Armchair Graphene Nanoribbons Research Articles

Lightwave-driven scanning tunnelling spectroscopy of atomically precise graphene nanoribbons

Atomically precise electronics operating at optical frequencies require tools that can characterize them on their intrinsic length and time scales to guide device design. Lightwave-driven scanning tunnelling microscopy is a promising technique towards this purpose. It achieves simultaneous sub-ångström and sub-picosecond spatio-temporal resolution through ultrafast coherent control by single-cycle field transients that are coupled to the scanning probe tip from free space. Here, we utilize lightwave-driven terahertz scanning tunnelling microscopy and spectroscopy to investigate atomically precise seven-atom-wide armchair graphene nanoribbons on a gold surface at ultralow tip heights, unveiling highly localized wavefunctions that are inaccessible by conventional scanning tunnelling microscopy. Tomographic imaging of their electron densities reveals vertical decays that depend sensitively on wavefunction and lateral position. Lightwave-driven scanning tunnelling spectroscopy on the ångström scale paves the way for ultrafast measurements of wavefunction dynamics in atomically precise nanostructures and future optoelectronic devices based on locally tailored electronic properties.

Density functional theory study of carbon monoxide adsorption on transition metal doped armchair graphene nanoribbon

Density Functional Theory calculations are used to study adsorption of carbon monoxide gas on transition metal doped armchair graphene nanoribbon. Sensing behaviour of pristine armchair graphene nanoribbon (Pr-AGNR) and different geometries of osmium doped AGNR such as one-edge doped AGNR, center doped AGNR and both-edge doped AGNR have been investigated. Most stable adsorption geometry, adsorption energy, binding distance, band structure and density of states of different geometries using in our work have been discussed. Electronic properties of different sensing materials can be interpreted in terms of band structure and density of states. It was studied that a weak type of physical adsorption process took place when CO molecule interacts with pristine AGNR with a very small amount of adsorption energy of amount −0.22 eV. This suggests that pristine graphene is not a good adsorber for CO adsorption. In contrast, doping of graphene with osmium atom enhances the adsorption energy of Pr-AGNR to greater extent. Computational results concluded that osmium doping in graphene nanoribbon at one-edge and both-edge position increases the adsorption energies with values −6.77 eV and −9.53 eV respectively which is 30 times and 43 times greater than in comparison to Pr-AGNR. Energy gap calculations proves that large band gap variations are observed for both-edge doped graphene nanoribbon. Sharp and intense peaks at various energy levels from density of states analysis of both-edge doped geometry confirms adsorption of CO. Our results predicts that both-edge osmium doped sensing material can be considered as promising sensing material for CO adsorption.

Multiphoton intersubband transitions in an armchair graphene nanoribbon

We present an analytical approach to the problem of the interband transitions in an armchair graphene nanoribbon (AGNR), exposed to the time-periodic electric field of strong light wave, polarized parallel to the ribbon axis. The two-dimensional Dirac equation for the massless electron subject to the ribbon confinement is employed. In the resonant approximation the probability of the transitions between the valence and conduction size-quantized subbands are calculated in an explicit form. We trace the dependencies of the Rabi frequency for these transitions on the ribbon width and electric field strength for both the multiphoton-assisted and tunneling regimes relevant to the fast oscillating and practically constant electric field, respectively. Estimates of the expected experimental values for the typically employed AGNR and laser technique facilities show that the Rabi oscillations can be observed under laboratory conditions. The data, corresponding to the intersubband tunneling, makes the AGNR a 1D condensed matter analog, in which the quantum electrodynamic vacuum decay can be detected by the employment of the attainable electric fields.

Peeling and sliding of graphene nanoribbons with periodic van der Waals interactions

The peeling and sliding behaviors of graphene nanoribbons (GNRs) atop a graphene substrate are studied by a continuum model and numerical simulations. A periodic potential energy function is used to simulate the van der Waals interactions between the GNRs and the substrate in both the normal and tangential directions, coupling adhesion and friction within the same model. Numerical simulations and associated analyses reveal remarkably rich dynamics in peeling and sliding of GNRs. It is found that the simple 90-degree peeling of a GNR depends primarily on the normal (adhesive) interactions, with negligible sliding or shear interactions. In contrast, peeling with the end fixed in the in-plane directions leads to stick-slip sliding, with a higher peeling force and a critical peeling angle depending on both adhesion and friction. Notably, the stick-slip sliding is facilitated by formation and gliding of strain solitons in GNRs, and different types of strain solitons may form in the zigzag, armchair and chiral GNRs, including tensile, shear and mixed types. Unconstrained sliding is typically accompanied by both lateral and normal displacements, while constraining the displacements in the normal or lateral directions would generally lead to a higher pulling force for sliding. The peak pulling force as a measure of the sliding friction depends on the ribbon width quasi-linearly but becomes nearly independent of the ribbon length for relatively long GNRs (L >20 nm). Finally, two cases with coupled peeling and sliding of GNRs are considered, and a simple analysis is proposed to simultaneously determine the adhesion and friction properties of GNRs from measurable quantities in potential experiments.

Electrically Induced Dirac Fermions in Graphene Nanoribbons.

Graphene nanoribbons are widely regarded as promising building blocks for next-generation carbon-based devices. A critical issue to their prospective applications is whether their electronic structure can be externally controlled. Here, we combine simple model Hamiltonians with extensive first-principles calculations to investigate the response of armchair graphene nanoribbons to transverse electric fields. Such fields can be achieved either upon laterally gating the nanoribbon or incorporating ambipolar chemical codopants along the edges. We reveal that the field induces a semiconductor-to-semimetal transition with the semimetallic phase featuring zero-energy Dirac fermions that propagate along the armchair edges. The transition occurs at critical fields that scale inversely with the width of the nanoribbons. These findings are universal to group-IV honeycomb lattices, including silicene and germanene nanoribbons, irrespective of the type of edge termination. Overall, our results create new opportunities to electrically engineer Dirac semimetallic phases in otherwise semiconducting graphene-like nanoribbons.

Order from a Mess: The Growth of 5-Armchair Graphene Nanoribbons

The advent of on-surface chemistry under vacuum has vastly increased our capabilities to synthesize carbon nanomaterials with atomic precision. Among the types of target structures that have been synthesized by these means, graphene nanoribbons (GNRs) have probably attracted the most attention. In this context, the vast majority of GNRs have been synthesized from the same chemical reaction: Ullmann coupling followed by cyclodehydrogenation. Here, we provide a detailed study of the growth process of five-atom-wide armchair GNRs starting from dibromoperylene. Combining scanning probe microscopy with temperature-dependent XPS measurements and theoretical calculations, we show that the GNR growth departs from the conventional reaction scenario. Instead, precursor molecules couple by means of a concerted mechanism whereby two covalent bonds are formed simultaneously, along with a concomitant dehydrogenation. Indeed, this alternative reaction path is responsible for the straight GNR growth in spite of the initial mixture of reactant isomers with irregular metal-organic intermediates that we find. The provided insight will not only help understanding the reaction mechanisms of other reactants but also serve as a guide for the design of other precursor molecules.

Hydrogen sulfide gas sensor using osmium doped graphene nanoribbon: An insights from DFT study

First Principles adsorption phenomenon of Hydrogen Sulfide (H2S) gas on surface of undoped armchair graphene nanoribbon (AGNRs) and Osmium doped AGNRs (Os-AGNRs) is reported. Adsorption energies and electronic properties of AGNRs doped with osmium atom in three possible configurations like one-edge doped, center doped and both-edge doped has been investigated theoretically using DFT. Results of adsorption energies revealed that undoped AGNR is insensitive to H2S gas with adsorption energy of about −0.25 eV. Whereas, osmium doped AGNRs at one-edge and both–edge doped position is having adsorption energies −6.67 eV and −9.67 eV, which is nearly 26 times and 38 times more than undoped AGNR. The both edges doped configuration shows significant variation in band gap after adsorption of gas. The study suggests that this novelboth edges doped Os-AGNRmaterial can be highly suitablefor H2S gas adsorption and sensing applications.

Novel hydrogen cyanide gas sensor: A simulation study of graphene nanoribbon doped with boron and phosphorus

Hydrogen Cyanide (HCN) gas is a toxic gas generated by burning of material. In the present study, the sensing material for this gas is optimized with simulation software. The adsorption analysis, stability analysis, structural and electronic properties of armchair graphene nanoribbon (ArGNR) and its doped system has been examined for sensing of HCN using Density Functional Theory with Non Equilibrium Green's Function. The BP co-doped ArGNR is explored for the first time for sensing of HCN gas in this work. The ArGNR studied is in the form of Pristine, Defective state, Boron doped, Phosphorus doped and Boron Phosphorus co-doped. The pristine ArGNR is not much sensitive to HCN gas molecule, whereas on introducing dopants, the sensitivity increased considerably. The changes are observed in optimized geometry and electronic properties of different variants. Among all the variants, Phosphorus doped and BP co-doped ArGNR results in strong adsorption and is most sensitive to the Cyanide molecule, showing adsorption energy 15 and 12 times more as compared to pristine ArGNR. It is proposed that Phosphorus doped and BP co-doped ArGNR may be considered for HCN gas sensor applications. • Boron–Phosphorus co-doped graphene nanoribbon as HCN gas sensing reported for the first time. • DFT analysis of pristine, defective and co-doped armchair graphene nanoribbon carried out. • Boron-Phosphorous co-doped armchair graphene nanoribbon emerges as promising material for sensing. • The large band gap variation from 1.91eV to 1.58eV shows sensitivity towards HCN gas. • The co-doped system shows adsorption energy of -1.23eV which is nearly 12 times higher than its pristine structure.

Engineering plasmon modes and their loss in armchair graphene nanoribbons by selected edge-extended defects

The effect of edge modification of armchair graphene nanoribbons (AGNRs) on the collective excitations are theoretically investigated. The tight-binding method is employed in conjunction with the dielectric function. Unconventional plasmon modes and their association with the flat bands of the specially designed AGNRs are thoroughly studied. We demonstrate the robust relationship between the novel collective excitations and both the type and period of the edge modification. Additionally, we reveal that the main features displayed in the (momentum, frequency)-phase diagrams for both single-particle and collective excitations of AGNRs can be efficiently tuned by edge-extended defects. Our obtained plasmon modes are found to be analogous to magnetoplasmons associated with collective excitations of Landau-quantized electrons. This work provides a unique way to engineer discrete magnetoplasmon-like modes of AGNRs in the absence of magnetic field.

Carbon nanotube-dependent synthesis of armchair graphene nanoribbons

Carbon nanotube-dependent synthesis of armchair graphene nanoribbons

Energy Gaps in BN/GNRs Planar Heterostructure.

Using the tight-binding approach, we study the band gaps of boron nitride (BN)/ graphene nanoribbon (GNR) planar heterostructures, with GNRs embedded in a BN sheet. The width of BN has little effect on the band gap of a heterostructure. The band gap oscillates and decreases from 2.44 eV to 0.26 eV, as the width of armchair GNRs, , increases from 1 to 20, while the band gap gradually decreases from 3.13 eV to 0.09 eV, as the width of zigzag GNRs, , increases from 1 to 80. For the planar heterojunctions with either armchair-shaped or zigzag-shaped edges, the band gaps can be manipulated by local potentials, leading to a phase transition from semiconductor to metal. In addition, the influence of lattice mismatch on the band gap is also investigated.

A Scheme of Quantum Tunnel Field Effect Transistor Based on Armchair Graphene Nano-Ribbon

We proposed a scheme of armchair graphene nanoribbon (AGNR) based tunnel field-effect transistor (TFET). The simulated device consists of two (AGNR) electrodes with zigzag termination that are separated by a narrow gap. The Fermi level of two electrodes is controlled with a common back gate. The main idea is based on taking advantage of the electronic effects of smooth edge atoms of (AGNR) and investigating the effect of applied small uniaxial tensile strain and gate voltage on the output characteristics of simulated TFET. Our analysis shows that the simulated device will have a pronounced negative differential conductance high peak to valley ratio at room temperature. We see that by applying the uniaxial tensile strain, this ratio upgrades to 70 for (AGNR) with width N = 13.

Modification of mechanical, electronic and transport properties of AB-stacked bilayer hybrid armchair graphene nanoribbons: Concurrent influence of chemical derivation and uniaxial strain

In this paper, we investigate the effect of chemical derivation and strain on the mechanical, electronic, and transport properties of AB-stacked bilayer hybrid AGNRs. The results show that the stresses as function of chemical derivation classes are remarkably changed with increasing of width under 10%. Moreover, the failure characteristics of all systems are found under about 11–13% strain. The systems derived by GA have the largest value of Young's modulus. The systems with width of 5 are semiconductors with wide band gap of 3.15 eV–3.81 eV within the strain range −10% to 10%. Interestingly, the 7-GA(FGA)NRs transform the semiconductor into metallic under sufficient tensile strain (over 8% strain). Especially, the 9-GANR may experience a direct gap→indrect gap transition at 8%. The p-orbital of C atoms in purity b-GNRs of edge parts is crucially contributed to the electronic properties near Fermi level. The I-V curve of 7-GANR system exhibits the semiconducting behavior with wide band gap under −8% strain, while for 8% it reveals metallic ones.

Switching Behavior of a Heterostructure Based on Periodically Doped Graphene Nanoribbon

We theoretically propose a switching device that operates at room temperature. The device is an in-plane heterostructure based on a periodically boron-doped (nitrogen-doped) armchair graphene nanoribbon, which has been experimentally fabricated recently. The calculated $I$-$V$ curve shows that for a realistic device with interface width longer than $20$ nm, nonzero electric current occurs only in the region of bias voltage between $\ensuremath{-}0.22$ and $0.28$ V, which is beneficial to low-voltage operation. Furthermore, in this case, the electric current is robust against the change of the potential profile in the interface since the metallic impurity-induced sub-bands with delocalized wave functions contribute to the transmission exclusively. This also suggests the high response speed of the proposed device. We also discuss the temperature dependence, the output impedance, the effect of phonons, and the possible regimes to extend our work, which suggest that our model may have potential room-temperature nanoelectronics applications.

Optimized graphene electrodes for contacting graphene nanoribbons

Atomically precise graphene nanoribbons (GNRs) are a promising emerging class of designer quantum materials with electronic properties that are tunable by chemical design. However, many challenges remain in the device integration of these materials, especially regarding contacting strategies. We report on the device integration of uniaxially aligned and non-aligned 9-atom wide armchair graphene nanoribbons (9-AGNRs) in a field-effect transistor geometry using electron beam lithography-defined graphene electrodes. This approach yields controlled electrode geometries and enables higher fabrication throughput compared to previous approaches using an electrical breakdown technique. Thermal annealing is found to be a crucial step for successful device operation resulting in electronic transport characteristics showing a strong gate dependence. Raman spectroscopy confirms the integrity of the graphene electrodes after patterning and of the GNRs after device integration. Our results demonstrate the importance of the GNR-graphene electrode interface and pave the way for GNR device integration with structurally well-defined electrodes.

Real-time label-free detection of DNA hybridization using a functionalized graphene field effect transistor: a theoretical study.

Detection of DNA hybridization with high sensitivity and accuracy plays a major role in clinical diagnosis and treatment. Despite intense experimental studies of graphene field effect transistor as DNA hybridization detector, the mechanism of detection and changes in the electrical properties of the device is not investigated in detail. To this end, we have investigated an armchair graphene nanoribbon (AGNR) interconnected between gold electrodes as a detector of DNA hybridization. Using non-equilibrium Green’s function method and density functional theory, the effect of 1-pyrenebutanoic acid succinimidyl ester (PBASE) linker, probe, and target DNA on the electrical properties of the device has been investigated at zero bias voltage. The results show that, after functionalization of AGNR with PBASE, the conductance of the device increases while functionalization with probe and target DNA leads to a decrease in conductance. The changes in the projected density of states on the AGNR and transmission around Fermi energy are the reason for the change in conductance of the system. In all cases, both charge transfer and electrostatic gating are responsible for the change in the electrical properties of the system. The results show that our device detects DNA hybridization with a sensitivity of 10% at zero bias voltage, and by applying a suitable gate voltage, it can show higher sensitivity.Graphical abstract

Evaluation of electron transport capability of armchair graphene nanoribbons (AGNRs) by calculating exchange interaction between terminally attached radicals

Electron tunneling efficiency of armchair graphene nanoribbons (AGNRs) with the different widths N (N = 5–14) was evaluated by calculating decay constant βJ of the exchange interaction J between terminal nitronyl nitroxides using broken-symmetry unrestricted density functional theory (UDFT) calculations. The open-shell character of the AGNR moiety originating from the “edge state” was effectively suppressed by the addition of peri-fused benzene ring. Thus calculated J values followed exponential decay against the molecular length, and the βJ values were much smaller for AGNRs with N = 3n − 1 than AGNRs with N = 3n or 3n + 1.

Energy band engineering via “Bite” defect located on N = 8 armchair graphene nanoribbons

Energy band engineering via “Bite” defect located on N = 8 armchair graphene nanoribbons

Modulating the conductance in graphene nanoribbons with multi-barriers under an applied voltage

The electronic transmission and conductance of Dirac electrons in an armchair graphene nanoribbon under an external voltage are investigated with the transfer matrix method. We investigate the velocity and voltage of nanoribbons in the presence of single and multiple barriers and show that the transmission coefficients can be modulated by varying the order of the mode, the number of carbon atoms, and the barrier velocities. In particular, we find that the nanoribbon appears to be fully transparent when the barrier and Fermi velocities are equal. Our numerical results show that the electronic conductance is sensitive to the applied external voltage and number of carbon atoms, which can be used to tailor the electronic properties of graphene-based devices.

Electronic cloaking effect of localized states induced in graphene nanoribbons

The cloaking effect of electronic states was only reported in bilayer graphene. Here in this work we show that this effect can also be induced in armchair graphene nanoribbons (AGNRs), by potential barriers that modulate the chirality property of the system (correlation between pseudospins). These barriers manipulate the chirality and generates pseudospin polarizations on the sides of the barrier, which leaves spatial regions in evidence, in which states behave differently. In AGNRs the extended states (ES), associated with the tunneling of Klein, use only some sites in the nanoribbon lattice (sublattice of ES). On the other hand, the barrier applied in the nanoribbon, induces states totally localized within the region of the barrier, these states use only the sites not used by the sublattice of ES. The localized states remain invisible for electronic transport for all the energies and characteristics of the barrier in the region of the first effective transport band, the same as the states are changing. This electronic cloaking effect can be suppressed by the application of a magnetic field, detecting in the conductance the previously invisible states in the form of Fano resonances. We discuss here the possibility of using this cloaking effect to generate mechanisms that can hide information or to activate hidden system effects.