Fabrication of ZnCoO nanowires and characterization of their magnetic properties
© Kim et al.; licensee Springer. 2014
Received: 6 January 2014
Accepted: 2 April 2014
Published: 7 May 2014
Hydrogen-treated ZnCoO shows magnetic behavior, which is related to the formation of Co-H-Co complexes. However, it is not well known how the complexes are connected to each other and with what directional behavior they are ordered. In this point of view, ZnCoO nanowire is an ideal system for the study of the magnetic anisotropy. ZnCoO nanowire was fabricated by trioctylamine solution method under different ambient gases. We found that the oxidation of trioctylamine plays an essential role on the synthesis of high-quality ZnCoO nanowires. The hydrogen injection to ZnCoO nanowires induced ferromagnetism with larger magnetization than ZnCoO powders, while becoming paramagnetic after vacuum heat treatment. Strong ferromagnetism of nanowires can be explained by the percolation of Co-H-Co complexes along the c-axis.
Co-doped ZnO (ZnCoO) has been intensively studied because of its widespread applicability as a magnetic semiconductor[1–3]. Many studies have shown that its ferromagnetism depends on the fabrication method and the post-treatment conditions. A variety of theoretical models have been suggested to explain experimental results[2, 4–7]. However, the origin of ZnCoO ferromagnetism remains unclear.
Chemical fabrication of ZnCoO is greatly affected by experimental factors, compared with other deposition methods such as pulsed laser deposition and radio frequency (RF) sputtering[8–11]. Post heat treatment, used to eliminate organic residuals, can induce secondary phases and crystalline defects, which can interfere with the investigation of intrinsic properties[12–15]. Unwanted hydrogen contamination during fabrication, in particular, is known to create defects that degrade the physical properties of ZnO-based materials. However, many experimental results have consistently supported the model of magnetic semiconductors in which Co-H-Co complexes are created by hydrogen doping of ZnCoO[5, 13, 16–21].
ZnCoO nanowires have received extensive attention because of advantages such as high aspect ratio and widespread applicability[22–25]. However, determining the intrinsic properties has been difficult, and the performance and reliability of ZnCoO nanowire devices have been controversial because they are typically fabricated using chemical methods with non-polar solvents[23, 26].
ZnCoO nanowire fabrication with non-polar solvents is based on thermal decomposition via a well-known chemical mechanism[27–30]. The reported fabrication conditions, including temperature, additives, and reaction environment, vary[26, 31]. These factors affect not only the growth of the nanowires but also the physical properties of the final nanowires. Although ambient synthesis has been regarded as a significant condition in such chemical reactions, no one has yet reported on the properties of nanowires with respect to their synthesis environment. In this study, we examined the change in the nanowire morphology as a function of the fabrication conditions. This is the first report suggesting that the ambient gas should be carefully considered as one of the more important factors in the chemical synthesis of high-quality nanowires. The high-quality ZnCoO nanowires initially exhibited intrinsic paramagnetic behavior; however, following hydrogen injection, the nanowires became ferromagnetic. This finding is consistent with the hydrogen-mediation model. Additionally, this was the first observation of the superb ferromagnetism of the nanowire, compared with powders, reflecting the favored direction of the ferromagnetism along the c-axis of the nanowires.
Controlling ambient gas by gas distinction
Argon gas (99.999%, continuous flow)
Air gas (99.999%, continuous flow)
Air gas (99.999%, non-continuous)
The change in nanowire morphology and the secondary phase were investigated by field-emission scanning electron microscopy (FE-SEM, S-4700, Hitachi, Tokyo, Japan) and X-ray diffraction (XRD, Empyrean series2, PANalytical, Almelo, The Netherlands). Magnetic properties such as magnetization were measured using a vibrating sample magnetometer (VSM, model 6000, Quantum Design, San Diego, CA, USA) attached to a physical property measurement system.
Results and discussion
Figure4c shows color changes during the reaction, as the solution turned brown after the synthesis of nanowires under each ambient gas. Generally, such browning reaction results from the oxidation of the chemical specimen. Because the color brightness is dependent on the oxygen content during the synthesis reaction, we assumed that the browning originated from the creation of the oxidized specimen in the presence of trioctylamine. The formation of an amine oxide specimen can be a contributing factor in the determination of the ZnCoO nanowire morphology. Therefore, we suppose that the variation in the synthesized ZnCoO nanowires shown in Figure2 is the result of different amine oxide contents generated under different ambient gases.
High-quality ZnCoO nanowires were obtained by the aqueous solution method. The ambient gas affected the magnetic properties of the fabricated samples, and the oxidation of trioctylamine solution played an important role. The generation of an appropriate amount of amine oxide due to a limited oxygen supply enhanced the growth of ZnCoO nanowires because the amine oxide acted as a surfactant. However, excessive oxygen inhibited the growth by changing the polarity of the solution. The as-grown ZnCoO nanowires exhibited magnetic properties, but these properties were extrinsic due to the thermal heat treatment process. Intrinsic ferromagnetism in ZnCoO nanowires was only obtained after hydrogen treatment. The room-temperature ferromagnetism of nanowires grown along the c-axis was larger than those of the nano- and micro-powders.
We suggest that the magnetic units of Co-H-Co formed in ZnCoO percolated efficiently along the c-axis. Furthermore, we expect that the nanowire structure of ZnCoO will enable further studies of magnetic anisotropy.
BSK, WKK, and JHP are graduate students of the Department of Cogno-Mechatronics Engineering, Pusan National University, Republic of Korea. SL is a research professor at the Institute of Basic Science, Korea University, Republic of Korea. YCC is a research professor at the Crystal Bank Institute, Pusan National University, Republic of Korea. JK is an associate professor at the Department of Physics, University of Ulsan, Republic of Korea. CRC is an associate professor at the Department of Nano Fusion Technology, Pusan National University, Republic of Korea. SYJ, the corresponding author, is a professor at the Department of Cogno-Mechatronics Engineering, Pusan National University, Republic of Korea.
This research was supported by the Converging Research Center Program through the Ministry of Science, ICT, and Future Planning, Korea (MSIP) (2013K000310), by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2011-0016525).
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