The aim of this paper was to improve the fire retardancy of beech wood by graphene. Six fire properties, namely time to onset of ignition, time to onset of glowing, back-darkening time, back-holing time, burnt area and weight loss were measured using a newly developed apparatus with piloted ignition. A set of specimens was treated with nano-wollastonite (NW) for comparison with the results of graphene-treated specimens. Graphene and NW were mixed in a water-based paint and brushed on the front and back surface of specimens. Results demonstrated significant improving effects of graphene on times to onset of ignition and glowing. Moreover, graphene drastically decreased the burnt area. Comparison between graphene- and NW-treated specimens demonstrated the superiority of graphene in all six fire properties measured here. Fire retardancy impact of graphene was attributed to its very low reaction ability with oxygen, as well as its high and low thermal conductivity in in-plane and cross-section directions, respectively. The improved fire-retardancy properties by the addition of graphene in paint implied its effectiveness in hindering the spread of fire in buildings and structures, providing a longer timespan to extinguish a fire, and ultimately reducing the loss of life and property. Based on the improvements in fire properties achieved in graphene-treated specimens, it was concluded that graphene has a great potential to be used as a fire retardant in solid wood species. Wood is a versatile material with a myriad of applications and therefore, its plantation and harvesting are vastly studied all over the world ,2. The iImproving the effects of nanotechnology on different materials has been vastly and intensively elaborated ,19,20,21In this connection, graphene is a one-atom-thick planar sheet of a hexagonally arranged carbon atoms and therefore, it is considered a nano-material. These sheets are densely packed in a honeycomb crystal lattice. Graphene has got appreciable attention over the last two decades due to its special structure and exceptional properties ,35. ThesFagus orientalis) were purchased from the Khavaran Wood Bazar . The density of beech specimens was measured to be 0.62 g·cm−3. Boards were kept in the wood workshop of Shahid Rajaee Teacher Training University  for four months before cutting . They were then cut to size, 220 mm in length, 140 mm in width (plain sawn), and 5 mm in thickness. Specimens were free from any fungal or insect attack, checks or cracks, and knots. Twenty specimens were selected and divided into four treatments; for each treatment, five replicate specimens were prepared. Treatments included: control , painted, NW+painted (nano-wollastonite was mixed in paint and applied on the surface of specimens), and NG+painted  and the anode electrode of graphite foil (2 × 10 cm2). Two electrodes were placed at a distance of 2.7 cm from each other. NiCl2.6H2O powder (98.0% Merck) was dissolved in water  to prepare electrolyte. To provide expansion, exfoliation of graphite, and deposition of Ni, a voltage of 10 V was applied. The Pt electrode was washed every 20 min with HCl and water to avoid the accumulation of the product on the cathode. Finally, the products were collected using vacuum filtration, and then they were washed with water. The end product was dispersible for sonication in water. In the electrochemical exfoliation mechanism, hydroxyl ions (OH) were first produced in the cathode region by applying the voltage between the electrodes, and then the ions accelerate towards the anode and hit the graphite surface. The collision of OH ions with graphite and oxidation at the edge sides and grain boundaries lead to expansion of the graphite layers, penetration of Cl ions through the graphite layers, and reduction of Cl ions to finally produce Cl gas. An excessive force exerted to graphite layers upon Cl gas caused the separation of the graphite layers [2+ ions and proceeded towards the negative electrode under an electric field. Finally, black composite on the Pt electrode was created. Generation of hydroxyl at cathode via ionization of water is as follows [Electrochemical exfoliation of graphite was performed in a system with a cathode electrode of platinum 2 as crystalline layers were deposited on graphene flakes (G-flakes). The final product in form of powders was pressed (under a pressure of 5 MPa for 30 min) into pellets. The average dimensions were 6.5 mm (radius) and 1 mm (thickness), approximately.The OH generation together with other electrons and ions lead to formation of Ni and Ni(OH)2, Ni-oxides) were randomly distributed on graphene sheets. The size of Ni and Ni(OH)2 nanocrystals obtained by TEM and XRD patterns were found to be 30–40 nm. The brightness of graphene sheets appearing in TEM images showed a small thickness (1.2 μm) of graphene sheets [The characterization of Ni-graphene composite was done by X-ray diffraction  radiation, X-ray photoelectron spectroscopy , tunneling electron microscopy (TEM-Philips model CM120), and dynamic light scattering . TEM and XRD patterns of Ni-graphene represented that the nano-crystals e sheets .4 (90–98%). The producer reported that other hydrocarbons accompanied methane . The gas flew steadily at the rate of 0.097 lt/s through a Bunsen type burner hold vertically and the specimen is mounted at a 45° angle to it. The internal diameter of the burner was 11 mm. The Bunsen-type burner provides a fairly mild and localized fire exposure to the testing specimens. The time from the point of burner application that it takes for the specimen to develop a visible flame that is sustained for more than one second will be registered as the “time to onset of ignition”; and the time from the point of burner application that it takes for the specimen to sustain glowing for more than one second will be registered as the “time to onset of glowing”. The higher the amount of time of these two properties, the better from the viewpoint of fire-safety. As the burning continues, the back face of the specimen nearest to the flame of the burner starts blackening and after some more time, a small hole or split appears. These times are also registered as back-blackening and back-holing times. The test is terminated once the back-holing occurs. Then, the burnt area, as well as weight loss, is measured. The whole apparatus is put in a three-wall-compartment in order to protect the burning flame from wind and air movements.Fixed Fire Test Apparatus (FFTA) was designed and built, using piloted ignition, as depicted in Density functional theory (DFT) calculations were carried out for graphene and oxygen molecule with OpenMX3.8 package to calculate the adsorption energy of oxygen molecule on graphene ,45,46. TIn the present simulations, one model for pure graphene flake (G-flake) was investigated. The fully optimized atomic structure of pure G-flake is shown in The adsorption energy, Statistical analysis was carried out by a SAS software, version 9.2 . One-way analysis of variance (ANOVA) was performed on the average values to ascertain significant differences at the 95% level of confidence. Hierarchical cluster analysis, including dendrograms and Ward methods , was carried out using SPSS/18, version 18 . The scaled indicator on top of cluster analysis shows similarities and differences between treatments; lower scale numbers show more similarities while higher ones show dissimilarities. Contour and surface plots were designed in Minitab software, version 16.2.2 .Results demonstrated outstanding improving effects of graphene (NG) on both times to onset of ignition and to the onset of glowing . NG resu−1·K−1, at room temperature [−1·K−1, and that of a purified diamond can be up to 50% higher. However, the thermal conductivity of graphene in cross-section direction  is as low as only 6 W·m−1·K−1 [The improvement in fire properties by graphene can be elaborated from its thermal conductivity as well. With regard to its two-dimensional structure, the thermal conductivity of graphene can also be studied in two directions. The in-plane thermal conductivity of graphene is reported to be one among materials the highest thermal conductivity, about 2000–4000 W·mperature . For com·m−1·K−1 . This ex·m−1·K−1 ,38. OverPainted specimens showed a little bit of decrease in time values, though the decreases were not statistically significant. The decrease, which is not favorable as far as fire retardancy is concerned, was attributed to the easier ignitability of the chemical ingredients of the paint applied on the surface of specimens. The addition of NW to the paint resulted in an increase in both times to onset of ignition and glowing in comparison to the unpainted specimens, though the increase was not statistically significant in time to onset of glowing. The increase was partially attributed to the mineral and unignitable nature of wollastonite. In fact, NW acted as an insulating layer, too, towards the penetration of the piloted fire from the Bunsen burner, protecting the wood substrate beneath the paint.Results of assessing the back of specimens showed that both NG and NW increased the times to back-darkening and to back-holing , showingMeasurement of the burnt area illustrated that NG was very effective in hindering the spread of fire throughout specimens . The sprThe weight measurement of specimens before and after being exposed to fire showed that the lowest weight losses occurred in specimens treated with NG . The weiA significant direct relationship was calculated between most of the fire properties. The highest R-square (99%) was found between the two times to onset of ignition and glowing. Contour and surface plots also illustrated a direct trend among different properties; for instance, weight loss increased as the two times to onset of glowing and ignition increased A,B.The cluster analysis based on all six fire properties studied in the present project demonstrated the distinct difference of specimens treated with graphene in comparison to the other three treatments  . This inFire properties of beech specimens were tested by a newly developed apparatus using piloted fire. Specimens were prepared to be surface-treated by graphene to investigate its improving effects on fire properties. Graphene was mixed at 5% with water-based paint and applied on the front and back surfaces of specimens. As a basis for comparison, three separate groups of specimens were prepared, including specimens without any paint (the control group), painted with no nano-material mixed with it, and painted with 5% nano-wollastonite. Results showed significant improving effects of graphene on all six fire properties studied in the present project. Graphene demonstrated outstanding improvement in times to onset of ignition and glowing, as two very important properties that determine the ignitability of materials. Moreover, graphene illustrated a high impact on decreasing the burnt area. The decrease was also important from the viewpoint of the spread of fire to other surrounding parts in an area. Overall, the improved fire properties provide fire-fighters with a larger timespan to extinguish the fire, ultimately saving both life and property. It was concluded that graphene has a great potential to be used as an effective fire-retardant in wood and wood-composite materials for surface protection against fire. Therefore, further studies should be carried out to investigate graphene from other perspectives, including FT-IR analysis.


Ions in the electrolytes are used to configure electric-double-layers (EDLs) that induce p- and n-type regions in graphene. These EDLs are locked in place by two different electrolytes with distinct mechanisms: (1) a polyethylene oxide (PEO)-based electrolyte, PEO:CsClO4, is locked by thermal quenching ), and (2) a custom-synthesized, doubly-polymerizable ionic liquid (DPIL) is locked by thermally triggered polymerization that enables room temperature operation. Both approaches are gateless because only the source/drain terminals are required to create the junction, and both show two current minima in the backgated transfer measurements, which is a signature of a graphene p-n junction. The PEO:CsClO4 gated p-n junction is reconfigured to n-p by resetting the device at room temperature, reprogramming, and cooling to T < Tg. These results show an alternate approach to locking EDLs on 2D devices and suggest a path forward to reconfigurable, gateless lateral p-n junctions with potential applications in polymorphic logic circuits. A gateless lateral  Orization . After pp-n junction formation and ion-locking are shown in p-n junctions, the highest concentration of ions (~2.4 × 1021 cm−3 at 1.5 V) builds up near the electrode surface and then dissipates with distance away from the electrode, meaning that a more abrupt junction can be expected with decreasing channel length. Note that the device channel lengths reported here are 100 times larger than those modeled in reference [TFSI] (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide), which has been successfully used as an ion gate ,60, but BG, which means that the DPIL was causing an overall n-type doping of the graphene channel. Note that such an n-type shift was not observed on the bare graphene device , the signature of a p-n junction and an indication that the ions were locked into place. However, unlike the PEO:CsClO4-gated device, I12 also exhibited two current minima . At first blush, a p-n junction would not be expected between electrodes 1 and 2. However, as discussed above, the cations of the DPIL seemed to have affinity for the graphene surface, which means that they would also preferentially accumulate near electrode 1, which was floated during programming and locking. Thus, the preferential accumulation of cations to the graphene surface can explain the additional p-n junction located between electrodes 1 and 2. In the future, the p-n junction between electrodes 1 and 2 can be avoided by further material optimization to avoid preferential adsorption of one type of ion over the other. The corresponding band diagrams are shown in 4, except for the prominent n-type shift observed for DPIL. Focusing again on p-n junction serve as proof-of-concept that alternate triggers can be used to semi-permanently dope 2D devices for room temperature operation. In fact, we have also recently shown that incorporation of thermally-labile linkers into polymerizable ionic liquids allows ions to be released after the materials are polymerized, which we anticipate will enable DPILs that can be locked and then unlocked with a second trigger [n-type doping caused by DPIL polymerization, locking and unlocking the EDLs on demand to demonstrate reconfigurability, and extending the concept to 2D semiconductors to show rectifying behavior. The results of the chemically locked, gateless, graphene  trigger . This typ-n junction is demonstrated on 4-electrode graphene devices by ion-locking using two different solid polymer electrolytes. PEO:CsClO4 is locked by lowering the operating temperature to 220 K , while DPIL is locked by heating to 353 K to polymerize the anions and cations, and then operating the device at room temperature. In both cases, the signature of the p-n junction is double current minima—consistent with what is expected for graphene. For the PEO:CsClO4, reconfigurability of the p-n junction is also demonstrated by resetting the device at room temperature, reversing the polarity of the applied bias, and cooling the temperature to 220 K to create a n-p junction. While the chemical trigger using DPIL is more practical than the low-temperature approach, preferential n-type doping is observed and attributed to the affinity of the cation over the anion for the graphene surface. Overall, this study provides proof-of-concept that triggerable electrolytes can lock-in p-n junctions in graphene, and this approach can be extended to other 2D material, for electronic and optoelectronic applications. A gateless lateral


By exploiting the tunable functionality of graphene, a two-dimensional material, efficient deposition from a chemical was achieved. Ozone-generated defects in the graphene were shown to be required to enhance the morphology and quality of the material and comprehensive characterization of the seed layer and the Sb2S3 film were applied to design an optimal deposition process. The resulting material exhibits efficient carrier transport and high photodetector performance as evidenced by unprecedented responsivity and detectivity in semiconductor/graphene/glass vertical heterostructures. .Despite significant progress in the fabrication and application of semiconductor materials for optical emitters and sensors, few materials can cover the cyan-gap between 450 and 500 nm. We here introduce a robust and facile method to deposit amorphous Sb To fulfill the missing cyan emission component in the PL spectrum, it is urgent to find a semiconductor material with the bandgap at 2.8 eV that can fill the cyan gap presents in the high-quality white light. GaN possesses a bandgap at 2.8 eV but also has a 3.27 eV bandgap for the ultraviolet emission4. Zincblende ZnSs also has a 2.8 eV bandgap structure, but it needs to be under 10 K environment to adjust the bandgap from 2.7 to 2.8 eV4. MnS is another wide-gap semiconductor when its structure is rocksalt configuration, but the bandgap energy is a range of 2.8–3.2 eV5. Semiconductors mentions above have the necessary bandgap at 2.8 eV, but none of them have a good absorption coefficient in the cyan gap.Solid-state light emitters have improved the efficiency, availability, and comfort of lighting in our daily lives. Through a combination of red, green and blue emissions white light can be generated but the quality of color-rendering is not good enough due to the missing cyan emission around 450–500 nm, which is also called the cyan gap2S3 is a promising semiconductor material with different phase structures  depending on the process. Whereas the stibnite phase has a direct energy gap ranging from 1.7 to 1.8 eV, the amorphous phase has a direct energy gap ranging from 1.7 to 2.8 eV8. In the amorphous phase, it has a very high absorption coefficient (1.8 × 105 cm−1)9 at a wavelength of 450 nm. Different from many other semiconductors, Sb2S3 films can be deposited by a variety of facile, inexpensive and scalable methods such as thermal evaporation method10, sputtering method11, polyol reflux method12, chemical vapor deposition method13, spray-lysis method14, hydrothermal method15, and chemical bath deposition method19. However, the formation of amorphous material with stable band-gap properties remains elusive.In chalcogenide semiconductors, Sb2S3 from a chemical bath by utilizing functionalized graphene as a substrate. Since graphene has a poor lattice mismatch with crystalline Sb2S3, only amorphous Sb2S3 can be deposited on functionalized graphene21. To increase the efficiency of deposition, UV ozone irradiation is utilized to functionalize the surface of graphene. Upon optimized deposition, the amorphous Sb2S3 exhibits high morphological stability and good electrical properties. Optoelectronic characterization reveals a bandgap of 2.85 eV and high sensitivity to blue light irradiation yielding high photo-responsivity values (112 A/W), and detection rate . The presented robust and scalable deposition of Sb2S3 on graphene (SSG) opens up new routes for large bandgap optoelectronic devices in light emission and solar cells.We here demonstrate the robust deposition of amorphous Sb22. The growth pressure was kept constant at 10 Torr and the flow rate ratio of H2/CH4 was fixed at 20 resulting in identical partial pressures for various flow speeds. A standard wet-chemical transfer process was employed to remove graphene from the growth substrate and deposit it on a 300 nm Si/SiO2 substrate for further characterization23.Graphene was grown on untreated Copper foil  following previous reports2S3 film was made from the literature published by PK Nair et al. in 199722. We used sodium thiosulfate (Na2S2O3·5H2O) and antimony trichloride (SbCl3) as the source of sulfur ions and the source of cerium ions when antimony trichloride (SbCl3) is reacted with sodium thiosulfate (Na2S2O3) and deionized water (DI Water). Absorbance, A%, and Transmittance, T% were measured by Thermo Scientific UV–Vis spectrophotometer, I–V and I–T measurements were conducted in Keithley 2636B on macroscopic (1 cm2) samples using silver paint electrodes. Raman spectroscopy was carried out in a homebuilt confocal setup using a 532 nm excitation and PL spectra were measured in Acton Series (SP-2156) spectroscopy. X-ray diffraction analysis was carried out in BRUKER D2 PHASER X-ray diffractometer. SEM images were carried out in Hitachi S-480 Scanning Electron Microscope.SbIt can be seen that the surface morphology of graphene before and after UV irradiation is not much different in Figs. D/IG and the high I2D/IG ratio of unmodified graphene  corroborate the presence of single-layer graphene with few defects . The incident source of the excitation element is a 405 nm laser. The incident light intensity is 10–60 mW/cm2 and the measurement voltage is 1–15 V. The SSG composite components grown for 2–8 h were compared by 405 nm illumination I–V measurement, and it was found that the illumination current was significantly improved at a growth time of 6 h in Fig. 2S3 film on the UV irradiated graphene due to the oxygen functional group on the graphene surface, the graphene defects are increased, and the graphene high carrier mobility is sacrificed26. Therefore when an electron–hole pair is generated after illumination, the photocarrier is trapped by the modified graphene defect before being injected into the pure graphene layer, compared to no illumination condition, as schematically shown in Fig. The dimensions of the components are glass (1.5 cm × 1.5 cm), graphene (1 cm × 1 cm) and Sb27.A comparison of the signal peak, photocurrent and rise time of the atmosphere and vacuum in Fig. When unilluminated, part of the photocarriers are still trapped by the modified graphene defects, making the photocarriers difficult to recombine and prolonging the excitons recombination time, cause additional a few seconds needed for the rise and fall times to complete the on/off action in the SSG.2) in the atmosphere that will attract electrons that jump to high energy states, producing negatively charged oxygen atoms , P is the incident light intensity of the xenon lamp, and S is the effective illumination area. Because our growing material area is larger than the illumination spot area, we do not need the area item in this experiment.2. In the ultraviolet band, when the light of the wavelength of 350 nm is irradiated. The optical response of the component is also 69.76 AW−1. When the wavelength of light absorbed by Sb2S3 exceeds 568 nm, the photo-responsivity value immediately drops to zero and there is no absorption at all. Therefore, it can be found that Sb2S3 is only in the ultraviolet to the visible range for photo-responsivity to occur, and the best photo-responsivity value in the visible light band will also confirm the argument that the Sb2S3 enthalpy gap deposited in this study is about 2.2 eV. The detection rate corresponding to the response value can be calculated by the following formula.When the light of the wavelength of 450 nm is utilized, the optical response is 112 A/W when the optical power intensity is 1 μW/cm–19 C. After calculation by the above formula, it has the highest response value at 450 nm and the highest detection rate, which is 2.01 × 1012 Jones, shown as Fig. R is the spectral response value, Id is the dark current measured by Xenon lamp, S is the effective illumination area, and e is 1.609 × 102S3 can be deposited on graphene after UV irradiation for 5, 10, and 20minutes. In the measurement of photoelectric characteristics, no photocurrent and switching characteristics were found from 5 and 20 min of UV irradiation. Therefore, in this study, the modified 10-min graphene was used as the best parameter for Sb2S3 film deposition. The effects of different Sb2S3 film deposition time from 2 to 8 h on the photoelectric properties of graphene were compared. It was found that the film thickness was not proportional to the photocurrent increase. Due to the increase in film thickness, the charge was transferred during the transfer process due to the graphene defect trap produced via UV irradiation. Influenced by the scattering between the particles, thus increasing the chance of carrier recombination and slowing down the rise time. Finally, the UV ozone irradiation was modified for 10 min and after 6 h of Sb2S3 film deposition, which had the best rise time of 27.05 ms for the 405 nm laser. Optimal photo-responsivity value 112 A/W and detection rate 2.01 × 1012 Jones, demonstrating that semiconductor Sb2S3 materials can increase the photo-responsivity of graphene.Using graphene with different UV irradiation times to find the best growth parameters, we found that SbSupplementary information.


Hybrid transparent contacts based on combinations of a transparent conductive oxide and a few graphene monolayers were developed in order to evaluate their optical and electrical performance with the main aim to use them as front contacts in optoelectronic devices. The assessment of the most suitable strategies for their fabrication was performed by testing different protocols addressing such issues as the protection of the device structure underneath, the limitation of sample temperature during the graphene-monolayer transfer process and the determination of the most suitable stacking structure. Suitable metal ohmic electrodes were also evaluated. Among a number of options tested, the metal contact based on Ti + Ag showed the highest reproducibility and the lowest contact resistivity. Finally, with the objective of extracting the current generated from optoelectronic devices to the output pins of an external package, focusing on a near future commercial application, the electrical properties of the connections made with an ultrasonic bonding machine (sonic welding) between the optimized Ti + Ag metal contacts and Al or Au micro-wires were also evaluated. All these results have an enormous potential as hybrid electrodes based on graphene to be used in novel designs of a future generation of optoelectronic devices, such as solar cells. Although graphite, one of the crystallographic forms of carbon, has been relatively well known for a long time, its two-dimensional version—graphene—is a much more recent object of research. The lamellar structures of graphite and of thermally reduced graphite oxide were already known in the mid-19th century . Despite2 hybrid orbitals of the mutually-bonded carbon atoms. It is therefore a two-dimensional material (2D). Such an atomic arrangement and its two-dimensional structure are the factors conferring extraordinary properties to graphene.Graphene is defined as the material made of a single layer of carbon atoms distributed in the vertexes of a hexagonal network. This hexagonal shape (honeycomb) results from the covalent bonds generated by the overlap of the sp2⋅V−1⋅s−1. A single layer of graphene absorbs 2.3% of incident light, allowing around 97.7% to pass through [Graphene is also a zero-gap semiconductor. Its electron mobility is extremely high, of the order of 150,000–200,000 cm through . As a ru2:F) abbreviated as FTO, indium tin oxide (In2O3:SnO2) abbreviated as ITO, and aluminum doped zinc oxide (ZnO:Al2O3), abbreviated as AZO. They usually play the role as transparent electrodes, and thus, they must have n-type conductivity [Many of the described properties of this new material are outstanding in many senses, but for the purpose of the present work, the above-mentioned electrical and optical features are particularly relevant. The possibility of obtaining an excellent sheet conductance in combination with a very low absorption of light (for one or a few atomic layers), added to an extremely low reflection to visible light, all of this without adding more than a few angstrom to device thickness, makes graphene an excellent candidate for those optoelectronic applications in which low sheet resistances, exceptional responses at high frequencies, low contact resistances or reduced light absorption are required. A wide range of optoelectronic devices such as light emitting diodes, photodiodes, liquid-crystal displays, touch screens or solar cells require transparent or semi-transparent contacts having such properties . In addiuctivity . The TCOuctivity . Among tuctivity . For theAmong the TCO materials, ITO is currently the most commonly used in silicon heterojunction solar cell (SHJ) technology. This semiconductor material is well-known from its mass production and exhibits very suitable optoelectronic properties. Generally, ITO is deposited by direct current (DC) magnetron sputtering on large areas . Otherwi13 cm−2. Tests involving different metals, surface treatments and innovative architectures have also been reported. Among them, Cr/Pd/Au (1/15/50 nm) metal stack fabricated by e-beam evaporation on CVD graphene using a pre-plasma treatment presenting ~270 Ωµm contact resistivity [On the other hand, to efficiently extract the current in the SHJ device, a front metal grid must be evaporated finally. Therefore, it is essential to achieve low resistive metal contacts. As the graphene atomic layers are part of the hybrid electrode concept, appropriated metal stacks with ohmic character deposited on top of graphene also play a vital role in the device operation. In graphene-based technology, several investigations have been devoted to check different metals with proper work functions for electronic devices such as field-effect transistors . These sistivity , Ni/Au  or gold (Au) micro-wires to Ti + Ag front contacts could modify the contact resistance of the metal/graphene interface were also carried out. At this stage, the ideal Ti + Ag thickness layer and the setting of the parameters used in the semiautomatic wedge-wedge bonding machine were calculated to minimize the resistance of the sonic welding and to avoid interface damage. The results derived from this investigation have a considerable potential for the development of advanced transparent or semitransparent electrodes based on graphene and its application in novel designs of optoelectronic devices. The research of such a combination and of adequate preparation protocols is the kernel of the present paper.The hybrid transparent contacts described in this work are based on the combination of a common TCO and 1 to 3 graphene monolayers. In most optoelectronic devices, an ohmic low-contact-resistance metal electrode is essential in order to extract current with the lowest power losses. Hence, this metal stack plays a relevant role in device operation. The choice of the metallization is a critical step in outlining the cost and efficiency of solar cells and is considered as a vital factor for achieving high efficiencies. For this reason, strategies to reduce the metal contact resistance were also developed in this work. 4 precursor, prepped for transfer with polymethyl methacrylate (PMMA) coating, and finally transferred to the desired substrate. The CVD fabrication technique was preferred because it can produce relatively high-quality and high-purity graphene and potentially on a large scale. More details about the graphene fabrication can be found in reference [The graphene monolayers were grown by the Spanish company Graphenea S.L. by chemical vapor deposition (CVD) on copper foil from CHeference .2O3  commercial target subject to radio-frequency (RF), and indium tin oxide (ITO) was obtained from a ceramic 4-inch In2O3:SnO2  commercial target subject to direct current (DC). Argon of a purity of 99.999% was used as the inert gas in the sputtering process, and the control of its flux was carried out by means of a mass-flow controller. All the TCO films were deposited at low temperature in order to work in conditions compatible with the preparation of different kinds of optoelectronic devices having a particular sensitivity to high temperatures.The TCO materials were fabricated using a commercial Leybold UNIVEX 450B magnetron sputtering system . Aluminum-doped zinc oxide (AZO) was deposited by using a ceramic 4-inch ZnO:AlDifferent combinations of TCO and graphene were tested, with different deposition and transfer sequences in order to assess technological paths taking into account the mutual compatibility of the processes involved as well as the properties of the resulting multilayers.2 quartz to make electrical measurements, 2 cm × 2 cm Corning glass to produce samples for testing transparency, and 2 cm × 2 cm silicon substrates yielding samples suitable for the assessment of their antireflecting capability. The latter is considered an essential property in the specific case of front transparent contacts for silicon-based solar cells.Three different substrates were used: 1 × 1 cmThe metal stacks were evaporated by thermal evaporation on the graphene-based transparent contacts by using a commercial UNIVEX 300 system. The metals tested in the different schemes were aluminum (Al), palladium (Pd), nickel (Ni), titanium (Ti) and silver (Ag), respectively. This choice was done based on metal costs, except for Pd, which was used because it is one of the most commonly used metals in high-efficiency photovoltaic device technology. cρ) and the current transfer length (TL) were determined for the metal stacks, as well as the sheet resistance (sR) of the transparent contacts, by using the transmission line method (TLM) [TR. Then, TR was plotted as function of the contact spacing d to extract parameters such as the TL, measured from the intersection of the TR curve for TR = 0, and the ρc, usually obtained from the plot using the extracted TL and the sheet resistance sR [To determine the electrical parameters, four metal coplanar parallel electrodes of 0.1 cm width and 1 cm length unequally spaced  were deposited by thermal evaporation onto the samples. These monolayers had been previously transferred onto 1 cm × 1 cm quartz substrates. These measurements were carried out by testing 4-point electrical resistance between all the possible electrode pairs with two contacts on each electrode. The electrodes were contacted by means of 4 micro-positioners, a power supply was used to bias the samples, an electrometer measured currents and a voltmeter provided voltages. The overall system is pictured in od (TLM) . These Ttance Rs .To determine the fraction of light transmitted by the different layers that compose the transparent contact, optical transmission maps were obtained by using an experimental optical-transmittance-mapping (OTM) system that combines a focused white-light lamp, an X-Y linear-positioner set, a pair of current pre-amplifiers, a reference photodiode and a pair of digital voltmeters. Finally, the optical total  reflectance of the transparent contact as function of the wavelength was measured using a UV/Visible/NIR Perkin Elmer Lambda 1050 spectrophotometer, equipped with a 6-mm-diameter integrated sphere.The ultrasonic wire bonding technique is considered as one of the most flexible and cost-effective to assemble the vast majority of semiconductor packages. It commonly uses either Au or Al wires with tens of micrometers of diameter to interconnect integrated circuits (IC) or other semiconductor devices, and to package during the semiconductor device fabrication. Hence, it can be very useful to connect an IC to other electronics, or to connect from one printed circuit board (PCB) to another.To extract the current generated by the optoelectronic device is mandatory to interconnect the optoelectronic device to the supporting board, as it is depicted in In this work, the ultrasonic wire bonding technique (labeled as sonic welding) was chosen to make the connections. For this purpose, the bonding machine used was the 5632DA model from F&S BONDTEC Semiconductor GmbH  . Three mTo measure sonic welding contact resistance, a four-wire technique was used with the home-made system pictures in c of the conventional metal stacks previously described was determined for the different metal stacks tested. The metal stacks were evaporated at room temperature on graphene monolayers, bilayers and trilayers, respectively, previously transferred onto quartz substrates at the common transfer process conditions used by the supplier of the graphene monolayers (www.graphenea.com) [The contact resistivity ρnea.com) . The metnea.com) . Al and cρ and transfer length TL depending on the metal stack used. In addition, the sheet resistance sR of the graphene material underneath was estimated in each case.It was found that the contact resistivity to graphene monolayers was lower if using Ti + Ag and Ni + Al metallic combinations. Pd and Al contacts were much more resistive. The sheet-resistance data obtained by the graphene monolayer using Al for the contact were very disparate, as seen in the out-of-range value of 843 Ω/sq and its wide error bar. This value was far from that given by the supplier  and thosT calculated, the longest value was reached by the Ti + Ag contacts. That implies that electrons travel a longer distance in the graphene beneath the contact before flowing up into it. A possible explanation about why the metal stack based on Ti presents a better electrical performance could be related to the positive difference of the work function at the graphene/Ti interface, close to 0.15 eV [From the results in  0.15 eV . A charg2 plasmas and UV-O3 gentle activation steps. The high temperature regime includes processing steps at a temperature range from 200 to 450 °C, that is, annealing in vacuum at 200 °C, the use of temperature up 450 °C in inert atmosphere, or both; while the low temperature regime avoids those steps and the temperature is not superior to 120 °C [sR, measured with the system presented in 2/Si substrates [The compatibility between the conditions used in the graphene transfer method  and the temperature applied to fabricate the cell material was evaluated. It is very important to perform these analyses because the hybrid transparent contact based on graphene is incorporated at the last manufacturing step. Hence, parameters such as the temperature and the environment used in the transfer process could negatively affect the performance of the material located underneath and that constitutes the device. For this reason, low- and high-temperature transfer procedures were mutually compared; avoiding Oo 120 °C . Table 2bstrates .sR, a slight electrical improvement was observed in the graphene material transferred at low temperature. This work undergone so far clearly suggests the possibility of reducing the temperature in that process while the material performance is maintained. In addition, the low substrate temperature would also benefit the deposition of the hybrid transparent contacts based on a TCO by reducing the possible damage to it.It can be noticed that the average white-light transmission values were in agreement with the theoretical predictions made according to the number of graphene monolayers . TherefoThe second step consisted of evaluating the position where the graphene monolayers should be placed within the hybrid transparent contact structure. This required the study of the possible damage to graphene material produced by the bombardment of the high-energy sputtered particles during the TCO deposition. For that, three graphene monolayers were placed in two positions: (i) transferred onto the already TCO-coated resistive glass  where the graphene material was transferred at low temperature. AZO and ITO materials were incorporated into the contact to enhance both the optical and electrical device performance. From the optical point of view, the main role of the TCO is to act as an antireflection coating diminishing the amount of light reflected on the solar-cell surface; that means, to eliminate unwanted reflection and increase the overall transparency. Hence, the thickness of the TCO must be fitted so that the wavelength in the dielectric material is one quarter the wavelength of the incoming wave, according to formula (1)spectrum . TherefoFrom the electrical point of view, the TCO must present the lowest possible resistivity to reduce device resistance and to extract the current in the most efficient way. Under these considerations, the performance of the hybrid transparent contacts was evaluated from the figure of merit (FOM) y Haacke  given by2-size TCO-coated silicon wafers, and the TLM mask used in this work with the further TLM structure based on Ti + Ag evaporated on graphene monolayer transferred onto quartz  for the bare ITO layer. This almost negligible difference was considered within the measurement error; therefore, the addition of the graphene material into the transparent contact would not affect its AR capability. 2 substrate were around 40 ± 12 mΩ. These values are found to be typical for sonic welding at ambient temperatures using AlSi wires and Ag substrates; this is a very promising result for the applications of the graphene-based technology. So far, no tests have been done on solar cells with the hybrid transparent electrodes, but they are foreseen in the near future. In depth studies about damage produced by the wire bonding on graphene/metal interface should be done.With the future goal of integrating the optoelectronic device based on the optimized hybrid graphene/TCO transparent electrode described above into a PCB, it is necessary to minimize the resistance connections of the wire bonding used. For this reason, the contact resistance of the welding Ti + Ag metal contact-AlSi wire, made with the ultrasonic wire bonding machine showed in This work presents the optimization of a metal stack combination being a candidate to make ohmic contacts for silicon-based photovoltaic devices. The metal combinations tested were Ti + Ag, Al, Ni + Al and Pd, being the first one that showed the most reliable and reproducible data with the lowest specific contact-resistance value.The evaluation of the preparation protocols used in the fabrication of the hybrid transparent contacts based on a common TCO and graphene monolayers was also studied. The first parameter evaluated was the temperature used during the graphene transfer process. Reduced sheet-resistance values of the graphene monolayers were obtained at low temperature. The optimum spatial configuration of the layers forming the hybrid transparent contacts was also determined. A strong effect on electrical properties of graphene due to the bombardment of the highly-sputtered atoms during the sputtering process of the TCO thin film was observed. Hence, adequate preparation conditions should include transferring the graphene monolayers onto the TCO and not the opposite. A correlation between the avoidance of aluminum  and good and repeatable electrical properties was found. Combinations of graphene with ITO were systematically better than those of graphene and AZO.Therefore, a suitable combination of graphene and ITO thin film in a hybrid transparent contact deposited on silicon was found, showing appropriate performance for it to be used as an electrode in optoelectronic devices, more specifically, for silicon-heterojunction solar cells. Finally, 25 µm-thick AlSi bonding wires were used to connect the samples with Ti + Ag contacts to a PCB. The contact resistances of such wires were around 40 ± 12 mΩ, resulting in suitable values for this type of wire bonding at ambient temperature.All these approaches have an enormous potential to open new horizons to achieve the definitive take-off of graphene technologies for electrical standard and transparent contacts.


A dual-controlled tunable broadband terahertz absorber based on a hybrid graphene-Dirac semimetal structure is designed and studied. Owing to the flexible tunability of the surface conductivity of graphene and relative permittivity of Dirac semimetal, the absorption bandwidth can be tuned independently or jointly by shifting the Fermi energy through chemical doping or applying gate voltage. Under normal incidence, the device exhibits a high absorption larger than 90% over a broad range of 4.06–10.7 THz for both TE and TM polarizations. Moreover, the absorber is insensitive to incident angles, yielding a high absorption over 90% at a large incident angle of 60° and 70° for TE and TM modes, respectively. The structure shows great potential in miniaturized ultra-broadband terahertz absorbers and related applications. Terahertz (THz) absorbers have been widely investigated due to their wide applications in electromagnetic radiation, radiation, sensation, and thermal imaging ,2,3. A cIn this work, a dual-controlled ultra-broadband THz absorber based on a hybrid graphene-Dirac semimetal multilayer structure is proposed and studied. By simply shifting the Fermi levels of graphene and Dirac semimetal, the proposed structure shows tunable absorption over a broadband wavelength range. The simulated results show that under normal incidence, the device exhibits a high absorption larger than 90% over a broad range of 4.06–10.7 THz for both TE and TM polarizations. Moreover, the absorber is insensitive to incident angles, yielding a high absorption over 90% at a large incident angle of 60° and 70° for TE and TM modes, respectively. The physical mechanisms are also elucidated by the impedance matching theory and electric field analyses.It is known that graphene is a potential material to design reconfigurable THz devices due to the tunable surface conductivity  formula :(1)σg(ω)In the THz range, the relative permittivity of BDS can be expressed as :(2)ε=εb+σ can be written as [The dynamic conductivity itten as :(3)Reσ=erated as :(5)θ(t)=From Equations (2)–(5), we can see that the permittivity of BDS can also be controlled by Fermi energy. 2O3 layer with a relative permittivity of 2.28 and loss tangent of 0.04, BDS complementary pattern layer, lower Al2O3 layer, and gold layer with conductivity σ = 4.56 × 107 S/m from top to bottom. In this work, AlCuFe is used as the BDS material [The schematic diagram of the dual-controlled ultra-broadband THz absorber is illustrated in material .2O3 layer is to provide resonance absorption space, and the bottom gold layer is used to reflect the incident wave. In order to satisfy the polarization and incident angle insensitivity, we have designed a symmetrical structure as shown in The top graphene patterned layer is to match the impedance of free space at a specific working frequency band, so as to achieve no reflection of the incident wave. The BDS pattern of the middle layer is to expand the absorption bandwidth. The basic function of the Al1, w2, R1, R2, and R3 to construct circular rings and square channels in the graphene pattern. (2) In the design of the patterned BDS structure, on the one hand, the method of designing multiple small channels similar to the graphene layer is continued to be used; on the other hand, the structure is supposed to complement to the upper graphene pattern. In this case, the two absorption layers would resonate at different frequencies, thus extending the absorption bandwidth of the structure. In our proposed structure, the BDS pattern is controlled by w1, w2, a, and b. (3) In the conventional absorbing material model, in order to ensure that the wavelength in the dielectric layers forms a stable standing wave of electromagnetic waves, the thickness of the dielectric layer satisfies h = (2n + 1) λ/4, where n = 0, 1, 2.... In that case, the incident electromagnetic wave would interfere destructively with the reflected wave from the bottom metal layer, leading to the absorption of the electromagnetic wave energy by the structure. But for the metamaterial absorber, the thickness of the dielectric layer does not need to be a quarter wavelength as the dielectric constant and permeability of the metamaterial are controllable, which facilitates the realization of the absorber structure. In our work, the size of the two dielectric layers of graphene-BDS hybrid absorber is 4 μm and 2.8 μm, which are obtained from scanning optimization in the simulation software in consideration of the requirements of structural miniaturization and high absorption performance.The reasons for designing this structure are as follows: (1) In the patterned graphene structure, due to the localized surface plasmon resonance, most of the electric field will be limited to the edge of the annular graphene. This resonance can effectively capture light energy to enhance the absorption rate. More circular channels in the graphene pattern can be arranged to limit the electric field distribution on the edge of the graphene, thereby increasing the absorption rate. Therefore, in our proposed structure, we use w21 = 0 and absorbance A = 1 − |S11|2. The detailed dimensional parameters of the proposed absorber are listed in Ansys HFSS is used for the simulation of the proposed structure. The THz wave impinges on the graphene pattern from the air. Periodic linked boundary conditions (primary and secondary) are adopted in the x- and y-directions and Floquet port excitation in the z-direction. To better describe a graphene film, the graphene layer is considered as a two-dimensional conductive surface with the impedance boundary of resistance and reactance in the software simulations. For BDS material, the relative permittivity is created by importing relevant data obtained from Equations (2)–(5). Since the thickness of the bottom gold film is considerably larger than the skin depth, the electromagnetic waves can hardly be transmitted through the bottom metal plate, leading to SThe absorption properties of different structures under normal incident TE and TM polarizations are shown in Next, we studied the influence of graphene Fermi energy on absorption performance. BDS offers another possibility to tune the absorption performance of the hybrid structures. To sum up, owing to the tunability of absorption bandwidth by graphene Fermi energy and the control of resonance absorption peaks by BDS Fermi energy, our proposed absorber combines the advantages of being broadband-controlled and frequency-adjusted.For THz absorbers, the sensitivity of polarization and incident angle is a significant issue in practical applications. Here, the absorption properties of the proposed structure under different polarization waves and incident angles are investigated, as shown in d is the travelled distance and k0 is the propagation constant of the wave at free space. The calculated values of the constitutive parameters for three absorption peaks are listed in To clarify the working principle, the absorption peaks located at 4.49, 6.738, and 9.66 THz are explained according to the impedance matching theory. The relative impedance (Zr) is calculated using Equations (6)–(10), where x and y directions and the internal oblique gaps, while the electric field distribution of the BDS layer is mainly concentrated in the wide channels along the x direction. At 6.73 THz, the electric field of the graphene layer is concentrated in each slit except for the four internal oblique slits, while the electric field of the BDS layer is scattered in the square blank channels. At 9.66 THz, the electric field of the graphene layer is concentrated in each slit except for the four slits in the x direction, and the electric field of the BDS layer is concentrated on the four sides of the x direction of the outer frame.In order to illustrate the working principle more intuitively, the electric field distributions on the x-y plane at frequencies of 4.49, 6.73, and 9.66 THz corresponding to absorption peaks I-III for TE polarization are shown in x-axis and induce tangential electric fields on both graphene and BDS layers, which will cause energy loss. The energy consumption inside the loss materials including graphene, BDS and Al2O3 can be calculated by:V is the volume of lossy material, and On the whole, as the frequency increases, the intensity of the electric field first increases and then decreases as the frequency increases in the graphene layer. As for the BDS layer, the change trend is opposite and firstly decreases and then increases. This is because that the incident wave excites carriers to oscillate along the 2O3 layer play an important role in absorption. The absorption contribution rate of each part is shown in 2O3 layer, which is consistent with the absolute electric field distributions in 2O3 layer in the THz frequency range. Due to the symmetry of the structure, the TM polarization is the same as the TE polarization.In order to understand how the energy is located at the resonance frequency, the electric field distribution in the y-z plane is simulated to reveal where absorption mainly occurs. In summary, a tunable hybrid graphene-BDS broadband THz absorber is designed and studied. Due to the combination of advantages of graphene and BDS, the structure can achieve a high absorption exceeding 90% over a broad wavelength range of 6.64 THz. By adjusting the Fermi energy level of graphene and BDS, the absorption peaks and bandwidth can be dynamically tuned without reconstructing the structure. Besides, the absorber is insensitive to incident angles, yielding a high absorption over 90% at a large incident angle of 60° and 70° for TE and TM modes, respectively. Owing to its excellent performance, the proposed absorber has great potential in miniaturized ultra-broadband THz devices and microsystems.


Nature-based recreation in urban areas is essential for the well-being of citizens. Park green space (PGS) is a necessary urban infrastructure and a critical step of urban planning and policy-making. The existing research on PGS only focuses on service allocation problems existing in the current urban development, ignoring changes in residential communities accessibility. This research provides new ideas to evaluate PGS. Based on parks and residential communities’ data, we adopt an improved Gaussian-based two-step floating catchment area (2SFCA) method to evaluate PGS accessibility in Ningbo (China) and its matching with different levels of residential areas. We present a case study in Ningbo, and discuss its implications for PGS management. This study contains two elements: (a) Compare the current and initial PGS accessibility of each community to accurately identify the communities with PGS vacancies. (b) Analyze and discuss the association between community accessibility and residential house prices. Compare the PGS coverage ratios of communities at different levels to determine the equity of PGS planning in Ningbo. We found that the level of PGS allocation in the central area of Ningbo is high. Obviously, high-value clusters are formed in Sanjiangkou, Zhenhai New Town, Southern and Eastern Yinzhou. The accessibility level in the middle area of Yinzhou is low, and there are super high accessibility residential communities in the outer city area. There is an exact period of green space vacancy in the middle and the outer area. The residential areas with ultra-high accessibility did not configure PGS services at the beginning of their construction. There is no noticeable difference in PGS accessibility of residential communities of different levels at present, but 149 low- and middle-income residential communities lack green space service when the construction was completed. High-end residential communities have priority on enjoying park green space services. Our study suggests that PGS accessibility should be studied temporally and spatially for each residential community. The Ningbo government should strengthen the balanced construction of green space in parks and guarantee green space services for low-end residential communities to improve green space equity. Half of the world’s population lives in cities, and the urban population’s rapid growth places higher demands on infrastructure . The disSince the 1980s, green space research has increased, mainly using green space rate and green area to reflect the overall green service level. However, there is not enough consideration at the micro-level. In 1998, Chinese scholars first proposed landscape accessibility, which became the main content of green space research . ResearcResearch on PGS equity is usually based on spatial accessibility. Euclidean distance or road network analysis in geographic information systems (GIS) is often used to examine the shortest distance to the nearest green space. However, people may choose other green spaces over the nearest one because of social preferences and PGS size. Accordingly, the gravity model is employed to address this issue. Radke and Mu first proposed a two-step floating catchment area (2SFCA) approach to measure spatial accessibility that takes into account interactions . Luo furResearch on the equity of green spaces has focused on spatial equity and social equity. Spatial equity focuses on differences in the spatial distribution and quality of green spaces. The research found significant differences in the number of users for parks of different sizes . RigolonIn these previous studies, we found a hidden problem. The accessibility evaluation in the green space equity study focused on PGS’s service level at the present stage but neglected the PGS matching with the residential community in its development . The conBeing an economic, political, and cultural center in the Yangtze River Delta China and currently experiencing unprecedented growth and expansion, Ningbo is selected for analysis in this study. Moreover, this city has experienced a tremendous green space loss, similar to many other cities in China. Therefore, we select Ningbo as the research area to enrich the local green space service research cases in China. On the research scale, Chinese studies are mostly evaluated on the sub-districts scale . The evaChina’s research is mostly concentrated in first-tier cities, such as Beijing , ShanghaBecause of its openness and publicity, PGS has become the primary way for Chinese residents to obtain green space services. Take PGS as the service supply source, and use the park level and green space area to reflect the supply capacity. According to the “2014 Ningbo Ecological Green Space System Special Plan”, map the distribution of parks in the study area a, Includhttps://nb.anjuke.com/), “Lianjiawang” (https://nb.lianjia.com/), and “Ningbo Housing Management Bureau,” 2225 residential communities in the study area were collected. The main information includes location, number of households, average price, and year of construction. According to the “Ningbo statistical yearbook 2019”, the average population per household is 2.50. Based on this, calculate the total population of each residential community. Furthermore match the location of it on the map  method. In recent years, 2SFCA has gradually become the mainstream because of its rich expansion forms for different cities. Radke and Mu first proposed 2SFCA, and then the extension of the distance attenuation function was introduced . The priThe extended form of the 2SFCA method has not been adjusted according to the research content in the current study. With the aim in mind, in this paper, we present an improved Gaussian-based 2SFCA for PGS. Add a time limit to the 2SFCA: when the park is built later than the community, it will not be counted in the service radius. Through this restriction, PGS accessibility in the present and at the beginning of the construction is calculated. By comparing the PGS accessibility at different time points, identify people with green space service vacancies. The specific process is as follows:d0  for each park green space j to form a spatial scope. For the community population k distributed in the spatial scope, we use Gaussian equation to give weight. The weighted population is summed to get all the potential users of the green space j, and then the green space is divided by the potential users to get the supply-demand ratio jR.First of all, we set a space distance threshold kC is the number of community population in the spatial scope of green space j; kjd is the spatial distance from the center of the community to the center of green space j; jS is the capacity of green space, where the area of park green space reflects the supply capacity of the park. G is the Gaussian equation, see Equation (2):i, given a spatial distance threshold d0  to form another spatial scope, The supply-demand ratio of each green area l, lR is weighted by the Gaussian equation, and then the weighted weekly supply-demand ratio is summed to obtain the park green space accessibility of each residential community iA, the size of iA can be understood as the threshold per capita green space in each district in the area:In the second step, for each residential community lR represents the supply-demand ratio of green space l within the spatial function threshold of residential community i, lT represents the opening year of green space l, and iT represents the construction year of residential community i. The description of other indicators is the same as above. The accessibility value can be understood as the per capita green area after a unique weighting process. With this formula, we can find out some communities. These communities were not served by parks and green spaces at the time they were first built. We call them unserved communities.In the formula: d0 according to the PGS type, and there are six residential communities in the spatial scope of the PGS. For each of them, the spatial distance from the PGS can be measured, and then the corresponding Gaussian equation can be established according to Equation (2), respectively. The PGS area and the number of people represented by each community are also known, so the corresponding supply and demand ratios can be derived from Equation (1). In d0 is set to 2500 m depending on the residents’ travel speed. According to Equation (3), the PGS accessibility of the residential community is obtained by accumulating the supply ratio to demand for each community.The accessibility calculation result is a spatial latitude value. To make the results easy to understand and analyze, we use house prices to represent the socio-economic attributes of residents. We obtain the prices of houses in the residential neighborhoods within the study area and then match them with the accessibility. According to the average housing price, the residential communities are divided into six levels: first, second, third, fourth, fifth, and unit communities. Respond to the disparity in residents′ access to green space services by counting the number of unserved communities in different classes of neighborhoods. The ratio of green space coverage in Ningbo is based on the greening requirements of the city. Compare the current differences in urban green space coverage.Cr is the coverage ratio of park green space. Ni is the number of i-level communities within 500 m of the park green space. Siis the total number of In Equation (4), At present, the accessibility of PGs in Ningbo is high, but there is imbalance among regions. The Sanjiang area has high-value clusters. Zhenhai New Town, Yinzhou Southern Business District, and Eastern New Town maintain a high level. Meanwhile, there are ultra-high accessibility residential areas in the outer area . SanjianThere are few parks in Zhenhai District. However, two high-value gathering areas are evident. High-access residential communities are mainly distributed near the Ningbo Botanical Garden and Zhenhai Green Axis Sports Park. Because this is the central area in the Zhenhai region. Ningbo Botanical Garden exceeds 10 hectares, which is significantly improving the service level of PGS in surrounding areas. Surprisingly, some residential communities in the outer area of the city have extremely high PGS accessibility. Combined with the population density map, we find that the super high accessibility residential areas in the city’s outer area are its low population density. There is pretty good PGS scattered around them. Taking Beilun ultra-high accessibility level residential areas as an example, the region’s total population is only. There are seven community parks within its radius of 2.5 km, including Xinmo Park, Meixu Park, Ningbobang Park, and Dadongjiang park. The total green area is more than 20 hectares.To further study urban parks and residential communities’ spatial and temporal distribution, Ningbo city is divided into three parts: the inner area, middle area, and the outer area . We count the year that each residential community was built and compare the time that parks were built. Due to the lack of construction years in some communities, 1514 communities have been counted. Equation (3) is used to calculate PGS accessibility when it has just been constructed . By compThere are 45 parks in the inner area. The parks were generally built earlier, with the average year of 1997. The number of parks built before 2000 exceeded 20. Early completion of PGS met the needs of residents. The number of communities without service is the smallest; the percentage is only 2.07%. Simultaneously, the number of residential communities in the inner ring has slowed down, and the entire area’s living space is becoming saturated. The residential communities in the inner ring area have been well supplied with infrastructure throughout the city′s development, and residents of this area enjoy some of the best PGS services in Ningbo. The middle area has the most unserved communities with a percentage of 18.61% . Over thThe outer area is a typical urban-rural junction, with 57 unserviced communities mainly located in Zhenhai, Beilun, and the west of Haishu. There is a significant PGS vacancy in this area, with nearly one-third of communities not having a corresponding PGS at the time they were built . Based oSuch a result is not found in previous surveys, but it is an element that cannot be ignored. Although the current service level is good, we found that the planning and construction process of green spaces in Ningbo has not been timely in meeting residents’ green space needs in the middle and outer area. There is a shortcoming in evaluating the service level of PGS: it ignores the sequence of residents enjoying green space services. Through the space-time matching evaluation of residential communities and PGS in the central area of Ningbo, we can accurately identify the green space vacant area and the residential community with service vacancy, which provides a more accurate judgment for future targeted solutions.In 1998, China implemented the housing commercialization reform. Taking 1998 as the starting year and counting the commercial housing built after that, the socio-economic status of buyers is reflected through house prices. Excluding the houses before 1998 and the houses whose construction year is not available, a total of 1334 commercial communities are counted. According to the average housing price, the residential communities are divided into six levels: first, second, third, fourth, fifth, and unit communities. The higher the group, the lower the income level of the residents of the residential areas. Calculate the average accessibility of each group of residential communities .The results show that: (1) The average accessibility of the unit communities is the highest. At present, the average accessibility of different levels of residential communities in the central urban area of Ningbo is similar, and there is no evident gentrification of PGS. The construction years of the unit communities were all before 1998. They were planned and constructed by the government and distributed in the old city of the Sanjiang area, so they have good green space services. (2) In terms of the average accessibility at the beginning of construction, the unit communities are the highest, followed by the first-level residential communities, and the fifth-level residential communities are the lowest. High-income groups have the smallest difference in PGS accessibility between the two stages, and green facilities can be matched in time. (3) In terms of the number of unserviced communities at the beginning of construction, there is no unserviceable situation in the first and second-level residential communities. Main unserviced communities are concentrated in low—income communities such as the third and fourth levels. High-end residential PGS services have a good match, and they have a priority on enjoying PGS services. There are extensive green space service vacancies in low- and middle-end residential communities. The financial capacity of residents directly affects the level of green space services they receive. Lower-income residents may not have immediate access to PGS when they move into a neighborhood. There are significant social inequalities in the planning process of green spaces in Ningbo.We compared the spatial distribution of different residential communities’ levels and the density of the parks . RegardiIn general, there is a temporal and spatial misalignment in the planning process of Ningbo Greenland, which leads to PGS inequities. There is a period of obvious green space service vacancies in the middle and low-end communities, and high-income communities have priority in enjoying park services. At the same time, the first-level and fifth-level communities have the lowest percentage of seeing gardens in 500 m.The first-level communities have natural and superior green space conditions, while the fifth-level communities have much lower percentages than the second, third, and fourth-level communities. It shows that the lowest-income communities have the lowest level of green space coverage, which is unfair. The unit community maintains a relatively high level in terms of accessibility and the ratio of 500 m garden communities. Planners should formulate reasonable greening policies according to the needs of different types of communities to avoid the waste of PGs resources.The study of PGS in central Ningbo shows that the residential accessibility in the inner ring is higher than that of the middle and outer rings, and that residents in affluent communities benefit more from the green accessibility than those in low—and middle-income communities. This difference is partly attributed to the spatial reconstruction of the green infrastructure in Ningbo. Urban development in the inner ring basically covers all parts of facilities such as medical care, education, and green space. Traffic problems have been alleviated after the opening of urban inner ring viaducts and expressways. Therefore, the urban PGS service level in the inner ring region is the highest. Along with industrial suburbanization, Ningbo takes tertiary industry as the key point of urban development. The development key of the town gradually shifts. The construction of green infrastructure starts, and good green infrastructure in the central area contributes to better accessibility. In the Master Plan of Ningbo city (2006–2020), it is clearly pointed out that the Sanjiang Area need to form a commercial center with dual-core spatial structure and the eastern region should create an ecological leisure axis based on water and green space along the river. In this case, the configuration of urban parks in Sanjiangkou and Eastern New Town is far better than the suburban fringe area.Our results are similar to those of previous studies, which found that compared with disadvantaged communities, more affluent communities are more likely to acquire green space ,52. In tManagers and planners should be sensitive to the inequalities in urban PGS. The attribute of PGS is not only reflected in the quality and spatial distribution. Besides, the urgency of different social groups′ demands for green space in parks is also an important indicator of green planning. To ensure equal opportunities, PGS services should be provided to the social groups living in the middle- and low-end communities. The spatio-temporal inequality of PGS accessibility in Ningbo can help municipalities and planners determine where further efforts should be made to improve green space equity. The time and space mismatch between housing prices and park green space accessibility help to inspire greening strategies. Planning should provide funds for green space around low-income communities in suburban areas to meet the needs of residents for green infrastructure. We are marking residential areas where green spaces have been vacant in the past in the hope that they will be taken seriously by planners. Meanwhile, in the Special Plan for Ningbo Ecological Green space System, it is proposed that the service radius of park green space 500 m reaches over 90%. As shown in This work provides comparative study on the individual accessibility of residential communities so that the method can more accurately evaluate the rationality of urban green space planning. We hope to objectively evaluate the fairness of PGS in this way. Meanwhile, we offer a new object of study for future research. That is to study the micro-perceptions of people in unserved communities. Residents′ perceptions of green space vacancies may not be very sensitive, so our future research will be to investigate residents′ satisfaction with PGS. We would like to know the maximum amount of time residents have allowed PGS vacancies. Setting a maximum threshold for vacancy periods in the future plan can alleviate residents′ frustration with the problem. Although we improve the methods and provide ideas, there are still some limitations. First of all, housing prices are used to reflect the economic conditions of residents without considering relevant factors, including external environmental information such as location, environment, and traffic. In future work, these factors can be combined to reflect changes in socio-economic conditions. Secondly, the green space service level measurement based on the Gaussian-based 2SFCA method takes the spatial distance as the reference, without considering the barrier function of traffic trunk lines, and considering that green accessibility of the Ningbo community may be affected by specific environmental factors, such as the distance to the central business district. It needs to be defined as a control variable in future studies. Thirdly, as for the population of residential areas, there is a certain vacancy rate of houses in the residential areas, so there is a deviation between the population data and the actual situation. We use park green space data, urban block green space and so on are not included in the study. In the future, based on information technology and social software to obtain spatial population data, we should further study the fairness of green space.Based on the data of PGS and residential communities, this paper adopts an improved Gaussian-based 2FCA method to calculate the accessibility of PGS in Ningbo. We integrate the year of construction and housing price information to evaluate the equity of PGS in Ningbo. The results show that the accessibility of PGs in Ningbo is good, but there are still regional differences. The middle part of Yinzhou is an obvious low value area, and there are high accessibility communities in the outer ring. By comparing the initial and current accessibility, we found that 181 residential communities with PGS service vacancies. There are significant spatial differences with initial PGS accessibility in the middle and outer Rings. The current high accessibility communities in outer ring lack PGS service at the beginning of their construction. Unserved communities are concentrated in low-end and mid-range neighborhoods. High-end communities have absolute priority access to the park green space service sequence. The highest end and lowest end communities have the lowest percentage of garden visits at 500 m. The most expensive communities have a very privileged natural environment inside and they don′t need PGS services. While the lowest end of communities have the lowest level of green area coverage, which is unfair.In the past, Chinese residents generally paid more attention to educational resources, commercial facilities, and transportation facilities. With the improvement of living standards, they care more about their health and PGS facilities have also attracted more and more attention. The evaluation of PGS accessibility based on spatial distribution cannot accurately identify communities with green space vacancies, which will impact future in-depth research. Government planning could lead to 5–10 years of residents not getting PGS services. This is unacceptable for a family. Therefore, it is very necessary to identify these groups, because they may dominate the future urban green space planning. That is why we do this research. Our research has identified these groups and demonstrated that social inequality is the main cause. In the future, planners may not be able to avoid this phenomenon. Therefore, future research will focus on the perception of green space among residents of unserved communities. Based on the discussion and conclusion of our study, we put forward the following relevant policy suggestions to the future planning: (a) Adjust the allocation of resources according to the needs of the population, focus on vulnerable areas, and add new PGS. We suggest to create a new comprehensive park in middle area of Yinzhou to meet the recreational needs of a large population. (b) The amount of green space within different communities varies, thus planners should implement appropriate greening strategies for different type communities. (c) Gaining awareness to the green space vacancy in the process of planning, the people without park green space service can provide meaningful ideas for the future planning.


The air gap thus decreases leading to increase in the tunneling current across the junction. The gap however is independent of temperature fluctuations as both patches expand or contract by near equal amounts. Humidity sensor action demonstrates an unassisted reversible resistance reduction Rmax/Rmin ~105 when the device is exposed to 20–90 RH% at a standby DC power consumption of ~0.4 pW. The observed resistance change when subject to a temperature sweep of 25–60 ° C @24% RH was ~0.0025% of the full device output range.We present the design, fabrication and response of a humidity sensor based on electrical tunneling through temperature-stabilized nanometer gaps. The sensor consists of two stacked metal electrodes separated by ~2.5 nm of vertical air gap. Upper and lower electrodes rest on separate 1.5 μm thick polyimide patches with nearly identical thermal expansion but different gas absorption characteristics. When exposed to a humidity change, the patch under the bottom electrode swells but the patch under the top electrode does not, as it is covered with a water-vapor diffusion barrier ~8 nm of Al Kharaz and Jones were one of the first to report optical methods of humidity measurement6. First introduced by Wohltjen in 1984, SAW sensors were widely used as hygrometers, where shift in resonant frequencies was a direct indication of mass of adsorbed water vapor7. The two most common type of hygrometer devices are resistive and capacitive type.Humidity sensors can be realized using various technologies8. These devices are used in approximately 75% of applications3. In these devices, the dielectric properties of a hygroscopic layer  change when exposed to humidity. This change may also be accompanied by swelling of the hygroscopic layer. Micro-cantilevers have also been used as capacitive type humidity sensors. In these devices, a thin polyimide film is patterned on the cantilever and when exposed to humidity, the polymer film tends to expand thereby exerting mechanical stress on the cantilever causing it to bend9. Capacitive humidity sensors display linear operation and can accurately measure from 0–100% relative humidity with an output range typically ~30% of the default capacitance. These devices consume very little electrical power and are relatively cheap to fabricate3.Capacitive humidity sensors are the most common realizations of microfabricated hygrometers2O310, conductive polymers such as PVA with graphitized carbon11 and polyelectrolytes12 have been used as functional materials for resistive humidity sensors. In contrast to capacitive devices, resistive humidity sensors are highly nonlinear and capable of providing several orders of magnitude change in resistance. An advantage of resistive RH sensors is their interchangeability, typically within ±2% RH. Operating temperatures of resistive sensors ranges from −40 °C to 100 °C; however these devices have significant temperature dependencies. For example, a typical temperature coefficient for a resistive RH sensor is −1% RH / °C.Resistive humidity sensors are also extensively used. In these devices the resistance of the sensing material changes when exposed to humidity. Ceramics such as AlIn this work we present a new type of microfabricated humidity sensor that is able to provide large output range and a low temperature dependence. The device utilizes the expansion of a polymer that swells when exposed to humidity as in the capacitive device, but it produces a resistive output that measures the polymer expansion through tunneling current across a humidity dependent, thermally stabilized nanogap. The tunneling current changes many orders of magnitude providing similar output as the resistive type device with a low temperature dependence. Table In the first part of the article, we focus on the fabrication and electrical characterization of the device. The second part of the paper will describe the device response to change in ambient RH% and temperature response of the device. The third part of the article deals with relevant analysis of the current conduction across the nanogap and a mathematical model describing current flow as well as an equivalent electrical circuit. This section also includes a discussion of the sorption kinetics of the device and analysis of the absorption-desorption phenomenon.16. Similar to other devices built by Mastrangelo et al.19, these can be used as low-power and highly sensitive tunneling hygrometers for IoT applications. In this article, we show the working of a new type of tunneling hygrometer. The device consists of a pair of upper and lower electrodes, separated by an air-gap of ~2.5 nm as shown in Fig. Nanogap electrodes (electrodes separated by a gap of a few nanometers), have been previously used for highly sensitive bio-sensor applications20. Figure 21 hence the device resistance goes asA(T) and B are fitting parameters. In the simple configuration shown in Fig. Δ is also affected by the thermal expansion of the polyimide, which makes the coefficient A and the resistance of the device strongly dependent of the ambient temperature.The upper electrode rests at a fixed height. The bottom electrode rests on top of a hygroscopic polymer, polyimide that swells when humidity is absorbed; thus reducing the gap between the two electrodes. The swelling of the polyimide with humidity is linear corresponding to its humidity coefficient of expansion or CHE, which is 60–75 ppm/% RH2O3 diffusion barrier whereas that under the lower electrode is exposed to ambient atmosphere.To eliminate this strong temperature dependence we utilize the differential device arrangement as shown in the schematic of Fig. We thus achieve nanogap temperature stabilization by using cancelation of a common mode thermal expansion of both patches and humidity signal extraction by differential response to humidity between the two patches. Since both the top and bottom electrodes are standing on near identical polyimide patches, any increase in ambient temperature would lead to both the patches expanding almost equally. This ensures that in the event of temperature fluctuation, the nanogap distance between the top and bottom electrode will remain unchanged and the electrical response will be negligible in comparison to sensor response.I-V characteristics of the device after fabrication of the device. The junction I-V characteristics are typical of very low current on account of very low electron tunneling across an air-gap of ~2.5 nm. The average DC resistance of the junction is measured to be ~271 GΩ. This shows an extremely low leakage current under low biasing conditions.The simplified fabrication process is shown in Fig. 2O3 acts as a diffusion barrier and prevents the polyimide patch under the upper electrode from absorbing water molecules22. This differential swelling of the polyimide patches results in the inter-electrode distance to reduce below 2.5 nm. Therefore, after absorption of water vapor molecules, the tunneling distance for the conduction electrons reduces. Since tunneling current exhibits exponential dependence on tunneling distance, increase in humidity levels lead to an exponential increase in tunneling current. In other words, junction resistance decreases exponentially when exposed to increased humidity levels.After the device is fabricated, the upper and lower electrodes are separated by an air-gap of ~ 2.5 nm. Therefore current flow across the nanogap junction involves conducting electrons tunneling through 2.5 nm of air-gap. However, when the device is exposed to an increase in humidity, only the un-protected patch of polyimide which is beneath the lower electrode absorbs water-vapor molecules and swells. This is because the AlI-V plots, the increasing RH% leads to an increase in magnitude of current flow for the same voltage bias. Figure 3/%RH with an average standard deviation of 6.16 × 10−3 which corresponds to an equivalent noise current of ~34.93 fA. The limit of detection of the device that is defined as three times the noise floor is calculated to be 3 ppb. However, the authors would like to clarify that the calculated limit of detection of the device is based on a linear sensitivity approximation of a non-linear device23. Further device characterization and calibration is required to accurately quantify the sensor parameters. Figure Figure 9. Essentially, temperature compensation can be achieved using passive and active methods. In passive temperature compensation, passive electrical elements such as resistors are employed to compensate pre-determined fluctuations in sensor readout due to temperature changes. Active temperature compensation involves an active temperature feedback to the transducer’s signal processing circuit in order to compensate for the temperature drift. However, all these methods require external power that limit their application in power-scarce environments25. Recently, Rinaldi26 and Mastrangelo27 used self-levelling beams in order to achieve temperature compensation in micromachined sensors. In our proposed tunneling hygrometer, we achieve passive temperature compensation without consuming any external power by using a non-differential polymer expansion design when exposed to temperature fluctuations. As shown in Fig. 2O3, absorbs the molecules and expands, but the protected polymer patch does not. Therefore, air-gap between the electrodes reduces and tunneling current flow between them increases. However, when the device is subjected to an increase in temperature, due to near identical thermal mass and negligible thickness of the Al2O3 layer in comparison with the platform, both polyimide patches expand equally. This ensures that the air-gap between the electrodes remains almost constant.Researchers have always tried to neutralize the undesired temperature response of polymer based sensorsFigure 28. Figure The current conduction through the nanogap junction is considered to be electron tunneling. Essentially, current flow across the air-gap is possible only when conduction electrons tunnel between the metal electrodes. As explained above, when the device is exposed to an increasing RH%, the gap between the electrodes reduce. Tunneling current exponentially depends on the distance through which the electrons have to tunnel through. Therefore, exposure to rising levels of humidity leads to an exponential reduction of junction resistance. The air-gap between the upper and lower electrode can then be considered as a function of RH% and coefficient of hygroscopic expansion of polyimide. The current conduction can be modelled by using the generalized Simmons’ expression for electron tunneling current densityIT = current across the air-gap as a function of applied voltage and relative humidity of the testing chamber, V = Biasing voltage across device, Cg = Relative humidity of the testing chamber, β is a fitting parameter proportional to the coefficient of hygroscopic expansion for polyimide, tp = thickness of polyimide platform, Φa is the mean barrier potential of the air-gap, ds = original air-gap, m = effective mass of tunneling electrons, ħ = reduced Planck’s constant, q = charge of electron and GSO is a conductance fitting factor proportional to the overlap area and having units of S.For intermediate voltages, current flowing through the nanogap junction can be written as:I-V plots of the device after being exposed to varying levels of relative humidity curve fitted with Eq. β to be ~2.78 ppm/RH% and GSO to vary from 5.2 × 10−4 Ω−1 for the I-V curve corresponding to 24 RH% to ~10 Ω−1 for the I-V curve corresponding to 83 RH%. The root-mean-square-error of the curve-fitting plots shown in Fig. Figure 29. Fick’s law of diffusion has been extensively used to describe the diffusion process and to experimentally determine diffusion constant of various substances. The desorption process ideally consists of excess water molecules in the polyimide desorbing back into the atmosphere into water-vapor molecules. This typically is characteristic of a simple first-order desorption process where the absorbed gas-molecule on the surface of the solid simply desorbs back into its gaseous form. The kinetic process is mathematically described by the Polanyi-Wigner equation and is used as the theoretical basis for thermal desorption spectroscopy process30. Figure Since sensor dynamics is heavily dependent on the diffusion of water vapor molecules into the polyimide patch causing expansion of the polymer, the dynamic response of the absorption cycle of the sensor can therefore by mathematically described using a modified version of Fick’s second law2 on a 4-inch Si wafer. Polyimide was then diluted by dissolving uncured HD4104 polyimide (purchased from HD Microsystems) in N-Methyl-2-pyrrolidone (NMP) solvent in the ratio of 1:0.5 (wt/wt). This mixture was then dissolved by using a stirring it with a magnetic stirrer at 300 rpm for 2 hours to ensure perfect mixing of the polyimide and the NMP solvent. This mixture was then spin-coated on the sample at 2000 rpm following the standard procedures for polyimide processing. For curing the polyimide, the sample was kept in a nitrogenized environment oven for three hours at a temperature of 300 °C. This procedure resulted in polyimide thickness of ~1.5 μm. Following this, we sputtered 200 nm of Al on the sample at 50 W and used it as a hard-mask to pattern the underlying polyimide. O2 plasma dry etching for 10 minutes at 100 W was sufficient to remove the unwanted polyimide from the sample. Next, the remnant Al was stripped off by using commercially available aluminum etchant. Following this, we sputtered ~100 nm of Cr at 50 W on the sample and lithographically patterned it to define the lower electrodes. After this, we deposited about 2.5 μm of PECVD α-Si on the sample to form the sacrificial bridging supports for the upper electrode. This was followed by thermal ALD of ~8 nm Al2O3 and its lithographic patterning using BOE. Then, we sputtered ~2.5 nm of α-Si on the sample at 50 W to pattern the sacrificial spacer layer to define the thickness of the eventual nanogap between the electrodes. We next deposited ~1 μm of Al on the sample and patterned it to form the upper electrode. Finally, we sacrificially etched away the α-Si using XeF2. A total of 1200 minutes of etching was required to completely etch away the sacrificial Si and release the upper electrode.The fabrication process started by growing ~1 μm of thermal SiOHigh resolution SEM imaging was done at an accelerating voltage of 15.0 kV by the FEI Nova NanoSEM to visually inspect the fabricated device. I-V characteristics of the device were measured using the Keithley 4200A-SCS Semiconductor Parameter Analyzer. Tunneling current measurements were performed in a dark room, where the device under test was placed on a probe-station kept inside an electrically shielded enclosure to ensure low-noise and high fidelity electrical signals. The enclosure had feedthroughs for providing electrical connections to the probe-station. An inlet from a commercially available humidifier with restricted flow was used to control and maintain the humidity levels in the enclosure. An Arduino powered BME280 chip was placed inside the chamber for serial monitoring of the relative humidity levels.We presented the design, fabrication, electrical characterization and working of a temperature compensated tunneling humidity sensor. Sensor response shows a completely reversible reduction of junction resistance of ~five orders of magnitude with a standby DC power consumption of ~0.4 pW when the device is exposed to an increasing RH% of ~ 20–90 RH%. Passive temperature compensation was also achieved using a non-differential swelling design for the polymer patches. Temperature response showed a reduction of ~2.5 times when the device was exposed to a temperature sweep of 25 °C – 60 °C, which is 0.0025% of the maximum sensor output when exposed to rising levels of humidity. A mathematical model and equivalent electrical circuit was also presented to accurately describe the current conduction mechanism responsible for sensor action. Finally, sensor response dynamics was also investigated and established sorption analytical models were used to describe the time dependent sensor action.