Tuning electronic and optical properties of a new class of covalent organic frameworks

Abstract

Covalent organic frameworks (COFs) are the new emerging functional materials for constructing novel electronic and optoelectronic devices. However, designing COFs with tunable electronic and optical properties is still a critical challenge of paramount significance. In this work, we demonstrate a novel and simple approach to tuning the electronic and optical properties of a new class of three-dimensional covalent organic frameworks – (X₄Y)(O₂B–C₆H₄–BO₂)₃. Boronic acid (O₂B–C₆H₄–BO₂) being the linker group and X₄Y being the node, we show that the band gap of these COFs can be tuned desirably via node-alteration in (X₄Y)(O₂B–C₆H₄–BO₂)₃, viz., by changing the elemental combinations in (X₄Y). Using density functional theory, these COFs with X = C and Si and Y = C, Si, Ge, Sn, and Pb are predicted to have a high thermodynamic stability, suggesting that these structures can be experimentally accessible under suitable conditions. Lattice parameter, bulk modulus, formation enthalpy, chemical bonding, band gap and optical properties are shown to systematically vary as a function of X and Y. All COFs in the series are found to be semiconductors with band gaps ranging from 2.7 to 3.8 eV. The bulk moduli of the current COFs are found to be larger than that of MOF-5, indicating a robust framework stability of the predicted COFs. Chemical bonding analysis indicates a predominantly covalent bonding along with a modest ionic bonding, which can be tuned from X₄C to X₄Pb. The optical response of current COFs can be systematically tuned from the UV to the visible spectrum. These findings will pave the way for the utilization of COFs in various applications such as in photovoltaics, photocatalysts, hybrid solar cells, electroluminescence cells and light-emitting, optoelectronic and nanoelectronic devices.