Diamanes are 2D diamond-like films that are nanometers in thickness. Diamanes can exist as bilayer or multilayer graphene with various modes of stacking and interlayer covalent sp3 bonds. The term “diamane” is used broadly for a variety of diamond-like materials at the nanoscale, from individual diamond clusters to nanocrystal films. A short overview of recent progress in the investigation of diamanes, starting from the first theoretical predictions to practical realization, is presented. The results of both theoretical and experimental studies on diamanes with various atomic structures and types of functionalization are considered. It is shown that diamanes are stronger than graphene and graphane and have wide bandgaps ranging from 3.1 to 4.5 eV depending on the structure. Diamane-like structures have been obtained using different experimental techniques, and their structures have been determined by Raman spectroscopy. The potential applications of these carbon nanostructures are briefly reviewed.

Band gap control by an external field is useful in various optical, infrared and THz applications. However, widely tunable band gaps are still not practical due to a variety of reasons. Using the orthogonal tight-binding method for π-electrons, we have investigated the effect of the external electric field on a subclass of monolayer chevron-type graphene nanoribbons that can be referred to as jagged graphene nanoribbons. A classification of these ribbons was proposed and band gaps for applied fields up to the SiO2 breakdown strength (1 V nm(-1)) were calculated. According to the tight-binding model, band gap opening (or closing) takes place for some types of jagged graphene nanoribbons in the external electric field that lies on the plane of the structure and perpendicular to its longitudinal axis. Tunability of the band gap up to 0.6 eV is attainable for narrow ribbons. In the case of jagged ribbons with armchair edges larger jags forming a chevron pattern of the ribbon enhance the controllability of the band gap. For jagged ribbons with zigzag and armchair edges regions of linear and quadratic dependence of the band gap on the external electric field can be found that are useful in devices with controllable modulation of the band gap.

The mechanism of translation symmetry breakdown in newly proposed low-dimensional carbon pentagon-constituted nanostructures (e.g., pentagraphene) with multiple sp(2)/sp(3) sublattices was studied by GGA DFT, DFTB, and model potential approaches. It was found that finite nanoclusters suffer strong uniform unit cell bending followed by breaking of crystalline lattice linear translation invariance caused by structural mechanical stress. It was shown that 2D sp(2)/sp(3) nanostructures are correlated transition states between two symmetrically equivalent bent structures. At DFT level of theory the distortion energy of the flakes (7.5 × 10(-2) eV/atom) is much higher the energy of dynamical stabilization of graphene. Strong mechanical stress prevents stabilization of the nanoclusters on any type of supports by either van der Waals or covalent bonding and should lead to formation of pentatubes, nanorings, or nanofoams rather than infinite nanoribbons or nanosheets. Formation of two-layered pentagraphene structures leads to compensation of the stress and stabilization of flat finite pentaflakes.

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Moiré diamanes (Dnθ) based on bigraphenes (BGθ) with a layer rotation angle (θ) of about 30° are considered by ab initio methods. The adsorption of hydrogens or fluorines on the bigraphene surface leads to the formation of interlayer covalent bonds. The resulting structure has only sp3-hybridized atoms, which leads to the appearance of a wide bandgap. Bandgaps of hydrogenated Dn29.4 and Dn27.8 and fluorinated F-Dn29.4 and F-Dn27.8 are 3.6, 3.3, 4.1, and 4.5 eV, respectively, which are larger than the dielectric gaps of ordinary diamanes based on the non-twisted AA- or AB-bigraphenes (≈3 eV). Possible applications of these 2D wide-gap dielectrics were also discussed.