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Tunable Ferromagnetism accompanied by Morphology Control in Li-doped Zn0.97Ni0.03O O. D. Jayakumar,† C. Sudakar,‡,# C. Persson,§ V. Sudarsan,† R. Naik,‡ and A. K. Tyagi*,† Chemistry DiVision, Bhabha Atomic Research Centre, Mumbai 400085, India, Department of Physics and Astronomy, Wayne State UniVersity, Detroit, Michigan 48201, and Department of Materials Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden ReceiVed: June 14, 2010; ReVised Manuscript ReceiVed: September 9, 2010
We report morphological and ferromagnetic property control in ZnO nanorod structures by an optimum doping of Ni and Li. Nanostructures of Zn0.97-xNi0.03LixO (x ) 0, 0.03, 0.05, 0.08, and 0.10) are prepared by a solvothermal method. High aspect ratio (5-15) ZnO nanorods transform to particles (with 1-3 aspect ratio) influenced by 3 at. % Ni substitution in ZnO (Zn0.97Ni0.03O). It is remarkable to note that the Zn0.97Ni0.03O particles completely retain the nanorod shape with significantly increased aspect ratio (15-30) when 3 at.% Li ions are codoped in Zn0.97Ni0.03O (Zn0.94Li0.03Ni0.03O). Li substitution also enhances ferromagnetism with largest magnetization (0.8 emu · g-1) observed for Zn0.94Li0.03Ni0.03O. For Li concentration >3 at.%, the aspect ratio as well as the magnetization decreased considerably. These experimental observations are explained by first-principles modeling. At low Li-on-Zn acceptor concentrations, the total magnetization is increased by lower Ni d-state populations, whereas at higher Li concentrations the population of ZnO host states decreases the ferromagnetism by induced magnetic moments on the oxygens. We discuss the significant implications of these results on the nanorods structures of room temperature ferromagnetic materials, which are expected to play pivotal role in developing spintronic devices. 1. Introduction One-dimensional (1D) nanostructures of wide band gap ZnO have attracted considerable attention because of their various remarkable physical and chemical properties distinctive from conventional bulk materials. ZnO nanoparticles show excellent chemical and thermal stability. Its electrical and optoelectronic properties of being II-VI semiconductor with a large exciton binding energy (60 meV) can be used for a broad range of high technology applications.1 In addition, transition metal (TM)doped ZnO is a potential candidate material for spintronic applications.2-5 Nanoscale dilute magnetic semiconductors (DMS), including quantum wires and rods, are envisioned as pivotal architectural elements in several spintronics devices. This has triggered significant research interest worldwide on the synthesis and characterization of various wide band gap semiconductor nanorods/nanowires. There is also a considerable interest in understanding the spin effects in nanoscale magnetic semiconductors, and many essential advances in this field would get impetus by the development of facile methods for the preparation of high-quality DMS nanostructures like TM-doped ZnO nanorods/nanowires. Li doping has proved to be an effective way to improve many properties like dielectric constant, ferroelectricity, and magnetic properties of undoped and TM-doped ZnO thin films and powders.6-10 Lin et al.9 and Jayakumar et al.10 observed enhanced room-temperature ferromagnetism (RTFM) in Co-doped ZnO when codoped with Li. In this manuscript we report the synthesis of nanorod structures of ZnO by a solvothermal method and the morpho-
logical stability of these ZnO nanorods when doped with 3 at.% Ni (Zn0.97Ni0.03O) and codoped with 3, 5, 8, and 10 at.% Li in Zn0.97Ni0.03O (Zn0.94Li0.03Ni0.03O, Zn0.92Li0.05Ni0.03O, Zn0.89Li0.08Ni0.03O, and Zn0.87Li0.10Ni0.03O, respectively). In general, the homogeneous distribution of transition metal (TM) dopants such as Mn, Co, Ni, Cr in ZnO system show maximum solubility and large magnetization independent of the contributions from the secondary phases for TM dopant concentrations of 3 at.%) adversely affected the aspect ratio and magnetization. To the best of our knowledge and for the first time, these results simply demonstrate the tunability of ferromagnetism accompanied by morphology control and codoping of Li in a transition metal (Ni)-doped ZnO. 2. Experimental Methods
* To whom correspondence should be addressed. E-mail: aktyagi@ barc.gov.in. Tel: +91 22-2559 5330. Fax: +91 22-2550 5151. † Bhabha Atomic Research Centre. ‡ Wayne State University. § Royal Institute of Technology. # Present address: Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India.
In a typical synthesis to prepare ZnO nanorods, zinc acetate dihydrate (10 mmol) (99.99%) was mixed with 15 mL of trin-octylamine in a round-bottomed flask. The mixture was rapidly heated to 320 °C and maintained at that temperature for 2 h with refluxing and cooled to room temperature. The obtained
10.1021/jp105457j 2010 American Chemical Society Published on Web 09/28/2010
Tunable Ferromagnetism in Li-doped Zn0.97Ni0.03O white precipitate was washed several times with acetone and absolute ethanol and dried by a rotoevaporator. The same procedure has been followed to prepare Ni-doped ZnO and Ni and Li-codoped ZnO by taking Ni acetate and Li acetate in appropriate proportions. The phase purity and crystal structure of the samples were analyzed using a Philips Diffractometer (model PW 1071) using Cu KR radiation fitted with graphite crystal monochromator. High-resolution transmission electron microscopy (HRTEM) imaging and selected area electron diffraction (SAED) studies were carried out with a JEOL JEM 2010 transmission electron microscope. DC magnetization measurements were carried out using an EG&G PAR vibrating sample magnetometer (model 4500). Photoluminescence (PL) studies were carried out using a Hitachi instrument (F-4010) with 150 W xenon lamp as the excitation source. All the emission spectra were corrected for the detector response. XPS measurements were made using an Escalab MK2 spectrometer equipped with an Al KR (1486.6 eV) source. The C 1s peak set at 284.6 eV was used for charge referencing. Calculated magnetic properties are determined from the projector augmented wave (PAW) method11 with the density functional of the local density approximation (LDA). This LDA functional is corrected by an on-site Coulomb potential within the LDA+U method with Ud(Zn) ) 6 eV and Ud(Ni; Li) ) 4 eV.11,12 This LDA+U modeling has been found13,14 to improve significantly the Zn-d-O-p hybridization at ∼7 eV below the valence band maximum. The Zn0.97-xNi0.03LixO compounds are modeled by 72 atoms 3 × 3 × 2 wurtzite structures (see comparable modeling in ref 12). Thus, the variations of the Lidoping concentrations in the Zn31-32xNiLi32xO32 supercell (∼3 at.% Ni) are obtained by x ) 0, 1/32, 2/32, and 3/32 (i.e., about 0, 3, 6, and 9 at.% Li, respectively). Modeling nanorods with bulk systems are justified by the primarily bulklike property of the relatively large 1D rods. Experimental lattice constants are used, and ions are fully relaxed by means of both the conjugate gradient algorithm and the quasi-Newton algorithm, and the convergence is 0.1 meV for the total energy of the unit cell and 8 meV/Å for the forces of each atom. Total energy, local charge distribution, density-of-states (DOS) as well as magnetic moment are obtained from the tetrahedron integration method with Blo¨chl corrections using a 6 × 6 × 6 Γ-centered k-mesh and the default muffin-tin radii of 1.27, 0.82, 1.28, and 1.36 Å for Zn, O, Ni, and Li, respectively. 3. Results and Discussion X-ray diffraction (XRD) patterns of undoped, Ni, and Ni + Li-doped ZnO samples showed that they are monophasic with wurtzite structure in the concentration range we have studied (Figure 1). The lattice parameters of these samples are plotted as a function of Li-doping concentration (at.%) and are shown as an inset in Figure 1. It can be seen that the cell parameters of the Ni-doped ZnO is slightly decreased compared to pristine ZnO nanorod samples (inset of Figure 1), which is due to the difference in ionic radii between Zn2+ (0.060 nm) and Ni2+ (0.055 nm) in tetrahedral co-ordination. However, Li-codoped ZnO showed marginal increase in lattice parameter values (Figure 1 and inset) due to the difference in ionic radii of Ni2+, Zn2+ and Li+ (0.059 nm) compared to Ni-doped ZnO. The microstructural and morphological studies carried out using TEM on these samples indicate that pure ZnO samples show nanorod morphology (Figure 2a) having aspect ratio in the range of 5-15. HRTEM and SAED images indicate that the particles are highly crystalline and devoid of defects and impurity phases. Figure 2b depicts the TEM of Zn0.97Ni0.03O. It
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Figure 1. Room temperature Powder XRD patterns of (a) ZnO, (b) Zn0.97Ni0.03O, (c) Zn0.94Ni0.03Li0.03O, (d) Zn0.92Ni0.03Li0.05O, (e) Zn0.89Ni0.03Li0.08O, and (f) Zn0.87Ni0.03Li0.10O. Inset shows the change in lattice parameter values as a function of Li-codoping concentration (at.%) in Zn0.97Ni0.03O.
is clear from the image that the aspect ratio of Zn0.97Ni0.03O nanorod has reduced significantly (1-3), and develops hexagonal faceted platelet structures while retaining the crystallinity of the particles. However, Zn0.97Li0.03O sample without Ni doping showed the nanorod shape with marginal increase in aspect ratio as is evident from Figure 2c. Interestingly, the Zn0.97Ni0.03O particles with low aspect ratio is reverted back to nanorods with significantly increased aspect ratio (15-30) on codoping of Li (3 at.%) in Zn0.97Ni0.03O i.e. (Zn0.94Li0.03Ni0.03O) (Figure 2d). On further increase in Li concentration to x > 3 at.% (x ) 5, 8, and 10 at.%), the aspect ratio decreased considerably (shown for 5 at.% Li-codoped Zn0.97Ni0.03O, that is, Zn0.92Li0.05Ni0.03O in Figure 2f). This clearly indicates that while Ni tries influence to destabilize the nanorod shape of the ZnO, Li tries to restabilize the impact of Ni in the nanorod structure. EDS analysis was carried out using a focused electron probe at a number of locations throughout the specimens. The analysis indicates the homogeneity of Ni substitution in the samples and the Ni concentration is 2.95 at.%, which is consistent with the 3 at.% nominal concentration. The HRTEM and SAED images of Zn0.97Ni0.03O and Zn0.94Li0.03Ni0.03O confirm that the samples are highly crystalline without any impurity phases. The reason for the morphology change is not understood at present and some of the possible reasons are discussed below. Nanorod formation during the synthesis arises due to the involvement of anisotropic growth mechanism. The anisotropic growth mechanism in nanocrystals can be brought about by either the pH variation, preferential adsorption of cations and anions on the growing crystals or by the energy minimization of the surfaces due to the defects induced by doping ions in the lattice. Generally, at low pH values during the precipitation of oxides, anisotropic growth is favored leading to the formation of elongated structures. For example, Li et al.15 observed during the synthesis of GaOOH nanorods that anisotropic growth resulted in the formation of nanorods at lower pH, whereas at high pH values an isotropic growth mechanism is observed. However, in the present case the pH is not changing significantly for Ni2+ doping. The dopants are uniformly distributed in the particles that exclude any preferential adsorption on the crystal
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Figure 2. TEM images of (a) ZnO, (b) Zn0.97Ni0.03O, (c) Zn0.97Li0.03O, and (d) Zn0.94Ni0.03Li0.03O; (e) HRTEM and SAED patterns of Zn0.94Ni0.03Li0.03O; (f) TEM image of Zn0.92Ni0.03Li0.05O; and (g) HRTEM and SAED patterns of Zn0.92Ni0.03Li0.05O.
Figure 3. The XPS spectra of (a) Zn2p, (b) O1s, and (c) Ni2p recorded for representative sample Zn0.94Ni0.03Li0.03O.
surfaces. Hence, the possible reason for the lack of anisotropic growth for Ni-doped ZnO (Zn0.97Ni0.03O) samples is the disorder in the ZnO structure brought about by the Ni2+ doping. This is understandable as Ni2+ is a d8 system and prefers square planar geometry whereas Zn2+ in ZnO mainly has got a tetrahedral environment. Probably, the competing coordination geometries are responsible for the change in the growth mechanism when Ni2+ ions are doped in the ZnO lattice. In tetrahedrally coordinated undoped ZnO structure, lowest density of surface energy is along c-axis (9.9 eV nm-2).16 This favors the growth of ZnO along the c-axis to form elongated structures, thereby minimizing the overall free energy of the system. Ni2+ doping can raise the overall free energy of the system and this might be responsible for the isotropic growth resulting in the formation of nanoparticles rather than the nanorods. With Li+ codoping in the Zn0.97Ni0.03O sample, oxygen vacancies could be generated, which are known to stabilize the lattice of ZnO. This facilitates the anisotropic growth (situation similar to that of undoped ZnO) and leads to the formation of nanorods. However, with the increase in Li+ concentration (5, 8, and 10 at.%) in Zn0.97Ni0.03O, the further growth kinetics are affected unfavorably and results in the growth of particles with lower aspect ratio as seen for 5, 8, and 10 at.% Li-doped Zn0.97Ni0.03O. It is noteworthy that the decrease in the aspect ratio is both due to the decrease in the average length of the particles and the increase in the width of the particles compared the pure ZnO. To confirm the oxidation state of Ni ions in ZnO, XPS spectra of Ni2p, Zn2p, and O1s (Figure 3) has been recorded for the representative sample Zn0.94Li0.03Ni0.03O, which showed maximum magnetization and morphology change. The Ni2p peak at ∼852.5 eV and O1s peak at ∼529.3 eV confirmed that the Ni ions exist in the 2+ oxidation state.17 We have not observed any peak due to Ni3+ or Ni metal clusters. As these transition metal-doped ZnO nanostructures are promising materials for spintronics devices, we studied the magnetic properties by using vibrating sample magnetometer. DC magnetization loops recorded at room temperature for Zn97-xNi0.03LixO (x ) 0, 0.03, 0.05, 0.08 and 0.10) are depicted in
Figure 4. M vs H curves measured at room temperature for ZnO, Zn0.97Li0.03O, Zn0.97Ni0.03O, Zn0.94Ni0.03Li0.03O, Zn0.92Ni0.03Li0.05O, Zn0.89Ni0.03Li0.08O, and Zn0.87Ni0.03Li0.10O (Inset of Figure 4 shows magnetization values in µB/Ni for (Ni,Li)-doped ZnO samples as a function of Li concentration in at.%.).
Figure 4. Inset of Figure 4 shows magnetization values in µB/ Ni for (Ni,Li)-doped ZnO samples as a function of Li concentration in at.%. While pristine ZnO and Zn0.97Li0.03O showed a diamagnetic behavior, Ni-doped ZnO (Zn0.97Ni0.03O) showed ferromagnetism with saturation magnetization (Ms) ∼ 0.55 emu g-1 (0.26 µB/Ni) at 300 K. We observe that the saturation magnetization, coercivity (Hc) and remanence (Mr) induced by Ni substitution in ZnO is further enhanced (Ms ) 0.8 emu g-1 (0.39 µB/Ni), Hc ) 175 Oe and Mr ) 0.145 emu g-1) at room temperature by 3 at.% of Li incorporation in Zn0.97Ni0.03O (Zn0.94Li0.03Ni0.03O). Increasing the Li concentration above 3 at.% (5, 8, and 10 at.%) shows a decrease in saturation magnetization. The enhanced RTFM has been reported by the Li codoping (10 at.%) in Co-doped (5 at.%) ZnO nanoparticles10 and the increase in magnetization are explained by the assumption that the injection of additional carriers in ZnO is required
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Figure 6. Photoluminescence measurements done at room temperature for ZnO, Zn 0.97Ni0.03O and Zn0.94Ni0.03Li0.03O after exciting at 345 nm.
Figure 5. (a) M vs T curve measured at an applied field of 200 Oe for Zn0.97Ni0.03O. (b) M vs T curve measured at an applied field of 200 Oe for Zn0.94Ni0.03Li0.03O.
in addition to the randomization of the magnetic ions and defects. This was in agreement with the studies of Sluiter et al.18 and Kittilstved et al.19 on cobalt-doped ZnO. However, in our present study increasing Li concentration to 10 at.% decreased the Ms value, and it has reached the lowest value of 0.2 emu g-1 (0.09 µB/Ni), compared to other Li-codoped Zn0.97Ni0.03O samples. In addition, we show that Li substitution controlled the ZnO morphology. ZnO nanorods with high aspect ratio (5-15) convert to particles with very low aspect ratio (1-3) on doping with 3 at.% Ni. Interestingly, the Zn0.97Ni0.03O nanoparticles with low aspect ratio are reverted back to nanorods with significantly increased aspect ratio (15-30) and saturation magnetization on codoping of Li (3 at.%) in Zn0.97Ni0.03O, that is, (Zn0.94Li0.03Ni0.03O). On further increasing the Li concentration to 5, 8, and 10 at.%, the aspect ratio as well as the magnetization decreased considerably. Magnetization as a function of temperature in the field cooled (FC) and zero-field cooled conditions (ZFC) are very sensitive to impurities and other magnetic phases like paramagnetic component present in the samples. Further, it is also used for the confirmation of the room temperature ferromagnetism of the samples under investigation. Figure 5a,b shows the magnetization as a function of temperature for the representative samples Zn0.97Ni0.03O and Zn0.94Li0.03Ni0.03O, measured at an applied field of 200 Oe, in FC and ZFC conditions. The magnetizations for both samples exhibit very similar behavior and are almost temperature independent at higher temperatures
(>290 K) with both the samples showing a noticeable increase only at temperatures below 25 K. We attribute this upturn in FC curve to a Curie tail arising from paramagnetic Ni ions in the sample. In ZFC curves, such an upturn is not seen even at the lowest temperature of measurement indicating that there may be some decreasing component as generally seen in the case of many of the diluted magnetic semiconductors. More systematic study to enumerate the magnetic nature of these materials at very low temperatures needs to be carried out for further understanding. From these curves, it is also clear that the temperature curve (Tc) of these samples is well above room temperature. To investigate the effect of dopants on the optical properties, photoluminescence studies are carried out at room temperature. Figure 6 shows the emission spectrum of ZnO, Zn0.97Ni0.03O, and Zn0.94Ni0.03Li0.03O nanorods obtained after exciting the samples at 345 nm. Pure ZnO nanorods are characterized by a sharp emission peak around 382 nm along with a broad peak centered around 500 nm. On the basis of the previous studies, the sharp peak around 382 nm has been attributed to the near band edge emission from the sample and the broad peak to the defects brought about by the zinc vacancies present in the sample. With incorporation of Ni2+ in the lattice, the defect emission gets completely quenched and only the near band edge emission is observed. The pattern remained more or less the same with Li+ codoping in Ni2+-doped ZnO samples. Doping transition metal ions in ZnO increases the nonradiative process20 and thereby the luminescence intensity from ZnO is reduced, which has been attributed to the trapping of the electron by the transition metal ions thereby preventing the recombination of electrons and holes in the lattice. By applying the same argument here, the lack of emission from Ni2+-doped sample can be attributed to the reduction of the extent of electron hole recombination by trapping of the electron by the Ni2+ centers present in the lattice. Thus, luminescence quenching in (Ni,Li+)doped ZnO samples can be attributed to the nonradiative process arising due to these impurity-doping of Ni and Li+. To further substantiate the experimental observations and to understand the origin of the doping induced impacts on the magnetic moment, detailed theoretical calculations were carried out. The calculations of Ni-on-Zn-doped (3 at.%) ZnO show as expected from Hund’s rule a total magnetic moment 2.0 µB with a distinct local magnetic effect of 1.6 µB from the d-like state
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Figure 7. Calculated magnetic moment of Li-Ni-codoped ZnO (3 at.% Ni) as function of Li percentage content using the 72-atom supercell Zn31-xLi32xNiO32; thus Li concentrations x ) 0, 1/32, 2/32, and 3/32 ≈ 0, 3, 6, and 9%, respectively. Li prefers a location close to the Ni dopant, and the open rectangles in the figure represent configurations with all Li atoms next to Ni, whereas filled circles represent configurations with only one Li close to the Ni dopant and remaining Li atoms randomly distributed. Cross marks indicate magnetic moment of charged Zn31NiO32 system having the same number of valence electrons as Zn31-xLi32xNiO32.
at the Ni2+. The four neighboring O atoms to the Ni dopant are induced with only ∼0.07 µB/oxygen on the p-like states, thus the ferromagnetism in Zn0.97Ni0.03O is caused by the magnetic moments of Ni2+. Moreover, Ni marginally distorts the ZnO host with a Ni-O bond length of δNi-O ≈ 0.199 nm to be compared with the bond length of δZn-O ) 0.198 nm in an intrinsic ZnO. Additional codoping by Li-on-Zn (i.e., 3 at.% Li and 3 at.% Ni) affects the magnetic properties in a similar way whether Li is located near or away from the Ni dopant. The total energy calculations demonstrate that Li prefers a Zn site next the Ni atom, even though the Li-O bond lengths are relatively large δLi-O ≈ 0.203-0.232 nm. The total energy Et is ∼0.2 eV/lithium lower for the system where Li is located close to Ni. Li-on-Zn acts as an acceptor and thus decreasing the valence electrons. Li (3 at.%) (that is, one Li per Ni atom) will thereby empty the Ni d-states in the ZnO gap region (Figure 8). Also, Li induces a somewhat larger magnetic moment of Ni (now 1.9 µB) plus additional magnetic moment of the O by (0.16-0.23) µB/oxygen. This additional magnetization occurs on the four O p-like states next to the Ni atom through hybridization with the Ni d-like states (see DOS in Figure 8), thus Ni 3d8 to 3d7-like. The total magnetic moment increases thereby by 1.0-3.0 µB. The same effect occurs by varying the valence population by ionization (cross marks in Figure 7) instead of Li incorporation. Because of the Ni-d-O-p hybridization, the magnetic moment is less localized, but the system is a semiconductor with a rather local magnetic moment and thus not hole mediated ferromagnetism. As a consequence of the completely filled or empty Ni d-bands, the filled Ni spin-down states become more localized well below the ZnO host topmost bands. The total energy calculations also reveal that further codoping by Li-on-Zn (i.e., 6 and 9 at.% Li) preferably will form Li cluster around the Ni dopant, however, the energy cost for Li to be
Figure 8. Spin-dependent and atomic resolved DOS of bulk ZnO and Zn31-xLi32xNiO32 (3 at.% Ni). DOS for host ZnO is shown as gray area, whereas blue and red lines represent Ni and Li, respectively. The DOS is in units of 1/(eV · atoms) to better visualize the Ni and Li contribution. Highest occupied energy state is indicated by dashed vertical lines. The sharp blue lines are local/partial DOS of Ni, and red lines show Li DOS.
away from Ni-O-Li cluster is low (
