Single-layer Graphene Structure Research Articles

Terahertz bandwidth absorption metamaterial based on composite structure of single layer graphene

With the development of science and technology, people have realized the value of studying the terahertz band, in this paper, a terahertz bandwidth tunable metamaterial based on composite structure of single layer graphene is proposed, which can have an absorption higher than 0.9 at 3–6.7 THz and exhibit polarization insensitivity. By employing parameter inversion to calculate the relative impedance, the metamaterial adheres to the principles of impedance matching theory. The metamaterial exhibits considerable chemical and physical tuning capabilities, and it can be substantiated that the enhanced absorption rate of the metamaterial stems from a multitude of graphical configurations. Simultaneously, the metamaterial demonstrates a notable level of stability towards the incidence angle of electromagnetic waves and can function reliably at an incidence angle of 30°. In conclusion, the metamaterial holds promising potential for applications in radar stealth, communication, and other related domains.

Ferrotronics for the creation of band gaps in graphene

We experimentally demonstrate a simple graphene/ferrolectric device, termed ferrotronic (electronic effect from ferroelectric) device in which the band structure of single-layer graphene is modified. The device architecture consists of graphene deposited on a ferroelectric substrate which encodes a periodic surface potential achieved through domain engineering. This structure takes advantage of the nature of conduction through graphene to modulate the Fermi velocity of the charge carriers by the variations in surface potential, leading to the emergence of energy minibands and a band gap at the superlattice Brillouin zone boundary. Our work represents a simple route to building circuits whose functionality is controlled by the underlying substrate.

Experimental observation of spin−split energy dispersion in high-mobility single-layer graphene/WSe2 heterostructures

Proximity-induced spin–orbit coupling in graphene has led to the observation of intriguing phenomena like time-reversal invariant {{mathbb{Z}}}_{2} topological phase and spin-orbital filtering effects. An understanding of the effect of spin–orbit coupling on the band structure of graphene is essential if these exciting observations are to be transformed into real-world applications. In this research article, we report the experimental determination of the band structure of single-layer graphene (SLG) in the presence of strong proximity-induced spin–orbit coupling. We achieve this in high-mobility hexagonal boron nitride (hBN)-encapsulated SLG/WSe2 heterostructures through measurements of quantum oscillations. We observe clear spin-splitting of the graphene bands along with a substantial increase in the Fermi velocity. Using a theoretical model with realistic parameters to fit our experimental data, we uncover evidence of a band gap opening and band inversion in the SLG. Further, we establish that the deviation of the low-energy band structure from pristine SLG is determined primarily by the valley-Zeeman SOC and Rashba SOC, with the Kane–Mele SOC being inconsequential. Despite robust theoretical predictions and observations of band-splitting, a quantitative measure of the spin-splitting of the valence and the conduction bands and the consequent low-energy dispersion relation in SLG was missing—our combined experimental and theoretical study fills this lacuna.

Electrically Tunable Nonequilibrium Optical Response of Graphene.

The ability to tune the optical response of a material via electrostatic gating is crucial for optoelectronic applications, such as electro-optic modulators, saturable absorbers, optical limiters, photodetectors, and transparent electrodes. The band structure of single layer graphene (SLG), with zero-gap, linearly dispersive conduction and valence bands, enables an easy control of the Fermi energy, EF, and of the threshold for interband optical absorption. Here, we report the tunability of the SLG nonequilibrium optical response in the near-infrared (1000–1700 nm/0.729–1.240 eV), exploring a range of EF from −650 to 250 meV by ionic liquid gating. As EF increases from the Dirac point to the threshold for Pauli blocking of interband absorption, we observe a slow-down of the photobleaching relaxation dynamics, which we attribute to the quenching of optical phonon emission from photoexcited charge carriers. For EF exceeding the Pauli blocking threshold, photobleaching eventually turns into photoinduced absorption, because the hot electrons’ excitation increases the SLG absorption. The ability to control both recovery time and sign of the nonequilibrium optical response by electrostatic gating makes SLG ideal for tunable saturable absorbers with controlled dynamics.

Colorimetric and Fluorescent Dual-Modality Sensing Platform Based on Fluorescent Nanozyme.

Compared with natural enzymes, nanozymes based on carbonaceous nanomaterials are advantages due to high stability, good biocompatibility, and the possibility of multifunctionalities through materials engineering at an atomic level. Herein, we present a sensing platform using a nitrogen-doped graphene quantum dot (NGQD) as a highly efficient fluorescent peroxidase mimic, which enables a colorimetric/fluorescent dual-modality platform for detection of hydrogen peroxide (H2O2) and biomolecules (ascorbic acid-AA, acid phosphatase-ACP) with high sensitivity. NGQD is synthesized using a simple hydrothermal process, which has advantages of high production yield and potential for large-scale preparation. NGQD with uniform size (3.0 ± 0.6 nm) and a single-layer graphene structure exhibits bright and stable fluorescence. N-doping and ultrasmall size endow NGQD with high peroxidase-mimicking activity with an obviously reduced Michaelis–Menten constant (K m) in comparison with natural horseradish peroxidase. Taking advantages of both high nanozyme activity and unique fluorescence property of NGQD, a colorimetric and fluorescent dual-modality platform capable of detecting H2O2 and biomolecules (AA, ACP) with high sensitivity is developed as the proof-of-concept demonstration. Furthermore, the mechanisms underlying the nanozyme activity and biosensing are investigated.

Water-soluble pillar[5]arene modified amphiphilic graphene quantum dots with excellent penetration for intracellular imaging

Amphiphilic graphene quantum dots (A-GQDs) reveal prodigious potential application in bioimaging, resulting from their hydrophilic and lipophilic properties, as well as their good biocompatibility and negligible cytotoxicity. Herein, green fluorescent A-GQDs with single-layer graphene structure were prepared by one step of hydrothermal method. The A-GQDs with the average size of 3.5 nm displayed excitation-independent emission due to uniform size. A-GQDs emitted bright fluorescence on the surface of the cell membrane of MCF-7 cells when they were co-cultured with MCF-7 cells, which was similar to that of QSG-7701 cells. The water-soluble pillar[5]arene (WP5) was then incorporated on to the A-GQDs via π-π interaction and electrostatic interaction. There was different labeling imaging effect of MCF-7 and QSG-7701 cells achieved by combining the merits of WP5 and A-GQDs compared with individual A-GQDs. The synthesized composites not only had a certain enhancement in fluorescence, but also constructed a penetrating channel through the cell membrane, so that the entire cell had a distinct imaging. Therefore, these composites might be a potential candidate for applications in visualization of cell membrane and bioimaging.

Theoretical Study on Proton Permeation Ability of Modified Single-layer Graphene

The proton permeation ability of graphene has been highlighted for use as proton exchange membranes in electrochemical devices. However, the tunability of proton permeation is still limited. We have developed molecular models for the theoretical study of the proton permeation ability of modified single-layer graphene structures. Energy calculations were employed for the evaluation of the barrier height and permeation rate. The proton permeation is tunable by the presence of covalently bonded molecules, which is useful for the design of tunable interfaces for proton permeation.

All-Solid-State Optical Phased Arrays of Mid-Infrared Based Graphene-Metal Hybrid Metasurfaces.

Optical phased arrays (OPAs) are essential optical elements in applications that require the ability to manipulate the light-wavefront, such as beam focusing and light steering. To miniaturize the optical components, active metasurfaces, especially graphene metasurfaces, are used as competent alternatives. However, the metasurface cannot achieve strong resonance effect and phase control function in the mid-infrared region only relying on a single-layer graphene. Here we present a graphene-metal hybrid metasurface that can generate a specific phase or a continuous sweep in the range of a 275°-based single-layer graphene structure. A key feature of our design is that the phase adjustment mainly depends on the combination mechanism of resonance intensity and frequency modulation. An all-solid-state, electrically tunable, and reflective OPA is designed by applying the bias voltage to a different pixel metasurface. The simulation results show that the maximum deflection angle of the OPA can reach 42.716°, and the angular resolution can reach 0.62°. This design can be widely applied to mid-infrared imaging, optical sensing, and optical communication systems.

A Tunable Perfect THz Metamaterial Absorber with Three Absorption Peaks Based on Nonstructured Graphene

In this paper, a graphene-based tunable multi-band terahertz absorber is proposed and numerically investigated. The proposed absorber can achieve perfect absorption within both sharp and ultra-broadband absorption spectra. This wide range of absorption is gathered through a unique combination of periodically cross- and square-shaped dielectrics sandwiched between two graphene sheets; the latter enables it to offer more absorption in comparison with the traditional single-layer graphene structures. The aforementioned top layer is mounted on a gold plate separated by a Topas layer with zero volume loss. Furthermore, in our proposed approach, we investigated the possibility of changing the shapes and sizes of the dielectric layers instead of the geometry of the graphene layers to alleviate the edge effects and manufacturing complications. In numerical simulations, parameters, such as graphene Fermi energy and the dimensions of the proposed dielectric layout, have been optimally tuned to reach perfect absorption. We have verified that the performance of our dielectric layout called fishnet, with two widely investigated dielectric layouts in the literature (namely, cross-shaped and frame-and-square). Our results demonstrate two absorption bands with near-unity absorbance at frequencies of 1.6–2.3 and 4.2–4.9 THz, with absorption efficiency of 98% in 1.96 and 4.62 THz, respectively. Moreover, a broadband absorption in the 7.77–9.78 THz is observed with an absorption efficiency of 99.6% that was attained in 8.44–9.11 THz. Finally, the versatility provided by the tunability of three operation bands of the absorber makes it a great candidate for integration into terahertz optoelectronic devices.

Laser generation on weak modes in graphene structure with asymmetric unit cell

Problem formulating. Lasing on strong («radiative») plasmon resonance mode in graphene structure with dual grating gate with asymmetric unit cell requires strong gain. It is possible to achieve lasing in a weak ("non-radiative") mode at a lower gain rate. Goal. Theoretical study of laser generation on weak plasmonic resonance mode in single layer graphene structure screened by dual grating gate with asymmetric unit cell. Result. Laser generation on weak plasmonic resonance mode in single layer graphene structure screened by dual grating gate with asymmetric unit cell is reached. Excitation of a weak plasmon resonance mode requires less gain than excitation of a radiative one. Practical meaning. Results can be used to create sources of terahertz waves.

Porous Graphene Composite Polymer Fibres.

Since the isolation of graphene, there have been boundless pursuits to exploit the many superior properties that this material possesses; nearing the two-decade mark, progress has been made, but more is yet to be done for it to be truly exploited at a commercial scale. Porous graphene (PG) has recently been explored as a promising membrane material for polymer composite fibres. However, controlling the incorporation of high surface area PG into polymer fibres remain largely unexplored. Additionally, most polymer-graphene composites suffer from low production rates and yields. In this paper, graphene-loaded microfibres, which can be produced at a very high rate and yield have been formed with a carrier polymer, polycaprolactone. For the first time, PG has been incorporated into polymer matrices produced by a high-output manufacturing process and analysed via multiple techniques; scanning electron microscopy (SEM), Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). Raman spectra showed that single layer graphene structures were achieved, evidence for which was also backed up by the other techniques. Fibres with an average diameter ranging from 3–8 μm were produced with 3–5 wt% PG. Here, we show how PG can be easily processed into polymeric fibres, allowing for widespread use in electrical and ultrafiltration systems

Effect of irradiation upon single layer graphene on SiO2/Si substrate using electron beam irradiation (EBI)

Graphene is of particular interest in utilizing it as a carbon-based radiation device due to its unique electronic properties. The understanding of radiation damage mechanism in graphene is crucial for applications in radiation harsh environments. In this paper, we investigate the influence of high energy (MeV) electron beam irradiation on structural and electrical properties of single layer graphene (SLG) prepared by Chemical Vapor Deposition (CVD) on SiO2/Si substrate using Raman Microscopy and Current-Voltage (I-V) measurement. The samples were irradiated at doses of 50 kGy, 100 kGy and 200 kGy with high energy voltage of 3 MeV. It was found that as irradiation dose increases to 100 kGy, the 2D bands and G bands shift to lower frequency energy and surprisingly no appearance of D band. As dose increases to 200 kGy, the G band shifts to higher frequency energy and 2D band shifts back to position similar with the pristine graphene as before irradiation. Only a small D band appear at 1349 cm−1 after 200 kGy of irradiation. This means that only small number of defects formed in SLG structure even after it is irradiated at higher electron energy (MeV), indicating stability of the SLG used in this study. I-V analysis shows non-monotonic behaviour of graphene which electrical conductance increase at 50 kGy, decrease at 100 kGy and increase significantly at 200 kGy. The major mechanism is probably related to the charge-transfer doping due to the high dose of electron irradiation and low defect scattering.

Spatial 3d Direct Visualization of Atoms, Molecules and Chemical Bonds

The paper presents the further development of direct visualization of individual atoms, molecules with chemical bonds by spatial 3d electron cloud densitometry. Direct visualization of small objects studied by chemistry is made possible by the discovery of the plane wave amplitude deformation in accordance with the electron cloud density. Accordance with this affect, an atom begins to illuminate, depicting its own form! A quantum mechanical theory of the effect is given. As a result, it was possible to trace a chemical reaction with a change in the chemical bonds, geometry molecules, and distances between the atoms. The spatial 3d picoscope is used to show the structure of crystalline graphite formed by the <i>sp²</i> chemical bonds of carbon atoms, and a system of weak <i>π</i>-bonds that connects the layers of graphite. This device confirmed the flat structure of single-layer graphene and allowed to detect a two-layer hexagonal diamond system that was previously un known. This material is proposed to designate gubanite. A scan be seen from the spatial 3d snapshot, as the graphene layers converge, the upper graphene π-electron clouds connect to the lower graphene π-electron clouds to form a two-layer diamond gubanite, the spatial 3d model of which is shown. It should lead to the appearance of visual chemistry.

Amphiphilic graphene quantum dots as a new class of surfactants

Graphene quantum dots (GQDs) are the latest addition to nanocarbon materials family and promise a wide spectrum of novel applications. Herein, we demonstrate amphiphilic GQDs as a new class of zero-dimensional (0D) inorganic surfactants. Amphiphilic GQDs are synthesized using a facile one-step hydrothermal process and exhibit single-layer graphene structure, high crystallinity, uniform size and bright photoluminescence. Owning to ultrasmall size and unique surface-active characteristics, amphiphilic GQDs are able to stabilize Pickering emulsion, self-assemble into self-supporting monolith, disperse hydrophobic carbon nanotubes to form ultra-light aerogel, promote fast spreading of water on superhydrophobic surface. Furthermore, GQD surfactants can mediate Pickering interfacial reactions, such as, polymerization to form hollow microspheres and chemical conversion catalyzed by acidic functional groups on GQD surface.

Frequency-Reconfigurable Wide-Angle Terahertz Absorbers Using Single- and Double-Layer Decussate Graphene Ribbon Arrays.

We propose and numerically demonstrate two novel terahertz absorbers made up of periodic single- and double-layer decussate graphene ribbon arrays. The simulated results show that the proposed absorbers have narrowband near-unity terahertz absorption with ultra-wide frequency reconfiguration and angular stability. By tuning the Fermi level of graphene ribbons, the over 90% absorbance peak frequency of the absorber with single-layer graphene structure can be flexibly adjusted from 6.85 to 9.85 THz for both the transverse magnetic (TM) and transverse electric (TE) polarizations. This absorber with single-layer graphene demonstrates excellent angular stability with the absorbance peaks of the reconfigurable absorption bands remaining over 99.8% in a wide angle of incidence ranging from 0 to 70°. The tuning frequency can be significantly enhanced by using the absorber with double-layer graphene structure from 5.50 to 11.28 THz and 5.62 to 10.65 THz, approaching two octaves under TM and TE polarizations, respectively. The absorbance peaks of the reconfigurable absorption band of this absorber for both polarizations maintain over 70%, even at a large angle of incidence up to 70°. Furthermore, an analytical fitting model is also proposed to accurately predict the absorbance peak frequencies for this variety of absorbers. Benefitting from these attractive properties, the proposed absorber may have great potential applications in tunable terahertz trapping, detecting, sensing, and various terahertz optoelectronic devices.

Heat flux induced coherent vibration of H-shaped single layer graphene structure.

We report an exciting behavior of H-shaped single layer graphene structures subject to a heat flux. H-shaped graphene structures with certain particular dimensions were observed to be able to establish coherent mechanical vibrations in the out-of-plane direction directly from a steady state heat flux flowing from the centerline of the structure towards the two ends. Molecular dynamics (MD) simulation methods together with phonon spectral energy density (SED) analysis techniques were used to obtain the phonon dispersion profiles and identify the low frequency high energy phonon modes. Based on these calculations, we proposed a hypothesis that the mechanical vibration is the result of the superposition of the high energy out-of-plane acoustic (ZA mode) phonons traveling in opposite directions via scattering mechanisms. By using a theoretical model based on the superposition principle, we reproduced the vibrational mode of the graphene structure observed in the MD simulation and verified the proposed hypothesis.

Resonant tunneling though an asymmetrical two-magnetic-barrier structure on single layer graphene

The transmission probability against the electron normal incident energy through an asymmetrical two-magnetic-barrier structure of single layer graphene has been investigated and presented. One barrier’s height and width are varied while those of the other are kept at constant values. The effect of the structural asymmetry on the resonance energy and the resonant transmission peak value are also systematically studied with the help of numerical calculations based on the transfer matrix method. Investigations show that the unity transmission occurs in an asymmetrical two-magnetic-barrier structure when both the phase difference condition for resonance (PDCR) and the maximum condition for the resonance peak value (MCPV) are satisfied simultaneously. However, PDCR underestimates the resonance energy of an asymmetrical two-magnetic-barrier structure, and the deviation becomes more obvious as the structure progressively becomes less symmetrical. These findings are believed to be of significant importance for the design of single layer graphene based devices.

Facile synthesis of sulfur-doped graphene quantum dots as fluorescent sensing probes for Ag+ ions detection

Sulfur-doped graphene quantum dots (S-GQDs) with bright blue emission have been prepared by a facile one-pot hydrothermal treatment. A specific compound, 1,3,6-trinitropyrene, which has a mother nucleus structure similar with graphene, was chosen as the carbon source and 3-mercaptopropionic acid (MPA) was employed for S-doping and carboxyl groups modification. The synthesized S-GQDs were characterized by atomic force microscopy (AFM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and fluoscence (FL) spectrum. Results indicated that S-GQDs possessed single layer graphene structure with mean size of about 2.5nm and presented an excitation-independent photoluminescence behavior with maximum excitation/emission wavelength at 360/450nm, respectively. The sulfur-doping of GODs drastically improved their electronic and chemical properties, which afforded the S-GQDs a sensitive response to Ag+ ions. Furthermore, the S-GQDs were successfully explored as a sensing probe for Ag+ detection with high sensitivity and selectivity. A wide linear range of 0.1-130.0μM with a low detection limit of 30nM was obtained. The facile preparation method and the high performace of the as-prepared S-GQDs present promising potential for their applications in sensing, biological imaging and catalysis.

Gate-Voltage Tunability of Plasmons in Single-Layer Graphene Structures—Analytical Description, Impact of Interface States, and Concepts for Terahertz Devices

The strong light-matter interaction in graphene over a broad frequency range has opened up a plethora of photonics applications of graphene. The goal of this paper is to present the voltage tunability of plasmons in gated single- and multi-layer graphene structures. Device concepts for plasmonic interconnects and antennas and their performance for THz communication are presented. For the first time, the role of gate voltage and the thickness of the gate dielectric on the characteristics of plasmon propagation in graphene are quantified by accounting for both the interface trap capacitance and the quantum capacitance. The gate voltage serves as a powerful knob to tweak the carrier concentration and allows building electrically reconfigurable terahertz devices. By optimizing the gate voltage to maximize the plasmon propagation length in a gated multi-layer graphene geometry, we derive simple scaling trends that give intuitive insight into device modeling and design.

Li Storage Feasibility of Defected Single- and Bi-Layer Graphene

The limited capacity of the intercalation-type graphitic materials has lead to much effort to explore new electrode materials. A natural choice among the carbonaceous materials, graphene, has attracted increased attention for obtaining higher Li storage capacity. The outstanding mechanical strength, electronic conductivity, and ready availability of extra storage sites for Li, has suggested that this material could be an ideal platform for storing more Li. However, for wide range of Li coverages, the calculations predict that defect-free single layer graphene is not thermodynamically favorable for lithiation compared to bulk metallic Li. Thus, exploring ways with which graphene surfaces can be activated such that Li storage capacity can be improved has become a significant focus of research in the field. One such strategy is to activate graphene surface with structural defects. Generally speaking, structural defects can be found nearly in all carbonaceous materials. The presence of such defects can be detrimental for the performance of graphene-based devices. In contrast, however, deviations from the perfection of atomic lattice can also be useful for some applications, as they make it possible to tailor the properties leading to new functionalities. Following this approach, we performed an extensive series of density functional theory (DFT) calculations for Li adsorption and intercalation in single and bi-layer graphene, which are activated by topological defects for improving Li adsorption. The results confirm that Li adsorption on defect-free single layer graphene is not thermodynamically favorable compared to bulk metallic Li. However, graphene surfaces activated by structural defects are generally found to bind Li more strongly, with the interaction strength sensitive to both the nature of the defects and their densities. The interaction of Li with a one-dimensional extended defect is additionally found to be strong, improving Li storage capacity. The theoretical Li storage capacities of the defected single-layer graphene structures are evaluated with a rigorous thermodynamic analysis, which establishes, in some cases, that these capacities approach, although not exceed, those of graphite. Li storage in bi-layer graphene with and without defects are evaluated in a similar way, and indicate more favorable interaction, and promise for higher Li storage than that of single layer graphene. We will present performance comparisons between defected single- and bi-layer graphene structures, and bulk-graphite for Li storage capacities. A detailed analysis of the effect of the van der Walls (vdW) interactions will also be presented.