Rapid, Low-Temperature Growth of Sub-10 nm Silica Nanowires

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Rapid, Low-Temperature Growth of Sub-10-nm Silica Nanowires through Plasma Pretreatment for Antireflection Applications Hong-Yi Lee, Bo-Wen Huang, Yi-Ci Tsai, and Jiann Shieh ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00301 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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ACS Applied Nano Materials

Rapid, Low-Temperature Growth of Sub-10-nm Silica Nanowires through Plasma Pretreatment for Antireflection Applications Hong-Yi Lee, Bo-Wen Huang, Yi-Ci Tsai, Jiann Shieh* Department of Materials Science and Engineering, National United University, Miaoli 36063, Taiwan KEYWORDS: silica, nanowires, nanopillars, active oxidation, plasma, low reflectance

ABSTRACT: Silica nanowires were rapidly grown at the Pt–Si eutectic temperature (830 C) through active oxidation. Because a silica source with a high amount of SiO is required for high solubility in a small droplet, we developed a plasma treatment that was conducted on a platinum thin film prior to nanowire growth to incorporate a high amount of silica source into the droplets. After pretreating the samples with hydrogen and argon plasma, sub-10-nm silica nanowires were successfully grown through 1.8-nm Pt thin films on silicon wafers. This is an effective method to grow thin nanowires on nanopillars with low surface reflection at low temperatures.

INTRODUCTION

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Because silica nanowires have large surface areas, easily modifiable surfaces, and are entirely compatible with Si-based microelectronic processes, they have attracted considerable attention in various applications such as biosensing and optoelectronic devices.13 Nanowires are conveniently grown through the vapor–liquid–solid (VLS) method,4 in which metal droplets catalyze incorporation of reaction gas into nanowire growth. During the VLS method, as the temperature increases above the eutectic temperature, the solid elements that are being alloyed may melt to form liquid droplets. Thus, the subsequent incorporation of precursor enables the precipitation of solid when the droplet is supersaturated. For silicon-based nanowires, a convenient source of silicon is silane gas, which easily disintegrates into silicon and hydrogen at high temperatures. However, silane gas is highly explosive when brought into contact with air and should be handled with appropriately. Recently, a clean and safe process based on active oxidation has been reported.510 In this process, a hazardous reaction gas is not required. Silicon can be obtained from volatile silicon monoxide through active oxidation. Silica will be precipitated instead of silicon because of the extremely low solubility of silica in alloying droplets. However, this process has several limitations. First, the nanowire diameter, which is determined from the droplet size, is usually large because the diameter of the droplets obtained using thin-film dewetting is considerably larger than the film thickness. Moreover, the droplet size increases from ripening because long-term annealing was performed during the nanowire growth. Therefore, the nanowire diameter is usually larger than 50 nm when the droplets were prepared using thin-film dewetting.10,11 To reduce the nanowire diameter, Gurylev et al. prepared small Pt nanoparticles by atomic layer deposition;8 Gómez-Martínez et al. adopted microsphere with curved surface for thin film dewetting.12 Furthermore, the growth temperature of the nanowires is high (usually above 1000 C) because Si is obtained using solid silicon monoxide


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instead of silane gas.6,8,13 By analyzing the phase diagram of the active oxidation process,57 we observed that a low partial oxygen pressure is required for a low growth temperature. When oxygen is deficient during growth, however, the nanowire growth may be terminated because of insufficient supersaturation. Therefore, few studies have reported the growth of silica nanowire with a thin diameter near the eutectic temperature though active oxidation. To fabricate silica nanowires at lower temperature, Chen et al. used oxygen plasma to produce sufficient silicon vapor by etching silicon wafer.14 We reported here a method of using plasma to pretreat the Pt thin film for growing nanowires. This pretreatment is conducted to obtain a sufficient silica source and grow thin nanowires at severe conditions that involve a thin catalyst film and have a low temperature. The results reveal that sub-10-nm silica nanowires can be efficiently obtained by conducting plasma pretreatment on a 1.8-nm-thick metal film at the eutectic temperature of Pt–Si (830 C). We also performed hydrogen fluoride (HF) cleaning and investigated plasma parameters to demonstrate that plasma pretreatment is an efficient method to promote silica nanowire growth through active oxidation. These silica nanowires grown from the nanoparticles further reduced the surface reflection of silicon nanopillars, which have potential widespread applications in fields such as solar cells15,16 and light emitting diodes.17

EXPERIMENTAL SECTION We used a p-type (100) silicon wafer that contained a nanopillar array as a substrate for the nanowire growth. The diameter, spacing, and height of the nanopillars on the silicon wafer were 400, 400, and 1000 nm, respectively. The nanopillars were prepared through i-line lithography and plasma etching16 and were used for the lateral growth of the nanowire. The lateral growth


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contributes to the characterization of the nanowire diameter. The Pt thin film was coated on a silicon substrate by sputtering for 5–60 s. The fabrication systems of nanowires are displayed schematically in Fig. 1. Before growing the nanowires, the substrate was pretreated by inductively coupled plasma (ICP) at a frequency of 13.56 MHz. The ICP power was set at 80 W, and the bias power was maintained at 40 W. The operation pressure was 50 mtorr, and the plasma process was conducted at room temperature for 520 min under 50 and 25 sccm of Ar and H2, respectively. After the plasma treatment, the sample was placed into a tube furnace and heated to 830 C at a heating rate of 0.4 C s−1 under an Ar–H2 mixture (25: 40 sccm) at a pressure of 5 torr. The residual oxygen in ultrahigh purity Ar was 0.66 ppm. As the temperature reached 830 C, the heat was immediately turned off. Subsequently, the sample was removed from the furnace as the temperature was reduced to 500 C at a cooling rate of 0.3C s−1. The thickness of the Pt film was measured using an atomic force microscope (Bruker Dimension Icon). Scanning electron microscopy (SEM) was performed using a JEOL 6700F scanning electron microscope. Transmission electron microscopy (TEM) was performed using JEOL 2100F and Tecnai F20 G2. For conducting TEM, samples were prepared by dispersing the nanowires and nanopillars on a copper grid. X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Theta Probe) was conducted to analyze the chemical bonding of Si and O. Hydrogen bonding on the plasma treated Pt/Si wafer were examined by an attenuated total reflection (ATR) Fourier-transform infrared spectroscopy (FTIR, Bruker Vertex 80v). The total reflection was estimated using a spectrometer with an integrating sphere setup above the samples to detect all the reflective light.


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Figure 1. Schematic representation of the systems for growing the silica nanowires.

RESULTS AND DISCUSSION Figure 2a presents a typical SEM image of the nanowires grown from 10-s Pt at 830 C. Another Si wafer was placed near the substrate to generate more SiO source, which enables the elongation of nanowires for TEM characterization. Fig. 2b shows a TEM image of a single nanowire, and the corresponding energy dispersive X-Ray spectroscopy (EDS) mapping images of Si, O, and Pt, are presented in Figs. 2c2e, respectively. The selected area diffraction (SAD) pattern presented in Fig. 2f shows that the particle was single crystal with a face-centered cubic structure, and was mostly composed by Pt (Fig. S1). The snake-like morphology indicates that the nanowire is not rigid, and the SAD pattern presented in Fig. 2g reveals that the structure of the nanowire is amorphous because the diffraction spots were absent. By integrating EDS mapping and TEM analysis, a cap that was present at the tip of nanowire indicates that the VLS method caused tip-led growth of the nanowires.


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Figure 2. (a) SEM of nanowires grown on flat and nanopillar surface. (b) TEM image of a single nanowire and the corresponding EDS mapping images of (c) silicon, (d) oxygen, (e) platinum elements. (f, g) SAD patterns from (f) nanoparticle and (g) nanowire.

Figure 3a shows a high-angle annular dark-field (HAADF) image of nanowires grown on a nanopillar, which provides the spatial distributions of atoms with various atomic numbers around the nanopillar. Figures 3b3d show the corresponding EDS elemental mapping images of Si, O, and Pt, respectively, revealing that abundant silica nanowires uniformly covered the nanopillar.


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Figure 3. (a) HAADF image of nanowires grown on nanopillar and the corresponding EDS mapping images of (b) Si, (c) O, (d) Pt elements.

To further characterize the nanowire, we performed XPS to analyze the atomic bonding. Figure 4a shows that the binding energy of the fabricated Si nanowire was 103.6 eV, which corresponds to the binding energy at the oxidation state of Si in fully oxidized SiO2 (Si4+). The silica structure was verified by analyzing the structure of the O 1s binding energy (532.8 eV), as presented in Fig. 4b. The broad peak was attributed to that of SiOSi and SiOH bindings. The silanol groups were formed on the surface of silica,18 and the results support the fact that the amorphous nanowires were made of silicon oxide.


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Figure 4. XPS spectra of (a) Si 2p and (b) O 1s.

TEM and XPS results revealed that the silica nanowires were precipitated from Pt–Si droplets. In the phase diagram of the Pt–Si system, the eutectic point was estimated to be 830 C (Fig. S2).19 The formation of nanowires were not observed at lower temperature (Fig. S3). This observation supports our inference that the nanowires were precipitated above the eutectic temperature from the droplets. Note that a large number of nanowires were formed despite the heat being turned off immediately when the temperature reached 830 C. This observation suggests that supersaturation was easily achieved for precipitation, thus facilitating rapid growth of nanowires even when the liquid catalyst droplets had just formed. The interaction of oxygen with silicon, determined by the oxygen partial pressure, may result in the formation of volatile SiO vapor or SiO2 film:20

𝑃𝑐 = 𝑃0exp (

―∆𝐸 𝑘𝑇 )

(1)

where Pc is a critical pressure, below which SiO will be produced by active oxidation mechanism; P0 = 2.01012 torr for (100) Si; ∆E = 3.83 eV; k is Boltzzman’s constant and T is temperature, respectively. Accordingly, lower oxygen pressure is required for active oxidation at


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lower temperature. The lower growth temperature in this study than most results in the literature can be attributed to the low oxygen partial pressure (approximately 0.25 ppm). Notably, because the required oxygen pressure is extremely low, it is hard to etch Si by active oxidation at lower temperature even using other metals with a lower eutectic temperature. For example, when we used Au instead of Pt as the catalyst, we did not observe nanowires grown at 500 C (Fig. S4a). Moreover, Si could be etched by Au at 830 C (Fig. S4b), revealing that Pt is more suitable than Au for silica nanowire growth. The diameter of the nanowires grown through the VLS method was determined by the droplet size, and thus was determined by the film thickness. Figure 5 presents the top-view SEM images of the nanowires grown from Pt films with various thicknesses by controlling the deposition time from 5 to 60 s. The thickness of the Pt film was evaluated using atomic force microscopy and increased with the deposition time. The thickness of the Pt film was 1.8, 2.6, 3.8, 6.2, 8.1, and 10.6 nm for a deposition time of 5, 10, 15, 30, 45, and 60 s, respectively (Fig. S5). When the film thickness decreased below the critical thickness value, no nanowires were formed, although the film agglomerated into droplets by dewetting (Figure 5a, b). The nanowires began to grow and thickened as the thickness of the Pt film increased (Figure 5c–5g), thus suggesting that the droplets act as reservoirs for the incorporation of silica. Figure 5h shows a lowmagnification SEM image of the sample prepared from 10.6-nm Pt, which presents the feature of large-diameter nanowires grown on nanopillars array. Consequently, the larger droplets accumulated from the thicker films formed wider nanowires.


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Figure 5. Top-view SEM images of nanowires grown at the various deposition thicknesses of Pt thin films: (a) 1.8 nm, (b) enlarged view of (a), (c) 2.6 nm, (d) 3.8 nm, (e) 6.2 nm, (f) 8.1 nm, (g) 10.6 nm and (h) overview image of (g).

Figure 6 presents the nanowire diameter as a function of the film thickness. In general, the size of droplet by thermal dewetting decreases with film thickness.21 When the film thickness was reduced to 2.6 nm, 15-nm silica nanowires were successfully synthesized. The smaller diameter is this study may have arisen from the thin and weak adhesion of the sputtering thin film. Moreover, hydrogen gas may also contribute to the thinning of silica nanowires. Because the nanowire diameter is based on the size of the droplet, the droplet size can be reduced using a thin metal film through thermal dewetting. The driving force for dewetting increases as the film thickness decreases;21 smaller droplets can be formed by limiting the thickness of the film. However, when the film thickness was further reduced, the nanowire growth terminated. For a 1.8-nm-thick Pt film, the droplets formed by dewetting are globular and have an average


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diameter of ca. 15–20 nm (Fig. 5b). Because nanowires are not formed when ultrathin films are used, there is a threshold Pt thickness for nanowire growth even after the droplets have been formed.

Figure 6. Relation between nanowire diameter and Pt film thickness.

The barriers for growing thin nanowires can be explained by the Gibbs−Thomson effect, which describes the effects of curved surface on the vapor pressure of small particles:22 ∆μ =

2𝛾 𝑟

(2)

where ∆ is the chemical potential difference between curved and flat surface,  is the surface energy, and r is the radius of the particle, respectively. The energy ∆ required to form curved surface is inversely proportional to the particle size, such that the vapor pressure of catalyst droplets increases with decreasing size. Accordingly, more supplements from vapor are required to attain supersaturation for precipitating nanowires from smaller droplets. To grow thin nanowires using Pt nanoparticles that are smaller than the cutoff value, we must develop a method to generate a high amount of silica source. We now consider the role of


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metal catalysts during active oxidation. Metal catalysts act as a eutectic reservoir for nanowire growth in the VLS method, dissociate oxygen molecules, and decompose native oxide.7 Therefore, when ultrathin catalysts are used, the amount of solute is insufficient, and the undecomposed native oxide may retard the active oxidation and diffusion between Si and Pt, thus restricting the growth of nanowires. Hydrogen radicals that are generated from plasma can assist the growth of nanowires by heating and roughening the metal catalysts.23 Thus, we applied plasma treatment on the metal film prior to heating. The plasma treatment removes partial native oxide, enables silicon evaporation, and assists the metal catalyst for initiating nanowire growth. As observed in Figs. 5a and 5b, no nanowires were generated from 1.8-nm-thick Pt. By contrast, the plasma treatment induced nanowire growth. Figure 7a shows the nanowires grown after conducting a 10-min plasma treatment on the film under a H2–Ar atmosphere. In this figure, we can see some silica nanowires that have grown laterally across the nanopillars, and the average diameter of the nanowires was reduced to approximately 10 nm (Fig. 7b). Higher magnification TEM image also shows that the nanowire diameter can be reduced to less than 10 nm via plasma pretreatment (Fig. 7c).

Figure 7. (a, b) SEM images (scale bar: 100 nm) and (c) TEM image of sub-10-nm nanowires obtained after conducting plasma pretreatment on Pt thin film for 10 min.


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Through plasma pretreatment, a higher amount of silica was introduced into the Pt nanoparticles. A schematic of this plasma-assisted process is presented in Figure 8. In this process, native oxide is removed through the sputter-off mechanism of Ar ions and the penetration of hydrogen atoms into bulk Si during plasma irradiation.24 The hydrogen plasma not only removes the native oxide but also retards the oxidation of silicon by forming Si–H termination on the surface.25 The Si–H groups were detected on Pt coated Si wafer after plasma treatment, as can be seen from the ATR-FTIR analysis shown in Fig. S6.26 Thus, we can transfer the samples after plasma cleaning into the furnace for nanowire growth on time. We also conducted water contactangle measurement on the samples before and after plasma treatment; the angles were 75.60 and 46.42, respectively. This presents the high ability of plasma cleaning to remove contamination.27 Moreover, the Pt and Si surface can be etched using Ar–H2 plasma. 28,29 This ensures that when the etched Si surface is exposed to diluted oxygen, SiO could form and evaporate easily. Because the population of volatile SiO increased when the partial native oxide was removed, a high amount of SiO could be incorporated into a small droplet for the precipitation of silica nanowire. Note that after plasma cleaning, volatile hydride such as SiH4 may also be formed by the interaction between Si and H2, which may provide a precursor for the growth of silicon nanowires. However, the solubility of Si is much higher than that of SiO2 in catalyst droplets; thus, silicon nanowires were not observed in this study.


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Figure 8. Schematic of plasma-assisted growth of silica nanowires. To appropriately analyze the effects of plasma on nanowire growth, we extended the plasma treatment time on Pt thin films before nanowire growth. By analyzing the images from SEM (Figs. 9a9d), we observed that the diameters varied from 9.3 nm for the sample with a treatment time of 5 min to 16.2 nm for the sample with a treatment time of 20 min. The corresponding plot of the diameter variation is presented in Figure 9e. The diameters of the nanowires increase when the treatment time increases, which can be attributed to the continuous coarsening of droplets during the longer plasma treatment time. From the results, we see that a short treatment is required to inhibit droplet accumulation.


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Figure 9. (a-d) Top-view SEM images of 1.8-nm thick Pt with plasma pretreatment for (a) 5 min, (b) 10 min (c) 15 min (d) 20 min. (e) Plot of nanowire diameter vs. plasma irradiation duration.

Because the interface between the silicon substrate and Pt thin films affects the growth of nanowires, we further performed HF cleaning to remove the native oxide. Figure 10 presents the SEM pictures of the samples after conducting various pretreatments. After the pretreatments, all the samples were placed into the furnace for conducting the experiment. Here, the nanowires were not generated when native oxide was not removed (Figure 10a). When HF cleaning was conducted, short nanowires were seen on the surface (Figure 10b). However, compared with the number of nanowires obtained after conducting plasma pretreatment (Figure 10c), the number of the nanowires obtained after conducting HF cleaning was substantially lower. The difference can be attributed to the retardation effect of oxidation and the accumulation of droplets by plasma, which makes plasma superior to HF cleaning for nanowire growth. When the sample was first cleaned with HF and was then exposed to plasma for 10 min, a high number of fine nanowires were generated over silicon nanopillars, as shown in Figure 10d. The higher magnification SEM images shows that the diameters of nanowires are also close to 10 nm with HF cleaning (Fig. S7). The synergy effects of plasma and HF cleaning on nanowire growth support the method illustrated in Figure 8. The method suggests that removing native oxide is beneficial for growing nanowires from thin metal films.


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Figure 10. SEM images of the samples after conducting various pretreatments: (a) no pretreatment, (b) HF cleaning, (c) plasma treatment, and (d) HF cleaning, followed by plasma treatment.

These silica nanowires can reduce the surface reflection of silicon wafer by light trapping. Figure 11a presents the reflectance spectra of flat Si wafers covered by silica nanowires grown from various Pt thicknesses. The average reflectance in the range 400–900 nm reduced from 35.9% to 14.2% as the Pt thickness used for nanowire growth increased from 1.8 nm to 10.6 nm. The antireflection properties can be explained by the smaller effective refractive index (ERI) made of nanowires and air, which reduced the retardation as the incident light entered the materials.30 There was no significant difference in the reflectance between some samples, which may be due to their similar ERI. The silicon nanopillars also exhibited low reflectance performance (8.9%), as shown in Fig. 11b. When the silica nanowires were grown on these silicon nanopillars, the corresponding reflections were reduced further. For example, the average reflectance reduced to 4.1% as nanowires were grown from 30-s (6.2-nm) Pt on silicon nanopillars. The effects of plasmaactivated nanowires on reflectance are presented in Fig. 11c. The average reflectance of nanoparticledecorated nanopillars was reduced to 6.7%. It is suggested that nanoparticles are able


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to suppress the light reflection over a broad spectral range by plasmonic effects.31 Here the further suppression in reflection was found by growing nanowires with plasma activation from these nanoparticles. The optimal antireflection performance was observed on the sample after 10-min plasma pretreatment, which had an average reflectance of 4.8%. The average reflectances are summarized in Fig. 11d. These results suggest that growing silica nanowires should be useful in reducing the surface reflection of materials.

Figure 11. (a) Reflectance spectra of silica nanowires grown on flat Si wafer from various thicknesses of Pt. (b) Reflectance spectra of silica nanowires grown on Si nanopillars from various thicknesses of Pt. (c) Reflectance spectra of silica nanowires grown on Si nanopillars with and without plasma treatment. (d) Summary of average reflectances for the different samples with nanowire decoration.


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CONCLUSION We developed a rapid method to grow silica nanowires through active oxidation at a low temperature (830 C) without maintaining the temperature. Moreover, we used plasma pretreatment for improving the growth of thin silica nanowires. The diameters of silica nanowires grown through active oxidation decreased with a reduction in the thickness of Pt thin films. However, when the film thickness was reduced to approximately 2 nm, the growth of the nanowires ceased because of insufficient catalysts and vapor precursors. To generate a high amount of silica for inducing supersaturation of smaller droplets, we conducted plasma treatment under H2–Ar atmosphere on Pt thin films before growing the nanowires. Nanowires with diameters below 10 nm can be produced through the proposed method. Furthermore, by conducting additional HF cleaning, more nanowires were produced. This implies that interface engineering is essential to facilitate the sub-10-nm nanowire growth. The generated nanowires exhibited low reflection performance, and we believe that these thin nanowires would benefit applications that demand high surface area and light trapping.

SUPPORTING INFORMATION EDS spectra of nanowires, partial Pt-Si phase diagram, SEM image of the sample grown at lower temperature, SEM images with Au catalyst, AFM images of the samples with various Pt thickness, FTIR spectra with and without plasma treatment and SEM images with HF cleaning can be found in the Supporting Information.


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AUTHOR INFORMATION Corresponding Author *Jiann Shieh. E-mail: [email protected] Author Contributions J.S. conceived this study. H.Y.L. B.W.H and Y.C.T. conducted the experiments. All authors contributed to the preparation of this manuscript. All authors have approved the final version of the manuscript.

ACKNOWLEDGMENT This study received financial support from the Ministry of Science and Technology, Taiwan, (grant nos. MOST 104-2628-E-239-001-MY3 and MOST 107-2221-E-239-005). We thank the Precision Instruments Center (National Sun Yat-sen University) for the TEM analyses.

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19. Tanner, L. E.; Okamoto H. The Pt-Si (Platinum-Silicon) System. J. Phase Equilibria. 1991, 12, 571–574. 20. Smith, F.W.; Ghidini, G. Reaction of Oxygen with Si(111) and (100): Critical Conditions for the Growth of SiO2. J. Electrochem. Soc., 1982, 129, 1300–1306. 21. Thompson, C. V. Solid-State Dewetting of Thin Films. Annu. Rev. Mater. Res. 2012, 42, 399– 434. 22. Choi, H. J. Vapor–Liquid–Solid Growth of Semiconductor Nanowires. in Semiconductor Nanostructures for Optoelectronic Devices, Springer Berlin Heidelberg, 2012, 1–36. 23. Jeon, M. S.; Tomitsuka, Y.; Aoyagi, M.; Kamisako, K. Effects of Hydrogen Radical Treatment on Fabrication of Catalyst Nanoparticles from Metal Oxide Film at Low Temperature and Synthesis of Silicon Nanowires. Jap. J. Appl. Phys. 2009, 48, 015002. 24. Chang, R. H. P.; Chang, C. C.; Darack, S. Hydrogen Plasma Etching of Semiconductors and Their Oxides. J. of Vac. Sci. and Technol. 1982, 20, 45–50. 25. Ishii, M.; Nakashima, K.; Tajima, I.; Yamamoto, M. Properties of Silicon Surface Cleaned by Hydrogen Plasma, Appl. Phys. Lett. 1991, 58, 1378. 26. Kroll, U.; Meier, J.; Shah, A.; Mikhailov, S.; Weber, J. Hydrogen in Amorphous and Microcrystalline Silicon Films Prepared by Hydrogen Dilution, J. Appl. Phys. 1996, 80, 4971– 4975. 27. Gupta, V.; Madaan, N.; Jensen, D. S.; Kunzler, S. C.; Linford, M. R. Hydrogen Plasma Treatment of Silicon Dioxide for Improved Silane Deposition, Langmuir 2013, 29, 3604–3609.


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Graphic for Manuscript Rapid, Low-Temperature Growth of Sub-10-nm Silica Nanowires through Plasma Pretreatment for Antireflection Applications Hong-Yi Lee, Bo-Wen Huang, Yi-Ci Tsai, Jiann Shieh* Department of Materials Science and Engineering, National United University, Miaoli, Taiwan E-mail: [email protected]

Silica nanowires grown from platinum nanoparticles by plasma pretreatment. 1


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Rapid, Low-Temperature Growth of Sub-10 nm Silica Nanowires

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