Formation window of gas bubble superlattice in molybdenum under ion implantation

Self-assembly of defects in materials can create novel physical properties with potential applications in various technological fields. Here, we studied the physical mechanism of self-assembly of helium gas bubbles in molybdenum under ion implantation and unified the formation window of gas bubble/void superlattice in terms of irradiation temperatures and helium-atomic parts per million/displacements per atom damage levels. The ion fluence and temperature-dependent formation of gas bubble superlattice in molybdenum was examined via both transmission electron microscopy and synchrotron-based small-angle x-ray scattering. The formation of gas bubble superlattice is linked with specific implantation conditions, including ion fluence and implantation temperature. The bubble lattice constant increases with increasing the implantation temperature from 150 to 450 E-C. Once the gas bubble superlattice forms, increasing fluence has no effect on the bubble lattice constant. Both experiments and atomic kinetic Monte Carlo modeling indicate a three-stage formation process of gas bubble superlattice, from random bubbles to planar ordering and then to three-dimensional superlattices, suggesting that one-dimensional diffusion of self-interstitial atoms can cause the formation of gas bubble superlattice. Our study advances the understanding of defect self-assembly in materials in nonequilibrium states and provides an approach of managing the defect formation and transforming them from a liability into an asset in a controllable way.