When 'smaller is stronger' no longer holds

'Smaller is stronger' has been a well-established tenet for the mechanical strength of small-volume single crystals. However, emerging evidence suggests that this trend does not always hold in the deep sub-micrometre regime (<similar to 10(2)nm) when surface-mediated displacive and diffusive processes dominate. Here we present a perspective to highlight typical scenarios in which the single-crystal strength is influenced by surface stresses, diffusion-mediated surface creep, or dislocation-starvation-engendered high-stresses in the crystal interior, leading to 'smaller is weaker' or 'plateau limited by ideal strength'. Such an unusual size dependence/independence of strength has implications for the reliability and stability of na no-structured devices, [GRAPHICS] IMPACT STATEMENT Due to significant surface effects, single crystals in the deep sub-micron regime (< similar to 10(2) nm) can be 'smaller is weaker' or reach a'saturated strength plateau', defying the well-known 'smaller is stronger' trend.