In this system, the enhanced capacitance of the hybrid structure can be understood by the formation of a space charge layer at the graphene/Al2O3 interface. The electrical properties of the interface can be further explained by the electrical double layer (EDL) model dominated by the diffuse layer.Graphene heterostructures are of considerable interest as a new class of electronic devices with exceptional performance in a broad range of applications has been realized. Here, we propose a graphene-embedded Al However, the leakage current due to the quantum tunneling effect has become a significant problem, which limits further scale down of the SiO2 layer. Alternatively, the use of high-k dielectric materials such as hafnium oxide (HfO2), zirconium oxide (ZrO2), and aluminum oxide (Al2O3) that may provide high capacitance without scaling down to nano-sizes has received a great deal of interest24567911121315171819The fabrication of field effect transistors (FETs) with low operating voltage and input power has been researched for decades. According to the drain current equation of FETs, reduction of operating voltage and input power can be achieved with enhancement of the mobility of channel materials or the capacitance of the gate dielectricxide Al2O that mayet al.Meanwhile, graphene, which is well known for its unique properties due to carbon-based two-dimensional structures, has a relatively low density of states compared to other metals because of its linear energy-momentum relationship near the Dirac point2223242O3 capacitors deposited by atomic layer deposition (ALD) with a relatively high dielectric constant and negligible dielectric loss. For the characterization of the interface between graphene and Al2O3, we measured the frequency and gate voltage dependence of capacitance by varying the thickness of the Al2O3 layer and the number of graphene layers for further analysis of the space charge layer. With the obtained results, we also suggest a reason for enhanced capacitance, which is the formation of an electrical double layer (EDL) due to the space charge at the interface.Here, we propose thermal chemical vapor deposition (TCVD)-grown graphene-embedded Al2O3 films were deposited on cleaned ITO-coated glass using the ALD process and the graphene was synthesized utilizing the same method as previous works (see Methods)2O3 film by a poly(methyl methacrylate) (PMMA) assisted wet transfer method. The graphene was patterned by Al pre-patterning and subsequent O2 plasma etching. The lateral dimension of patterned graphene was 0.8 × 1.0 mm. On the transferred graphene, Al2O3 was deposited again through the ALD process. Finally, the top electrode (Cr/Au of 5 nm/70 nm) was deposited by thermal evaporation with a shadow mask for capacitance measurements. The chemical and structural characterization of Al2O3 and the interface properties between graphene and Al2O3 were investigated using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and transmission electron microscopy (TEM). As shown in the cross-sectional TEM image . The RMS roughness was about 0.305 nm, which was compared with Al2O3 on ITO  and the space charge polarization (right semicircle) at the interface. The relaxation time, τ0, equals the reciprocal of the frequency at the local maxima of the semicircles; hence, the calculated relaxation time of space charge polarization is about 5.78 μs, which means that the response time is appropriate for practical applications. The correlation between ac-conductivities and frequency for both capacitors is plotted in −10 S/m at 100 Hz, we can safely assume that actual leakage is even lower and conclude that leakage is negligible.2O3, charge carrier concentration is rather diffuse, the capacitance from the diffuse layer is expected to be limiting. 2O3 capacitor and a graphene-embedded capacitor. A frequency of 1 kHz was adopted for this measurement because we expect to see the EDL behavior only at a frequency lower than 10 kHz. We observed gate voltage-independent behavior in capacitance for the Al2O3 capacitor without graphene, which is similar to the graphene-embedded capacitor at frequencies of 100 kHz and higher , l is the geometrical thickness, a is the applied potential. The total capacitance of the graphene-embedded capacitor consists of two diffuse layer capacitances in series because two interfaces are formed: one between the upper Al2O3 layer and graphene, and another between graphene and the lower Al2O3 layer. Thus, total capacitance in the low frequency region is given by:where εCD1 and CD2 are the diffuse layer capacitances of the upper and the lower layer, respectively. The total capacitance was slightly asymmetric with the direction of dc bias, causing a slight shift in the C–V curve. There are two possible reasons for this; one is that the formation of a direction-dependent potential due to the work function difference between the upper (Au) and lower (ITO) electrodes. Another is that the difference in dielectric constants of upper and lower Al2O3 layers is presumably due to the difference in surface conditions when depositing the layers. To extract the value of 2O3 layer. The result was fitted by combining 2O3 layers by fitting are approximately 19 and 27 nm, and dielectric constants are 4 and 8, respectively. The dielectric constant of upper Al2O3 layer is lower than that of the lower layer292O3 layer decreases below the calculated where 2O3 to explore the effects of the number of graphene layers to the diffuse layer formed at the interface, where a PMMA layer-by-layer transfer technique2O3 interface. It was reported in previous literature that the hole concentration in graphene is proportional to the number of graphene layersD calculated from Single, bi-, and tri-layer graphene sheets were inserted in Al2O3 to charge carrier transfer at the graphene/Al2O3 interface. The charge carrier transport will induce a highly intense accumulation of negative charges on graphene (inner Helmholtz layer); then, the positive charges form a diffuse layer at the interface between the graphene and Al2O3 due to the Coulombic attraction, forming the EDLs as stated above. Since Al2O3 is an insulator with low carrier concentration, the diffuse layer is typically broad and becomes the limiting factor in induced capacitance. When the graphene layer is increased from a single to a tri-layer, the diffuse layer becomes narrow, indicating that charge carrier concentration within the diffuse layer has increased. Such an increase in charge carrier concentration occurs because electrostatic interaction has become stronger at the graphene/Al2O3 interface due to increasing carrier density within graphene caused by band structure overlap.We attribute the formation of EDL in graphene-embedded Al2O3 capacitor with a relatively high dielectric constant of 15.5 and low leakage current, especially with insertion of tri-layer graphene. For characterization of the interface between graphene and Al2O3, we measured the frequency and gate voltage dependence of capacitance by varying the thickness of the Al2O3 layer and varying the number of graphene layers for further analysis of the space charge layers formed on both sides of the graphene. Results obtained from various thicknesses of Al2O3 indicate that such enhancement of capacitance occurs due to the formation of a pair of EDLs at the graphene/Al2O3 interface. Capacitance dependence on applied gate voltage measurements suggest that the diffuse layer of EDL is the limiting and controlling factor of the capacitance enhancement, which is reasonable since Al2O3 can be considered as a very diffuse system with low carrier concentration. We also calculated the width of the diffuse layer for single, bi-, and tri-layer graphene-embedded capacitors, and they were 27.0, 24.5, and 23.3 nm, respectively. This result was explained by the change of carrier density in graphene. Even though the proposed dielectric material is not suitable for high-frequency applications such as radio frequency devices due to the limitation of response time, the enhancement of capacitance for a gate dielectric material with such a simple fabrication method can be considered a valuable asset in rapid-free device applications such as sensors and integrated circuit for display. Other properties of graphene, such as its superior mechanical strength, can make this method applicable in devices for various purposes, making graphene-insulator heterostructures even more appealing.In summary, we proposed a graphene-embedded Al2 (2 Torr) atmosphere at 1050 °C for 30 min, after which 1:50 mixture of CH4 and H2 was introduced for 20 min. The samples were cooled down to room temperature to complete the synthesis.Graphene was synthesized on 25 μm-thick copper (Cu) foils (Sigma Aldrich) using conventional TCVD. Cu foils were located in the quartz tube and pre-annealed under H2O3, trimethylaluminum (TMA) was employed as the precursor with H2O reactant as the oxidant. Each cycle consists of the TMA exposure of 0.3 s, Ar purging of 10 s, water vapor exposure of 1 s, and another Ar purging of 10 s in sequence at the substrate temperature of 100 °C. Total of 300 cycles was performed.For the deposition of Al2O3 capacitors with/without graphene sheet was conducted over the frequency range of 100 Hz–1 MHz with ac amplitude of 50 mV.All room temperature dielectric measurements were performed using an Agilent 4284A impedance analyzer. The measurement of dielectric performance of the AlHow to cite this article: Ki Min, B. et al. Electrical Double Layer Capacitance in a Graphene-embedded Al2O3 Gate Dielectric. Sci. Rep.5, 16001; doi: 10.1038/srep16001 (2015).


Characterization of the electrodes was performed by scanning electron microscopy (FESEM and SEM). The electrochemical properties of the modified graphene electrodes were inspected by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and amperometry.A rapid prototyping of an inexpensive, disposable graphene and copper nanocomposite sensor strip using polymeric flexible substrate for highly sensitive and selective nonenzymatic glucose detection has been developed and tested for direct oxidization of glucose. The CuNPs were electrochemically deposited on to the graphene sheets to improve electron transfer rates and to enhance electrocatalytic activity toward glucose. The graphene based electrode with CuNPs demonstrated a high degree of sensitivity (1101.3±56 μA/mM.cm Current glucose sensor technology relies mostly on enzymatic sensing strips that require repeated painful pricking methods to measure glucose concentration. The typical range of sensitivity for a blood glucose sensor is from 1.0 mM to 60 mM  + 2mM K4[Fe(CN)6] in the frequency range of 0.1 Hz to 100 kHz was used to obtain information on electron transfer rates between the electrolyte and the electrode surface. EIS normally includes a semicircular part and a linear part. The semicircular part at higher frequencies corresponds to the electron transfer limited process, and the diameter is equivalent to the electron transfer resistance . The processing method for the prototyping requires only several minutes of processing time at room temperatures, enables direct fabrication on flexible polymer substrates without an additional transfer step, and delivers strong surface adhesion between the few graphene layers and the flexible polymer substrates. Following the fabrication of the SCT/PRG base, the electrode was rinsed to remove loose graphene flakes and dried with low pressure argon gas to prepare for electrochemical deposition of copper nanoparticles.For this investigation, simple processing integrates inexpensive materials  to form the Graphene three electrode base. First, a PVC mask was prepared by removing a three-electrode channel into the PVC sheet using a laser engraving machine. Second, one side of the PVC sheet was taped using regular scotch tape, forming a flexible substrate with an adhesive three-electrode channel. Third, the graphene powder (≈0.25 mg for making a series of 4 strips) was placed on the three-electrode adhesive channel and physically rubbed against the surface of the adhesive Scotch Tape (SCT) in a gentle manner to form the uniform Physically Rubbed Graphene (PRG) electrodes. In the next two steps, electrodes were copper taped and then passivated by using a thin Kapton tape. Silver glue was used to enhance better connectivity in the copper-graphene intersection. An Ag/AgCl reference electrode was formed by gluing the tip of the electrode on the left side of the strip using a silver-silver chloride paste. The strip was stored in an ambient temperature for one day, for the paste to completely dry. An illustration of layers and the final strip is shown in the CuNPs were electrochemically deposited on the surface of the working electrode. Since the sensitivity and the linear range of the sensor is highly dependent on the size and population density of CuNPs, the deposition of Copper nanoparticles on the working electrode is optimized systematically to achieve the smallest possible nanoparticles with a great population density. A large copper population will maximize the output signal of the biosensor due to an increased chemical reaction between the D-Glucose molecule and the CuNPs. The applied voltage during deposition improves the graphene surface and controls the electrodeposition parameters that contribute to optimal NP population with uniform distribution . The forct for the modified electrode with CuNps (SCT/PRG/CuNPs) was smaller than the bare graphene electrode (SCT/PRG); demonstrating that the SCT/PRG/CuNPs electrode has a higher charge transfer rate than the bare graphene electrode. The copper nanoparticles are acting as electron mediators in the electron transfer process . Subsequently, CuO is electrochemically oxidized to Cu(III) species such as CuOOH or Cu(OH)4− [CuO + OH− →CuOOH or CuO + H2O + 2OH− → Cu(OH)4− + e−]. Finally, glucose is oxidized by the Cu(III) species and forms hydrolyzate gluconic acid [Cu(III) + glucose → gluconolactone + Cu(II)] [Gluconolactone → gluconic acid (hydrolysis)]  , 14. Forolysis)] –26; seveThe effect of 250 μM Ascorbic Acid (AA), 250 μM Uric Acid (UA), 100 μM dopamine (DA), and 100 μM acetaminophen (ACT), as electrochemically active interferents found with glucose in human serum, was investigated on the sensor. This was carried out by systematically examining the amperometric response of the sensor upon successive addition of these interferents with their respective concentrations, as compared to 0.3 mM glucose in the order shown in 2 and a standard deviation of 37.7 was calculated from the 9 sensor samples  that was used as the stability measure of the sensor shows only a 17.2% of signal loss in the sensor strip  bio-sensing strip by the GOx as the functionalizing agent. The use of the GOx instead the CuNPs is expected to allow a wider detection and linear range while delivering high level of sensitivity in the sensorFurthermore, it should be considered that the compatibility of the sensor’s fabrication method with the screen printing technology would create opportunities for further development of the sensor in the following ways. First, the construction of a microfluidic channel on the strip would allow for a safe tear sample collection directly from the eye. Second, an accurate passivation and padding on the sensor strip would be expected to eliminate the human error during the fabrication step and increase the reproducibility rate of the sensor to percentages even higher than what is reported in this study. Third, mass production for commercialization of the biosensor.The behavior of most nonenzymatic glucose sensors is a direct function of the electrode material on which the glucose is oxidized. Recent efforts have exploited different materials and fabrication processes for these electrodes. Nanocomposite structure of graphene-metals or metal oxides offer large surface area for the oxidation of glucose. In this research, an inexpensive, disposable graphene and copper nanocomposite through a rapid prototyping process for highly selective nonenzymatic glucose detection was developed and tested for direct oxidization of glucose. The CuNPs effectively improve electron transfer rates and enhance electrocatalytic activity toward glucose. The graphene based electrode with CuNPs displayed good sensitivity, stability, linear range, detection limit, and selectivity against interfering molecules.S1 File(XLSX)Click here for additional data file.


In this work, the piezoresistive effects of defective graphene used on a flexible pressure sensor are demonstrated. The graphene used was deposited at substrate temperatures of 750, 850 and 1000 °C using the hot-filament thermal chemical vapor deposition method in which the resultant graphene had different defect densities. Incorporation of the graphene as the sensing materials in sensor device showed that a linear variation in the resistance change with the applied gas pressure was obtained in the range of 0 to 50 kPa. The deposition temperature of the graphene deposited on copper foil using this technique was shown to be capable of tuning the sensitivity of the flexible graphene-based pressure sensor. We found that the sensor performance is strongly dominated by the defect density in the graphene, where graphene with the highest defect density deposited at 750 °C exhibited an almost four-fold sensitivity as compared to that deposited at 1000 °C. This effect is believed to have been contributed by the scattering of charge carriers in the graphene networks through various forms such as from the defects in the graphene lattice itself, tunneling between graphene islands, and tunneling between defect-like structures. Since the discovery of graphene by Novoselov and Geim in 2004, immense efforts have been focused on potential applications of graphene in optoelectronics, flexible electronics, transparent conducting electrodes, and sensors2/Vs), very large surface area of 2630 m2/g and high elastic stiffness of 340 N/m with a stretchability of up to 20%2Graphene, a two-dimensional 2D) carbon material consisting of hexagonally packed carbon atoms bonded by spD carbon x) suspended membrane pressure sensor with graphene meander patterns, working in a broad pressure regime of 0–70 kPa, was fabricated by Zhu et al.Graphene has been popularly exhibited as a sensing material for sensor devices due to its excellent mechanical and electrical properties. A miniature fiber tip pressure sensor that uses graphene as a diaphragm was demonstrated in order to enhance the sensitivity. However, only the pressures in a small range 0 to 13 kPa) could be measured due to leakage, which resulted in the non-ideal adhesion of the graphene layer to the silica capillary endface kPa coulPereira et al.10Recently, piezoresistive sensing mechanism, which translates a mechanical displacement into an electrical signal, has been widely used because of its advantages, which include feasible preparation, low cost, and easy signal collection7In this context, we chose to explore the role of graphene defects on the pressure sensing performance and incorporate these results to produce highly sensitive flexible pressure sensors. The graphene was synthesized using a hot-filament thermal chemical vapor deposition (HFTCVD) technique by varying the deposition temperature  to induce defects in the graphene films in a controlled manner. This technique was utilized because of the higher temperature ramping rate, which has great potential for the large-scale, energy efficient, and rapid manufacture of graphene, and also for producing excellent step coverage and uniform films12−5 mbar. Prior to the fabrication process, the W hot-filament was set at ~1750 °C by using an external optical pyrometer and substrate temperature at 1000 °C. A cleaning treatment was performed on the Cu foil in H2 at a flow rate of 50 sccm at 3.3 × 10−1 mbar for 20 min to remove the native oxide and increase the Cu grain boundaries. Then, the filament temperature was adjusted to the required temperature of ~1550, ~1650 or ~1750 °C to set the substrate temperature at 750, 850 or 1000 °C, respectively, for the three sets of films studied in this work. The growth of the graphene films was carried out in a CH4/H2 (50 sccm for CH4 and 10 sccm for H2) gas mixture for 30 min at a pressure of 3.7 × 10−1 mbar. Finally, the samples were slowly cooled to ~200 °C (cooling rate: ~2 °C) by reducing the external power supply and were convectively cooled to room temperature in N2 gas at 100 sccm.Graphene was fabricated on Cu foils  in a custom-made HFTCVD system2CO3). The undeveloped photoresist was etched away. Then, the unwanted, exposed portions of the Cu were removed in a 1.0 mol ferric chloride etching solution. After the exposed Cu was completely etched away, the remaining photoresist was dissolved slowly in a 3 wt% NaOH stripping solution for 120 seconds to create an IDE array with 500-μm tracks and 200-μm gap widths and two terminal electrodes. The Cu patterns were then finalized with Au electroplating with 0.3 ± 0.02 μm to provide a substantial stable nature towards oxidation.The fabrication method of interdigitated electrode (IDE) array on polyimide film substrate is presented in −1 in the range of 1000–3000 cm−1 and were calibrated using the 520.5 cm−1 line of a silicon wafer.The surface morphology of fabricated graphene-based flexible pressure sensor was characterized by optical microscopy (Olympus MX80), field emission scanning electron microscopy (JEOL JSM-7500F) at a 2-kV accelerating voltage, and atomic force microscopy in semi-contact mode . In addition, a Raman spectroscopy system (Renishaw InVia micro Raman) was utilized to obtain the Raman spectra of the films to determine the thickness and crystalline quality of the graphene layers. The Raman spectra were excited by a 514-nm laser (~2 μm spot size) at a resolution of 2 cm2 with patterned IDE/polyimide film substrate. To demonstrate the pressure sensing performance, the fabricated sensor was initially attached to the substrate with a square cavity area of 5 × 5 mm2 using epoxy. It was then sealed and clamped completely on a test jig by epoxy bonding and carbon tape to prevent gas leakage . The length of pipeline from the valve to the fabricated pressure sensor and the commercial pressure sensor was fixed at 20 cm. In this case, the applied pressure on the fabricated pressure sensor was assumed to be equal to the measurement of the reference pressure sensor. Both the diaphragm and graphene experienced deformation under the applied pressure , and the resistance changes were simultaneously measured using a source-meter  at ambient conditions.In order to develop a flexible pressure sensor, the as-transferred graphene sheet was incorporated onto an effective sensing area of 5 × 5 mmkage see . A diffe2O, which acts as nucleation site for graphene4−1, respectively . From the results, it can be seen that the evolution of the D peak shows the strong temperature dependence of the substrate temperature , suggesting a major change in the defect concentration in the graphene. The intensity of the ID/IG ratio has often been used to identify the defect density inside graphene, given that a higher ratio indicates more defects and vice versa. This is probably due to the evolutionary kinetics of the graphene formation on the copper substrate as a result of the effects of the adsorption, dissociation, dehydrogenation and copper sublimation2D/IG ratio value, where an I2D/IG larger than 1.5 indicates monolayer graphene growth on copper substrate, and good correlations have been reported in the literature14Further, these three morphologies of graphene films were selected to develop flexible pressure sensors. The morphological characteristics of the resultant transferred graphene on the sensor platform are shown in 2/Si (100) substrate with patterned Pt electrodes  were studied using the Van der Pauw method. The graphene channel’s width and length were fixed equally at 1000 μm. The contact resistance of the Pt electrodes was found to be ~3.0 Ω. The transferred graphene for a growth temperature of 750 °C exhibited a higher sheet resistance of about 1222 ± 18.3 Ω/sq. (ohms per square), but this decreased to 166 ± 6.4 Ω/sq. as the growth temperature increased to 1000 °C, as listed in 17Gao et al.The transport properties of the transferred graphene on the SiOI-V curve was performed, where the current increased linearly as the bias voltage increased from 0 to 2 V, as shown in S = (ΔR/R0)/ΔP) and the unit is kPa−1, in which ΔR is the change in resistance (Ω), R0 is the initial resistance (kΩ), and ΔP is the pressure difference (kPa). The sensitivities for graphene grown at 750, 850 and 1000 °C based on the slope of the fitted lines can be calculated as 0.0045, 0.0025 and 0.0012 kPa−1, respectively. These experimental results demonstrate that this piezoresistive effect in the graphene networks could be improved by lowering the graphene growth temperature.Prior to testing the performance of the sensor devices to applied pressure see , their i−1)−1)−1)−1, which is comparable to the sensitivity of a discrete pressure sensor. The results of repeated cycling are shown in In the present work, it has been successfully demonstrated that a higher sensitivity of sensor device can be achieved by utilizing highly-dense graphene defects as compared to previous works demonstrated for flexible pressure sensor such as the rGO-polyurethane sponge (0.001 kPaa to b) or their overlap area (from c to d); resistance by tunneling or electron hopping effect between neighboring graphene islands (from b to c); resistance by tunneling effect through defects-like line disruptions (from e to f); and resistance by point defects-like vacancies or substitutional impurities (from g to h). In view of that, measurable changes in the resistance can be predicted by exploiting the aforementioned mechanisms which could result from even small modifications to the lattice distortion in the graphene itself, contact area or tunneling distance between neighboring graphene islands with structural defects.For pristine graphene grown at a higher growth temperature, the piezoresistive effect under deformation is limited to the structure deformation-dependence of the graphene lattice distortion, which only leads to a modified electronic band structure, so the overall variation of resistance is smalleri), tunneling between the defect-like line disruptions structure (Rd), defects in the graphene lattice itself (Rg) and the contact between the graphene and IDE array (Rc). Hence, any changes to these parameter as a function of applied pressure would induce strained in graphene lattice structure and scattering of charge carriers from the increased in tunneling distance, thus resulting in a significant resistance change  to be at 0.30, 0.20 and ~0.05 μm at 750, 850 and 1000 °C, respectively. From the graph, the concentration of the defects within the graphene layer is inversely proportional to the growth temperature. It has been established that based on the tunneling mechanism, a larger tunneling distance under applied strain or pressure would enhance the change in the resistance of the graphene material. In this work, it has been demonstrated that the same characteristic is also valid for the fabricated graphene-based pressure sensor, but with further scattering effects on localized defects supporting line disruptions and dislocations. At a growth temperature of 750 °C, the deformation of the high defect density had a more prominent influence on the resistance changes than that of perfect graphene due to the quantum confinement effects in the presence of point defects and line disruption defects. The strong dependence of the resistance changes in the high defect density of the graphene networks on the structural deformation could be attributed to the creation of charge carrier scattering. Under the application of pressure, the tunneling distortion among overlapped and adjacent graphene islands and defect-like structures became more significant, which in turn increased the density of the tunneling per graphene area. This led to a higher probability of electron scattering in the percolating networks, which further reduced the density of the conducting paths, thereby effectively increasing the rate of the resistance change. On the other hand, different percolation schemes through specific type of defects  may have a significant impact to the piezoresistive effect of graphene-based flexible pressure sensors which can be further study, but these cases exceed the scope of this work.The concentration of the defects within the graphene layer which relates to the nanoparticle density, overlapping region and I−1 has been attained for the sensor using graphene grown at the lowest substrate temperature of 750 °C. However, the sensitivity could be further enhanced by patterning smaller IDE channel or lowering the thickness of the diaphragm. This investigation provided a better understanding of the use of graphene in highly sensitive piezoresistive-based flexible pressure sensors.In this work, a fabricated flexible pressure sensor incorporating graphene synthesized at various substrate temperatures of 750, 850 and 1000 °C through the hot-filament thermal chemical vapor deposition (HFTCVD) technique has been successfully demonstrated. The experimental results show that the graphene grown at a lower temperature has a higher structural-defect density as compared to those grown at a higher substrate temperature. A linear piezoresistive effect has been observed for the applied pressure within the range of 0 to 50 kPa. The sensitivity of the fabricated pressure sensor could easily be modified by varying the state of the substrate temperature. In this work, the highest sensitivity of 0.0045 kPaHow to cite this article: Mohammad Haniff, M. A. S. et al. Piezoresistive effects in controllable defective HFTCVD graphene-based flexible pressure sensor. Sci. Rep.5, 14751; doi: 10.1038/srep14751 (2015).


Structural inhomogeneities, such as the wrinkles and ripples within a graphene film after transferring the free-standing graphene layer to a functional substrate, degrade the physical and electrical properties of the corresponding electronic devices. Here, we introduced titanium as a superior adhesion layer for fabricating wrinkle-free graphene films that is highly applicable to flexible and transparent electronic devices. The Ti layer does not influence the electronic performance of the functional substrates. Experimental and theoretical investigations confirm that the strong chemical interactions between Ti and any oxygen atoms unintentionally introduced on/within the graphene are responsible for forming the clean, defect-free graphene layer. Our results accelerate the practical application of graphene-related electronic devices with enhanced functionality. The large-area monolayer graphenes were prepared by a simple attachment of the Ti layer with the multi-layer wrinkle-free graphene films. For the first time, the graphene films were addressed for applications of superior bottom electrode for flexible capacitors instead of the novel metals. Since its initial preparation by mechanical exfoliation, the chemical vapor deposition (CVD) of graphene onto Ni23478911121315161718202/Si, glass, and polymers2223Platinum (Pt) is commonly used as the bottom electrode in thin-film devices due to its high work function (5.12–5.93 eV) and excellent chemical stability at high temperatures. A titanium (Ti) layer is generally used to improve the adhesion between Pt and oxygen-containing or oxygen-philic substrates, such as Si (with native oxide), SiOHere, we hypothesize that Ti interacts with the oxygen in the substrates to form strong Ti-O chemical bonds and utilize this concept to transfer wrinkle-free graphene films to various functional substrates by introducing a Ti adhesion layer. We demonstrate that defect-free graphene films on Ti adhesion layers can be used as transparent, reliable bottom electrodes without degrading the performance, even with mechanical bending, unlike conventional Pt electrodes.x phases such as TiO, TiO2, Ti2O3, and Ti3O5 as well as metallic Ti were found at the Ti-SiO2 (main phase of glass) interface, suggesting that a large portion of the Ti interacted chemically with oxygen (oxidized), which might be from the glass substrate  after an exposure at an air atmosphere. Various TiOubstrate  and 1b. ubstrate  were connterface  and 1b. nterface . These rnterface .Chemically synthesized graphene films are inevitably exposed to oxygen-rich conditions during post processing via physical or chemical treatments. Therefore, the final graphene products incorporate oxygen species2021In these contexts, we used the Ti layer as a binding layer between the graphene film and a transferred substrate. Naturally existing oxygen species at the graphene film surface provide the oxygen for the formation of strong Ti-O chemical bonds, which benefits the adhesion between graphene and the substrate.2 using graphene films transferred to the 10 nm-thick Ti layers, respectively. Compared with the image observed at a small area  images for a graphene layer transferred to a glass substrate without a Ti adhesion layer showed severe wrinkling , which i–4.0 nm, , which i–4.0 nm, . Our finWe used density functional theory (DFT) calculations and studied the Ti-graphene interface morphology to determine the role the Ti–O chemical bonds and under-coordinated Ti sites play in forming a wrinkle-free graphene layer on the Ti adhesion layer . The DFTx) interface were confirmed via composition sensitive annual dark field (ADF) imaging and electron energy loss spectroscopy (EELS) mapping. The transferred graphene films consisted of approximately 5-6 layers in the ADF image  of the Ti-C(O) is higher than that of the C-C bonding  between the layers of the graphene. Accordingly, Ti adhesion layer coated on the glass substrate may strongly bind with the first layer of the graphene by a mechanical attachment. If this concept is possibly realized, actual monolayer graphene can be obtained from the multi-layer graphenes transferred to the substrates as well as an additional demonstration for a strong adhesion between Ti layers with the graphene. The preparation of large-area monolayer graphene from the wrinkle-free multi-layer graphene films transferred to the Ti (10 nm)/glass substrate was performed using the method shown in an inset of 2Mg2/3Nb4/3O7 (BMNO) dielectric materials with pyrochlore structures were chosen to provide a high dielectric permittivity in amorphous films grown at room temperature, which is critical for flexible devices−8 A/cm2 at 10 V and a dielectric permittivity of approximately 40–60 at 100 kHz2829Wrinkle-free graphene films on a Ti adhesion layer were addressed for the bottom electrode applications in transparent and flexible thin film capacitors. For thin film capacitors, Bi5 Hz because of the high resistivity (~10−4 Ω-cm) of graphene. Using oxide electrodes, such as indium and Al-doped ZnO (AIZO)30.5Sr0.5CoO3 (LSCO)5 Hz due to their higher resistivity (~10−4 Ω-cm) relative to the metals 2) using an applied voltage of ±5 V (−8 A/cm2 at ±5 V), which approached those of conventional capacitors with a Pt/Ti bottom electrode. The capacitors containing the Ti adhesion layers exhibited stable electrical properties (approximately 3 × 10−7 A/cm2) even at a voltage range of ±20 V /Si substrates using dc sputtering under argon atmosphere at room temperature with a 2 inch Ti metal target (purity: 99.99%). DC power for the thin film deposition was maintained at 20 W. Thin film morphologies were characterized using atomic force microscopy  and field-emission scanning electron microscope ). The interface of the Ti substrates has been investigated using X-ray photoelectron spectroscopy . To rationalized the effect of oxygen involved in the substrate, the adhesion test of the Ti layer deposited onto the oxide layer (SiO2/Si) and non-oxide layer (CrN/Si) was performed by the scratch test using a scratch tester , and the amount of the residual Ti in the scratched regions was measured using energy dispersive spectroscopy (EDS).In order to investigate the role of the adhesion layer of the Ti films, 50 nm-thick Ti thin films were deposited onto the glass and SiO3) solution was used as an oxidizing etchant to remove the nickel layersGraphene films were grown at 900°C onto the Ni (250 nm)/Si substrate via rapid-thermal pulse chemical vapor deposition  and an aqueous iron chloride (1 M FeClThe new Ti/glass substrates (top sample) were mechanically putted on the multi-layer graphene films/Ti/glass (bottom sample) in a vacuum atmosphere to prevent the oxidation of the Ti layer deposited on the glass substrate. Here, the Ti layer in a new Ti/glass substrate was attached to the graphene films transferred to the Ti/glass substrate. The schematic structure for the preparation of the new graphene films from the wrinkle-free graphene films was inserted at 2 sample was fixed and the other side was pushed for 30 s at each point. Variations in rms roughness of the samples after bending at each point were measured by AFM. The 150 nm-thick Ni top electrodes (100 × 100 μm2 size) were deposited for the electrical measurement after the bending of the BMNO/graphene/Ti/PET.200 nm-thick BMNO high dielectric constant thin films were deposited at room temperature under an oxygen atmosphere via pulsed laser deposition (PLD) using a BMNO target (purity of 99.99%) with a diameter of one in. onto the graphene/Ti/glass  and PET (200 μm) substrates. The dielectric properties of the BMNO thin films were evaluated as a function of frequency using an impedance gain phase analyzer (HP4194A). The leakage current characteristics of the BMNO capacitors were investigated via HP4145B semiconductor parameter analysis. Bending tests of the graphene/Ti (10 nm)/PET and the 200 nm-thick BMNO films/graphene/Ti /PET were performed by the method shown in the inset of −4 eV and 0.01 eV/Å, respectively. We used the Gaussian smearing method with a finite temperature width of 0.1 eV in order to improve convergence of states near the Fermi level. A single layer graphene with 72 carbon atoms and a 5 × 5 Ti (0001) slab with three Ti layers was used to describe a graphene film and a Ti adhesion layer, respectively. The bottom Ti layer was fixed during structural optimization whereas the others were released. Initially, a graphene film was oxidized with 25 oxygen atoms to produce a graphene oxide layer and the graphene-oxide film was subsequently attached to the Ti slab. Seven more oxygen atoms were appropriately added to the Graphene-Ti interface to fill out the Ti vacancies formed upon the graphene-Ti interaction.We performed GGA-level spin-polarized DFT calculations with the VASP codeRealization of Large-Area Wrinkle-Free Monolayer Graphene Films Transferred to Functional Substrates


Silicene is an exciting two-dimensional material that shares many of graphene’s electronic properties, but differs in its structural buckling. This buckling allows opening a bandgap in silicene through the application of a perpendicular electric field. Here we show that this buckling also enables highly effective modulation of silicene’s conductance by means of an in-plane electric field applied through silicene side gates, which can be realized concurrently within the same silicene monolayer. We illustrate this by using silicene to implement Self-Switching Diodes (SSDs), which are two-dimensional field effect nanorectifiers realized within a single silicene monolayer. Our quantum simulation results show that the atomically-thin silicene SSDs, with sub-10 nm dimensions, achieve a current rectification ratio that exceeds 200, without the need for doping, representing a 30 fold enhancement over graphene SSDs. We attribute this enhancement to a bandgap opening due to the in-plane electric field, as a consequence of silicene’s buckling. Our results suggest that silicene is a promising material for the realization of planar field effect devices. Graphene has long been envisaged as a promising candidate for post-CMOS electronics. However, graphene lacks an electronic bandgap, making it difficult to turn a graphene device off effectively by means of an electric field. Accordingly, many efforts have been redirected towards finding other two-dimensional materials that have an electronic bandgap but still exhibit many of graphene’s desirable electronic properties, such as its high electron mobility. Amongst numerous candidates, silicene, a new material composed of a monolayer of silicon atoms, has attracted the interest of theorists123567891215161822Here we implement SSDs using silicene and study their transport properties using Non-Equilibrium Green’s Function (NEGF) formalismOur results suggest a similarity between the electronic properties and behaviour of silicene SSDs and graphene SSDs, but show superior performance of silicene SSDs, up to a 30 fold enhancement in rectification ratio, suggesting that an in-plane electric field has much stronger control on the conductivity of silicene nanoribbons in comparison to graphene nanoribbons. We attribute this enhanced performance in silicene SSDs to a bandgap opening that results due to silicene’s buckled structure, as will be discussed later on in the discussion section. Our findings suggest that silicene could potentially be a very promising material for the realization of planar field effect devices.In the next section we present the simulation results for the transport properties of silicene SSDs followed by their discussion in the final discussion section. The calculation method is described separately in the methods sections at the end of the paper.A self-switching diode is formed by etching two L-shaped trenches through a two-dimensional material. These two L-shaped trenches define a nanoribbon in between them, which acts as the nano-channel of the device, through which conduction occurs. This nano-channel is surrounded by two side gating nanoribbons, which are used to apply a self-induced electric field onto the channel in order to control its conductance. The two side gates are illustrated through With no bias voltage applied to the device, as shown in Since the channel in a silicene SSD is a nanoribbon, it can have either armchair or zigzag edges depending on the orientation of the etched L-shaped trenches. The channel of the device illustrated in In this sub-section we investigate the transport properties of silicene SSDs with armchair nanoribbon channels. One important observation in The presented findings on silicene SSDs with armchair channels are consistent with our previous findings for graphene SSDs with armchair channelsIn this next subsection we investigate a special class of silicene SSDs; the all-silicene self-switching MISFED, Metal-Insulator-Semiconductor Field Effect Diode.p and 3p + 1 atoms, where p is an integer, have sizable bandgaps making them semiconductingp + 2 atoms have vanishingly small bandgaps, making them behave as if they were metallicIn graphene, armchair nanoribbons have a unique property, in which the bandgap of the nanoribbon varies with its width27sg), marked on p + 2 atoms. This class of graphene SSDs achieves superior rectification when compared to the other two classes of graphene SSDs with semiconducting side-gatesBy utilizing the property of being able to tune armchair graphene nanoribbons from metallic to semiconducting by width variation, we had previously proposed the all-graphene self-switching MISFED The operation principle of this class of SSDs is illustrated through the schematic diagrams in sg), are 5 (3p + 2), 6 (3p) and 7 (3p + 1) atoms respectively. The device in p + 2 type, which are expected to have metallic behaviour, and therefore, the device is expected to behave in a similar fashion to a graphene self-switching MISFEDBuilding upon the similarity between graphene and silicene nanoribbons1The calculated current-voltage (I–V) characteristics of the three devices are plotted in In order to quantify the performance of the three devices of 2, bulk materials such as compound semiconductor heterostructures30In contrast, the achieved rectification ratio for a silicene self-switching MISFED , withoutin situ using Al2O3. The Al2O3/silicene/Ag film stack is then transferred to a device substrate and patterned into the device structure using electron beam lithography (EBL) and etching, with the native Ag(111) film patterned into electrodes for contacting the device.While silicene synthesis and its subsequent stability in air has proven to be a challenge, the recently demonstrated synthesis-transfer-fabrication method named ‘silicene encapsulated delamination with native electrodes’ (SEDNE) processWhile EBL would not be able to define nanoribbons with atomic precision, narrow nanoribbons with sub-20 nm widths and rough edges can be achieved, and these would behave as semiconducting nanoribbons required to construct the channel of the SSD device. To produce metallic side gates, for the silicene MISFEDs, nanoribbons with a width greater than 50 nm can be implemented for the side gates, which would ensure that they are semi-metallic, achieving stronger control over the channel’s conductivity.Finally it is worth mentioning that with such a process, the fabricated device may not be stable in air for a long period of time, however, the planar 2D architecture of SSD allows further capping, lamination or deposition of other material layers onto silicene, which may be able to help stabilize the silicene lattice. The challenge of silicene’s stability remains to be an open area that needs further investigation, however promising progress is being made towards achieving this endeavour.Silicene shares many of graphene’s electronic properties, but differs in the fact that it exhibits a buckled structure. When a perpendicular electric field is applied to silicene, as shown in When an in-plane electric field is applied to an armchair silicene nanoribbon by two side gates with equal voltage, as in a SSD, the electric field lines between the side gates and the channel would have the distribution shown in In summary, we have investigated the effect of an in-plane electric field on silicene and showed that it results in strong control over its conductivity. The control that is achieved by the in-plane electric field on silicene is much stronger than what can be achieved on graphene due to the buckled structure of silicene, which allows bandgap tunability. We have demonstrated how this effect can be utilized in order to realize high performance planar silicene self-switching diodes. Quantum simulation results show that silicene SSDs can achieve current rectification ratios that exceed 200, without the need for doping or special passivation. This is a 30 fold improvement over what can be achieved with similar graphene devices. Our findings suggest that silicene might offer an attractive platform for the realization of next generation planar field effect devices.In order to capture silicene’s buckling property, its atomic structure was optimized using the Density Functional Theory (DFT) as implemented in Atomistix Toolkit (ATK) package137Transport calculations were conducted on each device using the Extended Huckel methodL), central scattering region (C), and semi-infinite right electrode (R). The mesh points in real space calculation were defined as uniformly spaced k points of 1 × 10 × 50, with 50 sample points along the transport direction, and 10 points along the width (induced electric field direction). In the used tight-binding model the tight-binding Hamiltonian is parameterized using a two-centre approximation, where the matrix elements are described in terms of overlaps between Slater orbitals at each site. The weighting scheme used for the orbital energies of the offsite Hamiltonian was according to WolfsburgL) and (R) with Fermi levels μL and μR through (C) according to the Landauer formulaThe device structure was partitioned as three regions: semi-infinite left electrode  is the transmission probability of incident electrons with energy E from (L) to (R), f0(E − μL(R)) is the Fermi-Dirac distribution function of electrons in (L) and (R) respectively, and V = (μR − μL)/e is the potential difference between (L) and (R).where T, is correlated with L) and (R) to (C) respectively, according to the following equation:Transmission probability, L(R) are the electrodes’ self-energies describing coupling with (C).where ∑I(V) with the summed energy dependent transmission probability, T, we need to express local current components at the atomic level along the chemical bonds to describe the variation of coherent electron transport through the system. Local current components may be investigated by extracting local transmission components. The total transmission coefficient can be split into local bond contributions, Tij, which are represented in ATK by lines along the bond lengths, called transmission pathways. The relationship between the total transmission coefficient and the local bond contributions is:As the Landauer approach only connects the external electrode current A and B represent pairs of atoms separated by an imaginary surface perpendicular along the bond length. The total transmission coefficient is the sum of the local bond contributions between all pairs of atoms A and BTij corresponds to back scattered electrons along the bond, whilst a positive value corresponds to transmitted electrons. The same calculation method was used and is described in greater details in previous work2223where How to cite this article: Al-Dirini, F. et al. Highly Effective Conductance Modulation in Planar Silicene Field Effect Devices Due to Buckling. Sci. Rep.5, 14815; doi: 10.1038/srep14815 (2015).


The LNDR strongly depends on temperature (T) as well as on SQD size, and the T dependence is consistent with photocurrent (PC)-decay behaviors. With increasing light power, the PC-voltage curves are more structured with peak-to-valley ratios over 2 at room temperature. The physical mechanism of the LNDR, governed by resonant tunneling of charge carriers through the minibands formed across the graphene/SQDs:SiO2 MLs and by their nonresonant phonon-assisted tunneling, is discussed based on theoretical considerations.One of the interesing tunneling phenomena is negative differential resistance (NDR), the basic principle of resonant-tunneling diodes. NDR has been utilized in various semiconductor devices such as frequency multipliers, oscillators, relfection amplifiers, logic switches, and memories. The NDR in graphene has been also reported theoretically as well as experimentally, but should be further studied to fully understand its mechanism, useful for practical device applications. Especially, there has been no observation about light-induced NDR (LNDR) in graphene-related structures despite very few reports on the LNDR in GaAs-based heterostructures. Here, we report first observation of LNDR in graphene/Si quantum dots-embedded SiO Their device performances, however, are still far below commercial standards. For example, the quantum efficiency, on/off ratio, and responsivity of SQD PDs are currently as low as ∼1% at 530 nm/5 V23567Si is a principal material in semiconductor industries, but is of limited use in its photonic device applications due to its small- and indirect-bandgap nature. Si quantum dots (SQDs)235679102 (SQDs:SiO2) multilayers (MLs)-heterojunction tunneling diodes2 MLs heterojunction diodes.Since the advent of graphene in 2004, its high optical transparency, large carrier mobility, and easy tuning of work function have made it play key roles as transparent electrodes and others in various kinds of graphene-based device structures such as heterostructures with two-dimensional materials1315172 MLs heterojunction tunneling diodes. The LNDR behaviors strongly depend on SQD size (d), temperature (T), and irradiance power (P), and are well explained by resonant tunneling of charge carriers through the miniband formed across the graphene/SQDs:SiO2 MLs and by their nonresonant phonon-assisted tunneling. The T dependence of the LNDR properties is consistent with that of the lifetimes found in photocurrent (PC)-decay curves. As P increases, the photo I-V curves are more structured with the peak-to-valley ratios from ∼1.5 to ∼2.2 at room temperature, possibly originating from electric-field screening due to space charge buildup and state filling.Negative differential resistance (NDR) has a long history as one of the important tunneling phenomena not only under dark202122242527282931342 MLs tunneling diodes, composed of SQDs:SiO2 MLs with a total thickness of ∼100 nm, between monolayer graphene sheet and n-type Si wafer  SiO2 barriers, and broaden into minibands (extended Bloch-type states) if the mean free path of the carriers exceeds the ML period, as shown in 2 MLs become localized in the SQDs along the direction perpendicular to the layers and conduction proceeds by phonon-assisted tunneling (hopping between adjacent wells)V > 0 on graphene), the barrier oxide field increases linearly with the applied electric field2 MLs from the substrate (n-Si) to graphene by electron tunneling through minibands, thereby producing a dark current (DC). The rapid increase of current at larger d indicates the current is mainly governed by the tunneling probability that is enhanced at larger d due to the reduced barrier (SiO2) thickness, as shown in V < 0) the hole carriers give rise to the main current through phonon-assisted-tunneling conduction  curves under reverse bias for various SQD sizes. Note that the negligible PC is observed for the forward bias. This can be understood in terms of the slow transit of the electrons in the region of the SQDs:SiO2 MLs 2122T as well as d. In contrast, such RTD behaviors are not observed in dark I-V curves under forward bias as well as under reverse bias, irrespective of T and d  until the voltage reaches the valley voltage VV, as indicated in VV, the current through the device increases again as in conventional diodes. The T dependence of the peak voltages for each d are summarized in The most striking feature of the measured PCs in  T and d , possiblI-V curves more structured with peaks and valley were obtained for d = 2.5 nm, as shown in P-dependent monotonic shift of VP, as shown for d = 3.4 nm in d = 2.5 nm, the 1st peak voltage VP1, the 2nd peak voltage VP2, and VV are indicated in P-dependences are summarized in The NDR behaviors seem to strongly depend on the density of charge carriers, judging from no finding of the phenomenon without illumination. And there have been several reports on the dependence of the NDR behaviors on the carrier density in III-V superlattices25To probe the charge-transfer dynamics, transient PCs were recorded for various bias voltages and temperatures, as shown in 2 MLs structure, as explained above. Since at the valley voltage VV the electron wave functions become strongly localized within each SQD region, the overlapping of the wave functions decreases and the current is therefore reduced, which results in NDR. The threshold condition for the decrease in the overlapping of the wave functions is given approximately by eVd ~ ΔE. Here, Vd is the voltage drop across the ML period  and ΔE is the width of the miniband. Thus, the peak voltage VP ≈ ΔEN/e. As a result, the NDR in the SQDs:SiO2 MLs occurs when the potential energy difference between two adjacent SQDs in the MLs exceeds ΔE, which corresponds to the transition from band-like conduction to phonon-assisted-tunneling (hopping) conduction between the localized states of the SQDs. Clearly, this gives rise to NDR because the spatial overlap between the states of neighboring QDs decreases with increasing bias voltage. For example, no NDR is observed at temperatures below 200 K for the smallest SQD system (d = 2.1 nm), as shown in V curves show well-defined NDR behaviors. This phenomenon can be explained by the decrease of the recombination lifetime in the non-resonant tunneling regions at high temperatures.The observed NDR phenomena can be understood by considering the miniband formation in the SQDs:SiOE(k) = E0 [1 − cos(kds)], where 2E0 (=ΔE) is the band width and ds is the average distance between SQD layers . E0 (=ΔE/2) is proportional to the tunneling probability (TP) through the barrier which can be approximated by TP ≈ exp Vb is the barrier height and w is the width of the potential barrier. For larger w, TP is exponentially suppressed, as described in E becomes smaller. As summarized in VP increases with increasing temperature T for the four different SQD sizes. The current increases with increasing the bias voltage up to VP by the miniband conduction, as shown in VP to VV, the current decreases through the phonon-assisted tunneling. The resonance through the miniband occurs again at a higher bias voltage, resulting in a current increase above VV, consistent with the results in T is increased, the phonon scattering becomes stronger due to larger population of optical phonons at higher T. The phonon-assisted tunneling gives rise to the shift of VP, which is more sensitive to T at smaller d, as shown in d. The PC decay time or the life time decreases with increasing the temperature due to the enhanced phonon-assisted tunneling, as shown in T-dependent variations of the peak voltages, as shown in The energy dispersion relation of the miniband is given by the simple one-dimensional model as P, the plateau between VP1 and VP2 shrinks, and finally, it disappears as the two peaks merges at P = 3.0 mW. In contrast, the slope between VP2 and VV expands as P increases. The peak (VP2)-to-valley (VV) ratio varies from 1.57 to 1.84 in the range of P between 0.5 and 2.5 mW. These evolutions suggest that a blocking process in carrier transport exists around extrema VP1, VP2, and VV under high carrier densities. The shape of the photo I-V curve as a function of P is thought to be strongly affected by electric-field screening due to space charge buildup and state filling25VP1corresponding to the resonant tunneling through the miniband moves toward a higher voltage as the excited carrier density is increased. Simultaneously, valley VV, which is thought to correspond to increasing nonresonant phonon-assisted tunneling, also moves towards higher bias voltage. These behaviors can be explained by the electric-field screening generated by the space charges. Even when the carrier transport may be dominated by the resonant-tunneling-transfer path, it is considered that the carriers cannot enter into the miniband states in adjacent SQD filled by the optical excitation at point VP1. This will saturate the current up to VP2. Note that the position of VP2 is almost invariant. This means that the state filling is reached faster when the carrier density is higher, thereby causing such a pinning of VP2 for various light powers. The P-dependent monotonic shift of VP for d = 3.4 nm, as shown in VP)-to-valley (VV) ratio varies from 1.53 to 2.21 in the range of P between 0.5 and 3.0 mW.As shown in In these tunneling diodes we use graphene as an electrode. In addition to the electronic/optical advantages of graphene, graphene as a transparent electrode plays an important role in the observation of NDR in photo I-V curves. Unlike the devices with metal electrodes, the tunneling diodes with graphene electrodes give rise to the NDR behaviors in the wide range of light wavelength if the photon energy is greater than the band gap of SQDs. We also mention that the NDR behaviors can be adjusted by tuning the Fermi energy of graphene, which can be achieved by doping (or gating). The density of states of graphene affects the peak to valley ratio because the density of states of graphene is zero (or minimum) at the Dirac point. The shift of Dirac point with varying the applied bias gives a strong effect on the observed current because the current directly depends on the density of states.The achievements in this work  will enhance the possibilities that the heterojunction diodes can be employed in optoelectronic devices such as optical modulators, optical switches, optical memories, and photodetectors for imaging, sensing, recording, and communications.x/SiO2 MLs with 50 periods of 2 nm thin layers were grown on n-type (100) Si wafers at room temperature (RT) using an Ar+ beam with an ion energy of 750 eV and a Si target under oxygen atmosphere in a reactive ion beam sputtering system with a Kaufman type DC ion gun . HRTEM proved regularly-distributed SQDs within SiO2 matrix  in a graphite-heater-based chemical-vapor-deposition quartz tube furnace at a growth temperature of 1000 oC with 10-sccm H2 and 20-sccm CH4 flowing at a pressure of 3 Torr412 MLs/n-type Si wafers and blow-dried with dry N2. The PMMA/graphene/SQDs:SiO2 MLs/n-Si stack was then heated on a hot plate in air at 180 oC for 2 h to cure the PMMA. After the samples were cooled to RT, the PMMA was stripped by soaking them in acetone for 1 h at RT. Subsequently, the graphene/SQDs:SiO2 MLs/n-Si stack was put in isopropyl alcohol for 10 min and dried by blowing N2 to minimize the water traps possibly present at the graphene/SQDs:SiO2 MLs interface, and annealed at 400 oC for 1 h in vacuum to remove the surface adsorbates2 MLs and the graphene/SQDs:SiO2 MLs, respectively by AFM height profiles (G/I2D) of the G and 2D bands peaked at ~1585 and ~2683 cm−1, respectively, was about 0.47 on the graphene/SQDs:SiO2 MLs  was used to measure the average power of the laser pulses. The laser pulse temporal profiles and transient PC were monitored with the 50-ohm-terminated, 500-MHz–bandwidth digital oscilloscope (Tektronix DPO 4054)How to cite this article: Lee, K. W. et al. Light-induced negative differential resistance in graphene/Si-quantum-dot tunneling diodes. Sci. Rep.6, 30669; doi: 10.1038/srep30669 (2016).


Programmed cell death-ligand 1 (PD-L1) and driver mutations are commonly seen in non-small-cell lung cancer (NSCLC). However, the prevelance of PD-L1 over-expression and its prognostic value in Epstein–Barr virus (EBV) associated pulmonary lymphoepithelioma-like carcinoma (LELC) remains poorly understood.A total of 214 NSCLC patients and 113 surgically treated pulmonary LELC patients were included. Paraffin-embedded tumor sections were stained with PD-L1 antibody. Correlations between PD-L1 expression and clinicopathological features as well as survival outcomes were analyzed.p = 0.008). Multivariate analysis shows that PD-L1 expression level, N stage and M stage were independent prognostic factors for DFS. N stage and M stage but not PD-L1 expression level were significantly associated with overall survival (OS).The frequency of PD-L1 over-expression in NSCLC was 51.4%. No significant association was observed between common driver mutations and PD-L1 over-expression. Remakably, the positive rate of PD-L1 in pulmonary LELC was 74.3%. High PD-L1 expression was associated with impaired diseas-free survival (DFS) compared with low PD-L1 expression (PD-L1 over-expression was not related to common driver mutations in NSCLC. Pulmonary LELC have remarkably high incidence of PD-L1 expression. PD-L1 was a negative prognostic factor for DFS in surgically resected pulmonary LELC. These findings may provide a rationale for immunotarget therapy in this virus-associated lung cancer. Lung cancer is a leading cause of cancer-related mortality worldwide . Non-smaAnother less understood etiology of NSCLC is Epstein–Barr virus (EBV) infection . This raIn vitro studies have shown that driver mutations not only directly promote the proliferation of cancer cells but also indirectly induce immune evasion via the up-regulation of PD-L1 [Recently, immunotherapy has intensively been studied in a variety of cancers and is emerging as a promising treatment option . Evolutiof PD-L1 . Howeverof PD-L1 –12. Alsoof PD-L1 –16. HoweTherefore, the present study aimed to prospectively explore the association between PD-L1 expression and common driver mutations in NSCLC. Moreover, we investigated the prevalence and prognostic role of PD-L1 in a large cohort of surgically resected pulmonary LELC.To avoid selection bias, the first cohort prospectively enrolled 214 non-selective NSCLC patients. Baseline characteristics of these patients are presented in Table p = 0.034), tumor differentiation (p < 0.001) and gender (p = 0.010). However, no significant association was observed between PD-L1 expression and age (p = 0.398), smoking status (p = 0.372), stage (p = 0.548), EGFR mutations (p = 0.611), ALK rearrangements (p = 0.099) or KRAS mutations (p = 0.199). The most striking phenomenon was the PD-L1 expression in pulmonary LELC. In the 11 pulmonary LELC patients enrolled, 10 (90.9%) of them demonstrated PD-L1 positivity with a median H-score of 150 . Pulmonary LELC showed 9 times higher chance of having PD-L1 over-expression than non-LELC did . The remarkable phenomenon led us to expand this cohort of patients to study the overall prevalence and prognostic role of PD-L1 in pulmonary LELC.For PD-L1 immunohistochemical staining, we defined cases with more than 5% expression as positive ones. Thus, a total of 110 (51.4%) patients were positive for PD-L1 with a median H-score of 15 . The correlation between PD-L1 expression and patients' characteristics is shown in Table The second cohort involved 113 consecutive pulmonary LELC patients who were surgically treated in Sun Yat-sen University Cancer Center. The baseline characteristics of these patients are shown in Table p = 0.004). T stage was also significantly associated with PD-L1 over-expression. Other clinicopathological variables, including gender, smoking history, lymph node stage (N stage), metastasis and pathological stage were not significantly associated with PD-L1 over-expression. Due to the rare mutation rate of EGFR, ALK and KARS, the assessments of the association between PD-L1 over-expression and driver mutations are infeasible.As is shown in Table p = 0.008)   and those with high PD-L1 expression (H-score > 30). Kaplan–Meier analysis revealed that patients with high PD-L1 expression had shorter DFS than those with low PD-L1 expression . For OS, advanced N stage and advanced M stage remained the independent risk factor for poor prognosis Table .p = 0.008). The mutation rate in common driver genes including EGFR, KRAS and ALK was extremely low in pulmonary LELC. These results imply that pulmonary LELC is a very distinct subtype of lung cancer in terms of genetic aberrations and immune status.The present study reveals that the overall incidence of PD-L1 over-expression in non-selective NSCLC patients was 51.4% (110/214). There was no significant association between common driver mutations  and PD-L1 over-expression. However, PD-L1 over-expression was significantly associated with gender, tumor differentiation and pathological types. Remarkably, in pulmonary LELC, a rare subtype of primary lung cancer that is closely related to EBV infection, we found a very high incidence of PD-L1 over-expression . This phenomenon led us to expand this cohort of lung cancer and to investigate the clinical significance of PD-L1 in pulmonary LELC. In the expanded cohort involving 113 surgical LELC samples, the incidence of PD-L1 over-expression was 74.3%, which was consistent with the study by Chang et al. . High PDThe incidence of PD-L1 over-expression in NSCLC has been reported to vary from 19.6% to 65.9% and its association with clinicopathological features as well as patients' prognosis is very controversial –21. In Cin vitro studies provide solid evidence that PD-L1 could be regulated by oncogenic pathways. However, in tissue level, the association between PD-L1 and driver mutations is rather controversial. In the present study, we found no significant association between PD-L1 expression and EGFR mutations, KRAS mutations or ALK rearrangements, consistent with a previous study by Yang et al [in vivo studies on the association between oncogenes and PD-L1 regulation. In addition, the expression patterns of PD-L1 deserve deeper investigation. Yet some studies have shown that PD-L1 was positively associated with driver mutations in NSCLC samples [PD-1/PD-L1 pathway has been recognized as a key mediator of immune suppression . Cancer ng et al . Howeverng et al . One pos samples , 30. TheAnother mechanism of PD-L1 up-regulation is called adaptive immune resistance. In cancers with chronic virus infection, the anti-cancer and/or antiviral immune response could induce the secretion of important inflammatory factors like interferon gamma (IFN-γ), which may be utilized by cancer cell itself to maintain immune suppressive milieu. Our previous study have indicated that the up-regulation of PD-L1 in EBV infected NPC was partly mediated by IFN-γ. Pulmonary LELC is a rare subtype of primary lung cancer that is morphologically indistinguishable from NPC. It is also closely related to EBV infection indicated by positive in situ hybridization of EBERs and is characterized by intensive lymphoid infiltration. Unlike lung adenocarcinoma, pulmonary LELC rarely presents with EGFR mutations, KRAS mutations or ALK rearrangements. In a study by Liang et al. , they foAll in all, we show here that PD-L1 expression was not related to common driver mutations in NSCLC tissues and PD-L1 over-expression is a hallmark of pulmonary LELC. We further propose that the PD-1/PD-L1 axis might play a critical role both in the persistent infection of EBV and the resistance to immune elimination during malignant progression in pulmonary LELC. These findings extend those recently reported in NPC, hepatocellular carcinoma, which are virus-associated cancers and has also been considered as “immunogenic.” Our studies support a rationale for administering anti-PD-1 and anti-PD-L1 therapies to the EBV infected pulmonary LELC population. Future studies will need to characterize the genomic signatures of this tumor and identify additional factors responsible for inducing PD-L1 expression and hence immunosuppression within the tumor microenvironment.This study enrolled two cohorts of NSCLC patients. The first cohort prospectively enrolled newly diagnosed NSCLC patients in Sun Yat-sen University Cancer Center (SYSUCC) from January 2014 to December 2014. Patients who were pathologically diagnosed as NSCLC and provided sufficient tumor tissue for genomic analysis were included. The second cohort focused on surgically resected pulmonary LELC. NSCLC patients who were surgically treated in SYSUCC between January 2008 and December 2012 were retrospectively reviewed. Those who were diagnosed as pulmonary LELC with positive EBER results were eligible. All patients underwent endoscopic examination of the nasopharynx to rule out metastatic NPC. Detailed process of patient selection is shown in Patients' medical records were reviewed for the collection of clinicopathological information including gender, age, smoking status, tumor size, staging and treatments. Pathologic or clinical staging was determined according to the seventh edition of American Joint Committee on Cancer (AJCC)/International Union against Cancer (IUCC) staging system, which is based on tumor size, location, and the extent of lymph node or distant metastases. OS was defined as the time between surgery and death from any causes. Disease-free survival (DFS) was defined as the time from surgery until recurrence or death. Patients were censored at their last known alive date. Follow-up information was obtained from patients' medical records or by telephone interview.H = 7.4) and cooled slowly at room temperature for 20 minutes. After blocking the activity of endogenous peroxidase with 3% hydrogen peroxide for 8 minutes, the sections were treated with anti-PD-L1 antibody and incubated overnight. Subsequently, the slides were rinsed in PBS three times and incubated in HRR-linked secondary antibodies. After incubation, slides were washed again with PBS and then visualized using diaminobenzidine. The sections were counterstained with hematoxylin and then mounted.Immunohistochemical staining was performed using rabbit monoclonal anti-PD-L1 antibody  to determine the expression of PD-L1. Four-μm-thick sections from formalin-fixed, paraffin-embedded (FFPE) tissue blocks were de-waxed and rehydrated accordingly. For antigen retrieval, slides were heat-treated for 30 minutes in citrate buffer solution . Cases with more than 5% expression of PD-L1 were considered positive according to previous reports.EGFR mutations and KRAS mutations were detected using PCR-based direct sequencing. Briefly, genomic DNA was extracted from tumors embedded in paraffin blocks. PCR amplification was done using HotStarTaq DNA polymerase  using specific primers. PCR products were sequenced directly using Applied Biosystems PRISM dye terminator cycle sequencing method  with ABI PRISM 3100 Genetic Analyzer . ALK rearrangements were detected by Fluorescence in situ Hybridization (FISH) using a break-apart probe to the ALK gene  per manufacturer's instructions. At least 100 representative tumor cells were counted and then analyzed by using an Olympus fluorescence microscope equipped with orange, green, and 4′, 6-diamidino-2-phenylindole filters. Images were captured using the Video Test Image Analysis System. FISH-positive cases were defined as ≥ 15% of the tumor cells that showed a split red and green signal and/or an isolated (single) red signal. Otherwise, the specimen was classified as ALK FISH negative.p-value < 0.05 was deemed significant. Optimal cutoff value of PD-L1 H-score for survival discrimination was determined by X-tile software  as previously described [All the statistical analyses were performed using SPSS version 22.0 . Pearson's chi-squared test or Fisher's exact test was used to assess the correlations between PD-L1 expression and categorical variables. Median H-scores of PD-L1 was compared between groups using Mann-Whitney U test. The survival analysis was performed with Kaplan–Meier method and log-rank test was used to evaluate survival difference. Subsequently, the PD-L1 expression status was entered into multi-variate Cox regression analysis. Hazard ratios (HR) and 95% confidence intervals (CI) were used to determine the prognostic value of PD-L1 expression. A two sided escribed .The sample size of the first cohort was calculated prior to the initiation of the present study. Sample size was calculated to obtain an accurate estimate of the proportion of patients with PD-L1 positive tumors. Assuming a percentage of 50%, more than 193 samples were required to ensure a 95% CI of less than ± 7% (Wilson score method). Taking into account patients with tumors of undetermined status, an overall sample size of 214 was chosen.


Chordomas are primary malignant tumors of the notochord that are resistant to conventional chemotherapy. Expression of programmed cell death ligand 1 (PD-L1), prevalence of tumor-infiltrating lymphocytes (TILs), and their clinical relevance in chordoma remain unknown. We evaluated PD-L1 expression in three chordoma cell lines and nine chordoma tissue samples by western blot. Immunohistochemical staining was performed on a chordoma tissue microarray (TMA) that contained 78 tissue specimens. We also correlated the expression of PD-L1 and TILs with clinical outcomes. PD-L1 protein expression was demonstrated to be induced by IFN-γ in both UCH1 and UCH2 cell lines. Across nine human chordoma tissue samples, PD-L1 protein was differentially expressed. 94.9% of chordoma samples showed positive PD-L1 expression in the TMA. The expression score of PD-L1 for metastatic chordoma tumors was significant higher as compared with non-metastatic chordoma tumors. Expression of PD-L1 protein significantly correlates with the presence of elevated TILs, which correlates with metastasis. In summary, our study showed high levels of PD-L1 are expressed in chordoma, which is correlated with the prevalence of TILs. The current study suggests targeting PD-L1 may be a novel immunotherapeutic strategy for chordoma clinical trials. Chordoma, an extremely rare cancer presumably originating from notochord, accounts for 20% of primary spine tumors and 1- 4% of primary malignant bone tumors . The staThere is growing interest in the immunoregulatory receptor programmed cell death 1 (PD-1) and the corresponding B7 family of ligands as a pivotal mechanism of tumor immune tolerance and escape in cancer. PD-1 is a member of the B7-CD28 family that co-regulates T-cell receptors and contributes inhibitory signals to mediate physiological immune escape and tolerance , 7. PD-1Tumor-infiltrating lymphocytes (TIL) in many cancers express PD-1 and have been shown to be correlated with high PD-L1 expression in different cancers -15. In tThe aim of our study is to investigate the expression level of PD-L1, TILs, and clinicopathological parameters in chordoma, with a particular focus on any potential correlation with prognosis.Morphology of chordoma cells usually consist of small non-vacuolated cells, intermediate cells with a wide range of vacuolization, and large heavily vacuolated  cells , 25. UCHTo validate the chordoma cell line data, the PD-L1 western blot assay was performed on total protein isolated from 9 chordoma human tumor samples Figure . BrachyuP = 0.0057; Figure PD-L1 immunoreactivity was found in the cytoplasm of tumor cells. PD-L1 staining was detected in 78 TMA samples (including recurrence or metastasis) from 56 patients. The median age was 62 years (range: 24 - 85), and predominantly male (69.6% of patients). Of the 78 chordoma samples, 94.9% were positive for PD-L1 expression. Based on PD-L1 staining intensities in tumor samples, no staining (0) and weak staining (1+) specimens were classified as PD-L1-low patients (57.1%); moderate staining (2+) and intense staining (3+) specimens were classified as PD-L1-high patients (42.9%) Figure . We evalP = 0.0532; Figure P = 0.0325).The median overall survival for patients with low PD-L1 expressing tumors was 145 months compared with 81 months for patients with high PD-L1 expressing tumors, exhibiting a borderline trend, but not achieving statistical significance .Representative pictures of chordoma cases showing different levels of TILs (scores 0 - 2+) were depicted in Figure P = 0.0415, Figure We evaluated the clinicopathologic features of chordoma tumor samples and found no significant relationship between PD-L1 expression and age at surgery, gender, or tumor location. The presence of increased TILs was not significantly associated with survival. The median overall survival for TILs-few patients compared with TILs-prominent patients exhibited a borderline trend, but did not achieve statistical significance Figure . There wPD-L1 has been shown to be expressed in different solid tumors -23. Our Loss of tumor suppressor phosphatase and tensin homolog (PTEN) had been shown to increase PD-L1 expression and immunoresistance in glioma . In primIn order to characterize the relationship between the expression level of PD-L1 protein and clinical behavior, we used a chordoma TMA that allowed the simultaneous characterization of the expression status of PD-L1 in 78 chordoma samples. Our study showed that 42.9% samples were classified as staining highly for PD-L1. In our analyses, there was no correlation of PD-L1 expression with age at surgery, gender, or tumor location. Our study showed the median PD-L1 expression score for the group with metastsis was significantly higher compared with the PD-L1 expression score for the group without metastasis. Furthermore, previous studies have demonstrated a strong correlation between the expression of PD-L1 and poor prognosis in different solid tumors, including glioblastoma, pancreas, ovarian, breast, renal cell carcinomas, head and neck squamous cell carcinomas, esophageal, and non-small cell lung cancers , 37-41. TILs induce expression of PD-L1 through up regulation of cytokines, such as IFN-γ . In accoMultiple agents targeting the PD-1/PD-L1 system in cancer are currently at different stages of clinical development -46. Thesin vitro [Since these initial reports of clinical activity of PD-L1 blockade therapy in several tumor types, including advanced melanoma, lung, and renal cell carcinomas in 2012, there has been a growing interest in immunotherapy to see if these agents could be applied to other tumor subtypes. Furthermore, PD-L1 silencing with siRNA has been shown to inhibit colon cancer proliferation, migration, and invasion in vitro . Increasin vitro . On the in vitro . In conc2-95% air atmosphere at 37°C and passaged every 3 - 4 days using trypsin-EDTA when cell confluence reached 90–100%.UCH1 and UCH2 are established human chordoma cell lines and were kindly provided by Dr. Silke Bruderlein  , 50. Ano5 cells were incubated at 37°C for 48 hours in medium supplemented with 100 U/mL, and then washed with excess culture medium.Cells were treated with recombinant human interferon IFN-γ , as previously described . BrieflyExpression of PD-L1 protein was evaluated by Western Blot analysis. Protein lysates from chordoma cell lines were extracted using 1× Cell Lysis Buffer . The protein concentrations were determined using Protein Assay Reagents  and a SPECTRAmax Microplate Spectrophotometer from Molecular Devices . The primary antibodies for PD-L1 (1:1000 dilution), brachyury (1:1000 dilution), and β-actin  were purchased from Abcam, Santa Cruz, and Sigma-Aldrich, respectively. The secondary antibodies were bound to IRDye1 800CW or IRDye1 680LT . Western blots were carried out as previously described . NormaliNine of the chordoma tissue samples Tissue1–Tissue9) were obtained from the Massachusetts General Hospital Sarcoma Tissue Bank and were used in accordance with the policies of the institutional review board of the hospital (IRB protocol # 2007P-002464). Written informed consent was obtained from all patients whose specimens and clinical information were used for this research study. All tissue diagnoses were confirmed histologically, and proteins were extracted from these frozen tissues. The data of each patient's age, gender, metastasis, recurrence, tumor location(s), follow up months, and disease status were collected . A retrospective study of 78 samples (including recurrent and metastatic) from 56 chordoma patients was identified for the tissue microarray (TMA) immunohistochemical staining as previously reported . The cliThe expression level of PD-L1 was determined based on the Immunohistochemistry Protocol (Paraffin) from Cell Signaling Technology . Briefly, 5-μm-thick array sections were baked at 60°C for 1h, dewaxed with xylene (three times for 5 minutes each), transferred through 100% ethanol (twice for 5 minutes each), rehydrated through graded alcohol, and then immersed in deionized water for 10 minutes. Antigen retrieval was processed with Target Retrieval Solution . After antigen retrieval, the slide was washed with PBS twice for 5 minutes. Following the process of antigen retrieval, endogenous peroxidase activity was quenched by incubation in 3% hydrogen peroxide. After protein blocking with blocking solution  for 1 hour at room temperature, primary antibody  was applied at 4°C overnight in a humidified chamber. Primary PD-L1 antibody was probed at 4 °C overnight. Each step was succeeded by three Tris-buffered saline (TBS) rinses, and the bound antibody on the array was detected by using SignalStain® Boost Detection Reagent  and SignalStain® DAB . Finally, sections were counterstained with Hematoxylin QS (VectorLaboratories) and the slide was mounted with VectaMount AQ (Vector Laboratories) for long-term preservation. The immunostaining intensity pattern of PD-L1 was assessed on a scale semi-quantitatively as follows: 0, no staining; 1+, weak staining; 2+, moderate staining; and 3+, intense staining. Scoring was calculated from the mean of the two independently conducted assessments. PD-L1 staining images were obtained by using a Nikon Eclipse Ti-U fluorescence microscope (Nikon Corp) with a SPOT RT digital camera (Diagnostic Instruments Inc.).The TIL analysis were performed on the hematoxylin and eosin-stained chordoma TMA slide. Scores of the TMA were evaluated for the presence of TILs at 200× magnification and semi-quantitatively as follows: score 0, no TILs; 1, rare/few TILs; 2, brisk/prominent TILs.P values < 0.05 were considered as statistically significant.All statistical analyses were performed using GraphPad Prism software 5.0. Kaplan-Meier survival curves were generated to examine the relationship between the expression levels of PD-L1 and patients' survival rate. Survival time was calculated from the date of tumor diagnosis to the date of death or last follow-up. The statistical significance between two groups was determined using unpaired Student's t-test. Data were expressed as mean ± standard error of the mean (SEM). For comparison between expression of PD-L1 and immune infiltrates, a one-way ANOVA analysis was used. The statistical significance is described in figures and in legends.


Stacking various two-dimensional atomic crystals is a feasible approach to creating unique multilayered van der Waals heterostructures with tailored properties. Herein for the first time, we present a controlled preparation of large-area h-BN/graphene heterostructures via a simple chemical deposition of h-BN layers on epitaxial graphene/SiC(0001). Van der Waals forces, which are responsible for the cohesion of the multilayer system, give rise to an abrupt interface without interdiffusion between graphene and h-BN, as shown by X-ray Photoemission Spectroscopy (XPS) and direct observation using scanning and High-Resolution Transmission Electron Microscopy (STEM/HRTEM). The electronic properties of graphene, such as the Dirac cone, remain intact and no significant charge transfer i.e. doping, is observed. These results are supported by Density Functional Theory (DFT) calculations. We demonstrate that the h-BN capped graphene allows the fabrication of vdW heterostructures without altering the electronic properties of graphene. Low-dimensional materials have been emerging recently as new systems with unique structure and size-dependent properties. While having lateral dimensions typically at the micron scale, the thickness of layers is at the atomic scale, rendering them “true” nanoscale materials with significant anisotropic properties. While graphene is undoubtedly the archetypical 2D material, inorganic layers derived from layered transition metal oxides or dichalchogenides have the potential to increase as well as diversify the portfolio of 2D systems available to date2), allowing the use of h-BN as an alternative gate dielectric. Moreover, the surface optical phonon modes of h-BN have energies double that of similar modes in SiO2, offering the possibility of improving the high-temperature and high-electric field performances of h-BN/graphene based devices compared to the usual oxide/graphene counterparts.Hexagonal boron nitride (h-BN) promises to be an ideal dielectric substrate for improved graphene-based devices24et al. have so far focused on 2D materials  on epitaxial graphene/SiC112 on epitaxial graphene by CVD and performed TEM and Raman spectroscopy to investigate the interface of this heterostructure. Very recently, Yuan et al.h-BN has proved to be an excellent substrate for monolayer graphene by improving the performance of the quantum yield, for example6782 and ii) a continuous h-BN layer on the graphene surface. The h-BN/graphene heterostructure was also studied by Scanning and High-Resolution Transmission Electron Microscopy (STEM/HRTEM), Near Edge X-Ray Absorption Fine Structure (NEXAFS) and X-ray as well as angle-resolved X-ray Photoemission Spectroscopy (XPS and ARPES). Measurements of electronic structure were corroborated by Density Functional Theory (DFT) calculations.Here, we study the interface properties of h-BN on epitaxial graphene on 4H-SiC(0001). We demonstrate that the h-BN flakes can be deposited on the graphene in various sizes. Due to the inertness of both surfaces, van der Waals forces play a major role in the h-BN layer organization. Two distributions were observed using micro-Raman mapping: i) isolated h-BN thin flakes of 2×2 μm3N) micropowders to liquid-state polyborazylene (PBN)3N as a crystallization promoter allows h-BN to crystallise at a lower temperature (1200°C) than under classical conditions (1800 °C)ex situ from the growth chamber. After ultrasonication of the h-BN suspension, a few drops of solution were deposited on the epitaxial graphene. Raman spectroscopy was carried out to characterize the morphology of h-BN on graphene. −1. Several intense peaks are visible in this range; these correspond to second-order Raman bands originating in the SiC substrate−1, ii) the G band at 1595 cm−1 and iii) the 2D band at 2720 cm−1; the small D peak indicates the high quality of the sample. In the case of h-BN/graphene, we note a new peak at ∼1367 cm−1 originating from the E2g mode of h- BN vibration192g vibration mode for two representative areas of the h-BN/graphene heterostructure. As it is shown by the different colour scale in 21Thermal decomposition of undoped 4H-SiC(0001) can be used to produce large graphene films with a long-range order and high electron mobilitynditions 00 °Cex serature 10°C than s, Si 2s, and Si 2p peak intensities were not notably affected by the h-BN deposition (s and N 1s core level peaks observed on the h-BN/graphene spectrum. The different contributions to the spectra were extracted by a curve fitting procedure (see Methods). The depth position of the corresponding species across the h-BN/graphene interface was identified by varying the incident photon energy (hv = 510 and 825 eV) and thus changing the surface and bulk sensitivity. The experimental data points are displayed as dots. The solid line is the envelope of the fitted components. The C 1s and Si 2p spectra of the h-BN graphene heterostructure are shown in s spectra show the conventional deconvolution expected for epitaxial graphene on SiC(0001)232426p doublet was reconstituted with a 2p1/2:2p3/2 ratio of 0.5 and a spin-orbit splitting of 0.6 eVp peak at 101.5 eV. The small shoulder at 102.1 eV was attributed to an interface layer area of the SiC(0001) substrate, while the component at 100.9 eV was attributed to the presence of Si clusters formed when Si-C bonds were broken during graphitization31s and N 1s are shown in s and N1s spectra exhibited three peaks fitted with Gaussian components . The main peak at binding energy BE ~ 191 eV for the B 1s and BE ~ 398.6 eV for the N 1s were attributed to B-N bonding, matching those reported for bulk h-BN333435et al.i and the nitrogen vacancy. Interstitial nitrogen binds strongly with a N atom of the lattice forming a N-N pairet al.2O and B-NO2, respectively, (one and two N vacancies are replaced by one and two oxygen). In the case of N 1s, the components at higher BE ~ 399.8 eV and ~401.2 were attributed to O-B-N40s, B 1s and N 1s NEXAFS spectra were measured for different incidence angles of the linear polarized synchrotron light. As shown in s NEXAFS spectra. These spectra were characterized by two sharp resonances at 285.3 eV and 291.7 eV and a broad structure at 292.7 eV. These features were characteristic of pristine graphene on SiC24*, π* resonance intensity was strongly enhanced. This feature decreased with an increased incident angle and was almost suppressed for normal incidence (θ = 85°). As expected for a planar π conjugated system such as graphene, the σ* resonances revealed the opposite angular dependence behaviour. The sharpness of the NEXAFS features indicated a well-defined bonding environment and a long-range periodic order in the electronic structure. Thus, the NEXAFS spectra unequivocally proved the conservation of sp2 bonds between carbon atoms in the h-BN/graphene heterostructure. The fact that no modifications were induced on the C 1s NEXAFS spectra with respect to pristine graphene, confirmed the dominant role of van der Waals interactions between graphene and h-BN layers. s and N 1s NEXAFS spectra. In addition to the characteristic B(1s → π*) excitonic transition of h-BN at ~192.0 eV  and two σ* peaks at ~198.2 eV and ~199.5 eV, two other excitonic π* peaks were observed in the B 1s x-ray absorption edge at ~192.7 eV and ~193.3 eV3 and B-O3 environments, i.e., B-N2-O and B-NO2, respectivelys NEXAFS spectra displayed an excitonic peak with a maximum centred at ~401.5 eV and two σ* transitions at ~405.8 eV and ~408.1 eV, as found in the literature for a bulk h-BN43i at around 0.6 eV above the valence band maximum (VBM) of h-BN, i.e. just above 397 eV for the 4 eV energy gap of h-BN37s XPS spectra. For the orientation of the h-BN flakes on graphene we clearly saw that the flakes were not all perfectly flat on the graphene substrate, as observed in the HRTEM image. In fact, for normal incidence of light and a polarization vector almost in the plane of the sample (θ = 85°), the π* resonance was still visible, even if attenuated with respect to the grazing incidence (θ = 18°).To investigate the atomic composition, as well as the chemical bonding environment of our samples, XPS measurements were carried out for the pristine graphene and the h-BN/graphene heterostructure, as shown in position , whose pθ = 18°, ° and 85°K point of the first graphene Brillouin zone, for the pristine graphene layer and the h-BN/graphene heterostructure. On the pristine graphene layer  at −0.3 eV below the Fermi level (EF). This n-type doping is typical of epitaxial graphene and is due to the charge transfer from the SiC substrate. After h-BN deposition  of the Dirac cone branches probably increased because of the interaction between the graphene and h-BN layers. It has been proposed that a bandgap would be induced in graphene aligned in a commensurate way to a h-BN substrateIn order to clearly highlight the effect of h-BN on graphene’s electronic structure, ARPES measurements were carried out. ne layer , the linposition , these π3N) micropowders in order to obtain highly crystallized material. The epitaxial h-BN/graphene domains were shown by micro- Raman spectroscopy. More importantly, the STEM/HRTEM results showed that the h-BN layers are highly crystalline without any interdiffusion. XPS and ARPES spectroscopy shows a sharp interface between graphene and h-BN layers. This demonstrates that epitaxial graphene may be a suitable template to combine with alternative vdW solids due to its lack of dangling bonds, chemical inertness, and ability to remain intact under high stress. This study demonstrates a simple and efficient route for the controlled fabrication of multilayered graphene/h-BN heterostructures with promising applications in nanoelectronics.In summary, we are able to synthesize a high-quality multilayer h-BN/graphene heterostructure. The h-BN is synthesized by using the PDCs route, from a liquid-state polyborazylene as polymeric precursor additivated with lithium nitride (Li−5 mbar) and then further heated to 1550 °C in an Ar atmosphere for 10 mn. It was then cooled down to room temperature and transferred ex-situ from the growth chamber to the analysis chamber. The h-BN layers were synthesized using the Polymer Derived Ceramics route by adding lithium nitride (Li3N) micropowders to liquid-state polyborazylene (PBN), as recently describedThe h-BN on graphene/4H-SiC(0001)  (Si-face) was created in two stages: first few-layer graphene was produced on a 4H-SiC(0001) commercial wafer off-axis. The sample was heated to 1100 °C in a UHV  and α = 0 eV (SiC an IL). The graphene thickness was calculated from the ratio between the intensity of the G and SiC components48IG and ISiC represented the area of the G and SiC components of the C 1s peak extracted from the deconvolution. For epitaxial graphene on SiC, the areal density of the C atom in graphene NG was about three times that of C atoms in SiCd = 14.9 Å corresponding to about 6 ML of graphene. The Si 2p spectra were fitted by sums of Voigt curves, that is, the convolution of a Gaussian  by a Lorentzian . Where the LW are fixed at 45 meVs and N 1s, the peaks were fitted by the sum of Gaussian functions . For ARPES measurements, the photon energy (hν = 60 eV) and sample orientation were set in order to explore the k-space region around the K point in the ΓK direction of the Brillouin zone. The spot size was 100 × 40 (H × V) μm.XPS/ARPES experiments were carried out on the TEMPO beamlineAb initio calculations to describe the electronic structure of the h-BN/graphene van der Waals heterostructure were performed using a very efficient DFT localized orbital molecular dynamic technique (FIREBALL)sp3d5 for B, sp3 for C, N and Si, and s for H were used with cutoff radii (in atomic units) s = 4.5, p = 4.9, d = 5.2 (B), s = 4.5, p = 4.5 (C), s = 4.2, p = 4.2 (N), s = 4.8, p = 5.4 (Si) and s = 4.1 (H)et al.How to cite this article: Sediri, H. et al. Atomically Sharp Interface in an h-BN-epitaxial graphene van der Waals Heterostructure. Sci. Rep.5, 16465; doi: 10.1038/srep16465 (2015).