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Nanometer particles: A new frontier for multidisciplinary research
Pergamon
J. Aerosol Sci. Vol. 28, No. 4, pp. 539 544, 1997
PII: S0021-8502(96)00495-8
GUEST
x 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0021 8502/97 $17.00 + 0.00
EDITORIAL
NANOMETER PARTICLES: A NEW FRONTIER FOR MULTIDISCIPLINARY RESEARCH David Y. H. Pui* and Da-Ren Chen Particle TechnologyLaboratory, Mechanical Engineering Department, Universityof Minnesota, 111 Church Street S.E., Minneapolis, MN 55455, U.S.A.
INTRODUCTION "Nanometer particles" are currently defined as particles with diameters below about 50 nm (Op < 50 nm). A lower size limit for these nanoparticles depends somewhat on the criteria applied and is sometimes operational, since basic properties and measurement methods are still in a state of flux. The "size" measured by an aerosol process may not be entirely correlated with such other properties as catalytic activity or magnetic structure. However, stable molecular clusters, biological molecules, fullerene and oligonucleotides, may serve as examples of nanoparticles near the lower size limit. As a new frontier in particle and aerosol research, nanoparticle research has significant potential for scientific breakthrough and new technological innovations. The cooperation of scientists in the materials and aerosol fields, for example, are already beginning to bear fruit. In the following paragraphs, therefore, we outline nanoparticle applications in several scientific disciplines, covering a broad spectrum from the production of nanophase materials with superior properties to studies of human exposure by inhalation of nanometer particles. At the recent Annual Conference of the American Association for Aerosol Research (AAAR) in Orlando, FL, Professor S. K. Friedlander eloquently stated some of these--and other--aspects of nanoparticle applications, and drew attention to the need for increasing infusions of molecular science and solid state physics to nanoparticle research. Studies of nanoparticles would involve multidisciplinary approaches because of the broad nature of their properties (e.g. physical, chemical and biological) and the diverse possibilities for their applications. New instrumentation for nanoparticle characterization is developing at a rapid pace which will accelerate nanoparticle studies and further expand the scope of their applications. We feel that periodic, timely publications of the results and implications of nanoparticle studies will stimulate cross-fertilization of knowledge among scientists and engineers in many different fields. With this in mind, the publication of a special issue of the Journal of Aerosol Science is proposed, providing a focal point for current thinking and research directions in this highly topical area.
Nanometer particles versus ultrafine particles The terms "ultrafine particles" and "ultrafine aerosols" were used by some aerosol scientists during a 1979 Workshop on Ultrafine Aerosols (WUFA) in Vienna (Liu et al., 1982). The purpose of the Workshop was to evaluate the counting efficiencies of a number of condensation nuclei counters available at that time. Ultrafine aerosols were defined at that time as those characterized by particle diameters less than 0.1 #m (Dp < 100 nm). Earlier, Fuchs and Sutugin (1971) had used the term "highly-dispersed aerosol" to define aerodisperse systems with particle diameters less than 100 nm. Recently, the U.S. Environmental
*Author to whom all correspondenceshould be addressed. 539
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Protection Agency used the term "ultrafine particles in a biological context" to characterize particle size distributions with mass median diameter (MMD) below about 0.1 ym (U.S. EPA, 1996). From the preceding, it becomes clear that the working definition of"nanometer particles" (Dp < 50 nm) as stated earlier makes such an aerosol a subgroup of the "ultrafine particle" class. We have found much inconsistency in the use of the term "ultrafine particles". "'Ultrafine'" is of course intended to signify "very, very fine," but such a statement can evoke very different meanings between disciplines. For example, the term "ultrafine particles" has been used in some powder technology publications to denote particles smaller than 1.0 lLm in diameter (e.g. Kanda et al., 1989; Ng-Yelim et al., 1989). Some workers have even defined them as particles with diameters below 45 l~m (e.g. Ridder et al., 1988). "Nanometer particles" can and should be much more narrowly defined. The term itself is explicit in that it actually identifies the order of magnitude of the particles. The convention proposed here, D v < 50 nm, is consistent with this rationale. It should also be noted that the terms "nanostructure" and "nanophase'" materials used in materials community has come to refer to clusters consisting of primary particles below 100 nm diameter (e.g. Gurav et al., 1993). At a recent Joint NSF/JSPS U.S. Japan Workshop on Nanoparticle Synthesis and Applications (1996), it was proposed that nanoparticles be more narrowly defined as those which show quantum confinement effects, generally with Dp < 30 nm. M U L T 1 D I S C I P L I N A R Y RESEARCH I N V O L V I N G N A N O M E T E R P A R T I C L E S M a t e r i a l s synthesis
Production of nanostructure materials is an exciting new area for the materials industry. Materials in the nanostructure form possess many desirable properties, including improved hardness, reduced internal friction (i.e. improved ductility), reduced melting point, and special optical and magnetic properties (Nieman et al., 1990, 1991; Siegel et al., 1988; Karch et al., 1987; Sherby and Wadsworth, 1989; Futaki et al., 1992; Kofman et al., 1994; Endo et al., 1996; Lin et al., 1995; Yao et al., 1995; Sankaranarayanan et al., 1993; Taketomi et al., 1993; Gangopadhyay et al., 1992, 1993). These properties are the basis of many so-called "high-tech" applications, including quantum dots, drill bit coatings, fuel cells, and tunable laser, and so they promise to revolutionize the materials industry. Production of such materials via the aerosol route provides enhanced levels of materials purity and production control. As such, it is the topic of several recent symposia and workshops (e.g. Third International Conference on Nanostructured Materials, 1996: Joint NSF/JSPS U.S.-Japan Workshop on Nanoparticle Synthesis and Applications, 1996), bringing together researchers from the aerosol and materials communities to investigate new production and evaluation methods. H e a l t h eJfects
While nanostructure materials are produced in increasing quantities, there is an increased awareness in the occupational health community on the possible adverse health effects of exposure to nanometer particles (Cheng et al,, 1993, 1996; Ferin, 1994; Ferin et al., 1990: Oberd6rster et al., 1992, 1995; Martonen and Zhang, 1992; Gradon and Yu, 1989). it has been speculated that nanometer particles depositing in the human respiratory tract may enter the interstitial spaces and may penetrate through the cell membrane, so that even relatively insoluble material may pass beyond the respiratory tract. Those which are retained in the lung may become sequestered and remain there for very long periods. Recent intratracheal injection studies using rats, reported by Donaldson et al. (1996), have shown that material which is relatively inert for micrometer-sized particles (e.g. titanium dioxide) can be highly inflammogenic for particles in the nanometer size range. It is clearly suggested, therefore, that nanometer particles may, depending on their chemical composition, be associated with the possibility of serious ill health. At present, however, there is a lack of exposure data or epidemiology studies to either support or reject such a
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hypothesis. Undoubtedly, this is an area which will provide research opportunities for aerosol scientists, toxicologists, epidemiologists and other health researchers for the years to come.
M icrocontamination
The Semiconductor Industry Association (SIA) estimates that the next generation of 1-gigabit DRAM devices will have a minimum feature size of smaller than 0.18/xm. Currently, the majority of the product yield losses can be attributed to the deposition of particles on the semiconductor wafers. Using the common 1/3 rule for microcontamination control, particles in the nanometer size range will therefore need to be measured and controlled. Recent microcontamination research has focused on understanding particle formation and deposition mechanisms in the process equipment (Ye et al., 1993; Ziemann et al., 1995; Viner et al., 1989), and in cleaning methods (Futatsuki et al., 1993). Some of the challenges will be to measure these particles under the conditions of high vacuum and high temperature, and in aggressive gases and plasma, and to remove them by deposition techniques (Wang and Kasper,1991). It is believed that contamination as small as molecules need to be considered to achieve high yield in next several generations of semiconductor devices (Kinkead, 1996). Particles are also used in a beneficial way for the production of semiconductor wafers. In the chemical mechanical polishing (CMP) processes, slurry consisting of fine particles is used to polish wafer surfaces. To produce the desirable smooth surface on the wafer, uniform size particles below 100 nm are used in increasing quantity (Cook et al., 1995; Schlueter, 1996; Burggraaf, 1995). Microcontamination provides opportunities for both instrument designers and fundamental researchers, including for example studies of nucleation phenomena in complex systems and particle behavior in vacuum.
Biotechnology
Determination of the molecular weights of biopolymers, including oligonucleotides and proteins, is of fundamental importance to biotechnology development. Mass spectrometry has been used to perform such measurements by transforming individual molecules into ions and deflecting them in electric and magnetic fields. One of the challenges is to ionize the molecules without causing catastrophic decomposition. Various "soft" ionization methods have been used with limited success (Fenn et al., 1989). An exciting new area is to ionize the biomolecules using an electrospraying technique followed by mass spectrometry (Smith et al., 1990). These biomolecules with molecular weights up to 130,000 are, in essence, nanometer particles. Aerosol researchers can therefore participate and contribute in this exciting field by providing technologies for dispersing and ionizing of such nanometer-sized biomolecules. One example of such development is the electrospray-condensation particle counter for detecting macromolecules as proposed by Lewis et al. (1994).
Emission control
Energy saving and the environmental consequences of using Diesel engines are well known. Diesel engine exhaust aerosols are made up from clusters 9 f individual nanometer particles. Understanding the dynamics of soot formation and developing monitoring sensors for Diesel engines will therefore help in minimizing their environmental impact. Aerosol technologists have been at the forefront of such development and will no doubt continue to contribute to this effort (Amann and Siegla, 1982; Luo et al., 1989). "High-tech" coating technologies (e.g. coatings applied to deep-sea oil platforms and to engine cylinders) often produce nanoparticles as a by-product (Chan et al., 1995). Further, emissions from semiconductor processing equipment often consist of nanoparticles of exotic materials. Emission control technologies will no doubt play an important role in these areas.
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D.Y.H. Pui and Da-Ren Chen
Instrumentation
Interests in instrumentation development for nanoparticle measurements have increased significantly during the past five years. Two focused areas include generation of monodisperse nanometer aerosols for instrument calibrations, and measurement of the particle size distributions of nanometer aerosols. The traditional method involves using a differential mobility analyzer (DMA) to either measure or classify aerosols. However, the sizing resolution and detection sensitivities of the current generation of DMAs deteriorate drastically in the nanometer particle size range, due to the effects of diffusion broadening and losses. This realization has led to a series of publications on comparing available DMAs in terms of their suitability for nanometer aerosol measurements (Fissan et al., 1996), and the development of novel D M A s for nanometer size range (Pourprix, 1994; Zhang et al., 1995: Chen et al., 1996). A new approach of using electrospraying techniques for generating monodisperse nanometer aerosols has also been developed and has yielded excellent results (de la Mora et al., 1990; Chen et al., 1996). Another exciting possibility is to make use of mass spectrometers for detecting and analyzing chemical compositions of nanometer particles (Fenn et al., 1989; Smith et al., 1990). In addition, a variety of new tools are being developed for analysis of individual nanofeatures including atomic force microscopy, high resolution transmission electron microscopy, and near field optical microscopy. N a n o m e t e r particle physics and c h e m i s t r y
There is a great deal of interest in understanding the physics of the transfer processes of nanometer aerosol systems, including mass transfer, heat transfer, m o m e n t u m transfer, and charge transfer. Theories for describing these phenomena are relatively simple when the Knudsen number (Kn) is large (i.e. "free-molecule" theory) and small (i.e. "'continuum" theory). The greatest difficulty occurs for the cases of intermediate Kn. Fuch's "limitingsphere" approach provides reasonable but not rigorous results. Accurate calculation requires solving the Boltzmann collision equation. Significant efforts have been made towards this approach (e.g. Williams and Loyalka, 1991; Marlow and Brock, 1975: Huang et al., 1990). All these studies are based on linearized approximations of the Boltzmann equation, and are far from solving the actual nonlinear Boltzmann equation. New and solar not observed phenomena may become important, such as "thermal rebound" (Wang and Kasper, 1991) which may influence measurement techniques near the limit between particles and molecules. Meanwhile, new instrumentation and experimental techniques are becoming available which are stimulating more studies on nanometer particles. Some recent research topics reported include quantum confinement effects of nanometer particles (Vahala, 1988; Wiedensohler and Hansson, 1992: Alivisatos, 1996), the decrease of the melting point of nanometer particles (Goldstein et al., 1992), ultrafast dynamics in nanoparticle/nanocrystal (Mittleman et al., 1994), and nanoparticle bonds in agglomerates (Weber and Friedlander, 1994). SUMMARY In summary, it is clear that exciting new possibilities now exist for research into, and applications of, nanometer particles. The proposed special issue of the Journal o f Aerosol Science will draw together current knowledge on these subjects and describe the state-ofthe-art research, providing a platform for future exploration. REFERENCES Alivisatos, A. P. (1996) Semiconductor clusters, nanocrystals, and quantum dots. Science 271,933 937. Amann, C. A. and Siegla, D. C. (1982) Diesel particulates whatthey are and why. Aerosol Sci. Technol, 1, 73 101. Burggraaf, P. (t995) CMP: suppliers integrating, applications spreading. Proc. Semiconducu)r International, November, pp. 74-82. Chan, T. L., Rouhana, S. W., Mulawa, P. A. and Reuter, R. J. 11995) Occupational health assessmentof the high velocity oxy-fuelthermal metal spray process. Appl. Occup. Environ. Hy~t. 10, 482 487.
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Chen, D., Pui, D. Y. H., Hummes, D., Fissan, H., Quant, F. R. and Sere, G. J. (1996) Nanometer differential mobility analyzer (Nano-DMA): design and numerical modeling. J. Aerosol Sci. 27, S137-138. Chen, D., Pui, D. Y. H. and Kaufman, S. L. Electrospraying of conducting liquids for monodisperse aerosol generation in the 4 nm to 1.8 #rn diameter range. J. Aerosol Sci. 26, 963-977. Cheng, Y. S., Su, Y.-F., Yeh, Hsu-Chi and Swift, D. L. (1993) Deposition of thoron progeny in human head airways. Aerosol Sci. Technol. 18, 359-375. Cheng, Y. S., Yeh, H. C., Guilmette, R. A., Simpson, S. Q., Cheng, K. H. and Swift, D. L. (1996) Nasal deposition of ultrafine particles in human volunteers and its relationship to airway geometry. Aerosol Sci. Technol. 25, 274 291. Cook, L. M., Wang, J. F., James, D. B. and Sethurarnan, A. R. (1995) Theoretical and practical aspects of dielectric and metal CMP. Proc. Semiconductor International, November, pp. 141-144. De la Mora, F., Navascues, J., Fernandez, J. and Rosell Llompart, R. (1990) Generation of submicron monodisperse aerosol by electrosprays. J. Aerosol Sci. 21, $673-676; also Loscertales, I. G. and de la Mora, F. (1993) Characterization of electrospray-generated nanoparticles in a hypersonic impactor. Presented at the Workshop on Ultrafine Particles, Delf, The Netherlands, May 1993. Donaldson, K,, Gilmour, P., Brown, D. M., Beswick, P. H., Benton, E. and MacNee, W. (1996) Surface free radical activity of PM 10 and ultrafine titanium dioxide: a unifying factor in their toxicity. Paper presented at the 8th Int. Syrup. Inhaled Particles, Cambridge, U.K., August 1996. To be published in Inhaled Particles V I I I (Edited by Ogden, T. L.), Pergamon, Oxford. Endo, Y., Ono, M., Yamada, T., Kawamura, H., Kobara, K. and Kawamura, T. (1996) Study of antireflective and antistatic coating with ultrafine particles. Adv. Powder Technol. 7, 131-140. Fenn, J. B., Mann, M., Meng, C. K., Wong, S. F. and Whitehouse, C. M. (1989) Electrospray ionization for mass spectrometry of large biomolecules. Science 246, 64-71. Ferin, J. (1994) Pulmonary retention and clearance of particles. Toxicol. Lett. 72, 121-125. Ferin, J., Oberdoerster, G., Penney, D. P., Soderholm, S. C., Gelein, R. and Piper, H. C. (1990) Increased pulmonary toxicity of ultrafine particles I. Particle clearance, translocation, morphology. J. Aerosol Sci. 21, 381-384. Fissan, H., Hummes, D., Stratmann, F., Buscher, P., Neumann, S., Pui, D. Y. H. and Chen, D. (1996) Experimental comparison of four differential mobility analyzers for nanometer aerosol measurements. Aerosol Sci. Technol. 24, 1 13. Fuchs, N. A. and Sutugin, A. G. (1971) High-dispersed aerosol. In Topics in Current Aerosol Research (Edited by Hidy and Brock), pp. 1-59. Pergamon Press, New York. Futatsuki, T., Morinaga, H., Ohmi, T., Fuchita, E., Oda, M. and Hayashi, C. (1993) Mechanism of particle contamination removal from Si wafers. Proc. 39th 1ESAnnual TechnicalMeeting, Mount Prospect, IL, U.S.A., pp. 282-287. Futaki, Shoji, Shiraishi, Katsuzo and Uemura, Mikio (1992) Ultrafine refractory metal particles produced by hybrid plasma process. J. Japan Inst. Metals. 56, 464-471. Gangopadhyay, S., Hadjipanayis, G. C., Sorensen, C. M. and Klabunde, K. J. (1992) Magnetic properties of ultrafine Co particles. IEEE Trans. Maynetics 28, 3174 3176. Gangopadhyay, S., Hadjipanayis, G. C., Sorensen, C. M., and Klabunde, K. J. (1993) Effect of particle size and surface chemistry on the interactions among fine metallic particles. IEEE Trans. Magnetics 29, 2619-2621. Goldstein, A. N., Echer, C. M. and Alivisatos, A. P. (1992) Melting in semiconductor nanocrystals. Science 256, 1425-1427. Gradon, L. and Yu, C. P. (1989) Diffusional particle deposition in the human nose and mouth. Aerosol Sci. Technol. 11, 213-220. Gurav, A., Kodas, T., Pluym, T. and Xiaong, Y. (1993) Aerosol processing of material. Aerosol Sci. Technol. 19, 411-542. Huang, D. D., Seinfeld, J. H. and Marlow, W. H. (1990) BGK equation solution of coagulation for large Knudsen number aerosol with a singular attractive contact potential. J. Colloid Inter, Sci. 140, 258-276. Joint NSF/JSPS Japan-US workshop on Nanoparticle Synthesis and Application. (10 October-I November 1996) Mechanical Engineering Laboratory (MEL), AIST-MITI Research Center, Tsukuba, Japan. Proceedings to be published. Kanda, Y., Abe, Y., Hosoya, T., Honma, T. and Yashima, S. (1989) Consideration of ultrafine grinding based on experimental result of single particle crushing. Powder Technol. 58, 137-143. Karch, J., Birringer, R. and Gleiter, H. (1987) Ceramic ducile at low temperature. Nature 330, 556-558. Kinkead, D. (1996) Airborne molecular contamination: a roadmap for the 0.25 mm Generation. Semiconductor International (June) 231 238. Kofman, R., Cheyssac, P., Aouaj, A., Lereah, Y., Deutscher, G., Ben-David, T, Penisson, J. M. and Bourret, A. (1994) Surface melting enhanced by curvature effects. Surf Sci. 303, 231-246. Lewis, K. C., Dohmeier, D. M., Jorgenson, J. W., Kaufman, S. L., Zarrin, F. and Dorman, F. D. (1994) Electrospray-condensation particle counter: a molecule-counting LC detector for macromolecules. Analyt. Chem. 66, 2285 2292. Lin, H., Hsu, C. M., Yao, Y. D., Chen, Y. Y., Kuan, T. T, Yang, F. A. and Tung, C, Y. (1995) Magnetic study of both nitrided and oxidized Co particles. Nanostruct. Mat. 6, 977-980. Liu, B. Y. H., Pui, D. Y. H., McKenzie, R. L., Agarwal, J. K., Jaenicke, R., Pohl, F. G., Preining, O., Reischl, G., Szymanski, W. and Wagner, P. E. (1982) Intercomparison of different absolute instruments for measurement of aerosol number concentration. J. Aerosol Sci. 13, 429-450. Luo, L., Pipho, M. J., Arabs, J. L. and Kittelson, D. B. (1989) Particle growth and oxidation in a direct-injection diesel engine. S A E Paper # 890580. Marlow, W. H. and Brock, J. R. (1975) Unipolar charging of small aerosol particles. J. Colloid Interface Sci. 50, 32-38. Martonen, T. B. and Zhang, Z. (1992) Comments on recent data for particle deposition in human nasal passages. J. Aerosol Sci. 23, 667-674.
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Mittleman, D. M., Rosenthal, S. J., Schoenlein, R. W., Yeh, A. T., Shank, C. V., Shiang, J. J., Colvin, V. L., Grubbs, R. A. and Alivisatos, A. P. (1994) Ultrafast dynamics in CdSe nanocrystals, Springer Series in Chemical Physics, 60: 351-353. Proc. 9th Internat. Conf. Ultrafast Phenomena, Dana Point, CA, U.S.A. Morisaki, H., Hashimoto, H., Ping, F. W., Nozawa, H. and Ono, H. (1993) Strong blue light emission from an oxygen-containing Si fine structure. J. Appl. Phys. 74, 2977-2979. Ng-Yelim, J., Roy, C. and Morel, C. (1989) TEM characterization of a rapidly solidified ultrafine AI-Cu alloy powder. J. Mat. Sci. Lett. 8, 86 90. Nieman, G. W,, Weertman, J. R. and Siegel, R. W. (1990) Tensile strength and creep properties of nanocrystalline palladium. Scripta Metallurgica et Materialia 24, 145 150. Nieman, G. W., Weertman, J. R. and Siegel, R. W (1991) Mechanical behavior of nanocrystalline Cu and Pal. J. Mat. Res. 6, 1012 1027. Oberd6rster, G., Ferin, J., Finkelstein, J., Baggs, R., Stavert, D. M. and Lehnert, B. E. (1992) Potential health hazards from thermal degradation events: particulate vs. gas phase effects. SAE Technical Paper Series, pp. 1- 15. SAE, Warrendale, PA, U.S.A. Oberd/Jrster, G., Ferin, J. and Lehnert, B. E. (1995) Correlation between particle size, in vivo particle persistence and lung injury. Environ. Health Perspectives 102 (Suppl 5), 173 179. Pourprix, M. (1994) S61ecteur de particules charg6es, a haute sensibilite. Brevet francais No. 9406273, 24 Mal. Ridder, S. D. and Biancaniello, F. S. (1988) Process control during high pressure atomization. Mater. Sci. Enq. 98, 47-51. Sankaranarayanan, V. K. and Gajbhiye, N. S. (1990) Magnetization and magnetic resonance studies of uttrafine Ho3Fe5012 and Yb3Fe5Ol2. J. Ma~jnetism Magnetic Mater. 92, 217-227. Sankaranarayanan, V. K., Pankhurst, O. A., Dickson, D. P. E. and Johnson, C. E. (1993) Investigation of particle size effects in ultrafine barium ferrite. J. Magnetism Magnetic Mater. 125, 199-208. Sasajima, Yasushi, Arakawa, Takushi, Ichimura, Minoru and Imabayashi, Mamoru (1989) Molecular dynamics study of the structural changes of ultra-fine particles. Japan. J. Appl. Phys. 28, 1669 1672. Schlueter, J. (1996) Controlling contamination during CMP porcesses. Polishin9 Equipment (May) $8-S14. Sherby, O. D. and Wadsworth, J. (1989) Superplasticity. Recent advances and future directions.. Pro.q. Mater. Sci. 33, 169 221. Siegel, R. W., Ramasamy, S., Hahn, H., Zongguan, L., and Ting, L. (1988) Synthesis, characterization, and properties of nanophase TiO 2. J. Mater. Res. 3, 1367 1372. Smith, R. D., Loo, J. A., Edmonds, C. G., Barinaga, C. J. and Udseth, H. R. (1990) New developements in biochemical mass spectrometry elctrospray ionization. Analyt. Chem. 62, 882 899. Taketomi, S., Ozaki, Y., Kawasaki, K., Yuasa, S. and Miyajima, H. (1993) Transparent magnetic fluid: preparation of YIG ultrafine particles. J. Magnetism Magnetic Mater. 122, 6--9. Third International Conference on Nanostructured Materials, 8 12 July 1996, Kona, Hawaii, U.S.A. Proc. Elsevier (to be published). U.S. Environmental Protection Agency (1996) Air quality criteria for particulate matter. Officle of Research and Development, Washington, DC 20460, EPA/600/P-95/001af, April 1996. Vahala, K. J. (1988) Quantum box fabrication tolerance and size limits in semiconductors and their effect on optical gain. IEEE J. Quantum Electron. 24, 523-530. Viner, A. S., Periasamy, R., Johnson, E. W., Donovan, R. P. and Ensor, D. S. (1989) Measurement of ultrafine particles from processing equipment. Proc. 35th IES Annual Meeting, pp. 414420. Inst. of Environmental Sciences, Mount Prospect, IL, U.S.A. Wang, H. and Kasper, G. (1991) Filtration of efficiency of nanometer-size aerosol particles. J. Aerosol Sci. 22, 31 41. Weber, A. P. and Friedlander, S. K. (1994) Measurement of the bond strength between nanosized aerosol particles. J. Aerosol Sci. 25, 103- 104. Wiedensohler, A. and Hansson, H. (1992) Nanometer patterning of INP using aerosol and plasma etching techniques. Appl. Phys. Lett. 61, 837 839. Williams, M. M. R. and Loyalka, S. K. (1991) Aerosol Science: Theory and Practice. Elsevier Science, New York. Yao, Y. D., Chen, Y. Y., Hsu, C. M., Lin, H. M., Tung, C. Y., Tai, M. F., Wang, D. H., Wu, K. T. and Suo, C. T. (1995) Thermal and magnetic studies of nanocrystalline Ni. Nanostruct. Mater. 6, 933-936. Ye, Y., Liu, B. Y. H. and Pui, D. Y. H. (1993) Condensation-induced particle formation during vacuum pump down. J. Electrochem. Soc. 140, 1463 1468. Zhang, S.-H., Akutsu, Y., Russell, L. M., Flagan, R. C. and Seifeld, J. H. (1995) Radial differential mobility analyzer. Aerosol Sci. Technol. 23, 357 372. Ziemann, P. J., Liu, P., Nijhawan, S., Kittelson, D. B., McMurry, P. H. and Campbell, S. A. (1995) Particle beam mass spectrometry of submicron particles formed during LPCVD. Proc. 41st IES Annual Technical Meeting, Anaheim, CA, U.S.A.
J. Aerosol Sci. Vol. 28, No. 4, pp. 539 544, 1997
PII: S0021-8502(96)00495-8
GUEST
x 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0021 8502/97 $17.00 + 0.00
EDITORIAL
NANOMETER PARTICLES: A NEW FRONTIER FOR MULTIDISCIPLINARY RESEARCH David Y. H. Pui* and Da-Ren Chen Particle TechnologyLaboratory, Mechanical Engineering Department, Universityof Minnesota, 111 Church Street S.E., Minneapolis, MN 55455, U.S.A.
INTRODUCTION "Nanometer particles" are currently defined as particles with diameters below about 50 nm (Op < 50 nm). A lower size limit for these nanoparticles depends somewhat on the criteria applied and is sometimes operational, since basic properties and measurement methods are still in a state of flux. The "size" measured by an aerosol process may not be entirely correlated with such other properties as catalytic activity or magnetic structure. However, stable molecular clusters, biological molecules, fullerene and oligonucleotides, may serve as examples of nanoparticles near the lower size limit. As a new frontier in particle and aerosol research, nanoparticle research has significant potential for scientific breakthrough and new technological innovations. The cooperation of scientists in the materials and aerosol fields, for example, are already beginning to bear fruit. In the following paragraphs, therefore, we outline nanoparticle applications in several scientific disciplines, covering a broad spectrum from the production of nanophase materials with superior properties to studies of human exposure by inhalation of nanometer particles. At the recent Annual Conference of the American Association for Aerosol Research (AAAR) in Orlando, FL, Professor S. K. Friedlander eloquently stated some of these--and other--aspects of nanoparticle applications, and drew attention to the need for increasing infusions of molecular science and solid state physics to nanoparticle research. Studies of nanoparticles would involve multidisciplinary approaches because of the broad nature of their properties (e.g. physical, chemical and biological) and the diverse possibilities for their applications. New instrumentation for nanoparticle characterization is developing at a rapid pace which will accelerate nanoparticle studies and further expand the scope of their applications. We feel that periodic, timely publications of the results and implications of nanoparticle studies will stimulate cross-fertilization of knowledge among scientists and engineers in many different fields. With this in mind, the publication of a special issue of the Journal of Aerosol Science is proposed, providing a focal point for current thinking and research directions in this highly topical area.
Nanometer particles versus ultrafine particles The terms "ultrafine particles" and "ultrafine aerosols" were used by some aerosol scientists during a 1979 Workshop on Ultrafine Aerosols (WUFA) in Vienna (Liu et al., 1982). The purpose of the Workshop was to evaluate the counting efficiencies of a number of condensation nuclei counters available at that time. Ultrafine aerosols were defined at that time as those characterized by particle diameters less than 0.1 #m (Dp < 100 nm). Earlier, Fuchs and Sutugin (1971) had used the term "highly-dispersed aerosol" to define aerodisperse systems with particle diameters less than 100 nm. Recently, the U.S. Environmental
*Author to whom all correspondenceshould be addressed. 539
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D.Y.H. Pui and Da-Ren Chcn
Protection Agency used the term "ultrafine particles in a biological context" to characterize particle size distributions with mass median diameter (MMD) below about 0.1 ym (U.S. EPA, 1996). From the preceding, it becomes clear that the working definition of"nanometer particles" (Dp < 50 nm) as stated earlier makes such an aerosol a subgroup of the "ultrafine particle" class. We have found much inconsistency in the use of the term "ultrafine particles". "'Ultrafine'" is of course intended to signify "very, very fine," but such a statement can evoke very different meanings between disciplines. For example, the term "ultrafine particles" has been used in some powder technology publications to denote particles smaller than 1.0 lLm in diameter (e.g. Kanda et al., 1989; Ng-Yelim et al., 1989). Some workers have even defined them as particles with diameters below 45 l~m (e.g. Ridder et al., 1988). "Nanometer particles" can and should be much more narrowly defined. The term itself is explicit in that it actually identifies the order of magnitude of the particles. The convention proposed here, D v < 50 nm, is consistent with this rationale. It should also be noted that the terms "nanostructure" and "nanophase'" materials used in materials community has come to refer to clusters consisting of primary particles below 100 nm diameter (e.g. Gurav et al., 1993). At a recent Joint NSF/JSPS U.S. Japan Workshop on Nanoparticle Synthesis and Applications (1996), it was proposed that nanoparticles be more narrowly defined as those which show quantum confinement effects, generally with Dp < 30 nm. M U L T 1 D I S C I P L I N A R Y RESEARCH I N V O L V I N G N A N O M E T E R P A R T I C L E S M a t e r i a l s synthesis
Production of nanostructure materials is an exciting new area for the materials industry. Materials in the nanostructure form possess many desirable properties, including improved hardness, reduced internal friction (i.e. improved ductility), reduced melting point, and special optical and magnetic properties (Nieman et al., 1990, 1991; Siegel et al., 1988; Karch et al., 1987; Sherby and Wadsworth, 1989; Futaki et al., 1992; Kofman et al., 1994; Endo et al., 1996; Lin et al., 1995; Yao et al., 1995; Sankaranarayanan et al., 1993; Taketomi et al., 1993; Gangopadhyay et al., 1992, 1993). These properties are the basis of many so-called "high-tech" applications, including quantum dots, drill bit coatings, fuel cells, and tunable laser, and so they promise to revolutionize the materials industry. Production of such materials via the aerosol route provides enhanced levels of materials purity and production control. As such, it is the topic of several recent symposia and workshops (e.g. Third International Conference on Nanostructured Materials, 1996: Joint NSF/JSPS U.S.-Japan Workshop on Nanoparticle Synthesis and Applications, 1996), bringing together researchers from the aerosol and materials communities to investigate new production and evaluation methods. H e a l t h eJfects
While nanostructure materials are produced in increasing quantities, there is an increased awareness in the occupational health community on the possible adverse health effects of exposure to nanometer particles (Cheng et al,, 1993, 1996; Ferin, 1994; Ferin et al., 1990: Oberd6rster et al., 1992, 1995; Martonen and Zhang, 1992; Gradon and Yu, 1989). it has been speculated that nanometer particles depositing in the human respiratory tract may enter the interstitial spaces and may penetrate through the cell membrane, so that even relatively insoluble material may pass beyond the respiratory tract. Those which are retained in the lung may become sequestered and remain there for very long periods. Recent intratracheal injection studies using rats, reported by Donaldson et al. (1996), have shown that material which is relatively inert for micrometer-sized particles (e.g. titanium dioxide) can be highly inflammogenic for particles in the nanometer size range. It is clearly suggested, therefore, that nanometer particles may, depending on their chemical composition, be associated with the possibility of serious ill health. At present, however, there is a lack of exposure data or epidemiology studies to either support or reject such a
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hypothesis. Undoubtedly, this is an area which will provide research opportunities for aerosol scientists, toxicologists, epidemiologists and other health researchers for the years to come.
M icrocontamination
The Semiconductor Industry Association (SIA) estimates that the next generation of 1-gigabit DRAM devices will have a minimum feature size of smaller than 0.18/xm. Currently, the majority of the product yield losses can be attributed to the deposition of particles on the semiconductor wafers. Using the common 1/3 rule for microcontamination control, particles in the nanometer size range will therefore need to be measured and controlled. Recent microcontamination research has focused on understanding particle formation and deposition mechanisms in the process equipment (Ye et al., 1993; Ziemann et al., 1995; Viner et al., 1989), and in cleaning methods (Futatsuki et al., 1993). Some of the challenges will be to measure these particles under the conditions of high vacuum and high temperature, and in aggressive gases and plasma, and to remove them by deposition techniques (Wang and Kasper,1991). It is believed that contamination as small as molecules need to be considered to achieve high yield in next several generations of semiconductor devices (Kinkead, 1996). Particles are also used in a beneficial way for the production of semiconductor wafers. In the chemical mechanical polishing (CMP) processes, slurry consisting of fine particles is used to polish wafer surfaces. To produce the desirable smooth surface on the wafer, uniform size particles below 100 nm are used in increasing quantity (Cook et al., 1995; Schlueter, 1996; Burggraaf, 1995). Microcontamination provides opportunities for both instrument designers and fundamental researchers, including for example studies of nucleation phenomena in complex systems and particle behavior in vacuum.
Biotechnology
Determination of the molecular weights of biopolymers, including oligonucleotides and proteins, is of fundamental importance to biotechnology development. Mass spectrometry has been used to perform such measurements by transforming individual molecules into ions and deflecting them in electric and magnetic fields. One of the challenges is to ionize the molecules without causing catastrophic decomposition. Various "soft" ionization methods have been used with limited success (Fenn et al., 1989). An exciting new area is to ionize the biomolecules using an electrospraying technique followed by mass spectrometry (Smith et al., 1990). These biomolecules with molecular weights up to 130,000 are, in essence, nanometer particles. Aerosol researchers can therefore participate and contribute in this exciting field by providing technologies for dispersing and ionizing of such nanometer-sized biomolecules. One example of such development is the electrospray-condensation particle counter for detecting macromolecules as proposed by Lewis et al. (1994).
Emission control
Energy saving and the environmental consequences of using Diesel engines are well known. Diesel engine exhaust aerosols are made up from clusters 9 f individual nanometer particles. Understanding the dynamics of soot formation and developing monitoring sensors for Diesel engines will therefore help in minimizing their environmental impact. Aerosol technologists have been at the forefront of such development and will no doubt continue to contribute to this effort (Amann and Siegla, 1982; Luo et al., 1989). "High-tech" coating technologies (e.g. coatings applied to deep-sea oil platforms and to engine cylinders) often produce nanoparticles as a by-product (Chan et al., 1995). Further, emissions from semiconductor processing equipment often consist of nanoparticles of exotic materials. Emission control technologies will no doubt play an important role in these areas.
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Instrumentation
Interests in instrumentation development for nanoparticle measurements have increased significantly during the past five years. Two focused areas include generation of monodisperse nanometer aerosols for instrument calibrations, and measurement of the particle size distributions of nanometer aerosols. The traditional method involves using a differential mobility analyzer (DMA) to either measure or classify aerosols. However, the sizing resolution and detection sensitivities of the current generation of DMAs deteriorate drastically in the nanometer particle size range, due to the effects of diffusion broadening and losses. This realization has led to a series of publications on comparing available DMAs in terms of their suitability for nanometer aerosol measurements (Fissan et al., 1996), and the development of novel D M A s for nanometer size range (Pourprix, 1994; Zhang et al., 1995: Chen et al., 1996). A new approach of using electrospraying techniques for generating monodisperse nanometer aerosols has also been developed and has yielded excellent results (de la Mora et al., 1990; Chen et al., 1996). Another exciting possibility is to make use of mass spectrometers for detecting and analyzing chemical compositions of nanometer particles (Fenn et al., 1989; Smith et al., 1990). In addition, a variety of new tools are being developed for analysis of individual nanofeatures including atomic force microscopy, high resolution transmission electron microscopy, and near field optical microscopy. N a n o m e t e r particle physics and c h e m i s t r y
There is a great deal of interest in understanding the physics of the transfer processes of nanometer aerosol systems, including mass transfer, heat transfer, m o m e n t u m transfer, and charge transfer. Theories for describing these phenomena are relatively simple when the Knudsen number (Kn) is large (i.e. "free-molecule" theory) and small (i.e. "'continuum" theory). The greatest difficulty occurs for the cases of intermediate Kn. Fuch's "limitingsphere" approach provides reasonable but not rigorous results. Accurate calculation requires solving the Boltzmann collision equation. Significant efforts have been made towards this approach (e.g. Williams and Loyalka, 1991; Marlow and Brock, 1975: Huang et al., 1990). All these studies are based on linearized approximations of the Boltzmann equation, and are far from solving the actual nonlinear Boltzmann equation. New and solar not observed phenomena may become important, such as "thermal rebound" (Wang and Kasper, 1991) which may influence measurement techniques near the limit between particles and molecules. Meanwhile, new instrumentation and experimental techniques are becoming available which are stimulating more studies on nanometer particles. Some recent research topics reported include quantum confinement effects of nanometer particles (Vahala, 1988; Wiedensohler and Hansson, 1992: Alivisatos, 1996), the decrease of the melting point of nanometer particles (Goldstein et al., 1992), ultrafast dynamics in nanoparticle/nanocrystal (Mittleman et al., 1994), and nanoparticle bonds in agglomerates (Weber and Friedlander, 1994). SUMMARY In summary, it is clear that exciting new possibilities now exist for research into, and applications of, nanometer particles. The proposed special issue of the Journal o f Aerosol Science will draw together current knowledge on these subjects and describe the state-ofthe-art research, providing a platform for future exploration. REFERENCES Alivisatos, A. P. (1996) Semiconductor clusters, nanocrystals, and quantum dots. Science 271,933 937. Amann, C. A. and Siegla, D. C. (1982) Diesel particulates whatthey are and why. Aerosol Sci. Technol, 1, 73 101. Burggraaf, P. (t995) CMP: suppliers integrating, applications spreading. Proc. Semiconducu)r International, November, pp. 74-82. Chan, T. L., Rouhana, S. W., Mulawa, P. A. and Reuter, R. J. 11995) Occupational health assessmentof the high velocity oxy-fuelthermal metal spray process. Appl. Occup. Environ. Hy~t. 10, 482 487.
Nanometer particles
543
Chen, D., Pui, D. Y. H., Hummes, D., Fissan, H., Quant, F. R. and Sere, G. J. (1996) Nanometer differential mobility analyzer (Nano-DMA): design and numerical modeling. J. Aerosol Sci. 27, S137-138. Chen, D., Pui, D. Y. H. and Kaufman, S. L. Electrospraying of conducting liquids for monodisperse aerosol generation in the 4 nm to 1.8 #rn diameter range. J. Aerosol Sci. 26, 963-977. Cheng, Y. S., Su, Y.-F., Yeh, Hsu-Chi and Swift, D. L. (1993) Deposition of thoron progeny in human head airways. Aerosol Sci. Technol. 18, 359-375. Cheng, Y. S., Yeh, H. C., Guilmette, R. A., Simpson, S. Q., Cheng, K. H. and Swift, D. L. (1996) Nasal deposition of ultrafine particles in human volunteers and its relationship to airway geometry. Aerosol Sci. Technol. 25, 274 291. Cook, L. M., Wang, J. F., James, D. B. and Sethurarnan, A. R. (1995) Theoretical and practical aspects of dielectric and metal CMP. Proc. Semiconductor International, November, pp. 141-144. De la Mora, F., Navascues, J., Fernandez, J. and Rosell Llompart, R. (1990) Generation of submicron monodisperse aerosol by electrosprays. J. Aerosol Sci. 21, $673-676; also Loscertales, I. G. and de la Mora, F. (1993) Characterization of electrospray-generated nanoparticles in a hypersonic impactor. Presented at the Workshop on Ultrafine Particles, Delf, The Netherlands, May 1993. Donaldson, K,, Gilmour, P., Brown, D. M., Beswick, P. H., Benton, E. and MacNee, W. (1996) Surface free radical activity of PM 10 and ultrafine titanium dioxide: a unifying factor in their toxicity. Paper presented at the 8th Int. Syrup. Inhaled Particles, Cambridge, U.K., August 1996. To be published in Inhaled Particles V I I I (Edited by Ogden, T. L.), Pergamon, Oxford. Endo, Y., Ono, M., Yamada, T., Kawamura, H., Kobara, K. and Kawamura, T. (1996) Study of antireflective and antistatic coating with ultrafine particles. Adv. Powder Technol. 7, 131-140. Fenn, J. B., Mann, M., Meng, C. K., Wong, S. F. and Whitehouse, C. M. (1989) Electrospray ionization for mass spectrometry of large biomolecules. Science 246, 64-71. Ferin, J. (1994) Pulmonary retention and clearance of particles. Toxicol. Lett. 72, 121-125. Ferin, J., Oberdoerster, G., Penney, D. P., Soderholm, S. C., Gelein, R. and Piper, H. C. (1990) Increased pulmonary toxicity of ultrafine particles I. Particle clearance, translocation, morphology. J. Aerosol Sci. 21, 381-384. Fissan, H., Hummes, D., Stratmann, F., Buscher, P., Neumann, S., Pui, D. Y. H. and Chen, D. (1996) Experimental comparison of four differential mobility analyzers for nanometer aerosol measurements. Aerosol Sci. Technol. 24, 1 13. Fuchs, N. A. and Sutugin, A. G. (1971) High-dispersed aerosol. In Topics in Current Aerosol Research (Edited by Hidy and Brock), pp. 1-59. Pergamon Press, New York. Futatsuki, T., Morinaga, H., Ohmi, T., Fuchita, E., Oda, M. and Hayashi, C. (1993) Mechanism of particle contamination removal from Si wafers. Proc. 39th 1ESAnnual TechnicalMeeting, Mount Prospect, IL, U.S.A., pp. 282-287. Futaki, Shoji, Shiraishi, Katsuzo and Uemura, Mikio (1992) Ultrafine refractory metal particles produced by hybrid plasma process. J. Japan Inst. Metals. 56, 464-471. Gangopadhyay, S., Hadjipanayis, G. C., Sorensen, C. M. and Klabunde, K. J. (1992) Magnetic properties of ultrafine Co particles. IEEE Trans. Maynetics 28, 3174 3176. Gangopadhyay, S., Hadjipanayis, G. C., Sorensen, C. M., and Klabunde, K. J. (1993) Effect of particle size and surface chemistry on the interactions among fine metallic particles. IEEE Trans. Magnetics 29, 2619-2621. Goldstein, A. N., Echer, C. M. and Alivisatos, A. P. (1992) Melting in semiconductor nanocrystals. Science 256, 1425-1427. Gradon, L. and Yu, C. P. (1989) Diffusional particle deposition in the human nose and mouth. Aerosol Sci. Technol. 11, 213-220. Gurav, A., Kodas, T., Pluym, T. and Xiaong, Y. (1993) Aerosol processing of material. Aerosol Sci. Technol. 19, 411-542. Huang, D. D., Seinfeld, J. H. and Marlow, W. H. (1990) BGK equation solution of coagulation for large Knudsen number aerosol with a singular attractive contact potential. J. Colloid Inter, Sci. 140, 258-276. Joint NSF/JSPS Japan-US workshop on Nanoparticle Synthesis and Application. (10 October-I November 1996) Mechanical Engineering Laboratory (MEL), AIST-MITI Research Center, Tsukuba, Japan. Proceedings to be published. Kanda, Y., Abe, Y., Hosoya, T., Honma, T. and Yashima, S. (1989) Consideration of ultrafine grinding based on experimental result of single particle crushing. Powder Technol. 58, 137-143. Karch, J., Birringer, R. and Gleiter, H. (1987) Ceramic ducile at low temperature. Nature 330, 556-558. Kinkead, D. (1996) Airborne molecular contamination: a roadmap for the 0.25 mm Generation. Semiconductor International (June) 231 238. Kofman, R., Cheyssac, P., Aouaj, A., Lereah, Y., Deutscher, G., Ben-David, T, Penisson, J. M. and Bourret, A. (1994) Surface melting enhanced by curvature effects. Surf Sci. 303, 231-246. Lewis, K. C., Dohmeier, D. M., Jorgenson, J. W., Kaufman, S. L., Zarrin, F. and Dorman, F. D. (1994) Electrospray-condensation particle counter: a molecule-counting LC detector for macromolecules. Analyt. Chem. 66, 2285 2292. Lin, H., Hsu, C. M., Yao, Y. D., Chen, Y. Y., Kuan, T. T, Yang, F. A. and Tung, C, Y. (1995) Magnetic study of both nitrided and oxidized Co particles. Nanostruct. Mat. 6, 977-980. Liu, B. Y. H., Pui, D. Y. H., McKenzie, R. L., Agarwal, J. K., Jaenicke, R., Pohl, F. G., Preining, O., Reischl, G., Szymanski, W. and Wagner, P. E. (1982) Intercomparison of different absolute instruments for measurement of aerosol number concentration. J. Aerosol Sci. 13, 429-450. Luo, L., Pipho, M. J., Arabs, J. L. and Kittelson, D. B. (1989) Particle growth and oxidation in a direct-injection diesel engine. S A E Paper # 890580. Marlow, W. H. and Brock, J. R. (1975) Unipolar charging of small aerosol particles. J. Colloid Interface Sci. 50, 32-38. Martonen, T. B. and Zhang, Z. (1992) Comments on recent data for particle deposition in human nasal passages. J. Aerosol Sci. 23, 667-674.
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D . Y . H . Pui and Da-Ren Chen
Mittleman, D. M., Rosenthal, S. J., Schoenlein, R. W., Yeh, A. T., Shank, C. V., Shiang, J. J., Colvin, V. L., Grubbs, R. A. and Alivisatos, A. P. (1994) Ultrafast dynamics in CdSe nanocrystals, Springer Series in Chemical Physics, 60: 351-353. Proc. 9th Internat. Conf. Ultrafast Phenomena, Dana Point, CA, U.S.A. Morisaki, H., Hashimoto, H., Ping, F. W., Nozawa, H. and Ono, H. (1993) Strong blue light emission from an oxygen-containing Si fine structure. J. Appl. Phys. 74, 2977-2979. Ng-Yelim, J., Roy, C. and Morel, C. (1989) TEM characterization of a rapidly solidified ultrafine AI-Cu alloy powder. J. Mat. Sci. Lett. 8, 86 90. Nieman, G. W,, Weertman, J. R. and Siegel, R. W. (1990) Tensile strength and creep properties of nanocrystalline palladium. Scripta Metallurgica et Materialia 24, 145 150. Nieman, G. W., Weertman, J. R. and Siegel, R. W (1991) Mechanical behavior of nanocrystalline Cu and Pal. J. Mat. Res. 6, 1012 1027. Oberd6rster, G., Ferin, J., Finkelstein, J., Baggs, R., Stavert, D. M. and Lehnert, B. E. (1992) Potential health hazards from thermal degradation events: particulate vs. gas phase effects. SAE Technical Paper Series, pp. 1- 15. SAE, Warrendale, PA, U.S.A. Oberd/Jrster, G., Ferin, J. and Lehnert, B. E. (1995) Correlation between particle size, in vivo particle persistence and lung injury. Environ. Health Perspectives 102 (Suppl 5), 173 179. Pourprix, M. (1994) S61ecteur de particules charg6es, a haute sensibilite. Brevet francais No. 9406273, 24 Mal. Ridder, S. D. and Biancaniello, F. S. (1988) Process control during high pressure atomization. Mater. Sci. Enq. 98, 47-51. Sankaranarayanan, V. K. and Gajbhiye, N. S. (1990) Magnetization and magnetic resonance studies of uttrafine Ho3Fe5012 and Yb3Fe5Ol2. J. Ma~jnetism Magnetic Mater. 92, 217-227. Sankaranarayanan, V. K., Pankhurst, O. A., Dickson, D. P. E. and Johnson, C. E. (1993) Investigation of particle size effects in ultrafine barium ferrite. J. Magnetism Magnetic Mater. 125, 199-208. Sasajima, Yasushi, Arakawa, Takushi, Ichimura, Minoru and Imabayashi, Mamoru (1989) Molecular dynamics study of the structural changes of ultra-fine particles. Japan. J. Appl. Phys. 28, 1669 1672. Schlueter, J. (1996) Controlling contamination during CMP porcesses. Polishin9 Equipment (May) $8-S14. Sherby, O. D. and Wadsworth, J. (1989) Superplasticity. Recent advances and future directions.. Pro.q. Mater. Sci. 33, 169 221. Siegel, R. W., Ramasamy, S., Hahn, H., Zongguan, L., and Ting, L. (1988) Synthesis, characterization, and properties of nanophase TiO 2. J. Mater. Res. 3, 1367 1372. Smith, R. D., Loo, J. A., Edmonds, C. G., Barinaga, C. J. and Udseth, H. R. (1990) New developements in biochemical mass spectrometry elctrospray ionization. Analyt. Chem. 62, 882 899. Taketomi, S., Ozaki, Y., Kawasaki, K., Yuasa, S. and Miyajima, H. (1993) Transparent magnetic fluid: preparation of YIG ultrafine particles. J. Magnetism Magnetic Mater. 122, 6--9. Third International Conference on Nanostructured Materials, 8 12 July 1996, Kona, Hawaii, U.S.A. Proc. Elsevier (to be published). U.S. Environmental Protection Agency (1996) Air quality criteria for particulate matter. Officle of Research and Development, Washington, DC 20460, EPA/600/P-95/001af, April 1996. Vahala, K. J. (1988) Quantum box fabrication tolerance and size limits in semiconductors and their effect on optical gain. IEEE J. Quantum Electron. 24, 523-530. Viner, A. S., Periasamy, R., Johnson, E. W., Donovan, R. P. and Ensor, D. S. (1989) Measurement of ultrafine particles from processing equipment. Proc. 35th IES Annual Meeting, pp. 414420. Inst. of Environmental Sciences, Mount Prospect, IL, U.S.A. Wang, H. and Kasper, G. (1991) Filtration of efficiency of nanometer-size aerosol particles. J. Aerosol Sci. 22, 31 41. Weber, A. P. and Friedlander, S. K. (1994) Measurement of the bond strength between nanosized aerosol particles. J. Aerosol Sci. 25, 103- 104. Wiedensohler, A. and Hansson, H. (1992) Nanometer patterning of INP using aerosol and plasma etching techniques. Appl. Phys. Lett. 61, 837 839. Williams, M. M. R. and Loyalka, S. K. (1991) Aerosol Science: Theory and Practice. Elsevier Science, New York. Yao, Y. D., Chen, Y. Y., Hsu, C. M., Lin, H. M., Tung, C. Y., Tai, M. F., Wang, D. H., Wu, K. T. and Suo, C. T. (1995) Thermal and magnetic studies of nanocrystalline Ni. Nanostruct. Mater. 6, 933-936. Ye, Y., Liu, B. Y. H. and Pui, D. Y. H. (1993) Condensation-induced particle formation during vacuum pump down. J. Electrochem. Soc. 140, 1463 1468. Zhang, S.-H., Akutsu, Y., Russell, L. M., Flagan, R. C. and Seifeld, J. H. (1995) Radial differential mobility analyzer. Aerosol Sci. Technol. 23, 357 372. Ziemann, P. J., Liu, P., Nijhawan, S., Kittelson, D. B., McMurry, P. H. and Campbell, S. A. (1995) Particle beam mass spectrometry of submicron particles formed during LPCVD. Proc. 41st IES Annual Technical Meeting, Anaheim, CA, U.S.A.