Abstract
Foam-structured ZnO films were formed on c-axis Al2O3 substrates by using a catalyst-free carbothermal method. The morphological properties of the films strongly depended on their substrate position in the reactor, implying that the quantity of Zn vapor transported played a crucial role in determining the film structure. The formation mechanism of the films is proposed to be as follows: with increased transport of Zn vapor, vaporized Zn elements transformed into liquid Zn nanoparticles, and the liquid Zn elements on the rods promoted the new nucleation of ZnO nanorods. The repeated growth of ZnO rods at the nucleation sites led to the foam-like structure. The as-formed foam-structured ZnO films exhibited the wurtzite crystal structure, and the atomic composition ratio of Zn to O was 1 : 0.85. The main photoluminescence emissions were centered at 3.3 and 2.5 eV, and they were attributed to band-to-band and defect-related transitions, respectively.
Highlights
Zinc oxide (ZnO) has been receiving increasing attention owing to its promising properties, such as a bandgap that is direct and wide (≈3.4 eV), an exciton binding energy that is large (≈60 meV), high transparency in the visible region, high stability against high-energy radiation, and ease of etching in both acid and base media [1,2,3,4,5]
We focused on the formation of foam-structured ZnO (FS-ZnO) films, which are characterized by a network of ZnO NRs, on c-axis Al2O3 substrates in a one-step VLS process
FS-ZnO films were formed on c-axis Al2O3 substrates (4 cm × 4 cm) by using a catalyst-free carbothermal method
Summary
Zinc oxide (ZnO) has been receiving increasing attention owing to its promising properties, such as a bandgap that is direct and wide (≈3.4 eV), an exciton binding energy that is large (≈60 meV), high transparency in the visible region, high stability against high-energy radiation, and ease of etching in both acid and base media [1,2,3,4,5]. ZnO nanostructures have been attracting considerable interest because of their peculiar properties. They have been grown using various techniques such as chemical vapor deposition (CVD), physical vapor deposition, and solution techniques [1,2,3, 6,7,8,9]. In CVD methods, vapor-liquid-solid (VLS) and vapor-solid (VS) processes have been used widely to produce ZnO nanorods (NRs) [4, 10,11,12]. We focused on the formation of foam-structured ZnO (FS-ZnO) films, which are characterized by a network of ZnO NRs, on c-axis Al2O3 substrates in a one-step VLS process. In addition to a description and discussion of the aforementioned study, we propose a formation mechanism for the FS-ZnO films
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