Graphene Quantum Rings Research Articles

Electronic Properties of Hexagonal Graphene Quantum Rings from TAO-DFT.

The reliable prediction of electronic properties associated with graphene nanosystems can be challenging for conventional electronic structure methods, such as Kohn-Sham (KS) density functional theory (DFT), due to the presence of strong static correlation effects in these systems. To address this challenge, TAO (thermally assisted occupation) DFT has been recently proposed. In the present study, we employ TAO-DFT to predict the electronic properties of n-HGQRs (i.e., the hexagonal graphene quantum rings consisting of n aromatic rings fused together at each side). From TAO-DFT, the ground states of n-HGQRs are singlets for all the cases investigated (n = 3-15). As the system size increases, there should be a transition from the nonradical to polyradical nature of ground-state n-HGQR. The latter should be intimately related to the localization of active TAO-orbitals at the inner and outer edges of n-HGQR, which increases with increasing system size.

Thermal properties of quantum rings in monolayer and bilayer graphene

In this work, we use two different statistical mechanics formalisms like, Shannon and Tsallis formalisms (extensive and non-extensive), to study the thermodynamic properties of quantum rings (QRs) in monolayer and bilayer graphene under a uniform external magnetic field. To this goal, we first employ a simplified model to obtain energy eigenvalues of monolayer and bilayer graphene quantum rings. Then, by applying the Shannon and Tsallis formalisms, we have calculated the entropy, specific heat, magnetization, and magnetic susceptibility of monolayer and bilayer graphene. Our results show that thermodynamic quantities in the Shannon formalism have oscillation behaviors with increasing external magnetic fields. Unlike the Shannon formalism, the obtained properties by the Tsallis formalism show a peak structure. Also, the peak location depends on the number of graphene layers.

Trivial and Topological Bound States in Bilayer Graphene Quantum Dots and Rings

Trivial and Topological Bound States in Bilayer Graphene Quantum Dots and Rings

Trivial and Topological Bound States in Bilayer Graphene Quantum Dots and Rings

Herein, two different types of confinement in bilayer graphene by top and bottom gating with symmetrical microelectrodes are discussed and compared. Trivial confinement corresponds to the same polarity of all top gates, which is opposed to that of all bottom ones. Topological confinement requires the polarity of part of the top–bottom pairs of gates to be reversed. It is shown that the main qualitative difference between trivial and topological bound states manifests itself in the magnetic field dependence. The finding is illustrated with an explicit calculation of the energy spectrum for quantum dots and rings. Trivial confinement shows bunching of levels into degenerate Landau bands, with a non‐centered gap, while topological confinement shows no field‐induced gap and a sequence of state branches always crossing zero energy.

Isolated and hybrid bilayer graphene quantum rings

Using the continuum model, we investigate the electronic properties of two types of bilayer graphene (BLG) quantum ring (QR) geometries: (i) an isolated BLG QR and (ii) a monolayer graphene (MLG) with a QR put on top of an infinite graphene sheet (hybrid BLG QR). Solving the Dirac-Weyl equation in the presence of a perpendicular magnetic field and applying the infinite-mass boundary condition at the ring boundaries, we obtain analytical results for the energy levels and corresponding wave spinors for both structures. In the case of isolated BLG QR, we observe a sizeable and magnetically tunable band gap which agrees with the tight-binding transport simulations. Our analytical results also show the intervalley symmetry $ E^K_e (m) = -E^{K'}_h(m) $ between the electron (e) and hole (h) states ($ m $ being the angular momentum quantum number) for the energy spectrum of the isolated BLG QR. The presence of interface boundary in a hybrid BLG QR modifies drastically the energy levels as compared to that of an isolated BLG QR. Its energy levels are tunable from MLG dot, to isolated BLG QR, and to MLG Landau energy levels as magnetic field is varied. Our predictions can be verified experimentally using different techniques such as by magnetotransport measurements.

Modulation of persistent current in graphene quantum rings

We investigate the effect of long-range impurity potentials on the persistent current of graphene quantum rings in the presence of an uniform perpendicular magnetic field. The impurity potentials are modeled as finite regions of the ring with a definite length. We show that, due to the relativistic and massless character of the charge carriers in graphene, the effect of such non-uniform potentials on the energy spectrum and on the persistent current of the rings can be reliably modeled by assuming a non-perturbed ring and including an additional phase due to the interaction of the charge carriers with the potential. In addition, the results show the presence of localized states in the impurity regions. Moreover, we show that for the case of a potential created by a p-n-p junction, the persistent current can be modulated by controlling the voltage at the junction.

On-Surface Synthesis and Characterization of [7]Triangulene Quantum Ring.

The ability to engineer geometrically well-defined antidots in large triangulene homologues allows for creating an entire family of triangulene quantum rings (TQRs) with tunable high-spin ground state, crucial for next-generation molecular spintronic devices. Herein, we report the synthesis of an open-shell [7]triangulene quantum ring ([7]TQR) molecule on Au(111) through the surface-assisted cyclodehydrogenation of a rationally designed kekulene derivative. Bond-resolved scanning tunneling microscopy (BR-STM) unambiguously imaged the molecular backbone of a single [7]TQR with a triangular zigzag edge topology, which can be viewed as [7]triangulene decorated with a coronene-like antidot in the center. Additionally, dI/dV mapping reveals that both inner and outer zigzag edges contribute to the edge-localized and spin-polarized electronic states of [7]TQR. Both experimental results and spin-polarized density functional theory calculations indicate that [7]TQR retains its open-shell septuple ground state (S = 3) on Au(111). This work demonstrates a new route for the design of high-spin graphene quantum rings for future quantum devices.

The effects of inner radius and width on the energy spectrum and persistent current in zigzag hexagonal graphene quantum rings

The effects of inner radius and width on the energy spectrum and persistent current in zigzag hexagonal graphene quantum rings

Quasibound states in single-layer graphene quantum rings

We study the quasi-bound state (QBS) spectra of the graphene quantum rings created by an axially symmetric electrostatic potential. Detailed calculations are carried out for the case of rectangular confinement potentials using the T-matrix and/or the local density of states approaches. Obtained results are analyzed in detail with respect to the two principal characters of QBSs, the (resonant) level position and the level width. A unique relation is established between the QBS-spectrum of a quantum ring under study and the resonant levels formed in the corresponding one-dimensional rectangular potential barrier. Studies are realized in both cases of zero and non-zero mass.

Spin-active devices based on graphene/WSe2 heterostructures

We consider graphene on monolayer ${\mathrm{WSe}}_{2}$ and the spin-orbit coupling induced by the transition-metal dichalcogenide substrate for application to spin-active devices. We study quantum dots and graphene quantum rings as tunable spin filters and inverters. We use an atomistic tight-binding model as well as the Dirac equation to determine stationary states confined in quantum dots and rings. Next, we solve the spin-transport problem for dots and rings connected to nanoribbon leads. The systems connected to zigzag nanoribbons at low magnetic fields act as spin filters and provide strongly spin polarized current.

On the density of states of circular graphene quantum dots

We suggest a simple approach to calculate the local density of states that effectively applies to any structure created by an axially symmetric potential on a continuous graphene sheet such as circular graphene quantum dots or rings. Calculations performed for the graphene quantum dot studied in a recent scanning tunneling microscopy measurement (Gutierrez et al 2016 Nat. Phys. 12 1069–75) show an excellent experimental-theoretical agreement.

Interferometry of Klein tunnelling electrons in graphene quantum rings

We theoretically study a current switch that exploits the phase acquired by a charge carrier as it tunnels through a potential barrier in graphene. The system acts as an interferometer based on an armchair graphene quantum ring, where the phase difference between interfering electronic wave functions for each path can be controlled by tuning either the height or the width of a potential barrier in the ring arms. By varying the parameters of the potential barriers, the interference can become completely destructive. We demonstrate how this interference effect can be used for developing a simple graphene-based logic gate with a high on/off ratio.

A comparative study on the edge states in phosphorene quantum dots and rings

Using the tight-binding Hamiltonian approach, we comparatively investigate the energy spectrums of triangular zigzag phosphorene quantum dots (PQDs) and rings (PQRs), as well as their potential applications. In comparison with the outer edge states in the PQD, new extra inner edge states can be produced in the PQR by its internal hole. A transition from the uncoupled to coupled edge states can be induced by decreasing the width between the outer and inner edges of the PQR. Also, the edge states in PQD/PQR are all anisotropically localized in one side, rather than three sides as in triangular graphene quantum dots (QDs) and rings (QRs). Furthermore, the PQD/PQR energy levels can be anisotropically manipulated by the external electric fields and strains, clearly demonstrating their potential applications in field effect transistors or electromechanical devices. In the meanwhile, we also consider the electron probability distributions corresponding to the different energy levels, clearly exposing the characteristics of the PQD/PQR energy levels. The comparison between the asymmetrical triangular PQDs/PQRs and the symmetrical triangular QDs/QRs in graphene should be instructive for understanding the similar triangular QDs/QRs in other two-dimensional layered materials, as well as other types of QDs/QRs of different shapes.

Electronic confinement in graphene quantum rings due to substrate-induced mass radial kink

We investigate localized states of a quantum ring confinement in monolayer graphene defined by a circular mass-related potential, which can be induced e.g. by interaction with a substrate that breaks the sublattice symmetry, where a circular line defect provides a change in the sign of the induced mass term along the radial direction. Electronic properties are calculated analytically within the Dirac–Weyl approximation in the presence of an external magnetic field. Analytical results are also compared with those obtained by the tight-binding approach. Regardless of its sign, a mass term is expected to open a gap for low-energy electrons in Dirac cones in graphene. Both approaches confirm the existence of confined states with energies inside the gap, even when the width of the kink modelling the mass sign transition is infinitely thin. We observe that such energy levels are inversely proportional to the defect line ring radius and independent on the mass kink height. An external magnetic field is demonstrated to lift the valley degeneracy in this system and easily tune the valley index of the ground state in this system, which can be polarized on either K or valleys of the Brillouin zone, depending on the magnetic field intensity. Geometrical changes in the defect line shape are considered by assuming an elliptic line with different eccentricities. Our results suggest that any defect line that is closed in a loop, with any geometry, would produce the same qualitative results as the circular ones, as a manifestation of the topologically protected nature of the ring-like states investigated here.

Graphene Quantum Rings Doped PEDOT:PSS Based Composite Layer for Efficient Performance of Optoelectronic Devices

The method of synthesis has a profound influence on the properties of carbon nanostructures endowed with quantum confinement. Here, we report synthesis method of the graphene quantum rings (GQRs) from pitch carbon fiber by a thermal hydrolysis process and their application in optoelectronic devices. Synthesized GQRs with excitation-dependent photoluminescence and excellent electrical conducting properties have a work function of −4.16 eV suitable for optoelectronic applications. The electrical properties of the GQRs are studied by metal–semiconductor–metal (MSM), field-effect transistor (FET), and hybrid (inorganic and organic) solar cell devices comprising the GQRs as the p-type channel and hole transporting material, respectively. The current–voltage (I–V) characteristic from a diode-type MSM device shows a conductance of 1.5 mA at 3.5 V in the dark. Under an illumination of 1.5 AM the current increases to 5.28 mA at 3.5 V, showing that these GQRs are very good candidates for photodiodes and other optoe...

Electronic and magnetic properties of zigzag-edged hexagonal graphene ring nanojunctions

The zigzag-edged hexagonal graphene quantum rings (ZHGQRs) are sandwiched between two Au electrodes to construct nanojunctions, and their electronic and magnetic properties are investigated by the first-principles method. It shows that there are always significant transmission gaps (semiconductor behavior) for all ZHGQRs but changing in an obvious oscillating manner with variations of the edge width. The mechanism for such a feature is analyzed deeply, and it is found that the weak localization for the quantum interference correction to conductance and the parity of wave function play the crucial roles. No intrinsic magnetism is observed in the ZHGQRs, this is because there always exist local armchair-edge-like defects at its corners to weaken the spin polarization strongly. And also shown is that the magnetic field can effectively suppress the weak localization behaviors leading to a negative magnetoresistance for the ZHGQR, which is in good agreement with experimental findings in graphene. These findings are of very important for understanding the electromagnetic properties of graphene quantum ring devices.

Fano resonances in hexagonal zigzag graphene rings under external magnetic flux

We study transport properties of hexagonal zigzag graphene quantum rings connected to semi-infinite nanoribbons. Open two-fold symmetric structures support localized states that can be traced back to those existing in the isolated six-fold symmetric rings. Using a tight-binding Hamiltonian within the Green's function formalism, we show that an external magnetic field promotes these localized states to Fano resonances with robust signatures in transport. Local density of states and current distributions of the resonant states are calculated as a function of the magnetic flux intensity. For structures on corrugated substrates we analyze the effect of strain by including an out-of-plane centro-symmetric deformation in the model. We show that small strains shift the resonance positions without further modifications, while high strains introduce new ones.

Enhancing thermoelectric properties of graphene quantum rings

We study the thermoelectric properties of rectangular graphene rings connected symmetrically or asymmetrically to the leads. A side-gate voltage applied across the ring allows for the precise control of the electric current flowing through the system. The transmission coefficient of the rings manifests Breit-Wigner line-shapes and/or Fano line-shapes, depending on the connection configuration, the width of nanoribbons forming the ring and the side-gate voltage. We find that the thermopower and the figure of merit are greatly enhanced when the chemical potential is tuned close to resonances. Such enhancement is even more pronounced in the vicinity of Fano like anti-resonances which can be induced by a side-gate voltage independently of the geometry. This opens a possibility to use the proposed device as a tunable thermoelectric generator.

Electric field control of spin-resolved edge states in graphene quantum nanorings

The electric-field effect on the electronic and magnetic properties of triangular and hexagonal graphene quantum rings with zigzag edge termination is investigated by means of the single-band tight-binding Hamiltonian and the mean-field Hubbard model. It is shown how the electron and spin states in the nanoring structures can be manipulated by applying an electric field. We find different spin-depolarization behaviors with variation of electric field strength due to the dependence of spin densities on the shapes and edges of this kind of nanorings. In the case of triangular quantum rings, the magnetization on the inner and outer edges can be selectively tuned and the spin states depolarize gradually as the field strength is increased, while in the case of hexagonal nanorings, the transverse electric field reduces the magnetic moments on both inner and outer edges symmetrically and rapidly.

Geometry and edge effects on the energy levels of graphene quantum rings: A comparison between tight-binding and simplified Dirac models

We present a systematic study of the energy spectra of graphene quantum rings having different geometries and edge types in the presence of a perpendicular magnetic field. Results are obtained within the tight-binding (TB) and Dirac models and we discuss which features of the former can be recovered by using the approximations imposed by the latter. Energy levels of graphene quantum rings obtained by diagonalizing the TB Hamiltonian are demonstrated to be strongly dependent on the rings geometry and the microscopical structure of the edges. This makes it difficult to recover those spectra by the existing theories that are based on the continuum (Dirac) model. Nevertheless, our results show that both approaches (i.e., TB and Dirac model) may provide similar results, but only for very specific combinations of ring geometry and edge types. The results obtained by a simplified model describing an infinitely thin circular Dirac ring show good agreement with those obtained for hexagonal and rhombus armchair graphene rings within the TB model. Moreover, we show that the energy levels of a circular quantum ring with an infinite mass boundary condition obtained within the Dirac model agree with those for a ring defined by a ring-shaped staggered potential obtained within the TB model.