Optical Properties Of Graphene Research Articles

A Tunable “Ancient Coin”-Type Perfect Absorber with High Refractive Index Sensitivity and Good Angular Polarization Tolerance

In this paper, we design and present a graphene-based “ancient coin”-type dual-band perfect metamaterial absorber, which is composed of a silver layer, silicon dioxide layer, and a top “ancient coin” graphene layer. The absorption performance of the absorber is affected by the hollowed-out square in the center of the graphene layer and geometric parameters of the remaining nano disk. The optical properties of graphene can be changed by adjusting the voltage, to control the absorption performance of the absorber dynamically. In addition, the centrally symmetric pattern structure greatly eliminates the polarization angle dependence of our proposed absorber, and it exhibits good angular polarization tolerance. Furthermore, the proposed “ancient coin”-type absorber shows great application potential as a sensor or detector in biopharmaceutical, optical imaging, and other fields due to its strong tunability and high refractive index sensitivity.

Tuning the hybrid borophene−/graphene-ionic liquid interface: Effect of metal cations on the electronic and photonic properties

A density functional theory study of the effect of the adsorption of a mixture of potassium salt and an imidazolium-based ionic liquid on the electronic and optical properties of graphene and borophene is performed. This addition leads to a downward shift of the Dirac cone for both structures due to the electron transferred from potassium to the graphene and borophene sheets. The transferred charge to the wall and the binding energy of potassium are bigger than in the case of the adsorption of potassium on pristine graphene. The Bader analysis shows that the presence of potassium makes the charge distributions in graphene and the layers of boron atoms in borophene more homogenous and the electronic distribution in the sheets is consequently less affected by the electromagnetic interaction with the ions in the mixture. Moreover, the results show that the optical properties are altered by the metallic cation, especially at low frequencies where ionic liquid-enhanced 2D plasmons are registered due to the doping of 2D sheets by electrons transferred from the potassium atoms leading to near zero values of the parallel components of the dielectric function and refractive index.

Broadband Ultraviolet Photodetector Based on Graphene/β-Ga<sub>2</sub>O<sub>3</sub>/GaN Heterojunction

Broadband ultraviolet (BUV) photodetectors are widely used in military and civil fields. A high performance BUV photodetector based on graphene/β-Ga2O3/GaN heterojunction is proposed and realized by semiconductor micro-fabrication techniques in this paper. The β-Ga2O3 and GaN films are grown by metal organic chemical vapor deposition (MOCVD), and the graphene is also used as a transparent electrode. The device exhibits a broad response band from 230 nm to 368 nm with responsivity exceeding 0.4A/W at -5 V bias voltage and a peak responsivity of 0.53 A/W at 256 nm. These performances can be attributed to the internal gain mechanism of graphene/β-Ga2O3/GaN heterojunction and the optical properties of graphene. Our work provides an efficient method to realize a high-performance BUV photodetector for photoelectric applications.

Electronic and optical properties of borophene and graphene with an adsorbed ionic liquid: A density functional theory study

We present a comparative density functional theory study of the electronic and optical properties of graphene and borophene with an adsorbed ionic liquid. The Dirac cones of these two-dimensional nanostructures are not affected by the presence of the ionic liquid close to these sheets, not even in the borophene one in which a very small charge transfer to the ionic liquid anion is detected. However, Bader analysis and charge density calculations showed that the polarization of the ionic liquid induces a redistribution of the electric charge in both sheets as a consequence of the electromagnetic interaction between the anion and cation of the ionic liquid and the electrons on the two-dimensional materials, the effect being notably stronger in graphene than in borophene. Our calculations reveal that several changes take place in the dielectric constant and electron energy loss spectra, including shifts and damping of plasmon bands, related to electrostatically induced changes in the adsorbent surface electron charge distributions and to plasmon loss channels associated to excitations of electrons in the energy levels of the adsorbed ionic liquid species close to the Fermi level of the surface material. While only minor changes are registered in borophene plasmon spectra and in the IR and visible regions of the graphene spectra, important changes in the graphene ultraviolet C and X-ray spectra are registered due to the existence of IL electronic energy levels close to the Fermi level of graphene that are absent in borophene.

Effects of lanthanides doping on the optical properties of graphene/WSe2 heterostructure based on ab-initio calculations

Based on the density functional theory, we discussed the electronic and optical properties of graphene/ WSe2 (GW) heterostructure after lanthanides doping. Red shift appears and the optical parameter values are improved in the low energy region after the lanthanides are doped. Different doping types are also discussed. In the case of single doping, substitute Yb atom on W site will improve the peak values of the optical parameters greatly. In the case of co-doping, it is found that the effect will be more obvious when the two doped lanthanide atoms are located in the second neighboring positons. These results suggest that lanthanides doping does adjust the electronic structure and improve the optical properties of GW heterostructures, which providing useful guidance for the design of novel optical nanodevices based on two-dimensional materials.

Electronic, thermal, and optical properties of graphene like SiCx structures: Significant effects of Si atom configurations

We investigate the electronic, thermal, and optical characteristics of graphene like SiCx structure using model calculations based on density functional theory. The change in the energy bandgap can be tuned by the Si atomic configuration, rather than the dopants ratio. The effects of the concentration of the Si atoms and the shape of supercell are kept constant, and only the interaction effects of two Si atoms are studied by varying their positions. If the Si atoms are at the same sublattice positions, a maximum bandgap is obtained leading to an increased Seebeck coefficient and figure of merit. A deviation in the Wiedemann-Franz ratio is also found, and a maximum value of the Lorenz number is thus discovered. Furthermore, a significant red shift of the first peak of the imaginary part of the dielectric function towards the visible range of the electromagnetic radiation is observed. On the other hand, if the Si atoms are located at different sublattice positions, a small bandgap is seen because the symmetry of sublattice remains almost unchanged. Consequently, the Seebeck coefficient and the dielectric function are only slightly changed compared to pristine graphene. In addition, the electron energy loss function is suppressed in Si-doped graphene. These unique variations of the thermal and the optical properties of Si-doped graphene are of importance to understand experiments relevant to optoelectronic applications.

Graphene-Enabled, Spatially Controlled Electroporation of Adherent Cells for Live-Cell Super-resolution Microscopy.

The incorporation of exogenous molecules into live cells is essential for both biological research and therapeutic applications. In particular, for the emerging field of super-resolution microscopy of live mammalian cells, it remains a challenge to deliver tailored, often cell-impermeable, fluorescent probes into live cells for target labeling. Here, utilizing the outstanding mechanical, electrical, and optical properties of graphene, we report a facile approach that enables both high-throughput delivery of fluorescent probes into adherent mammalian cells and in situ super-resolution microscopy on the same device. Approximately 90% delivery efficiencies are achieved for free dyes and dye-tagged affinity probes, short peptides, and whole antibodies, thus enabling high-quality super-resolution microscopy. Moreover, we demonstrate good spatiotemporal controls, which, in combination with the ready patternability of graphene, allow for the spatially selective delivery of two different probes for cells at different locations on the same substrate.

Utilization of Synergistic Effect of Dimension‐Differentiated Hierarchical Nanomaterials for Transparent and Flexible Wireless Communicational Elements

AbstractThe demand of emerging transparent and flexible wireless electronic devices is ever‐increasing for Internet of Things (IoT) scenarios, like noninvasive healthcare, real‐time wearable electronics, etc. However, as an essential part of the IoT wireless communicational devices, radio frequency (RF) antennas are still hampered by poor‐flexibility, low‐conductivity, and weak‐transparency. Here, based on the unique electronic and optical properties of graphene, a method to obtain these appealing features concurrently through promoting synergistic effect between two‐dimensional (2D) and one‐dimensional (1D) materials is studied. It is found that this method could not only successfully maintain transparency and flexibility, but also greatly enhance the overall performance of the antenna. The fabricated antenna exhibits a 75% light transmittance, from 5.6 to 12.8 GHz ultrawide bandwidth and outstanding durability and stability. Moreover, a transparent and flexible radio frequency identification (RFID) tag is also designed and demonstrated with a remarkable reading distance. These findings show that the method by promoting synergistic effect of hybrid materials has great potential in the design of next generation novel and high‐performance wireless electronics.

Photoninduced charge redistribution of graphene determined by edge structures in the infrared region.

By using the ab initio density-functional theory method, we investigated the charge redistribution of monolayer graphene with ZigZag and/or ArmChair edges upon infrared excitation. The photoinduced charge redistribution is strongly dependent on edge types. The priority of electrons transfer has been revealed by charge density difference. To further investigate the influence of edge types on optical properties, the dielectric constants and absorption coefficient of graphene with various edge types have been calculated. The edge types have a non-negligible influence on optical properties of graphene, and the Zigzag edge graphene owns stronger optical absorption in infrared region. Our results are potentially beneficial for designing graphene nanodevices in the infrared region.

Ultracompact and chipless terahertz identification tags using multi-resonant metasurface based on graphene

Traditional identification tags in the radio or optical domain are unsatisfactory in terms of size and security. In this paper, we theoretically demonstrate an ultracompact and chipless identification tag in the terahertz band using multi-resonant metasurfaces based on graphene. Benefiting from the exceptional electrical and optical properties of graphene, the proposed tags encoding different bit sequences have a uniform geometric shape and size, which is significant for large-scale fabrication processes. Also, their operating frequency could be independently tuned by altering the doping level of each graphene loop without changing their geometric parameters. Tag principles are revealed by absorption spectra and the physical mechanism is investigated with electric field distributions. In addition, we further evaluate the multilayer graphene-based terahertz identification tag for multi-bit coding application, which breaks the spatial constraints in a single layer. Therefore, the proposed terahertz identification tags based on graphene may have remarkable potential values in future chipless identification applications.

Manipulation of epitaxial graphene towards novel properties and applications

Abstract The integration of epitaxial graphene on 4H-SiC with different metals may allow tunability of electronic and optical properties of graphene, enabling novel high-performance devices. Here we present a Raman spectroscopy study on epitaxial graphene decorated with electrodeposited Pb and Li adatoms and with magnetron sputtered 5 nm-thick Ag nano-island films. We find that the presence of metals on the epitaxial graphene surface generates defects and induces n-type doping, which is evidenced by the observation of the defect related Raman modes (namely D, D′ and D + G) and systematic red-shift of the main characteristic modes of graphene. In-depth statistical analysis of the Raman data before and after metal deposition complemented by density functional theory (DFT) calculations allowed to link the interaction strength between the three selected metals and graphene with the metal-induced changes in the vibrational/electronic properties of graphene. Large-area uniform electron doping of epitaxial graphene and surface-enhanced Raman scattering (SERS) effect are reached by room temperature deposition of Ag nano-island films. Very promising results have been obtained from graphene subjected to electrochemical intercalation by Li, which can serve as prerequisites of the construction of Li batteries. The strong interaction between Li or Pb with graphene implies the possibility to exploit the epitaxial graphene as an efficient material for energy storage or for heavy metal sensing, while predominant van der Waals interaction between Ag and graphene favors the formation of extremely thin silver coatings towards two-dimensional metal systems. The present results give better understanding of the nature of epitaxial graphene response to metal deposition and can be useful to design high-performance energy storage devices, optical sensors and heavy metal detection systems.

Nonlinear optical properties of graphene and pyrolytic carbon: A comparative study

We report on [Formula: see text]-scan investigations of the nonlinear optical properties of thin pyrolytic carbon films and two- and three-layer graphene films, utilizing an 800[Formula: see text]nm pulsed laser generating ultrashort 180[Formula: see text]fs pulses. We analyze the sign and magnitude of nonlinear refraction and absorption in these materials, given contradictory results in this respect in the literature. In particular, we find that nonlinear refraction of graphene changes sign with changing light intensity with concomitant subtle changes in nonlinear absorption, a behavior not reported previously that suggests an interplay of several mechanisms that depend on intensity. Pyrolytic carbon films, on the contrary, exhibit positive refraction, whose magnitude (nonlinear refractive index [Formula: see text][Formula: see text]cm2/W) is comparable to that of graphene, and strong negative absorption, i.e., bleaching, contrary to previously reported results. We discuss implications and possible mechanisms of the observed phenomena.

Graphene and silicene nanodomains in a ultra-thin SiC layer for water splitting and hydrogen storage. A first principle study

First-principles calculations within the density functional theory (DFT) have been addressed to investigate the energetic stability, electronic and optical properties of graphene and silicene nanodomains in a SiC single layer (h-SiC). We observe that graphene domains form a planar structure and give rise to an occupied and an empty electronic levels inside the h-SiC band gap, leading the h-SiC to present a strong optical absorption peak in the visible region. On the other hand, when a silicene nanodomain is present the system is no longer planar and present a corrugated structure similar to the silicene structure. The silicene nanodomain introduce three empty electronic levels within the band gap, leading the h-SiC with optical absorption in the visible region. These results show that a graphene nanodomain in h-SiC is appropriate for optical devices, while silicene nanodomains form almost sp3 quantum dots. This finding suggest that the graphene and silicene nanodomains in a SiC single layer increase the possibility to use h-SiC to produce new electronic and optical devices as well for energy storage by hydrogen adsorption. In fact, we study the H2 and O2 adsorption on the pristine system and on the nanodomains, we observe that the presence of the nanodomais increase the binding energies of the adsorbed molecules.

Large-Area Synthesis of Superclean Graphene via Selective Etching of Amorphous Carbon with Carbon Dioxide.

Contamination commonly observed on the graphene surface is detrimental to its excellent properties and strongly hinders its application. It is still a great challenge to produce large-area clean graphene film in a low-cost manner. Herein, we demonstrate a facile and scalable chemical vapor deposition approach to synthesize meter-sized samples of superclean graphene with an average cleanness of 99 %, relying on the weak oxidizing ability of CO2 to etch away the intrinsic contamination, i.e., amorphous carbon. Remarkably, the elimination of amorphous carbon enables a significant reduction of polymer residues in the transfer of graphene films and the fabrication of graphene-based devices and promises strongly enhanced electrical and optical properties of graphene. The facile synthesis of large-area superclean graphene would open the pathway for both fundamental research and industrial applications of graphene, where a clean surface is highly needed.

Large‐Area Synthesis of Superclean Graphene via Selective Etching of Amorphous Carbon with Carbon Dioxide

AbstractContamination commonly observed on the graphene surface is detrimental to its excellent properties and strongly hinders its application. It is still a great challenge to produce large‐area clean graphene film in a low‐cost manner. Herein, we demonstrate a facile and scalable chemical vapor deposition approach to synthesize meter‐sized samples of superclean graphene with an average cleanness of 99 %, relying on the weak oxidizing ability of CO2 to etch away the intrinsic contamination, i.e., amorphous carbon. Remarkably, the elimination of amorphous carbon enables a significant reduction of polymer residues in the transfer of graphene films and the fabrication of graphene‐based devices and promises strongly enhanced electrical and optical properties of graphene. The facile synthesis of large‐area superclean graphene would open the pathway for both fundamental research and industrial applications of graphene, where a clean surface is highly needed.

Impacts of uniaxial strain on optical properties of graphene by first principles

The impacts of strain along the zigzag and armchair directions on optical properties of graphene have been investigated theoretically by first principles. The relative changes of the refractive index achieve up to 1 with increasing polarized angle at some frequency points for both cases. It must be noted that strain along the armchair direction results in an additional infrared absorption peak at 1511[Formula: see text]cm[Formula: see text], because the electron concentration is inhomogeneous at the direction of atomic vibration. Thus, this work paves the way of strained graphene application in identifying polarization of light and infrared sensors in near future.

Direct band-gap crossover in epitaxial monolayer boron nitride

Hexagonal boron nitride is a large band-gap insulating material which complements the electronic and optical properties of graphene and the transition metal dichalcogenides. However, the intrinsic optical properties of monolayer boron nitride remain largely unexplored. In particular, the theoretically expected crossover to a direct-gap in the limit of the single monolayer is presently not confirmed experimentally. Here, in contrast to the technique of exfoliating few-layer 2D hexagonal boron nitride, we exploit the scalable approach of high-temperature molecular beam epitaxy to grow high-quality monolayer boron nitride on graphite substrates. We combine deep-ultraviolet photoluminescence and reflectance spectroscopy with atomic force microscopy to reveal the presence of a direct gap of energy 6.1 eV in the single atomic layers, thus confirming a crossover to direct gap in the monolayer limit.

Electronic and optical properties of vacancy and B, N, O and F doped graphene: DFT study

Structural and optical properties of graphene with a vacancy and B, N, O and F doped graphene have been investigated computationally using density functional theory (DFT). Absorption spectrum of all considered structures for both cases of in plane E ⊥ c and out of plane E ‖ c polarization of light are illustrated in figure a and b respectively. Optical properties of O and F doped graphene were calculated for the first time and other result compared with experimental and theoretical reports. • In all cases, the graphene with a vacancy and doped graphenes remain planar so that new structures like pure graphene have sp 2 hybridization. • It was found that 3.125% boron doped graphene is a p-type and oxygen and nitrogen doped graphene are a n-type semiconductor. (a) For requesting structures, band gap opened and for F doped graphene band gap was different for spin up and down that is caused this structure could be a good candidate for spintronic device design. the magnetic moment of this structure is 0.71 μB/cell. • The graphene with a vacancy undergoes a John-Teller distortion which leads to the saturation of two of the three dangling bonds toward the missing atom. Graphene with a vacancy has magnetic moment of 0.89 μB/cell and two of the carbon atoms close to the vacancy site move towards each other forming a C–C distance of 2.226Å. • All absorption peaks for different structures have shifted toward larger energies (blue shift) and peak intensities decrease with increasing number of electrons entering the structure. Structural and optical properties of graphene with a vacancy and B, N, O and F doped graphene have been investigated computationally using density functional theory (DFT). We find that B is a p-type while N, O and F doped graphene layers, as well as graphene with a vacancy are n-type semiconductors. Optical properties for both cases of in plane E ⊥ c and out of plane E ‖ c polarization of light are investigated. It is observed that with the increase in the number of electrons entering the supercell, the amount of absorption of the system decreases and the absorption peaks are transferred to higher energies (blue shift).

Impact of phosphorous and sulphur substitution on Dirac cone modification and optical behaviors of monolayer graphene for nano-electronic devices

Abstract In this work, pristine, S- and P-doped monolayer graphene are studied by employing full-potential linear augmented plane-wave method within the framework of density functional theory. One and two C atom in the 2 × 2 supercell of monolayer graphene are replaced by S or P atoms to explore the impact of increasing doping concentration of these reactive nonmetal dopants on the electronic and optical properties. It is found that the incorporation of single S and P dopants in monolayer graphene causes the conduction band minimum at the high symmetry K-point to shift below the Fermi level along with opening the energy states at the Dirac cone; transforming the semi-metallic nature of pristine graphene to an extrinsic n-type semiconductor. Furthermore, we show that increasing the doping concentration of S and P atoms by doping them in nearest-neighbour sites in the honeycomb graphene lattice causes conduction band states to overlap with the valence band of the undoped system; allowing the re-emergence of Dirac cone. The effect of increasing S and P doping concentration on the optical properties of graphene are also studied. Our results indicate that overall absorption spectra of monolayer graphene is red shifted towards lower energy photons upon doping with P and S atoms, while refraction index, extinction coefficient and reflectivity of monolayer graphene are also significantly altered. The electron energy loss function and X-ray absorption spectroscopy results for the S- and P-doped monolayer graphene also show significant changes when compared with pristine system owing to the shifts in the π* and σ* anti-bonding states. Our results provide useful information for tailoring the electronic structure and optical properties of monolayer graphene through S and P doping for its applications in contemporary nano-electronic technologies.

Graphene oxide/ZnO nanorods/graphene oxide sandwich structure: The origins and mechanisms of photoluminescence

In this paper, we present the structural and optical properties of Graphene oxide/ZnO nanorods/graphene oxide (GO/ZnO nanorods/GO) nanocomposites prepared via a hydrothermal method on Si (100) substrate. The X-ray diffraction measurements (XRD) confirm that the prepared samples are of hexagonal wurtzite structure with crystallite size around 50–60 nm. It was obvious from scanning electron microscopy (SEM) that by incorporating the ZnO nanorods between the inter-layer of GO confirmed the formation of sandwich-like nanocomposites structure. ZnO nanorods interaction with GO is displayed by the different vibrational frequencies in fourier transform infrared spectroscopy (FTIR). The UV–Vis spectrum reveals the strongest absorption was observed around 370 nm, while calculating optical band gap energy (Eg) of GO/ZnO NRs/GO was found to be 3.15 eV. The photoluminescence (PL) measurements indicates that the ZnO nanorods have a strong visible emission centered at 559 nm attributed to the presence of impurities in the form of oxygen vacancies After the nanorods were covered with GO layers, the PL intensity of the nanocomposite is quenched and shifted due to charge-transfer process. Consequently, the obtained results may lead to better performance for the optoelectronic applications.