Morphology and optical study of dye-doped TiO2 ...

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a. Department of Physics, National Institute of Technology, Hamirpur. Himachal Pradesh 177 005 India b. Department of Physics, Indian Institute of Technology, ...

Morphology and optical study of Dye doped TiO2-SiO2 thin films Arvind K. Gathania*a, Naresh Dhimana, Ankita Sharmaa , B.P. Singhb a Department of Physics, National Institute of Technology, Hamirpur Himachal Pradesh 177 005 India b

Department of Physics, Indian Institute of Technology, Bombay Powai, Mumbai 400076, India ABSTRACT

In the present work, we have prepared functional dye doped TiO2-SiO2 thin films by vertical sedimentation technique. Thin film samples are annealed at different temperature from 50oC to 850oC. Morphology and chemical bonding information is analysed using atomic force microscopy (AFM), and Fourier transform infrared spectroscopy (FTIR) respectively. Optical properties are characterized by using UV-visible spectroscopy. Effect of annealing temperature on the photonic forbidden band gap is also presented. The experimental measured values are compared with theoretical estimated results. Keywords: Vertical sedimentation technique, fourier transform infrared spectroscopy, UV-visible spectroscopy, annealing temperature, photonic forbidden band gap.

1. INTRODUCTION In recent years, microspheres of silica, polystyrene have been studied extensively because of their applications in photonic devices [1-5]. The idea of the periodic photonic crystal was proposed by Yablonovitch [6,7] and John [8]. The photonic crystals are in analogy with crystalline solid: electromagnetic waves propagating in the photonic crystal experienced periodic spatial variation of dielectric constant, as the de Broglie wave propagation in crystalline solids experienced periodic variation of the atomic potential. As a result, allowed and forbidden electromagnetic states with respective energy bands occur in the materials [9, 10]. This means that photonic crystal reflects incident electromagnetic modes with energy belonging to forbidden photonic band gap and the spontaneous emission process could also be controlled. Fabrication of colloidal multilayer structures in the visible and infrared region of the electromagnetic spectrum has been proposed by several differential routes. These structures of colloidal particles have been proved good candidate for photonic materials. These structures are developed by the gravity sedimentation technique from the aqueous solution. The drawback of this technique is the development of polycrystalline domains in the structure. To overcome the above problem, vertical sedimentation technique [11,12] that assembles the microspheres themselves to a regular crystalline order. The film thickness can be controlled through the microsphere diameter as well as by the volume fraction. The objective of the present work is to develop the microsphere with higher refractive contrast than those obtained with SiO2 and polystyrene opals in order to enhance the photonic characteristics. For this purpose, we synthesized TiO2 coated silica microspheres using dye as stabilizer and colloidal crystal thin films were fabricated from ethanol solution by the vertical sedimentation technique. Topography and chemical structures were characterized by atomic force microscopy and FTIR. To evaluate the effect of thermal treatment on the optical properties of the thin film samples were annealed at different temperatures.

Smart Nano-Micro Materials and Devices, edited by Saulius Juodkazis, Min Gu, Proc. of SPIE Vol. 8204, 82043N · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.904869

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Theoretical Background The propagation of electromagnetic waves in a Photonic crystal can be governed by the four macroscopic Maxwell equations as: · ·





Where E and H are the macroscopic electric and magnetic fields and B and D are the magnetic flux density and displacement fields. ρ and J are the charge density and current density respectively. The electric field for linear, lossless photonic crystal can be obtained by solving Maxwell’s equations is ′


Where is the average and be written as

is the periodic or fluctuating part of the dielectric constant and average dielectric constant can

Or 1 1 Where




is the dielectric contrast defined as the ratio of the dielectric constant of microspheres to the dielectric

constant of the background and f is the volume fraction of the crystal structure. The photonic forbidden band gap obtained by solving above equations is given as [13] : ∆




1 and α = (3)1/2 for T to L direction in the forbidden band. The variation of fcc

photonic forbidden band gap calculated from equation (4) at different volume fraction for dielectric contrast (6.25) is shown in figure1.

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0.00 0.0







Figure1. Variation of fcc photonic forbidden band gap with volume fraction of microspheres for fcc (111) from T to L direction. 2.EXPERIMENTAL 2.1 Preparation of dye doped TiO2-SiO2 microspheres The silica microspheres have been synthesized by sol–gel method. The detailed procedure is discussed elsewhere [1417]. The titania-silica microspheres have been prepared [17, 18-20] by doping with Rhodamine B (RB) dye. RB dye acts as doped agent as well as stabilizer for the formation of dye doped TiO2-SiO2 microspheres. It also promotes the uniform absorption of TiO2 on silica core. Dye molecules carry positive charge whereas silica core having negative charge on its surface, which results electrostatic force of attraction between them. The oxide groups of TiO2 form the hydrogen bond with the molecular chain of dye. To prepare TiO2-SiO2 microspheres 0.5g silica powder was added with 50 ml ethanol (99%) containing RB dye. This solution was sonicated for about 20 minutes. Secondly, 6M HCl (35% GR from MerckIndia) mixed with 0.5M TiCl4 (From Merck-India) and this solution was mixed with above solution agitated for 4h under water bath followed by centrifugation for 20 minutes to separate dye doped TiO2-SiO2 microspheres. Finally the sample is dried at 50℃ in vacuum oven. 2.2Preparation of TiO2-SiO2 thin films TiO2-SiO2 thin films were prepared using the suspension of TiO2-SiO2 microspheres in ethanol on quartz substrate by using vertical sedimentation method in which TiO2-SiO2 microspheres were self-assembled on the substrate with the evaporation of ethanol and then dried at 75oC in vacuum oven. Finally they were annealed at various temperatures (300, 500, 700 and 850oC). The topography of the samples were carried out at room temperature using AFM ( SPM5500 Angilient). FTIR(Perkin Elmer Model 2000) measurements were performed in the transmission mode in the range 400-4000 Cm-1. The nature of

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optical properties at different annealing temperatures has been analyzed using UV-visible spectroscopy (Shimadzu, Japan) performed in the range of 280-550 nm.

3. Results and discussion 3.1 Atomic Force Microscopy (AFM) Figure 2 shows 3-dimensional topographic image of the film surface taken by atomic force microscope. From the AFM image we can conclude that the TiO2 coated silica spherical particles having diameter of order of 150 nm and hexagonal closed packed arrangement. Following experimental height parameters are obtained from the AFM mrasurement: square root mean height (Sq) = 55.7 nm, Skewness of the height distribution (SsK) = 0.251, Kurtosis of the height distribution(Sku) = 3.05, Maximum height( Sp ) = 197 nm, Maximum depth(Sv) = 192 nm, Difference between highest and lowest valley(Sz) = 389 nm, Airthematic mean deviation(Sa) = 44.4 nm.

Figure 2. AFM image of as grown dye-doped TiO2-SiO2 thin film. 3.2 Fourier Transform Infrared Spectroscopy (FTIR) FTIR study gives the information of chemical bonding in the TiO2-SiO2 thin films. Figure 3 shows the FTIR spectra of dye-doped TiO2-SiO2 microsphere at room temperature and 850℃ temperature in the wavenumber range from 400 cm-1 to 4000 cm-1. At room temperature the characteristic bands can be observed around 3368.88, 1630.93, 1401.02, 1106.76, 944.93, 799.37, 588.77 and 470.85 cm-1. At 850℃ the band corresponds to 1118.83, 806.89, 475.11 cm-1. A band around ~ 800 and 1100 cm-1 can be attributed to asymmetric stretching and symmetric stretching due to Si-O-Si bond. The bands around ~ 3300 and 1600 cm-1 are due to the stretching vibration of hydroxyl group (OH) and C-H bond. Peaks around ~ 1400 cm-1 and 470-550 cm-1 are exhibiting Ti-O-Ti stretching mode for the sample at room temperature. Upon annealing, a significant change in FTIR spectra, i.e. the disappearance of bands around ~ 3399.86, 1870.46, 1630.93 (adsorbed water), 1401.02 (Ti-O-Ti), and 944.93 (Si-OH) and has been observed [21].

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Transmittance (%)

o 850 C

as grown











-1 Wavenumber (cm )

Figure 3 FTIR spectrum of dye-doped TiO2-SiO2samples. 3.3 UV-Visible Spectroscopy Figure 4 presents the UV-Visible transmission spectra of as grown and annealed samples at 300oC, 500oC, 700oC and 850oC temperatures. It is interesting to know that the transmission dip exhibits the red shift i.e. 320 nm to 337 nm below 500oC annealing temperature. On the other hand, transmission dip shift from 337 nm to 328 nm in the temperature regime 500oC to 850oC. The red shift is noticed up to 500oC temperature it may be due to the transformation of TiO2 from amorphous to crystalline anatase phase which reflects the increase in refractive index. Above 500oC, the blue shift appears due to the shrinkage of the core of the composite microspheres[22] and annealing temperature effect on the position of the transmission dip at the corresponding wavelength is summarized in table1. Table 1.Effect of annealing temperature on position of transmission dip. S.No. 1 2 3 4 5

Annealing temperature (inoC) As grown 300 500 700 850

Wavelength(nm) 320 321 337 333 328

The photonic forbidden band gap around the central frequency is calculated by equation (4), where ∆ω = ω+ - ω- is the full width at half maxima of the transmission dip and ωo is the central frequency. The experimental calculated photonic forbidden band gap for dye –doped TiO2-SiO2 microsphere having fcc photonic structure (f = 0.74) for T to L direction is ~ 0.16, which is fairly agreement with theoretical values as shown in figure 1.

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o 850 C

Transmission (arb. units)

o 700 C

o 500 C o 300 C RT




Wavelenth (nm)

Figure 4 UV-Visible Spectrum of TiO2-SiO2 thin films at different annealing temperatures. Conclusions: Dye doped TiO2-SiO2 thin films were fabricated from ethanol solution using the vertical deposition technique. The AFM of the thin film reflects a closed packed hexagonal arrangement of the sample. FTIR spectroscopy showed the Si-O-Si, Ti-O-Ti, Si-OH bonds were formed. Optical properties of the samples were characterized by measuring transmission spectra at different temperatures. The photonic forbidden band gap experimentally measured from transmission dip is close to what is predicted from equation (4).

Acknowledgements Author (Naresh Dhiman) is thankful to National Institute of Technology, Hamirpur (HP), India for providing financial assistance.

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