▎ 摘　　要

A high-symmetry D-6h form of graphene monoxide, labeled 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 D-2h form of two-dimensional (2D) solid CO, now relabeled as alpha-GmO, this work demonstrates that multiple solid phases are possible in 2D- and 3D-CO. A unit of the beta-GmO primitive cell - 1,4,7-trioxacyclononane - is composed of three 1,3-dioxetane units of the alpha-GmO primitive cell. The invariance of the SAED spots as the sample is tilted prove that beta-GmO is composed of large area monolayers, randomly translated with respect to each other, as they form a multilayer stack. In contrast, 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 beta-GmO monolayers generally do not have preferred stacking order and thus do not form three-dimensional (3D) crystal structure. The beta-GmO monolayer is predicted to be a 1.2 eV direct band-gap semiconductor and generally softer than both graphene and alpha-GmO.