Structure Of Graphene Nanoribbons Research Articles

Tailored Synthesis of the Narrowest Zigzag Graphene Nanoribbon Structure by Compressing the Lithium Acetylide under High Temperature

Scientists are searching for the goal-directed methods to synthesize graphene nanoribbons (GNRs) with a particular edge type and width, which determines their electronic transport properties. A series of Li zigzag GNRs (ZGNRs) with different widths were predicted under high pressure with a stoichiometric ratio of Lin+1C2n, which indicates a route to prepare ultranarrow GNRs. Here, with thermodynamics and ab initio Gibbs free-energy calculations by quasi-harmonic approximation, we investigated the phase stabilities of the Li GNR compounds under high pressure and high temperature. We have also identified Li graphenide LiC2 (n = ∞) and Li polyacenide Li3C4 (n = 2) experimentally at the predicted pressure and temperature conditions using in situ X-ray diffraction, which can be recognized as the two end members of Lin+1C2n with the widest and narrowest ZGNR structures. High temperature and the temperature gradient increased the degree of polymerization and facilitated the formation of wider GNR or carbon slice...

Electronic properties of graphene nanoribbons doped with zinc, cadmium, mercury atoms

The effect of substitutional impurities as Zinc (Zn), Cadmium (Cd) and Mercury (Hg) on electronic properties of graphene nanoribbons (GNRs) was investigated by using Density Functional Theory (DFT). A substantial change in the electronic properties of GNR structures was observed while changing the position of dopant atom from the edge to the center of armchair graphene nanoribbons (AGNRs) and zigzag graphene nanoribbons (ZGNRs). The calculations are shown that the electronic band gap of GNRs can be controlled depending on the position of dopant atoms. The calculated electronic band structures for both AGNRs and ZGNRs show spin-dependent metallic or semiconductor behavior according to the position of dopant atoms. From the Density of States (DOS) information, quasi-zero-dimensional (Q0D) and quasi-one-dimensional (Q1D) type behaviors are observed. It is shown that because the doped ZGNRs had the lowest total energies, ZGNRs are energetically more stable than AGNRs.

Graphene-based switched line phase shifter in THz band

In switched line phase shifters, which are used mostly in microwave frequency range, diodes and transistors are used as switching element. In this paper, a switched line phase shifter at the THz band with a graphene nanoribbon structure is proposed. According to the graphene electrical conductivity in the THz band, this material is used to design the switch element. The purpose is designing a 90° phase shifter at 2 THz frequency. 90° phase shifter is analyzed and 22 dB return loss, 6.5 dB insertion loss are achieved at 2 THz. The procedure is repeated to design phase shifter having 22.5°, 45°, 180° and 270° phase changes. The results are promising in terms of phase change, accuracy, return loss and insertion loss.

Bottom-Up Synthesis of Heteroatom-Doped Chiral Graphene Nanoribbons.

Bottom-up synthesis of graphene nanoribbons (GNRs) has significantly advanced during the past decade, providing various GNR structures with tunable properties. The synthesis of chiral GNRs, however, has been underexplored and only limited to (3,1)-GNRs. We report herein the surface-assisted synthesis of the first heteroatom-doped chiral (4,1)-GNRs from the rationally designed precursor 6,16-dibromo-9,10,19,20-tetraoxa-9a,19a-diboratetrabenzo[ a, f, j, o]perylene. The structure of the chiral GNRs has been verified by scanning tunneling microscopy, noncontact atomic force microscopy, and Raman spectroscopy in combination with theoretical modeling. Due to the presence of oxygen-boron-oxygen (OBO) segments on the edges, lateral self-assembly of the GNRs has been observed, realizing well-aligned GNR arrays with different modes of homochiral and heterochiral inter-ribbon assemblies.

Energetics and electronic structures of corrugated graphene nanoribbons

On the basis of the density functional theory with the generalized gradient approximation, we investigated the energetics and electronic structure of graphene nanoribbons (GNRs) with structural corrugation in terms of their edge shape. Although the corrugation gradually increases, the corrugation energy remains constant under small corrugation angles for zigzag GNRs. In contrast, for armchair GNRs, the energy rapidly increases with increasing corrugation angle. The structural corrugation does not affect the local atomic structure of GNRs except that with armchair edges. The electronic structure of GNRs is insensitive to the structural corrugation up to the corrugation angle of 20° for all GNRs.

Electronic Properties of Substitutionally Boron-Doped Graphene Nanoribbons on a Au(111) Surface

High quality graphene nanoribbons (GNRs) grown by on-surface synthesis strategies with atomic precision can be controllably doped by inserting heteroatoms or chemical groups in the molecular precursors. Here, we study the electronic structure of armchair GNRs substitutionally doped with di-boron moieties at the center, through a combination of scanning tunneling spectroscopy, angle-resolved photoemission, and density functional theory simulations. Boron atoms appear with a small displacement towards the surface signaling their stronger interaction with the metal. We find two boron-rich flat bands emerging as impurity states inside the GNR band gap, one of them particularly broadened after its hybridization with the gold surface states. In addition, the boron atoms shift the conduction and valence bands of the pristine GNR away from the gap edge, and leave unaffected the bands above and below, which become the new frontier bands and have negligible boron character. This is due to the selective mixing of boron states with GNR bands according to their symmetry. Our results depict that the GNRs band structure can be tuned by modifying the separation between di-boron moieties.

Concentration Dependence of Dopant Electronic Structure in Bottom-up Graphene Nanoribbons.

Bottom-up fabrication techniques enable atomically precise integration of dopant atoms into the structure of graphene nanoribbons (GNRs). Such dopants exhibit perfect alignment within GNRs and behave differently from bulk semiconductor dopants. The effect of dopant concentration on the electronic structure of GNRs, however, remains unclear despite its importance in future electronics applications. Here we use scanning tunneling microscopy and first-principles calculations to investigate the electronic structure of bottom-up synthesized N = 7 armchair GNRs featuring varying concentrations of boron dopants. First-principles calculations of freestanding GNRs predict that the inclusion of boron atoms into a GNR backbone should induce two sharp dopant states whose energy splitting varies with dopant concentration. Scanning tunneling spectroscopy experiments, however, reveal two broad dopant states with an energy splitting greater than expected. This anomalous behavior results from an unusual hybridization between the dopant states and the Au(111) surface, with the dopant-surface interaction strength dictated by the dopant orbital symmetry.

Bandgap Engineering of Graphene Nanoribbons by Control over Structural Distortion.

Among organic electronic materials, graphene nanoribbons (GNRs) offer extraordinary versatility as next-generation semiconducting materials for nanoelectronics and optoelectronics due to their tunable properties, including charge-carrier mobility, optical absorption, and electronic bandgap, which are uniquely defined by their chemical structures. Although planar GNRs have been predominantly considered until now, nonplanarity can be an additional parameter to modulate their properties without changing the aromatic core. Herein, we report theoretical and experimental studies on two GNR structures with "cove"-type edges, having an identical aromatic core but with alkyl side chains at different peripheral positions. The theoretical results indicate that installment of alkyl chains at the innermost positions of the "cove"-type edges can "bend" the peripheral rings of the GNR through steric repulsion between aromatic protons and the introduced alkyl chains. This structural distortion is theoretically predicted to reduce the bandgap by up to 0.27 eV, which is corroborated by experimental comparison of thus synthesized planar and nonplanar GNRs through UV-vis-near-infrared absorption and photoluminescence excitation spectroscopy. Our results extend the possibility of engineering GNR properties, adding subtle structural distortion as a distinct and potentially highly versatile parameter.

Structural evolution dynamics in fusion of sumanenes and corannulenes: defects formation and self-healing mechanism

The fusion processes of structures consisting of various combinations between sumanene and corannulene, leading to the formation of graphene nanoribbons (GNRs) under heating are simulated by density-functional-based tight-binding molecular dynamics. Distinct stages are unraveled in the course of GNR formation. Firstly, the carbon fragments coalescence into highly strained framework. Secondly, structural reconstruction invokes breaking most strained bonds to form a GNR structure containing numerous defects. Lastly, defects are remedied by the delicate ‘edge-facilitated self-healing’ process through two synergized edge-related effects: elevated mobility of defects and promoted structure reconstructions owing to the remarkable dynamics associated with edges. Importantly, detailed dynamics in the course of forming GNRs with defects and grain boundaries simulated in this work is valuable to provide better understanding at the atomistic scale of defect formation as well as self-healing in the context of the sp2 carbon network. In particular, edges play important roles in not only generating Stone–Wales (SW), 5-8-5 types of defects, 8-5-5-8 and pentagon–heptagon grain boundaries. In addition, our simulations predict the existence of one novel defect, coined as the Inverse SW defect, which is to be confirmed in future experimental studies. This study of dynamic structural evolution reveals that edges are prone to intrinsic and extrinsic modifications such as atomic-scale defects, structural distortions and inhomogeneity.

Effect of edge defects on band structure of zigzag graphene nanoribbons

In this article, we report band structure studies of zigzag graphene nanoribbons (ZGNRs) on introducing defects (sp3 hybridized carbon atoms) in different concentrations at edges by varying the ratio of sp3 to sp2 hybridized carbon atoms. On the basis of theoretical analyses, bandgap values of ZGNRs are found to be strongly dependent on the relative arrangement of sp3 to sp2 hybridized carbon atoms at the edges for a defect concentration; so the findings would greatly help in understanding the bandgap of nanoribbons for their electronic applications.

Atomistic full-quantum transport model for zigzag graphene nanoribbon-based structures: Complex energy-band method

Using a non-equilibrium Green’s function framework in combination with the complex energy-band method, an atomistic full-quantum model for solving quantum transport problems for a zigzag-edge graphene nanoribbon (zGNR) structure is proposed. For transport calculations, the mathematical expressions from the theory for zGNR-based device structures are derived in detail. The transport properties of zGNR-based devices are calculated and studied in detail using the proposed method.

The spin-dependent Seebeck effect and the charge and spin figure of merit in a hybrid structure of single-walled carbon nanotubes and zigzag-edge graphene nanoribbons.

A hybrid structure of carbon nanotubes and graphene nanoribbons was predicted and synthesized (Y. Li et al., Nat. Nanotechnol., 2012, 7, 394-400; P. Lou, J. Phys. Chem. C, 2014, 118, 4475-4482). Herein, using the non-equilibrium Green's function (NEGF) combined with density functional theory (DFT), the thermal spin transport properties and the figure of merit (a material constant proportional to the efficiency of a thermoelectric couple made with the material) of a composite of single-walled carbon nanotubes and zigzag-edge graphene nanoribbons, labeled (6,6)SWCNT/n-ZGNR, are investigated for n = 1, 2, 3, and 8. The results manifest that spin-dependent currents with opposite flow directions were generated when a temperature gradient was applied between two electrodes, indicating the occurrence of the spin-dependent Seebeck effect (SDSE). Remarkably, when n = 3, the charge current is equal to zero, meaning that a perfect SDSE is observed. Moreover, a pure spin-dependent Seebeck diode (SDSD) effect can be observed. Finally, we notice that the device presents an n-type characteristic when n = 1, while the device has a p-type feature when n = 2. In particular, the spin-up thermopower is equal to the spin-down thermopower when n = 3; as a consequence, the charge thermopower is equal to zero, further demonstrating that a perfect SDSE is generated. These discoveries indicate that the (6,6)SWCNT/n-ZGNR is a promising candidate for spin caloritronics devices.

Theoretical Study of Electronic Band Structure of Dumbbell-Shape Graphene Nanoribbons for Highly-sensitive and Stable Strain Sensors

Theoretical Study of Electronic Band Structure of Dumbbell-Shape Graphene Nanoribbons for Highly-sensitive and Stable Strain Sensors

171 Theoretical Study of Electronic Band Structure of Dumbbell-shape Graphene Nanoribbons for Highly-sensitive and Stable Strain Sensors

171 Theoretical Study of Electronic Band Structure of Dumbbell-shape Graphene Nanoribbons for Highly-sensitive and Stable Strain Sensors

Adsorption of gaseous air pollutants over Ti-doped ZGNR structures: a DFT study

In this study, we employed the density functional theory (DFT) to study the interactions between gaseous air pollutants, including di-atomic (CO and NO), tri-atomic (CO2 and HCN), and poly-atomic (HCHO and COCl2) species, and Ti-doped zigzag graphene nanoribbon (ZGNR) structures. During this, two types of doped structures are considered, i.e., SV-ZGNR in which Ti replaces one carbon atom and DV-ZGNR, in which Ti atom replaces two adjacent carbon atoms. All gases under consideration are observed to exhibit significant adsorption energy over doped ZGNR except COCl2 for which a catalytic dissociation is observed. Our results indicate that doped ZGNR is better for CO, NO, and HCHO adsorption as compared to that for reported doped graphene sheet. Also, DV-ZGNR is preferred over SV-ZGNR in terms of adsorption. Moreover, due to the adsorption, changes in the density of states are observed which confirms that the Ti-doped ZGNR has potential to be used as the sensing platform for these gases.

Adsorption of gaseous air pollutants over Ti-doped ZGNR structures: a DFT study

In this study, we employed the density functional theory (DFT) to study the interactions between gaseous air pollutants, including di-atomic (CO and NO), tri-atomic (CO2 and HCN), and poly-atomic (HCHO and COCl2) species, and Ti-doped zigzag graphene nanoribbon (ZGNR) structures. During this, two types of doped structures are considered, i.e., SV-ZGNR in which Ti replaces one carbon atom and DV-ZGNR, in which Ti atom replaces two adjacent carbon atoms. All gases under consideration are observed to exhibit significant adsorption energy over doped ZGNR except COCl2 for which a catalytic dissociation is observed. Our results indicate that doped ZGNR is better for CO, NO, and HCHO adsorption as compared to that for reported doped graphene sheet. Also, DV-ZGNR is preferred over SV-ZGNR in terms of adsorption. Moreover, due to the adsorption, changes in the density of states are observed which confirms that the Ti-doped ZGNR has potential to be used as the sensing platform for these gases.

Tuning the band structure of graphene nanoribbons through defect-interaction-driven edge patterning

We report a systematic analysis of pore-edge interactions in graphene nanoribbons (GNRs) and their outcomes based on first-principles calculations and classical molecular-dynamics simulations. We find a strong attractive interaction between nanopores and GNR edges that drives the pores to migrate toward and coalesce with the GNR edges, which can be exploited to form GNR edge patterns that impact the GNR electronic band structure and tune the GNR band gap. Our analysis introduces a viable physical processing strategy for modifying GNR properties by combining defect engineering and thermal annealing.

Enhancement of the Seebeck effect in bilayer armchair graphene nanoribbons by tuning the electric fields

The Seebeck coefficient in single and bilayer graphene sheets has been observed to be modest due to the gapless characteristic of these structures. In this work, we demonstrate that this coefficient is significantly high in quasi-1D structures of bilayer armchair graphene nanoribbons (BL-AGNRs) thanks to the open gaps induced by the quantum confinement effect. We show that the Seebeck coefficient of BL-AGNRs is also classified into three groups 3p, 3p + 1, 3p + 2 as the energy gap. And for the semiconducting BL-AGNR of width of 12 dimer lines, the Seebeck coefficient is found as high as 707 μV/K and it increases up to 857 μV/K under the impact of the vertical electric field. While in the semimetallic structure of width of 14 dimer lines, the Seebeck coefficient remarkably enhances 14 times from 40 μV/K to 555 μV/K. Moreover, it unveils an appealing result as the Seebeck coefficient always increases with the increase of the applied potential. Such BL-AGNRs appear to be very promising for the applications of the next generation of both electronic and thermoelectric devices applying electric gates.

Влияние адсорбции атомов и молекул кислорода на электронное строение графеновой наноленты

Влияние адсорбции атомов и молекул кислорода на электронное строение графеновой наноленты

Confinement and fermion doubling problem in Dirac-like Hamiltonians

We investigate the interplay between confinement and the fermion doubling problem in Dirac-like Hamiltonians. Individually, both features are well known. First, simple electrostatic gates do not confine electrons due to the Klein tunneling. Second, a typical lattice discretization of the first-order derivative $k\ensuremath{\rightarrow}\ensuremath{-}i{\ensuremath{\partial}}_{x}$ skips the central point and allow spurious low-energy, highly oscillating solutions known as fermion doublers. While a no-go theorem states that the doublers cannot be eliminated without artificially breaking a symmetry, here we show that the symmetry broken by the Wilson's mass approach is equivalent to the enforcement of hard-wall boundary conditions, thus making the no-go theorem irrelevant when confinement is foreseen. We illustrate our arguments by calculating the following: (i) the band structure and transport properties across thin films of the topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, for which we use ab initio density functional theory calculations to justify the model; and (ii) the band structure of zigzag graphene nanoribbons.