Toward Free-Standing Lonsdaleite and Diamond Few Layers: The Nitrogen Effect

Strong, directional, N bonding led surface stabilization is induced to obtain free-standing two-dimensional (2D) layered assemblies of Lonsdaleite and diamond phase carbon by surface termination at three coordinated C sites. These assemblies have strong bonding and surface N lone pair induced self-protection and achieve respective bulk-like electronic, mechanical, and crystallographic properties at as low as seven atomic layer thickness. Lonsdaleite phase subnano thickness free-standing as small as three layered assembly shows bulk-like thermal stability up to 2900 K. Size confinement effect dominates up to four layered assembly, and then electronic and crystallographic properties are dominated by bulk stabilization which saturates toward respective bulk-like features from seven layered assembly. Similarly, assemblies of Lonsdaleite phase carbon with BeF (C5NBeF) and BS (C5NBS) layer are also proposed along with low symmetric Cm-C3N4 layered assembly. The combination of N and O is shown to stabilize the surface of nanodiamond but shows significant variation in structural and electronic properties, which are principally governed by the confinement effect because of weak surface stabilization. Our results propose that, in principle, 2D free-standing layered assemblies of the spa hybridized phases of carbon with surface terminated by only three coordinated C sites can be stabilized and engineered by substitutional doping of N, and in some cases, respective bulk-like electronic and crystallographic properties at nanosize can be achieved.