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Density functional theory has been used to investigate the behavior of the π electrons in bilayer graphene and graphite under compression along the c axis. We have studied both conventional Bernal (A-B) and A-A stackings of the graphene layers. In bilayer graphene, only about 0.5% of the π-electron density is squeezed through the sp 2 network for a compression of 20%, regardless of the stacking order. However, this has a major effect, resulting in bilayer graphene being about six times softer than graphite along the c axis. Under compression along the c axis, the heavily deformed electron orbitals (mainly those of the π electrons) increase the interlayer interaction between the graphene layers as expected, but, surprisingly, to a similar extent for A-A and Bernal stackings. On the other hand, this compression shifts the in-plane phonon frequencies of A-A stacked graphene layers significantly and very differently from the Bernal stacked layers. We attribute these results to some sp 2 electrons in A-A stacking escaping the graphene plane and filling lower charge-density regions when under compression, hence, resulting in a nonmonotonic change in the sp 2-bond stiffness.

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On a long path of finding appropriate materials to store hydrogen, graphene and carbon nanotubes have drawn a lot of attention as potential storage materials. Their advantages lie at hand since those materials provide a large surface area (which can be used for physisorption), are cheap compared to metal hydrides, are abundant nearly everywhere, and most importantly, can increase safety to existing storage solutions. Therefore, a great variety of theoretical studies were employed to study those materials. After a benchmark study of different van-der-Waals corrections to Generalized Gradient Approximation (GGA), the present Density Functional Theory (DFT) study employs Tkatchenko–Scheffler (TS) correction to study the influence of vacancy and Stone–Wales defects in graphene on the physisorption of the hydrogen molecule. Furthermore, we investigate a large-angle (1,0) grain boundary as well as the adsorption behaviour of Penta-Octa-Penta (POP)-graphene.

Density functional theory has been employed to study graphene on the (111), ( 100) and ( 110) surfaces of bare silicon (Si) substrates, which provide three different densities of surface atoms. There are several interesting findings. First, carbon atoms in graphene can form covalent bonds with Si atoms, when placed close enough on Si (111) and ( 100) surfaces, but not on the (110) surface. The Si (111) surface shifts the Fermi level of graphene into its conduction band, resulting in an increase of the electron density by three orders of magnitude. The work function of graphene is increased by 0.29 eV on the (111) surface, likely due to the surface dipole from the re-distribution of π-orbitals. The change in the number of available states below the Fermi level of graphene due to its interaction with the Si surface, is the main cause for the unconventional doping reported in this paper. The electron density can also be increased by eighty times on a Si (100) substrate without the shift of Fermi level, which is another clear example of the proposed novel doping mechanism. These striking effects that different orientations of a silicon substrate can have on the properties of graphene are related to the surface atom density of the substrate. These results provide valuable guidance to the growth of graphene on Si for various purposes for electronic devices.