Plasmon Coupling in Two-Dimensional Arrays of Silver Nanoparticles: I. Effect of the Dielectric Medium Mark K. Kinnan, † Svetlana Kachan, ‡ Courtenay K. Simmons, † and George Chumanov* ,† 235 Hunter Laboratories, Clemson UniVersity, Clemson, South Carolina 29634 and B. I. StepanoV Institute of Physics, 68 NezaVisimosti AVenue, 220072 Mink, Belarus ReceiVed: January 5, 2009; ReVised Manuscript ReceiVed: January 28, 2009 The effect of the dielectric medium between the particles on plasmon coupling was studied experimentally and theoretically. Two-dimensional arrays of silver nanoparticles adsorbed on glass slides were immobilized using poly(methylmethacrylate). The method provides an efficient way for stabilization of particles on surfaces. It was concluded that increasing the dielectric function of the surrounding medium promotes plasmon coupling between silver nanoparticles. On the basis of the quasi-crystalline approximation, the optimum refractive index was calculated that yielded the strongest and highest Q extinction band due to coherent plasmon coupling. Introduction Optical properties of silver nanoparticles (SNPs) are deter- mined by excitation of the plasmon resonances that are the collective oscillations of the free electron density. 1 The optical excitation of plasmon resonances in SNPs represents the most efficient mechanism by which light interacts with matter. By varying the particle size, shape, and surrounding dielectric medium the plasmon resonance can be tuned across the near- UV, visible, and near-infrared spectral range. The high efficiency for interaction with light, tunability of the optical resonance, as well as photochemical robustness makes SNPs attractive for applications in optical filters, 2 plasmonic waveguides, 3 substrates for surface-enhanced spectroscopies, 4-6 and biosensors. 7,8 Excitation of plasmon resonances produces an electromag- netic field localized around SNPs that is enhanced as compared to the incident field. Evanoff et al. reported that the local electromagnetic field extends approximately 40 nm from the surface of 84 nm diameter particles. 9 When several particles are put into close proximity so that the local fields associated with electron oscillations in individual particles overlap, the system undergoes plasmon coupling and new plasmon modes result. The plasmon coupling was observed in 2D arrays of SNPs as a sharp band in the blue spectral range of the extinction spectra representing a coherent plasmon mode. 10 Drying of these arrays changed the refractive index of the medium between the particles from that of water (1.33) to that of air (1) and resulted in a loss of the sharp band. Additionally, the drying lead to the aggregation of the SNPs due to the forces associated with the surface tension of water as it dries. 11 Malynych et al. reported that the sharp band was preserved when the arrays were embedded into the poly(dimethylsiloxane) polymer matrix. 11 Even though embedding the SNPs into polymer matrixes proved to stabilize SNPs from aggregation, the particles are covered with a polymer and their surface is not accessible to the environmentsa feature desired for sensing applications of these arrays. It was previously shown that the coupled SNP arrays that are not embedded into polymers can be used for sensing of the refractive index when immersed into different solvents. 12 However, the coherent plasmon mode of the embedded arrays was insensitive to changes of the dielectric medium. A new approach for stabilizing SNP arrays based on im- mobilizing the particles with a thin layer of poly(methyl- methacrylate) (PMMA) was developed in this work. The key advantage of this approach relates to the ability of casting thin PMMA layers in the space between SNPs and not on their surface. This is achievable because PMMA does not wet the silver surface due to poor interaction between the two. The uncoated silver surface remained accessible to the interaction with various molecules, and this principle can be potentially utilized in sensing applications of SNP arrays. In addition, this approach allowed addressing a fundamental question about the effect of the dielectric medium on plasmon coupling. Experimental Section Materials. Poly(methylmethacrylate) (PMMA) [MW ≈ 996 000], anisole, 4-aminothiophenol, and poly(4-vinylpyridine) (PVP) were purchased from Sigma Aldrich. Silver(I) oxide and USP Absolute-200 Proof ethanol were acquired from Alfa Aesar and Aaper Alcohol & Chemical Co., respectively. All chemicals were used as received. Microscope glass slides were obtained from VWR. Deionized water with a nominal resistivity of 18 MΩ · cm came from a Millipore Milli-Q water purification system. Ultra-high-purity nitrogen and ultra-high-purity hydro- gen were purchased from Air Gas. PMMA and PVP solutions were made by dissolving a weighed quantity of PMMA or PVP into anisole and ethanol, respectively. Instrumentation. UV-vis absorption spectra were recorded using a Shimadzu UV-2501PC spectrophotometer. Electron microscopy images were taken with a Hitachi SEM-4800. Spin drying was performed using a spin coater WS-400B-6NPP/LITE (Laurell Technologies Corp.). Topographical images were recorded using an AutoProbe CP AFM equipped with Mikro- Masch Noncontact silicon cantilevers (NSC11/50) in noncontact mode. The AFM images were processed using MatLAB 2007b. All spectra were processed and figures prepared using Spectra- Solve for Windows (LasTek Pty. Ltd.). The Feret’s diameter of the particles was calculated using ImageJ for Windows (http:// rsbweb.nih.gov/ij/). Synthesis of Silver Nanoparticles. Silver nanoparticles were synthesized using the hydrogen reduction method. 13 Briefly, the * To whom correspondence should be addressed. E-mail: gchumak@ clemson.edu. † Clemson University. ‡ B.I. Stepanov Institute of Physics. J. Phys. Chem. C 2009, 113, 7079–7084 7079 10.1021/jp900090a CCC: $40.75  2009 American Chemical Society Published on Web 04/03/2009