Surface plasmon resonance method for probing interactions in nanostructures: CdS nanoparticles linked to Au and Ag substrates by self-assembled hexanedithiol and aminoethanethiol monolayers

Abstract

Self-assembled hexanedithiol (HDT) and aminoethanethiol (AET) monolayers (SAMs), ∼0.4–0.8 nm in thickness, are used to link ∼5 nm diam CdS nanoparticles covalently and electrostatically onto Au and Ag substrates. The resulting nanostructures are probed with scanning electron microscopy (SEM) and surface plasmon resonance (SPR) measurements. The CdS nanoparticle–SAM–substrate interactions manifesting themselves in the dielectric functions of the multilayered systems, are detected in the SPR data, and are discussed in terms of a phenomenological six-layer model. The SPR response of the Ag substrate is more sensitive to neighboring interactions than the Au substrate. The SEM images show that the CdS, connected either by HDT or AET onto Ag substrates, forms crystalline structures. The interactions responsible for this crystallization are absent in samples employing Au substrates, in which case only ∼5 nm diam CdS nanoparticles are detected by SEM. The experimental results of the present article, analyzed in detail using Fresnel and Maxwell equations, demonstrate how the SPR technique can be used to characterize layered nanostructured materials.