XANES, EXAFS, EPR, and First-Principles Modeling on Electronic

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

XANES, EXAFS, EPR and First Principles Modeling on Electronic Structure and Ferromagnetism in Mn Doped SnO Quantum Dots 2

Manikandan Dhamodaran, Ashok Kumar Yadav, Shambhu Nath Jha, Dibyendu Bhattacharyya, Boukhvalov Danil. W, and Ramaswamy Murugan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09937 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019

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The Journal of Physical Chemistry

XANES, EXAFS, EPR and First Principles Modeling on Electronic Structure and Ferromagnetism in Mn doped SnO2 Quantum Dots Dhamodaran Manikandan1, A.K. Yadav2, S.N. Jha2, D.Bhattacharyya2, D. W. Boukhvalov3, 4 and Ramaswamy Murugan*1 1

Department of Physics, Pondicherry University, Puducherry, India -605 014. Atomic& Molecular Physics Division, Bhabha Atomic Research Centre, Mumbai, India– 400 085 3 College of Science, Institute of Materials Physics and Chemistry, Nanjing Forestry University, Nanjing 210037, P. R. China 4 Theoretical Physics and Applied Mathematics Department, Ural Federal University, Mira Street 19, 620002 Ekaterinburg, Russia 2

*

Corresponding Author (R. Murugan) Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT The effect of Mn dopant on electronic structure and magnetic properties of SnO2 quantum dots was investigated using X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), electron paramagnetic resonance and first principles modeling. The results demonstrated that dilute Mn atoms substituted for Sn generates numerous oxygen vacancies. Interestingly, lower Mn doping concentration (2%) favored the formation of Mn3+ structures and increase of Mn doping to higher concentration (10% Mn) led to predominance of Mn3+ with a small fraction of Mn2+ configuration. The slight increase in bond length observed for 10% Mn doped SnO2 QDs in EXAFS also corroborates with the XANES results where the absorption edge was shifted to lower energy giving an indication of very small presence of Mn in +2 oxidation state. Electron paramagnetic resonance studies revealed the exchange coupled Mn interactions and also changes in distribution of local magnetic configurations with the increased dopant level of Mn. The considerable enhancement in the spin carrier density of states (DOS) and changes in the configuration of Mn upon variation in doping concentration in SnO2 QDs demonstrate the effective role of Mn in modulating the electronic structure.


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1. INTRODUCTION SnO2 diluted magnetic semiconductor has drawn attention in recent due to the richness in spin polarized carrier density and optical transparency in the visible region. Doping of transition metals in SnO2 activates the spin polarization through sp-d exchange interactions between host band structure electrons and the localized d electrons of transition metals.1 Among the various transition metals, Mn received considerable attention due to its large equilibrium solubility and nearly the same ionic radii (Mn3+ ionic radius of 0.65 Å) as compared to Sn4+ (ionic radius of 0.69 Å) for substitution.2,3 Sabergharesou. et. al investigated the local structure and magnetic properties of Mn doped SnO2 and reported room-temperature ferromagnetism with the saturation magnetic moment of 0.27µB/Mn for nanocrystalline films containing high fraction of Mn2+ dopant.4 Recently, we have demonstrated the presence of room temperature ferromagnetic order due to multiple configuration of Mn (Mn2+ and Mn3+) and oxygen vacancies in Mn doped SnO2 quantum dots (QDs) synthesized via high pressure microwave technique.5 Despite numerous efforts, the fundamental understanding of the origin and control of ferromagnetism in transition metal doped SnO2 QDs is still under debate and highly desirable.4 In order to substantiate further and to address the correlation of electronic structure with the ferromagnetic properties of synthesized SnO2 and Mn doped SnO2 QDs investigations using experimental techniques; X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS) and electron paramagnetic resonance (EPR) spectroscopy in support with density functional theoretical (DFT) calculations were carried out in the present work. 2. EXPERIMENTAL



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The high pressure microwave technique was used for synthesis of SnO2 and Mn doped

SnO2 of different Mn doping concentration (2%, 4%, 6% and 10%). The stoichiometric solution was prepared by dissolving required amount of SnCl4.5H2O (Sigma Aldrich), MnCl2.4H2O (Merck) and sodium dodecyl sulfate (Merck) in 200 ml premixed ethanol and water (1:1). The final precursor solution was added in the PTFE vessels of the microwave synthesizer (Anton Paar Multiwave PRO). The microwave synthesizer was programmed in the temperature controlled mode with temperature and power of 200 °C and 900 W, respectively, for 1 hour microwave irradiation time with maintained pressure of 30 bar. The detailed synthesis procedure of the investigated QDs was discussed earlier. 5 The XANES and EXAFS measurements have been performed at the Energy-Scanning

EXAFS beamline (BL-9) in fluorescence mode at the Indus-2 Synchrotron Source (2.5 GeV, 200 mA) at Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India.6,7 This beamline operates in the energy range of 4-25 KeV. The beamline optics consist of an Rh/PT coated collimating meridional cylindrical mirror and the collimated beam reflected by the mirror monochromatized by a Si (111) (2d=6.2709 Å) based double crystal monochromator (DCM). The second crystal of the DCM is a sagittal cylinder used for horizontal focusing while an Rh/Pt coated bendable post mirror facing down was used for vertical focusing of the beam at the sample position. For measurements in the fluorescence mode, the sample was placed at 45o to the incident X-ray beam and the fluorescence signal (If) was detected using a Si drift detector placed at 90o to the incident X-ray beam. An ionization chamber detector was used prior to the sample to measure the incident X-ray flux (I0). Absorbance of the sample (µ

) was obtained as a

function of energy by scanning the monochromator over the specified energy range. The electron


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paramagnetic resonance measurements were performed at room temperature using X-band (9.2 GHz) (JEOL Model JES FA200) spectrometer in the field range from 0 to 800 mT. 2.1 COMPUTATIONAL METHODS For the modeling of the energetics of the formation of defects in Mn-doped SnO2 quantum dots we have performed DFT calculations using the QUANTUM-ESPRESSO code8 and the GGA–PBE approximation for the exchange-correlation functionals9 feasible for the modeling of impurities in oxides.10 We used energy cutoffs of 25 Ry and 400 Ry for the planewave expansion of the wave functions and the charge density, respectively, and the 4×4×2 Monkhorst-Pack k-point grid for the Brillouin sampling.11 Similar to our previous calculations10 we used the slab of 96 atoms within periodic boundary conditions for SnO2 surfaces. Since the modeling of the realistic (110) and (101) surfaces require larger supercell building we performed calculations for two simpler cases (001) and (100). Taking into account our previous experience of the modeling of various impurities in oxides we consider not only substitutions (S) but also interstitial (I) impurities and their combinations with oxygen vacancies. 3. RESULTS AND DISCUSSION The procedure for synthesis of homogeneous SnO2 and Mn doped SnO2 QDs using the high pressure microwave technique and their structure, optical and magnetic properties were reported earlier.5 The synthesized SnO2 and Mn doped SnO2 QDs exhibit room temperature ferromagnetic (RTFM) behavior.5 The increase of Mn dopant concentration in SnO2 QDs offers not only an increase of total magnetic moment but also alters the magnetic interactions between magnetic moments on Mn ions and oxygens.5 3.1 Electronic structure



The XANES spectrum is quite sensitive to the local environment and the oxidation state of the absorbing atom. The normalized XANES spectra of Mn doped SnO2 QDs measured at the Mn K-edge are shown in Figure 1(a) along with that of Mn metal foil, MnO, Mn2O3 and MnO2 standard samples having 0, +2, +3 and +4 oxidation states of Mn, respectively. In general the maxima of the first derivative of the absorption edge was taken as the edge position. Figure 1(b) shows the first and second peaks in the derivative spectra vis-a-vis the XANES spectra of the samples. The transition metal oxides generally show two peaks in derivative spectra of absorption edge, one due to the main absorption peak (second maximum) and another due to the presence of small humps at the rising portion of the absorption edge (first maximum). Table 1 shows the values of first and second maxima of the derivative of absorption edge for Mn doped SnO2 QDs and standard reference samples. An error bar of ±0.25 eV has been taken for the measurement of the absorption edge position since this was the lowest energy step of the DCM used to record the XANES spectrum. The values of the first and second maxima of the first derivative of the absorption edge of 2% Mn doped SnO2 QDs suggested that Mn was in the +3 oxidation state. This first and second maxima position showed a slight shift in the peak positions (shown in Table 1) towards lower energy values with an increase in Mn doping concentration and it has been found that the shift being maximum for 10% Mn doped SnO2 QDs. The observed shift of the absorption edge towards lower value reveals that increase in Mn+2 species with an increase in Mn doping concentration. Figure 1(c) shows the variation of first and second maxima energy positions of the first derivative of absorption edge of Mn doped SnO2 QDs along with the reference Mn metal, MnO, Mn2O3 and MnO2 powder samples. It can be clearly seen from the Figure1(c), the peak position is in between +2 and +3 oxidation state for all the doped samples and minute decrease in peak energy was observed for the doped samples


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with an increased doping concentration. For more clarification the linear combination fitting (LCF) was carried out and an individual contribution of +2 and +3 oxidation state was plotted as shown in Figure 2(a), which clearly revealed the increasing trend of Mn+2 oxidation state with increasing doping concentration. From the observed shift in the first and second maximum of derivative spectra as well as the linear combination fitting, shown in Figure 1(c) and Figure 2(a), the obtained +2 oxidation contribution was very small (93%) was present, the absorption edge behaves more like the predominant state but with slightly lower energy position or slight change in the absorption edge slope. Figure 2(b) shows the result of LCF fitting of the Mn edge XANES data of 10% Mn doped SnO2 sample. It is quite clear that the edge position was not fitted well unless the small contribution of MnO was considered. Thus it clearly showed that the 10% Mn doped sample definitely has contribution of MnO. The observed trend was quite different from the earlier report on Mn doped SnO2 nanocrystals and nanowires, where the oxidation state of Mn was found to increase from +2 to +3 with an increase in Mn doping concentration.4 The inset of Figure 1(a) shows the pre-edge portion of the spectra in an expanded scale. The observed pre-edge was a contribution of the transition of the 1s electron to hybridized Mn3d/O2p states. The pre-edge position and intensity remains almost the same for all the QDs irrespective of Mn doping concentrations. Thus the XANES investigations on the present QD samples indicated that over the whole Mn concentration range, Mn remains predominantly in Mn3+ with a small fraction of Mn2+ appearing in samples with high Mn doping concentration. The analysis of the EXAFS data of Mn doped SnO2 QDs measured at Mn K-edge has been carried out following the procedure outlined earlier12,13 using the IFEFFIT software package14. This includes data reduction and Fourier



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transform to derive the  ( R ) versus R plots from the absorption spectra, generation of the theoretical EXAFS spectra starting from an assumed crystallographic structure and finally fitting of the experimental versus R data with the theoretical ones using the FEFF 6.0 code. The structural parameters (atomic coordination and lattice parameters) of Mn doped SnO2 used for simulation of theoretical EXAFS spectra of the QDs have been obtained from Ref.(15) and the best fit  (R ) versus R plots of the QDs have been shown in Figure 3 along with the experimental plots. The coordination number (N), bond distances and disorder (Debye-Waller) factors (  2 ), which give the mean square fluctuations in the distances, have been used as fitting parameters. The goodness of the fit in the above process is generally expressed by the Rfactor which is defined as:

R factor  

[Im( dat (ri )   th (ri )] 2  [Re(  dat (ri )   th (ri )] 2 [Im( dat (ri )] 2  [Re(  dat (ri )] 2

(1)

where, χdat and χth refer to the experimental and theoretical  (R ) values, respectively and Im and Re refer to the imaginary and real parts of the respective quantities. The fitting has been carried out in the R range of 1-3.5 Å and the number of fitting parameters which can be varied freely during fitting has been decided following the Nyquist criterion N Free  (2kr /  )  1 .13 The best fitted results are shown in Table 2. The first maxima at 1.5 Å shown in Figure 3 of phase uncorrected spectra corresponded to the first coordination shell and it was associated with oxygen atoms at the bond length of 2.04 Å for 2% Mn doped QDs. The region between 2.0 Å to 3.5 Å was fitted using the three coordination shells Mn-Sn, Mn-O and longer Mn-Sn. The variation in coordination numbers,


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bond lengths and 2 values of the different shells with doping concentration are shown in Figure 4. It was found that the first Mn-O bond length decreased for the 2%, 4% and 6% Mn doped QDs. Conversely, the bond length of 10% Mn doped SnO2 QDs was slightly increased. The MnO bond length obtained here was in good agreement with the Mn3+-O bond length obtained by Sabergharesou et al.4 These results confirm the finding of the XANES observations that Mn was present in +3 oxidation state. The obtained EXAFS results at Mn site indicating the similar coordination environment of Sn in SnO2 confirm that Mn occupied at the Sn site in SnO2 lattice. Thus led to a decrease in the Mn-O bond length with doping since the ionic radius of Mn+3 was smaller than the ionic radius of Sn4+ in similar coordination.16 The slight increase in the bond length for 10% Mn doped QDs also corroborated with the XANES results discussed above where the absorption edge was shifted to lower energy by -0.5 eV giving an indication of very small presence of Mn in +2 oxidation state. It should be noted here that the ionic radius of Mn+2(0.83Å) was higher than that of Mn+3 (0.65Å). However the oxygen coordination in the nearest neighbor coordination shell around Mn site remained constant over the whole Mn doping concentration range. Since Mn+3 replaced Sn+4 in the present QDs, it can be expected to have oxygen vacancy in the lattice to maintain charge balance. However, as has been seen in other cases also oxygen vacancies were generally created near the host sites rather than near the dopant sites.17,

18

It was also manifested by the fact that there was a monotonic decrease in oxygen

coordination of the far Mn-O shell. The disorder (Debye-Waller) factors (  2 ) is the measure of mean square fluctuations in the distances. An increase in  2 value of the different shells with an increase in Mn doping concentration, as shown in Table 2, indicated that the extend of local spin carrier density in the lattice might led to higher distortion.



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3.1.1 Computational Modeling DFT modeling was performed in order to understand the electronic structure of investigated QDs and the theoretical results were correlated to the experimental findings. Figure 5 shows the schematic representation of the optimized atomic structure of (001) surface with single substitutional Mn in the locality of the oxygen vacancy (MnSn + VO) (Figure 5(a)) and (100) surface with two substitutional and one interstitial Mn (2MnSn + Mni) of SnO2 (Figure 5(b)). Different combinations of substitutional and interstitial defects provide formation of various MnxOy configurations (see Table 3). In the case of single substitutional defects (MnSn) was realized further considering the scenario Mn + SnO2 ≈ MnO2. Combination of substitutional and interstitial defects (MnSn + Mni) led to the scenario: MnSn + Mni + SnO2 ≈ MnO. The presence of the oxygen vacancies also influenced the local configurations (SnO2 – O ≈ SnO + Mn → MnO or 2SnO2 –O ≈ Sn2O3 + 2Mn ≈ Mn2O3). The formation energies of the surfaces were calculated by the formula: Eform= (Esurf – Ebulk)/n, where Ebulk, Esurf, n can be related to the total energies of the supercell of bulk and slab that contain the same number of atoms, number of SnO2 units per surfaces of the slab, respectively. Results of calculations (Table 3) demonstrated that the formation of (001) surface was much more energetically favorable than (100) surface.5 Therefore, (001) surface was employed as the model for bulk-like parts of the surface of QDs and (100) surface for defect-rich areas (edges, steps, corners). The formation energy was calculated using the Eq. (2), which indicated that the formation energy of oxygen vacancies was higher in the case of the stable (001) surface and lower for (100) surface due to instability. Eform = (Esurf+Vo – (Esurf +μO2/2),

(2)


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where Esurf and Esurf+Vo is the total energies of pristine and defective surfaces, respectively, and μO2 – is the total energy of molecular oxygen in ground state. The formation energies of different configurations of Mn-impurities has been verified and corresponding formation energies of the defects were estimated by using Eq. (3): Eform = (Ehost+defects – (Ehost + nμMn – mμSn),

(3)

where Ehost and Ehost+defects is the total energies of the host (pristine or defected surfaces) before and after inclusion of n manganese atoms and removal of m tin atoms, respectively. Results of first principles modeling (Table 3) demonstrated that the less stable (001) surface achieve less formation energy and rising of formation energies for all configuration of Mn when stabilization of this surface by oxygen vacancies. Since the formation of oxygen vacancies in (100) surface require lower energies we considered this surface as rich in defects. Thus we concluded that the formation of MnO-like structures was occurring on defect-rich area. This result was in qualitative agreement with XANES results (Table 1 and Figure 1(a)) which also demonstrated only the traces of MnO-like features. Electronic structures of these configurations has the sharp first peak at -2 eV (Figure 6) that corresponded with the MnO-peak. The formation of MnO-like structure inside SnO2 matrix was not exactly coinciding with the spectrum of pure MnO peaks at 6554 and 6553 eV (Figure 1(a)). The second peak in the spectrum of pure MnO located about 15 eV after the first peak and corresponding with O 2p states (not shown in Figure 1(a)). In the case of 2% Mn doped sample additional peak separated from the first appeared by about 3 eV (Figure 1(a)). Theoretical spectrum for MnSn+VO configurations demonstrated the presence of the second peak 2 eV below the first (Figure 6). Thus at the lowest concentration of Mn the spectrum reflected only the MnO-like features.



The less energetically favorable surfaces led to saturation of all available spots when doping the different concentration of impurities and after that substitution of Mn into high stable surfaces. The configuration that consists of two or more Mn atoms associated to the stable (100) surface with lower density of oxygen vacancies and lowest formation energies. These (MnSn+Mni, MnSn+Mni+VO, 2MnSn, 2MnSn+Mni) configurations was compared to different local configurations like – MnO, Mn2O, Mn2O3 and Mn3O4. This result was also in agreement with the XANES measurements (shown in Figure 1(a)), which also demonstrated the changes of the ratio of MnO, Mn2O, and Mn2O3-like configurations with increasing of Mn doping concentrations. The calculated electronic structures also show the difference between configurations corresponding with various MnxOy-like local configurations (shown in Figure 6). In the XANES measurements increasing of the concentration of Mn provide shift of the edges at about 1 eV (from 6550 to 6549 eV) that is corresponding with appearance of the features between 0 and -1 eV for MnSn+Mni, MnSn+Mni+VO, 2MnSn, 2MnSn+Mni configurations. The shift of the main (highest) peaks for these configurations was shifted about 2~3 eV from the similar peak for MnSn+VO configuration that corresponded with the broadening of the experimental spectrum between 6555 and 6562 eV (Figure 1(a)). Note that increasing of the concentration of Mn provides smooth broadening of experimental spectrum with almost elimination of the second peak (at 6562 eV) for 10% Mn. The present theoretical modeling proposed an explanation of this observation as increase in diversity of MnxOy configurations for the increased dopant level of Mn. Furthermore, DFT calculations were performed to estimate the distances between Mnimpurities and the first and second oxygen and Sn-neighbors. Results of the calculations (Table 3) show that quantitative agreement with EXAFS experimental measurements (Table 2) for the


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first nearest oxygen and tin neighbors. For the second neighbors difference between calculated and measured distances could be explained by limitation of used model. We used a slab instead real nanoparticles and the distribution of the impurities (single or clusters was homogeneous). Some lattice distortions caused by the shape of the real nanoparticles and inhomogeneous distribution of impurities could explain why measured distances between Mn-impurities and second O and Sn neighbors was smaller than calculated. From the other hands quantitative agreement with the measured and calculated distances of first neighours and good enough agreement for the second neigbours demonstrate feasibility of used model for description of QDs. The comprehensive XANES, EXAFS and DFT calculations signaled that the investigated QDs were in a single phase with no evidence of Mn and Sn oxide related clusters. The considerable enhancement in the spin carrier density of states (DOS) and changes in the configuration of Mn upon Mn doping in SnO2 QDs demonstrated the role of Mn in modulating the electronic structure. These results of experimental and DFT calculations shown in Table 2 and Table 3 correlate unambiguously to the configuration of Mn and distance between the local configurations of Mn neighbors. 3.2 Electron paramagnetic resonance spectroscopy analysis Electron paramagnetic resonance is one of the most sensitive technique to investigate the spin dynamics of paramagnetic centers originates from Mn ions incorporated in the SnO2 QDs and oxygen vacancies. EPR spectra of synthesized QDs recorded at room temperature are shown in Figure 7(A). The EPR signals (ferromagnetic resonance (FMR)) appeared in the investigated QDs indicated the existence of ferromagnetic order at room temperature. The FMR, g-factor, line width (ΔH) and number of spins (Nspins) estimated from EPR spectra are shown in Table 4.



The observed EPR signal in undoped SnO2 QDs can be attributed to unpaired electrons from the oxygen vacancies.19,20 Further, the estimated g (2.0165) of SnO2 (shown in Table.4) also in agreement with the results of earlier reports.21 So the observed EPR signal in the undoped SnO2 can be indisputably associated to the oxygen vacancies. Ferromagnetic exchange between oxygen imperfectness in SnO2 nanoparticles have been also revealed in DFT modeling.5 In X-band (9-10 GHz) EPR spectra, the hyperfine splitting of Mn3+ ( S=2) was silent due to large zero- field splitting.22 So, the observed broad resonance line for 2 % Mn doped SnO2 samples clearly indicated the presence of Mn3+ state without any hyperfine splitting lines.23 At higher dopant level, the Mn-Mn interactions originates from the configuration of Mn which might be the possible reason for the absence of hyperfine splitting. The absence of hyperfine structure in Mn doped SnO2 QDs indicated the presence of exchange coupled Mn interactions.24 These exchange interactions could be as within MnxOy clusters and also between different clusters on the surface or in bulk of QDs. The broad EPR signal attributed to intrinsic anisotropy probably caused by inhomogeneous distribution of Mn-impurities in QDs, multiplicity of ferromagnetic domain ground state and inhomogeneity of the domain size.25 The changes in the line width (ΔH) with increase in Mn doping can be related to characteristic spin-spin or spin-lattice relaxation time.26 The intensity of the EPR signal mostly depends on the concentration of paramagnetic centers and Figure 7(B) (Integration curves) clearly indicated the intensity of EPR signal increases with increase in Mn doping. The intensity of 10% Mn doped SnO2 QDs was found to be higher and this might be due to the presence of multiple configuration of Mn. The area of integration curves were analogous to the number of spins participated in the FMR. Number of spins (Nspins) participated in the ferromagnetic resonance was estimated by using the formula in Eq.4.27, 28


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(4) Where I is the peak-to-peak height of EPR signal (arbitrary units) and ΔH is the line width (in mT). Figure 7(C) and 7(D) clearly indicated the correspondence between saturation magnetization (Ms) with area of integration curves and the number of spins participated in the FMR, respectively. As the Mn doping concentration increases, the number spins participated in the ferromagnetic resonance (FMR) increases linearly. This might be the cause for enhancement in EPR absorption.27 This observed tendency can be related to the presence of oxygen vacancies and configuration of Mn.5, 29 The EXAFS spectra and DFT results of Mn doped SnO2 samples also supports the conclusion of EPR via the formation of oxygen vacancies near the host sites. DFT calculations also suggested the presence of FM-order5 and additionally demonstrated the changes of the oxidation states and magnetic moments on Mn-impurities. The comprehensive EPR and EXAFS investigations clearly indicated the formation of oxygen vacancies in the investigated samples, and the observed RTFM was correlated to the effective exchange interactions between the configuration of Mn and oxygen vacancies. 4. CONCLUSIONS The electronic structure and magnetic properties of Mn doped SnO2 QDs synthesized using high pressure microwave synthesis technique were investigated using the DFT calculations and experimental (XANES, EXAFS and EPR) studies. The XANES and EXAFS studies signaled that the investigated QDs were in a single phase with no evidence of Mn and Sn oxide related clusters. Results of XANES, EXAFS and DFT calculations demonstrated that the local configurations of Mn depend upon the Mn concentration in SnO2 QDs. The XANES investigations indicated that the increase of Mn doping to higher concentration, led to predominance of Mn3+ with a small fraction of Mn2+ at the Sn lattice site. Moreover, EXAFS



results established that the first Mn-O bond length decreases for the 2%, 4% and 6% Mn doped SnO2 QDs. On the other hand, it was slightly increased for 10% Mn doped SnO2 QDs. The slight increase in the Mn-O bond length observed for the 10% Mn doped SnO2 QDs observed in EXAFS also corroborated with the XANES results giving an indication of very small presence of Mn in +2 oxidation state. The observed room temperature ferromagnetism in these quantum dots was correlated to the effective exchange interaction between configuration of Mn and the oxygen vacancies. The considerable enhancement in the spin carrier density of states (DOS) and changes in the configuration of Mn upon variation in doping concentration in SnO2 QDs demonstrated the effective role of Mn in modulating the electronic structure. The comprehensive experimental and first principles modeling studies elucidated the correlation between the enhanced spin carrier density of states in the electronic structure and the observed room temperature ferromagnetism. AUTHOR INFORMATION *

Corresponding Author (R. Murugan) Email: [email protected]

ACKNOWLEDGEMENTS DM acknowledge UGC, India for BSR fellowship (SRF). RM thank BRNS, DAE, Mumbai, India [No. 37(3)/14/42/2014-BRNS] for providing fund for the microwave synthesizer facilities. DM acknowledge SAIF, IIT Madras, Chennai, India for the EPR measurements.


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Table 1. The energy of pre edge peak and first and second maxima of the first derivative of absorption edge (in eV).

Sample

First maxima

Second maxima

Pre-edge position

2% Mn

6548.76±0.25

6553.27±0.25

6538.50±0.25

4% Mn

6548.53±0.25

6552.80±0.25

6538.75±0.25

6% Mn

6548.84±0.25

6552.80±0.25

6538.79±0.25

10% Mn

6548.36±0.25

6552.31±0.25

6538.51±0.25

MnO

6544.85±0.25

6550.55±0.25

6539.82±0.25

Mn2O3

6548.52±0.25

6553.58±0.25

6540.13±0.25

MnO2

6552.43±0.25

6558.75±0.25

6540.69±0.25

Mn metal

6539±0.25

6548.59±0.25



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Table 2. Bond length, coordination number and disorder factor obtain by EXAFS fitting for Mn doped SnO2 QDs at Mn K edge. Path

Parameter

Mn-O

R (Å)

2.04±0.01

1.95±0.01

1.92±0.01

2.01±0.01

N

4.2±0.22

4.2±0.22

4.2±0.28

4.2±0.30

2 (Å2) Mn-Sn

0.0076±0.001 0.0135±0.001

0.0097±0.001

3.05±0.02

3.05±0.02

2.99±0.02

3.05±0.02

N

2.61±0.39

1.56±0.18

1.34±0.13

1.14±0.11

0.0046±0.002

0.0034±0.001 0.002±0.001

0.0018±0.001

R (Å)

3.68±0.06

3.45±0.02

3.45±0.03

3.61±0.05

N

3.48±0.52

3.12±0.36

2.68±0.27

2.28±0.22

2 (Å2) Mn-Sn

0.0030±0.001

4%Mn-SnO2 6% Mn-SnO2 10% Mn-SnO2

R (Å)

2 (Å2) Mn-O

2% Mn-SnO2

0.0228±0.002

0.002±0.001 0.0063±0.002

0.0073±0.003

R (Å)

4.20±0.04

3.59±0.02

3.56±0.02

4.06±0.04

N

6.96±1.04

6.24±0.72

5.36±0.53

4.56±0.47

2 (Å2)

0.0165±0.008

0.0126±0.002 0.0223±0.003


0.0492±0.01

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Table 3. Results of the calculation of formation energies (Eform, in eV per Mn impurity) and average Mn-O and Mn-Sn first and the second distances (in Å) for various configurations of substitutional (MnSn) and interstitial (Mni) defects with different local MnxOy structures in absence and the presence of oxygen vacancies (VO, numbers in parenthesis) for two different types of surfaces. For the pure structure Sn-O and Sn-Sn distances for the atoms on the surfaces are reported. The most energetically favorable configurations are marked by bold. Local Configuration

Surface (001)

(100)

Eform

Mn-O

Mn-Sn

Eform

Mn-O

Mn-Sn

Pure (+VO)

——

+3.14 (+0.26)

2.13, 4.30 (2.09, 4.30)

3.40, 4.09 (3.52, 4.16)

+1.13 (+2.99)

2.10, 4.20 (2.12, 4,16)

3.30, 4.08 (3.33, 4.15)

MnSn (+VO)

MnO2 (MnO)

-2.62 (-1.08)

1.79, 3.82 (1.88, 3.88)

3.14, 4.02 (3.19, 4.08)

+2.26 (+1.46)

1.84, 3.76 (1.88, 3.81)

3.10, 3.84 (3.12, 3.87)

MnSn +Mni (+VO)

MnO (Mn2O)

-0.53 (+1.66)

1.82, 3.76 (1.84, 3.90)

3.10, 3.81 (3.12, 3.84)

+1.37 (+0.89)

1.92, 3.72 (1.98, 3.74)

3.06, 3.60 (3.08, 3.62)

2MnSn (+VO)

MnO2 (Mn2O3)

-0.50 (+0.87)

1.84, 3.70 (1.86, 3.71)

3.08, 3.76 (3.00, 3.68)

+1.86 (+1.03)

1.96, 3,62 (1.98, 3.60)

3.14, 3.78 (3.10, 3.66)

2MnSn + Mni (+VO)

Mn3O4 (MnO)

+0.62 (+1.81)

1.96, 3.74 (2.02, 3.79)

0.818 (0.926)

+1.53 (+1.17)

2.02, 3.59 (1.90, 2.72)

3.12, 3.81 (3.18, 3.86)



Page 20 of 32

Table 4. Summary of the ferromagnetic resonance field (FMR), g-factor, line width (ΔH) and number of spins (Nspins) estimated from EPR. Samples

FMR field (mT)

g-factor

∆H (mT)

Nspins (×106)

SnO2

328.45

2.0165

55.45

0.30

2% Mn

324.11

2.0297

50.73

0.81

4% Mn

322.56

2.0395

56.78

0.96

6% Mn

321.57

2.0457

56.62

1.69

10% Mn

320.79

2.0507

54.81

2.01


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Figure 1. (a) Normalized XANES spectra of Mn doped SnO2 QDs measured at Mn K-edge along with the spectra for reference Mn metal, MnO, Mn2O3 and MnO2 powder samples ( Inset shows the pre-edge portion of the XANES spectra in an expanded scale), (b) First and second peaks in the derivative spectra are indicated as below vis-a-vis the XANES spectra of the samples, and (c) Variation of first and second maxima energy positions of the first derivative of absorption edge of Mn doped SnO2 QDs along with the reference Mn metal, MnO, Mn2O3 and MnO2 powder samples.



Figure 2. (a) Linear combination fitting results for the Mn K-edge XANES spectra of Mn doped SnO2 QDs and (b) Linear combination fitting of 10% Mn doped SnO2.The weighted components used for LCF is also plotted to show their contributions.


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Figure 3. Fourier transformed EXAFS spectra of Mn doped SnO2 QDs measured at Mn K-edge (Black solid line) along with the theoretical fit (Red solid line): (a) 2% Mn, (b) 4% Mn, (c) 6% Mn and (d) 10% Mn.



Figure 4. Variation of disorder (Debye-Waller) factors (  2 ), coordination number and bond length with Mn concentration obtained from EXAFS fitting.


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Figure 5. (a) Schematic representation of optimized atomic structure of (001) surface of SnO2 with single substitutional Mn in the locality of the oxygen vacancy (1MnSn + VO) and (b) (100) surface with two substitutional and one interstitial Mn (2MnSn+Mni).



Figure 6. Partial densities of states (PDOS) for 3d orbitals of Mn-atom in the most energetically favorable configurations corresponding with formation of local MnxOy structures.


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Figure 7. (A) EPR spectrum of a) Undoped SnO2 b) 2% Mn doped SnO2 c) 4% Mn doped SnO2 d) 6% Mn doped SnO2 and e) 10% Mn doped SnO2 QDs measured at room temperature, (B) EPR spectra in absorption mode, (C) and (D) shows the dependence of area of the integration curves and number of spins (Nspins) participated in the FMR with saturation magnetization (Ms) of the investigated QDs, respectively.



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TOC Graphic:


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XANES, EXAFS, EPR, and First-Principles Modeling on Electronic

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