Synthesis, characterization and low power Z-scan study of bandgap-engineered N, S-co-doped carbon quantum dots using lasers of different colors

Abstract

The tailoring of electronic states in carbon-based nanostructures through heteroatom doping remains a critical frontier for advanced optoelectronic applications. In this work, N, S-co-doped carbon quantum dots (N, S-CQDs) were synthesized via a facile hydrothermal method, and their linear and thermos-optical (TO) properties were systematically compared with those of pristine CQDs (S1). Upon N, S-co-doping, an apparent discrepancy was observed: The Tauc plot revealed an intrinsic core bandgap widening (blue-shift), whereas the photoluminescence (PL) spectra and TO responses shifted toward longer wavelengths (red-shift). This behavior is elucidated by a model where doping simultaneously widens the core bandgap and introduces dense, lower-energy surface states within the forbidden zone, dominating the radiative recombination. TO properties were investigated using the Z-scan technique under continuous wave (CW) laser excitation at multiple wavelengths: violet (405 nm), green (532 nm), and red (650 nm). The experimental results revealed that sample S2 exhibits significantly enhanced nonlinear absorption coefficients ($\beta$) and nonlinear refractive indices ($n_2$) compared to S1, identifying a robust Reverse Saturable Absorption (RSA) and self-defocusing behavior. Notably, the TO response reached its maximum in the red region, where the excitation energy enters into resonance with the dopant-induced surface levels. These findings provide a comprehensive framework for bandgap engineering in CQDs and demonstrate their high efficacy as versatile materials for multi-wavelength optical limiting and nonlinear photonic devices.