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Low pressure DMA studies of laser ablation aerosols
1. Armsol SC;. Vol. 29, Suppl. I. pp. S52%S530,
1998
0 1998 Published by Elsevier Science Ltd. All rights reserved
Pergamon
Printed in Great Britain 0021.8502/98
LOW PRESSURE
R.P. CAMATA$
DMA STUDIES
M. HIRASAWAa’,
OF LASER ABLATION
K. OKUYAMAb’
$19.00 + 0.00
AEROSOLS
and K. TAKEUCHIa’
“‘The Institute of Physical and Chemical Research (RIKEN) 2-l Hirosawa, Wako-shi, Saitama 351-0198, Japan “Department of Chemical Engineering, Hiroshima University, 4- 1 Kagamiyama 1 chome, Higashi-Hiroshima 739, Japan KEYWORDS Laser Ablation,
Differential
Mobility
Analyzer,
Cesium Iodide Aerosol
The generation of particulate material due to the interaction of intense laser beams with solid surfaces is a widely observed phenomenon. In numerous applications of laser ablation these particulates are an undesired feature to be minimized or, if possible, eliminated. From the point of view of aerosol science, however, laser ablation in background gases provides an efficient way of producing aerosols with a broad range of properties. Few studies to date have concentrated on the use of this technique as a source to intentionally generate a stable, controlled and well characterized aerosol of ultrafine particles for nanocomposite and particulate engineering applications (Whitlock and Frick, 1994, Gaumet et al., 1996). Characteristics such as experimental simplicity, materials versatility, and reproducibility of complex target stoichiometry, afford interesting prospects for the use of laser ablation aerosols in engineering systems. Nonetheless, many details involved in the behavior of these aerosols are not understood. For one thing, multiple excitation pathways resulting from the laser-target interaction lead to the generation of a complex initial state comprised of ions, clusters, particles and droplets. Fundamental understanding of the processes involved in the formation and evolution of this aerosol is necessary if laser ablation is to be optimized and tailored to the synthesis of nanostructured materials with desired properties and function. The use of aerosol instrumentation in the study of laser ablation aerosols has been limited, in part by the lack of reliable tools to perform measurements at reduced pressure, where process manipulation is more effective. In this work we attempt to overcome this limitation by using a differential mobility analyzer (DMA) calibrated for operation under low pressure conditions (Seto et al., 1997). We have chosen to investigate a cesium iodide (CsI) aerosol. Besides the importance of CsI as an optical material and its role as an archetype for the study of ionic solid aerosols, understanding of the behavior of CsI aerosols is also relevant to prevent their release into the environment in the event of a severe accident in a nuclear reactor. The CsI aerosol was obtained by the ablation of a solid CsI target in a mixed flow reactor using the 1064 nm emission of a pulsed Nd3+:YAG laser (10 Hz, 10 ns pulse). The low pressure DMA was used to measure the aerosol size distribution in the l-20 nm range. Transmission electron microscopy (TEM) on particles deposited on a substrate S529
s530
Abstracts
of the 5th International
Aerosol Conference
1998
allowed estimation of particle diameters beyond the limit imposed on the DMA voltage due to the reduced pressure. Electron diffraction was used for structural characterization. Measurements with the DMA were performed during experiments with nominal laser fluences between 4 and 11 J/cm2 and argon background pressure in the range of 75 500 Torr. Within the size range of the DMA, data inversion reveals approximately lognormal size distributions with mean particle diameter D,, around 10 nm and geometric standard deviation a, - 1.6. Total number concentrations span the lo6 - lo7 cm1‘3range as laser fluences are varied from 4 to 11 J/cm2 . Small shifts in Drs as a function of pressure suggest some degree of coagulation growth as D,, scales with the system residence time. Transmission electron microscopy analysis was carried out on particles produced with nominal laser fluence of 11 J/cm2 in 100-500 Torr of argon. In addition to the lonm-scale particles detected with the DMA, TEM shows the presence of an abundance of much larger particles. Samples generated at 300 and 500 Torr are dominated by isolated CsI particles with D,, - 80 nm. These particles are spherical and compact. The particle mean diameter is apparently pressure-independent in this regime. At 100 Ton-, however, increased particle number concentration with consequent coagulation enhancement is observed. At this pressure, micron-sized agglomerates are the most abundant objects, with partial particle coalescence evident throughout the sample. On all samples analyzed by TEM (in which diffraction is dominated by large isolated particles or agglomerates) indexing of electron diffraction patterns yield all lattice planes of salt-like crystalline CsI, with straightforward generation of crystal-like dark-field contrast. Compact particles larger than about 200 nm exhibit tendency toward faceting. The measurements performed suggest that in addition to gas-phase condensation and coagulation, processes involving droplet generation and detachment from the target, such as hydrodynamic sputtering, may also play a role in particle formation (Kelly and Miotello, 1994). As previously demonstrated for selected metals (Gaumet et al., 1996), the aerosol concentration is strongly dependent on the target surface state. The overall number concentration tends to increase with surface roughness, thereby complicating the interpretation of size distribution data as a means for formation mechanism inferences. REFERENCES Gaumet, J.-J., Membrey, F., and Chambaudet, A. (1996) Characterization of aerosols induced by laser irradiation of material: significance of the target surface state, C.R. Acad. Sci. Paris 322, Series II b, 507-5 14. Kelly, R. and Miotello, A. (1994) Mechanisms of pulsed laser sputtering, Pulsed laser ofthinfilms, ed. D. B. Chrisey and G. K. Hubler, John Wiley & Sons, New York, 1994, pp. 55-87.
deposition
Seto, T., Nakamoto, T., Okuyama, K., Adachi, M., Kuga, Y., and Takeuchi, T. (1997) Size Distribution Measurement of Nanometer-Sized Aerosol Particles Using DMA Under Low-Pressure Conditions, J. Aerosol Sci. 28, 193-206. Whitlock, R.R. and Frick, G.M. (1994) Particle size distributions of aerosols formed by laser ablation of solids at 760 Torr, J. Mafer. Res. 9,2868-2872.
1998
0 1998 Published by Elsevier Science Ltd. All rights reserved
Pergamon
Printed in Great Britain 0021.8502/98
LOW PRESSURE
R.P. CAMATA$
DMA STUDIES
M. HIRASAWAa’,
OF LASER ABLATION
K. OKUYAMAb’
$19.00 + 0.00
AEROSOLS
and K. TAKEUCHIa’
“‘The Institute of Physical and Chemical Research (RIKEN) 2-l Hirosawa, Wako-shi, Saitama 351-0198, Japan “Department of Chemical Engineering, Hiroshima University, 4- 1 Kagamiyama 1 chome, Higashi-Hiroshima 739, Japan KEYWORDS Laser Ablation,
Differential
Mobility
Analyzer,
Cesium Iodide Aerosol
The generation of particulate material due to the interaction of intense laser beams with solid surfaces is a widely observed phenomenon. In numerous applications of laser ablation these particulates are an undesired feature to be minimized or, if possible, eliminated. From the point of view of aerosol science, however, laser ablation in background gases provides an efficient way of producing aerosols with a broad range of properties. Few studies to date have concentrated on the use of this technique as a source to intentionally generate a stable, controlled and well characterized aerosol of ultrafine particles for nanocomposite and particulate engineering applications (Whitlock and Frick, 1994, Gaumet et al., 1996). Characteristics such as experimental simplicity, materials versatility, and reproducibility of complex target stoichiometry, afford interesting prospects for the use of laser ablation aerosols in engineering systems. Nonetheless, many details involved in the behavior of these aerosols are not understood. For one thing, multiple excitation pathways resulting from the laser-target interaction lead to the generation of a complex initial state comprised of ions, clusters, particles and droplets. Fundamental understanding of the processes involved in the formation and evolution of this aerosol is necessary if laser ablation is to be optimized and tailored to the synthesis of nanostructured materials with desired properties and function. The use of aerosol instrumentation in the study of laser ablation aerosols has been limited, in part by the lack of reliable tools to perform measurements at reduced pressure, where process manipulation is more effective. In this work we attempt to overcome this limitation by using a differential mobility analyzer (DMA) calibrated for operation under low pressure conditions (Seto et al., 1997). We have chosen to investigate a cesium iodide (CsI) aerosol. Besides the importance of CsI as an optical material and its role as an archetype for the study of ionic solid aerosols, understanding of the behavior of CsI aerosols is also relevant to prevent their release into the environment in the event of a severe accident in a nuclear reactor. The CsI aerosol was obtained by the ablation of a solid CsI target in a mixed flow reactor using the 1064 nm emission of a pulsed Nd3+:YAG laser (10 Hz, 10 ns pulse). The low pressure DMA was used to measure the aerosol size distribution in the l-20 nm range. Transmission electron microscopy (TEM) on particles deposited on a substrate S529
s530
Abstracts
of the 5th International
Aerosol Conference
1998
allowed estimation of particle diameters beyond the limit imposed on the DMA voltage due to the reduced pressure. Electron diffraction was used for structural characterization. Measurements with the DMA were performed during experiments with nominal laser fluences between 4 and 11 J/cm2 and argon background pressure in the range of 75 500 Torr. Within the size range of the DMA, data inversion reveals approximately lognormal size distributions with mean particle diameter D,, around 10 nm and geometric standard deviation a, - 1.6. Total number concentrations span the lo6 - lo7 cm1‘3range as laser fluences are varied from 4 to 11 J/cm2 . Small shifts in Drs as a function of pressure suggest some degree of coagulation growth as D,, scales with the system residence time. Transmission electron microscopy analysis was carried out on particles produced with nominal laser fluence of 11 J/cm2 in 100-500 Torr of argon. In addition to the lonm-scale particles detected with the DMA, TEM shows the presence of an abundance of much larger particles. Samples generated at 300 and 500 Torr are dominated by isolated CsI particles with D,, - 80 nm. These particles are spherical and compact. The particle mean diameter is apparently pressure-independent in this regime. At 100 Ton-, however, increased particle number concentration with consequent coagulation enhancement is observed. At this pressure, micron-sized agglomerates are the most abundant objects, with partial particle coalescence evident throughout the sample. On all samples analyzed by TEM (in which diffraction is dominated by large isolated particles or agglomerates) indexing of electron diffraction patterns yield all lattice planes of salt-like crystalline CsI, with straightforward generation of crystal-like dark-field contrast. Compact particles larger than about 200 nm exhibit tendency toward faceting. The measurements performed suggest that in addition to gas-phase condensation and coagulation, processes involving droplet generation and detachment from the target, such as hydrodynamic sputtering, may also play a role in particle formation (Kelly and Miotello, 1994). As previously demonstrated for selected metals (Gaumet et al., 1996), the aerosol concentration is strongly dependent on the target surface state. The overall number concentration tends to increase with surface roughness, thereby complicating the interpretation of size distribution data as a means for formation mechanism inferences. REFERENCES Gaumet, J.-J., Membrey, F., and Chambaudet, A. (1996) Characterization of aerosols induced by laser irradiation of material: significance of the target surface state, C.R. Acad. Sci. Paris 322, Series II b, 507-5 14. Kelly, R. and Miotello, A. (1994) Mechanisms of pulsed laser sputtering, Pulsed laser ofthinfilms, ed. D. B. Chrisey and G. K. Hubler, John Wiley & Sons, New York, 1994, pp. 55-87.
deposition
Seto, T., Nakamoto, T., Okuyama, K., Adachi, M., Kuga, Y., and Takeuchi, T. (1997) Size Distribution Measurement of Nanometer-Sized Aerosol Particles Using DMA Under Low-Pressure Conditions, J. Aerosol Sci. 28, 193-206. Whitlock, R.R. and Frick, G.M. (1994) Particle size distributions of aerosols formed by laser ablation of solids at 760 Torr, J. Mafer. Res. 9,2868-2872.