Composition Of Graphene Research Articles

Highly sensitive and selective room-temperature nitrogen dioxide sensors based on porous graphene

Development of high-performance nitrogen dioxide sensors is extremely important not only for industrial applications but also for environmental monitoring and health protection. For practical applications, gas sensors with high sensitivity for low concentrations of nitrogen dioxide are desired. Here, a simple process to fabricate high-performance gas sensors based on porous reduced graphene oxide (PRGO) through a simple drop-casting technique is presented. The PRGO used as active sensing material is prepared through a scalable solution etching approach. This sensor exhibited a high sensitivity in the sub-0.5 ppm range with good selectivity and reversibility, as well as a low experimental detection limit of 20 ppb, which is much lower than the threshold exposure limit proposed by the National Ambient Air Quality Standard (53 ppb). Furthermore, it can be performed at room temperature without UV/IR light illumination or thermal assistance. This superior performance could be attributed to the high adsorption site density due to the edge sites of porous graphene. The relationship between the sensing performance and the microstructure or chemical composition of porous graphene were systematically investigated. This work paves the way for a facile tailoring of the sensor behavior as a function of graphene micromorphology.

STUDY ON OPTICAL PROPERTIES OF GRAPHENE-TIO2 NANOCOMPOSITE AS PHOTOANODES LAYER IN DYE SENSITIZED SOLAR CELL (DSSC)

Dye Sensitized Solar Cell (DSSC) using titanium dioxide (TiO2) has begun to play a significant role in future solar energy since it is known as cost effective and highly efficient. DSSC is the third generation of photovoltaic cells that have been widely investigated as a promising replacement of current commercial solar cell. However, the highest efficiency of DSSC still has not achieved the minimum requirement so that it can be commercialize. Much research has been done to improve DSSC performance by focusing on photoanodes layer. In this study, graphene was employed into TiO2 photoanode to increase the efficiency and to enhance the performance of dye sensitized solar cell. Four different samples of nanocomposites paste were prepared by varying the graphene composition of 0.00, 0.30, 0.50 and 0.70 wt%. The prepared samples were coated on Fluorine-Doped Tin Oxide (FTO) conductive glass substrates by a doctor blade method and annealed at 450oC for 30 minutes. The morphology and structure of the graphene-TiO2 nanocomposites layer were characterized by using Field Emission Scanning Electron Microscope (FESEM). The optical properties were studied by using UV-visible spectroscopy. Based on the result show that addition of graphene into TiO2 have provide larger surface area compared to pure TiO2. The optical properties of Graphene-TiO2 nanocomposites also improved as the fundamental of absorption edge has shifted toward longer wavelength and reduce the optical band gap.

Co-sensitization of graphene/TiO2 nanocomposite thin films with ruthenizer and metal free organic photosensitizers for improving the power conversion efficiency of dye-sensitized solar cells (DSSCs)

In this work, the power conversion efficiency (PEC) of dye-sensitized solar cells has been improved by using co-sensitized nanocomposite photoanodes. In the first step, mix dye solutions of ruthenizer (N719) and organic dye (RK-1) have been prepared for the sensitization of pristine TiO2 films. Among these dye solutions, 0.2 mM N719/0.3 mM RK-1 solution gives the highest PCE of 8.45%, which is superior to that of individually sensitized solar cells (N719 = 6.54% and RK-1 = 5.69%). In the second step, nanocomposites of TiO2 and graphene have been prepared by changing the composition of graphene from 0.05% to 0.2%. The dispersion of graphene in TiO2 has been confirmed by transmission electron microscopy (TEM). The optimized dye solution of 0.2 mM N719/0.3 mM RK-1 has been used to sensitize TiO2/graphene thin films. The solar cell based on TiO2/0.1%GR nanocomposite film shows the power conversion efficiency of 9.45%, which is about 45% and 66% higher than TiO2/N719 and TiO2/RK-1 based DSSCs, respectively. The overall enhancement in PCE can be credited to increase light absorption, improve light harvesting efficiency and fast electron transport, which ultimately enhance the current density of solar cells.

In vitro effect of graphene structures as an osteoinductive factor in bone tissue engineering: A systematic review.

Graphene and its derivatives have been well-known as influential factors in differentiating stem/progenitor cells toward the osteoblastic lineage. However, there have been many controversies in the literature regarding the parameters effect on bone regeneration, including graphene concentration, size, type, dimension, hydrophilicity, functionalization, and composition. This study attempts to produce a comprehensive review regarding the given parameters and their effects on stimulating cell behaviors such as proliferation, viability, attachment and osteogenic differentiation. In this study, a systematic search of MEDLINE database was conducted for in vitro studies on the use of graphene and its derivatives for bone tissue engineering from January 2000 to February 2018, organized according to the PRISMA statement. According to reviewed articles, different graphene derivative, including graphene, graphene oxide (GO) and reduced graphene oxide (RGO) with mass ratio ≤1.5 wt % for all and concentration up to 50 μg/mL for graphene and GO, and 60 μg/mL for RGO, are considered to be safe for most cell types. However, these concentrations highly depend on the types of cells. It was discovered that graphene with lateral size less than 5 µm, along with GO and RGO with lateral dimension less than 1 µm decrease cell viability. In addition, the three-dimensional structure of graphene can promote cell-cell interaction, migration and proliferation. When graphene and its derivatives are incorporated with metals, polymers, and minerals, they frequently show promoted mechanical properties and bioactivity. Last, graphene and its derivatives have been found to increase the surface roughness and porosity, which can highly enhance cell adhesion and differentiation. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 2284-2343, 2018.

Distribution-dependent capacitive and magnetic properties of Mn3O4 nanoparticles on reduced graphene oxide

The composition of nanoparticles (NPs) and graphene has aroused tremendous attention because it can synergistically combine the advantages of NPs and graphene. However, manipulating the distribution of NPs and identifying the effect of the distribution of NPs on the properties of the composites remain challenges. Here, we used graphene oxide (GO) of different oxidation degrees as the platform to prepare the Mn3O4/reduce GO composites. We found that the distribution of Mn3O4 NPs could be manipulated by adjusting the oxygen content of GO. Then we investigated the effect of Mn3O4 distribution on the capacitive and magnetic properties of the composites. The results showed that the distribution of Mn3O4 NPs could change the active sites, charge transfer, uncompensated spins, and spin coupling of the composites, resulting in the distribution-dependent capacitive and magnetic properties. Especially, with discrete Mn3O4 distribution, the composites exhibited the high specific capacitance of 1626.0 F g−1 and mass magnetization of ca. 85.3 emu g−1 normalized to the portion of Mn3O4. This work will help in understanding the interaction of NPs and graphene for fundamental research, and push the way for potential capacitive and magnetic applications of graphene-based composites.

Ultrafast, Reversible Transition of Superwettability of Graphene Network and Controllable Underwater Oil Adhesion for Oil Microdroplet Transportation

AbstractRecently, reversible surface superwettability has attracted enormous interest, and methods to shorten the cycle time of transition have also garnered the attention of researchers. Herein, a superhydrophobic, open‐cell graphene network (OCGN) is fabricated via self‐assembly of graphene oxide and vapor ejection. Owing to the special open‐cell microstructure, the OCGNs can be transformed to be superhydrophilic rapidly within only 1 s by air plasma treatment. Moreover, the OCGNs with pure graphene composition have a high conductivity and show an ultrafast Joule heating rate of up to 20 °C s−1 at a voltage of 20 V. By means of this property, for the first time an ultrafast recovery of the superhydrophobicity for OCGNs by self‐induced Joule heating with the shortest time of 1 min is reported. The mechanism of ultrafast, reversible transition is also explored specifically in this study. In addition, the superhydrophilic OCGNs show superoleophobicity in water and their underwater adhesion for oil droplets can be controlled by plasma treatment. Finally, the OCGNs with different oil adhesion properties are fabricated and the underwater oil microdroplet transportation is realized using OCGNs. Therefore, the OCGNs with smart surface can be an excellent candidate for achieving multifunctional superwettability of surfaces.

Hybrid graphene oxide/polysaccharide nanocomposites with controllable surface properties and biocompatibility

Herein, a strong interdependence between the composition of hybrid graphene oxide/hyaluronan/chitosan GO/HA/Chi multilayers and their surface properties and biocompatibility was demonstrated that can be used to build up coatings with desirable and precisely tunable properties. Both the position and the abundance of GO-layers into the polymer matrix were systematically varied to draw interconnection with the growth type, thickness, morphology, roughness, hydrophilicity and biocompatibility. It was found that when deposited in-between the HA and Chi layers GO forms diffusion barrier, hindering the mobility of Chi-chains and changing the exponential film growth to linear. Incorporation of GO-layers into the biodegradable and highly hydrated HA/Chi matrix does not affect the final thickness, but has a dramatic impact on the surface morphology and roughness, which in turn tunes the hydrophilicity, protein adsorption and platelets adhesion. This provides an opportunity for various biomedical applications of the studied hybrid films as coatings with controllable surface properties.

Preparation of nanoporous graphene sheets via free radical oxidation of graphene oxide and their application in lithium ion battery

Graphene is an attractive candidate for use as an electrode material in electrochemical energy storage due to its unique structure and excellent properties. Compared with graphene, nanoporous graphene is a superior electrode material, owing to the porous structure of its graphene sheets, which facilitates cross-plane lithium ion transportation and provides more binding sites for the lithium ions during the lithiation/delithiation process. In this work, we demonstrate a simple and efficient strategy for obtaining nanoporous graphene on a large scale. Nanoporous graphene can be generated through the oxidation of graphene oxide by H2O2 under high-power UV irradiation with a subsequent reduction process. The morphology, chemical composition and defects of the as-generated nanoporous graphene were studied. The electrochemical evaluation of the nanoporous graphene sheets showed that it delivered higher specific capacity and better charge/discharge rate capability compared with chemically reduced graphene sheets for use as an anode material in lithium ion batteries.

Thermal Properties of Graphene Filled Polymer Composite Thermal Interface Materials

Recent years have witnessed a staggering escalation in the power density of modern electronic devices. Because increasingly high power density accumulates heat, efficient heat removal has become a critical limitation for the performance, reliability, and further development of modern electronic devices. Thermal interface materials (TIMs) are widely employed between the two solid contact surfaces of heat sources and heat sinks to increase heat removal for electric devices. Composites of graphene and matrix materials are expected to be the most promising TIMs because of the remarkable thermal conductivity of graphene. Here, the recent research on the thermal properties of graphene filled polymer composite TIMs is reviewed. First, the composition of graphene filled polymer composite TIMs is introduced. Then, the synthetic methods for graphene filled polymer composite TIMs are primarily described. This study focuses on introducing the methods for improving and characterizing the thermal properties of graphene filled polymer composite TIMs. Furthermore, the challenges facing graphene filled poly­mer composite TIMs for thermal management applications in the modern electronic industry and the further progress required in this field are discussed. image

FTIR Spectrum Investigation of Thionine-Graphene Nanocomposite

Graphene is a material that has been heavily investigated in many researches due to its beneficial characteristics such as large surface area, low manufacturing cost, high electro conductivity and incredible mechanical strength. Applying the graphene in water-based solvents however can cause agglomeration due to its hydrophobic properties. Researchers have composited the graphene with other materials in overcoming its hydrophobicity. In this research, graphene was nanocomposited with thionine to make it disperse well in water-based solvents while preserving its intrinsic properties. The nanocomposition process involves mixing of both graphene oxide with thionine and were reduced by hydrazine hydrate while reflux heating. The produced mixture was then filtered to obtain the Thionine-Graphene nanocomposite. The obtained sample was then characterized to confirm the composition of both graphene and thionine. Fourier transfer infrared spectroscopy was operated to investigate the chemical bonds and hence concluding the presence of both graphene and thionine in the sample. The preservation of the intrinsic properties of graphene was also investigated through observing the absence of functionalized graphene bonds. Post-investigation reports that the chemical bonds from both of the materials, graphene and thionine were detected confirming the successfulness of the nanocomposition.

Variation of the resistivity and chemical composition of CVD graphene under annealing in a reductive atmosphere

Graphene samples synthesized by chemical vapor deposition, transferred onto Si/SiO2 substrates by the standard technology using PMMA, and then annealed in a reductive atmosphere have been characterized by XPS observations and measurements of the resistance temperature dependence. It was shown, that most of as-transferred samples exhibit a metallic-like resistivity dependence but some of them can demonstrate a non-metallic one. A comparative analysis of the results obtained by the XPS study and resistance measurements allowed us to conclude that the conduction process in CVD graphene is directly affected by functional groups adsorbed on the sample surface. Annealing in a mixture of H2 and Ar at T = 250–750°C is shown to result in a cleaning of the graphene surface from adsorbed contaminations and in a modification of the resistance temperature dependence which demonstrates a higher slope in case of a metallic-like behavior for as-transferred samples and a suppression of the activation-type dependence in case of a non-metallic one.

Enhancement of the Mechanical Properties of PEBAX Graphene Nanocomposite Using Supercritical Fluid Assisted Extrusion Polymer Processing Technique

Medical tubing used in minimally invasive devices presents a number of design considerations depending on the material used, design requirements (such as sufficient stiffness, flexibility and biocompatibility) and processing conditions. Currently, manufacturing industries adopt co-extrusion systems to meet design specifications, by using multilayer configuration leading to higher cost per device and increased complexity. This paper investigates the mechanical performance of nanocomposites using supercritical carbon dioxide assisted polymer processing technique. The use of innovative medical compounds such as PEBAX graphene nanocomposites have resulted in measurable improvements in mechanical properties. This study also presents the effect of supercritical carbon dioxide on the mechanical and physical properties of the polymer matrix. The mechanical properties have been investigated using dynamic mechanical analysis (DMA) and mechanical tensile test, where sufficient reinforcement was observed depending on the composition of graphene within PEBAX matrix. ATR-FTIR was used to further analyze the effect of supercritical carbon dioxide and interactions within the polymer composite matrix.

Graphene oxide layers modified by light energetic ions

In this paper, the effect of light ion irradiation on graphene oxide foil structure and composition was studied. Due to the excellent properties of graphene based materials suitable for application in electronics, optoelectronics, micro-mechanics and space technologies, the interaction of energetic ions with graphene based structures is worth studying. From the fundamental point of view, it is also interesting to get information about graphene oxide structure modification and the possible functional properties after irradiation by energetic ions. The light ion irradiation of graphene oxide (GO) foil was performed using 2.5 MeV H+ and 5.1 MeV He2+ ions. The change in the elemental composition of the GO foils after ion irradiation was investigated using Rutherford Backscattering Spectrometry and Elastic Recoil Detection Analysis. The influence of ion irradiation was further studied by microscopy methods. The chemical composition and structural changes of the GO foil surface were characterized by spectroscopy techniques including XPS, FTIR and Raman spectroscopy. Although the results of ion beam analysis indicated no significant compositional changes in the bulk of GO foils connected to ion irradiation, XPS, ATR-FTIR and Raman spectroscopy revealed reduction and removal of oxygen functionalities on the surface of graphene oxide. This reduction leads to a surface resistivity decrease after ion irradiation dependent on the ion species, fluence and energy.

A facile route to synthesize hollow Si-Ni3Sn2 nanospheres@ reduced graphene oxide nanocomposites for high performance lithium-ion batteries

In this paper, we report the original synthesis of hollow Si-Ni3Sn2 @ graphene composite and its high reversible capacity as lithium-ion battery anodes. To build the new architecture, three strategies of hollow architectures, porous surface and specific composition of graphene are attempted to develop lithium storage with far greater energy density and outstand rate performance. The results show that as-designed hollow Si-Ni3Sn2@graphene composites exhibit outstanding reversible capacity of 855.7 mAh g−1 during the first cycle, and 576.6 mAh g−1 can be maintained during a cycling test of 50 cycles at 300 mA g−1. The rate capability is also enhanced, delivering reversible capacity of 449.8, 420.4, 392.7 and 377.1 mAh g−1 at current densities of 600, 800, 1200 and 1800 mA h g−1, thus exhibiting great potential as an anode material for lithium-ion batteries.

SYNTHESIS AND PROPERTIES OF Ag2O/RGO NANOCOMPOSITE AS CATHODE MATERIALS FOR ZINC - SILVER BATTERIES

In this article, the effects of Ag2O/reduced graphene oxide (RGO) nanocomposite has been studied by the X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), the field emission scanning electrode microcopy (FESEM) and Raman methods. The discharge/charge property of the Ag2O/RGO nanocomposite was studied by galvanostatic charge-discharge measurement as a cathode material, CR2032 cells. The results indicated temperature and time for annealing to treated Ag2O/RGO nanocomposite and graphene composition have effect to properties of electrochemical electrode. The Ag2O/RGO nanocomposite electrode may be discharge/charge at hight current density (3 C) and used as a cathode for zinc- silver batteries.

Detection of Heavy Metal Ions Using a Long-Life Electrochemical Sensor Containing Conducting Polymer-Graphene Oxide Composite

Exposure to high levels of trace heavy metal ions (HMIs, e.g., Zn(II), Cd(II), Pb(II), Cu(II), and Hg(II)) poses a variety of health and environmental problems, because they tend to accumulate in the body and quality water, are toxic, and have a low rate of clearance. Therefore, it is critical to minimize the exposure of these contaminants to human and this can be ensured through effective water quality monitoring. Conductive polymers (CPs) have broad applications in various fields. We have effectively utilized various types of CPs for different electrocatalytic applications, such as polymer dyes for solar cell and sensor substrates [1-2]. To enhance the performance of the CPs, composition of graphene has been receiving much attention for sensor applications, due to their excellent conductivity because of π-π stacking and synergetic effects with other materials. Especially, one advantage of using a functionalized CP, aminopyrimidine terthiophene having selective metal-complexing property can be used to overcome the limitations of conventional stripping voltammetry that preconcentrates metal ions by applying a potential for a while. Thus, in the present work, a fast transient technique chronocoulometry (CC) is introduced for metal ions sensor. The synthesized monomer(PATT) was reacted to the GO suspension, which was then sonicated to obtain a homogeneous mixture of PATT and GO suspension. An optimized ratio of PATT and GO was used to obtain a homogeneous solution without signicant agglomeration of the GO with the PATT. Electrochemical polymerization was achieved in an acetonitrile solution containing a mixture of PATT monomer and GO. The schematic presentation of the sensor probe fabrication is presented in Figure 1. SWASV requires a long analysis time due to slow scan rate, whereas, CC is advantageous in achieveing quick analysis of HMIs without a preconcentration process. In addition, being an integrated technique, CC shows less kinetic effect while applying potential and if in a low cost technique due to the simple design of electronics and algorithm as compared to SWASV. Although it cannot detect each HMIs in one scan, the CC can monitor the total amount of metal ions in one scan and also each metal species can be detected using the step by step scan. Therefore, the proposed sensor probe using CC method can be applicable in the real time point of care analysis. The dynamic ranges of CC and SWASV were between 1.0 ppb and 10.0 ppm and between 1.0 ppb and 10.0 ppm, respectively, Here, the detection limits for Zn(II), Cd(II), Pb(II), Cu(II), and Hg(II) were 0.2 (±0.08), 0.8 (±0.20), 0.6 (±0.15), 0.5 (±0.12), and 0.1 (±0.05) ppb for the CC method without preconcentration, and 0.9 (±0.20), 0.6 (±0.15), 0.4 (±0.10), 1.0 (±0.25), and 0.7 (±0.15) ppb, respectively, for the SWASV method with 300 s of deposition time (n = 3). The reliability of the method for point of care analysis was evaluated by analyzing different water samples. The stability test was performed in a 1.0 ppm HMIs solution under the optimized pH and deposition time for a period of 3 months. The sensor lost only 1.7% of the initial response after approximately 45 continuous measurements. HMIs can selectively coordinate with nitrogen atom (-NH) of the pPATT ligand due to the complexation between metal ions and nitrogen compounds. In addition, negatively charged oxygenated groups of GO also interact with the HMIs. The proposed sensor was applied for detecting HMIs in real water (tap and local surface (Pusan National University, South Korea)), and the presence of metal ions was analyzed using the CC and SWASV methods. The observed values of Zn(II), Cd(II), Pb(II), Cu(II), and Hg(II) ions were 0.5 (± 0.1), 12.0 (± 0.4), 5.2 (± 0.3), 1.4 (± 0.2), and 0.9 (± 0.2) ppb using the CC method and 0.4 (± 0.1), 11.8 (± 0.5), 5.0 (± 0.2), 1.5 (± 0.6), and 10.7 (± 0.1) ppb, respectively, from SWASV method.[1] Chandra, P.; Shim, Y.-B.; et al., Chem. Comm., 2012, 49, 1900-1902.[2] Choi, S.-M.; Shim, Y.-B.; et al,. Anal. Chim. Acta., 2015, 892, 77-87. Figure 1

Graphene-based multilayer resonance structure to enhance the optical pressure on a Mie particle

We theoretically investigate the optical force exerted on a Mie dielectric particle in the evanescent field of a graphene-based resonance multilayer structure using the arbitrary beam theory and the theory of multilayer films. The resonance structure consists of several thin films including a dielectric film ( MgF 2 ), a metal film (silver or gold), and several graphene layers which are located on a prism base. The effects of the metal film thickness and the number of graphene layers on the optical force are numerically investigated. The thickness of the metal layer and the number of graphene layers are optimized to reach the highest optical force. The numerical results show that an optimized composition of graphene and gold leads to a higher optical force compared to that of the graphene and silver. The optical force was enhanced resonantly by four orders of magnitude for the resonance structure containing graphene and a gold film and by three orders of magnitude for the structure containing graphene and a silver film compared to other similar resonance structures. We hope that the results presented in this paper can provide an excellent means of improving the optical manipulation of particles and enable the provision of effective optical tweezers, micromotors, and microaccelelators.

Boron-doped graphene as promising support for platinum catalyst with superior activity towards the methanol electrooxidation reaction

We report the synthesis of boron-doped graphene by thermally annealing the mixture of graphene oxide and boric acid, and its usage as the support of Pt catalyst towards the methanol oxidation reaction. The composition, structure and morphology of boron-doped graphene and its supported Pt nanoparticles (Pt/BG) are characterized by transmission electron microscopy, inductively coupled plasma mass spectrometry, Raman spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. It is revealed that boron atoms are doped into graphene network in the form of BC2O and BCO2 bonds, which lead to the increase in defect sites and facilitate the subsequent deposition of Pt nanoparticles. Therefore, the Pt/BG catalyst presents smaller particle size and narrower size distribution than the graphene supported Pt (Pt/G) catalyst. When evaluated as the electrocatalyst for the methanol oxidation reaction, the Pt/BG catalyst exhibits excellent electrochemical activity and stability demonstrated by cyclic voltammetry and chronoamperometry tests. The enhanced activity is mainly ascribed to the electronic interaction between boron-doped graphene and Pt nanoparticles, which lowers the d-band center of Pt and thus weakens the absorption of the poisoning intermediate CO. Our work provides an alternative approach of improving the reaction kinetics for the oxidation of small organic molecules.

Solar-induced photoelectrochemical sensing for dopamine based on TiO2 nanoparticles on g-C3N4 decorated graphene nanosheets

Abstract A novel photoelectrochemical (PEC) sensing platform is fabricated using the composition of graphitic carbon nitride (g-C3N4), TiO2 nanoparticles (TiO2 NPs) and graphene (G) on an indium tin oxide (ITO) electrode. The graphene layer is acting as the supporter for g-C3N4 to form tightly G-C3N4 thin nanosheets, and behaves as an electron transfer medium to enhance the electron transport ability. Then, TiO2 NPs tightly anchor onto the G-C3N4 nanosheets to form G-C3N4/TiO2 nanocomposite. The photocurrent of G-C3N4/TiO2 drastically enhances compared to G-C3N4 and G-TiO2, which is attributed to the life improvement of the electron–hole pairs and the synergistic amplification between TiO2 NPs and C3N4. Interestingly, the photocurrent of G-C3N4/TiO2 nanocomposite is notably improved upon the addition of dopamine (DA). Based on the enhanced photocurrent signal, a PEC methodology for ultrasensitive determination of DA has been developed, which showed linearly enhanced photocurrent by increasing the DA concentration from 0.1 to 50 μM with a detection limit of 0.02 μM (S/N = 3) under optimized conditions. Therefore, the fabricated photoelectrochemical sensor provides a promising method in the sensing of biomolecules and extended the applications of C3N4-based nanocomposite in the field of PEC sensing.

Observation of negative refraction in the graphene/ferrite composite

An observation of negative refraction in the naturally obtained composition of graphene and barium ferrite is reported. The capacitance and inductance measurements revealed the electric and magnetic resonances accompanied with the negative values of permittivity and permeability in the overlapped frequency range. According to the “left‐handed” media approach such a material is characterized by negative refraction. The derived values of the real part of refractive index are negative at the frequencies above 500 MHz. magnified image(© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)