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Investigation of Nucleation Mechanism and Tapering Observed in ZnO Nanowire Growth by Carbothermal Reduction Technique

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ABSTRACT

ZnO nanowire nucleation mechanism and initial stages of nanowire growth using the carbothermal reduction technique are studied confirming the involvement of the catalyst at the tip in the growth process. Role of the Au catalyst is further confirmed when the tapering observed in the nanowires can be explained by the change in the shape of the catalyst causing a variation of the contact area at the liquid–solid interface of the nanowires. The rate of decrease in nanowire diameter with length on the average is found to be 0.36 nm/s and this rate is larger near the base. Variation in the ZnO nanowire diameter with length is further explained on the basis of the rate at which Zn atoms are supplied as well as the droplet stability at the high flow rates and temperature. Further, saw-tooth faceting is noticed in tapered nanowires, and the formation is analyzed crystallographically.

No MeSH data available.


a ZnO nanowires with uniform diameters grown on p-type Si (100) substrates. b–d show nanowires with a constant decrease in diameter along the axis grown at a higher flow rate and temperature. The growth time for figures (b) and (c) were 10 min and that for figures (e) and (f) were 25 min. Scales in all the figures are 2 μm.
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Figure 3: a ZnO nanowires with uniform diameters grown on p-type Si (100) substrates. b–d show nanowires with a constant decrease in diameter along the axis grown at a higher flow rate and temperature. The growth time for figures (b) and (c) were 10 min and that for figures (e) and (f) were 25 min. Scales in all the figures are 2 μm.

Mentions: The shape of the semiconductors at the nanoscale is another decisive factor for the properties, and the shape controlled growth of semiconductors can find unique applications in electronics and photonics. Until now, tapered ZnO nanowires have only been produced by chemical synthesis or electrochemical deposition method [11]. Here, we report the observation of tapered ZnO nanowires grown using the carbothermal reduction method in a furnace. Figure 3a shows nanowires with a uniform diameter throughout their axis, grown on p-type silicon substrates at a temperature of 900°C and argon flow rate of 100 sccm. Tapered nanowires are formed at a growth temperature of 980°C and an Ar flow rate of 160 sccm as seen in Figure 3b–e. The growth times were 10 min for Figure 3b, c and 25 min for Figure 3d, e. At a lower flow rate of 140 sccm but at the same temperature of 980°C, nanowires with just a tapered base but uniform long stems are seen in Figure 4b. The illustration in Figure 4a is used in order to understand the tapering mechanism. To put things into perspective, the growth of tapered nanowires can be categorized as a special case of cylindrical nanowires with the flank angle δ = 0. In fact, diameters of epitaxially grown Si nanowires have been shown [12] to vary, especially in the region close to the substrate where the nanowires exhibit larger diameters just as seen here for ZnO nanowires in Figures 3b–e and 4b. One first guess would be that this large diameter is created by radial overgrowth of the nanowire after axial growth. Especially at elevated temperatures, surface diffusion and vapor-solid growth might influence the shape and result in enlargement of the nanowire base. In particular, a faceting of the nanowire base expansion, often observed at high temperatures, might occur after growth by surface diffusion. Such faceting of the base is seen in Figure 4b as indicated by arrows.


Investigation of Nucleation Mechanism and Tapering Observed in ZnO Nanowire Growth by Carbothermal Reduction Technique
a ZnO nanowires with uniform diameters grown on p-type Si (100) substrates. b–d show nanowires with a constant decrease in diameter along the axis grown at a higher flow rate and temperature. The growth time for figures (b) and (c) were 10 min and that for figures (e) and (f) were 25 min. Scales in all the figures are 2 μm.
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Related In: Results  -  Collection

License
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Figure 3: a ZnO nanowires with uniform diameters grown on p-type Si (100) substrates. b–d show nanowires with a constant decrease in diameter along the axis grown at a higher flow rate and temperature. The growth time for figures (b) and (c) were 10 min and that for figures (e) and (f) were 25 min. Scales in all the figures are 2 μm.
Mentions: The shape of the semiconductors at the nanoscale is another decisive factor for the properties, and the shape controlled growth of semiconductors can find unique applications in electronics and photonics. Until now, tapered ZnO nanowires have only been produced by chemical synthesis or electrochemical deposition method [11]. Here, we report the observation of tapered ZnO nanowires grown using the carbothermal reduction method in a furnace. Figure 3a shows nanowires with a uniform diameter throughout their axis, grown on p-type silicon substrates at a temperature of 900°C and argon flow rate of 100 sccm. Tapered nanowires are formed at a growth temperature of 980°C and an Ar flow rate of 160 sccm as seen in Figure 3b–e. The growth times were 10 min for Figure 3b, c and 25 min for Figure 3d, e. At a lower flow rate of 140 sccm but at the same temperature of 980°C, nanowires with just a tapered base but uniform long stems are seen in Figure 4b. The illustration in Figure 4a is used in order to understand the tapering mechanism. To put things into perspective, the growth of tapered nanowires can be categorized as a special case of cylindrical nanowires with the flank angle δ = 0. In fact, diameters of epitaxially grown Si nanowires have been shown [12] to vary, especially in the region close to the substrate where the nanowires exhibit larger diameters just as seen here for ZnO nanowires in Figures 3b–e and 4b. One first guess would be that this large diameter is created by radial overgrowth of the nanowire after axial growth. Especially at elevated temperatures, surface diffusion and vapor-solid growth might influence the shape and result in enlargement of the nanowire base. In particular, a faceting of the nanowire base expansion, often observed at high temperatures, might occur after growth by surface diffusion. Such faceting of the base is seen in Figure 4b as indicated by arrows.

View Article: PubMed Central - HTML

ABSTRACT

ZnO nanowire nucleation mechanism and initial stages of nanowire growth using the carbothermal reduction technique are studied confirming the involvement of the catalyst at the tip in the growth process. Role of the Au catalyst is further confirmed when the tapering observed in the nanowires can be explained by the change in the shape of the catalyst causing a variation of the contact area at the liquid–solid interface of the nanowires. The rate of decrease in nanowire diameter with length on the average is found to be 0.36 nm/s and this rate is larger near the base. Variation in the ZnO nanowire diameter with length is further explained on the basis of the rate at which Zn atoms are supplied as well as the droplet stability at the high flow rates and temperature. Further, saw-tooth faceting is noticed in tapered nanowires, and the formation is analyzed crystallographically.

No MeSH data available.