Edges Of Graphene Nanoribbons Research Articles

A comprehensive comparative study of the performance of carbon‐ and copper‐based interconnects in ultra‐large‐scale integrated circuits

AbstractContinuous increase of transistor count in ultra‐large‐scale integrated (ULSI) circuits demands denser interconnection networks to guarantee normal operation. As the distances between adjacent interconnect lines shrinks, severe crosstalk effects appear. New interconnect materials and technologies are proposed to alleviate deteriorating crosstalk effects and maintain overall circuit performance at a desirable level. Carbon‐based materials including carbon nanotubes (CNTs) and graphene nanoribbons (GNRs) are among promising candidate materials to replace copper in future ULSI interconnects. In this work, carbon‐based interconnects including multiwall carbon nanotubes (MWCNTs) and multilayer graphene nanoribbons (MLGNRs), both horizontal (HMLGNRs) and vertical (VMLGNRs), are compared with traditional copper interconnects in terms of crosstalk delay and noise. Due to lower surface roughness, boron nitride (BN) is considered as substrate material for carbon‐based interconnects. To obtain more accurate and reliable results, layer number dependence of dielectric constant of BN multilayers and surface roughness dependence of the resistivity of HMLGNR interconnects are considered. The role of interconnect length, line spacing, and interlayer distance (substrate thickness) in crosstalk delay and noise of coupled carbon‐ and Cu‐based interconnect lines is investigated. It is shown that VMLGNR interconnects with perfect GNR edges exhibit the least crosstalk delay while MWCNT interconnects show far less crosstalk noise than other materials.

On-surface activation of benzylic C-H bonds for the synthesis of pentagon-fused graphene nanoribbons

Graphene nanoribbons (GNRs) have potential for applications in electronic devices. A key issue, thereby, is the fine-tuning of their electronic characteristics, which can be achieved through subtle structural modifications. These are not limited to the conventional armchair, zigzag, and cove edges, but also possible through incorporation of non-hexagonal rings. On-surface synthesis enables the fabrication and visualization of GNRs with atomically precise chemical structures, but strategies for the incorporation of non-hexagonal rings have been underexplored. Herein, we describe the on-surface synthesis of armchair-edged GNRs with incorporated five-membered rings through the C-H activation and cyclization of benzylic methyl groups. Ortho-Tolyl-substituted dibromobianthryl was employed as the precursor monomer, and visualization of the resulting structures after annealing at 300 °C on a gold surface by high-resolution noncontact atomic force microscopy clearly revealed the formation of methylene-bridged pentagons at the GNR edges. These persisted after annealing at 340 °C, along with a few fully conjugated pentagons having singly-hydrogenated apexes. The benzylic methyl groups could also migrate or cleave-off, resulting in defects lacking the five-membered rings. Moreover, unexpected and unique structural rearrangements, including the formation of embedded heptagons, were observed. Despite the coexistence of different reaction pathways that hamper selective synthesis of a uniform structure, our results provide novel insights into on-surface reactions en route to functional, non-benzenoid carbon nanomaterials.

Magnetic localized states and tunable magnetism of single vacancies in generalized chiral graphene nanoribbons

In this paper, we obtain the generalized chiral edges of graphene nanoribbons, through longitudinal unzipping of carbon nanotubes. After analysing the stability and magnetic localized states of the generalized chiral edges based on first-principles calculations, we find the novel phenomena will arise, i.e., antiferromagnetic order in one edge and ferromagnetic order between different edges. And furthermore, the vacancy in the bulk can induce or enhance the magnetic states in the edges.

Термофорез углеродных наночастиц (нанолент и нанотрубок) на плоской многослойной подложке гексагонального нитрида бора (h-BN)

Using the method of molecular dynamics and a 2D chain model, it is shown that thermophoresis of carbon nanoparticles (nanoribbons and nanotubes) on a flat multilayer substrate (on a flat surface of a hexagonal boron nitride crystal) has high efficiency. Placing a nanoparticle on a flat surface of a substrate involved in heat transfer leads to its movement in the direction of the heat flow. The heat flow along the substrate leads to the formation of constant forces acting on the nanoparticle nodes (thermophoresis forces). The main effect of the force is exerted on the edges of graphene nanoribbons, exactly where the main interaction of the nanoribbon with the bending phonons of the substrate occurs. These phonons have a long free path, so the effective transfer of nanoparticles using thermophoresis can occur at sufficiently large distances. The motion of carbon nanoparticles under the action of a heat flow has the form of particle motion in a viscous medium under the action of a constant force. Over time, the nanoparticles always enter the mode of movement at a constant speed. The velocity of the stationary motion is almost the same for all sizes and types of carbon nanoparticles, which is explained by the fact that the thermophoresis force and effective friction have the same source – the interaction of the nanoparticle with the bending thermal vibrations of the substrate layers.

Graphene-based nano-devices: high spin Seebeck and pure spin photogalvanic effects.

We investigate the magnetic, thermoelectric transport, and photogalvanic effect (PGE) properties of two nano-devices based on sawtooth edged graphene nanoribbons (SGNRs). It is found that a robust spin-semiconducting property exists in SGNRs. When SGNRs are arranged in a configuration, a large spin Seebeck coefficient is obtained, indicating a high Seebeck effect under a temperature difference. In addition, we also propose a new spatial inversion symmetry nano-device, which is constructed by two head to head semi-infinite SGNRs in a configuration. The results show that spin-up and spin-down currents are generated by the PGE with opposite flowing directions and the same magnitude. As a result, only a finite pure spin current arises without an accompanying charge current. More importantly, the pure spin current is robustly induced by photons and is independent of the photon energy, polarization angle and the model of polarization (linear or elliptical polarization), which is attributed to the symmetry of the spatial inversion and anti-symmetry of the spin density inversion. The results presented here provide a useful insight into the real application of both spin caloritronics and photoelectric carbon-based nano-devices.

How Do the Coadsorbates Affect the Oxygen Reduction Reaction Activity of Undoped and N-Doped Graphene Nanoribbon Edges? A Density Functional Theory Study

Nitrogen-doped graphene represents a good alternative and a promising catalytic structure for the oxygen reduction reaction (ORR) in fuel cell applications compared with the more costly Pt-containi...

Even\u2013odd conductance effect in graphene nanoribbons induced by edge functionalization with aromatic molecules: basis for novel chemosensors

We theoretically investigate the electron transport in armchair and zigzag graphene nanoribbons (GNRs) chemically functionalized with p-polyphenyl and polyacene groups of increasing length. Our nearest-neighbor tight-binding calculations indicate that, depending on whether the number of aromatic rings in the functional group is even or odd, the resulting conductance at energies matching the energy levels of the corresponding isolated molecule is either unaffected or reduced by exactly one quantum as compared to the pristine GNR, respectively. Such an even–odd effect is shown to originate from a subtle interplay between the electronic states of the guest molecule that are spatially localized on the binding sites and those of the host nanoribbon. We next generalize our findings by employing more accurate tight-binding Hamiltonians along with density-functional theory calculations and critically discuss the robustness of the observed physical effects against the level of theory adopted. Our work offers a comprehensive understanding of the influence of aromatic molecules bound to the edge of graphene nanoribbons on their electronic transport properties, an issue which is instrumental to the prospective realization of graphene-based chemosensors.

Origin of nonlinear current-voltage curves for suspended zigzag edge graphene nanoribbons

The structure-dependent electronic properties of suspended graphene nanoribbon (GNR) have still been important issues. However, these properties have been rarely investigated experimentally due to technical difficulties to produce high-quality suspended GNRs and the identification of atomic structures. Here, we report the electronic properties of suspended GNRs by in situ transmission electron microscopy observations to simultaneously obtain the structural information while measuring the current-voltage (I-V) curves. The suspended GNRs could be obtained by convergent electron beam nanosculpting after careful cleaning by current annealing. The I-V curves were measured for GNRs with a mixture of both zigzag and armchair edges (MGNR) with widths of 3.7 to 1.9 nm and also zigzag edge GNRs (ZGNR) with widths of 1.7 to 1.2 nm. The I-V curves for the ZGNRs were different from those of the MGNRs, as follows. (1) The ZGNRs showed a sharp increase at the threshold voltage in differential conductance-voltage curves. (2) The band gaps measured for ZGNRs were smaller than the band gaps calculated using the GW approximation. (3) The threshold voltage increased with the GNR length. These findings support magnetic-insulator and nonmagnetic-metal nonequilibrium phase transitions as theoretically predicted. Narrow and short ZGNR represents the potential for further nanosized switching devices.

Adiabatic and non-adiabatic quantum charge and spin pumping in zigzag and armchair graphene nanoribbons

We propose a graphene nanoribbon pumping device and study its quantum charge and spin pumping properties for both adiabatic and non-adiabatic regimes by using the Keldysh non-equilibrium Green's function and renormalization procedure. We show that the adiabatic regime is suitable for the generation of high charge current, while the non-adiabatic regime is appropriate for the generation of fully spin polarized and pure spin currents. Also, it is shown that the proposed device can act as a perfect and controllable spin filter. Moreover, we investigate the effects of width and edge of graphene nanoribbons and show that the pumped charge current in the zigzag graphene nanoribbon (ZGNR) strongly depends on nanoribbon width so that the maximum pumped current for width with even numbers of carbon chains is about one order of magnitude larger than that with odd numbers. In contrast with ZGNR, in armchair graphene nanoribbon, the pumped currents with even and odd numbers have the same order of magnitude.

Atomically Dispersed Iron-Nitrogen Sites on Hierarchically Mesoporous Carbon Nanotube and Graphene Nanoribbon Networks for CO2 Reduction.

Atomically dispersed metal and nitrogen co-doped carbon (M-N/C) catalysts hold great promise for electrochemical CO2 conversion. However, there is a lack of cost-effective synthesis approaches to meet the goal of economic mass production of single-atom M-N/C with desirable carbon support architecture for efficient CO2 reduction. Herein, we report facile transformation of commercial carbon nanotube (CNT) into isolated Fe-N4 sites anchored on carbon nanotube and graphene nanoribbon (GNR) networks (Fe-N/CNT@GNR). The oxidization-induced partial unzipping of CNT results in the generation of GNR nanolayers attached to the remaining fibrous CNT frameworks, which reticulates a hierarchically mesoporous complex and thus enables a high electrochemical active surface area and smooth mass transport. The Fe residues originating from CNT growth seeds serve as Fe sources to form isolated Fe-N4 moieties located at the CNT and GNR basal plane and edges with high intrinsic capability of activating CO2 and suppressing hydrogen evolution. The Fe-N/CNT@GNR delivers a stable CO Faradaic efficiency of 96% with a partial current density of 22.6 mA cm-2 at a low overpotential of 650 mV, making it one of the most active M-N/C catalysts reported. This work presents an effective strategy to fabricate advanced atomistic catalysts and highlights the key roles of support architecture in single-atom electrocatalysis.

Engineering Edge States of Graphene Nanoribbons for Narrow-Band Photoluminescence.

Solid-state narrow-band light emitters are on-demand for quantum optoelectronics. Current approaches based on defect engineering in low-dimensional materials usually introduce a broad range of emission centers. Here, we report narrow-band light emission from covalent heterostructures fused to the edges of graphene nanoribbons (GNRs) by controllable on-surface reactions from molecular precursors. Two types of heterojunction (HJ) states are realized by sequentially synthesizing GNRs and graphene nanodots (GNDs) and then coupling them together. HJs between armchair GNDs and armchair edges of the GNR are coherent and give rise to narrow-band photoluminescence. In contrast, HJs between the armchair GNDs and the zigzag ends of GNRs are defective and give rise to nonradiative states near the Fermi level. At low temperatures, sharp photoluminescence emissions with peak energy range from 2.03 to 2.08 eV and line widths of 2-5 meV are observed. The radiative HJ states are uniform, and the optical transition energy is controlled by the band gaps of GNRs and GNDs. As these HJs can be synthesized in a large quantity with atomic precision, this finding highlights a route to programmable and deterministic creation of quantum light emitters.

High gas-sensing selectivity of bilaterally edge-doped graphene nano-ribbons towards detecting NO2, O2 and SO3 gas molecules: Ab-initio investigation

The adsorption and gas-sensing properties of B/N edge-doped graphene nano-ribbons (GNRs) are investigated using state-of-the-art computational technique, which is based on a combination of density-functional theory (DFT) and non-equilibrium Green’s functions (NEGF) formalism. First, the assessment of the effects dopants’ positions, with respect to edges of GNR, on the transport properties has revealed that the bilaterally B/N edge-doping of GNR would yield negative-differential resistance (NDR) IV-characteristics, due to the back-scattering events. Then, the double-edge-doped GNR:B and GNR:N were used to study the gas-sensing properties. The results of adsorption tests show that chemisorption processes can be attained for NO2 and O2 molecules on GNR:B and SO3 molecule on GNR:N. Furthermore, the results of calculations of transport properties show that the chemisorption processes of these molecules can yield enormous rectifications to the IV-characteristics to sweep the NDR behaviors and should consequently yield large sensors responses in GNR-based devices. Comparison to many other gases is performed and it is concluded that edge-doping in both GNR:B and GNR:N would yield exceptionally high selectivity towards detecting toxic NO2 and SO3 gases, respectively. The combined GNR:B- and GNR:N-based sensors are suggested to be used as gas-sensor and alarm-sensor for NO2 gas, respectively. Our theoretical findings are corroborated with available experimental data.

Band Depopulation of Graphene Nanoribbons Induced by Chemical Gating with Amino Groups.

The electronic properties of graphene nanoribbons (GNRs) can be precisely tuned by chemical doping. Here we demonstrate that amino (NH2) functional groups attached at the edges of chiral GNRs (chGNRs) can efficiently gate the chGNRs and lead to the valence band (VB) depopulation on a metallic surface. The NH2-doped chGNRs are grown by on-surface synthesis on Au(111) using functionalized bianthracene precursors. Scanning tunneling spectroscopy resolves that the NH2 groups significantly upshift the bands of chGNRs, causing the Fermi level crossing of the VB onset of chGNRs. Through density functional theory simulations we confirm that the hole-doping behavior is due to an upward shift of the bands induced by the edge NH2 groups.

Investigating electrical properties of controllable graphene nanoribbon field effect transistors

It is always an important issue to open the energy gap for graphene while retaining its high carrier mobility. Narrowing down large-area graphene into graphene nanoribbons (GNRs) is one of the ways to open the energy gap to achieve field-effect transistors (FET) suitable for logic circuits. But it has been a problem to obtain GNRs with controllable width and specific boundary structure. In this work, we proposed a simple high-precision preparation method for GNRs. In the process, GNRs with a width of 200 nm and smooth edges were prepared by a focused ion beam (FIB) etching process. Graphene nanoribbon field-effect transistors (GNR-FET) were fabricated by electron beam lithography (EBL). In this device, due to the proximity of the edge of the He+ during the FIB etching process, the carbon atom structure of the GNRs edge was changed, resulting in variations in electron transport properties. The electrical performance test demonstrated that the on/off current ratio of the GNR-FET device was up to 103 at room temperature. The structural defects of the GNRs caused the device carrier mobility down to 371.6 cm2V−1s−1. The structural defects in the GNRs edge introduced by FIB can improve the on/off current ratio of the device and hence enhanced its electrical regulation performance. Our work provides a simple method of making controllable GNRs to fabricate field effect transistors.

Electrostatic forces above graphene nanoribbons and edges interpreted as partly hydrogen-free.

Graphene nanoribbons' electronic transport properties strongly depend on the type of edge, armchair, zigzag or other, and on edge functionalization that can be used for band-gap engineering. For only partly hydrogenated edges interesting magnetic properties are predicted. Electric charge accumulates at edges and corners. Scanning force microscopy has so far shown the centre of graphene nanoribbons with atomic resolution using a quartz crystal tuning fork sensor of high stiffness. Weak long-range electrostatic forces related to the charge accumulation on the edges of graphene nanoribbons could not be imaged so far. Here, we show the electrostatic forces at the corners and edges of graphene nanoribbons are amenable to measurement. We use soft cantilevers and a bimodal imaging technique to combine enhanced sensitivity to weak long-range electrostatic forces with the high resolution of the second-frequency shift. Additionally, in our work the edges of the nanoribbons are mainly hydrogen-free, opening to the route to investigations of partly hydrogenated magnetic nanoribbons.

Vibrational signature of the graphene nanoribbon edge structure from high-resolution electron energy-loss spectroscopy

Bottom-up approaches exploiting on-surface synthesis reactions allow atomic-scale precision in the fabrication of graphene nanoribbons (GNRs); this is essential for their technological applications since their unique electronic and optical properties are largely controlled by the specific edge structure. By means of a combined experimental-theoretical investigation of some prototype GNRs, we show here that high-resolution electron energy-loss spectroscopy (HREELS) can be successfully employed to fingerprint the details of the GNR edge structure. In particular, we demonstrate how the features of HREEL vibrational spectra - mainly dictated by edge CH out-of-plane modes - are unambiguously related to the GNR edge structure. Moreover, we single out those modes which are localized at the GNR termini and show how their relative intensity can be related to the average GNR length.

Analysis of Temperature-Dependent Crosstalk for Graphene Nanoribbon and Copper Interconnects

In this article, we propose a temperature-based crosstalk analysis model for rough edge graphene nanoribbon (GNR) and Cu interconnect. Temperature-dependent crosstalk delay and noise analysis for 16 nm technology node (ITRS-2011) is performed for three different chip-operating temperatures are −40°C (233 K), i.e. low temperature, 27°C (300 K), i.e. room temperature, and 105°C (378 K), i.e. high temperature. Our analysis shows that, first by increasing the temperature from 233 to 378 K and also by increasing interconnect length from 10 to 100 µm, the GNR shows less amount of crosstalk-induced delay compared with Cu interconnect and crosstalk noise increase slightly which indicates the performance degradation and reliability issues, second the GNR-based interconnect produce (%) variation in delay due to crosstalk and crosstalk noise is lesser than Cu interconnect when interconnect length increase from 10 to 100 µm by increasing temperature. Hence, GNR interconnect is much more thermally viable as compared with traditional Cu-based interconnect and it is very much useful for nano-scale interconnect modeling.

Room-temperature magnetism and tunable energy gaps in edge-passivated zigzag graphene quantum dots

Graphene is a nonmagnetic semimetal and cannot be directly used as electronic and spintronic devices. Here, we demonstrate that zigzag graphene nanoflakes (GNFs), also known as graphene quantum dots, can exhibit strong edge magnetism and tunable energy gaps due to the presence of localized edge states. By using large-scale first principle density functional theory calculations and detailed analysis based on model Hamiltonians, we can show that the zigzag edge states in GNFs ({mathrm{C}}_{6n^2}H6n, n = 1–25) become much stronger and more localized as the system size increases. The enhanced edge states induce strong electron–electron interactions along the edges of GNFs, ultimately resulting in a magnetic configuration transition from nonmagnetic to intra-edge ferromagnetic and inter-edge antiferromagnetic, when the diameter is larger than 4.5 nm (C480H60). Our analysis shows that the inter-edge superexchange interaction of antiferromagnetic states between two nearest-neighbor zigzag edges in GNFs at the nanoscale (around 10 nm) can be stabilized at room temperature and is much stronger than that exists between two parallel zigzag edges in graphene nanoribbons, which cannot be stabilized at ultra-low temperature (3 K). Furthermore, such strong and localized edge states also induce GNFs semiconducting with tunable energy gaps, mainly controlled by adjusting the system size. Our results show that the quantum confinement effect, inter-edge superexchange (antiferromagnetic), and intra-edge direct exchange (ferromagnetic) interactions are crucial for the electronic and magnetic properties of zigzag GNFs at the nanoscale.

Spin polarization in graphene nanoribbons functionalized with nitroxide.

Fine-tuning of magnetic states via an understanding of spin injection on the edge of graphene nanoribbons should allow for greater flexibility of the design of graphene-based spintronics. On the basis of calculations, we predict that coupling constants of the exchange interaction in the series of nitroxide-functionalized ribbon compounds are antiferromagnetic across the ribbons with values 0.2-0.4cm-1 and ferromagnetic along the ribbon with absolute values from 0.05 to 0.07cm-1. Such interacting nitroxide groups induce spin polarization of the edge states of stable graphene nanoribbons. Graphical abstract Exchange coupling constants inducing spin polarization in graphene nanoribbons functionalized with nitroxides.

Thermoelectric properties of nanostructured systems based on narrow armchair graphene nanoribbons

Thermoelectric properties of hybrid systems composed of graphene nanoribbons (GNRs) coupled to rectangular rings or functionalized with aromatic carbon molecules are theoretically addressed here. Graphene-based nanostructures are designed with the purpose of enhancing thermopower responses compared to the thermal performance of pristine GNRs. The electronic transport is calculated using standard tight binding models and the Landauer transport formalism. We found that both semiconducting and metallic armchair nanoribbons coupled to rings exhibit a pronounced enhancement of the thermoelectric responses with comparable intensities, due to Fano antiresonance and Breit–Wigner-like resonances in the electronic transport. As expected, details of the ring geometry and ribbons are important in determining the precise chemical potential values for optimal performance. Different configurations of attached aromatic molecules (single and double molecules) at the graphene nanoribbon edges are addressed. Our findings show that the presence of a molecule induces a gap formation in the metallic pristine GNRs, and a pronounced peak of the Seebeck coefficient is revealed for low chemical potential values, independent of the molecule length. Other features on the Seebeck spectra are found to depend on the electronic nature of the GNRs and on the molecule length and distribution. We have shown that by playing with them, it is possible to design better thermoelectric devices based on GNRs.