Optical and Magnetic Properties of Zn1âxMnxO Nanorods Grown by

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Optical and Magnetic Properties of Zn MnO Nanorods Grown by Chemical Vapour Deposition The-Long Phan, and Seong-Cho Yu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp312080v • Publication Date (Web): 28 Feb 2013 Downloaded from http://pubs.acs.org on March 17, 2013

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The Journal of Physical Chemistry

Optical and Magnetic Properties of Zn1-xMnxO Nanorods Grown by Chemical Vapour Deposition The-Long Phan, and S. C. Yu* Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea

Abstract We prepared successfully Zn1-xMnxO nanorods (x = 0, 0.008, 0.013, and 0.020) by using chemical vapour deposition. Their crystal-structural, optical and magnetic properties at room temperature were then investigated in detail. Experimental results have revealed that all the samples have a single phase with the wurtzite structure, and the lattice parameters slightly increase with increasing Mn-doping content. This is ascribed to an incorporation of Mn2+ into the Zn site of the ZnO host lattice. The Mn incorporation also creates more lattice defects and disorders, which influence directly characteristic photoluminescence (PL) and Raman scattering (RS) spectra of Zn1-xMnxO nanorods. For example, there are the blue shift of ultra-violet emission, and the rapid decline of visible PL emission when Mn-doping content (x) in Zn1-xMnxO increases. Besides conventional phonon-vibration modes, there are additional ones associated with defects and Mn impurities. The origin and nature of these additional modes is identified and assessed carefully. Interestingly, under the resonant-RS condition we have recorded the overtones/replicas of the longitudinal-optical phonon up to the seventh order. Magnetic measurements have been also performed, which reveal Zn1-xMnxO nanorods with x = 0.013 and 0.020 exhibiting the room-temperature ferromagnetic order. We believe that the ferromagnetic nature is related to both Mn2+ ions and intrinsic defects of zinc vacancies, rather than Mn-related secondary phases.

Keywords: Zn1-xMnxO nanorods; Photoluminescence; Raman scattering; Magnetism *Corresponding author: Electronic mail: [email protected] Phone: +82-43-261-2269; Fax: +82-43-275-6416 1

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1. INTRODUCTION Concerning a new research direction of dilute magnetic semiconductors (DMSs), recently theoretical and experimental studies have revealed that room-temperature ferromagnetism can be found in wide band-gap semiconductors (such as GaN and ZnO) doped with transition metals.1-4 These metals have partially filled 3d states, and contain unpaired electrons, which are thus responsible for the magnetic behaviour in DMSs. Basically, when a transition-metal ion substitutes for the cation of a semiconductor host lattice, the resultant electronic structure is strongly influenced by the hybridization between the 3d orbitals of the magnetic ion and the p orbitals of neighbouring anions. This hybridization can lead to a magnetic interaction between the localized 3d electrons and the carriers in the valence band of the host lattice.2 The finding of effective and optimal mechanisms to achieve the ferromagnetic (FM) order with a great magnetization value at room temperature in this material system is currently an issue of interest. Due to many pre-eminent properties of ZnO (such as a wide bandgap energy of Eg ≈ 3.37 eV, a large exciton binding energy of 60 meV, a high luminescence efficiency, transparency, piezoelectricity, simple growth/fabrication, and the exhibition of many novel nanostructures),2,5-7 ZnO-based DMSs have attracted intensive interest, particularly for two material systems of Mn- and Co-doped ZnO.1,2,4,7-12 Reviewing earlier reports on these materials, one can see that their magnetic properties shown out by different research groups are very contradictory. The origin and nature of ferromagnetism in Mn- and Co-doped ZnO materials accordingly becomes more complicated and unclarified. For Mn-doped ZnO, the Mn2+ substitution for Zn2+ could give a great magnetization value if this material system exhibits the FM behaviour since Mn2+ in the half-filled 3d-shell has 5 spins, which can give a maximum moment value of 5µB/Mn (µB is the Bohr magnetron).13 However, the magnetism observed in Mn-doped ZnO depends remarkably on sample types (i.e., bulk single crystal,

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polycrystalline ceramics, thin films or nanostructures), fabrication and processing conditions (such as the heat treatment, and annealing ambient and pressure), and Mn-doping concentration. In general, polycrystalline ceramics processed at temperatures below 700 oC exhibits the room-temperature FM order,4,9,11,14-19 and at higher annealing temperatures (T > 700 oC)4,9,15-17,20-22 become paramagnetic (PM), anti-FM, or spin-glass. For Mn-doped ZnO thin films prepared by RF-magnetron sputtering,2329

and pulsed laser deposition,4,9,11,13,29-36 most of them show the room-temperature FM order. For

nanostructured samples (such as nanoparticles, nanowires, nanorods, tetrapods, etc.), their magnetic properties reported vary from the diamagnetic (DM),37,38 through low-temperature FM (below 100 K),39-47 to high-temperature FM order (T > 300 K).48-57 As indicated above, many research works were performed to prepare and investigate the optical, magnetic and magneto-transport properties of Mn-doped ZnO. However, most of them focused on Mn-doped ZnO bulks, films and nanoparticles, there are not many works focusing on the Mn-doped ZnO nanowires/nanorods.38,42,43,49-56 Furthermore, their magnetic mechanism and optical properties (particularly for Raman scattering spectra associated with additional modes, and longitudinal-optical phonon replicas) have been not assessed in detail. To get more insight into this material system (i.e., Mn-doped ZnO nanowires/nanorods), we have prepared Zn1-xMnxO nanorods, and then studied carefully their structural characterization, and optical and magnetic properties. Our experimental results reveal an existence of the room-temperature ferromagnetism in Zn1-xMnxO nanorods with x = 0.013 and 0.02. The FM order increases with increasing Mn-doping content. Basing on the results obtained from analyzing the structural and optical properties, and comparing with recent reports on ZnO-based DMS materials,58,59 we believe that the ferromagnetism in the nanorods is mainly due to exchange interactions related to Mn2+ ions and intrinsic defects of zinc vacancies. Interestingly, the Raman scattering (RS) spectra under the resonant condition introduce the replicas of the longitudinaloptical (LO) phonon up to the seventh order. This is related to a strong decrease in the intensity of near-band-edge luminescence emission, and to intrinsic defects created massively by Mn-doping. 3



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2. EXPERIMENTAL SECTION 2.1. Initial materials. Powdered initial chemicals including Zn (99.99%, melting point ~420 o

C), and MnCl2⋅4H2O (99 %, melting point ~58 oC, and boiling point ~198 oC) were purchased from

Aldrich, and used as received from commercial sources, without further purification and/or treatment. 2.2. Synthesis of Zn1-xMnxO nanorods. Zn1-xMnxO nanorods were prepared from Zn and MnCl2⋅4H2O powders by using chemical vapor deposition (CVD). Weight ratios of Zn to MnCl2⋅4H2O with Zn:MnCl2⋅4H2O = 95:05, 85:15, and 75:25 were mixed to fabricate ZnO nanorods doped with various Mn concentrations; pure ZnO nanorods were prepared by using Zn powders only. These mixtures (vapor sources) were arranged in ceramic boats, and in turn placed at the centre of a horizontal quartz tube furnace, which was connected to a vacuum-diffusion pump. Clean Si(001) substrates were arranged around the vapor source. Ar gas was introduced to the tube furnace as a carrier at a flow rate of 150 sccm (standard cubic centimeters per minute) to keep a working pressure of about 150 Torr (~0.2 bar). Before the deposition, air in the tube was removed by backfilling several times with Ar gas and then pumping out until the pressure in the tube was reduced to ~10-3 Torr. The nanorod growth was carried out at temperatures of 450~500 oC for 60 minutes. During the growth of Zn1-xMnxO nanorods, the following reactions could be taken place Zn (solid) → Zn (vapour)

(1)

MnCl2⋅4H2O → MnCl2 + 4H2O ↔ Mn (vapour) + 2HCl + 2H2O + O2

(2)

(1-x)Zn (vapour) + xMn(vapour) + ½O2 → Zn1-xMnxO (seed)

(3)

Zn1-xMnxO (seed) → Zn1-xMnxO (crystal).

(4)

For the first process (Eqs. 1 and 2), metal vapors of Zn and Mn are created due to the vaporization of Zn, and decomposition of MnCl2⋅4H2O under thermal activation energy. The decomposition of MnCl2⋅4H2O also creates oxygen, which leads to combustion reaction with Zn and Mn vapors in

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order to form Zn1-xMnxO seeds (Eq. 3). In favorable conditions of temperature, pressure and substrates, Zn1-xMnxO nanorods will develop directly from these seeds (Eq. 4). 2.3. Measurements of the crystal structural, optical and magnetic properties. After fabrication, the structural characterization and surface morphology of Zn1-xMnxO nanorods were checked by an X'Pert Philips diffractometer (used an X-ray radiation source of Cu-Kα, with λ = 1.5406 Å), field-emission scanning electron microscopy (FE-SEM, JEOL-JSM 6330F), and transmission electron microscope (TEM, JOEL 2010). The concentration of Mn in nanorods was determined by energy dispersive X-ray (EDX) spectroscopy linked with both TEM and FE-SEM. Photoluminescence (PL) and Raman scattering (RS) measurements were performed on Renishaw spectrometers by using two wavelengths of 488 and 325 nm as excitation sources. Magnetic measurements were performed on a superconducting quantum interference device (SQUID) magnetometer. All measurements were carried out at room temperature.

3. RESULTS AND DISCUSSION 3.1. SEM, TEM and XRD studies. With the above weight ratios and fabrication conditions, we collected Mn-doped ZnO nanorods, which deposited on Si substrates. In Figure 1, it shows representative FE-SEM images with different magnifications for Mn-doped ZnO nanorods with the weight ratio of Zn:MnCl2⋅4H2O = 85:15. Vertically well-aligned nanorods with 100~150-nm diameters grown on the substrates are observed clearly. Basing on TEM, the study of electrondiffraction patterns taken for individual nanorods near the [11-20] zone axis, Figure 2(a), confirmed that nanorods with ~500-nm lengths and a hexagonal structure developed along the c = [0001] axis. A continuous variation in the contrast between light and shade regions for bright- and dark-field TEM images (as titling the sample holder about few degrees) demonstrated the nanorods having good quality in single crystal. No trace of Mn-related secondary phases was detected. The incorporation of

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Mn into ZnO nanorods was confirmed by EDX spectroscopy, as shown in Figure 2(b). By recording EDX spectra statistically at different positions and large area of nanorods, we obtained averaged Mn concentrations in Zn1-xMnxO nanorods to be x = 0.008, 0.013, and 0.020 for Zn:MnCl2⋅4H2O = 95:05, 85:15, and 75:25, respectively. These values are in good agreement with those obtained from EDX spectroscopy equipped with FE-SEM. For each nanorod, the change in Mn concentration measured at different points is about 5 %, revealing fairly uniform distribution of Mn. Here, one can see that estimated Mn amounts in ZnO nanorods are much lower than nominal doping rates. Such situation was also observed by Liu el al.,44 which is due to the loss of sublimated Mn vapors caused by carrying out the growth process at reduced pressure.43-45 In addition to FE-SEM and TEM studies, structural characterization of Zn1-xMnxO nanorods was further investigated by X-ray diffraction (XRD). Their XRD patterns recorded are shown in Figure 3. There is a strong peak at about 34.3o, which corresponds to the (002) plane of wurtzite ZnO. This one again proves that nanorods grow along the c = [0001] axis, and are mostly perpendicular to the Si substrates. Particularly, with increasing Mn-doping concentration, the (002) peak slightly shifts towards lower diffraction angles (see the inset of Figure 3). This is assigned to the substitution of Mn2+ for Zn2+ in the ZnO host lattice because the ionic diameter of Mn2+ (0.80 Å) is greater than that of Zn2+ (0.74 Å).7,56,60 Basing on the XRD data, we also determined the lattice parameter c for the samples with x = 0.008, 0.013, and 0.020 to be 5.233, 5.235 and 5.239 Å, respectively, while that of pure ZnO nanorods is about 5.232 Å. Clearly, the parameter c slightly increases with increasing the Mn content in Zn1-xMnxO nanorods. 3.2. Optical properties. Figure 4 shows photoluminescence (PL) spectra of Zn1-xMnxO nanorods excited by 325 nm (3.84 eV) from a helium-cadmium (He-Cd) laser. For the nanorods with x = 0, its PL spectrum consists of two ultra-violet (UV) and visible emissions peaked at 385 and 525 nm, respectively. Intrinsically, the former is associated with excitonic near-band-edge luminescence emission while the latter is associated with lattice defects and impurities.50,54,55 The relative intensity 6


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of the UV emission is about twice greater than that of the visible one. However, with increasing the Mn-doping content in Zn1-xMnxO nanorods, the visible emission intensity quickly decreases and becomes zero as x ≥ 0.013. Meanwhile, dominant UV emission gradually decreases. This is due to the fact that the incorporation of Mn dopants into the ZnO host lattice creates more lattice defects, which act as electron trappers and color centers with energy levels lower than the band-gap energy Eg. Both the UV and visible emissions are re-absorbed massively by these trappers/centers. In addition to the decrease in luminescent intensity, the UV-peak position is shifted towards shorter wavelengths when Mn concentration in Zn1-xMnxO increases (the UV emission is about 374 nm for x = 0.020). As shown in the inset of Figure 4, while there is a significant shift of the UV peak with the Mn doping increase from x = 0 to 0.008, a slower shift happens to the samples with x ≥ 0.013. This is related to a strong decrease in the visible emission intensity induced by a high defect density, which is assigned to be a nonlinear function of Mn-doping concentration in Zn1-xMnxO nanorods. Herein, the blue shift of the UV emission is ascribed to an expansion of the band-gap width that its physical mechanism is still an issue for debate.61-64 It was suggested that the band-gap expansion in Zn1-xMnxO was due to the presence of MnO, where its Eg ≈ 4.2 eV is greater than that of ZnO.63 However, this supposition is not reasonable to our case because no Mn-related secondary phases were found from the TEM and XRD studies. Taking into account for recent reports on Mg-doped ZnO materials,58,64 the band-gap expansion has been also observed, and is demonstrated to be the formation of zinc vacancies, which act as acceptors and cause the change of band structure. We thus believe that the same situation happen to our Zn1-xMnxO nanorods. These zinc vacancies play an important role in establishing a weak FM order in both doped and undoped ZnO materials.58,64-66 Notably, for the samples with x = 0.013 and 0.02, besides the UV emission, there are some narrow peaks, see Figure 4. They are resonant RS lines, as being discussed below.

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In an attempt to understand the influence of Mn-doping content on lattice vibrations of Zn1xMnxO,

we have investigated their RS spectra in the back-scattering configuration. Under an

excitation energy of 2.54 eV (488 nm) less than Eg values of Zn1-xMnxO (i.e., the off-resonant RS condition), the RS spectrum of pure ZnO (x = 0) exhibits conventional vibration modes peaked at 332, 380, 411, 438 and 583 cm-1 corresponding to processes of E2(H)-E2(L), A1(TO), E1(TO), E2(H) and 1LO (longitudinal-optical phonon), respectively,67,68 see Figure 5. Among them, E2(H) characteristic of the wurtzite ZnO structure has the strongest intensity. These modes persist when ZnO is doped with a small Mn amount of x = 0.008. Concurrently, there is the presence of some additional modes in the wavenumber range of 450-600 cm-1 peaked at about 473 (AM1), 523 (AM2) and 570 cm-1 (AM3). With higher Mn-doping concentrations (x ≥ 0.013), excepting E2(H) and 1LO, the other conventional modes are almost invisible. Particularly, E2(H) with the decreased intensity broadens asymmetrically and shifts towards lower frequencies. Meanwhile, the intensity of the 1LO and AM1-3 modes rapidly increases. Basically, the phenomena observed for E2(H) could be explained by the spatial correlation (SC) model.69 The SC function of phonons in a perfect crystal with the translational symmetry is infinite and in a form of the plane wave, which obeys normal selection rules with q = 0. However, a random substitution of Mn in the ZnO host lattice causes microscopictopological and structural disorders in the periodic Zn-atomic sub-lattice (partly due to zinc vacancies as mentioned above), and broke translational symmetry.28,70 Consequently, the mode correlations become finite and the q = 0 selection rule is relaxed. The contribution of phonons with q ≠ 0 to Raman scattering leads to the red-shift, asymmetrical broadening, and intensity decrease of E2(H). This phenomenon was also observed in some transition-metal-doped ZnO materials.8,71-73 Concerning the AM1-3 modes, their accurate origin is still a matter of controversy. It has been assigned AM1 to a Mn-related local, surface/interface or disorder-activated phonon mode.8,62,74,75 In fact, this mode is also observed in Co-doped, and (Mn, Co)-, (Li, Na)- and (Co, Al)-codoped ZnO

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materials,8,76 which is probably related to dopants located in interstitial sites of the ZnO lattice. For the AM2 mode, it is observed in all Mn-doped ZnO materials and considered as an indication for the Mn incorporation into the ZnO host lattice,35,38,73,75,77 which is assigned to an impurity-related vibration, depending on the nature of the Mn dopant. For AM3, many authors7,38,73,75,77 suggested it to be the red shift of 1LO (consisted of two vibrations E1(LO) = 583 cm-1 and A1(LO) = 574 cm-1, see Ref. 67). However, three following important features make us believed that AM3 is not related to the red shift of 1LO: (i) a large deviation between AM3 and 1LO (~13 cm-1), (ii) a small red shift of ~4 cm-1 in the LO-phonon position observed in resonant RS spectra (as indicated below), and (iii) it usually appears ZnO materials doped with Mn, Co and/or Ni,38,50,71,73,74,78-80 but not in those doped with Cu, Al, and Fe.71,81,82 This reflects that AM3 is a vibration activated by lattice defects and/or disorders, which is associated with the intrinsic of impurity types. If Zn1-xMnxO nanorods are excited by 3.84 eV (325 nm) above their band gap, resonant Raman scattering (RRS) occurs. Under this condition, incident and scattered phonons resonate with real electronic states in materials.83-85 It is different from the normal RS spectra, the RRS spectra exhibit only the LO phonon and its overtones (replicas), as shown in Figure 6, which have a broad background associated with excitonic near-band-edge luminescence emission. The first LO mode (1LO, n = 1) of pure ZnO is peaked at ~580 cm-1, and shifts about 4 cm-1 towards lower frequencies for the x = 0.020 sample. This shift is about three times shorter than the case of the AM3 mode in the normal RS spectra (as mentioned above), proving an independent appearance of the 1LO and AM3 modes. Interestingly, while pure ZnO nanorods exhibit only three LO overtones (n = 1-3), the Mncontent increase in Zn1-xMnxO enhances the number of multiple LO-phonon lines up to the seventh order (n = 1-7) for the x = 0.020 sample, see Fig. 6. The distance between two neighboring LOphonon modes is about 70 meV (~573 cm-1), which is very close to values reported recently on ZnOrelated materials.71,84,86,87 If reviewing carefully previous reports on the RRS spectra of ZnO-based materials, it can be realized that the number of multiple LO-phonon lines depend on many factors, 9



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including temperature measurement,83,84,88 crystal structures and morphology (i.e., a single-crystal or polycrystalline bulk, thin films, nanoparticles, nano-wires/rods, crystallite sizes ect.),89-92 the heat treatment conditions,84,90,91 concentration of carriers,84 the density of defects,93 and concentration and nature of impurities.71 For pure ZnO materials, it is usually observed LO-phonon replicas with n = 15.83,84,89-91 Very few works recorded LO-phonon replica up to the eighth order (n = 6-8) in a ZnO single crystal at room temperature, and thin films at low temperatures.88,94 The presence of dopants (such as N, Mn, Cu, Co, and Ni) and surface-coating layer (related to plasmon-related surface scattering) in ZnO materials stimulate the LO-phonon replicas higher than the fifth order (n > 5).71,72,85-87,92,95,96 Concerning Mn-doped ZnO materials, very few previous works recorded the highest replica with n = 6.85,86 However, with an appropriate Mn-doping content of x = 0.020 in Zn1xMnxO

nanorods, we have recorded the LO-phonon replicas up to the seventh order. It is suggested

that an adjustment in impurity concentration (associated to the change in the density of lattice defects and disorders) and in the luminescence intensity to an appropriate threshold is necessary to observe high LO-phonon replicas in Mn-doped ZnO materials. Concerning the RRS spectra, their feature is explained within the cascade model97 in which the cascade relaxation of an exciton with three sequential steps takes place as follows: (i) the creation of an exciton via absorption of the incident photon (ħωi = 3.84 eV), (ii) the non-ratiative relaxation of this exciton into lower energetic levels with successive emission of LO phonons, and (iii) the radiative recombination of the exciton with the emission of scattered phonons (ħωs ≈ ħωi - nħωLO). Such the processes are also known as exciton-mediated multiple LO-phonon resonant scattering, which is tightly related to electron-phonon interaction. The influence of phonons on the motion of electrons is thus considered as a perturbation. For the wurtzite Mn-doped ZnO system, two important types of electron-phonon interaction are the Fröhlich interaction with LO phonons, and deformationpotential interaction with both optical and acoustic phonons; piezoelectric interaction with acoustic

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phonons can be ignored because of the absence of an external force. The Fröhlich interaction is associated with the Coulomb interaction between electrons and the longitudinal electric field produced by LO phonons. Meanwhile, the deformation-potential interaction is caused by the displacement of parent atoms (i.e., Zn and O) from their equilibrium positions due to Mn impurities, which affects directly the electronic band structure. It has been pointed out that the number of LOphonon replicas (n) is proportional to the polaron coupling constant (α) of a material, and the maximum frequency shift (nωLO) is comparable to the deformation potential ( 12 αħωLO).94 A large frequency shift of n = 5-7 demonstrates a great deformation potential existing in our Zn1-xMnxO nanorods. On the other hand, more Mn impurities, and lattice defects and disorders introduced to the ZnO host lattice cause the displacement of parent Zn and/or O atoms from their equilibrium positions. Additional contributions of Mn- and defect/disorder-related scatters influence strongly the intensity of LO-phonon replicas (as well as Mn-related lattice vibrations shown in Fig. 5, as discussed above). Within the Franck-Condon approximation, the electron-phonon coupling strength is proportional to the Huang-Rhys factor (S) in an expression of In = I0Sne-S/n!, where In and I0 are the integrated intensities of the nth LO-phonon replica and main emissions, respectively.98 This reflects that the electron-phonon coupling strength can be also accessed through the ratio of I2LO/I1LO (meaning S ∝ 2I2LO/I1LO, for n = 1 and 2). In the upper inset of Figure 6, it plots the dependence of I2LO/I1LO on Mn content in Zn1-xMnxO nanorods. Excepting the x = 0 and 0.008 samples, I2LO/I1LO (and thus the electron-phonon coupling strength) decreases gradually with increasing Mn content. This is due to the fact that Mn impurities, and lattice defects and disorders introduced to Zn1-xMnxO nanorods act as electron-trapping centers to obstruct the electron-phonon coupling. Similar results have been recently found in Mn-diffused ZnO nanorods, Co-doped ZnO ceramics, and N-doped ZnO thin films.50,79,95 However, for ZnO nanorods doped with a low Mn content (x = 0.008), there is an increasing trend of the electron-phonon coupling (I2LO/I1LO ≈ 4.0) compared to that of pure ZnO nanorods (I2LO/I1LO ≈ 11



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3.5), see the upper inset of Figure 6. Such the scenario has been recently found in Mn-doped ZnO nanowires,92 and is thought to the nonlinear dependence of the electron-phonon coupling versus the defect density. Clearly, there is a concentration threshold of the Mn doping in Zn1-xMnxO that the Mn-doping at concentrations higher than the threshold value obstructs the electron-phonon coupling strength. More recently, Phan and Du et al.72,91 studied carefully pure ZnO and Mn-doped ZnO nanostructures annealed different temperatures, and found an gradual increase in the electron-phonon coupling (i.e., I2LO/I1LO) with increasing the annealing temperature (Tan) from 300 to ~800 oC due to the improvement of the crystal quality versus the annealing. The Tan increase to higher temperatures (> 800 oC), however, creates more lattice defects, and thus decreases the electron-phonon coupling.99 The crystallite-size decrease also results in a similar phenomenon, as discussed in detail by Cheng and co-workers.89 3.3. Magnetic properties. The above structural and optical analyses have revealed the presence of Mn2+ ions and lattice defects in Zn1-xMnxO nanorods. Their concentration increases with increasing Mn-doping content, influencing directly the magnetic properties. To perform in-plane magnetic measurements, two sample pieces of ~3.5×3.5 mm2 and a piece of ~2.5×2.5 mm2 (corresponding to a total sample area of ~154×10-6 cm2) were combined carefully and put in a plastic straw. Because there is not straightforward to determine the volume of nanorods accurately, we thus estimate an average volume for the total measured sample area, with a thickness of ~500 nm. In Figure 7, it shows magnetic-field dependences of magnetization, M(H), in the emu/cm3 unit for the samples recorded at 300 K; the measured data in the emu unit is also attached for reference. There is a remarkably change in magnetism as varying Mn-doping concentration in Zn1-xMnxO. While the samples with x = 0 and 0.008 are almost DM, the others with x = 0.013 and 0.02 exhibit weak FM order (where the PM contribution was subtracted). The existence of the DM properties in the x = 0 and 0.008 samples are completely understandable because both Zn2+ and O2- are non-magnetic

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ions.100 With a small doping amount of x = 0.008, Mn2+ ions mainly exist as PM isolated centers. Very weak FM coupling between Mn2+ ions via zinc vacancies are available and responsible for the nonlinear M(H) curve at low applied fields, as can be seen in Figure 7. Recent reports have also recorded FM order in pure ZnO nanostructures, and assigned it to be generated from intrinsic defects of zinc vacancies.65,101,102 The absence of defect-induced FM order in pure ZnO nanorods once again proves their high crystal quality. In regard to the magnitude of two FM samples, the saturation magnetization (Ms) slightly increases with increasing Mn content in Zn1-xMnxO. The values of Ms ≈ 1.0 emu/cm2 (1.4×10-5 emu, ~0.012 µB/Mn) for x = 0.013, and Ms ≈ 1.3 emu/cm3 (1.7×10-5 emu, ~0.014 µB/Mn) for x = 0.02 are comparable to those obtained for Mn- and Co-doped ZnO samples.7,103,104 Meanwhile, the coercive field (Hc) values obtained for x = 0.013 and 0.02 are about 165 and 382 Oe. As shown in Table 1 for Mn-doped ZnO nanorods/nanowires reported recently, their magnetic property varies from the PM,50,105,106 through the anti-FM and low-temperature FM (< 50 K),38,42,43,47,50 to the hightemperature FM (≥ 300 K),7,49,51-55,107,108 depending on sample fabrication routes and conditions. A change related to surface coating is an effective way to cause FM order.105 Such the similar circumstance also happens for Co-doped ZnO nanorods/nanowires,104,106,108-111 as shown in Table 1 for the comparison. Their Ms and Hc values are also in the magnitude scale of the Mn-doped ZnO system. In general, the Ms values obtained from Mn-doped ZnO nanorods/nanowires at room temperature are much smaller than the theoretical value of 5 µB/Mn for a free Mn2+ ion.13 This demonstrates anti-FM interactions usually taking place between Mn ions in Zn1-xMnxO nanorods/nanowires. To get high Ms and Hc values, more recently, Wang and co-workers112 have recommended another DMS system of Nd-doped ZnO nanowires. Their experimental and theoretical works pointed out that Nd-doped nanowires exhibited stable room-temperature ferromagnetism with

13



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Page 14 of 33

Ms = 4.1 µB/Nd and Hc = 780 Oe, which are much higher than those shown in Table 1 for both systems of Mn- and Co-doped ZnO nanorods/nanowires. Concerning the origin of the room-temperature FM order in our samples, it could not be from Mn-related secondary phases because no trace of their presence was confirmed by the structural analyses, to the limit of the TEM and XRD tools. If Mn-related secondary phases (such as MnO2, Mn2O3, Mn3O4, MnO, (Mn,Zn)Mn2O4 spinels, and/or cubic ZnMnO3) are present in our Zn1-xMnxO nanorods, they only exhibit a weak FM order at temperatures below 130 K, as can be seen in Table 2. It is thought that the Mn-doping at high concentrations (x = 0.013 and 0.020) induces more zinc vacancies to the ZnO host lattice. This leads to the band-gap expansion (as mentioned above), and promotes the FM couplings associated with super-exchange interaction of Mn2+-Mn2+ pairs, and with weak double-exchange interaction of Mn2+ ions mediated by zinc vacancies. Recently theoretical and experimental evidences have also indicated the vital role of zinc vacancies in introducing the FM order in ZnO-based DMSs.58,59,65,112 Higher Mn-doping concentrations in Zn1-xMnxO nanorods are thus expected to give higher Ms values, making them more suitable to spintronic applications.

Table 1: Magnetic properties of Mn-doped ZnO nanorods/nanowires reported in recent studies. Some Co-doped ZnO nanorods/nanowires are shown for the comparison. Composition

Preparation method

Preparation temperature

Zn0.992Mn0.008O Zn0.987Mn0.013O Zn0.98Mn0.02O

CVD

450~500 oC

Mn-doped ZnO

CVD

Mn-doped ZnO

CVD

700 oC

Treatment

-

Magnetic property

Magnetization

Hc

Ref.

(Oe)

DM FM > 300 K FM > 300 K

0.014 (µB/Mn)

165 382

Our work

0.012 (µB/Mn)

-

FM > 300 K

0.273 (µB/Mn)

70

53

o

-

FM > 300 K

0.39 (µB/Mn)

50

54

o

0.87 (µB/Mn)

50

55

650 C

Mn-doped ZnO

CVD

650 C

-

FM > 300 K

Mn-doped ZnO

CVD

600 oC

-

Anti-FM

Zn1-xMnxO (x = 0.01-0.04)

CVD

-

-

FM > 300 K

0.3-1.2 (µB/Mn)

Zn0.983Mn0.017O

CVD

850 oC

-

FM < 44 K

1.88 (µB/Mn)

14


-

38 40~50

51

90

43

Page 15 of 33

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Zn0.87Mn0.13O Zn0.9Mn0.1O


CVD CVD

500 oC

o

600-700 C o

Zn0.95Mn0.05O

Solvothermal

160 C

Zn0.95Mn0.05O

Solvothermal

-


Hydrothermal (Nanoparticles)

FM < 37 K

No coating PVP coating AOT coating

>1.5 (emu/g) -6

~1×10 (emu)

80

49

PM FM > 300 K FM > 300 K

~0.005 (emu/g) ~0.02 (emu/g)

104

105

-

106

280 255 203

7

PM

0.021 (µB/Mn)

180 C o

Zn1-xMnxO (x = 0.01-0.1)

Hydrothermal

95 C

Zn0.986Mn0.014O

Aqueous solution

85-90 oC

Zn0.937Mn0.063O

Aqueous solution

o


Aqueous solution and thermal diffusion

Zn0.97Mn0.03O Zn0.95Mn0.05O

Nonaqueous reaction

310 C

o

95 C

o

Dried at 60 C in air o

FM > 300 K

0.014 (µB/Mn) 0.011 (µB/Mn)

Dried at 60 C in air

FM > 300 K

Dried at RT in air

FM > 300 K

~0.01 (emu/g)

-

108

Annealed at 500 or 800 oC in air

FM > 300 K

~1.7 (µB/Mn)

20~260

107

100 210

50

Annealed at 680850 oC

-

o

42

FM > 300 K

o

386

0.01~0.11 (emu/g)

56

PM FM < 5 K FM < 5 K

6.1×10 (emu/cm )

FM < 10 K

4.4 (µB/Mn)

5.3×10-5 (emu/cm2) -5

2

47

3.8 (µB/Mn)

-

Zn0.97Mn0.07O

Seed-mediated solution

83 C

As-prepared Annealed at 900 oC

FM > 300 K

0.005 (emu/g) 0.014 (emu/g)

110 210

52

Zn0.979Mn0.021O

Aqueous solution

310 oC

Annealed at 500 or 800 oC in air

FM > 300 K

~1.3 (µB/Mn)

>140

107

Zn0.98Co0.02O

Aqueous solution

85-90 oC

Dried at RT in air

FM > 300 K

~0.13 (emu/g)

-

108

FM > 300 K FM > 300 K PM PM

-4

1.5×10 (emu/g) -

40 25 -

0.0125 (emu/g) 0.0221 (emu/g)

98 36

111

-

106 104

Zn0.97Co0.03O Zn0.95Co0.05O Zn0.92Co0.08O Zn0.9Co0.1O

Aqueous solution

Ambient RT

Dried at RT in air

Zn0.971Co0.029O Zn0.944Co0.056O

Hydrothermal

175 oC

Dried at 40 oC

Zn0.95Mn0.05O

Solvothermal

-

-

Zn0.95Co0.05O Zn0.9Co0.1O Zn0.8Co0.2O

Aqueous solution

70 oC

Dried at 60 oC

Electrochemical

90 oC

-

Zn0.99Co0.01O Zn0.98Co0.02O

FM > 300 K PM

15

3.6×10 (emu/g) -4

-

PM PM FM > 300 K

0.74 (emu/cm3)

27

FM > 300 K

0.006 (emu) ~0.025 (emu)

131 189


110

109


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Page 16 of 33

Table 2: Magnetic properties of some Mn-related secondary phases, which could be present in Mndoped ZnO materials. Composition

TC or TN

Magnetic property

Ref.

(K)

MnO2

Mn2O3

Mn3O4

MnO

(Mn,Zn)Mn2O4

ZnMnO3

Anti-FM

84

2,113

Anti-FM

85

114

Weak FM

44

115

Anti-FM

80

116

FM

40 ~ 45

117

FM

42

118

Ferrimagnetic

46

48

Anti-FM

122

113

116

118

Anti-FM

22

119

Ferrimagnetic

40

120

Spin-glass

-

20

4. CONCLUSION Zn1-xMnxO nanorods with x = 0, 0.008, 0.013, and 0.020 grown by CVD were studied the structural characterization, and optical and magnetic properties at room temperature. The structural analyses revealed the nanorod samples having a single phase in a wurtzite structure. There was an incorporation of Mn2+ ions into the Zn site of the ZnO host lattice. This created more disorders and defects of zinc vacancies, which influenced strongly the features of the PL and RS spectra of Zn1xMnxO

nanorods. For the PL spectra, there are the shift of the UV emission towards shorter

wavelengths caused by the band-gap expansion, and the rapid quenching of visible luminescence 16


Page 17 of 33

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when Mn-doping concentration in Zn1-xMnxO is increased. Meanwhile, for the normal RS spectra, besides conventional lattice vibrations, there are additional modes of AM1-3 associated with Mn impurities, and defects and disorders. Basing on some results reported earlier on normal RS spectra of ZnO-based materials, and on the features of the RRS spectra, we assessed carefully the origin and nature of the AM1-3 modes. Particularly, under the RRS condition, the RRS spectra exhibit only the LO-phonon and its replicas up to the seventh order. This indicates a great deformation potential existing in Zn1-xMnxO nanorods in order to cause the displacement of parent Zn and/or O atoms from their equilibrium positions. Room-temperature magnetic measurements proved Zn1-xMnxO nanorods with x = 0.013 and 0.020 having the FM order. It is believed that the FM nature is due to exchange interactions of Mn2+-Mn2+ pairs, and of Mn2+ ions mediated by zinc vacancies. There is no evidence showing the room-temperature FM origin generated from Mn-related secondary phases. It is necessary to emphasize that the successful preparation of such Zn1-xMnxO-based ferromagnets is considered as an important clue to develop next-generation spintronic devices.

ACKNOWLEDGEMENTS

This research was supported by the Converging Research Center Program funded by the Ministry of Education, Science and Technology (2012K001431). References (1)

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Figure captions Figure 1. Representative SEM images with different magnifications for CVD-grown Zn1-xMnxO nanorods with the weight ratio of initial powders of Zn:MnCl2⋅4H2O = 85:15. Figure 2. (a) Micro-electron diffraction patterns for Mn-doped ZnO nanorods near the [11-20] zone axis; (b) a representative EDS spectrum for Zn1-xMnxO nanorods with x = 0.02 (two additional peaks generated from a copper mesh grid are also recorded). Figure 3. XRD patterns recorded from CVD-grown Zn1-xMnxO nanorods. The inset is an enlarged view for the (002) peak of the samples. Figure 4. PL spectra for CVD-grown Zn1-xMnxO nanorods excited by a wavelength λexc. = 325 nm. The inset plots the dependence of the UV-emission peak on Mn-doping content (x). Figure 5. Normal RS spectra of CVD-grown Zn1-xMnxO nanorods excited by 2.54 eV (488 nm) lower than band-gap energies of Zn1-xMnxO. The solid lines show conventional modes while the dotted ones indicate additional modes (AM1-3). Figure 6. RRS spectra of CVD-grown Zn1-xMnxO nanorods excited by 3.84 eV (325 nm) above the band-gap energy. The upper inset shows the dependences of the I2LO/I1LO ratio on Mn-doping content (where I1LO and I2LO are integrated Raman intensities of the first and second LO phonons, respectively). Figure 7. M(H) curves for CVD-grown Zn1-xMnxO nanorods recorded at 300 K.

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