F. Keith Perkins

Reduced Graphene Oxide Molecular Sensors

We demonstrate reduced graphene oxide as the active material for high-performance molecular sensors. Sensors are fabricated from exfoliated graphene oxide platelets that are deposited to form an ultrathin continuous network. These graphene oxide networks are tunably reduced toward graphene by varying the exposure time to a hydrazine hydrate vapor. The conductance change of the networks upon exposure to trace levels of vapor is measured as a function of the chemical reduction. The level of reduction affects both the sensitivity and the level of 1/ f noise. The sensors are capable of detecting 10 s exposures to simulants of the three main classes of chemical-warfare agents and an explosive at parts-per-billion concentrations.

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.

Quantum linear magnetoresistance in multilayer epitaxial graphene.

We report the first observation of linear magnetoresistance (LMR) in multilayer epitaxial graphene grown on SiC. We show that multilayer epitaxial graphene exhibits large LMR from 2.2 K up to room temperature and that it can be best explained by a purely quantum mechanical model. We attribute the observation of LMR to inhomogeneities in the epitaxially grown graphene film. The large magnitude of the LMR suggests potential for novel applications in areas such as high-density data storage and magnetic sensors and actuators.

Heterojunction fabrication by selective area chemical vapor deposition induced by synchrotron radiation

We have fabricated a B5C, boron‐carbide/Si(111) heterojunction diode by the synchrotron radiation‐induced decomposition of orthocarborane. This diode can be compared with similar boron‐carbide/Si(111) heterojunction diodes fabricated by plasma enhanced chemical vapor deposition. The synchrotron radiation induced chemical vapor deposition is postulated to occur via the decomposition of weakly chemisorbed species and the results suggest that ‘‘real‐time’’ projection lithography (selective area deposition) of boron‐carbide devices is possible.

Fabrication of 15 nm wide trenches in Si by vacuum scanning tunneling microscope lithography of an organosilane self‐assembled film and reactive ion etching

Organosilane precursor molecules, here (aminoethylaminomethyl)phenethyltrimethoxysilane, or PEDA, are chemisorbed onto a Si surface forming a monolayer thick film. These films are patterned using the scanning tunneling microscope to locally modify the chemical reactivity and are then used as a template for selective electroless plating of a thin Ni film. Reactive ion etching in a C2F6/O2 mixture transfers this pattern into the substrate. Improvement over previous lithographic performance is achieved by growing the films on a passivated and lightly oxidized Si surface, optimizing the patterning conditions, and improving the metallization and etch chemistry. In this way, we have generated deep trenches on the order of 15±4 nm width, with edge roughness of 3 nm. We believe this demonstrates the resolution limiting factors of this lithographic process.

Functionalized Ga2O3 nanowires as active material in room temperature capacitance-based gas sensors.

We report the first evidence for functionalization of Ga(2)O(3) nanowires (NWs), which have been incorporated as the active material in room temperature capacitance gas-sensing devices. An adsorbed layer of pyruvic acid (PA) was successfully formed on Ga(2)O(3) NWs by simple room temperature vapor transport, which was confirmed by Fourier transform infrared spectroscopy. The effect of the adsorbed PA on the surface properties was demonstrated by the change in the response of the NW gas-sensing devices. Results indicate that the adsorption of PA reduced the sensitivity of the Ga(2)O(3) NW device to common hydrocarbons such as nitromethane and acetone while improving the response to triethylamine by an order of magnitude. Taking into account the simplicity of this functionalization together with the ease of producing these capacitance-based gas-sensing devices, this approach represents a viable technique for sensor development.

Proximal probe study of self‐assembled monolayer resist materials

A scanning tunneling microscope (STM) has been used to investigate organosilane self‐assembled monolayer films (SAMs) as imaging layers for low voltage e‐beam lithography. We have studied three different SAMs [(aminoethylaminomethyl)phenethyltrimethoxysilane (PEDA), 4‐chloromethylphenyltrichlorosilane (CMPTS), and n‐octadecyltrichlorosilane (OTS)] deposited on the native oxide of Si. We have found OTS to act as a positive resist in wet etch processing upon exposure to 50 keV electrons, in agreement with Lercel [J. Vac. Sci. Technol. B 11, 2823 (1993)]. Identically processed samples patterned with low voltage (−10 to −25 V tip–sample bias) electrons in the STM exhibit a negative tone. STM biases below −10 V were insufficient to expose the film. An improved etch for STM‐generated patterns, leading to smoother surfaces, was developed. STM exposure of PEDA and CMPTS SAMs leads to a destruction of ligand functionality. An exposure threshold of −4 V bias for CMPTS and −8 V for PEDA has been established. As an e...

Disordered Nanomaterials for Chemielectric Vapor Sensing: A Review

Although robust chemical vapor detection by chemielectric point sensors remains as a largely unmet challenge at present, the best performance to date and the most likely avenue for future progress is with sensor designs in which the transductive element is a disordered nanostructured material. We here review the evidence for this claim, with illustrations drawn from recent work on sensors made from gold nanoparticles, carbon nanotubes, and reduced graphene oxide nanoplatelets. These examples can be regarded as being prototypical of disordered nanostructured films formed of primitive objects that are nanoscopic in 3-D, 2-D, and 1-D, respectively.

Extraordinary magnetoresistance in shunted chemical vapor deposition grown graphene devices

We report gate tunable linear magnetoresistances (MRs) of ∼600% at 12 T in metal-shunted devices fabricated on chemical vapor deposition (CVD) grown graphene. The effect occurs due to decreasing conduction through the shunt as the magnetic field increases (known as the extraordinary magnetoresistance effect) and yields an MR that is at least an order-of-magnitude higher than in un-shunted graphene devices.

Determination of acid diffusion rate in a chemically amplified resist with scanning tunneling microscope lithography

A scanning tunneling microscope (STM) was employed to investigate properties of the chemically amplified negative electron beam resist MICROPOSIT■ SAL‐601■, from Shipley Company. The usual processing sequence of the resist includes a postexposure bake (PEB) to promote the cross‐linking action of the acid catalyst. However, it has been proposed that PEB can induce thermally activated diffusion of the acid catalyst as well. Using the STM to generate a spatially localized beam of low kinetic energy electrons, we have investigated the effects of exposure dose and voltage and PEB time on linewidth in thin (50 nm) resist films. Even with no PEB, insoluble resist features were observable. Energy calculations do not support significant local heating. Therefore, cross‐linking must be due to the electron exposure. Dose and PEB times under 10 min have only a small effect on linewidth. From the variation of feature size with PEB time, an upper limit to the diffusion coefficient was measured to be 0.3 nm2/s. This valu...