Comparison of the Deposition Behavior of Charged Silicon

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Comparison of the Deposition Behavior of Charged Silicon Nanoparticles between Floating and Grounded Substrates Woong-Kyu Youn,† Sung-Soo Lee,† Jae-Young Lee,† Chan-Soo Kim,‡ Nong-Moon Hwang,*,† and Sumio Iijima*,§ †

Department of Materials Science and Engineering, Seoul National University, Daehak-dong, Gwanak-gu, Seoul 151-744, Republic of Korea ‡ Marine Energy Convergence & Integration Laboratory, Korea Institute of Energy Research, Gimnyeong-ri, Gujwa-eup, Jeju-si, Jeju-do, 695-971, Republic of Korea § Meijo University, Faculty of Science and Technology (University Professor of Nagoya University, NEC Senior Research Fellow & Director, AIST/Nanotube Research Center) 1-501, Shiogamaguchi, Tenpaku, Nagoya, Aichi 468-8502, Japan ABSTRACT: Nanoparticle-based crystallization has become an important issue since the direct observation of the growth of nanorods or crystals from nanoparticles as the building block in a liquid cell by transmission electron microscopy. The growth of crystals by the fusion or coalescence of nanoparticles requires an unusually high rate of atomic diffusion. Evidences indicate that such a high rate of diffusion may be realized from the electric charge carried by the nanoparticles. In this work, the effect of nanoparticle charge on nanoparticlebased crystallization was studied by comparing the deposition behavior of charged silicon nanoparticles between electrically floating and grounded silicon substrates. Under the same processing condition, nanowires were grown on the floating substrate and nanoparticles were grown on the grounded substrate, or a dense film was grown on the floating substrate and a porous film was grown on the grounded substrate.

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and SnO2.3 Although nonclassical crystallization is a relatively new and revolutionary concept in crystal growth, it has now become so established that related books11,12 have been published and a new session about it has been included in the spring meeting of the European Materials Research Society 2014. Therefore, particle-based crystallization is considered general in the nanostructure evolution of various materials, including Si. Although these studies on nonclassical crystallization deal with crystal growth in solutions, Hwang et al.13−28 extensively studied the nonclassical crystallization of thin films and nanostructures in gas phase synthesis such as chemical vapor deposition (CVD). They emphasized that the charge of nanoparticles plays a critical role in the evolution of dense films.24 They claimed that many thin films and nanostructures that have been believed to have grown by individual atoms or molecules by CVD actually grow by charged nanoparticles (CNPs) that are generated in the gas phase of the CVD reactor. Ostrikov29 and Cabarrocas30 also suggested the possibility of incorporating nanoparticles into films or nanostructures in the plasma-enhanced CVD process. Furthermore, electrically floating, grounded, and biased surfaces are common in plasma-based deposition because of the large amounts of

INTRODUCTION Nonclassical crystallization, which refers to crystal growth by the building unit of nanoparticles, in contrast to classical crystallization where individual atoms or molecules are the building unit, has been studied extensively.1−7 Recently, crystal growth by nanoparticles in solution was directly observed by transmission electron microscopy (TEM).8 Using a silicon nitride liquid cell for in situ TEM observation, Liao et al.9 carried out detailed real-time imaging to show how Pt3Fe nanorods grow by nanoparticles in solution. First, winding polycrystalline nanoparticle chains were formed by shape-directed nanoparticle attachment. These nanochains were straightened and then reoriented. Therefore, even when nanoparticles attach without orientation, single-crystalline nanorods are formed eventually. By in situ TEM observation using graphene liquid cells, Yuk et al.10 carried out direct atomic-resolution imaging to show how Pt crystals grow in solution. The imaging clearly revealed that Pt nanocrystals grow not only by monomers but also by the coalescence of nanoparticles. Here, the kinetics is most notable. The neck growth of nanoparticles after coalescence occurs in less than several seconds. Spherical nanoparticles before coalescence change to faceted hexagonal nanoparticles after coalescence in less than 60 s. Nanoparticle-based crystal growth in metals has been observed directly by in situ TEM, and it has also been observed in many nonmetallic systems such as biominerals2,5 © 2014 American Chemical Society

Received: January 6, 2014 Revised: April 12, 2014 Published: May 15, 2014 11946

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electrons, ions, and CNPs that are generated in the plasmaenhanced CVD process.31,32 Although the charging behavior of nanoparticles is different between plasma-enhanced and thermal CVD processes, both positively and negatively CNPs are generated in these processes. In the case of the DC plasma CVD process for producing diamond, a positively biased substrate produces diamond, whereas a negatively biased substrate produces an amorphous or graphite phase.33 Homan et al.34 studied the difference between positively and negatively charged carbon particles by time-of-flight measurements. Based on these results, Hwang and Kim20 suggested that negatively charged carbon nanoparticles have a diamond structure and positively CNPs have an amorphous structure with a hydrogenated surface. Such a difference in structure between positively and negatively CNPs seems to be pronounced only when the size of the nanoparticles is sufficiently small. When the size is larger than a few nanometers, the structure does not seem to depend on the sign of the charge. In many CVD processes, however, there can be a big difference in the number density between positively and negatively CNPs. In the hot filament CVD process for producing diamond, almost all nanoparticles were negatively charged.17,19 In the thermal CVD process for producing carbon, ZnO and GaN, the number density of negatively CNPs is often more than 10 times that of positively CNPs. In many CVD processes, however, CNPs have been neglected because of their size being much smaller than the wavelength of visible light. Crystallization by the building unit of CNPs implies that CNPs should have a somewhat liquidlike property, which in turn implies that charge should enhance the atomic diffusion. In relation to the possibility that the atomic diffusion can be enhanced by charge, Iijima and Ichihashi35 showed by highspeed videotaped recording that nanoparticles become quasisolid during TEM observation. TEM observation showed that clusters of ∼460 gold atoms fluctuate and continue to change their structure at a fraction of a second. They further observed that the fluctuation and structural change become sluggish or even stop when conducting substrates are used and also the illuminating electron beam intensity is decreased. The dependence of the fluctuation rate on the conductivity of the substrate implies that the enhancement of the atomic diffusion is not due to the electron beam heating effect but due to the electric charging from ionization. Iijima et al.’s various TEM observations36−39 imply that charging should enhance the atomic diffusion of nanoparticles. Although charge appears to play a critical role in nanoparticle-based crystallization, its effect has not been systematically studied. The purpose of this paper is to examine the role of charge in nanoparticles-based crystallization. For this, under the process condition where charged silicon nanoparticles were generated in the CVD process, the deposition behavior of silicon was compared between electrically floating and grounded silicon substrates, which had maximum and minimum accumulation effects of charge, respectively. Silicon nanowires or dense films grew on the floating substrate whereas nanoparticles or porous films grew on the grounded substrate. These results indicate that charge plays a critical role in the growth of nanowires and films by nanoparticles.

an in situ differential mobility analyzer (DMA, TSI model 3081) system40 combined with a Faraday cup electrometer (FCE), designed to measure sizes in the range of 10−1000 nm, was connected to the CVD reactor. The amount of CNPs, which were size-classified by DMA, was measured as a current on the FCE. Our measurement system did not use the charging system because nanoparticles are self-charged in the quartz reactor of the CVD system22,23,25−27 The number concentration and size distribution of the CNPs were measured during the syntheses of silicon nanowires and films, respectively, at N2 gas flow rates of 500 and 1000 sccm. Electrically neutral nanoparticles were expected to exist in the CVD reactor; however, their fraction was unknown in the present experiment. The nanoparticles were sampled just above the substrate. Silicon nanowires and films were deposited on a silicon substrate without a catalyst for 2 h using a typical atmospheric pressure CVD process at a deposition temperature of 900 °C. Silane (10%) (SiH4 diluted in helium (He)), hydrogen (H2), and nitrogen (N2) were used to synthesize silicon. The flow rates of helium-diluted silane and hydrogen (99.9999%) were fixed at 5 standard cubic centimeters per minute (sccm) and 50 sccm, respectively, whereas two nitrogen flow rates (99.9999%) of 500 and 1000 sccm were used. The silicon substrate of 10 × 10 × 1 mm3 was placed at the center zone of the quartz tube. The deposition behavior of silicon was compared between the electrically floating and grounded substrates on a stainless substrate holder plate of 12 × 12 × 6 mm3. To ground the silicon substrate, the stainless holder was connected to the external ground via a stainless rod. The surface morphologies and cross sections of the nanowires and films were observed by field-emission scanning electron microscopy (FESEM, JSM7500F). The microstructure of the nanowires was characterized by high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) in the high-resolution mode (FEI, Tecnai-F20).

EXPERIMENTAL PROCEDURES To detect the CNPs generated in the gas phase during the CVD process, where silicon nanowires and films were grown,

Figure 1. Number concentrations and size distributions of negatively (open) and positively (closed) CNPs at reactor temperature of 900 °C with N2 flow rates of 500 and 1000 sccm, respectively, at a SiH4 flow rate of 5 sccm and H2 flow rate of 50 sccm.

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RESULTS Figure 1 shows the number concentrations and size distributions of negatively and positively charged silicon nanoparticles, which were measured by the DMA−FCE system, at N2 gas flow rates of 500 and 1000 sccm. The number

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For the TEM observation of the silicon nanowires in Figure 2b, the sample was immersed in ethanol for ultrasonic treatment. The suspended nanowires, which had been captured on a TEM copper grid membrane of holey carbon, were observed by high-resolution transmission electron microscopy (HRTEM) and HAADF−STEM in the high-resolution mode. Figure 3a shows a TEM image of a silicon nanowire. Figure 3b

concentrations and size distributions of negative and positive nanoparticles at N2 gas flow rate of 1000 sccm are larger and broader, respectively, than those at 500 sccm. At 1000 sccm of N2, the number concentrations of negative and positive nanoparticles are almost the same. At 500 sccm of N2, however, the number concentration of negative nanoparticles is larger, below 80 nm, and smaller, above 80 nm, than that of positive nanoparticles. When the N2 gas flow rate is 500 sccm, CNPs start to be measured at ∼20 nm. After confirming the generation of CNPs, we compared the deposition behavior of silicon between the floating and grounded silicon substrates. Figures 2a and b show FESEM

Figure 2. FESEM images: (a) low-magnification and (b) highmagnification images of a floating silicon substrate and (c) lowmagnification and (d) high-magnification images of a grounded silicon substrate at a N2 flow rate of 500 sccm. Figure 3. TEM images of silicon nanowires deposited on a floating silicon substrate at a N2 flow rate of 500 sccm: (a) low-magnification TEM image, (b) high-magnification TEM image, with the inset of an enlarged HRTEM image of the square-enclosed area, (c) dark-field HAADF image for elemental analysis, (d) EDX analysis of the squareenclosed area marked in (c), and (e) line elemental profiles of Si and O in (c).

images of lower and higher magnifications, respectively, for the 2 h deposited surface microstructure of the floating substrate without a catalyst at the substrate temperature of 900 °C with gas flow rates of 5 sccm 10% SiH4−90% He, 50 sccm H2, and 500 sccm N2. Figures 2c and d show FESEM images of lower and higher magnifications, respectively, for the surface microstructure of the grounded substrate with the deposition conditions being the same as those of Figures 2a and b. Silicon nanowires grew extensively on the floating substrate as shown in Figures 2a and b, whereas no silicon nanowire grew but only silicon nanoparticles were deposited on the grounded substrate as shown in Figures 2c and d. The nanowires in Figure 2b had diameters of about 10−30 nm. Since neither catalytic metal nor seed of silicon oxide was used in our experiments, the growth of silicon nanowires in Figure 2b cannot be explained by the vapor−liquid−solid (VLS)41 or oxide-assisted growth (OAG)42 mechanism. Charge buildup would be maximized on the floating substrate and minimized on the grounded substrate. Since the only difference in the processing condition between Figures 2b and d was the floating and grounding of the substrate, charge buildup should be responsible for the growth of silicon nanowires in Figures 2a and b. The results indicate that the electrostatic interaction between the CNPs and the substrate was responsible for the growth of the silicon nanowires.

shows an enlarged image of the box marked in the central part of Figure 3a. The nanowire has a core−shell structure with an inner crystalline silicon core and an outer amorphous oxide (SiOx) layer. The diameter of the silicon core is 5 nm, and the thickness of the outer oxide layer is 7 nm. The inner silicon core in Figure 3b has the same lattice fringe, indicating that it is a single crystal with ⟨220⟩ growth direction. The inset in the upper right corner of Figure 3b shows the image of the lattice, which is magnified in the box marked in the lower right. The nanowires observed by TEM were single crystalline. The spacing between the parallel lattice fringes was measured to be 0.19 nm, which is close to the spacing of the (220) planes of silicon. The same lattice fringe as that shown in the inset of Figure 3b was observed along each nanowire. Based on this consistent observation, we concluded that each nanowire was a single crystal with the ⟨220⟩ growth direction. Figure 3c shows the dark-field HAADF image, and Figure 3d shows the energy-dispersive X-ray (EDX) analysis of the box 11948

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marked in Figure 3c. The EDX analysis shows that the nanowires consisted of Si, O, and Cu, which would have come respectively from the silicon core, the outer oxide layer, and the copper mesh of the TEM grid. Figure 3e shows the line profile for the elemental analysis, which was scanned along the line marked in Figure 3c. The line profile shows that the Si peak is strong and the O peak is weak. The oxygen peak came from the outer oxide layer. In order to examine the crystallinity of the nanoparticles formed in the gas phase, nanoparticles were captured on the carbon membrane of the Cu TEM grid for 30 s in the CVD reactor. Although the temperature at the hot zone of the CVD reactor was 900 °C, nanoparticles were captured in the zone at 500 °C to minimize the damage to the membrane of the TEM grid. Nanoparticles were also captured for 2 h on the carbon membrane of the TEM grid in the Faraday cup after the DMA. The TEM images are shown in Figure 4.

Figure 5. FESEM images: (a) plane view and (b) cross section of films deposited on a floating silicon substrate and (c) plane view and (d) cross section of films deposited on a grounded silicon substrate at a N2 flow rate of 1000 sccm.

deposited on a grounded substrate. The deposition condition was the same as that of Figure 2 except that the gas flow rate of N2 in Figure 5 was 1000 sccm. The film on the grounded substrate in Figures 5c and d was much more porous than that on the floating substrate in Figures 5a and b. These results show that drastically different microstructures evolved between the floating and grounded substrates, indicating that the dense film in Figures 5a and b resulted from the electrostatic interaction between the CNPs and the growing surface. The film thicknesses of Figures 5b and d were ∼220 and ∼190 nm, respectively, indicating that the growth rate of the film on the floating substrate was higher than that on a grounded substrate. Besides, the film in Figure 5b is much denser than that in Figure 5d. Therefore, the growth rate of the film on the floating substrate in Figure 5b is much higher than that on the grounded substrate in Figure 5d.

Figure 4. TEM images of silicon nanoparticles captured on the carbon membrane of the TEM grid in the CVD reactor at 500 °C for 30 s (a, b) or in the Faraday cup after DMA at room temperature for 2 h (c, d): (a) low-magnification TEM image, (b) high-magnification TEM image, with the inset of an enlarged HRTEM image of the squareenclosed area, (c) low-magnification TEM image, and (d) enlarged HRTEM image of the square-enclosed area in (c).

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DISCUSSION If we compare the size distribution of CNPs shown in Figure 1 with the diameter of nanowires shown in Figures 2 and 3, we find that the diameter of the nanowires is smaller than the average size of CNPs measured by DMA. This difference indicates that nanoparticles larger than several tens of nanometers did not contribute to the growth of the nanowires but only small nanoparticles did. One possibility as to why the larger nanoparticles did not contribute to the growth of the nanowires would be that the drag force imposed on nanoparticles increases for larger nanoparticles. The drag force, which is imposed on a particle in a medium like gas, increases with the gas flow rate and the cross-sectional area of the particle.43 Therefore, if the drag force of nanoparticles is larger than the Coulomb attraction between CNPs and the tips of nanowires, the nanoparticles would not contribute to the growth of nanowires. Then, only small nanoparticles whose drag force is small enough would contribute to the growth of nanowires, whose diameter would be smaller than the average size of nanoparticles measured by DMA.

Figure 4a is the low-magnification image of isolated nanoparticles captured in the reactor on the membrane of the TEM grid. Figure 4b is the magnified image of a nanoparticle in the box marked in the upper right of Figure 4a. The HRTEM image of the lattice is shown in the inset of Figure 4b. The nanoparticles captured for 2 h in the Faraday cup aggregated, as shown in Figure 4c. The HRTEM image of the box marked in the center of Figure 4c is shown in Figure 4d. These results indicate that nanoparticles formed in the gas phase are not amorphous but crystalline. A film deposited by these nanoparticles would have a crystalline structure, which was confirmed by the Raman spectra of the film deposited under the same processing condition.28 Therefore, the silicon shown in Figure 2d would be crystalline. Figures 5a and b show FESEM images of the surface and the cross section of a film, respectively, deposited on a floating silicon substrate; Figures 5c and d show those of a film 11949

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When the flow rate of N2 increased from 500 to 1000 sccm, nanowires did not grow but films grew, as shown in Figure 5. Figure 1 shows that the number density of CNPs is larger at 1000 sccm than at 500 sccm in the overall size range. It is commonly observed that nanoparticles tend to be lost to the quartz tube wall near the position at which the precursor decomposes actively. The amount of nanoparticles lost to the wall was much less at 1000 sccm than at 500 sccm, which might explain the larger number density of CNPs at 1000 sccm than at 500 sccm. Then why did nanowires grow at 500 sccm and films grow at 1000 sccm? In relation to this question, we observed consistently that nanowires tend to grow when nanoparticles are relatively small and their number density is also relatively small.25,27 One possibility to explain this observation would be that the probability of CNPs to land on nanowires in the radial direction increases with increasing number density of nanoparticles, so the tendency for exclusive attachment of CNPs at the tips of nanowires would decrease. It should be noted that the radial direction to the tips of the nanowires has an advantage over the axial direction to the tips in that the former provides a larger area for attachment of CNPs than the latter. Considering these observations, the growth of nanowires can be regarded as a kind of instability phenomenon, where a certain type of growth is accelerated under a suitable condition. Previously, we studied the effect of the N2 flow rate on the microstructure evolution of silicon deposited on the quartz substrate.44 At 300, 500, 700, and 1000 sccm of N2, distinctively different microstructures were evolved. Nanowires attached with nanoparticles, typical nanowires as shown in Figure 2, porous aggregates, and the film shown in Figure 5 were grown respectively at 300, 500, 700, and 1000 sccm. The mass of the deposit on the substrate was so small that it could not be measured by a microbalance. Although the mass change of the deposit for the different N2 flow rates could not be determined quantitatively, the FESEM images indicated that the amount of the deposit was decreased with decreasing flow rate of N2. However, the dependence of the amount of the deposit on the gas flow rate should be understood with caution. The gas flow rate has two different effects in relation to the amount of the deposition. As the gas flow rate decreases, the amount of deposit loss to the tube wall increases. This effect decreases the amount of the deposit on the substrate. However, as the gas flow rate decreases, the drag force also decreases. This effect increases the amount of the deposit. Our previous observation that the amount of the deposit was decreased with decreasing flow rate44 indicates that with decreasing flow rate, the amount of the deposit lost to the tube wall outweighs the amount gained from decreased drag force. The comparison of the silicon deposition behavior between floating and grounded substrates reveals many important aspects of nonclassical crystallization by CNPs. First, it reveals that the silicon nanowires in Figures 2a and b grew by the electrostatic self-assembly of charged silicon nanoparticles. Furthermore, the comparison suggests that charge enhances atomic diffusion because self-assembly alone would produce a “pearl-necklace” aggregate. The growth of the nanowires as a single crystal with a smooth surface indicates not only that each nanoparticle underwent epitaxial growth but also that the atomic diffusion was so much enhanced as to produce the smooth surface of the nanowires. These phenomena are difficult to explain without assuming that the CNPs acted like a “quasi-solid”, whose atomic diffusion was enhanced to a

degree that it almost seemed like a liquid. This enhanced diffusion seemed to have come from the charge carried by the nanoparticles. Second, the comparison indicates that the dense film shown in Figures 5a and b grew also by the electrostatic self-assembly of CNPs. Mere self-assembly would produce a regular array of nanostructures with nanosized voids, considering that the densest packing of monodisperse hard spheres is 74%. Instead, a dense film without voids grew with grain sizes much larger than the individual nanoparticles. This means that most of the nanoparticles underwent epitaxial growth with the atomic diffusion enhanced so much that no voids were left behind. These phenomena strongly imply that the CNPs acted like a quasi-solid. Particle-based crystallization, which is similar to the result observed in Figures 2 and 4, is relatively well-established in a colloidal solution. The so-called colloidal crystallization can yield nanostructures of various morphologies that cannot be grown by classical atom- and molecule-based crystallization. Tang et al.45 reported the self-assembly of nanoparticles into nanowires very similar to those of Figures 2a and b. They observed that monodisperse CdTe nanoparticles in solution were spontaneously reorganized into crystalline nanowires upon controlled removal of the protective shell of the organic stabilizer. Although they occasionally observed pearl-necklace aggregates, they observed nanowires with a smooth surface in the standard dispersions of CdTe. Since then, various nanostructures in solution were reported to grow by selfassembly of nanoparticles.46−49 In most of these reports, the role of charge was neglected. However, based on the observation that the self-assembly was affected by the dielectric constant of the solution, Zhang and Wang50 suggested that the electrostatic energy from CNPs may be involved in the selfassembly. Then, why is the film growth rate on the floating substrate higher than that on the grounded one? This result can be explained by the image force between the CNPs and the growing surface. When two charged conducting spherical particles approach each other, the Coulomb interaction is described by the following equation51 F=

q1q2 4πε0d 2

−

q22r1d 4πε0(d 2 − r12)2

−

q12r2d 4πε0(d 2 − r22)2

+ ... (1)

where the sphere of radius r1 has a net charge q1 and the other of radius r2 has charge q2, d is the distance between the particle centers, and 1/4πεo the permittivity. The first term is the wellknown Coulomb equation, which can be attractive or repulsive depending on the signs of the two interacting particles. The second and third terms come from the image force, which are attractive regardless of the sign of the charge. This equation reveals that if a large spherical particle has a large amount of excess charges, it will attract a small singly charged particle of like sign. This aspect can be analyzed more specifically by inserting numerical values as follows. Writing q1 = n1e and q2 = n2e, where n1 and n2 are integers representing the excess charges per sphere and e is the electron charge, then F=

n1n2e 2 4πε0d

2

−

n22e 2r1d 2

4πε0(d −

r12)2

−

n12e 2r2d 4πε0(d 2 − r22)2

+ ... (2)

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If r1 = 10 nm, r2 = 2 nm, d = 20 nm, 4πεo = 1.11 × 10−10, and e = 1.6 × 10−19, then

will be repelled much further away to the opposite side of the rod and the electrostatic interaction would be attractive. Therefore, once two CNPs are attached to each other to form a short chain in the initial stage, newly incoming CNPs will be attracted more to the axial direction than to the radial direction. This tendency will increase as the chain length increases, producing a chainlike structure. Such a onedimensional chainlike structure was reported.45 The reason why the rod shape instead of a chainlike structure is evolved in many cases would be due to the charge-enhanced diffusion. Since the image force between the charged nanoparticle and the cylindrical rod is much less repulsive or much more strongly attractive in the axial direction than in the radial direction of the rod, the electrostatic interaction between the charged nanoparticle and the rod is highly anisotropic. This anisotropy increases with a longer rod, promoting one-dimensional nanowire growth. Such anisotropic electrostatic interaction would not be induced on the grounded substrate, where charge buildup is minimized. Hwang et al.20,52 and Ostrikov53 suggested that the growth of silicon nanowires in the absence of a metallic catalyst can be explained by the electrostatic interaction of CNPs in thermal CVD and plasma CVD, respectively. Especially in the plasma CVD, relatively lots of charges can be generated which is favorable for one-dimensional growth and other various nanostructures, which cannot be obtained by classical crystallization based on atomic or molecular growth. Ostrikov et al. suggested that the nanoparticles could be utilized to synthesize various nanostructures by the plasma-aided nanofabrication technique29,54 and that the nonequilibrium and kinetic properties of the localized plasma are very effective for the production of metastable nanosolids which are far from the equilibrium state.55

F(newtons) = 5.8 × 10−13n1n2 − 5.1 × 10−13n22 − 0.59 × 10−13n12 + ...

(3)

If a large sphere with r1 carries 10 excess charges and a small sphere with r2 carries a single charge, eq 3 reveals that the large sphere attracts the small sphere even if both carry charges of the same sign. If the incoming particle with r2 is neutral with n2 being zero, the first and second terms vanish but the remaining third term exerts an attractive force. This attraction can be large when n1 is large. The magnitude of attraction decreases in the order of CNPs of opposite signs, neutral nanoparticles, and CNPs of like signs. Based on eq 3, the deposition behaviors of incoming charged and neutral nanoparticles are compared between floating and grounded substrates. Let us assume that the same amount of silicon was deposited initially on both floating and grounded substrates. The initially deposited silicon can be regarded as a very large particle with r1 in eq 3. However, n1 in eq 3 will be much larger on the floating substrate than on the grounded substrate because of the charge buildup on the floating substrate. Therefore, the initially deposited silicon on the floating substrate will attract nanoparticles more strongly than that on the grounded substrate regardless of the sign of the charge carried by the nanoparticles. Therefore, the result of Figure 5, which showed that the floating substrate has a higher growth rate than the grounded substrate, can be explained. A similar logic based on the image force can be applied to explain the growth of nanowires on the floating substrate shown in Figure 2. Consider the electrostatic interaction between an incoming nanoparticle and a rod, both of which are positively charged, as shown schematically in Figures 6a and b.

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CONCLUSION This study demonstrates that the deposition behavior of charged silicon nanoparticles differs dramatically between floating and grounded substrates. These results imply that CNPs are critical in the evolution of films and nanowires, which are made by the self-assembly of CNPs through their electrostatic interaction with the growing surface and by the charge-enhanced atomic diffusion.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +82-2-880-8922. Fax:+822-883-8197. *E-mail: [email protected]. Tel./Fax: +81-(0)52-834-4001. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Brain Korea (BK plus) program, Republic of Korea and the Global Frontier R&D Program (2013-073298) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning.

Figure 6. Schematics for Coulomb interaction between an incoming positively charged nanoparticles and a rod: (a) repulsive force in radial direction and (b) attractive force in axial direction.

When the nanoparticle approaches the rod in the radial direction (Figure 6a), the positive charge in the rod will be repelled to the opposite side of the rod. Since the repelled distance is not much, the electrostatic interaction would be repulsive. When the nanoparticle approaches in the axial direction (Figure 6b), however, the positive charge in the rod

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REFERENCES

(1) Alivisatos, A. P. Naturally Aligned Nanocrystals. Science 2000, 289, 736−737.

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