Ferromagnetism in ZnO Nanocrystals: Doping and Surface Chemistry

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J. Phys. Chem. C 2010, 114, 1451–1459

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Ferromagnetism in ZnO Nanocrystals: Doping and Surface Chemistry Darshana Y. Inamdar,† Amit D. Lad,†,§ Arjun K. Pathak,‡ Igor Dubenko,‡ Naushad Ali,‡ and Shailaja Mahamuni*,† DST Unit on Nanoscience, Department of Physics, UniVersity of Pune, Pune 411007, India, and Department of Physics, Southern Illinois UniVersity, Carbondale, Illinois 62901 ReceiVed: September 19, 2009; ReVised Manuscript ReceiVed: December 5, 2009

Room temperature ferromagnetic ordering is observed in chemically grown ZnO nanocrystals. Nanocrystals are simultaneously capped by two different organic molecules inducing p-type and n-type defects. A saturation magnetization of 0.008 emu/g is achieved at 300 K. Incorporation of Mn2+ ions at substitutional sites in nanocrystals gives rise higher saturation magnetic moment at lower doping level. ZnO nanocrystals codoped with Mn2+ and Co2+ and prepared under identical conditions revealed an increase in saturation magnetization. However, the saturation magnetic moment remains lower than that obtained for Co2+-doped ZnO nanocrystals prepared by the same method. An increase in saturation magnetization was invariably associated with quenching of photoluminescence emission. These findings reaffirm that magnetism in nanocrystals is a defect induced phenomenon that can be controlled by choice of capping agent as well as incorporation of the transition metal impurity. Magnetization as a function of temperature [M(T)] curve is discussed in view of available reports on the global exchange mechanism in these ferromagnetic nanocrystals. 1. Introduction Recent reports showing ferromagnetic ordering in oxide nanocrystals open up a new challenging research field. Even undoped nanocrystals of diamagnetic materials are reported to be ferromagnetic.1-10 Nanocrystals can be obtained by a variety of chemical routes. The choice of a capping agent is important in its own right. For instance, Au nanocrystals have shown1 a signature of ferromagnetic ordering stabilized due to chemisorbed thiol capping. Sundaresan et al.2 proposed the origin of ferromagnetism to be the exchange interaction between localized electron spin moments resulting from oxygen vacancies at the surface of uncapped ZnO nanocrystals. Wang et al.7 reported that Zn vacancies are responsible for observed ferromagnetism. Schoenhaltz et al.8 proposed that ferromagnetism in undoped nanocrystals should be mediated by extended defects such as surfaces and grain boundaries. Xu et al.10suggest that controlled defects can induce room tempearature ferromagnetism in ZnO. On the other hand, Garcia et al.3 argue that capping nanocrystals with organic molecules such as trioctylphosphine oxide, dodecylamine, and dodecanethiol causes alteration in their electronic configuration and hence yield long-range ferromagnetic ordering in ZnO nanocrystals. Magnetism is observed to be the strongest when capped with thiol group. Until recently, it was believed4 that ZnO nanocrystals become ferromagnetic when coated with N and S ligands. Complementary calculations show4 that N or S atoms of the ligands bind Zn sites at the surface. Subsequent changes in Zn-O bond length and hybridization of Zn 4s levels with ligands redistribute the charges between Zn and O atoms that result in the ferromagnetism. Followed by these reports, we observed stabilization of ferromagnetic ordering manifested by attaching * To whom correspondence should be addressed. E-mail: [email protected]. † University of Pune. ‡ Southern Illinois University. § Present address: Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India.

ligands carrying P atom along with Zn-N bond. It may be worthwhile to note here that Meulenberg et al.11 observed ligand induced paramagnetism in CdSe quantum dots capped by HDA and TOPO. Earlier, Kittilstved et al.12 have vividly shown appearance of ferromagnetism in Co2+-doped ZnO nanocrystals by chemical perturbations (n-type defects) produced by capping agent trioctylphosphine oxide and in Mn2+-doped ZnO nanocrystals by perturbations (p-type defects) due to capping trioctylamine. Further, reversal of capping destroyed the ferromagnetic ordering. The magnetic moment of Mn2+-doped ZnO nanocrystals found to be higher than Co2+-doped ZnO nanocrystals when it is capped with dodecylamine.12 However, Coey et al.13 report that the high-temperature ferromagnetism is essentially defect mediated ferromagnetic ordering. Ferromagnetism was observed in carbon-doped14 and hydrogendoped15 ZnO in the bulk phase as well. These findings make the puzzle even more complex. In short, the scenario is very complex although defects generated by capping or passivating organic molecules seem to be the origin of ferromagnetism in nanocrystals of otherwise diamagnetic materials. We observed the feeble ferromagnetism in tert-butylphosphonic acid (TBPA) and 1-hexadecylamine (HDA) capped ZnO nanocrystals. Capping agent containing phosphorus (TBPA) is playing an important role in observed ferromagnetism. Co2+ incorporation in these ZnO nanocrystals yield high saturation magnetic moments. Inclusion of Mn2+ also leads to higher saturation magnetic moment albeit less than that observed in case of Co2+-doped case. However, an attempt to obtain higher saturation magnetization (due to presence of n-type and p-type defects simultaneously) by codoping the nanocrystals by Mn2+ and Co2+ was not materialized. In the present work, our experimental data provide an evidence of possibility of magnetic ordering due to (a) defects and (b) charge distribution caused by ligands passivating the surface atoms. An anticorrelation of magnetic moment and band edge luminescence is observed.

10.1021/jp909053f  2010 American Chemical Society Published on Web 01/04/2010

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TABLE 1: Concentration of Co in ZnO Nanocrystals, Ratio of Intensity of OH- to Intensity of Zn-O Band from Fourier Transform Infrared Spectra, Coercivity (Hc), Remanence Magnetization (Mr), and Saturation of Magnetization (Ms) of ZnO Nanocrystals nanocrystals

concentration by ICP-AES (Mn %, Co %)

average diameter (nm)

IOH-/IZn-O

Hc (Oe)

Mr (emu/g)

Ms (emu/g)

ZnO ZnO:Mn ZnO:Mn ZnO:Mn:Co ZnO:Co

(0.0%, 0.0%) (0.2%, 0.0%) (1.8%, 0.0%) (0.8%, 0.6%) (0.0%, 1.9%)

8.7 ( 1.0 7.6 ( 0.6 7.4 ( 0.6 6.8 ( 0.5 5.5 ( 0.5

7.5 2.4 2.4 1.4 2.1

96 138 105 148 398

0.002 0.006 0.001 0.013 0.221

0.008 0.051 0.008 0.228 1.064

2. Experimental Section 2+

2+

Undoped, Co -doped, Mn -doped, and (Mn, Co)-codoped ZnO nanocrystals were grown by thermal decomposition16,17 of zinc acetate [(CH3CO2)2Zn]. Zinc acetate and cobalt(II) acetate tetrahydrate [(CH3CO2)2Mn · 4H2O] for Co2+-doped samples, zinc acetate and manganese acetate tetrahydrate [(CH3CO2)2Mn · 4H2O] for Mn2+-doped samples, and zinc acetate, manganese acetate tetrahydrate, and cobalt(II) acetate tetrahydrate [(CH3CO2)2Co · 4H2O] for (Mn, Co)-codoped samples are used as precursors in the mixture of HDA (C16H33NH2) and TBPA [C4H9PO(OH)2] at high temperatures (300 °C). Nanocrystals were separated by centrifugation and washed several times with hexane. Finally nanocrystals were obtained in the form of a free-flowing powder. The chemicals were purchased from Aldrich and used without further purification. Maximum Mn2+ and Co2+ ion concentration in the solution was deliberately limited to 5% in order to avoid the impurity phase formation. Dopant concentration in ZnO nanocrystals are shown in Table 1 X-ray diffraction (XRD) analysis was carried out on a BRUKER AXS D8 ADVANCE powder X-ray diffractometer with Cu KR (1.541 Å) as the incident radiation. The average size of nanocrystals was estimated using the Scherrer formula.18 Transmission electron microscopy (TEM) measurements were carried out using a Philips CM200 microscope operating at 200 kV. Dopant concentration was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) employing LABTAN-8440 Plasma Lab. Room-temperature optical absorption spectra were recorded using Perkin-Elmer Lambda 950 spectrophotometer. Fourier transform infrared spectra (FTIR) were recorded using an FTIR - 6100 spectrophotometer. Photoluminescence (PL) measurements were performed using Perkin-Elmer LS 55 spectrophotometer with excitation wavelength of 325 nm. Electron paramagnetic resonance (EPR) measurement were carried out using a Bruker EMX spectrometer operating at 9.1 GHz (X band) at 77 K. Magnetization of the samples were studied with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design, Inc.) in the temperature (T) range of 5 to 300 K and magnetic fields (H) of (-50 to +50) kOe.

affect the growth behavior and hence responsible for reduction in size after doping. A subtle increase in the lattice parameters a by 0.7% and c by 0.2% for Mn2+-doped ZnO (1.8%) nanocrystals indicates expansion of the unit cell due to Mn2+ inclusion in ZnO nanocrystals. Such an increase in lattice parameter with doping is observed in earlier studies20-22 as well. The ionic radius21,22 of Zn2+ is (0.74 Å) smaller than Mn2+ (0.80 Å). Thereby incorporation of Mn2+ at Zn2+ site generates strain and hence leads to an increase in the lattice constant. These findings essentially indicate the presence of Mn2+ ions at cationic substitutional sites retaining the wurtzite structure. Similarly change in lattice parameters is observed due to codoping in ZnO nanocrystals is about 0.2% [a as well as in c for (Mn 0.8%, Co 0.6%)]. Co2+ incorporation in ZnO causes increase in crystallographic orientation of wurtzite nanocrystals along the c axis as seen from Figure 1a. No foreign phase was observed. There are several reports in which formation of metallic nanoclusters or metal oxides were detected in ZnO matrix.23-25 Such peaks are completely absent in the present case.

3. Results and Discussion Structural and Optical Properties of ZnO Nanocrystals. XRD patterns of undoped and doped ZnO nanocrystals are displayed in Figure 1a. The diffraction features match with those of wurtzite ZnO phase. Average size as estimated for undoped ZnO nanocrystals using Scherrer formula18 is found to be 8.7 ( 1.0 nm. A decrease in size is observed due to doping Mn2+ or Co2+ or codoping of (Mn, Co) in ZnO nanocrystals (Table 1). The present observation agrees with the proposition that addition of impurity in the semiconductor nanocrystals is known19 to

Figure 1. (a) XRD patterns of undoped, Mn2+-doped, (Mn, Co)codoped, and Co2+-doped ZnO nanocrystals of various doping level with y axis on logarithmic scale. (b) Electron diffraction pattern of undoped, Mn2+-doped, (Mn, Co)-codoped, and Co2+-doped ZnO nanocrystals.

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Figure 2. (a) Optical absorption spectra for Mn2+-doped, (b) (Mn, Co)-codoped ZnO, (c) Co2+-doped nanocrystals with varying doping levels indicating band-edge and Mn2+ and Co2+ ligand field absorption features. FTIR spectra of (d) Mn2+-doped, (Mn, Co)-codoped, and Co2+-doped ZnO nanocrystals.

Electron diffraction patterns of Mn2+-doped, (Mn, Co)codoped ZnO, and Co2+-doped nanocrystals (Figure 1b) do not show any feature that provides evidence of Mn or Co metal cluster formation or oxide cluster formation. No other impurity phase is observed in the electron diffraction as well. Optical absorption spectra of undoped and doped ZnO nanocrystals with varying dopant ion concentrations are presented in Figure 2. To measure the exact location of absorption peaks in the spectra, minimum of second order derivative of the curve is recorded. Excitonic feature in optical absorption is observed at about 355 nm (3.5 eV) in the case of undoped ZnO nanocrystals. As seen from Figure 2a, a blue shift of 25 nm is observed for 1.8% Mn2+-doped ZnO nanocrystals (330 nm, 3.7 eV). Such a blue shift in band gap was observed by several groups.26,27 Large band gap of MnO (4.2 eV) may be responsible for observed blue shift in band gap. A broadband from 415 to 470 nm corresponds27 to d-d transitions between 6A1g ground states to excited 4T, 4E, and 4A1 states of tetrahedral Mn2+ ions. In the case of undoped ZnO nanocrystals, a hump is observable in this spectral regime due to scattering from powder samples. The room-temperature optical absorption spectra of (Mn, Co)codoped ZnO nanocrystals along with undoped ZnO nanocrystals are shown in Figure 2b. We observe a blue shift commensurate with the concentration of dopant ions. Absorption spectra (Figure 2b) also reveal the additional features at 565, 611, and 655 nm similar to Co2+-doped ZnO nanocrystals with no discernible change in the two cases. The intensity of these features increases with increase in dopant concentration. These subforbidden gap features are28 the ligand field d-d transitions of Co2+ in tetrahedral environment and are ascribed as 4A2 (F) f 2A1 (G), 4A2 (F) f 4T1 (P), and 4A2 (F) f 2E (G) transitions. The appearance of these transitions also implies a high spin state of Co2+ (d7). Observed broad feature in the range 415 to 500 nm correspond to Mn2+ ligand field d-d transitions in (Mn, Co)-codoped ZnO nanocrystals. As expected, the spectral

features due to d-d transitions of Mn and Co are concomitantly observed in codoped samples. These ligand field transitions do not reveal appreciable change with an increase in doping level. As seen from Figure 2c, Co2+ doping in ZnO nanocrystals leads to bleaching of an excitonic feature associated with red shift in absorption. It is now well accepted that Co2+ in ZnO nanocrystals allows tuning the energy gap toward low energy.20,28-31 The shrinkage of energy gap due to Co2+ doping in ZnO is a manifestation of the sp-d exchange interactions20,30,31 between the band electrons and the localized d electrons of the Co2+ ions substituting for Zn2+ ions. Note that doped and undoped samples are prepared under exactly the same experimental conditions. Co2+ ligand field absorption features28 are clearly visible (Figure 2c) at 567, 610, and 654 nm in the doped nanocrystals. These are cobalt-related 4A2(F) f 2A1(G), 4A2(F) f 4T1(P), and 4 A2(F) f 2E(G) ligand field transitions28 for tetrahedral symmetry of transition metal ions. In short, the optical absorption study confirms presence of high spin Co2+ (d7) state in ZnO lattice. The intensity of d-d transitions increases with the increase in number of Co2+ ions in ZnO nanocrystals.31 The substitutional incorporation of Co2+ ions indicated by XRD is corroborated by the optical absorption measurements. It is well-known that Mn2+ doping in ZnO nanocrystals is responsible for the shift in band gap toward higher energy side,27 while shrinkage of band gap is observed in Co2+-doping in ZnO nanocrystals.20 In Mn2+-doped ZnO nanocrystals, we observed a blue shift of 25 nm, while a blue shift of 18 nm is observed in (Mn, Co)-codoped ZnO nanocrystals. Co2+-doped ZnO nanocrystals exhibit a red shift in the forbidden gap with increase in dopant concentration. The intermediate blue shift for (Mn, Co)-codoped ZnO nanocrystals is thus understandable. FTIR spectra of ZnO nanocrystals are shown in Figure 2d. Undoped and doped ZnO nanocrystals are capped by both nitrogen (HDA) and phosphorus (TBPA) bonds. The N-H bond in HDA breaks and nitrogen attaches to zinc ion located at

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Figure 3. The PL spectra for undoped, Mn2+-doped, (Mn, Co)codoped, and Co2+-doped ZnO nanocrystals.

surface.32 Nitrogen bonding leads to p-type defects in ZnO nanocrystals. The band at 525 cm-1 is assigned17 to the stretching vibrations of Zn-O. A broad feature centered at 3418 cm-1 due to hydroxyl ions in ZnO nanocrystals is clearly visible. In the case of ZnO nanocrystals, surface capping reduces the OH- concentration. Kittilstved et al.12 report that surface capping strongly influences the defect characteristics. OH- is a potential n-type defect in ZnO. The intensity ratio of OH- to Zn-O stretching bond for varying amount of Co2+ and Mn2+ in ZnO nanocrystals is presented in Table 1. OH- ions that may otherwise introduce n-type defects are suppressed with increase in Co2+ concentration. Other features in FTIR spectra are identical for undoped and doped ZnO nanocrystals ruling out the possibility of complex formation of transition metal and capping agent. This is crucial since the presence of impurity phase may affect the ferromagnetism. PL spectra were recorded to get information of defects present in undoped and doped ZnO nanocrystals. PL emission spectra of ZnO nanocrystals excited at 325 nm show near band edge luminescence at 385 nm Figure 3 with quantum efficiency of 0.1% with respect to laser dye polypropyleneoxide (PPO). The Stokes shift (shift between optical absorption and PL emission) is about 30 nm, which is rather high and indicates the presence of shallow defect levels. Defect emissions at 424, 468, and 521 nm were also observed in undoped ZnO nanocrystals. The appearance of blue luminescence at 424 nm is attributable to the transition from Zn interstials to valence band,33 while most probably the oxygen vacancies are responsible for blue luminescence at 468 nm and green luminescence at 521 nm in undoped ZnO nanocrystals.34 In fact, in case of nanocrystals, it is shown3 that the intensity of green luminescence depends on surface properties, in particular on the capping agent. The noteworthy finding is that TBPA and HDA capping quenches the overall luminescence, profoundly green luminescence. It is observed that the band edge luminescence is predominantly quenched for Co2+-doped ZnO nanocrystals followed by that of (Mn, Co)-codoped and least for Mn2+-doped ZnO nanocrystals. Weak green luminescence at 521 nm disappears, while intensity of blue luminescence increases with increase in dopant concentration. Interestingly, similar observations for Co-doped ZnO films was reported by Wang et al.35 The PL quantum efficiency can not be reliably determined in doped ZnO nanocrystals. These results suggest that defects such as Zn interstials and oxygen vacancies are present in TBPA and HDA capped ZnO nanocrystals. Transition metal doping can introduce shallow donor or acceptor levels, which may be responsible for

Figure 4. EPR spectra for (a) Mn2+-doped and (b) (Mn, Co)-codoped ZnO nanocrystals with varying doping levels.

quenching of band edge luminescence in case of doped ZnO nanocrystals. An anticorrelation of PL intensity along with ferromagnetic ordering is also observed36 in case of doped ZnO nanowires. EPR Measurements of Doped ZnO Nanocrystals. EPR measurements were carried out on Mn2+-doped and (Mn, Co)codoped ZnO nanocrystals at 77 K (parts a and b of Figure 4). Six line hyperfine splitting was observed for low Mn2+ concentration (0.2% Mn2+). Signal corresponds to splitting constant lAl ) 76 × 10-4 cm-1, and g ) 2.0049 is comparable27,37-39 to reported values for substitutional incorporation of Mn2+ at Zn sites. It suggests that Mn2+ is in tetrahedral environment rather than the usually preferred octahedral environment. The broad spectra as shown in Figure 4b with g value 2.0786 for higher concentration (1.8%) of Mn2+ in ZnO nanocrystals is due to the superposition of several spectral features corresponding to different Mn sites at surface.27 The g value observed for 1.8% Mn concentration in ZnO nanocrystals is higher than reported value.27,38,39 An increase in Mn2+-Mn2+ dipolar interaction with an increase in Mn concentration may be responsible for this shift. The nature of the M(H) curve for higher Mn concentration (1.8%) is also different than that for low concentration. An increase in paramagnetic and a decrease in ferromagnetic contribution for higher concentration of Mn also support increase in Mn2+-Mn2+ interactions. Yeom et al.40 also observed such a broad EPR spectrum in the case of Mndoped ZnS powder, which was attributed to strong magnetic interactions between Mn ions at higher concentration. Kremer et al.41 and Soskic et al.42 observed a shift in g values for higher concentration of Mn in CdTe and ZnTe. The notable fact is that the signal corresponding to Mn2+ in the Zn(OH)2 shell37 of the ZnO nanocrystals (g ) 2.001 and |A| ) 89 × 10-4 cm-1) is not observable in the present case. For (Mn, Co)-codoped ZnO nanocrystals (0.8% Mn and 0.6% Co), g ) 2.0269 (Figure 4b). Observed splitting constant |A| ) 79 × 10-4 cm-1 is higher than that reported values for

Ferromagnetism in ZnO Nanocrystals

Figure 5. M vs H curves (a) for undoped ZnO measured at 300 K. Inset displays magnetization curve in low field regime. (b) M vs T curves under field-cooled (FC) and zero field-cooled (ZFC) conditions of undoped ZnO nanocrystals.

substitutional incorporation of Mn2+ in ZnO nanocrystals. Moreover, it is also slightly higher than Mn2+-doped ZnO nanocrystals in the present case. Mn2+ ions could be located at different surface sites in this case. Only a few of them occupy substitutional sites. Probably presence of Co along with Mn is affecting substitutional incorporation of Mn in codoped samples. Co-related spectral features are not observed in EPR spectra. Magnetic Measurements of ZnO Nanocrystals. Magnetic hysteresis [M(H)] curve of undoped ZnO nanocrystals recorded at 300 K is shown in Figure 5a. The sample shows weakly ferromagnetic behavior that is superimposed on the diamagnetic background (diamagnetic background from sample holder is not subtracted from data). Coercivity and saturation magnetization is observed to be 96 Oe and 0.008 emu/g at 300 K (Table 1). Quite a few reports1-6,9,43 indicate occurrence of ferromagnetic ordering in nanocrystals of otherwise nonmagnetic solids due to surface defects. Although there is a possibility of presence of Zn interstials and oxygen vacancies at surface of undoped ZnO nanocrystals, these defects are not large enough to induce ferromagnetism in undoped ZnO nanocrystals.8 In particular, it is shown4 that the ligand capping causes redistribution of charges at Zn, O, and N sites. The resulting unpaired p electrons of O or N (or perhaps P in the present case) lead to the magnetic moments and make the system ferromagnetic. First-principle total energy calculations by Zuo et al.9 on ZnO surfaces adsorbed with different molecules also suggests that electronegativity of atoms in molecules adsorbed at surface of oxide nanoparticles may be an important factor for the origin of magnetic moments. Adsorbed molecules such as NH3 on Zn sites lead to attractive interaction between H in NH3 and O atoms at surface. Charge transfer from NH3 to ZnO surface is responsible for ferromagnetism in undoped ZnO. The present work gives evidence that the ferromagnetic ordering is observed in ZnO nanocrystals involving TBPA- and HDA-related bonds on the surface as well, which is confirmed from FTIR spectra. However size of TBPA molecule is large. Because of a lack of well passivation of surface of nanocrystals by TBPA molecule, surface defects in

J. Phys. Chem. C, Vol. 114, No. 3, 2010 1455 the present case may play an important role in observed ferromagnetism. Notably, ferromagnetic ordering exists even above room temperature. The saturation magnetization is stronger (Table 1) in these ZnO nanocrystals as compared to the thiol capped ZnO nanocrystals3 reported earlier. Magnetization is almost at the detection limit of SQUID, and hence magnetization as a function of temperature curve is noisy with distorted shape. Approximately, the shape lies between outwardly convex and linear nature as displayed in Figure 5b. Application of the percolation of bound magnetic polaron (BMP) is highly questionable in this case as the sample is free from transition metal impurity. However, notable prediction of the BMP theory that the defect induced stabilization of ferromagnetism is reflected in curvature of M(T) curve. Figure 6a reveal hysteresis with enhancement in saturation magnetization for 0.2% Mn2+-doped ZnO nanocrystals compared to undoped ZnO nanocrystals. On the other hand, 1.8% Mn2+-doped ZnO nanocrystals show a hysteresis at low field, which indicates the presence of small ferromagnetism (Ms ) 0.008 emu/g) along with large paramagnetic contribution (Figure 6b). That is, increase in concentration of Mn2+ ions in ZnO nanocrystals leads to an increase in paramagnetic and decrease in ferromagnetic contribution. The magnetic moment in the present case is calculated by linear subtraction.44 Jayakumar et al.44 and Chu et al.45 also observed coexistence of ferromagnetic and paramagnetic contributions in transition metal doped ZnO nanocrystals. This paramagnetic contribution may be due to presence of Mn at near surface sites. Broad EPR spectra (Figure 4a) for 1.8% Mn-doped ZnO nanocrystals also support enhancement in paramagnetic contributions. The decrease in ferromagnetic contribution for 1.8% Mn2+-doped ZnO nanocrystals may be due to an increase in (Mn2+-Mn2+) antiferromagnetic interaction21 with an increase in dopant concentration. Higher concentration of Mn2+ in ZnO, leads to the nearest neighbor super exchange interaction. Such an interaction between the nearest neighbors is antiferromagnetic in nature21 and hence can cause decrease in magnetization. Temperature dependent magnetization [M(T)] was studied from 5 K through 300 K. Figure 6c shows distinct splitting between corresponding ZFC and FC curves recorded at an applied field of 1000 Oe up to room temperature. These measurements indicate occurrence of ferromagnetism.44 The splitting between ZFC and FC curves is negligible in the case of 1.8% Mn2+-doped ZnO nanocrystals (Figure 6d) due to large paramagnetic contribution The feature around 60 K in FC and ZFC curves may appear because of contamination from molecular oxygen.5 Field-dependent magnetization curve of (Mn, Co)-codoped ZnO nanocrystals at temperature 300 K is shown in Figure 7a. The hysteresis curve superimposed on the linear increase in magnetization at higher value of applied field is observable. Such a behavior is an indication of the presence of isolated ions, not participating in the magnetic ordering, while giving rise to observed paramagnetic contribution at higher field. Coercivity and remanence magnetization are given in Table 1. The saturation magnetization observed for codoped (Mn 0.8%, Co 0.6%) nanocrystals at 300 K is 0.228 emu/g (Table 1), while for 1.8% Mn2+-doped ZnO nanocrystals (with same dopant concentration in reaction solution) it is 0.008 emu/g at 300 K (Table 1). It is clear that the magnetic moment in the case of codoped samples is higher as compared to Mn2+-doped sample. That is inclusion of Co along with Mn can increase the net magnetization.

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Figure 6. M vs H curves (a) for 0.2% Mn2+-doped ZnO nanocrystals and (b) for 1.8% Mn2+-doped ZnO nanocrystals recorded at 300 K. Inset displays magnetization curve in low field regime. M vs T curves under FC and ZFC conditions of (c) 0.2% Mn2+-doped and (d) 1.8% Mn2+-doped ZnO nanocrystals.

Figure 7. M vs H curves for (a) (Mn 0.8%, Co 0.6%) (Mn, Co)-codoped nanocrstals and (b) 1.9% Co2+-doped ZnO nanocrystals at 300 K. Inset displays magnetization curve in low field regime. M vs T curves under FC and ZFC conditions for (c) (Mn 0.8%, Co 0.6%) (Mn, Co)-codoped nano crystals and (d) 1.9% Co2+-doped ZnO nanocrystals.

The nature of ZFC and FC curves is similar to that observed for Mn2+-doped ZnO nanocrystals. M(T) curves confirm the appearance of room temperature ferromagnetism for (Mn, Co) codoped ZnO nanocrystals as shown in Figure 7b. Magnetization curve for 1.9% Co2+ in ZnO nanocrystals is shown in Figure 7c at 300 K. The coercivity and saturation magnetization of Co2+-doped ZnO nanocrystals is the highest (Table 1) for 1.9% doping level. Incorporation of higher amount

of Co2+ in ZnO nanocrystals was deliberately avoided in order to restrain from impurity phase formation. In the case of Co2+-doped ZnO nanocrystals, the saturation magnetization is substantially high compared to the undoped ZnO nanocrystals. At the same time, gigantic change in structural properties is not evident from XRD, electron diffraction, and FTIR studies.

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Figure 8. M/Ms vs H/T curves at various temperatures for undoped, Mn2+-doped, (Mn, Co)-codoped, and Co2+-doped ZnO nanocrystals.

M(T) curves (Figure 7d) show branching between ZFC and FC curves for the applied field of 1000 Oe. The existence of net magnetization even above room temperature implies the ferromagnetic behavior of the sample. For instance, Chu et al.45 also reported ferromagnetic ordering in Fe-doped In2O3 nanocrystals. Under certain experimental conditions, a hysteresis curve was observed without saturation magnetization. However, the corresponding ZFC and FC measurements show a distinct splitting suggesting the presence of ferromagnetism. The observed magnetization is sequentially decreasing from ZnO:Co to ZnO:Mn:Co followed by ZnO:Mn for same dopant concentration. The plots of (M/Ms) vs (H/T) curves are not superimposed at various temperatures as can be seen from Figure 8a-d. These results support ferromagnetic like behavior for undoped and doped ZnO nanocrystals in present case. Similar findings are reported by various groups.46,47 Karmakar et al.46 studied ferromagnetic Fe-doped ZnO nanocrystals. Co2+-doped ZnO nanocrystals prepared by Martinez et al.47 using vaporization condensation methods are superparamagnetic above room temperature and have low coercivity and saturation magnetization. The ferromagnetic ordering is found to be intimately related to the structural perfection. Incorporation of carbon causing ferromagnetic ordering as argued earlier14 may not be feasible in the present case. An increase in magnetic moment in Co2+-doped ZnO nanocrystals does not reveal any substantial difference in carbon related bonds. Even though the nanocrystals are prepared in organic media and are capped, FTIR spectra do not support this argument. The present data, however, is not sufficient to rule out the effect of carbon, if any. Fonin et al.49 proposed the appearance of defect induced ferromagnetism in Co2+-doped ZnO thin films. It is observed that oxygen rich Co2+-doped ZnO films are paramagnetic while oxygen poor films are ferromagnetic. Nanocrystals have large surface to volume ratio. Consequently, large number of surface defects is generated and plays

a profound role. Carriers associated with the defect levels lead to the quasi-continuous band like structure. Spin-alignment of transition metal cations is understood in the framework of overlap of defect band with the d orbital. M(T) curves are outwardly concave in the present case. Such a behavior is characteristic50,51 of DMS with localized charge carriers. Note that with an increase in Co2+ concentration in ZnO nanocrystals, ferromagnetic ordering is getting stronger at the cost of paramagnetic contribution. Coey et al.52 suggested that the ferromagnetic ordering is stabilized at critical doping concentration in dimethyl sulfide (DMS). Such a critical doping concentration required to induce the ferromagnetism cannot be determined in the present case as all Co2+ ions do not take part in the ferromagnetic ordering. Moreover, even undoped ZnO nanocrystals are ferromagnetic. The donor impurity band exchange model explained48 that point defects such as oxygen vacancies are responsible for formation of BMP. Defect-induced shallow levels hybridize with d level of transition metal ion and subsequently stabilize ferromagnetic ordering in diluted magnetic oxides. The present work suggests that the primary reason to observe ferromagnetism is defects, most probably located at surface and generated by the capping agents. Additional effects due to dopant ions are also observable. FTIR spectra show reduction in amount of hydroxyl ions with increase in Co2+ ions. As discussed earlier, chemisorbed OH- at surface gives rise to n-type defects in ZnO. Reduction in OH- ions is therefore indicative of reduction in carrier concentration. ZnO nanocrystals are simultaneously capped with organic molecules inducing p-type and n-type states. The experiments support the conjecture that the defects, especially surface defects (in case of nanocrystals) are playing the crucial role. 4. Conclusions Ferromagnetic ordering was observed in chemically prepared, undoped ZnO nanocrystals capped with organic molecules such

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as TBPA and HDA. In the present case, ZnO nanocrystals are capped with organic molecules leading to n-type and p-type defects simultaneously. Indeed electron energy levels are getting modified to the extent that long-range ferromagnetic ordering is stabilized. Interestingly, a signature of anticorrelation between luminescence and ferromagnetism is clearly observed supporting the existence of defect-induced ferromagnetic ordering. Co2+ doping in these ZnO nanocrystals yields magnetic moment higher by 2 orders of magnitude. Substitutional incorporation of Co2+ ions in ZnO alters the growth behavior, that generates defects and quench PL intensity. Co2+-doped ZnO nanocrystals exhibit concave M(T) curves indicative of localized carrier concentration and can be understood on the basis of percolation of BMP. Extending the studies further for Mn2+ and codoping Mn2+ along with Co2+ does retain the ferromagnetic ordering albeit with decrease in saturation magnetization. However, an increase in Mn ion concentration (1.8%) leads to antiferromagnetic coupling between Mn ion pairs resulting in a decrease in the saturation magnetic moment. It is worthwhile to note here that incorporation of Mn2+ ions show saturation magnetization in between that for undoped and Co2+-doped case. Moreover, band-edge luminescence is also observable in Mn2+-doped ZnO nanocrystals, which is a signature of relatively less defect states. Codoping ZnO nanocrystals with Mn2+ ions and Co2+ ions yields higher saturation magnetization than just Mn2+-doped case. Concomitant decrease in band edge luminescence due to the defect levels is observable. In short, the present work provides an additional evidence for the defect induced ferromagnetism in undoped and transition metal doped ZnO nanocrystals. Incorporation of Mn2+ ions in ZnO is affecting the forbidden gap. In bulk DMS systems, it is believed that p-d and s-d hybridization changes with amount of Mn2+ ions. Thereby, the forbidden gap of Mn2+-doped ZnO increases. Concomitantly, the magnetic properties also sensitively depend on the p-d and s-d hybridization. The notable fact is that in the case of Mn2+-doped ZnO nanocrystals, even though, appreciable change in forbidden gap is observable, magnetization depends on the induced defects in the nanocrystal. Co2+ incorporation leads to large number of defects in ZnO nanocrystals as experimentally evident from strong quenching of photoluminescence. Simultaneous increase in magnetization is observed due to Co2+ in ZnO. These facts clearly reveal presence of ferromagnetic ordering in nanocrystals due to defects (mostly present at the surface). Choice of capping agent and transition metal ion can control the defects and thereby magnetic ordering as well as luminescence of ZnO nanocrystals. Acknowledgment. The financial support by University Grants Commission (UGC) to University of Pune is gratefully acknowledged. D.I. and A.D.L. are thankful to UGC and DST for the financial support. TEM, ICP-AES, and EPR measurements are carried out at S.A.I.F., I.I.T., Mumbai. Absorption and PL measurements are carried out at DST nanoscience unit. We thank Prof. C. N. R. Rao for his useful suggestions. Discussions with S. V. Ghaisas, S. M. Yusuf, S. V. Bhoraskar, and S. K. Date are gratefully acknowledged. The work in USA was supported by the Research Opportunity Award from Research Corporation (RA-0357) and by the Office of Basic Energy Sciences, Material Sciences Division of the U.S. Department of Energy (Contract No. DE-FG0206ER46291).

Inamdar et al. References and Notes (1) Garcı´a, M. A.; Ruiz-Gonza´lez, M. L.; Quesada, A.; Costa-Kra¨mer, J. L.; Ferna´ndez, J. F.; Khatib, S. J.; Wennberg, A.; Caballero, A. C.; Martı´nGonza´lez, M. S.; Villegas, M.; Briones, F.; Gonza´lez-Calbet, J. M.; Hernando, A. Phys. ReV. Lett. 2005, 94, 217206. (2) Sundaresan, A.; Bhargavi, R.; Rangarajan, N.; Siddesh, U.; Rao, C. N. R. Phys. ReV. B 2006, 74, 161306(R). (3) Garcia, M. A.; Merino, J. M.; Fernandez, P. E.; Quesada, A.; Venta, J. de la; Ruiz Gonzalez, M. L.; Castro, G. R.; Crespo, P.; Llopis, J.; Gonzalez-Calbet, J. M.; Hernando, A. Nano Lett. 2007, 7, 1489. (4) Wang, Q.; Sun, Q.; Jena, P. J. Chem. Phys. 2008, 129, 164714. (5) Potzger, K.; Zhou, S.; Grenzer, J.; Helm, M.; Fassbender, J. Appl. Phys. Lett. 2008, 92, 182504. (6) Sundarasen, A.; Rao, C. N. R. Nano Today 2009, 4, 96. (7) Wang, Q.; Sun, Q.; Chen, G.; Kawazoe, Y.; Jena, P. Phys. ReV. B 2009, 77, 205411. (8) Schoenhalz, A. L.; Arantes, J. T.; Fazzio, A.; Dalpian, G. M. Appl. Phys. Lett. 2009, 94, 162503. (9) Liu, En. Zuo.; Jiang, J. Z. J. Phys. Chem C 2009, 113, 16116. (10) Xu, Q.; Schmidt, H.; Zhou, S.; Potzger, K.; Helm, M.; Hochmuth, H.; Lorenz, M.; Setzer, A.; Esquinazi, P.; Meinecke, c.; Grudmann, M. Appl. Phys. Lett. 2008, 92, 082508. (11) Meulenberg, R. W.; Lee, J. R.; McCall, S. K.; Hanif, K. M.; Haskel, D.; Lang, J. C.; Terminello, L. J.; Buuren, T. V. J. Am. Chem. Soc. 2009, 131, 6888. (12) Kittilstved, K. R.; Norberg, N. S.; Gamelin, D. R. Phys. ReV. Lett. 2005, 94, 147209. (13) Coey, J. M. D.; Chambers, S. M. MRS Bull. 2008, 33, 1053. (14) Pan, H.; Yi, J. B.; Shen, L.; Wu, R. Q.; Yang, J. H.; Lin, J. Y.; Feng, Y. P.; Ding, J.; Van, L. H.; Yin, J. H. Phys. ReV. Lett. 2007, 99, 127201. (15) Wang, Z. H.; Geng, D. Y.; Guo, S.; Hu, W. J.; Zhang, Z. D. Appl. Phys. Lett. 2008, 92, 242505. (16) Kshirsagar, S. D.; Inamdar, D.; Gopalakrishnan, I. K.; Kulshreshtha, S. K.; Mahamuni, S. Solid State Commun. 2007, 143, 457. (17) Cozzoli, P. D.; Curri, M. L.; Agostiano, A.; Leo, G.; Lomascolo, M. J. Phys. Chem. B 2003, 107, 4756. (18) Cullity, B. D.; Stock, S. R. Elements of X-Ray Diffraction; PrenticeHall: NJ, 2001; ps 170. (19) Mahamuni, S.; Lad, A. D.; Patole, S. J. Phys. Chem. 2008, 112, 2271. (20) Kim, K. J.; Park, Y. R. Appl. Phys. Lett. 2001, 81, 1420. (21) Xu, H. Y.; Liu, Y. C.; Xu, C. S.; Liu, Y. X.; Shao, C. L.; Mu, R. J. Chem. Phys. 2006, 124, 074707. (22) Duan, L. B.; Rao, G. H.; Yu, J.; Wang, Y. C.; Chu, W. G.; Zhang, L. N. J. Appl. Phys. 2007, 102, 103907. (23) Zhou, S.; Potzger, K.; Borany, J. V.; Gro¨tzschel, R.; Skorupa, W.; Helm, M.; Fassbender, J. Phys. ReV. B 2008, 77, 035209. (24) Liu, Q.; Gain, C. L.; Yuan, C. L.; Han, G. C. Appl. Phys. Lett. 2008, 92, 032501. (25) Zhou, S.; Potzger, K.; Talut, G.; Borany, J. V.; Skorupa, W.; Helm, M.; Fassbender, J. J. Appl. Phys. 2008, 103, 07D530. (26) Wang, Y. S.; Thomas, P. J.; O’Brien, P. J. Phys. Chem. B 2006, 110, 21412. (27) Viswanatha, R.; Sapra, S.; Gupta, S. S.; Satpati, B.; Satyam, P. V.; Dev, B. N.; Sarma, D. D. J. Phys. Chem. B 2004, 108, 6303. (28) Koidl, P. Phys. ReV. B 1977, 15, 2493. (29) Yoo, Y. Z.; Fukumura, T.; Jin, Z.; Hasegawa, K.; Kawasaki, M.; Ahmet, P.; Chikyow, T.; Koinuma, H. J. Appl. Phys. 2001, 90, 4246. (30) Deka, S.; Joy, P. A. Appl. Phys. Lett. 2006, 89, 032508. (31) Gaudon, M.; Toulemonde, O.; Demourgues, A. Inorg. Chem. 2007, 46, 10996. (32) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. B 1998, 102, 3655. (33) Lin, B.; Fu, Z.; Jia, Y. Appl. Phys. Lett. 2001, 79, 943. (34) Irimpan, L.; Nampoori, V. P. N.; Radhakrishnan, P.; Deepthy, A.; Krishnan, B. J. Appl. Phys. 2007, 102, 063524. (35) Wang, A.; Zhang, B.; Wang, X.; Yao, N.; Gao, Z.; Ma, Y.; Zhang, L.; Ma, H. J. Phys. D. Appl. Phys. 2008, 41, 215308. (36) Cui, J.; Gibson, U. Phys. ReV. B 2006, 74, 045416. (37) Zhou, H.; Hofmann, D. M.; Hofstaetter, A.; Meyer, B. K. J. Appl. Phys. 2003, 94, 1965. (38) Dorain, P. B. Phys. ReV. 1958, 112, 1058. (39) Diaconu, M.; Schmidt, H.; Po¨ppl, A.; Bo¨ttcher, R.; Hoentsch, J.; Klunker, A.; Spemann, D.; Hochmuth, H.; Lorenz, M.; Grundmann, M. Phys. ReV. B 2005, 72, 085214. (40) Yeom, T. H.; Lee, Y. H.; Oh, M. H.; Choh, S. H. Appl. Phys. 1996, 79, 1004. (41) Kremer, R. H.; Furdyna, J. K. Phy. ReV. 1985, 32, 5591. (42) Soskic, Z.; Stojic, B. B.; Stojic, M. J. Phys.: Condens. Matter 1994, 6, 1261. (43) Madhu, C.; Sundaresan, A.; Rao, C. N. R. Phys. ReV. B 2008, 77, 201306(R).

Ferromagnetism in ZnO Nanocrystals (44) Jayakumar, O. D.; Salunke, H. G.; Kadam, R. M.; Mohapatra, M.; Yaswant, G.; Kulshreshtha, S. K. Nanotechnol. 2006, 17, 1278. (45) Chu, D.; Zeng, Y. P.; Jiang, D.; Ren, Z. Appl. Phys. Lett. 2007, 91, 262503. (46) Karmakar, D.; Mandal, S. K.; Kadam, R. M.; Paulose, P. L.; Rajarajan, A. K.; Nath, T. K.; Das, A. K.; Dasgupta, I.; Das, G. P. Phys. ReV. B 2007, 75, 144404. (47) Martı´nez, B.; Sandiumenge, F.; Balcells, L.; Arbiol, J.; Sibieude, F.; Monty, C. Phys. ReV. B 2005, 72, 165202. (48) Mukadam, M. D.; Yusuf, S. M.; Sasikala, R.; Kulshreshtha, S. K. J. Appl. Phys. 2006, 99, 034310.

J. Phys. Chem. C, Vol. 114, No. 3, 2010 1459 (49) Fonin, M.; Mayer, G.; Biegger, E.; Janβen, N.; Beyer, M.; Thomay, T.; Bratschitsch, R.; Dedkov, Y. S.; Ru¨diger, U. J. Phys. Conf. Ser. 2008, 100, 042034. (50) Das Sarma, S.; Hwang, E. H.; Kaminski, A. Phys. ReV. B 2003, 67, 155201. (51) Calderon, M. J.; Das Sarma, S. Ann. Phys. 2007, 322, 2618. (52) Coey, J. M. D.; Venkatesan, M.; Fitzgerald, C. B. Nat. Mater. 2005, 4, 173.

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