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We report a method to introduce direct bonding between graphene platelets that enables the transformation of a multilayer chemically modified graphene (CMG) film from a paper mache-like structure into a stiff, high strength material. On the basis of chemical/defect manipulation and recrystallization, this technique allows wide-range engineering of mechanical properties (stiffness, strength, density, and built-in stress) in ultrathin CMG films. A dramatic increase in the Young s modulus (up to 800 GPa) and enhanced strength (sustainable stress /=1 GPa) due to cross-linking, in combination with high tensile stress, produced high-performance (quality factor of 31000 at room temperature) radio frequency nanomechanical resonators. The ability to fine-tune intraplatelet mechanical properties through chemical modification and to locally activate direct carbon carbon bonding within carbon-based nanomaterials will transform these systems into true materials-by-design for nanomechanics., Published in NANO Letters, p4212-, 5 Jul 2012. Sponsored in part by NRL s Nanoscience Institute.

In this article, we review our efforts to continuously tune mechanical and thermal properties in multilayer chemically modified graphene (CMG) films. An alteration of the graphene lattice by functional groups, by defects created during reduction, or by defect re-crystallization is used to control CMG mechanical and thermal properties. We attribute a notable increase in Young s modulus and film strength to an emerging network of sp2 sp3 crosslinks established between graphene layers. Control over the film stress and strength enabled us to dramatically improve the performance of radio frequency CMG resonators by fine tuning the fabrication process., Pub. in Solid State Communications, v152, p1990-1998, 2012.

Polymer nanostructures were directly written onto substrates in ultra-high vacuum. The polymer ink was coated onto atomic force microscope (AFM) probes that could be heated to control the ink viscosity. Then, the ink-coated probes were placed into an ultrahigh vacuum (UHV) AFM and used to write polymer nanostructures on surfaces, including surfaces cleaned in UHV. Controlling the writing speed of the tip enabled the control over the number of monolayers of the polymer ink deposited on the surface from a single to tens of monolayers, with higher writing speeds generating thinner polymer nanostructures. Deposition onto silicon oxideterminated substrates led to polymer chains standing upright on the surface, whereas deposition onto vacuum reconstructed silicon yielded polymer chains aligned along the surface., Published in Beilstein Journal of Nanotechnology v3, p52-56, 19 Jan 2012. The original document contains color images.

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., Published in NANO Letters, v10 p1559-1562, 2010.

Graphene films grown on Cu foils have been fluorinated with xenon difluoride (XeF2) gas on one or both sides. When exposed on one side the F coverage saturates at 25% (C4F), 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 C4F configuration is lowest in energy and that the calculated band gap increases with increasing coverage, becoming 2.93 eV for one C4F configuration. During defluorination, we find hydrazine treatment effectively removes fluorine while retaining graphene's carbon skeleton. The same films may be fluorinated on both sides by transferring graphene to a silicon-on-insulator substrate enabling XeF2 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., Published in NANO Letters, v10 p3001-3005, 2010.

Nanoparticle polymer composites containing metal, semiconductor, magnetic, and optically active nanoparticles were deposited onto multiple substrates from a heatable atomic force microscope tip. The nanoparticle nanostructures were functional as deposited or could be etched with an oxygen plasma, revealing single nanoparticle lithographic resolution. Many types of nanoparticles can be patterned with the same technique, without the need to tailor the substrate chemistry and without solution processing., Published in Nano Letters, 2009. Prepared in collaboration with University of Illinois, Urbana-Champaign. Sponsored in part by DARPA, by Office of Naval Research (ONR), and by National Science Foundation (NSF).