A Novel Route to the Synthesis of Silica Nanowires without a Metal

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A Novel Route to the Synthesis of Silica Nanowires without a Metal Catalyst at Room Temperature by Chemical Vapor Deposition Sanghyun Park,† Jaeyeong Heo,‡ and Hyeong Joon Kim*,† †

Department of Materials Science and Engineering and Inter-University Semiconductor Research Center, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-744, Korea ‡ Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States ABSTRACT: Silica nanowires were synthesized by employing inherent directionality of chemical vapor reaction between bis(ethylmethylamino)silane (H2Si[N(C2H5)(CH3)]2) precursor and water without a metal catalyst at room temperature. The difference in the oxidation reactivity between Si-H and Si-N bonds with water leads to the formation of silica nanowires. The mean diameter and length of the silica nanowires grown for 10 min were 60-80 nm and 1.9 μm, respectively. Transmission electron microscopy revealed that the obtained nanowires had the concave tip, differing from other silica nanowires produced by a conventional vaporliquid-solid method, and were amorphous. Energy dispersive X-ray spectroscopy, Fourier transform infrared, and X-ray photoelectron spectroscopy results also proved that the nanowires have a close composition to stoichiometric SiO2. Silica nanowires were successfully synthesized on a poly(ethylene terephthalate) film. The nanowires can emit strong blue light and ultraviolet light under excitation at 266 nm. KEYWORDS: Silica nanowire, chemical vapor deposition, transmission electron microscopy, heteroleptic precursor

ollowing the discovery of carbon nanotubes,1 one-dimensional nanostructures, such as nanotubes, nanowires (nanofibers), and nanoribbons (nanobelts), have attracted considerable attention owing to their fascinated chemical and physical properties and their great potential for nano/bio/optoelectronics.2-7 Many material systems of nanowires including carbon, metals, oxides, nitrides, and carbides have been fabricated by a variety of methods.8-12 Among them, silica nanowires have been the subject of intense study owing to their unique properties of blue light emission and easy surface functionalization.13,14 Since Yu et al.13 synthesized silica nanowires by a laser ablation method, demonstrating intense blue light emission from the silica nanowires, several other methods, such as sol-gel, thermal evaporation, thermal annealing, and chemical vapor deposition, have been applied to the synthesis of silica nanowires.13,15-18 Vaporliquid-solid (VLS), solid-liquid-solid, and solution-liquidsolid mechanisms have been used to explain the growth characteristics of these silica nanowires. However, these techniques require relatively high growth temperatures up to ∼1000 °C and long growth durations of several hours or even days to grow micrometer-long nanowires. Moreover, addition of metal catalysts, which is essential for initiating the synthesis of silica nanowires, could potentially pose a concern of metal contamination. The high growth temperature and metal contamination by the catalysts limits their applications to extended areas, such as display devices and bioelectronics. This paper reports a novel route to the synthesis of predominantly straight and long silica nanowires by employing chemical vapor reaction between heteroleptic bis(ethylmethylamino)silane

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r 2011 American Chemical Society

(BEMAS, H2Si[N(C2H5)(CH3)]2) and water at room temperature. This simple process is relatively fast (several minutes), compatible with conventional Si device technologies, and possible to synthesize silica nanowires even on flexible substrates without metal contamination issues. Furthermore patterned silica nanowires could be fabricated by a conventional lift-off process using photoresist by virtue of a low growth temperature, which is expected to provide the methodology to assemble opto-/bioelectronic devices. Also, it could be used as a sacrificial template for the formation of metallic nanotubes.19-21 In addition, it may be possible to extend this approach to the growth of other oxide nanowires. Experimental Methods. The growth of the nanowires was performed in a conventional chemical vapor deposition (CVD) reactor (TES Co.). A 4 in. bare Si wafer without a metal catalyst was used as the substrate. A poly(ethylene terephthalate) (PET) film (CG3300, 3M Co.) was also used as a substrate to confirm the possibility of silica nanowire formation at low temperatures on organic polymer substrates. Before being loaded into the growth chamber, the substrate was ultrasonicated in ethanol for 5 min followed by rinsing in deionized water for 10 min. HF cleaning was not performed in order to enhance the nucleation of the nanowires on the Si substrate. The BEMAS precursor was vaporized and carried with inert Ar gas at a flow rate of 25 sccm (standard cubic centimeters per minute) from a thermostatic Received: November 4, 2010 Revised: December 20, 2010 Published: January 10, 2011 740

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Figure 2. Cross-sectional SEM images of the silica nanowires grown on bare Si substrates under different growth pressures, i.e., (a) 1, (b) 2, (c) 7, and (d) 50 Torr (all scale bar 500 nm). Figure 1. Cross-sectional SEM images of the grown silica nanowires using (a) bis(ethylmethylamino)silane and (d) tetrakis(ethylmethylamino)silane (scale bar 500 nm). The molecular structures of the precursors used in this study: (b) bis(ethylmethylamino)silane and (c) tetrakis(ethylmethylamino)silane.

bubbler maintained at 25 °C. Figure 1b shows the molecular structure of the precursor, of which the vapor pressure at 25 °C was approximately 7 Torr. The vaporization rate of the water used as an oxidant was 200 mg/min. Ar gas carrying vaporized water was flown at 100 sccm as a dilute gas. The growth pressure was varied from 1 to 50 Torr. The optimized growth pressure to obtain the straight nanowires was 7 Torr. The deposition temperature was fixed at 25 °C. When the deposition temperature was increased above room temperature, thin film formation as in a normal CVD reaction started to occur instead of nanowire formation. The growth duration was varied from 1 to 10 min. Homoleptic tetrakis(ethylmethylamino)silane (TEMAS, Si[N(C2H5)(CH3)]4) was also used as a precursor to prove the hypothesis adopted for the formation mechanism of silica nanowires. Figure 1c shows the molecular structure of TEMAS. When TEMAS precursor was used, the source canister was heated to 93 °C to maintain a vapor pressure of approximately 7 Torr to afford a direct comparison of the results. The grown products were characterized by scanning electron microscopy (SEM, S4800, Hitachi). Before being loaded into the SEM chamber, the samples were coated with Pt for 100 s since the nanowires were not electrically conducting. Transmission electron microscopy (TEM, F20, FEI) and energy dispersive X-ray spectroscopy (EDX) were used to examine the crystallinity and composition of the synthesized nanowires, respectively. For TEM sample preparation, the synthesized nanowires dispersed in methanol were ultrasonicated for 10 min and one drop of the solution was placed on a Cu mesh grid. The grid was dried in a vacuum oven for 12 h before being loaded into the TEM apparatus. The chemical states of the grown nanowires were characterized by X-ray photoelectron spectroscopy (XPS, AXIS, KRATOS). The structure of the grown nanowires was also analyzed by Fourier transform infrared (FTIR, FT/IR-660 plus, Jasco) spectroscopy in absorption mode with a resolution of 4 cm-1. The photoluminescence (PL) measurements were performed on an RF 5301 PC spectrofluorophotometer (Shimadzu Corp.) at room temperature under excitation at 266 nm. Result and Discussion. Figure 1a shows a cross-sectional SEM image of the silica nanowires on a Si substrate using

Figure 3. Vertical and lateral growth behaviors of the silica nanowires. The closed squares and open triangles represent the mean length and diameter of the nanowires, respectively.

BEMAS as a precursor. The growth pressure, temperature, and reaction time were 7 Torr, 25 °C, and 10 min, respectively. As shown in Figure 1b, the heteroleptic BEMAS precursor has four ligands: two amide and two hydrogen ligands. On the other hand, the homoleptic TEMAS precursor consists of four amide ligands without hydrogen ligands, as shown in Figure 1c. The reaction of TEMAS with water resulted in the formation of a two-dimensional film with sparse nucleation, as shown in Figure 1d, while other growth conditions were maintained. This result clearly shows that the heteroleptic nature of the BEMAS precursor plays a key role in the formation of nanowires instead of forming thin films. It is believed that the different reactivities of the amide and hydrogen ligands in the heteroleptic precursor with water vapor induce the growth of one-dimensional structures. The resulting shape and the growth rate of the silica nanowires were largely affected by the growth pressure. Figure 2 shows cross-sectional SEM images of representative silica nanowires produced at various growth pressures. Here, the growth duration and temperature were 10 min and 25 °C, respectively. When the growth pressure was 1 Torr (Figure 2a), the early stage of nanowire growth was observed on limited nucleation sites, which may be due to limited precursor injection. The growth rate was estimated to be ∼10 nm/min. As the growth pressure increased to 2 Torr (Figure 2b), the growth rate increased to ∼43 nm/min. The nanowires grown under these conditions have thick tips tapering toward a toe near substrates, even though the vertical growth is still dominant. The uniformity of the diameter along one wire (∼71 ( 9 nm) was remarkably improved when the pressure increased to 7 Torr (Figure 2c). It also accompanied an 741

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Scheme 1. The Proposed Growth Mechanism of Silica Nanowires: (a) Vertical Growth by Hydrolysis and Condensation; (b) Nucleation on Hydroxylated Surfaces by Chemisorption Reaction; (c) Cross-Linking of Adsorbed Intermediates for Lateral Growth by Hydrolysis and Dehydrogenation; (d) Brief Growth Schematics of the Cone-Shaped (at Low Pressure) and CylinderShape Nanowires (at High Pressure)

enhanced growth rate of ∼190 nm/min due to the increased supply of sources. When the pressure was further increased to 50 Torr (Figure 2d), long but highly curled and tangled silica nanowires were synthesized. The diameter decreased slightly to ∼35 ( 10 nm with a dramatic increase in growth rate, which indicates that lateral growth is somewhat suppressed compared to the case of 7 Torr (Figure 2c). The increased supply of sources appears to be quickly consumed for the rapid growth in a vertical direction. Figure 3 shows the changes in the diameter near the distal tip region and the length of the nanowires grown for 10 min as a function of the growth pressure from 1 to 8 Torr. The length of the silica nanowires was largely affected by the growth pressure, but the diameter near the tip region was less affected. This suggests that vertical (length) and lateral (diameter) growth occur through different mechanisms. Scheme 1 summarizes the proposed reaction path of the dominant one-directional growth by the difference in oxidation reactivity after surface nucleation. Vertical growth occurs with the condensation reaction of the water and amide ligand. First, the lone pair of electrons of the amide group induces a preferential reaction with water, becoming an intermediate with one hydroxyl group. The two intermediates may react with each other or one intermediate may react with another amide ligand to form a siloxane bridge (SiO-Si), resulting in water or amine condensation (Scheme 1a). Heterogeneous nucleation on a hydroxylated substrate, in this case Si, occurs by the chemisorption of intermediates (Scheme 1b). The lateral growth of silica nanowires results from the crosslinking of chemisorbed intermediates. A dehydrogenation reaction of hydrogen bonds on the chemisorbed intermediates and water produces a hydroxyl group. Cross-linking is achieved by

Figure 4. (a) Bright field TEM image with the corresponding SAED pattern (scale bar 50 nm). (b) High-resolution TEM image (scale bar 5 nm). (c) Line profile of the EDX spectrum tracing the dotted line shown in panel a. The closed squares and circles represent Si and O, respectively. (d) FTIR spectrum of the silica nanowires grown on a Si substrate.

the elimination of hydrogen (H2) from a hydroxyl group and adjacent hydrogen bonding. The elimination of water between two adjacent newly formed hydroxyl groups also results in a 742

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Figure 5. The XPS spectra of the silica nanowires grown at 7 Torr for 10 min: (a) survey scan, (b) Si 2p, (c) O 1s, and (d) N 1s narrow scans.

siloxane bridge (Scheme 1c). The cross-linking rate of chemisorbed intermediates does not depend on the rate at which source arrives at the surface of a growth region. It is believed that vertical and lateral growth proceeds on a concave tip of the nanowires because the hydrogen-terminated side surface of the nanowire is chemically less active than the hydroxylated tip surface. Scheme 1d shows the final shapes of the nanowires formed at different vertical growth rates. At low process pressure, relatively similar growth rates of lateral and vertical directions can result in cone-shaped nanowires (panels a and b of Figure 2), whereas long and cylinder-shape nanowires are achieved at relatively high pressure (panels c and d of Figure 2), where the growth in vertical direction exceeds the one in lateral direction. Structural and chemical analyses of the grown nanowires were carried out for more detailed investigation. A representative TEM image of the silica nanowires grown at 7 Torr is shown in Figure 4a. The homogeneous contrast through the entire wire proves that the products are not nanotubes but nanowires. Notably, the concave tip region shows a different contrast from the bulk region. This peculiar shape differentiates it from other silica nanowires produced by a conventional VLS method, in which the region underneath the metal catalysts is completely filled with grown materials.22 From this figure, it is evident that further growth initiates from the outer side of nanowire and it proceeds centripetally, filling the remaining inside part of the wire. In addition, no metal catalyst was observed at the tip, indicating that the growth of the nanowires is governed by a different mechanism from the VLS growth. The crystallinity of the silica nanowires was characterized by selected area electron diffraction (SAED), as shown in the inset of Figure 4a. The highly diffusive and dispersed diffraction pattern suggests that the nanowire is in an amorphous state. The high-resolution TEM image shown in Figure 4b also confirms the amorphous nature of the grown silica nanowires. Figure 4c shows the EDX line profile of the nanowire. Here, the scan region is indicated by the dotted line in Figure 4a. The detected elements from the scan are Si, O, Cu, and C. The copper signal originates from a TEM grid. Considering that carbon was not detected by FTIR and XPS as discussed later, the carbon signal in the spectrum can be assigned to adventitious hydrocarbon from air exposure. The elemental profiles clearly show that the nanowire has a homogeneous distribution of Si and O. The

Figure 6. (a) The SEM image of the silica nanowires grown on a PET film substrate (scale bar 1 um). (b) Ester groups on the PET film surface. (c) The nucleation sites of the PET film surface: carboxyl group, alcohol by the reduction of a carboxyl or ester group, and hydroxyl group.

convex shape of the Si and O profiles is attributed to the insidefilled nature of the nanowires, which differentiates it from nanotubes. The estimated composition was similar to stoichiometric SiO2. No other metal signals except Cu were detected in the EDX spectrum near the tip region, which demonstrates that nanowire growth proceeds without the assistance of a metal catalyst. Figure 4d presents the FTIR spectrum of the as-grown silica nanowires on a Si substrate grown at 7 Torr. An absorption band between 837 and 885 cm-1 is assigned to the bending modes of H-SiO.23 These bending modes may be originating from the H-terminated outer surface of the silica nanowires (Scheme 1d). No carbon-related peaks such as Si-CH3 vibrations (∼12501270 cm-1) were observed.23,24 The strong peaks at ∼1078 and 743

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of the chemical vapor reaction between heteroleptic bis(ethylmethylamino)silane and water without the aid of a metal catalyst at room temperature. The shape of the grown nanowire was strongly affected by the growth pressure. The optimized pressure to obtain straight silica nanowires was 7 Torr. The diameter and length of the silica nanowires grown at 7 Torr for 10 min were 60-80 nm and ∼1.9 μm, respectively. TEM analysis revealed the grown nanowires to be amorphous. EDX, FTIR, and XPS results also proved that the nanowires have a close composition to stoichiometric SiO2. The possible growth mechanism of the silica nanowires was also proposed. This simple chemical route at room temperature without a metal catalyst enables the direct growth of silica nanowires on various materials including thermally stable polymers, and it may also be extended to the formation of other oxide nanowires. The PL measurement also suggested the grown silica nanowires to be applied for viable blue light/ultraviolet emitters in integrated optics.

Figure 7. The PL spectrum of the silica nanowires measured at room temperature under excitation at 266 nm.

1180 cm-1 come from the stretching vibrations of Si-O-Si, and the one at 1107 cm-1 originates from the antisymmetric vibration of the SiO2 complex.23,24 The FTIR analysis shows that the nanowires are mainly comprised of Si and O. Figure 5a shows the survey scan XPS spectrum of the nanowires grown at 7 Torr for 10 min and panels b-d of Figure 5 show its narrow scan spectra of the Si 2p, O 1s, and N 1s peaks, respectively. No signals for carbon (∼285 eV) and nitrogen (∼398 eV) peaks were detected in the spectrum in Figure 5a, except for Si and O. This result again suggests that the nanowires consist of Si and O, as indicated by the EDX result in Figure 4. In Figure 5b, the strong peak at ∼103.2 eV of Si 2p corresponds well to the binding energy of SiO2.25,26 The small peak at 99.2 eV is assigned to bulk Si since the entire probing area was not fully covered by grown nanowires for a growth duration of 10 min. A prominent O 1s peak at a binding energy of ∼532.6 eV is also observed in Figure 5c and it also fits well with the binding energy of SiO2.25,26 Quantitative analysis confirmed that the nanowires are composed of Si and O with the ratio of about 1:2. The signal level of nitrogen is insignificant, as shown in Figure 5d, which means that even at room temperature the ligand exchange reaction between the BEMAS precursor and water is almost complete. To determine whether this process could be extended to polymer substrates, silica nanowires were synthesized on a polyethylene terephthalate (PET) film. The growth pressure, temperature and duration time were 7 Torr, 25 °C and 10 min, respectively. The SEM image of Figure 6a clearly shows that long and predominantly straight silica nanowires were successfully grown on the polymer substrate. The PET film surface is basically covered with many ester groups (Figure 6b), but a few polar groups (carboxyl groups, alcohol functional groups by reduction of carboxyl or ester groups, and hydroxyl bonds, Figure 6c) are known to naturally appear.27 These polar groups appear to act as nucleation sites of the silica nanowires on a PET film surface. Up to now it is known that the most possible and immediate application of silica nanowires is the viable blue light/ultraviolet emissions.13,28-31 The photoluminescence (PL) properties were measured at room temperature under excitation at 266 nm to characterize the optical properties of the silica nanowires grown at 7 Torr for 10 min. Three strong PL peaks at ∼362 nm (3.42 eV), 423 nm (2.93 eV), and 459 nm (2.70 eV) are detected, as shown in Figure 7. This result suggests that the grown silica nanowires could be used as blue light/ultraviolet emitters. In summary, silica nanowires were successfully synthesized by chemical vapor deposition employing the inherent directionality

’ AUTHOR INFORMATION Corresponding Author

*E-mail: thinfi[email protected].

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