Improving ski-turn skills is of interest to both competitive and recreational skiers, but it is not easy to improve on one’s own. Although studies have reported various methods of ski-turn skill evaluation, a simple method that can be used by oneself has not yet been established. In this study, we have proposed a comfortable method to assess ski-turn skills; this method enables skiers to easily understand the relationship between body control and ski motion. One expert skier and four intermediate skiers participated in this study. Small inertial measurement units (IMUs) and mobile plantar pressure distribution sensors were used to capture data while skiing, and three ski-turn features—ski motion, waist rotation, and how load is applied to the skis—as well as their symmetry, were assessed. The results showed that the motions of skiing and the waist in the expert skier were significantly larger than those in intermediate skiers. Additionally, we found that the expert skier only slightly used the heel to apply a load to the skis (heel load ratio: approximately 60%) and made more symmetrical turns than the intermediate skiers did. This study will provide a method for recreational skiers, in particular, to conveniently and quantitatively evaluate their ski-turn skills by themselves. Alpine skiing, in which skiers make turns to descend a snow-covered slope, is one of the most popular winter sports. Improving one’s skill in making ski turns comfortably is beneficial for both competitive and recreational skiers. Turns performed by a highly skilled skier are, on a flat surface, for example, symmetric ,2,3. SucTo capture data to evaluate ski-turn skills in terms of the relationship between the physical movement of a skier and the interaction of the skis with the snow’s surface, photos, videos, and inertial measurement units (IMUs) are commonly used ,7,8,9,10However, inexpensive and convenient (such as mobile or wearable) devices have been commercialized for recreational skiers. One such product assesses skiing skills by using IMUs that are attached to the skis and chest . There iThe main purpose of this study is to propose a convenient methodology that is able to assess ski-turn skills using the features of the skill, especially for non-professional athletes. To extract the features, we measured and compared the ski-turn movements of advanced and intermediate skiers. For convenience, small wearable IMUs were used. Mobile plantar pressure distribution sensors were also used to capture the relationship between the load on the skis and the ski motion. This experiment was conducted on an actual piste, and the motion and load during ski turns were measured.One expert skier  and four intermediate skiers  participated in this study. The expert skier was officially ranked at Technical Prize by the Ski Association of Japan; see ® system, Novel GmbH, München, Germany)—were used with a temporal synchronization device; see ® wireless communication range, the sensor measurement status could be monitored on the laptop computer. Data generated by each sensor were time-stamped and stored in the sensor’s own memory. After the experiment, the data were extracted from each sensor’s memory for offline analysis.Sensors—IMU  and plantar pressure distribution . Each participant was asked to ski for two runs and, on each run, to ski down the piste in six parallel turns while their data were recorded. In this study, to eliminate the effects of different skis on turn performance, participants used the same skis , which were provided to each participant for use during the experiment.Before the start of the measurements, participants waited on a flat area at the top of the slope. Once the measurement began, the participants held still for 5 s to calibrate the sensors, standing in a neutral position—relaxed, with knees slightly bent; see Based on our previous work , in thisy-axes of the IMUs attached to the skis were used to partition the data; see y-axes of the IMUs pointed in the direction of travel , and thus, the angular velocity about these axes represented the edge rotation speed in a turn; see y-axis angular velocity data, wherein the peaks indicate the instant of the change in motion direction during the ski turn, the data between consecutive peaks, which included the end of the previous turn and the beginning of the next turn, were extracted. Therefore, the interval between those peaks corresponds to one turn. In the coordinate system used in this experiment, angular velocity time-series data from positive to negative peaks corresponded to a left turn, and those from negative to positive peaks corresponded to a right turn. We eliminated the first and last turns in each trial as they included starting and stopping motions in addition to turning motions. Thus, four sets of each turn-direction data were analyzed.In preprocessing, changes in angular velocity about the y-axis of the IMUs attached to skis was used to measure ski motion. The angular velocity time-series were preprocessed with a 5-Hz low-pass filter, and each turn was extracted as one interval. Third-order spline interpolation was then applied to the filtered data and resampled to 3000 points to yield the same number of data points for each turn interval, which allowed us to treat intervals of the same turn direction as the same type of interval. The resampled data during one interval were first evaluated qualitatively. Then, the sum of the squares of angular velocity v during each interval was calculated and evaluated as the quantitative feature for edge rotation:n is the number of data points.Edge rotation angular velocity about the maxx is the maximum coordinate value during a turn and minx is the minimum coordinate value.For waist rotation, we used data from the IMU worn at the waist to measure changes in posture. Quaternions outputted from the sensor were used, and the rotation was represented as the change from the position of the unit normal vector (with the sensor center as the origin) during calibration to that of the unit vector at each time point. Therefore, the change in the posture was expressed as coordinates on a unit sphere at a distance of 1 norm from the origin. Third-order spline interpolation was applied, and the data for each turn (based on the partitions determined from the synchronized ski IMU data) were resampled to 3000 points to yield the same number of data points for each turn interval. Resampled data during one interval were first evaluated qualitatively. Then, the peak-to-peak value of posture change during one interval was calculated and evaluated as a feature quantifying waist rotation,F is the force generated from the target area (the load to the skis), A is the area of each sensor that divided the foot into 99 segments , and P is the pressure measured by each sensor. Then, load data extracted at each turn were preprocessed with a 5-Hz low-pass filter. Third-order spline interpolation was applied to the filtered data, and the data were resampled to 500 points to yield the same number of data points for each turn. To examine whether the load was applied by the toe or the heel to the skis, the heel load ratio was calculated as the heel load divided by the entire load. Heel load ratio time-series data during one interval were evaluated qualitatively, and then, the mean heel load ratio during one interval was calculated and evaluated as a quantitative feature of the load applied to the skis:Plantar pressure distribution sensors were used to record plantar forces in regions of the foot. Sensor placement was divided into two regions—toe and heel; see p < 0.05. Finally, the left–right symmetry was calculated for general ski-turn skill assessment. For the three quantitative features respectively, the mean value of the left-turn features divided by that of the right-turn features was calculated as the feature of the left–right symmetry,For the three quantitative features above, the differences between skiers and turn directions were analyzed using a two-way repeated measures mixed model , and the post-hoc tests showed that the sum of the square of edge rotation angular velocity of the expert was significantly greater than those of the intermediate skiers; see p = 0.530, partial η2 = 0.027), and there was no significant interaction between skiers and turn directions  = 2.002, p = 0.146, partial η2 = 0.348). These results indicate that differences between the expert and intermediate skiers’ ski motion characteristics—i.e., changes in angular velocity at the beginning and the end of a turn, and constant during the middle of a turn—were significant.The quantitative feature of ski motion, the sum of the square of edge rotation angular velocity, is shown in p < 0.001, partial η2 = 0.792), and the post-hoc tests demonstrated that the peak-to-peak waist rotation of the expert was significantly greater than those of intermediate skiers 1, 3, and 4; see p = 0.086, partial η2 = 0.184), and there was also no significant interaction between skiers and turn directions  = 0.815, p = 0.535, partial η2 = 0.179). Waist rotation time-series data are shown in Heel load ratio time-series data are shown in p < 0.001, partial η2 = 0.919), and the post-hoc tests showed that the heel load ratio of the expert was significantly smaller than that of intermediate skier 1 and significantly greater than that of intermediate skier 4; see p = 0.181, partial η2 = 0.116), and there was also no significant interaction between skiers and turn directions  = 0.808, p = 0.539, partial η2 = 0.177). These results indicate that the expert skier only slightly tends to use the heel to apply loads to skis.The mean heel load ratio during one interval is shown in The left–right symmetry results for ski-turn skill are shown in We used a comfortable method to capture ski-turn feature data on actual alpine pistes. Three features that could be intuitively interpreted by skiers were extracted to assess differences in skill levels. The results showed that the motions of ski and waist in the expert skier were significantly larger than those in intermediate skiers. Additionally, the expert skier showed approximately a 60% heel load ratio and a symmetric left–right ratio in the evaluated features. These results indicate that an expert skier largely moves their waist and only slightly tends to use the heel to apply loads to skis, resulting in an aggressive ski motion and highly symmetric left–right turns.Because it has previously been reported that skilled skiers make symmetric turns  and, witWe suggest that the proposed method can be useful for recreational skiers. Our method consists of three small IMUs and a pair of plantar pressure distribution sensors, which is similar to off-the-shelf configurations that can be used by recreational skiers ,22. In tThe present study had a limitation that should be considered. Our proposed method mainly focused on sensors that could be worn comfortably, while data collection and analysis were limited to manual methods. However, from a practical perspective, development of an automatic evaluation system will be considered.There are few previous studies that used a comfortable measurement method to extract ski-turn features and evaluate skier skill level. Our method can quantitatively assess ski-turn features in relation to those of an expert skier, allowing skiers to be aware of the differences between themselves and expert skiers. We believe that our technique could lead to the development of an effective and convenient training system, especially for self-taught recreational skiers.In this study, we proposed a comfortable measurement method that can be easily and conveniently used by skiers themselves. This proposed method extracted features of ski-turn motion representative of differences in skill level. The measurement system consisted of three small IMUs and mobile plantar pressure distribution sensors that do not interfere with skiing. One expert skier and four intermediate skiers participated in this experiment, and ski motion, waist rotation, and how load is applied to the skis were measured on an actual piste. Three features that would be easy for skiers to interpret and that represent differences in skills were extracted and used to measure general ski-turn skill. The results indicated that an expert skier performs symmetric turns with aggressive motions. This study will contribute to the development of a method for recreational skiers to conveniently and quantitatively evaluate their own skiing skill level on actual pistes.


We report on spin transport in state-of-the-art epitaxial monolayer graphene based 2D-magnetic tunnel junctions (2D-MTJs). In our measurements, supported by ab-initio calculations, the strength of interaction between ferromagnetic electrodes and graphene monolayers is shown to fundamentally control the resulting spin signal. In particular, by switching the graphene/ferromagnet interaction, spin transport reveals magneto-resistance signal MR > 80% in junctions with low resistance × area products. Descriptions based only on a simple K-point filtering picture (i.e. MR increase with the number of layers) are not sufficient to predict the behavior of our devices. We emphasize that hybridization effects need to be taken into account to fully grasp the spin properties (such as spin dependent density of states) when 2D materials are used as ultimately thin interfaces. While this is only a first demonstration, we thus introduce the fruitful potential of spin manipulation by proximity effect at the hybridized 2D material / ferromagnet interface for 2D-MTJs. 2D materials are foreseen as an opportunity to tailor spintronics devices interfaces, a.k.a spinterfaces. Here, using state-of-the-art large-scale integration in spin-valves, authors demonstrate that hybridization of graphene with a metallic spin source results in strong spin filtering effects. Introducing novel 2D materials in MTJs is particularly attractive as these materials define sharp interfaces with unique properties, and the performance of spintronics devices is heavily dependent on interfacial properties. However, vertical spin transport through 2D materials has only just started to be studied. The role of many experimental parameters remains yet to be understood. There have been several initial reports on the behavior of spintronic devices incorporating 2D materials7 , but many experimental parameters are still to be studied in these systems. The expanding 2D materials family is now the focus of several on-going vertical spin transport studies, taking advantage of, for instance, tunneling 9, semiconducting  and magnetic  behaviors. Interestingly, a phenomenon of “bulk” K-point spin filtering  minority spin bands) has been early described by Karpan and co-workers15. Experimental exploration of this phenomenon has started (see review3) with interesting fundamental and technological consequences: for instance ref. 16 discusses K-point spin filtering with a weak spin-splitting of the Dirac cone and shift of the Fermi level due to charge transfer. However, a major difficulty concerns the integration of these 2D materials with FM without deteriorating their properties. While many 2D-based heterostructures can be readily fabricated through exfoliation or wet transfer techniques, these ambient processes usually result in degraded interfaces with reduced spin polarizations. Illustratively, the tunneling magnetoresistance (MR) values obtained for FM/monolayer Gr/FM MTJs structures have so far only reached 3.4%, most probably as a result of these integration issues19. A solution has been found with direct chemical vapor deposition (CVD) that has shown to provide clean and well-defined interfaces, but its exploration is still at its infancy as it requires the processes to be carefully adapted to each particular system23.Spintronics has demonstrated its potential with widely distributed applications such as hard disk drives, leading to the big data era. Its flagship application is now magnetic random access memories (MRAMs) based on magnetic tunnel junctions (MTJs) toward faster and greener electronics15. In particular, we fabricate complete MTJs where the potential of graphene as a spin filtering interface is studied by magneto-transport measurements. In between the Co top spin analyzer and the graphene layer, we introduce an interfacial Al2O3 layer either well-wetted or just below the wetting limit to modulate the Co/graphene interface from a tunnel to a metallic contact. The fabricated Ni(111)/Gr/Al2O3/Co junctions with a tunnel Al2O3/Co analyzer reveal a negative MR = −12%, characteristic of Ni/graphene spin filtering  are not sufficient to predict the behavior of our devices. Spin-dependent hybridization, required to understand the strong spin signal measured in our Ni(111)/Gr/Co MTJs, is further discussed in light of first-principles calculations /monolayer graphene system defined by a direct CVD step on a well-crystalized Ni(111) electrode  layer is patterned to define stripes using laser lithography and dry ion beam etching with ionized Ar gas. Monolayer graphene is then epitaxially grown by CVD on top of the Ni(111) electrodes in a low-pressure customized cold-wall reactor using ethylene at 600 °C. The process consists of a pre-annealing step in H2 followed by ethylene exposure at 10−5 mbar and a cooling step in vacuum. Further details and analysis of the CVD process used for graphene growth on nickel are given in “Methods” section following refs. 26. We have made in-depth reports on the quality of graphene in these systems and our study successfully relies on these results. As measured by Raman, and other techniques , the graphene in use is of high quality and homogeneity, and epitaxial on the Ni(111) underlying structure. The preservation of the chemical state of the Ni surface against oxidation diffusion has been confirmed by XPS27. Junctions are defined on the Ni(111)/Gr stripes by creating small openings (about 10 µm²) in a UVIII resist layer by laser lithography. A thin Al2O3 layer of 1 nm is deposited using a customized atomic layer deposition (ALD) growth system, directly integrated into a spintronic material deposition cluster. Following refs. 10 cycles of trimethylaluminum (TMA) and ozone are delivered at 80 °C. We have studied carefully the conditions for the wetting of ultrathin alumina on graphene, see our previous works in particular28 with similar problematics on different 2D materials29. For a first set of samples, a well-wetted ALD alumina tunnel barrier is deposited using the previously demonstrated ozone process on graphene (see “Methods” section)5. For another set of samples, we decrease the wetting in order to grow a slightly discontinuous Al2O3 film, without changing other fabrication parameters. This allows a direct contact to the graphene layer over a drastically reduced surface compared to the UVIII window size, aiming at a high enough junction resistance. The Co (15 nm) ferromagnetic top electrode is finally evaporated on the micrometric junctions and capped with Au (80 nm). In summary, we work here with monocrystalline fcc Ni(111) with epitaxially grown graphene and a top contact of amorphous alumina barrier and polycrystalline Co with nm-scale domains. After junction bonding to a measurement chip, we use a low-noise AC + DC transport setup at 4 K to characterize the spin valves, using a Signal Recovery 5210 lock-in amplifier, two Kethley K2182A nanovoltmeters, and a Yokogawa 7651 source connected to the junction.In Fig. 2O3/Co junctions that incorporate a continuous 1 nm Al2O3 tunnel barrier  measurements show the typical tunneling behavior across the Al2O3 tunnel barrier , validate the high quality of the ALD-grown tunnel barrier. The top Al2O3/Co layer can thus be used as a tunnel spin analyzer to probe the spin properties of the Ni(111)/Gr electrode. Magneto-transport measurements through the junction reveal a negative MR = −12%  produced using a similar direct CVD process27. It also presents a five-fold to ten-fold improvement compared to junctions fabricated with graphene transfer processes19. In previous experimental studies, it has been demonstrated that graphene layers grown on a ferromagnetic polycrystalline Ni electrode by direct CVD are able to induce a spin filtering effect32, even when only a monolayer graphene is formed27. This spin filtering likely results from the predicted band structure matching between Ni minority spins and graphene34, further demonstrated by the experimentally observed increase of spin polarization with the number of graphene layers3. This spin filtering effect is however expected to be very sensitive to properties of the ferromagnetic layer in contact35. In the framework of analysis introduced by Karpan et al.15, this improvement can be related to epitaxial alignment between the graphene and Ni(111), given their closely matched lattice parameters, that results in strong spin filtering. Comparatively, in the polycrystalline case the spin polarization is averaged over many different and less commensurate facets. In this device, with an Al2O3/Co top tunnel contact, a spin polarization of 36 and the previously evaluated 2O3/Co tunnel analyzer interface31.In Fig. ier Fig. . The I. Thus, for the Ni(111)/Gr/Co device with direct metallic contact, it is reasonable to deduce that the spin polarization of the Ni(111)/Gr interface Strikingly, this average spin polarization is well above that of entclass1pt{minima34. MR up to 33% was expected in the optimistic case for Ni/Gr/Ni monolayer graphene in ref. 15. Large MR values were later predicted for the monolayer by ref. 34. Interestingly, the >80% MR value falls in the range of values reported by Yazyev and Pasquarello34 for fcc Co/Gr/Co (60%), fcc Ni/Gr/Ni (17%), and hcp Co/Gr/Co (86%) junctions based upon quantum transport (QT) calculations. Still, the results of MR for Ni-based junctions are consistently lower: at 17% and 33% versus 60% and 86% for Co-based junctions. Hence one could be surprised that we achieve 82%  while working with a combination of a Ni and Co electrode. But more strikingly, the MR was predicted to drastically increase with the number of layers with up to 1012% MR for thick graphene multilayers33. This pointed to a strong band structure spin filtering effect of graphene—often referred to as a K-point filtering—that shifted the interest towards multilayers. Extensions to this simple K-point spin filtering picture used to interpret the results of the quantum transport calculations of refs. 33 have also been discussed (see for instance ref. 37). Here we focus on the proximity effect related to the monolayer case, where the band structure filtering effect is expected to be minimal. Additional mechanisms must be expected at the interface emanating from the proximity of a ferromagnet with graphene layers. Indeed, proximity effects are expected to play a major role in spin-dependent graphene band structure modifications. It is now well accepted that such hybridization can lead to strong modifications of the interfacial spin properties, as initially proposed more generally for organic materials by Barraud et al.35.Pioneering theoretical studies have highlighted the strong potential of graphene-based spin valves33 and Yazyev and Pasquarello34. The geometries considered in our calculations are illustrated in Fig. 38 with the exchange modified by J. Klimes et al.39 implemented within the SIESTA code40. Basis sets of numerical atomic orbitals  and is presented here as a reference to establish the impact of proximity with the top Co electrode in the full spin valve structure.The instrumental role of graphene hybridization and its chemical interactions with Ni/Co is confirmed here by first-principles calculations. The spin-dependent properties of the proximitized graphene interlayer are expected to be strongly modulated by the Gr–Ferromagnet layer separation/orientation, but we do not have experimental access to this information. Pioneering theoretical investigations have treated the epitaxial stacking of graphene on Co and Ni FM cases, corresponding to the ground-state and conveniently allowing for small computational cells. However, depending on the fabrication conditions, real devices are likely to present more complex interfaces corresponding to local energy minima. To achieve the closest estimate, we rely on ab-initio calculations of relaxed structures see Fig.  highligh16. The proximity effect of the FM on graphene’s electronic structure is further illustrated in Fig. 33 and more specifically for monolayer in Yazyev and Pasquarello34) will enable further clarification and exploitation of this phenomenon.The computed spin-resolved projected density of states (PDOS) on carbon atoms for the Ni/Gr and Co/Gr interfaces Fig.  illustrasee Fig. , b. This45. The large MR > 80% measured here in the case of a monolayer graphene-based MTJ illustrates the strong impact of hybridization with ferromagnetic electrodes on the extracted spin polarization. For comparison, previous reports of 2D-MTJs with monolayer graphene showed at best MR = 3.4%19, indicating a very large increase of spin signal output with our direct CVD approach to form epitaxial Gr/FM interfaces. While in our experiments the Co is polycrystalline and the spin polarization is averaged over many facets, we expect even higher TMR spin signals to be obtained if ideal situations like the ones depicted in theoretical studies such as ref. 34 can be reproduced. Graphene potential as a spin-filtering interface thus appears very promising, and well beyond initial predictions15. Interestingly, beyond our results, we note that many parameters remain uncharted territory in these 2D-MTJs systems. In particular we believe that increased performances and novel functionalities should arise from the study of spin-polarized transport dependence with some of the system parameters, such as the underlying FM crystallography and the related epitaxy/rotation of the graphene layer, the graphene layers stacking sequence and their integration in heterostructures with other 2D materials, etc. This offers a large playground, unseen with conventional oxides, towards more control to tailor spin transport with 2D layers.Alongside our experimental observations, this reveals that spin-dependent band structure modifications of the graphene layer due to the proximity of ferromagnetic electrodes should not be neglected in these ultrathin barrier MTJs. Clearly, the graphene interlayer presents different properties when the interaction with Co is weakened due to the presence of a tunnel barrier, compared to when there is a hybridization resulting from intimate contact with Co. This situation is reminiscent of the case of the 2D insulator h-BN, whose properties have been shown to drastically change, becoming metallic when it hybridizes with a ferromagnetic electrode15. Surprisingly, our values are on par with best figures of works of Karpan et al.15 and Yazyev and Pasquarello34 for monolayers of Co/Gr/Co and Ni/Gr/Ni and exceed naïve expectations that could be derived for Co/Gr/Ni. It evades simple analysis using the Jullière model24. We show here that monolayer graphene spin transport properties are strongly dependent on its hybridization with FM metals which are in close proximity (here top Co). This opens further potential for 2D-MTJs, as such proximity effects on 2D materials provide the opportunity to modify and tune spin properties (in a “spinterface” way46) beyond their intrinsic expectations. This additional degree of freedom, illustrated by our present results, offers a rich assortment of 2D material-based MTJ configurations for future theoretical and experimental studies.Our study exposes the significant potential of 2D hybridization for spin filtering in MTJs. The use of a tunnel analyzer confirms the negative polarization induced by graphene spin filtering at the Ni/Gr interface, with a strong enhancement shown here in the case of an epitaxial interface. Furthermore, the very large MR of >80% measured in a low resistance × area Ni(111)/Gr/Co magnetic tunnel junction highlights that much more can be exploited from monolayer graphene than the already impressive band-structure spin-filtering expected for thicker layersA 80 nm Ni(111) layer is epitaxially grown by sputtering on monocrystalline sapphire (0001) in a customized PLASSYS MP-900S system. The growth is carried at 600 °C in a 80 W Ar plasma and at a rate of 1.89 A/s. The resulting film is patterned into stripes using laser lithography with SPR700 resist and dry Ar ion beam etching.−7 mbar. Samples are heated to 600 °C at 300 °C/min in 1 mbar of H2 and annealed for 15 min. The H2 is then removed and the samples are exposed to a 10−5 mbar of C2H2 at 600 °C. Finally, the samples are left cooling in vacuum at ∼100 °C/min.Monolayer graphene is epitaxially grown by CVD on top of the Ni(111) electrodes in a customized cold-wall reactor whose base pressure is 5 × 102O3 layers of 1 nm. Depending on the targeted junction (tunnel Al2O3 layer or direct Co contact), a wetting step is initially carried using a 60 s ozone atmosphere. The Al2O3 growth is achieved by 10 cycles of TMA and ozone delivered at 80 °C sequentially, with a growth rate of 1 A per cycle. The sample is then partly protected by a shadow mask to preserve bonding pads. Using an e-beam evaporator, a gold capped 15 nm Co ferromagnetic top electrode is deposited on the samples. The devices are then bonded in a ceramic chip for measurements.Samples are covered with UVIII resist and small windows of about 10 µm² are opened above the Ni(111)/Gr stripes by laser lithography. A customized BENEQ atomic layer deposition (ALD) growth system, directly integrated into the spintronic material deposition cluster, is used to grow Al


The sensor devices were investigated in regard to their response towards NH3 as a typical reducing gas and CO, C6H6, CH2O, and NO2 as gases of interest for air quality monitoring. Moreover, the impact of operating temperature, relative humidity, and additional UV irradiation as changes in the sensing environment have been explored towards their impact on sensing properties. Finally, a cross-laboratory study is presented, supporting stable sensor responses, and the final data is merged into a simplified sensor array. This study shows that sensors can be tailored not only by using different materials but also by applying different working conditions, according to the requirements of certain applications. Lastly, a combination of several different sensors into a sensor array leads to a well-performing sensor system that, with further development, could be suitable for several applications where there is no solution on the market today.In this manuscript, we explore the sensor properties of epitaxially grown graphene on silicon carbide decorated with nanolayers of CuO, Fe It is estimated that people in North America and Europe spend 90% of their time indoors, where the air quality is usually worse than in outdoor air. Poor indoor air quality is responsible for about 4 million premature deaths and costs about USD5 trillion in welfare costs per year ,2. One o3O4 or TiO2 nanoparticles to decorate epitaxial graphene has very promising results regarding sensitivity [2O5 [2 is used in many catalytic applications, such as the lambda sensor, and CuO was also found to be a very promising material for gas sensors [Two-dimensional materials such as graphene have been shown to exhibit outstanding sensitivity if used as gas sensors . Howeversitivity ,11. Howeity [2O5 . ZrO2 is sensors ,14. FurtThe graphene surface was epitaxially grown on silicon carbide (SiC) through a silicon sublimation process . As a se2O5), and zirconium dioxide (ZrO2) have been used as target materials. A krypton fluoride (KrF) excimer laser COMPexPro 205  at a wavelength of 248 nm and laser fluences between 3–7 J/cm2 have been used for ablation. The number of laser pulses was adjusted to achieve a layer thickness of around 0.5–1 nm. The sensor substrates were kept at room temperature during deposition.The pulsed laser deposition (PLD) process used in this work is described in . Copper In this manuscript, pristine epitaxial graphene on SiC will be referred to as PEG and decorated epitaxial graphene as DEG.gasR is the saturated/absolute highest resistance signal during the gas exposure, and R0 corresponds to the baseline resistance before gas exposure.As mentioned before, the sensor resistance is measured over time, and the response is defined as−1, respectively [−1 could be misinterpreted as the D peak, which is related to defects of the graphene lattice. Instead, these features, arising above 1280 cm−1 and extending into the G peak, are related to the interfacial buffer layer between the graphene and the SiC substrate [−1. This could be due to different levels of doping in the graphene due to the charge transfer between the graphene and the various metal oxide decoration materials or simply due to different levels of strain in the graphene lattices [To ensure that the integrity of the graphene lattice is still given after the surface decoration, Raman spectroscopy measurements have been performed see . For a bectively . The peaubstrate . The smaubstrate . After tlattices . It can 3) gas was used to generally study the response of the different hybrid sensors towards influences of operating temperature, relative humidity, and irradiation with UV light during the gas measurements. In addition, the sensor response to benzene (C6H6), formaldehyde (CH2O), nitrogen dioxide (NO2), and carbon monoxide (CO), i.e., gases of interest for, e.g., air quality monitoring, is discussed. The results were obtained over a period of about two years with the same sensors, showing that the sensors work without significant degradation during this time span if the operating temperature is kept at 150 °C or lower.Ammonia , also at 50% RH. The resistance over time is exemplarily shown for the Fe3O4 NL DEG sensor in 2 has its highest response at 125 °C, while all other sensors exhibit their highest response at 50 °C. However, this rather untypical behavior with increasing temperature could be due to a reaction with OH groups or be products of reactions from OH groups and NH3 instead, leading to a higher overall response at the lower temperature, which is not necessarily related only to the NH3 exposure itself. This phenomenon was shown to occur for metal oxide gas sensors when operating them at relatively low temperatures in a humid environment [63 is extrapolated for all four operating temperatures using an exponential fit. It continuously increases, from approximately 150 s at 125 °C up to 550 s at 50 °C on average. Furthermore, the sensors are not always able to relax back to the baseline after 90 min when operated at lower temperatures. As a compromise between the level of response, response shape, and relaxation, all further measurements with ammonia were conducted at 75 °C except for ZrO2, which was operated at 125 °C. The overall highest response was observed for ZrO2 with a change of −21.2% for 25 ppm NH3 at 125 °C. Moreover, ZrO2 and Fe3O4 NL DEG sensors exhibit n-type behavior, i.e., resistance is reduced during exposure to reducing gases, while V2O5 exhibits p-type behavior. Presumably, this difference is due to different charge transfers between the metal oxide layer and the graphene, which will depend on the work function difference between the materials. CuO, on the other hand, shows a positive response at 125 °C and then switches to a negative response at lower temperatures. This change can also be induced at 125 °C when using a higher concentration (100 ppm) instead. The reason for this change in response direction is likely due to increasing gas adsorption, resulting in sufficient electrons being donated to the graphene for the Fermi level to move from below the Dirac point (p-type conductivity) to above the Dirac point (n-type conductivity). The change with temperature could also partly explain why the absolute response increases with lower temperatures as temperature and NH3 concentration work against each other in terms of charge carrier generation.All sensors have been tested at 125, 100, 75, and 50 °C at 50% relative humidity (RH) and exposed to 25 and 100 ppm NHironment ,24. Whil6H6 and CH2O at 50, 100, and 150 °C. Only CuO DEG was able to detect gas pulses down to 50 °C. While the ZrO2 DEG sensors showed a response only at 150 °C, Fe3O4 and V2O5 DEG sensors also exhibited a response at 100 °C.Other gases like benzene or formaldehyde, both volatile organic compounds (VOC), on the other hand, normally require a higher temperature to interact with the sensor surface. In a dry ambient, the four sensors were exposed to 200 ppb of CBesides the operating temperature, relative humidity in the ambient can be another critical parameter for sensor response. It was shown that many metal oxides exhibit a cross-sensitivity towards RH, and a higher level of RH in the ambient usually results in a lower gas sensitivity, as the target gas molecules compete with water molecules for available adsorption sites.3 at different levels of RH. The sensors were tested at approximately 0%, 20%, 40%, and 60% RH, and the operating temperature was kept at 125 °C (ZrO2) and 75 °C , respectively. Comparing the response at 0% to 20% RH, all sensors exhibit a decrease in response except V2O5, which not only increases for the first 20% RH but progressively increases from 8.8% at 0% RH to 13.8% at 60% RH. On the contrary, the response of the Fe3O4 NL DEG sensor decreases progressively with an increase in humidity, from initially −8.4% at 0% RH down to −4.3% at 60% RH. CuO, on the other hand, starts with a decrease in response for the first 20% RH but then stays almost constant for 40% and 60% RH. After the first drop in response at 20% RH compared to 0% RH, the ZrO2 NL DEG sensor increases its response for 40% and 60% RH again, eventually exceeding the response observed at 0% RH. This increase in response with an increase of relative humidity in the environment could be due to an additional sensor reaction with OH groups or with products resulting from reactions of OH groups with NH3, which can occur for metal oxide gas sensors when operated in a humid environment at relatively low temperatures [eratures ,24. An a2O for gas from gas bottles (6.0) and <63.2 ppm for gas from a 0-air generator; see 6H6, or CH2O in the presence of humidity. Similar to the observations for NH3, relative humidity enhances or hinders the response towards NO2. It greatly increases the response for V2O5 by almost 50% but only marginally increases it for ZrO2 by about 8% of the original response. The high response of the Fe3O4 NL DEG sensor decreases by about one-third. The response for CuO does not only decrease strongly, but it also changes direction from an increase in resistance towards NO2 exposure to a decrease in resistance if relative humidity is added. This is similar to what was observed for this sensor when exposed to NH3 + 50% RH at a temperature below 125 °C.The sensitivity of a chemical gas sensor was shown to be enhanced through UV irradiation for many different material/gas combinations, and, normally, the irradiation also decreases time constants, hence speeding up the sensor response . Moreove3 at 50% RH without and with the influence of UV irradiation. The sensors were operated at 75 °C except for ZrO2, which was operated at 125 °C. A clear trend can be observed as the relative sensor response is highest with 355 nm UV irradiation and decreases with no irradiation for all sensors except ZrO2. For ZrO2, on the other hand, a small decrease is observed with UV irradiation (−19.3%) compared to −21.3% without irradiation. When changing the UV irradiation from 355 to 265 nm, the sensor response decreases slightly. Both wavelengths have a strong impact on the time constants, which are lower with additional UV light compared to no irradiation. With UV irradiation, the time constants range between 350–500 s, whereas without irradiation, they go up to 550–1000 s. In contrast to the small effect on the response when adding UV irradiation, the ZrO2 DEG sensor shows the highest enhancement in time constant from 1000 s down to 450 s.2, with its comparably large bandgap (>5 eV), shows no significant difference in response magnitude under UV irradiation, while all other sensors did exhibit a significant difference .The complete sensing mechanism of DEG is not fully understood yet, and the introduction of one more variable like UV irradiation results in additional complexity. It is known that if the bandgap is below the photon energy , there will be charge excitation, changing the charge density in the surface modification and graphene as well . This ef2O with humidity; compare 6H6 and CH2O, the response measured at UdS is lower for all sensors except for V2O5. Especially for the CuO DEG sensor, the response varies a lot. The reported response for ZrO2 towards C6H6 was only observed at LiU, but the response (0.02%) was very small. Therefore, the measurements conducted with VOCs do not result in a clear outcome for the interlab studies. On the other hand, measurements with exposures towards NO2 show the same trend for three of the four sensors: the response increases with an increase in concentration and decreases with the introduction of relative humidity, indicating that the sensors show very similar performance at both laboratories. The only exception is the exposure of the CuO NL towards 200 ppb at 25% RH at LiU, where the response direction changed compared to the exposure at 0% RH or towards 500 ppb at 50% RH at UdS, which could be explained by the competing effects of NO2 and RH on the sensor surface.To verify that measurements performed are valid regardless of the lab-setting, some measurements were first performed in the gas sensing lab at Linköping University (LiU) and then repeated in another lab at Saarland University (UdS). All measurements were performed at 150 °C operating temperature, but the tested gas concentrations and levels of relative humidity were not always the same due to some restrictions of the respective systems. For example, the level of humidity and the gas concentrations changed between 25% RH and 200 ppb for LiU and 50% RH and 500 ppb for UdS, respectively. This means that a higher sensor response is expected for measurements performed at UdS as a higher concentration was used. Nevertheless, we tried to keep everything as close as possible to support the comparison. Measurements that did not show a response at both labs are not included , while the background gas at UdS comes from a 0-air generator. Since only relative responses and not absolute values are compared, the comparison is still valid. A variation due to the very small difference in background humidity is not expected  used for mixing of test gases still contains approximately 1 ppm of contaminants, or the system could be contaminated by prior gas tests . This ca compare .Nevertheless, we think that interlab studies should be conducted to verify sensor performances, at least qualitatively, and to raise awareness that published results may only be accurate for the specific setup used for the investigation, which itself can have an influence on sensor response. Indepth studies, comparing their gas measurements with certified labs, have shown that even there, unexpected deviations can occur, which opens the possibility for more insight into sensor behavior ,30. UnfoAs we have seen in the earlier sections, the different NL DEG sensors respond differently to different gases and environmental influences. This is summarized in 6H6, CH2O, and NO2) applied to all sensors during several single gas exposures at 0% RH. Each gas was applied three times, with a concentration of 200 ppb and a sensor operating temperature of 150 °C. The gas cycle was repeated twice, resulting in six exposures per gas in total. DAV3E, a MATLAB toolbox [2 and the two VOCs, as the response towards NO2 is much higher in comparison. It is harder, but still not impossible, to discriminate between benzene and formaldehyde. This example is very much oversimplified as, for example, similar concentrations of different gases might give very different magnitudes of response . Another problem is the high time constants of the sensor, and, therefore, each measurement point was used and not the slope of the change of the signal during exposure. One way to overcome this issue would be the application of cycled methods like temperature cycled operation (TCO) to create steep changes and more transient data for evaluation [However, one sensor alone would not be able to distinguish between different gases very effectively. Therefore, using a sensor array with all or some of the used sensors could lead to a better classification of exposed gases . A simpl toolbox , was usealuation ,36,37. A3, depending on the material. Moreover, illumination with UV light, in general, results in increased sensor response and decreased time constants, although some decoration materials are not significantly affected by UV. Investigating gases of interest for AQM, the influence of relative humidity is large as it inhibits the response towards C6H6, CH2O, and CO completely, at least at the low concentrations tested here. Comparing the sensors in an interlab study, the responses vary strongly for the measured VOCs but stay very comparable for NO2. This could indicate significant variations in either the concentrations of the source gases or, more likely, the concentrations of impurities in the source gas bottles, which, in turn, is an important aspect to consider for sensor calibration. Combining all four sensors into a sensor array, it was possible to distinguish between C6H6, CH2O, and NO2 in a dry ambient using an LDA.In conclusion, we have shown that nanolayer-decorated epitaxial graphene sensors perform differently depending on the decoration material used and the measurement conditions. Increased operating temperature and level of relative humidity were found to either increase or decrease the sensor response towards NH


Walter Everaerd, former President of the International Academy of Sex Research, World Association for Sexual Health (WAS) gold medalist, and internationally acclaimed sex researcher, died on October 4, 2019, at the age of 82. Walter had been suffering from bladder cancer for several years. He is survived by his wife Bets, his three children, and eight grandchildren.Walter was a pioneer in experimental sexology in the Netherlands. At the age of 21, after two years of military service, Walter began his studies at Utrecht University, where he specialized in the new and promising fields of experimental clinical psychology and psychotherapy. In an academic environment that was at the time mainly oriented toward psychoanalysis and introspection as scientific methods, Walter approached clinical problems as an experimental psychologist and behavioral therapist. He became a full professor in Psychology at the age of 37 at Utrecht University, and he moved to the University of Amsterdam 12 years later. Over the years, with his students and collaborators, he received numerous grants in fields as diverse as symptom perception, interoception, vaginismus, sexual arousal, stress and health, cancer, asthma, and traumatic memory. Walter’s contributions to the field of sexology, both theoretically and experimentally, have significantly changed and advanced how we define sexuality and treat sexual problems. Influenced by Walter’s work, the field of sexuality has now moved away from drive and libido as the main explanatory constructs for sexual response and sexual expression toward information processing models of sexual response. Walter was the first scholar to propose that sexual arousal should be considered an emotion. Also, current views and knowledge of sexual desire and sexual arousal problems, as reflected by the DSM-5, have, in more direct and indirect ways, been informed and inspired by Walter’s theoretical and experimental work.Walter was a modest and kind man who took his task as an academic mentor as seriously as his scientific endeavors. He mentored almost 60 Ph.D. students, many of whom became full professors. It is probably no exaggeration to say that he raised several generations of Dutch sexologists. His former Ph.D. students include Stephanie Both, Peggy Cohen-Kettenis, Joost Dekker, Jos Frenken, Luk Gijs, Erick Janssen, Bram Kuiper, Ellen Laan, Anne Oldenhave, Theo Sandfort, Mark Spiering, Arno Toorians, Adriaan Tuiten, Daan van Beek, Janneke van der Velde, and Jacques van Lankveld. Walter was honorary member, scientific program chair, and host and president of several national and international psychological and sexological societies, including the International Academy of Sex Research. He also served as department chair and as a board member and chair of various university bodies and national organizations, such as the Dutch Association for Gay and Lesbian Studies and the Dutch Health Council. He was the recipient of many awards, including the Heymans Award of the Dutch Psychological Association, the Sexologist of the Year Award of the Polish Academy of Sexology, the van Emde Boas–van Ussel Award of the Dutch Society of Sexology, and the WAS Gold Medal.We are grateful for Walter’s many contributions, and we will miss him dearly.


This performance outlines the optimized operation of this spectrometer-free sensor that simplifies its design and can bring terahertz sensing one step closer to its practical realization, with promising applications in biosensing and/or gas sensing.We propose a spectrometer-free refractive index sensor based on a graphene plasmonic structure. The spectrometer-free feature of the device is realized thanks to the dynamic tunability of graphene’s chemical potential, through electrostatic biasing. The proposed sensor exhibits a 1566 nm/RIU sensitivity, a 250.6 RIU Optical refractive index (RI) sensors are widely used due to their high sensitivity. Different structures for optical RI sensors have been developed over the past years, such as sensors based on optical fibers  and optiSeveral of these sensors involve more than one optical phenomenon to improve the capabilities  For instance, the sensor proposed in ,6 combinIn our study, we propose a spectrometer-free design using graphene SPR-based structures. In fact, since the first isolation of graphene ,10, thisIn this work, we propose a practical spectrometer-free RI sensor design using the electrical tunability of graphene sheet. The performance of the sensor is studied both by analytical calculations and numerical simulations. The results show that this design is flexible, robust, and with better combined sensing characteristics than comparable sensors. The paper is organized as follows. In the Results section, the sensing mechanism and the design of the proposed sensor are illustrated. After that, the sensor’s performance for different chemical potentials is analyzed, including the performance-boosting methods. Then, the sensing mechanism is investigated in the optical mode of operation  and electrical mode of operations (working as a spectrometer-free sensor). In a sense, electrical or optical mode of operation refers to the mode of operation of the sensor, i.e., whether the frequency or the chemical potential of graphene are held constant. In the Discussion part, the effect on the sensitivity of the device by the change of the grating size are briefly discussed and the proposed sensor is compared with other related works and shown to outperform them in terms of the combined sensitivity and figure of merit. Last, the Conclusion gives a summary of the findings of this contribution.The sensing mechanism of the proposed sensor in this work is based on graphene surface plasmon resonance (GSPR). Under the SPR, a strong electromagnetic field, highly confined around graphene leads to a high absorption peak. Thanks to the GSPR mode’s sensitivity to small changes of the surrounding optical properties, the proposed graphene sensor may sense the refractive index of the neighboring medium.Consider a typical GSPR based structure, e.g., a thin layer of graphene located in the middle and covered by two dielectric materials from the top and the bottom  By assuming a simple plane wave solution form and applying boundary conditions, we can obtain the dispersion relation of this sandwiched structure  See App. Also, gAssuming that the medium to be characterized is on top of the graphene layer, Equation (A1) See  can be gIn Equation (1), Generally, coupling techniques are required to overcome the wavenumber mismatch between incident electromagnetic (EM) wave and GSPR mode. In our study, we make use of gratings in order to realize the required coupling . Owing tn are chosen, the propagation constant is fixed, which leads to a specific GSPR mode. With a fixed In Equation (2), Equation (3) shows the direct relationship between the incident wave frequency Thanks to the tunability of graphene conductivity, one has extra free variables and parameters in the dispersion relation of the GSPR structure. In , the conHere In Equation (5), The graphene sensor structure is shown in 2/(V·s), which is a reasonable value for graphene with good quality  and finite-elements based simulations (using COMSOL MultiphysicsS-parameters) are calculated to evaluate the relationship between incident and reflected wave intensity. The simulations are performed both with reflector and without reflector, respectively. For the simulations with a reflector, a preliminary simulation is done in order to optimize the distance between graphene and the reflector so that we obtain the highest absorption peak.For the simulations, we treat the whole sensor structure as a repeated sequence of unit-cells shown in . To charAs can be seen from FWHMs, FoMs for both optical mode of operation and electrical mode of operation under different graphene chemical potentials are calculated and shown in FoMs increase linearly with the graphene’s chemical potential. Different from the sensitivities for both modes, the FWHMs and FoMs become slightly deteriorated due to the existence of the reflector.Afterwards, all the simulated sensitivities and FWHM is also simulated versus different graphene chemical potentials. FWHM decrease as FWHM of the electrical mode of operation is almost independent of the change of The sorption . GeneralFoMs for both optical mode of operation and electrical mode of operation and better sensitivity for the electrical mode of operation, while decreasing the sensitivity for the optical mode of operation.  The inclusion of the reflector will not affect the sensitivity and FoM alike, but it helps to realize stronger absorption peak (or weak reflection dip), which may reduce the constraint on sensitivity of the detecting devices such as photodiodes. Therefore, for the final design, graphene chemical potential is chosen to be 1500 meV with a gold reflector on the bottom structure. Summarizing all the observations above, a higher graphene’s chemical potential leads to better FoM from FoM for the optical mode of operation are 1566.03 nm/RIU and 250.6 RIU−1, respectively. For the electrical mode of operation, the sensitivity and FoM are 713.21 meV/RIU and 246.8 RIU−1, respectively. The simulation results of the chosen design, based on the abovementioned assumptions, are shown in To further investigate the feasibility of this device for a realistic configuration, an additional simulation is performed with a finite device structure (finite number of unit-cells of the grating). In this scenario, the device consists of only 40 units. A background normally-incident plane wave impinges on the device and the total scattered power density is calculated (by integrating the Poynting vector in the far-field over a closed surface enclosing the structure). The simulation results for the scattering cross-section (SCS) are shown in FoMs calculated from −1, and 713.4 meV/RIU, 250.5 RIU−1, respectively, which are very close to those calculated from FoM.)From n = 1.87), the absorption strength drops, because the reflector position is fixed and optimized to the original design. In other words, we can optimize the reflector distance according to different sensing cases. In order to investigate the sensor’s performance under significant RI changes, an additional simulation is performed with large analyte medium refractive index change. In the previous sections, we analyzed the role of graphene’s chemical potential and reflector position on the GSPR sensing mechanism. The objective of the following discussion is to understand the influence of the geometric parameters of the grating on the sensor’s performance.FWHM, that is halved (from 6.3 nm to 3 nm) when the grating’s width is almost halved (from 40 nm to 18 nm), which is a huge improvement. However, the same geometrical change leads to an improvement of only around 5% of the sensitivity  layer, due to the GSPR effect. Thus, the electromagnetic fields do not sense, in a certain way, the thickness of the grating, if these are larger than the confinement distance . Therefore, it can be concluded that the performance of the sensor is insensitive to the grating height, and this parameter can be flexibly designed, according to the nanofabrication process.In the same vein, we investigate the influence of the grating’s height and the corresponding optimized reflector distance on the spectrometer-free GSPR sensor. The results are summarized in FoM, while in Ref. [FoM of 690 RIU−1. In [FoM.To demonstrate the advantage of this sensor, we make a comparison of the device’s performance with other state of the art recent works. Since the novelty of this sensor comes from its electrical mode of operation, and due to the lack of sensors to compare with, in this specific regime, and as our sensor operates in both modes, this comparison is done in the optical mode of operation, without loss of generality. For instance, in , a termi in Ref. , the opp in Ref.  featuresIU−1. In , the perIU−1. In ,38,39, tFoMs result in sharper peak and less overlap between peaks under small RI change, which relaxes the demand on the distinction of the peaks. Usually, the peak distinction process is done by peak-fitting or other algorithms that require data processing. As can be seen, the sensor proposed in our study has a more balanced operation , which relaxes both the requirements of the spectrometer and peak distinction process. This comparison illustrates that our sensor has a comparable/better performance even in the traditional optical mode of operation, in addition to its operation in the electrical (spectrometer-free) mode.The performance data for the considered sensors are plot in 2/(V·s) was assumed in this work [FoM. Also, the size of the gratings has an effect on the device performance. Since the SPR mode is highly confined around the graphene layer, the height of the grating will not have an obvious effect on the device operation. However, the width of the grating will influence the SPR mode. Therefore, the grating width used in the simulations is well traded-off between device performance and nanofabrication technology limits.A mobility of 50,000 cmhis work ,40,41,42Furthermore, an important practical aspect of the proposed sensing method is the control of graphene’s Fermi level . One possible way may be to use the metal-reflector as a second electrode and that this may result in high voltages. However, we are inspired by the experimental setup demonstrated in  where siAdditionally, in practical applications, the laser source used to excite our device has a finite linewidth of its power spectrum rather than an ideal monochromatic spectrum. Therefore, we qualitatively examine the device performance under the excitation of a finite linewidth source See . The cal(1)Spectrometer-free: The design does not require the use of a spectrometer, which relaxes the design and reduces its complexity, via the electrical mode of operation.(2)Higher performance: The GSPR sensor possesses simultaneously a high FoM and sensitivity. This equips the proposed design with a higher precision in detection, in the terahertz range.(3)Geometry robustness: The operation is insensitive to the height of the grating, which gives more flexibility in the design and robustness regarding the imperfections in nanofabrication.(4)Dynamic tunability: The spectral location of the EM absorption peak can be tuned by changing To sum-up, the proposed sensor has performance that overcomes classical  biosensors, and its advantages over the state-of-the-art sensors are summarized below:FoM of 1566.03 nm/RIU and 250.6 RIU−1, respectively were demonstrated. On the other hand, for the electrical mode of operation, these were 720.75 meV/RIU and 287.2 RIU−1. In addition, a finite-size device (consisting of only 40 unit-cells) is also characterized and it is subsequently shown that it has almost the same performance as the infinitely periodic structure. The robustness of the sensor with respect to geometric variations was further investigated, and the results show that this device can be further optimized with a smaller width but at the expense of the complexity of the fabrication process. Last but not least, a comparison of the proposed sensor’s capabilities with similar works is drawn, and it is demonstrated numerically that the combination of the four advantages (mentioned in the discussion section) significantly increases the sensitivity and FoM of the proposed device in comparison with the listed designs.In this paper, a spectrometer-free sensing design is proposed and characterized through optical numerical simulations. This sensor consists of a GSPR structure with gratings for coupling to incident EM waves and a metallic reflector to boost the performance to have a higher absorption peak. The sensor can operate in both optical mode of operation and electrical mode of operation. The device’s performance for different graphene chemical potentials is investigated. These simulations assume a relative permittivity of the analyte medium varying around 3.5 and a chemical potential The new proposed device made of graphene is an important step for sensing for several reasons. On the one hand, this simple technique shows that it is possible to perform a complex analysis on a single device, without the need of a spectrometer, whereas it usually requires many different processes. On the other hand, it shows the unprecedented potential of graphene in sensing, which may lead to several possible applications in biosensing, e.g., for biomolecules, polymers, and many other substances.


We experimentally and theoretically investigated the effects of ionizing radiation on a stack of graphene sheets separated by polymethyl methacrylate (PMMA) slabs. The exceptional absorption ability of such a heterostructure in the THz range makes it promising for use in a graphene-based THz bolometer to be deployed in space. A hydrogen/carbon ion beam was used to simulate the action of protons and secondary ions on the device. We showed that the graphene sheets remain intact after irradiation with an intense 290 keV ion beam at the density of 1.5 × 10 THz fingerprints of space objects carry information of tremendous importance enabling insight into the past and future of the Universe  [In this paper, we focus on the robustness against the ionizing radiation of the graphene/ polymer multilayer that can be employed as a “thermal reservoir” to be heated up due to THz absorption. This thermal reservoir complemented with an element capable to change its electrical resistivity with temperature can be seen as a generic THz bolometer device . From thrim.org) ,27.We aim at the evaluation of the graphene/PMMA sandwich sustainability at the geostationary orbit (GEO), which is the focus orbit for THz space telescopes. At the GEO, the protons at the flux density is (1÷10)×10Fabrication of the graphene/PMMA sandwiches have been described elsewhere . BrieflyUsing graphene/PMMA sandwich structures that combine highly absorptive graphene sheets and PMMA slabs, simplifies the bolometer fabrication process because the PMMA is conventionally utilized for CVD graphene transfer and handling. Since PMMA is widely employed in nano- and microfabrication, the effect of ion bombardment on this polymer has been extensively studied ,29. In pIn order to reveal how the ion bombardment changes the THz performance of the PMMA/graphene sandwich we employ the home-made TEMP-4 ions  capable In order to visualize the effect of the ion bombardment on the macroscopic continuity, homogeneity, and defectiveness of the PMMA/graphene sandwich, we employed Nanofinder High End  confocal Raman system with 600 lines per mm grating and 532 nm laser excitation.The electromagnetic response of graphene/PMMA sandwiches was investigated using transmission mode of time-domain THz spectrometer described in .To study the dependence of graphene/PMMA sandwich properties on the radiation dose we initially irradiated the whole sample with two ion pulses. After that, we protected half of the sample from the ionizing radiation with a copper foil and irradiated the unprotected sample area again by two ion pulses see a.One can observe from graphene . The speIn practice, the ratio of posed in  the defeWe also observed a significant decrease in Raman signal intensity when the irradiation dose increases. Moreover, the reduction in the relative peak intensity of the PMMA bands after irradiation with four ion pulses indicates breaking of chemical bonds within PMMA and decreasing of hydrogen contents in the polymer layer . On the One may expect that since the THz response of the graphene/PMMA sandwich is governed by graphene sheets and silica substrate rather than polymer slabs, the structure should withstand radiation influence as graphene is radiative resistant. However, our experiments reveal that THz transmittance increases by 10–20% after four pulses. Specifically, after irradiation with four ion pulses the THz transmittance of five graphene/PMMA bi-layers is close to that of virgin three graphene/PMMA bi-layers, while transmittance of three irradiated graphene/PMMA bi-layers becomes comparable with that of a single graphene/PMMA bi-layer see d. In acchttp://www.srim.org/) as a function of the ion energy. One can observe from In order to understand the influence of the ions beam on the graphene/PMMA sandwich structure we calculate the range of both protons and carbon ions in PMMA using the SRIM-2013 package ∂E∂x=The solution of Equation  gives thAs anticipated see , the carBy comparing One can observe from Vacancies distribution in the samples are demonstrated in It is worth noting that the electron density at GEO is about an order of magnitude higher than that of protons. However, since the proton is three orders of magnitude heavier than the electron, one may expect from Equation  that theQ is the number of ions per unit surface, E is the ion energy, L are the PMMA density, specific heat capacity and the ion mean-free pass. By using the PMMA parameters Although a low-density proton beam at GEO unlikely increases the medium temperature due to its very low density, the intense beam, especially its carbon part, in our experiment may result in the heating of the irradiated structure. The increase equation:QE=ρcυ△TL compare e. Due to4 meV/A2  this locThe dependence of the THz transmittance and Raman spectra on the radiation dose demonstrates that the irradiation with a beam comprising 30% of hydrogen and 70% of carbon ions having the energy of 290 keV at a density of Our numerical simulations reveal that at the geostationary orbit, the ions bombardment is insufficient to produce observable damage to the graphene/PMMA multilayer, which should ensure a stable operation of the graphene-based THz bolometer ,37 for m


Surface plasmon polaritons (SPPs) can be generated in graphene at frequencies in the mid-infrared to terahertz range, which is not possible using conventional plasmonic materials such as noble metals. Moreover, the lifetime and confinement volume of such SPPs are much longer and smaller, respectively, than those in metals. For these reasons, graphene plasmonics has potential applications in novel plasmonic sensors and various concepts have been proposed. This review paper examines the potential of such graphene plasmonics with regard to the development of novel high-performance sensors. The theoretical background is summarized and the intrinsic nature of graphene plasmons, interactions between graphene and SPPs induced by metallic nanostructures and the electrical control of SPPs by adjusting the Fermi level of graphene are discussed. Subsequently, the development of optical sensors, biological sensors and important components such as absorbers/emitters and reconfigurable optical mirrors for use in new sensor systems are reviewed. Finally, future challenges related to the fabrication of graphene-based devices as well as various advanced optical devices incorporating other two-dimensional materials are examined. This review is intended to assist researchers in both industry and academia in the design and development of novel sensors based on graphene plasmonics. Plasmonics is an important technology that permits the manipulation of photons beyond the diffraction limit ,2,3. SurAs shown in In particular, graphene can serve as a plasmonic material operating in the mid-IR to THz regions of the spectrum and its optical constants (such as permittivity and refractive index) can be adjusted by applying a gate voltage , which cRecent advances in graphene plasmonics can be roughly classified into three areas, which involve the application of—(i) intrinsic plasmons in graphene , (ii) the interactions between graphene and SPPs induced by metallic nanostructures and (iii) the electrical control of SPPs induced by metallic nanostructures. Each of these fields of study can be applied to the design of specific sensors and, in the case of (ii) and (iii), various metamaterials ,33 or meThis section briefly discusses the theoretical background of graphene SPPs so as to clarify the advantages of this phenomenon in graphene and the aspects in which it differs from that in conventional noble metals. p, of metals can be written as:N, e, ε0 and e*m are the free carrier concentration, the electron charge, the permittivity of free space and the electron effective mass, respectively. The SPP frequency is q, dε and mε are the SPP wave vector and the permittivity values of the background dielectric and the metal, respectively. It is helpful to begin by examining conventional SPPs, because intrinsic graphene SPPs can be best understood by comparison to SPPs induced in noble metals. The bulk plasma frequency, ωmaterial . The plagσ, can be described as in Equations (3)–(7), which are derived from the Kubo formula [intraσ) and interband (interσ) transitions, such that:At this point, we can examine intrinsic graphene SPPs. The optical conductivity of graphene,  formula ,42,43,44Υ,ℏ, Bk, T and FE are the relaxation rate, the reduced Plank constant, the Boltzmann constant, temperature and the Fermi energy, respectively.Here,In case of de model  as:(8)σgFE [6 m/s and n is the free carrier concentration). At higher energy qSPPs in conventional plasmonic metals such as Ag and Au and Al occur at visible and near-IR, and ultraviolet wavelengths, respectively . Figure Equation (9) can be used to express the bulk plasma frequency based on the Drude model if the loss of graphene is ignored ωpω values. Specifically, pω depends on q, is proportional to This equation demonstrates why graphene and metals have different ene SPPs . FE value of graphene can be tuned by electrostatic gating. As a result, the optical constant of graphene can also be adjusted. In addition, SPPs are sensitive to changes in the refractive index of the surrounding materials and so the interaction between the structure into which the graphene is integrated and the SPPs is an important aspect of sensor applications. In particular, this phenomenon may permit the development of electrically-tunable graphene-based plasmonics-type sensors. cμ, of graphene at a temperature of 300 K and a wavelength of 1.55 μm [0σ. The chemical potential of graphene which can be adjusted by electrostatic gating and is calculated as [gV and DiracV are the applied voltage and the voltage at the Dirac point, respectively, and 0a is a constant with a value of approximately 9 × 10−16 m−2V−1.The  1.55 μm . Note thAs shown in 2/Si substrates. Applying a voltage between source and drain electrodes and a back gate results in changes in the channel that are correlated with incident light, while the back gate voltage controls the Fermi level of the graphene.One of the drawbacks of graphene is its low absorbance of approximately 2.3% and this low responsivity must be addressed to allow graphene to be used in optical sensors. Various approaches to mitigating this problem have been proposed, including thermoelectric systems using hetero-electrodes ,57, bolo2 layer on a Si substrate. SiO2 generates phonon polaritons that can couple with graphene SPPs, as shown in 2 phonons. opω is the graphene optical phonon energy. The sp0, sp1 and sp2 phonons are polar SiO2 phonons that interact with graphene plasmons. Anti-crossing behavior clearly appears in the SiO2 phonon modes, which is attributed to phono-plasmon interactions [Incident light cannot directly couple with graphene SPPs and so GNRs or micropatches are commonly used for the excitation of graphene SPPs, which can enhance the responsivity of GNRs-based photodetectors. ractions ,74. Thisractions . The resractions ,76,77.2, which has a high k value, have also demonstrated room temperature operation and broadband detection from the visible to mid-IR regions [2. Moreover, GNRs combined with nanodisks have demonstrated higher performance at room temperature in the case that mid-IR SPPs are employed [GNRs-based photodetector can also operate in THz region . GNRs co regions . This peemployed . It shouThe integration of graphene with plasmonic structures such as periodic metal patches and metal-insulator-metal (MIM) units has been widely investigated as a means of enhancing the responsivity of graphene-based photodetectors or realizing wavelength- or polarization-selective functions ,81,82. VIn addition to enhancement of the absorption of graphene by localized SPR, the thermoelectric effect induced by the metal nanostructures may contribute to the high responsivity and room temperature operation because of strong localization of the electromagnetic field in SPR .Another approach to improving the responsivity of graphene based on the use of plasmonic structures is to integrate plasmonic waveguides ,94 or naGraphene has also shown promise as a component of biological sensors, as it has a large surface area and is biocompatible with antibodies, enzymes, DNA, cells and proteins . In partOther well-known types of graphene-based biological sensors comprise graphene integrated with metasurfaces ,103 and As discussed in this section, biological sensors using graphene SPPs operate on the same principles as other optical sensors or devices. Therefore, present-day optical units based on graphene SPPs have significant potential in drug discovery, food safety analysis and the monitoring of bio-reactions.Graphene-based absorbers are very similar to graphene-based photoelectric sensors in terms of the enhancement of the absorbers. Graphene-based emitters are also important light sources in various analytical applications, such as gas analysis systems. Both graphene-based emitters and absorbers operate on the same principle based on Plank’s law  and the PMAs have attracted significant interest because they exhibit near-unity absorbance along with wavelength selectivity while allowing small device dimensions ,109,110.Interestingly, the absorbance of such devices will change according to the location of the graphene layers, indicating that the localized SPPs (LSSPs) induced by the MIM-based PMA can enhance the absorption of the graphene. It is important to note that the positioning of the graphene and the thickness of the insulator layer are also important factors, in addition to the chemical potential of the graphene resulting from matching of the surface optical impedance . These dAs discussed in There is presently a high demand for electrically-tunable optical devices, especially for reconfigurable mirrors suitable for solid-state light detection and ranging (LIDAR) applications . ConsequThe majority of metasurfaces used for the purpose of beam steering are based on the concept of gradients, meaning that specific surface patterns are topologically engineered using the general version of Snell’s law . Figure This same concept can be employed in other wavelength regions, such as the visible and near-IR. However, there are still challenges associated with this technology, including achieving high efficiencies, the suppression of higher order diffractions, variations in the uniformity of the graphene and the appropriate fabrication of the metallic nanostructures. Control of the transmission of light  or lightGraphene SPPs are significantly affected by the quality of the graphene ,153. TheThe combination of 2D materials such as hBN , transitThis review examined graphene plasmonics with regard to potential sensors applications. This phenomenon has opened up new fields concerning the use of SPPs in the mid-IR and THz regions, as a direct result of the Dirac cone band structure of graphene. As a consequence, high-performance optical sensors operating at IR wavelengths and at ambient temperature can be realized using graphene SPPs induced by GNRs. Graphene-based optical sensors with plasmonic nanostructures also show promise as high-responsivity wavelength- or polarization selective functional devices. Biological sensors based on graphene SPPs also exhibit higher performance and wavelength-tunable operation. In addition, work to date shows that graphene-based absorbers and reconfigurable absorbers will be important components of future sensing systems with applications in gas analysis, chemical analysis, alcohol detection and LiDAR. Taking advantage of graphene enables these optical components to operate at specific desired wavelengths as a result of electrostatic gating. Recent advances in other 2D materials as well as in the fabrication of heterostructures and in all-dielectric metasurfaces are expected to address the intrinsic loss effects associated with graphene SPPs and enable the development of next generation sensors having a wide range of uses, such as for the analysis of images, biological compounds, gases and chemicals,. It is hoped that this review will contribute to the development of advanced sensors using graphene plasmonics and the expansion of the range of associated applications.