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Monolayer transition-metal dichalcogenide (TMD) semiconductors exhibit strong excitonic effects and hold promise for optical and optoelectronic applications. Yet, electron doping of TMDs leads to the conversion of neutral excitons into negative trions, which recombine predominantly non-radiatively at room temperature. As a result, the photoluminescence (PL) intensity is quenched. Here we study the optical and electronic properties of a MoS2/WSe2 heterostructure as a function of chemical doping by Cs atoms performed under ultra-high vacuum conditions. By PL measurements we identify two interlayer excitons and assign them to the momentum-indirect Q-Gamma and K-Gamma transitions. The energies of these excitons are in a very good agreement with ab initio calculations. We find that the Q-Gamma interlayer exciton is robust to the electron doping and is present at room temperature even at a high charge carrier concentration. Submicrometer angle-resolved photoemission spectroscopy (micro-ARPES) reveals charge transfer from deposited Cs adatoms to both the upper MoS2 and the lower WSe2 monolayer without changing the band alignment. This leads to a small (10 meV) energy shift of interlayer excitons. Robustness of the momentum-indirect interlayer exciton to charge doping opens up an opportunity of using TMD heterostructures in light-emitting devices that can work at room temperature at high densities of charge carriers.

A precise control over the depth distribution of major, minor and trace elements in a structure is crucial for practical applications of semiconductors. Thus, reliable characterization tools capable of measuring depth distribution of electrically active impurities are essential to advance currently existing growth technologies. Secondary Ion Mass Spectrometry (SIMS) is frequently used to measure depth profiles of impurity atoms; however it provides only total elemental concentration distribution. In contrast techniques capable of measuring dopant profiles typically omit information concerning electrically inactive impurities. In this work, we propose a method for obtaining quantitative profiles of both active and hydrogen-passivated (inactive) zinc in a GaAs/AlGaAs sample using Ultra Low Impact Energy SIMS (ULIE-SIMS). The key to the procedure is the ability of ULIE-SIMS to measure complex As3Zn and hydrogen signals. The zinc–arsenic signal is a marker of the active Zn profile, whereas the hydrogen signal delivers information about the hydrogen-passivated impurity profile. Annealing of the sample provides further validation of the method as the observed increase of active Zn and decrease of passivated Zn signals is associated with hydrogen escaping from the sample surface. The presented approach can potentially be used in optimization of growth processes and controlling the level of impurity activation in semiconductor materials.

Nanoscale-resolution magnetic field sensing plays a significant role in fundamental research in a wide range of fields, from material science to biology. Magnetoresistors in particular offer simplicity of construction and easy adaptation for fabrication based on two-dimensional materials, together with the ability to detect small and medium magnetic fields. Among the wide range of magnetoresistance effects, extraordinary magnetoresistance is unique because it does not require a magnetic material as part of its structure, which may be advantageous in certain applications. In this paper we demonstrate that a recently proposed planar configuration of extraordinary magnetoresistance is naturally suited to the fabrication of devices based on two-dimensional materials. Several operating magnetoresistors have been made for this purpose, using epitaxial mono- and bilayer graphene as well as Bi 2 Se 3 microflakes. Besides the implementation of the extraordinary magnetoresistance in a planar configuration, we demonstrate experimentally the consequences of geometry optimization for future applications. Using the finite element method to simulate the extraordinary magnetoresistance effect in a specific hybrid structure containing a 2D sheet, our research goes beyond the experimental framework, and the efficiency of the applied optimization method can be compared to the best results achieved so far in the field.

We report on a comparative structural characterization of two types of high quality epitaxial graphene layers grown by CVD on 4H-SiC(0001). The layers under study are a single layer graphene on top of a buffer layer and a quasi-free-standing graphene obtained by intercalation of hydrogen underneath the buffer layer. We determine the morphology and structure of both layers by different complementary in-situ and ex-situ surface techniques. We found the existence of large islands in both samples but with different size distribution. Photoemission electron microscopy (PEEM) measurements were performed to get information about the chemical environment of the different regions. The study reveals that monolayer graphene prevails in most of the surface terraces, while a bilayer and trilayer graphene presence is observed at the steps, stripes along steps and islands., Financial support from EU Horizon 2020 research and innovation program under grant agreement No. 785219 (Graphene Flagship Core2-Graphene-based disruptive technologies) and Spanish MINECO (grants MAT2017-85089-C2-1-R, MAT2014-52405-C2-2-R, RYC-2014-16626) is acknowledged.

This paper presents recent progress in the development of the new type of Hall-effect sensor with the graphene layer acting as sensing material. Newly developed Hall-effect sensor is made of quasi-free-standing bilayer graphene structure. The graphene structure is placed inside the standard QFN-32 package for integrated circuits and mounted on the miniature printed circuit board with four leads, which allowed obtaining operational magnetic field sensor. Basic functional properties of the developed sensor were investigated as well as time drift and temperature dependence of these parameters. Performed investigation indicates high linearity of the sensor within the tested range of an external magnetic field. The measured current-related sensitivity of the sensor is about 50 V/AT, without any amplification of the output signal from the graphene structure. Also, long-term tests of sensitivity and offset voltage were performed. The results indicate high stability of the investigated parameters within long time. Obtained results are very promising and indicate the possibility of utilization of the developed sensor in measurement application.

The structure of magnetoresistance curves as a function of magnetic field from 0 to 14 T at temperatures from 0.4 to 6.0 K for macroscopic samples of the quasi-free-standing (QFS) graphene monolayer on SiC substrate, are observed and analyzed, and also the spatial and depth frequency distribution of phonons have been measured using the micro-Raman spectroscopy (MRS). That one enables us to interpret the obtained resonance magnetoresistance curves based on the electron-phonon (e-p) interaction taking into account the actually observed phonon spectrum in researched samples: in the case of a linear e-p interaction the observation of the corresponding peaks on the ${R}_{xx}(B)$ curves is difficult because an uninterrupted background is created. While nonlinear MPR with simultaneous G-phonon emission and D-phonon absorption occur in magnetic fields below 5 T against the background of MPR due to linear e-p interaction as well as Shubnikov--de Haas oscillations.

Graphene metrology needs to keep up with the fast pace of developments in graphene growth and transfer. Terahertz time-domain spectroscopy (THz-TDS) is a non-contact, fast, and non-destructive characterization technique for mapping the electrical properties of graphene. Here we show several case studies of graphene characterization on a range of different substrates that highlight the versatility of THz-TDS measurements and its relevance for process optimization in graphene production scenarios.

In this work, we report the impact of substrate type on the morphological and structural properties of molybdenum disulfide (MoS2) grown by chemical vapor deposition (CVD). MoS2 synthesized on a three-dimensional (3D) substrate, that is, SiO2, in response to the change of the thermodynamic conditions yielded different grain morphologies, including triangles, truncated triangles, and circles. Simultaneously, MoS2 on graphene is highly immune to the modifications of the growth conditions, forming triangular crystals only. We explain the differences between MoS2 on SiO2 and graphene by the different surface diffusion mechanisms, namely, hopping and gas-molecule-collision-like mechanisms, respectively. As a result, we observe the formation of thermodynamically favorable nuclei shapes on graphene, while on SiO2, a full spectrum of domain shapes can be achieved. Additionally, graphene withstands the growth process well, with only slight changes in strain and doping. Furthermore, by the application of graphene as a growth substrate, we realize van der Waals epitaxy and achieve strain-free growth, as suggested by the photoluminescence (PL) studies. We indicate that PL, contrary to Raman spectroscopy, enables us to arbitrarily determine the strain levels in MoS2.

Tissue engineering allows to combine biomaterials and seeded cells to experimentally replace urinary bladder wall. The normal bladder wall however, includes branched neuronal network propagating signals which regulate urine storage and voiding. In this study we introduced a novel biocomposite built from amniotic membrane (Am) and graphene which created interface between cells and external stimuli replacing neuronal network. Graphene layers were transferred without modifying Am surface. Applied method allowed to preserve the unique bioactive characteristic of Am. Tissue engineered constructs composed from biocomposite seeded with smooth muscle cells (SMC) derived from porcine detrusor and porcine urothelial cells (UC) were used to evaluate properties of developed biomaterial. The presence of graphene layer significantly increased electrical conductivity of biocomposite. UCs and SMCs showed an organized growth pattern on graphene covered surfaces. Electrical filed stimulation (EFS) applied in vitro led additionally to increased SMCs growth and linear arrangement. 3D printed chamber equipped with 3D printed graphene based electrodes was fabricated to deliver EFS and record pressure changes caused by contracting SMCs seeded biocomposite. Observed contractile response indicated on effective SMCs stimulation mediated by graphene layer which constituted efficient cell to biomaterial interface.

Micro four-point probes (M4PP) provide rapid and automated lithography-free transport properties of planar surfaces including two-dimensional materials. We perform sheet conductance wafer maps of graphene directly grown on a 100 mm diameter SiC wafer using a multiplexed seven-point probe with minor additional measurement time compared to a four-point probe. Comparing the results of three subprobes we find that compared to a single-probe result, our measurement yield increases from 72%-84% to 97%. The additional data allows for correlation analysis between adjacent subprobes, that must measure the same values in case the sample is uniform on the scale of the electrode pitch. We observe that the relative difference in measured sheet conductance between two adjacent subprobes increase in the transition between large and low conductance regions. We mapped sheet conductance of graphene as it changed over several weeks. Terahertz time-domain spectroscopy conductivity maps both before and after M4PP mapping showed no significant change due to M4PP measurement, with both methods showing the same qualitative changes over time.

Applied Sciences 10(6), 2183 (2020). doi:10.3390/app10062183, Published by MDPI, Basel

In the framework of the European project Flagship Graphene, we have developed several process technologies, for fabricating graphene field-effect transistors, for radio frequency (RF) applications. Depending on the technique used to synthesize graphene, different transistors topologies were designed, given rise to different applications. Graphene materials under consideration include graphene growth on silicon carbide, graphene growth by chemical vapor deposition (CVD) on copper foil. After fabrication of transistors on rigid and on flexible substrates, high frequency characterization of devices is made. Based on the performance of transistors, RF circuits where designed and fabricated.

In this work, we report the impact of the substrate type on the morphological and structural properties of molybdenum disulfide (MoS2) grown by chemical vapor deposition (CVD). MoS2 synthesized on ...

Integration of graphene with the existing fabrication lines may be possible only if the growth process fulfills the high purity requirements of the front-end-of-line integration approaches. While many analytical techniques may provide information about the concentration of contaminants, their dimensional sensitivity is usually poor and thus it is not possible to assess which contaminants are located above/below graphene and which are incorporated into its structure. In this work, an optimized secondary ion mass spectrometry measurement procedure that combines excellent detection limits (0.8 – 2.9 ppm) and subnanometer depth resolution is presented. It confirms the presence of sodium and sulfur at the surface of the graphene layer, the formation of graphene-copper composite, and allows the detection of molybdenum and tungsten at the interface between graphene and the germanium substrate. Such precision may prove to be valuable for further optimization of the growth process of 2D materials.

Graphene is considered a record-performance nonlinear-optical material on the basis of numerous experiments. The observed strong nonlinear response ascribed to the refractive part of graphene’s electronic third-order susceptibility χ(3) cannot, however, be explained using the relatively modest χ(3) value theoretically predicted for the 2D material. Here we solve this long-standing paradox and demonstrate that, rather than χ(3)-based refraction, a complex phenomenon which we call saturable photoexcited-carrier refraction is at the heart of nonlinear-optical interactions in graphene such as self-phase modulation. Saturable photoexcited-carrier refraction is found to enable self-phase modulation of picosecond optical pulses with exponential-like bandwidth growth along graphene-covered waveguides. Our theory allows explanation of these extraordinary experimental results both qualitatively and quantitatively. It also supports the graphene nonlinearities measured in previous self-phase modulation and self-(de)focusing (Z-scan) experiments. This work signifies a paradigm shift in the understanding of 2D-material nonlinearities and finally enables their full exploitation in next-generation nonlinear-optical devices., Graphene enables extraordinary nonlinear-optical refraction, far exceeding predictions based on conventional nonlinear-susceptibility theory. Here, Vermeulen et al. show that rather than the nonlinear susceptibility, a complex saturable refraction process is central to graphene’s unusual behavior.

Hydrogen intercalation is widely used to weaken graphene-substrate interactions and as a result enhancing graphene’s electronic properties. This paper presents the study of microstructure of hydrogen intercalated graphene grown on Ge(0 0 1)/Si(0 0 1) substrate using Low-Voltage Scanning Electron Microscopy. The findings reveal a significant change in the surface morphology of germanium substrate: faceting structure disappears almost completely, the germanium surface flattens, and steps/terraces are formed. This leads to degradation of graphene’s electronic properties, which shows the negative impact of hydrogen intercalation when graphene is grown on the germanium substrate.

We report a calculational study of electron states and the resulting electrochemical properties of uniaxially strained graphene with point defects. For this study the reduction of ferricyanide to ferrocyanide serves as a benchmark electrochemical reaction. We find that the heterogeneous electron transfer activity of the perfect graphene electrode rises under uniaxial strain. However, evolution of the cathodic reaction rate depends on the direction of strain. For moderate lattice deformations, the zigzag strain improves electrochemical performance better than the armchair strain. Standard rate constant increases by 50% at the zigzag strain of 10%. Vacancies, covalently bonded moieties, charged adatoms and substitutional impurities in the zigzag strained graphene induce changes in the shape of the curve of the cathodic reaction rate. However, this changes do not translate into the electrocatalytic activity. Vacancies and covalently bonded moieties at concentration of 0.1% do not affect the electrochemical performance. Charged adatoms and substitutional impurities give a slight increase in the standard rate constant by, respectively, 2.2% and 3.4%.

The aim of the present study was to evaluate the cytotoxicity of pristine graphene monolayer and its utility as a scaffold for murine fibroblast L929 cell line. Cell viability, morphology, cytoskeleton architecture (microfilaments and microtubules), cell adhesion and migration into the scratch-wound area were determined using pristine graphene-coated microscopic slides. We found that fibroblasts cultured on pristine graphene monolayer exhibited changes in cell attachment, motility and cytoskeleton organization. Graphene was found to have no cytotoxicity on L929 fibroblasts and increased cell adhesion and proliferation within 24 h of culture. The area of cells growing on graphene was comparable to the area of fibroblasts cultured on glass. Migration of cells on the surface of graphene substrate appeared to be more regular in comparison to uncoated glass surface, however in both control (glass) and experimental (graphene) groups the scratch wound was closed after 48 h of culture. Taken together, our results indicate that pristine graphene monolayer is non-toxic for murine subcutaneous connective tissue fibroblasts and could be beneficial for recovery of damaged tissues after injury. These studies could be helpful in evaluating biocompatibility of graphene, which still remains ambiguous.

In this paper we present the CMOS-compatible procedure for graphene-based planar electronic devices (i.e. magnetic field sensor) fabrication in the hundreds of micrometer scale by using of: combined photolithography, Ar + ion sputtering and DC magnetron metal evaporation processes. In our approach, the conductive channels based on the single graphene layer on the SiC substrate have been fabricated by Ar + ion sputtering, instead of reactive - ion etching techniques. The proposed solution prevents against semiconductor surface oxidation and therefore is much more compatible with the standard CMOS technology. The processed graphene channels and devices demonstrate similar properties as fabricated by means of the early used procedures. That underline usefulness of the presented approach in many fields of applications, i.e.: the devices which contain graphene channels on standard reactive, semiconductor surfaces.

New possibilities are presented for the characterization ofAIIIBVmixed superlattice compounds by the complementary use of synchrotron diffraction topography and rocking curves. In particular, using a synchrotron white beam and the section diffraction pattern of a 5 µm slit taken at a 10 cm film-to-crystal distance, it was possible to reproduce a set of stripes corresponding to interference fringes. These are analogous to the interference maxima revealed in high-resolution rocking curves, but are created by the changes in orientation of the planes inclined to the surface which are induced by unrelaxed strain. The section diffraction topographic method enabled examination of the sample homogeneity along the narrow intersecting beam. This was important in the case of the present sample containing a twin lamella in the InP substrate wafer. Both the section and projection Bragg case topographic methods enabled the crystallographic identification of the twin lamella. Another characteristic feature indicated in the section topography was the bending of the stripes corresponding to the superlattice peaks close to the boundaries of the twin lamella. The most probable interpretation of this phenomenon is an increase in the thickness of the deposited layers close to the lamella, together with possible changes in the chemical composition, leading to a decrease in the mean lattice parameter in the superlattice.

The interaction between graphene and germanium surfaces was investigated using a combination of microscopic and macroscopic experimental techniques and complementary theoretical calculations. Density functional theory (DFT) calculations for different reconstructions of the Ge(001) surface showed that the interactions between graphene and the Ge(001) surface introduce additional peaks in the density of states, superimposed on the graphene valence and conduction energy bands. The growth of graphene induces nanofaceting of the Ge(001) surface, which exhibits well-organized hill and valley structures. The graphene regions covered by hills are of high quality and exhibit an almost linear dispersion relation, which indicates weak graphene–germanium interactions. On the other hand, the graphene component occupying valley regions is significantly perturbed by the interaction with germanium. It was also found that the stronger graphene–germanium interaction observed in the valley regions is connected with a lower local electrical conductivity. Annealing of graphene/Ge(001)/Si(001) was performed to obtain a more uniform surface. This process results in a surface characterized by negligible hill and valley structures; however, the graphene properties unexpectedly deteriorated with increasing uniformity of the Ge(001) surface. To sum up, it was shown that the mechanism responsible for the formation of local conductivity inhomogeneities in graphene covering the Ge(001) surface is related to the different strength of graphene–germanium interactions. The present results indicate that, in order to obtain high-quality graphene, the experimental efforts should focus on limiting the interactions between germanium and graphene, which can be achieved by adjusting the growth conditions.

This paper presents the study on power scaling of an all-polarization maintaining (PM) fiber mode-locked laser using the multilayer chemical vapor deposition graphene/poly(methyl methacrylate) composite as a saturable absorber by changing the coupling ratio of the laser output coupler. Pulses with the duration of 148 fs were generated using 10% output coupler, which are the shortest pulses obtained from mode-locked erbium doped fiber laser working in anomalous dispersion regime, additionally the highest average output power of 15.66 mW was achieved by applying 70% output coupler. All-PM fiber resonator configuration provides self-start and stable operation. To our knowledge, this is the first comprehensive study of power scaling of erbium doped fiber laser with graphene saturable absorber. Conducted experimental work is in very good agreement with numerical simulations.

Secondary ion mass spectrometry is applied to the depth profiling of the superlattice active region of lattice matched (∼9.2 μm) Al0.48In0.52As/In0.53Ga0.47As/InP quantum cascade lasers. The developed measurement procedure is capable of characterizing the quality of each individual layer in the superlattice region, including layers as thin as 0.7 nm. The oxygen level for AlInAs and InGaAs layers is in the range of 1-3 × 1017 atms per cm3 and below the detection limit (∼1 × 1016 atms per cm3), respectively. Oxygen is not uniformly distributed in the AlInAs layers - more oxygen is embedded into the structure during the very first stage of the growth of the AlInAs layer and thus the corresponding interface is 1.83 ± 0.31 times more contaminated than the other. The procedure can also be operated in 3D imaging mode which proves to be invaluable for failure analysis.

The main idea behind this work was demonstrated in a form of a new thermoelectrochromic sensor on a flexible substrate using graphene as an electrically reconfigurable thermal medium (TEChrom™). Our approach relies on electromodulation of thermal properties of graphene on poly(ethylene terephthalate) (PET) via mechanical destruction of a graphene layer. Graphene applied in this work was obtained by chemical vapor deposition (CVD) technique on copper substrate and characterized by Raman and scanning tunneling spectroscopy. Electrical parameters of graphene were evaluated by the van der Pauw method on the transferred graphene layers onto SiO2 substrates by electrochemical delamination method. Two configurations of architecture of sensors, without and with the thermochromic layer, were investigated, taking into account the increase of voltage from 0 to 50 V and were observed by thermographic camera to define heat energy. Current-voltage characteristics obtained for the sensor with damaged graphene layer are linear, and the resistivity is independent from the current applied. The device investigated under 1000 W/m2 exhibited rise of resistivity along with increased temperature. Flexible thermoelectrochromic device with graphene presented here can be widely used as a sensor for both the military and civil monitoring.

For the largest European event in Graphene and 2D materials, Car2TERA project partners from ENT- Wlodek Strupinski and Iwona Pasternak held a presentation and represented the Ca2TERA project with the topic: Emerging technologies for future THz car sensors and networks. GRAPHENE 2019 was hosted in Rome, Italy on 25-28, June 2019. Partners shared that the conference was successfully completed and they exchanged great ideas and information about the latest technological advances.

Non-invasive measurement techniques are of utmost importance for characterization of atomically thin materials to speed up the measurement process while avoiding mechanical damage or contamination of the fragile materials. Terahertz time-domain spectroscopy (THz-TDS) provides non-contact measurement of the frequency dependent conductivity of thin films. Here, we expand the applicability of THz-TDS by spatially mapping the carrier density and mobility of epitaxial graphene grown on silicon carbide. The extracted values are compared to Hall measurements and agrees well for homogeneously conducting samples.

[Image: see text] We performed scanning thermal microscopy measurements on single layers of chemical-vapor-deposited (CVD) graphene supported by different substrates, namely, SiO(2), Al(2)O(3), and PET using a double-scan technique to remove the contribution to the heat flux through the air and the cantilever. Then, by adopting a simple lumped-elements model, we developed a new method that allows determining, through a multistep numerical analysis, the equivalent thermal properties of thermally conductive coatings of nanometric thickness. In this specific case we found that our CVD graphene is “thermally equivalent”, for heat injection perpendicular to the graphene planes, to a coating material of conductivity k(eff) = 2.5 ± 0.3 W/m K and thickness t(eff) = 3.5 ± 0.3 nm in perfect contact with the substrate. For the SiO(2) substrate, we also measured stacks made of 2- and 4-CVD monolayers, and we found that the effective thermal conductivity increases with increasing number of layers and, with a technologically achievable number of layers, is expected to be comparable to that of 1 order of magnitude-thicker metallic thin films. This study provides a powerful method for characterizing the thermal properties of graphene in view of several thermal management applications.

Kelvin Probe Force Microscopy (KPFM) in combination with Atomic Force Microscopy (AFM), Scanning Tunneling Microscopy/Spectroscopy (STM/STS), and X-ray Photoelectron Spectroscopy (XPS) were used to study early stages of oxidation of germanium in the graphene/Ge(001) system exposed to atmospheric environment. KPFM measurements allowed to distinguish nanoscale regions, which are not covered by graphene, as a result of graphene domain misorientation in the growth process. In this area, corrosion process penetrated the region underneath graphene, which can be observed at the nano- and microscale. Therefore, the electronic properties of graphene/germanium hybrid system are modified in the regions around defects. Whereas graphene can protect surfaces against oxidation, the described processes have impact of electronic properties of the sample in a long time scale. We showed that early oxidation stages can be identified in nanoscale even when macroscopic techniques such as XPS do not show signs of degradation. The obtained results are important for assessing the need of protection of graphene/Ge(001) devices.

Epitaxial Chemical Vapor Deposition growth of graphene on silicon carbide offers the maturity and reliability expected for large-scale fabrication of graphene-based devices. In particular, the ultimate challenge of graphene synthesis on SiC, i.e. quasi-free-standing monolayer graphene which comes through hydrogen atom intercalation of the sole buffer layer grown on the Si-face of SiC, offers high carrier mobility (as high as 6600 [cm2/Vs]) and electrical stability throughout the device processing cycle. In this report, we present extensive statistics of the electrical properties of QFS-monolayer graphene grown on 4H(0001) and 6H(0001) semi-insulating 10 mm × 10 mm substrates, being a result of 110 individual processes. The adopted explanation for the origin of the as-grown doping level in epitaxial graphene based on the spontaneous polarization of hexagonal SiC and its most up-to-date values is reaffirmed. We introduce the issue of the step-edge-induced offset voltage radial dependence and confront it with the morphological analysis of the average step edge height and terrace width, all related to the place of origin of a specific sample within a 4-inch SiC wafer. Finally, we conclude that within the step edge area QFS-monolayer graphene is statistically nearly half as resistive as the previously reported QFS-bilayer graphene.

We demonstrate the demodulation of a multi-Gb/s ON–OFF keying (OOK) signal on a 96 GHz carrier by utilizing a 250-nm graphene field-effect transistor as a zero bias power detector. From the eye diagram, we can conclude that the devices can demodulate the OOK signals up to 4 Gb/s.

The experimental and theoretical investigations of morphological and electronic properties of nitrogen-doped epitaxial graphene grown by chemical vapor deposition on 4H-SiC(0001) are discussed. It is shown that presence of nitrogen significantly affects the graphene growth process and leads to an increase in the concentration of defects (in the form of holes). Macro- and nanoscale investigations confirm the formation of pyridinic-N, pyrrolic-N and graphitic-N configurations within graphene layers. The relative concentrations of these configurations change as a function of global nitrogen concentration. Additionally, it is reported that the incorporated nitrogen results in inhomogeneous doping and a few nanometers wide spatial modification of the local density of states. Finally, the SiC substrate is also modified during the nitrogen doping process. To quantify the impact of the substrate modification on electronic structure of graphene the non-intercalated and hydrogen-intercalated doped graphene layers are compared. The presented complementary study sheds light on properties of N-doped graphene and its dependence on nitrogen concentration.

Epitaxial Chemical Vapor Deposition growth of graphene on silicon carbide is one of the most promising technologies to realize graphene-based electronics. Particularly, the quasi-free-standing bilayer which comes through hydrogen atom intercalation of monolayer graphene grown on the Si-face of SiC offers high carrier mobility (as high as 5000 [cm2/Vs]) and electrical stability throughout the device processing cycle. In this report, we present a statistical perspective on transport properties of QFS-bilayer graphene grown on 4H(0 0 0 1) and 6H(0 0 0 1) SiC, being a result of 460 individual processes on semi-insulating 10 mm × 10 mm substrates. The mutual relation between charge carrier concentration and mobility is determined and analyzed to raise the awareness of the effect of SiC hexagonality on charge transport in graphene. Special attention is paid to the applicability of quasi-free-standing bilayer graphene on SiC, primarily in magnetic field detection. The issue of the step-edge-induced offset voltage anisotropy in Hall effect sensors is introduced and a method to minimize it is presented.

The use of graphene in optical and photonic applications has gained much attention in recent years. To maximize the exploitation of graphene's extraordinary optical properties, precise control over its Fermi level (e.g. by means of chemical doping) will be of vital importance. In this work, we show the usage of a versatile p-doping strategy based on the incorporation of bis(trifluoromethanesulfonyl)amide (TFSA), functioning as an active p-dopant molecule, into a poly(2,2,3,3,4,4,5,5-octafluoropentyl methacrylate) (POFPMA) polymer matrix. The TFSA/POFPMA dopant can be utilized both onto large size graphene regions via spin coating and on small predefined spatial zones of micrometer dimension by localized inkjet printing. Whereas pure TFSA suffers from a clustered layer deposition combined with environmental instability, the application of the POFPMA polymer matrix yields doping layers revealing superior properties counteracting the existing shortcomings of pure TFSA. A first key finding relates to the optical quality of the dopant layer. We obtain a layer with an extremely low surface roughness (0.4-0.8 nm/25 μm 2) while exhibiting very high transparency (absorbance

The effects of humidity on the electronic properties of quasi-free standing one layer graphene (QFS 1LG) are investigated via simultaneous magneto-transport in the van der Pauw geometry and local work function measurements in a controlled environment. QFS 1LG on 4H-SiC(0001) is obtained by hydrogen intercalation of the interfacial layer. In this system, the carrier concentration experiences a two-fold increase in sensitivity to changes in relative humidity as compared to the as-grown epitaxial graphene. This enhanced sensitivity to water is attributed to the lowering of the hydrophobicity of QFS 1LG, which results from spontaneous polarization of 4H-SiC(0001) strongly influencing the graphene. Moreover, the superior carrier mobility of the QFS 1LG system is retained even at the highest humidity. The work function maps constructed from Kelvin probe force microscopy also revealed higher sensitivity to water for 1LG compared to 2LG in both QFS 1LG and as-grown systems. These results point to a new field of applications for QFS 1LG, i.e., as humidity sensors, and the corresponding need for metrology in calibration of graphene-based sensors and devices., 16 pages, 4 figures

We solve the long-standing discrepancy between the theoretically predicted and experimentally observed performance of nonlinear-optical refraction in graphene. Hereto we study self-phase modulation in silica-core waveguides covered with graphene.

Ion bombardment of graphene leads to the formation of defects which may be used to tune properties of the graphene based devices. In this work, however, we present that the presence of the graphene layer on a surface of a sample has a significant impact on the ion bombardment process: broken sp2 bonds react with the incoming ions and trap them close to the surface of the sample, preventing a standard ion implantation. For an ion bombardment with a low impact energy and significant dose (in the range of 1014 atoms cm-2) an amorphization of the graphene layer is observed but at the same time, most of the incoming ions do not penetrate the sample but stop at the surface, thus forming a highly doped ultra-thin amorphous carbon layer. The effect may be used to create thin layers containing desired atoms if no other technique is available. This approach is particularly useful for secondary ion mass spectrometry where a high concentration of Cs at the surface of a sample significantly enhances the negative ionization probability, allowing it to reach better detection limits.

Epitaxial graphene is a promising route to wafer scale production of electronic graphene devices. Chemical vapor deposition of graphene on silicon carbide offers epitaxial growth with layer control, but is subject to significant spatial and wafer-to-wafer variability. We use terahertz time-domain spectroscopy and micro four-point probes to analyze the spatial variations of quasi-free-standing bi-layer graphene grown on 4 inch silicon carbide (SiC) wafers, and find significant variations in electrical properties across large regions, which are even reproduced across graphene on different SiC wafers cut from the same ingot. The DC sheet conductivity of epitaxial graphene was found to vary more than one order of magnitude across a 4 inch SiC wafer. To determine the origin of the variations, we compare different optical and scanning probe microscopies with the electrical measurements from nano- to millimeter scale and identify three distinct qualities of graphene, which can be attributed to the microstructure of the SiC surface.

In this article an additional annealing step was used to increase the quality of gate dielectric layers on silicon carbide obtained by thermal oxidation. The mixture of POCl3, N2 and O2 gases was used within temperature range of 950 °C–1100 °C. Abnormal oxide growth rate was observed during the annealing process. Significant improvement of trap density was achieved however the best results were obtained for lower range of annealing temperatures (950 °C–1000 °C).

CVD is currently the most popular and widelyused of these techniques, owing to its low cost, good yield,and simple scalability.Among many CVD growth conditions for CNTs, thecarbon precursor is likely to play a crucial role indetermining the structural properties of the final material.Many gaseous precursors, such as methane, acetylene,carbon monoxide, and others, are widely used for high-quality CNT synthesis.