Defect healing and doping of CVD graphene by thermal sulfurization.

CVD graphene layers are intrinsically polycrystalline; depending on grain size, their structure at the atomic level is scarcely free of defects, which affects the properties of graphene. On the one hand, atomic-scale defects act as scattering centers and lead to a loss of carrier mobility. On the other hand, structural disorder at grain boundaries provides additional resistance in series that affects material conductivity. Graphene chemical functionalization has been demonstrated to be an effective way to improve its conductivity mainly by increasing carrier concentration. The present study reports the healing effects of sulfur doping on the electrical transport properties of single-layer CVD graphene. A post-growth thermal sulfurization process operating at 250 °C is applied on single layers of graphene on Corning-glass and Si/SiO2 substrates. XPS and Raman analyses reveal the covalent attachment of sulfur atoms in graphene carbon lattice without creating new C-sp3 defects. Measurements of transport properties show a significant improvement in hole mobility as revealed by Hall measurements and related material conductivity. Typically, Hall mobility values as high as 2500 cm2 V-1 s-1 and sheet resistance as low as 400 Ohm per square are measured on single-layer sulfurized graphene.

Design of optically transparent metasurfaces based on CVD graphene for mmWave applications.

We propose and numerically investigate a smart, optically transparent digital metasurface reflective in the mmWave range, based on CVD graphene programmable elements. For both TM and TE polarizations, we detail the optimization of the unit cells, designed to exhibit two distinct states that correspond to those of binary encoding. The whole metasurface encoding can be customized to provide different electromagnetic functions, such as wide-band beam splitting at a controlled angle and reduction of the Radar Cross Section. Optically transparent metasurfaces could be integrated and exploited in windows and transparent surfaces in future Beyond-5G and 6G ecosystems.

Au nanoparticles decorated nanographene oxide-based platform: Synthesis, functionalization and assessment of photothermal activity.

A novel hybrid nanocomposite formed of carboxylated Nano Graphene Oxide (c-NGO), highly densely decorated by monodisperse citrate-coated Au nanoparticles (c-NGO/Au NPs), is synthesized and thoroughly characterized for photothermal applications. A systematic investigation of the role played by the synthetic parameters on the Au NPs decoration of the c-NGO platform is performed, comprehensively studying spectroscopic and morphological characteristics of the achieved nanostructures, thus elucidating their still not univocally explained synthesis mechanism. Remarkably, the Au NPs coating density of the c-NGO sheets is much higher than state-of-the-art systems with analogous composition prepared with different approaches, along with a higher NPs size dispersion. A novel theoretical approach for estimating the average number of NPs per sheet, combining DLS and TEM results, is developed. The assessment of the c-NGO/Au NPs photothermal activity is performed under continuous wave (CW) laser irradiation, at 532 nm and 800 nm, before and after functionalization with PEG-SH. c-NGO/Au NPs composite behaves as efficient photothermal agent, with a light into heat conversion ability higher than that of the single components. The c-NGO/Au NPs compatibility for photothermal therapy is assessed by in vitro cell viability tests, which show no significant effects of c-NGO/Au NPs, as neat and PEGylated, on cell metabolic activity under the investigated conditions. These results demonstrate the great potential held by the prepared hybrid nanocomposite for photothermal conversion technologies, indicating it as particularly promising platform for photothermal ablation of cancer cells.

Effective hole conductivity in nitrogen-doped CVD-graphene by singlet oxygen treatment under photoactivation conditions.

Nitrogen substitutional doping in the π-basal plane of graphene has been used to modulate the material properties and in particular the transition from hole to electron conduction, thus enlarging the field of potential applications. Depending on the doping procedure, nitrogen moieties mainly include graphitic-N, combined with pyrrolic-N and pyridinic-N. However, pyridine and pyrrole configurations of nitrogen are predominantly introduced in monolayer graphene:N lattice as prepared by CVD. In this study, we investigate the possibility of employing pyridinic-nitrogen as a reactive site as well as activate a reactive center at the adjacent carbon atoms in the functionalized C-N bonds, for additional post reaction like oxidation. Furthermore, the photocatalytic activity of the graphene:N surface in the production of singlet oxygen (1O2) is fully exploited for the oxidation of the graphene basal plane with the formation of pyridine N-oxide and pyridone structures, both having zwitterion forms with a strong p-doping effect. A sheet resistance value as low as 100 Ω/□ is reported for a 3-layer stacked graphene:N film.

Electrochromic evaluation of airbrushed water-dispersible W18O49nanorods obtained by microwave-assisted synthesis.

Motivated by the technological relevance of tungsten oxide nanostructures as valuable materials for energy saving technology, electrochemical and electrochromic characteristics of greener processed nanostructured W18O49-based electrodes are discussed in this work. For the purpose, microwave-assisted water-dispersible W18O49nanorods have been synthesized and processed into nanostructured electrodes. An airbrushing technique has been adopted as a cost-effective large-area scalable methodology to deposit the W18O49nanorods onto conductive glass. This approach preserves the morphological and crystallographic habit of native nanorods and allows highly homogeneous transparent coating where good electronic coupling between nanowires is ensured by a mild thermal treatment (250 °C, 30 min). Morphological and structural characteristics of active material were investigated from the synthesis to the nanocrystal deposition process by transmission and scanning electron microscopy, x-ray diffraction, atomic force microscopy and Raman spectroscopy. The as-obtained nanostructured film exhibited good reversible electrochemical features through several intercalation-deintercalation cycles. The electrochromic properties were evaluated on the basis of spectro-electrochemical measurements and showed significant optical contrast in the near-infrared region and high coloration efficiency at 550 nm.

In-plane Aligned Colloidal 2D WS2 Nanoflakes for Solution-Processable Thin Films with High Planar Conductivity.

Two-dimensional transition-metal dichalcolgenides (2D-TMDs) are among the most intriguing materials for next-generation electronic and optoelectronic devices. Albeit still at the embryonic stage, building thin films by manipulating and stacking preformed 2D nanosheets is now emerging as a practical and cost-effective bottom-up paradigm to obtain excellent electrical properties over large areas. Herein, we exploit the ultrathin morphology and outstanding solution stability of 2D WS2 colloidal nanocrystals to make thin films of TMDs assembled on a millimetre scale by a layer-by-layer deposition approach. We found that a room-temperature surface treatment with a superacid, performed with the precise scope of removing the native insulating surfactants, promotes in-plane assembly of the colloidal WS2 nanoflakes into stacks parallel to the substrate, along with healing of sulphur vacancies in the lattice that are detrimental to electrical conductivity. The as-obtained 2D WS2 thin films, characterized by a smooth and compact morphology, feature a high planar conductivity of up to 1 µS, comparable to the values reported for epitaxially grown WS2 monolayers, and enable photocurrent generation upon light irradiation over a wide range of visible to near-infrared frequencies.

Reconfigurable and optically transparent microwave absorbers based on deep eutectic solvent-gated graphene.

Electrolytically tunable graphene "building blocks" for reconfigurable and optically transparent microwave surfaces and absorbers have been designed and fabricated by exploiting Deep Eutectic Solvents (DESs). DESs have been first explored as electrolytic and environmentally friendly media for tuning sheet resistance and Fermi level of graphene together with its microwave response (reflection, transmission and absorption). We consider the tunability of the reconfigurable surfaces in terms of transmittance, absorption and reflectance, respectively, over the X and Ku bands when the gate voltage is varied in the -1.4/+1.4 V range. The numerical simulations and experimental measurements also show the ability of the absorber, in the Salisbury screen configuration, to achieve near perfect absorption with a modulation of about 20%. These results could find applications in several technological fields, ranging from electromagnetic pollution to integrated multi-physical regulation systems, thereby helping the advance of the performance of microwave cloaking systems, stealth windows, frequency selective surfaces, modulators and polarizers.

Tailoring water stability of cellulose nanopaper by surface functionalization.

Cellulose nanopaper (CNP) features appealing properties, including transparency, flatness, a low thermal expansion coefficient and thermal stability, often outperforming conventional paper. However, free-standing crystalline cellulose films usually swell in water or upon moisture sorption, compromising part of their outstanding properties. This remains a major problem whenever working in a water environment is required. Freestanding cellulose nanopaper is prepared by solution casting water suspensions of cellulose nanocrystals with an average width of 10 nm and an average aspect ratio of 28, isolated from Avicel by acid hydrolysis and extensively characterized by AFM and FE-SEM measurements and GPC detection of their degree of polymerization. We demonstrate by elemental analyses, FT-IR, Raman spectroscopy, XRD measurements and water contact angle detection that wet treatment with lauroyl chloride results in surface hydrophobization of nanopaper. The hydrophobized nanopaper, C12-CNP, shows a more compact surface morphology than the starting CNP, due to the effect of chemical functionalization, and presents enhanced resistance to water, as assessed by electrochemical permeation experiments. The new hydrophobized nanopaper is a promising substrate for thin film devices designed to work in a humid environment.

Laser-patterned functionalized CVD-graphene as highly transparent conductive electrodes for polymer solar cells.

A five-layer (5L) graphene on a glass substrate has been demonstrated as a transparent conductive electrode to replace indium tin oxide (ITO) in organic photovoltaic devices. The required low sheet resistance, while maintaining high transparency, and the need of a wettable surface are the main issues. To overcome these, two strategies have been applied: (i) the p-doping of the multilayer graphene, thus reaching 25 Ω□-1 or (ii) the O2-plasma oxidation of the last layer of the 5L graphene that results in a contact angle of 58° and a sheet resistance of 134 Ω□-1. A Nd:YVO4 laser patterning has been implemented to realize the desired layout of graphene through an easy and scalable way. Inverted Polymer Solar Cells (PSCs) have been fabricated onto the patterned and modified graphene. The use of PEDOT:PSS has facilitated the deposition of the electron transport layer and a non-chlorinated solvent (ortho-xylene) has been used in the processing of the active layer. It has been found that the two distinct functionalization strategies of graphene have beneficial effects on the overall performance of the devices, leading to an efficiency of 4.2%. Notably, this performance has been achieved with an active area of 10 mm2, the largest area reported in the literature for graphene-based inverted PSCs.

Optically Transparent Microwave Polarizer Based On Quasi-Metallic Graphene.

In this paper, we report on the engineering and the realization of optically transparent graphene-based microwave devices using Chemical Vapour Deposition (CVD) graphene whose sheet resistance may be tailored down to values below 30 Ω/sq. In particular, we show that the process was successfully used to realize and characterize a simple, optically transparent graphene-based wire-grid polarizer at microwave frequencies (X band). The availability of graphene operating in a quasi-metallic region may allow the integration of graphene layers in several microwave components, thus leading to the realization of fully transparent (and flexible) microwave devices.

Surface chemical functionalization of single walled carbon nanotubes with a bacteriorhodopsin mutant.

In this work, single walled carbon nanotubes (SWNTs) have been chemically functionalized at their walls with a membrane protein, namely the mutated bacteriorhodopsin D96N, integrated in its native archaeal lipid membrane. The modification of the SWNT walls with the mutant has been carried out in different buffer solutions, at pH 5, 7.5 and 9, to investigate the anchoring process, the typical chemical and physical properties of the component materials being dependent on the pH. The SWNTs modified by interactions with bacteriorhodopsin membrane patches have been characterized by UV-vis steady state, Raman and attenuated total reflection Fourier transform infrared spectroscopy and by atomic force and transmission electron microscopy. The investigation shows that the membrane protein patches wrap the carbon walls by tight chemical interactions undergoing a conformational change; such chemical interactions increase the mechanical strength of the SWNTs and promote charge transfers which p-dope the nano-objects. The functionalization, as well as the SWNT doping, is favoured in acid and basic buffer conditions; such buffers make the nanotube walls more reactive, thus catalysing the anchoring of the membrane protein. The direct electron communication among the materials can be exploited for effectively interfacing the transport properties of carbon nanotubes with both molecular recognition capability and photoactivity of the cell membrane for sensing and photoconversion applications upon integration of the achieved hybrid materials in sensors or photovoltaic devices.