Bottom Graphene Layers Research Articles

Ta2NiSe5/MoTe2/Graphene van der Waals Heterostructures Toward Ultrabroadband and Polarization‐Sensitive Imaging

AbstractUltrabroadband and polarization‐sensitive imaging are essential for pioneering advancements in intelligent technology, offering a pivotal pathway to multidimensional information extraction. However, the dearth of appropriate photosensitive semiconductors in the infrared region and the lack of suitably efficient device architecture for the separation and collection of photocarriers significantly impede the development of ultra‐broadband and polarization‐sensitive photodetectors. To address these challenges, a sandwiched Ta2NiSe5/MoTe2/Graphene heterostructure device is engineered. In this device, efficient and polarization‐sensitive photocarriers are generated in the top Ta2NiSe5 layer, rapidly separated through the middle MoTe2 layer, and effectively collected by the bottom graphene layer. As a result, the developed photodetector exhibits an ultra‐broadband and polarization‐sensitive photoresponse that extends from visible light (520 nm) to short‐wave infrared (2200 nm). At 2200 nm, the device displays a notable responsivity of 5.79 A W−1, a specific detectivity of 1010 Jones, and an anisotropic ratio of 1.44. Furthermore, this device successfully demonstrates high‐resolution ultra‐broadband and polarized light imaging capabilities. This study thus presents an intriguing blueprint for the development of advanced 2D imaging platforms for future‐generation intelligent systems.

Spin and valley-polarized multiple Fermi surfaces of α -RuCl3/bilayer graphene heterostructure

We report the transport properties of α-RuCl3/bilayer graphene heterostructures, where carrier doping is induced by a work function difference, resulting in distinct electron and hole populations in α-RuCl3 and bilayer graphene, respectively. Through a comprehensive analysis of multi-channel transport signatures, including Hall measurements and quantum oscillation, we unveil significant band modifications within the system. In particular, we observe the emergence of spin and valley-polarized multiple hole-type Fermi pockets, originating from the spin-selective band hybridization between α-RuCl3 and bilayer graphene, breaking the spin degree of freedom. Unlike the α-RuCl3/monolayer graphene system, the presence of different hybridization strengths between α-RuCl3 and the top and bottom graphene layers leads to an asymmetric behavior of the two layers, confirmed by effective mass experiments, resulting in the manifestation of valley-polarized Fermi pockets. These compelling findings establish α-RuCl3 proximitized to bilayer graphene as an outstanding platform for engineering its unique low-energy band structure.

Ultrathin All-2D Lateral Diodes Using Top and Bottom Contacted Laterally Spaced Graphene Electrodes to WS2 Semiconductor Monolayers

The ultrathin nature of two-dimensional (2D) materials opens up opportunities for creating devices that are substantially thinner than using traditional bulk materials. In this article, monolayer 2D materials grown by the chemical vapor deposition method are used to fabricate ultrathin all-2D lateral diodes. We show that placing graphene electrodes below and above the WS2 monolayer, instead of the same side, results in a lateral device with two different Schottky barrier heights. Due to the natural dielectric environment, the bottom graphene layer is wedged between the WS2 and the SiO2 substrate, which has a different doping level than the top graphene layer that is in contact with WS2 and air. The lateral separation of these two graphene electrodes results in a lateral metal-semiconductor-metal junction with two asymmetric barriers but yet retains its ultrathin form of two-layer thickness. The rectification and diode behavior can be exploited in transistors, photodiodes, and light-emitting devices. We show that the device exhibits a rectification ratio up to 90 under a laser power of 1.37 μW at a bias voltage of ±3 V. We demonstrate that both the back-gate voltage and laser illumination can tune the rectification behavior of the device. Furthermore, the device can generate strong red electroluminescence in the WS2 area across the two graphene electrodes under an average flowing current of 2.16 × 10-5 A. This work contributes to the current understanding of the 2D metal-semiconductor heterojunction and offers an idea to obtain all-2D Schottky diodes by retaining the ultrathin device concept.

Design of graphene-based tunable ultra-thin UWB metasurface for terahertz regime

This work presents a simple graphene-based tunable ultra-wideband (UWB) metasurface for terahertz application. The unit cell comprises a silicon dioxide (SiO2) substrate sandwiched between the top patterned graphene layer and the bottom continuous graphene layer, where each graphene layer is 1 nm thick. A separate DC bias configuration is also introduced, consisting of another SiO2 substrate backed by a gold plate to achieve independent biasing of both graphene layers. The metasurface is ultra-thin with a thickness of only 5.1μm, i.e. λ0588.235, where λ0 is calculated at the lowest operating frequency. The proposed absorber provides an absorption bandwidth (BW) of 13.9 THz (Fractional BW = 197.16%), covering the electromagnetic spectrum from 0.1 to 14 THz with an absorptivity of more than 90%. The periodicity of the proposed unit cell is found to be only 13.5μm, i.e. λ0222.22. Hence, it satisfies the effective homogeneity condition. The designed unit cell has structural symmetry, which makes the proposed absorber polarization insensitive to the normal incidence of the plane wave. The structure is also examined under a different incident angle (θ) for both TE and TM polarization. It is found to have absorptivity of more than 80% in the frequency range 0.1–14 THz for incidence angle up to 60∘. The variation in the chemical potential (μc) of top Gpat provides tunable absorption characteristic. The increase in μc of Gpat leads to the blue shift in the absorptivity response. The proposed absorber covers the entire ”Terahertz Gap” of the electromagnetic spectrum and is suitable for various terahertz regime applications.

Electric field tunable layer polarization in graphene/boron-nitride twisted quadrilayer superlattices

The recently observed unconventional ferroelectricity in AB bilayer graphene sandwiched by hexagonal Boron Nitride (hBN) presents a new platform to manipulate correlated phases in multilayered van der Waals heterostructures. We present a low-energy continuum model for AB bilayer graphene encapsulated by the top and bottom layers of either hBN or graphene, with two independent twist angles. For the graphene/hBN heterostructures, we show that twist angle asymmetry leads to a layer polarization of the valence and conduction bands. We also show that an out-of-plane displacement field not only tunes the layer polarization but also flattens the low-energy bands. We extend the model to show that the electronic structures of quadrilayer graphene heterostructure consisting of AB bilayer graphene encapsulated by the top and bottom graphene layers can similarly be tuned by an external electric field.

Two-step method for growth of adlayer-free large-area monolayer graphene on Cu foil

Chemical vapor deposition is the most promising approach for synthesis of large-area monolayer graphene on Cu foil. However, numerous factors can result in formation of adlayers, such as the morphology of the Cu foil, methane concentration, and growth temperature. Here, we report atmospheric pressure chemical vapor deposition growth of large-area adlayer-free monolayer graphene by the two-step ‘bottom-up-etching’ method. The experimental results showed that a temperature increase in the second step can dramatically accelerate etching of the bottom graphene layer. A growth model for adlayer-free monolayer graphene on Cu foil is proposed.

Van der Waals integration of GaN light-emitting diode arrays on foreign graphene films using semiconductor/graphene heterostructures

We report the van der Waals integration of micropatterned GaN light-emitting diodes (LEDs) onto foreign graphene films. GaN micro-LEDs were selectively grown on a graphene substrate using a patterned SiO2 mask, and then the whole device structure was laterally fixed by a polyimide insulator to form a united layer. After device fabrication, the LED/graphene heterostructure device was piled on the foreign graphene layers using a typical wet transfer technique of 2D crystals where the bottom graphene layer of the heterostructure was adhered to the foreign graphene only by van der Waals interactions. The transferred micro-LEDs showed well-aligned crystallographic orientations as well as reliable device performances, including strong light emissions, good rectifying behaviors of the current density–voltage curve, and good simultaneity between the electroluminescence intensity and the applied currents, ensuring reliable electrical connections and mechanical adhesions of the light-emitting layer to the foreign graphene films. Furthermore, the reliable adhesiveness allowed us to achieve device wearability, while the LEDs exhibited homogeneous light emissions under various bending conditions because of negligible external stress in the discrete micro-LEDs.

Electrochemical sensors for sulfamethoxazole detection based on graphene oxide/graphene layered composite on indium tin oxide substrate

BackgroundSulfamethoxazole (SMX) can produce many side effects in the human body and several hematological abnormalities. Therefore, the analytical determination of SMX in pharmaceutical formulations and biological fluids is essential. MethodsA biocompatible composite of graphene oxide/graphene (GO/G) layered structure was synthesized to develop electrochemical biosensors for SMX detection. Two layers of single-layer graphene grown by chemical vapor deposition were stacked and annealed to form bilayer graphene (BLG). The BLG was modified by a low-damage plasma treatment—atomic layer oxidation. The top graphene layer was modified to graphene oxide, while the bottom graphene layer was conserved to safeguard its conductive properties and form a layered composite on an indium tin oxide (ITO) substrate. Significant findingsThe GO/G layered composite on the ITO glass substrate, plasma-treated for 15 min exhibited optimized redox properties, with a relatively low charge-transfer resistance. At pH 6, the relationship between the oxidation peak current and the SMX concentration (0.1–50 μM) was linear with a sensitivity of 0.262 μA/μM. With the addition of cetylpyridinium chloride, the sensing performance of the GO/G-based electrochemical sensor was ameliorated to 0.529 μA/μM. The GO/G electrode also exhibited high selectivity and stability to SMX.

Graphene‐Mercury‐Graphene Sandwich Electrode for Electroanalysis

AbstractWe present a new class of hybrid 2D electrodes, where mercury is incorporated between two graphene monolayers, prepared by bottom‐up assembly. First, the bottom graphene layer is electrochemically modified leading to the creation of fine mercury nanodroplets of variable size on the graphene surface. Although such electrodes show good sensitivity to heavy metal ions, their stability is limited due to the outgassing of mercury over time. After coverage with a top monolayer, the graphene surface is rendered with the favorable properties of mercury such as the high overpotential for hydrogen evolution, the ability to work at a broader cathodic potential range and higher sensitivity towards heavy metal ions such as Cd2+ and Pb2+. Most importantly, the outgassing of mercury is completely hindered by the top layer, which yields a stable mercury‐like electrode but with a carbonaceous non‐toxic interface. We attribute the favorable properties of the sandwich electrode to the subsurface mercury present below the top graphene sheet, which renders it with new electrochemical properties.

Graphene/Silver Nanowires/Graphene Sandwich Composite for Stretchable Transparent Electrodes and Its Fracture Mechanism.

Polycrystalline graphene grown by chemical vapor deposition (CVD) is characterized by line defects and disruptions at the grain boundaries and nucleation sites. This adversely affects the stretchability and conductivity of graphene, which limits its applications in the field of flexible, stretchable, and transparent electrodes. We demonstrate a composite electrode comprised of a graphene/silver nanowires (AgNWs)/graphene sandwich structure on a polydimethylsiloxane substrate to overcome this limitation. The sandwich structure exhibits high transparency (>90%) and excellent conductivity improvement of the graphene layers. The use of AgNWs significantly suppresses the conductivity loss resulting from stretching. The mechanism of the suppression of the conductivity loss was investigated using scanning electron microscopy, atomic force microscopy, and lateral force microscopy. The results suggest that the high surface friction of the sandwich structure causes a sliding effect between the graphene layers would produce low crack or hole formation to maintain the conductivity. In addition to acting as conductive layers, the top and bottom graphene layers can also protect the AgNWs from oxidation, thereby enabling maintenance of the electrical performance of the electrodes over a prolonged period. We also confirmed the applicability of the sandwich structure electrode to the human body, such as on the wrist, finger, and elbow.

In Operando Angle‐Resolved Photoemission Spectroscopy with Nanoscale Spatial Resolution: Spatial Mapping of the Electronic Structure of Twisted Bilayer Graphene

To pinpoint the electronic and structural mechanisms that affect intrinsic and extrinsic performance limits of 2D material devices, it is of critical importance to resolve the electronic properties on the mesoscopic length scale of such devices under operating conditions. Herein, angle‐resolved photoemission spectroscopy with nanoscale spatial resolution (nanoARPES) is used to map the quasiparticle electronic structure of a twisted bilayer graphene device. The dispersion and linewidth of the Dirac cones associated with top and bottom graphene layers are determined as a function of spatial position on the device under both static and operating conditions. The analysis reveals that microscopic rotational domains in the two graphene layers establish a range of twist angles from 9.8° to 12.7°. Application of current and electrostatic gating lead to strong electric fields with peak strengths of 0.75 V/μm at the rotational domain boundaries in the device. These proof‐of‐principle results demonstrate the potential of nanoARPES to link mesoscale structural variations with electronic states in operating device conditions and to disentangle such extrinsic factors from the intrinsic quasiparticle dispersion.

Transport through vertical graphene contacts under intense laser fields

We theoretically study the electronic and transport properties of two graphene layers vertically coupled by an insulating layer under the influence of a time-periodic external light field. The non-adiabatic driving induces excitations of electrons and a redistribution of the occupied states which is manifested in the opening of gaps in the quasienergy spectrum of graphene. When a voltage is applied between the top and bottom graphene layers, the photo-induced nonequilibrium occupation modifies the transport properties of the contact. We investigate the electronic and transport properties of the contact by using the nonequilibrium Green's function formalism. To illustrate the behavior of the differential conductance of the vertical contact under the light illumination, we consider two cases. First, we assume that both the bottom and top layers consist of graphene and second we consider a finite mass term in the bottom layer. We obtain that the differential conductance is strongly suppressed due to the opening of gaps in the quasienergy spectrum in graphene. Additionally, the conductance shows features corresponding to the tunneling of photoexcited electrons at energies of the van Hove singularity for both the top and bottom layers. In the case of a finite mass term in the bottom layer, the differential conductance can be directly related to the tunneling of photoexcited electrons.

Fast Response Characteristics of Flexible Ultraviolet Photosensors with GaN Nanowires and Graphene.

We report the fast response characteristics of flexible ultraviolet photosensors with GaN nanowires (NWs) and a graphene channel. The GaN NWs used as light-absorbing media are horizontally and randomly embedded in a graphene sandwich structure in which the number of bottom graphene layers is varied from zero to three and the top is a fixed single layer of graphene. In the response curve of the photosensor with a double-layer bottom graphene, as obtained under pulsed illumination with a pulse width of 50 ms and a duty cycle of 50%, the rise and decay times were measured as 24.1 ± 0.1 and 28.2 ± 0.1 ms, respectively. The eye-crossing percentage was evaluated as 52.1%, indicating no substantial distortion of the duty cycle and no pulse symmetry problem. The rise and decay times estimated from an equivalent circuit analysis represented by resistances and capacitances agree well with the measured values. When the device was under the bending condition, the rise and decay times of the photosensor were comparable to those in the unbent state.

Passivation of Layered Gallium Telluride by Double Encapsulation with Graphene.

Layered semiconductor gallium telluride (GaTe) undergoes a rapid structural transition to a degraded phase in ambient conditions, limiting its utility in devices such as optical switches. In this work, we demonstrate that the degradation process in GaTe flakes can be slowed down dramatically via encapsulation with graphene. Through examining Raman signatures of degradation, we show that the choice of substrate significantly impacts the degradation rate and that the process is accelerated by the transfer of GaTe to hydrophilic substrates such as SiO2/Si. We find that double encapsulation with both top and bottom graphene layers can extend the lifetime of the material for several weeks. The photoresponse of flakes encapsulated in this way is only reduced by 17.6 ± 0.4% after 2 weeks, whereas unencapsulated flakes display no response after this time. Our results demonstrate the potential for alternative, van der Waals material-based passivation strategies in unstable layered materials and highlight the need for careful selection of substrates for 2D electronic devices.

Tuning Adsorption of Methylamine and Methanethiol on Twisted-Bilayer Graphene

The influence of a second graphene layer on the adsorption of methanethiol and methylamine is studied using density functional theory. Exploring different adsorption sites, we compare adsorption energies of both molecules on the monolayer, A–B stacked bilayer, and three different twisted-bilayer graphene systems. Results show that both molecules are physisorbed with energies larger than room temperature on determined sites, where methylamine is stronger adsorbed than methanethiol. In A–B stacking, adsorption energies are always smaller than those found in the monolayer. However, at some relative angles between the top and bottom graphene layers, the adsorption energy increases suggesting a molecular-adsorption tuning effect. Although the electronic charge at the interlayer and vacuum regions suffer a rearrangement, there is no charge transfer between molecules and graphene layers. It is expected that these results motivate further studies of molecule adsorption using as additional parameter the relative a...

Symmetry-Controlled Reversible Photovoltaic Current Flow in Ultrathin All 2D Vertically Stacked Graphene/MoS2/WS2/Graphene Devices.

Atomically thin vertical heterostructures are promising candidates for optoelectronic applications, especially for flexible and transparent technologies. Here, we show how ultrathin all two-dimensional vertical-stacked type-II heterostructure devices can be assembled using only materials grown by chemical vapor deposition, with graphene (Gr) as top and bottom electrodes and MoS2/WS2 as the active semiconductor layers in the middle. Furthermore, we show that the stack symmetry, which dictates the type-II directionality, is the dominant factor in controlling the photocurrent direction upon light irradiation, whereas in homobilayers, photocurrent direction cannot be easily controlled because the tunnel barrier is determined by the doping levels of the graphene, which appears fixed for top and bottom graphene layers due to their dielectric environments. Therefore, the ability to direct photovoltaic current flow is demonstrated to be only possible using heterobilayers (HBs) and not homobilayers. We study the photovoltaic effects in more than 40 devices, which allows for statistical verification of performance and comparative behavior. The photovoltage in the graphene/transition-metal dichalcogenide-heterobilayer/graphene (Gr/TMD-HB (MoS2/WS2)/Gr) increases up to 10 times that generated in the monolayer TMD devices under the same optical illumination power, due to efficient charge transfer between WS2 and MoS2 and extraction to graphene electrodes. By applying external gate voltages ( Vg), the band alignment can be tuned, which in turn controls the photovoltaic effect in the vertical heterostructures. The tunneling-assisted interlayer charge recombination also plays a significant role in modulating the photovoltaic effect in the Gr/TMD-HB/Gr. These results provide important insights into how layer symmetry in vertical-stacked graphene/TMD/graphene ultrathin optoelectronics can be used to control electron flow directions during photoexcitation and open up opportunities for tandem cell assembly.

Gate-tunable Thermoelectric Effects in a Graphene/WS2 van der Waals Heterostructure

Thermoelectric effects of Dirac fermions through a van der Waals (vdW) heterostructure consisting of graphene and tungsten diselenide (WS2) are theoretically investigated. When the lattice temperature of the top graphene layer differs from that of the bottom graphene layer, thermally excited Dirac fermions can be transferred through the WS2 layer, generating tunnel current. This thermoelectric tunnel current shows drastic changes in its characteristics as a consequence of gatevoltage tuning. The thermoelectric power of the proposed graphene–WS2 vdW heterostructure is characterized by examining the Seebeck coefficient.

Growth of graphene/Ag nanowire/graphene sandwich films for transparent touch-sensitive electrodes

Abstract In this paper, a novel graphene/Ag nanowires/graphene (G/Ag NWs/G) sandwich film was directly grown on a conventional glass substrate by use of a technique of low temperature (600 °C) plasma-enhanced chemical vapor deposition (PECVD) combined with solvothermal and modified Hummers methods. The as-grown G/Ag NWs/G/glass sandwich film is uniform, smooth, touchable, transparent and conductive with an optical transmission 77.6% and a low sheet resistance ∼10.8 Ω/sq. The top graphene layer exhibits an effective role for improving the touchable and antioxidant properties of Ag NWs, while the bottom graphene layer enhances the binding stability of Ag NWs with substrate. The sheets resistance of G/Ag NWs/G/glass film exposed to the air for 72 h at 95 °C increases from 10.8Ω/sq to 14.6Ω/sq. These indicate a great promising in replacing the expensive and fragile indium tin oxide (ITO) glass and ITO film for touchable transparent electrode applications.

Critical Annealing Temperature for Stacking Orientation of Bilayer Graphene.

It is rarely reported that stacking orientations of bilayer graphene (BLG) can be manipulated by the annealing process. Most investigators have painstakingly fabricated this BLG by chemical vapor deposition growth or mechanical means. Here, it is discovered that, at ≈600 °C, called the critical annealing temperature (CAT), most stacking orientations collapse into strongly coupled or AB-stacked states. This phenomenon is governed (i) macroscopically by the stress generation and release in top graphene domains, evolving from mild ripples to sharp billows in certain local areas, and (ii) microscopically by the principle of minimal potential obeyed by carbon atoms that have acquired sufficient thermal energy at CAT. Conspicuously, evolutions of stacking orientations in Raman mappings under various annealing temperatures are observed. Furthermore, MoS2 synthesized on BLG is used to directly observe crystal orientations of top and bottom graphene layers. The finding of CAT provides a guide for the fabrication of strongly coupled or AB-stacked BLG, and can be applied to aligning other 2D heterostructures.

Fluctuation-induced tunneling conduction in iodine-doped bilayer graphene

Intrinsic bilayer graphene is a semimetal. Upon applying an electric field, the potential difference between top and bottom layers can open an energy gap and tune bilayer graphene to an insulating state at the charge neutrality point. Here, we demonstrate that the properties of semimetallic bilayer graphene can be controllably tuned to either metallic or insulating by a simple way of iodine molecular doping. The transport properties of iodine-doped bilayer graphene have been systematically investigated. At high iodine doping concentrations, the Fermi level shifts by approximately 0.35 eV to the metallic region because of the symmetric doping on the top and bottom bilayer surfaces. At low iodine doping concentrations, small energy gaps open in local areas due to the asymmetric doping between the top and the bottom graphene layers. In this case, an insulating behavior at low temperatures is observed, which can be well explained by employing the fluctuation-induced tunneling (FIT) model. At medium iodine doping concentrations, both metallic and insulating behaviors can be observed at different temperatures, implying that both FIT and metallic mechanisms take effect. Our work may have potential applications in on/off controllable electronic devices, gas sensors, and transparent flexible electrode in optoelectronics.