Chemical Vapor Deposited Monolayer Graphene Research Articles

(Digital Presentation) Deconstructing Proton Transport through Atomically Thin Monolayer CVD Graphene Membranes

Selective proton (H+) permeation through the atomically thin lattice of graphene and other 2D materials offers new opportunities for energy conversion/storage and novel separations. Practical applications necessitate scalable synthesis via approaches such as chemical vapor deposition (CVD) that inevitably introduce sub-nanometer defects, grain boundaries and wrinkles, and understanding their influence on H+ transport and selectivity for large-area membranes is imperative but remains elusive. Using electrically driven transport of H+ and potassium ions (K+) we probe the influence of intrinsic sub-nanometer defects in monolayer CVD graphene across length-scales for the first time. At the micron scale, the areal H+ conductance of CVD graphene (∼4.5–6 mS cm−2) is comparable to that of mechanically exfoliated graphene indicating similarly high crystalline quality within a domain, albeit with K+ transport (∼1.7 mS cm−2). However, centimeter-scale Nafion|graphene|Nafion devices with several graphene domains show areal H+ conductance of ∼339 mS cm−2 and K+ conductance of ∼23.8 mS cm−2 (graphene conductance for H+ is ∼1735 mS cm−2 and for K+ it is ∼47.6 mS cm−2). Using a mathematical-transport-model and Nafion filled polycarbonate track etched supports, we systematically deconstruct the observed orders of magnitude increase in H+ conductance for centimeter-scale CVD graphene. The mitigation of defects (>1.6 nm), wrinkles and tears via interfacial polymerization results in a conductance of ∼1848 mS cm−2 for H+ and ∼75.3 mS cm−2 for K+ (H+/K+ selectivity of ∼24.5) via intrinsic sub-nanometer proton selective defects in CVD graphene. We demonstrate atomically thin membranes with significantly higher ionic selectivity than state-of-the-art proton exchange membranes while maintaining comparable H+ conductance. Our work provides a new framework to assess H+ conductance and selectivity of large-area 2D membranes and highlights the role of intrinsic sub-nanometer proton selective defects for practical applications.

Graphene as effective anti-fading agent for the protection of artworks

Since all art materials in time are prone to degradation due to environmental conditions, a solution is needed to delay or even to prevent such an interference. Graphene, the pioneer of 2D materials, seems to pave the way to this end either as a protective layer on artworks or as an inclusion in paints, dyes, pigments acting as protective additive, respectively. Regarding the former, the proposed invention provides a method for applying a protective graphene coating onto a surface, such a two- or three-dimensional artwork or other coloured surface. The steps followed within this method are: Deposition of Chemical Vapor Deposition (CVD) produced graphene onto a supporting substrate in order to produce a graphene/substrate composite; Removal the graphene membrane from the supporting substrate; and Deposition the graphene membrane onto the surface of interest All the pre-mentioned steps are performed following the roll-to-roll method (Choi et al., 2015), with the assistance of a tailor-made laminator. However, the direct deposition of graphene onto artworks may not be straightforward due to the wet transfer and hot lamination conditions that can be catastrophic for fragile painted substrates. The novel strategy designed exploits the high-quality CVD monolayer graphene and uses a pressure-sensitive adhesive film (PSAF) in the transfer process that has been demonstrated to enable graphene wetting on the final target-substrate (Kim et al., 2015), thus maximizing the contact area and van der Waals interactions. The low work of adhesion between graphene and the PSAF, compared to the corresponding between graphene and the destination surface, promotes the transfer that can be performed at mild pressure and temperature conditions, without using a surface modifier or adhesives (Martins et al. 2013). Additionally, the proposed roll-to-roll method does not affect the mechanical integrity of the typical paper substrates used in graphic arts. As for the protective additive case, graphene is employed in the form of an aqueous dispersion in media such as paints, dyes, inks, pigments and/or varnishes used for artistic purposes in order to create indelible colours to capable to prevent degradation, attributed to UV radiation, moisture and oxidizing agents. The method presents a new approach for obtaining aqueous dispersions for the hydrophobic graphene and thus, protecting artworks and painted indoor/outdoor wall surfaces taking advantage of the optical and barrier properties of graphene.

Quantum Hall Effect across Graphene Grain Boundary.

Charge carrier scattering at grain boundaries (GBs) in a chemical vapor deposition (CVD) graphene reduces the carrier mobility and degrades the performance of the graphene device, which is expected to affect the quantum Hall effect (QHE). This study investigated the influence of individual GBs on the QH state at different stitching angles of the GB in a monolayer CVD graphene. The measured voltage probes of the equipotential line in the QH state showed that the longitudinal resistance (Rxx) was affected by the scattering of the GB only in the low carrier concentration region, and the standard QHE of a monolayer graphene was observed regardless of the stitching angle of the GB. In addition, a controlled device with an added metal bar placed in the middle of the Hall bar configuration was introduced. Despite the fact that the equipotential lines in the controlled device were broken by the additional metal bar, only the Rxx was affected by nonzero resistance, whereas the Hall resistance (Rxy) revealed the well-quantized plateaus in the QH state. Thus, our study clarifies the effect of individual GBs on the QH states of graphenes.

Nanopatterning of monolayer graphene by quantum optical lithography.

Quantum optical lithography, a diffraction-unlimited method, was applied to pattern monolayer graphene at 10 nm resolution. In our tests with chemical vapor deposition monolayer graphene samples, we have succeeded in producing flat surfaces of a sandwich of monolayer graphene-resist on Si, Si3N4, or glass substrates. Complex patterns have been written on monolayer graphene samples by a nanoablation process. The method could be used to realize monolayer graphene nanodevices.

Gas Barrier Properties of Chemical Vapor-Deposited Graphene to Oxygen Imparted with Sub-electronvolt Kinetic Energy.

Graphene gas-barrier performance holds great interest from both scientific and technological perspectives. Using in situ synchrotron X-ray photoelectron spectroscopy, we demonstrate that chemical vapor-deposited monolayer graphene loses its gas-barrier performance almost completely when oxygen molecules are imparted with sub-electronvolt kinetic energy but retains its gas-barrier performance when the molecules are not energized. The permeation process is nondestructive. Molecular dynamics-based simulation suggests kinetic energy-mediated chemical reactions catalyzed by common graphene defects as a responsible mechanism.

Stretchability of PMMA-supported CVD graphene and of its electrical contacts

The remarkable mechanical robustness and excellent electrical/thermal properties make graphene a promising candidate for future flexible, stretchable and bio-integrated electronics. In practice, many soft electronics such as the graphene electronic tattoos (GETs) demand the chemical vapor deposited (CVD) graphene to be supported by a deformable substrate. Moreover, various conductive overlayers need to directly laminate on graphene to make electrical contacts. To investigate the mechanical reliability of CVD graphene in these situations, we fabricated CVD monolayer graphene supported by ultrathin poly(methyl methacrylate) (PMMA) substrate and also placed gold/polyethylene terephthalate (Au/PET) and graphene/PMMA (Gr/PMMA) overlayers on graphene. The stretchability of the Gr/PMMA and the overlayer-Gr/PMMA interface was characterized by electrical resistance change during uniaxial tensile tests. Combined with in situ microstructure and Raman investigation, we identified four deformation/fracture stages of Gr/PMMA—pre-cracking elastic deformation, limited micro-cracking in graphene, extensive cracking in graphene, and macro-cracking in PMMA. While micro-cracks emerged in graphene at very small strain (~0.9%), the electrical conductivity of the Gr/PMMA specimen remained up to tensile strains of ~14.5%. In contrast, 100 nm-thick Au film supported by the same PMMA substrate fully ruptured after tensile strains of ~1%. When laminating Au/PET and Gr/PMMA over Gr/PMMA, we found that the Au/PET- Gr/PMMA interface is very vulnerable but the Gr/PMMA- Gr/PMMA interface behaves very similar to intact Gr/PMMA electromechanically. The cyclic behaviour of Gr/PMMA, the effects of PMMA thickness and adhesion are also briefly discussed. The present experimental study provides fundamental insight into the failure of ultrathin polymer-supported graphene and its electrical contacts, which is critical for designing future graphene-based soft electronics.

Graphene Sensor Based on Surface Plasmon Resonance for Optical Scanning

The experimental and theoretical evaluation of a simple, low cost, and compact footprint, as well as a high resolution graphene plasmonics sensor are presented here for optical scanning. The strong coupling condition between incident light and excited surface plasmon resonance (SPR) of the graphene-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si structure is thoroughly utilized to assay the proposed model. The substrate thickness, incident angle, and coupling conditions are considered to obtain optimal results including the highest resolution, and physical and optical properties including graphene thickness. The results show that high-resolution condition can be provided when the chemical vapor deposition monolayer graphene thickness is 0.335 nm, SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si thickness is 300 nm, and incident angle is θ = 62.5 °. Moreover, atomic force microscopy (AFM) and scanning electron microscopy (SEM) are employed to obtain and investigate the physical structures. Finally, it is shown that the highest sensitivity can be achieved by a strong coupling condition.

Electro‐oxidized Monolayer CVD Graphene Film Transducer for Ultrasensitive Impedimetric DNA Biosensor

AbstractWe report the effect of electrochemical anodization on the properties of monolayer graphene as the main aim of this research and consequently using the resulting label‐free impedimetric biosensor for DNA sequences detection. Monolayer graphene was grown by chemical vapor deposition (CVD) with methane as precursor on copper foil, transferred onto a glassy carbon electrode and electrochemically anodized. Raman spectroscopy and X‐Ray photo electron spectroscopy revealed enhancement of defect density, roughness and formation of C−O−C, C−O−H and C=O functional groups after anodization. Amine‐terminated poly T probe was linked covalently to the carboxylic groups of anodized graphene by the zero‐length linker to fabricate the impedance‐based DNA biosensor. The anodized graphene electrode demonstrated a superior performance for electrochemical impedance detection of DNA. The DNA biosensor showed a large linear dynamic range from 2.0×10−18 to 1.0×10−12 M with a limit of detection of 1.0×10−18 M using electrochemical impedance spectroscopy (EIS) method. Equivalent circuit modeling shows that DNA hybridization is detected through a change in charge transfer resistance.

Continuous-wave and chemical vapor deposition graphene-based passively Q-switched Er:Y2O3 ceramic lasers at 2.7 μm.

We report on 976-nm diode-pumped Er:Y2O3 ceramic lasers in continuous-wave and passively Q-switched regimes. The maximum output power of continuous-wave laser operation is about 0.78W with slope efficiency of about 11.8% at 2.7μm. Passively Q-switched Er:Y2O3 ceramic laser operation with chemical vapor deposition (CVD) graphene as the saturable absorber is also demonstrated for the first time, to our knowledge. Using a monolayer CVD graphene, the achieved shortest pulse width is about 408ns, while the shortest pulse width reduces to about 296ns with pulse energy of 2.59μJ and peak power of 8.77W by using a three-layer CVD graphene. The results reveal that graphene is a very promising saturable absorber operating in the middle infrared spectral region.

Fabrication of highly conductive graphene/ITO transparent bi-film through CVD and organic additives-free sol-gel techniques

Indium tin oxide (ITO) still remains as the main candidate for high-performance optoelectronic devices, but there is a vital requirement in the development of sol-gel based synthesizing techniques with regards to green environment and higher conductivity. Graphene/ITO transparent bi-film was synthesized by a two-step process: 10 wt. % tin-doped ITO thin films were produced by an environmentally friendly aqueous sol-gel spin coating technique with economical salts of In(NO3)3.H2O and SnCl4, without using organic additives, on surface free energy enhanced (from 53.826 to 97.698 mJm−2) glass substrate by oxygen plasma treatment, which facilitated void-free continuous ITO film due to high surface wetting. The chemical vapor deposited monolayer graphene was transferred onto the synthesized ITO to enhance its electrical properties and it was capable of reducing sheet resistance over 12% while preserving the bi-film surface smoother. The ITO films contain the In2O3 phase only and exhibit the polycrystalline nature of cubic structure with 14.35 ± 0.5 nm crystallite size. The graphene/ITO bi-film exhibits reproducible optical transparency with 88.66% transmittance at 550 nm wavelength, and electrical conductivity with sheet resistance of 117 Ω/sq which is much lower than that of individual sol-gel derived ITO film.

Versatile Water-Based Transfer of Large-Area Graphene Films onto Flexible Substrates

ABSTRACTNext-generation electronic devices are expected to demonstrate greater utility, efficiency and durability. Meanwhile, plastics such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN) and variety of poly(para-xylylene) polymers enable transformational advantages to device shape, flexibility, weight, transparency and recyclability. Exhibiting a combination of outstanding mechanical, electrical, optical, and chemical properties of graphene with the plastic substrates could propose ideal material for the future flexible electronics. Chemical vapor deposition (CVD) allows cost-effective fabrication of a high-quality large-area graphene films, however, the critical issue is clean and noninvasive transfer of the films onto a desired substrate. The water-based delamination of CVD grown graphene on Cu can be considered as a “green” transfer process utilizing only hot deionized water. We investigated a method requiring only two essential steps: coating of 6-inch monolayer CVD graphene with transparent and flexible polymer, and Cu delamination in hot water. Proposed method is inexpensive, reproducible, environmentally friendly, waste-free and suitable for large-scale, high quality graphene. The transfer process demonstrated films with enhanced charge carrier mobility, high uniformity, free of mechanical defects, and sheet resistance as low as ∼50 Ω/sq with 96.5 % transparency at 550 nm wavelength.

Q-factors of CVD monolayer graphene and graphite inductors

A carbon-based inductor may serve as an important passive component in a carbon-based radio-frequency (RF) integrated circuit (IC). In this work, chemical vapor deposition (CVD) synthesized monolayer graphene and graphite inductors are fabricated and their Q-factors are investigated. We find that the large series resistance of signal path (including coil resistance and contact resistance) in monolayer graphene inductors causes negative Q-factors at the whole frequency range in measurement. Comparatively, some of the graphite inductors have all of their Q-factors above zero, due to their small signal path resistance. We also note that some other graphite inductors have negative Q-factor values at low frequency regions, but positive Q-factor values at high frequency regions. With an equivalent circuit model, we confirm that the negative Q-factors of some graphite inductors at low frequency regions are related to their relatively large contact resistances, and we are able to eliminate these negative Q-factors by improving the graphite-metal contact. Furthermore, the peak Q-factor (Qp) can be enhanced by lowering down the resistance of graphite coil. For an optimized 3/4-turn graphite inductor, the measured maximum Q-factor (Qm) can reach 2.36 and the peak Q-factor is theoretically predicted by the equivalent circuit to be as high as 6.46 at a high resonant frequency, which is beyond the testing frequency range. This research indicates that CVD synthesized graphite thin film is more suitable than graphene for fabricating inductors in carbon-based RF IC in the future.

Graphene-based optically transparent dipole antenna

We fabricated an optically transparent dipole antenna based on chemical vapor deposition (CVD)-grown monolayer graphene on an optically transparent quartz substrate and characterized its properties in microwave bands. The measurements of the reflection coefficients for the dipole antenna revealed that ∼90% of the microwave power transmitted to the CVD monolayer graphene of the antenna element. By measuring transmission coefficients, we demonstrated that the graphene dipole antenna radiated microwave power around the operational frequency (∼20.7 GHz). The operational frequency of the graphene dipole antenna (∼20.7 GHz) shifted to a higher frequency than that of the Au dipole antenna with the same structure (∼9.2 GHz), which suggests that monolayer graphene behaves not as a metal but as a dielectric material.

Characterization of CVD graphene permittivity and conductivity in micro-/millimeter wave frequency range

The permittivity and conductivity of chemical vapor deposited monolayer graphene are investigated up to 40 GHz. The characterization method is based on a coplanar waveguide transmission line structure that is fabricated on a multilayer substrate of Si/SiO2/graphene/Al2O3 from the bottom up. The effective relative permittivity of the coplanar waveguide transmission line is extracted using Thru-Reflect-Line calibration and scattering parameter measurements, and then the relative permittivity and corresponding conductivity of graphene are characterized using partial capacitance techniques. The results demonstrate that the conductivity and sheet resistance are remarkably frequency-dependent and that the complex relative permittivity is consistent with the Drude model.

Graphene Conductivity Control By Friction Induced Charge Trapping

Generally, t he resistivity of chemical vapor deposition (CVD) based large-area graphene films is much high er than that of single crystalline mechanically exfoliated graphene . Therefore, the performance of CVD graphene based electronic devices has various limitations. Here we introduce a novel method for reducing the electrical resistivity of CVD graphene through triboelectrification. T riboelectri c charges, generated by the friction between a Pt tip and graphene , reduced the resistance of CVD graphene film/SiO2 system dramatically . The charges were localized only in the friction area and survived more than 12 hours. The graphene film on the localized charges has an ~ 80% reduced resistance without damage and without resort to any chemical surface treatments. T he localization of triboelectric charges was successfully achieved in a monolayer CVD graphene film/SiO2 system, but the charges were not localized when we used mechanically exfoliated graphene flake or highly ordered pyrolytic graphite (HOPG) film. Moreover, the resistance of the graphene film on the localized charges can be modified by controlling the localized charge density and polarity .

Chemical vapor deposition monolayer graphene functionalization by the Bingel reaction

ABSTRACTGraphene and its derivatives are being studied in nearly all fields of science and engineering. Much effort has been devoted to exploring the fundamental physical and chemical properties of graphene. This paper focuses on the chemical vapor deposition (CVD) monolayer graphene functionalized by the Bingel reaction (Cyclopropylation). From AFM test, the thickness of the sample increased from 0.48 nm to 1.19 nm after reaction for 24 h. According to Raman test, the intensity ratio between D and G peaks (ID/IG) increased from 0.01 to nearly 0.1 after reaction for 24. The peak area ratio between C‒O and C=C increased from 0.39 to 1.02, the peak area ratio between O‒C=O and C=C increased from 0.15 to 0.29 based on XPS results after reaction for 24 h. Therefore, this functionalization reaction for CVD monolayer graphene is successful and can be used in further research for graphene and its derivatives.

Generation of 30 fs pulses from a diode-pumped graphene mode-locked Yb:CaYAlO4 laser.

Stable 30 fs pulses centered at 1068 nm (less than 10 optical cycles) are demonstrated in a diode pumped Yb:CaYAlO4 laser by using high-quality chemical vapor deposited monolayer graphene as the saturable absorber. The mode-locked 8.43 optical-cycle pulses have a spectral bandwidth of ∼50 nm and a pulse repetition frequency of ∼113.5 MHz. To the best of our knowledge, this is the shortest pulse ever reported for graphene mode-locked lasers and mode-locked Yb-doped bulk lasers. Our experimental results demonstrate that graphene mode locking is a very promising practical technique for directly generating few-cycle optical pulses from a laser oscillator.

Scanning microwave microscope imaging of micro-patterned monolayer graphene grown by chemical vapor deposition

Characterization of micro-patterned chemical vapor deposited monolayer graphene using a scanning microwave microscope has been presented. Monolayer graphene sheets deposited on a copper substrate were transferred to a variety of substrates and micro-patterned into a periodic array of parallel lines. The measured complex reflection coefficients exhibit a strong dependency on the operating frequency and on the samples' electrical conductivity and permittivity. The experiments show an extremely high sensitivity by detecting image contrast between single and double layer graphene sheets. Correlating the images recorded at the half- and quarter-wavelength resonant frequencies shows that the relative permittivity of the single layer graphene sheet is above 105. The results are in good agreement with the three dimensional numerical electromagnetic simulations. This method may be instrumental for a comprehensive understanding of the scanning microwave microscope image contrast and provide a unique technique to estimate the local electrical properties with nano-meter scale spatial resolution of two dimensional materials at radio frequency.

Charge carrier concentration and offset voltage in quasi-free-standing monolayer chemical vapor deposition graphene on SiC

Epitaxial Chemical Vapor Deposition growth of graphene on silicon carbide offers the maturity and reliability expected for large-scale fabrication of graphene-based devices. In particular, the ultimate challenge of graphene synthesis on SiC, i.e. quasi-free-standing monolayer graphene which comes through hydrogen atom intercalation of the sole buffer layer grown on the Si-face of SiC, offers high carrier mobility (as high as 6600 [cm2/Vs]) and electrical stability throughout the device processing cycle. In this report, we present extensive statistics of the electrical properties of QFS-monolayer graphene grown on 4H(0001) and 6H(0001) semi-insulating 10 mm × 10 mm substrates, being a result of 110 individual processes. The adopted explanation for the origin of the as-grown doping level in epitaxial graphene based on the spontaneous polarization of hexagonal SiC and its most up-to-date values is reaffirmed. We introduce the issue of the step-edge-induced offset voltage radial dependence and confront it with the morphological analysis of the average step edge height and terrace width, all related to the place of origin of a specific sample within a 4-inch SiC wafer. Finally, we conclude that within the step edge area QFS-monolayer graphene is statistically nearly half as resistive as the previously reported QFS-bilayer graphene.

Raman Mapping Characterization of All-Fiber CVD Monolayer Graphene Saturable Absorbers for Erbium-Doped Fiber Laser Mode Locking

We present a study on the characterization and quality evaluation of all-fiber chemical vapor deposition (CVD) graphene saturable absorber samples for Erbium-doped fiber laser (EDFL) mode locking. Optical fiber end faces covered by CVD graphene were analyzed by Raman spectroscopy, and classification criteria related to graphene Raman bands properties were adopted in order to evaluate samples quality. The analysis revealed that for all samples, the fiber core was 100% covered by graphene, with 88% average monolayer graphene coverage, and 82% low defect graphene coverage. Graphene samples were used as saturable absorbers in EDFL, and soliton-like pulses as short as 599 fs were generated, with average pulse duration of 620 ± 10 fs among all samples.