DOI: 10.1007/s11090-005-4991-4Plasma Chemistry and Plasma Processing, Vol. 25, No. 5, October 2005 (© 2005)

System for In Situ Characterization of NanoparticlesSynthesized in a Thermal Plasma Process

X. Wang,1 J. Hafiz,1 R. Mukherjee,1 T. Renault,1,2

J. Heberlein,1 S. L. Girshick,1 and P. H. McMurry1,3

Received November 1, 2004; revised February 15, 2005

We have designed a particle diagnostic system that is able to measure particlesize and charge distributions from low stagnation pressure (≥746 Pa) and hightemperature (2000–4000 K) environments in near real time. This system utilizesa sampling probe interfaced to an ejector to draw aerosol from the low pressurechamber. Particle size and charge distributions are measured with a scanningmobility particle sizer. A hypersonic impactor is mounted in parallel with thescanning mobility particle sizer to collect particles for off-line microscopic anal-ysis. This diagnostic system has been used to measure size and charge distri-butions of nanoparticles (Si, Ti, Si–Ti–N, etc.) synthesized with our thermalplasma reactor. We found that the mean particle size increases with operatingpressure and reactant flow rates. We also found that most particles from ourreactor are neutral for particles smaller than 20 nm, and that the numbers ofpositively and negatively charged particles are approximately equal.

KEY WORDS: Nanoparticles; plasma synthesis; particle diagnostics; size dis-tribution; charge distribution.

1. INTRODUCTION

The need to measure aerosol size distributions in low pressure envi-ronments that may also be at elevated temperatures is encountered inaerosol synthesis reactors, semiconductor processing equipment, etc. Avariety of approaches have been used for such measurements, includinglaser light scattering,(1–3) sampling from the exhaust of turbomolecularpumps,(4,5) mobility analysis at low pressure,(6–9) and particle beam massspectrometry (PBMS).(10–12) There are limitations to each of these meth-ods. While laser light scattering is fast and can provide information on

1Department of Mechanical Engineering, University of Minnesota, 111 Chruch St. S.E.,Minneapolis, MN 55455, U.S.A.

2Current address: Thermal Dynamics Corporation, 82 Benning St., West Lebanon, NH03784, U.S.A.

3To whom correspondence should be addressed. E-mail: wxl@me.umn.edu

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0272-4324/05/1000-0439/0 © 2005 Springer Science+Business Media, Inc.

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aerosols throughout a reactor, the light scattering signals are complex con-volutions of particle size, shape, concentration and refractive index. It isnot trivial to deconvolute these parameters to obtain accurate informationon particle size distributions. Measurements made in the exhaust stream ofa vacuum pump are subject to changes in size distributions that occur dueto deposition, coagulation or nucleation as the aerosol travels through thepump. Mobility analyzers that operate at pressures as low as 200 Pa havebeen reported, but these instruments are constrained to measurements ofparticle size distributions in the 3–20 nm diameter range. The PBMS wasdesigned to sample from reactors at ∼133 Pa and can measure size distri-butions of particles in the 5–500 nm diameter range. A drawback of thePBMS is that it is a prototype instrument that requires more expertise tooperate than commercially available instrumentation.

In this paper we describe a sampling scheme that combines an airejector with a scanning mobility particle sizer(13) to measure particle sizeand charge distributions from high temperature and low pressure envi-ronments. This system is similar to the one described previously.(14,15)

We made two major modifications, First, a new type of ejector (Air-Vac,UV143H) is used. This modification enables us to sample particles fromthe supersonic jet at a chamber pressure of 266 Pa, while the old sys-tem only works at chamber pressures of 2666 Pa or higher. Second, abypass route was added to the bipolar charger, which enables us to mea-sure the charge distribution of aerosol sampled from the reactor. In brief,this system utilizes an ejector-dilutor system to sample particles fromthe hot vacuum zone through a water-cooled sampling probe and toadjust particle concentrations to a suitable level for measurements withthe scanning mobility particle sizer.(13) The scanning mobility particlesizer consists of a differential mobility analyzer (DMA)(16) to select par-ticles of a given mobility, and an ultrafine condensation particle counter(UCPC)(17) to detect them. Size distributions are determined by carry-ing out UCPC measurements for a range of mobilities. Both a regu-lar DMA(16) and a Nano-DMA(18) are used in our scanning mobilityparticle sizer to measure size distributions of particles in the 3–200 nmdiameter range with a time resolution of ∼2 min. We refer these measure-ments as “in situ” meaning that the particle measurement system samplesparticles directly from the aerosol jet. The measurements are “near realtime” because the aerosol residence time from the sampling inlet to themeasurement instruments is typically less than 5 s. A hypersonic impac-tor(19) with cut size of about 6 nm silicon particles samples the aerosolin parallel with the scanning mobility particle sizer. Particles are col-lected on TEM support grids for off-line measurements of morphologyand chemical composition. We report here on the performance of this

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Fig. 1. Schematic diagram of hypersonic plasma particle deposition apparatus.

nanoparticle characterization system and its use for measuring size andcharge distributions of nanoparticles produced by our hypersonic plasmaparticle deposition (HPPD) apparatus.(15,20–22)

The experimental apparatus for particle synthesis and deposition withthe HPPD process is shown in Fig. 1. Vapor phase reactants are injectedinto a DC thermal plasma and dissociate due to the high temperature(about 4000 K) in the reaction zone. The reactant mixture then expandsfrom a static pressure of about 67 kPa through a converging nozzle toa deposition chamber maintained at approximately 266 Pa. Rapid coolingduring expansion drives nucleation of nanoparticles in the nozzle. Particlesare then accelerated to high velocities in the nozzle and in the downstreamfree expansion region. Finally particles impact on substrate 1 hypersoni-cally and form a nanostructured film. We refer to this process as “highrate” deposition. In a complementary process, substrate 1 is removed andparticles are passed through an aerodynamic lens assembly to form atightly collimated particle beam.(23,24) The particle beam can be used todirectly “write” micropatterns with characteristic dimensions of a few tensof microns on the computer controlled substrate 2. We refer to this pro-cess as “high definition” deposition.

The size and charge distributions of the synthesized particles are ofprimary importance in the HPPD process. Many mechanical properties of

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the nanostructured films and micropatterns are directly related to particlesize. Electrical charges carried by particles enable the use of electrostaticforces to increase impaction velocity, enhance deposition efficiency andmanipulate the deposition pattern. Furthermore, knowledge of particlesize and charge distributions provides insights into the formation pro-cess for these nanoparticles, which in turn helps us to optimize operat-ing conditions such as reactant flow rates, operating pressures and plasmaconditions. Therefore, it is desirable to measure particle size and chargedistributions in real time. It is not straightforward to measure size andcharge distributions of particles under our experimental conditions. Thenormal operating pressure in the deposition chamber is low (∼266 Pabackground pressure and 3600 Pa stagnation pressure in the jet); the tem-perature at the sampling location is high (2000–4000 K); and the particleconcentration is high (107–109 cm−3). We show in the following sectionsthat our particle diagnostic system can successfully overcome these diffi-culties and measure quantitatively particle size and charge distributions innear real time.

2. EXPERIMENTAL SETUP

A schematic of the particle diagnostic system is shown in Fig. 2. Par-ticles exiting the nozzle are sampled through a water-cooled molybdenum

Fig. 2. Schematic diagram of the particle diagnostic system.

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probe connected to a two-stage ejector. For the data reported in this paperthe probe was located 2 cm downstream of the nozzle, where the “highrate” deposition of films takes place, but it can be moved to sample par-ticles from different axial locations or be rotated to sample from off-axislocations. A high flow rate (123.5 slm) of particle-free nitrogen is fed to theejector to create the reduced pressure that draws aerosol from the sam-pling probe and delivers it to atmospheric pressure where measurementsare carried out using the scanning mobility particle sizer and the hyper-sonic impactor. The scanning mobility particle sizer is used to measureparticle size distributions and charge state, and the hypersonic impactor isused to collect nanoparticle samples for analysis by electron microscopy.

The sampling probe is a short molybdenum capillary with an innerdiameter of 3 mm. The first half length of the probe is cooled by water toprevent melting. Note that the sampling is subisokinetic, which results inmodest enrichment of the larger particles in the sampled flow. The ejectoris a two-stage Venturi tube that can sample aerosol from a 746 Pa stag-nation pressure. Although our reaction chamber operates at about 266 Pa,a normal shock forms in front of the sampling probe and the static pres-sure behind the shock (and just in front the sampling probe) rises back toabout 3600 Pa,(22) which enables us to sample particles from the chamber.The ejectors have simple flow paths that we believe will lead to minimalperturbation on sampled particle size distributions.

The sampling extraction system was designed to minimize changes insampled aerosol size distributions due to coagulation prior to measure-ment. The aerosol residence time from the sampling inlet to the pointwhere nitrogen dilution occurs inside the ejector is less than 10 ms. Par-ticle coagulation is negligible for aerosol concentrations up to ∼1010 cm−3

in such a short time. The primary nitrogen dilution flow reduces concen-trations to ∼108 cm−3 during the 0.29 s transport time to the point wheresecondary dilution occurs, again helping to suppress coagulation. Thepressure-dependent dilution factor of the ejector is obtained by calibratingthe flowrate through the sampling probe as a function of chamber pres-sure and nitrogen flowrate. A portion of the flow leaving the ejector passesthrough a laminar flowmeter prior to further dilution by clean nitrogen.The dilution factor of this second dilution stage can be adjusted by con-trolling the aerosol flow through the laminar flowmeter or the dilutionnitrogen flow so that particle concentrations are brought to the rangethat can be measured with minimal coincidence by the UCPC(17) (about104 cm−3), which counts particles individually. Particle coagulation is neg-ligible during the 4 s transport time from the second dilution stage to theUCPC due to the low concentration. The data reported in this paper wascorrected for both ejector and second stage nitrogen dilutions.

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Prior to entering the scanning mobility particle sizer particles eitherpass through a 210Po bipolar charger, or through a “dummy” charger thatis geometrically identical to the bipolar charger but that does not con-tain a source of ionizing radiation. Particles that pass through the bipo-lar charger are brought to a well-defined stationary charge distribution(25)

from which the total (neutral and charged particles) size distribution canbe inferred. The 210Po sources used in our system had an initial activity of0.5 mCi and were replaced every 90 days. The efficacy of a bipolar chargeris determined by the product of the concentration of positive and negativeions that it produces, N , and the time to which particles are exposed tothese ions as they flow through the charger, t . Based on rates of ion pro-duction and recombination from this source and on the aerosol residencetime in the source, we estimate that the “Nt product” for this charger isequal to or greater than ∼5×108 ions/cm3 s. This Nt value is large enoughto ensure particles passing through the charger will achieve the station-ary charge distribution, independent of their initial charge state.(26) The“dummy” route is used to measure the unaltered charge distribution ofthe aerosol as it is delivered to the scanning mobility particle sizer, inan effort to obtain information about the unaltered charge distribution ofthe sampled aerosol. The possibility that this charge distribution under-goes changes during transport through the sampling and dilution systemis discussed later. When using the scanning mobility particle sizer to mea-sure charge distributions, either a positive or a negative classifying voltageis applied to the DMA when aerosol is sampled through the “dummy”charger. By using the UCPC to measure concentrations for a range ofDMA classifying voltages, it is possible to infer the relative abundances ofpositively and negatively charged particles as a function of size.

To obtain accurate size distribution data, it is necessary to know thesize dependent particle losses in the sampling line (including the ejec-tor) and the DMAs, the fraction of particles that are charged and theUCPC counting efficiency. In this work, particle transport losses by diffu-sion and inertial deposition were accounted for.(27) Both the regular DMAand the Nano-DMA operate at 1.5 slm aerosol flow and 15 slm sheathflow. The size-dependent particle losses inside the regular DMA are esti-mated by a 13-meter effective diffusion length,(28) and the losses in theNano-DMA are estimated from the experimental data provided by Chenet al.(18) Since half of the sampling probe length is water cooled, thereis a large temperature gradient inside the probe. Thermophoretic lossesmight be important. However, there is currently no model available to esti-mate the thermophoretic losses in such under-developed and high temper-ature gradient flows. Hence we do not account for thermophoretic lossesfor the data reported in this paper, Fortunately, as thermophoretic losses

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Fig. 3. Fuchs’ stationary charge distribution in N2 at atmospheric pressure. The curves areobtained by assuming the listed properties of charging agents.(25,29) It approximates thecharge distribution that particles acquire after passing through the bipolar charger. q is thenumber of charges on a particle, dp is the diameter of the particle, and MI+, MI−, ZI+, ZI−are mass and electrical mobility of positive and negative ions.

are independent of size for the particle Knudsen numbers in our experi-ment, neglecting them does not skew the size distribution. Particles can beassumed to achieve a stationary charge distribution after passing througha bipolar charger.(25) The stationary charge distribution in nitrogen isshown in Fig. 3.(29) Note that the fractions of doubly charged particlesare at least about one order of magnitude lower than the singly chargedparticles for particles smaller than 100 nm, which is approximately the sizerange of synthesized particles in our experiments. Therefore, the effect ofmultiply charged particles is not considered in the size distribution inver-sion in this paper. The counting efficiency of the UCPC was obtained bycomparing concentrations of DMA classified “monodisperse” particles bythe UCPC and by an electrometer(30) located in parallel. Particle transportefficiencies through the DMAs and the counting efficiency of the UCPCare shown in Fig. 4. Also shown is a curve of particle transport efficiencyin the sampling line under our typical operating conditions (266 Pa back-ground reactor chamber pressure, 1.5 slm aerosol flow through the laminarflowmeter and 38.5 slm clean nitrogen in the second stage dilution).

A hypersonic impactor is used to collect representative samples foranalysis by electron microscopy.(19) Under our experimental conditions, thepressure ratio across the 0.34 mm diameter orifice is about 390. Hence thesampled aerosol expands supersonically through the orifice. The particleladen hypersonic free jet impinges against the collection plate. The cut size

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Fig. 4. The size dependent counting efficiency of the UCPC and particle transportefficiencies in the sampling line, the regular DMA and the Nano-DMA at typical operatingconditions.

of this impactor is about 6 nm for silicon particles when the orifice-to-platedistance is about 4 orifice diameters.(31)

Figure 5(a) and (b) show pictures taken when we were synthesizingand analyzing titanium particles. Figure 5(a) shows an argon plasma beforehydrogen and reactants were injected. Figure 5(b) shows a bow shock cov-ering the sampling probe. The bow shock is made visible by photoemissionfrom titanium particles.

3. RESULTS

3.1. Results of Particle Size Distribution Measurements

The effects on particle size distributions of varying chamber pressure,reactant flow rates, and the method used to inject reactants are addressedin this section.

Figure 6 shows silicon particle size distributions at different cham-ber pressures with the same reactant flow rate. The first point to noteis that at normal operating chamber pressure (266 Pa), most particles aresmaller than 10 nm. Second, it is clear that particles grow to larger sizesas the chamber pressure increases. Similar results were obtained for par-ticles of other materials (Ti, Si–Ti–N, etc.). We believe that particle sizeincreases with pressure because both particle concentrations and transport

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Fig. 5. Photographs of the plasma-expansion process: (a) Argon plasma before hydrogen andreactants were injected; (b) Particle sampling. The sampling probe is covered by a bow shockcontaining titanium particles.

Fig. 6. Silicon particle size distributions at different chamber pressures with a constantreactant flow rate. n is the particle number concentration.

times increase with increased pressure. Hence more particles coagulate athigher pressures, and particles become larger.

Figure 7 shows the dependence of particle size distributions onreactant flow rates for a fixed chamber pressure. A background size

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Fig. 7. Dependence of silicon particle size distributions on reactant (SiCl4) flow rate at afixed chamber pressure.

distribution was measured before reactants were injected. Note that forthis particular run, virtually no particles were detected when the SiCl4flow rate equaled 20 sccm. This is presumably because the reactant sat-uration ratio was too low for particles to nucleate. For 30 and 40 sccmSiCl4, bimodal distributions were observed. Also note that the mode parti-cle size increases as reactant flow rates increase. This observation is consis-tent with a previous aerosol simulation on a similar system, which showsthat higher initial reactant concentrations result in larger particles due tofaster condensation and coagulation.(32)

In our system, we are able to inject different reactants separately orpremixed into the reaction zone as shown in Fig. 8 for Si–Ti–N synthesis.Without premixing, the reactants are injected separately from two injec-tion ports; with premixing, reactants are mixed before they are injected.As is seen in Fig. 9, the particle size distributions are not significantlydifferent for these two cases. However, significant differences in film prop-erties were found, as is shown in the X-ray diffraction (XRD) spectra inFig. 10. Note that we obtained multiple silicide peaks in the film withpremixed reactants, while these peaks were not observed in the unmixedfilms. It seems that when reactants are injected separately, they do not mixwell in the reacting zone. Silicon and titanium particles nucleate separatelybefore they react further with nitrogen. However, when SiCl4 and TiCl4

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Fig. 8. Schematic of different reactant injection methods: (a) Un-premixed; (b) premixed.

Fig. 9. Si-Ti-N particle size distributions with reactants injected premixed or un-premixed.

are premixed, they dissociate in the hot plasma gas and co-nucleate duringexpansion cooling to form silicide phases. The fact that the particle sizedistributions in the two cases are so similar is surprising, and currentlyunexplained.

3.2. Results of Particle Charge Distribution Measurements

As was mentioned earlier, when particles pass through a bipolarcharger, they achieve a stationary charge distribution at atmospheric pres-sure (Fig. 3).(25) This known charge fraction can be used to obtainthe total particle (positive + negative + neutral) size distribution fromthe measured distributions of charged (positive or negative) particles. Onthe other hand, if the bipolar charger is bypassed, the SMPS measures the

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Fig. 10. X-ray diffraction (XRD) spectra for unmixed and premixed Si-Ti-N films.

charged particles directly from the reactor. The particle charge distribution(size dependent charge fraction) can be obtained by dividing the size dis-tribution of these charged particles by the aforementioned total size distri-bution.

Figure 11 shows the size distributions of total particles and parti-cles that acquired charges within the reactor during titanium nanoparticlesynthesis. The size dependent fractions of charged particles are shown inFig. 12. Note that the positive and negative particles have almost identi-cal size distributions in the size range we measured. Furthermore, the frac-tions of charged particles are very small, especially for smaller sizes. Thisindicates that most particles from the reactor are neutral for sizes less than20 nm.

Particles in a plasma gain charges by attachment of electrons andions and by thermionic emission of electrons. When emission is not impor-tant, the equilibrium charge is negative due to the higher flux of elec-trons than positive ions. However, when electron emission is significant,the equilibrium charge may be positive.(33) Therefore, it is not surprisingthat our measured charge distributions have almost equal fractions of pos-itive and negative particles. It is not yet clear which charging mechanisms

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Fig. 11. Size distributions of positively and negatively charged titanium particles directlyfrom the reactor and total particle size distributions.

Fig. 12. Fraction of particles that obtained +1 and −1 charge within the reactor.

are important in our system. Furthermore, we have made the question-able assumption that the particle charge distribution does not change dur-ing transport. An equilibrium calculation shows that the concentrationsof positive (Ti+) and negative (electron) charge agents are on the orderof 1012 cm−3, when 40 sccm TiCl4 is disassociated in an Ar-H2 plasma at

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4000 K and 266 Pa. Assuming the electron and ion concentrations are fro-zen before the dilution in the ejector, the Nt value is ∼1010 ions/cm3 s fromthe sampling inlet to the ejector dilution (with a residence time ∼10 ms).This Nt value might have been sufficiently high to affect the charge stateof the sampled aerosol. Therefore, a detailed study of the role of possi-ble charging mechanisms (diffusion, thermionic emission, secondary elec-tron emission, etc.) and the electron-ion-particle interaction dynamics isstill required to quantitatively explain the measurements.

4. SUMMARY

A diagnostic system was developed to characterize nanosize particlessynthesized with a thermal plasma process. This system can measure parti-cle size and charge distributions in near real time from a low pressure andhigh temperature environment. It can also collect particles for subsequentmicroscopic analysis.

The effects of chamber pressure (266–5332 Pa) and precursor flowrates (20–80 sccm) on particle size distributions were investigated. It wasfound that the particle mean size increased with the chamber pressuredue to longer residence times for coagulation at higher chamber pressures.At lower chamber pressures (<1333 Pa), most particles were smaller than10 nm. The mean size also increased with reactant flow rates. SMPS mea-surements for particles of both positive and negative polarity were carriedout with and without the bipolar charger upstream of the DMA to deter-mine the charge distribution of the sampled aerosol. It was found thatwhile most sub-20 nm particles from the reactor were neutral, the popula-tions of positively and negatively charged particles were almost the same.These results are of foremost interest in the modeling and understandingof nucleation and coagulation phenomena in our thermal plasma process.

ACKNOWLEDGMENTS

This work was supported by NSF (Grant No. DIM-0103169).

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