Investigation of Magnetic and Optical Properties of Mn-Doped SnO2 at Different Annealing Temperatures

In this paper, a systematic investigation has been done to study the effect of variation in annealing temperature on magnetic and optical properties of Mn-doped SnO2. Coprecipitation technique has been used to synthesize Sn1-xMnxO2 samples with low Mn concentration (2 %). Pristine SnO2 is sintered at 350 C. Mn-doped SnO2 samples are sintered at 350, 450, 550 and 650 C. The structures of tetragonal rutile of SnO2 and Mndoped SnO2 have been confirmed by the XRD pattern. Typical ferromagnetic ordering has been observed for Mn-doped SnO2 samples annealed at different temperatures. However, on increasing the annealing temperature saturation magnetization as well as the coercive field decreases, indicating a crucial phase transformation from ferromagnetism to paramagnetism. UV-Vis spectroscopy shows increase in absorbance and a slight shift in the absorption edge with Mn-doping at increasing annealing temperature indicating shifting of absorption peak towards higher wavelength. Tauc’s plot has been used to find the optical band gap (Eg).


Introduction
Recently, nanoscale materials have gained enormous interest due to their interesting multifunctional properties which can be harnessed at nanometric scale. Among various nanomaterials, diluted magnetic semiconductors (DMS) have attracted attention owing to their unique property of room temperature ferromagnetism (RTFM). These materials have gained importance since their spin degree of freedom and charge carrier properties can be coupled together to tune them for future spintronic applications, thus for fabrication of multifunctional devices [1,2]. Investigations have been extensively focussed on various II-VI semiconducting oxide DMS materials like ZnO, TiO 2 , CuO, SnO 2 etc. [3]. In particular, a wide band gap of 3.6 eV at room temperature of the n-type SnO 2 has made it a promising material for a wide variety of optoelectronic devices [4,5] gas sensors [6], transparent conducting electrodes [7], catalyst supports [8] and spintronics [9]. Its high optical transparency, good electrical conductivity, low defect density and chemical sensitivity properties can be harnessed for many practical applications like solar cells, light emitting diodes, lithium ion batteries, gas sensing and field effect transistors etc. [10][11][12].
The multifunctionality of SnO 2 can be improved extensively by impurity doping with transition materials (TM) like Fe, Co, Ni, Cu, Mn etc. Ahmed et al. [13] studied that varying concentration of TM doping can alter the room temperature electrical and dielectric properties. In similar manner, it has been observed by many research groups that room temperature ferromagnetism, structural and optical properties of SnO 2 can be tuned by doping it with transition metals [14][15][16][17][18][19][20]. Optoelectronic properties like band gap and photoluminescence can also be tuned by impurity doping in SnO 2 [21] . Spin induced RTFM was predicted for doped SnO 2 with TMs V, Fe, Ni, Co and Cr by first principle study and experimentally as well [22][23][24] [27]. Various groups remarked about the effect of Ni doping in SnO 2 for various spintronic applications [28]. Researchers have shown that several dopants (Co, Fe, Cu) can lead to an increase in surface area of SnO 2 by reducing grain size and crystallinity [29]. It can be concluded that doping proves to be an impressive technique in controlling the electronic, spintronic and optical properties of semiconductors [30]. This approach brings about structural and compositional alterations in semiconductors by changing their particle or grain size. Recently, Mn doping with variation in concentration has been much emphasized since it can tailor the structure, morphology, crystallinity and thereby chemical properties of SnO 2 [31,32]. Mn ions proved to be a good candidate for replacing Sn ions in lattice of SnO 2 due to its large equilibrium solubility and nearly same ionic radii as compared to Sn 4+ ion for substitution, which can thereby provide increased spins and carriers for spintronic applications. By doping Mn the electronic configuration of SnO 2 modulates and gives rise to a huge amount of oxygen vacancies [33]. Mn ions exhibit giant sp-d exchange coupling and interesting properties like non-linear band splitting along with spin polarized bands [34]. This ability of tuning of position of Mn optical bands has stimulated great interest in the magneto-optical properties of Mn-doped SnO 2 .
Till now, much concern has remained on the introduction of Mn ions into the SnO 2 lattice with the variation in Mn concentration and synthesis techniques adopted [15,18,[31][32][33][34]. However, not much study has been done on the investigation of effect of annealing temperature on the growth of bulk materials of nano-sized dimensions. In the present work, we have chemically synthesized Mn-doped SnO 2 powders by co-precipitation technique and annealed at different temperatures. A detailed study has been done to investigate various reasons for the origin of RTFM and change in the band positions in Mn-doped SnO 2 powders annealed at different temperatures.

Experimental
Pristine and Mn-doped SnO 2 nanoparticles have been synthesized by co-precipitation technique. Appropriate amounts of SnCl 2 .2H 2 O and MnCl 2 .4H 2 O in proportion of 50:1 were taken and dissolved in 100 mL of deoxygenated distilled water. The sol was prepared by stirring the solution for 2 h at 80 C and then 50 mL of NH 4 OH was mixed drop-wise under magnetic stirring to get the precipitates. The solution was then stirred for 15 min. The precipitates were continuously washed at least 2-3 times and then filtered, and dried in air for several hr. The dried precipitates were then annealed for 2 h at different temperatures between 350 to 650 C to obtain the final Mn-doped SnO 2 powder. The samples were then characterized by X-ray diffraction (XRD), SEM and EDAX for structural, morphological and compositional investigations. The magnetic investigations of the Mn-doped SnO 2 powders were carried out with Vibrating Sample Magnetometer (VSM). UV-Vis-NIR spectroscopy was done to study the proper Mn ion substitution in SnO 2 lattice.

Structural investigations
Pristine SnO 2 and Mn-doped SnO 2 powders have been analyzed for the crystal structure and phase purity by the X-ray diffraction patterns. Fig. 1 shows the XRD pattern of pristine SnO 2 annealed at 350C, Mn-doped SnO 2 samples annealed at 350, 450, 550and 650C. Peaks have been observed corresponding to the (110), (101), (200), (211), (220), (002), (112) and (301) planes and compared with the XRD pattern of pristine SnO 2 on the basis of JCPDS file no. 88-0287 [35]. The prominent peaks indexed correspond to the tetragonal rutile structure of pristine SnO 2 and Mn-doped SnO 2 . No additional phases like orthorhombic SnO 2 , metallic Mn or other SnO-based phases are observed in the XRD patterns. Hence, it can be concluded that the transition metal ions are successfully substituted at the Sn site and Mn impurities do not affect the tetragonal structure of SnO 2 . From the XRD patterns, it is also observed that with increasing annealing temperature, the intensities of the XRD peak of the Mn-doped SnO 2 samples increases. This shows that though the crystallite size decreases with introduction of Mn dopant but with increasing annealing temperature the crystallite size increases. The reason is that with Mn doping the oxygen vacancies increases because of smaller ionic radius of Mn 3+ ion (0.65 Å) in comparison to Sn 4+ ion (0.69Å) which inhibits the growth of the crystal grains due to the presence of Mn ions in SnO 2 . Hence, the crystallite size decreases due to disturbance in the long range crystallographic ordering. But annealing the samples at higher temperature improves the crystallinity of the sample. With increasing annealing temperature, sharpness of the diffraction peaks intensifies and broadening of peak goes on decreasing. A broad diffraction peak indicates smaller particle size but with increase annealing temperature, broadening decreases enhancing the particle size. This is because as the annealing temperature increases, the lattice of SnO 2 is modified since the Sn 4+ ions are replaced by Mn 3+ ions which increases the intensity of peaks in diffraction patterns. This leads to more disorders in the crystal lattice due to change of the interstitial sites of Sn 4+ ions [36]. Thus, doping Mn on SnO 2 reduces the crystallite size but annealing at higher temperatures increases the crystallite size and hence improves the crystallinity of the material [37]. The average crystallite size (D) has been determined by Scherrer's formula [38] using the diffraction peaks (110) and (101) In this Eq. (1), k=0.9 , λ represent the wavelength of the X-ray radiation, β is the full width at half maximum of the diffraction peak (in radians) and θ the Bragg diffraction angle at full width half maximum (FWHM) of the diffraction peak.
For tetragonal system, the d-spacing (d hkl ) and lattice constants, 'a' and 'c', are calculated using Eq. (2) where'd' is the interplanar distance, (hkl) are the miller indices and 'a' and 'c' are the lattice constants.
In Table 1, we present the variation of the crystallite size (D), FWHM (β), lattice parameters 'a' and 'c' with variation in annealing temperature. The given parameters in Table 1 are in accordance with the conclusions from the XRD patterns. It is clear that the full width at half maxima (FWHM) values change with annealing. The FWHM of 2θ are related to the crystalline quality of the bulk samples. For pristine SnO 2 annealed at 350 C, the FWHM value is (0.0041). With Mn doping the FWHM value increases at 350 C (0.0073), indicates smaller crystallite size at 350 °C. With increase in annealing temperature from 350 to 650 C, the FWHM values decreases from 0.0073 to 0.0038. At 650 C, the sample shows sharper and intense peak with a smaller FWHM. The lattice parameters also show the same trend. For Mn-doped SnO 2 sample annealed at 350 C, the lattice parameters 'a' and 'c' increases compared to pristine SnO 2 but decreases with increasing annealing temperature. Both the FWHM and lattice parameter values indicate increase in particle size and therefore crystallinity with increase annealing temperature.

Magnetic investigation
Magnetic measurements for Mn-doped SnO 2 have been done at room temperature by VSM in varying magnetic field. Fig. 4 shows the magnetization vs. magnetic field (MH) curve at room temperature for Mn-doped SnO 2 samples annealed at temperatures 350, 450, 550, and 650 C. Pristine SnO 2 exhibits a diamagnetic behaviour at 350 C. The diamagnetic behaviour of SnO 2 sample at low annealing temperature maybe because of distortion created in the lattice structure of SnO 2 due to low diffusion of oxygen vacancies into the tin lattice. This distortion in lattice structure decreases the overlapping of bound magnetic polarons (BMP's) which leads to weak ferromagnetic interaction. It is further suppressed by the diamagnetic ordering which is the intrinsic property of SnO 2 [1]. On increasing the annealing temperature, slight ferromagnetic ordering maybe introduced or the system may show paramagnetic behaviour. Typical ferromagnetic ordering (magnetic saturation) has been observed for Mn-doped SnO 2 samples annealed at 350, 450, 550 and 650 C. However, on increasing the annealing temperature, saturation magnetization decreases at high magnetic field. Origin of RTFM has always remained a puzzling question for DMS materials. It has been predicted that exchange interaction between spin states of Mn 4+ , Mn 3+ and Sn 4+ cations are responsible for RTFM in Mn-doped SnO 2 samples. Strong ferromagnetic coupling is experienced by the Mn ions into the SnO 2 lattice. This type of coupling arises due to exchange of F-centres of doped-ion of transition metal and the oxygen vacancies present in SnO 2 lattice. At the sites of oxygen vacancies, one electron of transition metal gets trapped and creates the F-centres. The Fcentre electron then couples with neighbouring d-shell orbital of Mn 3+ ions. According to Pauli's exclusion principle and Hund's rule, in order to achieve ferromagnetic ordering, parallel spin orientation is observed between the trapped electrons and neighbouring Mn ions. Another reason ascribed to ferromagnetic ordering of Mn-doped SnO 2 may be due to overlapping of BMP's. These BMP's are formed when bound electrons at oxygen vacancy sites coupled with Mn ions spins incorporated in SnO 2 lattice. The ferromagnetic coupling is also strongly influenced by the variation in structural parameters like interatomic distances (lattice parameters). The calculated magnetic parameters are presented in Table 2 which shows that the saturation magnetization, coercivity and remnant magnetization decreases as the particle size increases with increase the annealing temperature. At higher magnetic field, the coupling due to antiferromagnetic exchange interaction enhances which decreases the saturation magnetization level [40]. This may be ascribed to the reason that the F-centre electron that occupies the d shell of Mn atom coupled with the Mn ion spin states becomes antiparallel giving rise to antiferromagnetic coupling thereby lowering the saturation magnetization. This might be because of quantum confinement effect in particle size which arises due to increasing annealing temperature. Fig. 5 shows the magnified magnetic measurements of Mn-doped SnO 2 samples annealed at different annealing temperatures. It suggests that increasing the annealing temperature diminishes the width of the ferromagnetic loop. This can be explained that as the annealing temperature increases, the ferromagnetic ordering as well as the coercive field decreases, indicating a crucial phase transformation from ferromagnetism to paramagnetism. This phase transformation might be due to lesser direct and indirect exchange interactions which might arise because of diffusion of dopant ions which cause distortion in the crystal lattice at higher annealing temperatures [1].

Optical investigations
The confirmation of Mn ion substitution in SnO 2 lattice has been obtained from UV-Vis-NIR spectroscopy. In Fig. 6 a sharp absorption edge has been observed for pristine SnO 2 annealed at 350 C at 284 nm. With Mn-doping a slight shift in the absorption edge has been observed for 350 °C (291 nm), 450 C (295 nm), 550 C (299 nm) and 650 C (306 nm) indicating shifting of absorption peak towards higher wavelength. For Mn-doped SnO 2 samples annealed at 350 C, the absorption peak decreases slightly. But as the annealing temperature increases, particle size increases and a red shift in absorption edges as well as increase in absorption has been observed for the samples annealed at 450, 550 and 650 C. In UV-Vis spectroscopy, absorption coefficient α is obtained from Beer Lambert's law. According to this law Absorbance is related to incident intensity I o and transmitted intensity I by relation Finally, Tauc's relation was used to determine the optical band gap, E g , of Mn-doped SnO 2 samples in Eq (5). [41]: where α is the absorption coefficient, A* isa constant, (hν) is the photon energy and E g is the energy band gap. The variation in (αhν) 2 with hν is observed for the Sn 1-x Mn x O 2 samples in Fig. 7.
The linear portion of plot (αhν) 2 versus (hν) is extrapolated to intercept the photon energy values to obtain the energy band gap. The (αhν) 2 versus hν curve gives direct optical band gap for Mn-doped SnO 2 samples since the plot asymptotically tend towards a linear portion. The measured band gap for pristine SnO 2 sample was found to be 3.05 eV. The measured band gap value is thus smaller than the value of bulk SnO 2 (3.6 eV) reported. This might be due to the quantum confinement effect.
The calculated E g values are given in Table 3 which shows that the band gap decreases as the particle size increases with increase in annealing temperature. Various factors like oxygen deficiency, impurity centres, surface roughness etc. may be attributed for the change in absorbance spectra. It has been understood from the local density approximation (LDA) [42] that the defects caused by Sn interstitial sites are mainly responsible for conduction than oxygen vacancies in pristine SnO 2 . It is assumed that due to loosely bound outer electrons in interstitial sites of Sn, a new donor level is created in the conduction band.  On the other hand, oxygen vacancies are produced in the band gap. But the presence of Sn interstitial sites in the conduction band supresses the formation of oxygen vacancies. This decreases the inter-conduction band absorption in SnO 2 leading to large band gap that inhibits transitions in the visible region. Mn-doping leads to modification in conduction bands due to surface diffusion of Mn ions and Mn d-state orbitals into the band gap. This leads to overlapping of functions leading to change in band gap. A red shift is observed in band edges of Mn-doped SnO 2 in comparison to pristine SnO 2 . This observation cannot be explained with quantum confinement phenomenon normally. It is assumed that band gap renormalization effect might be responsible for the red shift of the band edge due to doping. With increase in annealing temperature, particle size increases which leads to more diffusion of Mn ions and Mn d-state orbitals, thereby declining the band gap. Actually with Mn-doping, there are interactions due to sp-d exchange between the band electrons of the host (Sn 4+ ) and d electrons of the dopant (Mn 4+ ). This leads to change in the edges of the conduction and valence bands resulting into narrowing of band gap.

Conclusion
Summarizing, pristine SnO 2 and Mn-doped SnO 2 samples have been chemically synthesized by co-precipitation technique with low Mn concentration (2 %) at different annealing temperatures 350, 450, 550 and 650 C. It was observed from the XRD data that though the crystallite size decreases with introduction of Mn dopant but with increasing annealing temperature, the crystallite size increases. Morphological and compositional formation of Mn-doped SnO 2 samples were examined by SEM and EDX respectively. Magnetic investigations show typical ferromagnetic ordering (magnetic saturation) for Mn-doped SnO 2 samples. On increasing the annealing temperature, saturation magnetization decreases at high magnetic field due to antiferromagnetic coupling. The width of the ferromagnetic loop shows a phase transformation from ferromagnetism to paramagnetism due to distortion in the crystal lattice with diffusion of dopant ions at higher annealing temperature. A red shift is observed in the band edges of Mn-doped SnO 2 samples due to band gap renormalization effect resulting into narrowing of band gap. From the results it was concluded that annealing temperature plays a major factor in the magnetic and optical properties of Mn-doped SnO 2 samples.