Dark excitons and tunable optical gap in graphene nanodots

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Chemistry

Background:

• Graphene nanodots (GNDs) are zero-dimensional carbon nanomaterials with tunable electronic and optical properties.
• Understanding excitonic effects is crucial for applications in optoelectronics and photonics.
• Configuration interaction (CI) methods are essential for accurately describing electron-electron interactions in quantum systems.

Purpose of the Study:

• Investigate the nature and origin of excitonic effects in the optical absorption spectra of graphene nanodots.
• Analyze the influence of geometric symmetry and electron-electron interactions on exciton properties.
• Determine the role of dielectric screening and geometric factors in tuning the optical gap of GNDs.

Main Methods:

• Employed a configuration interaction (CI) approach, including up to fifth-order excitations.
• Studied triangular and trapezoidal graphene nanodot systems to probe symmetry effects.
• Analyzed the impact of varying electron-electron interaction strengths, including dielectric screening.

Main Results:

• Identified optically dark singlet excitons in triangular graphene nanodots, primarily due to geometric symmetry.
• Demonstrated that dark excitons persist even with broken electron-hole or sublattice symmetry.
• Observed sensitivity of exciton state ordering to electron-electron interactions and dielectric screening.
• Showed lifting of degeneracies in trapezoidal nanodots, with the first excited state remaining dark under strong screening.

Conclusions:

• Geometric symmetry is a primary factor governing the presence of dark excitons in graphene nanodots.
• Electron-electron interactions and dielectric environment significantly influence the excitonic spectrum.
• Geometric symmetry is critical for efficiently tuning the optical gap of graphene nanodots via their dielectric environment.