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Control of nanoparticle charge via condensation magnification
Aerosol Science 37 (2006) 1876 – 1882 www.elsevier.com/locate/jaerosci
Technical note
Control of nanoparticle charge via condensation magnification Dae Seong Kim1 , Dong Suk Lee, Chang Gyu Woo, Mansoo Choi∗ National CRI Center for Nano Particle Control, Institute of Advanced Machinery and Design, School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-742, Korea Received 21 July 2006; received in revised form 8 August 2006; accepted 14 August 2006
Abstract Previously, we reported a method for preparing highly charged nanoparticles via condensation magnification [Suh, J., Han, B., Kim, D. S., & Choi, M. (2005). A method for enhanced charging of nanoparticles via condensation magnification. Journal of Aerosol Science, 36, 1183–1193]. In the present work, we attempt to control the charge level of nanoparticles by varying the droplet size in the condensation magnification method. We also investigate whether both metallic and dielectric nanoparticles could be highly charged and their charge level can be controlled. Three different cases of 12 and 25 nm gold particles and 22 nm polystyrene polymer (PSP) particles were examined and for all three cases, the number of elementary charges could be successfully controlled by varying droplet sizes. The maximum charges of 12 and 25 nm gold nanoparticles were about 15 and 57 units, respectively, and that of 22 nm polystyrene polymer (PSP) particles was 43 units. Surface electric field strength corresponding to these maximum charges was found to be approximately independent of particle sizes indicating that ion evaporation model could be valid in our charging process. 䉷 2006 Elsevier Ltd. All rights reserved. Keywords: Nanoparticle; Condensation; Nanoparticle charging; Ion evaporation
1. Introduction Nanoparticles could be applied to fabricate nanodevices, such as quantum device, field emission display, singleelectron transistor, data storage devices, etc. These devices need nanoscale structure which can be built by using charged nanoparticles. In the nanoparticle patterning process, the charging of particles plays an important role for positioning nanoparticles onto the desired area (Choi, 2005; Deppert, Schmit, Krinke, Dixkens, & Fissan, 1996; Kang et al., 2004; Krinke & Fissan, 2001) Highly charged nanoparticles may be desirable to enhance the deposition efficiency and to control particle trajectories. However, it is well known that nanoparticles are hard to charge. For example, a high-efficiency unipolar charger using corona discharge could not reach a level of more than two elementary charges on particles smaller than 20 nm (Adachi, Kousaka, & Okuyama, 1985; Biskos, Reavell, & Collings, 2005; Pui, Fruin, & McMurry, 1988). Previously, we proposed and demonstrated a method for preparing highly charged nanoparticles by utilizing a combination of a particle magnifier (PM), corona charger, and drier (Choi, 2003; Suh, Han, Kim, & Choi, 2005). ∗ Corresponding author. Tel.: +82 2 880 7128; fax: +82 2 878 2465.
E-mail address: [email protected] (M. Choi). 1 Present address: R&D Center, Hyundai Calibration & Certification Technologies Co., Icheon-si, Gyeonggi-do 467-701, Korea.
0021-8502/$ - see front matter 䉷 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaerosci.2006.08.003
D.S. Kim et al. / Aerosol Science 37 (2006) 1876 – 1882
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In this study, we report that the charge of nanoparticles can be controlled by changing droplet sizes during this condensation magnification and charging method. We also investigate whether this method can be applied to dielectric nanoparticles in addition to metallic nanoparticles. 2. Experiment This experimental setup is almost the same as that of Suh, Han, Kim, et al. (2005) except droplet size measurement system and variable temperature control of liquid reservoir (see Fig. 1). The system consisted of an electrospray, a PM by condensation, a corona discharger, a drier, and measurement systems for charge and size distributions. In this work, monodiperse gold and polystyrene polymer (PSP) particles were prepared by electrospraying of colloidal gold and PSP suspensions (Sigma Aldrich Chemie Gmbh for 10 and 20 nm gold colloids and Duke Science Corp. for 20 nm PSP colloid). A positive DC voltage of 2.5–3 kV was applied to the metal capillary tube (0.2 mm O.D. and 0.1 mm I.D.) to assure cone-jet mode for electrospray. Flow rates of a carrier gas (N2 ) and colloidal solution were 1.7 l/min and 25 l/h, respectively. Generated nanoparticles were fully neutralized in a Po-210 neutralizer (Suh, Han, Okuyama, & Choi, 2005). The generated monodisperse particles were enlarged by evaporation and condensation of ethylene glycol in a particle size magnifier (PM) which is the same one used in Suh, Han, Kim, et al. (2005) . In this experiment, ethylene glycol was used as a condensable vapor. The temperature of the condensing zone was controlled with cooling water set to 30 ◦ C (Tc ), and the temperature of the liquid reservoir (containing ethylene glycol) was controlled from 50 to 180 ◦ C to vary droplet sizes. Droplet sizes were measured by a Grimm Dustmonitor 1.109 (Grimm Aerosol Technik). Filtered nitrogen flows into the liquid reservoir, where nitrogen becomes saturated with vapor. The nanoparticles introduced into the PM were mixed with super-saturated vapor and the size of the particles was enlarged by condensation in the condensing zone. To investigate the charge states of the nanoparticles, the electrical mobility was analyzed by a nanoDMA (Model 3085, TSI Corp.) and a faraday cup electrometer (FCE, Wyckoff Corp., Japan). The size distribution of fully dried
Fig. 1. Experimental setup for nanoparticle charging via condensation magnification, size and charge measurement systems.
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nanoparticles was also measured by a nanoDMA and a condensation particle counter (CPC, Model 3022A, TSI Corp.) in order to confirm whether the particles return to the initial sizes. In this work, the aerosol flow rate of the DMA was set to be 2.0 l/min while the sheath air flow rate was maintained at 20 l/min. The measured values, dg and qg , correspond to the geometric mean diameter of nanoparticles and the geometric mean number of charges, respectively. The particle charge can be estimated as q = Zp /Zp1 , where Zp and Zp1 are the mobility of the highly charged particles and a singly charged particles, respectively. 3. Results and discussion In this experiment, as written above, we used three kinds of particles. The morphology of the colloidal gold nanoparticles of 10 and 20 nm (nominal value) was spherical and their diameters were about 12 and 25 nm, respectively (Suh, Han, Kim, et al., 2005) . In addition, TEM analysis was done for investigating the size and morphology of 20 nm colloidal PSP nanoparticles, which was prepared for the charging experiment of dielectric nanoparticles. Nominal 20 nm PSP nanoparticles were spherical and particle diameter was about 22 nm. The droplet size distributions were measured before and after the charger. Fig. 2 shows one example of size distributions of droplets containing 12 nm gold particles before and after the corona charger. The filled circle shows the droplet size distribution right after particle magnification system (PM) before the corona charger (geometric mean diameter, Dg = 0.96 m), and the blank circle shows that after the charger (Dg = 0.77 m). As shown in the figure, the distributions are quite different before and after the charger. Overall, droplet size and number concentration decreased significantly after their passing through the corona charger. Some larger droplets are caught, though smaller droplets can pass through the charger, because the corona charger may play a particle collector like the electrostatic precipitator in this case. We adopted the droplet sizes measured after the charger as a variable for showing the controllability of nanoparticle charging. It is noted that measured sizes would be less than actual droplet sizes of ethylene glycol since evaporation occurs during the measurement, and Grimm dustmonitor was calibrated for polystyrene latex (PSL). However, estimation of relative sizes is sufficient in the present study to show the controllability of particle charges by varying droplet sizes. The droplet size can be varied by adjusting the temperature of ethylene glycol inside the PM. Figs. 3 and 4 show the distributions of the droplet size normalized by total number concentration (N0 ) and the resultant charges for 12 (Fig. 3) and 25 nm gold nanoparticles (Fig. 4), which were prepared by electrospraying of nominal 10 and 20 nm gold colloid, respectively. It is clearly seen in Figs. 3 and 4 that final charges of nanoparticles after drying are controllable by varying droplet sizes. The geometric mean number of elementary charges (qg ) of 12 nm gold nanoparticles was increased from 5.9 to 15.4 units, when the geometric mean diameters of the droplets (Dg ) were varied from 0.4 to 2.3 m (see Fig. 3).
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Droplet size (µm) Fig. 2. Droplet size distributions of 12 nm gold particles before and after the charger (temperature at PM was about 80 ◦ C).
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0
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Fig. 3. Distributions of the droplet size and the number of elementary charge for 12 nm gold nanoparticles when the droplet diameters (Dg ) were 0.4 (a) and 2.3 m (b).
Temperature of liquid reservoir of PM was changed from 65 to 120 ◦ C for obtaining droplet sizes of 0.4 and 2.3 m. In the case of 25 nm gold nanoparticles, the geometric mean number of elementary charges (qg ) was varied from 10.0 to 53.7 units, as the geometric mean diameter of the droplets (Dg ) increased from 0.5 to 2.1 m (see Fig. 4). Fig. 5 represents the distributions of the droplet size and its resultant charges for 22 nm PSP nanoparticles, which were prepared by electrospraying of 20 nm colloidal PSP nanoparticles. As shown in the figure, the geometric mean number of elementary charges (qg ) of 22 nm PSP nanoparticles was changed from 12.2 to 40.7 units, respectively, when the geometric mean diameters of the droplets (Dg ) were varied from 0.4 to 2.1 m. It was demonstrated that PSP particles (non-conducting particles) could be also highly charged like gold particles (metallic particles) by condensationmagnification charging technique. In addition, it is clear that the number of elementary charges for PSP nanoparticles depends on the droplet size as significantly as that of gold nanoparticles. Fig. 6 shows the number of elementary charges depending on different droplet sizes. Overall, the number of elementary charges increased with the increase of the droplet size. Eventually, the charge levels were saturated with further increase of droplet size. In the case of 12 nm gold particles, the number of elementary charges was varied from 2 to 15 units with different droplet sizes. Also, the average number of charges for other nanoparticles was varied: 3–57 units for 25 nm gold particles and 5–43 units for 22 nm PSP particles. This demonstrates that the number of elementary charges for nanoparticles can be controlled by varying the droplet size and dielectric particles (PSP) can be also highly charged like metallic particles (gold particles) by condensation-magnification charging technique. The nanoparticles have different maximum charges, which depend on nanoparticle size as shown in Fig. 6. The maximum charges of 12 and 25 nm gold nanoparticles were about 15 and 57 units, respectively, and that of 22 nm PSP particles was about 43 units. Therefore, the maximum charge levels are proportional to about Dp2 and it agrees well with the result of Suh, Han, Kim, et al. (2005) . Surface electrical field strength corresponding to saturated charge may be obtained as E∗ =
ne , 0 Dp2
(1)
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Fig. 4. Distributions of the droplet size and the number of elementary charge for 25 nm gold nanoparticles when the droplet diameters (Dg ) were 0.5 (a) and 2.1 m (b).
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Fig. 5. Distributions of the droplet size and the number of elementary charge for 22 nm PSP nanoparticles when the droplet diameters (Dg ) were 0.4 (a) and 2.1 m (b).
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Number of charges, q
100
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1
0
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Droplet size (µm) Fig. 6. The number of elementary charges vs droplet sizes.
Table 1 Calculated E ∗ values from experiments and corresponding ER Dp (nm)
n (saturated charge)
E ∗ (V/nm)
ER (V/nm)
12 (Gold) 22 (PSP) 25 (Gold)
15 43 57
0.5999 0.5117 0.5252
1.8951 1.3996 1.3130
Note: ER is electric field at Rayleigh limit calculated from ER =
8/0 Dp where = 47.70 mN/m at 20 ◦ C for ethylene glycol.
where n is the number of electrical charges, e is the elementary unit of charge (1.602 × 10−19 C) and 0 is electric permittivity constant (8.854 × 10−12 C/Vm). E ∗ s for three different particles are listed in Table 1 and the electric field strength corresponding to Rayleigh limit (ER ) is also given for comparison. Table 1 not only shows the discrepancy between E ∗ and ER , but also gives relatively independent values regardless of particle sizes. This indicates that our charging process would be governed by ion evaporation (not by Rayleigh fission) where field emission of ions occurs at a certain critical field strength (Loscertales & Fernandez de la Mora, 1995) which is fixed for a given liquid, for example, ethylene glycol in the present study. E ∗ is a function of the electrical property of liquid such as surface tension and dielectric constant (Gamero-Castano & Fernandez de la Mora, 2000; Guevremont, Le Blanc, & Siu, 1993; Labowsky, Fenn, & Fernandez de la Mora, 2000; Loscertales & Fernandez de la Mora, 1995). Suh, Han, Okuyama, et al. (2005) used a mixture of gold colloidal suspension and methanol (50/50, v/v, measured =50 mN/m) and the E ∗ was found to be about 1.2 V/nm. For ethylene glycol, Eq. (1) and our data of saturated charges give E ∗ to be about 0.5 V/nm. This indicates that a proper selection of condensable liquid would change the charging limit in the method. 4. Conclusions In this study, we demonstrated that the charge of nanoparticles can be controlled by varying droplet sizes in condensation magnification method. In addition, it was found that dielectric particles as well as metallic particles can be highly charged by this charging method and its charge level can be controllable. The maximum charges of 12 and 25 nm gold
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nanoparticles were about 15 and 57 units, respectively, and that of 22 nm PSP particles was 43 units. Therefore, the maximum charge states are proportional to about Dp2 and it agrees well with the result of Suh, Han, Kim, et al. (2005) and supports ion evaporation model. Acknowledgment This work was funded by Creative Research Initiatives program sponsored by Korea Ministry of Science and Technology. References Adachi, M., Kousaka, Y., & Okuyama, K. (1985). Unipolar and bipolar diffusion charging of ultrafine aerosol particles. Journal of Aerosol Science, 16, 109–123. Biskos, G., Reavell, K., & Collings, N. (2005). Unipolar diffusion charging of aerosol particles in the transition regime. Journal of Aerosol Science, 36, 247–265. Choi, M. (2003). Progress report of national CRI center for nano particle control (R16-1997-014-01001-3) (pp. 12). Ministry of Science and Technology, Korea. Choi, M. (2005). Controlled synthesis and patterning of nano particle in gas phase. In Abstract of the fourth Asian aerosol conference, Mumbai, India. Deppert, K., Schmit, F., Krinke, T., Dixkens, J., & Fissan, H. (1996). Electrostatic precipitator for homogeneous deposition of ultrafine particles to create quantum dot structures. Journal of Aerosol Science, 27, S151–S152. Gamero-Castano, M., & Fernandez de la Mora, J. (2000). Kinetics of small ion evaporation from the charge and mass distribution of multiply charged clusters in electrosprays. Journal of Mass Spectrometry, 35, 790–803. Guevremont, R., Le Blanc, J. C. Y., & Siu, K. W. (1993). Electrospray mass spectrometry: Ethylene glycol as a solvent and its effects on ion desorption. Organic Mass Spectrometry, 28, 1345–1352. Kang, M., Kim, H., Han, B., Suh, J., Park, J., & Choi, M. (2004). Nanoparticle pattern deposition from gas phase onto charged flat surface. Microelectronic Engineering, 71, 229–236. Krinke, T. J., & Fissan, H. (2001). Positioning of nanometer-sized particles on flat surfaces by direct deposition from the gas phase. Applied Physics Letters, 78, 3708–3710. Labowsky, M., Fenn, J. B., & Fernandez de la Mora, J. (2000). A continuum model for ion evaporation from a drop: Effect of curvature and charge on ion solvation energy. Analytica Chimica Acta, 406, 105–118. Loscertales, I. G., & Fernandez de la Mora, J. (1995). Experiments on the kinetics of field evaporation of small ions from droplets. Journal of Chemical Physics, 103, 5041–5060. Pui, D. Y. H., Fruin, S., & McMurry, P. H. (1988). Unipolar diffusion charging of ultrafine aerosol. Aerosol Science and Technology, 8, 173–187. Suh, J., Han, B., Kim, D. S., & Choi, M. (2005). A method for enhanced charging of nanoparticles via condensation magnification. Journal of Aerosol Science, 36, 1183–1193. Suh, J., Han, B., Okuyama, K., & Choi, M. (2005). Highly charging of nanoparticles through electrospray of nanoparticle suspension. Journal of Colloid and Interface Science, 287, 135–140.
Technical note
Control of nanoparticle charge via condensation magnification Dae Seong Kim1 , Dong Suk Lee, Chang Gyu Woo, Mansoo Choi∗ National CRI Center for Nano Particle Control, Institute of Advanced Machinery and Design, School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-742, Korea Received 21 July 2006; received in revised form 8 August 2006; accepted 14 August 2006
Abstract Previously, we reported a method for preparing highly charged nanoparticles via condensation magnification [Suh, J., Han, B., Kim, D. S., & Choi, M. (2005). A method for enhanced charging of nanoparticles via condensation magnification. Journal of Aerosol Science, 36, 1183–1193]. In the present work, we attempt to control the charge level of nanoparticles by varying the droplet size in the condensation magnification method. We also investigate whether both metallic and dielectric nanoparticles could be highly charged and their charge level can be controlled. Three different cases of 12 and 25 nm gold particles and 22 nm polystyrene polymer (PSP) particles were examined and for all three cases, the number of elementary charges could be successfully controlled by varying droplet sizes. The maximum charges of 12 and 25 nm gold nanoparticles were about 15 and 57 units, respectively, and that of 22 nm polystyrene polymer (PSP) particles was 43 units. Surface electric field strength corresponding to these maximum charges was found to be approximately independent of particle sizes indicating that ion evaporation model could be valid in our charging process. 䉷 2006 Elsevier Ltd. All rights reserved. Keywords: Nanoparticle; Condensation; Nanoparticle charging; Ion evaporation
1. Introduction Nanoparticles could be applied to fabricate nanodevices, such as quantum device, field emission display, singleelectron transistor, data storage devices, etc. These devices need nanoscale structure which can be built by using charged nanoparticles. In the nanoparticle patterning process, the charging of particles plays an important role for positioning nanoparticles onto the desired area (Choi, 2005; Deppert, Schmit, Krinke, Dixkens, & Fissan, 1996; Kang et al., 2004; Krinke & Fissan, 2001) Highly charged nanoparticles may be desirable to enhance the deposition efficiency and to control particle trajectories. However, it is well known that nanoparticles are hard to charge. For example, a high-efficiency unipolar charger using corona discharge could not reach a level of more than two elementary charges on particles smaller than 20 nm (Adachi, Kousaka, & Okuyama, 1985; Biskos, Reavell, & Collings, 2005; Pui, Fruin, & McMurry, 1988). Previously, we proposed and demonstrated a method for preparing highly charged nanoparticles by utilizing a combination of a particle magnifier (PM), corona charger, and drier (Choi, 2003; Suh, Han, Kim, & Choi, 2005). ∗ Corresponding author. Tel.: +82 2 880 7128; fax: +82 2 878 2465.
E-mail address: [email protected] (M. Choi). 1 Present address: R&D Center, Hyundai Calibration & Certification Technologies Co., Icheon-si, Gyeonggi-do 467-701, Korea.
0021-8502/$ - see front matter 䉷 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaerosci.2006.08.003
D.S. Kim et al. / Aerosol Science 37 (2006) 1876 – 1882
1877
In this study, we report that the charge of nanoparticles can be controlled by changing droplet sizes during this condensation magnification and charging method. We also investigate whether this method can be applied to dielectric nanoparticles in addition to metallic nanoparticles. 2. Experiment This experimental setup is almost the same as that of Suh, Han, Kim, et al. (2005) except droplet size measurement system and variable temperature control of liquid reservoir (see Fig. 1). The system consisted of an electrospray, a PM by condensation, a corona discharger, a drier, and measurement systems for charge and size distributions. In this work, monodiperse gold and polystyrene polymer (PSP) particles were prepared by electrospraying of colloidal gold and PSP suspensions (Sigma Aldrich Chemie Gmbh for 10 and 20 nm gold colloids and Duke Science Corp. for 20 nm PSP colloid). A positive DC voltage of 2.5–3 kV was applied to the metal capillary tube (0.2 mm O.D. and 0.1 mm I.D.) to assure cone-jet mode for electrospray. Flow rates of a carrier gas (N2 ) and colloidal solution were 1.7 l/min and 25 l/h, respectively. Generated nanoparticles were fully neutralized in a Po-210 neutralizer (Suh, Han, Okuyama, & Choi, 2005). The generated monodisperse particles were enlarged by evaporation and condensation of ethylene glycol in a particle size magnifier (PM) which is the same one used in Suh, Han, Kim, et al. (2005) . In this experiment, ethylene glycol was used as a condensable vapor. The temperature of the condensing zone was controlled with cooling water set to 30 ◦ C (Tc ), and the temperature of the liquid reservoir (containing ethylene glycol) was controlled from 50 to 180 ◦ C to vary droplet sizes. Droplet sizes were measured by a Grimm Dustmonitor 1.109 (Grimm Aerosol Technik). Filtered nitrogen flows into the liquid reservoir, where nitrogen becomes saturated with vapor. The nanoparticles introduced into the PM were mixed with super-saturated vapor and the size of the particles was enlarged by condensation in the condensing zone. To investigate the charge states of the nanoparticles, the electrical mobility was analyzed by a nanoDMA (Model 3085, TSI Corp.) and a faraday cup electrometer (FCE, Wyckoff Corp., Japan). The size distribution of fully dried
Fig. 1. Experimental setup for nanoparticle charging via condensation magnification, size and charge measurement systems.
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nanoparticles was also measured by a nanoDMA and a condensation particle counter (CPC, Model 3022A, TSI Corp.) in order to confirm whether the particles return to the initial sizes. In this work, the aerosol flow rate of the DMA was set to be 2.0 l/min while the sheath air flow rate was maintained at 20 l/min. The measured values, dg and qg , correspond to the geometric mean diameter of nanoparticles and the geometric mean number of charges, respectively. The particle charge can be estimated as q = Zp /Zp1 , where Zp and Zp1 are the mobility of the highly charged particles and a singly charged particles, respectively. 3. Results and discussion In this experiment, as written above, we used three kinds of particles. The morphology of the colloidal gold nanoparticles of 10 and 20 nm (nominal value) was spherical and their diameters were about 12 and 25 nm, respectively (Suh, Han, Kim, et al., 2005) . In addition, TEM analysis was done for investigating the size and morphology of 20 nm colloidal PSP nanoparticles, which was prepared for the charging experiment of dielectric nanoparticles. Nominal 20 nm PSP nanoparticles were spherical and particle diameter was about 22 nm. The droplet size distributions were measured before and after the charger. Fig. 2 shows one example of size distributions of droplets containing 12 nm gold particles before and after the corona charger. The filled circle shows the droplet size distribution right after particle magnification system (PM) before the corona charger (geometric mean diameter, Dg = 0.96 m), and the blank circle shows that after the charger (Dg = 0.77 m). As shown in the figure, the distributions are quite different before and after the charger. Overall, droplet size and number concentration decreased significantly after their passing through the corona charger. Some larger droplets are caught, though smaller droplets can pass through the charger, because the corona charger may play a particle collector like the electrostatic precipitator in this case. We adopted the droplet sizes measured after the charger as a variable for showing the controllability of nanoparticle charging. It is noted that measured sizes would be less than actual droplet sizes of ethylene glycol since evaporation occurs during the measurement, and Grimm dustmonitor was calibrated for polystyrene latex (PSL). However, estimation of relative sizes is sufficient in the present study to show the controllability of particle charges by varying droplet sizes. The droplet size can be varied by adjusting the temperature of ethylene glycol inside the PM. Figs. 3 and 4 show the distributions of the droplet size normalized by total number concentration (N0 ) and the resultant charges for 12 (Fig. 3) and 25 nm gold nanoparticles (Fig. 4), which were prepared by electrospraying of nominal 10 and 20 nm gold colloid, respectively. It is clearly seen in Figs. 3 and 4 that final charges of nanoparticles after drying are controllable by varying droplet sizes. The geometric mean number of elementary charges (qg ) of 12 nm gold nanoparticles was increased from 5.9 to 15.4 units, when the geometric mean diameters of the droplets (Dg ) were varied from 0.4 to 2.3 m (see Fig. 3).
Number Concent ration (#/cc)
1.2e+6
1.0e+6
Before the charger After the charger
8.0e+5
6.0e+5
4.0e+5
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0.0 0.1
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Droplet size (µm) Fig. 2. Droplet size distributions of 12 nm gold particles before and after the charger (temperature at PM was about 80 ◦ C).
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4 8 12 16 Number of charges, q
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1 Droplet size (µm)
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0
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Fig. 3. Distributions of the droplet size and the number of elementary charge for 12 nm gold nanoparticles when the droplet diameters (Dg ) were 0.4 (a) and 2.3 m (b).
Temperature of liquid reservoir of PM was changed from 65 to 120 ◦ C for obtaining droplet sizes of 0.4 and 2.3 m. In the case of 25 nm gold nanoparticles, the geometric mean number of elementary charges (qg ) was varied from 10.0 to 53.7 units, as the geometric mean diameter of the droplets (Dg ) increased from 0.5 to 2.1 m (see Fig. 4). Fig. 5 represents the distributions of the droplet size and its resultant charges for 22 nm PSP nanoparticles, which were prepared by electrospraying of 20 nm colloidal PSP nanoparticles. As shown in the figure, the geometric mean number of elementary charges (qg ) of 22 nm PSP nanoparticles was changed from 12.2 to 40.7 units, respectively, when the geometric mean diameters of the droplets (Dg ) were varied from 0.4 to 2.1 m. It was demonstrated that PSP particles (non-conducting particles) could be also highly charged like gold particles (metallic particles) by condensationmagnification charging technique. In addition, it is clear that the number of elementary charges for PSP nanoparticles depends on the droplet size as significantly as that of gold nanoparticles. Fig. 6 shows the number of elementary charges depending on different droplet sizes. Overall, the number of elementary charges increased with the increase of the droplet size. Eventually, the charge levels were saturated with further increase of droplet size. In the case of 12 nm gold particles, the number of elementary charges was varied from 2 to 15 units with different droplet sizes. Also, the average number of charges for other nanoparticles was varied: 3–57 units for 25 nm gold particles and 5–43 units for 22 nm PSP particles. This demonstrates that the number of elementary charges for nanoparticles can be controlled by varying the droplet size and dielectric particles (PSP) can be also highly charged like metallic particles (gold particles) by condensation-magnification charging technique. The nanoparticles have different maximum charges, which depend on nanoparticle size as shown in Fig. 6. The maximum charges of 12 and 25 nm gold nanoparticles were about 15 and 57 units, respectively, and that of 22 nm PSP particles was about 43 units. Therefore, the maximum charge levels are proportional to about Dp2 and it agrees well with the result of Suh, Han, Kim, et al. (2005) . Surface electrical field strength corresponding to saturated charge may be obtained as E∗ =
ne , 0 Dp2
(1)
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Fig. 4. Distributions of the droplet size and the number of elementary charge for 25 nm gold nanoparticles when the droplet diameters (Dg ) were 0.5 (a) and 2.1 m (b).
0.3
0.30 0.25
0.2 N/N0
N/N0
0.20
0.1
0.15 0.10 0.05
0.0 0.1
10
0.20
0.3
0.15 N/N0
0.4
0.2
0.0 0.1
5
10
15
20
25
Number of charges, q
0.1
(b)
0
Droplet size (µm)
(a)
N/N0
0.00 1
0.10 0.05 0.00
1 Droplet size (µ µm)
10
20
30
40
50
60
70
80
Number of charges, q
Fig. 5. Distributions of the droplet size and the number of elementary charge for 22 nm PSP nanoparticles when the droplet diameters (Dg ) were 0.4 (a) and 2.1 m (b).
D.S. Kim et al. / Aerosol Science 37 (2006) 1876 – 1882
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Number of charges, q
100
10
Gold 12 nm Gold 25 nm PSP 22 nm
1
0
1
2
3
4
5
Droplet size (µm) Fig. 6. The number of elementary charges vs droplet sizes.
Table 1 Calculated E ∗ values from experiments and corresponding ER Dp (nm)
n (saturated charge)
E ∗ (V/nm)
ER (V/nm)
12 (Gold) 22 (PSP) 25 (Gold)
15 43 57
0.5999 0.5117 0.5252
1.8951 1.3996 1.3130
Note: ER is electric field at Rayleigh limit calculated from ER =
8/0 Dp where = 47.70 mN/m at 20 ◦ C for ethylene glycol.
where n is the number of electrical charges, e is the elementary unit of charge (1.602 × 10−19 C) and 0 is electric permittivity constant (8.854 × 10−12 C/Vm). E ∗ s for three different particles are listed in Table 1 and the electric field strength corresponding to Rayleigh limit (ER ) is also given for comparison. Table 1 not only shows the discrepancy between E ∗ and ER , but also gives relatively independent values regardless of particle sizes. This indicates that our charging process would be governed by ion evaporation (not by Rayleigh fission) where field emission of ions occurs at a certain critical field strength (Loscertales & Fernandez de la Mora, 1995) which is fixed for a given liquid, for example, ethylene glycol in the present study. E ∗ is a function of the electrical property of liquid such as surface tension and dielectric constant (Gamero-Castano & Fernandez de la Mora, 2000; Guevremont, Le Blanc, & Siu, 1993; Labowsky, Fenn, & Fernandez de la Mora, 2000; Loscertales & Fernandez de la Mora, 1995). Suh, Han, Okuyama, et al. (2005) used a mixture of gold colloidal suspension and methanol (50/50, v/v, measured =50 mN/m) and the E ∗ was found to be about 1.2 V/nm. For ethylene glycol, Eq. (1) and our data of saturated charges give E ∗ to be about 0.5 V/nm. This indicates that a proper selection of condensable liquid would change the charging limit in the method. 4. Conclusions In this study, we demonstrated that the charge of nanoparticles can be controlled by varying droplet sizes in condensation magnification method. In addition, it was found that dielectric particles as well as metallic particles can be highly charged by this charging method and its charge level can be controllable. The maximum charges of 12 and 25 nm gold
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nanoparticles were about 15 and 57 units, respectively, and that of 22 nm PSP particles was 43 units. Therefore, the maximum charge states are proportional to about Dp2 and it agrees well with the result of Suh, Han, Kim, et al. (2005) and supports ion evaporation model. Acknowledgment This work was funded by Creative Research Initiatives program sponsored by Korea Ministry of Science and Technology. References Adachi, M., Kousaka, Y., & Okuyama, K. (1985). Unipolar and bipolar diffusion charging of ultrafine aerosol particles. Journal of Aerosol Science, 16, 109–123. Biskos, G., Reavell, K., & Collings, N. (2005). Unipolar diffusion charging of aerosol particles in the transition regime. Journal of Aerosol Science, 36, 247–265. Choi, M. (2003). Progress report of national CRI center for nano particle control (R16-1997-014-01001-3) (pp. 12). Ministry of Science and Technology, Korea. Choi, M. (2005). Controlled synthesis and patterning of nano particle in gas phase. In Abstract of the fourth Asian aerosol conference, Mumbai, India. Deppert, K., Schmit, F., Krinke, T., Dixkens, J., & Fissan, H. (1996). Electrostatic precipitator for homogeneous deposition of ultrafine particles to create quantum dot structures. Journal of Aerosol Science, 27, S151–S152. Gamero-Castano, M., & Fernandez de la Mora, J. (2000). Kinetics of small ion evaporation from the charge and mass distribution of multiply charged clusters in electrosprays. Journal of Mass Spectrometry, 35, 790–803. Guevremont, R., Le Blanc, J. C. Y., & Siu, K. W. (1993). Electrospray mass spectrometry: Ethylene glycol as a solvent and its effects on ion desorption. Organic Mass Spectrometry, 28, 1345–1352. Kang, M., Kim, H., Han, B., Suh, J., Park, J., & Choi, M. (2004). Nanoparticle pattern deposition from gas phase onto charged flat surface. Microelectronic Engineering, 71, 229–236. Krinke, T. J., & Fissan, H. (2001). Positioning of nanometer-sized particles on flat surfaces by direct deposition from the gas phase. Applied Physics Letters, 78, 3708–3710. Labowsky, M., Fenn, J. B., & Fernandez de la Mora, J. (2000). A continuum model for ion evaporation from a drop: Effect of curvature and charge on ion solvation energy. Analytica Chimica Acta, 406, 105–118. Loscertales, I. G., & Fernandez de la Mora, J. (1995). Experiments on the kinetics of field evaporation of small ions from droplets. Journal of Chemical Physics, 103, 5041–5060. Pui, D. Y. H., Fruin, S., & McMurry, P. H. (1988). Unipolar diffusion charging of ultrafine aerosol. Aerosol Science and Technology, 8, 173–187. Suh, J., Han, B., Kim, D. S., & Choi, M. (2005). A method for enhanced charging of nanoparticles via condensation magnification. Journal of Aerosol Science, 36, 1183–1193. Suh, J., Han, B., Okuyama, K., & Choi, M. (2005). Highly charging of nanoparticles through electrospray of nanoparticle suspension. Journal of Colloid and Interface Science, 287, 135–140.