Stabilizing the hexagonal diamond metastable phase in silicon nanowires

International audienceAt the nanoscale, the proportion of atoms at the surface of solids becomes significant, which may change the equilibrium of atomic edifices. In the case of silicon, experimental observations have evidenced indeed the presence of the hexagonal diamond (HD) metastable structure in as-grown nanowires of a few nm in diameter, as if that phase could become the stable one in such small objects. We present ab initiocalculations that demonstrate the existence of stable domain for the HD structure in silicon nanowires. Surfaces of HD Si are first studied without and with hydrogen, including possible relaxations, and compared to CD Si surfaces, with a globally favourable energy ratio for HD surfaces. The energies of several plausible HD and CD Si NWs of different thicknesses are then calculated and compared to estimate their relative phase stabilities, again in favour of HD NWs, without and with hydrogen at surfaces. Analytically extrapolating theab initioresults as functions of bulk, surface and edge energy contributions, the main result is that HD Si NWs are intrinsically stable with respect to CD Si NWs for effective diameters up to only about 15 nm, for pure Si NWs as well as for surface hydrogenated NWs. Thicker HD NWs can thus only be metastable. This may explain why Si HD NWs are so difficult to grow. The diameter size limit for germanium, silicon's big brother, is three times smaller, making HD Ge NWs much less likely, in agreement with recent experimental attempts to grow Ge NWs

Twin-interface interactions in nanostructured Cu/Ag: Molecular dynamics study

International audienceThe interaction of deformation twins with interfaces in nanostructured Cu/Ag is studied using molecular dynamics simulations. The influence of the interface structure on twin nucleation, propagation and thickening is analysed, and the role of the misfit interfacial dislocations mesh is detailed. In particular, we show that the interface can induce, directly or indirectly via Lomer dislocations, the nucleation of twinning dislocations. A thorough description of the involved mechanisms is given. Through this atomic scale approach, our study offers some useful understanding of the mechanical twinning process in nanolamellar composites, where twinning appears to be a common plasticity mechanism

The effect of surface step and twin boundary on deformation twinning in nanoscale metallic systems

International audienceThe effect of surface step and twin boundary on the mechanical twinning process in nanoscale face-centred cubic metallic films was studied using atomistic simulations. Aluminium was considered as a model material but comparisons were made with silver and copper. Surface steps were identified as privileged sites for twin nucleation at lower stresses, leading to the formation of only one large twin in defect-free films. In presence of a coherent twin boundary which acts as a strong barrier to the propagation of dislocations, the extension of nucleated twins is much more limited but the density of secondary twin boundaries is found higher. The key role played by Lomer dislocations, resulting from the interaction between incipient twins and the coherent twin boundary, on the nucleation of new twins was demonstrated. These findings shed light on some elementary mechanisms that can be involved in the elaboration of nanotwinned materials with interesting mechanical properties