Synergistic effects of size and nitrogen doping in modulating the optical and electronic properties of biphenylene quantum dots: a theoretical study

Size modulation and nitrogen doping are considered potential strategies for tuning the optical and electronic properties of biphenylene quantum dots (BPQDs), yet the underlying synergistic mechanisms between these two factors remain unclear. In this work, density functional theory (DFT) and time-dependent DFT (TDDFT) were employed to systematically investigate the combined effects of nine sizes, three nitrogen doping types (edge, surface and pyridinic) and three doping concentrations (2N, 4N, 6N) on the structural, optical and electronic behaviour of BPQDs. The results reveal distinct dependencies of optical and electronic modulation on dopant type and quantum dot size. Edge and surface nitrogen doping consistently lead to red-shifted absorption and band-gap narrowing across all sizes and concentrations, with modulation strength strongly dependent on both size and doping concentration. A blue shift appears exclusively in the large-sized, high-concentration surface-doped system (surf-6N-C96). By contrast, pyridinic doping exhibits a uniformly weak effect, leading to slight red shifts and band-gap reductions that are essentially independent of both size and concentration. These findings highlight the type-specific and size-dependent nature of doping in BPQDs and provide mechanistic insights into the synergistic regulation of their optoelectronic properties, offering theoretical guidance for the rational design of carbon-based quantum materials.