Subsurface Driven Size-Dependent Elasticity in Nanodiamond

Diamond, an archetype of hardness and brittleness, can nonetheless display striking elastic compliance at the nanoscale, in sharp contrast to its ultrastiff, fracture prone bulk behavior. Despite recent experimental progress, the atomic scale origin of this anomalous elasticity remains unresolved. Here, we uncover a pronounced size- dependent softening in nanodiamond, with the effective axial Young’s modulus decreasing from about 1000 to about 700 GPa as the particle diameter shrinks from 13 to 4 nm. By integrating transmission electron microscopy nanomechanical testing with first principles calculations and molecular dynamics simulations, we show that this stiffness reduction does not arise from the surface atomic layers, which remain comparatively stiff, but instead from weakened bonding in a distinct interfacial region between the surface and the crystalline core. These under-coordinated regions localize strain and accommodate large elastic deformation. The results uncover a previously overlooked interfacial mechanism that governs elasticity in brittle nanomaterials and offer design principles for nanomechanical resonators, quantum devices, and thermal transport applications with tunable stiffness.