Ballistic transport in graphene nanostrips in the presence of disorder: importance of edge effects

Thermoelectric Limitations of Graphene Nanodevices at Ultrahigh Current Densities

Graphene is atomically thin, possesses excellent thermal conductivity, and is able to withstand high current densities, making it attractive for many nanoscale applications such as field-effect transistors, interconnects, and thermal management layers. Enabling integration of graphene into such devices requires nanostructuring, which can have a drastic impact on the self-heating properties, in particular at high current densities. Here, we use a combination of scanning thermal microscopy, finite element thermal analysis, and operando scanning transmission electron microscopy techniques to observe prototype graphene devices in operation and gain a deeper understanding of the role of geometry and interfaces during high current density operation. We find that Peltier effects significantly influence the operational limit due to local electrical and thermal interfacial effects, causing asymmetric temperature distribution in the device. Thus, our results indicate that a proper understanding and design of graphene devices must include consideration of the surrounding materials, interfaces, and geometry. Leveraging these aspects provides opportunities for engineered extreme operation devices.

Thermoelectric properties of armchair graphene nanoribbons with array characteristics

The thermoelectric properties of armchair graphene nanoribbons (AGNRs) with array characteristics are investigated theoretically using the tight-binding model and Green's function technique. The AGNR structures with array characteristics are created by embedding a narrow boron nitride nanoribbon (BNNR) into a wider AGNR, resulting in two narrow AGNRs. This system is denoted as w-AGNR/n-BNNR, where 'w' and 'n' represent the widths of the wider AGNR and narrow BNNR, respectively. We elucidate the coupling effect between two narrow symmetrical AGNRs on the electronic structure of w-AGNR/i-BNNR. A notable discovery is that the power factor of the 15-AGNR/5-BNNR with the minimum width surpasses the quantum limitation of power factor for 1D ideal systems. The energy level degeneracy observed in the first subbands of w-AGNR/n-BNNR structures proves to be highly advantageous in enhancing the electrical power outputs of graphene nanoribbon devices.

Nonmonotonic dependence of thermal conductivity on surface roughness: A multiparticle Lorentz gas model

Utilizing surface roughness to manipulate thermal transport has aided important developments in thermoelectrics and heat dissipation in microelectronics. In this paper, through a multiparticle Lorentz gas model, it is found that thermal conductivity oscillates with the increase of surface roughness, and the oscillating thermal conductivity gradually disappears with the increase of nonlinearity. The transmittance analyses reveal that the oscillating thermal conductivity is caused by localized particles due to boundary effects. Nonlinearity will gradually break the localization. Thus, localization still exists in the weak nonlinear system, where there exists an interplay between nonlinear interaction and localization. Furthermore, it is also found that boundary shapes have a great influence on the oscillating thermal conductivity. Finally, we have also studied the oscillating thermal rectification effects caused by rough boundaries. This study gains insight into the boundary effect on thermal transport and provides a mechanism to manipulate thermal conductivity.

An epitaxial graphene platform for zero-energy edge state nanoelectronics

Graphene's original promise to succeed silicon faltered due to pervasive edge disorder in lithographically patterned deposited graphene and the lack of a new electronics paradigm. Here we demonstrate that the annealed edges in conventionally patterned graphene epitaxially grown on a silicon carbide substrate (epigraphene) are stabilized by the substrate and support a protected edge state. The edge state has a mean free path that is greater than 50 microns, 5000 times greater than the bulk states and involves a theoretically unexpected Majorana-like zero-energy non-degenerate quasiparticle that does not produce a Hall voltage. In seamless integrated structures, the edge state forms a zero-energy one-dimensional ballistic network with essentially dissipationless nodes at ribbon-ribbon junctions. Seamless device structures offer a variety of switching possibilities including quantum coherent devices at low temperatures. This makes epigraphene a technologically viable graphene nanoelectronics platform that has the potential to succeed silicon nanoelectronics.

Contact Effects on Thermoelectric Properties of Textured Graphene Nanoribbons

The transport and thermoelectric properties of finite textured graphene nanoribbons (t-GNRs) connected to electrodes with various coupling strengths are theoretically studied in the framework of the tight-binding model and Green's function approach. Due to quantum constriction induced by the indented edges, such t-GNRs behave as serially coupled graphene quantum dots (SGQDs). These types of SGQDs can be formed by tailoring zigzag GNRs (ZGNRs) or armchair GNRs (AGNRs). Their bandwidths and gaps can be engineered by varying the size of the quantum dot and the neck width at indented edges. Effects of defects and junction contact on the electrical conductance, Seebeck coefficient, and electron thermal conductance of t-GNRs are calculated. When a defect occurs in the interior site of textured ZGNRs (t-ZGNRs), the maximum power factor within the central gap or near the band edges is found to be insensitive to the defect scattering. Furthermore, we found that SGQDs formed by t-ZGNRs have significantly better electrical power outputs than those of textured ANGRs due to the improved functional shape of the transmission coefficient in t-ZGNRs. With a proper design of contact, the maximum power factor (figure of merit) of t-ZGNRs could reach 90% (95%) of the theoretical limit.

Effects of Different Phonon Scattering Factors on the Heat Transport Properties of Graphene Ribbons

Understanding the effect of phonon scattering is of primary significance in the study of the thermal transport properties of graphene. While phonon scattering negatively affects the thermal conductivity, the exact effect of microscopic phonon scattering is still poorly understood when full phonon dispersions are taken into account. The heat transport properties of graphene ribbons were investigated theoretically by taking into account different polarization branches with different frequencies in order to understand the physical mechanism of the thermal transport phenomenon at the nanoscale. The effects of grain size, chiral angle, Grüneisen anharmonicity parameter, specularity parameter, and mass-fluctuation-scattering parameter were evaluated, taking into account of the restrictions imposed by boundary, Umklapp, and isotope scattering mechanisms. The contribution from each phonon branch was estimated, and the anisotropic coefficients were determined accordingly. The results indicated that the graphene ribbons are very efficient at conducting heat in all the cases studied. All the acoustical branches contribute significantly to the heat transport properties, and the temperature strongly affects the relative contribution of the phonon branches. The lattice thermal conductivity varies periodically with the chiral angle. The maximum thermal conductivity is achieved in the zigzag direction, and the minimum thermal conductivity is obtained in the armchair direction. The lattice thermal conductivity and anisotropic coefficient depend heavily upon the roughness of the edges and the width of the ribbons. The specularity parameter and mass-fluctuation-scattering parameter significantly affect the lattice thermal conductivity, and the effect arising from isotope scattering is significant in the context of natural isotopic abundance. The dependence of the Grüneisen anharmonicity parameter on phonon branches must be taken into account when making predictions. The results have significant implications for the understanding of the relations between phonon scattering and thermal properties.

Impedance Spectroscopy of Encapsulated Single Graphene Layers

In this work, we demonstrate the use of electrical impedance spectroscopy (EIS) for the disentanglement of several dielectric contributions in encapsulated single graphene layers. The dielectric data strongly vary qualitatively with the nominal graphene resistance. In the case of sufficiently low resistance of the graphene layers, the dielectric spectra are dominated by inductive contributions, which allow for disentanglement of the electrode/graphene interface resistance from the intrinsic graphene resistance by the application of an adequate equivalent circuit model. Higher resistance of the graphene layers leads to predominantly capacitive dielectric contributions, and the deconvolution is not feasible due to the experimental high frequency limit of the EIS technique.

Emergence of carbon nanoscrolls from single walled carbon nanotubes: an oxidative route

Carbon nanoscrolls (CNS), a one dimensional (1D) helical form of carbon, have received enormous attention recently due to their unique structure, superior properties and potential applications. In this work, radial merging of HiPCO single walled nanotube (SWNT) bundles and emergence of CNS are reported following a reflux action involving wet oxidation, HCl washing and annealing at 900 °C. We observe macroscopic quantities of graphene sheets (GS) in the post-treated sample and beautiful manifestation of curling and folding of the GS into CNS. Here, a simple solution based oxidative route for successful merging and exfoliation of SWNT bundles and subsequent formation of CNS are demonstrated and discussed in view of Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) studies. Direct evidence of emergence of CNS from SWNTs via synthesis of GS through a simple oxidative method is reported for the first time.

Electronic properties of one-dimensional pentacene crystals

The electronic properties of an infinite row of freestanding, aligned side-by-side, pentacene molecules are derived as a function of the intermolecular overlap integral and the chemical potential shift. We use a semiclassical approximation and a first principles tight binding method to obtain conductance and mobility of this one-dimensional crystal as a function of temperature and gate voltage. For two values of the intermolecular overlap, energy bands show a metallic behavior. For all the other values, a bandgap is present and evolves with the intermolecular overlap following a typical modulation. The magnitude of the scattering parameters estimated by the observed conductivity is coherent with the existing literature values. These findings could be relevant for the implementation of organic-based sensors.

Enhanced thermoelectric properties in anthracene molecular device with graphene electrodes: the role of phononic thermal conductance

Density functional theory (DFT) and the non-equilibrium Green's function (NEGF) formalism in the linear response regime were employed to investigate the impact of doping on the electronic and phononic transport properties in an anthracene molecule attached to two metallic zigzag graphene nanoribbons (ZGNRs). Boron (B) and nitrogen (N) atoms were used for doping and co-doping (NB) of carbon atoms located at the edge of the anthracene molecule. Our results show that B doping enhances the electronic transport in comparison with the other dopants which is due to its ability to increase the binding energy of the system. The chemical doping of the anthracene molecule mainly impacts on the thermopower which results in a significantly enhanced electronic contribution of the figure of merit. On the contrary, considering the effect of phononic thermal conductance suppresses the figure of merit. However, by taking into account the effect of both electron and phonon contributions to the thermal conductance, we find that the thermoelectric efficiency can be improved by B doping. The potential role of the phononic thermal conductance in shaping the thermoelectric properties of molecular junctions has been ignored in numerous studies, however, our findings demonstrate its importance for a realistic and accurate estimation of the thermoelectric figure of merit.

Intrinsic electronic and transport properties of graphene nanoribbons with different widths

The intrinsic electronic and transport properties of a series of zigzag graphene nanoribbons (ZGNRs) have been investigated systematically using density functional theory coupled with the non-equilibrium Green's function (NEGF) method. It is found that ZGNRs with various ribbon widths have completely different transport properties under bias, depending on whether the number of zigzag chains is odd or even. ZGNRs with odd number except 1-ZGNR possess small current regardless of the bias applied. In contrast, ZGNRs with even number have much larger current and behave as a resistor having a linear current-voltage relationship. The results also reveal that the narrow ZGNRs, e.g., 1-ZGNR and 2-ZGNR, have different transport behaviors, which are governed by the edge-effect and the unique electronic structure.

Topological and transport properties of graphene-based nanojunctions subjected to a magnetic field

We study the topological properties of finite-size S-shaped graphene junctions with distinctive edge features subjected to the perpendicular magnetic field, using the tight-binding model. The quantum confinement and edge effects induced by the specific junction give rise to significant modifications in the Hofstadter spectra of the bent flakes, when compared to those of their perfect forms. Moreover, the results show that in absence of a magnetic field, the sharpest zigzag-edged corners support the edge states rather than the others, but the magnetic field leads to the localization of the edge states along the whole perimeter of the flakes. Furthermore, based on the Green's function method, we investigate the electron transport through our proposed junctions. We show that, under magnetic flux, one can effectively control the energy gap and the conductance around the Fermi energy. Moreover, the transitions between metallic, semimetallic, and semiconducting phases are possible by the magnetic flux in the S-shaped junctions.

Perturbation Electrochemiluminescence Imaging to Observe the Fluctuation of Charge-Transfer Resistance in Individual Graphene Microsheets with Redox-Induced Defects

Here, the fluctuation of charge-transfer resistance in individual reduced graphene oxide (rGO) microsheets with more redox-induced defects is unprecedentedly visualized using a perturbation electrochemiluminescence (ECL) imaging. This perturbation uses a short and low potential to recover defect-covered rGO microsheets slightly and then introduces a high potential to form more redox-induced defects resulting in an increase of charge-transfer resistance. Also, these defects at rGO microsheets enhance their catalytic feature and the resultant ECL intensity so that the temporal resolution in ECL imaging is improved to 30 ms. Aided by this fast imaging approach, the exponential decrease of ECL intensity at individual graphene microsheets after the oxidation is observed, which reflects the increase of their charge-transfer resistances. Since the charge-transfer resistance at electrode surfaces is mainly affected by the conductivity of electrode materials, the result provides the dynamic information to support the reduction of the electrical conductivity in graphene with more defects.

Building and exploring libraries of atomic defects in graphene: Scanning transmission electron and scanning tunneling microscopy study

The presence and configurations of defects are primary components determining materials functionality. Their population and distribution are often nonergodic and dependent on synthesis history, and therefore rarely amenable to direct theoretical prediction. Here, dynamic electron beam-induced transformations in Si deposited on a graphene monolayer are used to create libraries of possible Si and carbon vacancy defects. Deep learning networks are developed for automated image analysis and recognition of the defects, creating a library of (meta) stable defect configurations. Density functional theory is used to estimate atomically resolved scanning tunneling microscopy (STM) signatures of the classified defects from the created library, allowing identification of several defect types across imaging platforms. This approach allows automatic creation of defect libraries in solids, exploring the metastable configurations always present in real materials, and correlative studies with other atomically resolved techniques, providing comprehensive insight into defect functionalities.

Electron Traversal Times in Disordered Graphene Nanoribbons

Using the partition-free time-dependent Landauer-Büttiker formalism for transient current correlations, we study the traversal times taken for electrons to cross graphene nanoribbon (GNR) molecular junctions. We demonstrate electron traversal signatures that vary with disorder and orientation of the GNR. These findings can be related to operational frequencies of GNR-based devices and their consequent rational design.

Atomic-Scale Structural Modification of 2D Materials

2D materials have attracted much attention since the discovery of graphene in 2004. Due to their unique electrical, optical, and magnetic properties, they have potential for various applications such as electronics and optoelectronics. Owing to thermal motion and lattice growth kinetics, different atomic-scale structures (ASSs) can originate from natural or intentional regulation of 2D material atomic configurations. The transformations of ASSs can result in the variation of the charge density, electronic density of state and lattice symmetry so that the property tuning of 2D materials can be achieved and the functional devices can be constructed. Here, several kinds of ASSs of 2D materials are introduced, including grain boundaries, atomic defects, edge structures, and stacking arrangements. The design strategies of these structures are also summarized, especially for atomic defects and edge structures. Moreover, toward multifunctional integration of applications, the modulation of electrical, optical, and magnetic properties based on atomic-scale structural modification are presented. Finally, challenges and outlooks are featured in the aspects of controllable structure design and accurate property tuning for 2D materials with ASSs. This work may promote research on the atomic-scale structural modification of 2D materials toward functional applications.

New Frontiers in Electron Beam-Driven Chemistry in and around Graphene

Modern aberration corrected transmission electron microscopes offer the potential for electron beam sensitive materials, such as graphene, to be examined with low energy electrons to minimize, and even avoid, damage while still affording atomic resolution, and thus providing excellent characterization. Here in this review, the exploits in which the electron beam interactions, which are often considered negative, are explored to usefully drive a wealth of chemistry in and around graphene, importantly, with no other external stimuli. After introducing the technique, this review covers carbon phase reactions between amorphous carbon, graphene, fullerenes, carbon chains, and carbon nanotubes. It then explores different studies with clusters and nanoparticles, followed by coverage of single atom and molecule interactions with graphene, and finally concludes and highlights the anticipated exciting future for electron beam driving chemistry in and around graphene.

Bipolaron Dynamics in Graphene Nanoribbons

Graphene nanoribbons (GNRs) are two-dimensional structures with a rich variety of electronic properties that derive from their semiconducting band gaps. In these materials, charge transport can occur via a hopping process mediated by carriers formed by self-interacting states between the excess charge and local lattice deformations. Here, we use a two-dimensional tight-binding approach to reveal the formation of bipolarons in GNRs. Our results show that the formed bipolarons are dynamically stable even for high electric field strengths when it comes to GNRs. Remarkably, the bipolaron dynamics can occur in acoustic and optical regimes concerning its saturation velocity. The phase transition between these two regimes takes place for a critical field strength in which the bipolaron moves roughly with the speed of sound in the material.

The electronic transport efficiency of a graphene charge carrier guider and an Aharanov-Bohm interferometer

The electrostatic gating defined channel in graphene forms a charge carrier guider. We theoretically investigated electronic transport properties of a single channel and an Aharanov-Bohm (AB) interferometer, based on a charge carrier guider in a graphene nanoribbon. Quantized conductance is found in a single channel, and the guider shows high efficiency in the optical fiber regime, in good agreement with the experiment results. For an AB interferometer without a magnetic field, quantized conductance occurs when there are a few modes inside the channel. The local density of states (LDOS) inside the AB interferometer shows quantum scars when the scattering is strong. At low magnetic field, a periodical conductance oscillation appears. The conductance has a maximum value at zero magnetic field in the absence of intravalley scattering. The mechanism was investigated by LDOS calculations and a toy model.

Graphene Nanoribbons with Atomically Sharp Edges Produced by AFM Induced Self-Folding

The ability to create graphene nanoribbons with atomically sharp edges is important for various graphene applications because these edges significantly influence the overall electronic properties and support unique magnetic edge states. The discovery of graphene self-folding induced by traveling wave excitation through atomic force microscope scanning under a normal force of less than 15 nN is reported. Most remarkably, the crystallographic direction of self-folding may be either along a chosen direction defined by the scan line or along the zigzag or armchair direction in the presence of a pre-existing crack in the vicinity. The crystalline direction of the atomically sharp edge is confirmed via careful lateral force microscopy measurements. Multilayer nanoribbons with lateral dimensions of a few tens of nanometers are realized on the same graphene sheet with different folding types (e.g., z-type or double parallel). Molecular dynamics simulations reveal the folding dynamics and suggest a monotonic increase of the folded area with the applied normal force. This method may be extended to other 2D van der Waals materials and lead to nanostructures that exhibit novel edge properties without the chemical instability that typically hinders applications of etched or patterned graphene nanostructures.

Breakdown of the Law of Reflection at a Disordered Graphene Edge

The law of reflection states that smooth surfaces reflect waves specularly, thereby acting as a mirror. This law is insensitive to disorder as long as its length scale is smaller than the wavelength. Monolayer graphene exhibits a linear dispersion at low energies and consequently a diverging Fermi wavelength. We present proof that for a disordered graphene boundary, resonant scattering off disordered edge modes results in diffusive electron reflection even when the electron wavelength is much longer than the disorder correlation length. Using numerical quantum transport simulations, we demonstrate that this phenomenon can be observed as a nonlocal conductance dip in a magnetic focusing experiment.

Bandgap and pseudohelicity effects over conductance in gapped graphene junctures

We study the conductance in gapped single-layer graphene junctures as a function of bangap, pseudohelicity and charge carriers density. To do it, we first calculate the transmission coefficients of massive charge carries for p-n and n-p-n junctures of gapped single-layer graphene. Next, we calculate the conductance for these two systems using the Landauer formula. Only for the p-n juncture case and non-zero bandgap values, we find the existence of a contribution to the conductance from pseudohelicity inversion states, which is small compared to the contribution from pseudohelicity conservation states. Also, we find for both type of junctures that there exists a window of charge carriers densities values where the conductance is zero (conductance gap), in such a way that the size of this window depends on the squared of the bandgap. We observe that the existence of a bandgap in the system leads to valley mixing and this fact could be useful for the future design of devices based on single-layer graphene.

The Edge Stresses and Phase Transitions for Magnetic BN Zigzag Nanoribbons

The edge states are of particular importance to understand fundamental properties of finite two-dimensional (2D) crystals. Based on first-principles calculations, we investigated on the bare zigzag boron nitride nanoribbons (zzBNNRs) with different spin-polarized states well localized at and extended along their edges. Our calculations examined the edge stress, which is sensitively dependent on the magnetic edge states, for either B-terminated edge or N-terminated edge. Moreover, we revealed that different magnetic configurations lead to a rich spectrum of electronic behaviors at edges. Using an uniaxial tensile strain, we proposed the magnetic phase transitions and thereby obtained the metallic to half-metallic (or reverse) phase transitions at edges. It suggests zzBNNR as a promising candidate for potential applications of non-metal spintronic devices.

Flexible electrochemical biosensors based on graphene nanowalls for the real-time measurement of lactate

We demonstrate a flexible biosensor for lactate detection based on l-lactate oxidase immobilized by chitosan film cross-linked with glutaraldehyde on the surface of a graphene nanowall (GNW) electrode. The oxygen-plasma technique was developed to enhance the wettability of the GNWs, and the strength of the sensor's oxidation response depended on the concentration of lactate. First, in order to eliminate interference from other substances, biosensors were primarily tested in deionized water and displayed good electrochemical reversibility at different scan rates (20-100 mV s^-1), a large index range (1.0 μM to 10.0 mM) and a low detection limit (1.0 μM) for lactate. Next, these sensors were further examined in phosphate buffer solution (to mimick human body fluids), and still exhibited high sensitivity, stability and flexibility. These results show that the GNW-based lactate biosensors possess important potential for application in clinical analysis, sports medicine and the food industry.

A first-principles study on the electronic transport properties of zigzag graphane/graphene nanoribbons

The electronic transport properties of hybrid nanoribbons constructed by substituting zigzag graphane nanoribbons (ZGaNRs) into zigzag graphene nanoribbons (ZGNRs) are investigated with the non-equilibrium Green’s function method and the density functional theory. Both symmetric and asymmetric ZGNRs are considered. The electronic transport of symmetric and asymmetric ZGNR-based hybrid nanoribbons behave distinctly differently from each other even in the presence of the same substitution positions of ZGaNRs. Moreover, the electronic transport of these hybrid systems is found to be enhanced or weakened compared with pristine ZGNRs depending on the substitution position and proportion. Our results suggest that such hybridization is an effective approach to modulate the transport properties of ZGNRs.

Self-Assembled Three-Dimensional Graphene-Based Polyhedrons Inducing Volumetric Light Confinement

The ability to transform two-dimensional (2D) materials into a three-dimensional (3D) structure while preserving their unique inherent properties might offer great enticing opportunities in the development of diverse applications for next generation micro/nanodevices. Here, a self-assembly process is introduced for building free-standing 3D, micro/nanoscale, hollow, polyhedral structures configured with a few layers of graphene-based materials: graphene and graphene oxide. The 3D structures have been further modified with surface patterning, realized through the inclusion of metal patterns on their 3D surfaces. The 3D geometry leads to a nontrivial spatial distribution of strong electric fields (volumetric light confinement) induced by 3D plasmon hybridization on the surface of the graphene forming the 3D structures. Due to coupling in all directions, resulting in 3D plasmon hybridization, the 3D closed box graphene generates a highly confined electric field within as well as outside of the cubes. Moreover, since the uniform coupling reduces the decay of the field enhancement away from the surface, the confined electric field inside of the 3D structure shows two orders of magnitude higher than that of 2D graphene before transformation into the 3D structure. Therefore, these structures might be used for detection of target substances (not limited to only the graphene surfaces, but using the entire volume formed by the 3D graphene-based structure) in sensor applications.

Electronic and optical properties of graphene nanoribbons in external fields

A review work is done for the electronic and optical properties of graphene nanoribbons in magnetic, electric, composite, and modulated fields. Effects due to the lateral confinement, curvature, stacking, non-uniform subsystems and hybrid structures are taken into account. The special electronic properties, induced by complex competitions between external fields and geometric structures, include many one-dimensional parabolic subbands, standing waves, peculiar edge-localized states, width- and field-dependent energy gaps, magnetic-quantized quasi-Landau levels, curvature-induced oscillating Landau subbands, crossings and anti-crossings of quasi-Landau levels, coexistence and combination of energy spectra in layered structures, and various peak structures in the density of states. There exist diverse absorption spectra and different selection rules, covering edge-dependent selection rules, magneto-optical selection rule, splitting of the Landau absorption peaks, intragroup and intergroup Landau transitions, as well as coexistence of monolayer-like and bilayer-like Landau absorption spectra. Detailed comparisons are made between the theoretical calculations and experimental measurements. The predicted results, the parabolic subbands, edge-localized states, gap opening and modulation, and spatial distribution of Landau subbands, have been identified by various experimental measurements.

Role of Kekulé and Non-Kekulé Structures in the Radical Character of Alternant Polycyclic Aromatic Hydrocarbons: A TAO-DFT Study

We investigate the role of Kekulé and non-Kekulé structures in the radical character of alternant polycyclic aromatic hydrocarbons (PAHs) using thermally-assisted-occupation density functional theory (TAO-DFT), an efficient electronic structure method for the study of large ground-state systems with strong static correlation effects. Our results reveal that the studies of Kekulé and non-Kekulé structures qualitatively describe the radical character of alternant PAHs, which could be useful when electronic structure calculations are infeasible due to the expensive computational cost. In addition, our results support previous findings on the increase in radical character with increasing system size. For alternant PAHs with the same number of aromatic rings, the geometrical arrangements of aromatic rings are responsible for their radical character.

Graphene ballistic nano-rectifier with very high responsivity

Although graphene has the longest mean free path of carriers of any known electronic material, very few novel devices have been reported to harness this extraordinary property. Here we demonstrate a ballistic nano-rectifier fabricated by creating an asymmetric cross-junction in single-layer graphene sandwiched between boron nitride flakes. A mobility ∼200,000 cm(2) V(-1) s(-1) is achieved at room temperature, well beyond that required for ballistic transport. This enables a voltage responsivity as high as 23,000 mV mW(-1) with a low-frequency input signal. Taking advantage of the output channels being orthogonal to the input terminals, the noise is found to be not strongly influenced by the input. Hence, the corresponding noise-equivalent power is as low as 0.64 pW Hz(-1/2). Such performance is even comparable to superconducting bolometers, which however need to operate at cryogenic temperatures. Furthermore, output oscillations are observed at low temperatures, the period of which agrees with the lateral size quantization.

Contact conductance of a graphene nanoribbon with its graphene nano-electrodes

Electronically contacted between two graphene nano-electrodes, the contact conductance (G0) of a graphene nanoribbon (GNR) molecular wire is calculated using mono-electronic Elastic Scattering Quantum Chemistry (ESQC) theory. Different nano-electrode contact geometries are considered ranging from a top face to face van der Waals contact to an adiabatic funnel like planar chemical bonding. The Tamm state contributions to the GNR-graphene nano-electrode electronic interactions are discussed as a function of the molecular orbital hybridization. Contrary to the common belief, the adiabatic-like triangle shaped contact nano-graphene electrode does not provide a large G0 as compared to the abrupt contact geometry. The abrupt contact geometry is even worth than a top face to face van der Waals electronic contact with a metal.

Armchair-edged nanoribbon as a bottleneck to electronic total transmission through a topologically nontrivial graphene nanojunction

It is currently a promising approach to experimentally realize the topological insulator phase transition of graphene by introducing the extrinsic spin-orbit coupling (SOC). Then, electronic total transmission through various topological nontrivial graphene nanojunctions (GNJs) is obtainable, if the electronic transport is supported by the helical edge states. Though the bulk graphene is a gapless semiconductor, the inter-valley scattering could introduce a topological trivial gap in semiconducting armchair-edged graphene nanoribbon (GNR). The SOC should be strong enough to reopen a topological nontrivial gap before close such a trivial gap. Therefore, our theoretical study indicates that a semiconducting armchair-edged graphene nanoribbon (GNR) can not develop the helical edge states when the SOC strength is lower than a threshold, though the bulk phase is topological nontrivial. This implies a competition between the SOC and the inter-valley scattering. However, for a metallic armchair-edged GNR, a small SOC can always open a nontrivial gap. Nevertheless, the helical edge state is much less localized than that in a zigzag-edged GNR of the same width. As a result, and by numerically calculating the electronic transmission spectrum of step- and L-shaped GNJs, we conclude that when an armchair-edged GNR is a part of a GNJ, it is the weak point to realize the electronic total transmission even though the bulk phase of graphene is topologically insulating.

Electronic Properties of Zigzag Graphene Nanoribbons Studied by TAO-DFT

Accurate prediction of the electronic properties of zigzag graphene nanoribbons (ZGNRs) has been very challenging for conventional electronic structure methods due to the presence of strong static correlation effects. To meet the challenge, we study the singlet-triplet energy gaps, vertical ionization potentials, vertical electron affinities, fundamental gaps, and symmetrized von Neumann entropy (i.e., a measure of polyradical character) of hydrogen-terminated ZGNRs with different widths and lengths using our recently developed thermally-assistedoccupation density functional theory (TAO-DFT) [Chai, J.-D. J. Chem. Phys. 2012, 136, 154104], a very efficient method for the study of large strongly correlated systems. Our results are in good agreement with the available experimental and high-accuracy ab initio data. The ground states of ZGNRs are shown to be singlets for all the widths and lengths investigated. With the increase of ribbon length, the singlet-triplet energy gaps, vertical ionization potentials, and fundamental gaps decrease monotonically, while the vertical electron affinities and symmetrized von Neumann entropy increase monotonically. On the basis of the calculated orbitals and their occupation numbers, the longer ZGNRs are shown to possess increasing polyradical character in their ground states, where the active orbitals are mainly localized at the zigzag edges.

Thermal treatment-induced structural changes in graphene nanoribbons obtained from partially unzipped double-walled carbon nanotubes

Graphene nanoribbons were synthesized by chemically unzipping double-walled carbon nanotubes followed by evaluation of their nanostructural changes upon thermal annealing.

The green reduction of graphene oxide

Graphene is an ultra-thin material, which has received broad interest in many areas of science and technology because of its unique physical, chemical, mechanical and thermal properties.

Electronic and optical properties of surface-functionalized armchair graphene nanoribbons

The functional groups on armchair graphene nanoribbons affect the spatial distribution of the wavefunction and influence the electronic and optical properties as well.

Electronic and Optical Properties of the Narrowest Armchair Graphene Nanoribbons Studied by Density Functional Methods

In the present study, a series of planar poly(p-phenylene) (PPP) oligomers with n phenyl rings (n = 1–20), designated as n-PP, are taken as finite-size models of the narrowest armchair graphene nanoribbons with hydrogen passivation. The singlet-triplet energy gap, vertical ionization potential, vertical electron affinity, fundamental gap, optical gap, and exciton binding energy of n-PP are calculated using Kohn-Sham density functional theory and time-dependent density functional theory with various exchange-correlation density functionals. The ground state of n-PP is shown to be singlet for all the chain lengths studied. In contrast to the lowest singlet state (i.e., the ground state) of n-PP, the lowest triplet state of n-PP and the ground states of the cation and anion of n-PP are found to exhibit some multi-reference character. Overall, the electronic and optical properties of n-PP obtained from the ωB97 and ωB97X functionals are in excellent agreement with the available experimental data.

A facile approach for the synthesis of copper(ii) myristate strips and their electrochemical activity towards the oxygen reduction reaction

Copper(ii) myristate strips, an inexpensive, straight chain compound of copper act as active electrocatalyst in oxygen reduction reaction.

Tailoring the transmission lineshape spectrum of zigzag graphene nanoribbon based heterojunctions via controlling their width and edge protrusions

We report a first-principles analysis of electron transport through narrow zigzag graphene nanoribbon (up to 2.2 nm) based wedge-shaped heterojunctions. We show that the width difference between the electrode and the scattering region and the edge protrusion of heterojunctions can be tuned to endow the system's transmission spectrum with distinctive features. In particular, transport through junctions with a one sided protrusion in the scattering region is always dominated by a Breit-Wigner-type resonance right at the Fermi level, regardless of the large or small width difference. On the other hand, a junction with protrusions on both sides of the scattering region shows insulating behaviour near the Fermi level for a large width difference but weak transmission channels are formed at the core of the scattering region for a small width difference. When the protrusion is absent in the junction, transmission functions display rather complex structures: double peaks situating nearly symmetrically away from the Fermi level and a strongly asymmetric profile in the vicinity of the Fermi level are observed for large and small width differences, respectively. These results may shed light on the design of real connecting components in nanocircuits.

Conductance of graphene flakes contacted at their corners

Linear conductance of junctions formed by graphene flakes with the order of the nanometer-thick electrodes attached at the corners of the flakes is studied. The explored structures have sizes up to 20,000 atoms and the conductance is studied as a function of applied gate voltage varied around the Fermi level. The finding, obtained computationally, is that junctions formed by armchair-edge flakes with the electrodes connected at the acute-angle corners block the electron transport while only junctions with such electrodes at the obtuse-angle corners tend to provide the high electrical conductance typical for metallic GNRs. The finding in the case of zig-zag edges is similar with the exception of a relatively narrow gate voltage interval in which each studied junction is highly conductive as mediated by the edge states. The contrast between the conductive and insulating setups is typically several orders of magnitude in terms of ratio of their conductances. The main results of the paper also remain to a large extent valid in the presence of edge disorder.