The optical properties of carbon quantum dots (CQDs) have garnered significant attention due to their unique photoluminescence (PL) behavior, which distinguishes them from conventional semiconductor quantum dots. Unlike traditional quantum dots, whose emission is primarily governed by quantum confinement effects, CQDs exhibit complex PL mechanisms influenced by multiple factors, including their carbon core structure, surface functional groups, and chemical environment. Understanding these mechanisms is critical for optimizing their optical performance in various fields.

One of the most intriguing aspects of CQDs is their excitation-dependent PL behavior. Many CQDs display a shift in emission wavelength as the excitation wavelength changes, a phenomenon attributed to the presence of multiple emissive states. These states can arise from different surface functional groups, varying degrees of oxidation, or size distribution within the CQD ensemble. For example, CQDs with abundant oxygen-containing groups often exhibit red-shifted emission when excited at longer wavelengths due to transitions involving surface-related energy levels. In contrast, some CQDs demonstrate excitation-independent emission, suggesting a more uniform electronic structure or dominant emission from a single type of emissive center.

The quantum yield (QY) of CQDs, which measures the efficiency of photon emission relative to photon absorption, varies widely depending on synthesis methods and post-treatment processes. While some CQDs achieve QYs exceeding 80%, others may exhibit much lower values, often below 10%. The QY is strongly influenced by surface passivation—introducing nitrogen or sulfur doping, for instance, can significantly enhance PL efficiency by reducing non-radiative recombination pathways. Additionally, the presence of molecular fluorophores, either bound to the CQD surface or embedded within the carbon matrix, can contribute to high QYs in certain systems.

A key feature of CQD PL is the Stokes shift, the energy difference between the absorption and emission peaks. The magnitude of this shift provides insights into the relaxation processes occurring after photoexcitation. In CQDs, large Stokes shifts are often observed, indicating substantial structural reorganization or energy transfer between different emissive sites before radiative decay occurs. This behavior contrasts with traditional quantum dots, where Stokes shifts are typically smaller due to more rigid electronic structures.

Several theories have been proposed to explain the PL mechanisms in CQDs. The surface state emission model suggests that PL originates from electronic transitions involving surface functional groups such as carboxyl, hydroxyl, or amine groups. These groups introduce localized energy states within the bandgap, leading to tunable emission depending on their chemical nature and density. Another theory emphasizes the role of carbon core states, where sp2-conjugated domains act as primary luminescent centers. The size and distribution of these domains determine the emission wavelength, with larger sp2 clusters generally associated with longer-wavelength emission.

A third perspective attributes PL to molecular fluorophores either adsorbed on the CQD surface or formed during synthesis. These fluorophores, which may include polycyclic aromatic hydrocarbons or other small organic molecules, can dominate the optical properties if present in sufficient quantities. Distinguishing between these mechanisms remains challenging, as multiple contributions often coexist in a single CQD sample.

The size of CQDs plays a crucial role in their optical properties, though the relationship is less straightforward than in traditional quantum dots. While quantum confinement effects are observable in very small CQDs (below 5 nm), the influence of surface chemistry often overshadows size-dependent trends. Larger CQDs may exhibit red-shifted emission due to extended π-conjugation, but surface oxidation or functionalization can override this trend.

Surface groups are among the most significant factors governing PL behavior. Oxygen-rich CQDs tend to emit in the blue to green range, while nitrogen or sulfur doping can shift emission toward longer wavelengths. Protonation or deprotonation of surface groups can also alter PL, enabling pH-sensitive emission in some cases. Doping with heteroatoms like boron, phosphorus, or metals further modifies electronic structure, introducing new energy levels that affect both emission color and QY.

Compared to traditional semiconductor quantum dots (G35), CQDs offer distinct advantages and differences in optical properties. Traditional quantum dots exhibit narrow, size-tunable emission peaks due to quantum confinement, with high QYs and minimal Stokes shifts. In contrast, CQDs often show broad emission spectra, excitation-dependent behavior, and larger Stokes shifts. While traditional quantum dots rely on heavy metals like cadmium or lead, CQDs are composed of non-toxic carbon, making them more environmentally friendly. However, the PL mechanisms in CQDs are less predictable and more sensitive to environmental factors such as solvent polarity or temperature.

The structural complexity of CQDs also leads to varied responses to external stimuli. For example, some CQDs exhibit PL quenching in the presence of specific metal ions or organic molecules, enabling sensing applications. Others show enhanced emission under certain conditions due to aggregation-induced emission effects. The interplay between intrinsic carbon core states and extrinsic surface states creates a rich optical landscape that is still being deciphered.

In summary, the photoluminescence of carbon quantum dots arises from a combination of factors, including surface states, carbon core configurations, and embedded molecular fluorophores. Their excitation-dependent or independent behavior, variable quantum yields, and significant Stokes shifts reflect the heterogeneity of their structure and composition. While traditional quantum dots offer precise control over emission through size manipulation, CQDs provide a more complex but versatile system where surface chemistry and doping play pivotal roles. Continued research into these mechanisms will further elucidate the relationship between structure and optical properties, paving the way for tailored designs in optoelectronics, sensing, and beyond.