The process is triggered by a self-assembled copper faceting process during high-temperature graphene chemical vapor deposition, which defines interfaces between different graphene doping levels at the atomic level.The exceptional electronic properties of monatomic thin graphene sheets triggered numerous original transport concepts, pushing quantum physics into the realm of device technology for electronics, optoelectronics and thermoelectrics. At the conceptual pivot point is the particular two-dimensional massless Dirac fermion character of graphene charge carriers and its volitional modification by intrinsic or extrinsic means. Here, interfaces between different electronic and structural graphene modifications promise exciting physics and functionality, in particular when fabricated with atomic precision. In this study we show that quasiperiodic modulations of doping levels can be imprinted down to the nanoscale in monolayer graphene sheets. Vicinal copper surfaces allow to alternate graphene carrier densities by several 10 However, the crucial step towards a targeted realisation of heterogeneous graphene properties, mostly relying on lithography techniques, systematically faces spurious degradation of the structure and performance of devices. Yet, heterogeneous properties majorly widen the options for electronics and for experiments on exciting fundamental physics: 1D grain boundaries between different honeycomb lattice orientations can be exploited to achieve variable bandgaps for optoelectronics in otherwise semi-metallic graphenep−n junctions as a basis for transistor functionality5p−n transitions is governed by doping levels and their spatial abruptness8910Immense progress was achieved in recent years on fabricating high-quality homogeneous graphene sheets with small defect densities, reaching high carrier mobilities up to several 100,000 cmSupporting metallic surfaces are rich playgrounds for these concepts, moreover offering the prospect of large scale production of high-quality graphene via chemical vapor deposition (CVD). Indeed, metals may exhibit coexisting surface terminations with different interaction potentials and the potential to trigger variations in graphene doping15k-PEEM). Sampling local topography and diffraction, we show that a nanometer-scale periodic structuration and electronic doping of several 1013 carriers per cm2 can be achieved straightforwardly in graphene, as-grown by CVD on high-index vicinal copper. The pattern consists of a roof-top-like alternation of Cu facets of distinctive symmetries, formed by surface energy minimization at the atomic scale, which drives copper and carbon mass-transfers during high-temperature CVD. The general concept of this work, which avoids any lithography processing steps, can be extended towards other chemical vapor deposited 2D systems of current interest such as semiconducting transition metal dichalcogenides, e.g. MoS2, insulating hexagonal boron nitride (h-BN) monolayers, and respective hybrid structures.In this article we report an unprecedented 1D quasiperiodic modulation of graphene electron doping, probed by spatial mapping of the electronic band structure in wave-vector-resolved photoemission microscopy , low-energy electron diffraction (LEED), and X-ray photoemission spectroscopy (XPS) measurements under ultra-high vacuum (UHV) conditions, the samples were annealed k-PEEM pattern in + ion sputtering followed by 300 °C annealing in UHV. The Fermi surface cut is tilted towards the labeled hν = 21.2 eV, the Mahan cone of the Cu(111) surface state (SS) is detected, which is shifted away from the center of the (111) orientation against the tilting direction18Electron backscattering diffraction (EBSD) in Pulsed CVD leads to a characteristic graphene island morphology described in 20k-PEEM reveal further information on the graphene morphology and its domain orientations. Analysing energy-dependent LEED patterns shown in n1, n2, and n3 aligned perpendicular to the n1 has by far the largest intensity and thus dominates the surface area. Position-dependent LEED showed that the local surface normals are homogeneously oriented over millimeter scales on the copper foil. The diffraction LEED signal (first order scattering) in n13n1 and n2.Reciprocal space methods LEED and The facets are resolved in detail by scanning tunneling microscopy (STM) in n1 facet. In contrast to the other two facets it generates a distinct background LEED pattern, which corresponds to an oxygen p(2×2) superlattice, 30° rotated with respect to the Cu(111) reciprocal lattice (see red unit cell vectors in the LEED image), recently reported by Gottardi et al. in 2015The influence of oxygen is directly evident for the dominant n2 and n3 with respect to n1|| Cu[111] with good accuracy. The rotation angles defined by the rotation vector n3 with respect to (111) is consistent with that of a From the distance and alignment of the three specular spots on the LEED screen we can estimate the relative inclination angles of k-PEEM images in μm-wide spot. They confirm that although graphene rotations φ locally vary by significant relative angles, in this case 0°, ±8° and +18°, the replica spots remain oriented perpendicular to the k-space. In accordance with ref. k-PEEM pattern will be discussed in more detail below.Our model for the nanofaceted roof-top structure in μm to about 30 μm in width. They appear bright against the dark copper oxide background due to the work function contrast in the energy-filtered PEEM imaging mode. All islands obey a two-fold mirror symmetry, and their elongation along an axis oriented parallel to the μm scale.We are interested in understanding the local influence of the faceting process on graphene growth at the earliest stage. et al. from phase-field models212930μm scale roof-top modulation. This process is facilitated by high temperatures close to the Cu melting temperature, and most likely counterbalanced by the built-up of elastic energy.Our island shapes strongly resemble those predicted recently by Meca μm width. Using the spatially resolved k-PEEM mode of our NanoESCA instrument, the ARPES signal of the particular graphene island is captured at the Fermi level EF, revealing the rather complex pattern shown in φ = 0° oriented graphene domain indicated in white dashes, which exhibits the above discussed triplet replicas  according to the local facet planes n1, n2 and n3. A second graphene domain (denoted as 4) with minor intensity appears at φ = 30°, again locked to the symmetry of the underlying Cu(111) intensity indicated by the arrow in The self-organised faceting process during CVD is accompanied by a strong modulation of the electronic properties at the nanoscale. E(k) of the three replicas at their respective K-points  are shown as -space cuts in the direction K–K′. For flat 2D graphene k-space information can be reduced to k||. The linear dispersion of valence and conduction bands touching at the so-called Dirac point (DP) is evident, typical of free-standing graphene with a dispersion E(k) = ℏνF · |k|, where νF is the Fermi velocity. At variance with free-standing graphene however, EDP is not located at EF but shifted to higher binding energies, signifying electron transfer towards the graphene system, so-called n-doping, in accordance to previous results on graphene on homogeneous single crystalline Cu(001) and Cu(111) surfacesE = (EDP − EF) coexist as a consequence of the three supporting facets with different interaction potentials. Starting from the hexagon indicated as 1 with a n-doping of ΔE = (0.4 ± 0.1) eV, the energy position of the DP with respect to the Fermi level EF shifts to larger values ΔE = (0.6 ± 0.1) eV and ΔE = (0.8 ± 0.1) eV for replica K-points 2 and 3, respectively. The differences in doping levels within the graphene island correspond to spatial modulations of the areal carrier densities, which due to the linear dispersion close to the DP are determined by the simple relation for the density of states D(ΔE) = D0 · |ΔE|, where D0 is a function of the Fermi velocity νF  × 106 m/s, giving D0 = 0.085 per (eV2 and graphene unit cell area) (see D(ΔE) then allows to calculate the transferred carrier densities per area in units cm2 via the relation n(ΔE) = D0 · ΔE2/2. Doping levels at points 1, 2, and 3 thus correspond to carrier densities of 1.6, 3.3, and 5.7 × 1013 cm−2, respectively. The large modulation Δn by several 1013 cm−2 evidences the high efficiency of the nanofaceting process as a stamp for carrier modulations.In y νF see . From threa) see . Integrak-space can be made. So far we showed bright-field PEEM images of graphene islands, where for each spot the entire k-space signal contributes for imaging. The electron optics of our NanoESCA instrument, however, also allows for dark-field (DF) imagingk-space region of interest is selected by a narrow aperture. Switching back to real space, the spatial origin of this k-space signal can be traced back in the respective PEEM image. φ = 0°) and 4 (φ = 30°) in k-space, while the rest of k-space intensity is blocked. In φ = 30° domain seed in a φ = 0° graphene host structure. Measurements on many different islands on our copper foils indeed confirm that triangular seeds are oriented along the same direction perpendicular to μm scale continuous graphene domains on a nanoscale faceted surface.The findings raise the question if even more detailed assignments between real space and  to  see . DF imag13 carriers per cm2 in a periodic manner. The concept is powerful since faceting geometry and associated directional modulation of doping levels is predefined by the vicinal orientation of the catalytic metal via the conservation of total substrate symmetryC6 rotational symmetry the faceting effect is absent, which leads to homogeneously doped graphene sheets only. A targeted manipulation of graphene based upon the choice of vicinal symmetry can thus be envisioned, using the knowledge on various surfactant-induced faceting phenomena on different surface materials at different temperaturesA is the area and γ the area dependent specific surface free energy, precise predictions are often hampered by the fact that systems not always reach their thermodynamic equilibrium due to kinetic barriers in the faceting process. Nevertheless, due to the vast available parameter space in surface science our concept is potent and can be generalized to other chemical vapor deposited 2D systems in the focus of present research, such as semiconducting transition metal dichalcogenides, e.g. MoS2, insulating hexagonal boron nitride (h-BN) monolayers.The results of this study put forward a concept to achieve nanoscale doping modulations in chemical vapor deposited single layer graphene, exploiting surface energy driven faceting processes of supporting catalytic metals. For vicinal Cu(111) we show the self-assembly of coexisting copper (111), (110), and C2 rotational symmetry similar to those predicted recently by theory on CVD kinetics under anisotropic growth conditionsk-PEEM) technique, which enables to detect both real space and k-space signals of a particular graphene island of a micrometer scale. Boundaries between rotational graphene domains resolved in the dark-field mode of k-PEEM involve abrupt changes of the faceting morphology  studied in this work, we propose that the complex bottom-up faceting process is the result of anisotropic mobilities of carbon and copper atoms during high-temperature CVD close to the copper melting temperature. Mobilities and thus mass transport are asymmetric along and perpendicular to the vector logy see , which uμm thick copper foils . A protective oxide is stripped by electrolysis in a copper sulfate solution before graphene grains are grown by CVD using the pulsed method as described in ref. For the CVD growth of graphene islands we use 25 k-PEEM, LEED, and STM the samples were annealed under UHV conditions for 30 min at temperatures of 400 °C to ensure cleanliness. If needed, graphene was removed by mild Ar+ -ion sputtering at room temperature  followed by 400 °C annealing under UHV.Prior to surface sensitive UHV analysis techniques XPS, k-PEEM measurements were done using an Omicron NanoESCA instrument with laboratory light sources. The photoemission spectrometer is based on a PEEM column and a imaging double hemispherical energy filterk-PEEM mode, which allows to detect classical X-ray photoemission spectra with monochromatized Al Kα radiation, as well as an energy dependent mapping of the Brillouin zone using a helium discharge lamp at hν = 21.2 eV with a resolution of ΔE ≈ 0.2 eV. In the k-PEEM mode the Fermi edge EF is derived from the kinetic energy at which the k-PEEM intensity is cut off. We define this energy as zero binding energy and expect the error of this Fermi level estimation to be ±0.05 eV.XPS and μm were positioned to select regions of interest in k-space. Thereafter, dark field PEEM images were taken in the respective telescopic mode. In order to minimize parasitic signals, the iris aperture was narrowed down to encompass the graphene island of choice. See also ref. For dark field measurements, apertures sizes 150 EDAX camera and software on a Tescan FERA 3 instrument. Detailed technical information can be found in ref. Bruker Dimension Icon was employed. Both EBSD and AFM were performed under ambient conditions. STM was done at room temperature under UHV conditions (p ≈ 5 × 10−11 mbar) using an Omicron VT-STM.EBSD data was acquired with an How to cite this article: Vondráček, M. et al. Nanofaceting as a stamp for periodic graphene charge carrier modulations. Sci. Rep.6, 23663; doi: 10.1038/srep23663 (2016).


The increasing incidence and poor outcome associated with MPM requires finding effective treatment for this disease. PD1/PD-L1 pathway plays a central role in tumor immune evasion and appears to be predictive and prognostic marker. PD-L1 is expressed in many different human cancers but its role in MPM has yet to be established. The aim of this study is to evaluate the expression of PD-L1 in MPM.119 MPM patients (p) from two institutions between November 2002 and February 2014 were reviewed. Formalin-fixed, paraffin-embedded tissue was stained with anti-PD-L1 (clone E1L3N). Cases showing more than 1% of tumor cells expression of PD-L1 were considered positive.PD-L1 was analyzed in 77 p with tumor tissue available and was positive in 20.7% p . PD-L1 intensity was weak in 56.2%, moderate in 25% and strong in 18.7% p. There was a significant relationship between PD-L1 expression and histology . The median survival in p PD-L1 positive was 4.79 vs 16.3 months in p PD-L1 negative (p=0.012).We have shown PD-L1 is expressed in 20% of patients, associated with no epithelioid histology and poor prognostic in MPM. Our results suggest PD-L1 warrants further exploration in selecting p for immunotherapy. Malignant pleural mesothelioma (MPM) is a locally aggressive malignancy arising from the mesothelial cells lining the pleura with a median survival time for untreated patients ranging from 4 to 12 months . MultimoThe increasing incidence and poor outcome associated with MPM require urgently novel therapeutic strategies to improve the prognosis. There is some evidence that MPM is an immunogenic tumor that induces immune recognition, infiltration of immune cells and death mediated by autoimmunity –8. CliniThe programmed cell death (PD-1/PD-L1) pathway plays a critical role in to limit the activity of T cells in peripheral tissues at the time of an inflammatory response to infection and to limit autoimmunity. In tumors this pathway controls the tumor immune escape. PD-1 receptor is a negative regulator of T-lymphocyte and acts as a coinhibitory receptor to prevent off target immune activation . PD-1 biIt has been reported that PD-L1 is broadly expressed in several malignant tumors including carcinomas of the esophagus, kidney, lung cancer and brain tumors among others –21. MoreNewly developed immune checkpoint inhibitors have shown promising results in phase I trials in several tumor types –25. PrelThere are limited data on the prevalence and the prognostic role of PD-L1 expression in MPM. A mouse model reported that PD-L1 is highly expressed in the tumor cells and within tumor stroma and PD-L1 blockade results in T cell activation . A recenTo further explore the prevalence and roles of PD-L1 in MPM, we measured the levels of PD-L1 protein using E1L3N antibody in 119 samples from two retrospective MPM cohorts. The aim of this study is to investigate the association between PD-L1 and clinicopathological parameters in MPM and the potential association with prognosis.th classification and sub-classified histologically according to WHO guidelines [One hundred nineteen consecutive cases of MPM were collected from January 2000 to April 2014 at Vall d´Hebron University Hospital and 12 de Octubre University Hospital. All patients presented histologically proven diagnosis of malignant pleural mesothelioma. Clinicopathologic information gathered included complete history, age, sex, performance status (PS), asbestos exposure, tumor stage and histology subtype. The tumor stage was defined according to the International Union Againts Cancer´s tumor-node-metastasis 7idelines .Tissue specimens were obtained from the primary mesothelioma at the time of diagnosis. No patients received prior neoadjuvant therapy. Cases included epithelioid, sarcomatoid and biphasic subtypes of malignant mesothelioma. All of the tumor samples from the 119 patients were obtained from Department of Pathology. The same method was used for each patient. Approval for the use of the tissue used in research was obtained from the ethical local committee from both hospitals and all patients gave written inform consent before enrollment. Each sample was assessed histologically for tumor tissue by two pathologists (IS and AA)Immunohistochemistry analysis was carried out in sections that were deparaffinied (EZprepTMx10) in an oven for 30 minutes at 60ºC followed by three serial xylenes incubations. Sections were then rehydrated in grades alcohols and subjected to antigen retrieval using XS Tris Buffered Saline with Tween 20 and boiled for 20 minutes. IHC using rabbit monoclonal primary PD-L1 antibody (cloneE1L3N)XP Cell Signaling at 1:1200 dilution) was carried out using 4 mm-thick FFPE tissue sections on a Benchmark XT autostainer  with standard antigen retrieval methods. The SignalStain DAB substrate kit (#8959) was used according to the manufacturer´s instructions. Human placenta was included as positive control for endogenous PD-L1. The antibodies used for tumor-infiltrating lymphocytes (TILs) were anti-human CD4  and anti-human CD8 . For anti-human CD8 deparaffinized sections were immersed into 10mmol/L of pretreated citrate buffer (pH6.0), incubated at 95ºC for 20 minutes, and allowed to cool to room temperature. For anti-human CD4, pretreated Tris-EDTA buffer (pH0.9) was used for antigen retrieval at 95º for 20 minutes.Before scoring, specimens with no tumor cells or questionable cells from inflammatory cells were excluded from analysis. All IHC stained sections were initially evaluated and scored by two pathologist and discrepancies in interpretation of scoring were resolved by consensus. Tumors with ≥1% of tumor cells stained either in membrane or cytoplasm were considered positive for PD-L1. The expression of PD-L1 was evaluated according to the intensity of the staining and scored in a furthered system as follows: 0, negative;1, weak expression;2, moderate expression but weaker than placenta; and 3, equivalent or stronger expression than placenta. To examine TILs, the number of cells per microscopic with immunoreactivity to CD4 and CD8 were counted and we defined the percentage average media in the slide as the number of TILs for each case.Data were censored at last follow up for patients without relapse or death. Associations of PD-L1 expression with clinicopathologic features were evaluated with Fisher exact tests and Wilcoxon Mann-Whitney tests. Overall survival was calculated from diagnosis of malignancy until death due to any cause or until the date of last follow-up visit for still alive patients. Survival analysis that compared PD-L1 expressing tumors was carried out using the Kaplan-Meier curves and the significance was verified by a log-rank test. All p values were determined by two-sided tests and p values <0.05 were considered significant. Multivariate analysis was done using the Cox regression model including only the clinical variables and antibody expression markers that showed significance in univariate analysis. Data analysis and summary graphs were produced by the R statistical software version 3.0.1.We studied 119 patients with MPM whose clinicopathologic characteristics are summarized in The median follow-up time for the total cohort of 119 patients was 15.1 months (m) (range 0.2–99 m). Median survival of the entire group was 13.8 m (95% CI 9.6–20). There was an improved survival rate in patients with good PS, epithelioid subtype histology and patients with response to chemotherapy. Patients with epithelioid subtype presented a median survival of 16.8 m versus 0.8 m sarcomatoid and 13.2 m no other specify (p<0.001). Median survival for patients with PS 0, 1, 2 and 3 was 26.7, 16.5, 2.5 and 0.9 months respectively (p<0.001). Also we found significant differences in survival according to response to chemotherapy. Patients with partial response had a survival of 26.2m vs 17.5 m patients with stable disease and 7.8 m patients with progressive disease (p = 0.003). Stage III-IV and patients older than 75 were associated with worsened survival (OS 13.2 months in stage III-IV and 7.9 months in patients >75 years (p>0.05 in both cases). We did not found differences in survival according to gender, smoking, asbestos exposure and tumor localization (right or left).Of the initial cohort of 119 patients the immunohistochemical analysis for PD-L1 was available in the FFPE of 77 patients. Among the 77 MPM samples examined in our study 16 (20.8%) expressed PD-L1 and 61 (79.2%) were negative. Overall, all of these positive cases displayed cytoplasmic staining and in 14 cases the localization was in membrane. In addition expression was also detected in infiltrating lymphocytes in 4 patients, and all of these patients were also stained in membrane and cytoplasm .The percentage of tumor cells was low in the majority of positive cases. We found 10 (62.5%) PD-L1 positive patients had <5% tumor cells, 3 patients (18.7%) 5–10% tumor cells and 3 patients (18.7%) more than 10% of tumor cells. Analyzing the intensity of staining from the 16 positive patients PD-L1 was weak in 9 patients (56.2%), moderate in 4 patients (25%) and strong in 3 patients (18.8%) .We evaluated the presence of TIL and we observed all patients had TIL in the tumors without a predominant immune infiltrate CD4 or CD8. No association was found between expression of PD-L1 and TIL (p = 0.075). In the PD-L1 positive patients we detected an increased infiltration of TILs above average in 61% of patients for CD8+ and in 53% of patients for CD4+.The univariate relationship between clinical variables and PD-L1 was investigated and a significant correlation between the PD-L1 expression and histology was found . We deteExpression of PD-L1 in the tumor by either cytoplasmic, membrane or immune infiltrating cells did not correlate with patient’s gender, asbestos exposure, clinical stage, chemotherapy regimen, response to treatment or TILs.In our series, PD-L1 expression was associated with outcomes. Patients with PD-L1 positive expression presented shorter survival than PD-L1 negative patients . Median Additionally, we analyzed the outcomes for survival according to the intensity of staining and the percentage of positive tumor cells. Median survival for patients with 1–5%, 5–10% and more 10% tumor cell stained was 4.8, 5.6 and 2.6 months respectively (p = 0.9). Median survival for patients with weak, moderate and strong PD-L1 staining was 6.1, 5.2 and 2.6 months respectively (p = 0.9). However, this was an exploratory analysis with a small number of patients and the interpretation of the results must be considered carefully.Using multivariate analysis with a Cox regression model that included significant variables in the univariate model, we found that PD-L1 remain significant prognostic factor for survival .The aim of this study is to investigate the baseline expression of PD-L1 in patients with advanced MPM and to correlate the expression with the outcome. Our results show that MPM express PD-L1 and is associated with poor prognostic.PD-L1 regulates the cellular immune response and has been shown to be expressed in different tumors, including glioblastoma, pancreas, ovarian, breast, renal, head and neck, esophageal and non-small cell lung cancer (NSCLC) –21,23. RWe found that PD-L1 is expressed in 20% of MPM and no epithelioid subtype expresses PD-L1 more frequently than epithelioid. In this study we evaluate a novel antibody (E1L3N) using a threshold of 1% positive staining of malignant cells to determine whether the tumors were scored as positive or negative for PD-L1. With this threshold, an association between cases scored as positive for PD-L1 expression and clinical responsive to PD-L1 has been reported with the MPDL3280A and 2CC3 antibodies ,26. HowePD-L1 expression is measured most commonly by IHC however no test is uniformly accepted as the standard for quantitating PD-L1 expression. Multiple, distinct, companion assays for PD-L1 positivity have been developed, but there is not yet comparison, standardization, or prospective validation of these assays. Thus it is hard to determine whether there is consistency in the tumors that are declared to be PD-L1 positive. Additionally a different and more reproducible methodology for evaluating PD-L1 has been evaluated with the measurement of mRNA in breast cancer and lung cancer showing association with better outcomes ,34.Previous studies have shown that PD-L1 can be expressed by multiple components of the tumor microenvironment, including tumor cells themselves and infiltrating immune cells. The biological consequences of PD-L1 expression depend on cell membrane localization and cytoplasmic staining may represent intracellular stores of PD-L1 which may be developed to the cell surface depending on appropriate stimulation . HoweverAn association of PD-L1 expression with histology has been reported by other authors. Mansfield, in the series of MPM reported that every case of MPM with sarcomatoid differentiation expressed PD-L1 . In a stA strong correlation between PD-L1 expression on tumor cells and prognosis has been observed in some cancer –19,22. OOn the basis of the current status of MPM, immunotherapy has also been considered as one of the novel therapeutic approaches. Recently, tumor-specific immunotherapy using vaccination with antigen peptides of tumor associated antigens (TAA) has been conducted although the number of patients is small. Mesothelin is an immunogenic glycoprotein highly expressed in MPM. In preclinical studies and two phase I trials targeting mesothelin showed clinical activity –44. WilmIn summary, this is the first report measuring PD-L1 protein levels in MPM with the E1L3N antibody. Our data suggest association between the presence of PD-L1 with histology and survival. The measurement of PD-L1 has the potential to identify subsets of MPM and also may predict for response to PD-1/PD-L1 pathway blocked. We suggest that PD-L1 expression in MPM is a candidate molecular marker that warrants further exploration for use in selecting MPM patients for immunotherapy.


This paper presents a wireless humidity sensor tag for low-cost and low-power applications. The proposed humidity sensor tag, based on radio frequency identification (RFID) technology, was fabricated in a standard 0.18 μm complementary metal oxide semiconductor (CMOS) process. The top metal layer was deposited to form the interdigitated electrodes, which were then filled with polyimide as the humidity sensing layer. A two-stage rectifier adopts a dynamic bias-voltage generator to boost the effective gate-source voltage of the switches in differential-drive architecture, resulting in a flat power conversion efficiency curve. The capacitive sensor interface, based on phase-locked loop (PLL) theory, employs a simple architecture and can work with 0.5 V supply voltage. The measurement results show that humidity sensor tag achieves excellent linearity, hysteresis and stability performance. The total power-dissipation of the sensor tag is 2.5 μW, resulting in a maximum operating distance of 23 m under 4 W of radiation power of the RFID reader. Humidity sensors usually measure relative humidity (RH) rather than absolute humidity. Relative humidity is the ratio of the moisture level to the saturated moisture level at the same temperature and pressure and expressed as a percentage. The main mechanisms to sense and measure RH include detecting changes in the optical [Humidity measurement is essential for a wide range of applications in many fields including meteorology, agriculture, industrial control, medical instruments,  optical ,2, mecha optical ,4 and el optical ,6 proper optical . Fabricaetc. [Radio frequency identification (RFID), as a wireless automatic identification technology, is widely applied in traffic management, logistics transportation, medicine management, food production, etc. . Passiveetc. . The pasetc. ,11,12. Tetc., fully integrated humidity sensors were recently introduced [Previous research on capacitive CMOS humidity sensors was mainly focused on the humidity sensor element ,15,16,17troduced ,19,20. Ttroduced ,19 reporOur motivation in this work has been to develop a wireless humidity sensor operating at 900 MHz frequency. For low-cost applications, the humidity sensor element is incorporated with the wireless transceiver blocks and the humidity sensor element is fabricated in standard CMOS technology without any post-processing. An ultra-low power sensor interface is introduced to ensure that the wireless sensor can work in passive mode. The rest of the paper is organized as follows: tP is calculated from the Friis transmission equation [rE is the effective isotropic radiation power of a reader, aG is the tag antenna gain, rη is the RF-to-DC power conversion efficiency of the rectifier, and d is the communication distance. From this equation, it can be concluded that in order to achieve a longer communication distance, lower tag power and higher rη of the rectifier are critical for the UHF RFID tag design because rE is limited by regional regulations (4 W is the maximum transmitted power) and aG is roughly determined by the allowable antenna area (1.64 for the λ/2 dipole antenna).The operating frequency plays a very important role in RFID systems. In general, the operating frequency defines the data rate of the system and the tag size. Ultra-high frequency (UHF) RFID systems are a better solution for next generation auto-ID applications because they have advantages including fast transmission speed, long communication distance and simple modulation schemes. The theoretical practicable operating power of an RFID tag equation :(1)Pt=EinR = antR) and then the tag entirely absorbs the electromagnetic wave from RFID reader. If the tag intends to send a digital “0” to the RFID reader, the SW is closed which leads to the complete mismatch (inR = 0) and then the tag entirely backscatters the electromagnetic signal to the reader.For low-cost and small-size applications, the meandered dipole antenna is a natural choice for UHF RFID taga . The modThe reported CMOS humidity sensor designs ,14,15,16L is the length of metal electrodes, S is the width of each electrode, and W is the distance between adjacent electrodes. The thickness of the polyimide layer is H, which is generally larger than metal thickness h. For an N finger array sensor, the total sensor capacitance can be expressed as [essed as :(2)Csenh and the electrode width S are limited by the CMOS process. The thickness of the polyimide layer H is a trade-off of between the sensitivity and response time. The selection of H has great influence on the sensitivity and response time of the sensor. Generally, a humidity sensor with a thicker polyimide layer has better sensitivity and longer response times. Considering the factors including sensor capacitance, chip area sensitivity and etc., this work chooses N = 40, L = 200 µm, W = 2.5 µm, S = 2.5 µm, h = 1 µm and H = 2 µm.The metal thickness outP is the average DC output power generated at the output of the rectifier and inP is the average RF power available at the input of the rectifier. From a circuit-level point of view, rη is mainly degraded due to the forward drop of the switch diodes or transistors [thV transistors and dual-poly floating gate transistors, which all require advanced CMOS processing at an additional cost [rη of this architecture. The classic differential-drive architecture [rη curve of this design drops rapidly on the two sides of the optimal point.A popular performance metric of a rectifier is its power conversion efficiency which is defined as:nsistors . There anal cost ,28. In sitecture  is an exrη curve. When the input power is smaller (or larger) than the optimal point of the rη curve, an extra bias-voltage is added to positively (or negatively) boost the effective gate-source voltage of the switches. When input power is at the optimal point, the extra bias-voltage equals to zero. Thus, the bias-voltage is dynamic. According to the rules discussed above, N11-22 and PMOS transistors MP11-22 form the differential-drive switch. The bias-voltage VN1,2 and VP1,2 are employed to boost the gate-source voltage of NMOS switch and PMOS switch, respectively. The large resistor SR is added to block the AC component of VN1,2 and VP1,2. In CMOS process, SR could be replaced by a PMOS transistor that operates in the cut-off region to avoid the large silicon area. O. The transistors M1-M5 form the core of the reference voltage generator and the other transistors form the current mirrors. The required bias-voltage VN1,2 and VP1,2 can be achieved through by proper sizing of transistors.Inspired by Kamalinejad , this woThe capacitive humidity sensor acts as a capacitor when it works with the sensor interface. There are several popular techniques to perform the capacitance-to-digital conversion function. The traditional conversion starts with a capacitance-to-voltage converter, which is then followed by a voltage-to-digital converter ,32. ThissC acts as the variable load on a single stage of the SCO, thereby generating a sensor-controlled frequency sf. The DCO is steered by the PD output signal ob, which is a representation of the phase difference between the SCO and the DCO. The variable capacitive load on a single stage of the DCO consists of two capacitors, oC and mC. The capacitor oC, designed equal to the quiescent value of sC, always connected to the DCO. But the capacitor mC, designed slightly larger than the maximum variation of sC, is swapped in or out of the DCO depending on the feedback from the PD. The PD is simply composed of a single-bit d-flip-flop. When the entire feedback loop is locked, the average digital frequency df will correspond to the sensor frequency sf. Therefore, ob represents the digital value of the sensor capacitance. This paper employs current-starved ring oscillators for the design of SCO and DCO. As shown in 1-M6 form the 3-stage inverter-based ring oscillator, which current is constrained by the current mirror M7-M12. Although the extra transistors of M7-M12 need a higher supply voltage, the current flowing through the inverters can be constrained much smaller, resulting in lower power-dissipation and higher temperature stability.This work employs a phase-locked loop (PLL)-based architecture which can directly achieve the capacitance-to-digital conversion . The prooC and mC are selected as 5 pF and 2 pF respectively. Due to the fully digital architecture, the power supply of the interface is selected as 0.5 V. This architecture has an inherent immunity to supply voltage variations due to the fact that the digital output ob is determined by the phase difference between the DCO and the SCO, resulting in the exclusion of a voltage regulator in the proposed wireless humidity sensor tag (seen in For this work, the humidity sensor’s capacitance varies from 5 to 6.5 pF within the relative humidity range. Hence the capacitor  seen in .2 and the packaged sensor tag chip covers around 7 × 3 mm2. The humidity sensor performance was measured in a Votsch VCL4003 temperature and humidity chamber. As shown in Within the relative humidity range from 10% to 90% RH, this humidity sensor achieves highly linear performance. Due to the temperature dependence of the dielectric constant of polyimide, the average duty cycle shows an acceptable maximum temperature variation of 10% from 0 to 60 °C. The humidity sensor achieves a sensitivity of 18.75 fF/%RH at 25 °C. The hysteresis performance of the sensor at 25 °C is shown in rη curve of the proposed rectifier is shown in rη curve. When input power is 13 dBm, the two curves both reach the optimal point 69%.For a 900 MHz input frequency and 50 kΩ load, the rη is above 60%, the proposed rectifier and the conventional rectifier achieve −19 dBm and −7 dBm range, respectively. As for the input power range which The sensor interface performance was measured under a constant 0.5 V supply voltage from the rectifier. The performance comparison with previous designs is shown in The measured minimum power dissipation of the wireless humidity sensor tag is 2.5 µW, resulting in a maximum operating distance of 23 m under 4 W RFID reader radiation power conditions. Finally, the measured backscattering performance of the sensor tag is shown in We have developed a humidity sensor tag for wireless applications. This wireless humidity sensor tag operates in the UHF band and employs an ASK modulator. The humidity sensor element is designed in standard CMOS technology without any post-processing, which results in integration with other tag blocks and low fabrication cost. A two-stage rectifier adopts a dynamic bias-voltage generator to boost the effective gate-source voltage of the switches in differential-drive architecture, resulting in a flat power conversion efficiency curve. The sensor interface, based on phase-locked loop theory, employs a fully-digital architecture. Despite the moderate ENOB, this interface can operate on ultra-low supply voltage and then reduces the power dissipation significantly compared to previous designs. The total power-dissipation of the sensor tag is 2.5 µW, resulting in a maximum operating distance of 23 m under 4 W RFID reader radiation power conditions.


Distinct differences in community composition were found between 13C- and 12C-DNA-based communities and between two substrate-supplemented 13C-DNA-based communities; these differences distinctly varied with depth and site. In most cases, there were more genera with relative abundances of >0.1% in D-Glc-incorporating communities than in D-GlcN-incorporating communities. The Roseobacter clade was one of the prominent actively substrate-incorporating bacterial populations in all 13C-DNA-based communities. Vibrio was another prominent actively D-GlcN-incorporating bacterial population in most incubations. However notably, different OTUs dominated this clade or genus in different treatments at different depths. Altogether, these results suggested that there were taxa-specific differences in DOC assimilations and, moreover, their differences varied among the typical water masses, which could have been caused by the variant compositions of original bacterial communities from different hydrological environments. This implies that ecologically, the levels of labile or recalcitrance of DOC can be maintained only in a specific environmental context with specific bacterial community composition.To determine the adapted microbial populations to variant dissolved organic carbon (DOC) sources in the marine environment and improve the understanding of the interaction between microorganisms and marine DOC pool, field-based incubation experiments were carried out using supplemental  D-amino acid -AGAGTTTGATCMTGGCTCAG-3') and the unlabeled reverse primer 927R (5'-ACCGCTTGTGCGGGCCC-3') were used to amplify bacterial 16S rRNA genes [http://trex.biohpc.org/) [13C- and 12C-fractions were chosen for pyrosequencing.The 2g et al. . BrieflyNA genes , 35. PCRpc.org/) , 36 for 5'-AGAGTTTGATCCTGGCTCAG-3') and 534R (5'-ATTACCGCGGCTGCTGG-3'). In addition, a unique 10 bp barcode was used to tag each sample to enable multiplex sequencing. Each sample was amplified in triplicate to weaken the influence of specific amplification and with a negative control for each barcoded primer pair to verify samples were uncontaminated. Triplicate PCR products from each sample were combined, checked by electrophoresis on 1% agarose gel and purified with an agarose gel DNA purification kit . The quantity and quality of the combined products were checked using a NanoDrop device . Pyrosequencing was carried out on a 454 Genome Sequencer GS-FLX Titanium instrument  at the Chinese National Human Genome Center .PCR reaction mixtures and amplifying conditions for the V1–V3 region of bacterial 16S rRNA genes were in accordance with Zhang et al., using twww.mothur.org/wiki/Schloss_SOP) [screen.seqs” command was applied to remove those sequences whose lengths were outside the desired range and “filter.seqs” command applied to remove those columns with only gaps. Those sequences within 2 bp of difference to a more abundant sequence were merged with “pre.cluster” command for reducing sequencing error. Chimeras were identified and removed using “chimera.uchime” and “remove.seqs” commands, respectively. These processes avoided superfluous computations, as much as possible, for the following commands below. Finally, classification was carried out using the MOTHUR version of the “Bayesian” classifier with the SILVA reference sequences and taxonomic outline. The confidence cut-off was set to 60%. The sequences that were classified as “Cyanobacteria_Chloroplast” or “Mitochondria” or could not be classified at the kingdom level were removed from the data set.The criteria previously described  were useoss_SOP) . Brieflyoss_SOP)  using “aSequences were further clustered into Operational Taxonomic Units (OTUs) using the furthest-neighbor algorithm and the cutoff value set at 0.03 . All theRoseobacter and Vibrio in the present study) and its relative abundance was >1% of total reads in any 13C-DNA library were phylogenetically analyzed. Sequences were compared to known 16S rDNA sequences in the database using the BLASTN search (http://www.ncbi.nlm.nih.gov/BLAST/) to aid the selection of the closest reference sequences. Sequences were aligned and compiled using the MEGA5 program and neighbor-joining phylogenetic trees were constructed.Representative sequences from each OTU that fell in the most prominent actively substrate-incorporating bacterial populations  + 1.66.13C-DNA from incubations with 13C-labeled substrate supplementation peaked in the density range from 1.73 to 1.743 g∙mL-1, corresponding to the 4th–5th density fractions, while 12C-DNA peaked from 1.693 to 1.705 g∙mL-1, corresponding to the 10th–11th fractions  to 11th (light) density gradient fractions were analyzed with T-RFLP. In general, the community structure showed distinct differences between heavy and light fractions of each sample. In the 13C-D-GlcN supplemented seawater-incubation experiment at 0 m of site D001, for instance, the cluster analysis of communities from the 10 density gradient fractions revealed three distinct groups at 44.07% Bray-Curtis similarity level: heavy group , light group , and middle group  density ranges, the typical middle DNA  between them, and the extremely heavy DNA  were selected for 454 pyrosequencing and further analysis. The selected density fractions for each sample according to the same means are shown in 13C-labeled substrates, only the H2 fraction from each sample was selected and analyzed together with the light fraction.Among the 12 density gradient fractions, the 213C-labeled D-Glc at 0 m of site D001, were generally higher than heavy fractions (H2s) in each incubation  and from 74.4% to 94.5% , respectively. Diversity indexes  showed that the bacterial diversity of light fractions (L), except for the incubation with supplemental cubation  Tables. 13C-labeled substrates and light fractions representing populations not incorporating 13C-labeled substrates were analyzed with dendrogram. At the coastal site D001, 12C-DNA-based communities clustered together with in situ communities, there being two subclusters at 0–25 m and 70 m, respectively. In general, 13C-DNA-based communities clustered separately according to the two supplemented 13C-labeled substrates, but D-Glc-incorporating communities at 0 m separated from those at 25 and 70 m and D-GlcN-incorporating communities at 70 m separated from those at 0 and 25 m  where two 12C-DNA-based communities and the in situ community clustered together. In addition, 12C-DNA-based community at 200 m separated from 13C-DNA-based communities at 5 and 200 m within the D-Glc cluster; D-GlcN-incorporating communities at 5 m distinctly different from that at 200 m within the D-GlcN cluster  were the most abundant populations incorporating D-Glc at all three depths for site D001. Rhodobacteraceae Donghicola also accounted for >10% of 13C-DNA sequences at 0 and 25 m. For D-GlcN-incorporating organisms, the Roseobacter clade was a highly active population, accounting for >80% of total 13C-DNA sequences at 0 and 25 m and >15% at 70 m of site D001. Vibrio also accounted for >10% at 70 m. It was clear that there were more genera with relative abundances of >0.1% in D-Glc-incorporating communities than in the D-GlcN-incorporating communities  and many unclassified taxa belonging to Rhodobacteraceae (>10%) were the most abundant populations incorporating D-Glc at 5 and 200 m depths. However, for D-GlcN-incorporating organisms, Vibrio was the most abundant population  at the two depths. The Roseobacter clade and Thalassobius (>10%) were also relatively active at 5 m. There were more genera with relative abundances of >0.1% in D-Glc-incorporating communities than in D-GlcN-incorporating communities at 5 m, and on the contrary at 200 m  were used in incubation experiments for stimulating cellular growth. However, high substrate concentrations might lead to a fundamentally different metabolic characteristic of DOC incorporation compared with in situ or lower concentrations [12C-DNA-based communities showed relatively high similarities with their corresponding in situ communities after incubation suggesting that the results on differential incorporation of 12C vs. 13C into microbial community are reliable. In addition, the isotope fractionation effect in the incubation systems are neglectable in the concentrations of supplemented isotopic substrates. Secondly, the incubation time was set at 3 d, during which a cross-feeding effect might have occurred among the organisms. However, 3 d of incubation time has been normally used in seawater SIP experiments for obtaining sufficient 13C-labeled DNA [As sufficient trations , 44. Eveeled DNA , 44. In eled DNA , 31. How12C- than 13C-DNA-based communities in each incubation suggested that microbial populations that were adapted to the supplemented organic substrate accounted for only a small part of total bacterial community composition. Also, 12C-DNA-based communities showed relatively high similarities with their corresponding in situ communities and were distinctly separated from 13C-DNA-based communities. As revealed by the dendrograms, there were distinct differences in community composition between 13C-DNA-based communities in incubations supplemented with 13C-labeled D-Glc and D-GlcN  were found in D-Glc-incorporating communities than in D-GlcN-incorporating communities. The Roseobacter clade was one of the prominent actively substrate-incorporating bacterial populations in 13C-DNA-based communities. This result was consistent with the conclusion that Roseobacter dominates the incorporation of high concentration glucose and amino acids in the German Bay of the North Sea [Roseobacter must bloom when incubated because of a “bottle effect” [Roseobacter has revealed that they have versatile mechanisms for energy and carbon acquisition in various environments, behaving as an “ecological generalist” [Roseobacter thrived in all of the present incubations. It was clear that Roseobacter might have played an important role here in DOC transformation and the oceanic carbon cycle. However notably, different OTUs dominated this clade in different incubations from different depths.In most cases, there were more genera with relative abundances of >0.1% in orth Sea . Thus fa effect” . Genomiceralist” –48. ThisVibrio was another prominent actively D-GlcN-incorporating bacterial population, as expected. Vibrio is an important oceanic chitinolytic bacteria, depolymerizing chitin with cell surface hydrolases to GlcNAc for better use [Vibrio plays a significant role in oceanic nutrient cycling [Vibrio did not thrived at 0 and 25 m of site D001 among all 13C-D-GlcN supplemented incubations of the two sites. This could be caused by distinctly different original bacterial communities Click here for additional data file.S2 Table(DOC)Click here for additional data file.S3 Table(DOC)Click here for additional data file.


The adhesive system of geckos has inspired hundreds of synthetic adhesives. While this system has been used relentlessly as a source of inspiration, less work has been done in reverse, where synthetics are used to test questions and hypotheses about the natural system. Here we take such an approach. We tested shear adhesion of a mushroom-tipped synthetic gecko adhesive under conditions that produced perplexing results in the natural adhesive system. Synthetic samples were tested at two temperatures (12 °C and 32 °C) and four different humidity levels . Surprisingly, adhesive performance of the synthetic samples matched that of living geckos, suggesting that uncontrolled parameters in the natural system, such as surface chemistry and material changes, may not be as influential in whole-animal performance as previously thought. There was one difference, however, when comparing natural and synthetic adhesive performance. At 12 °C and 80% RH, adhesion of the synthetic structures was lower than expected based on the natural system’s performance. Our approach highlights a unique opportunity for both biologists and material scientists, where new questions and hypotheses can be fueled by joint comparisons of the natural and synthetic systems, ultimately improving knowledge of both. Principally known for strong, reversible, and dry adhesion134678910111213141516et al. tested whole animal shear adhesion of live geckos at two temperatures and a series of humidity levels3520and temperature, and this debate has been relatively quiet for years.One major challenge to fully resolving how geckos stick is also the most obvious: geckos are biological organisms and thus have inherent variability and complexity. Specifically, the biological adhesive system is likely adapted to work in multiple contexts and under a variety of circumstances, which may in turn mean that the system is not optimized to work maximally in any one condition. This versatility can be a useful property for synthetic adhesives, but can also be a central problem when designing tests to better elucidate the properties of the natural adhesive system. Take the case of capillary adhesion. The role of capillary adhesion has surfaced in the literature multiple times. In 2002, Autumn and colleagues dispelled the idea that capillary adhesion could be playing a significant role in gecko adhesion by showing that geckos were able to stick to both hydrophobic and hydrophilic substrates equally well during shear sliding27Using the capillary adhesion example, it is quite clear that the context in which adhesion occurs is critical. Load, sliding velocity, level of humidity, temperature, presence of water, dynamic surface groups, and other parameters can impact adhesion. Furthermore, whole animal studies are consistently different than those testing single spatula, setae, and setal arrays37383940et al. used for whole animal experiments, which consisted of measuring maximum shear adhesion of live geckos on a glass plate at two temperatures (12 °C and 32 °C) and four different humidity levels In this study, we tested shear adhesion of a high performing mushroom-tipped gecko-inspired synthetic and used it to probe the effect of humidity and temperature on a dry fibrillar adhesive. We replicated the conditions Niewiarowski 8,39 = 11.47, p < 0.0001; There was a significant effect of temperature and humidity on shear adhesion of the gecko-inspired synthetic microfibers . When investigating temperature and humidity independently, we found that the two temperatures tested had significantly different behavior , where samples tested at 32 °C had all three behavior types  and those tested at 12 °C only had two (SS and V). Humidity had no significant effect on adhesion behavior when considered independently .We defined three adhesion behavior types from the experimental force profiles. First, some force profiles exhibited a “U” shape (U), where maximum adhesion occurred at some point in a smooth “U” shaped curve. This profile suggests that sliding friction is greater than static friction. Second, some force profiles exhibited a “V” shape (V), where maximum adhesion occurred at the apex of the “V”. This profile implies static friction is stronger than sliding friction. Finally, the third type of force profile was “Stick-Slip” (SS), where maximum adhesion occurred during periodic stick-slip events. In this profile static and sliding fiction are intermittent and dynamic, where in periods of stick, static friction is dominant, and in periods of slip, sliding friction is dominant. Some samples showed multiple behaviors in one test, thus behavior was assigned based on where the maximum force value occurred. Examples of all three force profiles are shown in et al.Our work represents one of very few examples using a gecko-inspired synthetic microfiber array to probe questions about the natural gecko adhesive system. Results from both the natural system and the synthetic system are strikingly similar: adhesion only increases with increasing humidity at cool temperatures. In the biological system several hypotheses have been proposed, but not yet resolved, for the effect of humidity on adhesion. First, the reduction of setal modulus increases available contact area by allowing the setae to become more compliant and flexible, thus adhesion increases at high humidity (>80% RH). Second, surface restructuring causes an increase in adhesion in the presence of water . Finally, non-independent from the change in modulus hypothesis, material swelling may occur , causing an increase in contact area and therefore adhesion. Unfortunately, none of these hypotheses can be completely independently tested in the natural system , where temperature was the only significant effect driving this difference (p = 0.0007). Modulus at the 12 °C temperature set point was significantly higher than the modulus measured at 32 °C . This means that overall, temperature influences the synthetic modulus but humidity does not, suggesting that the temperature by humidity interaction observed in adhesion tests of the synthetic is not driven by changes in modulus. Furthermore, given the increase in modulus at 12 °C, we would expect the opposite adhesion results, where shear adhesion would reduce at cold temperatures rather than improve. It is important to note the modulus of gecko setae is higher than the synthetic (~2 GPa)The first, and most dominant hypothesis, is the reduction of setal modulus in high humidity (>80%)3536423,45 = 0.35, p = 0.7929), suggesting that no significant surface restructuring occurred in the synthetic adhesive. Contact angle for a smooth sheet of the fiber material yields a contact angle of 78.5 ± 1.4° at room temperature and humidity, which is within the range of what is expected for the natural system based on measurements and theoretical calculations (70–90°)19et al.Similar to the first hypothesis, we expected the second and third hypotheses (surface group restructuring and swelling) to have no major effect on adhesive performance of the synthetic. To test this, we again utilized the synthetic and investigated changes in contact angle and weight in high humidity to confirm that the synthetic material was stable during adhesion tests. To test the second hypothesis, we measured the contact angle of water on the smooth backing material in four of the eight treatments used in adhesion tests (12 °C and 32 °C at ~35% RH and 80% RH). Contact angle was averaged over three measurements per sample using ImageJ to analyze images from a custom goniometer. At least 10 samples were used per treatment and all were held in the test environment for at least 1 hour. We found no significant change in water contact angle across the treatments . One alternative may be to reconsider the influence of adsorbed water on shear adhesion. Water can influence shear adhesion by two main mechanisms. First, adsorbed water layers may lubricate the contact interface and reduce shear adhesion and friction. Secondly, capillary bridges may form between the contact elements and the substrate and increase normal adhesion. Both mechanisms can change as humidity increases, however, why increased shear adhesion of the natural and synthetic systems occurs only at cool temperatures is not explained completely by either mechanism. In whole animal experiments surface water on a hydrophilic glass surface, like the one used here, produced significantly reduced adhesion at room temperature484950While there are startling similarities between the natural and the synthetic systems, is also important to investigate where these two systems differ. One clear difference between the synthetic and natural system adhesion results is at 12 °C and 80% RH. In the natural system, adhesion was almost twice as high at 12 °C and 80% RH as compared to the low humidity trials at 12 °C and 35% RHFinally, this experiment also highlights the ability of a synthetic gecko-inspired mushroom-tipped adhesive to adhere in a variety of environmental conditions. Specifically, while temperature effects on pressure sensitive adhesives (PSAs) have been investigated previously53In summary, by comparing the natural and synthetic systems’ performance and adhesion behavior, we are able to offer a new perspective on experiments focusing on gecko adhesion at the whole animal scale. First, we found that several of the prevailing hypotheses about how geckos stick in humid environments may not be viable. Our results once again call into question the roles of capillary adhesion and interfacial water layers in the gecko adhesive system. Secondly, by investigating the particular temperature and humidity regime where the systems differ, our understanding of the natural adhesive system will improve, as well as produce ideas for design parameters that can increase synthetic performance in conditions where the natural system has higher performance. Thirdly, the stability of this adhesive in challenging environmental conditions, such as high temperature and humidity, has several applications to real-world adhesive use and commercialization. Finally, we hope our work inspires further investigations into how synthetic designs can improve the type of questions we ask about the natural adhesive system, creating an “idea feedback loop” between the natural and synthetic system, both of which have been generally tested independently.et al.2. Each sample was imaged independently both before and after experimental trials to check for defects and experimental damage. Samples were attached to a glass slide with mushroom caps facing upward using a cyanoacrylate gel adhesive that was slightly cured prior to attachment, making sure the gel adhesive did not cover the sample. A controlled weight (174.5 g) was placed on the sample before a protective backing layer was removed from the pillar caps. This pre-weight was used to ensure that the samples had been fully pressed into the glue and were horizontally level. No sample was used more than once and samples that had premature loading (prior to testing or during testing in a non-controlled way) were discarded.A polyurethane elastomer mushroom-tipped synthetic gecko-inspired adhesive was obtained from Carnegie Mellon University. Samples were manufactured using methods similar to those outlined by Murphy et al.et al. for a schematic). The weighted glass side was pulled across the sample in the shear direction for about 4 cm at a controlled rate. The glass slide was cleaned with ethanol and water after each sample and dried with a Kimwipe. Force was recorded over time and maximum force was determined from the force curve post-experiment. Adhesion behavior was also characterized post-experiment using the force curves. All experiments were conducted in an environmentally controlled walk-in chamber set to either 12 °C or 32 °C and all experiments were conducted within ±2 °C of this range. Humidity was controlled within ±2% of the following set points: 30%, 55%, 70% and 80% RH. Each of the four humidity set points were tested at the two temperature settings, totaling eight different treatments. All samples were equilibrated to the treatment temperature and humidity for at least 1 hour prior to experiments.Each sample slide was placed in a test arena as described in Badge Samples were tested semi-randomly. The effect of temperature, humidity, and their interaction on shear adhesion was tested using an analysis of covariance (ANCOVA) where area was used as a covariate. Individual comparisons across groups were done using a Tukey HSD test to control for multiple tests. To compare adhesion behavior across temperature and humidity we used a Pearson Chi-square test. Means are reported mean ±1 s.e.m.The raw data are provided in the How to cite this article: Stark, A. Y. et al. The effect of temperature and humidity on adhesion of a gecko-inspired adhesive: implications for the natural system. Sci. Rep.6, 30936; doi: 10.1038/srep30936 (2016).


Recently, quantum Hall state analogs in classical mechanics attract much attention from topological points of view. Topology is not only for mathematicians but also quite useful in a quantum world. Further it even governs the Newton’s law of motion. One of the advantages of classical systems over solid state materials is its clear controllability. Here we investigate mechanical graphene, which is a spring-mass model with the honeycomb structure as a typical mechanical model with nontrivial topological phenomena. The vibration spectrum of mechanical graphene is characterized by Dirac cones serving as sources of topological nontriviality. We find that the spectrum has dramatic dependence on the spring tension at equilibrium as a natural control parameter, i.e., creation and annihilation of the Dirac particles are realized as the tension increases. Just by rotating the system, the manipulated Dirac particles lead to topological transition, i.e., a jump of the “Chern number” occurs associated with flipping of propagating direction of chiral edge modes. This is a bulk-edge correspondence governed by the Newton’s law. A simple observation that in-gap edge modes exist only at the fixed boundary, but not at the free one, is attributed to the symmetry protection of topological phases. Graphene47891011131415161718192022In this paper, a honeycomb spring-mass model1827mR denotes a lattice point, and a is a sublattice index.Mechanical graphene consists of mass points with mass nts. See . Motion l is the length of the spring at that moment, In mechanical graphene, the elastic energy cost for finite η controls the relation between the restoring forces for with η. In contrast, the motion in parallel with the spring direction, which corresponds to a longitudinal mode, is not affected by η as far as good linearity of the springs is assumed, since with good linearity, the elastic energy has a constant second derivative with respect to the spring deformation.The physical meaning of the last statement is illustrated in Having this in mind and assuming the periodicity of the system, we introduce new dynamical variables where ω-spectrum is obtained by diagonalizing and nd . See . Practicη in a wide range in real graphene. Furthermore, the effective model for graphene phonon does not respect the “chiral symmetry”, which plays important roles in topological characterization. This is because if the vibration in graphene is modeled by a spring-mass model, it requires springs connecting the next nearest neighbor sites and more.Now, the problem is quite similar to the phonon problem in graphene30313233η dependence of the ω-spectrum [p-orbital honeycomb lattice modelω-dispersion for η becomes less than 2/3, two new Dirac cones other than those at the K- and K’- points appear on M-K lines. [See η approaches to zero. For small η where the new Dirac cones get close to the K- and K’-points, the dispersion relation looks like the one for bilayer graphene with trigonal warpingx- and y-direction decouple, which leads to the two-fold degeneracy.Now, we follow the spectrum . ThroughSee Ref. . Note thlat. At  , the gapnes. See . Due to  warping , the newThe transition at d is spatial dimension, where π for the spinless casewhere Therefore, the Herring number has an ability to detect the number of Dirac cones. This idea has been applied to the 2D organic Dirac fermion systems41at the M-point are indicated for each band. We notice that at Next, we see edge states using ribbon geometry with zigzag edges. In this case, the equation to be solved is similar to equation The difference in momenta along the edge is reflected in the oscillation pattern in the real space. In order to convince this expectation we perform a calculation using a rectangular system with long zigzag edges and short armchair edges. In ω-spectrum for nonuniform diagonal elements in In order to see the importance of the boundary condition, the Lastly, we break the time reversal symmetry by uniform rotation as a whole with angular frequency Ω, which brings two new forces, the centrifugal and the Coriolis force. For simplicity, only the Coriolis force, which is first order in Ω, is taken account of since the centrifugal force, which is second order in Ω, is negligible for small Ω. The equation of motion becomeswhereω-spectrum is obtained by evaluating ω, η less than It is no longer possible to reduce the problem to diagonalization of C asFor the topological analysis of this gap closing and opening, we define the Chern number C can be evaluated with a well established numerical methodη is reduced, the Chern number jumps from 1 to −2 at The summation is taken over In order to have a vivid view on this topological transition, we perform a calculation on a triangular system with zigzag edges. The Chern number for the mechanical system itself is not a physical observable but the corresponding edge states reflecting nontrivial bulk can be observed experimentallywhere and which is explicitly evaluated by diagonalizing The selected snapshots of the time evolution for Before closing, we make a comment on the effects of dissipation. Assuming that the frictional force is proportional to the velocity, the effects of friction are easily incorporated by slightly modifying To summarize, we first propose a simple and feasible way, uniform stretch of the system, to control the vibration spectrum of mechanical graphene. An interesting point of the proposed procedure is it does not involve symmetry breaking, but we observe generation, migration, and creation of Dirac cones. It is also shown that the transitions associated with the motion of Dirac cones are detected by observing real space pattern of the edge modes. For the case without the time reversal symmetry, an intimate relation between the bulk topological number and chiral edge modes in a mechanical system is established, which indicates universality of the bulk-edge correspondence, which is one of the most important concepts in the topological approach. In addition, the boundary condition dependence of the edge modes is explained in terms of the symmetry protection of the topological state, which reveals new aspect of the symmetry in classical mechanical systems.How to cite this article: Kariyado, T. and Hatsugai, Y. Manipulation of Dirac Cones in Mechanical Graphene. Sci. Rep.5, 18107; doi: 10.1038/srep18107 (2015).


We investigate the interactions between two identical magnetic impurities substituted into a graphene superlattice. Using a first-principles approach, we calculate the electronic and magnetic properties for transition-metal substituted graphene systems with varying spatial separation. These calculations are compared for three different magnetic impurities, manganese, chromium, and vanadium. We determine the electronic band structure, density of states, and Millikan populations (magnetic moment) for each atom, as well as calculate the exchange parameter between the two magnetic atoms as a function of spatial separation. We find that the presence of magnetic impurities establishes a distinct magnetic moment in the graphene lattice, where the interactions are highly dependent on the spatial and magnetic characteristic between the magnetic and carbon atoms, which leads to either ferromagnetic or antiferromagnetic behavior. Furthermore, through an analysis of the calculated exchange energies and partial density of states, it is determined that interactions between the magnetic atoms can be classified as an RKKY interaction. For the last century, the vast increase in technological capabilities has been largely governed by the ability of devices to manipulate and control charge in various materials. However, charge is not the only property of electrons that can be exploited for technological use. Within the last decade, there has been a surge in the number of devices and materials that work to control, not only charge, but also the electron spin degree of freedom. This evolution has brought about the area of spintronics1234567The use for spintronic devices varies dramatically due to the multitude of interesting phenomena they exhibit. This includes spin relaxation and transport that may be used for quantum computation671012The most popular example of a 2D material is that of graphene1517181920214914p-orbital (π) bondingK-point of the Brillouin zone  methods. The process of removing individual carbon atoms from the graphene lattice was recently demonstrated by Gomes In this article, we present a comparative computational study examining the changes in the electronic and magnetic properties of transition-metal substituted graphene. Starting from a 128-atom supercell of graphene, two carbon atoms are replaced with either vanadium (V), chromium (Cr), or manganese (Mn) impurities. The magnetic impurities are considered for various spatial separations of N carbon (C) atoms (N = 1–6) in between the magnetic atoms along the zig-zag chain direction in graphene. Using density functional calculations with a full geometry optimization and spin polarization, we performed calculations to determine the total energy ground state, electronic band structure, density of states (DOS), and magnetic moment for each spin configuration. Overall, it is determined that the presence of magnetic atoms induces a magnetic state in the graphene, where the partial density of states shows a distinct increase in electron mobility at the Fermi level producing a metallic state. Additionally, we map the magnetic moment for each atom in superlattice to examine the magnetic exchange (or super-exchange) interaction characteristics throughout the graphene layer. Overall, this study provides clarity on how the magnetic atoms are communicating through graphene layer as well as helps in the understanding of the super-exchange interactions between the impurity atoms.Furthermore, through an analysis of the calculated exchange energy between the magnetic atoms, we find that the interaction alternates between a ferromagnetic and antiferromagnetic state as the magnetic atoms are separated spatially. This, in combination with the metallic state in the carbon, illustrates and suggests the presence of an RKKY exchange interaction. An RKKY (Ruderman-Kittel-Kasuya-Yosida) interaction is an indirect exchange coupling that is governed through interaction of localized spin moment and conduction electrons3839We conducted simulations on magnetically-substituted graphene lattice to see how the presence of magnetic atoms would affect the electronic and magnetic structure in the system. Therefore, we replaced two carbon atoms with vanadium, chromium, or manganese in a symmetric dimer configuration. Through DFT simulation of the formation energy, it was determined that graphene with magnetic atom substitution is energy favorable over a pure graphene state. To examine the formation energy, we determined the total energy for a pure graphene sheet with a unbound magnetic atom  and compared that to a graphene sheet with a bound magnetic atom and a free carbon . Here, it is found that the magnetic substitution is energy favored by about 8 eV. This is most likely due to the number of electrons that can be shared by the magnetic atoms over the carbon.To examine the electronic and magnetic interactions, we started with two atoms separated by a single carbon atom, and then separating them by one carbon along the zig-zag chain of graphene until there were six carbon atoms between the magnetic atoms . This alThe presence of the magnetic atoms produce a distinct distortion in the graphene lattice. This distortion is shown in 21Focusing mainly on chromium, μB respectively. Here, μB is the Bohr magneton. μB, which is mainly produced by the vanadium atoms maintaining a constant ferromagnetism. However, the net magnetic moment of the entire lattice structure tends to fluctuate for chromium and manganese as the magnetic atoms flip between FM and AFM ground states. With respect to the electronic bandstructure, the competition between the interactions of the magnetic atoms and carbon atoms have distinct consequences on the presence of Dirac cone, which is the staple characteristic of pure graphene. It is clear that all of the odd carbon spacing , Mn4+ (high-spin state S = 3/2), and V4+ (S = 1/2) produced mainly from sharing three valance electrons with the graphene lattice. These spin states are minimum approximations for the local magnetic moments. The use of an on-site potential may increase these values. However, this shows that magnetic moment is shared with the surrounding carbon atoms.From Therefore, the addition of magnetic atoms produces a net magnetic moment in the graphene lattice around the atoms, which leads to better conduction and an overall super-exchange. This exchange can be quantified by examining the change in the total energy of the FM (↑↑) state and the AFM (↑↓) state. Using a standard Heisenberg Hamiltonian for an independent dimerSi is the quantum mechanical spin for sites i = 1, 2, it can be determined that the change in energy (ΔE) from the FM and AFM state is simply proportional to the exchange energy J42where JV−C (the exchange between vanadium and carbon) is dominant over JC−C (the exchange between carbon atoms). Therefore, vanadium is able to force the carbon moments into a configuration that works to maintain JV−V > 0.Depending on the coupling between the magnetic atoms and the carbons, magnetic frustration could be the cause of the variation between the FM and AFM characterization. As shown in JC−C seem to dominate JCr−C, since the JCr−Cr alternates through between FM and AFM, which is similar for manganese. These interactions seems to indicate the presence of an RKKY exchange through the graphene lattice. However, in order to have an RKKY interaction, the graphene needs to have a metallic DOS at the Fermi level, which is established in This effect is different for chromium  and manget al. using a standard exchange model with the spin-orbit couplingd transition-metal atoms, spin-orbit coupling can be assumed to be negligible. Therefore, we find that the presence of magnetic transition-metal atoms produces multiple electron bands at the Fermi level beyond the standard Dirac cone crossing, and that there are distinct correlations between magnetic atoms  and separated until there are six carbons in between (6C) the magnetic atoms. Using a generalized-gradient approximation density functional theory with a full geometry optimization and spin polarization, we performed calculations to determine the total energy ground state, electronic band structure, density of states (DOS), and magnetic moment for each configuration.Through an analysis of the total energy for the different magnetic configurations, the super-exchange interaction between magnetic atoms is shown to alternate between antiferromagnetic and ferromagnetic state as a function of impurity distance. Additionally, we examine the spatially dependent magnetic moment for each atom in the superlattice and discover that the carbon atoms within close proximity to the magnetic atoms gain a distinct magnetic moment. The magnetic carbon atoms produce a complex network of exchange interactions. Through an analysis of the partial density of states for the carbon atoms, a distinct increase in electron mobility at the Fermi level is shown. Therefore, the calculation of a variable super-exchange with spatial distance and the transition to a more metallic state for the carbons in between the magnetic atoms indicates the presence of an RKKY-like super-exchange interaction through the graphene atoms.in-situ substitution and induced magnetism in graphene shows the increased possibility of using graphene for spintronic materials. These calculations are meant as the motivation for the use of magnetically-doped graphene and other 2D materials for the possible advancement for spintronic devices and quantum computation.Further investigation is needed to understand the induced magnetic moment in carbon and the exchange network between the magnetic atoms. Given the advancements in STM and nanoscopic enhancements of atomic substitutions in graphene, experimental exploration of these interactions is a distinct possibility. The presence of variable magnetic exchange due to k-sampling was for purposes of computational time given that extent and number of calculations being performed. Geometry optimizations performed with the higher k-point sampling showed no significant difference.Using Atomistix Toolkit4546μeV with an energy cut off of 2 keV. As shown in Santos et al.We implemented initial spin states of up-up (↑↑), up-down (↑↓), and down-down (↓↓) configurations on the magnetically-substituted atoms and zero spin on the carbon atoms, which was established through non-substituted test runs on pure graphene. As expected, the configurations of ↑↑ and ↓↓ were found to be degenerate. In all, we perform calculations on 54 total configurations . The DFT calculations were run using a tolerance of 0.3 K point in the Brillouin zone, and we also map the induced magnetic moments for each atom to examine the interactions throughout the graphene supercell.In all, we calculated the total energy of each system to find the ground state configuration assuming collinear polarized spins and estimated the exchange parameters between the two magnetic atoms. Furthermore, we determined the electronic band structure and density of states (DOS) to investigate the magnetic effect on the graphene Dirac cone at the How to cite this article: Crook, C. B. et al. Proximity-induced magnetism in transition-metal substituted graphene. Sci. Rep.5, 12322; doi: 10.1038/srep12322 (2015).


We investigate the absorption properties of graphene-based anisotropic metamaterial structures where the metamaterial layer possesses an electromagnetic response corresponding to a near-zero permittivity. We find that through analytical and numerical studies, near perfect absorption arises over an unusually broad range of beam incidence angles. Due to the presence of graphene, the absorption is tunable via a gate voltage, providing dynamic control of the energy transmission. We show that this strongly enhanced absorption arises due to a coupling between light and a fast wave-mode propagating along the graphene/metamaterial hybrid. These artificial hybrid structures have been shown to allow propagation of electromagnetic (EM) waves through narrow369111214E) or magnetic (H) responses.With the refinement and advancement of fabrication techniques and theoretical methods, it is now possible to successfully create anisotropic metamaterial structures that have at least one vanishing principal component of the permeability EF. When EM waves interact with a graphene sheet, the gate voltage can tune the amount of energy that gets absorbed, depending on the relevant scattering processes, temperature, relaxation time, frequency of the incident beam and its polarization.These developments have led to a significant number of recent works devoted to studying graphene as a component in hybrid metamaterial designsε describing the anisotropic response has components that are near zeroBy placing graphene in contact with an anisotropic metamaterial that has vanishing principal components of the permittivity and permeability tensors over the frequencies of interest, one could expect the interplay between the two samples to present additional possibilities regarding the transmission of the light. More specifically, we pose the question if it is possible to utilize the dynamic and tunable conductivity of graphene to yield greater control over the way energy of EM waves is absorbed or reflected compared to when the materials are in isolation. Motivated by recent experimental and theoretical developments, we seek to establish if one may obtain tunable wide angle absorption in hybrid graphene/metamaterial layers where the tensor ε near-zero (ENZ) response, where the metamaterial has a vanishingly small value of the real part of ε. In contrast to numerous past works in the RF and visible light regime, we focus here on operational wavelengths that correspond to the less exploited THz regime, where many new technologies are emerging. To accurately determine the absorption as a function of the geometrical and material parameters that describe graphene and the metamaterial, Maxwell’s equations are solved using a transfer matrix approach that accounts for the presence of graphene through boundary conditions imposed on the magnetic field. We discuss the behavior of the absorption in terms of the energy flow throughout the entire structure.To this end, we consider here the absorption characteristics of a layered system, comprised of monolayer graphene overlaying a metamaterial slab that is connected to a perfectly conducting metal. In accordance with the discussion above, we will consider metamaterial structures that can possess an EF, we are able to determine experimentally relevant regimes where near perfect absorption can occur over an extremely wide range of incident angles for an ENZ metamaterial layer that has a small component of the permittivity tensor. We show that the enhanced absorption occurs due to coherent perfect absorption which couples light to a fast wave propagating along the graphene/ENZ hybrid.Our main result is that by tuning p-polarized incident light  ≠ 0. In the anisotropic medium (region 2), the nonzero components to the tensors are . It is backed by a perfect electric conductor (PEC) in region 1, where no electric or magnetic fields exist. The finite surface conductivity σ of graphene will be effectively taken into account in the boundary condition for the magnetic field H at the interface between air (region 3) and the anisotropic medium, whereThe principal coordinate axes are chosen so that both the permittivity and permeability tensor in the anisotropic medium are both diagonal. A diagram of our setup is shown in EF ≫ kBT, ħω, we have,When T of 300 K, and THz frequencies, we have kBT = 25 meV, and ħω ~ 1 meV, meaning doping levels should exceed EF = 100 meV when using EF is the Fermi energy, ω is the frequency of the EM wave, and trel is the relaxation time. For frequencies in the THz region and lower, Thus, at a temperature p-polarized incident light and diagonal tensors, we obtain the following set of equations determining the components of the fields when the time-dependence is harmonic (et−iω):We now consider Maxwell’s equation (in SI units) in the incident air region and in the anisotropic medium, both having no free charges or currents. For kx is conserved due to translational invariance. The Methods section contains additional details of the numerical and analytical solutions of the above equations.These equations are solved with a plane-wave ansatz for the field components where A2 and B2 are associated with the transmitted and reflected electric field wave in region 2 whereas B3 ≡ r is the coefficient for the reflected wave in region 3. Since there is no transmission into the PEC layer, the absorbance r is obtained asOnce the equations have been solved, one may compute the absorption where we have definedB2 = −A2. We are now interested to see how the presence of the graphene layer modifies the absorption properties of the metamaterial, and in particular determine the influence of the gate-controlled Fermi level which is tunable in situ. The analytical expression for the reflection coefficient r allows us to estimate which order of magnitude EF should have in order to have any influence on the results. If the terms proportional to the conductivity σ are to be comparable in magnitude with the other terms, we see that e.g. εx,3ω ~ kz,3σ needs to be satisfied. Since we are considering an EM wave incident from air, εx,3 = εz,3 = ε0. Inserting the expression for σ, one finds that the above equation corresponds to a Fermi level in graphene ofThe coefficients for the other waves are given as: EF. We also note that when σ → ∞, r → 1 such that the absorption vanishes, S = E × H. We write E = E0et−iω and H = H0et−iω. For future use, we define the normalization constant for the power flow as the time-averaged incident power at z = τ: S0 = ε0c/(2 cos θ).For grazing incidence θ ~ π/2, the graphene layer thus has no influence for any realistic values of EF, graphene-dielectric stacks can exhibit a wavevector dispersion that traces out a hyperbolic contour at a given frequency21EF. Specifically, we focus on gate-tunable HMM structures in the ENZ limit, whereby the tensor components εx,2 and εz,2 are opposite in sign. We set μy,2 = μ0, and the relaxation time in graphene to τ = 10−13 s. For incident light in the THz regime, with EF = 100 meV and λ = 1.6 mm, this leads to a surface conductivity of graphene σ = (1.2 × 10−3 + 1.37i × 10−4) A ⋅ C/J.By appropriately tuning We first consider the standard Drude-like frequency response in the metamaterial, such thatz and f = 0.02, where λz is the characteristic wavelength, with λz = 1.6 mm. Designing a metamaterial within effective medium theory to possess the prescribed dielectric response in the THz regime can be challenging with current fabrication and materials capabilities. It may be possible however to design a multilayer dielectric structure or rod lattice56Here, α = λ/λz,2, determined by the frequency of the incident wave, and also its magnitude. We set x,2 → 0, the reflection coefficient simplifies toIt will also be of interest to see how the absorption properties depend on the sign of εr|2 = 1. However, when graphene is present (σ ≠ 0) an interesting opportunity arises. It is seen from the above equation that r = 0, meaning perfect absorptionIn the absence of graphene (σ = 0), we have zero absorption since |In the low THz regime and below (ω ≪ 10 THz), we have ω ≪ τ and full absorption takes place for angles Although this equation represents the criterium for 100% absorption, we show in the next section that the absorption in fact is close to 100% for an extremely wide range of incident angles.kx = β + iα to the complex plane and solve the pole dispersion equation (see the Methods section) for β and α (at a given ω). This dispersion equation admits four types of solutions for the complex wavevectorkx/k0 < 1, and ℑm(kx) = 0. These modes represent a coherent superposition of waves that propagate without loss along the surface. Indeed, only when the incident wave is incident at the perfect absorption angle, Sx/Sz), precisely coincide with the incident angle r to be single-valued, the appropriate branch cuts at the points ±k0 need to be made for kz,3. When examining the perfect absorption phenomena, we utilize a simple yet useful property of the reflection coefficient: r into its reciprocal:To establish how perfect absorption can be achieved, we discuss how the incident beam can couple to the intrinsic EM modes of the system. Our approach involves finding the poles of the reflection coefficient, kz,3 = k0 cos θ. This property simplifies the solution process since finding a complex pole, related to −kz,3, is equivalent to finding the perfect absorption modes for kz,359with x,2 > 0 and εz,2 < 0, or type II if εx,2 < 0 and εz,2 > 0. The operating wavelength of the incident beam is tuned around λz to control the sign of εz,2. The component of ε parallel to the interface always has the same magnitude, but can differ in sign according to εx,2 = (±4 + 0.1i)ε0.We now introduce more possibilities for enhanced absorption, and investigate anisotropic systems where the permittivity components perpendicular and parallel to the interfaces are of opposite sign. The corresponding HMM dispersion relations are then identified as either type I if εEF is seen to improve the absorption dramatically not only in magnitude, but also in terms of which angles of incidence that become absorbed. Remarkably, the absorption coefficient can be tuned to be nearly perfect, for an unusually broad range of angles. This is distinct from a scenario involving conventional dielectrics where isolated absorption peaks can appear at specific angles of incidence.Extremely wide-angle absorption for type-I and type-II anisotropic structures is demonstrated clearly in kx of the incident EM wave with the permitted EM modes of the graphene-based ENZ structure, the critical coupling responsible for perfect absorption can be identified. Since kx/k0 < 1, and ℑm(kx) = 0, we search for the allowed fast-wave EM modes (whereby the phase velocities exceed c) that the structure supportsR+ = 0 [kx, are converted to a critical coupling angle via θ = arcsin(kx/k0), giving the necessary incident beam angles for fast-wave mode coupling to occur. To illustrate this, in R+ = 0) are shown as a function of EF and θ. The wavelength considered λ = 1.01λz corresponds to a negative real part of εz,2 that is also near zero. In (a) εx,2 = (4 + 0.1i)ε0 corresponding to a type I HMM, while in (b) both permittivity components are of the same sign, since now εx,2 = (−4 + 0.1i)ε0. The dispersion curves are shown together with density plots of the absorption EF and θ. Clearly the fast-wave dispersion curves overlap with the regions of perfect absorption. A broader range of incident angles leading to perfect absorption is found in the graphene/HMM structure (a) compared to (b) where both kx below the light line. These modes are distinct from conventional guided wave modes that have kx/k0 > 1, and consequently the EM fields would be evanescent in the air region near the graphene layer. They also differ from leaky wave modesx since in that case, ℑm{kx} ≠ 0. Therefore, by coupling the incident beam to fast wave modes of the structure, the enhanced absorption features provided by the HMM in the ENZ regimeEF afforded by graphene, generates a broad range of angles for perfect absorption to arise.To pinpoint the mechanism of the observed perfect absorption, the mode characteristics of the system should be identified. By correlating the longitudinal wavevector component tsR+ = 0 , we findSx and Sz are shown within the air (z/τ > 1) and metamaterial (z/τ ≤ 1) regions for the structure considered in Sx and Sz are largest near the interfaces, except for near normal angles of incidence, whereby the energy flows along the metal backing. The flow of Sx in the metamaterial is opposite to the incident beam, demonstrating negative refraction that arises from the small negative permittivity component εz,2. The perfect absorption features seen in The behavior of the energy flow is exhibited in Ex and Ez take the same form in region j = 2, 3:We here provide some details for how to obtain the solution of the Maxwell equations in the presence of graphene. The wave-equations for j is then written: j which takes into account both transmitted and reflected waves:The dispersion relation then reads: j = kxεx,j/kz,jεz,j. Since the light is incident from air, we take A3 = 1. The magnetic field is obtained as where we defined κA2, B2, B3}. To do this, we need to specify the boundary conditions at the air/anisotropic medium interface (z = τ) and the anisotropic medium/PEC interface (z = 0). Since the tangential component of E is always continuous, it follows that H by writing the free current as Jf = σE: aM = b whereThere are three coefficients to be determined: {a = M−1b, which in turn fully determines E and H throughout the entire structure.where we defined We also provide an explicit expression for the time-averaged Poynting vector kz,3. The corresponding transcendental equation determines the allowed ω and kx pair that a source field can excite in the metamaterial system. From the many possible solutions, we consider only fast-waves corresponding to kx < k0, and ℑ{kx} = 0. The dispersion equation that must be solved is,To determine the fast-wave modes of the system, we solve Maxwell’s equations for the EM fields in the absence of an incident field, thereby effectively treating the structure as a waveguide. This is equivalent to finding the poles of the reflection coefficient for a given sign of kz,3 can be either positive or negative. When the system under consideration is isotropic, the transcendental equation for the modes in the ENZ limit , reduces to,where kx is converted into a coupling angle θc that the incident beam must be directed at in order for fast-wave modes to be excited: θc = arcsin(kx/k0). What constitutes a zero numerically in the root finding algorithm that solves The z is relatively close to 1. For large deviations from λ/λz = 1, a conventional scenario with isolated absorption peaks arises.For completeness, we also briefly show results for the spectral dependence of the absorbance, i.e. when moving away from the ENZ regime by varying the wavelength λ of the incident wave. This is shown in z,2, has a large imaginary component. Although the wide-angle absorption properties are seen to be reduced in the type-II case, the top panels (b) and (c) reveal that remarkably for the type-I HMM, wide-angle absorption remains intact when considerable losses are present.We also examine the effects of loss on the absorption properties of the graphene-based metamaterial. In How to cite this article: Linder, J. and Halterman, K. Graphene-based extremely wide-angle tunable metamaterial absorber. Sci. Rep.6, 31225; doi: 10.1038/srep31225 (2016).