Graphene Ribbons Research Articles

Ultrafast terahertz conductivity in epitaxial graphene nanoribbons: an interplay between photoexcited and secondary hot carriers

Abstract Optical pump-terahertz probe spectroscopy has been used to investigate ultrafast photo-induced charge carrier transport in 3.4 µm wide graphene ribbons upon scaling the optical pump intensity. For low pump fluences, the deposited pump energy is rapidly redistributed through carrier-carrier scattering, producing secondary hot carriers: the picosecond THz photoconductivity then acquires a negative sign and scales linearly with an increasing pump fluence. At higher fluences, there are not enough equilibrium carriers able to accept the deposited energy, directly generated (excess) carriers start to contribute significantly to the photoconductivity with a positive sign leading to its saturation behavior. This leads to a non-monotonic variation of the carrier mobility and plasmonic resonance frequency as a function of the pump fluence and, at high fluences, to a balance between a decreasing carrier scattering time and an increasing Drude weight. In addition, a weak carrier localization observed for the polarization parallel to the ribbons at low pump fluences is progressively lifted upon increasing the pump fluence as a result of the rise of initial carrier temperature.

Four ribbons of double-layer graphene suspending masses for NEMS applications

Graphene ribbons with a suspended proof mass for nanomechanical systems have been rarely studied. Here, we report three types of nanomechanical devices consisting of graphene ribbons (two ribbons, four ribbons-cross and four ribbons-parallel) with suspended Si proof masses and studied their mechanical properties. The resonance frequencies and built-in stresses of three types of devices ranged from tens of kHz to hundreds of kHz, and from 82.61 MPa to 545.73 MPa, respectively, both of which decrease with the increase of the size of proof mass. The devices with four graphene ribbons featured higher resonance frequencies and spring constants, but lower built-in stresses than two ribbon devices under otherwise identical conditions. The Young’s modulus and fracture strain of double-layer graphene were measured to be 0.34 TPa and 1.13% respectively, by using the experimental data and finite element analysis (FEA) simulations. Our studies would lay the foundation for understanding of mechanical properties of graphene ribbons with a suspended proof mass and their potential applications in nanoelectromechanical systems.

Canonical scattering problem in topological metamaterials: Valley-Hall modes through a bend

We study the amount of backscattering of Valley-Hall modes in a classical topological insulator. In reciprocal systems, the conservation of the valley index has been argued to be at the root of the high transmission of Valley-Hall modes observed in many experimental realizations. Here, we reconsider this hypothesis by quantitatively analysing the canonical scattering problem of interface Valley-Hall modes impinging on sharp bends that may or may not conserve the valley index. We consider a tight binding model of graphene ribbons with an interface and compute the reflection and transmission coefficients using the transfer matrix formalism. We find that, in all configurations considered, the transmission of Valley-Hall modes is close to being maximal, even in cases where the valley index is not conserved. Hence, there appears to be no correlation between valley conservation and good transmission. Our results serve as a reference case for the design of Valley-Hall type metamaterial.

Compact Topological Edge States in Flux-Dressed Graphenelike Photonic Lattices.

Flat band lattice systems promote the appearance of perfectly compact bulk states, whereas topology favors edge localization. In this work, we report the existence of compact topological edge states on flux-dressed photonic graphene ribbons. We found that robust localization is achieved through a synergy of Aharonov-Bohm caging and topological protection mechanisms. The topological nontriviality of the compact edge states is characterized through both theoretical derivations and experimental observations of an integer Zak phase obtained from the mean chiral displacement. Experiments are performed using direct laser writing of a graphene ribbon photonic lattice having 0 or π effective magnetic fluxes. Mode stability is demonstrated by the exceptional localization of the edge compact mode and its resilience to fabrication tolerances and input phase deviations. Our findings demonstrate the existence of perfectly compact topological edge states, as a concrete and promising example of synergy in between flat band physics and topology.

Peculiarities of the Landau level collapse in graphene ribbons in crossed magnetic and in-plane electric fields

Employing the low-energy effective theory alongside a combination of analytical and numerical techniques, we explore the Landau level collapse phenomenon, uncovering previously undisclosed features. We consider both finite-width graphene ribbons and semi-infinite geometries subjected to a perpendicular magnetic field and an in-plane electric field, applied perpendicular to both zigzag and armchair edges. In the semi-infinite geometry the hole (electron)-like Landau levels collapse as the ratio of electric and magnetic fields reaches the critical value +(−)1. On the other hand, the energies of the electron (hole)-like levels remain distinct near the edge and deeply within the bulk, approaching each other asymptotically for the same critical value. In the finite geometry, we show that the electron (hole)-like levels become denser and merge, forming a band. Published by the American Physical Society 2024

Ultrafast reaction kinetic of polyaniline in flexible hydrogel electrodes facilitated by graphene ribbons

Abstract By loading energy storage active materials on hydrogel which is inherently flexible, the flexibility of electrode materials can be simply realized, thereby achieving the flexibility of energy storage devices. However, the polymer network that constructs the three-dimensional skeleton of the hydrogel is not conductive, which inhibits the redox ability of the active material. If a high speed conductive structure can be added to the colloidal phase, the performance of the flexible electrode material can be greatly improved. Here, we introduce redox graphene ribbons into the polyvinyl alcohol hydrogel loaded with polyaniline. The in situ three-dimensional conductive graphene network greatly enhanced the conductivity of the hydrogel electrode, thus increasing the specific capacitance to as high as 1117 F g−1 at 2 mg cm−2 mass loading, with a retention ratio of 66.96% from 0.5 A g−1 to 20 A g−1. These highlighted properties enable the PRP hydrogel as an electrode for flexible supercapacitors, which provides a promising possibility for the practical application of wearable electronics.

Automated and parallel transfer of arrays of oriented graphene ribbons

The transfer of two-dimensional materials from their growth substrate onto application wafers is a critical bottleneck in scaling-up devices based on such nanomaterials. Here, we present an innovative approach to achieve the automated and simultaneous transfer of arrays of graphene ribbons, with precise control over their orientation and alignment onto patterned wafers. The transfer is performed in a simple, yet efficient apparatus consisting of an array of glass columns, strategically shaped to control ribbon orientation and arranged to match the destination wafer, coupled to a dual inflow/outflow pumping system. This apparatus enables the transfer of a custom array of parallel graphene ribbons in a fraction of the time required with traditional methods. The quality of the transferred graphene was evaluated using optical imaging, scanning electron microscopy, hyperspectral Raman imaging, and electrical transport: all consistently indicating that the transferred graphene exhibits excellent quality, comparable to a manual transfer by an expert user. The proposed apparatus offers several competitive advantages, including ease of use, high transfer throughput, and reduced nanomaterial consumption. Moreover, it can be used repeatedly on the same wafer to assemble arrays of overlayed materials with controlled relative orientations. This approach thus opens promising opportunities for the large-scale fabrication of various heterostructures and devices based on vertical assemblies of 2D nanomaterials.

Graphene ribbons based THz toxic gas sensing

In this paper, an optical chemical sensor is proposed to detect some toxic gases such as Methane (CH4), Nitrogen (N2), Nitrogen dioxide (NO2), and Carbon monoxide (CO). This type of chemical sensor consists of graphene ribbons and Kapton materials as sensing elements. Also, exploits electromagnetic properties such as absorption in terms of signal transducing. These kinds of small-scale, flexible architectures and advanced detection techniques are in demand to identify toxic gases as well. To develop the proposed chemical sensor, this study describes the structure in the aspect of an equivalent circuit model (ECM) mathematically, while the full-wave simulation (FEM) is performed as the reference. Acceptable agreement between the ECM and FEM simulations is shown while an interesting tuning capability against external stimulation is obtained. It should be noted that the ECM approach is performed in just a few seconds while the FEM simulation takes more than 3 h to produce results. In addition, the maximum error is around the second absorption peak and is less than 4%.The main contribution of this work is introducing a simple structure to distinguish several toxic gases in the sub-THz gap (0.1–2 THz). Additionally, ample simulations are performed to verify the sensor's reliability. According to the simulation results, the proposed meta-structure can appropriately show different peak frequencies and even different numbers of absorption peaks against different concentrations of toxic gases. Additionally, due to the ultra-thin nature of the graphene and the flexibility of the Kapton, the proposed sensor can be wearable while it is considered non-invasive testing.

Tunable triple plasmon-induced transparency in E-type graphene metamaterials.

Enhancing light-matter interaction is crucial for boosting the performance of nanophotonic devices, which can be achieved via plasmon-induced transparency (PIT). This study introduces what we believe to be a novel E-type metamaterial structure crafted from a single graphene layer. The structure, comprising a longitudinal graphene ribbon and three horizontal graphene strips, leverages destructive interference at terahertz frequencies to manifest triple plasmon-induced transparency (triple-PIT). Through a comparison of simulations using the finite difference time domain (FDTD) method and theoretical coupled-mode calculations, we elucidate the physical mechanism behind triple-PIT. Our analysis shows that the PIT effect arises from the interplay between two single-PITs phenomena, further explored through field distribution studies. Additionally, we investigate the impact of varying Fermi levels and carrier mobility on the transmission spectrum, achieving amplitude modulation in photoelectric switches of 85.5%, 99.2%, and 93.8% at a carrier mobility of 2 m2/(V·s). Moreover, we explore the relationship between Fermi levels and carrier mobility concerning the slow light effect, discovering a potential group index of up to 1021 for the structure. These insights underscore the significant potential of this graphene-based metamaterial structure in enhancing optical switches, modulators, and slow light devices.

Room temperature thermal rectification in suspended asymmetric graphene ribbon

Thermal rectifiers are essential in optimizing heat dissipation in solid-state devices to enhance energy efficiency, reliability, and overall performance. In this study, we experimentally investigate the thermal rectification phenomenon in suspended asymmetric graphene ribbons (GRs). The asymmetry within the graphene is introduced by incorporating periodic parallel nanoribbons on one side of the GR while maintaining the other side in a pristine form. Our findings reveal a substantial thermal rectification effect in these asymmetric graphene devices, reaching up to 45% at room temperature and increasing further at lower environmental temperatures. This effect is attributed to a significant thermal conductivity contrast between pristine graphene and nanoribbon graphene within the asymmetric structure. We observe that the incorporation of nanoribbons leads to a notable reduction in thermal conductivity, primarily due to phonon scattering and bottleneck effects near the nanoribbon edges. These findings suggest that graphene structures exhibiting asymmetry, facilitated by parallel nanoribbons, hold promise for effective heat management at the nanoscale level and the development of practical phononic devices.

Different Cr(VI)-sequestration behaviors between carbon nanoparticles and their agglomerates

Aggregation of carbon nanoparticles (CNPs) exerts significant influence on its surrounding environment. To comprehensively evaluate their environmental impacts, it is essential to clarify the relationship between CNP monomers, agglomerates, and their sorption and photocatalysis behaviors. In this work, three agglomerates were prepared by crosslinking one-dimensional oxidized carbon nanotubes (OCNTs), quasi one-dimensional oxidized graphene ribbons (OGRs), and two-dimensional graphene oxide sheets (GOs) with polyacrylamide hydrochloride (PA). Notably, the sorption and photo-reduction behaviors of Cr(VI) are different on the surfaces of CNPs monomers and agglomerates. Overall, all the agglomerates demonstrated higher Cr(VI)-sequestration capacities than the corresponding monomers regardless of dark environment and light irradiation. For CNPs monomers, chromium species were scarcely adsorbed on the particle surface in the dark; the sunlight illumination promoted significantly the transformation of Cr(VI) ions to solid Cr(OH)3, resulting in sequestration capacities of 16.7, 47.7, and 115.4 mg/g in OCNTs, OGRs and GOs, respectively. After crosslinked by PA, the total sequestration capacity of Cr(VI) pollutants in the agglomerates increased by 3.4 ∼ 21.4 times compared to their corresponding monomers. It was noteworthy that the agglomerates almost converted all of Cr(VI) to Cr2O3 under the illumination. This study highlights the discrepancies in Cr(VI)-sequestration behavior between CNPs and their agglomerates, which provides a novel insight into environmental impacts of CNPs and their agglomerates in nature.

Interface friction between neighbor diamondene nanoribbons: A molecular dynamics study

Due to its unique carbon-carbon bond topology, diamondene exhibits different frictional properties compared to pristine graphene. This study evaluates the friction properties of diamondene ribbon (DR) for potential applications in the design of dynamic nanodevices. For a DR sample on a DR substrate at a specific temperature, it has a maximum static friction force (MSFF) that is about 38 times greater than that of a graphene counterpart with the same size. The rate of MSFF to the contact surface demonstrates the shear strength of a material. As an intrinsic property, the shear strength between DRs measures approximately 3.58 MPa at 10 K, which is much greater than that between graphene ribbons. When the pulling force on the DR sample exceeds MSFF, the sample undergoes a period of acceleration before reaching a stable sliding velocity. The viscous damping coefficient between DRs increases with increasing pulling force on the DR sample while decreasing with increasing temperature. The dynamic friction force behaves in a temperature-dependent manner due to the unstable bond topology of DRs at high temperatures. These studies address previous research lack regarding friction between carbon materials.

Edge-channel transport and spin-polarized interface states in folded graphene ribbons

The fabrication of devices that integrate two or more topological states simultaneously and achieve precise control over their edge state and interface states holds paramount importance. We report the quantized edge-channel transport and spin-polarized interface states in folded graphene ribbons under a magnetic field and predict that fully spin-polarized states can be generated at the interface. Our results also suggest that the system has the potential to exhibit the quantum spin Hall phase, the quantum Hall phase, and a coexistence of both phases. These phases can be manipulated by applying a bias potential and Zeeman splitting. Furthermore, the quantized conductance demonstrates a plateau at 3e2/h, which can be attributed to the presence of fully spin-polarized states. These outcomes provide a foundation for future explorations and manipulations of this novel form of edge-channel transport.

Building a cm2 scale CVD graphene-based gas sensor: modelling the kinetic with a three-site adsorption/desorption Langmuir model

In this article, we aim to develop and study a highly sensitive and selective cm2 scale graphene-based gas sensor. We present the technology used to fabricate sensors which integrate monolayer chemical vapour deposition graphene: photolithography and transfer of layers. Characterization techniques (optical microscopy, AFM, micro-Raman spectroscopy, transport electrical measurements) ensure a diagnosis of graphene ribbons and allow good reproducibility of technological processes. We present the results of gas characterizations after a 200 ppm NO2 exposure. We propose a novel approach for the modelling of the sensor response with a three-site adsorption/desorption Langmuir model. This innovative way of modelling the sensor response should provide a better understanding of the sensor’s kinetic and help to overcome the long response time observed with graphene gas sensors.

High Spatial Resolution 2D Imaging of Current Density and Pressure for Graphene Devices under High Pressure Using Nitrogen-Vacancy Centers in Diamond.

Current density imaging is helpful for discovering interesting electronic phenomena and understanding carrier dynamics, and by combining pressure distributions, several pressure-induced novel physics may be comprehended. In this work, noninvasive, high-resolution two-dimensional images of the current density and pressure gradient for graphene ribbon and hBN-graphene-hBN devices are explored using nitrogen-vacancy (NV) centers in diamond under high pressure. The two-dimensional vector current density is reconstructed by the vector magnetic field mapped by the near-surface NV center layer in the diamond. The current density images accurately and clearly reproduce the complicated structure and current flow of graphene under high pressure. Additionally, the spatial distribution of the pressure is simultaneously mapped, rationalizing the nonuniformity of the current density under high pressure. The current method opens a significant new avenue to investigate electronic transport and conductance variations in two-dimensional materials and electrical devices under high pressure as well as for nondestructive evaluation of semiconductor circuits.

Design constraints for Unruh-DeWitt quantum computers

The Unruh-DeWitt particle detector model has found success in demonstrating quantum information channels with non-zero channel capacity between qubits and quantum fields. These detector models provide the necessary framework for experimentally realizable Unruh-DeWitt quantum computers with near-perfect channel capacity. We propose spin-qubits with gate-controlled coupling to Luttinger liquids as a laboratory setting for Unruh-DeWitt detectors and explore general design constraints that underpin their feasibility in this and other settings. We present several experimental scenarios including graphene ribbons, edges states in the quantum spin Hall phase of HgTe quantum wells, and the recently discovered quantum anomalous Hall phase in transition metal dichalcogenides. Theoretically, through bosonization, we show that Unruh-DeWitt detectors can carry out quantum computations and identify when they can make perfect quantum communication channels between qubits via the Luttinger liquid. Our results point the way toward an all-to-all connected solid state quantum computer and the experimental study of quantum information in quantum fields via condensed matter physics.

Modeling and Simulation of Graphene-Based Transducers in NEMS Accelerometers.

The mechanical characteristics of graphene ribbons with an attached proof mass that can be used as NEMS transducers have been minimally studied, which hinders the development of graphene-based NEMS devices. Here, we simulated the mechanical characteristics of graphene ribbons with an attached proof mass using the finite element method. We studied the impact of force, residual stress, and geometrical size on displacement, strain, resonant frequency, and fracture strength of graphene ribbons with an attached proof mass. The results show that the increase of width and thickness of graphene ribbons would result in a decrease of the displacement and strain but also an increase of resonant frequency. The increase of the length of graphene ribbons has an insignificant impact on the strain, but it could increase the displacement and decrease the resonant frequency. The increase of residual stress in the graphene ribbons decreases its strain and displacement. The estimated fracture strength of graphene shows limited dependence on its thickness, with an estimated value of around 148 GPa. These findings contribute to the understanding of the mechanical characteristics of graphene ribbons with an attached proof mass and lay the solid foundation for the design and manufacture of high-performance graphene-based NEMS devices such as accelerometers.

All-functionalized-graphene ribbon films for flexible asymmetric supercapacitors with ultrahigh energy and power densities

Asymmetric (hybrid) supercapacitors (ASCs) based on different positive and negative pseudocapacitance materials can deliver more energy density than electric double layer capacitors (EDLCs). Yet, few pseudocapacitive materials can provide high energy and power density, simultaneously, due to their poor conductivity and unfavorable reaction kinetics under fast charge/discharge rates. Here, we graft sulphur- and oxygen- functionalized into interconnected graphene ribbons to improve the accessibility of ion and enhance redox-active sites. The prepared sulphur- and oxygen- functionalized interconnected graphene ribbon (S-IGR and O-IGR) films as positive electrode and negative electrode, respectively, for high energy density flexible ASCs. Benefiting from the more “active sites” for the graphene ribbons, the S-IGR and O-IGR films connect more sulphur and oxygen groups. As a result, the S-IGR and O-IGR films exhibit ultra-high specific capacitance of 1660 F g−1 (1.66 F cm−2) and 428 F g−1 (0.43 F cm−2), respectively. More importantly, the assembled O-IGR//S-IGR ASCs based on aqueous and gel electrolytes exhibit ultra-high energy densities of 106.6 Wh kg−1 and 35.6 Wh kg−1, respectively. With the ultrafast charging and discharging capability, the ASCs based on aqueous electrolytes can be charge/discharged within 0.53 s to deliver high energy density of 15.0 Wh kg−1 and ultrahigh-power density of 101.4 kW kg−1. Thus, this strategy could possibly be used as designing and fabricating new generation of all grphene-based ASCs with high energy and power densities.

Hyperbolic phonon-plasmon polaritons in a hBN-graphene van der Waals structure

In this paper, a thorough theoretical study of a class of collective excitations, dubbed hyperbolic surface phonon-plasmon polaritons, is performed. These types of light-matter excitations are shown to have unique properties that allow to explore them both as the basis of ultra-sensitive devices to the dielectric nature of their surroundings. The system is a van der Waals heterostructure–a layered metamaterial, composed of different 2D materials in direct contact one with another, namely, graphene ribbons and hexagonal boron nitride slabs of nanometric size. In the paper, we discuss the spectrum of these classes of excitations, the associated electromagnetic fields, the sensitivity to the dielectric function of its surroundings, and the absorption spectrum. All of this is accomplished using an analytical model that considerably diminishes the computational burden, as well as elucidates the underling physical mechanism of the excitations supported by the device.

An ultra-thin meta-material including graphene patterns: Coupling application

Here a novel ultra-thin meta-material structure is proposed, including periodic arrays of graphene rings, disks, and ribbons and SiO2 dielectric as spacer between graphene patterns layers at the terahertz (THz) range. The introduced device can couple electromagnetic waves by considering reflection and transmission channels as outputs. Electromagnetic wave coupling depends on the parameters design and the device thickness. The proposed structure can couple electromagnetic waves in multi-band and close frequencies including 2 THz, 4 THz, 6 THz, 7.5 THz, and 9.5 THz. By considering the impedance matching concept, an equivalent circuit model (ECM) is developed for the proposed meta-material. Also, the device stability is investigated in various physical coefficients, geometrical parameters, and incident wave angles to ensure optical applications such as sensors, indoor communications, security, and medical imaging.