Cliff Orori Mosiori1, Duke Ateyh Oeba2
2Department of Physics, Kenyatta University, P. O. Box 43844 – 00100 Nairobi, Kenya.
Abstract: Light sensing forms a basis of fabricating solar cells which in turn promise to avail clean and sustainable electrical suitable to make worthwhile contributions to solving global renewable-energy challenges. Current solar cell technology heavily relies on crystalline silicon wafers.Due to its challenges, great research interest is now directed towards thin-film solar cells. An attempt on organic semiconductors materials has presented much poorer charge transports and shorter exciton diffusion lengths than inorganic ones do. These has greatly limit the thickness of the photoactive layer in case of organic materials. Theoretically, materials with high electrical conductivity such as metal nanoparticles are opaque. They also offer lower optical transparency. Therefore, finding materials that are both transparent to visible light and electrically conductive is a continues study so that today’s popular devices such as liquid-crystal displays and organic light-emitting diodes in televisions, touch screens in phones or tablet computers, electrophoretic displays in e-readers, or solar cells are visible.in this work, silver nanoparticles were used to dope cerium titanium dioxide by using a laser source at low pressure oxygen atmosphere irradiations. It was found that this proceedure generated CexAg0.02xTiO2 composite when pure cerium oxide, silver metal and titanium dioxide composites were used. The resulting thin films were found to possess higher absorption coefficients in the UV-visible spectral region accompanied with a tunable optical band gap varying between 3.42 eV to 3.78 eV. It was proposed to be suitable for wind band gap optical applications like in lasers design and applications.
Keywords: CeTiO2, Composite, CexAg0.02xTiO2 thin films, laser radiation.
- INTRODUCTION:
Silver nanoparticles have proved to have various effective applications in technology. Dispersed Ag NPs on TiO2 thin films alone was found to enhance the photocurrent generation with five folds as compared to TiO2 under UV light irradiation. Silver modifies many nano-composites under visible light irradiation for complete inhibition of microorganism activities. Cerium titanium dioxide trioxide has shown some useful functional properties promising technological applications especially in gas sensor technology and electrochromic devices. By including metal nanoparticles silver or gold in a gas sensor, it is found to modify in their physical, chemical and structural properties. It has been found that gas sensor activity increase three times higher when loaded Ag nanoparticles under visible-light irradiation. It has also been found to effective enhance sensitivity and selectivity electrochromic devices. They have also been proposed as effective in reducing the response and recovery times in gas sensors. It is applications include removing biological impurities from drinking underground water supplies, modifying nanostructures through different processing methods like sol-gel procedures, magnetron sputtering, direct femtosecond laser irradiation or pulsed laser deposition. In this study, pulsed laser deposition technique was used and we present results on the effect of silver nanoparticles CeTiO2 thin films. laser.
- EXPERIMENTAL DETAILS:
2.1 Cleaning of Substrates:
The SiO2 quartz substrates were cleaned with acetone in a ultrasonic bath. The target was thoroughly cleaned before any preliminary trials were done and also after the trials.
2.2 preparation of Vacuum Chamber:
Prior to each irradiation the vacuum chamber was evacuated down to a residual pressure of 10-4 Pa. This pressure was maintained constant during the thin films synthesis process. High purity oxygen (99.9%) was then circulated inside the irradiation chamber through a calibrated gas inlet.
2.3 Reagents and Chemicals:
The chemicals and reagents included cerium, titanium, silver, silicon substrate, high purity oxygen (99.9%) among other laboratory chemicals and apparatus.
2.4. Growth procedures:
The growth of silver doped CeTiO2 thin films was performed inside a stainless-steel reaction chamber with a pulsed frequency laser fixed at λ = 255 nm, ~ 8 ns, n = 20 Hz; laser fluency fixed at 5 J/cm2 at 0.1 MPa pressure. Silver nano-particle concentration was varied between 1.5 to 3 wt.% range and sintered at 1000 °C for 2 hours. The substrates were positioned at a separation distance 30 mm from the target surface and parallel to it. The substrate temperature was fixed at about 630 0C during the film growth and only 10,000 laser pulses were applied to each thin film. The films were cooled down with a ramp of about 10 0C per minute at 0.5 MPa that used during the deposition experiments.
2.5 Thin film characterization:
The optical measurements were performed with a double beam Perkin Elmer Lambda 19 spectrophotometer in the wavelength range of 300nm to 1100 nm while compositional and crystallinity status were investigated by XRD and EXRD measurements by configuring the X-ray diffraction in θ-2θ configuration with a Philips MRD diffractometer (CuKα, λ=1.5418 Å radiation).
- RESULTS AND DISCUSSION:
3.1 Elemental Composition of CeTiO2:
The obtained SAED pattern resulted into inter-planar distances of 0.385 and 0.192 nm assigned to the (001) and (002) lattice plane reflections of the orthorhombic CeTiO2 phase, with lattice parameters a = 7.318 Å, b = 7.451 Å, and c = 3.684 Å, as referred in the JC-PDS 20-1324. Hence, there was no indication of the presence of silver or silver oxide observed.
3.2 Structural Properties of CeTiO2 Thin films:
The crystalline structure of the thin films was investigated using the XRD technology. The observed diffractogram curves composed of diffraction lines at 23.1°, 47.2° and 50.4° attributed to the (001), (002) and (112) lattice plane reflections. This implied an orthorhombic phase with a preferred orientation along the [001] crystal direction. With increase in Ag concentration, the intensity of the 23.1° line corresponding to (001) lattice plane reflection decreased gradually with full width at half maximum increasing.
3.2.1 Structural Properties silver doped of CeTiO2 Thin films:
Adding silver nanoparticles to the film, the XRD peaks were modified. It was attributed the line 23.6° that appeared at (020) lattice plane reflection. About 0.2°-degree shift towards higher values on the line was observed that corresponded to (001) lattice plane reflection. This was also attributed to the slight change of the d-spacing lattice parallel to the surface of substrate. It was concluded that silver nanoparticles as a dopant lead to an increase in the average dimensions’ crystals.
3.3. Optical properties:
3.3.1 Chromaticity coordinates and tristimulus:
It is known that chromaticity coordinates are used to specify the color observed according to the CIE chromaticity diagram. Equally, standard tristimulus value is used as a measure of the brightness of a surface or colour. By using Pyton 2.7 program, optical transmittance spectra, chromaticity coordinates and standard tristimulus value were calculated and tabulated as shown in table I. The dark blue colour was attributed to oxygen deficiency in the thin films. This further implied that CeTiO2 underwent partial decomposition at the target as it was being irradiated by the subsequent laser pulses.
Table I: Chromaticity coordinates and tristimulus of doped thin films
Ag conc. wt.%] | Chromaticity coordinates (x, y) | Tristimulus Y | Colour Status |
3.0 | (0.35, 0.34) | 817 | Dark blue |
2.5 | (0.29, 0.32) | 321 | slightly Transparent |
2.0 | (0.23, 0.16) | 19 | Fairly transparent |
1.5 | (0.18, 0.16) | 9 | Transparent |
1.0 | (0.14, 0.13) | 3 | Very transparent |
0.5 | (0.12, 0.11) | 1 | Very transparent |
0.0 | (0.11, 0.1) | 1 | Very transparent |
3.3.2 Optical Transmittance:
The average transmittance of the un-doped CeTiO2 film was close to 90 % in the visible spectral range while the one containing silver (Ag) had an average decrease of greater than 20 %. This was attributed to the formation of oxygen vacancies at low supply oxygen since this low supply pressure does not stabilize stroichiometric formation of CeTiO2 crystals.
3.3.3 Optical Absorption:
Optical absorption increased as Ag concentration increased. This was attributed to the surface plasmon resonance absorption resulting from the embedded Ag nanoparticles as the absorption was observed to shift towards higher wavelengths as silver concentration increased. SPR peak positions below 410 nm were observed at very low Ag nano-particle concentrations and thus optical absorption coefficient, a, was calculated from;
where A stands for the absorbance and d for the thickness of the films.
3.3.4 Determination of thickness:
Using surface profilometry data, the average thickness of the un-doped and doped thin films was evaluated to be around 98 nm.
3.3.5 Optical Band Gap, Eg:
Fundamentally absorption coefficient follows the relation in Eq. 4 where m = 2 for the indirect transitions and ½ for directly allowed transitions;
The Scout Software was used determine the possible transitions by simulating (ahn)1/m against hn, photon energy for m = 2 and for m = ½. Table II shows the obtained band gaps. This was similar to determining the band gap from plots of Eq. 4 for direct band gap values can be estimated;
Table II: Band gaps for thin films
CeTiO2 | CexAg0.02xTiO2 |
2.66 | 3.42 |
2.78 | 3.55 |
2.87 | 3.76 |
3.03 | 3.79 |
Band structure calculations were done on CeTiO2 thin films (reported elsewhere– Mosiori et al., 2016) revealed oxygen deficiency. The narrow changes observed in band gaps was attributed to charge carrier transfer from Ag to CeTiO2 layer that might have resulted into a downward shift of the conduction band and at the same time, an upward shift of the valence band hence increasing the band gap, Eg.
3.3.6 Urbach energy, E0:
As observed in Mosiori et al. 2016, at photon energy hv, below the absorption edge, it is possible to evaluate the Urbach energy or the Urbach tail. This is because the structural disorders observed generate band tails with localised states and therefore the Urbach energy, E0, can easily be estimated from;
Table IV: Current-Voltage curve by CSAFM technique
This means that the dependence of Urbach energy on Ag concentration may introduce some lattice disorder in the composite material. If this expression is used to plot a curve, then the inverse of the slope will be a linear fit to the logarithmic plot of the absorption coefficient. The value of Urbach energy calculated in this work as shown in table III was fairly similar for undoped CeTiO2 thin films as reported in literature elsewhere by other growth methods.
Table III: Average size and Urbach Energy
Ag concentration [wt.%] | Nano-crystallites size [nm] | Urbach energy [meV] |
3.0 | 85 | 297 |
2.5 | 63 | 361 |
2.0 | 51 | 398 |
1.5 | 41 | 421 |
1.0 | 33 | 441 |
0.5 | 29 | 496 |
0.0 | 17 | 573 |
3.4 Electrical properties:
3.4.1 Current-Voltage curve by CSAFM on CeTiO2 thin films:
CSAFM technique was used to determine the local conductivity of CeTiO2 thin films using biased Pt-Ir coated silicon tips in contact mode. All the curves were non-ohmic as depicted by table IV. It was observed that the turn-on voltage value ranged between 0.1V to 0.3V while their corresponding resistance at 0 V range from about 0.4 GW to 2.5 GW. The small element of conduction was attributed to the semiconducting properties of CeTiO2 thin films.
3.4.2 Current-Voltage curve by CSAFM on Ag doped CeTiO2 thin films:
Almost similar I-V curves were obtained for Ag doped CeTiO2 thin films, but the curves were slightly or nearly-ohmic in character. The slight deviation from the linearity was attributed to the contribution of the semiconducting behavior of CeTiO2 while the ohmic behavior was attributed to the presence of evenly dispersion Ag nanoparticles present. From table IV, the gradual transition behavior of I-V curve is a property that can be used in sensor applications to ensure better selectivity and sensitivity.
Table IV: Current-Voltage curve by CSAFM technique
Thin Film | CSAFM Measurements |
|
|
| ||||||||
Voltage (µV) | -0.5 | -0.4 | -0.3 | -0.2 | -0.1 | 00 | 0.1 | 0.2 | 0.3 | 0.4 | 0.5 | |
CeTiO2 | Current (nA) | -6 | -4 | -2 | -1.5 | -0.1 | 0.0 | 0.1 | 1.1 | 2.1 | 3.9 | 7.3 |
CeAgTiO2 | Current (nA) | – | – | -6.1 | -4 | -1 | 0.0 | 1.9 | 3.9 | 7.1 | – | – |
- CONCLUSIONS:
A nano-composite films of CexAg0.02xTiO2 were grown by pulsed laser deposition. The effect of Ag concentration on crystalline status and optical properties was investigated. It was observed that there was a gradual band gap increase. This was attributed to the thin film’s change in chemical composition as Ag was introduced. Electrical properties portrayed promising features for the design of new composite materials for applications electrochromic devices and gas sensors.
Acknowledgements
The authors thank Department of Geology, Ministry of environment, Nairobi for performing the X-RD and EXRD analysis, the University of Nairobi for allowing us to use the spectrophotometer for transmission and absorbance measurements and the Department of Mathematics and Physics of Technical University of Mombasa.
REFERENCES:
- Ritala, M., Leskelä, M., Nykänen, E., Soininen, P., & Niinistö, L. (1993). Growth of titanium dioxide thin films by atomic layer epitaxy. Thin Solid Films, 225(1), 288-295.
- Arabatzis, I. M., Stergiopoulos, T., Bernard, M. C., Labou, D., Neophytides, S. G., & Falaras, P. (2003). Silver-modified titanium dioxide thin films for efficient photodegradation of methyl orange. Applied Catalysis B: Environmental, 42(2), 187-201.
- Machida, M., Norimoto, K., & Kimura, T. (2005). Antibacterial activity of photocatalytic titanium dioxide thin films with photodeposited silver on the surface of sanitary ware. Journal of the American Ceramic society, 88(1), 95-100.
- Hass, G. (1952). Preparation, properties and optical applications of thin films of titanium dioxide. Vacuum, 2(4), 331-345.
- Justicia, I., Ordejón, P., Canto, G., Mozos, J. L., Fraxedas, J., Battiston, G. A.,… & Figueras, A. (2002). Designed Self‐Doped Titanium Oxide Thin Films for Efficient Visible‐Light Photocatalysis. Advanced Materials, 14(19), 1399-1402.
- Argall, F. (1968). Switching phenomena in titanium oxide thin films. Solid-State Electronics, 11(5), 535-541.
- Sberveglieri, G., Comini, E., Faglia, G., Atashbar, M. Z., & Wlodarski, W. (2000). Titanium dioxide thin films prepared for alcohol microsensor applications. Sensors and Actuators B: Chemical, 66(1), 139-141.
- Suzuki, T., Kosacki, I., Anderson, H. U., & Colomban, P. (2001). Electrical conductivity and lattice defects in nanocrystalline cerium oxide thin films. Journal of the American Ceramic Society, 84(9), 2007-2014.
- Keomany, D., Poinsignon, C., & Deroo, D. (1994). Sol gel preparation of mixed cerium—titanium oxide thin films. Solar Energy Materials and Solar Cells, 33(4), 429-441.
- Younis, A., Chu, D., & Li, S. (2012). Oxygen level: the dominant of resistive switching characteristics in cerium oxide thin films. Journal of Physics D: Applied Physics, 45(35), 355101.
- Yoshimoto, M., Shimozono, K., Maeda, T., Ohnishi, T., Kumagai, M., Chikyow, T.,… & Koinuma, H. (1995). Room-temperature epitaxial growth of CeO2 thin films on Si (111) substrates for fabrication of sharp oxide/silicon interface. Japanese journal of applied physics, 34(6A), L688.
- Bhuiyan, M. S., Paranthaman, M., & Salama, K. (2006). Solution-derived textured oxide thin films—a review. Superconductor Science and technology, 19(2), R1.
- Keomany, D., Poinsignon, C., & Deroo, D. (1994). Sol gel preparation of mixed cerium—titanium oxide thin films. Solar Energy Materials and Solar Cells, 33(4), 429-441.
