Fabrication of SiO 2 -TiO 2 Double Layers Anti-Reflection Coating by Sol-Gel Method and Study of its Optical Properties

. An inexpensive and easily adaptable to industry scale method is Sol-Gel used to get a double layer anti-reflection coating thin film from SiO 2 /TiO 2 by dip coating technique and we found that the Q H thickness SiO 2 /TiO 2 Double layer thin film gets the best results so, we get a reflectance 0.002-0.02 for ultraviolet region, 0-0.04 for visible region, and 0.004-0.04 for Near infra-red region, and it is better than a thin film from SiO 2 mono layer or thin film from TiO 2 mono layer individually.


INTRODUCTION
Coatings have been applied on glass to obtain a glass with more useful properties, such as anti-reflection(AR),anti-static, defogging, anti-abrasion, self-cleaning (SC), solar control and electrical conductivity. compounds are coated as a thin film on glass by a variety of techniques [1][2]. SiO 2 coatings, for example, have attracted attention due to their transmittance of glass as a result of their low refractive index and low surface scattering [3][4]. SiO 2 -coated anti-reflective glasses are currently used commercially in TV screens, picture framing, optical lenses and solar panels [5][6]. TiO 2 coatings, have attracted attention in many uses such as in photo voltaic and electro chromic applications [7].
Sol-Gel method is inexpensive and easily adaptable to industry scale and mass production comparing with other physical and chemical vapor methods,. It is possible to work in normal atmospheric conditions and get high homogeneity of the final coatings. The process can be controlled by temperature or through the chemical contents and their molar ratios. There are a variety of options for depositing the coatings, called dip-coating, spin coating, or spray coating on the substrate with the sol.

Substrate and cleaning procedure [8]:
We used glass substrates Soda-lime that has 13% Na 2 O which dissolved in water and converting to NaOH as a thin film making a collapse in preparing procedure so we can etch this film by washing the substrate with distilled water and apply the cleaning procedure as follows: 1

Dip coating Sol-Gel Method and Thickness Measurements:
We immersed the cleaned substrate into the Sol and get it up with a controlled with-drawal speed thus, the thickness is related with withdrawal speed and we can calculate it from modified landaulevich equation [8]:  where: h is the thickness; v is withdrawal speed;  viscosity ;  is density;  is surface tension between vapor and liquid; and g is the gravity acceleration As we know that to get a best anti reflection coating we have to put double layer where this equation is incontestable: where : n 1 is refractive index for the upper layer SiO 2 ( n 1 =1.51) n 2 is refractive index for the inner layer TiO 2 ( n 2 =2.25) n s is refractive index for the soda-lime substrate ( n s =1.5) n 0 is refractive index for the medium ( n 0 =1.00) So, we make three types of double layer anti reflection coating to use it in the seberal range of the spectrum the first for Ultra Violet ( 250-400 nm ) and we choose =300 nm is the center wavelength, the second for Visible region ( 400-800 nm) and we choose =550 nm is the center wavelength, the third for Near Infra-red region (800-1200 nm) and we choose =1050 nm is the center wavelength From each type we make three films depending on its optical thickness; where optical thickness calculated from the equation: ℎ = . 4 where: k=1,2,3,4…..;  is the center wavelength; n:refractive index So, when k=1 that we get a quarters' thickness film k=2 that we get a half thickness film k=3 that we get a three quarters' thickness film and so on… To get a desire thickness probably we have to repeat the procedure of precipitation more than one time or we apply the application one time with different withdrawal speed as it shown in table .2 International Letters of Chemistry, Physics and Astronomy Vol. 56 In the second step, ethanol was added to the solution so that the molar ratios became TIVBT:EtOH:CH 3 COOH=1:150:0.5, After this, the solution was stirred for 1 h more still at room temperature then the solution came out homogeneous and transparent. [12][13] We prepare thin films from TiO 2 by immersing the cleaned substrate into the Sol and get it up with a controlled withdrawal speed then we put it in the furnace at 100 C for 15 min, annealed it at 500 C for 1 hour then we cooled it to room temperature which we called procedure No.1 [14][15][16] The SiO 2 sol was prepared by tetra ethyl ortho silicate (Si(OC 2 H 5 ) 4 [18][19].
We prepare thin films from SiO 2 by immersing the cleaned substrate into the Sol and get it up with a controlled withdrawal speed then we put it in the furnace at 100 C for 15 min ,annealed it at 450 C for 1 hour then we cooled it to room temperature, which we called procedure No.2 [20] Now we get a cleaned substrate by cleaning procedure and apply first procedure No.1 then the procedure No.2 to get a Double Layer Anti-Reflection Coating to study its optical properties

RESULTS AND DISCUSSION
We measure the transmittance and the reflectance and calculate the absorbance for the prepared film with three types ( Q Q, Q H , Q 3Q) and get the follows:

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ILCPA Volume 56 As we see the film gets a high transmission upper than 95-99% and low reflection in the whole studied spectrum.
Then we compared our film with theoretical mathematical data for double layer anti-reflection coating in visible region and get the result as follows: We found that there is a good match between theoretical and practical results.
Then we study the reflection for each group (region in the studied spectrum ) and the results were:

1-Ultra Violet region:
We measure the reflectance for the first group (Double Layer Anti-Reflection Coating) which will be used for the ultra-violet region and we get:  We can see that the best film for SiO 2 /TiO 2 (Q H) Double Layer Anti-Reflection Coating for the ultra-violet region

2-Visible region:
We measure the reflectance for the second group (Double Layer Anti-Reflection Coating) which will be used for the Visible region and we get: We can see that the best film for SiO 2 /TiO 2 (Q H) Double Layer Anti-Reflection Coating for the visible region.

3-Near Infra-red region:
We measure the reflectance for the third group (Double Layer Anti-Reflection Coating) which will be used for the Near Infra-red region and we get: 148 ILCPA Volume 56 We can see that the best film for SiO 2 /TiO 2 (Q H) Double Layer Anti-Reflection Coating for the Near Infra-red region.
As a result, we see that is the film SiO 2 /TiO 2 (Q H) is the best for our aims. Finally, we study the reflectance for double layer SiO 2 /TiO 2 (Q H) as a comparison with a glass, Q thickness SiO 2 mono layer, H thickness TiO 2 mono layer and we get the follows: