Phase diagram of quasi-two-dimensional carbon, from graphene to diamond

Ionic Graphitization of Ultrathin Films of Ionic Compounds

On the basis of ab initio density functional calculations, we performed a comprehensive investigation of the general graphitization tendency in rocksalt-type structures. In this paper, we determine the critical slab thickness for a range of ionic cubic crystal systems, below which a spontaneous conversion from a cubic to a layered graphitic-like structure occurs. This conversion is driven by surface energy reduction. Using only fundamental parameters of the compounds such as the Allen electronegativity and ionic radius of the metal atom, we also develop an analytical relation to estimate the critical number of layers.

Intense, stable and excitation wavelength-independent photoluminescence emission in the blue-violet region from phosphorene quantum dots

Nanoscale phosphorene quantum dots (PQDs) with few-layer structures were fabricated by pulsed laser ablation of a bulk black phosphorus target in diethyl ether. An intense and stable photoluminescence (PL) emission of the PQDs in the blue-violet wavelength region is clearly observed for the first time, which is attributed to electronic transitions from the lowest unoccupied molecular orbital (LUMO) to the highest occupied molecular orbital (HOMO) and occupied molecular orbitals below the HOMO (H-1, H-2), respectively. Surprisingly, the PL emission peak positions of the PQDs are not red-shifted with progressively longer excitation wavelengths, which is in contrast to the cases of graphene and molybdenum disulphide quantum dots. This excitation wavelength-independence is derived from the saturated passivation on the periphery and surfaces of the PQDs by large numbers of electron-donating functional groups which cause the electron density on the PQDs to be dramatically increased and the band gap to be insensitive to the quantum size effect in the PQDs. This work suggests that PQDs with intense, stable and excitation wavelength-independent PL emission in the blue-violet region have a potential application as semiconductor-based blue-violet light irradiation sources.

Two-dimensional cyanates: stabilization through hydrogenation

According to first-principles calculations, it should be possible to grow two-dimensional (2D) forms of copper thio-cyanate (CuSCN) and copper seleno-cyanate (CuSeCN) since their energies are only marginally higher than those of their most stable three-dimensional (3D) wurtzite structures.

Modification of mechanical properties of vertical graphene sheets via fluorination

Fluorination can significantly enhance the reduced modulus of surfaces comprised of vertical graphene sheets.

Interfacing graphene and related 2D materials with the 3D world

An important prerequisite to translating the exceptional intrinsic performance of 2D materials such as graphene and transition metal dichalcogenides into useful devices precludes their successful integration within the current 3D technology. This review provides theoretical insight into nontrivial issues arising from interfacing 2D materials with 3D systems including epitaxy and ways to accommodate lattice mismatch, the key role of contact resistance and the effect of defects in electrical and thermal transport.

Spontaneous graphitization of ultrathin cubic structures: a computational study

Results based on ab initio density functional calculations indicate that cubic diamond, boron nitride, and many other cubic structures including rocksalt share a general graphitization tendency in ultrathin films terminated by close-packed (111) surfaces. Whereas such compounds often show an energy preference for cubic rather than layered atomic arrangements in the bulk, the surface energy of layered systems is commonly lower than that of their cubic counterparts. We determine the critical slab thickness for a range of systems, below which a spontaneous conversion from a cubic to a layered graphitic structure occurs, driven by surface energy reduction in surface-dominated structures.

Graphitic Phase of NaCl. Bulk Properties and Nanoscale Stability

We applied the ab initio approach to evaluate the stability and physical properties of the nanometer-thickness NaCl layered films and found that the rock salt films with a (111) surface become unstable with thickness below 1 nm and spontaneously split to graphitic-like films for reducing the electrostatic energy penalty. The observed sodium chloride graphitic phase displays an uncommon atomic arrangement and exists only as nanometer-thin quasi-two-dimensional films. The graphitic bulk counterpart is unstable and transforms to another hexagonal wurtzite NaCl phase that locates in the negative-pressure region of the phase diagram. It was found that the layers in the graphitic NaCl film are weakly bounded with each other with a binding energy order of 0.1 eV per stoichiometry unit. The electronic band gap of the graphitic NaCl displays an unusual nonmonotonic quantum confinement response.

Radiation-Induced Nucleation of Diamond from Amorphous Carbon: Effect of Hydrogen

Electron irradiation of anthracite functionalized by dodecyl groups leads to recrystallization of the carbon network into diamonds. The diamonds range in size from ∼2 to ∼10 nm and exhibit {111} spacing of 2.1 Å. A bulk process consistent with bias-enhanced nucleation is proposed in which the dodecyl group provides hydrogen during electron irradiation. Recrystallization into diamond occurs in the hydrogenated graphitic subsurface layers. Unfunctionalized anthracite could not be converted into diamond during electron irradiation. The dependence of the phase transition pressure on cluster size was estimated, and it was found that diamond particles with a radius up to 20 nm could be formed.

Lonsdaleite Films with Nanometer Thickness

We investigate the properties of potentially the stiffest quasi-2-D films with lonsdaleite structure. Using a combination of ab initio and empirical potential approaches, we analyze the elastic properties of lonsdaleite films in both elastic and inelastic regimes and compare them with graphene and diamond films. We review possible fabrication methods of lonsdaleite films using the pure nanoscale "bottom-up" paradigm: by connecting carbon layers in multilayered graphene. We propose the realization of this method in two ways: by applying direct pressure and by using the recently proposed chemically induced phase transition. For both cases, we construct the phase diagrams depending on temperature, pressure, and film thickness. Finally, we consider the electronic properties of lonsdaleite films and establish the nonlinear dependence of the band gap on the films' thicknesses and their lower effective masses in comparison with bulk crystal.