Drosophila and mice, as loss of the scaffolding factor SIN3 or the associated HDAC results in lethality. The objective of this study is to elucidate contributions of SIN3 complex components to these essential processes. We used the Drosophila model organism to carry out a systematic functional analysis of the SIN3 complex. We find that SIN3 associated proteins are essential for viability and cell proliferation during development. Additionally, tissue specific reduction of SIN3 complex components results in abnormal wing development. Interestingly, while knockdown of each factor resulted in similar phenotypes, their individual effects on recruitment of SIN3 to polytene chromosomes are distinct. Reduction of some factors leads to large changes in the morphology of the chromosome and/or greatly reduced SIN3 binding. These findings suggest that while individual SIN3 complex components work through distinct molecular mechanisms, they each make a substantial contribution to the overall function of this highly conserved histone deacetylase complex.Establishment and maintenance of histone acetylation levels are critical for metazoan development and viability. Disruption of the balance between acetylation and deacetylation by treatment with chemical histone deacetylase (HDAC) inhibitors results in loss of cell proliferation, differentiation and/or apoptosis. Histone deacetylation by the SIN3 complex is essential in  Histone acetylation is one of the earliest recognized modifications to correlate with transcriptional activity2. The enzymes that control acetylation levels, histone lysine acetyltransferases (KATs) and histone deacetylases (HDACs), typically assemble into large protein complexes3. Proteins within these enzymatic complexes serve different roles. In some cases, they are critical for recruitment to specific loci. For example, the ING2 subunit of the mammalian SIN3 complex contains a binding domain that recognizes histone H3 trimethylated at lysine 4 (H3K4me3) and is important for recruitment of HDAC1 to target genes in response to DNA damage4. In other cases, the complex subunit interacts with a transcription factor allowing for recruitment to specific gene regulatory regions. Mammalian SIN3 was first isolated as a binding partner of the DNA-binding transcriptional repressor Mad5.The template for eukaryotic transcription is chromatin, comprised of nucleosomes, DNA wrapped around a core of histone proteins. Histones are subject to post-translational chemical modifications, which impact nucleosome structure and protein accessibility to the chromatin templateDrosophila melanogaster, is found in multiple multi-subunit protein complexes. These complexes include the nucleosome remodeling (NuRD) complex, corepressor for REST (CoREST) and the SIN3 complex, named for the scaffolding factor SIN36. To add to the complexity, multiple distinct SIN3 complexes are present in a single organism7. These SIN3 HDAC complexes vary in subunit isoform specificity and accessory factor composition. Efforts to interrogate the complex components through a set of identical assays to examine the possible roles of individual factors in governing specific biological processes are limited.The histone deacetylase HDAC1, formerly named RPD3 in 5. Drosophila have a single gene, Sin3A, which has more sequence similarity to mammalian SIN3A compared to SIN3B, and from which multiple protein isoforms are produced8. Due to the presence of multiple paired amphipathic helical domains, known protein-protein interaction modules, SIN3 is believed to serve as the scaffold for assembly of the SIN3 HDAC complex9. While an HDAC was the first histone modifying enzyme found to interact with SIN310, subsequent work from multiple laboratories demonstrated that the histone demethylase KDM5 in mammals, dKDM5/LID in Drosophila, interacts with a subset of SIN3 complexes11. In studies performed by independent research groups using different organisms, a number of accessory factors including BRMS1, PF1, FAM60A, ING1/2, MRG15, RbAp46/48, RBP1, SAP180, SAP130, SAP30, SAP18 and SDS3 have been reported as members of a SIN3 complex, in addition to the two enzymatic components13. In an earlier proteomic study to identify isoform-specific SIN3 complexes, we found that the following proteins interact with each of the two major Drosophila SIN3 isoforms: HDAC1, CG14220 (SDS3), hat-trick (htk) (ARID4B), CG7379 (ING1/2), Sap130, CG3815 (PF1) and BRMS114. Additionally, we determined that Caf1-55, dKDM5/LID and CG15356 (EMSY) are factors that predominantly interact with the largest and most widely expressed SIN3 isoform, SIN3 220. Apart from the enzymatic activity of HDAC1 and dKDM5/LID, little is understood about the role of the accessory factors in the context of SIN3 complex function.SIN3 is a highly conserved protein that is expressed in organisms from yeast to mammals. Mammals have two genes encoding highly similar SIN3 isoforms, SIN3A and SIN3BDrosophila melanogaster. We sought to identify common and unique phenotypes resulting from reduced expression of complex components, either individually or in combination with reduction of the SIN3 scaffolding factor. We also set out to determine which factors participate in recruitment and/or stabilization of SIN3 onto chromatin, which is believed to be a pre-requisite for complex regulatory activity. Our data indicate that while reduced expression of the majority of the subunits results in highly similar biological phenotypes, only a subset of the factors impact chromatin recruitment. These findings highlight functional specialization of SIN3 complex components.To address this lack of comprehensive understanding, the objective of this study was to perform a systematic analysis of a defined SIN3 complex in 14 and are listed in Table 15. This dsRNA is processed by the RNAi machinery and will lead to degradation of the endogenously expressed target mRNA; ultimately resulting in reduced protein expression16. For these studies, we utilized a variety of GAL4 driver lines to express GAL4, and thus induce RNAi knock down, in different tissues. For all experiments, we compared the knockdown fly with a control that was the progeny of the same GAL4 driver crossed to either UAS-GFPRNAi or UAS-mCherryRNAi. These control flies have expression of GAL4 and induction of the RNAi pathway, but no specific Drosophila gene is targeted for knock down. As an additional control, to reduce the possibility that observed phenotypes are due to an off-target effect, when available, we utilized more than one RNAi line to target different regions of the targeted RNA. To validate the efficiency of knockdown, we performed RT-qPCR analysis  gene upstream of the GAL4 coding sequence. Ser has a complex expression pattern with prominent expression in the dorsal compartment of the wing imaginal disc18. We previously determined that this selected driver results in expression of GAL4 in the wing imaginal disc throughout larval development in a pattern that is largely consistent, however that is somewhat broader than, the reported expression of Ser19. We typically observed RNA levels reduced to about 60-70 % of the level in wild type wing discs in these heterogeneous cell populations.To examine functions of SIN3 complex subunits, we measured a number of phenotypic traits in flies with reduced levels of SIN3 complex components compared to wild type controls. The complex components investigated included factors previously determined to associate with the largest isoform of SIN3Drosophila viability. We used the Act5C-Gal4 driver line for ubiquitous expression of GAL4. Progeny of the cross of this driver to the UAS-targetRNAi line results in ubiquitous knockdown of the targeted complex component. Knockdown of each gene coding for a SIN3 complex component resulted in no adult progeny , Sap130, CG15356 (EMSY), CG3815 (PF1), CG7379 (ING1/2) and CG14220 (SDS3), are also required for viability.We first tested whether each of the complex components is essential for ny Table . Thus, eDrosophila wing is a nonessential tissue and has been used by us and others to investigate factors involved in development and cell cycle control25. We previously determined that SIN3 and dKDM5/LID are necessary for development of a normal wing21. For this study, we have repeated those experiments to allow for direct comparison with knockdown of the other complex components. As in those prior studies, here we used the Ser-Gal4 driver to induce RNAi knockdown of the SIN3 complex components in wing imaginal discs and observed the resulting adult wing morphology.Because ubiquitous RNAi knockdown resulted in loss of viability, we utilized a conditional knockdown system to further investigate SIN3 complex components. The Sin3A or lid. HDAC1 and Caf1-55 are each found in additional complexes apart from SIN3. HDAC1 is a component of other HDAC complexes including NuRD and CoREST6. Caf1-55 associates not only with HDACs, but also KATs and chromatin assembly factors26. It is possible that the loss of each of these factors impacts the function of not only the SIN3 complex but also the other chromatin organization complexes in which these factors are found. The severe disruption of wing morphology is thus likely due to effects on multiple developmental pathways. The other factor found to yield the blistered phenotype upon knockdown is CG3815 (PF1). Little is known about PF1 in flies and work in other organisms has focused on the role of this factor as a component of the SIN3 complex. In triple negative breast cancer cells, disruption of the interaction between PF1 and SIN3A altered the expression of cancer-related genes and importantly, decreased the metastatic potential of cancer cells in a mouse tumor model27. These authors additionally found that knockdown of PF1 phenocopies the effects of disruption of the PF1 and SIN3A interaction, indicating that a main role for PF1 is as part of the SIN3 complex. The data from the fly wing development assay suggests that, in flies, PF1 might function apart from SIN3. Interestingly, the blistered wings are only found in the male CG3815 (PF1) knockdown flies. The wings of the female CG3815 (PF1) knockdown flies are curved, similar to those of the Sin3A knockdown flies  and Brms1 knockdown female flies had curved wings, some had straight wings supporting the idea that females are less sensitive to reduced SIN3 complex function as compared to males. Like wild type strains, progeny of the control cross had straight wings. When multiple RNAi lines were tested for an individual gene, the resulting phenotypes were similar across the lines. These results suggest the phenotypes are not due to off-target effects but rather are due to reduced expression of the targeted gene.We first tested whether the individual complex component was necessary for wing development. Individual reduction of HDAC1, Caf1-55 or CG3815 (PF1) resulted in severely malformed wing phenotypes Fig. . The winSin3A wing specific knockdown suppresses the curved wing phenotype in female flies by not in males21. Additionally, male flies that carry a mutation in lid have shorter life spans and a higher sensitivity to the presence of paraquat-induced reactive oxygen species as compared to the females with the mutation28. Mutations in HDAC1 have also been found to yield sex specific phenotypic variation. Flies carrying certain specific alleles of HDAC1 survive at significantly lower rates as compared to their genetically identical females counterparts29. We do not have a definitive answer as to the mechanism controlling the sex specific differential sensitivity to mutation in SIN3 complex components. It is possible that the differences are related to the epigenetic variation linked to the presence of the heterochromatic Y chromosome in male flies and/or the single X chromosome, which is subject to dosage compensation controlled in part by histone acetylation30. In addition to the possible impact of dosage compensation mechanisms, male and female flies have differences in their genome-wide chromatin landscape31. It is possible that the SIN3 histone modifying complex controls some of these sex specific differences, which then differentially affect genes important for normal wing morphology.We note that sex specific phenotypic differences comparing flies with mutation or reduced expression of SIN3 complex components to wild type have been reported previously. For example, we previously observed that overexpression of dKDM5/LID in the context of GAL4 expression pattern controlled by the Ser-Gal4 driver is complex and thus the amount of knockdown is likely variable across the wing imaginal discs cells, with some cells affected very little and others a lot.An additional observation is that although we determined efficiency of knockdown in the wing imaginal discs at approximately 60 and 70% of control levels  knockdown was observed when htk (ARID4B) and Sin3A were simultaneously reduced by RNAi. These data suggest that the majority of the factors work through a similar mechanism to impact the same pathways for wing development. Reduction of HDAC1, CAF1-55 and CG3815 (PF1) leads to flies with a more severe phenotype compared to the Sin3A knockdown curved wing, which is not modified when combined with SIN3 reduction. This finding suggests that those three factors have function outside of the SIN3 complex. As noted, HDAC1 and CAF1-55 are part of multiple non-SIN3 containing chromatin modification complexes26. The number of male flies for the Caf1-55KK, Sin3A double knockdown genotype is low, possibly due to lethality in the pupal stage as we observed pupal cases from which flies did not eclose. We do not have an explanation as to reason why the wing specific knockdown would result in male lethality. Taken together, the results indicate all tested components of the SIN3 complex are required for normal wild type development of the Drosophila wing. Because wing specific RNAi of each factor results in a similar curved wing phenotype and they were found to biochemically associate in a previously conducted co-immunoprecipitation experiment14, it is possible that most of the components work as part of a single complex. Because of the distinct phenotypes observed in the wing specific RNAi knockdown flies, HDAC1, CAF1-55 and CG3815 (PF1) likely have some activity outside of their roles as members of the SIN3 complex. Whether other complex factors work outside of their association with SIN3 cannot be ruled out and future work identifying possible non-SIN3 containing additional complexes will shed light on this possibility.We next examined the effect of reducing levels of SIN3 along with a second component of the complex. We previously determined that dual knockdown of eny Fig. . The winDrosophila S2 cultured cell line32. It was subsequently determined that SIN3, HDAC1, the histone demethylase dKDM5/LID and CAF1-55 are also critical for cell proliferation and cell survival during Drosophila development35. To investigate if other components of the complex are necessary for cell proliferation during development, we performed clonal analysis in wing imaginal disc cells. For these studies, we utilized the heat shock “flip-out” GAL4 driver (Actin > GAL4) to activate expression of a UAS-linked RNAi target along with a GFP marker in random clones of cells via the heat-shock induction of FLP recombinase at a precise time point in larval development. Once activated, cell clones will continue to have reduced target gene expression and the effect of this reduced expression on cell proliferation can be measured by the size and number of the GFP positive cells. Reduction of all SIN3 complex components resulted in fewer GFP positive cells. In some instances, the clones are smaller in size and for others, the clones are fewer in number, relative to the clones produced in the mCherry RNAi controls  and Brms1 led to reduced cell proliferation in the developing wing tissue. This finding suggests that their function in these cells is similar to that of other complex components; they are important for cell proliferation rather than functioning as tumor suppressors.The human homologs of a number of SIN3 complex factors, including ARID4B, EMSY, PF1, ING1/2 and BRMS1, are known to function in cell proliferation and the level of their expression has been linked to growth of cancer cells. Elevated expression of ARID4B was found to promote metastasis in a mouse model of human breast cancer9. We asked if any of the complex components are critical for recruitment of SIN3 to chromatin. For this analysis, we used polytene chromosomes prepared from Drosophila larval salivary glands. We utilized the feb36-GAL4 driver line to induce RNAi and reduce the level of the SIN3 complex factor in salivary glands45. Chromosome preparations from third instar larval salivary glands were probed with antibody to SIN3 and also stained with DAPI to visualize the DNA.SIN3 is described as corepressor as it does not contain a DNA-binding domain for gene targeting. To recruit the complex to promoters where it functions to regulate gene expression, sequence-specific DNA-binding transcription factors interact with SIN3 and/or components of the complexHDAC1 led to disruption of polytene chromatin structure. The salivary glands were smaller and overall DAPI staining of nuclei was low compared to wild type controls. The chromosomes lack integrity; the banding pattern is disrupted and less distinct as compared to control chromosomes . cDNA was generated from total RNA using the ImProm-II Reverse Transcription System (Promega) with random hexamers. The cDNA was used as template in a quantitative real-time PCR (qPCR) assay. The analysis was performed using ABsolute SYBR Green ROX master mix (Fisher Scientific) and carried out in a Stratagene Mx3005P real-time thermocycler. Primers used for analysis are given in Supplemental Table Whole flies were imaged at 30x magnification using an Olympus DP72 camera coupled to an Olympus SZX16 microscope. Wings were imaged at 80x magnification using a SPOT RT color camera coupled to a Leica MZ125 microscope.mCherryRNAi-TRiP males or UAS-SIN3 complex component RNAi to generate random GFP positive clones via the heat shock flip-out system. Embryos were collected for four hours and then after 48–52 hours, 2nd instar larvae were subjected to heat shock at 37 °C for two hours. After returning to 27 °C, wandering 3rd instar larvae were dissected and immunostained 120 hours after egg laying19. Antibody against GFP  followed by sheep anti-mouse Alexa 488  was used for staining. Visualization and imaging was done using an Olympus BX53 compound microscope with a DP72, 12.8 megapixel camera. Images were processed using Olympus CellSens software. Clones were analyzed in a minimum of 20 discs per genotype using Photoshop CS to count the GFP positive pixels for each immunostained disc and comparing that number to the total number of pixels in the DAPI-stained disc48.hsFLP;Act5C > CD2 > Gal4,UAS-EGFP virgin females were crossed to 49. A primary antibody against SIN3 (1:1000)50 was followed by a secondary antibody Alexa Fluor 594 (1:400) (Life Technologies). Chromosome spreads were prepared from a minimum of three independent parental crosses and representative images are shown. A minimum of four slides of each control and experimental genotype were prepared at the same time and photographed using identical exposure times. Chromosomes were imaged using an Olympus BX53 compound microscope with a DP72, 12.8 megapixel camera at 400x. Five to ten chromosomal spreads were chosen from each slide for imaging, and images were processed identically using Olympus CellSens software. To quantify SIN3 immunofluorescence signals on polytene chromosomes relative to DAPI intensity, we used a program developed in Matlab 7.4.0 from the protocol outlined here51. Briefly, batches of control and experimental images, processed identically, are input into the program, which uses two fluorescent channels (DAPI and Alexa Fluor 594). The program applies a mask to each chromosome, removing non-chromosomal antibody staining from the pixel-based quantification.Polytene chromosome preparation and staining methods were modified from the protocol outlined heret test using GRAPHPAD software. http://www.graphpad.com/quickcalcs/index.cfm.All significance values were calculated by the two sample Student’s Supplementary Information


Graphene transparent conductive electrodes are highly attractive for photodetector (PD) applications due to their excellent electrical and optical properties. The emergence of graphene/semiconductor hybrid heterostructures provides a platform useful for fabricating high-performance optoelectronic devices, thereby overcoming the inherent limitations of graphene. Here, we review the studies of PDs based on graphene/semiconductor hybrid heterostructures, including device physics/design, performance, and process technologies for the optimization of PDs. In the last section, existing technologies and future challenges for PD applications of graphene/semiconductor hybrid heterostructures are discussed. A photodetector (PD) is a device for detecting light, the underlying mechanism is conversion of light to an electrical signal called a photocurrent (PC). PDs are used in a wide range of academic and industrial fields, including image sensing, optical communications, environmental monitoring, and chemical/biological sensing ,2,3. In A wide variety of optoelectronic devices based on graphene are still being studied, and some of them have already reached a level of competitiveness comparable to conventional semiconductor devices ,14. HoweThe emergence of the graphene/SiC interface in 2009  as the pChemical vapor deposition (CVD)-grown graphene was first employed in graphene/Si heterojunctions, resulting in 65% external quantum efficiency (EQE) , which sB) to prevent the electrons from flowing from the semiconductor to the graphene. Depending on the energy of the radiation (E) = hν (h = Planck’s constant and ν = photon frequency), PC is generated based on two main mechanisms. (1) When ΦB < hν < semiconductor band gap (gE), the electrons are excited from the graphene and are injected into the semiconductor; (2) When hν > gE, the electron-hole pairs are generated in the depletion layer of the semiconductor. The PDs are operated based on the photovoltaic or photogating effect depending on their device configurations.Graphene/semiconductor hybrid heterostructures can be formed by directly combining graphene with one-, two-, and three-dimensional semiconductors through relatively simple manufacturing processes. When graphene and a semiconductor material form a junction, charge transfer occurs until the Fermi energy levels of the two materials coincide because their work functions mismatch. Meanwhile, the free charge region inside the semiconductor near the interface is depleted by the charge transfer, resulting in the formation of the built-in electric field. In case of graphene/n-type semiconductor junction, the immobile positive charge formed by the depletion of electrons bends the semiconductor band near the interface upward. More details are given in previous reports ,102. In J-V) characteristics under dark. A PD operating in this mode is named as a photodiode, whose linearity, detectivity, and sensitivity are maximized due to the lowest dark current. In contrast, the responsivity (R) is usually low due to the lack of internal gain. The PD can be also operated in reverse bias (photoconductive mode), where the built-in external electromagnetic field enhances the separation efficiency of the electron-hole pairs, thereby reducing the response time due to the shortening of the passage time and the decrease of the diode capacity. The photovoltaic mode is more suitable for precise photodetection with the photoconductive mode being better suited for high-speed applications.The PD can be operated in a photovoltaic mode (=zero bias). The heterostructures generally exhibit rectifying behaviors, i.e., nonlinear current density-voltage  curve of the graphene transistor upon illumination. In addition, the direction of the curve shift represents the polarity of the captured carriers, and the photogate effect can induce positive or negative PC by gV.Photogating effects occur in the heterostructures-based phototransistors. The operating mechanism of the phototransistors is as follows. Once electron-hole pairs are generated in semiconductors, one type of carrier is transferred to the graphene, and the opposite-type carrier is trapped in the semiconductor. The trapped charges act as a local gate, thereby effectively modulating the Fermi level of graphene through capacitive coupling. This induces more carriers and consequently modulates the electrical conductance. In addition, the transferred free carriers circulate many times in the graphene channel within their lifetime, which contributes to higher photoconduction gain. The gain is expressed as gain = lifetime/transit, where transit is the transit time of the photogenerated carriers through the graphene channel. Shorter transit time and longer lifetime should be obtained to achieve higher gain. Nevertheless, the lifetime of the photocarrier is determined by the response time, which is directly related to the carrier recombination process. Thus, a high gain will extend the response time. A phototransistor can show much higher R is defined as the ratio of PC to incident light power, which indicates how efficiently a PD responds to an optical signal, as expressed by the equation R = ph/LlightI, in a unit of A/W, where phI is the PC and lightL is the incident-light power. R is proportional to EQE of a PD, meaning the conversion rate from photons to electrons/holes. Therefore, EQE is related to R based on the following equation: λ is the incident-light wavelength, q is the absolute value of electron charge, h is the Planck constant, and c is the speed of light.D* is the weakest level of light which can be detected by the device, and is determined by R and noise of a PD. D* is given by D* = (A1/2·∆f)n× R/i. Where A is the effective area of a PD, ∆f is the electrical bandwidth, and ni is the noise current. The performance of D* is limited by three kinds of noise such as dark current (DC), Johnson noise, and thermal fluctuation noise , w, w24], wR of 0.28 A/W, corresponding to an internal quantum efficiency of 10%, much higher than a typical Schottky junction (~1%). Usually, the R in the UV region is at least 1–2 times lower than that in the visible-near infrared (NIR) region due to the weak light absorption and relatively short carrier lifetime of the graphene. TiO2 [2O3 anti-reflection layers [2O3 antireflective layer has been fabricated for UV photodetection, resulting in 0.2 A/W R, 5 ns response time, 1.6 × 1013 cm·Hz1/2/W D* in wavelength range from 200 to 400 nm, comparable to those of state-of-the-art Si, GaN, and SiC Schottky PDs. The gapless and semi-metallic properties of graphene are very promising for its application for broadband photodetection from microwave to UV wavelengths. Graphene/Si Schottky PDs are also very efficient in fast carrier separation due to the presence of the built-in electric field without suffering from a sharp reduction in the lifetime of photocarriers under 1550 nm illumination , therebyne. TiO2  and Al2OR and response time of the PD at the same time, as shown in 3) has been widely used for improving the performance of the PD in organic electronics due to its high work function [2. The functionalization of the MoO3 layer on graphene led to a significant performance enhancement of the graphene/Si PDs [12 cm·Hz1/2/W D*. However, the interface between graphene and Si normally contains relatively-high density of surface states, resulting in large leakage current, which can limit the overall D* of the graphene/Si PD [2/Si PD showed 0.0078 pW·Hz−1/2 NEP at room temperature, resulting in D* up to 4.08 × 1013 cm·Hz1/2/W. Unlike traditional transparent electrodes, such as indium tin oxide or ultrathin metals, unique absorption properties of graphene in the UV range contribute to increasing the lifetime of hot electrons, thereby leading to large PC. Recently, a variety of graphene/Si-based structures have been proposed to improve the device performance. A new approach of integrating graphene/Si PD with two-dimensional (2D) Pt nanoparticles (NPs) has proven to enhance the function ,105 and e/Si PDs , based one/Si PD ,25. Recene/Si PD . The gra2/Si capacitors for high-performance optical detection [R/D* of 3 A·W−1/3.5 × 1012 cm·Hz1/2·W−1 in the visible range. Here, the PC exceeded the forward current because the photogenerated minority carriers accumulated in the Si/SiO2 interface diffused to the graphene/Si junction. According to another approach, the PC in graphene-based hybrid photodiodes was not only very efficiently generated in the graphene/Si Schottky barrier region but also in the adjacent graphene/SiO2/Si region due to the reverse bias voltage [2/Si region rose sharply by about one order of magnitude at voltages exceeding a certain threshold bias, regardless of the incident laser power. In Si under SiO2, the inversion layer not only provided a low resistance path for minority charge transport, but also acted as a passivation layer on the surface states of SiO2 to effectively collect the charge carriers, resulting in improved PC. These results provide guidelines for establishing the fundamental mechanism of PC generation in graphene-based hybrid optoelectronic devices and for designing hybrid photodetectors based on combinations of two- and three-dimensional materials. The PD was also fabricated by directly growing graphene nanowalls on a Si substrate instead of using graphene layers CVD-grown via a metal catalyst [D* was 5.88 × 1013 cm·Hz1/2/W, higher than ever reported in the planar graphene/Si based PDs, thanks to the ultra-low current noise of 3.18 fA·Hz1/2, which can be attributed to the excellent junction quality of 0.69 eV barrier height and 1.18 ideal factor. Other principal PD parameters were 107 PC/DC (light/IdarkI) ratio, 40 μs response time, 8.5 kHz cutoff frequency, and 0.52 A/W R. In addition, graphene/Si Schottky diodes were fabricated in parallel with graphene/SiOetection , thereby voltage . The PC catalyst . Resulti6light/IdarkI ratio and high response speed , better than those of pure graphene/Si NWs heterojunction [Two-dimensional Si nanostructures such as porous Si (PSi) and Si NWs have high ratio of surface to volume/light absorption, very useful for their application in graphene-based PDs. junction , as show2 matrix allow for large light absorption in the visible range based on the quantum confinement effect [3-doped graphene/SQDs: SiO2 multilayers (MLs)/Si PDs were successfully fabricated to show 0.35 A/W R and 109 cm·Hz1/2/W D* near the wavelength of 600 nm, indicating considerable performance improvement in the visible region due to the strong photoresponse of SQDs [3 doping concentration (Dn). However, the performance of the PD is limited by the surface of the AuCl3-doped graphene on which light is first irradiated due to large reduction in transmittance by the Au NPs formed by the doping. Recently, long-term stabilities have been remarkably enhanced by employing bis(trifluoromethanesulfonyl)-amide (TFSA) with low resistance and high transparency as a dopant for graphene, as shown in R and D* of the TFSA-doped graphene/SQD: SiO2 MLs/Si PDs with 0.413 A/W and 1010 Hz1/2/W at the peak wavelength of 630 nm under low voltage (−2 V), respectively, significantly better than the AuCl3-doped graphene PDs, indicating minimization of the optical loss by the use of TFSA. On the other hand, graphene/Si Schottky junction PDs with SQDs on top of the graphene, as show in −1/2 NEP, 7.4 × 109 cm·Hz1/2/W D*, and response time ≤ 25 ns, possibly resulting from significant reduction of the optical reflections, contributed by the SQDs, as shown in Si is widely used as a broadband PD, but is limited in its use for wider applications due to the inherent indirect bandgap nature. Especially, graphene/Si-based PD exhibits fast photoresponse in the UV and NIR regions, but not in the visible region. One way to overcome this drawback is to form Si quantum dots (SQDs) ,108. SQDt effect ,38. Rece of SQDs ,38. The light/IdarkI ratio (105) under UV illumination. In another approach, UV PDs were successfully fabricated by transferring large-area graphene TCE on vertically-standing GaN NW array as a light absorbing media, as shown in R of 25 A/W at 357 nm. These results represent a potential application of the graphene/GaN junctions in various optoelectronic devices operating in the UV region.GaN is a wide direct bandgap semiconductor with excellent optical/electrical properties and high chemical stabilities at high temperature, and has been employed in various PD applications. Graphene/GaN heterostructures are useful for light detection in the UV/visible region based on Schottky diode junction ,43. Grap4light/IdarkI ratio, 1.73 mA/W R, and 1011 cm·Hz1/2/W under 850 nm illumination. The PD also showed fast response speed with rise/fall times of 72/122 μs, as shown in x thin layer was employed for optimizing R of the graphene/GaAs NIR PD. Here, the AlOx serves not only as a surface protective layer for suppressing the carrier recombination, but also as a barrier for reducing the current leakage [x layer, the R and specific D* increased by 4 times to 5 mA/W and 2.8 × 1011 cm·Hz1/2/W, respectively, whilst rise/fall times sharply decreased to 320/380 ns.GaAs has long been considered as an ideal candidate material for NIR PD applications due to its striking electrical/optical properties such as high carrier mobility, direct bandgap of ~1.4 eV, and large optical absorption coefficient. As shown in  leakage , thereby7 through further device optimization, as shown in ZnO has received considerable attention over the past two decades due to its unique properties such as direct bandgap of 3.2 eV, large exciton binding energy of ~60 meV, high transmittance (>80%) in the wavelength range of 400–800 nm, and wide-variable conductivity in the metallic to insulating range ,110. To 2/Si substrate [2, thereby greatly reducing the time of the electron transfer in the channel. As a result, the photogenerated electrons transferred from the QD to the graphene are recycled several times before recombination, resulting in the increase of the photoconductivity gain. The phototransistors exhibited ~108 A/W R corresponding to ~3 × 109 gain, ~103 UV/visible rejection ratio, and ~5.1 × 1013 cm·Hz1/2/W specific D*. More recently, phototransistors consisting of graphene FET and ZnO QDs have been fabricated based on surface active process [D* increased up to 7.5 × 1014 Jones due to the efficient transfer of charge carriers from ZnO QDs to graphene. These results suggest how important the van der Waals interface is for the efficient optoelectronic processes in heterostructure PDs. On the other hand, in graphene/ZnO heterostructures photodiodes [D* [In recent years, ZnO NW or nanorod (NR) arrays have been combined with various graphene sheets on rigid or flexible substrates to form UV phototransistors ,52,53,54ubstrate . Surpris process . Resultitodiodes ,65,66,67odes [D* ,64,66. Todes [D* ,60,62,63odes [D* . R and 385 gains at −1 V [A Schottky-junction-type UV PD fabricated by depositing graphene layer on ZnO NR array was shown to be highly sensitive to UV illumination with excellent operational stability and reproducibility, resulting in 113 A/W  at −1 V . The res at −1 V  and by m at −1 V  or exter at −1 V ,62. The  at −1 V . The h-BR, measured under an external strain of 5 to 30%, showed maximum 17 μA/W and 3.8 μA/W at a strain of 30% for UV and visible light, respectively [Recently, the effect of strain modulation on PD performance was studied for a flexible graphene/ZnO nanostructure ,65. The 2 [2O3 [2 is a promising material for deep-UV detection applications. Recently, in the ZrO2 QDs-graphene heterostructure, R was shown to be as high as 22 A/W at a deep-UV wavelength of 254 nm [2 layer due to the high mobility of graphene. This explains why R is so extremely highLike graphene/ZnO hybrid structures, other semiconductor metal oxides including ZrO2 , Ga2O3 [2 [2O3 ,70 have f 254 nm . In the 2O3 junctions, as shown in R and D* of 39.3 A/W and 5.92 × 1013 Jones, respectively, comparable to, or even better than, Ga2O3 nanostructure-based PDs. The PD parameters were reproducible and stable even by repeated measurements under UV illumination, showed maxima at ~220 nm, but were almost blinded to light at wavelengths longer than 280 nm. Most recently, graphene/Ga2O3/graphene PD showed apparent rectifying characteristics and excellent solar-blind UV photoresponse [light/IdarkI ratio up to 82.88 under 254 nm illumination. As another approach, a deep UV photodiode was fabricated based on multilayer graphene/Garesponse  with fasR about 10 times larger on a piezoelectric substrate than on a SiO2 substrate [Organic semiconductors including small molecules and polymers are very attractive in practical view due to their simple solution production processes at low temperatures , low cosubstrate . The impR and specific D* being 700 A/W and of 1013 Jones, respectively, as shown in 4 s and PC being invariant for more than 200 cycles  hybrid, exhibiting the R of several hundreds of A/W and relatively-rapid response time shorter than 1 s in the UV-visible range [Perovskites have recently received strong attention as next-generation PD materials because of their high adsorption coefficient, tunable bandgap, small exciton binding energy, long carrier diffusion length, ambipolar charge transport, and high carrier mobility ,120,121.le range ,75. Espe3 hybrid phototransistors with those of graphene-based other semiconductor hybrid PDs operated based on similar mechanisms [3 stack was integrated with Au NPs having a plasmon resonance peak at ~530 nm, as shown in R and faster response speed [R was attributed to the light-harvesting effect enhanced by the near-field of the perovskite, induced by the plasmonic coupling of Au NPs. Furthermore, the time for these carriers to diffuse into graphene is shorter than that for carriers generated far from the perovskite–graphene interface, thus leading to the faster operation in the PD by the plasmonic effect.It is currently difficult to compare the performances of the graphene/MAPbIchanisms ,22,122, chanisms . The enhse speed . The hig3 films on graphene, thereby forming a compact heterostructure for efficient light harvesting and exciton separation, as shown in 7 A/W R and 2 × 1015 Jones D*, several times larger than those of PD containing a MAPbI3 film prepared by conventional spin-coating method. Very recently, a thin layer of P3HT has been inserted between MAPbI3−xClx perovskite and graphene as a hole transport layer for high-performance phototransistors [3−xClx perovskite was shown to be larger than that of the MAPbI3 perovskite, resulting in more efficient transfer of the photo-generated holes to the graphene side. In addition, the separation of the photo-generated electrons and holes was more effective due to the presence of the P3HT layer, thereby reducing the recombination of the optical carriers, resulting in the transport of high-density electrons to the perovskite layer. The gain of this PD was close to 1010, at least one order of magnitude higher than those of other PDs without the P3HT layer [As an advanced step, the sequential vapor deposition method has been developed to grow ultra-flat MAPbInsistors . In thisHT layer ,22,122. 3 NWs hybrid phototransistor was fabricated to show an R as large as 106 A/W [3−xIx nanocrystals (NCs) was done to fabricate phototransistors showing high R [D* (2.4 × 1016 Jones) thanks to the extremely-high carrier mobility of the graphene, very promising for the application of the perovskite NCs in next-generation high-performance optoelectronics.Highly-crystalline perovskite nanostructures are advantageous for the photo-detection application because the recombination rate of the photo-generated carriers is much smaller in the bulk than in the thin film. Recently, the graphene/MAPbI 106 A/W . As anotg high R . This PDLow-dimensional carbon materials such as fullerenes, carbon QDs, carbon nanotubes (CNTs), and graphene have attracted much interest over the past several decades due to their unique structural and physical properties ,125,126.2 on Si was employed to investigate the properties of the intrinsic charge transfer between graphene and graphite QDs in the graphene FET, as shown in DSI-gV characteristics of the graphene FET before and after the deposition of the graphite QDs. The Dirac points are 1.6 V and 48 V before and after doping the graphite QDs, respectively. In other words, a higher gV is required to access the Dirac point of the graphite QDs/graphene FET because the net negative charge of the graphite QDs can attract more light-induced free holes in the graphene film. Such carrier transfer is known to enable more effective charge separation in graphite QDs and consequently suppress the electron-hole recombination [DSV) under different optical powers, showing a linear dependence on bias. Here, the photoconduction gain is enhanced by the recycling of holes through the graphene channel within the lifetime of the trapped electrons. Zero-dimensional carbon materials including graphite QDs, graphene QDs, and C60 were deposited on the graphene by spin coating or thermal evaporation methods to form hybrid structures for high-performance phototransistor applications ,83,84,85bination ,127. Fig3 photoconductive gain in the UV range. Graphene/graphene QDs/graphene PD was also fabricated to form hybrid-type PD that operates over a wide spectral range from UV to NIR [R, 1011 cm·Hz1/2/W D*, ~95 dB linear dynamic range (LDR), and 100 μs response time. Recently, PDs composed of silica NPs and graphene QDs as donors and acceptors were well operated based on Förster resonance energy transfer (FRET) mechanism, as shown in R of the FRET system was more than three times greater than that of the PD containing only graphene QDs, as shown in Graphene was hybridized with graphene QDs on a rippled poly(dimethylsiloxane) substrate to fabricate stretchable PD , whose PV to NIR ,85. ThisR under different back-gate voltages. The R was shown to be ~120 A/W and ~40 A/W at 650 and 1550 nm, respectively.CNTs can be metallic or semiconducting depending on their respective chiral vector . Metallilight/IdarkI ratio, R, and specific D* of the PD are 240, 0.21 A/W, and 4.87 × 1010 Jones, respectively, much larger than other CNTs-based PDs. The PD also exhibited a fast response rate of 68/78 μs rise/fall times and 5400 Hz 3dB bandwidth. More recently, there has been a report on novel broadband graphene/CNT thin film photodiode , where t2, [2 [The van der Waals interaction between neighboring layers without dangling bonds in 2D materials allows for their integration with other materials, including graphene, to form a variety of planar or vertical functional heterostructures with fundamentally different properties. As a prototype geometry, 2D layered semiconductors such as InSe , WS2, [92, , MoTe2 [2, [2 , and gra3 A·W−1 and fast rise/fall times of 1/10 ms [R on incident light power was observed in graphene/WS2/graphene heterostructure device [R increases as the optical power increases at gV = 0 V (off state), considered to be due to an efficient photo-induced charge transfer process in which electrons reside in the graphene while the holes are located at WS2. On the other hand, at gV = 30 V (on state), the R decreases as the optical power increases. When on state in the phototransistor, the photo-gating effect can increase the Fermi level and reduce the contact resistance. As a result, the influence of illumination on the Fermi level of graphene is reduced, thereby causing different power dependence of R. Recently, the planar graphene/n-InSe/graphene van der Waals heterostructures showed high R of 4 × 10 1/10 ms . The elee device . In this2 hybrid heterostructure phototransistors exhibited a high R of 5 × 108 and 1 × 1010 A·W−1 at 130 K and room temperature, respectively [2 by local conditions, and the electrons are transferred to the graphene with the help of the gate electric field, resulting in high R. On the other hand, the trapped holes act as local gates, thereby causing a significant photogating effect on graphene through capacitive coupling. Eventually, the response time is slow because of the long life of the trapped holes. In another approach, a similar phototransistor was fabricated using CVD-induced graphene and MoS2 [7 A·W−1 due to the low carrier mobility of CVD-graphene and very slow response times of several-hundred seconds, but seemed to be better suited for practical applications by easy preparation in large areas. In device structures dominated by photogating effects, 2D semiconductor serves as a medium for harvesting light, whereas graphene serves as a conduction channel for carrier transport and circulation. Recently, mechanically exfoliated graphene-MoSectively . Here, tand MoS2 . This hy2/graphene stack heterostructures were useful for efficient light–matter interactions, leading to improved photon absorption and electron-hole generation [R increases from ~10−2 to ~10−1 A·W−1 by integrating plasmonic metal nanostructures and heterostructures to improve the light absorption. It is worth noting that these results can be used for designing the dual-gate-type graphene/WS2/graphene heterostructure to control the polarity and amplitude of the PC by adjusting the direction and intensity of the built-in electric field [2/graphene employing 2.2 nm trilayer WSe2, as shown in Recently, graphene/transition metal dichalcogenide (TMD) heterostructures have also exhibited significant functionalities in the photodiode type. Carrier generation, separation, and transport processes were shown to be adjusted by easily modulating the Fermi level of graphene and the height of the Schottky barrier in graphene/TMD/graphene vertically-stacked heterostructures. For instance, graphene/WSneration . Scanninic field . A typicR photodiodes were fabricated by integrating graphene with TMD stacked semiconductors such as MoS2, MoTe2, and WSe2 [2 Schottky junction photodiode exhibited unique optical response over a wide spectral range from 400 to 1500 nm [R values of 0.52 A·W−1 at 590 nm/1.26 A·W−1 at 1440 nm were obtained when operating in the energy gap excitation mode/when internal light emission dominated the PC generation in graphene, respectively. On the other hand, metal-insulator-semiconductor-like photodiodes with an insulator layer between graphene and MoS2 showed improved current rectification and higher current flow compared to 2D-based metal-semiconductor diodes and p-n junctions based on TMDs [2/graphene/MoS2 gate tunable rectification behavior and the broadband response ranging from 400 to 2400 nm. [R and D* of up to 105 A·W−1 and 1015 Jones in the visible range, respectively, but they sharply decrease to 10−1 A·W−1 and 109 Jones at 2400 nm IR wavelength, as shown in 2, graphene, and MoS2), resulting in a large number of photocarriers, contributing to significant increase of PC. However, when the IR light having energy smaller than the band gap of WSe2 is irradiated, only graphene can absorb light, resulting in very low PC. These results suggest that the van der Waals junctions based on 2D materials are very promising in future critical optoelectronic device applications.As another approach, relatively-low-and WSe2 ,97,98,99 1500 nm . Maximum2400 nm. . The devGraphene/semiconductor hybrid heterostructures showed their tremendous potential for PD applications. Particular attention was paid to several important issues related to device design, device performance, and processing technology to enhance figure-of-merit parameters of the PDs. By the optimization through various approaches, the PDs typically showed some new features that could open up the way to many conventional and advanced applications. The work function, surface resistance, and optical transparency of the graphene TCEs were successfully tuned by adjusting the number of layers, chemical doping, photo-induced doping, or electric field gating doping, which strongly affected the performance of the PDs. To meet the requirements for the practical applications of the PDs, some of the properties of the sensing materials should be optimized. Because the heterogeneity of the nanostructured materials can greatly affect the uniformity of the device performance, more precise control of their morphology, crystallinity, and orientation is highly needed. Some emerging methods such as plasmonic technology and integration of optical waveguides/micro cavities are also useful for improving the photo-material interactions, resulting in the increase of the light absorption. In particular, special efforts should be made for high response speed of the PDs by increasing the charge carrier mobility of the sensing materials or introducing a vertical electric field for the transfer and separation of photo-carriers. Broadband, spectrally selective, or flexible PDs should also be developed for specific applications. In addition to the improvements of the figure-of-merit parameters, long-term reliability and durability, environmentally-friendly and cost-effective machining technologies, large-scale production and consolidation are also important issues for various applications of the PDs in upcoming new situations.


Silicone rubber (SIR) is widely used as an insulation material in high voltage cable accessories. Electrical tree is a typical electrical degradation and is easily initiated because of the distorted electric field. In this study, graphene nanoplatelets at contents of 0.001–0.010 wt % (0.00044–0.00436 vol %) were added into SIR to improve the electrical tree inhibiting ability. Scanning electron microscopy, conductivity and surface potential decay tests were conducted to analyze the characteristics of graphene/SIR nanocomposites. The typical electrical treeing experiment was employed to observe the electrical tree inhibition of graphene in SIR. The results show that graphene nanoplatelets were well dispersed in SIR. The conductivity was higher after the addition of graphene nanoplatelets, and the trap distribution was affected by graphene nanoplatelets. The tree was changed from a bush-branch structure to a bush structure after the addition of graphene. Tree inception voltage improved and reached the highest mean value at 0.003 wt %. The tree length was inhibited at 0.001 to 0.007 wt % and the lowest tree length occurred at 0.005 wt %. Silicone rubber (SIR) is usually employed as insulation in both alternating current (AC) and direct current (DC) power cable accessories, forming a multilayer insulation with cross-linked polyethylene (XLPE), polypropylene (PP) or other polymers ,2,3. Giv2) and alumina (Al2O3) [Nanodielectrics with filler contents of several wt % have been proven to have a high electrical tree-inhibiting ability ,12,13,14 (Al2O3) ,16,17. W (Al2O3) ,16,17. T (Al2O3) . Accordi (Al2O3) ,20. FromGraphene is a typical 2D structure with a giant specific surface area. The effects of graphene nanoplatelets on SIR properties have been researched ,22,23,24In this work, graphene/SIR nanocomposites were prepared with filler contents of 0, 0.001, 0.003, 0.005, 0.007 and 0.010 wt %. Scanning electron microscopy (SEM), conductivity and trap distribution were tested. The electrical treeing experiment was conducted and the inhibition effect of graphene was discussed.4 g/mol, and the content of vinyl groups was 0.13–0.20 mol %. The graphene used for the sample was purchased from Tanfeng Graphene Technology Co., Ltd. . The diameter of graphene nanoplatelets was 0.5–5 μm, the thickness was 0.5–3.0 nm, with 1–2 layers and an SSA of 1000–1217 m2/g. Before mixing, a surface treatment was applied on the graphene.High temperature vulcanized methyl vinyl SIR (MVQ 110-2) was provided by Dongjue Silicone Group Co. Ltd. . Its average molecular weight was 65 × 10Zhang et al. reported that graphene modified with vinyltrimethoxysilane exhibited improved dispersing and banding properties with SIR . Thus, vThe weight content of graphene in nanocomposites was 0, 0.001, 0.003, 0.005, 0.007 and 0.010 wt % . The required graphene was weighed by a microbalance with a sensitivity of 0.01 mg . A twin-screw continuous mixer was employed to mix the graphene and SIR, with a rotation speed of 40 rpm at room temperature for 15 min. Then 2,5-dimethyl-2,5-di(tert-butylperoxy) hexane  was added into the mixture as the vulcanizing agent at 1.5 phr. After the mixing process, the compounds were placed into the moulding in the plate vulcanizing machine  at a temperature of 165 °C and pressure of 10 MPa for 10 min [A high-resolution scanning electron microscope  was employed to observe the cross-section of the graphene/SIR nanocomposites. The samples were cryo-fractured in liquid nitrogen and the surface of the cross-section was painted with platinum before scanning.DC conductivity was tested on a typical three-electrode system as in . The samI is the current in A, U is the applied voltage in V, l is the thickness of sample in m and r is the radius of sample in m.After 10,000 s, the conductivity was calculated as follows:To calculate the trap distribution in SIR, surface potential decay (SPD) measurement was used. The measurement setup was shown in our previous work . The samET is the trap energy in eV; T is the temperature in K; vATE is the attempt to escape frequency in s–1; kB is the Boltzmann’s constant in eV/K; h is the Planck constant in eV·s; NT is the trap density in eV–1m–3; ε0 and rε are the permittivity of vacuum and the relative permittivity of SIR; q is the elementary charge in C; L is the sample thickness in m; sU is the surface potential in V and t is the decay time in s.The potential decay rate is related to the trap distribution in samples. The trap energy and density can be calculated by these three equations ,30:(2)ETThe treeing test setup is shown in The graphene nanoplatelets distribution in the samples for all graphene contents were analyzed with SEM. The SEM pictures of samples with 0.001, 0.005 and 0.010 wt % are shown in −13 S/m. From 0.005 to 0.010 wt %, a sharp increase (1.55 × 10−11 S/m) in conductivity occurred, potentially caused by the shorter distance between graphene nanoplatelets at higher graphene contents.The conductivity of graphene/SIR nanocomposites is shown in The potential decay in graphene/SIR nanocomposites is related to the charge migration from SIR bulk ,32. Duri20 eV–1m–3 to lower than 2.7 × 1020 eV–1m–3 and the density of deep trap decreased from 2.2 × 1020 eV–1m–3 to lower than 1.7 × 1020 eV–1m–3 after the addition of graphene.The trap distribution in the nanocomposites can be calculated according to Equations (2)–(4). The relative permittivity used here is shown in During the experiment, two different tree structures were observed, defined as bush-branch tree and bush tree. As shown in To observe the inhibition effect of graphene, the tree inception voltage was tested. The RMS of the AC voltage was set to 5–10 kV. In the test, the voltage was increased in 0.2-kV steps and was maintained for 30 s at each step to check if the tree was triggered or not. If an electrical tree with a length of 20 μm was observed, the RMS was recorded as the tree inception voltage. Ten samples were tested for each graphene content.The average inception voltages from 10 tests and the standard deviations are shown in The treeing process was recorded using a microscope and computer. Typical tree growth processes in samples with different graphene contents are shown in As shown in As shown in As shown in The tree length of a typical tree without breakdown at different times was calculated as shown in During the experiments, nine samples of each graphene content were tested for 60 min. The tree length at 60 min was calculated to analyze the inhibition effect of graphene. If the breakdown occurred within 60 min, the length was calculated as 2 mm. In the whole experiment, breakdown only occurred in samples with 0 and 0.010 wt %. The box-plot of tree length at 60 min is shown in Under AC voltage, there is an electric field with altering direction. According to the theory proposed by Tanaka et.al., it is believed that there is an electron injection-extraction process near the needle tip . Given tIn the case of nanocomposites, the trapping-detrapping process is similar to what occurs in undoped SIR. When the electrons are injected into the sample, they will be captured by the traps around needle tip induced by the interface between SIR and graphene ,34. HoweAccording to the model proposed by Danikas et al. , the treHowever, the shorter distance between the nanoplatelets lower the dielectric strength in some regions inside the nanocomposites with higher content . BecauseGraphene nanoplatelets and SIR were employed to prepare nanocomposites with graphene content lower than 0.010 wt %. SEM, conductivity and trap distribution characteristics were tested to examine the nanocomposites.The results showed that the conductivity of SIR can be increased by the addition of graphene nanoplatelets. The trap distribution in SIR changes with the addition of graphene nanoplatelets. The shallow and deep trap levels in samples increase in nanocomposites with content ranging from 0.001 to 0.007 wt %.The electrical degradation was inhibited by the addition of graphene nanoplatelets at low contents. The average tree inception voltage increased after the addition of graphene nanoplatelets at contents of 0.001 to 0.007 wt %. The tree structure in undoped SIR was a bush-branch tree, which changed to the bush tree structure in graphene/SIR nanocomposites. The tree length in graphene/SIR nanocomposites was inhibited at contents of 0.001 to 0.007 wt % and increased at 0.010 wt %.


We numerically propose a dual-band absorber in the infrared region based on periodic elliptical graphene-black phosphorus (BP) pairs. The proposed absorber exhibits near-unity anisotropic absorption for both resonances due to the combination of graphene and BP. Each of the resonances is independently tunable via adjusting the geometric parameters. Besides, doping levels of graphene and BP can also tune resonant properties effectively. By analyzing the electric field distributions, surface plasmon resonances are observed in the graphene-BP ellipses, contributing to the strong and anisotropic plasmonic response. Moreover, the robustness for incident angles and polarization sensitivity are also illustrated. Graphene is a two-dimensional material with carbon atoms arranged in a honeycomb lattice , 2. VariIn our work, an anisotropic dual-band infrared absorber is numerically proposed using periodic elliptical graphene-BP pairs, which is ease of fabrication. The independent tunability of resonance by geometric size and doping level is demonstrated. Electric field distributions are plotted to reveal the physical mechanism. The incident angle tolerance and polarization sensitivity are also illustrated.2 layer as shown in Fig. 2 and hBN are obtained from Ref. x- and y- directions. A port with infrared wave excitation is set upon the top surface of the computational domain, while perfect electric conductor (PEC) boundary condition is set on the bottom surface. Tetrahedral meshes with user-controller mesh density are applied for the entire domain.The proposed absorber is made up of transverse and longitudinal elliptical graphene-BP pairs deposited on a SiOIn the simulation, both graphene and BP are treated as two-dimensional surface with surface conductivities instead of bulk materials with permittivity tensors. This assumption solves the problems of thickness definition for ultrathin materials and low computational efficiency .σ(ω), we use the well-known Kubo formulas as below [To describe the surface conductivity of graphene as below :1\documσ(ω) consists of the intraband and interband counterparts, namely σintra and σinter. ω is the radian frequency, μc is the chemical potential, Г is the scattering rate, and T is the Kelvin temperature. ħ, e, ξ, and kB are the reduced Planck constant, electron charge, electron energy, and Boltzmann constant, respectively.According to Eq. c ≫ kBT, Kubo formulas can be further simplified to Eq. ω, Г, and μc. Here, Г is assumed as 0.3 meV and μc is assumed to be 0.7 eV according to the previous work [In the infrared region, since the incident photon can hardly excite the interband transition, the light-graphene interaction is dominated by the intraband transition. Particularly, when μous work , 26.σj of BP with a simple semi-classical Drude model [ns is the carrier density relating with the doping level. We choose ns = 1.9 × 1013 cm−2 and ГBP = 10 meV according to the previous reference [j is the concerned direction, so σx and σy are determined by the electron mass along x- and y-direction, respectively. mx and my can be further calculated by:m0 is the standard electron mass, and Δ and a are the band gap and scale length for BP monolayer, respectively. By substituting Eqs. x-) and zigzag (y-) direction. The discrepancy between them contributes to the anisotropic surface conductivity of BP.On the other hand, we calculate the surface conductivity de model :6\documeference . j is thmentclasspt{minimaR1/R0), where R1 and R0 denote the reflectance  of different polarizations at the same wavelength, then the maximum PER of each resonance can reach up to 23 dB and 25 dB at λ = 9.5 μm and λ = 14.1 μm, respectively. Therefore, the proposed absorber can be utilized as a dual-band reflective polarizer with high performance.To illustrate the anisotropic absorption characteristic of the proposed absorber, we first simulate and compare the absorption spectra with individual graphene layer, individual BP layer, and graphene-BP pairs. As can been observed in Fig. a increases from 42 to 52 nm for both polarizations, while second resonant frequencies are almost unchanged. On the other hand, as shown in Fig. b, the second resonances are redshifted as well, while the first absorption peaks remain constant for TE and TM polarization. Therefore, the dual absorption peaks can be tuned independently by varying the corresponding axis length in the elliptical graphene-BP pairs. Moreover, the thickness of dielectric layer also plays a critical role in the performance for the proposed device, which acts as a Fabry-Perot resonator formed by the graphene-BP metasurface and the PEC substrate. Thus, the absorption spectra with different td are plotted in Fig. td increases from 0.95 to 1.75 μm, the first absorption peaks for TE and TM polarization have a dramatic drop, while the second peaks increase at first then decrease sharply. As a consequence, there is an optimal thickness td that maximizes the dual absorption peaks of the proposed absorber.We next analyze the absorption spectra with different geometric configurations to demonstrate the tunable dual-band absorption property in Fig. x-) direction. At the first peak (λ = 8.8 μm), the incident infrared light can excite electrons in graphene and BP to oscillate in the transverse direction, leading to the concentration of electric field at the short axis ends of the longitudinal ellipse as shown in Fig. λ = 14.1 μm, the localized electric field is enhanced at the long axis ends of the transverse ellipse. On the other hand, TM incidence with electric field in the zigzag (y-) direction can excite electrons to vibrate along the longitudinal direction at the absorption peak of 9.5 μm, leading to concentrated field distributions at the short axis ends of the transverse ellipse. Besides, at λ = 15.4 μm, the enhancement of electric field is focused at the long axis ends of the longitudinal ellipse. Therefore, the resonance wavelengths are directly related to the finite oscillation length of the induced dipoles in both transverse and longitudinal elliptical graphene and BP pairs.In order to elucidate the physical insight, we further reveal the electric field intensity distributions at different wavelengths in Fig. c and ns according to graphene and BP model formulas as mentioned above. μc and ns represent the doping level of graphene and BP that can be altered after geometric fabrication. Thus, performances of the proposed absorber with different μc and ns are depicted in Fig. μc is chosen between 0.4 and 0.8 eV from the previous work verified by experiments [ns of BP was demonstrated to be 2.6 × 1014 cm−2, so a moderate ns is chosen between 1013 cm−2 and 1014 cm−2 in the simulation. In Fig. μc = 0.4 eV, the first absorption peak is located at 10.9 μm and the second one is located at 17.1 μm. As μc increases to 0.8 eV, the two resonant wavelengths are blueshifted to 8.4 μm and 13.4 μm. Similarly for TM polarization, the dual absorption peaks are blueshifted from 12.4 and 19.8 μm to 8.9 and 14.4 μm, respectively, with μc increasing from 0.4 to 0.8 eV as shown in Fig. λp can be calculated as L is the effective oscillation length [L is fixed, the absorption spectra exhibit an obvious blueshift as ns increases for TE polarization as plotted in Fig. ns increases from 1013 cm−2 to 1014 cm−2 as demonstrated in Fig. One can tune the anisotropic dual-band absorption performance effectively by varying the geometric dimensions as demonstrated in Fig. eriments . In the eriments , the maxn length . Thus, iθ increases to 52°, while the second absorption peak maintains above 80% even when θ increases to 80°. When θ > 46°, the second resonant wavelength is redshifted gradually as θ becomes larger. For TM incidence, when θ is less than 62°, the absorption rate at the first peak maintains larger than 90%, while the resonant wavelength keeps constant at λ = 9.5 μm as shown in Fig. θ up to 60°, then drops slightly with the increase of θ. The excellent angular stability originates from the common feature of Fabry-Perot resonators, which are robust for oblique incident angles [In the practical applications, tolerance of wide incident angles is preferred for infrared absorbers. Therefore, absorption spectra under oblique incidences are elaborated. In Fig. t angles .Fig. 5Aφ are presented in Fig. φ increases from 0 to 90°, the absorption spectrum turns out to be the same as the TM polarization in Fig. φ < 90°, the incidence will excite electrons in BP to oscillate in both armchair and zigzag directions due to its x- and y- components of the incident electric field. Consequently, surface plasmon resonances can be induced simultaneously in armchair and zigzag directions of BP.Absorption spectra under normal incidence with different polarization angles In conclusions, we have proposed an anisotropic dual-band infrared absorber consisting of periodic transverse and longitudinal graphene-BP ellipses. The maximum PER at each resonance can reach up to 23 dB and 25 dB. The dual anisotropic resonances are attributed to the induced electric dipoles located at the ends of short and long axes. By adjusting the lengths of short axis and long axis, the first and second absorption peaks can be independently tuned, respectively. Moreover, the resonant absorption bands can also be tuned by changing the corresponding doping level of graphene and BP. Besides, high absorption rates at both peaks can be achieved under oblique incidence for any polarization. The proposed absorber can be utilized as a tunable reflective polarizer and novel infrared sensor.


Metal films deposited on graphene are known to influence its electronic properties, but little is known about graphene’s interactions with very low work function rare earth metals. Here we report on the work functions of a wide range of metals deposited on n-type epitaxial graphene (EG) as measured by Kelvin Probe Force Microscopy (KPFM). We compare the behaviors of rare earth metals  with commonly used noble metals . The rare earth films oxidize rapidly, and exhibit unique behaviors when on graphene. We find that the measured work function of the low work function group is consistently higher than predicted, unlike the noble metals, which is likely due to rapid oxidation during measurement. Some of the low work function metals interact with graphene; for example, Eu exhibits bonding anomalies along the metal-graphene perimeter. We observe no correlation between metal work function and photovoltage, implying the metal-graphene interface properties are a more determinant factor. Yb emerges as the best choice for future applications requiring a low-work function electrical contact on graphene. Yb films have the strongest photovoltage response and maintains a relatively low surface roughness, ~5 nm, despite sensitivity to oxidation. These photonic properties, when considered in combination with graphene’s high Fermi velocity, high mobility, and relativistic quantum electronic transport, suggest the opportunity to create a variety of unique photonic devices9. Among these devices, graphene optical detectors have been demonstrated with light intensity modulations of 40 GHz without degradation of the photoresponse, and transfer data as fast as 12 Gbits s−1 12. Such devices have great potential for use in the development of faster data transfer networks for optical telecommunications. Graphene optoelectronic devices operating in the terahertz gap between 0.1 and 10 THz are even more promising16, as THz technology in this range is emerging as a potential solution for a variety of problems in fields as diverse as cancer detection, atmospheric monitoring, and security screening19. Graphene’s two-dimensional lattice affords limited opportunities for lattice-electron energy loss22, thereby causing absorbed THz radiation to induce a hot-carrier effect, a useful mechanism for THz detection22. Graphene-based THz detectors currently achieve responsivities as high as 400 V W−1 and there are good indications for potential improvement23.Graphene, a single atomic layer of sp23. Metal films can dope graphene, causing junctions to form at the edge of each contact, thus generating a photoresponse when illuminated24. Scanning photocurrent measurements across metal-graphene-metal junctions reveal that the photoresponse is maximized near each metal-graphene junction, but vanishes in the center of the channel26. The photoresponse is dependent upon a combination of photovoltaic effects from the induced electric field and photothermoelectric effects from the Seebeck coefficient differences of differently doped graphene29. The mechanism is similar to the hot-carrier dominated photoresponse at graphene P-N junctions28. Moreover, the amount of doping induced by the metal is largely determined by its work function, ϕ24. This is important in detectors because using different metals at the source and drain creates an asymmetry that induces a net photoresponse, with the magnitude determined by the Δϕ of the two metals; this means that the maximum response should be obtained using contacts with the largest work function difference. Photodetectors operating in both the telecommunication and THz spectrum have already been developed using this type of architecture23. While back gating can improve performance, passive THz devices have responsivities as high as 200 V W−1 23. This asymmetric contact design enables rapid and wafer-scale fabrication of graphene devices by minimizing processing steps and reducing the need for split gating.While conventional semiconducting photodiodes utilize a P-N junction, efficient graphene devices can be fabricated with a single strip using different metals for the source and drain contacts35. Therefore, a preliminary study of the qualitative interactions between graphene and metals with a large range of ϕ is essential for various applications including photodetectors utilizing metal contacts that induce a voltaic asymmetry. There is very limited research on low work function thin films on graphene. The majority of graphene studies involving erbium and ytterbium metals use the metals as dopants in laser fibers37, while europium oxide films on graphene are used for spintronics due to EuO’s ferromagnetic capabilities40, and yttrium oxide is used as a high κ dielectric42. However, to our knowledge there are no studies in the literature that characterize these metals or their oxides for their capability to function as an electrical source/drain contact. For example, pairing a low work function contact with a noble metal should result in improved performance of graphene-based two-terminal photodetectors.Despite the usefulness of metal-graphene junctions, little published work exists that quantifies the behavior of different work function metals on graphene that can aid the optimization of device design and fabrication. While there has been extensive work on graphene-metal contacts, these studies largely pertained to achieving low contact resistance and have focused on metals with high work functionsϕ for the low work function category, with the measured ϕ consistently higher than expected. A number of metals introduce significant processing challenges and unique characteristics when deposited on graphene. Er and Pr films form micrometer-sized topographic features likely due to adhesion issues and complications from volumetric changes during rapid oxidation. Eu films exhibit unique phonon signatures when on graphene. A thorough examination of the graphene-metal interface at the Er film edge shows evidence of disorder in the graphene lattice, suggesting that sp3 bonding or oxidation of the graphene lattice may occur during metal oxidation. For Eu deposition, we also report on a redshift in the graphene Raman spectrum, possibly due to Eu-adsorption or Eu intercalation with the graphene lattice4. A preliminary analysis of photoresponse uncovers no correlation between photovoltage and surface potential, with the photovoltage values largely determined by the physical characteristics of the metal-graphene interface. Of the rare earth films studied, Yb films are the most encouraging because they exhibit the strongest photoresponse and smallest surface roughness.In this study, we deposit metals with work functions ranging between 2.5 eV and 5.8 eV on graphene grown epitaxially on SiC via Si sublimation. We find a discrepancy between the reported and measured values of 44.Figure We note here an important characteristic of KPFM scans. For the metal thicknesses used, KPFM measurements are not sensitive to sample  topography and, in the absence of chemical reactions, only depend on the material properties near the surface. This characteristic is exemplified by the Er scans. The interface where the Er pad overlaps the Au pad is not visible in KPFM Fig. . Yet, thϕR) and calculated work functions (ϕM) of the metals deposited in this study, as well as the measured SPD between the metal film and graphene (SPDM-G) is found in Table 146. The measured values and variance presented in Table ϕAu, is 5.10 eV based on referenced values of Au and the SPD between Au and graphene48. We use this value as our comparative reference to calculate the work functions for graphene and the other metals using the measured surface potential. For example, to calculate the work function of EG on a terrace using data found in Fig. q) and the value of ϕAuto find ϕG. A summary of the reported , Au(110), and Au(111) have work functions of 5.47, 5.31, 5.37 eV, respectively47. Similarly, the work function of Cu varies with orientation, ranging from 4.48–4.98 eV47. In any case, we emphasize that our KPFM results for the high work function metals are reproducible over the course of months.For the high work function metals, the discrepancy from literature values cannot be due to substrate effects since, as we discussed earlier, KPFM is sensitive to the surface properties of the metal; we also specifically tested and confirmed this characteristic using a different substrate. Moreover, the KPFM system is likely not the source of the discrepancy since we obtain reproducible results over the course of months and the calculated work function of graphene is the same as other reports that use calibrated KPFM tips5/2) peak was located at 185.87 eV indicative of Ytterbium oxide. Because of the metal oxidation, the surface potential measurements are expected to differ from the pure metals. For the case of Pr, the calculated ϕ is expected to increase with increasing oxygen exposure, due to the presence of Pr2O353, and this appears to be consistent with KPFM measurements in Fig. 2O3 having reported ϕ values of 1.8 and 2.0 eV, respectively56. And the case of EuO is further complicated because the work function is strongly dependent on orientation, e.g., the work function of EuO(111) is 7.5 eV, which is considerably higher than 1.8 eV of EuO(100)54. In general, oxidation of the deposited metal will change ϕ and the data imply that it generally increases. The experimental conditions used in this work are similar to those encountered in processing actual devices, including exposure of the devices to ambient during the device fabrication and subsequent possibility of oxidation as exhibited here.The discrepancy from literature values for the low work function metals, in addition to the purity issues noted above, is likely due to oxidation. These metals are known to oxidize rapidly when exposed to the atmosphere. Despite measuring the surface potential directly after metal deposition, the metals Eu, Y, Er, Yb, and Pr, begin oxidizing upon removal from the deposition chamber. The oxidation is optically apparent on the samples because the appearances of the deposited rare earth metal films change from reflective, to opaque, and then to semi-transparent or transparent within minutes to hours. We confirmed the oxidation by XPS measurements; for example, XPS showed that the Yb . Note that the Y work function does not change when overlapping gold, as is expected. In Fig. 2CrO3 has been reported to form during joint sputtering of Y and Cr films58. In that case, the overlapping region in Fig. 2CrO3 mixed with Y and Cr and we propose 3.93 ± 0.09 eV as the mixture’s previously unreported work function. Of the eleven metals investigated in this work, this is the only instance of a metal’s work function changing when overlapping another metal.To help guide utilization of the five rare earth metals for graphene applications, we look more closely at the film characteristics after deposition on EG. Y films were deposited after Cr causing Y to overlap Cr, and one such intersection is shown in Fig. Er and Pr deposited films on EG have similar morphology and surface potential characteristics. Deposition of both metals (100 nm) on EG results in raised hemispherical features (bubbles) Fig. . These bϕ changes over time. Moreover, as noted earlier for the Pr deposited films, the magnitude of ϕ is consistent with the formation, at least in part, of Pr2O3. Figures Since the bubble morphology evolves with exposure to the ambient atmosphere over a time span of 1–3 hours, this characteristic is likely due to oxidation of the Er and Pr films. This mechanism is supported by KPFM measurements, summarized in Table ϕPr has an unusual dependence on the underlying material; it is the only metal whose surface potential is dependent on whether or not the scan is taken with respect to Au or Graphene. From Table Pr-G is +0.10 ± 0.03 eV at t = 0 and +0.54 ± 0.03 eV at t = 60 min. However, when measured against Au the SPDPr-Au is −0.76 ± 0.02 eV at t = 0 and −0.48 ± 0.02 eV at t = 60 min. Using ϕAu = 5.10 eV, this corresponds to a ϕPr of 4.34 ± 0.02 eV and 4.62 ± 0.02 eV at t = 0 and t = 60 respectively. This is a 0.53–0.69 ± 0.06 eV difference than when measured against graphene. It is unclear why this discrepancy occurs with Pr, but one possibility might be that the Pr is charging during measurement. Pr2O3 is known to be a high-κ dielectric oxide, and the graphene-Pr interface is likely very inconsistent considering the high surface roughness and tall features59. Alternatively, the Pr film is much smoother when overlapping Au, which perhaps discourages charging, resulting in a lower measured surface potential.For the Pr deposited films, −1, respectively. The other peaks are from incompletely subtracted contributions of the SiC substrate. In the Raman maps, Fig. −1 in the unaffected graphene to 2685 ± 5 cm−1 along the perimeter. Likewise, the G peak in Fig. −1 to 1585 ± 5 cm−1. Spectroscopic shifts in these peaks are consistent with EG lattice strain and/or doping60. From Fig. 2 symmetry62. We propose that during oxidation of the Er pad the EG along the perimeter of the film is affected, reacting with the metal and/or oxide resulting in the formation of erbium carbide or forming graphene oxide. Furthermore, the Raman shifts in the G and 2D peaks are the result of strain created by the reactions at the perimeter62.To evaluate the effect of Er metal deposition on the underlying EG, including areas underneath the hemispherical features, the sample was sonicated in de-ionized water for 5 seconds. This removed the Er pads but not the Au pads, consistent with the hypothesis of poor adhesion. The exposed region was probed by Raman spectroscopy. Figure Another deposited metal that shows evidence of altering EG’s Raman signature is Eu. Figure −1, 2473 cm−1, 1990 cm−1, and 1893 cm−1 labeled α, β, γ, and Δ, respectively. Also, the spectrum is approximately ten times stronger in intensity. This suggests that the deposited Eu film results in resonant Raman scattering when 532 nm excitation is used, possibly due to electronic transitions associated with Eu oxides and/or other compounds created by reaction with the EG. Our results are comparable to a report of EuO deposition on graphene show strong EuO Raman signatures with similar peak positions when using a 535 nm laser40.We used Raman spectroscopy to characterize the EG covered by Eu. Figure 2 bonds. We propose that this is due to Eu-C bonding and possibly Eu and/or EuO intercalation. Eu and EuO films on graphene are known to intercalate under the graphene lattice, and can increase graphene conductivity, even inducing a band gap from the hybridization of EG/Eu/SiC buffer layer63.Using the 455 nm probe, we acquire a Raman spectrum at a white spot in the Eu film, shown in Fig. except the entire region under the deposited Eu is affected. For example, the 2D and G peak positions exhibit a red shift towards lower wavenumbers, see Fig. −1 in uncovered EG to 2715 ± 5 cm−1 for the deposited region. Likewise, the G peak shifts from 1595 ± 1 cm−1 in uncovered EG to 1577 ± 3 cm−1 for the deposited region. By contrast, studies of ultra-high vacuum deposition of EuO on CVD graphene result in a blue shift in the Raman spectrum40. Lastly, we find regions of substantially high D/G peak ratios in Eu covered EG, see Fig. Raman spectroscopic maps of the 2D peak position, G peak position, and D/G ratio of the graphene underneath the deposited Eu film are shown in Fig. As discussed for the case of deposited Er, as well as in the discussion in the paragraph above, the spectroscopic shifts in these peaks are consistent with damage to the graphene lattice, strain, oxidation, and intercalation.The last of the rare earth metals, Yb, was comparatively unremarkable. Upon deposition, the film was relatively smooth with a surface roughness of only 5.0 ± 0.3 nm, see Table VOC, when the metal-EG interface is illuminated as a scanned 532 nm laser is moved across the pads; this approach is similar to other reports26. Figure VOC for the difference between the reported and calculated work functions for Eu, Yb, Er, Rh, Ni, Au and Pt, and Fig. VOC as a function of surface roughness. Since the metal-graphene perimeter interface can vary from sample to sample due to various effects  epitaxial graphene (EG) was synthesized by Si sublimation, in a controlled Ar ambient, on nominally on-axis semi-insulating (0001) 6H-SiC using a commercial CVD reactor; additional details are found elsewhere−6 Torr. Unless otherwise noted, 25–30 nm of metal was deposited. We used a shadow mask to avoid photoresist contamination for each deposition and the deposition rate was 2–3 Å s−1, with exceptions for Yb and Eu. For Yb and Eu, the deposition occurred at a rate much greater than 2–3 Å s−1 even at the lowest beam power due to the high evaporation rate of the source material. Three metals  were deposited on each graphene sample in overlapping but shifted patterns.Metals of varying work functions  and Kelvin probe force microscopy (KPFM) with a Bruker Dimension Icon. Bruker SPCM-PIT tips with a lift height of 90 nm were used and scans were 20–50 μm wide to prevent scanning interference effects at the graphene-metal film interface. The first scan was taken within ten minutes after removal from the evaporation chamber. The surface roughness (RMS) was extracted from AFM scans over areas of 10 μm × 10 μm; before metal deposition the graphene films had a roughness of 1.7 ± 0.2 nm. Optical images of the surface were taken with either the camera embedded in the Bruker AFM or an Olympus BX60M optical microscope. The temperature and humidity of the KPFM lab space were approximately 72 °C and 38% respectively.The surface morphology and Photovoltage measurements were obtained by illuminating the sample with a 1 mW, 532 nm laser using a 1 μm spot size while recording the open circuit voltage measured by Be-doped Cu probe tips with a 50 μm spacing making contact with two identical deposited metal pads. A ThermoFisher DXRxi using a 9.6 mW, 532 nm or 6 mW, 455 nm confocal probe having a ~0.5 μm spot size was used to acquire the Raman spectroscopy data. X-ray Photoelectron Spectroscopy (XPS) using a Thermo Scientific K-Alpha system with a monochromatic Al source (Kα = 1486.6 eV) was used to analyze the chemical states of the rare earth metals. A low-energy electron flood gun was used for charge compensation and all core level peaks were fitted using a Shirley background subtraction.


Graphene has made significant contributions to neural tissue engineering due to its electrical conductivity, biocompatibility, mechanical strength, and high surface area. However, it demonstrates a lack of biological and chemical cues. Also, it may cause potential damage to the host body, limiting its achievement of efficient construction of neural tissues. Recently, there has been an increasing number of studies showing that combining graphene with other materials to form nano-composites can provide exceptional platforms for both stimulating neural stem cell adhesion, proliferation, differentiation and neural regeneration. This suggests that graphene nanocomposites are greatly beneficial in neural regenerative medicine. In this mini review, we will discuss the application of graphene nanocomposites in neural tissue engineering and their limitations, through their effect on neural stem cell differentiation and constructs for neural regeneration. Peripheral nerve injury (PNI) has been widely reported to be an essential cause of incapability in the clinical setting. Patients suffering from this disease encounter many challenges including pain, sensory loss, impaired movement, and cold intolerance, which have a significant adverse impact on their daily life . The mossp2 hybridization, forming a honeycomb pattern throughout the 2D plane [Recent advances in tissue engineering and regenerative medicine have paved the way for the development of many strategies in peripheral nerve repair and various functional topographical scaffolds such as gels, films, electro spun fibers, and grooves which have been successfully demonstrated to support neuronal cellular attachment and migration . Due to 2D plane . Highly 2D plane . Notably2D plane ,14. Comp2D plane . However, while pristine graphene can induce synaptogenesis and sustain neuronal growth, pure graphene sheets fail to recapitulate the intrinsic neural microenvironment in vivo due to its smoothness, chemical inertness and lack of biological cues ,19,20. IIn this mini review, we will first touch upon the structure and functions of graphene nanocomposites. The main focus will be on recent research advances in graphene-based nanocomposites for application in neural tissue engineering. Finally, we will provide a summary and perspectives for using graphene nanocomposites as the next generation of materials for neural tissue engineering.Graphene-based nanomaterials (GNs) can be subdivided according to the number of layers, sheet dimensions, surface chemical modification status, and defect density. The most widely applied ones are single-layer graphene, few-layered graphene, GO, as well as reduced graphene oxide (rGO), as shown in Inspired by their fascinating physical and chemical properties, the GNs have been widely applied to neural tissue engineering; however, some substantial problems have significantly hindered their application. For example, most of the above GNs have been demonstrated to possess an aggregation tendency, cytotoxicity and insufficient bioactivity in physiological environments because of the strong interaction among nano-sized graphenes, uncleared biocompatibility and their inherent bio-inert nature . Thus, fUtilizing graphene’s unique surface and electrical properties to mimic in vivo cues to enhance the differentiation of stem cells into neurons is an extremely attractive strategy for the treatment of neuronal injuries and diseases. Reports have shown that GN-based materials promote stem cells to differentiate into neurons, as well as improve their proliferation rate ,49,50. HIn one study, Akhavan et al. cultured neural stem cells (NSCs) on ginseng-reduced GO nanosheets to improve the material’s biocompatibility and capability for electron transfer . Using gTo investigate the effect of the physical cues of graphene composites on human neural stem cells (hNSCs), Solanki et al. functionalized laminated GO nanoparticles on charged glass to form aligned patterns, matching the extracellular matrix (ECM) protein patterns in the human body . GO was However, the aforementioned studies were only able to capture the differentiation of NSCs on a 2D plane, which is very different from the three-dimensional (3D) ECM nanostructure. The signaling between cell-microenvironment interaction and attachment of cells onto a 3D dynamic network of macromolecules is essential to maintaining stem cell homeostasis . In addi2 matrix containing TiO2 nanoparticles. After 3 weeks of differentiation, the number of cells increased up to ~26.8-fold. Notably, the nano-grids showed lowered electrical resistance due to the connection by neurite network, implying that formation of cell-to-cell electrical couplings existed in the differentiated network. However, the neural network formed by nano-grids was limited to the 2D plane. Hence, researchers have been searching for substitutes such as nanofiber scaffolds for cultivating neural cells [Other than promoting neuronal differentiation to replace damaged tissues, GNs can also directly stimulate neural regeneration as electrodes. Electrical stimulation is vital to promoting cell proliferation and mimicking in vivo neural networks as neural cells are interconnected via neurites in native tissues . For exaal cells . Electrically conductive nanofibers additionally provide cells with favorable 3D architecture and appropriate flexibility similar to neurites in the human body. The attractiveness of using graphene to fabricate nanofibers instead of traditional conductive materials such as carbon nanotubes and poly comes from graphene’s superior electrical properties and mechanical strength. While the smooth transition from graphene’s 2D sheet to 3D nanofibers may prove to be difficult, Feng et al. circumvented the problems by assembling a GO sheet onto the surface of electro-spun poly (vinyl chloride) nanofibers, allowing the surface of the nanofiber to be wrapped tightly with graphene . The fibIn conditions like glaucoma, high intraocular pressure induces lesions in the optical nerves. While reduction of pressure can halt the damage done to retinal ganglion cells, their function cannot normally be recovered. However, the construction of nanofibrous scaffold using graphene, conductive polymer polypyrrole and polylactic-co-glycolic acid (PLGA) allowed for a high surface area for cell attachment as well as high capacitance for electrical stimulation, guiding cell neurites to regenerate along aligned fiber patterns. Through electrospinning, a fibrous mat structure with diameter at 110 nm was collected. It was shown that the fibers have tunable electroactivity by varying the ratios of graphene and polypyrrole in PLGA, allowing for a flexible therapy for optical nerve-related diseases. Results also showed the importance of electrical stimulation to the anti-aging ability of retinal ganglion cells. Without electrical stimulation, the cells showed decreased size, increased roundness and number of tentacles, resulting in mass apoptosis and lowered cell viability to 40% of the original. The ganglion cells, however, remain healthy after 10 days when electrical stimulation was applied, showing the essentiality of electrical stimulation to neural constructs. Reports have showed that electrical stimulation is essential to calcium-dependent neurite outgrowth and immediate gene expression, and that regeneration rate is frequency-dependent . Hence, While graphene itself can be used as a substrate, platelets of graphene can also be infused with other materials such as polymers to exhibit specific bulk properties while retaining its conductivity and chemical cues . AlthougAdditionally, GNs were applied in neural regeneration through drug loading and delivery through the blood brain barrier, which is normally impassable for drug molecules ,64. A naNeurodegenerative and neurological diseases like Alzheimer’s disease and Parkinson’s disease threaten an ever-increasing percentage of our ageing population. Studies have shown that deep brain stimulation and recovery of sensory capabilities by electrical stimulation based on neural recordings are extremely beneficial to neural regeneration . Hence sIn one study, Kuzum et al. fabricated transparent graphene electrodes for simultaneous electrophysiology recording and neuro imaging using graphene and polyimide . The fleGraphene has been widely proved to play a vitally important role in the proliferation and differentiation of neural cells. Significant progress has been made during the past decades in the development of graphene-based materials ,55. Up tWith the increasing utilization of graphene in a variety of biomedical applications, the in vitro and in vivo biocompatibility of graphene and its derivatives and their probable toxicities must be taken into consideration. The critical parameters which result in toxicity using graphene-based materials are: (1) The size, concentration, and shape. Smaller sizes and higher dose could lead to significant toxicities, giving rise to the fragmentations of DNA and/or chromosomal aberrations in living cells. The potentially sharp edges of graphene flakes or platelets would also result in physical damage of the wall membrane of cells and even nuclei through the direct contact interaction with cells; (2) Their possible aggregation, which may bring about adverse effects  to the blood circulation or immune system ; (3) IntAdditionally, most studies only focus on the exterior cell behaviors upon graphene exposure, thus, the interior effect of intracellular processes should be more emphasized ,75. For 2 nanoparticles and near-infrared ray-assisted photodegradation to provide a noninvasive method for circumventing this limitation [The possibility of biodegradation of graphene-based materials is another challenging issue, which should be solved especially when graphene impedes the effective interaction of cells and/or exchange of ions . Severalmitation ,79. HoweOverall, although there are still various unresolved issues and challenges, GNs are promising substances to pave the way for a real breakthrough in future studies of neural regenerative medicine.