Theoretical investigation of functionalized diamond-like carbon with COOH, OH and NH2: a comprehensive DFT-D study

Abstract

In this study, the functionalization of diamond-like carbon (DLC) with carboxyl (COOH), hydroxyl (OH) and amine (NH₂) groups was investigated to understand its impact on the structural, electronic and nonlinear optical (NLO) properties. Dispersion-corrected density functional theory (DFT-D) calculations using the B3LYP-D3(BJ) exchange–correlation functional were performed in all calculations. The results indicated that functionalization with these groups enhanced the reactivity of the DLC surface. Molecular reactivity descriptors revealed that COOH − DLC exhibited the highest softness (S = 0.25 eV), significant electrophilicity (ω = 2.55 eV) and a reduced energy gap (∆Eg = 3.97 eV). Time-dependent DFT (TD-DFT) analysis showed that COOH − DLC achieved the maximum absorption wavelength among the systems investigated. Additionally, functionalization improved the NLO properties, including increased polarity, with COOH − DLC displaying the highest first hyperpolarizability value. Natural bond orbital (NBO) analysis indicated significant orbital delocalization between the functional groups and the pristine DLC surface. Quantum theory of atoms in molecules (QTAIM) and non-covalent interaction (NCI) analyses, based on the reduced density gradient (RDG), provided a detailed characterization of interactions, highlighting the presence of van der Waals forces