From metastable to stable: Possible mechanisms for the evolution of W ₁₈O₄₉ nanostructures

The evolution of nanoscale W₁₈O₄₉ from nanowires into nanosheets is investigated. FESEM and HRTEM images of self-synthesized W ₁₈O₄₉ nanostructures obtained from sputter-deposited tungsten films after thermal annealing at various temperatures (550-900°C) and times (0.5-3 h) are presented and discussed. W₁₈O₄₉ nanostructures obtained from annealed samples with sufficient thermal budgets exhibit Moiré fringes with various periodicities along the growth direction. Experimental results confirm that the evolution of nanostructures from nanowires into nanosheets can be attributed to two mechanisms. One is the formation of an amorphous buffer layer on the (502) planes between two neighboring nanowires that facilitates twisting and/or rotation of the two wires and re-crystallization of the buffer layer. The other is the assembly of two neighboring nanowires into a quasi-single crystal by coherent their (010) planes, followed by the rolling of subgrain boundaries to form single-crystalline nanosheets.