A DFT study to unravel the fluorescence mechanisms of APTES-modified carbon dots

Surface passivation is a well-established method for modifying carbon dots (CDs), intended to improve their properties. We present a theoretical study employing density functional theory (DFT) and time-dependent-DFT (TD-DFT) to explain the photoluminescence (PL) mechanism of amine-modified carbon dots (CD-NH₂) [CDs modified with (3-Aminopropyl) triethoxy silane (APTES)] considering their local geometry at the terminal ends; the zig-zag (CD_ZZ-NH₂) and armchair (CD_AC-NH₂) structural orientations. The experimental evidence from our previous report suggests that the amine groups were tethered on the surface of CDs through a Si-O bond realized by the silane coupling reaction between the ethoxy group of APTES and the hydroxyl group of the CDs. The effect of pH in tweaking the PL of these systems is scrutinized in the present study. The influence of pH and structure on the bandgap of CD-NH₂ is demonstrated by analyzing the difference in HOMO-LUMO energies, the density of states (DoS) spectra, and electrostatic potentials (ESP).