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Chemical vapor deposition (CVD) has been widely adopted as the most scalable method to obtain single layer graphene. Incorporating CVD graphene in planar devices can be performed via well-established wet transfer methods or thermal adhesive release. Nevertheless, for applications involving 3D shapes, methods adopted for planar surfaces provide only a crude solution if a continuous, tear-free, wrinkle-free graphene layer is required. In this work, we present the fabrication and characterization of Polydimethylsiloxane-supported 3D graphene probes. To accommodate 3D geometries, we perform CVD on catalysts possessing a non-trivial 3D topology, serving to mold the grown graphene to a final non-trivial 3D shape. This advance overcomes challenges observed in standard transfer processes that can result in uneven coverage, wrinkles, and tears. To demonstrate the potential of our different transfer approach, we apply it to fabricate graphene electrical probes. Graphene, due to its flexibility, transparency, and conductivity, is an ideal material with which conventional metal based probes can be replaced. In particular, with a contact impedance on the order of 10 kΩ, our graphene probes may find applications, such as in electrophysiology studies. [ABSTRACT FROM AUTHOR]

Multiple sub-band transport in bilayer graphene nanoribbons (GNRs) with localized charge states has not been experimentally investigated owing to the difficulty of accessing into the upper sub-band. In this letter, we utilize current annealing to heavily p-dope graphene from a capping hydrogen-silsesquioxane layer. As a result, Fermi energy can be tuned into the upper sub-valence band with reasonably small gate voltage. Owing to the localized charge states, quantum dot-like characteristics are measured in bilayer GNRs, as a result of the tunnelling transport of holes in the upper sub-band through these charge puddles. In contrast to monolayer GNRs, this phenomenon appears in a considerable conductive regime since the carrier transport in the lower sub-band is still diffusive. Moreover, by electrically tuning the Fermi energy, the localized charge puddles in the upper sub-band can be resized and isolated from each other. [ABSTRACT FROM AUTHOR]

Graphene-semiconducting single-wall carbon nanotubes' (graphene-s-SWCNTs) thin-film photodetector based on a double-layer stacked heterostructure was fabricated. The carbon-based heterostructure exhibits excellent long-range van der Waals interactions. The as-fabricated device was demonstrated with an ultra-broadband photodetection characteristic with a high responsivity of 78 A/W at a visible wavelength and a fast response time of 80 μs. Moreover, the high photoconductive gain based on the photogating effect for the graphene-s-SWCNTs device was realized. In addition, the temperature-dependent photoresponse performance was also demonstrated. Benefiting from the high photoconductive gain, ultra-fast response time, and high stable quality of carbon materials, our devices exhibit great potential applications for high sensitivity detection to weak target signals in extreme environments. [ABSTRACT FROM AUTHOR]

Engineered substrates offer a promising avenue towards graphene devices having tunable properties. In particular, topographically patterned substrates can expose unique behavior due to their ability to induce local variations in strain and electrostatic doping. However, to explore the range of possible science and applications, it is important to create topographic substrates that both have tunable features and are suitable for transport measurements. In this letter, we describe the fabrication of tunable, topographically patterned substrates suitable for transport measurements. We report both optical and transport measurements of graphene devices fabricated on these substrates and demonstrate the characteristic strain and local doping behavior induced by the topographic features. [ABSTRACT FROM AUTHOR]

We fabricated luminescent chemical vapor deposition-grown monolayer graphene sheets with an adsorbed europium complex, EuLC18, and characterized their luminescence properties. The EuLC18/graphene sheets clearly showed several photoluminescence peaks in a wavelength region from 580 to 694 nm, which were attributed to the ff transitions of the Eu ion. Luminescence was obtained via a photo-antenna effect, in which the ligands of EuLC18 absorbed the photo-excitation energy and transported it to the Eu excitation. Although the absolute luminescence quantum yield of the EuLC18/graphene sheet was as low as 0.5% due to the interaction between graphene and EuLC18, we demonstrated that graphene sheets can be made luminescent simply through adsorption of the luminescent Eu complex on the graphene surface. [ABSTRACT FROM AUTHOR]

The standard theory of thermionic emission developed for three-dimensional semiconductors does not apply to two-dimensional materials even for making qualitative predictions because of the vanishing out-of-plane quasiparticle velocity. This study reveals the fundamental origin of the out-of-plane charge carrier motion in a two-dimensional conductor due to the finite quasiparticle lifetime and huge uncertainty of the out-of-plane momentum. The theory is applied to a Schottky junction between graphene and a bulk semiconductor to derive a thermionic constant, which, in contrast to the conventional Richardson constant, is determined by the Schottky barrier height and Fermi level in graphene. [ABSTRACT FROM AUTHOR]

Bubbles generated during electro-delamination and chemical etch during large-area two-dimensional (2D) material transfer has been shown to cause rippling, and consequently, results in tears and wrinkles in the transferred film. Here, we demonstrate a scalable and reusable method to remove surface adhered micro-bubbles by using hydrophobic surfaces modified by self-assembled monolayers (SAMs). Bubble removal allows the 2D film to flatten out and prevents the formation of defects. Electrical characterization was used to verify improved transfer quality and was confirmed by increased field-effect mobility and decreased sheet resistance. Raman spectroscopy was also used to validate enhanced electrical quality following transfer. The bubble removal method can be applied to an assortment of 2D materials using diverse hydrophobic SAM variants. Our studies can be integrated into large scale applications and will lead to improved large-area 2D electronics in general. [ABSTRACT FROM AUTHOR]

Electronic properties of graphene edges under a lateral electric field were theoretically studied in regard to their edge shapes and terminations to provide a theoretical insight into their field emission properties. The work function and potential barrier for the electron emission from the graphene edges are sensitive to their shape and termination. We also found that the hydrogenated armchair edge shows the largest emission current among all edges studied here. The electric field outside the chiral edges is spatially modulated along the edge because of the inhomogeneous charge density at the atomic sites of the edge arising from the bond alternation. [ABSTRACT FROM AUTHOR]

We investigated the growth of boron-doped few-layer graphene on α-Al2O3 (0001) substrates by molecular beam epitaxy using two different growth approaches: one where boron was provided during the entire graphene synthesis and the second where boron was provided only during the second half of the graphene growth run. Electrical measurements show a higher p-type carrier concentration for samples fabricated utilizing the second approach, with a remarkable modulation in the carrier concentration of almost two orders of magnitude in comparison to the pristine graphene film. The results concerning the influence of the boron flux at different growth stages of graphene on the electrical and physicochemical properties of the films are presented. [ABSTRACT FROM AUTHOR]

We demonstrate that a graphene-based attenuator on a half-mode substrate integrated waveguide (HMSIW) can achieve dynamically tunable attenuation. This attenuator is produced by depositing a graphene sandwich structure (GSS) on the substrate of a HMSIW. By applying a bias voltage on the GSS, the surface impedance of graphene and the attenuation of the attenuator can be tuned. A prototype was fabricated and measured to verify the analysis. The measurement results manifest that as the bias voltage increases from 0 V to 4 V, the attenuation can be increased from 3 dB to 14 dB at the frequency from 7.7 GHz to 19 GHz, while |S11| of the attenuator can always remain at a low level smaller than −20 dB. [ABSTRACT FROM AUTHOR]

We report results of a systematic analysis of thermal transport in electron-irradiated, including irradiation-induced amorphous, graphene sheets based on nonequilibrium molecular-dynamics simulations. We focus on the dependence of the thermal conductivity, k, of the irradiated graphene sheets on the inserted irradiation defect density, c, as well as the extent of defect passivation with hydrogen atoms. While the thermal conductivity of irradiated graphene decreases precipitously from that of pristine graphene, k0, upon introducing a low vacancy concentration, c<1%, in the graphene lattice, further reduction of the thermal conductivity with the increasing vacancy concentration exhibits a weaker dependence on c until the amorphization threshold. Beyond the onset of amorphization, the dependence of thermal conductivity on the vacancy concentration becomes significantly weaker, and k practically reaches a plateau value. Throughout the range of c and at all hydrogenation levels examined, the correlation k=k0(1+αc)-1 gives an excellent description of the simulation results. The value of the coefficient α captures the overall strength of the numerous phonon scattering centers in the irradiated graphene sheets, which include monovacancies, vacancy clusters, carbon ring reconstructions, disorder, and a rough nonplanar sheet morphology. Hydrogen passivation increases the value of α, but the effect becomes very minor beyond the amorphization threshold. [ABSTRACT FROM AUTHOR]

The article examines a study which analyzes the nonlinear properties of graphene using general qualitative analysis method. Topics discussed include the high-frequency response of graphene, the nonlinear index for the interband transitions, and the nonlinearities of graphene which is strong at low electron densities.

We report the observations of electronic properties of graphene oxide and reduced graphene oxide, performed with electron paramagnetic resonance technique in a broad temperature range. Both materials were examined in pure form and saturated with air, helium, and heavy water molecules. We show that spin localization strongly depends on the type and amount of molecules adsorbed at the graphene layer edges (and possible in-plane defects). Physical and chemical states of edges play crucial role in electrical transport within graphene-based materials, with hopping as the leading mechanism of charge carrier transport. Presented results are a good basis to understand the electronic properties of other carbon structures made of graphene-like building blocks. Most active carbons show some degree of functionalization and are known of having good adsorptive properties; thus, controlling both phenomena is important for many applications. Sample treatment with temperature, vacuum, and various adsorbents allowed for the observation of a possible metalinsulator transition and sorption pumping effects. The influence of adsorption on the localization phenomena in graphene would be very important if to consider the graphene-based material as possible candidates for the future spintronics that works in ambient conditions. [ABSTRACT FROM AUTHOR]

We demonstrate few-layer molybdenum diselenide (MoSe2) dual-gated field effect transistors (FETs) with few-layer hexagonal boron nitride (hBN) flakes as encapsulation and multilayer graphene as electrical contacts. A high current on/off ratio of up to ~108, a two-terminal electron mobility of 38.5 cm2/V.s at room temperature, and negligible hysteresis are achieved in hBN encapsulated MoSe2 FETs. Our results also indicate that the flicker (1/f) current noise in hBN encapsulated MoSe2 transistors is governed by Hooge’s carrier mobility fluctuation and the normalized current noise in the dual-gated configuration can be dramatically reduced by applying a positive bias on the bottom gate. All these suggest that dual-gated MoSe2 FETs are very promising candidates for sensing applications. [ABSTRACT FROM AUTHOR]

Understanding the charge carrier transport characteristics at the graphene-GaN interface is of significant importance for the fabrication of efficient photoresponsive devices. Here, we report on the temperature dependent diode and photovoltaic characteristics of a graphene/n-GaN heterostructure based Schottky junction. The graphene/n-GaN heterojunction showed rectifying diode characteristics and photovoltaic action with photoresponsivity in the ultra-violet wavelength. The current-voltage characteristics of the graphene/n-GaN heterojunction device were investigated under dark and light illumination with changes in temperature. Under dark conditions, an increase in the forward bias current as well as saturation current was observed, and a decrease in the device ideality factor was obtained with an increase in temperature. Under illumination of light, a decrease in the open circuit voltage (Voc) and an increase in the short circuit current density (Jsc) was obtained with an increase in temperature. The increase in saturation current and carrier recombination with the increase in temperature leads to a reduction in Voc, while the photo-generated carrier increases in the heterojunction interface at higher temperatures contributing to the increase in Jsc. The observed temperature dependent device characteristics of the graphene/n-GaN heterojunction can be significant to understand the junction behavior and photovoltaic action. [ABSTRACT FROM AUTHOR]

The thermoelectric properties of graphene are strongly related to the defect density, and as such, these can be used to investigate carrier scattering. In this study, the defect density was controlled by the use of oxygen plasma treatment. Oxygen plasma introduces structural defects into graphene, initially introducing sp³ defects that transform into vacancy-type defects with further exposure, as indicated by XPS analysis, and these transitions cause substantial changes in both the electrical and thermoelectric properties of graphene. In this work, we estimate the effects of both defect density and species, analyzed by Raman spectroscopy, on the thermoelectric power of graphene, and find that the maximum thermoelectric power decreases with increasing defect density. We also find, from Ioffe's semiclassical approximation, that at the lower defect densities, phonons are the predominant source of carrier scattering, while at higher defect densities, the scattering is mainly caused by charged impurities, which corresponds to a change in defect population from the sp³-type to vacancies. [ABSTRACT FROM AUTHOR]

Polymer residue from Polymethyl methacrylate (PMMA) on transferred graphene is a common issue for graphene devices. This residue affects the properties of graphene. Herein, we have introduced an improved technique to reduce the effect of this residue by deep UV (DUV) exposure of PMMA coated graphene samples within the wet transfer process. This technique has systematically been evaluated by optical microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and electrical measurements. The results show that this residue is effectively reduced on the graphene surface after DUV treatment. In addition, the electrical characteristics of transferred graphene confirm that the sheet resistance and contact resistance are reduced by about 60 and 80%, respectively, after the DUV exposure. Electrical current transport characteristics also show that minimizing this residue on the graphene surface gives less hysteresis of electronic transport in back-gate graphene field-effect transistors. Furthermore, repeating electrical tests and aging shift the neutral point voltage of graphene. We attribute these improvements to cleaving of the chemical bonds in PMMA by DUV exposure and hence increasing the solubility of PMMA in acetone for subsequent processing steps. This work provides a unique route to enhance the electrical properties of transferred graphene after the fabrication process. [ABSTRACT FROM AUTHOR]

It was recently shown that the electronic band alignment in lateral two-dimensional heterostructures is strongly dependent on the system geometry, such as heterostructure width and layer thickness. This is so even in the absence of polar edge terminations because of the appearance of an interface dipole between the two different materials. In this study, this work is expanded to include two-dimensional materials that possess an electronic dipole over their surface, i.e., in the direction transverse to the crystal plane. To this end, a heterostucture consisting of polar hydrofluorinated graphene and non-polar graphane layers is studied with first-principles calculations. As for nonpolar heterostructures, a significant geometry dependence is observed with two different limits for the band offset. For infinitely wide heterostructures, the potential step in the vacuum is equally divided over the two sides of the heterostructure, resulting in a finite potential step in the heterostructure. For infinitely thick heterostructure slabs, on the other hand, the band offset is reduced, similar to the three-dimensional case. [ABSTRACT FROM AUTHOR]

Using first-principles calculations, we systematically investigated the electronic properties of graphene/g-GaN van der Waals (vdW) heterostructures. We discovered that the Dirac cone of graphene could be quite well preserved in the vdW heterostructures. Moreover, a transition from an n-type to p-type Schottky contact at the graphene/g-GaN interface was induced with a decreased interlayer distance from 4.5 to 2.5Å. This relationship is expected to enable effective control of the Schottky barrier, which is an important development in the design of Schottky devices. [ABSTRACT FROM AUTHOR]

In this letter, we report a method to control the reflection phase of microwaves using electrically tunable graphene devices. The device consists of mutually gated large-area graphene layers placed at a quarter-wave distance from a metallic surface. This device structure yields electrically tunable resonance absorbance and step-like phase shift around the resonance frequency when the impedance of graphene matches with the free space impedance. Electrostatic control of charge density on graphene yields unprecedented ability to control both intensity (>50 dB) and phase (~π) of the reflected electromagnetic waves with voltage. Furthermore, using the asymmetry of the doping at opposite polarity of the bias voltages, we showed bidirectional phase control with the applied voltage. [ABSTRACT FROM AUTHOR]

In this work, flexible and energy-efficient transparent heaters based on graphene and aligned silver nanowire (G-ASNW) hybrid structures are fabricated by thermal evaporation of silver on the aligned electrospun nanofiber templates and subsequent transfer of monolayer graphene onto the ASNWs. The G-ASNW films exhibit few wire-wire junctions and low resistance along the aligned direction, which are favorable for low-voltage transparent heater applications. Coating the ASNW network with monolayer graphene increases the saturated temperature of the hybrid heater due to the high thermal conductivity and low convective heat-transfer coefficient of graphene. Meanwhile, G-ASNW films show excellent electromechanical stability under cyclic bending because the graphene anchoring on the top surface of ASNWs could share tensile stress and serve as local conducting pathways at break-points even if small cracks were generated. The G-ASNW hybrid structures present a perspective on wearable transparent heaters. [ABSTRACT FROM AUTHOR]

Graphene encapsulated between flakes of hexagonal boron nitride (hBN) demonstrates the highest known mobility of charge carriers. However, the technology is not scalable to allow for arrays of devices. We are testing a potentially scalable technology for encapsulating graphene where we replace hBN with Parylene while still being able to make low-ohmic edge contacts. The resulting encapsulated devices show low parasitic doping and a robust Quantum Hall effect in relatively low magnetic fields <5 T. [ABSTRACT FROM AUTHOR]

Direct growth of graphene nanostructures by using concepts of self-assembly and intercalation without further lithography and transfer processes is beneficial for their integration into device applications. In this letter, we report on bilayer graphene nanoribbons, typically 100 nm in width, grown along step edges of SiC(0001) substrates. The ribbons are electrically decoupled from the substrate by an oxygen treatment. By means of a 4-tip STM system, the microscopic structure and transport properties were comprehensively studied. The ribbons reveal a robust hole concentration of around 1 x 1013 cm-2 and mobilities up to 700 cm2/Vs at room temperature. The comparably high mobilities are a consequence of interlayer hopping of the charge carriers. The transport is not limited by the step roughness; thus, this scalable process can be easily extended to arbitrarily shaped structures. [ABSTRACT FROM AUTHOR]

MoTe2 is an emerging two-dimensional layered material showing ambipolar/p-type conductivity, which makes it an important supplement to n-type two-dimensional layered material like MoS2. However, the properties based on its van der Waals heterostructures have been rarely studied. Here, taking advantage of the strong Fermi level tunability of monolayer graphene (G) and the feature of van der Waals interfaces that is free from Fermi level pinning effect, we fabricate G/MoTe2/G van der Waals heterostructures and systematically study the electronic and optoelectronic properties. We demonstrate the G/MoTe2/G FETs with low Schottky barriers for both holes (55.09 meV) and electrons (122.37meV). Moreover, the G/MoTe2/G phototransistors show high photoresponse performances with on/off ratio, responsivity, and detectivity of ~105, 87A/W, and 1012 Jones, respectively. Finally, we find the response time of the phototransistors is effectively tunable and a mechanism therein is proposed to explain our observation. This work provides an alternative choice of contact for high-performance devices based on p-type and ambipolar two-dimensional layered materials. [ABSTRACT FROM AUTHOR]

We report the formation of superstructures induced by Mg adatoms on a single layer graphene (SLG) formed on Ni(111) substrate, where a strong metallic parabolic band is found near the Fermi level at the Γ-point of the Brillouin zone. Our valence band and core level data obtained by using synchrotron photons indicate that Mg adatoms intercalate initially to lift the SLG from the Ni substrate to produce a well-defined π-band of SLG, and then the parabolic band appears upon adding extra Mg atoms on the Mg-intercalated SLG. Our scanning tunneling microscopy images from these systems show the presence of superstructures, a 2√3√2√3 phase for the intercalated Mg layer below the SLG and then a √7√√7 phase for the Mg overlayer formed on the Mg-intercalated SLG. We discuss the physical implications of these superstructures and the associated parabolic band in terms of a possible graphene-based two-dimensional superconductivity. [ABSTRACT FROM AUTHOR]

A graphene/LiNbO3 hybrid device is used to investigate the acoustic induced charge transport in chemical doped graphene. The chemical doping of graphene via its physisorption of gas molecules affects the surface acoustic wave (SAW) charge carrier transport in a manner different from electric field drift. That transport induces doping dependent macroscopic acoustoelectric current. The chemical doping can manipulate majority carriers and induces unique acoustoelectric features. The observation is explained by a classical relaxation model. Eventually the device based on acoustoelectric current is proved to outperform the common chemiresistor for chemicals. Our finding provides insight into acoustic charge carrier transport during chemical doping. The doping affects interaction of carriers with SAW phonon and facilitates the understanding of nanoscale acoustoelectric effect. The exploration inspires potential acoustoelectric application for chemical detection involving emerging 2D nanomaterials. [ABSTRACT FROM AUTHOR]

We demonstrate that the charge density wave (CDW) phase transition occurs on the surface of electronically doped multilayer graphene when the Fermi level approaches the M points (also known as van Hove singularities where the density of states diverge) in the Brillouin zone of the graphene band structure. The occurrence of such CDW phase transitions is supported by both the electrical transport measurement and optical measurements in electrostatically doped multilayer graphene. The CDW transition is accompanied with the sudden change of graphene channel resistance at Tm=100 K, as well as the splitting of Raman G peak (1580 cm-1). The splitting of Raman G peak indicates the lifting of in-plane optical phonon branch degeneracy, and the nondegenerate phonon branches are correlated to the lattice reconstructions of graphene--the CDW phase transition. [ABSTRACT FROM AUTHOR]

We report a self-powered photodetector based on the graphene/GaAs nanowire Schottky junctions. The device is fabricated by transferring a monolayer graphene onto an n-doped GaAs nanowire array. The nanowires are grown by Au-catalyzed metal organic chemical vapor deposition, and the Au particles are subsequently removed by wet etching to achieve close contact between graphene and GaAs. The device exhibits a responsivity of 1.54 mA/W at zero bias and a short response/ recover time of 71/194 μs at room temperature, showing a strong possibility for high-speed nearinfrared applications. [ABSTRACT FROM AUTHOR]

Nanomaterials hold great promise for applications in thermal management and thermoelectric power generation. Defects are important as they can be either inevitably present during fabrication or intentionally introduced to engineer properties. Here, we investigate how thermal conductance responds to edge defects in narrow graphene, silicene, and boron nitride nanoribbons (NRs), from first principles using non-equilibrium Green's function method. Geometric distortions, phonon conductance coefficients, and local densities of states are analyzed. Hydrogen absences produce similar reductions in conductance in planar graphene and boron nitride NRs with larger reductions in buckled silicene NRs. Large atom vacancies affect all systems similarly. Emerging flexible and stiff scattering centers, depending on bond strengths, are shown to cause thermal conductance reduction. This knowledge suggests that inferences on unknown thermal properties of novel defected materials can be made based on understanding how thermal transport behaves in their analogues and how bond characteristics differ between the systems. [ABSTRACT FROM AUTHOR]

Heterostructure of graphene nanowalls (GNW) supported Ni thin-layer was fabricated to form an on-chip pseudocapacitor via a standard microelectromechanical system process. Beyond a highrate capability of the micro-supercapacitors, a large specific energy density of 2.1 mWh cm-3 and power density up to 5.91Wcm-3 have been achieved, which are two orders of magnitude higher than those commercial electrolytic capacitors and thin-film batteries, respectively. Rational analysis revealed a rapid GNW growth originated from the Pt current collector embedment by catalyzing hydrocarbon dissociating. The unique concept in our design includes that Ni was evaporated onto GNW to serve as both the shadow mask for microelectrode patterning and subsequently a precursor to be in-situ electrochemically converted into pseudo-capacitive Ni(OH)2 for capacitance enhancing. Addressing the challenge to uniformly coat in complex nanoporous structures, this strategy renders a conformal deposition of pseudo-capacitive material on individual graphene nanoflakes, leading to efficient merits harnessing of huge accessible surfaces from the conductive GNW networks and great capacitance of the Ni-based active materials for high performance delivery. The proof of concept can be potentially extended to other transition metals and paves the way to further apply GNW hybrids in diverse microsystems. [ABSTRACT FROM AUTHOR]

Graphene is a promising material for applications in aqueous electrolyte environments. To explore the impact of such environments on graphene's electrical properties, we performed Hall bar measurements on electrolyte-gated graphene. Assuming a Drude model, we find that the room temperature carrier mobility in water-gated, SiO2-supported graphene reaches 7000 cm2/Vs, comparable to the best dry SiO2-supported graphene devices. Our results show that the electrical performance of graphene is robust, even in the presence of dissolved ions that introduce an additional mechanism for Coulomb scattering. [ABSTRACT FROM AUTHOR]

We investigated the electronic properties, stability, and transport of graphene under c-HfO2(111) layers by performing first-principles calculations with special attention to the chemical bonding between graphene and HfO2 surfaces. When the interface of HfO2/graphene is terminated by an O layer, the linear dispersion of graphene is preserved and the degradation of transport is suppressed. For other interface structures, HfO2 is tightly adsorbed on graphene and the transport is strictly limited. In terms of the stability of the interface structures, an O-terminated interface is preferable, which is achieved under an O-deficient condition. [ABSTRACT FROM AUTHOR]

Vertical spin valve devices with junctions of single- and bi-layer graphene interlayers sandwiched with Fe20Ni80 (Permalloy) electrodes were fabricated by exploiting the direct growth of graphene on the Permalloy. The linear current-voltage characteristics indicated that ohmic contacts were realized at the interfaces. The systematic characterization revealed the significant modification of the electronic state of the interfacial graphene layer on the Permalloy surface, which indicates the strong interactions at the interface. The ohmic transport was attributable to the strong interfaceinteraction. The vertical resistivity of the graphene interlayer and the spin asymmetry coefficient at the graphene/Permalloy interface were obtained to be 0.13 Ω cm and 0.06, respectively. It was found that the strong interface interaction modifies the electronic structure and metallic properties in the vertical spin valve devices with bi-layer graphene as well as single-layer graphene. [ABSTRACT FROM AUTHOR]

Atom (or molecule) intercalations and deintercalations have been used to control the electronic properties of graphene. In general, finite energies above room temperature (RT) thermal energy are required for the intercalations and deintercalations. Here, we demonstrate that alkali metal atoms can be deintercalated from epitaxial graphene on a SiC substrate at RT, resulting in the reduction in density of states at the Fermi level. The change in density of states at the Fermi level at RT can be applied to a highly sensitive graphene sensor operating at RT. Na atoms, which were intercalated at a temperature of 80 °C, were deintercalated at a high temperature above 1000 °C when only a thermal treatment was used. In contrast to the thermal treatment, the intercalated Na atoms were deintercalated at RT when tetrafluorotetracyanoquinodimethane (F4-TCNQ) molecules were adsorbed on the surface. The RT deintercalation occurred via the formation of charge-transfer complexes between Na atoms and F4-TCNQ molecules. [ABSTRACT FROM AUTHOR]

Liquid-based bio-applications of graphene require a quantitative understanding of the grapheneliquid interface, with the surface charge density of adsorbed ions, the interfacial charge transfer resistance, and the interfacial charge noise being of particular importance. We quantified these properties through measurements of the zero-bias Faradaic charge-transfer between graphene electrodes and aqueous solutions of varying ionic strength using a reproducible, low-noise, minimally perturbative charge measurement technique. The measurements indicated that the adsorbed ions had a negative surface charge density of approximately -32.8 mC m-2 and that the specific charge transfer resistance was 6.560.3 MΩ cm2. The normalized current noise power spectral density for all ionic concentrations tested collapsed onto a 1/fα characteristic with α=1.160.2. All the results are in excellent agreement with predictions of the theory for the graphene-solution interface. This minimally perturbative method for monitoring charge-transfer at the sub-pC scale exhibits low noise and ultra-low power consumption (∼fW), making it suitable for use in low-level bioelectronics in liquid environments. [ABSTRACT FROM AUTHOR]

We performed a temperature dependent study of the charge and spin transport properties of epitaxial graphene on the C-terminated (0001¯) face of silicon carbide (SiC), a system without a carbon buffer layer between the graphene and the SiC. Using spin Hanle precession in the nonlocal geometry, we measured a spin relaxation length of λS=0.7 μm at room temperature, lower than in exfoliated graphene. We show that the charge and spin diffusion coefficient, DC and DS, respectively, increasingly deviate from each other during electrical measurements up to a difference of a factor 4. Thus, we show that a model of localized states that was previously used to explain DC ≠ DS, can also be applied to epitaxial graphene systems without a carbon buffer layer. We attribute the effect to charge trap states in the interface between the graphene and the SiC. [ABSTRACT FROM AUTHOR]

CdS nanocrystals are attached on graphene nanosheets and their nonlinear optical properties are investigated by picosecond Z-scan technique at 532 nm. We found that synergistic effect between the graphene and CdS makes a major enhancement on the nonlinear optical absorption of graphene/ CdS nanohybrid in comparison with cooperative effect, and the synergistic improvement is restricted by nonradiative defects in hybrid. The synergistic mechanism involving the local field theory and charge transfer evolution is proposed. [ABSTRACT FROM AUTHOR]

Despite its superb electronic properties, the semi-metallic nature of graphene with no band gap (Eg) at the Dirac point has been a stumbling block for its industrial application. We report an improved means of producing a tunable band gap over other schemes by doping low energy (10 eV) potassium ions (K+) on single layer graphene formed on 6H-SiC(0001) surface, where the noble Dirac nature of the π-band remains almost unaltered. The changes in the π-band induced by K+ ions reveal that the band gap increases gradually with increasing dose (θ) of the ions up to Eg=0.65 eV at θ=1.10 monolayers, demonstrating the tunable character of the band gap. Our core level data for C 1s, Si 2p, and K 2p suggest that the K+-induced asymmetry in charge distribution among carbon atoms drives the opening of band gap, which is in sharp contrast with no band gap when neutral K atoms are adsorbed on graphene. This tunable K+-induced band gap in graphene illustrates its potential application in graphene-based nano-electronics. [ABSTRACT FROM AUTHOR]

The biggest challenge in the resistive random access memory (ReRAM) technology is that the basic operational parameters, such as the set and reset voltages, the current on-off ratios (hence the power), and their operational speeds, strongly depend on the active and electrode materials and their processing methods. Therefore, for its actual technological implementations, the unification of the operational parameters of the ReRAM devices appears to be a difficult task. In this letter, we show that by fabricating a resistive memory device in a thin film transistor configuration and thus applying an external gate bias, we can control the switching voltage very accurately. Taking partially reduced graphene oxide, the gate controllable switching is demonstrated, and the possible mechanisms are discussed. [ABSTRACT FROM AUTHOR]

A hybrid structure composed of Ag nanoparticles and graphene sheets has been synthesized by one-pot co-reduction process at room temperature. Compared to the isolated Ag nanoparticles, the presence of graphene enhanced the photoluminescence originating from the plasmon radiative decay of Ag nanoparticles as results of the charge injection and the near-field confinement resulting from the coupled graphene sheets. [ABSTRACT FROM AUTHOR]

Graphene devices have been constructed on silicon mirrors, and the graphene is optically probed through an evanescent wave interaction in an attenuated total reflectance configuration using an infrared spectrometer. The graphene is electrically biased in order to tune its optical properties. Exposure of the device to the chemicals iodine and ammonia causes observable and reversible changes to graphene's optical absorption spectra in the mid to near infrared range which can be utilized for the purpose of sensing. Electrical current measurements through the graphene are made simultaneously with optical measurements allowing for simultaneous sensing using two separate detection modalities. Our current results reveal sub-ppm detection limits for iodine and approximately 100 ppm detection limits for ammonia. We have also demonstrated that this approach will work at 1.55 μm, which opens up the possibility for graphene optical sensors that leverage commercial telecom light sources. [ABSTRACT FROM AUTHOR]

Epitaxial silicene, which forms spontaneously on ZrB2(0001) thin films grown on Si(111) wafers, has a periodic stripe domain structure. By adsorbing additional Si atoms on this surface, we find that the domain boundaries vanish, and a single-domain silicene sheet can be prepared without altering its buckled honeycomb structure. The amount of Si required to induce this change suggests that the domain boundaries are made of a local distortion of the silicene honeycomb lattice. The realization of a single domain sheet with structural and electronic properties close to those of the original striped state demonstrates the high structural flexibility of silicene. [ABSTRACT FROM AUTHOR]

Optical isolators are an important building block in photonic computation and communication. In traditional optics, isolators are realized with magneto-optical garnets. However, it remains challenging to incorporate such materials on an integrated platform because of the difficulty in material growth and bulky device footprint. Here, we propose an ultracompact integrated isolator by exploiting graphene's magneto-optical property on a silicon-on-insulator platform. The photonic nonreciprocity is achieved because the cyclotrons in graphene experiencing different optical spins exhibit different responses to counterpropagating light. Taking advantage of cavity resonance effects, we have numerically optimized a device design, which shows excellent isolation performance with the extinction ratio over 45 dB and the insertion loss around 12 dB at a wavelength near 1.55 μm. Featuring graphene's CMOS compatibility and substantially reduced device footprint, our proposal sheds light on monolithic integration of nonreciprocal photonic devices. [ABSTRACT FROM AUTHOR]

We report an observation of a scrolled graphene nanoribbon (sGNR) produced via a chemical vapor deposition. The sandwiched sGNR between Au nanoparticle and Au thin film system can be identified by the remarkable enhancement of G peak accompanied with a subsequent splitting (G+ and G-) with strong Radial Breading Like Mode enhancement. Because the weak adhesion force between graphene monolayer and target Au substrate during transfer maybe result in a sparse distribution of sGNR with a z-directional curvature-induced G peak splitting. Reproducibility and mass production with a nanometer scale circuit devices may be anticipated from this work. [ABSTRACT FROM AUTHOR]

We develop a high pressure-assisted (approximately 1000 kPa) transfer method to remove polymer residues and effectively reduce damages on the surface of graphene. By introducing an ethanol pre-dehydration technique and optimizing temperature, the graphene surface becomes nearly free of residues, and the quality of graphene is improved obviously when temperature reaches 140 °C. The graphene obtained using the high pressure-assisted transfer method also exhibits excellent electrical properties with an average sheet resistance of approximately 290 Ω /sq and a mobility of 1210 cm2/V•s at room temperature. Sheet resistance and mobility are considerably improved compared with those of the graphene obtained using the normal wet transfer method (average sheet resistance of approximately 510 ohm/sq and mobility of 750 cm2/V•s). [ABSTRACT FROM AUTHOR]

The spatial and spectral characteristics of mid-infrared thermal emission from devices containing a large area multilayer graphene layer, encapsulated using hexagonal boron nitride, have been investigated. The devices were run continuously in air for over 1000 h, with the emission spectrum covering the absorption bands of many important gases. An approximate solution to the heat equation was used to simulate the measured emission profile across the devices yielding an estimated value of the characteristic length, which defines the exponential rise/fall of the temperature profile across the device, of 40 μm. This is much larger than values obtained in smaller exfoliated graphene devices and reflects the device geometry, and the increase in lateral heat conduction within the devices due to the multilayer graphene and boron nitride layers. [ABSTRACT FROM AUTHOR]

Ethylene-dinitrobenzoyl (EDNB) linked to graphene oxide has been shown to improve the performance of graphene/polymer organic photovoltaics. Its binding conformation on graphene, however, is not yet clear, nor have its effects on work function and optical absorption been explored more generally for graphene quantum dots. In this report, we clarify the linkage of EDNB to GQDs from first principles and show that the binding of the molecule increases the work function of graphene, while simultaneously modifying its absorption in the ultraviolet region. [ABSTRACT FROM AUTHOR]

The properties of Few-Layer Graphene (FLG) change with the number of layers and Amplitude Modulation (AM) Atomic Force Microscopy (AFM) is commonly used to determine the thickness of FLG. However, AFM measurements have been shown to be sensitive to environmental conditions such as relative humidity (RH). In the present study, AM-AFM is used to measure the thickness and loss tangent of exfoliated graphene on silicon dioxide (SiO2) as RH is increased from 10% to 80%. We show that the measured thickness of graphene is dependent on RH. The loss tangent values of the graphene and oxide regions are both affected by humidity, with generally higher loss tangent for graphene than SiO2. As RH increases, we observe the loss tangent of both materials approaches the same value. We hypothesize that there is a layer of water trapped between the graphene and SiO2 substrate to explain this observation. Using this interpretation, the loss tangent images also indicate movement and change in this trapped water layer as RH increases, which impacts the measured thickness of graphene using AM-AFM. [ABSTRACT FROM AUTHOR]

Low temperature pulsed laser deposited (PLD) ultrathin boron nitride (BN) on SiO2 was investigated as a dielectric for graphene electronics, and a significant enhancement in electrical transport properties of graphene/PLD BN compared to graphene/SiO2 has been observed. Graphene synthesized by chemical vapor deposition and transferred on PLD deposited and annealed BN exhibited up to three times higher field effect mobility compared to graphene on the SiO2 substrate. Graphene field effect transistor devices fabricated on 5 nm BN/SiO2 (300 nm) yielded maximum hole and electron mobility of 4980 and 4200 cm2/V s, respectively. In addition, significant improvement in carrier homogeneity and reduction in extrinsic doping in graphene on BN has been observed. An average Dirac point of 3.5V and residual carrier concentration of 7.65x1011cm-2 was observed for graphene transferred on 5 nm BN at ambient condition. The overall performance improvement on PLD BN can be attributed to dielectric screening of charged impurities, similar crystal structure and phonon modes, and reduced substrate induced doping. [ABSTRACT FROM AUTHOR]

The dielectric functions of atomically thin graphene, silicene, and arsenene have been investigated as a function of temperature. With zero energy gap, more carriers in graphene and silicene are thermally excited as temperature increases and intraband transition strengthens, resulting in the strengthened absorption peak. Yet with large energy gap, interband transition dominates optical absorption of arsenene but it reduces as lattice vibration enhances, inducing the redshift and decreased absorption peak. To validate the theoretical method, the calculated optical constants of isolated graphene are compared with ellipsometry results and demonstrate good agreement. [ABSTRACT FROM AUTHOR]