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Study of nanostructure growth with nanoscale apex induced by femtosecond laser irradiation at megahertz repetition rate.

Patel NB, Tan B, Venkatakrishnan K - Nanoscale Res Lett (2013)

Bottom Line: We have recently developed this unique technique to grow leaf-like nanostructures with such interesting geometry without the use of any catalyst.It was found to be possible only in the presence of background nitrogen gas flow.We observed a clear transformation in the kind of nanotips that grew for the given laser conditions.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Aerospace Engineering, Ryerson University, Victoria Street, Toronto, ON M5B 2K3, Canada. tanbo@ryerson.ca.

ABSTRACT
Leaf-like nanostructures with nanoscale apex are induced on dielectric target surfaces by high-repetition-rate femtosecond laser irradiation in ambient conditions. We have recently developed this unique technique to grow leaf-like nanostructures with such interesting geometry without the use of any catalyst. It was found to be possible only in the presence of background nitrogen gas flow. In this synthesis method, the target serves as the source for building material as well as the substrate upon which these nanostructures can grow. In our investigation, it was found that there are three possible kinds of nanotips that can grow on target surfaces. In this report, we have presented the study of the growth mechanisms of such leaf-like nanostructures under various conditions such as different laser pulse widths, pulse repetition rates, dwell times, and laser polarizations. We observed a clear transformation in the kind of nanotips that grew for the given laser conditions.

No MeSH data available.


Related in: MedlinePlus

Effect of excessive machining of irradiation spot corresponding to various repetition rates. Nanostructures generated at the dwell time of 0.75 ms for the repetition rates of (a) 4, (b) 8, and (c) 13 MHz for 214 fs.
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Figure 9: Effect of excessive machining of irradiation spot corresponding to various repetition rates. Nanostructures generated at the dwell time of 0.75 ms for the repetition rates of (a) 4, (b) 8, and (c) 13 MHz for 214 fs.

Mentions: The ablation mechanism somewhat changes from one repetition rate to another due to the difference in threshold energy-per-pulse requirement. The incoming pulses also interact differently with plasma generated from previous pulses for each repetition rate. Thus, the nanostructures generated for even the same dwell time differ for different repetition rates, as seen in Figures 6 and 9. Figure 9 shows SEM images of the glass target irradiated with 4-, 8-, and 13-MHz repetition rates for a dwell time of 0.75 ms. For 8- and 13-MHz repetition rates, the number of nanotips produced is much less compared to 0.50 ms, as seen in Figures 6 and 7. Instead, the presence of many spherical micronanoparticles and molten droplets is observed. This phenomenon can better be understood from the stage 4 of the schematic representation depicted in Figure 8. When the irradiated spot is bombarded with too many pulses as in the case of high repetition rates and high dwell time, an excessive amount of material is added to the plasma. The incoming subsequent pulses also interact with the plasma species elevating their temperature and giving them high kinetic energy. As a result, the plasma expands outward faster and to the larger radius exerting more pressure in the surrounding including onto the redeposited plasma vapor condensates on the target surface. This creates the external pressure approximately similar to or higher than the internal pressure of the redeposited material, hence hindering the formation of stems, stage 4 of Figure 8. The excessive temperature of the plasma species and the target can also remelt the deposited material as well as previously grown stems and tips. The SEM image of the target irradiated with 13-MHz repetition rate for the dwell time of 0.75 ms depicted in Figure 9c is the perfect example of the stage 4 illustrated in Figure 8. For 8-MHz repetition rate at 0.75-ms dwell time, most of the redeposited material must be experiencing approximately equal internal and external pressure resulting in the formation of just circular micronanoparticles rather than the formation of stems. There is an evident of the formation of very few tips from bulk droplets in Figure 9b. If we follow the above four stages, there should not be any tip growth for 13-MHz repetition rate for the dwell time of 0.75 ms. However from Figure 9c, it can be seen that a significant number of nanotips grew on the target. This happened because the 13-MHz repetition rate provides a much larger number of pulses and the machining is performed way beyond stage 4 of the growth mechanism. When the plasma reaches stage 4, it will exert excessive pressure and temperature on previously deposited material resulting in remelting and formation of micronanoparticles. But at the same time, since plasma is continuously being heated by incoming pulses, plasma will rapidly expand outward. There will be a point in time where the plasma has expanded far enough from the redeposition site relieving excessive pressure and temperature. From this point onward, the transmission of the subsequent laser pulses will improve, and the new material will be ablated from the target forming new plasma over the target surface. This whole phenomenon must be occurring in the last part of the 0.75-ms dwell time during which the growth mechanism starts back at stage 1 and forms nanotips on previously deposited material, as seen in Figure 9c.


Study of nanostructure growth with nanoscale apex induced by femtosecond laser irradiation at megahertz repetition rate.

Patel NB, Tan B, Venkatakrishnan K - Nanoscale Res Lett (2013)

Effect of excessive machining of irradiation spot corresponding to various repetition rates. Nanostructures generated at the dwell time of 0.75 ms for the repetition rates of (a) 4, (b) 8, and (c) 13 MHz for 214 fs.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3643873&req=5

Figure 9: Effect of excessive machining of irradiation spot corresponding to various repetition rates. Nanostructures generated at the dwell time of 0.75 ms for the repetition rates of (a) 4, (b) 8, and (c) 13 MHz for 214 fs.
Mentions: The ablation mechanism somewhat changes from one repetition rate to another due to the difference in threshold energy-per-pulse requirement. The incoming pulses also interact differently with plasma generated from previous pulses for each repetition rate. Thus, the nanostructures generated for even the same dwell time differ for different repetition rates, as seen in Figures 6 and 9. Figure 9 shows SEM images of the glass target irradiated with 4-, 8-, and 13-MHz repetition rates for a dwell time of 0.75 ms. For 8- and 13-MHz repetition rates, the number of nanotips produced is much less compared to 0.50 ms, as seen in Figures 6 and 7. Instead, the presence of many spherical micronanoparticles and molten droplets is observed. This phenomenon can better be understood from the stage 4 of the schematic representation depicted in Figure 8. When the irradiated spot is bombarded with too many pulses as in the case of high repetition rates and high dwell time, an excessive amount of material is added to the plasma. The incoming subsequent pulses also interact with the plasma species elevating their temperature and giving them high kinetic energy. As a result, the plasma expands outward faster and to the larger radius exerting more pressure in the surrounding including onto the redeposited plasma vapor condensates on the target surface. This creates the external pressure approximately similar to or higher than the internal pressure of the redeposited material, hence hindering the formation of stems, stage 4 of Figure 8. The excessive temperature of the plasma species and the target can also remelt the deposited material as well as previously grown stems and tips. The SEM image of the target irradiated with 13-MHz repetition rate for the dwell time of 0.75 ms depicted in Figure 9c is the perfect example of the stage 4 illustrated in Figure 8. For 8-MHz repetition rate at 0.75-ms dwell time, most of the redeposited material must be experiencing approximately equal internal and external pressure resulting in the formation of just circular micronanoparticles rather than the formation of stems. There is an evident of the formation of very few tips from bulk droplets in Figure 9b. If we follow the above four stages, there should not be any tip growth for 13-MHz repetition rate for the dwell time of 0.75 ms. However from Figure 9c, it can be seen that a significant number of nanotips grew on the target. This happened because the 13-MHz repetition rate provides a much larger number of pulses and the machining is performed way beyond stage 4 of the growth mechanism. When the plasma reaches stage 4, it will exert excessive pressure and temperature on previously deposited material resulting in remelting and formation of micronanoparticles. But at the same time, since plasma is continuously being heated by incoming pulses, plasma will rapidly expand outward. There will be a point in time where the plasma has expanded far enough from the redeposition site relieving excessive pressure and temperature. From this point onward, the transmission of the subsequent laser pulses will improve, and the new material will be ablated from the target forming new plasma over the target surface. This whole phenomenon must be occurring in the last part of the 0.75-ms dwell time during which the growth mechanism starts back at stage 1 and forms nanotips on previously deposited material, as seen in Figure 9c.

Bottom Line: We have recently developed this unique technique to grow leaf-like nanostructures with such interesting geometry without the use of any catalyst.It was found to be possible only in the presence of background nitrogen gas flow.We observed a clear transformation in the kind of nanotips that grew for the given laser conditions.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Aerospace Engineering, Ryerson University, Victoria Street, Toronto, ON M5B 2K3, Canada. tanbo@ryerson.ca.

ABSTRACT
Leaf-like nanostructures with nanoscale apex are induced on dielectric target surfaces by high-repetition-rate femtosecond laser irradiation in ambient conditions. We have recently developed this unique technique to grow leaf-like nanostructures with such interesting geometry without the use of any catalyst. It was found to be possible only in the presence of background nitrogen gas flow. In this synthesis method, the target serves as the source for building material as well as the substrate upon which these nanostructures can grow. In our investigation, it was found that there are three possible kinds of nanotips that can grow on target surfaces. In this report, we have presented the study of the growth mechanisms of such leaf-like nanostructures under various conditions such as different laser pulse widths, pulse repetition rates, dwell times, and laser polarizations. We observed a clear transformation in the kind of nanotips that grew for the given laser conditions.

No MeSH data available.


Related in: MedlinePlus