Understanding Metal Nonoclusters through Ultrafast and Nonlinear Spectroscopy.

Abstract

In the past 20 years, nanomaterials studies have uncovered many new and interesting properties not found in bulk materials. Extensive research has focused on metal nanoparticles (> 2 nm) because of their potential applications, such as molecular electronics, image markers and catalysts. Moreover, the discovery of metal nanoclusters (< 2 nm) has greatly expanded the horizon of nanomaterial research. These nanosystems exhibit molecular-like characteristics as their size approaches the Fermi-wavelength of an electron. The relationships between size and physical properties of nanomaterials are intriguing. Metal nanosystems in this size regime have electronic properties that are determined by both size and shape. Remarkably, changes in the optical properties of nanomaterials have provided tremendous insight into the electronic structure of nanoclusters. The success of synthesizing monolayer protected clusters (MPCs) in the condensed phase has allowed scientists to study the metal core directly.

Spectroscopic studies are carried out on two different metal nanosystems, gold and silver. Gold nanosystems are known for their high stability. Detailed characterization of gold nanosystems allows for modeling of the electronic and optical properties. Major optical and electronic differences between gold nanoparticles and nanoclusters can be observed around 2.2 nm, which was not known previously. Gold MPCs also exhibit emissions that are five orders of magnitude larger than bulk gold. Chemical dynamics such as electron-electron scattering and electron-phonon coupling can be used to explain the subtle differences between nanosystems. Silver and gold nanosystems are compared because of the similarity between their bulk properties. Silver MPCs exhibit similar optical properties as gold MPCs, but differ in key electronic transitions.

The study of nanosystems aims to answer a few major questions. First, what is the effect of size on the electronic and optical properties of metal nanosystems? Second, what are the fundamental mechanisms that govern the electronic excitation? Can we take advantage of these new properties for optical and electronic applications? Finally, can we build better models to predict the properties of metal nanoclusters made and yet to be made? Nanosystem presents a new frontier in material science to be explored and exploited.