Structural, optical, and magnetic studies of manganese-doped zinc oxide hierarchical microspheres by self-assembly of nanoparticles
© Hao et al; licensee Springer. 2012
Received: 11 November 2011
Accepted: 2 February 2012
Published: 2 February 2012
In this study, a series of manganese [Mn]-doped zinc oxide [ZnO] hierarchical microspheres [HMSs] are prepared by hydrothermal method only using zinc acetate and manganese acetate as precursors and ethylene glycol as solvent. X-ray diffraction indicates that all of the as-obtained samples including the highest Mn (7 mol%) in the crystal lattice of ZnO have a pure phase (hexagonal wurtzite structure). A broad Raman spectrum from as-synthesized doping samples ranges from 500 to 600 cm-1, revealing the successful doping of paramagnetic Mn2+ ions in the host ZnO. Optical absorption analysis of the samples exhibits a blueshift in the absorption band edge with increasing dopant concentration, and corresponding photoluminescence spectra show that Mn doping suppresses both near-band edge UV emission and defect-related blue emission. In particular, magnetic measurements confirm robust room-temperature ferromagnetic behavior with a high Curie temperature exceeding 400 K, signifying that the as-formed Mn-doped ZnO HMSs will have immense potential in spintronic devices and spin-based electronic technologies.
KeywordsMn-doped ZnO hierarchical microspheres optical properties magnetic properties
Zinc oxide [ZnO] exhibits many fascinating properties [1–7]. It is an intrinsic n-type II-VI semiconductor with a wide bandgap of 3.37 eV at room temperature [RT] and a large excitation binding energy of 60 meV. Because of these properties, ZnO presents a strong excitonic UV light emission at RT [2–6]. It also presents a high photoconductivity and considerable piezoelectric and pyroelectric properties . Because of these properties, ZnO has attracted much attention for potential applications in various electronic and optoelectronic devices. In particular, the interest in ZnO has significantly been increased in the last decade [7–21] since the theoretical prediction of above RT ferromagnetism [RTFM] in Mn-doped ZnO diluted magnetic semiconductors [DMSs] by Dietl et al. . DMSs are materials that simultaneously exhibit ferromagnetic and semiconducting properties. In DMS materials, magnetic transition ions substitute a small percentage of cation sites of the host semiconductor and are coupled with free carriers to yield ferromagnetism via indirect interaction [9–12]. DMSs are considered to be very important materials in future semiconductor spintronic applications due to the simultaneous control of 'electron' charge and spin [9–12]. Among all the magnetic transition ion-doped ZnO systems, Mn doping is usually the single most concerned mainly because of the fact that the thermal solubility of metallic Mn is larger than 10 mol% in ZnO, and the 'electron effective mass' is as large as approximately 0.3 me, where 'me' is the free-electron mass . Therefore, injected spins and carriers in the nanostructures can be large, thus making Mn-doped ZnO ideal for the fabrication of spintronic nanodevices. For practical applications, a high-performance DMS with a high Curie temperature [TC] and saturation magnetic moments [MS] is required. However, until now, the mechanism involved in ferromagnetism [FM] is complicated and the reproducibility of ferromagnetic behavior is still a challenging problem. Because several groups have obtained different properties such as paramagnetism, anti-FM, and FM in Mn-doped ZnO [12, 13, 19–21], these magnetic properties are strongly dependent on the sample preparation conditions. Therefore, the development of a more controllable and repeatable synthetic route for RTFM Mn-doped ZnO nano/microstructures is crucial to their practical applications.
The fabrication of hierarchical and self-assembly (self-aggregation) micro-/nanostructures using nanoparticles, nanorods, nanoplatelets, etc. as building blocks at different levels have become a hot topic in recent material research fields [14–16]. Self-assembly and/or self-aggregation are fundamental mechanisms by which different nanoparticle assembly motifs or even close-packed periodic structures form in materials through spatial arrangement of their fundamental building blocks. The forces that controlled the assembly are determined by competing noncovalent intramolecular or intraparticulate interactions. The hierarchical structures obtained through the assembly of nanocrystalline building blocks provide new opportunities for optimizing, tuning, and/or enhancing the properties and performance of the materials [14–16]. So far, considerable efforts have been devoted to synthesize Mn-doped ZnO systems with RTFM including nanoparticles, nanowires, and thin films using different methods such as pulsed laser deposition, magnetron co-sputtering, and chemical vapor deposition [17–19]. However, there are very few reports on the synthesis of hierarchical spherical superstructures of Mn-doped ZnO DMS materials in solution phase. Herein, we only use zinc acetate and manganese acetate as precursors and ethylene glycol [EG] as solvent to synthesize Mn-doped ZnO hierarchical microspheres [HMSs] by self-assembly of nanoparticles. In particular, magnetic measurements confirm robust RTFM behavior with a high TC over 400 K. To our knowledge, there is no report on the Mn-doped ZnO hierarchical spherical structures showing a robust RTFM behavior with a high TC.
Meanwhile, optical properties of Mn-doped ZnO are currently the subject of numerous investigations in response to a strong demand for nano-/microscale magneto-optic devices in the future. However, it is unfortunate that although most of the Zn/Mn bulk and nanostructure materials exhibit RTFM, strong UV photoluminescence [PL] is hardly achieved. This may be due to the difficulty in controlling the interaction between the Mn dopant and intrinsic defects such as oxygen vacancies during the fabrication process [22–26]. So far, its luminescence mechanism has still been in discussion [22–26]. With the aim of providing further understanding of the optical nature of Mn-doped ZnO HMSs, UV-visible [vis] and PL spectra are used to study their optical characteristics, and the corresponding mechanism has been discussed. Also, the surface morphology of the products was investigated by scanning/transmission electron microscopy [SEM/TEM] or by high-resolution TEM [HRTEM]. The structure was studied by X-ray diffraction [XRD] and using Raman and Fourier transform infrared spectroscopy [FTIR] spectra.
Zinc acetate [Zn(CH3COO)2·2H2O], manganese acetate [Mn(CH3COO)2·2H2O], and EG are analytic grade reagents and purchased without further treatment. In a typical process, 4 mmol of mixed reactants of [Zn(CH3COO)2·2H2O] with different amounts of [Mn(CH3COO)2·2H2O] was dissolved in 30 mL of EG. The mixture was stirred vigorously for 1 h, sealed in a Teflon-lined stainless steel autoclave of 50-mL capacity kept at 180°C for 5 h, and then allowed to cool to RT naturally. Yellow Mn-doped ZnO precipitates were centrifugally collected and rinsed with absolute ethanol several times. Finally, the precipitates were dried in air at 60°C overnight.
The morphologies and microstructures of these as-fabricated specimens were investigated by SEM/TEM (JSM5600LV, JEOL 2010, JEOL Ltd., Akishima, Tokyo, Japan), selected area electron diffraction [SAED], and HRTEM (JEOL Ltd, Akishima, Tokyo, Japan). The sample phases, crystal structures, chemical compositions, and element valences of the Mn-doped ZnO samples were detected by XRD (Phillips X'Pert Pro MPD, PANalytical B.V., Almelo, The Netherlands), Raman spectroscopy (microscopic confocal Raman spectrometer, RW-1000, Renishaw, Wotton-under-Edge, UK), and X-ray photoelectric spectrum [XPS] (KRATOS AXIS ULTR, Kratos Analytical, Ltd., Manchester, UK), respectively. RTPL measurement was carried with a fluorescence spectrophotometer (SPEX F212, Spex Industries, Metuchen, NJ, USA) with an Xe lamp as the excitation light source (330 nm). UV-vis absorption was carried with a UV-vis absorption spectrometer (Lambda35, PerkinElmer, Boston, MA, USA). The molecular structure of the as-synthesized samples was studied using FTIR spectra (Shimadzu-8700, Shimadzu Corporation, Nakagyo-ku, Kyoto, Japan). Magnetic properties were carried out using Quantum Design's superconductor quantum interference device [SQUID] (MPMS XL7, San Diego, CA, USA).
Results and discussion
The unit cell parameters
The composition, quality, and molecular structure of the product were analyzed by FTIR spectroscopy. FTIR measurements of all the samples were performed in the wave number range from 400 to 4,000 cm-1 using the KBr method at RT as shown in Figure 1c. The broad absorption peak around 3,500 cm-1 represents the stretching vibration of the O-H group. The absorption peaks observed between 2,300 and 2,400 cm-1 are assigned to the CO2 mode . The CO2 modes are present in the FTIR spectra not owing to the serious contamination in Mn-doped ZnO HMSs, but these modes may be due to atmospheric CO2 in the samples. Samples might have trapped some CO2 from the atmosphere during FTIR characterization which might have given such modes . Two principal absorption peaks at 1,580 and 1,400 cm-1 with a wave number separation of 180 cm-1 correspond to the asymmetric stretching υas (COO-) and symmetric stretching υs (COO-) vibrations of unidentate acetate species . These characteristic bands of 3,126, 1,090, and 1,050 cm-1 are attributed to a neutrally adsorbed EG on the surface without an obvious shift in wave numbers compared with pure EG. The band at 1,330 cm-1 is assigned to the stretching vibrations of δ (CH3) . The bending vibration of the interlayer water molecule appeared with the typical band at 1,635 cm-1. The absorption band at 431 cm-1 is assigned to the stretching mode of ZnO . However, in the case of the Zn1-xMn x O HMSs (x = 0.02, 0.05, and 0.07), the values of absorption bands are found to be blueshifted at 438, 445, and 456 cm-1, respectively. The enlarged spectrum in the wave number range is shown in the inset of Figure 1c. The change in the peak position of the ZnO absorption bands reflects that the Zn-O-Zn network is perturbed by the presence of Mn in its environment.
Based on the mentioned analysis, EG plays at least a triple role. Firstly, it acts as a solvent, providing a medium for the reagents. Secondly, it serves as the reductant which reduces Zn(Ac)2/Mn(Ac)2 to Mn-doped ZnO crystalline nanoparticles. Finally, it coordinates Mn-doped ZnO nanocrystals to direct the formation of Mn-doped ZnO HMSs.
Figure 5b shows the UV-vis absorption spectra of ZnO with different Mn concentrations. The absorption edges of Zn1-xMn x O (x = 0, 0.02, 0.05, and 0.07) are 370, 363, 358, and 355 nm, respectively. The position of the absorption spectra is observed to shift towards the lower wavelength side with increasing Mn-doped concentration in ZnO. This indicates that the bandgap of the ZnO material increases with the doping concentration of the Mn2+ ion. The increase in the bandgap or blueshift can be explained by the Burstein-Moss effect . This is the phenomenon that the Fermi level merges into the conduction band with increasing of the carrier concentration. Thus, the low energy transitions are blocked. The results are in good agreement with the results reported by Sakai and Rekha [40, 41].
The optical emission properties of the Zn1-xMn x O HMSs (x = 0, 0.02, 0.05, and 0.07) were investigated by PL spectroscopy (Figure 5c) using a 330-nm excitation wavelength of Xe laser at RT. Typically, the following two bands have appeared in the PL spectra: the near-band edge emission in the UV region, which originates due to the recombination of free excitons through an exciton-exciton collision process, and the deep level emission in the visible region, caused by impurities and structural defects of the crystal. Herein, we found that our samples exhibit a UV emission peak at 395 nm and three defect peaks corresponding to blue emissions near 425 and 475 nm and to green emission near 490 nm in the PL spectra. The origin of the peaks at 425 and 475 nm could be ascribed due to the transition occurring from Zn interstitials to the valence band, and the peak of 425 nm may be the result of the singly ionized oxygen vacancy . The increase of the Mn concentration leads to the intensity reduction of both UV and blue emissions, which is mainly due to the increase of defect concentration induced by Mn doping. Besides, the worse crystallization caused by Mn doping is another reason for the rapid decrease in UV emission intensity . This is in agreement with the Raman spectra.
To summarize, Zn1-xMn x O HMSs (x = 0, 0.02, 0.05, and 0.07) with the hexagonal wurtzite structure are synthesized by a simple hydrothermal method. The results of XRD, XPS, and Raman spectrum confirm that Mn2+ ions are successfully incorporated into the ZnO host lattice at the Zn2+ site. The doping of Mn ions suppressed both near-band edge UV emission and defect-related blue emission, which could be mainly caused by the lattice defect increase due to Mn doping into ZnO lattice. In particular, the magnetic measurements reveal that the as-formed Mn-doped ZnO HMSs have above RTFM.
This work was partially supported by the program for Science and Technology Innovation Talents in Universities of Henan Province (no. 2008 HASTIT002) and by the Natural Science Foundation of China under grant no. 20971036.
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