Monolayer Graphene Nanoribbons Research Articles

Modulating Kerr nonlinearity and optical bistability in a monolayer graphene nanoribbon nanoresonator coupled to a metallic nanoshell

A scheme for modulating Kerr nonlinearity and optical bistability (OB) based on a monolayer graphene nanoribbon nanoresonator (NR) and a metallic nanoshell (MNS) is proposed. The MNS is made of an Au nanoshell and a dielectric core. Here, we not only demonstrate that the spectrum of the Kerr coefficient can switch from a four-peaked structure to a two-peaked structure by increasing the shell thickness of the MNS, but also plot bistability phase diagrams in the system’s parameter subspace, revealing that the bistable effect can be controlled via zero-channel, one-channel or two-channels by only changing the shell thickness in a strong exciton-phonon coupling regime. Additionally, the calculated results also show that the material of the core of MNS plays an essential role in tuning OB when the shell thickness is fixed. The findings pave a new way for developing optical Kerr switches and high-flexible bistable nanodevices.

Valley-dependent transport in a mescoscopic twisted bilayer graphene device

We study the valley-dependent electron transport in a four-terminal mesoscopic device of the two monolayer graphene nanoribbons vertically stacked together, where the intersection forms a bilayer graphene lattice with a controllable twist angle. Using a tight-binding lattice model, we show that the longitudinal and transverse conductances exhibit significant valley polarization in the low energy regime for small twist angles. As the twist angle increases, the valley polarization shifts to the high energy regime. This arises from the regrouping effect of the electron band in the twisted bilayer graphene region. But for relatively large twist angles, no significant valley polarization is observed. These results are consistent with the spectral densities of the twisted bilayer graphene.

Effect of doping on vibration frequency of graphene nanoribbons

Graphene has excellent mechanical properties and vibration frequency is an important mechanical property. In this paper, the effects of doping elements and doping concentrations on the vibration characteristics of graphene nanoribbons were investigated by molecular dynamics simulation. The vibration characteristics are influenced by the doping element, depending on the bond structural difference in bond structure between doping atoms and carbon atoms. The vibration frequencies of Nitrogen (N)-doped and Nitrogen–Nitrogen (N–N)-doped graphene nanoribbons are slightly lower than those of pure graphene nanoribbons. However, the vibration frequencies of Sulfur (S)/Phosphorus (P)-doped and Sulfur–Sulfur (S–S)/Phosphorus–Phosphorus (P–P)-doped graphene nanoribbons are significantly lower than those of pure graphene nanoribbons. In addition, with the increase of S (P) doping concentration, the vibration frequency of monolayer graphene nanoribbons decreases gradually.

Linear and Nonlinear Optical Properties of a Doubly Clamped Suspended Monolayer Graphene Nanoribbon Nanoresonator

We studied the optical properties of a hybrid structure that was composed of a semiconductor quantum dot and a doubly clamped suspended graphene nanoribbon nanoresonator. We obtained analytical results for the linear and third-order optical susceptibilities of the hybrid system. The spectrum of the linear susceptibility exhibited a single resonance, and its position depended on the value of the on-resonance exciton energy and the exciton–nanoribbon resonator coupling strength coefficient; the amplitude of the resonance was independent of the values of these parameters. The third-order optical susceptibility spectrum exhibited a sharp resonance arising at low frequencies of the probe field, the position of which depended only on the frequency of the fundamental flexural phonon mode. It also presented a broader resonance arising at higher frequencies of the probe field, the position of which was determined both by the coupling strength coefficient and by the exciton frequency; its amplitude depended solely on the exciton–photon coupling strength.

High-quality multi-band absorber in a hybrid all-dielectric system using the surface plasmon polaritons

High confinement of surface plasmon polaritons in graphene have important applications in many aspects. In this paper, we have demonstrated that high-quality multi-band absorption can be achieved by using a hybrid system where periodic monolayer graphene nanoribbons is coated on an all-dielectric nanostructure. The effect of the monolayer graphene nanoribbons on plasmonic resonances absorption is investigated theoretically and numerically. It is found that the high tunability and large near field enhancement of the system is observed by changing the grating period and Fermi level. After delicate optimization, the system can achieve high-Q resonances in the vicinity of the bound states in the continuum, and the Q factors of the two resonance modes are respectively significantly enhanced (＞450, 600). In addition, the multi-band light absorption enhancement with high quality can be observed by altering the carrier mobility of graphene. Wherein the optimized absorption efficiency up to 97% is reached. More interestingly, the proposed structure puts up palpable advantage in absorption stability when the incident angle is varied from 0° to 60°. Thus, this work may facilitate the design of high-performance optoelectronic devices based on all-dielectric configuration, such as tunable optical sensors and perfect absorbers.

Effect of Vacancy Defects on the Vibration Frequency of Graphene Nanoribbons.

Graphene is a type of two-dimensional material with special properties and complex mechanical behavior. In the process of growth or processing, graphene inevitably has various defects, which greatly influence the mechanical properties of graphene. In this paper, the mechanical properties of ideal monolayer graphene nanoribbons and monolayer graphene nanoribbons with vacancy defects were simulated using the molecular dynamics method. The effect of different defect concentrations and defect positions on the vibration frequency of nanoribbons was investigated, respectively. The results show that the vacancy defect decreases the vibration frequency of the graphene nanoribbon. The vacancy concentration and vacancy position have a certain effect on the vibration frequency of graphene nanoribbons. The vibration frequency not only decreases significantly with the increase of nanoribbon length but also with the increase of vacancy concentration. As the vacancy concentration is constant, the vacancy position has a certain effect on the vibration frequency of graphene nanoribbons. For nanoribbons with similar dispersed vacancy, the trend of vibration frequency variation is similar.

Ultra-strong optical four-wave mixing signal induced by strong exciton-phonon and exciton-plasmon couplings.

We propose a scheme to generate ultra-strong four-wave mixing (FWM) signal based on a suspended monolayer graphene nanoribbon nanomechanical resonator (NR) coupled to an Au nanoparticle (NP). It is shown that, the FWM spectrum can switch among two-peaked, three-peaked, four-peaked or five-peaked via the modulation of exciton-phonon and exciton-plasmon couplings. This is mainly attributed to the vibrational properties of NR related to the exciton-phonon coupling, and the energy-level splitting of the localized exciton correlated to three classes of resonances consisting of three-photon resonance, Rayleigh Resonance, and AC-Stark atomic resonance. Especially, in a dual-strong coupling regime, the gains for these peaks can be as high as nine orders of magnitude (∼ 109) around the lower bistable threshold due to a combined effect of two couplings. Our findings not only offer an efficient way to measure the vibrational frequency of NR and the exciton-phonon coupling strength but also provide a possibility to fabricate high-performance optoelectronic nanodevices.

Self-Referenced Refractive Index Biosensing with Graphene Fano Resonance Modes.

A hybrid structure composed of periodic monolayer graphene nanoribbons and a dielectric multilayer structure was designed to generate a Fano resonance (FR). The strong interaction between the surface plasmon resonance of graphene and the dielectric waveguide mode results in the FR. The finite element method is utilized to investigate the behaviors of the FR, and it matches well with the theoretical calculations using rigorous coupled wave theory. The results demonstrate that the profile of the FR can be passively tuned by the period of the graphene nanoribbons and actively tuned by the Fermi level of the graphene. The decoupled nature of the FR gives it potential applications as a self-calibrated refractive index biosensor, and the sensitivity can reach as high as 4.615 μm/RIU. Thus, this work provides a new idea for an excellent self-referencing refractive index biosensor.

Selective gas sensor based on bilayer armchair graphene nanoribbon

Abstract Detection and sensing of toxic gases are vital in many areas such as in environmental and safety monitoring, industrial applications and etc. Fundamental requirements for modern gas sensors typically are small size, low power consumption, high sensitivity and good selectivity towards various gases. Recently, graphene-based gas sensors have attracted significant attention due to their potential applications. Based on published works, the energy band gap of monolayer graphene nanoribbon (GNR) varies by changing its ribbon width. However, in the case of bilayer GNR, the energy band gap varies via three ways: the ribbon width, the layers separation and the symmetry between the layers. Therefore, it is interesting to investigate the characteristics of bilayer armchair graphene nanoribbon (B-AGNR) as gas nanosensor in presence of CO, O2 and CO2 gas molecules. In this study, we report a theoretical investigation and numerical simulation of the electronic properties of monolayer and bilayer AGNR (with 10 atoms in width) and study their sensitivities in presence of CO, O2 and CO2 gas molecules. Using Quantum espresso-5.3.0 package, the structure optimizations are done based on density functional theory (DFT). Moreover, we have evaluated the current-voltage (I-V) characteristics of the aforementioned structures using the open source package TRAN SIESTA which is based on non-equilibrium Green's function (NEGF) method. The obtained results show that the adsorption of CO, O2 and CO2 gas molecules onto AGNR can be modified significantly in bilayer AGNR with respect to the monolayer. These gas molecules show weak interaction with the AGNR atomic structure due to large interaction distances. Thus, it is deduced that the graphene nanoribbon (AGNR) shows weak gas sensing in presence of CO, O2 and CO2 molecules. Meanwhile, bilayer AGNR shows different electronic properties compared to monolayer AGNR which makes it highly sensitive and selective to the presence of O2 and CO gases.

Gap opening in graphene nanoribbons by application of simple shear strain and in-plane electric field

The effects of shear strain and applied in plane electric field on the electronic properties of monolayer graphene nanoribbons (GNRs) are theoretically investigated. Band structures and the probability densities are calculated within the tight-binding model and the mechanical stresses submitted to the GNRs are taken into account by using the theory of linear elasticity with joint modifications in the elongation of the nearest-neighbor vectors and the modification of the hopping parameters. The energy gaps for specific widths of (semiconducting) armchair nanoribbons are verified also in the presence of either strain or field, whereas zigzag nanoribbons are metallic for any value of strain and exhibit a small gap for any value of field. However, our results demonstrate that when both strain and electric field are combined, a significant energy gap is always observed in the band structure, for any width or edge type of the ribbon. Moreover, the obtained total wave function is asymmetric along the ribbon width due to the applied electric field that pushes the electrons to one side of the ribbon and, under shear strain, a peak at the center of the ribbon in the spatial distribution is also observed owing to the preferable localization around the almost undeformed carbon bonds at ribbon center.

In-situ electrical conductance measurement of suspended ultra-narrow graphene nanoribbons observed via transmission electron microscopy

Graphene nanoribbon is an attractive material for nano-electronic devices, as their electrical transport performance can be controlled by their edge structures. However, in most cases, the electrical transport has been investigated only for graphene nanoribbons fabricated on a substrate, which hinders the appearance of intrinsic electrical transport due to screening effects. In this study, we developed special devices based on silicon chips for transmission electron microscopy to observe a monolayer graphene nanoribbon suspended between two gold electrodes. Moreover, with the development of an in-situ transmission electron microscopy holder, the current–voltage characteristics were achieved simultaneously with observing and modifying the structure. We found that the current–voltage characteristics differed between 1.5 nm-wide graphene nanoribbons with armchair and zigzag edge structures. The energy gap of the zigzag edge was more than two-fold larger than that of the armchair edge and exhibited an abrupt jump above a critical bias voltage in the differential conductance curve. Thus, our in-situ transmission electron microscopy method is promising for elucidating the structural dependence of electrical conduction in two-dimensional materials.

Investigation on the Effect of Gate Dielectric and Other Device Parameters on Digital Performance of Silicene Nanoribbon Tunnel FET

In this article, silicene nanoribbon (SiNR)based tunneling field effect transistor (TFET) is investigated for low power digital applications. A comprehensive study on the role of gate dielectric and device parameters, such as drain doping, channel length of SiNR TFETs, are carried out to see the performance of digital applications. To achieve higher ION/IOFF ratio and lower subthreshold swing (SS), a detailed investigation is carried out on the equivalent oxide thickness (EOT), channel length, and doping of drain. A digital performance metric comparison of SiNR TFETs with germanium-silicon (Ge-Si), monolayer graphene nanoribbon (MLGNR), and bilayer graphene nanoribbon (BLGNR) TFETs has been performed. It is concluded that SiNR TFETs made of Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> gate dielectric with 0.86 nm effective oxide thickness, 0.5 mM (8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ) drain doping concentration, and 5 mM (8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ) source doping concentration exhibits lower intrinsic delay, higher ION, higher ION/IOFF ratio, and lower SS at 25 nm channel length.

Dual-channel bistable switch based on a monolayer graphene nanoribbon nanoresonator coupled to a metal nanoparticle.

We theoretically propose a dual-channel bistable switch based on a monolayer Z-shaped graphene nanoribbon nanoresonator (NR) coupled to a metal nanoparticle (MNP). We show that the bistable nonlinear absorption response can be realized due to a competition and combination of the exciton-plasmon and exciton-phonon interactions. We map out two-dimensional and three-dimensional bistability phase diagrams, which reveal clearly the dynamical evolution of the roles played by these two interactions in managing optical bistability (OB) at all stages. Specifically, the bistable switch proposed can be controlled via a single channel or dual channels by only adjusting the intensity or frequency of the pump field. In/outside these channels, the switch will be turned on/off. The results obtained here not only can be employed to measure precisely the distance between the MNP and the NR but also provide promising applications in optical switching and optical storage.

Investigation of electrical properties in AB-Stacked bilayer Graphene-DNA nanostructures

Abstract In this paper, we study electrical properties of AB-stacked bilayer graphene-DNA hybrid nanostructure connected to two semi-infinite monolayer graphene nanoribbon leads. We investigated two types of devices with both zigzag and armchair edges and the results are evaluated by replacing the junction position. Our calculations are based on the tight-binding approximation and the Green's function method in which the electrical transmission, the density of states (DOS) and the local density of states (LDOS) are calculated numerically. In the metallic bilayer graphene device via creating a nanopore and translocating the DNA molecule, the electron transmission reduces and an energy gap opens, so that, the metal-semiconductor phase transition occurs. Also, our results indicate that the model AC leads to a more considerable energy gap than model AB. Our results have important applications in DNA sensing.

Effect of acoustic phonon scattering on impact ionization rate of electrons in monolayer graphene nanoribbons

In this paper, we have studied the behavior of impact ionization rate of electrons in monolayer graphene nanoribbons due to various scattering mechanisms. The effects have been analyzed using the comprehensive analytical expressions, which result from all the possible inelastic interactions before the ionizing collision inside the graphene nanoribbons structures. These interactions include scattering of electrons due to acoustic phonon, inter-carrier scattering and optical phonon. Also, the obtained results are compared with previously reported data. The overestimation in earlier data has been analyzed in detail through the developed analytical expression. In addition to this, the percentage change in electron ionization rate has been calculated by considering it with and without acoustic phonon scattering.

Competing Gap Opening Mechanisms of Monolayer Graphene and Graphene Nanoribbons on Strong Topological Insulators.

Graphene is a promising material for designing next-generation electronic and valleytronic devices, which often demand the opening of a bandgap in the otherwise gapless pristine graphene. To date, several conceptually different mechanisms have been extensively exploited to induce bandgaps in graphene, including spin-orbit coupling and inversion symmetry breaking for monolayer graphene, and quantum confinement for graphene nanoribbons (GNRs). Here, we present a multiscale study of the competing gap opening mechanisms in a graphene overlayer and GNRs proximity-coupled to topological insulators (TIs). We obtain sizable graphene bandgaps even without inversion symmetry breaking and identify the Kekulé lattice distortions caused by the TI substrates to be the dominant gap opening mechanism. Furthermore, Kekulé distorted armchair GNRs display intriguing nonmonotonous gap dependence on the nanoribbon width, resulting from the coexistence of quantum confinement, edge passivation, and Kekulé distortions. The present study offers viable new approaches for tunable bandgap engineering in graphene and GNRs.

Effects of vacancies on spin-dependent behavior of monolayer and bilayer graphene nanoribbons

In this work, the effect of vacancies on magnetic properties and spin-dependent behaviors of monolayer and bilayer armchair and zigzag graphene nanoribbons is investigated using first principles calculations based on density functional theory (DFT). The armchair and zigzag graphene nanoribbons are composed of 6 rows and 4 rows of carbon atoms with the edges closed by the hydrogen atoms, respectively. Our results show that vacancies affect the magnetic properties and spin polarization of the graphene nanoribbons. It is seen that the monolayer armchair graphene nanoribbon with one vacancy in its supercell (24 carbon sites+8 hydrogen sites) gives the magnetic moment of 0.79 μB, while magnetic moment in the monolayer zigzag graphene nanoribbon with one vacancy in its supercell (24 carbon sites+6 hydrogen sites) is 1.72 μB (for site α) and 1.84 μB (for site β). The highest and lowest values of magnetic moment in different configurations of the bilayer armchair (zigzag) graphene nanoribbons with one vacancy in each layer of the supercell give 1.54 μB and 1.29 μB (3.51 μB and 2.72 μB), respectively. Numerical values of the magnetic moment in different configurations depended on the distance of vacancies from each other and from nanoribbon’s edge as well as their orientations.

Performance Evaluation of Bilayer Graphene Nanoribbon Tunnel FETs for Digital and Analog Applications

The recent advancement in the fabrication of narrow graphene nanoribbon with smooth edges has catalyzed the growth of nanoribbon based transistors. Motivated by these advances, we suggest bilayer graphene nanoribbon tunnel field-effect transistors (BLGNR-TFETs) for low voltage digital and analog applications. The device performance is analyzed by quantum transport simulation, based on self-consistent solutions of two-dimensional Poisson's equation and nonequilibrium Green's function formalism. The results indicate that the use of BLGNR-TFET instead of monolayer graphene nanoribbon TFET with very narrow width, increases the ON current and reduces the intrinsic device delay without lowering the ON/OFF current ratio. The BLGNR-TFET for analog/RF applications has provided higher intrinsic gain at very narrow ribbon width; however, very narrow width reduces the cutoff frequency. Furthermore, RF figure of merits are investigated in the presence of external parasitics.

The effect of the SiC(0001) surface morphology on the growth of epitaxial mono-layer graphene nanoribbons

Graphene nanoribbons (GNRs) are promising for applications in nanoelectronics due to their unique properties. Therefore, achieving the controlled and high-quality synthesis of GNRs is anticipated to be of great importance. One of the methods which shows great potential is the growth of GNRs on surface facets of SiC(0001) by the surface graphitization method. In this report we studied the dependency of the GNR width on growth temperature and SiC substrate miscut angle (or initial step height). While a linear growth rate best describes the growth in lower step heights, a nonlinear rate is observed for substrates with higher steps, which is also associated with the formation of few-layer graphene on the step edges. The structural characterization of the samples was performed by means of atomic force microscopy, scanning electron microscopy, and Raman spectroscopy.

Patterning monolayer graphene with zigzag edges on hexagonal boron nitride by anisotropic etching

Graphene nanostructures are potential building blocks for nanoelectronic and spintronic devices. However, the production of monolayer graphene nanostructures with well-defined zigzag edges remains a challenge. In this paper, we report the patterning of monolayer graphene nanostructures with zigzag edges on hexagonal boron nitride (h-BN) substrates by an anisotropic etching technique. We found that hydrogen plasma etching of monolayer graphene on h-BN is highly anisotropic due to the inert and ultra-flat nature of the h-BN surface, resulting in zigzag edge formation. The as-fabricated zigzag-edged monolayer graphene nanoribbons (Z-GNRs) with widths below 30 nm show high carrier mobility and width-dependent energy gaps at liquid helium temperature. These high quality Z-GNRs are thus ideal structures for exploring their valleytronic or spintronic properties.