Skills and Expertise
Research Items (165)
Self-powered photodetectors operating in the UV-visible-NIR window made of environmentally friendly, earth abundant, and cheap materials are appealing systems to exploit natural solar radiation without external power sources. In this study, we propose a new p-n junction nanostructure, based on a ZnO-Co3O4 core-shell nanowire (NW) system, with a suitable electronic band structure and improved light absorption, charge transport, and charge collection, to build an efficient UV-visible-NIR p-n heterojunction photodetector. Ultrathin Co3O4 films (in the range 1-15 nm) were sputter-deposited on hydrothermally grown ZnO NW arrays. The effect of a thin layer of the Al2O3 buffer layer between ZnO and Co3O4 was investigated, which may inhibit charge recombination, boosting device performance. The photoresponse of the ZnO-Al2O3-Co3O4 system at zero bias is 6 times higher compared to that of ZnO-Co3O4. The responsivity ( R) and specific detectivity ( D*) of the best device were 21.80 mA W-1 and 4.12 × 1012 Jones, respectively. These results suggest a novel p-n junction structure to develop all-oxide UV-vis photodetectors based on stable, nontoxic, low-cost materials.
Within the class of two-dimensional materials, transition metal dichalcogenides (TMDs), are extremely appealing for a variety of technological applications. Moreover, the manipulation of the layered morphology at the nanoscale is a knob for further tailoring their physical and chemical properties towards target applications. Here, the combination of atomic layer deposition (ALD) and chemical vapour deposition (CVD) is presented as a general approach for the fabrication of TMD layers arranged in arbitrary geometry at the nanoscale. Indeed, following such all-chemical based approach, high-resolution electron microscopy shows the conformal growth of MoS2 to nano-trench pattern obtained in SiO2 substrates on large area. Growth is uniform not only in the flat region of the pattern but also at the hinges and throughout vertical faces, without rupture, all along the rectangular shape profile of the trenches. Furthermore, MoS2 bending dramatically affects the electron-phonon coupling as demonstrated by resonant Raman scattering. The proposed approach opens the door to the on-demand manipulation of the TMDs properties by large-scale substrate pattern design.
Hydrogen evolution reaction through electrolysis holds a great potential as a clean, renewable and sustainable energy source. Platinum-based catalysts are the most efficient to catalyze and convert water into molecular hydrogen, however their large-scale application is prevented by scarcity and cost of Pt. In this work, we propose a new ternary composite of Ag2S, MoS2, and reduced graphene oxide (RGO) flakes via one pot synthesis. The RGO support assists the growth of 2D MoS2 nanosheets partially covered by Silver sulfides as revealed by HR-TEM. As compared to the bare MoS2 and MoS2/RGO, the Ag2S/MoS2 anchored on RGO surface (the ternary system Ag2S/MoS2/RGO) demonstrated a high catalytic activity towards hydrogen evolution reaction (HER). Its superior electrochemical activity towards HER is evidenced by the positively shifted (-190 mV vs RHE) over potential at a current density of -10 mA/cm², and a small Tafel slope (56 mV/dec) compared to bare and binary system. The Ag2S/MoS2/RGO ternary catalyst at overpotential of -200 mV demonstrated a turnover frequency equal to 0.38 s⁻¹. Electrochemical impedance spectroscopy (EIS) was applied to understand the charge transfer resistance. The ternary sample shows a very small charge transfer resistance (98 Ohm) at -155 mV vs RHE. Such large improvement can be attributed to the synergistic effect resulting from the enhanced active site density of both sulfides, and to the improved electrical conductivity at the interfaces between MoS2 and Ag2S. This ternary catalyst opens up further optimization strategies to design a stable and cheap catalyst for hydrogen evolution reaction, which holds a great promise for the development of a clean energy landscape.
During the last decade, electrochemical exfoliation of graphite has aroused great interest from both academia and industry for mass production of graphene sheets. Electrochemically exfoliated graphene oxide (EGO) features a much better tunability than chemically exfoliated GO, or even than graphene obtained with ultrasonic exfoliation. Chemical and electrical properties of EGO can be modified extensively thanks to its step-controllable oxidation process, varying the electrolytes and/or the applied potential. It is thus possible, using tunable electrochemical oxidation, to produce low-defect EGO sheets featuring both good electric conductivity and good dispersibility in common solvents (e.g. acetonitrile or isopropanol). This greatly facilitates its application in several fields, for example in flexible electronics. In this work, we correlate the dispersion behavior of EGO to its chemical properties using the relative Hansen solubility parameter, zeta potential values, XPS and Raman analysis. A surface morphology study by AFM and TEM analysis also reveals that EGO sheets are multiple structures of (partially oxidized) graphene bilayers. Conductive EGO films could be easily prepared by vacuum filtration on different substrates, obtaining electrical conductivity values of up to ~10⁴ S/m with no need for further reduction procedures.
This communication describes the bionic preparation of a composite material in which fluorescent magnetite nanoparticles are included within the calcitic skeletal structure of the foraminifera Amphistrigina lessoni. This research represents...
Bright and non-toxic quantum dots (QDs) are highly desirable in a variety of applications, from solid-state devices to luminescent probes in assays. However, the processability of these species is often curbed by their surface chemistry, which limits their dispersibility in selected solvents. This renders a surface modification step often mandatory to make the QDs compatible with the solvent of interest. Here, we present a new synthetic approach to produce CuInS2 QDs compatible with organic polar solvents and readily usable for the preparation of composite materials. 3-mercaptopropyl trimethoxysilane (MPTS) is used simultaneously as solvent, sulfur source, and capping agent for the QD synthesis. The synthesized QDs possessed a maximum photoluminescence quantum yield around 6% - reaching approximately 55% after growing a ZnS shell. The partial condensation of MPTS molecules on the surface of QDs was probed by solid-state nuclear magnetic resonance, whose results were used to interpret the interaction of the QDs with different solvents. To prove the versatility of the developed QDs – imparted by the thiolated silane molecules – we prepared via straightforward procedure two nanocomposites of practical interest: (i) silica nanoparticles decorated with QDs and (ii) an inexpensive polymeric film with embedded QDs. We further demonstrate the potential of this composite film as a luminescence thermometer operational over a broad temperature interval, with relative thermal sensitivity above 1% K⁻¹ in the range of biological interest.
A novel and enhanced electrosynthesis protocol that allows the deposition of thin films of Co/Al and Co/Fe layered double hydroxides (LDHs) on different supports is hereby proposed. The approach is based on a potentiodynamic cathodic reduction. All the films have been characterized by CVs, powder X-ray diffraction, scanning electron microscopy, and Raman and atomic emission spectroscopies. Moreover, the LDHs electrosynthesized on carbonaceous materials have been also investigated by X-ray absorption spectroscopy to analyze their local metal structure. These substrates are particularly interesting for the plethora of LDHs applications ranging from energy storage, sensing, electrocatalysis, also of industrial importance, due to their low cost, ecocompatibility, and easy handling. In particular, the material was exploited for the 5-(hydroxymethyl)furfural (HMF) electro-oxidation.
Overall water splitting represents one of the most promising approaches toward solar energy conversion and storage, which is, however, severely challenged by the four-electron/four-proton nature of the oxygen evolution reaction (OER). One option to overcome this issue is to replace OER with a more useful reaction, for simultaneous production of both hydrogen and chemicals of interest. For the purpose, in this paper a cheap, hydrothermally prepared Ti-doped nanostructured hematite photoanode was employed for the first time as highly stable, heterogeneous catalyst for the low bias, efficient and highly selective photoinduced oxidation of benzylamine to N-benzylidenebenzylamine, and for the simultaneous production of hydrogen in a double solvent/environment cell. A preliminary estimate indicates the possibility to obtain a ∼150 μmol h ⁻¹ H 2 production, with the contemporary production of stoichiometric benzylidene N-benzylamine in a 5 × 5 cm ² area electrode. This study contributes to overcome the 40-year lasting issues limiting the use of hematite in industrial photoelectrochemical sunlight conversion and storage, due to poor performance of hematite and lack of economic value of oxygen production, providing solid evidence for the use of hematite in alternative oxidation reactions of industrial importance.
Major breakthroughs in batteries would require the development of new composite electrode materials, with a precisely controlled nanoscale architecture. However, composites used for energy storage are typically a disordered bulk mixture of different materials, or simple coatings of one material onto another. We demonstrate here a new technique to create complex hierarchical electrodes made of multilayers of vertically aligned nanowalls of hematite (Fe 2 O 3 ) alternated with horizontal spacers of reduced graphene oxide (RGO), all deposited on a 3D, conductive graphene foam. The RGO nanosheets act as porous spacers, current collectors and protection against delamination of the hematite. The multilayer composite, formed by up to 7 different layers, can be used with no further processing as an anode in Li-ion batteries, with a specific capacity of up to 1175 μA h cm ⁻² and a capacity retention of 84% after 1000 cycles. Our coating strategy gives improved cyclability and rate capacity compared to conventional bulk materials. Our production method is ideally suited to assemble an arbitrary number of organic-inorganic materials in an arbitrary number of layers.
Metallic nanoparticles (NPs), either supported on a porous oxide framework or finely dispersed within an oxide matrix, find applications in catalysis, plasmonics, nanomagnetism and energy conversion, among others. The development of synthetic routes that enable to control the morphology, chemical composition, crystal structure and mutual interaction of metallic and oxide phases is necessary in order to tailor the properties of this class of nanomaterials. With this work, we aim at developing a novel method for the synthesis of metal/oxide nanocomposites based on the assembly of NPs formed by gas phase condensation of metal vapors in a He/O2 atmosphere. This new approach relies on the independent evaporation of two metallic precursors with strongly different oxidation enthalpies. Our goal is to show that the precursor with less negative enthalpy gives birth to metallic NPs, while the other to oxide NPs. The selected case study for this work is the synthesis of a Fe-Co/TiOx nanocomposite, a system of great interest for its catalytic and magnetic properties. By exploiting the new concept, we achieve the desired target, i.e., a nanoscale dispersion of metallic alloy NPs within titanium oxide NPs, the structure of which can be tailored into TiO1- or TiO2 by controlling the synthesis and processing atmosphere. The proposed synthesis technique is versatile and scalable for the production of many NPs-assembled metal/oxide nanocomposites.
Hydrogen production as alternative energy source is still a challenge due to the lack of efficient and inexpensive catalysts, alternative to platinum. Thus, stable, earth abundant and inexpensive catalysts are of prime need for hydrogen production via hydrogen evolution reaction (HER). Herein, we present an efficient and stable electrocatalyst composed of earth abundant TiO2 nanorods decorated with molybdenum disulfide thin nanosheets, a few nanometers thick. We grew rutile TiO2 nanorods via hydrothermal method on conducting glass substrate, and then we nucleated the molybdenum disulfide nanosheets as top layer. This composite possesses excellent hydrogen evolution activity both in acidic and alkaline media at considerably low overpotential (350mV and 700 mV in acidic and alkaline media, respectively) and small Tafel slopes (48 and 60 mV/dec in acidic and alkaline conditions, respectively), which are better than several transition metal dichalcogenides, such as pure molybdenum disulfide and cobalt diselenide. A good stability in acidic and alkaline media is reported here for the new MoS2/TiO2 electrocatalyst. These results demonstrate the potential of composite electrocatalysts for HER based on an earth abundant, cost effective and environmentally friendly materials, which can also be of interest for a broader range of scalable applications in renewable energies, such as lithium sulfur batteries, solar cells and fuel cells.
There are tens of industrial producers claiming to sell graphene and related materials (GRM), mostly as solid powders. Recently the quality of commercial GRM has been questioned, and procedures for GRM quality control were suggested using Raman Spectroscopy or Atomic Force Microscopy. Such techniques require dissolving the sample in solvents, possibly introducing artefacts. A more pragmatic approach is needed, based on fast measurements and not requiring any assumption on GRM solubility. To this aim, we report here an overview of the properties of commercial GRM produced by selected companies in Europe, USA and Asia. We benchmark: (A) size, (B) exfoliation grade and (C) oxidation grade of each GRM versus the ones of 'ideal' graphene and, most importantly, versus what reported by the producer. In contrast to previous works, we report explicitly the names of the GRM producers and we do not re-dissolve the GRM in solvents, but only use techniques compatible with industrial powder metrology. A general common trend is observed: Products having low defectivity (%sp ² bonds >95%) feature low surface area (<200 m ² g ⁻¹ ), while highly exfoliated GRM show a lower sp ² content, demonstrating that it is still challenging to exfoliate GRM at industrial level without adding defects.
- Jan 2019
We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt an "hands-on" approach, providing practical details and procedures as derived from literature as well as from the authors' experience, in order to enable the reader to reproduce the results.
Layered double hydroxides (LDHs) have been combined with graphene and/or carbon nanotubes to prepare new composite materials with fascinating electrochemical features. For the first time, this work describes the development of an electrosynthesis protocol that allows the deposition of thin films of a Ni/Al LDH on glassy carbon electrodes previously modified with carbon nanomaterials. Three different approaches (potentiostatic, galvanostatic and potentiodynamic) were investigated in order to identify the best procedure. In all cases the potentiodynamic synthesis exhibits better reproducibility than the potentiostatic one which is the most used in literature. The reliability of the synthesis protocol was evaluated by performing the LDH electrodeposition using glassy carbon electrodes modified with multiwalled carbon nanotubes and/or electrochemically reduced graphene oxide arranged in five configurations. XRD and SEM analysis confirmed the LDH formation. Cyclic voltammetry shows the graphene presence ensured a large electrochemically active area with values 3 times higher than the one observed for an LDH deposited on a bare glassy carbon. Moreover, impedance electrochemical spectroscopy highlights that carbon nanomaterials play a key role in reducing the charge transfer resistance. In fact, it decreases from 2800 KΩ recorded for LDH deposited on bare glassy carbon to about 600 Ω for the best composite material. The materials were tested for glucose electrooxidation which was exploited for the fabrication of a sensor with high sensitivity (2.6 A M⁻¹ cm⁻² for the best device) and low limit of detection (0.6 µM for the best device).
Metal-organic composites are of great interest for a wide range of applications. The control of their structure remains a challenge, one of the problems being a complex interplay of covalent and supramolecular interactions. This paper describes the self-assembly, thermal stability and phase transitions of ordered structures of silver atoms and thiol molecules spanning from the molecular to the mesoscopic scale. Building blocks of molecularly defined clusters formed from 44 silver atoms, each particle coated by a monolayer of 30 thiol ligands, are used as ideal building blocks. By changing solvent and temperature it is possible to tune the self-assembled 3D crystals of pristine nanoparticles or, conversely, 2D layered structures, with alternated stacks of Ag atoms and thiol monolayers. The study investigates morphological, chemical and structural stability of these materials between 25 and 300 °C in situ and ex situ at the nanoscale by combining optical and electronic spectroscopic and scattering techniques, scanning probe microscopies and density-functional theory (DFT) calculations. The proposed wet-chemistry approach is relatively cheap, easy to implement, and scalable, allowing the fabricated materials with tuned properties using the same building blocks.
- Nov 2018
The development of novel materials for enhanced electrochemical energy storage applications, in particular for the fabrication of supercapacitors (SCs) displaying increased properties, is a milestone of both fundamental and technological relevance. Among nanostructured materials, polyoxometalates (POMs) combined with various carbon-based nanostructures represent a very promising class of hybrid systems for energy storage, yet, guidelines for their rational design and synthesis leading to high-performance SCs is still lacking. Here, we have produced a novel hybrid architecture based on Keplerate type POM (Mo132) functionalized with dodecyltrimethylammonium bromide (DTAB), which upon mixing with electrochemically exfoliated graphene (EEG) nanosheets results in the formation of porous 3D superstructures. Mo132-DTAB-EEG combines the redox activity of POMs and high electrical conductivity of graphene, all synergically mediated by the surfactant-assisted porosity enhancement, to form new electrode materials for SCs. Cyclic voltammetry and galvanostatic charge/discharge electrochemical studies on Mo132-DTAB-EEG performed in aqueous H2SO4 electrolyte revealed that the unique combination of these three components yields highly efficient energy storage materials. In particular, our highly porous hybrids system exhibits high specific capacitance of 65 F g⁻¹ (93 F cm⁻³, 93mFcm⁻²) combined with excellent stability (99% of specific capacitance retained) after 5000 charge/discharge cycles at different current densities, overall displaying significantly improved performance compared to pristine electrochemically exfoliated graphene material. Strong non-covalent interactions between Keplerate type polyoxometalate Mo132-DTAB and graphene surface offer higher stability compared to other hybrid POM/carbon-based systems, and unique electrical properties of the multicomponent structure, thereby paving the way towards the development of novel, and potentially multifunctional, POM-based architectures to be exploited as SC electrode materials.
- Sep 2018
Magnetic Fe3O4 (magnetite) nanoparticles are synthesized via a chemical precipitation route in different alkaline environments (NH3 or NaOH) and subsequently functionalized with a (propynylcarbamate)triethoxysilane moiety, with the aim of promoting the nucleation and subsequent stabilization of gold nanoparticles. The propynylcarbamate group is able to capture the gold precursor (HAuCl4), spontaneously reduce it, and stabilize the resulting Au nanoaggregates. The obtained results show that though the dimensions of the starting magnetite substrate depend on the base used in the preparation, they remain unaltered upon the subsequent modification. Conversely, the average Au nanoparticle dimensions can be conveniently tailored as a function of the base used in Fe3O4 preparation and the presence/absence of the organic functionalization. The smallest dimensions (15 nm) are obtained for AuNP supported on propynylcarbamate-functionalized Fe3O4 prepared in the presence of ammonia. Magnetization measurements highlight that all the Au/Fe3O4 nanocomposites display a superparamagnetic behavior and those obtained using ammonia showed consistently smaller Hc and Mr values (av. values of 7.4 Oe and 0.8 emu/g) than those prepared with sodium hydroxide (av. values of 28 Oe and 2.8 emu/g).
Graphene and graphene substrates display properties that have a huge potential as material interface for device and biomedical tools targeting the modulation or recovery of brain functionality. However, to be considered a reliable neural interface, graphene derived substrates should properly interact with astrocytes, favoring their growth and avoiding gliotic adverse reaction. Indeed, astrocytes are the most numerous cells in the human brain and they have crucial physiological role to maintain its homeostasis and to modulate synaptic transmission. In this work, we describe a new strategy based on the chemical modification of GO with a synthetic phospholipid (PL) to improve interaction of graphene oxide (GO) with brain astroglial cells. The PL moieties were grafted on GO sheets through polymeric brushes obtained by atom-transfer radical-polymerization (ATRP) between acryloyl-modified PL and GO nanosheets modified with a bromide initiator. The adhesion of primary rat cortical astrocytes on GO-PL substrates increased of about three times with respect to that on glass substrates coated with standard adhesion agents (i.e. poly-D-lysine, PDL) as well as with respect to that on non-functionalized GO. Moreover, we show that astrocytes seeded on GO-PL did not display significant gliotic reactivity, indicating that the material interface did not cause inflammatory detrimental reaction when interacting with astroglial cells. Our results indicated that the reported biomimetic approach could be applied to neural prosthesis to improve cell colonization and to avoid glial scar formation in brain implants. Additionally improved adhesion might be extremely relevant in device targeting neural cell sensing/modulation of physiological activity.
Three-dimensional (3D) graphene based architectures can combine the two-dimensional properties of graphene with the high surface-to-volume ratio required for a large variety of technological applications. We present a spectro-microscopy study of stable micro-porous 3D few-layer graphene structures with a very low density of defects/edges and of unsaturated bonds, as deduced by Raman and core level photoemission spectroscopy. These qualities make these interconnected graphene networks ideal candidates to accommodate lithium adatoms, with a high density of Li per unit volume and a Li uptake per C atom higher than the value observed for graphite, as confirmed by core level photoemission spectroscopy.
- Jul 2018
- 9TH INTERNATIONAL CONFERENCE ON “TIMES OF POLYMERS AND COMPOSITES”: From Aerospace to Nanotechnology
Graphene oxide was used as charge able to confer high selectivity to the final product. A self-assembling technique, namely layer-by-layer has been developed to stratify graphene-based coating on polymeric films; this coating is composed by nanolayers of graphene oxide alternated with polymers, bonded each other by electrostatic forces. Permeability measurement on layered Matrimid®, a commercial polyimide, showed incredibly high selectivity values to small particle mixtures, as O2, CO2, He and H2. Through simple post-treatments the selective performance was also improved, as demonstration of potentiality of the well-ordered bi-dimensional system: improvement on the coating would make this material one of the viable solution for industrial separations, e.g. hydrogen purification in sustainable energy production. A further investigation on similar structures obtained by other strategies shall demonstrate the peculiar mechanism occurring in this material for high selective performance.
The development of next-generation electronics is very dependent on the discovery of materials with exceptional surface-state spin and valley properties. Bismuth has most of the characteristics required for technological development in this field. Thin films of this element have shown nontrivial topology enabling their definition as a topological insulator and a distribution of spin states and valleys in the band diagram that are suitable for both spintronics and valleytronics applications. In some cases, these properties depend on the quantum confinement of the related particles or quasi-particles; hence they can be tuned by varying the thickness in the ultrathin film range. Finally, thin films of Bi can be processed by electrochemical lithographic methods. Bismuth ultrathin films can be obtained by techniques that require vapor phase with different degrees of vacuum (e.g., PVD and CVD). These methods are efficient for producing flat polycrystalline thin films of Bi. Similarly, Bi thin films were electrodeposited from an aqueous solution containing organic additives with multiple morphologies and textures according to the different surface effect promoted by these substances.These studies have demonstrated that Bi thin films are among the wide range of technologically interesting coatings that cannot be easily obtained from aqueous solutions without interference from metal oxide growth. In most cases, these processes lead to films with uncontrolled morphology.
- Jun 2018
Oligothiophenes are π‐conjugated semiconducting and fluorescent molecules whose self‐assembly properties are widely investigated for application in organic electronics, optoelectronics, biophotonics, and sensing. Here an approach to the preparation of crystalline oligothiophene nano/microfibers is reported based on the use of a “sulfur overrich” quaterthiophene building block, T4S4 , containing in its covalent network all the information needed to promote the directional, π–π stacking‐driven, self‐assembly of Y‐T4S4‐Y oligomers into fibers with hierarchical supramolecular arrangement from nano‐ to microscale. It is shown that when Y varies from unsubstituted thiophene to thiophene substituted with electron‐withdrawing groups, a wide redistribution of the molecular electronic charge takes place without substantially affecting the aggregation modalities of the oligomer. In this way, a structurally comparable series of fibers is obtained having progressively varying optical properties, redox potentials, photoconductivity, and type of prevailing charge carriers (from p‐ to n‐type). With the aid of density functional theory (DFT) calculations, combined with powder X‐ray diffraction data, a model accounting for the growth of the fibers from molecular to nano‐ and microscale is proposed. The change of the aryl groups attached to the inner quaterthiophene core—promoting the directional, π–π‐stacking‐driven, self‐assembly into fibers—enables an unprecedented tuning of various properties within a set of structurally comparable crystalline, electroactive, self‐assembled nano/microfibers. In particular, changes in polarity of major charge carriers from p‐type to n‐type and light emission from visible to NIR are described.
- May 2018
A controlled and eco-friendly, scalable CVD method for the production of single and few layer MoS2 crystals is proposed. The MoS2 crystals are fabricated at atmospheric pressure through the reaction of pre-deposited sodium molybdate (Na2MoO4) in solution and elemental sulfur at 800 °C, offering the flexibility to achieve two growth regimes -either homogeneously distributed single layer MoS2 crystals or continuous MoS2 films- by varying the Na2MoO4 solution concentration. In particular, for low precursor concentrations, isolated single layer MoS2 crystals with controllable mean lateral size were produced. Higher concentrations resulted in continuous single layer films grown in tandem with highly oriented few layer epitaxial domains. The area of the monolayer relative to the few-layer domains can be adjusted. The significant impact on the optical properties of single layer MoS2 crystals due to the growth induced strain is also examined. The grown monolayer crystals are found to experience ∼0.3% biaxial tensile strain relative to the exfoliated ones, while a strain relief of 0.6% is measured when these CVD crystals are transferred to another plastic substrate. Moreover, in their photoluminescence (PL) spectra, the neutral exciton and negative trion peaks are shifted linearly with biaxial strain. By correlating PL and Raman spectroscopies the deformation potential of the direct optical transition in single layer MoS2 can be determined.
The efficient catalysis of oxidative alkylation and fluoroalkylation of aromatic C-H bonds is of paramount importance in pharmaceutical and agrochemical industry, and requires the development of convenient Ag0-based nano-architectures with high catalytic activity and recyclability. We prepared Ag-doped silica nanoparticles (Ag0/+@SiO2) with specific nano-architecture, where ultra-small sized silver cores are immersed into silica spheres, 40 nm in size. The nano-architecture provides efficient electrochemical oxidation to Ag+@SiO2 without any external oxidant. In turn, Ag+@SiO2 5 mol. % results in 100 % conversion of arenes into their alkylated and fluoroalkylated derivatives in a single step at room temperature in nanoheterogeneous electrochemical conditions. Negligible oxidative leaching of silver from Ag0/+@SiO2 is recorded during the catalytic coupling of arenes with acetic, difluoroacetic and trifluoroacetic acids, which enables the good recyclability of the catalytic function of the Ag0/+@SiO2 nanostructure. The catalyst can be easily separated from the reaction mixture and reused a minimum of five times upon electrochemical regeneration. The use of the developed Ag0@SiO2 nano-architecture as heterogeneous catalyst facilitates the aromatic C-H bonds substitution by alkyl and fluoroalkyl groups, which are privileged structural motifs in pharmaceuticals and agrochemicals.
- Apr 2018
We report the use of microfluidics to functionalize suspended reduced graphene oxide flakes through the addition of aryl radical, generated in situ by reaction between aryl amines and isopentyl nitrite. Microfluidic enabled a tight control of temperature, reaction times and stoichiometric ratios, making it possible to tune the growth of oligomers on the surface of the flakes, which in turn affects the interactions of the functional material with the surrounding environment. The results suggest that shear stress phenomena within the reactor may play a role in the chemistry of graphene materials, by providing a constant driving force towards exfoliation of the layered structures. Scale up of the functionalization process is also reported, along with the grafting of dyes based on squaric acid cores. Photophysical characterization of the dye-modified flakes proves that the microfluidic approach is a viable method towards the development of new materials with tailored properties.
- Mar 2018
The performance of membranes for gas separation is currently limited by the Robeson limit, stating that it is impossible to have high gas permeability and high gas selectivity at the same time. We describe the production of membranes based on graphene oxide (GO) and poly(ethyleneimine) (PEI) multilayers able to overcome such limit. The PEI chains acts as molecular spacers in between the GO sheets, yielding a highly reproducible, periodic multi-layered structure with constant spacing of 3.7 nm, giving a record combination of gas permeability and selectivity. The membranes feature a remarkable gas selectivity (up to 500 for He/CO2), allowing to overcome the Robeson limit. The permeability of these membranes to different gases depends exponentially on the diameter of gas molecule, with a sieving mechanism never obtained in pure GO membranes, where a size cut-off and a complex dependence on the chemical nature of the permeant is typically observed. The tunable permeability, the high selectivity and the possibility to produce coatings on a wide range of polymers represent a new approach to produce gas separation membranes for large-scale applications.
We fabricated novel composite (mixed matrix) membranes based on a permeable glassy polymer, Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), and variable loadings of few-layer graphene, to test their potential in gas separation and CO2 capture applications. The permeability, selectivity and diffusivity of different gases as a function of graphene loading, from 0.3 to 15 wt %, was measured at 35 and 65 °C. Samples with small loadings of graphene show a higher permeability and He/CO2 selectivity than pure PPO, due to a favorable effect of the nanofillers on the polymer morphology. Higher amounts of graphene lower the permeability of the polymer, due to the prevailing effect of increased tortuosity of the gas molecules in the membrane. Graphene also allows dramatically reducing the increase of permeability with temperature, acting as a “stabilizer” for the polymer matrix. Such effect reduces the temperature-induced loss of size-selectivity for He/N2 and CO2/N2, and enhances the temperature-induced increase of selectivity for He/CO2. The study confirms that, as observed in the case of other graphene-based mixed matrix glassy membranes, the optimal concentration of graphene in the polymer is below 1 wt %. Below such threshold, the morphology of the nanoscopic filler added in solution affects positively the glassy chains packing, enhancing permeability and selectivity, and improving the selectivity of the membrane at increasing temperatures. These results suggest that small additions of graphene to polymers can enhance their permselectivity and stabilize their properties.
The present work introduces optimal modifiсation of core-shell composite nanomaterial, where small (2–8 nm) Ag⁰ nanoparticles are deposited onto large (about 140 nm) silica spheres for application in oxidative catalysis. The size of Ag⁰ and density of its deposition onto silica spheres was modified by the post treatment of initially deposited Ag⁰ (about 30 nm) by hydrogen peroxide in specific conditions. The comparison of catalytic effect of the post-treated and initial SN-Ag⁰ in electrochemical phosphonation of benzo(thia)oxazoles by diethyl phosphite in oxidative conditions revealed the difference between the composite nanoparticles. In particular, the post-treated SNs-Ag⁰ nanoparticles exhibit efficient catalytic effect in oxidative conditions resulting in facile and green method for synthesis of phosphonated benzooxa(thia)zoles, while no catalytic effect is observed under the use of larger Ag⁰ nanoparticles deposited onto silica spheres. The use of Ag⁰-based nanomaterial in oxidative catalysis had been never demonstrated before.
- Sep 2017
Proteins are hetero-polymers made-up of single building blocks (aminoacids) whose composition determines folding and final architecture. Some proteins are able to undergo self-assembly process enabling the formation of ordered molecular aggregates that in some cases assume conformations particularly suitable to nanotechnological applications. In this work we describe the properties of a ring-like decameric protein, Peroxiredoxin (Prx), to build composite materials interacting with or catalyzing the formation of selectively metal nanoparticles that can be trapped over the surface of nanostructured graphene oxide (GO) sheets. We demonstrate furthermore the ability of Prx to guide the formation of 3D layers of GO embedding metal nanoparticles in the composite material. These composites are discussed as possible precursors to electronic and chemical devices.
Graphene foams grown by CVD on commercially available Ni foam templates were first reported in 2011. Since then they have been investigated widely due to their ability to transfer many of the unique properties of graphene to the macroscopic scale, with high surface area, high electrical conductivity and good structural integrity. However, the pore-size range is typically 200–400 μm, so much of the volume is unoccupied by the functional graphene material. We report a new synthesis procedure that produces graphene foams with pore sizes in the range of 1–10 μm, by using a sacrificial template of metal nanoparticles sintered together to form a network. These materials could have wide-ranging applications in fields such as high-density energy storage, membranes and sensing.
- Sep 2017
Photoluminescence (PL) spectra were measured for dodecene-capped Si nanocrystals with a wide range of average diameter, from 1.8 nm to 9.1 nm. Nanocrystals larger than 3 nm exhibited relatively high PL quantum yields of 30% to 45%. Smaller nanocrystals exhibited lower quantum yields that decreased significantly with reduced size. Because smaller nanocrystals also have lower optical absorption there is a significant biasing of the PL spectra by the larger nanocrystals. We show that with proper accounting of polydispersity and size-dependent quantum yields and optical absorption, the effective mass approximation (EMA) accurately estimates the average diameter of silicon (Si) nanocrystals from experimentally-determined PL emission peak energies. A finite confinement model is presented that explains the decreased PL quantum yields of the smaller diameter nanocrystals.
The morphological investigation at the micrometric scale of a graphene - ZnO nanorods hybrid structure is performed by scanning electron microscopy. When operated in the scanning-transmission imaging mode, the detection strategy allows implementation of a tomographic approach to recover the three dimensional spatial arrangement of the sample constituents. This tomographic approach complements the serial-sectioning imaging methods and is suitable for thin, self-standing specimens.
- Aug 2017
Controlled and reversible anisotropic self-assembly of gold nanoparticles (GNPs) is achieved by ligand exchange with concentrations of ligand below that needed for their complete capping, i.e. using substoichiometric ligand coverages. A thoughtful rationale of the ligand chemical structure and control over exchange conditions have been pursued to foster a reversible process grounded on the cooperative balance between attractive, and repulsive interactions. The gradual replacement of the electrostatic and steric repulsions between the GNPs with attractive dipolar interactions and enhanced steric repulsions is accomplished by ligand exchange with an oligo (ethylene glycol) alkanethiol. The modulation of interparticle interactions is driven by the extent of ligand substitution and allows the reversible assembly of nanoparticles and tuning of their optical properties.
We demonstrate room-temperature ferromagnetism in germanium counter-doped with manganese and arsenic at concentrations up to approximately 2.1 × 10²⁰ at/cm³: these values are one order of magnitude lower than those at which ferromagnetic behavior has previously been observed. Synthesis proceeded by ion implantation at 513 K followed by annealing in argon at 673 K. High resolution TEM, STEM, and EDX show single-phase diamond cubic material lacking Mn or As precipitates. These findings are consistent with the prediction of Chen et al. that counter-doping with approximately equal concentrations of a single-electron donor permits Mn, a two-electron acceptor, to be incorporated at high enough concentrations to yield a diluted magnetic semiconductor with a Curie temperature above room temperature.
- Jul 2017
Titanium dioxide (TiO2) is a wide gap semiconductor suitable for many applications. In this work, TiO2 nanostructured thin films are deposited by a plasma assisted supersonic deposition technique on silicon and on conductive glass substrates. Optical Emission Spectroscopy (OES) is used to monitor plasma conditions and precursor dissociation reactions. The influence of deposition parameters on TiO2 structure, uniformity, grain size, and optical properties are investigated by atomic force microscopy (AFM), mechanical profilometer, scanning electron microscopy (SEM) and spectroscopic ellipsometry (SE). Experimental results show how employed technique allows obtaining uniform films, with a tunable deposition range. Grains size could be chosen varying precursor flux during the deposition process. Films nanostructure and porosity result to be affected by grains size. Substrate roughness results to affect film morphology.
Charge carriers' long diffusion length in the CH3NH3PbI3 perovskite is one of the most relevant properties for explaining the high photovoltaic efficiency of perovskites solar cells. As possible mechanism for the large diffusion length of electrons and holes, several authors suggested a reduced coulomb attraction of the carriers due to the formation of polarons. Here we performed continuous wave far infrared photoinduced absorption (PIA) experiments on CH3NH3PbI3; spectral changes are associated to local deformation of the lattice around the photogenerated long lived charges, a typical signature of photo-induced polarons. Ab initio calculations show confinement of charge carriers at the interface between structural domains characterized by a different tilting of the PbI6 octahedra. Differential IR spectrum between unperturbed and perturbed simulated structures shows a close pattern to the experimental PIA. Positive and negative charges are confined in different varieties of the perovskites coherent with the low recombination and long diffusion length of photogenerated carriers.
We demonstrate a facile, scalable and tunable method to produce a composite material based on graphene multilayers and Fe2O3, combining the good conductivity and 2D layered structure of the former and the lithium storage capacity of the latter. The composite was obtained directly from bulk graphite, exploiting the fast electrochemical intercalation of tetrachloroferrate (III) anions (FeCl4⁻) and nitromethane molecules between the graphene sheets. Then, irradiation with microwaves triggered the simultaneous exfoliation of graphite and its functionalization with Fe2O3 nanocrystals, produced by the thermal hydrolysis of the FeCl4⁻. This process was monitored in real time using thermal gravimetry and mass spectrometry. X-rays diffraction, Raman spectroscopy, scanning electron and transmission microscopies confirmed the final structure of the composite formed by conductive 2D nanosheets coated by Fe2O3 crystals, featuring both high crystallinity and nanometric size. The composite could be used directly as an anode in Li-ion batteries, demonstrating the viability of this approach for high yield and scalable production of graphene/metal oxide composites.
- May 2017
Reversible hydrogen sorption coupled with the MgH2↔Mg phase transformation was achieved in the remarkably low 340 - 425 K temperature range using MgH2-TiH2 composite nanoparticles obtained by reactive gas-phase condensation of Mg-Ti vapours under He/H2 atmosphere. The equilibrium pressures determined by in situ measurements at low temperature were slightly above those predicted using enthalpy ΔH and entropy ΔS of bulk magnesium. A single van ‘t Hoff fit over a range extended up to 550 K yields the thermodynamic parameters ΔH = 68.1±0.9 kJ/molH2 and ΔS = 119±2 J/K·molH2 for hydride decomposition. A desorption rate of 0.18 wt% H2/min was measured at T=423 K and p(H2)≈ 1 mbar, i.e. close to equilibrium, without using a Pd catalysts. The nanoparticles displayed a small absorption-desorption pressure hysteresis even at low temperatures. We critically discuss the influence exerted by nanostructural features such as interface free energy, elastic clamping, and phase mixing at the single nanoparticle level on equilibrium and kinetic properties of hydrogen sorption.
Graphene has been acclaimed as a cutting edge material since its discovery . Because of its noteworthy properties  it has attracted and is still attracting major interest. Moreover, graphene's benefits have further expanded after obtaining related materials such as graphene oxide (GO)  that features tunable electro-optical and chemical properties, high hydrophilicity and ease of production and functionalization. Yet the possibility of obtaining reduced GO (rGO) by processing contributed greatly to its outstanding potential, as rGO can partially restore the properties of graphene while enabling its dispersion in surfactant-free solution . However, using graphene for practical applications remains to be fully realized. One main issue is the integration of its 2D structure into accessible and scalable 3D materials, a need that has inspired a growing field of research . We accomplish the task of building 3D GO structures by Peroxiredoxins (Prx), a family of multi-tasking enzymes with ring-like architectures. Taking advantage of the proteins capabilities of interacting with both GO and rGO through weak interactions due to their exposed surfaces, as well as of their symmetric structure, 3D rGO-based composites are hereby built up. The Prx rings adhere flat on single GO layers and induce partial reduction, driving their stacking into 3D multi-layer rGO-Prx composites, even when using very few amounts of GO. Further, protein engineering allows divalent metal ions to bind the Prx's lumen and this is exploited to capture pre-synthesized gold nanoparticles (AuNPs) and grow in situ palladium nanoparticles (PdNPs) using the protein ring as physical confinement, thus paving the way to straightforward and "green" routes to 3D rGO-metal composites. GO quickly gets clumped in the presence of Prx during mixing experiments in solution. Such clumps progressively push together leading to a soft colloid which can be hanged as compact material. The colloid can form again even after breaking by shaking, hence suggesting that a reversible self-assembly process occurs. The so-formed rGO-Prx colloid can be easily dried as a free-standing material by freeze-drying while keeping a microporous internal architecture. Addition by protein engineering of metal-binding sites on the lumen of the Prx ring allows the protein to bind divalent metal ions. By this way, Prx can trap Ni(II)-functionalized AuNPs with ~2 nm diameter and carrier them inside the multi-layer rGO-based composite. The versatility of the system is also demonstrated for Pd(II) and for Co(II) .
- Feb 2017
We describe the preparation of poly(3-hexylthiophene-S,S-dioxide) nanoparticles using Rozen’s reagent - HOF•CH3CN - either on poly(3-hexylthiophene) (P3HT) or on preformed P3HT nanoparticles (P3HT-NPs). In the latter case core-shell nanoparticles (P3HT@PTDO-NPs) are formed, as confirmed by X-ray Photoelectron Spectroscopy measurements indicating the presence of oxygen on the outer shell. The different preparation modalities lead to a fine tuning of the chemical-physical properties of the nanoparticles. We show that absorption and photoluminescence features, electrochemical properties, size and stability of colloidal solutions can be finely modulated by controlling the amount of oxygen present. Atomic Force Microscopy measurements on the nanoparticles obtained by nanoprecipitation method from preoxidized P3HT (PTDO-NPs), display spherical morphology and dimensions down to 5 nm. Finally, Kelvin Probe measurements show that the coexistence of p- and n-type charge carriers in all types of oxygenated nanoparticles makes them capable to generate and separate charge under illumination. Furthermore, in core-shell nanoparticles the nanosegregation of the two materials, in different region of the nanoparticles, allows a more efficient charge separation.
- Jan 2017
This book presents selected papers from the fourth edition of the GraphX conference series, GraphITA 2015. Its content range from fundamentals to applications of graphene and other 2D material such as silicene, BN and MoS2. The newest technological challenges in the field are described in this book, written by worldwide known scientists working with 2D materials. The chapter 'Morphing Graphene-Based Systems for Applications: Perspectives from Simulations' is published open access under a CC BY 4.0 license.
- Dec 2016
- European Microscopy Congress 2016: Proceedings
This paper summarizes the achievements in the 3D reconstruction of microscopic specimens through the tomographic algorithm applied to a set of projection\images obtained in the SEM. This approach is complementary to the serial-sectioning and the slice-and-view methods presently implemented in the SEM platform, and benefits from a compressed sensing approach to refine the reconstruction from a limited number of projections. A Si-based electron detector has been specifically developed for the purpose of operating the microscope in the scanning-transmission imaging mode for the tomographic application, and the detection strategy has been tailored in order to maintain the projection requirement over the large tilt range, a requirement needed for the reconstruction workflow . Either inorganic or biological samples have been investigated to demonstrate the adaptability of the compressed sensing refinement to the specimen characteristics: the former system is formed by cobalt particles within a carbon tube and the latter features collagen fibrils in dermal tissue. Figure 1 shows a STEM image from the tilt series of Co nanoparticles inside a carbon tube. The contrast in the STEM image is determined by local specimen thickness and composition, the Co particles being visible with the highest contrast. The reconstruction has been obtained starting from 53 projections at 2°steps, and refined through compressive sensing with regularization parameters emphasizing sparsity in the gradient domain. Figure 2 highlights the complex structure of the dermal tissue as revealed by the STEM imaging mode in the SEM. Cellular membranes and circular structures are mixed with bundles of collagen fibrils. The bundles were truncated by the fine sectioning and their disposition is clearly visible. A small bundle of collagen was selected as the region of interest for the tomographic reconstruction. Starting from 91 projections at 40.000× magnification and ranging between −50° to +40°. Compressed sensing was adapted to deal with the inherent complexity of biological images, and the final tomogram turned out to preserve the finest details of the fibrils. The known periodical striation (about 60 nm periodicity) of collagen was indeed recovered with adequate spatial resolution. The proposed system exploits the capability of the STEM imaging mode, which can be applied for both biological and physical science for the 3D analysis of volumes below 100 mm3. The limit in resolution is posed by the probe size of the microscope, specimen composition and thickness, and the number of projections that can be acquired without significant beam damage of the sample. Compressed sensing is effective in improving the quality of the reconstruction. Owing to the flexibility of the SEM platform, cryo-preservation of the specimen as well as site-selective sample preparation could be pursued within the proposed approach for tomography in the SEM.
Three-dimensional (3D) graphene-based structures combine the unique physical properties of graphene with the opportunity to get high electrochemically available surface area per unit of geometric surface area. Several preparation techniques have been reported to fabricate 3D graphene-based macroscopic structures for energy storage applications such as supercapacitors. Although reaserch has been focused so far on achieving either high specific capacitance or high volumetric capacitance, much less attention has been dedicated to obtain high specific and high volumetric capacitance simultaneously. Here, we present a facile technique to fabricate graphene foams (GF) of high crystal quality with tunable pore size grown by chemical vapor deposition. We exploited porous sacrificial templates prepared by sintering nickel and copper metal powders. Tuning the particle size of the metal powders and the growth temperature allow fine control of the resulting pore size of the 3D graphene-based structures smaller than 1 μm. The as-produced 3D graphene structures provide a high volumetric electric double layer capacitance (165 mF cm⁻³). High specific capacitance (100 Fg⁻¹) is obtained by lowering the number of layers down to single layer graphene. Furthermore, the small pore size increases the stability of these GFs in contrast to the ones that have been grown so far on commercial metal foams. Electrodes based on the as-prepared GFs can be a boost for the development of supercapacitors, where both low volume and mass are required.
The templating method has always been considered an efficient method to design new morphologies for old materials. We have already demonstrated that it is possible to generate by templating method, a new class of alumina meso-foams showing peculiar conductive properties that strictly depend on the size of the bubbles generated by polymeric templating beads. In this work we definitively demonstrate by the “high resolution transmission electron microscopy” and by the “Energy Dispersive X-ray Spectrometry (EDS)” that the alumina meso-foams are actually composites of conductive amorphous carbon and γ-alumina nanoparticles. This evidence supports the hypothesis that the conductive properties of the meso-foams are mainly due to the un-combusted amorphous carbon imbedded in the alumina frame. We believe that our innovative approach based on in-situ carbon-composite formation, could be potentially extended to several other non-conductive oxides shaped through the old templating method.
The interfacial structure in “giant” PbS/CdS quantum dots (QDs) was engineered by modulating the Cd:S molar ratio during in situ growth. The control of the gradient interfacial layer could facilitate hole transfer, regulate the transition from double- to single-color emission, as a consequence. These QDs are optically active close-to-the near-infrared (NIR) spectral region and are candidates as absorber materials in solar energy conversion. Photoinduced charge transfer from “giant” QDs to electron scavenger can still take place despite the ultra-thick (~5 nm) shell. The hybrid architecture based on a TiO2 mesoporous framework sensitized by the “giant” QDs with alloyed interface can produce a saturated photocurrent density as high as ~5.3 mA/cm² in a photoelectrochemical (PEC) cell under 1 Sun illumination, which is around 2 times higher than that of bare PbS and core/thin-shell PbS/CdS QDs sensitizer. The as-prepared PEC device presented very good stability thanks to the “giant” core/shell QDs architecture with tailored interfacial layer and a further coating of the ZnS shell. 78% of the initial current density is kept after 2-hour irradiation at 1 Sun. Engineering of electronic band structure plays a key role in boosting the functional properties of these composite systems, which hold great potential for H2 production in PEC devices.
The three-dimensional tomographic reconstruction of a biological sample, namely collagen fibrils in human dermal tissue, was obtained from a set of projection-images acquired in the Scanning Electron Microscope. A tailored strategy for the transmission imaging mode was implemented in the microscope and proved effective in acquiring the projections needed for the tomographic reconstruction. Suitable projection alignment and Compressed Sensing formulation were used to overcome the limitations arising from the experimental acquisition strategy and to improve the reconstruction of the sample. The undetermined problem of structure reconstruction from a set of projections, limited in number and angular range, was indeed supported by exploiting the sparsity of the object projected in the electron microscopy images. In particular, the proposed system was able to preserve the reconstruction accuracy even in presence of a significant reduction of experimental projections.
Zinc oxide nanorods (ZnO-NRs) with high density and chemical purity were grown onto unsupported graphene nanoplatelets (GNPs) in aqueous suspensions, using two different growth approaches namely: a hydrothermal method and ultrasonic probe sonication. The size and density of the ZnO-nanorods grown onto graphene nanoplatelets were controlled through seed layer deposition and through the proper setting of the process parameters, in particular through the control of the fluidodynamics of the colloidal suspension during the growth. The highest growth density of the ZnO nanorods having a diameter of ∼45 nm was obtained onto GNPs seeded by the probe sonication method and through the hydrothermal method in dynamic conditions. XRD and XPS investigations confirmed that all produced ZnO-GNP composites are characterized by high crystallinity and purity, although solution dynamics affected their UV luminescence. The proposed approaches enable the controlled high-density growth of crystalline ZnO-NRs onto GNPs in an aqueous suspension, at a low cost, and are suitable for large scale production.
Unlabelled: The development of efficient charge transport layers is a key requirement for the fabrication of efficient and stable organic solar cells. A graphene-based derivative with planar resistivity exceeding 10(5) Ω/□ and work function of 4.9 eV is here produced by finely tuning the parameters of the chemical vapor deposition process on copper. After the growth, the film is transferred to glass/indium tin oxide and used as hole transport layer in organic solar cells based on a PBDTTT-C-T:PCBM blend. The cells attained a maximum power conversion efficiency of 5%, matching reference cells made with state-of-the-art Pedot: PSS as the hole transport layer. Our results indicate that functionalized graphene could represent an effective alternative to Pedot: PSS as hole transport/electron blocking layer in solution-processed organic photovoltaics.
we report a new fabrication strategy to obtain large area continuous NPGs-on-substrate combining graphene-on-substrate industrial techniques and swift-ion beam irradiation (SHI). Graphene membranes were synthesized on the Cu substrate and afterwards a 600 nm layer of PMMA was spin-coated on the surface to complete the PMMA-Graphene-Cu stack. The PMMA-graphene-Cu trilayer was exposed to a flow of Au heavy ions that penetrate through the entire thickness of both polymer layer and the graphene sheet creating ion-tracks and damages. A consecutive track-etching technique is used with an adequate revealing agent for PMMA, IsoPropyl Alcohol (IPA), to selectively dissolve the latent tracks and damages created during SHI irradiation in the insulating material and the graphene sheet. Resulting from SHI irradiation and track-etching, the graphene nanopores are thus perfectly aligned to the PMMA nanopores, providing, after cupper dissolution, a composite that features both well-defined and truly 2-dimensional nanopores in the graphene layer but that can be handled as a normal polymer film.
- Jun 2016
Graphene has unique physical and chemical properties making it appealing for a number of applications in opto-electronics, sensing, photonics, composites, smart coatings, just to cite a few. These require the development of production processes that are inexpensive and up-scalable. These criteria are met in liquid-phase exfoliation (LPE), a process that can be enhanced when specific organic molecules are used. Here we report the exfoliation of graphite in N-methyl-2-pyrrolidinone, in the presence of heneicosane linear alkanes terminated with different head groups. These molecules act as stabilizing agents during exfoliation. The efficiency of the exfoliation in terms of concentration of exfoliated single- and few-layer graphene flakes depends on the functional head group determining the strength of the molecular dimerization through dipole-dipole interactions. A thermodynamic analysis is carried out to interpret the impact of the termination group of the alkyl chain on the exfoliation yield. This combines molecular dynamics and molecular mechanics to rationalize the role of functionalized alkanes in the dispersion and stabilization process, which is ultimately attributed to a synergistic effect of the interactions between the molecules, graphene, and the solvent.
Multifunctional materials can be engineered by combining multiple chemical components, each conferring a well-defined function to the ensemble. Graphene is at the centre of an ever-growing research effort due to its combination of unique properties. Here we show that the large conformational change associated with the trans-cis photochemical isomerization of alkyl-substituted azobenzenes can be used to improve the efficiency of liquid-phase exfoliation of graphite, with the photochromic molecules acting as dispersion-stabilizing agents. We also demonstrate reversible photo-modulated current in two-terminal devices based on graphene-azobenzene composites. We assign this tuneable electrical characteristics to the intercalation of the azobenzene between adjacent graphene layers and the resulting increase in the interlayer distance on (photo)switching from the linear trans-form to the bulky cis-form of the photochromes. These findings pave the way to the development of new optically controlled memories for light-assisted programming and high-sensitive photosensors.
Most methods used for the characterization of graphene produced by liquid phase exfoliation require the deposition of the liquid sample on a substrate and subsequent drying. Because of this, or other post-synthetic treatmement, the reliability of the data in describing the actual features of the graphene particles in the pristine solution becomes questionable. Hence there is a need for new methods that permit the study of graphene directly in solution. Fluorescence imaging is at present the most convenient and sensitive method to visualize nanosized objects in solution. Here we report the development of a new method for visualizing and tracking exfoliated graphene directly in solution using a conventional set-up for fluorescence microscopy. We functionalized a fluorescent surfactant typically used for exfoliating graphite in aqueous phase (Pluronic P123) with two different fluorophores, in order to make graphene detectable by fluorescence microscopy. The photophysical interactions between the fluorescent surfactant and graphene was investigated at the bulk level. Finally, fluorescence microscopy allowed us to track the carbon particles produced and to identify two different populations of particles with sizes 265±25 and 1100±200 nm respectively. Correlation of these results with TEM and DLS data is discussed.
Dispersing graphite in water to obtain true (single layer) graphene (SLG) in bulk quantity in a liquid has been an unreachable goal for materials scientists in the last decade. Likewise, a diagnostic tool for identifying solubilized SLG in situ has been long awaited. Homogeneous stable dispersions of SLG in water are obtained by mixing graphenide (negatively charged graphene) solutions in tetrahydrofuran (THF) with degassed water and evaporating the organic solvent. In situ Raman spectroscopy of these aqueous dispersions shows all the expected characteristics of single layer graphene, including an intense 2D band of Lorentzian shape and linewidth of 28 cm-1. Transmission electron and atomic force microscopies on deposits confirm SLG character. The resulting additive-free stable water dispersions contain 0.16 mg/mL of SLG.
- Feb 2016
Graphene oxide (GO) is rapidly emerging worldwide as a breakthrough precursor material for next-generation devices. However, this requires the transition of its two-dimensional layered structure into more accessible three-dimensional (3D) arrays. Peroxiredoxins (Prx) are a family of multitasking redox enzymes, self-assembling into ring-like architectures. Taking advantage of both their symmetric structure and function, 3D reduced GO-based composites are hereby built up. Results reveal that the “double-faced” Prx rings can adhere flat on single GO layers and partially reduce them by their sulfur-containing amino acids, driving their stacking into 3D multi-layer reduced GO-Prx composites. This process occurs in aqueous solution at very low GO concentration, i.e. 0.2 mg ml-1. Further, protein engineering allows the Prx ring to be enriched with metal binding sites inside its lumen. This feature is exploited to both capture pre-synthesized gold nanoparticles and grow in situ palladium nanoparticles paving the way to straightforward and “green” routes to 3D reduced GO-metal composite materials.
Graphene oxide (GO) is rapidly emerging worldwide as a breakthrough precursor material for next-generation devices. However, this requires the transition of its two-dimensional layered structure into more accessible three-dimensional (3D) arrays. Peroxiredoxins (Prx) are a family of multitasking redox enzymes, self-assembling into ring-like architectures. Taking advantage of both their symmetric structure and function, 3D reduced GO-based composites are hereby built up. Results reveal that the “double-faced” Prx rings can adhere flat on single GO layers and partially reduce them by their sulfur-containing amino acids, driving their stacking into 3D multi-layer reduced GO-Prx composites. This process occurs in aqueous solution at a very low GO concentration, i.e. 0.2 mg ml−1. Further, protein engineering allows the Prx ring to be enriched with metal binding sites inside its lumen. This feature is exploited to both capture presynthesized gold nanoparticles and grow in situ palladium nanoparticles paving the way to straightforward and “green” routes to 3D reduced GO–metal composite materials.
Semiconducting nanocrystals optically active in the infrared region of the electromagnetic spectrum enable exciting avenues in fundamental research and novel applications compatible with the infrared transparency windows of biosystems such as chemical and biological optical sensing, including nanoscale thermometry. In this context, quantum dots (QDs) with double color emission may represent ultra-accurate and self-calibrating nanosystems. We present the synthesis of giant core/shell/shell asymmetric QDs having a PbS/CdS Zincblende (Zb)/CdS Wurtzite (Wz) structure with double color emission close to the near-infrared (NIR)region. We show that the double emission depends on the excitation condition and analyze the electron-hole distribution responsible of the independent and simultaneous radiative exciton recombination in the PbS core and in the CdS Wz shell, respectively. These results highlight the importance of the driving force leading to preferential crystal growth in asymmetric QDs, and provide a pathway for a rational control of the synthesis of double color emitting giant QDs, leading to the effective exploitation of visible/NIR transparency windows.
Graphene has unique physical and chemical properties, making it appealing for a number of applications in optoelectronics, sensing, photonics, composites, and smart coatings, just to cite a few. These require the development of production processes that are inexpensive and up-scalable. These criteria are met in liquid-phase exfoliation (LPE), a technique that can be enhanced when specific organic molecules are used. Here we report the exfoliation of graphite in N-methyl-2-pyrrolidinone, in the presence of heneicosane linear alkanes terminated with different head groups. These molecules act as stabilizing agents during exfoliation. The efficiency of the exfoliation in terms of the concentration of exfoliated single- and few-layer graphene flakes depends on the functional head group determining the strength of the molecular dimerization through dipole−dipole interactions. A thermodynamic analysis is carried out to interpret the impact of the termination group of the alkyl chain on the exfoliation yield. This combines molecular dynamics and molecular mechanics to rationalize the role of functionalized alkanes in the dispersion and stabilization process, which is ultimately attributed to a synergistic effect of the interactions between the molecules, graphene, and the solvent.
- Nov 2015
Mg-Ti nanostructured samples with different Ti contents were prepared via compaction of nanoparticles grown by inert gas condensation with independent Mg and Ti vapour sources. The growth set-up offered the option to perform in situ hydrogen absorption before compaction. Structural and morphological characterization was carried out by x-ray diffraction, energy dispersive spectroscopy and electron microscopy. The formation of an extended metastable solid solution of Ti in hcp Mg was detected up to 15 at.% Ti in as-grown nanoparticles, while after in situ hydrogen absorption, phase separation between MgH2 and TiH2 was observed. At a Ti content of 22 at.%, a metastable Mg-Ti-H fcc phase was observed after in situ hydrogen absorption. The co-evaporation of Mg and Ti inhibited nanoparticles coalescence and crystallite growth in comparison with the evaporation of Mg only. The in situ hydrogen absorption was beneficial to subsequent hydrogen behaviour, studied by high pressure differential scanning calorimetry and isothermal kinetics. A transformed fraction of 90% was reached within 100 s at 300 °C both during hydrogen absorption and desorption. The enthalpy of hydride formation was not observed to differ from bulk MgH2.
The scanning-transmission imaging mode in the SEM allows for the threedimensional tomographic reconstruction of a specimen, starting from a set of projection images. Compressed sensing was used to solve the undetermined problem of structure reconstruction and was proven capable of overcoming the limitations arising from the sampling scheme. Reconstructions of cobalt particles within a carbon nanotube and collagen fibrils in a dermal tissue are presented, demonstrating the potential of this technique in the set of 3-D electron microscopy methods for both physical and biological science.
- Oct 2015
We investigated the effect of annealing on the crystalline quality of Ge epilayers grown on low porosity porous silicon (pSi) buffer layer and on bulk Si by LEPECVD. High-resolution XRD analysis indicates that during annealing, Ge grown on pSi undergoes a stronger reorganization compared to Ge grown on Si in terms of strain distribution and mosaic broadening. Strong morphological reorganization of the pSi buffer during annealing leads to a stronger reduction in Ge mosaicity as compared to annealed Ge on bulk Si. This improvement is attributed to bending of threading dislocations in a plane parallel to the growth interface, which is attributed to a strain field introduced by pSi within Ge during their simultaneous reorganization at high-temperature. After cyclic annealing at 750 °C, plan view transmission electron microscopy analysis revealed a threading dislocation density for Ge on pSi which is about one order of magnitude smaller than for annealed Ge on bulk Si. Ge on pSi virtual substrates thus represent a promising platform for the growth of III–V and GeSn semiconductors on Si with a low cost and high-throughput technique.
Graphene platelets were dispersed into photocurable SU-8 resin. A strong increase of the Tg value as a function of the graphene content was observed and attributed to a mobility hindering effect on the polymeric chains caused by the graphene filler. A significant increase of electrical conductivity is achieved for composites containing functionalized graphene sheets (FGS) between 3 and 4 wt%. The thermal diffusivity of the polymer was observed to increase as a function of filler content in the nanocomposites confirming the conducting nature of the polymeric coating with incorporation of graphene.
Building three-dimensional (3D) graphene oxide (GO) as breakthrough precursor for next-generation devices is spreading worldwide. We accomplish this task by Peroxiredoxins (Prx), a family of multi-tasking enzymes with ring-like architectures. Taking advantage of either the protein's symmetric structure and function, 3D rGO-based composites are hereby built up. The " Janus " Prx rings adhere flat on single GO layers and induce partial reduction, driving their stacking into 3D multi-layer rGO-Prx composites, even when using very few amounts of GO. Further, protein engineering allows divalent metal ions to bind the Prx's lumen and this is exploited to capture pre-synthesized gold nanoparticles (AuNPs) and grow in situ palladium nanoparticles (PdNPs) using the protein ring as physical confinement, thus paving the way to straightforward and " green " routes to 3D rGO-metal composites.
- Jul 2015
The use of graphene as transparent conducting layer in devices that require high temperature processing is proposed. The material shows stability upon thermal treatments up to 1100 °C if capped with a sacrificial silicon layer. The use of Cu foil or evaporated Cu as catalysts in Catalytic-Chemical Vapor Deposition growth gives rise to graphene of similar properties, which represents a promising result in view of its direct integration in microelectronic devices. Photovoltaic p–i–n thin film devices were fabricated on the as-deposited or annealed graphene membranes and compared with similar devices that incorporate as-deposited Indium Tin Oxide. No degradation in series resistance is observed for the annealed device. A 3.7% and 2.8% photovoltaic conversion efficiency is observed on the devices fabricated on as-transferred and on annealed graphene respectively. The major limitation derives from the high sheet resistance of the as-transferred state-of-the-art material. The results opens the way to the use of graphene in applications that require transparent conducting layers resistant to high temperature processing.
In this work we investigate the use of graphene as transducer in a novel biosensor for biomedical uses, in which electroactive membrane proteins would serve as biological recognition elements. Membrane proteins maintain their functionalities only if embedded in the cell membrane, so it is necessary to develop a system, which mimics their native environment. This study is focused on surface treatments of graphene to improve its biocompatibility and a first investigation of its interaction with liposomes, which rupture and spread to form a Supported Lipid Bilayer under specific surface conditions. The first step involved the characterization of the graphene membranes synthesized by Chemical Vapor Deposition, using several techniques to determine their morphological and structural properties. From these investigations, the CVD-synthesized graphene resulted to be mono- to few-layer. Next, the interaction of graphene with lipids (1,2-dioleoyl-sn-glicero-3-phosphocholine), in particular the formation of a supported lipid bilayer due to the liposome spreading, was investigated via electrochemical impedance spectroscopy. This indicated the presence of a stable insulating lipid layer on the graphene surface after liposome incubation.
The three-dimensional reconstruction of a microscopic specimen has been obtained by applying the tomographic algorithm to a set of images acquired in a Scanning Electron Microscope. This result was achieved starting from a series of projections obtained by stepwise rotating the sample under the beam raster. The Scanning Electron Microscope was operated in the scanning-transmission imaging mode, where the intensity of the transmitted electron beam is a monotonic function of the local mass-density and thickness of the specimen. The detection strategy has been implemented and tailored in order to maintain the projection requirement over the large tilt range, as required by the tomographic workflow. A Si-based electron detector and an eucentric-rotation specimen holder have been specifically developed for the purpose.
Graphene–epoxy flexible capacitors are obtained by graphene–polymer transfer and bonding via UV-cured epoxy adhesive. Ceramic fillers are dispersed into the epoxy resin with the aim of enhancing the capacitive behavior of the final device. Parallel plate capacitors in which epoxy resin is filled with ceramic nanoparticles demonstrate superior performance, up to two orders of magnitude better than unfilled samples. Zirconia, showing the highest dielectric constant, fails to give a stable output in the whole frequency range, as some competing phenomena occur and reduce the overall polarization of the system. Boehmite appears to be a better choice and gives reasonable performances, better than gibbsite.
Reduction of carbon nanohorn (CNH) aggregates by potassium naphthalenide resulted in their dismantling and individualization. Furthermore, the reduced CNHs were functionalized by addition of electrophiles.
Graphene–metal composites have potential as novel catalysts due to their unique electrical properties. Here, we report the synthesis of a composite material comprised of monodispersed platinum nanoparticles on high-quality graphene obtained by using two different exfoliation techniques. The material, prepared via an easy, low-cost and reproducible procedure, was evaluated as an electrocatalyst for the hydrogen evolution reaction. The turnover frequency at zero overpotential (TOF0 in 0.1 m phosphate buffer, pH 6.8) was determined to be approximately 4600 h−1. This remarkably high value is likely due to the optimal dispersion of the platinum nanoparticles on the graphene substrate, which enables the material to be loaded with only very small amounts of the noble metal (i.e., Pt) despite the very highly active surface. This study provides a new outlook on the design of novel materials for the development of robust and scalable water-splitting devices.
- Dec 2014
A supramolecular approach for producing homogeneous dispersions of graphene nanosheets in various solvents is described by A. Ciesielski, P. Samorì, and co-workers on page 1691. Comparative studies of the liquid-phase exfoliation of graphene in the presence of linear alkanes of different lengths terminated by a carboxylic-acid head group reveal that they molecules act as graphene dispersion-stabilizing agents during exfoliation. The efficiency of exfoliation in terms of the concentration of exfoliated graphene is found to be proportional to the length of the fatty acid. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
- Nov 2014
The growth of graphene by chemical vapor deposition on metal foils is a promising technique to deliver large-area films with high electron mobility. Nowadays, the chemical vapor deposition of hydrocarbons on copper is the most investigated synthesis method, although many other carbon precursors and metal substrates are used too. Among these, ethanol is a safe and inexpensive precursor that seems to offer favorable synthesis kinetics. We explored the initial stages of the growth of graphene on copper from ethanol, investigating the produced material by electron microscopy, Raman and x-ray photoemission spectroscopy. A graphene film with high crystalline quality was found to cover the entire copper catalyst substrate in just 20 s, making ethanol appear as a more efficient carbon feedstock than methane and other commonly used precursors.
We present the science and technology roadmap (STR) for graphene, related two-dimensional (2d) crystals, and hybrid systems, targeting an evolution in technology, with impacts and benefits reaching into most areas of society. The roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. In this document we provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlithing the roadmap to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries: from flexible, wearable and transparent electronics to high performance computing and spintronics.
We report on the epitaxial growth of Ge virtual substrates directly on Si (001) and on different porosity porous silicon (pSi) buffers. Obtained results indicate that Ge grown on low porosity (22%) pSi buffer has a better crystalline quality compared to Ge grown on bulk Si and on higher porosity buffers. This result is attributed to the compliant nature of pSi and to its reduced Young's modulus, which leads to plastic tensile deformation of the 22% porosity buffer under the in-plane tensile stress introduced by Ge lattice. The same result is not observed for higher porosity buffers, this effect being attributed to the higher buffer fragility. A low porosity pSi layer can hence be used as buffer for the growth of Ge on Si virtual substrates with reduced dislocation content and for the growth of Ge based devices or the successive integration of III-V semiconductors on Si.
The basic concept for efficiency improvement in dye-sensitized solar cells (DSSC) is limiting the electron-hole recombination. One way to approach the problem is to improve the photogenerated charge carriers lifetime and consequently reduce their recombination probability. We are reporting on a facile posttreatment of the mesoporous photoanode by using a colloidal solution of TiO 2 nanoparticles. We have investigated the outcome of the different sintering temperature of the posttreated photoanodes on their morphology as well as on the conversion efficiency of the DSSC. The DSSCs composed of posttreated photoanodes at 450°C showed an increase in J SC and consequently an increase in efficiency of 10%. Investigations were made to determine the electron recombination via the electrolyte by the OCVD technique. We found that the posttreatment has the effect of reducing the surface trap states and thus increases the electron lifetime, which is responsible for the increase of the overall cell efficiency.
The main advantage for applications of graphene and related 2D materials is that they can be produced on large scales by liquid phase exfoliation. The exfoliation process shall be considered as a particular fragmentation process, where the 2D character of the exfoliated objects will influence significantly fragmentation dynamics as compared to standard materials. Here, we used automatized image processing of Atomic Force Microscopy (AFM) data to measure, one by one, the exact shape and size of thousands of nanosheets obtained by exfoliation of an important 2D-material, boron nitride, and used different statistical functions to model the asymmetric distribution of nanosheet sizes typically obtained. Being the resolution of AFM much larger than the average sheet size, analysis could be performed directly at the nanoscale and at the single sheet level. We find that the size distribution of the sheets at a given time follows a log-normal distribution, indicating that the exfoliation process has a "typical" scale length that changes with time and that exfoliation proceeds through the formation of a distribution of random cracks that follow Poisson statistics. The validity of this model implies that the size distribution does not depend on the different preparation methods used, but is a common feature in the exfoliation of this material and thus probably for other 2D materials.
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.
- Mar 2014
A facile and efficient method based on electrochemistry for the production of graphene-based materials for electronics is demonstrated. Uncharged acetonitrile molecules are intercalated in graphite by electrochemical treatment, owing to the synergic action of perchlorate ions dissolved in acetonitrile. Then, acetonitrile molecules are decomposed with microwave irradiation, which causes gas production and rapid graphite exfoliation, with an increase in the graphite volume of up to 600 %. Upon further processing and purification, highly dispersible nanosheets are obtained that can be processed into thin layers by roll-to-roll transfer or into thicker electrodes with excellent capacitance stability upon extensive charging/discharging cycles. The good exfoliation yield (textgreater50 % of monolayers), minimal oxidation damage and good electrochemical stability of the nanosheets obtained were confirmed by scanning force and electron microscopy, as well as Raman spectroscopy and galvanostatic analyses.
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 Pt2+ complexation in the dendritic architecture and as photosensitizer units in the photocatalytic production of H-2 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.
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.
A simple, microwave-assisted route for producing Au/Ag concentred sols by glucose reduction in water was developed. Ag-Au bimetallic nanoparticles stabilized by polyvinylpyrrolidone (PVP) were characterized and their catalytic activity was studied in the reduction of 4-nitrophenol (4-NP) with NaBH 4 . Moreover they were tested as red ceramic inks for ink-jet printing technology. Since the research was developed in collaboration with a company interested in the large scale production of the suspensions, some fundamental properties for an industrial scale up were developed: high metal concentration, long time stability. The Au-Ag core-shell structures were prepared by a two-step process. Particle size-control and colloidal stability were achieved thanks to the accurate reaction optimization, combined with microwave heating, that allows the intensification of process even on large scale production. Prepared Au, Ag and Au/Ag nanocrystals acted as effective catalyst for the reduction of 4-NP.
We report on the tailoring of quantum dot (QD) emission efficiency by localized surface plasmon polaritons in self-organized mesoscopic rings. Ag nanoparticles (NPs) with CdSe QDs embedded in a polymeric matrix are spatially organised in mesoscopic rings and coupled in a tuneable fashion by breath figure formation. The mean distance between NPs and QDs and consequently the intensity of QD photoluminescence, which is enhanced by the coupling of surface plasmons and excitons, are tuned by acting on the NP concentration.
Chemical vapor deposition (CVD) is widely utilized to synthesize graphene with controlled properties for many applications, especially when continuous films over large areas are required. Although hydrocarbons such as methane are quite efficient precursors for CVD at high temperature (1000 °C), finding less explosive and safer carbon sources is considered beneficial for the transition to large-scale production. In this work, we investigated the CVD growth of graphene using ethanol, which is a harmless and readily processable carbon feedstock that is expected to provide favorable kinetics. We tested a wide range of synthesis conditions (i.e., temperature, time, gas ratios), and on the basis of systematic analysis by Raman spectroscopy, we identified the optimal parameters for producing highly crystalline graphene with different numbers of layers. Our results demonstrate the importance of high temperature (1070 °C) for ethanol CVD and emphasize the significant effects that hydrogen and water vapor, coming from the thermal decomposition of ethanol, have on the crystal quality of the synthesized graphene.
- Sep 2013
- European Congress and Exhibition on Advanced Materials and Processes (EUROMAT 2013), Oral contribution
- European Congress and Exhibition on Advanced Materials and Processes
The elastic properties of graphene crystals have been extensively investigated, revealing unique properties in the linear and nonlinear regimes, when the membranes are under either stretching or bending loading conditions. Nevertheless less knowledge has been developed so far on folded graphene membranes and ribbons. It has been recently suggested that fold-induced curvatures, without in-plane strain, can affect the local chemical reactivity, the mechanical properties, and the electron transfer in graphene membranes. This intriguing perspective envisages a materials-by-design approach through the engineering of folding and bending to develop enhanced nano-resonators or nano-electro-mechanical devices. Here we present a novel methodology to investigate the mechanical properties of folded and wrinkled graphene crystals, combining transmission electron microscopy mapping of 3D curvatures and theoretical modeling based on continuum elasticity theory and tight-binding atomistic simulations.
- Apr 2013
The different exfoliation routes of graphite to produce graphene by sonication in solvent, chemical oxidation and electrochemical oxidation are compared. The exfoliation process and roughening of a flat graphite substrate is directly visualized at the nanoscale by scanning probe and electron microscopy. The etching damage in graphite and the properties of the exfoliated sheets are compared by Raman spectroscopy and X-ray diffraction analysis. The results show the trade-off between exfoliation speed and preservation of graphene quality. A key step to achieve efficient exfoliation is to couple gas production and mechanical exfoliation on a macroscale with non-covalent exfoliation and preservation of graphene properties on a molecular scale.