Nanowires and ultra-thin films have wide applications in the quickly developed nanotechnology and nanoscience. However, their Young’s modulus varies with the size, which is seemingly contradictory to the conventional continuum elasticity. Investigating and understanding the underlying mechanism of the size-dependent elastic properties in nanomaterials is of both academic and practical significance. In this work, both theoretical modeling and virtual experiments have been made on this issue. A nanoelement, from the traction free bulk lattice, undergoes an initial relaxation, during which its morphology changes and energy reduces, which is an emphasis in this developed methodology and is a distinction from almost other existing models. With different definitions of surfaces and edges, two...[ Read more ]

Nanowires and ultra-thin films have wide applications in the quickly developed nanotechnology and nanoscience. However, their Young’s modulus varies with the size, which is seemingly contradictory to the conventional continuum elasticity. Investigating and understanding the underlying mechanism of the size-dependent elastic properties in nanomaterials is of both academic and practical significance. In this work, both theoretical modeling and virtual experiments have been made on this issue. A nanoelement, from the traction free bulk lattice, undergoes an initial relaxation, during which its morphology changes and energy reduces, which is an emphasis in this developed methodology and is a distinction from almost other existing models. With different definitions of surfaces and edges, two models for a nanomaterial – a nanowire or an ultra-thin film – are derived based on the same thermodynamics framework. Comparing with most of others’ treatment, Model I has an advantage to mathematically treat a surface phase to be two-dimensional and an edge phase to be one-dimensional. Under external loadings, the initial relaxed state is taken as the reference. Experimentally, relaxation and tension/compression tests in different loading directions have been conducted on SiC, Si and Cu crystalline nanowires with different cross-sectional sizes and ultra-thin films with different thicknesses by Molecular Dynamics (MD) simulations. This systematic study successfully illustrates the intrinsic mechanism of the size-dependent Young’s modulus in nanomaterials and the proposed methodology facilitate characterizing mechanical properties of nanomaterials to some extent when continuum concepts, such as, surface energy and surface elastic constants, are used.