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Enhanced Ordering in Gold Nanoparticles Self-Assembly through Excess Free Ligands Cindy Y. Lau,† Huigao Duan,‡ Fuke Wang,‡ Chao Bin He,‡ Hong Yee Low,‡ and Joel K. W. Yang*,‡ † ‡
Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544, United States Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602 ABSTRACT: Self-assembly of nanometer-sized particles is an elegant and economical approach to achieve dense patterns over large areas beyond the resolution and throughput capabilities of electron-beam lithography. In this paper, we present results of self-assembly of oleylaminecapped gold nanoparticles with 8.0 ( 0.3 nm diameter into densely packed and well-ordered monolayers with center-to-center distance of ∼11 nm. Self-assembly was done in a Langmuir-Blodgett trough and picked up onto Si substrates. The nanoparticles undesirably assembled within micrometer-sized “droplets” that were organic in nature. However, within these droplets, we observed that the addition of the excess ligand, oleylamine, drastically enhanced the self-assembly of the nanoparticles into monolayers with near-perfect ordering. This approach has the potential use in templated self-assembly of nanoparticles for rearranging poorly ordered assembly into a commensurate prepatterned substrate.
’ INTRODUCTION New approaches are actively being explored to meet the resolution needs for future magnetic storage applications using bitpatterned media. One promising approach to achieve the required high-resolution ordered patterns over large areas is templated selfassembly of block copolymers. It was demonstrated that sparse patterns defined by electron-beam lithography guided block copolymers to micro phase separate into ordered dense arrays of dots with densities of ∼1 Tdots/in2.1,2 Alternatively, nanoparticle systems provide a promising route to high packing density and etch durability, but analogous demonstrations of templated self-assembly have not been achieved. Difficulties lie in (1) the formation of large-area monolayer coverage of nanoparticles and (2) the rearrangement of nanoparticles into ordered arrays, which is analogous to the annealing step in block copolymer systems. The self-assembly of gold nanoparticles (AuNPs) into thin films has gained increasing interest because of its potential use in electronics, data storage, and plasmonic applications.3-9 However, achieving large-area ordering of AuNPs as required in these applications has proven to be challenging.10-13 Methods in synthesis and self-assembly appear to play important roles in the quality of the self-assembled monolayers. For instance, using dodecanethiol-capped AuNPs, Bigioni et al. demonstrated wellordered AuNP self-assembly by dropcasting a solution of AuNPs in an organic solvent containing excess dodecanethiol.14 Excess ligands have also been found to promote ordering in the selfassembly of binary systems of gold nanorods and nanospheres in nanowires15 and quantum dot-gold nanoparticle systems.16 Several groups have shown the possibility of creating NP monolayers using the Langmuir-Blodgett technique (LB).17-20 r 2011 American Chemical Society
However, the range of order tends to be rather small, with domain size in the submicrometer scale. In this paper, we demonstrate the self-assembly of oleylaminecapped gold nanoparticles using the Lagmuir-Blodgett technique (LB) and report a method to address the challenge of nanoparticle rearrangement. We observed that these oleylamine-capped AuNPs were undesirably confined within disk-like droplets, instead of the expected large-area monolayer coverage as reported by some.13 However, within these droplets, the formation of monolayer and large-area ordering was drastically improved upon addition of excess oleylamine to the LB trough surface. Our results suggest that the excess oleylamine, added after the AuNPs had self-assembled in a disordered fashion, assisted the rearrangement of the AuNPs into highly ordered areas. This result is notably different from previous work that reported the enhancing properties of excess ligands,14-16,21 in that in our case, excess ligands were added after most of the solvents had evaporated, showing evidence of the ability of excess ligands to recrystallize an initially disordered arrangement of particles. Nevertheless, we also demonstrate a case where excess ligands in the solvents do improve the ordering of AuNPs in agreement with previous reports.
’ EXPERIMENTAL SECTION Synthesis of Nanoparticles. Oleylamine- (OA-) capped AuNPs were synthesized using a modified procedure.22 Briefly, AuNPs with uniform sizes were prepared using organic amines both as sizes control Received: December 1, 2010 Revised: January 21, 2011 Published: February 24, 2011 3355
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Langmuir reagents and reducing reagents.23 In a typical synthesis, 50 mg of HAuCl4 was dissolved in 5 mL of oleylamine, together with 5 mg of octa-ammonium polyhedral oligomeric silsesquioxane (OA-POSS), which increased the reaction rate while simultaneously achieving a narrow distribution of particle sizes. The reaction mixture was then ultrasonicated for 15 min to achieve a homogeneous orange solution that was next transferred to a reaction flask equipped with a nitrogen gas inlet and condenser. Under nitrogen environment, the reaction temperature was increased to 80 °C and maintained for 3 h. During this time, a gradual color change from orange to colorless to pink, and finally to red was observed. Particle purification was accomplished by repeated centrifugation of the solution: the reaction mixture was first diluted with hexane in a 50/50 volume ratio, followed by addition of ethanol to precipitate the particles. The precipitates were centrifuged for 10 min and the centrifuge pellet was redispersed into hexane. This process was repeated three times to remove free OA and OA-POSS. The solvent (hexane) was removed from the supernatant by rotary evaporation and the residue was checked for the success of purification using Fourier transform infrared spectroscopy (FTIR). Purified particles were redissolved in hexane or toluene and cast on a transmission electron microscopy
Figure 1. (a) Representative bright-field TEM image of the synthesized oleylamine-capped gold nanoparticles drop-casted onto a TEM grid. (b) Schematic (not to scale) showing the capping of the gold nanoparticles with oleylamine, rendering the particles soluble in organic solvents.
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(TEM) grid for sizes analysis (Figure 1). A representative bright-field TEM image of the nanoparticles shows uniform size distribution, displaying an average diameter of 8.0 ( 0.3 nm. The uniform sizes of resultant AuNPs was also confirmed by dynamic light scattering (DLS), which showed a similarly narrow particle-size distribution. The concentration of the AuNPs is approximately 1.4 1013 particles per mL of solution. Self-Assembly of Gold Nanoparticles. The schematics of the self-assembly of AuNPs in a simplified LB trough is shown in Figure 2. Deionized (DI) water was added to a Teflon trough with dimensions of ∼14 cm 6 cm 0.8 cm (depth). About 125 μL of AuNPs in hexane (H-AuNP) was mixed with ∼500 μL of chloroform and dispensed onto the water surface in a dropwise manner using a micropipet over a period of >10 min. The spreading coefficient S, which compares the energies of cohesion and adhesion, can be calculated with the equation Ssolvent on water = γwater-air - (γsolvent-air þ γsolvent-water), where γ is the surface tension. The spreading coefficients of hexane and chloroform on water at room temperature, which was our experimental temperature, are 4.0 and 12.9 mN/m respectively, where γwater-air = 72.8 mN/m, γhexane-air = 18.0 mN/m, γchloroform-air = 27.1 mN/m, γhexane-water = 51.0 mN/m, and γchloroform-water = 32.8 mN/m.24 The lower S value of hexane indicates that it does not spread well on water, which is consistent with our experimental observation. However, the gold nanoparticles were not stable in pure chloroform, therefore we used a mixture of hexane and chloroform to improve the spreading while maintaining nanoparticle stability. The solvent was allowed to evaporate for >10 min. A substrate was then immersed vertically into the water at one side of the trough. The barrier was manually moved to compress the AuNPs on the water surface. Finally, AuNPs were transferred onto the substrate as it was manually lifted out of the water subphase. Both barrier compression and substrate lifting rates were high (∼10 mm/s). Lower rates did not have significant effects to the self-assembly results. The effect of excess OA on AuNP self-assembly was investigated as follows: immediately after the AuNPs were picked up and without
Figure 2. Schematics and representative results of AuNP self-assembly using LB method comparing results with and without the additional OA: (a) Schematics of simplified Langmuir-Blodgett trough; (b) SEM image of AuNPs on a Si substrate without the addition of free OA showing poor ordering with low monolayer coverage, defects such as voids, and multilayers of AuNP; (c) SEM image of AuNPs on a Si substrate after free OA addition showing defect-free coverage of highly ordered monolayer. 3356
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Figure 3. AuNPs in droplet-like features. (a, b) SEM images and (c) schematics of deposition without additional OA resulting in multilayer at ring edge and disordered monolayer with multilayer at ring center; (d, e) SEM images and (f) schematics of deposition with additional OA resulting in multilayer at ring edge and long-range ordered monolayer covering the whole drop. further perturbations on the AuNP layer on the water surface, a 1 μL drop of OA was added onto the water surface. By adding only this drop of OA and keeping everything else constant, we isolated the effect of OA on the rearrangement of the AuNPs. The OA drop was observed to be mostly localized to the position of dispension on the water surface, thus allowing the drop and its surrounding AuNPs to be picked up by the substrate. The effect of solvent was also investigated. For comparison with the H-AuNP system, we also investigated the self-assembly of AuNPs dispersed in toluene (T-AuNP). In these experiments, AuNPs were redispersed in toluene after the purification steps. The self-assembled nanoparticles from T-AuNP were transferred onto Si substrates using the same method as described above. As toluene was found to spread well on the water surface (Stoluene on waterr = 8.3 mN/m, with γtoluene-air = 28.4 mN/m and γtoluene-water = 36.1 mN/m25), no premixture with chloroform was necessary. The self-assembly of H-AuNP and T-AuNP were also performed using a full-sized NIMA LB trough (Model 622). The barrier compression and substrate dip rate could be controlled and recorded in this system. The surface pressure, measured with a Wilhelmy-plate tensionmeter, was plotted against the surface area of the compressed region. Characterization of Deposited AuNPs. The AuNPs deposited on substrates was imaged using an Elionix ESM-9000 field-emission scanning electron microscope (FESEM) at 5 kV acceleration voltage. The center-to-center distance of the AuNP array and the degree of ordering were obtained through a fast-Fourier transform (FFT) of the SEM image using ImageJ (National Institutes of Health). A code written in Matlab was used to identify the presence of defects in the large-area self-assembled nanoparticles. The surface composition of the droplet material was analyzed using energy-dispersive X-ray spectroscopy (EDX).
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’ RESULTS AND DISCUSSIONS Although we expected the gold nanoparticles to form largearea monolayer on the substrate, we observed instead that the nanoparticles were confined only in micrometer-sized droplets. Figure 3 shows examples of such droplets. In these SEM images, the droplets appeared as darker regions compared to the surrounding Si substrate. The nanoparticles within these droplets appeared as relatively brighter patches. Furthermore, multilayers of nanoparticles could be distinguished from the monolayers as they appeared brighter due to the higher efficiency of secondary-electron emission in these regions. The exact nature or cause of the droplets is unclear. However, we observed a strong carbon signal in the energy-dispersive X-ray spectroscopy (EDX) analysis, suggesting that the droplet was organic in nature. We further observed that these droplets were found on samples prepared with various substrate functionalizations, barrier compression rates, substrate dip rates, elapsed time between AuNPs dispensing and monolayer trasnfer, amount and type of AuNPs solution. It is possible that the droplets consisted of reaction products from the synthesis that could not be removed by the purification steps, including trace amount of OA as indicated in our FTIR results. These nonvolatile additives were also known to slow the evaporation rate of solvents.26,27 Although the presence of these droplets was undesirable for our targeted bit-patterned media application, the ability to achieve a long-range ordered monolayer within these droplets is valuable and is the focus of the present work. The use of diethylene glycol (DEG) instead of water as the subphase has shown promise in achieving large surface coverage of monolayer nanoparticles.15,28,29 We will investigate the suitability of this subphase for our system of nanoparticles in future studies. It is evident in Figure 2b and 3b that the self-assembly of AuNPs resulted in poor ordering before the addition of OA. Here, small regions (∼100 nm) of monolayer and multilayer were deposited onto the substrate with a large number of particles collecting around the edges of the droplets (Figure 3b), forming so-called coffee-stain-like ring appearance. This pattern is characteristic of a far-from-equilibrium state where the particles are kinetically trapped, thus unable to reach their thermodynamically lowest energy state.30 One effect that contributes to the coffeering arrangement in Figure 3b is the capillary flow during evaporation, which results in multilayer formation at the edge of the droplet. As the solvent evaporates while the contact line is pinned, more individual particles from both the bulk and the interfaces diffuse toward the edge of the droplet, hence forming disordered multilayer around the drop edge. As shown in Figure 2c and 3e, the addition of OA to a compressed monolayer of AuNPs on water surface resulted in a drastic improvement in the monolayer coverage and AuNP ordering. These results were reproducible when the experiment was repeated with different synthesis batches of AuNPs and chemical functionalizations of substrates. To demonstrate the high degree of ordering, we performed image analysis of an SEM image of the self-assembled AuNPs shown in Figure 4. Figure 4a shows the original SEM image with AuNPs packed in a hexagonal close-packed array with a center-to-center distance of 11.5 ( 0.4 nm (density of 6.2 Tdots/in2). The darker sites in this SEM are due to smaller AuNPs occupying these lattice sites. This SEM was taken at an operating voltage of 5 kV after beam focus and stigmation were highly optimized. The fast-Fourier transform (FFT) image in the inset confirmed that the AuNPs assembled 3357
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Figure 4. Large-area self-assembled AuNP monlayer after adding excess OA. (a) SEM image of AuNP monolayer with center-to-center distance of 11.5 ( 0.4 nm. Inset: FFT of the SEM image showing the high degree of hexagonal ordering. (b) Voronoi diagram indicating defects (blue, red, and green dots).
into a single hexagonally packed lattice within the image analyzed, as indicated by the six bright spots being equidistant from the center and from each other, i.e., at the vertices of a virtual hexagon. The long-range order was evident by the multiple concentric virtual hexagons marked by bright spots in the FFT image. Figure 4b shows a Voronoi diagram generated from this SEM (Figure 4a) by the image analysis software that we developed.1 Here, the blue, green and red dots are defect sites corresponding to AuNPs coordinated by 4, 5, and 7 nearest neighbors, respectively. The value of performing image analysis using the Voronoi diagram is evident from the defects that are highlighted in Figure 4b, which would otherwise be difficult to pick out from the SEM image in Figure 4a. As can be seen, the number of defects present is low and is measured to occupy only 1.7% of total area in this particular example. This analysis will enable quantitative analysis and process optimization, e.g. the study of defect density as a function of a parameter in the experiment. As the OA was added after the solvents had mostly evaporated and self-assembly completed, our results suggested that the additional OA caused a rearrangement of the disordered AuNPs into well-ordered monolayers. While the effect of excess ligands was clearly observed also by others, the actual mechanism for the ordering is arguably still not well established and depends on the specific system under study. Descriptions of these ligands as behaving as a high boiling-point solvent,16 enhancing the interparticle interactions,15 and promoting depletion forces31,32 are all possible explanations for the effect of excess ligands. One mechanism for this rearrangement as observed in our system is hypothesized as follows: the OA forms a monolayer on the water surface due to its ampiphilic nature and spread across the LB surface. While spreading, the OA molecules also dissolve in the organic droplets that contain the AuNPs. This excess OA allows the remobilization of the AuNPs trapped within the droplet to move to the surface and reorganize. Effectively, excess OA gets in between nanoparticles, thus reducing their attractive forces. As a result the system then moves from a kinetically trapped state to an ordered state that is more thermodynamically favorable. With the addition of free OA in the solvent, the attraction of an OAcapped AuNP at the bottom of a multilayer to another AuNP at the surface was reduced, hence reducing the formation of multilayers (Figure 3f).
Figure 5. Long-range ordered monolayer from T-AuNP without additional excess free OA. (a) SEM image and (b) schematics of T-AuNP with long-range ordering, presumably due to free OA extracted from AuNP by toluene. Detachment of OA from AuNP was evident from the precipitation of T-AuNP.
When we studied the AuNPs in toluene (T-AuNP), we observed the formation of long-range ordered monolayers without the addition of excess free ligands, although still within the droplets (Figure 5a). In addition to the slower evaporation rate of toluene,14 we attributed this better ordering of T-AuNP mainly to the higher solubility of OA in toluene than hexane. Although to our best knowledge there lacks literature value of OA solubility in the two solvents, the 10-fold better solubility of 1-otadecylamine, the saturated version of oleylamine, in toluene than hexane is in line with our hypothesis.33 This increased solubility caused the OA molecules that were originally attached to the AuNPs to gradually detach from the AuNPs, which is supported by our observations in poor AuNP stability and surface-pressure isotherms (Figure 6). Understandably, amines form weaker bonds with gold than thiols, thus resulting in a higher tendency to detach.34,35 The detachment led to two major effects: first, the decrease in OA on AuNP surface destabilized the AuNPs in toluene. The destabilized nanoparticles were observed in the form of black precipitates in the solution, i.e. aggregation of AuNPs, after about 1 day of synthesis. The amount of black precipitates continued to increase and the solution eventually turned from red to clear. Second, the detached OA effectively 3358
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the water and hydrocarbon termination in the air, and occupied a larger area to exert a greater effect on the increase of surface pressure than when they were attached to AuNPs as depicted in the schematics in Figure 6. Furthermore, an additional shoulder, which commonly appears in surface pressure isotherms of surfactants, was seen in the T-AuNP isotherm. Such a shoulder was also observed in the isotherm of a mixture of toluene and OA (T-OA), which again supported that the shoulder in the isotherms were due to the presence of free OA.
Figure 6. Surface pressure-area isotherms for two solvents (blue, hexane (H); pink, toluene (T)). Dashed curve: pure solvent (hexane and toluene). Solid curve: AuNPs in solvent (H-AuNP and T-AuNP). Dotted curve: mixture of OA and solvent (H-OA and T-OA). Curve for H-AuNP (blue solid) resembles the curve of pure hexane with a slight increase in the takeoff area (Ah) and maximum surface pressure, while curve for T-AuNP (pink solid) resembles the curve of T-OA with a much larger takeoff area (At) and maximum surface pressure than that of pure toluene. Schematics depict the larger value of At than Ah as a result of detached OA.
became excess free OA in the solution. The schematic in Figure 5b shows the possible T-AuNP solution where the detached OA caused AuNPs to precipitate but simultaneously acted as excess ligands to the remaining AuNPs. Unlike the case of H-AuNP where free OA helped the rearrangement of AuNPs after the solvents had evaporated, the free OA molecules were present in T-AuNP solution before it was dispensed onto the water surface. Hence, the OA molecules had a higher tendency to get in between AuNPs to improve interparticle interactions and prevented aggregation. The slightly larger center-to-center distance in the T-AuNP sample (11.7 ( 0.3 nm) supports this hypothesis. We could therefore observe better ordering in the self-assembly of T-AuNP than the H-AuNP before OA addition. To further support the hypothesis of detachment of OA from AuNPs that became free OA in solution, we studied the surfacepressure isotherms of H-AuNP, T-AuNP and other control solutions (Figure 6). The T-AuNP solution had a lower concentration of AuNP than the H-AuNP solution as most of the particles had precipitated out. The isotherms were significantly different between H-AuNP and T-AuNP. The isotherm for H-AuNP resembled that of pure hexane, except with a larger takeoff area (Ah), as indicated, and a larger maximum surface pressure. Both of these effects indicated the presence of H-AuNP on the water subphase. On the other hand, the isotherm for T-AuNP showed a much larger increase in the takeoff area (At) and maximum surface pressure, though the amount of AuNPs in the T-AuNP was significantly lower than the equivalent volume of H-AuNP added. This larger takeoff area could not be attributed to the larger monolayer coverage of AuNPs as these monolayers were observed (in SEM) to also form within droplets with a lower area coverage than in the case of H-AuNP. Hence, the observation that At > Ah suggests that there was a larger area coverage by OA on the water subphase from T-AuNP than H-AuNP. Hence the increased takeoff area and surface pressure were not due to the AuNPs. On the other hand, the excess OA molecules behaved like a surfactant, with its amine termination in
’ CONCLUSION Through the addition of excess free ligands, we showed a significant improvement in the ordering and monolayer coverage of self-assembled OA-capped AuNPs. Experiments indicate that the excess OA that was added after an initially disordered arrangement of AuNPs caused their rearrangement into ordered monolayers. A nearly defect-free monolayer with long-range order was found in a droplet up to ∼1.7 μm in diameter. We also attributed the better ordering in the self-assembly of OAcapped AuNPs in toluene over hexane to the detachment of OA on AuNPs, which acted as excess ligands in toluene. Hence we demonstrated that the addition of excess ligands before and after the self-assembly of AuNPs resulted in increased ordering and monolayer formation. Our techniques can be applied to rearrange nanoparticle arrays into commensurate prepatterned substrates to achieve long-range ordered superlattices, as is necessary for bit-patterned media. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the MRSEC Grant at Princeton University NSF DMR 0819860, and the Science and Engineering Research Council (SERC) grant of the Agency for Science, Technology and Research (A*STAR) Singapore. ’ REFERENCES (1) Bita, I.; Yang, J. K. W.; Jung, Y. S.; Ross, C. A.; Thomas, E. L.; Berggren, K. K. Graphoepitaxy of Self-Assembled Block Copolymers on Two-Dimensional Periodic Patterned Templates. Science 2008, 321 (5891), 939–943. (2) Cheng, J. Y.; Mayes, A. M.; Ross, C. A. Nanostructure engineering by templated self-assembly of block copolymers. Nat. Mater. 2004, 3 (11), 823–828. (3) Huang, D.; Liao, F.; Molesa, S.; Redinger, D.; Subramanian, V. Plastic-Compatible Low Resistance Printable Gold Nanoparticle Conductors for Flexible Electronics. J. Electrochem. Soc. 2003, 150 (7), G412–G417. (4) Kolliopoulou, S.; Tsoukalas, D.; Dimitrakis, P.; Normand, P.; Paul, S.; Pearson, C.; Molloy, A.; Petty, M. C. Gold Langmuir-Blodgett deposited nanoparticles for non-volatile memories. Mater. Res. Soc. Symp. Proc. 2004, 2004, D6.7.1–D6.7.6. (5) Liu, Z.; Lee, C.; Narayanan, V.; Pei, G.; Kan, E. C. Metal nanocrystal memories. I. Device design and fabrication. IEEE Trans. Electron Devices 2002, 49 (9), 1606–1613. (6) Liu, Z.; Lee, C.; Narayanan, V.; Pei, G.; Kan, E. C. Metal nanocrystal memories-part II: electrical characteristics. IEEE Trans. Electron Devices 2002, 49 (9), 1614–1622. 3359
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