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Evidence for surface states in pristine and Co-doped ZnO nanostructures: magnetization and nonlinear optical studies
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2011 Nanotechnology 22 095703 (http://iopscience.iop.org/0957-4484/22/9/095703) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 22 (2011) 095703 (5pp)
doi:10.1088/0957-4484/22/9/095703
Evidence for surface states in pristine and Co-doped ZnO nanostructures: magnetization and nonlinear optical studies Ramakrishna Podila1 , Benoy Anand2, J Palmer West3 , Reji Philip4 , S Siva Sankara Sai2 , Jian He1 , Malcolm Skove1, Shiou-Jyh Hwu3 , Sumanta Tewari1 and Apparao M Rao1,5,6 1
Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, USA Department of Physics, Sri Sathya Sai Institute of Higher Learning, Prashanti Nilayam, Andhra Pradesh, 515134, India 3 Department of Chemistry, Clemson University, Clemson, SC 29634, USA 4 Light and Matter Physics Group, Raman Research Institute, C V Raman Avenue, Sadashivanagar, Bangalore 560080, India 5 Center for Optical Materials Science and Engineering Technologies, Clemson University, Clemson, SC 29634, USA 2
E-mail:
[email protected]
Received 5 October 2010, in final form 5 January 2011 Published 24 January 2011 Online at stacks.iop.org/Nano/22/095703 Abstract An unexpected presence of ferromagnetic (FM) ordering in nanostructured ZnO has been reported previously. Recently, from our detailed magnetization studies and ab initio calculations, we attributed this FM ordering in nanostructured ZnO to the presence of surface states, and a direct correlation between the magnetic properties and crystallinity of ZnO was observed. In this study, through a systematic sample preparation of both pristine and Co-doped ZnO nanostructures, and detailed magnetization and nonlinear optical (NLO) measurements, we confirm that the observed FM ordering is due to the presence of surface states. S Online supplementary data available from stacks.iop.org/Nano/22/095703/mmedia (Some figures in this article are in colour only in the electronic version)
In this study, we investigate the magnetic properties of pristine and Co-doped ZnO nanostructures. Previously, homogeneous thin films of Zn1−x Cox O were found to be anti-ferromagnetic (AFM) [6], while inhomogeneous films exhibited FM, implying that the FM results from the presence of Co clusters [7–9]. In addition, it is widely believed that FM in many metal oxides originates from defects such as oxygen vacancies and is closely related to the electronic structure of the magnetic impurity ions [2–5, 10–12]. The nature and concentration of defects are dependent on synthesis conditions and therefore controversies regarding reproducibility are commonplace [13–16]. In our earlier studies on the origin of FM in pristine microand nanostructured ZnO, we observed the changes in magnetic
1. Introduction Wide interest in the scientific community to integrate both charge and spin into micro- and opto-electronic devices began with the observation of hole-mediated ferromagnetism (FM) in (Ga, Mn)As [1]. The low Curie temperature of (Ga, Mn)As (∼170 K) led to a search for alternate materials. Interestingly, high temperature FM in nanophase oxides was found in HfO2 and CuO, which are known to be diamagnetic (DM) in the bulk forms [2]. Subsequently, high temperature FM in several nanophase oxides, such as CaO, MgO, ZnO, CeO2 , Al2 O3 , In2 O3 and SnO2 , was reported in the literature [3–5]. 6 Author to whom any correspondence should be addressed.
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properties with varying crystallinity of ZnO. We attributed the FM to the presence of surface states [17]. Here, through a systematic sample preparation of both pristine and Co-doped ZnO nanostructures, and detailed magnetization and nonlinear optical (NLO) measurements, we confirm that the observed FM results are due to surface defects.
2. Experimental details For synthesizing pristine ZnO nanostructures, a 50 mM aqueous solution of ZnCl2 was injected (rate of 0.1 ml min−1 ) into a 1 diameter quartz tube reactor maintained at 550 ◦ C. An appropriate amount of Co(CH3 COO)2 was added to the ZnCl2 to prepare the 2% Co-doped nanostructures. A constant flow of O2 and H2 (10:1) was maintained at 500 sccm. Detailed characterization using electron microscopy, x-ray diffraction and Raman spectroscopy is provided in the supplementary section (available at stacks.iop.org/Nano/22/ 095703/mmedia) [17]. Magnetization studies were performed using a Quantum Design SQUID MPMS (model 5S) and the following samples were measured: (i) as-prepared pristine and Co-doped ZnO (denoted AP-CVD and AP-CoZnO); (ii) pristine and Co-doped ZnO samples annealed in an O2 atmosphere at 100 ◦ C for 5.5 h (O2 -100-5.5 h and Co–O2 -100-5.5 h); (iii) pristine and Codoped ZnO samples annealed in an Ar atmosphere at 400 ◦ C for 5 min (Ar-400-5 m and Co–Ar-400-5 m). The NLO measurements were done using the open aperture Z -scan technique [18], with linearly polarized 5 ns optical pulses from a Q -switched frequency-doubled Nd:YAG laser at 532 nm. The sample solutions in isopropyl alcohol were contained in 1 mm thick quartz cells for measuring the change in transmittance.
Figure 1. (a) SEM images of as-prepared CVD ZnO nanostructures on Si(100) substrates. The conical Co-doped ZnO nanostructures vary in diameter from 50 to 100 nm along the length and are typically ∼1–2 μm long. The inset shows EDX spectra of pristine ZnO nanowires. (b) SEM images of 2% Co-doped ZnO nanostructures on Si(100) substrates. The insets show the EDX of AP-CoZnO samples. Along with Zn and O, uniform distribution of Co was confirmed, as seen in the lower right panel.
3. Results and discussion fields (∼1 T) indicate the presence of reasonably large spin clusters. This FM ordering may be understood by considering chemisorption of O2 to form O− 2 . Since the growth occurs in an O-rich environment, it is probable that O2 is chemisorbed preferentially near a cluster of Zn vacancies. Given such a possibility one may visualize a dynamic exchange of electrons between O2 and O− leading to FM ordering [17, 19–21]. Furthermore, this FM ordering and hence the sharp transition is absent in AP-CVD samples after annealing in Ar at 400 ◦ C for a short time (Ar-400-5 m, figure 2(a)). In our previous studies, we observed that annealing in Ar presumably leads to (i) the merging of grains in our polycrystalline ZnO nanostructures, reducing the net surface/interfacial area [17], and (ii) reducing the amount of chemisorbed O2 in the sample, resulting in a DM response. As shown in figure 2(b), FM observed in AP-CoZnO samples also saturates at low magnetic fields (∼1 T) and is embedded in a background DM response similar to pristine ZnO. Unlike AP-CVD samples, a sharp transition from FM to diamagnetism is absent in AP-CoZnO samples (figure 2(b)). The absence of a sharp transition indicates that the FM ordering and diamagnetism originate from two different sources, namely, FM from Co dopants and diamagnetism
3.1. Electron microscopy Figure 1 shows electron microscope images of pristine and 2% Co-doped ZnO nanostructures prepared by the CVD technique. As shown in figure 1(a), pristine ZnO forms conical nanostructures that vary in diameter from 50 to 100 nm along the length and are typically ∼1–2 μm long. Energy dispersive x-ray spectroscopy (EDX) was employed to confirm the presence of the Zn, Co and O atoms in the nanostructures (inset of figures 1(a) and (b)). The distribution of Co dopants was observed to be fairly uniform for 2% Co-doped ZnO nanostructures (figure 1(b)). 3.2. Magnetization studies As shown in figure 2, clear evidence for FM is observed in the AP-CVD samples. This FM saturates at low magnetic fields (∼1 T) and is embedded in a DM background response. Upon annealing in O2 at 100 ◦ C (O2 -100-5.5 h), the FM signature is greatly enhanced as seen in figure 2(a). It is noteworthy that the sharp transition from FM to DM response in the insets in figure 2(a) implies that the measured magnetization is not due to a simple superposition of FM ordering and a DM response. The signs of saturation at relatively low magnetic 2
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Annealing effects on FM for AP-CoZnO samples were found to be similar to AP-CVD samples. We observed an increase in the net normalized magnetic moment from ∼25 to ∼45 memu g−1 upon annealing the AP-CoZnO samples in O2 at 100 ◦ C (Co–O2 -100-5.5 h; figure 2(b)). We also observed a DM response in AP-CoZnO samples annealed at 400 ◦ C in Ar for 5 min (Co–Ar-400-5 m; figure 2(b)). 3.3. Theoretical studies Previously, we showed using ab initio calculations that the net magnetization in ZnO nanostructures could arise from two possible contributions: one due to localized O p orbitals and the other due to delocalized O p orbitals in the ZnO lattice. In our model, the localized orbitals (px ) are preferentially aligned perpendicular to the wire direction, whereas the delocalized orbitals (pz ) point in a direction parallel to the wire direction. This would imply that the localized orbitals contribute a local moment, forming a paramagnetic center that can interact at a longer range with other such centers through the delocalized orbitals. This results in a net magnetization. Thus, annealing in O2 leads to an enhancement in the FM signal observed in the O2 -100-5.5 h and Co–O2 -100-5.5 h samples (cf figures 2(a) and (b)). From our previous calculations, for pristine ZnO nanostructures, the net computed magnetic moment was found to be strongly localized near the surface of the nanowires (FM ordering), while the interior of the wire did not show a similar strong contribution (DM response; cf figure 4 in [17]). Also, the origin of magnetization in O p orbitals was strongly supported by the presence of unpaired oxygen p levels at the Fermi energy in the projected density of states (PDOS) of pristine ZnO [17]. Earlier reports on Co-doped ZnO showed that new surface states are introduced above the valence band maximum in the gap region of ZnO when the Co atoms are incorporated [22]. Such new surface states introduced by Co would enhance the FM in ZnO nanostructures as observed in figure 2(b).
Figure 2. (a) M – H curves of CVD prepared ZnO nanostructures. As-grown nanostructures show a weak, but clear (top inset), FM embedded in a dominant DM response. The bottom inset shows enhancement in FM ordering by at least an order of magnitude when annealed in O2 at 100 ◦ C. (b) M – H curves of CVD prepared Co-doped ZnO nanostructures at room temperature. As-grown Co-doped ZnO nanostructures (top inset) exhibit at least three times stronger magnetization than pristine ZnO nanostructures. The bottom inset shows enhancement in FM ordering when annealed in O2 at 100 ◦ C.
3.4. Nonlinear optical studies Here we show that the presence of such surface states in pristine and AP-CoZnO can be confirmed from another independent measurement: the open aperture Z -scan. The Z -scan is a nonlinear optical measurement, in which the light-induced change in transmittance (T ) of a medium due to optical nonlinearity is measured as a function of input light energy density (fluence) or intensity. From this one can calculate the nonlinear absorption and refraction coefficients. In the experiment, a continuous variation of the input fluence is achieved by translating the sample under study through the focal region of a focused laser beam (the sample position is taken as z with z = 0 being the focal point for the incident laser beam; hence the name ‘ Z -scan’). A detailed description of the Z -scan technique can be found elsewhere [23]. The key point to note here is that, except when excited by ultrafast laser pulses, T strongly depends on the excited state population density. In view of this, we have measured the nonlinear transmission of the present samples in the nanosecond excitation regime, using linearly polarized 5 ns
from ZnO. Furthermore, in the case of AP-CoZnO samples, the transition from FM to DM response does not occur until ∼2–3 T, suggesting that the robust FM is due to the presence of Co atoms in the ZnO lattice. It is noteworthy that the net normalized magnetic moment for AP-CoZnO (∼25 memu g−1 ) samples is at least three times larger than the pristine ZnO nanostructures (∼7 memu g−1 ), confirming the enhancement of FM due to Co doping. Thus the FM ordering in AP-CoZnO samples may be understood in terms of Co atoms substitutionally doping the Zn sites. The formation of Co clusters in our samples is highly unlikely, since the nominal doping concentration (2 at.%) is so low. Also, EDX confirms uniformly distributed Co in AP-CoZnO samples (figure 1(b)). In addition, no signs of Co clusters were observed in either TEM or x-ray diffraction. 3
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Figure 3. (a) Z -scan data for AP-CoZnO samples fitted to a three-photon absorption (3PA) equation. The inset is a schematic depicting effective 3PA. When the intermediate states are virtual the effect is genuine 3PA, but when they are real it is an effective 3PA arising mostly from excited state absorption. (b) Z -scan data for pristine ZnO samples fitted to a 3PA equation. (c) Z -scan data for AP-CoZnO samples at different incident laser energies. (d) Variation of the 3PA coefficient (γ ) as a function of the incident laser pulse energy for AP-CVD, AP-CoZnO, O2 -100-5.5 h and Ar-400-5 m samples.
optical pulses from a Q -switched frequency-doubled Nd:YAG laser operating at 532 nm. The Z -scan data obtained for AP-CoZnO and pristine ZnO samples (figures 3(a) and (b) respectively) are found to be best fitted numerically by a threephoton absorption (3PA) process. Detailed NLO studies will be reported elsewhere. The nonlinear absorption coefficient α in the presence of 3PA is given by α(I ) = α0 + γ I 2 , where I is the intensity of the laser, α0 the unsaturated linear absorption coefficient, and γ the 3PA coefficient. The value of γ is obtained by fitting the Z -scan curve to the 3PA propagation equation, given by ddzI = −α0 I − γ I 3 , where z is the propagation distance within the sample. The values of γ numerically obtained from the fits indicate that nonlinear absorption in the present case arises not only from genuine 3PA, but also from ‘effective’ three-photon nonlinearity [23, 24]. Such effective 3PA originates from sequential excited state absorption as shown in the inset of figure 3(a). In the case of genuine three-photon absorption (3PA), where the transition states involved are virtual, the 3PA coefficient (γ ) is a constant and is independent of the incident laser fluence. All genuine ground state absorption coefficients (such as 2PA or 3PA) are dependent on the ground state population N , since the absorption coefficient is obtained by multiplying the corresponding absorption cross section (σ ) by N . The absorption cross section is a microscopic parameter,
independent of N , that characterizes the average twophoton/three-photon absorbability per molecule. However, the change in γ will be evident only when there is a substantial change in the ground state population due to the absorption. Therefore, if the absorptions are weak, then the coefficients can be considered to be almost constant, since there is a negligible change in N . Genuine 2PA and 3PA are usually very weak phenomena, and hence the corresponding coefficients can be considered as material constants at a given wavelength and concentration. But strong excited state absorption (ESA) during an effective 3PA depletes the ground state population significantly so that the absorption coefficient is no longer a constant. Hence a change in γ w.r.t. incident laser intensity can help confirm the presence of surface states in the forbidden gap of ZnO. Both two-photon absorption and/or sequential absorption of two photons at 532 nm (2.33 eV; inset figure 3(a)) will result in a real terminal level which is above the lowest excitonic state, that lies around 363 nm (3.4 eV) for ZnO, and another one-photon absorption to go from this level to the band edge. Figures 3(c) and (d) show the variation of γ for AP-CoZnO, AP-CVD, O2 -100-5.5 h and Ar-400-5 m respectively, as a function of incident laser energy. While γ remains almost a constant with input energy in AP-CVD, there is a pronounced increase in γ with incident energy in 4
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AP-CoZnO and O2 -100-5.5 h samples. Moreover, the 3PA coefficient (and hence the optical limiting efficiency) for APCoZnO samples is obviously higher than that of pristine ZnO. These observations concur with the enhancement seen in the magnetic moment of AP-CoZnO and O2 -100-5.5 h samples and indicate the presence of enhanced surface state density due to Co atoms in the ZnO lattice. Thus the increase of γ with the incident laser energy confirms effective 3PA due to the existence of a wide range of surface states in AP-CoZnO and O2 -100-5.5 h samples. Furthermore, the lowest γ values and a constant trend with input fluence were observed for Ar400-5 m samples, indicating the absence of any surface states. Thus, we believe that an enhanced surface state density in APCoZnO due to Co doping results in stronger FM and better optical limiting compared to pristine ZnO nanostructures.
[5] Belghazi Y, Schmerber G, Colis S, Rehspringer J L, Dinia A and Berrada A 2006 Extrinsic origin of ferromagnetism in ZnO and Zn0.9 Co0.1 O magnetic semiconductor films prepared by sol–gel technique Appl. Phys. Lett. 89 122504 [6] Kim J H, Kim H, Kim D, Ihm Y E and Choo W K 2002 Magnetic properties of epitaxially grown semiconducting Zn1−x Cox O thin films by pulsed laser deposition J. Appl. Phys. 92 6066 [7] Song C, Geng K W, Zeng F, Wang X B, Shen Y X, Pan F, Xie Y N, Liu T, Zhou H T and Fan Z 2006 Giant magnetic moment in an anomalous ferromagnetic insulator: Co-doped ZnO Phys. Rev. B 73 024405 [8] Sati P et al 2006 Magnetic anisotropy of Co+ 2 as signature of intrinsic ferromagnetism in ZnO:Co Phys. Rev. Lett. 96 017203 [9] Venkatesan M, Fitzgerald C B, Lunney J G and Coey J M D 2004 Anisotropic ferromagnetism in substituted zinc oxide Phys. Rev. Lett. 93 177206 [10] Hong N H, Sakai J and Brize V 2007 Observation of ferromagnetism at room temperature in ZnO thin films J. Phys.: Condens. Matter 19 036219 [11] Gacie M, Jakob G, Herbort C and Adrian H 2005 Magnetism of Co-doped ZnO thin films Phys. Rev. B 75 205206 [12] Kittilstved K R, Liu W K and Gamelin D R 2006 Electronic structure origins of polarity dependent high- TC ferromagnetism in oxide diluted magnetic semiconductors Nat. Mater. 5 291 [13] Diebold U 2003 Structure and properties of TiO2 surfaces: a brief review Appl. Phys. A 76 681 ¨ ur U, Alivov I Y, Liu C, Teke A, Reshchikov M A, [14] Ozg¨ Dogan S, Avrutin V, Cho S J and Morkoc¸ A 2003 Comprehensive review of ZnO materials and devices J. Appl. Phys. 98 041301 [15] McCluskey M D and Jokela S J 2009 Defects in ZnO: focused review J. Appl. Phys. 106 071101 [16] Schmidt-Mende L and MacManus-Driscoll J L 2007 ZnO nanostructures, defects, and devices Mater. Today 10 40 [17] Podila R et al 2010 Origin of FM ordering in pristine ZnO micro- and nanostructures Nano Lett. 10 1383 [18] Sivaramakrishnan S, Muthukumar V S, Reppert J, Anija M, Sivasankara Sai S, Philip R, Venkataramaniah K, Kuthirummal N and Rao A M 2007 Nonlinear optical scattering and absorption in bismuth nanorod suspensions Appl. Phys. Lett. 91 093104 [19] Tam K H et al 2006 Defects in ZnO nanorods prepared by a hudrothermal method J. Phys. Chem. B 110 20865 [20] Fan Z, Chang P C, Lu J G, Walter E C, Penner R M, Lin C and Lee H P 2004 Photoluminescence and polarized photodetection of single ZnO nanowires Appl. Phys. Lett. 85 6128 [21] Lee H J, Helgren E and Hellman F 2009 Gate-controlled magnetic properties of the magnetic semiconductor (Zn, Co)O Appl. Phys. Lett. 94 212106 [22] Wang Q, Sun Q and Jena P 2010 First principle studies on magnetic properties of Zn1−x Cox O thinfilms Nanotechnology 22 076002 [23] Van Stryland E W and Sheik-Bahae M 1998 Z -Scan: Characterization Techniques and Tabulations for Organic Nonlinear Materials ed M G Kuzyk and C W Dirk (New York: Dekker) pp 655–92 [24] Sutherland R L 1996 Handbook of Nonlinear Optics (New York: Dekker)
4. Conclusions In summary, pristine ZnO nanostructure (AP-CVD) and 2% Co-doped ZnO nanostructure (AP-CoZnO) samples showed similar FM signatures. The magnitude of the observed change in magnetization was pronounced for AP-CoZnO samples and is presumably due to the presence of Co-induced intermediate states. As expected, a sharp transition from FM to DM was not observed in AP-CoZnO samples, indicating the presence of Co dopants in the ZnO lattice. Lastly, we confirmed the presence of intermediate states in the gap region of ZnO nanostructures using Z -scan nonlinear optical measurements.
Acknowledgments RP and AMR thank Professor Amar Nath, University of North Carolina at Asheville, and are grateful for encouragement from Professor K Venkataramaniah of Sri Satya Sai Institute of Higher Learning. SSSS and BA acknowledge funding from NRB, DST and UGC, India. ST acknowledges funding from DoE.
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