In this work we present a novel route to produce a graphene-based film on a polymer substrate. A transparent graphite colloidal suspension was applied to a slat of poly(methyl methacrylate) (PMMA). The good adhesion to the PMMA surface, combined with the shear stress, allows a uniform and continuous spreading of the graphite nanocrystals, resulting in a very uniform graphene multilayer coating on the substrate surface. The fabrication process is simple and yields thin coatings characterized by high optical transparency and large electrical piezoresitivity. Such properties envisage potential applications of this polymer-supported coating for use in strain sensing. The electrical and mechanical properties of these PMMA/graphene coatings were characterized by bending tests. The electrical transport was investigated as a function of the applied stress. The structural and strain properties of the polymer composite material were studied under stress by infrared thermography and micro-Raman spectroscopy. Many materials have been proposed for strain sensing applications including metals, silicon, carbon nanotubes and graphene. The unique thermal, mechanical and electrical properties of graphene  have insIn this work we describe a simple and direct fabrication process based on a new micrographite colloidal suspension to produce thin coatings with large electrical piezoresitivity. When applied to a slat of poly(methyl methacrylate) (PMMA), the adhesion to the PMMA surface, combined with the shear stress, allowed for the uniform and continuous spreading of the graphite nanocrystals on the substrate surface with formation of a very uniform graphene multilayer coating.R, to the mechanical strain, ε, namely the GF is given as ΔR/(εR0), where R0 is the unstrained resistance. The values of GF for graphene range from 1.9–2.4 [The electrical response to the mechanical deformation of a strain sensor is generally quantified by the gauge factor (GF), that is, the ratio of the relative change in electrical resistance,  1.9–2.4 . The GF  1.9–2.4  or graph 1.9–2.4 –8. The vI–V) measurements and micro-Raman spectroscopy (μ-RS) during bending tests. Moreover, infrared thermographic technique (IRT) was used to assess the sensitivity of the PMMA/graphene slats to bending deformation.The electrical resistance and mechanical properties of the graphene-coated PMMA slat were investigated by current–voltage (3), which was dried in air at room temperature to give the nanostructured graphite powder. Then a concentrated nanostructured graphite colloid was prepared by accurately dispersing the graphite in ethanol  under sonication. The nanostructured graphite colloid was gently rubbed on the surface of a slat of PMMA, by hand, using a low-density polyethylene (LDPE) film to achieve a very uniform coating of the substrate surface. The adhesion to the PMMA surface, combined with the applied shear stress, allowed a uniform and continuous spreading of the graphite nanocrystals on the substrate surface with formation of a very uniform graphene multilayer coating. The obtained PMMA slats coated by this very thin and optically transparent layer of graphene were rinsed in acetone and dried in air at room temperature.The graphite used for this strain sensor was prepared according to –10 as foThe graphene compound layer on the top of the PMMA surface is clearly visible in the images of The bending stress was applied to the sample by tuning a screw located on one side of the specimen as reported in An estimate of the applied load during the bending test can been obtained by using the follow relation :f is the displacement induced by the applied force F. E is the Young modulus and Ics the moment of inertia of the cross section. The lengths l and b are defined in where N is enough to induce a slat deflection f = 10 mm, that corresponds to the maximum bending of the slat during the experiment. The force F was estimated by assuming E = 3.8 GPa for the Young’s modulus value of PMMA [Ics = wh3/12, where w = 0.02 m and h = 3 × 10 −3 m are the width and thickness of PMMA/graphene sample, respectively, and l = b = 75 mm.In our case a force of 6  of PMMA , and a mThe upside surface of the PMMA/graphene material was monitored with an infrared camera , equipped with a quantum well infrared photon detector working in the 8–9 μm infrared band cooled at 70 K by a Stirling cooler, a noise equivalent differential temperature <35 mK, spatial resolution of 640 × 512 pixels (full frame) with pixel size 25 × 25 μm. In the present application, thermal images in time sequence at 60 Hz were collected and after were subjected to postprocessing procedures .t = 0) of the sequence  was subtracted from each subsequent image in order to determine a map of temperature variations:Firstly, to account for thermal changes with respect to the ambient temperature, the first image . Therefore, a time sequence of ΔT images are created. These images are further analyzed to bring out information on the temperature variations along the specimen surface and to provide structural analysis of the material under investigation. Infrared thermographic tests (IRT) were carried out on the PMMA/graphene coatings and compared to the IRT measurements performed on the various composite polymer-based systems. A sketch of the setup for the thermographic measurements is shown in where I−V) curves between the electrodes located on A1 and A2 areas, as shown in The composites under test include: glass-reinforced plastic (GRP), nanocomposites, and PMMA. The nanocomposite sample is made using an epoxy resin EC01 and filled with multiwalled carbon nanotubes (MWCNT) provided by Nanocyl with a purity grade >95% with an average diameter ≈10 nm and an average length of 1.5 μm. Further details on the nanocomposite fabrication are reported elsewhere –15. The F = 0 and F = 6.9 N. Structural deformation of graphene induced by bending the PMMA slat has been investigated by means of micro-Raman Spectroscopy (μ-RS). The μ-RS measurements were performed by using a Jobin-Yvon system from Horiba ISA, with a TriAx 180 monochromater, equipped with a liquid nitrogen-cooled charge-coupled detector. The grating with 1800 grooves/mm allows for a final spectral resolution of 4 cm−1. The spectra were recorded in air at room temperature using a 17 mW He–Ne (λ = 632.8 nm) laser source. The laser light was focused to a 10 μm spot size on the samples through an Olympus confocal microscope with a 50× optical objective.The same setup was used to monitor time changes of the current for T depends on the sign of strain.During the bending stress cycles between unload and maximum load conditions, the temperature variation on the sample surface is measured by the IRT technique over a surface area of (10 × 10 mm) as illustrated by A1 and A2 in T to the amplitude variation of mean applied stress Δσ (pressure) by the simple relation ΔT = −ΓTασ where T is the absolute temperature of the sample, Γ is the material thermoelastic constant, given by Γ = α/ρcp where α is the thermal expansion coefficient, ρ the density and cp the specific heat at constant pressure. For graphene we obtain ΓTα = 0.19 GPa−1 at room temperature (Tα = 300 K) using the thermal expansion coefficient α = −3 × 10−6 K−1 [3 Kg m−3 and cp = 2.125 × 103 J·kg−1·K−1 [T ≈ 0.1 °C) corresponds to a maximum value of applied stress Δσ of about 0.53 GPa. Accordingly, we obtain Δε/Δσ ≈ 0.28% GPa−1 which turns out to be about 0.1% higher than the value reported in [Indeed, using the thermoelastic effect –17 under10−6 K−1 , ρ = 2.2orted in , probablT for several composites is reported. Here, the different thermal behavior of the new material measured by the IRT technique is shown. The carbon nanotubes (CNTs) can be used as a filler in the polymer composite system to obtain ultralight structural materials (nanocomposites) with enhanced electrical, thermal and optical characteristics. Glass-reinforced plastic (GRP) is used for the realization of the components for use in the automotive and aerospace industry.In the Thermographic measurements were carried out across the area A1 see  in the sI–V) using the electrical connection shown The effect of the bending on the electrical properties of the PMMA/graphene compound were investigated by measuring the current–voltage  and maximum load (F = 6.9 N) conditions. Even after several cycles between the unload and loaded state, the electrical resistance value show no hysteretic effect, i.e., the PMMA/graphene does not store stress and the maximum load implemented can be considered as a reversible stress condition.Noteworthy, the graphene/PMMA sample exhibits a good Ohmic behavior with a linear increase in electrical resistance to value of about 0.3 MΩ upon application of load up to In order to evaluate the microscopic effect of applied stress on sample micro-Raman spectroscopy (μ-RS) has been performed in the region A2 see  during tR/R0 is plotted as a function of the applied strain ε  embedded in an insulating matrix and its electrical conduction properties were analyzed within the framework of a simple electron hopping mechanism. The high sensitivity in addition to high stability of the graphene/PMMA structure and the high thermal sensitivity in the infrared wavelength region are interesting aspects that enable us to propose this polymer/graphene supported coating as a new kind of stress sensor useful for applications in different fields from civil, to automotive and aerospace engineering.


High-harmonic generation in condensed-matter systems is both a source of fundamental insight into quantum electron motion and a promising candidate to realize compact ultraviolet and ultrafast light sources. While graphene is anticipated to efficiently generate high-order harmonics due to its anharmonic charge-carrier dispersion, experiments performed on extended samples using THz illumination have revealed only a weak effect. The situation is further complicated by the enormous electromagnetic field intensities required by this highly nonperturbative nonlinear optical phenomenon. Here we argue that the large light intensity required for high-harmonic generation to occur can be reached by exploiting localized plasmons in doped graphene nanostructures. We demonstrate through rigorous time-domain simulations that the synergistic combination of strong plasmonic near-field enhancement and a pronounced intrinsic nonlinearity result in efficient broadband high-harmonic generation within a single material. Our results support the strong potential of nanostructured graphene as a robust, electrically tunable platform for high-harmonic generation. High-harmonic generation is a nonlinear optical phenomenon that may be harnessed towards realisation of ultrafast light sources. Here, the authors theoretically show that localized plasmons in graphene nanodisks result in broadband and electrically tunable high-harmonic generation. High-harmonic generation (HHG) is an extreme nonlinear optical phenomenon first observed by driving atomic gases with intense ultrashort light pulses124578Recent observations of HHG from condensed-matter systems12131415171213151920The linear, gapless dispersion relation of graphene electrons22242526272830When resonantly driven by optical fields, plasmons, the collective oscillations of electrons in conducting media, concentrate the incident electromagnetic energy into extremely subwavelength volumes, generating intense local electric fields that are essential to trigger nonlinear optical phenomena. In graphene, plasmons offer an efficient way to couple the atomically thin carbon layer with impinging light34353637383931414344454634353639Here we predict that highly efficient HHG takes place in doped graphene nanostructures when the incident light is tuned to their localized plasmons. Specifically, we obtain harmonic intensities that are orders of magnitude higher than in other materials. In addition, no sharp cutoff is observed with harmonic order. Our results are based on nonperturbative time-domain numerical simulations of the nonlinear optical response of graphene using two complementary approaches: a random-phase approximation (RPA) description of the single-particle density matrix within a tight-binding (TB) model for the electrons of ribbons and finite islandsin situ electric field enhancers for HHG in gaseous media484950k at low energies, ɛk=ħvF|k|, where vF≈c/300 is the Fermi velocity. In the single-particle MDF description of doped monolayer graphene, neglecting interband electronic transitions, this linear dispersion relation leads to a maximum achievable surface current density Jmax=−envF sign{sin(ωt)} when illuminated by a monochromatic field E(t)=E0 cos(ωt) in the E0→∞ limit30n. This square-wave profile of the induced current density under intense illumination translates into efficient generation of odd-ordered harmonics  shows a prominent dipolar plasmon . In 12 W m−2 peak intensity as those considered in the upper panel of The marked increase in HHG from localized plasmons in graphene nanoribbons is clearly shown in EF=0.4 eV and illuminated with pulses resonant with the dominant, low-energy plasmon mode (λF∼10 nm (ref. λF) are then absent. In the undoped nanotriangle, a relative increase in harmonic generation appears at low energies, which we attribute to an overlap of the generated harmonic frequency with higher-energy electron-hole transitions in the discrete electronic spectrum of the nanotriangle , φ is the self-consistent electric potential including both the impinging light and the Hartree interaction, and a phenomenological relaxation is assumed to bring the system to the relaxed state ρ0 at a rate ħHTB, where ρjj′ are the sought-after time-dependent expansion coefficients. In particular, we have fj are Fermi-Dirac occupation numbers at the initial temperature T=300 K. For ribbons, the states are treated as Bloch waves, arranged in bands as a function of their momentum along the direction of translational invariance, and the calculation is simplified by the orthogonality of different bandsRl is then constructed as ajl represent the change of basis set between state j and site l representations. Finally, the time-dependent induced dipole and surface current are given by where π=p+(e/c)A, where p is the unperturbed electron momentum, E is the classically calculated in-plane electric field . Electron dynamics is governed by the Dirac equation for massless fermions, which can be recast in the form of Bloch equations as303157In a complementary approach, we model electron dynamics in graphene within the MDF picture by adopting a nonperturbative semi-analytical modelnp and Γp represent the population inversion and the interband coherence, respectivelyħ−1=50 meV is the same as in the TB-RPA approach. These equations describe both inter- and intraband transitions. We solve N=15. The current is then parallel to the local electric field where ω<J>|2, where J is the time-Fourier transform of J, and <…> denotes the space average over the graphene structure under examination.Finally, the far-field power spectrum of the emitted light is proportional to |s=0.5 nm) of permittivity σ(ω) is the local-RPA conductivity4059E of the MDF-CEM approach, with the carrier component classically calculated as explained above.The classical response of graphene nanostructures is simulated by numerically solving Maxwell's equations using the boundary-element methodThe data that support the findings of this study are available from the corresponding authors upon request.How to cite this article: Cox, J. D. et al. Plasmon-assisted high-harmonic generation in graphene. Nat. Commun.8, 14380 doi: 10.1038/ncomms14380 (2017).Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.Supplementary Figures


R), absorption (A), and transmission (T) of electromagnetic waves is a key objective in quantum optics. To this end, one often utilizes hybrid structures comprised of elements with different optical properties in order to achieve features such as high R or high A for incident light. A desirable goal would be the possibility to tune between all three regimes of nearly perfect reflection, absorption, and transmission within the same device, thus swapping between the cases R → 1, A → 1, and T → 1 dynamically. We here show that a dielectric interfaced with a graphene layer on each side allows for precisely this: by tuning only the Fermi level of graphene, all three regimes can be reached in the THz regime and below. Moreover, we show that the inclusion of cylindrical defects in the system offers a different type of control of the scattering of electromagnetic waves by means of the graphene layers.Exerting well-defined control over the reflection ( EF, which allows the graphene layer to act as an effective sluice for the incident waves which depends on factors such as the electromagnetic frequency and polarization. In particular, several studies have been dedicated to how the radiation transmittance through graphene for normal incidence can be altered2324252830There has recently been a surge of interest in the use of graphene as an active component in structures where the aim is to control the propagation and energy flow of electromagnetic waves123456789101112131415161718192021Parallel with this development, there has also been a strong advancement in experimental techniques used to tailor metamaterial structures where either the permittivity tensor ε1 and permeability μ1) flanked by graphene layers on each surface, we show that one can operate the device in the THz regime and lower in three distinct regimes of high-reflectance, high-transmittance, and high-absorptance by tuning the Fermi levels in graphene. Dynamically swapping between these three regimes within a single device is highly unusual and could find use in a number of optics-related applications. We also comment in particular on limiting cases of special interest such as ENZ (ε1 → 0), and MNZ (μ1 → 0) metamaterials. Moreover, we include the role of cylindrical rods, or “defects” embedded in a matched impedance zero-index material (MIZIM) (μ1 → 0 and ε1 → 0), and show that defects interestingly allow for a different type of control of electromagnetic waves due to the presence of the graphene layers. The main novelty of our results compared to previous literature is that our proposed device, including only graphene and a dielectric layer, can be used to tune between the three distinct regimes of nearly perfect reflection, absorption, and transmission of electromagnetic waves, as opposed to operating in merely one of these regimes. Due to the simplicity of the structure, our results should be feasible to check experimentally and may find use in applications requiring a strong element of control of the reflection, absorption, and transmission of electromagnetic waves.One of the most fundamental challenges in quantum optics is the task of designing hybrid structures that offer tunable reflection, absorption, and transmission dynamically. Our work aims at delivering a progression toward this ambitious and important goal by combining graphene with a dielectric metamaterial. By considering such a material (characterized by a permittivity H) and electric (E) fields in each of the regions 0–3. The graphene layers are taken into account through the boundary condition n × (Hi+1 − Hi) = σjE⊥ where n is a unit vector pointing from region i to (i + 1), and E⊥ is the component of the electric field perpendicular to the interface. The correctness of modeling graphene as a surface conductivity sheet as opposed to using a slab model to treat the optical properties of graphene has recently been discussed and confirmed in ref. σj  can vary depending on their respective gate voltages. Additional details of the calculations are given in the Methods section. We present here the final result for the reflection and transmission coefficients:We consider first the system shown in R), transmission (T), and absorption (A) probabilities according to R = |r|2, T = |t|2, and A = 1 − |r|2 − |t|2 ≡ |a|2. Above, l = k1d where d is the length of the dielectric region, qi = σi/(cε0) is proportional to the conductivities at the interfaces, ω is the frequency.From t is invariant when interchanging the graphene layers, q1 ↔ q2, whereas the reflection coefficient is not, except in certain limiting cases, such as the limit of electrically small layers d/λ ≪ 1, leading to qi) ≪ Re(qi), allowing us to consider limits such as ωtrel ≪ 1 in the plots (see below for definition of trel), which provides precisely Im(qi) ≪ Re(qi).Before turning to our main results, some observations can be made regarding s → 0 and s → ∞, respectively. In the MNZ (s → ∞) case, we find d/λ ≫ 1, or when q1,2 ≫ 1, there is no transmission and perfect reflection. For the ENZ (s → 0) scenario, we findThe ENZ and MNZ cases are of particular interest, which correspond to andμ1 = μ0. Therefore if the conductivity in the first graphene sheet is large, q1 ≫ 1, we get total reflection. Increasing q2 to large values, again results in no transmission, however the reflection is tunable via q1: d/λ ≫ 1, we get t = 0. Here, the relevant variations in R or A are due to σ1. Indeed complete absorption can occur when q1 = 1, or equivalently when σ1 = cε0. This result is consistent with an incident beam normally incident on a graphene-based ENZ structureqi to tune between strong reflection, transmission, or absorption.where R, A, and T, as we now proceed to show. For a given structure, the value of d and s are locked and likely to be very difficult to tune dynamically. We thus fix the values of these quantities in what follows. To illustrate the versatility offered by the graphene layers in an analytically clear way, we first consider the case where l = k1d = π/2 + 2nπ, n = 0, ±1, ±2, …. Note that this means that we are not limiting the consideration to only one frequency ω: the transition between the R, A, and T regimes can be obtained for a range of frequencies due to the periodic nature of l. We provide results for the more general case where l is unrestricted in the Methods section, where the system can also transit between nearly perfect R, A, and T. We obtain with l = k1d = π/2 + 2nπ that:Arguably the most interesting regime somewhat surprisingly turns out to be the most “conventional” one, where the dielectric has a relative permittivity and permeability close to 1, or impedance matched with EFi is the Fermi energy of a given graphene sheet , ω is the frequency of the EM wave, EF ≫ kB ħω, we can write simply,The choice Perfect transmission: |t| → 1 when q1 ≪ 1 and q2 ≪ 1.Perfect reflection: |r| → 1 when q1 ≫ 1.Perfect absorption:t → 0, r → 0 when q2 ≫ 1 and From l, in which case the analytical expressions are more comprehensive (see Methods section). It is remarkable that one can dynamically tune between all three regimes in a single structure and for a range of frequencies. This is shown graphically in r, t, a) is shown. The two paths on the sphere depict how to acheive the desired admixtures of R, T, and A, by simply tuning the relative Fermi levels in the graphene sheets in the graphene/dielectric/graphene system. The first path (green curve) begins with the system in a highly reflective state as both graphene layers are gate tuned to equal levels of 800 meV. As the Fermi level in each graphene sheet is reduced by equal amounts, the structure then permits substantial transmission of the incident beam. Finally, by tuning EF1 and EF2 to be considerably different, we end up with a system that is a near perfect absorber. In panel (a) the reflectance is shown while the Fermi energies in the graphene layers are each varied. A wide range of energies are considered, from 0 meV, up to 800 meV. In this plot, it is evident that R can evolve from a state of no reflection to one of nearly perfect reflection when just increasing EF1 and keeping EF2 fixed. The conductivity of the first graphene layer nearest the incident beam clearly plays the bigger role here, as increases in EF1 cause σ1 to become large . The corresponding impedance mismatch between the two regions results in the observed enhanced reflectivity. Next, in σ1, σ2 → 0, or equivalently when q1 ≪ 1 and q2 ≪ 1. Next, for the structure to be a perfect absorber, it is of course necessary for there to not be any reflection or transmission of the incident wave. EF2 (so that q2 ≫ 1), the transmission of waves can then be restricted from entering the vacuum region 3 by the second graphene layer.One may also write down the conditions for the same three scenarios without limiting iωSx/∂x = 0. The quantity uE and uH, respectively: uE − uH, and the x-component of the time averaged complex Poynting vector Sx is given by Sx represents the energy flow within that segment, and the time averaged energy densities uE and uH correspond to the respective electric and magnetic field contributions, with uE = 1/4ε1|Ey|2, and uH = 1/4μ1|Hz|2. The net energy density is due to the energies of the forward and backward waves, whereas x direction (towards the second graphene sheet), with no net flow across the graphene sheets. This is consistent with the expression for The distribution of energy can be accounted for by using Poynting’s theorem for time harmonic fieldsSx/∂x.demonstrating from the conservation equation, the vanishing of the real part of ∂ε1 → 0 and μ1 → 0, the E and H fields become spatially uniform: If the region 1 is a MIZIM, whereby r and t are each independent of the material thickness, and equivalent to an electrically small region discussed above. We consider here a frequency dispersive permittivity and permeability in region 1, with ω → ωp, where ωp is a characteristic frequency of the Drude responses: q1,2 = 0, the system has complete transmission. By increasing the gate voltage so that the conductivities increase to levels corresponding to q1,2 ≫ 1, we get t → 0 and r → 1.Therefore We now consider the insertion of dielectric rods, or “defects” into the central metamaterial MIZIM region, where a different type of control over the transmission properties of the TEM mode can be achieved. Considering the system shown in The physical effect of the cylindrical defects is captured in the quantity Σ, which is purely real and can take any sign, defined as ref. w is the width of the sample, and the summation is taken over the number of defects. A given defect m has radius Rm, and wavenumber J0 and J1 are Bessel functions of the first kind of order 0 and 1, respectively.Here, km2Rm corresponds to a zero of the J0 or J1, then q1 ↔ q2. The reflection coefficient is, on the other hand, not invariant under the exchange q1 ↔ q2 except when Σ = 0 corresponding to an absence of defects, or if defects are present, each one satisfies J1(km2Rm) = 0. This is consistent with For a given frequency, the value of Σ is assumed fixed in what follows. If the quantity t as long as q1,2 ≠ 0. Interestingly, we can swap between perfect reflection and perfect absorption for a given Σ. To see this, consider the case Σ ≫ 1, which occurs when any defect has a radius and material parameters that satisfy J0(km2Rm) = 0. In that case, t → 0, q1 is tuned such that q1 = 1.From R, T, and A are shown in the left column of t = 0 and r = 1 in the limit q1 ≫ 1. It follows then that when the defects are designed so that Σ ≫ 1, the first graphene sheet described by q1 completely controls the reflection and absorption of EM waves.The relevant parameters r and t coefficients revert to the expressions shown in r and t are directly tunable via manipulating only q1 and q2. In the right column of q1,2 = 0, there is total transmission. Increasing the Fermi levels in one or both graphene sheets then increases the reflections of waves until q1,2 ≫ 1, at which point nearly complete reflection occurs. As the right column of q1 or q2 is small, we find qi is small). This quantity has a maximum when qi = 2, which gives A = 0.5.If the cylinders are now chosen so that Σ = 0, then the q1 and q2 that results in no reflections from the first graphene sheet. For example we get r = 0 if q2 = 0, and We can moreover control the reflectivity of the system in situations when the rods are not excited at resonance. In particular, if we relax the restriction that Σ ≫ 1, setting the numerator of e.g. three graphene layers separated by a dielectric spacer and an ENZ metamaterial4647492O3 and polytetrafluoroethylenet| → 0.8 instead of |t| → 1 in the transmission regime.Other works have shown high-absorption of normal incidence light by using δEF in the local Fermi level which typically are a few tens of meV. However, recent work using both single-layerEF considered in our work, the temperature needs to be less than 10 K in order to satisfy the requirement EF ≫ kB  Regarding the tunable Fermi level of graphene, one should acknowledge the experimental difficulty in minimizing fluctuations ε → 0 and μ → 0 in the microwave and infrared regimes5356If the central region is a MIZIM , there aWe here provide details for the solution of the Maxwell equations and the scattering coefficients.Dielectric case: we here provide some technical details regarding the calculation of the reflectance, transmittance, and absorptance of an air/graphene/dielectric/graphene/air region. For an incident TEM mode, we use the following fields where l is arbitrary, we find ∇ × Hj reveals that for the MIZIM region where ε1 → 0, we must have ∇ × H1 vanish so that the electric field remains finite. Therefore in region 1, we write simply H1 = h1, where h1 is a constant ∇ × Ej shows that ∇ × E1 must also vanish, for physical reasons that are similar to those requiring the vanishing of the curl of H1 above. If we apply Stoke’s theorem to the outer boundary ∂C of the MIZIM structure, which includes the PEC layers and graphene sheets, as well as the circular boundaries ∂Cm2 of each of the defects, the contributing paths in the line integral lead to the following expression involving the corresponding E field:Since within the MIZIM region we have a vanishing permeability E fields for the ∂C segments has been given above in Em2 involves a sum of Bessel functions of the first kindThe rm is the radial coordinate relative to the center of each defect of radius Rm, and mth cylinder. Inserting the electric fields into the line integral in where r or t, yielding the results found in where we have used EF ≫ kBT, i.e. the role of temperature. This is shown in Finally, we also provide plots that show the effect of relaxing the criterion How to cite this article: Linder, J. and Halterman, K. Dynamical tuning between nearly perfect reflection, absorption, and transmission of light via graphene/dielectric structures. Sci. Rep.6, 38141; doi: 10.1038/srep38141 (2016).Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.


Supplemental Digital Content is available in the text An increasing number of studies have examined the ability of programmed death-ligand 1 (PD-L1) to function as a marker for tumor prognosis. However, whether PD-L1 expression is a prognostic factor for the poor outcomes in many human cancers remains controversial. This study aims to investigate the prognostic role of PD-L1 expression through a meta-analysis update of 60 studies.The studies were identified by searching PubMed, Embase, Google Scholar, and Cochrane Library, and were assessed by further quality evaluation. The pooled hazard ratios (HRs) with 95% confidence intervals (CIs) for total and stratified analyses were calculated to investigate the association between the PD-L1 expression and the overall (OS) and disease-free (DFS) or progression-free survivals (PFS) of cancer patients. Heterogeneity and publication bias were also investigated.P <.000) and DFS/PFS . Subgroup analyses showed that PD-L1 overexpression was significantly related to the poor OS in patients with breast , urothelial , renal , and gastric cancers . Furthermore, PD-L1 overexpresion was significantly associated with poor DFS/PFS in patients with hepatocellular carcinoma (HCC) , melanoma , and renal carcinoma, . The adverse prognostic impact of PD-L1 was observed in patients of different ethnicities.The results indicated that PD-L1 overexpression can predict a poor OS (HR = 1.58, 95% CI = 1.38–1.81, The findings of this meta-analysis suggest the correlation of PD-L1 overexpression with worse OS in patients with solid tumors. However, the correlations differed according to tumor types. Although PD-1 cell surface protein expression can be discovered within 24 hours of stimulation, the functional effects of PD-1 ligation are observed within a few hours following T-cell activation. Tumors express antigens recognized by host T cells, although the immunologic clearance of tumors is rare. Part of this failure is attributed to the immune suppression by the tumor microenvironment. PD-1 interacts with 2 ligands, PD-L1  and PD-L2. The PD-L1 expression on many tumors is an integrant part of this suppressive milieu and may act in cooperation with other immunosuppressive signals. PD-L1 is expressed in several tumor types, such as melanoma, glioblastoma, and cancers in the lung, kidney, head and neck, stomach, colon, pancreas, breast, cervix, and ovary. PD-L1 inhibits T-cell proliferation and adhesion, as well as cytokine production. Furthermore, it appears to regulate T-cell function in peripheral tissues.Programmed cell death 1 (PD-1), an immunoinhibitory receptor was first described in 1992 by Ishida et al.–7 However, the prognostic role of PD-L1 remains unclear. Other studies have found that PD-L1 expression is correlated with both poor prognosis and no prognostic significance, making it difficult for researchers to present definitive conclusions. Thus, the PD-L1 expression in cancers and its prognostic significance are still uncertain.PD-L1-positive tumors may indicate immune active tumors that can be a response to anti-PD-1 and/or PD-L1 therapies because they are correlated with the poor prognosis of many of these malignancies, including lung adenocarcinoma, melanoma, and refractory Hodgkin's lymphoma.In this study, we conducted a systematic review and meta-analysis to summarize the global findings in using PD-L1 for the prediction of clinical results for cancer patients.22.1We performed a network search using PubMed, Embase, Google Scholar, and Cochrane Library for original articles that analyzed the prognostic value of PD-L1 in various cancers. We selected studies using different combinations of the following keywords: “Neoplasm,” “cancer,” “carcinoma,” “PD-L1,” “programmed cell death ligand 1,” “B4-H1,” and “CD-274.” The final search was performed on November 2015. We increased the integrity and accuracy of the search process by manually screening the reference lists for included articles to explore potential studies.As the present meta-analysis was performed based on previous published studies,thus no ethical approval and patient consent are required.2.2The studies were considered eligible if they met all of the following inclusion criteria: diagnosis of solid tumor was proven in human; the expression of PD-L1 through immunochemistry was evaluated; PD-L1 was correlated with overall survival (OS), disease-free survival (DFS), or progression-free survival (PFS); and hazard ratio (HR) and their corresponding 95% confidence intervals (CI) were estimated by sufficient data. Articles were excluded based on the following criteria: review or laboratory articles, as well as case studies or letters; articles that described the survival outcome of other indicators; articles from 1 author and studies with repeated samples of the same patients; and unpublished studies.2.3Two investigators (Qianqian Wang and Fang Liu) independently extracted data from each eligible study. The following information were extracted in each study: author, year of publication, country, number of patients, median age, median follow-up period, binded reading, tumor stage, PD-L1 overexpression cut-off value, prognostic outcomes of interest, analytical method, and HR with its 95% CI. Disagreements were resolved through discussion, and consensus between the 2 authors.2.4 Williamson et al, and Tierney et al. Statistical heterogeneity between studies was quantified using Cochran's Q test and Higgins I-squared statistic. Heterogeneity was defined as P <.05 or I2 >50%. If there was statistically significant heterogeneity, a random effects model was selected to combine the data. Otherwise, a fixed effects model was used. By convention, poor outcome for PD-L1 overexpression was considered when the described HR >1, and would be considered statistically significant if the 95% CI did not overlap 1. Subgroup analysis was further performed to explore the heterogeneity source. Publication bias was investigated by visually assessing the asymmetry of an inverted funnel plot. Moreover, Begg's and Egger's tests were conducted to quantitatively support the publication bias. All analyses were performed using STATA 12.0. A P-value <.05 was considered statistically significant.Positive or negative of PD-L1 was defined according to the cut-off values provided by the authors. The impact of PD-L1 overexpression on prognosis in solid tumors was estimated by HR and its 95% CI. We directly used crude values when HRs were reported in the original studies. Otherwise, the values were calculated using Kaplan–Meier curves according to the methods described by Parmar,33.1 Thus, 61 studies were included in this meta-analysis . Furthermore, significant heterogeneity was noted among the studies , as shown in Fig. P = .001), which suggested that PD-L1 overexpression was a poor prognosis indicator for multiple solid cancers, and significant heterogeneity was found across the studies , as shown in Fig. The combined HR of 60 included studies, which included 10,310 cancer patients, showed that the PD-L1 overexpression was associated with poor OS , urothelial , renal , and gastric cancers . No significant correlation between PD-L1 and OS was found in the other subgroup analyses, as shown in Fig. P = .003), melanoma , and renal carcinoma,  or Caucasian patients  and Caucasian patients   Fig. A. Sensit4,71 To evade from the immune system's monitoring, tumor cells in tumor microenvironment can upregulate the expression of PD-L1 through a variety of mechanisms, and bind themselves with negative immune checkpoint PD-1 on the surface of T cells, thereby inhibiting T cells function, losing its killing effect on tumor cells.PD-1 and its ligands, PD-L1 and PD-L2 expressed within the tumor microenviroment, can modulate the balance between T-cell activation, tolerance, and immunopathology during long-term antigen exposure.,73 Several clinical trials, which combine anti-PD-1 mAbs with cancer vaccines , antitumor mAbs (lymphoma), or chemotherapies (pancreatic cancer and NSCLC), have been planned or are in progress. Thus, PD-L1 may be used as a prognostic marker and therapeutic target for those cancers, which provides impetus for the further investigation of its functions.The correlations between PD-L1 expressions and different tumors have been studied by numerous researches. However, the results were inconsistent and conflicting. To arrive at a reasonable conclusion, we performed this meta-analysis and aimed to evaluate the relationship between PD-L1 overexpression and the prognosis of solid tumor patients. Our analysis included 60 studies with 10,310 cases. The results showed that PD-L1 overexpression is a poor prognostic factor in multiple solid tumors with statistical significances for OS , and DFS/PFS . We conducted subgroup analyses according to different tumor types and ethinicities. For tumor type, the analysis indicated a statistically significant detrimental effect of PD-L1 on the OS in breast, urothelial, renal, and gastric cancers. Furthermore, PD-L1 overexpression was also significantly associated with worse PFS in HCC, melanoma, and renal carcinoma. For ethnicities, the adverse prognostic impact of PD-L1 was retained in patients with different ethnicities. Other studies showed that antibodies blocking PD-1 or PD-L1 are likely to provide a new benchmark for antitumor activity in immunotherapy.The robustness of the results in our meta-analysis was confirmed by a sensitivity analysis, which did not significantly alter the pooled effect size for OS and DFS/PFS. Furthermore, HRs extracted from Kaplan–Meier curves seemed to be less reliable than those reported directly. However, we did not discover any obvious primary deviation in the publications when comparing our extracted OS HRs and 95% CIs with the outcomes of the published studies. Nevertheless, this meta-analysis still has many deficiencies. First, only published studies and studies in English were included. We may have missed unpublished and non-English studies which are unsuitable for meta-analysis, meanwhile there might be selection bias. Second, high heterogeneity was observed among studies, which will influence the analysis, interpretation, and conclusions of this study. To minimize the effect of the heterogeneity on OS and DFS/PFS, we used a random effect model. Nevertheless, the heterogeneity within and between studies only disappeared in urothelial cancer, melanoma, and renal carcinoma subgroups, we could not determine the origin of the heterogeneity. Third, no consistent standard for cut-off values were found in our included studies, which can cause clinical and statistical heterogeneity. Finally, the publication bias between the relationship between PD-L1 overexpression and OS has been shown on Egger tests and funnel plots, and we found evidence of the publication biases across the included studies. However, our finding that the corrected pooled effect size remained statistically significant after using the “Trim and Fill” method to adjust for the effect of potential publication bias on the results of the meta-analyses. Thus, the reliability of our results could be confirmed. In addition, studies with different follow-up times can also result in bias.5In conclusion, we demonstrated that PD-L1 overexpression is significantly associated with poor OS and DFS/ PFS, especially in renal carcinoma and urothelial cancer. PD-L1 overexpression may predict poor prognosis in different cancers. Furthermore, the PD-L1 target can become an effective anti-cancer therapy as well.Authors thank all the authors whose publications were included in our meta-analysis.


PD-1 and PD-L1 mRNA-expression in a cohort of ovarian cancer (OC) patients in relation to tumor mutations. We analyzed mRNA expression of PD-1, PD-L1 and IFNG by quantitative real-time PCR in tissue of 170 patients with low grade-serous (LGSOC), high-grade serous (HGSOC), endometrioid and clear cell OC compared to 28 non-diseased tissues  in relation to tumor protein 53 (TP53) and breast cancer gene 1/2 (BRCA1/2) mutation status. TP53-mutated OC strongly expressed PD-L1 compared to TP53 wild-type OC (p = 0.028) and BRCA1/2-mutated OC increasingly expressed PD-1 (p = 0.024) and PD-L1 (p = 0.012) compared to BRCA1/2 wild-type OC. For the first time in human, we noted a strong correlation between tumoral IFNG and PD-1 or PD-L1 mRNA-expression, respectively (p < 0.001). OC tissue increasingly expressed PD-1 compared to healthy controls . PD-1 and PD-L1 mRNA-expression increased with higher tumor grade  and younger age . Finally, in the major subgroup of our cohort, FIGO stage III/IV HGSOC, high PD-1 and PD-L1 mRNA-expression was associated with reduced progression-free (p = 0.024) and overall survival (p = 0.049) but only in the univariate analysis. Our study suggests that in OC PD-1/PD-L1 mRNA-expression is controlled by IFNγ and affected by TP53 and BRCA1/2 mutations. We suggest that these mutations might serve as potential predictive factors that guide anti-PD1/PD-L1 immunotherapy.Checkpoint molecules such as programmed cell death protein-1 (PD-1) and its ligand PD-L1 are critically required for tumor immune escape. The objective of this study was to investigate tumoral  Ovarian cancer (OC) is the major cause of death among gynecological cancer entities . In recePD-L1 is expressed by tumor cells  to inactThe rationale to test the efficacy of checkpoint inhibitors in OC arises from the observation that intratumoral T-cells directly correlate with clinical outcome  and thatPD-1 and PD-L1 mRNA expression in 170 epithelial OC in relation to 28 non-neoplastic tissues and to clinicopathological features to identify a subgroup of patients which may profit by checkpoint inhibitors. Since IFNγ was found to play an essential role in the adaptive immune resistance of tumors as an inducer of PD-L1 on tumor cells  were determined with the assistance of the computer program Primer Express . BLASTN searches were conducted to confirm the total gene specificity of the nucleotide sequences chosen for the primers and probes. PD-L1 forward primer: 5′-AATGATGGATGTGAAAAAATGTGG-3′; PD-L1 reverse-primer: 5′-AATGCTGGATTACGTCTCCTCC-3′; PD-L1 TaqMan probe: 5′-FAM-TCCAAGATACAAACTCAAAGAAGCAAAGTGATACACATT-3′-TAMRA. To prevent amplification of contaminating genomic DNA, the probe was placed at the junction between exons 6 and 7. Primers and probe for IFNG and PD-1 were purchased from Applied Biosystems . PCR reactions were performed as previously described [Primers and probes for the TATA box-binding protein . For the particular primary antibodies the following procedures were used: PD-L1  diluted at 1:100 was incubated for 32 minutes and the OptiView™ DAB detection kit system CC1 was used for 36 minutes. PD-1  ready to use incubated for 32 minutes followed by the UltraView™ DAB detection kit CC2 for 44 minutes. For counterstaining hematoxylin was used. The evaluation of immunohistochemistry was performed semiquantitatively.® and the NextSeq system . After sequencing, mutations in BRCA1, BRCA2 and TP53 were identified with the help of NextGene and Geneticist Assistant softwares. Pathogenicity for new mutations that were not found in the database was determined and categorized using prediction tools like SIFT, alignGVGD, mutation taster.Genomic DNA from 158 pulverized, quick-frozen OC specimens was isolated using the DNeasy tissue-kit . Targeted NGS was performed using the TruSight Cancer sequencing panel . The analyses were performed on the Illumina MiSequU test or Kruskal–Wallis test were applied to test for statistical significance between two groups or more than two groups, respectively. For parametric sample sets student’s two-tailed t-test was applied to test for statistical significance between two groups. The correlations between PD-1, PD-L1 and IFNγ mRNA expression were assessed by Spearman rank correlation analyses . Progression free survival (PFS) was defined as the time from diagnosis of the primary to tumor to the histopathological confirmation of recurrence or metastases and overall survival (OS) as the time from diagnosis of the primary to tumor to death from any cause or to the last clinical inspection. Univariate Kaplan-Meier analyses and multivariable Cox survival analyses were used to explore the association of PD-1 and PD-L1 expression with PFS and OS . For survival analyses, patients were dichotomized into low and high mRNA expression level groups by the optimal cut-off expression value calculated by the Youden’s index [P-values less than 0.05 were considered as statistically significant. Statistical analysis was performed using SPSS statistical software .The non-parametric Mann-Whitney ’s index . P-value


Solanum lycopersicum that contain different, defined chromosomal segments derived from the wild tomato relative, S. pennellii. Among these lines, IL4-4, in which the bottom of S. lycopersicum (cv. M82) chromosome 4 is replaced by the corresponding S. pennellii segment, is altered in many primary and secondary metabolites, including many related to fruit flavor and nutritional quality. Here, we provide a comprehensive profile of IL4-4 ripe fruit metabolites, the transcriptome and fine mapping of sub-ILs. Remarkably, out of 327 quantified metabolites, 185 were significantly changed in IL4-4 fruit, compared to the control. These altered metabolites include volatile organic compounds, primary and secondary metabolites. Partial least squares enhanced discriminant analysis of the metabolite levels among sub-ILs indicated that a genome region encompassing 20 putative open reading frames is responsible for most of the metabolic changes in IL4-4 fruit. This work provides comprehensive insights into IL4-4 fruit biochemistry, identifying a small region of the genome that has major effects on a large and diverse set of metabolites.A major resource for tomato quality improvement and gene discovery is the collection of introgression lines (ILs) of cultivated  Solanum pennellii, a wild South American relative, has a complete genome sequence  is a barrier to utilizing the genetic variation of wild relatives at the bottom of chromosome 4 for improving tomato fruit quality  (Agilent 5975 GC/MS)1. The list of quantified volatile compounds is presented in Supplementary Table Volatile organic compounds (VOCs) were collected from ripe fruits of sub-ILs R401, R2174, R2075, R100, R434 as well as the parent lines, IL4-4 and M82, grown in randomized, replicated plots. Each biological replicate was a mixture of five to six individual fruit at the red ripe stage. Collection of VOCs was performed as described previously . In brieThe extraction method was performed as described by Sugars  and acids  were quantified in Florida as previously described .For secondary metabolites the rest of polar phase (see above extraction protocol) was dried and residue was suspended in 200 μl of 80% methanol water (80:20). The extracts were then subjected to LC-MS analysis using a high-performance liquid chromatography , coupled to a Finnigan LTQ-XP system , Metabolite identification and annotation were performed using a combination of standard compounds and tomato metabolomics databases .2 in positive and negative ionization mode. Processing of chromatograms, peak detection and integration were performed using REFINER MS® 6.0 (GeneData)3. Processing of mass spectrometry data included the removal of the fragmentation information, isotopic peaks, as well as chemical noise. Obtained features (m/z at a certain retention time) were queried against an in-house lipid database for further annotation.For lipid extraction, the lipophilic phase (see extraction protocol above) was collected and vacuum-dried. Samples were processed using ultra-performance liquid chromatography coupled with Fourier transform mass spectrometry .Five ripe tomato fruits were homogenized in a blender for 30 s and frozen at -80°C until analysis. Samples were thawed and 1.5 mL was centrifuged at 16 000 × p < 0.01) was used to determine the QTL controlling metabolite content. All lines were compared with M82 and each other. If metabolite level of the line was significantly different from the M82 control (indicated by Dunnett’s t-test p < 0.01), a line was considered as harboring a QTL. A list of QTLs within the sub-ILs can be found at Supplementary Table To map the metabolite variation in the sub-ILs, a one-way analysis of variance  was performed by using the Excel add-in Multibase package .Unpaired Student’s Total RNA was extracted from frozen pericarp tissue of ripe fruits as described in S. lycopersicum and S. pennellii were aligned using ClustalW4, and polymorphic sequences were submitted to SIFT5 to predict the impact of amino acid substitutions on protein function . A dot represents the log2 of the RPKM ratio of a transcript between IL4-4 and control M82.Gene ontology enrichment analyses of DEGs were conducted with PANTHERT-test.Twenty fruits harvested from R2174, R434, R2075, IL4-4, and M82 were subjected to sensory evaluation. Samples were cut into wedges and labeled with random numbers. Sample taste tests were performed on two occasions with two separate harvests. Each taste panel had 14 panelists. Each sample was rated on a 10-point hedonic scale . Taste preference scores are an averaged score of all panelists. Significant differences were calculated using Student’s p < 0.05) . Chemicals with altered contents included 21 VOCs, 44 hydrophilic primary metabolites, 31 hydrophilic secondary metabolites and 89 lipids (Table 1). Consistent with previous results (Table 1). Interestingly, out of 18 quantified sugars and sugar alcohols, 12 were elevated, including the most important contributors to sweetness, glucose, and fructose, while only one unknown sugar was significantly decreased in IL4-4. As key precursors for phenylpropanoids biosynthesis, levels of all three AAA were increased in IL4-4, indicating potential regulators of the shikimate pathway may exist within the S. pennellii genomic segment included in IL4-4. In addition, 31 out of 49 hydrophilic secondary metabolites and 89 out of 170 lipids that were quantified were significantly altered (p < 0.05) (Table 1). These results together indicate a locus or loci at the bottom of S. pennellii chromosome 4 regulating multiple independent metabolic pathways during fruit ripening.To obtain an overview of the IL4-4 fruit metabolome, we quantified a set of metabolites that included VOCs, hydrophilic primary, hydrophilic secondary and lipophilic metabolites by utilizing targeted gas chromatography (targeted GC), gas chromatography-mass spectrometry (GC-MS) and high/ultra-performance liquid chromatography mass spectrometry (HP/UPLC-MS). An overlay heat map shows the metabolite contents in IL4-4 with respect to M82. The fully annotated data set is provided in Supplementary Table  results , many VO results , includiP-value < 0.05, we identified 2307 more abundant transcripts and 1635 less abundant transcripts in IL4-4 fruit tissue compared to M82 . GO analysis showed that the largest groups in biological process and molecular function were metabolic process (47.4%) and catalytic activity (45%), respectively.To gain insight into the molecular mechanisms underlying the observed metabolite changes, gene expression in ripe pericarp tissues of IL4-4 and M82 was determined using RNA-Seq. A total of 26,147 expressed genes were detected in IL4-4 and M82 . The mapped DEGs are involved in main primary and secondary metabolic biosynthetic processes such as sugar, acid, amino acid, phenylpropanoid, and lipid metabolism. Notably, light reaction-associated genes were significantly more abundant. This pattern suggests a possible increase in photosynthesis in IL4-4 fruit, which in turn could provide precursors for multiple metabolic pathways.In order to further investigate the potential roles of DEGs in metabolism, we generated a snapshot of these genes over the main metabolic pathways facilitated by MapMan. An overview of the map suggested a global transcriptional regulation of metabolism-associated genes by IL4-4 . We observed significantly higher levels of TomLOXB (Solyc01g099190), TomLOXF (Solyc01g006560) and HPL (Solyc07g049690) transcripts in IL4-4 fruit tissue while other members of the 13-LOX family remained unchanged . Increased transcript abundance of these genes is consistent with the higher levels of C5 and C6 volatiles in IL4-4 fruit .To better understand the relationship between transcription and metabolite synthesis in IL4-4, we examined transcript content of genes encoding key enzymes related to FA metabolism and the shikimate-phenylpropanoid pathway. 13-lipoxygenases (13-LOXs) and hydroperoxide lyase (HPL) are the main enzymes catalyzing conversion of C18 polyunsaturated FAs to C5 and C6 volatiles in tomato fruit . The transcripts associated with most genes in the shikimate pathway were not significantly different while there was a significant decrease in shikimate dehydrogenase (Solyc10g038080) in IL4-4. Transcripts encoding three genes in the tryptophan synthesis pathway  were elevated in IL4-4, while ADT (arogenate dehydratase: Solyc11g072520) in the phenylalanine pathway was reduced. In addition, we observed significantly higher transcript contents of PHENYLALANINE AMMONIA-LYASE (PAL: Solyc09g007910), which catalyzes the first step in phenylpropanoid synthesis from L-phenylalanine . A previous study suggested that expression of this PAL is the primary determinant of commitment to phenylpropanoid synthesis . Taken together, these results suggest that genetic factors within IL4-4 regulate metabolite levels via their influence on transcription of genes involved in multiple biosynthetic pathways.We observed significant increases in AAAs as well as nutrition and flavor contributing phenylpropanoids and glycoalkaloids in IL4-4. Therefore, we examined the transcript levels of genes encoding key steps in the shikimate and phenylpropanoid pathways . VOCs were collected from fruits of five selected sub-ILs  and the parents, M82 and IL4-4, while additional metabolite data were collected from all sub-ILs shown in Figure 5A. Partial least squares enhanced discriminant analysis (PLS-EDA) models were generated based on the metabolome data collected from the sub-ILs . The distribution patterns of different lines with and without the M genomic region suggest that the overall metabolic traits of all lines can be divided to two main clades, IL4-4-like and M82-like. Sub-ILs that contain S. pennellii genomic region M (defined by marker C2147 and C200952) are closer to IL4-4, while lines without the M region resemble M82. Thus, the S. pennellii M genome fragment contains the major locus affecting a wide range of metabolic changes in tomato fruit.IL4-4 includes a genomic region of ∼3.4Mb . To fineTable 1, we identified 185 significantly altered metabolites (p < 0.05). To further understand how the M genomic region regulates fruit quality, we reexamined all of these metabolites in all of the lines for significant effects using ANOVA. Using a statistical threshold of p < 0.01, we identified 72 metabolite QTLs in IL4-4. Of these QTLs, 43 were located within the M region . QTLs within this region affect metabolites synthesized from a wide range of both primary and secondary metabolism pathways. Interestingly, QTLs correlated with fructose, glucose, soluble solids (brix) and C5 volatile contents are located within this small segment, all traits likely to affect consumer taste preferences . There are also major effects on potential nutrition-associated metabolites, including flavonoids . To address the effects of these chemical alterations on flavor quality, we conducted a consumer panel where individuals rated fruits from M82, IL4-4, and sub-ILs. Consumers significantly preferred IL4-4 and a sub-IL containing the M region (R2174) over M82 or sub-ILs absent the M region . In the red fruit pericarp tissue, transcript levels of only three of these genes were significantly altered between IL4-4 and M82: ORF2 (Serine/threonine protein kinase), ORF10  and ORF17 . There were no premature stop codons in any open reading frames of the genes in either ortholog. To investigate the possible effects of amino acid sequence substitutions on the metabolic phenotype in IL4-4, a comparison of protein sequences of the 20 candidate genes from S. pennellii and S. lycopersicum located in the mapped M genome region was performed  responsible for altered metabolite biosynthesis.There are 20 annotated genes within the pe fruit . That skS. pennellii . Localization of the reticulated peel phenotype caused by CWP1  gene in the M region. Trehalose, which is regulated by T6PP, has been proposed to be a central signaling sugar that impacts sucrose content (Table 2). It is possible that altered expression of one or more of these genes could cause at least some of the observed metabolic changes.High-resolution fine mapping studies with tomato populations to identify loci regulating multiple fruit quantitative traits have been performed e.g., . Yates e by CWP1  to the Me fruits , higher  content . AlteredLOX transcripts as well as HPL were more abundant. For phenylpropanoids, the transcript encoding PAL (Solyc09g007910) was significantly higher in IL4-4. This transcript was previously shown to be significantly up-regulated in transgenic tomato fruits over-expressing the MYB transcription factor PhODO1  and the mechanism of action. Although the skin reticulation phenotype is not itself desirable, it may be possible to eliminate or minimize this effect while maximizing the overall effects on fruit metabolite contents.ZL, HK, AF, JG, and ZF organized and coordinated aspects of the research in their respective laboratories. ZL, SA, YB, DT and YZ performed experiments and data analysis. ZL, SA, HK, and AF wrote the manuscript.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


By using a highly doped p-Si/SiO2 substrate as the back gate, we analyzed the transport properties of the device and the dependence on the pressure and on the electron bombardment. We demonstrate herein that low energy irradiation is detrimental to the transistor current capability, resulting in an increase in contact resistance and a reduction in carrier mobility, even at electron doses as low as 30 e−/nm2. We also show that irradiated devices recover their pristine state after few repeated electrical measurements.We studied the effects of low-energy electron beam irradiation up to 10 keV on graphene-based field effect transistors. We fabricated metallic bilayer electrodes to contact mono- and bi-layer graphene flakes on SiO Graphene is a promising candidate for future nanoelectronics and has been attracting an enormous amount of attention from the scientific community since 2004, when graphene flakes were exfoliated from graphite for the first time in Manchester . Due to From an experimental viewpoint, the use of scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron beam lithography (EBL), and focus ion beam (FIB) processing in an ultra-high vacuum represents a necessary step for the fabrication and characterization of graphene-based devices. Consequently, graphene devices during fabrication or under test are necessarily exposed to a high vacuum and electron irradiation, which may considerably affect their electronic properties.Several experiments have shown that the irradiation of energetic particles, such as electrons ,14,15,16Raman spectroscopy has been largely used to study electron-beam induced structural modifications ,20,21, o2 and a carrier mobility as high as 4000 cm2·V−1·s−1 on a Si/SiO2 substrate. Electron irradiation affects the transistor current drive capability by reducing the carrier mobility and increasing the channel and contact resistance. We also show that, for low energy electron irradiation, the conditions of pristine devices are almost restored by successive gate voltage sweeps.In this paper, we study the modification of electronic transport properties of GFETs upon exposure to electron beam irradiation for scanning electron microscopy imaging with an acceleration energy up to 10 keV. An optimized fabrication process has been developed to obtain devices characterized by specific contact resistivity 2 (300-nm-thick) substrates. After optical identification, the mono- or bi-layer nature of the flakes were confirmed by Raman spectroscopy. Metal contacts to selected graphene flakes were realized by means of electron beam lithography and magnetron sputtering techniques. Spin coating of approximately 400 nm PMMA-A7 (poly-methyl methacrylate) at 4000 rpm was performed on the sample, and it was successively exposed by an EBL system, Raith Elphy Plus . Methyl isobutyl ketone and then isopropanol was used as a developer. The metal electrodes were fabricated by a three cathode RF Sputtering Magnetron  for in-situ multilayer deposition working at 10−7 mbar base pressure. The graphene flakes were contacted by a Nb/Au metallic bilayer (15 nm Nb/25 nm Au) with niobium contacting the graphene and gold working as a cap layer to prevent Nb oxidation and favor electrical connection with the probe tips. Metallic leads were sputtered at low power density (<0.7 W·cm−2) and small deposition rates (0.3 nm/s for Nb and 1.2 nm/s for Au) to prevent graphene damages.Graphene flakes were obtained from highly oriented pyrolytic graphite (from NGS Naturgraphit GmbH) by a scotch tape method and were placed on standard p-Si/SiOElectrical characterization was performed by means of a Janis Research ST-500 cryogenic probe station  connected to a Keithley 4200  Semiconductor Characterization System (SCS) working in wide ranges of current (100 fA to 0.1 A) and voltage (10 μV to 200 V). To study the effect of e-beam irradiation on transistors, the SCS was connected to a scanning electron microscope Zeiss LEO 1430  equipped with Kleindeik nanomanipulators MM3A , which allowed in-situ electrical measurements with the sample inside the high-vacuum SEM chamber to prevent adsorbate contamination.In order to characterize the contact resistance, we designed a device with standard geometry to apply the transfer length method (TLM), a structure consisting of a series of spaced electrodes up to 10 µm apart a.IDS vs. VDS) has been measured for each possible combination of contacts: the drain current (IDS) linearly increases with source-drain voltage (VDS), which is a typical behavior at low bias  indicate width and length of each contact, respectively. From the linear fitting of Reff vs. L . The negative value indicates that the graphene is n-doped. The device was measured as produced, without any electrical annealing (stress), which is suitable for inducing the desorption of surface contaminants and improving the metal-graphene coupling, thus reducing the contact resistance [IDS vs. VDS for several VGate values in the range −60 V to +60 V) and the transfer characteristic (at fixed VDS) measured before and after an electrical stress event that stabilize the device improving its performances. The black arrow in the figure identifies the voltage at which the device was suddenly modified, switching from a total resistance of about 250 kΩ to 150 kΩ, for the effect of current annealing. After such modification, the device was routinely measured, showing extreme stability without further modification of the total resistance RDS, which we report as a function of VGate in the insets of RDS is the series of the contact resistance and the channel resistance, RDS = RC + Rchannel, where the channel resistance can be expressed as L and W the length and width of the channel, respectively, μ is the carrier mobility, and q is the unit charge [n0 is the intrinsic carrier concentration, and nind is the carrier concentration induced by the back gate. nind can be expressed in terms of gate oxide capacitance as R vs. VGate allows for the extraction of the contact resistance and carrier mobility as fitting parameters.We also tested the current modulation of this device when used as a field effect transistor with the Si substrate as the back-gate electrode. In t charge . The tot2·cm−1·s−1 to 3900 V2·cm−1·s−1. The electrical stress increased the graphene–metal coupling and worked to clean the channel. The mobility values are comparable to values already reported for Nb-contacted GFETs [p-branch clearly away from the expected theoretical behavior. This can be explained in terms of the reduced coupling between the Nb electrode and the graphene channel (corresponding to large contact resistance), a situation that can cause asymmetry, a double dip, or both in such curves as reported in [RDS − Rcontact we also confirmed the improvement of the channel resistance.Using the transfer characteristics measured before and after the electrical stress, we found that the contact resistance improved (reduced from 200 kΩ to 90 kΩ), while the carrier mobility increased from 3600 Ved GFETs . We alsoorted in ,34. The −7 mbar) for different gate voltage values. The ohmic nature of the contacts is confirmed by the linearity of such characteristics.In VDS = 1 mV. Remarkably, the current annealing and the long high vacuum storage produced very stable devices with low contact resistance (5.0 kΩ < Rcontact < 5.5 kΩ). The high fabrication quality is confirmed by the small contact resistance, the low noise, and the high carrier mobility, which is 4000 V2·cm−1·s−1 < µ < 4400 V2·cm−1·s−1. The Dirac point at a bias between −40 V and −60 V indicates a strong n-doping that is favored by the vacuum and the electron irradiation .In p-dopants, shift the Dirac point towards positive biases, increase the contact resistance and reduce the carrier mobility [Rcontact ≈ 6.7 kΩ a value 35% larger than the value in the high vacuum, while the carrier mobility was reduced to µ ≈ 4100 V2·cm−1·s−1. The inset shows the evolution of the Dirac point from −55 V in the high vacuum to −30 V in air. This observation confirms the importance of performing the electrical measurements in-situ when studying irradiation effects to distinguish the electron beam from other environment-induced phenomena.As soon as the devices are exposed to air, the graphene collects adsorbates that, generally acting as mobility ,37,38,39Ibeam = 0.2 nA. We used an exposure time of 10 s, which resulted in an electron irradiation dose of about 30 e−/nm2. Differently from other works [2·cm−1·s−1 to about 3600 V2·cm−1·s−1 (as obtained from the first sweep measurement). The initial value is restored by the successive sweeps. A consistent behavior is shown by the total resistance, which is increased by the irradiation and recovers with an increasing number of sweeps. The increase in total resistance, as a consequence of the e-beam irradiation, has also been observed on chemical vapor deposition (CVD) grown graphene [0. Mobility and resistance degradation can be explained as increased long-range coulomb scattering [In the following, we consider the effect of electron beam irradiation on the GFETs. In particular, we consider electron beam energy up to 10 keV, i.e., the energy range typically used for SEM imaging. Larger energy (about 30 keV) is normally used for e-beam lithography or imaging in STEM mode. The irradiation was performed on an area of 20 µm × 20 µm, covering most of the graphene channel, with constant beam current er works , we perfer works ,41. By cer works  to estimgraphene . Figure attering  by elect2 substrate with Nb/Au metallic bilayers as contacting electrodes. Electrical characterization evidenced high-quality devices with carrier mobility as high as 4000 cm2·V−1·s−1 and specific contact resistivity of about 19 kΩ·µm2. The effect of 10 keV electron irradiation, with a dose of 30 e−/nm2, on the transport properties has been reported, evidencing a significant reduction in carrier mobility and an increase in contact resistance. Finally, we show here that, for low energy irradiation, the pristine conditions are almost restored after several electrical sweeps, which we explain as a gradual removal of electrons piled up in the gate oxide during e-beam exposure.We realized graphene-based field effect transistors on a Si/SiO


Polyurethane/graphene nanocomposites were synthesized using commercial thermoplastic polyurethane , and two types of graphene derivatives: graphene nanoplatelets (GNP) and reduced graphene oxide (RGO). Fourier Transformation Infrared Spectroscopy Fourier Transformation Infrared Spectroscopy (FTIR) spectroscopy, TEM, and SEM microscopy and XRD techniques were used to chemically and structurally characterize GNP and RGO nanofillers. The properties of the new TPU nanocomposite materials were studied using thermal analysis techniques , Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TG)) to describe the influence of graphene nanofillers on polyurethane matrix. Our investigation describes the comparison of two types of graphene derivatives, commercial one (GNP) and synthesized (RGO) on thermoplastic polyurethanes. These nanofillers provides opportunities to achieve compatibility with the TPU matrix. The property enhancements are attributed commonly to high aspect ratio of graphene nanoplatelets and filler–polymer interactions at the interface. The obtained nanocomposites exhibit higher thermal and mechanical properties due to the good dispersion of both nanofillers into TPU matrix. It was found that the addition of 2 wt % of the nanofiller could lead to a significant reinforcement effect on the TPU matrix. Also, with high content of nanofiller (GNP and RGO), the Payne effect was observed. Since the first scientific on the mechanical exfoliation of graphite into graphene monolayers by scotch-tape, several interesting studies have been published on graphene ,2,3. GraBottom-up methods allow for the production of wafer graphene, which covers the demand of the electronic industry. The high quality of graphene monolayer shows the high electrical conductivity and transparency of this material. Quality of graphene produced by bottom-up approach shows well-builded one-layer structure without atomic defects . The metTop-down approach of obtaining graphene is based on graphite as a raw material. In this way, graphite is exfoliated as the result of breaking inter-plane π-π interactions of stacked graphene in graphite crystal. Exfoliation can be carried in a mechanical, chemical, electrochemical, and plasma assisted way. According to top-down production methods, the obtained graphenes can differ in chemical and physical properties. The main differences are in crystal defects, various multi-layer stacked structure, polydispersity of basal plane, and thickness. The monolayer structure of top-down graphene can be folded and sharp-edged. We can also add function groups by various chemical modifications. These modifications have a strong effect on the electrical, mechanical, and optical properties of graphene obtained by top-down ways.Top-down graphene is promising nanofiller for segmented polyurethane matrix. It can be introduced to matrix via in-situ polymerization, solvent-methods and during extrusion ,11,12, oElastomeric materials can be successfully modified with many nanofillers , especiaSegmented polyurethanes contain flexible and rigid chains alternately in the polymer back bone. The precursor of soft flexible segments is polymeric macromolecules with hydroxyl-end groups, while rigid segments are obtained because of reaction diisocyanate with low-molecular diol or diamine. Due to the chemical incompatibility between the two types of segments (soft and hard), segmented polyurethanes are phase separated on a nanoscale. This phenomenon leads to the creation hard and soft domains, in which hard domains (HS) are rich in rigid segments and soft domains (SS) are rich in flexible chains. Hard domains are responsible for mechanical strength and hardness, and on the other hand, soft domains provide elasticity . FurtherThe aim of this study is to explore the influence of graphene-like nano structured carbon fillers, RGO (reduced graphene oxide), and GNP (graphene nanoplatelets) on the morphology and proprieties of segmented polyurethane thermoplastic elastomers. The filler dispersion, filler orientation and filler-filler networking (Payne effect) have been carefully studied and compared. Morphology-property correlation has been established. Techniques such as high-resolution transmission electron microscopy (HRTEM), DSC, TGA, XRD, Fourier Transformation Infrared Spectroscopy (FTIR), and UTM have been employed to characterize the morphology and proprieties of the system. Theoretical models have been used to fit the experimental data.4, Stanlab, >99%), hydrogen peroxide , hydrochloric acid , phosphoric acid , sulfuric acid , ethanol , and diethyl ether ((C2H5)2O), all from POCh S.A., Poland, were used in GO synthesis. Compressed argon  was used in thermal reduction of GO to reduced graphene oxide (RGO). Graphene platelets (GNP) were supplied by ACS Materials . Thermoplastic polyester urethane (TPU) 52DE55  was used as a nanocomposite polymer matrix. N,N-Dimethylmethanamide (DMF) ((HCON(CH3)2, 99%, POCh S.A., Gliwice, Poland) was used as a solvent in composite preparation.In the following research graphite flakes, Micro 850  was used as a precursor to obtain Graphite Oxide (GO). Also, potassium permanganate (KMnOReduced graphene oxide (RGO) was obtained in two-step synthesis. On the first step, graphite oxide (GO) was obtained using modified Hummer’s method described in the following article . On the Nanocomposites were prepared by the solvent blending method. TPU was dissolved by adding it to DMF at temperature of 80 °C with continuously stirring, the final concentration of TPU/DMF solutions was 25% by weight. Next, the proper amounts of nanofillers was added to prepared solution and was homogenized by high-shear mixer  with 2500 rpm for 30 min. Obtained polymer nanocomposite solutions, with good quality of dispersion, were poured onto Petri pans and the DMF was fully evaporated at 70 °C for 72 h. The obtained nanocomposites were compression molded (under a pressure of 1 ton) to thin film with 0.7 mm thickness at the temperature of 195 °C. Press molded films  was used−1. The measurement was carried in standard conditions .Fourier Transformation Infrared Spectroscopy of polyurethane materials has been carried using ThermoElectron Corporation  Nicolet 8700 Spectrometer in the Attenuated Total Reflectance  mode in the range of 500–4000 cmWide-angle X-ray scattering measurements of nanofillers and polyurethane composites were performed using a Bragg–Brentano X’PERT PHILIPS diffractometer , equipped with a Cu Anode X-ray tube and diffracted beam monochromator . TPU samples were scanned in 2θ range from 5° to 35°.Microscopy analysis (for nanofiller GNP and RGO) was performed by means of STEM-EDX technique using Transmission Electron Microscope FEI Europe , Tecnai F20 X-Twin coupled with EDX Spectrometer  (samples were cut in cryo-mode using ultramicrotome). HRTEM (for nanocomposite systems) is from JEOL , model JEM-2100. The samples were sliced with LEICA EM UC7 Ultra Microtome with cryo-mode  using glass knife. The thickness of the samples was 150 nm and the slices were deposited on a 200 mesh Cu Grid.Thermogravimetry  ISO 11358) of obtained materials was conducted by Netzsch TG209F3 TG analyzer . Degradation process has been performed in the temperature range from 35 °C to 700 °C, nitrogen atmosphere with gas flow rate 40 mL/min, and heating rate 20 °C/min.Differential Scanning Calorimetry  measurements have been carried by Netzsch DSC209F1 instrument  in the temperature range from −85 °C to 250 °C. The samples weight was about 10 mg, and analysis was conducted in nitrogen atmosphere with gas flow 40 mL/min. The samples were pre-heated (1st-heating) to eliminate thermal history, then cooling and 2nd-heating at rate 10 °C/min.3. Tensile (in static mode) tests were carried out using universal testing machine (UTM) Zwick/Roell Z020 . All of the tests were performed according to ISO 527-1 standard.Dynamical Mechanical Analysis  ASTM D4065, D7028-07) has been carried by TA Instruments DMA Q800 analyzer . Temperature investigation mode has been performed in uniaxial tension mode, in the temperature range −100–100 °C, 1Hz frequency amplitude of deformation 20 µm and heating rate 4 °C/min using liquid nitrogen as cooling medium. Strain sweep analysis for Payne effect investigation has been conducted in three different temperatures , in the same tension mode and frequency of deformation, but in the amplitude range from 0.25 to 170 µm. The dimensions of thin samples were 5.0 × 5.0 × 0.7 mmPrepared reduced graphene oxide (RGO) nanofiller possess good exfoliated structures a. Our pr−1, were assigned to stretching vibrations of N–H groups that were present in both types of TPU spectra. The wide base of the identified peak of NH stretching may be related to the presence of hydrogen bonds in the TPU and GNP or RGO modified TPU structure. The very strong carbonyl stretching, in the case of obtained TPUs and nanocomposite systems, appeared at 1726 cm−1. The FTIR analysis confirmed that the performed synthesis lead to the generation of obtain TPUs . For PURE TPU polyurethane matrix, DSC curve indicates that there are no crystalline regions within the soft domain, while at 216 °C there is a maximum corresponding to the melting of crystalline hard domains (Tm). Thus, the polyurethane matrix is composed of two amorphous regions, which are arranged in rigid segments consisting of hard domains. Polyurethane matrix possesses soft and hard segments what was also confirmed using XRD investigation and correlate with DSC results. The visibility of diffraction maxima’s in XRD studies (c), which is related to the GNP activity as nucleation process in the heterocrystallization by increasing its temperature. In the TPU/RGO nanocomposites, the crystallization temperatures have practically remained unchanged, indicating its less influential factor.For many group of polymers, which possess semicrystalline structure, the incorporation of nanofiller causes changes in the crystallization degree, size of the crystalline aggregates, and changes in crystallization rate and in the maximum crystallization temperature (Trmograms . What isrmograms . Additiormograms . It was 4–40 °C)  were regMA study  [Tg ≈ −2 studies  indicate studies , where m studies . The DSCDynamic mechanical properties were analyzed for all of the obtained materials. Based on curve behavior, it was visible high hard segments (HS) content for non-modified material (PURE TPU) . For modg region. There is only a slight change of Tg value of TPU nanocomposite in comparison to the PURE TPU  of nanocomposite systems increased with nanofiller (RGO or GNP) loadings , Table 5Based on this study it was successfully obtained polyurethane nanocomposites containing reduced graphene oxide (RGO) or graphene nanoplatelets (GNP). The addition of RGO or GNP undoubtedly influenced investigated properties of the polyurethane matrix.g), respectively, than those of the PURE TPU material. It is seen (maxδ) that the addition of the nanofillers improves the mechanical strength of TPU matrix. Enhanced mechanical properties were observed when add maximum 2 wt % of nanofiller and higher tensile strength properties obtain for RGO nanofiller in comparison to the GNP modificator.The authors showed the interaction behavior of the nanofillers with the polyurethane matrix by using spectroscopic and diffraction investigation. Morphology investigation confirmed good exfoliation nanoparticles into the polyurethane matrix and shows a better compatibly of RGO nanofiller to the matrix, in comparison to the GNP  and synthesized reduced graphene oxide (RGO) nanofiller. We could observe agglomeration (strong filler/filler interaction) at higher filler loading, which was evidenced by the high Payne effect. When comparing with results in literature  based on


Ultrathin metal layers can be highly active carbon dioxide electroreduction catalysts, but may also be prone to oxidation. Here we construct a model of graphene confined ultrathin layers of highly reactive metals, taking the synthetic highly reactive tin quantum sheets confined in graphene as an example. The higher electrochemical active area ensures 9 times larger carbon dioxide adsorption capacity relative to bulk tin, while the highly-conductive graphene favours rate-determining electron transfer from carbon dioxide to its radical anion. The lowered tin–tin coordination numbers, revealed by X-ray absorption fine structure spectroscopy, enable tin quantum sheets confined in graphene to efficiently stabilize the carbon dioxide radical anion, verified by 0.13 volts lowered potential of hydroxyl ion adsorption compared with bulk tin. Hence, the tin quantum sheets confined in graphene show enhanced electrocatalytic activity and stability. This work may provide a promising lead for designing efficient and robust catalysts for electrolytic fuel synthesis. Ultrathin metal layers can be highly active carbon dioxide electroreduction catalysts but may also be prone to oxidation. Here, the authors report the fabrication of reactive tin quantum nanosheets confined in graphene and demonstrate their enhanced electrocatalytic activity and stability. Furthermore, the intimate contact between ultrathin metal layers and graphene in a sandwich-like fashion could ensure excellent long-term stability.Motivated by the above consideration, we construct a material model of ultrathin metal layers confined in graphene, in which the protection of graphene could avoid the oxidation of the highly reactive metals at ambient atmosphere . The met2 to formate582 reduction performances. Therefore, it is highly warranted for controllably synthesizing Sn ultrathin layers in the confined space of graphene. Herein, metallic Sn quantum sheets confined in graphene were first successfully synthesized through a spatially confined reduction strategy  oscillation curves in 713To further identify the phase and unravel the local atomic arrangements of the synthetic Sn quantum sheets, synchrotron radiation XAFS measurements were performed. As shown in 2 electroreduction performances in comparison with the Sn nanoparticles as well as the bulk counterpart. To verify the higher activity of the unique sandwich-like 2D structure, CO2 reduction activities for the Sn quantum sheets confined in graphene, 15 nm Sn nanoparticles mixed with graphene, 15 nm Sn nanoparticles and bulk Sn were determined in CO2-saturated 0.1 M NaHCO3 solution. As revealed by linear sweep voltammetry (LSV) in −2 at −1.8 V versus SCE, which was roughly 2, 2.5 and 13 times larger than that of the 15 nm Sn nanoparticles mixed with graphene, 15 nm Sn nanoparticles and bulk Sn, respectively. In addition, the Sn quantum sheets confined in graphene exhibited an onset potential of −0.85 V versus SCE, which was much positive relative to other three samples  may be one of the main contributors since larger ECSA could afford more catalytically active sites−1. The remarkably increased ECSA for the Sn quantum sheets confined in graphene could afford larger amount of active sites to efficiently adsorb CO2, further confirmed by their 1.75, 2 and 9 times higher CO2 adsorption capacity compared with the 15 nm Sn nanoparticles mixed with graphene, 15 nm Sn nanoparticles and bulk Sn. The lowest interfacial charge-transfer resistance, benefiting from the highly-conductive graphene, facilitates the rate-limiting electron transfer from CO2 to CO2− intermediate. In addition, compared to the 15 nm Sn nanoparticles mixed with graphene, 15 nm Sn nanoparticles and bulk Sn, the 0.05, 0.06 and 0.13 V lowered potential of OH− adsorption indicate that the Sn quantum sheets confined in graphene possess the strongest adsorption affinity of CO2− intermediate, and hence they efficiently stabilize the CO2− intermediate, thus definitely lowering the overall reaction barrier. As an outcome, the Sn quantum sheets confined in graphene display a current density of 21.1 mA cm−2 at −1.8 V versus SCE, which is roughly 2, 2.5 and 13 times larger than that of the 15 nm Sn nanoparticles mixed with graphene, 15 nm Sn nanoparticles and bulk Sn. Also, the formate Faradaic efficiency for Sn quantum sheets confined in graphene is always larger than 85% during the long test period of 50 h. Thus, the present work demonstrates that the unique structure of highly reactive metal quantum sheets confined in graphene could fully optimize CO2 electroreduction performances.In conclusion, we first put forward an ideal material model of ultrathin metal layers confined in graphene, in which the protection of graphene could avoid the oxidation of the highly reactive metals in air. As an example, highly reactive metallic Sn quantum sheets, confined in few-layered graphene, are first successfully synthesized through a spatially confined reduction strategy. Synchrotron radiation XAFS results disclose that Sn–Sn coordination numbers for the Sn quantum sheets confined in graphene reduce from 2 and 4 to 1.4 and 2.7 compared with that of bulk counterpart, which implies the former's higher intrinsic catalytic activity, further demonstrated by their lowered Tafel slope from 176 to 83 mV dec2 layers, synthesized according to our previous work , were dispersed in 40 ml 0.03 M aqueous glucose solution under mild stirring for 30 min. Then the above mixture was transferred into a 50 ml Teflon-lined autoclave, sealed and heated at 180 °C for 10 h. After the initial hydrothermal method, the ultrathin SnO2 layers were homogenerously encapsulated in the carbon layer. Then, the carbon-coated ultrathin SnO2 layers were initially annealed at 500 °C for 2 h, and then immediately annealed at 1,000 °C for 5 min, and hence cooled down rapidly (within 1 min) to room temperature under the protection of argon atmosphere. During this temperature-programed process, the amorphous carbon turned into graphene, while the ultrathin SnO2 layers simultaneously reduced into monodispersed Sn quantum sheets in the confined space of graphene, thus forming a sandwich-like structure. By contrast, for the synthesis of 15 nm Sn nanoparticles mixed with graphene, the 15 nm Sn nanoparticles, synthesized according to the ref. of Nanotechnology, 2011, 22, 22,5701, were physically mixed with the commercial graphene under mild stirring for 1 h. Then, the 15 nm Sn nanoparticles could be dispersed on the surface of graphene.In a typical synthesis, 10 mg ultrathin SnO2 layers, synthesized according to ref. Angew. Chem. Int. Ed. 2013, 52, 10,569, were annealed at 500 °C for 5 h in H2 atmosphere and then cooled down to room temperature. The obtained powders were collected for further characterization.In a typical synthesis, 50 mg ultrathin SnOK-edge XAFS was measured at the Shanghai Synchrotron Radiation Factory and the National Synchrotron Radiation Laboratory, China. Thermal gravimetric analysis (TGA) of the as-synthesized samples were carried out on a Shimadzu TA-50 thermal analyser at a heating rate of 10 °C min−1 from room temperature to 700 °C in air. The liquid products were quantified by nuclear magnetic resonance (Bruker AVANCE AV III 400) spectroscopy. CO2 adsorption isotherms measurements for all the synthetic samples were carried out by using an automatic microporous physical and chemical gas adsorption analyser (ASAP 2020M+C).TEM images and high-resolution TEM image were performed by using a JEOL-2010 TEM with an acceleration voltage of 200 kV. The Sn 2 reduction experiments, LSV with a scan rate of 20 mV s−1 was carried out in CO2-saturated 0.1 M NaHCO3 solution (60 ml) (The NaHCO3 electrolyte was purged with CO2 for 30 min prior to the measurement.). ECSA was determined by measuring the capacitive current associated with double-layer charging from the scan-rate dependence of CVs. The Cdl was estimated by plotting the Δj (ja−jc) at −0.75 V versus SCE against the scan rate, where the Δj could be acquired by Cyclic voltammetry measurement under the potential windows of −0.8∼−0.7 V versus SCE (0.1 M NaHCO3 solution).Electrochemical measurements were implemented in a three-electrode system at an electrochemical station (CHI660E). The working electrode was glassy carbon electrode. The platinum gauze and the saturated calomel electrode (SCE) reference electrode served as counter and the reference electrodes, respectively. In a typical prepared procedure of the working electrode, 3 μl of the homogeneous ink, which was prepared by dispersing 5 mg sample and 40 μl Nafion solution (5 wt%) in 1 ml water-ethanol solution with volume ratio of 3:1, was loaded onto a glassy carbon electrode with 3 mm diameter. For COSample of 20 mg was homogeneously mixed with 100 mg graphite and hence pressed into circular pellets with a diameter of 10 mm for further extended X-ray absorption fine structure (EXAFS) measurement under ambient conditions. Then, the XAFS measurements were performed at BL14W1 station in Shanghai Synchrotron Radiation Facility, China. The storage rings of SSRF were operated at 3.5 GeV with the maximum current of 210 mA. Si(311) double-crystal monochromator crystals were used to monochromatize the X-ray beam. The energy resolution at Sn K-edge was about 2 eV. XAFS data were collected in transmission in the energy range from −200 below to 1,000 eV above the Sn K-edge. The detuning was done by 30% to remove harmonics.The authors declare that the data supporting the findings of this study are available within the article and its How to cite this article: Lei, F. et al. Metallic tin quantum sheets confined in graphene toward high-efficiency carbon dioxide electroreduction. Nat. Commun. 7:12697 doi: 10.1038/ncomms12697 (2016).Supplementary Figures 1-4