Freestanding graphene/MnO2 cathodes were manufactured through a vacuum filtration process. The structure of the graphene/MnO2 nanocomposites was characterized using X-ray diffraction (XRD) and Raman spectroscopy. The surface and cross-sectional morphologies of freestanding cathodes were investigated by scanning electron microcopy (SEM). The charge–discharge profile of the cathodes was tested between 1.5 V and 4.5 V at a constant current of 0.1 mA cm−2 using CR2016 coin cells. The initial specific capacity of graphene/α-, β-, and γ-MnO2 freestanding cathodes was found to be 321 mAhg−1, 198 mAhg−1, and 251 mAhg−1, respectively. Finally, the graphene/α-MnO2 cathode displayed the best cycling performance due to the low charge transfer resistance and higher electrochemical reaction behavior. Graphene/α-MnO2 freestanding cathodes exhibited a specific capacity of 229 mAhg−1 after 200 cycles with 72% capacity retention.Different polymorphs of MnO Therefore, researchers have been developing different cathode materials such as LiMn2O4 and LiFePO4, which have a capacity of merely 150 mAh/g and 170 mAh/g, respectively [Nowadays low cost, clean and sustainable energy storage requirements with high performance are of great need because of rapid improvement of mobile and stationary electronic applications. Lithium-ion batteries have been one of the key energy storage devices to meet these energy demands since the last century . However2 , and Si 2 , the perectively .2) is one of the most promising metal oxide as a replacement for the Li-ion electrode material owing to its high theoretical capacity (308 mAh/g), environmental friendliness and low cost [2 has been fabricated and used with carbon materials to achieve excellent conductivity with a large specific surface area [Manganese dioxide  atomic-scale honeycomb lattice made of carbon atoms. Its unique properties such as high electrical and thermal conductivity, high chemical stability, large specific surface area and high mechanical strength have made graphene one of the most promising materials for energy storage devices . In recee cycles .2 has many crystallographic polymorphs including α-, β-, δ-, γ-, ε-, and λ-MnO2. The electrochemical characteristics of MnO2, such as electrocatalytic activity, specific capacity and oxygen reduction reaction, vary according to its crystalline structure and morphology [MnOrphology . However2  were produced by a microwave hydrothermal method. Freestanding graphene/MnO2 cathodes were manufactured through a vacuum filtration process without using any additional additives such as a binder. The effect of the different polymorphs, α-, β-, and γ-MnO2, on the structural and electrochemical properties of the manufactured, freestanding graphene/MnO2 cathodes was investigated. To the best of our knowledge, this study is the first to investigate the electrochemical performance of freestanding graphene/MnO2 cathodes for Li-ion batteries. The freestanding graphene/MnO2 cathodes exhibit a high specific capacity and excellent electrochemical cycling performance.In this work, different polymorphs of MnO2 phases were synthesized by a microwave-assisted hydrothermal method. α-MnO2 nanowires and β-MnO2 nanorods were prepared according to our previous report [2, 1.83 mg of (NH4)2S2O8, 1.35 mg of MnSO4 and 3 mmol were dissolved in 80 mL of distilled water. Then, the resulting solution was transferred into a Teflon (PTFE)-lined autoclave, sealed and placed in a microwave oven (Milestone ROTOSYNTH). The hydrothermal reaction was carried out for 60 min at 90 °C. Then the autoclave was cooled down to room temperature and the as-prepared black precipitate was filtered and washed several times with distilled water. γ-MnO2 was obtained after drying at 80 °C in a vacuum oven for 12 h.The α-, β-, and γ-MnOs report . To prep2 cathodes were prepared via a vacuum filtration technique. Firstly, 30 mg of GO was dissolved in 50 mL distilled water by ultrasonication for 1 h to obtain GO. Then 30 mg of as-synthesized MnO2 was added to the solution and ultrasonicated for another 1 h. The GO/MnO2 solution was filtered on a PVDF membrane by a vacuum filtration technique. In order to reduce the GO to graphene, the as-synthesized GO/MnO2 was subjected to a hydrazine solution after filtration of GO. 5.6 mL of a hydrazine solution was slowly poured onto GO/MnO2 and filtered. Then the obtained solid was peeled-off from the PVDF membrane and the freestanding graphene/MnO2 was obtained (approximate thickness is 15 μm). This process was carried out for all MnO2 phases.Graphite oxide (GO) was synthesized according to a modified Hummers’ method  by using2 composite cathodes was characterized using scanning electron microscopy (SEM). The structural and phase investigation of the freestanding cathodes was tested using X-ray diffraction (XRD) and Raman spectroscopy.The microstructural morphology of the freestanding graphene/α-, β-, and γ-MnO6) was dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) , which was used as the electrolyte. In order to separate the electrodes, a microporous polypropylene membrane was used. Electrochemical tests of the cathodes were implemented between 1.5 and 4.5 V at a constant current density of 0.1 mA cm−2. The specific capacity of the freestanding graphene/MnO2 cathodes was calculated depending on the active mass of the graphene/MnO2 composite (about 20 mg) on Al foil. The resistance of the electrodes was evaluated via electrochemical impedance spectroscopy (EIS) using a Nyquist curve in the frequency range 1000 kHz–0.1 Hz with an AC amplitude of 10 mV with fresh electrode applied before the electrochemical cycling test.A CR2016 coin cell was used to investigate the electrochemical performance of the produced freestanding composite cathodes, assembled in an Ar-filled glove box. In this coin cell, the produced cathodes were used as a working electrode, and lithium foil was used as an anode. 1 M lithium hexafluorophosphate , cross-sectional characterization of graphene/MnO2 composite layers was implemented using SEM. As it can be seen from 2, β-MnO2 and γ-MnO2 structures were homogenously distributed and fixed between graphene layers.The surface morphologies of α-, β-, and γ-polymorphs of MnO. β-MnO2  shows the γ-MnO2  exhibits2. The typical reflection peaks of α-MnO2 are observed at 2θ values of 12.7°, 18.0°, 28.6°, 36.7°, 38.6°, 41.9°, 49.7o, 56.4° 60.2°, 65.4°, 69.6°, and 72.9° corresponding to (110), (200), (310), (400), (211), (420), (301), (600), (521), (002), (541), and (312) planes of α-MnO2 crystals [2, reflection peaks were observed at 2θ values of 28.7o, 37.4o, 41.0o, 42.9o, 46.1o, 56.7o, 59.4o, 65.0o, 66.8o, 67.3o, 72.3o, 79.7o and 86.6o corresponding to (110), (101), (200), (111), (210), (211), (220), (002), (310), (301), (202) and (321) planes of β-MnO2 [2, reflection peaks were observed at 2θ values of 22.0°, 34.8°, 37.0°, 38.5°, 42.2°, 57.0°, 65.4° and 68.6°, corresponding to (101), (130), (210), (400), (211), (402), (020), (421) planes of γ-MnO2 [2, graphene/β-MnO2 and graphene/γ-MnO2 composite structures. The graphene peak observed at a 2θ value of 25.8o indicates the (002) plane of carbon. However, there are still some remaining graphene oxide phases observed at 2θ values of 10.9o in graphene/α-MnO2 and graphene/β-MnO2, while almost all graphene oxide is transformed to graphene in the graphene/γ-MnO2 structure [o (110), 0, (310),, (400), 1, (420),, (211), 0, (002),f γ-MnO2 . Figure tructure –26.2, graphene/β-MnO2 and graphene/γ-MnO2 composites was performed via Raman spectroscopy using a 785 nm laser and the results are shown in 2 is generally used to characterize MnO2 structures, MnO2 structures may show different characteristic peaks due to different laser wavelengths and energy. Generally, in the Raman spectra of MnO2, the peaks between 500 and 700 cm−1 are attributed to the stretching mode of MnO6 octahedra [−1 originate from the formation of Mn2O3 or Mn3O4 and correspond to the bending mode of O–Mn–O [2 composite, α-MnO2 shows three weak peaks at 289 cm−1, 319 cm−1 and 376 cm−1 and one strong peak observed at 661 cm−1. In graphene/β-MnO2 composites, three weak peaks at 230, 330 and 385 cm−1 and two strong peaks at 562 and 648 cm−1 are observed. Graphene/β-MnO2 exhibited two weak peaks at 314 and 367 cm−1 and one strong peak at 658 cm−1. The observed peaks at around 1320 and 1590 cm−1 are related to the D- and G-bands of graphene [2 composite structures.Further phase characterization of graphene/α-MnOctahedra  and the f O–Mn–O . In the graphene  in the g2 in the graphene/MnO2 composites on the resistance of the cell, electrochemical impedance spectroscopy (EIS) measurements were performed and results are shown in Rct) of the graphene/α-MnO2, graphene/β-MnO2 and graphene/γ-MnO2 cathodes [2 composite cathode has the largest width, showing Rct = 102 Ω. Graphene/α-MnO2 with a Rct = 42 Ω has a smaller width than that of graphene/γ-MnO2 with Rct = 90 Ω. These Rct values indicate that the graphene/α-MnO2 composite cathode has better electronic contact and conductivity among the produced freestanding graphene/MnO2 cathodes [In order to investigate the effect of different crystal structures of MnOcathodes . As seencathodes .−1 in a voltage range from 1.5 to 4.5 V. In 2, graphene/β-MnO2 and graphene/γ-MnO2 cathodes are given for the 1st, 50th, 100th and 200th cycles. As shown in 2 cathode exhibited a specific capacity of 321 mAhg−1 upon first discharge with an open-circuit potential of about 3.2 V and an average voltage of approximately 2.25 V. It can also be seen that the capacity of the graphene/α-MnO2 cathode was sustained with a small amount of capacity loss. This could be attributed to the wire-like structure of α-MnO2 allowing ions to pass from the cathode. When the graphene-supported β-MnO2 cathode was investigated (2. While the first discharge capacity of graphene/β-MnO2 cathode was 198 mAhg−1, the graphene/γ-MnO2 cathode displayed a specific discharge capacity of 251 mAhg−1 (2 and γ-MnO2 electrodes decreased dramatically with increasing number of cycles. This could be attributed to the poor electrical conductivity and the textural modification during Li+ intercalation and de-intercalation processes. Cheng et. al. [2 cathodes and demonstrated a discharge capacity of 204.4 mAhg−1 for the first discharge with a constant current of 50 mAg−1. In our work, the as-prepared α-MnO2/graphene cathode was reached a specific capacity of 318 mAhg−1. This is probably due to graphene reinforcement, which increases the conductivity and electrochemical reaction of α-MnO2 with Li ions, as is reported in previous studies [The electrochemical performance of the as-synthesized cathodes was first evaluated by galvanostatic charge/discharge cycling at a constant current density of 254 mA gstigated , it can 1 mAhg−1 . The spe studies .2, graphene/β-MnO2, and graphene/γ-MnO2 cathodes. A remarkable result is obtained from the graphene/α-MnO2 cathode which has an initial capacity of 321 mAhg−1. It can be seen that there is no sudden loss of capacity and between cycles 2 and 44 it exhibits almost a stable capacity of 305 mAhg−1. The total capacity loss is 27% during 200 cycles. Graphene/β-MnO2 and graphene/γ-MnO2 cathodes were also cycled until the 200th cycle but they exhibited very poor capacity retention when compared with the graphene/α-MnO2 cathode. Although both of these cathodes display a high capacity during the first cycle, the capacity value decreases dramatically during the second cycle. While the total capacity loss for the graphene/β-MnO2 cathode was 61%, the graphene/γ-MnO2 cathode showed a 55% capacity loss after 200 cycles. Tu et al. [2-graphene cathodes and a γ-MnO2 cathode, and they observed huge capacity reduction due to the formation of Li2MnO3.u et al.  also rep2 phases. Freestanding graphene/MnO2 was successfully prepared with no further additives. The prepared nanocomposite samples were operated as positive electrodes for Li-ion batteries. The SEM images showed that α-MnO2 nanowires and β-MnO2 nanorods were homogenously dispersed not only at the surface, but also in the interlayer space of grapheme layers. Moreover, urchin-like γ-MnO2 microspheres were found wrapped by graphene nanosheets. The electrochemical cycling results demonstrated that the graphene/α-MnO2 cathode showed the best electrochemical performance among all prepared samples with an achieved initial capacity of 321 mAhg−1 and maintained its remarkable performance after many cycles. This study proved that α-MnO2 nanowires with graphene reinforcement could be promising cathodes for Li-ion batteries due to the high capacity and long cycle life.A facile and rapid microwave-assisted hydrothermal method was demonstrated to synthesize α-, β-, and γ-MnO


Furthermore, a significant increase of PD-1 in Treg cells was found inPIH compared with NP. In addition, PD-L1 Fc, an activator of PD-1/PD-L1 pathway,increased Treg cell percentage, enhanced Foxp3 mRNA expression, and elevatedlevels of TGF-β, IL-10, and IL-35 in PBMCs. However, anti-PD-L1 mAb exerted areverse effect. These findings revealed that PD-L1 Fc had a favorable effect onTreg cell differentiation, indicating a potential therapeutic value ofPD-1/PD-L1 pathway for PIH treatment.Pregnancy-induced hypertension (PIH) causes significant maternal and fetalmorbidity and mortality. A decreased number of regulatory T (Treg) cells isassociated with the pathogenesis of PIH. The programmed cell death-1(PD-1)/PD-ligand 1 (PD-L1) pathway is critical to normal pregnancy (NP) bypromoting Treg cell development. However, the relationship between PD-1/PD-L1and Treg differentiation in PIH has not been fully elucidated. In this study,venous blood was obtained from 20 NP and 58 PIH patients. Peripheral bloodmononuclear cells (PBMCs) were isolated from venous blood. The levels ofTreg-related cytokines  in serum and PBMCs weremeasured by ELISA. The percentage of Treg cells in PBMCs was assessed by flowcytometry. The mRNA levels of Treg-specific transcription factor Foxp3 in PBMCs,and PD-1 and PD-L1 in Treg cells were detected by qRT-PCR. The protein levels ofPD-1 and PD-L1 in Treg cells were evaluated by western blot. The serum levels ofTGF-β, IL-10, IL-35, and Foxp3 mRNA expression andCD4 Pregnancy-induced hypertension (PIH) syndrome complicates 6–10% of pregnancies andcauses significant maternal and fetal morbidity and mortality  andincl+ T lymphocytes, themajor cell population involved in the cell-mediated immune response, play a crucialrole in the development of PIH. Depending on the changes in differentcytokine-induced microenvironments, CD4+ T lymphocytes can differentiateinto T helper type 1 (Th1), Th2, Th17 or CD4+CD25+ regulatoryT (Treg) subsets performing inflammatory, regulatory or suppressor functions , indicating that Tregwas implicated in the development and progression of PIH. This provides insight intothe mechanism of PIH, providing Treg as a potential target for clinical treatment.However, the direct evidence of the role of PD-1/PD-L1 and the relationship betweenPD-1/PD-L1 and Treg in PIH requires further research. Accordingly, in this study, wehypothesized that PD-1/PD-L1 might alleviate PIH by regulating Treg differentiation.First, we explored cell quantities and function of Treg in PIH. Then, weinvestigated the relationship between the PD-1/PD-L1 pathway and Tregdifferentiation in PIH.The programmed cell death-1 (PD-1)/PD-ligand 1 (PD-L1) pathway is critical to immunehomeostasis. Recently, the relevance of Treg cells and PD-1/PD-L1 pathway incontrolling immune responses has been highlighted. Of note, PD-L1 enhances thestability of Treg cells and promotes Treg cell differentiation –15. ByAll study subjects were recruited from the Department of Obstetrics andGynecology at Anhui Provincial Hospital of Anhui Medical University. This studyincluded 20 normal pregnancies (NP) and 58 pregnant women suffering from PIHsyndrome including severe PE (n=20), mild PE (n=18), and HDCP (n=20). There wasno significant difference in age and gestational age between groups. Thepresence of PE was assessed according to ACOG guidelines . All pregnancies were singleton gestations, andnone of the participants had active labor at the time of enrollment and bloodsampling. Women with pre-existing renal diseases, chronic hypertension beforepregnancy, diabetes, recurrent miscarriage, disorders of the immune system, orusing immune suppressing medication were excluded from this study.This study was reviewed and approved by the Clinical Trial Ethics Committee ofAnhui Provincial Hospital of Anhui Medical University. All procedures werecarried out in strict accordance with the approved guidelines and regulations.Written informed consent was obtained from each participant prior to enteringthe study.Blood samples (5 mL) were obtained by venipuncture from severe PE, mild PE, HDCP,and NP women. Two milliliters was used for the preparation of serum, while theremaining 3 mL was heparinized for the isolation of peripheral blood mononuclearcells (PBMCs) by density gradient centrifugation. The serum was separated andstored at −70°C until required for cytokine determination using an enzyme-linkedimmunosorbent assay (ELISA). To illustrate the effect of the PD-1/PD-L1 pathwayon Treg cell differentiation, PD-L1 Fc  and anti-PD-L1 mAb  were added to the PBMCs from NPs and PIHpregnancies for incubation of 3 days. PBMCs were isolated for flow cytometry,quantitative polymerase chain reaction (qPCR), western blot, and ELISA. Allblood samples were obtained before the PE patients received treatments such assteroids or antihypertensive drugs.'sprotocol.The levels of transforming growth factor β (TGF-β), interleukin-10 (IL-10), andIL-35 in serum or cell culture fluid of PBMCs were measured with an ELISA kit, according to the manufacturer6PBMCs were resuspended and stained with anti-CD4-FITC monoclonal antibodies(mAbs) and anti-CD25-PE mAbs for surface antigens (eBioscience) according to themanufacturer's instructions. The cells were then permeabilized withpermeabilization/fixation buffer (eBioscience). The cells were resuspended in300 μL of PBS for subsequent flow cytometric analysis. Data were acquired usinga FACScalibur , and processed using the CellQuest program.Treg differentiation was performed by flow cytometry. In brief, 2×10+CD25+ Regulatory T Cell Isolation Kit, with a Midi&Mini MACS instrument (Miltenyi Biotec)following the manufacturer's instructions.Treg cells were isolated from PBMCs of the women by multi-step magnetic sortingusing a human CD4−ΔΔCT, andGAPDH was used for normalization.Total RNA was extracted and purified using TRIzol reagent , andan equal amount of total RNA (1 μg) was used for cDNA synthesis . The primer sets used in this study were designed using Primer-BLASTsoftware and were as follows: PD-1 Forward: 5′-ACCCTGGTGGTTGGTGTCGT-3′, Reverse:5′-CCTGGCTCCTATTGTCCCTC-3′; PD-L1 Forward: 5′-TTTGCTGAACGCCCCATA-3′, Reverse:5′-TGCTTGTCCAGATGACTTCG-3′; Foxp3 Forward: 5′-CACTGACCAAGGCTTCATCTG-3′, Reverse:5′-GGAGGAACTCTGGGAATGTG-3′; GAPDH Forward: 5′-GGACCTGACCTGCCGTCTAG-3′, Reverse:5′-GTAGCCCAGGATGCCCTTGA-3′. The cDNA (2 μL) was subjected to qRT-PCRamplification analysis using SYBR Green PCR mix . Theamount of target relative to a calibrator was computed by 2Protein was isolated from Treg cells that were lysed in radioimmunoprecipitationbuffer (RIPA) containing protease inhibitors at 4°C for 30 min. Cell lysateswere prepared with a RIPA lysis buffer kit , Treg cell:lysis buffer of 1:2 (v:v), and the protein concentrations werequantified using a Bio-Rad protein assay .Subsequently, equal proteins were separated by 10% SDS-PAGE gels and transferredonto PVDF membranes . After blocking with 5% fat-free milk,primary antibodies against PD-1  and PD-L1 were added, followed bysecondary antibody horseradish peroxidase-conjugated goat anti-rabbit IgG. GAPDHwas used as the loading control. The protein was detected with an enhancedchemiluminescence kit  and the band intensity wasquantified with Image-Pro Plus 6.0 software.'s t-testbetween two groups and reported as means±SD. All experiments were repeated atleast three times. P<0.05 was considered statistically significant.All statistical analyses were performed using SPSS 16.0 . Thedata were analyzed using StudentTreg cells maintain immune homeostasis and tolerance by cell-to-cell contact andby secretion of cytokines ,18. As sFlow cytometry analysis showed that, similar to the expression pattern ofTreg-related cytokines, the percentage of Treg cells in isolated PBMCs of HDCP,mild PE, and severe PE groups was significantly reduced compared with the NPgroup. Moreover, the percentage of Treg cells in isolated PBMCs of severe PEwomen was significantly decreased compared with that of HDCP pregnancies . FurtherThe mRNA and protein levels of PD-1 and PD-L1 were assessed by qRT-PCR andwestern blot, respectively. As shown in We found that compared with NP, HDCP showed significantly decreased Tregexpression in PBMCs. However, PD-L1 Fc elevated the reduced Treg expression inPBMCs of HDCP. In contrast, anti-PD-L1 mAb further reduced Treg expression inHDCP . SimilarIn line with the above results, HDCP showed reduced concentration of Treg-relatedcytokines in cell culture fluid of PBMCs compared with NP. Similarly, theseTreg-related cytokines in PBMCs were increased with PD-L1 Fc treatment anddecreased with anti-PD-L1 mAb .+CD25+ cells. With suppressive capacity and regulatoryfunction, Treg cells are major contributors to the tolerance maintenance of thefetus and the maintenance of normal pregnancy (Treg cells are a specialized subset of T cells characterized byCD4regnancy ,20,21. Tregnancy . TGF-β, regnancy .Accumulregnancy ,25. For regnancy . Consist+ Tregpercentage might account for the reduction of Treg cells in PE (PD-1 suppresses signaling through the T-cell receptor, resulting in reducedproliferation, cytokine production, and cytotoxic activation of T cells. PD-1 isconsidered a dominant responsive inhibitory receptor among all of the inhibitoryreceptors expressed in T cells . Apartfls in PE . Thus, w+ T cells in the presence ofanti-CD3 and TGF-β (As an activator of PD-1/PD-L1 pathway, PD-L1 Fc has been reported to significantlyalleviate symptoms and suppress disease progress in mice suffering from auto-immunedisorders by promoting the development of Treg cells . PD-L1 Fnd TGF-β . Furthernd TGF-β . As expe+ T lymphocytes can differentiate into the Th1, Th2, Th17 orCD4+CD25+ Treg subsets (+CD25+ T lymphocytes to specify Treg cells in thisstudy. Although we have detected mRNA expression of Treg-specific transcriptionfactor Foxp3 in isolated PBMCs of NPs, HDCP, mild PE, and severe PE groups, whichcould indirectly reflect the expression of Treg cells, other studies have usedCD4+CD25+ Foxp3+ T cells to specify Tregsubsets. Thus, this might be one of the limitations of this study. Furthermore,recent data showed that CD28 is a major target for PD-1 inhibition both invitro and in vivo (+ T cells, or Tregcells in PIH patients compared to NPs. These limitations should be investigated inour future study.As CD4 subsets ,7, we us in vivo ,32. In tIn summary, a reduction of Treg cells was associated with the pathogenesis of PIH.Interestingly, PD-1/PD-L1 pathway promoted Treg cell differentiation in PIH,indicating a potential therapeutic value of PD-L1 Fc for PIH treatment.


Have patience with everything unresolved in your heart and … try to love the questions themselves as if they were locked rooms or books written in a very foreign language. Don’t search for the answers, which could not be given to you now, because you would not be able to live them. And the point is to live everything. Live the questions now. Perhaps then, someday far in the future, you will gradually, without even noticing it, live your way into the answer.– Rainer Maria RilkeI entered university brimming with idealism and the desire to help. My premedical studies only partially satisfied this desire. I wanted to go beyond North America and do global health work; my heroes were Paul Farmer and other pioneers of social medicine. Motivated by their examples, I applied for some research funds from my university, and at 21 years of age found myself heading for Kampala, Uganda to gather data for an ambitious qualitative research project, on university students’ attitudes towards prevention of HIV/AIDS. Immediately after I touched down at Entebbe Airport I was confronted with some uncomfortable realities. I noticed the privilege and financial security I enjoyed compared to my Ugandan research subjects – who, as university students, were also my peers. I marveled at the incredible opportunity I had to do cross-cultural research as a mere university student, and realized my audacity in attempting this work despite my relative lack of experience and training. I was also acutely aware of the political and moral implications of my research topic. As I was embarking on this research, the reasons behind the success of Uganda’s HIV/AIDS prevention strategy were being hotly debated by the international academic community, large international funding organizations, and the Ugandan public.When, two months later, I unpacked my bags back home in Chicago, I found that my uncomfortable questions had travelled home with me as well. Ultimately, these questions left me so tangled in self-doubt and uncertainty that I was unable to finish the research, which then led to a sense of unfilled moral responsibility to my Ugandan friends. I feared that I had done more harm than good – primarily by wasting time and resources better spent elsewhere. I have since realized that my experience was not unique amongst learners from North America travelling to the Global South for research or educational opportunities. Hanson et al.So I packed my research notebooks away, and shelved my questions as well, apologizing to my Ugandan friends for leaving the project unfinished. I resolved to focus on local work and imagined that answers would come more easily in that setting. Eventually this work led me to medical school. Medical training promised a method of finding answers, through careful empirical observation and application of the best evidence to the patient problem at hand. I learned to search for randomized controlled trials or, better yet, meta-analyses, to aid my clinical decision-making. Yet, I remained perplexed. I encountered many patients that did not experience meaningful improvements in their health despite evidence-based care. Worse, there were many complex patients for whom no evidence seemed to apply.6My questions did not abate when I began my residency training in psychiatry. I learned many evidence-based interventions for psychotic disorders, but the problem of how to help a chronically homeless, socially isolated person with schizophrenia presenting to the ER was often more complicated than a reductionist meta-analysis could address. It seemed that the empirical evidence, while helpful, was not enough.Somewhat to my surprise, I found the questions that I faced daily in my practice drawing me yet again to global health. I was intrigued by reports of better outcomes of severe and persistent mental illness in lower/middle-income countries,Soon after I began residency, our department was invited by colleagues at Mbarara University of Science & Technology (MUST) in western Uganda to support their emerging postgraduate psychiatry program. This partnership, inspired by the educational collaboration between the psychiatry departments of Addis Ababa University and the University of Toronto,As I became involved in our partnership planning, all of my old questions and misgivings emerged: the fear of doing harm; mistrust of my own motives; concern about reinforcing existing unjust hegemonies. I realized that my questions about global health were intertwined with my questions about psychiatry. I wondered if the questions themselves could be a starting point for further engagement rather than a barrier to participation.I confess that I am still often confused about how to ethically engage in global health work, and find that over time my questions are only deepening and multiplying. The words of RilkeI am partially driven by a sense of moral responsibility and an awareness of my privilege in having received an expensive and lengthy education. I look to the available global health literature for guidance on ways to do more good than harm.In the end, however, the most persistent motivators are the same which fuel my clinical work in Canada: stories and relationships. The rich stories of our Ugandan colleagues, of Ugandans living with mental illness, and of our emerging partnership itself are a salve for the painful uncertainty of unanswered questions.


Raman spectroscopy, scanning electron microscopy, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy and Near Edge X-ray-absorption fine-structure spectroscopy were used to probe the morphological, chemical and microstructural features of the produced material. The method described here is scalable and show a potential for controllable, large-scale fabrication of other graphene derivatives and promotes microwave plasmas as a competitive, green, and cost-effective alternative to presently used chemical methods.One of the greatest challenges in the commercialization of graphene and derivatives is production of high quality material in bulk quantities at low price and in a reproducible manner. The very limited control, or even lack of, over the synthesis process is one of the main problems of conventional approaches. Herein, we present a microwave plasma-enabled scalable route for continuous, large-scale fabrication of free-standing graphene and nitrogen doped graphene sheets. The method’s crucial advantage relies on harnessing unique plasma mechanisms to control the material and energy fluxes of the main building units at the atomic scale. By tailoring the high energy density plasma environment and complementarily applying  Research conducted both on fundamental and application levels of graphene and its derivatives led to an impressive number of scientific reports on the topic e.g., refs 8. Preserving graphene’s large electric double-layer capacitance, N-graphene offers improved wettability and pseudocapacitance7. Lower resistivity and a specific capacitance of 282 Fg−1 at 1 Ag−1 was reported for N-graphene based supercapacitor’s electrode with excellent lifetime , high power capability, and compatibility with flexible substrates9. By influencing atomic charge distribution N atoms create “activate sites” on the graphene scaffold that can participate in catalytic reaction directly, such as the oxygen reducing reaction in fuel cells or anchor metal nanoparticles used in the catalytic reaction7, making N-graphene a potential substitute for expensive metal catalysts. The use of N-doped graphene also improves the rate capabilities of graphene-based electrodes in lithium-ion and lithium-sulfur (Li-S) batteries. It exhibits strong-couple interactions for anchoring sulfur-containing species, thus achieving higher stability and reversibility when compared with pristine graphene11, with an expected specific energy density of ~600 W∙h∙kg−1. Additional reviews on the progress of graphene/N-graphene and graphene based materials for energy storage and conversion devices are presented in14.Carbon-based 2D nanostructures have attracted an outstanding research interest due to their extraordinary properties15. Graphene’s properties vary strongly as a function of its fabrication method and many of graphene and N-graphene’s outstanding properties are drastically dependent on its structural quality, i.e., sp3 carbons, defects, etc. Currently available commercial products (e.g.: https://graphene-supermarket.com/) exhibit a large variability of their physical properties, which substantially affects their usability.One of the greatest challenges on the path towards graphene commercialization is the production of high quality material on a large scale at low cost and in a reproducible manner3, wet chemistry reduction techniques that employ graphite oxide (GO)16, liquid-phase exfoliation17 and chemical vapour deposition (CVD)19. Generally, these methods allow the synthesis of large-area graphene films and micron/submicron sized graphene sheets. CVD allows the production of large area sheets , but its efficiency depends on the quality of the underlying polycrystalline metallic film  and it requires multiple processing steps to obtain transferable sheets19. Furthermore, CVD in the presence of carbon and nitrogen containing precursors  is the most common direct method to prepare N-graphene20 on a nickel or copper substrate at a temperature of 1000 °C. It is worth noting that the presence of metal impurities can affect the real performance of the synthetized material. N-graphene sheets can also be produced via solvothermal methods21 and using an arc discharge with graphitic electrodes22. Post-synthesis methods such as thermal and chemical reduction of GO, along with simultaneous N-doping25 and plasma treatment of graphene27 are also applied to produce N-graphene.Multiple processes have been reported for graphene synthesis, including mechanical exfoliation of natural or synthetic graphite30, N-graphene like films33 on various types of substrates , dielectric  and 2D substrates (h-BN)) at lower temperatures, nearly all of the current plasma techniques involve plasma enhanced chemical vapor deposition (PECVD)34. The presence of energetic electrons in plasma environment boost the ionization, excitation and dissociation processes of carbon precursors at relatively low temperatures36. The plasma systems comprise thermal and chemical reactor functions, as well as catalytic properties. Therefore, plasma assisted growth of nanostructures can be achieved without using catalysts at lower substrate temperatures due to the plasma’s unique ability to activate the surface, thus creating favourable conditions for nucleation and growth processes37. Unique aligned growth of vertical graphene on Ni foam and silicon support has been achieved39. However, PECVD methods typically require low-pressure environments (<10 Torr) with the synthesis and growth of the structures occurring as in a conventional CVD proceed via reactions on the surface which depend on the substrate properties, often with low growth rates34. These methods struggle in producing high quality continuous and uniform graphene/N-graphene films on large areas.While promoting controllable, catalyst-free growth of graphene13. Such self-standing graphene sheets are an alternative to surface-supported horizontal graphene because both surfaces can be utilized, while surface bound counterparts can effectively only use one, and allow the use of at least three open edges in applications. Some analysts foresee that free-standing sheets will dominate the market by 2025 (http://www.idtechex.com). The approaches commonly used to generate free standing graphene sheets are not focused on plasma methods, such as the ones based on colloids or thermal/chemical exfoliation of GO17. While being recognized as a versatile and low-cost method for the preparation of graphene sheets at a relatively large scale, chemical reduction of GO (widely used for batteries/supercapacitors’ electrodes preparation) requires pre-production of GO from natural graphite via Hummers method and relies on the use of toxic chemical agents. The final products also exhibit a moderate electrical conductivity 42. To the best of our knowledge, only a couple of examples in the literature focus on the production of isolated free standing graphene sheets using plasmas at atmospheric pressure conditions. In the work of Dato43 free-standing graphene sheets synthesis using microwave plasma is reported without justification for the process that is simply referred as gas-phase synthesis. Moreover, the synthesized carbon material contains a large amount of amorphous carbon along with some individual graphene sheets as described in a related patent44. Selective synthesis of only high quality free-standing graphene sheets has been reported in our previous works but this was at the expense of quantity, i.e. at very low yield of about 0.01 mg/min47.Many applications  require graphene nanosheets in the form of free-standing graphene/N-graphene sheets consisting of a few atomic monolayersin situ infrared (IR) and ultraviolet (UV) radiation are applied. Besides Hirracane Cyclone system (http://www.acsystems.pt) is used for graphene sheet collection. With the patent describing the method recently issued, it becomes necessary to further investigate the scale-up of the process as well as a direct, single step N-graphene synthesis. The possibility to tune the density and energy of the building units, i.e., C2 radicals, C and N atoms, in the high energy-density plasma environment, which translates in an effective control over the energy and material fluxes towards growing nanostructures in the assembly zone of the reactor, is the most crucial advantage of this method. The plasma properties have been tailored to achieve selective synthesis of free-standing graphene/N-graphene sheets. Scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) as well as X-ray photoelectron spectroscopy (XPS) and Near Edge X-ray-absorption fine-structure (NEXAFS) spectroscopy48 were used to study the morphological, chemical and microstructural features of the synthesized nanostructures. The effect of N doping on the sample structure was studied by Raman and Fourier Transform Infrared (FT-IR) spectroscopies and XPS. Van der Pauw method was also applied to determine the electrical conductivity of the assembled graphene sheets49.Here we intend to provide substantial evidence that microwave plasma technologies, and in particular these based on wave-driven plasmas, can be used as a competitive and disruptive alternative to chemical methods in the controlable fabrication of free-standing graphene sheets. The novelty of the plasma approach used is based on the process scale-up using large-scale configuration of microwave driven discharges and complementary engineering to control morphologies and structural qualities of targeted nanostructures. This is achieved via synergistic tailoring of the plasma environment and the “cold” outlet gas flow, where 46. The microwave power is provided by a 2.45 GHz generator, whose output power was varied from 500 to 2000W. The generated plasma has an extended active zone outside the wave launcher, as seen in Fig. http://www.acsystems.pt). A methanol glass container is connected to the exit of the Hurricane system to capture the smallest particles, capable to escape the system with the gas flow.A waveguide-surfatron based setup was used to create a surface wave (SW) induced microwave plasma at atmospheric pressure conditions 2H5OH, take place. The created radicals become precursors of chain reactions leading to creation of the main building blocks of carbon nanostructures, i.e. carbon atoms and C2 radicals50. It is worthy to note, that the assembling process requires very fast delivering and stacking of building units and equally fast supply of energy sufficient to overcome all the actual potential barriers. In the “mild” plasma zone, which includes also the “near” plasma afterglow, gas temperature drops from about 2000 K to 500 K. Note that the gas temperature changes also in radial direction, following closely a parabolic profile, due to the radial thermal losses. The transport of gas-phase carbon atoms/molecules into a colder zones through the so-called surface of vaporization  of the reactor results in formation of solid carbon nuclei that are gradually withdrawn in the outlet plasma stream where kinetic processes of assembly and growth of “flowing” carbon nanostructures take place. Nitrogen atoms can be incorporated within the carbon nuclei via careful plasma tailoring and their density can be effectively controlled by the percentage of N2 flux in the gas mixture51. Given the fact that the nucleation and growth processes are determined by the interplay of kinetic and thermodynamic factors, the engineering of structural qualities of targeted nanostructures was achieved via synergistic tailoring of the “hot” plasma environment and thermodynamic conditions in the “mild” zone of the plasma reactor. Furthermore, the flowing structures can be a subject of infrared  in the third, post-plasma zone and soft UV irradiation treatment, to “kick-off” the epoxy oxygen groups and sp3 carbons during the flight to a Hirracane Cyclone collecting system.The plasma reactor can be considered as the assemblage of three different zones, as presented in Fig. These three zones can be clearly distinguished in a photograph of the plasma reactor Fig. . The cor2, C) with sufficiently high energy that will then flow towards the “mild” plasma zone, i.e. the assembly zone of the reactor. Once in the larger volume, the departure from the supersaturation conditions of the environment, prone to foster nanosheets assembling, is preserved46. That is, the existence of a smaller amount of nuclei surpassing the critical size for nucleation and growth and conditions in which no supersaturation of species is possible promote the growth of existing nuclei rather than creation of new seeds. The reduction of Gibbs free energy of the system is behind the processes of nucleation and growth. This reduction of the Gibbs free energy of a saturated environment (e.g. plasma + solid phase) is proportional to the gas temperature. Lower gas temperatures, as well as larger densities of building units, foster supersaturation conditions in the environment and thus promote the synthesis of plenty nuclei and amorphous structures are more likely to be created. Contrarily, lower density of nuclei and higher temperatures lead to a deviation from supersaturation conditions, more prone to generate graphitic sheets. As expected, in the larger volume, the density of carbon nuclei decreases, and, as a result, the conditions favor the creation of planar nanostructures. Selective synthesis of graphene sheets only is achievable in a very narrow range of discharge operational conditions for the reactor design considered.Using a cylindrical reactor with expanding radius allows the injection of large power density in the smaller radius section to achieve larger fluxes of building units  and to determine important plasma parameters such as the gas temperature in the discharge “hot” zone, plasma emission spectroscopy has been applied. Optical emission spectroscopy permits to identify the active carbon species in the plasma that are the “building units” for the synthesis and growth of the carbon nanostructures in the mild plasma stream. The detected argon/ethanol plasma emission spectra in the visible range (240–750 nm) are shown in Fig. 2Σ → X2Σ), C2 , C atoms (247.9 nm), the hydrogen Balmer-alpha line Hα (6563 Å) and several Ar lines. These species are formed as a result of ethanol decomposition. As mentioned, the experiments were carried out in the open atmosphere so that some nitrogen containing species also appear. As it can be seen in Fig. 2 molecular band, i.e. the Swan system, with heads band at 473.7, 516.5 and 558.6 nm were detected. The typical C2 emission is generated by the radiative decay of the ext = 2.4 eV), ground state C2 molecules can easily be excited to this level either by electron impact or by three body recombination processes involving carbon (C) and argon (Ar) atoms50. CN species are formed in a three-body recombination reaction: C + N + Ar → CN*(B2Σ) + Ar. The inset in the Fig. To identify the species of interest  and precursor fluxes in the range 15–30 sccm, remains nearly constant in the central “hot” zone with variation from about 4000 K to 3000 K. It should be mentioned, that this temperature is associated with the axis of the “hot” plasma column, since the radiation collected originates mainly from this region. Having a much lower value close to the wall, the gas temperature exhibits a sharp radial gradient. The 2D map of the temperature as detected by an infrared-sensitive thermal imager (FLIR camera) is shown in Fig. The gas temperature in the “hot” plasma zone is a key parameter concerning ethanol decomposition. The collision rates in the plasma at atmospheric pressure are high enough to ensure local thermodynamic equilibrium. Therefore, the rotational temperature can be taken as a good approximation to the gas kinetic temperature. The emission band from the CN species corresponding to violet https://graphene-supermarket.com).The range of operational conditions, i.e. background and precursor flow, microwave power etc. fostering selective synthesis (only sheets) is very limited. Outside of this range different carbon allotropes, i.e. amorphous carbon, 2D sheets etc, at the same time can be synthesized. Pure graphene sheets collected at background argon flow of 1200 sccm, power of 2 kW and ethanol flow of 30 sccm are produced at a rate of 2 mg/min. Figure In Fig. −1 (in λL = 633 nm spectra) up to 1367 cm−1 (in λL = 458 nm spectra). Its relative peak intensity (according to the G line (−1 (in λL = 633 nm spectra) up to 2730 cm−1 (in λL = 458 nm spectra). Its relative peak intensity  the obtained results show that the samples contain graphene sheets with single or few mono-layers of carbon atoms56.Raman spectroscopy is the most efficient way to provide a quick and easy structural and quality characterization of the samples. Typical Raman spectra of the synthesized carbon sheets produced at 2 kW microwave power, 1200 sccm Ar flow and precursor flow 30 sccm are shown in Fig. In addition to the recorded Raman spectra, the powder deposited on TEM grids allow the identification of monolayer and multilayer sheets Fig. . Less tr2. Even though this is not the area ratio, the results give an indication about graphene sheets thicknesses and allow the calculation of the fraction of monolayers. For a set of randomly chosen samples, the percentage of counts of monolayers is about 40%. The investigated samples contained also bilayer, trilayer and higher order multilayer sheets. Approximately 15% of the counted multilayer sheets are composed by 5 to 10 monolayers while 10% contain 10 to 15 monolayers. The lateral size of the flakes is in the range 200–700 nm as seen from Fig. A number of HRTEM images obtained from different randomly chosen graphene samples regions were analyzed. Considering that graphene sheets are crumpled, overlapped and/or entangled, the area ratio between monolayers graphene and the total graphene area is very difficult to determine. Therefore, we counting the number of monolayers/bilayers/multilayers from visual observation of HRTEM images. Analyzing a number of graphene areas (at 5 different positions), we produced the statistics presented in Fig. 2 (284.4 eV), sp3 (285.2 eV) and C-O-C or C-OH (286.3 eV) bonds57, while the wide fourth peak represents π- π* shake up satellite (290.6 eV). Analysis of O 1s line fully support these findings, with the peak being well reproduced as the sum of aliphatic C-O-C bond (532.4 eV)57 and PbO (529.3 eV)58 contributions after taking into consideration different C 1s positions assumed for adventitious carbon . The representative XPS spectrum in Fig. bon Fig. . The latwall = 150 oC) was measured at the reactor’s outlet gas zone  and it is indicative of the amount of IR irradiation that is simultaneously applied . The combination of UV irradiation and modest heating (i.e. 150 oC) significantly increases sp2/sp3 ratio to about 15, while in the absence of both, annealing and UV irradiation, the same ratio is 10. This is probably due to the breaking of sp3 bonds by UV photons with an energy of about 3 eV (300–400 nm). The change in the amount of oxygen is, however, negligible (from 2.3% to 2.2%). At higher precursor fluxes (QEt = 36 sccm) and annealing temperature (260 oC), keeping the other operational parameters constant , the relative amount of oxygen is two times smaller (~1.1%), whilst the sp2/sp3 ratio (15) is preserved. Under these conditions, defects are activated and annealed out by reopening the bonds and letting them reorganize to the perfect “crystalline” structure. This can be achieved only for a limited range of temperatures. Thus, the control/tuning of the thermodynamic conditions in the third (gas phase) zone of the reactor results in additional structural quality improvements of “flowing” graphene sheets.The wall temperature  S/m, which is much higher than the reported values of graphene sheets powder obtained by chemical methods64 and comparable with a reported value65. This could be explained with highly conductive sheets having multiple interfaces between them.To measure the electrical conductivity of the graphene sheets, 0.1 g of the graphene powder was pressed into a disc of 8 mm diameter and 1.2 mm thickness. The electrical conductivity was measured applying Van der Pauw method at room temperature2 in respect to the background argon flow results in a high dissociation degree of N2 molecules therefore generating a significant amount of reactive nitrogen atoms (~1022 m−3) that can be incorporated into the growing carbon lattice structure. However, the dependence of N atoms density on gas mixture composition is to be noticed51. The rate of pure N-graphene sheets collected at a background argon flow of 1200 sccm, power of 2 kW, ethanol flow of 15 sccm and N2 flow of 5 sccm was about 0.5 mg/min. In the next Fig. In order to test the direct route for N-graphene synthesis as well as to demonstrate potential of the method, nitrogen gas flow (5–10 sccm) has been added to the ethanol flow. A small percentage of the N2 flow. Laser emitting at 633 nm has been used. As discussed, the three typical graphene peaks are attributed to the G band at ~1583 cm−1, 2D band at ~2658 cm−1, and D band at about ~1332 cm−1. The D band, i.e., “disorder” band, is related to a series of defects: bond-angle and bond-length disorder as well as hybridization that are caused by hetero atoms, i.e., nitrogen doping. The presented spectra are averaged over different locations. The most notable spectral change is the increase in the D band to G-band intensity ratio when N2 is included in the gas mixture. As seen from the figure, the D-peak intensity rises with the introduction of N2 gas. The nitrogen atoms constitute defects within the graphene lattice and consequently contribute to the D band intensity, yielding the observed increase of the D band to G-band intensity ratio.Furthermore, Fig. 2 carbon atoms bound to carbon or hydrogen atoms. The position of this peak was reported in our previous studies on free-standing graphene sheets synthesis induced by microwave plasma50 and was obtained with no need of binding energy (BE) correction since no charge accumulation exists in this sample. Peaks centered at higher BE are assigned to sp3carbon atoms bound to other carbons or hydrogen atoms (285.0 eV), carbon bound to nitrogen or singly bound to oxygen (peaks at 285.7 and 286.6 eV), carbon in epoxide and/or carbonyl groups (287.6 eV) and carbon in carboxylate groups (289.0 eV)57. The spectral features detected at BE >290 eV are energy losses due to π-π* excitations typical of carbonaceous systems with delocalized π electrons such as graphene. The peak centered at 289 eV may be overlapping some of these features. Peaks found around 286 eV, which are, in a first approach, identified as C-N and C-O, are most probably superposing some aliphatic carbon atoms (C) bound to other carbons (C-C<) from more electronegative vicinities, for instance C-COO− or C-(C = O) 57. XPS confirms undoubtedly the doping of graphene by nitrogen atoms. The N 1s region was fitted with two peaks, one centered at a binding energy equal to 399.4 ± 0.2 eV assigned mainly to pyridinic nitrogen57, and a minor peak centered at 401.2 ± 0.2 eV, corresponding to graphitic nitrogen66. Pyrrolic nitrogen, which is likely to exist in these structures, must be close to 399.9 eV67, i.e. between the two fitted components. The shoulder found around 398.6 eV (not fitted) can be attributed also to pyridinic nitrogen in a slightly different chemical environment than the N 1s main peak , and 1550 cm−1 in the spectrum of N-graphene, can be attributed to C-N stretching vibrations and N–H band, respectively72. Additionally, the most intense peak at 1390 cm−1 (C–O–H stretching vibrations) is detected along with a wider band at around 3000 cm−1 (C-H)76.FT-IR analysis was additionally employed to investigate the chemical bonding differences when graphene sheets are N doped Fig. 69. The Fsee Fig. 70, 71. IProcessing graphene sheets is a delicate task since the charged sheets can reply in an ambiguous way during their treatment. Two photos of graphene sheets suspended on a glass substrate are shown in Fig. −1 and slight decrease of the G and 2D peaks. Moreover, a small frequency shift of the D-line of about 2 cm−1 towards smaller wavenumbers along with widening of the line is observed. The enhancement of the D-line, both absolute and relative to the G and 2D lines, may be simply related to the cutting-up of graphene sheets. However, the process may also force the parallel orientation of the sheets in respect to the supporting dielectric surface and lead to the formation of a network of electrically-oriented connected graphene sheets. In this case, the reduced G and 2D peak intensity could be linked to an increase in the conductivity of the whole layer and a corresponding decrease of the scattering volume.The corresponding Raman spectra are shown in Fig. 77, is observed, and second dominant feature, the double-structured resonance, is observed at around 292 eV, corresponding to C 1s - σ*. This double resonance comes from excitonic , from defects in the top most graphene layers78 or O-containing functional groups, resulting from oxidation (as it is most probably in our case).To test this hypothesis, traces of graphene powder were mechanically smeared over silicon substrate (simple Si wafer as used for microelectronics) and analyzed by means of NEXAFS spectroscopy. Measurements were performed at different beam incident angles (θ) relative to the substrate surface as shown in Fig. 79. There is a clear angular dependence of the C1s → π* resonance peak at 285.2 eV, with its intensity increasing towards the plateau regions and towards the intensity of excitons, as the incident angle relative to the supporting surface decreases. The differences are strongly correlated with orientation of the bonds relative to the surface and therefore support the existence of self-organization of the graphene sheets with respect to the surface of the substrate. In graphene sheets, the π bond deriving from pz orbitals is oriented perpendicular to graphene plane (out-of-plane) and the σ bond is oriented along the intermolecular bonding axes (in-plane). Thus, as the angle between incident beam and surface increases out-of-plane excitation, i.e., π* resonance becomes weaker, and the ratio between σ* and π* resonances at 291.7 eV and 285.1 eV increases, as it is expected for highly oriented structures61 with a preferential orientation of the graphene sheets parallel to the substrate. It is to be noted that this alignment does not disappear over time or after functionalization by means of for example ammonia or nitrogen plasma.NEXAFS is a powerful tool to observe molecular orientations due to the high linearly polarized nature of the synchrotron radiation, specifically for 1s → 2p excitations, where the product of the orbital symmetry axis and the electric dipole leads to simple intensity correlations between incidence angle and molecular orientation of the radiation2/sp3 carbons ratio (~15) has been achieved. For a randomly chosen samples, the percentage of counts of monolayers in all graphene sheets is about 40%. The method is rapid, highly cost-efficient and environmentally friendly, since it does not require the use of catalysts and noxious chemicals. It is also customizable and versatile, allowing the synthesis of different types of 2D nanostructures  in the same reactor. Furthermore, the high energy density of the generated plasma environment allows the use of gaseous, liquid or solid precursors. Moreover, self-organisation of the graphene sheets in respect to the supporting surface after mechanical treatment was observed.In summary, microwave-driven plasmas were successfully applied for the first time in the selective synthesis of high quality graphene and N-graphene sheets via a single step process at atmospheric pressure. A high level of control over oxygen functionalities and spTo this end, our key enabling technology provides a rapid, single-step, cost-efficient and environmentally friendly method for selective synthesis of tailored graphene/N-graphene sheets at high yield and at atmospheric ambient. It is non-destructive, free of toxic chemicals, metal catalysts and substrates, and allows the use of carbon precursors in solid, liquid or gas state. The main advantage of our approach is the achivement of a very high and extremely controllable energy density in the plasma reactor, which allows effective control over the energy and material fluxes towards growing nanostructures at the atomic level via proper reactor design and tailoring of the plasma environment in a synergistic way. The ability to control the amount and localization of energy and matter delivered from the plasma bulk to the developing nano-structures is the key to achieve the desired morphological, structural and functional properties of targeted materials.Further scale-up along with better understanding of nucleation/growth processes in a multiphase environment will certainly lead to a substantial increase of the yield and a reduction of the production costs. Besides, mastering will be focused on reducing the multilayer (5–10 single layers) sheets percentage below 1%, controllable increasing of N-doping and optimization of collecting system for extraction of nanoparticles with dimensions less than 20 nm, i.e. graphene quantum dots. The use of large-scale configurations of wave-driven discharges paves the way for further scale-up and a higher level of customization. The exclusive plasma mechanisms that rule the distributions of energy and matter at atomic scale endow the method significant potential for the synthesis of other 2D materials . The method has also a potential for producing unique graphene-metal nanocomposities with well-designed nano-architecture. The expanding pool of prospect applications will most certainly continue to drive forward the research in this cutting-edge field.Supplemetary Material


During the treatment, NH3 plasma radicals not only flattened the Cu substrate such that the root-mean-square roughness value gradually decreased from 51.9 nm to 15.5 nm but also enhanced the nitrogen content in the Cu substrate. The smooth surface of copper enables good control of graphene growth and the decoupling of height fluctuations and ripple effects, which compensate for the Coulomb scattering by nitrogen incorporation. On the other hand, the nitrogen atoms on the pre-treated Cu surface enable nitrogen incorporation with low defect densities, causing less damage to the graphene structure during the process. Most incorporated nitrogen atoms are found in the pyrrolic configuration, with the nitrogen fraction ranging from 1.64% to 3.05%, while the samples exhibit low defect densities, as revealed by Raman spectroscopy. In the top-gated graphene transistor measurement, N-doped graphene exhibits n-type behavior, and the obtained carrier mobilities are greater than 1100 cm2·V−1·s−1. In this study, an efficient and minimally damaging n-doping approach was proposed for graphene nanoelectronic applications.N-doped graphene with low intrinsic defect densities was obtained by combining a solid source doping technique and chemical vapor deposition (CVD). The solid source for N-doping was embedded into the copper substrate by NH To obtain these promising properties and applications, researchers have continuously endeavored to synthesis N-doped graphene without damaging the graphene crystal structure and deteriorating its carrier mobility.Graphene is a promising next generation electronic material, and its electronic properties can be modified by the substitutional doping of heteroatoms  pos3 plasma ,15,16,17lasma [3  and (ii)4/H2/NH3  and pyri4/H2/NH3  are simu4/H2/NH3 ,21. Ther3, covalent C–N bonds form at intrinsic defects, vacancies or sites with weak C–C bonds in which the carbon atoms can be easily replaced by nitrogen atoms. However, considering the mismatch of the thermal expansion coefficients between graphene and the underlying substrate and the electrical disturbance induced by the closer contact between graphene and the underlying substrate after high-temperature post-annealing, extensive research must be devoted to its transport performance after high-temperature post-annealing [x group-based etching effect that induces increased defect densities [x groups was limited during the growth process [To obtain N-doped graphene with low defect densities, Cory et al. determined the ion energy window for nitrogen doping in the graphene lattice with the balance between the substitutional reaction efficiency and the vacancies or defect formation . Moreovennealing . For theensities . The sec process ,25. The 3 plasma at room temperature, and atomic force microscopy (AFM) was performed to examine the surface morphology variation. This process embeds nitrogen atoms into the Cu substrate, which are subsequently used as the solid-state doping source. Then, N-doped graphene was grown by CVD on the nitrogen-doped copper substrate. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were used to analyze the bonding and lattice properties of N-doped graphene. Top-gated graphene field-effect transistors (GFETs) were fabricated using N-doped graphene as the channel, and 1100 cm2·V−1·s−1 carrier mobilities were achieved without carrier mobility degradation, which is comparable to the corresponding pristine graphene.To overcome the bottleneck in the direct synthesis of N-doped graphene in the conventional chemical vapor deposition (CVD) process and to obtain high transport properties for N-doped graphene, a solid state doping technique was introduced in this study. The solid state doping technique is broadly used for efficient and uniform doping with low amounts of structural defects in low dimensional materials, such as nanowires and conducting polymers ,27,28; h3 pretreatment and N-doped graphene synthesis. As illustrated in 3 plasma treatment, nitrogen radicals are embedded into the Cu surface, and N–Cu bonds are formed after the surface oxidation and after the organic residues were reduced by hydrogen radicals in the NH3 plasma [3 plasma treatment [3 plasma pretreatment were loaded into a low-pressure chemical vapor deposition (LPCVD) chamber for graphene growth separately, as shown in 3 plasma ,31. Mean3 plasma . The crireatment ,31, and reatment . Then, t3 plasma pretreatment times, acquired in a 5 μm × 5 μm area prior to graphene CVD growth, are shown in 3 plasma pretreatment exhibits a periodically corrugated structure with a maximum height of 202.4 nm and a surface roughness with a root mean square (RMS) value of 51.9 nm. Samples with an NH3 plasma pretreatment of 1, 3, and 5 min were labelled as NG1, NG3, and NG5, respectively. The Cu surface morphology becomes less rough as the NH3 plasma pretreatment time increases. For the Cu with a 1, 3, and 5 min plasma treatment, the height of the Cu surface corrugation structure and surface roughness decrease to 46.2 nm and 15.5 nm, respectively. The surface-flattening effect of the plasma treatment is comparable to the effects of other Cu pretreatment processes, such as electrochemical polishing or pre-annealing [2 substrate for further measurement and analysis .The distinct apparent morphologies of these Cu foil samples with different NHnnealing . After t2 substrates. The deconvoluted C1s spectra of pristine graphene and NG1 are shown in 3 plasma pretreatment on the Cu surface. Note that the hydrogen annealing process may remove some surface nitrogen atoms. However, based on these results, the amount of surface nitrogen is sufficient for the graphene dopants. As shown in 3 plasma. According to the recent report on atomic-level scanning tunneling microscopy (STM) analysis, nitrogen atoms are preferentially located at the graphene grain boundaries, while there is no indication of nitrogen in the atomically flat graphene regions [To investigate the bonding configurations of N atoms in the graphene lattice, XPS was performed on pristine graphene and N-doped graphene samples after transfer onto the SiO regions . Thus, t−1 range as shown in −1 (defect mode), (b) a G peak at 1586 cm−1 , (c) a D’ peak at 1620 cm−1 (disorder mode), and (d) a 2D peak at 2707 cm−1 (two-phonon vibration mode). The locations of the G peak and the 2D peak of pristine graphene and N-doped graphene are shown in −1, respectively, and a downshift of the 2D peak from 2707 to 2701, 2701, and 2701 cm−1, respectively, indicating n-type doping from pristine graphene to NG1, NG3, and NG5 [I2D/IG ratios for both pristine graphene and N-doped graphene were greater than 2, as shown in ID/IG ratios of pristine graphene and N-doped graphene range from 0.168 to 0.171, 0.173, and 0.191. Based on the ID/IG ratio, the defect density for each sample can be calculated using the average defect distances based on the Tuinstra-Koenig (TK) relation of [−10)λ4(ID/IG)−1 as 71.4 nm (pristine graphene), 70.1 nm (NG1), 69.3 nm (NG3), and 62,8 nm (NG5). The slightly increasing trend of defect mode intensity is consistent with our XPS survey because more structural defects are induced by pyrrolic N than graphitic N. Moreover, the D’ peak  is weak compared to the G peak, indicating that a relatively low disorder accompanies the nitrogen doping [2-hybridized carbon neighbors with adjacent carbon vacancies. In addition, nitrogen incorporation may influence and even interact with the defect formation process during the kinetic growth of graphene because the nitrogen atoms are predominantly distributed at grain boundary regions where defects and vacancies are also easily formed to release the interfacial stress originating from the lattice mismatch between Cu and graphene [4 gas in the processes of gas phase deposition, molecular decomposition, and long-range Cu surface kinetic diffusion.N-doped graphene samples were also analyzed using their Raman spectra in the 1200–3000 cm and NG5 . The I2Dn doping . The incgraphene . The int2 substrates. The graphene samples without and with NH3 plasma in 1, 3, and 5 min treatments were also examined by Raman spectroscopy, and the Raman spectra are shown in 3 plasma treatments, strong lattice structural damage was found in which splitting was observed for the D and G peaks, and the 2D peak showed an obvious decrease. For the graphene samples under a 5 min NH3 plasma treatment, the typical vibration modes of graphene  almost disappeared. During the NH3 plasma treatment directly on graphene, high additional plasma energy after nitrogen displacement remained in the graphene lattice and transferred into in-plane collisions [For comparison, the same plasma treatment process was directly applied to the graphene samples after the transfer onto SiOllisions . Thus, tIdVd curves in IdVg measurement results and the capacitance of graphene transistors shown in Cg(Vg − VDirac)/e), where Cg is the gate dielectric capacitance [2·v−1·s−1 (at 3 × 1012/cm2), respectively. The electron mobilities of pristine graphene, NG1, NG3, and NG5 were 1060, 1061, 1121, and 1151 cm2·v−1·s−1 (at 3 × 1012/cm2), respectively. The opposite ambipolar mobility behaviors of pristine graphene and N-doped graphene can be clarified using Novikov’s calculation showing that attractive scattering is dominant relative to the repulsive scattering for Dirac fermions in graphene [3 plasma treatments was also fabricated, and no Id–Vg curves were obtained because the graphene was severely damaged during the direct high-energy plasma treatment.Finally, to measure the electrical properties, a top-gated GFET was fabricated, as shown in acitance . As showgraphene . Therefographene . For comID/IG ratios, Dirac points, and carrier mobilities. Starting with exfoliated graphene, followed by ion implantation and high-temperature NH3 annealing, N-doped graphene exhibits a high mobility, reaching 6000 cm2·V−1·s−1. However, this post-annealing doping method for pristine graphene results in a relatively low doping concentration, possibly due to the low probability of graphene interface hybridization between nitrogen atoms and the continuous atomic lattice structure [ID/IG ratio to values ranging from 0.3 to 2 . Both nitrogen incorporation  [2·v−1·s−1 (CVD with a mixed gas precursor) [2·v−1·s−1 (plasma enhanced CVD-grown) [2·v−1·s−1  [2·v−1·s−1 (substrate doping) [2 as the gate dielectric, which is more compatible with the integrated circuit (IC) architectures and operates in the low gate voltage mode, but the trade-off is the series resistance in the access region.A brief comparison of the N-doped graphene with different CVD synthesis approaches is presented in tructure . For CVDttering)  and its ttering)  degrade ttering) . Recentlttering) . Comparettering) . In thisttering) ,34, compecursor) , 100–400D-grown) , 365 cm2g on Cu) , and 105 doping) . Note th3 plasma pretreatment of the Cu foil catalyst was proposed. Approximately 80% of the nitrogen dopant atoms in the graphene lattice are present in the pyrrolic configuration, and the nitrogen concentration was negatively correlated with Cu surface roughness. For synthesis on the pre-treated Cu catalyst, N-doping does not induce a high density of defects or disorder in the graphene lattice, as revealed by the Raman spectra. The electrical transport measurement shows that N-doping effectively modulates the electrical properties of graphene to n-type behavior with high conductivity and carrier mobility. This study provides a low defect density approach for N-doped graphene synthesis and significantly advances the modulation and improvement of the performance of graphene in nanoelectronic applications.In summary, a CVD synthesis process for low defect density N-doped graphene based on the NH3 flowed into the chamber at 20 sccm while the pressure was maintained at 0.5 Torr, and radio frequency power of 100 W was applied to generate the NH3 plasma. The degree of plasma treatment of each sample was adjusted by controlling the exposure time (from 1 min to 5 min) after the plasma was ignited. The Cu surface morphology was scanned using an Innova AFM  equipped with an Olympus AC160TS probe . The AFM tip was operated in tapping mode and set to map a 5 × 5 μm2 area across the samples.Cu foil  was loaded into the plasma-enhanced CVD (PECVD) system, and the chamber was evacuated to 2 mTorr at room temperature. NH3 plasma pretreatment were loaded into a low-pressure tube furnace separately [2 flow rate of 50 standard cubic centimeters per minute  to remove the native Cu oxide. Then, 20 sccm CH4 was introduced to grow graphene on the Cu foils for 20 min. After the graphene synthesis was complete, the furnace was cooled to room temperature at a rate of ~5 °C/s. A layer of polymethyl methacrylate (PMMA) was spin-coated onto the graphene on the Cu surface. The films were then treated with a Cu etchant and rinsed with deionized (DI) water before they were transferred to the 300 nm SiO2/Si substrates. The PMMA support layer was removed from each film using acetone, and the remaining graphene films were rinsed with isopropyl alcohol.Copper foils with and without NHparately . The gro−1 was used as the calibration signal for each sample. For Raman probing, each sample was exposed for 3 s, which was repeated 3 times to obtain an average value and to eliminate unintentional cosmic ray effects. The chemical configurations were determined using an X-ray photoelectron spectrometer  with an Auger electron beam gun (LEG 300-F electron column). The energies were calibrated relative to the C1s peak to eliminate the charge distribution of the sample during analysis.Raman spectra were collected using a RAMaker confocal Raman microscope system . The Raman peak of Si at 520 cm2/Si substrate, 50 nm of Ni was deposited as the source and drain (100 μm × 100 μm). The graphene channel (20 µm in width and 50 µm in length) was patterned using lithography methods followed by an O2 plasma etching step; finally, 1500 nm of Al was deposited followed by a 1.5-nm Al deposition as a buffer layer. Both of these Al layers were exposed to air for more than 8 h to form a thin AlOx dielectric layer between the graphene channel and the Al gate [After transferring the graphene onto the 300 nm SiO Al gate . Graphen


The effects of graphene stacking are investigated by comparing the results of methane adsorption energy, electronic performance, and the doping feasibility of five dopants  via first-principles theory. Both zigzag and armchair graphenes are considered. It is found that the zigzag graphene with Bernal stacking has the largest adsorption energy on methane, while the armchair graphene with Order stacking is opposite. In addition, both the Order and Bernal stacked graphenes possess a positive linear relationship between adsorption energy and layer number. Furthermore, they always have larger adsorption energy in zigzag graphene. For electronic properties, the results show that the stacking effects on band gap are significant, but it does not cause big changes to band structure and density of states. In the comparison of distance, the average interlamellar spacing of the Order stacked graphene is the largest. Moreover, the adsorption effect is the result of the interactions between graphene and methane combined with the change of graphene’s structure. Lastly, the armchair graphene with Order stacking possesses the lowest formation energy in these five dopants. It could be the best choice for doping to improve the methane adsorption. Formaterial . Yang etAr, etc. . HoweverGenerally, graphene can be stacked in various ways to form different multilayered graphenes (MLG), which have different properties. Typical graphene stacking includes Order, Bernal (AB), and Rhombus stacking (ABC) ,16,17. IWhile the effects of graphene stacking on MLG’s properties had been well acknowledged, a comparative study of different ways of stacking is still lacking. Although the stacking effects on the adsorption, the band gap, and the doping of MLG are considered important, they are not yet understood. For instance, some research has shown that the stacking possessed a nonnegligible influence on electrical performance, but the changes are really diverse and messy. Furthermore, the studies of graphene stacking effects on methane adsorption are still lacking. In addition, doping is good for methane adsorption, such as the dopants of B, N, Al, Si, and P. The doping of Al and P would lead to a dramatic increase in methane adsorption energy, but some research revealed that the doping of Al and P in graphene is difficult. However, there are no reports on the graphene stacking effects on doping. Adsorption energy is an important and direct criterion to evaluate the adsorption effect. Band gap can partly reflect the electrical properties of graphene. In addition, doping is often used as a means to improve the performance of the target material. However, the studies of graphene stacking in these aspects are still lacking, which is exactly what we need to know. This is why we investigate the effects of graphene stacking on graphene’s properties, methane adsorption, and doping.In this paper, the effects of graphene stacking on methane adsorption and doping are first reported using the first-principles theory. The DFT-D has demonstrated that it is more appropriate for the multilayered graphene calculations than DFT. Both the zigzag and armchair graphenes are considered. First, the effect of graphene stacking on the adsorption is studied. It is found that the AB-stacked graphene has the highest adsorption energy on methane, while the adsorption effect of the ABC-stacked graphene is the worst. Second, the changes of band gap related to different number layers are compared. The effect of stacking on band gap is significant, but it does not cause big changes to band structure and density of states. Then, the distances of methane–graphene systems are also analyzed. In the comparison of distances, the average interlamellar spacing of the Order stacked graphene is the largest. Moreover, the adsorption effect is the result of the interactions between graphene and methane combined with the change of graphene’s structure. Third, the effect of graphene stacking on the doping is discussed. The formation energies of the five dopants of B, N, Al, Si, and P in different stacked MLG are reported. The armchair graphene with Order stacking possesses the lowest formation energy in these five dopants, which is the best choice for doping to improve the methane adsorption. 3 code base on density functional theory (DFT) [RI and r are the coordinates of the atomic nucleus and the electron, respectively. Equations (1) and (2) describe a time-independent and solvable Schrödinger equation, where the first two terms of Equation (2) represent the kinetic energy of atoms and electrons, respectively, and the latter three items denote the system Coulomb exclusion potential. The generalized gradient approximation (GGA) and local density approximation (LDA) are the exchange–correlation functionals commonly used in quantum mechanics calculation. GGA was chosen in this theoretical study and can be described in Equation (3):−6 eV, and the MAX force is 0.002 Ha/Å.All the calculations were performed in Dmolry (DFT) , which iry (DFT) ,27. DFT ry (DFT) . The Schrst–Pack  k-point rst–Pack  smearingBecause of the sheet structure of graphene, the multilayered graphene should be restricted by Van der Waals force. However, DFT calculations cannot estimate the weak interaction, which would lead to the poor calculation accuracy of methane adsorption. In order to solve this problem, the dispersion correction in DFT (DFT-D) was introduced in our work .C6,ijRij−6 term, where C6,ij is a material-specific, so-called dispersion coefficient between any atom pair i and j at distance Rij. At short range, the long-range expression is matched to the DFT potential by multiplication with a damping function f , which reduces the additional dispersion contribution to zero, subject to a cutoff defined by some suitably calculated combination Rij° of the vdW radii of the atom pair. The dispersion-corrected exchange-correlation functional is then formed by simply adding the correction potential to the ordinary DFT exchange-correlation functional. As C6,ij coefficients are additive, the dispersion-corrected total energy Etot can be written as:EDFT is the standard DFT total energy and the sums go over all N atoms in the system.In the semiempirical dispersion-correction approach, the missing dispersion contribution to the interatomic interaction is approximated by a simple isotropic potential. At long range, this potential is given by the Much research has demonstrated that DFT-D can improve the calculation accuracy. Dappe et al. found that the interlayer equilibrium distance of graphene calculated by DFT-D is more closely with the experimental evidence . Vanin eEa) is defined in Equation (5):Egraphene + gas molecule is the total energy of graphene adsorbed system, Egraphene is the energy of pristine graphene, and Egas molecule is the energy of the gas molecule. Generally speaking, adsorption energy can be a criterion to evaluate the adsorption effect, namely, the value of adsorption energy is bigger, and the adsorption effect is better. The adsorption energy (The adsorption energy and the distances including the average interlamellar spacing of optimized graphene (d1) and optimized methane-graphene (d2) and the distance between graphene and methane of optimized methane-graphene (d3) are all calculated by DFT and DFT-D (PBE-grimme) and are listed in It is well known that the adsorption site has some important impacts on adsorption effects. Therefore, different adsorption models were firstly discussed in our calculations. In Taking an example of zigzag graphene, Graphene has a large surface area and porosity. As a result, it has excellent adsorption properties of gases and incomparable advantages in the production of gas sensors. Graphene has a better adsorption effect on methane compared with other gases. We know that the natural graphene often has many stacked ways, such as Order, AB, and ABC, so what are the effects of stacking on graphene-based methane sensor? A comprehensive research of stacking effects on methane adsorption of graphene is learned in this section. Model 2 is selected as the reference standard. Graphene adsorption is closely related to the change of electronic properties. In this section, the electronic properties of the graphene with different stacking are analyzed. As shown in To understand the mechanism of stacking effect on methane adsorption, some distances in six types of stacked graphene are recorded in Eformation is the formation energy, Egraphene is the energy of pristine graphene, and EC and Ed are the chemical potentials determining carbon and doping atoms. The three-layered graphene and the atoms of B, N, Al, Si, and P are chosen to calculate their formation energy, as shown in Previous studies have found that the doping in graphene is helpful to the adsorption of gases ,38, suchObviously, the graphene stacking methods have great influence on the properties of graphene, methane adsorption, and doping. As for methane adsorption and doping, AB stacked zigzag graphene with four layers has the best adsorption capacity, while Order stacked armchair graphene has the better doping performance. In addition, the armchair graphene possesses better electrical properties, but it has larger layer spacing than that of zigzag graphene. Therefore, the type of graphene should be selected according to the purposes and requirements of the sensing material.In this paper, the effects of Order, AB, and ABC stacking on the properties of graphene were investigated comprehensively via first-principles theory. Comparing the adsorption sites of methane, model 2 is the best choice for methane adsorption. The adsorption energy of the Z-AB graphene is the largest among the six types of stacked graphenes; the maximum value is 0.357 eV. Both Z-Order and Z-AB stackings are good for the methane adsorption. As for the band gap, it would be smaller in armchair graphene, which means that the electronic performance of armchair graphene is better than zigzag graphene. The stacking does not cause big changes to band structure and DOS; it only makes some changes in the band gap. In the comparison of distance, both d1 and d2 of the Order stacked graphene are the largest among these three types of stacked graphenes. The distances of d1 and d2 in the AB-stacked graphene are the smallest. The distances of d1 and d2 in the ABC-stacked graphene are close to the AB-stacked graphene. The adsorption effect is the result of the interactions between graphene and methane combined with the change of graphene’s structure. The decrease of d2 can strengthen the adsorption effect of methane. Lastly, A-Order stacking processes the lowest formation energy in all types of stacked graphene. It is the best graphene structure for doping. In addition, it is also the best choice for methane adsorption. In summary, model 2 and Z-AB stacking are the best choices for the methane adsorption. When doping is needed to improve the methane adsorption effect, the A-Order stacking would be a good choice. These discoveries will contribute to the design and production of the graphene-based methane sensor.


From Raman spectroscopy, it was concluded that the PLD process also induced defects in graphene. Compared to unmodified graphene, the obtained chemiresistive sensor showed considerable improvement of sensing ammonia at room temperature. In addition, the response time, sensitivity and reversibility were essentially enhanced due to graphene functionalisation by laser deposited V2O5. This can be explained by an increased surface density of gas adsorption sites introduced by high energy atoms in laser ablation plasma and formation of nanophase boundaries between deposited V2O5 and graphene.Graphene has been recognized as a promising gas sensing material. The response of graphene-based sensors can be radically improved by introducing defects in graphene using, for example, metal or metal oxide nanoparticles. We have functionalised CVD grown, single-layer graphene by applying pulsed laser deposition (PLD) of V Graphene, being a thin (semi)conducting material, is a promising gas sensing system. Highly sensitive response, down to single molecule resolution, has been demonstrated with graphene-based devices under laboratory conditions –3. Howev2O5) – a good oxygen transfer catalyst that is  stable in air and vacuum [Transition metal oxides constitute an important class of catalysts and photosensitizers. Apart from the very first and last 3d elements, scandium and zinc, the rest of the metals possess several oxidation states. The presence of several stable oxidation states serves as a basis of catalytic activity in redox reactions and is most noticeable for vanadium, chromium, and manganese. In particular, vanadium has the highest oxidation state in vanadium pentoxide  sensing properties of chemical vapour deposition (CVD) grown, single-layer graphene in our previous work, using ZrO2 and Ag for functionalisation [PLD is a highly versatile method for relatively well-controlled preparation of thin films, and many advanced composite materials have been produced for diverse applications . The poslisation .2O5. The amount and chemical state of vanadium oxide on graphene was characterized by X-ray photoelectron spectroscopy and X-ray fluorescence. The impact of the PLD process on graphene defect structures was investigated using Raman spectroscopy. Based on the electrical conductivity modulation, the room temperature gas sensing properties of the manufactured sensor structure towards ammonia (NH3) and (for comparison) nitrogen dioxide (NO2) gases were investigated.In the present work, we demonstrate functionalisation of single-layer CVD graphene with a few layers of laser deposited V−1 and ≈2690 cm−1, and have full-widths at half-maximum of 11 cm−1 and 29 cm−1, respectively. These characteristics, together with the ratio of G to 2D band peak intensities of approximately 1:3, correspond to single-layer graphene [−1 indicates extremely low defect density [2O5 onto graphene , the defect-related D and D’ bands emerge in the Raman spectrum, and at the same time, the G and 2D bands decrease in intensity  lattice plane layers is about 2.5.The amount of vanadium that was deposited on the sensor substrate was evaluated using X-ray fluorescence (XRF) analysis by measuring the amount of vanadium in the film deposited onto a fused quartz substrate under the identical PLD procedure. According to XRF, the mass thickness of vanadium on the substrate was 0.11 µg/cm38 g/cm3 , an esti1/2 region is not usable for the analysis, owing to the overlap with the Mg Kβ satellite with O 1s peak. The main component of V 2p3/2 at a binding energy of 517.4 eV was assigned to V5+, whereas a faint component at 513.3 eV belongs to V2+/1+. The O 1s peak component at 530.9 eV was identified as oxygen bound to vanadium, in agreement with Biesinger’s XPS measurements on vanadium oxide [2 in the silica substrate that can possibly overlap with the C–O component of (partly oxidized) graphene at the same binding energy (533.1 eV) [2O5, with an average thickness of about 0.6 nm. Among other vanadium oxides, V2O5 is a relatively stable compound that can be easily deposited by the PLD method [The oxidation state of vanadium was determined by means of X-ray photoelectron spectroscopy (XPS). um oxide . The mai33.1 eV) . Thus, aD method .2 and NH3. All the gas measurements in this work were recorded under continuous excitation with ultraviolet (UV) light (λ = 365 nm) at room temperature (RT). Illumination by UV light can enhance the sensing performance of graphene-based gas sensors. An increased gas response and fast recovery under UV irradiation has been demonstrated for pristine and functionalised graphene-based sensors, possibly as a result of cleaning the surface of interfering or passivating gases [ng gases –26. The 2O5, the conductivity of the sensor decreases by a factor of 30. According to the Raman spectra  into the test chamber. Graphene is typically a p-type conductor under ambient conditions due to chemical doping by adsorbed oxygen and water molecules [3 acts as a hole acceptor and NO2 as a hole donor [2O5 functionalised sensor to 100 ppm CO. Although the sensor is sensitive to CO and a ≈7% decrease in conductivity is observed, the sensitivity to CO is much lower than to NH3.As compared to the pristine sensor, the response to both gases is clearly improved after the functionalisation by PLD. The response to 1 ppm NOolecules –28. Bear2O5 under NH3 gas at different gas concentrations. The functionalisation with V2O5 clearly improves the performance of the sensor. The response and recovery times of pristine and functionalised sensors for 8 ppm NH3 gas were determined by fitting the time curves with the suitable functions. Either single or double exponential functions were used in the following form:S(t) is the relative change of conductance and t0 is the initial time when the respective stepwise change in gas composition was introduced. The characteristic times t1 and t2 are related to the rate constants of adsorption and desorption of gas molecules onto pristine or V2O5-functionalised graphene. In principle, a single exponential type of response  for gases at room temperature. The introduction of defects and dopant atoms into graphene can drastically increase both the adsorption of pollutant molecules and the influence of gas adsorption on the electronic properties of graphene [Ea) of an NH3 molecule on regular graphene is relatively small , but 0.11 eV ,31) or i 0.11 eV ,30,32). not used ,31. Howen et al.  for carbn et al. . The bense of Ag .2 molecules than to any other gas, including NH3. The binding energy is large and intermolecular charge transfer can occur when NO2 adsorbs on either pristine, defective, or doped graphene [2 and NH3 responses of our devices, and also of several other sensors reported in the literature, all based on a single-layer graphene. Only qualitative discussion is possible, because in most cases, the gas concentrations differ. It can be seen that most sensors are more responsive to NO2, as compared to NH3. However, concerning the relative responses, the results of Yavari et al. [2O5-functionalised sensor. According to the measured Raman spectra, the CVD grown graphene used in these experiments contains a relatively large concentration of defects of unspecified nature [3 sensing is well illustrated by Lee et al. [3 gas increased by 600% after creation of defects in the pristine defect-free graphene by reactive ion etching. At the same time, the response to NO2 gas increased only by 33% [2 in our previous work [2 than to 20 ppm NH3  NOgraphene ,31. Tabli et al.  are compd nature . The rolous work  showed aThe introduction of dopants and clusters of atoms into graphene strongly increases the interaction with adsorbing molecules, due to an increased charge transfer and formation of chemical bonds between the dopant and adsorbate ,32. More2O5 cluster growth at the defect site as the PLD process continues. The V2O5 clusters are probably chemically bound to the defect sites of the graphene sheet, yielding a strong and stable contact between the two phases. Chemical bonding is accompanied by charge transfer between V2O5 clusters and graphene. A similar situation was modelled previously by Lim and Wilcox [2 molecule onto a platinum nanocluster which was chemically bound to graphene resulted in a large influence on the charge density distribution of the system [According to the Raman analysis shown in d Wilcox  where lae system .2O5 is known for its catalytic properties and as a good NH3 adsorber [3 gas sensing properties of V2O5 thin films and nanofibers have been demonstrated by Huotari et al. [3 with two V2O5 adsorption sites, one with a surface OH group, forming positively charged NH4+, and the other with oxygen vacancy, forming species denoted as “coordinated NH3”. At the same time, the intensity of the V5+=O related band in the infrared reflectance spectra decreases, which is an indication of the reduction of V2O5 [2O5 catalytic surface. A variety of redox processes are possible, for example:Vadsorber –8. The gi et al.  and Modai et al. . Two str of V2O5 ,9. The cAmbient oxygen can render the process reversible, as desired for sensor reset, for example:2O5 can modify the charge distribution in the graphene–V2O5 system, thus modulating its conductivity.Obviously, strongly bound adsorbate species and the accompanying reduction of V2 can adsorb to V2O5 to form nitrato groups (V5+–NO3), and this process is reversible:According to Gao et al. , NO2 can2 adsorption on the conductivity of the V2O5–graphene system is lower, as expected. Additionally, other energetically favourable adsorption sites can exist at phase boundaries or at the point defects in the parts of graphene exposed to the gas, since the V2O5 material is on average only 2.5 layers thick and can cover the graphene surface unevenly.As there are no redox reactions involved in this adsorption process, the influence of NO2O5. The emergence of defect-related D and D’ peaks and the suppression of the 2D peak in the Raman spectrum suggest that a high concentration of defects was introduced during the deposition. According to XPS, the deposited vanadium was in the state V5+, which is the highest stable oxidation state of this element. As a result, an enhancement in the sensing properties of graphene towards the reducing NH3 gas was achieved. Such an improvement can arise from the strong adsorption ability of NH3 on V2O5 and possible redox reactions on the surface of V2O5, together with the activation of surface processes by using UV light illumination. The changing oxidation state of vanadium can modulate the electrical conductivity of a strongly coupled graphene–V2O5 system. Additionally, gas adsorption may be enhanced at the phase boundaries between a very thin nanostructured V2O5 layer and graphene substrate.CVD graphene was functionalised by laser deposition of a sub-nanometre layer of catalytically active V2 flow  for 60 min, and then exposed to the mixture of 10% CH4  in Ar at the same temperature for 120 min. The sample was allowed to cool slowly, 15 °C/min in Ar flow. The as-grown graphene film was transferred onto a Si/SiO2 substrate by using poly(methyl methacrylate)  as a supporting material. The PMMA solution (1% in chlorobenzene) was spin-coated onto graphene/Cu, dried, and the Cu foil was dissolved in ammonium persulfate solution overnight. The floating PMMA/graphene film was rinsed with deionized water and transferred onto the Si/SiO2 substrate  using a home-built CVD reactor. First, the foil was annealed at 1000 °C in Ar/Hrate see  equipped2O5 pellet was used as an ablation target. The sensor substrate was held in place by a shadow mask through which V2O5 was deposited onto graphene. A KrF excimer laser  was used for deposition. For the PLD target, a fine microcrystalline powder of V2O5  was pelleted at 740 MPa and subsequently baked at 550 °C in air for 5 h. Before starting the PLD procedure, the PLD chamber was evacuated and the sensor substrates were heated in situ at 150 °C for 1.5 h and then cooled down to room temperature. The heating procedure was established in order to clean the graphene surface and minimise the effect of trace contaminants left from the manufacturing process of the graphene sensor. The V2O5 target was ablated by using a laser pulse energy density of 5.0 J/cm2 in the presence of 5 × 10−2 mbar of O2. During the deposition, the sensor substrates were kept at room temperature, with other typical process parameters as follows: laser pulse repetition rate 5 Hz, number of laser pulses 120, and the distance between the substrate and the target 75 mm. Under exactly the same experimental conditions, another V2O5 layer was deposited onto a fused quartz substrate for the evaluation of the mass thickness of the deposited layer in the XRF experiment.In the PLD process, a ceramic VThe structural characterization of graphene was performed by using a micro-Raman spectroscopic system  with an excitation wavelength of 514 nm. The vanadium concentration in the deposited layer was analysed with X-ray fluorescence . The oxidation state of the deposited vanadium was also determined by X-ray fluorescence. The XPS spectra were acquired using a Scienta SES-100 spectrometer. The excitation source was a polychromatic twin-anode X-ray tube  with Mg Kα (1253.6 eV) 300 W irradiation at an analyser–source angle of 45°. The semi-quantitative analysis was conducted using the methods described by Seah et al.  for quan3 equipped with a gas mixing system based on mass flow controllers . The voltage applied between the electrodes was 100 mV. The gases used in our measurements N2, O2, CO/N2, NO2/N2, and NH3/N2 were all 99.999% pure. A synthetic N2 and O2 air mixture  was used as a carrier gas and the relative concentration of N2 and O2 gases was held constant during measurements. The gas flow through the sample chamber was kept constant at 200 sccm while the concentration of the test gas was regulated by the ratio of the flow rates of individual gas components. The nominal relative humidity of the testing gas was 20% during the measurements. All the sensor measurements were done at room temperature. The sensor system was exposed to light from a Xe/Hg high pressure lamp (Hamamatsu). The UV wavelength of 356 nm was selected with a narrow-band interference filter (Andover). The light intensity on the sample was 10–20 mW/cm2.The measurements of the electrical characteristics and gas sensitivity were carried out with a source meter  in a sample chamber with a volume of 7 cm