The integration of graphene and 2D materials into device technologies requires a detailed understanding of how intrinsic and extrinsic forces impact their properties, as well as the development of engineering strategies to vary their properties for a specific response. In this paper we describe and review our efforts for hybridizing graphene in different ways so as to modify or enhance a range of properties. This hybridization comes in the form of chemical or electronic modification for use in applications ranging from chem/bio sensors to nanoelectronics. We discuss results on exploiting chemistry and defects in graphene for chemical vapor sensing, on hybridizing graphene with fluorine atoms for potential use in nanoelectronics, and on electronically hybridizing graphene in multilayer stacks that give rise to new optical and surface properties.

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.

Introduction: Semiconducting nanowires and nanotubes have great potential as chemical and biological ChemFET-type sensors in a variety of military, industrial, and commercial applications. For example, the proliferation of improvised explosive devices (IEDs) and recent terrorist attempts demonstrate the need for portable and sensitive sensors for detecting explosives vapors and toxic industrial chemicals (TICs). The most common ChemFET configurations use electrical conductivity measurements across single nanowires or a network of nanowires placed randomly on a surface. Nanowire-based ChemFETs are extremely sensitive to vapors because of their large surface-to-volume ratios; however, they have underperformed as portable, field-deployable sensors. The lower than anticipated performance is due to a combination of factors including variable or uncontrolled doping and dimensions, substrate effects such as adsorption/desorption and trapping on the supporting substrate surface, variable surface structure, occlusion of the analyte by the electrodes, 1/f noise from a reduced number of charge carriers in single nanowire configurations, and shot noise from wire-to-wire junctions in networks of overlapping nanowires.

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 31,000 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.

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.

Graphene is one of the most interesting materials synthesized in the last years which two-dimensional nature along with the fascinating electronic properties attracts the great attention from the scientific community. Graphene has an unique electronic properties such as the linear dispersion law leading to zero effective mass for electrons and holes. It already applied as solar cell [1], liquid crystal device [2], molecular sensor [3] and nano-sized transistor prototype [4].

Molecular adsorption of nitroaromatic molecules on single-walled carbon nanotubes (NTs) was investigated using state-of-the-art ab initio calculations and conductivity measurements. In the calculations we considered both armchair and zigzag NTs of several diameters and treated adsorption both on pristine sidewalls and oxygenated defects. The resulting adsorption energies on the pristine sidewalls were found to increase linearly with the number of NO2 groups and with the NT diameter. The adsorption mechanism is predominantly physisorption through pi-pi stacking, with additional contributions from the NO2 groups. The calculated adsorption energies at oxygenated defects were found to be comparable with those on the pristine sidewall. In both cases the functional groups give rise to local electronic polarization and to shifts in vibrational lines due to small charge transfers. The dependence of adsorption energies on the number of NO2 groups was found to be consistent with conductivity measurements on NT arrays exposed to trace amounts of nitroaromatic derivatives.

We have measured internal friction and shear modulus of both reduced graphene oxide and chemical-vapor deposited graphene films measuring as thin as 5nm. Graphene oxide films were deposited from solutions by spin-coating, and graphene films were synthesized by chemical-vapor deposition (CVD) on Ni thin films. In both cases, these films were transferred from their host substrate into a water bath, then re-deposited onto to a high-Q single crystal silicon mechanical double-paddle oscillator. A minimal thickness dependence of both internal friction and shear modulus was found within the experimental uncertainty for reduced graphene oxide films varying thickness from 5 to 90nm. The internal friction of all films exhibits a temperature independent plateau below 10K. The values of the plateaus are similar for both the reduced graphene oxide films and CVD graphene films, and they are as high as the universal “glassy range” where the tunneling states dominated internal friction of amorphous solids lies. This result shows that from a mechanical loss point of view, both graphene oxide and CVD graphene films have high and similar level of disorder. Raman measurements performed on the same samples show higher structure order in CVD graphene films than in graphene oxide films. Our results suggest that internal friction probes different sources of disorder from those by Raman, and the disorder is not directly related to the existence of C–O binding in the graphene oxide films. The shear modulus averages 53GPa after subtracting Young's modulus component from the vibration mode used in experiments.

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.

Graphene represents an important new material with potential Department of Defense sensor applications. At the Naval Research Laboratory we have developed three techniques to produce large-area graphene films. We have used this material to construct chemical and radio-frequency electromagnetic sensors. Here we report the initial results of this effort.

Based on first-principles calculations we devise an alternative approach to p-type doping in AlN, ZnO, and ZnMgO. Instead of searching for acceptors on the left of the host atoms in the Periodic Table, we propose to search on the far right. We find that F placed at interstitial sites in AlN, ZnO, and ZnMgO acts as a shallow acceptor, leaving a hole in an effective-mass state near the valence-band maximum. We investigate the stability of F impurities and propose a procedure to selectively introduce F in the interstitial lattice sites of the above wide-band-gap semiconductors.

: Chemically modified graphene (CMG) has emerged as a new material whose many attractive properties complement those of pure graphene. Graphene, a single atomic sheet of carbon bonded in a honeycomb lattice, has remarkable physical properties ranging from near-ballistic electron conduction to extremely high mechanical stiffness (more than five times that of steel). Such extreme properties motivate researchers to investigate these materials for use in applications ranging from high-frequency, low-power electronics, to flexible displays, chemical/biological sensors, and high-frequency electromechanical devices. We have developed a process to form large-area, ultra-thin CMG films that enable us to investigate CMG properties and to explore prototype devices. Using these films we have fabricated state-of-the-art chemical sensors and nanomechanical resonators. For chemical sensors, we have increased the sensitivity and reduced the level of noise by tuning the CMG film chemistry. These optimized sensors are capable of real-time detection of explosives and the three main classes of chemical-warfare agents at parts-per-billion concentrations. For nanomechanics, we have utilized chemical modification to produce suspended films under high tension. These high-stiffness, low-mass resonators display quality factors (up to 4000) and figures of merit well exceeding those of pure graphene resonators and are comparable to diamond thin films. Together, these results demonstrate that CMG is an inexpensive, high-performance material that will find application in a wide range of defense and commercial applications.

Abstract : Chemically modified graphene (CMG) has emerged as a new material whose many attractive properties complement those of pure graphene. Graphene, a single atomic sheet of carbon bonded in a honeycomb lattice, has remarkable physical properties ranging from near-ballistic electron conduction to extremely high mechanical stiffness (more than five times that of steel). Such extreme properties motivate researchers to investigate these materials for use in applications ranging from high-frequency, low-power electronics, to flexible displays, chemical/biological sensors, and high-frequency electromechanical devices. We have developed a process to form large-area, ultra-thin CMG films that enable us to investigate CMG properties and to explore prototype devices. Using these films we have fabricated state-of-the-art chemical sensors and nanomechanical resonators. For chemical sensors, we have increased the sensitivity and reduced the level of noise by tuning the CMG film chemistry. These optimized sensors are capable of real-time detection of explosives and the three main classes of chemical-warfare agents at parts-per-billion concentrations. For nanomechanics, we have utilized chemical modification to produce suspended films under high tension. These high-stiffness, low-mass resonators display quality factors (up to 4000) and figures of merit well exceeding those of pure graphene resonators and are comparable to diamond thin films. Together, these results demonstrate that CMG is an inexpensive, high-performance material that will find application in a wide range of defense and commercial applications.

We investigate molecular adsorption on single-walled carbon nanotubes (NTs) by ab initio calculations and by measurements of the conductivity response of NT arrays to trace vapors for a range of linear chain molecules. Experiment and calculations show that the short-time responses give adsorption energies that increase linearly with the molecular length. This systematic dependence indicates that the initial adsorption occurs on the defect-free regions of NTs and that these regions can play an important role in the adsorption process on NTs.

The growing threat of chemical, biological and radiological attack has created a demand for sensors that are capable of monitoring a large number of facilities for the preemptive detection or potential release of toxic agents. Such applications are highly demanding, requiring inexpensive sensors tha...