This investigation reports on effect of C ion implantation on structure and optical properties of CeO2 thin films deposited on quartz substrates by the radio frequency (RF)-sputtering method. X-ray diffraction analysis shows that the face-centered cubic (FCC) structure corresponds to CeO2 . The lattice parameter of C ion implanted CeO2 films was found to decrease compared to that of the pristine CeO2 film. The shift in the peak positions after C ion implantation indicates changed lattice parameters. The observed values of strain are found to be positive and decreasing value of strain after C ion implantation indicates tensile strain in all of the prepared CeO2 films. The Raman spectra further confirm the formation of phase and also indicate the presence of defects in these films. The F2g peak intensity is decreasing with C ion fluence compared with that of the pristine film which is due to defects created after C ion implantation. Uv-Vis spectra shows that energy band gap decreases with C ion implantation which indicates increase in conductivity with C ion implantation in the films.
Keywords: CeO2 , Ion implantation, Band gap.
Ceria based metal oxide materials have several technological applications and has been extensively studied [1-5]. These materials are important because of their potential applications such as UV absorbents, electronic ceramic, polishing(ultra-precise), UV filters, fuel cell technology , engine exhaust catalysts and oxidation of water(photocatalysis). Most recently, free-radical scavenging ability of CeO2 nanoparticles has allowed their testing for serving as scavengers for free radicals[7-8]. In this backdrop the synthesis and study of CeO2 has assumed urgent importance for furthering the research and applications in this area [9-10]. Cerium oxide (CeO2 ) shows good optical properties and is a semiconductor with wide band gap energy (3.19eV).
The material’s modification with energetic ion beams shows interesting technological applications. Ion implantation and swift heavy ion (SHI) irradiation are the two common methods of material modification employing high energy ion beams. The energy and fluence of incident ions in these two techniques play significant role . The ion implantation technique uses incident ions of low energy (from few tens to few hundreds of keV). The can be The implantation of desired ions in the suitable material can be customized by controlling the various implantation parameters, for example energy of ions, type of ion, ion fluence and ion range etc. and the materials properties get modified due to the presence of implanted ions. There are several reports available on the formation of embedded nano phases of target ions and and effects of such nano phases on the various properties of host material [12-13]. In this context we have performed ion implantation experiment with different fluencies on CeO2 thin films.
Considering the importance of these materials, present study focused on understanding of optical and structural characteristics of quartz substrate based C ion implanted(deposited by RF sputtering) CeO2 thin films.
The radio frequency (RF)-sputtering technique was employed to deposit CeO2 thin films on quartz substrates. The technique was carried out in an argon (Ar) gas atmosphere at a temperature of 250C with sputtering power of 150W. Afterwards, implantation on deposited films was carried out by using 70 keV C ions. For this purpose the services of Low Energy Ion Beam Facility (LEIBF) at the Inter University Accelerator Center (IUAC), New Delhi, with fluencies of 1×1016 ions cm-2 and 3×1016 ions cm-2 were used. The C implanted films were then studied for optical and structural characteristics. The technique of X-ray diffraction (XRD) measurement (Bruker D8 x-ray diffractometer (CuKα radiation; λ=1.54 Å) was used for studying lattice structure of these films. Raman measurement using a Renishaw inVia Raman microscope (λ=514 nm) was employed to confirm the phase and defects such as oxygen vacancies in these films. Further, the optical properties were examined using a UV-Vis (Hitachi U-3300) spectrometer at 250C.
The XRD patterns (Figure 1) of all the films exhibited the fluorite-like Face Centered Cubic structure of CeO2 . Implantation with C ion led to increase in intensity of significant (111) planes, pointing towards increased crystallinity of implanted films as compared to pristine film. Scherrer’s equation  was used to calculate mean crystallite size based on most prominent (111) planes.
Where D is the size of the crystallite, λ (1.54 Å) indicates the wavelength of x-rays, β represents full width at half maximum (FWHM) and θ is the angle of reflection (Bragg). Table 1 details the crystallite size and other parameters of implanted and pristine films. Pristine CeO2 film exhibit a larger lattice constant (5.478 Å) than that for bulk CeO2 (5.411 Å) (JCPDS- 75-0390). Furthermore, a decrease in lattice parameter of implanted CeO2 films was observed as against pristine CeO2 films. XRD patterns clearly demonstrate shift in the peak positions after implantation thereby indicating changed lattice parameters. The decrease in the lattice parameters due to lattice contraction following C ion implantation resulted from difference in the ionic radii of C (0.260 nm) and O (0.140 nm) . These resultant changes in the lattice parameters brought about lattice strain, thereby the modifying various physical properties of implanted films. Following equation was used to calculate the strain ‘ε’ ,
Table 1 shows observed values of strain, which were found to be positive and decreased following implantation. The positive strain (ε) values, which may be due to difference between the thermal coefficient of substrate and that of deposited material indicates tensile strain in all films.
Table 1: Various calculated parameters of pristine and N ion implanted CeO2 thin films.
|Composition||Lattice Parameter, a (nm)||Crystallite size (nm)||Strain(ε)×10-4lin-2m-4||Band gap Energy (eV)|
Figure 2 details the effect of implantation on the local structure, stress/strain and defects in implanted films studied through Raman spectroscopy at 250C and compares same with pristine films .
Figure 2 indicates the formation of FCC structure for CeO2 . Raman-active vibrational mode (F2g ) of CeO2 is supposed to be reason for the peak observed at 443–463 cm-1 , which parallels the symmetrical breathing mode of oxygen ions around each Ce4+ cation (O–Ce–O) . Implantation brings about small shift in the F2g peak towards higher wavelength and broadening in the FWHM. These resultant structural changes in the Raman spectra are brought about by inhomogeneous strain and defects . In contrast with pristine films, C ion fluence leads to decrease in the F2g peak intensity (Figure 2)
Table 1 evidently shows that implantation leads to decrease in energy band gap, indicating that there is increase in conductivity with C ion fluence. Decreasing band gap causes increase in the density of states implying that the correlation length in the conducting network is increasing. This observation shows that due to implantation in CeO2 , the electronic property in the system is caused by the induced disorder effect.
In this work, we studied the optical and structural characteristics of quartz substrate based thin films of CeO2 after C ion implantation deposited by the radio frequency (RF)-sputtering method. Implantation with C ion was carried out at fluencies of 1×1016 and 3×1016 ions cm-2 . Techniques like XRD, Raman spectroscopy and Uv-Vis spectroscopy were employed to study the films. The results confirmed the Formation of FCC structure in all these CeO2 films was evidenced by XRD patterns and Raman spectra. Raman results also indicated presence of defects in pristine and C ion implanted films. C ion implantation was found to causing a decrease in the energy band gap as well.