Optical transmission in thin films

The effects of the heat-treatment temperature and Co Doping for different number of dipping on the
Structural, Optical and Electrical Characterizations of Tio2 Thin Films prepared by sol–gel dip coating
The effect of the heat-treatment temperature and doping level on the structural, optical and electrical films
characterisation has been focused in This study. . Pure and CO-doped, TiO2 thin films (Co: TiO2, Co: 0-
3-5 and 7 at. %) were grown onto glass substrates at room temperature by sol–gel method. The gained
films got heated for two hours at three different temperatures: 400, 450, and 500ºC. Different methods
such as Differential scanning Calorimetry, Spectroscopic ellipsometry (SE), X–ray diffraction (XRD) and
atomic force microscopy (AFM) were used in order to characterise the films. The results Confirmed that
all Co: TiO2 films are polycrystalline with a tetragonal anatase with (101) preferential orientation and
orthorhombic brookite type\'s structures. Moreover , it was noticed that there was a reduction in the grain
size with increase in annealing temperature and doping level. The topography of the surface was
observed by atomic force microscopy (AFM) exhibits surfaces of CO-doped TiO2 films are smoother than
undoped TiO2 films. .Also, the films’ refractive index increases simultaneously with a decrease in the
band gap of the films from 3.03 to 2.96 eV; in parallel, the porosity decreases. Keys words: TiO2 ,
anatse, Annealing temperature, optical band gap, Refractive index, sol-gel, morphology, optical
transmittance.

1. INTRODUCTION

The titanium oxide is an ideal n-type semiconductor [1], and a large band gap of 3- 3.2eV. Its characteristics depend on the crystalline phase. It reigns three different crystalline structures which are rutile, brookite, and anatase [2].The first structure of them is the most prevalent in nature. Brookite is quite rare. Rutile is stable at high temperature while anatase and brookite do not present this quality. [3, 4].Nowadays, Titanium oxide is largely used as a photo-catalyst because of its important characteristics such as high oxidation efficiency, high photo stability, corrosion protection, and non-toxicity… etc. [5]In general, TiO2 has a variety of applications that involve photo catalytic activity for water and air purification induced by UV radiation, self-cleaning surfaces, antibacterial activity, and super hydrophilic. In addition, its applications range from light detecting and emitting devices to transparent electronics and sensor devices in telecommunications. [6–8]Cobalt doping is important for photo catalysis and other electromagnetic application.   In fact, the incorporation of metal ions into the titanium crystal lattice is able to increase the extension of the absorption edge in the visible region, and anatase structure. At ambient temperature, Co-doped TiO2 layers will have ferromagnetic (FM). [9]Sol – gel method is one of the well-established synthetic approaches to prepare molecular scale homogeneity of the starting solution, and combine different types of metal dopants into distinct ceramics matrices [10–14]. Also, this process is not expensive, and accessible. Moreover, the sol–gel thin films show stellar anti friction reduction performances under low capacity [15–16] and facilitate the manipulation of cobalt.In this paper, we display the disposition of the thin film of TiO2 onto glass by using sol-gel method in order to study its physical properties. Also, we went through Transmittance, gap, refractive and index of the films. We used different types of physical, analytical methods such as Scanning Electron Microscope (SEM), Atomic Force Microscopy (AFM), X- ray Diffraction (XRD), Differential Scanning Calorimetry (DSC) to examine the structural evolution with annealing temperature.

2. EXPERIMENTAL DETAILS

Cobalt doped TiO2 thin film was incorporate on glass substrates (15mm×15mm×2 mm) by sol-gel process.The sol-gel solution [17] is prepared at ambiant temperature in the following  manner: 2 cm3 of titanium isopropoxide (Fluka, 99.9) were considered, to which 0.7 cm3 of isopropanol were added drop by drop. The solution was stirred for 10 minutes. Then, 2.2 cm3 of acetic acid were poured, stirred pending15 minutes. Cobalt acetate tetra hydrate, 98+% powder was dissolved in methanol was added to prepare the Co: TiO2 for various  at. % of cobalt concentrations Ti1-xCoxO2 (x =0.00, 0.03, 0.05, 0.07) finally, 5.2 cm3 of methanol were added and stirred pending1 hour. The samples were immersed in the sol-gel solution with drawing it at 12 cm/min speed, dried at 100°C during 15 min, and finally annealed at 300; 400 and 500°C for 2 h.The structural Properties of the films were studied by X-Ray Diffraction (XRD), using Cu-ka radiation of wavelength λ=0.15418 nm and microstructure by using scanning electron microscopy. The Optical transmittance was measured on a UV-VIS spectrophotometer. Atomic force microscopy (AFM) was used for the observation of surface morphology for Co: TiO2 films deposited onto glass substrates in a region of 4.68×4.68 µm2. The electrical properties of the layers were measured using I (V) characteristics by the two probes technique in a coplanar structure with two evaporated gold electrodes.

3. RESULTS AND DISCUSSION

3.1. Structural properties

3.1. 1.Differential scanning

Calorimetry The xerogel thermal curves in Fig. 1 shows two singularities: firstly, an endothermic peak spreading from 50 to 250°C, which corresponds to the evaporation of water [18], the thermal decomposition of isopropanol as well as the carbonization or the combustion of the acetic acid and certain elements which constitute our alkoxide. Secondly, an exothermic peak spread from 290 to 400°C, which corresponds to the crystallization of titanium oxide.  The low temperature and the increase of the cobalt content move the two picks lightly. This analysis proves that an annealing at a temperature equal or higher than 400°C would be largely enough  to form titanium oxide completely.

3.1. 2.SEM investigations   

SEM testing was aimed to analyzed the morphology and pore size in bone scaffolds. The morphology of material was important to study because it is related to the ability of cells to attach and grow in materials, especially those categorized as semiconductor. The TiO2 thin layers obtained: after a) 1 dipping; b) 2 dippings and c) 3dippings annealing at 500°C for 3 % at. Co (Figure 2) shows that the film was homogeneous without any visual cracking over a wide area. The increase in the treatment temperature, did not affect the uniformity of the film.

3.1. 3.X-ray diffraction

Fig. 3(a),( b), (c) and (d) displays the XRD patterns of  TiO2 thin films obtained after annealing: a) at 400, 450 and 500°C for 3 %At. Co and 3 dipping;  b) at 500°C for 5 %At. Co and 1, 2 and 3 dipping; c) at 350, 400 and 450°C; d) at 500°C for 2 dipping and 0, 3, 5and 7 %At. Co. As seen from this figure, the films exhibit a dominant anatase phase indicating the polycrystalline nature of the obtained films. At 400 ◦C the films show furthermore a preferential orientation along the [101] direction with unusually high intensity at about 25, 3° .Fig.3 (a). At elevated temperatures (450°C) for 2 layers and 3%At.Co, The major peak at 2θ=31° corresponds to the (121) reflection of the brookite phases simultaneous the anatse is disappeared.  At 500°C, there is the dissipated of the brookite and the increase in the intensity the (101) line of the anatse. For 3% At. Co, we notice the presence of two concurrent phases with two layers, while for one layer, there is brookite and for three layers, there is only anatse Fig. 3(b). Regardless of growth dipping number, no evidence for other orientation except the peak of Brookit relative intensity decreases with the increasing. Fig. 3(C).Fig. 3(d) shows that the (100) peak intensity increases with increasing in the level doping and for 7% At. The peak is very broad and can be identified as due to combination   of Brookit and anatase. Anatase (100) plane diffracts at 25.3°, and brookit at 31.7° and 31.8°, corresponding to the (121) and (211) planes respectively and shifting of peaks to larger angles which means that improving crystallinity of the films. The average grain size of Co: TiO2 thin films were evaluated for all the viewed planes by using the Scherer’s formula [19].                                                        (1)where ‘k’ varies from 0.89 to 1.39. ‘λ’ is wavelength of X-ray, ‘β’ is the full-width at half of the peak maximum in radians and ‘θ’ is Bragg’s angle.The values deducted are given in Table 1.. It could be noticed that the crystallite size of anatase and brookite increases with the increase in the annealing temperature, Co-doping and decrease with dipping number.Fig.4 shows variation of the grain size of the oxide layer with the annealing temperature. As we raise heat-treatment, TiO2 crystallites continue to develop. The crystallite size of anatse and brookite increases with the raise in the temperature thermal treatment from 6.7 to 15.4 nm for anatse, and from 18.8 to 23.5 nm for brookite. This is a general trend observed by many authors.

3.1. 4.Atomic force microscopy      

The thin layer structure and morphological evolution with co concentration and heat treatments were studied by AFM to confirm the crystallized structure studied in the preceding paragraph by DRX. fig. 5a shows the 3d AFM images of the Ti1-xCoxO2 (x=0 0.03 0.05 and 0.07) thin layers prepared onto glass substrates correlated to annealed at 500 °c for 2 hours. The size of the scanned area was 10.14 x 10.14 μm2. It appears that average mean square roughness rms increases from 8 to 16 nm of tio2 films can be explained by the deterioration of the optical and electrical characteristics [20]. However, these discrepancies in mean roughness have indicates effect  on optical characteristics of the films considered in  the present study  in parallel the grain size decreases with the cobalt content. This indicates that the rather stretched form of the films suggests that the density of the conducting changes drastically was in one dominant direction. These results, together with the XRD analysis, well confirm that the crystallinity is influenced by the cobalt doping. [21]Fig. 5(b) exhibits AFM images for a samples annealed at 400;450 ◦C. and 5 00 ◦C. for 7% at Co. As indicated by the images, the post-annealing process preserves and even improves the superficies morphology, average mean square roughness being reduced from 0.18 nm down to 0.8 nm after annealing. However at 400 ◦C (Fig. 5(b)), larger anatase crystal grains are observed. It can be seen that the samples annealed from 450 to 500 ◦C (Fig. 5(c)–(e)) show the same grain shape, but an increase in the particle size. The figures show as well the increase of the porosity and roughness with growth temperature.The root mean square (RMS) roughness analysis of TiO2 films has been carried out. As shown in Fig. 4, the RMS follows a similar evolution as the surface morphology observations. The RMS roughness increases slowly from 0.880 to 4.235nm when the annealing temperature increases from 400 to 500 ◦C, then reaches 7.270nm at 500 ◦C. The increase of roughness can be interpreted as the phase change, and the increase of grain size (Table 1).

3.2. Optical properties

We characterized optical of the Co: TiO2 using UV–Vis diffuse reflecting spectrometry. We calculate the band gaps by the Kubelka–Munk equations [22] as follows Eq (2):                                                                (2)Where R is the reflection coefficient of the sample, R=10−A, A is an optical absorption. Since TiO2 is an indirect gap semiconductor [22], the optical band gap (Eg) of the TiO2 films can be related to absorption coefficient (α) by Eq                                               (3)The transmittance spectra of TiO2 layers varying with co concentration and annealing temperatures after three dippings registered in the range from 300 to 900 nm shown in. figure .6. a and.6 b. obviously that the films are absolutely transparent in the visible region and start absorbing in between 300 and 350 nm. The observed oscillations, i.e. interference fringes, are due to multiple reflections in the layer. The influences of co doping into the tio2 lattice are clearly observed in the optical transmission spectra shown in. figure .6 as the co-doped tio2 thin films annealed at temperatures from 450°c to500°c are used as a transparent conductor in thin films solar cells [27] so one of the most important points is to obtain the films with a property of high transparency in the visible region range of 400-800nm. As we rise in temperature of annealing, the absorption edge shifts towards diminished energy side signifying the decrease in the band gap of the films. the decrease of transmittance in these films may be a result of the insufficient incorporation of oxygen in the films through deposition since the condensation of oxygen decreases at rised substrate temperature This is a general trend observed by many authors [24-25]. The typical variation of ah 2 dependence of photon energy h used for optical band gap is depicted in fig. 7. Extrapolating the straight line portion of this plot to the energy axis permits to determine Eg. It varied between 3.65 at 400°c to 3.55 ev at 500°c. It means that the band gap value decreases with the raise in temperature can be related with an increase in the grain size and the change in film density. The porosity of the thin films could be calculated from the following equation Eq. (4). [26]. Porosity = nd and n are, respectively, the refractive index of pore-free anatse nd= 2.52 [27] and the porous thin films respectively. The resultant values of band gap energy Eg , the refractive index (n) and porosity (p ) for TiO2 thin films as a function of Co doping level, dipping number and Temperature at 650 nm wave is reported in Table 2. From our results, we can deduce that when the refractive index of the films increases the optical band gap decrease at the same time. Here, it is worth noticing that the refractive index of the films increases with increasing cobalt doping concentration, treatment temperature and the number of dipping. in addition porosity decreases. That result in to film densification and pore destruction in films through different treatment temperatures. 

3. 3. Electrical conductivity

Fig. 8(a) presents the evolution of the electrical conductivity σ of undoped TiO2 and doped films with Co as a function of doping level. As can be seen, the electrical conductivity increases slightly first and then diminishes, achieving a minimum value of 7.9 10-9 (Ω cm)-1 at 450°C for 5%At.Co can be elucidated  by an increased in free carriers (electrons) concentration (density) there are coming from the ions Co2+ donors in the substitution sites of Ti3+[57].Fig. 8(b) shows the evolution of the conductivity of Co doped TiO2 films with heat-treated temperature from400°C to 500°C .as can be seen the conductivity raises with the increase in the heat-treated temperature which can be explained by the upgraded in crystallographic quality.

4. CONCLUSION

In conclusion, cobalt can be easily included through the sol-gel method.  The as deposited sample is amorphous and it crystallizes to anatase phase starting from 400°C with a preserve their (101) preferential orientation. In addition, we verify that the anatse and brookite phases are very unsettled, the crystalline size of anatse and brookite increases with the increase in the heat-treated temperature and dipping number. The mean grains size and the roughness of the samples rinse with increasing in annealing temperatures, dipping number and Co concentration. The analysis of the transmission spectra shows that TiO2 thin films are transparent in the visible and opaque in UV, as well as the transmission and the calculated optical band gap decrease with increasing in temperature, the number of dipping and Co concentration. The values of the conductivity decrease with the increase of Co doping level and annealing temperatures. The refractive index of the thin films of titanium oxide increases with the increasing in the Co concentration; however, the temperatures, the number of dipping in parallel the porosity decrease. The relationship    between the outcome gained by X-ray diffraction and optical properties of TiO2 thin films synthesise by sol-gel method.

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