pubs.acs.org/Langmuir © 2010 American Chemical Society
Characterization of Flowerlike Silicon Particles Obtained from Chemical Etching: Visible Fluorescence and Superhydrophobicity Peng Shen,* Norihisa Uesawa, Susumu Inasawa, and Yukio Yamaguchi Department of Chemical System Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received April 23, 2010. Revised Manuscript Received July 5, 2010 Flowerlike silicon particles are obtained by chemical etching of polycrystalline silicon polyhedrons using a mixture of hydrofluoric acid and nitric acid. The etched flowerlike particles show stable bright red photoluminescence under UV irradiation. The formation of pores with diameters of 3, 5.5, and 20 nm is revealed during etching. The etched particles exhibit superhydrophobic behavior with a contact angle of 158 because of the sharp tips of their “petals”. The source silicon polyhedrons are shown to possess radial grain boundaries. Preferential etching along the radial grain boundaries of the polyhedrons is thought to be the key reason for the formation of flowerlike porous silicon particles.
Introduction Photoluminescent (PL) low-dimensional silicon materials have been investigated since the 1990s because of their unique optical and electronic properties.1 Their confined dimensionality and unique surface states are used to explain their PL properties at room temperature. The optoelectronic and bioimaging applications of zero-dimensional quantum dots2 and one-dimensional nanowires3 have been widely studied. In addition, wafer-based porous silicon also has potential applications in the fields of health and energy such as bioimaging,4,5 drug carriers,6 cancer therapy,7sensors,8 lithium ion batteries,9 photovoltaics,10 and bioactive surfaces.11,12 In principle, the main processes to obtain porous silicon can be classified into two categories: etching of silicon wafers13-18 and *To whom correspondence should be addressed. E-mail: shenpeng@ chemsys.t.u-tokyo.ac.jp. (1) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046. (2) Erogbogbo, F.; Yong, K. T.; Roy, I.; Xu, G. X.; Prasad, P. N.; Swihart, M. T. ACS Nano 2008, 2, 873. (3) Mu, L.; Shi, W.; She, G.; Chang, J. C.; Lee, S. T. Angew. Chem., Int. Ed. 2009, 48, 3469. (4) Tasciotti, E.; Liu, X.; Bhavane, R.; Plant, K.; Leonard, A. D.; Price, B. K.; Cheng, M. M.; Decuzzi, P.; Tour, J. M.; Robertson, F.; Ferrari, M. Nature Nanotechnol. 2008, 3, 151. (5) Park, J.; Gu, L.; Maltzahn, G. V.; Ruoslahti, E.; Bhatia, S. N.; Sailor, M. J. Nature Mater. 2009, 8, 331. (6) Wu, E. C.; Park, J. H.; Park, J.; Segal, E.; Cunin, F.; Sailor, M. J. ACS Nano 2008, 2, 2401. (7) Lee, C.; Kim, H.; Hong, C.; Kim, M.; Hong, S. S.; Lee, D. H.; Lee, W. I. J. Mater. Chem. 2008, 18, 4790. (8) Harper, J.; Sailor, M. J. Anal. Chem. 1996, 68, 3713. (9) Kim, H.; Han, B.; Choo, J.; Cho, J. Angew. Chem., Int. Ed. 2008, 47, 10151. (10) Sun, W.; Kherani, N. P.; Hirschman, K. D.; Gadeken, L. L.; Fauchet, P. M. Adv. Mater. 2005, 17, 1230. (11) Canham, L. T. Adv. Mater. 1995, 7, 1033. (12) De Stefano, L.; Rea, I.; Giardina, P.; Armenante, A.; Rendina, I. Adv. Mater. 2008, 20, 1529. (13) Cullis, A. G.; Canham, L. T. Nature 1991, 353, 335. (14) Lockwood, D. J. Solid State Commun. 1994, 92, 101. (15) Wang, F.; Song, S.; Zhang, J. Chem. Commun. 2009, 28, 4239. (16) Dubbelday, W. B.; Szaflarski, D. M.; Shimabukuro, R. L.; Russell, S. D.; Sailor, M. J. Appl. Phys. Lett. 1993, 62, 1694. (17) Kelly, M. T.; Chun, J. K. M.; Bocarsly, A. B. Appl. Phys. Lett. 1994, 64, 1693. (18) Noguchi, N.; Suemune, I. Appl. Phys. Lett. 1993, 62, 1429. (19) Richman, E. K.; Kang, C. B.; Brezesinski, T.; Tolbert, S. H. Nano Lett. 2008, 8, 3075. (20) Hai, N. H.; Grigoriants, I.; Gedanken, A. J. Phys. Chem. C 2009, 113, 10521.
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reduction of ordered silica.19,20 The former method is commonly adopted because silicon wafers are readily available as a source material, and their inherent crystallinity is useful for optoelectronic applications. Mechanical ball milling or strong sonication is often conducted after etching to break the products down into microsized pieces. In general, traditional stain chemical etching lacks reproducibility and uniformity in the formed porous structures.21 As a result, electrochemical and metal-assisted electroless etchings are now widely used. Electrochemical etching is conducted under a lateral electric field, which greatly enhances the etching rate. Using this technique, luminescent porous silicon has been formed on both p-type13-15 and n-type22 silicon wafers. On another front, metal-assisted electroless etching allows control over the etching depth. Depending on the etching conditions, ordered arrays of silicon nanowires and porous silicon structures can be formed on the top surface of silicon wafers using noble metals such as gold23,24 or silver.25,26 In essence, this is a galvanic displacement process, in which silicon is spontaneously oxidized and metal ions are reduced to metallic particles. Metal-assisted chemical etching can be used to produce mesoporous silicon nanowires.27 Although a high pore ratio can be realized using a high electric field or metal particles, it should be noted that these etching methods are expensive because porous silicon films are formed only on the surface of the wafer. It is well-known that silicon wafers have a single crystalline structure with/without dopants. The single crystalline nature of silicon wafers causes common chemical etching to proceed almost homogeneously so the formation of a porous structure is unusual. This is one of the reasons that chemical stain etching gives poor reproducibility of pore formation. Innovative procedures, such as (21) Cullis, A. G.; Canham, L. T.; Calcott, P. D. J. J. Appl. Phys. 1997, 82, 909. (22) Li, S. Q.; Wijesinghe, T. L. S. L.; Blackwood, D. J. Adv. Mater. 2008, 20, 3165. (23) Buttner, C. C.; Langner, A.; Geuss, M.; Muller, F.; Werner, P.; Gosele, U. ACS Nano 2009, 3, 3122. (24) Zhu, J.; Bart-Smith, H.; Begley, M. R.; Zangari, G.; Reed, M. L. Chem. Mater. 2009, 21, 2721. (25) Peng, K. Q.; Hu, J. J.; Yan, Y. J.; Wu, Y.; Fang, H.; Xu, Y.; Lee, S. T.; Zhu, J. Adv. Funct. Mater. 2006, 16, 387. (26) Peng, K. Q.; Zhang, M. L.; Lu, A.; Wong, N. B.; Zhang, R.; Lee, S. T. Appl. Phys. Lett. 2007, 90, 163123. (27) Hochbaum, A. I.; Gargas, D.; Hwang, Y. J.; Yang, P. Nano Lett. 2009, 9, 3550.
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electrochemical etching and metal-assisted electroless etching, need to be developed to obtain porous silicon from silicon wafers easily. If polycrystalline silicon is used as a starting material, the situation changes because polycrystalline silicon has a large number of grain boundaries. These boundaries are etched more rapidly than the crystalline regions, resulting in a rough/porous structure being formed. In fact, chemical stain etching of a polycrystalline silicon film causes pores to form at the grain boundaries, giving a surface-roughened silicon film displaying PL.28,29 Although etching procedures have been thoroughly developed to form porous silicon from wafers, polycrystalline silicon should also be considered as a source material. It is expected that simple chemical etching can be used to readily form reproducible porous structures if the etching rate for crystal defects, such as grain boundaries and dislocations, is significantly different from that for crystal surface. Herein, we present flowerlike porous silicon particles that are formed reproducibly via simple chemical etching using a mixture of hydrofluoric acid (HF) and nitric acid (HNO3). The silicon particles used were formed by gas phase reduction of silicon tetrachloride with zinc vapor. These particles are polycrystalline and have crystal boundaries that are radially oriented. Because each “petal” in a flowerlike particle is a single crystal, preferential etching along radially oriented grain boundaries is thought to be one of the main mechanisms for the formation of porous flowerlike silicon particles. Furthermore, the porous particles show superhydrophobicity and stable visible PL under UV irradiation. Porous structures with three different pore sizes, 3, 5.5, and 20 nm, are formed after etching, and these nanopores are considered to be the origin of visible PL and superhydrophobicity in the etched particles.
Experimental Section Chemicals. Silicon tetrachloride (SiCl4) (99.9999%) was purchased from Tri Chemical Laboratories Inc., Japan. Zinc metal (99.995%) was purchased from Mitsui Kinzoku, Japan. Ethanol (HPLC grade), hydrofluoric acid (47 wt % in water), and nitric acid (69 wt % in water) were purchased from Wako Pure Chemical Industries, Japan. All of the chemicals were used as received without further purification. Synthesis of Silicon Particles. The same reactor system was used as reported in ref 30. In brief, silicon particles were synthesized in a horizontal quartz reactor placed within an electric furnace. The length and diameter of the reactor were 500 mm and 50 mm, respectively. Two different lines of argon gas (research grade 99.9995%) carried SiCl4 and zinc vapor into the reactor, respectively. To make zinc vapor, zinc was heated to 885 C in a quartz heating vessel, and the zinc vapor formed at the surface of the liquid zinc was transported using argon carrier gas. The total flow rate is a summation of the SiCl4, Zn, and Ar flow and was fixed at 1106 sccm. A quartz boat was set into the reactor to collect silicon particles with a production rate of about 400 mg/h. The concentration of the introduced SiCl4 gas was measured by gas chromatography (Shimadzu GC-8A). The amount of zinc vapor introduced was measured by considering the change in mass of the zinc solids before and after the reaction. The ratio of the introduced gas was SiCl4:Zn:Ar = 1:5:9. The reaction temperature was fixed at 920 C. The total reaction pressure was 1 atm. Using these conditions, silicon solids with a variety of morphologies, such as silicon nanowires,30 were obtained depending on the position in the reactor. Particles generally (28) Kageyama, Y.; Murase, Y.; Tsuchiya, T.; Funabashi, H.; Sakata, J. J. Appl. Phys. 2002, 91, 9408. (29) Steckl, A. J.; Xu, J.; Mogul, H. C. Appl. Phys. Lett. 1993, 62, 2111. (30) Uesawa, N.; Inasawa, S.; Tsuji, Y.; Yamaguchi, Y. J. Phys. Chem. C 2010, 114, 4291.
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formed around the gas outlet of the reactor, which is on the opposite side from the gas inlet, allowing nanowire-free particles to be collected. The particles seldom contained silicon solids with other morphologies. Etching of Silicon Particles. In a typical procedure, 20 mg of Si particles was immersed in 5 mL of HCl solution (1 M) for 15 min with ultrasonication. The amount of added HCl acid was calculated as a minimum of 6 times the stoichiometric ratio, assuming that the particles present consisted entirely of zinc. This step allows the residue Zn on the sample surface to be removed completely. The particles were then collected on a PTFE filter (100 nm pore size) using vacuum filtration and washed three times with deionized water to remove residual acid. The washed particles were resuspended in ethanol for further etching. Typically, the etchant was prepared by mixing HF and HNO3 solutions. The molar ratio was HF/HNO3 = 80/3. This procedure was used to etch silicon nanocrystals embedded in amorphous oxides.31 The etching time was in the range of 5-30 min. The etched particles were transferred onto a PTFE filter (100 nm pore size) and then washed with deionized water at least four times. For characterization, the particles were dried in a vacuum chamber at room temperature. Contact Angle Measurements. Particles were attached onto carbon tape that was adhered to a glass slide. The top surface of the tape was completely covered with particles. For comparison, a p-type silicon wafer that had been etched with HF was also prepared. Before measuring the contact angle of water, the etched particles and HF-treated wafer were exposed to air for 3 h to induce natural oxidation. The contact angle was measured by dropping 4 μL of deionized water onto the sample surface, and images were recorded using a CCD camera. Characterizations. Particles were directly observed with a SEM (Hitachi S-900 with an accelerating voltage of 6 kV) and a TEM (JEOL 2010-HC with accelerating voltage of 200 kV). Cross-sectional TEM images were taken using the same procedure at that reported in ref 32. EDS was used to analyze the elemental compositions of the samples. Size distribution histograms of the particles were obtained from SEM images by measuring the size of more than 200 particles. Raman spectra were obtained using a micro-Raman scattering spectrometer (Olympus BX-41, Horiba HR-800) with an Arþ laser (488 nm) as the excitation source. The level of zinc impurity in the particles was measured using ICP-OES (Perkin-Elmer, Optima 4300 DV). Nitrogen adsorption-desorption isotherms of the calcined samples were measured at 77 K with an Autosorb-1 (Quantachrome Instruments). The BET surface area was calculated from the adsorption branch in the relative pressure (P/P0) range from 0.05 to 0.25. The amount of particles used (∼400 mg) was enough to measure the surface area of the samples with negligible error. The samples for BET measurement were set into a quartz cell and degassed for 7 h at 250 C before determining the mass of the samples. The pore-size distribution was evaluated using the BJH method (N2 at 77 K on silica, cylindrical pore). PL spectra of the etched particles under a hand-held UV illuminator were recorded with a multichannel photodetector (Hamamatsu PMA-11). A sharp cut filter was used to remove higher energy UV light. Fourier transform infrared spectrometer (Shimadzu, IRPrestige-21 with attenuated total reflection accessories) measurement was carried out in air. The etched sample was dried in a nitrogenfilled desiccator before measurement.
Results and Discussion Characterization and Formation Mechanism of Flowerlike Si Particles. Figure 1a,b shows scanning electron microscope (SEM) images of the as-synthesized particles. These particles are (31) Li, X. G.; He, Y. Q.; Talukdar, S. S.; Swihart, M. T. Langmuir 2003, 19, 8490. (32) Tsuji, Y.; Noda, S.; Yamaguchi, Y. J. Vac. Sci. Technol. B 2007, 25, 1892.
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Figure 1. SEM images of particles before (a, b) and after (c, d) chemical etching.
polyhedral in shape, which is different from silicon microspheres obtained from disilane gas decomposition by chemical vapor deposition.33 The particles are highly crystalline, as shown in Figure S1 of the Supporting Information. The surface of the particles shown in Figure 1b consists of small facets, which implies that they are polycrystalline. Energy-dispersive X-ray spectroscopy (EDS) mapping (Supporting Information Figure S2) showed that the surface of the as-synthesized particles contains silicon, oxygen, and zinc. The particles were washed with HCl to remove the zinc deposited on the surface prior to use. Both the size and morphology of the particles hardly change after washing with HCl. Inductively coupled plasma optical emission spectrometry (ICP-OES) showed that the level of zinc impurity in the particles after washing with HCl was 83 ppm. After washing with HCl, the particles were etched with a mixture of HF and HNO3. Figure 1c,d shows that the surface morphology of the particles changed drastically upon etching to “flowerlike”. Despite the significant change in the morphology of the particles after etching for 10 min, the average size of the particles hardly changed (see Supporting Information Figure S3). Etching occurs preferentially toward the center of the particles rather than over the surface. The detailed structure of the flowerlike particles was observed by transmission electron microscopy (TEM). Figure 2a,b shows dark- and bright-field TEM images of “petals” of a silicon flower after etching. In Figure 2a, the bright part indicates the crystals in the petals. An electron diffraction pattern of the “petal” is shown in Figure 2c. Clear diffraction spots imply that the “petal” is a single crystal, and the growth direction is Æ110æ. The highresolution image of the edge of a “petal” in Figure 2d shows pores on the crystal surface. Nanometer-sized pores appear to form during chemical etching. Figure 3 shows micro-Raman spectra of the samples before and after etching. The spectrum of the as-synthesized sample shows a peak at 521 cm-1 which corresponds to the phonon vibration of bulk silicon. After etching, this peak shifts by 2 cm-1 to a shorter wavenumber (inset in Figure 3), consistent with previous results from Raman experiments on porous silicon.34,35 In these publications, the shift in the Raman spectrum is explained by phonon confinement caused by the nanosized pores in the silicon. This result is in agreement with the TEM image in Figure 2c; however, it should be noted that the Raman shift only supplies an indirect rough statement to nanostructures of etched silicon. No peaks
were observed for amorphous silicon at around 480 cm-1, suggesting that amorphous silicon is not present in the particles. Brunauer-Emmett-Teller (BET) N2 adsorption-desorption isotherms were used to determine the surface properties of the etched particles. The measured nitrogen adsorption and desorption isotherms are shown in Figure 4a. The adsorption and desorption curve of the etched sample shows hysteresis, but the as-synthesized sample does not. This means that mesopores are present in the etched sample. Furthermore, abrupt increases in the adsorption curve of the etched sample are seen at P/P0 = 0.3, 0.6, and 0.9, indicating that three types of pores of different sizes are formed. Multipoint BET analysis was used to obtain the mean surface area of the samples. The values obtained are 9.6 and 41.4 m2 g-1 for the samples before and after etching, respectively. The pore size distribution of the etched particles was calculated using the Barrett-Joyner-Halenda (BJH) method, and the result is shown in Figure 4b. Three sizes of pores, with diameters of 3, 5.5, and 20 nm, are formed in the particles during etching. The surface area is not very large after etching compared with other porous structures.36,37 The pores are formed only on the surface
(33) Fenollosa, R.; Meseguer, F.; Tymczenko, M. Adv. Mater. 2008, 20, 95. (34) Tsu, R.; Shen, H.; Dutta, M. Appl. Phys. Lett. 1992, 60, 112. (35) Islam, M. N.; Kumar, S. Appl. Phys. Lett. 2001, 78, 715.
(36) Xue, C.; Tu, B.; Zhao, D. Adv. Funct. Mater. 2008, 18, 3914. (37) Mukti, R. R.; Hirahara, H.; Sugawara, A.; Shimojima, A.; Okubo, T. Langmuir 2010, 26, 2731.
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Figure 2. TEM images of the “petals” of flowerlike silicon particles: (a) dark-field and (b) bright-field. (c) Electron diffraction pattern of the petal which is indicated by the dashed square in (b). The single crystal petal grows along Æ110æ direction. (d) Highresolution image of the edge of a petal.
Figure 3. Raman spectra of the particles before (black) and after (red) etching. The inset highlights the peak shift after etching.
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Figure 6. (a) Cross-sectional TEM image of an as-synthesized silicon particle and (b) high-magnification image of part of (a). The white dashed lines emphasize the crystal boundaries.
Figure 4. (a) N2 adsorption and desorption isotherms of the sample before (] and O) and after ([ and þ) etching. (b) BJH pore size distribution of the etched flowerlike silicon particles.
Figure 5. SEM images of the time evolution of chemical etching. HCl-washed particles before etching (a) and after chemical etching for 5 min (b) and 25 min (c). Insets show a water drop on each particulate film.
of the “petals”. Two of the pore sizes obtained from BET analysis, 3-5 and 20 nm, roughly agree with pore sizes observed by TEM (Figure 2c). We concluded that due to chemical etching by a mixture of HF and HNO3, the polygonal silicon particles were etched and nanopores were formed on particle surface. To understand the formation mechanism of flowerlike particles, SEM images of the particles were taken at different etching times. Figure 5a shows a silicon particle before etching, while Figures 5b and 5c show particles after etching for 5 and 25 min, respectively. Initially, etching proceeds into the particle and narrow trenches are formed (Figure 5b). Further etching results in large pits because etching causes the pits to become both deeper and wider as shown in Figure 5c. As clearly shown in Figure 5, etching occurs preferentially into the particles. In chemical stain etching, the formation of NOþ ions or a strong oxidizing agent is a critical factor for pore generation.17 In general, a solid, such as NaNO2, is added to etchants to control the concentration of the oxidizing agent.17 In our method, the etchant or etching procedure is not the key issue in the formation of flowerlike silicon particles because a dilute solution of acid was used as the etchant. Luminescent flowerlike particles were also obtained using another etchant with a different molar ratio of HF and HNO3 (HF/HNO3 = 40/3). The difference in the ratio of the acid solution affects only the etching rate of the particles. Langmuir 2010, 26(16), 13522–13527
Because a small amount of zinc (83 ppm) is present as an impurity in the particles, there is a possibility that metal-assisted electroless etching is the mechanism of pore formation. In metalassisted electroless etching, metal ions are reduced to metal on the surface of silicon while silicon is etched. Metal particles, such as Ag, Au, and Pt, have been reported to etch silicon wafers in HF solution containing metal ions.25 These metals have positive redox potentials and hence can remain as metals even in solutions of HF. However, the redox potential of zinc (Zn2þ/Zn) is negative (-0.76 V versus a normal hydrogen electrode)38 so zinc metal is easily oxidized in acid. Because a strong oxidizing agent (HF/HNO3) was used, reduction of zinc ions to metallic zinc is unlikely. Furthermore, even if all of the zinc impurity has dissolved into the etchant, the concentration of zinc ions is estimated to be 2 10-6 M at most, which is ca. 4 orders of magnitude lower than the concentration of metal ions used in typical metal-assisted electroless etching conditions.25,27 This precludes the possibility of metal-assisted electroless etching occurring in the present system. Figure 6 shows a cross-sectional TEM image of an as-synthesized particle. The crystal clearly shows radial grain boundaries. The mechanism for the formation of radially oriented polycrystalline silicon particles is an interesting topic that is currently under investigation. The radial grain boundaries are thought to be the main reason for the formation of flowerlike particles through HF/ HNO3 etching because grain boundaries, which generally contain many dislocations or defects, are etched faster than crystals.39,40 To test this hypothesis, a p-type silicon wafer was etched using the same etchant. No structural changes were found for the wafer sample, even after etching for longer than 30 min. Therefore, the radial orientation of the crystals in the silicon particles allows the formation of flowerlike particles by etching. Optical Property of Flowerlike Si Particles. PL was observed from the etched particles upon UV irradiation. The particles are gray under ambient light (Figure 7a). After etching for a few minutes, the particles exhibit yellow PL under UV light as shown in Figure 7b. However, the yellow PL is not stable. After filtration of etched particles with yellow PL, the PL color changed from yellow to red. Furthermore, the PL of the particles also changed from yellow to red by additional etching with HF/ HNO3. The etched particles were collected on a polytetrafluoroethylene (PTFE) filter by filtration and then washed several times with water. The visible PL and its spectra obtained from the (38) Lide, D. R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002. (39) Jenkins, M. W. J. Electrochem. Soc. 1977, 124, 757. (40) Blauw, M. A.; Zijlstra, T.; Bakker, R. A.; Van der Drift, E. J. Vac. Sci. Technol. B 2000, 18, 3453.
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Figure 8. FT-IR spectra of the samples. From top to bottom: before etching, as-etched, 3 days, and half year after etching.
Figure 7. Digital photographs of the particles in a PTFE beaker during chemical etching under ambient lighting (a) and UV irradiation (b). (c) Red PL from particles collected on a PTFE filter after etching. (d) PL spectra of the etched sample shown in (c). Two different excitation wavelengths, 302 nm (red line) and 365 nm (black line), were used.
collected particles are shown in Figure 7c,d. The PL maximum is at 700 nm when excited at 302 nm. The peak exhibits a slight red shift when excited at 365 nm. The visible PL from porous silicon obtained via anodic etching has been well studied and is often attributed to a quantum confinement effect.1,13 The origin of the confinement is thought to be nanoscale silicon islands. Contrarily, some other groups report that red PL from porous silicon films may come from oxide or oxide-related species.41,42 Silicon is easily oxidized when it is exposed to air, causing its PL properties to change.43 Thus, despite many studies on the visible PL from porous silicon materials, the origin of this PL remains controversial. As a primitive experiment, the effect of oxide-related species on the PL properties of the silicon particles was tested. The flowerlike particles with red PL were again etched with HF solution to remove silicon oxides. Red PL was observed from the particles in HF solution, suggesting that this red PL does not originate from oxide-related species. In addition, the flowerlike particles show stable red PL in air for at least 6 months after etching. To understand the surface chemistry of the luminescent flowerlike particles, FT-IR was taken from the samples exposed to air for different time of oxidations. Figure 8 shows spectra of the sample before etching, as-etched, 3 days after etching, and a half year after etching. The sample before etching only shows a weak Si-O-Si peak at ≈1100 cm-1, indicating that particles surface was dominantly oxidized. After etching, the as-etched sample shows Si-Hx stretching vibration at ≈2100 cm-1 and the Si-O-Si peak was hardly observed. On exposure to air for 3 days, Si-Hx peak decreases and the Si-O-Si peak increases. This suggests that oxidation surely occurs due to exposure to air. Finally, the etched sample exposed to air for 6 months showed a strong Si-O-Si peak and two weak peaks around 2200-2300 cm-1, which could be (41) Wolkin, M. V.; Jorne, J.; Fauchet, P. M.; Allan, G.; Delerue, C. Phys. Rev. Lett. 1999, 82, 197. (42) Li, P.; Wang, G.; Ma, Y.; Fang, R. Phys. Rev. B 1998, 58, 4057. (43) Xu, Y. K.; Adachi, S. J. Appl. Phys. 2009, 105, 113525.
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attributed to O2SiH2 and O3SiH, respectively.44,45 Hydrogentermination on the etched silicon particles is gradually replaced by oxygen through natural oxidation in air. We found both etched particles and sufficiently oxidized sample show red PL, and hence red PL emission is scarcely affected by subsequent surface oxidation after etching. In addition, PL was not observed from the particles shown in Figure 5a,b under UV irradiation, but it was found for the particles that were etched for 25 min. This indicates that sufficient etching is necessary to produce particles with visible PL. We consider that formation of nanosized pores would be involved in the visible PL. Another point in our results is the PL color change from the yellow to red. As already described, the yellow PL is not stable and changes into red by further chemical etching with HF/HNO3. In addition, filtration and subsequent exposure to air also change the PL color into red. The color change in both cases is irreversible. Because this unstable yellow PL was observed only at the initial stage of visible PL emission during etching, irreversible change of nanosized pores by etching or oxidation could be an origin of the color change. Further investigation should be done to clarify the color change. Supherhydrophobicity of Etched Flowerlike Si Particles. The unique flowerlike particles contain many deep trenches (as shown in Figure 1c,d), which inspired us to investigate their wettability. It is well-known that naked silicon surfaces are hydrophobic in nature because of surface Si-H bonds, and this usually disappears upon oxidation as surface Si-O bonds form. It has been reported that sharp tips or a suitably rough surface can increase the hydrophobicity of a wide variety of materials.46,47 For example, wormlike surfaces,48 needlelike surfaces,49 and surface coatings with organic materials50 have been shown to exhibit superhydrophobicity with a contact angle of water of larger than 150. Superhydrophobic materials provide biomimetic advantages for applications including self-cleaning surfaces and anisotropic dewetting behavior.47,48,51 Charge-coupled device (CCD) images of the contact angle measurements of water on each sample are shown in Figure 9. While the contact angles of water on the as-synthesized particles (Figure 9a) and p-type silicon wafer etched with HF (Figure 9b) were almost the same, (44) Ling, L.; Kuwabara, S.; Abe, T.; Shimura, F. J. Appl. Phys. 1993, 73, 3018. (45) Liptak, R. W.; Kortshagen, U.; Campbell, S. A. J. Appl. Phys. 2009, 106, 064313. (46) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818. (47) Sun, T.; Feng, L.; Gao, X.; Jiang, L. Acc. Chem. Res. 2005, 38, 644. (48) Gao, L.; McCarthy, T. J. J. Am. Chem. Soc. 2006, 128, 9052. (49) Winkleman, A.; Gotesman, G.; Yoffe, A.; Naaman, R. Nano Lett. 2008, 8, 1241. (50) Liao, C.-S.; Wang, C.-F.; Lin, H.-C.; Chou, H.-Y.; Chang, F.-C. Langmuir 2009, 25, 3359. (51) Qi, D.; Lu, N.; Xu, H.; Yang, B.; Huang, C.; Xu, M.; Gao, L.; Wang, Z.; Chi, L. Langmuir 2009, 25, 7769.
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becomes larger after longer etching time, which also supports that many petals are necessary to show superhydrophobicity. This hydrophobicity is comparable to those of surface-treated films50,52 and fiberlike surfaces.53,54 In addition, the contact angle of water hardly changed on the etched particles even after annealing at 250 C for 7 h, indicating that the flowerlike structure is thermally durable. These superhydrophobic particles could be used as durable antifouling coating for surface.
Conclusions Flowerlike silicon particles were formed by HF/HNO3 chemical etching. The source silicon particles are polyhedral in shape and contain unique radial grain boundaries, which is the key factor in the formation of the flowerlike structures. The etched particles show bright red PL and superhydrophobicity with a contact angle of 158. Pores with diameters of 3, 5.5, and 20 nm are formed during etching in the flowerlike particles. We consider that the formation of nanosized pores would be involved in the visible PL emission and durable superhydrophobicity. Figure 9. Digital photographs of a 4 μL water drop on (a) assynthesized particles, (b) HF-treated Si wafer, and (c) etched flowerlike particles. It should be noted that samples (b) and (c) were adequately oxidized before measurements. Cross-sectional SEM images of the particulate films for superhydrophobicity tests are shown in (d) before etching and (e) after etching with their enlarged pictures (f and g, respectively).
a much larger contact angle was observed for the etched particles as seen in Figure 9c. The contact angle of water on the etched particles is 158. We also observed that a water drop bounced on the surface of a film of etched flowerlike particles. It should be noted that both the etched wafer and particles were exposed to air for 3 h to form an oxide layer before performing contact angle measurements. Crosse-sectional SEM images of particulate films are shown in Figures 9d and 9e. Both films have almost the same roughness on this length scale; however, as shown in the Figures 9f and 9g (enlarged parts of Figures 9d and 9e, respectively), surface roughness of each particle is different. Because hydrogen termination of silicon surface also brings hydrophobicity, we should consider the effect of surface bonding. We used etched particles stored in air for 6 months for contact angle measurement. The observed contact angle of water was almost same as shown in Figure 9c. This means that the sample still shows superhydrophobicity even after sufficient oxidation and formation of hydrophilic oxide surface as seen in Figure 8. Thereby, formation of submicrometer “petals” is the main origin that develops superhydrophobicity. The contact angle was also measured for the particles etched at different time. As shown in the insets in Figure 5, the contact angle
Langmuir 2010, 26(16), 13522–13527
Acknowledgment. The authors are grateful to H. Tsunakawa and K. Ibe for technical support for TEM, S. Ohtsuka for SEM mapping, and J. Wang, M. Kubo, and Prof. T. Okubo for BET analysis and kind discussion. We also thank Prof. Y. Tsuji and S. Nakamura for their technical support for XRD and crosssectional TEM observation. We are grateful to Dr. S. Hanada and Prof. K. Yamamoto for ATR FT-IR measurement. TEM-EDS analyses were conducted in Center for Nano Lithography & Analysis, The University of Tokyo, supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. P. Shen and N. Uesawa thank the University of Tokyo Global COE Chemistry Innovation through Cooperation of Science and Engineering and Mechanical System Innovation for financial support. This work was supported by the Industrial Technology Research Grant Program in 2009 from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. Supporting Information Available: XRD and size distribution of the silicon particles before and after chemical etching, EDS mapping, and spectrum of as-synthesized silicon particles. This material is available free of charge via the Internet at http://pubs.acs.org. (52) Li, Y.; Huang, X. J.; Heo, S. H.; Li, C. C.; Choi, Y. K.; Cai, W. P.; Cho, S. O. Langmuir 2007, 23, 2169. (53) Pastine, S. J.; Okawa, D.; Kessler, B.; Rolandi, M.; Llorente, M.; Zettl, A.; Frechet, J. M. J. J. Am. Chem. Soc. 2008, 130, 4238. (54) Lu, P.; Huang, Q.; Liu, B.; Bando, Y.; Hsieh, Y.-L.; Mukherjee, A. K. J. Am. Chem. Soc. 2009, 131, 10346.
DOI: 10.1021/la102516g
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