Wafer-scale graphene quality assessment using micro four-point probe mapping.

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.

Formation of a highly doped ultra-thin amorphous carbon layer by ion bombardment of 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.

Amplification of noise-like pulses generated from a graphene-based Tm-doped all-fiber laser.

We report on the generation of noise-like pulse (NLP) trains in a Tm-doped fiber laser mode-locked by multilayer graphene saturable absorber. The spectral bandwidth obtained directly from the oscillator exceeds 60 nm, centered at 1950 nm, with 23.5 MHz repetition rate. The pulses were also amplified in a fully fiberized amplifier based on a double-cladding Tm-doped fiber. The system was capable of delivering 1.21 W of average power, which corresponds to 51.5 nJ energy stored in the noise-like bundle. We believe that the presented source might serve as a pump for supercontinuum generation in highly nonlinear fibers.

All-fiber Ho-doped mode-locked oscillator based on a graphene saturable absorber.

In this Letter, we demonstrate a graphene mode-locked, all-fiber Ho-doped fiber laser generating 1.3 nJ energy pulses directly from the oscillator. The graphene used as a saturable absorber was obtained via chemical vapor deposition on copper substrate and immersed in a poly(methyl methacrylate) support. The laser generated ultrashort soliton pulses at 2080 nm with bandwidth up to 6.1 nm. The influence of the output coupling ratio and the SA modulation depth on the mode-locking performance was also investigated.

Sub-90 fs a stretched-pulse mode-locked fiber laser based on a graphene saturable absorber.

In this paper a stretched-pulse, mode-locked Er-doped fiber laser based on graphene saturable absorber (SA) is presented. A 60 layer graphene/polymer composite was used as a SA. The all-fiber dispersion managed laser resonator with the repetition frequency of 21.15 MHz allows for Gaussian pulses generation with the full width at half maximum (FWHM) of 48 nm. The generated chirped pulses were compressed outside the cavity to the 88 fs using a piece of standard single mode fiber. The average output power and pulse energy were of 1.5 mW and 71 pJ, respectively.

All-polarization maintaining, graphene-based femtosecond Tm-doped all-fiber laser.

We report an all-fiber, all-polarization maintaining (PM) ultrafast Tm-doped fiber laser mode-locked by a multilayer graphene-based saturable absorber (SA). The laser emits 603 fs-short pulses centered at 1876 nm wavelength with 6.6 nm of bandwidth and 41 MHz repetition rate. Graphene used as saturable absorber was obtained via chemical vapor deposition (CVD) on copper substrate and immersed in a poly(methylmethacrylate) (PMMA) support, forming a stable, free-standing foil containing 12 graphene layers, suitable for the use in a fiber laser. The generated 603 fs pulses are the shortest reported pulses achieved from a Tm-doped laser mode-locked by graphene saturable absorber so far. Additionally, this is the first demonstration of an all-PM Tm-doped fiber laser incorporating a graphene-based SA. Such cost-effective, compact and stable fiber lasers might be considered as sources usable in nonlinear frequency conversion, mid-infrared spectroscopy and remote sensing.

260 fs and 1 nJ pulse generation from a compact, mode-locked Tm-doped fiber laser.

We report on generation of 260 fs-short pulses with energy of 1.1 nJ from a fully fiberized, monolithic Tm-doped fiber laser system. The design comprises a simple, graphene-based ultrafast oscillator and an integrated all-fiber chirped pulse amplifier (CPA). The system generates 110 mW of average power at 100.25 MHz repetition rate and central wavelength of 1968 nm. This is, to our knowledge, the highest pulse energy generated from a fully fiberized sub-300 fs Tm-doped laser, without the necessity of using grating-based dispersion compensation. Such compact, robust and cost-effective system might serve as a seed source for nonlinear frequency conversion or mid-infrared supercontinuum generation.

Laser ablation- and plasma etching-based patterning of graphene on silicon-on-insulator waveguides.

We present a new approach to remove monolayer graphene transferred on top of a silicon-on-insulator (SOI) photonic integrated chip. Femtosecond laser ablation is used for the first time to remove graphene from SOI waveguides, whereas oxygen plasma etching through a metal mask is employed to peel off graphene from the grating couplers attached to the waveguides. We show by means of Raman spectroscopy and atomic force microscopy that the removal of graphene is successful with minimal damage to the underlying SOI waveguides. Finally, we employ both removal techniques to measure the contribution of graphene to the loss of grating-coupled graphene-covered SOI waveguides using the cut-back method.

Passive synchronization of erbium and thulium doped fiber mode-locked lasers enhanced by common graphene saturable absorber.

In this work we present for the first time, to the best of our knowledge, a passively synchronized thulium (Tm) and erbium (Er) doped fiber laser mode-locked by a common graphene saturable absorber (GSA). The laser consists of two ring resonators combined with a 90 cm long common fiber branch incorporating the saturable absorber (SA). Such laser generates optical solitons centered at 1558.5 nm and 1938 nm with pulse durations of 915 fs and 1.57 ps, respectively. Both laser loops were passively synchronized at repetition frequency of 20.5025 MHz by nonlinear interaction (cross phase modulation, XPM) in common fiber branch between generated pulses. The maximum cavity mismatch of the Er-laser in synchronization regime was 0.78 mm. The synchronization mechanism was also investigated. We demonstrate that the third order nonlinearities of graphene enhance the synchronization range. In our case the range was increased about 85%. The integrated RMS timing jitter between the synchronized pulses was 67 fs.

Mapping nanoscale carrier confinement in polycrystalline graphene by terahertz spectroscopy.

Terahertz time-domain spectroscopy (THz-TDS) can be used to map spatial variations in electrical properties such as sheet conductivity, carrier density, and carrier mobility in graphene. Here, we consider wafer-scale graphene grown on germanium by chemical vapor deposition with non-uniformities and small domains due to reconstructions of the substrate during growth. The THz conductivity spectrum matches the predictions of the phenomenological Drude-Smith model for conductors with non-isotropic scattering caused by backscattering from boundaries and line defects. We compare the charge carrier mean free path determined by THz-TDS with the average defect distance assessed by Raman spectroscopy, and the grain boundary dimensions as determined by transmission electron microscopy. The results indicate that even small angle orientation variations below 5° within graphene grains influence the scattering behavior, consistent with significant backscattering contributions from grain boundaries.

Simultaneous mode-locking at 1565 nm and 1944 nm in fiber laser based on common graphene saturable absorber.

We present for the first time to the best of our knowledge an all-fiber thulium (Tm) and erbium (Er) doped fiber laser simultaneously mode-locked by a common graphene saturable absorber. The laser consists of two ring resonators combined with a common saturable absorber (SA). The generated optical solitons have a full width at half maximum (FWHM) of 3.9 nm and 4.2 nm for Tm- and Er-doped laser, respectively. The used graphene layers were grown on copper foils by chemical vapor deposition (CVD) and transferred onto the fiber connector end. Broadband and flat absorption spectrum of used SA supports mode-locked operation at 1565 nm and 1944 nm. The repetition frequency of the resonator with Er-doped fiber was 20.19 MHz while the Tm-doped resonator was around 1 m longer and resulted with repetition rate of 18.43 MHz. The reported experiment unambiguously confirms one of the biggest advantage of the carbon nanomaterial (in this case graphene) SAs over semiconductor saturable absorption mirrors (SESAM), which is broadband operation range, allowing to mode-lock two lasers spectrally separated by almost 400 nm.

Thulium-doped all-fiber laser mode-locked by CVD-graphene/PMMA saturable absorber.

We report an all-fiber Tm-doped fiber laser mode-locked by graphene saturable absorber. The laser emits 1.2 ps pulses at 1884 nm center wavelength with 4 nm of bandwidth and 20.5 MHz mode spacing. The graphene layers were grown on copper foils by chemical vapor deposition (CVD) and transferred onto the fiber connector end. Up to date this is the shortest reported pulse duration achieved from a Tm-doped laser mode-locked by graphene saturable absorber. Such cost-effective and stable fiber lasers might be considered as sources for mid-infrared spectroscopy and remote sensing.

Selective Growth of van der Waals Heterostructures Enabled by Electron-Beam Irradiation.

Van der Waals heterostructures (vdWHSs) enable the fabrication of complex electronic devices based on two-dimensional (2D) materials. Ideally, these vdWHSs should be fabricated in a scalable and repeatable way and only in the specific areas of the substrate to lower the number of technological operations inducing defects and impurities. Here, we present a method of selective fabrication of vdWHSs via chemical vapor deposition by electron-beam (EB) irradiation. We distinguish two growth modes: positive (2D materials nucleate on the irradiated regions) on graphene and tungsten disulfide (WS2) substrates, and negative (2D materials do not nucleate on the irradiated regions) on the graphene substrate. The growth mode is controlled by limiting the air exposure of the irradiated substrate and the time between irradiation and growth. We conducted Raman mapping, Kelvin-probe force microscopy, X-ray photoelectron spectroscopy, and density-functional theory modeling studies to investigate the selective growth mechanism. We conclude that the selective growth is explained by the competition of three effects: EB-induced defects, adsorption of carbon species, and electrostatic interaction. The method here is a critical step toward the industry-scale fabrication of 2D-materials-based devices.

Graphene 2D platform is safe and cytocompatibile for HaCaT cells growing under static and dynamic conditions.

The study concerns the influence of graphene monolayer, as a 2 D platform, on cell viability, cytoskeleton, adhesions sites andmorphology of mitochondria of keratinocytes (HaCaT) under static conditions. Based on quantitative and immunofluorescent analysis, it could be stated that graphene substrate does not cause any damage to membrane or disruption of other monitored parameters. Spindle poles and cytokinesis bridges indicating proliferation of cells on this graphene substrate were detected. Moreover, the keratinocyte migration rate on the graphene substrate was comparable to control glass substrate when the created wound was completely closed after 38 hours. HaCaT morphology and viability were also assessed under dynamic conditions (lab on a chip - micro scale). For this purpose, microfluidic graphene system was designed and constructed. No differences as well as no anomalies were observed during cultivation of these cells on the graphene or glass substrates in relation to cultivation conditions: static (macro scale) and dynamic (micro scale). Only natural percentage of dead cells was determined using different methods, which proved that the graphene as the 2 D platform is cytocompatible with keratinocytes. The obtained results encourage the use of the designed lab on a chip system in toxicity testing of graphene also on other cells and further research on the use of graphene monolayers to produce bio-bandages for skin wounds in animal tests.

Cytocompatibility of Graphene Monolayer and Its Impact on Focal Cell Adhesion, Mitochondrial Morphology and Activity in BALB/3T3 Fibroblasts.

This study investigates the effect of graphene scaffold on morphology, viability, cytoskeleton, focal contacts, mitochondrial network morphology and activity in BALB/3T3 fibroblasts and provides new data on biocompatibility of the "graphene-family nanomaterials". We used graphene monolayer applied onto glass cover slide by electrochemical delamination method and regular glass cover slide, as a reference. The morphology of fibroblasts growing on graphene was unaltered, and the cell viability was 95% compared to control cells on non-coated glass slide. There was no significant difference in the cell size (spreading) between both groups studied. Graphene platform significantly increased BALB/3T3 cell mitochondrial activity (WST-8 test) compared to glass substrate. To demonstrate the variability in focal contacts pattern, the effect of graphene on vinculin was examined, which revealed a significant increase in focal contact size comparing to control-glass slide. There was no disruption in mitochondrial network morphology, which was branched and well connected in relation to the control group. Evaluation of the JC-1 red/green fluorescence intensity ratio revealed similar levels of mitochondrial membrane potential in cells growing on graphene-coated and uncoated slides. These results indicate that graphene monolayer scaffold is cytocompatible with connective tissue cells examined and could be beneficial for tissue engineering therapy.

Substrate-Induced Variances in Morphological and Structural Properties of MoS2 Grown by Chemical Vapor Deposition on Epitaxial Graphene and SiO2.

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.

Graphene as a Schottky Barrier Contact to AlGaN/GaN Heterostructures.

Electrical and noise properties of graphene contacts to AlGaN/GaN heterostructures were studied experimentally. It was found that graphene on AlGaN forms a high-quality Schottky barrier with the barrier height dependent on the bias. The apparent barrier heights for this kind of Schottky diode were found to be relatively high, varying within the range of φb = (1.0-1.26) eV. AlGaN/GaN fin-shaped field-effect transistors (finFETs) with a graphene gate were fabricated and studied. These devices demonstrated ~8 order of magnitude on/off ratio, subthreshold slope of ~1.3, and low subthreshold current in the sub-picoamperes range. The effective trap density responsible for the 1/f low-frequency noise was found within the range of (1-5) · 1019 eV-1 cm-3. These values are of the same order of magnitude as reported earlier and in AlGaN/GaN transistors with Ni/Au Schottky gate studied as a reference in the current study. A good quality of graphene/AlGaN Schottky barrier diodes and AlGaN/GaN transistors opens the way for transparent GaN-based electronics and GaN-based devices exploring vertical electron transport in graphene.

Chemical-Vapor-Deposited Graphene as a Thermally Conducting Coating.

We performed scanning thermal microscopy measurements on single layers of chemical-vapor-deposited (CVD) graphene supported by different substrates, namely, SiO2, Al2O3, 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 SiO2 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.

The effects of graphene and mesenchymal stem cells in cutaneous wound healing and their putative action mechanism.

This study provides a review of the therapeutic potential of graphene dressing scaffolds and mesenchymal stem cells (MSCs) and their synergistic effects with respect to cutaneous wound healing. This study also considers their putative action mechanism based on the antibacterial, immunomodulating, angiogenic, matrix remodeling effects of materials belonging to the graphene family and MSCs during the wound healing process. In addition, this study discusses the cytocompatibility of graphene, its uses as a platform for skin substitutes, the properties it possesses with respect to providing protection against microbial invasion as well as strategies aimed at minimizing the chance of the occurrence of sepsis. MSCs are capable of secreting several factors that exert a therapeutic impact on reparative processes and tissue regeneration. In light of experiments conducted to date, graphene combined with MSCs appears to have the potential to enhance both the wound healing process and infection control at the injury site.

Biocompatibility of pristine graphene monolayer: Scaffold for fibroblasts.

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.