Search

The mechanical characteristics of graphene ribbons with an attached proof mass that can be used as NEMS transducers have been minimally studied, which hinders the development of graphene-based NEMS devices. Here, we simulated the mechanical characteristics of graphene ribbons with an attached proof mass using the finite element method. We studied the impact of force, residual stress, and geometrical size on displacement, strain, resonant frequency, and fracture strength of graphene ribbons with an attached proof mass. The results show that the increase of width and thickness of graphene ribbons would result in a decrease of the displacement and strain but also an increase of resonant frequency. The increase of the length of graphene ribbons has an insignificant impact on the strain, but it could increase the displacement and decrease the resonant frequency. The increase of residual stress in the graphene ribbons decreases its strain and displacement. The estimated fracture strength of graphene shows limited dependence on its thickness, with an estimated value of around 148 GPa. These findings contribute to the understanding of the mechanical characteristics of graphene ribbons with an attached proof mass and lay the solid foundation for the design and manufacture of high-performance graphene-based NEMS devices such as accelerometers. [ABSTRACT FROM AUTHOR]

Quantifying the intrinsic properties of 2D materials is of paramount importance for advancing their applications. Large‐scale production of 2D materials merits the need for approaches that provide direct information about the role of growth substrate on 2D material properties. Transferring the 2D material from its growth substrates can modify the intrinsic properties of the asgrown 2D material. In this study, suspended chemical vapor deposition (CVD) graphene films are prepared directly on their growth substrates in a high‐density grid array. The approach facilitates the quantification of intrinsic strain and doping in suspended CVD graphene films. To achieve this, transmission electron microscopy and large‐area Raman mapping are employed. Remarkably, the analysis reveals consistent patterns of compressive strain (≈−0.2%) both in the diffraction patterns and Raman maps obtained from these suspended graphene films. By conducting investigations directly on the growth substrates, the potential influences introduced during the transfer process are circumvented effectively. Consequently, the methodology offers a robust and reliable means of studying the intrinsic properties of 2D materials in their authentic form, uninfluenced by the transfer‐induced alterations that may skew the interpretation of their properties. [ABSTRACT FROM AUTHOR]

Abstract Quantifying the intrinsic properties of 2D materials is of paramount importance for advancing their applications. Large‐scale production of 2D materials merits the need for approaches that provide direct information about the role of growth substrate on 2D material properties. Transferring the 2D material from its growth substrates can modify the intrinsic properties of the asgrown 2D material. In this study, suspended chemical vapor deposition (CVD) graphene films are prepared directly on their growth substrates in a high‐density grid array. The approach facilitates the quantification of intrinsic strain and doping in suspended CVD graphene films. To achieve this, transmission electron microscopy and large‐area Raman mapping are employed. Remarkably, the analysis reveals consistent patterns of compressive strain (≈−0.2%) both in the diffraction patterns and Raman maps obtained from these suspended graphene films. By conducting investigations directly on the growth substrates, the potential influences introduced during the transfer process are circumvented effectively. Consequently, the methodology offers a robust and reliable means of studying the intrinsic properties of 2D materials in their authentic form, uninfluenced by the transfer‐induced alterations that may skew the interpretation of their properties.

The unique mechanical and electrical properties of graphene make it an exciting material for nanoelectromechanical systems (NEMS). NEMS resonators with graphene springs facilitate studies of graphene's fundamental material characteristics and thus enable innovative device concepts for applications such as sensors. Here, we demonstrate resonant transducers with ribbon-springs made of double-layer graphene and proof masses made of silicon and study their nonlinear mechanics at resonance both in air and in vacuum by laser Doppler vibrometry. Surprisingly, we observe spring-stiffening and spring-softening at resonance, depending on the graphene spring designs. The measured quality factors of the resonators in a vacuum are between 150 and 350. These results pave the way for a class of ultraminiaturized nanomechanical sensors such as accelerometers by contributing to the understanding of the dynamics of transducers based on graphene ribbons with an attached proof mass. [ABSTRACT FROM AUTHOR]

A highly sensitive nanoscale mass sensor was developed for weight measurement of single microparticles using a suspended graphene structure. The sensor is composed of an array of holes covered with suspended monolayer graphene. Based on the shift of the 2D Raman peak in graphene, originating from the elongation of carbon–carbon bonds under pressure, the mass of microparticles in suspended graphene can be measured. The results show that the sensor can detect microparticles with mass ranging from 0.1 ng to 3 ng. The peak shift ratio is −69.8 cm−1 per 1% strain for the experimental value and −72.3 cm−1 per 1% strain for the calculated value. The demonstrated concept presents a promising path for nano-mass measurement applications. [ABSTRACT FROM AUTHOR]

The mechanical characteristics of graphene ribbons with an attached proof mass that can be used as NEMS transducers have been minimally studied, which hinders the development of graphene-based NEMS devices. Here, we simulated the mechanical characteristics of graphene ribbons with an attached proof mass using the finite element method. We studied the impact of force, residual stress, and geometrical size on displacement, strain, resonant frequency, and fracture strength of graphene ribbons with an attached proof mass. The results show that the increase of width and thickness of graphene ribbons would result in a decrease of the displacement and strain but also an increase of resonant frequency. The increase of the length of graphene ribbons has an insignificant impact on the strain, but it could increase the displacement and decrease the resonant frequency. The increase of residual stress in the graphene ribbons decreases its strain and displacement. The estimated fracture strength of graphene shows limited dependence on its thickness, with an estimated value of around 148 GPa. These findings contribute to the understanding of the mechanical characteristics of graphene ribbons with an attached proof mass and lay the solid foundation for the design and manufacture of high-performance graphene-based NEMS devices such as accelerometers.

In this study, the hydrogen gas (H2) sensing mechanism of suspended graphene (Gr)/ Poly(3,4‐ethylene dioxythiophene): Poly(styrene sulfonate) – Polyethylene oxide (PEDOT: PSS‐PEO) composite nanoscale channels precisely patterned with near‐field electrospinning is investigated. Suspended Gr/PEDOT: PSS‐PEO nanoscale channels not only have a higher surface‐to‐volume ratio for easy diffusion in/out of the composite but also show enhanced response due to effective charge transfer at the interface of suspended graphene and PEDOT: PSS‐PEO nanofiber. A sensing response of 2% for 1 ppm of H2 concentration with good linearity over a wide dynamic range is achieved. Arrays of nanoscale channels for enhanced sensitivity are also implemented and microheaters for effective and fast device recovery are integrated. Moreover, the sensor response is also characterized at various conditions such as channel materials and sizes, temperatures, and gas concentrations. The demonstrated performance, with low power consumption and small form factor, promises a facile and low‐cost suspended graphene/PEDOT: PSS‐PEO sensor solution with enhanced sensitivity. [ABSTRACT FROM AUTHOR]

In this paper, we proposed and designed a plasmonic filter based on a suspended graphene sheet using ring resonator to operate in terahertz range of frequencies. The effective index method is applied to simulate and analyze the structure in two-dimension modeling. We evaluated the transmission coefficient and tunability behavior of the proposed structure via applying finite element method. It is found that the characteristic of the structure can be adjusted through two techniques; (1) controlling the resonant frequencies by fine-tuning the Fermi level of the graphene and (2) tuning the resonant wavelengths by changing the ring resonator dimensions and shape (making a gap in the ring). Also the simulated results show that the terahertz wave transmission ratio can be adjusted from 33.64 to 97.17% by changing the chemical potential of the suspended graphene sheet. These properties are very suitable for adjustable multi-resonance applications such as wavelength based electro-optical switching in terahertz range of operation. [ABSTRACT FROM AUTHOR]

In the present study we investigated the nanostructuring processes in locally suspended few-layer graphene (FLG) films by irradiation with high energy ions (Xe, 26–167 MeV). For such an energy range, the main channel of energy transfer to FLG is local, short-term excitation of the electronic subsystem. The irradiation doses used in this study are 1 × 1011–5 × 1012 ion/cm2. The structural transformations in the films were identified by Raman spectroscopy and transmission electron microscopy. Two types of nanostructures formed in the FLG films as a result of irradiation were revealed. At low irradiation doses the nanostructures were formed preferably at a certain distance from the ion track and had the form of 15–35 nm "bunches". We assumed that the internal mechanical stress that arises due to the excited atoms ejection from the central track part creates conditions for the nanodiamond formation near the track periphery. Depending on the energy of the irradiating ions, the local restructuring of films at the periphery of the ion tracks can lead either to the formation of nanodiamonds (ND) or to the formation of AA' (or ABC) stacking. The compressive strain value and pressure at the periphery of the ion track were estimated as ~0.15–0.22% and ~0.8–1.2 GPa, respectively. The main novel results are the first visualization of ion tracks in graphene in the form of diamond or diamond-like rings, the determination of the main condition for the diamond formation (the absence of a substrate in combination with high ion energy), and estimates of the local strain at the track periphery. Generally, we have developed a novel material and have found how to control the film properties by introducing regions similar to quantum dots with the diamond interface in FLG films. [ABSTRACT FROM AUTHOR]

Based on the Guyer–Krumhansl equation, a new form of governing equation for phonon hydrodynamic transport in two-dimensional non-metallic materials is derived, which is only related to heat flux. The solution of this governing equation shows that there is a new wave phenomenon in the phonon hydrodynamic transport, which is called heat flow wave, that is, the damped oscillation of heat flux. The occurrence conditions of the heat flow wave and resonance are analytically deduced. These conditions are different from that of second sound in phonon hydrodynamic transports. The occurrence conditions of the heat flow wave indicate that a small normal scattering time is a necessary condition for the occurrence of the heat flow wave, and the shape and size of suspended graphene have a significant influence on the occurrence of the heat flow wave. It is found that the frequency of the heat flux wave is lower than that of the second sound. [ABSTRACT FROM AUTHOR]

In this paper, we propose a reversible Feynman gate utilizing the interference effect for optical communications and computing. The plasmonic waveguides are created by placing a suspended graphene sheet, held by two SiO2 ridges, 10 nm above the Si ribs. The Finite-difference time-domain (FDTD) method is used to simulate the proposed gate in frequency and time domains. Simulation results show that high extinction ratios as much as (15.12 dB) and 13 dB are achievable at the wavelength of 10 μm for the output bits P and Q, respectively. The device is immune against ± 20% variations in the width of the Si ribs due to fabrication errors and its performance can be controlled by setting the chemical potential of the graphene sheet to a suitable value. The device has an ultra-compact footprint of 3 μm2, suitable for use in on-chip digital photonic applications. [ABSTRACT FROM AUTHOR]

Nanoscale light sources with high speed of electrical modulation and low energy consumption are key components for nanophotonics and optoelectronics. The record-high carrier mobility and ultrafast carrier dynamics of graphene make it promising as an atomically thin light emitter, which can be further integrated into arbitrary platforms by van der Waals forces. However, due to the zero bandgap, graphene is difficult to emit light through the interband recombination of carriers like conventional semiconductors. Here, we demonstrate ultrafast thermal light emitters based on suspended graphene/hexagonal boron nitride (Gr/hBN) heterostructures. Electrons in biased graphene are significantly heated up to 2800 K at modest electric fields, emitting bright photons from the near-infrared to the visible spectral range. By eliminating the heat dissipation channel of the substrate, the radiation efficiency of the suspended Gr/hBN device is about two orders of magnitude greater than that of graphene devices supported on SiO _2 or hBN. We further demonstrate that hot electrons and low-energy acoustic phonons in graphene are weakly coupled to each other and are not in full thermal equilibrium. Direct cooling of high-temperature hot electrons to low-temperature acoustic phonons is enabled by the significant near-field heat transfer at the highly localized Gr/hBN interface, resulting in ultrafast thermal emission with up to 1 GHz bandwidth under electrical excitation. It is found that suspending the Gr/hBN heterostructures on the SiO _2 trenches significantly modifies the light emission due to the formation of the optical cavity and showed a ∼440% enhancement in intensity at the peak wavelength of 940 nm compared to the black-body thermal radiation. The demonstration of electrically driven ultrafast light emission from suspended Gr/hBN heterostructures sheds the light on applications of graphene heterostructures in photonic integrated circuits, such as broadband light sources and ultrafast thermo-optic phase modulators.

An electrochemical micro‐reactor sealed with a single‐layer graphene (SLG) membrane is demonstrated that allows straightforward measurement with established scanning probe microscopies. SLG serves as a working electrode which separates the liquid electrochemical environment from the ambient to enable direct energy‐level determination. Kelvin probe force microscopy (KPFM) thereby reveals the shifts in Fermi‐level of suspended SLG under electrochemical reaction conditions in aqueous alkaline media. Polymer‐free transfer to create suspended SLG minimizes contributions to doping related to any support or contaminants, such that changes in work function (WF) relate predominantly to the electrochemical system under study. These WF changes are rationalized in the context of a simple model of electrochemical gating, providing insight into the interplay between electronic and electrochemical doping (through redox of water) of suspended graphene. Further changes in WF are attributable to the reversible functionalization of graphene during the oxygen evolution reaction. Mechanical changes in the suspended graphene in the form of bulging also occur, which are attributed to electro‐wetting of graphene induced by charge‐carrier doping. [ABSTRACT FROM AUTHOR]

Freestanding (suspended) graphene films, with high electron mobility (up to ~200,000 cm2V−1s−1), good mechanical and electronic properties, could resolve many of the current issues that are hampering the upscaling of graphene technology. Thus far, attempts at reliably fabricating suspended graphene devices comprising metal contacts, have often been hampered by difficulties in exceeding sizes of 1 µm in diameter, if using UV lithography. In this work, area of suspended graphene large enough to be utilized in microelectronic devices, have been obtained by suspending a CVD graphene film over cavities, with top contacts defined through UV lithography with both wet and dry etching. An area of up to 160 µm2 can be fabricated as backgated devices. The suspended areas exhibit rippling of the surfaces which simultaneously introduces both tensile and compressive strain on the graphene film. Finally, the variations of the Fermi level in the suspended graphene areas can be modulated by applying a potential difference between the top contacts and the backgate. Having achieved large area suspended graphene, in a manner compatible with CMOS fabrication processes, together with enabling the modulation of the Fermi level, are substantial steps forward in demonstrating the potential of suspended graphene-based electronic devices and sensors.

To achieve high-quality chemical vapor deposition of monolayer graphene electrodes (CVD-MG), appropriate characterization at each fabrication step is essential. In this article, (1) Raman spectroscopy/microscopy are employed to unravel the contact effect between the CVD-MG and Cu foil in suspended/supported formation. (2) The Surface-Enhanced Raman spectroscopy (SERS) system is described, unveiling the presence of a z-directional radial breathing-like mode (RBLM) around 150 cm−1, which matches the Raman shift of the radial breathing mode (RBM) from single-walled carbon nanotubes (SWCNTs) around 150 cm−1. This result indicates the CVD-MG located between the Au NPs and Au film is not flat but comprises heterogeneous protrusions of some domains along the z-axis. Consequently, the degree of carrier mobility can be influenced, as the protruding domains result in lower carrier mobility due to flexural phonon–electron scattering. A strongly enhanced G-peak domain, ascribed to the presence of scrolled graphene nanoribbons (sGNRs), was observed, and there remains the possibility for the fabrication of sGNRs as sources of open bandgap devices. (3) Electrostatic force microscopy (EFM) is used for the measurement of surface charge distribution of graphene at the nanoscale and is crucial in substantiating the electrical performance of CVD-MG, which was influenced by the surface structure of the Cu foil. The ripple (RP) structures were determined using EFM correlated with Raman spectroscopy, exhibiting a higher tapping amplitude which was observed with structurally stable and hydrophobic RPs with a threading type than surrounding RPs. (4) To reduce the RP density and height, a plausible fabrication could be developed that controls the electrical properties of the CVD-MG by tuning the cooling rate.

Suspended graphene can perfectly present the excellent material properties of graphene, which has a good application prospect in graphene sensors. The existing suspended graphene pressure sensor has several problems that need to be solved, one of which is the fabrication of a suspended sample. It is still very difficult to obtain large-size suspended graphene films with a high integrity that are defect-free. Based on the simulation and analysis of the kinetic process of the traditional suspended graphene release process, a novel setup for large-size suspended graphene release was designed based on the inverted floating method (IFM). The success rate of the single-layer suspended graphene with a diameter of 200 μm transferred on a stainless-steel substrate was close to 50%, which is greatly improved compared with the traditional impregnation method. The effects of the defects and burrs around the substrate cavity on the stress concentration of graphene transfer explain why the transfer success rate of large-size suspended graphene is not high. This research lays the foundation for providing large-size suspended graphene films in the area of graphene high-precision sensors.

Freestanding (suspended) graphene films, with high electron mobility (up to ~200,000 cm2V−1s−1), good mechanical and electronic properties, could resolve many of the current issues that are hampering the upscaling of graphene technology. Thus far, attempts at reliably fabricating suspended graphene devices comprising metal contacts, have often been hampered by difficulties in exceeding sizes of 1 µm in diameter, if using UV lithography. In this work, area of suspended graphene large enough to be utilized in microelectronic devices, have been obtained by suspending a CVD graphene film over cavities, with top contacts defined through UV lithography with both wet and dry etching. An area of up to 160 µm2 can be fabricated as backgated devices. The suspended areas exhibit rippling of the surfaces which simultaneously introduces both tensile and compressive strain on the graphene film. Finally, the variations of the Fermi level in the suspended graphene areas can be modulated by applying a potential difference between the top contacts and the backgate. Having achieved large area suspended graphene, in a manner compatible with CMOS fabrication processes, together with enabling the modulation of the Fermi level, are substantial steps forward in demonstrating the potential of suspended graphene-based electronic devices and sensors. [ABSTRACT FROM AUTHOR]

Graphene-based optical switches are one of the promising building blocks for future optical integrated circuits. For the first time in this paper, a novel all-optical graphene-based cross-bar switch is proposed. The structure is based on our recently reported suspended graphene plasmonic coupler. The high carrier mobility of the suspended graphene layer results in long propagation lengths of surface plasmon polaritons that are essential for realizing cross-bar optical switching. Dispersion relations of surface plasmon polaritons of a simple suspended graphene-based structure are derived in the nonlinear state. The relations are employed to analyze the device using the effective index method (EIM) that reduces the time and memory requirements, significantly. The switching length at the wavelength of 10 μm is as short as 2.6 μm. The required optical pump intensity is calculated as approximately 77 MW/cm2. The switching operation of the structure is investigated using the finite-difference time-domain method. According to the presented results, extinction ratios of as high as 11.18 and 11.1 dB are obtained at the bar and cross output ports, respectively. A wide spectral width of more than 1 μm is also calculated. Finally, the transient response of the structure is investigated and the switching time of less than 3 ps is predictable. [ABSTRACT FROM AUTHOR]

Suspended graphene flakes are exposed simultaneously to fluorine and sulfur ions produced by the μ-wave plasma discharge of the SF 6 precursor gas. The microscopic and spectroscopic analyses, performed by Raman spectroscopy, scanning electron microscopy and photoelectron spectromicroscopy, show the homogeneity in functionalization yield over the graphene flakes with F and S atoms covalently bonded to the carbon lattice. This promising surface shows potential for several applications ranging from biomolecule immobilization to lithium battery and hydrogen storage devices. The present co-doping process is an optimal strategy to engineer the graphene surface with a concurrent hydrophobic character, thanks to the fluorine atoms, and a high affinity with metal nanoparticles due to the presence of sulfur atoms. [ABSTRACT FROM AUTHOR]

The electrical performance of graphene-based devices is largely limited by substantial contact resistance at the heterodimensional graphene-metal junctions. A laser-assisted nanowelding technique is developed to reduce graphene-metal (G-M) contact resistance and improve carrier injection in suspended graphene devices. Selective breakdown of CC bonds and formation of structural defects are realized through laser irradiation at the edges of graphene within the G-M contact regions in order to increase the chemical reactivity of graphene, facilitate G-M bonding, and, therefore, maximize interfacial carrier transportation. Through this method, significantly reduced G-M contact resistances, as low as 2.57 Ω µm are obtained. In addition, it is demonstrated that the location of laser-induced defects within the contact areas significantly impacts the interfacial properties and the carrier mobility of graphene devices. A fourfold increase in photocurrent is observed in the suspended graphene photodetectors with treated G-M interfaces as compared to ordinary ones. This contact-free and position-selective technique minimizes the degradation of the graphene channels and maintains the superior performance of graphene devices, making it a promising approach for reducing G-M resistance in the fabrication of graphene-based devices. [ABSTRACT FROM AUTHOR]

Graphene has extraordinary mechanical and electronic properties, making it a promising material for membrane-based nanoelectromechanical systems (NEMS). Here, chemical-vapor-deposited graphene is transferred onto target substrates to suspend it over cavities and trenches for pressure-sensor applications. The development of such devices requires suitable metrology methods, i.e., large-scale characterization techniques, to confirm and analyze successful graphene transfer with intact suspended graphene membranes. We propose fast and noninvasive Raman spectroscopy mapping to distinguish between free-standing and substrate-supported graphene, utilizing the different strain and doping levels. The technique is expanded to combine two-dimensional area scans with cross-sectional Raman spectroscopy, resulting in three-dimensional Raman tomography of membrane-based graphene NEMS. The potential of Raman tomography for in-line monitoring is further demonstrated with a methodology for automated data analysis to spatially resolve the material composition in micrometer-scale integrated devices, including free-standing and substrate-supported graphene. Raman tomography may be applied to devices composed of other two-dimensional materials as well as silicon micro- and nanoelectromechanical systems. [ABSTRACT FROM AUTHOR]

High-field-effect mobility and the two-dimensional nature of graphene films make it an interesting material for developing sensing applications with high sensitivity and low power consumption. The chemical vapor deposition process allows for producing high-quality graphene films in a scalable manner. Considering the significant impact of the underlying substrate on the graphene device performance, methods to enhance the field-effect mobility are highly desired. This work demonstrates a simplified fabrication process to develop suspended, two-terminal chemical vapor deposition (CVD) graphene devices with enhanced field-effect mobility operating at room temperature. Enhanced hole field-effect mobility of up to ∼4.8 × 10 4 cm 2 /Vs and average hole mobility >1 × 10 4 cm 2 /Vs across all of the devices is demonstrated. A gradual increase in the width of the graphene device resulted in the increase of the full width at half-maximum (FWHM) of field-effect characteristics and a decrease in the field-effect mobility. Our work presents a simplified fabrication approach to realize high-mobility suspended CVD graphene devices, beneficial for developing CVD graphene-related applications.

Using nanoparticles to impart extrinsic rippling in graphene is a relatively new method to induce strain and to tailor the properties of graphene. Here, we study the structure and elastic properties of graphene grown by chemical vapour deposition and transferred onto a continuous layer of SiO2nanoparticles with diameters of around 25 nm, prepared by Langmuir–Blodgett technique on Si substrate. We show that the transferred graphene follows only roughly the morphology induced by nanoparticles. The graphene membrane parts bridging the nanoparticles are suspended and their adhesion to the atomic force microscope tip is larger compared to that of supported graphene parts. These suspended graphene regions can be deformed with forces of the order of 10 nN. The elastic modulus of graphene was determined from indentation measurements performed on suspended membrane regions with diameters in the 100 nm range. [ABSTRACT FROM AUTHOR]

The electronic transport properties of graphene and suspended (intrinsic) graphene sheets are studied using an ensemble Monte Carlo (EMC) technique. The combined scattering mechanisms that are taken into account for both cases are nonpolar optic and acoustic phonons, ionized impurity, interface roughness, and surface polar phonon scatterings. The effect of screening is also considered in the ionized impurity and surface polar phonon scatterings of electrons. A rejection technique is used in EMC simulations to account for the occupancy of the final states. Velocity-field characteristics of graphene and suspended graphene sheets are obtained using various values of acoustic deformation potential constants. The variation of electron mobility of graphene is studied as a function of electron concentration and its variation as a function of temperature are investigated for the case of suspended graphene. For the former case, the mobility increases with electron concentration first and after a certain value of electron concentration it begins to decrease, while for the latter case the mobility decreases almost linearly with temperature. The mobility results from EMC simulations are compatible with the existing experimental studies for the unsuspended graphene case. [ABSTRACT FROM AUTHOR]

We investigated the evolution of the electronic transport properties of a suspended disordered graphene device by systematically controlling the bias voltage. Previously, the application of high bias voltage resulted in the sudden constriction of a graphene device. During the high bias voltage process, we utilized an electronic feedback loop to prevent the sudden change in resistance to ensure that the constriction proceeded sufficiently slowly. We performed a total of 137 sequential steps of a controlled high bias voltage exposure on the device in a cryostat. After each exposure, we measured the electronic transport properties and observed the transformation between various intriguing quantum transport phenomena. When the overall conductance was suppressed below approximately 10% of the single quantum conductance, superimposed Coulomb diamond patterns appeared at low temperatures, exhibiting parallel double quantum dot behaviors. We also found that the parallel double quantum dots were transformed into a single quantum dot and then into a single tunnel barrier as the high-voltage exposure was applied repeatedly. The transformation of the electronic transport properties along with the high bias voltage exposure processes can be attributed to the gradual reduction of the graphene width. [ABSTRACT FROM AUTHOR]

Graphene has extraordinary mechanical and electronic properties, making it a promising material for membrane based nanoelectromechanical systems (NEMS). Here, three methods for direct transfer of chemical vapor deposited graphene onto pre-fabricated micro cavity substrates were investigated and analyzed with respect to yield and quality of the free-standing membranes on a large-scale. An effective transfer method for layer-by-layer stacking of graphene was developed to improve the membrane stability and thereby increase the yield of completely covered and sealed cavities. The transfer method with the highest yield was used to fabricate graphene NEMS devices. Electrical measurements were carried out to successfully demonstrate pressure sensing as a possible application for these graphene membranes. [ABSTRACT FROM AUTHOR]

In this work we present combined Kelvin probe force microscopy and Raman spectroscopy studies of supported and suspended structures formed out of chemical vapor deposition (CVD) grown graphene. Work function of both suspended and supported graphene was -4.81 ± 0.06 eV and -4.92 ± 0.06 eV respectively. By G and 2D modes correlation we showed, that CVD graphene was influenced by biaxial strain. Increased contact potential difference (CPD) on the suspended graphene in comparison with the areas of the supported graphene was the sign of increased strain (from 0.05% to ~ 0.12%) rather than decreased doping (p-doping decreased from ~ 5.5 × 10 12 cm -2 to ~ 4.5 × 10 12 cm -2 ). [ABSTRACT FROM AUTHOR]

Interfacial water molecules affect carrier transportation within graphene and related applications. Without proper tools, however, most of the previous works focus on simulation modeling rather than experimental validation. To overcome this obstacle, a series of graphene field-effect transistors (GFETs) with suspended (substrate-free, SF) and supported (oxide-supported, OS) configurations are developed to investigate the graphene-water interface under different hydrophilic conditions. With deionized water environments, in our experiments, the electrical transportation behaviors of the graphene mainly originate from the evolution of the interfacial water-molecule arrangement. Also, these current-voltage behaviors can be used to elucidate the first-water layer at the graphene-water interface. For SF-GFET, our experimental results show positive hysteresis in electrical transportation. These imply highly ordered interfacial water molecules with a separated-ionic distributed structure. For OS-GFET, on the contrary, the negative hysteresis shows the formation of the hydrogen-bond interaction between the interfacial water layer and the SiO 2 substrate under the graphene. This interaction further promotes current conduction through the graphene/water interface. In addition, the net current-voltage relationship also indicates the energy required to change the orientation of the first-layer water molecules during electro-potential change. Therefore, our work gives an insight into graphene-water interfacial evolution with field-effect modulation. Furthermore, this experimental architecture also paves the way for investigating 2D solid-liquid interfacial features.

We report on strong interference effects on the ratio of the integrated intensities of the Raman 2D- and G-mode features (herein denoted [ABSTRACT FROM AUTHOR]

To address the need for large area dry graphene transfer techniques in Nanomechanical applications we use transfer printing to suspend large areas of graphene on pre-patterned substrates with cavities. We find that using this method, clean suspended graphene can be produced with areas up to 15 × 15 μm2. [ABSTRACT FROM AUTHOR]

We report an improved recipe for synthesizing high quality graphene through chemical vapor deposition (CVD), scanning electron microscopy (SEM) characterization of CVD graphene, and optimized SEM imaging conditions for efficient visualization of surface features in CVD graphene. We have developed an optimized graphene growth recipe by characterizing the quality of as-grown graphene using Raman spectroscopy and SEM. We have examined graphene samples both on copper (Cu) and silicon dioxide (SiO 2 ) substrate using SEM. We have found that features on the samples are highly sensitive to both SEM imaging conditions and the type of detector used. With low acceleration voltage (1 keV), immersion lenses, and through the lens detector, we have clearly observed fine features including wrinkles, folding lines, defects, and different layer numbers of graphene, many of which are not visible in un-optimized SEM images. Further, we demonstrate mechanical bulging of suspended CVD graphene membranes covering microtrenches by using electron beam to activate the trapped gas underneath. Our findings and techniques can lead to improved characterization, understanding, and manipulation of graphene and other two-dimensional materials. [ABSTRACT FROM AUTHOR]

In this work, we report the electrical field tuning of magneto-phonon resonance in monolayer graphene under magnetic fields up to 9 T. It is found that the carrier concentration can drastically affect the G (E) phonon response to a varying magnetic field through a pronounced magneto-phonon resonance (MPR). In charge neutral or slightly doped monolayer graphene, both the energy and the line width of the E phonon show clear variation with magnetic fields. This is attributed to magneto-phonon resonance between magnetoexcitations and the E phonons. In contrast, when the Fermi level of the monolayer graphene is far away from the Dirac point, the G band shows weak magnetic dependence and exhibits a symmetric line-shape. This suggests that the magneto-phonon coupling around 4 T has been switched off due to the Pauli blocking of the inter-Landau level excitations. Moreover, the G band asymmetry caused by Fano resonance between excitonic many-body states and the E phonons is observed. This work offers a way to study the magnetoexcitation phonon interaction of materials through magneto-Raman spectroscopy with an external electrical field. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

We study the optical response of a suspended, monolayer graphene field-effect transistor structure in magnetic fields of up to 9 T (quantum Hall regime). With an illumination power of only 3 μ W, we measure a photocurrent of up to 400 nA (without an applied bias) corresponding to a photo-responsivity of 0.13 A W ^−1 , which we believe to be one of the highest values ever measured in single-layer graphene. We discuss possible mechanisms for generating this strong photo-response (17 electron–hole pairs per 100 incident photons). Based on our experimental findings, we believe that the most likely scenario is a ballistic two-stage process including carrier multiplication via Auger-type inelastic Coulomb scattering at the graphene edge.

• Suspended graphene can reduce scattering effectively. • Fabrication of suspended graphene arrays by silver liquid dynamics. • Suspended graphene improves detection sensitivity for nitrogen dioxide. Graphene, with excellent electrical and thermal properties, is far from its theoretical limit because of substrate scattering which can be improved by suspending the graphene. Presently, the suspended graphene is generally fabricated by etching, which requires complicated operation. Here, we propose a strategy to prepare suspended graphene arrays by using the surface motion of molten silver. The prepared suspended graphene arrays exhibit stable contact and low contact resistance with electrodes. A gas sensor is demonstrated by using the suspended graphene, which shows high performance in detecting harmful gas nitrogen dioxide. [ABSTRACT FROM AUTHOR]

The interactions between phonons and electrons induced by the dopants or the substrate of graphene in spectroscopic investigation reveal a rich source of interesting physics. Raman spectra and surface-enhanced Raman spectra of supported and suspended monolayer graphenes were measured and analyzed systemically with different approaches. The weak Raman signals are greatly enhanced by the ability of surface-enhanced Raman spectroscopy which has attracted considerable interests. The technique is regarded as wonderful and useful tool, but the dopants that are produced by depositing metallic nanoparticles may affect the electron scattering processes of graphene. Therefore, the doping and substrate influences on graphene are also important issues to be investigated. In this work, the peak positions of G peak and 2D peak, the I2D/ IG ratios, and enhancements of G and 2D bands with suspended and supported graphene flakes were measured and analyzed. The peak shifts of G and 2D bands between the Raman and SERS signals demonstrate the doping effect induced by silver nanoparticles by n-doping. The I2D/ IG ratio can provide a more sensitive method to carry out the doping effect on the graphene surface than the peak shifts of G and 2D bands. The enhancements of 2D band of suspended and supported graphenes reached 138, and those of G band reached at least 169. Their good enhancements are helpful to measure the optical properties of graphene. The different substrates that covered the graphene surface with doping effect are more sensitive to the enhancements of G band with respect to 2D band. It provides us a new method to distinguish the substrate and doping effect on graphene. PACS: 78.67.Wj (optical properties of graphene); 74.25.nd (Raman and optical spectroscopy); 63.22.Rc (phonons in graphene) [ABSTRACT FROM AUTHOR]

The authors present the blue-shift of Raman 2D and G peak of a single layer graphene film, which is fabricated via chemical vapor deposition, on the relatively even Cu foil domain with nearly uniform thickness. This can be interpreted that rather mechanically compressive strain is exerted between graphene and the even Cu domain compared to the uneven. We observe that relaxing compressive strain to minimize the perturbation from Cu surface to the graphene film by virtue of the uneven Cu domain has a critical role in forming a suspended graphene. Furthermore, mechanical strain influence derived from the underlying substrate morphology can be suggested as one of the crucial elements in evaluating high quality graphene film considering suspended shape. [ABSTRACT FROM AUTHOR]

This study reports on controlling the formation of nanoimpurities on suspended graphene to investigate the inelastic scattering of electrons using a two-phonon Raman process. Results were analyzed by transmission electron microscopy (TEM) and scanning Raman spectroscopy in the same region of suspended graphene. The findings revealed that the area with a higher concentration of impurities shown in the TEM image corresponds directly to the area with a lower integrated intensity and a wider full width at half maximum in the Raman mapping of the 2D band and vice versa. The same trend is also apparent in the 2D′ and D + D″ bands. In conclusion, the results are explained by an increase in the electronic scattering rate due to impurities, which affects two-phonon Raman scattering. Combining the TEM image and Raman mapping image effectively demonstrates how electron behavior is affected by the distribution of impurities in graphene systems. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The above question is frequently asked by theorists who are interested in graphene as a model system, especially in context of relativistic quantum physics. We offer an experimental answer by describing electron transport in suspended devices with carrier mobilities of several 106 cm2 V–1 s–1 and with the onset of Landau quantization occurring in fields below 5 mT. The observed charge inhomogeneity is as low as ≈108 cm–2, allowing a neutral state with a few charge carriers per entire micrometer-scale device. Above liquid helium temperatures, the electronic properties of such devices are intrinsic, being governed by thermal excitations only. This yields that the Dirac point can be approached within 1 meV, a limit currently set by the remaining charge inhomogeneity. No sign of an insulating state is observed down to 1 K, which establishes the upper limit on a possible bandgap. [ABSTRACT FROM AUTHOR]

Narrow gaps are formed in suspended single- to few-layer graphene devices using a pulsed electrical breakdown technique. The conductance of the resulting devices can be programmed by the application of voltage pulses, with voltages of 2.5 to ∼4.5 V, corresponding to an ON pulse, and ∼8 V, corresponding to an OFF pulse. Electron microscope imaging of the devices shows that the graphene sheets typically remain suspended and that the device conductance tends to zero when the observed gap is large. The switching rate is strongly temperature dependent, which rules out a purely electromechanical switching mechanism. This observed switching in suspended graphene devices strongly suggests a switching mechanism via atomic movement and/or chemical rearrangement and underscores the potential of all-carbon devices for integration with graphene electronics. [ABSTRACT FROM AUTHOR]