Graphene, a novel form of the hexagonal honeycomb two-dimensional carbon-based structural material with a zero-band gap and ultra-high specific surface area, has unique optoelectronic capabilities, promising a suitable basis for its application in the field of optical fiber sensing. Graphene optical fiber sensing has also been a hotspot in cross-research in biology, materials, medicine, and micro-nano devices in recent years, owing to prospective benefits, such as high sensitivity, small size, and strong anti-electromagnetic interference capability and so on. Here, the progress of optical fiber biochemical sensors based on graphene is reviewed. The fabrication of graphene materials and the sensing mechanism of the graphene-based optical fiber sensor are described. The typical research works of graphene-based optical fiber biochemical sensor, such as long-period fiber grating, Bragg fiber grating, no-core fiber and photonic crystal fiber are introduced, respectively. Finally, prospects for graphene-based optical fiber biochemical sensing technology will also be covered, which will provide an important reference for the development of graphene-based optical fiber biochemical sensors.  In 2004, graphene was separated by the 2010 Nobel Prize winners Andre Geim and Konstantin Novoselov [Graphene is a two-dimensional single-crystal functional material with a hexagonal honeycomb lattice composed of carbon atoms with spovoselov . Moreoveovoselov . Becauseovoselov ,14,15,16Optical biochemical sensors have broad development prospects because of their advantages of non-destructive detection, high sensitivity and fast detection speed, etc. . The opt2/Vs) at room temperature, a high specific surface area and a unique Dirac cone structure leading to zero-band gaps. All these contribute to absorption on the surface of biomolecules, leading to extremely sensitive biomolecule sensing. In recent years, graphene biochemical sensors have been explored by numerous scholars due to their obvious advantages, high sensitivity, and label-free, real-time detection of bio-logical molecules  [As a new functional 2D material, graphene can be easily integrated into optical fiber because of its high electrical conductivity, flexibility, strength, and lack of dangling bonds. Then, graphene has no mass and has an ultra-fast migration speed  ,28,29. FA, etc.) . In 2017A, etc.) . In 2021A, etc.) . In 2019A, etc.) . In 2017A, etc.) . MoreoveA, etc.) . ThroughIn this paper, development of optical fiber biochemical sensors based on graphene is reviewed. Firstly, four basic methods of preparing graphene, including mechanical the exfoliation method, chemical vapor deposition method, epitaxial growth method, and oxidation-reduction method are introduced, and the advantages and disadvantages of each method are stated. The next section describes the graphene-based optical fiber-sensing mechanism, including sensing properties of graphene and the graphene optical fiber-sensing mechanism. In the following sections, we review recent progresses in different various typical types of optical fiber biochemical sensors, covering the long-period fiber-grating sensor, Bragg fiber-grating sensor, no-core fiber sensor, and photonic crystal fiber sensor. Finally, the issues of the graphene optical fiber biochemical sensor are proposed and the progress to be made in the future is described, which not only clearly clarifies the current development stage, but also provides an efficient feasible direction and guidance for the future exploration of graphene-based op-tical fiber biochemical sensors.Graphene, as one of the 2D-layered nanomaterials, can be synthesized in two techniques: top-down exfoliation and bottom-up growth. The top-down method, including the mechanical exfoliation method, liquid phase exfoliation technique and oxidation-reduction method (redox), is used to prepare single or few-layer 2D nanomaterials by destroying the van der Waals forces between the layers of 2D materials. The bottom-up method is the synthesis of 2D nanomaterials at the molecular level by chemical means, including chemical vapor deposition, epitaxial growth method, hydrothermal method, and pulsed magnetron sputtering, pulsed laser deposition, etc. ,38,39,40The mechanical exfoliation method, also known as the micromechanical cleavage technique, is an effective way to exfoliate graphene into single- or few-layer nanomaterials by applying mechanical force on layered bulk crystals. 2SO4, NaNO3, and KMnO4. Then, reduction can obtain graphene by using hydrazine hydrate and ammonia water as reducing agents and mixing the reducing agent with the GO solution by ultrasonic dispersion. Redox has the advantages of low environmental pollution, high efficiency, high feasibility, low cost, and large-scale production in the industry, etc. However, it is easy to produce the pollution of liquid waste during the synthesis process and to destroy the electronic structure of the graphene crystal due to strong oxidizers, which affects the performance of graphene and limits its development in the field of microelectronics.At present, the redox method is a relatively commonly used method for preparing graphene . Figure CVD synthesizes graphene by the chemical decomposition material of carbon source under a certain temperature or in an external field and deposition on the substrate surface ,48,49; iIn 2006, Bergeretal C. et al. used the epitaxial growth method to prepare monolayer and multilayer graphene sheets . The thiGraphene has been considered as a multifunctional material because of its 2D planar structure. It has excellent potential applications in wide range of fields, such as photonics, optoelectronics, sensors, flexible electronics and nanocomposites, etc. . In part2/(Vs). Because of its excellent carrier mobility and mechanical properties, graphene is widely favored by researchers in condensed matter physics and materials science. Graphene has been widely used in the field of biochemical sensing based on its good photoelectric characteristics and excellent high-energy transfer efficiency, large surface area, and biocompatibility [The structure of graphene is shown in tibility . This seGraphene has excellent optical absorption characteristics. The absorption rate of monolayer graphene to visible light is only 2.3% , and thePhotoluminescence (PL) is an optical phenomenon that semiconductors launch light emissions by absorbing incident light, in which semiconductors emit light by absorbing incident light with energy higher than the energy band gap of the semiconductor. In the mechanism of PL, the excited electrons generated by optical excitation will return to the ground state with the emission of photons . Graphen5 cm2/(V·s), which is about 100 times that of the electron in silicon and the resistivity is about 10−6 Ω·cm, which is lower than that of metallic copper or silver. Therefore, it is the material with the lowest resistance at room temperature, and its conductive density is one million times that of copper. As the number of graphene layers increases, its conductivity gradually decreases, and then tends to remain unchanged at eight monolayers [Graphene’s unique structure enables it to have special properties, such as the perfect quantum tunneling effect and quantum Hall effect, etc. The optical conductivity of graphene depends on electron density, layer number and width at low temperatures. When electrons are transmitted in the graphene layer, the interference is very small and it is not easy to be scattered. The mobility can reach 2 × 10nolayers . The optnolayers ; the optGraphene is an excellent plasmon material , which iThe energy band of graphene has a linear dispersion relation, and its Dirac cone structure provides a unique zero band gap, large specific surface area, ultra-broadband optical spectrum, and strong nonlinear optical properties, which enable it to have a certain optical transmission ability. In this section, we will mainly introduce the energy band theory and sensing mechanism of graphene.2 hybrid orbital. Kx and Ky are the components of the wave vector k. As k and k’ are symmetric, and the conduction and valence bands at two points in the Brillouin zone are degenerate, the linear dispersion relationship of the energy bands of graphene is obtained. Hence, electrons can be considered to be massless relativistic particles, namely Dirac fermions, and the dispersion relation of the two-dimensional electron energy is isotropic, and is called Dirac cones. [It can be observed from c cones. ,82. Figuc cones. ,84,85,86c cones. ,88.(1)σ0 = πe2/2/π, which can be controlled by chemical doping or applied voltage. There is no intra-band contribution in the µ = 0 of pristine graphene. It can be observed from the theoretical expression and experimental results of optical absorption that the intra-band photoconductivity σ in the terahertz and far-infrared bands dominates due to the appearance of the zero-band gap, while the total conductivity variation in the near-infrared and visible light bands mainly depends on the transition process between the bands. Particularly, the propagation of surface plasmons in graphene has relations to the intra-band contribution.From the above formular, it can be observed that the intra-band and inter-band optical conductivities of graphene are concerned with μ and ω. The μ of doped graphene is controlled by the carrier concentration nThe optical fiber evanescent wave biochemical sensor works based on the principle of the evanescent wave . The evaIt is well known that biochemical sensors have been widely used in medical diagnosis, environmental monitoring, food safety, genetic engineering and other fields, and the accuracy and requirements of detection are becoming higher and higher with the development of artificial intelligence technology. Graphene and its derivatives (graphene oxide-GO), as new sensitive materials, are rapidly becoming a popular material for fiber-optic biochemical sensors. Based on the two-dimensional monolayer honeycomb lattice structure, GO is formed by enriching oxygen-containing functional groups (hydroxyl and carboxyl groups) on the surface of graphene through oxidation. Compared with graphene, the functional groups of GO are highly dispersed, hydrophilic and modifiable. Its unique two-dimensional structure and abundant oxy-gen-containing functional groups on the surface have presented unique advantages in biochemical-sensing applications. As shown in In recent years, researchers have discovered that the change in the effective refractive index of the optical fiber evanescent field caused by the combination of the substance to be tested; and the molecular film can be used to achieve detection, which involves using ultraviolet light to modify the refractive index of the fiber core on a regular basis in order to create a spatial phase grating . Fiber gLong-period fiber gratings are also called transmission fiber gratings, and their period is relatively long, which is generally hundreds of microns. In 2017, Liu Chen’s research team studied biosensors based on graphene oxide nanosheets that functionalized dual-peak long-period grating (dLPG), and proposed a label-free biosensor based on GO-coated dLPG for the real-time detection of the biological affinity between the antibody and the antigen; this was through a new GO deposition technology that combines chemical bonding and physical adsorption, enabling GO to bind to dLPG firmlyand uniformly with good stability and high repeatability. −1 to the change of methane gas concentration, which is 2.96 times and 1.31 times higher than the traditional long-period fiber-grating sensor and the silver-plated long-period fiber-grating SPR sensor, respectively. The performance of the LPFG SPR sensor can be improved by covering the graphene. It has good response characteristics and repeatability, which provides a broad prospect for high-sensitivity graphene gas sensors [In 2017, Wei Wei’s research team proposed a SPR sensor based on graphene LPG for high-sensitivity gas-sensing detection. A layer of single-molecule graphene is coated on the surface of the Ag film of the long-period optical fiber SPR sensor to enhance the evanescent electric field intensity on the surface of the optical fiber, thereby enhancing the interaction between the SPR wave and the molecule.  sensors .In 2020, Wang’s research team developed a graphene oxide micro-tapered long-period fiber-grating (MTLPG) biosensor for the detection of hemoglobin molecules . The resIn 2021, Esposito’s research team developed a long-period fiber-grating biosensor based on the graphene oxide double-clad structure for the detection of C-reactive protein (CRP) in human serum . The minBragg fiber gratings are also called reflective or short-period fiber gratings, and the grating period is only a few hundred nanometers. In 2014, Sridevi et al. developed a graphene oxide-etched Bragg fiber-grating biosensor for the detection of the protein concanavalin A (Con A) . In 20152 gas in room temperature sensing. 2 gas-sensing mechanism on RGO coated eFBG. 2 gas molecules on RGO changes the refractive index by increasing and decreasing the local carrier concentration. The RGO-coated eFBG sensor can detect sub-ppm level NO2 gas. By using multiple gratings in the same fiber, the performance of the sensor can be improved to sub-ppb level, and the response time can be shortened, providing more effective biochemical-sensing reference data [In 2016, the Sridevi research team studied an etched fiber Bragg grating (eFBG) chemical sensor based on the redox graphene (RGO) coating for NOnce data .The classic approach of improving the sensitivity of optical fiber sensors involve corroding the structure of fiber cladding . HoweverYang HongYan’s research team designed a single-mode no-core single-mode (SNS) fiber step-index distribution sucrose sensor for glucose detection. Photonic crystal fiber (PCF) has been proven to be a good prism. Single-mode transmission, excellent birefringence, and tunable dispersion characteristics are all advantages of its unique structure and flexible design . The com−1 and a resolution of 3.97 × 10−5 RIU [−1 in the refractive index range of 1.39–1.42 and the maximum sensitivity reaches 10,000 nmRIU−1; the resolution is 1 × 10−6 RIU in the range of 1.43–1.46 [In 2014, Dash et al. proposed a birefringent PCF-SPR biochemical sensor based on a graphene-silver layer ; graphen10−5 RIU . In the .43–1.46 .−1, resolution is 7.1 × 10−5 RIU, wavelength sensitivity is 2000 nmRIU−1, and resolution is 5 × 10−5 RIU. Lou et al. proposed an eccentric PCF-SPR biochemical sensor based on graphene-gold coated on the outer surface of PCF in 2019 [−1; the maximum sensitivity is 8600 nmRIU−1.Rifat et al. coated graphene copper on the outer surface of PCF in 2016, and constructed a PCF-SPR biochemical sensor based on the graphenecopper layer . Within  in 2019 . After gIn 2018, Wang qi’s research team proposed and developed a sensitivity-enhanced SPR biochemical sensor based on the GO complex and staphylococcal protein A (SPA) co-modified PCF for detecting human IgG. . The schCompared with circular-shaped PCF, D-shaped PCF has an asymmetric cross-sectional structure and a wide and flat side polishing surface, which is not only conducive to the transfer and deposition of graphene, but also make it easy to reprocess the graphene on the side-polishing surface . Chemica−1, and a resolution of 2 × 10−5 RIU. The sensor can be applied to real-time detection of biomolecules and tiny drug molecules due to the long-distance transmission, high sensitivity and resolution.In 2018, Tong Kai’s research team proposed a D-shaped PCF biosensor based on SPR graphene coating . Graphen−1 and a maximum amplitude sensitivity of 192 RIU−1 in the refractive index range of 1.27–1.37 in the internal filling detection scheme; the maximum sensitivity is 22,400 nmRIU−1 and the maximum quality factor (FOM) is 127 in the external-filling detection scheme.Yashar et al. proposed a D-shaped PCF biochemical sensor based on SPR in 2021 . They st−1 and a maximum resolution of 8.33 × 10−6 RIU in the refractive index range of 1.21–1.32. This type of chemical sensor offers good sensing performance and a lot of economic promise in biochemical sensing and environmental monitoring and assessment. However, it cannot obtain excellent sensing performance in the entire effective sensing range which should be the direction of our future work.Instead of using metal layers as plasma materials, some researchers employed two-dimensional materials and put a layer of graphene on the film for study. In 2021, Tian et al. proposed a graphene-coated D-shaped PCF chemical sensor based on SPR, which uses indium tin oxide (ITO) instead of metal as the plasma material . The senFor the past few years, graphene-based fiber-optic biochemical sensors have demonstrated excellent performance in genetic engineering, medical diagnosis, environmental monitoring, and food safety, etc. However, the widespread application of graphene-based commercial fiber biochemical sensors still faces challenges, and the following key issues need to be solved: Firstly, the current research on graphene is still limited to theories and laboratories-the existing preparation methods of graphene are not perfect enough. How to prepare high-quality graphene with controllable size and thickness, no defect, no pollution, and uniform distribution at a low cost is one of the urgent problems to be solved. Secondly, when graphene is transferred to the surface of the fiber, it is easy to be contaminated and damaged. How to realize the lossless and high-quality coupling between graphene and fiber is the second key problem to be solved. Thirdly, graphene’s flexible photoelectric characteristics make it difficult to find a perfect solution to the cross-sensitive detection. For example, temperature can interfere with the gas sensor when measuring gas concentrations. Finally, the repeatability and stability of graphene sensors are not optimistic either. When the external environment causes the electronic structure of graphene to change, it will result in graphene taking a certain time to recover. It is difficult to automatically restore it to the original state, which leads to the poisoning of the sensor when graphene adsorbs chemical molecules.As the world is suffering from COVID-19, the public health security system of mankind is facing unprecedented challenges. If the research of the graphene fiber biochemical sensor can break through the limitation of theory and experiment and put it in to large-scale industrial production, it will rapidly improve the pace of human intelligent life. Here, based on the above reasons, by choosing three graphene-based fiber biochemical sensors with special fiber structures  as breakthrough points, we reviewed the new research development and existing problems of the graphene-based fiber biochemical sensor in detail, which provides reliable data and novel ideas for the practical application of the graphene-based fiber biochemical sensor in order to achieve a major breakthrough in the fiber biochemical sensor. 2) and black phosphorus (BP), etc., which is the multifunctional integration with optical fiber to realize biochemical sensing. Moreover, graphene, as a modifier combined with semiconductor materials, in order to realize the application of graphene photoelectric sensors and graphene-based flexible implantable biochemical sensors, is mushrooming development. In the future, graphene will present extraordinary talents in the field of artificial intelligence and wearable sensing as a new material for fiber sensors.Although much work still need to be conducted for the application of the graphene-based fiber biochemical sensor, the future for graphene-sensing devices appears very exciting. In order to improve the performance of graphene-based fiber-optic biosensors, there have been many improved progresses in the preparation of graphene, the structural form of graphene and the design of graphene-based fiber-optic biosensors. For example, functionalized graphene is modified by other molecules or atoms, such as doping molecules and adding substrates, etc., in order to improve the performance of graphene. So far, in addition to graphene and its derivatives, there have been many reports on novel two-dimensional materials, such as molybdenum disulfide (MoS


Vascular smooth muscle cells (VSMCs) are stromal cells of the vascular wall and are continually exposed to mechanical signals. The loss of VSMCs is closely related to the occurrence of many vascular diseases, such as aortic aneurysms and aortic dissection. The proliferation and apoptosis of VSMCs are mechanically stimulated. Yes-associated protein (YAP), one of the core components of the Hippo pathway, plays a key role in the response of VSMCs to mechanical signals. In this study, we tested the impact of different intensities of mechanical stretch on the proliferation and apoptosis of VSMCs, as well as YAP. We tested VSMCs’ proliferation and apoptosis and YAP reaction via immunocytochemistry, western blotting, CCK-8 and flow cytometric analysis. We found that 10% elongation could increase the phosphorylation of YAP and prevent it from entering the nucleus, as well as inhibit cell proliferation and promote apoptosis. However, 15% elongation reduced YAP phosphorylation and promoted its nuclear entry, thereby promoting cell proliferation and inhibiting apoptosis. Accordingly, YAP knockdown suppressed the phenotype of VMSCs induced by 15% elongation. Taken together, YAP regulates proliferation and apoptosis of VSMCs differently under different intensity of mechanical stretch. Mechanical stretch with appropriate intensity can promote the proliferation and inhibit apoptosis of VSMCs by activating YAP. Aortic aneurysms are associated with aortic dissection and rupture. There is currently no effective treatment to prevent or cure aortic aneurysms. At present, both the pathogenesis and pathophysiology of ascending aortic aneurysms are not entirely clear. Vascular smooth muscle cells (VSMCs) have been recognized as the most important factor in the development of ascending aortic aneurysms. Aortic aneurysms can occur due to loss of VSMCs in the media layer of the aortic wall, leading to progressive aortic dilation –3. We obAortic walls are subjected to various mechanical stimuli from the bloodstream, such as shear and mechanical stretch. Stretch sensing is generally known as an integrin-mediated pathway, which is coupled to cell contractile activity, and thus shares many mechano-transduction pathways with the rigidity sensing process in translating mechanical stimuli into intracellular biochemical signals –6. The rDrosophila by genetic mosaic screening following identification of a loss-of-function mutation of Hippo that led to a strong overgrowth phenotype , anti-Lats , anti-p-Lats , anti-Rho , anti-ROCK , and anti-β-actin ). The membranes were incubated with IRDye 800CW goat anti-rabbit IgG (H+L)  at room temperature for 1 h. Bands were detected using the chemiluminescent imaging system.P values < 0.05 were considered to indicate statistical significance.One-way analysis of variance was performed in SPSS software  to evaluate group differences. Data are expressed as means ± standard deviation, and All data generated or analyzed during this study are included in this published article.To determine the changes in proliferation and apoptosis of VSMCs under different intensities of mechanical stretch, we applied 0, 10, 15, and 20% elongation forces to VSMCs . We usedTo test whether YAP was involved in the effects of mechanical stress on VSMCs, we first performed immunofluorescence staining to determine the intracellular localization of YAP. YAP was both in the nucleus and the cytoplasm under 0% elongation –2C. YAP To determine the effects of the Hippo pathway in VSMCs, we knocked down YAP by siRNA. We tested the efficiency of different YAP-targeting siRNAs using western blotting to generate optimal knockdown of YAP. Our results showed that YAP siRNA1 (ACAGCAGGAGTTATTTCGG) resulted in the greatest knockdown of YAP . To testTo better understand the role of the Rho-ROCK-Hippo pathway in the effects of mechanical stretch on YAP, we inhibited the Rho-ROCK pathway using Y27632. We performed immunocytochemistry staining to determine the intracellular localization of YAP. The results showed that more YAP stayed in the cytoplasm under 0% elongation following inhibition of the Rho-ROCK pathway (group 2) compared with 0% elongation (group 1) , 4B. Y27One emerging concept is that changes in the VSMCs phenotype in response to mechanical forces are important in vascular diseases, such as hypertension, atherosclerosis, aortic aneurysms, and age-dependent arterial stiffening , 26, 27,Studies have found that mechanical stimulation can inhibit the Hippo pathway, increase nuclear levels of YAP, promote cell proliferation, and inhibit apoptosis. , 28–31 YIn addition, we tested the activity of the Rho-ROCK pathway and found that 10% elongation stimulated the expression of related proteins to decrease, while 15% elongation stimulated the expression of related proteins to increase. After blocking the Rho-ROCK pathway, the effects of different intensities of stretch on YAP were weakened and phosphorylated YAP was decreased in each group. Overall, inhibition of the Rho-ROCK pathway combined with 10% mechanical stress led to activation of the Hippo pathway, which in turn reduced nuclear localization of YAP. YAP failed to bind to TEAD; this led to a decrease in ccnd1 expression, inhibition of VSMCs proliferation, and promotion of apoptosis. When the mechanical stress was increased to 15%, we observed the opposite effects.Through our experiments, we found that the mechanism underlying the effects of mechanical stress on VSMCs is as follows: after being stimulated by physiological 10% elongation stretch, VSMCs are inhibited in various ways. Inhibition of Rho-ROCK leads to a decrease in LATS1/2 levels, which in turn leads to an increase in the phosphorylation of YAP. Phosphorylated YAP cannot enter the nucleus nor bind to TEAD. On the other hand, our ongoing experiment shows that 10% elongation stretch may downregulates the expression of miR130a, which increases the amount of VGLL4. In the nucleus, VGLL4 may competes with YAP for the opportunity to bind TEAD. As a result of these two mechanisms, 10% elongation reduced ccnd1, resulting in decreased VSMCs proliferation and increased apoptosis, while a stimulus that is lower or higher than physiological elongation resulted in opposing effects, eventually promoting the proliferation of VSMCs and reducing apoptosis.On conclusion, our study demonstrates the possibility of preventing or treating diseases such as aortic aneurysms by controlling blood pressure. The Rho-ROCK pathway can be activated by mildly adjusting the blood pressure. Activation of the Rho-ROCK pathway, in turn, inhibits the Hippo pathway and ultimately promotes VSMCs proliferation and inhibits apoptosis. Our findings may provide insight into the pathogenesis prognosis of these diseases, as well as into therapeutic interventions.


In situ intercalative polymerization has been investigated as a strategic way to obtain poly (PPF) and poly (PHF) nanocomposites with different graphene types and amounts. Graphene (G) has been dispersed in surfactant stabilized water suspensions. The loading range in composites was 0.25–0.75 wt %. For the highest composition, a different type of graphene (XT500) dispersed in 1,3 propanediol, containing a 6% of oxidized graphene and without surfactant has been also tested. The results showed that the amorphous PPF is able to crystallize during heating scan in DSC and graphene seems to affect such capability: G hinders the polymer chains in reaching an ordered state, showing even more depressed cold crystallization and melting. On the contrary, such hindering effect is absent with XT500, which rather induces the opposite. Concerning the thermal stability, no improvement has been induced by graphene, even if the onset degradation temperatures remain high for all the materials. A moderate enhancement in mechanical properties is observed in PPF composite with XT500, and especially in PHF composite, where a significative increase of 10–20% in storage modulus E’ is maintained in almost all the temperature range. Such an increase is also reflected in a slightly higher heat distortion temperature. These preliminary results can be useful in order to further address the field of application of furan-based polyesters; in particular, they could be promising as packaging materials. Nowadays, there is a growing interest in the preparation of new chemicals and materials based on renewable resources, as biomass-derived fuel and chemicals are a promising alternative to fossil-based materials. Chemicals from vegetable feedstocks like sugars, vegetable oils, organic acids, glycerol, and others have been proposed as monomers for polymers production ,2,3,4. 2Concerning composites, the state of the art reports furan-based resin composites with clays, basalt, glass, or flax fibers ,16,17,182 carbons organized in a honeycomb structure. It has been considered the building block of all other graphitic carbon allotropes such as graphite, fullerenes, carbon nanotubes (CNT), and nanoribbons, even if, however, these carbon allotropes are not synthesized from graphene (except for nanoribbons). Graphite indeed is a naturally occurring material, single-walled CNT was first synthesized in 1991 following the discovery of fullerenes in 1985. With Young’s modulus of 1 TPa and ultimate strength of 130 GPa, single-layer graphene is the strongest known material. Moreover, single-layer graphene has very high electrical conductivity, up to 6000 S/cm, and a thermal conductivity of 5000 W/mK. These excellent properties, in addition to extremely high surface area (2600 m2/g) and gas impermeability indicate graphene as the perfect candidate for improving mechanical, electrical, thermal, and gas barrier properties of polymers [Among fillers commonly used in composites, graphene, with its combination of extraordinary physical properties and ability to be dispersed in various polymer matrices, is a promising candidate for a new class of furan-based polymer nanocomposites. Graphene is an atomically thick, two-dimensional sheet composed of space area 00 m2/g aace area 00 m2/g apolymers ,28,29,30ace area 00 m2/g apolymers , nylon 6polymers , poly, diffractometry (XRD), thermogravimetry (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMTA), and scanning electron microscopy (SEM).In particular, in the present work, poly (PPF) and poly (PHF) have been taken into consideration. PPF is a promising sustainable alternative of poly (PET), while PHF, in comparison with PEF, PPF, and PBF, shows minor tensile modulus and strength. Jiang et al.  indeed, 2O3, purity 99%) was supplied by Carlo Erba. The reactants have been used as received. Graphene (G) dispersions in water, exfoliated using a proprietary surfactant, were kindly provided by Graphene-XT . The industrial grade graphene G from Graphene-XT had an average lateral width of about 0.2 μm and 2–10 nm thickness. The concentration of the undisclosed suspending agent was below 1% w/w; for further information, refer to the technical data sheet [1,3-propanediol  and 1,6-hexanediol  were purchased from Sigma-Aldrich; 2,5-furandicarboxylic acid  was provided by Aldrich; antimony trioxide (Sbta sheet . Graphen−3 mol) of Sb2O3 corresponding to 1 wt % respect to the theoretical final polymer, were charged into a glass reactor (250 mL capacity). The ratio PD/FDCA was 1.8. The reactor was closed with a three-necked flat flange lid equipped with a heating band (T = 80 °C), a mechanical stirrer and a torque-meter (which gave an indication of the viscosity of the reaction melt). The system, under nitrogen flux, was then connected to a water-cooled condenser and immersed in a thermostatic salt-bath at 190 °C, while the stirrer was switched on at 200 rpm. The temperatures of band and bath were gradually increased up to 100 and 220 °C, respectively. All the water distilled off after less than 2 h. The reactor was then connected to a liquid nitrogen-cooled condenser. Dynamic vacuum was applied in 2 h down to 0.14 mbar, while the temperature was gradually increased up to 250 °C for the salt-bath and 120 °C for the band. When the torque of the melt was around 4 mN, a viscous, brown and transparent melt was discharged from the reactor. The molecular structure of PPF was confirmed by 1H-NMR.The polycondensation procedure was described in previous papers ,44,45. 21 H NMR , 600 MHz): δ 2.28 , 4.55 , 7.31 . The carbon labels are referred to the chemical structure reported in The composites have been similarly prepared by adding the dispersion containing graphene into the reactor. The compositions have been calculated in wt % respect to the theoretical final polymer. The samples are denoted PPF-XYw(d), where X is the wt % of graphene, Y indicate the graphene type G or XT500, and w or d indicates a water or diol dispersion.−3 mol) of Sb2O3, were charged into a glass reactor. The ratio HD/FDCA was 1.2. The reactor was closed with a three-necked flat flange lid equipped with a heating band (T = 80 °C), a mechanical stirrer, a torque-meter (which gave an indication of the viscosity of the reaction melt). The system was kept under nitrogen flow, then connected to a water-cooled condenser and immersed in a thermostatic salt-bath at 180 °C, while the stirrer was switched on at 200 rpm. The temperature was gradually increased up to 220 °C, as well as the heating band (100 °C), until almost all the water distilled off, after less than 2 h. The reactor was then connected to a liquid nitrogen-cooled condenser. Dynamic vacuum was applied in 2 h down to 0.23 mbar, while the temperature was increased up to 250 °C and the lid was set at 120 °C. When the torque of the melt was around 8 mN, a viscous, dark brown and transparent melt was discharged from the reactor. The molecular structure of PHF was confirmed by 1H-NMR.28.3 g (0.181 mol) of FDCA, 25.7 g (0.218 mol) of HD and 407 mg , 600 MHz): δ 1.49 , 1.82 , 4.41 , 7.30 . The carbon labels are referred to the chemical structure reported in The composite was similarly prepared by adding the water dispersion containing G into the reactor. The sample is named PHF-0.75Gw and corresponds to 0.75 wt % of graphene respect to the theoretical final polymer.1 H NMR spectra were recorded on Varian Inova 600 spectrometer ; the solvent used was CDCl3.3 and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) 95/5 as eluent and to dissolve polymer samples. A UV detector was used and a calibration plot was constructed with polystyrene standards.Gel permeation chromatography (GPC) measurements were performed on a HP 1100 Series using a PL gel 5 µm Minimixed-C column with a solution of CHCl−1 heating rate from 50 to 900 °C. The onset degradation temperatures (Tonset) were taken from the intersections of the tangents of the initial points and the inflection points. The 10% mass loss temperatures (T10D) and the temperature at maximum process rate (Tmax) were also measured.Thermogravimetric analysis (TGA) was performed in nitrogen atmosphere, using a Perkin Elmer TGA7 apparatus (gas flow 40 mL/min) at 10 °C min−1 from 20 to 220 °C, kept at high temperature for 2 min and then cooled to 0 °C at 10 °C min−1. After this thermal treatment, the samples were analyzed by heating from 0 °C to 220 °C at 10 °C min−1 (second scan). The PHF samples were heated at 20 °C min−1 from 0 to 180 °C, kept at high temperature for 2 min, then cooled to −20 °C at 10 °C min−1, and heated from −20 °C to 180 °C at 10 °C min−1 (second scan). During the cooling scan the crystallization temperature (Tc) and the enthalpy of crystallization (∆Hc) were measured. During the second scan, the glass transition temperature (Tg), the melting temperature (Tm) and the enthalpy of fusion (∆Hm) were measured. Tg was taken as the midpoint of the heat capacity increment associated with the glass-to-rubber transition.The calorimetric analysis (DSC) was carried out by means of a Perkin-Elmer DSC6. Measurements were performed under nitrogen flow. In order to delete their previous thermal history, the PPF samples (ca. 10 mg) were first heated at 20 °C minThe X-ray diffraction (XRD) analysis was carried out at room temperature by means of a PANalytical X’Pert PRO diffractometer equipped with an X’Celerator detector. Data were acquired by exposing the samples to Cu-Kα X-ray radiation, which has a characteristic wavelength (λ) of 1.5418 Å. X-rays were generated from a Cu anode supplied with 40 kV and a current of 40 mA. The 2θ plots of X-ray spectra have been obtained over 2θ range of 2.1–35°. For each step of 0.067°, the signal has been counted for 41 s, to increase its intensity. PPF film samples, pressed at 200 °C were analyzed. PHF samples, were pressed at 100 °C, obtaining samples with almost 1 mm of thickness and a smooth and regular surface to analyze.−1 in the temperature range starting from −120 °C to 110 °C.Physical and mechanical properties were determined using a Rheometric Scientific DMTA IV Dynamic Mechanic Thermo analysis instrument with a dual cantilever testing geometry. Typical test samples were bars obtained by injection molding at 190 °C using a Minimix Molder with PHF. PPF sample bar have been obtained by compression molding at 200 °C. The analysis was carried out at a frequency of 3 Hz, at 3 °C minFracture surfaces of samples were obtained under cryogenic fracture by liquid nitro-gen to avoid plastic deformations. Sections were analyzed by scanning electron microscopy  to investigate their morphology.In a previous work by the same Authors , it was 1 H NMR. PPF and PHF homopolymers spectra are showed in The molecular structure was checked by The samples have high molecular weight within the range 50,000–70,000 g/mol  with a pg) of 53 °C. During the cooling scan from the melt at 10 °C/min, PPF is unable to crystallize; it only presents, during the second heating scan (at 10 °C/min) a cold crystallization process at 138 °C and a melting peak at 172 °C. All the PPF composites with G are substantially amorphous too, but some observations can be done. The samples PPF-Gw during the second heat scan present low enthalpy values (ΔHcc and ΔHm) with respect to the homopolymer. Moreover, the shape of the peaks become broader, showing cold crystallization and melting processes more and more depressed. The polymer chains in such samples have even more difficulties in reaching an ordered state; therefore, Gw hinders the crystallization of the matrix occurring during heating. This behavior can be due to the presence of surfactant used to stabilize the G suspension. On the other hand, in PPF-0.75XT500d, cold crystallization and melting processes are more intense than those of PPF and of the corresponding PPF-0.75Gw. The graphenic sample XT500 seems to favor the rearrangement of the polymer chains during heating, probably due to the absence of surfactant and/or to the presence of 6 wt % of graphene oxide, and/or to the larger lateral width respect to G. In contrast, PHF is semicrystalline (c) and melts at 145 °C (Tm). The glass transition temperature is 13 °C. No significant differences are present in the composite: graphene does not seem to affect the thermal behavior of PHF. The DSC data are consistent with literature [The thermal behavior of the investigated samples was assessed. Calorimetric data are summarized in stalline : it crysterature ,47. Moreg, no substantial changes respect to both PPF and PHF were highlighted. Graphene does not seem to affect the mobility of the chains neither as plasticizing neither as nucleating agent.Concerning the T2O3 around 2θ = 28°. The reflection attributed to (002) diffraction peak of graphene, at 26.6° [2O3 is similarly visible around 2θ = 28°, as well as the (002) diffraction peak of graphene in PHF-0.75Gw [at 26.6° , is visiat 26.6° . The catF-0.75Gw .onset (369 °C) and Tmax (391 °C). Tonset, Tmax, and T10D of all the composites remain almost similar to the respective homopolymers, no significant differences are present. Thus, graphene does not seem to improve the thermal stability of the polyester matrix: indeed, decomposition curves shift toward slightly lower temperature region. On the contrary, slight retardation in decomposition were reported by Paszkiewicz et al. for PPF composites with few layer graphene [The thermal stability of homopolymers and their nanocomposites with graphene was investigated by TGA under inert nitrogen atmosphere. The thermogravimetric curves are shown in graphene , while mgraphene . The nongraphene ,42 and agraphene .Temperature and frequency dependent mechanical relaxation data for amorphous PPF and semicrystalline PHF were recorded via storage modulus (E′) and tan δ, which is the ratio of the loss modulus to storage modulus. Such data can provide indications about the mechanical reinforcement, which is correlated to a good interface interaction filler/matrix due to the presence of van der Waals forces or π–π interactions . In FiguSimilarly, a slight reinforcing effect of graphene in E’ was reported with PMMA . The autIn the zoomed image of For all samples, the glass transition temperature is recognized by the large drop in storage modulus and by the corresponding peak maximum in tan δ c. The vag, all samples are substantially similar to the homopolymer.Above TCrystallization is observed by the increase in storage modulus around 100 °C for all samples. As already expressed, PPF is able to crystallize under specific conditions, as for example during DMTA scan at 3 °C/min, after the glass-rubber transition. Such phenomenon is more pronounced for PPF-0.75XT500d, probably due to a considerably higher crystallization rate induced under heating by graphene XT500, as already evidenced by DSC analysis.In case of PHF samples , the comMoreover, heat distortion temperature (HDT), corresponding to log E’ = 8.9 (1.82 MPa) according to Scobbo  was calcPPF and PHF composites containing graphene were prepared by in situ polymerization. Graphene (G) was dispersed in surfactant stabilized water suspensions. For one PPF composition, a different type of graphene was also tested, XT500, dispersed in 1,3-propanediol without any surfactant. In this case, it is notably to observe that 1,3-propanediol is also a monomer for the PPF synthesis. PPF samples result to be amorphous, while PHF samples are semicrystalline materials. Graphene seems to affect the ability of PPF to crystallize during heating: the nanocomposites containing G show a depressed cold crystallization and melting processes. On the contrary, XT500 seems to help the rearrangement of the polymer chains in an ordered state.-affects the ability of PPF to crystallize during heating, while it does not affect the thermal transitions of PHF;-does not affect the thermal stability neither of PPF neither of PHF;-determines a mechanical reinforcement especially in PHF.Concerning the thermal stability, no improvement has been induced by graphene; however, the onset degradation temperatures still remain high for all the materials, around 370 °C. A moderate enhancement in mechanical properties is observed in PPF-0.75XT500d (around 6–12% of E’ increase), and especially in PHF composite, where an adequate increase of 10–20% in E’ is maintained in almost all the temperature range. Such increase is also reflected in a slightly higher HDT (38 vs. 34 °C). In terms of mechanical performances, for PPF composites 1,3-propanediol results to be a better medium to suspend graphene respect to surfactant stabilized water dispersions. PD in fact, is a monomer in PPF polycondensation and this helps in preventing the restacking of the sheets. In summary, graphene:Such preliminary results can be useful in order to further address the field of application of furan-based polyesters. Indeed, the above materials could be potentially used for packaging application, where high barrier properties—potentially further improved by graphene—are required. Moreover, it is noteworthy that the in situ polymerization procedure avoids the use of powders, employed in the melt compounding techniques, and the use of a large amount of solvent, employed for solvent casting methods, representing an advantage in term of safety and sustainability.


The effect of electrolyte concentration wasstudied, as low concentrations do not lead to a favorable graphiteexfoliation and high concentrations produce fast graphite expansionbut poor layer-by-layer delamination. The optimal concentrations are0.25 M H3PO4 and 0.05 M Na3PO4, which also exhibited the highest phosphorus contents of2.2 and 1.4 at. %, respectively. Furthermore, when PFG-acid at 0.25M and PFG-salt at 0.05 M were tested as an electrode material forcapacitive energy storage in a three-electrode cell, they achieveda competitive performance of ∼375 F/g (540 F/cm3) and 356 F/g (500 F/cm3), respectively. Finally, devicesmade up of symmetric electrode cells obtained using PFG-acid at 0.25M possess energy and power densities up to 17.6 Wh·kg–1 (25.3 Wh·L–1) and 10,200 W/kg; meanwhile,PFG-salt at 0.05 M achieved values of 14.9 Wh·kg–1 (21.3 Wh·L–1) and 9400 W/kg, with 98 and99% of capacitance retention after 10,000 cycles, respectively. Themethodology proposed here also promotes a circular-synthesis processto successfully achieve a more sustainable and greener energy-storagedevice.Phosphate-functionalizedcarbon-based nanomaterials have attractedsignificant attention in recent years owing to their outstanding behaviorin electrochemical energy-storage devices. In this work, we reporta simple approach to obtain phosphate-functionalized graphene (PFG)via anodic exfoliation of graphite at room temperature with a highyield. The graphene nanosheets were obtained via anodic exfoliationof graphite foil using aqueous solutions of H Nonetheless, producinggraphene nanosheets at a mass scale and low cost is still a big challenge.Many works have studied and developed several top-down and bottom-upproduction methods to overcome this challenge. Nonetheless, nowadaysthe electrochemical exfoliation12 (anodicor cathodic), the graphene oxide route,13 and the direct liquid-phase exfoliation method,14 are the most promissory methodologies15 to produce single- or few-layered graphene nanosheets inbulk quantities.Sustainableenergy storage has recently arisen as a need to satisfythe global energy demand in the near future.17 The exfoliation occurs when theions from the electrolyte are intercalated between graphite layersin the graphite electrode, during the feed of an electrical currentin an electrolytic cell. As a result, the interlaminar distance isincreased, leading to exfoliated graphene layers with a high yield.In particular, for electrochemical exfoliation under cathodic conditions,the graphite electrode acts as a cathode and the cations intercalatingfrom the electrolyte can be molten salts, salts in organic solvents,ionic liquids, etc.20 In contrast, anodic electrochemical exfoliation occurswhen the graphite works as the anode, and anions from the aqueouselectrolyte intercalate in the anode. It is worth noting that anodicexfoliation is considered a greener and environmentally respectfulprocess, due to the use of water as the electrolyte which leads toa high yield of exfoliated layers; nevertheless, it can produce oxygen-centeredradicals from the anodic oxidation of water that attack the exfoliatedlayers.16Specifically, the electrochemical exfoliation has become an attractive methodologyby its operative simplicity, easy scalability, and versatility touse a wide variety of electrolytes to get exfoliated graphene nanosheetswith high yields.22 since the new functional groups promote localized effects on thegraphene layer, thus enhancing their physicochemical properties. Inparticular, several heteroatoms, including N, P, S, and B, have beenincorporated into the graphene lattice26 (doping), and nitrogen-, sulfur-,and phosphorus-based functional groups  have been used to chemically functionalize the graphene nanosheets.28 Among them, phosphorus-based functional groups are highly desirablefor electrochemical energy-storage devices since they possess an excellentelectron-donating ability improving the band gap of the functionalizedlayers. In this sense, many methodologies to functionalize or dopegraphene nanosheets with phosphorus groups have been reported; however,they are usually time-consuming or multistep processes that use expensivereagents and high-temperature post-treatments, or involve processesat high pressures.30On the other hand, thechemical surface modification of grapheneprevents the restacking of the exfoliated layers and can potentiallyimprove their electrochemical performance,31 Nonetheless, studies focused on electrochemical energystorage are scarce.32 Here, graphene nanosheets electrochemically exfoliated from graphitefoil and phosphate-functionalized ) were obtained in an expeditious and single-step processvia anodic exfoliation at room temperature. The underlying exfoliation/functionalizationmechanisms by electrochemical exfoliation of graphite using phosphoricacid (H3PO4) or sodium phosphate (Na3PO4) are proposed. Three concentrations of H3PO4  and Na3PO4 , both as the phosphate sources and the electrolytesin an aqueous solution, were studied. On the other hand, regardlessof the phosphate source , as the electrolytemolar concentration decreased, nanosheets with higher phosphate groupswere obtained. Thus, PFG-acid at 0.25 M and PFG-salt at 0.05 M exhibitedthe highest phosphorus contents ,and its specific capacitance was investigated with three- and two-electrodecells, achieving a competitive performance as a more sustainable andgreener energy-storage device.In this context, anodic exfoliation appears potentially asa powerful,affordable, and greener technique to produce exfoliated and phosphorus-containingnanosheets in one-step using mild reaction conditions without complexpost-treatments, as evidenced by recent studies.3PO4, 85% w/w aqueous solution, ACS grade), andsodium phosphate dodecahydrate  were purchased from Alfa Aesar, and sulfuric acid(1 M H2SO4) was purchased from VWR Chemicals.Deionized water was used throughout the experiments.A 500 μm-thick high-purity graphitefoil (Papyex I980) was purchased from Mersen. Phosphoric acid (H2O and deposited by drop-casting onto preheated (50–60°C) circular stainless-steel sample-holders (12 mm in diameter)for analysis by Raman spectroscopy and X-ray photoelectron spectroscopy(XPS). UV–vis absorption spectra of aqueous PFG dispersionswere obtained with a double-beam Heλios α spectrophotometer(Thermo Spectronic). Raman analysis was carried out by using a HoribaJobin-Yvon LabRam instrument at an incident laser power of 2.5 mWand a laser wavelength of 532 nm. The XPS measurements were accomplishedon a SPECS apparatus at 10–7 Pa with a nonmonochromaticMg Kα X-ray source . The recorded spectra werecurve-fitted using the AAnalyzer software, and a Shirley-Sherwoodbackground function and Voigt curve type were used. The thermogravimetricanalysis was carried out in a TA Instruments SDT Q600 apparatus, heatingthe samples from room temperature to 900 °C at 10 °C/minunder ultra-high-purity nitrogen flow (100 mL/min) and using platinumcrucibles of 90 μL as the sampler container. Scanning electronmicroscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyseswere performed using a Quanta FEG 650 apparatus from FEI Company.The samples were supported onto metallic holders with carbon adhesivetape and analyzed at a 20–25 kV voltage. Atomic force microscopy(AFM) imaging was performed in a Nanoscope IIIa Multimode system inthe tapping mode of operation and using rectangular silicon cantileverswith resonance frequencies of 250–300 kHz and nominal springconstants of 40 N m–1. Specimens for AFM were preparedby drop-casting small volumes (∼20 μL) of an aqueousdispersion of the graphene sample (∼0.01 mg mL–1) onto freshly cleaved highly oriented pyrolytic graphite (HOPG)substrates the were preheated to 50–60 °C. Transmissionelectron microscopy (TEM) and selected area electron diffraction (SAED)measurements were carried out using a JEOL JEM 2100F microscope atan acceleration voltage of 200 kV on specimens prepared by drop-casting20 μL of an aqueous dispersion of the sample (0.01 mg·mL–1) onto a copper grid (200 square mesh) covered witha lacey carbon film and allowing them to dry at room temperature.A graphene slurry was immersed in liquid nitrogen (−176 °C)and freeze-dried for 24 h using a Telstar Cryodos apparatus. The N2 adsorption isotherms were recorded at −196 °Cin an ASAP 2420 volumetric apparatus (Micromeritics), after degassingthe resulting powder overnight under vacuum at 110 °C. The specificsurface areas were obtained from the adsorption branch of the N2 isotherms by the standard Brunauer–Emmett–Teller(BET) method in the relative pressure range 0.05–0.35. PFGfilms were prepared by vacuum-filtering aqueous dispersions of thecorresponding sample through PTFE membrane filter. The electricalconductivity of the PFG films was determined through measurement oftheir resistance with a Fluke 45 digital multimeter. To this end,the films were cut into rectangular strips about 10 × 25 mm largeand their thickness was measured with a thickness gauge.PFG nanosheets were dispersed in DIH3PO4 or Na3PO4 ata specific concentration  as the anode and platinum foil(2.5 cm × 2.5 cm × 25 μm) as the cathode. Both electrodeswere located in a face-to-face parallel position at 2 cm from eachother. The electrodes were immersed in 80 mL of an aqueous solutionof Htion see 1. Then, 2SO4 (as the electrolyte). On the other hand, in the two-electrodeconfiguration the PFG nanosheets were used as both the WE and CE inthe same electrolyte (1 M H2SO4). The graphene-basedWE and CE were prepared by pressing a certain amount of dried PFGsample onto a circular piece of graphite foil (1 cm2),to a mass loading of around 1 mg cm–2. The carbonfiber-based CE was incorporated in paste form, which comprised activatedcarbon fiber as the active component, polytetrafluoroethylene as abinder, and carbon black as a conductive additive, in a weight ratioof 90:5:5, respectively. Two stacked nylon membrane filters  were used as the electrode separator.Before the cell was assembled, the WE and CE were individually soakedin 1 M H2SO4, and the assembled cell was vacuum-degassed.The energy storage performance measurements were carried out in aVSP potentiostat (Bio-Logic Science Instruments), recording cyclicvoltammograms at different sweep rates (3–500 mV s–1) and galvanostatic charge–discharge curves at different potentialcurrent densities (0.1–20 A g–1), using anelectrochemical window between −0.2 and 0.9 V vs Ag/AgCl formeasurements in the three-electrode configuration and between 0 and1.0–1.1 V in the two-electrode configuration. Electrochemicalimpedance spectroscopy (EIS) measurements were carried out for thefully discharged cell using a potential amplitude of 10 mV in a frequencyrange from 100 kHz to 0.1 Hz.The electrochemicalenergy storage experiments were performed in both three-electrodeand two-electrode configurations with a Swagelok-type cell. In thethree-electrode configuration PFG nanosheets were used as the workingelectrode (WE), commercial activated carbon fiber as the counter electrode(CE), and Ag/AgCl as the reference electrode (RE) in 1 M H3PO4 or Na3PO4 as the electrolyte and phosphate source, asshown in Graphenenanosheets decorated with phosphate groups were readily obtained ina single step via anodic exfoliation at room temperature in a shorttime using aqueous solutions of H31 concluded that using a graphite source withcompact layer stacking (HOPG) and H3PO4 as theelectrolyte, it was impossible to achieve phosphate-functionalizednanosheets by anodic exfoliation; nonetheless, Zu et al.32 achieved the phosphate functionalization viaanodic exfoliation using a graphite rod and ammonium phosphate asthe electrolyte. Moreover, Munuera et al.16 demonstrated that effective electrochemical exfoliation of graphitein different electrolytes is strongly dependent on the type of graphitesource used . In the same way, we found that the use ofgraphite with noncompact stacking such as graphite foil, readily affordedthe exfoliation and functionalization of the material via anodic exfoliationwith a phosphate-based aqueous electrolyte. As is well known, theelectrolyte concentration is also a critical factor in the anodicexfoliation process.16 For this reason,to understand the influence of this parameter on the chemical andmorphological characteristics of the resulting PFG nanosheets andtheir energy-storage behavior, both electrolytes (H3PO4 and Na3PO4) were used at differentconcentrations.A few works have made use of anodicexfoliation as a powerful toolto obtain phosphate-based functional groups on graphene nanosheetsor reduced graphene, owing to the fact that this technique is simple,fast, environmentally friendly, and scalable. Lee et al.,3PO4 and Na3PO4 electrolytes tested here. We determined the concentrationrange where the exfoliation process can be successfully achieved forboth electrolytes. As a general rule, when the electrolyte concentrationis too low, no exfoliation process is attained; this phenomenon wasobserved for H3PO4 at 0.10 M and Na3PO4 at 0.01 M. On the other hand, if the electrolyte concentrationis too high, a very fast expansion and detachment of graphite sheetsfrom the anode takes place. As a result, the attainment of well-exfoliatedgraphene nanosheets is not possible, probably because the fast detachmentof graphite fragments, leaves no time for an efficient layer-by-layerintercalation of the material to occur.16 This behavior was observed for H3PO4 at concentrationsabove 1 M. For the Na3PO4, its solubility inwater is not possible at concentrations higher than 0.25 M, so thiswas the upper concentration limit for this electrolyte. It is alsoworth noting that increasing the electrolyte molar concentration promotesan increment in the yield and consequently an improvement in the solutionconductivity.33 Thus, a good formationof intercalating aqueous ions and a better electrical exfoliationare obtained.2 , C-sp3 , C–O/C–OP (285.7 eV), epoxy , carbonyl , and carboxyl .35 The composition values obtained by the curve-fitting are shown in Table S1 (Supporting Information). It is worthhighlighting that regardless of the electrolyte type or concentrationused to obtain PFG-acid or PFG-salt, the final materials possess oxygen-basedfunctional groups. The O/C ratio without contribution from phosphategroups was calculated to determine the presence of the oxygen-basedfunctional groups, which suggested an oxidized material with an oxygencontent within the common range for anodically exfoliated graphene.37High-resolution XPS (HR-XPS) analysis of C 1sand P 2p correspondingto PFG-acid and PFG-salt at different concentrations were carriedout to examine the new chemical states. 38 Using H3PO4 or Na3PO4 as electrolytes at different concentrationsresults in exfoliated graphene with different O/C ratios, as shownin In fact, the oxidation obtained is similar thatof graphene oxideafter being subjected to an efficient reduction process.3.39 This curve comprises the 2p1/2 and 2p3/2 contributions with a gap of 0.95 eV of separationand a ratio of 2:1 (2p3/2: 2p1/2), which istypically observed for phosphate compounds.39 It is worth noting that all characterizations performed here werecarried out after a thorough purification process of all PFG samplesto remove remnants of electrolyte as well as free, nonbonded chemicalspecies. Therefore, it is reasonable to assume that the phosphategroups observed into graphene samples by XPS are those that are chemicallybonded to the graphene nanosheets.Furthermore,to get a more comprehensive insight into the phosphatespecies chemically bonded to graphene layers, HR-XPS P 2p analysiswas performed, as shown in 3PO4 possesses the highest phosphorus content, and indeed it has thehighest oxygen concentration . The oxidized species reduction suggeststhat when the electrolyte concentration is increased, it promotesa faster intercalation, minimizing the graphene oxidation. Thus, usinglow electrolyte concentration (0.25 M H3PO4 and0.05 M Na3PO4) allows a better intercalationand aqueous ions diffusion. As a result, a slower detachment and layer-by-layerexfoliation was obtained. For this reason, the PFG nanosheets producedusing low electrolyte concentrations were exposed to oxidizing conditionsfor longer times, and as such they were more functionalized with oxygen-and phosphate-based functional groups, as shown in Nonetheless, the use of lowermolar concentrations of electrolytesleads to PFG nanosheets richer in phosphorus. Also, as revealed in 3PO4 or Na3PO4 as electrolytes at different concentrationsproduce PFG materials with different thermostability and hygroscopicproperties (weight loss around 100 °C), as shown in On the other hand, thethermostability and the structural informationon the exfoliated material were obtained by thermogravimetric analysis(TGA) and Raman spectroscopy, respectively, as shown in 40 In particular, desorptionof chemisorbed water around this temperature can be reasonably relatedto water molecules associated to the phosphate groups23 and decomposition of nearby-lying oxygen groups locatedon the basal surface of graphene to give CO and CO2 alsotakes place at temperatures around 150–250 °C.41 Decomposition of oxygen groups bound to edges,rather than basal surfaces, leads to CO/CO2 evolution andhence to weight losses across the whole temperature range above ∼300°C. When the electrolyte concentration was increased, a decrementin the weight loss, both at around 200 °C and at higher temperatures,was generally observed for the PFG-acid and PFG-salt materials. Thisresult was consistent with the lower extent of functionalization (oxidationand phosphatation) of the graphene nanosheets with increasing electrolyteconcentration that the XPS measurements revealed. Moreover, the presenceof oxygen-containing and phosphate functional groups favors the interactionof the graphene materials with the atmospheric water molecules, implyingthat the amount of absorbed water will increase; this phenomenon isobserved in both PFG  materials. The wettability isan essential property in the materials applied in the electrochemicalenergy storage since this property promotes a better interaction ofthe aqueous ions with the electrode surface.It is well known that chemisorbed water moleculesand many oxygen-containingfunctional groups in highly oxidized graphenes can be removed at temperaturesaround 200 °C.–1, respectively,37 appeared in the first-order region of the spectra. Also, the 2Dband was seen in the second-order region of the spectra at ∼2700cm–1. The G band corresponds to C–C bond-stretchingvibrations in the sp2 lattice graphite/graphene materials,whereas D and D′ are defect-related bands.42 The intensity ratio between D and G bands (ID/IG ratio) can be used asa quantitative measure of the structural defects and imperfectionspresent in the lattice.42 As is well known,the intensity of the 2D band is also correlated with the amount ofdefects in the graphene lattice:43 a lowintensity of the 2D band implies a high concentration of defects.The presence ofdefects of the PFG nanosheets was evaluated byRaman spectroscopy, as shown in ID/IG ratio and anincrement in the 2D band intensity were observed, indicating moderatelyoxidized graphene materials. As revealed by the ID/IG ratio and the 2D band,the use of H3PO4 as electrolyte leads to PFGmaterials having slightly higher structural disorder than those observedusing Na3PO4-derivated materials. These resultsare in accordance with the O/C ratio behavior. Additionally, the PFGmaterials were analyzed by UV–vis spectroscopy. It can be observedthat the peak for the different PFG-acid or PFG-salt samples is locatedbetween 260 and 270 nm, as shown in Figure S1a,b (Supporting Information). This absorption peak corresponds to π→ π* transitions from electronicallyconjugated domains in carbon materials,21 and its positionat 260–270 nm is associated to graphitic/graphene materialswith low to moderate oxidation levels.37 These results are thus consistent with the O/C ratios obtained byXPS for the present graphene samples  compared with the PFG-salt (22 m2 g–1), as shown in Figure S2 (Supporting Information),which is related to the exfoliation level. The ultrasonic/centrifugationtreatment can be expected to select those nanosheets with similar lateral sizes  and also with a surfacechemistry that has a high affinity toward water , eliminating those (hydrophobic)nanosheets that are not functionalized, if the latter are really presentin the particles. The introduction of a small amount of functionalgroups to graphene nanosheets, it is possible to increase the waterdispersion stability without disturbing the inherent properties.28 On the other hand, it is very difficult to actuallydetermine whether different particles detached from the graphite anodepossess somewhat different surface chemistries. However, this is possiblynot the case: the graphite foil electrode is rather compact and shouldnot be easy to be infiltrated by the aqueous electrolyte. This meansthat, at any given time, only the graphite microparticles that arestrictly located on the surface of the foil will be expanded and finallydetached from the electrode. Only after this happens, new deeper-lyingmicroparticles will become exposed to the electrolyte, so that theycan be expanded by the electrolytic treatment and detached. Such acourse of action implies that the time any given graphite microparticleis actually subjected to the electrolytic treatment before detachingwill be more or less similar for all the particles, and thereforetheir overall characteristics  should also be similar.As shown in the AFM images 4c,d the 44 Specifically, the green and red dotted circles in the reciprocalspace corresponded to electron diffraction by crystallographic planeswith separations of 2.13 and 1.23 Å, respectively, which couldbe assigned to the {10–10} and {11–20} planes. Theseresults confirmed that PFG-acid and PFG-salt possessed substantialcrystallinity in their basal planes,45 eventhough they were functionalized to a significant extent with oxygenand phosphate groups. It is worth highlighting that PFG possessesa competitive electrical conductivity with other materials obtainedvia anodic exfoliation,37 in which thePFG-acid and PFG-salt yielded values of 8300 and 6600 S m–1, respectively, which is related to the substantial crystallinitypreviously observed.Furthermore, TEMand electron diffraction were used to evaluatethe crystalline quality of the as-prepared PFG sheets. A careful examinationon the edge structure confirmed that the PFG-salt and PFG-acid materialswere well exfoliated and appeared as continuous objects with uniformthickness over the area, as shown in Figure S3d,h, Supporting Information) was uniformlyincorporated throughout the nanosheets. The EDS-derived atomic percentagecorresponding to phosphorus was 1.5 and 1.6 at. % for PFG-acid andPFG-salt, respectively, which was comparable to the values determinedby XPS (2.2 and 1.4 at. %). Oxygen was also uniformly distributedthroughout the graphene samples, as expected .In addition, the presence of the phosphate-basedfunctional groupsin the nanosheets was evidenced by EDS mapping of phosphorus (P Ka1).As evidenced by the corresponding EDS elemental maps, phosphorus  undergoes exfoliationby intercalation of ions  with cointeractionof water molecules from an aqueous electrolyte solution.16 Specifically, the H3PO4 and Na3PO4 electrolytes possess differentspecies; X2PO4–, XPO42–, and PO43– . Since H3PO4 and Na3PO4 are a weak acid and weak base, respectively, the predominantspecies are and H2PO4–, andPO4–3 and HPO42–. The second process is electrogeneration of oxygen radicals: the use of a large anodic overpotential ; active intermediatesfrom water decomposition  are obtained at the graphiteinterface.46 These radical oxygen speciestend to oxidize the subsequent exfoliated graphene nanosheets. Onthe other hand, we know from previous electrochemical exfoliationexperiments carried out in our laboratory with different aqueous electrolytesthat working at bias voltages of 8 or even 6 V still affords exfoliationof the graphite anode, although at somewhat lower yields, but exfoliationbecomes inefficient at even lower voltages. Therefore, a bias voltageof 10 V appears as a reasonable compromise between efficiency of exfoliationand energy consumption. The third process is oxidation byradicals: Radical species oxidize the graphene layers16 and increase the aromatic ring reactivity.47 More specifically, the -OH substituent possessa high resonance donator effect, favoring further functionalizationin ortho- or para-substitutions.48 Furthermore, the presence of •OH radicals can be expectedto induce the formation of phosphate radical species (from phosphateanions), thus increasing the reactivity of this species.50 The fourth process is phosphate functionalization. The radical phosphate functional groups react with carbon atomsfrom the graphene in an addition reaction.51Para-substitution wouldbe favored since the phosphate radicals possess a significant volumeand a steric hindrance phenomenon could be present with -OH substituent.The functionalization routes for both materials are slightly different,since the H3PO4 electrolyte route is an acidmedium and the Na3PO4 electrolyte a basic medium.The formation of one C–O–P is supported by the atomicratio Na/P (2:1) obtained by EDX.The additionof phosphorus-based functional groups to graphenenanosheets is potentially attractive due to their electron-donatingability.39 The presentPFG materials were explored as an electrochemical energy-storage electrodein aqueous 1 M H2SO4 electrolyte using boththree- and two-electrode configurations. The characterization by cyclicvoltammetry(CV) curves and galvanostatic charge–discharge (GCD)profiles at different sweep rates and current densities, as well asby EIS, was carried out for the different electrolyte concentrationsin a three-electrode configuration. The recorded CV curves are shownin Figure S4 (Supporting Information) andexhibited approximately rectangular shapes for a voltage window of1.1 V , which is indicative of a charge storage behaviordominated by capacitive processes.39 All the materialsexhibited pseudocapacitive (Faradaic) reactions in the form of a pairof redox peaks in the 0.3–0.6 V range (vs Ag/AgCl), attributedto reversible C=O/C–O– reactions inquinones and P=O/P–O– in phosphategroups on the carbon surface.25 Remarkably,the samples with higher capacitance were the samples with more functionalgroups. There are two main reasons for this result, owing to the increaseof functional groups: (i) an increment in the hydrophilicity of thenanosheets and (ii) increased redox pseudocapacitance. This phenomenonwas observed for PFG nanosheets obtained with both phosphate sources.Previousstudies on PFG materials as an electrode for electrochemical capacitorsdemonstrated enhanced energy-storage performance owing to the presenceof the phosphate functional group.26 as shown in 3Na2 , whichpermits its use as an efficient auxiliary dispersant in aqueous solutions.52 In the same way, this property allows watermolecules and electrolyte ions to diffuse easier throughout the activematerial, compared with the C–O–PO3H2 functional groups (PFG-acid). This observation is consistentwith the Bode plot analysis , which shows a change in the capacitive processat medium and low frequencies (capacitive and resistance behaviorregion) according to the content of phosphorus-centered functionalgroups.Similarly, EIS measurements can be used to explorethe resistance and capacitive properties of the PFG-acid and PFG-saltsamples from their corresponding Nyquist plots,Figure S6 (SupportingInformation). All profiles exhibited an essentially triangular shape,which is characteristic of a dominantly capacitive process.53 Nonetheless, they showed a very slight slopechange at 0.4–0.5 V (vs Ag/AgCl), associated with the above-mentionedpseudocapacitive reactions. These results are in good agreement withthe observed in CV curves. The specific capacitance values derivedfrom the GCD profiles for the different graphene samples at differentcurrent density are provided in 54 Moreover,in the PFG-salt samples, the material with a higher functional groupscontent is the sample that exhibits a higher specific capacitance(356 F/g), in contrast to materials obtained using other concentrations,where the PFG-salt nanosheets possessed a shorter process (1–2Ω), compared with PFG-acid, as shown in the insets of 39 developed a graphene-based material with a high phosphorus content(3.2 at. %); nonetheless, the energy storage capacitive was 260 F/cm3.The capacitivebehavior of the PFG-acid and PFG-salt is competitivecompared with other products 2, whose In this work, we have obtained graphenematerials with substantialphosphorus content in the form of phosphate groups via a greener,one-step synthesis strategy that is attractive compared to other morecomplex approaches, such as those based on the graphite oxide route.Furthermore, the present graphene materials exhibited good capacitiveperformance. These results arise from: (i) good wettability  and 10,200W·kg–1. On the other hand, the electrode withPFG-salt at 0.05 M yielded values of 14.9 Wh·kg–1  and 9400 W·kg–1.The performance of the PFG devices was investigated by GCDtestsat different current densities at a voltage window of 1.1 and 1 Vfor PFG-acid and PFG-salt, respectively. In this case, the devicesobtained using PFG-acid at 0.25 M possess energy and power densityvalues up to 17.6 Wh·kg–3) together with the eqs S2 and S3. The results found here for thesymmetric device compared favorably to results reported in the literatureusing carbon or graphene materials. For example, the power and energydensities delivered by the PFG materials were comparable to or evenhigher than those of phosphorus-doped graphene,25 as shown in 32 or reduced graphene oxide23 have notreported results for two-electrode devices (only for their three-electrodecounterparts).The gravimetricand volumetric energy and power densities werecalculated using the total electrode mass and the electrode density and 14.9 Wh·kg–1 (21.3 Wh·L–1). Overall, the straightforwardsynthesis conditions as well as fast and facile recovery of electrolytessuggest a circular synthesis, promoting sustainable capacitive energy-storagedevices, combined with a competitive performance in two-electrodesymmetric devices and affordable synthesis.We have successfullydeveloped a straightforward approach to obtainphosphate-functionalized graphene (PFG) in a single-step via anodicexfoliation of graphite. Aqueous H


In wave physics and engineering, directional emission sets a fundamental limitation on conventional simple sources as their sizes should be sufficiently larger than their wavelength. Artificial metamaterial and animal biosonar both show potential in overcoming this limitation. Existing metamaterials arranged in periodic microstructures face great challenges in realizing complex and multiphase biosonar structures. Here, we proposed a physical directional emission model to bridge the gap between porpoises’ biosonar and artificial metamaterial. Inspired by the anatomical and physical properties of the porpoise's biosonar transmission system, we fabricated a hybrid metamaterial system composed of multiple composite structures. We validated that the hybrid metamaterial significantly increased directivity and main lobe energy over a broad bandwidth both numerically and experimentally. The device displayed efficiency in detecting underwater target and suppressing false target jamming. The metamaterial-based physical model may be helpful to achieve the physical mechanisms of porpoise biosonar detection and has diverse applications in underwater acoustic sensing, ultrasound scanning, and medical ultrasonography. Directional emitters can transmit waves to form a narrow beam, increasing the detection resolution and energy in the direction of interest. They have been widely used in applications, such as wireless communications, weather radars, quantum emitters, underwater sonars, medical ultrasonography, etc ,2. ExistMetamaterials provide a promising tool to design physical properties through the programmable design of microstructures . AcoustiHere, we propose a physics-based porpoise model (PPM) via hybrid metamaterial. Based on the complex anatomical and acoustical properties of the porpoise's head, we composed a PPM with steel maxilla, air structures, steel–water and aluminum–water composites. A physical porpoise model with programmable microstructure has been used for the first time to reconstruct complex biosonar components. In comparison with omni-directional systems, the PPM device numerically and experimentally increased directivity and main lobe energy over a broad bandwidth. Its application in underwater target detection is also investigated.Porpoises develop highly sophisticated biosonar for spatial orientation and food acquisition Fig. . Accordiφ = 0.44, aluminum–water composite with φ = 0.62, and aluminum–water composite with φ = 0.66 mimicked the melon, muscle, and connective tissue, respectively. The sound speeds of the system are tunable by changing the filling fraction of these metamaterials. Clearly, the effective medium theory provides a programmable way to design PPM by parameterizing the sound speeds in the porpoise  was placed 1 m away from the source, which was also located along the main beam axis. A second aluminum target, Object 2,  was placed 10 cm behind Object 1 as a jamming object. The hydrophone was placed 10 cm away from the object, between Object 1 and the transducer. In the experiment, the motion of Object 2 was centered on the transducer Fig. . The sout 1 Fig. . The dirNeophocaena phocaenoides) inhabiting Indo-Pacific coastal waters produce directional echolocation clicks to track prey [Finless porpoises (ack prey . Their cack prey . During ack prey . The genack prey  is appliack prey , and thaIn this paper, we have proposed a hybrid metamaterial device inspired by porpoise biosonar. In comparison with existing artificial devices ,24–26 us


In this paper, a switchable and dual-tunable terahertz absorber based on patterned graphene and vanadium dioxide is proposed and analyzed. By controlling the Fermi level of graphene and the temperature of vanadium dioxide, the device’s function can be switched and its absorbing properties can be tuned. When the vanadium dioxide is in an insulator state, the device can be switched from near-total reflection (>97%) to ultra-broadband absorption (4.5–10.61 THz) as the Fermi level of graphene changes from 0 to 0.8 eV. When the vanadium dioxide is changed to a metal state, the device can act as a single-band absorber (when the Fermi level of graphene is 0 eV) and a dual-band absorber with peaks of 4.16 THz and 7.3 THz (when the Fermi level of graphene is 0.8 eV). Additionally, the absorber is polarization-insensitive and can maintain a stable high-absorption performance within a 55° incidence angle. The multilayered structure shows great potential for switchable and tunable high-performance terahertz devices. The efficient absorption of incident waves in terahertz (THz) bands is of significance for a wide range of applications such as imaging, food quality control, communication, spectroscopy, and sensing devices ,3,4,5,6.2) is an important phase change material that represents an insulator-to-metal transition. Its conductivity can be increased by four to five orders of magnitude under light, thermal approaches, and external stimuli [5 S/m is attained. This phase change leads to a reversible transition of infrared light from transmission to reflection [2 can undergo remarkable changes in its electromagnetic characteristics and can also be applied in tunable broadband THz absorber designs [2. By controlling the Fermi levels of graphene and the temperature of VO2, the device’s function can be switched and its absorbing properties can be tuned. When VO2 is in an insulator state, the device can be switched from near-total reflection (>97%) to ultra-broadband absorption 6.11 THz (4.5–10.61 THz) as the Fermi level of graphene changes from 0 to 0.8 eV. When VO2 is transmitted to a metallic state, the device can act as a single-band absorber with an absorption band of 0.84 THz (when the Fermi energy level of graphene is 0 eV) and a dual-band absorber with absorption peaks of 4.16 THz and 7.3 THz (when the Fermi energy level of graphene is 0.8 eV). Moreover, the absorber is polarization-insensitive and can maintain a stable high-absorption performance within a 60° incidence angle. In this work, the absorption mechanism is analyzed first with an impedance matching concept [An efficient approach for realizing switchable and tunable terahertz absorbers is to combine other materials with graphene. This achieves switchable functionality and provides extra tunability. Vanadium dioxide σgσgμc=ℏv follows :(4)Zg=1σIn our simulations, we assumed a relaxation time of 2 can be described by the Drude model [−1 expresses the damping frequency, and 2 spans several orders of magnitude when it is changed from an insulator state to a metallic state by thermal and electrical stimuli [2 is difficult to control during its transition, two stable states of VO2 were taken into consideration for investigating the absorber. For the conductivity of VO2 εVO2 which is supported by a back plate. The thicknesses of the SiO2 layers are h1, h2, and h3 from bottom to top, respectively. Here, the dielectric layers act as resonance absorption spaces. The bottom of the structure is a gold mirror with a thickness of d5 = 0.2 μm. This gold mirror is thick enough to block the propagation of electromagnetic waves and reflect the energy reduced in graphene. Absorption is strengthened by this part of the gold mirror. The first layer is an ultra-thin patterned graphene film, formed by subtracting four rectangular films from a disk with a radius of 2.7 μm. The width of the interval between these different parts is d1. For the second layer, eight rectangular films with a side length of l1 and interval width d2 were subtracted from a square patterned graphene. The third layer consists of a crossed-shaped VO2 with four gold parts in the corners of the square. The square has a thickness of d4, and the side length of the outsider square is l1 and the side length of the hollow part is l4. The absorption of the different layers affects each other, which enhances overall absorption and expands bandwidth. The electric field of the patterned graphene design is concentrated to the edge because of the local surface plasmon resonance that contributes to the absorption. Narrow intervals added to the designed pattern can increase the edge of the graphene and can enhance its absorption. Moreover, the whole structure is symmetrical to ensure that the absorber is insensitive to polarizations and incident angles within a large range. The parameters of the absorber proposed are introduced below. 2’s transition is temperature. It can be tuned using electrifying and terahertz radiation. Adulteration is sometimes used to change the transition temperature in order to suit practical working conditions. of 2.25 . Each laμc = 0.8 eV. VO2 is in an insulator state with a conductivity of 10 S/m. Both the finite element method and ANSYS HFSS are applied to analyze the absorption characteristics of the absorber. A THz wave is applied perpendicular to the upper graphene surface. The periodic boundary conditions are used for the x-direction and the y-direction, and the Floquet ports are set in the z-direction. The absorption of the absorber can be calculated as follows:S parameters are obtained from ANSYS HFSS. Due to the ground gold mirror, the The initial chemical potential of the graphene is assumed as 2–gold layers and the thickness of the dielectric layers can affect the input impedance of the whole structure. To facilitate simulation work, the selection and optimization of the proposed structure parameters were conducted by using the Optimetrics function in ANSYS HFSS. The parameters proposed in Therefore, a relatively high absorption can be obtained when the reflection efficiency is close to zero. Furthermore, the equivalent input impedance (fc) is 7.56 THz, and the fractional bandwidth is 80.87%. It can be concluded that the real part of the impedance is close to 1 and that the imaginary part is close to 0, which demonstrates that the impedance of the absorber matches the free space. To further discuss the function of multilayered structures in achieving broadband absorption, the absorption curves of both single-layer and multi-layer structures are displayed in In this section, both the broadband absorption of the proposed structure and the absorptions of different parts of the structure are displayed and analyzed in In order to further disclose the mechanism of the absorber, electric field distributions of the absorber have been plotted. The frequencies corresponding to the absorption peaks of the curves plotted in In order to further discuss the mechanism of the broadband absorption mode of the absorber, the electric field distributions from both an x-y perspective and y-z perspective are given in c is the speed of light in a vacuum. The electric field is trapped in graphene layers, in the VO2 layer, and in the SiO2 dielectric layers. However, the absorption effect of the dielectric layers is less effective than other parts, corresponding to electric distribution. This can be explained by Equation (9); the imaginary parts of graphene and VO2 permittivity are larger than the dielectric layers within the THz range. In resonant frequencies of 4.91 THz and 6.86 THz, energy consumption mainly occurs in the top and second graphene layers. As the resonant frequency rises to 9.85 THz, the energy consumption of the VO2 layer is strengthened since the electric field intensity increased.The electric field intensity distributions in the y-z plane of the absorber at each peak frequency are plotted in the right side of 2. The structure acts as a broadband THz absorber when μc is 0.8 eV and VO2 is in an insulator state and as a dual-band absorber when μc is 0.8 eV and VO2 is in a metallic state. Additionally, when the voltage applied to the graphene layers is set to 0, the device acts as a single-band absorber when the VO2 is in a metallic state and as a broadband reflector when the VO2 is in an insulator state. The reflectivity is greater than 97% because the transmission coefficient of the absorber is 0 and the absorption is less than 0.03. As shown in The switchable function of the THz absorber is achieved by tuning both the Fermi energy level and the phase transition property of the VOθ) range. In this section, absorptions of the proposed absorber are plotted in contour maps to analyze their insensitivities in both the incident angles and polarization angles. In φ and incident angles θ. In φ. From our results, it can be concluded that the absorber is polarization-insensitive. This can be ascribed to the asymmetrical design of the structure. For broadband absorption, spectra with TE and TM polarized waves are represented in It is of great importance to design absorbers equipped with polarization insensitivity and tolerance within a wide incident angle (μc) for two graphene layers are hard to implement. Therefore, the Fermi energy was set the same for each graphene layer. In μc are set to 0.2 eV, 0.5 eV, and 0.8 eV, respectively. Because the relaxation time is in direct proportion to the chemical potential μc, the following expression is given: μ is the electron mobility decided by the quality of the graphene and is fixed on 1250 cm2/Vs. Therefore, the relaxation time in 2 is in an insulator state. The absorption curves of both TE and TM polarized waves are then calculated. The results indicate that the structure proposed can support different polarized incident waves. As μc increases, the absorption increases gradually and absorption peaks start to appear. Additionally, the quantity of the absorption peaks increases and the broadband absorption appears. The relationship between the Fermi energy μc and the conductivity of graphene is introduced in μc, the greater the surface impedance of the graphene. This change leads to an increase in absorption. Moreover, the relationship between central frequency (fc) and consecutive Fermi energy levels varies within a range of 0.75 eV–0.85 eV. Tuning μc in the range of 0.8 eV ± 0.05 eV, the fc of the broadband absorber can be tuned linearly. It can be concluded that the graphene also plays an important role in controlling the structure. The relaxation time (τ) of the patterned graphene is investigated because it can be changed by the quality of the graphene [Finally, we discuss the tunable function for the broadband absorber of Fermi energy level of the graphene and the relaxation time τ. In practical situations, different energy levels  to high absorption over a wide frequency range (4.5–10.61 THz) as the Fermi level of graphene changed from 0 to 0.8 eV. When VO2 was transmitted to a metal state, the device acted as a single-band absorber with absorption band 0.84 THz (when the Fermi energy level of graphene is 0 eV). When VO2 was in a metal state and the Fermi energy level of graphene was tuned to 0.8 eV, the device acted as a dual-band absorber with bands of 3.78–4.61 THz and 6.75–7.94 THz. The absorber was polarization-insensitive and could maintain a stable high absorption performance within a 55° incidence angle. Moreover, the broadband structure could maintain a stable absorption within a τ change of ±50%. With respect to the large absorption bandwidth, flexible tunability, and switchable functionality, the device is promising for various terahertz applications.In summary, a switchable, dual-tunable THz absorber based on patterned graphene and phase-change VO


Graphene is a type of two-dimensional material with special properties and complex mechanical behavior. In the process of growth or processing, graphene inevitably has various defects, which greatly influence the mechanical properties of graphene. In this paper, the mechanical properties of ideal monolayer graphene nanoribbons and monolayer graphene nanoribbons with vacancy defects were simulated using the molecular dynamics method. The effect of different defect concentrations and defect positions on the vibration frequency of nanoribbons was investigated, respectively. The results show that the vacancy defect decreases the vibration frequency of the graphene nanoribbon. The vacancy concentration and vacancy position have a certain effect on the vibration frequency of graphene nanoribbons. The vibration frequency not only decreases significantly with the increase of nanoribbon length but also with the increase of vacancy concentration. As the vacancy concentration is constant, the vacancy position has a certain effect on the vibration frequency of graphene nanoribbons. For nanoribbons with similar dispersed vacancy, the trend of vibration frequency variation is similar. The mechanical property of graphene is crucial for its reliable application, both from a technical point of view and from the perspective of understanding its fundamental interest in physics deformation [Graphene has excellent elastic properties and high intrinsic strength ,2,3,4, aormation ,21,22. Hormation , vacancyormation , grain bormation , and adsormation  are inevormation . Wang etormation  found thormation  investigormation  investigormation  studied ormation  conducteormation ,34 used ormation  measuredIn this paper, the vibration frequencies of monolayer graphene nanoribbons with different defect concentrations and different defect locations were calculated using the Forcite Tools module of MS  software and the molecular dynamics method. It is predictable that ideal graphene nanoribbons and graphene nanoribbons with different vacancy defects produce different vibrations under the same simulation conditions. The structure of graphene nanoribbons was fixed with the width (3.935 nm) and the thickness (2.0 nm), and the length increased from 8.379 nm to 23.718 nm.i  indicates the type of vacancy and i is the number of missing atoms in a supercell. For example, 3(2 + 1) indicates that the number of missing atoms in the supercell is three, and (2 + 1) indicates the combination of missing atoms. The structure of graphene nanoribbons was obtained by expanding the supercells of the above model according to the requirement. The types of vacancy defects in graphene we discuss in this paper (based on a supercell with the number of atoms 8 × 8 × 1) are shown in The structure of different vacancy supercells was expanded to study the effect of the vacancy concentration of graphene nanoribbons with different lengths on their vibration frequencies. As the vacancy concentration was certain, the effect of the vacancy position of graphene nanoribbons on their vibration frequency was also explored.The vibration frequency of graphene nanoribbons with different vacancy concentrations were obtained from the energy of the simulation results using molecular dynamics method, as shown in Therefore, the variation of bond length and bond angle mades the C-C bond energy of graphene vancancy-containing nanoribbons smaller than the ideal structure, which meant that the graphene nanoribbons containing vacancies acquired less deformation energy than that of the ideal graphene nanoribbons after bending. In addition, the deformation energy of graphene nanoribbons with multiple vacancies was converted to smaller vibration energy in the NVE system synthesis. Since the vibration energy was proportional to the square of the vibration frequency, the vibration frequency of graphene nanoribbons containing vacancies was less than that of ideal graphene nanoribbons.In contrast, the inset shows that the vibration frequency of graphene nanoribbons containing different vacancy concentrations decreased with increasing length as the nanoribbon length increased from 13 nm to about 17 nm. In particular, the vibration frequency of the graphene nanoribbons containing five vacancies decreased sharply; the reduction was greater than that of other vacancies. When the length exceeded 20 nm, the reduction in the vibration frequency changed slowly. The vibration frequencies of the nanoribbons containing single vacancies were significantly higher than those of other vacancies, mainly because the interactions between atoms around the vacancies changed little. In addition, the bond lengths and bond angles between the carbon atoms in nanoribbons containing single vacancies varied less, which had less influence on the vibration frequency.Except for a single vacancy, the decreasing trend of vibration frequency of other vacancies was not obvious. In general, compared with ideal graphene nanoribbons, the vibration frequency of nanoribbons containing vacancies decreased with increasing vacancy concentration. Therefore, vacancy concentration has a certain effect on the vibration frequency of nanoribbons.In addition, dynamic simulation of graphene nanoribbons with different vacancy positions was carried out while the vacancy concentration was kept constant to further understand the influence of different vacancy positions on the vibration frequency of nanoribbons. As shown in As the number of the vacancy in a supercell was four (V-4), the effect of different vacancy positions on the vibration frequency of graphene nanoribbons with (V-4) were discussed, as shown in Compared with other vacancies, the vibration frequency of graphene nanoribbons containing vacancies (V-6) is significantly smaller than that without vacancies  when the nanoribbon length is small, as shown in Defects are inevitable in the actual preparation and processing of graphene. The effects of vacancy defect concentrations and vacancy positions on the vibration frequency of monolayer graphene were studied using molecular dynamics simulations. The results show that vacancy defects reduce the vibration frequency of the graphene nanoribbon, and both vacancy concentration and vacancy position have an effect on the vibration frequency of graphene nanoribbons. Regardless of the presence of vacancy defects, the vibration frequency of graphene nanoribbons decreases with the increase of length. Compared with ideal nanoribbons, nanoribbons with vacancies have smaller vibration frequencies. The vibration frequency of nanoribbons decreases with increasing vacancy concentration, especially when the length of the nanoribbon is small.Furthermore, the vibration frequency of graphene nanoribbons with different vacancy positions showed little difference. The vibration frequency of the nanoribbon with dispersed vacancy was less than that of the nanoribbon with concentrated vacancy. As the vacancy positions were similar, the vibration frequency of graphene nanoribbons was close. In conclusion, both vacancy concentration and vacancy position have a certain effect on the vibration frequency of graphene nanoribbons, but the size effect of nanoribbons is the main factor.