In recent years, optoelectronics and related industries have developed rapidly. As typical optoelectronics devices, photodetectors (PDs) are widely applied in various fields. The functional materials in traditional PDs exhibit high hardness, and the performance of these rigid detectors is thus greatly reduced upon their stretching or bending. Therefore, the development of new flexible PDs with bendable and foldable functions is of great significance and has much interest in wearable, implantable optoelectronic devices. Graphene with excellent electrical and optical performance constructed on various flexible and rigid substrates has great potential in PDs. In this review, recent research progress on graphene-based flexible PDs is outlined. The research states of graphene conductive films are summarized, focusing on PDs based on single-component graphene and mixed-structure graphene, with a systematic analysis of their optical and mechanical performance, and the techniques for optimizing the PDs are also discussed. Finally, a summary of the current applications of graphene flexible PDs and perspectives is provided, and the remaining challenges are discussed. With the development of modern communication technology and the Internet of Things (IoT), devices with high foldability, wearability, and bending resistance have been increasingly developed, and there are in-depth studies on flexible optoelectronic devices. Flexible photodetectors (PDs) can be bent, folded, or even stretched, and their applications in imaging, display, optical communications, medical care, and other fields are drawing increasing attention, as they are highly useful in electronic skin, smart textiles, electronic eyes, and flexible cameras ,2,3,4. TAnother important strategy is to develop graphene hybrid structures. Graphene can be used as a light-absorbing layer or a transparent conductive layer in devices . It can Carbon-based materials, such as diamond, graphite, and amorphous carbon, have broad applications. New carbon materials, including zero-dimensional (0D) fullerenes, one-dimensional (1D) carbon nanotubes (CNTs), and 2D graphene, have gradually evolved into the most promising carbon nanomaterials. Graphene is a single-layer sheet of graphite, which is the matrix of other allotropes of carbon, such as fullerenes, CNTs, and graphite ,19,20,212/(V·s) at low temperatures [2/(V·s) [Due to its 2D structure, graphene has unique physical properties, such as high electrical conductivity and high light transmission. Ultrathin graphene also exhibits high chemical stability, high ductility, and high mechanical strength, e.g., monolayer graphene has a high Young’s modulus of 1.0 TPa and a tensile strength of 130 GPa . Grapheneratures , a value[2/(V·s) ,26. Grap[2/(V·s) ,23, prov2), it is difficult to realize uniform deposition on the required substrate. Therefore, the efficiency of graphene-based (Transparent Conductive Films) TCFs is generally lower than that of commercially available transparent conductive oxides. Graphene films used in photoelectric detection not only need to be highly efficient but must also be uniform without agglomeration [At present, the crystal domain size of graphene films is mostly from micrometers to millimeters, and it can even reach the centimeter level . The curmeration . Recentlmeration .Although the development prospects of graphene films are great, the large-area and high-quality preparation of graphene films remains a critical challenge. Graphene was originally obtained via the mechanical exfoliation of graphite flakes. At present, a series of preparation methods for graphene have been developed, including liquid-phase exfoliation, reduction of graphene oxide, epitaxial growth of silicon carbide (SiC) or metal single crystals, molecular assembly, and CVD, among others ,31. The These methods are thus not suitable for application in flexible electronic devices. CVD is another effective technology for synthesizing large-area graphene films. At present, this technology is being widely studied in the field of graphene film preparation . In the Thermal CVD of graphene involves the use of first-row transition metals, such as iron (Fe) , cobalt The preparation of high-quality graphene is an important issue in the practical applications of graphene. Simultaneously, transfer of graphene is also an indispensable process, which is closely related to the realization of large-scale production of graphene films. For application of graphene in PDs, it needs to be transferred to a target substrate that is suitable for the device. As mentioned in Using a carrier material as a supporting layer, after the graphene is transferred from the growth substrate to the target substrate, the carrier material is removed through physical or chemical methods to complete the transfer. The common method is using polymethyl methacrylate (PMMA) as the carrier to obtain the transfer of graphene grown on the surface of Cu foil. PMMA has high flexibility and high solubility in a variety of organic solvents, but its most important property is high transparency, which is beneficial for clearly observing the removal process of the Cu foil . HoweverUsing this method, graphene is directly attached to the target substrate and peeled off from the growth substrate to complete the transfer without the need for any carrier materials. The principles behind this method are that after the target substrate is processed, the binding force between graphene and the target substrate is much larger than the binding force between graphene and the growth substrate, with the growth substrate then being removed via etching or direct peeling. In 2012, Yoon et al.  used epoFor the transfer of graphene, the solution processing of graphene offers various facile processes, such as spin, dip, and spray coating. The production cost of this method is relatively low and involves the use of fewer chemicals. However, overall, the high-quality, uniform, easy-to-process, and low-cost transfer of graphene is still challenging and requires further exploration to meet industrial and market demands .The basic principles of traditional PDs are straightforward and are described as follows. A reverse bias is applied to PN or PIN junctions; when incident light with a higher energy than the bandgap of the SC absorption layer irradiates the depletion region, the light is absorbed, and photogenerated carriers are generated. The photogenerated electron–hole pairs are separated under the action of an external electric field and quickly reach the electrode on both sides, thereby generating a photocurrent to detect optical signals .11, and the maximum photoconductivity gain is 1.6 × 1011. These results are orders of magnitude higher than previously reported results  [In 2009, Xia et al.  develope7 × 104) .7 A/W, but it will directly affect the response time, and the overall effect is not ideal [However, graphene alone does not have a bandgap, since its unique structure causes photogenerated electron–hole pairs to recombine too fast, meaning that they cannot be effectively separated, limiting its light absorption and EQE. This zero bandgap will also cause the dark current of the graphene device to be too high, indicating that graphene PDs do not achieve a high degree of detection. In addition, although graphene absorbs over the entire electromagnetic spectrum, due to its extremely low light absorption coefficient, intrinsic graphene can only absorb 2.3% of incident light for graphene with a single-layer thickness of 0.34 nm . These wot ideal . Theoretot ideal . Howeverot ideal , resonanot ideal , and hetot ideal  are wideot ideal . On the ot ideal  used hexD*, which reflects the sensitivity of the PD. The realization of high D* relies on a high light response rate and very low noise [2-graphene-WSe2 heterostructure for wide-band optical detection in the visible short-wavelength IR region of the electromagnetic spectrum at room temperature was prepared. The bandwidth of this device reached 400–2400 nm, and the D* of the heterojunction in the near-IR region reached 1011 Jones [3, respectively. Such hybrid PDs that have undergone doping or heterogeneous recombination exhibit greatly improved light responsivity and D* values. The following section gives an introduction to graphene hybrid PDs, including their chemical doping and heterostructure composites.As we know, the most important parameter for evaluating a PD is the specific detectivity, ow noise . In 2013ow noise  used metow noise  coupled 11 Jones  Niu et a11 Jones  used CMOThe room for the development of hybrid PDs is huge. The resistivity of this type of detector is low, and the carrier mobility is extremely high. The response rate and speed of the device are also high, which is exactly what is required for PDs. This is also the reason why the performance of graphene hybrid PDs is higher than those of traditional (semiconductor photodetectors) SCPDs .60 exhibits significant absorption from the ultraviolet (UV) to near-IR regions. Its medium bandgap allows the suppression of dark current and low noise photoelectric detection. Due to its excellent optoelectronic properties and mechanical properties, it has a wide range of potential applications in low-cost flexible optoelectronic devices. The combination of C60 and graphene, a material with low light absorption, overcomes the defects of all-graphene PDs, and the unique semiconductor characteristics of C60 also promote the performance enhancement of all-carbon flexible PDs. Qin et al. [60 film on a hexagonal boron nitride template. Two graphene sheets were used as the source and drain electrodes to prepare a graphene-C60-graphene PD operating over 200–800 nm. As an all-carbon device, it has an operating bandwidth of approximately 5 kHz and an extremely high responsivity of 5510 A/W at 360 nm and 2280 A/W at 405 nm. The graphene-C60 interface barrier with a tunable gate significantly affects the responsivity of the device, with the principles behind this shown in 60 heterostructure on a plastic substrate. 60 device on a PET substrate. It is considered the first all-carbon flexible PD fabricated using a graphene-C60 hybrid structure that exhibits long-term stability. The tight electronic coupling at the all-carbon interface in such devices leads to improved charge transfer efficiency. Due to the excellent light absorption capability of C60, the resultant detector can be used to detect bright signals, including fluorescent lamps, and the light response in the UV to near-IR regions is relatively significant. The mechanical flexibility of the detector is also very high, as shown in As a type of low-dimensional carbon material, SC Cn et al.  obtained2/(V·s), and their on/off ratio is approximately 105 [−2 A/W), excellent mechanical performance, and stable performance after heavy crumpling (≈50% strain). The experimental findings show the great potential of this device as a wearable sunlight sensor that can be used on human skin and other curved surfaces.As one of the allotropes of carbon, 1D CNTs are rolled from a single-layer graphene sheet. They are hollow cylinders with a diameter of approximately 0.5–2 nm that are very flexible in the radial direction and have great potential in high-performance nanoelectronics . The mobtely 105 . These ctely 105 ,74,75,76tely 105 . Liu et tely 105  reportedtely 105  modifiedAll-carbon hybrid films are highly robust films. Their hybrid structures are fully compatible with graphene-based electronic components and can facilitate the integration of large-scale optoelectronics. The hybridization of low-dimensional carbon nanomaterials also allows the realization of extremely flexible, stretchable, and transparent electronic devices. Moreover, the relatively low cost of flexible all-carbon nanomaterials means that they have considerable potential for applications in future wearable and foldable PDs. However, the high optical transmittance of such all-carbon devices has a negative effect on their light response across the visible spectrum, demonstrating that effective methods are required to increase the light-absorption rate while ensuring the transparency of these devices. These methods include decorating or integrating microcavities and waveguides with nanoparticles (NPs) or graphene/SWNTs to improve the photoresponse of carbon nanomaterials ,79,80. F2Te3), as a typical topological insulator, is similar to graphene, exhibiting hexagonal symmetry, a narrow bandgap, high carrier mobility (104 cm2 V−1 s−1), high stability, and great prospects in PDs [2Te3 nanocrystals on single-layer graphene and fabricated a PD based on this graphene/Bi2Te3 heterostructure. 2Te3 and graphene, a light-carrying current is effectively generated and transferred without sacrificing the width of the detection spectrum to effectively increase the photocurrent of the device. It is found that the photoresponsivity and sensitivity of the graphene/Bi2Te3 PD are higher than those of the pure graphene-based PD , and the photocurrent of the heterostructure-based PD is approximately 10 times that of the pure graphene-based PD without applying a grid bias [2Te3/graphene heterostructures by CVD is relatively complicated, suggesting that these materials are not suitable for large-scale preparation. Some reported results demonstrated that this heterostructure-based PD can be manufactured by a facile one-pot hydrothermal process, with the prepared composite material as an electrode material in a photoelectrochemical (PEC) PD. As described in the photoelectrochemical results, the prepared Bi2Te3/graphene heterostructure-based PD exhibited photoresponse performance when exposed to sunlight. Moreover, as the PEC PD is self-powered, it can operate under a bias potential of 0 V, and its light response is very obvious, and can reach 2.2 mA/W. At the same time, upon an increase in the irradiation power density, there is a linear increase in the photocurrent density of Bi2Te3/graphene heterostructure-based PD. Even when the light intensity is reduced, the photocurrent density still reaches a high level, proving that the Bi2Te3/graphene heterostructure-based PD still shows strong light detection ability under low light intensity [2Te3, Yoo et al. [2Te3 nanowires (NWs) to modify graphene-based PDs that delivered 3 × 104 high gain and a wide bandwidth window of 400–2200 nm. Compared with the PD using a Bi2Te3/graphene nanoplate junction, the photoconductive gain of the device can be improved by two orders of magnitude. Using a low bandgap Bi2Te3 NW and graphene junction, the light response rate at 2200 nm (~0.09 mA/W) can be increased by 200%. These Bi2Te3 NW/graphene heterostructures have the advantages of high flexibility, low cost, and facile preparation, and are highly suitable for applications in wearable pressure and light sensors. There are many optoelectronic materials with this topology, such as 2D covalent organic frameworks (COFs), which can also be combined with graphene to form heterostructures. COFs have a high specific surface area and positive selectivity, which enables the photosensitive characteristics of PD to be reversibly adjusted using specific target molecules; therefore, the performance of the device is improved [Topological insulators that have a conductive surface state and exhibit overall insulation properties have been widely investigated for various applications. A large number of studies have shown that topological insulators exhibit excellent photoelectric, high light absorption, and strong photoelectric conversion properties ,83,84. Bs in PDs ,86. Qiaos in PDs  used CVDrid bias . Experimntensity . In addio et al.  recentlyimproved . 6 A/W and a photoconductivity gain of 8.3 × 106. These high values can be attributed to the ZnO nanorods and 5 V grid modulation. This 5 V grid modulation allows the quasi-Fermi level of ZnO to approach the conduction band of graphene, reducing the energy barrier between the materials and enhancing the transfer of photogenerated electrons from ZnO NRs to graphene [3, an order of magnitude superior to that of a photomultiplier without CsPbBr3. The performance of this flexible PD is stable, with its photocurrent exceeding 93% of the maximum value after being bent 500 times [As a transparent conductive semiconductor, ZnO has a wide bandgap of ≈3.37 eV and a high exciton binding energy of 60 meV . The novgraphene . This stgraphene  producedgraphene  introduc00 times .A < 1015 cm−3, which is obtained via the thermal oxidation of copper and exhibits a high light absorption coefficient, high electrical conductivity, and high hole mobility [2O to prepare a graphene/Cu2O QD hybrid PD, which showed an ultrahigh responsivity of ≈1010 A/W at room temperature and achieved fW-scale optical detectivity. This device is flexible, with a responsivity that increases linearly with increasing bias, as shown in 6 A/W at 72.3 pW. However, the device cannot detect weak light intensity at bending states. Even so, the close-to-inversion relationship between the responsivity and incident light intensity still holds, as shown in 2O surface through carbon powder-assisted thermal oxidation. Thus, Cong et al. [2O/Cu-based visible light PD. Under zero bias, this graphene/Cu2O/Cu PD exhibited an ultrafast response time of 4.1 μs at 530 nm, a responsivity at 550 nm of 86 mA/W, and high selectivity for visible light. It is important that the graphene/Cu2O/Cu PD can be bent to a radius of curvature of 30 mm, and even if it is bent thousands of times, it still maintains a high response rate.Cuprous oxide is a non-toxic, naturally p-doped material with a doping level of Nmobility . Liu et g et al.  preparedx3−P) NCs  exhibited localized surface plasmon resonance and ultrafast carrier relaxation. Based on this, Sun et al. [x3−P QDs. This PD delivered a broadband light response in the range of 400–1550 nm. At a visible wavelength of 405 nm, the responsivity of the device is 1.59 × 105 A/W, its photoconductor gain is as high as 6.66 × 105, and it exhibits a high responsivity of 9.34 A/W at 1550 nm. The surface potential of the hybrid structure was studied, and it was found that the surface potential of the area covered by the NCs (doped graphene) was 175 meV higher than the one of undoped graphene, which reduced the work function of graphene. Comparative experiments proved that Cux3−P NC surface ligands play a key role in determining the charge transfer efficiency of Cux3−P to graphene. Through testing, the strain of the device is 1.95%, and after 5000 cycles of bending, the photocurrent and responsivity of the PD remain the same.Graphene-based composites composed of graphene and graphene QDs on stretchable substrates have been studied as sensitizing materials in hybrid structures. The combination of graphene and colloidal QDs with tunable light absorption also provides new possibilities for optoelectronic applications. This hybrid system can be used to address the issues associated with the number and mobility of photoelectric carriers . The repn et al.  produced2) and molybdenum disulfide (MoS2) are the most researched materials for applications in optoelectronic devices in recent years. The reason for this is that their energy gap lies in the visible region of the electromagnetic spectrum, and they exhibit relatively high chemical stability [2 has excellent photoelectric properties, with a bandgap that can be turned from indirect to direct by adjusting the thickness of its layers. Photoelectric devices based on monolayerMoS2 nanosheets have a direct bandgap of 1.8 eV [2 can also be combined with graphene to form a Schottky barrier. Roy [2 to prepare photosensitive memory devices and photoconductive detectors, respectively. In these devices, the MoS2 layer is employed as the active layer to absorb photons and generate carriers. Under the application of a bias voltage, a light response of ~5 × 108 A/W is achieved, which is currently the highest response rate exhibited for a graphene/TMD device. Similarly, limited by the trapped state inside the material and the defects at the interface, the response of the device is ~1 s. Recently, Zhou et al. [2/WS2 is the active semiconductor layer in the middle, with a type II energy band formed between WS2 and MoS2. The photogenerated electrons (holes) are quickly separated and transferred from WS2 (MoS2) to MoS2 (WS2) under 532 nm light, thereby generating photovoltaic effects in the device. In addition, it is also found that the stacking symmetry determines that the type II directionality is the main factor controlling the direction of photocurrent under illumination, while the direction of photocurrent in a homogeneous double-layer film is not easy to control, as the doping level of graphene determines the tunnel barrier [Due to their flexibility and transparency, atomically thin vertical heterostructures have broad prospects in optoelectronic applications. The TMDs that are used to form these heterostructures are layered materials with strong in-plane and weak out-of-plane bonding. There are no dangling bonds on the surface of these materials. The atoms in the layers of the materials are connected via covalent bonds, and the layers are combined with van der Waals forces. They can be peeled off into a 2D layer with a thickness of a single cell. These materials have a unique layered structure and exhibit strong light–matter interactions, with high carrier mobility and a narrow bandgap ,108. Thetability . MoS2 haf 1.8 eV . MoS2 caier. Roy , Zhang [ier. Roy  and cowou et al.  successf barrier . Due to  barrier ,114.2/graphene PD, which exhibited a high light responsivity of 6.3 A/W. As shown in 2 are transferred by a soft lithographic patterning process so that the two materials are stacked in a cross arrangement -based high-performance PDs with horizontal heterostructures and vertical heterostructures have been extensively studied. The atoms in the layers of these heterogeneous structures are connected with covalent bonds, and the layers are combined through van der Waals forces. The core of the device is a Schottky junction formed between MoS2 and graphene, which shortens the transmission distance of photogenerated carriers between the source and drain electrodes and reduces the cutoff current. Compared with graphene- and MoS2-based biophotoelectric detectors, the response speed of GMG PDs is greatly accelerated. For example, the GMG lateral heterojunction PD synthesized by Liu et al. [6. The responsivity is more than 2 × 103 mA/W, and the maximum D* is as high as 1013 Jones. The test results show that in this GMG structure, the photocurrent increases with an increase in the illumination power and the grid voltage, and the transmission curves of the GMG PD under dark conditions and different illumination powers have almost the same characteristics , the R value of the flexible PD was still maintained at approximately 32% (Doping graphene with trifluoromethanesulfonyl amide (TFSA) and triethylenetetramine (TETA) molecules results in changes in its electronic composition. Unlike doping with metal particles, these two molecules tend to accept/donate electrons from/to the host materials, respectively, which induces p-/n-type doping . Jung ettely 32% l, and it2 has a high optical absorption coefficient in the visible region and excellent photoconductive properties. It is an n-type semiconductor with a layered cadmium iodide (CdI2) structure and a bandgap of approximately 2.2 eV [2 has received an extensive amount of attention, especially in photoelectric detection. Using a facile chemical bath deposition method (CBD) to prepare a vertically grown SnS2 nanosheet array film, Fang et al. [2 nanosheet array thin-film heterojunction PD. Then, they adopted a direct transfer using a wet chemical method to introduce a transparent graphene film as a hole-transport layer in a PD. The composition and microstructure of the obtained samples were analyzed by Raman spectroscopy, and the results indicated that a graphene/SnS2 composite was successfully fabricated. The detector exhibits strong photoresponsivity, with a measured optical on/off ratio of 1.53, which is higher than that of pure SnS2 film of 1.1. The response time (rise time/decay time) was 5 s/4.9 s, which was better than that of pure SnS2 array membranes (5.4 s/5.1 s). The improvement in photoresponse indicated that the effective heterojunction formed between the vertical SnS2 nanosheet array film and graphene suppresses the recombination of photogenerated carriers [Among the various 2D layered metal disulfides, SnSy 2.2 eV . Owing tg et al.  obtainedcarriers . This su2-graphene lateral heterostructure PD on a PEN substrate, which failed during strain measurements. Later, Pataniya et al. [2 to decorate pencil trace on ordinary cellulose paper to prepare a high-performance WS2/graphene heterostructure PD. It is noteworthy that cellulose paper is readily available, light weight, and environmentally friendly. The production of such paper-based flexible equipment is not only a huge advantage in terms of cost, but also allows environmentally friendly and super large-area equipment to be prepared. The fabricated device uses large-area electrophoretic deposition technology to decorate the pencil trace conductive film with liquid-phase exfoliated WS2 nanosheets. The resultant device exhibited excellent photoresponse and temporal photoresponse characteristics over broad spectral regions, from the visible to near-IR region. This is due to the WS2 nanosheets having the ability to absorb photons over a broad spectral region and the high electrical conductivity of graphene promoting the unimpeded flow of photogenerated carriers [10 Jones, the EQE is 81.39%, and the response time is 2.1 s. As shown in 2 on the substrate via a hydrothermal method to prepare a SnS2/graphene broadband heterojunction PD. Using first principles calculations, the bandgap and electron affinity values are theoretically estimated. The test results showed that under visible and UV light, the response rates of the device were 6.98 and 3.67 mA/W, respectively, and the response times were 4.53 and 5.41 s, indicating that the response of the device to visible light was stronger than that to UV light . However, due to the long lifetime of photogenerated carriers, the response speed of the device increased . The PD is shown to withstand 3000 cycles of tensile strain with a radius of curvature of 12mm, with no obvious changes in photocurrent, indicating reliable light response stability after successful bending  driven by a 3 V bias voltage, a high light response rate (≈1017.1 A/W), and high light detection performance (≈2.02 × 1013 Jones) under 532 nm laser irradiation. As shown in 3. The energy band diagram of the device under illumination is shown in Perovskite materials exhibit high light absorption characteristics. The photoluminescence of CsPbX3 nanocrystals is characterized by narrow emission line widths of 12–42 nm, wide color gamut covering up to 140% of the NTSC color standard, high quantum yields of up to 90%, and radiative lifetimes in the range of 1–29 ns . In part bending b,c. UndeR is 5.90 × 104 A/W, with a D* of 1.31 × 1013 Jones, the highest detection value of a perovskite-functionalized graphene PD reported [In optoelectronic devices, the synergistic effects and complementarity of perovskite hybrid materials will result in improved performance of each component. However, the formation of spatially inefficient perovskite islands on graphene leads to the formation of perovskite grains with lower crystallinity, which reduces the photogenerated carrier density compared with the perovskite layer and remains an issue to be solved. A previously studied dual hybrid system did not exhibit a synergistic effect, which led to insufficient improvement in device performance . Thereforeported . In addi3 NC/graphene hybrid PD that can be used to effectively detect UV light, in which CsPbBr3 NCs are used as a light absorber and graphene as a carrier transport channel. The hole and electron mobilities of the device in the dark are 1.7 × 103 cm2/(V·s) and 6.4 × 103 cm2/(V·s), respectively. Under the illumination of 405 nm light, the device exhibited high responsivity, D*, and EQE, with maximum values of 3.4 A/W, 7.5 × 108 Jones, and 103%, respectively, and short rise/decay times of 7.9/125 ms, respectively. Although the response speed, responsivity, and stability of flexible graphene/perovskite hybrid PDs need to be further improved, this facile and low-cost manufacturing method provides the possibility for producing devices for large-scale applications in sensitive broadband detection [Recently, Che et al.  demonstretection . These n2 (SQD:SiO2) ML heterojunction PDs, Shin et al. [R value of 0.413 A/W at a doping concentration of nD = 20 nm, a linear dynamic range of 92 dB, a detection efficiency of 1.09 × 1010 cmHz1/2/W, and an EQE of 81.33% at a peak wavelength of 630 nm. In air for 700 h, the device has almost no loss in the R value. These characteristics are comparable with those of commercially available Si-based PDs and are superior to those of previously reported graphene/Si PDs.The design of hybrid PDs aims to improve the sensitivity of graphene PDs, but the integration process of these hybrid systems is complicated. Combining graphene with QDs can also extend the lifetime of induced photon carriers, which helps to obtain devices that can achieve a high-response light detection distribution. Hybrid graphene–QDPDs combine the strong light absorption of QDs with the high mobility of graphene, in which the QDs absorb light and generate photogenerated carriers efficiently transported by graphene ,140,141.n et al.  employedf noise. Tian et al. [11 Jones. Compared with the results measured by the source-drain electrode, D* is increased by approximately 500 times. The reason for this result is that the Schottky barrier at the graphene/SC interface shields the process of carrier capture and release. In addition, the rise and fall times of the PD are 4 and 37 ms, respectively, and the response speed is increased by two orders of magnitude. By analyzing the rectification characteristics and photovoltaic characteristics of this type of PD, it is found that at 300 K, this type of PD is highly sensitive to visible/near-IR light (405–850 nm). These results further confirmed the great potential of the graphene/GaAs heterojunction as a high-performance broadband self-powered PD [Graphene/SCs are another hybrid PD structure. In these structures, a heterojunction is formed between graphene and an SC. This heterojunction simultaneously has excellent light absorption of SCs and an ultrahigh migration rate of graphene, in which the photogenerated carriers are separated at the graphene/SC interface. This unique heterostructure breaks the limitations of previous epitaxial growth, such as point defects and dislocations caused by interatomic diffusion and lattice mismatch ,144. Hown et al.  combinedThe current status and research progress of different graphene hybrid PDs are discussed. Through the combination of graphene and other materials, some traditional inorganic materials can be used in PDs, which exhibit excellent optical and electrical performance. The unique 2D structure of graphene makes an important contribution to the improvement of the detectivity and responsivity of these devices. 3NH3PbI3 microwires and graphene as the matrix. Stretchability of the device is achieved by releasing and crumpling the perovskite/graphene matrix. The device performs well after being stretched to 50% strain 100 times. When stretched to 80% or even 100%, the photocurrent does not disappear completely, and the response is faster.The design of flexible electronic devices needs to overcome the mechanical mismatch between rigid inorganic SCs and elastic and curved surfaces. There are two ways by which to build a structure that can bend: using flexible materials or making a flexible structure. When the material is thin enough, the bending strain decreases as the thickness of the material decreases. For example, single-layer or few-layer graphene atoms are thin and exhibit high flexibility. Graphene-based devices can be bent and cycled many times without affecting their performance by placing them on flexible substrates ,155. Rec2-graphene heterostructure to be softly implanted into the retina of the human eye. MoS2 is transferred to the graphene electrode as a light-absorbing layer and can be used to effectively detect light signals and apply programmed electrical stimulation to the optic nerve, with minimal mechanical side effects on the retina, as shown in Although the manufacturing process of flexible stretchable devices is complicated, they have multiple functions. For example, a flexible PD can be placed on the surface of skin to form a wearable patch for the monitoring of human health. Choi et al.  develope. Paper-based PDs make full use of the natural moisture absorption and flexibility of paper, exhibiting high flexibility. Recently, a new type of wearable paper-based PD was developed, which is the first PD that can be used to simultaneously measure the intensity and dose of UV light. Even after 1000 bends, the photoelectric signal of this PD remains stable. Part of the properties are shown in As a very low-cost soft material, the light weight, flexibility, and environmental friendliness of paper have made it the focus of many studies . Paper-bThe development of PDs has grown very rapidly over the last few years. Traditional graphene PDs based on composites have been extensively studied. With the ability to integrate them with soft materials and bendable surfaces, flexible electronic products surpass traditional rigid electronic device technology and have broad development prospects in wearable devices and the Internet of Things. This review discusses and summarizes the latest developments in graphene flexible PDs. Graphene can be used not only as a transparent conductive layer, but also as a light-absorbing layer. By combining graphene with different materials, the sensitivity of the resulting device can be significantly improved, and its detection spectrum can be extended from the UV to mid-IR or even THz bands. Graphene PDs can be applied to other types of sensing elements, such as gas sensors or biosensors, to achieve multifunctional sensing. Although great progress has been made in the photodetection of heterostructures, the charge transfer in heterojunctions still needs to be studied in depth. Although one of the performance indicators of a device can reach an ultrahigh level, the overall performance of PDs still needs to be improved. For applications in different fields, the focus on improving device performance is also different. For example, imaging applications require high responsiveness, while optical communication fields require devices that exhibit fast response. In practical applications, factors such as manufacturing costs and the size of packaging should also be taken into consideration. The quality of graphene presents a huge challenge in the large-scale production of graphene PDs. Although many methods can be used in the large-scale preparation of graphene films (as mentioned above), many challenges still exist. The preparation technology of graphene films is an issue that requires attention, in addition to the complexity and cost of the processes. Although the performances of graphene-based hybrid devices are far superior to those of traditional PDs, they are still not currently used in large-scale commercial applications. Research on new layered materials with suitable bandgaps and effective light absorption thus remains a critical challenge.


The graphene/C4F2 vdW heterostructure is structurally stable at room temperature. In the ground state, the graphene/C4F2 heterostructure forms n-type Schottky contact with a Schottky barrier height of 0.46/1.03 eV given by PBE/HSE06. The formation of the graphene/C4F2 heterostructure tends to decrease in the band gap of the semiconducting C4F2 layer, suggesting that such a heterostructure may have strong optical absorption. Furthermore, the electronic properties and contact types of the graphene/C4F2 heterostructure can be adjusted by applying an external electric field, which leads to the change in the Schottky barrier height and the transformation from Schottky to ohmic contact. Our findings reveal the potential of the graphene/C4F2 heterostructure as a tunable hybrid material with strong potential in electronic applications.Motivated by the successful exfoliation of two-dimensional F-diamane-like C 4F2 heterostructure.We perform a first principles study to investigate the atomic structure, electronic properties and contact types of the graphene/F-diamane-like C Graphene, a single-atom-thick 2D material was successfully discovered by mechanical exfoliation of graphite in 2004.7 After such a discovery, a lot of novel 2D materials with unusual physical and chemical properties and wide range of potential applications, were discovered and studied, including transition metal dichalcogenides,8,9 phosphorene,10,11 MXenes12,13 and Janus-like 2D materials.14,15 Owing to their outstanding physical and chemical properties, 2D materials are promising candidates for electronics, optoelectronics and photocatalytics.16–18 However, the aforementioned 2D materials have some disadvantages that hinder their application in many advanced technologies. For instance, the lack of a band gap in graphene limits its application in high-performance nanodevices like electronics and photoelectronics.19 A small carrier mobility in 2D TMDs like MoS2  materials. Through the application of modern science and technology, such as exfoliation21–24 functionalization25–27 and constructing 2D van der Waals heterostructures (vdWH).28–31 For instance, Pierucci et al.21 demonstrated that the structural and electronic properties of the MoS2 monolayer can be modulated by chemical doping of hydrogen atoms, which results in the transition from an n-type to p-type semiconductor. Furthermore, Muniz et al. predicted that the band gap of twisted bilayer graphene can be tuned by the substitution of chemisorbed H by F atoms,32 by the formation of diamond superlattices33 and by the formation of fullerene superlattices.34 Sun et al.35 predicted that the ability to absorb the sunlight of 2D blue phosphorene is enhanced upon defects. Very recently, F-diamane-like C4F2 2D materials, a new type of carbon allotrope have been successfully synthesized by a CVD method36 and liquid-phase exfoliation.37 The F-diamane-like C4F2 monolayer has been predicted to be stable under an ambient atmosphere.36 The F-diamane-like C4F2 possesses a semiconducting characteristic and exhibits superior carrier mobility as well as high mechanical strength and thermal conductivity.38 Furthermore, the electronic and transport properties of the F-diamane-like C4F2 monolayer are very sensitive to strain engineering38,39 and chemical functionalization.40 These aforementioned properties of the F-diamane-like C4F2 material make it a promising candidate for high-performance electronic and optoelectronic applications.Currently, there have been many effective strategies for modulating the properties of 2D materials, such as doping,41 or by exfoliation.42 A plethora of vdW heterostructures composed of two or more 2D materials have been fabricated experimentally and predicted theoretically, such as TMDs-based vdW heterostructures,43–46 phosphorene-based vdW heterostructures47–49 and graphene-based vdW heterostructures.50–56 Among these, graphene-based vdW heterostructures have received considerable interest owing to the existence of new properties different to the constituent materials. For instance, Aziza et al.57 showed that the Dirac cone of graphene is shifted by 100 meV toward lower binding energy upon contact between graphene and GaSe. Nguyen et al.56 predicted that the contact between graphene and a BiI3 monolayer gives rise to the formation of an n-type Schottky contact. To date, the interfacial characteristics and the electronic properties of the contact between graphene and an F-diamane-like C4F2 monolayer, have not yet been investigated.As we discussed above, the construction of vdW heterostructures is known to be one of the most effective tools to adjust the electronic properties of 2D materials. In particular, 2D-based vdW heterostructures can be fabricated in experiments by transfer methods4F2 monolayers and the superior properties of graphene-based vdW heterostructures, in this work, we perform a first principles study to investigate the atomic structure, electronic properties and contact types of the graphene/F-diamane-like C4F2 (graphene/C4F2) vdW heterostructure. The graphene/C4F2 vdW heterostructure is structurally and thermodynamically stable at room temperature. The formation of graphene/C4F2 vdW heterostructure gives rise to the Schottky contact, which can be adjusted by an external electric field. Our findings reveal the potential of graphene/C4F2 heterostructure as a tunable hybrid material with strong potential in electronic applications.Motivated by the successful exfoliation of 2D F-diamane-like C24F2 vdWH are calculated using first-principles calculations. All calculations are carried out in the Vienna ab initio simulation (VASP)58 and Quantum Espresso 59,60 simulation packages. The generalized gradient approximation (GGA)61 by Perdew–Burke–Ernzerhof (PBE) is employed to describe the electronic exchange and correlation. The projector augmented wave (PAW) approach62 is chosen to treat the core and valence electrons. According to the weak vdW interactions between layered 2D materials, the DFT-D3 method of Grimme 63 is adopted to better describe these interactions. The underestimation of the traditional PBE method on the band gap of 2D materials leads us to use the Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional to obtain a more accurate value for the band gap. The cut-off energy for the plane-wave expansion is set to be 510 eV with a 12 × 12 × 1 k-point mesh. To avoid interactions between periodical slabs, we set a large vacuum thickness of 30 Å along the z direction. The convergence of energy and force are set to be 10−6 eV and 10−3 eV Å−1, respectively.In this work, our results for the geometric optimization, electronic properties and the band alignment of the graphene/C34F2 monolayer. After geometric optimization, the atomic structure of single-layered F-diamane-like C4F2 is as depicted in 4F2 monolayer has a layered buckling structure. The lattice constant of F-diamane-like C4F2 monolayer is calculated to be 2.54 Å, which shows good agreement with the experimental value37 and theoretical reports.39 The electronic band structures as well as the weighted band structure of the F-diamane-like C4F2 monolayer are depicted in 4F2 monolayer exhibits a semiconducting characteristic with a direct band gap. Both the valence band maximum (VBM) and conduction band minimum (CBM) of the F-diamane-like C4F2 monolayer are located at the Γ point for both PBE and HSE06 method. The calculated band gap of F-diamane-like C4F2 monolayer is 4.03 eV, which shows good agreement with the previous calculations.39,64 The traditional PBE method is known to underestimate the band gap of 2D materials, we thus perform HSE06 calculations to obtain a more accurate band gap for the F-diamane-like C4F2 monolayer. The HSE06 band gap of the F-diamane-like C4F2 monolayer is calculated to be 5.66 eV. The weighted band structure of F-diamane-like C4F2 monolayer in 4F2 monolayer, as depicted in Fig. S1 of the ESI.4F2 monolayer is structurally stable in the ground state. The electrostatic potential of the F-diamane-like C4F2 monolayer is illustrated in 4F2 monolayer is calculated to be 8.07 eV.We first explore the atomic structure and electronic properties of the F-diamane-like C4F2 vdW heterostructure by placing graphene on top of the C4F2 layer. The same lattice parameter of graphene (2.46 Å) and C4F2 (2.54 Å) gives rise to a small lattice mismatch of about 2% in the graphene/C4F2 vdW heterostructure. The optimized atomic structure of the graphene/C4F2 vdW heterostructure for different stacking configurations, namely SC-I, SC-II, SC-III and SC-IV are depicted in 4F2 layer is defined by D as shown in 4F2 vdW heterostructure are insensitive to the stacking configuration. Furthermore, we find that this value of the interlayer distance is the same as that of graphite65 and other graphene-based vdW heterostructures.56,66–70 This finding suggests that the graphene/C4F2 vdW heterostructure is mainly contributed by weak vdW interactions. Such vdW interactions keep the system stable and can be obtained in future by common methods such as CVD. Furthermore, it should be noted that the weak vdW interactions between graphene and the F-diamane-like C4F2 monolayer keeps the graphene/C4F2 vdWH stable and makes the graphene surface flat. However, in reality the graphene surface may be corrugated when it is deposited on the C4F2 monolayer. The surface corrugation may affect the change in the barrier height of the contact types. However, we believe that the surface corrugation does not change the band shapes and contact types of vdWHs.Now, we construct the atomic structures of the graphene/CEH, EG and EC are the total energy of the vdW heterostructure, isolated graphene and C4F2 monolayers, respectively. N stands for the number of carbon atoms in such vdW heterostructures. The binding energy of the graphene/C4F2 heterostructure is calculated to be −45.32 meV per C atom for SC-I, −43.96 meV per C atom for SC-II, −40.38 meV per C atom for SC-III, and −44.38 meV per C atom for SC-IV. The SC-I configuration has the lowest binding energy, indicating that it is the most energetically favorable stacking configuration. We further consider the effect of the interlayer coupling and external electric field on the electronic properties and contact types of the graphene/C4F2 vdW heterostructure for stacking SC-I configuration.To confirm the stability of such a heterostructure, we calculate the binding energy as follows:4F2 vdW heterostructure for all stacking configurations are illustrated in 4F2 monolayers. The reason of such a combination is due to the weak vdW interactions between graphene and C4F2 monolayers. Moreover, such band structures demonstrate that the stacking configurations do not affect the electronic properties of the heterostructure. Graphene maintains the metallic characteristic of a Dirac cone at the k point. While, the F-diamane-like C4F2 layer is the semiconductor with a direct band gap of 3.90/5.53 eV for PBE and HSE06 calculations for all stacking configurations. The contributions of each atom in the graphene/C4F2 vdW heterostructure for different stacking configurations are depicted in Fig. S2 ESI.4F2 layer is still smaller than that of the freestanding monolayer. This finding suggests that the graphene/C4F2 heterostructure may have a stronger optical absorption than that of its constituent monolayers.The projected band structures of the graphene/C4F2 monolayer gives rise to the formation of metal–semiconductor contact, which is a crucial component of high-performance electronic and optoelectronic devices. Depending on the position of the VBM and CBM of semiconducting C4F2 relative to the Fermi level of metallic graphene, the graphene/C4F2 heterostructure may form either Schottky contact or ohmic contact. The schematic diagram of the graphene/C4F2 heterostructure is illustrated in 4F2 heterostructure forms the Schottky contact. The Schottky contact is mainly characterized by an energy barrier, namely the Schottky barrier height (SBH). Regarding the Schottky–Mott rule,71,72 the n-type SBH (Φn) and p-type (Φp) can be calculated as: Φn = ECBM − EF and Φp = EF − ECBM. The Φn and Φp are calculated to be 0.46/1.03 eV and 3.43/4.50 eV, given by PBE and HSE06 methods, respectively. This finding demonstrates that the graphene/C4F2 heterostructure exhibits an n-type Schottky contact for both PBE and HSE06 methods.More interestingly, the contact between metallic graphene and the semiconducting C4F2 heterostructure is always subjected to an electric field when used as a component of electronic and optoelectronic devices. Therefore, it is important to check the effect of an external electric field on the electronic properties and contact types of the graphene/C4F2 heterostructure. The schematic model of applying an external electric field is depicted in 4F2 monolayer graphene is defined as a positive direction. The change in the band gap as well as the SBH of the Schottky contact is illustrated in −1, the Φn increases, while the Φp decreases accordingly. The change of the SBH of the graphene/C4F2 heterostructure under the positive electric field is almost linear. With the range of the positive electric field from 0 to +3 V nm−1, the Φn is still smaller than the Φp, indicating that the n-type Schottky contact is maintained in the graphene/C4F2 heterostructure. On the other hand, when the negative electric field is applied, the Φn decreases, while the Φp increases. At the critical negative electric field of −3 V nm−1, the Φn has already fallen to nearly zero, indicating a transformation from n-type Schottky contact to n-type ohmic contact.Furthermore, the graphene/C4F2 heterostructure under an electric field are plotted to provide a better understanding of the change in the electronic properties, as depicted in 4F2 layer moves toward the Fermi level, while its CBM comes upward from the Fermi level. Such shifts cause the change in the Φn and Φp as we have discussed above. When the negative electric field is applied, the CBM is shifted downward to the Fermi level, while the VBM is moved far from the Fermi level. At the negative electric field of −3 V nm−1, the CBM of the semiconducting C4F2 layer crosses the Fermi level, suggesting a transformation from Schottky contact to an ohmic one. The nature of the change of band types of vdWH can be described by the change in the PFOS of all atoms, as depicted in Fig. S3 of the ESI.4F2 heterostructure as a tunable hybrid material with strong potential in electronic applications.The projected band structures of the graphene/C44F2 heterostructure as well as the contact types and the effect of an external electric field. The graphene/C4F2 heterostructure is mainly characterized by weak vdW interactions with the interlayer distance of 3.35 Å and the binding energy of −45.32 meV per C atom for the most energetically favorable stacking configuration. The formation of the graphene/C4F2 heterostructure gives rise to a decrease in the band gap of semiconducting C4F2 layer and tends to the formation of the n-type Schottky contact with an SBH of 0.46/1.03 eV given by the PBE/HSE06 method. Both the contact type and SBH of the graphene/C4F2 heterostructure can be modified by applying an external electric field, which gives rise to the transformation from a Schottky contact to an ohmic one, and tends to a variation in the SBH. Our findings reveal the potential of the graphene/C4F2 heterostructure as a tunable hybrid material with strong potential in electronic applications.In summary, we have performed first-principles calculations to study the structural and electronic properties of the graphene/CThere are no conflicts to declare.RA-011-D1RA06986A-s001


Passer domesticus) can adjust colouration when faced with varying humidity conditions. We exposed birds to either a dry (humidity 45%) or a wet environment (70%) six months before their moult, and measured colouration in newly developed feathers in five parts of the body . As predicted by Gloger’s rule, birds in wet conditions developed darker (bib and belly) and larger (bib) melanised plumage patches, than birds in dry conditions. Our result provides the first unequivocal evidence that the ability of individual birds to adjust their colouration may be a potential adaptation to climatic changes in endotherms.Knowing how animals adapt their phenotype to local temperature and humidity is key to understanding not only ecogeographical rules, but also how species will manage climate change, as current models predict changes in global patterns of temperature and precipitation. In endotherms, colour adaptations in response to climate have been under investigated, and their acclimatization-the individual capacity to reversibly adjust phenotype in response to different environments-is unknown. Geographic trends can provide clues about abiotic variables involved in colouration, as postulated by Gloger’s rule, which predicts darker individuals in warm and humid regions. We tested whether house sparrows ( One type of phenotypic plasticity is acclimatization, which refers to reversible, temporary, and repeatable phenotypic adjustments within a single individual in response to changing environmental conditions2. Such an ability is an important component of fitness for organisms that face variable environments3. Understanding how acclimatization generates functional variability has become increasingly important in the context of climate change, as current models predict changes in global patterns of temperature and precipitation5.Phenotypic plasticity is the ability of a single genotype to express different phenotypes depending on environmental conditions7. Whereas both rules account for geographical variation in colour due to the differential deposition of melanic pigments, their predictions contradict each other: Bogert´s rule, initially proposed for ectothermic animals, predicts darker individuals in cold environments, based on the thermal advantages of darker phenotypes which generally heat more and faster than paler ones8. In contrast, Gloger’s rule, initially proposed for endothermic animals, predicts darker individuals in warm and humid environments7. Although the mechanism behind Gloger’s rule is not clear, thermal regulation may not be the only explanation as endotherms mainly rely on internal metabolic processes to maintain their body temperature. Currently, selection for more cryptic-darker phenotypes and increased protection against keratinolytic bacteria by melanin deposition in warm humid environments are two main explanations when this biogeographical pattern is found9.Animal colouration is one of the most diverse phenotypic traits and is known to vary along climatic gradients, as described by two ecogeographical rules: Bogert and Gloger’s rules10. This is particularly true in birds, as this taxon shows the most diverse colours among animals. Recently, it has been shown that patterns predicted by Gloger’s rule are mainly associated with variation in humidity rather than temperature9. Further, the effects of temperature on colouration have been found to oppose Gloger’s rule predictions, namely that species/individuals with darker plumage are often found in colder rather than in warmer regions16. Therefore, it has become necessary to disentangle the effects of temperature and humidity on melanin deposition.However, colouration has several other functions, from cryptic camouflage to conspicuous visual signals. So, explanations for its variation are far from simple and individual colouration does not necessarily follow the patterns one would expect from Gloger’s rulePasser domesticus and common kestrels Falco tinnunculus18. Also, a recent work with nestling griffon vultures Gyps fulvus19 suggested birds have the capacity to respond to local sunlight conditions by adjusting melanin deposition in growing feathers, which influences fitness. Yet, experimental support for such capacity is lacking, and to our knowledge, only two outdated works suggest the idea of humidity and temperature may promote darker colours, one conducted in birds and one in mammals21. Such relevant findings urge confirmation with a solid experimental design.A main point in understanding the underlaying mechanism of Gloger’s rule is to test the ability of individuals to adjust their colouration to fit their environment. Knowing how populations adapt their colour to local temperature and humidity conditions is key to understanding colour-related biogeographical patterns. The observed colouration in a population may be the product of microevolution (gene frequency) or of phenotypic plasticity, including acclimatization. Unlike ectotherms, colour adaptations in endotherms in response to climatic changes have been under investigated, and their phenotypic plasticity associated with humidity and temperature acclimatization is currently unknown. It has been experimentally shown that environmental growth conditions can vary the melanic colouration in nestlings of house sparrows 22 suggesting a role for phenotypic plasticity. We exposed house sparrows to either a dry or a humid environment six months before their moult, and measured plumage colouration in newly developed feathers. If humidity promotes a higher deposition of melanin pigments, as assumed by Gloger’s rule, birds housed in the humid environment should be darker than birds housed in the dry environment.The purpose of our study was to test phenotypic plasticity in melanin colouration in endotherms in relation to variation in climatic conditions. Specifically, we tested whether birds may acclimatize their plumage colouration after moult in response to varying conditions of humidity, since humidity appears to be the core driver behind Gloger’s rule. We used the house sparrow as a model species because its plumage is coloured through melanin deposition and because it has been reported that introduced house sparrow populations have apparently rapidly changed their coloration to local climatic variationad libitum with water, a commercial mixture of seeds for canaries (VINCI) and cuttlefish bones to meet calcium needs . To ensure room ventilation, aviary windows were opened twice a day (at 9:00 and at 18:00 h) for one hour. Aviaries were checked daily, and the feeders and drinkers were cleaned every day. A veterinary revision was carried out four days a week.In February–March 2021, 54 adult male house sparrows were captured with mist nests from a wild population at the ZOO Aquarium in Madrid. All individuals were marked with metallic rings. After capture, birds were transported to the Foundation for Research in Ethology and Biodiversity (FIEB Foundation) and confined randomly in 4 indoor aviaries  that were built with insulating material and had climate-controlled housing. Birds in the aviary were maintained at ambient temperature/humidity and under a natural photoperiod. All aviaries had a north-facing window (1 m × 1 m) and contained plants and shrubbery to simulate a natural environment. After one month of habituation to captivity, the experiment was performed. During both habituation and experimental periods, birds were provided 24 thus as total reflectance increases, plumage tends to get lighter . Also, to measure the area of the black chest bib, digital photographs from the ventral side of the bird were taken while the bird was held at an angle of 90° between the objective of the camera (Canon EOS 77D) and the surface of the body. Photographs were taken under a sunshade in the morning. Images were imported into Adobe Photoshop CS6, where the size of the bib was measured by selecting the total black area on throat/chest and converting pixels to cm226. The repeatability27 of bib size based on 20 males measured twice was high .Measurements of tarsus length, body mass and plumage colouration were taken at the beginning of the experiment. Tarsus and body mass measurements were taken with a digital calliper and balance, respectively. Plumage colouration was measured in 5 different parts of the body  using a MINOLTA spectrophotometer (CM26d) which measures the reflectance from 360 to 700 nm in intervals of 10 nm. Reference calibrations were performed against zero and a white standard tablet associated with the apparatus according to the instructions provided by the maker. Reflectance spectra were automatically produced as means of 3 sequential measurements. The SPECTRAMAGIC software (MINOLTA) was used to analyse spectra. Total Reflectance was obtained as the summed reflectance at each 10-nm interval from 360 to 700 nm. Total Reflectance is a measure of the total amount of light reflected by the feathersp > 0.16, n = 54). Constant temperature was achieved using air heaters on thermostats in each aviary. Humidity conditions were controlled either by humidifiers in the wet aviaries, and by sorption dehumidifiers in the dry aviaries, both with humidity sensors to precisely maintain humidity. Two thermohydrometers located inside each aviary allowed us to monitored temperature and humidity daily (at 8:00 and 19:00 h). The relative humidity values we chose are within the natural range where the birds were captured from . The duration of the experimental treatment was 6 months. In the house sparrow, the moulting period takes place at the time our experiment was conducted , so, after the end of the treatment (14 October 2021) all birds had developed a new plumage. On 14 October 2021, body mass and plumage colouration were measured. When the experiment concluded, birds were released at their capture place.To test the effect of ambient humidity on plumage colouration, on 14 April 2021 we exposed house sparrows to either a “Dry”  or a “Wet”  environment at 20ºC. The birds were housed in each of the four aviaries (two for the wet treatment and two for the dry treatment) as they were being released one by one from the collectors to maintain a similar number of animals in each aviary after each release. In this way we ensured a random accommodation of individuals in relation to size, body mass and starting plumage colouration among groups  that included colour traits after treatment as the dependent variables, treatment as a fixed factor and body mass as a covariate. The initial colour (or size) of each plumage trait was also included as a covariate to increase the precision of the estimatesAll methods of this research were carried out in accordance with the ethical guidelines proposed for the Spanish Royal Decree 53/2013 which establishes the rules applicable for the protection of animals used in experiments and scientific research. All experimental protocols were approved by the Research Ethics Committee of the Rey Juan Carlos University, as the organism authorized by the Comunidad Autónoma de Madrid for evaluation of projects based on what is stated in RD 53/2013 (Ref:1011202020220). All methods were carried out in accordance with ARRIVE guidelines. Permissions to conduct the study were granted by the Comunidad Autónoma de Madrid and Junta de Castilla la Mancha. The experimental work was supervised by the veterinary staff from FIEB.During the experiment 7 males from the wet treatment died (4 from one aviary and 3 from the other one). The corpses were analysed by the FIEB veterinarians who determined a possible mycosis as the cause of death. Prior to the date of death, no birds (except one) showed apparent signs of disease in their routine health assessments done by the veterinarians (birds were checked four days a week). The only bird with disease signs was observed perched in the ground showing no interest in food or water. Immediately after this observation the experiment was finished for this bird and it was isolated in an individual cage at room temperature and “normal” humidity conditions. The bird died within a few hours, before the veterinarians could administrate any antifungal medication. No deaths were recorded in the dry treatment. Thus, final samples sizes are 27 individuals in the dry treatment and 20 individuals in the wet treatment.As this is the first work in which humidity was manipulated in these ranges-which are within the natural variation of humidity faced by house sparrows in their original population-we could not have foreseen this outcome. Yet, because mortality only occurred in the high humidity group, and because the suspected cause of death is likely linked to the treatment, we consider this information very important for future researchers attempting similar experiments.p = 0.30).Controlling for initial colouration, no differences between groups were found after exposure to the humidity treatments in the lightness of the grey crown, the rufous lateral crown stripe or the grey rump Table ; Fig. 1.1,45 = 5.74, p = 0.02). Although body mass had no significant effect on the grey crown, the rufous lateral crown stripe, the grey rump, the black bib or the bib size  is consistent with both the pioneering work of Beebe20. Despite the small sample sizes of Beebe´s experiments (Hylocichla mustelina (n = 2), Zonotrichia albicollis (n = 1) and Columbina inca  and the lack of real controls, his study was innovative in suggesting plasticity in plumage colouration. Since then, the ability of birds to adjust their plumage colouration in response to climatic variables had only been tested twice, first by Fargallo et al.19 showing the potential role of solar exposure in nestling feathers colouration in vultures, and now this study in captive adult sparrows, in which we provide the first unequivocal evidence of colour acclimatization in response to humidity.In relation to the former, Beebe found that individuals of three different species that were kept in indoor aviaries in wet conditions developed darker feathers after 2–3 years of exposure to humidity, when compared to those that remained outdoors7. Sparrows in the wet treatment developed not only darker (belly and bib) but also larger traits (bib) than those in the dry environment, indicating more melanin deposition, as the bib black colouration is produced by melanic pigments30. Melanin pigments are grouped in two main chemical variants: eumelanin and pheomelanin which in different combinations produce a broad range of black, brown, gray and reddish colors31. To date, it is not really understood how the final coloration derives from both pigments and physical disposition and shape of melanosomes influences feather coloration32, yet many authors have assumed that black/gray is eumelanin dependent; and yellow, brown, and reddish are pheomelanin dependent, considering the most prevalent pigment in each case (e.g. 38.). Gloger’s rule, can be defined in a simple or a complex way. In its simple version, it, states that endothermic animals are predicted to be darker in warmer and humid areas due to the increased deposition of melanin pigments. In a more complex version, climatic variables are supposed to correlate in different ways with the deposition of the two melanin pigments: whereas the deposition of eumelanins is predicted to increase with humidity and decrease only at extreme low temperatures, the deposition of pheomelanin is higher in dry and warm regions and decrease rapidly with lower temperatures. Currently, however, we know that pale and dark coloration does not necessarily have to be associated with the type of melanin pigment. For example, brown-reddish coloration of feathers has more eu- than pheomelanin38, the degree of reddishness is associated to the concentration of both pheo- and eumelanin pigments37, grey coloration has a higher eu/pheo ratio than the black coloration18, and darkness in feathers is explained by the total amount of melanin rather than by the concentration of one of the two pigments38. So, in the absence data quantifying the deposition of both eu- and pheo-melanin the evidence from the rule refers to the simple version. Interestingly, among all plumage traits measured, darker colouration in the black bib is particularly relevant since it is known that male house sparrows with larger bibs are dominant in aggressive interactions, have better body condition and a higher lifetime reproductive success30. Thus, such acclimatization of endotherms through the modification of colouration in a socially selected trait may affect their sexual and non-sexual interactions, and ultimately their fitness.In relation to the predictions of Gloger’s rule, this study supports the idea that darker individuals are expected in humid conditions due to the increased deposition of melanin pigments41. In birds, melanogenesis is influenced by a variety of intrinsic and extrinsic factors such as hormonal changes, exposure to UV light, food quality and nutrition, or parasite infestation46. Whether these stimuli may be affected by humidity and thus influence the different pathways in melanogenesis warrant further investigation.The physiological mechanism(s) underlying the change in plumage colouration in response to humidity is unknown. The types and amounts of melanins produced by melanocytes during melanogenesis are genetically and environmentally controlled47. This provides a plausible explanation for our finding that birds confined to a humid environment produce more melanin, as this may be to counteract melanin degradation in humid regions. In fact, the idea behind Gloger’s rule that dark colouration confers selective advantages can be explained by different selective forces that are not necessarily mutually exclusive. For example, individuals may be darker in wetter habitats because melanin pigments play a role in thermoregulation and birds with heavily pigmented feathers repel water better, allowing the body to dry more rapidly50. Also, darker animals in humid environments could benefit from camouflage, since in more humid regions, the soil is darker (rich in organic matter), and the vegetation is denser, which limits the entry of light and produces a dark and shadowy environment51. Parasites and pathogens living in humid habitats may also exert a selective pressure, as darker birds have been shown to be more resistant to them due to increased melanin pigments that protect the integument against ectoparasites52 and feather degrading bacteria54. In fact, there is evidence that the melanin deposited in the feathers confers the feathers strength and makes them more resistant to feather-degrading bacteria54. Finally, evolution might favour melanic coloured traits to be phenotypically integrated with other traits, such as immunity and hormonal profiles via pleiotropy, genetic or functional correlations56. Hence, if the prevalence of parasites and pathogens varies along humidity gradients, selection would favour stronger immune responses in warm-humid zones57 which darker individuals may possess. Whatever the mechanism, it is known that that house sparrow coloration rapidly adapts to local climatic conditions after colonization22. So, the results of our experiment would suggest that a component for such adaptation might be the plasticity observed in this study, which is an important but ill-understood component promoting rapid phenotypic changes.Recently, it has been shown that the increase in ambient humidity accelerates melanin photodegradation58. The proximate explanation for this association may be that humidity can impact heat retention and fuel composition which can, in turn, impact lean mass60. Differences in body mass did not affect plumage colouration. Yet, an interesting result from our study is the fact that that body mass differed between treatments: birds in the wet treatment were heavier than those in the dry treatment. Differences in body mass in relation to treatment is an important result since it adds experimental evidence to a recent work that found that increased humidity was associated with increased body condition in 24 of 46 passerine bird speciesIn summary, our results show birds have the capacity to respond to local conditions of humidity by changing plumage colouration due via adjustments in melanin deposition between moults. Both the proximate and the ultimate causes of such acclimatization in plumage colour traits in response to humidity, as well as its ecological implications remain unsolved. Yet we provide the first unequivocal evidence that the ability to adjust individual colouration may be a potential adaptation to climatic changes in endotherms.Supplementary Information.


Communities across the nation have outgrown their capacities to meet the transportation needs of older people. While a struggle in the past, for many communities it now is a crisis given an increase in older people with impairments in critical driving skills that lead to driving cessation and/or limitations that preclude them from successfully navigating public transit. It is time for new strategies and innovations to meet these growing challenges. Jansen will describe a pilot study of a regional transit authority’s use of customized software with four Councils on Aging to increase riders’ trips and destinations, improve efficiencies in reporting and dispatching, and increase shared rides across the communities. Gleason will present results of a qualitative study of paratransit managers across the U.S. and their attempts to innovate and adapt to evolving market expectations spurred on by the emergence of transportation network companies (TNC) like Uber and Lyft. Schwartz describes a mixed-methods study of Project TRIP, a rural transportation program designed to increase access to healthcare and other locations for low income and vulnerable populations in rural eastern North Carolina. Lynott introduces AARPs RideSheet, an open-source ride scheduling software application designed for small, demand-responsive transportation providers that incorporates a transactional data specification (TDS) enabling two or more providers to interoperate more efficiently, improving service for their clients. Bayne concludes as discussant bringing in her own work with the CDC on barriers and facilitators to ride sharing and reflecting on the themes presented. Transportation and Aging and Qualitative Research IGs collaboration.


Direct determination of lead trace concentration in the presence of relatively complex matrices is often a problem. Thus, its preconcentration and separation are necessary in the analytical procedures. Graphene-based nanomaterials have attracted significant interest as potential adsorbents for Pb(II) preconcentration and removal due to their high specific surface area, exceptional porosities, numerous adsorption sites and functionalization ease. Particularly, incorporation of magnetic particles with graphene adsorbents offers an effective approach to overcome the separation problems after a lead enrichment step. This paper summarizes the developments in the applications of graphene-based adsorbents in conventional solid-phase extraction column packing and its alternative approaches in the past 5 years. Among the heavy metals, lead is extremely toxicant, affecting multiple body systems, and is particularly harmful to young children. Exposure to lead can result in significant health issues, such as damage to brain, kidney, liver, bones, blood and the nervous system ,2. The mDespite the high sensitivity provided by the atomic and mass spectrometric techniques in trace analysis, a preconcentration step is very often needed to improve their limits of detection further. Additionally, matrix components which cause spectral interference can be removed during this step. Solid phase extraction (SPE) is commonly used as a preconcentration technique owing to its simple procedure, high preconcentration factor and minimal regeneration costs ,5. The aIndustrial effluents can contain dissolved lead as well as its various compounds, such as lead salts, oxides, and sulfides. Lead accumulation over time can cause severe ecological problems in water reservoirs due to their toxicity and biocumulation in food chains . It is aOver the last decade, various nanomaterials have been widely applied as powerful adsorbents for trace metal preconcentration and removal of environmental contaminants as they exhibit large surface area, fast adsorption capability and easy functionalization or coatings ,18,19,20This review presents the last five years of achievement in the use of graphene-based nanomaterials in solid phase extraction for preconcentration of lead ions in analytical processes as well as its removal from aqueous solutions such as sewage. Different works that provide interesting results for this application are summarized. Interested readers could find more details concerning earlier contributions in the review papers ,29,30,312-hybridized carbon atoms arranged in a planar honeycomb structure. It can be stacked to form graphite and rolled to form carbon nanotubes , graphene oxide (GO) and reduced graphene oxide (rGO), have become one of the research hotspots. They exhibit high surface area, unique structural regularity, chemical inertness, electrical conductivity, thermal and mechanical stability. Graphene is considered the fundamental structure of all carbon allotropes and much research has been done in the field of analytical extraction processes involving graphene and its derivatives as the main material ,32. Grapanotubes  [32]. AcHowever, the agglomeration of graphene sheets due to the strong van der Waals interactions causes a great loss of accessible surface area for the adsorbates. It can be avoided selecting suitable dispersion methods and functionalizing the fillers .2SO4, KMnO4 and NaNO3 x+[(An−)x/nmH2O], where M2+ and M3+ are the divalent and trivalent cations, respectively, while An−1 represents the intercalating anions. Several possible mechanisms are proposed for adsorption of metal ions, such as surface complexation, isomorphic substitution, surface precipitation, and electrostatic interaction and chelation. The combination of LDH with GO can increase the the adsorption capacity and selectivity [3 eluent volume), a preconcentration factor of 10 was obtained and adsorbent capacity was 16.9 mg/g. It should be mentioned that the removal of Cu(II), Cr(VI) and Fe(III) was similar to Pb(II). However, as was examined, these ions did not disturb Pb(II) sorption due to its higher adsorption capacity of GO/ZnCr LDH.Other interesting adsorptive materials include layered double hydroxides (LDHs), two-dimensional nanostructures that consist of layers of metal oxides with a positive charge and charge balancing hydrated anions between the layers . They haectivity ,78,79. A60(OH)22 has been proposed for efficient removal of Pb(II) ions in complex matrix samples [60(OH)22 toward Pb(II) ions of 1307 mg/g at pH 5.5 and high resistance to ionic strength up to 1 mol/L. The experments indicated that the main mechanism of adsorption was an inner-sphere model based on surface complexation with different types of coordination of Pb(II) to the oxygen functional groups.Recently, GO decorated with fullerenol nanoparticles C samples . The lowExamples of the recent applications of graphene-based materials for the preconcentration of Pb(II) using the SPE technique are presented in It is noteworthy to add that graphene-based materials are also used in membrane preconcentration techniques for removal of metal ions . GO-baseTogether with the fast development of the new nanomaterials for preconcentration purposes, great importance is nowadays attached to the quality of the measurement data in analytical methodologies. The validation of the proposed procedures for Pb(II) determination after an enrichment step was reported in terms of sensitivity, linearity and precision in many of the cited papers. Accuracy of the developed methods was assessed by analyzing appropriate certified reference materials.The removing of toxic lead ions from industrial and mining waste effluents has become a key concern ,15,84,85Various parameters such as temperature, contact time, lead ion concentration, the chemical structure of the adsorbent, its amount, and the presence of other pollutants influence the adsorption process for lead removal. However, the type and chemical structure of an adsorbent and its adsorption capacity are the most critical factors in successful performance.Graphene oxide functionalized with polyethylenimine (GO-BPEI) in the form of highly porous foams has been proposed for removal of Pb(II) from waters at a large scale . A schemPiperazine-modified magnetic graphene oxide (Pip@MGO) nanocomposite also exhibits high adsorption capacity for lead ions, taking advantage of the coordinating capability of piperazine for metal ions and also the high surface area of graphene oxide . At pH 6Nickel ferrite based reduced graphene oxide (NFrGO) nanocomposite was used by Lingamdinne et al. for the removal of Pb(II) ions from aqueous solution ,88. For Amongst the adsorbents that have been developed to solve water pollution, metal-organic frameworks and their composites have drawn great attention due to their high surface area, ordered porosity and adjustable functionalization ,91. VariThe incorporation of melamine into Zr-MOFs was successfully performed by the oven-promoted method . It has Graphene-based nanomaterials, such as graphene oxide and reduced graphene oxide, have attracted significant interest in recent years for preconcentration and removal of heavy metal ions, including toxic Pb(II), and the number of scientific reports on this subject is constantly growing. They exhibit huge surface area, good thermal and mechanical stability, and ease of surface modification. Similar to carbon nanotubes, graphene-based nanoparticles can be modified using polymers, metal oxides as well as doping with heteroatoms. Functionalization with magnetic nanoparticles facilitates the separation of Pb loaded adsorbent from the solution. In the described works, a particular focus has been placed on the factors affecting the adsorption process; factors such as adsorbent dose, pH, initial concentration, and contact time were addressed, which determine the performance of graphene-based materials as useful adsorbents.However, it should be noticed that while graphene derivatives have advantages such as high adsorption capacity or selectivity, some drawbacks can also be found for their application in lead preconcentration and removal. The synthesis of the proposed adsorbents is often time-consuming due to multi-stage processes and it requires appropriate high skills.Future research on graphene-based materials for preconcentration purposes in analytical procedures should concentrate on molecularly imprinted polymers (MIP) due to their excellent selectivity and specific separation. Molecular imprinting has been widely recognized as a potential technique for the synthesis of tailor-made recognition materials through the formation of a polymer network around a template molecule. They are designed with a particular cavity of a suitable size and structure to remove a given analyte by chemisorption or physicosorption process . AlthougThe application of natural components with several hydroxyl, amine and carboxyl groups, such as chitosan, cellulose, β-cyclodextrin and amino acids, in the green synthesis of functionalized graphite-based nanoparticles makes an attractive material in both an economic and an environmental perspective. The control of solution pH can change the surface charge of these functional groups. Thus, by choosing appropriate conditions, the electrostatic interaction with Pb(II) ions could be enhanced.Despite the unique properties of MOFs, such as tunable porosity, large surface area and chemically stability, the number of their applications in wastewater remediation is still limited . TherefoIt should be noticed that most research studies only examine generally carbon-based nanomaterials under laboratory conditions, while wastewaters usually contain different pollutants in different combinations. Thus, further studies are required in large-scale practical environments.


This performance exceeds that of the majority of previously reported graphene oxide (GO)- or rGO-based humidity sensors. In addition, the polyelectrolyte deposition time is shown to be key to controlling the humidity sensing kinetics. The as-developed rapid sensing system is expected to provide useful guidance for the tailorable design of fast humidity sensors.Fast humidity sensors are of interest due to their potential application in new sensing technologies such as wearable personal healthcare and environment sensing devices. However, the realization of rapid response/recovery humidity sensors remains challenging primarily due to the sluggish adsorption/desorption of water molecules, which particularly impacts the response/recovery times. Moreover, another key factor for fast humidity sensing, namely the attainment of equal response and recovery times, has often been neglected. Herein, the layer-by-layer (LbL) assembly of a reduced graphene oxide (rGO)/polyelectrolyte is demonstrated for application in fast humidity sensors. The resulting sensors exhibit fast response and recovery times of 0.75 and 0.85 s (corresponding to times per RH range of 0.24 and 0.27 s RH Humidity sensing is crucial in a wide range of fields, such as biological science , foods , electroHerein, rGO/polyelectrolyte multilayer films are prepared via layer-by-layer (LbL) assembly for application in a fast humidity sensor. The sensor exhibits fast and almost identical response and recovery times, thus outperforming the majority of previously reported GO- or rGO-based humidity sensors. In addition, the humidity-sensing kinetics are shown to be controlled by the deposition time of the polyelectrolyte. Thus, the present study opens new possibilities for employing graphene-based materials in fast-response humidity sensors.Materials. Poly  was purchased from Sigma-Aldrich . Graphite  was purchased from Alfa Aesar . Slide glasses  were used as the substrates for LbL deposition. Potassium permanganate (KMnO4) and hydrogen peroxide  were purchased from Samchun, Korea. Hydrochloric acid (HCl) was obtained from Duksan, Korea. Deionized (DI) water  was used for all experiments.Synthesis of graphene oxide. Graphene oxide was synthesized using the modified Hummers method, as described in previous work. In detail, graphite powder (3 g), concentrated H2SO4 (120 mL), and NaNO3 (2.5 g) were added to a round-bottomed flask in an ice bath. The mixture was stirred with a Teflon-coated magnetic stir bar at 350 rpm for 1 h, and then KMnO4 (15 g) was gradually added over a period of 2 h. The temperature of the mixture was kept below 20 °C by cooling it in an ice bath. The mixture was stirred at 500 rpm and 35 °C for a further 3 h. After mixing, 200 mL of DI water were added to the flask over a period of 5 h, followed by an additional 750 mL of DI water over a period of 0.5 h. Next, H2O2  was added dropwise, and the color of the mixture was seen to turn brown. The resulting GO dispersion was left to stand overnight. Then, the supernatant was decanted to remove the small GO particles. The sediment was then re-dispersed with dilute HCl  and washed three times with DI water using a centrifuge at 6000 rpm for 5 min. The resulting GO was then re-dispersed in DI water and dialyzed against DI water for 7 days to remove any residual metal ions and acids. The DI water for the dialysis was replaced with clean DI water every 24 h. The dialyzed GO dispersion (roughly 600 mL) was then mixed with DI water (2400 mL) and tip-sonicated  at 375 W for 10 min to obtain exfoliated GO sheets in water.Preparation of the reduced graphene oxide/polyelectrolyte multilayers. The PDAC and GO sheets were diluted to a concentration of 0.5 mg mL−1 in DI water to obtain a pH of 5. The glass slides were cut into 12.5 mm × 50 mm pieces, cleaned sequentially for 10 min each with water, isopropyl alcohol, and acetone via bath sonication, and then dried with air. The substrates were then subjected to plasma treatment  for 3 min under vacuum, then immediately immersed in PDAC solution for the desired deposition time . After rinsing three times  with DI water, the substrates were immersed in GO dispersion for 10 min (unless otherwise stated), then rinsed again three times  with DI water. Each cycle (immerse in PDAC/rinse/immerse in GO/rinse) is termed as a layer pair. The same procedure was repeated as necessary to obtain the desired number  of layer pairs (LPs). At this stage, the electrical conductivities of the assembled films were below the detection limit of the tester because both the GO and the polyelectrolyte were electrically insulating. The GO/polyelectrolyte multilayer samples were then reduced to 220 °C under vacuum for 30 min to obtain rGO/polyelectrolyte samples with increased electrical conductivities (resistance = 10–20 MΩ/cm).Preparation of the humidity test cells. The rGO/polyelectrolyte sample was cut into 5 mm × 20 mm pieces. After placing 5 mm wide tape across the rGO/polyelectrolyte film, copper wires were connected to the end of the film with silver paste , followed by vacuum drying overnight. The tape was then gently removed, and the copper wires were connected to a multimeter  to measure the resistance change according to the humidity. Unless stated otherwise, the 10-LP rGO/polyelectrolyte multilayer that was prepared using 2 min PDAC deposition time was used in the experimental tests.Characterization. Scanning electron microscopy  was used to investigate the morphologies of the multilayer films. Ultraviolet-visible (UV–vis) spectroscopy  was conducted over a wavelength range of 350–900 nm. The humidity response of the rGO/polyelectrolyte multilayer device was tested in a lab-built chamber at 22–23 °C. The humidity in the chamber was controlled in the test range of 20–70% RH by either bubbling air in DI water or filtering air through dried silica gel. The relative humidity in the chamber was recorded using an Arduino Uno R3 control board  with a commercial humidity sensor module  having an accuracy of ±2%, while the resistance change was measured simultaneously using a multimeter . The humidity environment was stabilized by waiting for 30 s after each adjustment. The humidity response and recovery times were evaluated by measuring the time required to reach a normalized response (RN) of 0.9 from an RN of 0.1 and vice versa, with humid airflows of 0.2, 0.5, 1.0, and 2.0 Hz using an electric fan. A plot of ln[(R1 − R0)/(R1 − Rt)] vs. time was then used to determine the adsorption and desorption rate constants (ksorp and kdesorp) according to the resistance change due to water adsorption or desorption from the relationship:R1 and R0 are the respective final and initial resistances of the multilayer cell, and Rt is the resistance at time t. The sensitivity is defined by (R1 − R0)/(RH0 − RH1), where RH0 and RH1 are the respective initial and final RHs (%) of the multilayer cell. The limit of detection (LOD) is defined as the lowest detectable humidity and is calculated from the relation LOD = 3SD/m  against time in −1, respectively, which are comparable with those obtained in previous work on ultrafast humidity sensors [The response and recovery times of the multilayers were evaluated with the interruption of the humid airflow at a frequency of 0.2 Hz in the RH range of 57 to 61% using an electric fan. The results in ksorp values of 2.7, 2.0, and 1.7 s−1, respectively. This indicates that an increase in the PDAC deposition time leads to a decrease in the rate constant, thus confirming the crucial role of PDAC in humidity sensing. Based on these results, it appears that the change in interlayer spacing is the predominant response/recovery mechanism. Moreover, the as-fabricated sensors exhibit fast humidity sensing capabilities at various airflow modulation frequencies of up to 1.0 Hz  of 0.1 s (corresponding to 0.03 s RH−1), thus indicating that the recovery and response times are nearly identical. This result is compared with those of previous studies in 2 [2 [2 [As noted in the Introduction, achieving equal response and recovery times is another key requirement that has often been ignored, especially for a rapid response towards dynamic changes in humidity. In the present work, the rGO/polyelectrolyte multilayer exhibits an absolute difference between the recovery and response times /polyelectrolyte multilayers prepared via layer-by-layer (LbL) assembly. The as-fabricated sensors exhibited fast and nearly identical response and recovery times, exceeding those of most previously reported GO- or rGO-based humidity sensors. In addition, the deposition time of the polyelectrolyte was shown to be a key factor in controlling the humidity sensing kinetics, with a short polymer deposition time leading to a faster response. Hence, in future work, the humidity response of the sensor might be further modulated and intensified by optimizing the LbL assembly process in order to identify the minimum amount of PDAC needed to form the multilayer structure while achieving the fastest possible response/recovery times. In addition, the underlying mechanism of the reversible resistance change must be elucidated, with two possible contributions being a change in the interlayer spacing due to the water-sensitive polymer ,70 and a


We demonstrate a reconfigurable beam steerable meta-surface through a graphene-biased slot-array over a grounded quartz substrate. More specifically, the graphene meta-elements can be dynamically tuned to program the radiations by applying adequate DC bias voltages to different gating pads, capable of turning on or off the releasing slots of the guided fields as adjustable switches. In particular, such a graphene-biased terahertz meta-surface will achieve a wide-angle steerable beam at a fixed frequency and the scanning directions can further be modulated when varying the frequency at a certain state of the graphene, thus should pave the way for building up more advanced reconfigurable transceivers and sensors in terahertz wireless electronics. Graphene is a two-dimensional material composed of carbon atoms arranged in a hexagonal lattice. The extraordinary electronic, optical, and thermal characteristics of graphene as well as the possibility of facile integration have enabled the great applications of terahertz graphene meta-surfaces. In particular, graphene meta-surfaces have been widely applied to the reconfigurable electronics  and provOn the other hand, coding meta-surfaces with dual states of “0” and “1” can sufficiently control the scattering characteristics, where PIN-diodes or varactors integrated with meta-units are usually employed to build up the reconfigurable meta-arrays mostly in the microwave regime. For example, Cui et al. initially proposed the concept of digital coding and programmable meta-surface composed of two types of units, with 0 and  targets . Li et a targets . Tao et  targets . Yurduse targets  and also targets . Li et a targets . However targets ,15. Grap targets ,17. Base targets . Especiaubstrate . Consideubstrate . The proℏ is the reduced Planck constant, The surface conductivity of graphene in the low terahertz band can be described by the Kubo formalism ,(1)σ=−jThe chemical potential The far-field radiation pattern of the graphene-biased terahertz meta-surface can be approximated by accumulating the fields released from all the meta-slots : (4)AF=∑z-direction which generates a highly directive beam as the numerical analysis. The HPBW of the simulated meta-surface is x-polarized and the cross polarization ratio is above 30 dB. The total energy provided in the simulation is 0.5 W, the energy lost in graphene is 0.31 W, and the radiation energy accounts for 0.08 W. The rest of the energy is reflected at the input port or absorbed by the terminal matching load. The slight discrepancy of far field patterns between the theoretical and simulated results is mainly attributed to the coupling between meta-slots.Full-wave simulations (CST Microwave Studio) are carried out in In conclusion, we have demonstrated a wide-angle scanning graphene-biased terahertz coding meta-surface in this paper. Especially, the proposed graphene-biased terahertz meta-surface has achieved a wide-angle steerable beam at a fixed frequency and the scanning directions can further be modulated when varying the frequency at a certain coding state of the meta-slot array. Excellent agreement is obtained between the theoretical results and simulated results. The graphene meta-elements can be dynamically tuned to program the radiations by applying adequate DC bias voltages to different gating pads, capable of turning on or off the releasing slots of the guided fields as adjustable switches. Such an amplitude modulation process does not require active phase shifters, but can also include the phase information, where the guided wave controls the phase of each unit. We expect the design strategy of using the graphene as adjustable switches to build up a programmable beam scanner in terahertz wireless electronics, paving the way for the quest of tangible applications and more advanced meta-devices in controlling the electromagnetic field with more freedom.