We review graphene gas sensors functionalized by different materials, including metallic nanoparticles, metal oxides, organic molecules, and conducting polymers. The various sensing mechanism of functionalized graphene gas sensors are explained and compared. Meanwhile, some existing challenges that may hinder the sensor mass production are discussed and several related solutions are proposed. Possible opportunities and perspective applications of the graphene NH3 sensors are also presented.Since the first graphene gas sensor has been reported, functionalized graphene gas sensors have already attracted a lot of research interest due to their potential for high sensitivity, great selectivity, and fast detection of various gases. In this paper, we summarize the recent development and progression of functionalized graphene sensors for ammonia (NH Most of NH3 in our life environment is emitted directly or indirectly by agricultural sector, waste handling, road transportation, industrial applications, and human activities. NH3 plays a decisive role in particulate matter (PM) formation in polluted environments [3 is caustic and hazardous to human health. Specifically, it injures the human eyes, skin, respiratory tract, liver, and kidneys beyond a concentration of 25 parts per million (ppm) [3 sensing and monitoring are essential for industrial emission control, environment conservation, and human safety. Recently, NH3 sensors have found new potential in medical applications, because NH3 is a biomarker for renal and ulcer diseases [3 sensors can be used to diagnose these diseases through detecting NH3 concentration from human breath. For a healthy person, the mean concentration of NH3 in the breath exhalation is about 0.83 ppm [3 concentration in the range from 0.82 to 14.7 ppm  [3 sensors have been investigated in the literature [3 intelligent sensors, which can be integrated with complementary metal-oxide semiconductor (CMOS) circuits and used in the context of the Internet of Things (IoT). Metal oxides, such as SnO2, V2O2, WO3, ZnO, In2O3, TiO2, and Cu2O, are very popular compounds for NH3 sensing [3 sensing are based on conducting polymers such as polypyrrole or polyaniline [Ammonia (NHronments . NH3 is diseases . Therefo.88 ppm) . Althougterature , it is s sensing ,11,12. Hyaniline ,16,17. Dyaniline ,19,20. T2 molecule by using pristine graphene in pure helium or nitrogen at atmospheric pressure. Their results also revealed that pristine graphene sensor responds not only to NO2, but also to NH3, CO, and H2O. Subsequently, graphene-based sensors were widely exploited for detecting various types of gases, for example NH3, CO, O2, NO2, H2, CH4, SO2, H2S, and volatile organic compounds (VOCs) [2 and NH3. The detection limit reached 100 and 500 parts-per-billion (ppb) for NO2 and NH3, respectively. These values are markedly superior to commercially available sensors. However, the full recovery of pristine graphene sensors requires them to be heated at 200 °C in inert gases. Dan et al. [3 at room temperature. The first-principles calculation also predicts that pristine graphene is chemically inert [3 molecules in ambient condition. As a result, interest in the functionalization to generate specific links between pristine graphene and NH3 molecules is highlighted. Generally, graphene functionalization can be accomplished by metallic nanoparticles, metal oxides, organic molecules, or conducting polymers. This article does not review all the functionalization methods presented in the literature but mainly focuses on the graphene functionalized by ultrathin conducting polymers for NH3 sensing. Since the discovery of graphene (G) in 2004, it has been researched and developed as a gas-sensing material because of its extremely large surface-to-volume ratio and high carrier mobility. One adsorbed molecule can cause a notable change in the graphene resistance, allowing the sensing ability to single molecule level ,22. In 2s (VOCs) ,25,26,27s (VOCs)  used chen et al.  found ouly inert . These f3 detection, including mass sensitive, Schottky/semiconductor diode, chemiresistor, and field-effect transistor (FET) sensors. In 3 sensors are first briefly described, followed by the presentation of their main properties and their related sensing mechanisms. The graphene sensors functionalized by ultrathin polymers are presented in detail. In 3 sensors are presented, which may provide future research directions. The article is arranged as follows. In Graphene is one of the most promising materials for gas-sensor applications. In this section, we present the distinctive band structure and outstanding physical properties of graphene. Then, we briefly review the graphene preparation methods.sp2-bonded carbon atoms. Electrons are delocalized and free to move in this 2D sheet. The energy band structure of graphene is illustrated in p-type doping [p-type or n-type semiconductor.Graphene is a two-dimensional (2D) material with a monolayer of e doping . The zere doping . HoweverDue to this structure, graphene is very sensitive to Fermi level changes. Its conductivity can be easily modified by doping or a back gate voltage when the graphene sheet is fabricated as a field effect transistor sensor .2/g [2/s.V) [Single-layer graphene has the largest surface-to-volume ratio of 2630 m2/g  and expo [2/s.V)  and exce [2/s.V)  of graph [2/s.V) . For gra2/Si wafer) is dipped in the acetone and then washed with plenty of water and propanol, which removes mostly thick flakes. In this case, some thin flakes are captured on the target substrate’s surface due to van der Waals and/or capillary forces. Although mechanical exfoliation can provide high-quality single-layer graphene, it is not a scalable technique, and other techniques were proposed.Historical graphene was first prepared by mechanical exfoliation of graphite ,40. For 4) on catalytic metal substrates at high temperature (about 1000 °C) in a controlled atmosphere or atmospheric pressure. Cu, Ni, Ti, Ru, Pd, Pt, Ir, Co, Au, and Rh can be used as the catalytic metal substrates [Chemical vapor deposition (CVD) holds great promises for large-scale production of high-quality graphene ,42. Concbstrates . Among tGraphene has also been grown on the surface of single-crystal silicon carbide (SiC) by thermal decomposition . Namely,4 and NaNO3 in concentrated H2SO4. This method has broadly been used in gas-sensor fabrication because of its high efficiency. However, it still has a few drawbacks. For instance, the oxidation procedure releases toxic gasses and the control of the graphene oxide sheets is difficult. Recently, several modified methods were proposed in the literature to improve the control of the GO and rGO synthesis [For large-scale production of low-cost graphene, one of the most popular approaches is the use of strong oxidizing agents to obtain GO and then convert GO to reduced graphene oxide (rGO) by thermal treatment or chemical reduction. In 1958, Hummers  reportedynthesis ,47. It iynthesis ,49.3. Graphene and rGO can be fabricated by various methods. CVD can provide large-scale production of high-quality graphene. However, high power consumption and waste catalytic metals limit its production cost. The rGO is considered as a more practical method due to its large-scale and low-cost production.In brief, the outstanding properties of graphene make it suitable for developing highly sensitive sensors for NH3 detection. It includes mass sensitive sensors, Schottky/heterojunction diode sensors, chemiresistor sensors, and field-effect transistor (FET) sensors.In this section, we briefly review the working principle and operation mode of various functionalized graphene sensors for NH3 molecules adsorb on the functionalized graphene surface over the quartz crystal surface, the resonant frequency of the resonator varies proportionally with the mass of NH3 adsorbed on the sensing layer. The change in the resonance frequency is used to measure the mass of adsorbed NH3 by using the Sauerbrey equation [3, respectively. A is the surface area of the electrode. According to the chemical equilibrium, one can obtain the 3 adsorption and increases the resonance frequency change, thereby improving the sensitivity of the QCM.Mass sensitive sensor is a transducing device with a sensitive layer. After the sensitive layer adsorbing the target gas, the device gives a signal related to the mass uptake of this layer. The sensors are divided in quartz crystal microbalance (QCM) sensors  and surfequation :(1)Δf=−23 detection. This is attributed to the abundant oxygen-containing groups on the surface of rGO film, which strongly adsorb NH3 molecules and increase the film stiffness.The surface acoustic wave (SAW) sensor is more common than the QCM sensor. Its working principle is based on the elastic loading effect. ψSBH) can lead to a huge difference in the diode current–voltage (I–V) characteristics. n-type silicon interface for the pristine graphene state (middle), the graphene electrode exposed to electron-donor gas (left), and the graphene electrode exposed to electron-acceptor gas (right) [3 molecule is strongly nucleophilic. Once it is attached to graphene, it donates electrons to the latter via coupling π bonds. One NH3 molecule can donate 0.03 electrons onto graphene [3 can be described as follows: when the graphene electrode surface is exposed to NH3, the electron transfer from NH3 to graphene causes a shift of the graphene Fermi level toward the Si conduction band, lowering the ψSBH (as shown in I) across the graphene/n-type Si interface is determined by the ψSBH through formula [V is the applied voltage, A is the area of the Schottky junction, A** is the Richardson constant, T is temperature, q is electric charge, k is the Planck constant, ψSBH is the Schottky barrier height, n is the ideality factor. For the pristine graphene/n-type silicon, n = 1.41 and ψSBH = 0.79 eV. For the pristine graphene/p-type silicon, n = 1.31 and ψSBH = 0.74 B eV [Graphene  and semiconductor Schottky diode sensor is a promising candidate for quickly sensing low concentration gas. When the Schottky diode is exposed to the target gas, a small change in the graphene electrode composition and Schottky barrier height ( (right) . NH3 molgraphene ,57. The  formula :(2)I= AA.74 B eV .3 directly impacts the current of the Schottky diode. More specifically, a small change in the ψSBH caused by NH3 adsorption on the graphene electrode can exponentially change the current of the Schottky diode through Equation (2). As a consequence, the current change can be used to identify various adsorbents and determine their concentration. I–V curves of a graphene/n-type Si Schottky diode sensor before and after exposing to electron-donor gas and electron-acceptor gas.The chemical or physical modification of the graphene electrode by NHn-Si Schottky diode for sensing NH3. Their results indicated that 200 ppm NH3 caused the ψSBH change of 0.1 meV. In order to engender selectivity of the graphene/Si Schottky diode sensor, functionalizing graphene is necessary. Biswas et al. [3 in ambient air. They obtained a detection limit of ppb level with the response and recovery times of 95 and 101 s, respectively.Polichetti et al.  reporteds et al.  used rGO2) layer on top of Si wafer. The SiO2 thickness is 90 or 300 nm for easily observing graphene with an optical microscope [Chemiresistor sensor is popular for the successful detection of a wide range of gases. Chemiresistor sensor is easy to fabricate, has relatively low cost, and enables direct measurement. The typical structure of the graphene chemiresistor sensor is shown in croscope . The usep-type semiconductor containing many holes [3) are adsorbed on the graphene surface, they donate the weakly bonded electrons to graphene, causing the Fermi level to rise by small increments. In this case, the donor electrons deplete the holes in the p-type graphene, resulting in the resistance increase in graphene. Conversely, the electron-acceptor gas molecules (such as NO2) are adsorbed on the graphene surface, they take the electrons from graphene, lowering the Fermi level. The extra holes increase the holes concentration in the p-type graphene, leading to the resistance decrease in graphene. Therefore, the gas concentration can be quantitatively analyzed by directly measuring the change in the sensor resistance under the interaction with the target gas [n-type graphene can be described by only changing sign referred to in In ambient condition, graphene behaves like a ny holes , as showrget gas . The casThe electron-donating or electron-accepting nature of the gas is related to the relative position between the graphene Fermi level and the electron energy level of the highest occupied molecular orbital (HOMO) or the electron energy level of the lowest unoccupied molecular orbital (LUMO).Many research groups used graphene to fabricate field-effect transistors (FET) . Howeverp-type and n-type, for which the current flow is due to electron and hole transport, respectively. Hereafter, we will limit our analysis and discussion to the p-type sensing layer, since the principle is the same for n-type sensing layer but changing sign. When the target gas is off, the current into the sensing layer is only controlled by the back gate voltage (VG). As shown in p-type sensing layer is exposed to the electron-donor gas under the negative gate voltage, three possibilities exist: (i) the channel holes induced by negative gate voltage are equal to the donated electrons by the target gas, the sensing layer current keeps unchanged (keeps on baseline); (ii) the induced channel holes are more than the donated electrons, the sensing layer current decreases; (iii) the induced channel holes are less than the donated electrons, the sensing layer current increases. In the case of the p-type graphene FET sensor with electron-donor gas, it is better to use high positive gate voltage for the measurement. Such choice ensures that the graphene FET sensor would still be with electron conduction under electron-donor exposure. Otherwise, the case will be very complicated. It was reported [3, the p-type graphene FET sensor exhibits superior performance operated at high positive gate voltage rather than at negative gate voltage. On the contrary, for the electron-acceptor gas, it is better to use n-type graphene and high negative gate voltage.The detection mechanism of the graphene FET sensor to gas depends on the synergistic effect: the current of the sensing layer is modified (i) by the field strength through the gate voltage and (ii) the electron transfer with the gas molecules. reported  that forID) of the graphene FET sensor is expressed by [µ is the carrier mobility, W and L are the width and length of the sensing layer, respectively, COX is the unity gate capacitance, VD and VG are the drain voltage and the gate voltage, respectively. When the sensing layer is exposed to the target gas and the modification of the graphene surface is taken into account in the above relationship, the following equation is obtained [Qm (C.cm−2) is the interface charges induced by the gas molecules. Importantly, Qm is not zero when electrons transfer between the gas molecules and graphene channel. This is the reason why the graphene FET sensor can detect the gas concentration when it reacts with an electron-donor or electron-acceptor gas. From Equation (4), we can clearly see the synergistic effect: for a given sensing layer and drain voltage (VD), ID depends on not only the gate voltage (VG), but also the adsorbed gas molecules (Qm).When the target gas is off, the simplified drain current  is applied to the gate of the graphene FET sensor in order to simultaneously excite the perturbation of target molecular dipoles and modulate the channel charge carriers. As discussed above, graphene-based materials are suitable for various mode NH3 detection, because it does not possess any functional groups or defects. Monocrystalline graphene of mechanical exfoliation is well known as pristine graphene. However, it includes few isolated point defects. When NH3 molecules are attached on the isolated point defects, the resistance of the pristine graphene sensor does not significantly change. Since the donor electrons of NH3 can transfer through low resistance pathways around the isolated point defects [3 sensor. Thanks to the abundant defects  and functional groups on it, they strongly adsorb NH3 molecules. Around such defects, no low-resistance pathways exist. So, the resistance change caused by NH3 adsorption is significant, leading to a higher sensitivity.Pristine graphene without bandgap behaves like a semimetal . It is n defects . On the p-type graphene, the adsorption of a reducing gas (either NH3 or H2S) donates electrons to the graphene and depletes the concentration of holes, increasing the graphene resistance. The adsorption of an oxidizing gas (either NO2 or SO2) accepts electrons from the graphene and enhances the concentration of holes, decreasing the graphene resistance. As a consequence, different gases may generate the same sensing signals [3, but also CO, H2O, and NO2 in the same conditions as shown in It is mentioned above that non-functionalized graphene is nonselective to gases . A varie signals . Indeed,sp2 into sp3, disrupting graphene electronic and mechanical properties. Whereas, the latter provides functional groups to the graphene surface by electrostatic or π–π interactions, enhancing the graphene bonding ability and simultaneously preserving the graphene original properties (the high carrier mobility and favorable noise characteristics). It is, thus, important to optimize and improve non-covalent functionalization techniques.The functionalization (generating specific links and increasing the adsorbing sites) is a viable method to improve chemical inert and poor selectivity of graphene. The graphene functionalization can be classified to covalent and non-covalent strategies . The for3 sensing. According to the interaction nature between the target molecule and the receptor attached to graphene (or rGO), sensing reactions are distinguished to physisorption and chemisorption. The physisorption depends on the van der Waals forces of attraction with low binding energy, leading to full and fast recovery, but low sensitivity and poor selectivity. The chemisorption depends on the formation of chemical bonds between NH3 molecules and the sensing layers, resulting in good selectivity, but slow and incomplete recovery. For certain sensing layers, physisorption and chemisorption can happen at the same time. Both adsorptions can cause charge transfer, yielding p-doping or n-doping of the graphene film.Over the past few years, a number of functionalization procedures have been developed. Various materials, from metallic nanoparticles and metal oxides to organic molecules and conducting polymers, are used to accomplish the graphene functionalization for promoting a high sensitivity and great selectivity to NH2) [2 sensors with high performance [One of the strategies is to modify graphene by decorating its surface with metallic nanoparticles (NPs). They were widely used for detecting hydrogen (H2) ,79,80. Rformance .3 at room temperature. While the rGO sensor without Ag nanoparticles only shown a response of 0.2%. Karaduman et al. [3 sensors based on rGO decorated by metallic nanoparticles  by using a single-step chemical reduction. They obtained the highest sensitivity of 6.52%/ppm with Ag-NPs/rGO sensor. This sensor exhibited fast response/recovery of 5 s and great selectivity compared to CO, CO2, and H2 at room temperature. Gautam et al. [3 detection. The other functionalization methods of metallic nanoparticles can be found in reference [Cui et al.  reportedn et al.  fabricatm et al.  and Songm et al.  also preeference .3 sensing mechanism. When the Pt nanoparticles are weakly bonded with graphene by van der Waals force, the electronic structure of graphene is still preserved [p-type graphene is 5.1 [2/Si substrate behaves like a p-type semiconductor. After the Pt nanoparticle decoration, the graphene resistance is decreased, since the work function of Pt is larger than that of the p-type graphene. In the other words, the Fermi level of Pt is lower than that of the p-type graphene. This is beneficial towards the electron transfer from the p-type graphene to Pt nanoparticles, enhancing the hole density in the p-type graphene. When the Pt-decorated graphene sensor is exposed to NH3 at room temperature, NH3 molecules are dissociated at the surface of the Pt nanoparticles to form the Pt-NH3 phase, which exhibits a work function smaller than the Pt one, as shown in 3 phase to the graphene, reducing the hole density in the p-type graphene.Now, we take the Pt-decorated graphene sensor as an example to explain the NHreserved . As showe is 5.1  and 4.7 e is 5.1 , respect3 chemisorption, improving the sensor selectivity. The Pt-decorated graphene surface can be considered as a graphene film with p-type doping. It is equivalent to the hole reservoir and pathway to accelerate the sensor response and recovery. The issue for the functionalization by noble metal NPs, including Pt, Au, Ag, Pd, Ti, and so on [In fact, the Pt nanoparticles act as catalysts to concentrate the reactants by adsorption to increase the probability of interaction, thereby enhancing the sensor sensitivity. Moreover, the Pt nanoparticles may also provide number of specific reaction sites for NHnd so on ,91,92, iIn general, metal-oxide-sensing materials have relatively poor selectivity and need a high operating temperature. To overcome these shortages, heating or illumination is required. This results in a complex fabrication process, high power consumption, high cost, and the size increase in the overall device. It is interesting that the hybrid of metal oxides with graphene can significantly improve the sensing performance, particularly, the selectivity and response/recovery times at room temperature ,94.2 sensor presented a negligible response even for 100 ppm NH3 [2 hybrid sensor yielded a response of 21% for 50 ppm NH3, with response/recovery times of 15/30 s and without the interference of VOCs [2 solution at 70 °C to form C-O-Sn- bonds. When the hybrid sensor was exposed to NH3, the donor electrons of NH3 transferred from Sn to C of graphene through O. High sensitivity of the hybrid sensor is attributed to high electrical conductivity of graphene. Sn is a transition element, and it can easily accept the electron into its “d” orbital. Therefore, the rapid response was obtained in the hybrid sensor during exposure to NH3, while the rapid recovery is due to the weak bonding between NH3 and Sn. Kodu et al. [3 sensors functionalized by V2O5. The response of the epitaxial graphene sensor was one order of magnitude higher than the one of the CVD-G sensor. The authors believed that the adsorption sites provided by V2O5 nanophase were similar for both, but the response difference was due to the larger initial free charge carrier in the CVD-G sensor.At room temperature, a pure SnO ppm NH3 . However of VOCs . The hybu et al.  compared3 at room temperature. Meanwhile, our previous review raised the improved experimental schemes and the future research directions. The functionalization methods of metal oxides are also described in the review paper [More work on graphene mixed by metal oxides can be found in our previous work , which sew paper .3. The combination included a lot of functional groups for selectively binding NH3 molecules. Therefore, the Rose Bengal/rGO sensor exhibited an enhanced response to NH3 compared with the rGO sensor without Rose Bengal. The detection limit of the Rose Bengal/rGO sensor was 0.9 ppm.One of the advantages of graphene is its compatibility with organic molecules . Graphen3 in ambient condition. Their sensor has dual-mode  response to NH3 and high stability under mechanical bending. Their results reveal that BPB aromatic molecules can be anchored and concentrated at the edges and defects of the rGO layer. Interactions between the sensing layer and NH3 molecules can be increased on rough surfaces. This is why the BPB/rGO sensor has an increased sensitivity. On the other hand, the BPB/rGO sensor is similar to pH papers, which can qualitatively measure the presence of NH3. Specifically, when the sensor was exposed to NH3, the color change was clearly observed with naked eyes.Duy et al.  demonstr3 sensors [3 compared to non-functionalized graphene [3 (~2.5% and 45 s for 100 ppb NH3) [3  [Besides, porphyrins and phthalocyanines with different coordinated metals were used to functionalize graphene for improving the performance of the NH sensors . For exagraphene . The Co-ppb NH3) . In addi and CO) .However, most of dye molecules are toxic. In the case of high concentration, Rose Bengal is toxic on the human corneal epithelium , and broThe selectivity of gas sensors can be achieved with conducting polymer/graphene composites at room temperature. The synergistic effect of the conducting polymer/graphene is highly significant towards enhancing the sensor performance. Conducting polymers include polypyrrole (PPy), polyaniline (PANI), polythiophene (PTh), poly (butyl acrylate) (PBuA), and poly (vinylidene fluoride) (PVDF) . Hereaft3 sensor with PPy as sensing material goes back to 1983 by Nylander et al. [3 [2 [2S [3, the high contact resist (Schottky contact) between PANI and electrode materials was an issue. General problems in the use of conducting polymers for gas detection are: (i) sensing response and recovery are usually slow [In fact, pure conducting polymer gas sensors have attracted great interest due to the high sensitivity, repeatable response, and low-cost production . The firr et al. . To datet al. [3 , they cal. [3 [2 , H2S [123 [2 [2S , amines 3 [2 [2S , polar v3 [2 [2S , and so lly slow , (ii) thlly slow .3 sensor was built in 2012 based on PANI/graphene hybrids with a response of 59.2% for 50 ppm NH3 [In 2009, Dan et al.  first fo ppm NH3 .The easily tunable electrical properties of PPy make it a very popular conducting polymer ,128. PPy3 detection by in situ oxidative polymerization. The sensor exhibited a response three times larger than the pure PPy thin film sensor. Wang et al. [3 is nearly consistent with Tiwari’s group research result. Hu et al. [Most of PPy/graphene sensors used the chemical synthesis PPy. Tiwari et al.  preparedg et al.  developeu et al.  reported3 sensors based on the composites of CVD-G or rGO with conducting polymers, operating at room temperature. As we can see from the According to the literature survey, we summarized in the 3 is presented at room temperature. A repeatable response of 12.6 ± 0.8% is obtained after 10 injections. 3 concentrations from 1 to 4 ppm, with a sensitivity of 6.1%/ppm. The key parameters of two sensors are added in 3, high sensitivity, quick and reversible response, good stability, and immunity to humidity. These performances are associated with the synergistic effect of the ultrathin PPy layer and graphene.In 3 is described as follows. The PPy layer behaves like a p-type semiconductor [3, its resistance increases due to the fact that the donor electrons of NH3 molecules deplete the holes in the p-type PPy. The donor electrons can also transfer to p-type CVD-G film through the ultrathin PPy layer, in certain way thereby increasing the p-type CVD-G film resistance. On the other hand, the p-type CVD-G film is not only the growth support of the PPy layer, but also the conductive pathway of the electron transfer. The high conduction of the CVD-G film accelerates the sensor response and recovery. Additionally, the porous nature of the ultrathin PPy layer makes a major contribution to the sensor selectivity, response speed, and immunity to humidity. The synergistic effect of both materials promotes the sensor performance.The sensing mechanism of the PPy/CVD-G sensor for NHonductor . When it3. Nonetheless, the PPy synthesis or growth is more effective on the rGO film than on the CVD-G film. The explanation is that the hydrophilic oxygen groups (hydroxyl and epoxy groups) in the rGO provide more nucleation sites for the PPy layer during the electropolymerization. The PPy/rGO sensor, thus, has a higher availability of the surface activation for NH3. Moreover, the formation of π–π stacking between rGO and PPy results in a faster transport of electrons [The PPy/rGO sensor and the PPy/CVD-G sensor have the same sensing mechanism when exposed to NHlectrons . These f3 at room temperature. Therefore, the study on the functionalization materials is critical to fabricate high performance graphene sensors for NH3 detection. The synergistic effect of graphene and functional materials significantly improves the sensor performance. As discussed above, there is no unified sensing mechanism for the graphene NH3 sensors functionalized by different materials. However, all of the sensor responses depend on their resistance change caused by electrons transfer between NH3 and the related sensing layers. High conductance of graphene provides an efficient pathway for electrons transfer, thereby accelerating response and recovery of the sensor. Each functional material plays a key role in the sensitivity and selectivity of the sensors, as listed in In summary, non-functionalized graphene is chemically inert and nonselective to NH3 sensors have been studied and developed, great challenges still exist for their practical applications. In this section, we will analyze the hindering factors and propose related solutions.Although a lot of graphene-based NH3. They found out that the number of layers of graphene has no obvious influence on the sensor’s sensitivity. On the contrary, Crowther et al. [3 molecules. Therefore, the multiple-layer graphene has poor sensing performance.Hwang et al.  studied r et al.  indicater et al.  explain r et al.  in a rev3 molecules adsorbed on the cluster top cannot influence electric current passing through the cluster bottom. This means that the resistance modulation is not very effective, degrading the sensor performance.Our research results reveal that the PPy/rGO sensor is superior to the PPy/CVD-G in terms of performance. In essence, the PPy/rGO sensor can be produced more easily and at a cheaper cost. However, the PPy/rGO sensor reproducibility is still a key issue. Unlike CVD-G, the size, thickness, and uniformity of the GO dispersions are not well controlled presently. As shown in 3 sensors does not absolutely need high-quality single-layer graphene, such as the CVD-G. However, for the integration of the NH3 sensors with graphene electron devices in CMOS circuits, CVD-G is a good candidate. rGO is an abundant material and it is very easy to be prepared. However, the sensitivity and response time obviously degrade as the rGO thickness increases. For mass production and commercial application, we have to find appropriate methods to well control the thickness of the rGO for improving the sensor reproducibility. Recently, new approaches for large-scale production of high-quality graphene are being developed and studied, such as industrially viable water-phase exfoliation method [Our results suggest that the unique application of the NHn method  and non-n method .3 molecules and graphene. During functionalization, the specific surface area of pristine graphene should be kept unchanged or even increased for obtaining high sensitivity. Tamersit et al. [3 by a factor of 2.The functionalization is the core of the graphene sensor development. It provides specific links and binding sites. The specific links act as bridges to enhance the temporary connection between NHt et al.  establist et al.  also fou3 sensing, phenyl was used. Designing appropriate structures (such as nanomesh or nanoribbon) can also increase the specific surface area of graphene or the edges of graphene. Based on the CVD-G film, Paul et al. [3 with a limit detection of 160 ppb at room temperature. This value is significantly better than its film counterpart due to more edges of the nanomesh graphene. Three-dimensional (3D) structures may be another pathway to increase the specific surface area of graphene [Usually, the specific surface area of thick GO dispersions is degraded. Teradal et al.  proposedl et al.  construcgraphene ,162.3 sensor, which exhibited fast response (30 s) and high sensitivity (3.8%/ppm) at room temperature. The fluorine atoms interact with the graphene matrix to form covalent C−F bonds, which enhance physisorption of NH3 molecules, thereby improving the sensor performance. Kim et al. [3 sensor based on chemically fluorinated GO flakes. The sensor achieved the theoretical detection limit of ~6 ppb at room temperature. The other trends are to synthesis novel multicomponent materials and hybrid nanostructures for improving sensor performance. Su at al. [3 sensing, such as the ternary nanocomposite film of Pd-NPs/SnO2/rGO. Recently, they reported the results of the ternary film (Pd-NPs/TiO2/rGO) for NH3 sensing [3 at room temperature. For 50 ppm NH3, the response of the PPy/TiO2/graphene nanoparticles sensor was 102.2% [Besides the functionalization methods discussed above, the fluorination or fluorine doping is the other functionalization technology to increase the sensitivity and selectivity of the graphene sensors. Zhang et al.  used a pm et al.  exploiteu at al.  proposed sensing . This ses 102.2% , while ts 102.2% ,126. The2, talc (Mg3Si4O10(OH)2), and hexagonal boron nitride (hBN). They found out that for the NH3 detection the FET sensor on G/hBN-substrate exhibited higher response and faster recovery than the FET sensor on G/SiO2-substrate. The electron transfer from NH3 molecules to the graphene sensing layer is strongly dependent on the distance between the sensing layer and substrate. Aziza et al. [2-3T4O10X2) and SiO2 substrates. The sensitivity of the sensor on G/mica-substrate is higher than that of the sensor on G/SiO2-substrate, because the mica substrate induces more p-doping in graphene. Therefore, the substrate engineering should be put under consideration for the further improvement of the sensor performance, particularly, in the choice of the flexible substrates.The sensor substrate also plays an important role for the interaction of the sensing layer with target gas. Cadore et al.  compareda et al.  compared3 sensors are still in the laboratory. To move them from the laboratory level to commercial and practical applications, the following challenges exist.Up to now, the graphene NH3 sensors first needs large-area few-layer graphene with controlled structural quality. CVD-G is the most appropriate choice. However, high growth temperature (above 1000 °C), transferring from metal foils to target substrates, and catalytic metal foil waste make the CVD process complex, expensive, and have a negative impact on the environment. By contrast, rGO can be produced in large quantities and converted from GO by thermal treatment or chemical reduction. Particularly, rGO can be deposited by simple methods such as drop coating, spin coating, and inkjet printing. Although the rGO synthesis is quite easy and cheap, the rGO properties  are difficult to be well controlled. In addition, high contact resistance between rGO and metal electrodes degrades the sensor sensitivity.Mass production of graphene NH3 sensing applications. A detection limit of 1 ppm and response time of 30 min were observed for a typical sensor at room temperature. However, the characterization of the batch sensors has not been reported yet. Our PPy/G and PPy/rGO sensors were fabricated on 3-inch wafers. However, the electropolymerization was a post process step, which was carried out one sensor by one sensor at a time. To move the present process from laboratory concept to industrial application, it is necessary to functionalize all the sensors on one wafer once. In reality, the technique to produce the graphene NH3 sensors with a high yield was not been found due to reproducibility issues. In addition, for the feasibility of a sensing system, uniformity across the batch is a more relevant parameter than the sensor itself [Deokar et al.  synthesir itself ,171. FurFrom what has been discussed above, although CVD-G presents high quality, it requires high growth temperature and catalytic metal foils. The rGO is an easily synthesized, cheap, and defective graphene. However, the properties of the GO dispersions should be well controlled. The suspended graphene and designed structures can increase the specific surface area or the edges of graphene. New and innovative functionalization technologies, such as fluorination and graphene-based ternary composites, are other ways for improving sensing properties. In addition, the substrate choice is also important for the further improvement of the sensor performance. For mass production and practical applications, the sensor reproducibility, uniformity, stability, and immunity to humidity should be further exploited and investigated.3 sensors can be assembled with commercially available electronic devices and modules, as well as previously fabricated chips to build a prototype. With application software, the prototype would wirelessly transfer the measurement data to a mobile phone or computer. Thus, one could continuously and remotely monitor the NH3 concentration in a real-life environment.Graphene NHOur research indicates that the combination of graphene and conducting polymers is very promising as a chemical sensing material. As long as the electropolymerization could be carried out on wafer scale, it could be possible to integrate the conducting polymer/graphene sensors with CMOS circuits onto the same chip for monitoring different gases. The electronic nose consists of a gas-sensor array to detect various gases through multiple sensors. Graphene could be used as the sensing layers in the array. Then, the sensing layer of each sensor is functionalized by different materials or methods according to the requirements of target gases. This could be of great interest for fabricating the electronic nose fully integrated with CMOS circuits for accurately detecting different gases and biochemical and chemical molecules. It also could be of great interest for fabricating lab-on-chip in the field of environmental monitoring. Due to the current boom of IoT and connected devices, there is a growing demand for low-power devices. Graphene-based gas sensors with good performance are more advantageous in this topic than gas sensors based on heated metal oxides in the market, since the former requires less energy than the latter.3) sensing due to its large specific surface area and high carrier mobility. However, the non-functionalized graphene is nonselective and weakly binding with NH3 molecules at room temperature. Fortunately, it can be easily modified by functional groups. Functionalized graphene will occupy an important position in the future of gas-sensing materials. This paper summarizes the graphene sensors functionalized by different materials for NH3 detection, including metallic nanoparticles, metal oxides, organic molecules, and conducting polymers. Among them, the graphene functionalized by conducting polymers has a long lifetime, low cost, operation at room temperature, and compatibility with CMOS technology. Recently, we functionalized graphene and reduced graphene oxide by ultrathin polypyrrole (PPy) through electropolymerization. The synergistic effect of graphene and the ultrathin PPy significantly improves the sensor performance. Graphene increases the conductivity of the polymer film, accelerating the sensor response and recovery. On the other hand, the polymer film provides the specific reaction sites for NH3 adsorption, defining the sensor selectivity and enhancing the sensor sensibility. Overall, the sensor performance is strongly influenced by the charge transfer mechanisms, the functional molecules, and graphene itself. The functionalized graphene NH3 sensors will have a bright perspective due to the advantages of high sensitivity, fast response, great selectivity, low power consumption, cost-effectiveness, and operation in real-life condition. They are expected to be applied in the fields of lab-on-chip, electronic nose, Internet of Things for industrial control, agricultural storage, environmental monitoring, disease diagnosis, wearable health, ecological protection, and public safety.Graphene is a potential material for ammonia (NH


Supercell atomic models of composite films on the basis of graphene and single-wall carbon nanotubes (SWCNTs) with an irregular arrangement of SWCNTs were built. It is revealed that composite films of this type have a semiconducting type of conductivity and are characterized by the presence of an energy gap of 0.43–0.73 eV. It was found that the absorption spectrum of composite films contained specific peaks in a wide range of visible and infrared (IR) wavelengths. On the basis of calculated composite films volt-ampere characteristics (VAC), the dependence of the current flowing through the films on the distance between the nanotubes was identified. For the investigated composites, spectral dependences of the photocurrent were calculated. It was shown that depending on the distance between nanotubes, the maximum photocurrent might shift from the IR to the optical range. An impoficiency .2-hybridized carbon nanostructures and the availability of experimental technologies for the chemical and structural modification of graphene and SWCNTs, it is predicted that an all-carbon graphene/SWCNT hybrid material may become a promising candidate for the creation of phototransistors with tunable bandwidth. In particular, a high-performance photodetector based on graphene/SWCNT van der Waals heterostructures obtained by transferring the CVD-grown graphene onto an ultrathin layer of SWNTs formed on a SiO2/Si substrate has already been implemented in practice. The created device exhibited a photoconductive gain of 105 and a fast response time (~100 μs), which indicates the potential use of the graphene/SWCNT composite for light harvesting applications [Due to the existing electronic coupling between two spications . Liu et ications . The comications . The synications . Based oications . A graphications . This deNevertheless, studies of the photoelectric properties of graphene/SWCNT composite films with covalent bonding of carbon components are currently fragmentary and need further deepening. In this paper, using in silico methods, we predict the electroconductive and photovoltaic properties of composite films based on two graphene monolayers and SWCNTs located irregularly relative to each other between the layers and covalently bonded to them.x and Ly, in the direction of the X (perpendicular to the SWCNT) and Y  axes were introduced, respectively. Optimization of the extended supercell structure geometry in this case was performed by the quantum method. Herewith, the lengths of the translation vectors Lx and Ly, as well as coordinates of atoms, were optimized. Due to the polyatomic nature of the resulting supercells (400–800 atoms), we applied the self-consistent charge density functional tight-binding (SCC DFTB) method [To find supercells of graphene/SWCNTs films with irregular arrangement of nanotubes between graphene monolayers we used the original “magnifying glass method” that was earlier successfully applied for search of supercells of graphene/SWCNTs films with regular arrangement of nanotube graphene monolayers . The “ma) method , which, c—energy of composite graphene/SWCNT, Egr—total energy of graphene monolayers in a composite, Etube—total energy of nanotubes in a composite, and Natom—number of the composite’s atoms. If the value of ∆E was negative, such a topological configuration of the supercell was considered energetically favorable.Using the “magnifying glass method”, we built topological models of supercell of graphene/SWCNT composite films with tubes , where m = 10, 12, 14, 16. Non-chiral zigzag-type SWCNTs were selected since it was previously shown that only tubes of this topological type can form energy-stable covalent bonds with graphene monolayers . The diax and Ly, film thickness h, changes in total energy ∆E, Fermi level Ef, and energy gap Egap of the band structure. According to the data in T(E)—transmission function that determines the total quantum mechanical transparency of the conducting structure over all independent conduction channels for an electron with energy E; f1 and f2—Fermi–Dirac functions that characterize the energy levels of the source and drain corresponding to the energy level of the conducting structure. The VAC was calculated for the direction of current transport along the SWCNTs, since there is no current in the perpendicular direction. Further, in order to identify patterns of current transfer, the volt-ampere characteristics (VAC) were calculated for all the considered models of graphene/SWCNT composite films. For the calculation of current transfer in the structure, the Landauer–Butticker formula was used :(2)I=eh∫αβ(ω)Z0/2 + σαβ(ω)Z0/2—the reflection coefficient, T(ω) = 2/2 + σαβ(ω)Z0/2—transmission coefficient, Z0—wave resistance, and σαβ(ω)—the optical conductivity tensor. The value Z0 was calculated by the formula Z0 = Ex/Hy = ηcosθ—for waves of S-polarization (the electric field vector E was perpendicular to the plane of the wave incidence) and Z0 = Ex/Hy = η/cosθ—for waves of P-polarization . Coefficient η = 120π Ohm expressed the impedance of the vacuum. Note that sunlight is not polarized, so we calculated the average value of a(ω) for S- and P- polarization waves. The incidence of light waves on the film was considered as normal, i.e., θ = 90°. The complex optical conductivity tensor was calculated by the Kubo–Greenwood formula [To study the photovoltaic properties of composite films, it is necessary to know, first of all, the spectrum of the absorption coefficient a(ω). For this purpose, we applied quasi-classical approach, in which the formula for the absorption coefficient and the formulas for the reflection and transmission coefficients were determined based on the classical theory of electrodynamics. To calculate the dynamic conductivity, the quantum theory of irreversible Kubo processes was used. Previously, authors developed the formula for calculating the reflection coefficient of thin films whose thickness was much less than the wavelength of incident radiation . In our  formula .The calculated absorption spectra for different variations of the distances between the SWCNTs are shown in To explain the appearance of peaks at certain wavelengths, we calculated the absorption spectra of films, which are only irregularly arranged nanotubes, and the spectra of films made of curvilinear graphene. in—the power of incident solar radiation, e—electron charge, and hω is the energy of a quantum of solar radiation. Formula (4) allows one to obtain the maximum photocurrent spectrum, since it does not take into account a partial electron–hole recombination. Thus, Formula (4) is applicable for the case of an internal quantum efficiency of 100%, when each absorbed photon generates an electron. Further, on the basis of calculated spectrum, the integral value of the photocurrent was calculated as an integral over the entire considered frequency range. It should be noted that the power of solar radiation is unevenly distributed over the wavelengths. The peak of the maximum intensity of solar radiation is located in the visible light region; then, the intensity decreases exponentially, reaching a minimum in the infrared region [The spectrum of the photocurrent maximum value and the integral value of the photocurrent were calculated on the basis of absorption spectra. The photocurrent spectrum was calculated by the following formula:d region .−2, respectively  has an absorption peak at the wavelength of ~1500 nm (see The spectra of the photocurrent with the distribution of solar radiation power over wavelengths on the earth’s surface (AM1.5) and outside the atmosphere (AM0) (in inserts)  are illuvely see b. These e −2 see c. With a0 nm see c. Howevesum calculated for the range 0.2–2 µm for all film topological models with different distances between tubes H1/H2. The values of the photocurrent for the visible range of the solar spectrum of 380–780 nm and the integral value of the photocurrent for the IR range of 780–2000 nm are also given.To determine the efficiency of the considered composite films as materials for photovoltaic applications, it is necessary to identify the integral value of the photocurrent. 2 (3.9 mA/cm2), MoSe2 (4.6 mA/cm2) and WS2 (2.3 mA/cm2) [The analysis of the obtained data presented in  mA/cm2) . Based oThus, we have shown that all the topological models of graphene/SWCNT composite films considered in this paper are semiconductors with an energy gap in the range of 0.43–0.73 eV, which is an important condition for the potential use of such films in photovoltaic applications. According to the simulation results, the electric current flowing through the nanotubes of graphene/SWCNT composite films can reach values of ~50–65 µA at a voltage of 2 V. In this case, the current value is affected by the regularity of the nanotube locations relative to each other and the distance between them. This fact indicates the possibility of topological controlling the electrophysical properties of the considered graphene/SWCNT composite films. Thus, for example, the change in the distance between the SWNTs by just one hexagon leads to a change in the current by 8–17%. Thus, by regulating the distance between the SWCNTs, one can change the behavior of the current–voltage characteristics and the current value. Controlling the nanotube diameter allows us to control the profile of the absorption spectrum of graphene/SWCNT composite films by shifting the maximum peaks. Controlling the distance between the SWCNTs and the regularity of their location allows us to control the absorption intensity without significantly shift of the peaks. It was found that the integral photocurrent for all the considered graphene/SWCNT composite films with an irregular arrangement of SWCNTs is practically independent of the nanotube diameter and the distance between them, which is important in terms of the potential practical application of such films, since it is practically impossible to achieve the same CNT diameters and the distance between them in a real device.


To access the properties of theoretical graphene, it is crucial to manufacture layers with a defect-free structure. The imperfections of the structure are the cause of deterioration in both electrical and mechanical properties. Among the most commonly occurring crystalline defects, there are grain boundaries and overlapping zones. Hence, perfect graphene shall be monocrystalline, which is difficult and expensive to obtain. An alternative to monocrystalline structure is a quasi-monocrystalline graphene with low angle-type boundaries without the local overlapping of neighboring flakes. The purpose of this work was to identify factors that directly affect the structure of graphene grown on a surface of a liquid metal. In the article the growth of graphene on a liquid copper is presented. Nucleating graphene flakes are able to move with three degrees of freedom creating low-angle type boundaries when they attach to one another. The structure of graphene grown with the use of this method is almost free of overlapping zones. In addition, the article presents the influence of impurities on the amount of crystallization nuclei formed, and thus the possibility to order the structure, creating a quasi-monocrystalline layer. HighlightsMetallurgical graphene is grown on a liquid metal surfaceGraphene grain boundaries obtained in the research are of low-angle typeThe lower the nucleation rate, the more ordered the quasi-monocrystalline structureA large number of impurities prevents the formation of an ordered  structureImpurities initiate heterogeneous nucleation of graphene nucleiGraphene is one of the carbon derivatives with sp2 hybridization. In graphene, carbon atoms are arranged in a honeycomb structure, which provides the material with its outstanding properties, such as chemical stability, optical transparency, enormous mechanical strength and electrical characteristics . Graphen2 to even 1 m2 and bigger) is a much safer form of graphene, since it is highly unlikely to be inhaled. Processing large-area graphene, namely transferring it to desired substrates, requires the usage of metal etchants (wet transfer) or electrolyte solutions (hydrogen delamination) [However, as a nanomaterial, especially in the powder form, graphene can cause a serious health hazard through pulmonary, oral or dermal exposure, where inhalation causes the greatest risk ,7,8,9. Wination) , that agFor the commercial implementation of graphene, a scalable, repeatable and efficient production technique must be developed . ProductAnother approach is to slow down the nucleation rate, to achieve a limited number of graphene nuclei. That will allow the formation of larger single crystals before neighboring islands merge ,26,27. IThe graphene synthesis on liquid metal takes advantage of the quasi-atomically smooth surface of the liquid, which prevents the negative influence of defects and grain boundaries present in solids . The graLiquid metals have demonstrated a great potential to synthesize large-area, uniform and almost defect-free graphene sheets. Liquid can play the role of a matrix for carbon deposition in a CVD process, or a solvent for carbon atoms to dissolve in, from which the graphene precipitates out further in the process in the process the graphene precipitates out . Using aSeveral liquid metal substrates have been reported suitable for graphene growth, including copper, gallium and silver ,37,38,39In this report, an investigation of graphene growth on liquid copper via precipitation, and how this growth is affected by solid impurities, is presented. Liquid copper was chosen because the liquid is perfectly smooth at the sub-micrometer level with no surface defects, which is an issue in the case of solid substrates. Graphene grown on a solid metal substrate reproduces the surface defects of the growth substrate. In addition, the liquid surface allows the formed nuclei to move and rotate, which creates the possibility of creating an ordered structure, as demonstrated by the article. Another raised issue was the dependence of graphene nuclei mobility on their number and density. The aim of this research was to obtain a monolayer of graphene with low-angle type boundaries in which properties are closer to the theoretical ones, as it was described by Kula et al. .Graphene was grown on a liquid copper surface inside a molybdenum crucible. As mentioned before, copper can spread only over several metals, among which nickel it the most suitable. Since it has better wettability than molybdenum, the nickel layer was deposited on the crucible. A nickel layer with a thickness of 0.2 µm was applied on a molybdenum crucible by the Radio Frequency Physical Vapor Deposition (RF PVD) method. Before putting on the nickel coating, the surface of the samples was subjected to argon plasma etching with a pressure of 2 Pa, auto-polarization potential of −800 V and a time of 10 min. Plasma etching was applied to remove any impurities from the crucible surface. Then, the coating was applied using DC pulse magnetron sputtering  with a power of 1.0 kW, a pressure of 0.5 Pa, a potential of −50 V and a time of 30 min. The dimensions of the crucible were 53 mm in diameter and 5 mm in height. Then, a copper disc  with a 52 mm diameter and a 1.0 mm thickness was put into the crucible. The copper purity was 99.9995%.The graphene synthesis was conducted in a vacuum furnace . After inserting the samples, the furnace was pumped to a pressure of 10 Pa. The next step of the process consisted of heating the substrate to the temperature of 1100 °C under argon with a 3% hydrogen atmosphere. The pressure was maintained at 10 kPa. The growth substrate was kept at these conditions for 10 min. Eventually, the system was cooled down to 1050 °C with a cooling rate of 0.5 °C/min in the same atmosphere. The source of carbon was the mixture of acetylene, ethylene and hydrogen in a mass flow proportion of 2:2:1, respectively. The gas mixture was simultaneously added into the reaction chamber for 10 min with a flow rate of 5 L/min during the initial thermal stage of 1100 °C. The synthesis was conducted according to the patent .A Hitachi scanning electron microscope , working in secondary electron (SE) mode at 5 kV accelerating voltage, was used for the qualitative morphology analysis of the metallurgical graphene layers.Raman spectroscopy  with an excited wavelength of 532 nm was used to characterize the graphene.A graphene layer was separated from the growth substrate using the electrochemical method and transferred into a copper mesh. Quality measurements of the obtained graphene were conducted with a transmission electron microscope (TEM). For this purpose, an HR TEM  was used with an atomic resolution microscope) with a maximum accelerating voltage of 200 kV.The most common structural defects are graphene grain boundaries ,47,48,49By measuring the misorientation angles, we proved that in the case of high-strength metallurgical graphene, there are low-angle grain boundaries at the misorientation angle below 10°. This phenomenon was a consequence of the growth mechanism on a liquid phase. Graphene islands appearing on a liquid surface have three degrees of freedom: they are able to move and rotate in the XY plane, which is shown in The process purity is one of the main factors which determines the number of appearing nuclei and their sizes. Each foreign particle on the growth surface acts as an active site, favoring the heterogenous graphene nucleation starting with those particles. It is clearly visible that the particle is in the center of the forming graphene flakes. The same impurity particle can be an active site for another graphene nuclei. Based on the EDS spectrum , the parA transmission electron microscope (TEM) was used for grain boundary examination. Measurement was conducted for graphene grown on a clean surface. This study, as shown in A low-angle boundary is formed on the contact surface of two grains with a misorientation angle smaller than a few degrees. −1), G (1580 cm−1) and 2D (2690 cm−1). Among others, it is possible to evaluate the number of layers and structural defects by the analysis of the peak area, position, intensity and intensity ratios of spectrum peaks. In the spectrum analysis, the relative intensities of individual peak values are important, not the absolute counts’ values. The D peak intensity and ID/IG intensities ratio increases with the increase in the number of graphene layer defects [Additionally, Raman spectroscopy was used for the further investigation of the prepared samples. The analysis of the graphene Raman spectra requires three main peaks to take into consideration: D (1350 cm defects ,61. Figu2D/IG ratio is 1.52, proving the monolayer character of the sample. No additional peaks are visible in the spectra. On the other hand, the spectrum of locally overlapped flakes represented in D/IG ratio equal to 0.37, shows significant defects in the graphene structure. A more defective structure is also supported by the presence of the D′ peak, which results from disorders in the graphene layer and appears around 1620 cm−1 [2D/IG ratio is equal to 1.16, which could even represent a monolayer, but the shape of the 2D peak is more characteristic of a multilayer structure. This can be explained by the local overlapping of the graphene flakes.The Raman spectrum of the sample with aligned graphene flakes shown a, is typGraphene growth on a liquid metal surface was examined and presented in this paper. The study shows that the obtained graphene grains were of low-angle type due to their ability to move on the surface of the substrate leading to a controlled flakes arrangement. The study also indicates that a lower number of graphene nuclei is favorable for the formation of a monolayer without overlapping zones. The fewer the nuclei, the more possibilities for movement and rotation, as well as a higher probability of perfect alignment. The increasing number of nuclei may result in mutual interference between flakes, which will immobilize them and therefore, prevent them from creating a continuous sheet. The clean and smooth surface of liquid allows graphene flakes to rearrange and lean towards each other creating a quasi-monocrystalline structure. However, when the surface of the growth substrate is contaminated with foreign particles, a perfect alignment is impossible due to the heterogeneous nucleation of graphene on those particles. This leads to a higher nucleation rate, the immobilization of occurring graphene flakes and the growth of additional layers. In order to obtain a monolayer graphene, a fundamental issue is to keep the substrate and process purity at the highest level.


After the injection of TDP-43 PFFs, the elevated levels of pTDP-43 were present in several pyramidal tract-associated regions of Atg5+/- mice. Additionally, the occurrence of spontaneous potentials detected by electromyography demonstrates evidence of lower motor neuron dysfunction in M1-TDP-43 PFFs-injected Atg5+/- mice, and prolonged central motor conduction time detected by motor evoked potentials provides evidence of upper motor neuron dysfunction in C7-TDP-43 PFFs-injected Atg5+/- mice. These results show that injection of TDP-43 PFFs into the M1 or C7 of Atg5+/- mice induces the spreading of pathological TDP-43 along corticospinal tract axons in both an anterograde and retrograde manner. Importantly, TDP-43 PFFs-injected Atg5+/- mice also display ALS-like motor dysfunction. Taken together, our findings provide direct evidence that TDP-43 PFFs-injected Atg5+/- mice exhibited ALS-like neuropathology and motor phenotypes, suggesting that autophagy deficiency promotes the formation and spreading of pathological TDP-43 in vivo.Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease, characterized by phosphorylated TDP-43 (pTDP-43)-positive inclusions in neurons and glial cells. However, the pathogenic mechanism that underlies ALS remains largely unknown. To investigate the effects of autophagy deficiency in the formation and spreading of pathological TDP-43 along corticospinal tract axons, TDP-43 preformed fibrils (PFFs) were prepared and unilaterally injected into the fifth layer of the left primary motor cortex (M1) or the left anterior horn of the seventh cervical spinal cord segment (C7) of Atg5 After the injection of TDP-43 PFFs, the elevated levels of pTDP-43 were present in several pyramidal tract-associated regions of Atg5+/- mice. In addition, the occurrence of spontaneous potentials detected by electromyography (EMG) demonstrates evidence of lower motor neuron (LMN) dysfunction in M1-TDP-43 PFFs-injected Atg5+/- mice, and prolonged central motor conduction time (CMCT) detected by motor evoked potentials (MEPs) suggests upper motor neuron (UMN) lesion in C7-TDP-43 PFFs-injected Atg5+/- mice. Moreover, behavioral analysis indicates that both M1- and C7-TDP-43 PFFs-injected Atg5+/- mice developed an ALS-like syndrome in a time-dependent manner.In our present study, TDP-43 preformed fibrils (PFFs) were prepared and unilaterally injected into the fifth layer of the left primary motor cortex (M1) or the left anterior horn of the seventh cervical spinal cord segment (C7) of Atg5+/-) and C57BL/6 mice weighing 18-20 g were used. Animals were kept under standard laboratory conditions with free access to standard laboratory food and water . All experiments were performed in accordance with the Guide for the Care and Use of Laboratory Animals. The protocols were approved by the Institutional Ethics Committees of the Zhengzhou University.Eight-week-old male B6: 129-Atg5<tm1Nmz> (RBRC02231) . Unilateral injections were made into the fifth layer of the left M1 or left anterior horn of C7 at the following coordinates: M1: anteroposterior, 0.7 mm; mediolateral, 1.5 mm; dorsoventral, 1.4 mm; and C7: mediolateral, 0.8 mm; dorsoventral, 1.2 mm. Each injection site received 5 µl of TDP-43 PFFs or PBS at a rate of 0.2 µl/minute using a Hamilton Syringe .One hundred and twenty Atg5Date and cause of death were recorded for each mouse. For ethical reasons, animals were closely monitored and sacrificed as moribund prior to actual death using criteria for severe illness. To determine the duration of survival reliably and humanely, the moribund state, defined as the inability of mice to right themselves 30 seconds after being placed on their side, was used. The moribund mice were scored as “dead” and were euthanized.After euthanasia and perfusion, the brain and spinal cord from perfused animals were fixed in 30% sucrose solution containing 4% paraformaldehyde. Tissues were embedded in paraffin blocks, cut into 4 µm using a Rotary Microtome , and mounted on glass slides. Sections of spinal cord and brain tissues were then deparaffinized and rehydrated in a graded ethanol series. Immunohistochemistry was performed as previously described Frozen spinal cord or brain samples were thawed on ice and sonicated in 5× v/w RIPA buffer  containing protease inhibitor cocktails (Thermo Fisher Scientific). Samples were centrifuged for 30 minutes at 4 °C, 100,000 g. The supernatant was taken as RIPA-soluble fractions. The pellet was sonicated with 5× v/w RIPA buffer as above. Then, the pellet was sonicated in 2× v/w urea buffer  and centrifuged for 30 minutes at 22 °C, 100,000 g. This supernatant was collected as the RIPA-insoluble/urea-soluble fractions. Samples were analyzed by 10% SDS-PAGE with polyvinylidene fluoride (PVDF) membranes. Following transfer to the PVDF membranes, the immunoblot was sequentially probed with antibodies that recognize pTDP-43  or LC3A/B . The densities of the proteins were normalized to GAPDH.°C. A standard EMG apparatus  was applied. To determine UMN and LMN deficits, we collected motor unit action potentials (MUAPs) and spontaneous activities including fibrillation potentials, positive sharp waves and fasciculation potentials. A sweep speed of 10 ms/div, a gain of 100 uv/div and a band pass filter with low and high cut-off frequency of 20 and 10,000 Hz were performed. The mice were electrically grounded through a disposable needle electrode implanted subcutaneously at the abdominal wall. Next, we used concentric needles  to detect the following muscles: bilateral biceps brachii, bilateral T10 paraspinal muscles, bilateral tibialis anterior, and bilateral gastrocnemius. Spontaneous activities were recorded for 120 seconds and MUAPs were recorded at different sites of each muscle until 20 MUAPs were obtained.Mice were anesthetized with isoflurane and placed in the prone position on a thermostatically controlled warming plate to maintain body temperature at 37 Cortical MEP (cMEP) and spinal MEP (sMEP) were obtained using two monopolar needle electrodes. For cMEP, the cathode was positioned at the midline of the interaural line through the scalp; the anode was positioned 4-5 mm lateral and anterior to the cathode. For sMEP, two monopolar needle electrodes were inserted into C7, close to the emergence of the spinal nerve root. cMEP and sMEP were recorded with the active needle electrode inserted into the muscle of the forelimb footpad and reference needle electrode inserted under the skin of the second digit. The onset of the first, generally negative, deflection was taken as the MEP latency. The MEP with the shortest latency was considered as the final evaluation criteria of MEP latency, the peak-to-peak amplitude of the motor response was used for the mean MEP amplitude. The CMCT is the propagation time between motor cortex and spinal cord, which is the difference between the cMEP latency and the sMEP latency.Biopsied biceps brachii were dissected out from the forelimb of an anesthetized animal and immersed immediately in isopentane cooled in liquid nitrogen. Serial frozen sections were cut at 10 μm thickness and stained by hematoxylin and eosin (H&E). All steps were performed according to the standard procedures To evaluate behavioral performance, 15 mice from each group were weighed weekly and underwent a battery of behavioral tests every two weeks starting from the second month post-surgery. Baseline performance was measured before surgery. The experimenter was blinded to the treatment group for all behavioral tests.Rotarod test was performed using the rotarod apparatus . The test began by placing the mice on the rod rotating at 30 rpm. The latency to fall off the rotarod within the maximum time (180 seconds) was recorded as a measurement of the competence of their motor function. Three trials were performed per day with a 15-minute inter-trial interval. The mean latency to fall off the rotarod was calculated for the data analysis.The hanging wire test was performed to evaluate general muscle strength. Each mouse was placed on a horizontally positioned screen with many 1 cm × 1 cm grids. The period until the mouse left the screen was recorded, with a maximum time of 180 seconds. Three trials were performed per day, and the mean latency was recorded for the data analysis.To obtain footprints, paws of the mice were painted with two different non-toxic paints (fore-paws in red and hind-paws in green). The mice were then allowed to walk along a restricted cardboard tunnel  into an enclosed box and a sheet of white paper was placed on the floor of the tunnel, and one set of footprints were collected for each mouse. Three steps from the middle portion of each run were measured for the following parameters (cm): (1) stride length (front and hind legs). (2) The front- and hind-base width. The mean value of each set of values was used for the data analysis.2 value to test for significance. For all statistical tests, significance was taken as *p < 0.05, **p < 0.01, ***p < 0.001.The statistical analyses were performed using SPSS 24.0 . Behavioral and electrophysiological data of mice were presented as mean ± standard deviation (SD), and two-way ANOVA was employed for comparison of multiple groups. Disease onset and survival statistics were performed by Kaplan-Meier survival curves and the data were analyzed using the log-rank test, generating a χFigure ) into the fifth layer of the left M1 or the left anterior horn of C7 of Atg5+/- mice . Diagrams of estimated conduction pathway for pathological TDP-43 spreading were shown in Figure . Firstly, pTDP-43 immunoactivity was detected at the injection sites using an anti-pTDP-43 (phosphorylated at Ser409/Ser410) antibody. As early as 2 months post-injection (mpi), pTDP-43-immunoreactive (pTDP-43-ir) pathology was detected in the neurons and glial cells in the second, third and fifth layers of the M1 around the injection region of M1-TDP-43 PFFs-injected Atg5+/- mice ; however, the staining was not observed on the contralateral side . In the cervical enlargement of C7-TDP-43 PFFs-injected Atg5+/- mice, pTDP-43-ir pathology was found in the neurons of the left spinal cord anterior horn adjacent to the injection sites at 2 mpi . No pTDP-43-ir pathology was detected in the age-matched PBS-injected Atg5+/- mice . Thus, these data indicated that TDP-43 PFFs could induce the formation of pTDP-43 pathology in vivo. These data supported that the injection of TDP-43 PFFs into the brain or spinal cord induce pTDP-43-ir pathology deposits in situ.To investigate whether pathological TDP-43 could spread along corticospinal tract axons, we injected TDP-43 PFFs , medulla oblongata , and cervical enlargement  of M1-TDP-43 PFFs-injected Atg5+/- mice. In the cortex, neuronal and glial cytoplasmic pTDP-43-ir pathology mainly deposited in the fifth layer of the left M1 , while no pTDP-43 pathology was observed on the contralateral side . In the CA1 region of the hippocampus, the positive neuronal and glial cytoplasmic pTDP-43 staining was more intense on the left side  than the contralateral side . In medulla oblongata, the positive neuronal and glial cytoplasmic pTDP-43 pathology was detected in both sides of hypoglossal nucleus  and the pTDP-43-ir nerve fibers in the left corticospinal tract , and the pTDP-43 staining was stronger in the left side of the hypoglossal nucleus , however, the pTDP-43-ir staining was not detected in the contralateral side of the corticospinal tract . Furthermore, neuronal and glial cytoplasmic pTDP-43 was deposited in the right side of anterior horn of the cervical enlargement . However, no pTDP-43 pathology was detected in both sides of M1-PBS-injected Atg5+/- mice .At about 6 mpi, pTDP-43-ir staining was detected in the hippocampus , CA1 region of hippocampus , and the fifth layer of the M1  compared to the left side  at about 5 mpi. The pTDP-43-ir nerve fibers were detected in the left corticospinal tract at about 5 mpi . As with M1-PBS-injected Atg5+/- mice, no pTDP-43 pathology was detected in both sides of C7-PBS-injected Atg5+/- mice . In addition, we also injected TDP-43 PFFs into the C7 of C57BL/6 mice. However, the formation of TDP-43-ir neuronal and glial cytoplasmic pathology in the fifth layer of the M1 was not detected until 18 months . Taken together, these data suggest that the injection of TDP-43 PFFs into motor cortex or ventricornu of Atg5+/- mice induces the spreading of TDP-43 pathology along corticospinal tract axons anterogradely and retrogradely in vivo.In C7-TDP-43 PFFs-injected Atg5+/- mice, the results showed that LC3A/B-II/LC3A/B-I levels were significantly decreased in these regions of adult Atg5+/- mice compared with age-matched C57BL/6 mice . To further demonstrate whether there was an increase of pTDP-43 pathology in TDP-43 PFFs-injected Atg5+/- mice, immunoblotting analysis was also performed in the abovementioned regions of M1-TDP-43 PFFs- and M1-PBS-injected Atg5+/- mice at 6 mpi. In the soluble fractions, no significant difference of pTDP-43 levels was found between M1-TDP-43 PFFs- and M1-PBS-injected Atg5+/- mice . Nevertheless, in the insoluble fractions, pTDP-43 levels were increased in all examined M1-TDP-43 PFFs-injected Atg5+/- mice, while it was faintly detected in age-matched PBS-injected Atg5+/- mice . The same results were also found in C7-TDP-43 PFFs-injected Atg5+/- mice at 5 mpi (data not shown). These results support that the injection of TDP-43 PFFs into M1 or C7 of Atg5+/- mice could induce the spreading of pathological TDP-43 along corticospinal tract axons anterogradely and retrogradely.To evaluate whether autophagy deficiency can affect the aggregation of pathological TDP-43, we first detected LC3 expression levels in M1, hippocampus, medulla oblongata, and cervical enlargement of C57BL/6 and Atg5+/- mice exhibited any neurophysiological abnormality. Spontaneous activity, motor unit potential recruitment, and MUAPs were used to quantify LMN dysfunction. MEPs were used to evaluate potential impairment of the corticospinal tract. Firstly, needle EMGs were recorded from the biceps brachialis, T10 paraspinals, tibialis anterior, and gastrocnemius muscles. In M1-TDP-43 PFFs-injected mice, abnormal spontaneous activity including fibrillation potentials , fasciculation potentials , and positive sharp waves  were detected on the right side of biceps brachialis, T10 paraspinals, tibialis anterior, and gastrocnemius muscles as early as 2 mpi. At 5 mpi, abnormal spontaneous activities were found in both side of biceps brachialis, T10 paraspinals, tibialis anterior, and gastrocnemius muscles, and there was no significantly difference between the two sides . Significantly, the abnormal spontaneous activities were detected in the right biceps brachialis of all the M1-TDP-43 PFFs-injected mice, however, the M1-PBS-injected Atg5+/- mice only showed mild abnormal spontaneous activity in right biceps brachialis muscles at 7 mpi . The frequency of abnormal spontaneous activity was developed in a time-dependent manner as shown in Figure . Secondly, we collected 20 MUAPs in each mouse for the motor unit measurement. The findings showed a significantly increase in the mean amplitude and duration of MUAPs at 2, 5, and 7 mpi in M1-TDP-43 PFFs-injected Atg5+/- mice, compared to age-matched M1-PBS-injected Atg5+/- mice . Furthermore, we performed a biopsy in both sides of the biceps brachialis in M1-TDP-43 PFFs-injected Atg5+/- mice at 7 mpi. H&E staining showed muscle atrophy, round muscle fibers, and muscle fibers with central nuclei in the right biceps brachialis Figure K, while no morphological changes were found in the left biceps brachialis . Taken together, these findings suggest LMN dysfunction in M1-TDP-43 PFFs-injected Atg5+/- mice.Besides the morphological evidence of TDP-43 PFFs-induced pathology, we further investigated whether the M1- and C7-TDP-43 PFFs-injected Atg5+/- mice to quantify UMN impairment . At about 2 mpi, CMCT was significantly prolonged in the injection side of C7-TDP-43 PFFs-injected Atg5+/- mice compared to age-matched C7-PBS-injected Atg5+/- mice , and the CMCT increased in a time-dependent manner  in C7-TDP-43 PFFs-injected Atg5+/- mice. Moreover, cMEP amplitude decreased in C7-TDP-43 PFFs-injected Atg5+/- mice compared to age-matched C7-PBS-injected Atg5+/- mice  at about 2 mpi, and this decline became more significant over time . At the end stage, responses to cortical stimulation were absent in C7-TDP-43 PFFs-injected Atg5+/- mice, indicating devastating damage of the corticospinal tract. Taken together, the quantitative analyses of CMCT and cMEP amplitude indicated UMN dysfunction in C7-TDP-43 PFFs-injected Atg5+/- mice.Furthermore, we performed cMEP and sMEP in C7-TDP-43 PFFs-injected Atg5+/- mice exhibit ALS-like symptoms. First, the lifespan of TDP-43 PFFs-injected Atg5+/- mice was significantly shorter compared with PBS-injected controls, and the Kaplan-Meier curve showed overall survival in Figure . To assess the motor function of TDP-43 PFFs-injected Atg5+/- mice, we performed a series of behavioral tests. At approximately 8 weeks post-injection, C7-TDP-43 PFFs-injected Atg5+/-mice gradually lost weight  and performed poorly on the rotarod test  and hanging wire test , revealing a significant deficit in muscle strength, motor coordination, and balance. In addition, C7-TDP-43 PFFs-injected Atg5+/-mice showed significantly shorter stride length and wider base width compared to PBS-injected controls in the footprint test . As for M1-TDP-43 PFFs-injected Atg5+/-mice, they began to lose weight at around 10 weeks post-injection , and they also showed deficient motor performance in the rotarod test and hanging wire test compared to age-matched PBS-injected Atg5+/-mice . However, the severity of motor disability was much higher in C7-TDP-43 PFFs-injected Atg5+/-mice compared to age-matched C7-TDP-43 PFFs-injected Atg5+/-mice . Moreover, in the footprint test, M1-TDP-43 PFFs-injected Atg5+/-mice also showed significantly shorter stride length and wider base width similar to those found in C7-TDP-43 PFFs-injected Atg5+/-mice . In conclusion, these changes suggest that the M1- and C7-TDP-43 PFFs-injected Atg5+/- mice exhibited ALS-like symptoms, including short survival, weight loss, abnormal gait, and motor dysfunction.We further investigated whether M1- and C7-TDP-43 PFFs-injected Atg5in vivo, we injected TDP-43 PFFs into the cortex or spinal cord anterior horn of Atg5+/- mice. The results show that unilateral injection of TDP-43 PFFs into the fifth layer of the left M1 or the left anterior horn of C7 of Atg5+/- mice could induce the formation and spreading of pathological TDP-43 along the corticospinal tract. Moreover, we also found the presence of TDP-43 pathology in our animal models at CA1 region of hippocampus and hypoglossal nucleus in M1-TDP-43 PFFs-injected Atg5+/- mice at 6 mpi and in C7-TDP-43 PFFs-injected Atg5+/- mice at 5 mpi. This supports the hypothesis that TDP-43 pathology could spread via axonal pathways To investigate whether inoculation of TDP-43 PFFs can transmit pathological TDP-43 along corticospinal tract axons in vivo recapitulates ALS-like pathology. Furthermore, M1-TDP-43 PFFs-injected Atg5+/- mice showed abnormal spontaneous activity in both sides of biceps brachialis, T10 paraspinals, tibialis anterior, and gastrocnemius muscles at around 5 mpi, and C7-TDP-43 PFFs-injected Atg5+/- mice showed prolonged CMCT and decreased cMEP amplitude at around 2 mpi, which recapitulated the electrophysiologic changes found in ALS patients. On the other hand, these results also reflect the anterograde and retrograde spreading of pathological TDP-43.It mimics the TDP-43 pathology in patients with ALS. These data indicate that TDP-43 seeding in vitro evidence has suggested that autophagy induction improves TDP-43 metabolism and clearance and prevents death in cultured neurons and astrocytes expressing mutant TDP-43 Drosophila models for TDP-43 proteinopathy, evidence shows that inducing autophagy effectively diminishes TDP-43 aggregation and finally rescues behavior deficits +/- mice, and pTDP-43 levels were significantly increased in M1-TDP-43 PFFs-injected Atg5+/- mice compared with age-matched PBS-injected Atg5+/- mice. It indicated that autophagy deficiency may promotes the formation and spreading of pathological TDP-43 in vivo. Furthermore, our results showed that TDP-43 PFFs-injected Atg5+/- mice exhibits ALS-like neuropathology and motor phenotypes.It is known that autophagy is a vital degradation mechanism of dysfunctional organelles and large protein aggregates +/- mice to SOD1 G93A transgenic mice in further study, thus making our results more convincing.As we all know, SOD1 G93A transgenic mouse is a commonly used animal model for ALS. Previous studies have found increased autophagy in SOD1 G93A mice in vivo. These data suggest that promoting autophagy and inhibiting cell-to-cell transmission of pathological TDP-43 may have therapeutic potential in the treatment of ALS.In conclusion, we provided the evidence that TDP-43 PFFs could induce the transmission of pathological TDP-43 along corticospinal tract axons in both an anterograde and retrograde manner


These results provide the first quantitative description of the density of defects and transport properties, which plays an important role for potential applications.In recent years, graphene oxide has been considered as a soluble precursor of graphene for electronic applications. However, the performance lags behind that of graphene due to lattice defects. Here, the relation between the density of defects in the range of 0.2 % and 1.5 % and the transport properties is quantitatively studied. Therefore, the related flakes of monolayers of graphene were prepared from oxo‐functionalized graphene (oxo‐G). The morphologic structure of oxo‐G was imaged by atomic force microscopy (AFM) and scanning tunneling microscopy (STM). Field‐effect mobility values were determined to range between 0.3 cm The effect of defects: The room‐temperature mobility values of single‐layer graphene derived from oxo‐graphene with densities of defects between 0.2 % and 1.5 % are presented. The fundamental findings can explain the generally diverging results reported for reduced graphene oxide used in applications. In addition, we investigated the effect of structural defects on the electrical properties of monolayer oxo‐G by two‐probe measurements under ambient conditions. Our results quantify the correspondence relationship between structural defects and transport capacities in high‐defect single‐layer graphene materials.0 %  and graphene with various densities of defects  obtained by wet‐chemical preparation and reduction with hydroiodic acid (HI).0 % is determined to about 0.5 nm with lateral dimensions of 10–20 μm. Contaminants are visible at the edge of the G0 % sheet, which stems from the used tape during the exfoliation and transfer processes. In contrast, the roughness of GD is with 1.0 nm twice as high as for G0 % due to bitopic oxo‐groups at the carbon basal plane and possible adsorbates  was used. Comparing the atomically‐resolved honeycomb structure for defect‐free highly ordered pyrolytic graphite (HOPG) in Figure D reveals distinguishable topographical features with the appearance of islands and rows at bright spots, as depicted in Figure As shown in Figure 0 %, there are two distinct peaks, the G band (at 1570 cm−1), associated with the in‐plane stretching vibration of C−C bonds, and the 2D band (at 2670 cm−1), activated by a double‐resonant Raman scattering.−1 is noticed, probably evolving from grain boundaries or other lattice imperfections.−1) and D′ band (1620 cm−1). In addition, the broadening of all bands is observed with increasing density of defects. The full‐width‐at‐half‐maximum  of the 2D peak (Γ2D) increases from approximately 24 cm−1 in the single‐layer G0 % to about 178 cm−1 in the single‐layer G1.5 %. In Table 0 %, G0.2 %, G0.4 %, G0.5 %, G0.9 % and G1.5 %.The density of defects in single layers of graphene can be quantified by Raman spectroscopy. Figure  band at 570 cm−1,ID/IG is used for determining the density of defects in the GD samples. In the case of single‐layer G0 %, it contains an extremely low density of defects, which belongs to the low‐defect density regime according to the Raman model introduced by Lucchese, Cançado and Ferrari et al., with a ratio of ID/IG=0.2±0.07 corresponding to about 24 000 C atoms within the intact graphene lattice. According to Equation The intensity ratio of NC corresponds to intact carbon atoms and Acell=0.2462×sin (60°)=0.05239 nm2, the average distance between defects LD is about 25 nm. The related defect density (nD) is about 5.1×1010 cm−2, using nD (cm−2)=1014/(πLD2).D samples relate to the high‐defect density regime. In this regime, the ID/IG ratio increases with an increase of LD, based on the relation of ID/IG∝LD∝NC.ID/IG ratio decreases from 4.5 to 1.0 and the LD values gradually decrease from 3.5 nm to 1.3 nm, respectively. The related nD increases from about 2.6×1012 cm−2 to 1.9×1013 cm−2. As depicted in Figure ID/IG ratio versus Γ2D. With increasing the density of defects, the Γ2D values increase, which is consistent with our discussion above.in which the 0 % flakes were mechanically exfoliated from a bulk graphite and transferred to a heavily p‐doped Si substrate with a 300 nm thick SiO2 layer (Si/SiO2),Γ2D value of 23.6±2.6 and I2D/IG>4 was determined by Raman spectroscopy to prove the single‐layer nature of the produced flake , served as source and drain electrodes, were deposited onto single‐layer graphene channel materials by using e‐beam lithography and evaporation processes. Avoiding any thermal decomposition of chemically‐derived GD samples, no annealing process was performed for all prepared devices after the lift‐off process.Field‐effect transistors were fabricated by using the obtained monolayer graphene samples as conducting channels Figure  a. The mElectrical transport measurements were performed at ambient conditions in a two‐terminal configuration. The performance of transistors relies on the properties of channel materials, gate dielectrics, electrodes and test conditions. Therefore, to reliably compare electrical performances for the obtained monolayer graphene samples, all transistors were prepared with parallel electrodes, the same manufacturing processes and test conditions.IDS–VDS) are determined and visualized in the insets of Figure 0 % device in Figure IDS–VBG) with asymmetric Dirac voltage  located at +20 V. The observed p‐doping behavior was probably attributed to the heavily p‐doped Si/SiO2 substrate, impurity doping as a result of exfoliation and transfer processes or polar adsorbates like water or oxygen acting as charge traps between substrate and the graphene surface. Furthermore, a hysteretic behavior between forward and reverse sweeps are observed. For GD transistors  and distance between defects (LD), respectively. It is found that the mobilities follow, within experimental uncertainties, a nonlinear relationship with LD and NC, because the defect‐free area of graphene increases over‐proportionally with increasing LD.in which LD≤3 nm) in oxo‐G were identified by Raman spectroscopy. The isolation of these domains limits the charge transport in reduced oxo‐G. Therefore, the mobility values of charge carriers of graphene with densities of defects between 0.2 % and 1.5 %, change by three orders of magnitude, from 0.3 cm2 V−1 s−1 and 33.2 cm2 V−1s−1. More generally, the mobility of charge carriers varies by orders of magnitude, although it looks like that the density of defects varies only a little. The fundamental findings reported here can explain the generally diverging results often reported for reduced graphene oxide used in applications.In summary, we have studied the room‐temperature electrical properties of single‐layer graphene derived from oxo‐G containing defect densities varying from 0.2 % to 1.5 %. The defects give rise to a heterogeneous topographical morphology of oxo‐G. The isolated graphene domains (−10 mbar). All transport measurements were performed under ambient conditions by a two‐probe station and two source‐measurement units (Keithley 2450).AFM characterization was performed by using a JPK NanoWizard 4 Atomic Force Microscope in tapping mode at room temperature. Raman characterization was carried out with a Horiba Explorer spectrometer with a 532 nm laser for excitation under air conditions. Scanning tunneling microscopy (STM) was conducted by using Omicron‐STM1 microscope under ultra‐high vacuum . Reduction was performed by vapor of hydriodic acid (HI) and trifluoroacetic acid (TFA)  at 80 °C (10 min). Subsequently, the surface of GD was cleaned with doubly distilled water (Carl Roth) to remove iodine species. The density of defects of individual flakes was determined by Raman spectroscopy (Horiba Explorer spectrometer with a 532 nm laser for excitation under air conditions). Subsequently, flakes with defined density of defects were selected for FET device fabrication.The defective G0 % and GD flakes, respectively.Standard electron beam lithography procedure (Raith PIONEER TWO) was used to define and expose the geometry of metal contacts. Subsequently, a 5 nm/70 nm Cr/Au layer was deposited with thermal evaporation (Kurt J. Lesker NANO 36) and lifted off in acetone to make electrode contact to monolayer GThe authors declare no conflict of interest.As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.SupplementaryClick here for additional data file.