Room Temperature Ferromagnetism and Photoluminescence of Fe

Oct 25, 2011 - Undoped and iron doped ZnO nanocrystals are observed to be ferromagnetic at room temperature. The role of short-range ... Citation data...
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Room Temperature Ferromagnetism and Photoluminescence of Fe Doped ZnO Nanocrystals Darshana Y. Inamdar,† Arjun K. Pathak,‡ Igor Dubenko,‡ Naushad Ali,‡ and Shailaja Mahamuni*,† † ‡

Department of Physics, University of Pune, Pune 411007, India Department of Physics, Southern Illinois University, Carbondale, Illinois 62901, United States

bS Supporting Information ABSTRACT: Undoped and iron doped ZnO nanocrystals are observed to be ferromagnetic at room temperature. The role of short-range disorder in stabilizing ferromagnetism is evident from structural, optical, and electron paramagnetic resonance (EPR) measurements. The substitutional incorporation of Fe3+ at Zn sites is reflected in optical and EPR studies. Isolated as well as interacting Fe3+ ions are observed in EPR. An increase in saturation magnetization is associated with broadening of a ferromagnetic resonance feature with an increase in doping level. Co-, Mn-, and Fe-doped ZnO nanocrystals prepared by the same synthesis protocol confirm anticorrelation between photoluminescence efficiency and ferromagnetic ordering. M(T) curves indicate that the ferromagnetic nanocrystals are formed by the short-range exchange forces associated with the bound magnetic polaron at defect sites.

1. INTRODUCTION Studies of ferromagnetic nanocrystals are motivated by unique phenomena observable in these systems, such as superparamagnetism or defect induced ferromagnetism. Change in electron energy levels due to broken symmetry at the surface as well as site specific magnetic moment in capped nanocrystals is observed in the literature.1 10 In fact, defects are known to govern most of the physical properties and carry great importance in nanocrystals having high surface to volume ratio. Organic capping agents are usually used to passivate the defects as well as to form nanocrystals by chemical route avoiding aggregation. These capping agents have limitations in passivation of the defects due to steric hindrance. Subsequently, core/shell nanocrystals often show better passivation and hence higher photoluminescence efficiency. Nanocrystals also exhibit a tendency of self-annealing and purification. Thereby uniform doping of nanocrystals remains a challenging task. Presence or absence of ferromagnetic ordering in doped nanocrystals is observed to be depending on dopant distribution in nanocrystals.11,12 Jayakumar et al.11 reported that, random distribution of dopant ions at cationic sites, isolated ion pairs or any cluster formation can cause absence of ferromagnetic ordering in Fe-doped ZnO nanocrystals. Moreover, chemical routes to form doped nanocrystals are yet under development. No unique synthesis protocol exists for doping a particular type of nanocrystals. Methods of forming doped nanocrystals are element sensitive. For instance, undoped ZnO nanocrystals capped by tert-butylphosphonic acid (TBPA) and hexadecylamine (HDA) are found to be ferromagnetic.9 Doping Mn2+ and Co2+ ions as well as Fe3+ in ZnO nanocrystals are feasible by the r 2011 American Chemical Society

same route. However, Cu or Ni could not be doped by the same route. Charge transfer between surface of nanocrystals and capping molecule can alter the electronic structure of nanocrystals and hence can boost the ferromagnetism in ZnO nanocrystals. Presence of defects due to large surface to volume ratio of nanocrystals allows tuning various properties which can not be observed in their bulk form. Doping semiconductor nanocrystals provides an additional means to control the optical, electrical and magnetic properties. For instance, band gap engineering is feasible13 in transition metal ion doped ZnO leading to different colors useful for optical devices. Earlier, Inamdar et al.9 reported ferromagnetic ordering in undoped ZnO nanocrystals at room temperature, that increases by 1 order of magnitude on Mn doping and 2 orders of magnitude by Co doping. These studies indicate the possibility of tuning ferromagnetic ordering along with the color of doped ZnO nanocrystals.13 Incorporation of iron ions in ZnO nanocrystal lattice, clearly gives a signature of Fe3+ ion state in ZnO as observed by local probe such as EPR. Fe in 3+ state replacing Zn ion in 2+ state, reorganizes the charge distribution in Fe-doped ZnO nanocrystals resulting in subtle increase in the saturation magnetization. Keeping the same synthesis protocol for ZnO nanocrystal formation (and hence identical surface structure), here we explain change in the electron energy levels due to Fe3+ ion incorporation and try to understand the differences in Fe3+, Mn2+, and Co2+ ions in ZnO. Received: June 22, 2011 Revised: October 21, 2011 Published: October 25, 2011 23671

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2. EXPERIMENTAL SECTION Undoped and Fe3+-doped ZnO nanocrystals are grown by following the methods9,14involving thermal decomposition of zinc acetate [(CH3CO2)2Zn] and iron(II) acetate [(CH3CO2)2Fe] in the mixture of HDA [CH 3 (CH 2 )15 NH 2 ] and TBPA [C4H9PO(OH)2] at high temperature (∼300 °C). Nanocrystals were separated by centrifugation and washed several times with hexane. Finally nanocrystals were obtained in the form of a freeflowing powder. To study the phase of nanocrystals, X-ray diffraction (XRD) analysis was carried out on a Bruker AXS D8 advance powder X-ray diffractometer, with Cu Kα (λ = 1.5402 Å) as the incident radiation. The average size of nanocrystals was estimated using Scherrer formula.15 Dopant concentration was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) employing LABTAN-8440 Plasma Lab. Transmission electron microscopic (TEM) measurements were carried out using a Philips CM200 microscope operating at 200 kV. Room temperature optical absorption spectra were recorded using Perkin-Elmer Lambda 950 spectrophotometer while photoluminescence was measured with the aid of Perkin-Elmer LS 55 spectrometer. Fourier transform infrared spectra (FTIR)

Figure 1. (a) XRD patterns of undoped, Fe3+-doped ZnO nanocrystals with varying doping level. (b) TEM image and electron diffraction pattern of 3.21% Fe3+-doped ZnO nanocrystals.

were recorded using Thermo Nicolet FTIR-6100 spectrophotometer. EPR measurement were carried out using a Bruker EMX spectrometer operating at 9.1 GHz (X band) at 77 K. Magnetization of the samples was studied with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design, Inc. U.S.A.) in the temperature (T) range of 5 to 300 K and magnetic fields (H) of 20 to +20 kOe. Magnetization versus temperature [M(T)] measurements were carried out for zero field cooled (ZFC) and field cooled (FC) conditions in the applied magnetic field of 1000 Oe.

3. RESULTS AND DISCUSSION Figure 1a shows XRD pattern of ZnO and Fe-doped ZnO nanocrystals for various doping levels. Diffraction pattern confirms formation of ZnO without any impurity or secondary phase formation. Average size of undoped ZnO nanocrystals is estimated to be 8.7 ( 1 nm. However decrease in lattice parameter ‘a’ by 0.2% and ‘c’ by 1.4% is observed in case of 0.15% Fe-doped ZnO nanocrystals. Similarly for 3.21% Fe-doped ZnO nanocrystals decrease in lattice parameter ‘a’ by 0.8% and ‘c’ by 1.4% is observed. As given in Table 1, ionic radius of Fe3+ is smaller than that of Zn2+. And therefore, decrease in the lattice constant is plausible and is also reported in the literature.16,17 In fact, change in lattice constant of doped nanocrystals is commensurate with the dopant ionic radii of other transition metal ions as well. Moreover, decrease in lattice constant gives a clear clue regarding the occupation of mostly Fe3+ ions at substitutional sites. On the contrary, Fe2+ ions would result into an increase in the lattice constant. Figure 1b shows the TEM image of 3.21% Fe-doped ZnO nanocrystals. Size of nanocrystals is observed to be 8.9 ( 2 nm. Electron diffraction pattern did not show secondary phase formation on Fe doping in ZnO nancrystals and is in agreement with the XRD analysis. Optical absorption and photoluminescence of undoped and Fe-doped ZnO nanocrystals were recorded at room temperature. Figure 2a shows absorption spectra of undoped and Fe-doped ZnO nanocrystals. In order to measure the exact location of absorption peaks in the spectra, minimum of second order derivative of the curve were recorded. Excitonic feature of undoped ZnO nanocrystals appears at 355 nm (3.49 eV). A small blue shift as compared to undoped ZnO nanocrystals (355 to 345 nm) in absorption spectra is observed for lower doping levels. It is possible that at lower doping levels, most of the iron ions may be in Fe3+ ionic state. Fe3+ ions substituted at Zn2+ site act as donors. Subsequently Moss Burstein type shift18 in the forbidden gap is possible. Fe2+ ions coexist along with Fe3+ ions, but cannot be detected by the optical absorption measurements. Controversial results are reported regarding optical absorption of Fe-doped ZnO.19 21 Forbidden gap narrowing as well as widening is observed. Band gap narrowing is explained in terms of s d and p d exchange interactions between the host and dopant materials.19 22 A negative and positive correction in unoccupied

Table 1. Comparison between ZnO:Fe, ZnO:Mn and ZnO:Co Nanocrystals nanocrystals (concentration by ICP-AES)

ionic radius in Å 2+

change in “a”

change in “c”

Ms (emu/g) 300 K 0.105 0.052

ZnO Co (0.20%) ZnO Mn (0.20%)

Co 0.72 Mn2+0.80

0.7% decrease 0.4% increase

0.7% increase 0.2% increase

ZnO Fe (0.15%)

Fe3+ 0.64 Fe2+ 0.77

0.2% decrease

1.4% decrease

undoped ZnO

Zn2+ 0.74

0.012 0.008

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Figure 2. (a) Optical absorption spectra for undoped and Fe3+-doped ZnO nanocrystals with varying doping levels. (b) The PL spectra for undoped, Fe3+-doped ZnO nanocrystals with varying doping levels at excitation wavelength 325 nm. (c) Comparison between PL spectra of Co-, Mn-, and Fe-doped ZnO nanocrystals at excitation wavelength 325 nm.

and occupied levels leads to the forbidden gap narrowing. Red shift in the optical absorption spectra is vivid at higher doping level (3.21%). Red shift in absorption peak would be a result of increase in sp-d exchange interaction which becomes a dominant factor over Moss Burstein type shift at higher doping level. Absorption spectra show the additional features in the range from 410 nm through 600 nm. The intensity of these features increases with the doping level [Figure 2a]. These broad features correspond13,19 to d d transitions between 6A1g ground states to excited 4T, 4E, and 4A1 states of Fe3+ ions. The ground state electronic configuration of Fe3+ ion is (Ar) 3d5. Substitutional incorporation of Fe3+at Zn2+ sites can lead to tetrahedral crystal field around Fe3+ ions surrounded by 02‑ ions. These crystal field effects can split first excited state (4G) of d5 free electrons in to 4 T, 4E, and 4A1 states. Similar transitions are also observed for isoelectronic Mn2+ -doped ZnO and CdTe nanocrystals.13,23 Figure 2b shows PL spectra of undoped and doped ZnO nanocrystals at an excitation wavelength 325 nm. Undoped ZnO nanocrystals show emission maxima at 385 nm along with blue (424 nm, 468 nm) and green (521 nm) luminescence. Transitions from Zn interstitals to valence band are attributed to blue emission (424 nm) in PL spectra.24,25 Oxygen vacancies are also observed to be responsible for blue (468 nm) and green emission (521 nm).24,25 The quantum efficiency of emission for undoped ZnO nanocrystals prepared by the present route with respect to laser dye (PPO) is very low (0.1%). Presence of shallow defect levels is one of the reasons for such a low quantum efficiency of ZnO nanocrystals. Red shift from 385 (for undoped ZnO) to 394 nm (for 3.21% Fe in ZnO nanocrystals) as well as quenching

in emission maxima is observed with increase in Fe concentration in ZnO nanocrystals. Thus, both optical absorption and photoluminescence spectra suggest substitutional incorporation of Fe3+ at Zn sites. However, presence of Fe2+ at Zn sites can not be ruled out. Only optical measurements are not sufficient enough to detect these ions. It is clear that, substitutional incorporation of dopant ions at host sites can create defects in nanocrystals. Introduction of these defects in nanocrystals is not only depending on dopant concentration but also depend on type of dopant. PL spectra for Fe (0.15%), Co (0.20%), and Mn (0.20%) doped ZnO nanocrystals are compared in Figure 2c. It should be noted that, all other parameters such as concentration of nanocrystals, excitation wavelength, and growth conditions for Fe-, Co-, and Mn-doped ZnO nanocrystals are kept same. It is observed that, quenching of band edge emission is the highest in case of Co-doped ZnO nanocrystals while band edge to defect emission ratio is higher in Fe-doped ZnO nanocrystals compared to Mn-doped ZnO nanocrystals. This also suggests presence of relatively less number of defects in Fe3+-doped ZnO nanocrystals as compared to Co2+- or Mn2+-doped ZnO nanocrystals in spite of the fact that most of iron ions exist in Fe3+ state. EPR spectra are recorded on undoped and Fe-doped ZnO nanocrystals at 77 K. No paramagnetic resonance signal is observed for undoped ZnO nanocrystals (inset of Figure 3). EPR spectra of Fe3+-doped ZnO nanocrystals consist of three overlapping signals as shown in Figure 3. Signal 1 with g value 4.30 indicates isolated Fe3+ in distorted symmetry.26 28 A broad signal with g value 2.20 (signal 2) corresponds to ferromagnetic 23673

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The Journal of Physical Chemistry C resonance (FMR), and indicates ferromagnetic exchange between Fe3+ ions. Signal 3 with g value 2.16 is attributable to Fe3+ at distorted octahedral sites.22 24 Broadening of these features is observed with increase in Fe3+ % in ZnO nanocrystals. That is increase in intensity of FMR signal along with decrease in intensity of isolated Fe3+ signal is observed with increase in Fe3+ % in ZnO nanocrystals. Also a slight shift of g value for FMR signal toward lower resonance field is observed with increase in iron concentration in ZnO nanocrystals. In short, EPR spectra suggest presence of Fe3+ at substitutional Zn2+ sites along with the interacting Fe3+ ions. Occurrence of room temperature ferromagnetic ordering (RTFM) in undoped ZnO nanocrystals is discussed earlier.9 Sample shows weakly ferromagnetic behavior that is superimposed on the diamagnetic background. On subtracting diamagnetic background (see the Supporting Information), hysteresis curve [Figure 4a] shows coercivity of 96 ( 30 Oe and saturation magnetization about 0.008 ( 0.001emu/g at 300 K while at 5 K these values are 442 ( 11 Oe and 0.019 ( 0.001 emu/g, respectively. No trace of magnetic impurity in ZnO nanocrystals

Figure 3. EPR spectra for Fe3+-doped ZnO nanocrystals with varying doping levels. Inset shows the EPR spectra of undoped ZnO nanocrystals.

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is observed in ICP-AES analysis as well as X-ray photoelectron spectroscopy (data not shown). M(H) curve for undoped ZnO nanocrystals at 5 K show small remanence at H = 0 and large difference between curve with increasing H and decreasing H for higher fields. M(H) is generally determined by magnetocrystalline anisotropy and magnetic interactions existing in the system. Presence of large uniaxial anisotropy with random distribution of the direction of anisotropy axis or presence of inter/intraparticles exchange interactions with different sign and value may be responsible of such behavior.29 The primary cause of ferromagnetic ordering is defects. Several reports indicate, modification of surface states via capping agent can induce ferromagnetism in nanocrystals.1,3,7,9 In the present case, ZnO nanocrystals are capped by TBPA and HDA molecules. FTIR spectra reveal presence of P and N related bonds at the surface of nanocrystals (data not shown).9 Higher electro negativity of P and N attached to Zn can affect charge redistribution and hence can induce spin polarization in ZnO nanocrystals. Low PL intensity of ZnO nanocrystals suggests presence of shallow defect levels such as oxygen vacancies as well as Zn interstials at surface of nanocrystals. In the present case, surface defects such as oxygen vacancies, Zn interstials as well as ligand induced defects contribute to observed RTFM. M vs H curves measured at 5 and 300 K for various doping level of Fe3+ in ZnO nanocrystals are shown in Figure 4b d. Figure 4b shows enhancement in saturation magnetization (Ms) with Fe3+ doping in ZnO nanocrystals compared to undoped ZnO nanocrystals (Table 2).The saturation magnetization was calculated by linear subtraction of paramagnetic contribution.30 Further increase in saturation magnetization is observed with increase in doping level in ZnO nanocrystals. Observed saturation magnetization and coercivity for different doping level are given in Table 2. Increase in saturation magnetization with increase in doping level can be understood in terms of enhanced bound magnetic polarons. EPR spectra (Figure 3) also suggest

Figure 4. M versus H curves (a) for undoped ZnO measured, (b) for 0.15% Fe3+-doped ZnO nanocrystals, (c) for 2.18% Fe3+-doped ZnO nanocrystals, and (d) for 3.21% Fe3+-doped ZnO nanocrystals recorded at 5 and 300 K. Inset of (d) shows M versus H curves for 3.21% Fe3+-doped ZnO nanocrystals recorded at 300 K at low field. 23674

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Table 2. Concentration of Fe in ZnO Nanocrystals, Absoption Peak, Coercivity (Hc), and Saturation Magnetization (Ms) of ZnO Nanocrystals at 5 and 300 K iron concentration by ICP-AES %

absorption peak position (nm)

Hc (Oe) 300 K

Ms (emu/g) 300 K

Hc (Oe) 5 K

Ms (emu/g) 5 K

ZnO

0.00%

355

96

0.008

442

0.019

ZnO: Fe ZnO:Fe

0.15% 2.18%

347 345

60 126

0.012 0.080

106 115

0.032 0.401

ZnO:Fe

3.21%

369

94

0.140

400

0.484

nanocrystals

Figure 5. M versus T curves (a) for undoped ZnO, (b) for 0.15% Fe3+-doped ZnO nanocrystals, (c) for 2.18% Fe3+-doped ZnO nanocrystals, and (d) for 3.21% Fe3+-doped ZnO nanocrystals recorded in temperature range from 5 to 300 K with an applied field of 1000 Oe.

dominant ferromagnetic exchange between Fe3+ ions for higher concentration of Fe in ZnO nanocrystals. Enhancement in both ferromagnetic and paramagnetic contributions due to incorporation of Fe3+ ions in ZnO nanocrystals is observed and is also reported earlier.31,32 Magnetizations versus temperature [M(T)] curves are shown in Figure 5a d. These curves show splitting between zero field cooled (ZFC) and field cooled (FC) conditions for applied field of 1000 Oe and therefore are related to the ferromagnetic behavior of the sample. Hump at 64 K [in Figure 5, panels a and b] may be due to contamination5 with molecular oxygen. Nature of M(T) curves is observed to slightly vary with increase in dopant concentration. The shape of M(T) curves is almost concave suggesting predominance of bound magnetic polaron interaction.33 37 Coey et al.36 reported that an electron trapped in the defect (oxygen vacancies) level creates an F center. Exchange interaction between neighboring magnetic ions mediated by this F center forms a bound magnetic polaron. Overlapping of such polarons contributes to long-range ferromagnetic ordering in doped nanocrystals. In the case of Ga1‑xMnxAs system, it is shown32 that, in the situation with itinerant carriers in metallic DMS, the temperature dependent magnetization curves are almost linear at low temperatures. As carrier density increases, the linear magnetization curves evolve toward outwardly convex magnetization. Further,

it is shown that the concave magnetization behavior arises from a combination of the strongly localized nature of carriers and low carrier density. The two drastically different shapes are shown to be two extremes of a continuum of magnetization behavior in DMS materials.33 35 Figure 5d shows decrease in magnetization in ZFC condition at temperature below 54 K for 3.21% Fe-doped ZnO nanocrystals. Such a decrease in M(T) is typical for the blocking temperature of the superparamagnetic clusters originated from the increase in magnetocrystalline anisotropy at low temperature, although no drastic reduction in coercivity is observed for 3.21% Fe -doped ZnO nanocrystals (Table 2). Hence decrease in M(T) in this case may be due to freezing of domain walls at low temperature.38 XRD, optical absorption, PL and EPR measurements suggest substituitonal incorporation of Fe3+ at Zn sites. No secondary or impurity phase is observable. Correlation of these structural, optical and magnetic properties indicate that, the formation of defect related polarons are responsible for observed ferromagnetism in case of doped ZnO nanocrystals.36,37 These measurements also suggest presence of relatively less number of defect levels in Fe-doped ZnO nanocrystals compared to Mn or Co-doped ZnO nanocrystals. Correlation of defect concentration was observed on magnetic properties of Fe, Mn and Co-doped ZnO nanocrystals. Table 1 shows that the saturation magnetic moment at same doping level is the highest 23675

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The Journal of Physical Chemistry C in case of Co-doped ZnO nanocrystals showing large number of defect levels and the lowest for Fe-doped ZnO nanocrystals showing comparatively less number of defects (as evident from PL measurements).

4. CONCLUSIONS Earlier, we reported ferromagnetic ordering at room temperature in ZnO and Mn as well as Co-doped ZnO nanocrystals. Fe-doped ZnO nanocrystals prepared by the same route are found to be ferromagnetic at room temperature albeit with reduced saturation magnetization. Structural and optical analysis did not show any secondary phase formation. Observed d d transitions in optical spectra affirm substitutional incorporation of Fe3+ at Zn sites. Shift in absorption edge as well as emission maxima with increase in Fe3+doping levels gives clear evidence of exchange interaction between Fe3+ ions with increase in doping level in nanocrystals. EPR analysis confirms existence of interacting Fe3+ along with noninteracting Fe3+ ions. Subtle enhancement in the saturation magnetization in Fe3+doped ZnO nanocrystals can be understood in terms of formation of F centers to produce charge neutrality by Fe3+ substitution at Zn2+ sites. Thus F center exchange mechanism is responsible for enhanced saturation magnetization in Fe3+doped ZnO nanocrystals. In fact, all three dopants, viz. Fe, Mn and Co reveal the same mechanism underlying ferromagnetic ordering as deduced from M(T) curves. ’ ASSOCIATED CONTENT

bS

Supporting Information. Calculation of saturation magnetization after subtraction of diamagnetic contribution. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT D.Y.I. thanks CSIR for the fellowship. TEM, ICP-AES, and EPR measurements were carried out at S.A.I.F., I.I.T., Mumbai. Absorption, and PL measurements were carried out at the DST nanoscience unit. The work in the U.S.A. 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-FG02-06ER46291). ’ REFERENCES (1) Chaboy, J.; Boada, R.; Piquer, C.; Laguna-Marco, M. A.; GarciaHernandez, M.; Carmona, N.; Llopis, J.; Ruíz-Gonzalez, M. L.; Gonzalez-Calbet, J.; Fernandez, J. F.; Garcia, M. A. Phys. Rev. B 2010, 82, 064411. (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.

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