Topological Impact of Delocalization on the Stability and Bandgap of Partially Oxidized Graphene

Abstract

Strategic perturbations on graphene framework to inflict a tunable energy bandgap promises intelligent electronics that are smaller, faster, flexible, and much more efficient than silicon. Despite different chemical schemes, a clear scalable strategy for micromanaging the bandgap is lagging. Since conductivity arises from the delocalized π-electrons, chemical intuition suggests that selective saturation of some sp2 carbons will allow strategic control over the bandgap. However, the logical cognition of different 2D π-delocalization topologies is complex. Their impact on the thermodynamic stability and bandgap remains unknown. Using partially oxidized graphene with its facile and reversible epoxides, we show that delocalization overwhelmingly influences the nature of the frontier bands. Organic electronic effects like hyperconjugation, conjugation, aromaticity, etc., can be used effectively to understand the impact of delocalization. By keeping a constant C4O stoichiometry, the relative stability of various π-delocalization topologies is directly assessed without resorting to resonance energy concepts. Our results demonstrate that >C=C< and aromatic sextets are the two fundamental blocks resulting in a large bandgap in isolation. Extending the delocalization across these units will increase the stability at the expense of the band gap. The bandgap is directly related to the extent of bond alternation within the π-framework, with forced single/double bonds causing the large gap. Furthermore, it also establishes the ground rules for the thermodynamic stability associated with the π-delocalization in 2D systems. We anticipate our findings will provide the heuristic guidance for designing partially saturated graphene with desired bandgap and stability using chemical intuition.

Alternatives for Epoxides in Graphene Oxide

“…We hope that the planarity of the carbon framework and other structural requirements required for various functional groups unveiled here will provide valuable chemical insights into the structure of GO in its various avatars and the mechanistic pathways for the formation of different suspected functional groups. The conceptual framework provided for the binary C 2 O phase of GO is also generalizable for other compositions such as C 4 O without significant loss of generality 52 and will also provide the logical first step that will aid further studies on its hydrated products.…”

“…As in stoichiometric graphene epoxides, 3,6 steric repulsion from oxygen lone pairs correlates with their relative stability. As shown in Figure 6A, the epoxy groups are positioned in the trans arrangement within the hexagon, and the cis-epoxy interactions across the adjacent hexagonal rings are moderated by the splaying of the oxygen atoms.…”

“…Surprisingly, an isomer with isolated double bonds (Figure 10) is marginally more stable than the epoxide reference (Figure 7A) despite the short O•••O distances arising from its parallel arrangement of 1,3-dioxetane groups. The increased stability arises from the pronounced hyperconjugation involving σ-C−O → π*-C�C donation that switches on the pseudo-aromaticity 6 within the hexagonal ring. Its impact is reflected in its metallic band structure as the conduction π*bands cross the Fermi along the direction of the π-bonds, stealing electrons from the oxygen lone pairs.…”

“…Even if one overlooks the finite presence of hydrogen and its nonstoichiometry, the inherent hexagonal symmetry of its graphene backbone leads to isomers that grow exponentially with the unit cell size for any oxygen content. 3,6 Disorder originates not only from the various possible cyclic ether groups arising from oxygen bridges and carbonyls but also due to their relative placement on the lattice, leading to topological isomerism.…”

“…3,6,27−29 Earlier, we rationalized the structural preferences and their preferred distribution by chemical bonding analysis. 3,6 Intriguingly, there are no exclusive theoretical studies reported on 1,3-ethers. However, they are not ruled out experimentally 23 and there are lingering doubts about their finite presence in GO.…”