Monolayer Graphene Research Articles

Langmuir-Blodgett Monolayer of Cobalt Phthalocyanine as Ultralow Loading Single-Atom Catalyst for Highly Efficient H2O2 Production.

The electrochemical production of H2O2 via the two-electron oxygen-reduction reaction (2e- ORR) has been actively studied using systems with atomically dispersed metal-nitrogen-carbon (M-N-C) structures. However, the development of well-defined M-N-C structures that restrict the migration and agglomeration of single-metal sites remains elusive. Herein, we demonstrate a Langmuir-Blodgett (LB) monolayer of cobalt phthalocyanine (CoPc) on monolayer graphene (LB CoPc/G) as a single-metal catalyst for the 2e- ORR. The as-prepared CoPc LB monolayer has a β-form crystalline structure with a lattice space for the facile adsorption of oxygen molecules on the cobalt active sites. The CoPc LB monolayer system provides highly exposed Co atoms in a well-defined structure without agglomeration, resulting in significantly improved catalytic activity, which is manifested by a very high H2O2 production rate per catalyst (31.04 mol gcat-1 h-1) and TOF (36.5 s-1) with constant production stability for 24 hours. To the best of our knowledge, the CoPc LB monolayer system exhibits the highest H2O2 production rate per active site. This fundamental study suggests that an LB monolayer of molecules with single-metal atoms as a well-defined structure works for single-atom catalysts.

Characterization of Chiral Near Field Within Nanogaps Using Surface‐Enhanced Raman Scattering

AbstractChiral nanogap antennas, which exhibit specific chiroptical responses, hold exceptional potential in various chiral applications, including polarization converters, asymmetric catalysis, label‐free chiral recognition, and chiral sensing. However, characterizing the chiral optical fields within nanogaps presents considerable challenges owing to their complex light field responses and spatial dimensions. In this study, the strong modulation of helicity‐resolved surface‐enhanced Raman spectroscopy (SERS) signals in graphene by chiral plasmonic antennas, offering a promising approach to characterize chiral near fields within nanogaps is demonstrated. The SERS of achiral monolayer graphene is performed within plasmonic nanogap antennas, including single gold nanorods (GNRs) and GNR dimers on gold microflakes. The Raman scattering intensity of graphene within these nanogap antennas is enhanced approximately sixfold. Furthermore, the degree of cross‐circularly polarized Raman intensity reaches up to ≈45%, indicating a robust chiral optical field. Moreover, under opposite circularly polarized light excitation, the SERS of graphene reveals varying emission intensities attributed to the modulation arising from the chiral antennas in both the excitation and emission processes of Raman scattering. Furthermore, the degree of chirality depends on the antenna configuration and the excitation laser wavelength.

Solvent-free transfer of monolayer graphene with recrystallized cyclododecane

Graphene produced by chemical vapor deposition finds applications in a wide range of technologies. However, the transfer of monolayer graphene from the native substrate (commonly Cu foils) to a desired substrate remains challenging. In this study, we report a solvent-free transfer technique for monolayer graphene using a sublimable supporting coating, cyclododecane (CDD). After spin-coating CDD on graphene grown on Cu foil, we rapidly heat the stack at 80 °C to melt and recrystallize the CDD coating. The recrystallized coating top surface becomes composed of larger and interconnected CDD crystals, which form a mechanically strong coating that takes almost 2 h to sublimate completely. Having a bottom surface that closely conforms to graphene's topography, the CDD coating can effectively support graphene during the whole transfer process without compromising its structural integrity, maintaining its superior electrical and optical properties. In this way, CDD becomes capable of transferring monolayer graphene over square centimeter areas. Unlike traditional polymer-based techniques, after transfer, the CDD coating sublimates completely, without the need of any post-transfer cleaning treatment. Our work shows that the CDD coating can be an alternative transfer medium for the efficient and clean transfer of monolayer graphene. This technique paves the way for the widespread adoption of graphene in various applications, including those not compatible with solvents.

A tunable graphene-based dual PIT device with dual-polarization excitation capability

A simple dual plasmon induced transparency (PIT) device based on “Z” type and “circle-strip” combination is designed, which is composed of monolayer graphene. This structure has the capability to excite double PIT simultaneously in the x polarization and y polarization directions. The polarization direction of the incident light will change the resonance frequency of the horizontal and vertical bands, but the resonance frequency of the circular graphene in the middle is not affected by the polarization direction of the light. After comparing the transmission spectra of the single structure and the combined structure, combined with the analysis of the surface electric field, it is determined that the structure adopts the coupling method of bright-bright-bright mode to realize the double PIT function. In both x and y polarization directions, the three components of the device are capable of inducing dipole resonance. Additionally, simulation results fit well with the theoretical transmittance calculations. When the device is utilized as a light sensor, it has been observed that sensitivity tends to increase alongside Fermi level elevation. Specifically, under x polarization, sensitivity can reach 1.888THz/RIU with a maximum FOM of 9.28 when the refractive index of the analyte is 1.7. Meanwhile, under y polarization, sensitivity can reach 1.943THz/RIU with a maximum FOM of 7.61 when the refractive index of the analyte is 1.3. In its capacity as an optical switch, the device has demonstrated a maximum modulation depth of 80.6 % at 2.64THz under x polarization, and 73.29 % at 1.92THz under y polarization. All modulations are achieved by adjusting the Fermi level.

Ultra-broadband near-infrared absorption enhancement of monolayer graphene by multiple-resonator approach

We theoretically study how to achieve an ultra-broadband near-infrared absorption enhancement of monolayer graphene through the multiple-resonator approach. The monolayer graphene (about 0.34 nm in thickness) is on nanostructured metal surfaces with multiple one-dimensional nanogrooves. The investigated nanostructure has an advantage of easy fabrication, because the monolayer graphene needs not to be sandwiched between different material layers. The metallic nanogrooves are able to support magnetic plasmon resonances whose resonance wavelengths are largely tuned, by changing the nanogroove depth from 70 to 210 nm or the nanogroove width from 20 to 40 nm. In a very wide near-infrared wavelength range from 850 nm to 1500 nm, the light absorption efficiency of graphene is enhanced to be more than 60 %, by carefully designing multiple magnetic plasmon resonances to be partly overlapped in optical spectra. Our work is of interest for some photoelectric devices, for example infrared photodetectors.

The effect of non-metals (O, F) dopant on the electronic structure, Dirac cone, and optical characteristics of graphene sheets applicable for gas sensing

In order to gain a systematic comprehension of the impacts of low-level defects in graphene, ab-initio calculations were performed on monolayer graphene sheets to examine how oxygen and fluorine substitutional impurities affect their energetic stability, electronic and optical characteristics. The optimal configurations, structural stability, electronic band structures, spatial charge density, and optical spectra of graphene single-layer materials doped with oxygen and fluorine atoms are investigated. The fluorine-doped graphene, therefore, showed a lower formation energy than its oxygen-doped counterpart. Additionally, F-doped graphene shows a higher charge density around the fluorine atom than O-doped graphene. Based on band structure analysis, the lower conduction bands of graphene sheets shift below the Fermi state (EF) when two O or F impurities are substituted into it. With two O/F impurity atoms doped into graphene, the conduction states are farther displaced below the EF, resulting in n-type behavior. Consequently, altering graphene structure via an O/F-substituting scheme results from the disruption of the Dirac cone near EF, which has become flat along the high symmetry point (K-point) in the Brillouin zone. Furthermore, the substitution of graphene sheets via O or F impurities exhibit markedly modified optical spectra and refractive indices at low photon energies. As a result, with one or two O/F impurity elements inserted into graphene sheets, the reflectivity and optical conductivity spectra differ at the lowest electromagnetic radiation ranges. Additionally, the electron energy loss functions as well as X-ray absorption spectra of clean and doped graphene monolayers revealed spectral variations at a moderate energy span. These investigations surmise that novel chemical modifications to graphene sheets doped with light non-metallic elements (O/F) may offer the opportunity to tailor their optical characteristics in the infrared spectrum, which may be beneficial in sensor and electrocatalyst uses.

Dual-mode tunable absorber based on quasi-bound states in the continuum.

In this Letter, we propose a novel, to the best of our knowledge, dual-mode tunable absorber that utilizes quasi-bound states in the continuum (q-BIC) based on the periodically arranged silicon cylinders tetramer. By introducing asymmetry perturbation through manipulating the diameters of diagonal cylinders in the all-dielectric structure, the symmetry-protected BIC (SP-BIC) transforms into q-BIC, leading to the emergence of one transmission and one reflection Fano-like resonant mode. The relationship between the quality factor of each mode and the asymmetry parameter α is analyzed, revealing an exponential dependence with an exponent of -1.75, i.e., Q ∝ α-1.75. To explain the underlying physics, multipole decomposition analysis and Aleksandra's theory are applied. Subsequently, a monolayer graphene is introduced to the all-dielectric structure to demonstrate the application of the dual-mode tunable absorber. When the critical coupling condition is satisfied, each mode can achieve the theoretical maximum absorption, demonstrating the distinctive capability of our proposed absorber for tuning and efficient light absorption. This research provides valuable insights into light-matter interactions and opens up possibilities for optical modulation and the development of graphene-based devices.

Adsorption and Sensing Performances of a TiO2 Cluster-Doped Graphene Monolayer toward SOF2 and SO2F2.

Due to the high surface activity of metal atoms, it is difficult for doped metals to maintain long-term stability. Compared with other metal oxides, TiO2 graphene sensors have a higher sensing response and a larger specific surface area. However, there is currently no literature comparing and analyzing the effect of graphene-doped TiO2 on the adsorption performance of SF6 decomposition components. Therefore, this article investigated the adsorption performance of SF6 decomposition products: the distribution of SOF2 and SO2F2 on nTiO2 (n = 1,2,3)-doped graphene. In order to explore the interaction mechanism between gas molecules and these modified systems, their Eads, charge transfer, DOS, and PDOS were systematically studied. The doping of nTiO2 significantly improves the adsorption ability of graphene to gas molecules, discovers and enriches the online monitoring methods of GIS, and improves the engineering application value of GIS.

High performance sensor based on phase difference induced quasi-BIC and Fermi energy

We propose a dielectric corrugated structure surrounded by two monolayer graphene and find that the structure supports bound states in the continuum (BIC). By introducing a phase difference between the upper and lower surface of dielectric grating, the symmetry of the structure is broken, and the BIC turns into quasi-BIC. In addition, we find that the Fermi energy of graphene strongly affect the spectral line. By controlling phase difference and Fermi energy of graphene, the ultrahigh Q-factor can be achieved. Finally, introducing a sensing medium at the incident side, the high performance sensor is realized.

Application of Monolayer Graphene to Cryo-Electron Microscopy Grids for High-resolution Structure Determination.

In cryogenic electron microscopy (cryoEM), purified macromolecules are applied to a grid bearing a holey carbon foil; the molecules are then blotted to remove excess liquid and rapidly frozen in a roughly 20-100 nm thick layer of vitreous ice, suspended across roughly 1 µm wide foil holes. The resulting sample is imaged using cryogenic transmission electron microscopy, and after image processing using suitable software, near-atomic resolution structures can be determined. Despite cryoEM's widespread adoption, sample preparation remains a severe bottleneck in cryoEM workflows, with users often encountering challenges related to samples behaving poorly in the suspended vitreous ice. Recently, methods have been developed to modify cryoEM grids with a single continuous layer of graphene, which acts as a support surface that often increases particle density in the imaged area and can reduce interactions between particles and the air-water interface. Here, we provide detailed protocols for the application of graphene to cryoEM grids and for rapidly assessing the relative hydrophilicity of the resulting grids. Additionally, we describe an EM-based method to confirm the presence of graphene by visualizing its characteristic diffraction pattern. Finally, we demonstrate the utility of these graphene supports by rapidly reconstructing a 2.7 Å resolution density map of a Cas9 complex using a pure sample at a relatively low concentration.

Reflectionless graphene perfect absorber based on parity symmetric unidirectional guided resonance.

We propose a type of reflectionless graphene perfect absorber (GPA) in which the reflection channel is forbidden, while the transmission channel is open. Peak absorption of 99.97% in the near-infrared is numerically demonstrated for monolayer graphene loaded on a one-dimensional silicon photonic crystal slab with rhomboid cross sections that supports parity symmetric unidirectional guided resonances (UGRs). Based on the proposed GPA, a transmissive optical modulator with a modulation depth of about 28 dB and an insertion loss of 0.31 dB by varying the Fermi energy level graphene from 0.3 eV to 0.7 eV is numerically presented. Remarkably, the design strategy can be straightforwardly applied to other two-dimensional (2D) materials. Our study may find promising applications in 2D material-based optical modulators and filters.

Gaussian and Gaussian-pulsed-like Fermi velocity graphene structures

Gaussian and Gaussian-related structures are quite attractive due to its versatility to modulate the electronic transport, including its possibility as electron filters. Here, we show that these non-conventional profiles are not the exception when dealing with Fermi velocity barriers in monolayer graphene. In particular, we show that Gaussian Fermi velocity graphene barriers (G-FVGBs) and Gaussian-pulsed-like Fermi velocity graphene superlattices (GPL-FVGSLs) can serve as electron band-pass filters and oscillating conductance structures. We reach this conclusion by theoretically studying the transmission and transport properties of the mentioned structures. The study is based on the continuum model, the transfer matrix method and the Landauer–Büttiker formalism. We find nearly flat transmission bands or pass bands for G-FVGBs modulable through the system parameters. The pass bands improve as the maximum ratio of Fermi velocities () increases, however its omnidirectional range is reduced. These characteristics result in a decaying conductance (integrated transmission) with . The integrated transmission remains practically unaltered with the size of the system due to the saturation of the electron pass band filtering. In the case of GPL-FVGSLs the GPL profile results in regions of high transmission probability that can merge as flat transmission minibands if the pulse fraction and the superlattice parameters are appropriately tuned. The GPL profile also results in conductance (integrated transmission) oscillations that can be multiplied or reduced in number by adjusting the pulse fraction as well as the superlattice parameters.

Graphene-based ultralow-profile microwave Fresnel lens

The development of ultrathin Fresnel lenses has revolutionized the lens technology and provided great prospects for the miniaturization of electromagnetic systems. Despite considerable advances in microtechnology, a scalable fabrication strategy to achieve ultralow profile microwave lenses is yet to be developed. Here, we report on a novel strategy to create ultrathin and essentially flat Fresnel lenses using graphene rings. We show that monolayer or multilayer graphene can be microstructured into a Fresnel lens to utilize the unique transmission properties of graphene. We design different Fresnel lenses and investigate their performance in the microwave regime both numerically and experimentally. It is shown that one can change the intensity of the focused wave without shifting the focusing point by altering the number of the graphene layers. The good agreement between our experimental results and the results of numerical simulations demonstrates the feasibility of atom-thick (about 10−7 of the operating wavelength) graphene-based lenses and shows their potential for ultra-compact integration.

Morphology of Bi(110) quantum islands on epitaxial graphene

Proximitized 2D materials present exciting prospects for exploring new quantum properties, enabled by precise control of structures and interfaces through epitaxial methods. In this study, we investigated the structure of ultrathin coverages formed by depositing high-Z element bismuth (Bi) on monolayer graphene (MLG)/SiC(0001). By utilizing electron diffraction and scanning tunneling microscopy, ultrathin Bi nanostructures epitaxially grown on MLG were studied. Deposition at 300 K resulted in formation of needle-like Bi(110)-terminated islands elongated in the zig-zag direction and aligned at an angle of approximately 1.75∘ with respect to the MLG armchair direction. By both strain and quantum size effects, the shape, the orientation and the thickness of the Bi(110) islands can be rationalized. Additionally, a minority phase of Bi(110) islands orthogonally aligned to the former ones were seen. The four sub-domains of this minority structure are attributed to the formation of mirror twin boundaries, resulting in two potential alignments of Bi(110) majority and minority domains with respect to each other, in addition to two possible alignments of the majority domain with respect to graphene. Notably, an annealing step at 410 K or lowering the deposition temperature, significantly increases the concentration of the Bi(110) minority domain. Our findings shed light on the structural control of proximitized 2D materials, showcasing the potential for manipulating 2D interfaces.

Negative refraction achieved by inducing chirality in monolayer graphene through Raman gain mechanism

Negative refraction achieved by inducing chirality in monolayer graphene through Raman gain mechanism

Metallurgical slag modified monolayer graphene hybrid SA-based composite phase change materials for high thermal conductivity

Phase change materials are thermal energy storage materials with high heat storage density, low temperature loss and long cycle life. However, they also show a weak light absorption ability and low thermal conductivity, limiting their applicability for solar thermal conversion and storage technologies. In this paper, in order to improve the relatively low thermal conductivity of stearic acid and enhance its thermal conductivity, SGC-T composites with enhanced thermal conductivity were prepared using metallurgical slag, via the melt blending method. The SEM and EDS, surface scanning tests, were used to determine the morphology characteristics of the composite phase change materials and verify the addition of metallurgical slag. The physical phase composition, structure, and absorption coefficients were analyzed by XRD, FT-IR, and UV-Vis-NIR analyses. The thermophysical properties were analyzed by DSC. The thermal stability was explored using TG testing. The thermal conductivity was explored by measuring the thermal conductivity coefficient value of the samples at the testing temperature of 25 ℃ using the hot disk method/transient flat plate heat source method. The heat storage and exothermic capabilities of the samples were evaluated by the photo-thermal warming system/infrared thermography test. The results showed that the thermal conductivity of SGC was 0.663 W·(m·K)−1, and that of SGC-2 T, SGC-6 T, SGC-10 T, SGC-14 T, were 0.695 W·(m·K)−1, 0.758 W·(m·K)−1, 0.824 W·(m·K)−1 and 0.872 W·(m·K)−1, respectively, which are comparable to SGC. Compared with SGC, the thermal conductivity values of the four composite phase change materials increased by 4.8%, 14.3%, 24.3%, and 31.5%, respectively. SGC-14 T has the best heat transfer performance, which significantly improves the heat transfer performance during latent heat storage and release. Therefore, the SGC-14 T material reported in this work is a promising candidate in the field of energy-saving conversion, and further expands the applicability of phase change materials.

Ionic liquid/metal salt mixtures at the graphene interface: A density functional theory approach

The interaction of graphene sheets with metal cations is very relevant because of the modifications that are induced in the electrochemical properties of the 2D material. Ionic liquids are a promising kind of materials with several applications in electrochemical devices, so understanding how they affect the graphene-metal interaction is key for their practical implementation. Therefore, we have studied the adsorption of a mixture of an ionic liquid and a metal salt on a graphene surface by means of first-principles calculations. Several metals were chosen to analyze the effect they have on the optoelectronic properties of the graphene monolayer and to understand the trends in their adsorption behavior. We have characterized the ground state configurations in terms of their binding energies and the distance between the metal atom and the graphene layer. From the analysis of the charge transfer behavior, calcium and magnesium have been identified as the species that transfer the highest and the lowest amount of charge, respectively, which is related to their ionization energies. Band structure diagrams and projected density of states calculations also show that the energy shift of the Dirac cone increases with the degree of charge transfer. In addition, a stronger interaction of magnesium with the ionic liquid compared to that of other metal atoms was observed. The analysis of several electromagnetic parameters indicated an anisotropic behavior for electric fields polarized both perpendicular and parallel to the graphene layer. Our density functional theory study offers fundamental insights into the adsorption behavior of ionic liquids mixed with metal ions on monolayer graphene.

Graphene binding on black phosphorus enables high on/off ratios and mobility.

Graphene is one of the most promising candidates for integrated circuits due to its robustness against short-channel effects, inherent high carrier mobility and desired gapless nature for Ohmic contact, but it is difficult to achieve satisfactory on/off ratios even at the expense of its carrier mobility, limiting its device applications. Here, we present a strategy to realize high back-gate switching ratios in a graphene monolayer with well-maintained high mobility by forming a vertical heterostructure with a black phosphorus multi-layer. By local current annealing, strain is introduced within an established area of the graphene, which forms a reflective interface with the rest of the strain-free area and thus generates a robust off-state via local current depletion. Applying a positive back-gate voltage to the heterostructure can keep the black phosphorus insulating, while a negative back-gate voltage changes the black phosphorus to be conductive because of hole accumulation. Then, a parallel channel is activated within the strain-free graphene area by edge-contacted electrodes, thereby largely inheriting the intrinsic carrier mobility of graphene in the on-state. As a result, the device can provide an on/off voltage ratio of >103 as well as a mobility of ∼8000cm2 V-1 s-1 at room temperature, meeting the low-power criterion suggested by the International Roadmap for Devices and Systems.

High‐Quality van der Waals Epitaxial CsPbBr3 Film Grown on Monolayer Graphene Covered TiO2 for High‐Performance Solar Cells

Two‐dimensional materials have been widely used to tune the growth and energy‐level alignment of perovskites. However, their incomplete passivation and chaotic usage amounts are not conducive to the preparation of high‐quality perovskite films. Herein, we succeeded in obtaining higher‐quality CsPbBr3 films by introducing large‐area monolayer graphene as a stable physical overlay on top of TiO2 substrates. Benefiting from the inert and atomic smooth graphene surface, the CsPbBr3 film grown on top by the van der Waal epitaxy has higher crystallinity, improved (100) orientation, and an average domain size of up to 1.22 μm. Meanwhile, a strong downward band bending is observed at the graphene/perovskite interface, improving the electron extraction to the electron transport layers (ETL). As a result, perovskite film grown on graphene has lower photoluminescence (PL) intensity, shorter carrier lifetime, and fewer defects. Finally, a photovoltaic device based on epitaxy CsPbBr3 film is fabricated, exhibiting power conversion efficiency (PCE) of up to 10.64% and stability over 2000 h in the air.

Resonant amplification of slow surface plasmon polaritons in a DC current pumped semiconductor/graphene waveguide with a groove defect

We propose the principle of a planar surface plasmon polariton amplifier composed of a complex waveguide structure based on a semiconductor thin film separated from a dielectric substrate by a graphene monolayer. The amplification of surface plasmon polaritons in that waveguide in the terahertz regime is driven by a direct current in the graphene layer under a synchronism condition that allows an efficient energy exchange from the collective flux of charge carriers in graphene and the surface electromagnetic wave in the semiconductor film. Positive feedback required for resonant amplification is achieved through surface plasmon polariton reflections at the edges of the active waveguide, one of which is formed by a local thickness defect (a groove) at the upper surface of the semiconductor film that also provides an exit channel for the energy of the amplified surface wave to be transferred to an adjacent passive thin film semiconductor waveguide. We determine the conditions required for the resonant amplification of surface plasmon polaritons in that complex structure, which are essentially governed by the geometry of the groove as well as by the characteristic parameters of the active and passive waveguides.