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

Abstract

Strong, directional, N bonding led surface stabilization is induced to obtain free-standing two-dimensional (2D) layered assemblies of Lonsdaleite and diamondphase 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 bulklike 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, free-standing hybrid assemblies of Lonsdaleite phase carbon with BeF (C 5 NBeF) and BS (C 5 NBS) layer are also proposed along with low symmetric Cm-C 3 N 4 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 sp 3 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.

Two-Dimensional Anisotropic C10 Carbon Allotrope with Mechanically Tunable Band Gap

“…28 The principle difference is in reduction of the forbidden region from 4.11 eV (for diamond) to the present case 2.94 eV. 28 This reduction of ∼1 eV in the forbidden region can be ascribed to the bonding in deformed tetrahedral geometry of the TO-C 10 structure, wherein the bond angles and lengths are varied from 109.5°, 1.54 Å from the perfect tetrahedral structure, to that as shown in Table 1. The atoms in the ladder region of TO-C 10 (C 2 sites) have maximum deformation relative to the tetrahedral structure, and hence their states appear in the forbidden region causing reduction in the band gap.…”

“…The parabolic region of ±5% strain is used to extract the elastic constants by fitting second-order partial derivative of strain energy with respect to strain. 28 The strain is enforced by changing the respective lattice constant, and then the ions are relaxed with restricting the dimensions of the unit cell. Calculated elastic constants C 11 , C 22 , C 12 , C 66 , Young’s modulus, and Poisson’s ratio ( v ) of TO-C 10 are 320.7, 293.0, 11.7, 154.5, and 306.5 GPa nm, and 0.04, respectively; the detailed plot (Figure S6) along with relevant discussion can be found in the Supporting Information.…”

“…41 The calculated average Young’s modulus is 306.5 GPa nm and is significantly larger than penta-graphene (265 GPa nm) and comparatively close to that of graphene (338 GPa nm) among 2D carbon allotropes. 25,28 Strong sp 3 -hybridized structures govern the large Young’s modulus and can in principle enhance further by reducing inbuilt strain with appropriate engineering of the structure. In that direction, N is found to be a suitable candidate, and the resultant structure is found to show substantially higher Young’s modulus, and these results will be communicated separately.…”

“…Despite having a zigzag nature, the strain enforced along the b parameter endures less stretching because of bonds of C 2 (terminal carbon along z -axis) atoms with the basal plane C 1 site atoms. This behavior is the artifact of the large deviation of asymmetric bond angles of C 2 carbon atoms than the diamond-like sp 3 hybridized structure, and it results in decreased band gap than diamond (4.11 eV) 28 as evident by l-DOS and partial charge density analysis.…”

“…In this direction, recently, we have proposed free-standing few-layered thermomechanically robust N-terminated assemblies of sp 3 -bonded carbon phases, providing respective bulk-like electronic, mechanical, and thermal properties. 28…”