The paper was published in J. Phys. Chem. Lett.
The possibility of mild oxidation of sp2-hybridized carbon gives access to graphene oxide, one of the oldest and the most extensively studied graphene derivative. The relatively inexpensive and largely available GO is an attractive material for a variety of applications in sensing, energy storage, two-dimensional electronics and optoelectronics, photocatalysis and memristors etc.
Graphene oxide is a monolayer material which further development can be devoted with the study of the thicker structure such as bilayer graphene oxide. Hydrogenation or fluorination of bilayer graphene leads to the barrier-free bonding of the layers into a sp3-hybridized structure called diamane. This effect of chemically induced transition has been numerously confirmed in the experiment.
It is important to note that despite the successful synthesis of diamane by hydrogenation and fluorination processes, in the most papers binding of graphene is associated with deposition of oxide groups on its surface. In contrast to hydrogenated and fluorinated diamane, the structure of oxidized diamane has not been studied in detail yet. Only limited number of works exist where relatively simple models are proposed. This is an obstacle to further analysis and interpretation of the experimental data. The main problem is that graphene oxide (as well as diamane oxide) can be considered as two-dimensional solid solution of different functional groups statistically distributed on the graphene surface. This is probably valid for diamane oxide as well, so the description of its structure requires its consideration as a solid solution of different functional groups as we proposed for the graphene oxide.
In the presented paper we try to fill the gap and elucidate details of diamane oxide formation along with its properties. We study the idea that oxygen-containing groups are able to break the π-system and completely cover the outer surface of multilayer graphene changing the hybridization of carbon atoms from sp2 to sp3.
First, we found energetically favorable structures of diamanes with a full surface coverage by H, -OH or peroxide functional groups. Then we revealed thermodynamic stability range depending on the external pressure and chemical environment as denoted by precursor choice. In particularly, we found that commonly used source of oxygen, H2O, requires the pressure applying to form the stable oxidized diamane which is in a full accordance with experimental data (see the figure below).
The dependence of ΔG of diamane oxide [C16H4(OH)4]m film on pressure
Next, we studied the tuning of electronic properties in energetically favorable diamane films. We showed that depending on surface OH-groups concentration the band gap of diamane oxide can vary from 4.6 eV to 6.5 eV while the effective mass ranges from 1.1 m0 to 0.6 m0.
For two most representative films, namely H-diamane and OH-diamane, we studied how their electronic states are changed by the film thickness. We demonstrated that bilayer diamane behaves as uniform semiconductor while thicker films with more than 5 layers include surface and bulk-like regions with different conduction properties (see the figure below).
a) Band gap resolved from partial DOS in H-diamane (circles) and OH-diamane (rhombuses) depending on the inversed number of carbon layers. Band gap is calculated for surface carbon atoms (solid markers) and for inner carbon atoms in the middle of diamane (empty markers). Inset shows 5-layer H-diamane with inner carbon atoms indicated; Wave functions distribution for 2- and 9-layer H- (b) and OH- (c) diamanes are plotted for bottom conduction band with green color and top valence band and the with pink color
We believe that the present work will motivate experimental groups to discover novel approaches of ultrathin diamond films fabrication and their application.