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Structural Organization of Gold Nanoparticles onto the ITO Surface and Its Optical Properties as a Function of Ensemble Size Om P. Khatri,* Kuniaki Murase, and Hiroyuki Sugimura* Department of Materials Science and Engineering, Kyoto UniVersity, Sakyo-ku, Kyoto 606-8501, Japan ReceiVed December 14, 2007. In Final Form: January 28, 2008
Self-assembly of citrate-stabilized gold nanoparticles (AuNPs) onto an optically transparent indium tin oxide (ITO) surface followed by neutralization of these particles using dodecanethiol as a surfactant have been demonstrated. X-ray photoelectron spectroscopic (XPS) studies revealed the partial removal of citrate ions from the immobilized AuNPs, which advances the dilution of electrostatic attraction between AuNPs and the APS (amino-terminated monolayer)functionalized ITO surface. The resultant AuNPs restore their mobility to some extent and form small ensembles. Some of the immobilized AuNPs were completely removed from the surface due to neutralization, as confirmed by XPS studies. Interparticle distance and size of ensembles were manipulated by consecutive cycles of immobilization and neutralization of AuNPs. Controlled nanostructural fabrication progression, which leads to two-dimensional lateral growth of AuNPs, provides a method for systematically shifting the surface plasmon resonance band based on the increase in plasmon coupling among the closely placed AuNPs of an ensemble. The magnitude of shift increases with the size of ensemble. This manipulated chemical strategy offers a convenient and simple method to tune the optical properties of materials on a nanoscale.
Introduction Nanoarchitectures with different structures, organizations, and geometrics have been demonstrated for the fabrication of a wide range of optical and electronic devices using top-down and bottom-up approaches.1-3 In the past few years, extensive research on the size-dependent properties of small building blocks like nanoparticles, nanoclusters, and molecular domains has been undertaken. The integration of these building blocks is very challenging due to the complexity of selecting and/or manipulating the nanoobjects into the desired position with their ordered aggregation and organization. Noble metal nanoparticles are promising candidates and are playing vital roles in fabricating advanced optoelectronic devices. Nanoparticles interact strongly with visible light through the resonant excitation of the collective oscillations of their conduction electrons.4 As a result of these oscillations, local electromagnetic fields near the particle can be many orders higher than the incident fields and generate intense scattered light around the wavelength of the resonant peak. The magnitude and wavelength of these plasmon resonance bands are characteristic to the size, shape, composition of nanoparticles, andinterparticledistancesaswellasthesurroundingenvironment.5-13 * Corresponding authors. E-mail:
[email protected] (O.P.K.). Phone: +81 75 753 9130. Fax: +81-75-753-5484. Email: hiroyuki.
[email protected] (H.S.). (1) (a) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. AdV. Mater. 2000, 12, 693-713. (b) Shipway, A. N.; Lahav, M.; Willner, I. AdV. Mater. 2000, 12, 993-998. (c) Teo, B. K.; Sun, X. H. Chem. ReV. 2007, 107, 1454-1532. (d) Wouters. D.; Schubert, U. S. Angew. Chem., Int. Ed. 2004, 43, 2480-2485. (e) Jackson, A. M.; Myerson, J. W.; Stellacci, F. Nat. Mater. 2004, 3, 330-336. (f). Jeong, U.; Wang, Y.; Ibisate, M.; Xia, Y. AdV. Funct. Mater. 2005, 15, 1907-1921. (g) Graf, C.; Blaaderen, A. Langmuir 2002, 18, 524-534. (h) Quake, S. R.; Scherer, A. Science 2000, 290, 1536-1540. (i) Kim, B.; Tripp. S. L.; Wei, A. J. Am. Chem. Soc. 2001, 123, 7955-7956. (j) DeVries, G. A.; Brunnbauer, M.; Hu, Ying, Jackson, A. M.; Long, B.; Neltner, B. T.; Uzun, O.; Wunsch, B. H.; Stallacci, F. Science 2007, 315, 358-361. (k) Jackson, A. M.; Hu, Y.; Silva, P. J.; Stellacci, F. J. Am. Chem. Soc. 2006, 128, 11135-11149. (2) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem. 2000, 1, 18-52. (3) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293-346. (4) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (5) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668-677.
Short- and long-range interactions between nanoparticles furnish new optical properties14 due to near-field coupling (between close particles) and far-field dipolar interactions.15-17 Interparticle coupling between pairs of elliptical,10 angular,11 and spherical15 shaped metal nanoparticles has demonstrated that the resonant wavelength peak of two interacting particles is red shifted as a result of near-field coupling. If two nanoparticles are in close proximity, the coupling partner shifts the frequency of the surface plasmon band by affecting the conduction electron oscillations of each nanoparticle. Methods are continuously being developed and improved for the organization of nanoparticles into one-, two-, and threedimensional arrays for different optical properties, and these nanostructures have drawn much interest because of their potential applications as in sensors,18 photonic devices,14,19 medical science,20and catalysis,21 etc. Electron beam lithography10-12,16 (6) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 5599-5611. (7) Eustis, S.; El-Sayed, M. A. Chem. Soc. ReV. 2006, 35, 209-217. (8) Basu, S.; Ghosh, S. K.; Kundu, S.; Panigrahi, S.; Praharaj, S.; Pande, S.; Jana, S.; Pal, T. J. Colloid Interface Sci. 2007, 313, 724-734. (9) Chumanov, G.; Sokolov, K.; Gregory, B. W.; Cotton, T. M. J. Phys. Chem. 1995, 99, 9466-9471. (10) Su, K.-H.; Wei, Q.-H.; Zhang, X.; Mock, J. J.; Smith, D. R.; Schultz. S. Nano Lett. 2003, 3, 1087-1090. (11) Sung, J.; Hicks, E. M.; Van, Duyne, R. P.; Spears, K. G. J. Phys. Chem. C 2007, 111, 10368-10376. (12) Huang, W.; Qian, W.; Jain, P. K.; El-Sayed, M. A. Nano Lett. 2007, 7, 3227-3234. (13) Atay, T.; Song, J.-H.; Nurmikko, A. V. Nano Lett. 2004, 4, 1627-1631. (14) Hutter, E.; Fendler, J. H. AdV. Mater. 2004, 16, 1685-1706. (15) Nedyalkov, N. N.; Atanasov, P. A.; Obara, M. Nanotechnology 2007, 18, 305703. (16) Salerno, M.; Krenn, J. R.; Hohenau, A.; Ditlbacher, H.; Schider, G.; Leitner, A.; Aussenegg, F. R. Opt. Commun. 2005, 248, 543-549. (17) Felidj, N.; Aubard, J.; Levi, G.; Krenn, J. R.; Schider, G.; Leitner, A.; Aussenegg, F. R. Phys. ReV. B 2002, 66, 245407. (18) (a) Yonzon, C. R.; Jeoung, E.; Zou, S.; Schatz, G. C.; Mrksich, M.; Van Duyne, R. P. J. Am. Chem. Soc. 2004, 126, 12669-12676. (b) Yu, A.; Liang, Z.; Cho, J.; Caruso, F. Nano Lett. 2003, 3, 1203-1207. (19) (a) Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Nature 2003, 424, 824830. (b) Maier, S. A.; Brongersma, M. L.; Kik, P. G.; Meltzer, S.; Requicha, A. A. G.; Atwater, H. A. AdV. Mater. 2001, 13, 1501-1505. (c) Cao, Y. C.; Jin, R.; Mirkin, C. A. Science 2002, 297, 1536-1540. (20) (a) Alivisatos, P. Nat. Biotechnol. 2004, 22, 47-51. (b) El-Sayed, I. V.; Huang, X.; El-Sayed, M. A. Nano Lett. 2005, 5, 829-834.
10.1021/la7039042 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/14/2008
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Scheme 1. Schematic Illustration for Immobilization of Citrate-Stabilized AuNPs onto the ITO Surface Followed by Neutralization Using Dodecanethiol as Surfactanta
a Electrostatic charge due to citrate capping keeps AuNPs well spaced from each other, and van der Waals force between DDT-capped AuNPs advance them for close packing.
(EBL), nanosphere lithography,6,22 and the recently developed nanoskiving technique23 have been demonstrated as fabrication tools for immobilization and/or formulation of the nanoparticles into the manipulated position with controlled interparticle distance to observe the desired optical properties. EBL is widely used to fabricate gold nanoparticles (AuNPs) for studying surface plasmon coupling and subwavelength optical waveguides,10,12,16 but this method requires very sophisticated instrumentation with high operation costs, which limits its exploitation at the application level with mass productions. Nanosphere lithography6,22 and nanoskiving23 are more convenient and approachable, but they have particle dimension limitations. Chemical strategies have also been developed based on the interaction between AuNPs and thiols or amino groups24-27 but without monitoring the twodimensional growth and size of ensembles to tune the optical properties. A three-dimensional thin film of AuNPs has been reported using either dithiols28 or polyelectrolytes-assisted29 layerby-layer approach and red shifts due to nanoparticle aggregation were demonstrated, but this methodology leads to an increase in the thickness of the film, which alters the absorption coefficient. Here, we address a controlled immobilization of the AuNPs in two-dimensional arrays to monitor the collective optical properties of the substrate. The method described here is not intended to replace these existing methods but rather to provide a simple alternative for formulating the nanostructure relevant to controlled plasmonics properties. In the current study, we establish a simple and versatile wet strategy for controlled structural organization of AuNPs onto an optically transparent indium tin oxide (ITO) surface using a cyclic approach to immobilize the citrate-stabilized AuNPs, (21) (a) Guczi, L.; Peto, G.; Beck, A.; Frey, K.; Geszti, O.; Molnar, G.; Daroczi, C. J. Am. Chem. Soc. 2003, 125, 4332-4337. (b) Esumi, K.; Houdatsu, H.; Yoshimura, T. Langmuir 2004, 20, 2536-2538. (22) Zhang, X.; Yonzon, C. R.; Van Duyne, R. P.; J. Mater. Res. 2006, 21, 1083-1092. (23) Xu, Q.; Bao, J.; Capasso, F.; Whitesides, G. M. Angew. Chem., Int. Ed. 2006, 45, 3631-3635. (24) Sato, T.; Brown, D.; Johnson, B. F. G. Chem. Commun. 1997, 10071008. (25) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 11481153. (26) Liu, S.; Zhu, T.; Hu, R.; Liu, Z. Phys. Chem. Chem. Phys. 2002, 4, 6059-6062. (27) Weisbecker, C. S.; Merrritt, M. V.; Whitesides, G. M. Langmuir 1996, 12. 3763-3772. (28) (a) Abdelrahman, A. I.; Mohammad, A. M.; Okajima, T.; Ohsaka, T. J. Phys. Chem. B 2006, 110, 2798-2803. (b) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14, 5425-5429. (29) Vial, S.; Pastoriza-Santos, I.; Perez-Juste, J.; Liz-Marzan, L. M. Langmuir 2007, 23, 4606-4611.
followed by neutralization of these particles using dodecanethiol (DDT), as shown in Scheme 1. This consecutive cyclic approach is used to monitor the nanoparticle ensemble size, distribution, and the interparticle distances. Correlation was established between the manipulated structural organization of AuNPs and their plasmonic modes. Experimental Indium tin oxide (ITO, thickness ) 150 nm, rms roughness ) 4.7 Å, over 20 × 20 µm2) deposited on soda lime glass was procured from Kuramoto Co. Ltd., Japan. Aminopropyltriethoxysilane (APS, 97%) and 1-dodecanethiol (DDT, 98+%) were purchased from Gelest Inc. and Aldrich, respectively, and were used as received. Monodispersed gold nanoparticles (AuNPs, φ ) 20 ( 3 nm) were procured from Sigma. Ultrapure water (UPW) was used throughout the sample preparation. Toluene, ethanol, and other reagents were of analytical grade. ITO samples were sonicated with ethanol and UPW, respectively, to remove environmental and industrial dust. Subsequently, samples were immersed in a mixture of H2O and H2O2 (5:2, v/v) for 40 min at 100 °C and then washed with a copious amount of UPW. Before APS monolayer deposition, ITO samples were kept in a VUV/ozone treatment chamber for 20 min to burn off all carbonaceous contaminants. An APS monolayer was formed by the chemical vapor deposition method.30 To prepare the APS monolayer, ITO samples were placed in a Teflon container with a glass vessel under dry nitrogen atmosphere. The glass vessel contained the APS solution in toluene (10% v/v). The Teflon container was sealed with a cap and placed in an electric oven at 100 °C. After 2 h of deposition time, samples were sonicated successively in toluene, ethanol, and UPW for 5 min each. The water contact angle measured on APS modified ITO surface was 50° ( 2 (the modification resulted in the formation of an amino-terminated monolayer). To immobilize the AuNPs, the APS functionalized ITO samples were immersed in a suspension of citrate-stabilized AuNPs for 2 h and then rinsed thoroughly with UPW. Neutralization of AuNPs was performed by immersing the samples in a 10 mM DDT solution for 24 h followed by rinsing with ethanol and blowing with dry nitrogen. The above two steps were performed for ten successive cycles. X-ray photoelectron spectroscopic (XPS, Kratos Analytical Ltd., ESCA 3400) measurements were performed to monitor the elemental composition during each cycle of AuNPs deposition and neutralization on the ITO surface using unmonochromatized Mg KR radiation as a X-ray source. All measurements were executed at 10 mA and 10 kV. The vacuum level during the measurements was kept in the range of 10-7 Pa. Since a weak charging-up of the monolayer on (30) Sugimura, H.; Hozumi, A.; Kameyama, T.; Takai, O. Surf. Interface Anal. 2002, 34, 550-554.
Structural Organization of Gold Nanoparticles
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Figure 1. FESEM images of AuNPs supported on the ITO surface: (a) uniformly scattered AuNPs monitored by electrostatic repulsive force between citrate-stabilized AuNPs, and (b) neutralization of AuNPs using DDT as the surfactant, which leads them to form ensembles.
Figure 2. XPS spectra of (a) Au 4f and (b) S 2p regions for immobilized AuNPs on the ITO surface (solid lines). The effect of neutralization is represented by dotted lines. ITO was recognized, the binding energies were aligned with the In 3d5/2 binding energy, i.e., 445 eV.31 The chemically distinct species for C 1s were resolved using a Gaussian-Lorentzian fitting program. The JEOL JSM-7400F field emission scanning electron microscope (FESEM) was used to examine the structural organization of AuNPs onto the ITO surface. The AuNPs coverages for different samples were calculated from an area analysis of the FESEM images. The analysis is based on the total number of AuNPs, which are directly attached to ITO surface, and the area of the relevant FESEM image. The reported values are the average of three different measurements in each type of sample. UV-visible absorption spectra were recorded on a HITACHI U-3500 spectrophotometer. A thoroughly cleaned ITO sample was used as the background for all UV-visible spectra measurements.
Results and Discussion ITO surface functionalized with APS monolayer was used as the chemical template to immobilize the gold nanoparticles. Deposition of AuNPs on the ITO surface originates from the electrostatic attraction between citrate-stabilized AuNPs and amino groups of APS monolayer, resulting in the formation of well-spaced, scattered organization of AuNPs, as shown in Figure 1a. Electrostatically monitored organization due to trivalent citrate ions on AuNPs inhibits further immobilization and prevents them (31) Song, W.; So, S. K.; Wang, D.; Qiu, Y.; Cao, L. Appl. Surf. Sci. 2001, 177, 158-164.
for close packing to each other. The surface coverage, measured from the FESEM images, was 26.7 ( 1% with 10-25 nm interparticle distances. To accomplish the close packing, we carried out neutralization24 of the citrate coating employing DDT as a surfactant. Strong affinity between thiol molecules and gold replaces the loosely bound citrate capping from AuNPs as a result of Au-S chemisorption bonding.27 This neutralization process enhances the mobility of AuNPs and allows them to approach each other and form small-size ensembles, as shown in Figure 1b. The van der Waals attractions among adjacent AuNPs (driven by alkyl chains of DDT molecules) within an ensemble make them close to each other, with interparticle distances in the range of 4-6 nm. To gain a better understanding, the surface compositions of the functionalized ITO surface before and after the neutralization of AuNPs were examined using XPS. A representative set of XPS spectra are shown in Figures 2 and 3. This study is focused on Au 4f, S 2p, and C 1s spectral regions, since these elements are characteristic components of the AuNPs and DDT monolayer. The Au 4f signal, as shown in Figure 2a, appears at 83.9 and 87.6 eV as a doublet corresponding to 4f7/2 and 4f5/2. The binding energy of Au 4f7/2 is in agreement with results reported for AuNPs.32,33 The S 2p signal appeared after the neutralization at ∼163 eV, as shown in Figure 2b, and is attributed to the bound (32) Bourg, M.-C.; Badia, A.; Lennox, R. B. J. Phys. Chem. B 2000, 104, 6562-6567.
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Figure 3. XPS spectra of C 1s region for immobilized AuNPs on the ITO surface (a) before and (b) after the neutralization of AuNPs using DDT molecules to replace the citrate coating. Table 1. Chemical Quantification of AuNPs Immobilized on the ITO Surface elemental composition, % C 1s sample description
O 1s
Sn 3d
In 3d
N 1s
C-C
C-N
C-O/ COO
Si 2p
Au 4f
S 2p
citrate-stabilized AuNPs DDT-treated AuNPs
44.3 40.7
2.0 1.7
27.3 25.1
1.3 0.9
5.6 14.4
2.9 2.9
8.5 6.6
3.6 2.9
4.5 4.0
0.0 0.8
thiolate, which is shifted compared to that of elemental sulfur (164.2 eV).32 The C 1s peaks were processed with a GaussianLorentzian fitting program and were deconvoluted. Figure 3a shows that the C 1s signal is composed of four different types of carbons, C-C/C-H, C-NH2, CO, and COO which appear at 285, 286, 286.6, and 288.6 eV respectively.34 The C 1s peak at 286 eV is ascribed to the presence of the amino-terminated (C-NH2) monolayer, and its intensity remains the same after neutralization as shown in Table 1. The C 1s bands due to CO and COO groups contribute to 50% of the total carbon, and this reveals the presence of the citrate coating on the immobilized AuNPs. Since each citrate ion carries three carboxyl groups, they have the maximum contribution. The increase of C 1s intensity from 17 to 23.9% with maximum contribution from the C-C/C-H peak at 285 eV demonstrates the assembling of DDT molecules during neutralization, and the simultaneous decrease of CO and COO contributions from 8.5 to 6.6% reveals the desorption of citrate ions from AuNPs. However, the presence of significant amounts of CO and COO carbons after the neutralization illustrates that the citrate coating still remains, but some fraction of it has been replaced by DDT molecules. Additionally, we flushed the AuNPs-functionalized ITO sample with ethanol after the neutralization and found a slight decrease in 4f peak intensity (complete detail and results are provided in Supporting Information). This reveals that citrate ions are not being fully replaced by the DDT molecules on AuNPs; the resultant particles are still tethered to the ITO surface. DDT molecules show high affinity toward gold, and even the presence of a physisorbed citrate coating on gold nanoparticles (33) Joseph, Y.; Besnard, I.; Rosenberger, M.; Guse, B.; Nothofer, H.-G.; Wessels, J. M.; Wild, U.; Axel, K-G.; Su, D.; Schlogl, R.; Yasuda, A.; Vossmeyer, T. J. Phys. Chem. B 2003, 107, 7406-7413. (34) Briggs, D. Surface Analysis of Polymers by XPS and Static SIMS; Cambridge University Press: New York, 1998; p 65.
did not prevent them from assembling. However, chemisorption of DDT molecules on a gold surface is slow,35 possibly due to a decreased sticking coefficient of the alkanethiols on the gold in the presence of a physisorbed layer of citrate ions. This could be the reason for incomplete replacement of the citrate coating by DDT molecules, as illustrated by XPS studies. Figure 1b shows that AuNPs are still tethered to the ITO surface, and this is probably due to electrostatic interaction between the aminoterminated monolayer and the partial citrate coating that is present on AuNPs. The neutralization process advances the dilution of electrostatic attraction between AuNPs and the APS monolayer, and the amplitude of the localizing Brownian motion as well as the lateral diffusion of AuNPs become larger.26 Consequently, AuNPs restore their mobility and approached each other to form the small size ensembles. The decrease of Au 4f peak intensity after neutralization as shown in Figure 2a revealed that the citrate coating on some of the AuNPs might be completely replaced by DDT molecules. As a consequence, they lose their interaction with the APS monolayer and detach from the ITO surface. The assembling of DDT molecules on AuNPs allows them to attract each other via van der Waals interaction. However, this effect is still inadequate, and hence we observed small ensembles containing only 2-8 nanoparticles rather than complete aggregation of these nanoparticles. The vacant space formed after ensembles formation provides room for additional immobilization of AuNPs. Consecutive cycles of immobilization and neutralization of AuNPs lead to an increase in the AuNPs’ density on the ITO surface and an increase in ensemble size. Figure 4 shows the FESEM images after one, two, five, and ten cycles of immobilization assisted by neutralization of AuNPs. The small size ensembles, which are formed after the first cycle, either act as nuclei to grow the ensemble (35) Aslan, K. and Perez-Luna, V. H. Langmuir 2002, 18, 6059-6065.
Structural Organization of Gold Nanoparticles
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Figure 4. FESEM images of 2D arrays of AuNPs supported on ITO surface. Structural organizations of AuNPs were monitored using a cyclic approach of immobilization followed by neutralization. (A) Replacement of citrate coating using DDT molecules advances them to assemblage. Consecutive cycles of immobilization and neutralization of the AuNPs increase the size of ensembles, and this is illustrated in (B) after two cycles, (C) after five cycles, and (D) after ten cycles.
Figure 5. Surface coverage vs number of cycles for the immobilization and neutralization of AuNPs. The AuNPs surface coverage was calculated using FESEM images.
and/or merge with each other, and the resultant surface coverage increases with successive cycles as shown in Figure 5. Screening of the surface charge on AuNPs by DDT molecules leads them to form a hexagonally packed structure driven by the van der Waals interaction. Since short-range steric repulsion exists due
to the DDT molecules, AuNPs do not experience diffusionlimited aggregation,24,36 and they form a hexagonally packed structure with fractal morphologies as shown in the inset of Figure 4c. Surprisingly, we observed piling-up of AuNPs in some places instead of continuous lateral two-dimensional growth after ten cycles of immobilization of AuNPs. This is probably due to the adsorption of atmospheric impurities on the APS monolayer, which might deactivate or contaminate the amino groups during the cyclic process of immobilization and neutralization of AuNPs. The Schinmel group37 has conducted systematic studies on the effect of environmental impurities on freshly prepared APS monolayers. They observed that increasing the atmospheric exposure time for APS monolayer increased the water contact angle (hydrophobicity), and one of the estimates said that the advancing contact angle reached 70° after three weeks of exposure, which is very high compared to that of freshly prepared aminoterminated monolayer.37,38 Surface coverage of AuNPs reveals that the increment of surface coverage decreases with increasing number of cycles, as illustrated in Figure 5, which supports the deactivation of amino sites. During each cycle, the APS monolayer is exposed to (a) the ambient environment, (b) gold colloid solution which contains 0.04% sodium citrate, 0.02% sodium azide, and 0.01% HAuCl4, and (c) ethanolic solution of DDT. These environmental and chemical exposures probably reduce the activity of amino groups, and this effect becomes more prominent with increasing number of cycles. Hence after more number of (36) Weitz, D. A.; Lind, M. Y.; Sandroff, C. J. Surf. Sci. 1985, 158, 147-164. (37) Sequeira, Petri, D. F.; Wenz, G.; Schunk, P.; Schimmel, T. Langmuir 1999, 15, 4520-4523. (38) Tsukruk, V. V.; Bliznyuk, V. N. Langmuir 1998, 14, 446-455.
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Figure 6. XPS spectra of (a) Au 4f and (b) S 2p regions. Intensity of these elements changes with the number of deposition steps of AuNPs assisted by DDT assembly. Curves (A) APS monolayer, (B) uniformly scattered citrate capped AuNPs, (C) two cycles of immobilization of AuNPs assisted by DDT monolayer, (D) five cycles of immobilization of AuNPs assisted by DDT monolayer, and (E) ten cycles of immobilization of AuNPs assisted by DDT monolayer.
successive cycles, the active amino sites are reduced, which generally hold the AuNPs, and therefore these particles start to pile up on the ensembles of AuNPs instead of lateral twodimensional growth. To monitor the chemical/elemental change during consecutive cycles of immobilization and neutralization of AuNPs, we performed XPS measurements. The Au 4f7/2 peak appears at 83.9 eV which corresponds to Au (0), demonstrating the immobilization of AuNPs, as shown in Figure 6a. The intensity of the Au 4f peak increases with successive cycles of immobilization and neutralization, but the rate of increase decreases with the number of cycles, since the availability of active amino sites is decreases. The intensity of the S 2p signal increases with successive neutralization cycles, as shown in Figure 6b. This illustrates that the replacement of citrate coating by DDT molecules increases as the new AuNPs increases with successive cycles of immobilization and neutralization. Figure 7 illustrates the UV-visible spectra for different structural organization of AuNPs on the ITO surface. Curve A shows the spectra of uniformly scattered AuNPs with λmax ) 517 nm as the surface plasmon absorption peak. Subsequently, the functionalized chemical template was exposed to DDT molecules to screen the electrostatic charge, and a shifted surface plasmon resonance peak appeared at 566 nm. This plasmon shift was originated by ensembles of AuNPs with reduced interparticle distance, as illustrated in Figure 4a. The optical properties of AuNPs are mainly determined by two contributions: (a) the surface plasmon mode of the well-spaced isolated AuNPs, which provides individual identity as observed in curve A of Figure 7 with the corresponding structural organization as shown in Figure 1a and (b) the collective surface plasmon properties of the ensembles/domains. An ensemble is an assemblage of closely packed AuNPs. The oscillating electrons in one particle experience the electric field due to the oscillation of electrons in the surrounding particles within an ensemble, and this leads to the appearance of a plasmon band attributed to the coupled plasmon absorbance of AuNPs.8 These ensembles are well spaced, and hence their surface plasmon band will not interact or couple. Application of successive deposition and neutralization of AuNPs
Figure 7. UV-vis spectra for 2D arrays of AuNPs on the ITO surface. Variations in optical properties were manipulated by monitoring the nanostrutural organization of AuNPs using a cyclic approach of immobilization and neutralization of AuNPs. (A) Uniformly scattered citrate capped AuNPs, and (B) replacing of citrate coating by DDT molecules leads to plasmon coupling and resultant red shifts. Similarly, consecutive cycles approach of immobilization and neutralization of AuNPs strengthens the surface plasmon coupling, resulting in more bathochromic shifts. Curves (C), (D), and (E) illustrate two, five, and ten successive cycles, respectively.
increases the size of ensembles, while the interparticle distance remains the same, i.e., in the range of 4-6 nm.
Structural Organization of Gold Nanoparticles
Figure 8. Number of AuNPs in an ensemble vs bathochromic shift for AuNPs functionalized ITO surface. Number of AuNPs within an ensemble was calculated from FESEM images.
Figure 8 illustrates that the bathochromic shift increases with increasing number of AuNPs in an ensemble. Here, we demonstrate the collective optical properties of particle ensembles during each cycle of immobilization and neutralization of AuNPs. After the first cycle of neutralization, AuNPs in an ensemble are close to each other, with DDT hindered close packing and resultant strong electromagnetic interaction among adjacent AuNPs. This coupling leads to absorbance at 566 nm, which is 49 nm red shifted from that of the individual AuNPs. Successive cycles of deposition and neutralization of AuNPs lead to increasing ensemble size by the addition of new AuNPs and merging of small ensembles, which strengthens the plasmon coupling and results in more bathochromic shift as illustrated in Figure 8. The bathochromic shift increment decreases with increasing number of cycles.
Conclusion We have demonstrated a chemical strategy to manipulate the interparticle distance between AuNPs and the size of ensembles
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to tune the desired optical properties for ITO substrates. The electrostatic attraction between the APS monolayer and citratestabilized AuNPs allows them to be immobilized on the ITO surface. DDT molecules with strong affinity toward gold replace the citrate ion coating and form the ensembles. However, the incomplete replacement of the citrate coating restricts their detachment from ITO surface. Controlled nanostructural organization of AuNPs, monitored by successive immobilization and neutralization of AuNPs in a cyclic process, systematically provides bathochromic shifts as a result of surface plasmon coupling among adjacent AuNPs within an ensemble. We reported 100 nm bathochromic shifts in the visible region for the ITO surface in periodic increments. The methodology addressed in this study is very convenient and inexpensive for tuning the optical properties and can be applied to develop optical and plasmonic devices with monitored wavelengths. We believe that by applying different types of nanoparticles (both separate and mixed) with suitable linkers (surfactants) the shifts in visible regions can be broadened, and we are continuing to work on this task. Acknowledgment. The authors are grateful to Professor M. Oyama, International Innovation Center (IIC) and Prof. Y. Awakura, Kyoto University for providing FESEM and UV-vis spectroscopic facilities to perform this work. They also acknowledge the support of Mr. K. Adachi and Mr. D. Kasahara in utilizing the instrument facilities. We gratefully acknowledge the financial support from a Grant-in-aid for scientific research on priority area “Strong photons-molecules coupling fields” (No. 470) and scientific research category A (No. 17206073) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Author Om P. Khatri highly acknowledges the Japan Society for the Promotion of Science for the JSPS postdoctoral fellowship. Supporting Information Available: Details of ethanol flush test and XPS results of AuNPs on APS-functionalized ITO surface. This material is available free of charge via the Internet at http://pubs. acs.org. LA7039042
