Carbon quantum dots (CQDs) are tiny carbon-based nanomaterials that have attracted increasing attention as environmentally friendly alternatives to conventional heavy-metal quantum dots. They are lightweight, photostable, and potentially biocompatible, and their light absorption and emission properties can be tuned. These features make CQDs promising for a wide range of applications, including fluorescence sensing, bioimaging, cancer-related photothermal technologies, optoelectronic devices, and solar energy conversion.
However, a major challenge remains: researchers still lack a sufficient atomistic understanding of how CQDs interact with light and how this interaction can be precisely controlled. Conventional approaches, such as adjusting particle size or introducing surface functional groups, can tune optical properties to some extent. Yet a predictive framework for designing CQDs that operate at specific wavelengths, especially in technologically important visible and near-infrared regions, has not been fully established.
In recent years, "defect engineering" has emerged as a powerful strategy to address this challenge. Rather than treating defects as undesirable imperfections, researchers are beginning to use vacancies, nitrogen dopants, oxygen-containing surface functional groups, and combined defects as design elements for encoding optical functions into carbon nanomaterials. However, the fundamental mechanisms by which specific defects alter electronic structure, excitonic behavior, and optical response have remained insufficiently understood.
In this study, a research team at the Graduate School of Science and Engineering, Saitama University, led by Dr. Christian Ebere Enyoh and Professor Emeritus Wang Qingyue, aimed to establish a predictive framework linking structure and properties in defect-encoded CQDs. Using density functional theory, time-dependent density functional theory, and Hirshfeld population analysis, the team systematically compared eight CQD models: pristine CQDs, core vacancy, edge vacancy, graphitic nitrogen, pyridinic nitrogen, pyrrolic nitrogen, oxygen-containing surface functionalization, and a combined edge vacancy–graphitic nitrogen structure. Through this approach, the team clarified how the type and position of defects determine the electronic structure, excitonic behavior, and optical properties of CQDs. The study was published online on May 25, 2026, in Computational Materials Science.
Key findings of the study include:
- The type and position of defects substantially reshaped the electronic structure of CQDs, changing the HOMO–LUMO gap from 2.77 eV in pristine CQDs to 1.08 eV in CQDs with a core vacancy.
- The simulated optical response was tunable over a broad spectral range, from 313 nm in the ultraviolet region to 1193 nm in the near-infrared region, corresponding to an approximately 880 nm window.
- Each defect played a distinct electronic role. Graphitic nitrogen and edge vacancy–graphitic nitrogen behaved as n-type dopants, while vacancy defects introduced dangling-bond-derived states within the band gap. Pyridinic and pyrrolic nitrogen localized frontier orbitals at edge sites, and oxygen-containing surface functional groups generated trap states.
- The calculations identified three application-relevant optical regimes: UV-bright emitters, visible-light absorbers, and near-infrared-active materials.
- Exciton binding energies varied widely, indicating a transition from weakly bound or charge-transfer excitations in vacancy-containing structures to strongly localized excitons in nitrogen-doped CQDs.
"These results show that defects in carbon quantum dots should not simply be regarded as structural imperfections," says Dr. Enyoh. "By choosing the type, position, and combination of defects, we can design how CQDs absorb light, redistribute charge, and form excitons. In that sense, defect encoding offers a design language for carbon-based optical nanomaterials."
The research team emphasizes that, although this is a computational study, it has direct relevance to experimental materials design. By clarifying which defect structures are likely to produce ultraviolet, visible, or near-infrared optical responses, the study provides a roadmap for experimental researchers seeking to synthesize CQDs with targeted functions.
"Our work provides atomistic guidelines that can help reduce trial-and-error in CQD development," says Dr. Enyoh. "For example, edge-vacancy structures may be useful for bright UV-responsive systems, while pyridinic nitrogen defects may guide the design of near-infrared-active CQDs. Such knowledge is important for moving from empirical synthesis toward rational nanomaterial design."
Looking ahead, these findings may contribute to the development of CQD-based technologies over the next five to ten years. Because CQDs are composed mainly of carbon, they may offer advantages in applications where low toxicity, sustainability, and wavelength tunability are important.
"In the longer term, predictive defect engineering could advance the creation of metal-free nanomaterials for optical sensors, photocatalysts, light-harvesting systems, bioimaging probes, and photothermal technologies," says Dr. Enyoh. "If experimental synthesis can be guided by these computational design rules, CQDs may become more practical material components in healthcare, environmental monitoring, renewable energy, and next-generation optoelectronic industries."
