Article pubs.acs.org/JPCC
Effect of Electric Bias on the Deposition Behavior of ZnO Nanostructures in the Chemical Vapor Deposition Process Seong-Han Park,† Jin-Woo Park,† Seung-Min Yang,†,‡ Kwang-Ho Kim,*,§ and Nong-Moon Hwang*,† †
Department of Materials Science & Engineering, Seoul National University, Daehak-dong, Gwanak-gu, Seoul 151 744, Republic of Korea ‡ Korea Institute of Industrial Technology, 106-11 Gwahakdanji-ro, Gangneung-si, Gangwon-do 210-340, Republic of Korea § Global Frontier R&D Center for Hybrid Interface Materials, Pusan National University, Pusandeahak-ro 63 beon-gil 2, Geumjeong-gu, Busan 609 735, Republic of Korea ABSTRACT: Much evidence has been reported that charged nanoparticles are generated in the gas phase and contribute to film deposition or nanostructure formation in the chemical vapor deposition (CVD) process. Such a new way of crystal growth with nanoparticles as a building block is closely related to a recently discovered nonclassical crystallization, which has recently attracted great attention. It is often found that deposition does not occur on the substrate even when a huge amount of charged nanoparticles are generated in the gas phase. A drag force exerted on the nanoparticles would be one possibility for such nondeposition. To test such a possibility, an electric bias was applied to the substrate. Under the condition where no deposition occurred on a substrate at 450 °C, small ZnO nanoparticles were deposited on the substrate when 100 V of direct current (dc) was applied to the substrate. When the bias voltage was increased to 300 V, nanoparticles of a larger size were deposited. When the bias voltage was increased to 600 V, tetrapod ZnO nanoparticles of much larger size were deposited. These results indicate that the drag force becomes an important factor in deposition when nanoparticles are formed in the gas phase. field mechanism by assuming a strong electric field at the tip of silicon nanowires.16 Although many microstructures such as spontaneous self-coiling and 1D nanowire growth strongly imply that electrostatic energy might be involved in the growth, the origin of electrostatic energy is not clearly understood. In relation to an origin of electrostatic energy, Kim et al.17 showed that charged ZnO nanoparticles were generated in the gas phase under typical growth conditions of various ZnO nanostructures by measuring their size distribution using a nanodifferential mobility analyzer (nano-DMA). Park et al.18 showed further that tetrapod-shaped ZnO nanowire (T-ZnO), which had been believed to grow on the substrate surface, were actually formed in the gas phase by capturing them on a grid membrane for transmission electron microscopy (TEM) observation. The generation of charged nanoparticles in the gas phase was confirmed not only in the ZnO synthesis but also in many other CVD syntheses such as diamond films,19−21 carbon nanotubes,22,23 silicon nanowires,24 silicon films,25−27 and GaN crystals.28 Because the generation of charged nanoparticles in the gas phase was confirmed without exception via special apparatus to confirm their generation in the gas phase such as Wien filter, energy analyzer, nano-DMA, and particle beam mass spectroscopy (PBMS), the formation of
1. INTRODUCTION ZnO has attracted a lot of attention over the past decades because of a wide band gap (3.37 eV) and a large exciton binding energy (60 meV), which makes ZnO transparent in visible light and gives it efficient luminescence characteristics at room temperature.1,2 Because nanostructure shapes have a great effect on electrical and optical properties,3−5 ZnO nanostructures of various shapes have been synthesized by chemical vapor deposition (CVD) processes. For example, ZnO nanowires,6 nanobelts,7−9 nanocoils,10 nanorings,8,9 and nanocombs11 have been reported to be grown by CVD. However, their growth mechanism has not been clearly understood because the evolution of such peculiar microstructures is difficult to approach by classical crystallization based on the atomic or molecular unit. In contrast, many microstructural evolutions in ZnO nanostructure growth imply the involvement of electrostatic energy.8−10,12−14 For example, in order to explain the microstructure evolutions of ZnO nanobelts and nanorings, Kong et al. reported the polar-charging model.8,9 They suggested that the electrostatic energy plays a critical role in the growth of ZnO nanorings by a spontaneous self-coiling process of polar nanobelts. Parkansky et al. also reported a method for the growth of ZnO nanorods by applying an electric field to ZnO film at low temperature.15 Similarly, to explain the 1D growth of silicon nanowires without catalytic metal nanoparticles, Cheng and Cheung suggested an electric © XXXX American Chemical Society
Received: July 15, 2015 Revised: September 20, 2015
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DOI: 10.1021/acs.jpcc.5b06796 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. Schematic of the CVD reactor connected to a power supply for applying electric bias.
Considering these, one possibility for the nondeposition of ZnO structures under the abundant generation of charged ZnO nanoparticles would be the drag force. If the drag force is responsible for nondeposition, then it will be revealed by examining the deposition behavior under the electric bias applied to the substrate. Once the bias is applied, then charged particles that could not deposit on a substrate otherwise would be attracted toward the substrate. The purpose of this paper is to test this possibility by examining the bias effect on the deposition behavior in the CVD reactor where charged ZnO nanoparticles are generated in abundance in the gas phase.
charged nanoparticles seems to be very general in the CVD process. It is a great issue whether or not these charged nanoparticles are involved in the growth of thin films and nanostructures. In relation to this issue, Youn et al.29 compared the deposition behavior between electrically floating and grounded substrates. On the floating substrate, silicon nanowires were grown, whereas on the grounded substrate, silicon nanoparticles were grown. Under another deposition condition, a dense silicon film was grown on the floating substrate, whereas a porous film was grown on the grounded substrate. These results indicate that these charged nanoparticles are actively involved in the growth of films and nanostructures at least under certain conditions. A concept similar to the growth of films and nanostructures by nanoparticles formed in the gas phase has been suggested in the plasma-enhanced chemical vapor deposition (PECVD) process by Cabarrocas et al.,30,31 Ostrikov et al.,32−35 and Shiratani et al.36−38 In this process, the building block of nanoparticles can be utilized to synthesize various nanostructures by the plasma-aided nanofabrication technique. Recently, in relation to the growth of films and nanostructures by nanoparticles, nonclassical crystallization, which refers to crystal growth by the building unit of nanoparticles, has been studied extensively.39−45 Nonclassical crystallization was directly observed by in situ transmission electron microscopy in the growth of Pt3Fe nanorods46 and Pt crystals.47 Nanoparticle-based crystal growth has also been observed in many nonmetallic systems such as biominerals42,43 and SnO2.40 Although nonclassical crystallization is a relatively new and revolutionary concept in crystal growth, it has now become so established that related books48,49 have been published, and its tutorial and technical sessions have been included in the spring meetings of the Materials Research Society and the European Materials Research Society, respectively, in 2014. It was often found under the condition where charged nanoparticles were measured to exist in abundance in the gas phase that the deposition of films or nanostructures did not occur on the substrate in some locations of the CVD reactor. This aspect was also observed during the synthesis of ZnO nanostructures by the carbothermal reduction process. In comparison with the deposition of atoms or molecules, at least two additional interactions must be considered in the deposition of charged nanoparticles: electrostatic energy and drag force. Youn et al.29 clearly showed the effect of electrostatic energy by comparing the deposition behavior between floating and grounded substrates. They observed that large nanoparticles did not contribute to the growth of nanowires on the floating substrate. They suggested the possibility that such nondeposition of large nanoparticles is attributed to the drag force which increases with increasing size of nanoparticles.
2. EXPERIMENTAL SECTION ZnO nanoparticles and tetrapod nanowires were grown by using a typical carbothermal reduction process, where a powder mixture of ZnO and graphite is used.50−52 The role of graphite is to reduce ZnO and thereby to produce Zn, which vaporizes and reacts with oxygen to produce the ZnO vapor. From this vapor source of ZnO, various nanostructures can be synthesized at relatively low temperature less than ∼1000 °C.50−52 The source materials of 4 g of ZnO powder (99.9%; Sigma-Aldrich) and 0.8 g of graphite powder (99.99%; Sigma-Aldrich) were loaded on an alumina boat that was placed at the center of the highest temperature zone in a quartz tube reactor with the inner diameter and length of the tube being 5 and 100 cm, respectively. Nitrogen was supplied to the inlet of the reactor as a carrier gas at a flow rate of 500 standard cubic centimeter per minute (sccm). To minimize the contact of the oxygen gas with the powder mixture of ZnO and graphite, the oxygen gas was supplied at a flow rate of 500 sccm through a small quartz tube of 4 mm in inner diameter, which was placed 2 cm away from the powder mixture toward the outlet of the reactor tube. It took 60 min to heat the reactor for the maximum temperature zone to reach 1000 °C, which was maintained for another 60 min for the synthesis of ZnO nanostructures. To minimize the effects of thermal energy such as the Brownian motion of particles and the condensation of zinc, a silicon wafer substrate was placed at the zone of 450 °C. As shown in Figure 1, the substrate was on a stainless-steel holder that was connected to a power supply for applying electric bias. The bias was applied to the substrate holder with respect to the grounded plate, which is 1 cm away from the holder. Therefore, the bias produced an electric field between the holder and the grounded plate. The nanostructures deposited on the substrate were examined via field-emission scanning electron microscopy (FE-SEM, Hitachi SU70). To confirm the generation of charged ZnO nanoparticles in the gas phase during the CVD process, a nano-DMA (TSI model 3085) combined with a Faraday cup electrometer (FCE) was used. For measurements by the DMA, the substrate holder and the grounded plate were taken out of the reactor, and a small quartz tube with an inner diameter of 4 mm was placed in the outlet of the reactor at a distance of 11 cm away from the B
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The Journal of Physical Chemistry C source materials (800 °C) and was connected to the DMAFCE system at the outside of the reactor. The DMA-FCE system could not only confirm the generation of charged particles in the gas phase but also measure their size distribution.17,18,23−25,28 Normally, particles are electrically charged by artificial charging for measurements by the DMA. However, we did not use an artificial charger because ZnO nanoparticles were self-charged in the reactor.
Although there exist a large number of charged ZnO nanoparticles in the gas phase of the CVD reactor as shown in Figure 2, they do not land on the substrate placed in the reactor as shown in Figure 3a. One possibility for this nonlanding
3. RESULTS AND DISCUSSION Figure 2 shows the size distribution of ZnO charged nanoparticles measured by the DMA-FCE system. As to the
Figure 3. Deposition behavior of ZnO nanoparticles under the positive biases of (a) 0 V, (b) +100 V, (c) +300 V, and (d) +600 V. The scale bars of panels a−c are each 100 nm, and the scale bar of d is 1 μm. Figure 2. Size distribution of charged ZnO nanoparticles that were generated in the gas phase during the synthesis of ZnO nanostructures.
would be the drag force that is influenced by the relative velocity of particles in the medium.59 Considering the gas flow dynamics in the reactor, the hot gas that comes from the heated reactor zone would move upward over the colder gas in the unheated zone because of the density difference arising from the temperature gradient. This gas stream would exert a drag force on the nanoparticles that are generated in the hot zone so that they would move upward over the gas of 450 °C zone. To test this possibility, the electric bias was applied to the substrate holder. The result is shown in Figure 3. As previously reported, charged T-ZnOs as well as charged ZnO nanoparticles are generated in the gas phase.18 This fact helps us understand the results of Figure 3. When +100 V was applied, ZnO nanoparticles of ∼10 nm were deposited in not a little amount as shown in Figure 3b. When the bias was increased to +300 V, a slightly larger ZnO nanoparticles of ∼40 nm were deposited in an appreciable amount as shown in Figure 3c. When the bias was increased even further to +600 V, T-ZnOs were deposited as shown in Figure 3d. SEM observation at higher magnification revealed that T-ZnOs were deposited together with smaller ZnO nanoparticles. Figure 3b−d shows that as the bias voltage increased the size of particles that could be deposited increased. This bias experiment shows that the charged ZnO nanoparticles are under the influence of the drag force and explains why charged nanoparticles in Figure 2 did not land on the substrate in Figure 3a. Because the bias was positive, deposited particles would be negatively charged. A drag force exerting on particles by the gas flow is expressed as
charging mechanism of nanoparticles, the dominant mechanism was determined to be the contact charging with the quartz tube wall.53,54 According to our general observation in many CVD systems, the number concentration of both positively and negatively charged nanoparticles was normally similar.17,18,23−25 Under this synthesis condition, however, the number concentration of the negative nanoparticles was about two orders of magnitude higher than that of the positive ones, as shown in Figure 2. This might be due to the fact that p-type defects such as oxygen interstitials, zinc vacancies, and oxygen antisites are generated under the oxygen-rich processing condition and ZnO nanoparticles with p-type defects are more easily negatively charged because the work function of ptype materials is higher than that of the intrinsic and n-type ones.55−57 This possibility is supported by our observation that the number concentration of negative ZnO nanoparticles was similar to that of positive ones when the oxygen gas was not supplied.18 Considering the charge neutrality, an enormous amount of excess positive charges should build up on the quartz wall. It might be suspected that the insulating quartz wall could accommodate such an amount of excess charges. In relation to the possible accommodation of the charge, McCarty et al.58 measured the electrostatic charge on individual microspheres of 50−450 μm and found out that the charge on a microsphere was proportional to its surface area, with about 1 elementary charge per 2000 nm2, which corresponds to about 5 × 108 charges per square millimeter. This measurement indicates that the area of our quartz tube is large enough to accommodate the expected excess charges.
FD = 1/2C DAρv 2 C
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The Journal of Physical Chemistry C where FD = drag force, CD = drag coefficient, A = crosssectional area perpendicular to the flow, ρ = density of the medium, v = velocity of the body relative to the medium. This equation reveals that particles with a large area have a larger drag force and thereby a larger resistance to move toward the substrate than those with a smaller area. If particles have the same shape, then the area of particles would be proportional to the square of the size. Therefore, in order to deposit large particles, a high bias voltage is needed because of the drag force arising from the upward gas stream caused by the temperature gradient. Nondeposition in Figure 3a can be understood on the basis of the drag force: Under zero bias voltage, ZnO particles generated in the gas phase just flew out with the carrier gas because the drag force hindered the particles from moving toward the substrate. Also, under the bias of +100 V in Figure 3b, small particles of ∼10 nm were deposited, but large particles flew out with the carrier gas. Because the drag force for small charged particles is weak, only a small electric force is needed to overcome the drag force of the small particles, which results in the deposition of small charged particles on the substrate. Under the bias of +300 V in Figure 3c, slightly larger particles of ∼40 nm were deposited with much larger particles being flowed out with the carrier gas. Under the bias of +600 V in Figure 3d, however, even larger tetrapod particles with the leg length of ∼600 nm could be deposited on a substrate, indicating that the electric force is high enough to overcome the drag force. The gravity of these nanosized particles appeared to be much smaller than the electric force, and the gravity would be ignored. When the negative bias was applied, a similar result was obtained, but the amount of deposited particles was much less than that of Figure 3 because the number concentration of positive particles was much less than that of the negative ones as shown in Figure 4, which is consistent with Figure 2. When the alternative bias of 600 V was applied, the deposition behavior was affected by bias frequencies as shown in Figure 5a−d. For frequencies of 0.2 and 1 Hz (Figure 5a,b),
Figure 5. Deposition behavior of ZnO nanoparticles under an alternative bias fixing the bias strength at 600 V with the various bias frequencies of (a) 0.2 Hz, (b) 1 Hz, (c) 3 Hz, and (d) 5 Hz. The scale bar in each panel is 1 μm.
an appreciable amount of T-ZnOs was deposited similar to the deposition behavior for +600 V bias (Figure 3d). However, the amount of T-ZnOs was markedly decreased for 3 Hz (Figure 5c), and T-ZnOs were hardly deposited for 5 Hz (Figure 5d). This dependence of deposition behavior on the bias frequency might be related with the distance traveled by charged T-ZnOs until the bias was reversed. More specifically, the distance traveled by charged T-ZnOs during 5 s for 0.2 Hz or 1 s for 1 Hz is expected to be longer than the distance (1 cm) between the two electrodes, which results in deposition of T-ZnOs. In contrast, the distance traveled by charged T-ZnOs during 0.33 s for 3 Hz or 0.2 s for 5 Hz is expected to be shorter than ∼1 cm, which results in nondeposition. In this case, charged T-ZnOs would fluctuate above the substrate and flow out eventually with the carrier gas. Thus, nondeposition under the alternating bias of high frequency is attributed to the drag force that hindered the movement of the T-ZnOs. When the size of graphite powder was changed from less than ∼45 μm to less than ∼20 μm, very large T-ZnO was deposited on the substrate even without any bias, as shown in Figure 6a. In this case, the size of T-ZnO is so large that the gravity seems to outweigh the drag force arising from the upward gas stream. The formation of such a large T-ZnO might
Figure 4. Deposition behavior of ZnO nanoparticles under the negative biases of (a) 0 V, (b) −100 V, (c) −300 V, and (d) −600 V. The scale bars of panels a−c are each 100 nm, and the scale bar of d is 1 μm.
Figure 6. Deposition behavior under the positive biases of (a) 0 V and (b) +600 V during the synthesis of very large T-ZnO. The scale bar in each panel is 1 μm. D
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be attributed to the increase of the Zn flux due to the increased reduction rate achieved by decreasing the size of the graphite powder. When +600 V was applied to the substrate, smaller TZnOs were also deposited, as shown in Figure 6b. These smaller T-ZnOs could not be deposited under 0 V bias condition (Figure 6a) because the drag force would outweigh the gravity. From some SEM images such as those in Figures 4d, 5c, and 6a, the T-ZnOs seem to be standing, which implies that the TZnOs may be grown directly from the substrate rather than falling from the vapor phase. However, without applying the bias, neither these T-ZnOs nor nanoparticles were deposited. There might be a possibility that applying the bias should enhance the nucleation and growth of T-ZnOs from the substrate. Previously, however, Park et al.18 observed T-ZnOs captured by the DMA. With processing time, the size of TZnOs did not increase, but only the number density of T-ZnOs increased, indicating that T-ZnOs are formed in the gas phase instead of growing directly from the substrate. To make it clear that the T-ZnOs shown in Figures 4−6 are formed in the gas phase, we captured T-ZnOs in the exhaust gas as shown in Figure 7. Figure 7a,b, respectively, shows the T-
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This work was supported by the Brain Korea (BK 21 plus) program at Seoul National University (F15SN02D1702) and Pusan National University (21A20131900005) and also supported by the Global Frontier Program (2013M3A6B1078874) through the Global Frontier Hybrid Interface Materials(GFHIM) of National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, & Future Planning.
(1) Nickel, N. H., Terukov, E., Eds. Zinc Oxide - A Material for Microand Optoelectronic Applications; Springer: Dordrecht, The Netherlands, 2005. (2) Morkoç, H.; Ö zgür, Ü . Zinc Oxide: Fundamentals, Materials and Device Technology; Wiley-VCH: Weinheim, Germany, 2009. (3) Xu, C.; Sun, X.; Chen, B.; Shum, P.; Li, S.; Hu, X. Nanostructural Zinc Oxide and Its Electrical and Optical Properties. J. Appl. Phys. 2004, 95, 661−666. (4) Wang, Z. L. Zinc Oxide Nanostructures: Growth, Properties and Applications. J. Phys.: Condens. Matter 2004, 16, R829−R858. (5) Djurišić, A. B.; Leung, Y. H. Optical Properties of ZnO Nanostructures. Small 2006, 2, 944−961. (6) Yao, B. D.; Chan, Y. F.; Wang, N. Formation of ZnO Nanostructures by a Simple Way of Thermal Evaporation. Appl. Phys. Lett. 2002, 81, 757−759. (7) Ago, H.; Ohshima, S.; Uchida, K.; Yumura, M. Gas-Phase Synthesis of Single-Wall Carbon Nanotubes from Colloidal Solution of Metal Nanoparticles. J. Phys. Chem. B 2001, 105, 10453−10456. (8) Kong, X. Y.; Wang, Z. L. Polar-Surface Dominated ZnO Nanobelts and the Electrostatic Energy Induced Nanohelixes, Nanosprings, and Nanospirals. Appl. Phys. Lett. 2004, 84, 975−977. (9) Kong, X. Y.; Ding, Y.; Yang, R.; Wang, Z. L. Single-Crystal Nanorings Formed by Epitaxial Self-Coiling of Polar Nanobelts. Science 2004, 303, 1348−1351. (10) Korgel, B. A. Self-Assembled Nanocoils. Science 2004, 303, 1308−1309. (11) Wang, Z. L.; Kong, X. Y.; Zuo, J. M. Induced Growth of Asymmetric Nanocantilever Arrays on Polar Surfaces. Phys. Rev. Lett. 2003, 91, 185502. (12) Greene, L. E.; Law, M.; Tan, D. H.; Montano, M.; Goldberger, J.; Somorjai, G.; Yang, P. General Route to Vertical ZnO Nanowire Arrays Using Textured ZnO Seeds. Nano Lett. 2005, 5, 1231−1236. (13) Liu, J.; Lee, S.; Lee, K.; Ahn, Y.; Park, J. Y.; Koh, K. H. Bending and Bundling of Metal-Free Vertically Aligned ZnO Nanowires Due to Electrostatic Interaction. Nanotechnology 2008, 19, 185607. (14) Vayssieres, L. Growth of Arrayed Nanorods and Nanowires of ZnO from Aqueous Solutions. Adv. Mater. 2003, 15, 464−466. (15) Parkansky, N.; Shalev, G.; Alterkop, B.; Goldsmith, S.; Boxman, R. L.; Barkay, Z.; Glikman, L.; Wulff, H.; Quaas, M. Growth of ZnO Nanorods by Air Annealing of ZnO Films with an Applied Electric Field. Surf. Coat. Technol. 2006, 201, 2844−2848. (16) Cheng, S. W.; Cheung, H. F. Role of Electric Field on Formation of Silicon Nanowires. J. Appl. Phys. 2003, 94, 1190−1194. (17) Kim, C. S.; Chung, Y. B.; Youn, W. K.; Hwang, N. M. Generation of Charged Nanoparticles During Synthesis of ZnO
Figure 7. (a) SEM and (b) TEM images of T-ZnOs captured in the exhaust gas. The inset of b is the diffraction pattern showing the single crystalline ZnO. The scale bars in a and b are 1 μm and 200 nm, respectively.
ZnOs captured on a quartz substrate and on a TEM holeycarbon membrane in the exhaust gas. Because the substrate in Figure 7 was at room temperature, the T-ZnOs cannot grow on the substrate. This result is consistent with the previous work that T-ZnOs are formed in the gas phase.18 The charged nanoparticles measured in Figure 2 are not visible to the naked eye because they are much smaller than the wavelength for visible light (380−740 nm). Their generation in the CVD process would not have been noticed if they had not been measured by nano-DMA or if the bias had not been applied. To understand the deposition in the CVD process, great attention must be paid for the generation of invisible charged nanoparticles in the gas phase of the reactor.
4. CONCLUSIONS Under the synthesis condition of ZnO nanostructures by carbothermal reduction, charged ZnO nanoparticles including T-ZnOs are generated in the gas phase. These nanoparticles are shown to be subjected to the drag force through the bias effect on their deposition behavior. As the bias voltage was increased, the size of nanoparticles that could be deposited was increased. When the size of ZnO particles was sufficiently large, they could be deposited on a substrate even without a bias, which is attributed to the gravity outweighing the drag force. E
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Article
The Journal of Physical Chemistry C Nanowires by Carbothermal Reduction. Aerosol Sci. Technol. 2009, 43, 120−125. (18) Park, S. H.; Park, J. W.; Kim, C. S.; Hwang, N. M. Formation of Tetrapod-Shaped Nanowires in the Gas Phase During the Synthesis of ZnO Nanostructures by Carbothermal Reduction. J. Nanosci. Nanotechnol. 2013, 13, 7198−7201. (19) Jeon, I. D.; Park, C. J.; Kim, D. Y.; Hwang, N. M. Experimental Confirmation of Charged Carbon Clusters in the Hot Filament Diamond Reactor. J. Cryst. Growth 2000, 213, 79−82. (20) Jeon, I. D.; Park, C. J.; Kim, D. Y.; Hwang, N. M. Effect of Methane Concentration on Size of Charged Clusters in the Hot Filament Diamond CVD Process. J. Cryst. Growth 2001, 223, 6−14. (21) Ahn, H. S.; Park, H. M.; Kim, D. Y.; Hwang, N. M. Observation of Carbon Clusters of a Few Nanometers in the Oxyacetylene Diamond CVD Process. J. Cryst. Growth 2002, 234, 399−403. (22) Lee, J. I.; Hwang, N. M. Generation of Negative-Charge Carriers in the Gas Phase and Their Contribution to the Growth of Carbon Nanotubes during Hot-Filament Chemical Vapor Deposition. Carbon 2008, 46, 1588−1592. (23) Kim, C. S.; Chung, Y. B.; Youn, W. K.; Hwang, N. M. Generation of Charged Nanoparticles during the Synthesis of Carbon Nanotubes by Chemical Vapor Deposition. Carbon 2009, 47, 2511− 2518. (24) Kim, C. S.; Kwak, I. J.; Choi, K. J.; Park, J. G.; Hwang, N. M. Generation of Charged Nanoparticles during the Synthesis of Silicon Nanowires by Chemical Vapor Deposition. J. Phys. Chem. C 2010, 114, 3390−3395. (25) Kim, C. S.; Youn, W. K.; Hwang, N. M. Generation of Charged Nanoparticles and Their Deposition during the Synthesis of Silicon Thin Films by Chemical Vapor Deposition. J. Appl. Phys. 2010, 108, 014313. (26) Youn, W. K.; Kim, C. S.; Lee, J. Y.; Lee, S. S.; Hwang, N. M. Generation of Charged Nanoparticles and Their Deposition Behavior under Alternating Electric Bias during Chemical Vapor Deposition of Silicon. J. Phys. Chem. C 2012, 116, 25157−25163. (27) Hong, J. S.; Kim, C. S.; Yoo, S. W.; Park, S. H.; Hwang, N. M.; Choi, H. M.; Kim, D. B.; Kim, T. S. In-Situ Measurements of Charged Nanoparticles Generated during Hot Wire Chemical Vapor Deposition of Silicon Using Particle Beam Mass Spectrometer. Aerosol Sci. Technol. 2013, 47, 46−51. (28) Lee, S. S.; Kim, C. S.; Hwang, N. M. Generation of Charged Nanoparticles during the Synthesis of GaN Nanostructures by Atmospheric-Pressure Chemical Vapor Deposition. Aerosol Sci. Technol. 2012, 46, 1100−1108. (29) Youn, W. K.; Lee, S. S.; Lee, J. Y.; Kim, C. S.; Hwang, N. M.; Iijima, S. Comparison of the Deposition Behavior of Charged Silicon Nanoparticles between Floating and Grounded Substrates. J. Phys. Chem. C 2014, 118, 11946−11953. (30) Roca i Cabarrocas, P. Plasma Enhanced Chemical Vapor Deposition of Amorphous, Polymorphous and Microcrystalline Silicon Films. J. Non-Cryst. Solids 2000, 266-269, 31−37. (31) Roca i Cabarrocas, P. Plasma Enhanced Chemical Vapor Deposition of Silicon Thin Films for Large Area Electronics. Curr. Opin. Solid State Mater. Sci. 2002, 6, 439−444. (32) Vladimirov, S. V.; Ostrikov, K. Dynamic Self-Organization Phenomena in Complex Ionized Gas Systems: New Paradigms and Technological Aspects. Phys. Rep. 2004, 393, 175−380. (33) Ostrikov, K. Colloquium: Reactive Plasmas as a Versatile Nanofabrication Tool. Rev. Mod. Phys. 2005, 77, 489−511. (34) Ostrikov, K.; Murphy, A. B. Plasma-Aided Nanofabrication: Where Is the Cutting Edge? J. Phys. D: Appl. Phys. 2007, 40, 2223− 2241. (35) Ostrikov, K.; Neyts, E. C.; Meyyappan, M. Plasma Nanoscience: from Nano-Solids in Plasmas to Nano-Plasmas in Solids. Adv. Phys. 2013, 62, 113−224. (36) Nunomura, S.; Koga, K.; Shiratani, M.; Watanabe, Y.; Morisada, Y.; Matsuki, N.; Ikeda, S. Fabrication of Nanoparticle Composite Porous Films Having Ultralow Dielectric Constant Jpn. J. Appl. Phys. 2005, 44, L1509.
(37) Shiratani, M.; Maeda, S.; Koga, K.; Watanabe, Y. Effects of Gas Temperature Gradient, Pulse Discharge Modulation, and Hydrogen Dilution on Particle Growth in Silane RF Discharges. Jpn. J. Appl. Phys. 2000, 39, 287−293. (38) Shiratani, M.; Fukuzawa, T.; Watanabe, Y. Particle Growth Kinetics in Silane RF Discharges. Jpn. J. Appl. Phys. 1999, 38, 4542− 4549. (39) Alivisatos, A. P. Naturally Aligned Nanocrystals. Science 2000, 289, 736−737. (40) Leite, E. R.; Giraldi, T. R.; Pontes, F. M.; Longo, E.; Beltrán, A.; Andrés, J. Crystal Growth in Colloidal Tin Oxide Nanocrystals Induced by Coalescence at Room Temperature. Appl. Phys. Lett. 2003, 83, 1566−1568. (41) Zhang, Z.; Sun, H.; Shao, X.; Li, D.; Yu, H.; Han, M. ThreeDimensionally Oriented Aggregation of a Few Hundred Nanoparticles into Monocrystalline Architectures. Adv. Mater. 2005, 17, 42−47. (42) Niederberger, M.; Cölfen, H. Oriented Attachment and Mesocrystals: Non-Classical Crystallization Mechanisms based on Nanoparticle Assembly. Phys. Chem. Chem. Phys. 2006, 8, 3271−3287. (43) Gebauer, D.; Cölfen, H. Prenucleation Clusters and NonClassical Nucleation. Nano Today 2011, 6, 564−584. (44) Li, D.; Nielsen, M. H.; Lee, J. R. I.; Frandsen, C.; Banfield, J. F.; De Yoreo, J. J. Direction-Specific Interactions Control Crystal Growth by Oriented Attachment. Science 2012, 336, 1014−1018. (45) Teng, H. H. How Ions and Molecules Organize to Form Crystals. Elements 2013, 9, 189−194. (46) Liao, H. G.; Cui, L.; Whitelam, S.; Zheng, H. Real-Time Imaging of Pt3Fe Nanorod Growth in Solution. Science 2012, 336, 1011−1014. (47) Yuk, J. M.; Park, J.; Ercius, P.; Kim, K.; Hellebusch, D. J.; Crommie, M. F.; Lee, J. Y.; Zettl, A.; Alivisatos, A. P. High-Resolution TEM of Colloidal Nanocrystal Growth using Graphene Liquid Cells. Science 2012, 336, 61−64. (48) Cölfen, H.; Antonietti, M. Mesocrystals and Nonclassical Crystallization; John Wiley & Sons: Chichester, U.K., 2008. (49) Leite, E. R.; Ribeiro, C. Crystallization and Growth of Colloidal Nanocrystals; Springer: New York, 2011. (50) Kong, X. H.; Sun, X. M.; Li, Y. D. Synthesis of ZnO Nanobelts by Carbothermal Reduction and Their Photoluminescence Properties. Chem. Lett. 2003, 32, 546−547. (51) Wang, J.; Sha, J.; Yang, Q.; Ma, X.; Zhang, H.; Yu, J.; Yang, D. Carbon-Assisted Synthesis of Aligned ZnO Nanowires. Mater. Lett. 2005, 59, 2710−2714. (52) Park, J. H.; Park, J. G. Hierarchical Evolution of Arrayed Nanowires, Nanorods, and Nanosheets in ZnO. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 43−46. (53) Hwang, N. M.; Kim, D. Y. Charged Clusters in Thin Film Growth. Int. Mater. Rev. 2004, 49, 171−190. (54) Hwang, N. M.; Lee, D. K. Charged Nanoparticles in Thin Film and Nanostructure Growth by Chemical Vapour Deposition. J. Phys. D: Appl. Phys. 2010, 43, 483001. (55) Tuomisto, F.; Ranki, V.; Saarinen, K.; Look, D. C. Evidence of the Zn Vacancy Acting as the Dominant Acceptor in n-type ZnO. Phys. Rev. Lett. 2003, 91, 205502. (56) Janotti, A.; Van de Walle, C. G. Native Point Defects in ZnO. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 165202. (57) Liu, Y. Y.; Jin, H. J.; Park, C. B.; Hoang, G. C. Analysis of Photoluminescence for N-doped and Undoped P-type ZnO Thin Films Fabricated by RF Magnetron Sputtering Method. Trans. EEM 2009, 10, 24−27. (58) McCarty, L. S.; Winkleman, A.; Whitesides, G. M. Ionic Electrets: Electrostatic Charging of Surfaces by Transferring Mobile Ions upon Contact. J. Am. Chem. Soc. 2007, 129, 4075−4088. (59) Davies, C. N., Ed. Aerosol Science; Academic: New York, 1966.
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DOI: 10.1021/acs.jpcc.5b06796 J. Phys. Chem. C XXXX, XXX, XXX−XXX
