Interaction of graphene and WS2 with neutrophils and mesenchymal stem cells: implications for peripheral nerve regeneration

Graphene and bidimensional (2D) materials have been widely used in nerve conduits to boost peripheral nerve regeneration. Nevertheless, the experimental and commercial variability in graphene-based materials generates graphene forms with different structures and properties that can trigger entirely diverse biological responses from all the players involved in nerve repair. Herein, we focus on the graphene and tungsten disulfide (WS2) interaction with non-neuronal cell types involved in nerve tissue regeneration. We synthesize highly crystalline graphene and WS2 with scalable techniques such as thermal decomposition and chemical vapor deposition. The materials were able to trigger the activation of a neutrophil human model promoting Neutrophil Extracellular Traps (NETs) production, particularly under basal conditions, although neutrophils were not able to degrade graphene. Of note is that pristine graphene acts as a repellent for the NET adhesion, a beneficial property for nerve conduit long-term applications. Mesenchymal stem cells (MSCs) have been proposed as a promising strategy for nerve regeneration in combination with a conduit. Thus, the interaction of graphene with MSCs was also investigated, and reduced viability was observed only on specific graphene substrates. Overall, the results confirm the possibility of regulating the cell response by varying graphene properties and selecting the most suitable graphene forms.

Graphene-based nanomaterials for peripheral nerve regeneration

Emerging nanotechnologies offer numerous opportunities in the field of regenerative medicine and have been widely explored to design novel scaffolds for the regeneration and stimulation of nerve tissue. In this review, we focus on peripheral nerve regeneration. First, we introduce the biomedical problem and the present status of nerve conduits that can be used to guide, fasten and enhance regeneration. Then, we thoroughly discuss graphene as an emerging candidate in nerve tissue engineering, in light of its chemical, tribological and electrical properties. We introduce the graphene forms commonly used as neural interfaces, briefly review their applications, and discuss their potential toxicity. We then focus on the adoption of graphene in peripheral nervous system applications, a research field that has gained in the last years ever-increasing attention. We discuss the potential integration of graphene in guidance conduits, and critically review graphene interaction not only with peripheral neurons, but also with non-neural cells involved in nerve regeneration; indeed, the latter have recently emerged as central players in modulating the immune and inflammatory response and accelerating the growth of new tissue.

Improving the accuracy of the FMO binding affinity prediction of ligand-receptor complexes containing metals

Polarization and charge transfer strongly characterize the ligand-receptor interaction when metal atoms are present, as for the Au(I)-biscarbene/DNA G-quadruplex complexes. In a previous work (J Comput Aided Mol Des2022, 36, 851-866) we used the ab initio FMO2 method at the RI-MP2/6-31G* level of theory with the PCM [1] solvation approach to calculate the binding energy (ΔEFMO) of two Au(I)-biscarbene derivatives, [Au(9-methylcaffein-8-ylidene)2]+ and [Au(1,3-dimethylbenzimidazole-2-ylidene)2]+, able to interact with DNA G-quadruplex motif. We found that ΔEFMO and ligand-receptor pair interaction energies (EINT) show very large negative values making the direct comparison with experimental data difficult and related this issue to the overestimation of the embedded charge transfer energy between fragments containing metal atoms. In this work, to improve the accuracy of the FMO method for predicting the binding affinity of metal-based ligands interacting with DNA G-quadruplex (Gq), we assess the effect of the following computational features: (i) the electron correlation, considering the Hartree-Fock (HF) and a post-HF method, namely RI-MP2; (ii) the two (FMO2) and three-body (FMO3) approaches; (iii) the basis set size (polarization functions and double-ζ vs. triple-ζ) and (iv) the embedding electrostatic potential (ESP). Moreover, the partial screening method was systematically adopted to simulate the solvent screening effect for each calculation. We found that the use of the ESP computed using the screened point charges for all atoms (ESP-SPTC) has a critical impact on the accuracy of both ΔEFMO and EINT, eliminating the overestimation of charge transfer energy and leading to energy values with magnitude comparable with typical experimental binding energies. With this computational approach, EINT values describe the binding efficiency of metal-based binders to DNA Gq more accurately than ΔEFMO. Therefore, to study the binding process of metal containing systems with the FMO method, the adoption of partial screening solvent method combined with ESP-SPCT should be considered. This computational protocol is suggested for FMO calculations on biological systems containing metals, especially when the adoption of the default ESP treatment leads to questionable results.

Azobenzene-based optoelectronic transistors for neurohybrid building blocks

Exploiting the light-matter interplay to realize advanced light responsive multimodal platforms is an emerging strategy to engineer bioinspired systems such as optoelectronic synaptic devices. However, existing neuroinspired optoelectronic devices rely on complex processing of hybrid materials which often do not exhibit the required features for biological interfacing such as biocompatibility and low Young's modulus. Recently, organic photoelectrochemical transistors (OPECTs) have paved the way towards multimodal devices that can better couple to biological systems benefiting from the characteristics of conjugated polymers. Neurohybrid OPECTs can be designed to optimally interface neuronal systems while resembling typical plasticity-driven processes to create more sophisticated integrated architectures between neuron and neuromorphic ends. Here, an innovative photo-switchable PEDOT:PSS was synthesized and successfully integrated into an OPECT. The OPECT device uses an azobenzene-based organic neuro-hybrid building block to mimic the retina's structure exhibiting the capability to emulate visual pathways. Moreover, dually operating the device with opto- and electrical functions, a light-dependent conditioning and extinction processes were achieved faithful mimicking synaptic neural functions such as short- and long-term plasticity.

Strong Coupling of Coherent Phonons to Excitons in Semiconducting Monolayer MoTe2

The coupling of the electron system to lattice vibrations and their time-dependent control and detection provide unique insight into the nonequilibrium physics of semiconductors. Here, we investigate the ultrafast transient response of semiconducting monolayer 2H-MoTe2 encapsulated with hBN using broadband optical pump-probe microscopy. The sub-40 fs pump pulse triggers extremely intense and long-lived coherent oscillations in the spectral region of the A' and B' exciton resonances, up to ∼20% of the maximum transient signal, due to the displacive excitation of the out-of-plane A1g phonon. Ab initio calculations reveal a dramatic rearrangement of the optical absorption of monolayer MoTe2 induced by an out-of-plane stretching and compression of the crystal lattice, consistent with an A1g -type oscillation. Our results highlight the extreme sensitivity of the optical properties of monolayer TMDs to small structural modifications and their manipulation with light.

Sub-THz wireless transmission based on graphene-integrated optoelectronic mixer

Optoelectronics is a valuable solution to scale up wireless links frequency to sub-THz in the next generation antenna systems and networks. Here, we propose a low-power consumption, small footprint building block for 6 G and 5 G new radio wireless transmission allowing broadband capacity (e.g., 10-100 Gb/s per link and beyond). We demonstrate a wireless datalink based on graphene, reaching setup limited sub-THz carrier frequency and multi-Gbit/s data rate. Our device consists of a graphene-based integrated optoelectronic mixer capable of mixing an optically generated reference oscillator approaching 100 GHz, with a baseband electrical signal. We report >96 GHz optoelectronic bandwidth and -44 dB upconversion efficiency with a footprint significantly smaller than those of state-of-the-art photonic transmitters (i.e., <0.1 mm2). These results are enabled by an integrated-photonic technology based on wafer-scale high-mobility graphene and pave the way towards the development of optoelectronics-based arrayed-antennas for millimeter-wave technology.

Scalable High-Mobility Graphene/hBN Heterostructures

Graphene-hexagonal boron nitride (hBN) scalable heterostructures are pivotal for the development of graphene-based high-tech applications. In this work, we demonstrate the realization of high-quality graphene-hBN heterostructures entirely obtained with scalable approaches. hBN continuous films were grown via ion beam-assisted physical vapor deposition directly on commercially available SiO2/Si and used as receiving substrates for graphene single-crystal matrixes grown by chemical vapor deposition on copper. The structural, chemical, and electronic properties of the heterostructure were investigated by atomic force microscopy, Raman spectroscopy, and electrical transport measurements. We demonstrate graphene carrier mobilities exceeding 10,000 cm2/Vs in ambient conditions, 30% higher than those directly measured on SiO2/Si. We prove the scalability of our approach by measuring more than 100 transfer length method devices over a centimeter scale, which present an average carrier mobility of 7500 ± 850 cm2/Vs. The reported high-quality all-scalable heterostructures are of relevance for the development of graphene-based high-performing electronic and optoelectronic applications.

Industrial Graphene Coating of Low-Voltage Copper Wires for Power Distribution

Copper (Cu) is the electrical conductor of choice in many categories of electrical wiring, with household and building installation being the major market of this metal. This work demonstrates the coating of Cu wires-with diameters relevant for low-voltage (LV) applications-with graphene. The chemical vapor deposition (CVD) coating process is rapid, safe, scalable, and industrially compatible. Graphene-coated Cu wires display good oxidation resistance and increased electrical conductivity (up to 1% immediately after coating and up to 3% after 24 months), allowing for wire diameter reduction and thus significant savings in wire production costs. Combined spectroscopic and diffraction analysis indicates that the conductivity increase is due to a change in Cu crystallinity induced by the coating process conditions, while electrical testing of aged wires shows that graphene plays a major role in maintaining improved electrical performances over long periods of time. Finally, graphene coating of Cu wires using an ambient-pressure roll-to-roll (R2R) CVD reactor is demonstrated. This enables the in-line production of graphene-coated metallic wires as required for industrial scale-up.

Probing Enhanced Electron-Phonon Coupling in Graphene by Infrared Resonance Raman Spectroscopy

We report on resonance Raman spectroscopy measurements with excitation photon energy down to 1.16 eV on graphene, to study how low-energy carriers interact with lattice vibrations. Thanks to the excitation energy close to the Dirac point at K, we unveil a giant increase of the intensity ratio between the double-resonant 2D and 2D^{'} peaks with respect to that measured in graphite. Comparing with fully ab initio theoretical calculations, we conclude that the observation is explained by an enhanced, momentum-dependent coupling between electrons and Brillouin zone-boundary optical phonons. This finding applies to two-dimensional Dirac systems and has important consequences for the modeling of transport in graphene devices operating at room temperature.

Local dielectric function of hBN-encapsulated WS2flakes grown by chemical vapor deposition

Hexagonal boron nitride (hBN), sometimes referred to as white graphene, receives growing interest in the scientific community, especially when combined into van der Waals (vdW) homo- and heterostacks, in which novel and interesting phenomena may arise. hBN is also commonly used in combination with two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs). The realization of hBN-encapsulated TMDC homo- and heterostacks can indeed offer opportunities to investigate and compare TMDC excitonic properties in various stacking configurations. In this work, we investigate the optical response at the micrometric scale of mono- and homo-bilayer WS₂grown by chemical vapor deposition and encapsulated between two single layers of hBN. Imaging spectroscopic ellipsometry is exploited to extract the local dielectric functions across one single WS₂flake and detect the evolution of excitonic spectral features from monolayer to bilayer regions. Exciton energies undergo a redshift by passing from hBN-encapsulated single layer to homo-bilayer WS₂, as also confirmed by photoluminescence spectra. Our results can provide a reference for the study of the dielectric properties of more complex systems where hBN is combined with other 2D vdW materials into heterostructures and are stimulating towards the investigation of the optical response of other technologically-relevant heterostacks.

Strain-Induced Plasmon Confinement in Polycrystalline Graphene

Terahertz spectroscopy is a perfect tool to investigate the electronic intraband conductivity of graphene, but a phenomenological model (Drude-Smith) is often needed to describe disorder. By studying the THz response of isotropically strained polycrystalline graphene and using a fully atomistic computational approach to fit the results, we demonstrate here the connection between the Drude-Smith parameters and the microscopic behavior. Importantly, we clearly show that the strain-induced changes in the conductivity originate mainly from the increased separation between the single-crystal grains, leading to enchanced localization of the plasmon excitations. Only at the lowest strain values explored, a behavior consistent with the deformation of the individual grains can instead be observed.

Moiré-Induced Transport in CVD-Based Small-Angle Twisted Bilayer Graphene

To realize the applicative potential of 2D twistronic devices, scalable synthesis and assembly techniques need to meet stringent requirements in terms of interface cleanness and twist-angle homogeneity. Here, we show that small-angle twisted bilayer graphene assembled from separated CVD-grown graphene single-crystals can ensure high-quality transport properties, determined by a device-scale-uniform moiré potential. Via low-temperature dual-gated magnetotransport, we demonstrate the hallmarks of a 2.4°-twisted superlattice, including tunable regimes of interlayer coupling, reduced Fermi velocity, large interlayer capacitance, and density-independent Brown-Zak oscillations. The observation of these moiré-induced electrical transport features establishes CVD-based twisted bilayer graphene as an alternative to "tear-and-stack" exfoliated flakes for fundamental studies, while serving as a proof-of-concept for future large-scale assembly.

Ultra-clean high-mobility graphene on technologically relevant substrates

Graphene grown via chemical vapour deposition (CVD) on copper foil has emerged as a high-quality, scalable material, that can be easily integrated on technologically relevant platforms to develop promising applications in the fields of optoelectronics and photonics. Most of these applications require low-contaminated high-mobility graphene (i.e., approaching 10 000 cm² V^-1 s^-1 at room temperature) to reduce device losses and implement compact device design. To date, these mobility values are only obtained when suspending or encapsulating graphene. Here, we demonstrate a rapid, facile, and scalable cleaning process, that yields high-mobility graphene directly on the most common technologically relevant substrate: silicon dioxide on silicon (SiO₂/Si). Atomic force microscopy (AFM) and spatially-resolved X-ray photoelectron spectroscopy (XPS) demonstrate that this approach is instrumental to rapidly eliminate most of the polymeric residues which remain on graphene after transfer and fabrication and that have adverse effects on its electrical properties. Raman measurements show a significant reduction of graphene doping and strain. Transport measurements of 50 Hall bars (HBs) yield hole mobility μ_h up to ∼9000 cm² V^-1 s^-1 and electron mobility μ_e up to ∼8000 cm² V^-1 s^-1, with average values μ_h ∼ 7500 cm² V^-1 s^-1 and μ_e ∼ 6300 cm² V^-1 s^-1. The carrier mobility of ultraclean graphene reaches values nearly double than those measured in graphene processed with acetone cleaning, which is the method widely adopted in the field. Notably, these mobility values are obtained over large-scale and without encapsulation, thus paving the way to the adoption of graphene in optoelectronics and photonics.

Unexpected Electron Transport Suppression in a Heterostructured Graphene-MoS2 Multiple Field-Effect Transistor Architecture

We demonstrate a graphene-MoS2 architecture integrating multiple field-effect transistors (FETs), and we independently probe and correlate the conducting properties of van der Waals coupled graphene-MoS2 contacts with those of the MoS2 channels. Devices are fabricated starting from high-quality single-crystal monolayers grown by chemical vapor deposition. The heterojunction was investigated by scanning Raman and photoluminescence spectroscopies. Moreover, transconductance curves of MoS2 are compared with the current-voltage characteristics of graphene contact stripes, revealing a significant suppression of transport on the n-side of the transconductance curve. On the basis of ab initio modeling, the effect is understood in terms of trapping by sulfur vacancies, which counterintuitively depends on the field effect, even though the graphene contact layer is positioned between the backgate and the MoS2 channel.

Microscopic Understanding of Ultrafast Charge Transfer in van der Waals Heterostructures

Van der Waals heterostructures show many intriguing phenomena including ultrafast charge separation following strong excitonic absorption in the visible spectral range. However, despite the enormous potential for future applications in the field of optoelectronics, the underlying microscopic mechanism remains controversial. Here we use time- and angle-resolved photoemission spectroscopy combined with microscopic many-particle theory to reveal the relevant microscopic charge transfer channels in epitaxial WS_{2}/graphene heterostructures. We find that the timescale for efficient ultrafast charge separation in the material is determined by direct tunneling at those points in the Brillouin zone where WS_{2} and graphene bands cross, while the lifetime of the charge separated transient state is set by defect-assisted tunneling through localized sulphur vacancies. The subtle interplay of intrinsic and defect-related charge transfer channels revealed in the present work can be exploited for the design of highly efficient light harvesting and detecting devices.

Temperature-Dependent Bending Rigidity of AB-Stacked Bilayer Graphene

The change in bending rigidity with temperature κ(T) for 2D materials is highly debated: theoretical works predict both increase and decrease. Here we present measurements of κ(T), for a 2D material: AB-stacked bilayer graphene. We obtain κ(T) from phonon dispersion curves measured with helium atom scattering in the temperature range 320-400 K. We find that the bending rigidity increases with temperature. Assuming a linear dependence over the measured temperature region we obtain κ(T)=[(1.3±0.1)+(0.006±0.001)T/K]  eV by fitting the data. We discuss this result in the context of existing predictions and room temperature measurements.

Local Optical Properties in CVD-Grown Monolayer WS2 Flakes

Excitons dominate the light absorption and re-emission spectra of monolayer transition-metal dichalcogenides (TMD). Microscopic investigations of the excitonic response in TMD almost invariably extract information from the radiative recombination step, which only constitutes one part of the picture. Here, by exploiting imaging spectroscopic ellipsometry (ISE), we investigate the spatial dependence of the dielectric function of chemical vapor deposition (CVD)-grown WS2 flakes with a microscopic lateral resolution, thus providing information about the spatially varying, exciton-induced light absorption in the monolayer WS2. Comparing the ISE results with imaging photoluminescence spectroscopy data, the presence of several correlated features was observed, along with the unexpected existence of a few uncorrelated characteristics. The latter demonstrates that the exciton-induced absorption and emission features are not always proportional at the microscopic scale. Microstructural modulations across the flakes, having a different influence on the absorption and re-emission of light, are deemed responsible for the effect.

Hot-Carrier Cooling in High-Quality Graphene Is Intrinsically Limited by Optical Phonons

Many promising optoelectronic devices, such as broadband photodetectors, nonlinear frequency converters, and building blocks for data communication systems, exploit photoexcited charge carriers in graphene. For these systems, it is essential to understand the relaxation dynamics after photoexcitation. These dynamics contain a sub-100 fs thermalization phase, which occurs through carrier-carrier scattering and leads to a carrier distribution with an elevated temperature. This is followed by a picosecond cooling phase, where different phonon systems play a role: graphene acoustic and optical phonons, and substrate phonons. Here, we address the cooling pathway of two technologically relevant systems, both consisting of high-quality graphene with a mobility >10 000 cm² V^-1 s^-1 and environments that do not efficiently take up electronic heat from graphene: WSe₂-encapsulated graphene and suspended graphene. We study the cooling dynamics using ultrafast pump-probe spectroscopy at room temperature. Cooling via disorder-assisted acoustic phonon scattering and out-of-plane heat transfer to substrate phonons is relatively inefficient in these systems, suggesting a cooling time of tens of picoseconds. However, we observe much faster cooling, on a time scale of a few picoseconds. We attribute this to an intrinsic cooling mechanism, where carriers in the high-energy tail of the hot-carrier distribution emit optical phonons. This creates a permanent heat sink, as carriers efficiently rethermalize. We develop a macroscopic model that explains the observed dynamics, where cooling is eventually limited by optical-to-acoustic phonon coupling. These fundamental insights will guide the development of graphene-based optoelectronic devices.

Survival of Floquet-Bloch States in the Presence of Scattering

Floquet theory has spawned many exciting possibilities for electronic structure control with light, with enormous potential for future applications. The experimental demonstration in solids, however, remains largely unrealized. In particular, the influence of scattering on the formation of Floquet-Bloch states remains poorly understood. Here we combine time- and angle-resolved photoemission spectroscopy with time-dependent density functional theory and a two-level model with relaxation to investigate the survival of Floquet-Bloch states in the presence of scattering. We find that Floquet-Bloch states will be destroyed if scattering-activated by electronic excitations-prevents the Bloch electrons from following the driving field coherently. The two-level model also shows that Floquet-Bloch states reappear at high field intensities where energy exchange with the driving field dominates over energy dissipation to the bath. Our results clearly indicate the importance of long scattering times combined with strong driving fields for the successful realization of various Floquet phenomena.

Acoustic streaming of microparticles using graphene-based interdigital transducers

Surface acoustic wave (SAW) devices offer many benefits in chemistry and biomedicine, enabling precise manipulation of micro-droplets, mixing of liquids by acoustic streaming and pumping of liquids in enclosed channels, while presenting a cost-effective and easy fabrication and integration with electronic devices. In this work, we present microfluidic devices which use graphene-based interdigital transducers (IDTs) to generate SAWs with a frequency of 100 MHz and an amplitude of up to 200 pm, which allow us to manipulate microparticle solutions by acoustic streaming. Due to the negligible mass loading of the piezoelectric surface by graphene, the SAWs generated by these devices have no frequency shift, typically observed when metal IDTs are used.

Large-area, high-responsivity, fast and broadband graphene/n-Si photodetector

A graphene/Si heterojunction device has been realized to overcome many different requests necessary to make it a versatile, widely used and competitive detector. The obtained photodetectors, which operate at room temperature, are sensitive in the spectral region from ultraviolet (240 nm) to infrared (2000 nm) and they can be used in different configurations that allow a high responsivity up to 10⁷ A W^-1, a rise time of a few nanoseconds, an external quantum efficiency greater than 300%, and a linear response for different light sources. This is allowed by the high quality of the graphene deposited on a large area of 8 mm², and by the interdigitated design of the contacts, both preserving the excellent properties of graphene when switching from nanoscale to macroscopic dimensions of commonly used devices.

Synthesis of Large-Scale Monolayer 1T'-MoTe2 and Its Stabilization via Scalable hBN Encapsulation

Out of the different structural phases of molybdenum ditelluride (MoTe2), the distorted octahedral 1T' possesses great interest for fundamental physics and is a promising candidate for the implementation of innovative devices such as topological transistors. Indeed, 1T'-MoTe2 is a semimetal with superconductivity, which has been predicted to be a Weyl semimetal and a quantum spin Hall insulator in bulk and monolayer form, respectively. Large instability of monolayer 1T'-MoTe2 in environmental conditions, however, has made its investigation extremely challenging so far. In this work, we demonstrate homogeneous growth of large single-crystal (up to 500 μm) monolayer 1T'-MoTe2 via chemical vapor deposition (CVD) and its stabilization in air with a scalable encapsulation approach. The encapsulant is obtained by electrochemically delaminating CVD hexagonal boron nitride (hBN) from copper foil, and it is applied on the freshly grown 1T'-MoTe2 via a top-down dry lamination step. The structural and electrical properties of encapsulated 1T'-MoTe2 have been monitored over several months to assess the degree of degradation of the material. We find that when encapsulated with hBN, the lifetime of monolayer 1T'-MoTe2 successfully increases from a few minutes to more than a month. Furthermore, the encapsulated monolayer can be subjected to transfer, device processing, and heating and cooling cycles without degradation of its properties. The potential of this scalable heterostack is confirmed by the observation of signatures of low-temperature phase transition in monolayer 1T'-MoTe2 by both Raman spectroscopy and electrical measurements. The growth and encapsulation methods reported in this work can be employed for further fundamental studies of this enticing material as well as facilitate the technological development of monolayer 1T'-MoTe2.

Deterministic synthesis of Cu9S5 flakes assisted by single-layer graphene arrays

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 cm² V^-1 s^-1.

Wafer-Scale Integration of Graphene-Based Photonic Devices

Graphene and related materials can lead to disruptive advances in next-generation photonics and optoelectronics. The challenge is to devise growth, transfer and fabrication protocols providing high (≥5000 cm2 V-1 s-1) mobility devices with reliable performance at the wafer scale. Here, we present a flow for the integration of graphene in photonics circuits. This relies on chemical vapor deposition (CVD) of single layer graphene (SLG) matrices comprising up to ∼12000 individual single crystals, grown to match the geometrical configuration of the devices in the photonic circuit. This is followed by a transfer approach which guarantees coverage over ∼80% of the device area, and integrity for up to 150 mm wafers, with room temperature mobility ∼5000 cm2 V-1 s-1. We use this process flow to demonstrate double SLG electro-absorption modulators with modulation efficiency ∼0.25, 0.45, 0.75, 1 dB V-1 for device lengths ∼30, 60, 90, 120 μm. The data rate is up to 20 Gbps. Encapsulation with single-layer hexagonal boron nitride (hBN) is used to protect SLG during plasma-enhanced CVD of Si3N4, ensuring reproducible device performance. The processes are compatible with full automation. This paves the way for large scale production of graphene-based photonic devices.

Photo thermal effect graphene detector featuring 105 Gbit s-1 NRZ and 120 Gbit s-1 PAM4 direct detection

One of the main challenges of next generation optical communication is to increase the available bandwidth while reducing the size, cost and power consumption of photonic integrated circuits. Graphene has been recently proposed to be integrated with silicon photonics to meet these goals because of its high mobility, fast carrier dynamics and ultra-broadband optical properties. We focus on graphene photodetectors for high speed datacom and telecom applications based on the photo-thermo-electric effect, allowing for direct optical power to voltage conversion, zero dark current, and ultra-fast operation. We report on a chemical vapour deposition graphene photodetector based on the photo-thermoelectric effect, integrated on a silicon waveguide, providing frequency response >65 GHz and optimized to be interfaced to a 50 Ω voltage amplifier for direct voltage amplification. We demonstrate a system test leading to direct detection of 105 Gbit s^-1 non-return to zero and 120 Gbit s^-1 4-level pulse amplitude modulation optical signals.

Tuning the Doping of Epitaxial Graphene on a Conventional Semiconductor via Substrate Surface Reconstruction

Combining scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we demonstrate how to tune the doping of epitaxial graphene from p to n by exploiting the structural changes that occur spontaneously on the Ge surface upon thermal annealing. Furthermore, using first-principle calculations, we build a model that successfully reproduces the experimental observations. Since the ability to modify graphene electronic properties is of fundamental importance when it comes to applications, our results provide an important contribution toward the integration of graphene with conventional semiconductors.

Scanning Probe Spectroscopy of WS2/Graphene Van Der Waals Heterostructures

In this paper, we present a study of tungsten disulfide (WS2) two-dimensional (2D) crystals, grown on epitaxial Graphene. In particular, we have employed scanning electron microscopy (SEM) and µRaman spectroscopy combined with multifunctional scanning probe microscopy (SPM), operating in peak force-quantitative nano mechanical (PF-QNM), ultrasonic force microscopy (UFM) and electrostatic force microscopy (EFM) modes. This comparative approach provides a wealth of useful complementary information and allows one to cross-analyze on the nanoscale the morphological, mechanical, and electrostatic properties of the 2D heterostructures analyzed. Herein, we show that PF-QNM can accurately map surface properties, such as morphology and adhesion, and that UFM is exceptionally sensitive to a broader range of elastic properties, helping to uncover subsurface features located at the buried interfaces. All these data can be correlated with the local electrostatic properties obtained via EFM mapping of the surface potential, through the cantilever response at the first harmonic, and the dielectric permittivity, through the cantilever response at the second harmonic. In conclusion, we show that combining multi-parametric SPM with SEM and µRaman spectroscopy helps to identify single features of the WS2/Graphene/SiC heterostructures analyzed, demonstrating that this is a powerful tool-set for the investigation of 2D materials stacks, a building block for new advanced nano-devices.

Effect of Chemical Vapor Deposition WS2 on Viability and Differentiation of SH-SY5Y Cells

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.

Epitaxial Growth of Wafer-Scale Molybdenum Disulfide/Graphene Heterostructures by Metal-Organic Vapor-Phase Epitaxy and Their Application in Photodetectors

Van der Waals heterostructures have attracted increasing interest, owing to the combined benefits of their constituents. These hybrid nanostructures can be realized via epitaxial growth, which offers a promising approach for the controlled synthesis of the desired crystal phase and the interface between van der Waals layers. Here, the epitaxial growth of a continuous molybdenum disulfide (MoS₂) film on large-area graphene, which was directly grown on a sapphire substrate, is reported. Interestingly, the grain size of MoS₂ grown on graphene increases, whereas that of MoS₂ grown on SiO₂ decreases with an increasing amount of hydrogen in the chemical vapor deposition reactor. In addition, to achieve the same quality, MoS₂ grown on graphene requires a much lower growth temperature (400 °C) than that grown on SiO₂ (580 °C). The MoS₂/graphene heterostructure that was epitaxially grown on a transparent platform was investigated to explore its photosensing properties and was found to exhibit inverse photoresponse with highly uniform photoresponsivity in the photodetector pixels fabricated across a full wafer. The MoS₂/graphene heterostructure exhibited ultrahigh photoresponsivity (4.3 × 10⁴ A W^-1) upon exposure to visible light of a wide range of wavelengths, confirming the growth of a high-quality MoS₂/graphene heterostructure with a clean interface.

Ultrafast, Zero-Bias, Graphene Photodetectors with Polymeric Gate Dielectric on Passive Photonic Waveguides

We report compact, scalable, high-performance, waveguide integrated graphene-based photodetectors (GPDs) for telecom and datacom applications, not affected by dark current. To exploit the photothermoelectric (PTE) effect, our devices rely on a graphene/polymer/graphene stack with static top split gates. The polymeric dielectric, poly(vinyl alcohol) (PVA), allows us to preserve graphene quality and to generate a controllable p-n junction. Both graphene layers are fabricated using aligned single-crystal graphene arrays grown by chemical vapor deposition. The use of PVA yields a low charge inhomogeneity ∼8 × 10¹⁰ cm^-2 at the charge neutrality point, and a large Seebeck coefficient ∼140 μV K^-1, enhancing the PTE effect. Our devices are the fastest GPDs operating with zero dark current, showing a flat frequency response up to 67 GHz without roll-off. This performance is achieved on a passive, low-cost, photonic platform, and does not rely on nanoscale plasmonic structures. This, combined with scalability and ease of integration, makes our GPDs a promising building block for next-generation optical communication devices.

Low-voltage 2D materials-based printed field-effect transistors for integrated digital and analog electronics on paper

Paper is the ideal substrate for the development of flexible and environmentally sustainable ubiquitous electronic systems, which, combined with two-dimensional materials, could be exploited in many Internet-of-Things applications, ranging from wearable electronics to smart packaging. Here we report high-performance MoS2 field-effect transistors on paper fabricated with a "channel array" approach, combining the advantages of two large-area techniques: chemical vapor deposition and inkjet-printing. The first allows the pre-deposition of a pattern of MoS2; the second, the printing of dielectric layers, contacts, and connections to complete transistors and circuits fabrication. Average ION/IOFF of 8 × 103 (up to 5 × 104) and mobility of 5.5 cm2 V-1 s-1 (up to 26 cm2 V-1 s-1) are obtained. Fully functional integrated circuits of digital and analog building blocks, such as logic gates and current mirrors, are demonstrated, highlighting the potential of this approach for ubiquitous electronics on paper.

Graphene Promotes Axon Elongation through Local Stall of Nerve Growth Factor Signaling Endosomes

Several works reported increased differentiation of neuronal cells grown on graphene; however, the molecular mechanism driving axon elongation on this material has remained elusive. Here, we study the axonal transport of nerve growth factor (NGF), the neurotrophin supporting development of peripheral neurons, as a key player in the time course of axonal elongation of dorsal root ganglion neurons on graphene. We find that graphene drastically reduces the number of retrogradely transported NGF vesicles in favor of a stalled population in the first 2 days of culture, in which the boost of axon elongation is observed. This correlates with a mutual charge redistribution, observed via Raman spectroscopy and electrophysiological recordings. Furthermore, ultrastructural analysis indicates a reduced microtubule distance and an elongated axonal topology. Thus, both electrophysiological and structural effects can account for graphene action on neuron development. Unraveling the molecular players underneath this interplay may open new avenues for axon regeneration applications.

30°-Twisted Bilayer Graphene Quasicrystals from Chemical Vapor Deposition

The artificial stacking of atomically thin crystals suffers from intrinsic limitations in terms of control and reproducibility of the relative orientation of exfoliated flakes. This drawback is particularly severe when the properties of the system critically depends on the twist angle, as in the case of the dodecagonal quasicrystal formed by two graphene layers rotated by 30°. Here we show that large-area 30°-rotated bilayer graphene can be grown deterministically by chemical vapor deposition on Cu, eliminating the need of artificial assembly. The quasicrystals are easily transferred to arbitrary substrates and integrated in high-quality hexagonal boron nitride-encapsulated heterostructures, which we process into dual-gated devices exhibiting carrier mobility up to 10⁵ cm²/(V s). From low-temperature magnetotransport, we find that the graphene quasicrystals effectively behave as uncoupled graphene layers, showing 8-fold degenerate quantum Hall states. This result indicates that the Dirac cones replica detected by previous photoemission experiments do not contribute to the electrical transport.

Direct evidence for efficient ultrafast charge separation in epitaxial WS2/graphene heterostructures

We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to investigate ultrafast charge transfer in an epitaxial heterostructure made of monolayer WS₂ and graphene. This heterostructure combines the benefits of a direct-gap semiconductor with strong spin-orbit coupling and strong light-matter interaction with those of a semimetal hosting massless carriers with extremely high mobility and long spin lifetimes. We find that, after photoexcitation at resonance to the A-exciton in WS₂, the photoexcited holes rapidly transfer into the graphene layer while the photoexcited electrons remain in the WS₂ layer. The resulting charge-separated transient state is found to have a lifetime of ∼1 ps. We attribute our findings to differences in scattering phase space caused by the relative alignment of WS₂ and graphene bands as revealed by high-resolution ARPES. In combination with spin-selective optical excitation, the investigated WS₂/graphene heterostructure might provide a platform for efficient optical spin injection into graphene.

Wafer-Scale Synthesis of Graphene on Sapphire: Toward Fab-Compatible Graphene

The adoption of graphene in electronics, optoelectronics, and photonics is hindered by the difficulty in obtaining high-quality material on technologically relevant substrates, over wafer-scale sizes, and with metal contamination levels compatible with industrial requirements. To date, the direct growth of graphene on insulating substrates has proved to be challenging, usually requiring metal-catalysts or yielding defective graphene. In this work, a metal-free approach implemented in commercially available reactors to obtain high-quality monolayer graphene on c-plane sapphire substrates via chemical vapor deposition is demonstrated. Low energy electron diffraction, low energy electron microscopy, and scanning tunneling microscopy measurements identify the Al-rich reconstruction 31 × 31 R ± 9 ° of sapphire to be crucial for obtaining epitaxial graphene. Raman spectroscopy and electrical transport measurements reveal high-quality graphene with mobilities consistently above 2000 cm² V^-1 s^-1 . The process is scaled up to 4 and 6 in. wafers sizes and metal contamination levels are retrieved to be within the limits for back-end-of-line integration. The growth process introduced here establishes a method for the synthesis of wafer-scale graphene films on a technologically viable basis.

Waveguide-Integrated, Plasmonic Enhanced Graphene Photodetectors

We present a micrometer-scale, on-chip integrated, plasmonic enhanced graphene photodetector (GPD) for telecom wavelengths operating at zero dark current. The GPD is designed to directly generate a photovoltage by the photothermoelectric effect. It is made of chemical vapor deposited single layer graphene, and has an external responsivity ∼12.2 V/W with a 3 dB bandwidth ∼42 GHz. We utilize Au split-gates to electrostatically create a p-n-junction and simultaneously guide a surface plasmon polariton gap-mode. This increases the light-graphene interaction and optical absorption and results in an increased electronic temperature and steeper temperature gradient across the GPD channel. This paves the way to compact, on-chip integrated, power-efficient graphene based photodetectors for receivers in tele- and datacom modules.

Submicron Size Schottky Junctions on As-Grown Monolayer Epitaxial Graphene on Ge(100): A Low-Invasive Scanned-Probe-Based Study

We report on the investigation of the Schottky barrier (SB) formed at the junction between a metal-free graphene monolayer and Ge semiconductor substrate in the as-grown epitaxial graphene/Ge(100) system. In order to preserve the heterojunction properties, we defined submicron size graphene/Ge junctions using the scanning probe microscopy lithography in the local oxidation configuration, a low-invasive processing approach capable of inducing spatially controlled electrical separations among tiny graphene regions. Characteristic junction parameters were estimated from I-V curves obtained using conductive-atomic force microscopy. The current-voltage characteristics showed a p-type Schottky contact behavior, ascribed to the n-type to p-type conversion of the entire Ge substrate due to the formation of a large density of acceptor defects during the graphene growth process. We estimated, for the first time, the energy barrier height in the as-grown graphene/Ge Schottky junction (φ_B ≈ 0.45 eV) indicating an n-type doping of the graphene layer with a Fermi level ≈ 0.15 eV above the Dirac point. The SB devices showed ideality factor values around 1.5 pointing to the high quality of the heterojunctions.

High-speed double layer graphene electro-absorption modulator on SOI waveguide

We report on a C-band double layer graphene electro-absorption modulator on a passive SOI platform showing 29GHz 3dB-bandwith and NRZ eye-diagrams extinction ratios ranging from 1.7 dB at 10 Gb/s to 1.3 dB at 50 Gb/s. Such high modulation speed is achieved thanks to the quality of the CVD pre-patterned single crystal growth and transfer on wafer method that permitted the integration of high-quality scalable graphene and low contact resistance. By demonstrating this high-speed CVD graphene EAM modulator integrated on Si photonics and the scalable approach, we are confident that graphene can satisfy the main requirements to be a competitive technology for photonics.

Early-morning OFF and levodopa dose failures in patients with Parkinson's disease attending a routine clinical appointment using Time-to-ON Questionnaire

BACKGROUND AND PURPOSE

In patients with Parkinson's disease (PD) with motor fluctuations, total daily OFF time is comprised of both end-of-dose time and the time taken to turn ON with medication. However, little is known about the impact of delays in ON time.

METHODS

This was a single-visit pilot study of fluctuating patients with PD attending a routine appointment. During a single visit, adult patients with idiopathic PD who were treated with levodopa for at least 1 year completed a questionnaire evaluating the time waiting for ON and the symptoms experienced while waiting to turn ON. Patients then completed a 5-day home time-to-ON diary, where they documented how long it took to turn ON following their first morning dose of levodopa in 5-min increments.

RESULTS

A total of 151 consecutive patients completed the study survey, of whom 97 (64.2%) experienced motor fluctuations. Of the patients experiencing motor fluctuations, 54 (56%) reported delays in ON time (latency >30 min) following their first morning dose of levodopa. Half (51%) reported that they had experienced delayed ON at least once in the previous week and 21% reported having delayed ON during all seven mornings of the previous week. In addition, 10% of patients reported having dose failures on four or more mornings during the previous week. The most common symptoms experienced while waiting for ON were slowness (94.8%), fatigue (87.6%), reduced dexterity (82.5%), problems in walking (66.0%) and problems with balance (59.8%).

CONCLUSION

Early-morning OFF problems such as delays in time to ON and dose failures are common in levodopa-treated patients with PD.

A sensitive calorimetric technique to study energy (heat) exchange at the nano-scale

Every time a chemical reaction occurs, an energy exchange between reactants and the environment takes place, which is defined as the enthalpy of the reaction. During the last few decades, research has resulted in an increasing number of devices at the micro- or nano-scale. Sensors, catalyzers, and energy storage systems are more and more developed as nano-devices which represent the building blocks for commercial "macroscopic" objects. A general method for the direct evaluation of the energy balance of such systems is not available at present. Calorimetry is a powerful tool to investigate energy exchange, but it usually requires macroscopic sample quantities. Here, we report on the development of an original experimental setup able to detect temperature variations as low as 10 mK in a sample of ∼10 ng using a thermometer device having physical dimensions of 5 × 5 mm2. This technique has been utilized to measure the enthalpy release during the adsorption process of H2 on titanium-decorated monolayer graphene. The sensitivity of these thermometers is high enough to detect a hydrogen uptake of ∼10-10 moles, corresponding to ∼0.2 ng, with an enthalpy release of about 23 μJ. The experimental setup allows, in perspective, scalability to even smaller sizes.

Rippling of graphitic surfaces: a comparison between few-layer graphene and HOPG

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 bio-organic 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.

The ultrafast dynamics and conductivity of photoexcited graphene at different Fermi energies

For many of the envisioned optoelectronic applications of graphene, it is crucial to understand the subpicosecond carrier dynamics immediately following photoexcitation and the effect of photoexcitation on the electrical conductivity-the photoconductivity. Whereas these topics have been studied using various ultrafast experiments and theoretical approaches, controversial and incomplete explanations concerning the sign of the photoconductivity, the occurrence and significance of the creation of additional electron-hole pairs, and, in particular, how the relevant processes depend on Fermi energy have been put forward. We present a unified and intuitive physical picture of the ultrafast carrier dynamics and the photoconductivity, combining optical pump-terahertz probe measurements on a gate-tunable graphene device, with numerical calculations using the Boltzmann equation. We distinguish two types of ultrafast photo-induced carrier heating processes: At low (equilibrium) Fermi energy (E_F ≲ 0.1 eV for our experiments), broadening of the carrier distribution involves interband transitions (interband heating). At higher Fermi energy (E_F ≳ 0.15 eV), broadening of the carrier distribution involves intraband transitions (intraband heating). Under certain conditions, additional electron-hole pairs can be created [carrier multiplication (CM)] for low E_F, and hot carriers (hot-CM) for higher E_F. The resultant photoconductivity is positive (negative) for low (high) E_F, which in our physical picture, is explained using solely electronic effects: It follows from the effect of the heated carrier distributions on the screening of impurities, consistent with the DC conductivity being mostly due to impurity scattering. The importance of these insights is highlighted by a discussion of the implications for graphene photodetector applications.

Patterned tungsten disulfide/graphene heterostructures for efficient multifunctional optoelectronic devices

One of the major issues in graphene-based optoelectronics is to scale-up high-performing devices. In this work, we report an original approach for the fabrication of efficient optoelectronic devices from scalable tungsten disulfide (WS₂)/graphene heterostructures. Our approach allows for the patterned growth of WS₂ on graphene and facilitates the realization of ohmic contacts. Photodetectors fabricated with WS₂ on epitaxial graphene on silicon carbide (SiC) present, when illuminated with red light, a maximum responsivity R ∼220 A W^-1, a detectivity D* ∼2.0 × 10⁹ Jones and a -3 dB bandwidth of 250 Hz. The retrieved detectivity is 3 orders of magnitude higher than that obtained with graphene-only devices at the same wavelength. For shorter illumination wavelengths we observe a persistent photocurrent with a nearly complete charge retention, which originates from deep trap levels in the SiC substrate. This work ultimately demonstrates that WS₂/graphene optoelectronic devices with promising performances can be obtained in a scalable manner. Furthermore, by combining wavelength-selective memory, enhanced responsivity and fast detection, this system is of interest for the implementation of 2d-based data storage devices.

Peripheral Neuron Survival and Outgrowth on Graphene

Graphene displays properties that make it appealing for neuroregenerative medicine, yet its interaction with peripheral neurons has been scarcely investigated. Here, we culture on graphene two established models for peripheral neurons: PC12 cells and DRG primary neurons. We perform a nano-resolved analysis of polymeric coatings on graphene and combine optical microscopy and viability assays to assess the material cytocompatibility and influence on differentiation. We find that differentiated PC12 cells display a remarkably increased neurite length on graphene (up to 27%) with respect to controls. Notably, DRG primary neurons survive both on bare and coated graphene. They present dense axonal networks on coated graphene, while they form cell islets characterized by dense axonal bundles on uncoated graphene. These findings indicate that graphene holds potential for nerve tissue regeneration and might pave the road to novel concepts of active nerve conduits.

Electronic properties of single-layer tungsten disulfide on epitaxial graphene on silicon carbide

This work reports an electronic and micro-structural study of an appealing system for optoelectronics: tungsten disulfide (WS₂) on epitaxial graphene (EG) on SiC(0001). The WS₂ is grown via chemical vapor deposition (CVD) onto the EG. Low-energy electron diffraction (LEED) measurements assign the zero-degree orientation as the preferential azimuthal alignment for WS₂/EG. The valence-band (VB) structure emerging from this alignment is investigated by means of photoelectron spectroscopy measurements, with both high space and energy resolution. We find that the spin-orbit splitting of monolayer WS₂ on graphene is of 462 meV, larger than what is reported to date for other substrates. We determine the value of the work function for the WS₂/EG to be 4.5 ± 0.1 eV. A large shift of the WS₂ VB maximum is observed as well, due to the lowering of the WS₂ work function caused by the donor-like interfacial states of EG. Density functional theory (DFT) calculations carried out on a coincidence supercell confirm the experimental band structure to an excellent degree. X-ray photoemission electron microscopy (XPEEM) measurements performed on single WS₂ crystals confirm the van der Waals nature of the interface coupling between the two layers. In virtue of its band alignment and large spin-orbit splitting, this system gains strong appeal for optical spin-injection experiments and opto-spintronic applications in general.

Local anodic oxidation on hydrogen-intercalated graphene layers: oxide composition analysis and role of the silicon carbide substrate

We investigate nanoscale local anodic oxidation (LAO) on hydrogen-intercalated graphene grown by controlled sublimation of silicon carbide (SiC). Scanning probe microscopy was used as a lithographic and characterization tool in order to investigate the local properties of the nanofabricated structures. The anomalous thickness observed after the graphene oxidation process is linked to the impact of LAO on the substrate. Micro-Raman (μ-Raman) spectroscopy was employed to demonstrate the presence of two oxidation regimes depending on the applied bias. We show that partial and total etching of monolayer graphene can be achieved by tuning the bias voltage during LAO. Finally, a complete compositional characterization was achieved by scanning electron microscopy and energy dispersive spectroscopy.

Investigating the CVD Synthesis of Graphene on Ge(100): toward Layer-by-Layer Growth

Germanium is emerging as the substrate of choice for the growth of graphene in CMOS-compatible processes. For future application in next generation devices the accurate control over the properties of high-quality graphene synthesized on Ge surfaces, such as number of layers and domain size, is of paramount importance. Here we investigate the role of the process gas flows on the CVD growth of graphene on Ge(100). The quality and morphology of the deposited material is assessed by using μ-Raman spectroscopy, X-ray photoemission spectroscopy, scanning electron microscopy, and atomic force microscopy. We find that by simply varying the carbon precursor flow different growth regimes yielding to graphene nanoribbons, graphene monolayer, and graphene multilayer are established. We identify the growth conditions yielding to a layer-by-layer growth regime and report on the achievement of homogeneous monolayer graphene with an average intensity ratio of 2D and G bands in the Raman map larger than 3.

Morphological modulation of graphene-mediated hybridization in plasmonic systems

We investigated the plasmonic response of a 2-dimensional ordered array of closely spaced (few-nm apart) Au nanoparticles covered by a large-area single-layer graphene sheet. The array consisted of coherently aligned nanoparticle chains, endowed with a characteristic uniaxial anisotropy. The joint effect of such a morphology and of the very small particle size and spacing led to a corresponding uniaxial wrinkling of graphene in the absence of detectable strain. The deposition of graphene redshifted the Au plasmon-resonance, strongly increased the optical absorption of the array and, most importantly, induced a marked optical anisotropy in the plasmonic response, absent in the pristine nanoparticle array. The experimental observations are accounted for by invoking a graphene-mediated resistive coupling between the Au nanoparticles, where the optical anisotropy arises from the wrinkling-induced anisotropic electron mobility in graphene at optical frequencies.