Graphene Micro 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.

A Universal Approach for Maximizing Terahertz Wave Absorption in Graphene Cut-Wires

Graphene micro-/nanostructures and their arrays have attracted considerable attention in infrared (IR) and terahertz (THz) applications due to their strong plasmon responses. However, as too many parameters, including geometry, carrier concentration, frequency, and adjacent substrate, can affect the plasmonic behaviors of the micro-/nanostructures, the optimization of the THz-IR responses, such as absorption and reflection, of these structures and their arrays require tremendous computations on parameter scanning. Here, we propose a theoretical approach to design graphene cut-wires with maximized THz wave absorption. Analytical expression describing the THz absorption/reflection of graphene cut-wires is derived. Accordingly, a maximum THz wave absorption of the array, regardless of its operating frequencies and geometrical parameters, can be achieved by simply tuning the cut-wires duty ratio. The analytical results are further validated by numerical simulations. This intuitive design manner is of significance for the design of graphene arrays with high-efficiency THz responses as well as promoting their practical applications in THz functional devices.

Visualization of graphene grain boundaries through oxygen intercalation

Efficient control over the grain boundaries (GBs) is a vital aspect in optimizing the graphene growth conditions. A number of methods for visualization of GBs were developed for graphene grown on weakly interacting surfaces. Here, we utilize oxygen intercalation to reveal GBs and study their morphology for graphene strongly bound to the cobalt surface. We demonstrate that upon the intercalation of oxygen, GBs in polycrystalline graphene become easily detectable due to graphene cracking and selective oxidation of the substrate, thus giving a direct insight into the graphene micro- and nanostructure by means of different electron microscopy methods, including scanning electron microscopy, photoemission microscopy and low-energy electron microscopy.

Dynamic wave propagation in micro-torus structures: Implementing a 3D physically realistic theory

This paper makes strides to analyze the wave propagation in micro-torus structures with circular cross-sections using the modified couple stress theory (MCST). This physically realistic theory encompasses the material size effect The fundamental equilibrium equations of MCST in the mathematical framework of toroidal coordinates are first reformulated. Navier’s equations resulting from the implementation of MCST for micro-torus structures are treated by employing the Helmholtz decomposition technique. Several micro-torus problems using MCST in the 3D elasticity framework are solved numerically by a finite element method (FEM) to find eigenvalues and eigenfunctions from the elastic wave propagation standpoints. Wave propagation analysis within micro-torus structures and the effects of material length scale parameter on the vibrational response of various geometries, such as a quarter and a half torus along with a cylinder, are investigated. Opposed to the classical continuum theory that fails to describe the dispersion of waves at higher frequencies within micro-tours structures, the MCST can demonstrate the experimentally reported dispersive wave behavior. The findings can be potentially implemented to analyze and design a variety of sensors and actuators as well as medical, telecommunication, and electronic devices. This paper also develops an accurate estimation for the biharmonic and Laplace’s toroidal coordinates to study charge distribution in isolated conducting graphene micro-torus structures. Finally, the resulting electrostatic potential and the charge density in a conducting torus surface for electrostatics applications are determined in terms of Legendre functions. Regarding the different cases of arbitrary charge distribution, findings are helpful to control the electronic properties of graphene micro torus and rational design of nano-structures used in nanoscale transistor channels and superconductors. Treating the Laplace equation on a nano torus assists to more precisely realize the high-frequency localization of Laplacian eigenfunctions, i.e., how an eigenfunction is distributed in a small region of the torus domain and decays rapidly outside the region.

Definition of CVD Graphene Micro Ribbons with Lithography and Oxygen Plasma Ashing

This work presents the definition of CVD (Chemical Vapor Deposition) Graphene Micro Ribbons (GMRs) with traditional Photolithography and Oxygen Plasma Ashing, where: (i) CVD Graphene Micro Ribbons were defined in parallel and serpentine patterns; (ii) The defined width dimensions of GMRs are between 0.23 µm and 10 µm; (iii) The empirical calibration curve between physical and mask widths of GMRs was obtained; (iv) With our method, Graphene Field-Effect Transistors, GFETs, can be fabricated with parallel GMRs as conduction channel between source and drain. It is important to notice that parallel channels are used in Silicon Nano Wires 3D Transistors, such as FinFETs. Our method is based on low-cost lithography, compared to commonly found in the literature, based on the definition of graphene patterns by Electron Beam Lithography (high cost and low repeatability techniques). Also, graphene's good quality characteristics remain after GMRs fabrication, as proven by Raman spectroscopy. In conclusion, our method to define the GMRs is suitable for GFET technology.

Ultrathin Single‐Crystalline 2D Perovskite Photoconductor for High‐Performance Narrowband and Wide Linear Dynamic Range Photodetection

For next-generation Internet-of-Everything applications, for example, artificial-neural-network image sensors, artificial retina, visible light communication, on-chip light interconnection, and flexible devices, etc., high-performance microscale photodetectors are in urgent demands. 2D material (2DM) photodetectors have been researched and demonstrated impressive performances. However, they have not met the demands in filterless narrowband photoresponse, wide linear dynamic range (LDR), ultralow dark current, and large on/off ratio, which are key performances for these applications. 2D Ruddlesden-Popper perovskites (2D-RPPs) are recently highlighted photovoltaic and optoelectronic materials. Embedding ultrathin 2D-RPPs into 2DM photodetectors holds potentials to improve these performances. Herein, a single-crystalline ultrathin (PEA)2 PbI4 is integrated into a vertical-stacked graphene-(PEA)2 PbI4 -graphene micro photoconductor (V-PEPI-PC). V-PEPI-PC exhibits narrowband photoresponses at 517nm with a full-width-at-half-maximum of 15nm and a wide LDR of 122dB. Due to the multiple quantum wells in (PEA)2 PbI4 , V-PEPI-PC demonstrates an ultralow dark current of 1.1 × 10-14 A (44 pA mm-2 ), a high specific detectivity of 1.2 × 1013 Jones, and a high on/off ratio of 1.6 × 106 . Owing to the short vertical channel, V-PEPI-PC shows a fast response rise time of 486 µs. Therefore, the vertical-stacked photodetectors based on hybrid 2D-RPPs and 2DMs may have great potentials in future optoelectronics.

Physics-Based Analytical Engineering Models of Graphene Micro- and Nanostrip Lines

In this article, new approximate analytical models of graphene-based micro- and nanostrip transmission lines are given. These models are based on the representation of the mentioned lines by a parallel-plate waveguide embedded into an effective frequency-dependent permittivity media. Our theory allows analytical calculation of complex propagation constant and characteristic impedance of the main quasi-TM mode of graphene strip transmission lines. The developed approach is verified by comparisons with the thin-film lossy microstrips, for which the measurements and theory are available for frequencies up to 1 THz and with parallel-plate graphene waveguides. For very narrow lines, the developed model is improved by a correction tuning technique. The obtained analytical formulas for complex propagation constant and characteristic impedance are interesting in calculations of electronically controlled graphene-based interconnects, microwave and millimeter-wave attenuators, antennas, and transistors.

Sn-C and Se-C Co-Bonding SnSe/Few-Layered Graphene Micro-Nano Structure: Route to a Densely Compacted and Durable Anode for Lithium/Sodium-Ion Batteries.

Developing anodes with a high and stable energy density for both gravimetric and volumetric storage is vital for high-performance lithium/sodium-ion batteries. Herein, an SnSe/few-layered graphene (FLG) composite with a high tap density (2.3 g cm-3) is synthesized via the plasma-milling method, in which SnSe nanoparticles are strongly bound with the FLG matrix, owing to both Sn-C and Se-C bonds, to form nanosized primary particles and then assemble to microsized secondary granules. The FLG can effectively alleviate the large stress generated from the volume expansion of SnSe during cycling based on its superstrength. Furthermore, as demonstrated by the density-functional theory calculations, the Sn-C and Se-C co-bonding benefitting from the formation of substantial vacancy defects on the P-milling-synthesized FLG enables strong affinity between SnSe nanoparticles and the FLG matrix, preventing SnSe from aggregating and detaching even after long-term cycling. As an anode for lithium-ion batteries, it exhibits high gravimetric and volumetric capacities (864.8 mAh g-1 and 1990 mAh cm-3 at 0.2 A g-1), a high rate (612.6 mAh g-1 even at 5.0 A g-1), and the longest life among the reported SnSe-based anodes (capacity retention of 92.8% after 2000 cycles at 1.0 A g-1). Subsequently, an impressive cyclic life (capacity retention of 91.6% after 1000 cycles at 1.0 A g-1) is also achieved for sodium-ion batteries. Therefore, the SnSe/FLG composite is a promising anode for high-performance lithium/sodium-ion batteries.

Inductively coupled remote plasma-enhanced chemical vapor deposition (rPE-CVD) as a versatile route for the deposition of graphene micro- and nanostructures

Multiple layers of graphene thin films with micro-crystalline orientation and vertical graphene nano-sheets were grown on different substrates (i.e., polycrystalline nickel foil, Ni(111), highly oriented pyrolytic graphite) using a single-step process based on low-pressure remote Plasma-Enhanced Chemical Vapor Deposition (rPE-CVD). In contrast to previous studies, a novel basic approach to this technique including a new remote inductively coupled RF plasma source has been used to (i) minimize the orientational effect of the plasma electrical fields during the catalyst-free growth of graphene nano-sheets, (ii) warrant for a low graphene defect density via low plasma kinetics, (iii) decouple the dissociation process of the gas from the growth process of graphene on the substrate, (iv) tune the feedstock gas chemistry in view of improving the graphene growth, and (v) reduce the growth temperature as compared to conventional chemical vapor deposition (CVD). In order to study the various aspects of the rPE-CVD graphene growth modes and to assess the characteristics of the resulting graphene layers, Raman spectroscopy, XPS, SEM, and STM were used. The results give evidence for the successful performance of this new rPE-CVD plasma deposition source, that can be combined with in situ UHV-based processess for the production of, e. g., hybrid metal ferromagnet/graphene systems.

Nonlinear vibration analysis of MSGT boron-nitride micro ribbon based mass sensor using DQEM

In this research, the nonlinear free vibration analysis of boron-nitride micro ribbon (BNMR) on the Pasternak elastic foundation under electrical, mechanical and thermal loadings using modified strain gradient theory (MSGT) is studied. Employing the von <TEX>$K{\acute{a}}rm{\acute{a}}n$</TEX> nonlinear geometry theory, the nonlinear equations of motion for the graphene micro ribbon (GMR) using Euler-Bernoulli beam model with considering attached mass and size effects based on Hamilton's principle is obtained. These equations are converted into the nonlinear ordinary differential equations by elimination of the time variable using Kantorovich time-averaging method. To determine nonlinear frequency of GMR under various boundary conditions, and considering mass effect, differential quadrature element method (DQEM) is used. Based on modified strain MSGT, the results of the current model are compared with the obtained results by classical and modified couple stress theories (CT and MCST). Furthermore, the effect of various parameters such as material length scale parameter, attached mass, temperature change, piezoelectric coefficient, two parameters of elastic foundations on the natural frequencies of BNMR is investigated. The results show that for all boundary conditions, by increasing the mass intensity in a fixed position, the linear and nonlinear natural frequency of the GMR reduces. In addition, with increasing of material length scale parameter, the frequency ratio decreases. This results can be used to design and control nano/micro devices and nano electronics to avoid resonance phenomenon.

Interaction of terahertz electromagnetic waves with periodic gratings of graphene micro- and nanoribbons

An original mathematical model of the interaction of terahertz (THz) electromagnetic waves with periodic gratings of graphene micro- and nanoribbons is based on the solution to the boundary-value problem of diffraction for the Maxwell equations with electrodynamic boundary conditions and material equations. The electrodynamic calculations of the transmission coefficients of the TEM wave versus frequency are performed for the 2D grating of graphene micro- and nanoribbons at several chemical potentials, grating periods, and geometrical sizes of ribbons. The results of the calculations show that the transmission spectrum exhibits a minimum in the THz range if the electric field of the wave is perpendicular to the graphene ribbons. The minimum is due to the plasmon resonance of the fundamental mode in graphene, and the absorption peaks at higher frequencies in the upper part of the THz range are related to the highorder plasmon modes.

Plasma Protein Corona Reduces the Haemolytic Activity of the Graphene Oxide Nano and Micro Flakes

Intensive research on the bio-applications of graphene and its derivatives is leading to many technological applications. Graphene Oxide (GO), for its unique 2-D structure and its physical/chemical properties, has attracted increasing interest over the last few years in the fields of drug/gene delivery, biological imaging and antibacterial treatments. Together with these great potentials for biomedical applications, several aspects of graphene toxicity mechanisms including oxidative stress, cutting off intracellular metabolic routes and cell membrane rupture must be carefully evaluated. In this work we demonstrate that the GO flakes, able to disrupt the erythrocyte plasma membrane, greatly reduce their hemolytic activity after interacting with plasma proteins.

Tunable graphene metasurfaces by discontinuous Pancharatnam–Berry phase shift

Metal–dielectric–graphene three-layer structures are proposed to improve the interaction of graphene micro- and nanostructures with incident waves, as the upper graphene cut-wire layer introduces a discontinuous Pancharatnam–Berry phase profile. A semi-analytical method based on the Jones calculus is conducted to investigate the conversion efficiency of cross-polarized light on this graphene metasurface for circularly polarized wave incidence, which predicts that the physical constraint (25%) of cross-coupling conversion efficiency for individual graphene layers can be overcome. Numerical simulations confirm the conclusion and demonstrate an efficiency as high as 60%. Based on this mechanism, high-efficiency anomalous reflection surfaces and flat focal mirrors are designed with the tunability of reflection angles and one order of magnitude improved focusing intensity. This method paves the way to engineering high-efficiency graphene metasurfaces for tunable electromagnetic wave manipulation.

Terahertz Wavefront Control by Graphene Metasurface

ABSTRACTWe propose and numerically investigate a tunable metasurface made of an array of graphene ribbons to dynamically control terahertz (THz) wavefront. The metasurface consists of graphene micro ribbons on a silver mirror with a SiO2 gap layer. The graphene ribbons are designed to exhibit localized plasmon resonances depending on their Fermi levels to introduce abrupt phase shifts along the metasurface. With interference of the Fabry-Perot resonances in the SiO2 layer, phase shift through the system is largely accumulated, covering up to 2π range for full control of the THz wavefront. Numerical simulations prove that wide-angle reflected THz beam steering from -53° to +53° with a high reflection efficiency as high as 60% is achieved at 5 THz while the propagation direction of THz beam could be switched within 0.6 ps.

Plasma protein corona reduces the haemolytic activity of graphene oxide nano and micro flakes

GO flakes, able to disrupt the erythrocyte plasma membrane, greatly reduce their haemolytic activity after interacting with plasma proteins. Haemolysis activity increases inversely to the GO flakes size.

Scalable and Direct Growth of Graphene Micro Ribbons on Dielectric Substrates

Here we report on a scalable and direct growth of graphene micro ribbons on SiO2 dielectric substrates using a low temperature chemical vapor deposition. Due to the fast annealing at low temperature and dewetting of Ni, continuous few-layer graphene micro ribbons grow directly on bare dielectric substrates through Ni assisted catalytic decomposition of hydrocarbon precursors. These high quality graphene micro ribbons exhibit low sheet resistance of ~700 Ω −2100 Ω, high on/off current ratio of ~3, and high carrier mobility of ~655 cm2V−1s−1 at room temperature, all of which have shown significant improvement over other lithography patterned CVD graphene micro ribbons. This direct approach can in principle form graphene ribbons of any arbitrary sizes and geometries. It allows for a feasible methodology towards better integration with semiconductor materials for interconnect electronics and scalable production for graphene based electronic and optoelectronic applications where the electrical gating is the key enabling factor.

Three-dimensional graphene architectures

Graphene has been widely explored for applications in electronics, sensors, actuators, catalysis and bio- or energy related systems. For these purposes, graphene materials usually have to be fabricated or assembled into desired micro-/nano-architectures for tuning and/or controlling their electrical, optical, mechanical, chemical or electrochemical properties. In this feature article, we review recent achievements in the studies of three-dimensional (3D) graphene architectures. The methodology for preparing these 3D graphene micro-/nano-architectures and their potential applications have been summarized.

High-Resolution Separation of Graphene Oxide by Capillary Electrophoresis

Separation and purification of graphene oxide (GO) prepared from chemical oxidation of flake graphite and ultrasonication by capillary electrophoresis (CE) was demonstrated. CE showed the ability to provide high-resolution separations of GO fractionations with baseline separation. The GO fractionations after CE were collected for Raman spectroscopy, atomic force microscopy, and transmission electron microscopy characterizations. GO nanoparticles (unexfoliated GO) or stacked GO sheets migrated toward the anode, while the thin-layer GO sheets migrated toward the cathode. Therefore, CE has to be performed twice with a reversed electric field to achieve a full separation of GO. This separation method was suggested to be based on the surface charge of the GO sheets, and a separation model was proposed. This study might be valuable for fabrication of GO or graphene micro- or nanodevices with controlled thickness.

Direct patterning on reduced graphene oxide nanosheets using femtosecond laserpulses

Micro- and nanostructures were fabricated directly on graphene nanosheets by controllingthe conditions of femtosecond laser pulse etching. High quality graphene micro- andnanostructures with a minimum width of 492 nm were obtained as the graphene nanosheetsused in our experiments were large-scale, uniform and highly conductive. Variouscomplex patterns were successfully created through femtosecond laser etching.Furthermore, by managing the laser energy, the graphene under the Au electrodescould be completely or partly removed. This technology of direct patterning ofmicro- and nanostructures on graphene through femtosecond laser technologymight pave the way for the integration of graphene-based electronic microdevices.