Imaging mechanical vibrations in suspended graphene sheets

We carried out measurements on nanoelectromechanical systems based on multilayer graphene sheets suspended over trenches in silicon oxide. The motion of the suspended sheets was electrostatically driven at resonance using applied radio frequency voltages. The mechanical vibrations were detected using a novel form of scanning probe microscopy, which allowed identification and spatial imaging of the shape of the mechanical eigenmodes. In as many as half the resonators measured, we observed a new class of exotic nanoscale vibration eigenmodes not predicted by the elastic beam theory, where the amplitude of vibration is maximum at the free edges. By modeling the suspended sheets with the finite element method, these edge eigenmodes are shown to be the result of nonuniform stress with remarkably large magnitudes (up to 1.5 GPa). This nonuniform stress, which arises from the way graphene is prepared by pressing or rubbing bulk graphite against another surface, should be taken into account in future studies on electronic and mechanical properties of graphene.     

Stamp transferred suspended graphene mechanical resonators for radio frequency electrical readout

We present a simple micromanipulation technique to transfer suspended graphene flakes onto any substrate and to assemble them with small localized gates into mechanical resonators. The mechanical motion of the graphene is detected using an electrical, radio frequency (RF) reflection readout scheme where the time-varying graphene capacitor reflects a RF carrier at f = 5-6 GHz producing modulation sidebands at f ± f(m). A mechanical resonance frequency up to f(m) = 178 MHz is demonstrated. We find both hardening/softening Duffing effects on different samples and obtain a critical amplitude of ~40 pm for the onset of nonlinearity in graphene mechanical resonators. Measurements of the quality factor of the mechanical resonance as a function of dc bias voltage V(dc) indicates that dissipation due to motion-induced displacement currents in graphene electrode is important at high frequencies and large V(dc).     

Bending-mode vibration of a suspended nanotube resonator

We have used a suspended carbon nanotube as a frequency mixer to detect its own mechanical motion. A single gate-dependent resonance is observed, which we attribute to the fundamental bending mode vibration of the suspended carbon nanotubes. A continuum model is used to fit the gate dependence of the resonance frequency, from which we obtain values for the fundamental frequency, the residual and gate-induced tension in the nanotube. This analysis shows that the nanotubes in our devices have no slack and that, by applying a gate voltage, the nanotube can be tuned from a regime without strain to a regime where it behaves as a vibrating string under tension.     

A carbon nanotube field effect transistor with a suspended nanotube gate

We investigate theoretically field effect transistors based on single-walled carbon nanotubes (CNTFET) and explore two device geometries with suspended multiwalled carbon nanotubes (MWNT) functioning as gate electrodes. In the two geometries, a doubly or singly clamped MWNT is electrostatically deflected toward the transistor channel, allowing for a variable gate coupling and leading to, for instance, a superior subthreshold slope. We suggest that the proposed designs can be used as nanoelectromechanical switches and as detectors of mechanical motion on the nanoscale.     

Graphene gate electrode for MOS structure-based electronic devices

We demonstrate that the use of a monolayer graphene as a gate electrode on top of a high-κ gate dielectric eliminates mechanical-stress-induced-gate dielectric degradation, resulting in a quantum leap of gate dielectric reliability. The high work function of hole-doped graphene also helps reduce the quantum mechanical tunneling current from the gate electrode. This concept is applied to nonvolatile Flash memory devices, whose performance is critically affected by the quality of the gate dielectric. Charge-trap flash (CTF) memory with a graphene gate electrode shows superior data retention and program/erase performance that current CTF devices cannot achieve. The findings of this study can lead to new applications of graphene, not only for Flash memory devices but also for other high-performance and mass-producible electronic devices based on MOS structure which is the mainstream of the electronic device industry.     

悬浮石墨烯的制备、性能及应用研究

作为一种新型二维材料,石墨烯在电学、光学、传热学及力学性能等方面均表现出极其优异的特性,对石墨烯的研究也得到众多研究者的关注.同时,拥有桥状悬浮结构的悬浮石墨烯(Suspended graphene)以其杂质少、受外界干扰小等优点使得石墨烯的本征特性得到最大化施展.在研究石墨烯的电子迁移率、传热性、力学性能等方面,悬浮石墨烯有着独特的优势,并且对提升微电子器件的性能作用显著.综述了悬浮石墨烯的制备与性质研究及其在微电子领域的应用进展,并展望了悬浮石墨烯的应用前景.     

SWNT array resonant gate MOS transistor

We show that thin horizontal arrays of single wall carbon nanotubes (SWNTs) suspended above the channel of silicon MOSFETs can be used as vibrating gate electrodes. This new class of nano-electromechanical system (NEMS) combines the unique mechanical and electronic properties of SWNTs with an integrated silicon-based motion detection. Its electrical response exhibits a clear signature of the mechanical resonance of SWNT arrays (120-150 MHz) showing that these thin horizontal arrays behave as a cohesive, rigid and elastic body membrane with a Young's modulus in the order of 1-10 GPa and ultra-low mass. The resonant frequency can be tuned by the gate voltage and its dependence is well understood within the continuum mechanics framework.     

Extraction of the short-range defect potential parameters from available experimental data on the graphene resistance

Abstract

We consider a problem of obtaining information about the scattering potentials of the monolayer graphene sample using available experimental data on its resistance. For this purpose, we study theoretically the dependence of graphene resistance on Fermi energy having in mind to compare it with experimental data where super-high mobility electrons in suspended graphene samples without chemical doping were investigated. As far as practical absence of the doping impurities in this case makes the Coulomb scattering negligible, we consider models of the short-range scattering potentials. The model of short-range potential is assumed to be supported by the close vicinity of the ring or the circumference of a circle. The diameter of circles is supposed to be of the order of the crystal lattice spacing. The empty core of the model potential guarantees the suppression of nonphysical shortwave modes. Two models are investigated: the delta function on the circumference of a circle (delta shell) and the annual well. An advantage of the former is simplicity, while a virtue of the latter is regularity. We consider scattering of electrons by these potentials and obtain exact explicit formulae for the scattering data. We here discuss application of these formulae for calculation of observables. Namely, we analyze the contribution of this scattering into the graphene resistance and plot the resistivity as a function of the Fermi energy according to our theoretical formulae. The obtained results are consistent with experiment, where the resistance was measured as a function of the Fermi momentum on the suspended annealed graphene. This fact gives a possibility to find parameters of the modeled potential on the basis of the available experimental data on resistance of the suspended graphene sample with the gate voltage controlled Fermi level position. It is clear to be very important for applications.     

Large‐Scale Suspended Graphene Used as a Transparent Substrate for Infrared Spectroscopy

Due to weak interactions between micrometer‐wavelength infrared (IR) light and nanosized samples, a high signal to noise ratio is a prerequisite in order to precisely characterize nanosized samples using IR spectroscopy. Traditional micrometer‐thick window substrates, however, have considerable IR absorption which may introduce unavoidable deformations and interruptions to IR spectra of nanoscale samples. A promising alternative is the use of a suspended graphene substrate which has ultrahigh IR transmittance (>97.5%) as well as unique mechanical properties. Here, an effective method is presented for fabrication of suspended graphene over circular holes up to 150 µm in diameter to be utilized as a transparent substrate for IR spectroscopy. It is demonstrated that the suspended graphene has little impact on the measured IR spectra, an advantage which has led to the discovery of several missing vibrational modes of a 20 nm thick PEO film measured on a traditional CaF₂ substrate. This can provide a better understanding of molecules' fine structures and status of hanging bands. The unique optical properties of suspended graphene are determined to be superior to those of conventional IR window materials, giving this new substrate great potential as part of a new generation of IR transparent substrates, especially for use in examining nanoscale samples.     

Macroscopic graphene membranes and their extraordinary stiffness

The properties of suspended graphene are currently attracting enormous interest, but the small size of available samples and the difficulties in making them severely restrict the number of experimental techniques that can be used to study the optical, mechanical, electronic, thermal, and other characteristics of this one-atom-thick material. Here, we describe a new and highly reliable approach for making graphene membranes of a macroscopic size (currently up to 100 microm in diameter) and their characterization by transmission electron microscopy. In particular, we have found that long graphene beams supported by only one side do not scroll or fold, in striking contrast to the current perception of graphene as a supple thin fabric, but demonstrate sufficient stiffness to support extremely large loads, millions of times exceeding their own weight, in agreement with the presented theory. Our work opens many avenues for studying suspended graphene and using it in various micromechanical systems and electron microscopy.     

A High‐Performance Top‐Gated Graphene Field‐Effect Transistor with Excellent Flexibility Enabled by an iCVD Copolymer Gate Dielectric

A high‐performance top‐gated graphene field‐effect transistor (FET) with excellent mechanical flexibility is demonstrated by implementing a surface‐energy‐engineered copolymer gate dielectric via a solvent‐free process called initiated chemical vapor deposition. The ultrathin, flexible copolymer dielectric is synthesized from two monomers composed of 1,3,5‐trimethyl‐1,3,5‐trivinyl cyclotrisiloxane and 1‐vinylimidazole (VIDZ). The copolymer dielectric enables the graphene device to exhibit excellent dielectric performance and substantially enhanced mechanical flexibility. The p‐doping level of the graphene can be tuned by varying the polar VIDZ fraction in the copolymer dielectric, and the Dirac voltage (V_Dirac) of the graphene FET can thus be systematically controlled. In particular, the V_Dirac approaches neutrality with higher VIDZ concentrations in the copolymer dielectric, which minimizes the carrier scattering and thereby improves the charge transport of the graphene device. As a result, the graphene FET with 20 nm thick copolymer dielectrics exhibits field‐effect hole and electron mobility values of over 7200 and 3800 cm² V^?1 s^?1, respectively, at room temperature. These electrical characteristics remain unchanged even at the 1 mm bending radius, corresponding to a tensile strain of 1.28%. The formed gate stack with the copolymer gate dielectric is further investigated for high‐frequency flexible device applications.     

Probing mechanical properties of graphene with Raman spectroscopy

The use of Raman scattering techniques to study the mechanical properties of graphene films is reviewed here. The determination of Grüneisen parameters of suspended graphene sheets under uni- and bi-axial strain is discussed, and the values are compared to theoretical predictions. The effects of the graphene−substrate interaction on strain and to the temperature evolution of the graphene Raman spectra are discussed. Finally, the relation between mechanical and thermal properties is presented along with the characterization of thermal properties of graphene with Raman spectroscopy.     

温度对石墨烯膜光纤F-P声压传感器灵敏度影响研究

石墨烯具有优异的机械、电学与光学等传感特性,有希望成为下一代可穿戴电子设备的功能敏感材料。石墨烯膜Fabry-Perot(F-P)声压传感器具有高灵敏度、小型化和抗电磁干扰等优点,但会受到温度漂移的影响。温度对传感器的影响主要体现在F-P腔长变化,引起工作点漂移,导致传感器光学灵敏度发生变化,以及改变石墨烯膜预应力。本文制备了石墨烯膜光纤F-P声压传感器探头,通过声压测试表明,温度改变了悬浮石墨烯膜的机械力学特性,在1 kHz处使其机械灵敏度由1.80 nm/Pa提高至2.44 nm/Pa。     

Molecular beam epitaxy growth of free-standing plane-parallel InAs nanoplates

Free-standing nanostructures such as suspended carbon nanotubes, graphene layers, III-V nanorod photonic crystals and three-dimensional structures have recently attracted attention because they could form the basis of devices with unique electronic, optoelectronic and electromechanical characteristics. Here we report the growth by molecular beam epitaxy of free-standing nanoplates of InAs that are close to being atomically plane. The structural and transport properties of these semiconducting nanoplates have been examined with scanning electron microscopy, transmission electron microscopy, X-ray diffraction and low-temperature electron transport measurements. The carrier density of the nanoplates can be reduced to zero by applying a voltage to a nearby gate electrode, creating a new type of suspended quantum well that can be used to explore low-dimensional electron transport. The electronic and optical properties of such systems also make them potentially attractive for photovoltaic and sensing applications.     

Bottom-gated epitaxial graphene

High-quality epitaxial graphene on silicon carbide (SiC) is today available in wafer size. Similar to exfoliated graphene, its charge carriers are governed by the Dirac-Weyl Hamiltonian and it shows excellent mobilities. For many experiments with graphene, in particular for surface science, a bottom gate is desirable. Commonly, exfoliated graphene flakes are placed on an oxidized silicon wafer that readily provides a bottom gate. However, this cannot be applied to epitaxial graphene as the SiC provides the source material out of which graphene grows. Here, we present a reliable scheme for the fabrication of bottom-gated epitaxial graphene devices, which is based on nitrogen (N) implantation into a SiC wafer and subsequent graphene growth. We demonstrate working devices in a broad temperature range from 6 to 300 K. Two gating regimes can be addressed, which opens a wide engineering space for tailored devices by controlling the doping of the gate structure.     

High-frequency graphene voltage amplifier

While graphene transistors have proven capable of delivering gigahertz-range cutoff frequencies, applying the devices to RF circuits has been largely hindered by the lack of current saturation in the zero band gap graphene. Herein, the first high-frequency voltage amplifier is demonstrated using large-area chemical vapor deposition grown graphene. The graphene field-effect transistor (GFET) has a 6-finger gate design with gate length of 500 nm. The graphene common-source amplifier exhibits ～5 dB low frequency gain with the 3 dB bandwidth greater than 6 GHz. This first AC voltage gain demonstration of a GFET is attributed to the clear current saturation in the device, which is enabled by an ultrathin gate dielectric (4 nm HfO(2)) of the embedded gate structures. The device also shows extrinsic transconductance of 1.2 mS/μm at 1 V drain bias, the highest for graphene FETs using large-scale graphene reported to date.     

Clean Transfer of Large Graphene Single Crystals for High‐Intactness Suspended Membranes and Liquid Cells

The atomically thin 2D nature of suspended graphene membranes holds promising in numerous technological applications. In particular, the outstanding transparency to electron beam endows graphene membranes great potential as a candidate for specimen support of transmission electron microscopy (TEM). However, major hurdles remain to be addressed to acquire an ultraclean, high‐intactness, and defect‐free suspended graphene membrane. Here, a polymer‐free clean transfer of sub‐centimeter‐sized graphene single crystals onto TEM grids to fabricate large‐area and high‐quality suspended graphene membranes has been achieved. Through the control of interfacial force during the transfer, the intactness of large‐area graphene membranes can be as high as 95%, prominently larger than reported values in previous works. Graphene liquid cells are readily prepared by π–π stacking two clean single‐crystal graphene TEM grids, in which atomic‐scale resolution imaging and temporal evolution of colloid Au nanoparticles are recorded. This facile and scalable production of clean and high‐quality suspended graphene membrane is promising toward their wide applications for electron and optical microscopy.     

Graphene–Graphene Oxide Floating Gate Transistor Memory

A novel transparent, flexible, graphene channel floating‐gate transistor memory (FGTM) device is fabricated using a graphene oxide (GO) charge trapping layer on a plastic substrate. The GO layer, which bears ammonium groups (NH₃⁺), is prepared at the interface between the crosslinked PVP (cPVP) tunneling dielectric and the Al₂O₃ blocking dielectric layers. Important design rules are proposed for a high‐performance graphene memory device: i) precise doping of the graphene channel, and ii) chemical functionalization of the GO charge trapping layer. How to control memory characteristics by graphene doping is systematically explained, and the optimal conditions for the best performance of the memory devices are found. Note that precise control over the doping of the graphene channel maximizes the conductance difference at a zero gate voltage, which reduces the device power consumption. The proposed optimization via graphene doping can be applied to any graphene channel transistor‐type memory device. Additionally, the positively charged GO (GO–NH₃⁺) interacts electrostatically with hydroxyl groups of both UV‐treated Al₂O₃ and PVP layers, which enhances the interfacial adhesion, and thus the mechanical stability of the device during bending. The resulting graphene–graphene oxide FGTMs exhibit excellent memory characteristics, including a large memory window (11.7 V), fast switching speed (1 μs), cyclic endurance (200 cycles), stable retention (10⁵ s), and good mechanical stability (1000 cycles).     

Nonlinear damping in mechanical resonators made from carbon nanotubes and graphene

The theory of damping is discussed in Newton's Principia and has been tested in objects as diverse as the Foucault pendulum, the mirrors in gravitational-wave detectors and submicrometre mechanical resonators. In general, the damping observed in these systems can be described by a linear damping force. Advances in nanofabrication mean that it is now possible to explore damping in systems with one or more atomic-scale dimensions. Here we study the damping of mechanical resonators based on carbon nanotubes and graphene sheets. The damping is found to strongly depend on the amplitude of motion, and can be described by a nonlinear rather than a linear damping force. We exploit the nonlinear nature of damping in these systems to improve the figures of merit for both nanotube and graphene resonators. For instance, we achieve a quality factor of 100,000 for a graphene resonator.     

Scanning gate microscopy on graphene: charge inhomogeneity and extrinsic doping

We have performed scanning gate microscopy (SGM) on graphene field effect transistors (GFET) using a biased metallic nanowire coated with a dielectric layer as a contact mode tip and local top gate. Electrical transport through graphene at various back gate voltages is monitored as a function of tip voltage and tip position. Near the Dirac point, the response of graphene resistance to the tip voltage shows significant variation with tip position, and SGM imaging displays mesoscopic domains of electron-doped and hole-doped regions. Our measurements reveal substantial spatial fluctuation in the carrier density in graphene due to extrinsic local doping from sources such as metal contacts, graphene edges, structural defects and resist residues. Our scanning gate measurements also demonstrate graphene's excellent capability to sense the local electric field and charges.