Showing papers on "Bilayer graphene published in 2015"

Topological valley transport at bilayer graphene domain walls

Abstract: The bandgap of bilayer graphene can be tuned with an electric field and topological valley polarized modes have been predicted to exist at its domain boundaries; here, near-field infrared imaging and low-temperature transport measurements reveal such modes in gapped bilayer graphene. Bilayer graphene offers an interesting platform in which to observe novel electronic effects that are different to those in monolayer graphene because the bilayer has a bandgap that can be tuned with an electric field. Moreover, topological valley polarized modes have been predicted to exist at its domain boundaries and in this study Feng Wang and colleagues use near-field optical imaging and low-temperature transport measurements to reveal that such modes do exist in in gapped bilayer graphene. This finding opens up the possibility to explore topological states in bilayer graphene that can be tuned with an electric field. Electron valley, a degree of freedom that is analogous to spin, can lead to novel topological phases in bilayer graphene. A tunable bandgap can be induced in bilayer graphene by an external electric field1,2,3,4,5, and such gapped bilayer graphene is predicted to be a topological insulating phase protected by no-valley mixing symmetry, featuring quantum valley Hall effects and chiral edge states6,7,8,9. Observation of such chiral edge states, however, is challenging because inter-valley scattering is induced by atomic-scale defects at real bilayer graphene edges10. Recent theoretical work11,12,13 has shown that domain walls between AB- and BA-stacked bilayer graphene can support protected chiral edge states of quantum valley Hall insulators. Here we report an experimental observation of ballistic (that is, with no scattering of electrons) conducting channels at bilayer graphene domain walls. We employ near-field infrared nanometre-scale microscopy (nanoscopy)14,15,16 to image in situ bilayer graphene layer-stacking domain walls on device substrates, and we fabricate dual-gated field effect transistors based on the domain walls. Unlike single-domain bilayer graphene, which shows gapped insulating behaviour under a vertical electrical field, bilayer graphene domain walls feature one-dimensional valley-polarized conducting channels with a ballistic length of about 400 nanometres at 4 kelvin. Such topologically protected one-dimensional chiral states at bilayer graphene domain walls open up opportunities for exploring unique topological phases and valley physics in graphene.

Proximity-induced ferromagnetism in graphene revealed by the anomalous Hall effect.

Abstract: Placing graphene on an insulating magnetic substrate can make the material ferromagnetic without disturbing its exceptional conductivity.

Strong interface-induced spin-orbit interaction in graphene on WS2.

Abstract: Interfacial interactions allow the electronic properties of graphene to be modified, as recently demonstrated by the appearance of satellite Dirac cones in graphene on hexagonal boron nitride substrates. Ongoing research strives to explore interfacial interactions with other materials to engineer targeted electronic properties. Here we show that with a tungsten disulfide (WS2) substrate, the strength of the spin-orbit interaction (SOI) in graphene is very strongly enhanced. The induced SOI leads to a pronounced low-temperature weak anti-localization effect and to a spin-relaxation time two to three orders of magnitude smaller than in graphene on conventional substrates. To interpret our findings we have performed first-principle electronic structure calculations, which confirm that carriers in graphene on WS2 experience a strong SOI and allow us to extract a spin-dependent low-energy effective Hamiltonian. Our analysis shows that the use of WS2 substrates opens a possible new route to access topological states of matter in graphene-based systems.

Generation and detection of pure valley current by electrically induced Berry curvature in bilayer graphene

Abstract: Bilayer graphene can host topological currents that are robust against defects and are associated with the electron valleys. It is now shown that electric fields can tune this topological valley transport over long distances at room temperature. The field of ‘Valleytronics’ has recently been attracting growing interest as a promising concept for the next generation electronics, because non-dissipative pure valley currents with no accompanying net charge flow can be manipulated for computational use, akin to pure spin currents1. Valley is a quantum number defined in an electronic system whose energy bands contain energetically degenerate but non-equivalent local minima (conduction band) or maxima (valence band) due to a certain crystal structure. Specifically, spatial inversion symmetry broken two-dimensional honeycomb lattice systems exhibiting Berry curvature is a subset of possible systems that enable optical2,3,4,5, magnetic6,7,8,9 and electrical control of the valley degree of freedom10,11,12. Here we use dual-gated bilayer graphene to electrically induce and control broken inversion symmetry (or Berry curvature) as well as the carrier density for generating and detecting the pure valley current. In the insulating regime, at zero-magnetic field, we observe a large nonlocal resistance that scales cubically with the local resistivity, which is evidence of pure valley current.

Gate-tunable topological valley transport in bilayer graphene

Abstract: Bilayer graphene can host topological currents that are robust against defects and are associated with the electron valleys. It is now shown that electric fields can tune this topological valley transport over long distances at room temperature. Valley pseudospin, the quantum degree of freedom characterizing the degenerate valleys in energy bands1, is a distinct feature of two-dimensional Dirac materials1,2,3,4,5. Similar to spin, the valley pseudospin is spanned by a time-reversal pair of states, although the two valley pseudospin states transform to each other under spatial inversion. The breaking of inversion symmetry induces various valley-contrasted physical properties; for instance, valley-dependent topological transport is of both scientific and technological interest2,3,4,5. Bilayer graphene is a unique system whose intrinsic inversion symmetry can be controllably broken by a perpendicular electric field, offering a rare possibility for continuously tunable topological valley transport. We used a perpendicular gate electric field to break the inversion symmetry in bilayer graphene, and a giant nonlocal response was observed as a result of the topological transport of the valley pseudospin. We further showed that the valley transport is fully tunable by external gates, and that the nonlocal signal persists up to room temperature and over long distances. These observations challenge the current understanding of topological valley transport in a gapped system, and the robust topological transport may lead to future valleytronic applications.

Graphene on transition-metal dichalcogenides: A platform for proximity spin-orbit physics and optospintronics

Abstract: Hybrids of graphene and two-dimensional transition-metal dichalcogenides (TMDCs) have the potential to bring graphene spintronics to the next level. As we show here by performing first-principles calculations of graphene on monolayer ${\mathrm{MoS}}_{2}$, there are several advantages of such hybrids over pristine graphene. First, Dirac electrons in graphene exhibit a giant global proximity spin-orbit coupling, without compromising the semimetallic character of the whole system at zero field. Remarkably, these spin-orbit effects can be very accurately described by a simple effective Hamiltonian. Second, the Fermi level can be tuned by a transverse electric field to cross the ${\mathrm{MoS}}_{2}$ conduction band, creating a system of coupled massive and massless electron gases. Both charge and spin transport in such systems should be unique. Finally, we propose to use graphene/TMDC structures as a platform for optospintronics, in particular, for optical spin injection into graphene and for studying spin transfer between TMDCs and graphene.

Electronic properties of graphene-based bilayer systems

Abstract: This article reviews the theoretical and experimental work related to the electronic properties of bilayer graphene systems. Three types of bilayer stackings are discussed: the AA, AB, and twisted bilayer graphene. This review covers single-electron properties, effects of static electric and magnetic fields, bilayer-based mesoscopic systems, spin-orbit coupling, dc transport and optical response, as well as spontaneous symmetry violation and other interaction effects. The selection of the material aims to introduce the reader to the most commonly studied topics of theoretical and experimental research in bilayer graphene.

Metal Oxide Induced Charge Transfer Doping and Band Alignment of Graphene Electrodes for Efficient Organic Light Emitting Diodes

Abstract: The interface structure of graphene with thermally evaporated metal oxide layers, in particular molybdenum trioxide (MoO3), is studied combining photoemission spectroscopy, sheet resistance measurements and organic light emitting diode (OLED) characterization. Thin (<5 nm) MoO3 layers give rise to an 1.9 eV large interface dipole and a downwards bending of the MoO3 conduction band towards the Fermi level of graphene, leading to a near ideal alignment of the transport levels. The surface charge transfer manifests itself also as strong and stable p-type doping of the graphene layers, with the Fermi level downshifted by 0.25 eV and sheet resistance values consistently below 50 Ω/sq for few-layer graphene films. The combination of stable doping and highly efficient charge extraction/injection allows the demonstration of simplified graphene-based OLED device stacks with efficiencies exceeding those of standard ITO reference devices.

Quantum Monte Carlo calculation of the binding energy of bilayer graphene.

Abstract: We report diffusion quantum Monte Carlo calculations of the interlayer binding energy of bilayer graphene. We find the binding energies of the AA-and AB-stacked structures at the equilibrium separation to be 11.5(9) and 17.7(9) meV/atom, respectively. The out-of-plane zone-center optical phonon frequency predicted by our binding-energy curve is consistent with available experimental results. As well as assisting the modeling of interactions between graphene layers, our results will facilitate the development of van der Waals exchange-correlation functionals for density functional theory calculations.

Thermoelectric effects in graphene nanostructures.

Abstract: The thermoelectric properties of graphene and graphene nanostructures have recently attracted significant attention from the physics and engineering communities. In fundamental physics, the analysis of Seebeck and Nernst effects is very useful in elucidating some details of the electronic band structure of graphene that cannot be probed by conductance measurements alone, due in particular to the ambipolar nature of this gapless material. For applications in thermoelectric energy conversion, graphene has two major disadvantages. It is gapless, which leads to a small Seebeck coefficient due to the opposite contributions of electrons and holes, and it is an excellent thermal conductor. The thermoelectric figure of merit ZT of a two-dimensional (2D) graphene sheet is thus very limited. However, many works have demonstrated recently that appropriate nanostructuring and bandgap engineering of graphene can concomitantly strongly reduce the lattice thermal conductance and enhance the Seebeck coefficient without dramatically degrading the electronic conductance. Hence, in various graphene nanostructures, ZT has been predicted to be high enough to make them attractive for energy conversion. In this article, we review the main results obtained experimentally and theoretically on the thermoelectric properties of graphene and its nanostructures, emphasizing the physical effects that govern these properties. Beyond pure graphene structures, we discuss also the thermoelectric properties of some hybrid graphene structures, as graphane, layered carbon allotropes such as graphynes and graphdiynes, and graphene/hexagonal boron nitride heterostructures which offer new opportunities. Finally, we briefly review the recent activities on other atomically thin 2D semiconductors with finite bandgap, i.e. dichalcogenides and phosphorene, which have attracted great attention for various kinds of applications, including thermoelectrics.

Plasmon-driven reaction controlled by the number of graphene layers and localized surface plasmon distribution during optical excitation

Abstract: Graphene-plasmonic hybrid platforms have attracted an enormous amount of interest in surface-enhanced Raman scattering (SERS); however, the mechanism of employing graphene is still ambiguous, so clarification about the complex interaction among molecules, graphene, and plasmon processes is urgently needed. We report that the number of graphene layers controlled the plasmon-driven, surface-catalyzed reaction that converts para-aminothiophenol (PATP)-to-p,p′-dimercaptoazobenzene (DMAB) on chemically inert, graphene-coated, silver bowtie nanoantenna arrays. The catalytic reaction was monitored by SERS, which revealed that the catalytic reaction occurred on the chemical inertness monolayer graphene (1G)-coated silver nanostructures. The introduction of 1G enhances the plasmon-driven surface-catalyzed reaction of the conversion of PATP-to-p,p′-DMAB. The chemical reaction is suppressed by bilayer graphene. In the process of the catalytic reaction, the electron transfer from the PATP molecule to 1G-coated silver nanostructures. Subsequently, the transferred electrons on the graphene recombine with the hot-hole produced by the localized surface plasmon resonance of silver nanostructures. Then, a couple of PATP molecules lost electrons are catalyzed into the p,p′-DMAB molecule on the graphene surface. The experimental results were further supported by the finite-difference time-domain method and quantum chemical calculations. The introduction of a graphene coating on metal nanostructures can help control the efficiency of plasmon-driven chemical reactions. Xiang-heng Xiao and co-workers from China used an array of silver bowtie nanoantennas to perform a surface-enhanced photocatalytic reaction and convert para-aminothiophenol (PATP) into p,p′-dimercaptoazobenzene under optical excitation. The conversion was enhanced when the nanoantennas were coated with monolayer graphene, whereas it was suppressed when they were coated with bilayer graphene. The reaction was monitored in situ by capturing and analysing surface-enhanced Raman spectra. The enhanced reaction rate is thought to stem from the efficient transport of electrons from the PATP molecules to the nanoantennas coated with monolayer graphene and their subsequent recombination with hot-holes. Quantum chemical calculations and finite-difference time-domain modelling confirmed this scenario.

Gate-Tunable Resonant Tunneling in Double Bilayer Graphene Heterostructures

Abstract: We demonstrate gate-tunable resonant tunneling and negative differential resistance in the interlayer current–voltage characteristics of rotationally aligned double bilayer graphene heterostructures separated by hexagonal boron nitride (hBN) dielectric. An analysis of the heterostructure band alignment using individual layer densities, along with experimentally determined layer chemical potentials indicates that the resonance occurs when the energy bands of the two bilayer graphene are aligned. We discuss the tunneling resistance dependence on the interlayer hBN thickness, as well as the resonance width dependence on mobility and rotational alignment.

Semiconducting Graphene from Highly Ordered Substrate Interactions

Abstract: While numerous methods have been proposed to produce semiconducting graphene, a significant band gap has never been demonstrated. The reason is that, regardless of the theoretical gap formation mechanism, subnanometer disorder prevents the required symmetry breaking necessary to make graphene semiconducting. In this work, we show for the first time that semiconducting graphene can be made by epitaxial growth. Using improved growth methods, we show by direct band measurements that a band gap greater than 0.5 eV can be produced in the first graphene layer grown on the SiC(0001) surface. This work demonstrates that order, a property that remains lacking in other graphene systems, is key to producing electronically viable semiconducting graphene.

Quantum Hall resistance standard in graphene devices under relaxed experimental conditions

Abstract: The quantum Hall effect provides a universal standard for electrical resistance that is theoretically based on only the Planck constant h and the electron charge e. Currently, this standard is implemented in GaAs/AlGaAs, but graphene's electronic properties have given hope for a more practical device. Here, we demonstrate that the experimental conditions necessary for the operation of devices made of high-quality graphene grown by chemical vapour deposition on silicon carbide can be extended and significantly relaxed compared with those for state-of-the-art GaAs/AlGaAs devices. In particular, the Hall resistance can be accurately quantized to within 1 × 10−9 over a 10 T wide range of magnetic flux density, down to 3.5 T, at a temperature of up to 10 K or with a current of up to 0.5 mA. This experimental simplification highlights the great potential of graphene in the development of user-friendly and versatile quantum standards that are compatible with broader industrial uses beyond those in national metrology institutes. Furthermore, the measured agreement of the quantized Hall resistance in graphene and GaAs/AlGaAs, with an ultimate uncertainty of 8.2 × 10−11, supports the universality of the quantum Hall effect. This also provides evidence of the relation of the quantized Hall resistance with h and e, which is crucial for the new Systeme International d'unites to be based on fixing such fundamental constants of nature. Large-area graphene devices synthesized by chemical vapour deposition are used to develop electrical resistance standards, based on the quantum Hall effect, with state-of-the-art accuracy and under an extended range of experimental conditions of magnetic field, temperature and current.

Structural and Electrical Investigation of C60-Graphene Vertical Heterostructures.

Abstract: Graphene, with its unique electronic and structural qualities, has become an important playground for studying adsorption and assembly of various materials including organic molecules. Moreover, organic/graphene vertical structures assembled by van der Waals interaction have potential for multifunctional device applications. Here, we investigate structural and electrical properties of vertical heterostructures composed of C60 thin film on graphene. The assembled film structure of C60 on graphene is investigated using transmission electron microscopy, which reveals a uniform morphology of C60 film on graphene with a grain size as large as 500 nm. The strong epitaxial relations between C60 crystal and graphene lattice directions are found, and van der Waals ab initio calculations support the observed phenomena. Moreover, using C60–graphene heterostructures, we fabricate vertical graphene transistors incorporating n-type organic semiconducting materials with an on/off ratio above 3 × 103. Our work demonstrat...

Local spectroscopy of moiré-induced electronic structure in gate-tunable twisted bilayer graphene

Abstract: Twisted bilayer graphene (tBLG) forms a quasicrystal whose structural and electronic properties depend on the angle of rotation between its layers. Here, we present a scanning tunneling microscopy study of gate-tunable tBLG devices supported by atomically smooth and chemically inert hexagonal boron nitride (BN). The high quality of these tBLG devices allows identification of coexisting moir\'e patterns and moir\'e super-superlattices produced by graphene-graphene and graphene-BN interlayer interactions. Furthermore, we examine additional tBLG spectroscopic features in the local density of states beyond the first van Hove singularity. Our experimental data are explained by a theory of moir\'e bands that incorporates ab initio calculations and confirms the strongly nonperturbative character of tBLG interlayer coupling in the small twist-angle regime.

Strong Plasmonic Coupling between Graphene Ribbon Array and Metal Gratings

Abstract: The collective oscillation of the massless electrons in graphene ribbons can interact with photons to create graphene plasmon polaritons. The resonance-induced absorption is critical in signal detection and energy harvesting applications. However, because of their atomic thickness, high absorptance is difficult to achieve with graphene ribbons alone. In this work, a hybrid plasmonic system composed of an array of graphene ribbons over a periodic metal grating is theoretically investigated. It is shown that the localized resonances, that is, magnetic polaritons, in metal gratings can couple with the plasmonic resonance in graphene ribbons, resulting in significantly enhanced absorption in graphene. Moreover, the coupling phenomenon depends on the width of the ribbons and the relative positions of the ribbon and the grating. The coupling between the grating and a continuous graphene monolayer sheet is also investigated and the results are compared to those with graphene ribbons. The findings of this work ma...

Probing the role of interlayer coupling and coulomb interactions on electronic structure in few-layer MoSe₂ nanostructures.

Abstract: Despite the weak nature of interlayer forces in transition metal dichalcogenide (TMD) materials, their properties are highly dependent on the number of layers in the few-layer two-dimensional (2D) limit. Here, we present a combined scanning tunneling microscopy/spectroscopy and GW theoretical study of the electronic structure of high quality single- and few-layer MoSe2 grown on bilayer graphene. We find that the electronic (quasiparticle) bandgap, a fundamental parameter for transport and optical phenomena, decreases by nearly one electronvolt when going from one layer to three due to interlayer coupling and screening effects. Our results paint a clear picture of the evolution of the electronic wave function hybridization in the valleys of both the valence and conduction bands as the number of layers is changed. This demonstrates the importance of layer number and electron-electron interactions on van der Waals heterostructures and helps to clarify how their electronic properties might be tuned in future 2D nanodevices.

Controlled van der Waals Epitaxy of Monolayer MoS2 Triangular Domains on Graphene

Abstract: Multilayered heterostructures of two-dimensional materials have recently attracted increased interest because of their unique electronic and optical properties. Here, we present chemical vapor deposition (CVD) growth of triangular crystals of monolayer MoS2 on single-crystalline hexagonal graphene domains which are also grown by CVD. We found that MoS2 grows selectively on the graphene domains rather than on the bare supporting SiO2 surface. Reflecting the heteroepitaxy of the growth process, the MoS2 domains grown on graphene present two preferred equivalent orientations. The interaction between the MoS2 and the graphene induced an upshift of the Raman G and 2D bands of the graphene, while significant photoluminescence quenching was observed for the monolayer MoS2. Furthermore, photoinduced current modulation along with an optical memory effect was demonstrated for the MoS2-graphene heterostructure. Our work highlights that heterostructures synthesized by CVD offer an effective interlayer van der Waals interaction which can be developed for large-area multilayer electronic and photonic devices.

Electronic and thermoelectric properties of van der Waals materials with ring-shaped valence bands

Abstract: The valence band of a variety of few-layer, two-dimensional materials consist of a ring of states in the Brillouin zone. The energy-momentum relation has the form of a “Mexican hat” or a Rashba dispersion. The two-dimensional density of states is singular at or near the band edge, and the band-edge density of modes turns on nearly abruptly as a step function. The large band-edge density of modes enhances the Seebeck coefficient, the power factor, and the thermoelectric figure of merit ZT. Electronic and thermoelectric properties are determined from ab initio calculations for few-layer III–VI materials GaS, GaSe, InS, InSe, for Bi2Se3, for monolayer Bi, and for bilayer graphene as a function of vertical field. The effect of interlayer coupling on these properties in few-layer III–VI materials and Bi2Se3 is described. Analytical models provide insight into the layer dependent trends that are relatively consistent for all of these few-layer materials. Vertically biased bilayer graphene could serve as an experimental test-bed for measuring these effects.

van der Waals bilayer energetics: Generalized stacking-fault energy of graphene, boron nitride, and graphene/boron nitride bilayers

Abstract: The structure, thermodynamics, and band gaps in graphene/graphene, boron nitride/boron nitride, and graphene/boron nitride bilayers are determined using several different corrections to first-principles approaches to account for the dispersion interactions. While the density functional dispersion correction, van der Waals density functional, meta--generalized gradient approximation, and adiabatic fluctuation-dissipation theorem methods (ACFDT-RPA) all lead to qualitatively similar predictions, the best accuracy is obtained through the application of the computationally expensive ACFDT-RPA method. We present an accurate ACFDT-RPA-based method to determine bilayer structure, generalized stacking-fault energy (GSFE), and band gaps as a function of the relative translation states of the two layers. The GSFE data clearly identify all of the stable and metastable bilayer translations as well as the barriers between them. This is key for predicting the sliding, formation, and adhesion energies for homo- and hetero-bilayers, as well as for the determination of defects in such multilayer van der Waals systems. These, in turn, provide an accurate approach for determining and manipulating the spatial variation of electronic structure.

1/f noise in graphene nanopores

Abstract: Graphene nanopores are receiving great attention due to their atomically thin membranes and intrinsic electrical properties that appear greatly beneficial for biosensing and DNA sequencing. Here, we present an extensive study of the low-frequency 1/f noise in the ionic current through graphene nanopores and compare it to noise levels in silicon nitride pore currents. We find that the 1/f noise magnitude is very high for graphene nanopores: typically two orders of magnitude higher than for silicon nitride pores. This is a drawback as it significantly lowers the signal-to-noise ratio in DNA translocation experiments. We evaluate possible explanations for these exceptionally high noise levels in graphene pores. From examining the noise for pores of different diameters and at various salt concentrations, we find that in contrast to silicon nitride pores, the 1/f noise in graphene pores does not follow Hooge's relation. In addition, from studying the dependence on the buffer pH, we show that the increased noise cannot be explained by charge fluctuations of chemical groups on the pore rim. Finally, we compare single and bilayer graphene to few-layer and multi-layer graphene and boron nitride (h-BN), and we find that the noise reduces with layer thickness for both materials, which suggests that mechanical fluctuations may be the underlying cause of the high 1/f noise levels in monolayer graphene nanopore devices.

Laser-induced white-light emission from graphene ceramics–opening a band gap in graphene

Abstract: Graphene ceramics can have their band gaps opened and emit white light when excited by visible or infrared light, report scientists in Poland. Wieslaw Strek and co-workers from the Polish Academy of Sciences and Wroclaw University of Technology say that the broadband emission is centred at 650 nm and has an intensity that is strongly influenced by the excitation laser power with a clear threshold. Their analysis suggests that the origin of the emission is entirely electronic in nature and is not related to incandescence or blackbody radiation. White-light emission is observed for cryogenically cooled samples with temperatures as low as 10 K. The graphene ceramic was fabricated by placing graphene flakes in a calcium carbonate toroid and subjecting them to a high temperature (550 °C) and a pressure of 8 GPa for 1 min.

Graphene versus MoS2: A short review

Abstract: Graphene and MoS2 are two well-known quasi two-dimensional materials. This review presents a comparative survey of the complementary lattice dynamical and mechanical properties of graphene and MoS2, which facilitates the study of graphene/MoS2 heterostructures. These hybrid heterostructures are expected to mitigate the negative properties of each individual constituent and have attracted intense academic and industrial research interest.

Interlayer interaction in general incommensurate atomic layers

Abstract: We present a general theoretical formulation to describe the interlayer interaction in incommensurate bilayer systems with arbitrary crystal structures. By using the generic tight-binding description, we show that the interlayer coupling, which is highly complex in the real space, can be simply written in terms of generalized Umklapp process in the reciprocal space. The formulation is useful to describe the interaction in the two-dimensional interface of different materials with arbitrary lattice structures and relative orientations. We apply the method to the incommensurate bilayer graphene with a large rotation angle, which cannot be treated as a long-range moire superlattice, and obtain the quasi band structure and density of states within the first-order approximation.

Superconductivity in Ca-doped graphene

Abstract: Graphene, a zero-gap semimetal, can be transformed into a metallic, semiconducting or insulating state by either physical or chemical modification. Superconductivity is conspicuously missing among these states despite considerable experimental efforts as well as many theoretical proposals. Here, we report superconductivity in calcium-decorated graphene achieved by intercalation of graphene laminates that consist of well separated and electronically decoupled graphene crystals. In contrast to intercalated graphite, we find that Ca is the only dopant that induces superconductivity in graphene laminates above 1.8 K among intercalants used in our experiments such as potassium, caesium and lithium. Ca-decorated graphene becomes superconducting at ~ 6 K and the transition temperature is found to be strongly dependent on the confinement of the Ca layer and the induced charge carrier concentration. In addition to the first evidence for superconducting graphene, our work shows a possibility of inducing and studying superconductivity in other 2D materials using their laminates.

A graphene/GaAs near-infrared photodetector enabled by interfacial passivation with fast response and high sensitivity

Abstract: We report a simple AlOx passivation approach to optimize the device performance of a bilayer graphene/gallium arsenide (BLG/GaAs) Schottky junction based near infrared photodetector (NIRPD). The as-fabricated NIRPD is highly sensitive to NIR illumination at zero bias voltage, with a detectivity of 2.88 × 1011, which is much higher than that without passivation (7.3 × 109 cm Hz1/2 W−1). The corresponding responsivity is 5 mA W−1. Additionally, the surface passivation can substantially increase both the response rate (rise/fall time τr/τf from 32/48 μs to 320/380 ns), and lift time. It is expected that such a self-driven NIRPD with fast response and high detectivity will have great potential in the future optoelectronic devices.

Linear magnetoresistance in mosaic-like bilayer graphene

Abstract: Contrary to common belief, bilayer graphene is not defect-free: the abundance of partial dislocations leads to a mosaic-like network structure. As a result, as now shown, the magnetoresistance of bilayer graphene depends linearly, rather than quadratically, on the external magnetic field. The magnetoresistance of conductors usually has a quadratic dependence on magnetic field1, however, examples exist of non-saturating linear behaviour in diverse materials2,3,4,5,6. Assigning a specific microscopic mechanism to this unusual phenomenon is obscured by the co-occurrence and interplay of doping, mobility fluctuations and a polycrystalline structure7,8. Bilayer graphene has virtually no doping fluctuations, yet provides a built-in mosaic tiling due to the dense network of partial dislocations9,10. We present magnetotransport measurements of epitaxial bilayer graphene that exhibits a strong and reproducible linear magnetoresistance that persists to B = 62 T at and above room temperature, decorated by quantum interference effects at low temperatures. Partial dislocations thus have a profound impact on the transport properties in bilayer graphene, a system that is frequently assumed to be dislocation-free. It further provides a clear and tractable model system for studying the unusual properties of mosaic conductors.

Understanding the origin of band gap formation in graphene on metals: graphene on Cu/Ir(111)

Abstract: Understanding the nature of the interaction at the graphene/metal interfaces is the basis for graphene-based electron- and spin-transport devices. Here we investigate the hybridization between graphene- and metal-derived electronic states by studying the changes induced through intercalation of a pseudomorphic monolayer of Cu in between graphene and Ir(111), using scanning tunnelling microscopy and photoelectron spectroscopy in combination with density functional theory calculations. We observe the modifications in the band structure by the intercalation process and its concomitant changes in the charge distribution at the interface. Through a state-selective analysis of band hybridization, we are able to determine their contributions to the valence band of graphene giving rise to the gap opening. Our methodology reveals the mechanisms that are responsible for the modification of the electronic structure of graphene at the Dirac point, and permits to predict the electronic structure of other graphene-metal interfaces.