A Unified Guide to Two Opposite Size Effects in Nano Elastic Materials *

The microstructural variation near surface of nano elastic materials is analyzed based on different potentials. The atomic/molecular mechanism underlying the variation and its effect on elastic modulus are such that the nature of long-range interactions (attractive or repulsive) in the atomic/molecular potentials essentially governs the variation near surface (looser or tighter) and results in two opposite size effects (decreasing or increasing modulus) with decreasing size. PACS: 62. 25.−g, 68. 35. B−, 68. 35.Gy Recently, the rapid development of nanotechnolo-gies and extensive use of nano materials have made the understanding and characterizing of their mechan-ical properties become an urgent task. On the other hand, nano materials often exhibit behaviors very dif-ferent from their conventional counterparts, even their elastic modulus varies significantly when the sample size decreases down to nano-scale. Surprisingly, varia-tions of the modulus are radically different in differen