Single-layer CVD Graphene Research Articles

Layered Heterostructure of Graphene and TiO2 as a Highly Sensitive and Stable Photoassisted NO2 Sensor.

As an atomically thin electric conductor with a low density of highly mobile charge carriers, graphene is a suitable transducer for molecular adsorption. In this study, we demonstrate that the adsorption properties can be significantly enhanced with a laser-deposited TiO2 nanolayer on top of single-layer CVD graphene, whereas the effective charge transfer between the TiO2-adsorbed gas molecules and graphene is retained through the interface. The formation of such a heterostructure with optimally a monolayer thick oxide combined with ultraviolet irradiation (wavelength 365 nm, intensity <1 mW/mm2) dramatically enhances the gas-sensing properties. It provides an outstanding sensitivity for detecting NO2 in the range of a few ppb to a few hundred ppb-s in air, with response times below 30 s at room temperature. The effect of visible light (436 and 546 nm) was much weaker, indicating that the excitations due to light absorption in TiO2 play an essential role, while the characteristics of gas responses imply the involvement of both photoinduced adsorption and desorption. The sensing mechanism was confirmed by theoretical simulations on a NO2@Ti8O16C50 complex under periodic boundary conditions. The proposed sensor structure has significant additional merits, such as relative insensitivity to other polluting gases (CO, SO2, NH3) and air humidity, as well as long-term stability (>2 years) in ambient air. The results pave the way for an emerging class of gas sensor structures based on stacked 2D materials incorporating highly charge-sensitive transducer and selective receptor layers.

ISCOPEM2D V1.0: An In Situ Method to Characterize and Compare Chemical Vapor Deposition Graphene Films Using Quality Matrix Approaches

Single‐layer chemical vapor deposition (CVD) graphene films are commonly transferred from their native growth substrates during characterization. However, the transfer process influences their intrinsic material properties and growth substrate‐related material information is lost. Herein, an in situ method is presented to suspend CVD graphene films directly on their growth substrate. The approach makes the sample compatible for characterization across the widely used optical and electron microscopy techniques such as Raman spectroscopy and transmission electron microscopy. In addition, a quality matrix framework “ISCOPEM2D V1.0” is provided to support the quantification, comparison, and validation of various material properties of single‐layer graphene. The approach provides a holistic overview of various properties and facilitates quality control and reproducibility of single‐layer graphene‐based application development. Similar approaches can be useful to develop quality matrix‐based for other 2D materials.

Effect of the electrolyte composition on the corrosion resistance of single-layer CVD-graphene

The corrosion resistance of a single-layer graphene film synthetized via chemical vapor deposition (CVD) on a polycrystalline copper substrate is studied. The anticorrosive properties were identified by means of electrochemical tests of potentiodynamic polarization and electrochemical impedance spectroscopy in the following environments: (a) 0.5 M H2SO4, (b) 0.5 M HCl, (c) 3.5wt% NaCl + 0.5 M H2SO4, and (d) Na2S2O3 (10−4 mol / l) + 5% NaCl + (CH3COOH) 0.50%). For the characterization of the graphene and corrosion products, the following techniques were used: Raman spectroscopy, AUGER electron spectroscopy, X-ray energy dispersion probe (EDS), scanning electron microscopy (SEM), and confocal laser scanning microscopy (CLSM). The corrosion mechanism and the degree of degradation of the graphene are discussed as a function of the corrosive electrolyte.

Towards Perfect Absorption of Single Layer CVD Graphene in an Optical Resonant Cavity: Challenges and Experimental Achievements.

Graphene is emerging as a promising material for the integration in the most common Si platform, capable to convey some of its unique properties to fabricate novel photonic and optoelectronic devices. For many real functions and devices however, graphene absorption is too low and must be enhanced. Among strategies, the use of an optical resonant cavity was recently proposed, and graphene absorption enhancement was demonstrated, both, by theoretical and experimental studies. This paper summarizes our recent progress in graphene absorption enhancement by means of Si/SiO2-based Fabry–Perot filters fabricated by radiofrequency sputtering. Simulations and experimental achievements carried out during more than two years of investigations are reported here, detailing the technical expedients that were necessary to increase the single layer CVD graphene absorption first to 39% and then up to 84%. Graphene absorption increased when an asymmetric Fabry–Perot filter was applied rather than a symmetric one, and a further absorption increase was obtained when graphene was embedded in a reflective rather than a transmissive Fabry–Perot filter. Moreover, the effect of the incident angle of the electromagnetic radiation and of the polarization of the light was investigated in the case of the optimized reflective Fabry–Perot filter. Experimental challenges and precautions to avoid evaporation or sputtering induced damage on the graphene layers are described as well, disclosing some experimental procedures that may help other researchers to embed graphene inside PVD grown materials with minimal alterations.

The effect of quasi-free graphene layer on the electrical transport properties of sandwich-like graphene / Co nanoparticles / graphene structure

The paper reports on the charge transport in the sandwich-like graphene / Co nanoparticles / graphene structure (G/CoNPs/G). It is fabricated by combining the electrochemical deposition of Co nanoparticles onto a single layer CVD graphene and their capping with another graphene layer. The isolated semi-spherical, surface oxidized Co NPs of 250 nm in diameter serve as a support for the top graphene layer and provide its suspended state confirmed by atomic force microscopy revealing periodic ripples on its surface. A high optical transparency of 92.4% is found for the G/CoNPs/G sandwich. The observed twofold reduction in the sheet resistance with a ∼ 1.5 times increase in the positive magnetoresistive effect for G/CoNPs/G compared to the pristine graphene is attributed to the suppression of carrier scattering due to the substrate. From the Hall effect measurements, a predominant role of the top graphene layer in the charge transport in G/CoNPs/G is established, with a significantly increased carrier mobility µ∼1800 cm2/(V·s). An elastic intervalley scattering length estimated from the magnetoresistance results is found to be three times increased in G/CoNPs/G, indicating that the top graphene layer is almost unaffected by the substrate. The proposed technology looks promising for possible optoelectronic and magneto-optical applications.

Electrical magnetotransport properties of chlorinated CVD graphene

We report on electrical resistivity, magnetoresistance, and Hall-effect experiments in single-layer CVD graphene submitted to photochlorination. The resistivity and the ordinary Hall effect reveal a strong hole doping due to Cl adsorption. Intrinsic disorder of the as-grown system as well as that produced in the chlorination process lead to weak-localization effects, observed in the low-temperature magnetoresistance. The theoretically predicted regime where intervalley scattering fully restores the usual negative magnetoresistance is observed in a freshly chlorinated sample. Also observed are time-dependent effects in the electrical transport properties of the functionalized CVD-graphene samples due to a progressive loss of adsorbed Cl.

Rapid Proton Transmission through Nafion | Graphene | ALD Alumina | Nafion Membranes

Recent reports from several laboratories indicate that proton transmission through graphene and other 2D materials in contact with proton-conducting electrolytes can occur rapidly with a much lower activation energy than expected from a wide range of computational studies on the proton / graphene interaction energy, and a much higher selectivity relative to transmission of other ions than would be expected based only on ion size. It would be desirable to be able to control transport through the graphene, and many approaches have been tried to rationally control transport rates through graphene via chemical treatments applied to the graphene. One such approach involves use of atomic layer deposition (ALD), in which single layers of inorganic materials, often oxides, are coated in a self-limiting way onto a surface by a series of pulsed reaction steps delivered in series. A common ALD implementation involves coatings of aluminum oxide, or alumina, created by sequential dosing of a surface with trimethyl aluminum and water vapor. We report here on use of the ALD method to coat single-layer CVD graphene supported on copper foil or on Nafion membranes with layers of alumina. Coatings resulting from 50 ALD cycles are found to be thick enough and homogeneous enough to block photoelectron emission from the underlying substrates (copper or Nafion) when the coatings are analyzed by X-ray photoelectron spectroscopy. Electrochemical hydrogen pump experiments were then performed to assess the degree to which ALD treatment of the graphene on Nafion, delivered prior to preparing Nafion | graphene | Nafion sandwich membranes, could change the rate of proton transmission through the graphene that is embedded within the Nafion. As little as two ALD treatment cycles are sufficient to reduce proton transmission currents through graphene by approximately half relative to graphene that is not treated by ALD. Surprisingly, fifty ALD cycles has almost the same effect as two cycles. This finding indicates that that ALD alumina coating is a relatively good proton conductor which is thought to reflect the action of transport pathways on the hydrated surface of the porous ALD alumina layer. The insensitivity of proton transmission to the number of ALD cycles applied suggests that the function of the ALD is to change the transport rates at interfaces, perhaps by creating a new ALD alumina / Nafion interface that protons must traverse in order to pass through the membrane.

Влияние магнитных частиц Co-CoO на свойства электропереноса в однослойном графене

Electrochemical deposition of cobalt onto the single-layer CVD graphene draw to the formation of Co-CoO/graphene composites with increased electrical resistance and magnetoresistance. It is shown that magnetoresistance is governed with two competing mechanisms – negative (NMR) and positive (PMR). NMR at low magnetic fields could be well described with localized quantum corrections to the Drude conductivity in graphene. The enhancement of PMR at high magnetic fields could be associated with the influence of Lorentz mechanism in Co-CoO particles.

Graphene-Based Proton Transmission and Hydrogen Crossover Mitigation in Electrochemical Hydrogen Pump Cells

A monolayer of single-layer CVD graphene embedded between two Nafion® polyelectrolyte membranes (PEMs) allows for high proton transmission rates through the membrane while also restricting the rate of hydrogen gas crossover through the membrane. Proton transmission occurs through single-layer graphene embedded in Nafion® | graphene | Nafion® PEM sandwich structures with an area-specific resistance (ASR) less than 40 mΩ cm2 for the graphene layer alone at ambient temperature (near 20 C). This ASR value for graphene is less than the ASR of most PEM membranes alone. Hydrogen crossover rates in Nafion® | graphene | Nafion® PEM sandwich structures were measured by a limiting current method and found to be attenuated by more than eight times relative to values in similar membranes without graphene.

Studies on Proton Transmission across Graphene in Proton-Exchange Membrane Structures

This talk will cover a wide range of recent studies involving proton transmission across CVD graphene layers incorporated into proton-exchange-membrane (PEM) sandwich structures. Many low-temperature electrochemical systems including fuel cells and water electrolysis cells involve proton transmission across proton-exchange membranes integrated with electrodes. A key to optimizing such systems is the accomplishment of selective proton transmission at high rates, while simultaneously inhibiting crossover transmission of other molecular species such as water, hydrogen / oxygen, methanol, and other species. Single-layer CVD graphene has been shown to accomplish this. Proton transmission across graphene in Nafion | graphene | Nafion sandwich structures has been shown to occur with an area-normalized resistance of less than 0.05 ohm cm2 at room temperature, with high selectivity relative to other ions. The graphene layer is nearly defect free and should be an excellent barrier to other species besides protons. The talk will include studies of proton transmission rates using hydrogen pump cells, water electrolysis using PEM cells, and related work. Graphene characterization studies using confocal Raman spectroscopy and X-ray photoelectron spectroscopy, and graphene defect characterization using chemical etching of graphene on copper, will also be covered. Effects of the graphene structure (single vs double layer) and the nature of the 2D material (graphene vs. hexagonal boron nitride) on proton transmission rates, and the rates for other ions besides proton, will also be discussed.

Protons Are Transmitted across Single-Layer Graphene in Proton-Exchange-Membrane (PEM) Sandwich Structures More Than 100 Times Faster Than Other Cations

Proton transmission rates across single-layer CVD graphene embedded in sandwich structures with proton-exchange membranes made from the perfluorosulfonic acid (PFSA) ionomer Nafion were measured and found to occur more than 100 times larger than for any other cation. Measurements were made for protons and a series of other cations including Li+, Na+, K+, Rb+, Cs+ and NH4 + using a four-electrode method in which two platinum electrodes drive ionic current through the membrane and two reference electrodes installed in Luggin capillaries sense the transmembrane potential difference induced by the forced ion flow. Proton transmission rates across graphene were between 150 and 350 times larger than for any of the other cations studied. Characterization studies of graphene on Nafion using confocal Raman microscopy and X-ray photoelectron spectroscopy, and defect counting for graphene on copper by chemical etching through graphene on copper will also be reported. Findings are consistent with transport through physical defect structures for all cations except protons. Proton transmission occurs at very high rates and may involve sites that are more widely distributed and that have much higher proton selectivity than the sites responsible for transmission of other ions. Proton transmission rates across graphene were also measured using electrochemical hydrogen pump cells at variable temperature, which provides an activation energy for proton transmission across graphene. Rate constants for proton transmission are interpreted using a charge-transfer resistance model that allows for interpretation of ionic currents in terms of a rate constant for interfacial proton-transfer. Anrhenius analysis of the rate constants gives values for the activation energy and frequency factor for the interfacial proton-transfer reaction.

Single-Layer Graphene Sandwiched between Proton-Exchange Membranes for Selective Proton Transmission

Proton transmission through single-layer CVD graphene in graphene/proton-exchange-membrane (PEM) sandwich structures is found to be more than 100 times faster than for any other cation. Ion transmi...

Mobility enhancement of CVD graphene by spatially correlated charges

We present a strategy for enhancing the carrier mobility of single layer CVD graphene (CVD SLG) based on spatially correlated charges. Our Monte Carlo simulations, numerical modeling and the experimental results confirm that spatial correlation between defects with opposite charges can provide a means to control independently the carrier concentration and mobility of planar field effect transistors in which graphene is decorated with a layer of colloidal quantum dots (QDs). We show that the spatial correlation between electrically charged scattering centres close to the graphene/SiO2 interface and the localised charges in a QD layer can smooth out the electrostatic potential landscape, thus reducing scattering and enhancing the carrier mobility. The QD capping molecules influence the distribution and correlation of electrical charges in the vicinity of SLG and provide a means of tuning the carrier concentration and increasing the carrier mobility in graphene. These results represent a significant conceptual advance and provide a novel strategy for control of the electronic properties of 2D materials that could accelerate their utilization in optoelectronic devices.

Tunable Schottky barrier and high responsivity in graphene/Si-nanotip optoelectronic device

We demonstrate tunable Schottky barrier height and record photo-responsivity in a new-concept device made of a single-layer CVD graphene transferred onto a matrix of nanotips patterned on n-type Si wafer. The original layout, where nano-sized graphene/Si heterojunctions alternate to graphene areas exposed to the electric field of the Si substrate, which acts both as diode cathode and transistor gate, results in a two-terminal barristor with single-bias control of the Schottky barrier. The nanotip patterning favors light absorption, and the enhancement of the electric field at the tip apex improves photo-charge separation and enables internal gain by impact ionization. These features render the device a photodetector with responsivity ( for white LED light at intensity) almost an order of magnitude higher than commercial photodiodes. We extensively characterize the voltage and the temperature dependence of the device parameters, and prove that the multi-junction approach does not add extra-inhomogeneity to the Schottky barrier height distribution. We also introduce a new phenomenological graphene/semiconductor diode equation, which well describes the experimental I–V characteristics both in forward and reverse bias.

Contactless graphene conductance measurements: the effect of device fabrication on terahertz time-domain spectroscopy

We perform contactless full-wafer maps of the electrical conductance of a 4-inch wafer of single-layer CVD graphene using terahertz time-domain spectroscopy both before and after deposition of metal contacts and fabrication of devices via laser ablation. We find that there is no significant change in the measured conductance of graphene before and after device fabrication. We also show that precise terahertz time-domain spectroscopy can be performed when the beam spot is at sufficient distance (>1.2 mm) from metal contacts.

Electrical transport in graphene with different interface conditions

The effect of the interface on the electrical resistance of graphene chemically deposited from the gas phase (CVD graphene) has been studied in the range from room temperature to the nitrogen boiling temperature. The resistance of a single-layer CVD graphene in the cases of its contacting with Si/SiO2 and GaAs substrates demonstrates a nearly linear metallic temperature dependence with almost the same slope of the normalized curves. This slope corresponds to an increase in the graphene resistance by ~8% when graphene is heated from the boiling nitrogen temperature to room temperature. The four-layer graphene demonstrates a semiconducting temperature dependence in the same temperature range. It has been found that the sputtering of an organic insulator (parylene) on the four-layer graphene increases the slope of this dependence by ~5% and, at room temperature, increases the graphene resistance by ~20%.

Chemical hydrogenation of single-layer graphene enables completely reversible removal of electrical conductivity

The chemical modification of graphene greatly expands its potential applications in electronics, chemistry, and biology. Here, we report the rapid and extensive hydrogenation of single layer CVD graphene using the Birch reduction method. This method hydrogenates much more extensively than cold plasmas and electrochemical methods. Moreover, use of single layer graphene enables greater control of electronic conductivity than previously achieved with the Birch method using multilayer graphene or graphene oxide. Indeed, this method enables both the elimination of electronic conductivity through hydrogenation and the subsequent recovery of essentially pristine graphene with thermal annealing—a reversible >107 fold change in resistance. Raman and photoelectron spectroscopies show that the reaction is complete within 90s of immersion. Finally, we show that we can use the Birch reduction to functionalize graphene with tributyltin moieties.

High-yield fabrication of nm-size gaps in monolayer CVD graphene

Herein we demonstrate the controlled and reproducible fabrication of sub-5 nm wide gaps in single-layer graphene electrodes. The process is implemented for graphene grown via chemical vapor deposition using an electroburning process at room temperature and in vacuum. A yield of over 95% for the gap formation is obtained. This approach allows producing single-layer graphene electrodes for molecular electronics at a large scale. Additionally, from Raman spectroscopy and electroburning carried out simultaneously, we can follow the heating process and infer the temperature at which the gap formation happens.

Highly tunable local gate controlled complementary graphene device performing as inverter and voltage controlled resistor

Using single-layer CVD graphene, a complementary field effect transistor (FET) device is fabricated on the top of separated back-gates. The local back-gate control of the transistors, which operate with low bias at room temperature, enables highly tunable device characteristics due to separate control over electrostatic doping of the channels. Local back-gating allows control of the doping level independently of the supply voltage, which enables device operation with very low VDD. Controllable characteristics also allow the compensation of variation in the unintentional doping typically observed in CVD graphene. Moreover, both p–n and n–p configurations of FETs can be achieved by electrostatic doping using the local back-gate. Therefore, the device operation can also be switched from inverter to voltage controlled resistor, opening new possibilities in using graphene in logic circuitry.

Low-Bias Active Control of Terahertz Waves by Coupling Large-Area CVD Graphene to a Terahertz Metamaterial

We propose an hybrid graphene/metamaterial device based on terahertz electronic split-ring resonators directly evaporated on top of a large-area single-layer CVD graphene. Room temperature time-domain spectroscopy measurements in the frequency range from 250 GHz to 2.75 THz show that the presence of the graphene strongly changes the THz metamaterial transmittance on the whole frequency range. The graphene gating allows active control of such interaction, showing a modulation depth of 11.5% with an applied bias of 10.6 V. Analytical modeling of the device provides a very good qualitative and quantitative agreement with the measured device behavior. The presented system shows potential as a THz modulator and can be relevant for strong light-matter coupling experiments.