The topology of two-dimensional network materials is investigated by persistent homology analysis. The constraint of two dimensions allows for a direct comparison of key persistent homology metrics (persistence diagrams, cycles, Betti numbers) with more traditional metrics such as the ring-size distributions. Two different types of networks are employed in which the topology is manipulated systematically. In the first, comparatively rigid networks are generated for a triangle-raft model, which are representative of materials such as silica bilayers. In the second, more flexible networks are generated using a bond-switching algorithm, which are representative of materials such as graphene. Bands are identified in the persistence diagrams by reference to the length-scales associated with distorted polygons. The triangle-raft models with the largest ordering allow specific bands Bn (n = 1, 2, 3,. . . ) to be allocated to configurations of atoms separated by n bonds. The persistence diagrams for the more disordered network models also display bands albeit less pronounced. The persistent homology method thereby provides information on n-body correlations that is not accessible from structure factors or radial distribution functions. An analysis of the persistent cycles gives the primitive ring statistics, provided the level of disorder is not too large. The method also gives information on the regularity of rings that is unavailable from a ring-statistics analysis. The utility of the persistent homology method is demonstrated by its application to experimentally-obtained configurations of silica bilayers and graphene.
42In this paper the utility of persistent homology will 43 be assessed using two-dimensional amorphous materials.
44The interpretation of the properties of these reduced di-45 mensional systems is simpler compared to three dimen-46 sional networks, and should bring insight to the analysis 47 of the latter. The paper begins by outlining the relevant 48 theory to persistent homology, with the aid of small ex-49 ample systems. Persistent homology is then calculated 50 for triangle rafts (a proxy for amorphous bilayers of, e.g., 51 silica), systematically generated with increasing levels of 52 disorder. The results are contrasted with those obtained 53 from generic random-networks as produced from bond 54 switching. In addition, the persistence diagrams are dis-55 cussed for configurations obtained directly from experi-56 ments on silica bilayers and graphene. Finally, the con-57 clusions of these investigations are assessed in the con-58 text of more complex systems, and the potential utility 59 of persistent homology as an analytic tool for materials 60 characterisation is examined. 61 II. OVERVIEW OF TRADITIONAL METHODS 62 Amorphous network-forming materials are usually 63 built from simple local-structural-units such as the SiO 4 64 tetrahedron in amorphous silica. Bond-length and bond-65 bending flexibility leads, however, to structural complex-66 ity as these units connect to form configurations on lon...