Activating "Invisible" Glue: Using Electron Beam for Enhancement of Interfacial Properties of Graphene-Metal Contact.

Interfacial contact of two-dimensional graphene with three-dimensional metal electrodes is crucial to engineering high-performance graphene-based nanodevices with superior performance. Here, we report on the development of a rapid "nanowelding" method for enhancing properties of interface to graphene buried under metal electrodes using a focused electron beam induced deposition (FEBID). High energy electron irradiation activates two-dimensional graphene structure by generation of structural defects at the interface to metal contacts with subsequent strong bonding via FEBID of an atomically thin graphitic interlayer formed by low energy secondary electron-assisted dissociation of entrapped hydrocarbon contaminants. Comprehensive investigation is conducted to demonstrate formation of the FEBID graphitic interlayer and its impact on contact properties of graphene devices achieved via strong electromechanical coupling at graphene-metal interfaces. Reduction of the device electrical resistance by ∼50% at a Dirac point and by ∼30% at the gate voltage far from the Dirac point is obtained with concurrent improvement in thermomechanical reliability of the contact interface. Importantly, the process is rapid and has an excellent insertion potential into a conventional fabrication workflow of graphene-based nanodevices through single-step postprocessing modification of interfacial properties at the buried heterogeneous contact.

Controlling the physicochemical state of carbon on graphene using focused electron-beam-induced deposition.

Focused electron-beam-induced deposition (FEBID) is a promising nanolithography technique using "direct-write" patterning by carbon line and dot deposits on graphene. Understanding interactions between deposited carbon molecules and graphene enables highly localized modification of graphene properties, which is foundational to the FEBID utility as a nanopatterning tool. In this study, we demonstrate a unique possibility to induce dramatically different adsorption states of FEBID-produced carbon deposits on graphene, through density functional theory calculations and complementary Raman experiments. Specifically, an amorphous carbon deposit formed by direct irradiation of high energy primary electrons exhibits unusually strong interactions with graphene via covalent bonding, whereas the FEBID carbon formed due to low-energy secondary electrons is only weakly interacting with graphene via physisorption. These observations not only are of fundamental importance to basic physical chemistry of FEBID carbon-graphene interactions but also enable the use of selective laser-assisted postdeposition ablation to effectively remove the parasitically deposited, physisorbed carbon films for improving FEBID patterning resolution.

Chemical reduction of individual graphene oxide sheets as revealed by electrostatic force microscopy.

We report continuous monitoring of heterogeneously distributed oxygenated functionalities on the entire surface of the individual graphene oxide flake during the chemical reduction process. The charge densities over the surface with mixed oxidized and graphitic domains were observed for the same flake after a step-by-step chemical reduction process using electrostatic force microscopy. Quantitative analysis revealed heavily oxidized nanoscale domains (50-100 nm across) on the graphene oxide surface and a complex reduction mechanism involving leaching of sharp oxidized asperities from the surface followed by gradual thinning and formation of uniformly mixed oxidized and graphitic domains across the entire flake.

Competitive adsorption of dopamine and rhodamine 6G on the surface of graphene oxide.

Competitive adsorption-desorption behavior of popular fluorescent labeling and bioanalyte molecules, Rhodamine 6G (R6G) and dopamine (DA), on a chemically heterogeneous graphene oxide (GO) surface is discussed in this study. Individually, R6G and DA compounds were found to adsorb rapidly on the surface of graphene oxide as they followed the traditional Langmuir adsorption behavior. FTIR analysis suggested that both R6G and DA molecules predominantly adsorb on the hydrophilic oxidized regions of the GO surface. Thus, when R6G and DA compounds were adsorbed from mixed solution, competitive adsorption was observed around the oxygen-containing groups of GO sheets, which resulted in partial desorption of R6G molecules from the surface of GO into the solution. The desorbed R6G molecules can be monitored by fluorescence change in solution and was dependent on the DA concentration. We suggest that the efficient competitive adsorption of different strongly bound bioanalytes onto GO-dye complex can be used for the development of sensitive and selective colorimetric biosensors.

Written-in conductive patterns on robust graphene oxide biopaper by electrochemical microstamping.

The silk road: By employing silk fibroin as a binder between graphene oxide films and aluminum foil for a facile, highly localized reduction process, conductive paper is reinvented. The flexible, robust biographene papers have high toughness and electrical conductivity. This electrochemical written-in approach is readily applicable for the fabrication of conductive patterned papers with complex circuitries.

Star polymer unimicelles on graphene oxide flakes.

We report the interfacial assembly of amphiphilic heteroarm star copolymers (PSnP2VPn and PSn(P2VP-b-PtBA)n (n = 28 arms)) on graphene oxide flakes at the air-water interface. Adsorption, spreading, and ordering of star polymer micelles on the surface of the basal plane and edge of monolayer graphene oxide sheets were investigated on a Langmuir trough. This interface-mediated assembly resulted in micelle-decorated graphene oxide sheets with uniform spacing and organized morphology. We found that the surface activity of solvated graphene oxide sheets enables star polymer surfactants to subsequently adsorb on the presuspended graphene oxide sheets, thereby producing a bilayer complex. The positively charged heterocyclic pyridine-containing star polymers exhibited strong affinity onto the basal plane and edge of graphene oxide, leading to a well-organized and long-range ordered discrete micelle assembly. The preferred binding can be related to the increased conformational entropy due to the reduction of interarm repulsion. The extent of coverage was tuned by controlling assembly parameters such as concentration and solvent polarity. The polymer micelles on the basal plane remained incompressible under lateral compression in contrast to ones on the water surface due to strongly repulsive confined arms on the polar surface of graphene oxide and a preventive barrier in the form of the sheet edges. The densely packed biphasic tile-like morphology was evident, suggesting the high interfacial stability and mechanically stiff nature of graphene oxide sheets decorated with star polymer micelles. This noncovalent assembly represents a facile route for the control and fabrication of graphene oxide-inclusive ultrathin hybrid films applicable for layered nanocomposites.

A robust and facile approach to assembling mobile and highly-open unfrustrated triangular lattices from ferromagnetic nanorods.

A simple and widely applicable approach to assemble long-range two-dimensional mobile arrays of functionalized nickel nanorods with tunable and "highly open" lattice structures is presented. The magnetic assembly of uniformly oriented nanorods in triangular lattices was achieved by a phase separation of the surface confined yet mobile vertical nanorods driven by a gradient magnetic field. In contrast to known approaches, the unfrustrated lattices can be further locked in place allowing for the removal of the applied magnetic field and processing without disrupting the initial order with different symmetries precisely assembled and locked in their position on the same substrate. We suggest that the tunable assemblies of magnetic nanorods provide a versatile platform for downstream handling of open lattice arrays for eventual device integration.

A new twist on scanning thermal microscopy.

The thermal bimorph is a very popular thermal sensing mechanism used in various applications from meat thermometers to uncooled infrared cameras. While thermal bimorphs have remained promising for scanning thermal microscopy, unfortunately the bending of the bimorph directly interferes with the bending associated with topographical information. We circumvent this issue by creating bimorphs that twist instead of bending and demonstrate the superior properties of this approach as compared to conventional scanning thermal microscopy.

Thermally induced transformations of amorphous carbon nanostructures fabricated by electron beam induced deposition.

We studied the thermally induced phase transformations of electron-beam-induced deposited (EBID) amorphous carbon nanostructures by correlating the changes in its morphology with internal microstructure by using combined atomic force microscopy (AFM) and high resolution confocal Raman microscopy. These carbon deposits can be used to create heterogeneous junctions in electronic devices commonly known as carbon-metal interconnects. We compared two basic shapes of EBID deposits: dots/pillars with widths from 50 to 600 nm and heights from 50 to 500 nm and lines with variable heights from 10 to 150 nm but having a constant length of 6 µm. We observed that during thermal annealing, the nanoscale amorphous deposits go through multistage transformation including dehydration and stress-relaxation around 150 °C, dehydrogenation within 150-300 °C, followed by graphitization (>350 °C) and formation of nanocrystalline, highly densified graphitic deposits around 450 °C. The later stage of transformation occurs well below commonly observed graphitization for bulk carbon (600-800 °C). It was observed that the shape of the deposits contribute significantly to the phase transformations. We suggested that this difference is controlled by different contributions from interfacial footprints area. Moreover, the rate of graphitization was different for deposits of different shapes with the lines showing a much stronger dependence of its structure on the density than the dots.

Graphene oxide--polyelectrolyte nanomembranes.

Owing to its remarkable electrical, thermal, and mechanical properties, graphene, an atomic layer of carbon, is considered to be an excellent two-dimensional filler for polymer nanocomposites with outstanding mechanical strength along with the potential for excellent electrical and thermal properties. One of the critical limitations with conventional fillers is that the loading fraction required for achieving significant improvement in mechanical properties is relatively high, frequently reaching 50% for maximum strength. Here, we demonstrate that the mechanical properties of ultrathin laminated nanocomposites can be significantly enhanced by the incorporation of small amounts of a dense monolayer of planar graphene oxide (GO) flakes. Negatively charged functionalized graphene oxide layers were incorporated into polyelectrolyte multilayers (PEMs) fabricated in a layer-by-layer (LbL) assembly via Langmuir-Blodgett (LB) deposition. These LbL-LB graphene oxide nanocomposite films were released as robust freely standing membranes with large lateral dimensions (centimeters) and a thickness of around 50 nm. Micromechanical measurements showed enhancement of the elastic modulus by an order of magnitude, from 1.5 GPa for pure LbL membranes to about 20 GPa for only 8.0 vol % graphene oxide encapsulated LbL membranes. These tough nanocomposite PEMs can be freely suspended over large (few millimeters) apertures and sustain large mechanical deformations.