The surface plasmon resonance (SPR) biosensor has become a powerful analytical tool for investigating biomolecular interactions. There are several methods to excite surface plasmon, such as coupling with prisms, fiber optics, grating, nanoparticles, etc. The challenge in developing this type of biosensor is to increase its sensitivity. In relation to this, graphene is one of the materials that is widely studied because of its unique properties. In several studies, this material has been proven theoretically and experimentally to increase the sensitivity of SPR. This paper discusses the current development of a graphene-based SPR biosensor for various excitation methods. The discussion begins with a discussion regarding the properties of graphene in general and its use in biosensors. Simulation and experimental results of several excitation methods are presented. Furthermore, the discussion regarding the SPR biosensor is expanded by providing a review regarding graphene-based Surface-Enhanced Raman Scattering (SERS) biosensor to provide an overview of the development of materials in the biosensor in the future. Graphene, the mother of all carbon materials, has opened a new era in technological development because of its unique properties. Graphene is a single layer of carbon atoms with a 2D hexagonal crystal lattice and is the thinnest and strongest material that has existed to date . VariousThe Surface Plasmon Resonance (SPR) biosensor is a type of biosensor which is very powerful for detecting and determining the specificity, affinity, and kinetic parameters of macromolecular bonds. The working principle of this biosensor uses metals such as gold (Au), silver (Ag), and aluminum (Al) to excite surface plasmon waves which are very sensitive to changes in the refractive index on their surface . VariousThe sensitivity of the SPR biosensor is simply defined as the ratio between the shift in wavelength or angle and the change in the refractive index on the sensing surface. This sensitivity is influenced by many factors depending on the excitation method chosen. For the prism coupled SPR biosensor, the sensitivity of the SPR is influenced by the type of prism, metal and wavelength used. Islam et al. reported that for the angle investigation mode, the SPR structure has the best sensitivity with a gold metal with a thickness of 50 nm . For theThe challenge in biosensor development is to detect analytes with small molecular weight and extremely diluted concentrations . At this2 bond in the crystal plane which will make GO non-conductive and has lower mechanical properties than graphene . In addition, the manufactured chips also have excellent reproducibility. Over the twenty-day time span, there was no significant change in the SPR signal. A sensor interface with a long lifetime was obtained in this experiment  composite that functioned with amines to detect and measure the dengue virus. Gold film is inserted into succinimidyl undecanoate (DSU) solution to form a self-assamble monolayer (SAM) which functions for chemisorption of biomolecules through amide relations. After SAM was formed, rGO-PAMAM was deposited on the gold surface using the spin coating method and followed by immobilization of specific antibodies for DENV 2 E-Protein via EDC/NHS using the same method. From this experimental scheme, the SPR chip formed is called the Au/DSU/rGO-PAMAM/Ab chip.2 is 0.92577. The selectivity of the SPR chip was also tested by detecting other types of antigens, namely DENV E-proteins and ZIKV E-proteins. Based on The prism-coupled SPR biosensor is a popular platform and is used in many detection cases. The preceding discussion describes several approaches that have been investigated to improve the performance of SPR biosensor. The propagation of light in optical fiber also works based on total internal reflection (TIR). The prism in the prism-based SPR biosensor functions to produce TIR on the metal–prism surface. For this reason, the prism can be replaced with a core from an optical fiber to form a fiber-optic-based SPR biosensor . The senSimulation of the effect of graphene on conventional optical fiber SPR biosensors carried out by Fu et al. in 2012. Fu et al. investigated the sensitivity changes in fiber clading with sensing medium N = 5 mm, gold thickness 40 nm, number of graphene layers N = 5, diameter of fiber cores. D = 50 micrometers, and the refractive index of fiber core and clading are 1.451 and 1.45, respectively. The complete experimental scheme is shown in Another research group, Zhou et al. investigated the effect of single layer graphene on SPR sensitivity in end reflection optical fibers with a coroless fiber diameter of 600 micrometers and a silver thickness of 40 nm a. The re3) to etch the Cu foil, placing the PMMA/graphene film on the sensing surface, heating the graphene and removing PMMA by immersing the sensing probe in acetone. The fabricated probes are then tested by detecting NaCl with different concentrations. At the same concentration change, the shift in the SPR wavelength for graphene-based optical fibers shows a higher shift  of the probe was immersed in acetone to soften the clading layer so that the clading layer on the sensing part could be removed easily. After that, the sensing part was coated with Ag film by chemical method and followed by transferring graphene on the optical fiber. Briefly, the transferring process begins by immersing the PMMA/graphene/Cu film in ferric chloride . Li et al. investigated an H-formed photonic crystal fiber SPR biosensor with U-shaped grooves open structure for refractive index sensing. PCF biosensors are composed of two layers of air holes in a hexagonal layout. The center air outlet can lower the effective RI from core guiding mode to in phase with plasmon mode. There are two large air holes in the first layer, leading to the phenomenon of strong birefringence and a highly polarized light connection with the metal dielectric interface. In this investigation, the designed structure has a gap between two air holes −4/°C) and its refractive index decreases with increasing temperature [2 is 0.993 which indicates the biosensor has good repeatability. From the slope obtained from the fitting process, the sensitivity of the MMF–SMF–MMF structure with graphene-gold-Au@Ag NPs-PDMS is −1.02 nm/°C [The last configuration is optical fiber with a nano-structured coating to produce the localized surface plasmon. One of the studies using this configuration was carried out by Huang et al. in 2019. Huang et al. used a multimode fiber-single mode–multimode (MMF–SMF–MMF) structure coated with a graphene-metal hybrid for temperature sensor. The sensor sensitivity was further enhanced by the addition of core-shell gold–silver nanoparticles (Au@AgNPs) modified with polydimethylsiloxane (PDMS) as a material for temperature sensing. PDMS is a material with a high thermo-optical coefficient  = 56 nm, gold thickness (h) = 10 nm, horizontal diameter (T2) = 46 nm, and the height of the sensing medium (Ds) = 600 nm. Based on the profile of the extinction curve at wavelengths from 1500 nm to 2500 nm, the peak extinction is at a wavelength of 2001.2 nm. The performance of the SPR-based grating biosensor is influenced by several factors, including the shape of the grating, the operating wavelength, the period of the grating, the type of metal, and the refractive index of the analyte . Sadeghi2 nm/RIU . Sadeghi2 nm/RIU .In the mid infrared range, noble metal exhibits large ohmic losses due to its low charge-carrier mobility and large permittivity . Wei et ectively . The mosectively ,78.In 2013, Reckinger et al. fabricated holey gold films with colloid nanosphere lithography with grating parameter of 980 nm and holes diameter of 405 nm for ethanol detection. The fabricated film was then confirmed based on a Scanning Electron Microscope image a. Graphe2 laser pulses. The dimensions of the sensing medium (L), fiber core diameter (D), and metal thickness (T) are shown in 2 of LPFG, LPFG + Ag, LPFG + Ag + graphene being 0.975, 0.995, and 0.995. The sensitivity and detection limits obtained were 0.116 nm /% and 0.086% for LPFG, 0.262 nm /% and 0.038% for LPFG + Ag, and 0.34 nm /% and 0.029% for LPFG. The presence of graphene on the silver surface of LPFG can increase sensor sensitivity by 1.31 times that of bare silver LPFG [Wei et al. investigated the effect of graphene on LPG to detect methane. Long period fiber grating (LPFG) with a grating period of 600 micrometers was fabricated by the writing method of high frequency COver LPFG .Although SPR biosensors coupled with gratings are pioneers in the field of SPR biosensors, experimental studies related to the use of graphene for this platform are still rare. Another type of SPR biosensor is the SPR biosensor which uses nanoparticles as a signal amplifier. Of the various types of nanoparticles, plasmonic nanoparticles are the most preferred because of their properties that can produce localized surface plasmon resonance (LSPR) which can increase local electromagnetic fields  and incrTaking advantage of the unique properties of gold, silver and graphene, Zhang et al. decorated gold or silver nanoparticles with graphene to detect mouse IgG. To be able to capture the IgG mouse, the hybrid nanoparticles are functionalized with goat anti human IgG. Next, solutions containing different concentrations of target human IgG are flushed onto the sensing surface to evaluate the performance and detection limits of the system. Of the three fabricated systems, Ag-graphene hybrid nanoparticles showed the best response compared to Au-graphene hybrid nanoparticles and unmodified biosensors a. The deChiu et al. proposed a colorimetric LSPR immunoassay based on the AuNPs-GO hybrid to detect disease biomarkers and rapidly diagnose infectious diseases. The binding of AuNPs-GO- anti-BSA to GO-BSA is detected by monitoring changes in optical absorbance at a specific wavelength. 2 , CO , and NO2 (1 ppm). The results obtained indicate that the sensor shows a good and reversible response with fast kinetics to H2 and NO2, while no response was detected to CO [The same structure was also applied to gas detection by Cittadini et al. They developed gas sensor based on LSPR using rGO coupled with a gold monolayer. Gold monolayer was fabricated by spinning gold colloids with a concentration of 15 and 30 mM at a speed of 3000 rpm for 30 s. The gold monolayer produced from gold colloids for a concentration of 15 mM was indicated by L (AuL) and for a concentration of 30 mM indicated by H (AuH). The two monolayers of gold are then coated with rGO by the spin method and produce a sensing layer called AuL-rGO and AuH-rGO. The sensing layer was then tested by exposing different gas types, namely Hed to CO . A summaIn 2003, Lakowicz et al. developed the SPR biosensor with a fluorescence technique called surface plasmon coupled emission (SPCE) . The worMulpur et al. engineered and investigated the SPCE properties of the graphene–silver thin film stack as a function of the graphene thickness. Different types of graphene, both single layer and multilayer, are fabricated by the CVD method compared to graphene which is fabricated by the chemical exfoliation method. The resulting graphene is then grown on a silver surface using the spin coating method. Flourescence measurements were carried out at a wavelength of 532 nm with a reverse Kretschmann (RK) configuration a. FigureNanocubes (NCs) have a special edge structure and tip effect that can induce strong localization and increased localized surface plasmons resonance fields. Therefore, Xie et al. performed an enhanced fluorescence signal using silver nanocubes (Ag NCs) and GO on a gold film. The SPCE signal is measured in the reverse kretschmann (RK) configuration at a wavelength of 532 nm a. The re2 are 0.01–800 ng/mL and 0.993 for SPCE (Au + GO) and 1–100 ng/mL and 0.926 for SPCE (Au), respectively. The detection limits for SPCE (Au + GO) and SPCE (Au) were 0.006 ng/mL and 0.15 ng/mL [Xie et al. amplified the SPCE signal in gold using GO and applied it to detect human IgG. The fluorescence signal from SPCE was measured with a Reverse Kretschmann (RK) configuration at a wavelength of 532 nm. The results obtained indicate that GO can increase the fluorescence intensity up to seven times that of the SPCE structure without GO (SPCE (Au)) or twenty-five times that of the free space emission (FSE). Furthermore, different human IgG concentrations were measured by SPCE (Au + GO) and SPCE (Au) and the fluorescence intensity at each concentration was then measured. The results obtained indicate that the linear relationship is shown in SPCE (Au + GO) with a wider range compared to SPCE (Au) . At semi15 ng/mL .The previous discussion describes the working principle of SPCE and the use of graphene for this platform of biosensor. Until now, graphene has not been widely applied to this platform. Raman spectroscopy is a vibration spectroscopy technique that measures the inelastic scattering of light. The resulting Raman spectrum provides a specific fingerprint regarding the molecular structure and material composition . In biosThe latest development in analyte detection which utilizes Raman signals is the surface-enhanced Raman scattering (SERS) technique. This technique, first introduced by Fleischmann, Hendra, and McQuillan in 1973 , was acc2/Si surfaces to suppress photoluminescence. Graphene is fabricated by mechanical oxfoliation from graphite. 2/Si. To determine the performance of the two substrates, R6G was deposited on the two substrates using a solution-soaking method. As shown in 2/Si substrate. In addition, the Raman signal in single layer graphene can still be detected clearly even though the R6G concentration is as low as 1 nM [Graphene has been shown to be a very outstanding SERS active substrate due to its advantages over other traditional materials. By utilizing its large surface area and superior adsorption capabilities, graphene can be utilized to extinguish photoluminescence of fluorescent dyes and drastically eliminate the fluorescence background . Xie et  as 1 nM .−1 can be clearly observed. More importantly, the SERS intensity at 1078 cm−1 increases linearly with the logarithm of the target DNA concentration in the range 10−6–10−12 M. The detection limit obtained from this experiment is 10−14 M [Graphene only supports chemical enhancement and does not support electromagnetic enhancement due to its smooth surface and high optical transmission over the visible range . To over 10−14 M . In addi 10−14 M .2+ sensors [The second metal type that is widely used as a SERS substrate is gold. One of the advantages of this material is its higher stability than Ag NP. Possible interactions between graphene and gold can occur in several ways including covalent bonds when graphene, gold, or both are functionalized , non-covractions , and vanractions . Until nractions , electroractions , or in sractions  and has ractions , multiplractions , or as H sensors ,123. TabGraphene and its derivatives have proven to be one of the best materials to enhance the performance of SPR biosensors in many platforms. The simulation study of the graphene-based SPR biosensor has been proven experimentally. To date, experimental studies have reported that the graphene-based SPR biosensor is capable of detecting analytes up to nM or ng/mL levels. Some researchers have also claimed that they were able to detect analyte to smaller levels (fM or fg/mL and aM or ag/mL). This shows that graphene is a superior material and is very promising in the development of SPR biosensors in the future.One of the challenges in developing graphene-based biosensors is to reduce noise in the detection process of very complex human samples such as in the form serum, plasma, urine, and stool. Modification of the sensing surface is needed to reduce noise so that the detection process is very selective and accurate. For the commercialization of graphene-based SPR biosensors, a simple SPR chip fabrication method, which can control the chip dimensions precisely and accurately in large quantities, is required. This is very important to ensure the resulting performance is as desired.


Fano resonances in nanostructures have attracted widespread research interests in the past few years for their potential applications in sensing, switching and nonlinear optics. In this paper, a mid-infrared Fano resonance in a hybrid metal-graphene metamaterial is studied. The hybrid metamaterial consists of a metallic grid enclosing with graphene nanodisks. The Fano resonance arises from the coupling of graphene and metallic plasmonic resonances and it is sharper than plasmonic resonances in pure graphene nanostructures. The resonance strength can be enhanced by increasing the number of graphene layers. The proposed metamaterial can be employed as a high-performance mid-infrared plasmonic sensor with an unprecedented sensitivity of about 7.93 μm/RIU and figure of merit (FOM) of about 158.7. In the past decade, the so called Fano resonance—a type of resonance originated from the constructive and destructive interference of a narrow discrete resonance with a broad spectral line or continuum—has attracted wide spread research interests in the nanophotonics community . Fano reTraditionally, plasmonic nanostructures are built on noble metals such as silver and gold and they work mainly in the visible and near-infrared (IR) ranges. Meanwhile, graphene has recently been rising as a building block for plasmonic devices in the mid- and far-IR ranges ,22,23. GIn this paper, a hybrid metal-graphene metamaterial with a Fano-like resonance in the mid-IR range is proposed. The Fano resonance arises from the coupling of the narrowband plasmonic resonance of the graphene nanostructure to the broadband resonance of the metallic structure. Its linewidth is narrower than that of the graphene plasmonic resonance and the resonance strength can be controlled by changing the number of graphene layers. Its potential as a mid-IR refractive index sensor is studied.For the metallic structure without graphene, the calculated spectra are shown in An effective way to manipulate the mid-IR resonance is controlling the Fermi energy of graphene. However, it is technically challenging to realize Fermi engergy higher than 1 eV. Another way to enhance the plasmonic responses of graphene is using stacked graphene instead of monolayer. Previous studies have shown that infrared plasmonic response of a graphene multi-layer stack is analogous to that of a highly doped single layer of graphene . For simThe sharp Fano resonance of our proposed hybrid graphene-metal metamaterial can increase the figure of merit  of sensors and is suitable for mid-IR plamsonic sensing. To evaluate its sensing ability, we calculate the transmission spectra of the hybrid metamaterial when it is covered by a semi-infinite layer with different values of refractive indices . As the 93μm/RIU b and FOMIn the above, we have shown that the hybrid graphene-metal metamaterial exhibits sharp Fano resonance in the mid-IR range and it can be employed for high-performance sensing. The discussed structure possess several drawbacks in practical realization. First, the graphene nanodisks are electrically isolated and this makes it difficult for electrostatic doping and dynamic modulation of Fermic energy (electrostatic doping with ion-gel is possible but the cover layer will affect its sensing applications). So we may need to employ chemically doped graphene. Secondly, we have assumed that the graphene film is located at the same height of the top surface of the metallic grid which is difficult to fabricate. In order to solve these problems, modified structures can be employed. As an example, we show a hybrid metal-graphene metamaterial where graphene is located directly on the surface of the substrate and the graphene is electrically connected a. Such aThe numerical simulations are conducted using a fully three-dimensional finite element technique . In simulations, the monolayer graphene sheet is modeled as a conductive surface ,45 and tc doping ,49.The substrate is assumed to be lossless with the refractive index In summary, a mid-infrared Fano-like resonance in a hybrid metal-graphene metamaterial has been studied. The Fano resonance arises from the coupling of the broadband resonance of the metallic nanostructure and the narrowband plasmonic resonance of the graphene nanostructure. It is sharper than plasmonic resonances in pure graphene nanostructures. The resonant strength can be effectively enhanced by increasing the layer numbers of graphene. The sensing properties of the proposed metamaterial are studied and it shows a sensitivity of about e mid-IR ,50 while


Using this method, WS2 growth was achieved at temperatures as low as 700 °C – significantly less than the temperature required for conventional CVD. Achieving long-range uniformity remains a challenge, but this process could provide a route to synthesize a broad range of TMD/graphene van der Waals heterostructures with novel properties and functionality not accessible by conventional CVD growth.Van der Waals heterostructures of monolayer transition metal dichalcogenides (TMDs) and graphene have attracted keen scientific interest due to the complementary properties of the materials, which have wide reaching technological applications. Direct growth of uniform, large area TMDs on graphene substrates by chemical vapor deposition (CVD) is limited by slow lateral growth rates, which result in a tendency for non-uniform multilayer growth. In this work, monolayer and few-layer WS Furthermore, owing to broken inversion symmetry, monolayer TMDs also possess strongly spin-split valence states, where the splitting exhibits opposite signs at K and K’ valleys enabling optically induced spin and valley polarization5. In recent years, research has turned towards creating heterostructures of TMDs and graphene due to the appeal of capitalizing on the complementary properties of the two materials7. For example, the exceptional carrier mobility of graphene complements the direct optical bandgap of monolayer TMDs such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2)8. In addition, spin polarization in TMDs is complemented by long spin lifetimes in graphene10. When combined in a heterostructure these two materials can deliver outstanding performance in a range of applications including optoelectronics13, spintronics and valleytronics16, sensing18, and energy storage20.Transition metal dichalcogenides (TMDs) are a family of two-dimensional (2D) materials that exhibit an interesting range of layer dependent properties. Semiconducting TMDs such as MoX2, WS2, MoSe2 and WSe2 at the wafer scale on substrates such as SiO2/Si and sapphire22. Despite these successes there are still some limitations in extending this process to synthesizing large area monolayers on van der Waals substrates such as graphene. Studies of CVD growth of TMDs on graphene typically report limited monolayer domain size before the onset of multilayer growth28. A fundamental limitation of CVD growth of TMDs on graphene compared to traditional substrates (i.e. SiO2/Si or sapphire) is the weak adsorption of the precursors on the surface, which results in slow lateral growth rates and a tendency for multilayers to form29. Specifically, the adsorption energy of chalcogen adatoms is approximately half that of metal adatoms, which in turn is more than an order of magnitude larger than the diffusion energy barrier. Thus, at elevated growth temperatures, the chalcogen-deficient environment and large diffusion of metal adatoms results in growth of multilayer pyramids29. The upper limit of monolayer domain size for TMDs grown on graphene by CVD is currently in the 10 μm range28. Therefore, there is impetus to develop alternative approaches to synthesizing TMDs on graphene substrates.To date, chemical vapor deposition (CVD) of TMDs has been used to successfully to produce monolayers of MoS2/Si32. This has the advantage of allowing a control of the layer thickness and morphology directly according to the thickness of the precursor. For instance, it has previously been demonstrated that the thickness of a metal precursor layer can be used to grow TMD layers with the basal plane oriented either horizontally or vertically with respect to the substrate33. In this work, we have produced WS2/graphene/SiC van der Waals heterostructures by depositing WO−x3 thin films directly onto epitaxial graphene/SiC substrates followed by sulfurization to produce WS2/graphene/SiC van der Waals heterostructures. We have utilized scanning tunneling microscopy (STM), atomic force microscopy (AFM), photoemission spectroscopy and Raman spectroscopy to investigate the surface morphology and composition of the WS2 layers and have presented the electronic band alignment of the van der Waals heterostructure.Previous studies have demonstrated direct sulfurization of the metal precursor as an effective route to producing large-area TMDs on SiO2 layers were grown following the procedure shown schematically in Fig. 2 growth. The sample was then characterized by XPS, AFM and Raman spectroscopy to determine the surface chemistry and morphology (data is presented in the Supporting Information). Figure −x3 layer thickness, which was found to be 10.8 nm  the silicon peaks disappear due to the thickness of the WO−x3 plus graphene layers exceeding the sampling depth of elastically scattered photoelectrons. Inspection of the high-resolution C 1 s core level indicates that the carbon content is mostly graphene with a small amount of adventitious carbon on the surface. As mentioned previously, quantification of the W 4f and O 1 s core levels points to a sub-stoichiometric oxide layer WO−x3 where x = 0.5 ± 0.3. After sulfurization of the WO−x3 layer the XPS survey (purple) shows that the oxygen is almost completely removed, and the carbon and silicon peak intensity re-emerge. These trends are indicative of partial evaporation of the oxide layer during sulfurization to produce a WS2 thickness of only a few layers as observed by STM (discussed later). It is noted that calculation of the number of WS2 layers from the W 4f and S 2p core level intensities is not straightforward due to the inhomogeneity of the number of layers and coverage of WS2.Evolution of the surface chemistry during the WS2 growth, different samples of WO−x3/graphene/SiC were sulfurized at different temperatures under otherwise identical conditions, and the chemical composition was monitored by XPS. Figure −x3 before sulfurization. The samples sulfurized at 800 °C and 700 °C contain a single chemical state with the 4f7/2 peak at approximately 32.7 eV with a spin–orbit peak separation of 2.1 eV corresponding to fully sulfurized WS2. There is a shift in the peak positions to higher binding energies as the sulfurization temperature increases. The binding energy of W 4f electrons in WS2 has previously been shown to be sensitive to the stoichiometry34, however the S:W ratios for the samples sulfurized at 800 °C and 700 °C are 2.1 ± 0.1 and 2.2 ± 0.3, respectively, and cannot be differentiated within the bounds of uncertainty. In addition to the WS2 contribution, a second component is present in the sample sulfurized at 650 °C at 35.5/37.6 eV which is a slightly lower binding energy compared to the WO−x3 reference spectrum.To determine the lower temperature limit for WS3/2 and S 2p1/2, respectively, and at 530.4 eV in the O 1 s core level. We therefore ascribe the peak to a tungsten oxysulfide compound (WOxSy)37. Based on the peak area ratios from the respective core levels, the composition of the WOxSy compound was determined to be xSy is believed to occur in the initial stages of WO3 sulfurization where the oxide is reduced followed by incorporation of sulfur to form a tungsten oxysulfide intermediate. Previous work has demonstrated that complete sulfurization of crystalline WO3 can occur at temperatures as low as 400°C37. The incomplete sulfurization observed in this work is likely due to the sulfur feedstock becoming fully depleted before the reaction is complete. It is possible that sulfurization using a H2S sulfur precursor may allow longer sulfurization times at lower temperatures37.Figure 2 layers. The surface morphology was studied by STM as shown in Fig. 2. Figure 2 in the areas outlined by dashed lines. It contains both triangular domains (bottom-right and top-right), as is common for TMDs grown by CVD, and a larger irregularly shaped domain (left). A different region of the sample is shown Fig. 2 multilayers highlighting the inhomogeneity of the WS2 morphology across the sample. Additional STM data and discussion on the assignment of the WS2 regions can be found in Figure S4, Supporting Information. A histogram of the step heights extracted from multiple images of the WS2/graphene/SiC surface , shown in Fig. 2, respectively39.Next, we present a detailed surface characterization of the sample sulfurized at 800 °C which produced the thinnest WS2, and multilayer WS2 are shown in Fig. 40. The atomically resolved image of monolayer WS2 in Fig. 2 as well as a moiré pattern with 10.28 ± 0.26 Å periodicity indicated by the blue unit cell. This corresponds to a (4 × 4) moiré structure of graphene, and the angle between the moiré pattern and WS2 lattice is 23.6°. Moiré superstructures are known to occur in TMDs grown on graphene: they are usually associated with small rotational misalignment between the TMD and graphene lattices28. For example, Ugeda et al. observed in MoSe2 grown on epitaxial graphene that a slight rotation of 3° of the MoSe2 monolayer with respect to graphene produces a 9.87 Å moiré superstructure, which accommodates four graphene and approximately three MoSe2 unit cells41. Unstrained graphene and WS2 are non-commensurate, however it has previously been shown that the residual strain in epitaxial graphene can improve commensurability between graphene and MoS223. It has been demonstrated for both MoS2 and WS2 that the TMD preferred orientation with respect to graphene is either 0° or 30° with fluctuations of a few degrees39. Chen et al. related the misalignment of the moiré lattice , an additional peak is required for a good fit to the experimental data. It is likely that this peak is required only to compensate for an imperfect background subtraction. It cannot correspond to residual WO−x3 since there is minimal oxygen left of the surface after sulfurization, and the high-resolution O 1 s core level shows only a single component at 531.9 eV corresponding to C-Ox species rather than metal oxide . W 4f. and S 2p core levels reveals the ratio of S:W to be 1.97 ± 0.05 indicating that the WS2 is stochiometric. Figure 2 synthesis. In both cases peaks corresponding to the SiC substrate, graphene layer and buffer layer are present in the same ratio indicating that graphene is unaffected during the growth process. The change in the envelope shape is caused by a slight shift of the sp2 C–C component by −0.1 eV after WS2 growth, and may be caused by band bending of the C 1 s core level in the heterostructure27. Notably there are no components corresponding to C-W or C-S bonds, which means the WS2 is a true van der Waals layer with no chemical bonds or strong chemical interaction at the interface.Figure 2 were probed by UPS and are shown in Fig. 2/graphene/SiC, and the blue line shows the region for the bare epitaxial graphene before WS2 synthesis. The spectra show the valence band edge of WS2 superimposed on the continuum of states down to the Fermi level contributed by the graphene layer. The position of VBM for WS2 is located at 1.38 eV below the Fermi level and is consistent with the observed VBM for sulfur-deficient edges in monolayer WS2 (1.35 eV)44. Previous reports have placed the VBM for WS2 on graphene/SiC between 1.13 eV and 1.84 eV below the Fermi level43. Differences in the VBM can arise due to the intrinsic charge density, but it is also noted that the VBM shifts depending on the number of layers46. Due to the spot size of the UV source it is likely that the measurement is sampling many WS2 domains which may have different numbers of layers.Valence states in WS48, noting that the band structure is not significantly perturbed after WS2 growth by CVD27. The position of the WS2 conduction band minimum (CBM) is approximately 0.4 eV above the Fermi level and is based on time- and angle-resolved photoemission spectroscopy (TR-ARPES) measurements of monolayer WS249. The CBM of TMDs is dominated by d-states of the transition metal atom sandwiched between chalcogen atoms and is therefore less sensitive to number of layers than the VBM. Based on this measurement, the resulting WS2 bandgap is approximately 1.78 eV and is in good agreement with the bandgap measured by scanning tunneling spectroscopy (STS) of 3L MoS2 which is known to have a very similar electronic band structure24.Based on the UPS measurement we propose the heterostructure band alignment shown in Fig. 2/graphene/SiC is shown in Fig. 2 synthesis presented in the top right. The 532 nm excitation used to acquire the spectrum is in resonance with the B exciton of WS2 which produces a series of combination modes in addition the in-plane and out-of-plane vibrational modes52. The A1g(G) peak at 419 cm−1 corresponds to an out-of-plane vibration of S atoms in the S-W-S stack. The large feature at ~ 350 cm−1 can be deconvolved into components corresponding to the E2g1(G) in-plane vibration of W and S atoms at 358 cm−1, the 2LA(M) overtone of longitudinal acoustic phonons at 354 cm−1, and E2g1(M) mode at 347 cm−1. In addition, peaks at 325 cm−1 and 298 cm−1 correspond to the 2LA(M)-E2g2(G) and 2LA(M)-2E2g2(G) combination modes, respectively. The A1g(G) and E2g1(G) peak separation has been established as a useful parameter in determining the number of WS2 layers51. In our case, the peak separation is 61 ± 2 cm−1 which is indicative of 1–3 WS2 layers and is consistent with local observations of the number of WS2 layers by STM.The Raman spectrum of WS−1, 1606 cm−1 and 2732 cm−1, respectively. The broad band at 2893 cm−1 arises due to photoluminescence (PL) from WS213. In comparison to the Raman spectrum for epitaxial graphene  has increased from 0.64 ± 0.03 to 0.95 ± 0.08. This result suggests that the WS2 growth process has induced some defects in the graphene layer. The G and 2D bands of graphene are also observed to shift after WS2 synthesis. The G band frequency becomes redshifted by −4.8 ± 1.1 cm−1, and the 2D peak may slightly blueshifted although the shift is within the bounds of uncertainty (−1). Noting that a laser power of < 3.5 mW was used to prevent local heating and strain induced by the laser, the origin of the peak shift is due to charge transfer between graphene and WS2. This is consistent with laser-induced hole-doping of the graphene layer since epitaxial graphene is n-type  exceeds the bandgap of WS2, it will generate electron–hole pairs which are then separated at the interface between graphene and WS2 layers54. Shifts in the G and 2D indicate that the electrons remain in WS2 and holes move into graphene thereby reducing the electron concentration in the graphene layer.The graphene Raman spectrum consists of three modes: D, G and 2D, which are positioned at 1374 cm2 layers at temperatures as low as 700 °C, with an optimal growth temperature of 800 °C, by utilizing sulfurization of a pre-deposited WO−x3 layer. At this temperature we were able to produce WS2 layers down to monolayer thickness. Though we have locally identified WS2 with a slight misalignment of 5.4° with respect to the graphene lattice, on a macroscopic scale there is no preferred orientation as determined by low-energy electron diffraction . This may be due to the reduced growth temperature compared to the CVD growth used in other studies where there is a preferred orientation.The main result of this study is the growth of WS3 thin films to grow WS2 layers on epitaxial graphene presents two advantages over conventional CVD: (1) Sufficient supply of the tungsten precursor is guaranteed during growth, allowing for higher coverages of thin WS2 layers rather than thicker multilayer structures; and (2) the metal oxide precursor no longer needs to be volatilized thus allowing lower growth temperatures compared to CVD. As a comparison to the sulfurization method we also used CVD only to grow WS2 from WO3 and sulfur powder precursors following the procedure outlined in the Supporting Information. In this case, in order to produce WS2 layers a growth temperature of 1100 °C was required . Typically WS2 synthesis by CVD is more challenging than MoS2 synthesis due to the difficulty in volatilizing the WO3 precursor, which requires temperatures of exceeding 900 °C56. In comparison to WS2 grown by thin film sulfurization which results in numerous grains of thin (1–3 layers) WS2, conventional CVD resulted in the formation of isolated, thick multilayer (> 10 layers) WS2 crystals.Direct sulfurization of WO29. This presents a fundamental limitation of the CVD process since the metal oxide precursor cannot be sufficiently isolated from the chalcogen source, and the supply is therefore limited due to sulfurization of the source material. This matter becomes a significant problem in extending the CVD procedure to synthesize TMDs beyond the commonly studied molybdenum and tungsten disulfides and diselenides due to the high melting points of other metal oxide or metal precursors57. Recently developed molten salt assisted approaches can also reduce the required growth temperature for WS259, and has been extended to other TMDs that are not easily synthesized by CVD57. We envisage that the thin-film sulfurization approach can similarly provide a versatile method of synthesizing a broad range of TMDs, heterostructures or alloys based on the choice of precursors30. This approach may also lend itself to TMD nanostructure fabrication by pre-patterning the metal oxide precursor prior to sulfurization60.Due to the low diffusion barrier of the metal presursors on graphene, and the low adsorption energy of chalcogen species, slow growth rates over long time periods are required to produce extended TMD monolayers on epitaxial graphene2 and graphene layers61. Such approaches can be used to engineer ohmic contacts between TMD and graphene layers and optimize their photoconductive properties11.Future experiments will look to further optimize and extend the procedure to fabricate other TMDs and heterostructures. Further considerations will also be given to the role of the substrate in the growth, and in tailoring electronic properties of the heterostructures. For example, by employing hydrogen intercalation to release the buffer layer from the SiC, the quasi-freestanding epitaxial graphene becomes p-type which in turn is expected to result in charge transfer and an interface dipole between WS2 and epitaxial graphene have been synthesized by sulfurization of WO−x3 thin films deposited directly onto the epitaxial graphene/SiC substrate. The WS2 layer has a measured thickness of 1–3 layers and individual grains exhibit many rotational orientations with respect to the underlying graphene. As an alternative to conventional growth which requires growth temperatures in excess of 900 °C, WO−x3 thin-film sulfurization was demonstrated to produce WS2 at temperatures as low as 700 °C with no residual oxide. Below this temperature WS2 coexists with an intermediate WOxSy compound. We anticipate that this process could be utilized to produce novel heterostructures of graphene with TMDs that cannot be synthesized by a conventional CVD method.Van der Waals heterostructures of WS−10 mbar). Samples were degassed at 600 °C until the pressure recovered to the 10−10 mbar range, before annealing at 950 °C under Si flux to remove contaminants. The Si flux was generated by electron beam evaporation  of a high purity Si rod (n-doped (P), ρ = 10–100 Ω cm, Hans Holm GmbH). For graphene growth, the samples were flash annealed to 1400–1500 °C while maintaining the Si flux. The sample temperature was measured using an optical pyrometer with an emissivity of 0.9. The temperature uncertainty is estimated to be ± 25 °C.Graphene was grown epitaxially on vicinal 4H-SiC(0001) substrates  on the epitaxial graphene substrates by electron-beam physical vapor deposition  with a deposition rate of 0.5 Ås−1. The substrates were kept at room temperature and the chamber pressure was < 4.0 × 10−6 Torr during deposition. For WS2 synthesis, the samples were loaded into the downstream zone of the quartz tube furnace, and sulfur powder  was placed 20 cm upstream from the sample in a separately heated zone. Before the heating ramps were applied the tube was pumped to 10 Torr and purged with 600 sccm Ar for one hour. The Ar carrier gas flow rate was reduced to 400 sccm during sulfurization. The WO−x3/graphene/SiC sample was first heated to 150 °C for 20 min to remove adsorbed water, followed by a 60 min ramp to the sulfurization temperature, TS, chosen in the range 650–800 °C. The sulfur powder was heated to 200 °C in a 15 min ramp, so that it reached the evaporation temperature at the same time as the sample reached the sulfurization temperature. The total sulfurization time was 20 min, which was sufficient to completely deplete the sulfur feedstock, and after sulfurization the furnace was allowed to cool naturally to room temperature.WS63. X-ray photoemission spectroscopy (XPS) measurements were taken using a Kratos Axis Supra system with a monochromatic Al Kα source (1486.7 eV). Survey spectra were collected at 160 eV pass energy and high-resolution core level spectra were collected with a pass energy of 20 eV. The binding energy scale was calibrated by a rigid shift of the spectra to align the Si 2p3/2 core level of SiC to 101.6 eV. Ultraviolet photoemission spectroscopy (UPS) measurements were acquired on the same system using the HeI emission line (21.22 eV) and 10 eV analyzer pass energy. The binding energy scale was calibrated by aligning the Fermi level of an electrically contacted Au foil to 0 eV. XPS and UPS data were analyzed using CasaXPS64. Raman spectra were acquired using a Renishaw inVia Raman microscope equipped with a frequency doubled NdYAG laser (λ = 532 nm). The laser was focused on the sample with a 50 × objective lens giving a spot size of ~ 1 μm, and the laser power was limited to < 3.5 mW to prevent laser induced heating of the sample.The sample morphology was characterized with atomic force microscopy (AFM) and scanning tunneling microscopy (STM). AFM images were acquired on an NT-MDT Solver microscope in tapping mode under ambient conditions. STM measurements were taken in UHV at room temperature using a VT-AFM/XA system (ScientaOmicron) with electrochemically etched W tips. The bias voltage was applied to the sample while the tip remained grounded. Before imaging, the samples were degassed at 400 °C for 30 min to desorb contaminants. Scanning probe microscopy data were analyzed using Gwyddion and SPIPSupplementary file1


In recent years, the field of nanophotonics has progressively developed. However, constant demand for the development of new light source still exists at the nanometric scale. Light emissions from graphene-based active materials can provide a leading platform for the development of two dimensional (2-D), flexible, thin, and robust light-emitting sources. The exceptional structure of Dirac’s electrons in graphene, massless fermions, and the linear dispersion relationship with ultra-wideband plasmon and tunable surface polarities allows numerous applications in optoelectronics and plasmonics. In this article, we present a comprehensive review of recent developments in graphene-based light-emitting devices. Light emissions from graphene-based devices have been evaluated with different aspects, such as thermal emission, electroluminescence, and plasmons assisted emission. Theoretical investigations, along with experimental demonstration in the development of graphene-based light-emitting devices, have also been reviewed and discussed. Moreover, the graphene-based light-emitting devices are also addressed from the perspective of future applications, such as optical modulators, optical interconnects, and optical sensing. Finally, this review provides a comprehensive discussion on current technological issues and challenges related to the potential applications of emerging graphene-based light-emitting devices. Graphene is a single layer honeycomb structure of carbon lattice  with manIn the context of light-emitting diode (LEDs), the existing LEDs technology is considered quite mature, even at the consumer end. LEDs have already been applied in several fields, including signage, display backlight, general illumination, and communications ,21. LEDsMoreover, the light emission from different graphene structures, such as single and multilayer graphene , reduced−6 part) is converted into light emissions [7 A.cm−2) in micron-sized CVD graphene as compared to conventional tungsten filament (~100 A.cm−2) and with low thermal mass three times smaller in the magnitude of silicon cantilever offers the prospect of high-frequency operation. Likewise, there is constant demand for the development of new IR sources with low cost, safe, and portable safe gas sensors, particularly for mine security. The existing IR sensors use conventional incandescent light sources with several limitations such as lifetime, wavelength, time response, excessive power consumption, and the requirement of explosion-proof casing in a flammable environment. MEMS-based electromechanical silicon emitters, as an alternative IR source, also exhibit low response time up to ~100 Hz modulation frequencies [Thermal emission from electrically driven graphene devices was also reported in the spectral region from infrared (IR) to visible range . Thermalmissions ,45. The quencies . Solid-squencies . The infquencies . HoweverThe realization of ultrafast plasmons-based optical signal source at the nanoscale is considered as a longstanding goal, the potential of the graphene-based emitter to revolutionize optoelectronics, thus allowing ultrafast optical signal processing for communication . When thGraphene can also produce the luminescence effect by inducing an energy bandgap. Therefore, there are two possible ways to induce bandgap in graphene: the first is by cutting it into ribbons or quantum dots, and the second is by chemical or physical treatments by connectivity reduction of the π-electron network . The eleIn this article, we have extensively reviewed the recent progress in graphene-based light-emitting devices, including device structure, fabrication, and their optical and electronics properties related to device structure, material, and emission mechanism. The manuscript is subdivided into four sections based on the light emission mechanism from graphene-based light-emitting devices, where the light emissions are ascribed to thermal, electroluminescent (EL), and plasmons assisted emissions. We have also highlighted the future applications of graphene-based light emitters, such as a broadband light source for optical modulators, optical interconnect, and broadband spectrum light source for gas detection. This article also discussed the current technological issues and challenges related to the recent development of graphene-based light emitters.9 A/cm2 in nanoribbons) [7 A/cm2 reported in micron-sized graphene fibber synthesized by chemical vapor deposition (CVD) [2) tungsten filament light bulb, the graphene is as an attractive potential material for the development of incandescent light source [The high carrier mobility ,65,66,67ribbons) , particuon (CVD) , with hion (CVD) . When cot source . Besidest source . Besidest source . Signifit source ,77,78−6—was transformed into emission radiation and detected with the spectral resolution near-infrared region. Graphene shows high thermal conductivity at room temperature  [Characteristics of emissions and performance of light-emitting devices are strongly influenced by their thermal behavior, whereas distinct features of the biased graphene layer, such as spatial resolved thermal emissions, high carrier densities, and spatially located direct points can be utilized for extraction of temperature-dependent distributions. Besides, the electrically derived graphene layer depicts the gate-dependent location of a hot spot. Though the fixed location of temperature maxima was also reported, the infrared thermal emission from graphene provides an appropriate and noninvasive characterization tool for graphene-based devices. Marcus Freitag et al. reported spatial resolution thermal emission radiation from the graphene-based transistor, which can be utilized to obtain the carrier densities, temperature distribution, and spatially resolved location of the Dirac point in the graphene layer. Moreover, the applied gate voltage can also be utilized to control the temperature maxima and the corresponding location of the stationary hot spot, where the thermal emissions in the infrared region depict a non-invasive and convenient characteristic tool for graphene-based devices . To analm−1 K−1) , where t2 substrate [T is the temperature, hv the photon energy, and u is the density of spectral energy. The equivalent drain to source currents and voltage are shown in It has also been reported that the Umklapp scattering reduces heat dissipation at higher temperatures , and theubstrate . Moreove2/Si substrate, for the realization of device fabrication. The experimental setup for light emission from suspended graphene (trench-based structure) on SiO2/Si substrate under the high electric field is shown in −4 Torr). Therefore, the significant reduction in heat transport from suspended graphene structure was observed [DS exceeds the threshold value, the suspended graphene begins to emit light. The light-emitting area and brightness were observed to be propositional with VDS. The light-emitting spot was observed in the middle of the suspended graphene, and it overlaps with the position of maximum temperature. Moreover, reproducibility and stability of light emission have been experimented under vacuumed conditions. Besides, the light emission spectra with several peaks were observed from suspended graphene  ranging from 1.2 to 3 eV as depicted in Furthermore, the bright and visible light emission from a suspended-like graphene structure was also reported by Y.D. Kim and coworkers. The mechanically exfoliated graphene, low pressure, and plasma-assisted CVD grown single-layered graphene (on Cu foil) were deposited on a SiOobserved , where hAccording to Equation (2), D ≈ 1,000 nm for Δ ≈ 0.6 eV agrees with results c,d. The Moreover, the electrically biased layered graphene facilitates the accumulation of spatially localized hot electrons , which makes graphene an ideal material for the application of light-emitting device at the nanoscale. In addition, the tunability of broad-spectrum emission can be attained by manipulating the strong interference effect; particularly, the interference effect in an ultra-thin and flat layer of graphene could allow the realization of unique flexible, thin, transparent and large-scale display modules . In the 2/Si substrate. The large area graphene was defined by using electron beam lithography and Ar/O2 reactive ion etching, where the source and drain contacts were deposited by utilizing Au/Cr thermal evaporation, as presented in Thermal emission from large-area, single, and multilayered graphene, coupled with extraordinary conductivity, has been highly demanding for the development of nano-scaled high-frequency infrared sources. In this context, Lawton and co-workers have also investigated the thermal infrared emission from large-area CVD graphene. The device was fabricated by transferring the single and multi-layered graphene on a highly doped SiOHowever, in the case of a single layer, graphene emission is found to be more consistent with channel resistance and exposed to the whole area. The light emission from the hotspot is proportional to the square of the applied current, which indicates joule heating. Specifically, the thermal temperature of light-emitting graphene has been well-defined by the duration of the current pulse and dimension of the graphene channel. In the experimental description, the emission spectral was calibrated with a black body source at 673 K as shown in 2 (285 nm)/Si substrate by transferring the exfoliated monolayer graphene and hBN flakes. Followed by etching of the graphene-based heterostructure, the Cr/Pd/Au metal was deposited for the realization of source and drain contacts, as depicted in 8 A/cm2, a high electric field up to ~6.6 V/μm, and with zero gate voltage.Moreover, the graphene-based thermal emitter with large area characteristics depicts a relatively low modulation speed of 100 kHz as demonstrated under steady-state conditions . Young an = 2.2). The dielectric cavity formed by the formation of the hBN layer on a substrate, which can be utilized to tailor the thermal radiation by modification of the optical density of states [Additionally, remarkable stability has also been reported from encapsulation (with hBN) edges contact even under high current density, high electric field, and temperature. The light emission spectrum under vacuum conditions and in air is shown in f states . From thap both in hBN and graphene is almost the same but below the electronic temperature. 8 A/cm2), electric field (F = 4.2 V/μm), and in vacuum space (~10−5 Torr). The persistent performance of the device without any substantial degradation has been reported for ~106 s . Likewise, the reduction in emissivity (absorption) has been observed in the near-infrared regime, which was ascribed to the Pauli blocking. The current saturation was also reported under the high electric field for the hBN encapsulated graphene . The cur06 s see f, where 2. The conductivity of graphene can be modeled as a two-dimensional ohmic sheet as in Equation (4) [µc and Γ are chemical potentials and carrier scattering with the graphene layer. Furthermore, the first emission wavelength band was used as a reference, whereas the second wavelength band resonates with related absorption of the targeted gas being detected. The frequency and wavelength of the emission radiation correspond to the size of the circle resonators. The resonance frequency of the corresponding emission can be calculated by Equation (5):n is the mode number, c is the velocity of light in vacuum, and r is the averaged value for the outer and inner radius of the resonating ring, S-parameter calculations. The thermal energy radiation from the encapsulated graphene was observed upon the joule heating effect, whereas the embedded ring resonators on the frequency selective metamaterial surface resonate in phase with a resonant frequency. Moreover, enhanced emission radiation was observed due to the constructive interference of resonators rings with thermal radiation. Specifically, the destructive interference at other frequencies due to the out-of-phase resonance will reduce thermal emissions. For the direct comparison of thermal emission from graphene with and without metamaterial, the fabrication process was divided into four quadrants (250 μm × 250 μm), depicted in It has also been reported that the emission spectra can also be manipulated by the using sub-wavelength photonic crystals, where the enhancement in the narrowband thermal emission from Tungsten filament was achieved by the resonant coupling modes of 2D crystals ,100. Bestion (4) ,103:(4)σ2O3 insulator using ALD (atomic layer deposition) to inhibit the oxidation of the graphene layer. The conductance of the capped graphene layer is reduced to 50% because of the scattering of charged impurity and the scattering of surface polar phonons [DS = 8 V was observed. The time-resolved emissions depend on the amplitude and width of the input rectangle pulse. It shows the quick response time of ~0.4 ns, which dominates the signal generator with response time ~0.5 ns, as shown in 2O3 can operate for more than 100 h [The integration of conventional light-emitting sources for the semiconductor-based integrated system is a significant challenge due to their exertion of direct growth on the silicon-based substrate . Yusuke  phonons . The cor phonons  and maxian 100 h . Finally14–1015 Hz) light-emitting sources with relativity low energy electrons [There is increased interest on highly integrated optoelectronic devices with surface plasmons polarities and nanoscale light emitters . In recelectrons . Additiolectrons . Besides, the tunable, monochromatic, and highly directional light emission from the graphene layer with the interaction of electrons and plasmons has been reported by Liang and colleagues . The sch6 ms–1) is possible, even in graphene sheets larger than 10 μm large [In related research, Pavel A. Cherenkov showed light radiation from charged particles when the charge moves faster than the phase velocity of light in the medium. However, the requirement of relativistic partials makes light emission unreachable at nanoscale optoelectronic devices . Notablyμm large . Howeverμm large ,120. Theμm large ,121. F, which exceed the energy level of photon emission and enable plasmons emission from terahertz to infrared and possibly invisible emissions spectra. Likewise, the tunability of energy levels from the implication of external electrical excitations can improve radiation parameters such as direction, spectrum, and intensity. More significantly, the emission radiation behavior of graphene-based material such as high-frequency radians [Additionally, the surface plasmons create energy levels higher than 2E radians ,64,122,  radians ,123, bla radians , and tun radians  can be e radians ,127. In z-axis, which is in the direction of the hot carrier motion [Moreover, the coupling of plasmons and charge carriers inside the graphene layer enables the highly efficient two-dimensional Cerenkov emission, where versatile, tunable, and ultrafast conversion from electrical signal source can be used to overcome the limitations. r motion .According to the classical approximation, the charge carriers outside the graphene layer satisfy the actly as . Howeveractly as . The CE 2 (~300 nm) substrate was explained by the CˇE effect. The phenomenon of plasmonic coupled emission from GFET is further investigated by using spectroscopic technique and imaging. The light emission from the electrically excited graphene layer was observed as a yellow-orange spot located near the edge of the graphene channel near Au electrodes (see DS and VG), similar behavior from each device was observed, which implies that both charge carriers and current directions can be utilized for light excitations. Exceptionally, the physical location of the light-emitting spot was highly influenced by the scattering sites, nanoparticle, edges, and formation defects. However, it was also demonstrated in optical microscope color images of electrically excited devices that the position of light-emitting spot remained independent of the applied variation of VDS and VG. The SEM image as shown in Several theoretical studies reveal that visible and short-wavelength emission spectrum can be attained through the graphene layer by the interaction of surface plasmons and charged particle . Howeverodes see a. Likewiodes see b. Under Graphene has attracted plenty of attention from the perspective of optoelectronic applications due to its amazing optical and electronic properties, such as massless fermion, direct cons, tunable fermions, and nearly flat absorption ,130,131.B; the experiment setup is illustrated in Only a few studies have been explored EL emissions from graphene, where the EL measurement was either limited to the few-layered graphene or nano-scale photonic crystals . In relamissions . MoreoveU tip material, which raises the P-doping level, and secondly, the bias voltage VB, which can also induce the capacitive effect. These two possible doping effects can induce a bandgap from ~0.5 to 0.65 eV [F, as shown in F, then electrons lose their energy before the process of radiative decaying. The output radiation spectrum for single-layer graphene is shown in In addition, the change in the Fermi energy of the graphene layer can be due to two possible reasons: first, the high work function of the A 0.65 eV . The ind2V–1S1 that supports the carrier injection. Moreover, the emission spectrum can be controlled by the doping level or by the gating level from blue to red −2, but short lifetime and low efficiency ~1% [The established LEDs-based technology is the key component of today’s solid-state semiconductor industry , where ttum dots . The chaency ~1% .max/Imin < 2). The EL spectral can be tuned from light blue to dark red emission spectrum by the adjustment of the Fermi level, where the implication of side gate voltage was used to tune the Fermi level. Besides, the ambient doping can also be utilized for the tuning EL spectra. The variation in EL spectral under different gate voltage values is demonstrated in G = 0) to 470 nm (VG = 50 V) were observed. The temperature-dependent charge transportation was also analyzed by plotting drain-source current versus gate voltage (I-V), as shown in 1/2 indicates an agreement with Poole Frankel’s current law [Moreover, the light emission intensity as a function of the drain to source bias is shown in rent law . The Poorent law . Figure rent law . However7 photon/s (~1 pW at 30 nA). Moreover, the intensity of light emission from GNRs was found to be 100 times higher than that of the plasmonic emission reported from a single-molecule optoelectronic device [Several theoretical and experimental studies have been explored on the related energy bandgap of GNRs (graphene nanoribbons) with design configurations ,155, whec device .n/(dt dEd) (photons/second) and electron-volt were calculated from intensity I (count per second) as in Equation (6): λ is the bandwidth, E is the energy in eV, c the speed of light, s is the sensitivity in the diffraction mode, and h is the Planck constant in eV·s. Besides, the emission spectral curve can be deconvoluted into two Gaussians curves with center at ~1.35 and ~1.76 eV, with a full width at half-maximum (FWHM) of 0.28 and 0.24 eV, respectively. Moreover, a slight change in FWHM, and the value of peak energy was reported under an applied voltage. However, the exponential increment in emission intensity was also observed with the increment of bias voltage, as shown in E/2 ≈ 0.2 eV, and hence most likely can provide the phonon involved in the emission process. In contrast, the TO branch with ~0.17 eV at a wave vector 0.5|ΓK| is slightly lower in energy [F with respect to the charge neutrality point E0. At EF = E0, the EPC for the LO and TO phonons is large only at the Γ and K points, respectively [Essig and coworker reported phonon-assisted electroluminescence from a few-layer graphene (FLG) structure and metallic SWNT. The visible spectrum with peaks at 1.4 and 1.8 eV was reported from SWNT (diameter range of 0.7 to 1.5 nm); the comparable response was found in multiwall carbon nanotubes. The light emission spectra from SWNT and their evolution with applied voltage is illustrated in n energy . The eleectively . Lastly,Much interesting information has been put forward from recent progress in the field of graphene-based light emitters. Light emission has been reported from the different structures of graphene and related material, such as single/multi-layer graphene, reduced graphene oxide, and graphene-based heterostructures. Graphene light emitters may open the door to the development of mid-infrared to far-infrared light sources for gas sensing and infrared photodetection. The performance of light-emitting devices is highly influenced by different design parameters, such as ohmic contact, device, and material structure. The light emission radiations from graphene structures are explained in various theoretical aspects, including thermal emission, plasmons assisted emission, and electroluminescence, and have been extensively discussed. The wide range of emission spectrum has been demonstrated for visible to infrared wavelengths. Thermal emission from graphene follows a graybody/black body radiation curve, following Plank’s Law. Based on the review, none of the mechanisms can exactly explain the entire features of light emissions from graphene and related structures. G or VDS cannot control the position of the light emission spot. Thus, the controlled position of the light emission point in the graphene channel is missing under applied voltage conditions; it excludes the presence of thermal emissions. Certain facts have also been extracted. Firstly, ionization/excitation is a conceivable mechanism to explain light emission from the graphene-based structure. The negative temperature coefficient is always accompanied by impact ionization, but the reported positive coefficient rules out the presence of impact ionization. Secondly, the EL emission peaks from graphene correspond to the local temperature , if emission radiation is due to the Joule heating effect; this will nullify the light emission mechanism by thermal emission. In addition, in the electron tunneling effect, the charge carrier injection mechanism is responsible for the EL effect in the planner device, and light emissions are likely to occur at the drain terminal, which holds a wide bandgap. However, light emissions from the middle part of the graphene channel have also been addressed. The light emission from graphene supported on substrate localized at the center of the graphene channel and the position of the localized hot spot can be controlled by the source to drain or applied gate voltage. Some reports also addressed the fact that the emissions do not occur from the center of the graphene channel, and the V2O3) has shown an extended emission lifetime up to 1000 h. Further advancements in graphene-based light emitters can pave the way for highly efficient, flexible, transparent, compact, spectrally tunable light sources for future commercial applications. Graphene has been the subject of intense research on optical and electronic applications. Therefore, numerous future challenges for the realization of graphene-based light emitters at a commercial level have been discussed. The maximum efficiency < 1% and short emission times from the suspended and substrate support graphene layer < 2 and 20 min, respectively, in vacuumed conditions are reported. However, in several other reports, the coated graphene layer with hexagonal boron nitride (h-BN) and aluminum trioxide (AlThe graphene-based active layer exhibits different optical, electrical, and chemical behavior. Oxidation of the graphene layer highly influences the efficiency, emission spectra, and lifetime of the graphene light-emitting device. The protective coating of the light-emitting graphene thin film possibly stops structural damage.By introducing an intersystem crossing path, singlet excitation could be achieved. The light source with an efficiency upper limit of up to 25% can be realized.Graphene-based doped material and nanocomposite material have not been investigated for light-emitting devices. As a possible solution, the doped graphene and graphene-based composite materials with numerous polymers could be used to tune the optical bandgap and surface plasmonic effect as well as thermal stability in graphene layer for optical emissions. Some critical issues in this emerging field of graphene-based light emitters, including short plasmons lifetime, instability of the induced bandgap, oxidation of the light active layer, low efficiency, and short lifetime, can also be overcome.Future demands for flexible optical and electronics require deformable and flexible light-emitting devices. Hence, future studies should be focused on the development of flexible graphene-based materials for optoelectronic devices.In order to improve the stability and performance of future graphene-based light emitters, the following aspects should be considered:


Immune checkpoint inhibitors have great potential for the treatment of gliomas; however, their therapeutic efficacy has been partially limited by their inability to efficiently cross the blood–brain barrier (BBB). The objective of this study was to evaluate the capability of focused-ultrasound-mediated intranasal brain drug delivery (FUSIN) in achieving the locally enhanced delivery of anti-programmed cell death-ligand 1 antibody (aPD-L1) to the brain. Both non-tumor mice and mice transcranially implanted with GL261 glioma cells at the brainstem were used in this study. aPD-L1 was labeled with a near-infrared fluorescence dye (IRDye 800CW) and administered to mice through the nasal route to the brain, followed by focused ultrasound sonication in the presence of systemically injected microbubbles. FUSIN enhanced the accumulation of aPD-L1 at the FUS-targeted brainstem by an average of 4.03- and 3.74-fold compared with intranasal (IN) administration alone in the non-tumor mice and glioma mice, respectively. Immunohistochemistry staining found that aPD-L1 was mainly located within the perivascular spaces after IN delivery, while FUSIN further enhanced the penetration depth and delivery efficiency of aPD-L1 to the brain parenchyma. The delivered aPD-L1 was found to be colocalized with the tumor cells after FUSIN delivery to the brainstem glioma. These findings suggest that FUSIN is a promising technique to enhance the delivery of immune checkpoint inhibitors to gliomas. Immune checkpoint inhibitor (ICI) therapy has revolutionized the field of cancer immunotherapy. Under normal physiological conditions, immune checkpoints are crucial to maintaining immune tolerance. However, in the tumor environment, tumor cells hijack these inhibitory mechanisms to avoid antitumor immune responses. ICIs are monoclonal antibodies that disrupt the engagement of immune checkpoints, which enables tumor-reactive T cells to overcome inhibitory mechanisms and mount effective antitumor immune responses . PD-1 isclinicaltrial.gov; NCT03233152), allowing direct delivery of ICIs to the tumor. However, this delivery strategy is invasive, and the penetration depth of the delivered ICIs into the surrounding brain tissue is limited [Several strategies have been proposed to enhance the delivery of ICIs across the BBB. One strategy modified the aPD-L1 by attaching BBB targeting moieties on the antibody, which facilitated the trans-BBB delivery of systemically injected aPD-L1 by dopamine-receptor-mediated transcytosis . Another limited .Focused ultrasound (FUS)-mediated intranasal delivery (FUSIN) harvests the unique advantages of intranasal (IN) administration for direct nose-to-brain drug administration with minimized systemic exposure and of FUS in combination with microbubbles for targeted and enhanced delivery to the brain ,10,11,12The objective of this study was to demonstrate the potential of FUSIN in the noninvasive and targeted delivery of aPD-L1 to an intracranial murine glioma model. A near-infrared fluorescent dye (IRDye 800CW) was conjugated to aPD-L1 (800CW-aPD-L1) for direct visualization of the spatial distribution of aPD-L1 in ex vivo brain. The IRDye 800CW has low autofluorescence background, making it an attractive option for antibody labeling. FUSIN delivery of 800CW-aPD-L1 was first evaluated in non-tumor mice, followed by studies using mice with orthotopic implantation of murine glioma cells (GL261) at the brainstem. Treatment of the brainstem glioma is extremely challenging. The brainstem connects the brain to the spinal cord and contains densely packed ascending and descending tracts and nuclei carrying information to and from the brain ,17. The A280) and 780 nm (A780) with a UV spectrophotometer . The conjugate was diluted in a mixture of PBS and methanol (1:1), and the following formula was used for calculating the dye/protein (D/P in mol/mol) ratio,Dyeε and Proteinε are molar extinction coefficients for the dye and protein, respectively. Dyeε is 270,000 M−1 cm−1 and Proteinε is 203,000 M−1 cm−1 for typical immunoglobulin G antibodies (IgG) in a 1:1 mixture of PBS/methanol [Monoclonal aPD-L1 was obtained from Bio X Cell  and diluted to 2.4 mg/mL when ready to use. IRDye 800CW with NHS ester was purchased from LI-COR Biosciences . IRDye 800CW was conjugated to aPD-L1 following the manufacturer’s protocol for labeling high-molecular-weight proteins (LI-COR Biosciences). In brief, aPD-L1 was brought to pH 8.5 with 1 M potassium phosphate (pH 9) and mixed with the dye in a 3:1 dye/aPD-L1 mol/mol ratio. The antibody/dye solution was protected from light and incubated for 2 h at room temperature. The labeled aPD-L1 was purified using a desalting column  pre-equilibrated with 1× PBS. The conjugated aPD-L1 was collected in the 1× PBS and kept at 4 °C until further use. The dye-to-protein ratio of the conjugate was determined according to the manufacturer’s protocol (LI-COR Biosciences) by measuring the absorbance of 800CW-aPD-L1 at 280 nm . The gel was imaged with the Licor Pearl small animal imaging system (LI-COR Biosciences) using the 800 nm channel, then stained with Coomassie blue overnight and imaged with a ChemiDoc™ MP Imaging System  using white light.Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) was performed to confirm the conjugation of the aPD-L1 with IRDye 800CW. aPD-L1 and 800CW-aPD-L1 (10 μg of each) were boiled and reduced with 4× Laemmli sample buffer  containing β-mercaptoethanol. Electrophoresis was performed on 4–15% Mini-PROTEAN5) were collected and suspended with 100 μL FACS buffer (0.5% BSA + 2 mM EDTA/PBS). Cells were stained with 1 µg aPD-L1 and 800CW-aPD-L1 at 4 °C for 30 min. Cells without aPD-L1 staining were used as control. After that, all cells were then incubated with 1 µg Alexa Fluor® 647 conjugated anti-rat IgG  at 4 °C for 30 min. After incubation, cells were washed and resuspended with FACS buffer for measurement with flow cytometry. Data were acquired using MACSQuant  and analyzed using Flowjo software .To evaluate whether 800CW labeling affected the capability of aPD-L1 binding to PD-L1, GL261 cells (which had high surface PD-L1 expression ) were usAll animal procedures were reviewed and approved by the Institutional Animal Care and Use Committee in accordance with the National Institutes of Health guidelines for animal research . Cr. NIH Swiss mice  were purchased from Charles River Laboratory . The animals were housed in a room maintained at 22 °C and 55% relative humidity, with a 12-h/12-h light/dark cycle and access to standard laboratory chow and water. Both mice without and with glioma implantation were used in this study.2 incubator at 37 °C. Mice were anesthetized and their heads fixed on a stereotactic frame. A paramedian incision was made on the scalp, and a 1-mm burr hole was drilled 0.8 mm posterior and 1.0 mm lateral to the lambda. GL261-eGFP cells  were injected through the burr hole using a syringe. The burr hole was sealed with bone wax, and the skin incision was glued together with tissue glue. Post-surgery analgesia was provided by subcutaneous injection of buprenorphine  for 3 days. The FUSIN (n = 5) and IN (n = 5) delivery of 800CW-aPD-L1 to the glioma mice was conducted on day 14 post-implantation.For the glioma mice, the mouse glioma cell line GL261 with the expression of the enhanced green fluorescent protein (GL261-eGFP) was obtained from Dr. Dinesh Thotala  and cultured in Dulbecco’s Modified Eagle Medium (DMEM) with Nutrient Mixture F-12 1:1, 10% fetal bovine serum, and 1% sodium pyruvate  in a 5% CO8 #/mL; injection volume: 30 μL) manufactured in-house [Both non-tumor mice and glioma mice were used for FUSIN delivery following a similar procedure, as reported before . Brieflyin-house  were injin-house . Mice weAfter perfusion, the brains were excised and sliced into 2-mm coronal sections using a brain matrix  and examined by the Licor Pearl small animal imaging system, with acquisition using the 800 nm channel for 800CW-aPD-L1. The exposure time for fluorescence imaging was kept the same (30 s) for all groups. The fluorescence intensity of the brain slices was quantified using Licor’s Image Studio Lite software.n = 5) or FUSIN (n = 5) to verify the fluorescence quantification by the Licor Pearl system. After imaging by the Licor Pearl system, the mouse brainstem tissues were collected, weighed, and homogenized by adding 1× RIPA buffer at 10× tissue weight. The homogenized tissues were then centrifuged at 5000× g for 10 min at 4 °C and the supernatant was collected. The protein concentration of the supernatant was measured with BCA assay and diluted to 1 mg/mL using dilution buffer (PBS+/0.5% BSA+0.05% Tween-20). The ELISA standard was made using the aPD-L1 stock to generate a serial dilution in the dilution buffer. The ELISA plates  were coated with recombinant mouse PD-L1 protein  at 4 °C overnight. The plates were then washed with 300 µL washing buffer (PBS+0.05% Tween-20) three times and blocked with blocking buffer (PBS+2% BSA+0.05% Tween-20) for 1.5 h at room temperature. The tissue supernatant and the aPD-L1 standards were added to the pre-coated plate and incubated overnight at 4 °C, and the polyclonal horseradish peroxidase (HRP)-conjugated rabbit anti-rat antibodies  were then added and incubated for 1.5 h at room temperature. After the incubation, the plates were washed and added with 90 µL TBM substrates for 10 min, followed by 50 µL stop buffer. Data was acquired with SpectraMax® i3 Platform . The concentration of 800CW-aPD-L1 in brainstem tissue samples was calculated in reference to the 800CW-aPD-L1 standard curve.ELISA quantification of the 800CW-aPD-L1 delivery was performed for the non-tumor mice treated by IN only   solution overnight at 4 °C, washed with PBS, and then incubated in Alexa Fluor 594 AffiniPure donkey anti-rat IgG (H+L)  overnight at 4 °C for staining aPD-L1. After that, the slices were added with Alexa Fluor 488 donkey anti-rabbit IgG  and DyLight 649-labeled tomato lectin  and incubated in the dark for 2 h at room temperature. Finally, the slices were mounted with VECTASHIELD antifade mounting media (Vector Laboratories). For the tumor mice, the slices were only stained with Alexa Fluor 594 AffiniPure donkey anti-rat IgG (H+L), following the same procedure mentioned above.In order to further study the microscopic level distributions of 800CW-aPD-L1 after FUSIN, the mouse brainstem after FUSIN was harvested and stained for the aquaporin 4 (AQP4), the blood vessel, and the aPD-L1. AQP4 is the most prevalent aquaporin channel, specifically located at the perimicrovessel astrocytic endfeet and aligned with the perivascular spaces. The brainstems of tumor mice were also harvested and stained for aPD-L1 to image the spatial distribution of the delivered aPD-L1. All the brain samples were fixed in 4% paraformaldehyde (PFA) overnight and equilibrated in 30% sucrose for cryosectioning. The fixed brainstems were sectioned into 20 µm slices using a Leica CM3050 S cryostat (Leica Biosystems). The slices were preprocessed in 0.3% t-test. A p-value < 0.05 was used to determine statistical significance.Statistical analysis was performed using GraphPad Prism . Differences between the two groups (IN vs. FUSIN) were determined using an unpaired two-tailed Student’s The 800CW-aPD-L1 conjugation A had an p < 0.05). Moreover, the ELISA quantification of the same brainstem slices found a strong linear correlation (R2 = 0.9195) between the concentration of the 800CW-aPD-L1 and the fluorescence intensity  ( < 0.01) C.The efficient delivery of ICIs is critical for ICI therapy of the glioma. Although the glioma immune microenvironment is very complex, it has been demonstrated that the treatment efficacy of mice with gliomas could be significantly increased by enhancing the delivery of ICIs across the BBB ,6. The eThe mechanism of FUSIN delivery is still under investigation. The olfactory nerve and trigeminal nerve ending in the nose cavity provide a route for direct medication delivery into the brain in nose-to-brain delivery. Bulk transport along the channels surrounding the olfactory and trigeminal nerves is the most likely mechanism for nose-to-brain transport, based on the rapid speed of transport from the nose to the brain . Once inIn this study, aPD-L1 was fluorescently labeled by IRDye 800CW and used to evaluate the potential of FUSIN in ICI delivery to both non-tumor mice and glioma mice. The labeling did not alter the binding affinity of aPD-L1 with PD-L1 protein and enabled the quantification of the spatial distribution of aPD-L1 delivery by fluorescence imaging. FUSIN achieved significantly enhanced delivery of aPD-L1 to the brainstem of non-tumor mice and brainstem glioma in tumor mice. The delivered aPD-L1 penetrated deep into the brain parenchyma at the FUSIN-targeted brainstem with high local accumulation. The delivered aPD-L1 was also found to bind to the tumor cells when delivered to the brainstem glioma. These findings suggest that FUSIN is a promising technique to facilitate the delivery of immune checkpoint inhibitors to the brain.