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Mechanical stacking of two dissimilar materials often has surprising consequences for heterostructure behavior. In particular, a 2D electron gas (2DEG) is formed in the heterostructure of the topological crystalline insulator Pb0.24Sn0.76Te and graphene due to contact of a polar with a nonpolar surface and the resulting changes in electronic structure needed to avoid polar catastrophe. The spintronic properties of this heterostructure with non‐local spin valve devices are studied. This study observes spin‐momentum locking at lower temperatures that transitions to regular spin channel transport only at ≈40 K. Hanle spin precession measurements show a spin relaxation time as high as 2.18 ns. Density functional theory calculations confirm that the spin‐momentum locking is due to a giant Rashba effect in the material and that the phase transition is a Lifshitz transition. The theoretically predicted Lifshitz transition is further evident in the phase transition‐like behavior in the Landé g‐factor and spin relaxation time. [ABSTRACT FROM AUTHOR]

We demonstrate field emission from an integrated three-terminal device using a suspended planar graphene edge as the source of vacuum electrons. Energy spectra of the emitted electrons confirm the field-emission mechanism. The energy spectra produced by graphene grown by chemical vapor deposition and reduced graphene oxide are compared. The drain-source voltage required to produce a given drain current increases when negative voltages are applied to the gate, confirming field-effect transistor operation. The emission current rises exponentially with inverse voltage over the measured current range from 1 pA to 10 nA. The current-voltage characteristics are consistent with tunneling through barrier potentials calculated numerically from the device geometry. [ABSTRACT FROM AUTHOR]

The properties of reduced graphene oxide (GO) are reported from a non-chemical reduction method. Ultra-high vacuum annealing of GO films in the thickness of 1-80 nm was studied by XPS, AFM, UV-Vis-NIR, Raman and TEM to observe the controlled removal of oxygen. We observed the loss of hydroxyl (C-OH) at low temperatures (<600 °C) followed by the complete loss of carbonyls (C=O) and epoxy (C-O-C) species by 1200 °C. As oxygen was removed, we observed a decrease in the layer spacing between the GO sheets and a concurrent decrease in the film resistance. While the Raman spectroscopy showed no change with reduction, indicating no change in the overall defect density or the general structure of the GO, the transmission spectra showed a shift in the transmission minimum from 245 nm to 260 nm and a total decrease in transmission above 800 nm occurs as the films visibly darken. TEM indicated that there is turbostratic stacking of the graphene layers as the reduction occurs, leading us to conclude that at a certain threshold of reduction the film properties are similar to epitaxial graphene growth on the C-face of SiC, but that a reduction gone too far results in a layer spacing equivalent to graphite. [ABSTRACT FROM AUTHOR]

We demonstrate that unstained DNA nanostructures can be chemically mapped by using aberration-corrected scanning transmission electron microscopy (STEM) with energy-dispersive spectroscopy (EDS). Key to this measurement is the use of graphene supports whose reduced background scattering compared to thicker carbon supports allows EDS to image phosphorus and other elements in the DNA (or in attachments to the DNA) as well as to see the divalent cations used to stabilize the nanostructure. In addition, unlike in other EDS analyses, the chemical maps we obtain can be made quantitative in an absolute sense because each individual DNA nanostructure is in effect a very large macromolecule (∼4.7 MDa) with a known chemical composition. In this way, not only can STEM/EDS serve DNA nanotechnology as a characterization tool but also the DNA structures can function as molecular standards for improving mesoscale EDS techniques. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Tunnel barriers are key elements for both charge-and spin-based electronics, offering devices with reduced power consumption and new paradigms for information processing. Such devices require mating dissimilar materials, raising issues of heteroepitaxy, interface stability, and electronic states that severely complicate fabrication and compromise performance. Graphene is the perfect tunnel barrier. It is an insulator out-of-plane, possesses a defect-free, linear habit, and is impervious to interdiffusion. Nonetheless, true tunneling between two stacked graphene layers is not possible in environmental conditions usable for electronics applications. However, two stacked graphene layers can be decoupled using chemical functionalization. Here, we demonstrate that hydrogenation or fluorination of graphene can be used to create a tunnel barrier. We demonstrate successful tunneling by measuring non-linear IV curves and a weakly temperature dependent zero-bias resistance. We demonstrate lateral transport of spin currents in non-local spin-valve structures, and determine spin lifetimes with the non-local Hanle effect. We compare the results for hydrogenated and fluorinated tunnel and we discuss the possibility that ferromagnetic moments in the hydrogenated graphene tunnel barrier affect the spin transport of our devices. [ABSTRACT FROM AUTHOR]

2D materials play a special role in the race to make smaller and smaller devices. Their unique and strong in-plane bonding makes them impervious to diffusion into other layers and provides excellent thickness control. Their van der Waal's bonding with other monolayers or substrates allows for heterostructures unattainable by any other technique. This is reflected by the abundant popularity of research into graphene and other 2D materials. In this review article, we will describe the out-of-plane properties of graphene and functionalized graphene. We will use three specific examples to illustrate how these out-of-plane properties can be used in spintronic devices, in section "Graphene as a Tunnel Barrier" we will describe a magnetic tunnel junction (MTJ) based on graphene. Section "Graphene Based MTJs" will describe the spin injecting properties of a graphene tunnel barrier on silicon. Section "Graphene in Semiconductor Spintronic Devices" describes how you can use functionalized graphene to make a homoepitaxial graphene device. The second part of this article reviews monolayer transition-metal dichalcogenides (TMDs). First, we will show how TMDs are grown and specifically how we can grow large-area TMDs by chemical vapor deposition. Secondly, we will describe the optical properties of several TMDs and compare the results from several authors. Finally, we choose a chemical sensor as a specific example to show how TMDs can be used in a device. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

Heterostructures composed of multiple layers of different atomically thin materials are of interest due to their unique properties and potential for new device functionality. MoS2-graphene heterostructures have shown promise as photodetectors and vertical tunnel transistors. However, progress is limited by the typically micrometer-scale devices and by the multiple alignments required for fabrication when utilizing mechanically exfoliated material. Here, the synthesis of large-area, continuous, and uniform MoS2 monolayers directly on graphene by chemical vapor deposition is reported, resulting in heterostructure samples on the centimeter scale with the possibility for even larger lateral dimensions. Atomic force microscopy, photoluminescence, X-ray photoelectron, and Raman spectroscopies demonstrate uniform single-layer growth of stoichiometric MoS2. The ability to reproducibly generate large-area heterostructures is highly advantageous for both fundamental investigations and technological applications. [ABSTRACT FROM AUTHOR]

The growing field of spintronics relies on new techniques and technologies for injecting and detecting electron spins to generate spin-dependent signals and utilize spin as a new state variable. Magnetic tunnel junctions (MTJs) do this by employing thin oxide layers as insulating barriers between two ferromagnetic metals, but the oxides suffer from defects and material interdiffusion that limit device performance. In this work, we demonstrate that graphene, a material widely studied for its high lateral conductance, functions as a tunnel barrier in the out-of-plane direction. We fabricate graphene-based MTJs and characterize spin and charge transport as a function of bias and temperature from 4 to 425 K. The device behavior fits well with traditional charge and spin-polarized tunneling transport models. This result has implications for development of new, ultra-low power spin-based devices such as magnetic random access memory (MRAM), spin logic, and reconfigurable circuits. [ABSTRACT FROM AUTHOR]

We investigate total ionizing dose (TID) effects in graphene field effect transistors comprised of chemical vapor deposition grown graphene transferred onto trimethylsiloxy(TMS)-passivated SiO2Si substrates. TID exposure with a positive gate bias increases the concentration of positive oxide trapped charges near the SiO2/TMS/graphene interface making Coulomb-potential scatterer limited mobility more apparent. In particular, we observe asymmetric degradation in electron and hole mobility, the former degrading more rapidly. Consistent with the electron-hole puddle description, we observe an increase in intrinsic electron carrier density that varies linearly with the oxide trapped charge density, while the hole carrier density remains largely unaltered. These effects give rise to an increasing minimum conductivity. [ABSTRACT FROM AUTHOR]

Abstract: 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 sp 2–sp 3 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. [Copyright &y& Elsevier]

We report a study of the valence band dispersion of twisted bilayer graphene using angle-resolved photoemission spectroscopy and ab initio calculations. We observe two noninteracting cones near the Dirac crossing energy and the emergence of van Hove singularities where the cones overlap for large twist angles (>5°). Besides the expected interaction between the Dirac cones, minigaps appeared at the Brillouin zone boundaries of the moiré superlattice formed by the misorientation of the two graphene layers. We attribute the emergence of these minigaps to a periodic potential induced by the moiré. These anticrossing features point to coupling between the two graphene sheets, mediated by moiré periodic potentials. [ABSTRACT FROM AUTHOR]

We report a method to introduce direct bonding betweengrapheneplatelets that enables the transformation of a multilayer chemicallymodified graphene (CMG) film from a “paper mache-like”structure into a stiff, high strength material. On the basis of chemical/defectmanipulation and recrystallization, this technique allows wide-rangeengineering of mechanical properties (stiffness, strength, density,and built-in stress) in ultrathin CMG films. A dramatic increase inthe Young’s modulus (up to 800 GPa) and enhanced strength (sustainablestress ≥1 GPa) due to cross-linking, in combination with hightensile stress, produced high-performance (quality factor of 31 000at room temperature) radio frequency nanomechanical resonators. Theability to fine-tune intraplatelet mechanical properties through chemicalmodification and to locally activate direct carbon–carbon bondingwithin carbon-based nanomaterials will transform these systems intotrue “materials-by-design” for nanomechanics. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Graphene grown by chemical vapor deposition and supported on SiO2 and sapphire substrates was studied following the controlled introduction of defects induced by 35 keV carbon ion irradiation. Changes in Raman spectra for fluences ranging from 1012 to 1015 cm-2 indicate that the structure of graphene evolves from a highly ordered layer, to a patchwork of disordered domains, to an essentially amorphous film. These structural changes result in a dramatic decrease in the Hall mobility by orders of magnitude while, remarkably, the Hall concentration remains almost unchanged, suggesting that the Fermi level is pinned at a hole concentration near 1×1013 cm-2. A model for scattering by resonant scatterers is in good agreement with mobility measurements up to an ion fluence of 1×1014 cm-2. [ABSTRACT FROM AUTHOR]

From healthcare to environmental sensors, small operational devices are being developed to monitor and report in a host of new environments. These varied environments require widely varying material properties to meet operational challenges. Unfortunately, the tools used for device fabrication are not fully compatible with the breadth of substrates being utilized. The ability to transfer operational devices from a fabrication‐friendly substrate to any arbitrary substrate is essential to meet needs. Single layer hydrogenated graphene (HG) is an excellent candidate to support the transfer of operational devices. Hydrogenation of graphene weakens the film's van der Waals interaction with its substrate, allowing it to delaminate in water. The free‐floating HG layer can then be placed onto a new substrate. In this work, this property is exploited to transfer working field‐effect transistors (FETs) and gas‐sensing polymer films onto representative hard and flexible substrates. The transferred devices are operational in place and are spaced only one atomic layer away from their final substrate surface. The versatility of HG‐assisted device transfer is also demonstrated by transferring devices onto curved, irregular, and fragile surfaces that are otherwise not fabrication‐friendly, including an nuclear magnetic resonance (NMR) spectroscopy tube, plastic water bottle, waxy plant leaf, and a culture of live neurons. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editor discusses various reports within the issue on topics including two dimensional heterostructure material research, transition metal dichalcogenides, and graphene heterostructures and its mechanical properties.

The mechanisms leading to spin relaxation in graphene and its heterostructures continue to be debated. Control of the spin relaxation in graphene-based structures is necessary to achieve the envisioned utility of graphene in future spintronic devices beyond Moore's law. Proximity induced spin relaxation caused by contact to a high spin-orbit material, such as WS 2 , offers a promising avenue to manipulate the spin lifetime. We demonstrate the operation of WS 2 /graphene/fluorographene non-local spin valves and extract the spin lifetimes for a range of carrier concentrations by Hanle effect measurements. Four-terminal charge transport measurements allow us to calculate the momentum relaxation time as a function of carrier concentration and compare it to the spin lifetime. These data show that the D'yakonov-Perel’ mechanism is the dominant spin relaxation mechanism for WS 2 /graphene/fluorographene devices, while, for reference graphene/fluorographene devices, linear scaling between the spin and momentum lifetimes points to spin-flip scattering during strong elastic scattering events where the scattering event is strongly coupled to the electron spin. We attribute the change in spin relaxation type in part with the inclusion of WS 2 as a substrate to proximity induced spin-orbit coupling due to the adjacent WS 2 layer, and we compare our data to the literature. [ABSTRACT FROM AUTHOR]

Abstract: Spatial distribution of nitrogen functional groups on graphene by use of physical masks during plasma processing is demonstrated. Structural and chemical modifications are evidenced by micro-Raman and high-resolution X-ray photoelectron spectroscopy mapping, demonstrating a strong dependence on the processing conditions and physical mask configurations. The resulting nitrogen chemical moieties influence the electronic transport by affecting the mobility of carriers and consequently increasing the electron transfer kinetics, which provide localized chemical reactive sites. The chemical moieties demonstrate the ability to tailor the locality of the surface chemistry on graphene opening up a wide range of reactivity studies and synthesis capabilities, such as site-specific electrochemical deposition. [Copyright &y& Elsevier]

The article discusses the use of graphene in biosensing, particularly noting the importance of accurate measurements in industrial and medical technology. It addresses topics including label-free detection to determine the amount of proteins in a biological sample, advances in real-time monitoring of biomolecules, and the graphene production method.

The low-energy electronic dispersion of graphene is extremely sensitive to the nearest layer interaction and thus the stacking sequence. Here, we report a method to examine the effect of stacking misorientation in bilayer graphene by transferring chemical vapor deposited (CVD) graphene onto monolithic graphene epitaxially grown on silicon carbide (SiC) (0001). The resulting hybrid bilayer graphene displays long-range Moiré diffraction patterns having various misorientations even as it exhibits electron reflectivity spectra nearly identical to epitaxial bilayer graphene grown directly on SiC. These varying twist angles affect the 2D (G')-band shape of the Raman spectrum, indicating regions of both a monolayer-like single &pgr; state and Bernal-like split n states brought about by the differing interiayer interactions. This hybrid bilayer graphene fabricated via a transfer process therefore offers a way to systematically study the electronic properties of bilayer graphene films as a function of stacking misorientation angle. [ABSTRACT FROM AUTHOR]

Abstract: Graphene films grown by chemical vapor deposition on copper foils were hydrogenated using commercially viable methods. Parameters such as plasma power, plasma frequency, and sample temperature were varied to determine the maximum possible hydrogenation without etching the film. The kinetic energy of the ions inside the plasma is critical, in that higher kinetic energy ions tend to etch the film while lower kinetic energy ions participate in the hydrogenation process. The film sheet resistance was shown to increase, while the hole mobility was shown to decrease with increasing hydrogenation. Variable temperature measurements demonstrate a transition from semi-metallic behavior for graphene to semiconducting behavior for hydrogenated graphene. Sheet resistance measurements as a function of temperature also suggest the emergence of a bandgap in the hydrogenated graphene films. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Layered two-dimensional crystal systems can exhibit complex interlayer interactions, which are influenced by local crystal structure and/or electronic variations. Here, we study the influence of defects in twisted bilayer graphene (TBG) using Raman spectroscopy. We explore the varied influence of defects on three characteristic Raman modes of both fully-defected TBG, with defects introduced in both layers, and half-defected TBG, with defects introduced in only a single layer. The resonance condition responsible for a strong enhancement of the G peak is sensitive to structural disorder and is quenched within a radius ∼3 nm of defects, while the twist-angle dependence of the 2D peak is influenced only at the site of structural disorder (∼1 nm radius). [ABSTRACT FROM AUTHOR]

Graphene (Gr)–polystyrene (PS) and graphene (Gr)–ultra-high molecular weight polyethylene (UHMW PE) laminates were fabricated using a transfer print approach that relies on differential adhesion to remove graphene from Cu foil without chemical etching. The polymer surfaces were prepared using plasma functionalization followed by N-ethylamino-4-azidotetrafluorobenzoate (TFPA) deposition. Then, the graphene on Cu foil and the TFPA coated polymers were pressed at elevated temperature and mild pressure. Finally, they were separated by mechanical peeling. No additional processing was applied. Detailed chemical, structural, and morphological characterization of PS and UHMW PE before and after graphene transfer print was performed using a suite of complementary surface analysis techniques including X-ray Photoelectron Spectroscopy (XPS), Near Edge X-ray Absorption Fine Structure Spectroscopy (NEXAFS), Raman Spectroscopy, and Atomic Force Microscopy (AFM). The charge carrier density and charge carrier mobility of the transferred graphene were determined using Hall effect measurements. We found that graphene’s electrical properties were preserved and comparable to those of graphene on SiO 2 /Si. Furthermore, modulation of TFPA attachment to PS and UHMW PE led to different TFPA-layer microstructure and therefore to a different amount of functional azide groups available to form carbene bonds with graphene causing changes in graphene’s compressive strain, doping and mobility. [ABSTRACT FROM AUTHOR]

Heterogeneous engineering of two-dimensionallayered materials,including metallic graphene and semiconducting transition metal dichalcogenides,presents an exciting opportunity to produce highly tunable electronicand optoelectronic systems. In order to engineer pristine layers andtheir interfaces, epitaxial growth of such heterostructures is required.We report the direct growth of crystalline, monolayer tungsten diselenide(WSe2) on epitaxial graphene (EG) grown from silicon carbide.Raman spectroscopy, photoluminescence, and scanning tunneling microscopyconfirm high-quality WSe2monolayers, whereas transmissionelectron microscopy shows an atomically sharp interface, and low energyelectron diffraction confirms near perfect orientation between WSe2and EG. Vertical transport measurements across the WSe2/EG heterostructure provides evidence that an additional barrierto carrier transport beyond the expected WSe2/EG band offsetexists due to the interlayer gap, which is supported by theoreticallocal density of states (LDOS) calculations using self-consistentdensity functional theory (DFT) and nonequilibrium Green’sfunction (NEGF). [ABSTRACT FROM AUTHOR]

The addition of a single sheet ofcarbon atoms in the form of graphenecan drastically alter friction between a nanoscale probe tip and asurface. Here, for the first time we show that friction can be alteredover a wide range by fluorination. Specifically, the friction forcebetween silicon atomic force microscopy tips and monolayer fluorinatedgraphene can range from 5−9 times higher than for graphene.While consistent with previous reports, the combined interpretationfrom our experiments and molecular dynamics simulations allows usto propose a novel mechanism: that the dramatic friction enhancementresults from increased corrugation of the interfacial potential dueto the strong local charge concentrated at fluorine sites, consistentwith the Prandtl-Tomlinson model. The monotonic increase of frictionwith fluorination in experiments also demonstrates that friction forcemeasurements provide a sensitive local probe of the degree of fluorination.Additionally, we found a transition from ordered to disordered atomicstick–slip upon fluorination, suggesting that fluorinationproceeds in a spatially random manner. [ABSTRACT FROM AUTHOR]

Abstract: The chemical modification of graphene greatly expands its potential applications in electronics, chemistry, and biology. Here, we report the rapid and extensive hydrogenation of single layer CVD graphene using the Birch reduction method. This method hydrogenates much more extensively than cold plasmas and electrochemical methods. Moreover, use of single layer graphene enables greater control of electronic conductivity than previously achieved with the Birch method using multilayer graphene or graphene oxide. Indeed, this method enables both the elimination of electronic conductivity through hydrogenation and the subsequent recovery of essentially pristine graphene with thermal annealing—a reversible >107 fold change in resistance. Raman and photoelectron spectroscopies show that the reaction is complete within 90s of immersion. Finally, we show that we can use the Birch reduction to functionalize graphene with tributyltin moieties. [Copyright &y& Elsevier]

Graphene has been widely studied for its high in-planecharge carrier mobility and long spin diffusion lengths. In contrast,the out-of-plane charge and spin transport behavior of this atomicallythin material have not been well addressed. We show here that whilegraphene exhibits metallic conductivity in-plane, it serves effectivelyas an insulator for transport perpendicular to the plane. We reportfabrication of tunnel junctions using single-layer graphene betweentwo ferromagnetic metal layers in a fully scalable photolithographicprocess. The transport occurs by quantum tunneling perpendicular tothe graphene plane and preserves a net spin polarization of the currentfrom the contact so that the structures exhibit tunneling magnetoresistanceto 425 K. These results demonstrate that graphene can function asan effective tunnel barrier for both charge and spin-based devicesand enable realization of more complex graphene-based devices forhighly functional nanoscale circuits, such as tunnel transistors,nonvolatilemagnetic memory, and reprogrammable spin logic. [ABSTRACT FROM AUTHOR]

There has been considerable interest in chemically functionalizinggraphene films to control their electronic properties, to enhancetheir binding to other molecules for sensing, and to strengthen theirinterfaces with matrices in a composite material. Most reports todate have largely focused on noncovalent methods or the use of grapheneoxide. Here, we present a method to activate CVD-grown graphene sheetsusing fluorination followed by reaction with ethylenediamine (EDA)to form covalent bonds. Reacted graphene was characterized via X-rayphotoelectron spectroscopy (XPS), attenuated total reflectance Fouriertransform infrared spectroscopy (ATR-FTIR), and Raman spectroscopyas well as measurements of electrical properties. The functionalizationresults in stable, densely packed layers, and the unbound amine ofEDA was shown to be active toward subsequent chemical reactions. [ABSTRACT FROM AUTHOR]

Hyperthermal ion implantation offers a controllable method of producing high-quality substitutionally doped graphene with nitrogen, an n-type dopant that has great potential for graphene electronics and spintronics applications where high carrier concentration, uniform doping, and minimal vacancy defect concentration is desired. Here we examine the transport properties of monolayer graphene sheets as a function of implantation beam energy and dose. We observe a transition from weak to strong localization that varies as a function of carrier concentration. For nominally equivalent doses, increased N ion energy results in an increasing magnetoresistance magnitude, reaching a value of approximately -5.5% at 5000 Oe, which we discuss in the context of dopant concentration and defect formation. We use a model for the temperature dependence of the conductivity that takes into account both temperature activation, due to the formation of a transport gap, and Mott variable-range hopping, due to the formation of defects, to further study the electronic properties of the doped films as a function of dose and N ion energy. We find that the temperature activation component dominates the behavior. [ABSTRACT FROM AUTHOR]