Characteristics of CVD graphene nanoribbon formed by a ZnO nanowire hardmask

A graphene nanoribbon (GNR) is an important basic structure to open a bandgap in graphene. The GNR processes reported in the literature are complex, time-consuming, and expensive; moreover, the device yield is relatively low. In this paper, a simple new process to fabricate a long and straight graphene nanoribbon with a high yield has been proposed. This process utilizes CVD graphene substrate and a ZnO nanowire as the hardmask for patterning. 8 μm long and 50-100 nm wide GNRs were successfully demonstrated in high density without any trimming, and ～ 10% device yield was realized with a top-down patterning process. After passivating the surfaces of the GNRs using a low temperature atomic layer deposition (ALD) of Al(2)O(3), high performance GNR MOSFETs with symmetric drain-current-gate-voltage (I(d)-V(g)) curves were demonstrated and a field effect mobility up to ～ 1200 cm(2) V(-1) s(-1) was achieved at V(d) = 10 mV.     

Organic Mott insulator-based nanowire formed by using the Nanoscale-electrocrystallization

We obtained organic Mott insulator nanowire by using the Nanoscale-electrocrystallization. The ac electrocrystallization provided nanowires only in the gap between two electrodes though dc yielded them over the anode surface. These nanowires ranged in width from 50 nm to several hundred nm and were several μm in length. We also measured their i-V characteristics.     

Enhanced half-metallicity in edge-oxidized zigzag graphene nanoribbons

We present a comprehensive theoretical study of the electronic properties and relative stabilities of edge-oxidized zigzag graphene nanoribbons. The oxidation schemes considered include hydroxyl, lactone, ketone, and ether groups. Using screened exchange density functional theory, we show that these oxidized ribbons are more stable than hydrogen-terminated nanoribbons except for the case of the etheric groups. The stable oxidized configurations maintain a spin-polarized ground state with antiferromagnetic ordering localized at the edges, similar to the fully hydrogenated counterparts. More important, edge oxidation is found to lower the onset electric field required to induce half-metallic behavior and extend the overall field range at which the systems remain half-metallic. Once the half-metallic state is reached, further increase of the external electric field intensity produces a rapid decrease in the spin magnetization up to a point where the magnetization is quenched completely. Finally, we find that oxygen-containing edge groups have a minor effect on the energy difference between the antiferromagnetic ground state and the above-lying ferromagnetic state.     

Strain effects in graphene and graphene nanoribbons: The underlying mechanism

A tight-binding analytic framework is combined with first-principles calculations to reveal the mechanism underlying the strain effects on electronic structures of graphene and graphene nanoribbons (GNRs). It provides a unified and precise formulation of the strain effects under various circumstances-including the shift of the Fermi (Dirac) points, the change in band gap of armchair GNRs with uniaxial strain in a zigzag pattern and its insensitivity to shear strain, and the variation of the k-range of edge states in zigzag GNRs under uniaxial and shear strains which determine the gap behavior via the spin polarization interaction.      

Nonlocal exchange interaction removes half-metallicity in graphene nanoribbons

Band gap studies of zigzag-edge graphene ribbons are presented. While earlier calculations at LDA level show that zigzag-edge graphene ribbons become half-metallic when cross-ribbon electric fields are applied, our calculations with hybrid density functional demonstrate that finite graphene ribbons behave as half-semiconductors. The spin-dependent band gap can be changed in a wide range, making possible many applications in spintronics.     

Large intrinsic energy bandgaps in annealed nanotube-derived graphene nanoribbons

The usefulness of graphene for electronics has been limited because it does not have an energy bandgap. Although graphene nanoribbons have non-zero bandgaps, lithographic fabrication methods introduce defects that decouple the bandgap from electronic properties, compromising performance. Here we report direct measurements of a large intrinsic energy bandgap of approximately 50 meV in nanoribbons (width, approximately 100 nm) fabricated by high-temperature hydrogen-annealing of unzipped carbon nanotubes. The thermal energy required to promote a charge to the conduction band (the activation energy) is measured to be seven times greater than in lithographically defined nanoribbons, and is close to the width of the voltage range over which differential conductance is zero (the transport gap). This similarity suggests that the activation energy is in fact the intrinsic energy bandgap. High-resolution transmission electron and Raman microscopy, in combination with an absence of hopping conductance and stochastic charging effects, suggest a low defect density.     

Quantum inductance and high frequency oscillators in graphene nanoribbons

Here we investigate high frequency AC transport through narrow graphene nanoribbons with top-gate potentials that form a localized quantum dot. We show that as a consequence of the finite dwell time of an electron inside the quantum dot (QD), the QD behaves like a classical inductor at sufficiently high frequencies ω ≥ GHz. When the geometric capacitance of the top-gate and the quantum capacitance of the nanoribbon are accounted for, the admittance of the device behaves like a classical serial RLC circuit with resonant frequencies ω ～ 100-900 GHz and Q-factors greater than 10(6). These results indicate that graphene nanoribbons can serve as all-electronic ultra-high frequency oscillators and filters, thereby extending the reach of high frequency electronics into new domains.     

Excitonic effects in the optical spectra of graphene nanoribbons

We present a first-principles calculation of the optical properties of armchair-edged graphene nanoribbons (AGNRs) with many-electron effects included. The reduced dimensionality of the AGNRs gives rise to an enhanced electron-hole binding energy for both bright and dark exciton states (0.8-1.4 eV for GNRs with width approximately 1.2 nm) and dramatically changes the optical spectra owing to a near complete transfer of oscillator strength to the exciton states from the continuum transitions. The characteristics of the excitons of the three distinct families of AGNRs are compared and discussed. The enhanced excitonic effects found here are expected to be of importance in optoelectronic applications of graphene-based nanostructures.     

Synthesis of graphene nanoribbons encapsulated in single-walled carbon nanotubes

A novel material, graphene nanoribbons encapsulated in single-walled carbon nanotubes (GNR@SWNT), was synthesized using confined polymerization and fusion of polycyclic aromatic hydrocarbon (PAH) molecules. Formation of the GNR is possible due to confinement effects provided by the one-dimensional space inside nanotubes, which helps to align coronene or perylene molecules edge to edge to achieve dimerization and oligomerization of the molecules into long nanoribbons. Almost 100% filling of SWNT with GNR is achieved while nanoribbon length is limited only by the length of the encapsulating nanotube. The PAH fusion reaction provides a very simple and easily scalable method to synthesize GNR@SWNT in macroscopic amounts. First-principle simulations indicate that encapsulation of the GNRs is energetically favorable and that the electronic structure of the encapsulated GNRs is the same as for the free-standing ones, pointing to possible applications of the GNR@SWNT structures in photonics and nanoelectronics.     

Synthesis of carbon nanotubes by rolling up patterned graphene nanoribbons using selective atomic adsorption

We demonstrate a new method (U.S. Patent Appl., serial no. 60/908039) for synthesizing carbon nanotubes (CNTs), using first-principles and classical molecular dynamics simulations. The single-walled nanotubes (SWNTs) are formed by folding graphene nanoribbons patterned on graphite films through adsorption of atoms of varying coverage, which introduces an external stress to drive the folding process. The diameter and chirality of SWNTs can be a priori controlled by patterning graphene nanoribbons with predefined width and direction so that the postsynthesis sorting process is eliminated. Our method allows potentially mass production of identical tubes and easy integration into device structures on a substrate.     

Spin filtering in graphene nanoribbons with Mn-doped boron nitride inclusions

We investigate the spin filtering effects in graphene nanoribbons, where inclusions of hexagonal boron nitride were introduced together with substitutional magnetic impurities. The embedded Mn-doped boron nitride regions serve as quasi-0D islands of diluted magnetic semiconductor in the otherwise metallic graphene nanoribbon. Our first principle approach based on non-equilibrium Green's functions gives the polarization of the spin current for structures with one or two Mn impurities as a function of the applied bias. For the two impurity case, ferromagnetic and antiferromagnetic spin configurations of the magnetic impurities are considered. The spin resolved current indicates that the analyzed structures are suitable for spin filter applications or for spin current switching devices.     

Enhanced conductance fluctuation by quantum confinement effect in graphene nanoribbons

Conductance fluctuation is usually unavoidable in graphene nanoribbons (GNR) due to the presence of disorder along its edges. By measuring the low-frequency noise in GNR devices, we find that the conductance fluctuation is strongly correlated with the density-of-states of GNR. In single-layer GNR, the gate-dependence of noise shows peaks whose positions quantitatively match the subband positions in the band structures of GNR. This correlation provides a robust mechanism to electrically probe the band structure of GNR, especially when the subband structures are smeared out in conductance measurement.     

Ferromagnetism in graphene nanoribbons: split versus oxidative unzipped ribbons

Two types of graphene nanoribbons: (a) potassium-split graphene nanoribbons (GNRs), and (b) oxidative unzipped and chemically converted graphene nanoribbons (CCGNRs) were investigated for their magnetic properties using the combination of static magnetization and electron spin resonance measurements. The two types of ribbons possess remarkably different magnetic properties. While a low-temperature ferromagnet-like feature is observed in both types of ribbons, such room-temperature feature persists only in potassium-split ribbons. The GNRs show negative exchange bias, but the CCGNRs exhibit a "positive exchange bias". Electron spin resonance measurements suggest that the carbon-related defects may be responsible for the observed magnetic behavior in both types of ribbons. Furthermore, information on the proton hyperfine coupling strength has been obtained from hyperfine sublevel correlation experiments performed on the GNRs. Electron spin resonance finds no evidence for the presence of potassium (cluster) related signals, pointing to the intrinsic magnetic nature of the ribbons. Our combined experimental results may indicate the coexistence of ferromagnetic clusters with antiferromagnetic regions leading to disordered magnetic phase. We discuss the possible origin of the observed contrast in the magnetic behaviors of the two types of ribbons studied.     

Detection of acetone vapor using graphene on polymer optical fiber

Recently, many studies have been focused on the development of fiber optic sensor systems for various gases and vapors. In the present study, an intrinsic polymer optical fiber (POF) sensor using graphene is described for the purpose of acetone vapor sensing for the first time. Observations on the continuous measurement of acetone vapor in dehydrated air are presented. The principle of operation of sensor transduction relies on the dependence of the reflectance on the optical and geometric properties of the sensitive over layered when the vapor molecules are adsorbed on the graphene film. For the same purpose the CVD synthesized graphene film was transferred on the POF end. The synthesized graphene film thickness was evaluated using atomic force microscopy (AFM), Raman spectroscopy and transmission electron microscopy (TEM). For the preliminary evaluation using volatile organic compounds, we evaluated the sensor performance for acetone. Upon the interaction of the sensor with acetone vapor, the variation in the reflected light was monitored as a function of the acetone concentration. The sensor response shows a significant change in sensitivity as compared with the POF probe without a graphene coating. The present sensor shows a satisfactory response upon exposure to various concentrations of acetone vapor from 44 ppm to 352 ppm. To the best of our knowledge, the use of graphene film along with POF for the sensing of volatile organic compounds has not previously been reported.     

A bipolar spin-filtering effect in graphene zigzag nanoribbons with spin-orbit coupling

We predict a large spin-filtering effect in graphene zigzag nanoribbons in the presence of Rashba spin-orbit coupling. The spin polarization of the transmitted current reaches a maximum when the incoming electrons occupy only one subband and the outgoing electrons occupy two subbands (spin is not taken into account). This situation can be reached by applying a potential barrier or a width constriction to the incoming lead of the ribbon. A simple physical picture is provided to explain the spin-filtering effect. Because of the electron-hole symmetry and the time-reversal symmetry, the spin-filtering is antisymmetric for the hole when compared with that for the electron. So the bipolar spin-polarized current can be generated by tuning the Fermi energy across the Dirac point. Besides, the wedge-shaped constriction can modify the conductance spin polarization.