β-graphene monoxide: Two-dimensional crystal form of carbon monoxide

A high-symmetry ${\mathrm{D}}_{6h}$ form of graphene monoxide, labeled $\ensuremath{\beta}$-GmO, is proposed based on selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) experiments and density functional theory (DFT) modeling. Joining the previously observed lower symmetry ${\mathrm{D}}_{2h}$ form of two-dimensional (2D) solid CO, now relabeled as $\ensuremath{\alpha}$-GmO, this work demonstrates that multiple solid phases are possible in 2D- and 3D-CO. A unit of the $\ensuremath{\beta}$-GmO primitive cell -- 1,4,7-trioxacyclononane -- is composed of three 1,3-dioxetane units of the $\ensuremath{\alpha}$-GmO primitive cell. The invariance of the SAED spots as the sample is tilted prove that $\ensuremath{\beta}$-GmO is composed of large area monolayers, randomly translated with respect to each other, as they form a multilayer stack. In contrast, $\ensuremath{\alpha}$-GmO domains rotate with respect to each other and have smaller in-plane domain areas for similar synthesis conditions. Ab initio computations demonstrate that $\ensuremath{\beta}$-GmO monolayers generally do not have preferred stacking order and thus do not form three-dimensional (3D) crystal structure. The $\ensuremath{\beta}$-GmO monolayer is predicted to be a 1.2 eV direct band-gap semiconductor and generally softer than both graphene and $\ensuremath{\alpha}$-GmO.