Approaching ballistic transport in suspended graphene

The discovery of graphene raises the prospect of a new class of nanoelectronic devices based on the extraordinary physical properties of this one-atom-thick layer of carbon. Unlike two-dimensional electron layers in semiconductors, where the charge carriers become immobile at low densities, the carrier mobility in graphene can remain high, even when their density vanishes at the Dirac point. However, when the graphene sample is supported on an insulating substrate, potential fluctuations induce charge puddles that obscure the Dirac point physics. Here we show that the fluctuations are significantly reduced in suspended graphene samples and we report low-temperature mobility approaching 200,000 cm2 V-1 s-1 for carrier densities below 5 x 109 cm-2. Such values cannot be attained in semiconductors or non-suspended graphene. Moreover, unlike graphene samples supported by a substrate, the conductivity of suspended graphene at the Dirac point is strongly dependent on temperature and approaches ballistic values at liquid helium temperatures. At higher temperatures, above 100 K, we observe the onset of thermally induced long-range scattering.     

Spin transport in high-quality suspended graphene devices

We measure spin transport in high mobility suspended graphene (μ ≈ 10(5)cm(2)/(V s)), obtaining a (spin) diffusion coefficient of 0.1 m(2)/s and giving a lower bound on the spin relaxation time (τ(s) ≈ 150 ps) and spin relaxation length (λ(s) = 4.7 μm) for intrinsic graphene. We develop a theoretical model considering the different graphene regions of our devices that explains our experimental data.     

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.     

Discrete dynamics of nanoparticle channelling in suspended graphene

We have observed a previously undescribed stepwise oxidation of mono- and few layer suspended graphene by silver nanoparticles in situ at subnanometer scale in an environmental transmission electron microscope. Over the range of 600-850 K, we observe crystallographically oriented channelling with rates in the range 0.01-1 nm/s and calculate an activation energy of 0.557 ± 0.016 eV. We present a discrete statistical model for this process and discuss the implications for accurate nanoscale patterning of nanoscale systems.     

Thermal transport in suspended and supported few-layer graphene

We report thermal conductivity (κ) measurements from 77 to 350 K on both suspended and supported few-layer graphene using a thermal-bridge configuration. The room temperature value of κ is comparable to that of bulk graphite for the largest flake, but reduces significantly for smaller flakes. The presence of a substrate lowers the value of κ, but the effect diminishes for the thermal transport in the top layers away from the substrate. For the suspended sample, the temperature dependence of κ follows a power law with an exponent of 1.4 ± 0.1, suggesting that the flexural phonon modes contribute significantly to the thermal transport of the suspended graphene. The measured values of κ are generally lower than those from theoretical studies. We attribute this deviation to the phonon-boundary scattering at the graphene-contact interfaces, which is shown to significantly reduce the apparent measured thermal conductance of graphene.     

Fast and controllable fabrication of suspended graphene nanopore devices

A poly(methyl methacrylate) assisted dry transfer method was developed to transfer graphene microflake onto a suspended SiN chip in an effective and efficient way for further graphene nanopore drilling for DNA analysis. Graphene microflakes can be patterned by e-beam lithography to a designed shape and size on a large scale of a few thousands simultaneously. Subsequently, individual graphene microflakes can be picked up and transferred to a target hole on a suspended SiN membrane with 1?μm precision via a site-specific transfer-printing method. Nanopores with different diameters from 3 to 20?nm were drilled on the as-transferred graphene membrane in a transmission electron microscope. This method offers a fast and controllable way to fabricate graphene nanopores for DNA analyses.     

Carbon nanotube field effect transistors with suspended graphene gates

Novel field effect transistors with suspended graphene gates are demonstrated. By incorporating mechanical motion of the gate electrode, it is possible to improve the switching characteristics compared to a static gate, as shown by a combination of experimental measurements and numerical simulations. The mechanical motion of the graphene gate is confirmed by using atomic force microscopy to directly measure the electrostatic deflection. The device geometry investigated here can also provide a sensitive measurement technique for detecting high-frequency motion of suspended membranes as required, e.g., for mass sensing.     

Mapping the nanomechanical properties of graphene suspended on silica nanoparticles

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 SiO₂ nanoparticles 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.     

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).     

Influence of polymeric residue on the thermal conductivity of suspended bilayer graphene

The thermal conductivity (κ) of two bilayer graphene samples each suspended between two microresistance thermometers was measured to be 620 ± 80 and 560 ± 70 W m(-1) K(-1) at room temperature and exhibits a κ ∝ T(1.5) behavior at temperatures (T) between 50 and 125 K. The lower κ than that calculated for suspended graphene along with the temperature dependence is attributed to scattering of phonons in the bilayer graphene by a residual polymeric layer that was clearly observed by transmission electron microscopy.     

Fractional quantum Hall effect in suspended graphene probed with two-terminal measurements

Recently, fractional quantization of two-terminal conductance was reported in suspended graphene. The quantization, which was clearly visible in fields as low as 2 T and persistent up to 20 K in 12 T, was attributed to the formation of an incompressible fractional quantum Hall state. Here, we argue that the failure of earlier experiments to detect the integer and fractional quantum Hall effect with a Hall-bar lead geometry is a consequence of the invasive character of voltage probes in mesoscopic samples, which are easily shorted out owing to the formation of hot spots near the edges of the sample. This conclusion is supported by a detailed comparison with a solvable transport model. We also consider, and rule out, an alternative interpretation of the quantization in terms of the formation of a p-n-p junction, which could result from contact doping or density inhomogeneity. Finally, we discuss the estimate of the quasi-particle gap of the quantum Hall state. The gap value, obtained from the transport data using a conformal mapping technique, is considerably larger than in GaAs-based two-dimensional electron systems, reflecting the stronger Coulomb interactions in graphene.     

Room-temperature high on/off ratio in suspended graphene nanoribbon field-effect transistors

We have fabricated suspended few-layer (1-3 layers) graphene nanoribbon field-effect transistors from unzipped multi-wall carbon nanotubes. Electrical transport measurements show that current annealing effectively removes the impurities on the suspended graphene nanoribbons, uncovering the intrinsic ambipolar transfer characteristic of graphene. Further increasing the annealing current creates a narrow constriction in the ribbon, leading to the formation of a large bandgap and subsequent high on/off ratio (which can exceed 10(4)). Such fabricated devices are thermally and mechanically stable: repeated thermal cycling has little effect on their electrical properties. This work shows for the first time that ambipolar field-effect characteristics and high on/off ratios at room temperature can be achieved in relatively wide graphene nanoribbons (15-50 nm) by controlled current annealing.     

Thermal conductivity measurements of suspended graphene with and without wrinkles by micro-Raman mapping

The thermal conductivity (κ) of suspended graphene membranes made by chemical vapor deposition (CVD) was measured by micro-Raman mapping. Cracks and wrinkles present in these suspended graphene membranes were identified by micro-Raman mapping, and κ values and their statistics were obtained on membranes free of such imperfections in a single mapping. Based on this new technique, an average κ value of 1875?±?220?W?m(-1)?K(-1) at 420?K was measured on 26 suspended graphene membranes that were free of wrinkles, ～27% higher than the average value measured from 12 graphene membranes with wrinkles. These results suggest that the variation in published thermal conductivity values for suspended graphene samples could, at least in part, be due to the presence or absence of wrinkles.     

Studies of intrinsic hot phonon dynamics in suspended graphene by transient absorption microscopy

Correlated transient absorption and atomic force microscopy (AFM) measurements have been performed for monolayer graphene, both free-standing and supported on a glass substrate. The AFM images allow us to locate regions of the suspended graphene. The transient absorption traces show a fast instrument response limited decay, followed by a slower intensity dependent decay. The fast decay is assigned to a combination of coupling between the excited charge carriers and the optical phonon modes of graphene and the substrate, and diffusion of the charge carrier out of the probe region. The slow decay is due to the hot phonon effect and reflects the lifetime of the intrinsic optical phonons of graphene. The time constant for the slow decay is longer for suspended graphene compared to substrate-supported graphene. This is attributed to interactions between the excited charge carriers and the surface optical phonon modes of the substrate, which supplies an additional relaxation channel for supported graphene.     

Fluorination of Polymer Surfaces

Polyethylene greenhouse foils show an extraordinary prolonged lifetime at exposure to natural and artificial weathering if a gas‐phase fluorination under low‐pressure conditions was applied. The fluorination was performed by F₂/N₂ mixtures and provided ca. 50 F/100 C (fluorination degree ≈ 25%). The lifetime of PE greenhouse foils at exposure to artificial or natural weathering were increased by at least the factor 2–4 measured in terms of tensile strength and break at elongation.     

Fluorination of CNx Nanotubes

Multiwall CN_x nanotubes have been prepared by thermal decomposition of acetonitrile over Co/Ni catalytic particles. The fluorination of nanotubes was performed at room temperature by using a gaseous mixture of BrF₃ and Br₂. Transmission electron microscopy (TEM) and x-ray diffraction (XRD) indicated that only the outer shells of CN_x nanotubes were fluorinated, whereas the inner shells remained intact. X-ray photoelectron spectroscopy (XPS) showed an oxidation of pyridinic-type nitrogen with tube fluorination.     

Fluorination of CN x Nanotubes

Abstract

Multiwall CN _x nanotubes have been prepared by thermal decomposition of acetonitrile over Co/Ni catalytic particles. The fluorination of nanotubes was performed at room temperature by using a gaseous mixture of BrF₃ and Br₂. Transmission electron microscopy (TEM) and x‐ray diffraction (XRD) indicated that only the outer shells of CN _x nanotubes were fluorinated, whereas the inner shells remained intact. X‐ray photoelectron spectroscopy (XPS) showed an oxidation of pyridinic‐type nitrogen with tube fluorination.     

Fluorination of Underdoped Mercurocuprate Superconductors

We present the results of a study into the incorporation of fluorine into the mercurocuprate superconductors. In all cases, the effect of fluorination has been to increase the T_c of underdoped superconductors to the maxima reported. This has important ramifications for the role of the interstitial oxygen in the mercurocuprate superconductors. To assist us in our study, we have, additionally, developed a novel synthetic procedure for preparing HgBa₂CuO₄₊ in a highly underdoped state. Remarkably, this material shows an increase in T_c, upon fluorination, of 60K!