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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.


Related in: MedlinePlus

AFM images (5 μm × 5 μm in size) of Bi films deposited at a rate of (a) 2.7 Å/s and (c) 32.7 Å/s, after heat treatment at 270°C for 10 h, (b, d) SEM images of the respective Bi films, with no nanowires and dense nanowires on them, (e) Histograms of Bi nanowire densities depending on the deposition rates.
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Figure 2: AFM images (5 μm × 5 μm in size) of Bi films deposited at a rate of (a) 2.7 Å/s and (c) 32.7 Å/s, after heat treatment at 270°C for 10 h, (b, d) SEM images of the respective Bi films, with no nanowires and dense nanowires on them, (e) Histograms of Bi nanowire densities depending on the deposition rates.

Mentions: Figure 1b,c show X-ray diffraction (XRD) patterns of Bi thin films grown at deposition rates (γ) of 2.7 Å/s (RF power: 10 W) and 32.7 Å/s (RF power: 100 W), respectively, before and after thermal annealing. For both deposition rates, the identical 50-nm-thick Bi films were obtained by controlling the deposition time. From Figure 1b,c, it is evident that the Bi film grown at 100 W have preferential orientations of (003), (006), and (009) after heat treatment, while the film deposited at 10 W have additional orientations of (012) and (104). Interestingly, Bi nanowires grew from Bi films deposited at 100 W at far higher densities than from Bi films deposited at 10 W (see Figure 2). This implies that the preferential orientation (00ℓ) in a Bi film facilitates Bi nanowire growth. At a fixed growth temperature, the impinging flux of Bi atoms onto the surface of a substrate is expected to be higher for the higher RF power of 100 W, leading to a shorter time interval between encounters of adatoms, and in turn, creating a local excess of adatoms, called supersaturation [14]. This causes adatoms not to settle into possible equilibrium positions, resulting in the Bi film having a non-equilibrium microstructure and a non-uniform surface. In such a Bi film, Bi atoms are more likely to occupy unstable positions and are susceptible to migration upon thermal activation. This is why the grain orientations of the Bi film deposited at 100 W are redirected to the (00ℓ) through thermal annealing, as shown in Figure 1c.


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)

AFM images (5 μm × 5 μm in size) of Bi films deposited at a rate of (a) 2.7 Å/s and (c) 32.7 Å/s, after heat treatment at 270°C for 10 h, (b, d) SEM images of the respective Bi films, with no nanowires and dense nanowires on them, (e) Histograms of Bi nanowire densities depending on the deposition rates.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 2: AFM images (5 μm × 5 μm in size) of Bi films deposited at a rate of (a) 2.7 Å/s and (c) 32.7 Å/s, after heat treatment at 270°C for 10 h, (b, d) SEM images of the respective Bi films, with no nanowires and dense nanowires on them, (e) Histograms of Bi nanowire densities depending on the deposition rates.
Mentions: Figure 1b,c show X-ray diffraction (XRD) patterns of Bi thin films grown at deposition rates (γ) of 2.7 Å/s (RF power: 10 W) and 32.7 Å/s (RF power: 100 W), respectively, before and after thermal annealing. For both deposition rates, the identical 50-nm-thick Bi films were obtained by controlling the deposition time. From Figure 1b,c, it is evident that the Bi film grown at 100 W have preferential orientations of (003), (006), and (009) after heat treatment, while the film deposited at 10 W have additional orientations of (012) and (104). Interestingly, Bi nanowires grew from Bi films deposited at 100 W at far higher densities than from Bi films deposited at 10 W (see Figure 2). This implies that the preferential orientation (00ℓ) in a Bi film facilitates Bi nanowire growth. At a fixed growth temperature, the impinging flux of Bi atoms onto the surface of a substrate is expected to be higher for the higher RF power of 100 W, leading to a shorter time interval between encounters of adatoms, and in turn, creating a local excess of adatoms, called supersaturation [14]. This causes adatoms not to settle into possible equilibrium positions, resulting in the Bi film having a non-equilibrium microstructure and a non-uniform surface. In such a Bi film, Bi atoms are more likely to occupy unstable positions and are susceptible to migration upon thermal activation. This is why the grain orientations of the Bi film deposited at 100 W are redirected to the (00ℓ) through thermal annealing, as shown in Figure 1c.

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.


Related in: MedlinePlus