Preparation Of Gold Nanorods And Their Optical Properties

Nanoscale science is one of the important fundamental researches in modern science and technology. Mastery over the shape of a nanocrystal provides one important method in controlling the properties and, in turn expanding its potential application fields. Due to surface effect, small size effect and quantum size effect of nanometer sized materials, nanomaterials have fascinating properties which are different from corresponding bulk materials. For example, gold nanoparticles with diameter less than 3 nm are no longer golden in color, and their chemical activity is obviously enhanced. Depending on the size and shapes, gold colloids have novel optical properties, exhibiting red, blue and other colors. In recent years, much attention was paid on one dimensional mental nanostrucrures, such as nanowires and nanorods, which exhibit extraordinary properties and have promising applications, including electronics, optics, magnetics, sensing, biological compatibility and five-dimensional optical recording. Investigations on gold nanorods show that there are two (transverse and longitudinal) plasma modes, and the optical properties are determined mainly by their shape, aspect ratios, composition and crystallinity.In this work, our aim involves: 1) the synthesis of gold nanorods by chemical method, 2) discover the growth proceeds of nanocrystal and 3) understanding of their optical properties. Spectra, morphology and structure characterization are carried out by UV-vis-NIR spectrometer (sp-752), transmission electron microscopy (TEM: JEM-100), high-resolution transmission electron microscopy (HRTEM: JEM-200CX) and X-ray diffraction (XRD: Ultima-III, Rigaku).We begin with a brief introduction to synthesis methods, including templating, optical and electronic lithography, electrochemical method, photochemical reduction and seed-mediated method, in which the seed-mediated method are taken in our experiment due to easily fabricated and lower cost.We then focus on a variety of experimental parameters that have been explored to manipulate the growth of gold nanorods with different aspect ratios, and the strong longitudinal surface plasmon resonance (SPRL) shifts from the visible (700 nm) to the near-infrared (Vis–NIR) region (1100 nm). The optical properties of metallic nanoparticles were described by employing the quasistatic approximation theory, Mie theory and the discrete dipole approximation (DDA) theory. We also performed theoretical calculations of the absorption spectrum using Gan equations and finite difference time domain (FDTD) solutions, which were directly compared with experiments.Au dog-bones, dumbbells and nanorods were prepared respectively with CTAB as surfactant. To produce Au longer nanorods, a surfactant mixture contained CTAB and BDAC was applied in the growth solution. It was found that the aspect ratios can be tuned by the content of acid. The particles have been determined to be a single crystalline face-centred cubic structure from the measurements of HRTEM, selected area electron diffraction (SAED) patterns and XRD profile. The Au nanorod has an octagonal cross-section and the sidewalls of the nanorods are {110} and {100} facets. The nanorod is grown along [100] direction and both {111} and {110} facets bounded at the truncated corners. A growth model is proposed, in which kinetics and thermodynamics are employed.In addition, Au/Ag core/shell nanorods were prepared by the reduction of AgNO3 with ascorbic acid in the presence of Au nanorods. Upon the Ag overgrowth, it was observed the variation in the SPRL absorption bands. This work is valuable for the growth mechanism of nanostructures and their potential applications.