Abstract
In this study, zinc oxide (ZnO) nanorod arrays were synthesized using a simple hydrothermal reaction on ZnO seeds/n-silicon substrate. Several parameters were studied, including the heat-treatment temperature to produce ZnO seeds, zinc nitrate concentration, pH of hydrothermal reaction solution, and hydrothermal reaction time. The optimum heat-treatment temperature to produce uniform nanosized ZnO seeds was 400°C. The nanorod dimensions depended on the hydrothermal reaction parameters. The optimum hydrothermal reaction parameters to produce blunt tip-like nanorods (770 nm long and 80 nm in top diameter) were 0.1 M zinc nitrate, pH 7, and 4 h of growth duration. Phase analysis studies showed that all ZnO nanorods exhibited a strong (002) peak. Thus, the ZnO nanorods grew in a c-axis preferred orientation. A strong ultraviolet (UV) emission peak was observed for ZnO nanorods grown under optimized parameters with a low, deep-level emission peak, which indicated high optical property and crystallinity of the nanorods. The produced ZnO nanorods were also tested for their UV-sensing properties. All samples responded to UV light but with different sensing characteristics. Such different responses could be attributed to the high surface-to-volume ratio of the nanorods that correlated with the final ZnO nanorods morphology formed at different synthesis parameters. The sample grown using optimum synthesis parameters showed the highest responsivity of 0.024 A/W for UV light at 375 nm under a 3 V bias.
Highlights
Zinc oxide (ZnO) nanostructures are receiving considerable interest because of their excellent electronic, optical, and photonic properties, as well as a wide bandgap of 3.2 eV that benefits shortwavelength optoelectronic applications
Li et al [10] and Song et al [11] deposited ZnO on silicon (Si)- substrate by using RF sputtering followed by hydrothermal reaction without any heat treatment, and they obtained columnar ZnO nanorods. These results show the importance of the heat treatment of ZnO seed layers in forming ZnO nanorods using the hydrothermal method
The surface morphology of ZnO seed layer transforms from a flaky structure at 250uC to a circular structure with increased heat-treatment temperature from 300uC to 450uC
Summary
Zinc oxide (ZnO) nanostructures are receiving considerable interest because of their excellent electronic, optical, and photonic properties, as well as a wide bandgap of 3.2 eV that benefits shortwavelength optoelectronic applications. One-dimensional ZnO nanostructures (nanorods, nanowires, and nanotubes) can facilitate more efficient carrier transport because of decreased grain boundaries, surface defects, disorders, and discontinuous interfaces [1,2]. The simplest, most economical, most energy-efficient method for synthesizing ZnO nanorods is by using the hydrothermal method [3,4]. Seeded substrates are used in the hydrothermal technique to achieve well-controlled morphology and growth direction of ZnO nanorods. Heterogeneous nucleation favors the growth of ZnO nanorods because the interfacial energy between crystals and substrate is usually smaller than the interfacial energy between crystals and solution [5]. Pre-coating the substrate with seed materials similar to nanocrystals can effectively control nanorods growth and morphology. Several works have used seeded substrates to produce ZnO nanorods
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