Long INterspersed Element-1 (LINE-1 or L1) retrotransposons form the only autonomously active family of transposable elements in humans. They are expressed and mobile in the germline, in embryonic stem cells and in the early embryo, but are silenced in most somatic tissues. Consistently, they play an important role in individual genome variations through insertional mutagenesis and sequence transduction, which occasionally lead to novel genetic diseases. In addition, they are reactivated in nearly half of the human epithelial cancers, contributing to tumor genome dynamics. The L1 element codes for two proteins, ORF1p and ORF2p, which are essential for its mobility. ORF1p is an RNA-binding protein with nucleic acid chaperone activity and ORF2p possesses endonuclease and reverse transcriptase activities. These proteins and the L1 RNA assemble into a ribonucleoprotein particle (L1 RNP), considered as the core of the retrotransposition machinery. The L1 RNP mediates the synthesis of new L1 copies upon cleavage of the target DNA and reverse transcription of the L1 RNA at the target site. The L1 element takes benefit of cellular host factors to complete its life cycle, however several cellular pathways also limit the cellular accumulation of L1 RNPs and their deleterious activities. Here, we review the known cellular host factors and pathways that regulate positively or negatively L1 retrotransposition at post-transcriptional level, in particular by interacting with the L1 machinery or L1 replication intermediates; and how they contribute to control L1 activity in somatic cells. The Long INterspersed Element-1 (LINE-1 or L1) retrotransposon forms 17% of our genome , two open reading frames (ORF1 and ORF2) separated by a short inter-ORF spacer and a 3′ UTR Figure . As a coNew L1 copies are directly synthesized and inserted in the genome by a process called TPRT  some of the discovered partners become associated with L1 components as a result of L1 overexpression beyond physiological levels. (ii) L1 RNP stoichiometry is altered; (iii) the retrotransposition reporter cassette, which contains an intron, modifies L1 RNA cellular processing, and thus its binding partners.Poly(A) binding proteins (PABPs) bind mRNA poly(A) tails and are involved in mRNA stability and translation initiation -P putative phosphorylation sites for proline-directed protein kinases (PDPKs), such as mitogen-activated protein kinases and cyclin-dependent kinases. Mutations of S18, S27, T203, and T213, which are potential PDPK targets, decrease L1 retrotransposition; and these residues were found phosphorylated by mass-spectrometry in human cells  pathways act in somatic or embryonic cells to prevent the accumulation of L1 RNA, and eventually retrotransposition.in vivo, to reduce its abundance and to limit L1 retrotransposition. In addition, it can cleave various L1 RNA fragments derived from the L1 5′ UTR region in vitro, indicating that L1 RNA can be a direct Microprocessor substrate , a major nuclear complex implicated in microRNA (miRNA) biosynthesis through pri-miRNA processing, is also able to bind L1 RNA Second, the combined expression of sense and antisense L1 transcripts driven by L1 5′ UTR promoters reduces L1 RNA stability and L1 retrotransposition  response pathway plays a central role  cytidine deaminase family inhibit L1 retrotransposition   are also strong inhibitors of L1 retrotransposition  or DNA lesion signaling, and activation of subsequent DNA repair pathways. Conversely, these cellular processes could also participate in the resolution of L1 integration, through L1 second strand DNA synthesis or DNA ligation.The role of DSB signaling and non-homologous end-joining (NHEJ) pathways remains controversial. Ataxia-telangiectasia mutated (ATM) protein, a kinase activated upon DSB, was initially proposed to be required for L1 retrotransposition and L1-induced DSBs , the French Government  through the “Investments for the Future” , and the Fondation pour la Recherche Médicale (FRM DEP20131128533).The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


Furthermore, the magnitude of the elastic deformations can be controlled by changing the relative rotation angle between two layers. Magnitude of the out-of-plane displacements in graphene agrees well with those observed in experiments and can explain the experimentally observed bandgap opening in graphene. Upon increasing the relative rotation angle between the two lattices from 0° to 10°, the magnitude of the out-of-plane displacements decrease while in-plane strains peaks when the angle is ~6°. For large misorientation angles (>10°), the out-of-plane displacements become negligible. We further predict the deformation fields for MoS2-WS2 and MoSe2-WSe2 heterostructures that have been recently synthesized experimentally and estimate the effect of these deformation fields on near-gap states.Recent technological advances in the isolation and transfer of different 2-dimensional (2D) materials have led to renewed interest in stacked Van der Waals (vdW) heterostructures. Interlayer interactions and lattice mismatch between two different monolayers cause elastic strains, which significantly affects their electronic properties. Using a multiscale computational method, we demonstrate that significant in-plane strains and the out-of-plane displacements are introduced in three different bilayer structures, namely graphene-hBN, MoS Recent technological advancements in the isolation and transfer of different 2D materials without loss of material quality have led to the renewed interest in stacked/layered materials and heterostructures12345et al.2-graphene and graphene-MoS2-metal junctions can be tuned to obtain photocurrent with quantum efficiency as high as 55%. Similarly, graphene stacked on hBN has been shown to possess electron mobilities that are at least two orders of magnitudes higher compared to graphene on the conventional substrates such as silica or silicon9102D materials such as graphene, hexagonal boron nitride (h-BN), phosphorene, MoSet al.22 on WS215162  per percent of applied strain has been recently confirmed by photoluminescence experiments2 bilayer DFT calculations shows that the interlayer coupling varies with the twist angle and distance between two layers which increases the bandgap from 1.2 eV to 1.7 eV when interlayer distance is increased by 1 Å.Recently Woods ab-initio computational methods. Length scales associated with the Moiré patterns require consideration of large system sizes that are currently not possible using ab-initio simulations. Molecular dynamics simulation studies are also limited due to the difficulty in developing empirical force fields for many of the new materials. Recent attempts to study deformations of Moiré patterns are based on various poorly controlled approximations, whose validity is difficult to quantify. For example, Neek-Amal et al.et al.It is clear from the above discussion that elastic deformations play a crucial role in determining the electronic properties of vdW heterostructures. However, current theoretical studies to investigate such interlayer interaction induced deformations are limited in their predictive power mainly due to the large mismatch between the length scales of deformations (ten to hundred nanometers) and the length scales accessible to accurate i.e., continuum finite-deformation simulations informed by ab-initio calculations and infer possible consequences of these deformations on the electronic properties of these materials. vdW-corrected DFT simulations can be used to determine the interlayer interaction for any bilayer structures while the continuum model based on the non-linear theory of elastic plates can be used to study the deformation at larger length scales associated with the Moiré patterns. Continuum models have been successfully used to study the micrometer scale deformations such as rippling, scrolling and warping of single and multilayer graphene sheets and nanoribbons22232, WS2 and MoSe2 and estimate the change in bandgaps due to these elastic deformations. Furthermore, we also obtain analytical solutions to estimate in-plane strains and the out-of-plane displacements in 2D materials as a function of material and interaction parameters obtained from DFT calculations. These closed-form solutions can serve as a useful guide in determining appropriate material combinations to engineer strain at 2D interfaces.In this article, we develop a general framework to estimate the deformations in heterostructured bilayers with arbitrary misorientation angle using a multi-scale modeling approach, Determine the interaction energy of the unit cell of layer A as a function of its position relative to the unit cell of the substrate, layer B.Based on the above interaction energy, derive a continuum description of the spatial variation  of the interactions energy between the layer A and the substrate by inverse Fourier transformation.Obtain in-plane and out-of-plane forces acting on layer A from the spatial variation of the interlayer interaction energy.Use the forces in a non-linear elastic plate model to predict in-plane and out-of-plane deformations in layer A.Our DFT informed continuum approach involves the following steps  is the average energy of the unit cell, u1(z) is the magnitude of the energy modulations and ϕ(z) gives the energy difference between AB and BA stacked configurations and G1 is the magnitude of the reciprocal vector for substrate primitive unit cell. In the case of Graphene on h-BN, EXX + RPA (exact exchange and random phase approximation) level density functional theory energy calculations were used150 = 3.4 Å, this gives u0 (z) = –69.2 meV, u1 (z) = 2.26 meV and ϕ = –50.4°.Spatial variation of the interaction energy of the unit cell of layer A as a function of its position over the layer B unit cell can be written as (Step 1):AB can be written in terms of Moiré reciprocal lattice vectors Based on this functional form of the interaction energy of the primitive cells, spatial variation of the interaction energy of the two layers within a Moiré cell can be computed by calculating the local stacking of the each unit cell in layer A relative to the unit cell in layer B. Note that since the stacking configurations change only appreciably over the length scales of the Moiré patterns, the variations are gradual on the scale of the lattice spacing. Using this fact, spatial variation of the interaction energy between two layers  VAB can The numerical scheme to relate the unknown Fourier coefficients BN = ac (1+δ) and lattice misorientation angle is denoted by θ. Reciprocal lattice vectors Next, we relate the reciprocal lattice vectors of the Moiré cell to the lattice mismatch and the relative rotation of the two layers. For the case of graphene on hBN (with the same lattice symmetry), the lattice mismatch is defined through the a0 between the layers (Step 2)0 . Here, Fourier coefficient All the components of the force acting on layer A can be computed by taking the derivatives of interaction energies computed in the previous step. However, this will require the knowledge of the Fourier coefficients Derivatives of this energy give the components of the forces acting on the top layer due to the substrate (Step 3):αβ are related to the in plane strains by Hooke’s law:p = 2000 eV/nm2 and flexural rigidity to the bending modulus Eh3 /12(1– σ2) = 1eV. We solve these equations to find in-plane and out-of-plane displacements using the finite element methodSince the length scales over which the magnitudes of the forces vary are large (~14 nm) compared to atomic scales, the deformation of the two sheets can be computed using the von-Karman non-linear plate theoryFor a given bilayer combination, interlayer forces depend both on the magnitude of the lattice mismatch δ and the misorientation angle θ. These forces cause strains in both layers, which are partially relaxed through out-of-plane deformations, depending on the elastic properties of the layers and the nature of interlayer forces. For sake of simplicity we consider the case of layer A on a substrate (for example graphene on h-BN) and present the results for the case of free-standing bilayers in the SI.et al.Due to the forces from the substrate, the graphene monolayer deforms and corrugations develop across the sheet. Our calculations show that the out-of-plane displacements form Moiré patterns as shown in 126Forces arising due to weak vdW interaction between two layers are not strong enough to compensate the lattice mismatch strain; therefore domains of non-uniform strain distribution are created. Strain energy associated with such deformations consists of two parts; one due to curvature (bending) and one due to in-plane strains. In-plane displacements cause lowering of vdW energy by moving to more favorable energy configurations. Out-of-plane displacements are caused by buckling instabilities due to compressive in-plane strains as well as due to different equilibrium interlayer separations for different stacking configurations. Critical strain in each domain is determined by the competition between the penalty associated with the straining the layers and the gain in vdW energy.For a given bilayer with lattice mismatch δ, the period of the Moiré superlattice decreases sharply with the misorientation angle θ between two lattices and is given by:et al.As the cost of bending the graphene monolayer increases with the decrease in length scale over which curvature change appreciably, we find that the magnitudes of the out-of-plane displacements decrease with increase in misorientation angles θ between graphene lattice and hBN lattices . For theThe in-plane strains show a non-monotonic behavior with increasing misorientation angle. In the To gain further insight into the interplay between vdW interactions and elastic deformations, we develop an analytical model using elastic plate theory. Our goal is to determine the out-of-plane displacements w and in-plane displacements u for any given bilayer combination depending on the strength of vdW interactions, material properties and the lattice structure of two sheets. From the numerical solutions, we observe that the out- of-plane displacements do not have a strong dependence on the in-plane forces and similarly the contribution of the out-of-plane displacements to the in-plane strains is small in all cases. In this limit, we can decouple in-plane and out-of-plane displacements and independently determine in-plane displacements as a function of in-plane forces and out-of-plane displacements due to the out-of-plane forces. Governing equations for in-plane displacements are:α we get,Using this form of the displacements in Similarly, Fourier coefficient for the out-of-plane displacement can be obtained from Eq. (14)x,y vary as 1/G, out-of-plane displacement coefficients uz vary as 1/G4. This leads to the rapid decay of the magnitude of the out-of-plane displacements with increasing rotation angle between two sheets, as observed in m have been plotted in Replacing these Fourier coefficients in the 2-WS2 and MoSe2-WSe2 and present in Next, we applied the multiscale method on two model systems of vdW TMD heterostructures MoSK point with the conduction band minimum (CBM) derived from K point in the MoS2-WS2 system are nearly a sum of the electronic structures of the individual layers with the VBM in the W layer while the CBM in the Mo layer2 layer can induce a decrease of CBM and therefore, a band gap reduction. Our results show that the in-plane strains vary from 0.07% to 1.27% for the MoS2-WS2 bilayer structure with the misorientation angle changing from 0.5° to 10° and the band gap reduction due to such strains can be as large as ~190 meV , the direct gap opens at ~190 meV . Similar~130 meV . Non-uni2 bilayer. Our results show that the magnitudes of the out-of-plane displacements, for the vdW heterostructures studied here, are considerably large (~0.5 Å) and will significantly change the electronic properties. A simple estimate based on MoS2 bilayer studyIn addition to the in-plane strains, the out-of-plane displacements may also lead to significant change in the electronic properties. Out-of-plane displacements lead to spatially varying interlayer distances that have been theoretically shownMolecular mechanics simulations of graphene over hBN substrate were performed using the Adaptive Intermolecular Reactive Empirical Bond OrderA multiscale method to compute the elastic deformations for general bilayer structures has been established and validated via comparing the deformations with molecular dynamics simulations as well as with experiments for graphene/hBN bilayer heterostructures. This method is a convenient and precise way to predict the deformation and strains in bilayer 2D heterostructures, and hence paves the way of tuning the electronic and optoelectronic properties of these constructs via strain engineering.Closed form analytical solution to predict the in-plane and out-of-plane displacements are in good agreement with finite element results and remains valid for a vast range of interlayer interactions.2-WS2 and MoSe2-WSe2) have also been computed using our multi-scale method. Patterned strains due to these deformations are predicted to induce periodically localized electron states in the upper layer while the hole states remain extended in the substrate layer.Elastic deformation fields for the two newly synthesized vdW heterostructures .


We simulated and analyzed in detail the behavior of ultrashort optical pulses, which are typically used in telecommunications, propagating through graphene-based nanoribbon waveguides. In this work, we showed the changes that occur in the Gaussian and hyperbolic secant input pulses due to the attenuation, high-order dispersive effects and nonlinear effects. We concluded that it is possible to control the shape of the output pulses with the value of the input signal power and the chemical potential of the graphene nanoribbon. We believe that the obtained results will be highly relevant since they can be applied to other nanophotonic devices, for example, filters, modulators, antennas, switches and other devices. The production of smaller yet more simple and efficient electronic and photonic devices is the biggest concern of industry and commerce, given the increase in the user requirements for these devices. Meanwhile, the electrical and optical characteristics of graphene are attracting attention from researchers around the world. The plasmon technology on which this new material is based has enabled the development of nanoscale devices, which are also faster and more efficient than traditional devices found today. The nanoscale dimensions of graphene-based plasmonic devices allow for their integration into electronic and photonic integrated circuits.Since there are many nanophotonic devices based on nanophotonics waveguides, this study was focused on the simulation and analysis of the attenuation, dispersion and nonlinear effects occurring in signals propagating through a graphene-based waveguide. We considered a graphene nanoribbon located between similar dielectric layers, as will be described further. Previous reports illustrated several details regarding these waveguides –7; howevW) strongly influences the mode behavior that propagates through these graphene nanoribbons. In this sense, the smaller the nanoribbon width, the lower the number of modes that are present in the waveguide. Previous studies showed that in a graphene nanoribbon of width <50 nm, there exists only a single mode  [W < 10 nm, the classical theory can no longer predict the behavior of GSPPs in a graphene nanoribbon [Given that graphene surface plasmon polaritons (GSPPs) are highly confined in graphene nanoribbons acting as waveguides, it is possible to integrate these waveguides into photonic integrated circuits (PICs). However, the nanoribbon width  . Howevernoribbon .Recent reports showed that it is possible to couple the radiation emitted by a transmitter  with the surface plasmons (SPs) present on a graphene sheet . Other eThe guided TM polarized GSPPs modes were excited through a polarization maintaining single mode fiber, by the use of the end-fire coupling method.This paper is organized as follows: in the following section, we present the characteristics of waveguides composed of a single graphene nanoribbon. After the simulation and analysis is presented, the attenuation, dispersion and nonlinear effects occurring in a graphene-based waveguide are described in the Results and Discussion section. The last section summarizes the conclusions.intra + σinter, can be estimated by the Kubo formula, given by [The local conductivity of graphene, σ = σgiven by :intra is the intraband conductivity (due to electron–photon scattering), μg is the chemical potential of graphene, e is the electron charge, ω is the angular frequency of the light in air and τ is the phenomenological scattering rate. Moreover, the interband conductivity (due to the electron transition) is given by [where σgiven by :g >> kBT and T = 300 K (room temperature), thus, kBT ≈ 26 meV.It is worth noting that The imaginary part of the conductivity determines the type of the polarized modes that can be supported in a graphene nanoribbon. For σ′′ > 0 (where σ′′ is the complex part of the conductivity of graphene), TM modes can be supported, while for σ′′ < 0, TE modes can be supported .T = 0, kBT/μg = 0), the TM modes (s-polarization) and TE modes (p-polarization) are supported in graphene when  and , respectively. By numerical simulations it was proved that energy is absorbed or dissipated when . However, if we consider a fixed graphene chemical potential, the temperature increase causes a finite damping, which is smaller for TM vs TE modes. This is because the real part of the conductivity undergoes more changes in the region where the TE modes are located [In the limit without collision between electrons ( located .t) of the graphene nanoribbon, given as [Starting from Maxwell’s Equations, we arrive at the expression that relates the electric permittivity of a graphene nanoribbon to the angular frequency (ω), conductivity and graphene effective thickness , the equation for the dispersion relation for graphene nanoribbons is given by [In a graphene nanoribbon embedded in a substrate with relative permittivity εgiven by :r is the relative permitivity in which the graphene nanoribbon is embedded.where εThe dispersion relation for graphene nanoribbons for TM modes propagating along a graphene/dielectric interface is also given by :0 = 377 Ω is the air impedance and k0 = 2π/λ0. Similarly, the dispersion relation for the TE modes is given by:where ηSince the wave vectors of the surface plasmons polaritons in a graphene/dielectric surface (GSPPs) have high values, we can conclude that these modes are very well confined in the graphene nanoribbon.W, where W is the width of the graphene nanoribbon [Previous studies have shown that GSPPs can only propagate in graphene nanoribbons when the plasmon energy corresponds to −iσ(ω)/ωnoribbon .W < 10 nm, the classical theory can no longer determine the behavior of GSPPs in the graphene nanoribbon [W < 50 nm for our simulations.As previously mentioned, in a graphene nanoribbon of width <50 nm, only a single mode  exists . On the noribbon . Hence, W = 10 nm and W = 20 nm.W = 20 nm and W = 10 nm (μg = 0.5 eV).However, as we are interested in analyzing the behavior of nanophotonic waveguides with the smallest possible widths, we simulate these waveguides for widths 0 ≈< 1.55 μm in graphene nanoribbons with a width of 10 nm and 20 nm (µg = 0.5 eV). However, to work around this problem, we can increase the value of the graphene chemical potential. Note that the GSPP wavelengths for a graphene nanoribbon of width W = 20 nm are greater than the values for W = 10 nm, that is, the GSPPs modes in a narrower nanoribbons are more confined.From 0 < 2 μm and μg = 0.45 eV are shown in To simplify our calculations, we considered graphene nanoribbons suspended in air. The real and imaginary parts of the GSPP wave vector as a function of the wavelength in air (before coupling to the SPs) for 1 μm < λe of its initial intensity) is given by Lp = Im(q)/2, where q is the wave vector of the GSPP TM modes, we can see from the lower part of Lp remains constant for wavelengths up to 1.7 μm. Also note that in this wavelength range, the value of the ratio Re(q)/Im(q) = ωτ (where τ is the relaxation time) varies very little.Taking into account that the propagation length of the GSPP mode  the SiO2 substrate. Previous studies have defined the value of the mobility in graphene as μg = 60,000 cm2/Vs [2.Hexagonal boron nitride  is a graphite isomorphic insulator in which the boron and nitrogen atoms occupy positions A and B in the Bernal structure. Therefore, the atomic structure of h-BN is similar  to the structure of graphene. It is worth noting that graphene on a layer of h-BN has a charge carrier mobility value and homogeneity of almost an order of magnitude better than on SiO0 cm2/Vs . It is i2, so that a graphene nanoribbon is better positioned on the surface of a h-BN layer. Methods for the deposition of a h-BN layer on SiO2/Si already exist, as detailed in [The surface roughness of the h-BN layer is much smaller than the surface roughness of SiOailed in .The graphene/substrate geometry for a graphene-based waveguide in shown A = A × exp(−(t/τ0)2), where A is the pulse amplitude and τ0 is the temporal half width . To obtain more reliable results, we also considered hyperbolic secant pulses  = A × sech(t/τ0)) in some cases.Since the nanophotonic devices should operate in telecommunication networks, Gaussian pulses should be considered as the input: The GSPP mode wavelengths and the propagation distance can be determined by its wave vectors as given by –25:−1) referring to one propagation length, for λ0 = 1.55 μm and μg > 0.48 eV is illustrated in Using g ≈> 0.68 eV, the mode suffers virtually no attenuation. The attenuation constant values (μm−1) for one propagation length for λ0 = 3 μm and μg > 0.25 eV are given at the bottom of g ≈> 0.43 eV, the mode suffers almost no attenuation. Therefore, since the graphene-based waveguide we are analyzing has a length smaller than 1 Lp, we can neglect the attenuation inside the waveguide. However, we must consider the insertion loss. A schematic view for insertion of an optical signal in a nanophotonic waveguide is shown in Note that for μThe optical signal from an optical source is inserted into a monomode optical fiber via a connector with an insertion loss of 0.5 dB, mainly due to misalignment between the two portions of the optical fibers. The insertion loss of the optical fiber can be neglected due to its short length. The signal output to be coupled to the graphene-based waveguide  suffers I) at the input of the graphene-based waveguide. However, the electromagnetic flux coupled to the SPs (GSPPs) is subject to the electrical and optical graphene parameters. Therefore, the output pulse shape will suffer changes imposed by the graphene parameters.Considering that the modal area of the electromagnetic flux reaching the graphene nanoribbon center is approximately equal to the modal area of a single mode fiber, we can obtain the value of the intensity , as shown in the top of To prove that the attenuation can be neglected when the length of the graphene nanoribbon, It is well known that for short pulses the dispersion and nonlinearity effect should not be considered individually, that is, it is necessary to consider the combined effects of dispersion and nonlinearity.The behavior of the propagation of optical signals in a nonlinear medium can be determined by the following equation :A) is normalized such that represents the intensity (I) of the propagating signal, β1 = 1/Vg is the parameter which determines the group velocity (Vg), β2 is the second order dispersion parameter, β3 is the third order dispersion parameter, γ = n2ω0/cAeff is the nonlinear coefficient , α is the attenuation constant, and Tr ≈ 5 fs is a parameter related to the slope of the Raman gain. The second, third and fourth terms on the left side of where the pulse amplitude , which governs each frequency component with respect to the dispersive broadening, as the pulse propagates a certain distance [In a previous work, the Fourier transform was applied to find the broadening of a pulse that propagates on a nanophotonic metal/dielectric waveguide. Here, a multiplication factor, distance :L is the propagation distance (L < Lp). After applying I is equal to .In We applied n2 ≈ 10−7 cm2/W, which is much higher than the nonlinear refractive index of dielectrics typically used in optical communications (approximately 109 higher) [−7 × λ02 [The advantage of using graphene as a nonlinear medium is the high value of its nonlinear refractive index  higher) . MoreoveLp = 4.146 μm , as a function of time in the top part of L = 500 nm with an input signal power of P0 = 10−10 W, λ0 = 1.55 mm and μg = 0.80 eV) is represented by the solid line.We show the intensity, which was calculated in accordance with L = 500 nm, with an input signal power P0 = 10−10 W, λ0 = 1.55 mm and μg = 0.80 eV. Notice that the shape of the Gaussian pulse in the time domain undergoes a small change, but the output Gaussian pulse in the frequency domain remains unchanged.The intensity as a function of the frequency spectral component is shown at the bottom of 0 = 15 fs at the input (dotted line) and output (solid line) of the nanophotonic waveguide after propagating L = 500 nm, given an input signal power of P0 = 10−9 W . From P0 = 10−9 W suffer a moderate change in the time domain, but they are not changed in the frequency domain.In the upper part of After several simulations, we concluded that given the same propagation length, an increase in the signal power at the input results in strong nonlinear and dispersive effects, which distort the shape of the Gaussian pulses in the time and frequency domain at the output. Additionally, it was found that increasing the value of the chemical potential of the graphene resulted in dispersive effects that distort the shape of the Gaussian pulse in the time domain at the nanophotonic waveguide output.L = 500 nm for the input signal power P0 = 10−9 W . Shown at the bottom of To conclude our simulations regarding graphene-based nanophotonic waveguides, we used a hyperbolic-secant-shaped pulse in the time domain. The results of the intensity (calculated according to In this work we have shown the effects related to the attenuation, high-order dispersion and high-order nonlinearity in graphene-based nanophotonic waveguides. In particular, the changes in the shape of ultrashort Gaussian and hyperbolic secant input pulses due to the attenuation, dispersion and nonlinearities were analyzed. We concluded that it is possible to control the shape of the output pulse as a function of the input signal power and chemical potential of graphene. We believe that the obtained results will be very relevant to the development of graphene-based nanophotonic devices, such as filters, modulators, antennas, switches, and other devices.


The density of states for the gap states are in the range from the latter half of 1012 to 1013 eV−1cm−2. Although the large amount of gap states at the interface of high-k oxide/bilayer graphene limits the current on/off ratio at present, our results suggest that the reduction of gap states below ~1011 eV−1cm−2 by continual improvement of the gate stack makes bilayer graphene a promising candidate for future nanoelectronic device applications.The origin of the low current on/off ratio at room temperature in dual-gated bilayer graphene field-effect transistors is considered to be the variable range hopping in gap states. However, the quantitative estimation of gap states has not been conducted. Here, we report the systematic estimation of the energy gap by both quantum capacitance and transport measurements and the density of states for gap states by the conductance method. An energy gap of ~250 meV is obtained at the maximum displacement field of ~3.1 V/nm, where the current on/off ratio of ~3 × 10 N, εch, εox, tch, and tox are the effective gate number, dielectric constants for the channel and gate insulator, and thickness of the channel and the gate oxide. This perspective attracts great attention to two-dimensional (2D) layered channels in the FET application because of their rigidly controllable atomic thickness (tch < 1 nm), as well as the low dielectric constant (εch = ~4) for typical 2D layered channels35678mBLG = ~0.037mBP = 0.13mMoS2 = ~0.37The main issue of downscaling in the Si field-effect transistors (FETs) is the short channel effect in which the gate control is weakened by the drain bias. Based on an analysis of the electrical potential distribution in the channel region, it is well known that the short channel effect can be neglected when the channel length is ~6 times longer than the scaling length t εch = ~ for typiIon/Ioff) of ~106 is obtained only at the quite low temperature of 300 mK13151718b centers in the SiO2/Si system, assigned by the electron spin resonance measurement2829Dit) and the time constant (τit) for gap states have not been reported.The suppression of conductivity in bilayer graphene has so far been reported by many researchers by applying an external electrical field13141516171819202122231426CQ) through the capacitance measurements (C-V) of bilayer graphene provides direct information on the density of states (DOS) of bilayer graphene, consequently, the energy gap (EG), because it is regarded as the energy cost of inducing the carriers in graphene and is directly related as CQ = e2DOSCQ measurement for bilayer graphene3233EG estimated both from I-V and C-V has not been done yet. Moreover, in principle, the mobile carrier response to a small-signal alternating current voltage at a certain frequency is measured in the C-V measurement. The capture and emission process of mobile carriers at the trap levels distributed throughout the band gap can be extracted as a deviation from the ideal carrier response without any trap levels under the assumption of the equivalent circuit. This technique is known as the conductance methodContrary to the transport measurement, the extraction of the quantum capacitance  by annealing the SiO2/Si substrate in a 100% O2 atmosphere at 1000 °C prior to the graphene transfer2/n+-Si substrates by the mechanical exfoliation of Kish graphite. This device was annealed under Ar/H2 gas flow at 300 °C for 3 hours to remove the resist residue on the bilayer graphene channel. Subsequently, Y2O3, with a thickness of ~6 nm, was deposited on bilayer graphene FETs by the thermal evaporation of Y metal in the O2 atmosphere at PO2 = 10−1 Pa2 atmosphere at ~100 atm and 300 °C. Finally, the top gate electrode was patterned, followed by annealing at 300 °C for 30 s under 0.1% O2 gas flow. The lack of a Raman D band measured through Y2O3 indicated that no detectable defects were introduced into the bilayer graphene by the high-pressure O2 annealing, as shown in To suppress the hysteresis in drain current—gate voltage curves, the SiOEG through the CQ extraction. CTotal) between the source and top gate electrodes, obtained by sweeping VTG at different VBG, which were measured at the frequency of 1 MHz in a vacuum of ~1 × 10−5 Pa at 20 K. CTotal. The CTotal reduction at the Dirac point with increasing VBG indicates the decrease in the DOS by the gap formation because of the strong contribution of CQ. The hysteresis in the bidirectional C-V curves is quite small (~0.1 V for a VTG = ±4 V sweep)CTotal is clearly observed in the gap region, suggesting the existence of trap sites. The gradual saturation of the capacitance with increasing frequency from 200 kHz to 1 MHz suggests that CTotal measured at 1 MHz is close to the ideal capacitance without any response to the trap site. Therefore, the C-V measurement in We first focus on the capacitance measurement to determine CBG/CTG. Using CBG = 0.038 μFcm−2 for SiO2 with dBG = 90 nm and εBG = 3.9, CTG is estimated to be 0.93 μFcm−2. In the course of this study, the typical CTG value is ~1.2 μFcm−2, depending on the Y2O3 thickness. The CTG value is considerably high compared with those reported previously for both monolayer and bilayer graphene, as shown in CTG and CBG, the white dotted lines indicate the constant The slope of the dotted black line at the charge neutrality point of Vch and Cpara are the charging voltage and the parasitic capacitance. The Fermi energy (EF) and the band structure of bilayer graphene are independently controlled by changing VTG and VBG. It should be noted that the contribution of CBG is implicitly involved in CTotal through CQ and Vch, in CQ was extracted along the constant Cpara as a fitting parameter. CQ as a function of EF for different constant CQ is fitted reasonably well with the theoretical value for bilayer graphene calculated by the tight-binding modelCpara = 0.91 μFcm−2. The charging energy required to induce carriers in bilayer graphene is denoted by EF, which is expressed as EF = eVch. Vch can be expressed by the equation for a series combination of capacitors according to CTotal−Cpara and VTG−VDP, respectively. VDP is the Dirac point voltage. The detailed calculation method is explained in CQ value at the Dirac point for DOS for bilayer graphene at the Dirac point is larger than the residual carriers induced externally by the charged impurities at the SiO2 surfaceCQ is clearly observed because the scattering issue, which strongly contributes to the conductivity in I-V, can be neglected in C-V. It should be emphasized that the DOS within the gap region almost reaches zero, which is not observed in the previous report for bilayer graphene with the h-BN top gateEG as a function of absolute value for C-V data. EG was defined as the energy between inflection points for the conduction and valence sides in EG is roughly ~300 mV at The simplified equivalent circuit model of the device is shown in σ) measured as a function of VTG for different VBG at 20 K. The conductivity at the Dirac point decreases with increasing VBG, again indicating the band gap opening. The maximum Ion/Ioff is ~3 × 103 for pnp, ppp, and so on, whose positions are shown in σ = enμ), the carrier density (n) is estimated by the integration of the differential capacitance of CQ. Otherwise, n is overestimated. However, the carrier density at the access region between the source (drain) and topgate is assumed to be consistent with that at the main channel region just below the topgate electrode for the mobility estimation. Shortening this access region by the self-alignment processWe now consider the transport properties for the same device. S) is plotted as a function of temperature, as shown in S value (~600 mV/dec) extrapolated to room temperature is considerably larger than the theoretical lower limit of 60 meV/dec at room temperatureIon/Ioff at 20 K with previously reported data. All the data for Ion/Ioff are selected at 20 K from the literature, while the temperature for the mobility is not limited to 20 K. The mobility in the present study is not very high because it includes the contact resistance. In contrast, Ion/Ioff in the present study is quite high in spite of the direct deposition of high-k oxide on bilayer graphene without any organic buffer layer.Moreover, the subthreshold swing  at the high temperature region, nearest neighbor hopping (NNH) at the intermediate temperature range and variable range hopping (VRH) in a two-dimensional system of localized states at the low temperature region, as follows17To determine E0, T0 and kB are the activation energy, the hopping energy and the Boltzmann constant. Since the lowest temperature in this study is 20 K, the contribution of VRH may not be observed. The conductivity at the Dirac point is plotted as a function of temperature and fitted by two combinations of TA + NNH and TA + VRH, as shown in −1 for TA + NNH, while the plot with respect to T−1/3 does not show clear linear behavior below T = 100 K. The present data can be explained by TA + NNH for the temperature range above 20 K. This is consistent with the previous report where VRH was observed at below 5 K1317EG and E0 are plotted as a function of EG determined by I-V is reasonably consistent with the previous infrared absorption data, while EG determined by C-V is slightly larger. This difference is discussed later. Moreover, E0 increases with where Rit and Cit are the resistance and capacitance associated with the interface traps, their product CitRit is defined as τit, and Rs is the series resistance. It should be noted that Rit and Cit have been neglected due to roughly no electrical communication of carriers with trap sites at 1 MHz in Cp, in parallel with Gp, as shown in the right of Gp and Dit can be given as the followingTo gain insight into the interface properties, the gap states in the electrostatically opened-band gap are quantitatively analyzed using the conductance method. The equivalent circuit of the device is shown in the left of ω = 2πf (f: measured frequency). Finally, Dit and τit can be obtained as f0 is the frequency at Dit is possible because Gp/ω does not include CQ, which is the advantage of the conductance method. However, if there is leakage conductance (Gleak), indicated by the dotted line in the left of Dit quantitatively is the fabrication of a highly reliable top gate insulator on bilayer graphene without any leakage current. This has been achieved by the high-pressure O2 post-deposition annealing of Y2O3.where Gp/ω as a function of f for EF from −74 to −28 meV, where the band structure was fixed as constant at EG = ~150 meV. It should be emphasized that Gp was measured under the constant band structure along the constant VTG and VBG at the same time. To prevent the contribution of a minority carrier response to the conductance, especially for small band gap semiconductors, the measurement temperature was lowered as much as possible, i.e., 20 K (kBT = ~1.7 meV). This quite low temperature is only possible in bilayer graphene because the majority carriers do not freeze due to the doping by the external gate control, unlike substitutionally doped semiconductors. The more detailed measurement conditions are described in the method section. Clear peaks were observed in the Gp/ω-f relation, suggesting the detection of a trap-detrapping sequence of carriers with gap states. The τit value is evaluated to be ~4 μs. In Cit is roughly negligible in CTotal, measured at 1 MHz. According to this τit value, this assumption is roughly, but not completely, reasonable. Therefore, EG estimated by C-V may be slightly overestimated due to the contribution of Cit. Dit as a function of EG for different Dit is in the range from the latter half of 1012 to 1013 eV−1cm−2. The energy dependence of Dit is not so obvious. Although this value is slightly smaller than that for MoS2k oxide/bilayer graphene limits Ion/Ioff at present. The much lower Dit could be achieved for dual-gate bilayer graphene heterostructure with h-BN.Gp in the equivalent circuit. To avoid this, a device with a relatively short channel was used at the expense of the channel area, which increased Cpara in this study compared with typical value of ~0.2 μFcm−2 τit with the channel time constant , suggesting that τit is dominant. Therefore, the present Dit measurement is reliable.In general, the conductance method is applied to the metal-oxide-semiconductor (MOS) capacitor, where the current is injected from the back side through the semiconductor substrate, whose series resistance, including back side contact, can be removed by measuring the impedance at the high-frequency limit (see Method). The interface trap conductance can be precisely measured. In contrast, in the present FET structure, the carriers are injected from the side of the channel through the source and drain43EF stays within EG at some momentum space, while the real states exist at another momentum space, which may macroscopically act as gap states. Here, let us consider the dominant origin of the gap states. From the viewpoint of the time constant, the former origin will be a more time-consuming process than the latter because the rate limiting process is the tunneling between bilayer graphene and trap sites at the oxide surfaceDit should decrease with increasing Dit for different T0 in VRH is extracted at the low temperature region below 5 K, the Dit can be discussed more in detail because T0 is inversely proportional to the density of states for the localized statesDit. Although it is difficult to determine the dominant origin of the gap states at present, the quantitative recognition of the current status for Dit in this study does provide the realistic comparison with other semiconductor channels in terms of the interface quality.Finally, let us discuss the origin of the gap states. As mentioned before, no detectable defects are observed by the Raman D band. Therefore, the trap sites formed by defects are ignored in this discussion. Two possible origins are discussed here, as shown in 47Dit and τit for gap states. At the large displacement field of ~3 V/nm, they are in the range from the latter half of 1012 to 1013 eV−1cm−2 and ~4 μs, respectively. The large amount of gap states at the interface of high-k oxide/bilayer graphene limits the Ion/Ioff ratio at present. Many trials to characterize the interface properties for other 2D layered channels, as well as bilayer graphene, will help in understanding the origin of the gap states. The improvement of gap states below ~1011 eV−1cm−2 is the first step for bilayer graphene devices to be a promising candidate for future nanoelectronic applications.We have succeeded in extracting critical information on the interface quality, that is, −5 Pa. The capacitance measurement and conductance method under the constant Gp should be measured accurately by reducing the contribution of Rs in the equivalent circuit in the right side of Rs was estimated as follows. For the equivalent circuit in Z) is given by Rs at the accumulation side was estimated to be 29.4 kΩ by taking the high-frequency limit (ω→∞), as shown in Gp can be accurately estimated by removing Rs in the equivalent circuit in the right side of Dit measurement at the valence band side, EF is scanned along negative p-type .


The CR line-width is consistent with a high Dirac fermions mobility of ~3000 cm2·V−1·s−1 at 4 K.The van de Waals heterostructure formed by an epitaxial trilayer graphene is of particular interest due to its unique tunable electronic band structure and stacking sequence. However, to date, there has been a lack in the fundamental understanding of the electronic properties of epitaxial trilayer graphene. Here, we investigate the electronic properties of large-area epitaxial trilayer graphene on a 4° off-axis SiC(0001) substrate. Micro-Raman mappings and atomic force microscopy (AFM) confirmed predominantly trilayer on the sample obtained under optimized conditions. We used angle-resolved photoemission spectroscopy (ARPES) and Density Functional Theory (DFT) calculations to study in detail the structure of valence electronic states, in particular the dispersion of π bands in reciprocal space and the exact determination of the number of graphene layers. Using far-infrared magneto-transmission (FIR-MT), we demonstrate, that the electron cyclotron resonance (CR) occurs between Landau levels with a (B) K and K’ points of the Brillouin zone. Low-energy charge carriers therein obey the Dirac-Weyl equation and behave like massless fermionsπ and π* bands near the Fermi level in monolayer graphene are modified in multilayer graphene, showing a strong dependence on the stacking sequence as well as the layer number. Recently multilayer graphene has attracted considerable interest43Monolayer graphene is a two-dimensional materials with linear dispersions near the et al.et al.A viable method for obtaining a large-scale graphene production is the epitaxial approach based on SiC graphitization. The epitaxial growth of graphene on semi-insulating SiC has the potential to enable the next generation of ultra high frequency and low power electronic devices. The main advantage here is the possibility of obtaining a large graphene area directly on semi-insulating substrates which is one of the main requirements for high frequency devices, with a huge control of the number of graphene layers. Moreover, the choice of the appropriate SiC polytype also makes it possible to select for the formation of Bernal or rhombohedral multilayer grapheneThe Si-face 4 H-SiC substrate is preferred to the C-face for the growth of graphene. This is mainly due to the fact that graphene growth is easier to control on the Si-face, and under optimized conditions the thickness of the graphene can be limited to just a mono, bi and few layers over large areas. The off-axis 4H-SiC(0001) substrate could therefore, be a good candidate to obtain trilayer graphene. Although further, experiments are required to obtain large-area and high quality sample with high electron mobility.2·V−1·s−1 at 4 K) for the observation of the electron cyclotron resonance, despite their high doping level. By comparing ARPES measurements and DFT calculations, we show that the ordered trilayer graphene presents an ABA stacking. Using the near-edge X-ray absorption fine structure (NEXAFS) technique, we also measured the unoccupied electronic states (π* and σ*). These observations could be of prime importance, particularly regarding the development of multilayer graphene-based electronic devices.We demonstrate here the successful growth of ordered trilayer graphene layers on off-axis SiC(0001) substrates up to an unprecedentedly large area of hundreds of micrometers. The high quality of the graphene layers was further confirmed by AFM, micro-Raman mappings and X-ray photoelectron spectroscopy (XPS) measurements. Using FIR-MT measurements, we show that trilayer graphene exhibits high electron mobility  at 1550 °C in 800 mbar argon for 10 min . For Si During the graphitization process, graphene growth starts in the vicinity of the nanofacets regions consisting of step edgesfwhm on a 12 × 12 μm2 scan. The different color scale highlights a blue shift of the 2D peak from the terraces to the nanofacets, combined with an increase in the fwhm. This effect was not surprising, as the reduced bonding coordination of the nanofacets led to an easier Si sublimation from the substrate, which as explained before, facilitated graphene growth. This different growth rate between the terraces and the nanofacets may induce the formation of up to a trilayer along the nanofacetsfwhm of the 2D peak. As usual for epitaxial graphene on SiC, several intense peaks were visible in the wavenumber range between 1300–800 cm−1, corresponding to second-order Raman bands originating in the SiC substrate13In addition to AFM imaging, we carried out micro-Raman spectroscopy mapping. −1 (from 2700 to 2750 cm−1), with an increased fwhm of about 60 cm−1 (from 50 to 110 cm−1). At the same time, we also saw a shift in the G peak of about 20 cm−1 (from 1590 to 1610 cm−1). This shift shows a strong correlation between the strain and the charge density on epitaxial graphene. Thus, the observed blue-shift indicates that there is a difference in strain (associated with the shift of the G and the 2D lines) and carrier density (associated with the shift of the G line) between the (0001) terraces and (11–20) nanofacets.This is an indication of the high quality of epitaxial trilayer graphene on off-axis SiC under argon flux. As shown by the Raman maps, the 2D peak from the (11–20) nanofacets had blue-shifted with respect to those of the spectrum obtained for the (0001) terrace of about 50 cms spectra show the conventional deconvolution expected for epitaxial graphene on SiC(0001)12152 hybridized C–C bonds and it this is the signature of the graphene layers. This component was fitted using a Doniach-Sunjic line shape with an asymmetry factor α of 0.1 and a fwhm of 0.5 eV. The two other peaks at binding energies of 283.8 eV (SiC) and 285.3 eV (IL) are attributed to the SiC bulk and to the interfacial layer respectively. Due to the non-metallic nature of these components, the peaks are symmetric (asymmetric factor α = 0 eV). Assuming that the graphene-SiC sample can be modeled as a semi-infinite SiC substrate with a uniform graphene overlayer, the graphene thickness can be calculated from the ratio between the intensity of the G and SiC componentsWe probed the electronic properties of the sample further using XPS and NEXAFS experiments. The C1s spectra  were acq15 cm−2 is about three times of that of the C atoms in the SiC bilayer 15 cm−2. We obtained a thickness For a photon energy of 510 eV, we have an escape depth λ ~ 7.2 Å. In the geometry of our XPS experiment the take-off angle α was 0°, and Unoccupied electronic states of the trilayer graphene were also examined by NEXAFS spectroscopy. The C k-edge spectra were measured for different incidence angles of linear polarized synchrotron light. As shown in the insert of 12F = –1.32 eV. The three peaks observed in the MDC result from the presence of three bands in the ARPES spectrum in 7D) at –320 meV below the Fermi level (EF). This energy shift ΔE (ΔE = ED − EF) is known to be related to the charge transfer from the substrate to the graphene layersn ~ 9 × 1012 cm−2. Considering that the x-ray spot size in the ARPES measurements was about 180 × 80 (H × V) μm2, we can assume that we are able to obtain large areas of mono-domain trilayer with Bernal stacking, in contrast to other works, where the epitaxial graphene showed different domains with different numbers of layers or different stacking10The electronic structure was also probed using ARPES, which is a powerful method for this purpose, as it gives direct access to the spectral function containing the information on electron energy band dispersion. 2) was placed in a 15 T superconducting coil at 4 K and exposed to the radiation from a mercury lamp (Faraday geometry). The transmitted light was analyzed by a Fourier transform spectrometer using a Si composite bolometer directly below the sample for detection. The transmission at a given magnetic field T(B) was normalized by the zero field transmission T(0). Typical transmission spectra are shown in 24th level is the upper populated LL  substrate. Before graphitization, the substrate was hydrogen etched (100% H2) at 1550 °C to produce well-ordered atomic terraces of SiC. The sample was heated to 820 °C and deoxidized at 1100 °C in order to remove the native oxide and a possible surface contamination. The substrate was then heated to 1550 °C in an Ar atmosphere (800 mbar) for 10 min. The samples were cooled down to room temperature and transferred ex-situ to perform the different measurements.The AFM measurements were carried out under ambient conditions and the images recorded in tapping mode. The Micro-Raman mapping was performed at room temperature with a Renishaw spectrometer using 532 nm laser light auto-focusing on the sample by a DMLM Leica microscope with a 100× objective and a power of 5 mW with a spot size of about 1 μm.ν = 60 eV) and sample orientation were set in order to explore the k-space region around the K point in the ΓK direction of the Brillouin zone.The XPS, NEXAFS and ARPES experiments were carried out on the TEMPO beamlineHow to cite this article: Hajlaoui, M. et al. High Electron Mobility in Epitaxial Trilayer Graphene on Off-axis SiC(0001). Sci. Rep.6, 18791; doi: 10.1038/srep18791 (2016).


To date, there are no effective therapeutic targeting agents for triple-negative breast cancer (TNBC), and PD-L1 has presented potential as an effective marker of immunotherapeutic agents. The aim of this study was to evaluate the expression of PD-L1 by three different immunohistochemical antibodies in TNBC.Interpretation of all three PD-L1 antibodies showed good concordance among three readers  in both cancer cells and immune cells. Using a tissue microarray (TMA) constructed from 218 cases of TNBC, we performed immunohistochemical staining using three of the most popular commercially used PD-L1 monoclonal antibodies  in cancer cells and immune cells.Using various cut-off values of previous studies , the expression rates in cancer cells were: PD-L1 (E1L3N) , PD-L1 (28-8) , and PD-L1 (SP142) , respectively. At the 5 % cut-off value, the discordance rate among the three antibodies was 6.0–10.6 % and was highest between PD-L1 (SP142) and the other two antibodies. The expression rates in immune cells were PD-L1 (E1L3N) (37.6 %), PD-L1 (28-8) (36.7 %), and PD-L1 (SP142) (19.3 %), and the discordance rate among the three antibodies ranged from 13.8 to 24.8 % and was also highest between PD-L1 (SP142) and the other two antibodies. Among stromal histologic types, higher PD-L1 expression in cancer cells and immune cells was measured in inflammatory-type (p < 0.05). The absence of PD-L1 (28-8) staining in immune cells was associated with shorter disease free survival (DFS) and overall survival (OS)  in univariate analyses, and with shorter OS in multivariate Cox analysis .PD-L1 detection in cancer cells and immune cells varied by antibody clone. The greatest amount of staining occurred with PD-L1 (E1L3N), followed by PD-L1 (28-8) and PD-L1 (SP142). The concordance rate among monoclonal PD-L1 antibodies was higher between PD-L1 (28-8) and PD-L1 (E1L3N). To determine the gold standard antibody and the most appropriate cut-off value, further study of the clinical trial group treated with PD-L1 inhibitor is necessary.The online version of this article (doi:10.1186/s12967-016-0925-6) contains supplementary material, which is available to authorized users. Breast cancer is a heterogeneous disease that has several clinical, histological, and genetic forms. There have been many attempts to categorize this heterogeneous disease, and now molecular classification into five groups  can be accomplished by gene-expression profiling , 2.Among these molecular subtypes, basal-like breast cancer has a more aggressive clinical course than other subtypes and is commonly known as triple-negative breast cancer (TNBC), which is defined clinically as lacking estrogen receptor (ER) and progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2). TNBC accounts for 10–17 % of all breast cancers –8. HighlProgrammed death 1 (PD-1) is a check point molecule in immune reactions and can be expressed in various immune cells . PD-L1, Target therapy for PD-L1 in PD-L1-expressing cancers represents a possible treatment for inducing antitumor immune responses. PD-L1-targeted therapy has been investigated in preclinical and clinical trials in many tumors , 26–28 aThe aim of this study was to evaluate the expression of PD-L1 using different immunohistochemical antibodies in TNBC and associated clinical implications.We analyzed 218 patients with TNBC who underwent surgery at Severance Hospital between January 2000 and December 2006. This study was approved by the Institutional Review Board of Severance Hospital. All patients were diagnosed as having invasive ductal carcinoma, not otherwise specified (NOS) by pathologists. We defined TNBC as when IHC for ER, PR, and HER-2 and FISH for HER-2 were all negative.ER and PR immunohistochemistry signal were considered positive when more than 1 % of invasive tumor cells were expressed . HER-2 sdesmoplastic type, with cellular fibroblast/myofibroblast proliferation as the main stromal content; sclerotic type, which has a small cell component and fibrotic collagenous component as its main stromal content; or inflammatory type, with stroma mainly composed of inflammatory cells such as lymphocytes. Tumor staging was based on the 8th American Joint Committee on Cancer (AJCC) criteria. Disease-free survival (DFS) time was measured from the date of the first curative surgery to the date of the first locoregional or systemic relapse, or death without any type of relapse. Overall survival (OS) time was calculated from the date of the first curative operation to the date of the last follow-up or death from any cause. Histologic parameters were evaluated from the H&E-stained slides. Clinical parameters evaluated in each tumor included patient age at initial diagnosis, lymph node status, local recurrence, systemic recurrence, and patient’s survival.Formalin-fixed and paraffin-embedded tissue specimens from 218 cases of primary breast cancer were included. All archival hematoxylin and eosin (H&E)-stained slides for each case were reviewed by one pathologists (Koo JS). The histological grade was accessed using Nottingham grading system . AccordiOn H&E-stained slides of tumors, a representative area was selected and a corresponding spot was marked on the surface of the paraffin block. Using a punch machine, the selected area was punched out and a 3-mm tissue core was placed into a 6 × 5 recipient block. More than two tissue cores were extracted to minimize extraction bias. Each tissue core was assigned with a unique tissue microarray location number that was linked to a database containing other clinicopathologic data.The antibodies and dilution used for IHC are shown in Additional file We evaluated the expression of immunohistochemical markers in cancer cells and peri-tumoral immune cells by light microscopy. Expression in cancer cells was analyzed by various cut-off values  and expression in immune cells was analyzed as follows: negative, no immunostaining; low-positive, stained immune cells ≤30/high power field (HPF); and high-positive, stained immune cells >30/HPF.Interpretation of immunohistochemical staining was performed independently by three researchers . We investigated the proportion of expression (%) in cancer cells according to the above criteria, counted the number of expressed immune cells. Each of three pathologists analyzed the samples and reached a conclusion, which were then analyzed to determine inter-reader concordance. The final results were determined through discussion and multi-view microscopy in the cases that showed discrepancy among the three pathologists.t and Fisher’s exact tests were used for continuous and categorical variables, respectively. The inter-reader concordance rate and concordance rate among PD-L1 antibodies were analyzed using the Kappa-Cohen method. Statistical significance was when p < 0.05. Kaplan–Meier survival curves and log-rank statistics were employed to evaluate time to tumor recurrence and overall survival. Multivariate regression analysis was performed using Cox proportional hazards model.Data were analyzed using SPSS for Windows, Version 12.0 . For determination of statistical significance, Student’s Among the 218 TNBC patients, a desmoplastic type was observed in 138 cases (63.3 %), inflammatory type in 63 cases (28.9 %), and sclerotic type in 17 cases (7.8 %). There was no difference in clinicopathologic factors according to stromal type  showed the highest expression rate in cancer cells  and immune cells (37.6 %) and PD-L1 (SP142) showed the lowest expression rate in cancer cells  and immune cells (19.3 %) for all cut-off values   and PD-L1 (E1L3N) was 6 % (13 cases) and was higher in PD-L1 (28-8)-negative/PD-L1 (E1L3N)-positive (9 cases) than PD-L1 (28-8)-positive/PD-L1 (E1L3N)-negative (4 cases) cells. The discordance rate between PD-L1 (28-8) and PD-L1 (SP142) was 10.6 % (23 cases) and was higher in PD-L1 (28-8)-positive/PD-L1 (SP142)-negative (13 cases) than PD-L1 (28-8)-negative/PD-L1 (SP142)-positive (10 cases) cells. Likewise, discordance was higher in PD-L1 (E1L3N)-positive/PD-L1 (SP142)-negative (15 cases) than PD-L1 (E1L3N)-negative/PD-L1 (SP142)-positive (7 cases) cells  had the highest and PD-L1 (SP 142) had the lowest total positive expression rate. The low-positive rate was higher with PD-L1 (28-8) than PD-L1 (E1L3N); however, the high-positive rate was higher with PD-L1 (E1L3N) than PD-L1 (28-8). In the analysis for expression concordance among the three antibodies in immune cells, the discordance rate between PD-L1 (28-8) and PD-L1 (E1L3N) was 13.8 % (30 cases), with 14 PD-L1 (28-8)-positive/PD-L1 (E1L3N)-negative cases and 16 PD-L1 (28-8)-negative/PD-L1 (E1L3N)-positive cases. The discordance rate between PD-L1 (28-8) and PD-L1 (SP142) was 24.8 % (54 cases), with 46 PD-L1 (28-8)-positive/PD-L1 (SP142)-negative cases and 8 PD-L1 (28-8)-negative/PD-L1 (SP142)-positive cases. Finally, the discordance rate between PD-L1 (E1L3N) and PD-L1 (SP142) was 22.0 % (48 cases), with 44 PD-L1 (E1L3N)-positive/PD-L1 (SP142)-negative cases and 4 PD-L1 (E1L3N)-negative/PD-L1 (SP142)-positive cases  (p < 0.001). For PD-L1 SP142), the concordance rate was higher for positive staining in cancer cells and in negative staining in immune cells  negativity in immune cells was associated with shorter disease-free survival (DFS) and overall survival (OS)   and 64–8PD-L1 showed membranous expression in this study, in agreement with one previous study in TNBC , but theIn this study, the concordance rate between PD-L1 (28-8) and PD-L1 (E1L3N) was high in both cancer cells and immune cells while PD-L1 (SP142) showed low concordance rates with the other two antibodies. Previous studies reported poor concordance  between PD-L1 (E1L3N) and PD-L1 (SP142) in lung cancer , but a hPD-L1 positivity in immune cells (28-8 clone) was an independent prognostic factor in our study, in contrast to previous studies showing good prognosis –53 or poIn breast cancer, tumor infiltrating lymphocytes (TILs) have been linked with good prognosis , as haveClinical trials targeting PD-L1 are underway , 58–60 aThe limitation of this study was the potential difference in results between TMA and the whole cancer tissue section. Breast cancer also shows intracancer heterogeneity, like other types of cancer, and the expression of PD-L1 may cause differences between TMA and the whole cancer tissue section. PD-L1 expression in lung cancer showed a high discordance rate between TMA samples and whole tissue sections . In a prIn conclusion, staining with PD-L1 (clone 28-8), PD-L1 (clone E1L3N), and PD-L1 (clone SP142) monoclonal antibodies in cancer cells and immune cells varied, with the highest staining by PD-L1 (E1L3N) and the lowest by PD-L1 (SP142). The concordance rate among monoclonal PD-L1 antibodies was higher between PD-L1 (28-8) and PD-L1 (E1L3N). PD-L1 positivity in immune cells correlated with a favorable prognosis. To determine the gold standard antibody and the most appropriate cut-off value, further study of the clinical trial group treated with PD-L1 inhibitor is necessary.


The advent of 2D materials integration has enabled novel heterojunctions where carrier transport proceeds thrsough different ultrathin layers. We here demonstrate the potential of such heterojunctions on a graphene/dielectric/semiconductor vertical stack that combines several enabling features for optoelectronic devices. Efficient and stable light emission was achieved through carrier tunneling from the graphene injector into prominent states of a luminescent material. Graphene’s unique properties enable fine control of the band alignment in the heterojunction. This advantage was used to produce vertical tunneling-injection light-emitting transistors (VtiLET) where gating allows adjustment of the light emission intensity independent of applied bias. This device was shown to simultaneously act as a light detecting transistor with a linear and gate tunable sensitivity. The presented development of an electronically controllable multifunctional light emitter, light detector and transistor open up a new route for future optoelectronics. The discovery of graphene and other 2D materials has led to the emergence of a new class of hybrid materials that derive their properties from interfaces between layers of dissimilar materials123567891012131415A wide range of accessible wavelengths, for example, is required for lighting applications as well as wide-bandwidth optical communication. Furthermore, the increasing resolution of displays will necessitate high integration densities of emitters171819Current light emitters have severe limitations that prevent them from supporting these developments. While crystalline semiconductors exhibit composition-dependent emission properties, fabrication challenges arise for each new material combination2324We here demonstrate the potential of metal/insulator/semiconductor heterojunctions between 2D and thin film materials as a composited architecture for efficient and multifunctional optoelectronic devices as shown in 2 layer in the emission process, we first compare the result for both samples of graphene/SiO2/p-GaN and graphene/p-GaN. 2/p-GaN and graphene/p-GaN devices under forward source-drain-bias . The emission peak of graphene/SiO2/p-GaN device around 400 nm can be attributed to the transition from states near the conduction band to acceptor impurity states caused by interstitial Mg atoms near the valence band30312) is characterized by a broad band emission as shown in The fabricated vertical tunneling-injection light emitting transistor on sapphire substrate with the structure shown in SD > 0 V), the bands of p-GaN at the interface near the thin SiO2 are bent upwards and the work function of graphene changes to smaller values  and 5.4 nm blue shift of the p-GaN emission spectrum can be observed .G)G)  is changed from −20 V to 20 V. We observe an on/off ratio of 102 indicating effective suppression of current flow even at large forward biases. Previous reports have shown the high achievable switching speed for graphene-based tunneling transistors which indicates the potential of our approach for high speed emittersThe single layer graphene serves in a two-fold role in our architecture. First, its very small effective electron mass improves the efficiency of tunneling injection2G)G) . To compG). Under positive gate voltage, the device shows higher light emission than that at zero gate voltage. On the other hand, suppressed emission can be observed under negative gate voltage (VG). This behavior shows that our emitter can be operated in a three-terminal configuration as a light emitting transistor. This ability could be exploited in compact light emitters through integration in crossbar matrix arrays for high resolution displays.We now turn to identifying the effect of electrostatic gate control on the light emission process. 2 and p-GaN. This model as shown in the inset of G) is applied. As shown in G) induces hole doping in the graphene that increases its work function and then leads to a larger barrier height The demonstrated light emitting transistor is different from previous LET structures where a gate voltage was used to modulate the carrier transport along a semiconducting channelIn the tunneling-based operating mechanism electrostatic gating introduces a varying density of state in the graphene without affecting the condition of the luminescent material. This advantage is demonstrated when analyzing the bias dependence of the emission spectrum. Commonly, p-GaN exhibits a blue-shift in emission for high bias that is caused by electric field-induced polarizations in the materialThe light emitting transistor structure employed here was found to improve the performance of light emitters. Commercially available LEDs based on multiple GaN/InGaN/GaN quantum wells exhibit a turn on voltage around 2.75 V, which necessitates high operating voltages and increases power consumption. In our device, applying a gate voltage decreases the turn on voltage to 1 V under positive gate voltage and increases it to 12 V for under negative gate voltage. Application of a gate thus increases the efficiency to ~200% compared to ungated conductions. This novel operating mechanism yields unoptimized emission efficiencies of ~50% of commercial multiple quantum well light emitting diodes (MQW-LEDs) , which cIn addition to an enhanced emission, electrostatically controlled tunneling promises faster and more efficient transition between on- and off-states than conventional transistor operation mechanisms7SD > 0 V) and reverse bias (VSD < 0 V) (39D < 0 V) . We obseG) increases, the decrease of the work function of graphene will induce larger band bending of p-GaN. Due to the larger bending the excited electron and holes could be separated much easier and generate a higher photoresponse. The observed linearity in sensitivity with gate voltages improves the accuracy of the sensor compared to traditional photosensors which exhibit an exponential characteristic. The different dependences of photosensitivity on gate voltage (VG) and source-drain bias (VSD) could enable sensors that are adaptable to widely different illumination conditions (through control of VSD) while retaining ultrahigh sensitivity (by modifying VG). Compared with published reports of GaN based photodetectors, the phtoresponse shown here is more than a factor of 10 times higherIn conclusion, we have utilized a heterojunction of 2D materials and ultrathin films in a novel and multifunctional device that serves as light emitter, light detector and transistor. The integration of these unique properties requires less complex fabrication and lend themselves to new applications. It was observed that carrier injection from graphene into a luminescent material created emission from well-defined states and its intensity could be controlled by a gate. The mechanism was found to be based on tunneling through a barrier that could be modified by changing the graphene work function. The application of a gate was shown to enhance the emission efficiency and the performance of the emitter and also enabled multifunctional devices. The presented gate controlled carrier injection mechanism enabled fine control of the device operation, such as tunable detection sensitivity. Finally, the universal applicability of the underlying quantum tunneling mechanism is compatible with a wide range of 2D and quasi-2D materials which enables tailoring of device properties for a bright future for optoelectronics.2 film was deposited on the p-GaN surface as a tunneling barrier via radio frequency (RF) sputtering. Finally, graphene grown by chemical vapor deposition on copper foil was transferred onto the SiO2 with the aid of polymethylmethacrylate (PMMA) following previous reports4244−1 which is the characteristic phonon frequency of the GaN and at 737 cm−1 which was previously found to originate from Mg+2 dopants2 film that protects the graphene and then sputtered 300 nm thick SiO2 film by RF sputtering. To keep the quality of this stacking structure without the damage from one more water transfer of graphene, dry transfer process of graphene onto the thick SiO2 as a transparent top-gate electrode which was followed with previous report−7 Torr and at a constant sample rotation rate. To improve the deposited electrode’s contact resistance, annealing was carried out in a furnace at 400 °C for 20 minutes.A 10 nm thin SiOHow to cite this article: Tan, W.-C. et al. Tunneling-injection in vertical quasi-2D heterojunctions enabled efficient and adjustable optoelectronic conversion. Sci. Rep.6, 31475; doi: 10.1038/srep31475 (2016).


Thermal resistance probed by pulsed photothermal reflectance measurements demonstrated an improved thermal coupling due to functionalization on the graphene–graphene oxide interface. Three functionalization molecules manifest distinct interfacial thermal transport behaviour, corroborating our atomistic calculations in unveiling the role of molecular chain length and functional groups. Molecular dynamics simulations reveal that the functionalization constrains the cross-plane phonon scattering, which in turn enhances in-plane heat conduction of the bonded graphene film by recovering the long flexural phonon lifetime.The high thermal conductivity of graphene and few-layer graphene undergoes severe degradations through contact with the substrate. Here we show experimentally that the thermal management of a micro heater is substantially improved by introducing alternative heat-escaping channels into a graphene-based film bonded to functionalized graphene oxide through amino-silane molecules. Using a resistance temperature probe for  The high thermal conductivity of graphene is considerably reduced when the two-dimensional material is in contact with a substrate. Here, the authors show that thermal management of a micro heater is improved using graphene-based films covalently bonded by amino-silane molecules to graphene oxide. Gao et al.T=13 °C) for a heat flux of 430 W cm−2 after the introduction of a single-layer graphene heat spreader. Moreover, the simulations of graphene heat spreaders were also reported for silicon-on-insulator integrated circuitsR) at the graphene/FLG–substrate interfaceAnisotropic properties of two-dimensional 2D) layered materials make them promising in the application of next-generation electronic devices, among which graphene and few-layer graphene (FLG) have been most intensively studied for thermal management, due to their extraordinarily high in-plane thermal conductivity D layered−1 K−1  of a graphene/FLG–substrate interface is another limiting factor to their thermal performance in devices. Covalent functionalization has been proved to efficiently promote heat transfer between interfaces by introducing additional thermal pathways through the functionalizing molecules18192021222324252627282930313220182526The thermal boundary resistance  bonded to functionalized graphene oxide (FGO) through amino-silane molecules. We probed interface thermal resistance by photothermal reflectance measurements to demonstrate an improved thermal coupling due to functionalization on the graphene–graphene oxide interface and the graphene oxide–silica interface. Molecular dynamics simulations and 2. 3-Amino-propyltriethoxysilane (APTES) has three –Si–O– groups and one –NH2 end, as shown in 2H5 groups of APTES hydrolyse in water and form crosslink bonds with each other. The crosslinked Si–O structure acts as a strong bonding layer between the substrate and GBFs. On the other hand, the –NH2 end of APTES binds onto carboxyl groups on the functionalized graphene film. The FGO layer has a thickness of ∼5 nm. The graphene film was fabricated from chemically reduced GO and can recover relatively high in-plane thermal conductivity after thermal annealing2 substrate. The thermal evaluation device was integrated with micro Pt-based heating resistors as the hotspot and temperature sensorsA GBF bonded to the FGO substrate through silane molecules is shown in I was input into the circuit by applying an outer voltage V in P=V × I. Since the lateral dimension of the hotspot (A=400 × 400 μm2) is much larger than its thickness (260 nm), most of the heat is dissipated through the lateral direction of the hotspot. Hence the heat flux is defined as Q=P/A, and the direction is parallel to the substrate. The calibration relationship between the resistance RT (°C) of the thermal evaluation chip is R(T)=0.21T+112. The temperature measurement uncertainty is ε=±0.5 °C. in situ at the hotspot and compares the thermal performance of the graphene film with and without the functionalization. With a constant heat flux of 1,300±2.3 W cm−2 at the Pt chip, the hotspot temperature decreased from 135±1.2 to 118±1.1 °C (ΔT=17±2.3 °C) with a GBF deposited on non-functionalized GO compared with the case of a bare chip. Such a remarkable temperature decrease is far beyond the measurement uncertainty ΔT>>ε (see T=11±1.9 °C) thanks to the presence of the APTES functionalization. The heat-spreading performance is thus enhanced by ∼57% via the functionalization. We have implemented a finite-element model  GO layer, and the resistance R2 between  GO layer and GBF or SiO2 were measured. The experimental set-up and the procedure of thermal resistance extraction by fitting the photothermal response to the model  method36he model . The nor−1 was used for all three types of molecules. Eph) by atomistic Green's function to characterize the local heat conduction with and without the presence of the molecule (Eph) enables a precise measurement of the atomic-scale molecule-graphene heat transport that the conventional models fail to provide. The phonon transmission functions through different molecules and the associated thermal conductances versus temperature are shown in 11–Si–O3) has the same functional groups as APTES (–N–C3–Si–O3) but has a longer carbon backbone. Such a long chain length has a rather weak impact on the low-frequency phonons due to their very long wavelength but can strongly suppress the phonon transport at high frequencies. On the other hand, when comparing the phonon transmission through the junction of APTES and 3-(Azidopropyl)triethoxysilane (–N−N+N–C3–Si–O3), it is evident to identify a stronger transmission at all frequencies. By comparing the chemical structures in sp2-bonded carbon lattice of graphene to form three-membered heterocycle structures. Given the similar backbone structure, the phonon eigenmodes in the molecule have not been significantly altered, but the transmission is enhanced due to the stronger thermal coupling to the graphene reservoirs. However, the introduction of nitrogen atoms into the sp2 carbon structure can strongly scatter phonons by distorting the structure of the graphene sheet, which leads to a poor thermal performance of the heat spreader.To gain a deeper insight into the impact of molecular structure on the thermal transport along the molecules, we used APTES, 11-Aminoundecyltriethoxysilane and 3-(Azidopropyl)-triethoxysilane as different functional agent on the GO to evaluate their heat-spreading performance on the same thermal test platform. The same concentration of 0.1492, mol kgmolecule . Ξ(Eph) Eph) to characterize the local heat conduction with and without the presence of the APTES molecule. As shown in Eph) for the two adjacent graphene layers without any molecule displays a clear stepwise structure that provides the number of phonon channels. Low-energy phonons (Eph≤10 meV) dominate heat conduction since the adjacent graphene flakes interact only through weak van der Waals (vdW) forces that inhibit the transmission of high-frequency phononsGph, creating more phonon channels through the covalent bond vibrations. This is in line with the transmission calculation of Segal et al.4142Eph=20–100 meV, as was previously observed in an alkane SAM interfaceab initio calculation combined with Green's function  Functional group of the molecule: when the functional agent forms a stronger bond with its substrate than that of the amino-silane molecule, more severe static graphene distortion occurs thus increasing the critical number lc. (iii) Substrate of the graphene film: a highly mismatched substrate yields remarkable surface distortion of the graphene sheets in such a way that two layers of graphene are not sufficient to recover the high basal-plane thermal conductivity.It is clearly seen from y a role : (i) moln and n0 are the phonon occupation numbers out of and at thermal equilibrium. Under the single-mode-relaxation-time approximation, the thermal conductivity is given by Ci and vi are the specific heat per volume unit and the group velocity of the i-th phonon mode. The phonon dispersion of the supported graphene film for ρ=0.081 nm−2 and lG=2, and the extracted relaxation time R for ρ=0.081 nm−2 and lG≥2, as is shown in K is the average vdW coupling constant between the graphene film and its substrate. A previous calculationKZA largely depends on the substrate morphology. The presence of the molecule strongly decreases the film–substrate coupling KZA, thus yielding a longer time We investigate the microscopic origin of the thermal conductivity ion time . The pho0 K ref. . We attrR with the functionalized substrate versus the equivalent APTES molecule number density ρ in κ–ρ relation at low molecular densities). Therefore, the net effect is an increase in the cross-plane thermal resistance between the top graphene films and the lower GO layers compared to the non-functionalization case. In the limit of small number density, we consider the molecules as independent heat conductors connecting the film and the substrate. R agrees well with a reduced model of parallel thermal resistors. Thus, R decreases with the number of molecules. ρ, the interactions among the molecules gain importance and R starts to deviate from the prediction of the parallel-thermal-resistor model, as shown in ρ, the thermal resistance R is lower than that without molecules. However ρ effectively tunes the thermal conductivity of the supported graphene.The cross-plane heat conduction between the top graphene films and the lower FGO layers is governed by the competing effects of (i) the intercalation of the molecules that tends to weaken the interlayer coupling of graphene therefore increasing the cross-plane thermal resistance and (ii) the additional heat channels introduced by the molecules covalently bonding the graphene sheets that tend to facilitate the cross-plane thermal coupling. We investigated the in-plane thermal conductivity et al.et al.The correlation between the in- and cross-plane thermal transport of 2D materials has been recently studied from both theoretical151314−2 can be observed in microprocessors−1 K−1 for Al and Cu, and the low cost. However, for these technologically important metals, the thermal conductivity of a 100-nm-thick film constitutes only ∼20% of the thermal conductivity of the bulk52−1 K−1 , resulting in a large total temperature decrease of 28 °C when operating at 1,300 W cm−2. In comparison, the highest ΔT achieved so far at a similar high heat flux of 1,250 W cm−2 is 19 °C by using a miniaturized thermoelectric coolerT is robust especially at higher heat flux beyond 1,000 W cm−2, as shown in Our work tackles the key technological challenge of efficient thermal management in the industry of the next-generation integrated circuit. The excessive heat from the hotspots accelerates the failure rate and slows down the operating speed of microelectronic devices50K−1 ref. , which i2SO4 ; NaNO3 (2 g); and KMnO4 (12 g) were used to prepare GO dispersion by following Hummers' methodL-ascorbic acid, and polyvinyl alcohol was also added for better suspension. The GBF was prepared via vacuum filtration with polycarbonate filter paper with a pore size of 3 μm. The film thickness was controlled by the filtration volume and the graphene concentration in the suspension. After dissolving the filter paper in pure acetone, a freestanding GBF was obtained. The thickness of the GBF was measured as ∼20 μm. Raman spectroscopy data of the GBF before and after the spin-coating with FGO are shown in (i) Sample synthesis: graphite ; HClassical molecular dynamics simulations were performed using LAMMPSEph) the thermal conductance due to the phonon could be calculated as T refers to the mean temperature of the system, fBE is the Bose–Einstein phonon statistics, and ħ represents the reduced Plancks constant. The thermal conductance due to the electronsTel as fFD is the Fermi–Dirac electron statistics.Thermal conductance How to cite this article: Han, H. et al. Functionalization mediates heat transport in graphene nanoflakes. Nat. Commun. 7:11281 doi: 10.1038/ncomms11281 (2016).Supplementary Figures 1-19, Supplementary Table 1, Supplementary Notes 1-14 and Supplementary References.


With the recent development of commercial-level single-crystal graphene layers, the potential for manufacturing household graphene-based devices has improved, but significant challenges still remain with regards to patterning the graphene into devices. In the case of graphene supported on a substrate, traditional nanofabrication techniques such as e-beam lithography (EBL) are often used in fabricating graphene nanoribbons but the multi-step processes they require can result in contamination of the graphene with resists and solvents. In this letter, we report the utility of scanning helium ion lithography for fabricating functional graphene nanoconductors that are supported directly on a silicon dioxide layer, and we measure the minimum feature size achievable due to limitations imposed by thermal fluctuations and ion scattering during the milling process. Further we demonstrate that ion beams, due to their positive charging nature, may be used to observe and test the conductivity of graphene-based nanoelectronic devices  Moreover, as a direct-fabrication method, He+ milling requires no resists, developer or wash to fabricate features yet can achieve feature sizes comparable to or smaller than EBL. The smaller interaction volume of the helium ion beam with the sample results in reduced proximity effects compared to EBL, and improved sputtering yield in the surface layerScanning helium ion microscopy (SHIM) and ion milling (performed with the same instrument) has emerged as a tool capable of fabricating graphene devices1in situ, thereby allowing for the estimation of the quality of a graphene conductor. Furthermore, SHIM provides unprecedented high contrast images due to its shorter range and superior ion-generated secondary electron (iSE) yieldin situ by varying the size and width of the connecting strip in each device.By varying the length and width of the strip and then imaging the entire structure, we can directly observe how the electrical conductivity changes as a function of pad brightness. As the ion beam impinges on the pad, secondary electrons are ejected from the pad, yielding a significant net positive charge on the pad. Unlike in a scanning electron microscope (SEM) the positively charged ion beam is not capable of compensating for lost secondary electrons. While graphene’s work function can be tuned across the charge neutrality point by an electric field effect+ milled graphene pads (left panel) that have been acquired following the fabrication process. Corresponding SEM images of the exact same devices (right panel) were used to directly compare the effects of imaging with a negatively charged beam. In + do not neutralize at the graphene due to their velocity, but neutralize deeper in the Si wafer. In general, these strips are compensating for lost secondary electrons, and conducting around 1 pA. The transition from bright to dark occurs when the strip is no longer capable of carrying 1 pA. As the width of the strip is increased to 12 nm . Also, this noise (at room temperature) may place a limit on the smallest feature sizes that can be used for graphene circuitry supported by SiO2, which in our case is ~10 nm. This noise does not represent the distribution of electrons within the pad – rather, this noise reflects the instantaneous conductivity of the strip at the moment that pixel is being collected. Of particular interest is the subtle noise in the pad in A second class of graphene devices was explored in order to investigate the impact of the length of the conducting strip. These devices were fabricated to include two smaller regions (i and ii) that have been completely electrically isolated in order create conducting-insulating composites. Similar to in situ conductivity can be observed in micron-size devices . The minimum feature size achievable using SHIM is a product of several factors. There are the typical problems of instrumental drift and electronics stability in the raster scan. The SHIM described here is located in a cleanroom but not a low-noise microscopy suite, and undoubtedly experiences some mechanical noise. Also specific to this technique are the beam shape and ion backscattering which create a larger interaction volume between the beam and substrate than intended. The minimum width of the connecting strips reported here are likely determined by the density of defects along the edges of the strips, which are in turn determined by impingement by ions that escape outside of the illuminated region, either through an imperfect beam profile (tailing) or backscattering. Our experiments indicate that for a very short strip, the minimum width is about 10–12 nm. For longer strips, minimum width increases because the resistivity of strip is in part due to the sum of all defects along the length of the strip. This experiment has an additional limitation because as a focused ion beam technique, the quality of the device is dependent on operator skill in aligning and focusing the instrument.2 sample preparation method does not exclude the possibility of having contaminants from the residual PMMA resist that may be present after the transfer and annealing process (see Methods section). However, for non-CVD graphene, or any 2D material prepared without the need for transfer using a polymer, this approach may entirely exclude unnecessary exposure to solvents and resists.Grain boundaries, which are a common occurrence in CVD-grown graphene can also affect the conductivity of graphene12in situ. The minimum width of the conducting strip attainable with scanning helium ion lithography is limited by the coupled vibrational modes arising from thermal fluctuations within the device. Electrical conductivity through a supported graphene strip is limited by the width of the strip. As a direct-fabrication technique, helium-ion lithography will be a preferred method of patterning graphene-based circuits and devices smaller or comparable in size to EBL, and will eliminate the need for the multistep processes involved in EBL.In summary, we have demonstrated that positively charged ion beams from a SHIM may be used to fabricate devices in supported graphene on an insulator, and then detect conductivity in such devices et al.142 in Ar for 30 mins. Graphene growth was performed by addition of methane with a gradual increase in concentration from 10 to 20, to 40 ppm for 30 mins in each step. After growth, Microchem PMMA 495A4 solution was spin-coated at 2000 rpm on top of graphene on copper foil. Graphene from the back side of copper was etched away by oxygen plasma and copper was dissolved by 1 M FeCl3 in 3% HCl. Graphene/PMMA sandwich floating on water surface was washed by DI water and transferred onto the SiO2 substrate. PMMA was dissolved in acetone with subsequent annealing at 550 °C to remove the PMMA residue.Single layer graphene was synthesized using a method by Vlassiouk +/Ne+ beam at a field of view and pixel spacing that yielded a fluence of ~1 × 1019 ions/cm2 for He+ beam and ~5 × 1017 ions/cm2 for the Ne+ beam. Subsequent high-resolution images were acquired at the same field of view using a 50 μs dwell time.Scanning helium ion microscopy and lithography on single layer graphene was performed using a Zeiss ORION Nanofab He/Ne ion microscope, operating at an accelerating voltage of 30 kV and a beam current of 4.2 pA. All the graphene devices were fabricated using the ion microscope’s built-in patterning software and imported bitmaps. Each milled area was exposed to the HeScanning electron microscopy secondary electron images of the exact same graphene devices were obtained using a Zeiss MERLIN VP SEM equipped with an in-lens detector operating at 3 kV.How to cite this article: Iberi, V. et al. Maskless Lithography and in situ Visualization of Conductivity of Graphene using Helium Ion Microscopy. Sci. Rep.5, 11952; doi: 10.1038/srep11952 (2015).