Layers In Few-layer Graphene Research Articles

Charge distribution in turbostratic few-layer graphene studied by carbon isotope labeling

Elucidating layer-resolved charge distribution in van der Waals stacked crystals is crucial for electronic applications based on 2D materials. Here, we use CVD-grown few-layer graphene (FLG) labeled by carbon isotopes as a prototype, in which Raman features of two isotope components identify distinct layers. Electrical transfer characteristics of graphene field-effect transistors (GFETs) are employed to quantitatively calibrate the correspondence of Fermi level and G-phonon frequency that shifts with the charge-carrier concentration (n) variation. The isotope-assisted spectroscopy reveals previously unprobed characteristics that the n value of both top and bottom layers in FLG is close rather than following an exponential decay law. This negligible gradient likely benefits from the π−π interaction that favors the interlayer diffusion of charges. In addition, each extra layer reduces the degree of charge exchange at the graphene/dopant interface. These results have important implications for FLG nanoelectronics and provide a robust framework by which one can further investigate the critical properties of other 2D systems.

Polycrystalline Few-Layer Graphene as a Durable Anticorrosion Film for Copper

Corrosion of metals in atmospheric environments is a worldwide problem in industry and daily life. Traditional anticorrosion methods including sacrificial anodes or protective coatings have performance limitations. Here, we report atomically thin, polycrystalline few-layer graphene (FLG) grown by chemical vapor deposition as a long-term protective coating film for copper (Cu). A six-year old, FLG-protected Cu is visually shiny and detailed material characterizations capture no sign of oxidation. The success of the durable anticorrosion film depends on the misalignment of grain boundaries between adjacent graphene layers. Theoretical calculations further found that corrosive molecules always encounter extremely high energy barrier when diffusing through the FLG layers. Therefore, the FLG is able to prevent the corrosive molecules from reaching the underlying Cu surface. This work highlights the interesting structures of polycrystalline FLG and sheds insight into the atomically thin coatings for various applications.

Synthesis of transparent few layer graphene films using a dual flame approach for the ammonia gas sensor

With regard to graphene preparation, dual flame synthesis method has significant advantages from the up-scaling perspective in terms of cost-effectiveness and fast processing compared to other methods. In this study, a few layer graphene (FLG) films were prepared on quartz in only 1 min using a dual flame approach at different temperatures (600, 700 and 800 °C). The structural, morphological, and optical features of FLG films as well as its gas sensing ability of ammonia gas (NH3) were reported. Raman analyses prove the crystallinity of the prepared FLG with two characteristic bands namely ID and IG. X-ay diffraction (XRD) results present the FLG films have a hexagonal phase with a distinguished orientation along the direction (002). Optical spectra demonstrate the transmittance reduces and FLG layers increase as temperature increases. Transmission electron microscope (TEM) images display the FLG films have high quality with smooth surfaces and flat structures. The best data are obtained in graphene films, where few layers, high crystallinity, and highest sensitivity toward NH3 gas. The FLG-based sensors revealed high sensitivity and selectivity to NH3 over dimethylformamide, toluene, acetone, and carbon dioxide (CO2) at room temperature. These results verified that the preparation approach designed in this work yielded transparent FLG films with acceptable quality and functional performance.

Dispersion of graphene in ethanol by sonication

Graphene is prepared from natural graphite flakes by solvent dispersion and short-sonication time. In this study, we exfoliate few-layer graphene (FLG) directly in ethanol without adding high boiling point solvents, surfactants or polymers. Diverse characterization methods are employed to scrutinize the morphology and quality of the obtained graphene. UV–visible spectrum is characterized by a prominent peak at 269 nm, corresponding to the π to π∗ transitions of graphene. Raman spectroscopy show that exfoliated FLG to be free of basal-plane defects. Scanning electron microscopy (SEM) evidence semitransparent FLG layers. Transmission electron microscopy (TEM) presents folded (or folded edge) graphene sheets. As the sonication time is reduced, the proposed method shows a great promise for the industrial-scale synthesis of graphene for many applications, from high-frequency electronics to smart coatings.

Photoelectrochemical water splitting by engineered multilayer TiO2/GQDs photoanode with cascade charge transfer structure

Herein, for the first time, an efficient photoanode engineered with the cascade structure of FTO|c-TiO2|few graphene layers|TiO2/GQDs|Ni(OH)2 assembly (Ni(OH)2 photoanode) is designed. This photoanode exhibited much lower electron–hole recombination, fast charge transport, higher visible light harvesting, and excellent performance with respect to FTO|c-TiO2|TiO2 assembly (TiO2 photoanode) in the photoelectrocatalytic oxygen evolution process. The photocurrent density of Ni(OH)2 photoanode is 7 times (0.35 mA cm−2 at 1.23 V vs. RHE) greater than that of TiO2 photoanode (0.045 mA cm−2 at 1.23 V vs. RHE). The compact TiO2 (c-TiO2) layer in Ni(OH)2 photoanode plays a role of an effective hole-blocking layer. Few-layer graphene layer could speed up the transport of the photogenerated electrons from the conduction band of the TiO2/GQDs to FTO. Ni(OH)2 layer could transfer rapidly holes into electrolyte solution.

Polarity Tunable Trionic Electroluminescence in Monolayer WSe2.

Monolayer WSe2 exhibits luminescence arising from various types of exciton complexes due to strong many-body effects. Here, we demonstrate selective electrical excitation of positive and negative trions in van der Waals metal-insulator-semiconductor (MIS) heterostructure consisting of few-layer graphene (FLG), hexagonal boron nitride (hBN), and monolayer WSe2. Intentional unbalanced injection of electrons and holes is achieved via field-emission tunneling and electrostatic accumulation. The device exhibits planar electroluminescence from either positive trion X+ or negative trion X- depending on the bias conditions. We show that hBN serves as a tunneling barrier material allowing selective injection of electron or holes into WSe2 from FLG layer. Our observation offers prospects for hot carrier injection, trion manipulation, and on-chip excitonic devices based on two-dimensional semiconductors.

Epitaxial growth and characterization of GaN thin films on graphene/sapphire substrate by embedding a hybrid-AlN buffer layer

This study investigates that high-quality GaN thin films can be grown on a few-layer graphene (FLG)/sapphire substrate by embedding a hybrid AlN buffer layer (BL). The hybrid AlN BL is constructed by low-temperature AlN nucleation layer (LT-AlN NL) and high-temperature AlN BL grown respectively by sputtering and metal organic chemical vapor deposition (MOCVD). The high density of edge-type threading dislocation (TD) in the GaN sample without hybrid AlN BL provide current leakage paths, resulting in a symmetric and temperature-independent I-V characteristic curve for a Ni-based Schottky contact. The excellent adhesion and uniform coverage of LT-AlN NL on the FLG layer by sputtering can overcome the nucleation issue and prevent the thermal etching effect of graphene during MOCVD epitaxial process. The edge-type TD density and carbon concentration of the GaN thin films grown on the hybrid AlN BL/FLG/sapphire substrate can be reduced significantly, resulting in a lower intensity of blue, green, and orange luminescences on a 17-K photoluminescence spectrum. The Ni-based Schottky contact with a barrier height of 0.69 eV and leakage current density of 4.38 × 10−6 A/cm2 is obtained, which demonstrates that a high-quality GaN thin films can be grown onto an FLG substrate by embedding a hybrid AlN BL.

Hierarchical Hybrid of Few-Layer Graphene upon Tungsten Monocarbide Nanowires: Controlled Synthesis and Electrocatalytic Performance for Methanol Oxidation

Hierarchical hybrid of few-layer graphene (FLG)/tungsten monocarbide nanowires has been achieved by carbonization of metal tungsten nanowires under atmospheric pressure chemical vapor deposition. Morphology, structure, composition, element distribution, and surface microstructure variations of the hybrid nanostructure were characterized by field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy, scanning transmission electron microscope–energy dispersive X-ray spectroscopy (STEM-EDX) mapping, and Raman spectroscopy.The results indicate that the graphene nanosheets are made of FLG layers with good crystallinity and the nanowires can be characterized as single-phase tungsten monocarbide. In situ growth of the graphene layers on the WC nanowires presents a hierarchical architecture that exhibits notable electrocatalysis with enhanced methanol oxidation current and higher ratio of the forward a...

Modifying the electrical properties of graphene by reversible point-ripple formation

Strain, ripples and wrinkles in graphene reduce the charge-carrier mobility and alter the electronic behaviour. In few-layer graphene the anisotropy between the in-plane and cross-plane resistivity is altered and a band gap can be opened up. Here we demonstrate a method to reversibly induce point ripples in electrically isolated few-layer graphene with the ability to select the number of layers used for transport measurement down to single layer. During ripple formation the in-plane and cross-plane sheet resistances increase by up to 78% and 699% respectively, confirming that microscopic corrugation changes can solely account for graphene's non-ideal charge-carrier mobility. The method can also count the number of layers in few-layer graphene and is complimentary to Raman spectroscopy and atomic force microscopy when n ≤ 4. Understanding these changes is crucial to realising practical oscillators, nano-electromechanical systems and flexible electronics with graphene.

Determining the number of layers in few‐layer graphene by combining Raman spectroscopy and optical contrast

Raman spectroscopy is commonly used to determine the number of layers of few‐layer graphene (FLG) samples. In this work, we focus on the criteria based on the G‐band integrated intensity and on the laser optical contrast. Limitations due to stacking order are discussed and lead to the conclusion that it is necessary to combine Raman and optical contrast to avoid misinterpretation. Both methods enable to distinguish unambiguously between single layer graphene and multilayer graphene. However, neither each method separately nor the combination of the two enable a determination of the number of layers for all possible stacking orientations. Importantly, because the two methods always significantly disagree when they fail, the comparison of the values deduced by each method allows to discriminate if the determined number of layers can be specified or not. Other important parameters (substrate, laser wavelength, objective numerical aperture) are discussed to define a reliable method to determine the number of graphene layers in FLG and its domain of validity. The proposed method that combines Raman and optical contrast measurements, carried out with a 532 nm laser and using a 100× objective with a numerical aperture of 0.9, allows the determination of the number of layers for (up to 5) FLG on the following substrates: (1) glass (soda lime glass or similar with refractive index between 1.50 and 1.55) and (2) oxidized silicon (SiO2 on silicon, with a SiO2 thickness of 90 ± 5 nm). The method is however limited to high quality graphene and FLG with small defect density and low residue. Copyright © 2017 John Wiley & Sons, Ltd.

Light-assisted recharging of graphene quantum dots in fluorographene matrix

In the present study, the charge transient spectroscopy was used to analyze the transient relaxation of charges in graphene and bilayer-graphene quantum dot (QD) systems formed by chemical functionalization of graphene and few-layer graphene layers. A set of activation energies (one to three different values) for the emission of charges from QDs sized 50 to 70 nm, most likely proceeding via the thermal activation of charge carriers from QD quantum confinement levels, were deduced from measurements performed in the dark. Daylight illumination of samples during measurements was found to result in a strong decrease of the activation energies and in an involvement of an athermal process in the charge relaxation phenomenon. The time of the light-assisted emission of charge carriers from QDs proved to be two to four orders of magnitude shorter than the time of their emission from QDs under no-illumination conditions.

Nanoscale Adhesive Properties of Graphene: The Effect of Sliding History

Single‐asperity adhesion between nanoscale silicon tips and few‐layer graphene (FLG) sheets, as well as graphite, was measured using atomic force microscopy (AFM). The adhesion mechanism was understood through experiments and finite element method (FEM) simulations by comparing conventional pull‐forces measurements (contact and separation, without sliding) to those obtained after the tip was slid along the surface before separation (“pre‐sliding”). Without pre‐sliding, no variation in the pull‐off force was measured between consecutive measurements, and there was no observable dependence of the mean pull‐off force value on the number of FLG layers. However, when the tip was pre‐slid over a local area, the first pull‐off force was enhanced by 12–17%; subsequent pull‐off forces then relaxed to a lower, constant value. This occurred regardless of the number of layers, and occurred for aged graphite samples as well. Our analysis indicates that this is due to sliding‐induced changes of graphene's interfacial geometry, whereby local delamination of the top graphene layer occurs, provided there is sufficient atmospheric exposure of the surface after cleaving. This effect provides another unique feature of the nanotribological behavior of atomically‐thin sheets and is consequential for designing graphene‐based devices and coatings where adhesive interactions are important.

Terahertz wave responses of dispersive few-layer graphene (FLG) and ferroelectric film composite

Terahertz wave responses of few-layer graphene (FLG) and ferroelectric film (LiNbO3) composite are studied using our developed algorithm based on the spectral-domain exponential matrix (SDEM) technique. The electrical conductivity of such FLG is calculated by a set of closed-form equations with coupling effect between the bottom graphene layer (BGL) and the substrate treated in an appropriate way, while the dispersive properties of the LiNbO3 film are described by the Lorentz model over an ultra-wide THz band. It is shown that both transmittance and reflectance of the composite can be adjusted effectively by changing the number of layers in FLG, the optical pumping intensity and the transverse optical-phonon frequency of LiNbO3. Moreover, producing such FLG and ferroelectric film composite is technically feasible, which can be exploited for further developing some novel planar THz structures.

Raman Spectroscopy as a Tool to Address Individual Graphene Layers in Few-Layer Graphene

We developed a new general approach to address individual graphene sheets in few-layer graphene by Raman spectroscopy. Our method is based on isotope labeling of individual layers during their synthesis and subsequent transfer to form multilayered graphene. The power of the procedure is demonstrated in the analysis of the interactions of individual layers with the substrate and with the environment. In addition, we measured Raman spectra of individual graphene layers in 3-LG during electrochemical doping. We show that they do not exhibit the same level of doping as one another and the doping level is dependent on layer position with respect to the substrate.

Few-layer graphene growth on 6H-SiC(0001) surface at low temperature via Ni-silicidation reactions

Few-layer graphene (FLG) has been prepared by thermal annealing of SiC crystal via the surface Ni-silicidation reactions. Results reveal that the temperature plays an important role for the final FLG quality and the optimized annealing temperature is about 800 °C. The investigation of surface morphology and microstructure for the FLG sample indicates that after the rapid cooling, the carbon atoms will segregate to form the FLG layer and the NiSix particles will congregate on the top surface. The mechanism of the FLG formation on SiC surface assisted by the Ni ultra-thin layer is briefly discussed based on the experimental results.

Effect of substrate temperature on few-layer graphene grown on Al2O3 (0 0 0 1) via direct carbon atoms deposition

Few-layer graphene (FLG) was grown on Al2O3 (0001) substrates at different temperatures via direct carbon atoms deposition by using solid source molecular beam epitaxy (SSMBE) method. The structural properties were characterized by reflection high energy electron diffraction (RHEED), Raman spectroscopy and near-edge X-ray absorption fine-structure (NEXAFS). The results showed that the FLG started to form at the substrate temperature of 700°C. When the substrate temperature increased to 1300°C, the quality of the FLG was the best and the layer number was estimated to be less than 5. At higher substrate temperature (1400°C or above), the crystalline quality of the FLG would be deteriorated. Our experiment results demonstrated that the substrate temperature played an important role on the FLG layer formation on Al2O3 (0001) substrates and the related growth mechanism was briefly discussed.

Chirality and thickness-dependent thermal conductivity of few-layer graphene: A molecular dynamics study

The thermal conductivity of graphene nanoribbons (layer from 1 to 8 atomic planes) is investigated by using the nonequilibrium molecular dynamics method. We present that the room-temperature thermal conductivity decays monotonically with the number of the layers in few-layer graphene. The superiority of zigzag graphene in thermal conductivity is only available in high temperature region and disappears in multilayer case. It is explained that the phonon spectral shrink in high frequency induces the change in thermal conductivity. It is also reported that single-layer graphene has better ballistic transport property than the multilayer graphene.

Transparent conductive graphene electrode in GaN-based ultra-violet light emitting diodes

We report a graphene-based transparent conductive electrode for use in ultraviolet (UV) GaN light emitting diodes (LEDs). A few-layer graphene (FLG) layer was mechanically deposited. UV light at a peak wavelength of 368 nm was successfully emitted by the FLG layer as transparent contact to p-GaN. The emission of UV light through the thin graphene layer was brighter than through the thick graphene layer. The thickness of the graphene layer was characterized by micro-Raman spectroscopy. Our results indicate that this novel graphene-based transparent conductive electrode holds great promise for use in UV optoelectronics for which conventional ITO is less transparent than graphene.

Initial stages of few-layer graphene growth by microwave plasma-enhanced chemical vapourdeposition

A promising method for the production of few-layer graphene (FLG) is microwaveplasma-enhanced chemical vapour deposition (MW PECVD). However, the growthmechanism of PECVD-synthesized FLG is not completely understood. The aim of thiswork was to investigate the initial stages of the growth process of FLG deposited by MWPECVD on several substrates (quartz, silicon, platinum). The deposited thin films werecharacterized by angle-resolved x-ray photoelectron spectroscopy (ARXPS), electronbackscattered diffraction (EBSD), scanning electron microscopy (SEM) and x-raydiffraction (XRD). It was found that the initial stages of the deposition were different forthe three chosen substrate materials. However, the fully grown FLG layers were similar forall substrates.