Keith E. Whitener

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.

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.

Hydrogenated Graphene as a Homoepitaxial Tunnel Barrier for Spin and Charge Transport in Graphene

We demonstrate that hydrogenated graphene performs as a homoepitaxial tunnel barrier on a graphene charge/spin channel. We examine the tunneling behavior through measuring the IV curves and zero bias resistance. We also fabricate hydrogenated graphene/graphene nonlocal spin valves and measure the spin lifetimes using the Hanle effect, with spintronic nonlocal spin valve operation demonstrated up to room temperature. We show that while hydrogenated graphene indeed allows for spin transport in graphene and has many advantages over oxide tunnel barriers, it does not perform as well as similar fluorinated graphene/graphene devices, possibly due to the presence of magnetic moments in the hydrogenated graphene that act as spin scatterers.

Patterning Magnetic Regions in Hydrogenated Graphene Via E‐Beam Irradiation

Partially hydrogenated graphene is ferromagnetic and may be patterned by electron-beam irradiation. Sequential patterning produces a patterned magnetic array. Removal of the hydrogen atoms also can convert electrically insulating fully hydrogenated graphene back into conductive graphene, enabling the writing of chemically isolated, dehydrogenated graphene nanoribbons as narrow as 100 nm.

Robust reduction of graphene fluoride using an electrostatically biased scanning probe

AbstractWe report a novel and easily accessible method to chemically reduce graphene fluoride (GF) sheets with nanoscopic precision using high electrostatic fields generated between an atomic force microscope (AFM) tip and the GF substrate. Reduction of fluorine by the electric field produces graphene nanoribbons (GNR) with a width of 105-1,800 nm with sheet resistivity drastically decreased from >1 TΩ·sq.−1 (GF) down to 46 kΩ·sq.−1 (GNR). Fluorine reduction also changes the topography, friction, and work function of the GF. Kelvin probe force microscopy measurements indicate that the work function of GF is 180–280 meV greater than that of graphene. The reduction process was optimized by varying the AFM probe velocity between 1.2 μm·s−1 and 12 μm·s−1 and the bias voltage applied to the sample between −8 and −12 V. The electrostatic field required to remove fluorine from carbon is ∼1.6 V·nm−1. Reduction of the fluorine may be due to the softening of the C-F bond in this intense field or to the accumulation and hydrolysis of adventitious water into a meniscus.

Transfer of Chemically Modified Graphene with Retention of Functionality for Surface Engineering

Single-layer graphene chemically reduced by the Birch process delaminates from a Si/SiOx substrate when exposed to an ethanol/water mixture, enabling transfer of chemically functionalized graphene to arbitrary substrates such as metals, dielectrics, and polymers. Unlike in previous reports, the graphene retains hydrogen, methyl, and aryl functional groups during the transfer process. This enables one to functionalize the receiving substrate with the properties of the chemically modified graphene (CMG). For instance, magnetic force microscopy shows that the previously reported magnetic properties of partially hydrogenated graphene remain after transfer. We also transfer hydrogenated graphene from its copper growth substrate to a Si/SiOx wafer and thermally dehydrogenate it to demonstrate a polymer- and etchant-free graphene transfer for potential use in transmission electron microscopy. Finally, we show that the Birch reduction facilitates delamination of CMG by weakening van der Waals forces between graphene and its substrate.

Characterizing Multi-layer Pristine Graphene, Its Contaminants, and Their Origin Using Transmission Electron Microscopy

1. Materials Sci. and Tech. Division, U.S. Naval Research Laboratory, Washington, DC, USA 20375 2. Electronics Sci. and Tech. Division, U.S. Naval Research Laboratory, Washington, DC, USA 20375 3. Chemistry Division, U.S. Naval Research Laboratory, Washington, DC, USA 20375 4. NRC Postdoctoral Associate, U.S. Naval Research Laboratory, Washington, DC, USA 20375 5. Dept. of Physics and Nanoscience Technology Ctr., Univ. of Central Florida, Orlando, FL USA 32816 *current address: Dept. of Mat. Sci. and Eng., McMaster Univ., Hamilton, Ontario, Canada L9H 4L7

Activation of radical addition to graphene by chemical hydrogenation

We report several methods of chemical dehydrogenation of hydrogenated graphene (HG), characterizing the results using Raman, X-ray photoelectron spectroscopy, and electrical conductivity measurements. Notably, the hydrogen–graphene bonds appear to activate the graphene toward subsequent reaction such that, in several cases, the addition of the dehydrogenating agent to the graphene accompanies the removal of hydrogen. We compare the uptake of chemical groups on HG to those on pristine graphene and find that HG reacts more readily than pristine graphene with radical generators such as chlorine and AIBN.

Hydrogen-assisted graphene transfer: surface engineering for chemical, electronic, and biological applications

Functional surfaces find application in a number of areas, such as designing flexible electronic devices and integrating electronic systems with biological ones. However, the preparation of functional surfaces entails processing that is destructive to fragile polymer or biological substrates. A benign transfer method is thus needed to move pre-functionalized surfaces from a stable substrate to a fragile one. Chemical hydrogenation of graphene weakens the adhesion force between the graphene and its substrate. We exploit this phenomenon to construct a method for transferring graphene with pre-formed chemical, physical, and electronic functionalities from a heat-, vacuum-, and chemical-stable substrate such as silicon to several less robust ones, including polymers and living cells. We also discuss reversibility of graphene hydrogenation and the implications for re-adhering graphene securely to new substrates.

Protection from Below: Stabilizing Hydrogenated Graphene Using Graphene Underlayers

We show that dehydrogenation of hydrogenated graphene proceeds much more slowly for bilayer systems than for single layer systems. We observe that an underlayer of either pristine or hydrogenated graphene will protect an overlayer of hydrogenated graphene against a number of chemical oxidants, thermal dehydrogenation, and degradation in an ambient environment over extended periods of time. Chemical protection depends on the ease of oxidant intercalation, with good intercalants such as Br2 demonstrating much higher reactivity than poor intercalants such as 1,2-dichloro-4,5-dicyanonbenzoquinone (DDQ). Additionally, the rate of dehydrogenation of hydrogenated graphene at 300 °C in H2/Ar was reduced by a factor of roughly 10 in the presence of a protective underlayer of graphene or hydrogenated graphene. Finally, the slow dehydrogenation of hydrogenated graphene in air at room temperature, which is normally apparent after a week, could be completely eliminated in samples with protective underlayers over the course of 39 days. Such protection will be critical for ensuring the long-term stability of devices made from functionalized graphene.