Phase diagram of solid-phase transformation in amorphous carbon nanorods

The transformations of amorphous carbon nanorods with different diameters and densities upon heating up to different temperatures are studied with density-functional-based tight-binding molecular dynamics. Phase diagrams with assorted transformed sp2 nanostructures depending on both temperatures and line density, under different heating treatments, are presented to place the observations in perspective. Under instant heating, the lowest line density at which a carbon nanotube can form is 8 Å–1, while a double-walled carbon nanotube can form at a linear density of 19–20 Å–1 and higher. Under gradual heating, both partially unzipped carbon nanotubes and carbon nanoscrolls are formed as notable intermediate structural motifs. This work sheds light on the microscopic mechanism of various sp2 nanostructural formations with the featured motifs highlighted as important intermediates, which will serve as an important guide in producing graphene nanoribbons, single-walled and double-walled carbon nanotubes, and carbon nanoscrolls from amorphous carbon nanorods

The mechanism of transforming diamond nanowires to carbon nanostructures

The transformation of diamond nanowires (DNWs) with different diameters and geometries upon heating is investigated with density-functional-based tight-binding molecular dynamics. DNWs of 〈100〉 and 〈111〉 oriented cross-section with projected average line density between 7 and 20 atoms Å−1 transform into carbon nanotubes (CNTs) under gradual heating up to 3500–4000 K. DNWs with projected average line density larger than 25 atoms Å−1 transform into double-wall CNTs. The route of transformation into CNTs clearly exhibits three stages, with the intriguing intermediate structural motif of a carbon nanoscroll (CNS). Moreover, the morphology plays an important role in the transformation involving the CNS as one important intermediate motif to form CNTs. When starting with $\langle \bar {2}1 1\rangle$ oriented DNWs with a square cross-section consisting of two {111} facets facing each other, one interesting structure with 'nano-bookshelf' shape emerges: a number of graphene 'shelves' located inside the CNT, bonding to the CNT walls with sp3 hybridized atoms. The nano-bookshelf structures exist in a wide range of temperatures up to 3000 K. The further transformation from nano-bookshelf structures depends on the strength of the joints connecting shelves with CNT walls. Notably, the nano-bookshelf structure can evolve into two end products: one is CNT via the CNS pathway, the other is graphene transformed directly from the nano-bookshelf structure at high temperature. This work sheds light on the microscopic insight of carbon nanostructure formation mechanisms with the featured motifs highlighted in the pathways