We probe electron and hole mobilities in bilayer graphene under exposure to molecular oxygen. We find that the adsorbed oxygen reduces electron mobilities and increases hole mobilities in a reversible and activated process. Our experimental results indicate that hole mobilities increase due to the screening of long-range scatterers by oxygen molecules trapped between the graphene and the substrate. First principle calculations show that oxygen molecules induce resonant states close to the charge neutrality point. Electron coupling with such resonant states reduces the electron mobilities, causing a strong asymmetry between electron and hole transport. Our work demonstrates the importance of short-range scattering due to adsorbed species in the electronic transport in bilayer graphene on SiO2 substrates.

The temperature dependence of the photoluminescence properties of a thin film of poly[2-methoxy-5-(2(')-ethylhexyloxy)-p-phenylene-vinylene], MEH-PPV, fabricated by spin coating, is analyzed. The evolution with temperature of the peak energy of the purely electronic transition, of the first vibronic band, of the effective conjugation length, and of the Huang-Rhys factors are discussed. The asymmetric character of the pure electronic transition peak and the contribution of the individual vibrational modes to the first vibronic band line shape are considered by a model developed by Cury et al. [J. Chem. Phys. 121, 3836 (2004)]. The temperature dependence of the Huang-Rhys factors of the main vibrational modes pertaining to the first vibronic band allows us to identify two competing vibrational modes. These results show that the electron coupling to different vibrational modes depends on temperature via reduction of thermal disorder.

Graphene is regarded as the toughest two-dimensional material (highest in-plane elastic properties) and, as a consequence, it has been employed/proposed as an ultrathin membrane in a myriad of microfluidic devices. Yet, an experimental investigation of eventual variations on the apparent elastic properties of a suspended graphene membrane in contact with air or water is still missing. In this work, the mechanical response of suspended monolayer graphene membranes on a microfluidic platform is investigated via scanning probe microscopy experiments. A high elastic modulus is measured for the membrane when the platform is filled with air, as expected. However, a significant apparent softening of graphene is observed when water fills the microfluidic system. Through molecular dynamics simulations and a phenomenological model, we associate such softening to a water-induced uncrumpling process of the suspended graphene membrane. This result may bring substantial modifications on the design and operation of microfluidic devices which exploit pressure application on graphene membranes.

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We present a resist-free patterning technique to form electrically contacted graphene nanochannels via localized burning by a pulsed white light source. The technique uses end-point detection to stop the burning process at a fixed resistance to produce channels with resistances of 10 kX to 100 kX. Folding of the graphene sheet takes place during patterning, which provides very straight edges as identified by AFM and SEM. Electrical transport measurements for the nanochannels show a non-linear behavior of the current vs source-drain voltage as the resistance goes above 20 kX indicating conduction tunneling effects. Electrochemical gating was performed to further electrically characterize the constrictions produced. The method described can be interesting not only for fundamental studies correlating edge folded structures with electrical transport but also as a promising path for fabricating graphene devices in situ. Additionally, this method might also be extended to create nanochannels in other 2D materials. V C 2015 AIP Publishing LLC.Since the isolation of graphene in 2004 1 by micromechanical exfoliation of graphite, its electronic, mechanical, and structural properties have been studied extensively. [2][3][4][5][6] With the advent of chemical vapor deposition (CVD) techniques, large-area single layer graphene (SLG) became available, 7-9 making possible top-down device architectures where the graphene is patterned into desired shapes. Patterning graphene into nanochannels is a pathway to high performance electronics 10,11 and is also interesting for biosensing applications such as DNA sequencing. 12 One challenge is controlling the properties of the edges of these structures, which can lead to strong disorder. On the other hand, the production of folded edges has been predicted as an alternate way to modify graphene electronic structure and enhance its mechanical properties. [13][14][15][16][17][18][19][20] Recently, the ablation of graphene by ultra-short laser pulses has been demonstrated, and this technique often generates folded graphene edges; 21-25 however, so far the previous works have been focused on either the patterning of graphene into microribbons or on the understanding of the ablation process itself. Differently, here, we used this approach to achieve folded graphene nanochannels down to 30 nm in width with controllable resistance ranging from 10 kX to 100 kX. A focused pulsed laser is scanned along the graphene to define a cut while simultaneously employing end-point detection to turn off the laser when a desired resistance is reached. Additionally, we also have performed a detailed structural analysis (by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and TEM) of the morphology of the folded structures produced, as well as nonlinear electrical measurements and electrolyte gating to further characterized the nanochannels.We start by describing the sample preparation. Graphene is grown on Cu foils inside a CVD chamber at 1000 C and low pressure of H 2 /CH 4 (P total ¼ 0.12 To...

between water and graphene is crucial for building up novel and smart biointerfaces. [18,19] Additionally, the study of reactivity and structure of water at the graphene interface has also generated intriguing questions and controversial results. [20,21] For instance, several experimental works demonstrate that the charge transfer process that happens between graphene and water molecules is highly dependent on the underlying substrate. [20,22] Thus, it would be highly desirable to elucidate the above discussion by probing the electrical response of a suspended graphene membrane in contact with water without the presence of any substrate. We also believe that a precise understanding of the electrochemical behavior of water/graphene interface would be fundamental for developing novel and superior electrical, mechanical, and optical devices.In the present work, we develop a microfluidic platform that integrates suspended graphene membrane windows (with electrical contacts) with buried fluid channels to probe the electrical response of a graphene membrane in contact with water. The platform design provides a direct probing of the electrical response of the air/graphene/liquid interface without the presence of any underlying substrate. [23] Our results show a significant change of graphene resistivity (of about 25%) due to the presence of water in the microchannel. The identification of the physical mechanisms behind such strong change in resistivity is not A water-induced electromechanical response in suspended graphene atop a microfluidic channel is reported. The graphene membrane resistivity rapidly decreases to ≈25% upon water injection into the channel, defining a sensitive "channel wetting" device-a wetristor. The physical mechanism of the wetristor operation is investigated using two graphene membrane geometries, either uncovered or covered by an inert and rigid lid (hexagonal boron nitride multilayer or poly(methyl methacrylate) film). The wetristor effect, namely the water-induced resistivity collapse, occurs in uncovered devices only. Atomic force microscopy and Raman spectroscopy indicate substantial morphology changes of graphene membranes in such devices, while covered membranes suffer no changes, upon channel water filling. The results suggest an electromechanical nature for the wetristor effect, where the resistivity reduction is caused by unwrinkling of the graphene membrane through channel filling, with an eventual direct doping caused by water being of much smaller magnitude, if any. The wetristor device should find useful sensing applications in general micro-and nanofluidics.

A method based on the unidimensional gain equation has been developed in order to fit the experimental data due to amplification of spontaneous emission in a thin film of conjugated polymer waveguide. The results have confirmed not only a dependence of the gain coefficient on the laser intensity but also on the length of the excitation laser stripe. The results are presented as a function of the average intensity in W/cm2, which is a manner to express the threshold intensity for a direct comparison between different materials, independent of the setup used.

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