Continuous Monolayer Graphene Research Articles

Self-Assembly for Creating Vertically-Aligned Graphene Micro Helices with Monolayer Graphene as Chiral Metamaterials.

Graphene's emergence enables creating chiral metamaterials in helical shapes for terahertz (THz) applications, overcoming material limitations. However, practical implementation remains theoretical due to fabrication challenges. This paper introduces a dual-component self-assembly technique that enables creating vertically-aligned continuous monolayer graphene helices at microscale with great flexibility and high controllability. This assembly process not only facilitates the creation of 3D microstructures, but also positions the 3D structures from a horizontal to a vertical orientation, achieving an aspect ratio (height/width) of ≈2700. As a result, an array of vertically-aligned graphene helices is formed, reaching up to 4mm in height, which is equivalent to 4 million times the height of monolayer graphene. The benefit of these 3D chiral structures made from graphene is their capability to infinitely extend in height, interacting with light in ways that are not possible with traditional 2D layering methods. Such an impressive height elevates a level of interaction with light that far surpasses what is achievable with traditional 2D layering methods, resulting in a notable enhancement of optical chirality properties. This approach is applicable to various 2D materials, promising advancements in innovative research and diverse applications across fields.

Graphene-enhanced polarization-independent high-Q terahertz metamaterial absorber with tunability for sensing applications

Terahertz metamaterial absorber (TMMA) has broad prospects for sensing applications. To achieve high sensitivities and low detection limits in practice, a TMMA should exhibit a high Q-factor preferably independent of polarization, tunability, and consists of simple structures that are easy to realize. However, simultaneously achieving these is difficult. Here, we propose a graphene-enhanced TMMA that exhibits a high Q-factor independent of polarization with tunability, using couplings between a continuous monolayer graphene and the underneath metallic fractal structures with rotational symmetry. Simulation results show that the TMMA exhibits dual-band absorption with broad tunability from ∼1.5 to 3.0 THz. A maximum polarization-independent Q-factor of 133.12 with a maximum frequency sensitivity (S) of 0.42 THz/RIU and a figure of merit of 18.28 RIU-1 are achieved theoretically. The presented graphene-enhanced TMMA has many potential applications, particularly in the field of chemical and biological sensing.

Minute-level-fast and recyclable large-area monolayer graphene transfer onto polymer membranes

Monolayer graphene-based composites are of immense importance for research and applications in the microelectronic and thermal-management fields. For the current continuous production of monolayer graphene protocols, chemical etching of the catalyst layer acts as the bottleneck, which is tedious, expensive, and polluting. Thus in this work, by combining chemical etching and mechanical exfoliation, copper foil etching has been compressed from hours to 30 s, with the foil entirely recyclable and reusable. Furthermore, the whole transfer process could be confined within 7 min, including all pre- and post-treatments, without the usage of any intermediate, glue, electricity, vacuum, heat, or pressure. And this minute-level-fast monolayer graphene transfer could be performed continuously multiple times. The key to this novel transfer method is the superficial etching and the subsequent delamination of copper foil off graphene. In-situ OM, AFM, SEM, Raman, and UV–vis spectra have been employed to carefully inspect the transfer mechanism, effectiveness, and quality. The prepared monolayer graphene/polymer composite membrane is ultrathin (∼200 nm), conductive, highly transparent (93.9%), and flexible simultaneously. Its application as an ultrathin, efficient, and conformable Joule heating membrane-form device has been demonstrated. This minute-level-fast transfer is well-adapted to continuous monolayer graphene production and could considerably improve their practicality and efficiency. Additionally, this study provides more understanding of the interface between graphene-polymer and graphene-catalyst and a minute-level-fast route to fabricate 2D-based polymeric composite materials.

Asymmetric Growth of Monolayer Graphene on Cu Foil by Space‐Confined CVD

AbstractChemical vapor deposition (CVD) is the most efficient method for growing graphene while its performance heavily relies on the growth environment. In this work, we examine the growth of continuous monolayer graphene films using CVD in an asymmetric environment by placing the Cu foil substrate on a specially designed semicircular quartz boat with grooves. We find that the asymmetry plays a key role in the growth of graphene and the quality of graphene on the bottom surface of Cu foil is higher than that on the top surface. This difference is attributed to different oxygen and carbon source concentration on the two surfaces of Cu foil, which leads to different nucleation density, grain size, and growth rate. COMSOL simulation confirms this hypothesis and reveals that the lower gas flow velocity on the bottom is responsible for the higher oxygen content and less carbon source supply, promoting higher‐quality graphene. Based on this asymmetric mechanism, we achieve stable and efficient batch production of high‐quality graphene using a special substrate support. Our method has guiding significance for the stable and batch growth of high‐quality graphene or other two dimensional (2D) materials in spatial design strategies.

Linear indium atom chains at graphene edges

The presence of metal atoms at the edges of graphene nanoribbons (GNRs) opens new possibilities toward tailoring their physical properties. We present here formation and high-resolution characterization of indium (In) chains on the edges of graphene-supported GNRs. The GNRs are formed when adsorbed hydrocarbon contamination crystallizes via laser heating into small ribbon-like patches of a second graphitic layer on a continuous graphene monolayer and onto which In is subsequently physical vapor deposited. Using aberration-corrected scanning transmission electron microscopy (STEM), we find that this leads to the preferential decoration of the edges of the overlying GNRs with multiple In atoms along their graphitic edges. Electron-beam irradiation during STEM induces migration of In atoms along the edges of the GNRs and triggers the formation of longer In atom chains during imaging. Density functional theory (DFT) calculations of GNRs similar to our experimentally observed structures indicate that both bare zigzag (ZZ) GNRs as well as In-terminated ZZ-GNRs have metallic character, whereas in contrast, In termination induces metallicity for otherwise semiconducting armchair (AC) GNRs. Our findings provide insights into the creation and properties of long linear metal atom chains at graphitic edges.

Anti-dewetting of Cu thin film on nanostructured black Si template for continuous CVD growth of monolayer graphene

The growth of high-quality continuous film of graphene on less than [Formula: see text]m-thick Cu film is proven to be a challenging task due to the solid-state dewetting of Cu during the high-temperature chemical vapor deposition (CVD) process. In this paper, we introduce the use of nanostructured black Si (b-Si) as a template for Cu evaporation to mitigate the dewetting of Cu thin film. Using a cold-wall CVD system at a process temperature of 825[Formula: see text]C, even Cu thickness, [Formula: see text] nm on polished SiO2/Si substrate is poor for maintaining Cu as a continuous film. If the polished SiO2/Si is replaced with SiO2/b-Si, the minimum [Formula: see text] nm is sufficient. According to the Cu trapping mechanism, moving Cu particle is trapped in the nanostructured trenches of SiO2/b-Si during annealing and CVD growth processes. Continuous monolayer graphene with a grain size of [Formula: see text]m without defect is obtained on Cu/SiO2/b-Si substrate. The improved adhesion of Cu to the SiO2/b-Si enables dry-transfer of graphene by mechanical peeling using a polyvinyl alcohol (PVA) film. Our solution is promising for obtaining flat graphene and a recyclable Cu/SiO2/b-Si substrate.

Monolayer crumpled graphene-based mechanically and electrically tunable infrared absorbers for high-sensitivity sensing

Enhancing the absorption of infrared light in continuous monolayer graphene layer has received much attention in various optical devices. Here, based on the uniaxial and biaxial crumpled graphene structures, two mechanically and electrically tunable infrared absorbers have been proposed and investigated. We have demonstrated that the near-unity absorption spectra of proposed absorbers are ascribed to the electric resonance modes excited along the sidewalls of crumpled graphene structures, and a broad spectra tunability can be achieved through combing mechanical stretching/compression with the conventional electrostatic doping method. Furthermore, the refractive index sensing capacity of proposed biaxial absorber has been further investigated. Benefiting from the unique stereoscopic morphologies, the sensing sensitivity of each mode of the absorber can reach up to 5.546 and 8.251 µm per refractive index unit, respectively. The broad spectra tunability, high sensing sensitivity as well as flexible structures render it holding promising potentials in high-sensitivity flexible sensing applications.

Oxygen-mediated selection of Cu crystallographic orientation for growth of single-crystalline graphene

Grains of a polycrystalline Cu foil (CF) can be recrystallized into Cu(100) or Cu(111) by adequate thermal annealing near the surface melting temperature. However, the thermally driven recrystallization mechanism of CF remains elusive, which is connected to the uncontrollability of the orientation of recrystallized Cu surface. In this study, we ascertained that the unintentional presence of oxygen in CF grains acts a crucial role in determining the crystal orientation of CF. Comprehensive spectroscopic analysis coupled with density functional theory calculation was implemented to explore the correlation between the surface phase transition and the oxygen content of the surface of CFs. After annealing, oxygen-free Cu grains were explicitly recrystallized in the (111) crystal plane by minimizing their surface energy, whereas oxygen-contained Cu grains were recrystallized in the (100) orientation due to the elastic strain energy induced by oxygen. Notably, we arguably accomplished the recrystallized CF with (111) crystal plane can be transformed into a single crystal (111) CF by utilizing rationally designed cyclic heat treatment (CHT), however Cu(100) grains were not merged into a single crystal (100) CF presumably due to the presence of residual oxygen. We accomplished the synthesis of defect-less, continuous monolayer graphene on a single crystal (111) CF.

Giant and controllable Goos-Hänchen shift of monolayer graphene strips enabled by a multilayer dielectric grating structure.

The discovery of monolayer graphene allows the unprecedented chance for exploring its Goos-Hänchen (GH) shift. However, most of the pronounced GH shifts are achieved in various structures with two-dimensional continuous monolayer graphene. Here, we report on the giant GH shift of reflected wave in monolayer graphene strips by constructing the multilayer dielectric grating structure under them. The observed GH shift here is as high as 7000 times that of the incident wave at the near-infrared frequency region, whose magnification is significantly larger than that of the monolayer graphene ribbon array. We further elucidate that the enhanced GH shift originates from the guided mode resonance of the dielectric grating structure and its magnitude and sign can be manipulated by chemical potential of the monolayer graphene strip. Our work enables a promising route for enhancing and controlling the GH shifts of reflected wave in monolayer graphene strips, which might contribute to their applications in biosensors and detectors.

Rapid and facile fabrication of conducting monolayer reduced graphene oxide films by methane plasma-assisted reduction

In this research article, we report a method to fabricate conducting reduced graphene oxide (rGO) monolayer films reduced by methane plasma treatment within few minutes and the nanoscale characterization of local conductivity via conductive atomic force microscopy (C-AFM), realizing their applicability in highly transparent conducting electrodes. Continuous monolayer graphene oxide (GO) films were fabricated by a simple spin-coating method and reduction of oxygen-containing groups with simultaneous lattice defect restoration was done by methane plasma treatment. Current images obtained by C-AFM provide detailed information about the conductivity through the flakes depending on the plasma irradiation time. Moreover, lattice defect restoration was confirmed by the appearance of the G’-band in Raman spectra, which is in correlation with current images, whereas no G’-band or current image was observed in vacuum annealed samples at similar durations in the absence of methane. This rapid and facile method offers a way to fabricate conductive monolayer rGO films by a short-term plasma treatment allowing to utilize low melting point substrates to form flexible conductive graphene-based films.

Characterization of graphene grown by direct-liquid-injection chemical vapor deposition with cyclohexane precursor in N2 ambient

We synthesize graphene films by direct-liquid-injection chemical vapor deposition (DLI-CVD) method with cyclohexane precursor (C6H12) in N2 ambient. This process offers a safer, and more economical route for large-scale graphene production in which hydrogen gas is not required. Graphene films are grown on Cu foil substrates at 890 to 980 °C for 10 min in the total pressure of 2 mbar. The flow rate of cyclohexane is varied between 0.2 and 0.5 g/min. The Raman results shows continuous monolayer graphene films at growth temperature of 950 °C and a flow rate of 0.5 g/min. Hall and field-effect measurements show mobilities in the range of 450–800 cm2/V·s. The relatively low D peak intensity suggests that carrier mobility is likely limited by impurities introduced to the devices during transfer process and device fabrication.

Photonic crystal for graphene plasmons

Photonic crystals are commonly implemented in media with periodically varying optical properties. Photonic crystals enable exquisite control of light propagation in integrated optical circuits, and also emulate advanced physical concepts. However, common photonic crystals are unfit for in-operando on/off controls. We overcome this limitation and demonstrate a broadly tunable two-dimensional photonic crystal for surface plasmon polaritons. Our platform consists of a continuous graphene monolayer integrated in a back-gated platform with nano-structured gate insulators. Infrared nano-imaging reveals the formation of a photonic bandgap and strong modulation of the local plasmonic density of states that can be turned on/off or gradually tuned by the applied gate voltage. We also implement an artificial domain wall which supports highly confined one-dimensional plasmonic modes. Our electrostatically-tunable photonic crystals are derived from standard metal oxide semiconductor field effect transistor technology and pave a way for practical on-chip light manipulation.

Monolayer Graphene Coating of Intracortical Probes for Long‐Lasting Neural Activity Monitoring

The invasiveness of intracortical interfaces currently used today is responsible for the formation of an intense immunoresponse and inflammatory reaction from neural cells and tissues. This leads to a high concentration of reactive glial cells around the implant site, creating a physical barrier between the neurons and the recording channels. Such a rejection of foreign analog interfaces causes neural signals to fade from recordings which become flooded by background noise after a few weeks. Despite their invasiveness, those devices are required to track single neuron activity and decode fine sensory or motor commands. In particular, such quantitative and long-lasting recordings of individual neurons are crucial during a long time period (several months) to restore essential functions of the cortex, disrupted after injuries, stroke, or neurodegenerative diseases. To overcome this limitation, graphene and related materials have attracted numerous interests, as they gather in the same material many suitable properties for interfacing living matter, such as an exceptionally high neural affinity, diffusion barrier, and high physical robustness. In this work, the neural affinity of a graphene monolayer with numerous materials commonly used in neuroprostheses is compared, and its impact on the performance and durability of intracortical probes is investigated. For that purpose, an innovative coating method to wrap 3D intracortical probes with a continuous monolayer graphene is developed. Experimental evidence demonstrate the positive impact of graphene on the bioacceptance of conventional intracortical probes, in terms of detection efficiency and tissues responses, allowing real-time samplings of motor neuron activity during 5 weeks. Since continuous graphene coatings can easily be implemented on a wide range of 3D surfaces, this study further motivates the use of graphene and related materials as it could significantly contribute to reduce the current rejection of neural probes currently used in many research areas, from fundamental neurosciences to medicine and neuroprostheses.

Shuttle Suppression by Polymer-Sealed Graphene-Coated Polypropylene Separator.

"Shuttle effect" of lithium polysulfides (LiPS) leads to a poor performance and a short cycle life of the Li-S battery, thus limiting their practical application. We demonstrate here that after coating polypropylene (PP) separator with a continuous monolayer graphene, the shuttle effect can be significantly suppressed by limiting the passage of long-chain LiPS. The graphene/PP separator can be further modified by sealing the big holes or pores on graphene with in situ polymerized nylon-66 via an interfacial polymerization reaction between diamine and adipoyl chloride supplied by the aqueous and oil phase, respectively, from each side of the membrane. With this engineered membrane, an initial specific capacity of 1128.4 mAh g-1 at 0.05C is achieved after test in a coin cell, higher than that of 983.2 mAh g-1 with pristine PP, along with increased Coulombic efficiency from 96.0 to 99.9% and enhanced cycling durability. Molecular dynamics simulations attest that the nanopores with appropriate size and structure are effective in acting as a "sieve" to selectively allow only Li+ ions to pass through but prevent LiPS from migrating to the anode, consequently alleviating the shuttle effect. Our method provides a facile solution toward the mitigated shuttle effect and eventually contributes to the high performance of Li-S battery.

Chemical vapor deposition growth of scalable monolayer polycrystalline graphene films with millimeter-sized domains

The preparation of scalable graphene films has attracted an enormous attention due to its industrial importance for graphene applications. Here we present a synthesis method of high-quality continuous monolayer graphene films with millimeter-sized domains on ordinary Cu substrates using oxygen-assisted chemical vapor deposition. This method demonstrates a significantly improvement to the resultant graphene such as a largely decreased nucleation density from ∼106 to ∼101 nuclei/cm2, an increased average domain size from ∼0.004 to ∼1.5 mm and a film coverage from 64% to 100%. We attributed the success of this growth to the surface layer formed by bonded oxygen in Cu, which provides a higher priority for catalyzing the nucleation and growth of graphene domains.

Graphene on Cu(111) at the nonzero temperatures: Molecular dynamic simulation

We present results of molecular dynamic simulation of continuous graphene monolayer on Cu(111). In this paper, we investigate the dependencies of the average binding energy and the average binding distance on the temperature. The interaction between carbon and copper atoms was described by Lennard-Jones potential. It is shown that the binding energy practically remains constant in a wide range of temperatures 0–800 K. However, in the same temperature range, the binding distance of graphene on Cu(111) surface has a linear dependence on temperature. The dependence of the linear thermal expansion coefficient of the binding distance on Lennard-Jones parameters has been calculated. We suggest a simple theoretical model to explain this dependence qualitatively.

Tunable dualband light trapping and localization in coupled graphene grating-sheet system at mid-infrared wavelengths

This paper presents a comprehensive study of light trapping and localization in graphene grating-sheet structure by numerical simulation, and shows a strategy to realize controllable absorption enhancement and localized field enhancement in a large-area continuous graphene monolayer. For the structure with strong plasmons coupling, more than 95% of total absorption and 60% of optical absorption in graphene sheet are realized at two different infrared wavelengths over a wide incident angle range. Meanwhile, the enhanced E-field around the graphene sheet is over 40 times of magnitude stronger than that of the incident field. Additionally, by changing the Fermi energy of graphene or the distance between the sheet and the grating, adjustable light trapping and switchable infrared absorber/reflector can be obtained. These features enable the coupled graphene grating-sheet to serve as platform for infrared sensing, photodetection and light-matter interactions.

Massless Dirac fermions trapping in a quasi-one-dimensional npn junction of a continuous graphene monolayer

Massless Dirac fermions in graphene provide unprecedented opportunities to realize the Klein paradox, which is one of the most exotic and striking properties of relativistic particles. In the seminal theoretical work [Katsnelson et al., Nat. Phys. 2 620 (2006)], it was predicted that the massless Dirac fermions can pass through one-dimensional (1D) potential barriers unimpededly at normal incidence. Such a result seems to preclude confinement of the massless Dirac fermions in graphene by using 1D potential barriers. Here, we demonstrate, both experimentally and theoretically, that massless Dirac fermions can be trapped in quasi-1D npn junction of a continuous graphene monolayer. Because of highly anisotropic transmission of the massless Dirac fermions at n-p junction boundaries (the so-called Klein tunneling in graphene), charge carries incident at large oblique angles will be reflected from one edge of the junction with high probability and continue to bounce from the opposite edge. Consequently, these electrons are trapped for a finite time to form quasi-bound states in the quasi-1D npn junction. The quasi-bound states seen as pronounced resonances are probed and the quantum interference patterns arising from these states are directly visualized in our scanning tunneling microscope measurements.

Safe growth of graphene from non-flammable gas mixtures via chemical vapor deposition

Chemical vapor deposition has emerged as the most promising technique for the growth of graphene. However, most reports of this technique use either flammable or explosive gases, which bring safety concerns and extra costs to manage risk factors. In this article, we demonstrate that continuous monolayer graphene can be synthesized via chemical vapor deposition technique on Cu foils using industrially safe gas mixtures. Important factors, including the appropriate ratio of hydrogen flow and carbon precursor, pressure, and growth time are considered to obtain graphene films. Optical measurements and electrical transport measurements indicate graphene films are with comparable quality to other reports. Such continuous large area graphene can be synthesized under non-flammable and non-explosive conditions, which opens a safe and economical method for mass production of graphene. It is thereby beneficial for integration of graphene into semiconductor electronics.

Growth of continuous graphene by open roll-to-roll chemical vapor deposition

We demonstrate the growth of high-quality, continuous monolayer graphene on Cu foils using an open roll-to-roll (R2R) chemical vapor deposition (CVD) reactor with both static and moving foil growth conditions. N2 instead of Ar was used as carrier gas to reduce process cost, and the concentrations of H2 and CH4 reactants were kept below the lower explosive limit to ensure process safety for reactor ends open to ambient. The carrier mobility of graphene deposited at a Cu foil winding speed of 5 mm/min was 5270–6040 cm2 V−1 s−1 at room temperature (on 50 μm × 50 μm Hall devices). These results will enable the inline integration of graphene CVD for industrial R2R production.