Inelastic deformability of nanopillar by focused-ion-beam chemical vapor deposition

Pillars with nanosized diameter and microsized length can be constructed by chemical vapor deposition using a focused-ion beam. For pillars consisting of an outer amorphous carbon (a-C) ring and an inner gallium (Ga) core, we performed an internal structure analysis using a resonance vibration test and elementary composite theory. The ratio of the a-C outer ring to the Ga core increases as the pillar diameter—and thus, Young’s modulus of the whole pillar—increases, because of the much larger Young’s modulus of a-C than that of Ga. The bending test of the pillar grown onto the Si substrate was performed under the lateral load at the free end using the cantilever tip in the scanning electron microscope (SEM). The obtained load-deflection curve tells us that the pillar has a wide low-hardening region after a linear response and then becomes extremely hardened at the finite displacement. The pillar intrinsically possesses much more flexible deformability for bending than expected, in contrast to tensile deformation. This is caused by the atomistically bonding anisotropy of bond stretching, bond bending, and bond dihedral angular bending observed in covalent a-C. Additionally, the pillar can be regarded as a hollow circular cylinder because of the extremely low rigidity of the Ga core. Therefore, these factors give the pillar a finite inelastic deformability like rubber, which was experimentally observed in the SEM.