The Influences of CuO/ZnO Ratios on the Crystallization Characteristics Electrical and Magnetic Properties of Cu

1 The Instrument Center, National Cheng Kung University, Tainan 70101, Taiwan 2Department of Materials Science and Engineering, Institute of Nanotechnology and Microsystems Engineering, Center for Micro/Nano Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan 3 Institute of Microelectronics and Department of Electrical Engineering, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan

Recent reports on CuO-ZnO compound and CuO/ZnO heterojunction structures show the need for efficient characteristics [11,12]. A few Cu dopants (<10 at.%) doped ZnO samples have been investigated [13,14]. However, the effects of the higher Cu doping contents (>10 at.%) on electrical properties and magnetic behavior of ZnO were rarely reported. Thus, the electrical properties that dominate devices performance and the influences of higher dopant level on the electrical resistance of Cu Zn 1− O are topics worthy of further research. In addition, ZnO-diluted magnetic semiconductor (DMS) can enhance its magnetic behavior by Cu doping [15]. The influences of high CuO concentration on magnetic properties of ZnO are not clear.
The study uses an aqueous solution method to synthesize CuO

Experimental Procedures
The experiments in this study synthesized the CuO powder using an aqueous solution. This study investigates the structural characteristics of powders using X-ray diffraction (XRD, Siemens/D5000) and scanning electron microscopy (SEM, Hitachi/S-4100). To understand the contribution of CuO on the electrical and magnetic characteristics of Cu Zn 1− O powders, a semiconductor parameter analyzer (Agilent/4155-B) and superconducting quantum interference device vibrating sample magnetometer (SQUID VSM) were used, respectively. The composition and chemical bonding of the crystallization were analyzed using an electron spectroscopy for chemical analysis (ESCA, PHI 5000 VersaProbe).

Results and Discussions
This study uses DSC and TGA analysis to determine the crystallization conditions of CuO precursors (Figure 1)  specimens were heated from room temperature to 1000 ∘ C at a rate of 10 ∘ C/min in air. The TGA data reveals a sharp weight loss in the powder at 270 ∘ C because of the evaporation of water and organics [16]. This result is consistent with the DSC curve, which shows an exothermic peak at 280 ∘ C. An endothermic peak appears at 480 ∘ C and the powder weight (TGA curve) gradually increases and then stabilizes as the temperature exceeds 500 ∘ C. The main reason is that the CuO crystallization gradually forms with increasing temperature [17]. Based on these reasons, the CuO powder was annealed at 500 ∘ C to estimate the CuO crystallization. Figure 2(a) shows the XRD patterns of as-grown CuO precursors and CuO powder annealed at 500 ∘ C for 1 h. Both samples were polycrystalline and had a monoclinic structure. An additional diffraction peak of (CH 3 COO) 2 ⋅ Cu appeared in as-grown CuO precursors, indicating that a drying temperature of 120 ∘ C was insufficient for its evaporation. After thermal annealing at 500 ∘ C, the Cu(CH 3 COO) 2 phase disappeared and the (111) diffraction peak dominated the CuO crystallization. The intensity of the major diffraction peaks increased, indicating that the sufficient thermal energy improved CuO crystallization. Based on the Scherrer formula [16], the grain size of CuO nanoparticles can be estimated from the full width at half-maximum (FWHM) of the (111) diffraction peak. The average grain size of CuO powder increased from 8 nm to 21 nm after thermal annealing at 500 ∘ C. This result is associated with the grains growing more easily under the higher temperature [16]. The ZnO powder was also synthesized and compared to Cu Zn A comparison of the CuO revealed that the diffraction peak of (111) for Cu 0.33 Zn 0.67 O shifted to higher degree. In contrast, the diffraction peak of (101) for Cu 0.33 Zn 0.67 O shifted to lower degree comparing with ZnO. These results are associated with compressive strain in the crystallization [4]. In addition, the related (220) diffraction peak of the Cu 2 O phase was attributed to insufficient oxidation [2]. Notably, the (200) Figure 3 shows SEM images of Cu Zn 1− O (x = 0.33, 0.5, and 0.67) powders at an annealing temperature of 500 ∘ C. All powders displayed a particle-like structure, and the agglomeration of particles was randomly distributed. The morphology of Cu 0.5 Zn 0.5 O powder shows an irregular and its size for the agglomeration of particles is larger than that of Cu 0.67 Zn 0.33 O and Cu 0.33 Zn 0.67 O powders. The electrical properties of powder might be influenced by the particle size [18], therefore, the electrical resistance were examined.
Electrical measurements were conducted to determine the resistance value of CuO powder. The CuO powder was    Figure 4 shows the magnetization (M) as a function of magnetic field (H) at the temperature of 10 K for the Cu Zn 1− O powders. All samples have a linear-like M-H variation at a magnetic field of ±5000 Oe without measureable hysteresis, which indicates a paramagnetic behavior [21]. The magnetization of Cu 0.33 Zn 0.67 O sample was higher than that of Cu 0.5 Zn 0.5 O and Cu 0.67 Zn 0.33 O samples, which indicated that the Cu 0.33 Zn 0.67 O powder contains a better paramagnetism (PM). The increment of antiferromagnetic interaction possibly reduced the PM that resulted from the formation of CuO crystallization [21]. From XRD data (Figure 2(b)), the CuO phase dominated the Cu 0.5 Zn 0.5 O, Cu 0.67 Zn 0.33 O crystallization and affected their magnetic properties that were consistent with the result of M-H curves. In addition, the lower magnetization of Cu 0.5 Zn 0.5 O, Cu 0.67 Zn 0.33 O was also attributed to the secondary phase of Cu 2 O [22]. The low-field region of the hysteresis loops of Cu Zn 1− O powders was clearly observed (inset of Figure 4) which indicated that all Cu Zn 1− O powders also contained a diluted ferromagnetism (DFM). The FM could be developed by the distortion of ZnO structure by the substitution of remnant Cu 2+ ions into ZnO lattice [22,23]. It is found that the coercivity field of the Cu 0.33 Zn 0.67 O, Cu 0.5 Zn 0.5 O, and Cu 0.67 Zn 0.33 O powders is 75 Oe, 150 Oe, and 78 Oe, respectively. That is to say, the Cu 0.5 Zn 0.5 O powder has a good stability for thermal interference [24].
The chemical bonding of the Cu 0.5 Zn 0.5 O powder was examined by XPS with full region scanning from 0 eV to 1200 eV ( Figure 5). In Figure 5 (Figure 5(b)) indicates that the binding energy of 530.3 eV can be attributed to oxidized ions in the CuO particles [25]. Multipeak Gaussian fitting shows another O 1s peak located at 532.4 eV, indicating that this binding energy was dominated by Zn 2+ ion doping [23]. The high-resolution XPS spectrum of the Cu 2p3/2 mode ( Figure 5(c)) appears at 932.3 eV, indicating the presence of Cu 2+ ion [2]. The binding energy of 952.3 eV (Cu 2p1/2 mode) can be attributed to Cu + ions in Cu 0.5 Zn 0.5 O that resulted from Cu 2 O [26,27]. This result is consistent with the observation of XRD data (Figure 3(b)). Figure 5(d) shows two strong peaks at 1021.5 eV and 1044.7 eV which correspond to Zn 2p3/2 and Zn 2p1/2 , respectively. This is consistent with the Zn 2+ ion binding in previous reports [28]. Based on these results, the Zn 2+ ions were substituted by Cu + to form the Cu 2 O phase that affects the electrical and magnetic properties.
The correlation of the magnetic properties and resistivity of Cu Zn 1− O powders were summarized in Table 1

Conclusions
The stability of oxide (CuO or Cu 2 O) depends primarily on the intensity of oxidation (annealing temperature). When ZnO participated in the CuO system, the crystalline quality of CuO powder deteriorated. The ZnO mixing effect increased the crystallization size and induced a compressive stress in the particle. Although the presence of ZnO phases deteriorated Cu 0.5 Zn 0.5 O crystallization, the electrical conductance was improved. A lower Cu 2 O phase concentration and stable crystallization reduced the electrical resistance of Cu 0.5 Zn 0.5 O powder. The electrical resistance of Cu 0.5 Zn 0.5 O powder was the lowest and the magnetic behavior was the smallest because CuO and Cu 2 O contents were higher. XPS analysis reveals that the Zn 2+ ions were substituted by Cu 2+ and Cu + ions, forming CuO and Cu 2 O phases that confirmed the contribution of Cu 2 O on the electrical and magnetic properties.