Theoretic Insights of Design and Synthesis of Intrinsic Two-Dimensional Organic Topological Insulators

Two-dimensional organic topological insulators (OTIs) have attracted numerous attentions due to the incredible variety of organic compounds and their combinations. Up to now, designing OTIs with large gaps and realizing topological states in experiments remain as the biggest challenge in this realm. We investigate a total number of 232 honeycomb-Kagome metal-organic networks formed by two types of metal clusters and tetrahydroxybenzene (THB) molecules using high-throughput DFT calculations. Among the 232 MOFs, 16 intrinsic OTIs are discovered. Typically, the \alpha-Pb₃Zn₄-THB structure has a nontrivial gap of 97.5 meV, almost 4 times larger than its flat with bandwidth (22.5 meV), which can be an ideal platform for realizing fractional quantum Hall effect. This work provides a new avenue for designing 2D topological MOFs with large topological band gaps. To obtain topological materials with large nontrivial gaps, another strategy besides increasing SOC is to design materials with high-order topological states. Chiral symmetry breaking (CSB) is promising to realize a series of exotic topological phenomena due to the coupling of Dirac fermions at inequivalent valleys. We propose a strategy to introduce CSB by applying a novel Kekule distortion in a spinless honeycomb lattice, resulting in the emergence of second-order topological phase as the period of superlattice is 3m \times 3m (m =1, 2 ... ). Following the strategy, Si_xC_y and circumcoronene-based two-dimensional (2D) honeycomb lattices are predicted to be second-order topological insulators (SOTIs) that are identified via the topological invariants and the presence of in-gap topological corner modes. Although a few materials like Cu-dicyanoanthracene (Cu-DCA) have been experimentally synthesized, observation of topological states remain hard due to the interference of substrates. To discover suitable substrates for synthesizing Cu-DCA while preserving its topological band structures, we search for candidates in a database of 2D materials exfoliable from experimentally known compounds with 1825 2D materials. We find 35 semiconductors in the database having suitable lattice and interaction strength and 11 of them preserve the intrinsic kagome band of Cu-DCA, with transition metal dichalcogenides(TMD) materials like MoSe₂ as representative. Further ab-initial molecular dynamics simulations show that Cu-DCA remains dynamically stable on MoSe₂ substrate under 300K room temperature, and growth and expansion of the organic framework is also dynamically possible on the surface. This work provides guidance for experimental synthesis of 2D OTIs.<br/><br/>Reference:<br/>1. Yin, Y., Gao, Y., Zhang, L. <i>et al.</i> Prediction of intrinsic two-dimensional topological insulators in Y3-/X3Y4-tetrahydroxybenzene metal–organic networks. <i>Sci. China Mater.</i> <b>67</b>, 1202–1208 (2024).