Armchair-edge Graphene Nanoribbons Research Articles

Family behavior and Dirac bands in armchair nanoribbons with 4–8 defect lines

Bottom-up synthesis from molecular precursors is a powerful route for the creation of novel synthetic carbon-based low-dimensional materials, such as planar carbon lattices. The wealth of conceivable precursor molecules introduces a significant number of degrees-of-freedom for the design of materials with defined physical properties. In this context, a priori knowledge of the electronic, vibrational and optical properties provided by modern ab initio simulation methods can act as a valuable guide for the design of novel synthetic carbon-based building blocks. Using density functional theory, we performed simulations of the electronic properties of armchair-edged graphene nanoribbons (AGNR) with a bisecting 4–8 ring defect line. We show that the electronic structures of the defective nanoribbons of increasing width can be classified into three distinct families of semiconductors, similar to the case of pristine AGNR. In contrast to the latter, we find that every third nanoribbon is a zero-gap semiconductor with Dirac-type crossing of linear bands at the Fermi energy. By employing tight-binding models including interactions up to third-nearest neighbors, we show that the family behavior, the formation of direct and indirect band gaps and of linear band crossings in the defective nanoribbons is rooted in the electronic properties of the individual nanoribbon halves on either side of the defect lines, and can be effectively through introduction of additional ‘interhalf’ coupling terms.

First-principles simulations of scanning tunneling microscopy images exhibiting anomalous dot patterns on armchair-edged graphene nanoribbons

First-principles simulations of scanning tunneling microscopy images exhibiting anomalous dot patterns on armchair-edged graphene nanoribbons

Platinum contacts for 9-atom-wide armchair graphene nanoribbons

Creating a good contact between electrodes and graphene nanoribbons (GNRs) has been a long-standing challenge in searching for the next GNR-based nanoelectronics. This quest requires the controlled fabrication of sub-20 nm metallic gaps, a clean GNR transfer minimizing damage and organic contamination during the device fabrication, as well as work function matching to minimize the contact resistance. Here, we transfer 9-atom-wide armchair-edged GNRs (9-AGNRs) grown on Au(111)/mica substrates to pre-patterned platinum electrodes, yielding polymer-free 9-AGNR field-effect transistor devices. Our devices have a resistance in the range of 106–108 Ω in the low-bias regime, which is 2–4 orders of magnitude lower than previous reports. Density functional theory calculations combined with the non-equilibrium Green's function method explain the observed p-type electrical characteristics and further demonstrate that platinum gives strong coupling and higher transmission in comparison to other materials, such as graphene.

Twisted Armchair-Edged Graphene Nanoribbons

Key words graphene nanoribbons - deoxygenative coupling - armchair edges - copper catalysis

Quantum interference of electrons through electric-field-induced edge states in stacked graphene nanoribbons

In this work, we investigate, by means of numerical simulations, the quantum interference of electrons in stacked graphene structures consisting of two unequal width, armchair-edged graphene nanoribbons. Electronic states residing near the edges of the system are induced when an external electric field is applied normal to the ribbons. By reversing the direction of electric field in the central region, one can create an electronic analogue of the optical Fabry-Pérot (FP) interferometer. Electronic junctions formed at the boundaries between the central region and the left and right ones in the former play the role of the partially reflected mirrors in the latter. The observed conductance oscillations demonstrate that electrons in the edge states transporting through the system experience quantum interference similar to that of light waves passing through an optical FP interferometer. Moreover, electronic states formed at the junctions enhance inter-edge scattering which affects electron transmission significantly. The possibility to control electron transport via electric gates is also considered.

Bottom-Up Preparation of Twisted Graphene Nanoribbons by Cu-Catalyzed Deoxygenative Coupling.

Graphene nanoribbons (GNRs) are promising in organic optoelectronic materials, and their properties largely depend on the size, edge, and conformation. Herein, the fully armchair-edged GNRs (AGNRs) with lengths up to 2.65 nm by using a Cu-catalyzed deoxygenative coupling as a key step. The resulting AGNRs (2HBT, 3HBT, and 4HBT) possess highly twisted π-scaffolds, and the torsion angles between the adjacent triphenylene moieties are larger than 32°, as proved by crystallographic analyses. Theoretical and spectroscopic studies show that the butoxy groups endow AGNRs with electron-rich features, the extension of the π-system from 2HBT to 4HBT reinforces S0 →S1 excitation, and the distortion of the π-scaffold enhances the fluorescence quantum yield (ΦF ). In particular, 4HBT has the lowest oxidation potential (Eox 1 =0.55 V vs. SCE) and displays red fluorescence with a ΦF value of 81 %.

Unravelling the Complete Raman Response of Graphene Nanoribbons Discerning the Signature of Edge Passivation.

Controlling the edge morphology and terminations of graphene nanoribbons (GNR) allows tailoring their electronic properties and boosts their application potential. One way of making such structures is encapsulating them inside single-walled carbon nanotubes. Despite the versatility of Raman spectroscopy to resolve strong spectral signals of these systems, discerning the response of long nanoribbons from that of any residual precursor remaining outside after synthesis has been so far elusive. Here, the terrylene dye is used as precursor to make long and ultra-narrow armchair-edged GNR inside nanotubes. The alignment and characteristic length of terrylene encapsulated parallel to the tube's axis facilitates the ribbon formation via polymerization, with high stability up to 750°C when the hybrid system is kept in high vacuum. A high temperature annealing is used to remove the terrylene external molecules and a subtraction model based on the determination of a scaling factor related to the G-band response of the system is developed. This not only represents a critical step forward toward the analysis of the nanoribbon-nanotube system, but it is a study that enables unraveling the Raman signatures of the individual CH-modes (the signature of edge passivation) for GNR for the first time with unprecedenteddetail.

Multiple hydrogen bonds enhancing formaldehyde adsorption on functionalized zigzag graphene nanoribbons

In this work, based on density functional theory, the edge and the center of both armchair-edged graphene nanoribbons (AGNRs) and zigzag-edged graphene nanoribbons (ZGNRs) have been modified through five oxygen-containing groups (epoxy, methoxy, formyl, hydroxyl and carboxyl) to form functionalized AGNRs and ZGNRs, in which distorted surface of AGNRs can be observed. Computational results indicated that ZGNR is appealing choice to act as a candidate for designing formaldehyde (HCHO) sensor. Simulation results demonstrate that the interactions between HCHO and oxygen-containing ZGNRs were mainly determined by the multiple hydrogen bonds. The novel design of carboxyl modified ZGNRs on the center of surface with the largest adsorption energy of − 0.510 eV and the relatively long recovery time of 4.15 × 10-4 s could act as a physical sensor to detect HCHO molecules. Dynamics simulation results indicated that HCHO shows very strong hydrogen bonding interactions during competitive adsorption with N2 and O2 at 298.15 K and 308.15 K. Consequently, simulation results offer a molecular-level insight into the interactions between HCHO and functionalized ZGNR surfaces, and might provide useful information for the design of HCHO sensors.

Improved performance of sub-10-nm band-to-band tunneling n-i-n graphene nanoribbon field-effect transistors using underlap engineering: A quantum simulation study

In this study, we investigated the effect of underlapping on improving the performance of sub-10-nm band-to-band tunneling (BTBT) double-gate (DG) armchair-edge graphene nanoribbon field-effect transistors (GNRFETs). BTBT was adopted as an operating regime in n-i-n GNRFETs, where p-type doping is not required unlike the conventional p-i-n tunnel FETs. The quantum model employed was based on self-consistently solving the non-equilibrium Green's function and Poisson's equation while considering ballistic transport. Quantum simulation studies were conducted to analyze the impacts of underlapping on direct source-to-drain tunneling and BTBT, which are the dominant quantum mechanical mechanisms in n-i-n GNRFETs that operate in the BTBT regime, where the ambipolar property is induced by thermionic emission. The underlap-induced dilation in the potential barrier dramatically improved the device switching performance according to atomistic simulations. The computational investigation also considered how increasing the underlap length boosted the performance and scaling capacity of the BTBT GNRFETs. Nanodevices with sub-10-nm gate length and an underlap structure exhibited an improved sub-thermionic swing factor, reduced leakage current, and higher current ratios. Despite the slightly reduced on-current due to the underlap-induced dilation in BTBT windows, the recorded improvements in terms of the subthreshold swing, leakage current, and current ratio mean that the sub-10-nm DG BTBT n-i-n GNRFETs with an underlap configuration can potentially be used as steep ultra-scaled transistors for ultra-low power applications.

Gate potential-controlled current switching in graphene Y-junctions

In this work we investigate the ballistic transport of electrons through three-terminal graphene-based devices. The system consists of a Y-shaped junction formed by three armchair-edged graphene nanoribbons with a rectangular gate potential applied to one of the output branches, whereby current control can be established by the controlling of the refractive index in graphene p–n junctions. Transport properties are obtained by using the Landauer–Büttiker formalism and the tight-binding model within the nearest-neighbor approximation, which allows the calculation of the conductance as function of the Fermi energy, the applied potential, and the system size, as well as the current density. The results demonstrate that the applied electric field can tune the current transmission between the input and two output leads and, consequently, the proposed system acts as a current switch.

Atomic-scale insights into the origin of rectangular lattice in nanographene probed by scanning tunneling microscopy

We conducted atomic-scale scanning tunneling microscopy of a graphene nanosheet on graphite. In addition to a rhombus lattice representing the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R{30}^{\ensuremath{\circ}}$ superstructure, we resolved another quadrangle lattice similar to a rectangle in the sheet. Its lattice size was approximately $0.37\ifmmode\times\else\texttimes\fi{}0.22\phantom{\rule{0.16em}{0ex}}\mathrm{n}{\mathrm{m}}^{2}$. To clarify the origin of this unique rectangular lattice, the overlap of the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ superstructures along the direction of their long diagonals was theoretically examined using a simple model. The electron distribution with high energy in the occupied states of armchair-edged graphene nanoribbons (AGNRs) was calculated based on first principles. A rectangular lattice, resembling the one observed experimentally, was found to form on the AGNR under a specific width condition. This finding was also analyzed in terms of Clar's theory and the scattering of electron waves. We propose that wrinkles and adsorbates in graphene play a role similar to an armchair edge, resulting in the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ phase. If these local defects are in close proximity, the rhombus phases interact to generate electronic structures predicted for AGNRs. This is probably the reason why a rectangular lattice was imaged on the graphene sheet that is not an ideal AGNR.

Sensitive SO2 gas sensor utilizing Pt-doped graphene nanoribbon: First principles investigation

The objective of this work is to seek a highly sensitive gas sensor for the detection of sulfur dioxide (SO2) by investigating the impacts of passivation as well as metal doping of armchair-edge graphene nanoribbon (AGNR) on the adsorption capacity. Herein, two AGNR based sensor materials for SO2 are passivated with hydrogen and nitrogen (AGNRH and AGNRN) then investigated theoretically using density functional theory (DFT). The results demonstrate that AGNRH is more sensitive to SO2 gas than AGNRN with −0.532 eV adsorption energy (Ead) and 0.05 e charge transfer (ΔQ). Furthermore, two more sensor materials are examined via doping AGNRH and AGNRN with platinum: Pt-AGNRH and Pt-AGNRN. Interestingly, the adsorption energies and charge transfer increased remarkably to −6.327 eV (almost 12 times the non-doped case) and 0.373 e, respectively, for the case of the SO2/Pt-AGNRH system. The same trend is observed as well for the case of SO2/Pt-AGNRN. The significant improvement of the adsorption parameters specifies that Pt doping has a considerable impact on enhancing the sensing performance of passivated AGNR toward the detection of SO2 gas.

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.

Switching characteristics of anthraquinone molecular devices based on graphene electrodes

With the development of microelectronics and the miniaturization of electronic devices, the use of molecular materials to construct various components in electronic circuits has become a most likely development trend. Compared with silicon-based semiconductor components, molecular electronic device has the advantages of small size, high integration, low energy consumption and fast response. In recent years, more and more molecules have been used to design molecular devices such as molecular diodes, molecular switches, molecular field effect transistors and molecular memories. In this paper, sandwich structure devices based on graphene nanoribbon electrodes are constructed. The first-principles calculation method combining density functional theory and non-equilibrium Green’s function is adopted to design the molecular devices with functional characteristics. The effects of redox reactions on the electrical transport properties of molecular devices are systematically discussed. The main research contents of this paper are as follows. The switching characteristics of an anthraquinone molecular device based on graphene electrode are studied. The zigzag-edge nanoribbons and armchair-edge graphene nanoribbons are selected as electrodes. Considering the two isomers of anthraquinone (HQ) and anthraquinone (AQ) molecules in the redox reaction, the double electrode molecular junction is constructed. The effects of redox reaction and electrode structure on the switching characteristics of anthraquinone molecular devices are discussed. It is found that the current in the HQ configuration is significantly greater than that in the AQ configuration, regardless of the zigzag-edge graphene electrode or the armchair-edge graphene electrode. That is, under the redox reaction, the anthraquinone molecules show significant switching characteristics. The switching ratio of zigzag-edge graphene electrode is selected to reach a maximum of 3125, and that of armchair-edge graphene electrode is selected to maximum of 1538. In addition, when the armchair-edge graphene is used as an electrode in the HQ configuration, the negative differential resistance is obviously between 0.7 and 0.9 V.

Pure spin current generation via photogalvanic effect with spatial inversion symmetry

We propose a new idea to generate pure spin current with photogalvanic effect (PGE) by designing devices with spatial inversion symmetry based on two-dimensional spin semiconducting materials. Due to the preservation of spatial inversion symmetry, the electric current generated by the PGE must be zero. However, finite spin-dependent current ${I}_{\ensuremath{\uparrow}/\ensuremath{\downarrow}}$ may still be produced. Once an amount of spin-up electrons flow into the device region from lead $\ensuremath{\alpha}$ and flow out from lead $\ensuremath{\beta}$ under light irradiation, the same amount of spin-down electrons will flow into the device region from lead $\ensuremath{\beta}$ and flow out from lead $\ensuremath{\alpha}$, which results in ${I}_{c}^{\ensuremath{\alpha}/\ensuremath{\beta}}={I}_{\ensuremath{\uparrow}}^{\ensuremath{\alpha}/\ensuremath{\beta}}+{I}_{\ensuremath{\downarrow}}^{\ensuremath{\alpha}/\ensuremath{\beta}}=0$ and ${I}_{s}^{\ensuremath{\alpha}/\ensuremath{\beta}}={I}_{\ensuremath{\uparrow}}^{\ensuremath{\alpha}/\ensuremath{\beta}}\ensuremath{-}{I}_{\ensuremath{\downarrow}}^{\ensuremath{\alpha}/\ensuremath{\beta}}\ensuremath{\ne}0$ simultaneously for each lead and thus finite pure spin current arises. As a concrete example, this idea is demonstrated by calculating the PGE in a photoelectric device constructed with an armchair-edged graphene nanoribbon which is divided into two semi-infinite ribbons. The two semi-infinite ribbons are then periodically and symmetrically patterned with ferromagnetic triangle antidots in the form of ``$▹◃$'' to achieve spatial inversion symmetry of the device. We find that the pure spin current can be robustly generated, neither dependent on the photon energy and polarization/helicity angle, nor dependent on whether it is linearly, circularly, or elliptically polarized.

Probing the origin of photoluminescence blinking in graphene nanoribbons: Influence of plasmonic field enhancement

The photoluminescence from aligned 7-atom wide armchair-edge graphene nanoribbons coupled to plasmonic nanoantennas was recently observed to display blinking. Photoluminescence blinking is a hallmark of emission from single quantum emitters. Here we explore the origin of the blinking. We study the influence of the local field enhancement in the vicinity of nanoantennas on the photoluminescence blinking. We observe a clear correlation between the blinking amplitudes and the plasmonic enhancement. For non-resonant metal nanostructures the blinking vanishes almost completely. Our results allow us to conclude that the blinking is an intrinsic feature of the emission from the graphene nanoribbons. This is in contrast to the case of single-molecule surface-enhanced Raman scattering, where it is known that ballistic charge transfer between plasmonic nanoparticles and the molecule under study critically contributes to the blinking.

Photodetection Using Atomically Precise Graphene Nanoribbons

In the search for high-sensitivity, low-noise, and high-bandwidth photodetectors, materials are a key ingredient. One- and two-dimensional materials are of particular interest in this area due to their extraordinary properties such as ballistic transport. Here, we demonstrate nanoscale photoconductive photodetectors using aligned atomically precise seven-atom wide armchair-edge graphene nanoribbons. The detector responsivity is 0.035 mAW–1 at a bias voltage of 2 V. The dark current is below 30 pA for a bias voltage of 1.5 V, which is orders of magnitude lower than that of typical graphene photodetectors. The possibility to align the nanoribbons and to tune their optical and electronic properties by choice of ribbon width and edge structure enables nanoscale polarization-resolving photodetectors optimized for specific spectral ranges. Graphene nanoribbons with identical electronic and optical properties can be prepared on a large scale using bottom-up synthesis, making them a highly interesting material for electronics and optoelectronics.

Small bandgap in atomically precise 17-atom-wide armchair-edged graphene nanoribbons

Bottom-up synthesis of graphene nanoribbons (GNRs) may open new possibilities in future electronic devices owing to their tunable electronic structure, which depends strongly on their well-defined width and edge geometry. For instance, armchair-edged GNRs (AGNRs) exhibit width-dependent bandgaps. However, the bandgaps of AGNRs synthesized experimentally so far are relatively large, well above 1 eV. Such a large bandgap may deteriorate device performance due to large Schottky barriers and carrier effective masses. Here, we describe the bottom-up synthesis of AGNRs with smaller bandgaps, using dibromobenzene-based precursors. Two types of AGNRs with different widths, namely 17 and 13 carbon atoms, were synthesized on Au(111), and their atomic and electronic structures were investigated by scanning probe microscopy and spectroscopy. We reveal that 17-AGNRs have the smallest bandgap, as well as the smallest electron/hole effective mass, among bottom-up AGNRs reported so far. The successful synthesis of 17-AGNRs is a significant step toward the development of GNR-based electronic devices.

On-Surface Synthesis of Graphene Nanoribbons on Two-Dimensional Rare Earth-Gold Intermetallic Compounds.

Here, we demonstrate two reliable routes for the fabrication of armchair-edge graphene nanoribbons (GNRs) on TbAu2/Au(111), belonging to a class of two-dimensional ferromagnetic rare earth-gold intermetallic compounds. On-surface synthesis directly on TbAu2 leads to the formation of GNRs, which are short and interconnected with each other. In contrast, the intercalation approach-on-surface synthesis of GNRs directly on Au(111) followed by rare earth intercalation-yields GNRs on TbAu2/Au(111), where both the ribbons and TbAu2 are of high quality comparable with those directly grown on clean Au(111). Besides, the as-grown ribbons retain the same band gap while changing from p-doping to weak n-doping mainly due to a change in the work function of the substrate after the rare earth intercalation. The intercalation approach might also be employed to fabricate other types of GNRs on various rare earth intermetallic compounds, providing platforms to tailor the electronic and magnetic properties of GNRs on magnetic substrates.

Wavefunctions and energy eigenvalues of charge carriers in zigzag carbon nanotubes and in armchair nanoribbons in the vicinity of Dirac point under the influence of longitudinal electric field

Abstract Finite length graphene nanoribbon with armchair edges impacted by the external electrical field longitudinal to the edges was investigated, and approximate solutions for the electrons energy and wavefunctions in the neighborhood of the Dirac point were obtained. Matrix elements for the position operators are also calculated. Based on the four-point unit cell for elongated zigzag carbon nanotubes and staggered armchair-edge graphene nanoribbons under the influence of longitudinal electric field, the eigenvalues and eigenfunctions are obtained in vicinity of the Dirac point. It is shown that for the not-perturbed Hamilton operator H 0 the eigenvalues of armchair nanoribbon and zigzag nanotube coincide if their lengths are the same, while their spectrum is equidistant. Similarly to carbyne and zigzag edge nanotubes, for armchair edge nanoribbons based on the four-point unit phase cell in the vicinity of Dirac points, the wavefunction is expressed in the form of two-component Dirac function. It is shown that in the case of semiconducting N armchair-edge nanoribbons, where N + 1 = 3 p + δ , δ = ± 1 , N > > 1 , electronic properties can be described using the two uncoupled Majorana equations of 1 + 1 dimension for the particle with non-zero mass. Solutions of these equations are charge-neutral Majorana fields φ ± ( δ ) . These results are of practical interest in designing the supercomputers and experiments in this area. The possibility of generation of terahertz radiation by the Cherenkov mechanism and by analogy with cascade lasers is also discussed.