On the Development of Optical Properties during Thermal Coarsening of Gold Nanoparticle Composites

The changes in optical properties that occur as gold nanoparticles (AuNPs) are thermally converted to a continuous thin film were studied with the purpose of determining the roles of particle coarsening and temperature. In situ reflectance spectroscopy, electron microscopy, and synchrotron X-ray diffraction (XRD) were applied to provide complementary information on the changes in particle size and shape. The AuNPs studied were stabilized with 1-butanethiol, 1-octanethiol, oleylamine (OA), or 4-(pyren-l-yl)butane-1-thiol (PyBuSH). Initially, the films were dark brown or purple because of the plasmon resonance of the AuNPs. As the temperature was increased, the AuNPs started to coalesce and percolate, thereby changing the color of the films to that of bulk gold. Films of AuNPs stabilized with alkanethiols sintered very rapidly, measured as a rapid change in the reflectance spectrum. In contrast, films of AuNPs stabilized with OA or PyBuSH sintered more gradually and at a higher temperature. This permitted the transition to be studied in greater detail than for the alkanethiols. Red-shifted plasmon peaks and increased intensity in the reflectance data and XRD and electron microscopy measurements revealed that a prolonged process of nanoparticle coarsening occurred prior to sintering. The effect of temperature on the optical properties was isolated by monitoring samples as they cooled. The insulator-to-metal transition in these types of composites offers a very flexible platform for controlling spectral properties in the near-infrared region.