GCRIS

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Graphene-Based Integrated Electronic, Photonic and Spintronic Circuit
(SPIE, 2013) Potasz, P.; Güçlü, Alev Devrim; Özfidan, Işıl; Korkusinski, Marek; Hawrylak, Pawel
To create carbon-based nanoscale integrated electronic, photonic, and spintronic circuit one must demonstrate the three functionalities in a single material, graphene quantum dots (GQDs), by engineering lateral size, shape, edges, number of layers and carrier density. We show theoretically that spatial confinement in GQDs opens an energy gap tunable from UV to THz, making GQDs equivalent to semiconductor nanoparticles. When connected to leads, GQDs act as single-electron transistors. The energy gap and absorption spectrum can be tuned from UV to THz by size and edge engineering and by external electric and magnetic fields. The sublattice engineering in, e.g., triangular graphene quantum dots (TGQDs) with zigzag edges generates a finite magnetic moment. The magnetic moment can be controlled by charging, electrical field, and photons. Addition of a single electron to the charge-neutral system destroys the ferromagnetic order, which can be restored by absorption of a photon. This allows for an efficient spin-photon conversion. These results show that graphene quantum dots have potential to fulfill the three functionalities: electronic, photonic, and spintronic, realized with different materials in current integrated circuits, as well as offer new functionalities unique to graphene.
Citation - WoS: 26
Citation - Scopus: 26
Theory of Optical Properties of Graphene Quantum Dots
(John Wiley and Sons Inc., 2016) Özfidan, Işıl; Güçlü, Alev Devrim; Korkusinski, Marek; Hawrylak, Pawel
We present here a theory of the optical properties of graphene quantum dots (GQDs) with tunable band gaps by lateral size confinement, from UV to THz. Starting from the Hartree-Fock ground state, we construct the correlated many-body ground and excited states of GQDs as a linear combination of a finite number of electron-hole pair excitations. We discuss the evolution of the band gap with size and its renormalization by self-energy and excitonic effects. We calculate and analyze the dipole moments of graphene quantum dots that possess a degenerate valence and conduction band edge, and construct a characteristic exciton and biexciton spectrum. We find an exciton band consisting of a pair of robust, spin singlet bright exciton states and a band of dark, spin singlet and spin triplet, exciton states at lower energies. We predict a characteristic band of biexciton levels at the band edge, discuss the Auger processes and identify a biexciton-exciton cascade. Our theoretical results are compared with experimental linear absorption and non-linear transient absorption spectra of colloidal GQDs. We next discuss the optical properties of triangular GQDs with zigzag edges whose magnetic moment can be controlled by gates. The control over the magnetic moment through carrier density manipulation results in optical spin blockade and gate tunable optical properties over a wide range of photon energies.
Citation - WoS: 58
Citation - Scopus: 60
Microscopic Theory of the Optical Properties of Colloidal Graphene Quantum Dots
(American Physical Society, 2014) Özfidan, Işıl; Korkusinski, Marek; Güçlü, Alev Devrim; Mcguire, John A.; Hawrylak, Pawel
We present a microscopic theory of electronic and optical properties of colloidal graphene quantum dots (CGQDs). The single-particle properties are described in the tight-binding model based on the pz carbon orbitals. Electron-electron screened Coulomb direct, exchange, and scattering matrix elements are calculated using Slater pz orbitals. The many-body ground state and excited states are constructed as a linear combination of a finite number of excitations from the Hartree-Fock (HF) ground state (GS) by exact diagonalization techniques. HF ground states corresponding to semiconductor, Mott-insulator, and spin-polarized phases are obtained as a function of the strength of the screened interaction versus the tunneling matrix element. In the semiconducting phase of a triangular CGQD, the top of the valence band and the bottom of the conduction band are found to be degenerate due to rotational symmetry. The singlet and triplet exciton spectra from the HF GS are obtained by solving the Bethe-Salpeter equation. The low-energy exciton spectrum is predicted to consist of two bright-singlet exciton states corresponding to two circular polarizations of light and a lower-energy band of two dark singlets and 12 dark triplets. The robustness of the bright degenerate singlet pair against correlations in the many-body state is demonstrated as well as the breaking of the degeneracy by the lowering of symmetry of the CGQD. The band-gap renormalization, electron-hole attraction, fine structure, oscillator strength, and polarization of the exciton are analyzed as a function of the size, shape, screening, and symmetry of the CGQD. The theoretical results are compared with experimental absorption spectra.

Physics / Fizik

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