© Copyright 2006 American Chemical Society
APRIL 25, 2006 VOLUME 22, NUMBER 9
Letters Uniformly Sized Gold Nanoparticles Derived from PS-b-P2VP Block Copolymer Templates for the Controllable Synthesis of Si Nanowires Jennifer Q. Lu* and Sung Soo Yi Agilent Laboratories, 3500 Deer Creek Road, Palo Alto, California 94304 ReceiVed December 13, 2005. In Final Form: March 3, 2006 A monolayer of gold-containing surface micelles has been produced by spin-coating solution micelles formed by the self-assembly of the gold-modified polystyrene-b-poly(2-vinylpyridine) block copolymer in toluene. After oxygen plasma removed the block copolymer template, highly ordered and uniformly sized nanoparticles have been generated. Unlike other published methods that require reduction treatments to form gold nanoparticles in the zero-valent state, these as-synthesized nanoparticles are in form of metallic gold. These gold nanoparticles have been demonstrated to be an excellent catalyst system for growing small-diameter silicon nanowires. The uniformly sized gold nanoparticles have promoted the controllable synthesis of silicon nanowires with a narrow diameter distribution. Because of the ability to form a monolayer of surface micelles with a high degree of order, evenly distributed gold nanoparticles have been produced on a surface. As a result, uniformly distributed, high-density silicon nanowires have been generated. The process described herein is fully compatible with existing semiconductor processing techniques and can be readily integrated into device fabrication.
Interest in semiconducting nanowires has been fueled by continuous discoveries of their new and unique properties and demonstrations of nanowire-enabled devices for electronics, optoelectronics, photonics, and biosensing applications.1-8 It has been well recognized that the electrical and optical properties of these 1D nanostructures are size-dependent. Therefore, to commercialize devices based on their highly touted properties, it is imperative to develop a synthetic method that enables the growth of uniformly * Corresponding author. E-mail:
[email protected]. (1) Persson, A. I.; Larsson, M. W.; Stenstro¨m S.; Ohlsson B. J.; Samuelson L.; Wallenberg, L. R. Nat. Mater. 2004, 3, 677. (2) Thelander, C.; Mårtensson, T.; Bjo¨rk, M.; Ohlsson, B. J.; Larsson, M. W.; Wallenburg, L. R.; Samuelson, L. Appl. Phys. Lett. 2003, 83, 2052. (3) Nelson, J.; van Buuren, T.; Willey, T. M.; Bostedt, C.; Adams, J. J.; Schaffers, K. I.; Terminello, L. J.; Callcott. T. A. Nano Lett. 2004, 4, 137. (4) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D.; Leiber, C. M. Nature 2002, 415, 617. (5) Duan, X.; Huang, Y.; Agarwai, R.; Lieber, C. M. Nano Lett. 2002, 2, 487. (6) Zheng, G.; Patolsky, F.; Cui, Y.; Wang, W. U.; Lieber, C. M. Nat. Biotechnol. 2005, 23, 1294. (7) Law, M.; Goldberger, J.; Yang P. Annu. ReV. Mater. Sci. 2004, 34, 83. (8) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455.
sized nanowires with consistent properties at predefined locations. Over the years, chemical vapor deposition has gained in popularity as a method to grow nanowires selectively on the catalytically active sites. This method is based on the vapor-liquid-solid (VLS) scheme in which catalyst nanoparticles interact with a vapor source to form eutectic nanodroplets. The supersaturation of gaseous reactants on the eutectic nanodroplets leads to solidification and outward growth as perfect single-crystal nanowires.9,10 Thus, catalyst nanoparticle size determines nanowire diameter as has been verified experimentally.11 So far, gold is the most widely used catalyst material. There are two main methods to generate gold nanoparticles for nanowire growth. One is to use a thin deposited gold film. At elevated temperature, such a thin film breaks up to form gold nanoparticles. The other method is to coat surface with gold colloidal nanoparticles. The first method is capable of producing densely populated gold (9) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89. (10) Westwater, J.; Gosain, D. P.; Tomiya, S.; Usui, S.; Ruda, H. J. Vac. Sci. Technol., B 1997, 15, 554. (11) Cui, Y.; Lauhon, L.; Gudiksen, M.; Wang, Lieber, C. J. Appl. Phys. Lett. 2001, 78, 2214.
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Figure 1. (a) AFM height image of a PS475-b-P2VP141 film (1 µm × 1 µm scan, 10 nm in height). (b) AFM height image of surface micelles formed by gold-modified PS475-b-P2VP141 (1 µm × 1 µm scan, 20 nm in height). (c) AFM height image of the resulting Au nanoparticles, (1 µm × 1 µm scan, 10 nm in height). (d) Schematic diagram of gold-induced micellization.
nanoparticles but provides no means to control the size of the nanoparticles. Using the second method, the size of gold colloids is well defined, but the interparticle distance cannot be controlled. It is difficult to distribute colloidal nanoparticles uniformly on a substrate surface. To address the problem of the broad size distribution of nanowires produced by many existing catalyst systems, in this letter we report the use of a block copolymer template approach to produce nanometer-sized gold nanoparticles with uniform size, density, and distribution. Block copolymers can be considered to be homopolymers joined together by covalent bonds. To minimize the Gibbs free energy, the chemical and physical immiscibility among the constituent polymer segments gives rise to a variety of selfassembled nanoscaled morphologies. Self-assembled block copolymers have emerged as an elegant and simple nanoscale fabrication tool.12-14 Because block copolymers can offer a variety of nanomorphologies with very strict control of architecture in the bulk and in solution, they can thus be used as templates to fabricate nanostructures of interest. One approach to generate periodically ordered transition-metal nanoparticles is to form surface micelles by depositing metal-loaded solution micelles onto a surface.15,16 After the removal of all organics, metalloaded surface micelles convert to ordered nanoparticles with controlled size. It has also been demonstrated by Mo¨ller’s group that gold nanoparticles can be generated by this method. In addition, the size and spacing of gold nanoparticles can be tailored by adjusting the length of the blocks and the gold loading.15 In this letter, gold-loaded micelles were produced by selectively reacting tetrachloroauric acid with a preexisting block copolymer, polystyrene-b-poly(2-vinylpyridine) (denoted PS-b-P2VP) in solution, a scheme similar to that pioneered by Mo¨ller’s group.15-18 However, there are two major deviations. One is that we use a spin-coating method that is more compatible with conventional device fabrication to form a monolayer of surface micelles. The other is that a reducing step has not been used in (12) Park, M.; Harrison, C.; Chaikin, P.; Register, R. A.; Adamson, D. H. Science 1997, 276, 1401 (13) Lopes, W. A.; Jaeger, H. M. Nature 2001, 414, 735. (14) Thurn-Alberchet, T.; Schotter, G. A.; Kastle, N.; Emley, T.; Shiauchi, L.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126. (15) Glass, R.; Mo¨ller, M.; Spatz, J. P. Nanotechnology 2003, 14, 1153. (16) Spatz, J. P.; Roescher, A.; Mo¨ller, M. AdV. Mater. 1996, 8, 337. (17) Spatz, J. P.; Mo¨ssmer, S.; Mo¨ller, M. Chem.sEur. J. 1996, 2, 1552. (18) Lu, J.; Yi, S.; Kopley, T.; Gulari, E. J. Chem. Phys. B, in press.
the process described here to generate gold nanoparticles in the zero-valent state. Without any post or prior reduction treatments, the gold nanoparticles formed after treatment with oxygen plasma to remove the block copolymer template are metallic gold. Owing to the excellent processability of polymeric material, gold-loaded surface micelles can be distributed evenly by spin-coating, resulting in gold nanoparticles uniformly distributed on a surface. High density, uniform, small-diameter nanowires have been synthesized. Two PS-b-P2VP systems were purchased from Polymer Sources, PS475-b-P2VP141 and PS1814-b-P2VP314 (subscripts denote the degree of polymerization of each block). A gold salt, tetrachloroauric acid, was obtained from Aldrich. First, PS-b2PVP was dissolved in toluene to form a 0.1 wt % solution. The solution was spin-coated onto a silicon substrate with a 500nm-thick thermally grown silicon oxide layer. A DI-500 Digital Instruments atomic force microscope (AFM) was used to examine the surface morphology. Unlike polystyrene-b-poly(4-vinylpyrindine), the chemical dissimilarity between the two polymer segments is not sufficient to self-assemble in toluene, and thus no surface micelles were observed as shown in Figure 1a. The pyridine units in the P2VP block are Brønsted bases. Acids such as tetrachloroauric acid can react with pyridines by protonating the pyridine units.19-21 For the IR study, poly(2-vinylpyridine) homopolymer, purchased from Polymer Source, was dissolved in ethanol. Tetrachloroauric acid was then added to obtain a molar ratio of gold to pyridine of 0.25. Both solutions with and without the gold compound were then spotted onto NaCl windows, and the solvent was removed in vacuum at 80 °C. A Nicolet MagNA-IR 850 was used to determine whether the acid-base reaction between tetrachloroauric acid and P2VP occurs. Such a reaction was reported previously between tetrachloroauric acid and PS-b-P4VP.19 Figure 2 is a comparison of FTIR spectra of P2VP with and without the addition of tetrachloroauric acid. The weakening of the intensity of pyridine bands at 1569 and 1434 cm-1 and the appearance of a new band at 1620 cm-1 are characteristic of pyridinium salts formed by the protonation of pyridine.19 (19) Antonietti, M.; Wenz, E.; Bronstein, L.; Seregina, M. AdV. Mater. 1995, 7, 1000. (20) Spatz, J. P.; Mo¨ssmer, S.; Hartmann, C.; Mo¨ller, M.; Herzog, T.; Krieger, M.; Boyen, H.-G.; Ziemann, P.; Kabius, B. Langmuir 2000, 16, 407. (21) Valetsky, P. M.; Antonietti, M. Polym. Sci., Ser. A 1997, 39, 1249.
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Figure 2. FTIR spectra of P2VP before and after the addition of HAuCl4. The offset was applied for clarification.
The amount of tetrachloroauric acid added to PS-b-P2VP was adjusted to obtain a molar ratio of gold to pyridine of 0.35. After spin-coating the gold-modified PS-b-P2VP solution onto a surface, surface micelles were observed as shown in Figure 1b. Micelle formation is likely due to the increased polarity of the gold-modified P2VP relative to P2VP. The difference in solubility between PS and gold-modified P2VP in toluene, a nonpolar solvent, is sufficient that this gold-modified block copolymer spontaneously self-assembles into spherical micelles. The core of each solution micelle is composed of a gold-bearing P2VP that is encapsulated by the PS corona. By depositing the solution micelles, a monolayer of gold-containing surface micelles was formed as seen in Figure 1b. After UV ozonation, Au nanoparticles were produced as seen in Figure 1c. Figure 1d is a schematic illustration of the micellization of PS-b-P2VP induced by attaching gold selectively onto the P2VP segments. The AFM height analysis of gold nanoparticles prepared from PS475-b-P2VP141 and PS1814-b-P2VP314 is displayed in Figure 3a. The average gold nanoparticle diameters according to AFM height estimations were 4.7 and 8.2 nm, respectively. Assuming a Gaussian distribution of nanoparticle size, the standard deviation is less than 10%, indicating that this block copolymer template method is a very effective means to generate uniformly sized gold nanoparticles. Two-dimensional Fourier transform analysis indicated that the interparticle distances were 44 and 80 nm, respectively. This result confirms that by varying the chain lengths of the blocks the size and spacing of nanoparticles can be adjusted. A Quantum 2000 X-ray photoelectron spectrometer (XPS) was used to identify the chemical nature of the nanoparticles. An aluminum source was used, and the beam diameter was 100 µm. In Figure 4, Au 4f7/2 appears at 83.5 eV, indicating that Au is in the form of Au(0). Typically, the peak from Au2O3 is around 85.9 eV.22,23 This result can be explained by the facile reduction of an oxidized gold species to its zero-valent state. The most stable state for gold is the zero-valent state. Although Au(+3) was used as the gold precursor and UV ozonation, which can be viewed as a low-power oxidation process, was used in the subsequent process to remove all of the organic components, only Au(0) was detected. Contrary to other published methods that require a reduction step to obtain gold nanoparticles in the zero-valent state,15-17 the method described herein does not need such a reduction treatment. (22) Pireaux, W. A.; Leihr, M.; Thiry, P. A.; Delrue, J. P.; Caudano, R. Surf. Sci. 1984, 141, 221. (23) Du¨ckers, K.; Bonzel, H. P. Surf. Sci. 1989, 213, 25.
Figure 3. (a) AFM height image of gold nanoparticles produced from PS475-b-P2VP141 and PS1814-b-P2VP314 (1 µm × 1 µm scan, 10 nm in height). Summary of AFM image analysis results. (b) XPS spectrum of Au nanoparticles derived from PS475-b-P2VP141.
Silicon nanowire growth was carried out at 550 °C using 1% disilane in H2 as a silicon precursor. A Hitachi scanning electron microscope (SEM) was used to inspect the growth result. As expected, high density, uniformly distributed silicon nanowires were obtained by this approach. Figure 4a is a representative SEM image of Si nanowires formed using gold nanoparticles derived from gold-modified PS475-b-P2VP141 surface micelles. The diameter and crystal structure were studied by using a JEOL TEM-400FX transmission electron microscope (TEM). The TEM analysis indicates that the nanowire diameter distribution is narrow, ranging from 7 to 9 nm with an average of 8.1 nm and a standard deviation of 0.8 nm. Figure 4b is the summary of TEM diameter measurement results. The inset shows a representative TEM image used for diameter measurement. The tight
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Figure 4. (a) SEM micrograph of high-density silicon nanowires. (b) Diameter distribution of silicon nanowires based on TEM analysis. The inset is a representative TEM micrograph of a silicon nanowire. (c) High-resolution TEM image showing lattice fringes.
diameter distribution of silicon nanowires can be attributed to the narrow size variation of gold catalyst nanoparticles produced from the block copolymer template. Gold nanoparticles can be formed by annealing a thin gold film deposited on a substrate at elevated temperatures. When an ∼1-nm-thick gold film annealed at 550 °C was used for nanowire growth, the diameter distribution of the resulting nanowires was wide and ranged from 20 to 80 nm. Similar wide diameter distributions of Si nanowires grown using thin Au films have been reported recently.24,25A representative high-resolution TEM image in Figure 4c indicates that the diameter of the nanowires is 7.5 nm, which is 3.0 nm larger than the catalyst. This result is consistent with an earlier report that the diameter of nanowires is greater than that of catalyst nanoparticles.11 Most of the nanowires examined show [111] lattice fringes parallel to the nanowire growth axis with an average spacing of 3.15 Å. This polymer micelle system is compatible with photoresist processes, thus a patterned array with gold nanoparticles can be
produced for the lithographically controlled growth of silicon nanowires.26 In conclusion, a monolayer of gold-loaded surface micelles can be produced by spin-coating a gold-containing micelle solution. Highly ordered nanoparticles with controlled size and spacing over a large surface area can be obtained directly after oxygen plasma treatment without any reduction treatment. We have demonstrated that gold nanoparticles prepared from the PS-b-P2VP block copolymer template are excellent catalysts for growing silicon nanowires. Highly populated, evenly distributed, small-diameter silicon nanowires of uniform size have been synthesized. The block copolymer template approach is fully compatible with conventional device fabrication methodologies. Because gold is a widely used catalyst system to synthesize a variety of inorganic nanowires, this method of generating controlled gold nanoparticles should enable the controlled synthesis of a multitude of nanowires.
(24) Sharma, S.; Kamins, T. I.; Williams, R. S. Appl. Phys. A 2005, 80, 1225. (25) Schmidt, V.; Senz, S.; Go¨sele, U. Nano Lett. 2005, 5, 931.
(26) Lu, J.; Kopley, T.; Moll, N.; Roitman, D.; Chamberlin, D.; Fu, Q.; Liu, J.; Russell, T.; Rider, D.; Manners, I.; Winnik, M. Chem. Mater. 2005, 17, 2227.
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