composite films are of relatively high hardness. It is rather difficult to scrape the films from the substrate with a blade, suggesting that the thin films be promising for practical applications. 4. Conclusion In this work, it is demonstrated that, after a PSS/TiO2 thin film was layer-by-layer assembled onto the end face of an optic fiber, the reflected optic intensity periodically
oscillated as the bilayer number increased; the film could serve the function as a reflection-enhancing coating; the transmittance at 850 nm decreased more than 20% after a 24-bilayer thin film was assembled onto both sides of a glass slide. Furthermore, if the film-making process parameter is given, the thickness of the film can be controlled through controlling the bilayer number of the film, and subsequently, the optic properties of the films can be regulated. Colloidal TiO2 particles are mainly brookite nanocrystals, and the PSS/TiO2 thin films are polycrystalline thin films with relatively high hardness. The experimental results suggest that the LbL technique is promising to develop functional optic films and fiber optic devices. References [1] G. Decher, J.D. Hong, Ber. Bunsen Phys. Chem. 95 (1991) 1430. [2] Y. Liu, A. Wang, R.O. Claus, J. Phys. Chem. B 101 (1997) 1385. [3] F.J. Arregui, I.R. Matias, Y. Liu, K.M. Lenahan, R.O. Claus, Opt. Lett. 24 (1999) 596. [4] Y. Shen, L. Wang, Z. Lu, Y. Wei, Chin. J. Mater. Res. 9 (1995) 81 (in Chinese). [5] Z. Shen, G. Wang, Colloidal and Surface Chemistry, second ed., Chemical Industry Press, Beijing, China, 1997 (in Chinese).