Graphene field effect transistors are becoming an integral part of advanced devices. Hence, the advanced strategies for both characterization and tuning of graphene properties are required. Here we show that the X-ray irradiation at the zero applied gate voltage causes very strong negative doping of graphene, which is explained by X-ray radiation induced charging of defects in the gate dielectric. The induced charge can be neutralized and compensated if the graphene device is irradiated by X-rays at a negative gate voltage. Here the charge neutrality point shifts back to zero voltage. The observed phenomenon has strong implications for interpretation of X-ray based measurements of graphene devices as it renders them to significantly altered state. Our results also form a basis for remote X-ray tuning of graphene transport properties and X-ray sensors comprising the graphene/oxide interface as an active layer. One of the most appealing graphene attributes is the possibility of controlling the type and concentration of charge carriers via application of an electrostatic potential between a grounded graphene layer and a gate electrode, so called gate voltage. The high intrinsic charge carrier mobility in graphene implies a high application promise for use of gated graphene devices – graphene field effect transistors (GFETs) – as high speed electronic devices7. Although a direct application of GFETs in electronic circuits is largely hampered by the lack of a bandgap in a single-layer graphene8, there are advanced devices that do not require the bandgap for their functionality. In particular, graphene spintronic devices9, gas sensors with sensitivity down to the single molecule limit11, and photodetectors12 show great potential for future applications.Graphene has attracted an enormous attention for its unique mechanical, optical, and electronic properties with a wide technological perspective13, graphene-adsorbate charge transfer, molecular orientation, and magnetic properties naming only the most prominent14. However, the possible effect of ionizing X-ray radiation on the GFET properties should be considered. In this paper we show that the X-ray radiation induces strong changes in graphene transport properties via charging of intrinsic defects in the gate dielectric.The research and development of these devices is intimately connected with analysis of their structural, chemical and optical properties. In this respect, the characterization tools based on X-ray radiation are invaluable to determine bond specific chemical composition15. More particularly, defects within the dielectric layer behave like charge traps, which can be ionized, e.g., by electron or hole injection or X-ray radiation, rendering FET sensitive to ionizing radiation16. The effect of charged impurities and adsorbates is even more pronounced in graphene devices18. Recently, the photo-induced doping of graphene has been realized by visible or UV radiation exposure of GFETs26. Here, two distinct groups of GFET devices were introduced: in the first group the charges excited within the photoabsorbing medium  are transferred to graphene appearing as an increase of the graphene DC conductivity22. Within the second group the UV/Vis radiation ionize donor-like traps leaving the gate dielectric positively charged. This charge acts as a positive gate: it increases the electron concentration in graphene by capacitive coupling26. In contrast to direct graphene doping from adsorbed species causing also a decrease of carrier mobility27, the major advantage of “remote gating” is its minimal impact on the charge carrier mobility28.As the semiconductor field effect transistors (FETs) comprise the heart of a modern electronic industry the huge amount of work has been devoted to understanding their properties with respect to their further development. The quality of the gate dielectric has a profound impact on the long term stability of FETset al. studied the effect of the gate voltage on the shift of the core-level peaks associated with graphene and gate dielectric using X-ray photoelectron spectroscopy29. In a separate work, in pursuit for graphene application as an X-ray sensor, Cazalas et al. observed the change in graphene source-drain current of GFET upon hard X-ray irradiation (15 keV) of graphene on SiC30. While the latter work introduces the GFET as a device sensitive to X-ray irradiation, the basic description and understanding of the X-ray radiation effect on the GFET is still missing. Here we show that X-ray radiation induces the ionization of donor-like defects in the gate dielectric leading to a large increase of the electron concentration in graphene, i.e., to its strong negative electrostatic doping (n-doping). This results in an observed shift of the position of the charge neutrality point (CNP) to the large negative values. The succeeding irradiation and simultaneous application of negative gate voltage induces backward shift of the CNP as a result of the compensation of positively charged defects by photoexcited electron current in the gate dielectric, an effect which was not demonstrated for visible or UV irradiation so far.Surprisingly, only little attention was paid to reveal the influence of X-ray radiation on graphene in the GFET configuration. In this respect, Copuroglu 2 gate dielectric and Si substrate beneath as schematically shown in Fig. 31. Subsequently, the graphene was transferred on a p-doped silicon substrate with a 285 nm thick SiO2 layer and pre-fabricated Au/Ti electrodes. Two kinds of devices were used in this study – open and passivated ones. The passivated device comprised a 25 nm thick Al2O3 passivation layer covering the whole device surface whereas the open are left without the passivation layer. The completed GFET was inserted in an ultrahigh vacuum (UHV) chamber. The exposure to X-rays was carried out by Al Kα radiation generated by a standard X-ray source used for X-ray photoelectron spectroscopy. Both types of the devices showed the qualitatively same type of behavior unless explicitly specified.The effect of X-ray radiation was studied employing a graphene field effect transistor with a conventional structure consisting of SiOBG = 17 V. Hence, at the zero external bias the graphene layer is positively doped with holes being the majority charge carriers. For VBG larger than 17 V, electrons become the majority charge carriers. In the following we first describe an initial X-ray irradiation, which is fundamentally different from the subsequent ones and then we turn our attention to these “succeeding irradiations”.The resistivity of passivated graphene as a function of applied back-gate voltage  of the GFET device measured before X-ray irradiation is shown in Fig. BG as depicted in the inset of Fig. BG = −47 V. The subsequent turning of the X-ray source off portrays the CNP shifted to even larger negative values  only after a 24-hour long exposure of the sample to ambient conditions, which is consistent with the UV/Vis irradiation induced doping effect reported to be stable under vacuum at least for several days28.When the GFET is exposed to X-ray radiation at zero back-gate voltage we observe dramatic changes in graphene resistivity in time. As documented in Fig. ues Fig. , thus in26, the permanent change of resistivity upon initial X-ray irradiation can be explained by an accumulation of positive charge within the gate oxide, which via capacitance coupling induces negative graphene doping  interface. Since electrons possess significantly higher mobility in SiO2 than holes they reach either the graphene or the gate on the picosecond time scale whereas the holes diffuse relatively slowly through the oxide layer . Importantly, the SiO2 gate dielectric possesses a significant number of donor-like defects 2 interface33. Donor-like defects are neutral when they are occupied with electron and can be positively charged when one electron is released . These defect sites can be directly photoexcited to create charged defects  we can extrapolate the CNP position to ~−130 V. From this value we can determine the concentration of charge carries in graphene; this is equal to the concentration of charged defects 13 cm−2 taking into account the direct capacitance coupling of trapped charge in oxide with graphene charge carriers.Consistently with the recent reports on UV light induced graphene dopinging Fig. . Here, tBG electrons are driven out of the near G/SiO2 interface region. Hence, in region depleted from electrons there is a lower recombination rate of diffusing holes and, consequently, a higher number of holes approaches this region. The hole diffusion to this region is further promoted by direction of applied external electric field33. Both these effects will result in higher hole capture rate and a faster build-up of the positive charge within the oxide and its higher saturated concentration BG < 0, the electric field drives the electrons towards the G/SiO2 interface while the holes move in an opposite direction resulting in the exactly opposite effect. These effects are documented by observed increase (decrease) of the n-doping rate and saturated graphene doping level if a positive (negative) gate bias is applied , the system can be driven out of the equilibrium between BG = −70 V, the GFET resistivity rapidly increases  a partial decrease of the potential barrier and (ii) its transformation into a potential well  and the CNP shifts back only to −48 V instead to 5 V. If we consider that the charge in the SiO2 is fully compensated, then the CNP position at −48 V is determined by positive charge localized within the non-gated Al2O3 passivation layer. The associated defect concentration 12 cm−2; 2 after the initial irradiation therefore amounts to 6 × 1012 cm−2.The distinct behavior of open and passivated devices originates from the absence/existence of non-gated dielectric near graphene layer, i.e., Al33. To reveal the character of the involved defects we have measured the GFET response to the X-ray radiation as a function of time. The time traces measured on passivated devices for negative VBG are shown in Fig. 2  this gives a significant reduction of the 2 to 2 × 1012 cm−2. The associated potential barrier for the electron flux to graphene drops to ~0.5 eV, which is comparable to the value enabling electrons to flow to graphene. The positive VBG and simultaneous irradiation causes an additional significant shift of the CNP to larger negative values for both open and passivated devices . However, one should take this value with care as it was determined from extrapolated data.In addition to the charge compensation, the electrons can be directly captured by the positively charged donor-like defects  when the VInterestingly, when the succeeding X-ray irradiation was interrupted and after a short period of relaxation renewed, the resistivity quickly increases to the value measured before the turning the X-ray source off and continues as if there was no interruption Fig. . This is26. Since the remote doping does not impose degradation of the charge carrier mobility28, this approach possesses great potential in device tuning. We report on the induced doping level on standard SiO2 substrates that is much higher than has been observed for the UV doping of G/SiO2 or G/h-BN substrates28 and comparable both to chemical doping and UV-induced doping employing highly defective TiOx dielectric26.In this paper we have demonstrated that the X-ray irradiation of graphene field effect transistors causes a strong negative doping of the graphene layer as a result of charging of donor-like defects in the gate dielectrics similarly to recently reported graphene doping by visible and UV radiation2 interface volume, thus neutralizing the built-in positive charge rendering the graphene doping level in GFET devices close to intrinsic one.The strong sensitivity to X-ray irradiation has serious implications for long term stability of GFET devices and GFET characterization employing X-ray radiation as a probe. Concerning the later, the advanced strategies should be utilized to obtain the correct results from X-ray based measurements. In this context, a slight negative voltage and continuous sample irradiation will induce the electron accumulation in the near G/SiO31. Before the growth, the copper foil was heated up to 1000 °C in a home-built reactor and annealed for 30 minutes in a flow of hydrogen . The graphene layer was subsequently grown under the methane atmosphere  for 30 minutes. After cooling the sample to the room temperature, the graphene layer was removed from the bottom side of the copper foil by oxygen/argon plasma treatment .Polycrystalline graphene layers were grown by a standard low-pressure CVD method on ultrasmooth copper foil prepared by the template stripping method35, the graphene was transferred to a p-doped silicon substrate (resistivity of 0.00010–0.0015 Ω cm) covered with a 285 nm SiO2 thin film  and pre-arranged Au (80 nm)/Ti (3 nm) electrodes fabricated by e-beam lithography. Sputter deposition of metal electrodes before graphene transfer is an important step for fabrication of functional GFET structure. The metal contacts underneath graphene possess rough edges from lift-off process ensuring good graphene adhesion and electrical contact. The prepared structure was inserted into a chip holder and contacted by an ultrasonic wire bonder. The breakdown voltage of the dielectric layer (in full device configuration) in our experiments was higher than 4 MV/cm and leakage current through the oxide at VBG = 80 V was lower than 2 nA. The number of adsorbates attached to the graphene surface was decreased by vacuum annealing of the sample at 150 °C for 1 hour in an atomic layer deposition system , which was subsequently used for deposition of a 25 nm-thick Al2O3 passivation layer at the same temperature. The Al2O3 layer was deposited from Trimethylaluminum 3) and H2O precursors employing Ar as a carrier gas  chamber equipped with a manipulator providing five contacts to the sample: one used for the gate, two for the source and drain electrodes and the last two for sample heating.Using a PMMA-assisted wet transfer method−7 Pa) in two terminal geometry, which is sufficient for precise determination of charge neutrality point position. The resistance R of the graphene channel between the metal electrodes was measured using a lock-in amplifier SR830 (Stanford Research Systems) with a frequency of 1333 Hz and a fixed current of 100 nA. The graphene resistivity ρ was calculated from the channel geometry by the formula ρ  = RW/L where W is the channel width (400 μm) and L is its length (50 μm). According to the parallel-plate capacitor model, applying the back gate voltage VBG to a silicon substrate results in a change of the charge carrier concentration in graphene by n = (ε0εr/ed)VBG where ε0 is the vacuum permittivity, εr is the relative permittivity of SiO2 (3.9), e is elementary charge and d is the thickness of the SiO2 thin film. Measuring the graphene resistivity as a function of the back gate voltage (BG trace) then gives us an information about graphene doping and charge carrier mobility. Back gate voltage applied to the silicon substrate was changed during the measurements of BG traces with a sweeping rate of 0.7 V s−1.Electrical transport measurements were performed under the UHV conditions  attached to custom built UHV chamber standardly used for X-ray photoelectron spectroscopy was employed. The photon flux over the sample area was 10Supplementary Information


We report on an optimization of the commonly used wet transfer technique for large-area graphene, grown via chemical vapor deposition, onto aluminum nitride (AlN), which is mainly used as an active, piezoelectric material for acoustic devices. Today, graphene wet transfer is well-engineered for silicon dioxide (SiO2). Investigations on AlN substrates reveal highly different surface properties compared to SiO2 regarding wettability, which strongly influences the quality of transferred graphene monolayers. Both physical and chemical effects of a plasma treatment of AlN surfaces change wettability and avoid large-scale cracks in the transferred graphene sheet during desiccation. Spatially-resolved Raman spectroscopy reveals a strong strain and doping dependence on AlN plasma pretreatments correlating with the electrical conductivity of graphene. In our work, we achieved transferred crack-free large-area (40 × 40 mm2) graphene monolayers with sheet resistances down to 350 Ω/sq. These achievements make graphene more powerful as an eco-friendly and cheaper replacement for conventional electrode materials used in radio frequency resonator devices.The two-dimensional and virtually massless character of graphene attracts great interest for radio frequency devices, such as surface and bulk acoustic wave resonators. Due to its good electric conductivity, graphene might be an alternative as a virtually massless electrode by improving resonator performance regarding mass-loading effects Its higbstrates ,10. Sheeoducibly . Transfeuppliers , which puppliers ,14, but In this publication, we show an optimized wet transfer technique on AlN. As a two-dimensional conductive material, graphene can play an important role as an alternative electrode material regarding the elimination of viscous losses due to its virtually massless character. Using the wet transfer technique , the des2) as a target substrate is highly hydrophilic, therefore, no other surface treatment than standard substrate cleaning is necessary for a successful transfer [SR ≈ 350 Ω/sq as a best value, which is strongly comparable to commercially available graphene on SiO2.Commonly used silicon dioxide (SiOtransfer . Howevertransfer  using grtransfer . Additiotransfer . Our inv2 does not show cracks after transfer under the same transfer conditions (a). In order to investigate surface effects, SiO2 on Si and AlN on Si were characterized via atomic force microscopy (AFM). 2 surface (a) is explicitly smoother than the AlN surface (b). The calculated root mean square (RMS) as a surface roughness parameter averages between 0.9 ± 0.43 nm and 3.49 ± 0.88 nm for SiO2 and AlN, respectively , AlN after an Ar plasma. 2 plasma pretreatment. The effect of N2 and O2 (not shown) is fairly similar as the one for Ar. Comparing the corresponding roughness parameters we obtained RMS values for N2, O2 and Ar (1.1–1.2 nm), which are close to the one for SiO2 (0.9 nm)  measurements were performed. According to the sessile drop method  very sma(0.9 nm) i. For th2, O2 and Ar plasma pretreatments. In general, the contact angle between a fluid and a solid is described by the Young equation [r, which is the ratio between the real and the projected solid surface area [2, O2 and Ar physically damage the AlN structure and smoothen its surface, whereas H2 has almost no major physical impact on AlN due to its low atomic weight. The surface damage is clearly seen in Corresponding CA measurements show a strong change in surface properties for the differently treated AlN samples. The obtained CA a for meaequation :(1)γSA−γace area . The WenApart from this physical smoothing which influences the substrates wettability, there is another important effect responsible for the surface properties. Plasma treatment also leads to a change in the chemical composition of the surface. For all plasmas there is a cleaning effect of the surface. Contamination atoms like carbon and other carbonic composites, which tend to be hydrophobic, are both physically and chemically removed. These composites can readsorb on the AlN surface after the plasma pretreatment , which i2 plasma, there is an oxidation effect of the surface which in turn becomes more hydrophilic due to a smoothening. A certain amount of residual O2 within the reactor chamber might enable oxidizing reactions which results also in a smoothening of the AlN surface. A nanolayer of aluminum oxide allotropes directly increases the wettability. Depending on the fabricated application one needs to keep in mind that oxidation might cause undesired side effects on the device performance 3 and NH4NO3, due to the aforementioned, non-negligible amount of residual oxygen in the plasma chamber. Despite their negligible effect in physical surface smoothening, H2 atoms might create labile hydroxide compounds with the assistance of background oxygen, such as Al(OH)3. These chemical reactions can also activate the AlN surface due to the availability of aluminum atoms in the substrate material. The creation of nitrate components as intermediate products of chemical reactions increase, as well as oxides, the surface energy i.e., wettability [Physical damage in combination with Ntability . Nitrateθ* for all applied plasmas within the first minutes after plasma exposure. This drop in CA is rapidly recovered within one hour after pretreatment. The recovery effect is quickest for H2 plasma. There is a specific saturation CA value for each of the plasma pretreatments, which is time-independent and averages between 44° for H2 and 18° for N2, O2 and Ar (dashed lines). Hence these maximum values of t > 2 h for different plasmas represent the ones shown in 2, and O2 plasma several oxidation reactions superpose the cleaning effects, whereas for H2 plasma a stable oxidation apparently does not take place. For this purpose a profound analysis of the chemical compounds on the AlN surface after plasma treatment needs to be carried out, this is not within the scope of this publication.The “cleaning” phenomenon of the removal of surface contaminations can be seen in θ* < 20° improves the quality of transferred graphene. The Raman D/G ratio decreases below 0.2 and the crack density can be lowered significantly. As shown later on this influences directly the graphene sheet resistance.In a first summary we can conclude that plasma pretreatments impact the surface both physically and chemically, whereas a chemically activated surface shows an aging effect. Transfer experiments indicate that already a physical impact on the AlN surface resulting in as little as 2 and AlN on Si are shown as a reference. Although the two samples differ in their surface roughness (RMS = 4.4 ± 0.7 nm (1.8 ± 0.2 nm) for B1 (B2)) the CA is remarkably high for B2 (63 ± 6°). When an Ar plasma is applied on both samples RMS values do not change  line shows different levels of hole doping (strain). The slopes are obtained from [Additionally, the strain distribution across the transferred graphene layer was investigated to further understand the effect of plasma pretreatment. Using spatial Raman spectroscopy analysis of the 2D and G peak positions, ω2D modes . In Figuned from , as(3) would not hold. Hall measurements of graphene on AlN showed hole doping, which allows the use of the aforementioned equations. Das et al. ,25 revean et al.  calculatD2 versus ωG reveals a significant change for plasma pretreated AlN on Si. Strain and doping levels are labelled in 2 and Ar plasmas, whereas there is no dependency identifiable for untreated AlN and AlN pretreated with an H2 plasma. Graphene on Ar (O2) plasma pretreated AlN shows dominantly tensile strain between 0.1–0.3% (0.1–0.2%); the doping is significantly lower for the Ar plasma (n = 2 × 1011–1 × 1013 cm−1) compared to the O2 plasma (n = 2 × 1013–4 × 1013 cm−1). The same result is obtained for N2 (not shown). We can conclude that variations of strain and doping are strongly reduced applying an N2, O2 or Ar plasma instead of an H2 or no plasma pretreatment at all. To obtain a greater spatial distribution map measurements were done, shown in (e–g) for untreated and Ar plasma treated AlN substrate (AlN/Si and B1). Due to observable D peak intensities for graphene on untreated AlN (e) information about actual doping concentrations cannot be extracted due to overlapping effects in the Raman shifts. However, a clear reduction of strain variation is apparent for the Ar plasma for both AlN/Si and B1 . The same results hold for B2 (not shown). This indicates that the transfer-induced strain results not from surface roughness itself but from wettability effects on the graphene sheet during transfer. For the transfer of graphene on B1 we added exemplary AuCl3 during the Cu etching, which results in an additional doping of the transferred layer. This is directly visible in a shift of the 2D/G ratio (g). AuCl3 might play an important role regarding the decrease of sheet resistance due to the doping effect. This will be investigated further.In −1) vanishes almost completely, whereas the 2D/G intensity ratio increases up to 2.6. The 2D peak lies at 2772 cm−1. Both indicate the existence of mostly monolayer graphene on AlN [In order to quantify the enhanced graphene quality due to the applied plasma treatments the samples were investigated via optical microscopy and Raman spectroscopy. SR, shown in SR constantly reaches values below 1 kΩ/sq. Best values were obtained as SR = 350 Ω/sq. Sheet resistance measurements were homogenous on an area of 40 × 40 mm2. So far, we can exclude the effect of induced additional strain due to doping, as shown in SR seems to be promising. Additional experiments on this topic still need to be carried out.The obtained quality improvement is directly proven via four-point measurements. Due to plasma pretreatment, the sheet resistance 2 (1)) ensure a continuous desiccation via capillary forces in between the substrate and the graphene/PMMA (Poly (methyl methacrylate)) whereas hydrophobic surfaces (such as AlN (2)) form water droplets instead. These droplets either remain in between the substrate and the graphene layer (1) or they break the graphene layer for sublimation, creating cracks and discontinuities (2). As shown experimentally in this work, additional strain is applied on the transferred graphene sheets, which critically influences the sheet resistance SR of the graphene. The underlying effect of hydrophobic surfaces is the concept of surface and interfacial energies, which we identified indirectly via CA measurements. Hydrophobic surfaces feature a low surface energy and hydrophilic surfaces feature a high surface energy [SR. Problems regarding the transfer of graphene onto graphene will occur due to its extremely hydrophobic behavior. CA measurements revealed θ* to be very close to 90° .In this work, the synthesis of a graphene monolayer was achieved via chemical vapor deposition (CVD), a mature and commonly applied technique of growing graphene , on copp2 pieces for 10 × 15 mm2 graphene transfers. Solidly mounted BAW resonators (BAW-SMR) with sputtered AlN as a piezoelectric layer  and 1.5 µm AlN on MoAlN (B2)) were used for 40 × 40 mm2 graphene transfers ). The AlN wafers were cut into 20 × 20 mmransfers . Both Al−1 clearly identifies this as a monolayer structure [The graphene on Cu foil was routinely investigated by a Raman spectroscopy line scan of 121 data points , since it is an approved, easily applicable, non-destructive characterization method . The reptructure ,31. An a4)2S2O8, 0.5 mol/L). This turned out to result in a cleaner interface between AlN and graphene after transfer due to lower formation of precipitates and chemical byproducts than commonly used etching solvents like iron chloride (FeCl3) and iron nitrate (Fe(NO3)3). Wu et al. [4)2S2O8 as an etching solvent since it avoids additionally induced Fe compounds during etching, which reduce the quality of the grown graphene. The PMMA/graphene stack was then deposited onto different AlN target substrates. Before the graphene transfer, the AlN substrates were exposed to different plasmas  in order to vary the surface conditions regarding surface energy and roughness. The SiO2 substrates received no plasma treatment and were used as reference samples. For comparison purposes, graphene transferred on untreated AlN was also investigated. Plasma pretreatments of AlN samples were done in a direct current (DC) plasma reactor . The wettability of the AlN sample was characterized via contact angle measurements using the sessile drop method [After CVD synthesis, the graphene monolayers were transferred on differently pretreated AlN substrates using the wet transfer technique , describu et al.  suggest p method  in a sel2, O2 and Ar plasma pretreatments. Focusing on the chemical “cleaning” of the surface leads to a temporal but strong increase of wettability, which enables a defect-free graphene transfer without changing the AlN’s surface morphology. Sheet resistance measurements show a direct effect of this optimization regarding sheet conductivity. SR can reproducibly be reduced below 1 kΩ/sq for transferred graphene with the size of 40 × 40 mm2 and an Ar plasma pretreatment. The wet transfer of CVD graphene monolayers onto AlN was investigated. We identified a strong dependence of the wettability of AlN surfaces on different plasma pretreatments due to both a physical and a chemical effect on the AlN surface. AFM images and contact angle measurements reveal a change of surface properties resulting in spatially lower strain variation and a tremendous reduction of layer cracks and gaps for NWith this new knowledge, the wet transfer method for the assembly of multilayer graphene sheets for further sheet resistance reduction should be investigated further. A sensitive adjustment of the surface properties of graphene regarding its strong hydrophobic behaviour is essential to obtain even lower sheet resistances. Furthermore, the effect of substrate surface pretreatment is an interesting and inevitable topic for the transfer of high quality graphene onto other substrate materials. Our work clearly shows a strong improvement in the quality of graphene and therefore emphasizes the promising future of graphene as a virtually massless and ultrathin electrode material for high-frequency devices.


The commensurability condition is applied to determine the hierarchy of fractional fillings of Landau levels in monolayer and in bilayer graphene. The filling rates for fractional quantum Hall effect (FQHE) in graphene are found in the first three Landau levels in one-to-one agreement with the experimental data. The presence of even denominator filling fractions in the hierarchy for FQHE in bilayer graphene is explained. Experimentally observed hierarchy of FQHE in the first and second Landau levels in monolayer graphene and in the zeroth Landau level in bilayer graphene is beyond the conventional composite fermion interpretation but fits to the presented nonlocal topology commensurability condition.  SU(4) spin-valley symmetry is broken by a larger magnetic field, the new features for IQHE appear, corresponding to removal of LL subbands degeneracy [Recent progress in Hall experiments with graphene has revealed many new features in longitudinal and transversal resistivity in Hall configurations characteristic for fractional quantum Hall effect (FQHE), both in suspended graphene scrapings –3, and igeneracy . In the graphene . EspeciaIn the present paper we analyze the hierarchy for fractional fillings linked to strongly correlated multiparticle states in graphene using the topological commensurability approach developed earlier for the ordinary 2DEG Hall systems –10. In t2. N-fold normal product of the manifold N elements [N particle system resulting in special shapes of trajectories are not important here, unless they cause the topological nonequivalence. Only topology of trajectories on M decides whether one trajectory loop can be continuously transformed into another one without a cut or not. If a smooth continuous transformation is impossible between two trajectories, they are topologically nonequivalent and fall into distinct classes of the full braid group. Thus, the full braid group does not reflect the dynamics details, but rather identifies the topology restrictions imposed on the multiparticle system, which are associated to some global features of the system including the manifold type and only those aspects of interaction that might cause topological nonequivalence of trajectories. The Coulomb repulsion of electrons on the plane is a central prerequisite for the braid group definition at magnetic field presence, because the scale of interparticle separation defined by the Coulomb repulsion must interfere with the planar cyclotron orbits, discriminating in this way possible correlation types. As the loops from i-th particle on the manifold M, i.e., the classical position of the i-th particle on M) are changing along a selected loop from the The concept of the commensurability of cyclotron trajectories with interparticle spacing in a 2D charged system at strong magnetic field is born in relation to the braid group approach to multiparticle systems in the presence of a magnetic field, and employs the interaction of electrons, essential for formation of any correlated state, including states corresponding to FQHE. The collective behavior of a quantum multiparticle state must be assigned by the statistics phase shift acquired by the multiparticle wave function if one considers position exchanges of particle pairs ,13. The elements . The quoar braid . In thisnt 1DURs :(1) σi→ei-th and M the braid groups differ considerably from the permutation group. For where he 1DURs :(2) σi→ecorresponding to so-called anyons . This is illustrated in the central panel of Figure BA must be shared among three loops with the same surface A (i.e. BA for each loop). Thus, each loop accommodated to the flux quantum A and gives the contribution BA to the flux, resulting in the total flux 3BA per particle, as needed – this is schematically illustrated in Figure A in the right panel is equal to A in the left panel, which means that the three-loop orbits fit to the interparticle separation.In Figure As the braid group generator must be defined by the half of the cyclotron orbit . The genductors .x-th particle separation  is here 4. This corresponds to the ordinary Zeeman spin-splitting and to the so-called valley splitting (expressed often in terms of a pseudospin) due to doublet of inequivalent K points mixed with two sublattices for C atoms in crystal lattice of graphene [In graphene one deals with the relativistic version of LLs ,23. Thisnds meet . Thus thgraphene  and the rvations . The remThe Berry phase contribution is linked to an additional Similar analysis of the LLs can be done for the bilayer graphene ,24. Due  and n=1 ,24. Beca the LLs ,24.3.1. For the magnetic field strong enough that The cyclotron orbits in the LLL must be accommodated to the bare kinetic energy S is the sample surface). As this orbit size is lower than the interparticle separation l-th particles. In this way one can obtain the hierarchy of fillings for FQHE in this subband of the LLL, q-looped orbit x-th particle . For hoFor the completely filled subband x-th particle) we get Similarly one can consider fillings of the following LL with At the special case q-looped orbits, their size may be accommodated to every x-th particle in the subband, resulting in additional hierarchy in all subbands of the first LL, Moreover, for , ν=23,4,+xll3(q-1n), whereas the interparticle distances are measured with the plaque The following subbands are filled with electrons upon the similar scheme. For the subband 3.2. The special topology of the bilayer graphene creates the opportunity to verify the braid group based concept of the commensurability of cyclotron orbit size with the interparticle spacing in the planar system of interacting particles. Let us emphasize once more that the interaction is essential for correlation manifestation – the interaction prevents particles from approaching one another closer than the particle separation resulting from the planar density. This separation is rigidly kept by the interaction allowing the commensurability notion to be useful. Exclusively when the cyclotron orbits fit accurately to the particle spacing the exchange of neighboring particles in the presence of the perpendicular magnetic field is possible along specific braids named as the cyclotron braids.It must be emphasized that commensurability condition, as related to the braid group, concerns the classical archetype of the quantum system, thus is not addressed to real quantum particles which do not have any trajectories. This agrees with the spirit of the braid group approach in which classical trajectories in configuration space supply information on quantum statistics via 1DURs of the braid group. To organize any correlated collective state, the related statistics of particles must be determined first. In order to establish the statistics of quantum particles, the appropriate braid group must be defined whereas the statistics is governed by the selected 1DUR of this group .The bilayer graphene is, however, not strictly two dimensional and for the bilayer graphene the topological situation changes considerably. Two sheets of the graphene plane lie close to each other and the hopping can change the electron positions among the planes. Thus, we deal here with double the number of electrons residing on a two-sheet structure simultaneously instead of the single sheet as was the case for the monolayer graphene.doublelooped cyclotron orbits may have in bilayer graphene the same size as the single-looped orbit in opposition to the monolayer case. This follows from the fact that the second loop can be located in the opposite sheet of graphene than the first one and the external field passing through such doublelooped orbit is twice larger than the flux passing through the single-looped orbit. Each loop has in this case its own separate surface in contrary to the multilooped cyclotron orbit located on the purely 2D plane. Taking into account that in the bilayer system loops of the multilooped orbit may be located partly in both 2D sheets, the contribution of the one loop must be avoided whereas the remaining loops must share the same flux as passing through a single-looped orbit, independently of how loops are distributed among two sheets. Thus, one can write out the commensurability condition in the bilayer graphene for the case of too short single-looped cyclotron orbits in the following form , and p is an odd integer to assure that the half of the cyclotron orbit defines the braid.where, p-looped cyclotron orbit take part only The factor Equation  is causep – thus the generators of the corresponding cyclotron subgroup are of the form p exponent for the Jastrow polynomial. But due to the distinct commensurability of orbits with interparticle separation (Equation (p-odd) in the first particle-type subband of the LLL, i.e., in the subband Let us emphasize that for such multilooped orbits the total number of loops is still Equation ) the reln due to the same size of the cyclotron orbits.For holes in this subband  one can write Some novelty occurs, however, in the following two subbands of the LLL, The generalization of this main series for holes in the subband and to the full FQHE hierarchy in this subband is as follows: for subband holes, In the subband The last subband The evolution of the fractional filling hierarchy of subsequent LLs is illustrated in Figures For bilayer graphene the degeneration of One can consider also the situation in the LLL of bilayer graphene, when the degeneracy of ∗<1 , it wasce band) ,29. Both011 cm-2 ; and in FQHE in suspended graphene is observed at relatively high temperatures around 10 K , and eveThe competition between the FQHE state with the insulator state near the Dirac point, corresponding to a rapidly decreasing concentration has also been demonstrated ,33 Figu.From the perspective of cyclotron groups, experimental results on FQHE in graphene –31,34 se5-60 V ) allows 5-60 V ,29. For 5-60 V . In the 5-60 V  may exhiBN) in high magnetic fields of the order of 40 T  .The mobility of carriers in graphene is lower than in traditional 2DEG, but taking into account that the carrier concentration in graphene can be lower in comparison to semiconductor heterostructure ,38, the Energy gaps protecting incompressible FQHE states are larger in graphene than in traditional semiconductor materials, reaching a magnitude in order of 16 K  , the subn. The repeating doublet of filling ratios for If one compares the experimentally observed fractions for characteristic FQHE features in longitudinal and Hall conductivities measured on a variety of samples of graphene with the pattern of fractional hierarchy for two lowest LLs, as illustrated in Figure le-loop) –5. The n3), 133,13, , 3,113, -rates indistinguishable at the observation resolution. In higher LL the new features, however, occur in between the above-mentioned doublets, but with nonzero longitudinal resistivity contrary to other FQHE states. This suggests that not all particles participate in these correlated states and they may correspond to the multilooped correlations of every second or every third particles according the appropriate cyclotron commensurability. And indeed these new features in the first LL of monolayer graphene, recently reported  at ν=73,N, B)-plane in Figure N, B)-plane. Straight lines, which lie in a -plane and have a common point in a coordinate origin, are connected to a constant filling factor value; N, B)-plane can be described as n enumerating LL, b depending on n and LL degeneracy, first n and compressed to smaller periods for higher n, which also agrees with the details of the observation [Another experimental evidence for the above hierarchy follows from the measurement  of the tervation . SimilarThe most convincing evidence supporting the correctness of the commensurability condition is, however, the coincidence of the related predictions with the experimental observations in bilayer graphene. The pronounced feature of these observations is the occurrence of primary fractional features with even denominators also in the lowest subband of the LLL in bilayer graphene, oppositely to monolayer one . The comNote finally that the FQHE hierarchy in bilayer systems with the characteristic even denominators holds also for bilayer 2DEG conventional Hall setups and indeed the ed there ,41, whic5. The condition for commensurability of cyclotron orbits building cyclotron braid subgroups with interparticle spacing in homogeneous 2D charged systems is formulated in order to verify possibility of arrangement of correlated multiparticle Hall states. Using this commensurability condition based on the braid group approach to statistics of interacting many particle systems, the hierarchy of fractional fillings for LLs in graphene has been determined. The FQHE evolutionwith growing numbers of LLs has been described. In higher LLs the new opportunities for commensurability occur leading to the different than the ordinary FQHE(multiloop) correlated states. The fractional fillings of LLs related with this new commensurability opportunity have been identified, starting from the first LL. They are referred to FQHE(single-loop) because the related correlations are described by single-looped braids. Both the monolayer and bilayer graphene have been considered and the essential difference of related hierarchy structures has been demonstrated and described. The even denominator main line of the fractional filling hierarchy in bilayer graphene is found in agreement with the experimental observations. The confirmation of the presented hierarchy for the monolayer and bilayer graphene can be found in experimental data for both, the graphene on BN substrate as well as the suspended samples including bilayer graphene available for fillings up to sixth spin-valley subband.The success in description of hierarchy for FQHE using commensurability conditions is linked with the nonlocal and topological character of this approach. It is known that FQHE results from non-perturbative effects of the Coulomb interaction in 2D, which give rise to gapped many-body states at special filling factors. These states have long-range entanglement and cannot be explained in a simple single-particle picture. Similarly, the composite fermion theory, effective in the LLL of conventional 2DEG and in monolayer graphene, is in fact not strictly local, utilizing the auxiliary concept of flux tubes, or vortices, fixed to particles and allowing via this multiparticle effective trick to discriminate highly nonlocal effects. The commensurability approach presented in this paper bases on the topological braid group description of statistics in many body systems and falls to the same class of nonlocal multiparticle models. It elucidates artificial assumptions of composite fermion theory and allows for the generalization of the topological approach beyond the limit of the composite fermion model. In particular, the commensurability approach is effective also in higher LLs and allows the hierarchy of FQHE to be defined quite differently than that in the LLL – in agreement with experimental observations. Utilizing the energy minimization methods , the difference between FQHE states belonging to various Landau levels is referred to the different form of the effective Coulomb interaction (i.e. Coulomb interaction dressed by the form factor corresponding to a given Landau level). The energy minimization gives energy gaps for fractions related to special multiparticle correlations of FQHE. These fractions (and their hierarchy) can be, however, identified by a nonlocal effective-multiparticle topological method of commensurability braid approach, generalizing in this way the previous composite fermion approach from the LLL. The advantage of the commensurability condition is especially clearly visible in the case of bilayer graphene, where the states at fractions with even denominators cannot be explained by the simple composite fermion model, even in the LLL.


Smart actuators have many potential applications in various areas, so the development of novel actuation materials, with facile fabricating methods and excellent performances, are still urgent needs. In this work, a novel electromechanical bimorph actuator constituted by a graphene layer and a PVDF layer, is fabricated through a simple yet versatile solution approach. The bimorph actuator can deflect toward the graphene side under electrical stimulus, due to the differences in coefficient of thermal expansion between the two layers and the converse piezoelectric effect and electrostrictive property of the PVDF layer. Under low voltage stimulus, the actuator  can generate large actuation motion with a maximum deflection of about 14.0 mm within 0.262 s and produce high actuation stress (more than 312.7 MPa/g). The bimorph actuator also can display reversible swing behavior with long cycle life under high frequencies. on this basis, a fish‐like robot that can swim at the speed of 5.02 mm/s is designed and demonstrated. The designed graphene‐PVDF bimorph actuator exhibits the overall novel performance compared with many other electromechanical avtuators, and may contribute to the practical actuation applications of graphene‐based materials at a macro scale. The excellent performance of the bimorph actuator may provide a possible design for the construction of the robot. Subsequently, a fish‐like robot inspired by the dolphin swimming is designed, which is driven by the bimorph actuator upon electrical stimulus, and the fish‐like robot can swim at different frequencies, whose swimming speed is up to 5.02 mm s−1 when the voltage is 13.0 V.Herein, we demonstrate a novel electromechanical actuator which is composed of one layer of poly‐(vinylidene fluoride) (PVDF) and the other layer of flexible graphene paper, utilizing a facile process which only involves sequential filtration processing for graphene layer and drop‐coating processing for PVDF layer, respectively. PVDF is chosen as the polymer layer for several reasons: (i) PVDF as an electroactive polymer possesses excellent electrostrictive performance,22.1−6 K−1,−6 K−1.The graphene film was first fabricated by direct filtration of an aqueous suspension of reduced graphene oxide colloids followed by 400 °C annealing according to previous works.Figure2.2−1) of the β phase, as shown in Figure S1 (Supporting Information). Moreover, the graphene–PVDF bimorph actuator exhibits a good conductivity of ≈2500 S m−1. Figure I–V) curve of the graphene‐PVDF actuator. The linear line indicates that the resistance is relatively stable in the test range from 3 to 13 V. The square resistance of the graphene layer is also measured, about 58.79 Ω □−1, and this can be well in consistent with the data in the Figure Figure 2.3FigureTo investigate the actuation performance of the graphene‐PVDF bimorph actuator, a homemade setup was designed and demonstrated, as shown in D/L (displacement/length) ratio of samples with different sizes was similar. Therefore, the length–width ratio has no influence on the deflection angle of the graphene‐PVDF bimorph actuator. Subsequently, the samples with different thicknesses of PVDF layer were prepared while the sizes of the samples are uniform and the thickness of the graphene layer is 4–5 μm. The results are shown in Figure S3 (Supporting Information). The tip displacement D gets its maximum value when the thickness of the PVDF layer is 3–4 μm. So the samples in the following tests are all 4–5 μm for the graphene layer, and 3–4 μm for the PVDF layer.In order to investigate the electromechanical properties of our graphene‐PVDF bimorph actuator, the thickness and the size of the actuator were all varied and optimized. First, pieces of the bimorph actuator with the thicknesses of graphene and PVDF layers being 4–5 μm and 3–4 μm, respectively, were cut into different sizes. In Figure S2 (Supporting Information), the P =U2/R. Thus, the tip displacement variation has a mismatch with the variation of the sample temperature. Accordingly, it might be possible that the temperature change induced by the Joule effect was one of the motivations to the deflection due to the different coefficients of thermal expansion (CTE) of the two layers.When applied the direct current, as shown in Figure To further investigate the influence of the temperature change on the deflection, we extracted the temperature data from the infrared camera, and made the tip displacement of the cantilever beam as a function of the temperature. As a contrast, we also carried out a study on the deflection in which the graphene‐PVDF bimorph actuator was heated in an oven. As shown in Figure −1 under 17.0 V, which is larger than the tensile strength of traditional engineering polymer materials.To further evaluate the actuation performance of the graphene‐PVDF bimorph actuator, the electric‐induced stress of the actuator was investigated, and the results were depicted in Figure 2.4FigureThe real‐time displacement and temperature variation of the graphene‐PVDF bimorph actuator with the cycled voltage on and off were simultaneously monitored. When applied the square wave voltage 0–13.0 V), the graphene‐PVDF bimorph actuators could be triggered, and exhibited rapid and reversible swing motions, also shown in Video S1 (Supporting Information). The tip displacement variation and the temperature variation under different frequencies  were studied. .0 V, theThe frequency response and the cycle life are two significant indexes for evaluating the performance of actuators. Figure 2.5Figure−1, shown in Figure The graphene‐PVDF bimorph actuator with the ultra large displacement and fast response to the low electric voltage could provide a basis for the various robotic device applications. Herein, a fish‐like robot was designed to imitate fish swimming in petroleum ether. The fish‐like robot consists of an expandable polystyrene body and a tail which is made of the graphene‐PVDF actuator to offer the propulsion. As shown in 3−1). In addition to the conventional thermal‐induced mechanism (the different CTEs of two layers), other mechanisms including the good converse piezoelectric effect and electrostrictive performance of PVDF, also play a significant role in the fast and enhanced actuation. Interestingly, the actuator could exhibit fast and reversible swing motions without any downgrade performance due to the super mechanical properties of graphene. The actuation could response up to 14 Hz while the performance keeps unchanged with even high cycling times (over 3600 times). Finally, based on the graphene‐PVDF bimorph actuator, a fish‐like robot was built, and it could swim at the speed of 5.02 mm s−1. Note that better performance is expected with further material and device structure optimization. These results may have some important implications to design micro or light devices based on graphene for applications in various areas such as for sensors, microrobotics, switches, artificial muscle, and so on.In summary, the electric‐driven bimorph actuator comprised of one layer of graphene and another layer of PVDF was fabricated. The actuator could bend to the graphene layer side with a maximum deflection of about 14.0 mm within 0.262 s, it also could generate large stress  should be addressed to the authors.SupplementaryClick here for additional data file.SupplementaryClick here for additional data file.SupplementaryClick here for additional data file.


Herein, free-standing nitrogen-doped graphene aerogel has been fabricated via hydrothermal reaction as the potential anode material for sodium ion batteries. The three dimensional porous network structure of the graphene aerogel provides sufficient interstitial space for sodium ion accommodation, allowing fast and reversible ion intercalation/de-intercalation. The nitrogen doping could introduce defects on the graphene sheets, making the feasible transport of large-sized sodium ion. Benefiting from the effective structure and nitrogen doping, the obtained material demonstrates high reversible capacities, good cycling performance (287.9 mA h g Sodium ion batteries have been expected as the most competitive alternative due to their abundance recourse in the earth crust, low price, and similar chemical properties to lithium ion batteries6. One great challenge in achieving stable cyclability and rate capability for sodium ion batteries is making sure the reversible insertion/extraction of sodium ions, which have larger ionic radius than that of lithium ions7. Therefore, it is vital to fabricate a promising electrode material with large surface area to provide more active reaction sites and contact area for electrode materials and electrolyte, high electrical conductivity to ensure fast electron transport, as well as high porosity to access freely sodiation/de-sodiation and shorten the diffusion distance for sodium ions.Although lithium ion batteries have been regarded as the most competitive choice for high power sources and electric vehicles because of the high energy density, the limited lithium source driven researcher to explore the substitutes for lithium ion batteries8. To date, many carbonaceous material with different structures have been investigated in the electrochemical performance as anode materials for sodium ion batteries, such as carbon fibers10, nanowires11, carbon nanospheres12, reduced graphene oxide15, and porous carbon composites16. Among these materials, graphene is the most attractive anode material for sodium ion batteries due to its extraordinary electric conductivity, high surface area, and remarkable mechanical flexibility. Notably, three dimensional (3D) structured graphene with open channels allows free and fast sodium ion intercalation/de-intercalation. Furthermore, 3D graphene network acted as free-standing anode material can eliminate the loss of electrical contact between the electrode material and current collector, ensuring good electron transport.Carbonaceous materials, the common anode materials for lithium ion batteries, are considered as the attractive anode candidates for sodium ion batteries because of their good electrical conductivity, environmental friendly, low cost, and stable chemical and physical properties17. Firstly, nitrogen doping is believed to increase the electric conductivity by lowering the semiconducting gap19. Secondly, the nitrogen doping sites could attract a large number of positive ions due to the higher electronegativity around the doping area20, therefore, increase the specific capacity of graphene. Thirdly, nitrogen doping can introduce defects on the graphene sheets22. Based on the first-principle’s calculation, the defects can facilitate lithium ion diffusion from the vacancy and go through the graphene sheets due to the lower diffusion barrier around defect sites21. These appealing merits driven the study of nitrogen doped graphene as an anode material in the field of sodium ion batteries.In order to improve the specific capacity and energy density, chemical doping, especially nitrogen doping, has attracted great interests in the enhancement of the electrochemical properties of graphene−1 at a high current density of 5 A g−1 and even maintained a capacity of 287.9 mA h g−1 over 200 cycles at a current density of 100 mA g−1. The superior electrochemical performance makes the N-doped graphene aerogel material have a promising prospect in practical applications.With these motivations, nitrogen-doped graphene (N-doped graphene) aerogel was successfully synthesized via a facile hydrothermal method followed by an annealing treatment using ammonium bicarbonate as nitrogen source. Porous structure with heteroatom doping improves the interface interaction and provides efficient diffusion channels for sodium ions and electrons. The obtained material delivered a reversible capacity of 151.9 mA h g−1. After a typical hydrothermal reaction of the obtained GO aqueous solution, the reduced graphene oxide sheets assembled to form a black cylindrical hydrogel, which was then freeze-dried to form a 3D porous graphene aerogel. Finally, nitrogen doping in graphene aerogel was conducted by an annealing treatment using ammonium bicarbonate as the nitrogen source. The obtained 3D N-doped graphene aerogel is light and porous, with a low density of 0.034 g cm−3 and a good mechanical strength . Figure 2 adsorption-desorption curve presents a typical type-IV isotherm and H2 hysteresis loop with a BJH pore distribution ranged from 2.0 to 10.0 nm, which further confirms the mesoporous texture  . As shown in Fig. ure Fig. . The spe−1) Fig. .Figure 224. The graphene powder shows a broad diffraction peak owning to the stacking of graphene nanosheets. By contrast, the N-doped graphene aerogel exhibits a sharp diffraction peak with high intensity, which may be related to the fine changes of morphology on the surface25. Figure −1 and 1587 cm−1 can be observed, which correspond to D band and G band, respectively. The D band is resulted from the defects or structural disorder and G band is accordance with the E2g mode of sp2 hybridized carbon atoms26. The intensity ratio of the D to G band (ID/IG) for N-doped graphene aerogel is 1.42, which is higher than that of pristine graphene (1.40) and graphene powder (1.36). This suggests more defects were introduced on the graphene aerogel sheets after nitrogen doping. In addition, the 2D, 2D’ and the combination mode D + D’ are shown in this spectrum of N-doped graphene aerogel, indicating the existence of both monolayer and multilayers of graphene nanosheets27.X-Ray diffraction (XRD) patterns of graphene powder, graphene aerogel, and N-doped graphene aerogel are shown in Fig. 29. The C 1s peak of N-doped graphene aerogel can be fitted into three components at 284.4, 285.6 and 288.3 eV  was applied to analyze the element distribution and chemical composition on the surface layer of the N-doped graphene aerogel. Figure  eV Fig. , which a eV Fig. 31.−1. The initial capacity of N-doped graphene aerogel is 1013.8 mA h g−1. This high value in capacity is attributed to the electrolyte reductive decomposition, reactions between the oxygen-containing functional groups on the graphene sheets and sodium ions, and the formation of solid electrolyte interphase (SEI) layer35. The electrode of N-doped graphene aerogel delivered an average reversible capacity of 260.3, 242.9, 215.9, 191.1, and 151.9 mA h g−1 at the current densities of 1, 2, 3, 4, and 5 A g−1, respectively. It can be observed that there is a steady decrease in capacity from 0.1 to 2 A g−1. When the current density came back to 0.1 A g−1, the average capacity recovered to 268.3 mA h g−1, indicating a good reversibility of the composite. In comparison, the pristine graphene aerogel presents an inferior rate performance, delivering an average of 196.1, 145.5, 102.5, 73.1, and 53.7 mA h g−1 at the current density of 1, 2, 3, 4, and 5 A g−1, respectively. Figure st, 11th, 21st, 31st, 41st, and 51st cycles at respective current density, which correspond to Fig. 37.The behaviors of sodium ion intercalation/de-intercalation were investigated by a serial of electrochemical tests and shown in Fig. −1. The N-doped graphene aerogel electrode exhibits an excellent cycling stability over 200 cycles. The capacity underwent a decrease in the first three cycles and became stable from the 4th cycle, which remained a value of 287.9 mA h g−1 after 200 cycles with about 98.6% Coulombic efficiency, corresponding to 90.6% of that at the 4th cycle. In terms of the graphene aerogel sample, there is an obvious decaying in capacity during cycling. The specific capacity is 209.2 mA h g−1 at the 100th cycle with about 93.4% Coulombic efficiency, which is 71.6% of that at the 4th cycle. The cyclabilities of graphene aerogel and N-doped graphene aerogel were further evaluated a high current density of 5 A g−1 for 100 cycles, as shown in Fig. −1 at the 100th cycle.Figure The superior electrochemical performance of N-doped graphene aerogel can be attributed to the following essential features. First, the obtained 3D porous framework of N-doped graphene aerogel can provide sufficient interstitial space and more electrochemical active sites for sodium ion reaction and storage, increase the surface area for the contact between active material of electrode and electrolyte, as well as shorten the diffusion distance for sodium ions. Second, the porous structure can accommodate volume change during the sodiation/de-sodiation processes, thus maintain the structural integrity of the electrode material. Third, the robust mechanical flexibility originated from free-standing structure guaranteed an improved cyclability even at high current densities. Forth, the defects introduced by nitrogen doping make the feasible transport of large-sized sodium ion between graphene sheets and also increase the storage sites for sodium ion.−1 at a high current density of 5 A g−1 and even maintained a capacity of 287.9 mA h g−1 after 200 cycles at a current density of 100 mA g−1. The superb results demonstrate that the effective 3D framework of N-doped graphene aerogel can provide high porosity with good mechanism properties to tolerate the volume change during charging/discharging processes, increase reaction active sites for sodium ions, and facile sodium ion diffusion. The enhanced electrochemical performance can also be attributed to the nitrogen-doping introduced defects, which is beneficial to the diffusion of large-sized sodium ion during the intercalation/de-intercalation processes. This work proves that the free-standing and binder-free 3D N-doped graphene aerogel is a promising candidate anode material for sodium ion batteries with high energy storage.In summary, 3D free-standing nitrogen-doped graphene aerogel was synthesized by hydrothermal reaction and annealing treatment. The obtained materials exhibited good sodium storage performance, delivering a reversible capacity of 151.9 mA h g4HCO3), potassium permanganate (KMnO4), sodium nitrate (NaNO3), hydrogen peroxide (H2O2), and sulfuric acid (H2SO4) were analytical grade reagents received from Tianjin Chemical Reagent Technology Co., Ltd. All the chemicals were used without further purification.Graphite powder was purchased from Sinopharm Chemical Reagent Co., Ltd. Ammonium bicarbonate  graphene hydrogel was prepared through a hydrothermal process. In a typical experiment, 0.5 g of the obtained GO was dispersed in 100 mL of distilled water under ultrasonication for 4 h and subsequently treated via a hydrothermal reaction at 150 °C for 20 h to form a graphene hydrogel. The hydrogel was freeze-dried to obtain pristine graphene aerogel. To obtain the nitrogen-doped (N-doped) graphene aerogel, the pristine graphene aerogel was loaded in an alumina boat, which was placed in a larger, covered alumina boat containing excess amount of ammonium bicarbonate (6.0 g) outside of the inner alumina boat. Then the alumina boats were transferred to a quartz tube furnace under vacuum and annealed at 300 °C for 2 h. To study the effect of graphene structure on the electrochemical performance of sodium ion batteries, graphene powder without 3D structure was also prepared as a reference from the graphene oxide through a thermal reduction at 800 °C in N2 atmosphere with a heating rate of 10 °C min−1. The conductivity of N-doped aerogel measured by Four-Point Probe Meter at ambient temperature with a value of 2.4 × 10−2 S m−1, which is larger than that of graphene aerogel (2.9 × 10−3 S m−1).Graphene oxide (GO) was synthesized via a modified Hummers method−2 in the electrode. The electrochemical tests were carried out with CR2032 coin type cells. The cells were constructed with sodium foil as the anode, the prepared active material as the cathode, glass microfiber as the separator, and 1 M NaClO4 in a mixture of ethylene carbonate (EC) and propylene carbonate (PC) (1:1 by volume) as the electrolyte (about 4 drops). The whole assembly process was carried out in an argon-filled glove box. The charge-discharge cycling was performed within the voltage range of 0.01–3 V vs. Na+/Na on a battery test instrument  at ambient temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements of the electrodes were carried out on an electrochemical workstation (CHI660E). The cyclic voltammograms were obtained over the potential range from 0.01 to 3.0 V at varous scan rate from 0.1 to 5 mV s−1. The EIS was conducted by applying a dc potential equal to open circuit voltage over the frequency range from 100 kHz to 0.01 Hz with an amplitude of 5 mV.The as-obtained pristine and N-doped graphene aerogel were pressed under a pressure of 20 MPa and used as the working electrodes without any conductive agent or binder. The average weight of the active material is about 0.60 g cmθ = 10° to 90°. Raman spectra were recorded on a LabRAM HR Evolution with the laser excitation of 532 nm. X-ray photoelectron spectroscopy (XPS) experiments were conducted on an ESCALAB 250Xi X-ray photoelectron spectrometer. The electrical conductivities were measured using SZT-2A Four-Point Probe Meter at ambient temperature. Brunauer−Emmett−Teller (BET) measurement was carried out on a Quantachrome NOVA 1000e instrument by N2 physisorption at 77 K. Macropores distribution was analyzed on AutoPore IV 9500 instrument (Micromeritics).The morphologies and microstructures of the resulting materials were characterized by transmission electron microscope  and field-emission scanning electron microscope . X-ray diffraction analysis were performed on a PANalytica X’PERT PRO with Cu Kα (λ = 1.541 Å) over the range from 2Supplementary Information


Studies combining immune checkpoint inhibitors with external beam radiation have shown a therapeutic advantage over each modality alone. The purpose of these works is to evaluate the potential of targeted delivery of high LET radiation to the tumor microenvironment via an immune checkpoint inhibitor.111In-DTPA-anti-PD-L1-BC, an 111In-antibody conjugate targeted to PD-L1, was evaluated in an immunocompetent mouse model of breast cancer. 225Ac-DOTA-anti-PD-L1-BC was evaluated by both macroscale (ex vivo biodistribution) and microscale (alpha-camera images at a protein concentration determined by the 111In data.The impact of protein concentration on the distribution of 111In-DTPA-anti-PD-L1-BC at 1, 3, and 10 mg/kg highlighted the impact of protein concentration on the distribution of the labeled antibody, particularly in the blood, spleen, thymus, and tumor. Alpha-camera images for the microscale distribution of 225Ac-DOTA-anti-PD-L1-BC showed a uniform distribution in the liver while highly non-uniform distributions were obtained in the thymus, spleen, kidney, and tumor. At an antibody dose of 3 mg/kg, the liver was dose-limiting with an absorbed dose of 738 mGy/kBq; based upon blood activity concentration measurements, the marrow absorbed dose was 29 mGy/kBq.The evaluation of 225Ac-DOTA-anti-PD-L1-BC is capable of delivering high LET radiation to PD-L1 tumors. The use of a surrogate SPECT agent, 111In-DTPA-anti-PD-L1-BC, is beneficial in optimizing the dose delivered to the tumor sites. Furthermore, an accounting of the microscale distribution of the antibody in preclinical studies was essential to the proper interpretation of organ absorbed doses and their likely relation to biologic effect.These studies demonstrate that  The recent successes of novel immunomodulatory agents have brought immunotherapy up to the forefront as a possible treatment against various types of cancers –4. This A number of investigators have demonstrated the potential advantages of combined immune checkpoint inhibition therapy and external beam radiotherapy –11. RadiWe previously reported on the feasibility of modifying an anti-PD-L1 antibody to deliver an imaging radionuclide in an immune-intact mouse model . This wo111In]InCl3 was purchased from MDS Nordion . 225Ac was produced from a 229Th source as described in -trans-cyclohexane-1,2-diamine-N,N′,N′,N″,N″-pentaacetic acid (SCN-CHX-A″-DTPA) using previously described standard methods yielding DTPA-anti-PD-L1 [111In]InCl3 (37-74 MBq) was added to an acid washed 1.5 mL eppendorf tube containing 0.25 mL of 0.2 M HCl and 0.03 mL of 3 M NH4OAc, pH = 7. After a minute, 0.2 mg of DTPA-anti-PD-L1-BC Ab was added to the mixture. The mixture was allowed to set at room temperature for 45–60 min and then transferred to an Amicon Ultra 10K centrifugal filter device. Phosphate-buffered saline (PBS) was added and the device was centrifuged for 15 min at 3000 rpm to remove free [111In]InCl3 (1×). Radiochemical purity of >98% was determined by radio TLC, and the protein concentration was determined by Nanodrop .The anti-PD-L1-BC Ab was conjugated to ti-PD-L1 , 24. Bri225Ac-labeled anti-PD-L1-BC Ab conjugate was prepared using the high-efficiency one-step method described in PCT/US2010/042885. For the one-step method, p-SCN-Bn-DOTA was conjugated to the anti-PD-L1-BC Ab overnight and the resulting conjugate, DOTA-anti-PD-L1-BC, was purified prior to radiolabeling. DOTA-anti-PD-L1-BC (0.5 mg) was then radiolabeled with 225Ac (23 MBq) in 100 μL of 3 M sodium acetate at 37 °C for 90 min. The resulting radiolabeling conjugate 225Ac-DOTA-anti-PD-L1-BC was purified using a 10 DG desalting column  prepped with PBS containing 1% bovine serum albumin (BSA) followed by a PBS wash. The radiolabeled conjugate was eluted with PBS and collected in 0.5 mL fractions. TLC determined radiochemical purity, and the protein concentration was determined by Nanodrop .The neu-N mice [l-glutamine + 1% non-essential amino acids (NEAA) + 1% sodium pyruvate + 0.2% insulin + 0.02% gentamycin. Cells were cultured for a maximum of 4 weeks before thawing fresh, early passage cells.The NT2.5 cell line was established from spontaneous mammary tumors in female u-N mice , 26. Dr.neu-N) female mice, 4 to 7 weeks old, purchased from The Jackson Laboratory . All animal studies were approved by the Animal Care and Use Committee of the Johns Hopkins University, School of Medicine.Animal studies were performed using FVB/N-Tg(MMTVneu)202Mul/J (neu-N mice (n = 3–5/time point) were injected subcutaneously (s.c.) with NT2.5 cells (1 × 106) in the right flank. Following a growth period of 4 weeks, the tumor-bearing mice were injected intravenously (i.v.) with 111In-DTPA-anti-PD-L1-BC (370 kBq) at the following Ab protein concentrations: 1, 3, and 10 mg/kg as well as with 225Ac-DOTA-anti-PD-L1-BC (15 kBq) at 3 mg/kg. At 1, 6, 24, 72, and 144 h post-injection (p.i.), the mice were sacrificed. The blood, heart, lungs, liver, spleen, kidneys, stomach (with content), intestine (with content), bone, thymus, muscle, tumor, and brown adipose tissue (BAT) were harvested, weighed, and measured in a gamma well counter  using the 400 to 480 keV energy window for 213Bi (440.6 keV and yield 26.1%). The percent-injected activity per gram (%IA/g) was calculated by comparison to a weighed, diluted standard.Biodistribution experiments were conducted as previously described with minor modifications , 27, 28.225Ac-DOTA-anti-PD-L1-BC within the normal tissues and tumors. neu-N mice (n = 1/time point) were injected s.c. with NT2.5 cells (1 × 106) in the right flank. Following a growth period of 4 weeks, the tumor-bearing mice were injected i.v. with 225Ac-DOTA-anti-PD-L1-BC (67 kBq). The mice were sacrificed at 1, 6, 24, and 72 h, and the organs  were immediately removed, embedded in optimal cutting temperature (OCT) compound and frozen (−78.5 °C). Consecutive, 8-μm-thick cryostat sections of each tissue were obtained for alpha-camera imaging, hematoxylin and eosin (H&E) staining, and immunohistochemistry. Immunohistochemistry was performed for CD8 . Exposure times for alpha-camera imaging were in the range of 20 to 24 h. Alpha-camera images were analyzed using the software, ImageJ 1.49 g .The alpha-camera is an ex vivo digital autoradiography imaging technique dedicated to detecting emitted alpha-particles . The alp225Ac-DOTA-anti-PD-L1-BC biodistribution data. The radioactivity concentration vs time curves were fitted using the software SAAM II  and integrated from zero to infinity to calculate the time-integrated activity per unit mass. If the data could not be fitted, the integral was obtained as the sum of a numerical integration over the measured time period and an analytically integrated exponential function based on an exponential extrapolation beyond the last measure time point that used the log-linear slope of the last two time points as the exponential clearance rate. The mean absorbed dose Normal tissue and tumor mean absorbed doses were calculated using the Ã is the time-integrated activity, 225Ac, 221Fr, 217At, 213Bi, and 213Po, and for 213Bi, 225Ac leads to the release of free 213Bi, which concentrates in the kidneys [213Bi was added to the 225Ac-absorbed dose. The radioactivity in kidneys was measured using a gamma well counter. Counts detected over a 1-min interval were recorded for 4.5 h. The measurements were fitted with a double exponential function from which the activity per unit mass (Bq/g) of free 213Bi (obtained as the time-zero intercept) and 225Ac-anti-PD-L1-BC within the kidneys at the time of sacrifice  could be determined.where fraction . For 225J/(Bq s) . Only th kidneys , 32. Acc111In-DTPA-anti-PD-L1-BC 1, 3, and 10 mg/kg biodistribution data. These data were used to calculate the TIA for three different radionuclides that are candidates for alpha-particle emitter therapy: 225Ac, 212Pb, and 211At with half-lives of 10 days, 10.6 h, and 7.2 h, respectively.To assess the impact of protein amount administered on the optimal alpha-particle emitter choice, the time-integrated activity (TIA) for the tumor, blood, spleen, liver, kidneys, and thymus were calculated using the neu-N mice (n = 5 mice/group) were injected i.v. with 225Ac-DOTA-anti-PD-L1-BC (3 mg/kg) at the following doses: 15, 22, 30, 37, and 44 kBq. The dose range was based on previously performed MTD studies with 111In-DTPA-7.16.4 as well as organ and blood clearance rates of 111In-DTPA-anti-PD-L1-BC in comparison to 111In-DTPA-7.16.4 (data not shown) [Healthy non-tumor-bearing t shown) . The mict test and two-way and one-way ANOVAs. p values were considered significant if p ≤ 0.05.Statistical analysis was performed using the software, GraphPad . Groups were compared using multiple 111In-DTPA-anti-PD-L1-BC conjugate was radiolabeled in 45–60 min at room temperature at a specific activity of 11.8 MBq/nmol with >95% radiochemical purity following purification. The 225Ac-DOTA-anti-PD-L1-BC conjugate was radiolabeled in 90 min at 37 °C at a specific activity of 4.0 ± 0.7 MBq/nmol with >95% radiochemical purity following purification.The 111In-DTPA-anti-PD-L1-BC (approximately 370 kBq) at the following protein doses: 1 mg/kg (20 μg Ab), 3 mg/kg (60 μg Ab), and 10 mg/kg (200 μg Ab); the pharmacokinetics of 225Ac-DOTA-anti-PD-L1-BC (15 kBq on 3 mg/kg) were also examined  by a rapid  phase. A lower fraction of the 225Ac-DOTA conjugate is cleared rapidly , and correspondingly, a larger component (37%) clears with a 19-h half-life. The difference in blood kinetics is consistent with the PK differences of the DOTA- compared to the DTPA-conjugate seen in the liver, kidneys, and tumor  calculations  with activities ranging between 15 and 44 kBq. All the mice were sacrificed after a 100-day monitoring period except one, which was sacrificed on day 98. Over the 100-day period, the whole-body weight of the mice decreased in a dose-dependent manner with greater weight loss at the larger administered activities . Administered activity-dependent decreases in organ weights were also observed for the spleen, kidneys, liver, and thymus. Significant decreases occurred in the spleen at the 44-kBq dose compared to the 22-kBq dose (p ≤ 0.001), as well as in the liver at the 37-kBq dose and 44-kBq dose as compared to the 15-kBq dose (p ≤ 0.01). A 20 to 40% reduction in kidney mass occurred at 22 to 30 kBq. Relative to the 15-kBq dose, significant decrease were seen at the 30 (p ≤ 0.007), 37 (p ≤ 0.005), and 44 kBq (p ≤ 0.0001) dose. In addition, a significant decrease (p ≤ 0.006) was seen at 44 kBq relative to the 22-kBq dose. Mice reaching the 100-day endpoint had blood chemistries within the normal range . The ALT was over 1000 IU/L with the normal range being 17–77 IU/L. Pathology showed significant degenerative and proliferative changes in the liver and kidney, as well as hematopoietic neoplasia in this mouse.The single mouse (44 kBq) that was sacrificed at 98 days displayed signs of pain, particularly labored breathing and a hunched back. Upon necropsy, the mouse was found to have a visually enlarged thymus and spleen, fluid appeared to have accumulated in the chest cavity, and the liver was pitted. The blood chemistries of this mouse all fell within the normal range, except the ALP and ALT. The ALP (526 IU/L) was again elevated outside the normal range and was significantly higher compared to the other 44-kBq-dosed mice , significant liver toxicity was noted, including a significant reduction in liver mass and weight loss. In this case, the macroscopic absorbed dose calculation is consistent with the observed MTD. However, the activity distribution in the spleen and thymus was highly non-uniform; correspondingly, only the highest administered activity led to a significant reduction in spleen weight, and the thymus seemed to be insensitive to administered activity. The threshold for renal toxicity occurs at 18 to 23 Gy which corresponds to 130 to 170 kBq administered activity. As is evident from EMS. 8, renal toxicity as indicated by mass reduction occurs at far lower administered activities. This again highlights the importance of dosimetry calculations that account for the microscopic localization of alpha-particle emitters. In the case of kidneys, 63% of the absorbed dose is due to free l cortex . It shoul cortex . Preclinl cortex . Given t225Ac-DOTA-anti-PD-L1-BC, successfully targeted a PD-L1-positive tumor for delivery of 225Ac. However, the low absorbed dose and highly non-uniform distribution in the tumor supports the use of this agent to supplement anti-PD-L1 therapy through the recruitment of immune cells to the tumor microenvironment.Targeted alpha-particle emitting radionuclides are extremely potent over a short range, making it a valuable treatment for treating single cells and small metastatic cell clusters. The development of PD-L1-targeted high LET agents has the potential to eradicate small metastatic sites and theoretically has an immunostimulatory effect in larger PD-L1-positive tumor sites. However, these agents also target PD-L1-positive immune cells potentially limiting their use as a therapeutic agent. Both imaging and therapy using radiolabeled antibody-based checkpoint inhibition therapy will be influenced by binding to PD-L1-positive sites in normal tissues. Such normal organ uptake may be overcome to some extent by optimizing the protein amount of antibody administered for therapy, helping to protect normal tissues and potentially preserve immune cells by saturating non-tumor PD-L1 sites. An accounting of the microscale distribution of the antibody in preclinical studies is essential to the proper interpretation of organ absorbed doses and their likely relation to biologic effect. The agent developed here, The distinction between an imaging agent used to evaluate target expression for patient selection and therapeutic response monitoring and a surrogate agent used for treatment planning is important. For the latter, the surrogate-imaging agent that best matches the pharmacokinetics of the therapeutic agent is required. Alternatively, differences in pharmacokinetics must be modeled to derive the kinetics of the therapeutic agent. In the former, a low-molecular-weight imaging agent that rapidly images PD-L1 expression in tumor sites with quick clearance is appropriate. Such an agent was recently reported and its biodistribution investigated in immunodeficient NOD-SCID (NSG) mice with xenografted tumors .


The 3D lamellar graphene structures were formed due to the synergistic effect of the tremendous heat induced gasification as well as huge volume expansion. The 3D lamellar graphene/BaFe12O19 composites bearing 30 wt % graphene present the reflection loss peak at −27.23 dB as well as the frequency bandwidth at 2.28 GHz (< −10 dB). The 3D lamellar graphene structures could consume the incident waves through multiple reflection and scattering within the layered structures, prolonging the propagation path of electromagnetic waves in the absorbers.The synthesis of 3D lamellar graphene/BaFe Electromagnetic wave absorbing materials have attracted much attention in commercial and military applications and it is significant to produce microwave absorbing materials with excellent absorption properties ,2. High 12O19 is widely used as magnetic substrate because of its high coercivity, excellent magnetization, good magnetocrystalline anisotropy, easy preparation with low cost, and adjustable properties [12O19. Nevertheless, how to obtain ultra-light and economic graphene/BaFe12O19 composite has become the essential issue in the preparation process of the composite material. The free-standing three-dimensional (3D) graphene synthesized by self-assembled method with the characteristics of broad frequency bandwidth, ultrahigh compressibility and adjustable electrical conductivity , could boperties ,8,9,10. operties  ensures 12O19 composite was prepared by self-propagating combustion, triggered by the exothermic oxidation-reduction reaction between citric acid and nitrate. The high temperature generated by self-propagating combustion could vaporize the oxygen-containing functional group of graphite and promote the formation of BaFe12O19 crystal. Therefore, graphite sheets that can take advantage of the energy are largely expanded into large amounts of lamellar graphenes due to the sudden gasification and stupendous volume expansion effect. The two-step process of preparing stabilized 3D lamellar graphene and embedding BaFe12O19 between the graphite layers to synthesize the composite material is completed at one time, which lead to simple operation procedure. In this paper, the self-propagating combustion triggered synthesis process of 3D lamellar graphene/BaFe12O19 composites has been studied, which has not been reported before.Here, we report a potential use of the self-propagating combustion  process 12O19 with high permeability and coercivity can easily widen absorption bandwidth and further expand reflection loss as well as remedying the tremendous gap between low permeability and high complex permittivity. Such superiorities of self-propagating combustion with short reaction time; little pollution, high temperature and high energy triggered by low temperature and low energy; the capability of chemical exothermic maintenance by itself; and high quality of purity of the product attract much attention [The 3D independent graphene in this composite has a wide frequency band and high dielectric constant characteristic. Meanwhile, BaFettention ,15.12O19 composite is presented in 4 in order to activate the graphite layers. Second, 3D lamellar graphene/BaFe12O19 composite was prepared by self-propagating combustion using activated graphite sheets and citric acid compound with Fe(NO3)3 and Ba(NO3)3 as precursor at 350 °C. The formation of 3D lamellar graphene structures from graphite mainly based on two effects occurred in the self-propagating combustion reaction process. The self-propagating combustion that was triggered at lower temperature can produce tremendous heat to continue the reaction and form the crystal structure. The temperature during the reaction increased to about 1500 °C. Thus, the high temperature and tremendous heat can gasify the functional groups on the graphite layers. The instantaneous gas flow could easily expand the graphite layers into multilayered graphene. Furthermore, for the second aspect, the self-propagating combustion reaction also has volume expansion effect. The precursor firstly infiltrate into the graphite layers  was activated by concentrated acid for the later reaction. First, 1 g NFG was mixed with 4 g concentrated sulfhuric acid, 2 g concentrated nitric acid and 0.8 g KMnO12O19 composite was in situ prepared by self-propagating combustion method using the activated graphite sheets and citric acid mixed with Fe(NO3)3 and Ba(NO3)3 as precursor. The mass ratio of Fe(NO3)3, Ba(NO3)3 and citric acid is 10:1:(6–15). The mass fraction of activated graphite sheets are various from 10 to 40 wt %. The ingredients were first dissolved into distilled water and then ammonium hydroxide was added to adjust pH = 7. The obtained colloidal sol was continuously heated and stirred at 350 °C to evaporate water and give rise to the self-propagating combustion process. After the combustion reaction, 3D lamellar graphene/BaFe12O19 composite was prepared. The sample powders were collected and cleaned for further test and application.The 3D lamellar graphene/BaFe2 F30, FEI: Hillsboro, OR, USA), field emission scanning electron microscope , X-ray diffraction spectrometer  and the Raman spectrometer  are used for qualitative analyses on the morphology, structural characteristics and component composition of all samples. The as-obtained samples that were used for the characterization of electromagnetic wave absorbing properties were prepared by mixing 10 wt % 3D lamellar graphene/BaFe12O19 composite with 90 wt % molten paraffin, then made into a doughnut-shaped sample . The complex permittivity  and permeability  components as the function of frequency of a sample are measured using a MS4644A vector network analyzer .High resolution transmission electron microscope , and the others are rectangular-like shape. The graphene sheets interlinked BaFe12O19 particles for increasing the dielectric loss in the composite. The 3D structure of lamellar graphene/BaFe12O19 composite is beneficial to the pass through of electromagnetic wave in different directions. 12O19 and 3D lamellar graphene/BaFe12O19 composite. The BaFe12O19 peaks labeled with crystal indices in the two curves are coincident. The spectrum of 3D lamellar graphene/BaFe12O19 composite shows the graphene peak at around 18°, proving the formation of graphene and the graphite peak at around 26°. Compared with the graphite peak in the upper curve, the peak intensity of graphite is far lower in that of lamellar graphene/BaFe12O19 composite, indicating that there is only a little residual graphite after the self-propagating combustion process. The BaFe12O19 spectrum has sharp peaks showing the good crystallinity. 12O19 composite. The spectrum has two obvious peaks centered at 1324.7 and 1593.2 cm−1, namely D and G modes of graphene. The D peak at 1324.7 cm−1 stems from a symmetry-lowering effect owing to the defects of graphene sheets. The G peak at 1593.2 cm−1 indicates the graphite crystallinity of carbon material. The ratio of ID to IG is used to characterize the structure integrity of carbon materials. 3D lamellar graphene/BaFe12O19 composite shows both the characteristic peak of BaFe12O19 (labeled in the image) and graphene. The presence of 2D peak indicates the existence of graphene. Therefore, from above characterization and analysis, the graphene/BaFe12O19 composite have a 3D lamellar structure and the BaFe12O19 particles were embedded inside the graphene sheets after the self-propagating combustion process.ε′, ε″) and permeability  curves of 3D lamellar graphene/BaFe12O19 composites (with different graphene mass fraction) vs. frequency are shown in ε′) and permeability (μ′) represent the storage ability of electromagnetic energy, whereas the imaginary permittivity (ε″) and imaginary permeability (μ″) are connected with the energy dissipation and magnetic loss, respectively [ε′) decreases slightly with frequency increasing, which is also observed by other researchers [12O19, graphene and paraffin wax plays an important role in the formation of the grain boundaries. At low frequencies, the grain boundaries are more effective than the conducting grains. Owing to the high resistance of the grain boundaries, the hopping electrons will be stacked and produce polarization at these areas. The electronic conduction is high at low frequencies due to the polarization. However, the electron exchange between Fe3+ and Fe2+ ions cannot follow the fast alternating field, which decreases the probability of electrons reaching the grain boundaries and the interfacial polarization [ε′) and dielectric loss (ε″) of 3D lamellar graphene/BaFe12O19 composites enhance with the graphene mass fraction increasing. It is easy to understand that the interlinked graphene sheets can form conducting network to increase the conductivity. With the mass fraction of graphene increasing, the conductivity of the composite improves. Besides, the lamellar graphene/BaFe12O19/paraffin wax structure has a better dielectric polarization and relaxation effects. When the conductive phase is distributed in the insulating matrix to form composite materials, the free charge gathering will exist in the insulation/conductor interface due to the difference in the two phase conductive performance. The interlinked graphene conductive networks embedded with BaFe12O19 particles improved the dielectric polarization properties. In μ′ and μ″ in some lines, respectively, those obvious peaks for μ′ and μ″ can be ascribed to a resonance phenomenon, which is also shown in inZ is the normalized input impedance at free space and material interface, rε = ε′− jε" is the complex permittivity, rμ = μʹ − jμ" is the complex permeability of absorbers, f is the frequency of the microwave in free space, d is the thickness of the absorber, and c is the velocity of light in free space. The impedance matching condition is given by inZ = 1 to represent the perfect absorption properties. The surface reflectivity of an absorber was presented as a function of six parameters, ε′, ε″, μ′, μ″, f and d [ε′, ε″, μ′, μ″, f and d (see 12O19 composites with 30 wt % graphene presents the reflection loss peak at −27.23 dB as well as the frequency bandwidth at 2.28 GHz (<−10 dB). For contrast, under the same matching thickness (dm = 2.0 mm), the absorbing peaks of 20 wt % graphene sample presents a slight decrease in absorbing peak and absorbing bandwidth, which is −22.83 dB and 2.04 GHz, respectively. However, when the mass fraction of graphene is up to 40 wt %, the reflection loss and the frequency bandwidth are only 11.18 dB and 1.61 GHz, respectively. Here, a crucial weight value at 30 wt % appears. Based on the former analysis, the dielectric loss tangent of the composite with 40 wt % graphene is larger than that of 20 wt % and 30 wt %. The possible reason for the poor electromagnetic wave absorbing properties of 40 wt % might be the poor matching degree of dielectric loss and magnetic loss, the electromagnetic wave absorbing property of pure EG without BaFe12O19 was also tested. It can be seen from that the reflection loss peak is only about −5 dB. The high dielectric loss and the poor reflection loss further prove the assumption. Overall, the electromagnetic wave absorbing properties improved with the increasing of graphene mass fraction before 30 wt % and recede after that. The composite with 30 wt % graphene presents the best performance. 12O19 composites at various thicknesses in the frequency range of 2–18 GHz with 30 wt % graphene. This indicates that the microwave absorbing ability of the lamellar graphene/BaFe12O19 composite at different frequencies can be adjusted by controlling the thickness of the absorbents [12O19 composite. The good electromagnetic wave absorbing properties of 3D lamellar graphene/BaFe12O19 composite may be explained by the following several reasons. First, the layered structures with high density graphene sheets formed conductive networks leading to more physical contacts between conductive graphene sheets, therefore it increases the resonance circuit density, which is consistent with the good electrical conductivity and large dielectric loss tangent. The graphene possess excellent electrical conductivity. The conducting networks would interact and attenuate the electromagnetic radiation in the absorbers effectively. Second, the polarization relaxation of defects or π-electron and interfacial polarization between graphene and BaFe12O19 exist. The 3D lamellar graphene structures can consume the incident waves through multiple reflection and scattering inside the layered structures, prolonging the propagation path of electromagnetic waves in the absorbers. The multiple reflections of electromagnetic wave lead to the larger losses of electromagnetic energy. Furthermore, the intertwined layer structures spontaneously and intensely response to the broadband incident wave, presenting as tremendous resistance–inductance–capacitance coupled circuits. The interaction of electromagnetic waves with dielectric materials intensified the molecular motions such as ionic conduction, dipolar polarization relaxation, etc., leading to energy dissipation in the form of heat through molecular friction and dielectric loss and the highly conductive networks would also consume the electromagnetic wave as resistance heat.The complex permittivity  in the frequency range 2–18 GHz. The 3D lamellar graphene nanostructures could consume the incident waves through multiple reflection and scattering inside the layer structures, prolonging the propagation path of electromagnetic waves in the absorbers. The multiple reflections of electromagnetic wave lead to the larger losses of electromagnetic energy.The 3D lamellar graphene/BaFe