Positively spin coherent

98 nature materials | VOL 7 | FEBRUARY 2008 | www.nature.com/naturematerials fact that dislocation plasticity is much more discrete at the nanoscale. !eir observations of inhomogeneous dislocation nucleation under the surface of the loading platen — where the indenter contacts the specimen — and the occurrence of discrete dislocation bursts provide valuable insight into how plasticity occurs in these reduced volumes. Another advantage of direct observation is the ability to see and address experimental complications. !e presence of surface layers (oxides or focused-ion-beam-induced radiation damage) might be expected to inhibit dislocation exhaustion, but the observations presented by Shan and co-workers clearly indicate that this is not the case. By contrast, the smaller-is-stronger creed suggests that pillars should be stronger than the larger substrate that they rest on, and the in situ observations provide clear evidence of the pillar punching into the substrate. !ese observations also highlight the importance of geometry by showing that tapered specimens deform much less homogeneously than has previously been assumed. Such e"ects greatly reduce the #delity of ex situ experiments unless properly accounted for. !e emergence of microand nanoscale materials science has led to the discovery that the intrinsic strength of very small structures is higher than for bulk materials. !e in situ results reported by Shan and colleagues provide clear evidence of the importance of dislocation starvation and the discreteness of dislocation processes in understanding this phenomenon in nanopillars. More broadly, these experiments show that direct observations can provide much needed clarity in understanding complex material behaviour at the nanoscale.