Graphene Crystals Research Articles

Plasmonics in Atomically Thin Crystalline Silver Films

Light-matter interaction at the atomic scale rules fundamental phenomena such as photoemission and lasing while enabling basic everyday technologies, including photovoltaics and optical communications. In this context, plasmons, the collective electron oscillations in conducting materials, are important because they allow the manipulation of optical fields at the nanoscale. The advent of graphene and other two-dimensional crystals has pushed plasmons down to genuinely atomic dimensions, displaying appealing properties such as a large electrical tunability. However, plasmons in these materials are either too broad or lying at low frequencies, well below the technologically relevant near-infrared regime. Here, we demonstrate sharp near-infrared plasmons in lithographically patterned wafer-scale atomically thin silver crystalline films. Our measured optical spectra reveal narrow plasmons (quality factor of ∼4), further supported by a low sheet resistance comparable to bulk metal in few-atomic-layer silver films down to seven Ag(111) monolayers. Good crystal quality and plasmon narrowness are obtained despite the addition of a thin passivating dielectric, which renders our samples resilient to ambient conditions. The observation of spectrally sharp and strongly confined plasmons in atomically thin silver holds great potential for electro-optical modulation and optical sensing applications.

Remote plasma-assisted low-temperature large-area graphene synthesis

Graphene is typically grown using thermal chemical vapor deposition (CVD) on metallic substrates such as copper and nickel at elevated temperatures above 1000 °C. The synthesis of large-area graphene at low temperature is highly desirable for large volume industrial production. In this paper, the authors report a remote plasma-assisted CVD graphene synthesis at a reduced temperature of 600 °C in a relatively shorter duration of 15 min. Scanning electron microscopy reveals the formation of large graphene crystal with an approximate size of 100 × 100 μm2 over the entire 2 × 10 cm2 surface of copper foil substrates. Raman spectra recorded for graphene grown at 600 °C show the presence of a graphene characteristic “2D” peak, attesting to the formation of graphene. The results show that it is possible to grow horizontal graphene at low temperatures and transfer it to flexible polyethylene terephthalate substrates. The utility of the synthesized graphene is ascertained through the successful fabrication of a flexible graphene-based electrochemical sensor for the detection of glucose concentration. The present research will have a direct impact on flexible wearable biosensors.

Thermal properties of multilayer graphene and hBN reinforced copper matrix composites

The unique properties of graphene make it a very attractive application, although there are still no commercial products in which graphene would play a key role. Good thermal conductivity is undoubtedly one of the attributes which can be easily used both in materials involving large monoatomic layers, that are very difficult to obtain, as well as multilayer graphene flakes, which have been commercially available on the market for several years. The article presents the results of tests on the characteristic thermal properties of composites with the addition of 2–15% of multilayer graphene (MLG) crystals. The motivation of the study was literature reports showing the possibility of increasing the thermal conductivity of composites with MLG participation in the copper matrix. Since the production of composites with increased properties is associated with obtaining a strong orientation of the flakes in the structure, composites with hBN flakes exhibiting significantly worse but also directional thermal properties were produced for comparison. The paper showed a strong influence of flake morphology on the possibility of creating a directional structure. The obtained Cu/MLG composites with the addition of only 2% MLG were characterized by an increase in the thermal conductivity coefficient of about 30% in relation to sinters without the participation of MLG.

Cobalt Oxide Nanoparticles/Graphene/Ionic Liquid Crystal Modified Carbon Paste Electrochemical Sensor for Ultra-sensitive Determination of a Narcotic Drug.

Purpose: Drug-abuse, namely morphine (MO) affects the metabolism of neurotransmitters such as dopamine (DA). Therefore, it is crucial to devise a sensitive sensing technique to simultaneously determine both compounds in real samples. Methods: The fabrication of the sensor is based on in situ modification of a carbon paste (CP) electrode with cobalt oxide nanoparticles, graphene, and ionic liquid crystal in presence of sodium dodecyl sulfate; CoGILCCP-SDS. The modified sensor is characterized using scanning electron microscopy, electrochemical impedance spectroscopy and voltammetry measurements. Results: Electron transfer kinetics and analytical performance of the proposed sensor were enhanced due to the synergistic role of all the modifiers. The simultaneous determination of MO and DA achieved low detection limits of 0.54 nmol L−1 and 0.25 nmol L−1, respectively. Besides, a carbon-based electrochemical sensor is fabricated for the nano-molar determination of MO in real samples and formulations. The sensor showed fouling resistance and anti-interference ability in presence of other species in human fluids. The real sample analysis of MO was successfully achieved with good recovery results in urine samples and pharmaceutical tablets. Linear dynamic range, sensitivity, detection limit and quantification limit of MO in urine were 5 nmol L−1 to 0.6 μmol L−1, 6.19 μA/μmol L-1, 0.484 nmol L−1 and 1.61 nmol L−1, respectively. Conclusion: This sensor has great ability to be extended for electrochemical applications in assaying of many drugs.

Circularly polarized extreme ultraviolet high harmonic generation in graphene.

Circularly polarized extreme ultraviolet (XUV) radiation is highly interesting for investigation of chirality-sensitive light-matter interactions. Recent breakthroughs have enabled the generation of such light sources via high harmonic generation (HHG) from rare gases. There is a growing interest in extending HHG medium from gases to solids, especially to 2D materials, as they hold great promise to develop ultra-compact solid-state photonic devices and provide insights into electronic properties of the materials themselves. However, so far reported, HHG in graphene driven by terahertz to mid-infrared fields generates only low harmonic orders, and no harmonics driven by circularly polarized lasers have been reported. Here, using first-principles simulations within a time-dependent density-functional theory framework, we show that it is possible to generate HHG extending to the XUV spectral region in monolayer extended graphene excited by near-infrared lasers. Moreover, we demonstrate that a single circularly polarized driver is enough to ensure HHG in graphene with circular polarization. The corresponding spectra reflect the six-fold rotational symmetry of the graphene crystal. Extending HHG in graphene to the XUV spectral regime and realizing circular polarization represent an important step toward the development of novel nanoscale attosecond photonic devices and numerous applications, such as spectroscopic investigation and nanoscale imaging of ultrafast chiral and spin dynamics in graphene and other 2D materials.

Plasmons in realistic graphene/hexagonal boron nitride moiré patterns

Van der Waals heterostructures employing graphene and hexagonal boron nitride (hBN) crystals have emerged as a promising platform for plasmonics thanks to the tunability of their collective modes with carrier density and record values for plasmonics figures of merit. In this Article we investigate theoretically the role of moir\'e-pattern superlattices in nearly aligned graphene on hBN by using continuum-model Hamiltonians derived from ab initio calculations. We calculate the system's energy loss function for a variety of chemical potential values that are accessible in gated devices. Our calculations reveal that the electron-hole asymmetry of the moir\'e bands leads to a remarkable asymmetry of the plasmon dispersion between positive and negative chemical potentials, showcasing the intricate band structure and rich absorption spectrum across the secondary Dirac point gap for the hole bands.

Two-Dimensional Flexible High Diffusive Spin Circuits.

Owing to their unprecedented electronic properties, graphene and two-dimensional (2D) crystals have brought fresh opportunities for advances in planar spintronic devices. Graphene is an ideal medium for spin transport while being an exceptionally resilient material for flexible nanoelectronics. However, these extraordinary traits have never been combined to create flexible graphene spin circuits. Realizing such circuits could lead to bendable strain-spin sensors, as well as a unique platform to explore pure spin current based operations and low-power 2D flexible nanoelectronics. Here, we demonstrate graphene spin circuits on flexible substrates for the first time. Despite the rough topography of the flexible substrates, these circuits prepared with chemical vapor deposited monolayer graphene reveal an efficient room temperature spin transport with distinctively large spin diffusion coefficients ∼0.2 m2 s-1. Compared to earlier graphene devices on Si/SiO2 substrates, such values are up to 20 times larger, leading to one order higher spin signals and an enhanced spin diffusion length ∼10 μm in graphene-based nonlocal spin valves fabricated using industry standard systems. This high performance arising out of a characteristic substrate terrain shows promise of a scalable and flexible platform towards flexible 2D spintronics. Our innovation is a key step for the exploration of strain-dependent 2D spin phenomena and paves the way for flexible graphene spin memory-logic units and planar spin sensors.

Graphene Tamm plasmon-induced giant Goos–H?nchen shift at terahertz frequencies

In this Letter, we have shown that a giant Goos–Hanchen shift of a light beam reflected at terahertz frequencies can be achieved by using a composite structure, where monolayer graphene is coated on one-dimensional photonic crystals separated by a dielectric slab. This giant Goos–Hanchen shift originates from the enhancement of the electrical field, owing to the excitation of optical Tamm states at the interface between the graphene and one-dimensional photonic crystal. It is shown that the Goos–Hanchen shift in this structure can be significantly enlarged negatively and can be switched from negative to positive due to the tunability of graphene’s conductivity. Moreover, the Goos–Hanchen shift of the proposed structure is sensitive to the relaxation time of graphene and the thickness of the top layer, making this structure a good candidate for a dynamic tunable optical shift device in the terahertz regime.

Multifield-tunable magneto-optical effects in electron- and hole-doped nitrogen–graphene crystals

The magneto-optical effects play a prominent role in probing the exotic magnetism in 2D materials. Here, we present that the magneto-optical Kerr and Faraday effects in carrier-doped nitrogen–graphene crystals can be effectively mediated by electric, magnetic, and strain fields. Our results indicate that nitrogen–graphene crystals provide a novel 2D material platform for nano-spintronics and magneto-optical devices.

Effect of electrolytes and sonication times on the formation of graphene using an electrochemical exfoliation process

The exfoliation of graphite is an intriguing approach for simple, fast and large-scale production of graphene. In the present study, three types of electrolytes were investigated, namely an ionic liquid (H2SO4), aqueous acid (H3PO4) and inorganic salt ((NH4)2SO4). Different sonication times (15, 30, 45 and 60 min) at room temperature were used to assist the electrochemical exfoliation. The graphene that was produced was characterized by the zeta potential, surface electrical conductivity, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, field emission microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). From the XRD analysis, it was observed that the graphene flakes had a broad peak centred at 26.5°, which corresponds to the (0 0 2) plane of graphene crystal. The FTIR spectrum revealed that the prominent peak was in conjunction with exfoliated graphene. The characteristic Raman bands were observed for the D, G and 2D bands and the ratio of I2D/IG was 1.7, which indicated that the exfoliated graphene was a multilayer structure. From the results, it was found that graphene obtained with the H2SO4 electrolyte that used a 45 min sonication time had the highest stability, best electrical conductivity, lowest crystallite size, fewer defects and lowest interlayer distance.

Spontaneous Nucleation and Growth of Graphene Flakes on Copper Foil in the Absence of External Carbon Precursor in Chemical Vapor Deposition

In this work, we uncover a mechanism initiating spontaneous nucleation of graphene flakes on copper foil during the annealing phase of chemical vapor deposition (CVD) process. We demonstrate that the carbon in the bulk of copper foil is the source of nucleation. Although carbon solubility in a pure copper bulk is very low, excess carbon can be embedded inside the copper foil during the foil production process. Using time-of-flight secondary ion mass spectrometry, we measured the distribution profile of carbon atoms inside the copper foils and its variation by thermal annealing. We also studied the role of hydrogen in the segregation of carbon from the bulk to the surface of copper during annealing by scanning electron microscopy and Raman analysis. We found that carbon atoms diffuse out from the copper foil and accumulate on its surface during annealing in the presence of hydrogen. Consequently, graphene crystals can be nucleated and grown while “any external” carbon precursor was entirely avoided. To our knowledge, this is the first time that such growth has been demonstrated to take place. We believe that this finding brings a new insight into the initial nucleation of graphene in the CVD process and helps to achieve reproducible growth recipes.

Effect of Sonication Time on the Production of Graphene by Electrochemical Exfoliation Method

In this study, Graphene has been synthesized by using electrochemical exfoliation method. Effect of sonication times (15, 30 and 45 min) at room temperature on the production of graphene was investigated. The synthesized graphene were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Atomic Force Microscopy (AFM). The crystal structure of the exfoliated graphene were analyzed using XRD. The graphene flakes exhibit a sharp peak centered at 26.5°corresponding to the (002) plane of graphene crystal. It is found that, the crystallite sizes is gradually decreased with increased in sonication time. The FTIR spectrum reveals the prominent appearance of C-O-H group which indicate that OH− radicals exist in conjunction with exfoliated graphene sheets. Graphene flakes with average thickness of 0.2 nm to 0.8 nm were observed using AFM. The electrical conductivity results showed that 45 min sonication time produced the highest electrical conductivity (1.38×102sm−1).

Low-Temperature Heteroepitaxy of 2D PbI2 /Graphene for Large-Area Flexible Photodetectors.

Heterostructures based on graphene and other 2D atomic crystals exhibit fascinating properties and intriguing potential in flexible optoelectronics, where graphene films function as transparent electrodes and other building blocks are used as photoactive materials. However, large-scale production of such heterostructures with superior performance is still in early stages. Herein, for the first time, the preparation of a submeter-sized, vertically stacked heterojunction of lead iodide (PbI2 )/graphene on a flexible polyethylene terephthalate (PET) film by vapor deposition of PbI2 on graphene/PET substrate at a temperature lower than 200 °C is demonstrated. This film is subsequently used to fabricate bendable graphene/PbI2 /graphene sandwiched photodetectors, which exhibit high responsivity (45 A W-1 cm-2 ), fast response (35 µs rise, 20 µs decay), and high-resolution imaging capability (1 µm). This study may pave a facile pathway for scalable production of high-performance flexible devices.

Evolution of Dirac Cone in Disclinated Graphene

Abstract Graphene crystals, containing arrays of disclination defects, are modeled and their energies are calculated using molecular dynamics (MD) simulation technique. Two cases are analyzed in details: (i) pseudo-graphenes, which contain the alternating sign disclination ensembles and (ii) graphene with periodic distribution of disclination quadrupoles. Electronic band structures of disclinated graphene crystals are calculated in the framework of density functional theory (DFT) approach. The evolution of the Dirac cone and magnitude of band gap in the band structure reveal a dependence on the density of disclination quadrupoles and alternating sign disclinations. The electronic properties of graphene with disclination ensembles are discussed.

Exotic bilayer crystals in a strong magnetic field

Electron bilayers in a strong magnetic field exhibit insulating behavior for a wide range of interlayer separation $d$ for total Landau level fillings $\nu\leq 1/2$, which has been interpreted in terms of a pinned crystal. We study theoretically the competition between many strongly correlated liquid and crystal states and obtain the phase diagram as a function of quantum well width and $d$ for several filling factors of interest. We predict that three crystal structures can be realized: (a) At small $d$, the Triangular Ising AntiFerromagnetic (TIAF) crystal is stabilized in which the particles overall form a single-layer like triangular crystal while satisfying the condition that no nearest-neighbor triangle has all three particles in the same layer. (b) At intermediate $d$, a Correlated Square (CS) crystal is stabilized, in which particles in each layer form a square lattice, with the particles in one layer located directly across the centers of the squares of the other. (c) At large $d$, we find a Bilayer Graphene (BG) crystal in which the A and B sites of the graphene lattice lie in different layers. All crystals that we predict are strongly correlated crystals of composite fermions; a theory incorporating only electron Hartree-Fock crystals does not find any crystals besides the `trivial' ones occurring at large interlayer separations for total filling factor $\nu\leq1/3$ (when layers are uncorrelated and each layer is in the long familiar single-layer crystal phase). The TIAF, CS and BG crystals come in several varieties, with different flavors of composite fermions and different interlayer correlations. The appearance of these exotic crystal phases adds to the richness of the physics of electron bilayers in a strong magnetic field, and also provides insight into experimentally observed bilayer insulator as well as transitions within the insulating part of the phase diagram.

Out-of-plane interface dipoles and anti-hysteresis in graphene-strontium titanate hybrid transistor

The out-of-plane electric polarization at the surface of SrTiO3 (STO), an archetypal perovskite oxide, may stabilize new electronic states and/or host novel device functionality. This is particularly significant in proximity to atomically thin membranes, such as graphene, although a quantitative understanding of the polarization across graphene–STO interface remains experimentally elusive. Here, we report direct observation and measurement of a large intrinsic out-of-plane polarization at the interface of single-layer graphene and TiO2-terminated STO (100) crystal. Using a unique temperature dependence of anti-hysteretic gate-transfer characteristics in dual-gated graphene-on-STO field-effect transistors, we estimate the polarization to be as large as ≈12 μC cm−2, which is also supported by the density functional theory calculations and low-frequency noise measurements. The anti-hysteretic transfer characteristics is quantitatively shown to arise from an interplay of band bending at the STO surface and electrostatic potential due to interface polarization, which may be a generic feature in hybrid electronic devices from two-dimensional materials and perovskite oxides.

Tunability of temperature-dependent absorption in a graphene-based hybrid nanostructure cavity

Enhanced absorption is obtained in a hybrid nanostructure composed of graphene and one-dimensional photonic crystal as a cavity in the visible wavelength range thanks to the localized electric field around the defect layers. The temperature-induced wavelength shift is revealed in the absorption spectra in which the peak wavelength is red-shifted by increasing the temperature. This temperature dependence comes from the thermal expansion and thermo-optical effects in the constituent layers of the structure. Moreover, the absorption peaks can be adjusted by varying the incident angle. The results show that absorption is sensitive to TE/TM polarization and its peak values for the TE mode are higher than the TM case. Also, the peak wavelength is blue-shifted by increasing the incident angle for both polarizations. Finally, the possibility of tuning the absorption using the electro-optical response of graphene sheets is discussed in detail. We believe our study may be beneficial for designing tunable graphene-based temperature-sensitive absorbers.

Enhancing the response of NH3 graphene-sensors by using devices with different graphene-substrate distances

Graphene (G) is a two-dimensional material with exceptional sensing properties. In general, graphene gas sensors are produced in field effect transistor configuration on several substrates. The role of the substrates on the sensor characteristics has not yet been entirely established. To provide further insight on the interaction between ammonia molecules (NH3) and graphene devices, we report experimental and theoretical studies of NH3 graphene sensors with graphene supported on three substrates: SiO2, talc and hexagonal boron nitride (hBN). Our results indicate that the charge transfer from NH3 to graphene depends not only on extrinsic parameters like temperature and gas concentration, but also on the average distance between the graphene sheet and the substrate. We find that the average distance between graphene and hBN crystals is the smallest among the three substrates, and that graphene-ammonia gas sensors based on a G/hBN heterostructure exhibit the fastest recovery times for NH3 exposure and are slightly affected by wet or dry air environment. Moreover, the dependence of graphene-ammonia sensors on different substrates indicates that graphene sensors exhibit two different adsorption processes for NH3 molecules: one at the top of the graphene surface and another at its bottom side close to the substrate. Therefore, our findings show that substrate engineering is crucial to the development of graphene-based gas sensors and indicate additional routes for faster sensors.

Ultrathin Active Layer for Transparent Electromagnetic Shielding Window

Recently, electromagnetic (EM) shielding (EMS) window, especially the ultrathin, transparent EMS active layer, has been the primary objective of extensive studies because of its widely potential applications in stealth technology, high radiation pollution, and others. However, several defects, including opacity and large thickness, have severely restricted the application in optical EMS devices. Herein, we developed an ultrathin and highly transparent EMS active layer on a rigid glass and a flexible polyethylene terephthalate substrate by chemical doping CVD (chemical vapor deposition) graphene with nitric acid (HNO3) as the P-type dopant, which has a 91% transmittance and 1/1000 thickness compared to the conventional EMS active layer. The HNO3-treated graphene shows excellent EMS efficiency by a factor of 4.5, significantly compensating for the adverse effects of the grain boundaries between CVD graphene crystals. Additionally, 55% HNO3 is the most suitable for achieving high EMS effectiveness, which can be significantly improved by treating for only 5 min. This unique chemical-doping CVD graphene holds potential for being exploited as a transparent active layer in numerous EMS applications.

Surface plasmon resonance fiber optic biosensor-based graphene and photonic crystal

A new sensor — transverse electric (TE) polarized excite surface plasmon resonance (SPR) fiber optic biosensor is proposed. The graphene is the plasma layer. The transfer matrix method and the finite difference time domain method are applied to conduct the numerical simulation of the four layers (fiber core/photonic crystals/graphene/sample) of fiber optic biosensor. The results show that the relationship between refractive index and resonant wavelength is linear and the sensitivity of the fiber optic biosensor reaches 1942 nm/RIU.