Comparison on Photoluminescence and Magnetism between Two

Article pubs.acs.org/JPCC

Comparison on Photoluminescence and Magnetism between Two Kinds of Undoped ZnO Nanorods Xiaoyong Xu,†,‡ Chunxiang Xu,*,† Yi Lin,† Jitao Li,† and Jingguo Hu† †

State Key Laboratory of Bioelectronics, School of Electronic Science and Engineering, Southeast University, Nanjing 210096, China School of Physics Science and Technology, Yangzhou University, Yangzhou 225002, China

‡

ABSTRACT: We carried out a detailed comparison on the defect-related photoluminescence and magnetism in two kinds of undoped ZnO nanorods synthesized by low-temperature chemical bath deposition and high-temperature chemical vapor deposition methods to clarify further the nature of visible emission and d0 ferromagnetism in ZnO based on the fact that these two kinds of ZnO nanorods have significantly different crystallinity and defect states. The results obtained by analyzing X-ray diffraction, electron paramagnetic resonance, and Raman scattering show that the intrinsic ferromagnetism in ZnO nanorods is originated from the single ionized oxygen defects (vacancies and interstitials). Particularly, it is demonstrated that the lattice disorder along the c-axis along with the introduced oxygen interstitials can boost ferromagnetism and induce the red-shift in visible emission.

1. INTRODUCTION

structural dependence of RTFM and visible photoluminescence (PL) in undoped ZnO. In this work, we employ a comparative study on the defectrelated PL and FM in undoped ZnO NRs fabricated by two distinct methods. With the help of high-resolution transmission electron microscope (HRTEM) with selected electron diffraction (SAED), X-ray diffraction (XRD), Raman scattering (RS), and electron-spin-resonance (ESR) measurements, the correlations between microsturctural, optical, and magnetic properties in terms of defects and crystallinity were investigated in detail.

Diluted magnetic semiconductors (DMSs) have attracted great interest due to the meritorious collaboration between chargebased semiconductors and spin-based magnetism for developing spintronics.1,2 Following the theoretical prediction of roomtemperature (RT) ferromagnetism (RTFM) in wide band gap DMSs,3 transition metal (TM) doped ZnO as a particularly promising candidate has triggered extensive studies.4−6 Interestingly, unexpected RTFM has also been observed in various nondoped ZnO nanostructures,7−11 such as nanoparticles (NPs),7,8 nanorods (NRs),9,10 nanowires (NWs),11 and so on, although the origin of such a so-called d0 ferromagnetism (FM) still remains debatable.12 For example, Wu11 and Panigrahy et al.9 suggested a correlation between the FM and the oxygen vacancies (VO) by tuning the oxygen deficiency in ZnO NWs and NRs, respectively. However, Li13 and Zhang et al.14 claimed recently that the FM in undoped ZnO nanofilms may be attributed to the existence of zinc interstitials (Zni). Most recently, Yu et al.8 produced a high RTFM in pure ZnO NPs by mechanical milling and revealed that zinc vacancies (VZn) may be more effective in boosting FM. In addition, both the crystalline quality and orientation have also been found to have a great effect on the magnetic ordering in ZnO-based DMSs.15,16 Based on these representative reports, it can be concluded that the FM in undoped ZnO may be somehow related to crystalline quality, grain boundaries, and several intrinsic defects like VO, Zni, and VZn, etc. Some previous works showed that the ZnO nanorods synthesized by chemical bath deposition (CBD) and chemical vapor deposition (CVD) methods have markedly different crystallinity and defect states.10,17 This idea motivated us to utilize such a marked difference to investigate the micro© XXXX American Chemical Society

2. EXPERIMENTAL SECTION The two types of ZnO NRs, denoted S1 and S2, were fabricated respectively by CVD and CBD methods similar to the previous letters.18,19 Typically, in the CVD growth process, pure 0.1 g of ZnO powder maxed with graphite at the mass ratio of 1:1 was used as the source, and argon mixed with 5% oxygen was used as the carrying gas. A rinsed Si substrate placed downstream of gas at the distance of about 10 cm away from the source was used to collect the sample S1 when the source temperature was kept at 1150 °C for 30 min. In the CBD growth, the equimolar of 0.05 M aqueous solutions of pure Zn(CH3COO)2·2H2O and C6H12N4 were prepared and mixed together. A Si coated with Zn(CH3COO)2·2H2O seed layer was immersed into the above solution and kept at 90 °C for 2 h to collect the product. The previous studies have shown the as-grown ZnO NRs via lowtemperature CBD route have plenty chemical groups and diverse defects attached at the surface, and the annealing can Received: June 7, 2013 Revised: October 26, 2013

A

dx.doi.org/10.1021/jp405662y | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

Figure 1. (a c) SEM images, (b d) TEM, HRTEM images, and corresponding SAED patterns of S1 and S2. The HRTEM images of S1 and S2 show the (0002) and (101̅0) planes of wurtzite ZnO with the lattice spacing of 0.26 and 0.28 nm, respectively. A few deformed domains in S2 are marked by white circles. The SAED pattern of S1 displays a perfect single crystal lattice with [0002] growth direction, whereas irregular SAED pattern of S2 conforms further the existence of the lattice disorder.

Figure 2. (a) XRD patterns and (b) RS spectra excited by λ = 785 nm of S1, S2, and SC. The quenching of XRD (0002) peak and RS A1(TO) mode indicates the lattice deformation along the c-axis in S2.

remove these chemical bonds to transform the surface states.20 Thus, for the CBD NRs, a postgrowth thermal treatment was performed at 600 C in air for 2 h to exclude the effect of other chemical appendages. The morphologies and microstructures of as-synthesized samples were characterized by field-emission scanning electron microscopy (SEM, Hitachi S-4800), transmission electron microscopy (TEM, JEM-2100), and HRTEM equipped with SAED, X-ray diffractometer (Shimadzu XRD-7000) with Cu Kα radiation (1.5406 Å), and RS spectrometer (Renishaw in Via) with an excitation laser of 785 nm. PL measurements (FL4600) were performed at RT with a Xe lamp emitting at 340 nm as excitation source. ESR spectra (A300-10/12) were recorded at RT under an X-band with magnetic field modulation at 9.86 GHz. The magnetic properties of the samples were measured by a vibrating sample magnetometer (VSM) integrated in a physical property measurement system (PPMS-9, Quantum Design).

3. RESULTS AND DISCUSSION Figures 1a−d compare the morphologies and microstructures of S1 and S2 by showing the SEM, TEM, and HRTEM images along with the corresponding SAED patterns. The SEM and TEM images reveal that both S1 and S2 show the rod-like structures of similar size with 110 nm diameter. As shown in Figures 1a and 1c, the NRs in S1 are almost perpendicular to the substrate, while that in S2 are more randomly oriented. The HRTEM images of S1 and S2 show respectively the lattice spacing of 0.26 nm for the adjacent (0002) planes and that of 0.28 nm for the adjacent (101̅0) planes of wurtzite ZnO. These two crystalline planes are usually detected in the HRTEM characterization of ZnO nanocrystals, and it is worth noticing that a few damaged domains (marked by white circles) can be observed in the HRTEM image of S2. Such a lattice deformation may be mainly caused by the surface reconstruction accompanying by the 600 °C annealing in the CBD NRs because the previous works have demonstrated that the asB



Article

Figure 3. (a) PL spectra and (b) X-band (9.86 GHz) ESR spectra of S1, S2, and SC. The inset in (a) shows the digital photographs of S1 and S2 under the 365 nm UV irradiation. The broad visible emissions are fitted by P0 (478 nm), P1 (516 nm), P2 (569 nm), and P3 (629 nm). The g-factor can be calculated by the equation g = hv/μBB.

splits the transverse optical (A1(TO)) and longitudinal optical (A1(LO)) components with different frequencies because of the macroscopic electric fields correlated with the LO phonons.21 Among them, the A1(LO) mode generally cannot be detected for the excitation wavelengths longer than 407 nm.25 For the SC with only the c-axis orientation, it is reasonable that the polar A1(TO) mode also is not detected, whereas for the polycrystalline ZnO samples (S1 and S2), the A1(TO) mode should always appear and represent the lattice quality along the c-axis of ZnO crystals. Thus, the A1(TO) mode quenching in S2, as shown in Figure 2b, suggests further the presence of the lattice deformation along the c-axis, in agreement with the HRTEM and XRD results. To identify the intrinsic point defects in CVD and CBD NRs, we carried out the PL and ESR measurements. Figure 3a shows the PL spectra of S1, S2, and SC as a reference. The SC exhibits only one ultraviolet (UV) emission induced by the near-bandedge (NBE) exciton recombination of ZnO, where no obvious visible emission related to defects can be observed, confirming the scarcity of defects in the SC. While for the S1 and S2, the robust visible emissions peaked at different wavelengths observably appear, and their digital photographs under the 365 nm UV irradiation are shown in the inset of Figure 3a. The deep level visible emissions (DLE) in ZnO usually occur near the blue-green (480−550 nm), yellow (550−610 nm), and orange-red (610−750 nm) regions.26 Although their defect origins are still controversial, it is indicated gradually that the blue-green emission denoted P0 is related to the VZn, and the green emission denoted P1 originates from the singly charged oxygen vacancy (VO+), while the yellow one (∼580 nm) denoted P2 is assigned commonly to the doubly charged oxygen vacancy (VO2+) and the orange-red emission denoted P3 is associated with the interstitial oxygen (Oi) on the ZnO surface, which results mainly from the annealing or the surface modification.9,27,28 Thus, the strong green emission in S1 suggests that the VO+ defects are involved at a high concentration in the CVD NRs, whereas the broad emission containing multiple bands (P1, P2, and P3) in the PL spectrum of S2 indicates that the annealed CBD NRs may possess three main defects of VO+, VO2+, and annealing-introduced Oi. It can be prefigured that there are the rich defect states in the annealed CBD ZnO NRs due to the following two reasons: (i)

synthesized CBD ZnO NRs have the well-formed crystallization, but there are a large number of chemical groups attached on the surface of NRs.10,17 Correspondingly, the SAED pattern of S1 displays a perfect single crystal lattice with [0002] growth direction, whereas the irregular SAED pattern of S2 indicates further the existence of the lattice disorder. Such lattice distortion and defect propagation were also observed in ZnO nanocrystals by the mechanical milling,8,21 the ion implantation, or the oriented annealing,10,15 and they must have significant impacts on the magnetic and optical properties of ZnO. The XRD patterns of S1 and S2 as well as a commercial ZnO single crystal (SC) as a reference are shown in Figure 2a. All diffraction peaks can be indexed to the hexagonal ZnO wurtzite structure. No additional peaks from secondary phases are detected, ruling out the presence of any contaminants within the detection precision. The dominant ZnO (0002) peak of S1 similar to SC corresponds to the orderly arrangement of the CVD NRs along the c-axis. For the S2 NRs with the relatively low-order arrangement, the XRD pattern should exhibit the polycrystalline diffraction phases; however, the (0002) phase corresponding to the growth direction of NRs should also be most significant. However, the (101̅0) peak dominates the XRD pattern of S2 rather than the preferred (0002) phase, implying a possible lattice breakage along the c-axis in CBD NRs, in agreement with the observed lattice distortion in HRTEM and SAED images of S2. Such a microstructural difference between S1 and S2 can be further confirmed by analyzing their RS spectra. Figure 2b displays the RS spectra of S1, S2, and SC as a reference. The conventional vibration peaks at 100, 332, 380, and 438 cm−1 are assigned to the processes of E2(L), E2(H)−E2(L), A1(TO), and E2(H), respectively.22 The nonpolar E2 phonon modes as prominent peaks in these three samples have two frequencies: the low-frequency E2(L) mode is related to the vibration of the Zn sublattice, whereas the highfrequency E2(H) mode is associated with the vibration of O atoms.23 The polar A1 phonon mode corresponds to the displacement of ions parallel to the c-axis in wurtzite ZnO crystals, where close-packed layers of O and Zn are stacked alternately along the c-axis; the lattice irregularity induced by zinc or oxygen defects along the c-axis will directly affect the displacement of ions in the A1 mode.24 Usually, the A1 mode C



Article

and SC before and after the necessary background diamagnetic subtraction, respectively. The central parts of the hysteresis curves are amplified in the inset of Figure 4b. The reference SC hardly shows the magnetism, but both S1 and S2 display clear hysteresis loops, demonstrating the RTFM. For the S1, the saturation magnetization (Ms) and the coercive field (Hc) are about 0.031 emu/g and 75 Oe, respectively. The Ms of S1 is quite comparable to the values reported recently in the undoped ZnO nanoparticles8 and nanosheets.27 In contrast, the S2 exhibits a stronger FM with its Ms of 0.06 emu/g and Hc of 73 Oe. From the above analyses on microstructures and defects, the lattice deformation along the c-axis and defect propagation occur in the S2. In the BMPs framework, the observed VO+ and Oi− defects with unpaired electrons as the paramagnetic F+ centers can serve as the source of ferromagnetism, but a sufficient density of F+ centers is also necessary for magnetic percolation due to that only the overlapping of magnetic polarons can induce spin−spin interactions between F+ centers, stabilizing a long-range ferromagnetic ordering.33,34 The structural inhomogeneity in S2 can favor the overlapping of magnetic polarons and yield a greater overall volume occupied by BMPs and then promoting the creation and percolation of ferromagnetic domains. Therefore, the enhancement of RTFM in the S2 should be attributed to the lattice imperfection along the c-axis along with the annealing-introduced Oi− defects.

the low-temperature CBD grown ZnO NRs usually have poor crystallization, abundant defects, and strange surface composition; (ii) the microstructure of such ZnO NRs is particularly sensitive to the postannealing, which can reconstruct surface composition and transform defect states. For example, Yang et al.17 found that the annealing can largely remove the OH and H bonds and change the surface states using the chemisorbed oxygen at the surface of ZnO NRs grown by the lowtemperature CBD method. Lin et al.29 also reported that the high-temperature air annealing can lead to the reassignment of the single charged interstitial oxygen (Oi−) and VO+ defects based on the following reactions: ZnO → ZnO1 − x ·x VO+·Oi−

(1)

ZnO1 − x ·x VO+· Oi− + (y /2)O2 → ZnO1 − x + y · (x − y)VO+·xOi− − ye−

(2)

Among the above-mentioned defects, the VO+ and Oi− defects with unpaired electrons are paramagnetic and are responsible for the generation of the ESR signals, although the defectrelated paramagnetic centers still remain controversial.30−32 Figure 3b illustrates the ESR spectra of S1, S2, and SC as a reference. As expected, no ESR signal can be detected in the defect-rare ZnO SC. For the S1, there is a single high-field ESR signal, which should be assigned to the VO+ defects identified by P1 emission in Figure 3a. While the S2 shows another low-field ESR signal, which probably originates from the paramagnetic Oi− defects related to P3 emission according to the above defect and PL analyses. Therefore, it is strongly suggested that the VO+ and Oi− defects can serve as F+ centers to activate bound magnetic polarons (BMPs), giving rise to the ferromagnetic ordering. Figures 4a and 4b show the magnetization versus magnetic field (M − H) hysteresis curves measured at 300 K for S1, S2,

4. CONCLUSIONS In conclusion, a comparative study on the defect-related PL and FM in two kinds of undoped ZnO NRs synthesized by CVD and CBD methods reveals the correlations between microstructural, optical, and magnetic properties in terms of defects and crystallinity. The results suggest the single charged oxygen defects (VO+ and Oi−) with unpaired electrons as the paramagnetic F+ centers may be responsible for the observed RTFM; moreover, the lattice distortion along the c-axis along with the introduced oxygen interstitials can lead to the enhancement of RTFM and the red-shift in DLE. This study gets further insight into the defect origins of d0 RTFM in undoped ZnO and provides a significant way to boost such RTFM.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.X.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (Grants 11104240 and 60725413), “973” Program (Grants 2011CB302004 and 2013CB932903), MOE (Grant 20110092130006), JSIS (BE2012164), JGSOS, and SRFSU.

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Comparison on Photoluminescence and Magnetism between Two

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