H2O

Jul 18, 2014 - Sivakov , V.; Andra , G.; Gawlik , A.; Berger , A.; Plentz , J.; Falk , F.; Christiansen , S. H. Silicon Nanowire-Based Solar Cells on ...
0 downloads 0 Views 10MB Size
Download PDF
Letter pubs.acs.org/NanoLett

Metal-Catalyzed Electroless Etching of Silicon in Aerated HF/H2O Vapor for Facile Fabrication of Silicon Nanostructures Ya Hu,† Kui-Qing Peng,*,† Zhen Qiao,† Xing Huang,‡ Fu-Qiang Zhang,† Rui-Nan Sun,† Xiang-Min Meng,‡ and Shuit-Tong Lee*,§ †

Department of Physics and Beijing Key Laboratory of Energy Conversion and Storage Materials, Beijing Normal University, Beijing 100875, China ‡ Technical Institute of Physics and Chemistry and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Chinese Academy of Sciences, Beijing 100190, China § Institute of Functional Nano and Soft Materials (FUNSOM) and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, China S Supporting Information *

ABSTRACT: Inspired by metal corrosion in air, we demonstrate that metal-catalyzed electroless etching (MCEE) of silicon can be performed simply in aerated HF/H2O vapor for facile fabrication of three-dimensional silicon nanostructures such as silicon nanowires (SiNW) arrays. Compared to MCEE commonly performed in aqueous HF solution, the present pseudo gas phase etching offers exceptional simplicity, flexibility, environmental friendliness, and scalability for the fabrication of three-dimensional silicon nanostructures with considerable depths because of replacement of harsh oxidants such as H2O2 and AgNO3 by environmental-green and ubiquitous oxygen in air, minimum water consumption, and full utilization of HF. KEYWORDS: Silicon nanowire, electrochemistry, galvanic etching, gas phase etching

M

etal-catalyzed electroless etching (MCEE) of silicon or metal-assisted chemical etching (MacEtch or MACE), typically performed in an aqueous hydrofluoric acid (HF, highly hazardous and corrosive) solution, has been extensively investigated in the last decades.1−10 MCEE of silicon has been proven to be a low-cost, highly scalable, reliable top-down nanofabrication technique capable of producing a variety of silicon nanostructures.2,3,8−22 Silicon nanowires (SiNWs) are a particularly attractive nanostructure because of their high potential for technological applications23−36 including advanced energy conversion and storage, chemical sensing, and drug release. MCEE of silicon in aqueous HF solution proceeds via the concurrent action of electrochemical dissolution of Si and reduction of oxidizing agents (typically hydrogen peroxide, silver nitrate), both assisted by noble metal catalysts. During MCEE, a flux of electrons flows from the anode (silicon) to the cathode (noble metal catalyst), and a microscopic circuit is completed by proton transport in aqueous solution. In recent years, MCEE of Si in aqueous HF solution has been widely © 2014 American Chemical Society

used for the fabrication of three-dimensional high aspect-ratio Si nanostructures. Commonly used oxidation agents in MCEE are strong oxidants such as hydrogen peroxide and silver nitrate, which are expensive and hazardous. Silicon is widely known to be thermodynamically unstable and readily oxidized by oxygen in air or water,37 which is nontoxic, environmentally friendly, and ubiquitous in nature. Despite the advantages, it remains unclear how cathodic oxygen reduction effects MCEE of silicon is influenced due to sluggish kinetics arising from inherent difficulty of splitting oxygen molecule. Although large cathode surfaces can enhance oxygen reduction and silicon corrosion,38,39 the use of large cathodes is inconvenient and impractical. It is well recognized that atmospheric corrosion of metals is caused by air/oxygen transported to metals through Received: January 28, 2014 Revised: June 26, 2014 Published: July 18, 2014 4212

dx.doi.org/10.1021/nl500361u | Nano Lett. 2014, 14, 4212−4219

Nano Letters

Letter

ultrathin surface liquid film and is the more prevalent form of metal corrosion than corrosion occurring in aqueous solution. By the same token, atmospheric corrosion may similarly work for MCEE of Si. However, to our best knowledge MCEE of Si in HF/H2O vapor has never been reported despite the intense research of MCEE of Si in aqueous solution in recent years. Here, motivated by atmospheric corrosion of metals, we demonstrate the first utilization of enhanced MCEE of Si in HF/H2O vapor and oxygen ambient to fabricate SiNWs and nanoholes array. Compared to conventional MCEE process performed in aqueous HF solution, the benefits MCEE of silicon in HF/H2O vapor are remarkable. First, the reduction kinetics of oxygen is significantly enhanced in HF/H2O vapor due to high oxygen penetration rate so that oxygen can function like H2O2 as an efficient oxidant to prepare SiNWs and other complex silicon nanostructures. Second, the amount of HF usage can be considerably reduced. This new MCEE approach may serve as an attractive alternative MCEE process due to its remarkable simplicity, flexibility. and scalability. MCEE of Si in aerated HF/H2O vapor is simply performed in a polytetrafluoroethylene (PTFE) reactor filled with aqueous HF solution and air as schematically shown in Supporting Information Figure S1. The HF/H2O vapor is prepared by spontaneous evaporation of aqueous HF solution (14 mol L−1) at room temperature. According to previous studies,40,41 the presence of H2O enhances the dissolution of SiO2 produced during etching process. As MCEE of Si is an electrochemical process, silicon/metal must be exposed to an electrolyte, which is formed by HF/H2O condensed on silicon surface. The clean surface of Si substrate is coated with noble metal (silver, gold) nanoparticles via electroless metal deposition (EMD) or thermal deposition. The structure of the metal nanoparticle film would dictate the morphology of Si nanostructure subsequently produced. MCEE of Si in aerated HF/H2O vapor is performed by placing the metal-coated Si substrate on a PTFE platform above the aqueous HF solution. The MCEE process can be conveniently controlled by monitoring the color change of Si surface, which would turn black after 30 min etching due to formation of nanowires array. As a reference, two Si substrates coated with silver nanoparticles (AgNPs) are respectively placed in air outside the reactor (HF-free air) or inside the aqueous HF solution. Figure 1 shows the photographs and corresponding scanning electron microscope (SEM) images of AgNPs-coated Si substrates after 5 h MCEE placed at different positions shown in Supporting Information Figure S1. The images are visibly different, indicating different surface morphologies in the samples obtained. Little corrosion has occurred in Si sample exposed to HF-free air, whereas considerable corrosion is observed in Si samples placed in aerated HF/H2O vapor or aqueous HF solution. MCEE of Si in aqueous HF solution is relatively slow producing only shallow pits, while MCEE of Si in atmospheric HF/H2O vapor is significantly faster forming SiNWs with considerable lengths, indicating oxygen reduction kinetics is substantively enhanced due to faster oxygen transport to metal catalyst. MCEE of silicon in aerated HF/H2O vapor is intrinscially an electrochemical process analogous to MCEE of Si in aqueous HF solution except that the former process involves a much thinner electrolyte layer, which enables significantly faster oxygen penetration. Figure 2 shows the process of MCEE of Si in aerated HF/H2O vapor in forming three-dimensional nanostructures. First, upon exposure to aerated HF/H2O vapor, an “invisibly” thin electrolyte layer would form on

Figure 1. Photographs and SEM images of AgNPs-coated Si substrates after 5 h duration (a) in HF-free atmosphere, (b) submerged in 20% aqueous HF solution, and (c) in atmospheric HF/H2O vapor.

substrate surface (Supporting Information Figure S2) and oxygen in ambient air can readily reach silicon/metal surface. Because the redox potential of O2/H2O is more positive than that of H+/H2, oxygen will be cathodically reduced to water preferentially on metal catalysts by taking electrons from silicon while cathodic hydrogen evolution is not expected. The cathodic half-cell reaction in the shorted microscopically galvanic cell is described as the following eq 1 O2 + 4H+ + 4e− → 2H 2O

(1)

While gas bubbles could be clearly observed during by MCEE of silicon in HF-H2O2, in contrast no gas bubbles can be observed during MCEE of silicon in the present pseudo gas phase etching, indicating much less gas generation in the latter. Nevertheless, we believe similar gaseous reaction products should be generated in the present experimental system. Because cathodic hydrogen evolution is not expected at the catalytic metal surface as noted above, the gaseous products, especially hydrogen evolution, should be related to the anodic dissolution mechanism of silicon. Hydrogen in the gaseous reaction products is characterized using a gas chromatograph (GC) equipped with a thermal conduction detector and a 5Å molecular sieve column (Φ3 × 3m) with nitrogen serving as the carrier gas. The experimental setup for gas collection is schematically shown in Supporting Information Figure S3. The typical GC trace shown in Supporting Information Figure S4 clearly confirmed the collected gaseous product contains hydrogen, indicating the presence of divalent oxidation of silicon.36 The dissolution process of silicon via a divalent route can be described as a mixed electrochemical and chemical process as shown in eqs 2 and 3

4213

Si + 2F− + 2h+ → SiF2

(2)

SiF2 + 4F− + 2H+ → SiF6 2 − + H 2

(3)

dx.doi.org/10.1021/nl500361u | Nano Lett. 2014, 14, 4212−4219

Nano Letters

Letter

Figure 2. Schematics illustrating MCEE of Si in aerated HF/H2O vapor (a) and production of three-dimensional Si nanostructures such as SiNW array (b).

Figure 3. SEM images of SiNW arrays on p-type 1−10 Ω·cm Si(100) wafers after (a) 1 h and (b) 5 h MCEE in aerated HF/H2O vapor.

Besides the divalent electrochemical process, the tetravalent electrochemical process would oxidize silicon to SiO2, which is removed immediately by HF, as shown in the following eqs 4 and 5. This tetravalent electrochemical process has been previously proposed to explain electroless metal deposition on silicon and MCEE of silicon in HF solution.1,11 Si + 2H 2O + 4h+ → SiO2 + 4H+

(4)

SiO2 + 6HF → 2H+ + SiF6 2 − + 2H 2O

(5)

Si + 6HF +

⎡4 − n⎤ n n O2 → H 2SiF6 + H 2O + ⎢ H ⎣ 2 ⎥⎦ 2 4 2 (6)

Because of the large potential difference between metal and silicon, silicon would be electrochemically anodized, and local silicon dissolution underneath metal particles would start to produce “child” pits (Figure 2a). The processes are called metal-induced oxidation and metal-induced pitting, respectively. The continual cathodic oxygen reduction on metal catalysts surface and removal of anodic silcion oxide under metal nanoparticles would drive the metal-induced “child” pits inward to bulk silicon to a considerable depth, thereby producing three-dimensional Si nanostructures such as arrays of nanowires (Figure 1c). Supporting Information Figure S5 shows ordered silicon nanoholes can be fabricated by the combination of deep ultraviolet lithography (UVL)32 and MCEE of silicon in aerated HF/H2O vapor, revealing the versatility of the present MCEE method. In general, longer etching time would cause more silicon corrosion and longer nanowires, but silicon corrosion would subsequently be inhibited by the thickening electrolyte layer

We suggest that anodic dissolution of silicon during MCEE of silicon in aerated HF/H2O vapor involves both the divalent and tetravalent dissolution processes. Analogous to electrochemical etching of porous silicon at low applied bias,37 the divalent dissolution reaction may be the main anodic half-cell reaction in MCEE of silicon. Ignoring the intermediate steps involved in silicon dissolution, the balanced overall reaction can be expressed in the following equation (where n is the number of holes consumed by each dissolved silicon atom) 4214

dx.doi.org/10.1021/nl500361u | Nano Lett. 2014, 14, 4212−4219

Nano Letters

Letter

Figure 4. SEM images of SiNW arrays on p-type 1−10 Ω·cm Si(100) wafers after (a) 1, (b) 2, and (c) 6 wet/dry cycle corrosion in atmospheric HF/H2O vapor. (d) SEM image of SiNW array on n-type 2−3 Ω·cm Si wafer after 6 wet/dry cycle corrosion in atmospheric HF/H2O vapor. (e) HRTEM image of SiNW produced from p-type 1−10 Ω·cm Si(100) wafer. (f) HRTEM image of SiNW produced from n-type 2−3 Ω·cm Si(100) wafer.

MCEE of Si in aerated HF/H2O vapor. The wet/dry cycle is defined by subjecting Si surface to 30 min damping in aerated HF/H2O vapor, followed by 1−2 min rapid drying in air. Figure 4 shows the cross-sectional SEM images depicting the effect of wet/dry cycling on the MCEE of silicon. It clearly shows that wet/dry cycling significantly enhances MCEE of Si in aerated HF/H2O vapor. Specifically, after 6 wet/dry cycles of MCEE the length of SiNW array produced on a p-Si(100) substrate is 6 μm (Figure 4c), which is about 6 times of that obtained without cycling (Figure 4a). Figure 4d shows the SEM image of SiNW array similarly prepared on a n-Si(100) wafer after 6 wet/dry cycles of MCEE. The SEM observations described above show that the wet/dry cycling of MCEE is capable of producing three-dimensional Si nanostructure layers with variable lengths. Figure 4 panels e and f, respectively, show the high-resolution transmission electron microscopic (TEM) images of the as-prepared SiNWs on p-Si and n-Si wafers, revealing their microstructural features are identical to those prepared by conventional MCEE of silicon in aqueous HF solution. The above results also clearly show that the length of n-type SiNW array is much shorter than that of p-type SiNW array produced under the same MCEE condition, revealing that the etch rate of n-Si is much slower than that of p-Si. We suggest the distinctly different etch rate is attributable to the more abundant holes in p-Si than n-Si. It is widely accepted that holes are injected by strong oxidants into the valence band of silicon during electroless etching of silicon.11,42 In conventional

condensing on sample surface. Note that MCEE of Si in aerated HF/H2O vapor is considerably reduced when the “invisibly” thin electrolyte layer became visibly thick, indicating thicker electrolyte layer would lead to slower corrosion rate. Such corrosion behavior is analogous to that observed in metals. Figure 3a,b shows the cross-sectional SEM images of SiNW arrays on p-type Si wafers after 1 and 5 h etching in aerated HF/H2O vapor, respectively. The length of SiNW array obtained after 5 h etching almost equals to that of SiNW array obtained after 1 h etching, confirming that the visibly thick electrolyte layer formed during etching reduces MCEE of silicon to a negligible level. This is expected because the thickening electrolyte layer would increasingly hinder oxygen transport to metal nanoparticles and reduce oxygen reduction reaction. We further investigated the MCEE rate of silicon in different HF/H2O vapor environments by vaporizing aqueous HF solutions with different HF concentrations (Supporting Information Figure S6). It can be clearly seen that the etch rate of silicon can be effectively enhanced with increasing HF concentration for vaporization but is reduced for HF concentration higher than 14 mol L−1. It is well-known that atmospheric corrosion of iron is accelerated by wet/dry cycling. The same phenomenon may be expected for MCEE of silicon in aerated HF/H2O vapor; namely MCEE of Si in aerated HF/H2O vapor would be similarly enhanced by wet/dry cycling. On the basis of this consideration, we investigate the effect of wet/dry cycling on 4215

dx.doi.org/10.1021/nl500361u | Nano Lett. 2014, 14, 4212−4219

Nano Letters

Letter

Figure 5. (a) Schematics illustrating Si partly submerged vertically in aqueous HF solution. (b) Photographs and SEM images of A, B, and C zones on Si surface after 30 min MCEE in aqueous HF solution.

MCEE of silicon in aqueous HF/H2O2 solution, we have found that the etch rates of silicon are dependent on doping type and oxidant concentration. Supporting Information Figure S7 shows the MCEE rates of p- and n-type silicon samples versus H2O2 concentration in aqueous HF-H2O2 solution. The oxidant concentration-dependent etching behavior indicates H2O2 at high concentration can inject abundant holes into the valence band of silicon, thus suppressing the effect of charge carrier characteristics of silicon substrates. However, this is not the case for the vapor and wet etching of MCEE of silicon with oxygen as oxidant. We suggest that the difference is caused by the limited solubility of oxygen in both thin HF layer condensed on silicon surface and aqueous HF solution. We next evaluate the influence of electrolyte layer thickness on MCEE of silicon by monitoring the surface structure of a silicon wafer piece that is partly submerged vertically in aqueous HF solution, as schematically illustrated in Figure 5a. Silicon surface is divided into the following three distinct zones: (A) the exposed area above the meniscus; (B) the narrow meniscus area; and (C) the submerged area below the water line. Figure 5b shows the photograph of the p-type Si specimen after 30 min MCEE in the reactor and the typical crosssectional SEM images of the Si surface in A, B, and C zones. It clearly reveals that A, B, and C zones display distinctively different colors due to different surface microstructures. In

general, a stronger oxidizing condition would lead to more severe corrosion or MCEE. The black zone A consists of aligned SiNW array with a considerable and uniform length, indicating MCEE of zone A in aerated HF/H2O vapor is uniform and greatly accelerated due to faster transport of oxygen from vapor to Si surface. In contrast, MCEE of Si in zone B and C is comparatively slower due to the limited oxygen concentration in aqueous HF solution. Note that MCEE of the upper part of zone B close to the meniscus would be much faster than that of the lower part of zone B adjacent to zone C, because oxygen near the meniscus area (three phase boundary) could be replenished more quickly through the shorter path of the electrolyte. Extensive SEM observations show MCEE of Si below the waterline is almost independent of electrolyte thickness, indicating MCEE of Si below the waterline is limited by mass transport of oxygen, thus reducing the kinetics of oxygen reduction reaction. On the basis of SEM observations, the etch rate, represented by the etch depth or the length of silicon nanowires, versus distance to the waterline is given in Supporting Information Figure S8. The above experiments demonstrated that HF/H2O vapor allows oxygen to function as H2O2 and lead to enhanced silicon etching in the absence of large cathode, but the amount of HF used is not reduced. To date, reducing the amount of needed HF solution is a daunting challenge facing current MCEE 4216

dx.doi.org/10.1021/nl500361u | Nano Lett. 2014, 14, 4212−4219

Nano Letters

Letter

Figure 6. A schematic diagram of experimental setup used for MCEE of silicon in aerated HF/H2O vapor. Inset shows the photograph of wafer-scale SiNW array on a 4 in. silicon wafer.

aerated HF/H2O vapor for facile fabrication of three-dimensional silicon nanostructures such as silicon nanowires (SiNW) arrays with a considerable length. Compared to the conventional MCEE of silicon performed in aqueous HF solution, MCEE of silicon in aerated HF/H2O vapor offers the advantages of simplicity, flexibility, environmental friendliness, and scalability due to replacement of harsh and costly oxidants (such as H2O2 and AgNO3) by nontoxic, environmental-green, and ubiquitous oxygen in air and miniscule water consumption and waste production. The development of MCEE of silicon in aerated HF/H2O vapor and new insights of its reaction mechanisms should facilitate the production of a variety of high-quality silicon nanostructures on an industrial scale.

methods. We design an experimental setup schematically illustrated in Figure 6 to address this challenge. The setup mainly consists of a HF storage tank, clean air source, and a PTFE chamber for vapor etching. Inset shows the typical photograph of a 4 in. silicon wafer after MCEE in aerated HF/ H2O vapor. The entire wafer surface is dark due to excellent light trapping by the as-prepared vertically aligned SiNW arrays on surface. Compared with conventional wet MCEE process requiring a large amount of HF solution, the current design can be easily scaled up and requires considerably smaller amount of HF solution. Specifically, aqueous HF solution can be fully utilized for etching, whereas this is not the case for the wet MCEE methods, in which the HF solution after every MCEE process has to be replaced with fresh HF solution. It is important to point out that excessive hydrogen evolution and buildup accompanying silicon etching in a closed chamber may cause the possibility of an explosion in the presence of oxygen and metal. The following safety conditions and measures are always observed in performing MCEE of Si in the reactor shown in Figure 6. (1) Hydrogen evolution and buildup are small (see GC analysis below) due to slow MCEE rate of silicon in aerated HF/H2O vapor; (2) oxygen for MCEE of silicon is provided in the form of clean air and continually decreased in the reactor during silicon etching; (3) wet/dry cycle etching is used to produce silicon nanostructures, and the reactor is regularly purged after each etching cycle. As a result, GC analysis confirms that the collected gaseous products contain only 1.13% hydrogen, which is substantively lower than the flammability limits (4.0 to 75.0% by volume in air) or the explosion limits (18.3 to 59.0% by volume in air).43 While no explosion has ever occurred in our studies, we are quick to recommend that hydrogen generation should be monitored and kept below the danger thresholds during MCEE of silicon. In conclusion, we demonstrated that metal-catalyzed electroless etching (MCEE) of silicon can be performed simply in

■

ASSOCIATED CONTENT

S Supporting Information *

Sample fabrication and characterization and other supporting figures and plots. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We acknowledge financial support from the Major Research Plan of the National Natural Science Foundation of China (91333208), National Basic Research Program of China (2012CB93220), and the Fundamental Research Funds of the Central Universities (2012LZD02). We acknowledge XiaoFan Huang from Tsinghua University and Huadian Coal Industry Group Co. Ltd for GC characterization. 4217

dx.doi.org/10.1021/nl500361u | Nano Lett. 2014, 14, 4212−4219

Nano Letters

■

Letter

(22) Chern, W.; Hsu, K.; Chun, I. S.; de Azeredo, B. P.; Ahmed, N.; Kim, K. H.; Zuo, J.; Fang, N.; Ferreira, P.; Li, X. Nonlithographic Patterning and Metal-Assisted Chemical Etching for Manufacturing of Tunable Light-Emitting Silicon Nanowire Arrays. Nano Lett. 2010, 10, 1582−1588. (23) Peng, K. Q.; Xu, Y.; Wu, Y.; Yan, Y. J.; Lee, S. T.; Zhu, J. Aligned single-crystalline Si nanowire arrays for photovoltaic applications. Small 2005, 1, 1062−1067. (24) Tsujino, K.; Matsumura, M.; Nishimoto, Y. Texturization of multicrystalline silicon wafers for solar cells by chemical treatment using metallic catalyst. Sol. Energy. Mater. Sol. Cells. 2006, 90, 100−110. (25) Hochbaum, A. I.; Chen, R.; Delgado, R. D.; Liang, W.; Garnett, E. C.; Najarian, M.; Majumdar, A.; Yang, P. D. Enhanced thermoelectric efficiency of rough silicon nanowires. Nature 2008, 451, 163−167. (26) Peng, K. Q.; Jie, J. S.; Zhang, W. J.; Lee, S. T. Silicon nanowires for rechargeable lithium-ion battery anodes. Appl. Phys. Lett. 2008, 93, 033105. (27) Garnett, E. C.; Yang, P. D. Silicon Nanowire Radial p−n Junction Solar Cells. J. Am. Chem. Soc. 2008, 130, 9224−9225. (28) Hwang, Y.; Bukai, A.; Yang, P. D. High Density n-Si/n-TiO2 Core/Shell Nanowire Arrays with Enhanced Photoactivity. Nano Lett. 2009, 9, 410−415. (29) Sivakov, V.; Andra, G.; Gawlik, A.; Berger, A.; Plentz, J.; Falk, F.; Christiansen, S. H. Silicon Nanowire-Based Solar Cells on Glass: Synthesis, Optical Properties, and Cell Parameters. Nano Lett. 2009, 9, 1549−1554. (30) Peng, K. Q.; Wang, X.; Wu, X. L.; Lee, S. T. Platinum Nanoparticle Decorated Silicon Nanowires for Efficient Solar Energy Conversion. Nano Lett. 2009, 9, 3704−3709. (31) Chang, S. W.; Oh, J.; Boles, S. T.; Thompson, C. V. Fabrication of silicon nanopillar-based nanocapacitor arrays. Appl. Phys. Lett. 2010, 96, 153108. (32) Peng, K. Q.; Wang, X.; Li, L.; Wu, X. L.; Lee, S. T. HighPerformance Silicon Nanohole Solar Cells. J. Am. Chem. Soc. 2010, 132, 6872−6873. (33) Shen, X. J.; Sun, B. Q.; Liu, D.; Lee, S. T. Hybrid Heterojunction Solar Cell Based on Organic−Inorganic Silicon Nanowire Array Architecture. J. Am. Chem. Soc. 2011, 133, 19408− 19415. (34) Brammer, K. S.; Choi, C.; Oh, S.; Cobb, C. J.; Connelly, L. S.; Loya, M.; Kong, S. D.; Jin, S. Antibiofouling, Sustained Antibiotic Release by Si Nanowire Templates. Nano Lett. 2009, 9, 3570−3574. (35) Wang, X. T.; Shi, W. S.; She, G. W.; Mu, L. X. Using Si and Ge Nanostructures as Substrates for Surface-Enhanced Raman Scattering Based on Photoinduced Charge Transfer Mechanism. J. Am. Chem. Soc. 2011, 133, 16518−16523. (36) Wang, X.; Peng, K. Q.; Hu, Y.; Zhang, F. Q.; Hu, B.; Li, L.; Wang, M.; Meng, X. M.; Lee, S. T. Silicon/Hematite Core/Shell Nanowire Array Decorated with Gold Nanoparticles for Unbiased Solar Water Oxidation. Nano Lett. 2014, 14, 18−23. (37) Sailor, M. J. Porous Silicon in Practice: Preparation, Characterization, and Applications; Wiley-VCH: Weinheim: Germany, 2012. (38) Liu, L.; Peng, K. Q.; Hu, Y.; Wu, X. L.; Lee, S. T. Fabrication of Silicon Nanowire Arrays by Macroscopic Galvanic Cell-Driven Metal Catalyzed Electroless Etching in Aerated HF Solution. Adv. Mater. 2014, 26, 1410−1413. (39) Hu, Y.; Peng, K. Q.; Liu, L.; Qiao, Z.; Huang, X.; Wu, X. L.; Meng, X. M.; Lee, S. T. Continuous-flow Mass Production of Silicon Nanowires via Substrate-Enhanced Metal-Catalyzed Electroless Etching of Silicon with Dissolved Oxygen as an Oxidant. Sci. Rep. 2014, 4, 3667. (40) Watanabe, H.; Ohnishi, S.; Honma, I.; Kitajima, H.; Ono, H.; Wilhelm, R. J.; Sophie, A. J. L. Selective Etching of Phosphosilicate Glass with Low Pressure Vapor HF. J. Electrochem. Soc. 1995, 142, 237−243. (41) Habuka, H.; Otsuka, T. Reaction of Hydrogen Fluoride Gas at High Temperatures with Silicon Oxide Film and Silicon Surface. Jpn. J. Appl. Phys. 1998, 37, 6123−6127.

REFERENCES

(1) Morinaga, H.; Futatsuki, T.; Ohmi, T.; Fuchita, E.; Oda, M.; Hayashi, C. Behavior of ultrafine metallic particles on silicon wafer surface. J. Electrochem. Soc. 1995, 142, 966−970. (2) Li, X.; Bohn, P. W. Metal-assisted chemical etching in HF/H2O2 produces porous silicon. Appl. Phys. Lett. 2000, 77, 2572−2574. (3) Peng, K. Q.; Yan, Y. J.; Gao, S. P.; Zhu, J. Synthesis of large-area silicon nanowire arrays via self-assembling nanoelectrochemistry. Adv. Mater. 2002, 14, 1164−1167. (4) Mitsugi, N.; Nagai, K. Pit formation induced by copper contamination on silicon surface immersed in dilute hydrofluoric acid solution. J. Electrochem. Soc. 2004, 151, G302−306. (5) Huang, Z.; Geyer, N.; Werner, P.; Boor, J.; Gösele, U. MetalAssisted Chemical Etching of Silicon: A Review. Adv. Mater. 2011, 23, 285−308. (6) Peng, K. Q.; Wu, Y.; Fang, H.; Zhong, X. Y.; Xu, Y.; Zhu, J. Uniform, axial-orientation alignment of one-dimensional single-crystal silicon nanostructure arrays. Angew. Chem., Int. Ed. 2005, 44, 2737. (7) Lai, C. Q.; Cheng, H.; Choi, W. K.; Thompson, C. V. Mechanics of Catalyst Motion during Metal-Assisted Chemical Etching of Silicon. J. Phys. Chem. C 2013, 117, 20802−20809. (8) Weisse, J.; Kim, D.; Lee, C.; Zheng, X. Vertical Transfer of Uniform Silicon Nanowire Arrays via Crack Formation. Nano Lett. 2011, 11, 1300−1305. (9) Yae, S.; Morii, Y.; Fukumuro, N.; Matsuda, H. Catalytic activity of noble metals for metal-assisted chemical etching of silicon. Nanoscale Res. Lett. 2012, 7, 352−356. (10) Peng, K. Q.; Fang, H.; Hu, J. J.; Wu, Y.; Zhu, J.; Yan, Y. J.; Lee, S. T. Metal-Particle-Induced, Highly Localized Site-Specific Etching of Si and Formation of Single-Crystalline Si Nanowires in Aqueous Fluoride Solution. Chem. Eur. J. 2006, 12, 7942−7947. (11) Peng, K. Q.; Hu, J. J.; Yan, Y. J.; Wu, Y.; Fang, H.; Xu, Y.; Lee, S. T.; Zhu, J. Fabrication of Single-Crystalline Silicon Nanowires by Scratching a Silicon Surface with Catalytic Metal Particles. Adv. Funct. Mater. 2006, 16, 387−394. (12) Huang, Z. P.; Fang, H.; Zhu, J. Fabrication of silicon nanowire arrays with controlled diameter, length and density. Adv. Mater. 2007, 19, 744−748. (13) Peng, K. Q.; Zhang, M. L.; Lu, A. J.; Wong, N. B.; Zhang, R. Q.; Lee, S. T. Ordered silicon nanowire arrays via nanosphere lithography and metal-induced etching. Appl. Phys. Lett. 2007, 90, 163123. (14) Chen, C. Y.; Wu, C. S.; Chou, C. J.; Yen, T. J. Morphological control of single-crystalline silicon nanowire arrays near room temperature. Adv. Mater. 2008, 20, 3811−3814. (15) Huang, Z.; Zhang, X.; Reiche, M.; Liu, L. F.; Lee, W.; Shimizu, T.; Senz, S.; Gosele, U. Extended Arrays of Vertically Aligned Sub-10 nm Diameter [100] Si Nanowires by Metal-Assisted Chemical Etching. Nano Lett. 2008, 8, 3046−3051. (16) Choi, W. K.; Liew, T. H.; Dawood, M. K.; Smith, H. I.; Thompson, C. V.; Hong, M. H. Synthesis of Silicon Nanowires and Nanofin Arrays Using Interference Lithography and Catalytic Etching. Nano Lett. 2008, 8, 3799−3802. (17) Chang, S. W.; Chuang, V. P.; Boles, S. T.; Ross, C. A.; Thompson, C. V. Fabricated using block-copolymer lithography and metal-assisted etching. Adv. Funct. Mater. 2009, 19, 2495−2500. (18) Huang, Z.; Shimizu, T.; Senz, S.; Zhang, Z.; Zhang, X.; Lee, W.; Geyer, N.; Gosele, U. Ordered Arrays of Vertically Aligned [110] Silicon Nanowires by Suppressing the Crystallographically Preferred Etching Directions. Nano Lett. 2009, 9, 2519−2525. (19) Hochbaum, A.; Gargas, D.; Yang, P. Single Crystalline Mesoporous Silicon Nanowires. Nano Lett. 2009, 9, 3550−3554. (20) Geyer, N.; Huang, Z.; Fuhrmann, B.; Grimm, S.; Reiche, M.; Nguyen-Duc, T.; Boor, J.; De Leipner, H.; Werner, P.; Goesele, U. Sub-20 nm Si/Ge Superlattice Nanowires by Metal-Assisted Etching. Nano Lett. 2009, 9, 3106−3110. (21) Peng, K. Q.; Lu, A. J.; Zhang, R. Q.; Lee, S. T. Motility of Metal Nanoparticles in Silicon and Induced Anisotropic Silicon Etching. Adv. Funct. Mater. 2008, 18, 3026−3035. 4218

dx.doi.org/10.1021/nl500361u | Nano Lett. 2014, 14, 4212−4219

Nano Letters

Letter

(42) Kolasinski, K. W. Charge Transfer and Nanostructure Formation During Electroless Etching of Silicon. J. Phys. Chem. C 2010, 114, 22098−22105. (43) Lewis, B.; Von Elbe, G. Combustion, Flames and Explosions of Gases; Academic Press: New York, 1961.

4219

dx.doi.org/10.1021/nl500361u | Nano Lett. 2014, 14, 4212−4219