Optical and Magnetic Properties of Fe Doped ZnO Nanoparticles Obtained by Hydrothermal Synthesis

Diluted magnetic semiconductors Zn 1−x Fe x O nanoparticles with different doping concentration (x = 0, 0.01, 0.05, 0.10, and 0.20) were successfully synthesized by hydrothermal method.The crystal structure, morphology, and optical and magnetic properties of the samples were characterized by X-ray diffraction (XRD), energy dispersive spectrometer (EDS), high-resolution transmission electron microscopy (HRTEM), Raman scattering spectra (Raman), photoluminescence spectra (PL), and the vibrating sample magnetometer (VSM). The experiment results show that all samples synthesized by this method possess hexagonal wurtzite crystal structure with good crystallization, no other impurity phases are observed, and the morphology of the sample shows the presence of ellipsoidal nanoparticles. All the Fe successfully substituted for the lattice site of Zn and generates single-phase Zn 1−x Fe x O. Raman spectra shows that the peak shifts to higher frequency. PL spectra exhibit a slight blue shift and the UV emission is annihilated with the increase of Fe concentration. Magnetic measurements indicated that Fe-doped ZnO samples exhibit ferromagnetic behavior at room temperature and the saturationmagnetization is enhanced with the increase of iron doping content.


Introduction
Diluted magnetic semiconductors (DMSs) are referred to as nonmagnetic semiconductors in which a small fraction of host cations are replaced by transition metal or rare-earth ions, with both spin and charge degrees of freedom in a single material [1,2].In recent years, DMSs have been under intense research due to their potential applications in spinbased multifunctional electronic devices, such as ultraviolet absorbers, optoelectronics, spin field-effect transistors, fieldemission devices, gas sensors, nonvolatile memory devices, and quantum computer [3][4][5][6][7][8].Of all the oxide diluted magnetic semiconductors, the transition metal (TM) doped ZnO has been identified as a promising one, because the host material ZnO is a chemically and thermally stable -type II-VI compound semiconductor with a wide band gap energy (3.37 eV) and a large exciton binding energy (60 meV) [9][10][11].During the past years, transition metal doped ZnO has been doped into the ZnO lattice to modulate the local electronic structure and cause dramatic changes in their optical and electromagnetic properties [12][13][14][15].Various methods have been used to synthesize TM doped ZnO nanoparticles: some experimental results observed the room temperature ferromagnetism in ZnO : Fe system, but the experimental results were contradictory, and the theoretical explanation about the origin of the magnetism is still not mature [16][17][18][19][20].However, there are very few reports on the magnetic behavior of transition metal doped ZnO synthesized using hydrothermal method.Thus, in this paper, our aim is to synthesize Zn 1− Fe  O nanoparticles with different concentration using hydrothermal method and to study their structural, optical, and magnetic properties.
In this paper, diluted magnetic semiconductors Zn 1− Fe  O nanoparticles with different doping consistency ( = 0, 0.01, 0.05, 0.10, and 0.20) were synthesized by hydrothermal method.And the crystal structure,

Experimental
Zn 1− Fe  O ( = 0, 0.01, 0.05, 0.10, and 0.20) nanoparticles were prepared by hydrothermal method.All materials were of analytical grade and used without further purification in this study.In a typical experiment, zinc acetate (Zn(CH 3 COO) 2 ⋅2H 2 O, 99.0%, AR) and ferric nitrate (Fe(NO 3 ) 3 ⋅9H 2 O, ≥99.0%,AR) were mixed together according to the required amounts of the corresponding chemical reagents and dissolved into absolute alcohol under magnetic stirring at room temperature.Then cetyltrimethylammonium bromide (CTAB, ≥99.0%,AR) was added to the above solution; meanwhile, sodium hydroxide (NaOH, ≥96.0%,AR) was dissolved into absolute ethanol and introduced into the above solution.After being stirred for 30 min, the mixture was transferred into a 45 mL Teflon lined stainless steel autoclave.The hydrothermal synthesis was heated to 180 ∘ C and maintained for 24 h in an oven and then cooled down to the room temperature naturally.The obtained products were washed with distilled water and absolute ethanol several times and dried at 60 ∘ C for 24 h.Finally, the samples were obtained.
The crystalline structures of the samples were examined by a rotating-target X-ray diffractometer (Japan Rigaku D/Max-2400) equipped with graphite monochromatized CuK radiation ( = 1.54056Å, 40 kV, 100 mA).The samples were scanned in the angular range from 10 to 90 ∘ (2) with scanning rate 0.005 ∘ /s and step size 0.02 ∘ .The morphology and microstructure of the samples were observed through transmission electron microscopy (TEM, Japan JEOL JEM 1200EX), the corresponding selected-area electron diffraction (SAED), and high-resolution transmission electron microscopy (HRTEM, JEM-2010).The chemical composition of the products was verified by energy dispersive spectroscopy (EDS) at an acceleration voltage of 200 keV in TEM.Raman spectral measurements at room temperature in the energy region between 100 and 1500 cm −1 were carried out using a micro-Raman spectrometer (HR800) with 525 nm line of He-Ne ion laser as exciting light source.Photoluminescence (PL) spectra were performed at 266 nm wavelength using a PerkinElmer spectrophotometer with a He-Cd laser as excitation source.Magnetic hysteresis loops were investigated at room temperature using a vibrating sample magnetometer (VSM, Lakeshore 7304) with an applied field from −12000 Oe to 12000 Oe.

Results and Discussion
The phase purity and crystal structure of the Zn 1− Fe  O samples were examined by XRD. Figure 1  0.20) nanoparticles.It is clearly observed from the pattern that all the diffraction peaks can be well corresponding to the diffractions of ( 100), ( 002), ( 101), ( 102), ( 110), ( 103), ( 200), ( 112), (201), and (202) planes of single-phase hexagonal wurtzite structure with the space group 6 3 mc, respectively.All the diffraction peaks are in good agreement with the values of the standard spectrum (JCPDS 01-079-2205).In addition, there are no traces of impurity or secondary phases within the detection limit of XRD and there are no obvious diffraction patterns of Fe species such as Fe, Fe 2 O 3 , and Fe 3 O 4 in the samples where the doping atomic percentage is from 1% to 20%.These results indicate that all Fe ions were incorporated in the lattice of the host crystals, the products consist of pure phase, and no characteristic peaks can be found from other impurities.For Fe-doped ZnO dilute magnetic semiconductors, the peak position of the doped ZnO samples shifts to lower angles compared with pure ZnO, and also there is a decrease in the intensity of peaks with the Fe doping concentration.This shifting as well as decrease in intensity of the characteristic peak clearly indicates the successful incorporation of Fe in the ZnO matrix, which can be attributed to the difference of ion radius of Zn 2+ (0.74 Å) comparing with that of Fe 3+ (0.64 Å).It is indicated that Fe ions occupy the Zn ions sites in the hexagonal wurtzite structure and no impurity phase appears.
The morphology and microstructure of the samples were observed through HRTEM.The images of all samples reveal the presence of randomly distributed ellipsoidal shape nanoparticles.Figure 2(a) shows the HRTEM image of the Zn 0.95 Fe 0.05 O; it can be seen that the sample consists of ellipsoidal shape particles, and well dispersed with smooth surface and uniform size, the diameter of the nanoparticles ranges from 10 to 40 nm, with an average diameter approximately 25 nm.The crystalline quality improves since CTAB promotes the establishment of a net surface charge onto the nanocrystals preventing their aggregation.which is in good agreement with the d-spacing of (002) planes of hexagonal wurtzite ZnO structure.The value of lattice spacing is greater than that of pure ZnO (0.26 nm).The HRTEM analysis is consistent with the XRD results, which further indicates that all the Fe ions successfully substituted for the lattice site of Zn 2+ in the hexagonal wurtzite structure and no impurity phase appeared.
In order to determine the elements of the prepared samples, EDS analysis was performed.Figure 3 shows the EDS patterns of Zn 0.95 Fe 0.05 O sample.The EDS spectrum indicates that the as-prepared nanoparticles mainly contain Fe, Zn, and O three elements.The characteristic peaks of O appeared at 0.5 keV and the characteristic peaks of Zn appeared at 1 and 8.7 keV.The characteristic peaks of Cu appeared at 0.9, 8 and 8.9 keV.The Fe signal at 0.7, 6.4, and 7.1 keV was observed in the Fe-doped ZnO nanoparticles.Apart from that, the Cu and C peaks in the EDS originate from the TEM micromesh grid.The EDS results further verify XRD conclusion; this indicates that Fe-doped ZnO diluted magnetic semiconductor was successfully synthesized by hydrothermal method and Fe 3+ in the sample occupied the place of Zn 2+ .
Raman spectra are a versatile technique for detecting the incorporation of dopants and the resulted defects and lattice disorder in the host lattice.In order to further confirm the phase transition and the defect chemistry variation after Fe-doped ZnO, Raman spectra of the as-prepared products were measured with an excitation wavelength of 532 nm at room temperature.Wurtzie structure ZnO as a -type II-VI semiconductor with the space group 6 3 mc, has zone center optical phononmodes: Γ opt =  1 + 2 1 +  1 + 2 2 [21], where  1 ,  1 , and  2 are Raman active and  1 is Raman forbidden. 1 and  1 modes are polar and split into the transverse optical (TO) and longitudinal optical (LO) phonons, and  2 mode consists of two modes:  2 high which is associated with the vibration of oxygen atoms and  2 low attributed to the Zn sublattices.
Room temperature Raman spectrum of all the synthesized samples is presented in Figure 4.The spectra reveal that all the major peaks which are the characteristics of vibrational modes in ZnO exist in all samples.For all the samples, Raman  peak at 332 cm −1 is attributed to the second order Raman mode arising from zone boundary phonons  2 high- 2 low, the peak at 381 cm −1 is assigned to the fundamental optical mode  1 TO, and the peak at 444 cm −1 is the  2 high mode, whereas peak at about 478 cm −1 is thought as originated from two phonon process, which correspond to  1 TO +  2 low mode.Moreover, for the Zn 0.99 Fe 0.01 O and Zn 0.95 Fe 0.05 O samples, a broad hump at 574 cm −1 is  1 LO symmetry modes [11,22,23].And for the Zn 0.90 Fe 0.10 O and Zn 0.80 Fe 0.20 O samples, a broad peak centered at 640 cm −1 is related to intrinsic host-lattice defects after doping and it is also a powerful evidence to prove Fe 3+ occupation at Zn 2+ sites in the ZnO lattice [24,25].Besides, with increasing the content of Fe 3+ doping in the samples, there is shift in all peaks towards higher frequency region, broaden asymmetrically and weakened peak intensity.This shift can be due to strain, phonon confinement by boundaries, and the force constant changes caused by the less defects or impurities.
In these modes, the  2 high peak in all the samples is attributed to the standard mode in wurtzite structure, which indicates that Fe-doped ZnO nanoparticles keep a good hexagonal ZnO structure.Moreover,  2 high mode is very sensitive to the stress in the crystal [24].From Figure 4, the peaks at 444 cm −1 of  2 high mode are lowered in the intensity with the more Fe content, which due to the radius of Fe 3+ (0.063 nm) smaller than radius of Zn 2+ (0.074 nm), and after being doped with Fe 3+ , it can cause the lattice expansion, defects, and the stress increase [26].
Figure 5 depicts PL spectra of Fe-doped ZnO nanoparticles at room temperature.For comparison, PL spectrum of undoped ZnO sample is also given.The pure ZnO sample contains six main emission peaks, including three obvious broad bands centered at 390, 440, and 490 nm and three comparatively weak emission peaks at 420, 460, and 525 nm, respectively.The 1% sample exhibits similar emission to pure ZnO.This illustrates that the Zn 0.99 Fe 0.01 O sample displays nearly the same structure of pure ZnO.But the emission peak position of doped sample exhibits a slight blue shift with the increase of Fe 3+ concentration, and this is attributed to defects or the oxygen vacancies in ZnO induced by Fe 3+ doping.
In addition, the visible light (VL) emission band is also suppressed with the increase of the Fe 3+ concentration.The strong ultraviolet (UV) emission band at 390 nm (3.18 eV) and the weak violet emission peak located at 420 nm (2.95 eV) originate from excitonic recombination corresponding to the near-band-edge emission (NBE) of wide band gap of ZnO, because of the quantum confinement effect [27].The strong blue emission bands centered at 440 nm (2.8 eV) and the weak blue emission peak at 460 nm (2.69 eV) are attributed to the transitions between the shallow donor levels of the interstitial Zn to the top of the valence band [14] or the transitions between shallow acceptor levels of oxygen vacancies and shallow donor levels of zinc vacancy [28].The blue-green bands centered at 490 nm (2.55 eV) for samples are ascribed to the transition between the deep donor level of oxygen vacancy, which is close to the bottom of the conduction band of about 0.8∼0.9eV, to the valence band [28,29].The weak green emission peak at 525 nm (2.36 eV) is possibly assigned to positively charged electron transition and surface traps mediated by defects in the band gap [30].PL spectrum analysis further illustrates that Fe ions are probably incorporated into the ZnO host matrix in the doped ZnO sample.
The related PL intensity of doped sample decreases rapidly with the increase of Fe doping concentration compared with undoped ZnO, and the emission peak position intensity of 20% Fe-doped ZnO sample was annihilated; this is because the photogenerated electrons preferentially occupy the Fe ion induced trap centers, resulting in the quenching of luminescence [14].
Figure 6 shows the magnetic hysteresis (M-H) loops of the Zn 1− Fe  O nanoparticles with different doping consistency ( = 0, 0.01, 0.05, 0.10, and 0.20) measured at room temperature.The magnetic hysteresis loops of ZnO and lower doped ZnO ( = 0.01 and 0.05) samples pass origin of coordinates, and the remanent magnetization (M r ), and coercivity (H c ) are zero.It suggests that these samples show paramagnetic behaviors at room temperature.While the higher doped ZnO ( = 0.10 and 0.20) samples exhibit obvious ferromagnetic behaviors.The saturation magnetization values are 0.74 emu/g and 1.74 emu/g and the coercive force values are 90 Oe and 78 Oe, respectively, for ZnO ( = 0.10 and 0.20).It is evident from Figure 6 that a transition from the paramagnetic state to the ferromagnetism state occurs.The saturation magnetization of Zn 1− Fe  O nanoparticles increases with the increasing of Fe doping concentration compared with pure ZnO.
The room temperature ferromagnetism of the Zn 1− Fe  O nanoparticles could arise from two possible sources.One is extrinsic magnetism and the other is intrinsic magnetism.Extrinsic source includes the formation of clusters of transition elements or secondary phase.Exchange interactions come under intrinsic source of magnetism.But the XRD results of the Zn 1− Fe  O nanoparticles suggest no traces of impurity or secondary phases (these parasitic phases include the Fe clusters and any other phases like Fe 2 O 3 , Fe 3 O 4 , etc.).Thus, the possibility of ferromagnetism due to the clusters of transition elements or secondary phases in the samples could be ruled out.Hence, the obtained ferromagnetism is an intrinsic magnetic property of the Zn 1− Fe  O nanoparticles.It is evident from the XRD analysis that Fe is incorporated into the ZnO lattice.In view of the Fe 3+ ions substituted into ZnO lattice, the origin of magnetism in the samples is due to the exchange interaction between local spinpolarized electrons (such as the electrons of Fe 3+ ions) and the conductive electrons.Such interaction can lead to the spin polarization of conductive electrons.Consequently, the spin-polarized conductive electrons undergo an exchange interaction with local spin-polarized electrons of Fe 3+ ions.Thus, after a successive long-range exchange interaction, almost all Fe 3+ ions exhibit the same spin direction, resulting in the ferromagnetism of the material [31].

Conclusions
(1) Diluted magnetic semiconductors Zn 1− Fe  O nanoparticles with different concentration ( = 0, 0.01, 0.05, 0.10, and 0.20) have been synthesized by hydrothermal method.The samples possess hexagonal wurtzite crystal structure with good crystallization, no other impurity phase appeared, and the morphology of samples is ellipsoidal shape nanoparticles with good dispersion.
(2) All the Fe 3+ successfully substituted for the lattice site of Zn 2+ and generates single-phase Zn 1− Fe  O.The Raman spectra of the Zn 1− Fe  O nanoparticles occurs red shift, and the UV emission of photoluminescence spectra is annihilated with the increase of Fe 3+ concentration.The ferromagnetic behaviors are found for all doped samples at room temperature.
Figure 2(b) shows a typical HRTEM image of Zn 0.95 Fe 0.05 O nanoparticles.The resolved lattice plane extending through the image confirms that the particle is crystallized.

Figure 6 :
Figure 6: The magnetic hysteresis loops of the Zn 1− Fe  O nanoparticles.