Structural investigations of (Mn, Dy) co-doped ZnO nanocrystals using X-ray absorption studies

(Mn, Dy)-doped ZnO nanocrystals with Mn-concentrations of 0 and 2% and Dy-concentrations of 0, 0.5%, 1%, 2%, 4% and 6% have been prepared using a sol – gel technique. Preliminary structural characterisations of the samples have been carried out using X-ray di ﬀ raction (XRD), Transmission Electron Microscopy (TEM) and Fourier Transformed Infra-red (FTIR) spectroscopy. Changes in the luminescence characteristics of the samples due to rare earth doping have been investigated by Photoluminescence (PL) measurement and its e ﬀ ect on the magnetic properties of the samples has also been studied. The local structure at the host (Zn) and dopant (Mn and Dy) sites of the samples have been thoroughly investigated by synchrotron based X-ray absorption spectroscopy (XAS), which is an element speci ﬁ c microscopic technique comprising both X-ray near edge structure (XANES) and extended X-ray absorption ﬁ ne structure (EXAFS) measurements and the magnetic properties of the samples have been explained in light of the ﬁ ndings from XANES and EXAFS.


Introduction
In recent years, diluted magnetic semiconductors (DMS) have attracted great interest for their potential technological applications in the eld of spintronics and many other spin based devices. 1,2These materials are used in various applications because of their specic catalytic, optical, electrical and magnetic properties. 3,4DMS materials are normally formed through the introduction of transition metal (TM) ions, such as Fe, Ni, Mn, Cr or rare earth (RE) ions like Gd or Dy into a host semiconductor like ZnO.ZnO a transparent conducting oxide, has a direct and wide bandgap 5 and is widely used in semiconductor devices like light-emitting diodes 6 and photo voltaic cells. 7Considerable efforts were made to study TM ion doped ZnO aer the theoretical prediction of room temperature ferromagnetism by Dietl et al. in Mn doped ZnO. 8 Later, theoretical demonstration by rst-principles electronic structure calculations by Sato and Katayama-Yoshida 9 suggested that transition-metal (TM ¼ Ti, V, Cr, Mn, Fe, Co, Ni, Cu) doped ZnO compounds are ferromagnetic provided the carriers produced by TM doping formed a partially lled spin-split impurity band.
Transition metal doped ZnO exhibiting ferromagnetism at or above room temperature makes these materials very attractive for use as non-volatile switching elements, LEDs, sensors, electronic and magnetic devices. 10n TMs, the magnetization arises from partially lled 3d shells and most of the cases since total orbital magnetic moment is zero, the magnetic moment is only due to the spin component and hence total magnetic moment per atom is less.Thus the ferromagnetism observed in TM doped ZnO samples so far has never been very signicant.2][13] In pure form, RE elements exhibit magnetism only at low temperatures and the advantage of rare earth compounds over other magnets is that, these materials are easy to magnetise in one direction and resist magnetisation in the other direction.Recent results for Gd in GaN, indicating high magnetic moments, 14 has instigated investigations on ZnO nanocrystals doped with RE metal ions.Rare earth elements doped ZnO nanocrystals are technologically important for industrial applications also in the eld of optoelectronics, photo catalysis, ber amplier 15 etc.Another advantage of doping with RE ions is that these go into ZnO lattice in +3 oxidation state leading to enhancement of carrier density. 11mong various rare earth elements, (Er 3+ & Tb 3+ ) doped ZnO have been reported extensively [16][17][18][19] and doping of ZnO with Dy 3+ has also been reported. 19,20Dy 3+ ions are also well known for producing visible light by appropriately adjusting yellow and blue emissions 21 and these are also used in thermoelectric devices which directly convert waste heat from the surroundings into electricity. 22agnetic properties of a semiconductor can also be tuned by co-doping, i.e., simultaneous doping with two elements at the host site which can increase the carrier concentration and in turn enhance carrier-mediated room temperature ferromagnetism (RTFM).Jayakumar et al. reported that when Al is co-doped with ZnO:Co system, a systematic increase in ferromagnetic behaviour has been observed. 23Wu et al. reported that addition of Al to the Cu-doped ZnO nanorods increased the carrier concentration, and retained the ferromagnetic properties of the Zn:CuO nanorods. 24Wibowo also reported the presence of RTFM in ZnO:Fe nanoparticles with the additional doping of Cu and Ni. 25 Recently, RE-doped DMS materials together with TM dopants have been actively investigated to get benet of higher magnetic moment of RE ions as well as stronger exchange interaction of the TM ions. 26,27However, the results of co-doping of (Mn, Dy) into ZnO have not been reported yet in the literature.
Though a large volume of work also exists in the literature on room temperature ferromagnetism (RTFM) of doped ZnO systems, however, there are wide variations in the reported papers regarding the origin of RTFM observed in the samples which has been attributed to a variety of intrinsic and extrinsic reasons by various authors.Thus origin of RTFM in doped and co-doped ZnO systems is still a fairly unresolved question and further experimental and theoretical studies are required particularly to explore the local environment around the host and the dopant cations carefully to obtain unambiguous results on the above subject.
In this study we have prepared (Mn, Dy) co-doped ZnO nanocrystals through sol-gel route and characterised them thoroughly by several techniques with an emphasis on synchrotron based Extended X-ray Absorption Fine Structure (EXAFS), which is an element specic technique and can yield important information on local environments of dopants (Mn, Dy) and also host element (Zn) in ZnO nanocrystals and thus in turn can give useful insight into the origin of RTFM in these samples.

Preparation of samples
To synthesize (Mn, Dy) doped ZnO nanocrystals with Mnconcentrations of 0 and 2% and Dy-concentrations of 0%, 0.5%, 1%, 2%, 4% and 6%, we follow the sol-gel route.Appropriate proportions of powders of analytical grade metal nitrates Zn(NO 3 ) 2 $6H 2 O from Sigma-Aldrich (99.99%),Dy(NO 3 ) 3 $5H 2 O from Alfa Aesar (99.99%), and Mn(NO 3 ) 2 $4H 2 O from Merck, Germany (99.98%) were thoroughly mixed and dissolved in equimolar solution of ethylene glycol and poly(vinyl alcohol) (PVA) (99+ purity), prepared in double distilled water, while stirring to obtain a homogeneous precursor solution.The obtained solution was slowly heated on a hot plate at 200 C until a highly viscous gel precursor was obtained.The highly viscous gel was kept at 200 C in the oven for 12 h for complete drying.Aer grinding the powder is calcined at 500 C for 10 h.

Characterisation
Structural characterization of (Mn, Dy) doped ZnO nanocrystals was performed by X-ray diffractometer (Model: Miniex-II, Rigaku, Japan) with Cu Ka radiation (l ¼ 1.54 Å).TEM, HRTEM and Selected Area Electron Diffraction (SAED) measurements were done with Technai G 2 S-Twin (FEI, Netherlands).Fourier transmission infrared (FT-IR) spectra of the samples (as pellets with KBr) were obtained using FT-IR Spectrometer (Spectrum One, Perkin Elmer Instrument, USA) in the range of 400-4000 cm À1 with a resolution of 1 cm À1 .The photoluminescence measurements have been carried out using 355 nm radiation from a Nd:YAG laser source (Innolas, Spitlight 600, 7 ns pulse width) and a charge-coupled-device (CCD) camera has been used as the spectral detector.The magnetic (M-H) measurements were done using Superconducting Quantum Interference Device (SQUID) Magnetometer [Magnetic Property Measurement System (MPMS) XL-7, Quantum Design, Inc.].
XAS measurements on the samples were carried out at the Energy Scanning EXAFS beamline (BL-8) at the Indus-2 Synchrotron Source (2.5 GeV, 120 mA) at the Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India. 28The beamline uses a double crystal monochromator (DCM) which works in the photon energy range of 4-25 keV with a resolution of 10 4 at 10 keV.It uses one meridional cylindrical mirror before the DCM for collimation and another meridional cylindrical mirror aer the DCM for vertical focussing and the sagittally bent second crystal of the DCM is used for horizontal focussing of the beam at the sample position.For the present set of samples measurements at Zn K-edge has been carried out in transmission mode while measurements at the dopant Dy L 3 and Mn K-edges have been carried out in uorescence mode.
For measurements in the transmission mode, three ionization chamber detectors are used, the rst one for measuring the incident ux (I 0 ) and the second one for measuring the transmitted ux (I t ), the absorbance of the sample being obtained as m ¼ ln À I 0 I t , while a third ionisation chamber is used for measuring the XAS signal of a reference metal foil for calibration of the DCM.The EXAFS spectra of the samples at Zn K-edge were recorded in the energy range 9585-10310 eV in transmission mode.For measurement in uorescence mode, a Si dri detector placed at 90 to the incident X-ray beam is used for measurements in the uorescence signal (I f ) while an ionization chamber detector placed prior to the sample is used to measure the incident X-ray ux (I 0 ), the sample is placed at 45 to the incident X-ray beam and the absorbance of the sample is obtained as a function of energy by scanning the monochromator over the specied energy range.The EXAFS spectra of the samples at Mn K-edge were recorded in the energy range of 6470-7250 eV and at Dy L 3 -edge the measurements have been carried out in the range of 7675-8490 eV.

X-ray diffraction
Phase purity of the (Mn, Dy) co-doped ZnO samples has been checked by XRD measurement (Fig. 1) which shows that no secondary phase is present in the samples other than wurtzite hexagonal ZnO phase.Thus incorporation of Dy in ZnO:Mn does not change the structure of wurtzite ZnO.However, it can be seen from the Fig. 1 that there is a shi in the peaks towards lower 2q values upon Dy doping.Since it has been observed from the XANES studies on the samples at Dy L 3 -edge, as discussed later, that Dy goes to ZnO lattice as Dy 3+ , the above shi may be because of expansion of ZnO lattice since the ionic radii of Dy 3+ (0.91 Å) is higher than the ionic radii of Zn 2+ (0.74 Å), similar results being observed by Khataee et al. also for their Dy doped ZnO nanoparticles. 29The parameters obtained from the analysis of XRD data of the co-doped samples are shown in Table 1.

Transmission electron microscopy
Transmission electron microscopy (TEM) measurements have been carried out to nd out the morphology and microstructure of the nanoparticles.Moreover, it should be pointed out here that d-values of the (2% Mn, 2% Dy) co-doped ZnO determined from TEM measurements are found to be higher than that of pristine ZnO, which signies the presence of tensile strain in the (Mn, Dy) co-doped samples as observed from XRD measurements.The HRTEM pattern also indicates that all the nanoparticles are single crystalline in nature and are free from major lattice defects.Thus according to the results of XRD pattern and HRTEM images, we can say that the Mn and Dy have been well incorporated into the crystal lattice of ZnO.

Photo luminescence measurement
Photo luminescence is an important tool to study the optical properties and structural defects in the semiconductor material.To identify the point defects we have studied the PL spectra at room temperature for (Mn, Dy) co-doped ZnO samples, where the concentration of Mn is kept xed at 2%, while the concentration of Dy varies from 0.5 to 6%.Fig. 4 shows the room temperature PL spectra of co-doped ZnO samples along with the undoped one recorded between 400 nm to 900 nm range, at the excitation wavelength of 355 nm.It can be seen from the above gure that pure ZnO sample have four emission bands in the visible range centred at 472.1 nm, 481.6 nm, 588.4 nm and 635.6 nm.It can also be found that PL intensity at the above wavelengths increase with the doping of Dy in ZnO:Mn alongwith the appearance of a violet band at 420 nm.The blue emissions at $472 and 482 nm are characteristic emissions of some oxides like ZnO, TiO 2 , SnO 2 and are caused by oxygen defects and increase in their intensities manifest increase in these oxygen defects in the samples with increase in Dy doping Fig. 3 FTIR spectra of (Mn, Dy) co-doped ZnO nanocrystals.
This journal is © The Royal Society of Chemistry 2017 concentrations.As has been shown by various author 32 the luminescence lines in the wavelength range of 420-650 nm obtained from doped and undoped ZnO systems are due to defects related to oxygen vacancies, Zn vacancies, oxygen interstitials and Zn interstitials.In the present samples these defects increase with the increase in Dy doping concentration resulting in the increase of intensities of the above peaks.Thankgeeswari et al. have also observed quenching of UV emission and enhancement of green emission which they have attributed to the creation of oxygen vacancies on Dy doping. 13u et al. have observed a PL peak at 575 nm which is due to 4 F 9/2 -6 H 13/2 transition, characteristic of Dy 3+ ions apart from the near band edge transition peak at 384 nm for Dy doped ZnO nanowires. 21Yan et al. on the other hand observed an additional characteristic peak at 482 nm due to 4 F 9/2 -6 H 15/2 transition in their Dy complex doped ZnO/polyethylene glycol hybrid phosphors. 19We have not observed any of these bands in our samples manifesting that Dy has been successfully incorporated in the ZnO lattice in the samples.

Magnetic measurements
Fig. 5 shows the magnetization (M) vs. applied eld (H) curve of the M-H plots of 2% Mn doped and (2% Mn, 2% Dy), (2% Mn, 4% Dy), (2% Mn, 6% Dy) co-doped ZnO samples at room temperatures while that of pure ZnO exhibiting diamagnetic behaviour at room temperature is shown in the inset of the gure.It can be seen that Mn doped and (Mn, Dy) co-doped samples with lower concentration of Dy (2%) exhibit super paramagnetic/weak ferromagnetic behaviour.However as the concentration of Dy is increased, magnetic behaviour changes from weak ferromagnetic/super paramagnetic to ferromagnetic nature.Thangeeswari et al. 13 however, contrary to our result have observed a decrease in FM in their (Co, Dy) co-doped ZnO samples with an increase in Dy concentration and have attributed it to anti-ferromagnetic (AFM) interaction among the Dy ions.Subramanian et al. 11 and Vijayaprasath et al. 12 have also observed decrease in magnetization with an increase in Gd doping concentration in case of Gd doped ZnO samples possibly due to AFM interaction among Gd atoms.

X-ray absorption spectroscopy
To further investigate the origin of RTFM in the above samples XAS measurement have been carried out on Mn doped and (2% Mn, 2% Dy), (2% Mn, 4% Dy) and (2% Mn, 6% Dy) co-doped samples.
Fig. 6(a) represents the experimental EXAFS (m(E) versus E) spectra of (Mn, Dy) doped ZnO NCs measured at Zn K-edge.A set of EXAFS data analysis program available within the IFEFFIT soware package have been used for reduction and tting of the experimental EXAFS data. 33The ATHENA subroutine of the above soware package has been used for converting the (m(E) versus E) data to (c(E) versus E) where, 34 E 0 is the absorption edge energy, m 0 (E 0 ) is the bare atom background and Dm 0 (E 0 ) is the step in the m(E) value at the absorption edge.Subsequently, (c(E) versus E) data have been converted to c(k) versus k, where photoelectron wave number (k) is dened as: Finally the c(k) versus k data is Fourier transformed to derive the c(R) versus R spectra.The ARTEMIS subroutine of the IFEFFIT package has subsequently been used to generate the theoretical EXAFS spectra from an assumed crystallographic structure and to t the experimental data with the theoretical spectra using the FEFF 6.0 code.The bond distances (R), coordination numbers (including scattering amplitudes) (N) and disorder (Debye-Waller) factors (s 2 ), which give the meansquare uctuations in the distances, have been used as tting parameters.The goodness of the t in the above process is generally expressed by the R factor which is dened as: where, c dat and c th refer to the experimental and theoretical c(r) values respectively and Im and Re refer to the imaginary and real parts of the respective quantities.Fig. 6(b) shows the k 2 c(k) versus k plots for the samples derived from the experimental EXAFS spectra and Fig. 6(c) shows the Fourier transformed EXAFS (FT-EXAFS) c(R) versus R spectra of (2% Mn, 2% Dy), (2% Mn, 4% Dy) & (2% Mn, 6% Dy) doped ZnO samples at the Zn K-edge along with the best t theoretical spectra, k range of 3-11 ÅÀ1 being used for the Fourier transform.The rst and second major peaks in the radial distribution functions of the undoped and co-doped ZnO samples correspond to the nearest oxygen and the Zn/Mn/Dy shells respectively from the central Zn atom.The data have been tted between 1-3.5 Å in R space where the theoretical spectra have been generated assuming the model described by Kisi et al. 35 having the rst oxygen shell (Zn-O1) at 1.97 Å with coordination number (N) of 4 and the second Zn shell (Zn-Zn) at 3.27 Å with N of 12.The best t parameters have been shown in Table 2.For comparison in Fig. 6(d) we have also plotted the Fourier transformed EXAFS (FT-EXAFS) c(R) versus R spectra along with the best t theoretical spectra for only Mn doped ZnO samples and the best t parameters have been shown in Table 3 from where it can be seen that the results are similar for both Mn doped and (Mn, Dy) co-doped samples.Thus it has  been observed that Dy doping has not caused any additional changes around Zn sites.Fig. 7(a) shows the experimental EXAFS (m(E) versus E) spectra of (Mn, Dy) co-doped ZnO NCs at Mn K-edge while Fig. 7(b) shows the k 2 -weighed c(k) versus k plots of the samples derived from the experimental EXAFS spectra.At the Mn K-edge we have explored two possibilities of theoretical modelling to t the experimental data: (a) starting with the basic wurtzite ZnO structure and replacing the central Zn atom with Mn and (b) taking the initial model to be of cubic Mn 2 O 3 .Such an approach to modelling has been reported by other authors as well. 36For the second case, structural parameters of Mn 2 O 3 has been taken from ICSD database 36 and data has been tted by assuming the rst nearest oxygen at 1.89 Å with N of 4 and second nearest oxygen shell at 2.24 Å with N of 2. Fig. 7(c) and (d) show the FT-EXAFS spectra or c(R) versus R plots of (2% Mn, 2% Dy), (2% Mn, 4% Dy) and (2% Mn, 6% Dy) co-doped ZnO samples at the Mn K-edge, along with the best t theoretical spectra, where the ttings have been carried out by using (i) wurtzite ZnO structure (where Zn atoms are replaced by Mn atoms according to the doping concentration) and (ii) cubic Mn 2 O 3 structure respectively and the best t parameters have been given in Tables 4 and 5.It should be noted here that in the theoretical model, which is generated by assuming Mn 2 O 3 structure, the rst Mn-Mn path occurs at 3.1 Å, but in FTspectra of Mn-K edge, no signicant peak occurs corresponding to this Mn-Mn path.For comparison the c(R) versus R plots  of the 2% Mn doped and (2% Mn, 2% Dy) ZnO nanocrystals measured at Mn K-edge have been plotted in the inset of Fig. 7(c), which shows very broad and reduced Mn-Mn peak in case of the later compared to the former sample, manifesting that Dy doping introduces some distortion in and around Mn sites in the lattice.Thus during tting of the data for (Mn, Dy) co-doped samples only the contribution of nearest oxygen shells have been taken.It has been observed that the parameters obtained by the later approach viz., assuming Mn 2 O 3 structure yields more reasonable results as shown in Table 5 and it is also reected in the better quality of tting obtained in Fig. 7(d) compared to in Fig. 7(c).The above results indicate that Mn is going to ZnO lattice as Mn 3+ .For comparison, Fig. 8(a) shows the experimental FT-EXAFS data of the Mn doped samples at the Mn K-edge along with the best t theoretical plot where the tting has been carried out assuming Mn at Zn sites in tetrahedral ZnO structure and Fig. 8(b) shows the corresponding plot for the 1% Mn doped sample where tting has been carried out using Mn 2 O 3 structure at the Mn sites.The best t parameters of the above two cases have been shown in Tables 6 and 7.It is clear from the above gures and tables that the tting quality of the data with the second theoretical model is poor and it yeilds unreasonable results of very low coordination in the oxygen and Mn shells.Hence we can conclude that in case of Mn doped ZnO samples the EXAFS data is best tted with the model of Mn replacing Zn atoms in ZnO lattice.
Thus the above EXAFS measurements on the Mn doped and (Mn, Dy) co-doped samples show two striking similarities, viz., in case of Mn doped samples, the Mn-Mn peak at 3.1 Å is present in the FT-EXAFS spectra, while in (Mn, Dy) co-doped samples it is signicantly reduced due to the disorder introduced by Dy atoms.Secondly, in Mn doped samples Mn is going to the ZnO lattice as Mn 2+ while in case of (Mn, Dy) co-doped samples, Mn is going into the lattice in Mn 3+ oxidation state.Fig. 9 shows the XANES spectra of the (Mn, Dy) co-doped samples alongwith that of standard MnO 2 and Mn 2 O 3 commercial powder and Mn metallic foil.It shows that the Mn absorption edge positions of the samples lie just above that of Mn 2 O 3 showing that Mn goes into the samples as Mn 3+ .However it is evident from the above gure that the post edge features (shown with arrows in the gure) of Mn 2 O 3 do not exactly match with that of the samples.This manifests that though Mn goes into the ZnO lattice in the co-doped samples as Mn 3+ , however it does not exist as separate Mn 2 O 3 phase in the sample manifesting proper Mn doping at Zn sites in the samples.8.In this case, structural parameters of Dy 2 O 3 has been taken from ICSD database. 37Initially tting has been carried out assuming the two nearest oxygen shells (Dy-O1) at 2.24 Å with coordination number of 2 & (Dy-O2) at 2.35 Å with coordination number of 4 respectively.From the tting results it has been found that for all the co-doped ZnO samples both oxygens shells are almost at the same distance of 2.25 Å and 2.26 Å from the central atom Dy.Hence during tting we have combined the contributions of both oxygen shells at 2.24 Å with coordination number of 6 and tting has been carried out by assuming this single shell of oxygen.It can be seen from Fig. 10(d) and Table 8 that tting with this structure has yielded better results with reasonable values of the tting parameters.
Fig. 11 shows the Dy L 3 edge XANES spectra of the (Mn, Dy) co-doped ZnO samples alongwith that of standard Dy 2 O 3 commercial powder.It shows that the Dy L 3 absorption edge positions of the co-doped samples agree with that of Dy 2 O 3 powder.However the post edge features (like the peak at 7810 eV) of Dy 2 O 3 is not exactly matching with that of the co-doped samples showing that though Dy goes as Dy 3+ in the ZnO lattice, it is not present as a separate Dy 2 O 3 phase manifesting successful doping Dy in the ZnO lattice.
However, it can be seen from Table 8 that signicant oxygen vacancies are present near Dy sites since oxygen coordination is less than that expected which is corroborated by PL measurements also discussed above.Creation of oxygen vacancy at Dy sites takes place possibly to compensate for the charge neutrality at Zn 2+ sites occupied by Dy 3+ ions.Thus the FM  observed in the Dy doped samples can be attributed to the vacancy mediated exchange interaction between the Dy 3+ ions or in other words due to the formation of bound magnetic polarons or BMP's.Thangeeswari et al. have also attributed the FM observed in their (Co, Dy) co-doped nanoparticles to bound magnetic polarons 13 Oxygen vacancies have been considered to be responsible for FM observed in several RE doped CeO 2 systems also. 38[41]

Conclusion
Sol gel derived (Mn, Dy) doped ZnO nanocrystals with Mnconcentrations of 0 and 2% and Dy-concentrations of 0.5%, 1%, 2%, 4% and 6% have been subjected to various complementary characterisation techniques.XRD measurements show that incorporation of Dy in Mn doped ZnO does not change the structure of wurtzite ZnO though there is slight expansion of ZnO lattice to accommodate relatively larger Dy 3+ (0.91 Å) ions which is also conrmed by HRTEM measurement.TEM measurements show (100) oriented wurtzite structure and $22 nm and 12 nm sizes respectively for the undoped and codoped nanocrystals.FTIR study also corroborates the above results that upon rare earth doping ZnO lattice is not distorted signicantly.PL measurements however indicate creation of oxygen vacancies on Dy doping.Magnetic measurement shows that as doping concentration of Dy is increased, magnetic behaviour changes from weak ferromagnetic/super paramagnetic to ferromagnetic nature.EXAFS data analysis at Zn Kedge shows that the results are similar for both Mn doped and (Mn, Dy) co-doped samples corroborating the above results that Dy doping has not caused any additional changes around Zn sites.However, EXAFS results at Mn K-edge on the Mn doped and (Mn, Dy) co-doped samples show two striking dissimilarities, viz., in case of co-doped samples, the second shell Mn-Mn peak is signicantly reduced due to the disorder introduced by doping of Dy atoms and in only Mn doped samples Mn is going to the ZnO lattice is Mn 2+ oxidation state while in case of (Mn, Dy) co-doped samples, Mn is going into the lattice in Mn 3+ oxidation state.Dy L 3 -edge results also show that Dy is going to the ZnO lattice as Dy 3+ and signicant oxygen vacancies are created near Dy sites to compensate for the charge neutrality at Zn 2+ sites, a result also corroborated by PL measurements.Thus, the FM observed in the Dy doped samples can be attributed to the oxygen vacancy mediated exchange interaction between the Dy 3+ ions or in other words due to formation of bound magnetic polarons or BMP's.It has also been noted from the XANES measurements of the samples at Mn K-edge and Dy L 3 edge alongwith that of standard oxide samples that though Mn and Dy go into ZnO lattice as Mn 3+ and Dy 3+ , no separate Mn 2 O 3 or Dy 2 O 3 phase exists in the samples.

Fig. 2 (
a), (b), (d) and (e) represent the TEM images and SAED patterns of pure ZnO and (2% Mn, 2% Dy) codoped ZnO nanoparticles which show that the size of nanocrystals are $22 nm and 12 nm respectively.The HRTEM micrograph of a representative ZnO and (2% Mn, 2% Dy) codoped ZnO sample is shown in Fig. 2(c) and (f) which show dvalue of 0.272 and 0.284 nm for (100) plane of wurtzite ZnO.

Fig. 3
Fig.3represents FTIR spectra of undoped and (Mn, Dy) codoped ZnO NCs capped by PVA.It can be seen from Fig.3that all the samples exhibited absorption bands at 3444, 2928, 2856, 2360, 1626, 1534, 1387, 842, 440 cm À1 .The peak appeared around 440 cm À1 can be attributed to the Zn-O stretching mode in the ZnO lattice.30The peak found at around $3444 cm À1 can be assigned to the -OH mode while two other peaks observed around 2928 and 2856 cm À1 are due to CO 2 molecules present in the air.The absorption peak observed around 1626, 1534 and 1387 cm À1 are due to stretching vibrations of C]H, C]C and C]O groups in acetate species which maybe present on the surfaces of the undoped and co-doped (Mn, Dy) ZnO nanoparticles.It is clear from the Fig.3that with an increase in Dy
concentration, the intensity of the ZnO band decreases which manifests successful incorporation of Dy 3+ ions in ZnO lattice and the disorder produced thereof.Similar results have also been obtained by Soni et al. for their Mn doped ZnO nanoparticles prepared by microwave irradiation.31However, it can be seen from Fig.3that in our case there is no signicant shi in the Zn-O stretching mode frequency due to Dy incorporation showing that Zn-O bond length does not distort signicantly due to Dy incorporation, the result being corroborated from EXAFS measurements as discussed later.

Fig. 8
Fig. 8 (a) Experimental c(R) versus R data (scatter points) and best fit theoretical plots (solid line) of Mn doped ZnO samples at Mn K-edge (fitting has been done assuming Mn at Zn sites in wurtzite ZnO structure).(b) Experimental c(R) versus R data (scatter points) and best fit theoretical plots (solid line) of Mn doped ZnO samples at Mn K-edge (fitting has been carried out by assuming Mn 2 O 3 structure around Mn sites).

Fig. 10 (
Fig.9shows the XANES spectra of the (Mn, Dy) co-doped samples alongwith that of standard MnO 2 and Mn 2 O 3 commercial powder and Mn metallic foil.It shows that the Mn absorption edge positions of the samples lie just above that of Mn 2 O 3 showing that Mn goes into the samples as Mn 3+ .However it is evident from the above gure that the post edge features (shown with arrows in the gure) of Mn 2 O 3 do not exactly match with that of the samples.This manifests that though Mn goes into the ZnO lattice in the co-doped samples as Mn 3+ , however it does not exist as separate Mn 2 O 3 phase in the sample manifesting proper Mn doping at Zn sites in the samples.Fig. 10(a) shows the experimental EXAFS (m(E) versus E) spectra of (Mn, Dy) co-doped ZnO nanocrystals while Fig. 10(b) shows c(k) versus k plots for the samples derived from the experimental EXAFS spectra.In this case also to t the experimental FT-EXAFS data at the Dy L 3 -edge two possibilities were examined viz., (a) Dy at Zn sites in tetrahedral ZnO structure and (b) Dy 2 O 3 structure at the Dy sites.Fig. 10(c) shows the FT-EXAFS spectra or c(R) versus R plots of (2% Mn, 4% Dy) co-doped ZnO nanocrystals at the Dy L 3 -edge along with the best t theoretical plots where the tting has been carried out assuming wurtzite ZnO structure with Zn atoms replaced by Dy atoms according to doping concentration.The theoretical FT-EXAFS spectra have been generated assuming the wurtzite ZnO structure where Zn is replaced by Dy with the rst oxygen shell (Dy-O) at 1.97 Å with coordination number (N) of 4. It has been observed that the tting quality of the data with this theoretical model is poor and it yields unreasonable results of higher bond distance (2.31 Å) as compared to its theoretical value.Fig. 10(d) shows the FT-EXAFS c(R) versus R plots of (Dy, Mn) co-doped ZnO nanocrystals at the Dy L 3 -edge along with the best t theoretical plot where the tting has been carried out by assuming the 2 nd option viz., Dy 2 O 3 structure at the Dy sites and the best t parameters have been shown in Table8.In this case, structural parameters of Dy 2 O 3 has been taken from ICSD database.37Initially tting has been carried out assuming the two nearest oxygen shells (Dy-O1) at 2.24 Å with coordination number of 2 & (Dy-O2) at 2.35 Å with coordination number of 4 respectively.From the tting results it has been found that for all the co-doped ZnO samples both oxygens shells are almost at the same distance of 2.25 Å and 2.26 Å from the central atom Dy.Hence during tting we have combined the contributions of both oxygen shells at 2.24 Å with coordination number of 6 and tting has been carried out by assuming this single shell of oxygen.It can be seen from Fig.10(d) and Table8that tting with this structure has yielded better results with reasonable values of the tting parameters.Fig.11shows the Dy L 3 edge XANES spectra of the (Mn, Dy) co-doped ZnO samples alongwith that of standard Dy 2 O 3 commercial powder.It shows that the Dy L 3 absorption edge positions of the co-doped samples agree with that of Dy 2 O 3 powder.However the post edge features (like the peak at 7810 eV) of Dy 2 O 3 is not exactly matching with that of the co-doped samples showing that though Dy goes as Dy 3+ in the ZnO lattice, it is not present as a separate Dy 2 O 3 phase manifesting successful doping Dy in the ZnO lattice.However, it can be seen from Table8that signicant oxygen vacancies are present near Dy sites since oxygen coordination is less than that expected which is corroborated by PL measurements also discussed above.Creation of oxygen vacancy at Dy sites takes place possibly to compensate for the charge neutrality at Zn 2+ sites occupied by Dy 3+ ions.Thus the FM

Fig. 11
Fig. 11 XANES plots for (Mn, Dy) co-doped ZnO samples at Dy L 3 edge alongwith that of standard Dy 2 O 3 powder.

Table 1
Values of particle size, lattice parameters and interplanar spacing for different planes of the (Mn, Dy) co-doped samples

Table 2
Fit parameters for Zn K-edge data by assuming wurtzite ZnO structure for (Mn, Dy) co-doped ZnO samples

Table 3
Fit parameters for Zn K-edge data by assuming wurtzite ZnO structure for Mn doped ZnO samples

Table 4
Fit parameters for Mn K-edge data by assuming Mn at Zn sites in wurtzite ZnO structure for (Mn, Dy) co-doped ZnO samples

Table 6
Fit parameters for Mn K-edge data assuming Mn at Zn sites in wurtzite ZnO structure for Mn doped ZnO samples

Table 7
Fit parameters for Mn K-edge data assuming Mn 2 O 3 structure at Mn site XANES plots for (Mn, Dy) co-doped ZnO samples at Mn K-edge alongwith that of standard MnO 2 , Mn 2 O 3 powders and Mn foil.

Table 8
Fit parameters for Dy L 3 -edge data by assuming Dy 2 O 3 structure at Dy site for (Mn, Dy) co-doped ZnO samples