A semimetallic square-octagon (fes) two-dimensional polymer with high mobility

TL;DR

This study explores the electronic and topological properties of a square-octagon (fes) 2D polymer network using tight-binding calculations, revealing a semimetallic phase with high mobility and tunable band gaps.

Contribution

It demonstrates the feasibility of designing 2D polymers with fes structure that replicate the band features of the fundamental network, enabling targeted electronic property tuning.

Findings

Identified a semimetallic configuration with very low effective electron and hole masses.

Showed that linker length variations can tune band gaps in fes 2D polymers.

Found no large topologically non-trivial band gaps in the studied configurations.

Abstract

The electronic properties of $\pi$-conjugated two-dimensional (2D) polymers near the Fermi level are determined by structural topology and chemical composition. Thus, tight-binding (TB) calculations of the corresponding fundamental network can be used to explore the parameter space to find configurations with intriguing properties before designing the the atomistic 2D polymer network. The vertex-transitive \textbf{fes} lattice, which is also called square-octagon lattice, is rich in interesting topological features including Dirac points and flat bands. Herein, we study its electronic and topological properties within the TB framework using representative parameters for chemical systems. Secondly, we demonstrate that the rational implementation of band structure features obtained from TB calculations into 2D polymers is feasible with a family of 2D polymers possessing \textbf{fes} structure. A one-to-one band structure correspondence between fundamental network and 2D polymers is found. Moreover, changing the relative length of linkers connecting the triangulene units in the 2D polymers reflect tuning of hopping parameters in the TB model. These perturbations allow to open sizeable local band gaps at various positions in the Brillouin zone. From analysis of Berry curvature flux, none of the polymers exhibits a large topologically non-trivial band gap. However, we find a particular configuration of semimetallic characteristics with separate electron and hole pockets, which possess very low effective masses both for electrons (as small as $m^*_\mathrm{e} = 0.05$) and holes (as small as $m^*_\mathrm{h} = 0.01$).