Influence of Y-doped induced defects on the optical and magnetic properties of ZnO nanorod arrays prepared by low-temperature hydrothermal process
- Chung-Yuan Kung†1Email author,
- San-Lin Young†2Email author,
- Hone-Zern Chen2,
- Ming-Cheng Kao2,
- Lance Horng3,
- Yu-Tai Shih3,
- Chen-Cheng Lin1,
- Teng-Tsai Lin1 and
- Chung-Jen Ou4
© Lin et al.; licensee Springer. 2012
Received: 29 November 2011
Accepted: 22 June 2012
Published: 7 July 2012
One-dimensional pure zinc oxide (ZnO) and Y-doped ZnO nanorod arrays have been successfully fabricated on the silicon substrate for comparison by a simple hydrothermal process at the low temperature of 90°C. The Y-doped nanorods exhibit the same c-axis-oriented wurtzite hexagonal structure as pure ZnO nanorods. Based on the results of photoluminescence, an enhancement of defect-induced green-yellow visible emission is observed for the Y-doped ZnO nanorods. The decrease of E2(H) mode intensity and increase of E1(LO) mode intensity examined by the Raman spectrum also indicate the increase of defects for the Y-doped ZnO nanorods. As compared to pure ZnO nanorods, Y-doped ZnO nanorods show a remarked increase of saturation magnetization. The combination of visible photoluminescence and ferromagnetism measurement results indicates the increase of oxygen defects due to the Y doping which plays a crucial role in the optical and magnetic performances of the ZnO nanorods.
The II-VI semiconductor zinc oxide (ZnO) with a direct wide bandgap (3.37 eV) and a large exciton binding energy (60 meV) has attracted substantial attention in the research community [1–3]. Although ZnO had been researched for the past decades, the renewed interests are focused on the low-dimensional nanostructures, such as nanoparticles , nanowires , nanarods , and nanotubes , due to brand new fundamental physical properties and applications of nanodevices. Progressive studies on the performance improvement of these one-dimensional nanostructured ZnOs for optoelectronic device applications have been performed by various growth methods, such as hydrothermal methods , vapor–liquid–solid , metal organic vapor-phase epitaxy , and pulsed laser deposition  and doped with impurities, such as Ag , Li , and P . Besides, effective mass production process of ZnO nanowires by a modified carbothermal reduction method has been also reported . Recent researches further denoted that nanostructured ZnO with large effective surface area is suitable for ultraviolet devices and photovoltaic applications, such as light-emitting diodes , nanolasers , photodetectors , field emitters , chemical sensors , and photo-electrodes in dye-sensitized solar cells .
Recently, the observation of ferromagnetism with high Curie temperature in III-V and II-VI semiconductors has also attracted a great deal of attentions [22–24]. Room temperature ferromagnetism of ZnO doped with transition metals has been also theoretically predicted and experimentally confirmed for spintronics applications [25, 26]. In order to form diluted magnetic semiconductors, ZnO nanostructures have been doped with magnetic metal elements, such as Mn, Co, or Ni . Recently, some researches declared that ferromagnetism had been obtained from undoped nanostructured ZnO and suggested to be induced by defects . Non-magnetic elements, such Bi  or Li , have been doped into ZnO and room temperature ferromagnetism has been also observed. Therefore, ferromagnetism would not originate from the non-magnetic dopants since they do not contribute to ferromagnetism.
Based on the previous reports [1–30], one of the effective ways to approach the optical and magnetic properties of these nanostructured materials is the doping with selective elements. By choosing suitable rare-earth dopant, modification of the properties can be anticipated. In this present study, we will focus on the doping effect of larger non-magnetic element Y on the structural property of Y-doped ZnO (ZnO:Y) nanorods. In addition, the defect-related origin of optical and magnetic properties will be also discussed.
The ZnO and ZnO:Y nanorod arrays were fabricated by hydrothermal method on the ZnO-seeded silicon substrate. Pure ZnO seed layers for both nanorod compositions were firstly deposited on silicon substrate by spin coating technique. Then, the source solutions for ZnO and ZnO:Y nanorods growth were prepared using the precursors, zinc acetate dihydrate Zn(C2H3O2)22H2O and yttrium acetate hydrate Y(C2H3O2)34H2O, in stoichiometric proportions within a blending solvent of de-ionized water and HMTA ((CH2)6 N4). Then, the seeded substrate was placed upside down into the solution contained in a closed vial at 90°C for 3 h to grow the nanorods. Finally, the samples were rinsed with de-ionized water and dried in air for characterization.
The crystal structure was determined by X-ray diffraction (XRD) spectrum using a Rigaku D/max 2200 X-ray diffractometer (Rigaku Corporation, Tokyo, Japan) with Cu-Kα radiation. Morphological characterization was observed using a field emission scanning electron microscope (FE-SEM, JEOL JSM-6700 F, JEOL Ltd., Akishima, Tokyo, Japan) at 3.0 kV. Secondary ion mass spectrometry (SIMS) was utilized to identify the elemental distribution. Room temperature photoluminescence (RTPL) spectroscope was used to measure optical emissions from 350 to 645 nm using the He-Cd laser with wavelength of 325 nm. Raman spectra of the samples were measured with an excitation wavelength of 532 nm from argon laser. Finally, the magnetization measurements were performed using a MicroMag™ 2900 alternative gradient magnetometer (AGM) (Princeton Measurements Corp., Princeton, NJ, USA) at room temperature to investigate the magnetic properties of ZnO and ZnO:Y nanorods.
Results and discussion
Under the hydrothermal conditions, HMTA will hydrolyze and release NH3 to provide (OH)−. Finally, the reaction of Zn2+ and (OH)− brings the products, ZnO and H2O. It is obvious that HMTA plays a key role to form Zn-O bonds. The (002) plane of wurtzite-structured ZnO is terminated with Zn2+, resulting in polar top surfaces with positive charge. In the chemical solution, non-polar HMTA will precedently chelate the non-polar facets except the polar (002) plane for epitaxial method. Therefore, a preferential growth along (002) is reasonably observed. Meanwhile, substitution of Y for Zn during the growth of nanorods can be obtained, which is similar to the synthesized process of Ce-doped ZnO nanorods . Compared with ZnO nanorods, the XRD spectrum of ZnO:Y nanorods exhibits obvious single diffraction peak of (002) and two slight peaks of (101) and (100) as shown in the inset of Figure 1. The obvious decrease of (002) diffraction peak intensity for ZnO:Y nanorods shows the restrain of the crystallization compared with ZnO samples. The result also indicates the suppression of growth rate along (002) crystal plane and slight enhancement of growth rate along (101) and (100) crystal planes. Besides, the decrease of a-axle lattice constants from 3.2526 to 3.2576 Å and c-axle lattice constant from 5.1849 to 5.1904 Å is obtained from the 2θ angles of diffraction peaks measured from ZnO and ZnO:Y nanorods. The reason is that the radius of Y3+ ion (0.92 Å) is larger than that of Zn2+ ion (0.74 Å), and the doping of Y into ZnO nanorods should lead the increase of a- and c-axis lattice constants and, correspondingly, the shift of all diffraction peaks towards lower 2θ angle.
ZnO and ZnO:Y nanorods have been successfully synthesized at a low temperature by hydrothermal method. The correlation between the Y-doped induced defects in the ZnO:Y nanorods and their structural, optical, and magnetic properties was studied in details. It was found that the doping of Y results in the increase of defects which also affects the corresponding structural, optical, and magnetic properties of the nanorods. The XRD spectra show that crystallization is suppressed by the doping of Y in the nanorods. The increase of visible emission in RTPL spectra and E1(LO) intensity in Raman spectra demonstrates that the doping of Y will increase the doping-induced defects in the nanorods. Magnetization curve measurements show the room temperature ferromagnetism of both nanorods. Finally, the combination of the optical and magnetic measurement results reveals that the oxygen defects play a crucial role in introducing ferromagnetism which can be enhanced by the doping of Y in the ZnO nanorods.
C-YK is a professor of the Department of Electrical Engineering, National Chung Hsing University. S-LY, H-ZC, and M-CK are professors of the Department of Electronic Engineering, Hsiuping University of Science and Technology. LH and Y-TS are professors of the Department of Physics, National Changhua University of Education. C-CL and T-TL are Ph.D. students. C-JO is a professor of the Department of Electrical Engineering, Hsiuping University of Science and Technology.
This work was sponsored by the National Science Council of the Republic of China under the grant numbers NSC 101-2221-E-164-004 and NSC 99-2221-E-005-103.
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