Armchair Graphene Nanoribbons Research Articles

Integrating Silicon In Armchair Graphene Nanoribbon For Solar Cell Applications

Solar power is not only free but also infinite and use of solar energy indeed has many advantages. Thus, solar technologies are greatly studied for producing low cost and better versions of existing devices that can be integrated in the existing photovoltaic (PV) systems. Many such devices like the solar powered curtains and clothes, cases for laptop and other electronic products are being produced. Silicon today is of course being used rapidly as a PV material for solar cell. However, a continuous effort has been made to use graphene for high efficiency solar cell applications and its use. Graphene on the other hand, is not very good at collecting the electrical current produced inside the solar cell but researchers are looking appropriately on this material. Many ways to modify graphene for this purpose are being studied. Solar cell requires semiconductor materials for exhibiting photovoltaic effect. As graphene is zero band gap material, hence, armchair graphene nanoribbons (AGNR) that are semiconducting are being looked up for solar cell applications. Therefore, in this present work, various attempts have been made to investigate the electronic bandgap by using AGNRs in solar cells. The in-depth analysis of electronic properties has been done where band structure, density of states and geometrical stability on the basis of transmission of energy has been examined. Further, AGNR doping with silicon has been performed in which we have replaced the carbon with silicon. Initially, we started with one-silicon atom as a dopant and then went upto 4 silicon as dopant and the variations has been compared. Thus, through this analysis, the use of doped AGNRs in solar cells is investigated.

Strongly localized states and giant optical absorption induced by multiple flat-bands in AA-stacked multilayer armchair graphene nanoribbons

We propose an AA-stacked multilayer graphene nanoribbon with two symmetrical armchair edges as a multiple flat-band (FB) material. Using the tight-binding Hamiltonian and Green’s function method, we find that the FBs are complete and merged into many dispersive bands. The FBs cause multiple strongly localized states (SLSs) at the sites of the odd lines in every sublayer and a giant optical absorption (GOA) at energy point 2t, where t is the electronic intralayer hopping energy between two nearest-neighbor sites. By driving an electric field perpendicular to the ribbon plane, the bandgaps of the FBs are tunable. Accordingly, the positions of the SLSs in the energy regime can be shifted. However, the position of the GOA is robust against such field, but its strength exhibits a collapse behavior with a fixed quantization step. On the contrary, by driving an electric field parallel to the ribbon plane, the completeness of FBs is destroyed. Resultantly, the SLSs and GOA are suppressed and even quenched. Therefore, such ribbons may be excellent candidates for the design of the controllable information-transmission and optical-electric nanodevices.

Periodic Nanoarray of Graphene pn‐Junctions on Silicon Carbide Obtained by Hydrogen Intercalation

AbstractGraphene pn‐junctions offer a rich portfolio of intriguing physical phenomena. They stand as the potential building blocks for a broad spectrum of future technologies, ranging from electronic lenses analogous to metamaterials in optics, to high‐performance photodetectors important for a variety of optoelectronic applications. The production of graphene pn‐junctions and their precise structuring at the nanoscale remains to be a challenge. In this work, a scalable method for fabricating periodic nanoarrays of graphene pn‐junctions on a technologically viable semiconducting SiC substrate is introduced. Via H‐intercalation, 1D confined armchair graphene nanoribbons are transformed into a single 2D graphene sheet rolling over 6H‐SiC mesa structures. Due to the different surface terminations of the basal and vicinal SiC planes constituting the mesa structures, different types of charge carriers are locally induced into the graphene layer. Using angle‐resolved photoelectron spectroscopy, the electronic band structure of the two graphene regions are selectively measured, finding two symmetrically doped phases with p‐type being located on the basal planes and n‐type on the facets. The results demonstrate that through a careful structuring of the substrate, combined with H‐intercalation, integrated networks of graphene pn‐junctions could be engineered at the nanoscale, paving the way for the realization of novel optoelectronic device concepts.

Unconventional Metallicity in Graphene Nanoribbons with Armchair Edges

AbstractUsing first‐principles density functional theory calculations, 1D armchair graphene nanoribbons (AGNRs) and graphene with an extended line defect (ELD) mimicking a grain boundary (GB) are investigated. Two AGNRs with n and m indices are laterally coupled, generating an ELD with alternated four‐ and eight‐membered carbon rings. Band structure, electronic density of states, wave functions, scanning tunneling microscope image, and quantum transport are analyzed. Unlike the conventional semiconducting behavior of AGNRs, the authors demonstrated that AGNRs with a 4‐8‐ELD exhibit a metallic behavior for n = m = 3p + 2 (p ≥ 0). They further demonstrate that graphene with 4‐8‐ELDs is metallic compared to the semi‐metallic behavior shown by the pristine graphene. Additionally, they investigate the effect of a single vacancy placed at the 4‐8‐ELDs on the electronic properties of AGNRs. The present investigation provides a theoretical basis to tailor graphene structures with possible applications in electronic devices.

First-principles study on controlling transport gap of graphene nanoribbons using hybrid Armchair–Zigzag nanostructures

The electronic and transport properties of hybrid armchair–zigzag nanostructures, which include right-angles-shaped GNRs, U-shaped GNRs, and patterned nanopores structured GRNs, were studied by the combination of density functional theory and nonequilibrium Green’s function method. The density of state, electron transmission spectra, and molecular orbitals were analyzed. The obtained results show that right-angles-shaped GNRs junctions tend to open a transport gap when the numbers of right-angles are greater than or equal to 4. The gap increases insignificantly as the numbers of right-angles are greater than 4. It implies that the U-shaped GNR junction, corresponding to 4 right-angles structure, is a potential structure for controlling transport gap of GRNs. The transport gap of the U-shaped GNRs decreases as the length of horizontal edge increases. In contrast, the transport gap increases as the high of vertical edge increases. In addition, the patterned nanopore had an enormous influence on the electronic and transport properties though the armchair GNRs junctions, depending on the shape and size of nanopore. This research suggests that designed tailored GNRs based on hybrid armchair–zigzag nanostructures can be used to control the transport gap of graphene. The formation of quasi-bound states at zigzag edges of the hybrid nanostructures plays a key role.

An interface trap charge model for simulation of graphene-based synaptic field effect transistors

We propose a compact computational method based on the capacitance model for the efficient design of graphene-based synaptic field effect transistors (FETs), in which the hysteresis of conduction characteristics due to the channel–gate interface trap is used as synaptic plasticity. Using our method to calculate the conduction properties of graphene and armchair graphene nanoribbon (AGNR) superlattice FETs, it is shown that the AGNR can achieve an efficient conductance change rate Δw, which is approximately 7.4 times that of graphene. It was also found that Δw was the greatest when the gate oxide thickness was around 2–3 nm, which is near the limit of miniaturization. These results suggest that the proposed synaptic FETs are a promising approach to realize large scale integration chips for biological timescale computation.

A theoretical study of HCN adsorption and width effect on co-doped armchair graphene nanoribbon

Hydrogen cyanide (HCN) is one of the hazardous gas and requires sensitive detection material. In the present work stable structures, adsorption analysis, charge transfer and electronic properties of defective, pristine, Phosphorus doped, Boron doped and Boron Phosphorus co-doped ArGNRs are studied using first principles calculations combined with Density functional theory (DFT). In two models of ArGNRs with N = 6 and N = 8 atoms along width, it is observed that pristine ArGNR is not a good sensing material for HCN molecule however, on introducing defects and dopants its sensitivity drastically increases and results in chemisorption for all models of ArGNRs. Different doping site optimizations for all models of ArGNR are observed which reveal that defective, Phosphorous doped, Boron-Phosphorus co-doped ArGNR with width N = 6 shows higher negative value of adsorption energy with adsorption energy values are −5.81 eV, −1.27 eV and −1.06 eV. Band nature predicts that for BP-ArGNR with N = 6 shows large reduction as compared to co-doped system of N = 8 model system. A remarkable charge transfer occurs between the HCN molecule and BP-ArGNR as proved by the electronic charge densities. DOS analysis predicts that the defective, phosphorus doped and boron-phosphorus co-doped ArGNRs are good sensing materials with intense peaks after adsorption of cyanide molecule as compared to pristine ArGNR for N = 6 model system. This implies that defective, Phosphorus and B, P co-doped ArGNR with N = 6 has high sensitivity to cyanide molecule as compared to N = 8 model system. Results of our simulated work are not only helpful to evaluate sensing mechanism but also provide a potential sensor material detection of HCN.

Realizing Pure Spin Current by Photogalvanic Effect in Armchair Graphene Nanoribbons with Nano-Constriction Engineering

Realizing Pure Spin Current by Photogalvanic Effect in Armchair Graphene Nanoribbons with Nano-Constriction Engineering

Inducing a topological transition in graphene nanoribbon superlattices by external strain.

Armchair graphene nanoribbons, when forming a superlattice, can be classified into different topological phases, with or without edge states. By means of tight-binding and classical molecular dynamics (MD) simulations, we studied the electronic and mechanical properties of some of these superlattices. MD shows that fracture in modulated superlattices is brittle, as for unmodulated ribbons, and occurs at the thinner regions, with staggered superlattices achieving a larger fracture strain than inline superlattices. We found a general mechanism to induce a topological transition with strain, related to the electronic properties of each segment of the superlattice, and by studying the sublattice polarization we were able to characterize the transition and the response of these states to the strain. For the cases studied in detail here, the topological transition occurred at ∼3-5% strain, well below the fracture strain. The topological states of the superlattice - if present - are robust to strain even close to fracture. The topological transition was characterized by means of the sublattice polarization of the states.

Edge Engineered Graphene Nanoribbons as Nanoscale Interconnect: DFT Analysis

Density functional theory (DFT) with non-equilibrium Green's function (NEGF) formalism and HSPICE simulator have been used to analyse the effect of elemental oxygen atom passivation on the structural, electronic and transport properties of Graphene Nanoribbons (GNRs). The analysis of delay, power dissipation, crosstalk effect, stability and frequency analysis has also been performed to understand its interconnect application. The present study includes all the possible morphologies of zigzag and armchair edge states of GNRs with oxygen and hydrogen passivation. The structural stability of GNRs, analysed in terms of binding energy <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$(E_{binding})$</tex-math></inline-formula> observed that the Zigzag graphene nanoribbon (ZGNR) with both edge oxygen passivated, is the most stable configuration, where stability of the configuration increases with Oxygen concentration. Further, fro <uri xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">text</uri> m the bandstructure and density-of-states calculations, it has been observed that considered 6 atom wide and hydrogen passivated Armchair GNR (AGNR) is semiconducting in nature, whereas, with same width hydrogen passivated ZGNR is metallic. On the other hand, passivation of AGNR with increasing concentration of oxygen form them to metallic in nature. Based on the enhanced metallicity and increment in fermi velocity due to passivation of GNRs with oxygen, these structures may be a potential candidate for interconnects. Their computed electron transport properties, dynamical parameters, delay, power delay product and crosstalk induced delay confirms that the zigzag GNR with both the edges passivated with oxygen (O-ZGNR-O) can be considered as best contender for interconnect application due to its remarkable electrical and thermal transport in comparison to other GNR's. The O-ZGNR-O shows lowest value of kinetic inductance and quantum capacitance of. 01032H/m and 2.21 nF/m respectively with higher stability and higher immunity to crosstalk effect in comparison to other proposed GNRs, which is required for nanoscale interconnects. The relative stability of GNRs have been analyzed in terms of nyquist plot with different interconnect lengths. The results suggest that O-ZGNR-O have lowest delay and power dissipation with higher stability, hence defends its application for interconnect application.

Density Functional Theory Study of Manganese doped Armchair Graphene Nanoribbon for Effective Carbon Dioxide Gas Sensing

Density Functional Theory Study of Manganese doped Armchair Graphene Nanoribbon for Effective Carbon Dioxide Gas Sensing

Design of multifunctional spin logic gates based on manganese porphyrin molecules connected to graphene electrodes.

The spin-resolved transport properties of molecular logic devices composed of two Mn porphyrin molecules connected to each other via a six-carbon atomic chain were studied using the non-equilibrium Green's function combined with density functional theory. The molecules were symmetrically connected to armchair graphene nanoribbon electrodes through four-carbon atomic chains on the left- and right-hand sides. Our calculations revealed that the spin-resolved current-voltage curves depend on the initial spin setting of the transition metal Mn atoms and carbon atoms on the zigzag edges where the electrodes come in contact with the molecule. By simultaneously regulating the spin orientations of the intermediate functional molecules and the zigzag edges of the armchair graphene nanoribbon electrodes, seven spin polarization configurations were obtained. These configurations were examined in this study considering the spin-related symmetry of molecular junctions. By meticulously selecting different combinations according to the specific input and output signals, YES, NOT, OR, NOR, and XOR multifarious spin logic devices were created. The findings of this study are expected to contribute toward the extension of molecular junction functions in future spintronic integrated circuit design and further miniaturization.

Electronic properties and reactivity of oxidized graphene nanoribbons and their interaction with phenol.

The effect of the oxidized functional groups on the structural, electronic, and reactivity properties of armchair graphene nanoribbons has been investigated in the framework of the density functional theory. The presence of functional groups near the edges stabilizes the oxidized graphene nanoribbons (OGNRs) more than substituting near the center. Overall, we found slight differences in the electronic properties of OGNRs concerning the pristine ones. The oxygen contribution of functional groups to the DOS is found in the conducting energy bands far from the Fermi level. Consequently, the semiconducting behavior is maintained after doping. Based on the reactivity of OGNRs, the most promising nanostructures were proposed as adsorbents studying the interaction and complexation with phenol, a critical pollutant removed mainly by hydrotreating processes (HDO) to produce bio-oil. Parallel and perpendicular phenol conformations were found towards the OGNRs in the optimized complexes driven by a physisorption process. These results provide significant insights for catalytic processes that use biomass derivatives containing phenolic compounds. The physisorption of streams containing pollutants on OGNRs could be adapted to new technological applications for the remotion of aromatic compounds under environmentally friendly operational conditions.

Attached two folded graphene nanoribbons as sensitive gas sensor

Gas sensors based on Carbon Nanotube (CNT) can be used to detect harmful environmental pollutants. Also, CNT has some unique electrical properties as gas sensor application. In this research, we have presented a new structure that is constructed from joining two similar Folded Armchair Graphene Nanoribbons (FAGNRs). Two FAGNRs are joined from their open sides, which make a CNT like device with noncircular cross section and we call it Attached FAGNR tube (AFAGNT). In addition to the physical properties we have investigated gas sensing performances of CNT and AFAGNT in presence of CO, O2 and CO2 gases molecules. In this regards, our simulation results of AFAGNT show different significant sensitivities to CO gas molecule at various bias voltages, especially at 0.8V. On the other hand, CO2 gas molecule doesn't adsorb onto the AFAGNT which means it is not a CO2 gas sensor.

Armchair graphene nanoribbon-based spin caloritronics

Armchair graphene nanoribbons are considered unsuitable for spin caloritronic applications, due to the lack of intrinsic magnetism. Inspired by the progress on fabricating carbon nanotubes with Janus edges Bets et al. (2019) [26], we construct graphene nanoribbons with Janus edges, where one edge is armchair type and the other one possesses triangular protrusions with zigzag-armchair or zigzag-zigzag sub-edges. By first-principles calculations, Spin Seebeck effect is found. Further analysis shows that, it is resulted from the difference of Fermi distributions between the two electrodes, which is triggered by temperature difference, and the peculiar transmission spectra, where two transmission peaks with opposite spins reside discretely on both sides of EF. These two peaks are contributed by the corresponding bands with opposite spins above and below EF respectively, which are induced by the sub-edges. We believe our findings would be useful in developing graphene-based spin caloritronics.

Regio- and Steric Effects on Single Molecule Conductance of Phenanthrenes

Here, six phenanthrene (the smallest arm-chair graphene nanoribbon) derivatives with dithiomethyl substitutions at different positions as the anchoring groups were synthesized. Scanning tunneling microscopy break junction technique was used to measure their single molecule conductances between gold electrodes, which showed a difference as much as 20-fold in the range of ∼10-2.82 G0 to ∼10-4.09 G0 following the trend of G2,7 > G3,6 > G2,6 > G1,7 > G1,6 > G1,8. DFT calculations agree well with this measured trend and indicate that the single molecule conductances are a combination of energy alignment, electronic coupling, and quantum effects. This significant regio- and steric effect on the single molecule conductance of phenanthrene model molecules shows the complexity in the practice of graphene nanoribbons as building blocks for future carbon-based electronics in one hand but also provides good conductance tunability on the other hand.

THE EFFECT OF CRITICAL ELECTRIC FIELDS ON THE ELECTRONIC DISTRIBUTION OF BILAYER ARMCHAIR GRAPHENE NANORIBBONS

We employed tight-binding calculations and Green’s function formalism to investigate the effect of applied electric fields on the energy band and electronic properties of bilayer armchair graphene nanoribbons (BL-AGNRs). The results show that the perpendicular electric field has a strong impact on modifying and controlling the bandgap of BL-AGNRs. At the critical values of this electric field, distortions of energy dispersion in subbands and the formation of new electronic excitation channels occur strongly. These originate from low-lying energies near the Fermi level and move away from the zero-point with the increment of the electric field. Phase transitions and structural changes clearly happen in these materials. The influence of the parallel electric field is less important in changing the gap size, resulting in the absence of the critical voltage over a very wide range [–1.5 V; 1.5 V] for the semiconductor-insulator group. Nevertheless, it is interesting to note the powerful role of the parallel electric field in modifying the energy band and electronic distribution at each energy level. These results contribute to an overall picture of the physics model and electronic structure of BL-AGNRs under stimuli, which can be a pathway to real applications in the future, particularly for electronic devices.

Topologically protected edge and confined states in finite armchair graphene nanoribbons and their junctions

Topologically protected edge and junction states, previously predicted, have recently been observed in armchair graphene nanoribbon (AGNR) heterojunctions. Here, via tight-binding-based calculations, we explain the relation between the nature and number of the zero-mode edge states of finite-length AGNRs and their structure, topological invariants, and winding number. This allows us to rationalize the design of AGNR heterojunctions and superlattices with tailored phases. We show how the choice of widths, interface coupling geometry, and boundaries determines the emergence of topological states following patterns that depend on the structure and family of the constituent AGNRs. Furthermore, we prove that quantum-well-like states confined in one of the constituent ribbons develop in all the AGNR junctions irrespective of their trivial or topological character. The bipartite nature of the honeycomb lattice is determinant for the topological properties of the junctions: their electronic states can be topologically trivial or nontrivial depending on subtle differences at the boundaries of the ribbons.

Performance enhancement of armchair graphene nanoribbon resonant tunneling diode using V-shaped potential well

We study the electron transport in armchair graphene nanoribbon (AGNR) resonant tunneling diode (RTD) using square and V-shaped potential well profiles. We use non-equilibrium Green’s function formalism to analyze the transmission and I–V characteristics. Results show that an enhancement in the peak current (I p ) can be obtained by reducing the well width (W w ) or barrier width (W b ). As W w decreases, I p shifts to a higher peak voltage (V p ), while there is almost no change in V p with decreasing W b . It is gratifying to note that there is an enhancement in I p by about 1.6 times for a V-shaped well over a square well. Furthermore, in the case of a V-shaped well, the negative differential resistance occurs in a shorter voltage range, which may beneficial for ultra-fast switching and high-frequency signal generation. Our work anticipates the suitability of graphene having better design flexibility, to develop ideally 2D RTDs for use in ultra-dense nano-electronic circuits and systems.

Role Played by Edge-Defects in the Optical Properties of Armchair Graphene Nanoribbons.

We explore the implementation of specific optical properties of armchair graphene nanoribbons (AGNRs) through edge-defect manipulation. This technique employs the tight-binding model in conjunction with the calculated absorption spectral function. Modification of the edge states gives rise to the diverse electronic structures with striking changes in the band gap and special flat bands at low energy. The optical-absorption spectra exhibit unique excitation peaks, and they strongly depend on the type and period of the edge extension. Remarkably, there exist the unusual transition channels associated with the flat bands for selected edge-modified systems. We discovered the special rule governing how the edge-defect influences the electronic and optical properties in AGNRs. Our theoretical prediction demonstrates an efficient way to manipulate the optical properties of AGNRs. This might be of importance in the search for suitable materials designed to have possible technology applications in nano-optical, plasmonic and optoelectronic devices.