Monolayer Graphene Research Articles

Superconductivity in twisted double bilayer graphene stabilized by WSe2.

Identifying the essential components of superconductivity in graphene-based systems remains a critical problem in two-dimensional materials research. This field is connected to the mysteries that underpin investigations of unconventional superconductivity in condensed-matter physics. Superconductivity has been observed in magic-angle twisted stacks of monolayer graphene but conspicuously not in twisted stacks of bilayer graphene, although both systems host topological flat bands and symmetry-broken states. Here we report the discovery of superconductivity in twisted double bilayer graphene (TDBG) in proximity to WSe2. Samples with twist angles 1.24° and 1.37° superconduct in small pockets of the gate-tuned phase diagram within the valence and conduction band, respectively. Superconductivity emerges from unpolarized phases near van Hove singularities and next to regions with broken isospin symmetry. Our results show the correlation between a high density of states and the emergence of superconductivity in TDBG while revealing a possible role for isospin fluctuations in the pairing.

Fabrication of Graphene Optical Devices for Endoscope Calibration

Endoscopic devices used in the clinical examination of pathologies such as glottic tumors can record real-time images of the vocal cords during vibration. Quantitative analysis of the glottic surface requires the metric system to obtain the absolute dimensions of the larynx in a series of acquired images. However, so far, there is no effectively automated and stable calibration method for clinical use. Therefore, most metrics used by professionals are invasive and subjective. Thus, this study reports the microfabrication of a diffractive optical element (DOE) that could potentially quantify cancer cells from the glottic surface of patients. It has been observed that the image of the glottis seen through an endoscope is shaped by its geometric properties and by the shading of color and brightness. Therefore, tumor location and size can be quantified by overlaying the DOE metric with light spots on images that include the glottic region. DOE was developed from processes used in the fabrication of integrated circuits and MEMS, already quite mature. To obtain high-quality images from optical reconstruction of the glottis, the use of graphene monolayers grown by chemical vapor deposition (CVD) on copper (Cu) foil was investigated. This was transferred to a glass substrate by wet method (Fishing method) and the etching steps of the Cu substrate were carried out. In this method, polymethylmethacrylate (PMMA) was used as the supporting polymer through the spinner. Two lithographs were made under the glass substrate with different approaches (using a Karl Suss MJB3 mask aligner system with a maximum resolution of 1 µm). The first, which preceded the thin-film transfer, aimed to delineate the graphene transfer area using an aluminum (Al) deposition technique by sputtering. The second lithography, linked to oxygen Ashing plasma corrosion, made it possible to study the modulation of a microrelief containing parallel and serpentine patterns, of amplitude and phase, spaced from 1 to 10 μm. An optical system containing an EOD and a Fourier lens was attached to the endoscope end to project a laser line parallel to the vocal cords. Image processing methods automatically identify pixels in the image data that belong to the projected laser line. Thus, the detection of the diffractive pattern was based on segmentation of endoscopic images centered on Sobel filtering and on the method with active boundary regions of the watershed. These use successively robust temporal and spatial characteristics of the laser lines projected onto the endoscopic image sequence. In short, the combination of laser projection device and image processing allows calibration of endoscopic images and can provide quantitative metric data for tumors on the surface of the vocal cords. Therefore, the proposed calibration method can be considered a powerful tool to aid in the prognosis of laryngeal pathologies. Figure 1

(Invited) 2D-2D Heterostructures Beyond Van Der Waals

We will describe the covalent grafting of 2H-MoS2 flakes on graphene monolayers embedded in field-effect transistors.[1] A bifunctional molecule was used that features a maleimide and a diazonium functional group, known to connect to sulfide- and carbon-based materials, respectively. MoS2 flakes were first exfoliated, functionalized by reaction with the maleimide moieties, then anchored to graphene through the diazonium groups. This approach enabled the simultaneous functionalization of several devices. The electronic properties of the resulting heterostructure are shown to be dominated by the MoS2–graphene molecular interface.We will also discuss the journey that has led to these results, including the development of a “click” chemistry reaction for transition metal-dichalcogenides,[2-4] and insights into the covalent patterning of graphene.[5-8]

Brownian dynamics of Dirac fermions in twisted bilayer graphene

Brownian dynamics of Dirac fermions in twisted bilayer graphene is investigated within the framework of semiclassical relativistic Langevin equations. We find that under the influence of orthogonal, commensurate ac drives in the periodic ratchet potential of a substrate, the charge carriers in the system exhibit pronounced random dynamics, tuned by the twist angle, making twisted bilayer graphene distinct from monolayer graphene. It is shown that as threshold twist angle matches the optimal angle, deterministic running states appear in the limit of weak thermal noise where the diffusion rate is enhanced significantly compared to bare thermal diffusion. Analysis of the real space trajectories and diffusion coefficient illustrates the significant role of thermal noise in the random motion of Dirac fermions. In addition, we find that the Brownian particle shows remarkable ratchet effect as a net current.

Plasmonic heterostructure biosensor based on perovskite/two dimensional materials

We propose a bimetallic glucose sensor based on surface plasmon using three-dimensional metal halide perovskites and two-dimensional material. The transfer matrix method and density functional theory corroborated by finite-element simulation are used for numerical analysis for the first time. In the angle interrogation method, MAPbBr3 and monolayer graphene enhanced sensitivity by 254%. A conventional Ag-based sensor has a sensitivity of 113 deg/RIU, whereas the bimetallic/ MAPbBr3/ graphene stack has a sensitivity of 400.25 deg/RIU with 22 nm Ag/ 22 nm Au/ 17 nm MAPbBr3/ graphene. Furthermore, the signal-to-noise ratio, figure of merit, and detection range of the proposed structure are 0.153 deg− 1, 61.23 RIU− 1, and 1.33–1.345, respectively. We also investigate the ductility, temperature stability, thermodynamics, and mechanical stability of metal halide perovskites in detail. The results show that compared to published results, the proposed SPR sensor has significantly improved characteristics.

Proton transport through nanoscale corrugations in two-dimensional crystals

Defect-free graphene is impermeable to all atoms1–5 and ions6,7 under ambient conditions. Experiments that can resolve gas flows of a few atoms per hour through micrometre-sized membranes found that monocrystalline graphene is completely impermeable to helium, the smallest atom2,5. Such membranes were also shown to be impermeable to all ions, including the smallest one, lithium6,7. By contrast, graphene was reported to be highly permeable to protons, nuclei of hydrogen atoms8,9. There is no consensus, however, either on the mechanism behind the unexpectedly high proton permeability10–14 or even on whether it requires defects in graphene’s crystal lattice6,8,15–17. Here, using high-resolution scanning electrochemical cell microscopy, we show that, although proton permeation through mechanically exfoliated monolayers of graphene and hexagonal boron nitride cannot be attributed to any structural defects, nanoscale non-flatness of two-dimensional membranes greatly facilitates proton transport. The spatial distribution of proton currents visualized by scanning electrochemical cell microscopy reveals marked inhomogeneities that are strongly correlated with nanoscale wrinkles and other features where strain is accumulated. Our results highlight nanoscale morphology as an important parameter enabling proton transport through two-dimensional crystals, mostly considered and modelled as flat, and indicate that strain and curvature can be used as additional degrees of freedom to control the proton permeability of two-dimensional materials.

Detection of Interleukin-6 Protein Using Graphene Field-Effect Transistor.

Universal platforms to analyze biomolecules using sensor devices can address critical diagnostic challenges. Sensor devices like electrical-based field-effect transistors play an essential role in sensing biomolecules by charge probing. Graphene-based devices are more suitable for these applications. It has been previously reported that Graphene Field-Effect Transistor (GFET) devices detect DNA hybridization, pH sensors, and protein molecules. Graphene became a promising material for electrical-based field-effect transistor devices in sensing biomarkers, including biomolecules and proteins. In the last decade, FET devices have detected biomolecules such as DNA molecules, pH, glucose, and protein. These studies have suggested that the reference electrode is placed externally and measures the transfer characteristics. However, the external probing method damages the samples, requiring safety measurements and a substantial amount of time. To control this problem, the graphene field-effect transistor (GFET) device is fabricated with an inbuilt gate that acts as a reference electrode to measure the biomolecules. Herein, the monolayer graphene is exfoliated, and the GFET is designed with an in-built gate to detect the Interleukin-6 (IL-6) protein. IL-6 is a multifunctional cytokine which plays a significant role in immune regulation and metabolism. Additionally, IL-6 subsidizes a variability of disease states, including many types of cancer development, and metastasis, progression, and increased levels of IL-6 are associated with a higher risk of cancer and can also serve as a prognostic marker for cancer. Here, the protein is desiccated on the GFET device and measured, and Dirac point shifting in the transfer characteristics systematically evaluates the device's performance. Our work yielded a conductive and electrical response with the IL-6 protein. This graphene-based transducer with an inbuilt gate gives a promising platform to enable low-cost, compact, facile, real-time, and sensitive amperometric sensors to detect IL-6. Targeting this pathway may help develop treatments for several other symptoms, such as neuromyelitis optica, uveitis, and, more recently, COVID-19 pneumonia.

Amplifying sensing performance through gold micropatterns-induced modulation of graphene's vertical electron transfer

High-quality graphene monolayers exhibit limited vertical electron transfer properties due to their inert basal plan structure, hampering their use in electrochemical sensing devices. Overcoming this limitation while maintaining the graphene's basal plane electrical conductivity is crucial for developing sensitive and efficient electrochemical sensors. In this study, we demonstrate that incorporating a gold support beneath graphene enhances its electrochemical activity by facilitating effective hybridizations between carbon atoms in graphene and gold atoms. To achieve this, micrometer-sized gold micropatterns were fabricated on Si/SiO2 substrates to support the graphene monolayer. The presence of gold significantly improved the electrochemical activity of graphene towards redox probes in solution (ferri-/ferrocyanide) as well as for adsorbed ferrocene, indicating a synergistic effect between graphene and gold. We further investigated the adsorption effects of double-stranded DNA on pristine graphene, bare gold, and graphene with a gold underlayer, revealing that the graphene-gold combination achieved a significantly lower limit of detection for DNA hybridization events, surpassing the other configurations. These findings highlight the importance of appropriate substrate selection in the design of graphene-based electrochemical sensors, particularly in the context of DNA recognition events. The proposed approach of incorporating a gold support beneath graphene holds promise for enhancing the performance of electrochemical sensing devices without compromising the intrinsic properties of graphene.

An infrared passively Q-switched laser based on graphene/BN heterojunction

Passively Q-switched solid-state lasers (PQSL) can obtain output lasers with narrow pulse widths and high peak powers, where play a vital role in the information society. Saturable absorbers (SA) are an essential part of passively Q-switched solid-state lasers. In this paper, we demonstrated 2 μm passively Q-switched (PQS) solid-state laser based on a graphene/BN heterojunction saturable absorber. In the experiments, we used two output mirrors with transmittance of 1.5 %/2.5 % for comparative study. We also used monolayer graphene as saturable absorber for experiment to compared with graphene/BN saturable absorber. The experimental results showed that the best passively Q-switched operation was achieved with the graphene/BN heterojunction saturable absorber at output mirror transmittance of 1.5 %. When the maximum absorption pump power was 1.49 W, the maximum average output power was 38 mW, corresponding with a slope efficiency of 5.4 %. At this point, the minimum pulse width was 1.56 μs, corresponding with a repetition frequency of 52 kHz. The single pulse energy and the peak power were calculated to be 730.36 nJ and 466.96 mW, respectively. The center wavelength of the PQS laser was 2066.8 nm. The experimental results showed that graphene/BN heterojunction was a promise saturable absorber in the field of 2 μm passively Q-switched solid-state lasers.

Realistic systems testing Klein tunneling in graphene: An ab initio calculations

We investigate the tunneling in monolayer graphene (MLG) through the potential barrier generated by the ribbons of Na, Cl and h-BN using the density functional theory (DFT) approach. Such an approach to the problem resulted in determining the shapes and parameters of the potential barriers in individual nanodevices in contrast to the tight binding (TB) approach where these parameters are assumed. In all systems we could observe the Klein tunneling features although under complex potential barrier/well profiles for some cases. The double barrier created in systems consisting of h-BN ribbons as overlayers/underlayers on graphene (h-BN/MLG/h-BN) allowed to analyze Klein tunnelling depending on the distance between two triangular barriers. The presented concept gives the possibility to measure the angular dependence of tunnelling and as such is of particular relevance to alternative/supplementary experiments to those related to observations of Klein tunnelling in graphene through potential barriers generated by applying a gate voltage.

Tunable transparent terahertz absorber for sensing and radiation warming

The lack in efficient functional devices has hindered plenty practical applications in terahertz (THz) band. In this work, we developed transparent THz absorber by employing trilayer Salisbury screen structure. Monolayer graphene and indium tin oxide (ITO) glass were spaced by a thin dielectric film, forming a transparent THz absorber. Assisted by ion gel, the carrier density of graphene layer can be effectively tuned through small external electrical gating (−2.5 V-2.5 V), with a modulation depth reaching to 100%. Taking advantage of the strong π−π interaction between graphene and external molecules containing π electrons, our fabricated transparent THz absorber enabled diphenylamine (DPA) detection. The adsorbed DPA molecule moves Fermi level of graphene toward Dirac point, performing a limit of detection of 5.5 ng. Moreover, by transferring the absorbed radiation into heat, our fabricated absorber can serve as a radiation warming device. The surface temperature of transparent THz absorber is 0.7 °C higher than that of glass in IR radiation environment. Note that the absorber is highly transparent in visible band, it will not prevent photosynthesis process of plants. Thereby, the THz absorbers have potential practical applications in diverse fields including wave manipulation, molecule sensing, and radiation warming.

Structural Study of Diketopyrrolopyrrole Derivative Thin Films: Influence of Deposition Method, Substrate Surface, and Aging.

We report the morphology and microstructure of n-dialkyl side-chain-substituted thiophene DPP end-capped with phenyl groups (Ph-TDPP-Ph) thin films and compare the influence of deposition method and substrate surface using thermally oxidized Si and graphene substrates as well as monolayer graphene surfaces with an underlying self-assembled octadecyltrichlorosilane monolayer, complemented by an aging study of spin-coated films over a 2 weeks aging period. A distinct difference in morphology was observed between spin-coated and vacuum-deposited thin films, which formed a fiber-like morphology and a continuous layer of terraced grains, respectively. After an initial film evolution, all combinations of deposition method and substrate type result in well-ordered thin films with almost identical crystalline phases with slight variations in crystallinity and mosaicity. These findings point toward strong intermolecular forces dominating during growth, and the templating effect observed for other oligomer films formed on graphene is consequently ineffective for this material type. Upon aging of spin-coated films, a noticeable evolution involving two different morphologies and crystalline phases were observed. After several days, the thin film evolved into a more stable crystal phase and a fiber-like morphology. Moreover, slight variation in optical spectra were elucidated on the basis on density functional theory calculations. These results demonstrate that thin-film properties of DPP derivatives can be tailored by manipulating the film formation process.

Probing the wear characteristics of graphene on flexible polymer substrates using a heated atomic force microscopy tip

Graphene is a useful solid lubricant for flexible micro/nanoelectromechanical systems (MEMS/NEMS) because of its beneficial mechanical properties. However, understanding the tribological and wear characteristics of graphene on polymer substrates under high-temperature conditions is essential for predicting whether graphene will fail as a solid lubricant for flexible substrates in high-temperature environments. Herein, the friction and wear characteristics of monolayer graphene on flexible polydimethylsiloxane (PDMS) substrates are studied by atomic force microscopy using a heated tip. The friction of graphene before wear is more stable than that of PDMS, demonstrating that graphene can be used as a solid lubricant to stabilize the stick-slip motion of the tip on the flexible substrate at elevated temperatures. The friction of graphene shows an increasing trend with the heating temperature increasing due to an enlarged tip-graphene contact area. However, the friction of graphene considerably decreases with the tip sliding velocity increasing at a speed of less than 0.5 µm/s, resulting from a reduced tip-graphene contact area. The thermally-induced wear of graphene is caused by the coupling effect of the temperature and the sliding velocity. These results can provide an in-depth understanding of graphene as a solid lubricant for flexible polymer-based MEMS/NEMS devices in high-temperature environments.

Influence of Growth Time and Temperature on Optical Characteristics and Surface Wetting in Nano-Crystalline Graphene Deposited by PECVD Directly on Silicon Dioxide

Unique electronic properties of graphene offer highly interesting ways to manipulate the functional properties of surfaces and develop novel structures which are sensitive to physical and chemical interactions. Nano-crystalline graphene is frequently preferable to crystalline monolayer in detecting devices. In this work, nano-crystalline graphene layers were synthesized directly on SiO2/Si substrates by plasma-enhanced chemical vapour deposition (PECVD). The influence of the deposition time and temperature on the characteristics of the structures were studied. The optical properties and evaporation kinetics of pure water droplets were analysed, along with arrangement and composition of the grown layers. The nano-crystalline graphene layers grown at 500 °C were characterised by the refraction index 2.75 ± 0.35 and the normalised excess Gibbs free energy density 0.85/γwater 10−4 m, both being similar to those of the monolayer graphene. The changes in the refraction index and the excess Gibbs free energy were related to the parameters of the Raman spectra and a correlation with the technological variables were disclosed.

High-Density Vertical Transistors with Pitch Size Down to 20nm.

Vertical field effect transistors (VFETs) have attracted considerable interest for developing ultra-scaled devices. In particular, individual VFET can be stacked on top of another and does not consume additional chip footprint beyond what is needed for a single device at the bottom, representing another dimension for high-density transistors. However, high-density VFETs with small pitch size are difficult to fabricate and is largely limited by the trade-offs between drain thickness and its conductivity. Here, a simple approach is reported to scale the drain to sub-10nm. By combining 7nm thick Au with monolayer graphene, the hybrid drain demonstrates metallic behavior with low sheet resistance of ≈100 Ω sq-1 . By van der Waals laminating the hybrid drain on top of 3nm thick channel and scaling gate stack, the total VFET pitch size down to 20nm and demonstrates a higher on-state current of 730A cm-2 . Furthermore, three individual VFETs together are vertically stacked within a vertical distance of 59nm, representing the record low pitch size for vertical transistors. The method pushes the scaling limit and pitch size limit of VFET, opening up a new pathway for high-density vertical transistors and integrated circuits.

Multifunctional properties of alumina-based graphene nanocomposites as catalysts for esters of glycerol production

The properties of alumina-based graphene nanocomposites were fine-tuned using a directed self-assembly of graphene oxide and metal oxides mediated by a surfactant approach. The resultant nanostructured metal oxides were evaluated in the esterification of glycerol with acetic acid as catalysts for the esters of glycerol production. X-Ray Diffraction (XRD), High-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy measurements of the self-assembled nanocomposites revealed the nanocrystalline γ-Al2O3 phase between the separated graphene lamellar nanostructures. A crumpled monolayered graphene sheet along with well dispersed Ni and Co entities on solid surface appeared by Scanning electron microscopy coupled to energy dispersive X-ray spectroscopy (SEM-EDS) analyses. Electron paramagnetic resonance spectroscopy (EPR) and X-ray photoelectron spectroscopy (XPS) showed Ni2+, Co3+/Co2+and Al3+species and a more reduced surface oxygen species in the self-assembled solids. The addition of basic promoters such as La, Mg or Zn to the composites did not improve the catalytic properties of the solids, but combined with NiCo-alumina-based graphene nanocomposites, they had shown good activities for triacetin production.

Ag-doped Monolayer and Bilayer Graphene for the Gas Sensor and Purifier of Industrial Oils

Graphene sensors have high sensitivity and selectivity and are commonly used to detect and clean oilbreakdownresulting gas contaminants. Silver impurities on Graphene sheet vacancywere simulated for Methane interaction with vacant Graphene layers, with and without silver impurities, based on density functional theory with Quantum ESPRESSO.PBEexchange–correlation function withGGAfunction and vdW-DF corrections were used. Analyzing adsorption energies revealed theeffects of the additives on Graphene, andshowedGraphene with silver impuritieswasa good option. Analyzes of methane(CH4/MVG,CH4/BVG,CH4/MVG_Ag,andCH4/BVG_Ag)show that adsorption strengths of the structures were the relationship MVG_Ag > BVG > MVG > BVG_Ag. AddingAgimpurity to the foil greatly increases the CH4 adsorption strength of MVG_Ag and decreases the adsorption strength of BVG_Ag. Due to CH4′s symmetry, orientation relative to Graphene makes no difference as it interacts with carbon atoms from any side.

Spectral Tuning of a Nanoparticle-on-Mirror System by Graphene Doping and Gap Control with Nitric Acid.

Nanoparticle-on-mirror systems are a stable, robust, and reproducible method of squeezing light into sub-nanometer volumes. Graphene is a particularly interesting material to use as a spacer in such systems as it is the thinnest possible 2D material and can be doped both chemically and electrically to modulate the plasmonic modes. We investigate a simple nanoparticle-on-mirror system, consisting of a Au nanosphere on top of an Au mirror, separated by a monolayer of graphene. With this system, we demonstrate, with both experiments and numerical simulations, how the doping of the graphene and the control of the gap size can be controlled to tune the plasmonic response of the coupled nanosphere using nitric acid. The coupling of the Au nanosphere and Au thin film reveals multipolar modes which can be tuned by adjusting the gap size or doping an intermediate graphene monolayer. At high doping levels, the interaction between the charge-transfer plasmon and gap plasmon leads to splitting of the plasmon energies. The study provides evidence for the unification of theories proposed by previous works investigating similar systems.

Effect of doping on vibration frequency of graphene nanoribbons

Graphene has excellent mechanical properties and vibration frequency is an important mechanical property. In this paper, the effects of doping elements and doping concentrations on the vibration characteristics of graphene nanoribbons were investigated by molecular dynamics simulation. The vibration characteristics are influenced by the doping element, depending on the bond structural difference in bond structure between doping atoms and carbon atoms. The vibration frequencies of Nitrogen (N)-doped and Nitrogen–Nitrogen (N–N)-doped graphene nanoribbons are slightly lower than those of pure graphene nanoribbons. However, the vibration frequencies of Sulfur (S)/Phosphorus (P)-doped and Sulfur–Sulfur (S–S)/Phosphorus–Phosphorus (P–P)-doped graphene nanoribbons are significantly lower than those of pure graphene nanoribbons. In addition, with the increase of S (P) doping concentration, the vibration frequency of monolayer graphene nanoribbons decreases gradually.

Bag Boundaries for Quasispinor Confinement Within Nanolanes on a Graphene Sheet

AbstractThe problem of bag boundary conditions within a field‐theoretic approach is revisited to study confinement of massless Dirac quasispinors in monolayer graphene. While no‐flux bag boundaries have previously been used to model lattice termination sites in graphene nanoribbons, a generalized setting is considered in which the confining boundaries are envisaged as arbitrary straight lines drawn across a graphene sheet and the quasispinor currents are allowed to partially permeate (leak) through such boundaries. Specifically focus is on rectangular nanolanes defined as areas confined between a pair of parallel lines at arbitrary separation on an unbounded lattice. It is shown that such nanolanes exhibit a considerable range of bandgap tunability depending on their widths and armchair, zigzag, or intermediate orientation. The case of nanoribbons can be derived as a special limit from the nanolane model. In this case, certain inconsistencies are clarified in previous implementations of no‐flux bag boundaries and show that the continuum approach reproduces the tight‐binding bandgaps accurately (within just a few percent in relative deviation) even as the nanoribbon width is decreased to just a couple of lattice spacings. This accentuates the proper use of boundary conditions when field‐theoretic approaches are applied to graphene systems.