The team, led by Shaghayegh Khalilzadeh, Ehsan Koushki, and Mohammad-Reza Zamani-Meymian, synthesized two types of carbon quantum dots using simple thermal decomposition methods. The pristine sample, labeled S1, was made by heating 15 grams of citric acid at 250 degrees Celsius for 20 minutes in a solvent-free solid-phase process, yielding a carbonized powder with a consistent reaction mass yield of roughly 42 percent across three replicate batches. The co-doped sample, S2, was produced by dissolving citric acid and thiourea in water and heating the mixture at 180 degrees Celsius for 40 minutes, a gentler protocol chosen specifically to prevent the degradation of thiourea and to optimize the incorporation of nitrogen and sulfur into the carbon framework. Under ultraviolet illumination at 365 nanometers, the two samples glow distinctly: S1 emits blue light, while S2 glows blue-green.

Confirming that the doping had genuinely worked required a battery of characterization techniques. Energy-dispersive X-ray spectroscopy revealed that the pristine sample contained only carbon, oxygen, and sodium, with no detectable nitrogen or sulfur, while the co-doped sample showed weight percentages of 1.28 percent nitrogen and 3.95 percent sulfur. The carbon-to-sulfur weight ratio of approximately 2 to 1 matched the ideal values reported in prior literature. Elemental mapping went further, demonstrating that the nitrogen and sulfur signals overlapped precisely with the carbon framework, proving that the heteroatoms were chemically integrated into the lattice rather than sitting in separate clusters. X-ray diffraction backed this up: both samples showed the broad (002) peak of amorphous carbon at 2 theta equals 20 degrees, but the doped sample displayed new peaks at roughly 19, 28, and 32 degrees, signatures of lattice strain from larger sulfur atoms, graphitic carbon nitride-like domains from carbon-nitrogen bonding, and localized ordering at sulfur-decorated lattice edges.

High-resolution transmission electron microscopy of the pristine sample revealed well-dispersed spherical nanoparticles with an average diameter of about 4 nanometers and clear lattice fringes with a spacing of 0.34 nanometers, corresponding to the (200) plane of graphitic structure. Fourier-transform infrared spectroscopy completed the chemical picture. The pristine dots showed the broad hydroxyl stretching band between 3000 and 3500 inverse centimeters and carboxyl peaks expected of an oxidized carbon surface, while the co-doped sample developed intense new peaks between 1030 and 1150 inverse centimeters assigned to carbon-sulfur and sulfonic group vibrations, plus an enhanced signal at 1380 inverse centimeters from covalent carbon-nitrogen stretching. Together, these results describe what the authors call bond engineering: a stable hybrid nanostructure in which nitrogen and sulfur are covalently woven into the carbon matrix, fundamentally altering its electronic charge density.

The optical consequences of this structural surgery were striking. Ultraviolet-visible absorption spectra showed a dominant band near 230 nanometers from pi-to-pi-star transitions in the aromatic carbon domains, plus a shoulder between 270 and 320 nanometers from n-to-pi-star transitions involving lone-pair electrons of the heteroatoms. In the doped sample, the absorption edge red-shifted and a pronounced tail extended into the visible range, evidence that nitrogen and sulfur had introduced new intermediate energy states between the intrinsic frontier orbitals. Yet when the team quantified the optical bandgap using Tauc analysis with an allowed direct transition exponent, they found the gap had actually widened, from 3.20 electron volts in the pristine sample to 3.45 electron volts in the doped one. This apparent paradox, a blue-shifted bandgap alongside a red-shifted absorption tail, is explained by the Burstein-Moss effect, in which nitrogen and sulfur act as electron donors that fill the lower states of the conduction band, forcing valence electrons to absorb higher-energy photons to find unoccupied states. The larger atomic radius of sulfur also induces lattice strain that fragments the extended sp2 domains into smaller clusters, strengthening the quantum confinement effect and further widening the gap.

Photoluminescence measurements told an equally compelling story. The pristine sample displayed a dual-peak emission profile, with a higher-energy peak from the intrinsic graphitic core and a lower-energy peak from oxygen-containing surface states, plus a broad near-infrared band around 760 nanometers signaling deep defect traps. After co-doping, this complicated landscape collapsed into a single, intense green emission band centered near 500 nanometers. The dopant-induced surface states act as efficient carrier trapping centers that quench the intrinsic core emission and channel all radiative recombination through one low-energy pathway. The practical payoff was dramatic: the photoluminescence intensity of the doped sample increased more than 3.5-fold, and the fluorescence quantum yield, measured against a quinine sulfate standard, jumped from 4.2 percent to 32.5 percent. The nitrogen and sulfur atoms appear to heal surface defects and dangling bonds, suppressing non-radiative recombination and preserving excitation energy for light emission.

The centerpiece of the study, however, was the Z-scan investigation of nonlinear optics. In this technique, a sample is translated through the focus of a laser beam while its transmittance is recorded, allowing researchers to extract the nonlinear absorption coefficient and the nonlinear refractive index. The team used three continuous-wave lasers at 405, 532, and 633 nanometers, violet, green, and red, at powers of 5, 10, and 15 milliwatts, with peak intensities ranging from about 1.62 to 4.84 kilowatts per square centimeter. Control measurements on the pure solvent confirmed that the nonlinear signals originated from the nanodots themselves. In the open-aperture configuration, all samples showed positive nonlinear absorption, which the authors attribute to electrostriction: polarizable nanoparticles are drawn toward the center of the beam by the electric field gradient, increasing local concentration and absorption.

The doped sample outperformed the pristine one decisively. Under green excitation at 5 milliwatts, the nonlinear absorption coefficient of the co-doped dots reached 9.74 times ten to the minus three centimeters per watt, roughly five times higher than the pristine value and orders of magnitude above many recently reported carbon-based nanomaterials, including sugarcane-derived carbon dots and yttrium-doped carbon nanodots measured at the same wavelength. The closed-aperture measurements revealed negative nonlinear refractive indices in all cases, confirming self-defocusing behavior driven by thermal lensing, in which absorbed energy heats the solvent and lowers the refractive index at the beam center. The doped sample again showed larger on-axis phase shifts, thanks to the higher polarizability imparted by the bulky, electron-rich sulfur atoms.

Perhaps the most counterintuitive finding concerned wavelength. One might expect violet photons, with energy closest to the 3.45-electron-volt bandgap, to produce the strongest electronic response. Instead, the red laser generally yielded the largest nonlinear coefficients. The authors argue that at these low powers the response is not governed by electronic transitions at all, but by the classical mechanics of electrostriction: lower-frequency red light exerts its driving force on the nanoparticles over longer time periods, making it more effective at pulling the particles toward the beam center. This frequency matching to the mechanical response of nanometer-scale particles, rather than to their electronic energy levels, is a distinctive feature of the continuous-wave regime, where the measured coefficients are effective, thermally dominated quantities rather than the ultrafast electronic Kerr responses captured by pulsed lasers.

The implications reach well beyond the laboratory bench. Materials that change their absorption and refraction in response to light intensity are the working elements of optical limiters, which protect sensitive sensors and human eyes from damaging laser exposure, as well as optical switches and modulators for photonic circuits. Because the co-doped carbon dots deliver their strongest nonlinear response under cheap, low-power continuous-wave diode lasers, and because they are biocompatible, water-soluble, and free of the toxic heavy metals found in first-generation quantum dots, they are unusually well suited to practical, cost-effective photonic devices. The study also delivers a conceptual lesson for the field: doping does not just shift energy levels, it reshapes the entire interplay between electronic structure, polarizability, and the mechanical forces that light exerts on matter. By widening the core bandgap while simultaneously seeding dense sub-bandgap surface states, nitrogen and sulfur co-doping creates a material whose linear and nonlinear optical personalities can be tuned almost independently, a degree of control that could define the next generation of carbon-based photonics.

Subject of Research: Bandgap engineering and nonlinear optics of nitrogen and sulfur co-doped carbon quantum dots

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

Article References: Synthesis, characterization and low power Z -scan study of bandgap-engineered N, S-co-doped carbon quantum dots using lasers of different colors. (n.d.). https://doi.org/10.1016/j.rio.2026.101169

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101169

Keywords: carbon quantum dots, nitrogen sulfur co-doping, nonlinear optics, Z-scan, bandgap engineering, photoluminescence, electrostriction, thermal lensing, optical limiting, nanomaterials, photonics, Burstein-Moss effect