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The employment of two-dimensional materials, as growth substrates or buffer layers, enables the epitaxial growth of layered materials with different crystalline symmetries with a preferential crystalline orientation and the synthesis of heterostructures with a large lattice constant mismatch. In this work, we employ single crystalline graphene to modify the sulfurization dynamics of copper foil for the deterministic synthesis of large-area Cu9S5 crystals. Molecular dynamics simulations using the Reax force-field are used to mimic the sulfurization process of a series of different atomistic systems specifically built to understand the role of graphene during the sulphur atom attack over the Cu(111) surface. Cu9S5 flakes show a flat morphology with an average lateral size of hundreds of micrometers. Cu9S5 presents a direct band-gap of 2.5 eV evaluated with light absorption and light emission spectroscopies. Electrical characterization shows that the Cu9S5 crystals present high p-type doping with a hole mobility of 2 cm(2) V-1 s(-1).

In recent years, transition metal dichalcogenides have been attracting an increasing interest in the biomedical field, thus implying the need of a deeper understanding of their impact on cell behavior. In this study we investigate tungsten disulfide (WS2) grown via chemical vapor deposition (CVD) on a transparent substrate (sapphire) as a platform for neural-like cell culture. We culture SH-SY5Y human neuroblastoma cells on WS2, using graphene, sapphire and standard culture well as controls. The quality, thickness and homogeneity of the materials is analyzed using atomic force microscopy and Raman spectroscopy. The cytocompatibility of CVD WS2 is investigated for the first time by cell viability and differentiation assessment on SH-SY5Y cells. We find that cells differentiated on WS2, displaying a viability and neurite length comparable with the controls. These findings shine light on the possibility of using WS2 as a cytocompatible material for interfacing neural cells.

We report on the superlubric sliding of monolayer tungsten disulfide (WS2) on epitaxial graphene (EG) on silicon carbide (SiC). WS2 single-crystalline flakes with lateral size of hundreds of nanometers are obtained via chemical vapor deposition (CVD) on EG and microscopic and diffraction analyses indicate that the WS2/EG stack is predominantly aligned with zero azimuthal rotation. Our experimental findings show that the WS2 flakes are prone to slide over graphene surfaces at room temperature when perturbed by a scanning probe microscopy (SPM) tip. Atomistic force field based molecular dynamics simulations indicate that through local physical deformation of the WS2 flake, the scanning tip releases enough energy to the flake to overcome the motion activation barrier and to trigger an ultra-low friction roto-translational displacement, that is superlubric. Experimental observations indicate that after the sliding, the WS2 flakes rest with a rotation of npi/3 with respect to graphene. Atomically resolved investigations show that the interface is atomically sharp and that the WS2 lattice is strain-free. These results help to shed light on nanotribological phenomena in van der Waals (vdW) heterostacks and suggest that the applicative potential of the WS2/graphene heterostructure can be extended by novel mechanical prospects., 27 pages, main text and supplementary information

Monolayer tungsten disulfide ($WS_2$) has recently attracted large interest as a promising material for advanced electronic and optoelectronic devices such as photodetectors, modulators, and sensors. Since these devices can be integrated in a silicon (Si) chip via back-end-of-line (BEOL) processes, the stability of monolayer $WS_2$ in BEOL fabrication conditions should be studied. In this work, the thermal stability of monolayer single-crystal $WS_2$ at typical BEOL conditions is investigated; namely (i) heating temperature of $300$ $^\circ C$, (ii) pressures in the medium- ($10^{-3}$ mbar) and high- ($10^{-8}$ mbar) vacuum range; (iii) heating times from $30$ minutes to $20$ hours. Structural, optical and chemical analyses of $WS_2$ are performed via scanning electron microscopy (SEM), Raman spectroscopy, photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS). It is found that monolayer single-crystal $WS_2$ is intrinsically stable at these temperature and pressures, even after $20$ hours of thermal treatment. The thermal stability of $WS_2$ is also preserved after exposure to low-current electron beam ($12$ pA) or low-fluence laser ($0.9$ $mJ/\mu m^2$), while higher laser fluencies cause photo-activated degradation upon thermal treatment. These results are instrumental to define fabrication and in-line monitoring procedures that allow the integration of $WS_2$ in device fabrication flows without compromising the material quality., Comment: 21 pages, 5 figures

This work presents a comparison of the structural, chemical and electronic properties of multi-layer graphene grown on SiC(000-1) by using two different growth approaches: thermal decomposition and chemical vapor deposition (CVD). The topography of the samples was investigated by using atomic force microscopy (AFM), and scanning electron microscopy (SEM) was performed to examine the sample on a large scale. Raman spectroscopy was used to assess the crystallinity and electronic behavior of the multi-layer graphene and to estimate its thickness in a non-invasive way. While the crystallinity of the samples obtained with the two different approaches is comparable, our results indicate that the CVD method allows for a better thickness control of the grown graphene., 6 pages, 5 figures

Recently, hexagonal boron nitride (h-BN) has been shown to act as an ideal substrate to graphene by greatly improving the material transport properties thanks to its atomically flat surface, low interlayer electronic coupling and almost perfect reticular matching [1]. Chemical vapour deposition (CVD) is presently considered the most scalable approach to grow graphene directly on h-BN. However, for the catalyst-free approach, poor control over the shape and crystallinity of the graphene grains and low growth rates are typically reported [2–5]. In this work we investigate the crystallinity of differently shaped grains and identify a path towards a real van der Waals epitaxy of graphene on h-BN by adopting a catalyst-free CVD process. We demonstrate the polycrystalline nature of circular-shaped pads and attribute the stemming of different oriented grains to airborne contamination of the h-BN flakes. We show that single-crystal grains with six-fold symmetry can be obtained by adopting high hydrogen partial pressures during growth. Notably, growth rates as high as 100 nm/min are obtained by optimizing growth temperature and pressure. The possibility of synthesizing single-crystal graphene on h-BN with appreciable growth rates by adopting a simple CVD approach is a step towards an increased accessibility of this promising van der Waals heterostructure.

The surface structure of Few-Layer Graphene (FLG) epitaxially grown on the C-face of SiC has been investigated by TM-AFM in ambient air and upon interaction with dilute aqueous solutions of bioorganic molecules (L-methionine and dimethyl sulfoxide, DMSO). Before interaction with molecular solutions, we observe nicely ordered, three-fold oriented rippled domains, with a 4.7 +/- 0.2 nm periodicity (small periodicity, SP) and a peak-to-valley distance in the range 0.1-0.2 nm. Upon mild interaction with the molecular solution, the ripple periodicity `` relaxes'' to 6.2 +/- 0.2 nm (large periodicity, LP), while the peak-to-valley height increases to 0.2-0.3 nm. When additional energy is transferred to the system through sonication in solution, graphene planes are peeled off, as shown by quantitative analysis of Raman spectroscopy and X-ray photoelectron spectroscopy which indicate a neat reduction of thickness. Upon exfoliation rippled domains are no longer observed. In comparative experiments on cleaved HOPG, we could not observe ripples on pristine samples in ambient air, while LP ripples develop upon interaction with the molecular solutions. Recent literature on similar systems is not univocal regarding the interpretation of rippling. The ensemble of our comparative observations on FLG and HOPG can be hardly rationalized solely on the basis of the surface assembly of molecules, either organic molecules coming from the solution or adventitious species. We propose to consider rippling as the manifestation of the free-energy minimization of quasi-2D layers, eventually affected by factors such as interplanar stacking, and interactions with molecules and/or with the AFM tip.

Analytical formulas are derived describing the coherent absorption of light from a realistic multilayer structure composed by an optically conducting surface on a sup- porting substrate. The model predicts two fundamental results. First, the absorption regime named coherent perfect transparency theoretically can always be reached. Second, the optical conductance of the surface can be extrapolated from absorption experimental data even when the substrate thickness is unknown. The theoretical predictions are experimentally verified by analyzing a multilayer graphene structure grown on a silicon carbide substrate. The graphene thickness estimated through the coherent absorption technique resulted in good agreement with the values obtained by two other spectroscopic techniques. Thanks to the high spatial resolution that can be reached and high sensitivity to the probed structure thickness, coherent absorp- tion spectroscopy represents an accurate and non-destructive diagnostic method for the spatial mapping of the optical properties of two-dimensional materials and of metasurfaces on a wafer scale.

In recent years, various doping methods for epitaxial graphene have been demonstrated through atom substitution and adsorption. Here we observe by angle-resolved photoemission spectroscopy (ARPES) a coupling-induced Dirac cone renormalization when depositing small amounts of Ti onto epitaxial graphene on SiC. We obtain a remarkably high doping efficiency and a readily tunable carrier velocity simply by changing the amount of deposited Ti. First-principles theoretical calculations show that a strong lateral (non-vertical) orbital coupling leads to an efficient doping of graphene by hybridizing the 2pz orbital of graphene and the 3d orbitals of the Ti adsorbate, which attached on graphene without creating any trap/scattering states. This Ti-induced hybridization is adsorbate-specific and has major consequences for efficient doping as well as applications towards adsorbate-induced modification of carrier transport in graphene., 24 pages, 4 figures

We investigate the nonlinear transmission of a ~280-layer turbostratic graphene sheet for near-infrared amplifier laser pulses (775 nm, Ti:sapphire laser) with a duration of 150-fs and 20-fs. Saturable absorption is observed in both cases, however it is not very strong, amounting to ~13% transmittance change for the 20-fs (150-fs) pulses at a peak intensity of 30 GW/cm2 (4 GW/cm2). The dependence on incident peak intensity is reproduced well using a theoretical model for the time-dependent saturable absorption, where the excited carriers vacate the photo-excited energy range within 3-5 fs, which we attribute to energy redistribution due to carrier-carrier scattering. This is also supported by spectrally resolved measurements for the 20-fs pulses, which show a marked dependence of the degree of saturation on the photon energy. A key result is that the shorter pulses do not yield a lower saturation fluence, due to the combined effects of the broader excitation bandwidth, and the rapid and broad energy redistribution. We also predict the potential performance of multilayer graphene samples for removing pedestal and pre-pulse structure from ultrafast high-energy pulses.

Hydrogen adsorption on graphene-supported metal clusters has brought much controversy due to the complex nature of the bonding between hydrogen and metal clusters. The bond types of hydrogen and graphene-supported Ti clusters are experimentally and theoretically investigated. Transmission electron microscopy shows that Ti clusters of nanometer-size are formed on graphene. Thermal desorption spectroscopy captures three hydrogen desorption peaks from hydrogenated graphene-supported Ti clusters. First principle calculations also found three types of interaction: Two types of bonds with different partial ionic character and physisorption. The physical origin for this rests on the charge state of the Ti clusters: when Ti clusters are neutral, H2 is dissociated, and H forms bonds with the Ti cluster. On the other hand, H2 is adsorbed in molecular form on positively charged Ti clusters, resulting in physisorption. Thus, this work clarifies the bonding mechanisms of hydrogen on graphene-supported Ti clusters.

In this work we present a simple pathway to obtain large single-crystal graphene on copper (Cu) foils with high growth rates using a commercially available cold-wall chemical vapour deposition (CVD) reactor. We show that graphene nucleation density is drastically reduced and crystal growth is accelerated when: i) using ex-situ oxidised foils; ii) performing annealing in an inert atmosphere prior to growth; iii) enclosing the foils to lower the precursor impingement flux during growth. Growth rates as high as 14.7 and 17.5 micrometers per minute are obtained on flat and folded foils, respectively. Thus, single-crystal grains with lateral size of about one millimetre can be obtained in just one hour. The samples are characterised by optical microscopy, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy as well as selected area electron diffraction (SAED) and low-energy electron diffraction (LEED), which confirm the high quality and homogeneity of the films. The development of a process for the quick production of large grain graphene in a commonly used commercial CVD reactor is a significant step towards an increased accessibility to millimetre-sized graphene crystals., Comment: Article: 7 pages, 6 figures. Supplementary Information: 5 pages, 7 figures

Ultranarrow graphene nanoribbons (GNRs) with atomically precise structures are considered a promising class of materials for the realization of optoelectronic and photonic devices with improved functionalities. Here we report the optoelectronic characterization of a field-effect transistor device made of a layer of bottom-up synthesized GNRs contacted with multilayer graphene electrodes, showing high photoresponsivity of 5 × 105 A/W for small incident power in the visible-UV range. Our results show that combining the properties of intrinsic graphene with that of semiconducting GNRs is a viable route to realize novel devices for optoelectronic and sensing applications. © 2017 American Chemical Society.