Amorphization of Ge and Si nanocrystals embedded in amorphous SiO₂ by ion irradiation

Finite-size effects become significant in nanoscale materials. When a nanocrystal is surrounded by a host matrix of a different nature, the perfection of the crystal structure is distorted by the interface formed between the nanocrystal and the matrix. The larger the surface-to-volume ratio of the nanocrystal, the higher the influence of the interface defect states on its properties. The presence of defect states in the interface can also explain the different responses of the nanocrystals (NCs) on external influences. By the combination of molecular-dynamics simulations and x-ray absorption spectroscopy measurements, we show that the amorphization of Si and Ge nanocrystals is reached at doses roughly one order of magnitude lower than those for the bulk crystals. Examining nanocrystals in the size range from 2.4 to 9 nm, we also show that the susceptibility to the amorphization decreases with increasing nanocrystal size. The finite-size effect remains significant also for the largest nanocrystals of 9 nm.