FoM), ratio of normalized propagation length to localization length of the modes, can be obtained for SPPPs at 5–9 THz, as compared with SPPs of graphene on conventional glass substrate. The enhancement is efficiently tunable by varying the chemical potential of graphene. SPPPs with characteristics which strongly differ inside and around the polaritonic gap are found.Being one-atom thick and tunable simultaneously, graphene plays the revolutionizing role in many areas. The focus of this paper is to investigate the modal characteristics of surface waves in structures with graphene in the far-infrared (far-IR) region. We discuss the effects exerted by substrate permittivity on propagation and localization characteristics of surface-plasmon-polaritons (SPPs) in single-layer graphene and theoretically investigate characteristics of the hybridized surface-phonon-plasmon-polaritons (SPPPs) in graphene/LiF/glass heterostructures. First, it is shown how high permittivity of substrate may improve characteristics of graphene SPPs. Next, the possibility of optimization for surface-phonon-polaritons (SPhPs) in waveguides based on LiF, a polar dielectric with a wide polaritonic gap (Reststrahlen band) and a wide range of permittivity variation, is demonstrated. Combining graphene and LiF in one heterostructure allows to keep the advantages of both, yielding tunable hybridized SPPPs which can be either forwardly or backwardly propagating. Owing to high permittivity of LiF below the gap, an almost 3.2-fold enhancement in the figure of merit ( As a plasmonic material, graphene offers such intriguing properties as crystalline stability5, large optical nonlinearities8 and extremely high electromagnetic field concentration9. Recent demonstrations of surface plasmon polariton (SPP) excitation in graphene using near-field scattering of infrared light10, with all-optical mechanisms11, and by structure patterning13 have received intense interests. The surface conductivity of graphene (σg) can be effectively modulated via tuning of chemical potential (μ) through chemical doping or electrostatic/magnetostatic gating2. When Im(σg) > 0, graphene behaves like a very thin metal layer capable of supporting transverse-magnetic (TM) guided plasmonic modes24. Tunability of its plasmon resonance through the variation of μ together with a relatively large propagation length (PL) and a small localization length (LL) of SPPs in the mid-infrared (IR)20, far-IR and terahertz (THz) ranges26 are the key advantages of the graphene SPPs over those supported by noble metals like silver and gold27. It has been experimentally demonstrated that the plasmonic modes in graphene can be confined in extremely small volumes. The modal confinement can be ∝107 times smaller than free space wavelengths28. To compare, surface plasmons supported by normal metals typically display mode volumes that are ∝103 times smaller than free space with similar damping27. Therefore, graphene plasmons can couple stronger to electronic and vibronic excitations of their local environment than normal metal plasmons.After the rise of graphene, the two-dimensional periodic array of carbon atoms arranged in a honeycomb lattice29. SPhPs originate from the interaction of optical phonons with long-wavelength incident fields, creating a surface excitation mediated by the atomic vibrations. Depending on the type of the polar material, SPhPs can be recognized in a wide frequency range from mid-IR 31 and SiC33) to far-IR . Such SPhP modes can be stimulated between the longitudinal optical (LO) and transverse optical (TO) phonon frequencies. This spectral range is referred as the Reststrahlen (RS) band or polaritonic gap. According to the above-mentioned eye-catching characteristics of graphene SPPs and the SPhPs of polar dielectrics in the THz and IR regions, heterostructures combining advantages of graphene and polar dielectrics can yield unique and useful optical responses. Recent experimental and theoretical studies on graphene on SiO241 and SiC44 substrates have shown that the graphene dispersion relation in the mid-IR range can be significantly modified due to the substrate phonons with extra modes arising due to plasmon-phonon coupling. This coupling has been considered as surface-phonon-plasmon-polaritons, or SPPPs28. However, in those studies it was not tested whether the graphene plasmons were coupled to a large volume of phonons spread throughout the dielectric environment, or only to the phonons in the immediate vicinity of the graphene sheet. The recent experiments aimed to clarify the coupling between graphene plasmons and phonons of thin layers of PMMA show that the PMMA phonon spectral signature can be enhanced through graphene plasmon coupling, at least for the PMMA layers as thin as 8 nm45. Hexagonal boron nitride (hBN) is another polaritonic material that operates in the mid-IR and can be exfoliated as mono-/multi-layer atomically thin films47 or thick films (>50 nm)31. In47, by fabricating patterned graphene/monolayer-hBN and graphene/triple-layer-hBN structures, it was proved that graphene plasmons can couple to optical excitations occupying an atomically thin slice of volume near the graphene. In this case, two clearly separated hybridized SPPP modes can be formed. At the same time, the three-dimensional specimens of this layered anisotropic material are capable of supporting hyperbolic phonon polaritons31. Combining graphene with hBN films in multilayer structures, one can tune the hyperbolic dispersion of hBN phonons51. Moreover, the coupling of graphene plasmons and hyperbolic phonons in multilayer graphene-hBN metamaterials leads to the appearance of hybrid plasmon-phonon-polariton bands that makes this structure beneficial for waveguiding, negative refraction, and hyperlensing52.Alongside the noble metals and graphene, polar dielectrics also offer simultaneous sub-diffractional confinement, low optical losses, and operation in the mid-IR to THz spectral ranges through the stimulation of surface phonon polariton (SPhP) modes52 in the mid-IR region, similar effects are expected to be realizable but have not yet been studied in the far-IR range, in which graphene possess numerous potential applications34. However, it remains unclear whether the improvement of the resulting characteristics for the mid-IR by the use of hNB can be replicated in the far-IR region by using polar dielectrics, for which phonon-photon interaction occurs at lower frequencies. More general questions are (i) whether SPPPs obtained due to a polar dielectric substrate, which is highly dispersive, may have advantages over SPPs obtained for a conventional dielectric substrate, and (ii) which properties of polar dielectric are required to obtain better characteristics of SPPPs at the far-IR. One more important question is whether the properties of the SPPPs in case of polar-dielectric substrate can be predicted based on the characteristics of the SPPs in case of conventional dielectric substrate. Hence, possible effects of variations in permittivity of substrate should be clarified starting from the case of dispersion-free dielectric substrate.While plasmon-phonon coupling in graphene-polar structures has been studied inFoM) at f < 20 THz. Since polar dielectrics typically show strong variations in permittivity inside and around their polaritonic gap (RS band), they are good candidates to be high-permittivity substrates for single-layer graphene at the far-IR. On the other hand, they may themselves support SPhPs. Thus, we next investigate SPhPs supported by thin films of LiF. The capability of a 10 nm thick LiF waveguide on glass substrate in support of the highly confined and long-range phononic modes at 10–18 THz will be discussed in detail. Then, we combine single-layer graphene and LiF film in one structure in order to keep the advantages of both tunability and wide-range variations of permittivity, and study the propagation and localization characteristics of SPPPs of this heterostructure. It will be shown that the high-permittivity region of LiF can positively affect the SPPPs characteristics below the polaritonic gap, in coincidence with our results for SPPs, obtained in the case of dispersion-free lossless substrates. Moreover, it is found that transition from forward to backward SPPPs may occur inside and just below the polaritonic gap. To the best of our knowledge, the results presented in this paper is the first attempt to systematically study the effects of dispersion-free and dispersive polar substrates on graphene surface plasmons characteristics at the far-IR region.In this paper, we theoretically investigate the far-IR propagation and localization characteristics of SPPs of a single-layer graphene on dispersion-free dielectric substrates and SPPPs supported by the single-layer graphene on a thin film of LiF, a polar dielectric. First, we consider graphene SPPs for four different substrates and demonstrate importance of the choice of substrate permittivity and chemical potential of graphene on the resulting characteristics. In particular, we will show that high-permittivity substrates allow one to significantly improve figure of merit , is known for lossless cavities58. Prediction of scaling capability of an open resonance structure is a challenging task, since there is no exact boundary of the region occupied by the resonance field. A proportional change of geometrical  parameters enables scaling of an existing transmittive/reflective structure with respect to frequency, provided that dispersion and losses in the used materials are relatively weak59. Besides, partial scaling is possible by varying permittivity of substrate and other dielectric components, while all geometrical sizes are kept fixed60. Alongside the cavities and transmittive/reflective configurations, effect of dielectric substrate on propagation of plasmons in silver nanowires shows up as interesting characteristics53. Intuitively, increase of permittivity leads to downscaling (for fixed frequency) or redshift (for fixed geometry). However, the quantifying of these effects is impossible without a detailed numerical study. Therefore, in order to investigate influence of substrate on SPPs of a single-layer graphene, in this section we numerically study the basic effects of variation of permittivity of a lossless, dispersion-free dielectric substrate, εd, on the SPPs characteristics, i.e., propagation length, localization length, and figure of merit.The use of a dielectric substrate is unavoidable for feasibility of the design and often enables advanced physical regimes and operating modes. Various electromagnetic phenomena can be efficiently controlled by a proper choice of substratez = 0 on a semi-infinite lossless substrate with permittivity εs. Taking y component of the magnetic field as61, we arrive at the following well-known dispersion relation of graphene SPPs25:i = a, s), β = kx, β0 = ω/c, and α = σg/iωε0. The optical conductivity of graphene  and T = 300 K, respectively. Moreover, considering β = β′ + iβ″, we define wavelength, propagation length and localization length of the guided modes as λsp = 2π/β′, PL = 1/2β″ and LL = 1/2Re(qa + qs), respectively, and figure of merit as FoM = (PL/λsp)/LL(μm). For the sake of simplicity, we use the notation λsp for surface waves of all types studied here, i.e., SPPs, SPhPs and SPPPs.The schematic of the studied structure is shown in Fig. μ = 0.2 eV. The considered substrates include glass (εs = 2.25), Si (εs = 11.7), Ge (εs = 16), and a material with εs = 3560. It is seen in Fig. εs, graphene SPPs are supported for larger values of the wavenumber at fixed frequencies, and, consequently can propagate with smaller λsp and group velocity, vg. The scaling is applicable to the results in Fig. 57. Note that such large wavenumbers like those in Fig. εs = 35 can also be achieved using patterned graphene on conventional substrate12. Figure εs leads to decrease in PL and LL. However, as observed in Fig. FoM of the plasmonic guided modes becomes considerably larger at f > 5 THz, that originates from decrease in λsp and LL of the graphene SPPs. Thus, increase of FoM is coherent here with decrease of vg.First, we examine the effect of different, dispersion-free lossless substrates on the characteristics of graphene SPPs for μ. In Fig. ε = 35, we examine the SPP characteristics for different values of μ. We found that the effect of increase of μ is similar to the effect of decrease of εs for λsp, but the same cannot be said regarding FoM, for which an optimal value of εs can exist. Moreover, for a richer insight into the localization of the graphene SPPs, field profiles are also presented in Fig. It is known that one of the most impactful properties of the SPPs supported by graphene is their tunability by varying εs, the larger values of FoM are obtainable for the graphene SPPs. High-permittivity dispersion-free materials are not accessible for the far-IR. Hence, the choice of polar dielectrics like LiF35, NaCl34, and GaAs38 is natural. These materials are known to show a polaritonic gap (RS band), which appears due to phonon-photon interaction. Desired high permittivity values result from this interaction. Graphene SPPs can be hybridized at the far-IR with phonons in heterostructures composed of graphene and polar materials. From a fabrication point of view, those materials should be used as a buffer layer between a single-layer graphene and a thicker low-loss dielectric substrate. Therefore, a study of the waveguide structures comprising thin films of a polar dielectric on low-loss dielectric substrate is required prior to combining with graphene. In this section, we scrutinize propagation and localization characteristics of SPhPs supported by thin films of LiF on different lossless substrates, within 1 THz to 20 THz. LiF is particularly appropriate due to the wide RS band and a wide range of permittivity variation inside and around this band35.According to the results discussed in Sect. 2, the larger value of εs. The considered LiF has thickness t and permittivity given by34ε∞ = 2.027, ωTO = 2πfTO, ωLO = 2πfLO, fTO = 9.22 THz, fLO = 19.1 THz and γ = 2π × 0.527 THz. Here, fTO, fLO and γ are, respectively, transverse optical frequency, longitudinal optical frequency and damping factor. Real and imaginary parts of εLiF are shown in Fig. Re(εLiF) takes considerably large values at frequencies slightly smaller than fTO. A thin layer of LiF that has thickness t and is bounded by air half-space (εa = 1) on the one side and by a typical dielectric substrate (εs) on the other side can support SPhPs. Taking Hy asa = qLiFεa/qaεLiF, Γs = qLiFεs/qsεLiF, and Figure εs = 2.25). The obtained results are presented in Fig. Re(εLiF) < 0. At t = 10 nm, they can propagate with large wavenumbers, Re(β)/β0 < 640, i.e., with small phonon wavelengths (λsp). By increasing t, SPhPs with larger λsp can be supported, as occurs for t = 150 nm when Re(β)/β0 < 50. Figure PL is achievable for the SPhPs. In particular, the maximum PL is obtained at t = 150 nm, while the minimum value of PL belongs to the case of t = 10 nm. More investigations show that taking t = 200 nm, even larger values of PL can be obtained. In turn, the results presented in Fig. LL is achieved at t = 10 nm. The FoM is presented in Fig. FoM are obtained at t = 10 nm. By increasing t, one reduces the FoM value. Consequently, as far as supporting the phononic guided modes with lower losses  and stronger localization  within 10–18 THz are concerned, the smaller thickness of the phononic waveguide can be chosen. One more point that should be highlighted here is the support of phononic guided modes with negative slope of dispersion. These modes have been considered here as backward waves, being similar to the ones supported by the left-handed metamaterials63, and by metal-insulator-metal65 and insulator-metal-insulator66 structures. The total power Sx is x-component of the Poynting vector, S).First, we investigate characteristics of the SPhPs for different thicknesses of the LiF layer placed on a semi-infinite glass substrate  and εs are illustrated in Fig. t = 10 nm and t = 100 nm. As comprehensively discussed in the the Supporting Information, we found that the SPhPs of the 10 nm-thick LiF waveguide are as confined as the graphene SPPs in 10–18 THz.Similar to the the previous section, we have also investigated the effect of 6728. According to the results presented in the first part of the Results and Discussion section, we found that the SPPs of a single layer of graphene possess larger values of FoM for larger values of εs. Hence, due to the large values of Re(εLiF) at the frequencies below but close to fTO, unique modal characteristics are expected to appear. In this section, our consideration is restricted to the case when glass is the substrate of the graphene-LiF heterostructure. As discussed in the previous part, the 10 nm-thick LiF waveguide on glass substrate shows the largest FoM values due to the support of low-loss SPhPs with strong confinement [see Fig. t = 10 nm. In Fig. THz < f < fTO and 16.83 THz < f < 18.68 THz. These two features cannot be observed in the air/graphene/glass and air/LiF/glass waveguides separately, i.e., without combining them in one structure. Figure highly confined SPhPs of LiF with the localized graphene SPPs leads to a giant increase in the losses of the guided modes, so a considerable decrease/increase in the PL/LL values of the SPPPs occurs as compared to the graphene SPPs. As illustrated in Fig. FoM values than the graphene SPPs.To keep the advantages of tunability of graphene and a wide permittivity range of LiF, we combine them in one hybrid structure so that graphene SPPs could be hybridized with SPhPs. As schematically illustrated in Fig. see Fig. . TherefoFoM of the SPhPs is decreased due to the confinement reduction. Therefore, it is worth to investigate the specifics of coupling between the highly confined graphene SPPs and the lowly confined SPhPs of air/LiF/glass system for two larger values of the thickness of LiF; i.e. t = 100 nm and t = 200 nm. As it was expected from the results shown in Fig. f > 14.86 THz by the air/graphene/LiF/glass waveguides for both t = 100 nm and t = 200 nm, see Fig. t = 10 nm, for the larger t, the backward modes that occur for 8.63 THz < f < fTO are turned to be forwardly propagating SPPPs with very small vg and λsp in the vicinity of 8.63 THz. As is seen from Fig. f < fTO possess smaller values of PL than those of graphene SPPs, they are more strongly confined; i.e., they also possess smaller values of LL. This means that the hybridization of the highly confined graphene SPPs with the lowly confined SPhPs of air/LiF/glass structure leads to the extremely confined SPPPs at frequencies below fTO, where Re(εLiF) is large. This decrease in the localization length of the modes yields a considerable rise in the FoM of the SPPPs at 5.2 THz < f < fTO, compared to graphene SPPs illustrated in Fig. FoM is achieved around 8.5 THz. Therefore, hybridization of graphene SPPs with the lowly confined SPhPs leads to the appearance of SPPPs with larger FoM than a single layer of graphene. It should be noted that since LiF is a lossy material, its presence reduces PL, however decreases LL as well for the cases of t = 100 nm and 200 nm. Similar to first part of the Result and Discussion section, increase of FoM is associated in this regime with the slowing effect of the substrate. In the case we are only interested in supporting SPPPs with larger PL than for graphene SPPs, air/graphene/LiF/glass waveguides with t = 100 nm and t = 200 nm are the appropriate designs for the range of 9.45 THz < f < 13.77 THz. In line with the said above, support of SPPPs with significantly different propagation and localization characteristics within three neighboring frequency bands  makes the suggested air/graphene/LiF/glass heterostructure a distinguishable candidate for multifunctional applications including, for instance, waveguiding, sensing, and absorption purposes.From the results in Figs t = 100 nm, at four different frequencies. In agreement with Fig. f = 11 THz [Fig. f = 18 THz [Fig. f = 8 THz [Fig. f = 18 THz [Fig. t = 100 nm in Fig. f > fTO propagation in the waveguide with noticeably small confinement is possible, as shown in Fig. f < fTO are strongly confined inside the waveguide and show high FoM. As mentioned above, tunability of the optical properties via changing μ can be considered as the most advantageous characteristic of graphene over other plasmonic and phononic materials. Therefore, we finally investigate the effect of changes in μ on the characteristics of SPPPs supported by the air/graphene/LiF/glass waveguide with t = 100 nm. The results in Fig. μ, dispersion, propagation and localization characteristics of the SPPPs can be efficiently tuned at frequencies inside and outside of the RS band of LiF. In Fig. FoM of the forwardly propagating slow SPPPs at f < fTO can be significantly increased by decreasing μ, that is associated with decrease of vg and increase of wavenumber. Moreover, it is possible to tune the frequency of the forward-to-backward (FB) wave transition at fTO < f < fLO. In Fig. μ, it can be shifted toward lower frequencies and larger wavenumbers. From a practical point of view, the presence of substrate may add some additional effects, such as electron impurity and electron losses, on the electronic and finally optical properties of graphene. These effects can be included in the calculations by the phenomenological relaxation time (τ) or the electron mobility as τ can affect FoM of the hybrid guided modes supported by the graphene/LiF/glass waveguide for t = 100 nm. As it is observed from Fig. FoM of the modes. More investigations reveal that a reverse trend can be observed by decreasing τ. Moreover, for frequencies larger than the optical phonon frequency (f > 48.4 THz), electron-phonon scattering can considerably affect τ20. The inter-layer coupling is also another mechanism that can considerably amend the relaxation time69. One more important point should be highlighted here is that, in general, nonlocal optical conductivity of graphene can be considered in the calculations20. As explained in20, once the substrate of graphene is low-index and not strongly dispersive, taking the local optical conductivity of graphene in the calculations leads to a precise description of the modal characteristics of the guided waves for low frequencies and small values of the wavenumbers; i.e. for β/β0 < 500 and f < 20 THz. To the present, most of the studies on the investigation of plasmon-phonon hybridizations in the graphene/polar heterostructures have been done in the local regime52. However, in order to gain more practical insight into the hybridization mechanism and the modal characteristics of the guided waves supported by those systems, considering the nonlocal effects in the calculations will be the subject of our future studies.For a deeper insight into the features of the SPPPs, in Fig. THz Fig.  and f = THz Fig.  are veryTHz Fig.  and f = THz Fig. , the modTHz < f < 19.1 THz. Therefore, combining graphene and LiF components in one structure promises surface waves with the improved and unusual characteristics at the far-IR region. Moreover, from the investigation of graphene SPPs for different dispersion-free dielectric substrates, it has been understood that the use of lossless high-permittivity substrates allows us to considerably increase FoM of the structure. This effect is coherent with the slowing of surface plasmons due to increase of substrate permittivity. From the investigation of SPhPs supported by thin films of LiF on different substrates, it has been concluded that a 10 nm-thick film of LiF on glass substrate is a distinguishable candidate for obtaining highly confined and largely propagating SPhPs within 10–18 THz, when tunability is not required. Finally, we studied the effects that appear due to combining graphene with a thin film of LiF and a glass substrate in one structure, so that the effects of wide-range permittivity of LiF and tunability of graphene co-exist. We have shown that due to the hybridization of graphene SPPs and LiF SPhPs, SPPPs are supported by the graphene-LiF waveguide, which may show very different behavior within the neighboring frequency regions, i.e., below, inside, and above the polaritonic gap. This characteristic makes the graphene-LiF heterostructures beneficial for multifunctional applications. In particular, (i) due to the large values of permittivity of LiF just below the transverse optical phonon frequency, tunable , forwardly propagating, slow SPPPs with 3.2 times larger FoM than graphene SPPs are supported within 5–9 THz, when the thickness of the LiF layer is 100 nm; (ii) the coherence between the extent of wave slowing and the increase of FoM takes place; (iii) backward SPPPs with tunable propagation and localization characteristics can be supported. These features make graphene/LiF/glass heterostructures versatile candidates for waveguides, sensors and absorbers in the far-IR region. While LiF has been chosen owing to a wide polaritonic gap and relatively low losses at the far-IR, the potential of other polar dielectrics, e.g., GaAs, InP, and CaF2 in obtaining tunable hybrid surface waves with improved characteristics may be a subject of future studies. The obtained results can be viewed from the perspective of new advanced physical scenarios and applications for graphene, but simultaneously from the perspective of those ones for polar dielectrics. They indicate that radiation and manipulation of THz waves at the far-IR by using the hybridized SPPPs in graphene-polar dielectric heterostructures is a promising research direction.To summarize, we have theoretically studied two types of the structures with graphene and one auxiliary structure with LiF, a polar dielectric, which all may support surface waves in the far-IR region. Graphene is a well-known plasmonic material capable of supporting low-loss tunable SPPs in the THz and IR regions. On the other hand, LiF is a strongly dispersive phononic material which can support SPhPs within its Reststrahlen band (polaritonic gap), i.e., at 9.22 67 the dispersion relations of the guided modes supported by the aforementioned three structures were derived. Considering real values of ω and complex values of β in the calculations, dispersions, propagation and localization characteristics of the guided modes were extracted using custom-made MATLAB codes. The 1D and 2D mode profiles presented in the manuscript were also extracted by custom-made MATLAB codes and verified by 3D Finite Difference Time Domain (FDTD) calculations70. Taking the effective mode index obtained from the MATLAB codes, we performed the FDTD simulations using a mode source with the periodic boundary condition in the plane of graphene and applying perfectly matched layer boundary condition at the out of plane boundaries. Moreover, the graphene and LiF layers were, respectively, modeled by Eqs. (We first obtained the appropriate boundary conditions for TM polarization, then using a standard direct-matching technique by Eqs.  in the cSupplementary Information


A Si/graphene/Si planar double heterostructure has been fabricated by means of semiconductor wafer bonding. The interfacial mechanical stability and interlayer electrical connection have been verified for the structure. To the best of our knowledge, this is the first realization of a monolayer-cored double heterostructure. In addition, a double heterostructure with bilayer graphene has been prepared for bandgap generation and tuning by application of a bias voltage. These structures move towards the realization of versatile graphene optoelectronics, such as an electrically pumped graphene laser. Our Si/graphene/Si double heterostructure is positioned to form a new basis for next-generation nanophotonic devices with high photon and carrier confinements, earth abundance , environmental safety , and excellent optical and electrical controllability by silicon clads. Monolayer-material-based optoelectronic devices possess promising characteristics for high-density integration, low-power-consumption, and high-speed operation ,9,10,11.p-type Si <100> wafers doped with boron (doping concentration of ~1 × 1019 cm−3) and double-side polished, epi-ready p-type Si <100> wafers doped with boron (doping concentration of ~1 × 1016 cm−3). Large-area graphene sheets (~20 cm2) transferred onto the polished-side surface of the Si wafers was used in this study. In an attempt to create balance between ease in obtaining electric measurements from ohmic metal contacts and optical transparency for transmission measurements, the higher and lower doping-concentration Si wafers were used for monolayer and bilayer graphene, respectively. The Si wafer topped with a graphene layer was cut into ~1 cm2 area dies after being coated with a photoresist, in order to protect the bonding surface from particles generated during the cutting process. During cutting, no issues were observed for the graphene layer, which remained intact on the Si wafer and did not experience delamination. A bare Si wafer was also coated with a photoresist and then cut into ~0.64 cm2 area dies. Immediately before bonding, the photoresist on both dies was removed with acetone, along with degreasing of the bonding surfaces. The graphene-side surface of the graphene-on-Si wafer was brought into contact with the polished-side surface of a bare Si wafer with the same doping concentration, with their Si (011) edges aligned. The two die pieces were then bonded by annealing in ambient air for 3 h under a uniaxial pressure of 0.1 MPaG [We used commercially available monolayer and bilayer graphene materials  in this work. Silicon, a versatile semiconductor, was adopted as the cladding material. We used single-side polished, epi-ready 0.1 MPaG . The detV, were normalized to that without electrical bias .In the second part of this work, the functionality of the structure was demonstrated by attempting to generate and tune the graphene bandgap energy through application of an electrical bias voltage. Bilayer graphene  was usedInterfacial bonding strength as a function of temperature was first analyzed, where the temperature varied from room temperature (20 °C) to 500 °C. The measured interfacial bonding strength of the Si/graphene/Si structure (~30 kPa) was drastically reduced when compared to the Si/Si control structure without graphene (~650 kPa). To investigate the origin of this interfacial strength difference, an FTIR measurement was conducted on the Si/graphene/Si double heterostructure containing bilayer graphene, as the Si wafer used for monolayer graphene was not transparent. Absorption peaks for covalent Si–C and Si–O–C bonds  were not2, and comparable to that for the corresponding Si/Si control, as seen in 2O and CO2, respectively. As the applied voltage increases, oscillatory changes in the transmittances were observed for both the Si/bilayer graphene/Si and reference Si/Si samples. This spectral oscillation can be attributed to the optical interference at the bonded interfaces, which are magnified due to the existence of interfacial voids. These interfacial voids are known to exist partially at bonded interfaces, originating from airborne particles or partial missing bonds [ng bonds ,35,36,37kA is the amplitude, n is refractive index, voidh is the void height, h is the Planck constant, c is the speed of light, photonE is the photon energy, and Φ is the phase offset. For clearer quantitative examination of the transmission spectra, the interference effect was eliminated from the data using a mathematical model for the interference mechanism, as shown below. The change in transmittance is formulated, accounting for the voids of various sizes as:Our fabrication of the Si/graphene/Si structure is the first realization of a monolayer-cored double heterostructure, to the best of our knowledge. Importantly, the choice and use of the chemical elements of carbon  and silicon (Si) presented here have been proven suitable for future device use due to their low cost, earth abundance, and environmental friendliness. The Si/graphene/Si double heterostructures presented here can form a new basis for the next-generation nanophotonic devices with superb optical and electrical properties provided by the graphene ,7,9, andIn this work, we have fabricated, for the first time, a monolayer-cored double heterostructure, towards the realization of high-efficiency nano-optoelectronic devices. We prepared a Si/graphene/Si stack by means of graphene-mediated wafer bonding and verified the interfacial mechanical stability and interlayer electrical connection, demonstrating a new application of wafer bonding. The fabrication scheme and structure presented here provides a new device platform for functional monolayer materials.


Benefiting from unique planar structure, high flexibility, splendid thermal, and electric properties; graphene as a crucial component has been widely applied into smart materials and multi-stimulus responsive actuators. Moreover, graphene with easy processing and modification features can be decorated with various functional groups through covalent or non-covalent bonds, which is promising in the conversion of environmental energy from single and/or multi-stimuli, to mechanical energy. In this review, we present the actuating behaviors of graphene, regulated by chemical bonds or intermolecular forces under multi-stimuli and summarize the recent advances on account of the unique nanostructures in various actuation circumstances such as thermal, humidity, electrochemical, electro-/photo-thermal, and other stimuli. Typically, GO sheets are synthesized by Hummer's method, in which graphite is oxidized using strong oxidants such as KMnO−6 K−1) compared to graphene (7 × 10−6 K−1), further enhancing the contraction amplitude as the temperature increases , boron (B), sulfur (S), phosphorus (P), and iodine (I) could efficiently create a disordered surface topography and modulates local surface and electric features of the conjugated carbon-carbon structures  of graphene, driven by electrical charging and discharging, can be further enhanced or precisely controlled through rational design of the graphene and active responsive components . In orde2-bonded carbon network, an appropriate electric current or infrared light applied to a graphene structure would induce Joule heating that generates the electric current passing through the graphene plane or induced the enhanced disordering degree by infrared–phonon interactions during the forced resonance vibration process, leading to the conversion of kinetic vibration energy to heat motion energy  with rich edge functional oxygen-containing groups to obtain a maximum negative coefficient of thermal expansion of the GO sheets . This phenomenon is the same as that of the bilayer responsive structure of graphene and organic glass substrate which exhibited a reversible and large bending angle of 270° with a fast response of 8 s and recovery period of 19 s under the driving voltage of 65 V  stimuli (Hu et al., As actuation active components, graphene and its derivatives with the extraordinary conversion of kinetic vibration energy to heat energy of graphene sheets, induced by the infrared phonon strong interactions during the resonance vibration process, have been applied to infrared light-driving actuators (Zhao et al., The actuators with the graphene as active components show great potential in the various intelligent bionic devices. Particularly, the actuating behaviors of graphene regulated by chemical bonds or intermolecular forces under multi-stimulus are discussed, which endow graphene-based responsive units with response capability to single and/or multiple environment stimulus, including electro-thermal stimuli, humidity stimuli, temperature stimuli and so on. For instance, one way is to selectively decorate the graphene with heteroatoms, such as oxygen, in which graphene oxide is fast and sensitive to water molecules, showing excellent responsive behavior to humidity. The other way is to specifically modify the graphene with conducting polymers or thermoplastic polymer, making it a crucial part of smart switch system or intelligent robotics. These prominent features present promising applications in smart systems with complex functions.However, there are still some challenges that remain to be solved to meet future requests of graphene-based intelligent materials. The high manufacturing cost and delayed response time of graphene-based actuator have always been restricting the practice of advanced application in industry and intelligent home system. It is still an efficient way to introduce highly sensitive responsive polymers or gels into graphene systems to construct high performance bionic intelligent systems, such as modified PDMS, PMMA and other new designed organic polymers. Moreover, the mechanical properties and durability of the current graphene-based actuation system needs to be promoted, which involves preparation of high-quality graphene, matching of graphene with functional materials, and the design of specific graphene microstructures or assemblies. In addition, it is highly desirable to constitute smart graphene systems that handle multi-stimuli, have good sensitivity and high efficiency. Considering the promising potential application of bionic intelligent systems in the future, low cost biocompatible graphene-based actuators are also in great need of development, which remains a big challenge to overcome in the production process. There is no doubt that graphene-based smart devices will soon be playing an important role in modern intelligent systems, with various possibilities and continuous research efforts.JW and YX are responsible for the main text. VC provided the adjustment suggestions. CS was responsible for drawing some figures. LQ and YZ hold the overall ideas and corrections.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.


Strain relaxation caused by grain boundary diffusion is considered to be the reason and the mechanism is analyzed in detail.Strains in graphene play a significant role in graphene-based electronics, but many aspects of the grain boundary effects on strained graphene remain unclear. Here, the relationship between grain boundary and strain property of graphene grown by chemical vapor deposition (CVD) on the C-face of SiC substrate has been investigated by Raman spectroscopy. It is shown that abundant boundary-like defects exist in the graphene film and the blue-shifted 2D-band frequency, which results from compressive strain in graphene film, shifts downward linearly as 1/L Althobstrates ,9. Howevbstrates . Grain bbstrates . Such kibstrates ,13,14,15−1) and 2D-band (at about 2680 cm−1) are the fingerprints of pristine graphene. The G-band originates from in-plane vibration of sp2 carbon atoms and is a doubly degenerate (TO and LO) phonon mode at the Brillouin zone center. The 2D-band is the second order overtone of the D-band and originates from a two-phonon double resonance Raman process where momentum conservation is satisfied by two phonons with opposite wave vectors. In Raman spectra of strained graphene, the G-band and 2D-band peak frequencies shift due to phonon hardening or softening. Typically, compressive strain leads to phonon hardening (frequency upshift), while tensile strain results in phonon softening (frequency downshift). For defective graphene there will appear another two disorder-induced peaks at about 1350 cm−1 (D-band) and 1620 cm−1 (D’-band). The D-band involves an iTO phonon around the K-point and is an intervalley process, and the D’-band is an intravalley process connecting two points belonging to the same cone around K (or K’). Both the strain and grain size in materials can be probed by XRD and Raman spectroscopy ,17,18. HIn this work, Raman spectra of polycrystalline graphene, grown by CVD method on the C-face of SiC substrate (CVD-EG) using propane as the carbon source ,19, are 2 semi-insulating C-face 4H-SiC substrate was loaded into a commercial horizontal CVD hot-wall reactor (Aixtron VP508). The chamber was pumped down for 3 h to reach the pressure of 1 × 10−6 kPa for venting oxygen. Then the temperature was raised up to 1250 °C under an argon flow of 20 L/min. After temperature stabilized, 6 SCCM of propane was added to the argon to grow graphene. The growth time was 20 minutes and the growth pressure was kept at 40 kPa. After growth, the chamber was self-cooled down to room temperature under argon ambient. After ultrasonic cleaning by ethanol and acetone, the 2 × 2 cmThe Raman measurements were carried out at room temperature using a HORIBA LabRAM HR800 system. The excitation laser energy was 2.41 eV (514.5 nm) and the laser spot size was about in diameter of 1 μm focused by a 100× objective lens. Backscattering configuration was applied with low power of 1 mW to avoid laser induced heating. The atomic force microscopy (AFM) measurements were carried out using a Bruker Dimension Edge system and the AFM images were obtained in tapping-mode.−1 (D-band), 1586 cm−1 (G-band) and 2704 cm−1 (2D-band) and one weak peak at 1620 cm−1 (D’-band). The appearance of significant D-band and weak D’-band proves the presence of plenty of defects in the CVD-EG sample. Blue-shifts of G-band position (6 cm−1 shifted from 1580 cm−1 observed for monolayer exfoliated graphene at a laser wave-length of 514.5 nm) [−1 shifted from 2680 cm−1) are observed, which are attributed to compressive strain in graphene layer generated during the cooling down period, because of the large thermal expansion coefficient difference between SiC and graphene [Raman spectra were recorded at five different positions on the surface of the CVD-EG sample. 14.5 nm)  and espegraphene ,26.a are presented in a is the crystallite size of the grown graphene film, and can be calculated using the ratio of the D-band intensity (ID) to that of the G-band (IG) according to Equation (1) [a can be explained as follows [a. Since the FWHM is determined by lifetime effects in Raman bands involving resonance conditions, it can be assumed that FWHM is proportional to 1/τ and consequently FWHM is proportional to 1/La. The intercept for the FWHM(2D) is 44 cm−1, which means FWHM(2D) has a value of 44 cm−1 when the crystallite size of epitaxial graphene is large. This is consistent with the FWHM(2D) of single layer graphene epitaxially grown on SiC substrate observed by Lee et al. [The full width at half-maximum (FWHM) of the D-band (FWHM(D)), G-band (FWHM(G)) and 2D-band (FWHM(2D)) as a function of 1/Ltion (1) .(1)La and low sp3 amorphous carbon to high sp3 amorphous carbon (stage three). The evolution of the Raman spectrum in stage one is as follows: (a) D-band appears and ID/IG increases following Equation (1); (b) D’-band appears at about 1620 cm−1; (c) each FWHM for all bands is broadened due to disorder; (d) D+D’ appears. The transition between stage one and two usually occurs at ID/IG ≈ 3 (corresponding to La ≈ 5.5 nm) using excitation laser energy of 2.41 eV [Ferrari and Robertson  defined  2.41 eV .a in our graphene sample is about 25 nm, and it is believed that our CVD-EG sample is in stage one, according to the three-stage amorphization trajectory and the peak characteristics shown in L13 cm−2 [Although both charge and strain could affect the positions of G and 2D bands, the unintentional electron doping of epitaxial graphene on SiC substrate was found previously to be only about 1 × 1013 cm−2 . Such or13 cm−2 ,36. Thus13 cm−2 .m is the mode Grüneisen parameter. For 2D-band of graphene γ2D ≈ 2.7. −1/%) has opposite sign comparing with that of 2D-band (−72.4 cm−1/%). This phenomenon is interesting, since the lattice deforms and the G-band and 2D-band positions should shift in the same way as a consequence of phonon hardening or softening when strain occurs in single domain graphene. Bissett et al. [t et al.  also obsa. As mentioned above, the 2D-band keeps constant in stage one of the three-stage amorphization trajectory. So, it could not directly obtain the relation between 2D-band position and crystallite size from the effect of defects, which is attributed to the nonuniform distribution of compressive strain. In this case, the compressive strain has a larger value in the positions with larger crystallite size. This interesting property of polycrystalline graphene has never been reported before.Another significant observation in (1)lattice diffusion directly into the grains and(2)much faster diffusion along the grain boundary.It is assumed that the graphene is free of strain when the growth period is over . As the a/2) = C0. Here, it is assumed that the diffusion length L is much smaller than crystallite size La. Thus, A = C0 and B = 0. The solution of Equation (3) is written by Equation (5).The square shape grain model  inside the grain is given by Equation (6).max is the strain at the center of the grain. The average strain Integrating Equation (7), the average strain a is linear, thus the relation of 2D-band frequency shift Δω2D with 1/La is also linear according to Equation (2), which explains the Raman results in −1, which corresponds to a compressive strain of 0.47%. The value of 0.47% well matches with the results in Ref. [Equation 8) shows that the relation of the average strain  shows tha increases and the G-band frequency increases linearly as 1/La increases. The opposite variation direction of G and 2D bands results from the presence of grain boundaries in graphene with small crystallite size. The downshift of 2D-band is caused by strain relaxation due to grain boundary diffusion. Raman spectra of CVD-EG show that abundant boundary-like defects exist in the graphene film and a significant blue-shift of 2D-band position is observed. The frequency of the 2D-band decreases linearly as 1/L


For intermediate numerosities, errors vary directly with the numerosity, following Weber’s law, but for very high numerosities, with very dense patterns, thresholds continue to rise with the square root of numerosity. This suggests that three different mechanisms operate over the number range. In this study we provide further evidence for three distinct numerosity mechanisms, by studying their dependence on attentional resources. We measured discrimination thresholds over a wide range of numerosities, while manipulating attentional load with both visual and auditory dual tasks. The results show that attentional effects on thresholds vary over the number range. Both visual and auditory attentional loads strongly affect subitizing, much more than for larger numerosities. Attentional costs remain stable over the estimation range, then rise again for very dense patterns. These results reinforce the idea that numerosity is processed by three separates but probably overlapping systems.Humans can estimate numerosity over a large range, but the precision with which they do so varies considerably over that range. For very small sets, within the  Evidence for subitizing comes largely from a discontinuity in reaction times, response variability, and accuracy. These parameters are consistently lower for numbers of 1 to 4, with performance sharply declining for larger numbers outside the subitizing range  and higher densities, in which items are not segregable. Studies have shown clear differences in the psychophysical laws governing precision for relatively sparse as compared with packed dot patterns: For sparse patterns, the discrimination thresholds are higher and obey Weber’s law; at higher numerosities, they decrease with the square root of numerosity  with normal or corrected-to-normal vision were tested on the visual spatial attention task; five of these were also tested on the auditory time bisection task (two did not give consent for the whole protocol). All participants performed the single-task control. All participants gave written informed consent, and the experimental procedures were approved by the local ethics committee .Seven participants  could not be separated by less than 0.25°. In a particular session, one cloud of dots  maintained a particular numerosity across trials, whereas the other (the probe) varied around this numerosity. The number of dots in the probe patch varied according to the QUEST adaptive algorithm . In the main experiment, all trials started with a fixation point presented until the participant pressed a key to start the experiment, and then the primary and secondary stimuli were presented for 500 ms. Participants were tested with 14 different reference numerosity levels. The order with which each numerosity was tested was pseudorandom across participants and attentional conditions.Three sessions of 30 trials each were run for each numerosity level and each attentional condition, yielding a psychometric function for that condition. The function was plotted and inspected visually, to ensure that it was monotonically ascending and well behaved. We also checked the estimate of the standard error of the mean: If this was greater than 30% of the estimated just-noticeable difference (JND), we added another session of 30 trials. In practice this happened on only 4% of the psychometric functions. On average, each participant had 1,260 trials.For each participant, the proportion of trials in which the probe appeared more numerous than the reference was plotted against the number of reference dots on a logarithmic scale and was fit with a cumulative Gaussian error function. The median (the numerosity corresponding to 50% left responses) gave the point of subjective equality (PSE), and the difference in numerosity required to pass from 50% to 75% correct responses defined the JND, a measure of precision. The JND divided by the reference numerosity yields the coefficient of variation (CV), a dimensionless index of precision that allows comparison of performance across numerosities. Where performance was errorless (as often occurred in the subitizing range in the single task), the JND was arbitrarily assigned as 0.001 dots.Biases in PSE were tested by a series of Wilcoxon signed-rank tests (two-tailed) comparing, separately for each numerosity (14 levels) and attentional condition, the PSE shifts from the physical reference numerosity. The alpha level was Bonferroni corrected according to .05/14 (.0035).N ≥ 6) were fitted with two-segment piecewise linear fits, with slope of the first segment set to zero and the second left free to vary. Standard error estimates for all fit parameters were obtained by bootstrap resampling of participants  and fitting the data to the average group performance. The same iterations were used to calculate bootstrap sign-test p values. Residuals of the two-segment function  were compared to those of a simple linear fit (two parameters) by means of the Akaike information criterion (AIC). By definition, the AIC of each model isk is the number parameters in the model. The maximal of log-likelihood can be derived from the residual sum of squares according to the following formula:RSS is the residual sum of squares, n is the number of data points, and C is a constant that depends solely on the data and does not vary from model to model. Overall, save for a common constant term C + ln(n), the AIC of a model isTo model numerosity-dependent changes in thresholds, CV-versus-numerosity curves above the subitizing range  by resampling  participants and numerosities within the range . The proportion of times in which the cost was less than or equal to unity (null hypothesis) was taken as the BST p values represent the proportions of times the attentional cost of the estimation regime exceeded that in the other regime .The differential attentional cost between numerosity regimes was also measured by a similar procedure to yield average CVs for each numerosity range, which were then pitted against each other. By convention, the reported To determine the appropriate sample size, we ran two bootstrap power analyses for the two analyses of attentional costs. The first is a comparison of CVs of the single and dual tasks within one numerosity regime. To mirror our paradigm, we assumed each participant would be tested over a broad range of numerosities with a psychometric curve based on 90 two-alternative forced choice trials at each numerosity. Given the previous literature and the present choice of reference numerosities, it was reasonable to assume that at least three would fall in one regime and three into the other. Thus, conservatively, we assumed that the measure of attentional costs within one regime would be based on the average CVs in three psychometric curves in the single and dual tasks. Population variance was derived from the previous literature . However, this was to be expected, since the probe and reference stimuli were randomized in position.We tested the effect of attentional load on numerosity perception over a wide range of numerosities. We first examined whether the attentional manipulations affected PSEs. We found no significant deviation from the physical reference numerosity , indicating that the boundaries of the three regimes were similar in the two conditions.The precision for the two attentional conditions also followed a two-limbed function, with log–log slopes of – 0.47 ± 0.07 and – 0.65 ± 0.17. Interestingly, the knee points for the two conditions  fell close to that of the single-task condition , the visual dual-task raised the CV from ~ 0 to 0.22, a factor of 121 (BST p < .001), and the auditory task raised the CV by a factor of 11.2 . In the estimation range (6 < N < 60) the visual dual task had less effect than in the subitizing range, raising CVs from 0.16 to 0.25 . The auditory dual task had a negligible impact on CVs in this range . In the texture density regime (N > 75), attentional costs rose again .We calculated the visual and auditory attentional costs as the ratio of the dual to single CVs Fig. . At low t test of attentional costs revealed that the effects of the dual tasks in the three regimes were different from each other. In particular, the costs in the estimation and density regimes differed for both the visual distractor (p = .037) and the auditory distractor (p = .005). The attentional cost in the subitizing range was also markedly higher than in the estimation range .A bootstrap t tests revealed that none of these were statistically significant .To verify that the differences in attentional costs between ranges did not result from a change in the resources allocated to the primary task, we calculated the average accuracy in the three regimes for both types of distractors. Performance in the distractor visual task was 92%, 96%, and 96.2%, respectively, for subitizing, estimation, and density perception, and 98%, 97%, and 97% for the three regimes with the auditory distractors. Bootstrap Replicability is important. We therefore ran a replication study on nine new, naïve participants to verify the main results of this study: that attentional costs were different for the three regimes of numerosity perception. We tested three sample numerosities, representative of the subitizing, estimation, and texture ranges: 3, 24, and 150.Figure t test: t(8) = 6.278, p = .0013]. The trend of the results with the auditory distractor  = 2.89, p = .015].Similarly, the attentional cost in the texture range was more than twice that in the estimation range, a factor of 3.04 compared to 1.25. This difference was highly significant [one-tailed tor Fig.  was simiFigure Three separate regimes have been proposed for numerosity perception: subitizing, estimation, and texture density  had a greater effect on subitizing than on estimation, and increased again at higher densities. Numerosities higher than 60–80 dots were more affected by attentional load  than were lower (nonsubitizing) numerosities. This major result was confirmed on a replication of key numerosities with an additional nine naïve participants. These results reinforce suggestions of a third regime of numerosity perception. It is interesting that the mechanism that suffered least from depriving it of attentional resources was the “estimation range,” which suffered only a slight cost with the visual task, and no cost at all with the auditory task. Given that the two distractor tasks were different in nature , we cannot directly compare the modality-specific costs with each other. However, it is interesting that these diverse distractors led to qualitatively similar relative effects on thresholds over the three ranges.There is now a better understanding of the involvement of attentional and visual working memory in the judgment of numerosities within the subitizing range (Anobile et al., The discontinuity in psychophysical performance between estimation and texture density does not necessarily imply the existence of three totally independent systems. It is possible, indeed probable, that estimation mechanisms operate over the entire range, but that this system is supplemented by attentional mechanisms at low and very high numerosities. There is good evidence for an attention-dependent subitizing mechanism in the low range, allowing for perfect enumeration; but when attention is drawn from this mechanism by dual tasks, the estimation system continues to operate (Burr et al.,


In this study, poly(lactic acid) (PLA)/wood flour (WF) composites were prepared by first blending PLA with organo-montmorillonite (OMMT) at different contents . The physical and mechanical properties of the virgin and OMMT modified PLA and its WF composites were tested. The results showed that: (1) at low OMMT content (<1 wt %), OMMT can uniformly disperse into the PLA matrix with highly exfoliated structures. When the content increased to 1.5 wt %, some aggregations occurred; (2) after a second extruding process, the aggregated OMMT redistributed into PLA and part of OMMT even penetrated into the WF cell wall. However, at the highest OMMT content (2 wt %), aggregates still existed; (3) the highly exfoliated OMMT was beneficial to the physical and mechanical properties of PLA and the WF composites. The optimal group of OMMT-modified PLA was found at an OMMT content of 0.5 wt %, while for the PLA/WF system, the best properties were achieved at an OMMT content of 1.5 wt %. Sustainable and efficient resource utilization becomes more and more important due to the continuously growing demand for resources. As an alternative material for fossil fuels, wood–polymer composites (WPC) are a group of hybrid materials which are prepared from renewable waste wood fiber/flour (WF) and polymer. Compared with classic fibers, WF represents superior properties when they are used as reinforcing fillers in a polymer matrix, such as lost cost, non-toxicity, recyclability, high strength, and specific stiffness ,2,3. By + and Ca2+, showing a high polarity and difficulty of dispersion in non-polar polymers [Using nanoparticles as fillers in polymer composites has been achieved with improvements in stiffness, strength, and the glass transition temperature of the polymers due to the large interfaces developed with the dispersion of nanoparticles in the polymer matrix ,16,17. Tpolymers . Introdupolymers .In most studies, the introduction of OMMT into WPC was carried out by simple ternary blending with WF and polymers ,24,25. TPopulus tomentasa Carr.) with mesh size of 10–60 was kindly donated by Xingda Wood Flour Company, Gaocheng, China. It has an average length of 1.5 mm and an average diameter of 0.2 mm. PLA (AI-1001) (L content) was obtained from Esun, Shenzhen BrightChina Industrial Co., Ltd, Guangzhou, China. It has a density of around 1.25 g/cm3 and a melt flow index of about 12–15 g/10min at 190 °C with a load of 2.16 kg. The Mn of PLA is about 80,000 g/mol. Na-MMT  was purchased from East West Company, Beijing, China. It is a hydrophilic clay powder with a specific gravity of 2.6 and its pH at 5% w/w in water is 9–10. The cation exchange capacity (CEC) of Na-MMT is 145 mmol/100 g. DDAC with a concentration of 70% was supplied by Shanghai 3D, Bio-chem Co., Ltd., Shanghai, China. The polyfluotetraethylene (PTFE) films were used as demoulding materials to avoid mould sticking to the board during hot-pressing.The wood flour of poplar  powder were again mixed in the high-speed blender and extruded in the same manner as the process of PLA modification and broken into granules. A hot press  was used to produce the composites by compressing the mat at 180 °C with a pressure of 4 MPa for 6 min. About 300 g of granules were placed in a square mould with dimensions of 270 × 270 × 3 mmd001 is the interlayer distance between (001) planes; n refers to the integer wavelength number (n=1); λ refers to the X-ray wavelength; θ refers to the maximum diffraction angle.X-ray diffraction (XRD) analysis of oven-dried Na-MMT and OMMT powders, virgin and OMMT-modified PLA, and the composites were carried out on an X-ray 6000  machine. The X-ray beam was Cu-Kα (λ = 0.154 nm) radiation, operated at 40 kV and 30 mA. The scanning rate was 2 °/s and 2θ ranged from 1.5° to 40°. By considering Bragg’s law, see Equation (1) [The distribution of OMMT in PLA and the composites was observed by TEM  with an acceleration voltage of 100 kV. The samples were cut with an ultra-microtome diamond knife to obtain ultrathin (50 nm) sections. 3 with complete immersion in water at 20 ± 2 °C. The WU was calculated based on the weight percent gains at intervals of 6, 24, 48, 96, 144, 192, and 240 h. The weights were taken after the removal of excessive water on the sample surface. The TS was determined based on the mid-span thickness changes till the end of the test. The average WU, TS, and standard deviations were based on the values of six replicates. To further understand the water absorption process, the Fick’s diffusion coefficients of PLA and PLA composites were calculated according to Equation (2) [D is the diffusion coefficient, eqWU is the final equilibrium water uptake measured at the end of the test, t, and h is the thickness of the sample.The water uptake (WU) and thickness swelling (TS) tests were carried out according to the Chinese standard GB/T 17657-2013 on the samples with a size of 50 × 50 × 3 mmtion (2) .(2)D=π13 with a gauge length of 50 mm. The tensile test was carried out according to the Chinese standard GB/T 1040-92 at a testing speed of 2 mm/min. The sample dimension is 250 × 25 × 3 mm3 with a gauge length of 100 mm. The Notched Charpy impact test was carried out according to the Chinese standard GB/T 1043.1-2008 with a size of 80 × 15 × 3 mm3. Twenty replicates were used in flexural, tensile, and impact tests.The flexural test was carried out according to the Chinese standard GB/T 9341-2000, which involves a three-point bending test at a crosshead speed of 2 mm/min. The sample dimension is 60 × 25 × 3 mmThe morphologies of virgin and OMMT-modified PLA and the composites were observed by SEM  with an acceleration voltage of 5 kV. All the samples were sputter-coated with gold prior to observation.d001 value of 1.46 nm. After modifying with DDAC, the peak shifted to a lower location at 2θ = 3.98°, indicating the enlarged interlayer distance of MMT of 2.22 nm, see The XRD patterns of Na-MMT and OMMT powders, virgin and OMMT-modified PLA, and the WF composites are shown in The TEM images of virgin and OMMT-modified PLA and the composites are shown in In the case of the PLA/WF composites, the values of water uptake, diffusion coefficient, and thickness swelling increased sharply because the WF easily absorbed water and was quick to hydro-expand. At a certain content of WF (50 wt %), the differences among these composites can be owed to different OMMT contents. Similar to the results of virgin and OMMT-modified PLA, the addition of OMMT lead to decreases of these values. Furthermore, OMMT penetrated into the WF cell wall can reduce accessible hydroxyl groups of WF, which consequently improved the compatibility with the non-polar PLA matrix, restricting both the water absorption and dimensional expansion. However, the optimal group was obtained at an OMMT content of 1.5 wt %, which was different to OMMT-modified PLA at a content of 0.5 or 1 wt %. As mentioned in the TEM analysis, the second preparation process contributed to the redistribution of OMMT in both PLA and WF. Therefore, the properties of composites could be further improved. 2, respectively. MMT was a type of material with high strength and hardness, which could absorb more of the stress and energy applied to the PLA [2, respectively. Except for the flexural modulus and impact strength, all values obtained in this study were lower, which should be associated with the weak interface. Thus, to further improve the properties of the composites, WF modification should be taken into account. the PLA . Therefo the PLA , the OMMThe SEM images of the fracture surfaces can give supporting information for the mechanical properties. The modification of PLA with OMMT could improve the physical and mechanical properties of PLA and its resulting composites with WF. For PLA, the optimal sample of modified PLA was found at an OMMT content of 0.5 wt %, where good dispersion of OMMT was obtained with highly exfoliated structures. For PLA/WF composites, owing to the redistribution of OMMT during the second manufacturing process, the optimal group was found at an OMMT content of 1.5 wt %. The OMMT was not only dispersed into the PLA matrix but also penetrated into the WF cell wall. Thus, the interfacial adhesion between WF and PLA also improved. However, at the highest OMMT content (2 wt %), the physical and mechanical properties of PLA and the composites decreased due to the agglomeration of OMMT.


Papillomaviridae is a family of small, non-enveloped viruses with double-stranded DNA genomes of 5 748 to 8 607 bp. Their classification is based on pairwise nucleotide sequence identity across the L1 open reading frame. Members of the Papillomaviridae primarily infect mucosal and keratinised epithelia, and have been isolated from fish, reptiles, birds and mammals. Despite a long co-evolutionary history with their hosts, some papillomaviruses are pathogens of their natural host species. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the taxonomy of the Papillomaviridae, which is available at http://www.ictv.global/report/papillomaviridae.The  The non-enveloped viral capsid is ~600 Å in diameter and consists of 72 pentamers of the major capsid protein, L1, and ~12 molecules of the L2 minor capsid protein , Fig. 1 The viral genome varies from 5 748 to 8 607 bp. The genome comprises three functional regions. The early region encodes proteins involved in transcription, replication, and manipulation of the cellular milieu. The late region encodes the capsid proteins L1 and L2. The upstream regulatory region, located between the L1 and E6 open reading frames, contains the origin of replication as well as binding sites for viral and cellular transcription factors  [2].Papillomaviruses primarily infect epithelial cells. Following a micro-abrasion, the incoming virion complexes with extracellular heparin sulfate proteoglycans on the basement membrane. This interaction results in conformational changes in the L1 and L2 capsid proteins, in turn allowing for transfer of the virion to an unknown entry receptor. Following furin cleavage of L2, the virion becomes internalised using a process that shares similarities with macropinocytosis. The L2-DNA complex traffics to the trans-Golgi network, until mitosis. By metaphase, the viral DNA can be seen to be associated with host chromosomes . The virPapillomaviridae is based on sequence identity across the L1 open reading frame [Firstpapillomavirinae, which includes >50 genera and >130 species, and Secondpapillomavirinae, with a single genus and species. Genera are named according to the Greek alphabet (e.g. Alphapapillomavirus), with the prefixes ‘Dyo-’ and ‘Treis-’ indicating additional cycles through the alphabet (i.e. Greek for ‘a second or third time’).Classification of the ng frame . The famhttp://www.ictv.global/report/papillomaviridae.Full ICTV Online (10th) Report: http://pave.niaid.nih.gov.The Papillomavirus Episteme (PaVE): http://www.hpvcenter.se.The human papillomavirus reference center: http://www.animalpv.org.The animal papillomavirus reference center:


N,N-dimethyl acrylamide) (PDMA) on the optical characteristics and sensing properties of corresponding thin films is discussed. Series of hydrophilic PDMA-based copolymers of different chemical composition and chain architecture such as triblock, star-shaped, and branched were synthesized. The copolymers were characterized using conventional spectroscopic techniques as well as methods for characterization of copolymer macromolecular characteristics in solution, namely size-exclusion chromatography and static light scattering. Thin films of the copolymers of nanometer scale thickness were deposited on silicon substrates by the spin-coating method. The refractive index and extinction coefficient of the copolymer films were calculated from the reflectance spectra by using non-linear curve fitting methods and the composition-structure-optical properties relationships were evaluated. Humidity-sensing properties of the films were studied by measuring reflectance spectra of the films at a relative humidity range from 5 to 95%RH. The implementation of the copolymer films as optical sensors of humidity is justified and discussed.The influence of the macromolecular architecture of block copolymers containing poly( Complex macromolecular structures formed by covalently bonded blocks or material segments of different thermodynamic features are subject of immense research interest due to the unique properties in solution as well as at surface . In thisHumidity is present everywhere in our daily lives and it is our continuing companion. Moreover, humidity controls many environmental processes, because even at small variations in temperature, water evaporates or its vapors condense. At first glance it seems that humidity is harmless, but more in-depth analysis shows that in many cases humidity is undesirable and should be avoided. Humidity can damage sensitive materials and objects, disturb human comfort and negatively affect many manufacturing processes in food, pharmaceuticals and electronic industries. Thus, a demand for methods for precisely detecting and monitoring humidity is raised that explains the persistent scientific interest worldwide in this research field. Developing low cost materials through environmentally friendly methods of synthesis, on the one hand, and using simple methods of detection, on the other, will open new possibilities for sensitive and convenient detection of humidity with high resolution.N,N-dimethylacrylamide) (PDMA) [2 mesoporous nanoparticles [Due to some advantages over electronic sensors, such as room temperature operation, high accuracy, resistance to electromagnetic interference, and lack of explosion danger, optical sensing of humidity has gained increasing interest, and great research efforts have been made in recent years in this field ,5. The m) (PDMA) ; and, biarticles . Despite2 deposited by glancing angle deposition method [N-vinylcarbazole) [A relatively simpler alternative is color sensing of humidity where detection is performed by visual inspection of color, or color monitoring, by cheap camera ,16,17. Mn method ; alternarbazole) ; and, porbazole) .An even simpler approach for humidity detection is reported in ,22, wherN,N-dimethyl acrylamide)-poly(ethylene oxide) (PDMA-PEO) di- and triblock copolymers deposited by spin coating on silicon substrates [2 and Nb2O5 [In our previous studies, we have demonstrated the humidity-sensing ability of thin films from poly and branched are synthesized and characterized. The impact of macromolecular architecture on the optical and humidity-sensing properties of spin-on thin films is discussed and the implementation of polymer films for optical sensing of humidity is demonstrated.N,N-dimethyacrylamide (DMA); initiator ammonium cerium (IV) nitrate (NH4)2Ce(NO3)6; 1,1,1-tris(hydroxymethyl)propane (TMP); trimethylolpropane triacrylate (TMPTA); and, poly(ethylene glycol) diacrylate ) were purchased from Sigma-Aldrich . Poly(ethylene oxide) of molar mass 2000 g·mol−1 (PEO2000) was supplied from Fluka . DMA was purified from inhibitor by passing through a column of basic alumina. All solvents and other reagents were of standard laboratory reagent grade and used as received. The reagents used in the synthesis of polymers (monomer b-PEO-b-PDMA (P1) was carried out as follows: A reaction mixture composed of DMA and PEO2000  in deionized water was prepared and bubbled with nitrogen for 15 min for oxygen removal. A solution of the initiator (NH4)2Ce(NO3)6 (in equimolar ratio to the hydroxyl end groups of the precursor) in 1 N HNO3 was added and the reaction mixture was bubbled with nitrogen for another 15 min. The polymerization was implemented for 3 h at 35 °C under nitrogen atmosphere. The reaction scheme of the synthesis is shown in , and star-shaped polymer, P2, were purified by dialysis against deionized water and finally isolated by freeze-drying.All copolymers studied in this work were synthesized by means of redox polymerization of DMA in deionized water using ammonium cerium (IV) nitrate as initiator and suitable hydroxyl functionalized initiating moiety. The reaction for obtaining triblock copolymer PDMA-In order to synthesize branched copolymers of different macromolecular architecture, PEGDA  or TMPTA  were added to the initial reaction mixture following the same procedure as described above. The structures of synthesized branched copolymers P3 and P4 are presented schematically in Fourier Transform Infrared Spectroscopy: Attenuated total reflectance Fourier Transform Infrared (ATRFTIR) spectra were recorded using an IRAffinity-1 spectrophotometer  equipped with a MIRacleTM ATR accessory  providing depth of penetration of the IR beam into the sample of approximately 2 μm. All samples were scanned over a wavenumber range from 4000 to 600 cm−1 performing 50 scans at a resolution of 4 cm−1.Nuclear Magnetic Resonance (NMR) investigations were carried out on a Bruker Avance II + 600 NMR spectrometer  using deuterated water or dimethylsulfoxide-d6 as solvent. The copolymer molar mass and/or composition (mole ratio) were calculated based on the integrated proton spectra when comparing the integrals of the signals of PDMA and PEO repeating units.Size Exclusion Chromatography (SEC): The coupled size exclusion chromatography/static light scattering (SEC/SLS) analysis was carried out with an Alliance e2695 Separations Module  equipped with Optilab T-rEX refractive index detector and DAWN Heleos II MALS detector  Chromatographic separation was achieved by a set of two Waters Ultrahydrogel GPC columns with nominal pore sizes of 120 and 1000 Å. An aqueous solution of 0.1 M NaNO3 was used as an eluent at a flow rate of 0.8 mL/min at 35 °C. Polyethylene glycol standards (PSS GmbH) were used for calibration. The data were collected through an e-SAT/IN digital to analog converter and relative molecular mass characteristics  were calculated with the help of Empower 3 software (Waters Corp.).Static Light Scattering (SLS) measurements were carried out on a DAWN HELEOS-II multi-angle laser light scattering  detector (SLS detectors at 18 angles) equipped with a laser emitting at a wavelength of 658 nm, built-in QELS and COMET ultrasonic cleaning system (Wyatt Tech.). Analyses were performed in a micro batch mode at 25 °C. The specific refractive index increment dn/dc was measured at 25 °C on an Optilab T-rEX refractive index detector (Wyatt Tech.) and determined with the Wyatt dn/dc software. The data analysis was undertaken using the Rayleigh–Gans–Debye equation (Equation (1)), valid for small interacting particles in the form:c is concentration; and, gR, and the second virial coefficient, A2, were calculated using Berry or Zimm fit methods processed with ASTRA 6 (Wyatt Tech.) software. All stock solutions (5 g·L−1) were prepared directly by dissolving copolymer in bi-distilled water. Different formulations of the nanostructures were obtained by dilution of stock solutions with appropriate amounts of solvent and subsequent vortexing. The stock solutions for the light scattering measurements were purified from dust through polyvinylidene difluoride (PVDF) membranes of 0.45 μm pore size and diluted with a filtered solvent.Thin polymer films with thicknesses of 95 nm were prepared by the method of spin coating  on silicon substrates using P1 and P2 polymer solutions in methanol with concentrations of 0.5 wt %. Additionally, by varying the concentration of P1 solution to 0.25 and 1 wt %, films with thicknesses of 55 and 215 nm, respectively, were deposited for comparative purposes. In order to obtain films with good optical quality from polymer P3, a synthesis time of at least 5.5 h was required. After dilution of the initial solution with methanol 5, 7.5 and 10 times, films with thicknesses of 80, 135 and 305 nm were obtained, respectively, for postdeposition annealing of 30 min at 60 °C. When a post deposition annealing of 30 min at 180 °C was applied, thinner films were obtained: 70, 100 and 200 nm, respectively. Thin films from polymer P4 with thicknesses of 60 nm were deposited at synthesis time of 2, 5 and 24 h after dilution of the initial solution with methanol in the ratio 1:1.n, extinction coefficient, k and thickness, d, of the films were calculated from reflectance spectra measured with UV-VIS-NIR spectrophotometer  using a previously developed calculation procedure including two-stage nonlinear curve fitting [R is the change of film’s reflectance in % which is provoked by the alteration of relative humidity, Δ%RH.Refractive index,  fitting . The sen fitting . The senPDMA copolymers studied in this work were prepared by applying cerium ion initiated redox polymerization in mild and environmentally tolerant reaction conditions: Aqueous media, low reaction temperature, and easy products workup. The method is well known in the literature and widely used in the synthesis of graft copolymers mainly based on poly and other hydroxyl groups abundant polymers ,28,29. D−1 ascribed to the PDMA amide group and the peak at 1497 cm−1 assigned to H–C–H bending vibrations of the methyl group, attached to nitrogen. The band at approximately 1101 cm−1, assigned to the characteristic absorption of C–O–C groups, confirms the presence of PEO segments in P1, P3, and P4 copolymers.1H NMR spectra  as compared to the theoretical one, as well as to those determined by NMR (3) is much more preferable for DMA-based copolymers, despite the partial aggregation that might occur. This was the reason SEC analyses were performed in water as an eluent, which is a good solvent for all studied copolymers. Moreover, copolymers P2, P3, and P4 showed limited or poor solubility in THF.Macromolecular characteristics of the copolymers were determined by SEC. SEC analysis of the linear block copolymer, P1, by using organic eluent such as tetrahydrofuran (THF) showed underestimated molar mass  may no longer hold at higher angles, which supports the assumption of formation of aggregates in aqueous solution already observed by SEC. In contrast, the linear Zimm plot in accordance with first order polynomial fitting observed for the P4 copolymer, implies that an associative process occurs by a different mechanism as indicated by the SEC results presented and discussed above  for the thickest film. Interestingly, the thickness dependence of reflectance change (ΔR), when humidity changes from 5%RH to 95%RH, is very similar to the thickness dependence of the refractive index, and we may conclude that there is a relationship between them. Furthermore, it is well known that the refractive index is directly related to the density of material, and the low value of n for the thinnest film means that its density is the lowest. Therefore, we could make a hypothesis that the sensitivity of polymer films toward humidity is related to some extent to their density and is higher for denser films. The calculated refractive index of P1 films with different thicknesses and the reflectance change when relative humidity increases from 5%RH to 95%RH are shown in R is a function of both parameters. Therefore, if the dependence of reflectance on relative humidity is known, the sensing could be performed by monitoring film reflectance with time. In order to reveal the reflectance versus humidity dependence, we measure R at different values of relative humidity in increasing and decreasing humidity modes. The variation of reflectance of P1 film (thickness of 95 nm) with humidity is shown in When exposed to humidity, the polymer chains swell, resulting in an increase of film thickness and a decrease of refractive index, finally leading to a change of reflectance of the films, because n for P1 film with a similar thickness (95 nm). The next step of our investigation concerns star-shaped PDMA polymer (P2). In our previous study, when comparing the humidity changes of PDMA thin films to those of corresponding PDMA-PEO di- and triblock copolymers, we have obtained the strongest changes in refractive index and thickness for PDMA homopolymer thin films . ConsideR in the increasing mode of humidity are very close to the values for the decreasing mode, so the hysteresis is very weak. This is a very important characteristics of a sensor and could be regarded as a good prerequisite for using P2 polymer as an active medium for humidity sensing. In addition, two linear ranges for humidity higher than 40%RH are observed. The calculated sensitivity is 0.05% in the 40–75%RH range and increases to 0.46% for RH values higher than 80%RH. Therefore, for RH higher than 80%RH, the P2 thin film could resolve less than 1%RH. However, the P2 film is insensitive for relative humidity less than 40%RH, because no change in the measured signal is obtained in this humidity range. The variation of reflectance of P2 film with humidity is presented in The comparison of humidity sensing abilities of P1 and P2  shows thn and k) and thicknesses are determined from their reflectance spectra. Further, reflectance spectra of P3 polymer films with thickness in the range 70–200 nm are measured at low (5%RH) and high (95%RH) humidity levels. Reflectance change and refractive index are presented in Thin films from P3 with different thicknesses are deposited on silicon substrate by the method of spin coating. The films are annealed at 60 and 180 °C, and their optical constants  with relative humidity for P4 thin films is presented in The comparison between N,N-dimethylacrylamide)-based copolymers were shown to be advantageous new materials in designing thin polymer films for optical sensing of humidity. Ceric ion redox polymerization used for the synthesis of copolymers provided diverse linear block or branched copolymer architectures in effective one-pot one-stage reaction procedures in mild and environmentally tolerant reaction conditions . It was demonstrated that the chemical structure and composition, as well as macromolecular architecture of the copolymers plays a crucial role in their optical and humidity-sensing behavior. Macromolecular structure influences both the sensitivity and hysteresis level. It is demonstrated that the most suitable polymer for optical sensing of humidity is the branched polymer, P3, in which the optimal balance between sensitivity and hysteresis is achieved. The highest sensitivity is obtained for RH > 80%RH, where humidity is measured with a resolution of 1.4%. For a relative humidity range of 65–80%RH, 2% of humidity could be resolved, while for humidity less than 65%RH, the measurement resolution is 8.8%. All films have thicknesses less than 100 nm, which provides a fast and reversible response to humidity changes and guarantees sustained humidity-sensing properties.In this work, poly(


Three-dimensional (3D) graphene-based hydrogels have attracted great interest for applying in supercapcacitors electrodes, owing to their intriguing properties that combine the structural interconnectivities and the outstanding properties of graphene. However, the pristine graphene hydrogel can not satisfy the high-performance demands, especial in high specific capacitance. Consequently, novel graphene-based composite hydrogels with increased electrochemical properties have been developed. In this mini review, a brief summary of recent progress in the research of the three-dimensional graphene-based composite hydrogel for flexible supercapacitors electrodes materials is presented. The latest progress in the graphene-based composite hydrogel consisting of graphene/metal, graphene/polymer, and atoms doped graphene is discussed. Furthermore, future perspectives and challenges in graphene-based composite hydrogel for supercapacitor electrodes are also expressed. In recent years, supercapacitors have been attracted intensive investigation for sustainable energy application, because of their advantages in excellent power density and high charge/discharge rates  carbon material, graphene has gained tremendous attention in various application aspects due to its fast-charged carrier mobility, excellent conductivity and largely tunable surface area Sattar, . Based oAmong various 3D macroscopic structures, graphene hydrogels consist of interconnected porous networks with large specific surface areas have received particular attention 2, and etc. have been widely incorporated into graphene to form composite materials to obtain enhanced performance.Usually, metal oxides or hydroxides exhibit high pseudo-capacitance mainly due to their faradic reaction beyond formation of electrical double-layers in the charge-discharge processes. Thus, metal oxides or hydroxides including MnO2 with other materials . Tran et al. (2 nanowire hydrogel with a high specific capacitance (397 F/g) at 1.0 A/g. Meng et al. 2 nanoplate Besides, in consideration of the advantage of Co and Ni, the strategy of combining Ni with Co was developed. Hwang et al.  embedded2 has also been focused. Liu et al.  molecules onto the hydrogel to form an hybrid graphene hydrogel, and the hydrogel containing ~14.6% RuO2 also can show a superior specific capacitance of 450.8 F/g.In addition, as one of the most promising nanomaterials, TiOu et al.  obtainedg et al.  synthesim-phenylenediamine (mPD) to preserve the conjugated structure of PANI, and the composite hydrogel showed an improved capacity of 514.3 F/g. They also developed double-crosslinked network functionalized graphene/PANI hydrogel with high specific capacitance and mechanical strength , which has good environmental stability and high pseudocapcacitance. Various types of PANI including nanorods and nanowires have been used to form graphene/PANI composite hydrogel, and these hydrogels can show improved specific capacitance  has also attracted considerable attention. Wu and Lian  synthesi Aht-ong  controll Aht-ong . Recentlg et al. , and theExcept for PANI and PPy, biopolymer such as lignosulfonate that a derivate of lignin can be regarded as a candidate for electrode material due to its electroactive components. Xiong et al.  preparedAs known, chemical doping is an effective strategy to modify the intrinsic properties of the materials. For graphene materials, nitrogen doping has been found to be an important method to provide graphene composite with high capacitance (Jin et al., Compared to single-atom doping, multiple doping can exhibit a synergetic effect and further improve the capacitive behavior of the materials. Typically, nitrogen and sulfur can be doped into graphene concurrently. After co-doping, the pseudo capacitance of the graphene will be increase, because of the redox faradic reactions existed at nitrogen-containing groups and sulf-containing species. Tran et al.  outlinedIn this work, the recent advances in three-dimensional graphene-based composite hydrogel were reviewed in term of their use as flexible supercapacitors electrode materials. As can be seen, much effort has been devoted in the field of 3D graphene-based composite hydrogel electrode materials. A variety of materials including metals, polymers, dopants have been reported to construct high performance graphene-based composite hydrogel, and the excellent capacitive behavior of the typical composite hydrogel can be achieved . NeverthAll authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.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.