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Macroscopic graphene paper is fabricated by an electrospray deposition approach, and the microstructure can be controlled from highly porous to highly compact geometries by varying deposition parameters including graphene colloid concentration and deposition rate. Free-standing graphene films can be separated from substrates via a simple water exfoliation method in which the surface properties of graphene films and substrates control film exfoliation. Specifically, water exfoliation can be achieved when the contact angle of substrates is 64° or below. Thermal and electrical conductivities of the macroscopic graphene paper upon thermal annealing are measured, enabling the establishment of the process-microstructure-property correlation beneficial for further development and property manipulation of graphene-based materials. [ABSTRACT FROM AUTHOR]

Lateral orientation and aggregation of the graphene sheets limited field enhancement of graphene paper (GP). To improve the field enhancement of GP, argon plasma treatment was induced to destroy the aggregation and cause formation of surface protrusions. After Ar plasma treatment, turn-on field and threshold field of GP were reduced from 2.3 V/μm to 1.6 V/μm and 4.4 V/μm to 3.0 V/μm, respectively. The enhancement was attributed to the protrusions. Scanning electron microscopy and hydrophobicity had been used to prove the morphology change after plasma treatment. [ABSTRACT FROM AUTHOR]

Graphene plasmons (GPs) are broadband and electrically tunable mid-infrared (MIR)/terahertz (THz) excitations, exhibiting high confinement factors exceeding two orders of magnitude. Such highly confined modes are extremely attractive for nonlinear frequency conversion owing to the large inherent field enhancement. However, this high confinement is also accompanied by losses, and together with the centrosymmetric nature of graphene practical usage of its properties in second-order nonlinear processes remains hindered. In this paper, we introduce an approach for realizing quasi-phase-matching (QPM) of propagating GPs, by placing the graphene on an orientationally patterned GaAs substrate—a transparent material in the MIR/THz range with a large second-order nonlinear coefficient. We analyze the complete frequency/Fermi-level space for QPMed second-harmonic generation of GPs in the MIR and THz and demonstrate GP amplification and loss compensation. We find that our approach provides extended GP propagation lengths that are more than twice larger than the state-of-the-art cryogenic temperature propagation lengths. The approach is general to all second-order nonlinear processes, such as sum and difference frequency generation, thus opening a path for efficient and electrically tunable QPM nonlinear processes at the atomic scale. [ABSTRACT FROM AUTHOR]

With the development of neuromorphic electronics, much effort has been devoted to expand perception, memory, and computing integration capabilities. In this paper, an ionic-based graphene synaptic transistor with long-gate structure has been investigated to mimic memory, learning function and perceive humidity. By harnessing the tunable in-plane-field transport of charge carriers in graphene and ions motion in ion-gel, this transistor mimics various synaptic functionalities, including inhibitory postsynaptic current, excitatory postsynaptic current, paired-pulse facilitation, long-term depression, and long-term potentiation. Under short pules stimuli, the long-gate structure provides our transistor with an inertial assisted re-accumulation, generating two excitatory postsynaptic current peaks and enhanced paired-pule facilitation up to ∼265%. Furthermore, the presence of the long-gate structure enables our transistor to exhibit excellent learning and simulate Ebbinghaus' memory. In addition, physical mechanic about its humidity perception has been analyzed and discussed. This study provides a unique platform for designing high-performance carbon-based artificial synapses enabling integrated functions of sensing, storage, and computation for the neuromorphic system. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate realization of printable radio frequency identification (RFID) antenna by low temperature processing of graphene ink. The required ultra-low resistance is achieved by rolling compression of binder-free graphene laminate. With compression, the conductivity of graphene laminate is increased by more than 50 times compared to that of as-deposited one. Graphene laminate with conductivity of 4.3 × 104S/m and sheet resistance of 3.8 Ω/sq (with thickness of 6 μm) is presented. Moreover, the formation of graphene laminate from graphene ink reported here is simple and can be carried out in low temperature (100 °C), significantly reducing the fabrication costs. A dipole antenna based on the highly conductive graphene laminate is further patterned and printed on a normal paper to investigate its RF properties. The performance of the graphene laminate antenna is experimentally measured. The measurement results reveal that graphene laminate antenna can provide practically acceptable return loss, gain, bandwidth, and radiation patterns, making it ideal for low cost printed RF applications, such as RFID tags and wearable wireless sensor networks. [ABSTRACT FROM AUTHOR]

In electrode preparation, graphene-based electrodes usually form a uniaxially oriented stacking structure, which limits the Na+ diffusion in graphene electrodes for sodium-ion batteries (SIBs). In this work, a crumpled graphene (CG) anode with graded interlayer spacings was synthesized, and CG exhibits high-rate performance for SIBs. The CG has a folding microstructure like a paper clip that facilitates rapid adsorption/desorption of Na+ on its surface so that the diffusion kinetics of Na+ could be improved. The x-ray absorption fine structure and transmission electron microscope proved the mechanism of Na+ co-adsorption and explained the reason for its high-rate performance. When the three-dimensional CG is used as the anode of SIBs, it has a high-rate performance of 146 mAh g−1 with the current density increasing to 1 A g−1, and CG still maintained about 79 mAh g−1 after 1000 cycles at 0.5 A g−1 with good structural stability. [ABSTRACT FROM AUTHOR]

Plasmons in graphene (Gr) show many fascinating characteristics, such as dynamic tunability, strong field confinement of light-matter interaction, and highly responsive, which has been widely exploited for a number of applications, including photodetectors, optical modulators, and sensors. In this paper, graphene plasmons (GPs) were motivated by implanting Au nanoparticles (Au NPs) into Ta2O5 thin layers adjacent to the Gr film, and the strong localized surface plasmon resonance (LSPR) effect has been proposed and demonstrated by placing the GPs structure on a III–V semiconductor quantum well saturable absorber (SA). It has been substantiated that the heightened interaction between light and Gr via LSPR predominantly occurs through the mechanisms of resonant energy transfer and local electromagnetic field enhancement, rather than direct electron transfer. Significant improvement on the nonlinear characteristics of the GPs modulated III–V semiconductor SA has been observed with a 17.1% large modulation depth and obviously improved working stability. A 1550 nm passive mode-locked laser has been successfully constructed with a pulse width down to 523 fs by integrating the SA into the laser cavity. This work lays the foundation for the development of high-performance mode-locked lasers and also demonstrates the substantial enhancement of nonlinear optical properties of various materials not limited to III–V semiconductors provided by this GPs' modulated structure; hence, these findings offer extensive prospects for applications in various photonics and optoelectronic devices. [ABSTRACT FROM AUTHOR]

The search for materials with high thermal resistance has promising applications in thermoelectric devices and boiling crisis retardation. In this paper, we study the interfacial heat transfer between water and gold, nanostructuring the gold surface and coating it with graphene. By trapping air (or vacuum in our simulations) between graphene and the nanopatterned surface, we observe a considerable increase in the interfacial resistance compared to the planar gold situation, which is shown to scale with the effective graphene–gold contact surface for both monolayer and multilayer graphene. With the massive thermal resistances we predict (up to 200 nm in terms of Kapitza length), the system proposed here represents a robust alternative to superhydrophobic Cassie materials. Moreover, since the low thermal conductance is achieved primarily due to geometry (vacuum trapping), it is straightforward to extend our results to any material with a structure equivalent to that of the nanopatterned gold wall considered here. [ABSTRACT FROM AUTHOR]

Due to its unique features, laser-induced graphene (LIG) can be considered as disruptive technology for creating a few-layer graphene-based film that received much attention in the field of flexible electronics. Among all, energy storage, catalysis, sensing, and separation are the main applications that have been investigated in recent years with large improvements in the respective device performance. In particular, miniaturized supercapacitor—usually called a micro-supercapacitor (μSC)—is the most investigated field in which LIG can strongly provide outstanding results concerning the state of the art simplification of the fabrication procedure and intrinsically allowing the flexibility of the device. However, many open points still limit the possible full exploitation of this technology in the energy storage sector. This paper provides a concise overview of the LIG application in μSCs suggesting where the community should direct efforts to enhance the results together with associated challenges. [ABSTRACT FROM AUTHOR]

The direct coupling of material properties across a nanoscale interface is a promising route to achieve the functionality unavailable in bulk materials. Graphene is a kind of sp2 hybridized carbon monolayer and has been investigated in many applications due to its high charge-carrier mobility. In this paper, a type of enhanced light-induced resistance effect (LRE) is observed in the structure of Ag/graphene/n-type Si. This effect features a remarkable linear resistance change with a sensitivity of 4.39 kΩ/mm when a laser moves along the surface of the structure. With the optimal thickness of the Ag film, the resistance change ratio of LRE can reach 472%, which is significantly higher than the Ag/Si control sample (6.4%), showing an obvious graphene-induced enhancement. Photocarriers' diffusion and recombination at the heterojunction interface are crucial for the enhancement. These findings offer an effective way to study the carrier dynamics at the heterojunction interface and will be useful in the development of graphene-based optoelectronic devices, such as laser-controlled variable resistors, laser-induced diodes, and storage devices. [ABSTRACT FROM AUTHOR]

Piezoelectricity of two-dimensional layered materials is of great significance for electromechanical coupling applications in nanoelectromechanical systems. The nanoscale devices based on graphene have attracted intense interest due to its excellent electronic and mechanical properties. However, the centrosymmetric crystal structure in intrinsic graphene severely restricts its applications in mechanical sensors, transducers, and energy harvesters. In this paper, a self-powered vibration sensing element based on rolled-up single-layer graphene is proposed and demonstrated, which realizes the conversion from mechanical vibrations into electrical signals. In contrast to previous pioneering works requiring silica cavities or Si/SiO2 calibration grating substrate to support the graphene, a three-dimensional (3D) non-closed tubular structure is adopted to develop the piezoelectricity in single-layer graphene, where the inversion symmetry of single-layer graphene is broken via self-rolled-up process induced strain. Hence, apparent piezoresponse from the 3D non-closed tubular graphene field effect transistor is observed. Moreover, a peak-to-peak amplitude for the piezoelectric current up to 4.2 is achieved, corresponding to the periodicity of mechanical vibration, and the value can increase to 10.8 nA by applying a small source–drain voltage of 6 mV. The device is also sensitive to mechanical vibration with different frequencies and shows similar currents. The electromechanical coupling in rolled-up graphene provides a basis for the applications in sensing, actuating, and energy harvesting. [ABSTRACT FROM AUTHOR]

We demonstrate two-dimensional mapping of current flow in graphene devices by using a single-spin scanning magnetometer based on a nitrogen-vacancy defect center in diamond. We first image the stray magnetic field generated by the current and then reconstruct the current density map from the field data. We focus on the visualization of current flow around a small sized current source of ∼500 nm diameter, which works as an effective point contact. In this paper, we study two types of point-contacted graphene devices and find that the overall current profiles agree with the expected behavior of electron flow in the diffusive transport regime. This work could offer a route to explore interesting carrier dynamics of graphene including ballistic and hydrodynamic transport regimes. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate an ultraviolet photodetector (UV-PD) that uses coalesced gallium nitride (GaN) nanorods (NRs) on a graphene/Si (111) substrate grown by plasma-assisted molecular beam epitaxy. We report a highly sensitive, self-powered, and hybrid GaN NR/graphene/Si (111) PD with a relatively large 100 mm2 active area, a high responsivity of 17.4 A/W, a high specific detectivity of 1.23 × 1013 Jones, and fast response speeds of 13.2/13.7 μs (20 kHz) under a UV light of 355 nm at zero bias voltage. The results show that the thin graphene acts as a perfect interface for GaN NRs, encouraging growth with minimum defects on the Si substrate. Our results suggest that the GaN NR/graphene/Si (111) heterojunction has a range of interesting properties that make it well-suited for a variety of photodetection applications. [ABSTRACT FROM AUTHOR]

Realizing the reliable modulation of the carrier type and density in graphene is a challenging task, especially for epitaxial graphene (EG) grown in situ on SiC via thermal decomposition. In this process, doping is seldom adopted for substrate pretreatment, and the residual carriers in EG are determined by the properties of the SiC substrate and the random defects in EG. Here, we demonstrate a simple and effective technique for hole doping in EG on the Si-face of SiC (0001) implanted with boron ions. P-type EG with a carrier concentration of approximately 1–2 × 1012 cm−2 and a room-temperature mobility of 100–300 cm2/V s was achieved at the boron ion dose of approximately 1 × 1015 cm−2 at a centroid depth of approximately 140 nm from the SiC surface. These transport properties are comparable with the data reported for unintentionally doped P-type EG on the Si-face of SiC [Tedesco et al., Appl. Phys. Lett. 95, 122102 (2009)], for which the Hall device size was 40 times smaller compared to our device. The formation of P-type EG on the Si-face of SiC (0001) in this study was attributed to the synergetic effects of boron doping in EG and the transfer of electrons from EG into SiC. The detailed mechanisms are studied and discussed in this paper. The method demonstrated herein can be universally applied to downshift the Fermi level of EG on SiC, regardless of the crystallization orientation or polytype. The developed method is also compatible with modern semiconductor procedures. [ABSTRACT FROM AUTHOR]

Graphene superlattices have recently been attracting growing interest as an emergent class of quantum metamaterials. In this paper, we report the observation of nonlocal transport in bilayer graphene (BLG) superlattices encapsulated between two hexagonal boron nitride (hBN) layers, which formed hBN/BLG/hBN moiré superlattices. We then employed these superlattices to detect a long-range charge-neutral valley current using an all-electrical method. The moiré superlattice with broken inversion symmetry leads to a "hot spot" at the charge-neutral point (CNP), and it harbors satellites of the CNP. We observed nonlocal resistance on the order of 1 kΩ, which obeys a scaling relation. This nonlocal resistance evolves from an analog of the quantum Hall effect but without magnetic field/time-reversal symmetry breaking, which is associated with a hot-spot-induced topological valley current. This study should pave the way for developing a Berry-phase-sensitive probe to detect hot spots in gapped Dirac materials with inversion-symmetry breaking. [ABSTRACT FROM AUTHOR]

Cold field emission sources are capable of providing highly coherent and bright electron beams. However, they suffer from some well-known practical difficulties that have prevented their widespread use: unmanageably stringent ultrahigh vacuum requirements, relatively large current instabilities, and rapid emission decays in periods as short as 1–2 h, requiring regular flashing (Joule heating). This paper presents the microfabrication and a successful emission test of a micron-sized concentric ring-graphene cathode cold field emitter, without any observable wall collapse. The cathode is designed to have a nanometer size ring-edge, while its radius can measure several microns or larger, providing stable electron emission under high vacuum conditions. The turn-on electric field of a ring-cathode source can be dramatically lowered by introducing a thin layer of nickel nanoparticles by an in-situ focused ion beam process. The dependence of field enhancement on the anode-cathode distance and the ring-cathode radius has been systematically studied. [ABSTRACT FROM AUTHOR]

In this paper, the dependence of thermal conductivities of polycrystalline few-layer graphene (PFG) on the layer number and in-plane strain is systematically studied using the reverse non-equilibrium molecular dynamics method. The thermal conductivities are shown to decrease with the increased layer number, but the dependency on the layer number is anomalously less than what has been observed in single-crystalline few-layer graphene. Through a detailed analysis of the in-plane and out-of-plane phonon density of states, it is found that such a weak dependence can be explained by an interlayer/intralayer coupling mechanism, which is sensitive to the presence of grain boundaries. It is also found that this coupling mechanism can also be greatly influenced by in-plane tension, which indicates that thermal conductivities of PFG can be manipulated using in-plane tensile strains in addition to grain boundaries. The present study provides fundamental understanding and valuable guidelines for the design of graphene based flexible devices for efficient thermal management. [ABSTRACT FROM AUTHOR]

The paper presents the experimental and modeling results of a microwave slot antenna in a coplanar configuration based on graphene. The antennas are fabricated on a 4 in. high-resistivity Si wafer, with a ~300 nm SiO2 layer grown through thermal oxidation. A CVD grown graphene layer is transferred on the SiO2. The paper shows that the reflection parameter of the antenna can be tuned by a DC voltage. 2D radiation patterns at various frequencies in the X band (8-12 GHz) are then presented using as antenna backside a microwave absorbent and a metalized surface. Although the radiation efficiency is lower than a metallic antenna, the graphene antenna is a wideband antenna while the metal antennas with the same geometry and working at the same frequencies are narrowband. [ABSTRACT FROM AUTHOR]

We show that reversible two- and three-input logic gates, such as the universal Toffoli gate, can be implemented with three tilted gate electrodes patterned on a monolayer graphene flake. These reversible gates are based on the unique properties of ballistic charge carriers in graphene, which induce bandgaps in transmission for properly chosen potential barriers. The enhanced architectures for reversible logic gate implementation proposed in this paper offer a remarkable design simplification compared to standard approaches based on field-effect transistor circuits, as well as potential high-frequency operation. [ABSTRACT FROM AUTHOR]

This paper presents graphene field-effect transistor (GFET) based pressure sensors for tactile sensing. The sensing device comprises GFET connected with a piezoelectric metal-insulator-metal (MIM) capacitor in an extended gate configuration. The application of pressure on MIM generates a piezo-potential which modulates the channel current of GFET. The fabricated pressure sensor was tested over a range of 23.54–94.18 kPa, and it exhibits a sensitivity of 4.55 × 10−3 kPa−1. Further, the low voltage (∼100 mV) operation of the presented pressure sensors makes them ideal for wearable electronic applications. [ABSTRACT FROM AUTHOR]

In this paper, we numerically and experimentally demonstrate that few-layer Chemical Vapour Deposition graphene can be employed for the fabrication of fully optical transparent antennas for microwave applications. We show that planar graphene-based antennas, having a size of tens of square centimeters, can achieve relatively high gain over a wide operating bandwidth (>3.5 GHz) simultaneously covering the GPS, WiFi, Bluetooth, and 5G bands. The measured 3D radiation patterns show dipole-, quadruple-, and hexapole-behavior. These findings open up routes for the realization of innovative devices where “invisible and hidden” antennas could be integrated in smart windows or photovoltaic systems, fostering configurations for camouflage, and communications systems. Furthermore, the possibility to handle different radiation patterns could allow the engineering of complex systems such as antenna arrays devoted to beam-steering, beam-forming, and healthcare applications. Finally, combining graphene transparency and flexibility could also pave the way for the realization of wearable devices, demanding invisibility, which operate on the surface of the human body or can be integrated in transparent devices (for example, in contact lenses) reducing their invasiveness. [ABSTRACT FROM AUTHOR]

We theoretically investigate the ballistic thermoelectric performance of graphene nanoribbons with surface roughness using the nonequilibrium Green's function method. The results show that the surface roughness could dramatically reduce the thermal conductance of graphene nanoribbons, and thus lead to the boosting of thermoelectric performance of graphene (the figure of merit can be as high as 3.7 at room temperature). Meanwhile, the electron transport properties of different edged rough graphene nanoribbons exhibit distinctive anisotropic behaviors, i.e., the thermal power of armchair edged nanoribbons significantly increases, while that of zigzag edged remains nearly unchanged, which is mainly attributed to the edge effect. The findings presented in this paper qualify surface roughness as an efficient approach to enhance the thermoelectric performance of graphene nanoribbons. [ABSTRACT FROM AUTHOR]

Van der Waals heterostructures have exhibited interesting physical properties. In this paper, heat transfer in hybrid coplanar bilayer/monolayer (BL-ML) graphene, as a model layered van der Waals heterostructure, was studied using non-equilibrium molecular dynamics (MD) simulations. The temperature profile and inter- and intra-layer heat fluxes of the BL-ML graphene indicated that, there is no fully developed thermal equilibrium between layers and the drop in the average temperature profile at the step-like BL-ML interface is not attributable to the effect of Kapitza resistance. By increasing the length of the system up to 1 μm in the studied MD simulations, the thermally non-equilibrium region was reduced to a small area near the step-like interface. All MD results were compared to a continuum model and a good match was observed between the two approaches. Our results provide a useful understanding of heat transfer in nano- and micro-scale layered materials and van der Waals heterostructures. [ABSTRACT FROM AUTHOR]

Graphene has attracted enormous attention due to its extraordinary physical properties, which have potential for increasing the thermal conductivity of nanocomposites or nanofluids, and the thermal resistance between graphene and the surrounding matrices arises as an important issue. In this paper, the thermal transport at the graphene-water interface is investigated by molecular dynamics simulations. The interfacial thermal resistance decreases with the graphene layer number. Interlayer functionalization by oxygen atoms is applied to tune the interfacial thermal resistance. A peak thermal resistance reduction of nearly 50% is generated with the oxygen ratio of only 0.5% for two-layer graphene. Based on the analyses of vibrational density of states, it is found that lower thermal resistance is consistent with more vibrational density of states overlaps at the interface. Our results are instructive for improving the interfacial thermal transport in graphene-based nanocomposites and nanofluids. [ABSTRACT FROM AUTHOR]

A graphene-Si junction is an attractive system as it is both CMOS-compatible and representative of very interesting van der Waals (vdW) heterostructures. In this paper, the full spectral photoresponse of the graphene-Si heterojunction is investigated in the photovoltaic mode by using Fourier transform infrared photocurrent spectroscopy. Two photoresponse bands at 980 nm and 1550 nm are measured, which are attributed to the photocarrier generations in Si and in the graphene-Si vdW junction, respectively. Peak detectivities of the Si and the vdW junction photoresponses are measured to be 1.3×109 cm Hz1/2/W and 1.3×108 cm Hz1/2/W, respectively. The band diagram of the heterojunction suggests an indirect spatial transfer process from graphene to silicon. The results are indicative of great potential of the graphene-Si vdW junction for photodetection in the infrared region. [ABSTRACT FROM AUTHOR]

To extend the frequency range of transistors into the terahertz domain, new transistor technologies, materials, and device concepts must be continuously developed. The quality of the interface between the involved materials is a highly critical factor. The presence of impurities can degrade device performance and reliability. In this paper, we present a method that allows the study of the charge carrier velocity in a field-effect transistor vs impurity levels. The charge carrier velocity is found using high-frequency scattering parameter measurements followed by delay time analysis. The limiting factors of the saturation velocity and the effect of impurities are then analysed by applying analytical models of the field-dependent and phonon-limited carrier velocity. As an example, this method is applied to a top-gated graphene field-effect transistor (GFET). We find that the extracted saturation velocity is ca. 1:4 × 107 cm/s and is mainly limited by silicon oxide substrate phonons. Within the considered range of residual charge carrier concentrations, charged impurities do not limit the saturation velocity directly by the phonon mechanism. Instead, the impurities act as traps that emit charge carriers at high fields, preventing the current from saturation and thus limiting power gain of the GFETs. The method described in this work helps to better understand the influence of impurities and clarifies methods of further transistor development. High quality interfaces are required to achieve current saturation via velocity saturation in GFETs. [ABSTRACT FROM AUTHOR]

In this paper, we have developed a high-performance graphene electrothermal actuator (ETA). The fabrication method is easy, fast, environmentally friendly, and suitable for preparing both large-size and miniature graphene ETAs. When applied with the driving voltage of 65 V, the graphene ETA achieves a large bending angle of 270° with a fast response of 8 s and the recovery process costs 19 s. The large bending deformation is reversible and can be precisely controlled by the driving voltage. A simple robotic hand prepared by using a single graphene ETA can hold the object, which is more than ten times the weight of itself. By virtue of its large-strain, fast response, and easy-to-manufacture, we believe that the graphene ETA has tremendous potential in extensive applications involving biomimetic robotics, artificial muscles, switches, and microsensors in both macroscopic and microscopic fields. [ABSTRACT FROM AUTHOR]

Existing thermoacoustic devices are based on a complicated fabrication process, which extremely limits their practical applications. In this paper, we realize a flexible graphene sound device based on laser reduced graphene. The graphene oxide is converted into graphene by a 450 nm laser with a one-step process. The performance of the graphene sound device is affected by the laser power, the scanning speed, and the substrate thickness. The experimental results match well with the theoretical results. Besides, the sound device has the advantages of excellent flexibility, broad frequency spectrum (0-40 kHz), fast fabrication process, and low cost, which will become a promising alternative in the flexible electronic systems in the future. [ABSTRACT FROM AUTHOR]

The synthesis of piezoelectric two-dimensional (2D) materials is very attractive for implementing advanced energy harvesters and transducers, as these materials provide enormously large areas for the exploitation of the piezoelectric effect. Among all 2D materials, molybdenum disulfide (MoS2) has shown the largest piezoelectric activity. However, all research papers in this field studied just a single material, and this may raise concerns because different setups could provide different values depending on experimental parameters (e.g., probes used and areas analyzed). By using conductive atomic force microscopy, here we in situ demonstrate that the piezoelectric currents generated in MoS2 are gigantic (65 mA/cm2), while the same experiments in graphene just showed noise currents. These results provide the most reliable comparison yet reported on the piezoelectric effect in graphene and MoS2. [ABSTRACT FROM AUTHOR]

Injection pumping of the pristine graphene is a promising alternative to optical pumping, but it is not obvious how to properly combine electronic and plasmonic features in one and the same device in order to obtain the terahertz plasmon amplification in graphene with injection pumping. This paper shows that an amplified plasmon mode can be effectively excited at the plasmon resonance frequencies in an active ungated region graphene with injecting (absorbing) gated parts of the periodic graphene structure. Necessary conditions for the plasmon lasing in the structure are found. [ABSTRACT FROM AUTHOR]

Research on graphene/hexagonal boron nitride (h-BN) heterostructures has attracted much attention for band engineering and device performance optimization of graphene. However, the growth of graphene/h-BN heterostructure is still challenging, which usually requires high growth temperature and long growth duration. In this paper, we demonstrate graphene/h-BN heterostructures by growing graphene onto the substrates which consist of exfoliated h-BN flakes on Co thin films using molecular beam epitaxy. The heterostructure samples grown at different temperatures and growth times were characterized by Raman, optical microscopy, atomic force microscopy, microwave impedance microscopy, and scanning tunneling microscopy. It is found that the graphene/h-BN heterostructures were formed by the formation of graphene underneath rather than on top of the h-BN flakes. The growth mechanism is discussed. [ABSTRACT FROM AUTHOR]

Most applications of graphene need a suitable support substrate to present its excellent properties. But transferring graphene onto insulators or growing graphene on foreign substrates could cause properties diminishing. This paper reports the graphene growth directly out of diamond (111) by B doping, guided by first-principles calculations. The spontaneous graphene formation occurred due to the reconstruction of the diamond surface when the B doping density and profile are adequate. The resulting materials are defect free with high phase purity/carrier mobility, controllable layer number, and good uniformity, which can be potentially used directly for device fabrication, e.g., high-performance devices requiring good thermal conductivity. [ABSTRACT FROM AUTHOR]

In this paper, we studied stacked mL+nL graphene layers using Raman scattering spectroscopy. Our results indicate that the 2D band from stacked graphene can be considered as a superposition of those from the constituent nL and mL graphene layers, and a blueshift in the 2D band is observed when n or m=1. The blueshift increases with the number of stacked layers and can be well understood by the reduction of Fermi velocity in the single layer graphene, as studied in the 1L+1L (or twisted bilayer) case. As the number of stacked layers changes from 1 to 5, the Fermi velocity in the single layer graphene reduces to about 85% of its initial value. This study shows a convenient way to realize the modification of the Fermi velocity in free-stacking graphene and is of significance to the applications of graphene-based heterostructures.plications of graphene-based heterostructures. [ABSTRACT FROM AUTHOR]

In this paper, we propose a graphene rectenna that encompasses two distinct functions in a single device, namely, antenna and rectifier, which till now were two separate components. In this way, the rectenna realizes an efficient energy harvesting technique due to the absence of impedance mismatch between antenna and diode. In particular, we have obtained a maximum conversion efficiency of 58.43% at 897GHz for the graphene rectenna on n-doped GaAs, which is a very good value, close to the performance of an RF harvesting system. A comparison with a classical metallic antenna with an HfO2-based metal-insulator-metal diode is also provided. [ABSTRACT FROM AUTHOR]

In this paper, we report single layer graphene (SLG) hybridized with silver nanoplates, in which nanoplates act as either a charge doping or a field enhancement source for the SLG Raman spectrum. Surprisingly, the stiffening of both G and 2D peaks of more than 10 cm-1 was observed with no plasmonic enhancement of peaks, indicating that p-doping from nanoplates on SLG is occurring. Such observation is explained in terms of the contact separation distance between the graphene and the silver nanoplates being enough (~4Å ) to cause a Fermi level shift in graphene to allow p-doping. When nanoplates were modified in shape with laser irradiation by either photothermal plasmon printing or laser induced ablation, the charge doping was lifted and the strong plasmonic enhancement of Raman signals was observed, indicating that the separation distance is increased. Further, when the nanoplates are oxidized, the two effects on the Raman bands of SLG are turned off, returning the Raman signals back to the original SLG state. [ABSTRACT FROM AUTHOR]

Graphene nanostructures are potential building blocks for nanoelectronic and spintronic devices. However, the production of monolayer graphene nanostructures with well-defined zigzag edges remains a challenge. In this paper, we report the patterning of monolayer graphene nanostructures with zigzag edges on hexagonal boron nitride (h-BN) substrates by an anisotropic etching technique. We found that hydrogen plasma etching of monolayer graphene on h-BN is highly anisotropic due to the inert and ultra-flat nature of the h-BN surface, resulting in zigzag edge formation. The as-fabricated zigzag-edged monolayer graphene nanoribbons (Z-GNRs) with widths below 30 nm show high carrier mobility and width-dependent energy gaps at liquid helium temperature. These high quality Z-GNRs are thus ideal structures for exploring their valleytronic or spintronic properties. [ABSTRACT FROM AUTHOR]

The control of tunnel contact resistance is of primary importance for semiconductor-based spintronic devices. This control is hardly achieved with conventional oxide-based tunnel barriers due to deposition-induced interface states. Manipulation of single 2D atomic crystals (such as graphene sheets) weakly interacting with their substrate might represent an alternative and efficient way to design new heterostructures for a variety of different purposes including spin injection into semiconductors. In the present paper, we study by x-ray photoemission spectroscopy the band alignments and interface chemistry of iron-graphene-hydrogenated passivated silicon (001) surfaces for a low and a high n-doping concentration. We find that the hydrogen passivation of the Si(001) surface remains efficient even with a graphene sheet on the Si(001) surface. For both doping concentrations, the semiconductor is close to flat-band conditions which indicates that the Fermi level is unpinned on the semiconductor side of the Graphene/Si(001):H interface. When iron is deposited on the graphene/Si(001):H structures, the Schottky barrier height remains mainly unaffected by the metallic overlayer with a very low barrier height for electrons, a sought-after property in semiconductor based spintronic devices. Finally, we demonstrate that the graphene layer intercalated between the metal and semiconductor also serves as a protection against iron-silicide formation even at elevated temperatures preventing from the formation of a Si-based magnetic dead layer. [ABSTRACT FROM AUTHOR]

Rectifying properties of graphene-semiconductor junctions depend on the Schottky barrier height. We report an enhanced barrier lowering in graphene-Si junction and its essential doping dependence in this paper. The electric field due to ionized charge in n-type Si induces the same type doping in graphene and contributes another Schottky barrier lowering factor on top of the image force-induced lowering (IFIL). We confirm this graphene-doping-induced lowering (GDIL) based on well reproductions of the measured reverse current of our fabricated graphene-Si junctions by the thermionic emission theory. Excellent matching between the theoretical predictions and the junction data of the doping-concentration dependent barrier lowering serves as another evidence of the GDIL. While both GDIL and IFIL are enhanced with the Si doping, GDIL exceeds IFIL with a threshold doping depending on the as-prepared graphene itself. [ABSTRACT FROM AUTHOR]

The paper studied preparation and formation mechanism of free-standing 3D graphene nanoribbons (GNRs) from graphite by pulsed laser ablation in liquid. The method to fabricate freestanding graphene nanoribbons directly was simple and controllable, which does not need other precursor materials and has no byproducts. Prepared graphene nanoribbons are shown composed of up to 14 layers of graphene, spaced about 0.30-0.35 nm and have a length of hundreds of nanometers. Formation mechanism of graphene nanoribbons was proposed based on the interaction between laser and material which can be demonstrated that the exfoliation of GNRs is a carbon plasma collision connecting-graphene segments-graphene sheets-multilayer graphene-graphene nanoribbons process. The high degree of repeatability and particularity found in the obtained GNRs might suggest their unique advantages and potential applications in nano-devices and spin electronics. [ABSTRACT FROM AUTHOR]

From sensing perspective, smaller electromechanical devices, in general, are expected to be more responsive to the stimuli. This enhanced performance, however, is contingent upon the noise sources remaining unchanged and the onset of nonlinear behavior not being precipitated by miniaturization. In this paper, we study the effect of strain on the nonlinearities and dynamic range in graphene nanoresonators. The dynamic response and the onset of nonlinearity in these devices are sensitive both to the electrostatic field used to actuate the device and the strain. By tuning the strain of the device by two orders of magnitude, we observe an enhancement of 25 dB in the dynamic range leading to a mass resolution of 100 yoctogram. The increase in dynamic range in our devices is modeled as a combined effect of strain and partial cancellation of elastic and electrostatic nonlinearities. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the hyperelastic tensile behaviour of single layer graphene sheets (SLGSs). A one-term incompressible Ogden-type hyperelastic model is chosen to describe the mechanical response of C-C bonds. By establishing equality between the Ogden strain-energy and the variation of the Tersoff-Brenner interatomic potential, three different geometries of SLGSs are studied under tensile loading. We compute the Young's modulus, the finite-deformation Poisson's ratio, ultimate strains, total reactions, and the variation of the potential energy per carbon atom for large strains. Numerical simulations are compared with results obtained by molecular mechanics and molecular dynamics simulations, finite elements, continuum mechanics theory, and experiments. Our predictions are validated, revealing the potential predictive capabilities of the present hyperelastic framework for the analysis of graphene in the context of infinitesimal and large deformations. The good agreement found between our calculations and the published data suggests that graphene may be described as a hyperelastic material. [ABSTRACT FROM AUTHOR]

Graphene exhibits wavelength-independent, saturable optical absorption with fast response time, and large modulation depth. Thus, it is an attractive material for the saturable absorption of fiber lasers. In this paper, we report a simple method for the in-situ monitoring of the deposition of few-layers graphene in an optical fiber end by mechanical exfoliation. Saturable absorbers with different number of graphene layers (from 4 layers of graphene to few 10 s of layers) are prepared and low threshold, self-starting passive mode-locked operation of a fiber laser with sub-picosecond pulse duration is demonstrated. [ABSTRACT FROM AUTHOR]

The term ambipolar has been used extensively in association with carbon nanotube and graphene-based field effect transistors, often in a varied manner, leading to a confused understanding of the term. Through the use of established scientific definitions and theoretical work on device operation, this paper attempts to clarify the understanding of the term and present a discussion of its appropriate use. [ABSTRACT FROM AUTHOR]

In this paper, graphene/mica and graphene/SiO2 based ammonia gas sensors are compared. It is found that adsorbed NH3 molecules result in up-shifting of the Fermi level in graphene, leading to a significant increase in graphene resistance. In comparison with SiO2 supporting substrate, the mica supporting substrate is found to induce more p-doping in graphene, in favour of NH3 molecule adsorption, yielding a high sensitivity. These findings suggest that the substrate plays an important role in mediating the interaction between graphene and NH3 molecules and that mica can be an excellent underlying substrate for graphene for ammonia gas detection. [ABSTRACT FROM AUTHOR]

Graphene, a well-known two-dimensional sheet, has attracted strong interest for both fundamental studies and applications. Due to its high intrinsic thermal conductivity, graphene has many potential applications in thermal management, such as in heat spreaders and flexible heaters. In this paper, a graphene-based transparent flexible heat conductor for nanophotonic integrated devices is demonstrated. The graphene heat conductor is designed to deliver heat from a non-local traditional metal heater to nanophotonic integrated devices for realizing efficient thermal tuning. With the present graphene heat conductor, a thermally tuning silicon Mach-Zehnder interferometer and micro-disk have been realized with good performance in terms of heating efficiency and temporal response. This indicates that the present graphene-based transparent flexible heat conductor provides an efficient and beneficial heating method for thermally tuning nanophotonic integrated devices. [ABSTRACT FROM AUTHOR]

This paper studies the performance degradation of graphene field-effect transistors due to the presence of traps. The mobile charge modulation by gate voltage is degraded because of immobile trapped charges. As a result the current is reduced and the on/off ratio is decreased. Extracted mobility using transconductance method is shown to be underestimated considerably due to the effect of traps. [ABSTRACT FROM AUTHOR]

In this paper, heat flux in graphene nanoribbons has been studied by using molecular dynamics simulations. It is found that the heat flux runs preferentially along the direction of decreasing width, which demonstrates significant thermal rectification effect in the asymmetric graphene ribbons. The dependence of rectification ratio on the vertex angle and the length are also discussed. Compared to the carbon nanotube based one-dimensional thermal rectifier, graphene nanoribbons have much higher rectification ratio even in large scale. Our results demonstrate that asymmetric graphene ribbon might be a promising structure for practical thermal (phononics) device. [ABSTRACT FROM AUTHOR]

In this paper, we investigate on the capacitance behavior of radio-frequency interdigital capacitor with single- and multi-layer graphenes, 5-μm wide and 20-μm long, grown by chemical vapor deposition. From the obtained results, the self-resonance frequency of the capacitor alone is unaffected by adding single- and multi-layer graphenes. However, the capacitor with single-layer graphenes shows a lower capacitance peak compared to the two other configurations due to an increase of overall resistance by the single-layer graphene. As a result, we find that the performance of the overall capacitor can be dependent on the resistance of graphenes. [ABSTRACT FROM AUTHOR]

Graphite lubrication is not effective in vacuum, and the failure mechanism is still under debate. Here, we show that graphene as two-dimensional (2D) "graphite paper" can overcome this shortcoming of graphite. Graphene exhibits stable super-low friction in a vacuum environment at the engineering scale because it can self-form a highly ordered lamellar structure on the sliding interface during the friction process owing to its unique 2D nano-effects. Experimental observation of the layer-slip phenomenon on the low-energy-state outside layers provides direct evidence to understand the lubrication mechanism of graphitic materials. [ABSTRACT FROM AUTHOR]

In this paper, we report the alleviation of the Fermi-level pinning on metal/n-germanium (Ge) contact by the insertion of multiple layers of single-layer graphene (SLG) at the metal/n-Ge interface. A decrease in the Schottky barrier height with an increase in the number of inserted SLG layers was observed, which supports the contention that Fermi-level pinning at metal/n-Ge contact originates from the metal-induced gap states at the metal/n-Ge interface. The modulation of Schottky barrier height by varying the number of inserted SLG layers (m) can bring about the use of Ge as the next-generation complementary metal-oxide-semiconductor material. Furthermore, the inserted SLG layers can be used as the tunnel barrier for spin injection into Ge substrate for spin-based transistors. [ABSTRACT FROM AUTHOR]