The pooled relative risk (RR) value for objective response rate (ORR) was 1.53, . Further, subgroup analysis based on PD-L1 expression indicated that pooled HR for OS was significant with 0.66 for PD-L1≥1%. However, PD-L1 < 1% had HR value of 0.79 . Our study concluded that advanced NSCLC patients benefited more with PD-1/PD-L1 antibodies than docetaxel in the second line treatment. PD-L1≥10% in tumor tissues is sufficient to show significant improvement in patient's outcome with PD-1/PD-L1 antibodies compared to docetaxel. Moreover, PD-1/PD-L1 antibodies treatment showed significant decrease in conventional chemotherapy adverse events, but increased immune-associated adverse effects.Meta-analysis was conducted to systematically assess the effectiveness and safety of programmed cell death protein-1 or ligand-1 (PD-1 or PD-L1) antibodies versus docetaxel alone in advanced non small cell lung cancer (NSCLC). In addition, the prognostic significance of PD-L1 expression in advanced NSCLC was also investigated. 5 eligible studies including 3579 patients were identified through comprehensive search of multiple databases. The results showed that pooled hazard ratios (HR) for overall survival (OS) and progression free survival (PFS) were 0.69 (95% CI: 0.63-0.75;  Lung cancer ranks first in term of causing cancer associated deaths worldwide, with an estimation of 221,200 new cases and 158,040 deaths alone in the year 2015 . Lung caApparently the cost of immunotherapy for advanced NSCLC has been enormous  and is det al. had analyzed two dosage (2 and 10 mg/kg) of pembrolizumab. Furthermore, the Cochrane risk of bias tool was used to measure the quality of the included studies, and the results are shown in Figure Based on the search criteria, we initially identified 1011 studies from our database search. Among these, 858 studies were excluded because they did not fit our selection criteria. Of the remaining 153 studies, only 5 randomized controlled trials (RCTs) passed the inclusion criteria, and others were excluded because of repetitive or insufficient information Figure . Out of 2 = 0%). Further subgroup analysis of OS based on PD-L1 expression again revealed statistically significant advantage for PD-1/PD-L1 therapy as compared to docetaxel, with pooled HR values of 0.79  in subgroups with PD-L1 expression of < 1%,0.66  with PD-L1 expression of ≥1%, 0.55  with PD-L1 expression of ≥5%, 0.41  with PD-L1 expression of ≥10%, and 0.49  with PD-L1 expression of ≥50%. However, the pooled HR values were not statistically significant in subgroups with PD-L1 expression of < 5% [0.86], and < 10% [0.86]. In addition, we found very little overall heterogeneity for OS in all studies , but the heterogeneity at the PD-L1 expression subgroup levels was different. For instance, PD-L1 expression of ≥1%, 5%, 10%, 50% and < 1 %, displayed I2 values of 0% (p = 0.740); 10.0% (p = 0.343); 0% (p = 0.537); 0% (p = 0.811);18.5% (p = 0.298). respectively, and represented less heterogeneity. However other subgroups based on PD-L1 expression like, < 5% and < 10% showed I2 values of 56.1% (p = 0.131) and 56.5% (p = 0.129), respectively, and suggested high heterogeneity . The subgroup analysis for PFS based on PD-L1 expression showed different levels of heterogeneity, ranging from no to significant levels. For instance, PD-L1 expression with ≥1%, 5% ,10%, 50% and < 1% showed I2 values of 0% (p = 0.567); 6.2% (p = 0.362); 0% (p = 0.748); 0% (p = 0.993) and 42.8% (p = 0.155), respectively. But PFS analysis of PD-L1 subgroups with expression < 5% and < 10% showed I2 values of 83% (p = 0.015) and 84.8% (p = 0.010), respectively, and represented high heterogeneity.Similarly, forest plot analysis of PFS indicated better results with PD-1/PD-L1 antibodies than docetaxel in advanced NSCLC patients Figure . The PD-2 = 59.2%) in favor of PD-1/PD-L1 antibodies in NSCLC patients. Further subgroup analysis of ORR according to PD-L1 expression also indicated that the pooled RRs in subgroups were in favor of PD-1 antibodies treatment. For example subgroups displaying PD-L1 expression of ≥1%, 5%,10% and 50% had RR values of 1.70 ; 2.08 ; 2.75 ; and 3.55 , respectively. However, the pooled RRs of subgroups with PD-L1 expression of < 1%, < 5% and < 10% were 0.81 ; 0.79; and 1.01 , respectively, and were not statistically significant. The heterogeneity among other subgroups except overall RR was between 0 to 50% and was thus not a major factor effecting RR values.The comparison of overall response rate in all four RCTs also demonstrated that PD-1/PD-L1 antibodies have significantly better effect than docetaxel Figure . The ORRAs compared with standard second line docetaxel chemotherapy, the advanced stage NSCLC patients receiving PD-1/PD-L1 antibodies showed significant increase in the incidence rate of grade 1-4 adverse events (AEs). The overall RR value for AE was 0.77 , and specific RRs based on each event also showed significant effect, like for rashes, the RR value was 2.01, 95% CI: 1.14-3.51, P = 0.02; for hypothyroidism, the RR value was 23.36, 95% CI: 8.04-67.90; P < 0.001; for hyperthyroidism, the RR value was 5.1, 95% CI: 2.23-11.68, P < 0.001; for pneumonitis, the RR value was 3.19, 95% CI: 1.9-5.34, P < 0.001; and for colitis, the RR value was 4.99, 95% CI: 1.45-17.11, P = 0.011. But, we did not observe any significant difference for the incidence rate of interstitial lung disease between both groups . However the incidence rates of other grade 3-4 AEs including: nausea, rash and colitis, did not show any significant difference between PD-1 antibodies and docetaxel therapy  with nivolumab, for patients with NSQ tumors by 67%, and led to 40% reduction in the incremental costeffectiveness ratios (ICER) (from US$ 176K to 105K). In case of pembrolizumab, the use of 50% instead of 1% cutoff, helped to increase the incremental QALY by 18% and reduced ICER by 15% (from US Dollar163K to 138K) . ConsistHowever, in our meta, the OS of the patients was significantly elevated with PD-1/PD-L1 antibodies than docetaxel in subgroups with PD-L1 expression of < 1%, which is consistant with the rusults of several some studies –32. BrahThe adverse events due to immunotherapy have been because of disruption in immune tolerance. The immune-related adverse events (irAEs) are usually defined as any AE associated with drug exposure and consistent with an immune-mediated mechanism of action , 42. OurThus, in conclusion, our meta-analysis study indicated that PD-1/PD-L1 antibodies treatment indeed has beneficial effects on advanced NSCLC patients in comparison to docetaxel monotherapy, along with displaying few adverse events. In addition, the PD-L1 expression of more than 10% on tumor tissues can serve as a biomarker to identify the NSCLC patient populations that might respond positively to PD-1/ PD-L1 antibody therapy, and thereby helping many patients to make a informed decision about this high cost immunotherapy.All the random controlled trials with information about NSCLC, docetaxel, PD1 and PD-L1 antibodies, from January 1990 to January 2017, were searched using the following databases, Cochrane library, Embase, PubMed, China hospital knowledge database, China National Knowledge Infrastructure, Wangfang Data and Weipu Data. The medical subject heading (MeSH) terms included for searching the relevant studies were: lung neoplasm, pulmonary neoplasm, lung carcinoma, pulmonary carcinoma, lung cancer, pulmonary cancer and PD-1 or PD-L1, PD-1 or PD-L1 inhibitors and chemotherapy or docetaxel.The studies with the following information were selected for inclusion in our meta-analysis; (1) Phase II/III (randomized controlled trails) RCTs with primary endpoints as OS or PFS; (2) histological confirmed SQ and/or NSQ non small cell lung cancer; (3) having the information about OS, PFS and ORR, AEs, and PD-L1 expression; (4) published in English language; and (5) having some similarity between experimental design and methods across different studies. However, the studies were excluded if they were: (1) reviews, duplicate reports, letters, unfinished studies, or conference reports; (2) studies conducted with cell lines, animal models or other types of non-lung cancers; (3) studies where HR and 95% CI could not be determined due to insufficient survival data; (4) papers in other laungages except English; (5) methods or experimental design were substantially different from other selected RCTs; and (6) sample size was smaller than 100.Two reviewers (Qiang Su and Yanli Hou) independently searched all the relevant studies and read the titles, abstracts and full texts of the identified studies. Cases of disagreement were resolved through discussion with the third reviewer (Hongchao Zhen). The following information was extracted from the selected studies; year of publication, name of the journal, author's name, methods of randomization, OS, PFS, ORR, PD-L1 expression rate and adverse events (AEs) with grades 1-4 and 3/4.2 test was used to assess the heterogeneity among the RCTs. When I2 value was < 50%, the fixed-effects model (FEM) was employed for analysis, and if I2 value was ≥50%, random-effects model (REM) was used. The Begg's and Egger's tests were used to analyze the publication bias between different RCTs.In our meta-analysis, Risk of bias analysis was prepared using Review Manager 5.3 software . Two reviewers (Q.S. and C.G.Z.) independently assessed the quality of the included studies according to the Cochrane risk of bias tool, which assesses the following six domains: selection bias , performance bias, detection bias, attrition bias, reporting bias and other bias. The Stata version 12.0 statistical software  was used to conduct the meta-analysis. The HR and 95% CI values were collected and merged to estimate overall OS and PFS. The HR value of < 1.0 indicated reduced progression or death in the PD-1/PD-L1 antibody group. The RR value was used to estimate ORR and AEs for grade 1-4 /3-4, and RR value of >1.0 represented higher ORR or the incidence of grade 1-4 and 3/4 AEs in the PD-1/PD-L1 antibodies group. In addition, the Cochran's χ


Graphene, the thinnest, strongest, and stiffest material with exceptional thermal conductivity and electron mobility, has increasingly received world‐wide attention in the past few years. These unique properties may lead to novel or improved technologies to address the pressing global challenges in many applications including transparent conducting electrodes, field effect transistors, flexible touch screen, single‐molecule gas detection, desalination, DNA sequencing, osmotic energy production, etc. To realize these applications, it is necessary to transfer graphene films from growth substrate to target substrate with large‐area, clean, and low defect surface, which are crucial to the performances of large‐area graphene devices. This critical review assesses the recent development in transferring large‐area graphene grown on Fe, Ru, Co, Ir, Ni, Pt, Au, Cu, and some nonmetal substrates by using various synthesized methods. Among them, the transfers of the most attention kinds of graphene synthesized on Cu and SiC substrates are discussed emphatically. The advances and the main challenges of each wet and dry transfer method for obtaining the transferred graphene film with large‐area, clean, and low defect surface are also reviewed. Finally, the article concludes the most promising methods and the further prospects of graphene transfer. Besides, the methods of directly synthesizing graphene on target substrates are very immature now. So, the LAG films synthesized by both CVD method and the epitaxial growth method through SiC have to be transferred from the growth substrates to the target substrates in the applications of LAG films. Up to now, various methods have been developed to transfer the graphene film, many of which are of advantages for industrial applications. No doubt the development of the transfer methods will promote the study and application of LAG films. This work aims to give a comprehensive review of the method for transferring graphene from various growth substrates to the target substrates.TableIn this review, we first discuss the transfer methods of graphene grown on different kinds of metal substrates and focus on the transfer of graphene grown on Cu substrate. Then, the progress in each wet or dry transfer method is reviewed along with the properties of the transferred graphene film, respectively. In the third part, the transfer of epitaxial graphene grown on SiC is discussed. The properties of the transferred graphene film by different methods are listed in 24) at high temperature and then deposit the decomposition products on metal substrate to form single‐layer or few‐layers graphene. Graphene synthesized by CVD method has been realized on many metal substrates, such as Fe,Chemical vapor deposition method is believed to be an efficient method to synthesize LAG. The mechanism of CVD method is described as follows: decompose the hydrocarbon precursors  stamp as the supporting layer. Various sizes of graphene film can be transferred to arbitrary substrate. For graphene growing on Co film, Wang et al.3 solution as the Co etchant. After the removal of Co film, the graphene film was transferred to the target substrates.An et al.Among all of these methods for growth graphene, CVD synthesis of graphene on Cu foil is of many advantages. For example, the C solubility of Cu is low, the graphene layers can be controlled easily, and it can also be transferred easily with relatively low cost. Therefore, Cu has been often used as the metal substrate. Here, we mainly focus on the transfer of graphene synthesized on Cu substrate. The transfer methods for CVD graphene can be classified as wet and dry transfer, depending on the environment where graphene touches the target substrate.2.12.1.12 plasma in order to etch the Cu substrate; 2) PMMA layer is coated on the surface of the graphene/copper substrate and then cured; 3) the underlying copper substrate is etched away by the etchant, then washed by the deionized (DI) water; 4) transfer the PMMA/graphene sample to the target substrate; 5) the PMMA layer is removed by acetone, then washed by the DI water and dry.Generally, taking advantage of the polymer layer such as PMMA as a supporting layer is a necessary step for wet method of transferring graphene grown by CVD. PMMA has many prominent features, such as the relatively low viscosity, excellent wetting capability, flexibility, and good dissolubility in several organic solvents.Besides using PMMA as the polymer supporting layer, poly(bisphenol A carbonate) (PC)2.1.2Figure2/Si substrate to SiC substrate.The transfer of carbon nanotubes Figure137 hasLi et al.2/Si substrate.FigureIn order to make the graphene to be better contacted with the substrate, Li and co‐workers developed an improved method by adding a second PMMA coating after the PMMA/graphene was placed on the SiO2.1.3−1).PMMA layer was removed by acetone in the wet‐transfer method. Due to the high molecular weight and high viscosity of PMMA, it is inevitable to leave PMMA residues on the graphene surface without further annealing. However PMMA residues may trigger charged impurity scattering and unintentional graphene doping thus affect the property of graphene.FigureIt is believed that annealing in high temperature (250 °C–350 °C)For the above improved methods, the subsequent processes were added to remove the PMMA residues. However, the adding steps will make the manipulation more complex, which may introduce defects to the graphene and increase the cost. Many simple improved methods have been reported to reduce the PMMA residues. Taking advantage of the excellent solubility of the organic small molecular in the solvent, Han et al. inserted an organic small molecular buffer layer between the PMMA and graphene.22H14) thin film as the supporting layer had transferred the graphene to 6 in. Si wafer with clean surface and almost no crack. The 200 nm thick pentacene was thermally evaporated onto CVD‐grown graphene on a copper foil to be used as a supporting layer. After transferring the pentacene/graphene onto a target substrate, pentacene was removed by thermal evaporation (250 °C and 300 °C for 1 h) and chemical solvent (tetrahydrofuran (THF)). They also tried other solvent such as benzene, toluene, and chlorobenzene (CB). As a result, the THF was believed to be the best solvent for removing pentacene layer. The graphene transferred by this method showed excellent electrical properties with the hole and electron mobility observed in FETs were 8050 and 9940 cm2 V−1 s−1, respectively. These values were almost two times higher than the field‐effect mobility of the graphene transferred by the PMMA‐assisted method.Scientists also tried to look for the suitable replacer of PMMA layer to get the clean and doping‐free transfer of LAG graphene. Kim et al.Figure−1 and the mixed water/IPA solution was simultaneously injected at the same rate. The surface tension for graphene in the solution could be controlled by this way. The solution was pulled out with the syringe to lower the graphene onto the substrate when the etchant was replaced by the mixed solvents. Compared with the graphene transferred by the conventional polymer‐assisted methods, the graphene transferred by this polymer‐free method showed advanced electrical properties. The mobility of the polymer‐free monolayer graphene is as high as 63 000 cm2 V−1 s−1, which is 50% higher than the similar graphene transferred by the conventional method. But the shape and size of the transferred graphene films are limited by the graphite holder.Transferring graphene without using the polymer supporting layer may be the best way to reduce the polymer residues. These kinds of methods are called the “polymer‐free” methods.To transfer graphene with unrestricted shape, Pasternak and co‐workers used a waterproof marker‐frame to transfer graphene by “polymer‐free” method.2/Si or PET). The negative charges formed on the target substrates. The graphene was thus attracted to the target substrate by the electrostatic force during the transfer process. After the metal substrate removal, graphene was transferred onto the target substrate. Graphene can be transferred layer by layer in this way. These methods did not use the polymer supporting layer, which enabled the clean transfer of CVD‐grown graphene.Wang et al. developed a unique technique called the “clean‐lifting transfer” method by using the electrostatic force to transfer graphene onto target substrates.The “polymer‐free” methods that direct transfer the graphene from the growth substrate to the target substrate provide a way to transfer graphene without polymer residues. The interfacial bonding condition between the graphene and the target substrate is critical for transfer success when direct transfer the graphene film to the soft substrate. A key factor for high quality graphene transfer is the wettability of the target substrate. Martins et al.Etchants are efficient to remove the copper substrate, and the commonly used etchants are aqueous solutions of iron nitrate,2O/H2O2/NH4OH (5:1:1) solution; (2) the second step is oxide strip using a diluted H2O/HF (50:1) solution to remove a thin silicon dioxide layer where metallic contaminants may have accumulated as a result of step 1; (3) in this step, ionic and heavy metal atomic contaminants are removed by using a solution of H2O/H2O2/HCl (5:1:1).A cleaning technology called “RCA cleans” is used for Si wafer cleaning in the semiconductor manufacturing. The traditional RCA cleans involve three sequential steps: (1) the first step is intended for the removal of insoluble organic contaminants with a H2.1.4The traditional wet transfer methods that use the etchants to etch the metal substrates have many disadvantages, such as time‐consuming, high cost , the metal particulates residues and serious environmental pollution. These disadvantages will no doubt restrict the industrial scalability application of the wet transfer methods.FigureThe electrochemical delamination methodm NaOH aqueous solution under a constant current of 1 A, with the applied voltage of 5–15 V. Graphene film of 1 × 3 cm2 was separated from the Pt substrate in 30 s. What is more, the Pt substrate can be repeatedly used for graphene growth. But when taking the PMMA/graphene/Cu stack as the anode electrode,m H2SO4 electrolyte with a potential of 0.5 V and a current of about 0.03 A. It was enough to etch away the Cu foil rapidly. The whole process ended within several minutes, which was much faster than the conventional wet transfer methods. But the Cu substrate cannot be reused.Gao et al.The electrochemical delamination methods in aboveThe wet transfer method is an important way to transfer graphene. However, as we have mentioned above, it has many problems, for example, it is easy to form defects, easy to induce impurity residues on the graphene surface, time consuming, high cost, environment pollution. Etchant and chemical solvents are used in the wet transfer methods. The solvents will introduce the unwanted doping to graphene,2.1.5−1) and 2D peak (≈2700 cm−1) are the typical peaks for graphene  will emerge in the Raman spectrum  can be used to estimate the graphene layers and the doping level of graphene.I2D/IG indicates the increase of the graphene layers, while the increase of the I2D/IG usually indicates the lower doping level of graphene. Since D peak is associated with the defects of the graphene film, the lower ID/IG indicates the higher quality of graphene film. So, the values of I2D/IG and ID/IG can give us information of graphene quality. Raman spectroscopy is a very powerful tool to test the quality of graphene film. The development and application of the Raman technology will notably promote the study of the graphene film.Raman spectroscopy is usually used to characterize the graphene layers, defects, and the doping concentration of graphene. The G peak , atomic force microscope (AFM) and scanning electron microscopy (SEM) are also efficient tools to characterize the surface morphology of the transferred graphene. These images can give direct information to determine whether there are contaminants or wrinkles on the graphene film. And AFM can also be used to determine the number of graphene layers from the film thickness. But when taking for practice, it is only possible to distinguish between one and two layers by AFM if the graphene contain folds or wrinkles.2.2As we mentioned in the beginning of the section Figure−1 without additional doping. They also fabricated graphene‐based capacitive touch‐screen devices, which demonstrated the potential application of graphene as electrode in the capacitive multitouch devices installed in the most sophisticated mobile phones.  The adhesion of subsequent graphene and dielectric layers on flexible substrates is very poor, so it is difficult to fabricate graphene‐based flexible devices where more than one graphene layer is need. Bointon et al.The “Roll‐to‐roll” method of synthesizing and transferring the 30 in. graphene has been reported in 2010 Figure7a.28 FFigure−2). The TRT was also used as the supporting layer in this process. After the removal of Cu substrate, graphene on the TRT was inserted into two rollers to attach the target substrates (PET or SiO2/Si). The TRT lost the adhesion force after the process of hot pressing for less than 10 s, then graphene was transferred onto the target substrate. The improved method can transfer graphene to the flexible or rigid substrates. Compared with “Roll‐to‐roll” method, graphene transferred by the “hot pressing” method forms relatively less defects  and the graphene is very strong, so it is inevitable to leave adhesive residues on the graphene surface. In order to get a clean surface of the transferred graphene, Chen et al.−2 after etching away the copper substrate. The PSAF layer was peeled off from the graphene/target substrate with the speed and angle with respect to substrate surface were 2 mm s−1 and about 90°, respectively. The graphene transferred by this method had cleaner surface than transferred by the PMMA and TRT. The PSAF‐assisted transfer method is applicable to both rigid and flexible substrates without difficulty in scaling up for industrial application.Kim et al.In the dry transfer methods discussed above, the etchants have also been used to remove the Cu substrates, so Cu foil cannot be reused to grow graphene. Graphene is grown on both side of the Cu foil by CVD method. One side of the graphene must be removed in advance before etching away the Cu substrate. This process causes a lot of waste. So there are still many problems that restrict the industrial scalability application of these methods. Zaretski et al.−2. And they used the dry transfer method to transfer the graphene to the polyimide (PI) by the assistance of an epoxy layer. The Cu etchant was avoided in this process, and Cu substrate can be reused to grow graphene.To realize the real dry transfer process, the adhesion energy between the graphene and substrate should be measured for the precise control of the delamination process. Yoon et al.2 was transferred to the target substrate in short time. Juang et al.It is very important to achieve highly conformal contact between the graphene and the target substrate to obtain high adhesion energy between them. Jung et al. developed a mechano‐electro‐thermal method,FigureN‐ethylamino‐4‐azidotetrafluorobenzoate (TFPA‐NH2) was deposited on the polymer surface after plasma treatment. This process made the adhesion between the graphene and polystyrene to be much higher than that between the graphene and metal substrate. Graphene can be separated from the metal substrate by the assistance of the nanoimprinter. The whole process took less than 3 h. This method is also suitable for the industrial scalability application. The properties of the transferred graphene film by dry transfer methods are listed in Table Besides using the adhesives as the above methods done, introducing covalent bonds between the graphene and the substrates has also been adopted to increase the adhesion energy. Graphene can be transferred with high quality without using any chemical etchant. Lock et al.Figure2/Si substrate was pretreated by N2 plasma to convert the surface of the substrate to silicon oxynitride phases. This step was demonstrated to be helpful for facilitating the formation of capillary bridges. The “Face‐to‐face” method realized both the growth and transfer of graphene on one wafer, and this process did not have to be done by hand, which were different from the wet and dry transfer methods. The quality of the transferred graphene was better than the conventional transfer method.In above, we review the wet transfer and dry transfer method of graphene grown on metal substrates. Since the “Roll‐to‐roll” dry transfer method has succeeded transferring the 30 in. graphene, it seems to be a more available way for industrial application. But many defects are emerged after transferring. And the disadvantages we have mentioned above suggest that the wet transfer method is difficult to scale up. Gao et al. developed a “Face‐to‐face” transfer method Figure10.36 D3We have discussed the transfer methods of graphene grown on the metal substrates by CVD method in the part 2. CVD method is believed to be an efficient way to synthesis LAG. But the surface of the Cu substrate is rough, so graphene grown on Cu substrate is easy to form wrinkles. Besides, the orientation of the graphene cannot be controlled easily. Epitaxial growth of graphene on SiC surface is also an important way to synthesis LAG. High‐quality graphene can be grown on SiC wafer by self‐limiting sublimation of Si. And the synthesized graphene has a single orientation because of the self‐limiting growth process. SiC substrate is insulated, so in some cases it can be used directly without transfer. But the cost of SiC is very high and sometimes the graphene is needed to be transferred to other substrates. It is also necessary to develop the method for transferring graphene grown on SiC wafer. However, SiC is resisted to the chemical etchants. The traditional wet transfer method is not available here.2/Si substrate by the assistance of the adhesive tape. But the size of the transferred graphene was limited to be about 1 μm2 or less.Graphene grown on SiC substrate can be transferred onto SiOFigure2/Si substrate successfully. The PI and the metal layer were removed by O2 plasma reactive ion etching and chemical etchant (Transene), respectively. This method can transfer the graphene film layer by layer. It is also suitable to transfer LAG. But the results of Raman spectrum showed that more defects were formed after transferring  have been estimated by Hamada and Otani.2/Si substrate and then the PVA was dissolved by water. No chemical solvent was used during this process. The doping‐free graphene can be obtained by this method.The metal layers have been used in the above transfer methods. When the graphene was transferred to the target substrates, chemical etchants were used to remove the metal layer. Defects were introduced during the etching process easily. The residue metal particles and the doping of graphene introduced by the chemical etchants will influence the property of graphene. Tanabe et al.In above, we have reviewed the transfer methods of the epitaxial graphene grown on SiC substrate. The “Etch‐free” transfer method seems to be an efficient method because it does not use any chemical solvent. This method is a more environment friendly and economical way to transfer the graphene for applications. The dry transfer method4The transfer of graphene acts as the bridge between the production and application of graphene. In this work, we have reviewed the transfer methods of graphene grown on metal (mainly on Cu) and SiC substrates. All of these methods are aimed at transferring graphene with clean surface and little defect. Many transfer methods have achieved these goals. The success of the graphene transfer is of great significance for the application of graphene film, making it possible for the industrial scalability application of graphene. The “Roll‐to‐roll” dry transfer method can transfer the graphene film as large as 30 in., so it seems to be the efficient method for industrial application. Other methods also have their own advantages. Some of them are hopeful for the practical industrial application. Despite that, the transfer methods are still need to be optimized to transfer graphene with little defect and large area. There are still many challenges to transfer LAG films. (1) The first challenge is avoiding the crack during the transfer process while achieve large area transfer at the same time. Monolayer graphene itself can be easily cracked during the transfer process without the assisted of the supporting layer. The improper operation during the transfer process can also induce cracks on the graphene. (2) The second challenge is obtaining LAG film with clean surface. The contaminants on the graphene surface are also big problems to be solved because they may act as scattering centers and induce unintentional graphene doping thus affect the graphene property. It is necessary to obtain LAG film with clean surface when fabricate graphene‐based devices. (3) Simplifying the transfer process to reduce the cost is also a main challenge. The cost of the transfer process must be taken into consideration in the practical industrial application. The complex and time‐consuming transfer methods are unsuitable for transferring the LAG film. Besides, both the transfer of graphene and the synthesis methods of graphene are very important for the application of graphene. It is necessary to optimize the methods to product LAG films with high quality. In recent years, the transfer‐free growth methods where graphene grew directly on target substrates have been reported.


GII/GI, leading to lower adhesion toughness. For silicon oxide substrate and pressure loading, mode mixity decreases from 232% for monolayer films to 130% for multilayer films, causing the adhesion toughness Gc to decrease from 0.424 J m−2 to 0.365 J m−2. The mode I and II adhesion toughnesses are found to be GIc = 0.230 J m−2 and GIIc = 0.666 J m−2, respectively. With point loading, mode mixity decreases from 741% for monolayer films to 262% for multilayer films, while the adhesion toughness Gc decreases from 0.543 J m−2 to 0.438 J m−2.Interface adhesion toughness between multilayer graphene films and substrates is a major concern for their integration into functional devices. Results from the circular blister test, however, display seemingly anomalous behaviour as adhesion toughness depends on number of graphene layers. Here we show that interlayer shearing and sliding near the blister crack tip, caused by the transition from membrane stretching to combined bending, stretching and through-thickness shearing, decreases fracture mode mixity  The reason why the surface adhesion of a graphene monolayer is much greater than that of graphene multilayers remains unclear. Here, the authors build a model to show interlayer sliding and fracture mode mixity cause the decrease in adhesion toughness of multilayer graphene. Multilayer graphene films may conform less well to the substrate than monolayer ones. Koenig et al.1 made roughness measurements on the top surfaces of graphene films and found a large drop in roughness from monolayer to two layer; however, they also found a large drop from two layer to three layer. This suggests that the roughness of the substrate surface is unable to explain the large decrease in adhesion toughness. To investigate the effect of interface roughness further, Gao and Huang2 argued that the rough surface of silicon oxide causes graphene films to bend; hence, the total adhesion energy consists of both van der Waals interaction energy and a negative contribution of bending strain energy. By assuming the substrate to have a sinusoidal rough surface, they attempted to calculate the adhesion energy. They concluded that the large decrease in adhesion toughness from monolayer to multilayer graphene films is due to the increase in bending strain energy caused by the large increase in the bending stiffness. Jiang and Zhu3 measured the van der Waals interaction energy between monolayer graphene films and silicon oxide substrate using atomic force microscopy. Their measurements show, however, that the roughness increases the interaction energy. In contrast, He et al.4 studied the large decrease in adhesion toughness from another perspective. They proposed that the total adhesion energy consists of both van der Waals interaction energy and residual in-plane strain energy due to lattice mismatch strain at the graphene film-silicon oxide interface. Their results show that the van der Waals interaction energy remains nearly the same for graphene films with any number of layers, but that the residual in-plane strain energy and Young’s modulus decrease sharply from monolayer to multilayer graphene films. Koenig et al.1, however, reported convincing experimental results that show a constant Young’s modulus. This observation provided a solid foundation for their subsequent adhesion toughness calculations using a continuum mechanics approach.Koenig et al.1 also suggested possible sliding between graphene layers in multilayer graphene films. The present work follows Koenig et al.’s1 continuum mechanics approach but with consideration for the interlayer shearing and sliding effect. Furthermore, the present work considers the effect of shearing and sliding on the fracture mode mixity. This is an important consideration, since interface adhesion toughness is not a purely intrinsic material property, but instead also depends on the mode mixity.Koenig et al.10 and we argue that this has caused confusion when calculating adhesion toughness. Cao et al.12 did, however, recently report studies on adhesion toughness between photoresist films and copper substrates using blister tests and the finite element method. Two types of film are considered: One is pure photoresist film and the other is combined photoresist film and a monolayer graphene. Mode mixity is considered by using cohesive zone modelling.Note that the fracture mode mixity and the interlayer shear and sliding effect are not considered anywhere in the current analytical mechanical modelsGII/GI, consequently reducing the adhesion toughness Gc. By considering the interlayer shearing and sliding effect, the mode I and mode II toughnesses are shown to be independent of the number of graphene layers. Accounting for the interlayer shearing and sliding effect on the fracture mode mixity explains the behaviour reported in the literature1, where adhesion toughness measurements seemingly depend on the film thickness . Once the mode I and mode II adhesion toughnesses have been found, the linear failure criterion can accurately determine the adhesion toughness under general loading conditions for real-world applications of graphene film-substrate systems.The present work shows that adhesion toughness is mode mixity dependent, and that interlayer shearing and sliding near the blister crack tip, caused by the transition from membrane stretching to combined bending, stretching and through-thickness shearing, decreases the mode mixity RB, the thickness of the monolayer graphene is t, n represents the number of graphene layers and the Young’s modulus of graphene is E. In Fig. 1. According to Jensen14, the deflection δ at the centre of the blister in the membrane limit isp is the pressure load and f(ν) is given by Storåkers15 asf(1/3) = 0.645 obtained by Jensen13 since Storåkers’ formula15f(ν) = [3(1−ν)/(7−ν)]1/3 is approximate. The bending moment per unit width MB, in-plane force per unit width NB, and shear force per unit width PB, at the blister crack tip14 can be expressed in the following forms, respectively  isFigure entclass1pt{minimaentclass1pt{minima20ρ = GII/GI asλ parameter in Eqs. , which is assumed to take the following form:At this stage, the effect of interlayer shearing and sliding on the fracture mode mixity can be introduced. An introduction to mixed-mode partition theory is given in Supplementary Note mentclass2pt{minimsing Eq.  in conjuλ is given in Supplementary Note A more thorough and detailed explanation for the origin of GI in Eq. (GII in Eq. (Gc can now be determined by using the mode I and mode II adhesion toughnesses and a linear failure criterion in which Gc = (1 + ρ)/(1/GIc + ρ/GIIc). Note that GIc and GIIc are intrinsic interface material properties but Gc is not. One major aim of the present study is to determine values for GIc and GIIc based on Koenig et al.’s1 experimental results. Once these two properties are known, the adhesion toughness under other loading conditions can be readily calculated.The total ERR is simply the sum of the mode I ERR I in Eq.  and the I in Eq. . The modMB in Eq. , then Eqs. (f(0.16) = 0.6907, φ(0.16) = 0.3099, ζ(0.16) = 0.4502, respectively. Then, the essential equations above, namely Eqs. , 1 found tely Eqs. , 17),  = GJ(1 + η), thenNote that Koenig et al.e to Eq.  in the pe to Eq.  and 15)1 used Ge to Eq. , 21) an an1 usedp, the central deflection δ and the radius RB of the multilayer graphene film blisters are taken from figures in Koenig et al.’s1λ, η and ρ, respectively, for monolayer and multilayer graphene films are plotted based on the measured values of p and δ from Koenig et al.1. In Fig. Gc is plotted vs. the fracture mode mixity ρ. In Fig. p, δ and RB, and the present mechanical model for graphene films with different numbers of layers. Note that the ‘Theory’ curve in Fig. p; then for Fig. RB and δ. There is generally very good agreement between the present mechanical model and the experimental measurements1.In the following, the pressure 1, results are calculated using the pressure p and the central deflection δ meaning that GJ = 0.6517 pδ and λ parameter, based on Koenig et al.'s1 measurements, are recorded in Supplementary Tables λ parameter are very close to each other. This shows the typical interlayer shearing and sliding behaviour.The numerical data for Fig. rom Eqs.  and 20)1, resultGc = 0.424, 0.362, 0.389, 0.348 and 0.359 J m−2 for the mono-, two-, three-, four- and five-layer graphene film blisters, respectively, which correspond to the following mode mixities ρ = GII/GI = 2.319, 1.400, 1.259, 1.263 and 1.272. There is a large decrease in mode mixity for two-layer graphene film blisters in comparison to monolayer films, which results in a large decrease in the adhesion toughness. For higher numbers of graphene layers, the adhesion toughness does not change significantly from the two-layer case as there are no significant changes in mode mixity. An overall average adhesion toughness for multilayer graphene films blisters is Gc = 0.365 J m−2 with ρ = GII/GI = 1.299. These results are shown in Table The average adhesion toughnesses are GIc and GIIc, are considered. He et al.4 showed that the van der Waals interaction energy remains nearly the same for graphene films with any number of layers at 0.266 J m−2. This suggests that GIc and GIIc are the same for interfaces between monolayer graphene films and silicon oxide substrates, and between multilayer graphene films and silicon oxide substrates. As adhesion toughness is generally very small, a linear failure criterion can provide an accurate representation of the fracture mechanics in question18.Now the mode I and mode II adhesion toughnesses, Let subscripts ‘1’ and ‘2+’ represent values for monolayer and multilayer graphene films, respectively. Substituting the monolayer and multilayer results from Table GIc = 0.230 J m−2 is very close to He et al.’s4 theoretical calculation of the van der Waals interaction energy at 0.266 J m−2. In fact, the van der Waals interaction energy is essentially the same in concept as the mode I adhesion toughness. The mode I adhesion toughness GIc can be determined using atomic force microscopy measurements3 and JKR model asFadh = 378 nN is the van der Waals interaction force and Rtip = 405.4 nm is the radius of the microsphere tip used in the atomic force microscopy measurements. It is seen that the measured GIc = 0.198 J m−2 is very close to the present value of GIc = 0.230 J m−2.It is interesting to note that GIc = 0.230 J m−2 and GIIc = 0.666 J m−2 will be used to predict adhesion toughness under point loading in order to examine the validity of the approach.In the following section, the theory developed above for the circular blister test under pressure loading and the determined values of P (refs. 13) is shown in Fig. 13 Fig. 15 gives φ(ν) asA blister under a point load f(ν) now becomesThe function p can now simply be replaced everywhere with 20The pressure load  in Eqs.  to 5), , p can nIn addition, Eqs.  and 15)15) becomν = 0.16, then Eqs. (φ(0.16) = 0.4636, f(0.16) = 1.497, ζ(0.16) = 0.3743, respectively. Equations , ν = 0.16,quations –31) the theν = 05 in which they used nanoparticles to create a point load on five-layer graphene membrane blisters. The blisters typically possessed a radius RB in the range 250–300 nm and central deflection δ in the range 50–70 nm. They used the formula Gc = 0.0625nEt(δ/RB)4 with E = 0.5 TPa and nt = 1.7 nm. Note that Zong et al.’s5 value for E is half of that used by Koenig et al.1, and that n ≈ 5. Zong et al. reported the adhesion toughness as Gc = 0.151 J m−2 meaning that δ/RB = 0.2309. When using Koenig et al.’s1 value of E = 1.0 TPa, then Eq. , Boddeti et al. reported further studies on the adhesion toughness between monolayer graphene and silicon oxide substrates. The adhesion toughness was found to be Gc = 0.24 J m−2, which is significantly smaller than Gc = 0.45 J m−2, reported by Koenig et al.1. Boddeti et al.21 suggest that the difference arises from the differences in interface properties such as roughness and chemical reactivity between the samples in ref. 1 and the samples in refs. 22. In line with this suggestion, the present work suggests that the reduction is caused by reduction of the mode I and mode II adhesion toughnesses at the interface, GIc and GIIc, which are now estimated. Taking the Young’s modulus and Poisson’s ratio still as E = 1 TPa and ν = 0.16, Eq.  to determine the mode I and mode II adhesion toughness of thin films by using blister tests. The analytical predictions agree very well with the experimental results reported by Cao et al.It should be noted that a general methodology has been presented, and the substrate should not be restricted to silicon oxide substrates. Furthermore, the ‘adhesion energy’ commonly used in the literature is generally different from the adhesion toughness unless the mode I adhesion toughness is equal to mode II adhesion toughness, which is not generally the case. It is the adhesion toughness that matters for the design of graphene film-substrate material systems.The authors declare that the data supporting the findings of this study are available within the article and its Supplementary Information file.Supplementary InformationPeer Review File


Graphene aerogels have many advantages, such as low density, high elasticity and strong adsorption. They are considered to be widely applicable in many fields. At present, the most valuable research area aims to find a convenient and effective way to prepare graphene aerogels with excellent properties. In this work graphene/carbon nanotube aerogels are prepared through hydrothermal reduction, freeze-drying and high temperature heat treatment with the blending of graphene oxide and carbon nanotubes. A new reducing agent-ascorbic acid is selected to explore the best preparation process. The prepared aerogels have compression and resilience and oil absorption properties due to the addition of carbon nanotubes as designed. To take advantage of these properties, the monolayer graphene must be assembled into macroscopic materials, such as graphene films, graphene fibers and 3D graphene aerogels (GAs). GAs have attracted wide attention from scientists due to their ultra-low density blow 10 mg/cm0 mg/cm3 3)  and mech3) ,5,6,7,8,However, GAs are formed by self-assembly processes, which are mainly affected by Van der Waals forces and π–π bonds  between Carbon nanotubes (CNTs) have good mechanical properties ,15 and iIf CNTs can be added to GA, CNTs and graphene can play a synergistic role through their π–π bonds . Large lIn this paper, the two aspects are focused on: (1) CNTs are mixed in the solution of GO so that the aerogels prepared are homogeneous mixtures containing graphene and CNTs; (2) Ascorbic acid is chosen as a reducing agent to make the hydrothermal reduction reaction of GO take place under mild reaction conditions. The lamellar graphene layers are close to each other, and the hydrogel is formed on the macro surface. The water solvent is removed from the hydrogel to become an aerogel through a freeze-drying process. Then the structure and properties of graphene/CNTs aerogels are provided. Test results are consistent with the design results.The preparation process of graphene/CNT aerogels is shown in With the increase of hydrothermal reaction time, the volume shrinkage of the hydrogel becomes more and more serious. With the increase of ascorbic acid amount, the volume shrinkage is also more and more serious. Then graphene/CNTs hydrogel is dialysed for 24 h using ethanol/deionized water solution in order to prevent the frost heave phenomenon in the subsequent freeze-drying process which will make the aerogels crack and destroy the macrostructure integrity of the aerogels. Such a destruction would inevitably lead to a decline of the mechanical properties of the aerogels. Therefore, the dialysis process for graphene/CNTs hydrogels is a key step in the whole preparation process. After dialysis, the freeze-drying is used to remove water to guarantee that there is no volume reduction, structural deformation or collapse during drying. Then the aerogels are heated at 500 °C under an argon atmosphere. Through the process of heat treatment, the graphene is further reduced so that the six element ring structure is further repaired, and the superfluous ascorbic acid is decomposed at the same time. Finally, the pure reduced graphene/CNTs aerogels are obtained.3 and the density of aerogels after heat treatment is 3 mg/cm3. The density of aerogels after heat treatment is reduced by more than half compared with that before heat treatment. This is because the heat treatment makes the residual ascorbic acid decompose and some functional groups of the reduced graphene can be degraded at the same time. Therefore, to a great extent, the quality of aerogels is reduced. The pore structure of the aerogels is uniform and well-arranged under the compression of ice crystal structure during the freezing process. Therefore, the perfect 3D pore structure of aerogels with the joint action of the graphene and CNTs can withstand the high temperature without volume reduction or structural deformation. Therefore, the heat treatment can greatly reduce the density of graphene/CNTs aerogels.The density of aerogels has a very important impact on their properties and applications. Therefore, this paper explores the factors that influence the aerogel density during the preparation process. Firstly, the influence of final heat treatment on aerogel density during the preparation process is discussed and the result is illustrated in 3 to 8.5 mg/cm3, and the density after heat treatment increases from 3.0 mg/cm3 to 4.0 mg/cm3. The reason is that the increase of the reducing agent leads to the rate and degree of the hydrothermal reduction reaction increase and the force between the lamellae of graphene is increased, therefore, the porosity and pore size of hydrogel and aerogel and the density of the aerogel increases.In addition, as for the preparation process, the smaller the volume shrinkage during the hydrothermal reduction step, the greater density of the resulting aerogels. Therefore, we need to find a balance point that can form a complete hydrogel and guarantee a slight volume contraction. By controlling the reaction time and the dosage of ascorbic acid, the optimum reduction degree is finally found. The effect of ascorbic acid on aerogel density during preparation of graphene/CNTs aerogels is investigated and the result is shown in The macrostructure of graphene/CNTs aerogels is shown in The microstructure of graphene/CNTs aerogels was observed by scanning electron microscope (SEM) and the results are shown in The prepared aerogels have excellent compression and resilience due to the introduction of CNTs, as shown in During the compression process, the pore network structure of pure graphene aerogels is extruded into almost dense parallel structures. The storage energy is very low. From the macro structure viewpoint, there is an unrecoverable destruction. In the structure of graphene/CNTs aerogels, graphene is wrapped by CNTs wrapped around each other. There is a synergistic effect between graphene and CNTs during the compression process. Therefore, the external force can be transferred to the carbon nanotube through the graphene. In the process of compression, the slip of the graphene layer and the fracture of the connection point can be avoided by CNTs so that the porous network structure is very stable. Meanwhile, CNTs also store a lot of energy to ensure that the aerogels can restore the original shape when the external force is removed.3 as an example. In order to verify the stability of the elastic properties of aerogels, the compression fatigue tests of 1, 5 and 10 times are repeated at the compression speed of 10 mm/min. As is shown in Taking the aerogel with density of 5 mg/cm3 as an example. The maximum recoverable elastic deformation of aerogels with different CNTs content is investigated, and the result is shown in In order to study the influencing factors of the elastic properties of graphene/CNTs aerogels, taking the aerogel with density of 5 mg/cmThe excellent elastic properties of aerogels are due to the synergistic effect of graphene and CNTs. Therefore, in addition to the influence of the content of CNTs, the interaction force between graphene and CNTs also has a great affect. The greater the binding force between graphene and CNTs, the better this synergistic effect is, and the better the elastic properties of the aerogel. It can be seen from The hydrophilic properties of the prepared aerogel are tested, and the water contact angles of aerogels with different CNTs content are shown in The oil affinity test of the aerogels is carried out, and the results are that aerogels showed super strong affinity to oil, as shown in In order to explore the adsorption ability of aerogels to different oils, several different oil products are selected, and their basic properties are listed in We take aerogels of graphene and CNTs mass ratio is 3:1 as an example to explore the oil absorption of different oil products. The result is shown in As an oil absorbing material, aerogels have good compression resilience performance. After the adsorption of oil, the oil pollution can be discharged simply by extrusion, and the shape and structure of the aerogels are not destroyed during the extrusion process. Therefore, aerogels as an oil absorbing material can be recycled, and the amount of oil contaminated by extrusion can reach 63%, and the recovery of low density oil, such as gasoline, can be over 80%. The recovery and efficiency of adsorption materials are achieved.3 so they can be considered ultralight materials. However, when the aerogel with 50% CNTs content is compressed to its own height of 90%, it can still quickly rebound to its original height without structural collapse or permanent deformation, showing excellent compressibility. The oil absorption capacity reaches 100 g/g, showing excellent oil absorption properties. Graphene/CNTs aerogels are also hydrophobic so that they can be used can be used as materials for oil and water separation.In this paper, a method of preparation of graphene/CNTs aerogels is developed. Ascorbic acid is chosen as a reducing agent for hydrothermal reaction, and then aerogels undergo the process of freeze drying and heat treatment. When the mass ratio of graphene and ascorbic acid is 1:2, the volume shrinkage degree is minimum and the aerogel density is the smallest. When the CNTs content is 50%, aerogels have the best performance. The resulting graphene/CNTs aerogels show a density as 3 mg/cm


There are several issues in this study that must be commented.In a recent study, Satar et al.  have reported “the synthesis and characterization of graphene for the immobilization of  Moreover, it exhibited neither high thermal conductivity nor high-specific surface area as graphene. In addition, in Fig. 2, the scheme is related to graphene oxide not graphene. In Fig. 1, one can find “Transmission electron microscopy revealing the size of graphene”. There are two problems: (1) it is highly improbable to achieve such perfect surface of graphene oxide (it is not a TEM picture) and (2) this picture does not reveal the size of graphene. In our opinion, the presented graphene sheet is generated in silico.At first, it is necessary to address the problem of nomenclature, i.e., graphene and graphene/graphite oxide. Authors use these terms interchangeably which are highly misleading. Authors use “graphene”; however, they synthesized graphene oxide (in fact graphite oxide). Already in “Introduction”, graphene properties are described instead of quite different materials which were synthesized, i.e., graphene oxide (GO). The discrepancy lies in the fact that GO does not possess only spMisrepresentation of the experimental parts has become common in recent years, so it is not worth mentioning that the synthesis cannot be performed as it is described. Nevertheless, in the cited reference, the method of GO production proposed by Zhao et al.  is complHowever, more important shortcomings are presented further in this section:How is it possible to force ethanol to dissociation through simple bath in GO solution? Even if so, authors did not prove this. Moreover, what is the sense of incorporation of –OH into the material which possesses lots of such functionalities?The concentrations of GO at the level of few mg/mL cause that the solution possess very high viscosity. Thus, reasonably seems to be using 1 mg of GO. Nevertheless, it would be helpful to know how much enzyme was used, as well as the volume of the solution.β-Galactosidase was stirred slowly in modified GO (1.0 g) and kept for overnight”—the authors provide no information about the concentration/amount of protein added, no immobilization yield (was the whole protein adsorbed? How much enzyme was unbound and discarded?)“The yield calculated by authors (from equation) is nowhere mentioned in the manuscript. Why?Another chemical problem appears in “enzyme immobilization”. Authors unnecessarily describe it  as covalent bonding.The principle of direct alkylation of amines with alcohols, leading to the formation of the secondary amines from primary ones, is known in the literature e.g., . HoweverExtremely huge problem is with Fig. 3. In fact, this figure shows completely nothing and it was the inspiration for this comment. Let us first start from “Experiment”, and actually the lack of it. It is not so important to write the name of FTIR apparatus, as the obtained spectrum should be independent on it. More important is to describe the used technique, ATR, DRIFT, transmittance, etc. From “The nondestructive analysis”, readers know essentially nothing.− 1”: (1) cannot be attributed to –OH and (2) there is not such a band in Fig. 3. Similarly, “the broadening of peaks from 1200 to 1700 cm− 1”: (1) there are no such a peaks and (2) even if so, such effect cannot be attributed to protein attachment to the GO.The spectra in Fig. 3 are of extremely weak quality. Skipping the fact that there is no scale, and usually, it is impossible to obtain in transmittance mode the up-going bands . The bands “present at 3105 cmProblem with Figs. 4 and 5 and their description is that authors did not realize the real size of the graphene sheet. Omitting the fact that it is not a graphene oxide, the size of modelled sheet is extremely too small.β-galactosidase (IβG) showed improved stability against various physical and chemical denaturants” was not proved here; authors tested only the influence of the temperature and (2) these are not the conclusions, rather a summary.Summarizing, the analysis of catalytic activity was done on graphene oxide, not activated GO nor graphene. The material was not characterized and “The size of synthesized graphene was observed to be 25 nm by TEM analysis while interaction of enzyme with the nanosupport was observed by FTIR spectroscopy.” This sentence from Abstract is untruth. As we fully agree with authors’ statements written in the “Conclusion”, it is necessary to mention that (1) claim “Immobilized


We investigate theoretically the effects of a modulated periodic perpendicular magnetic fields and the Rashba spin-orbit coupling (RSOC) on the electronic states and optical absorption spectrum in a graphene monolayer. The magnetic fields and supperlattice geometry give rise to distinct Dirac cone shift and open a finite bandgap at the Dirac point. In contrast to the energy spectrum without the RSOC interaction, we find that the RSOC term will develop a spin-splitting energy-momentum dispersion relation in this graphene magnetic supperlattice. Anisotropic and spin-split group velocities, effective masses and the momentum-dependent carrier distributions along the magnetic strips are demonstrated. And the manipulations of these exotic properties by tuning the magnetic fields and the RSOC are addressed systematically. Accordingly, we find bright-to-dark transitions in the electron-hole pairs transition rate spectrum and absorption peak splitting in the optical absorption spectrum tuned by the RSOC interaction. This feature offers us a practical way to detect these band engineering effects especially the exotic spin splitting states by using the conductance and optical technique. In addition, within a very large wavelength range, graphene possesses large and almost-uniform absorption coefficients but small optical absorptance. The small optical absorptance makes graphene an attractive alternative for optoelectronic devices such as transparent electrodes and optical display materials4. People have studied the dynamics of the electrons in a nonuniform magnetic field applied perpendicular to a bilayer graphene sheet and showed that snake states in graphene exhibit peculiar properties related to the underlying dynamics of the Dirac fermions5. However, making high quality graphene based devices involves etching it into nanoscale structures, creating commensurate potentials on the lattice scale with acceptable disorders, et al., which are very experimentally challenging. Therefore the prospect of employing graphene as a building block in optoelectronic devices has stimulated an intense research activity addressing the problem of how to manipulate its peculiar electronic states in a practical way that is robust to the disorders. Proposal of synthetic superlattices by Esaki and Tsu by combining alternate nano-layers of materials6 offers us new way for band engineering and charge flow controlling. Recently a great deal of attention has been devoted to superlattice structures, where external spatially periodic electric12 and/or magnetic fields are applied to a graphene monolayer16. In these previous studies, people have found anisotropic energy dispersion and gap modulation by the external fields. However the optical response and transport property of such magnetic superlattice system have not been investigated thoroughly until now, especially for the experimental measurable quantities, like conductance, optical absorption spectrum. Additionally the hybrid impact of the periodic magnetic stripes in coexistence with the Rashba spin-orbit coupling (RSOC) has not yet been reported or fully understood in these researches.Graphene monolayer and few layers have been created by mechanical exfoliation of small mesas of graphite, CVD, graphite oxidation reduction and other chemical synthesis methods18 and experimentally observed19. Compared to the relatively small intrinsic SOC in graphene, the RSOC can be greatly enhanced as experimentally demonstrated in recent work with the order of 10 meV20. In crystals without an inversion center, electric energy bands are split by RSOC under the modulation of doped impurities or an external electric field. The exploration of the RSOC, including both physics and applications, is now a rapidly growing research field of spin-orbitronics21, a new direction of spintronics focusing on the manipulation of non-equilibrium materials properties using the RSOC.The Rashba spin-orbit coupling(RSOC) is a relativistic effect. The RSOC in graphene arising from crystal with structural inversion asymmetry by impurities or external electric fields has been theoretically predictedky) tend to spatially separated distributions in a superlattice cell. A bright-to-dark transition in optical transition rate of e-h pairs can be tuned by the RSOC interactions. We can also find the highly anisotropic behavior in transition rate spectrum and spin splitting states in optical absorption.In this work, we propose a magneto-optical instrument utilizing a graphene magnetic superlattice which can be realized by coating a periodic array of ferromagnetic stripes above it. we demonstrate a clear conception about the graphene monolayer under periodic magnetic stripes, which leads to a magnetic superlattice in graphene without the need for cutting or etching. We investigate the effect of a periodic magnetic field and the RSOC interaction on the energy dispersions and corresponding transport properties, like group velocity and effective mass. The longitudinal propagation of charge carriers through such a graphene magnetic superlattice can be tuned significantly by changing the incident Fermi energies, magnetic fields and the superlattice structures. The transverse propagation is highly anisotropic. The electrons of opposite wave vectors  are Pauli matrices, p = −iħ∇ is the momentum operator and vector potential A is related to the magnetic field B applied perpendicular to the film by ∇ × A = B. The magnetic strips induce periodic magnetic fields of pointing up and down together. Accordingly the vector potential A in each cell are given by:W is the width of magnetic stripes. This description is accurate theoretically7 and has also been proved experimentally. The second term HZ = gμBσ · B is induced by Zeeman splitting. The impact of Zeeman term in our calculation is negligibly small with splitting energy of EZ = 0.12 meV at B = 1T. The structural inversion asymmetry (SIA) terms couples the two blocks of H0 such that the axial spin symmetry is broken. Here, we include only the most important RSOC term23 which is linear in momentum giving rise to the following Hamiltonian matrix for the K valley in the basis |A ↑ 〉, |B ↑ 〉, |A ↓ 〉, |B ↓ 〉. A and B correspond to the sublattice of graphene and the arrows denote the electron spin,We consider a graphene monolayer coated by periodic magnetic stripes along K and K′ states. We can use this reduced 4 × 4 Hamiltonian since the matrix for both K and K′ valleys is block-diagonal with degenerated states. The Rashba term in HR only couples the electrons from the same valley. The value of the RSOC parameter R0 depends on the external electric field perpendicular to the graphene plane. We take R0 = 102 m/s for the calculations21. To solve the single particle problem we assume that the graphene structure is embedded in a large hard wall rectangular box with the sizes Li . The electron wave functions are expanded in a plane wave basis confined by the large hard wall box. The wave function Ψ for electron can be expanded askx (ky) is the wave vector in the x (y) direction, and the expansion coefficient Cn a four-component column vector. The wave function can be calculated numerically in the basis set with the periodic boundary conditions in the x (y) direction.Note that the electronic properties of graphene are invariant by interchanging the vf = 0.86 × 106 ms−1 is used24. The effective mass is defined as the 26,Ly is the sample size along the y direction which is much larger than L and a, FT(E − EF) = −df(E)/dE is the thermal broadening function, and f(E) is the Fermi-Dirac distribution function.Based on this Hamiltonian we denote the velocity operator of the carrier as 27, where the vector potential σ± circularly polarized light. |i〉 denotes the initial states in the lower cones that are hole or valence like, |f〉 denotes the final states in the upper cone states that are electron of conduction like. Now the electron-light interaction induces transition from |i〉 to |f〉, |i〉 and |f〉 are written as Ψe,h in Eq. k space andThe interaction Hamiltonian between the Dirac fermion and the photon within the electrical dipole approximation is x direction has a varying character where the graphene sheet is alternately uncovered or covered by a ferromagnetic strip of W = 50 nm, creating one unit cell with period of L = 100 nm as sketched in Fig. y direction is uniform with period of a = 0.246 nm. The magnetic stripes generate inhomogeneous magnetic fields cooperating with the RSOC, that change the band structure of graphene significantly as shown in Fig. E(kx) of the graphene magnetic superlattice in Fig. kx near the Γ point (kx = ky = 0). When a finite magnetic field is applied by the periodic magnetic stripes, the energy spectrum becomes parabolic as shown in Fig. k and vanishes when k = 0. When both a magnetic field and an electric filed are applied on the graphene layer, we can observe both parabolic energy dispersion and spin states splitting as shown in Fig. In our calculation the ky as shown in Fig. ky when the wave vector kx and the magnetic field are both zero, see Fig. ky feel different external fields and group velocities from the ones with positive ky as we will discuss in more details in subsection conductance. When we take the RSOC term into consideration without a magnetic field, we can find that the effect of the RSOC on the dispersion is quite different from the magnetic field. The degeneracy of excited states is lifted by the RSOC when ky ≠ 0, which can be verified by the optical measurement as illustrated subsection optical Absoeption Spectrum. The energy spectrum of spin-up carriers splits from the spectrum of spin-down carriers as shown in Fig. Similarly we also plot the energy spectrum of the graphene magnetic superlattice as dependence of x direction (vx) as a function of the wave vector kx with/without magnetic field and the RSOC term. For an electron in the Dirac cone the group velocity is approaching the Fermi velocity and is almost unchanged with wave vector kx. When a magnetic field is applied, the group velocity exhibits a sinusoidal shape arising from the parabolic energy spectrum as shown in Fig. x direction, the in-plane magnetic field tries to push the electron out of the plane, resulting in the reduced velocity along the x direction. Furthermore, we also calculate the group velocity of the second subband in the presence of both magnetic fields and the RSOC. The maximum velocity of electrons in the second subband is higher than that of ground-state electrons. Note that the first and second subband are spin-split subbands induced by the RSOC. Therefore, spin-polarized electrons in these two subbands are of different velocities. The relative difference, including magnitude and direction, in spin-up and spin-down-electron velocities varies with wave vector kx. This feature makes the graphene magnetic superlattice a proming building block of possible spin/momentum-filter devices. Correspondingly we plot the group velocity of the ground-state electrons along the y direction (vy) as a function of the wave vector ky in Fig. vx, vy exhibits a step-like shape and a lateral shift when a magnetic field is applied. The asymmetrical velocity component denotes the relocated Dirac cone and the anisotropic effective fields along y directions. Similar behavior of the suppression on vy by the RSOC is also observed. In Fig. x and the y directions (mxx and myy). We can find that the effective mass is approaching zero for a bare graphene layer near the band edge. Light effective mass results in high mobility. When a periodic magnetic field is applied, mxx can be increased effectively. When the RSOC is induced, it further increases the mxx, that is in consistent with the velocity drop. For the second subband, the effective mass mxx is reduced effectively, indicating a superior electron transport ability with opposite spin polarization. The myy shows asymmetrical profile when a magnetic field is applied also due to the introducing of vector potential in the Hamiltonian and the gauge we used. The RSOC term can enlarge effective mass and this effect becomes more significant with increasing the ky due to the RSOC term ±R0p± in Eq. The carrier transport properties in such graphene-based magnetic superlattice can be tuned dramatically according to the band engineering by the periodic magnetic fields as well as the RSOC. In Fig. kx. The electrons tend to concentrate in the center of a cell, i.e., in the magnetic barrier region. Since the perpendicular magnetic fields develop a circular motion of an electron and prevent it transmitting away. However the distribution is anisotropic for ky, when ky is negative, the electrons tend to distribute along the edge of a cell where the graphene is not coated with magnetic stripes above. On the other hand, if the ky is positive, the electrons tend to concentrate in the center of a cell where the graphene is coated with magnetic stripes. It indicates that the carriers moving in opposite (±y) directions are spatially separated in a periodic cell of the magnetic superlattice. The magnetic field has apparent effect on the election distributions with different ky. This behavior can be understood by the profile of the effective vector potential in the Hamiltonian, in which the effective vector potential acts as ky-related barrier altered by magnetic field. This behavior can also be interpreted semiclassically as the Lorenz force whose direction depends on the moving direction of electrons. The RSOC term tends to push the electrons out of the plane resulting in the squeezed distributions as shown in Fig. kx, the distributions of electrons with different sign of ky are less sensitive to the RSOC term. Note that the strength of the RSOC is adjustable, so it keeps the potential to control the electrostatic properties.In Fig. G accounting for the lowest subband of the graphene based magnetic superlattice structures, using Eq. G as a function of the electron incident Fermi energy. In contrast to the smooth variation of the conductance G for a single magnetic strip, G for the magnetic superlattice vanishes at the low energy region and turns to oscillate as we increase the energy. The vanished conductance arises from the evanescent modes with an imaginary wave vector in the superlattice. It indicates that a finite band gap is opened, which is consistent with the band structure as we have obtained. The conductance gap can be enlarged by reducing the distance between two adjacent magnetic stripes or by increasing the magnetic fields strength as expected. The oscillations are caused by the Fabry-P e′ rot resonant modes formed between two adjacent magnetic stripes due to the multiple reflections at the interfaces30. The resonances in G are more pronounced for larger distance L since more Fabry-Pérot modes are formed. We can therefore effectively tune the carrier transport properties in graphene by using the periodic magnetic stripes.The conductance is a quantity easier to measure than the group velocity or the effective mass. We have also calculated the conductance σ+ circularly polarized optical transition rate of e-h pairs and plot them as functions of the wave vector in Fig. ky = −7.5 × 10−4π/a accounting for the aforementioned Dirac cone shift in k space. The energies of the e-h pairs show trigonometric-function-like shapes with respect to the wave vector kx when magnetic field B = 1T, the RSOC strength R0 = 0 as read in Fig. kx  in Eq. ħω0 = 35 meV in the absence of the RSOC as denoted by the black arrow. This absorption peak agrees well with the optical transition rate diagram and is associated with the second subband. When the RSOC term is incorporated, the corresponding energy spectrum and transition rates shown in Fig. R0p± coupling spin-up and spin-down states as we discussed in the subsection Energy Spectrum. The distinct optical absorption spectra provide an effective way to control the optical properties by tuning the strength of the RSOC and magnitude of the magnetic field on graphene. The graphene magnetic supperlattice is a promising platform for potential application in anisotropic magneto-optical devices.Finally the optical absorption spectrum of such a graphene magnetic supperlattice is shown in Fig. In summary, we have investigated theoretically the energy spectrum, conductance and optical absorption spectrum of a graphene magnetic superlattice in the presence of the RSOC. We find that a finite gap is opened at the shifted Dirac point of the magnetic superlattice. The RSOC term splits the spin-degenerated energy level giving rise to a double Dirac cone. The group velocity, effective mass and charge distribution can be tuned by the magnetic fields and the RSOC effect. Importantly, anisotropic bright-to-dark transitions and peak splitting behavior in the transition rate spectra of e-h pairs are developed in the two directions respectively. As a clue for possible experimental verifications, our theoretical results demonstrate that the magnetic superlattice geometry and the RSOC effect play crucial and adjustable roles on the exotic massive spin-split states which determine the conductance gap and multiple peaks in optical absorption spectrum respectively. Our theoretical results shed new light on designing graphene-based magneto-optical devices.


However, graphene/metal oxide nanohybrid- based gas sensors still have several limitations in practical application such as the insufficient sensitivity and response rate, and long recovery time in some cases. To achieve higher sensing performances of graphene/metal oxides nanocomposites, many recent efforts have been devoted to the controllable synthesis of 3D graphene/metal oxides architectures owing to their large surface area and well-organized structure for the enhanced gas adsorption/diffusion on sensing films. This review summarizes recent advances in the synthesis, assembly, and applications of 3D architectured graphene/metal oxide hybrids for gas sensing.Graphene/metal oxide-based materials have been demonstrated as promising candidates for gas sensing applications due to the enhanced sensing performance and synergetic effects of the two components. Plenty of metal oxides such as SnO With the rapid development of modern industry, the detection of hazardous gases has become an important issue for human health and environmental protection. A variety of materials such as carbon-based materials, noble metals, metal oxides or sulfides and organic semiconductors have been explored to fabricate gas sensors ,13,14,152/g), and charge carrier mobility  at room temperature 4OH) and 2 was dispersed into multilayer GO solution for ion anchoring. Second, NaNO3 and NaAc were added to the mixed solution resulting in a precursor solution. Then, the obtained mixture was transferred to a Teflon-lined stainless-steel autoclave for the in-situ growth of the ZnO spheres under solvothermal treatment conditions, while the in-situ reduction of GO to rGO occurred simultaneously. Finally, a black integrated 3D graphene aerogel–ZnO was obtained via a freeze-drying process to maintain the 3D monolithic architecture [Construction of multilayer networks for GO can serve as a hierarchical space for the growth of MOx to form 3D architectured rGO/MOx hybrids ,99,100. itecture . The 3D 2, NH3, HCHO, H2S and so on [2 seems to be the most investigated room temperature sensing target, not only because its toxicity, but also its electrophilic characteristics that improve the gas chemical adsorption reaction on the surface of sensing materials. Many efforts have been dedicated to control the structures of 3D rGO/MOx sensing materials for enhanced NO2 sensing properties at room temperature [3D architectured graphene/MOx nanocomposites have been widely used for the fabrication of gas sensors to detect various hazardous gases such as NOnd so on ,104,105.perature ,107,108.2 sensing performance. In graphene/MOx composites, the MOx nanostructures act as key sensitive materials for the chemical adsorption of NO2; on the other hand, the highly conductive graphene can not only ensure the current flow across electrodes for fast response, but also conjugate with MOx to form Schottky-junctions enhancing the electron capture. Thus, many efforts have been dedicated to optimizing the components’ structures in 3D rGO/MOx hybrids to achieve enhanced sensing performance. The synergetic effects between graphene and MOx are the key factors for improving the room temperature NO2 gas sensor [2  and selectivity by decoration of ultrafine (3–6 nm) SnO2 nanocrystals [Controllable preparation of MOx with confined sizes in the nanohybrids could be one efficient way to contribute to enhancing sensing properties. Yang et al. developed a 3D nanoflower-like CuxO consisting of 5–9 nm ultrafine nanoparticles/multilayer graphene (CuMGC) composites as a room temperature NOs sensor . The 3D ensor [2 b. The encrystals . 2 sensing. For instance, a gas sensor based on 3D mesoporous rGO aerogels embedded with SnO2 or ZnO showed an enhanced response rate (190 s and 200 s to 10 and 100 ppm of NO2) [The design of porous structures was also proved to be feasible route to enhance the performance of 3D rGO/MOx nanohybrids for room temperature NO of NO2) ,44. Howe2 sensor based on 3D rGO/ultrathin ZnO nanorods (rGO/UT-ZNR) with simultaneous characteristics of size-confined ZnO nanorods (with average diameter of 12 nm) and mesoporous structure [2 sensor exhibited high sensitivity (119% to 1 ppm NO2), low LOD of 50 ppb and fast response and recovery (75 and 132 s to 1 ppm NO2). Meanwhile, the rGO/UT-ZNR sensor presented a favorable linearity, good selectivity and stability  sensitivity (149%) to 20 ppm NO2 and a short recovery time (56 s)  c. Furthee (56 s) d. The en3, HCHO, H2 and so on at room temperature [3O4 nanocrystals fabricated by using the electro-spinning method for room temperature NH3 sensing [3O4 hybrids showed nanofiber morphology and the Co3O4 nanocrystals were wrapped by rGO thin layers. The 3D rGO/Co3O4 nanofiber-based sensors exhibited a p-type semiconductor behavior and showed a high sensitivity (53.6%) and fast response (4 s), recovery rate (5 min) to 50 ppm of NH3 at room temperature. In addition, the sensor exhibited good selectivity to some potential interferents such as methanol, ethanol, formaldehyde, acetone, benzene, and methylbenzene. The enhanced room temperature sensing performance was attributed to the unique hierarchical wrapping microstructure and the selective NH3 adsorption at both the wrapping layer of rGO and the polarized C-Co3+ covalent centers within the nanofibers [3D graphene/MOx hybrids can be also used to detect other gases such as NHperature ,112,113. sensing . The 3D nofibers . In the nofibers . The prenofibers . Thus, tIn this review, the recent developments in the construction of 3D graphene/metal oxide-based hybrids, as well as their gas sensing application were discussed. A variety of logical strategies were adapted to design the desired 3D nanohybrids by combing rGO with dimensionally different MOx. In addition, as sensing materials, 3D graphene/metal oxide hybrid-based gas sensors exhibit enhanced room temperature sensing performance owing to their superior high surface area, porosity, functionalized structures and the synergistic effects among them, resulting in the improvement of efficient gas adsorption/diffusion and charge transportation. Therefore, such structure-controlled 3D graphene/metal oxide composites could play very important roles as promising candidates for room temperature hazardous gas sensing. However, further efforts are required to achieve green, cost-effective and commercial scale production methods for 3D graphene/metal oxide sensing materials. The controllable fabrication of 3D graphene/metal oxide such as regulation of the number of graphene layers and the achievement for the in-situ growth of MOx on graphene sheets remain unsolved challenges. Furthermore, the unclear interaction mechanisms between building units and gas adsorption/diffusion requires more systematic studies coupled with many fields, such as physics, chemistry, electronics and mathematics.4, C2H2 and so on at room temperature. We hope that this review will help motivate scientific interest in the controllable fabrication of graphene/MOx-based 3D hybrids nanostructures, as well as in their advanced room temperature gas sensing applications.In the future, the environmentally friendly, facile and industry-scale multistep synthesis of 3D architectured graphene/MOx-based highly-active sensing materials containing controllable porous structures, confined size of MOx and functionalized graphene would be highly desirable. In addition, the development of novel syntheses of 3D graphene/MOx nanostructures is also expected to lead to potential applications in detecting more gases such as CH