Optical Properties of Noncubic Au Microcrystallites

Abstract

Size-and shape-controlled synthesis of Au nanomaterials and their optical properties have been well explored in the past. However, the influence of crystallinity, particularly in the case of Au, which exists in the face-centered cubic (fcc) phase and is extraordinarily stable, is much unexplored. Recently, Au microcrystallites with unusually stable phases of body-centered tetragonal and body-centered orthorhombic lattices have been synthesized in the laboratory. These metastable crystallites possess unusual properties compared to their fcc counterparts. In this work, we have investigated the optical extinction properties of the microcrystallites while varying the noncubic mole fractions by carefully tuning the synthetic conditions. The crystallites show near-infrared plasmonic features associated with multipolar resonances. The experimental observations have been compared with calculations performed using the discrete dipolar approximation (DDA) method. While there is a reasonable agreement between the two, the noncubic nature of the lattice seems to have little influence on the optical properties investigated.

Tuning Noncubic Phases of Au at Microscale using Low-Energy Ar+ Ion Irradiation

“…In the quest for novel optical and catalytic properties, extensive studies on the phase engineering of noble fcc Au have led to the stabilization of noncubic phases: the hcp 4H-phase in Au nanowires or nanoribbons, , 2H- hcp Au square sheets with a thickness of ∼2–3 nm, and fcc / hcp heterophase nanostructures . While most studies investigate phase transitions in Au at the nanoscale, synthesis and metastability of unconventional non- fcc phases of Au at the microscale has fascinated us for quite some time. − These phases emerge from a constrained penta-twinned bipyramidal morphology (∼3 μm in length and ∼400 nm in width) and encompass locked strains in the elastic regime with respect to fcc Au. With small strains (<2%) in the elastic regime, the metastable non- fcc phases are expected to revert readily to the native fcc phase when subjected to external perturbations such as temperature, pressure, or exposure to a charged particle beam.…”