Monolayer Graphene Nanoribbons Research Articles

Electron transmission through graphene monolayer-bilayer junction: An analytical approach

A junction of monolayer and bilayer graphene nanoribbons is investigated using the tight-binding approximation. An external potential is applied on the bilayer graphene layers to control the electronic transport properties of the junction. The reflection and transmission probabilities for an incident electron at the junction are analytically calculated. The dependence of the reflection probability on the external potential, the wave vector of the incident electron and the width of the nanoribbon are evaluated.

Electron transmission through graphene monolayer-bilayer junction: An analytical approach

A junction of monolayer and bilayer graphene nanoribbons is investigated using the tight-binding approximation. An external potential is applied on the bilayer graphene layers to control the electronic transport properties of the junction. The reflection and transmission probabilities for an incident electron at the junction are analytically calculated. The dependence of the reflection probability on the external potential, the wave vector of the incident electron and the width of the nanoribbon are evaluated.

Temperature dependent transfer characteristics of graphene field effect transistors fabricated using photolithography

We report on the temperature dependent transport characteristics of graphene field effect transistors (G-FETs) fabricated using photolithographic technique. Monolayer graphene layers were selected for the fabrication of electronic devices and the fabricated devices were further annealed in Ar/H 2 at 200 °C. The temperature dependence of resistance of the graphene flake shows semiconductor-type behavior. The resistance increases about one order of magnitude upon cooling from 300 K to 8 K. Our observations are good in agreement with the previously reported temperature behavior of monolayer graphene nanoribbons and reduced graphene oxide. A higher drain-current modulation in negative back-gate field with current minimum (the Dirac point) is observed at V GS ∼ −2.75 V. The carrier mobilities were determined from the measured transconductance and obtained mobilities are less than the conductivity and mobility of pristine graphene. The reason could be discussed in detail with variable range hopping mechanism which is consistent to our resistance/temperature data.

Nonlinear Mechanical Properties of Graphene Nanoribbons

ABSTRACTBased on atomistic simulations, the nonlinear elastic properties of monolayer graphene nanoribbons under quasistatic uniaxial tension are predicted, emphasizing the effect of edge structures (armchair and zigzag, without and with hydrogen passivation). The results of atomistic simulations are interpreted using a theoretical model of thermodynamics, which enables determination of the nonlinear functions for the strain-dependent edge energy and the hydrogen adsorption energy, for both zigzag and armchair edges. Due to the edge effects, the initial Young’s modulus of graphene nanoribbons under infinitesimal strain varies with the edge chirality and the ribbon width. Furthermore, it is found that the nominal strain to fracture is considerably lower for armchair graphene nanoribbons than for zigzag ribbons. Two distinct fracture mechanisms are identified, with homogeneous nucleation for zigzag ribbons and edge-controlled heterogeneous nucleation for armchair ribbons.

Radiation-induced quantum interference in low-dimensionaln−pjunctions

We predict and analyze {\it radiation-induced quantum interference effect} in low-dimensional $n$-$p$ junctions. This phenomenon manifests itself by large oscillations of the photocurrent as a function of the gate voltage or the frequency of the radiation. The oscillations result from the quantum interference between two electron paths accompanied by resonant absorption of photons. They resemble Ramsey quantum beating and Stueckelberg oscillations well-known in atomic physics. The effect can be observed in one- and two-dimensional $n$-$p$ junctions based on nanowires, carbon nanotubes, monolayer or bilayer graphene nanoribbons.

Electronic transport in monolayer graphene nanoribbons produced by chemical unzipping of carbon nanotubes

We report on the structural and electrical properties of graphene nanoribbons (GNRs) produced by the oxidative unzipping of carbon nanotubes. GNRs were reduced by hydrazine at 95 °C and further annealed in Ar/H2 at 900 °C; monolayer ribbons were selected for the fabrication of electronic devices. GNR devices on Si/SiO2 substrates exhibit an ambipolar electric field effect typical for graphene. The conductivity of monolayer GNRs (∼35 S/cm) and mobility of charge carriers (0.5–3 cm2/V s) are less than the conductivity and mobility of pristine graphene, which could be explained by oxidative damage caused by the harsh H2SO4/KMnO4 used to make GNRs. The resistance of GNR devices increases by about three orders of magnitude upon cooling from 300 to 20 K. The resistance/temperature data is consistent with the variable range hopping mechanism, which, along with the microscopy data, suggests that the GNRs have a nonuniform structure.

Edge disorder and localization regimes in bilayer graphene nanoribbons

A theoretical study of the magnetoelectronic properties of zigzag and armchair bilayer graphene nanoribbons (BGNs) is presented. Using the recursive Green's function method, we study the band structure of BGNs in uniform perpendicular magnetic fields and discuss the zero-temperature conductance for the corresponding clean systems. The conductance is quantized as $2(n+1){G}_{0}$ for the zigzag edges and $n{G}_{0}$ for the armchair edges with ${G}_{0}=2{e}^{2}/h$ being the conductance unit and $n$ an integer. Special attention is paid to the effects of edge disorder. As in the case of monolayer graphene nanoribbons (GNR), a small degree of edge disorder is already sufficient to induce a transport gap around the neutrality point. We further perform comparative studies of the transport gap ${E}_{g}$ and the localization length $\ensuremath{\xi}$ in bilayer and monolayer nanoribbons. While for the GNRs ${E}_{g}^{\text{GNR}}\ensuremath{\sim}1/W$, the corresponding transport gap ${E}_{g}^{\text{BGN}}$ for the bilayer ribbons shows a more rapid decrease as the ribbon width $W$ is increased. We also demonstrate that the evolution of localization lengths with the Fermi energy shows two distinct regimes. Inside the transport gap, $\ensuremath{\xi}$ is essentially independent on energy and the states in the BGNs are significantly less localized than those in the corresponding GNRs. Outside the transport gap $\ensuremath{\xi}$ grows rapidly as the Fermi energy increases and becomes very similar for BGNs and GNRs.

Theoretical Evaluation of Channel Structure in Graphene Field-Effect Transistors

Energy band structures for monolayer and bilayer armchair graphene nanoribbons on SiC substrates are investigated using a nearest-neighbor tight-binding approximation with two distinct models (interlayer coupling and substrate-induced asymmetry models). Differences in the band structures and charge distributions based on the two models are clarified. The calculation results suggest that wide films of bilayer armchair graphene on SiC substrates should be used for electronics applications such as graphene-channel field-effect transistors.

Magnetoconductance of graphene nanoribbons

The electronic and transport properties of monolayer and AB-stacked bilayer zigzag graphene nanoribbons subject to the influences of a magnetic field are investigated theoretically. We demonstrate that the magnetic confinement and the size effect affect the electronic properties competitively. In the limit of a strong magnetic field, the magnetic length is much smaller than the ribbon width, and the bulk electrons are confined solely by the magnetic potential. Their properties are independent of the width, and the Landau levels appear. On the other hand, the size effect dominates in the case of narrow ribbons. In addition, the dispersion relations rely sensitively on the interlayer interactions. Such interactions will modify the subband curvature, create additional band-edge states, change the subband spacing or the energy gap, and separate the partial flat bands. The band structures are symmetric or asymmetric about the Fermi energy for monolayer or bilayer nanoribbons, respectively. The chemical-potential-dependent electrical and thermal conductance exhibits a stepwise increase behaviour. The competition between the magnetic confinement and the size effect will also be reflected in the transport properties. The features of the conductance are found to be strongly dependent on the field strength, number of layers, interlayer interactions, and temperature.

Molecular dynamics study of ripples in graphene nanoribbons on 6H-SiC(0001): Temperature and size effects

Using the classical molecular dynamics and the simulated annealing techniques, we show that monolayer graphene nanoribbons (GNRs) on 6H-SiC(0001) surface form atomic scale rippled structures. From the analysis of atomic configurations, two different types of rippled structures in GNRs can be identified, namely, the periodic rippled structure at room temperature or even at lower temperatures and random ripples at high temperatures. The dependence of microscopic roughness of the ripples on temperature and size are studied through analyzing the covalent bonding inhomogeneities in bond-length and bond-angle distributions. Our results provide atomic-level information about the rippled GNRs on SiC substrate, which is useful not only for understanding the structure and stability of monolayer GNRs but also for future applications of GNRs in nanoelectronics.

An ab initio study on energy gap of bilayer graphene nanoribbons with armchair edges

Dependency of energy bandgap (Eg) of bilayer armchair graphene nanoribbons (AGNRB) on their widths, interlayer distance (D), and edge doping concentration of boron/nitrogen was investigated using local density approximation and compare to the results of monolayer graphene nanoribbons (AGNRM). Although Eg of AGNRB, in general, is smaller than that of AGNRM, of AGNRB exhibits two distinct groups, metal and semiconductor, while AGNRM displays purely semiconducting behavior. Moreover, Eg of AGNRB is highly sensitive to D, indicating a possible application in tuning Eg by varying D. Finally, edge doping of both AGNR systems reduces Eg by 11%–17%/4%–10% for AGNRM∕AGNRB, respectively.