Tuning the optoelectronic properties of graphene quantum dots by BN-ring doping: A density functional theory study

Graphene monolayer is a material with zero band gap, because of which its applications in optoelectronics are limited. The question arises, can we modify the optoelectronic properties of graphene by doping it with other atoms? Synthesis of 2D monolayer of graphene doped with hetero-atoms such as boron and nitrogen, and a few computational studies of their structural and electronic properties were previously reported. In this work, we aim to answer this question for graphene quantum dots (GQDs) by replacing their carbon rings with $(BN)_3$ (borazine) hexagonal rings. We have studied in detail the geometry, electronic structure, and optical absorption spectra of fourteen different borazine-ring doped diamond-shaped GQDs using first-principles density functional theory (DFT). These BN-GQDs differ in the location, orientation, and the number of borazine rings. We computed their optical absorption spectra using time-dependent DFT (TDDFT) and examined: (a) for single-ring doped BN-GQDs the influence of ring location on optical properties, and (b) for double-ring doped systems, the influence of location, mutual distance and orientation of the rings on their absorption spectra. Frontier molecular orbitals are studied in detail to understand the nature of low-lying optical excitations. We also performed a group-theoretic analysis of the influence of their reduced symmetries on their optical properties. Our results indicate that BN-ring doping can achieve significant control over the optical properties of GQDs. The comparison of the optical absorption spectra of the BN-GQDs with the parent GQD shows remarkable spectral broadening with optical gap spanning over infrared to visible region. Thus, systematic BN-ring doping provides easy tunability of the electronic and optical properties of BN-GQDs, which is very promising for optoelectronic applications.

💡 Research Summary

This study investigates how the optoelectronic properties of graphene quantum dots (GQDs) can be systematically tuned by substituting carbon hexagonal rings with borazine ((BN)₃) rings. The authors selected a diamond‑shaped, D₂h‑symmetric pristine GQD consisting of 30 carbon atoms and 14 hydrogens (C₃₀H₁₄) as the host structure. Fourteen distinct doped configurations were generated, covering three doping categories: (i) single‑ring substitution (four positional isomers), (ii) two fused (BN)₃ rings (four positional isomers), and (iii) two separated (BN)₃ rings (six isomers, each with parallel ↑↑ or antiparallel ↑↓ orientation). All edge carbon atoms were hydrogen‑terminated to ensure chemical stability.

All calculations were performed with Gaussian 16 using an all‑electron DFT approach. The hybrid B3LYP functional was employed for geometry optimizations, electronic structure, and TDDFT‑based optical spectra, together with the 6‑31++G(d,p) basis set, which includes polarization and diffuse functions. Convergence criteria of 10⁻⁸ Hartree for SCF cycles and stringent force/displacement thresholds were applied. Vibrational frequency analyses confirmed the absence of imaginary modes, establishing that every structure is a true minimum on the potential energy surface. For validation, selected TDDFT spectra were recomputed with the screened hybrid HSE06 functional, yielding essentially identical results.

Geometrically, all doped structures remain planar; C–C bond lengths vary between 1.358 Å and 1.445 Å, while B–N, B–C and N–C bonds cluster around the ideal 1.40 Å. Mulliken charge analysis shows the expected polarity: boron atoms carry positive partial charges, nitrogen atoms negative, and carbon atoms display both signs depending on proximity to B or N. The total charge of each system remains zero, confirming overall neutrality.

Cohesive (binding) energies per atom were calculated as E_coh = (E_total – Σ n_i E_i)/N_total. All values are negative, indicating thermodynamic stability. Structures containing two fused (BN)₃ rings (models 5–8) exhibit the lowest magnitude (≈ –4.4 eV/atom), suggesting that higher local BN concentration stabilizes the lattice. The remaining systems, whether single‑ring or separated‑ring doped, have larger cohesive energies (≈ –6.5 eV/atom or more). Notably, single‑ring doped structures are about 0.2 eV/atom less stable than the separated‑ring counterparts.

Frontier molecular orbital (FMO) analysis reveals that both HOMO and LUMO retain a π/π* character delocalized over the carbon framework, with significant contributions from the B and N atoms of the borazine rings. Partial density of states (PDOS) confirms that B‑p and N‑p orbitals participate in both valence and conduction bands, effectively widening the band gap relative to the pristine GQD.

Time‑dependent DFT (TDDFT) was used to compute vertical excitation spectra up to ~5 eV. For single‑ring doped GQDs, the first intense absorption peak shifts modestly (≈ 0.2 eV) depending on the ring’s location (edge vs. central). In the double‑ring series, the optical response becomes highly sensitive to inter‑ring distance and relative orientation. Parallel orientation (↑↑) leads to constructive coupling of transition dipoles, producing strong absorption bands that span from the near‑infrared (≈ 1.5 eV) to the visible region (≈ 3.0 eV). Antiparallel orientation (↑↓) partially cancels dipole moments, resulting in weaker intensities and slight blue‑shifts. The mutual distance also modulates the exciton coupling: closer rings generate larger red‑shifts and higher oscillator strengths, whereas more separated rings resemble the additive spectra of two isolated single‑ring dopants.

A group‑theoretic analysis shows that pristine D₂h symmetry imposes strict selection rules (only certain irreducible representations are optically allowed). Doping reduces symmetry to C₂v, Cₛ, or lower, lifting many of these restrictions and allowing additional transitions. Consequently, the absorption spectra of BN‑GQDs are markedly broadened compared with the parent GQD, which exhibits a narrow, well‑defined peak.

Overall, the work demonstrates that (BN)₃ ring doping provides a versatile “knob” for engineering the electronic gap and optical absorption profile of graphene quantum dots. By judiciously choosing the number, placement, and orientation of borazine rings, one can tailor the material’s response across the infrared–visible spectrum, opening pathways for applications in light‑emitting diodes, photodetectors, solar energy conversion, and fluorescence‑based sensing. The authors suggest that future experimental synthesis and transport measurements will be essential to validate the predicted tunability and to explore charge‑carrier mobility in these BN‑doped nanostructures.