We report a unique study performed on the modal transition laser fluence of agglomerated nanoparticle size distributions and their averages in three-dimensional nanostructures that were formed on aluminosilicate ceramic using a megahertz femtosecond laser. At low repetition rates, bimodal particle distributions were obtained and changed to unimodal distributions with the increase in repetition rate. The distribution modals obtained depend only on the laser fluence and the presence of photoionized species were the possible reason for the formation of bimodal distributions. Laser fluence and heat accumulation could have played key roles in determining the average particle sizes. Our study would help to enhance the properties of 3-D agglomerated nanostructures.
©2012 Optical Society of America
Size plays a critical role in determining the properties of produced nanostructures. Previous studies have shown that the properties of nanomaterials, such as optical, magnetic, chemical and chemical transformation, and melting behaviour, are useful in applications like biomedical imaging, clinical diagnostics and therapeutics, which are related to the size of the nanostructures [1–10].
Pulsed laser ablation is one of the more popular methods for the synthesis of nanostructure. It involves several particle formation mechanisms, such as vapour condensation, spallation, hydrodynamic sputtering, solid exfoliation, and phase explosion. Even though several particle forming mechanisms occur simultaneously during pulsed laser ablation for different laser conditions, only vapour condensation forms particles in nanoscale (very much less than 100nm). Particles produced by all other mechanisms are in microscale (greater than 100nm) .
During vapour condensation, particles are formed from species such as atoms, molecules, clusters, and ions. The amount of supersaturation of plume vapour, determined by plume temperature, governs the size of the formed particles [12–15].
There are some other minor mechanisms known to take place during pulsed laser ablation, such as coalescence and fragmentation. These mechanisms change the size of the particles that have already formed within the plume. Studies have shown that the presence of coalescence of particles leads to the formation of microscale particles (greater than 100nm) . On the other hand, fragmentation leads to size reduction of the particles that have already formed .
In an attempt to improve the properties of nanostructures, many efforts have been carried out to study the effect of laser parameters, such as intensity, fluence, wavelength, pulse duration and power on the particle size distribution . S. Barcikowski et al. showed the relationship between pulse energy and pulse overlap, and how they affected the rate of production and the size distribution of the nanoparticles . A. Menendez- Manjon et al. showed that the laser pulse with Gaussian beam profile produced nanoparticles with broad, bimodal particle size distribution . Bimodal distributions with nano and micro scale particles have been obtained in most of the studies that showed a combination of two totally different mechanisms, such as melt ejection and vapour condensation.
3-D nanostructures were fabricated by our research team, in ambient air and with catalysts free conditions, on several different materials such as glasses, silicon, and metals [20,21]. To the best of our knowledge, no 3-D nanostructures on aluminosilicate ceramic were reported with pulsed laser. This study was performed on the agglomerated particles of the fabricated three dimensional nanostructures. No previous studies were performed on the agglomerated particle size distribution of the three dimensional nanostructures for varying megahertz repetition rates. Furthermore, our studies identified possibility of two different routes of particle generation within the vapour condensation process itself. A new explanation is given for the formation of bimodal particle size distributions during the vapour condensation process.
A better understanding of laser parameters and how they affect the particle size distribution would help to synthesize monodispersed particles, as well as high quality thin films . Recently, megahertz femtosecond lasers have emerged as a new tool for ultrafast laser ablation. This research was conducted to study the effect of laser repetition rate (in megahertz) on particle size distribution, the average particle size of the agglomerated particles in 3-D nanostructures as well as the modal transition laser fluence.
General purpose ceramic aluminosilicate thin plates, with a chemical composition of 29.2% (by weight) Al2O3, 59% SiO2 and trace of elements, was ablated by a femtosecond laser. A direct-diode-pumped, Yb-doped fiber amplified femtosecond laser system was used. The laser with Gaussian beam profile delivered a maximum output power of 11.0W, with a repetition rate ranging from 200kHz to 25.2MHz and with a pulse duration of 200fs. During the experiment, dwell time, pulse duration, and wavelength were kept constant. During the first set of experiments, aluminosilicate substrates were ablated at 2.1, 4.2, 8.4, 12.6 and 25.2MHz repetition rates while the applied power is kept at the constant value of 10.5W. The second set of experiments was performed to find out the threshold fluence of aluminosilicate at 2.1, 4.2, 6.3, 8.4, 12.6 and 25.2MHz repetition rates. Samples of aluminosilicate were ablated with decreasing laser power until threshold points of ablation were obtained and identified using optical microscope arrangement. Then aluminosilicate substrates were ablated by femtosecond laser at the constant fluence to threshold ratio (fluence ratio) of 3.2 for 4.2, 6.3, 8.4, 12.6 and 25.2MHz repetition rates. The generated nanoceramics were studied using a Transmission Electron Microscope (TEM). At a constant power and a constant fluence ratio, the particle size distributions of agglomerated particles in the 3-D nanostructures were obtained from the TEM images.
3. Results and discussion
3-D nanoparticles agglomerated structures were obtained in both first and second sets of experiments. Processes involved in the formation of structures are shown in Fig. 1 , which also shows the TEM and SEM images obtained from those structures at 25.2MHz repetition rate and 10.5W laser power. TEM images obtained from those structures confirmed that the 3-D nanostructures were made out of particle agglomeration.
3.1 Effects of repetition rate on the particle size distribution at a constant laser power
TEM images obtained at different repetition rates are shown in Fig. 2 . At each repetition rate, the sizes of agglomerated particles were measured from those images and the particle size distribution curves were obtained, which are shown in Fig. 3 .
The particle size distributions obtained at the repetition rates of 2.1, 4.2 and 8.4MHz are bimodal, while at 12.6 and 25.2MHz are unimodal. Studies have shown that the mechanism, which forms nanoscale particle size distributions during femtosecond laser ablation is the vapour condensation. The average particle size formed by vapour condensation is well below 100nm due to the high cooling rate of the expanding plasma .
The particle size distributions shown in Fig. 3 are in nanoscale, which indicate the occurrence of the vapour condensation process during particle formation. The chances of forming bimodal distribution decreased with the increase in repetition rate.
Formation of bimodal distribution is determined by how the particles are formed during nucleation. Previous studies have shown that the nucleation during vapour condensation can take place in two different routes, under the influence of ionized species and under the influence of neutral species and the growth rate of particles under the influence of ionized species is higher compare to the neutral species . At constant laser power, an increase in repetition rate decreases the laser fluence and thus the amount of ionized species formed in the laser plume . The presence of higher amount of ionized species in the plasma due to the increase in laser fluence increases the growth of particles, whereas neutral species in the same plume grow on their normal phase. The combined effect of particles from these two streams could be the reason for the formation of bimodal particle size distribution. If the amount of ionized species presence in the plume is not sufficient, due to the decrease in laser fluence, the particle growth is only influenced by the neutral species, which results in the formation of unimodal distribution. This study shows the decrease in ionized species presence in the plume; due to decrease in laser fluence, could be the possible reason for switching of bimodal to unimodal particle size distribution.
The average particle sizes at each repetition rate are plotted in Fig. 4 . A simple mathematical calculation is followed to obtain the average particle size, which is the addition of diameters of particles at each repetition rate divided by the total number of particles at that particular repetition rate. The rate of nucleation of vapour condensed particles increase faster than linear with the amount of supersaturation. However, the rate of condensational growth of particles is approximately linear with the supersaturation. Hence, the average size of formed particles decreases with an increase in the supersaturation . Furthermore, an increase in the plume temperature decreases the amount of supersaturation, which results in forming larger particles . Factors that determine the plume temperature for varying repetition rates could be the laser fluence and the amount of heat accumulation due to the decrease in pulse interval. An increase in the laser fluence as well as in heat accumulation results in the increase of plume temperature [26,27].
In Fig. 4, the average particle size is decreased with the increase in the repetition rate. At constant laser power, an increase in the repetition rate decreases the laser fluence, which decreases the plume temperature and increases the supersaturation. These changes lead to decrease the average size of particle formed with increasing repetition rate at constant laser power. This study shows, at constant laser power, laser fluence played a critical role in controlling the average particle size than the heat accumulation in the laser plume, when the repetition rate is increased in megahertz.
3.2 Effects of repetition rate on the particle size distribution at a constant fluence ratio
Agglomerated particle size distributions were studied in the 3-D nanostructures for different repetition rates at the constant fluence ratio of 3.2. The study was performed with the maximum possible fluence ratio that could be obtained from our laser system to enhance the chances of getting nanostructures at all repetition rates and to harvest maximum agglomerated particles on those structures. Figure 5(a) shows that the threshold power for 25.2MHz was 3.25W. But the maximum power from our laser system is closed to 10.5W. Hence, the fluence ratio was taken as 3.2 and this ratio was maintained at all repetition rates. The constant fluence ratio was taken to minimize the effect of change in threshold fluence with repetition rate. Initially, experiments were performed to study the threshold fluence of aluminosilicate ceramic at different repetition rates.
The above study was performed with laser parameters of 200fs pulse duration, 0.1ms dwell time, and 0.1mm laser spot separation. At each repetition rate, samples were ablated in atmosphere with decreasing laser power until the initial ablation point was obtained. Both threshold fluence and threshold power with repetition rates are shown in Fig. 5.
A decrease in threshold fluence was obtained with the increase in the repetition rate. At high repetition rate, ablation efficiency increases due to the hypothesis of heat accumulation on the surface of material with the number of pulses. It tends to decrease the ablation threshold . When the repetition rate is increased from 12.6MHz to 25.2MHz the curve approaches flat. It shows the existence of a minimum threshold fluence to start the ablation .
Then, the experiments were performed to study the agglomerated particle size distribution in the 3-D nanostructures at the constant fluence ratio of 3.2 for different repetition rates in megahertz. The sample of TEM image obtained at each repetition rate is shown in Fig. 6 .
In Fig. 7 , bimodal particle size distributions were obtained at 4.2MHz and 6.3MHz (at high laser fluence), while unimodal distributions were obtained at other repetition rates (at low laser fluence). As discussed in the first set of experiments, the reason for the formation of bimodal distribution could be the influence of ionized species during vapour condensation. The average particle size obtained at each repetition rate for constant fluence ratio of 3.2 is shown in the Fig. 8 .
When the repetition rate was increased from 4.2MHz to 8.4MHz, a decrease in the average particle size was obtained as shown in Fig. 8. The laser fluence decreased, when the repetition rate was increased from 4.2MHz to 8.4MHz. Even though heat accumulation increased during this change in repetition rate, decrease in the laser fluence played a critical role in determining the average particle size in that range. When the repetition rate was increased beyond 8.4MHz, the average particle size also increased. This change in average particle size shows that the heat accumulation could have played a major role than the laser fluence beyond 8.4MHz. In Fig. 9 , at constant fluence ratio, the change in laser fluence was minimum beyond 8.4MHz. This further shows that the decrease in influence of laser fluence beyond 8.4MHz at constant laser fluence ratio. Increase in repetition rate decreases the cooling time between each pulse. Hence, the total pulse interaction time of both plume and target material at a given dwell time was increased. Also the laser power applied was increased with repetition rate leads to increase the laser fluence. These effects results in increase in plume temperature, which decreases the amount of super saturation, decreases the nucleation rate, and increases the size of particles formed.
A comparison between the effects of laser fluence on the distribution modes is shown in Fig. 9. Bimodal distributions were obtained above the modal transition laser fluence, while unimodal distributions were obtained below modal transition laser fluence. These studies show that the distribution modals depend only on the laser fluence. Further, the amount of ionized species formed during laser ablation for a particular material solely depends on the laser fluence applied. This study also shows the possible influence of ionized species in the formation of bimodal distribution.
Our studies have also shown that the average size of agglomerated particles was influenced by the competing effects between the laser fluence and the heat accumulation for varying repetition rate in megahertz. At constant repetition rate, average particle size decreases with decrease in laser fluence . But in Fig. 8, our study in constant laser fluence ratio has shown that the average particle size increased with increase in repetition rate (decrease in fluence) beyond 8.4MHz. This study shows that both fluence and the repetition rate (in megahertz) have influence in determining the average sizes of particles obtained.
Our studies involved the analysis of particle size distribution modals and the average sizes of agglomerated particles in the 3-D nanostructure at varying megahertz repetition rates. Results have shown a change from bimodal to unimodal particle size distribution with increasing repetition rate. Furthermore, the modal transition laser fluence determines whether unimodal or bimodal distributions are obtained. The amount of ionized species formed during laser ablation could play a critical role in the formation of bimodal distributions. The average size of the agglomerated particles is determined by the repetition rate as well as the laser fluence. At constant laser power, laser fluence determined the average particle sizes, but at constant fluence ratio, fluence and the heat accumulation (repetition rate) were the determining factors. This study can be used to enhance the properties of fabricated 3-D nanostructures.
This research is funded by Natural Science and Engineering Research Council of Canada and Ministry of Research and Innovation, Ontario, Canada.
References and links
1. S. Stankic, M. Müller, O. Diwald, M. Sterrer, E. Knözinger, and J. Bernardi, “Size-dependent optical properties of MgO nanocubes,” Angew. Chem. Int. Ed. Engl. 44(31), 4917–4920 (2005). [CrossRef] [PubMed]
3. L. L. Shaw, D. Goberman, R. Ren, M. Gell, S. Jiang, Y. Wang, D. T. Xiao, and P. R. Strutt, “The dependency of microstructure and properties of nanostructured coatings on plasma spray conditions,” Surf. Coat. Tech. 130(1), 1–8 (2000). [CrossRef]
5. S. R. Emory, W. E. Haskins, and S. Nie, “Direct observation of size-dependent optical enhancement in single metal nanoparticles,” J. Am. Chem. Soc. 120(31), 8009–8010 (1998). [CrossRef]
7. C. L. Haynes and R. P. Van Duyne, “Nanosphere lithography: a versatile nanofabrication tool for studies of size-dependent nanoparticle optics,” J. Phys. Chem. B 105(24), 5599–5611 (2001). [CrossRef]
8. Y. Mao and S. S. Wong, “Size- and shape-dependent transformation of nanosized titanate into analogous anatase titania nanostructures,” J. Am. Chem. Soc. 128(25), 8217–8226 (2006). [CrossRef] [PubMed]
9. T. J. Park, G. C. Papaefthymiou, A. J. Viescas, A. R. Moodenbaugh, and S. S. Wong, “Size-dependent magnetic properties of single-crystalline multiferroic BiFeO3 nanoparticles,” Nano Lett. 7(3), 766–772 (2007). [CrossRef] [PubMed]
10. M. Zhang, M. Y. Efremov, F. Schiettekatte, E. A. Olson, A. T. Kwan, S. L. Lai, T. Wisleder, J. E. Greene, and L. H. Allen, “Size-dependent melting point depression of nanostructures: nanocalorimetric measurements,” Phys. Rev. B 62(15), 10548–10557 (2000). [CrossRef]
11. J. Koch, A. V. Bohlen, R. Hergenröder, and K. Niemax, “Particle size distributions and compositions of aerosols produced by near-IR femto- and nanosecond laser ablation of brass,” J. Anal. At. Spectrom. 19(2), 267–272 (2004). [CrossRef]
12. C. H. Hung, M. J. Krasnopoler, and J. L. Katz, “is Condensation of a supersaturated vapor. VIII. The homogeneous nucleation of n-nonane,” J. Chem. Phys. 90(3), 1856–1865 (1989). [CrossRef]
13. K. Y. Park and H. J. Jeong, “Effect of temperature on particle size for vapor - phase synthesis of Ultrafine Iron particles,” Korean J. Chem. Eng. 16(1), 64–68 (1999). [CrossRef]
14. H. Lihavainen, Y. Viisanen, and M. Kulmala, “Homogeneous nucleation of n-pentanol in a laminar flow diffusion chamber,” J. Chem. Phys. 114(22), 10031–10038 (2001). [CrossRef]
15. M. S. Tillack, D. W. Blair, and S. S. Harilal, “The effect of ionization on cluster formation in laser ablation plumes,” Nanotechnology 15(3), 390–403 (2004). [CrossRef]
16. F. Mafuné, J.-y. Kohno, Y. Takeda, and T. Kondow, “Dissociation and aggregation of gold nanoparticles under laser irradiation,” J. Phys. Chem. B 105(38), 9050–9056 (2001). [CrossRef]
17. W. Marine, L. Patrone, B. Luk’yanchuk, and M. Sentis, “Strategy of nanocluster and nanostructure synthesis by conventional pulsed laser ablation,” Appl. Surf. Sci. 154–155, 345–352 (2000). [CrossRef]
18. S. Barcikowski, A. Hahn, A. V. Kabashin, and B. N. Chichkov, “Properties of nanoparticles generated during femtosecond laser machining in air and water,” Appl. Phys., A Mater. Sci. Process. 87(1), 47–55 (2007). [CrossRef]
19. A. Menéndez-Manjón, S. Barcikowski, G. A. Shafeev, V. I. Mazhukin, and B. N. Chichkov, “Influence of beam intensity profile on the aerodynamic particle size distributions generated by femtosecond laser ablation,” Laser Part. Beams 28(01), 45–52 (2010). [CrossRef]
21. M. Sivakumar, K. Venkatakrishnan, and B. Tan, “Study of metallic fibrous nanoparticle aggregate produced using femtosecond laser radiation under ambient conditions,” Nanotechnology 21(22), 225601 (2010). [CrossRef] [PubMed]
22. T. Takiya, I. Umezu, M. Yaga, and M. Han, “Nanoparticle formation in the expansion process of a laser ablated plume,” J. Phys. 59, 445–448 (2007). [CrossRef]
23. F. Yu and R. P. Turco, “Ultrafine aerosol formation via ion-mediated nucleation,” Geophys. Res. Lett. 27(6), 883–886 (2000). [CrossRef]
24. N. A. Fuchs and A. G. Sutugin, Highly Dispersed Aerosols (Ann Arbor, 1970), Chap. 1.
25. A. C. Zettlemoyer, Nucleation, R. Andres, ed. (Marcel Dekker Inc., 1969), Chap. 2.
26. V. Piñon and D. Anglos, “Optical emission studies of plasma induced by single and double femtosecond laser pulses,” Spectrochim. Acta, B At. Spectrosc. 64(10), 950–960 (2009). [CrossRef]
27. S. M. Eaton, H. Zhang, P. R. Herman, F. Yoshino, L. Shah, J. Bovatsek, and A. Y. Arai, “Heat accumulation effects in femtosecond laser-written waveguides with variable repetition rate,” Opt. Express 13(12), 4708–4716 (2005). [CrossRef] [PubMed]
28. F. Brygo, C. Dutouquet, F. Le Guern, R. Oltra, A. Semerok, and J. M. Weulersse, “Laser fluence, repetition rate and pulse duration effects on paint ablation,” Appl. Surf. Sci. 252(6), 2131–2138 (2006). [CrossRef]
29. B. Tan, A. Dalili, and K. Venkatakrishnan, “High repetition rate femtosecond laser nano-machining of thin films,” Appl. Phys., A Mater. Sci. Process. 95(2), 537–545 (2009). [CrossRef]
30. M. Vitiello, S. Amoruso, C. Altucci, C. de Lisio, and X. Wang, “The emission of atoms and nanoparticles during femtosecond laser ablation of gold,” Appl. Surf. Sci. 248(1-4), 163–166 (2005). [CrossRef]