FCC-HCP phase transition in ultrathin gold films: A first-principles investigation

Abstract

Gold with the unconventional hexagonal close-packed (HCP) phase shows outstanding optical and mechanical properties, holding immense promises for optoelectronic applications. Ultrathin HCP gold films can be fabricated by reducing the thickness of the bulk face-centered cubic (FCC) precursor to a few nanometers, but the fundamental mechanism governing the thickness-dependent FCC→HCP phase transition remains insufficiently investigated. Herein, density functional theory (DFT) calculations and high-resolution transmission electron microscopy (HRTEM) are employed to elucidate the physical origin of the FCC→HCP phase transition in ultrathin gold films. DFT calculations demonstrate that in-plane compressive strain is a determining factor in facilitating the FCC→HCP phase transition, with a critical in-plane compressive strain of 2 ∼ 3 % (corresponding to a lattice constant of ∼ 2.8 Å). HRTEM measurements confirm the accessibility of such compressive strain. The intrinsic compressive strain in gold thin films is approximately 1 % and becomes more pronounced as the thickness decreases. The extrinsic compressive strain may arise from residual strain and substrate epitaxial strain induced during the fabrication process. Further DFT study of the kinetics of the phase transition reveals that in-plane compressive strain effectively modulates the stacking fault energy and transition pathways. Our findings establish a foundation for the structural manipulation and fabrication of advanced nanometals.