Enhancement of stiffness of vertically standing Si nanosprings by energetic ions

In this study, the compressive strength of individual Si nanosprings grown by glancing angle deposition was determined by atomic force microscope based force-distance spectroscopy. Different irradiation conditions were employed to separately investigate the role of inelastic electronic excitations and elastic nuclear stopping on the stiffness of Si nanosprings. This was achieved by using different ion beams at various energies such that the energy deposited per unit volume (ε) ranged from less than 1 eV nm−3 to ∼MeV nm−3. For moderate values of energy densities, a logarithmic dependence of the stiffness change on ε was observed. Interestingly, stiffness of the nanosprings increased up to 32% without any visible deformation and is explained in terms of densification of structures caused via electronic excitations induced by energetic ions. However, for very high energy densities (∼MeV nm−3), an anisotropic deformation of nanosprings initiated by nuclear energy losses occurred predominantly and the stiffness of the nanosprings was observed to increase up to 170%. The present study demonstrates the role of electronic excitations and nuclear stopping in enhancing stiffness of Si nanosprings, and indicates that higher surface-to-volume ratio and shape anisotropy of nanostructures aid in delaying the process of anisotropic deformation during irradiation.