Electronic Properties Of Graphene Nanostructures Research Articles

First-principles study of the electronic properties of graphene nanostructures for high current density cathodes

Graphene is a crystalline allotrope of carbon with 2D properties. Its carbon atoms are densely packed in a nanoscale hexagonal pattern. Graphene has many unusual properties. In this study, the authors study the electronic properties of graphene nanostructures using first-principles or ab initio calculations based on density functional theory as implemented in the Vienna ab initio simulation package in order to explore its applications in field-emission devices. The density of states and work function of graphene nanoribbons are calculated. The work function value is a key parameter in determining the field emission from a cathode surface according to the Fowler–Nordheim theory. For practical applications, the work functions of graphene nanoribbons with different widths and terminating edges, with and without passivation, have been investigated. Specifically, with the decoration of different alkali and alkaline earth metal species, the reduction of the work function has been systematically studied and determined for achieving higher current density emission.

Generalized Hamiltonian for a graphene subjected to arbitrary in-plane strains

The interplay between the linear elastic deformation up to 20% and the unique electronic properties of graphene nanostructures offers an attractive prospect to manipulate their properties by strain. Here we review the recent progress on the electronic response of graphene to the in-plane strains, including the strain-modulated electronic structure and the strain-modulated spin, valley and superconducting transports. A generalized Hamiltonian for a graphene was constructed subjected to arbitrary in-plane strains. The Hamiltonian is helpful to design and optimize the graphene-based nano-electromechanical systems (NEMS).

Role of edge geometry and chemistry in the electronic properties of graphene nanostructures.

The geometry and chemistry of graphene nanostructures significantly affects their electronic properties. Despite a large number of experimental and theoretical studies dealing with the geometrical shape-dependent electronic properties of graphene nanostructures, experimental characterisation of their chemistry is clearly lacking. This is mostly due to the difficulties in preparing chemically-modified graphene nanostructures in a controlled manner and in identifying the exact chemistry of the graphene nanostructure on the atomic scale. Herein, we present scanning probe microscopic and first-principles characterisation of graphene nanostructures with different edge geometries and chemistry. Using the results of atomic scale electronic characterisation and theoretical simulation, we discuss the role of the edge geometry and chemistry on the electronic properties of graphene nanostructures with hydrogenated and oxidised linear edges at graphene boundaries and the internal edges of graphene vacancy defects. Atomic-scale details of the chemical composition have a strong impact on the electronic properties of graphene nanostructures, i.e., the presence or absence of non-bonding π states and the degree of resonance stability.

Spin Filtering and Magneto-Resistive Effect at the Graphene/h-BN Ribbon Interface

Advances in the realization of hybrid graphene/h-BN materials open new ways to control the electronic properties of graphene nanostructures. In this paper, the structural, electronic, and transport properties of heterojunctions made of bare zigzag-shaped h-BN and graphene ribbons are investigated using first-principles techniques. Our results highlight the potential of graphene/h-BN junctions for applications in spintronic devices. At first, density functional theory is used to detail the role played by the edge states and dangling bonds in the electronic and magnetic behavior of h-BN and graphene ribbons. Then, the electronic conductance of the junction is computed in the framework of Green's function-based scattering theory. In its high-spin configuration, the junction reveals a full spin polarization of the propagating carriers around the Fermi energy, and the magnitude of the transmission probability is predicted to be strongly dependent on the relative orientation of magnetic momenta in the leads.

Optical Properties and Charge-Transfer Excitations in Edge-Functionalized All-Graphene Nanojunctions.

We investigate the optical properties of edge-functionalized graphene nanosystems, focusing on the formation of junctions and charge-transfer excitons. We consider a class of graphene structures that combine the main electronic features of graphene with the wide tunability of large polycyclic aromatic hydrocarbons. By investigating prototypical ribbon-like systems, we show that, upon convenient choice of functional groups, low-energy excitations with remarkable charge-transfer character and large oscillator strength are obtained. These properties can be further modulated through an appropriate width variation, thus spanning a wide range in the low-energy region of the UV-vis spectra. Our results are relevant in view of designing all-graphene optoelectronic nanodevices, which take advantage of the versatility of molecular functionalization, together with the stability and the electronic properties of graphene nanostructures.

Ballistic transport through graphene nanostructures of velocity and potentialbarriers

We investigate the electronic properties of graphene nanostructures when the Fermivelocity and the electrostatic potential vary in space. First, we consider the transmissionT andconductance G through single and double barriers. We show thatG for velocity barriers differs markedly from that for potential barriers for energies belowthe height of the latter and it exhibits periodic oscillations as a function of theenergy for strong velocity modulation. Special attention is given to superlattices(SLs). It is shown that an applied bias can efficiently widen or shrink the allowedminibands of velocity-modulated SLs. The spectrum in the Kronig–Penney limit isperiodic in the strength of the barriers. Collimation of an electron beam incidenton an SL with velocity and potential barriers is present but it disappears whenthe potential barriers are absent. The number of additional Dirac points maychange considerably if barriers and wells have sufficiently different Fermi velocities.

Designing All-Graphene Nanojunctions by Covalent Functionalization

We investigated theoretically the effect of covalent edge functionalization, with organic functional groups, on the electronic properties of graphene nanostructures and nano-junctions. Our analysis shows that functionalization can be designed to tune electron affinities and ionization potentials of graphene flakes, and to control the energy alignment of frontier orbitals in nanometer-wide graphene junctions. The stability of the proposed mechanism is discussed with respect to the functional groups, their number as well as the width of graphene nanostructures. The results of our work indicate that different level alignments can be obtained and engineered in order to realize stable all-graphene nanodevices.

Orientation tendency of PLD carbon films as a function of substrate temperature: A NEXAFS study

Electronic properties of graphene nano-structures are highly anisotropic and correlated to their basal plane orientation. In this work, we tried to study and explain how deposition temperature can draw a definite plane growth direction. Carbon films have been deposited by Pulsed Laser Ablation (Nd:YAG, 2nd harmonic: λ=532 nm, hν=2.33 eV, τ=7 ns, ν=10 Hz, Φ ≈7 J/cm 2), from a pyrolytic graphite target on Si <100> substrates. PLD depositions were performed in vacuum, at increasing substrate temperature, ranging from room temperature (RT) to 900 °C. Near-edge X-ray absorption fine structure (NEXAFS), SEM and Raman spectroscopy have been used to characterise the films. Using linearly polarized Synchrotron radiation and HOPG (Highly Oriented Pyrolytic Graphite) as a reference system, we investigated, ex situ, the occurrence of an average orientation distribution of the nano-sized graphene clusters, grown at high temperature. The results of the angle-dependent study by NEXAFS of film grown at 900 °C provide evidence that at high temperature the graphene sheets show a tendency to grow with basal planes oriented perpendicularly to the substrate surface. At low temperature, the film is less structured. The features ascribable to graphitic character are less pronounced and broadened; the absence of an angle dependence of NEXAFS measurements suggest that C-structures are randomly oriented. The corresponding Raman spectrum substantiates the prevailing amorphous nature of the film.