The paper was published in Nanomaterials.
The synthesis of a 2D diamond is the most challenging field since unlike graphene and many other two-dimensional materials, diamane cannot be cleaved from the bulk. Moreover, a thermodynamic analysis shows that a few-layered diamond film without coverage layer is simply unstable and decomposes into multilayered graphene because the diamond surface energy is higher than the one of graphite. This conclusion is well supported by experiment where the direct pressure in diamond anvil cells was used to induce conversion of the whole graphene-flake while the diamondization pressure was much higher than in the bulk case, and instability of the formed diamondized film was apparent after pressure release.
The most promising way to obtain a two-dimensional diamond seems to be the use of graphene as a precursor, by deposition of reference atoms (e.g. hydrogen) on its surface. In this case the thermodynamic stability of the material is reversed, the previously unstable diamond film becomes energy favorable and graphene layers tend to bond to each other. Despite a number of encouraging experimental results confirming such predictions, the question of diamane synthesis is far from being resolved. Indeed, the nucleation of the diamane in graphene is hindered by the high stability of the graphene π-system resisting attachment of reference atoms. As a result, only two layers of graphene can be connected relatively easily, and only in the case of using hydrogen plasma as a hydrogen source. In the case of using H2, we can expect the appearance of a significant nucleation barrier which can be overcome only by high pressure and temperature.
However, the real structure of graphene contains structural defects that can be used as nucleation centers which may allow the synthesis of diamane under less severe conditions. Here we investigated such an effect in detail. For this purpose, we studied ones of the most common structural defects in graphene and reveal their impact on diamond nucleation. We found that type and concentration of structural defects can sufficiently impact the initial stages of diamond nucleation. At the same time, it does not influence further diamond formation. Defects impact on C-H bonding strength is being disappeared at the second coordination sphere already. We show that vacancies agglomeration (that can be produced by low energy ion irradiation) can sufficiently expand the reactive region which vanishes the nucleation barrier for the first stages of nucleation (see the figure below).
Average binding energy as a function of numbers of atoms in H cluster for AB bilayer graphene contained 1 (blue line), 2 (red line), 3 (yellow line) and 4 (purple line) cross layer vacancies. In the inset top view of graphene with 4 vacancies agglomerate is presented. H bonding energies in the H2 molecule (εH2) and in infinite diamane (εb(∞)) are marked by dashed and solid horizontal lines, respectively. The εb(n) dependence for hydrogen on the surface of defectless AB bilayer graphene is shown by black
Stone-Wales defects impact is lower but still promotes the hydrogenation and bonding of the graphene layers. We show that 1D defect (dislocation) not only facilitates the diamondization but also may lead to the appearance of 2D diamond consisting of chemically connected grains of different crystallographic orientations. Therefore, polycrystalline graphene usually observed in the experiment can produce specific 2D diamond polycrystals containing different surfaces. Even hexagonal and cubic 2D diamonds can coexist together in the same film with grain boundary energy comparable with the same values for other two-dimensional carbon structures (see the figure below).
(left) the atomic structure of polycrystalline bilayer graphene containing symmetrically inclined grains by θ = 11.5° with 5|7 defects highlighted by blue and red colors corresponding to first and second layer, respectively; (right) atomic structure of polycrystalline diamane produced by hydrogenation of graphene presented in left consisting of grains with cubic diamond and lonsdaleite structures.