pubs.acs.org/Langmuir © 2009 American Chemical Society
Fractal Structure Formation from Ag Nanoparticle Films on Insulating Substrates Jing Tang, Zhiyong Li,* Qiangfei Xia, and R. Stanley Williams Information and Quantum Systems Laboratory, Hewlett-Packard Laboratories, 1501 Page Mill Road, Palo Alto, California 94304 Received March 25, 2009. Revised Manuscript Received May 11, 2009 Two dimensional (2D) fractal structures were observed to form from fairly uniform Ag island films (equivalent mass thicknesses of 1.5 and 5 nm) on insulating silicon dioxide surfaces (thermally grown silicon oxide on Si or quartz) upon immersion in deionized water. This result is distinctly different from the previously observed three-dimensional (3D) growth of faceted Ag nanocrystals on conductive surfaces (ITO and graphite) as the result of an electrochemical Ostwald ripening process, which also occurs on native oxide covered silicon surfaces as reported here. The fractal structures formed by diffusion-limited aggregation (DLA) of Ag species on the insulating surfaces. We present the experimental observation of this phenomenon and discuss some possible mechanisms for the DLA formation.
The unique plasmonic properties of Ag nanoparticles have attracted great interest and played an important role in the area of surface-enhanced optical processes, including metal-enhanced fluorescence, absorption, second-harmonic generation, and in particular surface-enhanced Raman spectroscopy.1 This has led to various potential applications of Ag nanostructures in areas such as sensors, solar cells, and light emitting diodes.2-5 However, it has been observed that Ag nanoparticles undergo morphological changes when exposed to water or other solvents,6-8 which pose as a serious challenge for their stability in practical applications. Exploring the morphological changes of Ag nanostructures and their underlying mechanisms can provide insight for the control of such reformations, as well as guide the future design of optical devices based on Ag and other metal nanostructures. Previously observed spontaneous Ag nanoparticle reformation can be understood as an electrochemical Ostwald ripening process, as reported by Brus et al.,8 where uniformly distributed Ag nanoparticles on conductive substrates can evolve into a smaller number of larger faceted crystallites. Herein, we report a drastically different type of Ag nanostructure reformation, where two-dimensional (2D) fractal structures evolved after immersion in water from a uniform distribution of nanoparticles in Ag island films (e5 nm equivalent continuous film) on Si substrates with thermally grown silicon dioxide as thin as 4 nm. This is in striking contrast to the faceted 3D Ag crystallite formation from similar Ag nanoparticles on native oxide covered silicon substrates as observed in our experiments, as well as previously reported on ITO and HOPG surfaces.8 The 2D fractal structures form via diffusion-limited aggregation (DLA) of Ag species on insulating surfaces, after which Ostwald ripening leads to coarsening within the structures. *To whom correspondence should be addressed. E-mail:
[email protected]. (1) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783. (2) Van Duyne, R. P.; Hulteen, J. C.; Treichel, D. A. J. Chem. Phys. 1993, 99, 2101. (3) Hayashi, S.; Kozaru, K.; Yamamoto, K. Solid State Commun. 1991, 79, 763. (4) Morfa, A. J.; Rowlen, K. L.; Reilly, T. H.III; Romero, M. J.; van de Lagemaat, J. Appl. Phys. Lett. 2008, 92, 013504. (5) Kwon, M.-K.; Kim, J.-Y.; Kim, B.-H.; Park, I.-K.; Cho, C.-Y.; Byeon, C. C.; Park, S.-J. Adv. Mater. 2008, 20, 1253. (6) Roark, S. E.; Semin, D. J.; Lo, A.; Skodje, R. T.; Rowlen, K. L. Anal. Chim. Acta 1995, 307, 341. (7) Murakoshi, K.; Tanaka, H.; Sawai, Y.; Nakato, Y. J. J. Phys. Chem. B 2002, 106, 3041. (8) Redmond, P. L.; Hallock, A. J.; Brus, L. E. Nano Lett. 2005, 5, 131.
7222 DOI: 10.1021/la9010532
The thin (∼4 nm) thermal oxide was grown on Si(100) wafers (4 in. diameter, p-type, resistivity 5-20 Ω 3 cm, Silicon Quest International, Inc.). The silicon wafers were first thoroughly cleaned by using a piranha solution (2:1 H2SO4/H2O2, 10 min) and then a 5:1:1 H2O/H2O2/HCl solution (70 °C, 10 min), followed by a thorough rinse with deionized (DI) water after each step. The native oxide on the silicon wafers was then stripped by dipping in a 10:1 H2O/HF solution for 10 s. The wafers were rinsed with DI water and spun dry. The cleaned Si wafers were immediately placed in a furnace to thermally grow silicon dioxide, aka thermal oxide, at 948.5 °C under flowing O2 for 30 s followed by annealing with N2 for 10 min at the same temperature. The growth temperature and time were chosen according to the kinetics of thermal oxidation of Si,9 and the actual thickness of the silicon dioxide was determined by ellipsometry (Stokes Ellipsometor LSE). All the substrates used in the study were thoroughly cleaned prior to Ag film deposition. For Si substrates with native or thermal oxide, the Radio Corporation of America method10 was conducted with a 1:1:5 NH4OH/H2O2/H2O solution at 75-80 °C for 10 min. They were then rinsed with DI water and spun dry. Polished fused quartz substrates (∼3 cm 3 cm, Delta Technologies, resistivity 7 107 Ω 3 cm) were ultrasonicated in acetone and isopropyl alcohol for 5 min each and then rinsed with DI water and blown dry with N2. Ag film depositions were carried out with an e-beam evaporator (CHA industries Mark50) under a base pressure of
