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Zinc oxide is one of the versatile and important oxide material because of its typical properties such as resistivity control over the range 10âˆ’3-10_5 cm, transparency in the visible range, high electrochemical stability, direct band gap (3.37 eV), absence of toxicity, abundance in nature, etc. . It crystallizes in a wurtzite structure and exhibits n-type conductivity . ZnO thin films have been used in varistors , gas sensors , solar cell transparent contact fabrications , surface acoustic wave systems , UV laser  etc. Another interesting application of ZnO is in the field of spintronics [8,9]. ZnO is one of the most studied materials of the II-VI oxide materials that derive continuous attention of the researchers worldwide since forties (Bunn 1935). Because of its current and possible applications in several novel devices, renewed interest has emerged and several reviews (Liu et al, 2005; Tsukazaki et al, 2005), and conference proceedings are published exclusively for ZnO nano crystallites similar systems at Singapore (2005), (2009) and Changchan, China (2006) to explore the feasibility of commercial application for future devices. Yet the realm of novel devices from this wonderful material is yet to be accomplished in full (Wellings, et al, 2008). With a wide band gap of 3.2 eV and a large exciton binding energy of 60 meV at room temperature, ZnO, line GaN, will be important for blue and ultraviolent optical devices.
Materials and Method
Mn doped ZnO films were deposited over the surface of ultrasonically cleaned glass substrate, using a home built spray pyrolysis unit. The details about the experimental set-up are explained in our earlier work [Durgajanani Sivalingam, Jeyaprakash Beri Gopalakrishnan, John Bosco Balaguru Rayappan, Influence of Precursor Concentration on Structural, Morphological and Electrical Properties of Spray Deposited ZnO Thin Films, Crystal Research and Technology, 1-6 (2011) / DOI 10.1002/crat.201000672]. Zinc acetate dihydrate (Zn (CH3COO) 2. (H2O) 2) of purity 99.9% from Merck and Manganese (II) Acetate tetrahydrate ((CH3COO)2 Mn. 4H2O) of 99.9% purity from Merck were used as precursor to coat nanostructured Mn doped ZnO films. Films of various precursor concentrations of Mn (20%, 40% and 60% Mn ) in 0.05 M of Zinc acetate dihydrate were coated under optimized deposition conditions like distance between substrate & spray gun nozzle, Angle of spray nozzle corresponding to the substrate, Spray time during each cycle, Time interval between successive sprays and carrier gas as reported in our previous work [ ]. The films deposited at a substrate temperature below 503 K were less adhesive and powdery in nature. Hence the substrate temperature was optimized and maintained throughout the deposition process at 503 K. Uniform films with mirror finish can be obtained, at this optimized deposition temperature,.
Structural analysis of the Mn doped ZnO thin films were studied using powder diffractometer (D8 Focus, Bruker, Germany) with Cu K¡ radiation of wavelength 1.5418 Å at 2Î¸ range between 20° and 70°. Field Emission Scanning Electron Microscope (FE -SEM, 6701F, JEOL, Japan) with an in-built energy dispersive X- ray (EDAX) analyzer was used to obtain the microstructure, chemical composition and thickness of the films. UV-VIS absorption spectrum of the films was obtained with an Oceanoptics (USB 4000) spectrophotometer. I-V characteristics of the deposited films were observed using electrometer (6517A, Kethiley, Germany). Further the sensing characteristics of the as deposited Mn doped ZnO films were studied using home-made VOC testing chamber of 5L capacity. In order to observe the sensing studies at a optimized operating temperature the chamber has digital thermostat coupled compact heater. A septum provision was there to inject desired concentration of VOCs using micro-syringe.
3. Results and Discussion
3.1. Structural and Morphological Studies of Mn Doped ZnO Thin Film
The crystal structure and orientation of the Mn doped ZnO thin films with a precursor solution concentrations of 2%, 4% and 6% Mn in 0.05 M of Zinc acetate dihydrate were investigated by X-ray diffraction studies shown in Fig. 1. The as deposited films were found to be polycrystalline in nature with hexagonal wurtzite structure. The peak position are found to be in agreement with the JCPDS [card 36-1451] of ZnO and were indexed to (100), (002), (101), (102) and (110) plane. It is evident from the XRD data that, there are no extra peaks due to Mn2+ ions substitution. This indicates that the substitution of Zn2+ ions by Mn2+ ions have been achieved without changing Wurzite structure of ZnO. At the same time due to the replacement of Zn2+ ions with small ionic radius (0.74 Ëš A) by Mn2+ ions of larger ionic radius (0.83ËšA), [Singh P, Kaushal A, Kaur D. J Alloys Compd 2009 ; 471 : 11.  Senthilkumaar S, Rajendran K, Banerjee S, Chini TK, Sengodan V. Mater Sci Semicond Process 2008 ; 11 : 6. Dantas NO, Damigo L, Qu F, Cunha JFR, Silva RS, Miranda KL, et al. J Noncryst Solids 2008 ; 354 : 4827] a slight shift in the lattice constant of Mn doped ZnO thin films was observed, compared to the undoped ZnO thin film which was observed in our previous work ( Crystal research tech.).
The average crystallite size (D) of Mn doped ZnO films were determined using Scherrer formula .
where k is the shape factor (0.9), ¬ is the wavelength of CuK¡, ¢ is full width at half maximum (FWHM) of the most intense peak and ± is the Bragg angle. An increase in grain size from 30 - 40nm was observed with an increase in the concentration of Mn from 20 to 60%. The same is confirmed through the secondary electron (SE) image obtained from SEM (Fig.2). The observed increase in grain size with an increase in Mn2+ doping concentration is in agreement with earlier report by Ubale et al. [A.U. Ubale, V.P. Deshpande, Effect of manganese inclusion on structural, optical and electrical properties of ZnO thin films, Journal of Alloys and Compounds 500 (2010) 138-143].
Fig. 2: SEM Micrograph of (a) 20% Mn (b) 40% Mn and (c) 60% Mn doped ZnO
From the SE image (Fig. 3) one can observe that the spherical shaped nanograins organized in fiber like morphology, is more prominent with an increase in the concentration of Mn 2+ ions. This shows the influence of Mn 2+ ion on the growth kinetics during deposition process. As the concentration of dopant precursor increased the metal nuclei centre also increased, which leads to the formation of dense and compact morphology with pours. The purity of as deposited Mn doped nanostructured ZnO thin film was confirmed through EDAX spectra shown in Fig. 4 (b)
Fig. 3: (a) Cross - Sectional SEM image (b) EDAX Spectrum of 4 % Mn doped ZnO
3.2 Optical properties
The optical absorbance and transmittance spectra of the as deposited Mn doped ZnO thin films coated using various precursor concentrations of Mn, were observed in the wavelength range of 350 - 750 nm. From the observed spectra shown in Fig. 4, a slight increase in the absorbance with an increase in the Mn doping concentration has been noted. This may be due to the introduction of Mn defects within the forbidden bands and in turn absorption of the incident photons. (V.R. Shinde, T.P. Gujar, C.D. Lokhande, R.S. Mane, Sung-Hwan Han, Mn doped and undoped ZnO films: A comparative structural, optical and electrical properties study Materials Chemistry and Physics 96 (2006) 326-330). Similarly, all the Mn doped ZnO thin films showed good optical transmittance in the visible region. The optical band gap for the films can be determined by relating the absorption coefficient (Î±) and the incident photon energy (hÎ½) as follows 
(Î±hÎ½) 1/n = A (hÎ½ - Eg), (1)
Fig. 4: Optical Absorbance and Transmittance Spectra of Various precursor Concentrations of Mn Doped ZnO (a) 20% (b) 40% (c) 60%where A is a constant, Eg is the band gap of the material and the exponent n depends on the type of transition. If the value of n= 1/2, 2, 3/2 and 3 then the band gap is of allowed direct, allowed indirect, forbidden direct and forbidden indirect type respectively. The absorption coefficient Î± is calculated from Lamberts law Î±= (2.303 A) / t .Where 'A 'is optical absorbance and't' is the film thickness.
The optical energy gap of the films were estimated by extrapolation of the linear portion of the graph plotted between hÎ½ and (Î±hÎ½)2 as shown in Fig. 5. Decrease in the bang gap with an increase in concentration of Mn was observed and this was attributed to the s-d and p-d interactions. This was theoretically explained using the second-order perturbation theory [P. Singh, A. Kaushal, D. Kaur, J. Alloys and Compounds 471, 11 (2009): R.B. Bylsma, W.M. Becker, J. Kossut, U. Debsta, Phys Rev B 33, 8207 (1986)]. The huge variation in the band gap of Mn doped ZnO compared to undoped ZnO [----] may be due to the spatial confinement. [R. Vishwanatha, S. Sapra, S.S. Gupta, B. Satpati, P.V. Satyam, B.N. Dev, D.D. Sarma, J. Phys. Chem. B 108 (2004) 6303.]
3.3 Electrical properties
Fig. : I-V Characteristics of Mn doped ZnOOhmic electrical contacts [P. P. Sahay, S. Tewari and R. K. Nath, Cryst. Res. Technol. 42, 723 (2007)] were made on the film using thin copper wires and highly conducting silver paste.
Further the n- type conductivity of all the nanostructured Mn doped ZnO thin films were observed using hot-probe method . The dependence of conductivity on Mn precursor concentration was studied by observing the I-V characteristics of the as-deposited films. The experiment was carried out in dark under air atmosphere at room temperature and the response is shown in Fig. 7. The I-V measurement of the Mn doped ZnO thin films were found to respond profoundly with the non- stoichiometry of the films. It is well known that the n-type conductivity in ZnO is due to the oxygen vacancies, which gives rise to the metal interstitials and oxygen deficiencies in the lattice, which leads to crystal defect. Thus the defect chemistry plays an important role for the increase in n-type conductivity of the ZnO thin film. The concentrations and mobilities of the ionic point defects play a decisive role in the transport properties as well as the kinetics of the solid-state reactions. Further control over the transport properties can be achieved by introducing, Mn as external doping element. It is observed that the conductance of the film increased with an increase in the concentration of Mn in ZnO thin film though the band gap of Mn doped ZnO decreases with an increase in Mn concentration. This may be attributed to the fact, Mn acts as a deep donor in ZnO, which modify the concentrations of intrinsic defects at the grain boundaries by decreasing the donor concentration and in turn significantly suppressed the concentration of charge carriers like intrinsic donors, interstitial Zinc or oxygen vacancy. [J. Han, P.Q. Mantas, A.M.R. Senos, J. Eur. Ceram. Soc. 22 (2002) 49-59.] The lattice distortion may also contribute to the change in resistivity. Thus a reverse trend in the electrical conductivity was observed in ZnO thin films with various precursor concentrations of Mn.