Luca Ortolani

Surfactant-free single-layer graphene in water

Dispersing graphite in water to obtain true (single-layer) graphene in bulk quantity in a liquid has been an unreachable goal for materials scientists in the past decade. Similarly, a diagnostic tool to identify solubilized graphene in situ has been long awaited. Here we show that homogeneous stable dispersions of single-layer graphene (SLG) in water can be obtained by mixing graphenide (negatively charged graphene) solutions in tetrahydrofuran with degassed water and evaporating the organic solvent. In situ Raman spectroscopy of these aqueous dispersions shows all the expected characteristics of SLG. Transmission electron and atomic force microscopies on deposits confirm the single-layer character. The resulting additive-free stable water dispersions contain 400 m2 l-1 of developed graphene surface. Films prepared from these dispersions exhibit a conductivity of up to 32 kS m-1.

Folded Graphene Membranes: Mapping Curvature at the Nanoscale

While the unique elastic properties of monolayer graphene have been extensively investigated, less knowledge has been developed so far on folded graphene. Nevertheless, it has been recently suggested that fold-induced curvature (without in-plane strain) could possibly affect the local chemical and electron transport properties of graphene, envisaging a material-by-design approach where tailored membranes are used in enhanced nanoresonators or nanoelectromechanical devices. In this work we propose a novel method combining apparent strain analysis from high-resolution transmission electron microscopy (HREM) images and theoretical modeling based on continuum elasticity theory and tight-binding atomistic simulations to map and measure the nanoscale curvature of graphene folds and wrinkles. If enough contrast and resolution in HREM images are obtained, this method can be successfully applied to provide a complete nanoscale geometrical and physical picture of 3D structure of various wrinkle and fold configurations.

Micron-sized [6,6]-phenyl C61 butyric acid methyl ester crystals grown by dip coating in solvent vapour atmosphere: interfaces for organic photovoltaics

We have devised a novel dip coating procedure to form highly crystalline and macroscopic pi-conjugated architectures on solid surfaces. We have employed this approach to a technologically relevant system, i.e. the electron-acceptor [6,6]-phenyl C61 butyric acid methyl ester molecule (PCBM), which is the most commonly used electron-acceptor in organic photovoltaics. Highly ordered, hexagonal shaped crystals of PCBM, ranging between 1 to 80 mum in diameter and from 20 to 500 nm in thickness, have been grown by dip coating the substrates into a solution containing the fullerene derivative. These crystals have been found to possess a monocrystalline character, to exhibit a hexagonal symmetry and to display micron sized molecularly flat terraces. The crystals have been prepared on a wide variety of surfaces such as SiO(x), silanized SiO(x), Au, graphite, amorphous carbon-copper grids and ITO. Their multiscale characterization has been performed by atomic force microscopy (AFM), Kelvin probe force microscopy (KPFM), X-ray diffraction (XRD), optical microscopy, scanning and transmission electron microscopy (SEM, TEM).To test the stability of these electron accepting PCBM crystals, they have been coated with a complementary, electron donor hexa-peri-hexabenzocoronene (HBC) derivative by solution processing from acetone and chloroform-methanol blends. The HBC self assembles in a well-defined network of nanofibers on the PCBM substrate, and the two materials can be clearly resolved by AFM and KPFM.Due to its structural precision on the macroscopic scale, the PCBM crystals appear as ideal interface to perform fundamental photophysical studies in electron-acceptor and -donor blends, as well as workbench for unravelling the architecture vs. function relationship in organic solar cells prototypes.

Green and easily scalable microwave synthesis of noble metal nanosols (Au, Ag, Cu, Pd) usable as catalysts

A green synthesis process was developed for the production of PVP-coated noble metal nanoparticles in the form of stable nanosols. Water is the environmentally benign solvent; glucose serves as a mild, renewable and non-toxic reducing agent and microwave irradiation is an effective and fast heating technique. The same green process has been optimized to obtain several metal nanoparticles (Au, Ag, Cu, Pd), and therefore encourages the easy preparation of bimetallic nanostructures. Nanosols were characterized by dynamic light scattering DLS, HR-TEM, UV-Vis spectroscopy, XRD and ICP-AES. The total reaction yield for all the samples was assessed, the prepared nanoparticles were spherical shaped with an average diameter ranging from 3 to 20 nm. Nanosols with excellent stability over several months, achieved even for high solid contents, were prepared. Additionally, it is shown that all of the synthesized nanoparticles can act as effective catalysts for the reduction of 4-nitrophenol (4-NP) in the presence of NaBH4 (which is otherwise unfeasible without a metal catalyst). This reduction was spectrophotocally followed and the rate constants were determined by measuring the change in absorbance at 400 nm (the wavelength typical of 4-NP) as a function of time. The following ranking of decreasing efficiency of the catalyst was found: Pd > Au > Ag > Cu.

Solutions of fully exfoliated individual graphene flakes in low boiling point solvents

Graphenide solutions (solutions of negatively charged graphene flakes) have been prepared in low boiling point solvents such as tetrahydrofuran (THF) by dissolution of the graphite intercalation compound (GIC) KC8. The presence of two-dimensional objects in solution, with an average lateral size of over one micron, is evidenced by light scattering analysis. High resolution transmission electron microscopy analysis shows that the solubilized graphene flakes are exclusively single and double layers with no evidence for thicker species. Molecular dynamics simulations support the graphene folding, observed in TEM, and suggest it is triggered by solvent nanodrops.

Graphene–organic hybrids as processable, tunable platforms for pH-dependent photoemission, obtained by a new modular approach

We describe a new approach to attach organic dyes to graphene oxide (GO) sheets with high loading and minimal perturbation of the electronic and optical properties of the dye. The dye unit used (a pH-sensitive terthiophene) is grafted to GO using a new modular synthetic approach, passing through a C6-aminic linker which makes GO more soluble in different organic solvents and allows straightforward attachment at high yield not only of terthiophene but of many commercially available amino-reactive dyes. The covalent engraftment to GO does not perturb the absorption and emission properties of the dye, and in particular the pH sensing capability through amidic group reversible protonation. This approach can allow (i) high solubility of the GO intermediate in organic solvents, (ii) convenient coupling with commercial, stable amino-reactive dyes under mild conditions, (iii) easy control of the spacer length between the GO and oligothiophene dye and finally (iv) high (up to 5 wt%) dye functionalization loadings.

Photoactive Dendrimer for Water Photoreduction: A Scaffold to Combine Sensitizers and Catalysts

We report on the synthesis and characterization of platinum nanoparticles (PtNps) inside the cavities of a PAMAM dendrimer decorated with [Ru(bpy)3](2+) units at the periphery. The phosphorescent ruthenium complexes are used as signaling units of the Pt(2+) complexation in the dendritic architecture and as photosensitizer units in the photocatalytic production of H2 from water. This is the first example of water photoreduction in which the catalyst and the sensitizer are anchored on a dendritic molecular scaffold. This study provides a new outlook in the design of new supramolecular systems and materials for developing artificial photosynthesis.

Graphene-epoxy flexible transparent capacitor obtained by graphene-polymer transfer and UV-induced bonding.

A new approach is reported for the preparation of a graphene-epoxy flexible transparent capacitor obtained by graphene-polymer transfer and UV-induced bonding. SU8 resin is employed for realizing a well-adherent, transparent, and flexible supporting layer. The achieved transparent graphene/SU8 membrane presents two distinct surfaces: one homogeneous conductive surface containing a graphene layer and one dielectric surface typical of the epoxy polymer. Two graphene/SU8 layers are bonded together by using an epoxy photocurable formulation based on epoxy resin. The obtained material showed a stable and clear capacitive behavior.

Synthesis and properties of ZnTe and ZnTe/ZnS core/shell semiconductor nanocrystals

We report the synthesis of spherical ZnTe nanocrystals and the successive coating with a ZnS shell to afford core/shell quantum dots. These nanocrystals can represent alternatives to cadmium-based quantum dots but their preparation and properties are challenging and relatively unexplored. The effect of various synthetic parameters on the reaction outcome was investigated, and the resulting nanocrystals were characterized by TEM, EDX, XPS, and spectroscopic measurements. The optical data indicate that these core/shell quantum dots belong to type I, i.e., both the electron and the hole are confined within the ZnTe core. Both the ZnTe core and ZnTe/ZnS core/shell quantum dot samples absorb in the visible region and are not luminescent. The ZnS shell preserves the optical properties of the core and improves the chemical and photochemical stability of the nanoparticles in air equilibrated solution, whereas they appear to be quite fragile in the solid state. XPS results have evidenced the distinct nature of core and core/shell QDs, confirming the formation of QDs with shells of different thicknesses and their evolution due to oxidation upon air exposure. Anodic photocurrent generation was observed when an ITO electrode functionalized with ZnTe/ZnS nanocrystals was irradiated in the visible region in a photoelectrochemical cell, indicating that the quantum dots perform spectral sensitization of the electron injection into the ITO electrode. Conversely, cathodic photocurrent generation was not observed; hence, the QD-modified electrode performs electrical rectification under a photon energy input.

Lateral epitaxial growth of germanium on silicon oxide

We have developed a method using local oxidation on silicon to create nanoscale silicon seeds for the lateral epitaxial overgrowth of germanium on silicon oxide. The germanium growth starts selectively from silicon seed lines, proceeds by wetting the SiO2 layer and coalesces without formation of grain boundary. Analysis by high resolution transmission electron microscopy have shown that Ge layers grown above silicon oxide are perfectly monocrystalline and are free of defect. The only detected defects are situated at the Ge∕Si interface. Geometrical phase analyses of the microscopy images have shown that the Ge layer is fully relaxed and homogeneous.