Graphene Microribbons Research Articles

High-efficiency light manipulation using a single layer of folded graphene microribbons

Since its one-atom thickness, it remains an open question to enhance light–matter interactions in graphene, which is usually implemented through external resonant structures such as Fabry–Perot cavity. Here, we propose an alternative scheme to enhance light–matter interactions in a single layer of folded graphene microribbons (FGMRs), and remarkably, for normal incidences rather than oblique incidences in most studies. By optimizing structural parameters (e.g., the location of folding axis and folding angle), three light manipulations such as perfect absorption, perfect reflection, and perfect transmission can be achieved independently. More interestingly, any one of the three functionalities can be actively switched to the other via changing material parameters (Fermi level and carrier mobility ), which is actually the most attractive feature of graphene plasmonics. Finally, we show FGMRs can also support triple functionalities, i.e., via changing material parameters, one of the three functionalities can be switched to the second one and then the third one. Our results will be of great interest to fundamental physics and pave the way for graphene plasmonic device applications.

Thermal rectification induced by phonon hydrodynamics in asymmetric 2D microstructures

A thermal diode/rectifier, where the thermal resistance is direction dependent, has always been highly sought after and may find applications in potential thermal logic devices and thermal management systems. By using a more accurate local spatial linear approximation (LSLA) when numerically solving the phonon Boltzmann transport equation (BTE) under Callaway's dual model, it is shown here for the first time that phonon hydrodynamics combined with geometric asymmetry can lead to thermal rectification (TR). Interestingly, in a T-shaped graphene microribbon heat preferentially flows from the narrow end to the wide end, which is opposite to what is observed in T-shaped graphene nanoribbons. Flexural acoustic (ZA) phonons play the dominant role in the TR effect, due to their prominent hydrodynamic behavior. By analysis of the temperature spatial distribution and the phonon mean free paths (MFP), the reasons for TR are explained. By adjusting geometric parameters, the 16 % TR ratio is reached. Furthermore, the impact of edge roughness, temperature difference and strength of phonon hydrodynamics on the TR effect are investigated. This work points to a new mechanism for enabling thermal rectification and may provide insight into the unique interplay between phonon hydrodynamics and engineered microstructures.

Исследование влияния краевых эффектов на свойства плазмонных мод в графеновой микроленте

The work devoted to theoretical studies of the excitation of plasmon modes in a graphene microribbon by an incident terahertz electromagnetic wave. The problem is solved in a three-dimensional formulation taking into account edge-field effects at the ends of graphene microribbon by using the integral equation method being developed by the authors. The influence of microribbon length (for constant width of the microribbon) on the properties of the fundamental plasmon mode is studied. Criteria for choosing either simplified two-dimensional or three-dimensional electromagnetic approaches for studying the properties of plasmon modes in graphene microribbons are formulated.

Wavefront reconfigurable metasurface through graphene micro-ribbons with resonant strategy

The majority of metasurfaces are static in their optical properties due to the principles of their construction. For the effect of wavefront reconfigurable, graphene provides the opportunity for adjustability via gate biasing. In this study, a high-efficiency wavefront reconfigurable metasurface is proposed with the feature of multiple optional functions, including perfect absorption, arbitrary multi-focus, and self-healing. Using a resonant strategy in the THz frequency range, the designed metasurface can shape the wavefront at will without reconstituting the device’s configuration. With the help of matrix analysis theory, we find that the reflected phase shifts show active adjustable characteristics as the Fermi level ranges from 0 eV to 0.6 eV, corresponding to the phase coverage of 0-2π. The proposed wavefront reconfigurable metasurface may open avenues for wave manipulation and have deep implications in optical communications.

Observation of hydrodynamic plasmons and energy waves in graphene.

Thermally excited electrons and holes form a quantum-critical Dirac fluid in ultraclean graphene and their electrodynamic responses are described by a universal hydrodynamic theory. The hydrodynamic Dirac fluid can host intriguing collective excitations distinctively different from those in a Fermi liquid1-4. Here we report the observation of the hydrodynamic plasmon and energy wave in ultraclean graphene. We use the on-chip terahertz (THz) spectroscopy technique to measure the THz absorption spectra of a graphene microribbon as well as the propagation of the energy wave in graphene close to charge neutrality. We observe a prominent high-frequency hydrodynamic bipolar-plasmon resonance and a weaker low-frequency energy-wave resonance of the Dirac fluid in ultraclean graphene. The hydrodynamic bipolar plasmon is characterized by the antiphase oscillation of massless electrons and holes in graphene. The hydrodynamic energy wave is an electron-hole sound mode with both charge carriers oscillating in phase and moving together. The spatial-temporal imaging technique shows that the energy wave propagates at a characteristic speed of [Formula: see text] near the charge neutrality2-4. Our observations open new opportunities to explore collective hydrodynamic excitations in graphene systems.

Modeling 2D Arrangements of Graphene Nanoribbons

In the last two decades, interest in graphene has grown extensively due to its extraordinary properties and potential for various applications such as sensing and communication. However, graphene is intrinsically a semimetal with a zero bandgap, which considerably delays its use where a suitable bandgap is required. In this context, quasi-one-dimensional counterparts known as graphene nanoribbons (GNRs) have demonstrated sizeable bandgaps and versatile electronic properties, which make them promising candidates for photonic and plasmonic applications. While progress has recently been made toward the synthesis of GNRs, theoretical models to envisage their electronic and optical properties have been restricted to ab initio approaches, which are not feasible for wide systems because of the large number of atoms tangled. Here, we use a semi-analytical model based on Dirac cone approximation to show the adjustable electronic and plasmonic characteristics of wide and experimental GNRs, both freestanding and non-freestanding. This approach utilizes the group velocity of graphene, which is calculated using density functional computations (vF=0.829×106 m s−1), as the primary input. Importantly, our research reveals that at the terahertz level, the plasmon-momentum dispersion is highly responsive to changes by varying the ribbon width or charge carrier concentrations, the other involved parameters can be manipulated by setting values from experiments or more sophisticated predictions. In particular, this model can replicate the electronic properties of GNRs on Ge(001) and GNRs on Au(111). From the plasmonic side, the plasmon spectrum of graphene microribbon arrays of 4 μm wide on Si/SiO2 and GNR arrays on Si are found in good agreement with experiments. The potential use of GNRs in sensing molecules such as chlorpyrifos-methyl is also discussed. Chlorpyrifos-methyl is chosen as the test molecule because it is a commonly used insecticide in agriculture, but its high toxicity to organisms and humans makes it a concern. It has been established that the plasmon resonances of all the studied GNRs occur at the same frequency as chlorpyrifos-methyl, which is 0.95 THz. Our findings can serve as a useful guide for future experiments.

Frequency conversion in time-varying graphene microribbon arrays.

We investigate the possibility of frequency conversion in time-varying metasurfaces, composed of graphene microribbon arrays (GMRAs) with time-periodic modulation of their conductivity. We present a quasi-static model for the interaction of light with a temporally modulated metasurface, as well as an accurate analytical treatment of the problem of time-varying GMRAs. Results coming from numerical simulations are also available. We provide corrections to a previous related proposal for frequency conversion and refute the possibility of attaining frequency shifts not equal to an integral multiple of modulation frequency. Contrary to the preceding results, our findings show that efficient frequency conversion demands more requisites than single-layer GMRAs can supply and that its requirements can be addressed successfully by a multi-layer design.

Dispersion Theory of Surface Plasmon Polaritons on Bilayer Graphene Metasurfaces.

Surface plasmon polaritons (SPPs) on the graphene metasurfaces (GSPs) are crucial to develop a series of novel functional devices that can merge the well-established plasmonics and novel nanomaterials. Dispersion theory on GSPs is an important aspect, which can provide a basic understanding of propagating waves and further guidance for potential applications based on graphene metamaterials. In this paper, the dispersion theory and its modal characteristics of GSPs on double-layer graphene metasurfaces consisting of the same upper and lower graphene micro-ribbon arrays deposited on the dielectric medium are presented. In order to obtain its dispersion expressions of GSP mode on the structure, an analytical approach is provided by directly solving the Maxwell’s equations in each region and then applying periodical conductivity boundary onto the double interfaces. The obtained dispersion expressions show that GSPs split into two newly symmetric and antisymmetric modes compared to that on the single graphene metasurface. Further, the resultant dispersion relation and its propagating properties as a function of some important physical parameters, such as spacer, ribbon width, and substrate, are treated and investigated in the Terahertz band, signifying great potentials in constructing various novel graphene-based plasmonic devices, such as deeply sub-wavelength waveguides, lenses, sensors, emitters, etc.

Coupling graphene microribbons with carbon nanofibers: New carbon hybrids for high-performing lithium and potassium-ion batteries

Carbon‑carbon allotropic hybrids exhibit remarkable properties, including exceptional electrochemical charge storage capacities. A novel hybrid material composed of 1D carbon nanofibers (CNF) and 2D graphene micro-ribbons (GMR) was synthesized and incorporated as anodes in Li-ion batteries (LIB) and Potassium-ion batteries (KIB) for improved storage capacity. CNF-GMR material was hybridized simultaneously by one-step chemical vapour deposition (CVD) synthetic process, wherein the CNF were grown on the graphene surface using an iron oxide catalyst. Meanwhile, the GMRs were formed by the catalytic cutting of few-layer graphene. This unique carbon‑carbon allotropic hybrid exhibits excellent structural integrity, good electrical conductivity (718 S/m) and high specific surface area (305.6 g/m2). The as-prepared materials, when used as an anode in batteries, exhibited a highly reversible capacity (598 mAhg−1 and 410 mAhg−1 at 0.10 Ag−1 for LIB and KIB, respectively) with fast charging and discharging capability, and long-term cycling stability with 99% Coulombic efficiency over 1000 cycles.

Definition of CVD Graphene Micro Ribbons with Lithography and Oxygen Plasma Ashing

This work presents the definition of CVD (Chemical Vapor Deposition) Graphene Micro Ribbons (GMRs) with traditional Photolithography and Oxygen Plasma Ashing, where: (i) CVD Graphene Micro Ribbons were defined in parallel and serpentine patterns; (ii) The defined width dimensions of GMRs are between 0.23 µm and 10 µm; (iii) The empirical calibration curve between physical and mask widths of GMRs was obtained; (iv) With our method, Graphene Field-Effect Transistors, GFETs, can be fabricated with parallel GMRs as conduction channel between source and drain. It is important to notice that parallel channels are used in Silicon Nano Wires 3D Transistors, such as FinFETs. Our method is based on low-cost lithography, compared to commonly found in the literature, based on the definition of graphene patterns by Electron Beam Lithography (high cost and low repeatability techniques). Also, graphene's good quality characteristics remain after GMRs fabrication, as proven by Raman spectroscopy. In conclusion, our method to define the GMRs is suitable for GFET technology.

Study on THz SPPs excitation and field distributions of graphene microribbons

Surface plasmon polaritons have wide applications in sensing, biasing, absorption, and beam splitting, etc. because they can break diffraction limits and have strong field localization. So far, most of the research on terahertz surface plasmon polaritons is in the far-field spectrum, and exploring near-field properties is further needed. In this paper, based on the graphene strip structure, the excitation and field distribution characteristics of terahertz surface plasmon polaritons were studied. Specifically, the graphene microribbons that could excite surface plasmon polaritons by terahertz waves were designed, and the field distribution was calculated. Then, we experimentally fabricated the designed structure and measured its surface plasmon polaritons by terahertz near-field microscope. Finally, the effects of resonance detuning on surface plasmon polaritons field distribution were investigated. Our results provide a research basis for the application of graphene surface plasmon polaritons components in terahertz detection, biomedicine, safety detection, and high data rate communication.

Оптимизация параметров и характеристик широкополосных терагерцовых поглотителей на основе 2d-решеток графеновых лент на многослойных подложках

A multilayer two-dimensional periodic arrays of graphene micro-ribbons, which absorbs almost 100% of the energy of incident wave at the resonance of the surface plasmon-polariton, has been theoretically investigated. To increase the bandwidth of the absorbers, it is proposed to place 2-3 parallel graphene ribbons with close lengths in the unit cell of the 2d array.

Tunable Optical Demultiplexer for Dense Wavelength Division Multiplexing Systems Using Graphene–Silicon Microring Resonators

An ultracompact tunable optical dense wavelength division demultiplexer (DWDM) compatible with complementary metal–oxide–semiconductor technology is proposed, consisting of a central bus waveguide coupled to two drop waveguides through two ring resonators, all made of silicon (Si). Each Si ring resonator is topped by a graphene microribbon whose chemical potential can be tuned as desired. The center wavelength of each drop channel is determined by the resonant wavelength of the adjacent ring resonator, which in turn is tuned by modifying the conductivity of the graphene. Simulations show that the proposed structure with two graphene–Si ring resonators (GSRRs) having a chemical potential difference of 210 meV demultiplexes a broad optical signal into two narrow channels centered at about 1552.42 nm and 1552.85 nm (with a channel spacing of less than 0.43 nm), each emerging from one drop waveguide, with a transmission efficiency above 64%. The full-width at half-maximum values of the channels are 160 pm and 250 pm, providing quality factors of ∼ 9705 and 6209 with crosstalk of less than − 13.3 dB. By exchanging the chemical potentials of the GSRRs, the drop channels can be swapped dynamically without mechanical manipulation of the device. The proposed ultracompact graphene-based demultiplexer enables electrically tuned optical demultiplexing for DWDM systems as a potential alternative to the thermooptically tuned demultiplexers employed in long-haul optical communications.

Multifunctional space–time phase modulated graphene metasurface

Metasurfaces provide special features for manipulating electromagnetic wavefronts that are not possible with conventional optical devices. A common approach in designing metasurfaces has been the use of spatially varying metallic and/or dielectric nanoantennas separated with a subwavelength distance to obtain the required local phase change yielding the desired optical performance. In this paper, we propose a space–time phase modulation technique based on a graphene metasurface with the possibility of actively manipulating the electromagnetic wavefront. In this technique, we utilize graphene microribbon arrays that exhibit resonant behavior at terahertz (THz) frequencies. By applying an alternating voltage with a particular modulation frequency and phase, the time-dependent changes in the complex refractive indices of the graphene ribbons can be induced. This phenomenon results in the active control of the reflection amplitude and phase and the generation of the harmonic frequencies in the output reflection spectra. Theoretically, by using the Floquet analysis, it is shown that the reflected wave has harmonic frequencies, and the phase of the reflection wave at each harmonic component changes through changing the modulation phase of each graphene ribbon. The performance of the wavefront manipulation technique is evaluated using the finite difference time domain method and the circuit model. The results of the proposed circuit model are in good agreement with those of the full-wave simulation. Additionally, the applications of the proposed space–time phase modulated graphene metasurface for realizing an anomalous reflector and a lens with a tunable focal length are explained in detail.

Morphological effect and conductivity tunability of different regions in a single graphene film by surface steps

Surface step is a fascinating aspect of chemical vapor-deposited graphene. Although the Cu surface steps influence graphene nucleation, they limit the domain expansion at lower temperatures. In this study, we report surface steps and nanoscale surface roughness that can be used to control the electrical properties of two regions in the same graphene film. Higher processing temperatures such as 1000 °C produced an ultra-smooth surface (below 0.1 nm roughness), whereas lower temperatures of 700 °C showed minimal step height due to lack of surface rearrangement. In order to introduce two different surface step conditions, the selected regions were individually subjected to electropolishing (smooth) and Ar plasma (nanoscale surface steps) treatments. At a moderate temperature of 800 °C, a smooth area eases diffusion-driven graphene growth, which results in a continuous film of monolayer graphene with better electrical properties (mobility ≈ 700 cm2/V). However, the region with surface steps showed no conductivity whereas Raman spectra confirmed the existence of multilayer graphene. This phenomenon was observed with fabricated graphene micro-ribbons at 800 °C; a 40 μm ribbon showed an approximate resistance of 3.5 kΩ mm−1. Most importantly, the resistance was precisely controlled by physical parameters of the ribbon or during graphene processing in a single film.

Giant enhancing Faraday Rotation of graphene microribbons by asymmetric geometry

We have designed a unit cell employing only a single sheet of graphene ribbons with an asymmetric geometry leading to the Faraday rotation (FR) up to 24 degree. Asymmetric geometry boosts plasmonic coupling between arrays as well as increasing transmission through structure. Placing a single layer of graphene with a glass dielectric on the structure results in enhancing FR more than 21%. This design opens up a way in ultrathin magneto-optical nanophotonic devices to realize polarization rotators such as isolator, circulator, modulator, etc.

On the Amplification of Terahertz Radiation by High-Q Resonant Plasmons in a Periodic Graphene Bilayer under Plasmon-Mode Anticrossing

The spectrum of amplification of terahertz radiation in a structured active graphene bilayer consisting of two identical periodic graphene microribbon arrays separated by a thin dielectric barrier layer is theoretically investigated. The system supports optical and acoustic plasmon modes. The resonant frequencies of the optical and acoustic modes change oppositely with the dielectric-layer thickness, which allows plasmon-mode anticrossing. It is shown that the investigated graphene structure is characterized by a strong plasmon response and giant terahertz-radiation amplification at plasma resonance frequencies in the vicinity of the anticrossing between the optical and acoustic plasmon modes at room temperature.

Freestanding Laser-Assisted Reduced Graphene Oxide Microribbon Textile Electrode Fabricated on a Liquid Surface for Supercapacitors and Breath Sensors.

Graphene microribbons (rGO-MRs) are highly desired for their high electrical conductivities and specific surface areas, which contribute to multiple applications in thin, flexible, textile supercapacitors, sensors, and actuators. Herein, we demonstrate a facile method for creating reduced graphene oxide microribbons with microscale architecture utilizing a simple blue-violet diode laser under ambient conditions. This method takes advantage of the photochemical reduction mechanism of self-assembled graphene oxide liquid crystals (GO-LC), allowing rGO-MR patterns to be directly printed on the solution surface. The rGO-MR films demonstrated tunable diameters and can be tailored into any geometries. A maximum intrinsic electrical conductivity for rGO-MR reaching 325.8 S/m was observed. The rGO-MR textile electrodes can be assembled into microsupercapacitors with a high areal specific capacitance of 14.4 mF/cm2, a low charge-transfer impedance, and an exceptional cycling performance with a retained 96.8% capacitance after 10 000 cycles. The rGO-MR films also experience changes in resistance in response to the moisture adsorption from human breaths and therefore can also be employed as a breathing sensor for health monitoring. The presented facile method for creating multilayered rGO-MR films directly on liquid surfaces can further expand the potential for three-dimensional printing graphitic materials for various multifunctional applications.

Nano-featured poly (lactide-co-glycolide)-graphene microribbons as a promising substrate for nerve tissue engineering

In this research, nanocomposite poly (lactide-co-glycolide)-Graphene (PLGA-Gr) microribbons were developed for neural tissue engineering. Moreover, the effects of Gr concentration (0, 0.1, 0.5 and 1 wt %) on the chemical and physical structure, mechanical properties, thermal stability and biological properties were evaluated. Our findings proved that incorporation of Gr nanosheets in the PLGA matrix resulted in the formation of aligned groove-shaped roughness on the surface of microribbons. In addition, Gr nanosheets could significantly promote the electrical conductivity and hydrophilicity of PLGA microribbons. In addition, the tensile strength and elastic modulus of the PLGA-Gr microribbons significantly promoted (upon 2 times and more than 3 times, respectively) compared to PLGA microribbons. The results demonstrated enhanced differentiation rate of SH-SY5Y cells to mature neurons on PLGA-Gr compared to PLGA. In summary, our findings discovered that aligned PLGA-Gr microribbons presented appropriate chemical, physical and mechanical properties to promote neuroblastoma cells. It is anticipated that the offered PLGA-Gr scaffolds might have great potential to develop a favorable construct for central nerve regeneration. However, further biological in vivo studies are required to assess the role of PLGA-Gr microribbons on the nerve regeneration.

Production and Mechanical Characterization of Graphene Micro-Ribbons

Patterning of graphene into micro- and nano-ribbons allows for tunability in emerging fields such as flexible electronic and optoelectronic devices, and is gaining interest for the production of more efficient reinforcement for composite materials. In this work we fabricate micro-ribbons from graphene synthesized via chemical vapor deposition (CVD) by combining ultraviolet (UV) photolithography and dry etching oxygen plasma treatments. We used Raman spectral imaging to confirm the effectiveness of the patterning procedure, which is suitable for large-area patterning of graphene on wafer-scale, and confirms that the quality of graphene remains unaltered. The produced micro-ribbons were finally transferred and embedded into a polymeric matrix and the mechanical response was investigated by in-situ mechanical investigation combining Raman spectroscopy and tensile/compressive tests.