Paul E. Sheehan

Properties of Fluorinated Graphene Films

Graphene films grown on Cu foils have been fluorinated with xenon difluoride (XeF(2)) gas on one or both sides. When exposed on one side the F coverage saturates at 25% (C(4)F), which is optically transparent, over 6 orders of magnitude more resistive than graphene, and readily patterned. Density functional calculations for varying coverages indicate that a C(4)F configuration is lowest in energy and that the calculated band gap increases with increasing coverage, becoming 2.93 eV for one C(4)F configuration. During defluorination, we find hydrazine treatment effectively removes fluorine while retaining graphenes carbon skeleton. The same films may be fluorinated on both sides by transferring graphene to a silicon-on-insulator substrate enabling XeF(2) gas to etch the Si underlayer and fluorinate the backside of the graphene film to form perfluorographane (CF) for which calculated the band gap is 3.07 eV. Our results indicate single-side fluorination provides the necessary electronic and optical changes to be practical for graphene device applications.

Nanotribology and Nanofabrication of MoO3 Structures by Atomic Force Microscopy

Atomic force microscopy was used to characterize the sliding of molybdenum oxide (MoO3) nanocrystals on single-crystal molybdenum disulfide (MoS2) surfaces. Highly anisotropic friction was observed whereby MoO3 nanocrystals moved only along specific directions of the MoS2 surface lattice. The energy per unit area to move the MoO3 nanocrystals along their preferred sliding direction was an order of magnitude less than required to slide macroscopic MoS2-bearing contacts. This extreme friction anisotropy was exploited to fabricate multicomponent MoO3 nanostructures. These reversibly interlocking structures could serve as the basis for devices such as mechanical logic gates.

The Assembly of Single-Layer Graphene Oxide and Graphene Using Molecular Templates

Charged molecular templates were created to direct the placement of single-layer graphite oxide (GO) sheets. The distribution of the GO sheets depended on the surface functionalization, background passivation, pH, and deposition time. Electrostatic attraction guides the templating of the GO sheets and, consequently, templating could be modulated by adjusting the pH of the deposition solution. In contrast to CNT immobilization, we find that the GO sheets do not adhere to the bare Au surface.

Conductance Anisotropy in Epitaxial Graphene Sheets Generated by Substrate Interactions

We present the first microscopic transport study of epitaxial graphene on SiC using an ultrahigh vacuum four-probe scanning tunneling microscope. Anisotropic conductivity is observed that is caused by the interaction between the graphene and the underlying substrate. These results can be explained by a model where charge buildup at the step edges leads to local scattering of charge carriers. This highlights the importance of considering substrate effects in proposed devices that utilize nanoscale patterning of graphene on electrically isolated substrates.

Chemical stability of graphene fluoride produced by exposure to XeF2.

Fluorination can alter the electronic properties of graphene and activate sites for subsequent chemistry. Here, we show that graphene fluorination depends on several variables, including XeF2 exposure and the choice of substrate. After fluorination, fluorine content declines by 50-80% over several days before stabilizing. While highly fluorinated samples remain insulating, mildly fluorinated samples regain some conductivity over this period. Finally, this loss does not reduce reactivity with alkylamines, suggesting that only nonvolatile fluorine participates in these reactions.

High-Density Amine-Terminated Monolayers Formed on Fluorinated CVD-Grown Graphene

There has been considerable interest in chemically functionalizing graphene films to control their electronic properties, to enhance their binding to other molecules for sensing, and to strengthen their interfaces with matrices in a composite material. Most reports to date have largely focused on noncovalent methods or the use of graphene oxide. Here, we present a method to activate CVD-grown graphene sheets using fluorination followed by reaction with ethylenediamine (EDA) to form covalent bonds. Reacted graphene was characterized via X-ray photoelectron spectroscopy (XPS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and Raman spectroscopy as well as measurements of electrical properties. The functionalization results in stable, densely packed layers, and the unbound amine of EDA was shown to be active toward subsequent chemical reactions.

Aminated graphene for DNA attachment produced via plasma functionalization

We demonstrate the use of a unique plasma source to controllably functionalize graphene with nitrogen and primary amines, thereby tuning the chemical, structural, and electrical properties. Critically, even highly aminated graphene remains electronically conductive, making it an ideal transduction material for biosensing. Proof-of-concept testing of aminated graphene as a bio-attachment platform in a biologically active field-effect transistor used for DNA detection is demonstrated.

Direct mechanochemical cleavage of functional groups from graphene

Mechanical stress can drive chemical reactions and is unique in that the reaction product can depend on both the magnitude and the direction of the applied force. Indeed, this directionality can drive chemical reactions impossible through conventional means. However, unlike heat- or pressure-driven reactions, mechanical stress is rarely applied isometrically, obscuring how mechanical inputs relate to the force applied to the bond. Here we report an atomic force microscope technique that can measure mechanically induced bond scission on graphene in real time with sensitivity to atomic-scale interactions. Quantitative measurements of the stress-driven reaction dynamics show that the reaction rate depends both on the bond being broken and on the tip material. Oxygen cleaves from graphene more readily than fluorine, which in turn cleaves more readily than hydrogen. The technique may be extended to study the mechanochemistry of any arbitrary combination of tip material, chemical group and substrate.

Van der Waals screening by single-layer graphene and molybdenum disulfide

A sharp tip of atomic force microscope is employed to probe van der Waals forces of a silicon oxide substrate with adhered graphene. Experimental results obtained in the range of distances from 3 to 20 nm indicate that single-, double-, and triple-layer graphenes screen the van der Waals forces of the substrate. Fluorination of graphene, which makes it electrically insulating, lifts the screening in the single-layer graphene. The van der Waals force from graphene determined per layer decreases with the number of layers. In addition, increased hole doping of graphene increases the force. Finally, we also demonstrate screening of the van der Waals forces of the silicon oxide substrate by single- and double-layer molybdenum disulfide.

Nanomachining, manipulation and fabrication by force microscopy

Force microscopy has been used to machine nanocrystals of and manipulate the resulting objects into new nanostructures. nanocrystals were grown on the surfaces of single crystals by controlled thermal oxidation. These nanocrystals may be moved and modified with a force microscope by controlling the applied load. The atomic structure of the nanocrystal-substrate interface is shown to constrain the motion of the nanocrystals to the lattice rows of the substrate, a phenomenon termed lattice-directed sliding. In addition, scanning in a direction off this preferred sliding direction can be used to machine the thus making it possible to selectively move and modify these nanocrystals. Significantly, our ability to machine and manipulate the nanocrystals has been exploited to fabricate interlocking nanostructures that can be reversibly assembled and disassembled.