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Structure-dependent growth control in nanowire synthesis via on-film formation of nanowires.

Shim W, Ham J, Noh JS, Lee W - Nanoscale Res Lett (2011)

Bottom Line: In this study, we investigated various experimental growth parameters such as deposition rate, deposition area, and substrate structure which modulate the microstructure and the magnitude of stress in the films, and thus significantly affect the nanowire density.We found that Bi nanowire growth is favored in thermodynamically unstable films that facilitate atomic mass flow during annealing.A large film area and a large thermal expansion coefficient mismatch between the film and the substrate were found to be critical for inducing large compressive stress in a film, which promotes Bi nanowire growth.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Materials Science and Engineering, Yonsei University, 134 Shinchon, Seoul 120-749, Korea. wooyoung@yonsei.ac.kr.

ABSTRACT
On-film formation of nanowires, termed OFF-ON, is a novel synthetic approach that produces high-quality, single-crystalline nanowires of interest. This versatile method utilizes stress-induced atomic mass flow along grain boundaries in the polycrystalline film to form nanowires. Consequently, controlling the magnitude of the stress induced in the films and the microstructure of the films is important in OFF-ON. In this study, we investigated various experimental growth parameters such as deposition rate, deposition area, and substrate structure which modulate the microstructure and the magnitude of stress in the films, and thus significantly affect the nanowire density. We found that Bi nanowire growth is favored in thermodynamically unstable films that facilitate atomic mass flow during annealing. A large film area and a large thermal expansion coefficient mismatch between the film and the substrate were found to be critical for inducing large compressive stress in a film, which promotes Bi nanowire growth. The OFF-ON method can be routinely used to grow nanowires from a variety of materials by tuning the material-dependent growth parameters.

No MeSH data available.


Schematics and histograms of Bi nanowire densities. (a) Schematics of Bi nanowires grown on different substrates. (b) Histograms of Bi nanowire densities depending on the substrate structures.
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Figure 4: Schematics and histograms of Bi nanowire densities. (a) Schematics of Bi nanowires grown on different substrates. (b) Histograms of Bi nanowire densities depending on the substrate structures.

Mentions: Finally, the effect of the substrate layer structure (α) on Bi nanowire density was investigated to elucidate the role of thermal expansion mismatch between the substrate and the film. For this study, two different film stack structures, Bi/SiO2/Si and Bi/Si, with different thermal expansion mismatches, were exploited. Here, Bi films were deposited at an identical rate of 32.7 Å/s for both stacks. Figure 4a schematically shows Bi nanowires grown on the Bi/SiO2/Si and Bi/Si stacks, illustrating that the nanowire density on a Bi/SiO2/Si stack is much larger than on a Bi/Si stack. In fact, the Bi nanowire density on the Bi/SiO2/Si stack was measured to be 5400 cm-2, which is much higher than that on the Bi/Si stack (240 cm-2), as shown in Figure 4b. The thermal expansion mismatch that causes compressive stress in a film results from the large difference in thermal expansion coefficients of Bi (13.4 × 10-6/°C) and SiO2 (0.5 × 10-6/°C) or Si (2.4 × 10-6/°C). It is inferred that the 20 times larger Bi nanowire density on the Bi/SiO2/Si stack results from the larger mismatch of thermal expansion coefficients between the substrate and the Bi film for the Bi/SiO2/Si stack than for the Bi/Si stack (note the difference in the thermal expansion coefficients of Si and SiO2). Therefore, the choice of a substrate structure that can maximize the thermal expansion mismatch with the film is a crucial parameter for optimizing nanowire growth. This principle may be universally applicable to nanowire growth based on any material systems, using the OFF-ON method.


Structure-dependent growth control in nanowire synthesis via on-film formation of nanowires.

Shim W, Ham J, Noh JS, Lee W - Nanoscale Res Lett (2011)

Schematics and histograms of Bi nanowire densities. (a) Schematics of Bi nanowires grown on different substrates. (b) Histograms of Bi nanowire densities depending on the substrate structures.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211252&req=5

Figure 4: Schematics and histograms of Bi nanowire densities. (a) Schematics of Bi nanowires grown on different substrates. (b) Histograms of Bi nanowire densities depending on the substrate structures.
Mentions: Finally, the effect of the substrate layer structure (α) on Bi nanowire density was investigated to elucidate the role of thermal expansion mismatch between the substrate and the film. For this study, two different film stack structures, Bi/SiO2/Si and Bi/Si, with different thermal expansion mismatches, were exploited. Here, Bi films were deposited at an identical rate of 32.7 Å/s for both stacks. Figure 4a schematically shows Bi nanowires grown on the Bi/SiO2/Si and Bi/Si stacks, illustrating that the nanowire density on a Bi/SiO2/Si stack is much larger than on a Bi/Si stack. In fact, the Bi nanowire density on the Bi/SiO2/Si stack was measured to be 5400 cm-2, which is much higher than that on the Bi/Si stack (240 cm-2), as shown in Figure 4b. The thermal expansion mismatch that causes compressive stress in a film results from the large difference in thermal expansion coefficients of Bi (13.4 × 10-6/°C) and SiO2 (0.5 × 10-6/°C) or Si (2.4 × 10-6/°C). It is inferred that the 20 times larger Bi nanowire density on the Bi/SiO2/Si stack results from the larger mismatch of thermal expansion coefficients between the substrate and the Bi film for the Bi/SiO2/Si stack than for the Bi/Si stack (note the difference in the thermal expansion coefficients of Si and SiO2). Therefore, the choice of a substrate structure that can maximize the thermal expansion mismatch with the film is a crucial parameter for optimizing nanowire growth. This principle may be universally applicable to nanowire growth based on any material systems, using the OFF-ON method.

Bottom Line: In this study, we investigated various experimental growth parameters such as deposition rate, deposition area, and substrate structure which modulate the microstructure and the magnitude of stress in the films, and thus significantly affect the nanowire density.We found that Bi nanowire growth is favored in thermodynamically unstable films that facilitate atomic mass flow during annealing.A large film area and a large thermal expansion coefficient mismatch between the film and the substrate were found to be critical for inducing large compressive stress in a film, which promotes Bi nanowire growth.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Materials Science and Engineering, Yonsei University, 134 Shinchon, Seoul 120-749, Korea. wooyoung@yonsei.ac.kr.

ABSTRACT
On-film formation of nanowires, termed OFF-ON, is a novel synthetic approach that produces high-quality, single-crystalline nanowires of interest. This versatile method utilizes stress-induced atomic mass flow along grain boundaries in the polycrystalline film to form nanowires. Consequently, controlling the magnitude of the stress induced in the films and the microstructure of the films is important in OFF-ON. In this study, we investigated various experimental growth parameters such as deposition rate, deposition area, and substrate structure which modulate the microstructure and the magnitude of stress in the films, and thus significantly affect the nanowire density. We found that Bi nanowire growth is favored in thermodynamically unstable films that facilitate atomic mass flow during annealing. A large film area and a large thermal expansion coefficient mismatch between the film and the substrate were found to be critical for inducing large compressive stress in a film, which promotes Bi nanowire growth. The OFF-ON method can be routinely used to grow nanowires from a variety of materials by tuning the material-dependent growth parameters.

No MeSH data available.