Monolayer Graphite Research Articles

Anomalous Hooke’s law in disordered graphene

The discovery of graphene, a single monolayer of graphite, has provided an experimental demonstration of stability of 2D crystals. Although thermal fluctuations of such crystals tend to destroy the long-range order in the system, the crystal can be stabilized by strong anharmonicity effects. This competition is the central issue of the crumpling transition, i.e., a transition between flat and crumpled phases. We show that anharmonicity-controlled fluctuations of a graphene membrane around equilibrium flat phase lead to unusual elastic properties. In particular, we demonstrate that stretching ξ of a flake of graphene is a nonlinear function of the applied tension at small tension: and for clean and strongly disordered graphene, respectively. Conventional linear Hooke’s law, , is realized at sufficiently large tensions: where depends both on temperature and on the disorder strength.

(Invited) Highly Functional Graphene Nano-Electromechanical (GNEM) Devices for Advanced Switch and Sensor Applications

Graphene, as a monolayer of graphite, has an ultra-high Young’s modulus of ~1 TPa, which makes it a promising candidate for Nano-Electromechanical (NEM) devices. The graphene NEM switches is expected to show minimized electrical leakage, sharp switching response, low actuation voltage, and high on/off ratio. A simple bottom-up procedure using a polymer sacrificial spacer is utilized to fabricate graphene electromechanical contact switch devices. Low pull-in voltage of below 5 V is achieved with good consistency [1], which is compatible with the conventional complementary metal-oxide-semiconductor circuit requirements. In addition, the formation of carbon-gold bonds at the contact position is proposed as another important mechanism for the irreversible switch other than the well-known irreversible static friction. In order to overcome the stiction issue and realize repetitive switching operations, we exploited a natural chromium oxide layer formed on the chromium electrode. We fabricated a doubly-clamped beam and cantilever graphene nanoelectromechanical switches with a local top actuation electrode. The low pull-in voltage below 5 V was realized in both of the switches. The naturally formed chromium oxide at the graphene-electrode contact interface prevents the formation of chemical bonds between graphene and metal, resulting in reversible switching operation successfully. Thanks to highly-stable SP2 bonds in a 2D honeycomb lattice, graphene exhibits an extremely suppressed electrical noise, making it possible to realize extremly-sensitive sensor. For the physisorbed CO2 molecules - graphene vdW complexes, we found it is possible to tune the vdW interaction with external electric fields. The field-dependent charge transfer in the CO2−graphene complex was unveiled along with small variations in the equilibrium CO2−graphene distance and the O−C−O bonding angle. By reversing the substrate bias polarity, the charge transfer direction also switched, signifying the role of physisorbed CO2molecules can be altered electrically between donor and acceptor. The range of such electrical tunability was found inherent in individual molecular species [2]. In recent years, direct milling of graphene by Helium ion beam (HIB) has been actively explored. The main advantages of HIB milling over typical gallium focused ion beam (FIB) milling are: high resolution and less damage to the graphene. This HIB milling can be actively explored to carve the suspended graphene into a very narrow beam, which will help to realize narrower suspended graphene beams, which can’t be realized with the standard electron beam lithography technique. Towards this direction, gas field ionization source (GFIS) system with Helium and Nitrogen beams is introduced. In order to use HIB milling, a suspended monolayer graphene beam device was first fabricated without a top gate. These devices were used for the further HIB milling. We successfully patterned a 60-nm-long and 15-nm wide suspended graphene beam. As the next step to utilize our unique graphene nano-electro-mechanical systems (NEMS) to realize a well challenging application, the NEMS sensor devices were explored to realize individual CO2 molecule detection. Detection of a single molecule’s adsorption and desorption, which determines the ultimate resolution of gas sensing, has rarely been achieved with conventional solid-state sensor devices. A doubly clamped bilayer graphene beam was first fabricated and then by applying a voltage to the lower gold electrode the graphene beam was electrostatically pulled down. It was found that despite the negligible charge transfer from a single very poor doping CO2 physisorbed molecule, it strongly affects the electronic transport in suspended graphene beam by inducing charged impurity, which can shut down part of conduction of the graphene channel with Coulomb impurity scattering. We detected each individual physisorption of CO2molecule as a step-wise resistance change with a quantized interval for the graphene channel [3].

Hybrid Structure of Atomically Thin Carbon Nanosheet/Au Particles and Its Application As Organic Solar Cells

Graphene with two-dimensional structure has attracted a significant interest due to its unique properties such as remarkable optical transmittance, electrical conductivity, and electron mobility. There are two representative methods to prepare the large-area graphene with properties suitable for organic electronics: chemical vapor deposition and solid carbon coating. In the former method, the graphene is synthesized using gasified carbon species (i.e. methane) deposited onto metal film as a catalyst at temperature as high as 800 oC. In the latter method, solid carbon sources such as polymer, graphite, and self-assembled monolayer are coated on the metal film, followed by thermal treatment at similar temperature to the former method. The graphene prepared by the methods exhibits superior properties to chemically converted graphene. However, the graphene grown on the metal substrate has to transfer on the other proper substrates such as Si wafer to apply for the electrodes of the organic electronics. The transfer process brings about the artificial defects, such as wrinkles, cracks, and vacancy, on the graphene, resulting in degradation of the graphene properties. In this study, we have introduced a novel and simple approach to prepare an atomically thin carbon nanosheet (CNS) similar to graphene properties. In addition, we have modulated the electronic properties the CNSs using chloride-based metal precursor. The CNSs were prepared using polyacrylonitrile (PAN) polymer spin-coated onto the quartz substrates. The polymer films were oxidized at 250 oC for 2 h in order to induce formation of ladder structure. The oxidized films were then heat-treated at 1150 oC under a H2/Ar mixture to promote inter- and intra-molecular bonding and remove heteroatoms bonded to the carbon atom. The structural and morphological properties of the CNS were analyzed by HR-TEM, Raman spectroscopy, XPS, and AFM. The CNS prepared using 0.5 wt% PAN solutions showed about 0.4 nm of thickness, 1600 S/cm of electrical conductivity, and 92% of transmittance. The CNS was mostly composed of sp2 carbon with a small amount of impurities such as oxygen and nitrogen analyzed by XPS. The metal ions in the precursors are easily reduced into the metal particles without post-treatment due to the electron transfer from the CNSs to the metal ions. The WFs of the CNSs doped with AuCl3, ZnCl2, and CoCl2 exhibit significantly higher values of 4.91, 4.77, and 4.69 eV, respectively, compared to the pristine CNSs (4.60 eV). The metal particles on the CNSs locally influence the electronic modification of the CNSs, leading to transparent conducting electrodes with high WFs and a high power conversion efficiency of ~ 2.01%. Therefore, we suggested that the CNS modified by metal precursor is a promising candidate as transparent and conductive electrodes for organic devices. The specific properties and the possibility of utilization of the CNS will be discussed.

Observation of Ferromagnetic Semiconductor Behaviors from Graphene Doped with Manganese-Oxide By Electrochemical Method

Ferromagnetic semiconductors have attracted much attention in the field of spintronic application, because future electronic devices will have a switchable magnetic ordering in semiconductors for exploiting spin rather than charge. The key importance of the magnetic semiconductor is the Curie temperature to maintain the ferromagnetic ordering at room temperature. A high Curie temperature above room temperature has been shown in carbon-based materials. Recently, graphene which is an atomically monolayer graphite, has been an important issue for future spin electronic devices due to the high mobility and superb material properties. Especially, a theoretical result showed that graphene with defects behaves like a diluted magnetic semiconductor (DMS). In general, sp2 hybridized carbon atoms in graphene have an unsaturated dangling bond, which induces various surface modifications. Therefore, doping onto graphene easily leads to modification of the electronic structure, magnetic property, and carrier density. In this work, we report an electrochemical doping of manganese-oxide onto graphene, and also investigate the ferromagnetic semiconductor behavior from the manganese-oxide doped graphene. We have doped a manganese-oxide onto graphene by an electrochemical method. Graphene showed a clear ferromagnetic semiconductor behavior after doping of manganese-oxide. The manganese-oxide doped graphene has a coercive field (Hc) of 232 Oe at 10 K, and has the Curie temperature of 270 K from the temperature-dependent resistivity using transport measurement. The ferromagnetism of manganese-oxide doped graphene attributes to the double-exchange from the coexistence of Mn3+ and Mn4+ on the surface of graphene. In addition, the semiconducting behavior is caused by the formation of manganese-oxide on graphene.

Strain-Mediated Modification of Phagraphene Dirac Cones

A first-principles study on the electronic and dynamical properties of phagraphene [Nano Lett. 2015, 15 (9), p 6182] is presented. This carbon allotrope exhibits a square unit cell, Dirac cones, and robustness against uniaxial deformation. By analyzing the contribution of each carbon atom orbital in the formation of the electronic states, we conclude that only the pz orbitals of 8 out of the 20 atoms in the square unit cell are responsible of the formation of the nanostructure Dirac cones. Spatial symmetry breaking of the underlying honeycomb-like network upon shear stress application leads to a band gap opening. The analysis of the phonon spectra demonstrates that the dynamical stability of phagraphene is guaranteed for small distortion angles. Phagraphene is identified here as the first all-C graphitic monolayer with Dirac cones modifiable by a small and realistic physical deformation. The analysis and conclusions of this study can be applied to other monolayered materials exhibiting Dirac cones in squa...

Adsorption of Papain on solid substrates of different hydrophobicity.

Adsorption properties of protein Papain at the solid|liquid (0.1 M KCl) interfaces of different hydrophobicity [highly oriented pyrolytic graphite (HOPG), bare gold, CH3, OH, and COOH-terminated self-assembled monolayers on gold] were studied by a combined quartz crystal microbalance and atomic force microscopy techniques. It was found that Papain forms an incomplete monolayer at hydrophobic interfaces (HOPG and CH3-terminated substrate), whereas on more hydrophilic ones, a complete monolayer formation was always observed with either the onset of the formation of a second layer (bare gold substrate) or adsorption in a multilayer fashion, possibly a bilayer formation (OH-terminated substrate). The surface concentration and compact monolayer film thickness was much lower on the COOH-terminated substrate compared to other surfaces studied. This result was explained by partial dissociation of the interfacial COOH groups leading to additional electrostatic interactions between the positively charged protein domains and negatively charged carboxylate anions, as well as to local pH changes promoting protein denaturation.

The Process of Dispersing Bismuth Atoms into Iridium via an Intermediate Monolayer of Graphite

It is first revealed by thermodesorption spectroscopy that when bismuth atoms diffuse the indium atoms through an intermediate graphite layer, they show certain characteristic features. In the Ir (111)-C system at 1400 K δ = 0.4 ± 0.05 and diffusion δBi = Ndif/N = 2.5 × 10-3 bismuth atoms in the Ir(111)-C system. The amount of intercalated and diffused bismuth depends on the electrical field, i.e., on the positive potential in the Ir(111)-C system. It has been noted that a considerable diffusion of bismuth into iridium started at 180 V and increased up to 3000 V. The activation energies for bismuth diffusion into and from iridium were calculated to be En1 = 6.05 ± 0.05 eV and E1n = 6.3 ± 0.1 eV, respectively.

Preparation of graphene thin films for radioactive samples

A new method for the preparation of conductive thin films is presented. The metallization of VYNS films guarantees the electrical conductivity but it results in the breaking of a high proportion of them. Graphene, a two-dimensional nanostructure of monolayer or few layers graphite has attracted a great deal of attention because of its excellent properties such as a good chemical stability, mechanical resistance and extraordinary electronic transport properties. In this work, the possibilities of graphene have been explored as a way to produce electrical conductive thin films without an extra metallization process. The procedure starts with preparing homogenous suspensions of reduced graphene oxide (rGO) in conventional VYNS solutions. Ultra-sonication is used to ensure a good dispersibility of rGO. Graphene oxide (GO) is prepared via oxidation of graphite and subsequent exfoliation by sonication. Different chemically rGO were obtained by reaction with hydrazine sulfate, sodium borohydride, ascorbic acid and hydroiodic acid as reducing agents. The preparation of the thin graphene films is done in a similar way as the conventional VYNS foil preparation procedure. Drops of the solution are deposited onto water.The graphene films have been used to prepare sources containing some electron capture radionuclides (109Cd, 55Fe, 139Ce) with an activity in the order of 3kBq. The samples have been measured to test the attainable low energy electron efficiency and the energy resolution of Auger and conversion electrons by 4π (electron capture)-γ coincidence measurements. The 4π (electron capture)-γ coincidence setup includes a pressurized proportional counter and a NaI(Tl) detector. Tests with different pressures up to 1000kPa were carried out. All these tests show similar values in both parameters (efficiency and resolution) as those obtained by using the conventional metallized films without the drawback of the high percentage of broken films.

Flame synthesis of graphene layers at low pressure

The synthesis of graphene layers on a nickel substrate in a butane–benzene–oxygen premixed flame at a pressure of 40–100 Torr is studied. It is demonstrated that, the temperature of 900–950°C and exposure time of 0.5 min are sufficient for synthesizing graphene layers on a nickel substrate. It is shown that, at a pressure of 45–55 Torr, single-layer graphene is predominantly formed. It is found that, at a pressure of 90 Torr and an exposure time of 0.5 min, monolayer graphite can be produced, but with a lower yield as compared to that prepared at 45–55 Torr. It is demonstrated that the degree of defectiveness of graphenes decreases with the exposure time, reaching a minimum value of I D /I G = 0.36.

Functional group changes of polyacrylonitrile fibres during their oxidative, carbonization and electrochemical treatment

A unique consideration is intended, concerning the creation of functional groups during the production of carbon fibres from polyacrylonitrile, i.e. oxidation/stabilization, carbonization/pyrolysis and electrochemical modification processes and their role for applications. Acrylic fibres of homopolymer PAN are subjected to thermo-oxidative treatment following a four-stage or a two-stage isothermal programme, and the resulting oxidized fibres are pyrolyzed up to 1000 °C for 30 min or they are flash-pyrolyzed at 1000 °C for 5 min, and the shrinkage and weight loss are determined. According to FTIR spectra, two indices were expressed to follow the dehydrogenations and cyclizations (nitrile groups) and the band intensity of carbonyl groups to follow the oxidation. The carbon fibres produced from the four-stage oxidation were examined using SEM/EDS. Based on SEM/EDS analysis, an oxygen content of approx. 8.7 at.% in the fibres is needed for the stabilization of the fibres during the oxidation by a four-stage treatment up to 300 °C. These carbon fibres were electrochemically modified by Cyclic Voltammetry in narrow and wide potential regions, using aqueous H2SO4. The electrochemical treatment of carbon fibres in the wide region leads to the introduction of an additional amount of oxygen of about 5 at.%, exhibiting also acidic and basic groups in adequate amount. The formation of oxygenous groups on carbon fibres during the three processes is indirectly followed by their ability to adsorb methylene blue or alizarin yellow dyes, based on the concept of “acid–base interaction”. Considering a model of a graphitic monolayer of a basic square that includes 9 rings, the O atoms introduced in this segment were estimated. Carbon fibre applications depend on the amount and type of oxygenous groups that can be targeted by selecting proper treatment conditions.

Preparation of colloidal graphene in quantity by electrochemical exfoliation

We reported the preparation of colloidal graphene in quantity via the anodic exfoliation of graphite in (NH4)2SO4 aqueous solution. In the currently designed electrochemical exfoliation route, mass high-quality graphene was produced within short reaction time, around 1h. The proposed electrochemical exfoliation mechanism showed that SO42− and H2O can be intercalated into those graphite sheets, monolayer and few-layer graphene were obtained by the formation of gaseous SO2 and O2 within graphite sheets. Stability evaluation showed that our exfoliated colloidal graphene can be perfectly stabilized in DMF solvent more than 1week. The colloidal graphene can be used to construct various simple and complex patterns by writing it on A4 paper, which can be applied to flexible printed electronic devices. Furthermore, colloidal graphene can show promising applications in the fabrication of binder- and additive-free electrodes for supercapacitors and lithium-ion batteries. Our present method shows huge potential for industrial-scale synthesis of high-quality graphene and further commercialization of graphene colloid for numerous advanced applications in flexible printed electronics and energy storage devices.

Direct torsional actuation of microcantilevers using magnetic excitation

Torsional mode dynamic force microscopy can be used for a wide range of studies including mapping lateral contact stiffness, torsional frequency or amplitude modulation imaging, and dynamic friction measurements of various materials. Piezo-actuation of the cantilever is commonly used, but it introduces spurious resonances, limiting the frequency range that can be sampled, and rendering the technique particularly difficult to apply in liquid medium where the cantilever oscillations are significantly damped. Here, we demonstrate a method that enables direct torsional actuation of cantilevers with high uniformity over wide frequency ranges by attaching a micrometer-scale magnetic bead on the back side of the cantilever. We show that when beads are magnetized along the width of the cantilever, efficient torsional actuation of the cantilevers can be achieved using a magnetic field produced from a solenoid placed underneath the sample. We demonstrate the capability of this technique by imaging atomic steps on graphite surfaces in tapping mode near the first torsional resonance of the cantilever in dodecane. The technique is also applied to map the variations in the lateral contact stiffness on the surface of graphite and polydiacetylene monolayers.

Determination of Graphene Sheets Content in Carbon Materials by Raman Spectroscopy

AbstractGraphene layer is a monolayer of graphite. Graphene single layer has confirmed to have a higher electron and hole mobility than silicon and has high heat conduction and special optical properties. And un‐perfect graphene sheets exist in any carbon materials. In our previous result to determine the graphene sheets content in any carbon material can be determined by X‐ray spectrum combination with a peak decomposition method. In this study, we have prepared two serious carbon materials from wood wastes, with and without MnO2 catalytic pyrolysis processes. Results show that all the G band intensity of all synthesized samples including a commercial graphene powder in Raman spectrum has a proportional relation with the graphene sheets content in these carbon materials.

Structural and elastic properties of hybrid bilayer graphene/h-BN with different interlayer distances using DFT

Importance of synthesizing graphene-substrate hybrid structure to open a band gap in graphene and apply them in novel nanoelectronic devices is undeniable. Graphene/hexagonal boron-nitride (h-BN) hybrid bilayer is an important type of these structures. The synthesized h-BN/graphene is found to have interesting electrical properties which is very sensitive to the change of the interlayer distance. This has encourages researchers to tune the energy and band gap of such structures. A change in the interlayer distance can also alter the mechanical properties, considerably, due to the variation of interaction energies. The current study is aimed to characterize the mechanical properties variation with interlayer distance change for h-BN/graphene hybrid bilayer structure. To this end, density functional theory calculations are employed within the generalized gradient approximation (GGA) framework. The results demonstrate that there are different possible equilibrium interlayer distances between layers related to two types of layer configuration, i.e. AA and AB. It is found that increasing the interlayer distance causes reduction of Young’s modulus. Also, Young’s modulus of hybrid structure is approximately between those of graphene/graphene and h-BN/h-BN bilayer structures and also lower than pristine monolayer graphene and graphite. Unlike the pure bilayer structures, Poisson’s ratio of hybrid bilayer structure is found to be higher than those of pristine monolayer graphene and h-BN nanosheets.

Coherent phonon and surface-enhanced Raman scattering dynamics in solids

We have demonstrated coherent phonon and surface-enhanced Raman scattering (SERS) dynamics of carbon-related materials: gold (Au) deposited graphite and benzenethiol (BT) self-assembled monolayer (SAM) adsorbed on Au film with roughness. Using ultrafast pump-probe spectroscopy with the enhanced electric field via Au hemispheres or Au film with nanoscale roughness, transient behaviors of high frequency phonons located at surface/interface were sensitively detected. In Au deposited graphite, the electric field near graphite surface around Au nanoparticles, whose typical diameter is ~10nm, is strongly enhanced. As a result, the disorder-induced mode (D-mode) near the surface was clearly observed. In BT-SAM, the nanoscale roughness of the Au film might contribute to the amplitude enhancement of coherent vibrations up to the detected level. From these results, we believe that coherent SERS spectroscopy will open the door to observe coherent phonon dynamics even at the level of monolayer and single molecules for future.

Driving the Diffusion of Cesium Atoms of Graphite Monolayers on Metal Surfaces into Rhenium (1010)

It has been shown experimentally for the first time that diffusion of cesium n rhenium takes place at irradiation of atoms flux heated up to Т ≤ 1800 K -C. Analysis of thermal desorption spectra showed that cesium forms two high-temperature phases during adsorption of cesium on graphite monolayer in rhenium heated up to 1800 K. The first, intercalated phase starts filling immediately and grows up to maximum concentration. The second, diffusion phase appears with slight delay and grows incessantly during adsorption. It is for the first time that the Fick Equation has been applied for such kind of diffusion. Correspondences and average displacement L = f(Т,t) have been obtained at diffusion of surface cesium in rhenium in the interval of temperatures 1600 K Т ν = 5 × 1013 cm-2·s-1, U = +2000 V, T = 1700 K, effectiveness of cesium diffusion δCs in the system of Re(1010)-C was identified with application of suggested methods, which showed δCs = 0.45. Activation energies of Сs atoms diffusion in rhenium from rhenium were found and values Е1p = 5.65 ± 0.05 eV and Еp1 = 5.85 ± 0.05 eV were obtained respectively. Effect of electrical field on intercalation and cesium diffusion in Re-C was observed.

Large-Area 2-D Electronics: Materials, Technology, and Devices

Recent experiments since the discovery of monolayer graphite or graphene have led to an exciting revival in the interest in the electronic applications for graphene, as well as other 2-D materials such as hexagonal boron nitride (hBN) and molybdenum disulfide (MoS <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{2}$</tex></formula> ). These layered materials serve as an exciting new platform for flexible and transparent electronics where surfaces can be enriched with new functionality. This paper aims to provide an overview behind these new class of materials ranging upon important issues for electronic integration including synthesis all the way to current state-of-the-art circuits and devices made from these materials.

Diffusion of potassium atoms into rhenium covered with two-dimensional graphite film

The effective diffusion of K atoms into Re (1010) stimulated by a graphite monolayer on its surface is discovered. The activation energies of the diffusion of potassium into rhenium and from it, as well as the effectiveness of the diffusion of potassium atoms, are determined. The following values are obtained: Ef1 = 5.4 ± 0.05 eV, E1f = 5.75 ± 0.05 eV, and δdif(K) ≈ 0.25. The difference in the energies of the diffusion of potassium into rhenium is lower than the energy of the diffusion of potassium into iridium. It is shown that the diffusion length in the process of the diffusion of potassium adatoms into rhenium is greater than in the case of diffusion into iridium. The noticeable diffusion of potassium atoms are observed when the voltage U ≥ +200 V is applied to the specimen and intensifies approximately following the linear law as U increases to 4000 V. The formation of a graphite monolayer on the rhenium surface due to the enormous prolongation of the lifetime of K adatoms under the graphite island compared to the lifetime of adatoms on the exposed rhenium surface increases greatly the effectiveness of potassium diffusion, which grows by ∼108 times, then approaches unity. This phenomenon can be used as a method for the active alloying of subsurface layers of solids by atoms.

Diffusion of indium atoms into iridium through an intermediate graphite monolayer

Peculiar features of diffusion of indium atoms into iridium through a graphite monolayer have been revealed for the first time by thermodesorption spectroscopy. The most intense diffusion of indium into iridium has occurred in the Ir(111)-C[plain] system at 1400 K < T < 1600 K. The fractions of the fluxes of intercalation \(\bar \delta = 0.3 \pm 0.05\) and diffusion δIn = Ndif/N = 2 × 10−3 indium atoms in the Ir(111)-C system have been found. It has been demonstrated that the amount of intercalated and diffused indium depends on the electrical field, i.e., on the positive potential in the Ir(111)-C system. A noticeable diffusion of indium into iridium started at 200 V and increased up to 2500 V. The activation energies for indium diffusion into and from iridium have been determined to be En1 = 5.05 ± 0.05 eV and En1 = 5.4 ± 0.1 eV, respectively.

Molecular beam growth of graphene nanocrystals on dielectric substrates

We demonstrate the growth of graphene nanocrystals by molecular beam methods that employ a solid carbon source, and that can be used on a diverse class of large area dielectric substrates. Characterization by Raman and near edge X-ray absorption fine structure spectroscopies reveal a sp2 hybridized hexagonal carbon lattice in the nanocrystals. Lower growth rates (below 0.5Å/min) favor the formation of layered, larger size multi-layer graphene crystallites (up to 22nm) on all investigated substrates. The surface morphology is determined by the roughness of the underlying substrate (RMS ∼8Å) and graphitic monolayer steps are observed by ambient scanning tunneling microscopy.