Morphology- and dehydrogenation-controlled mechanical properties in diamond nanothreads

Recently synthesized diamond nanothreads (DNTs), collecting desired properties of both inorganic nanostructures and hydrocarbon molecular structures, are an interesting group of carbon-based materials. Using full atomistic first-principles based ReaxFF molecular dynamics (MD) simulations, a comprehensive study on tensile and bending mechanical characteristics of fifteen energy-favorable DNTs is performed. All the DNTs show unique tensile and bending mechanical properties that markedly vary with morphology and arrangement of carbon polygons. A straight DNT composed of purely carbon hexagons shows brittle fracture in the temperature range of 1-2000 K, whereas with regard to another hexagon-dominated DNT and helically coiled DNT with the largest coiled radius, a thermal-induced brittle-to-ductile transition is uncovered at 2000 K. Particularly, the coiled DNT subjected to tensile loading/unloading shows a clear mechanical hysteresis loop. Dehydrogenation does not change the morphologies and stability of DNTs, but significantly affect the tensile mechanical responses; the tensile stiffness, toughness and ductility can be enhanced by approximately 1-fold, 2-folds and 3-folds as much of their pristine counterparts, respectively, however, the failure strain is reduced at any degree of dehydrogenation. Similarly, bending stiffness also closely connects with dehydrogenation. A transition of bending stiffness in two specific dehydrogenation-free DNTs occurring at critical curvatures is detected as a consequence of local bond transformations. Moreover, bending stiffness in different bending directions can differ by around 8-folds, originating from the distinct surface morphologies. The findings provide a critical knowledge of mechanical properties of DNTs for practical applications.

Diamond nanothreads (DNTs) show unique tensile and bending mechanical properties that markedly vary with morphology, arrangement of carbon polygons and dehydrogenation. Dehydrogenation has a strongly strengthen effect on the tensile stiffness, and dehydrogenated DNTs show change in fracture pattern from brittle to ductile.267