Reversing the size-dependence of surface plasmon resonances

Abstract

The size-dependence of surface plasmon resonances (SPRs) is poorly understood in the small particle limit due to complex physical/chemical effects and uncertainties in experimental samples. In this article, we report an approach for synthesizing an ideal class of colloidal Ag nanoparticles with highly uniform morphologies and narrow size distributions. Optical measurements and theoretical analyses for particle diameters in the d ≈ 2-20 nm range are presented. The SPR absorption band exhibits an exceptional behavior: As size decreases from d ≈ 20 nm it blue-shifts but then turns over near d ≈ 12 nm and strongly red-shifts. A multilayer Mie theory model agrees well with the observations, indicating that lowered electron conductivity in the outermost atomic layer, due to chemical interactions, is the cause of the red-shift. We corroborate this picture by experimentally demonstrating precise chemical control of the SPR peak positions via ligand exchange.T he ability to control surface plasmon resonances (SPRs) in metal nanostructures is critical for achieving advances in many areas, including chemical and biological sensing, imaging, optoelectronics, energy harvesting and conversion, and medicine (1-12). Metal nanoparticles (NPs), particularly those of the noble metals (e.g., Au and Ag) that exhibit strong SPRs, have been the focus of much work (13-17). SPRs have intense and broad optical absorption bands that arise from coherent oscillations of conduction electrons near the NP surfaces. In general, SPRs are influenced by their size, morphology, composition, surface chemistry, and surrounding environment (18,19). However, the size-dependence of SPRs that is important for the aforementioned applications (1-12) is poorly understood in the small particle limit due to complex physical and chemical effects as well as uncertainties in experimental samples. Here, we report an approach for synthesizing an ideal class of colloidal Ag NPs with highly uniform morphologies and narrow size distributions. The SPR absorption band for particles with diameters, d, in the range of 2-20 nm is found to exhibit an exceptional behavior: As size decreases from d ≈ 20 nm it blue-shifts but then turns over near d ≈ 12 nm and strongly red-shifts. We have developed a multilayer Mie theory model and the corresponding calculation results agree well with the observations, indicating that the lowered electron conductivity in the outermost atomic layer, due to chemical interactions, is the cause of the red-shift. We corroborate this picture by experimentally demonstrating precise chemical control of the SPR peak positions via ligand exchange. Such chemical control of the NP surface layer may provide a promising strategy for sensitively probing species strongly adsorbed (or bonded) to the NPs.For metal NPs with d or structural features larger than 20 nm, often a purely continuum-level, classical electrodynamics picture suffices for interpretation or prediction of their optical properties. In this approach the bulk dielectric constants for the vario...

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