Synthesis of Ag Nanoplates on GaAs Wafers: Evidence for Growth

Nov 19, 2009 - Nathan H. Mack , James A. Bailey , Stephen K. Doorn , Chien-An Chen , Han-Mou Gau , Ping Xu , Darrick J. Williams , Elshan A. Akhadov ,...
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J. Phys. Chem. C 2010, 114, 857–863

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Synthesis of Ag Nanoplates on GaAs Wafers: Evidence for Growth Mechanism Yugang Sun* Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass AVenue, Argonne, Illinois 60439 ReceiVed: September 28, 2009; ReVised Manuscript ReceiVed: NoVember 4, 2009

Direct synthesis of Ag nanoplates on GaAs wafers has been developed in our group through a simple solution/ solid interfacial reaction (SSIR) strategy, in which aqueous solutions of pure AgNO3 react with the GaAs wafers at room temperature [J. Phys. Chem. C 2009, 113, 6061; 2008, 112, 8928; Chem. Mater. 2007, 19, 5845]. However, a number of questions are still not clear yet regarding the roles of different possible pathways for reducing Ag+ ions in the growth of Ag nanoplates. In this article, we try to answer these remaining questions by specifically designing experiments and extracting direct evidence from systematic characterizations of different samples. It is conclusive that growth of high-quality Ag nanoplates on GaAs wafers is ascribed to the good separation between nucleation and growth steps, which are driven by two different reduction pathways. At the nucleation step, fast reduction of Ag+ ions with a high concentration of surface electrons is crucial for the formation of Ag nuclei with multiple (111) twin planes parallel to each other, and remaining the environment of a high concentration of surface electrons for a period long enough is also important to develop the Ag nuclei into stable seeds. At the growth step, a hole injection process is mainly responsible for reduction of Ag+ ions to enlarge the stable seeds into Ag nanoplates with controlled sizes by tuning the growth time. The paralleled multiple (111) twin planes provide a crystalline confinement to guide the growth of the seeds into nanoplates. Introduction Synthesis of metal nanoparticles with anisotropic shapes on large-area semiconductor substrates is interesting because of their potential applications in surface-enhanced Raman scattering (SERS),1-3 efficient photocatalysis,4 and photoelectrochemical cells for solar energy conversion and hydrogen evolution.5-10 Physical deposition (e.g., sputtering, thermal evaporation, electron-beam evaporation, etc.) of metals on appropriate substrates against well-defined templates represents the most straightforward strategy for growing anisotropic metal nanoparticles on semiconductors. For example, “nanosphere lithography”, pioneered by Prof. Van Duyne’s group, has been proven a successful approach to the fabrication of shaped metal nanoparticles over large substrates.11 In a typical fabrication process, monodispersed micrometer or submicrometer colloidal beads (for instance, particles made of silica and polystyrene with spherical shapes and uniform sizes) are assembled into a closely packed monolayer with an hexagonal lattice on an appropriate substrate. The arrayed particles serve as a shadow mask to prevent depositing metal materials underneath the particles in the following flood deposition process. After the colloidal beads are lifted off, metal nanoplates with triangular shapes are formed on the substrate only in the area opening to the holes defined by the colloidal particle arrays.12 This template-assisted physical deposition is, in principle, capable of generating nanoparticles made of any metal on many kinds of substrates, but the preparation of appropriate templates is nontrivial; the use of high-vacuum evaporation instruments results in high-cost and time-consuming processing. In contrast, we have recently developed a simple solution/solid interfacial reaction (SSIR) approach to efficiently grow Ag nanoplates on GaAs wafers through reactions of aqueous solutions of pure AgNO3 with the GaAs wafers themselves at room temperature. The resulting Ag * To whom correspondence should be addressed. E-mail: [email protected].

nanoplates have clean surfaces and protrude out of the surfaces of the GaAs wafers. The dimensions of the Ag nanoplates can be easily tuned by controlling the AgNO3 concentration and reaction time. According to the general understanding of the synthesis of nanoparticles with anisotropic shapes, reduction of Ag+ ions involved in the formation of Ag nanoplates should be dominated by two different pathways at nucleation and growth steps, that is, fast reduction for the nucleation step and relatively slow reduction for the growth step.13 The separation between nucleation and growth steps benefits anisotropic growth of Ag nanoparticles. We previously proposed that a high concentration of surface electrons of the GaAs wafers is responsible for the fast reduction of Ag+ ions to form stable Ag nuclei with an appropriate crystalline structures (i.e., seeds) and a hole injection process of Ag+ ions dominates the enlargement of the Ag seeds to Ag nanoplates with controlled sizes.14,15 In this article, we report the specifically designed experiments that allow us to extract the direct and convincing evidence to confirm the proposed mechanism. Experimental Section Growth of Ag Nanoplates. p-type GaAs wafers doped with Zn at a concentration of ∼1 × 1019 cm-1 were purchased from AXT (Fremont, CA). The mechanical grade flat surfaces of the wafers orientate along the (100) crystalline direction and are exposed to AgNO3 solution for growing Ag particles. For convenience, each wafer was cut into ∼1 cm × 1 cm square pieces along their cleavage planes (i.e., (01j1j) and (01j1))16,17 and were cleaned by immersing them in a 2% hydrofluoric acid (HF) aqueous solution (Fisher) for 5 min. Caution: personal protectiVe equipment is required to handle HF, which is highly corrosiVe toward tissues, bones in particular. The GaAs pieces were then thoroughly rinsed with deionized (DI) water, followed by drying with N2 blowing. The cleaned wafers were then ready to react with an aqueous solution of 2 M AgNO3 (Aldrich). In

10.1021/jp909312g  2010 American Chemical Society Published on Web 11/19/2009

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a typical synthesis, a droplet (∼60 µL) of AgNO3 solution was delivered to the central area of the flat surface of a cleaned GaAs substrate with a micropipettor. The reaction was allowed to continue either in the dark (with a cover) or under illumination of a linearly polarized, red laser beam provided by a laser pointer (Interlink Electronics, Inc., Camarillo, CA). In an illuminationdriven reaction, the illumination time was programmed with the assistance of a shutter. The laser beam illuminated partial reaction area (Figure 3A). The reaction was terminated by immersing the GaAs wafer in a large volume (∼1 L) of water to remove excess AgNO3. Finally, the wafer was thoroughly rinsed with DI water and dried with gentle N2 blowing. The growth of Ag nanoplates on n-type GaAs wafers was carried out by following the same procedure as that applied for the p-type GaAs wafers in the dark. The n-type wafers used in this work have flat surfaces orientated along the (100) direction, a thickness of ∼550 µm, and a doping (Si) concentration of ∼1 × 1018 cm-3. Characterization. Scanning electron microscopy (SEM) images of the as-grown samples were recorded on a Quanta 400F (FEI) microscope operated at 20 kV under high-vacuum mode. The high doping concentration of the GaAs wafers allowed us to clearly image the Ag nanoparticles on the wafers. Samples for transmission electron microscopy (TEM) evaluation were prepared by following the standard procedure.18 In a typical process, a sample was cut into a piece with a size smaller than the standard 3 mm TEM grids, followed by gluing a steel holder to the small sample. The surface with Ag nanoparticles was embedded in the glue to protect the Ag particles from being destroyed in the following milling process. The sample was then ground and ion milled to a thickness with electron transparency. The entire ion-milling procedure was carried out while the samples were cooled with liquid nitrogen to prevent the GaAs from becoming amorphous. The TEM images were collected on a JEOL 2010 LaB6 transmission electron microscope operated at 200 kV. Results and Discussion Reactions of aqueous solutions of concentrated AgNO3 (>0.3 M) with highly doped n-type GaAs wafers lead to the growth of Ag nanoplates on the GaAs surfaces that contact the AgNO3 solutions. Figure 1A,B presents typical SEM images of Ag nanoplates deposited on an n-type GaAs wafer after the wafer reacts with a 2 M AgNO3 solution for 3 min in the dark at room temperature. The nanoplates clearly exhibit smooth surfaces, a relatively uniform thickness of ∼26 nm, and high density (1.3 × 109 plates/cm2). The sizes, determined by measuring the lengths of the orthographic projections of individual nanoplates along their longitudinal axes in the surface of the substrate, of the nanoplates are 454 ( 121 nm. These nanoplates protrude from the surface of the GaAs substrate to expose most of their surfaces to the surrounding environment, leading to beneficial applications, such as catalysis.19 Detailed characterization of these nanoplates has been reported in our previous work.20-23 Each Ag nanoplate is terminated with two (111) basal surfaces and contains multiple twin planes parallel to its basal surfaces.20 In contrast, reaction of a 2 M AgNO3 solution with highly doped p-type GaAs wafers results in products significantly different from those shown in Figure 1A. Figure 1C gives an SEM image of Ag nanoparticles grown on a p-type GaAs wafer after reaction lasting 3 min in the dark, clearly showing the formation of nanocrystals with dominating sizes less than 10 nm. The morphologies of these nanocrystals are not well-defined. The difference of Ag particles grown on n- and p-type GaAs wafers originates from the availability of surface electrons when

Figure 1. SEM images of Ag nanostructures grown on (A, B) an n-type GaAs wafer and (C) a p-type GaAs wafer obtained through reactions of the wafers with a 2 M aqueous AgNO3 solution for 3 min in the dark at room temperature.

the wafers are in contact with AgNO3 solution. In an n-type GaAs wafer, its surface is occupied with surface electrons with a concentration, ns, determined by

(

ns ) n0 exp

e∆φSC kT

)

(1)

where n0 is the bulk electron concentration, e is the charge of an electron, ∆φSC represents the band bending at the GaAs/ solution interface, and k and T are the Boltzmann constant and

Synthesis of Ag Nanoplates on GaAs Wafers the temperature (in Kelvin scale), respectively.24 The surface of a p-type GaAs wafer, on the other hand, is occupied with positive charges (i.e., holes) with a concentration determined by the equation similar to 1. Surface electrons represent a class of strong reductant for quickly reducing Ag+ ions to Ag atoms, which are consequentially condensed into Ag nanocrystals. In contrast, surface holes with positive charges in the p-type GaAs wafers always prevent the Ag+ ions from being reduced. Formation of the small Ag nanocrystals (Figure 1C) grown on the p-GaAs substrate are believed to be initiated by the reduction of Ag+ ions with surface states of the GaAs substrate.15,25 The Ag nuclei formed through reduction of Ag+ ions with surface states seem to lack the crystalline structures that allow them to grow into large Ag nanoplates. The significant difference of the Ag particles grown on n- and p-type GaAs wafers indicates that a high concentration of surface electrons is crucial for the formation of nuclei (and seeds) with an appropriate crystalline structure, which consists of multiple twin planes parallel to (111) facets (Figure 5B), to enable growing them into Ag nanoplates with different sizes. Given the fact that the Ag nanocrystals (Figure 1C) deposited on the p-type GaAs wafers are much smaller than the Ag nanoplates (Figure 1B) on the n-type GaAs wafers even when the reaction times are the same, the first question is raised whether surface electrons are necessary in the growth step for growing the stable Ag nuclei (i.e., seeds) into large Ag nanoplates. If the Si dopants in the n-type GaAs wafers are completely ionized electron donors, the number of total free electrons in a GaAs wafer with size of 1 cm2 is ∼9.1 × 10-8 mol, higher than the available surface electrons. On the other hand, the atoms of the Ag nanoplates, as shown in Figure 1B, deposited on a 1 cm2 surface is approximately calculated as 5.5 × 10-7 mol by ignoring the variation of the profiles of the nanoplates and assuming all the plates with circular profiles and diameters same as their sizes. It is apparent that the surface electrons themselves cannot fully support the growth of Ag nanoplates, as shown in Figure 1B. In fact, the Ag nanoplates can grow even larger. Therefore, surface electrons of GaAs wafers are not necessary to support the growth of Ag seeds and an alternative pathway for reducing Ag+ is required in the growth step (answer for the first question). Because the equilibrium potential (0.858 V versus NHE, or normal hydrogen electrode for 2 M AgNO3 aqueous solution) of Ag+/Ag overlaps the valence band of GaAs (∼0.32 V versus NHE for the valence band edge of GaAs at pH ) 7),26 Ag+ ions can also be reduced through either a direct reaction with the surface GaAs lattice in a fashion similar to a conventional redox reaction between an oxidant and an reductant or a hole injection process highlighted in Figure 2. In a typical hole injection process for growing a Ag seed into a Ag nanoplate, each Ag+ releases a hole (h+), which is injected into the GaAs lattice through the seed. The Ag+ ion becomes a Ag atom, followed by moving on the highindex lateral surfaces of the seed to condense at a defect site with local maximum energy. The hole diffuses in the GaAs lattice to the surface area uncovered with the Ag seeds to oxidize the surface GaAs lattice with the assistance of water (from the aqueous AgNO3 solution) through reaction

2GaAs + 12h+ + 6H2O f 12H+ + Ga2O3 + As2O3 (2) The continuous hole injection process deposits more and more Ag atoms onto the lateral surface of the seed to enlarge it anisotropically into a nanoplate because these surfaces have higher surface energies than the basal (111) surfaces, which

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Figure 2. Schematic illustration of the hole injection process of Ag+ ions involved in the growth of a Ag seed with appropriate crystalline structure into a large Ag nanoplate on a GaAs substrate.

always represent the crystalline planes with the lowest surface energy for face-centered cubic (fcc) metals.27 The internal multiple (111) twin planes in the seed might provide crystalline confinement to direct the growth of the seed into a nanoplate with an appropriate size by controlling the growth time. At the same time, a layer of oxides of GaAs is generated on the GaAs surface, which prevents Ag+ ions from directly contacting the GaAs surface lattice to follow a direct redox reaction between Ag+ ions and the GaAs lattice

2GaAs + 12Ag+ + 6H2O f 12Ag + 12H+ + Ga2O3 + As2O3 (3) Therefore, the hole injection process and direct contact redox reaction are competitive. The second question is which pathway dominates the growth step inVolVed in the synthesis of Ag nanoplates? As indicated in Figure 1C, reaction induced by direct contact of AgNO3 solution with the p-type GaAs wafer cannot grow Ag nanoplates. If reaction conditions are specifically manipulated to drive reactions between AgNO3 solutions and p-type GaAs wafers to grow Ag nanoplates, the importance of the hole injection process for growing Ag nanoplates can be confirmed. It is well-known that illuminating a p-GaAs/electrolyte interface usually leads to the generation of surface electrons at the interface through charge separation and migration.28 When the illumination intensity is strong enough to produce a high concentration of surface electrons, Ag seeds with multiple (111) twin planes parallel to each other are formed at the nucleation step and further enlarged to Ag nanoplates with different sizes, depending on growth time. Figure 3A,B shows SEM images of a sample prepared by continuously illuminating a part of the surface area of a highly doped p-type GaAs wafer in contact with a 2 M AgNO3 solution with a polarized laser beam of ∼50 mW/cm2 in power density for 2 min (inset of Figure 3A). The size and shape of the laser beam is highlighted with the red frame in Figure 3A. The stark contrast between the areas inside and outside the red frame clearly shows that the strong photoillumination can significantly boost the growth of Ag particles into large sizes. The high-magnification image (Figure 3B) of the products in the illuminated area (i.e., inside the red frame) shows that the reaction between AgNO3 solutions and p-type GaAs wafers produces silver nanoplates with a quality

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Figure 3. SEM images of Ag nanoplates grown on p-type GaAs wafers obtained through reaction of the wafers with a 2 M AgNO3 solution at room temperature with the assistance of continuous laser illumination for 120 s (A, B) and short-time illumination of 15 s, followed by continuous reaction for an additional 105 s in the dark (C, D). The drawing in (C) illustrates how to control the laser illumination by programming a shutter. The red frame in (A) highlights the size and shape of the laser beam.

similar to those (Figure 1A) obtained from reaction of AgNO3 with n-type GaAs wafers. The density of Ag nanoplates on the p-type wafer is higher than those on the n-type wafer, and some nanoplates with the same orientations polymerize into oligomers with thicker thicknesses.21 The higher density is ascribed to the higher concentration of surface electrons, which leads to the formation of more Ag seeds, induced by strong laser illumination. To eliminate the possible influence of surface electrons in the growth step, we program a shutter to illuminate a p-type GaAs wafer for only a short time of 15 s, followed by continuous reaction in the dark for an additional 105 s (Figure 3C). This two-step reaction leads to the growth of Ag nanoplates (Figure 3D) essentially similar to those grown under continuous illumination (Figure 3B). This similarity confirms that surface electrons only account for the nucleation step and the hole injection process (Figure 2) is responsible for the growth step involved in the synthesis of Ag nanoplates on GaAs wafers (answer for both the first and the second questions). The third question is whether the fast reduction of Ag+ ions with a high concentration of surface electrons should remain long enough to deVelop stable seeds with appropriate crystalline structures for the growth step in the synthesis of Ag nanoplates. We can program the shutter opening time to control the number of available surface electrons in the p-type GaAs wafers at the nucleation step when they react with a 2 M AgNO3 solution. Figure 4 compares samples formed through reactions under illumination for different times, followed by continuous reaction in the dark to keep the total growth time of 2 min. The difference clearly shows that high density Ag nanoplates can only be grown with an illumination time long enough (i.e., 15 s and longer) (Figure 4C). When the illumination time is shorter, only sparse particles with large sizes and irregular shapes as well as dense Ag nanocrystals with small sizes, as shown in Figure 1C, are deposited on the p-type GaAs wafers after reaction (Figure 4A,B). The difference implies that forming Ag nuclei through fast reduction of Ag+ ions with a high concentration of surface electrons requires a reaction time long enough to develop the Ag nuclei into stable Ag seeds with multiple twin planes parallel to their (111) facets, which can be further grown into Ag

nanoplates driven by the hole injection process (answer for the third question). On the other hand, if the illumination time is short (e.g., < 15 s), the nuclei formed through reduction with surface electrons may fluctuate between varying crystalline structures due to their small sizes. Only when the sizes of the nuclei reach a critical value, their crystalline structures can be fixed to support the further growth of them into large particles with appropriate shapes.13 Therefore, a high concentration of surface electrons should be remained long enough to develop the Ag nuclei into stable seeds with multiple twin planes parallel to (111) facets at the nucleation step. The fourth question is why the nanocrystals shown in Figure 1C cannot grow as fast as Ag nanoplates through the hole injection process. Nanocrystals formed through reaction of a p-type GaAs wafer and a 2 M AgNO3 solution for 15 s under laser illumination with a power density of ∼50 mW/cm2 are carefully characterized with electron microscopy. Figure 5A-C shows typical SEM and TEM images of a typical as-grown sample. This sample includes two types of nanocrystals: nanocrystals with multiple twin planes parallel to each other (as highlighted by arrows in Figure 5A,B) and nanocrystals with multiple twin planes, which cross over at certain axes (as highlighted in Figure 5C). These two classes of nanoparticles are termed as laminar twinned seeds and angular twinned seeds, respectively. As indicated in Figures 3D and 4C, the laminar twinned seeds can continuously grow into large Ag nanoplates in the dark, driven by the hole injection process. When the illumination time is shorter, essentially no stable nanocrystals with multiple (111) twin planes parallel to each other are formed, as shown in Figure 5D for a sample formed with an illumination time of 10 s. It is apparent that almost all particles exhibit angular twinned structures. The only exception is highlighted by the red arrow, which is also shown in the inset with higher magnification, clearly showing that this particle includes two parallel twin planes and exhibits an ellipse shape. This particle is different from the particles highlighted by arrows in Figure 5B in terms of morphology and the number of twin planes. This difference may indicate that developing the nuclei into stable seeds with multiple (111) twin planes parallel to each other

Synthesis of Ag Nanoplates on GaAs Wafers

J. Phys. Chem. C, Vol. 114, No. 2, 2010 861 results indicate that angular twinned seeds are more difficult than laminar twinned seeds to grow into larger ones and require a higher driving force to reduce Ag+ ions at the growth step. As highlighted in Figure 6A, a laminar twinned seed contains two (or more) twin planes parallel to its (111) surfaces, leading to the existence of both reentrant and ridge structures at the lateral surfaces of the seed. The reentrant sites have high surface energies and represent nucleation sites for adding more Ag atoms to enlarge the seed laterally to a plate.29 The addition of Ag atoms to the reentrant sites will form new reentrant sides at the ridges to allow the ridge surfaces to grow laterally. This simultaneous multiplication of reentrant sites enables the laminar twinned seed to be easily grown into a large nanoplate. In contrast, an angular twinned seed has internal structural stress because of the crossover of the multiple twin planes. For instance, the 5-fold multiple twinned seed shown in Figure 6B represents a common class of Ag nanocrystals (e.g., the left particle shown in Figure 5C). The ideal angle between the twinned planes is 70.5° and five of these angles leave a 7.5° angle gap. Closing the gap requires that the 7.5° angle must be compensated by strain in the lattice, leading to higher strain in the position farther from the central axis. As a result, the angular twinned nanocrystals are difficult to grow into larger ones unless their symmetry is broken and their growth is driven by higher chemical force.30 As shown in Figure 6B, when the multiple twinned crystal is large enough, the crystal splits at one corner to release the increased internal stress, resulting in the formation of new facets, which serve as new nucleation sites for the formation of a new crystalline domain (as highlighted in gray). The right particle of Figure 5C represents a multiple twinned Ag nanoparticle derived from an angular twinned seed. The structural difference between the laminar twinned seeds and angular twinned seeds determines that a much higher driving force is required for enlarging the angular twinned seeds than the laminar twinned seeds. Therefore, the mild driving force provided by the hole injection process preferably grows the laminar twinned seeds into large Ag nanoplates at high speed, while this force cannot drive the angular twinned seeds to grow at the same speed. Conclusion

Figure 4. SEM images of Ag particles grown on p-type GaAs wafers obtained through illumination-assisted reaction for different times: (A) 5 s, (B) 10 s, and (C) 15 s, followed by continuous reaction in the dark. The total reaction times for all the samples are the same, 120 s. Other reaction conditions are the same as those for Figure 3.

needs the environment of a high concentration of surface electrons to last long enough. However, at the growth step, the hole injection process cannot drive the angular twinned seeds to grow as large as the Ag nanoplates, as reflected in Figure 3D, in which no large spherical particles are observed. The

The roles of varying pathways for reducing Ag+ ions involved in the reaction between aqueous solutions of AgNO3 and highly doped GaAs wafers have been determined for the formation of anisotropic Ag nanoplates. By systematically characterizing and comparing the Ag nanostructures obtained on both n- and p-type GaAs wafers synthesized through specifically designed reactions, it is concluded that the nucleation and growth steps involved in the synthesis of Ag nanoplates on GaAs substrates are wellseparated and driven by two different reduction pathways. In particular, four questions are answered: (i) a high concentration of surface electrons in the GaAs wafers is critical for the formation of Ag nuclei with multiple twinned planes parallel to each other, and they are not necessary at the growth step; (ii) remaining the environment of a high concentration of surface electrons is important to develop the Ag nuclei into stable seeds to support their further growth without structural variation; (iii) the hole injection process of Ag+ ions represents the major driving force responsible at the growth step for enlarging the seeds into Ag nanoplates; and (iv) the driving force provided by the hole injection process can selectively grow the laminar twinned seeds into nanoplates and is reluctant to enlarge the angular twinned seeds due to their internal stress. This understanding is important for reproducibly controlling the synthesis

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Figure 5. (A) SEM and (B, C) TEM images of the Ag nanocrystals formed through reaction of a 2 M AgNO3 solution with a p-type GaAs wafer for 15 s under continuous laser illumination at room temperature. The arrows in (A, B) highlight the existence of particles with multiple twin planes parallel to their (111) surfaces. The red lines in (C) highlight twin boundaries in the nanoparticles. (D) TEM image of the Ag nanocrystals formed through reaction of a 2 M AgNO3 solution with a p-type GaAs wafer for 10 s under continuous laser illumination at room temperature.

Figure 6. Illustration of structures of a (A) laminar twinned nanoparticle and (B) angular twinned nanoparticle. The drawings highlight the relationship between the twin planes within different particles. The red lines represent twin boundaries.

of Ag nanoplates on GaAs wafers as well as serves as a guideline to designing experiments to expand the SSIR strategy for synthesizing nanoplates made of other metals on different semiconductor substrates. The as-synthesized nanoplates have clean surfaces and protrude out of the substrate surfaces to expose their surfaces to the surrounding environment at the most, leading to beneficial applications, such as catalysis. Acknowledgment. Argonne National Laboratory, a U.S. Department of Energy, Office of Science, laboratory, is operated under Contract No. DE-AC02-06CH11357. Use of the Center for Nanoscale Materials and the Electron Microscopy Center for Materials Research at Argonne was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Characterizations were also carried out by partially using the Center for Microanalysis of Materials Facilities in Frederick Seitz

Materials Research Laboratory, University of Illinois, which is partially supported by the U.S. Department of Energy under Grant No. DEFG02-91-ER45439. The help from Dr. Changhui Lei in TEM characterization is greatly appreciated. References and Notes (1) Van Duyne, R. P.; Haushalter, J. P. J. Phys. Chem. 1983, 87, 2999. (2) Van Duyne, R. P.; Haushalter, J. P.; Janik-Czachor, M.; Levinger, N. J. Phys. Chem. 1985, 89, 4055. (3) Huang, B.-B.; Wang, J.-Y.; Huo, S.-J.; Cai, W.-B. Surf. Interface Anal. 2008, 40, 81. (4) Szklarczyk, M.; Bockris, J. O. M. J. Phys. Chem. 1984, 88, 1808. (5) Lewis, N. S. J. Electroanal. Chem. 2001, 508, 1. (6) Lombardi, I.; Marchionna, S.; Zangari, G.; Pizzini, S. Langmuir 2007, 23, 12413. (7) Nakato, Y.; Ueda, K.; Yano, H.; Tsubomura, H. J. Phys. Chem. 1988, 92, 2316. (8) Ueda, K.; Nakato, Y.; Suzuki, N.; Tsubomura, H. J. Electrochem. Soc. 1989, 136, 2280.

Synthesis of Ag Nanoplates on GaAs Wafers (9) Heller, A.; Aharon-Shalom, E.; Bonner, W. A.; Miller, B. J. Am. Chem. Soc. 1982, 104, 6942. (10) Waki, I.; Cohen, D.; Lal, R.; Mishra, U.; DenBaars, S. P.; Nakamura, S. Appl. Phys. Lett. 2007, 91, 093519. (11) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 5599. (12) Chan, G. H.; Zhao, J.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. C 2008, 112, 13958. (13) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int. Ed. 2009, 48, 60. (14) Sun, Y.; Lei, C.; Gosztola, D.; Haasch, R. Langmuir 2008, 24, 11928. (15) Sun, Y.; Wiederrecht, G. P. Small 2007, 3, 1964. (16) Sun, Y.; Khang, D.-Y.; Hua, F.; Hurley, K.; Nuzzo, R. G.; Rogers, J. A. AdV. Funct. Mater. 2005, 15, 30. (17) Sun, Y.; Rogers, J. A. Nano Lett. 2004, 4, 1953. (18) Lei, C. H. Thin Solid Films 2007, 515, 3584.

J. Phys. Chem. C, Vol. 114, No. 2, 2010 863 (19) Sun, Y.; Lei, C. Angew. Chem., Int. Ed. 2009, 48, 6824. (20) Sun, Y. Chem. Mater. 2007, 19, 5845. (21) Sun, Y.; Yan, H.; Wiederrecht, G. P. J. Phys. Chem. C 2008, 112, 8928. (22) Sun, Y.; Yan, H.; Wu, X. Appl. Phys. Lett. 2008, 92, 183109. (23) Sun, Y.; Qiao, R. Nano Res. 2008, 1, 292. (24) Oskam, G.; Long, J. G.; Natarajan, A.; Searson, P. C. J. Phys. D: Appl. Phys. 1998, 31, 1927. (25) Allongue, P.; Blonkowski, S. J. Electroanal. Chem. 1991, 317, 77. (26) Mandler, D.; Bard, A. J. Langmuir 1990, 6, 1489. (27) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (28) Sun, Y.; Pelton, M. J. Phys. Chem. C 2009, 113, 6061. (29) Hamilton, D. R.; Seidensticker, R. G. J. Appl. Phys. 1960, 31, 1165. (30) Lofton, C.; Sigmund, W. AdV. Funct. Mater. 2005, 15, 1197.

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