Carbon is one of the most abundant elements found in nature, and its compounds have wide existence on the Earth. As such, it remains as one the most common resource materials to form various nanostructured composites. The carbon-based compounds form the basis of all known lives in nature. Modern times witness the development of techniques to use the allotropes of carbon for varieties of needs. Owing to the versatile bonding ability of carbon, it possesses unique properties of reacting with many other elements, thereby making the carbon-based compounds to find a wide range of applications in everyday human life.
Within the context, graphene is a single tightly packed layer of carbon atoms – the thinnest known lattice structure of one atom thickness. Carbon being the chemical basis for life on the Earth makes graphene to be the most ecofriendly candidates for multifarious applications. Among the notable features, excellent conductor of heat and electricity allows graphene to be of great potentials in the present day thrust in nanotechnology research. This is evident from the significant amount of attention paid by the R&D investigators to the formation of novel graphene-based nanostructured mediums followed by the study of their characteristic features. Some of the examples to highlight the technology and/or consumer oriented applications of graphene would be – molecular electronic devices, highly flexible and lightweight supercapacitors for power storage, printed electrodes, consumer electronics products, and corrosion-free household items.
The current research trends of graphene technology involve designing and fabricating such mediums capable of manipulating electromagnetic waves. This is primarily because of the physical and/or chemical properties of graphene that can be affected in the presence of external forces. On the device/component level, graphene mediums of certain forms are generally introduced in host mediums to achieve the on-demand kind of spectral characteristics. Growing demands of graphene research made us to bring out a Special Issue of the Journal of Electromagnetic Waves and Applications (JEMWA), entitled Current Trends of Graphene Technology to showcase some of the recent research results highlighting the different dimensions of graphene-based theoretical and experimental investigations.
As the conductance of graphene is one of the most important parameters of interest, alterations in its value would introduce varieties of features that would be of great technological potentials. Interestingly, the conductivity would be affected in the presence of external fields. In view of this, Ganguly and Basu presented the behavior of charge and spin transport properties of graphene nanoribbon in the presence of magnetic field. They took into account Rashba spin–orbit interaction, and used the multi-terminal Landauer-Büttiker formula and Green’s function approach to determine the effect of spin–orbit coupling on the quantum Hall plateaus. The authors demonstrated the effect of magnetic field on the spin Hall conductance, which exhibits discrete step-like behavior.
The simulation-related aspects to model graphene structures has been of marked attention. In literature, graphene is modeled as a thin surface with the conductance as the algebraic sum of the interband and intraband conductivities. Within the context, the finite-difference time-domain (FDTD) technique has been widely recognized. However, the method inherits the lacuna of relatively longer computational time. To overcome the issue, Guo et al. put forward a hybrid implicit explicit-FDTD (HIE-FDTD) technique for the simulation of infinitely extended graphene layer, in order to extract accurate results in the far- and near-infrared frequency regimes of the electromagnetic spectrum. The authors compared the results with those obtained by the FDTD scheme, in order to validate the proposition. In yet another paper, Chen et al. of the same research group introduced a scheme – the alternating-direction implicit-FDTD (ADI-FDTD) method – to simulate infinite graphene sheet. The results reveal significantly larger computational efficiency than that of the FDTD technique. The authors considered several numerical examples to validate the obtained results.
Another specially classified FDTD-based simulation method – the dispersive weakly conditionally stable FDTD (WCS-FDTD) technique – is exploited by Xu et al. for the simulation of graphene-based absorbers in the terahertz regime. The obtained numerical results demonstrate high computational accuracy along with greatly reduced computational time, as compared to the situations when the dispersive FDTD and HIE-FDTD methods are used.
Graphene-based metamaterials can be engineered to achieve desired technology-oriented potentials through devising electronic (or rather optoelectronic) components. Within the context, Deng et al. in their paper put forth the design of graphene-based broadband terahertz metamaterial modulator. They proposed a dynamically tuneable structure composed of graphene layer that serves as reflector. In the structure, graphene medium is spaced by a SiO2/Si substrate and a patterned metallic split-ring resonator layer. The authors claimed to achieve fairly wide modulation bandwidth (of the structure) with tuneable transmission properties in the THz regime.
The features of optical bistability and group velocity in a dielectric medium with introduced defect of a single-layer graphene are presented by Jabbari. The author studied the proposed structure theoretically based on quantum coherence and interference mechanism in a defect dielectric slab with a monolayer of quantized four-level graphene system. The quantum mechanical density-matrix formalism is exploited, and the control over optical bistability for the transmitted light from the defect slab medium is analyzed. It is reported that the parameters such as incoherent pumping rate and thickness of graphene slab play vital roles to govern the switching mechanism. The investigations revealed the possibility of subluminal and superluminal light propagation in a specific situation of parametric conditions.
Ghosh and Mitra discussed the antenna-based aspects of graphene metasurface. More precisely, they worked on an effective approach to mitigate mutual coupling between two planar antennas operating at the lower THz frequency regime. This is achieved by incorporating a graphene-based metasurface between the radiating elements. The metasurface itself is comprised of a modified electric inductive capacitive resonator, which provides band-stop functionality in the operating bandwidth of the antenna. The tuneability of graphene conductivity and surface impedance are used for modeling the graphene metasurface. The authors claimed as achieving fairly good directive performance of the proposed antenna array.
In line of another antenna-based application of graphene medium, Basu et al. developed a conductive layer by printing graphene nanoparticles-based conductive ink on silk fabric, which is utilized to fabricate wideband antenna for aeronautical applications. Although the authors reported robust wideband antenna of excellent radiation performance, the conductivity of printed fabric, however, greatly depends on various factors such as the smoothness of textile surface, density of fibers, penetration depth of graphene ink and errors in the process of printing.
The mode filtering property of graphene-based silica-clad slab waveguide is discussed by Baqir and Choudhury highlighting the inherent slow-wave characteristics of the guide. In the attempt, they investigated slab-waveguide comprised of sub-wavelength-sized bulk gold, bounded with graphene and silica layers. The investigation revealed filtering characteristic of the guide by supporting only the fundamental transverse magnetic mode; the transverse electric and other higher order transverse magnetic modes are completely blocked (by the device). Furthermore, the silica–graphene–gold planar guide slows down the speed of waves in the visible range, which determines the existence of slow-waves in the guide – the feature that may be harnessed for sensing applications.
Vertically aligned graphene-based thin films are of much interest owing to their promising applications in sensing. This is because the alignment of graphene molecules can be affected by suitably applied external agitation. Furthermore, these can be exploited in field emission display technology as well, as demonstrated by Baek et al. in their paper. They investigated the field emission properties of vertically aligned graphene-based thin film, and could achieve uniform high current as well as current density. This could be realized by adopting certain mechanical and electrical treatments of the reduced graphene oxide film. They demonstrated experimentally the field emission performance of the synthesized graphene-based film that exhibits negligible current degradation (less than 5% of the average emission current) during the life-time test. This essentially opens up avenues of these in field emission displays, electron microscopy and terahertz vacuum electronic devices.
Another relevant work in the context of vacuum electron devices is presented by Barik et al. The use of thermionic cathode remains the key part in vacuum devices. Due to high temperature operation, the quality of the component becomes critical, and essentially depends on the robustness and yield factor. In their paper, the authors described synthesis of a brazing alloy based on nano-composites of Mo–Ru–Ni using the sol-gel route, and determined its brazing characteristics. They used carbon tape for the purpose of characterization, and found the developed alloy to induce significant improvements in the mechanical and thermal performance of the dispenser cathode.
Finally, the potentials of graphene-enhanced technology have significant impacts on the current nanotech-based R&D world. This is evidenced by the number of research papers appearing in pioneering journals and periodicals. The papers incorporated in this Special Issue of JEMWA are pivoted to the modeling aspects of graphene structure, and also, applications in a few different directions. The contributors presented theoretical and experimental findings. Though these papers represent only a few aspects of the ongoing research on graphene, and varieties of other dimensions are still untouched, the Editors-in-Chief expect the readers would find the Issue useful. We are hopeful to bring out another volume of work pivoted to grapheme technology throwing more interesting results and ideas to work on.
Joint Editors-in-Chief