Tuning topological states in a Ni-hexaaminobenzene framework by NO2 adsorption

Abstract

Exploring a quantum topological phase, such as the quantum spin Hall (QSH) effect and quantum anomalous Hall (QAH) effect in two-dimensional metal-organic frameworks (MOFs), has been attracting significant attention. In this study, we propose to realize a topological phase transition via NO2 adsorption in a recently synthesized two-dimensional Ni-hexaaminobenzene [Ni3(HAB)2]. First-principles calculations show a QSH state of the pristine Ni3(HAB)2 nanosheet. However, when NO2 molecules are adsorbed on the Ni3(HAB)2 surface, the system changes to a ferromagnetic state and the time-reversal symmetry is broken. We find that the NO2-adsorbed Ni3(HAB)2 exhibits an intrinsic QAH state with the Fermi level exactly located within a topologically nontrivial bandgap. The calculated gapless edge states and quantized Hall conductance further confirm the QSH-to-QAH phase transition. Our finding offers an alternative approach of tuning the topological phase transition in a real MOF material.