Implication of Mn concentration on the properties of cerium oxide thin films

: Uniform and adhesive Manganese doped cerium oxide (MDC) films are successfully deposited by Nebulizer Spray Pyrolysis (NSP) technique. The MDC films are characterized by XRD, FT-IR, UV-ViS, PL and I-V analysis. X-ray diffraction peaks reveal the single-phase polycrystalline cubic fluorite structure with preferential orientation along (2 0 0) direction. The broad bands observed at 695, 659, 538 and 517 cm -1 are due to the envelope of (Ce=O) symmetric, asymmetric terminal stretching and phonon band of metal oxide (Ce-O) network from FT-IR spectra. The transmittance decreases with Mn concentration due to the increase in scattering of photon by crystal defects created by doping and lower ionic size of Mn. The electrons in the outer orbits have travelled to the higher energy levels and have occupied vacant positions in energy bands. Some of the NBE emission and green emission peaks are vanished at higher doping level of Mn. The occurrence of a strong and broad blue emission peak at 15% for MDC films has been confirmed from emission spectra.


Introduction
II-VI, IV-VI transition metal nanocrystalline thin films have attracted considerable interest of investigators because of their optical, electrical parameters in the optoelectronic, solar cells, IR detectors and photo induced devices [1][2][3][4][5]. Manganese is considered to be one of the most useful VIIB group transition metal compounds, which has found applications in magnetic memory devices, ferrofluids, and other optoelectronic devices. Manganese oxide (MnO2) provides a lower cost and lower toxicity replacement for ruthenium oxide in supercapacitor applications [6]. There are several oxidation states, including Mn(0), Mn(II), Mn (III), Mn(IV), Mn(V), Mn(VI) and Mn(VII) for manganese oxides [7]. Composite electrodes based on MnO2 and containing carbon nanotubes, carbon blacks, polyaniline and other conducting materials are under investigation for application in super capacitors [8][9][10]. In the present work, an attempt has been made to prepare thin films of manganese doped ceria and investigate their structural, optical and electrical properties. The effect of manganese doping on the particle size, lattice parameter, electrical conductivity and band gap energy is also investigated.

Experimental details
Analytical grade cerium (III) nitrate, manganese (III) chloride are used for the film preparation. Mn doped CeO2(MDC) films deposited at an optimized substrate temperature 400 o C with film thickness 451, 489, 534, 579and 608 nm are prepared through Nebulizer Spray Pyrolysis (NSP) technique. The doping of Mn leads to the increase of film thickness and lattice parameter as the radius of Mn (0.57Å) is lower than that of Ce (0.97Å). The spray solution is prepared by dissolving 0.08 M cerium nitrate in 20 ml of de-ionized water and ultrasonicated for 10 minutes and then the solution is continuously stirred for 20 minutes using a magnetic stirrer. Similarly manganese chloride solution is also prepared at (5-25%) in step of 5% from 0.08 M cerium nitrate. The stirring is continued for 30 minutes to get clear and homogeneous spray solution. The nebulizer is connected to an air compressor. The prepared solution is sprayed onto the ultrasonically cleaned glass substrates maintained at 400 o C (controlled within ± 5 o C) by using Iron-Constantan thermocouple kept on the metallic hot plate surface of the oven. Fig. 1 shows the XRD patterns MDC thin films deposited at an optimized substrate temperature (Ts) 400 o C. Some of the Ce 4+ cations have been substituted easily by Mn 3+ cations due to the difference in their atomic sizes (Ionic radii of Ce and Mn atoms are Ce=0.97Å and Mn=0.53Å respectively) instigating the lattice distortion. The films thus formed are formulated as Ce1-xMnxO2-δ indicating the electrical neutrality by the O-vacancy formation mechanism. At lower Mn concentration (5-15%), the diffraction peaks indicate the polycrystalline nature with single phase cubic fluorite structure having preferred orientation along (2 0 0) direction. The additional peaks observed at 2θ=28.34, 47.12 and 56.35 due to (1 1 1), (2 2 0)and (3 1 1) hkl planes compared with JCPDS data (File No. 34-0394) confirm the cubic fluorite structure of MDC thin films [11]. Also a small hump is observed at diffraction angle 20-25 o corresponds to the amorphous glass substrates with films of low thickness. At higher Mn concentration, no prominent peaks are observed which indicates the amorphous nature of MDC thin films. This poor crystallinity may ascribed to the amount of solute reaching the surface of the substrate increases to form film and therefore the electrostatic interaction between solute particles becomes larger thereby increasing the probability of more solute particles to be gathered together forming a grains [12,13]. In addition, the width of the (2 0 0) peak decreases with increasing doping concentration having increased particle size. The manganese-doped film has less intensity peak corresponds to (2 0 0) plane compared to pure cerium oxide film. This indicates that dopant incorporation deteriorates the crystallinity of films due to the ion size difference between cerium and dopant atoms. Basically, the addition of a dopant into a crystalline structure will affect the crystalline growth kinetics [12]. The dopants first reach the grain boundaries and disturb the normal growth of CeO2 crystallites.

Structural properties a) XRD analysis
It is observed that the lattice constant varies between 5.3683 and 5.4057Å with the increase of Mn concentration from 5 to 25%. The average crystallite size is also found to be increased with the increase of Mn concentration and attains a maximum value 61 nm. Because of the smaller size of the dopants, there is a considerable decrease in the lattice parameter of the CeO2 lattice.  The peaks in the region 2900-2800 cm -1 correspond to the symmetric stretching and bending modes of the hydrocarbon chain of residual organic moieties in the films. The strong and broad absorption band at 1904 cm -1 is attributed to the C=O stretching modes of carbonate ions. It is also observed from the spectra that the surfaces are covered by several layers of carbonate-like species especially the bidentate carbonates which are characterized by the absorption bands at 1303, 1010 and 917 cm -1 assigned to the bending, OCO deformation and rocking of carbonate groups respectively. These carbonate species are coordinated on the film surfaces by unsaturated chemical bonding, which has some impact on the thermal behavior and surface structural characteristics. The broad bands observed at 695, 659, 538 and 517 cm -1 are due to the envelope of (Ce=O) symmetric, asymmetric terminal stretching and phonon band of metal oxide (Ce-O) network [14]. Significant broadening or sharpening of the peaks is due to the incorporation of Mn. At higher Mn concentration, some of the carbonation and hydrocarbon group peaks vanished whereas hydroxyl amine group peaks are enhanced.  Fig. 3a shows the transmittance spectra of MDC thin films. It shows that the transmittance decreases to zero in the UV region, the same increases up to 60% in the visible region and remains almost constant towards the NIR region. It also shows a sharp and strong absorption edge in the wavelength range 425 nm in the visible region. The transmittance decreases with Mn concentration due to the increase in scattering of photon by crystal defects created by doping and lower ionic size of Mn. The electrons in the outer orbits have travelled to the higher energy levels and have occupied vacant positions in energy bands. Thus, a part of incident light does not penetrate through it [15]. Reflectance decreases with the increase of Mn concentration as shown in Fig. 3b. The refractive index of these films is estimated from spectral transmittance and reflectance data for different Mn concentrations and is given in Fig.  3c. It increases up to a certain wavelength in UV region and then decreases exponentially and finally becomes constant in the visible region. It is also established that the refractive index increases from 2.23 to 2.62 with the increase of Mn concentration up to 20% and then decreases [16]. The increase of refractive index with Mn concentration is mainly due the increase of packing density, thickness and low extinction coefficient. The observed maximum refractive index is 2.62. In order to determine the values of optical band gap,(αhυ) n vs. hυcurves have been plotted for the prepared films and the values of n = 1/2, 3/2, 2 and 3 for direct allowed, direct forbidden, indirect allowed and indirect forbidden transitions respectively. The values of the tangents intercepting the energy axis give the values of optical band gap as shown in Fig. 3d.The calculated optical band gap is found to be in the range 2.90 to 3.81 eV for indirect allowed transitions. It is seen that the band gap decreases with the dopant concentration of manganese which can be explained as follows: the narrowing band gap energy is possibly due to the existence of more Mn impurities in the CeO2 crystallites, which induce the formation of new recombination centers with lower emission energy. This narrowing of the band gap with doping concentration is consistent with the observation of red-shift in the absorption tail of the transmittance spectra [17]. The optical conductivity decreases suddenly at 300 nm wavelength as shown in Fig. 3e. It is observed that the optical conductivity decreases with increasing Mn concentration upto 15% and then increases due to the increase of the contribution of electron transition between valance band and conduction band, which lead to reduction of energy gap as a result of sit level generation [18]. The calculated optical conductivity is found to be varied between 6.72x10 14 and12.20x10 14 (Ω-cm) -1 . The variation of optical parameters at 550 nm in the visible region with different Mn concentrations (Fig. 3f) is calculated and listed in Table 1.

b) PL analysis
Room temperature PL spectra of the ring and button shaped MDC thin films are obtained using Xenon laser of 325 nm as the excitation source and are shown in Fig. 4. The addition of trivalent ion Mn 3+ introduces oxygen vacancies in the CeO2thin films. The majority of luminescence peaks produced in these doped films is due to the presence of the oxygen vacancies. The emission peaks ranging from 400 to 500 nm can be related to the hopping from different defect levels to the O2p level [19]. The defects energy levels between Ce4f and O2p are dependent on the temperature and density of defects in the films. Three consistent sharp and broad peaks are observed at 394  These weak violet blue emissions are possible due to the surface defects in the CeO2 thin films. When doping concentration decreases, the intensity of violet and green emission reduces constantly but blue emission peak enhanced in the same position. The deep level emissions result from the recombination of electrons with holes trapped in singly ionized oxygen vacancies (Vo + ). A low intense green emission is also observed at 467 nm (2.65 eV),possibly due to low density of oxygen vacancies incorporated in the sample and the transition of conduction band to deeper acceptor level of oxygen antisite (Ozn) [20]. The emission peak at 369 nm can be related to electron hopping from the localized Ce4f state to the O2p valence band. The broad peak at 394 nm could be due to either 5d to 4for 4f to valence band energy gap of 3.1 eV obtained from optical absorption measurements on CeO2 reported in the literature [21]. Some of the NBE emission and green emission peaks are vanished at higher doping level of Mn. The occurrence of a strong and broad blue emission peak at 15% for MDC films has been confirmed.

Electrical properties
In order to investigate the rectifying behavior of the films at different temperatures, the current drop across the films for a constant voltage is measured. It is well-known that the resistivity of a semiconducting material strongly depends on the temperature, carrier concentration and mobility. I-V characteristics of MDC films (Fig. 5a-f) show sharp increase of conductivity with the increase of temperature and doping concentration that may be attributed to the presence of unevoprated precursor solvent and incorporation of Mn 3+ ions into the host CeO2 lattice. The calculated conductivity is found to be in the range 4.06x10 -8 -1.10x10 -5 S/cm for the corresponding temperature range 30-200 o C and are listed in Table 2 [22]. It is clear from Fig. 5f, the resistivity decreases with temperature showing the semiconducting behavior of MDC thin films. The resistivity of the MDC film deposited at 400 o C is in the order10 4 Ω/cm, which decreases to 10 2 Ω/cm for the films prepared at higher Mn doping and temperature (20% and 280 o C).

Conclusion
The results observed form the samples speak about the structural, optical and electrical properties modification due to the incorporation of Mn 3+ (low ionic size) into of cerium oxide thin films. The structural analysis proved that the MDC films are polycrystalline in nature with single phase cubic fluorite structure having preferred orientation along (200) direction. FT-IR analysis confirms the presence of terminal stretching (Ce=O) and phonon band (Ce-O) of metal oxide network. At higher Mn concentration, some of the carbonation and hydrocarbon group peaks found vanished whereas hydroxyl and amine group peaks are found to be enhanced. Emission analysis evidenced the presence of three broad peaks at 394, 425 and 467 nm due to the presence of DLE of violet, blue and green emission in the visible region. Optical analysis revealed that the prepared films are having 80% transparency in the visible region. Band gap energy is increased from2.90-3.81 eV with the addition of Mn. The calculated refractive index and optical conductivity are found to be in the range 2.23 to 2.62 and 6.72x10 14 to 12.20x10 14 (Ω-cm) -1 respectively. The other optical parameters strongly depend on Mn doping. Electrical analysis established that the room temperature resistivity of the films deposited at 400 o C is found to be in the order of 10 4 Ω/cm, which decreases to 10 2 Ω/cm for the films prepared at higher Mn doping and temperature. The calculated electrical conductivity and activation energy are found to be in the range 4.06x10 -8 -1.10x10 -5 S/cm and 1.258 to 0.390 eV. The electrical conductivity increased with the addition of Mn due to the reduction of band gap energy and activation energy. Out of the results obtained, it has been concluded that the prepared films can be applied to the development of optoelectronic devices due to high optical conductivity, electrochemical devices due to large charge recombination.