We systematically investigate the electronic structure and optical properties of edge-functionalized graphene quantum dots (GQDs) utilizing density functional and many-particle perturbation theories. A mechanism based on the competition and collaboration between frontier orbital hybridization and charge transfer is proposed. The frontier orbital hybridization of the GQD moiety and functional group reduces the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), while the charge transfer from the GQD moiety to the functional group enlarges it. Contrarily, frontier orbital hybridization and charge transfer collaborate to shift down the energy of the first bright exciton, the former through activation of low-lying dark excitons and the latter via increased exciton binding energy. Functional groups containing a carbon−oxygen double bond (CO), namely, aldehyde (−CHO), ketone (−COCH 3 ), and carboxyl (−COOH), are more favorable for tailoring the electronic and optical properties of pristine GQD among all the functional groups investigated here. The amino group (−NH 2 ), although frequently employed in experiments, has a much weaker influence on electronic structure since the large charge transfer cancels out the effect of frontier orbital hybridization.

The electronic structure and optical properties of hexagonal armchair and zigzag-edged graphene quantum dots (GQDs) are investigated within the framework of manybody perturbation theory. Many-body effects are significant due to quantum confinement and reduced screening. The quasi-particle corrections and exciton binding energies can be several eV, much larger than those of other carbon allotropes with higher dimensionality. All the GQDs show similar absorption spectra when electron−hole interaction is included, with a prominent peak emerging below the absorption onset of the noninteracting spectrum. This peak is contributed by a pair of double-degenerate excited states originating from the transitions between degenerate frontier orbitals. The spin singlet−triplet splitting is closely related to the electron−hole overlap, which can be approximately measured by the overlap between frontier orbitals involved in the optical transitions. The strong many-body effects in GQDs should be of great importance in optoelectronic applications.

Understanding electron transitions in black phosphorus nanostructures plays a crucial role in applications in electronics and optoelectronics. In this work, by employing time-dependent density functional theory calculations, we systematically study the size-dependent electronic, optical absorption, and emission properties of black phosphorus quantum dots (BPQDs). Both the electronic gap and the absorption gap follow an inversely proportional law to the diameter of BPQDs in conformity to the quantum confinement effect. In contrast, the emission gap exhibits anomalous size dependence in the range of 0.8-1.8 nm, which is blue-shifted with the increase of size. The anomaly in fact arises from the structure distortion induced by the excited-state relaxation, and it leads to a huge Stokes shift in small BPQDs.

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Nitrogen-doped graphene quantum dots (N-GQDs) hold promising application in electronics and optoelectronics because of their excellent photo-stability, tunable photoluminescence and high quantum yield. However, the absorption and emission mechanisms have been debated for years. Here, by employing time-dependent density functional theory, we demonstrate that the different N-doping types and positions give rise to different absorption and emission behaviors, which successfully addresses the inconsistency observed in different experiments. Specifically, center doping creates mid-states, rendering non-fluorescence, while edge N-doping modulates the energy levels of excited states and increases the radiation transition probability, thus enhancing fluorescence strength. More importantly, the even hybridization of frontier orbitals between edge N atoms and GQDs leads to a blue-shift of both absorption and emission spectra, while the uneven hybridization of frontier orbitals induces a red-shift. Solvent effects on N-GQDs are further explored by the conductor-like screening model and it is found that strong polarity of the solvent can cause a red-shift and enhance the intensity of both absorption and emission spectra.

The optical absorption spectra of hexagonal arsenene (β-As) and antimonene (β-Sb).

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Type-II heterostructures are appealing for application in optoelectronics due to their effective separation of photogenerated charge carriers.