Showing papers on "Bilayer graphene published in 2010"
TL;DR: Graphene devices on h-BN substrates have mobilities and carrier inhomogeneities that are almost an order of magnitude better than devices on SiO(2).
Abstract: Graphene devices on standard SiO(2) substrates are highly disordered, exhibiting characteristics that are far inferior to the expected intrinsic properties of graphene. Although suspending the graphene above the substrate leads to a substantial improvement in device quality, this geometry imposes severe limitations on device architecture and functionality. There is a growing need, therefore, to identify dielectrics that allow a substrate-supported geometry while retaining the quality achieved with a suspended sample. Hexagonal boron nitride (h-BN) is an appealing substrate, because it has an atomically smooth surface that is relatively free of dangling bonds and charge traps. It also has a lattice constant similar to that of graphite, and has large optical phonon modes and a large electrical bandgap. Here we report the fabrication and characterization of high-quality exfoliated mono- and bilayer graphene devices on single-crystal h-BN substrates, by using a mechanical transfer process. Graphene devices on h-BN substrates have mobilities and carrier inhomogeneities that are almost an order of magnitude better than devices on SiO(2). These devices also show reduced roughness, intrinsic doping and chemical reactivity. The ability to assemble crystalline layered materials in a controlled way permits the fabrication of graphene devices on other promising dielectrics and allows for the realization of more complex graphene heterostructures.
6,261 citations
TL;DR: This work reports on a novel and simple hydrothermal approach for the cutting of GSs into surface-functionalized GQDs, which were found to exhibit bright blue photoluminescence (PL), which has never been observed in GSs and GNRs owing to their large lateral sizes.
Abstract: 2010 WILEY-VCH Verlag Gm Graphene-based materials are promising building blocks for future nanodevices owing to their superior electronic, thermal, and mechanical properties as well as their chemical stability. However, currently available graphene-based materials produced by typical physical and chemical routes, including micromechanical cleavage, reduction of exfoliated graphene oxide (GO), and solvothermal synthesis, are generally micrometer-sized graphene sheets (GSs), which limits their direct application in nanodevices. In this context, it has become urgent to develop effective routes for cutting large GSs into nanometer-sized pieces with a well-confined shape, such as graphene nanoribbons (GNRs) and graphene quantum dots (GQDs). Theoretical and experimental studies have shown that narrow GNRs (width less than ca. 10 nm) exhibit substantial quantum confinement and edge effects that render GNRs semiconducting. By comparison, GQDs possess strong quantum confinement and edge effects when their sizes are down to 100 nm. If their sizes are reduced to ca. 10 nm, comparable with the widths of semiconducting GNRs, the two effects will become more pronounced and, hence, induce new physical properties. Up to now, nearly all experimental work on GNRs and GQDs has focused on their electron transportation properties. Little work has been done on the optical properties that are directly associated with the quantum confinement and/or edge effects. Most GNRand GQD-based electronic devices have been fabricated by lithography techniques, which can realize widths and diameters down to ca. 20 nm. This physical approach, however, is limited by the need for expensive equipment and especially by difficulties in obtaining smooth edges. Alternative chemical routes can overcome these drawbacks. Moreover, surface functionalization can be realized easily. Li et al. first reported a chemical route to functionalized and ultrasmooth GNRs with widths ranging from 50 nm to sub-10 nm. Very recently, Kosynkin et al. reported a simple solution-based oxidative process for producing GNRs by lengthwise cutting and unraveling of multiwalled carbon nanotube (CNT) side walls. Yet, no chemical routes have been reported so far for preparing functionalized GQDs with sub-10 nm sizes. Here, we report on a novel and simple hydrothermal approach for the cutting of GSs into surface-functionalized GQDs (ca. 9.6-nm average diameter). The functionalized GQDs were found to exhibit bright blue photoluminescence (PL), which has never been observed in GSs and GNRs owing to their large lateral sizes. The blue luminescence and new UV–vis absorption bands are directly induced by the large edge effect shown in the ultrafine GQDs. The starting material was micrometer-sized rippled GSs obtained by thermal reduction of GO sheets. Figure 1a shows a typical transmission electron microscopy (TEM) image of the pristine GSs. Their (002) interlayer spacing is 3.64 A (Fig. 1c), larger than that of bulk graphite (3.34 A). Before the hydrothermal treatment, the GSs were oxidized in concentrated H2SO4 and HNO3. After the oxidization treatment the GSs became slightly smaller (50 nm–2mm) and the (002) spacing slightly increased to 3.85 A (Fig. 1c). During the oxidation, oxygen-containing functional groups, including C1⁄4O/COOH, OH, and C O C, were introduced at the edge and on the basal plane, as shown in the Fourier transform infrared (FTIR) spectrum (Fig. 1d). The presence of these groups makes the GSs soluble in water. A series of more marked changes took place after the hydrothermal treatment of the oxidized GSs at 200 8C. First, the (002) spacing was reduced to 3.43 A (Fig. 1c), very close to that of bulk graphite, indicating that deoxidization occurs during the hydrothermal process. The deoxidization is further confirmed by the changes in the FTIR and C 1s X-ray photoelectron spectroscopy (XPS) spectra. After the hydrothermal treatment, the strongest vibrational absorption band of C1⁄4O/COOH at 1720 cm 1 became very weak and the vibration band of epoxy groups at 1052 cm 1 disappeared (Fig. 1d). In the XPS C 1s spectra of the oxidized and hydrothermally reduced GSs (Fig. 2a), the signal at 289 eV assigned to carboxyl groups became weak after the hydrothermal treatment, whereas the sp carbon peak at 284.4 eV was almost unchanged. Figure 2b shows the Raman spectrum of the reduced GSs. A G band at 1590 cm 1 and a D band at 1325 cm 1 were observed with a large intensity ratio ID/IG of 1.26. Second, the size of the GSs decreased dramatically and ultrafine GQDswere isolated by a dialysis process. Figure 3 shows typical TEM and atomic force microscopy (AFM) images of the GQDs. Their diameters are mainly distributed in the range of 5–13 nm (9.6 nm average diameter). Their topographic heights are mostly between 1 and 2 nm, similar to those observed in functionalized GNRs with 1–3 layers. More than 85% of the GQDs consist of 1–3 layers.
2,484 citations
TL;DR: This new form of hybrid h-BNC material enables the development of bandgap-engineered applications in electronics and optics and properties that are distinct from those of graphene and h-BN.
Abstract: (1) Department of Mechanical Engineering and Materials Science, Rice University, Houston, TX 77005, United States
1,995 citations
TL;DR: It is shown experimentally that κ of monolayer graphene exfoliated on a silicon dioxide support is still as high as about 600 watts per meter per kelvin near room temperature, exceeding those of metals such as copper.
Abstract: The reported thermal conductivity (κ) of suspended graphene, 3000 to 5000 watts per meter per kelvin, exceeds that of diamond and graphite. Thus, graphene can be useful in solving heat dissipation problems such as those in nanoelectronics. However, contact with a substrate could affect the thermal transport properties of graphene. Here, we show experimentally that κ of monolayer graphene exfoliated on a silicon dioxide support is still as high as about 600 watts per meter per kelvin near room temperature, exceeding those of metals such as copper. It is lower than that of suspended graphene because of phonons leaking across the graphene-support interface and strong interface-scattering of flexural modes, which make a large contribution to κ in suspended graphene according to a theoretical calculation.
1,708 citations
TL;DR: The existence of a bandgap opening in graphene is demonstrated, induced by the patterned adsorption of atomic hydrogen onto the Moiré superlattice positions of graphene grown on an Ir(111) substrate.
Abstract: Graphene, a single layer of graphite, has recently attracted considerable attention owing to its remarkable electronic and structural properties and its possible applications in many emerging areas such as graphene-based electronic devices. The charge carriers in graphene behave like massless Dirac fermions, and graphene shows ballistic charge transport, turning it into an ideal material for circuit fabrication. However, graphene lacks a bandgap around the Fermi level, which is the defining concept for semiconductor materials and essential for controlling the conductivity by electronic means. Theory predicts that a tunable bandgap may be engineered by periodic modulations of the graphene lattice, but experimental evidence for this is so far lacking. Here, we demonstrate the existence of a bandgap opening in graphene, induced by the patterned adsorption of atomic hydrogen onto the Moire superlattice positions of graphene grown on an Ir(111) substrate.
1,375 citations
TL;DR: It is shown that when immersed in an ionic solution, a layer of graphene becomes a new electrochemical structure that is called a trans-electrode, which is an ideal substrate for very high resolution, high throughput nanopore-based single-molecule detectors.
Abstract: Isolated, atomically thin conducting membranes of graphite, called graphene, have recently been the subject of intense research with the hope that practical applications in fields ranging from electronics to energy science will emerge. The atomic thinness, stability and electrical sensitivity of graphene motivated us to investigate the potential use of graphene membranes and graphene nanopores to characterize single molecules of DNA in ionic solution. Here we show that when immersed in an ionic solution, a layer of graphene becomes a new electrochemical structure that we call a trans-electrode. The trans-electrode's unique properties are the consequence of the atomic-scale proximity of its two opposing liquid-solid interfaces together with graphene's well known in-plane conductivity. We show that several trans-electrode properties are revealed by ionic conductance measurements on a graphene membrane that separates two aqueous ionic solutions. Although our membranes are only one to two atomic layers thick, we find they are remarkable ionic insulators with a very small stable conductance that depends on the ion species in solution. Electrical measurements on graphene membranes in which a single nanopore has been drilled show that the membrane's effective insulating thickness is less than one nanometre. This small effective thickness makes graphene an ideal substrate for very high resolution, high throughput nanopore-based single-molecule detectors. The sensitivity of graphene's in-plane electronic conductivity to its immediate surface environment and trans-membrane solution potentials will offer new insights into atomic surface processes and sensor development opportunities.
1,327 citations
TL;DR: It is demonstrated that large area, high-quality graphene with controllable thickness can be grown from different solid carbon sources—such as polymer films or small molecules—deposited on a metal catalyst substrate at temperatures as low as 800 °C.
Abstract: The past few years have seen spectacular growth of interest in graphene, the carbon monolayer with novel electronic properties. Efforts to produce large sheets of monolayer (or few-layer) graphene could receive a welcome boost from a simple new procedure. Just by baking various solid carbon sources deposited on a metal catalyst substrate at a relatively modest 800 °C, it is possible to produce either pristine graphene or doped graphene in a single step. Suitable starting materials include polymer films and various small molecules. The past few years have seen a spectacular growth of interest in graphene. Efforts to produce large sheets of monolayer (or few-layer) graphene could receive a welcome boost from the simple procedure reported by these authors. They show how baking various solid carbon sources (for example polymer films) deposited on a metal catalyst substrate can produce either pristine graphene or doped graphene in a single step. Monolayer graphene was first obtained1 as a transferable material in 2004 and has stimulated intense activity among physicists, chemists and material scientists1,2,3,4. Much research has been focused on developing routes for obtaining large sheets of monolayer or bilayer graphene. This has been recently achieved by chemical vapour deposition (CVD) of CH4 or C2H2 gases on copper or nickel substrates5,6,7. But CVD is limited to the use of gaseous raw materials, making it difficult to apply the technology to a wider variety of potential feedstocks. Here we demonstrate that large area, high-quality graphene with controllable thickness can be grown from different solid carbon sources—such as polymer films or small molecules—deposited on a metal catalyst substrate at temperatures as low as 800 °C. Both pristine graphene and doped graphene were grown with this one-step process using the same experimental set-up.
1,290 citations
IBM1
TL;DR: This demonstration reveals the great potential of bilayer graphene in applications such as digital electronics, pseudospintronics, terahertz technology, and infrared nanophotonics.
Abstract: Graphene is considered to be a promising candidate for future nanoelectronics due to its exceptional electronic properties. Unfortunately, the graphene field-effect transistors (FETs) cannot be turned off effectively due to the absence of a band gap, leading to an on/off current ratio typically around 5 in top-gated graphene FETs. On the other hand, theoretical investigations and optical measurements suggest that a band gap up to a few hundred millielectronvolts can be created by the perpendicular E-field in bilayer graphenes. Although previous carrier transport measurements in bilayer graphene transistors did indicate a gate-induced insulating state at temperatures below 1 K, the electrical (or transport) band gap was estimated to be a few millielectronvolts, and the room temperature on/off current ratio in bilayer graphene FETs remains similar to those in single-layer graphene FETs. Here, for the first time, we report an on/off current ratio of around 100 and 2000 at room temperature and 20 K, respectively, in our dual-gate bilayer graphene FETs. We also measured an electrical band gap of >130 and 80 meV at average electric displacements of 2.2 and 1.3 V nm(-1), respectively. This demonstration reveals the great potential of bilayer graphene in applications such as digital electronics, pseudospintronics, terahertz technology, and infrared nanophotonics.
1,259 citations
TL;DR: In this article, the authors provide an in-depth description of the physics of monolayer and bilayer graphene fermions, where the quasiparticles are massive chiral Dirac Fermions.
Abstract: The electronic properties of graphene, a two-dimensional crystal of carbon atoms, are exceptionally novel. For instance, the low-energy quasiparticles in graphene behave as massless chiral Dirac fermions which has led to the experimental observation of many interesting effects similar to those predicted in the relativistic regime. Graphene also has immense potential to be a key ingredient of new devices, such as single molecule gas sensors, ballistic transistors and spintronic devices. Bilayer graphene, which consists of two stacked monolayers and where the quasiparticles are massive chiral fermions, has a quadratic low-energy band structure which generates very different scattering properties from those of the monolayer. It also presents the unique property that a tunable band gap can be opened and controlled easily by a top gate. These properties have made bilayer graphene a subject of intense interest. In this review, we provide an in-depth description of the physics of monolayer and bilayer graphene f...
932 citations
TL;DR: The realization of a one-dimensional topological defect in graphene, containing octagonal and pentagonal sp(2)-hybridized carbon rings embedded in a perfect graphene sheet is reported, which acts as a quasi-one-dimensional metallic wire.
Abstract: Many proposed applications of graphene require the ability to tune its electronic structure at the nanoscale1,2. Although charge transfer3 and field-effect doping4 can be applied to manipulate charge carrier concentrations, using them to achieve nanoscale control remains a challenge. An alternative approach is ‘self-doping’5, in which extended defects are introduced into the graphene lattice. The controlled engineering of these defects represents a viable approach to creation and nanoscale control of one-dimensional charge distributions with widths of several atoms6. However, the only experimentally realized extended defects so far have been the edges of graphene nanoribbons7,8,9,10, which show dangling bonds that make them chemically unstable11,12,13. Here, we report the realization of a one-dimensional topological defect in graphene, containing octagonal and pentagonal sp2-hybridized carbon rings embedded in a perfect graphene sheet. By doping the surrounding graphene lattice, the defect acts as a quasi-one-dimensional metallic wire. Such wires may form building blocks for atomic-scale, all-carbon electronics. A stable extended defect in graphene consisting of octagonal and pentagonal rings produces one-dimensional charge localization, allowing it to act as a metallic wire embedded in an otherwise perfect graphene sheet.
916 citations
TL;DR: This work develops a theory of charge carrier transmission through grain boundaries composed of a periodic array of dislocations in graphene based on the momentum conservation principle and sheds light on the transport properties of large-area graphene samples.
Abstract: Most materials in available macroscopic quantities are polycrystalline. Graphene, a recently discovered two-dimensional form of carbon with strong potential for replacing silicon in future electronics, is no exception. There is growing evidence of the polycrystalline nature of graphene samples obtained using various techniques. Grain boundaries, intrinsic topological defects of polycrystalline materials, are expected to markedly alter the electronic transport in graphene. Here, we develop a theory of charge carrier transmission through grain boundaries composed of a periodic array of dislocations in graphene based on the momentum conservation principle. Depending on the grain-boundary structure we find two distinct transport behaviours--either high transparency, or perfect reflection of charge carriers over remarkably large energy ranges. First-principles quantum transport calculations are used to verify and further investigate this striking behaviour. Our study sheds light on the transport properties of large-area graphene samples. Furthermore, purposeful engineering of periodic grain boundaries with tunable transport gaps would allow for controlling charge currents without the need to introduce bulk bandgaps in otherwise semimetallic graphene. The proposed approach can be regarded as a means towards building practical graphene electronics.
TL;DR: The approach, which provides a physical mechanism for the introduction of defect and subsequent hetero dopant atoms into the graphene material in a controllable fashion, will be promising for producing graphene-based devices for multiple applications.
Abstract: Opening and tuning an energy gap in graphene are central to many electronic applications of graphene. Here we report N-doped graphene obtained by NH3 annealing after N+-ion irradiation of graphene samples. First, the evolution of the graphene microstructure was investigated following N+-ion irradiation at different fluences using Raman spectroscopy, showing that defects were introduced in plane after irradiation and then restored after annealing in N2 or in NH3. Auger electron spectroscopy (AES) of the graphene annealed in NH3 after irradiation showed N signal, however, no N signal was observed after annealing in N2. Last, the field-effect transistor (FET) was fabricated using N-doped graphene and monitored by the source−drain conductance and back-gate voltage (Gsd−Vg) curves in the measurement. The transport property changed compared to that of the FET made by intrinsic graphene, that is, the Dirac point position moved from positive Vg to negative Vg, indicating the transition of graphene from p-type to ...
TL;DR: In this article, the authors found flat bands near Fermi level in slightly twisted bilayer graphene as a signature of a transition from a parabolic dispersion to the characteristic linear dispersion of graphene.
Abstract: The presence of flat bands near Fermi level has been proposed as an explanation for high transition temperature superconductors. The bands of graphite are extremely sensitive to topological defects which modify the electronic structure. In this Rapid Communication, we found nondispersive flat bands no farther than 10 meV of the Fermi energy in slightly twisted bilayer graphene as a signature of a transition from a parabolic dispersion of bilayer graphene to the characteristic linear dispersion of graphene. This transition occurs for relative rotation angles of layers around $1.5\ifmmode^\circ\else\textdegree\fi{}$ and is related to a process of layer decoupling. We have performed ab initio calculations to develop a tight-binding model with an interaction Hamiltonian between layers that include the $\ensuremath{\pi}$ orbitals of all atoms and takes into account interactions up to third nearest neighbors within a layer.
TL;DR: Graphene layers with AB stacking, FLG with arbitrary stacking, is considered to possess distinct properties arising from its different crystalline structure and p electron interactions, and it has been observed that the electroand magnetotransport properties for folded graphene sheets are different to those ofAB stackedbilayers.
Abstract: Graphene is a two-dimensional material defined as a planar honeycomb lattice of close-packed carbon atoms, where the electrons exhibit a linear dispersion near Dirac K points and behave as massless Dirac fermions. However, the valence and conduction bands in an AB stacked graphene bilayer split into two parabolic branches near the K point originating from the interaction of p electrons, and the electrons are hence described by massive Dirac fermions. Moreover, a graphene bilayer is a tunable-gap semiconductor under electric-field biasing. With a further increase in the number of layers along with AB stacking, the electronic structure reveals stepwise variations that eventually approach that of the three-dimensional counterpart. Considering the close relation between the electronic properties and layer number of few-layer graphene (FLG), the ability to accurately determine the layer number and correlating this with the electronic structure is a prerequisite in understanding the evolution of the electronic properties from twoto threedimensional graphitic materials. In addition to graphene layers with AB stacking, FLG with arbitrary stacking (Figure 1) is considered to possess distinct properties arising from its different crystalline structure and p electron interactions. Experimentally, it has been observed that the electroand magnetotransport properties for folded graphene sheets are different to thoseofAB stackedbilayers. Furthermore,FLG grown on SiC, Ni, and Ru also have non-AB stacking order. Therefore, elucidating the detailed character-
Journal Article•
TL;DR: In this article, the fractional quantum Hall effect (FQHE) is observed in suspended sheets of graphene, probed in a two-terminal measurement setup, and it is shown that at low carrier density, graphene becomes an insulator with a magnetic-field-tunable energy gap.
Abstract: The fractional quantum Hall effect is a quintessential manifestation of the collective behaviour associated with strongly interacting charge carriers confined to two dimensions and subject to a strong magnetic field. It is predicted that the charge carriers present in graphene — an atomic layer of carbon that can be seen as the 'perfect' two-dimensional system — are subject to strong interactions. Nevertheless, the phenomenon had eluded experimental observation until now: in this issue two groups report fractional quantum Hall effect in suspended sheets of graphene, probed in a two-terminal measurement setup. The researchers also observe a magnetic-field-induced insulating state at low carrier density, which competes with the quantum Hall effect and limits its observation to the highest-quality samples only. These results pave the way for the study of the rich collective behaviour of Dirac fermions in graphene. The fractional quantum Hall effect (FQHE) is the quintessential collective quantum behaviour of charge carriers confined to two dimensions but it has not yet been observed in graphene, a material distinguished by the charge carriers' two-dimensional and relativistic character. Here, and in an accompanying paper, the FQHE is observed in graphene through the use of devices containing suspended graphene sheets; the results of these two papers open a door to the further elucidation of the complex physical properties of graphene. When electrons are confined in two dimensions and subject to strong magnetic fields, the Coulomb interactions between them can become very strong, leading to the formation of correlated states of matter, such as the fractional quantum Hall liquid1,2. In this strong quantum regime, electrons and magnetic flux quanta bind to form complex composite quasiparticles with fractional electronic charge; these are manifest in transport measurements of the Hall conductivity as rational fractions of the elementary conductance quantum. The experimental discovery of an anomalous integer quantum Hall effect in graphene has enabled the study of a correlated two-dimensional electronic system, in which the interacting electrons behave like massless chiral fermions3,4. However, owing to the prevailing disorder, graphene has so far exhibited only weak signatures of correlated electron phenomena5,6, despite intense experimental and theoretical efforts7,8,9,10,11,12,13,14. Here we report the observation of the fractional quantum Hall effect in ultraclean, suspended graphene. In addition, we show that at low carrier density graphene becomes an insulator with a magnetic-field-tunable energy gap. These newly discovered quantum states offer the opportunity to study correlated Dirac fermions in graphene in the presence of large magnetic fields.
TL;DR: Graphene paper, fabricated via flow-directed filtration of an electrostatically stabilized aqueous graphene dispersion that was pre-prepared via hydrazine reduction of graphene oxide sheets, has excellent electrical conductivity and similar mechanical properties as graphene oxide paper maintained at temperatures below 100 8C.
Abstract: 2010 WILEY-VCH Verlag Gm Two-dimensional graphene nanosheets and graphene-based materials have garnered significant attention in recent years due to their excellent materials properties. Many graphenebased materials can be conveniently synthesized from graphite oxide (GO), which can be prepared in bulk quantities from graphite under strong oxidizing conditions. GO is a layered material featuring a variety of oxygen-containing functionalities with epoxide and hydroxyl groups on the basal plane and carbonyl and carboxyl groups along the edges, which provide a platform for rich chemistry to occur both within the intersheet gallery and along sheet edges. In addition, GO can be easily exfoliated into individual graphene oxide sheets, which can be reassembled into thin films or paper-like materials. For the latter case, flow-directed filtration of an aqueous graphene oxide dispersion produces very large sheets of a free-standing, foil-like material known as graphene oxide paper. This paper retains all the functional groups found in GO, preserving all of its native chemistry. While graphene oxide paper can be chemically modified in a facile fashion and has goodmechanical properties, it was found to be electrically conductive only after thermal annealing, which presumably converts it into graphene paper. Unfortunately, this thermal treatment also degrades its structural integrity. Graphene paper, fabricated via flow-directed filtration of an electrostatically stabilized aqueous graphene dispersion that was pre-prepared via hydrazine reduction of graphene oxide sheets, has excellent electrical conductivity and similar mechanical properties as graphene oxide paper maintained at temperatures below 100 8C. However, the hydrazine reduction of graphene oxide sheets can remove a significant amount of oxygen-containing functionalities and lead to graphene papers with low functional-group content. To produce functionalized graphene paper from graphene oxide sheets, we envisioned two strategies: 1) preparing functionalized graphene sheets before assembling them into ‘‘paper’’ or 2) reducing a pre-assembled, functionalized graphene oxide paper. Here, we present the successful preparation of a conductive, ‘‘alkylated’’ graphene paper via the post-synthetic modification of ‘‘alkylated’’ graphene oxide paper. By treating pre-assembled graphene oxide paper with hexylamine prior to hydrazine reduction, we can convert this insulating paper into conductive ‘‘alkylated’’ graphene paper while maintaining its well-ordered structure and good mechanical properties. Since reduction in the absence of hexylamine affords a less-ordered material with inconsistent conductivity, we attribute the uniform conductivity we observe for the ‘‘alkylated’’ paper to the structure-stabilizing presence of the hexylamine. GO prepared using the Hummers method was sonicated to yield aqueous dispersions of graphene oxide sheets, which were vacuum-filtered through an Anodisc membrane to yield graphene oxide paper (see Supporting Information (SI) for further details). Hexylamine-modified (HA-) graphene oxide paper was prepared by flowing a methanol solution of the amine (100mM) through the as-prepared wet paper, which already has a ‘‘well-stacked’’ structure. In contrast, if graphene oxide sheets aremodified first with hexylamine, they become hydrophobic and quickly precipitate in water, precluding the formation of well-ordered paper (Fig. S1 in SI). HA-graphene paper was then obtained by flowing an aqueous hydrazine monohydrate solution (2 M), a commonly used reducing agent for graphene oxide, through the as-prepared, wet HA-graphene oxide paper at 90 8C under vacuum assistance. Unmodified graphene paper was prepared by a similar reduction of unmodified wet graphene oxide paper. As the structures of the papers were already established during the assembly, our method conveniently omits the use of ammonia andmineral oil stabilizing agents found in an alternative method for preparing graphene paper from aqueous dispersions of graphene sheets. Functionalization prior to reduction is key to the proper preparation of HA-graphene paper (Fig. S2 in SI); performing reduction first removes themajority of reactive oxygen-containing functionalities from graphene oxide and prevents any substantial hexylamine functionalization. Successful hexylamine functionalization and reduction of the graphene oxide paper were confirmed by elemental analysis (EA) and Karl–Fischer titration (Table S2 in SI). As fabricated, graphene oxide paper has a Cgraphene/O ratio of 2.9 with a water content of 17wt%. In contrast, the water content for the HA-graphene oxide paper is significantly decreased to 1.49wt%
TL;DR: It is shown, for the first time, that the reactsivity of edges is at least two times higher than the reactivity of the bulk single graphene sheet, as supported by electron transfer theory.
Abstract: The reactivity of graphene and its various multilayers toward electron transfer chemistries with 4-nitrobenzene diazonium tetrafluoroborate is probed by Raman spectroscopy after reaction on-chip Single graphene sheets are found to be almost 10 times more reactive than bi- or multilayers of graphene according to the relative disorder (D) peak in the Raman spectrum examined before and after chemical reaction in water A model whereby electron puddles that shift the Dirac point locally to values below the Fermi level is consistent with the reactivity difference Because the chemistry at the graphene edge is important for controlling its electronic properties, particularly in ribbon form, we have developed a spectroscopic test to examine the relative reactivity of graphene edges versus the bulk We show, for the first time, that the reactivity of edges is at least two times higher than the reactivity of the bulk single graphene sheet, as supported by electron transfer theory These differences in electron tr
TL;DR: In this article, the growth of epitaxial bilayer graphene on silicon carbide (SiC) wafers has been studied, where a carbon interface layer is introduced to compensate for the structural and electronic influence of the interface.
Abstract: Graphene, a monoatomic layer of graphite, hosts a two-dimensional electron gas system with large electron mobilities which makes it a prospective candidate for future carbon nanodevices. Grown epitaxially on silicon carbide (SiC) wafers, large area graphene samples appear feasible and integration in existing device technology can be envisioned. This paper reviews the controlled growth of epitaxial graphene layers on SiC(0 0 0 1) and the manipulation of their electronic structure. We show that epitaxial graphene on SiC grows on top of a carbon interface layer that—although it has a graphite-like atomic structure—does not display the linear π-bands typical for graphene due to a strong covalent bonding to the substrate. Only the second carbon layer on top of this interface acts like monolayer graphene. With a further carbon layer, a graphene bilayer system develops. During the growth of epitaxial graphene on SiC(0 0 0 1) the number of graphene layers can be precisely controlled by monitoring the π-band structure. Experimental fingerprints for in situ growth control could be established. However, due to the influence of the interface layer, epitaxial graphene on SiC(0 0 0 1) is intrinsically n-doped and the layers have a long-range corrugation in their density of states. As a result, the Dirac point energy where the π-bands cross is shifted away from the Fermi energy, so that the ambipolar properties of graphene cannot be exploited. We demonstrate methods to compensate and eliminate this structural and electronic influence of the interface. We show that the band structure of epitaxial graphene on SiC(0 0 0 1) can be precisely tailored by functionalizing the graphene surface with tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) molecules. Charge neutrality can be achieved for mono- and bilayer graphene. On epitaxial bilayer graphene, where a band gap opens due to the asymmetric electric field across the layers imposed by the interface, the magnitude of this band gap can be increased up to more than double its initial value. The hole doping allows the Fermi level to shift into the energy band gap. The impact of the interface layer can be completely eliminated by decoupling the graphene from the SiC substrate by a hydrogen intercalation technique. We demonstrate that hydrogen can migrate under the interface layer and passivate the underlying SiC substrate. The interface layer alone transforms into a quasi-free standing monolayer. Epitaxial monolayer graphene turns into a decoupled bilayer. In combination with atmospheric pressure graphitization, the intercalation process allows the production of quasi-free standing epitaxial graphene on large SiC wafers and represents a highly promising route towards epitaxial graphene based nanoelectronics.
TL;DR: In this paper, the authors report measurements of high-frequency conductivity of graphene from THz to mid-IR at different carrier concentrations, showing that the conductivity exhibits Drude-like frequency dependence and increases dramatically at THz frequencies.
Abstract: Electrons moving in graphene behave as massless Dirac fermions, and they exhibit fascinating low-frequency electrical transport phenomena. Their dynamic response, however, is little known at frequencies above one terahertz (THz). Such knowledge is important not only for a deeper understanding of the Dirac electron quantum transport, but also for graphene applications in ultrahigh-speed THz electronics and infrared (IR) optoelectronics. In this paper, we report measurements of high-frequency conductivity of graphene from THz to mid-IR at different carrier concentrations. The conductivity exhibits Drude-like frequency dependence and increases dramatically at THz frequencies, but its absolute strength is lower than theoretical predictions. This anomalous reduction of free-electron oscillator strength is corroborated by corresponding changes in graphene interband transitions, as required by the sum rule.
TL;DR: In this article, the authors report homogeneous bilayer graphene films over at least a 2 in. × 2 in area, synthesized by chemical vapor deposition on copper foil and subsequently transferred to arbitrary substrates.
Abstract: The discovery of electric field induced band gap opening in bilayer graphene opens a new door for making semiconducting graphene without aggressive size scaling or using expensive substrates. However, bilayer graphene samples have been limited to μm2 size scale thus far, and synthesis of wafer scale bilayer graphene poses a tremendous challenge. Here we report homogeneous bilayer graphene films over at least a 2 in. × 2 in. area, synthesized by chemical vapor deposition on copper foil and subsequently transferred to arbitrary substrates. The bilayer nature of graphene film is verified by Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. Importantly, spatially resolved Raman spectroscopy confirms a bilayer coverage of over 99%. The homogeneity of the film is further supported by electrical transport measurements on dual-gate bilayer graphene transistors, in which a band gap opening is observed in 98% of the devices.
TL;DR: In this paper, the authors demonstrate that the excess negative charge can be fully compensated by noncovalently functionalizing graphene with the strong electron-acceptor tetrafluorotetracyanoquinodimethane (F4-TCNQ) charge transfer complex.
Abstract: Epitaxial graphene on SiC(0001) suffers from strong intrinsic $n$-type doping We demonstrate that the excess negative charge can be fully compensated by noncovalently functionalizing graphene with the strong electron-acceptor tetrafluorotetracyanoquinodimethane (F4-TCNQ) Charge neutrality can be reached in monolayer graphene as shown in electron-dispersion spectra from angular-resolved photoemission spectroscopy In bilayer graphene the band-gap that originates from the SiC/graphene interface dipole increases with increasing F4-TCNQ deposition and, as a consequence of the molecular doping, the Fermi level is shifted into the band-gap The reduction in the charge-carrier density upon molecular deposition is quantified using electronic Fermi surfaces and Raman spectroscopy The structural and electronic characteristics of the graphene/F4-TCNQ charge-transfer complex are investigated by x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy The doping effect on graphene is preserved in air and is temperature resistant up to $200\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ Furthermore, graphene noncovalent functionalization with F4-TCNQ can be implemented not only via evaporation in ultrahigh vacuum but also by wet chemistry
TL;DR: Comparative quantum-mechanical calculations reveal that graphene fluoride is the most thermodynamically stable of five studied hypothetical graphene derivatives; graphane, graphene fluoride, bromide, chloride, and iodide.
Abstract: Stoichoimetric graphene fluoride monolayers are obtained in a single step by the liquid-phase exfoliation of graphite fluoride with sulfolane. Comparative quantum-mechanical calculations reveal that graphene fluoride is the most thermodynamically stable of five studied hypothetical graphene derivatives; graphane, graphene fluoride, bromide, chloride, and iodide. The graphene fluoride is transformed into graphene via graphene iodide, a spontaneously decomposing intermediate. The calculated bandgaps of graphene halides vary from zero for graphene bromide to 3.1 eV for graphene fluoride. It is possible to design the electronic properties of such two-dimensional crystals.
TL;DR: The chemical doping of monolayer and bilayer graphene with aluminium, silicon, phosphorus and sulfur was investigated in this paper, where the formation of interlayer bonds in bilayer GAs was investigated.
TL;DR: In this paper, the low-energy electronic spectra of rotationally faulted graphene bilayers are studied using a long-wavelength theory applicable to general commensurate fault angles, which can be interpreted as energy-renormalized forms of the spectra for the limiting Bernal and $AA$ stacked structures.
Abstract: The low-energy electronic spectra of rotationally faulted graphene bilayers are studied using a long-wavelength theory applicable to general commensurate fault angles. Lattice commensuration requires low-energy electronic coherence across a fault and pre-empts massless Dirac behavior near the neutrality point. Sublattice exchange symmetry distinguishes two families of commensurate faults that have distinct low-energy spectra which can be interpreted as energy-renormalized forms of the spectra for the limiting Bernal and $AA$ stacked structures. Sublattice-symmetric faults are generically fully gapped systems due to a pseudospin-orbit coupling appearing in their effective low-energy Hamiltonians.
TL;DR: In this paper, the synthesis and evidence of Graphene fluoride, a two-dimensional wide bandgap semiconductor derived from graphene, has been presented, which exhibits hexagonal crystalline order and strongly insulating behavior with resistance exceeding $10 at room temperature.
Abstract: We report the synthesis and evidence of graphene fluoride, a two-dimensional wide bandgap semiconductor derived from graphene. Graphene fluoride exhibits hexagonal crystalline order and strongly insulating behavior with resistance exceeding $10\text{ }\text{G}\ensuremath{\Omega}$ at room temperature. Electron transport in graphene fluoride is well described by variable range hopping in two dimensions due to the presence of localized states in the band gap. Graphene obtained through the reduction of graphene fluoride is highly conductive, exhibiting a resistivity of less than $100\text{ }\text{k}\ensuremath{\Omega}$ at room temperature. Our approach provides a pathway to reversibly engineer the band structure and conductivity of graphene for electronic and optical applications.
TL;DR: In this paper, local laser excitation and temperature readout from the intensity ratio of Stokes to anti-Stokes Raman scattering signals are employed to study the thermal properties of a large graphene membrane.
Abstract: Local laser excitation and temperature readout from the intensity ratio of Stokes to anti-Stokes Raman scattering signals are employed to study the thermal properties of a large graphene membrane. The concluded value of the heat conductivity coefficient κ ≈ 600 W/(m·K) is smaller than previously reported but still validates the conclusion that graphene is a very good thermal conductor.
TL;DR: In this article, a comparative study and Raman characterization of the formation of graphene on single crystal Ni (111) and polycrystalline Ni substrates using chemical vapor deposition (CVD) was performed.
Abstract: We report a comparative study and Raman characterization of the formation of graphene on single crystal Ni (111) and polycrystalline Ni substrates using chemical vapor deposition (CVD). Preferential formation of monolayer/bilayer graphene on the single crystal surface is attributed to its atomically smooth surface and the absence of grain boundaries. In contrast, CVD graphene formed on polycrystalline Ni leads to a higher percentage of multilayer graphene (≥3 layers), which is attributed to the presence of grain boundaries in Ni that can serve as nucleation sites for multilayer growth. Micro-Raman surface mapping reveals that the area percentages of monolayer/bilayer graphene are 91.4% for the Ni (111) substrate and 72.8% for the polycrystalline Ni substrate under comparable CVD conditions. The use of single crystal substrates for graphene growth may open ways for uniform high-quality graphene over large areas.
TL;DR: In this article, the electrical, mechanical and optical properties of few-layer graphene (FLG) synthesized by chemical vapor deposition (CVD) on a Ni-coated substrate were studied.
TL;DR: The fabrication of nanoperforated graphene materials with sub-20-nm features using cylinder-forming diblock copolymer templates across >1 mm(2) areas is demonstrated to facilitate applications of graphene in electronics, optoelectronics, and sensing.
Abstract: We demonstrate the fabrication of nanoperforated graphene materials with sub-20-nm features using cylinder-forming diblock copolymer templates across >1 mm2 areas Hexagonal arrays of holes are etched into graphene membranes, and the remaining constrictions between holes interconnect forming a honeycomb structure Quantum confinement, disorder, and localization effects modulate the electronic structure, opening an effective energy gap of 100 meV in the nanopatterned material The field-effect conductivity can be modulated by 40× (200×) at room temperature (T = 105 K) as a result A room temperature hole mobility of 1 cm2 V−1 s−1 was measured in the fabricated nanoperforated graphene field effect transistors This scalable strategy for modulating the electronic structure of graphene is expected to facilitate applications of graphene in electronics, optoelectronics, and sensing
TL;DR: In this article, the authors have fabricated transistor structures using fluorinated single-layer graphene flakes and studied their electronic properties at different temperatures, showing that fluorination creates a mobility gap in graphene's spectrum where electron transport takes place via localized electron states.
Abstract: We have fabricated transistor structures using fluorinated single-layer graphene flakes and studied their electronic properties at different temperatures. Compared with pristine graphene, fluorinated graphene has a very large and strongly temperature-dependent resistance in the electroneutrality region. We show that fluorination creates a mobility gap in graphene's spectrum where electron transport takes place via localized electron states.