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Optically Definable Reaction-Diffusion-Driven Pattern Generation of AgAu Nanoparticles on Templated Surfaces Sonit Kumar Gogoi,† Sankar Moni Borah,† Krishna Kanti Dey,‡ Anumita Paul,*,† and Arun Chattopadhyay*,†,‡ †
Department of Chemistry and ‡Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati 781039, India
bS Supporting Information ABSTRACT: We introduce a new lithographic method for the generation of 2D patterns of composite nanoparticles (NPs) of Ag and Au by taking recourse to combine top-down and bottom-up approaches. Micrometer-scale and submicrometer-scale patterned Ag foils of commercially available compact disks (CDs) and digital versatile disks (DVDs), respectively, were used as templates. The galvanic replacement reaction of Ag by HAuCl4 in the presence of the dye coatings on the foils led to the formation of patterned NP composites of Ag and Au, in addition to the formation of AgCl. The resultant structures appeared in the form of cross patterns of particles with micrometer and submicrometer dimensions. The AgCl crystals thus formed could be removed by using either a saturated NaCl solution or aqueous ammonia. In addition, AgCl could be converted to Ag by electrochemical reduction, thus generating Ag-coated Au NPs. Interestingly, the digital writing on CDs led to the formation of tertiary imprints on the patterns, based on the original writing patterns. This provided an additional handle in generating hierarchical patterns using light in combination with a chemical reaction diffusion process and the nearly parallel line patterns originally present in commercial CDs. The reactions could be carried out in aqueous solution, and the method does not require any additional curing. Also, the density of patterned particles is scalable on the basis of the choice of the original line patterns as present in CDs and DVDs.
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he marriage of top-down and bottom-up approaches in generating patterned surfaces constituting nanoparticles (NPs) of metal and semiconductors is increasingly being viewed as the best approach for incorporating the advantages of both methods while circumventing the limitations of either of them.1,2 This would potentially save time and increase the efficiency of pattern formation, at high resolution and on various length scales, in order to meet the demand of viable commercial applications. Controlled 2D patterned structures could be used in, among others, plasmonic wave guides,3 electrodes for sensors and actuators via increasing their efficiency,4 reusable catalysts,5 and the optimization of surface hydrophobicity6 and hydrophilicity.7 Furthermore, well-defined surface structures of noble metal NPs would provide additional collective optical properties such as Fano resonance and electromagnetically induced transparency, which may otherwise not be attainable using single NPs. 8 In addition, it has been demonstrated that plasmon coupling modifies the optical properties of fluorophores when present in the vicinity of the coupling structures.9,10 Fortunately, because of significant commercial potential and scientific interest a large number of methods are currently available for the generation of patterned structures using techniques such as photoinduced deposition,10 physical vapor deposition,11 r 2011 American Chemical Society
chemical deposition of metals,11 nanosphere lithography,9 adsorption of metal colloids on substrates,12 and several other lithography-based techniques.1316 In a clever design, Shankar et al. recently imprinted patterned bulk Ag metal on a 2D surface, followed by a galvanic replacement reaction with HAuCl4.17 The resultant structures consisted of patterned nanoscale alternating Ag and Au metals with characteristic optical properties. Compared to the monometallic nanostructures, bimetallic structures are of importance not only because of their optical properties but also for catalytic and other chemical properties providing additional advantages.18 However, there is still plenty of scope for development in this regard, especially in meeting the fundamental challenge of generating large-scale patterns of NPs in a short time. A way out of this impasse could lie in being able to modify micrometer-scale patterned structures, conventionally generated by a lithographic method, using reaction-diffusion waves.19 The emergence of elaborate spatiotemporal patterns as a result of competition between reaction and diffusion has been studied Received: June 28, 2011 Revised: September 6, 2011 Published: September 07, 2011 12263
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Langmuir over a significant period of time not only for their aesthetic appeal but also for the scientific challenges that they pose in understanding as well as controlling the patterns, not to mention the application potential that they possess.19,20 Furthermore, important developments have been made in understanding pattern formation and their control in excitable systems comprising gassolid and liquidsolid reactions.19,20 Additionally, heterogeneous reaction systems could generate products over large dimensions quickly, making the process less time-intensive. Therefore, in an ideal scenario the structures could be made of secondary patterns consisting of nanoscale particles, generated over primary micrometer-scale or submicrometer-scale patterns imprinted by lithography-based technology, using a gassolid or liquidsolid reaction, while taking advantage of reaction diffusion as the origin of a regular nanoscale pattern. Herein we report the generation of micrometer- and submicrometer-scale regular cross patterns consisting of composite particles of Au and Ag NPs in addition to AgCl by a galvanic replacement reaction of Ag, present in the metallic foils of commercially available compact discs (CDs) and digital versatile discs (DVDs), with HAuCl4. The micrometer- and submicrometer-scale line patterns present in the Ag foil of CDs and DVDs provided the basis of the top-down guiding of patterned particle formation along the lines. On the other hand, Au NPs (and AgAu NPs) generated by reaction of HAuCl4 with bulk Ag (in the foil) in the presence of a thin dye coating was the origin of the bottom-up-directed formation of patterned particles. Observations indicated that the reaction diffusion wave played an important role in the resultant pattern. The composite particles were on the order of a micrometer or lower in dimension and were separated from each other by about a micrometer. The particles could be converted so as to consist of AgAu bimetallic NPs with different compositions upon treatment with either a saturated NaCl solution or ammonia or by the electrochemical reduction of the AgCl that formed. Interestingly, when the CDs were “written” digitally by storing data and then the foil was treated with HAuCl4, distinct signatures of the written patterns were transferred to the patterns of NPs in the form of a variation in the density of particles formed in the written and unwritten parts. This provided an addition external handle in generating well-defined patterns in two dimensions. The basic structure of a CD and the procedure for the recovery of a Ag foil with the retention of the dye from commercial CDs are shown in Schemes S1 and S2, respectively (refer to Supporting Information, SI), and also have been described elsewhere.21 Briefly, the CD (or DVD) was cut into smaller pieces (2.5 cm 1.6 cm), and then the Ag-film-containing foil was removed from the polycarbonate part by using adhesive tape. The foil, which was attached to the tape, was then dipped into an aqueous HAuCl4 solution for the generation of patterned particles using a galvanic replacement reaction. (The detailed procedure is included in SI.) A typical pattern of the original metal foil, as obtained from scanning electron microscopy (SEM) imaging, is also shown in Scheme S2 and Figure S1A (in the SI). As is clear from the image, the foil typically consisted of parallel lines of “hills” and “valleys” that were separated from each other by about 1 μm, and this is consistent with the known structure of a CD foil. It may be mentioned here that these line (track) structures of the foils constitute the starting pattern as obtained indirectly from a top-down approach. A similar procedure was followed for obtaining Ag foils from DVDs and the reaction with HAuCl4. Also, the structure of the Ag foil obtained from a DVD was similar to that obtained from a CD, except for the fact that the tracks
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were separated by a smaller distance of about 400 nm (Figure S1B). The presence of silver in shiny layers on the CD and DVD was confirmed by energy dispersive X-ray (EDX) spectroscopy (Figure S1C,D) and X-ray diffraction (XRD) as shown in Figure S2A,B, respectively. When the Ag foil (2.5 cm 1.6 cm) from a CD was kept in an aqueous solution containing 2.98 104 M HAuCl4 for 5 h, the color of the foil changed from shiny silver to bluish red. Investigations by SEM revealed that the foil consisted of arrays of particles (Figure 1A) deposited in cross patterns. The average diameter of the particles was calculated to be 1.0 ( 0.2 μm, additional images of which are shown in Figure S3 in the SI. The particle size distribution, shown in Figure S3E, was calculated from the image in Figure 1A. Interestingly, the cross-patterns were formed out of particles organized in the form of parallel lines with one set of lines (marked in blue in Figure 1A) separated from one another by about 1.6 μm (typically that of CD lines) whereas the other set of lines (marked in red) was at an angle of 94° to the first one and the lines themselves were separated by about 1.9 μm. The separation between lines was thus also the separation between particles in perpendicular directions. It is important to mention here that the arrays of particles thus formed were nearly uniform over a larger distance in each direction and were largely devoid of defects, making the process a robust one in terms of the generation of organized arrays of particles. XRD patterns (Figure 1B) of the particle-containing surface (foil) consisted of peaks due to Au or Ag (or both) metal(s) with characteristics peaks occurring at 2θ values of 38.18, 44.42, and 64.70°, which correspond to diffractions from the (111), (200), and (220) planes of the metal(s), respectively.22 In addition, the pattern consisted of additional peaks occurring at 2θ values of 32.24 (200), 46.22 (220), 54.78 (311), and 57.46° (222) corresponding to AgCl attributed to lattice planes mentioned in parentheses.23 The UVvisible spectrum (recorded in reflectance mode) of the as-prepared patterned foil consisted of surface plasmon resonance (SPR) peaks due to Ag and Au NPs occurring at 469 and 564 nm, respectively, as shown in Figure 1C. The peak positions representing each type of NP (Ag and Au) indicated that particles formed on the surface were rather large and could even be made of an agglomeration of smaller NPs. The peaks may also be assigned to the SPR of AgAu alloy/coreshell NPs and monometallic Au NPs, where the alloy/coreshell NP exhibited a peak at a wavelength longer than that due to Ag NPs and shorter than that due to Au NPs.22 It may be mentioned here that typically the SPR peak due to small Ag NPs occurs in the region of 380420 nm. However, peaks at longer wavelengths indicate the formation of either larger particles or the assembly of particles. The peak at 469 nm observed here could possibly be due to larger particles of Ag NPs. However, the SPR peak due to Au NPs typically occurs at 530580 nm, with smaller particles giving rise to peaks at lower wavelengths. Thus, the observed peak at 564 nm indicated the formation of larger particles. It may further be mentioned here that peak at 469 nm could also be assigned to the formation of bimetallic nanoparticles of Au and Ag.22 Thus, the reaction of aqueous HAuCl4 with Ag(0) present in the foil produced nanoscale particles of Ag and Au and, in addition, AgCl, possibly in the form of microparticles. Because the standard reduction potential of AuCl4/Au (1.0 V vs the standard hydrogen electrode, SHE)24 is higher than that of Ag+/Ag (0.80 V vs SHE),24 the metallic Ag present in the foil from the CDs could be 12264
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Figure 1. (A) Representative SEM image consisting of arrays of microcrystals formed by the reaction of the Ag layer of a CD foil and 2.98 104 M HAuCl4 for 5 h with the conspicuous presence of a cross pattern of particles. Blue lines indicate the direction of the original CD tracks, and red lines indicate the direction of part of the wave formed after the reaction. The spacing between the red lines and blue lines is 1.9 and 1.6 μm, respectively. (B) Corresponding X-ray diffraction pattern and (C) UVvisible spectrum of the sample.
oxidized to Ag+ by an aqueous HAuCl4 solution according to the following galvanic replacement reaction.25,26 3AgðsÞ þ AuCl4 ðaqÞ f 3AgClðsÞ þ AuðsÞ þ Cl ðaqÞ
ð1Þ
This could account for the formation of three species constituting the particulate arrays. In other words, the galvanic replacement reaction produced Au and AgCl, and Ag was the remaining unreacted metal present in the original foil. Further treatment of the patterned surface with a saturated aqueous solution of NaCl led to the modification of the structure of the patterns (Figures 2A). The original pattern formed after the reaction with HAuCl4 was, however, retained. Similar observations were made with the patterned foil treated with aqueous ammonia (Figure S4 in the SI). Also, when the film was treated with a saturated NaCl solution the peak due to Au NPs occurring at 571 nm (Figure 2B) could be observed more prominently, in comparison to that in the as-prepared film (Figure 1C). Thus, Au NPs were probably formed and were present as one of the components of the arrays in the film. Furthermore, the XRD pattern of the samples recorded following
treatment with a saturated aqueous solution of NaCl indicated the removal of AgCl (Figure 2C).23 As is clear from the figure, the treatment led to the loss of peaks due to AgCl crystals whereas those due to Ag (Au) were prominent. Interestingly, the electrochemical reduction of the as-prepared patterned foil led to the generation of patterned surfaces (Figure 2D) consisting of porous particles of about 1 μm in diameter. Moreover, the UVvisible spectrum, as shown in Figure 2E, indicated the complete disappearance of the peak due to Au NPs whereas the peak due to Ag could still be observed at 448 nm. This could be due to the reduction of AgCl present on the surface of Au NPs to Ag and the product Ag being coated onto the surface of Au, leading to a loss of light extinction due to Au.22 The XRD pattern of the same sample (Figure 2F) consisted of peaks due to Ag (and Au) whereas the peaks due to AgCl disappeared,22,23 confirming the conversion of AgCl to Ag0. The percentage elemental compositions of the patterned particles after either NaCl/NH3 treatment or electrochemical reduction as obtained by EDX spectroscopy are shown in Figure S4A,B, respectively. Furthermore, transmission electron microscopy (TEM) measurements supported the formation of coreshell 12265
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Figure 2. (A) SEM image of a patterned CD foil after treatment with NaCl. The spacing between the white and black lines is 1.2 and 1.5 μm, respectively. (B) UVvisible spectrum and (C) XRD pattern of the sample in A. (D) SEM image, (E) UV-visible spectrum and (F) XRD pattern of the (Ag/Au)-containing patterned foil following electrochemical reduction. The spacing between white and black lines in (D) is 1.5 and 1.3 μm, respectively.
structure with Ag deposited on Au following electrochemical reduction (Figure S5 in the SI). However, upon reduction several Au NPs seem to have been fused together, with Ag being smeared around the Au NPs. It can be inferred from the combined results of XRD, TEM, UVvisible, and EDX analyses that initially after the reaction of the Ag foil from CD with HAuCl4 patterns of Ag NPs, Au NPs or AgAu NPs and AgCl microcrystals were formed, which were supported on the flexible protective layer of the CD. The treatment of this sample with a saturated solution of NaCl or dilute ammonia etched off the AgCl, leaving behind Ag NPs, Au NPs, or AgAu composite NPs with the disappearance of the peak due to Cl as observed in the EDX spectra (Figure S4A,B). However, in the electrochemical reduction process AgCl was reduced and deposited over the substrate (Figure S4C). It is to be noted here that EDX spectroscopy as such would not provide the absolute concentration of the species present on the substrate surface; however, it could still indicate the relative abundance of elements present in similar samples before and after treatment. The experimental evidence indicated that the pattern formation on the surface of the dye-coated Ag foil (from CD) was dependent on the concentration of HAuCl4. For example, at the
lowest concentration of HAuCl4 (3.45 106 M), smaller particles of about 100 nm in diameter were formed in 5 h along the hills of the CD lines. The particles consisted of elements Ag, Au, and Cl as evidenced by EDX spectroscopy. However, there was no discernible particle formation in the valley regions of the CD (Figure S3A in the SI). Also, the EDX spectrum showed the presence of primarily Ag in the valley, indicating no deposition of AgCl even though the reaction might have taken place there. In other words, the produced AgCl particles were deposited preferentially on the hills of the CD (Figure S3C,D in the SI). With increasing HAuCl4 concentration, the density of particles formed on the foil increased and the particles were increasingly spread over the entire foil. The cross pattern appeared clearly at the HAuCl4 concentration of 8.60 105 M. Further increases in the concentration of HAuCl4 led to the formation of larger particles; however, the cross pattern of deposition was retained. The details of the results are included in the SI (Figure S6AK). An investigation of the time-dependent growth process of the patterns indicated (Figure S7) an evolution of patterns suggesting the role of reaction diffusion in the presence of the dye coating the Ag foil; as in the absence of the dye (following removal using a solvent), pattern formation was not observed 12266
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Langmuir (Figure S8 in the SI). For example, 5 s after dipping the Ag foil into 2.98 104 M HAuCl4, AgCl crystals of approximately 100 nm and less were formed. The pattern appeared to be a series of comets separated by a distance of 1 μm from each other as shown Figure S7A. It is also interesting that most of the particles formed were in the hill region. The same pattern persisted for 30 s (Figure S7B) but with more crystals being formed in the intervening period. At 2 min, the cometlike pattern was enhanced with more particle deposition, and a clear direction toward crosspattern formation could be observed (Figure S7C). By 5 min, the initial pattern was superseded by the appearance of a new pattern formed by larger (∼1 μm) crystals separated from each other by distances (1.01.5 μm). The new pattern formed by the larger crystals appeared to be arrays of squarelike blocks, as could be seen in Figure S7HJ. From the time-dependent studies, it is apparent that the formation of patterns and crystals started early in the reaction of HAuCl4 with a dye-coated Ag foil consisting of parallel line patterns. Initially, smaller particles were formed in the hill region of the CD lines, and as time progressed, larger particles formed, eventually leading to cross-pattern formation. It is possible that the formation of (AgCl) crystals was followed by dissolution and recrystallization that occurred concurrently because of the high Cl concentration present in the medium.27,28 During the recrystallization process, the initially formed small AgCl crystals might act as seeds for relatively larger AgCl crystals. The cross patterns generated here might be the result of waves of AgCl crystal deposition nearly perpendicular to the CD tracks, along which the wavefront might have moved because of the availability of Ag. In other words, if the waves moved along the CD tracks then there would be valleys and hills present and there would be a periodic concentration variation along the wavefront due to the height difference. However, if the wave moved at an angle with respect to the tracks, then the concentration of Ag at an axis perpendicular to the motion (i.e., along the CD tracks) would be uniform and thus there would be a possible formation of larger crystals. Phrased another way, a reaction front arises possibly because it is preceded by a high concentration of reactant (i.e., Ag0 in the present case) and is immediately followed by a high concentration of product (i.e., AgCl in the present case). Thus, when a wavefront moves perpendicular to the CD tracks, the deposition of AgCl product that occurs preferentially on hill tops is broken up, resulting in a sharp front of AgCl deposition. In contrast, when the wavefront intercepts the CD track in a nearly parallel manner, the AgCl that deposits on the hills is nearly continuous and hence larger stretches of AgCl deposition are observed. Interestingly, at 8 h (i.e., beyond the time for the generation and development of cross patterns), the larger 1 μm crystals were somewhat broken into smaller crystals and little of the cross pattern was retained (Figure S7K). Furthermore, it is interesting that the content of Ag in the hills as well as in the valleys was similar, as is evident from the EDX results in Figure S3C,D. This indicates that even though the galvanic replacement reaction led to the removal of Ag from the original film, the formation of AgCl crystals and their depositions in the arrays might account for the near conservation of total Ag. A natural extension of this work would be the ability to have control over pattern formation with an additional handle such as light. As far as a CD is concerned, this can easily be achieved by “burning” the CD before dismantling, followed by treatment with HAuCl4. This was achieved by first digitally recording data to a CD such that a part of the CD would be written and the
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Figure 3. SEM images of crystals deposited along the patterned tracks in a partially written CD. (A) Image at low magnification showing both written and unwritten parts of the patterned CD generated by reaction with 3.46 104 M HAuCl4 for 5 h. (B, C) Exploded images of the written and unwritten parts, respectively, in A. (D) Image of the foil at 5 min of treatment with HAuCl4. The spacing between white and black lines is (B) 1.25 and 3.38 μm and (C) 1.40 and 1.40 μm, respectively.
remaining part would remain unwritten. This was followed by the removal of the Ag foil and treatment with HAuCl4 on the basis of the same procedure as described previously. Importantly, the reaction was carried out with the same foil containing both the written and unwritten parts. Interestingly, there were concrete and observable differences in the types of patterns generated on the written and unwritten parts of the CD foil. SEM images of typical patterns observed in burned CDs are shown in Figure 3. The images shown in the figure correspond to Ag foils reacted with 3.46 104 M HAuCl4. As is clear from Figure 3A, the AgCl crystals were deposited over the CD tracks on both the written and unwritten parts. However, the densities of particles and sizes were different as seen in the figures. Better views of the differences are shown in Figure 3B,C. In general, the crystals formed in the written part were typically smaller (∼800 nm) in comparison to those formed in the unwritten parts (∼1 μm). It was also observed that if the time period of the reaction was kept short then there was hardly any particle formation on the written part whereas a significant number of particles were formed in the unwritten part, as can be seen in Figure 3D. The difference in the distribution of crystals in the written and unwritten parts again indicated the importance of the photoactive dye. During the burning process of a CD-R, the photoactive dye, which is transparent, is converted to an opaque substance by the infrared laser.29 The density of the opaque dye and its structure (in thin foil) could be different and thus might be the origin of the difference in patterns. Also, the AFM measurement indicated that the height of the dye increased in the written part of the CD (Figure S9A,B), possibly leading to a lesser availability of AuCl4 on the Ag surface. Thus, the effective rates of reactions in the written and unwritten parts might be different, giving rise to the differences in particle formation and thus pattern formation. 12267
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Langmuir That there were distinct differences in particle as well pattern formation in the written and unwritten parts of the same Ag foil bodes well for controllable and well-defined pattern generation of metallic and other crystalline nanomaterials on 2D substrate surfaces. Also, the CD foil with its base being mechanically flexible adds value to the process of generating flexible devices with predetermined patterns. It may be mentioned here that pattern formation over the CD foils was observed for a variety of CDs obtained from different makers as available commercially (Figure S10 in the SI). This makes the current process robust and appealing for commercial exploitation. We were also interested in reducing the dimensions of the particles formed on the patterned surface. An obvious and easy choice was to use a commercially available DVD Ag foil for this purpose. Because the DVD tracks are approximately 400 nm wide contrary to the 1 μm tracks of CDs, the patterns generated are expected to be smaller in dimensions than those in CDs. Upon reaction with HAuCl4, the Ag foils of a DVD under similar reaction conditions as used for the CD Ag foil generated patterned structures with the formation of particles in a similar fashion. SEM images of typical patterns are shown in Figure 4A, B. The figure clearly shows that the particles were formed along the lines of the DVD. The average particle size was smaller than those generated in CDs (for example, 0.7 ( 0.1 μm as opposed to 1.0 ( 0.2 μm, Figure 4B). The average particle size and its distribution (Figure S11C) were calculated from Figure 4B. The EDX spectrum (Figure S11B) indicated the formation of AgCl in the process, being present along the DVD hill tracks. However, cross patterns of particles were not observed as clearly in comparison to those present in foils from CDs, although as shown in Figure S11A there was a certain degree of cross patterning in the foil. It is possible that the presence of the dye played a key role in the cross-pattern generation. The nature of the dye would therefore be crucial in that respect, as is evident from the difference in the observations. Typically, in a CD or DVD phthalocyanine-based dyes are used to imprint patterns optically. The exact nature of the dye may vary from model to model. The dye layer is generally coated onto the polycarbonate structure by spin coating. When the metal film is extracted from the CD, the remaining dye layer on top of the metal film would generally be thin, thus keeping the undulation present in the original structure.30 Atomic force microscopy (AFM) measurements (Figure S12) indicated that the typical estimated thickness of the dye in the hills was about 615 nm and that in the valleys was about 38 nm. This is reasonable considering that the hills and valleys in the metal foil would be opposite of those in the polycarbonate and thin dye layers may be transferred during the separation of the foil from the carbonate. Because the dye is water-insoluble (or sparingly soluble), the HAuCl4 solution may not be able to dissolve the dye as such. However, a thin film of the dye may be sufficiently porous to allow the liquid to come into contact with the Ag film. Also, because the deposited dye is not chemically bonded to the metal film it may slowly come off in the presence of the liquid. The contact of the liquid with the metal film would lead to a galvanic replacement reaction producing Au NPs. Now there are two extreme ways that this whole process can occur—one with the motion of a liquid front parallel to the hills as well as valleys and the other one perpendicular to that. It may also be possible that a combination of these two may result in a different pattern. A moving liquid front would not only etch off the Ag film but also produce Au NPs and AgCl in the process by the reaction of
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Figure 4. (A, B) SEM images at two magnifications of a DVD foil treated with 1.71 104 HAuCl4 for 5 h.
HAuCl4 with Ag. The dye would be crucial in controlling the velocity of the front (both in the plane and perpendicular to it), and the variation in its thickness would essentially change the diffusion rate of the waves affecting the curvature of the deposited pattern and the time taken for the local deposition. Furthermore, the modulation of the surface due to the periodic hills and valleys will affect the details of the pattern that is deposited. For example, deposition at the hills would be favored over that at valleys because of the motion of the wave front. In addition, deposition on valleys will be less favorable because of shadowing effects of the hills against the wave front that is advancing towards the hills. The combined effect of the reaction diffusion wave and the modulation of the surface results in the formation of arrays, with depositions occurring in a direction perpendicular to the velocity of the front. This would eventually result in the formation of arrays of particles with one axis being decided by the parallel hills, whereas the other one is due to depositions (and thereby the formation of crystals) in the immediate vicinity of the centers of reactions. Additionally, reactions may also occur, albeit at a slower rate because of the limited availability of fresh reactants in the valleys, and product may be deposited in such a fashion that they merge with or migrate to the crystals deposited on the tops of the hills in order to form larger deposited crystals. However, the thickness of the dye in the valley is typically less than that on the hill, which may contribute to a higher initial reaction rate in the valley. Furthermore, the differences in the sizes of the crystals and the patterns between films from CDs and DVDs could be due to the differences in the dyes and the dimensions of the hills and valleys, which would indicate a different diffusion and availability of Ag (and hence reaction rates) on the hills. With sharper hills present in the DVDs, the amount of Ag present per hill would be less, as would the shadowing effects on the reactants present in the wave front. Thus, all the products get deposited over a narrower region resulting in smaller crystals. In essence the size of the particles formed on the pattern would be dependent on the dimensions of the “master” lines, imposing a limit on cross-patterning and also on the nature of the dye present apart from the other factors such as reaction rates, diffusion rates, foil thickness, and so forth. In brief, we have been able to utilize the top-down corrugated patterns of lines of Ag (bulk) present in the foils of commercial CDs and DVDs in order to generate well-defined patterns of particles consisting of Ag NPs, Au NPs, and AgCl microcrystals. Under the appropriate parameters, some of the resultant patterns had the structure of cross depositions of particles in the foil from CDs. The thus-formed AgCl could be chemically removed or 12268
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Langmuir converted to Ag providing patterns of NPs of Ag and Au. The dimensions of particles in the arrays could be changed using Ag foils from DVDs instead of CDs. Finally, the nature of patterns with varying densities of particles could be further controlled by using foils from written CDs. The results reported herein could usher in a new method of 2D pattern formation of NPs with optical controllability. This may provide a viable alternative for fast pattern generation over larger dimensions consisting of nanoscale materials.
’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental section. Additional SEM images. AFM images. TEM image. EDX spectroscopy results. XRD patterns. Schematics. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
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(19) Grzybowski, B. A.; Bishop, K. J. M.; Campbell, C. J.; Fialkowski, M.; Smoukov, S. K. Soft Matter 2005, 1, 114–128. (20) Imbihl, R.; Ertl, G. Chem. Rev. 1995, 95, 697–733. (21) Chowdhury, D.; Paul, A.; Chattopadhyay, A. Nano Lett. 2001, 1, 409–412. (22) Chen, D. H.; Chen, C. J. J. Mater. Chem. 2002, 12, 1557–1562. (23) Zayat, M.; Einot, D.; Reisfeld, R. J. Sol -Gel Sci. Technol. 1997, 10, 67–74. (24) CRC Handbook of Chemistry and Physics, 85th ed.; Electrochemical Series;Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2005. (25) Sun, Y.; Xia, S. J. Am. Chem. Soc. 2004, 126, 3892–3901. (26) Zhang, Q.; Lee, J. L.; Yang, J.; Boothroyd, C.; Zhang, J. Nanotechnology 2007, 18, 245605. (27) Caley, E. R.; Shank, L. W. Ohio J. Sci. 1968, 68, 100–104. (28) Sun, Y.; Wiley, B.; Li, Z. Y.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 9399–9406. (29) Pohlmann, K. C. The Compact Disc Handbook; Oxford University Press: New York, 2001; pp 47102. (30) Konstantinov, I.; Sharlandjiev, P.; Babeva, Tz.; Kitova, S. J. Opt. A: Pure Appl. Opt. 2001, 3, 460–465.
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’ ACKNOWLEDGMENT We thank the Department of Science and Technology, Government of India (DST SR/S1/PC-30/2008 and SR/S5/ NM-108/2006), for funding. S.K.G. and K.K.D. thank CSIR, New Delhi, India, for fellowships (09/731(0043)/2006-EMR-I and (09/731(0051)/2007-EMR-I). ’ REFERENCES (1) Mazzola, L. Nat. Biotechnol. 2003, 21, 1137–1143. (2) Wang, Y.; Xia, Y. Nano Lett. 2004, 4, 2047–2050. (3) Quinten, M.; Leitner, A.; Krenn, J. R.; Aussenegg, F. R. Opt. Lett. 1998, 23, 1331–1333. (4) Bonroy, K.; Friedt, J.-M.; Frederix, F.; Laureyn, W.; Langerock, S.; Campitelli, A.; Sara, M.; Borghs, G.; Goddeeris, B.; Declerck, P. Anal. Chem. 2004, 76, 4299–4306. (5) Bansal, V.; Jani, H.; Plessis, J. D.; Coloe, P. J.; Bhargava, S. K. Adv. Mater. 2008, 20, 717–723. (6) Hosono, E.; Fujihara, S.; Honma, I.; Zhou, H. J. Am. Chem. Soc. 2005, 127, 13458–13459. (7) Drelich, J.; Chibowski, E. Langmuir 2010, 26, 18621–18623. (8) Niels Verellen, N.; Sonnefraud, Y.; Sobhani, H.; Hao, F.; Moshchalkov, V. V.; Van Dorpe, P.; Nordlander, P.; Maier, S. A. Nano Lett. 2009, 9, 1663–1667. (9) Stewart, M. E.; Anderton, C. R.; Thompson, L. B.; Maria, J.; Gray, S. K.; Rogers, J. A.; Nuzzo, R. G. Chem. Rev. 2008, 108, 494–521. (10) Geddes, C. D.; Parfenov, A.; Roll, D.; Fang, J.; Lakowicz, J. R. Langmuir 2003, 19, 6236–6241. (11) Cao, G. Nanostructures and Nanomaterials; Imperial College Press: London, 2004; pp 173228. (12) Xie, F.; Baker, M. S.; Goldys, E. M. Chem. Mater. 2008, 20, 1788–1797. (13) Wouters, D.; Schubert, U. S. Angew. Chem., Int. Ed. 2004, 43, 2480–2495. (14) Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1171–1196. (15) Rogers, J. A.; Paik, U. Nat. Nanotechnol. 2010, 5, 385–386. (16) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 18231848. (17) Shankar, S. S.; Rizzello, L.; Cingolani, R.; Rinaldi, R.; Pompa, P. P. ACS Nano 2009, 3, 893. (18) Toshima, N.; Yonezawa, T. New J. Chem. 1998, 22, 1179–1201. 12269
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