Electroless Gold Island Thin Films: Photoluminescence and ...
Electroless Gold Island Thin Films: Photoluminescence and Thermal Transformation to Nanoparticle Ensembles Wonmi Ahn,†,‡ Benjamin Taylor,‡ Analı´a G. Dall’Ase´n,‡ and D. Keith Roper*,‡ Department of Materials Science and Engineering, 304 CME, UniVersity of Utah, Salt Lake City, Utah 84112, and Department of Chemical Engineering, 3290 MEB, UniVersity of Utah, Salt Lake City, Utah 84112 ReceiVed October 3, 2007. In Final Form: December 21, 2007 Electroless gold island thin films are formed by galvanic replacement of silver reduced onto a tin-sensitized silica surface. A novel approach to create nanoparticle ensembles with tunable particle dimensions, densities, and distributions by thermal transformation of these electroless gold island thin films is presented. Deposition time is adjusted to produce monomodal ensembles of nanoparticles from 9.5 ( 4.0 to 266 ( 22 nm at densities from 2.6 × 1011 to 4.3 × 108 particles cm-2. Scanning electron microscopy and atomic force microscopy reveal electroless gold island film structures as well as nanoparticle dimensions, densities, and distributions obtained by watershed analysis. Transmission UV-vis spectroscopy reveals photoluminescent features that suggest ultrathin EL films may be smoother than sputtered Au films. X-ray diffraction shows Au films have predominantly (111) orientation.
Introduction Nanoscale gold (Au) structures on silicon (Si) substrates are important in electronics, MEMS devices, diagnostics, biosensing, spectroscopy, and microscopy. Au thin films are applied in gateable electronic transistors1,2 and conductors,3 optically induced thermoplasmonic gratings for nanomanipulation of picoliter droplets,4 surface plasmon resonance (SPR) sensors (45-60-nm thickness), and SERS-active substrates5,6 excited by visible (red), near-IR, and IR7 light. Au island films are substrates for SERS,8 attenuated total reflection surface-enhanced IR absorption (SEIRA)9 microspectroscopy,10 and transmission localized surface plasmon sensors11-13 and identify nanoscale structures in nearfield scanning optical microscopy (NSOM).14 Au nanoparticle (NP) assemblies on Si have uses in thermal nanoprocessing,15,16 nanoactuation,17 immunoassays,18 porphyrin-conjugated nano* Corresponding author: Tel:+1 801 585 9185. Fax: +1 801 585 9291. E-mail: [email protected]. † Department of Materials Science and Engineering. ‡ Department of Chemical Engineering. (1) Luo, X.; Orlov, A. O.; Snider, G. L. J. Vac. Sci. Technol. B 2004, 22, 3128-3132. (2) Scheible, D. V.; Weiss, C.; Kotthaus, J. P.; Blick, R. H. Phys. ReV. Lett. 2004, 93, 186801/1-186801/4. (3) Fedorovich, R. D.; Inosov, D. S.; Kiyaev, O. E.; Lukyanets, S. P.; Marchenko, A. A.; Tomchuk, P. M.; Bevzenko, D. A.; Naumovets, A. G. J. Mol. Struct. 2004, 708, 67-77. (4) Passian, A.; Lereu, A. L.; Farahi, R. H.; Ferrell, T. L.; Thundat, T. Trends in Thin Solid Films Research; Jost, A. R., Eds.; Nova Science Publishers: New York, 2007; Chapter 3. (5) Sockalingum, G. D.; Beljebbar, A.; Morjani, H.; Manfait, M. Proc. SPIEInt. Soc. Opt. Eng. 1998, 3260, 58-62. (6) Tolaieb, B.; Aroca, R. Can. J. Anal. Sci. Spectrosc. 2003, 48, 139-145. (7) Jennings, C. A.; Kovacs, G. J.; Aroca, R. Can. J. Phys. Chem. 1992, 96, 1340-1343. (8) Sockalingum, G. D.; Beljebbar, A.; Morjani, H.; Angiboust, J. F.; Manfait, M. Biospectrosc. 1998, 4, S71-S78. (9) Vongsvivut, J.; Itoh, T.; Ikehata, A.; Ekgasit, S.; Ozaki, Y. Sci. Asia 2006, 32, 261-269. (10) Sudo, E.; Esaki, Y.; Sugiura, M.; Murase, A. Appl. Spectrosc. 2007, 61, 269-275. (11) Kalyuzhny, G.; Vaskevich, A.; Schneeweiss, M. A.; Rubinstein, I. Chem. A Eur. J. 2002, 8, 3849-3857. (12) Ruach-Nir, I.; Bendikov, T. A.; Doron-Mor, I.; Barkay, Z.; Vaskevich, A.; Rubinstein, I. J. Am. Chem. Soc. 2007, 129, 84-92. (13) Lahav, M.; Vaskevich, A.; Rubinstein, I. Langmuir 2004, 20, 73657367. (14) Ianoul, A. Abstracts of the 35th Northeast Regional Meeting of the American Chemical Society, Binghamton, New York, 2006.
wires,19 surface-enhanced Raman (SER) scattering20 for singlemolecule spectroscopy,21,22 and SER nanosensors.23 Nanoparticles have been assembled on Si by deposition using pulsed lasers,21 electrochemistry,24 and microwaves;25 selfassembly on thiols,20,26 dendrimers,27 or other polymers;28 conjugation to porphyrin polymers;19 evaporation;29,30 and resistive31 or thermal32 heating of evaporated thin film. Metal islands and thin films have been formed by sputtering,33 vacuum evaporation8 and evaporation onto chemically modified glass substrates,11,34 nanomaching,4 electrodeposition, and electroless deposition.35 Electroless (EL) deposition of Au has the advantage of being able to rapidly coat fragile, 3D, or internal surfaces at ambient (15) Palpant, B.; Rashidi-Huyeh, M.; Gallas, B.; Chenot, S.; Fisson, S. Appl. Phys. Lett. 2007, 90, 223105/1-223105/3. (16) Numata, T.; Tatsuta, H.; Morita, Y.; Otani, Y.; Umeda, N. IEEJ Trans. Elect. Electron. Eng. 2007, 2, 398-401. (17) Mitsuishi, M.; Koishikawa, Y.; Tanaka, H.; Sato, E.; Mikayama, T.; Matsui, J.; Miyashita, T. Langmuir 2007, 23, 7472-7474. (18) Driskell, J. D.; Uhlenkamp, J. M.; Lipert, R. J.; Porter, M. D. Anal. Chem. 2007, 79, 4141-4148. (19) Ozawa, H.; Kawao, M.; Tanaka, H.; Ogawa, T. Langmuir 2007, 23, 63656371. (20) Toderas, F.; Baia, M.; Baia, L.; Astilean, S. Nanotechnology 2007, 18, 255702/1-255702/6. (21) Kneipp, K. Single Mol. 2001, 2, 291-292. (22) Domingo, C.; Resta, V.; Sanchez-Cortes, S.; Garcia-Ramos, J. V.; Gonzalo, J. J. Phys. Chem. C 2007, 111, 8149-8152. (23) Kneipp, J.; Kneipp, H.; Wittig, B.; Kneipp, K. Nano Lett. 2007, 7, 28192823. (24) Fang, J.; You, H.; Ding, B.; Song, X. Electrochem. Commun. 2007, 9, 2423-2427. (25) Huang, H.; Zhang, S.; Qi, L.; Yu, X.; Chen, Y. Surf. Coat. Technol. 2006, 200, 4389-4396. (26) Shih, T.-Y.; Requicha, A. A. G.; Thompson, M. E.; Koel, B. E. J. Nanosci. Nanotechnol. 2007, 7, 2863-2869. (27) Bar, G.; Rubin, S.; Cutts, R. W.; Taylor, T. N.; Zawodzinski, T. A., Jr. Langmuir 1996, 12, 1172-1779. (28) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629-1631. (29) Diao, J. J.; Sun, J.; Hutchison, J. B.; Reeves, M. E. Appl. Phys. Lett. 2005, 87, 103113/1-103113/3. (30) Roberson, L. J.; Ouzounova, T.; Umbach, C. NNIN REU Res. Accomplishments 2006, 82-83. (31) Xiao, M. Mater. Lett. 2002, 52, 301-303. (32) Spadavecchia, J.; Prete, P.; Lovergine, N.; Tapfer, L.; Rella, R. J. Phys. Chem. B 2005, 109, 17347-17349. (33) Khriachtchev, L.; Heikkila¨, L.; Kuusela, T. Appl. Phys. Lett. 2001, 78, 1994-1996.
10.1021/la703064m CCC: $40.75 © xxxx American Chemical Society Published on Web 03/07/2008 PAGE EST: 10.6
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conditions without requiring conductive substrates or expensive, sophisticated equipment. It has been used to create nanowires,36 nanocrystals,37 nanodomes,38 thin films on self-assembled Au NP colloid monolayers,39-41 electrodes,42 and derivatizable biosensor surfaces. It shows increasing promise for semiconductor applications. Effects of plating solution composition, temperature, pH, agitation, and reducing agent on deposition rate, bath stability, and resulting crystal structure have been summarized.43-45 Recent attention has been given to EL Au plating by galvanic substitution of less noble metals by gold,36,46-50 focusing on deposition of continuous Au structures like islands and thin films followed by electrical,31,51 thermal,52 or flame53 annealing to modify surface properties. However, optical features including plasmon resonant characteristics of EL films remain relatively uncharacterized, despite their potential in optoelectronic, optothermal, and spectroscopic applications. We present the first examination of photoluminescent properties of ultrathin and thin Au films deposited by EL plating on Si and evaluate a novel approach to creating Si-based nanoparticle (NP) ensembles with thermally tunable dimensions, densities, and distributions from EL-deposited Au island films. In our approach, silver is galvanically46,54 replaced by Au after Ag has been reduced onto a tin-sensitized55 Si surface. We control the deposition time of EL-plated Au thin films annealed at 250 °C and heated to 800 °C to produce monomodal ensembles of NPs from 9.5 ( 4.0 to 266 ( 22 nm at densities ranging from 2.6 × 1011 to 4.3 × 108 particles cm-2. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images reveal Au island film structures as well as NP sizes and particle densities of the ensembles obtained by watershed analysis. Transmission UVvis spectroscopy reveals photoluminescent features that suggest that ultrathin EL films may be smoother than sputtered Au films and scatter less in optoelectronic applications. X-ray diffraction (XRD) shows predominantly (111) orientation in Au film prepared (34) Doron-Mor, I.; Barkay, Z.; Filip-Granit, N.; Vaskevich, A.; Rubinstein, I. Chem. Mater. 2004, 16, 3476-3483. (35) Electroless Plating: Fundamentals and Applications; Mallory, G. O., Hajdu, J. B., Eds.; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990; Chapter 1. (36) Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67, 1920-1928. (37) Yasseri, A. A.; Sharma, S.; Jung, G. Y.; Kamins, T. I. Electrochem. Solid-State Lett. 2006, 9, C185-C188. (38) Zhao, L.; Siu, A. C.-L.; Petrus, J. A.; He, Z.; Leung, K. T. J. Am. Chem. Soc. 2007, 129, 5730-5734. (39) Brown, K. R.; Natan, M. J. Langmuir 1998, 14, 726-728. (40) Menzel, H.; Mowery, M. D.; Cai, M.; Evans, C. E. AdV. Mater. 1999, 11, 131-134. (41) Jin, Y.; Kang, X.; Song, Y.; Zhang, B.; Cheng, G.; Dong, S. Anal. Chem. 2001, 73, 2843-2849. (42) Hrapovic, S.; Liu, Y.; Enright, G.; Bensebaa, F.; Luong, J. H. T. Langmuir 2003, 19, 3958-3965. (43) Ali, H. O.; Christie, I. R. A. Gold Bull. 1984, 17, 118-127. (44) Okinata, Y. Electroless Plating: Fundamentals and Applications; Mallory, G. O., Hajdu, J. B., Eds.; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990; Chapter 15, p 401-420. (45) Bhuvana, T.; Kulkarni, G. U. Bull. Mater. Sci. 2006, 29, 505-511. (46) Nishizawa, M.; Menon, V. P.; Martin, C. R. Science 1995, 268, 700-702. (47) Hou, Z.; Abbott, N. L.; Stroeve, P. Langmuir 1998, 14, 3287-3297. (48) Hou, Z.; Dante, S.; Abbott, N. L.; Stroeve, P. Langmuir 1999, 15, 30113014. (49) Alvarez-Puebla, R. A.; Nazri, G.-A.; Aroca, R. F. J. Mater. Chem. 2006, 16, 2921-2924. (50) Ferralis, N.; Maboudian, R.; Carraro, C. J. Phys. Chem. C 2007, 111, 7508-7513. (51) Perea-Lopez, N.; Rakov, N.; Xiao, M. ReV. Sci. Instrum. 2002, 73, 43994401. (52) Dubrovsky, T. B.; Hou, Z.; Stroeve, P.; Abbott, N. L. Anal. Chem. 1999, 71, 327-332. (53) Lauer, M. E.; Jungmann, R.; Kindt, J. H.; Magonov, S.; Fuhrhop, J.-H.; Oroudjev, E.; Hansma, H. G. Langmuir 2007, 23, 5459-5465. (54) Kohli, P.; Harrell, C. C.; Cao, Z.; Gasparac, R.; Tan, W.; Martin. C. R. Science 2004, 305, 984-986. (55) Koura, N. Electroless Plating: Fundamentals and Applications; Mallory, G. O., Hajdu, J. B., Eds.; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990; Chapter 17, pp 441-462.
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by this approach, which has correlated with higher intensity and better resolution in previous SERS applications. Experimental Section (i) Materials. (a) Solutions were made using distilled, deionized water that was degassed (DDD-H2O) to mitigate undesired oxidation. HAuCl4‚3H2O (99.9+%), AgNO3 (99.0+%), anhydrous SnCl2 (99.9+%), trifluoroacetic acid (99.0%), and NaCl were obtained from Sigma-Aldrich (St. Louis, MO) and used as received. Ba(OH)2‚8H2O (98.9%), Na2SO3 (98.6%), HNO3, NaOH, and formaldehyde were obtained from Mallinckrodt (Phillipsburg, NJ). Ammonium hydroxide (EMD Chemicals Inc., Darmstadt, Germany) was handled in a chemical fume hood using gloves, lab coat, and safety glasses to prevent corrosion and/or burns. (b) Quartz slides (GE 124 fused quartz, Chemglass Inc., Vineland, NJ) were immersed in 25% HNO3 for 30 min to improve Au adhesion on the surface. After etching, quartz slides were rinsed throughoutly with DDD-H2O and dried with N2 gas. Quartz slides plated for 30 and 60 min were HF etched for 20 min before HNO3 etching and were not washed with DDD-H2O after Au plating, as described in the text. (ii) Synthesis. (a) Preparing Sodium Gold Sulfite Solution. The procedure for producing sodium gold sulfite solution was based on a method by Abys et al.56 Briefly, 0.1 g of HAuCl4‚3H2O was dissolved in 0.5 mL of aqua regia (HNO3:HCl ) 3:1) and boiled at 84 °C to evaporate HNO3 with stirring. A 0.03 g portion of NaCl was added and the mixture boiled at 94 °C until dry to produce Na(AuCl4). Na(AuCl4) was then dissolved in 1.8 mL of DDD-H2O and heated to 80 °C. Under stirring, 0.1 g of Ba(OH)2‚8H2O was added, and the color of the yellow solution changed to dark amber. After complete dissolution, a concentrated aqueous NaOH solution that was produced by dissolving 0.06 g of NaOH in 120 µL of DDD-H2O was added to form a greenish brown precipitate. The solution was boiled to dryness and slurried in 1.8 mL of cold DDDH2O, followed by filtration using a fine filter paper and washing with cold DDD-H2O. Then, the precipitate was slurried in 1.8 mL of cold DDD-H2O, heated to 50 °C with stirring, cooled, filtered, and washed with cold DDD-H2O, and the precipitate was collected. This slurry/wash was repeated three times. The final washed precipitate was slurried again in 2.3 mL of DDD-H2O and heated to 60-65 °C. With stirring, 0.36 g of Na2SO3 was dissolved under N2 gas feed. During ∼45 min of stirring at 60-65 °C under N2 gas feed, the color of the precipitate changed to purple or blue. Finally, the precipitate was filtered through the fine filter and washed with a little hot DDD-H2O, and the filtrate (sodium gold sulfite solution, Na3[Au(SO3)2]) was stored under a nitrogen overlayer in a brown vial at 4 °C. (b) Electroless Au Plating. To improve adhesion and uniformity of Au film on quartz slides, three steps were performed in electroless Au plating.36 First, quartz slides were sensitized by immersion in a solution of 0.026 M SnCl2 and 0.07 M trifluoroacetic acid for 3 min, followed by rinsing in warm DDD-H2O (50-55 °C) and drying with N2 gas. Stannous ions (Sn2+) sensitize the surface for subsequent adsorption of catalytic nuclei.57 Second, 0.029 M ammoniacal AgNO3 was prepared to activate the Sn2+-sensitized surface. A 1.0 mL portion of 10% AgNO3 was titrated with concentrated ammonium hydroxide drop by drop with stirring in a hood until the precipitate is completely dissolved, 3.33 mL of 3% NaOH was added, and ammonium hydroxide titration was performed again until the precipitate just dissolved. The solution was diluted and filtered using a fine filter paper.58 During Ag deposition, the reducing action of stannous ions adsorbed on the surface causes the deposition of isolated particles of silver-silver oxide material, which adhere strongly to the surface.58 After 2 min in Ag solution, quartz slides were rinsed with warm (56) Abys, J. A.; Maisano, J. J. U.S. Patent 6,126,807, 2000 (Process for making sodium gold sulfite solution). (57) McDermott, J. Plating of Plastics with Metals; Noyes Data Corp.: Park Ridge, NJ, 1974; p 180-182. (58) Accustain reticulum stain (procedure # HT 102) Sigma-Aldrich (2005) Preparation of ammoniacal silver nitrate solution.
Electroless Gold Island Thin Films DDD-H2O and dried with N2. Finally, Ag-derivatized quartz slides were immersed in a Au plating solution (Na3Au(SO3)2: a solution of 0.127 M Na2SO3 and 0.625 M formaldehyde ) 1:10) for the duration of desired deposition times. Au plated slides were thoroughly rinsed with 25 °C DDD-H2O and dried with N2 gas. (c) Drop Method. To reduce the amount of gold sulfide solution needed to plate a thin film, we devised an electroless drop method. After the proposed procedures of HNO3 etching and Sn and Ag deposition on quartz slides, a drop of 1-5 µL gold sulfide solution was placed on a slide. Images and spectra resulting from drop volumes in this range were similar. A polypropylene container allowed the drop to remain in a N2 environment during the deposition period. Afterward, the Au drop-plated slide was washed with DDD-H2O and dried with N2. (d) Thermal Treatment. After Au plating, slides were immediately brought into a 250 °C furnace (Lindberg/Blue BF51732BC, New Columbia, PA) for 3-h thermal annealing. At this temperature, the furnace had 13 min of ramp time for both heating and cooling, giving a ramp rate of ∼17 °C/min. This annealing process allowed deposited Au islands to rearrange, resulting in a more uniform Au surface. For some of the Au plated slides, further heating to 800 °C was performed for 20 min in a 9 × 9.5 × 13 in. furnace at a heating/ cooling ramp rate of ∼9 °C/min. The furnace was purged with a continuous N2 gas at 3.52 SLPM throughout the process. (e) Au Sputtering. In order to compare the morphological and optical quality of electroless plated Au films to the widely used simple method to produce thin Au film, the Au films were deposited by a Denton Discovery 18 sputter system (Moorestown, NJ). Quartz slides were etched with 25% HNO3 for 30 min to maintain the same initial slide conditions as the ones for electroless plating. In some slides, chromium (Cr) was precoated for 15 s followed by Au sputtering to increase adhesion. (iii) Characterization. (a) Transmission UV-Vis spectroscopy was recorded (Perkin-Elmer Lambda 35, Wellesley, MA) in a 350800-nm wavelength range. To isolate the Au-plated slide for analysis, a 4.5 × 4.7 cm black paper pierced by a 1.5 mm diameter hole was inserted between the incident light and sample inside the spectrophotometer. Etched quartz slides that had not been Au plated were used to obtain reference spectra. Spectra were fit to Gaussian curves [Origin(Pro) 7.5, OriginLab Corp., Northampton, MA] to calculate peak intensity and full width at half-maximum (fwhm) using the parameter W ∼ 0.849 × fwhm and the minimum extinction as a baseline offset, yo. (b) Scanning electron microscopy (SEM) was performed using a Philips XL30 ESEM FEG (FEI, Hillsboro, Oregon). Images were obtained by backscattered electron detector mode operating using a 15 kV electron accelerating voltage. The low vacuum setting was used and the pressure was varied independently for each image based on charging and desired resolution over a range of 0.4-1.2 Torr. Acquisition preprocessing software automatically stretches to accommodate low- and high-intensity in an image. This makes comparing contrast in SEM images with different ranges of intensity susceptible to error. (c) Atomic Force Microscopy (AFM). The surface morphology of the samples was analyzed in an air atmosphere by means of an AFM (WITec alpha 300, Ulm, Germany) with a maximum scan range of 100 µm × 100 µm × 20 µm. The 256 × 256 pixel images were captured for different scanning areas (from 1 × 1 µm2 up to 50 × 50 µm2) at a scan rate of 1 line/s. AFM contact and tapping modes were employed using highly doped single-crystal silicon tips with aluminum reflex coating (spring constant ) 3 µm; resonant frequency ) 60 kHz). Etched quartz slides that had not been Au plated were used to obtain reference images. Sample average roughness and cross-section curves in the AFM images were obtained with WITec software.59 (d) Energy dispersiVe X-ray spectroscopy spectrum (EDS) from EDAX (HIT S3000N, Mahwah, NJ) showed the proportion of the material on the prepared surface. (59) WITec Project User Manual, 2007, Chapter 7, p 66.
Langmuir C (e) X-ray diffraction pattern was obtained on a Philips X’Pert XRD (Almelo, Netherlands) to study the crystalline structure of the sample with a fixed anode Cu source. X-ray was generated at 40 mA and 45 kV, and the scanning position ranged from 20° to 99° (2θ). Collected data were analyzed using X-ray analysis software from PANalytical Inc. (Almelo, Netherlands). (f) Image Processing and Analysis. Distributions and surface densities of Au islands, particles, and voids in SEM and AFM images of Au-coated quartz slides were characterized using image processing tools (Image Processing Toolbox 6.0) in MATLAB (v. 7.4; MathWorks). The open source computer program GIMP (v. 2.2.11; www.gimp.org) was used to manually mark particles that were missed by the watershed algorithm and unmark sodium crystals that were selected as nanoparticles. The 95% confidence intervals reported for all particle sizes were produced using Calc from the OpenOffice package (v. 2.3.1). The standard deviation and particle count were input into the confidence function to produce the confidence intervals reported in the paper.
Results EL Plating Yields Au Thin Island Films. Figure 1 shows SEM images of Au thin films produced after immersing fused quartz slides in EL plating solution for (A) 0.33 min, (B) 1 min, and (C) 4 min, respectively. Because noble metals bind weakly to quartz surfaces, dissolved Au(I) reduces onto surface-associated Au(0) that has galvanically displaced Ag, producing an Au island film.39,40 Island film shapes are governed by underlying substrate composition (epitaxial) while their fractal dimension is determined by competition between terrace diffusion and edge diffusion of adatoms.60 The transmission UV-vis (T-UV) spectra inset in each image exhibit increased optical density (e.g., lower transmittance) and a maximum optical density at ∼650 nm in Figure 1A that upshifts in wavelength as film thickness increases. These features are consistent with previous descriptions of Au island films.8,33 We attribute the optical density maxima to excitation of localized surface plasmon polaritons (SPP). Comparable SPP spectral feature at ∼650 nm were observed from 2.5- and 5-nm-thick Au island films evaporated on (mercaptopropyl)trimethoxysilane (MPTS)-modified glass.11,61 Surface plasmon excitation and subsequent thermal dissipation can produce large nonlinear refraction in Au island films.62 EL Plated Thin Films Photoluminesce. The T-UV spectra in images A-C of Figure 1 show a feature appearing at ∼500 nm that increases in relative intensity and upshifts in wavelength as immersion time increases: (A) 467 nm, (B) 481 nm, and (C) 508 nm. We attribute this feature primarily to photoluminescence (PL) caused by electron transitions and recombination between filled d-bands and the Fermi-level conduction band in the gold (Au).63 Absorption of a photon at ∼530 nm is sufficient to promote an electron from the top d-band to the Fermi-level conduction band.64 PL recombination of a Fermi-level electron (i.e., a band-6 electron from an occupied sp conduction band) with the d-band (i.e., band-5) hole then follows.65 The shape and band gap energy of noble metal PL are predicted by the theory describing these transitions that occur at a quantum efficiency of ∼10-10. PL at ∼500 nm increases with thickness in “ultrathin” 2.5- and 5-nmthick Au island films11,61 but decreases with increasing thickness in films >60 nm.66 In Figure 1A-C the PL peak height increases (60) Ogura, S.; Fukutani, K.; Matsumoto, M.; Okano, T.; Okada, M.; Kawamura, T. Phys. ReV. B 2006, 73, 125442/1-125442/10. (61) Doron-Mor, I.; Cohen, H.; Barkay, Z.; Shanzer, A.; Vaskevich, A.; Rubinstein, I. Chem. A Euro. J. 2005, 11, 5555-5562. (62) Borshch, A. A.; Brodin, M. S.; Volkov, V. I.; Lyakhovetskii, V. P.; Fedorovich, R. D. JETP Lett. 2006, 84, 214-216. (63) Mooradian, A. Phys. ReV. Lett. 1969, 22, 185-187. (64) Boyd, G. T.; Yu, Z. H.; Shen, Y. R. Phys. ReV. B 1986, 33, 7923-7936. (65) Apell, P.; Monreal, R.; Lundquist, S. Phys. Scr. 1988, 38, 174-179. (66) Kim, J. H.; Moyer, P. J. Opt. Expr. 2006, 14, 6595-6603.
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Figure 1. SEM images of fused quartz slides after immersion (first row) in Au plating solution for (A) 0.33 min, (B) 1 min, and (C) 4 min; after respective immersed slides are annealed (second row, images D, E, and F) for 3 h at 250 °C; and after respective immersed, annealed slides are heated (third row, images G, H, and I) for 20 min at 800 °C. Inset are transmission UV-vis spectra corresponding to each image. Percent (%) transmission is shown in images A-C, E, and F. Extinction in arbitrary units (au) is shown in images D, G-I.
with plating time and film thickness, as would an island film. We anticipate that a second source for the ∼500-nm feature is “red photoluminescence”, which has been observed upon excitation of Au thin films at 488.0, 514.5, and 832.8 nm due to collective optical absorption by resonant surface plasmon coupling inside cavities and between surfaces on both sides of island structures.33,67 This plasmon absorption decays radiatively, producing a broad spectral emission (550-750 nm) in thin films.68 Our spectra would exhibit red PL at excitation, but not emmission, wavelengths since we use a 1-nm bandwidth light source and a broadband detector. SPP interactions due to red PL are excited more strongly in thin (e10 nm) Au films than thicker films.33 This trend is observed in Figure 1A-C as red PL intensity upon excitation at 488 nm decreases with increasing immersion time and film thickness. PL peaks in T-UV spectra from sputtered films do not exhibit red PL contributions to the same degree at 488 nm (see Figure 2). Red PL is not produced by nanoparticles on Si surfaces but appears when spheroid shapes coalesce due to EL plating.39 EL Plating Rate Is ∼8 nm/min and Nonlinear. Figure 2 shows T-UV spectra of Au thin films created by sputtering at a rate of 20 nm/min for times ranging from 0.33 min to 4 min. Values of extinction at 350 nm in sputtered film spectra increase in proportion to sputtering time up to ∼2 min (Extinction profiles are calculated from % transmission profiles shown in Figure 2 using the relation E ) 2 - log(%T)). Au deposits at an average rate of 40-50 nm/h in EL plating at the conditions we use.47,48 But EL deposition rates are nonlinear, due to acceleration of Au reduction at pre-existing molecule-scale Au surfaces followed by deceleration of deposition as plating proceeds because surface free energy decreases as the dimensions of deposited Au increase.41 No Au particles nucleated in solution or on uncoated surfaces at EL plating times e1 h in this study. We estimate an EL deposition rate of ∼8 nm/min during the first 4 min by comparing extinction values at 350 nm for EL plating with those for sputtering. (67) Xiao, M.; Rakov, N. J. Phys.: Cond. Matter 2003, 15, L133-L137.
Figure 2. Transmission UV-vis spectra showing percent transmission of Au-sputtered quartz slides after sputtering at 20 nm/min for 0.33 min (solid line), 0.67 min (dash), 1 min (dot), 2 min (dashdot), and 4 min (dash-dot-dot).
Ultrathin EL Plated Films Are Smooth Compared to Sputtered Films. Rough surfaces enhance scattering and broaden the PL feature, especially at energies lower (e.g., longer wavelengths) than the interband adsorption edge due to roughness quenching.63 This is due to electron-phonon and hole-phonon scattering processes after photon absorption and electron promotion that contribute to energy loss. As an example, in Figure 2 transmission PL maxima of sputtered films shift to lower energies as sputtering time decreases, and roughness increases. Rough, sputtered 20-nm Au films have been shown to exhibit 5-fold higher PL intensity at ∼500 nm than smooth evaporated 20-nm films.69 To compare the smoothness of EL films to sputtered films, we consider optical characteristics: peak height (PH) and fwhm of the PL feature at ∼500 nm. One minute of EL film deposition (∼8 nm thick) shown in Figure 1B resulted in PH of 7% and fwhm of 92.9 nm. These values are much smaller than (68) Vengurlekar, A.; Ishihara, T. J. Lumin. 2007, 122-123, 796-799. (69) Lin, H. Y.; Chen, Y. F. Appl. Phys. Lett. 2006, 88, 101914/1-101914/ 3.
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Figure 3. SEM image of fused quartz slide after sputtering at 20 nm/min for 0.33 min. The inset shows percent transmission with peak height of 15.5% and fwhm of 155 nm.
the PH (15.5%) and fwhm (155 nm) of the slide sputtered for 0.33 min (6.6 nm thick) shown in Figure 3. This difference occurs even though the contribution of red PL to the ∼500 nm feature in Figure 3 appears less than for the ultrathin EL film in Figure 1B. This suggests that ultrathin (
Introduction Nanoscale gold (Au) structures on silicon (Si) substrates are important in electronics, MEMS devices, diagnostics, biosensing, spectroscopy, and microscopy. Au thin films are applied in gateable electronic transistors1,2 and conductors,3 optically induced thermoplasmonic gratings for nanomanipulation of picoliter droplets,4 surface plasmon resonance (SPR) sensors (45-60-nm thickness), and SERS-active substrates5,6 excited by visible (red), near-IR, and IR7 light. Au island films are substrates for SERS,8 attenuated total reflection surface-enhanced IR absorption (SEIRA)9 microspectroscopy,10 and transmission localized surface plasmon sensors11-13 and identify nanoscale structures in nearfield scanning optical microscopy (NSOM).14 Au nanoparticle (NP) assemblies on Si have uses in thermal nanoprocessing,15,16 nanoactuation,17 immunoassays,18 porphyrin-conjugated nano* Corresponding author: Tel:+1 801 585 9185. Fax: +1 801 585 9291. E-mail: [email protected]. † Department of Materials Science and Engineering. ‡ Department of Chemical Engineering. (1) Luo, X.; Orlov, A. O.; Snider, G. L. J. Vac. Sci. Technol. B 2004, 22, 3128-3132. (2) Scheible, D. V.; Weiss, C.; Kotthaus, J. P.; Blick, R. H. Phys. ReV. Lett. 2004, 93, 186801/1-186801/4. (3) Fedorovich, R. D.; Inosov, D. S.; Kiyaev, O. E.; Lukyanets, S. P.; Marchenko, A. A.; Tomchuk, P. M.; Bevzenko, D. A.; Naumovets, A. G. J. Mol. Struct. 2004, 708, 67-77. (4) Passian, A.; Lereu, A. L.; Farahi, R. H.; Ferrell, T. L.; Thundat, T. Trends in Thin Solid Films Research; Jost, A. R., Eds.; Nova Science Publishers: New York, 2007; Chapter 3. (5) Sockalingum, G. D.; Beljebbar, A.; Morjani, H.; Manfait, M. Proc. SPIEInt. Soc. Opt. Eng. 1998, 3260, 58-62. (6) Tolaieb, B.; Aroca, R. Can. J. Anal. Sci. Spectrosc. 2003, 48, 139-145. (7) Jennings, C. A.; Kovacs, G. J.; Aroca, R. Can. J. Phys. Chem. 1992, 96, 1340-1343. (8) Sockalingum, G. D.; Beljebbar, A.; Morjani, H.; Angiboust, J. F.; Manfait, M. Biospectrosc. 1998, 4, S71-S78. (9) Vongsvivut, J.; Itoh, T.; Ikehata, A.; Ekgasit, S.; Ozaki, Y. Sci. Asia 2006, 32, 261-269. (10) Sudo, E.; Esaki, Y.; Sugiura, M.; Murase, A. Appl. Spectrosc. 2007, 61, 269-275. (11) Kalyuzhny, G.; Vaskevich, A.; Schneeweiss, M. A.; Rubinstein, I. Chem. A Eur. J. 2002, 8, 3849-3857. (12) Ruach-Nir, I.; Bendikov, T. A.; Doron-Mor, I.; Barkay, Z.; Vaskevich, A.; Rubinstein, I. J. Am. Chem. Soc. 2007, 129, 84-92. (13) Lahav, M.; Vaskevich, A.; Rubinstein, I. Langmuir 2004, 20, 73657367. (14) Ianoul, A. Abstracts of the 35th Northeast Regional Meeting of the American Chemical Society, Binghamton, New York, 2006.
wires,19 surface-enhanced Raman (SER) scattering20 for singlemolecule spectroscopy,21,22 and SER nanosensors.23 Nanoparticles have been assembled on Si by deposition using pulsed lasers,21 electrochemistry,24 and microwaves;25 selfassembly on thiols,20,26 dendrimers,27 or other polymers;28 conjugation to porphyrin polymers;19 evaporation;29,30 and resistive31 or thermal32 heating of evaporated thin film. Metal islands and thin films have been formed by sputtering,33 vacuum evaporation8 and evaporation onto chemically modified glass substrates,11,34 nanomaching,4 electrodeposition, and electroless deposition.35 Electroless (EL) deposition of Au has the advantage of being able to rapidly coat fragile, 3D, or internal surfaces at ambient (15) Palpant, B.; Rashidi-Huyeh, M.; Gallas, B.; Chenot, S.; Fisson, S. Appl. Phys. Lett. 2007, 90, 223105/1-223105/3. (16) Numata, T.; Tatsuta, H.; Morita, Y.; Otani, Y.; Umeda, N. IEEJ Trans. Elect. Electron. Eng. 2007, 2, 398-401. (17) Mitsuishi, M.; Koishikawa, Y.; Tanaka, H.; Sato, E.; Mikayama, T.; Matsui, J.; Miyashita, T. Langmuir 2007, 23, 7472-7474. (18) Driskell, J. D.; Uhlenkamp, J. M.; Lipert, R. J.; Porter, M. D. Anal. Chem. 2007, 79, 4141-4148. (19) Ozawa, H.; Kawao, M.; Tanaka, H.; Ogawa, T. Langmuir 2007, 23, 63656371. (20) Toderas, F.; Baia, M.; Baia, L.; Astilean, S. Nanotechnology 2007, 18, 255702/1-255702/6. (21) Kneipp, K. Single Mol. 2001, 2, 291-292. (22) Domingo, C.; Resta, V.; Sanchez-Cortes, S.; Garcia-Ramos, J. V.; Gonzalo, J. J. Phys. Chem. C 2007, 111, 8149-8152. (23) Kneipp, J.; Kneipp, H.; Wittig, B.; Kneipp, K. Nano Lett. 2007, 7, 28192823. (24) Fang, J.; You, H.; Ding, B.; Song, X. Electrochem. Commun. 2007, 9, 2423-2427. (25) Huang, H.; Zhang, S.; Qi, L.; Yu, X.; Chen, Y. Surf. Coat. Technol. 2006, 200, 4389-4396. (26) Shih, T.-Y.; Requicha, A. A. G.; Thompson, M. E.; Koel, B. E. J. Nanosci. Nanotechnol. 2007, 7, 2863-2869. (27) Bar, G.; Rubin, S.; Cutts, R. W.; Taylor, T. N.; Zawodzinski, T. A., Jr. Langmuir 1996, 12, 1172-1779. (28) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629-1631. (29) Diao, J. J.; Sun, J.; Hutchison, J. B.; Reeves, M. E. Appl. Phys. Lett. 2005, 87, 103113/1-103113/3. (30) Roberson, L. J.; Ouzounova, T.; Umbach, C. NNIN REU Res. Accomplishments 2006, 82-83. (31) Xiao, M. Mater. Lett. 2002, 52, 301-303. (32) Spadavecchia, J.; Prete, P.; Lovergine, N.; Tapfer, L.; Rella, R. J. Phys. Chem. B 2005, 109, 17347-17349. (33) Khriachtchev, L.; Heikkila¨, L.; Kuusela, T. Appl. Phys. Lett. 2001, 78, 1994-1996.
10.1021/la703064m CCC: $40.75 © xxxx American Chemical Society Published on Web 03/07/2008 PAGE EST: 10.6
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conditions without requiring conductive substrates or expensive, sophisticated equipment. It has been used to create nanowires,36 nanocrystals,37 nanodomes,38 thin films on self-assembled Au NP colloid monolayers,39-41 electrodes,42 and derivatizable biosensor surfaces. It shows increasing promise for semiconductor applications. Effects of plating solution composition, temperature, pH, agitation, and reducing agent on deposition rate, bath stability, and resulting crystal structure have been summarized.43-45 Recent attention has been given to EL Au plating by galvanic substitution of less noble metals by gold,36,46-50 focusing on deposition of continuous Au structures like islands and thin films followed by electrical,31,51 thermal,52 or flame53 annealing to modify surface properties. However, optical features including plasmon resonant characteristics of EL films remain relatively uncharacterized, despite their potential in optoelectronic, optothermal, and spectroscopic applications. We present the first examination of photoluminescent properties of ultrathin and thin Au films deposited by EL plating on Si and evaluate a novel approach to creating Si-based nanoparticle (NP) ensembles with thermally tunable dimensions, densities, and distributions from EL-deposited Au island films. In our approach, silver is galvanically46,54 replaced by Au after Ag has been reduced onto a tin-sensitized55 Si surface. We control the deposition time of EL-plated Au thin films annealed at 250 °C and heated to 800 °C to produce monomodal ensembles of NPs from 9.5 ( 4.0 to 266 ( 22 nm at densities ranging from 2.6 × 1011 to 4.3 × 108 particles cm-2. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images reveal Au island film structures as well as NP sizes and particle densities of the ensembles obtained by watershed analysis. Transmission UVvis spectroscopy reveals photoluminescent features that suggest that ultrathin EL films may be smoother than sputtered Au films and scatter less in optoelectronic applications. X-ray diffraction (XRD) shows predominantly (111) orientation in Au film prepared (34) Doron-Mor, I.; Barkay, Z.; Filip-Granit, N.; Vaskevich, A.; Rubinstein, I. Chem. Mater. 2004, 16, 3476-3483. (35) Electroless Plating: Fundamentals and Applications; Mallory, G. O., Hajdu, J. B., Eds.; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990; Chapter 1. (36) Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67, 1920-1928. (37) Yasseri, A. A.; Sharma, S.; Jung, G. Y.; Kamins, T. I. Electrochem. Solid-State Lett. 2006, 9, C185-C188. (38) Zhao, L.; Siu, A. C.-L.; Petrus, J. A.; He, Z.; Leung, K. T. J. Am. Chem. Soc. 2007, 129, 5730-5734. (39) Brown, K. R.; Natan, M. J. Langmuir 1998, 14, 726-728. (40) Menzel, H.; Mowery, M. D.; Cai, M.; Evans, C. E. AdV. Mater. 1999, 11, 131-134. (41) Jin, Y.; Kang, X.; Song, Y.; Zhang, B.; Cheng, G.; Dong, S. Anal. Chem. 2001, 73, 2843-2849. (42) Hrapovic, S.; Liu, Y.; Enright, G.; Bensebaa, F.; Luong, J. H. T. Langmuir 2003, 19, 3958-3965. (43) Ali, H. O.; Christie, I. R. A. Gold Bull. 1984, 17, 118-127. (44) Okinata, Y. Electroless Plating: Fundamentals and Applications; Mallory, G. O., Hajdu, J. B., Eds.; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990; Chapter 15, p 401-420. (45) Bhuvana, T.; Kulkarni, G. U. Bull. Mater. Sci. 2006, 29, 505-511. (46) Nishizawa, M.; Menon, V. P.; Martin, C. R. Science 1995, 268, 700-702. (47) Hou, Z.; Abbott, N. L.; Stroeve, P. Langmuir 1998, 14, 3287-3297. (48) Hou, Z.; Dante, S.; Abbott, N. L.; Stroeve, P. Langmuir 1999, 15, 30113014. (49) Alvarez-Puebla, R. A.; Nazri, G.-A.; Aroca, R. F. J. Mater. Chem. 2006, 16, 2921-2924. (50) Ferralis, N.; Maboudian, R.; Carraro, C. J. Phys. Chem. C 2007, 111, 7508-7513. (51) Perea-Lopez, N.; Rakov, N.; Xiao, M. ReV. Sci. Instrum. 2002, 73, 43994401. (52) Dubrovsky, T. B.; Hou, Z.; Stroeve, P.; Abbott, N. L. Anal. Chem. 1999, 71, 327-332. (53) Lauer, M. E.; Jungmann, R.; Kindt, J. H.; Magonov, S.; Fuhrhop, J.-H.; Oroudjev, E.; Hansma, H. G. Langmuir 2007, 23, 5459-5465. (54) Kohli, P.; Harrell, C. C.; Cao, Z.; Gasparac, R.; Tan, W.; Martin. C. R. Science 2004, 305, 984-986. (55) Koura, N. Electroless Plating: Fundamentals and Applications; Mallory, G. O., Hajdu, J. B., Eds.; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990; Chapter 17, pp 441-462.
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by this approach, which has correlated with higher intensity and better resolution in previous SERS applications. Experimental Section (i) Materials. (a) Solutions were made using distilled, deionized water that was degassed (DDD-H2O) to mitigate undesired oxidation. HAuCl4‚3H2O (99.9+%), AgNO3 (99.0+%), anhydrous SnCl2 (99.9+%), trifluoroacetic acid (99.0%), and NaCl were obtained from Sigma-Aldrich (St. Louis, MO) and used as received. Ba(OH)2‚8H2O (98.9%), Na2SO3 (98.6%), HNO3, NaOH, and formaldehyde were obtained from Mallinckrodt (Phillipsburg, NJ). Ammonium hydroxide (EMD Chemicals Inc., Darmstadt, Germany) was handled in a chemical fume hood using gloves, lab coat, and safety glasses to prevent corrosion and/or burns. (b) Quartz slides (GE 124 fused quartz, Chemglass Inc., Vineland, NJ) were immersed in 25% HNO3 for 30 min to improve Au adhesion on the surface. After etching, quartz slides were rinsed throughoutly with DDD-H2O and dried with N2 gas. Quartz slides plated for 30 and 60 min were HF etched for 20 min before HNO3 etching and were not washed with DDD-H2O after Au plating, as described in the text. (ii) Synthesis. (a) Preparing Sodium Gold Sulfite Solution. The procedure for producing sodium gold sulfite solution was based on a method by Abys et al.56 Briefly, 0.1 g of HAuCl4‚3H2O was dissolved in 0.5 mL of aqua regia (HNO3:HCl ) 3:1) and boiled at 84 °C to evaporate HNO3 with stirring. A 0.03 g portion of NaCl was added and the mixture boiled at 94 °C until dry to produce Na(AuCl4). Na(AuCl4) was then dissolved in 1.8 mL of DDD-H2O and heated to 80 °C. Under stirring, 0.1 g of Ba(OH)2‚8H2O was added, and the color of the yellow solution changed to dark amber. After complete dissolution, a concentrated aqueous NaOH solution that was produced by dissolving 0.06 g of NaOH in 120 µL of DDD-H2O was added to form a greenish brown precipitate. The solution was boiled to dryness and slurried in 1.8 mL of cold DDDH2O, followed by filtration using a fine filter paper and washing with cold DDD-H2O. Then, the precipitate was slurried in 1.8 mL of cold DDD-H2O, heated to 50 °C with stirring, cooled, filtered, and washed with cold DDD-H2O, and the precipitate was collected. This slurry/wash was repeated three times. The final washed precipitate was slurried again in 2.3 mL of DDD-H2O and heated to 60-65 °C. With stirring, 0.36 g of Na2SO3 was dissolved under N2 gas feed. During ∼45 min of stirring at 60-65 °C under N2 gas feed, the color of the precipitate changed to purple or blue. Finally, the precipitate was filtered through the fine filter and washed with a little hot DDD-H2O, and the filtrate (sodium gold sulfite solution, Na3[Au(SO3)2]) was stored under a nitrogen overlayer in a brown vial at 4 °C. (b) Electroless Au Plating. To improve adhesion and uniformity of Au film on quartz slides, three steps were performed in electroless Au plating.36 First, quartz slides were sensitized by immersion in a solution of 0.026 M SnCl2 and 0.07 M trifluoroacetic acid for 3 min, followed by rinsing in warm DDD-H2O (50-55 °C) and drying with N2 gas. Stannous ions (Sn2+) sensitize the surface for subsequent adsorption of catalytic nuclei.57 Second, 0.029 M ammoniacal AgNO3 was prepared to activate the Sn2+-sensitized surface. A 1.0 mL portion of 10% AgNO3 was titrated with concentrated ammonium hydroxide drop by drop with stirring in a hood until the precipitate is completely dissolved, 3.33 mL of 3% NaOH was added, and ammonium hydroxide titration was performed again until the precipitate just dissolved. The solution was diluted and filtered using a fine filter paper.58 During Ag deposition, the reducing action of stannous ions adsorbed on the surface causes the deposition of isolated particles of silver-silver oxide material, which adhere strongly to the surface.58 After 2 min in Ag solution, quartz slides were rinsed with warm (56) Abys, J. A.; Maisano, J. J. U.S. Patent 6,126,807, 2000 (Process for making sodium gold sulfite solution). (57) McDermott, J. Plating of Plastics with Metals; Noyes Data Corp.: Park Ridge, NJ, 1974; p 180-182. (58) Accustain reticulum stain (procedure # HT 102) Sigma-Aldrich (2005) Preparation of ammoniacal silver nitrate solution.
Electroless Gold Island Thin Films DDD-H2O and dried with N2. Finally, Ag-derivatized quartz slides were immersed in a Au plating solution (Na3Au(SO3)2: a solution of 0.127 M Na2SO3 and 0.625 M formaldehyde ) 1:10) for the duration of desired deposition times. Au plated slides were thoroughly rinsed with 25 °C DDD-H2O and dried with N2 gas. (c) Drop Method. To reduce the amount of gold sulfide solution needed to plate a thin film, we devised an electroless drop method. After the proposed procedures of HNO3 etching and Sn and Ag deposition on quartz slides, a drop of 1-5 µL gold sulfide solution was placed on a slide. Images and spectra resulting from drop volumes in this range were similar. A polypropylene container allowed the drop to remain in a N2 environment during the deposition period. Afterward, the Au drop-plated slide was washed with DDD-H2O and dried with N2. (d) Thermal Treatment. After Au plating, slides were immediately brought into a 250 °C furnace (Lindberg/Blue BF51732BC, New Columbia, PA) for 3-h thermal annealing. At this temperature, the furnace had 13 min of ramp time for both heating and cooling, giving a ramp rate of ∼17 °C/min. This annealing process allowed deposited Au islands to rearrange, resulting in a more uniform Au surface. For some of the Au plated slides, further heating to 800 °C was performed for 20 min in a 9 × 9.5 × 13 in. furnace at a heating/ cooling ramp rate of ∼9 °C/min. The furnace was purged with a continuous N2 gas at 3.52 SLPM throughout the process. (e) Au Sputtering. In order to compare the morphological and optical quality of electroless plated Au films to the widely used simple method to produce thin Au film, the Au films were deposited by a Denton Discovery 18 sputter system (Moorestown, NJ). Quartz slides were etched with 25% HNO3 for 30 min to maintain the same initial slide conditions as the ones for electroless plating. In some slides, chromium (Cr) was precoated for 15 s followed by Au sputtering to increase adhesion. (iii) Characterization. (a) Transmission UV-Vis spectroscopy was recorded (Perkin-Elmer Lambda 35, Wellesley, MA) in a 350800-nm wavelength range. To isolate the Au-plated slide for analysis, a 4.5 × 4.7 cm black paper pierced by a 1.5 mm diameter hole was inserted between the incident light and sample inside the spectrophotometer. Etched quartz slides that had not been Au plated were used to obtain reference spectra. Spectra were fit to Gaussian curves [Origin(Pro) 7.5, OriginLab Corp., Northampton, MA] to calculate peak intensity and full width at half-maximum (fwhm) using the parameter W ∼ 0.849 × fwhm and the minimum extinction as a baseline offset, yo. (b) Scanning electron microscopy (SEM) was performed using a Philips XL30 ESEM FEG (FEI, Hillsboro, Oregon). Images were obtained by backscattered electron detector mode operating using a 15 kV electron accelerating voltage. The low vacuum setting was used and the pressure was varied independently for each image based on charging and desired resolution over a range of 0.4-1.2 Torr. Acquisition preprocessing software automatically stretches to accommodate low- and high-intensity in an image. This makes comparing contrast in SEM images with different ranges of intensity susceptible to error. (c) Atomic Force Microscopy (AFM). The surface morphology of the samples was analyzed in an air atmosphere by means of an AFM (WITec alpha 300, Ulm, Germany) with a maximum scan range of 100 µm × 100 µm × 20 µm. The 256 × 256 pixel images were captured for different scanning areas (from 1 × 1 µm2 up to 50 × 50 µm2) at a scan rate of 1 line/s. AFM contact and tapping modes were employed using highly doped single-crystal silicon tips with aluminum reflex coating (spring constant ) 3 µm; resonant frequency ) 60 kHz). Etched quartz slides that had not been Au plated were used to obtain reference images. Sample average roughness and cross-section curves in the AFM images were obtained with WITec software.59 (d) Energy dispersiVe X-ray spectroscopy spectrum (EDS) from EDAX (HIT S3000N, Mahwah, NJ) showed the proportion of the material on the prepared surface. (59) WITec Project User Manual, 2007, Chapter 7, p 66.
Langmuir C (e) X-ray diffraction pattern was obtained on a Philips X’Pert XRD (Almelo, Netherlands) to study the crystalline structure of the sample with a fixed anode Cu source. X-ray was generated at 40 mA and 45 kV, and the scanning position ranged from 20° to 99° (2θ). Collected data were analyzed using X-ray analysis software from PANalytical Inc. (Almelo, Netherlands). (f) Image Processing and Analysis. Distributions and surface densities of Au islands, particles, and voids in SEM and AFM images of Au-coated quartz slides were characterized using image processing tools (Image Processing Toolbox 6.0) in MATLAB (v. 7.4; MathWorks). The open source computer program GIMP (v. 2.2.11; www.gimp.org) was used to manually mark particles that were missed by the watershed algorithm and unmark sodium crystals that were selected as nanoparticles. The 95% confidence intervals reported for all particle sizes were produced using Calc from the OpenOffice package (v. 2.3.1). The standard deviation and particle count were input into the confidence function to produce the confidence intervals reported in the paper.
Results EL Plating Yields Au Thin Island Films. Figure 1 shows SEM images of Au thin films produced after immersing fused quartz slides in EL plating solution for (A) 0.33 min, (B) 1 min, and (C) 4 min, respectively. Because noble metals bind weakly to quartz surfaces, dissolved Au(I) reduces onto surface-associated Au(0) that has galvanically displaced Ag, producing an Au island film.39,40 Island film shapes are governed by underlying substrate composition (epitaxial) while their fractal dimension is determined by competition between terrace diffusion and edge diffusion of adatoms.60 The transmission UV-vis (T-UV) spectra inset in each image exhibit increased optical density (e.g., lower transmittance) and a maximum optical density at ∼650 nm in Figure 1A that upshifts in wavelength as film thickness increases. These features are consistent with previous descriptions of Au island films.8,33 We attribute the optical density maxima to excitation of localized surface plasmon polaritons (SPP). Comparable SPP spectral feature at ∼650 nm were observed from 2.5- and 5-nm-thick Au island films evaporated on (mercaptopropyl)trimethoxysilane (MPTS)-modified glass.11,61 Surface plasmon excitation and subsequent thermal dissipation can produce large nonlinear refraction in Au island films.62 EL Plated Thin Films Photoluminesce. The T-UV spectra in images A-C of Figure 1 show a feature appearing at ∼500 nm that increases in relative intensity and upshifts in wavelength as immersion time increases: (A) 467 nm, (B) 481 nm, and (C) 508 nm. We attribute this feature primarily to photoluminescence (PL) caused by electron transitions and recombination between filled d-bands and the Fermi-level conduction band in the gold (Au).63 Absorption of a photon at ∼530 nm is sufficient to promote an electron from the top d-band to the Fermi-level conduction band.64 PL recombination of a Fermi-level electron (i.e., a band-6 electron from an occupied sp conduction band) with the d-band (i.e., band-5) hole then follows.65 The shape and band gap energy of noble metal PL are predicted by the theory describing these transitions that occur at a quantum efficiency of ∼10-10. PL at ∼500 nm increases with thickness in “ultrathin” 2.5- and 5-nmthick Au island films11,61 but decreases with increasing thickness in films >60 nm.66 In Figure 1A-C the PL peak height increases (60) Ogura, S.; Fukutani, K.; Matsumoto, M.; Okano, T.; Okada, M.; Kawamura, T. Phys. ReV. B 2006, 73, 125442/1-125442/10. (61) Doron-Mor, I.; Cohen, H.; Barkay, Z.; Shanzer, A.; Vaskevich, A.; Rubinstein, I. Chem. A Euro. J. 2005, 11, 5555-5562. (62) Borshch, A. A.; Brodin, M. S.; Volkov, V. I.; Lyakhovetskii, V. P.; Fedorovich, R. D. JETP Lett. 2006, 84, 214-216. (63) Mooradian, A. Phys. ReV. Lett. 1969, 22, 185-187. (64) Boyd, G. T.; Yu, Z. H.; Shen, Y. R. Phys. ReV. B 1986, 33, 7923-7936. (65) Apell, P.; Monreal, R.; Lundquist, S. Phys. Scr. 1988, 38, 174-179. (66) Kim, J. H.; Moyer, P. J. Opt. Expr. 2006, 14, 6595-6603.
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Figure 1. SEM images of fused quartz slides after immersion (first row) in Au plating solution for (A) 0.33 min, (B) 1 min, and (C) 4 min; after respective immersed slides are annealed (second row, images D, E, and F) for 3 h at 250 °C; and after respective immersed, annealed slides are heated (third row, images G, H, and I) for 20 min at 800 °C. Inset are transmission UV-vis spectra corresponding to each image. Percent (%) transmission is shown in images A-C, E, and F. Extinction in arbitrary units (au) is shown in images D, G-I.
with plating time and film thickness, as would an island film. We anticipate that a second source for the ∼500-nm feature is “red photoluminescence”, which has been observed upon excitation of Au thin films at 488.0, 514.5, and 832.8 nm due to collective optical absorption by resonant surface plasmon coupling inside cavities and between surfaces on both sides of island structures.33,67 This plasmon absorption decays radiatively, producing a broad spectral emission (550-750 nm) in thin films.68 Our spectra would exhibit red PL at excitation, but not emmission, wavelengths since we use a 1-nm bandwidth light source and a broadband detector. SPP interactions due to red PL are excited more strongly in thin (e10 nm) Au films than thicker films.33 This trend is observed in Figure 1A-C as red PL intensity upon excitation at 488 nm decreases with increasing immersion time and film thickness. PL peaks in T-UV spectra from sputtered films do not exhibit red PL contributions to the same degree at 488 nm (see Figure 2). Red PL is not produced by nanoparticles on Si surfaces but appears when spheroid shapes coalesce due to EL plating.39 EL Plating Rate Is ∼8 nm/min and Nonlinear. Figure 2 shows T-UV spectra of Au thin films created by sputtering at a rate of 20 nm/min for times ranging from 0.33 min to 4 min. Values of extinction at 350 nm in sputtered film spectra increase in proportion to sputtering time up to ∼2 min (Extinction profiles are calculated from % transmission profiles shown in Figure 2 using the relation E ) 2 - log(%T)). Au deposits at an average rate of 40-50 nm/h in EL plating at the conditions we use.47,48 But EL deposition rates are nonlinear, due to acceleration of Au reduction at pre-existing molecule-scale Au surfaces followed by deceleration of deposition as plating proceeds because surface free energy decreases as the dimensions of deposited Au increase.41 No Au particles nucleated in solution or on uncoated surfaces at EL plating times e1 h in this study. We estimate an EL deposition rate of ∼8 nm/min during the first 4 min by comparing extinction values at 350 nm for EL plating with those for sputtering. (67) Xiao, M.; Rakov, N. J. Phys.: Cond. Matter 2003, 15, L133-L137.
Figure 2. Transmission UV-vis spectra showing percent transmission of Au-sputtered quartz slides after sputtering at 20 nm/min for 0.33 min (solid line), 0.67 min (dash), 1 min (dot), 2 min (dashdot), and 4 min (dash-dot-dot).
Ultrathin EL Plated Films Are Smooth Compared to Sputtered Films. Rough surfaces enhance scattering and broaden the PL feature, especially at energies lower (e.g., longer wavelengths) than the interband adsorption edge due to roughness quenching.63 This is due to electron-phonon and hole-phonon scattering processes after photon absorption and electron promotion that contribute to energy loss. As an example, in Figure 2 transmission PL maxima of sputtered films shift to lower energies as sputtering time decreases, and roughness increases. Rough, sputtered 20-nm Au films have been shown to exhibit 5-fold higher PL intensity at ∼500 nm than smooth evaporated 20-nm films.69 To compare the smoothness of EL films to sputtered films, we consider optical characteristics: peak height (PH) and fwhm of the PL feature at ∼500 nm. One minute of EL film deposition (∼8 nm thick) shown in Figure 1B resulted in PH of 7% and fwhm of 92.9 nm. These values are much smaller than (68) Vengurlekar, A.; Ishihara, T. J. Lumin. 2007, 122-123, 796-799. (69) Lin, H. Y.; Chen, Y. F. Appl. Phys. Lett. 2006, 88, 101914/1-101914/ 3.
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Figure 3. SEM image of fused quartz slide after sputtering at 20 nm/min for 0.33 min. The inset shows percent transmission with peak height of 15.5% and fwhm of 155 nm.
the PH (15.5%) and fwhm (155 nm) of the slide sputtered for 0.33 min (6.6 nm thick) shown in Figure 3. This difference occurs even though the contribution of red PL to the ∼500 nm feature in Figure 3 appears less than for the ultrathin EL film in Figure 1B. This suggests that ultrathin (
