Width Of Graphene Nanoribbons Research Articles

Electron-Lattice Coupling in Armchair Graphene Nanoribbons.

We report the effects of electron-lattice coupling on the charge density distribution study of armchair graphene nanoribbons (GNRs). Here, we perform a theoretical investigation explaining the unexpected electronic density states observed experimentally. By means of a tight-binding approach with electron-lattice coupling, we obtained the same characteristic pattern of charge density along the C-C bonds suggested by both scanning tunneling and transmission electron microscopic measurements. Our results suggest electronic localized states whose sizes are dependent on the GNR width. We also show that our model rescues the quasi-particle charge-transport mechanism in GNRs. The remarkable agreement with experimental evidence allows us to conclude that our model could be, in many aspects, a fundamental tool when it comes to the phenomenological understanding of the charge behavior in this kind of system.

Electronic thermal conductivity and thermopower of armchair graphene nanoribbons

A theoretical investigation of the diffusion contribution to thermopower, Sd, and the electronic thermal conductivity, κe, of semiconducting armchair graphene nanoribbons (GNRs) is made for T⩽300K. Considering the electrons to be scattered by edge roughness, impurities and deformation-potential coupled acoustic phonons and optical phonons, expressions for Sd and κe are obtained. Numerical calculations of Sd and κe, as functions of temperature and linear carrier density, bring out the relative importance of the contributing scattering mechanisms. A GNR of width 5nm, supporting an electron density 2×108m−1, is found to exhibit room temperature values of Sd and κe as 42μV/K and 26.5W/mK, respectively. A decrease in armchair GNR width, is found to enhance Sd and reduce κe. The effect of varying the electron density is to increase their magnitude when Fermi energy moves into the second subband. An analysis of thermopower and thermal conductivity data in clean armchair GNR samples will enable better understanding of the electron–phonon interaction.

Theoretical investigation of the electronic structures and carrier transport of hybrid graphene and boron nitride nanostructure

Recently hybridgraphene and hexagonal boron nitride (C-BN) nanostructures receive much research interest due to the complementary electronic properties. Graphene is a zero-gap semiconductor, while hexagonal boron nitride (h-BN) is a wide gap semiconductor. Here we studied the electronic structures and carrier transport of hybrid C-BN nanostructures by using first principles calculations and deformation potential theory. We have found that the physical quantities in these systems under study, band gap, effective mass, deformation potential, and carrier mobility, can be categorised into three different families depending on the width of graphene nanoribbon. This family behavior is similar to pristine armchair graphene nanoribbon, but with slight difference from the individual component. New opportunities of designing nanoelectric devices are discussed by utilizing the quantum confinement effect based on such kind of hybrid nanostructures.

Monolayer Graphene Nanoribbon Homojunction Characteristics

One dimensional band structure limit is applied on monolayer graphene nanoribbon homo-junction. The parabolic band energy approximation on graphene nanoribbon in low energy limit is assumed which means carrier movement can be presumed as a ballistic transport. Two graphene nanoribbon with different width are projected as metallic and semiconducting components of a homo-junction schottky diode. Finally analytical model of junction current-voltage (I-V), graphene nanoribbon width effects on carrier transport also physical parameters, such as drain voltage, gate voltage are presented. Comparison study between presented model and conventional diodes indicates smaller effective turn-on voltage of the graphene nanoribbon schottky diode.

Dimension Dependence of Negative Differential Thermal Resistance in Graphene Nanoribbons

Negative differential thermal resistance (NDTR) in approximate graphene nanoribbons (GNRs) is investigated from one dimension to three dimensions by using classical molecular dynamics method. For single-layer GNRs, NDTR can not be observed for very narrow GNRs (one dimension), NDTR appears when the width of GNRs increases (two dimensions). However, NDTR disappears gradually on further increasing the width. For multiple-layer GNRs, when the number of the layers increases, GNRs becomes from two-dimensional system to three-dimensional system, NDTR regime reduces and eventually disappears. In addition, when the length of GNRs increases, NDTR regime also reduces and vanishes in the thermodynamic limit. These effects may be useful for designing thermal devices where NDTR plays an important role.

Graphene nanoribbon FETs for digital electronics: experiment and modeling

ABSTRACTOne‐dimensional nanostructures of graphene such as graphene nanoribbons (GNRs) can prove attractive for digital electronics in the form of interband tunneling transistors, as they are capable of high drive currents. Here, we report on the transport properties of p‐n junctions formed in GNR field effect transistors (FETs). It is found that the current density in the devices is indeed high; in the 1–1.5 A/mm range have been measured, comparable to Si‐MOSFETs and III‐V Nitride HEMTs. The observed unique current–voltage characteristics of the double‐gated GNR FETs having a lateral p‐n junction as their channel is explained by a field‐effect model. Due to the lack of sufficiently large bandgap in the 30 nm wide GNR, the device still cannot be turned off completely, but rectification is achieved. The results suggest that the fabrication of tunneling FETs made out of graphene is possible and their characteristics may meet the expectations. Copyright © 2012 John Wiley & Sons, Ltd.

Effect of an out-of-plane cross connection on the electronic transport of zigzag graphene nanoribbon

Electronic transport properties of out-of-plane graphene nanoribbon intersections have been investigated by using computational method. The inter-distance between two graphene nanoribbons is found to affect the transport properties strongly and its affection can be neglected for larger ones, even under an external bias. Wider graphene nanoribbons will bring stronger interaction into the system, and result in more transmission dips. Moreover, the stacking configuration between two graphene nanoribbons is found to be crucial for the electronic transport under an external bias, as it can affect the electronic transport strongly near the charge neutral point.

Tunable thermal rectification in graphene nanoribbons through defect engineering: A molecular dynamics study

Using non-equilibrium molecular dynamics, we show that asymmetrically defected graphene nanoribbons (GNR) are promising thermal rectifiers. The optimum conditions for thermal rectification (TR) include low temperature, high temperature bias, ∼1% concentration of single-vacancy or substitutional silicon defects, and a moderate partition of the pristine and defected regions. TR ratio of ∼80% is found in a 14-nm long and 4-nm wide GNR at a temperature of 200 K and bias of 90 K, where heat conduction is in the ballistic regime since the bulk effective phonon mean-free-path is around 775 nm. As the GNR length increases towards the diffusive regime, the TR ratio decreases and eventually stabilizes at a length-independent value of about 3%–5%. This work extends defect engineering to 2D materials for achieving TR.

Conductance of metallic nanoribbons with defects

The conductances of two typical metallic graphene nanoribbons with one and two defects are studied using the tight binding model with the surface Green's function method. The weak scattering impurities, U ∼ 1 eV, induce a dip in the conductance near the Fermi energy for the narrow zigzag graphene nanoribbons. As the impurity scattering strength increases, the conductance behavior at the Fermi energy becomes more complicated and depends on the impurity location, the AA and AB sites. The impurity effect then becomes weak and vanishes with the increase in the width of the zigzag graphene nanoribbons (150 nm). For the narrow armchair graphene nanoribbons, the conductance at the Fermi energy is suppressed by the impurities and becomes zero with the increase in impurity scattering strength, U > 100 eV, for two impurities at the AA sites, but becomes constant for the two impurities at the AB sites. As the width of the graphene nanoribbons increases, the impurity effect on the conductance at the Fermi energy depends sensitively on the vacancy location at the AA or AB sites.

Strain-activated edge reconstruction of graphene nanoribbons

The edge structure and width of graphene nanoribbons (GNRs) are crucial factors for the electronic properties. A combination of experiment and first-principles calculations allows us to determine the mechanism of the hexagonhexagon to pentagon-heptagon transformation. GNRs thinner than 2 nm have been fabricated by bombardment of graphene with high-energetic Au clusters. The edges of the GNRs are modified in situ by electron irradiation. Tensile strain along the edge decreases the transformation energy barrier. Antiferromagnetism and a direct band gap are found for a zigzag GNR, while a fully reconstructed GNR shows an indirect band gap. A GNR reconstructed on only one edge exhibits ferromagnetism. We propose that strain is an effective method to tune the edge and, therefore, the electronic structure of thin GNRs for graphene-based electronics.

Iron-phthalocyanine molecular junction with high spin filter efficiency and negative differential resistance

We investigate the spin transport properties of iron-phthalocyanine (FePc) molecule sandwiched between two N-doped graphene nanoribbons (GNRs) based on the density functional theory and nonequilibrium Green's function methods. Our calculated results clearly reveal that the FePc molecular junction has high spin-filter efficiency as well as negative differential resistance (NDR). The zero-bias conductance through FePc molecule is dominated by the spin-down electrons, and the observed NDR originates from the bias-dependent effective coupling between the FePc molecular orbitals and the narrow density of states of electrodes. The remarkable high spin-filter efficiency and NDR are robust regardless of the edge shape and the width of GNRs, and the N-doping site in GNRs. These predictions indicate that FePc junction holds great promise in molecular electronics and spintronics applications.

Metal contact to graphene nanoribbon

Metal to graphene nanoribbon (GNR) contact is explored by self-consistent numerical simulations. The Schottky barrier height of the GNR contact can be modulated by either the gate voltage or the workfunction of the metal contact. An Ohmic contact with multiple-subband injection can be achieved with increase of the GNR width. The contact resistance normalized by the GNR width is smaller than that of the two-dimensional graphene, and the effect of the quasi-one-dimensional subbands for GNRs is manifested when the GNR is narrower than about 5 nm.

Current-voltage Characteristics of Graphene Nanoribbon Schottky Diodes

Theoretical investigation demonstrates that the current-voltage characteristics of a metal/graphene nanoribbon with donor impurities contact under forward bias condition has typical properties of a metal/silicon contact. Assuming that the work function of graphene nanoribbon will not change with its width, the results demonstrate that under a given biasing voltage, the current density of the contact decreases with the decrement of the nanoribbon width. Such a current-density decrement is especially large when the graphene-nanoribbon width drops to the few nanometers range. Furthermore, the high-energy electron density increases with the decrement of the nanoribbon width, which also impacts the current density significantly. Therefore, the relationship between the decrement of the current density and the nanoribbon width is non-exponential. The results in this paper also illustrate that the current density of metal/graphene nanoribbon contact at a given biasing voltage increases slowly with the increment of the donor density.

From zigzag to armchair: the energetic stability, electronic and magnetic properties of chiral graphene nanoribbons with hydrogen-terminated edges

The energetic stability, electronic and magnetic properties of chiral graphene nanoribbons (GNRs) with hydrogen-terminated edges are investigated using density functional theory. Our calculations show that the percentage of carbon atoms at the zigzag sites (Pz) is the key factor determining the electronic and magnetic properties of chiral GNRs. Within the local spin density approximation, chiral GNRs with Pz ≥ 50% have a semiconducting antiferromagnetic ground state. Otherwise, chiral GNRs are spin degenerate semiconductors. Thus, the critical chiral angle for the occurrence of spin polarization is determined to be 13.9°. In contrast to the antiferromagnetic state that is independent of the width of GNRs investigated, size effects occur for the ferromagnetic metastable state. These findings are helpful for the design of GNR-based spintronic devices.

Helical Wrapping and Insertion of Graphene Nanoribbon to Single-Walled Carbon Nanotube

Molecular dynamics simulations are performed to demonstrate that the graphene nanoribbon (GNR) could helically wrap onto and insert into the single-walled carbon nanotube (SWNT) to form helical configurations, which are quite close to the helices found in nature. The steady decline of the potential energy suggests that the helical wrapping and insertion are spontaneous. The van der Waals interaction, π–π stacking interaction between the GNR and the SWNT, and dangling σ-orbitals on carbon atoms at open edges of GNR are all responsible for these unique phenomena. Two GNRs would form a DNA-like double-helix with the same handedness. The dependence of the diameter and chirality of SWNT and the width of GNR in the helix-forming process are investigated. The gaps on the tube walls are not good at the helical wrapping and insertion. In addition, we explore the possibility of using the spontaneous wrapping and insertion of GNRs to deliver substances. These unique properties are expected to provide novel strategie...

Electronic transport in large systems through a QUAMBO–NEGF approach: Application to atomic carbon chains

The conductance of single-atom carbon chain (SACC) between two zigzag graphene nanoribbons (GNR) is studied by an efficient scheme utilizing tight-binding (TB) parameters generated via quasi-atomic minimal basis set orbitals (QUAMBOs) and non-equilibrium Greenʼs function (NEGF). Large systems (SACC contains more than 50 atoms) are investigated and the electronic transport properties are found to correlate with SACCʼs parity. The SACCs provide a stable off or on state in broad energy region (0.1–1 eV) around Fermi energy. The off state is not sensitive to the length of SACC while the corresponding energy region decreases with the increase of the width of GNR.

Specific heat of graphene nanoribbons

We studied the specific heat of graphene nanoribbons (GNRs) using an extended force constant model. We found that at low temperature, the specific heat decreases, and its variation with temperature increases with increasing GNR width. However, the specific heat increases with increasing GNR width after crossing a chaotic region. Free boundary conditions, –CHOH-terminated and armchair-edge-induced phonon nondegeneracy, shift and distortion and localized vibrational modes significantly influence GNR specific heat compared with periodic boundary conditions and bare and zigzag edges in GNRs. Finally, we found a uniform expression for specific heat vs. width at every temperature except for the chaotic region.

Charge detection in a bilayer graphene quantum dot

We show measurements on a bilayer graphene quantum dot with an integrated charge detector. The focus lies on enabling charge detection with a 30 nm wide bilayer graphene nanoribbon located approximately 35 nm next to a bilayer graphene quantum dot with an island diameter of about 100 nm. Local resonances in the nanoribbon can be successfully used to detect individual charging events in the dot even in regimes where the quantum dot Coulomb peaks cannot be measured by conventional techniques.

Analytical Modeling of Graphene Nanoribbon Schottky Diodes Using Asymmetric Contacts

This paper presents graphene nanoribbon (GNR) Schottky diode through an analytical approach. To achieve an analytical relation for channel current, first an analytical equation for potential distribution within the GNR is offered. Then by using the WKB approximation, transmission probability through Schottky barriers is derived. Finally the channel current is analytically achieved, which is a function of physical and electrical parameters including gate insulator thickness, Schottky barrier height, drain bias voltage, gate bias voltage, GNR width, and subband number. To get a rectification behavior of presented device, two different metals have been considered at the two ends of a p-type semiconducting GNR resulting in asymmetric contacts. The effect of different parameters such as gate bias voltage, GNR width, and contact metals on the rectification behavior is investigated. We demonstrate that the rectification characteristic, threshold voltage, reverse saturation current, and reverse turn-on voltage can be tuned through the use of these parameters. The derived analytical current well describes the rectifying behavior of presented GNR Schottky diode.

Manifold electronic structure transition of BNC biribbons

Using first principles calculations, we investigate the electronic and magnetic properties of BNC biribbons, a laterally-heterostructured nanoribbon constructed by joining a graphene nanoribbon (GNR) and a BN nanoribbon (BNNR) with zigzag edges. We find that the spin-polarization and electronic structures of the biribbons can be well-tuned by changing the width of the GNR, undergoing manifold transitions from semiconducting to half-metal and ferromagnetic metal. The critical points of GNR width to induce the transitions depend on the interface type (B/C or C/N) rather than the BNNR width. The ground states of metallic BNC biribbons are spin-polarized, forming non-zero magnetic moments. The tunable electronic structures and ferromagnetic ground states make the BNC biribbons promising candidate nanomaterials for building nanoscaled spintronic devices.