Methods: Sixteen patients  with non–small cell lung cancer were recruited. Primary tumor PD-L1 expression was measured by immunohistochemistry. NM-01 was radiolabeled with [99mTc(OH2)3(CO)3]+ complex binding to its C-terminal hexahistidine tag. Administered activity was 3.8–10.4 MBq/kg, corresponding to 100 μg or 400 μg of NM-01. Whole-body planar and thoracic SPECT/CT scans were obtained at 1 and 2 h after injection in all patients, and 5 patients underwent additional imaging at 10 min, 3 h, and 24 h for radiation dosimetry calculations. All patients were monitored for adverse events. Results: No drug-related adverse events occurred in this study. The mean effective dose was 8.84 × 10−3 ± 9.33 × 10−4 mSv/MBq (3.59 ± 0.74 mSv per patient). Tracer uptake was observed in the kidneys, spleen, liver, and bone marrow. SPECT primary tumor–to–blood-pool ratios (T:BP) varied from 1.24 to 2.3  at 1 h and 1.24 to 3.53  at 2 h (P = 0.005). Two-hour primary T:BP ratios correlated with PD-L1 immunohistochemistry results . Two-hour T:BP was lower in tumors with ≤1% PD-L1 expression . Nodal and bone metastases showed tracer uptake. Heterogeneity (>20%) between primary tumor and nodal T:BP was present in 4 of 13 patients. Conclusion: This first-in-human study demonstrates that 99mTc-labeled anti–PD-L1-single-domain antibody SPECT/CT imaging is safe and associated with acceptable dosimetry. Tumor uptake is readily visible against background tissues, particularly at 2 h when the T:BP ratio correlates with PD-L1 immunohistochemistry results.Immunotherapy with checkpoint inhibitor programmed cell death 1 (PD-1)/programmed death ligand-1 (PD-L1) antibodies demonstrates improvements in treatment of advanced non–small cell lung cancer. Treatment stratification depends on immunohistochemical PD-L1 measurement of biopsy material, an invasive method that does not account for spatiotemporal heterogeneity. Using a single-domain antibody, NM-01, against PD-L1, radiolabeled site-specifically with  Non–small cell lung cancer (NSCLC) is the leading cause of cancer mortality, with nearly 1.6 million cancer-related deaths each year worldwide ,3. The rPD-1 receptor, expressed on the surface of activated T cells, is a negative costimulatory receptor, and its ligands, PD-L1 or PD-L2, are normally expressed on the surface of dendritic cells or macrophages ,13. CertDiscrepancies in treatment response have highlighted potential deficiencies in the current methods to evaluate PD-L1 expression in a clinical setting . ImmunohSingle-domain antibodies (sdAbs) are the smallest naturally derived antigen-binding fragments from camelid antibodies and demonstrate great potential for molecular imaging in both preclinical and clinical models –21. Beca99mTc-labeled sdAb (99mTc-NM-01) that specifically binds to human PD-L1 is feasible, is safe, and correlates with PD-L1 immunohistochemistry results. In this first-in-human study, we aimed to assess safety, radiation dosimetry, and imaging characteristics of 99mTc-NM-01 and sought to compare imaging and PD-L1 immunohistochemistry results in patients with advanced NSCLC.Our hypothesis was that SPECT imaging of NSCLC with a n = 16) between March and November 2018  and obtained approval from Shanghai General Hospital Ethics Committee . All patients enrolled into this study gave written informed consent to participate.This was an open-label, nonrandomized early phase I diagnostic study in patients with untreated NSCLC (99mTc(OH2)3(CO)3]+ complex (pH 7.0–7.5) was added to a sealed vial containing 200 μg of NM-01 in 100 μL of phosphate-buffered saline (pH 7.4), and the mixture was incubated at 37°C for 1 h . PD-L1 expression was determined in primary tumor tissue obtained by core needle biopsy.Patients aged between 18 and 75 y with histopathologically confirmed NSCLC and an Eastern Cooperative Oncology Group Performance Score of 1 or less were eligible to participate in this study  was also processed in parallel. After collection, formalin-fixed paraffin-embedded blocks were prepared according to instructions set in the PD-L1 IHC 22C3 pharmDx immunohistochemical assay kit manual . Briefly, each specimen was fixed in 10% neutral buffered formalin. After being rinsed, samples were dehydrated by sequential immersion in ascending concentrations of ethanol in water for 2 h at each concentration, starting at 80% and reaching 100% in 5% increments. Dehydrated samples were cleared in xylene and subsequently infiltrated with melted paraffin at 55°C.Formalin-fixed paraffin-embedded blocks were cut into 4-µm sections in an RM2235 Rotary Microtome (Leica Biosystems). Sections were mounted onto DAKO Flex IHC microscope slides , stored in the dark at 2°C–8°C and used within 30 d of preparation.Further processing and immunostaining of primary tumor samples and controls were performed in a DAKO Autostainer Link 48 SK006 immunohistochemistry stainer  using the pre-programmed PD-L1 IHC 22C3 pharmDx staining protocol and reagents, including the mouse anti–PD-L1 (clone 22C3), provided in the assay kit. Briefly, an automated 3-in-1 process of deparaffinisation, sample rehydration, and target retrieval was followed by an automated staining procedure with mouse anti–PD-L1 (clone 22C3) antibody, followed by hematoxylin (Baso Diagnostic Inc.) counterstaining. Microscopic and histopathologic evaluations of PD-L1 immunostaining were performed on a DAKO Autostainer Link 48  and results validated by a pathologist. Representative images were recorded using a Leica DM6000B microscope (Leica Biosystems) and Leica DFC 550 digital camera (Leica Biosystems).99mTc-NM-01 in an intravenous bolus (3.8–10.4 MBq/kg), corresponding to 100 µg  or 400 µg  of NM-01. Patients were asked to drink 300–500 mL of water after injection then void their bladder before imaging on a GE Discovery NM07 SPECT/CT scanner . Planar scans were acquired with the patient supine at 1 and 2 h after injection at 10 cm/slice/min. A CT scan was acquired at 60 min followed by local tomographic SPECT imaging focused around primary and suspected secondary as follows. All scans were acquired using low-energy high-resolution collimators in a 20% energy window centered around 140 keV, in a 256 × 1,024 matrix for planar images and 64 × 64 matrix for tomographic images. A 10% energy window centered around 120 keV was also collected during tomographic acquisitions for attenuation and scatter correction. SPECT images were acquired over 360° in 60 frames per full rotation with 20-s acquisition per frame.Patients were administered All scans were assessed by a single observer with more than 25 y of nuclear medicine and radiology experience. Maximum counts were recorded from regions of interest drawn manually on SPECT scans, using the CT for guidance, around the primary tumor; any lymph node metastases within the thoracic field of view; a 3-cm normal-lung ROI in the right upper lobe (or left upper lobe if pathology existed in the right upper lobe) for tumor-to-lung ratios (T:L); and a ROI, within the aortic arch avoiding the walls of the aorta, for calculation of tumor–to–blood-pool ratios (T:BP).Five patients underwent extra whole-body planar imaging at additional time points  to obtain radiation dosimetry data. A calibration source of 37 MBq at injection time was placed above the head of each patient to provide quantitative calibration of counts to activity. Visible organs were drawn on the initial anterior and posterior whole-body images for each patient at 10 min using OsiriX software (Pixmeo) by an experienced operator and transferred to all remaining time points. The remainder of body was assigned activity not accounted for by organ delineation or excretion. All organs for each patient were assumed to be of the same volume over the imaging time frame. Counts from the anterior and posterior images were geometrically averaged. Time–activity curves were converted to percentage injected dose and used as an input to the OLINDA/EXM dose calculation software (version 1.1) . Organ nU test, and Spearman correlation using IBM SPSS Statistics (version 24) software. A P value of less than 0.05 was taken for statistical significance.Differences and correlations between parameters were tested using the Wilcoxon signed-rank test, Mann–Whitney Average radiochemical purity of radiolabeled NM-01 was 96.9% ± 0.9% (95.4%–98.5%); purification after radiolabeling was not necessary . Endotoxin levels remained below 0.4 EU/mL.99mTc-NM-01 was 372 ± 62 MBq  in group 1 and 659 ± 25 MBq  in group 2  with histopathologically proven NSCLC  were included. Clinical stage and other characteristics are summarized in  group 2 .99mTc-NM-01 was administered in 16 patients; no drug-related adverse reactions were reported during the 7-d follow-up period. Vital signs were measured during the screening period, before injection and then every 60 min up to 5 h after injection and first follow up period (48 h after SPECT/CT scan). Clinical laboratory testing, including standard hematologic and comprehensive metabolic panels , was performed during the screening period and 48-h follow-up period. In an additional step to ensure the safety of patients, a phone interview was conducted 7 d after the SPECT/CT scan to follow-up on patients, and a questionnaire was filled out by the investigator as presented in the case report form. Symptoms related to advanced-stage lung cancer, that is, frequent coughing, dyspnea, and fatigue, reported by some patients were not attributed to the radiopharmaceutical by the supervising physician. Blood and urine clinical laboratory tests performed in the follow-up period were in the reference range of National Cancer Institute–Common Toxicology Criteria for Adverse Events .,29.28,29No noticeable differences in image quality and no statistically significant differences in tumor-to-background ratio were observed between group 1 and group 2 .n = 5). All patients had normal liver and kidney functions. The kidneys showed the highest organ dose (0.036 ± 0.018 mSv/MBq), followed by the bladder (0.026 ± 0.011 mSv/MBq), spleen (0.022 ± 0.011 mSv/MBq), and liver (0.011 ± 0.0025 mSv/MBq) and in line with the uptake and excretory pathway of the imaging agent. Time–activity curves for organs with highest radiotracer uptake are shown in P = 0.034), allowing reasonably good tumor conspicuity against normal background lung activity in patients with high PD-L1 expression (P = 0.005). Intratumoral heterogeneity of tracer uptake was noted in some primary tumors (P = 0.048). When receiver operating characteristic analysis is used, this gives an optimal threshold for the 2-h T:BP of 2.32 .The primary tumor T:L ratio was greater at 2 h than 1 h . At 2 h, primary tumor and maximum lymph node metastasis T:BP ratios (n = 13) showed a correlation . Intrapatient heterogeneity of T:BP ratios was noted between the primary tumor and lymph node metastases in several patients with a mean difference of 19% (±17%), with 4 of the 13 patients with lymph node metastases showing a greater than 20% difference . Statistically significant correlations were not found for T:L ratios or 1-h T:BP ratios.Thoracic lymph node metastases T:BP ratios were also greater at 2 h, although this difference did not reach statistical significance  and similar to other SPECT agents in clinical use. The kidneys received the highest organ dose (0.036 ± 0.018 mSv/MBq), being the major route of excretion, and although it remained tolerable, urinary tract dose could be further minimized by good hydration or coinjection of amino acids or peptides .Our initial results show promise that the simple measurement of T:BP ratios at 2 h correlates with PD-L1 expression and if confirmed in subsequent studies could potentially replace or complement immunohistochemistry in some circumstances. We noted heterogeneity of activity within primary tumors in some patients, and between the primary tumor and metastases within patients, suggesting that intra- and intertumoral heterogeneity of PD-L1 expression is relatively common and is another factor that could be measured noninvasively with imaging to allow more relevant information to be available for patient treatment decisions. Intratumoral PD-L1 expression heterogeneity may also explain the imperfect correlation with T:BP  could be improved, further scanning beyond 2 h could be considered in subsequent studies. The relatively low spatial resolution of SPECT does not allow more than gross visual intratumoral heterogeneity observations. Although a PET tracer, in theory, might allow more accurate and detailed heterogeneity analysis similar to what has been reported with in NSCLC , recent in NSCLC . In addi99mTc-labeled anti-PD-L1-sdAb SPECT/CT using 99mTc-NM-01 is a safe diagnostic procedure, delivering a tolerable radiation dose and presenting favorable biodistribution and image characteristics correlating with PD-L1 immunohistochemistry results in patients with NSCLC. As PD-1/PD-L1 immune checkpoint inhibitors are more frequently used in the treatment of NSCLC, a companion imaging methodology is becoming increasingly sought after for the staging and monitoring of patients. Anti–PD-L1-sdAb SPECT/CT using 99mTc-NM-01 has the potential to be used for the close monitoring of changes in PD-L1 expression levels during PD-L1 immunotherapy. In addition, the heterogeneous and dynamic nature of PD-L1 expression in most malignancies could make imaging a preferred method, compared with more invasive core needle biopsies, in diagnosing and staging patients with PD-L1–positive malignancies.This study demonstrates that This research is supported by Nanomab Technology Limited and Shanghai Shenkang Development Center for Clinical Skills Innovation (16CR3052A). The authors acknowledge financial support from the King’s College London/University College London Comprehensive Cancer Imaging Centres funded by Cancer Research U.K. and Engineering and Physical Sciences Research Council in association with the Medical Research Council and the Department of Health (C1519/A16463) and the Wellcome Trust EPSRC Centre for Medical Engineering at King’s College London (WT203148/Z/16/Z). No other potential conflict of interest relevant to this article was reported.Click here for additional data file.


Finally, decay rates for a dipole emitter are significantly enhanced in the proximity of finite-area biaxially crumpled graphene flakes. Our findings indicate that crumpled graphene provides a platform to engineer graphene-based plasmonics through broadband manipulation of strong plasmonic resonances.Graphene nanostructures with complex geometries have been widely explored for plasmonic applications, as their plasmonic resonances exhibit high spatial confinement and gate tunability. However, edge effects in graphene and the narrow range over which plasmonic resonances can be tuned have limited the use of graphene in optical and optoelectronic applications. Here we present a novel approach to achieve mechanically reconfigurable and strongly resonant plasmonic structures based on crumpled graphene. Our calculations show that mechanical reconfiguration of crumpled graphene structures enables broad spectral tunability for plasmonic resonances from mid- to near-infrared, acting as a new tuning knob combined with conventional electrostatic gating. Furthermore, a continuous sheet of crumpled graphene shows strong confinement of plasmons, with a high near-field intensity enhancement of ~1 × 10 An innovative approach for making tunable plasmonic nanoparticles using crumpled graphene opens the door to new optical and optoelectronic devices. Graphene is a promising plasmonic material due to its low optical loss, high spatial confinement, and tunable plasmonic resonances. However, its structural patterns fix the resonance wavelengths, making broadband tunability across near- to mid-infrared wavelengths very challenging, limiting its range of applications. This led Pilgyu Kang from the University of Illinois at Urbana-Champaign in America, in collaboration with fellow American and South Korean colleagues, to turn to crumpled graphene, a form of graphene with unique optical and physical properties. By modelling the plasmonic resonances of crumpled graphene, the researchers demonstrated mechanically reconfigurable structures that enable a much broader range of spectral tunability, leading to new applications from optical sensors to photovoltaic devices. Graphene-based plasmonic devices have shown potential in broad applications, including optical biosensing3, optical communications4, photodetectors5, and plasmonic metamaterials6. In particular, desired plasmonic resonances are easily excited in lithographically patterned graphene structures14 using a far-field coupling method. However, such structural patterns in graphene are not reconfigurable, thus fixing their resonance wavelengths. Although one can tune the resonance wavelength of graphene through electrical gating15, it is challenging to achieve broadband tunability across the near- to mid-infrared wavelength ranges. While graphene grating structures have been proposed for on–off switching and plasmonic modulation19, post-fabrication reconfiguration of plasmonic structures has not yet been realized.Graphene is a promising plasmonic material with superior properties that distinguish it from conventional noble metals22. However, the potential of crumpled graphene structures for reconfigurable plasmonic materials has not been explored. In this work, we report on the use of mechanically reconfigurable crumpled graphene structures to realize strong plasmonic resonances with broadband tunability. We introduce a resonant electrical inductor–capacitor (LC)-circuit model to describe the characteristics of plasmonic resonances in the crumpled graphene. By varying the geometry of the crumpled graphene structures, we show a tunable spectral range for plasmonic resonances that is fivefold broader than that achieved by conventional electrical gating. Our results show that mechanical reconfiguration of crumpled graphene serves as a new way to manipulate strong plasmonic resonances over a broad spectral range.It has recently been demonstrated that mechanical crumpling of graphene can be used to create different surface structures while maintaining reversibilityJs = σEt, where Js is the surface current, σ is the optical conductivity of graphene, and Et is the tangential component of the electric field to the graphene surface24. We used a mobility value of 10,000 cm2/(V·s), which is achievable for crumpled graphene based on the reported range of mobilities (μ) for flat graphene15 from ~1000 cm2/(V·s) for chemical vapor deposition grown graphene to 230,000 cm2/(V·s) for suspended exfoliated graphene. Moreover, the mobility of wrinkled graphene was shown to be similar to that of flat graphene26. For the optical absorption/extinction spectra of uniaxially and biaxially crumpled graphene in Figs. T/T0, where T and T0 are the transmittance values with and without crumpled graphene, respectively. To investigate substrate-induced effects on plasmonic resonances in Fig. 2) substrate27 was used as a model material. In Fig. We performed full-wave simulations using the finite element method  to investigate the plasmonic resonances of crumpled graphene structures. We modeled graphene as a two-dimensional surface current tangential to the crumpled structure: σ) for varying Fermi energy (EF) and carrier mobility (μ) values of graphene at a constant temperature of 300 K using a random phase approximation method29. The optical conductivity is expressed by σ(ω) = σintra(ω) + σinter(ω), where σintra(ω) and σinter(ω) are the intraband and interband conductivities, respectively23. They are expressed as below, where ω is the frequency of the incident light, e is the charge of an electron, τ is the Drude relaxation rate, T is the temperature, kB is the Boltzmann constant, and ℏ is the reduced Planck constant. EF is the Fermi energy, determined by EF = ℏvf(πn)1/2, where vf is the Fermi velocity of electrons (~106 m/s) and n is the carrier density of graphene. The Drude relaxation rate τ is determined by τ = µEF/evf2, where µ is the carrier mobility of graphene.ωF = EF/ℏ and ωT = kBT/ℏ.We calculated the optical conductivity of graphene (λc) and crumple height (h)  at a constant λc of 250 nm  redshifted from ~4.5 to ~11.3 µm with increasing h/λc from 0.25 to 2.0. The optical absorption and extinction (σabs and σext) also increased with higher h/λc from 0.25 to 1.0 but showed no significant change from h/λc = 1.0 to 2.0  and observed redshifts for λres from ~3.2 to ~13.8 µm and an increase in σabs and σext from ~0.12 to ~0.41 and from ~0.14 to ~0.50, respectively  is the curvature factor for the crumpled structures. By using Leff and Ceff, we derived the plasmonic resonance wavelength (λres) of the crumpled graphene expressed as λres = 2πc(LeffCeff)1/2 (see SI for detailed derivation).To describe the plasmonic resonance in crumpled graphene structures, we introduced a resonant ons Fig. . An effeLC-circuit model, we compared the value for λres predicted by the LC-circuit model with that predicted by numerical simulations. With varying λc from 50 nm to 1 μm and h/λc from 0.5 to 2, the λres obtained by the full-wave simulations (dots) agreed with that of our LC-circuit model (solid curves) , leading to a shift in λres. This implies that controlling the charge interactions on confronting graphene surfaces by varying h/λc enables the tunability of λres across the near- to mid-infrared wavelength ranges. The mechanically reconfigurable crumpled graphene structures are thus an excellent platform for broadband tunability of plasmonic resonances.To validate our es) Fig. . In contEF and carrier mobility µ  for varying EF was ~3.7 µm, while ∆λres for varying λc was ~20.1 µm . The redshift of λres is attributed to confinement of resonant plasmons exclusively in the apex of the crumpled graphene on the undulating substrate  were enhanced approximately twofold for the undulating substrate compared to the free-standing crumpled graphene (14.0 vs. 7.6% and 63.8 vs. 26.1)   Fig. . However6%) Fig. . These rλxc, = λyc,)  2) compared to uniaxially crumpled graphene  for the uniaxially crumpled graphene (∆λres ~ 6.1 µm)   Fig.  and an eµm) Fig. . Notablyµm) Fig.  and Q-faene Fig. . The abs·s) Fig. . The Q-fity Fig. . The biax and y directions. Plasmonic resonance peaks were observed for various values of h/λc. The near-field distributions at the plasmonic resonance peaks show that resonant plasmons were strongly confined to the center of the crumpled graphene flake  showed the same plasmonic resonance peak wavelengths as those with lower mobilities, including at 1000 and 4000 cm2/(V·s)  Fig. . Crumple39 of crumpled graphene flakes, we introduce a dipole emitter placed 40 nm above the center of a free-standing biaxially crumpled graphene flake. The orientation of the dipole moment for the emitter is parallel to the x crumpling direction  and total decay rates (Γ), which were normalized by those of the emitter in free space without the crumpled graphene flake (Γ0). Figure h/λc. We observed significant enhancements in the radiative decay rates, up to three orders of magnitude, at the plasmonic resonance peaks  of ~1.6 × 105  showed the same plasmonic resonance peak wavelengths as those with a lower mobility of 1000 cm2/(V·s)  Fig. . We also·s) Fig.  and tota·s) Fig.  for vari4 for biaxially crumpled graphene flakes. Furthermore, we showed strong light–matter interactions for the crumpled graphene flakes, which enabled large decay rate enhancements to yield a maximum Purcell factor of ~1.6 × 105. We believe that our crumpled graphene structures, with strong and broadly tunable plasmonic resonances in the near- to mid-infrared, can find broad applications43, including photonic and optoelectronic devices, ultrasensitive chemical and biological sensing, photovoltaic devices, and spectroscopy.In summary, we have demonstrated, for the first time, mechanically reconfigurable crumpled graphene exhibiting broadband tunability of strong plasmonic resonances from the mid- to near-infrared. The reconfigured crumpled graphene structures, with varying crumple wavelengths and heights, enabled the manipulation of plasmonic resonances over a tunable wavelength range five times broader than that afforded by conventional electrical tuning of graphene. We have also demonstrated efficient excitement of graphene plasmons in crumpled graphene structures by far-field coupling of light as well as strong confinement of the excited plasmons in crumpled graphene. The strong plasmonic resonances enabled optical absorption enhancements of up to ~0.46 as well as high near-field enhancements on the order of 1 × 10Supplementary InformationSupplementary Movie


Gases, such as nitrogen dioxide, formaldehyde and benzene, are toxic even at very low concentrations. However, so far there are no low-cost sensors available with sufficiently low detection limits and desired response times, which are able to detect them in the ranges relevant for air quality control. In this work, we address both, detection of small gas amounts and fast response times, using epitaxially grown graphene decorated with iron oxide nanoparticles. This hybrid surface is used as a sensing layer to detect formaldehyde and benzene at concentrations of relevance (low parts per billion). The performance enhancement was additionally validated using density functional theory calculations to see the effect of decoration on binding energies between the gas molecules and the sensor surface. Moreover, the time constants can be drastically reduced using a derivative sensor signal readout, allowing the sensor to work at detection limits and sampling rates desired for air quality monitoring applications. Several toxic air pollutants in more than 80% of the urban areas where air pollution is monitored exceed the World Health Organization’s (WHO) recommended safe exposure levels. Poor air quality has been associated with several negative health aspects ranging from less severe conditions, such as skin and eye irritation, to more acute respiratory problems, cancer, or death. Air pollution has been estimated to cause 8 million annual deaths and to financially burden the European region by about 1.6 trillion US dollars per year .2O) and benzene (C6H6), at levels of relevance to human health. Benzene is a genotoxic aromatic compound, especially associated with leukemia. There are no safe exposure limits for C6H6 according to the last published WHO recommendations [Air quality (AQ) monitoring and control using extremely sensitive sensors are crucial from the viewpoint of preventing further deaths and diseases correlated with toxic air substances. However, commercial sensors/instruments available today are either large, expensive, and complex or small but limited by poor selectivity, sensitivity, and a slow sampling rate . In addindations , howeverndations  recommenbenzene CH6, at leIn the future, AQ assessment should ideally be based on real-time monitoring of air pollutants with high spatial resolution , allowin6H6 at the concentrations of relevance for AQ monitoring [6H6 detection limit down to 0.5 ppb. Unfortunately, none of the sensors are selective to a particular VOC, making it impossible to distinguish, e.g., the C6H6 concentration. Some of the reviewed sensors also include a selective absorbing cartridge for benzene. Unfortunately, the best limit of detection for commercially available sensors selective to C6H6 was found to be 10 ppb for the Ion Science Tiger Select .In a report by Spinelle et al. from 2017, commercially available portable low-cost sensors for VOCs were reviewed, and it was found that few sensors can detect Cnitoring , with mo6H6 [Among state-of-the-art research studies on portable AQ sensors , there a6H6 . Both Si6H6  and meta6H6  operated6H6 , in whic6H6 . HoweverA low cost, portable sensor capable of detecting benzene at 1 ppb or lower concentrations would thus constitute a breakthrough in the field of air quality monitoring.2), C6H6 and CH2O [3O4) decorated multiwall carbon nanotubes [6H6. However, poor reproducibility and insufficient detection limits make the state-of-the-art sensors ill-suited for human safety applications. So far, these sensors were able to detect the mentioned gases down to tens of ppb, but not reliably lower. In this paper, which is an extension of a conference contribution [Using the unique properties of graphene as a transducer allows fabrication of sensor devices that can be used for gas detection where low concentrations can be detected, including air quality control for human health. Besides a high sensitivity, also interaction with specific target analytes and a good selectivity must be addressed to get a useful sensor device. It has already been shown that decoration of the graphene surface with metal/oxide nanoparticles can lead to a higher sensitivity and selectivity towards certain gases, e.g., nitrogen dioxide  through a sublimation process were an on-axis, semi-insulating (0001) 4H-SiC substrate (7 mm × 7 mm) is used for the formation of graphene in argon (Ar) at a temperature of 2000 °C and a pressure of 1 bar  was used3O4 was present on the surface.Before and after the deposition of nanoparticles, a series of characterization measurements was conducted to determine the graphene uniformity and quality, and to see if any damage of the graphene occurred during the deposition. Atomic Force Microscopy (AFM) (Quadrexed Dimension 3100 with a Nanoscope IVa controller) was used in tapping mode to obtain topography images of the sensing layers. The measurements were performed using silicon (Si) tips (PPP-NCHR-50 from Nanosensors) with a tip radius of curvature below 7 nm. X-ray photoelectron spectroscopy (XPS) studies using a Microlab 310-F spectrometer were performed to investigate possible alterations made to the sample after deposition of NPs and to establish if Fe2 and 20% O2 was used as a purging and carrier gas. A more detailed description of the measurement system can be found in a previous work [As a first step, titanium on gold  contact pads were thermally evaporated onto the epitaxially grown graphene on SiC (EG/SiC) before functionalization. The contacts have a size of 1 mm × 1 mm with a distance of 1 mm between them. The sensor chip and a Pt-100 resistance thermometer were glued (Aremco Ceramabond 571) onto a ceramic heater (Heraeus GmbH) to enable a controlled temperature loop, and welded to a 16 pin TO8 header to establish the electrical contacts. This device was mounted on top of a TO8-socket and connected to its pins using gold-wire bonding and silver glue (Epotek E3081). The final sensor was placed in a flow chamber connected to a gas mixing setup. A Keithley 2601B SourceMeter was used in a two-wire mode to measure the resistance between the contacts during gas exposure. The total gas flow was kept constant at 100 mL/min, and a dry mixture of 80% Nous work . 6H6 and CH2O) on pristine EG (PEG) and Fe3O4-decorated epitaxial graphene (DEG) on Si-face 4H-SiC was investigated based on hybrid gas-phase density functional theory (DFT) calculations performed by using Gaussian 16 Rev. B.01 program package [3O4 located on PEG. All dangling bonds are passivated by the hydrogen atoms. The calculations were carried out using M06-2X level of theory with consideration of split basis set [The adsorption of gas molecules (C package . As a moasis set . It is iasis set . A 6-31Gasis set . All atoEads) was calculated by using the following Equation:6H6 or CH2O), respectively, whereas The adsorption energy of gas molecules (r (BSSE)  was appl3O4 NPs. Neglecting the characteristic steps corresponding to the SiC step bunching , the as-grown graphene surface in 3O4 NPs onto graphene was investigated by means of XPS. The elemental compositions of as-grown EG/SiC prior to and after deposition of Fe3O4 NPs (Fe3O4/EG/SiC) were obtained to demonstrate the presence of Fe3O4 NPs on the latter sample. The XPS survey spectra for EG/SiC sample and Fe3O4/EG/SiC sample are shown in 3O4/EG/SiC showed the presence of iron and gold (Au) with XPS peaks Fe2p, Au4d, and Au4f found at binding energy position about 711 eV, 340 eV and 84 eV [3O4/EG/SiC are shown in 3O4/EG/SiC, based on the XPS O1s and C1s core level spectra has been performed. An increase of O, estimated to be a fivefold increase, and a decrease of C, reduced to about half for Fe3O4/EG/SiC, were observed. The strong increase in oxygen signal is shown for Fe3O4/EG/SiC sample, with an additional peak at binding energy position 530.0 eV, in good agreement with the presence of the chemical structure Fe-O in Fe3O4 NPs. XPS Fe2p core level spectra before and after deposition of Fe3O4 NPs are shown in 3O4/EG/SiC. The corresponding XPS C1s core level spectra for EG/SiC and Fe3O4/EG/SiC are presented in The effect of deposition of Fend 84 eV . The gol3O4 NPs, gas measurements were performed. 2O. The sensor was exposed to CH2O and C6H6 in a dry background of synthetic air using concentrations ranging from 5 parts per million (ppm) to 1 ppb, and a pulse duration of 30 min. Tests to identify a good operating temperature have been conducted and 150 °C, which is a good compromise between stability and high response, was chosen for all measurements shown here. Similar measurements with pure graphene did not show any response to both test gases. In an earlier work of ours [3O4 nanoparticle-decorated graphene sensors can be used for formaldehyde and benzene detection, but not over such a large range and down to a single ppb. The same measurement was repeated twice in the same environment and was repeated once more approximately one year later with the same sensor. The measured responses show some small deviations between the measurements without a clear trend of decreasing response over time. The sensor response shown here is from the first measurement. The response is defined as R is the saturated resistance signal and R0 corresponds to the baseline resistance before the gas exposure. The raw sensor signal in 6H6 at the sensor surface at 150 °C  is easily reached with a relative response above 0.1%.After the successful decoration of the graphene surface with Fe of ours , it has 0 °C see . The reldR/dt (after smoothening the data with a 500-point moving average filter), where the time derivative signal reaches its maximum value after about 50 s. It is evident from the insert that dR/dt exhibits distinct peaks in response to CH2O and the peak values are concentration dependent. Because of the smaller time constants needed to reach maximum dR/dt, the actual sensor signal does not have to be saturated when using the rate of change instead of the absolute value of resistance. It is therefore possible to significantly improve the speed of the sensor by utilizing dR/dt as a sensor parameter. Besides a very good sensitivity, also time constants of these measurements need to be addressed. For an application in indoor air monitoring, the time constants for the sensor should be in the range of half a minute to several minutes, depending on the application. However, as shown in 3O4-decorated epitaxial graphene was performed. Energetically favored adsorption configurations of considered gas molecules on PEG and DEG are illustrated in 6H6 on DEG, Eads = 1.795 eV, is significantly higher than on PEG with a value of Eads = 0.284 eV. In the absence of the Fe3O4 nanoparticle, C6H6 molecule adsorbs in the flat geometry, which is parallel to the surface. In this case, the center of C6H6 ring lays on top of a carbon atom belonging to graphene. On the other hand, a functionalization of graphene with Fe3O4 causes the formation of a strong chemical bond between C6H6 ring and one of the Fe atoms, facilitating the C6H6 adsorption in the tilted geometry regarding the graphene surface. Like the situation for C6H6, we found that CH2O molecule prefers to accommodate in a parallel geometry to graphene surface with weak adsorption energy of 0.149 eV, while the energetically preferred adsorption mode for CH2O on DEG is related to the formation of strong Fe-O covalent bond with a bond length of the 1.98 Å. The calculated adsorption energy of 1.870 eV is much higher than the value predicted for adsorption on pristine graphene sample, implying the strong chemisorption case. To summarize, one can conclude that weak physisorption of gas molecules on non-decorated graphene is immediately changed to strong chemisorption when iron oxide NPs are involved in adsorption phenomena. From an experimental point of view, the observed chemical similarity between adsorption configurations of benzene and formaldehyde enables us to anticipate comparable responses of Fe3O4-decorated epitaxial graphene to exposure of both gases, with slightly higher sensitivity towards formaldehyde.An efficiency of the decoration approach towards gas sensing can be directly demonstrated and validated by the DFT results. For this aim, a comparative analysis of the adsorption of benzene and formaldehyde onto non-decorated and Fe3O4 NPs. We could verify the decoration and the integrity of the graphene surface using AFM and XPS. With this sensor platform, concentrations down to a single ppb of toxic VOCs could be quantitatively measured, which makes it very promising for air quality monitoring. DFT calculations show that the gas molecules are more likely to bind to the decorated sensor surface. Moreover, by evaluating the first-order time derivative of the sensor signal, it was possible to significantly enhance the speed of the sensor, yielding response times of less than one minute, in turn allowing sampling rates desired in air quality monitoring.Benzene and Formaldehyde sensing properties were investigated using epitaxially grown graphene on silicon carbide decorated with Fe


These findings represent a major step forward toward the practical application of nanomaterials in civil engineering.This study reports on the development of a cementitious composite incorporating electrochemically exfoliated graphene (EEG). This hybrid functional material features significantly enhanced microstructure and mechanical properties, as well as unaffected workability; thus, it outperforms previously reported cementitious composites containing graphene derivatives. The manufacturing of the composite relies on a simple and efficient method that enables the uniform dispersion of EEG within cement matrix in the absence of surfactants. Different from graphene oxide, EEG is found to not agglomerate in cement alkaline environment, thereby not affecting the fluidity of cementitious composites. The addition of 0.05 wt% graphene content to ordinary Portland cement results in an increase up to 79%, 8%, and 9% for the tensile strength, compressive strength, and Young's modulus, respectively. Remarkably, it is found that the addition of EEG promotes the hydration reaction of both alite and belite, thus leading to the formation of a large fraction of 3CaO·2SiO Therefore, cement composites incorporating graphene oxide (GO), i.e., the most easily processable graphene derivative, have been intensively investigated in recent years.The cement–GO composites have emerged as materials with improved mechanical properties Table . Even in2+ in calcium hydroxide.Mechanical properties of cementitious composites should be assessed by means of their microstructure and composition of cement paste hydrates. As demonstrated in previous studies, the reinforcing mechanism of GO is attributed to the chemical reaction between GO nanosheets and cement hydration products.While the potential application of graphene oxide in cement composites has been extensively studied in recent years, similar research involving graphene, to date, remained limited. Some attempts have been made to fabricate cement composites with the addition of graphene nanoplatelets.Here we present an unprecedented cementitious composite incorporating few‐layer thick graphene nanosheets. We have devised a technologically simple yet efficient method for manufacturing a cement–graphene composite based on the use of graphene obtained via electrochemical exfoliation (EE) of graphite foil. Prior to being mixed with cement, the graphene aqueous suspension is dried and wiped through the set of sieves. Contrary to previous reports describing the generation of cementitious materials incorporating graphene derivatives, the preparation of our composite does not require special treatments or the use of surfactant to obtain uniform dispersion of graphene within cement matrix. We first evaluated the dispersion of graphene in cement alkaline environment and its effect on the consistency of cement mortar. Then we investigated the mechanical properties of produced composites to select the most advantageous content of graphene. The 0.05 wt% addition of graphene results in the highest increase of mechanical properties, namely, 79%, 8%, and 9% for tensile strength, compressive strength, and Young's modulus, respectively. Finally, we characterized the microstructure and composition of our mortars. As revealed by detailed characterization, the hydration rate of calcium silicates is remarkably increased in cement–graphene composites. The addition of electrochemically exfoliated graphene (EEG) accelerates the nucleation and growth of C‐S‐H phase, thus resulting in significant improvement of composites' strength. Subsequently, the fluidity of the processed composites remains unaltered compared to reference mortars, indicating that the concrete alkaline environment does not jeopardize the uniform dispersion of graphene within cement matrix. These results open new possibilities for the practical applications of graphene in high‐performance cementitious composites.22.1EEG has been prepared under anodic conditions by using a simple electrolytic cell.2 substrate by spin coating 200 µL of 1 mg mL−1 dispersion in dimethylformamide. FigureAtomic force microscopy (AFM) is employed to gain in‐depth insight into the surface morphology of EEG flakes. Toward this end, EEG flakes were deposited on SiO2.2FigureThe chemical composition of the as prepared EEG flakes was unraveled by X‐ray photoelectron spectroscopy (XPS) and compared with the starting material, i.e., the graphite foil, as well as with commercially available graphene oxide. Such analysis was performed aiming at following the changes in the chemical composition of the material during EE. 2.32+, Na+, and K+ and anions such as OH− and (SO4)2−. Ca2+ and OH− ions originate from calcium hydroxide, Na+ and K+ ions exist in sodium and potassium oxides present in cement, while sulfate ions can originate from gypsum, mixing water, or aggregate.2, NaOH, KOH, NaCl, CaCl2, and NH3 x H2O2, NaOH, and KOH.+ and K+ ions in the solution with pH value below 13.2+ ions. Toward this end, we added 100 µL of saturated Ca(OH)2 solution to 3.0 mL of aqueous dispersion of EEG or GO dispersion with concentration of 0.07 mg mL−1. Indeed, the addition of Ca(OH)2 results in rapid and notable flocculation of GO, conversely not affecting graphene dispersion .We first investigated the dispersion of EEG and GO in alkaline environment and their influence on the consistency of cement mortar. The solution present in concrete pores is highly alkaline due to the presence of cations, including CaFigureWe then attempted to manufacture several cement mortars made out of Portland cement Type I with various loadings of EEG and GO ranging from 0 wt% to 0.1 wt%. Noteworthy, the water‐to‐cement and sand‐to‐cement ratios remained 0.5 and 3.0, respectively. Moreover, in order to provide extensive benchmarking for samples with EEG, we have produced additional CEM I mortar incorporating 0.05 wt% of graphite flakes (GFs). However, as in the case of EEG, we have reported negligible effect of the graphite flakes addition on the consistency of cement mortar . Given a similar amount of water required to obtain the standard consistency for both types of cement (see Table S1 in the Supporting Information), as well as the same depth of plunger penetration noted for reference samples in consistency test, this phenomenon should be explained in terms of chemical composition of CEM I and CEM II . CEM II modified with granulated blast furnace slag contains less CaO than CEM I. This fact, combined with our consistency measurements, provides evidence that chemical cross‐linking of GO by calcium ions may account for the reduction of cement mortar fluidity, as revealed by previous studies.2.4FigureWe then performed compressive and tensile strength tests of all produced mortars at the age of 28 d. In order to obtain the most accurate results not affected by the end restraint effect, we have investigated mechanical properties on cylindrical samples with the height‐to‐diameter ratio of 2 .Figure On the other hand, with the introduction of 0.05 wt% of GO, the compressive strength is considerably decreased by 11% Figure b. This fFigureFurthermore, since it is of crucial importance to improve concrete performance in tension, we have then determined the tensile strength of produced mortars by means of a direct tension test Figure5a. HerSignificantly, the remarkable tensile strength values we achieved cannot be directly compared with those reported in the literature,Clearly, since large pores have been monitored in cement–GO composites Figure f, the teSimultaneously, CEM II composites incorporating EEG demonstrate slightly enhanced tensile strength  and, surprisingly, decreased compressive strength . The effect of GO addition on CEM II mortars strength and Young's modulus is, however, comparable with that noted for CEM I .2.5In order to understand the origin of the improvement of mechanical properties, as well as the underlaying strengthening mechanism of EEG, we investigated the microstructure and composition of produced cement mortars using various characterization techniques, including thermogravimetric analysis (TGA), X‐ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET) adsorption–desorption analysis.2, and calcium silicate hydrate, C‐S‐H phase. C‐S‐H amorphous phase, accounting for ≈50%–70% of cement structural components, is considered as the strongest phase in hardened cement paste and plays the major role in developing macroproperties of cementitious composites. Ca(OH)2 with its hexagonal plate‐shaped crystals, being a considerably weaker phase, is responsible for providing high alkalinity of concrete environment, thereby protecting steel reinforcement against corrosion. However, high amount of calcium hydroxide may contribute to leaching, carbonation, alkali aggregate reaction, or sulfate attack, thus deteriorating the durability of concrete structures.Fundamentally, mechanical properties of cementitious composites originate from microstructure and composition of cement paste hydrates, in particular, calcium hydroxide, Ca(OH)2 content is of significant importance in characterizing cementitious materials, we first performed thermogravimetric analysis to probe Ca(OH)2 amount in all produced mortars at the age of 28 d. TGA has been shown to be a powerful tool for estimating portlandite content in cement composites, considering the specimens weight losses in the dehydroxylation (≈400–550 °C) and decarbonation (≈600–800 °C) regions.mH2O, mCH, and mCO2 correspond to the molecular mass of H2O (18 g mol−1), Ca(OH)2 (74 g mol−1), and CO2 (44 g mol−1), respectively.Since the determination of C‐S‐H and Ca(OH)on ≈400–5 °C and dFigure2 content is drastically reduced, achieving the lowest value of 13.07% for CI‐G‐0.05 samples, while an average value for reference samples is 16.37%. It should be pointed out that the lowest portlandite content occurred in samples exhibiting the highest tensile strength. On the other hand, we also report the slight decrease of Ca(OH)2 amount in GO composites, i.e., from 16.37% to 15.17% for CI‐GO‐0.05 samples. Since these specimens possess the lowest compressive and tensile strength, these results indicate the opposite trend in Ca(OH)2 amount compared to graphene composites.2 presence in hardening cement matrix. First, at initial contact with water, cement grains start to dissolve, thus releasing, among others, calcium and hydroxide ions from their surface. At the end of dormant period, when the supersaturation point of Ca2+ ions is reached, Ca(OH)2 starts to precipitate from the saturated solution. Second, Ca(OH)2 is produced as a result of hydration reactions  and dicalcium silicate 2CaO·SiO2 (belite)To fully explain our observations, it is particular important to attain a deep understanding on the origin of Ca(OH)Equation  of two m2. Alite with its fast reaction and high release of heat is responsible for early hydration, thus contributing mainly to early‐age strength of concrete, whereas the reaction of belite being markedly slower releases less heat and contributes to long‐term strength.Noteworthy, although these two reactions result in the same hydration products, they differ considerably in hydration rate, heat, and the amount of produced Ca(OH)Since the formation of portlandite is a highly complex process extensively varying at the different stages of ongoing cement hydration, we have then performed XRD and FTIR analysis on samples cured for 28 d, in order to gain further insight onto the course of cement hydration reactions and their products. We have focused our attention on four representative samples made out of CEM I, namely, CI‐R, CI‐G‐0.05, CI‐GF‐0.05, and CI‐GO‐0.05.Θ = 26.6°.2 appear at 18.0° and 34.1°.2 amount in CI‐G‐0.05 samples as revealed previously by TGA. Basically, as C‐S‐H gel is an amorphous phase, it cannot be detected during XRD analysis. However, the hydration degree may be estimated on the basis of remaining peaks of alite and belite at the positions of 28.7° and 29.4°.Figure 2 at 3650 cm−1,3 at 874 and 1414 cm−1,−1 represents the stretching vibration of Si—O bonds of C‐S‐H gel revealing, in particular, higher formation of C‐S‐H phase in cement mortar reinforced with graphene and, simultaneously, restrained C‐S‐H growth in cement–GO composites.In the case of FTIR, the peaks attributed to —OH of Ca(OH)2 adsorption–desorption isotherms measurement at 77 K . CI‐R composite displays a specific surface area of ≈9 m2 g−1, while the specific surface area of CI‐GO and CI‐G is calculated with the BET model as 17 and 29 m2 g−1, respectively. Significantly, the results indicate the increased development of the highly porous phase, i.e., the C‐S‐H gel, resulting from the addition of EEG. Moreover, the average pore diameter calculated with the Barrett–Joyner–Halenda model for CI‐R, CI‐GO, and CI‐G amounts to ≈3.9 nm .Interestingly, the evaluation of the specific surface area of cementitious composites has been shown being a powerful tool for assessing the C‐S‐H phase development, which is strongly associated with the degree of cement hydration.2 formation at different stages of cement hydration n Figure d. In thiIn the case of GO, Figure Significantly, we also observed that no major differences occurred in the diffraction patterns and FTIR spectra of reference samples and samples with graphite flakes, pointing toward similar mineralogical compositions of both materials. This provides unambiguous proof that the addition of graphite flakes, in contrast to EEG, does not affect the microstructure and hydration products of cement composites, acting only as dispersed reinforcement.Figure2 can be easily observed in plain, GO, and graphite composites, while they do not occur in graphene mortar. The visible densification of microstructure provides a rational explanation of the remarkable performance of cement‐EEG mortars in mechanical properties tests.Our observations are further confirmed by SEM images . As in CEM I composites, the lowest value of Ca(OH)2 occurred in samples with the highest tensile strength. Nevertheless, the value changes only from 11.21% in reference samples to 10.55% in CII‐G‐0.05 specimens. Interestingly, the addition of GO results in intense formation of calcium hydroxide, achieving a value as high as 13.8% for CII‐GO‐0.05 samples. These observations are also supported by XRD  and FTIR  analysis. Moreover, XRD patterns reveal a slight decrease in alite and belite amount for samples with graphene, while FTIR spectra show a similar trend in C‐S‐H phase formation as in CEM I.In addition to our findings in terms of Portland cement Type I, structural characterization of CEM II composites was performed using the same techniques previously described. The Ca(OH)2 content after 1, 3, and 7 d of cement hydration. In Portland cement with the addition of granulated blast furnace slag, Portland cement components start to hydrate first. At this initial stage, the hydration of CEM II strongly resembles that of ordinary Portland cement.2 content for reference sample is noted, a fingerprint of the presence of granulated blast furnace slag. Basically, when Portland clinker reactions are in progress, granulated blast furnace slag is then activated by alkalis and starts to react with calcium hydroxide released by Portland cement. As a consequence, additional amount of C‐S‐H phase is formed.Particular attention should be paid to Ca(OH)3In summary, we have demonstrated a novel cementitious nanocomposite incorporating graphene that features remarkably enhanced mechanical properties and microstructure. We first established a technologically simple and efficient method for producing a cement–graphene composite, which can be easily employed in industrial scale applications. At the core of our approach is graphene obtained by electrochemical exfoliation of graphite, dried and wiped through the set of sieves, prior to mixing with cement. The preparation method described allows manufacturing a nanocomposite without the use of surfactants or any special treatment to obtain the homogenous dispersion of graphene within cement matrix. In this regard, our composite significantly outperforms previously reported composites with graphene derivatives. Electrochemically exfoliated graphene proved not to aggregate in alkaline environment and thereby does not reduce the fluidity and workability of cement mortars. We then investigated the mechanical properties of produced composites. Remarkably, the addition of 0.05 wt% of graphene to ordinary Portland cement results in significant enhancement of tensile strength up to 79% and, simultaneously, slight increase of compressive strength and Young's modulus by 8% and 9%, respectively. As revealed by further structural characterization, graphene promotes the hydration reactions of calcium silicates, thus resulting in much more intense formation of C‐S‐H phase, as well as regular and compact microstructure. Overall, such findings provide unambiguous evidence that our composite can boost the practical application of graphene in concrete technology. Moreover, the improved performance of cementitious composites incorporating graphene, in particular the significantly enhanced tensile strength, will allow designing lighter concrete structures with extended durability. Therefore, the consumption of concrete components may be reduced, thus mitigating the environmentally harmful impacts of concrete production.4Materials: Cement, sand, distilled water, graphene, graphene oxide, and graphite flakes were used in this study to fabricate cement mortar. In particular, two types of cement were employed, which were provided by Gorazdze Cement S.A.: CEM I 42.5R (Portland cement Type I) and CEM II/B‐S 32.5R‐NA (Portland cement Type II with granulated blast furnace slag). The chemical composition of the cement is presented in Table S2 (Supporting Information). The mechanical and other physical properties of the cement are reported in Table S1 (Supporting Information). The results of sand sieve analysis are plotted in Figure S11 (Supporting Information). Dispersion of graphene oxide sheets with concentration of 4 mg mL−1 was purchased from Graphenea Inc. Graphite flakes were purchased from Sigma‐Aldrich. Graphene was produced by electrochemical exfoliation of graphite foil  using a platinum wire  and aqueous solution of ammonium sulfate (NH4)2SO4 (Sigma‐Aldrich). To stop cement hydration, acetone (Carlo Erba Reagents) was used in a process of solvent replacement and liquid nitrogen in a process of freeze‐drying. Dispersions of graphene and graphene oxide in alkaline environment were investigated by using saturated aqueous solution of calcium hydroxide Ca(OH)2 (Sigma‐Aldrich).Electrochemical Exfoliation of Graphite Foil: Graphite foil cut into pieces of 2.5 cm x 6 cm was used as anode and platinum wire was used as cathode in an electrolytic cell. Both elements of the electrolytic cell were immersed in aqueous solution of (NH4)2SO4 with concentration of 0.1 m. A starting current of 0.4 A generated by the applied voltage of 15 V (ISO‐TECH IPS‐603 DC power supply) prompted the exfoliation of graphite foil. The exfoliation was conducted for ≈3 h, i.e., until the graphite foil was completely exfoliated. The resulting material was then rinsed several times and filtrated via polytetrafluoroethylene (PTFE) membranes (pores diameter of 5 µm). Finally, collected materials were dried at 60 °C for at least 48 h.Cement Mortar Preparation: Cement mortar was prepared by mixing cement, sand, water, and graphene/graphene oxide/graphite flakes. The water‐to‐cement ratio was kept at 0.5. The sand‐to‐cement ratio was also kept constant in all mixes and it was equal to 3.0. A plain mortar (labeled as R), five mixtures with EEG (labeled as G) and three mixtures containing GO were prepared for each type of cement (CI or CII). Graphene was incorporated in the cement mortar at five different percentages, namely, 0.01 wt%, 0.03 wt%, 0.05 wt%, 0.075 wt%, and 0.1 wt% (dosage by weight of cement). The content of graphene oxide was 0.01 wt%, 0.03 wt%, and 0.05 wt%. Cement mortar with a higher dosage of graphene oxide was not produced due to the reduction of fluidity and setting time, thus resulting in poor workability. Additionally, a reference mixture with 0.05 wt% dosage of GFs was fabricated using CEM I.For cement mortar containing graphene, dried graphene was wiped through the set of five sieves with the finest mesh sieve size being 250 µm. Graphene and cement were stirred sufficiently at low speed (≈140 rpm) using a hand mixer to obtain a homogenous dry mixture. Mixing procedure from PN‐EN 196‐1:2005 was then adopted. Water was added to cement with graphene and the mixer was immediately started at low speed for 30 s. After that, sand was steadily added during the next 30 s and the mixer was switched to high speed (≈285 rpm) for additional 30 s. Afterward, the mixer was stopped to remove all the mortar adhering the walls of the bowl and then mixing was continued at high speed for 60 s. For samples with graphite, graphite flakes were also added to cement prior to pouring water, whereas for composites with graphene oxide, the GO dispersion was added to cement simultaneously with water. All resulting cement composites were placed into steel cylindrical molds with the diameter of 60 mm and the height of 120 mm. Cement mortar was placed in the molds in a few layers and each layer was subjected to vibration on a vibration table for 1 min to ensure the compaction of the composite. All specimens were immediately covered by polyethylene foil to prevent loss of water. After 24 h, the hardened cement mortar samples were demolded and continued to be cured in water at 20 °C. All cylindrical samples were dried in the air for 24 h before performing mechanical tests.Characterization and Measurements: In order to investigate the dispersion of graphene and graphene oxide in alkaline environment, 3.0 mL of graphene and graphene oxide aqueous dispersions with concentration of 0.07 mg mL−1 were first prepared. Then 100 µL of saturated aqueous Ca(OH)2 solution was introduced into both dispersions.The consistency of fresh mortars was determined according to PN‐EN 1015‐4 by the plunger penetration method. The sample was placed in the vessel in two layers and each layer was subjected to vibration on a vibration table. The vessel was then put on the base plate under the plunger. The plunger was allowed to freely fall from the height of 100 mm above the vessel. The depth of the plunger penetration was determined as a consistency measure. Two measurements were performed for each cement mortar and the average was taken.−1. The compressive strength tests were performed using MEGA 3‐3000‐100 compressive strength tester (Form+Test Prüfsysteme) at a loading rate of 0.5 kN s−1. Five samples of cement mortar were prepared for each test and the average was taken. To obtain the compression stress–strain curve, linear polyester strain gauges with gauge factor of 2.13  were employed. Two pairs of strain gauges were attached to the two sides of the sample to measure axial and transverse strain. On the basis of the strain gauges measurements, the modulus of elasticity was calculated. According to PN‐EN 1992‐1‐1:2008 the Young's modulus was determined as a secant value between zero stresses and stresses equal to 40% of the average compressive strength.Mechanical properties tests were conducted on cylindrical samples  at the age of 28 d. The tensile strength was determined using ZD 10/90 tensile strength tester (Heckert) at a loading rate of 1.0 kN sInhibiting the cement hydration is essential to prepare cement composites' samples for microstructure and composition characterization by means of TGA, SEM, XRD, FTIR, or BET analyses. Two different methods for stopping the cement hydration in order to obtain the most satisfactory and precise results were employed: freeze‐drying to effectively preserve the composition of cement mortar for TGA, XRD, and FTIR analyses and solvent replacement method to avoid damage of pores and alternations to the cement mortar microstructure for SEM and BET.The microstructure of the fracture surface of hardened cement mortar was investigated by SEM using FEI Dual Beam 235 with the accelerating voltage of 5 keV incident beam energy. The pieces of crushed cement composites after the solvent replacement process were glued to a support with conductive carbon adhesive and the fracture surface was sputter‐coated with a thin layer of gold prior to SEM imaging.For TGA, XRD, and FTIR analyses, the crushed pieces of cement composites were, initially, grounded completely into fine powder and filtered through a 250 µm sieve to remove coarse grains of sand.−1. All experiments were performed under air atmosphere.TGA was performed using Mettler Toledo TGA/DSC 2 sensor using alumina crucibles. Three samples of ≈15 mg of each cement mortar were, first, kept isothermally at 30 °C for 30 min and then heated from 30 to 1000 °C at a heating rate of 10 °C minXRD was carried out on Bruker ASX D8 Advanced with Cu anode with Kα radiation (λ = 1.5418 Å). Diffraction patterns were collected at room temperature in the scattered angular range between 6° and 60°. An angular resolution and a typical counting time per each step were of 0.02° and 10 s, respectively.−1) by using a Perkin Elmer Spectrometer (Spectrum Two) equipped with ATR Diamond.FTIR spectra were obtained within the mid‐IR range (400–4000 cm−9 mbar and an Al anode as the X‐ray source (X‐ray radiation of 1486 eV). Spot size was of 400 µm. Pass energies of 200.00 eV for wide energy scans and 10.00–20.00 eV for scans were used.XPS analyses were performed on a photoelectron spectrometer with a basic chamber pressure of −10The morphology of electrochemically exfoliated graphene was investigated using a Veeco Dimension 3100 AFM with a Nanoscope IV control unit under ambient condition. Topographic imaging was carried out in tapping mode with the use of antimony (n) doped silicon cantilever.The specific surface area was measured using a Micromeritics ASAP 2050 surface area and porosity analyzer. Prior to the BET measurements, the samples were outgassed for 10 h at 100 °C. Adsorption isotherms were calculated for nitrogen adsorption at 77 K and pressures up to 1 bar.The authors declare no conflict of interest.SupplementaryClick here for additional data file.


Polarization-insensitive modulation, i.e., overcoming the limit of conventional modulators operating under only a single-polarization state, is desirable for high-capacity on-chip optical interconnects. Here, we propose a hybrid graphene-silicon-based polarization-insensitive electro-absorption modulator (EAM) with high-modulation efficiency and ultra-broad bandwidth. The hybrid graphene-silicon waveguide is formed by leveraging multi-deposited and multi-transferred methods to enable light interaction with graphene layers in its intense field distribution region instead of the commonly used weak cladding region, thus resulting in enhanced light–graphene interaction. By optimizing the dimensions of all hybrid graphene-silicon waveguide layers, polarization-insensitive modulation is achieved with a modulation efficiency (ME) of ~1.11 dB/µm for both polarizations (ME discrepancy < 0.006 dB/µm), which outperforms that of previous reports. Based on this excellent modulation performance, we designed a hybrid graphene-silicon-based EAM with a length of only 20 µm. The modulation depth (MD) and insertion loss obtained were higher than 22 dB and lower than 0.23 dB at 1.55 µm, respectively, for both polarizations. Meanwhile, its allowable bandwidth can exceed 300 nm by keeping MD more than 20 dB and MD discrepancy less than 2 dB, simultaneously, and its electrical properties were also analyzed. Therefore, the proposed device can be applied in on-chip optical interconnects. Silicon photonics based on the mature silicon-on-insulator (SOI) platform has aroused tremendous interest for building compact, high performance, and energy-efficient photonic integrated circuits (PICs), which has also promoted the rapid development of optical interconnects, especially for data center and telecom applications ,2,3,4. T2·V−1·s−1 at room temperature) , imaginary [Im(neff)]) of the hybrid graphene-silicon waveguide as a function of the graphene chemical potential (µc) for both transverse electric (TE) and transverse magnetic (TM) modes at λ = 1.55 µm where the modal calculation is based on the above-mentioned surface conductivity model of graphene. The calculation engine is available from a commercial software package  where the MD of the TM mode has a peak value at ΔH = −20 nm. Moreover, the polarization-insensitive property of the proposed EAM remains unaffected if ΔW and ΔH are kept within the ranges of  and , respectively, which gives a relatively large tolerance for fabrication. For the device length L, the variations of transmittance and MD of the TE and TM modes are nearly the same, showing excellent polarization-insensitive properties. In addition, a higher MD for this device can be achieved only by increasing the device length because its insertion loss is very low and increases slowly owing to the small absorption loss at the “ON” state for both polarizations. Finally, Next, to investigate the performance deterioration brought by fabrication imperfections in practice, we analyzed the fabrication tolerances of the width f3dB) and the energy consumption (E), which are associated with the total capacitance (C) and resistance (R) of the optical device, f3dB = 1/(2πRC), E = CV2/4 (V stands for the switching voltage between “OFF” and “ON” states) [C) is estimated to be ~326 fF. Thus, f3dB and E are derived to be ~6.1 GHz and ~7.8 pJ/bit, respectively. To further enhance the electrical properties of the proposed EAM, its total resistance and capacitance should be effectively reduced, especially for the capacitance, which can be realized by cutting down the overlapping area between two corresponding graphene layers, choosing the spacer/insulating layer with the lower dielectric constant, and increasing the thickness of the spacer/insulating layer. In addition, traveling-wave electrode designs could also be leveraged to enhance the modulation performance of the proposed EAM [Besides the above-analyzed optical properties of the proposed hybrid graphene-silicon-based polarization-insensitive EAM, its corresponding electrical characteristics are also critical, such as the 3 dB modulation bandwidth ( states) ,42. Here states) ,28,29. Bosed EAM .In summary, using multi-deposited and multi-transferred methods, we propose a highly-efficient, ultra-broadband, and polarization-insensitive EAM, the key part of which is a hybrid graphene-silicon waveguide formed by five inverted U-shaped graphene-silicon layers. In order to effectively enhance LGI, multi-deposited and multi-transferred graphene-silicon layers are employed such that light interacts with the graphene layers at high optical intensity in the interior of the hybrid graphene-silicon waveguide rather than at weak optical intensity in the cladding region, as commonly done. We then optimized the dimensions of each hybrid graphene-silicon layer based on the inverted U-shaped waveguide structure and achieve an ME of ~1.11 dB/µm and an ME discrepancy of lower than 0.006 dB/µm for both polarizations, which are significant improvements over previous results. By using this structure, we designed and analyzed a hybrid graphene-silicon-based polarization-insensitive EAM only 20 µm long. The MD and IL were calculated to be > 22 dB and < 0.23 dB at 1.55 µm, respectively, and the available bandwidth can be over 300 nm (from 1.367 to 1.668 µm) by simultaneously keeping MD greater than 20 dB and MD discrepancy less than 2 dB. The MD of the designed EAM can be further improved by increasing its length. The electrical properties of the EAM  were studied. Several methods were also proposed to enhance its electrical features. With high ME, ultra-broad bandwidth, polarization-insensitivity, compact footprint, and scalable performance, the proposed hybrid graphene-silicon-based EAM will find applications in the high performance hybrid silicon modulators for high capacity on-chip optical interconnects.


During the past decades, major advances have been made in both the generation and detection of infrared light; however, its efficient wavefront manipulation and information processing still encounter great challenges. Efficient and fast optoelectronic modulators and spatial light modulators are required for mid-infrared imaging, sensing, security screening, communication and navigation, to name a few. However, their development remains elusive, and prevailing methods reported so far have suffered from drawbacks that significantly limit their practical applications. In this study, by leveraging graphene and metasurfaces, we demonstrate a high-performance free-space mid-infrared modulator operating at gigahertz speeds, low gate voltage and room temperature. We further pixelate the hybrid graphene metasurface to form a prototype spatial light modulator for high frame rate single-pixel imaging, suggesting orders of magnitude improvement over conventional liquid crystal or micromirror-based spatial light modulators. This work opens up the possibility of exploring wavefront engineering for infrared technologies for which fast temporal and spatial modulations are indispensable. 2O3, an amorphous-Si layer and a rear reflector made of gold. The structure also features gate and drain electrodes to electrically tune the device. When constructed into a 6x6 pixel array the modulators allow the realization of a SLM for proof-of-principle high frame rate single-pixel imaging in the mid-IR.Ultrafast modulators and spatial light modulators (SLMs) for mid-infrared light have been realized by the use of an electrically-tunable graphene on a silicon-integrated metasurface. The modulators designed and built by Beibei Zeng and coworkers from Los Alamos National Laboratory, Duke University and Arizona State University in the US offer a depth of modulation of 90% and a modulation speed exceeding 1 GHz. The modulator consists of a sandwich structure consisting of an array of gold nanoantennas, a layer of graphene, atop a thin dielectric film of Al For example, solid-state infrared light modulators based on semiconductor quantum wells either have a small modulation depth or require sophisticated material growth and cryogenic temperatures7. In addition, state-of-the-art liquid crystal8 and micromirror9-based spatial light modulators (SLMs) suffer from limitations including slow modulation speeds of a few kilohertz and complex and expensive instrumentation. In this context, active metamaterials/metasurfaces composed of planar subwavelength resonators and reconfigurable functional materials/devices have provided an alternative approach10. In the microwave region, the integration of electronic devices  has resulted in reprogrammable coding metasurfaces13. In the far-infrared region, the integration of metasurfaces and compound semiconductors  has enabled the modulation of terahertz (THz) waves with high modulation depth and speed at room temperature17. However, such configurations are difficult to scale up to the mid- and near-infrared regions, where electronic devices fail to operate and the optical conductivity of conventional semiconductors can only be slightly adjusted19, resulting in limited dynamic modulations.Emerging infrared technologies lie at the core of advanced photonic research because of their great potential for numerous applications. Despite the tremendous efforts in the development of sources23. In addition, the compatibility of graphene with CMOS processing makes it an extremely promising material for cost-effective optoelectronics in high-frequency applications25. Most recently, hybrid graphene metasurfaces have shown unprecedented capabilities in developing electrically reconfigurable infrared optoelectronic devices47, although in these designs the modulation relies on a high voltage bias and the modulation speed is limited, and many studies have focused on a waveguide configuration31. In this paper, we present the experimental demonstration of hybrid graphene metasurface free-space mid-infrared modulators that enable a large intensity modulation depth of up to 90% and a high modulation speed exceeding 1 GHz over a broad bandwidth by tuning the Fermi level of graphene at a low gate voltage bias of ∼7 V. The low-voltage operation achieved by the design architecture of our device is essential for obtaining a high-quality modulation signal and an ultra-high modulation speed under ambient conditions. Furthermore, a prototype mid-infrared SLM is demonstrated to function as an electrically encoded aperture or mask, realizing single-pixel imaging with a high frame rate.In contrast, graphene has demonstrated exceptional electrical and optical properties and a widely tunable electro-optical response through the entire infrared region47, which also limits the modulation speed and the scope of applications. Operating at mid-infrared wavelengths, the high gate voltage is due to the thick dielectric spacer  between the metal ground plane and metallic resonator array32. When graphene is integrated into the resonator array, the metamaterial absorber is essentially a field-effect transistor (FET) structure in which the application of a gate voltage Vg tunes the graphene conductivity by varying the Fermi level μC according to33μC= ħv(πCVg/e)1/2. To lower the gate voltage, it is necessary to increase the capacitance C by decreasing the gate dielectric thickness . Although we cannot reduce the dielectric spacer thickness of a metamaterial absorber, an alternative solution is to replace a large portion of the dielectric spacer with a material that is conducting for the applied voltage bias but behaves as a dielectric in the mid-infrared range. This strategy is demonstrated by using the FET structure schematically shown in Fig. 2O3 layer to increase the capacitance. In this way, we are able to significantly reduce the gate voltage to a few volts to reach the graphene charge neutrality point and effectively tune the graphene conductivity, as shown in Fig. Ids and resistance Rds as functions of gate voltage Vg for a few different drain-source biases Vds.A major issue in frequency-tunable hybrid graphene metamaterial absorbers is the requirement of a high voltage bias34. That is, the incident light is trapped within the metamaterial cavity and eventually absorbed by the metallic structure, the lossy dielectric spacer, or both. Because the optical loss of a-Si is negligible in the mid-infrared region, the absorption mainly occurs via ohmic dissipation at the metallic resonator array integrated with graphene. These two facts give rise to the possibility of tuning the resonant dispersion of the nanoantenna array with the goal of satisfying the phase condition and thereby the absorption at a different wavelength by tuning the conductivity of the integrated graphene through the application of a gate voltage. In Fig. λ = 7.3 μm (reflection minimum ∼16%) to 8.3 μm (reflection minimum ∼5%) when the low gate voltage changes from +7 V to −3 V. The corresponding change of reflection ∆R(λ) = R+7 V(λ) − R−3 V(λ) and modulation depth M = |∆R(λ)|/max are plotted in Fig. λ = 8.5 μm and a modulation depth as high as ∼90% in this wavelength range.Following this strategy, our hybrid graphene metasurface structure is schematically shown in Fig. 36. The gate-tunable plasmonic modes are clearly shown in Fig. λ = 8.3 μm, corresponding to the reflection minimum in Fig. λ = 7.3 μm, as shown in Fig. λ = 7.3 μm, as shown in Fig. λ = 8.3 μm, as shown in Fig. The shifting of the plasmonic resonance dispersion can be visualized through near-field imaging of localized plasmonic modes in the hybrid graphene metasurface with different gate voltages15 with a resistance of either R0 = 2 kΩ or 4 kΩ. The cyan circles and green triangles in Fig. R0 = 2 kΩ and 4 kΩ external resistors, respectively, from which we can clearly observe the different modulation cut-off frequencies. Using the high-frequency circuit model27, the experimental data are fitted with a set of circuit parameters. The results are shown as the solid curves. Based on the same set of device circuit parameters, the intrinsic modulation is shown as the red curve in Fig. In addition to the high modulation depth, the hybrid graphene metasurface modulator enables an ultra-high modulation speed. Our characterization system, schematically shown in the inset to Fig. λ = 8.3 μm. By simultaneously turning on the functional pixels in the corresponding mask patterns, the red curve in Fig. λ = 7 μm and 8.5 μm due to their excellent modulation depth, in contrast to the lower imaging contrast at λ = 5.5 μm and 9.5 μm due to their reduced modulation depth  in the spacer of the metamaterial absorber. For the mid-infrared light, the a-Si layer can be considered as a dielectric material with negligible loss, but the semiconducting properties make it suitable to serve as part of the electrode for applying a gate voltage bias to tune the graphene conductivity, resulting in shifted/damped resonant dispersion of the nanoantenna array and thus frequency tuning of the metasurface absorption. The high-speed modulation is in part due to the low voltage operation and the significant reduction of parasitic capacitance from the electrodes.38. This work opens up the possibility of exploring wavefront engineering for infrared technologies for which fast temporal and spatial modulations are indispensable, and the demonstrated high frame-rate imaging approach holds great potential for many applications, such as the detection of transient thermal phenomena, real-time thermal imaging, security screening, navigation, and medical detection.The hybrid graphene metasurface modulator was then formed into a pixel array to serve as a prototype mid-infrared SLM. We showed that single-pixel imaging can be achieved by creating a series of spatial mask patterns, revealing orders of magnitude faster imaging speeds than those obtained using traditional liquid crystal and micromirror spatial light modulators. The speed of single-pixel imaging can be further enhanced by adopting a compressive single-pixel imaging algorithm39. We would like to emphasize that the prototype mid-infrared SLM can be scaled to operate at other frequencies, including terahertz, near-infrared, and even visible, since graphene has been proven to maintain the high optical conductivity and have opto-electronic responses at these frequencies due to the intra-band (terahertz and mid-infrared) and inter-band (near-infrared and visible) transitions42. The basic principle is the same, but when approaching shorter wavelengths, the concern is the increasing absorption in the amorphous silicon spacer layer, which can potentially reduce the modulation depth. Additionally, when scaling to operate at shorter wavelengths, the further reduced critical dimension of the metasurface structure and back gate array can potentially create fabrication challenges.We note that the optics in Fig. 2O3 gate dielectric grown by atomic layer deposition (ALD), two photolithography-defined 150 nm-thick gold strips serving as source and drain electrodes, and a transferred monolayer graphene grown by low-pressure chemical vapor deposition (LPCVD). Figure 43. Compared to the FET structure shown in Fig. 2O3 insulating layer and 3 nm/30 nm Ti/Au layers and a lift-off process. Graphene was then transferred and patterned to align with the electrode array, with graphene between electrodes removed by photolithography and O2 plasma etching, defining electrically isolated functional pixels as depicted by the optical micrograph in Fig. Figure 45. The copper foil was first loaded in a quartz tube and annealed at 1000 °C for 1 h while maintaining the H2/Ar pressure at 1 Torr under a 200 sccm flow. Graphene was then grown for 30 min by introducing 50 sccm CH4 at a total pressure of 1.7 Torr. The furnace was then rapidly cooled to room temperature. The growth conditions outlined above produced a large-area, continuous graphene film with very few multi-layer regions or cracks. To transfer the graphene layer to the substrate, PMMA (950 A4) was spin-coated onto one side of the graphene/copper/graphene foil and stored in vacuum overnight. The graphene on the backside was then removed with O2 plasma etching. The sample was then placed in an FeCl3-based etchant for 2 h, allowing the copper to completely dissolve. The remaining graphene/PMMA film was cleaned in HCl solution, rinsed with DI water, transferred onto the substrate, and put into acetone to remove the PMMA layer.Graphene was grown on a piece of copper foil  in a home-built LPCVD system36 equipped with a broadband infrared beam by difference frequency generation (DFG) to cover a wide mid-infrared range from 650 to 2400 cm−1. The raw data contain spectral information due to the use of broadband illumination, and we used Matlab programming to extract near-field images at specific wavelengths. The modulation speed was measured for individual pixels using the setup schematically shown in the inset to Fig. λ = 6.75 μm) was transmitted through a beam splitter (55/45) and focused onto the hybrid graphene metasurface modulator with a mid-infrared objective. The reflected light was collected by the same objective, reflected by the beam splitter, and focused into the TE-MCT detector using an off-axis parabolic mirror. A functional pixel of the device was applied with  a variable frequency sine voltage with offset 0 V and amplitude 1 V from a function generator integrated with an oscilloscope . The response of the TE-MCT detector was measured with the oscilloscope, determining the optical modulation signal of the device. Note that we were only able to measure the modulation frequency up to 1 GHz due to the limited frequency range of the function generator and TE-MCT detector and the low signal-to-noise ratio of our measurement setup. Reduction of the modulation cut-off frequency (at 3 dB) within the measurement range was achieved by inserting different series resistors  into the driving circuit15. The intrinsic modulation speed was then inferred using a high-frequency circuit model of the device. The spatial light modulation was characterized using the FTIR microscope by raster scanning the functional pixels—measuring the reflection intensity using a single-pixel detector as we turned only one pixel ‘ON’ or ‘OFF’ while all other pixels remained ‘OFF’. The imaging capability of the hybrid graphene metasurface SLM was demonstrated using the setup schematically shown in Fig. X) is projected onto the SLM through an infrared objective lens. The reflected light thus represents the image encoded by the SLM mask (forming the measurement matrix Φ) and is collected by the same objective and focused onto the MCT single-pixel detector (Y). For a proof-of-concept demonstration, a raster-scan measurement matrix Φ of the SLM mask was employed by sequentially turning on/off only one pixel for each measurement, resulting in a 36 × 36 identity matrix Φ. The original image X was then reconstructed by X = Φ−1Y, where Φ−1 is the inverse measurement matrix.Each pixel of the 6 × 6 pixel array was independently connected to one of the 36 channels  of a DAC evaluation board , which was then controlled by a microprocessor  using home-built Python codes. The reflection spectra of the individual pixels under different voltage biases were measured using a Fourier transform infrared spectroscopy microscope . The plasmonic resonance modes were visualized using a scattering-type near-field scanning optical microscope CE (CG) represents the capacitance between the top Au electrode (graphene) and the a-Si layer across the 750 nm-thick (6 nm-thick) Al2O3 insulating layer, RG and RS are the resistances of the graphene and a-Si, respectively, and R0 is the externally loaded resistance. The high-frequency modulation voltage from a function generator was applied to the device as the gate voltage Vg(ω), and the modulation voltage Vm(ω) at the functional pixel and the frequency-dependent modulation depth η(ω) of the device were derived asA simplified high-frequency circuit model of the device was developed, as shown in the inset to Fig.


The observed potential energy surfaces, representing the intermolecular interactions within the CH4/N2 gaseous mixtures and between these and the graphene layers, have been formulated by adopting the so-called Improved Lennard-Jones (ILJ) potential, which is far more accurate than the traditional Lennard-Jones potential. Previously derived ILJ force fields are used to perform extensive molecular dynamics simulations on graphene's ability to separate and adsorb the CH4/N2 mixture. Furthermore, the intramolecular interactions within graphene were explicitly considered since they are responsible for its flexibility and the consequent out-of-plane movements of the constituting carbon atoms. The effects on the adsorption capacity of graphene caused by introducing its flexibility in the simulations are assessed via comparison of different intramolecular force fields giving account of flexibility against a simplified less realistic model that considers graphene to be rigid. The accuracy of the potentials guarantees a quantitative description of the interactions and trustable results for the dynamics, as long as the appropriate set of intramolecular and intermolecular force fields is chosen. In particular it is shown that only if the flexibility of graphene is explicitly taken into account, a simple united-atom interaction potential can provide correct predictions. Conversely, when using an atomistic model, neglecting in the simulations the intrinsic flexibility of the graphene sheet has a minor effect. From a practical point of view, the global analysis of the whole set of results proves that these nanostructures are versatile materials competitive with other carbon-based adsorbing membranes suitable to cope with CH4 and N2 adsorption.We theoretically investigate graphene layers, proposing them as membranes of subnanometer size suitable for CH Graphene has often been investigated as a possible material for the separation of small gases  results as was explained in previous publications  from now on in this work  over the graphene sheet. The molecules were placed such that at least 5 Å were left in between the different molecules, the edges of the box and the graphene sheet to avoid instantaneous, strongly repulsive interactions at the start of the simulation. We then ran one NVE simulation to allow the system to relax to a physically viable conformation and used the resulting output as input for a first NVT simulation. After this NVT simulation, we kept the molecules that were adsorbed (see below) and deleted the ones that were not. Subsequently, we randomly distributed 100 new gas molecules (50 CH4 + 50 N2) over the remaining system from the previous simulation to run a new NVT simulation. This protocol was repeated until convergence of the amount of adsorbed molecules. As such, this protocol gives an indication of the amount of molecules that saturates the first adsorption layer and it allows comparison for the different intramolecular force fields and molecular models under study.Since the stronger affinity of graphene for methane than for nitrogen was clearly proven in this previous work, we expect that the graphene sheet could effectively separate this gas mixture. Therefore, in a first set of simulations we have adopted a protocol where we randomly allocated 100 gas molecules , while this fraction lowers in subsequent simulations. In the first simulations, the amount of molecules that is introduced in the system is not enough to fully saturate the graphene sheet and thus most molecules stay will adsorb onto the sheet. When more molecules are gradually introduced, the graphene sheet gets quickly saturated and more molecules will be forced to stay in gas phase in equilibrium with the adsorbed molecules. A closer inspection of the results for methane and nitrogen within the mixture reveals a changing methane/nitrogen ratio from one simulation to the next. Indeed, it can be seen that in the first simulations, a substantial amount of nitrogen molecules is adsorbed on the graphene sheet , while in subsequent simulations, they are steadily removed from the adsorbed layer. The methane molecules are adsorbed preferentially, but in the first simulations there are not enough molecules to completely saturate the graphene sheet allowing nitrogen molecules to adsorb as well. When more methane molecules are added, they occupy more and more space on the graphene surface allowing less space to nitrogen, which is forced to stay in gas phase. In the end, a situation is reached where about 70% of the molecules in the simulation are methane molecules and 30% are nitrogen, consistent over the four graphene sheets. Adding to this, the fraction of methane molecules that adsorbs is much larger than the fraction of nitrogen molecules, giving a first indication that indeed the graphene sheet is well capable of separating methane and nitrogen from each other.4/N2 mixture, this results is no surprise. Dreisbach et al.  is lower than when it is explicitly included (about 65%). As methane is dominantly present in the mixture, this result is clearly a reflection of the results for methane within the mixture. While the simulation predicts an adsorption of about 70% of the available methane , the flexible ones increases the prediction to up about 80% of the available methane. For nitrogen, the difference is smaller and anyway with less influence on the total result since the initial fraction is much lower. Keeping in mind that for the pure methane, the united-atom model predicted a lower uptake on the rigid sheet  a strong second adsorption layer is formed. As before, the assumed rigid sheet adsorbs slightly less nitrogen molecules than the flexible sheets.Looking at the simulations with the atomistic model, the behavior changes quite a bit: the first adsorption layer for nitrogen is smaller for the simulation with the most (350) molecules compared to the other simulations with 150 and 250 methane molecules. This is a consequence of the strong methane adsorption predicted by the atomistic model: the stronger competition allows less nitrogen molecules to enter the first adsorption layer. In the simulation with 150 molecules, the first adsorption layer is not yet completely saturated by methane and nitrogen is allowed space in the first adsorption layer. As the amount of molecules increases, less and less space remains available for nitrogen and the first adsorption peak of nitrogen decreases as a result. Already in the simulation with 150 molecules, there is a small second adsorption layer present, which increases strongly upon increasing the amount of molecules in the system. In the simulation with 350 nitrogen molecules, there are very little nitrogen molecules present in the first adsorption layer, but there is a relatively large second adsorption layer and subsequent gas phase.Due to the changing widths of the adsorption peaks—due to the internal graphene movement—in For nitrogen, it can be seen that the united-atom model predicts a converged adsorption within the investigated pressure range due to the strong methane competition. The rigid model predicts a slightly lower uptake and slower convergence than the flexible ones. For the atomistic model it is striking that, due to the very strong methane adsorption, the simulation with 350 molecules has very little nitrogen adsorbed on the sheet leading to a very low uptake. For this reason, we have fitted the Langmuir isotherm using only the first two simulation results as it is not intended to describe such a sudden drop in the uptake. Furthermore, the drop is caused by the methane more than by the nitrogen itself. The atomistic model predicts a very slightly lower uptake of nitrogen in the rigid simulation than when considering the flexible representations. As was expected, more nitrogen is adsorbed using the united-atom model than the atomistic model, because the methane is less strongly attracted to the graphene sheet in that case.Kumar and Rodríguez-Reinoso  have invFinally, we have calculated the selectivity as followsx is the mole fraction adsorbed of the specified molecule, while y is the mole fraction in the bulk of the specified molecule.where, Even though (Dreisbach et al., In this work, we have studied the influence of the flexibility of the graphene sheet on its separation ability of the methane/nitrogen gas mixture. The flexibility was introduced via three intramolecular force fields taken from the literature, two of which contained stretching, bonding and torsional terms, while a third one lacked the latter contribution. Two fields were specifically designed for graphene, while a third one was originally intended for use on carbon nanotubes. Furthermore we studied the different behavior of a united-atom model and an atomistic model during these simulations.In general, we have confirmed that graphene shows a strong preference for methane over nitrogen. Using both models, the methane posed a strong competition toward the nitrogen and pushed the nitrogen out of the first adsorption layer, forcing it to form a second adsorption layer or go into gas phase. This effect was found stronger for the atomistic gas models than for the united-atom models, which is expected to give a poorer representation as it does not takes into account orientation effects. The difference, which is at times large, calls for care when assuming united-atom models in these types of simulations.Concerning the treatment of flexibility, we showed that, indeed, it influences the behavior of the graphene sheet as a separator of the methane/nitrogen gas mixture. For the united-atom model, neglecting the flexibility of the graphene sheet leads to predict lower methane uptakes that if such flexibility is taken into account. The atomistic model on the other hand predicted a slightly higher methane uptake. Similar results were found for nitrogen, leading to the conclusion that for the united-atom model, a lower amount of gas molecules in general is predicted to be adsorbed if the sheet is supposed to be rigid. The atomistic model, on the other hand, predicts a larger general gas adsorption also on the considered rigid graphene sheet. Observing the combination of these results in the selectivity of the four models of the intramolecular motions of graphene, we find that the united-atom model in conjunction with the rigid representation predicts the lowest selectivity for the methane/nitrogen mixture in the largest portion of the pressure range investigated, while the atomistic model shows very similar behavior for the rigid and the flexible graphene models. Moreover, since graphene is, in fact, flexible, it is then clear that in order to safely disregard its internal movements an atomistic model is definitely required, the simplified united-atom model being absolutely inadequate. On the other hand, if the flexibility of graphene is explicitly accounted for by means of the appropriate force field -the field 2 m (Fthenakis et al., JV, IG, and AS ran the first principles calculations and developed the subsequent interaction force field. NF-L, AL, and MR developed and ran the theoretical and molecular dynamics calculations.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.