J. Phys. Chem. C 2007, 111, 5893-5897
5893
Synthesis of Room-Temperature Ferromagnetic Co-Doped ZnO Nanocrystals under a High Magnetic Field Dewei Chu,†,‡ Yu-Ping Zeng,*,† and Dongliang Jiang† Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China, and Graduate School of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ReceiVed: December 6, 2006; In Final Form: February 9, 2007
Transitional-metal cobalt-doped ZnO nanocrystals were synthesized under an external magnetic field. The growth behavior of cobalt-doped ZnO nanocrystals was greatly affected by the high magnetic field. The growth rate of the nanocrystals along (001) direction was significantly suppressed because of the magnetization energy differences in a, b, and c-axis. Robust room-temperature ferromagnetism of Co-doped ZnO nanocrystals can be activated by the external magnetic field. The characterization of the annealed samples combined with the analysis of Raman scattering suggested that the appearance of ferromagnetism cannot be contributed to the presence of oxygen vacancies but to the ferromagnetic coupling of Co2+ under an external magnetic field.
Introduction The developments of superconductive magnetic fields have made it possible to carry out many researches such as magnetic orientation, magnetic levitation, phase transition, Mosses effect,1-7 and so forth. Specially, many efforts have been done to realize magnetic orientation during the crystallization of polymers, proteins, and inorganic materials.8,9 Nowadays, the influence of magnetic fields on the growth behaviors of superconducting materials has been widely investigated, and the magnetic orientation has been proved to improve their crystallinity and grain connectivity.10 However, little attention has been paid to the crystallization behavior of one-dimensional (1D) nanocrystals under an external magnetic field. It is known that 1D nanocrystals usually possess anisotropic susceptibilities; therefore, an applied external magnetic field might induce oriented growth along the easiest magnetization axis. For example, magnetic field-induced fabrications of a few magnetic materials such as Fe3O4 nanowires and iron arborescences have been reported.11,12 However, less significant improvement of magnetic properties has been obtained by this method. The reason may lie in that the interactions between the nanocrystals and external magnetic field are less known and are very difficult to be observed because of the weak magnetic dipoles induced by the magnetic field.1 In view of this, it is worthwhile to explore new ways to observe the trivial interactions during nanocrystal growth process. Dilute magnetic semiconductors (DMSs) might be good research candidates because of their unique magnetic properties and potential spintronic applications.13 Among them, ZnO-based DMSs are proposed to show ferromagnetic behavior with a Curie temperature (Tc) above room temperature, which would allow spintronic devices to operate at room temperature.14,15 However, the reported magnetic properties of ZnO-based DMSs were inconsistent or controversial. Some investigations on ZnObased DMSs argued that robust ferromagnetism might occur if * To whom correspondence should be addressed.
[email protected]. † Shanghai Institute of Ceramics. ‡ Graduate School of the Chinese Academy of Sciences.
E-mail:
there were additional defect-induced carriers,16 while other results showed the absence of ferromagnetism even if additional free carriers were created.17 The possible origin of this disagreement may be associated with the energy differences between the ferromagnetic and antiferromagnetic spin alignment of doping ions.18 If the total energies for ferromagnetic coupling are higher, the transitional-metal-doped ZnO would not show ferromagnetism. It is imaginable that if a high magnetic field is applied during the growth process of ZnO-based DMSs, the energies of ferromagnetic spin alignment would decrease to a degree that ferromagnetism might be activated. For the requirement of device miniaturization, lots of efforts have been dedicated to synthesize nanoscale ZnO-based DMSs,19-26 while the fabrication of one-dimensional ZnO-based DMSs nanocrystals with room-temperature ferromagnetism remains a great challenge.27-33 In this letter, cobalt-doped ZnO nanorods with room-temperature ferromagnetism were fabricated with an external magnetic field applied, where the morphologies and optical and magnetic properties were investigated. Experimental Section All of the reagents are analytical grade and are used without any further purification. In this method, zinc acetate dihydrate (10 mmol) and cobalt(II) acetate tetrahydrate (0.6 mmol, a dopant concentration of 5.7 atom%) were mixed in a beaker with 100 mL absolute ethanol at 273 K. Fifty milliliters of ethanol solution of 1 M sodium hydroxide was slowly dripped into the former solution at 273 K under stirring till precipitate was found. The obtained mixture was transferred into a sealed autoclave and was appended in a 12 T superconducting magnet and then was maintained at 393 K for 5 h. The products were washed several times with deionized water and absolute ethanol to remove sodium acetate and other impurities and were subsequently dried at 393 K in air for 10 h. The superconducting magnet used in our experiments was made by Oxford Instrument Co. Ltd. Transmission electron microscopy (TEM) images and energy dispersive spectra (EDS) were taken on a JEM-200CX transmission electron microscope operated at 200 kV. The phase composition of the products was characterized by powder X-ray diffraction (XRD, D/max 2550V,
10.1021/jp0684067 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/29/2007
5894 J. Phys. Chem. C, Vol. 111, No. 16, 2007
Figure 1. (a) TEM image of Co2+ doped ZnO nanocrystals without magnetic field treatment. (b) HRTEM image of a nanocrystal without magnetic field treatment, with an accompanying electron diffraction pattern in the inset; similar images for nanocrystals treated in a 12 T magnetic field are shown in (c) and (d); (e) and (f) are TEM and HRTEM images of fused nanocrystals in 12 T magnetic field.
Rigaku, Cu KR, λ ) 0.15406 nm). The diffuse reflectance spectra of the products were measured using a Hitachi U-3010 spectrophotomer. Raman spectra were recorded at room temperature using a micro-Raman spectrometer (Renishaw InViaReflex) with the 785 nm line of diode lasers as excitation source. Magnetization measurements as a function of the field were carried out using a PPMS-9 physical property measurement system (Quantum Design). Results and Discussion Figure 1 shows the TEM images of nominal 5.7 atom % Co2+ doped ZnO samples obtained in the absence of magnetic field (named 0 T sample) and in 12 T magnetic field (named 12 T sample). Both of the two samples are in good rod shape but the average length of the 12 T sample significantly decreases (see Supporting Information). From the high-resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction patterns (inset), it can be seen that both of the samples are single crystalline and the preferred growth directions of the two samples are parallel to the [001] direction (c-axis). Well-resolved lattices with a measured interplane spacing of 0.52 nm is shown in 12 T sample, consistent with the distance between (002) crystal planes. An interesting phenomenon is found in the 12 T sample, where the lattice planes are extended out of the side edge of the nanorods. Furthermore, some nanorods are epitaxially fused together, which is shown in Figure 1e. Corresponding HRTEM image shows that the lattice planes go straight through the contacting
Chu et al. areas. Pacholski et al. have reported the formation of single crystalline ZnO nanorods by oriented attachment mechanism.34 That is to say, the jiggling of nanoparticles by the Brownian motion allows adjacent particle to collide and rotate to find a low-energy configuration, resulting in a coherent grain-grain boundary.35 In many cases, oriented attachments take place along the preferential growth direction, for ZnO, the c-axis [001] However, lateral oriented attachment parallel to the c-axis is observed in 12 T sample. The results show that the external magnetic field plays a key role in the experiment. When a nanocrystal is magnetized in magnetic field, magnetization energy of the nanocrystal is given by U ) -∫0B/µ0 M dBin. If the magnetic susceptibility of this nanocrystal is different in each crystal direction, according to the former equation, the value of the magnetic energy can be illustrutated by U ) -χB2/ [2µ0(1 + Nχ)2], where N is the demagnetization factor, and χ is the magnetic susceptibility.36 For ZnO, χc < χa,b (χc, χa,b are the magnetic susceptibility in c-axis and a, b-axis of a crystal, respectively), Uc > Ua,b, c-axis needs more magnetization energy and is less stable in magnetic field during crystal growth process. Therefore, the growth rate along c-axis is suppressed and the average length of the nanorods significantly decreases. Furthermore, the nanorods would partially fuse along the easiest magnetization axes when a strong magnetic field is applied. The XRD measurements are displayed in Figure 2a. In all cases, only the wurtzite structure of zinc oxide can be found. To ensure that the dopants are substituional in this method, the relationship between the cell volume and Co content is investigated. Figure 2b indicates that with increasing Co content, the cell volumes of 0 T samples decrease. The cobalt concentration in the nanorods is determined by energy-dispersive X-ray spectroscopy, as shown in Figure 2c. The average concentrations of cobalt atoms over groups of nanorods in 0 T and 12 T samples are uniform and near to the relative atomic concentrations of the cobalt(II) inserted into the precursor solution, indicating a homogeneous cobalt distribution in the as-prepared sample. Figure 2d shows the room-temperature UV-vis optical absorption spectra. For 12 T sample, the shape of the curve is very similar to that of 0 T sample but quite different from that of pure ZnO, as the fundamental absorption peak around 3.2∼3.4 eV is not observed. The result is consistent with the report of the abnormal band gap narrowing in Zn1-xCoxO nanorods.33 For x ) 0.075, the shift of band gap is about -0.5 eV, which might originate from the strong sp-d exchange interactions existing between the band electrons and the localized d electrons of dopant. On the other hand, defect-related tail of the fundamental absorption edge would also have some effects on the shapes of the spectra. The absorption peaks at 1.91, 2.03, and 2.18 eV in 0 T and 12 T samples can be assigned as 4A (F) f 2E(G), 4A (F) f 4T (P), and 4A (F) f 2A (G) of Co2+, 2 2 1 2 1 suggesting the crystal field transitions in the high spin state of Co2+ in the tetrahedral coordination. All these characterizations indicate that the dopants are substitutional in both 0T and 12T samples. Figure 3 shows magnetization (M) versus field (H) curves measured at 300 K. It can be seen that for the 0 T sample the magnetization is linear indicating its paramagnetic nature, while the sample prepared in a 12 T magnetic field shows hysteresis curves with the coercive field (Hc) of 140 Oe and saturation magnetization of 0.03 emu/g, indicating its ferromagnetic characteristic at 300 K. Cobalt oxide (CoO) is well-known to be paramagnetic above the Ne´el temperature of 291 K. Hence, ferromagnetic behavior of the 12 T sample might not be explained in terms of the formation of the cobalt oxide.
Synthesis of Co-Doped ZnO Nanocrystals
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Figure 2. (a) Powder X-ray diffraction patterns of pure and Co2+ doped ZnO nanocrystals. (b) Cell volumes of ZnO doped with different contents of cobalt under 0 T magnetic field. (c) Energy-dispersive spectrum of a single Co2+ doped ZnO nanocrystal treated with 12 T magnetic field. (d) Optical absorption spectra of pure ZnO nanocrystals, Co2+ doped ZnO nanocrystals treated in 0 T and 12 T magnetic field.
Figure 3. M-H curves of Co2+ doped ZnO: (a) 0 T sample; (b) 12 T sample; (c) 0 T sample annealed in air at 873 K for 2 h; (d) 12 T sample annealed in air at 873 K for 2 h.
The appearance of ferromagnetism on Co2+ doped ZnO is usually attributed to the oxygen vacancy, fusion of defects at the interfaces, and the randomization of the dopant ions.23
Theoretical studies suggested that oxygen vacancies can cause a marked change of the band structure of host oxides and can make significant contribution to the ferromagnetism.37 Hsu et
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Figure 4. Room-temperature Raman spectra of 0 T and 12 T samples.
al. reported the evidence of oxygen vacancy enhancing roomtemperature ferromagnetism in Co2+ doped ZnO.38 To exclude the effects of oxygen vacancies that are easily introduced during solution-based growth process, it is necessary to anneal samples in the presence of oxygen and to perform Raman scattering. Figure 3c and d shows the M-H curves of 0 T and 12 T samples annealed in air at 873 K for 2 h. The ferromagnetic ordering of annealed 12 T sample indicates that the oxygen vacancies should not be responsible for the observed magnetism. Raman spectra measured at room temperature are shown in Figure 4. For 0 T and 12 T samples, the sharpest peak at about 437 cm-1 can be assigned to the frequency branch of E2 mode of ZnO, which suggests the strongest mode in wurtzite crystal structure. The vibration centered at 538 cm-1 can be indicated to a local vibrational mode related to the donor defects bound on Co sites, such as oxygen vacancies.39,40 Although the donor defects are observed in the spectra, the similarity of the two peaks around 538 cm-1 in 0 T and 12 T sample shows that the ferromagnetism should not just be activated by these donor defects. In addition, the main difference in two spectra lies in that the sharp peak located at about 484 cm-1 in 12 T sample is stronger than that of 0 T sample. The similar peak (∼475 cm-1) was also observed in Co2+ doped, Li+ and Na+ codoped, and Co2+ and Al3+ codoped ZnO,17 and this vibration was not specific to the dopant and its intensity increased by adding doping contents. The vibrational mode was due to the insertion of dopant in interstitial sites, and there was no association between this vibrational mode and the appearance of ferromagnetism in Co2+ doped ZnO. On the basis of the above discussion, it is reasonable to assume that the external magnetic field instead of oxygen vacancy plays a crucial role in mediating the ferromagnetism. Without the application of magnetic filed, the spin alignment of the Co2+ might be random or opposite and the obtained samples might fail to exhibit ferromagnetism. Under the high magnetic field, the spin alignment of the Co2+ might be forced to turn to the same direction and to keep this state during the ZnO nanocrystal growth process. Figure 5 illustrates the effect of magnetic field treatment in activating the ferromagnetism. A careful analysis of magnetic field effects is intricate because of the lack of in-situ observation. Nevertheless, it can be concluded that the high-temperature ferromagnetism has been activated by the external magnetic field. Further studies will be done to completely understand the ferromagnetism origin. In summary, the external high magnetic field can significantly influence the growth behavior of Co2+ doped ZnO nanocrystals. TEM studies show that growth rate of the nanocrystals along (001) direction is greatly suppressed. High-Tc ferromagnetism in Co2+ doped ZnO nanocrystals has been activated by strong magnetic field. The possible effects of oxygen vacancies on magnetic properties are excluded by the analysis of the annealed
Figure 5. Schematic illustration of the effect of magnetic field on the magnetic state of the Co2+ doped ZnO. (a) Supercell of Co2+ doped ZnO with antiferromagnetic spin alignment (without magnetic field treatment); (b) supercell of Co2+ doped ZnO with ferromagnetic spin alignment (with a magnetic field applied). The blue atoms are Co2+, and they might occupy other sites, but there are also antiferromagntism or ferromagnetism states for each configuration.
samples and characterization of Raman scattering. The appearance of ferromagnetism may originate from the ferromagnetic coupling between Co atoms under high magnetic field. The method can also be extended for the synthesis of other transitionmetal-doped ZnO nanocrystals and the investigation of the interactions between the nanocrystals and the external magnetic field. Acknowledgment. The authors are thankful for the financial support from the “Plan of Outstanding Talents” of Chinese Academy of Sciences. The comments of the reviewers are greatly appreciated. Supporting Information Available: Figures show the distributions of the length of the 0 T sample and 12 T sample. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Takayama, T.; Ikezoe, Y.; Uetake, H. Appl. Phys. Lett. 2005, 86, 234103. (2) Kimura, T.; Yamato, M.; Koshimizu, W.; Koike, M.; Kawai, T. Langmuir 2000, 16, 858. (3) Kawai, T.; Iijima, R.; Yamamoto, T.; Kimura, T. J. Phys. Chem. B 2001, 105, 8077. (4) Uchikoshi, T.; Suzuki, T. S.; Iimura, S.; Tang, F.; Sakka, Y. J. Eur. Ceram. Soc. 2006, 26, 559. (5) Beaugnon, E.; Tournier, R. Nature 1991, 349, 470. (6) Masahashi, N.; Matsuo, M.; Watanebe, K. Mater. Res. 1998, 13, 457. (7) Hirota, N.; Homma, T.; Sugawara, H.; Kitazawa, K.; Iwasaka, M.; Ueno, S.; Yokoi, H.; Kakudate, Y.; Fujiwara, S.; Kawamura, M. Jpn. J. Appl. Phys., Part 2 1995, 34, L991. (8) Helseth, L. E. Langmuir 2005, 21, 7276. (9) Tolbert, S. H.; Firouzi, A.; Stucky, G. D.; Chmelka, B. F. Science 1997, 278, 264. (10) Ma, Y.; Watanabe, K.; Awaji, K.; Motokawa, M. J. Cryst. Growth 2001, 233, 483. (11) Wang, J.; Chen, Q.; Zeng, C.; Hou, B. AdV. Mater. 2004, 16, 137. (12) Boden, S.; Vignon, L.; Ballou, M. R. P. Phys. ReV. Lett. 1999, 83, 2612. (13) Ohno, H. Science 1998, 281, 951. (14) Dietl, T.; Ohno, H.; Matsukura, F.; Cibert, J.; Ferrand, D. Science 2000, 287, 1019. (15) Sato, K.; Katayama-Yoshida, H. Jpn. J. Appl. Phys., Part 2 2000 39, L555. (16) Rao, C. N. R.; Deepak, F. L. J. Mater. Chem. 2005, 15, 573. (17) Alaria, J.; Bieber, H.; Colis, S.; Schmerber, G.; Dinia, A. Appl. Phys. Lett. 2006, 88, 112503.
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