Nanoribbon Quantum Dot Research Articles

Tunable Magnetic Coupling in Graphene Nanoribbon Quantum Dots.

Carbon-based quantum dots (QDs) enable flexible manipulation of electronic behavior at the nanoscale, but controlling their magnetic properties requires atomically precise structural control. While magnetism is observed in organic molecules and graphene nanoribbons (GNRs), GNR precursors enabling bottom-up fabrication of QDs with various spin ground states have not yet been reported. Here the development of a new GNR precursor that results in magnetic QD structures embedded in semiconducting GNRs is reported. Inserting one such molecule into the GNR backbone and graphitizing it results in a QD region hosting one unpaired electron. QDs composed of two precursor molecules exhibit nonmagnetic, antiferromagnetic, or antiferromagnetic ground states, depending on the structural details that determine the coupling behavior of the spins originating from each molecule. The synthesis of these QDs and the emergence of localized states are demonstrated through high-resolution atomic force microscopy (HR-AFM), scanning tunneling microscopy (STM) imaging, and spectroscopy, and the relationship between QD atomic structure and magnetic properties is uncovered. GNR QDs provide a useful platform for controlling the spin-degree of freedom in carbon-based nanostructures.

Scalable fabrication of graphene nanoribbon quantum dot devices with stable orbital-level spacing

Large-scale integration of quantum-dot devices is essential for realizing various quantum devices. Graphene-based quantum dots provide a promising platform for spin qubits because of their low nuclear spin density and weak spin-orbit interaction. However, the integration of graphene-based quantum dots remains a challenge. Here, we demonstrate the scalable fabrication of graphene nanoribbon-based quantum-dot devices using a nickel nanobar technique. Fine structures formed in the middle of the nanoribbons exhibit quantum-dot behavior, and more than 56% of devices fabricated on the same substrate show Coulomb diamond features, indicating that large-scale integration of graphene nanoribbon quantum-dot devices is possible with our method. Cryogenic measurements reveal orbital-level spacings between the ground and excited states that are stable up to high-temperature conditions of ~20 K. We explain this stability in terms of the very fine structures formed in the middle of the nanoribbons and their relatively low effective mass.

A facile synthesis of graphene nanoribbon-quantum dot hybrids and their application for composite electrolyte membrane in direct methanol fuel cells

In this work, we report the concept of synthesizing graphene nanoribbon-graphene quantum dot (GNR-GQD) hybrids with even grafting of GQD on GNR sheet in a facile single step process under ultrasonication in chlorosulfonic acid. Further, diazotization route is followed for the preparation of 4-benzenediazonium sulfonate precursor to sulfonate both GNR and GQD to form sulfonated graphene nanoribbons-sulfonated graphene quantum dots (sGNR-sGQD) nanohybrids. Synergistic and structure dependent effect of nanohybrids is seen via its dispersion in sulfonated poly(ether ether ketone) (sPEEK) to form nanocomposite membrane. sPEEK/sGNR-sGQD (1.5 wt %) nanocomposite membrane shows remarkable direct methanol fuel cell (DMFC) performance compared to pristine sPEEK and Nafion 117 with 40% increment in peak power density along with higher durability up to 100 h due to better physicochemical properties like water uptake, ion exchange capacity, proton conductivity, and reduced methanol crossover to suggest its potential which includes diverse membrane applications.

Vacancy-induced shifts of edge states in Z-shaped graphene nanoribbon quantum dot

Quasi-one-dimensional quantum dot consisting of Z-shaped graphene nanoribbon junction, is known to create discrete edge states in the bandgap due to quantum confinement effect depending on the length of the quantum dot. To examine possible changes in the midgap states caused by introducing a carbon vacancy within the quantum dot, we calculated the band structures of the defective systems using density functional based tight-binding method, and also using a simplest tight-binding model for π-electrons. As the central result, we predict that the separation of midgap edge states are considerably variable, depending on the choice of a vacancy site. The transmission via midgap edge states and zero-mode state is shown.

Theoretical studies of graphene nanoribbon quantum dot qubits

Graphene nanoribbon quantum dot qubits have been proposed as promising candidates for quantum computing applications to overcome the spin-decoherence problems associated with typical semiconductor (e.g., GaAs) quantum dot qubits. We perform theoretical studies of the electronic structures of graphene nanoribbon quantum dots by solving the Dirac equation with appropriate boundary conditions. We then evaluate the exchange splitting based on an unrestricted Hartree-Fock method for the Dirac particles. The electronic wave function and long-range exchange coupling due to the Klein tunneling and the Coulomb interaction are calculated for various gate configurations. It is found that the exchange coupling between qubits can be significantly enhanced by the Klein tunneling effect. The implications of our results for practical qubit construction and operation are discussed.

Charging graphene nanoribbon quantum dots

We describe charging a quantum dot induced electrostatically within a semiconducting graphene nanoribbon by electrons or holes. The applied model is based on a tight-binding approach with the electron-electron interaction introduced by a mean field local spin density approximation. The numerical approach accounts for the charge of all the $p_z$ electrons and screening of external potentials by states near the charge neutrality point. Both a homogenous ribbon and a graphene flake embedded within the ribbon are discussed. The formation of transport gaps as functions of the external confinement potential (top gate potential) and the Fermi energy (back gate potential) are described in good qualitative agreement with the experimental data. For a fixed number of excess electrons we find that the excess charge added to the system is, - depending on the voltages defining the work point of the device: $(i)$ delocalized outside the quantum dot, - in the transport gap due to the top gate potential $(ii)$ localized inside the quantum dot, - in the transport gap due to the back gate potential or $(iii)$ extended over both the quantum dot area and the ribbon connections, - outside the transport gaps. The applicability of the frozen valence band approximation to describe charging the quantum dot by excess electrons is also discussed.

Nucleonic-resolution optical mass sensor based on a graphene nanoribbon quantum dot

The high frequency and ultrasmall mass of graphene make it an ideal material for ultrasensitive mass sensing. In this article, based on the all-optical technique, we propose a scheme of an optical mass sensor to weigh the mass of a single atom or molecule via a doubly clamped Z-shaped graphene nanoribbon (GNR). We use the detection of shifts in the resonance frequency of the Z-shaped GNR to determine the mass of an external particle landing on the GNR. The highly sensitive mass sensor proposed here can weigh particles down to the yoctogram and may eventually be enable to realize the mass measurement of nucleons.

Electron spin relaxation in graphene nanoribbon quantum dots

Graphene is promising as a host material for electron spin qubits because of its predicted potential for long coherence times. In armchair graphene nanoribbons (aGNRs) a small bandgap is opened, allowing for electrically gated quantum dots, and furthermore the valley degeneracy is lifted. The spin lifetime T_1 is limited by spin relaxation, where the Zeeman energy is absorbed by lattice vibrations, mediated by spin-orbit and electron-phonon coupling. We have calculated T_1 by treating all couplings analytically and find that T_1 can be in the range of seconds for several reasons: (i) low phonon density of states away from Van Hove singularities; (ii) destructive interference between two relaxation mechanisms; (iii) Van Vleck cancellation at low magnetic fields; (iv) vanishing coupling to out-of-plane modes in lowest order due to the electronic structure of aGNRs. Owing to the vanishing nuclear spin of 12C, T_1 may be a good measure for overall coherence. These results and recent advances in the controlled production of graphene nanoribbons make this system interesting for spintronics applications.

Electron Spectrum and Tunneling Current of the Toroidal and Helical Graphene Nanoribbon-Quantum Dots Contact

We study the electron spectrum and the density of states of long-wave electrons in the curved graphene nanoribbon based on the Dirac equation in a curved space-time. The current-voltage characteristics for the contact of nanoribbon-quantum dot have been revealed. We also analyze the dependence of the specimen properties on its geometry.

Resonant transport through graphene nanoribbon quantum dots

The transport properties of armchair graphene nanoribbons with two structural nanoconstrictions are studied by atomistic simulations. With the appropriate geometry and size, the central region between the two constrictions can behave as a single tunneling quantum dot. Within the low energy range around the Dirac point, the conductance of the structure shows strong resonances that are related to the quasi-bound states localized in the dot. The quasi-bound states are associated with the standing waves confined by the two constrictions, and can be effectively modulated by the geometric parameters of the structure.

Investigation of electronic transport through graphene nanoribbon quantum dots

Local density of state (LDOS), current–voltage characteristic ( I – V characteristic) and differential conductance (DC) of pure and defected zigzag and armchair graphene nanoribbons quantum dots (GNRQDs) are investigated. We found that for the pure systems of armchair GNRQDs, the DC has oscillation feature where these oscillations could be explained by 1D characteristic of atomic wires. In our samples, these wires constructed from sites with the same symmetry along the GNRQD length. Some of these wires have metallic features while some of them show semiconducting features. The zigzag GNRQDs have approximately ohmic behavior. Also it is found that the defected DC significantly depend on vacancy position.

Quantum chaos in graphene nanoribbon quantum dot

The electron transport property of a graphene nanoribbon （GNR） quantum dot in a dilution refrigerator at temperature 20 mK is studied. The Coulomb blockade diamonds and excited energy levels of the quantum dot are clearly observed. It is found that the statistical distribution of the spacing between the nearest-neighboring Coulomb blockade peaks and that of the individual peak height are consistent respectively with the typical Wigner-Dyson distribution and Porter-Thomas distribution as prescribed by the random matrix theory for a quantum chaotic system. Thus， our results demonstrate that the GNR quantum dot is a quantum chaotic system at low temperatures. The possible origin of this phenomenon is also discussed.

Z-shaped graphene nanoribbon quantum dot device

Stimulated by recent advances in isolating graphene, the authors discovered that a quantum dot can be trapped in a Z-shaped graphene nanoribbon junction. The topological structure of the junction can completely confine electronic states. By varying the junction length, the authors can alter the spatial confinement and the number of discrete levels within the junction. In addition, a quantum dot can be realized regardless of substrate induced static disorder or irregularities on the edges of the junction. The method can be used to easily design quantum dot devices. The authors also provide a platform to design zero-dimensional functional nanoscale electronic devices using graphene ribbons.