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The growing interest for the use of 4H-SiC in photonics is triggering the interest for more accurate characterizations of this semiconductor from the optical and opto-electronic point of view. In this work we report about new measurements run on an undoped 4H-SiC substrate, finalized at determining the precise dependence of its refractive index on temperature in the visible spectrum, and precisely at the wavelength of λ=632.8 nm, in a temperature range from room temperature (RT) to 400K. Measurements are performed by exploiting the properties of a Fabry-Perot cavity interrogated with a laser beam. It is known that the transmitted radiation intensity shows fringes that shift with temperature and the refractive index. By precisely monitoring the transmitted signal, the thermo-optic coefficient dn/dT can be determined with a resolution that approaches 10-6 K-1.

Abstract Aiming at the production of light, porous, conductive, biosafe composites, in this paper we are presenting a novel fabrication method for monolithic, three-dimensional (3D) graphene foam (GF)/porous polymer composites. The synthesis adopts a novel process architecture by using Ni foam templates in an inductive heating chemical vapor deposition growth process, and by removing Ni chemically while retaining graphene integrity by the reversible application of cyclododecane (CD); finally, nondestructive coating procedures with polycaprolactone (PCL) solutions have been developed. The composites can be optimized to enhance electrical conduction, flexibility and mechanical properties, while mixing PCL and CD allows to coat the GF with a novel mesoporous polymer coating. By tuning the GF properties, the typical electrical resistance of the 3D forms can be reduced to a few 10 s of Ohms, values that are maintained after the PCL coatings. The current study achieved a GF fraction ranging between 1 and 7.3 wt%, with even the lower graphene content composites showing acceptable electrical and mechanical properties. The properties of these conductive 3D-GF/PCL composites are in line with the requirements for applications in the field of nerve tissue engineering. Graphical abstract

Crystalline rocks can produce dangerous radiation levels on the basis of their content in radioisotopes. Here, we report radiological data from 10 metamorphic and igneous rock samples collected from the crystalline basement of the Peloritani Mountains (southern Italy). In order to evaluate the radiological properties of these rocks, the gamma radiation and the radon emanation have been measured. Moreover, since some of these rocks are employed as building materials, we assess the potential hazard for population connected to their use. Gamma spectroscopy was used to measure the 226Ra, 232Th and 40K activity concentration, whereas the radon emanation was investigated by using a RAD 7 detector. The results show 226Ra, 232Th and 40K activity concentration values ranging from (17 ± 4) to (56 ± 8) Bq kg−1, (14 ± 3) to (77 ± 14) Bq kg−1 and (167 ± 84) to (1760 ± 242) Bq kg−1, respectively. Values of the annual effective dose equivalent outdoor range from 0.035 to 0.152 mSv y−1, whereas the gamma index is in the range of 0.22–0.98. The 222Rn emanation coefficient and the 222Rn surface exhalation rate vary from (0.63 ± 0.3) to (8.27 ± 1.6)% and from (0.12 ± 0.03) to (2.75 ± 0.17) Bq m−2 h−1, respectively. The indoor radon derived from the building use of these rocks induces an approximate contribution to the annual effective dose ranging from 8 to 176 μSv y−1. All the obtained results suggest that the crystalline rocks from the Peloritani Mountains are not harmful for the residential population, even though they induce annual effective doses due to terrestrial gamma radiation above the worldwide average values. Moreover, their use as building materials does not produce significant health hazards connected to the indoor radon exposure.

This study is focused on the compositional and structural analysis of marine sediments from eight different sites of the Calabria region, south of Italy. For this purpose, a combination of spectroscopic techniques is utilised. X-ray fluorescence spectrometry is used for the quantitative elemental analysis of the investigated samples. To identify their crystalline mineral components and study their microstructure, X-ray diffraction and Raman scattering measurements are carried out. SiO2 and Al2O3 are the most abundant sediment components. Fe2O3, K2O, Na2O, MgO and CaO are also present in smaller amounts as well as TiO2 and P2O5. Sediments are characterised by a narrow range of Al2O3/SiO2 abundance ratio and a wider range of the overall Fe2O3 + MgO concentration. The analysis of the geochemical discriminating factors indicates that the marine sediments under study come from the dismantling and transportation of materials of the “Calabrian–Peloritan arc”, whose rocks are acidic intrusive igneous and metamorphic. From the environmental point of view, the obtained results can be used to assess changes in the pollution level due to geological processes.

In this article, an investigation of the natural radioactivity content of pyroclastic products from Mt. Etna, eastern Sicily, Southern Italy, was carried out. In particular, the assessment of the average activity concentration of the investigated radionuclides, related to the mineralogical phase composition of the analyzed samples, and the radiological health risk for the population, was performed. High Purity Germanium (HPGe) gamma-ray spectrometry was employed in order to quantify the average specific activity of 226Ra, 232Th, and 40K natural radioisotopes. The absorbed gamma dose rate (D), the radium equivalent activity (Raeq), the hazard indices (Hin and Hex), the annual effective dose equivalent outdoor (AEDEout), and the excess lifetime cancer risk (ELCR) were also estimated in order to assess any possible radiological hazard for the population. In our case, they were found to be lower than the maximum recommended values for the population members, thus reasonably excluding radiological hazard effects. Moreover, the identification of the source of the aforementioned naturally occurring radionuclides was attempted by X-ray Diffraction (XRD) and Micro-Raman Scattering (MRS), thereby recognizing the main radioisotope-bearing minerals present in the investigated pyroclastic products. Finally, Pearson correlation, Principal Component Analysis (PCA), and Hierarchical Cluster Analysis (HCA) were performed by processing observed radioactivity and radiological parameters in order to determine their correlation with the sampling locations.

Carbon-based nanomaterials, such as carbon dots (CDs) and graphene (Gr), feature outstanding optical and electronic properties. Hence, their integration in optoelectronic and photonic devices is easier thanks to their low dimensionality and offers the possibility to reach high-quality performances. In this context, the combination of CDs and Gr into new nanocomposite materials CDs/Gr can further improve their optoelectronic properties and eventually create new ones, paving the way for the development of advanced carbon nanotechnology. In this work, we have thoroughly investigated the structural and emission properties of CDs deposited on single-layer and bilayer graphene lying on a SiO2/Si substrate. A systematic Raman analysis points out that bilayer (BL) graphene grown by chemical vapor deposition does not always respect the Bernal (AB) stacking, but it is rather a mixture of twisted bilayer (t-BL) featuring domains with different twist angles. Moreover, in-depth micro-photoluminescence measurements, combined with atomic force microscopy (AFM) morphological analysis, show that CD emission efficiency is strongly depleted by the presence of graphene and in particular is dependent on the number of layers as well as on the twist angle of BL graphene. Finally, we propose a model which explains these results on the basis of photoinduced charge-transfer processes, taking into account the energy levels of the hybrid nanosystem formed by coupling CDs with t-BL/SiO2.

Chemical vapor deposition (CVD) on metal foils is regarded as the most effective method to produce large-area graphene with properties in line with the requirements of an extensive range of electronic and optical applications. For the CVD of graphene, ethanol is a versatile carbon source alternative to the widely used methane. In this review we report on the current progress in this field showing how the CVD parameters can be modulated to gain full microstructural control on graphene grown on Cu foils. Using ethanol vapor, graphene can be grown as a continuous film with mono- or multi-layer thickness, and also in the form of isolated crystals on pre-oxidized Cu substrates. Overall, ethanol-CVD allows a controllable degree of crystallinity and tunable electrical/optical characteristics in the grown samples. In turn, this control translates into a superior versatility for device design and related applications.

In the present article, a case study is reported regarding an investigation carried out in order to assess radioactivity concentration, heavy metals pollution and mineralogy of a beach stretch extending from Soverato to Squillace municipalities of the Ionian coast of Calabria, South of Italy, a popular tourist destination, especially in summer. The analysis of radionuclides contents was performed by using a High Purity Germanium (HPGe) gamma-ray detector, in order to quantify the average specific activity of 226Ra, 232Th and 40K natural radionuclides and 137Cs anthropogenic radioisotope. The absorbed dose rate and the annual effective dose equivalent radiological hazard indices were also estimated. Furthermore, X-ray Fluorescence (XRF) spectrometry measurements were carried out for the quantitative elemental analysis of the sand, in order to investigate any possible chemical pollution by heavy metals. For this aim, different indices such as Enrichment Factor (EF), Geoaccumulation Index (Igeo), Contamination Factor (CF) and Pollution Load Index (PLI) were applied to estimate the level of toxicity imposed on the ecosystem by the detected heavy metals. Finally, in order to identify the crystalline mineral components of the investigated sand samples, X-ray Diffraction (XRD) and Micro-Raman Scattering (MRS) measurements were carried out.

The knowledge of material physical properties is essential for a proper design of micro-electronic or micro-photonic devices. In this work, the thermo-optic effect, namely the refractive index dependence on temperature, was analized for 4H silicon carbide (4H-SiC) at the wavelength of interest for fiber-optic communication systems, $A$ = 1.55 µm, by using a simple direct technique based on the multiple beam interference in a Fabry-Perot cavity. Measurements of refractive index and extinction coefficient were carried out in a wide wavelength range by spectroscopic ellipsometry.

In this study, a novel nerve guide conduit was developed, based on a three-dimensional (3D) graphene conductive core grown, by chemical vapor deposition (CVD) coupled with a polycaprolactone (PCL) polymer coating. Firstly, the monolithic 3D-graphene foam (3D-GF) was synthesized on Ni foam templates via inductive heating CVD, subsequently, Ni/Graphene samples were dipped successively in PCL and cyclododecane (CDD) solutions prior to the removal of Ni from the 3D-GF/PCL scaffold in FeCl3. Our results showed that the electrical conductivity of the polymer composites reached to 25 S.m-1 after incorporation of 3D-GF. Moreover, the mechanical properties of 3D-GF/PCL composite scaffold were enhanced with respect to the same geometry of PCL scaffolds. The wettability, surface porosity, and morphology did not show any significant changes, while the PC12 cell proliferation and extension were increased for the developed 3D-GF/PCL nanocomposite. It can be concluded that 3D-GF/PCL nanocomposites could be good candidates to utilize as a versatile system for the engineering of peripheral nerve tissue.

The presence of heavy minerals, including radionuclides such as 238U and 232Th, in the beach placers outcropping along the western Calabrian coast (South Italy) has been investigated in order to single out the contribution from natural sources to the total radiation level. Assessing the human health radiation risk for these areas is of great importance due to their main touristic destination. With the aim of characterizing the natural radionuclides contents of the Calabrian sands, the correlations existing between their mineralogical and geochemical features and those of the parent rocks have been studied by measuring the sand radionuclides concentrations and decay products and by calculating the outdoor effective dose rate. The radioactivity in the Calabrian beach sediments is for the most part attributed to monazite and, in turn, to the high amount of Th (in the 409–464 ppm range), and is a function of the rock types and of the formation processes. The outdoor dose and effective dose rates for an exposure period of three months are in the range 1045–1240 nGy/h and 0.32–0.37 mSv/y, respectively. Although these values are remarkably higher than the corresponding average world values, they fall within the limits fixed by the Italian legislation, assuming individuals that spend three summer months on the beach. The results obtained for this area of the Southern Italy coastlines can be used as a baseline for future investigations concerning radioactivity background levels, also in other regions. Moreover, the coupling of the present findings with those of epidemiological studies will allow gaining a better evaluation of possible health effects on the population.

Low-dimensional carbon materials occupy a relevant role in the field of nanotechnology. Herein, the authors report a study conducted by atomic force microscopy and Raman spectroscopy on the deposition of carbon dots onto graphene surfaces. The study aims at understanding if and how the morphology and the microstructure of chemical vapor deposited graphene on Si/SiO2 may change due to the interaction with the carbon dots. Potential alteration in the graphene's electrical properties might be detrimental for optoelectronic applications. The deposition of carbon dots dispersed in water and ethanol solvents are explored to investigate the effect of solvents with different fluidic properties. The obtained results indicate that the carbon dots do not alter the quality of graphene.

Schottky-barrier solar cells (SBSCs) represent low-cost candidates for photovoltaics applications. The engineering of the interface between absorber and front electrode is crucial for reducing the dark current, blocking the majority carriers injected into the electrode, and reducing surface recombination. The presence of tailored interfacial layers between the metal electrode and the semiconductor absorber can improve the cell performance. In this work, the interface of a graphene/n-type Si SBSC by introducing a graphene-based derivative (GBD) layer meant to reduce the Schottky-barrier height (SBH) and ease the charge collection are engineered. The chemical vapor deposition (CVD) parameters are tuned to obtain the two graphene films with different structure and electrical properties: few-layer graphene (FLG) working as transparent conductive electrode and GBD layer with electron-blocking and hole-transporting properties. Test SBSCs are fabricated to evaluate the effect of the introduction of GBD as interlayer into the FLG/n-Si junction. The GBD layer reduces the recombination at the interface between graphene and n-Si, and improves the external quantum efficiency (EQE) with optical bias from 50 to 60%. The FLG/GBD/n-Si cell attains a power conversion efficiency (PCE) of ≈5%, which increase to 6.7% after a doping treatment by nitric acid vapor.

High-quality graphene can be produced in large scale by chemical vapor deposition (CVD). Ethanol is emerging as a versatile carbon source alternative to methane for the growth of graphene on a copper (Cu) foil catalyst. To date, rigorous studies of the ethanol-based process still lack, especially concerning the first stages of the growth, which ultimately determines graphene's properties, such as defect density and crystal size, and performance, such as electrical conductance and mechanical strength. In particular, so far the growth of isolated graphene grains by ethanol-CVD has been achieved only on preoxidized Cu foils folded in enclosures, in an attempt to limit the partial pressure of the precursor, and thus the nucleation rate. We systematically explored the process parameters of ethanol-CVD to obtain full control over the nucleation rate, grain size, and crystallinity of graphene on flat Cu foils, which are of interest for any realistic production in large scale. To limit the nucleation density and increase the grain size, preoxidized Cu foils (250 °C in air) were used as substrates, and the process parameters were thoroughly investigated and tuned. Ultimately, at an ethanol vapor flow of 1.5 × 10-3 sccm the nucleation density reduced to less than 3 nuclei/mm2 and isolated single-crystal grains grew with a lateral size above 500 μm. When transferred onto Si/SiO2 substrates, these grains showed field-effect mobility beyond 1300 cm2/(V s). Our results provide a step closer towards an affordable commercialization of electronic-grade, large-area graphene. © 2018 American Chemical Society.

The two most significant categories of physical and chemical pollutants in sediments (radionuclides and metals) were investigated in this article, in order to evaluate pollution levels in marine sediments from eight different selected sites of the Calabria region, south of Italy. In particular samples were analyzed to determine natural and anthropic radioactivity and metal concentrations, in order to assess any possible radiological hazard, the level of contamination and the possible anthropogenic impact in the investigated area. Activity concentrations of 226Ra, 232Th, 40K and 137Cs were measured by High Purity Germanium (HPGe) gamma spectrometry. The obtained results show that, for radium (in secular equilibrium with uranium), the specific activity ranges from ( $ 14 \pm 1$ ) Bq/kg dry weight (d.w.) to ( $ 54 \pm 9$ ) Bq/kg d.w.; for thorium, from ( $ 12 \pm 1$ ) Bq/kg d.w. to ( $ 83 \pm 8$ ) Bq/kg d.w.; for potassium, from ( $ 470 \pm 20$ ) Bq/kg d.w. to ( $ 1000 \pm 70$ ) Bq/kg d.w. and for cesium it is lower than the minimum detectable activity value. The absorbed gamma dose rate in air (D), the annual effective dose equivalent (AEDE) outdoor and the external hazard index ( $ H_{\rm ex}$ ) were calculated to evaluate any possible radiological risk, mainly due to the use of marine sediments for the beach nourishment. The results show low levels of radioactivity, thus discarding any significant radiological risk. Some metals (As, Cd, Crtot, Hg, Ni, Pb, Cu, Zn, Mn and Fe), that could be released into the environment by both natural and anthropogenic sources, were investigated through inductively coupled plasma mass spectrometry (ICP-MS) measurements and compared with the limits set by the Italian Legislation, to assess any possible contamination. Experimental results show that they are much lower than the contamination threshold value, thus excluding their presence as pollutants. The degree of sediment contaminations were quantified using enrichment factor (EF) and geoaccumulation index ( Igeo) for some potential hazardous elements. Results show that EF and Igeo values of As, Pb and Mn were the greatest among the studied metals. Data of this preliminary study could be helpful in the future to obtain background levels in marine sediments of the investigated region and to develop environmental regulatory frameworks.

This work demonstrates that upon anodic polarization in an aqueous fluoride-containing electrolyte, TiO

Close-packed carbon nanotubes, forming a three-dimensional nonwoven macroporous sponge-like network, are synthesized at 900 °C over a sol–gel Co–Mo–Mg catalyst using CH4 + He growth mixture with variable composition (CH4: 190–1000 sccm; He: 0–810 sccm). The samples consist of dense, randomly oriented bundles of tubes grown on and interconnected through catalyst fragments and entanglements of curly tubes. The relative amounts of the two network components depend on the gas-flowing setup selected. A systematic multiwavelength Raman spectroscopy analysis is carried out in order to investigate the tensional states arising from the interconnections among bundles. Results obtained are understood in terms of the morphology evolution produced by the catalyst phases with different activity involved in the growth process as an effect of the changes in the gas-flowing setup.

The functionalization by nitric acid vapors at azeotropic concentration has been recently proposed to eliminate drawbacks of the widely utilized liquid phase functionalization method. This work suggests to exploit the so-called “salt effect” to improve the vapor phase oxidation method in terms of safety and flexibility. Increasing the relative volatility of acid, the addition of Mg(NO 3 ) 2 salt to the HNO 3 + H 2 O solution allows (i) obtaining vapors with HNO 3 at the azeotropic concentration from a more diluted liquid solution (i.e. operating under safer conditions), and (ii) varying the concentration of HNO 3 in the vapor phase even above the azeotropic concentration limit (with improved process flexibility). High-resolution transmission electron microscopy, thermo-gravimetry, Raman spectroscopy and X-ray photoemission spectroscopy systematic analyses are carried out on pristine and oxidized nanotubes in order to assess their functionalization degree, surface chemistry and structural evolution. The most relevant finding of this preliminary study is that the nanotube functionalization extent increases linearly with the HNO 3 vapor concentration.

We present a graphene/n-Si Schottky junction for NH3 detection at level of few tens of parts-per-million (ppm). Graphene was synthesized by Liquid Phase Exfoliation and transferred onto the Si by drop casting. The Schottky barrier characterization towards NH3 was performed by volt-amperometric measurements in the range 10-200 ppm at bias of -3 V. The characterization in the test chamber simulated environmental conditions by Relative Humidity at 50% and temperature at 295 K. Results suggest that the NH3 induces a barrier height modulation with current variations up to 4% for 200 ppm. In environmental conditions, a spontaneous restoring is observed for the device.

Liquid phase functionalisation of carbon nanotubes is carried out via a H2SO4 + HNO3 mixture, and the effect of the sulphuric to nitric acid volume ratio (1:3–3:1) is systematically investigated by means of complementary techniques, observing the expected progressive downgrade of the crystalline quality, along with the increase of oxygenated functionality concentration. In addition, in contrast with common expectations, the results obtained demonstrate that the concentration of carboxylic groups (acids and anhydrides) never exceeds that of all other functionalities (lactones, phenols, quinones/carbonyls and sulphonic groups) introduced by chemical oxidation. Only by using equal volumes of sulphuric and nitric acids the concentrations of carboxylic and non-carboxylic groups become comparable. Raman analysis reveals that a change in the sample homogeneity accompanies the variations of the relative proportions of the various oxygenated groups, by the typology of which the vibration modes of carbon pairs and carbon rings appear to be affected to different extents. Copyright © 2012 John Wiley & Sons, Ltd.

Mg(Al)O supported Ru catalysts with low loading of active metal (0.6 wt.%), obtained from simple inorganic salts, were tested, for the first time, in the steam reforming (SR) of glycerol to hydrogen rich mixtures. The catalysts, obtained by simple wet impregnation of Ru salts on a Mg(Al)O mixed oxide followed by high temperature oxidation/reduction treatments, were systematically characterized by numerous complementary techniques, both before and after their use in SR reaction. The changes of the performances (activity, selectivity and stability) of Ru/Mg(Al)O catalysts were studied, produced by varying reaction temperature (450–650 °C) at fixed glycerol concentration (10 wt.% in water), and by varying glycerol concentration (10–40 wt.% in water) at fixed reaction temperature (550 °C). The best performances (in terms of glycerol conversion, H2 yield and CO2 selectivity), at lowest reaction temperature, were obtained operating at 10 wt.% glycerol and 550 °C. Correspondingly, catalysts showed glycerol conversion, H2 yield and CO2 selectivity close to 100%. Methane was practically absent, CO selectivity was lower than 3.5% and coke deposition quite scarce (1.1 mgC gcat−1 h−1). However, at this temperature, catalysts exhibited stable, although lower, performances up to 40 wt.% glycerol concentrations. Significantly, catalytic performances could be improved increasing reaction temperature up to 650 °C keeping other advantages.

Composite vanadium oxide (VOx)-based fibres were synthesised by the electro-spinning method combined with conventional sol–gel processing using polyvinyl acetate (PVAc) as a polymeric binder and vanadium oxytriisopropoxide as a vanadium oxide precursor. The microstructure and composition of as-spun and calcined (300–500 °C) VOx–PVAc fibres were systematically investigated by scanning electron microscopy, thermogravimetry, reflectance infrared Fourier transform, micro-Raman spectroscopy and photoluminescence in view of their possible use in gas sensor fabrication. The comparative discussion of the characterization results indicates that V2O5–PVAc fibres are obtained. Calcination gradually removes PVAc and promotes structural rearrangement with consequent fibre-morphology changes. With increasing calcination temperature, the crystallinity degree of V2O5 improves and a more oxygen-deficient substoichiometric surface layer forms. Calcination at 400 °C preserves the fibre integrity. Indeed, fibres calcined at this temperature appear as the most suitable ones for use as the active layer in gas-sensing devices. Copyright © 2011 John Wiley & Sons, Ltd.

Nanotubes-based nanocomposites to be used as polymer reinforcing/flame-retardant additives are synthesized by decomposition of isobutane at 600 °C. Catalytic chemical vapor deposition (CCVD) is carried out over 17 wt%Fe-catalysts supported on various oxides (Al 2 O 3 , MgO, CaO, SrO or BaO) reduced at 600 °C. Catalysts utilized and carbonaceous deposits obtained are systematically characterized by the use of several analysis techniques, in order to investigate the influence of catalyst specifics on reaction yield, selectivity and characteristics (crystallinity and purity) of the grown nanotubes. The results show that the support greatly influences the catalyst performance. The lack of metallic iron renders Fe/SrO- and Fe/BaO-catalysts inactive. Fe/Al 2 O 3 catalysts exhibit the highest catalytic activity, but give rise to scarce selectivity and large metallic impurity contents. Contrarily, using Fe/MgO and Fe/CaO catalysts leads to lower yields, but allows reducing impurities and remarkably improving selectivity and nanotube crystallinity.

Titanium oxide/carbon nanotubes-based nanocomposites (TiO2/CNTs, prepared by sol–gel method, and 2%Pt/TiO2/CNTs, obtained by wetness impregnation of the TiO2/CNTs base material with a solution of platinum acetylacetonate) have been recently used as active layer in hydrogen sensing devices at near room temperature, obtaining quite different responsiveness. The microstructure of these hybrid materials is here systematically investigated by micro-Raman spectroscopy at 2.41 eV. The results show that regardless of the nominal C/Ti molar ratio (3.6 or 17.0) only the anatase phase of titania is formed. Theoretical calculations demonstrate that phonon confinement is fully responsible for the large blue-shift (∼10 cm−1) and broadening (∼20 cm−1) of the lowest-frequency Raman mode with respect to bulk anatase. The average size (4.3–5.0 nm) of TiO2 crystallites, resulting from Raman spectra fitting, is in excellent agreement with those inferred from transmission electron microscopy and X-ray diffraction measurements.

Silicon carbide and silicon carbon nitride thin films were grown by pulsed laser ablation of SiC targets in vacuum or in the presence of a controlled N2 atmosphere. The influence of the substrate t...

Due to its exceptional physical properties, synthetic diamond is an ideal material for the realization of UV and X-ray detectors to be used for the characterization of laser-generated plasmas. Diamond detectors are able to operate at high temperatures and in the presence of high fluxes of ionizing radiation, where traditional silicon-based detectors usually fail. In this paper, we report on Raman and electro-optical characterization of a structure consisting of intrinsic diamond/boron-doped diamond homoepitaxially grown by chemical vapor deposition onto a commercial high pressure high temperature Ib-type diamond substrate using a 1% CH4/H2 gas mixture. A metal–insulator–semiconductor (MIS) diode was obtained by thermally evaporating an aluminum contact on the growth surface of the intrinsic layer. The detection capability of this device operating in transverse configuration was measured in the deep UV spectral range. The device sensitivity has been estimated at different biasing voltages and at two wavele...

Commercially available and laboratory-prepared multi-walled carbon nanotubes (MWCNTs) are systematically investigated by the use of micro-Raman spectroscopy (MRS), thermogravimetric analysis (TGA) and complementary techniques (scanning electron microscopy (SEM) and transmission electron microscopy (TEM)) with the aim of establishing a standardised post-growth diagnostic protocol for the assessment of their overall crystalline quality. By studying a set of ‘reference’ samples, clear correlations are evidenced between the Raman graphitisation indexes (D/G, G′/G and G′/D intensity ratios) commonly adopted to describe the crystalline arrangement of nanotubes, and their reactivity towards oxygen, as measured by the apparent activation energy needed for their oxidation, inferred from the kinetic analysis in quasi-isothermal conditions. The higher the crystalline perfection degree, the higher the energy needed for oxidising them. The efficacy of the found correlations in indirectly assessing the reactivity of nanotubes prepared under different conditions is successfully demonstrated by the use of a second set of samples. The physical meaning and range of validity of the shown correlations are further discussed. Copyright © 2010 John Wiley & Sons, Ltd.

The results of Raman analysis on multi-walled carbon nanotubes, prepared by catalysed chemical vapour deposition, are used as a guide for the calibration of the growth parameters, directed to improve crystalline quality and resulting thermal stability of nanotubes. Under selective growth conditions, the resistance to oxidation in air, as assessed by thermogravimetry measurements, is found to increase with the establishment of the long-range graphitic order in radial tube direction, as signalled by the Raman G′/G intensity ratio enhancement. In the range of parameters explored (synthesis temperature: 500–700 °C; growth atmosphere: 120 cc/min i-C4H10–H2–He mixture with He at 0–25%; i-C4H10/H2 flow ratio: 1–3; metal load and reduction temperature of Fe/Al2O3 catalysts: 17–40 wt%, and 500 and 700 °C, respectively), the best crystalline quality and the highest oxidative resistance are achieved by carrying out the synthesis reaction at 700 °C in 1:1:0 i-C4H10–H2–He atmosphere over 29 wt% Fe catalysts reduced at 700 °C. An additional relevant finding is the strong correlation evidenced between results of thermogravimetry and Raman analyses, suggesting the use of Raman spectroscopy for non-destructively evaluating the thermal stability of any graphitically ordered carbon species.

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 105 Ω/S 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:[70]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. © 2016 American Chemical Society.

The detection properties of heteropitaxial (polycrystalline, pCVD) and homoepitaxial (single crystal, scCVD) diamond films grown by microwave chemical vapor deposition (CVD) in the Laboratories of Roma “Tor Vergata” University are reported. The pCVD diamond detectors were tested with α -particles from different sources and 12 C ions produced by 15 MV Tandem accelerator at Southern National Laboratories (LNS) in Catania (Italy). pCVDs were also used to monitor 14 MeV neutrons produced by the D-T plasma at Joint European Torus (JET), Culham, U.K. The limit of pCVDs is the poor energy resolution. To overcome this problem, we developed scCVD diamonds using the same reactor parameters that optimized pCVD diamonds. scCVD were grown on a low cost (1 0 0) HPHT single crystal substrate. A detector 110 μ m thick was tested under α -particles and under 14 MeV neutron irradiation. The charge collection efficiency spectrum measured under irradiation with a triple α -particle source shows three clearly resolved peaks, with an energy resolution of about 1.1 % . The measured spectra under neutron irradiation show a well separated C ( n , α 0 ) 9 Be 12 reaction peak with an energy spread of 0.5 MeV for 14.8 MeV neutrons and 0.3 MeV for 14.1 MeV neutrons, which are fully compatible with the energy spread of the incident neutron beams.

This work reports on the development and characterization of bi-dimensional deep-UV sensor arrays based on synthetic diamond to address the requirements of space-born astrophysical experiments. The material was synthesized at the University of Rome “Tor Vergata” where both heteroepitaxial polycrystalline diamond films and homoepitaxial single-crystal diamonds are grown using a tubular MWCVD reactor. The quality of chemical vapour deposited diamond was characterized by cathodoluminescence, photoluminescence, Raman spectroscopy and thermally stimulated currents. Then, suitable samples were selected and used to fabricate photoconductive single-pixel and 2D array devices by evaporating metal contacts on the growth surface. The electro-optical characterization of the devices was carried out in a wide spectral region, ranging from 120 to 2400 nm. A deuterium lamp and a 0.5 m vacuum monochromator were used to measure the detector responsivity under continuous monochromatic irradiation in the 120–250 nm spectral range, while an optical parametric oscillator tunable laser producing 5 ns pulses was used as light source from 210 up to 2400 nm. Time response, signal-to-noise ratio, responsivity and visible rejection factor were evaluated and the results are hereafter summarized.

Polycrystalline (pCVD) and single crystal (scCVD) diamond films grown from Chemical Vapour Deposition (CVD), if sufficiently pure at Raman analysis, are very good materials for beam or flux monitors inside accelerators or nuclear reactors. This is because they are very hard to damage in high radiation fields and very resistant to high temperatures. Films of pCVD diamond are, however, not so good as spectroscopy detectors due to inhomogeneities induced by their growth in grains with the consequent presence of grain boundaries which worsen their energy resolution. The latter can be significantly improved by growing scCVD diamond films onto HPHT synthetic diamond substrates. We have shown that it is possible to measure the density of defects inside diamond specimens using as probes suitable penetrating nuclear radiations. With the preliminary results reported here we'll show that, bombarding CVD diamond films grown at Roma 'Tor Vergata' with energetic protons and 4 He, Li-6 and C-12 ions produced in the accelerators of Catania laboratories, the pulse height defects are higher than those in silicon detectors and likewise well described by a power law in the deposited energy. Furthermore, we'll show that pulse heights for the same particles seem to depend on the duration of the measurement, thus exhibiting a sort of depolarization of the insulator when exposed to the electric voltage which makes it a particle detector. (c) 2006 Elsevier B.V. All rights reserved.

The performance of detectors based on CVD diamond films strongly depends on the material quality, which, in turn, is extremely sensitive to even slight variations of the deposition conditions. In this work, a careful Raman and photoluminescence (PL) study has been carried out on diamond films grown on Si substrates by microwave plasma CVD under nominally identical deposition conditions, but exhibiting, quite surprisingly, different detection sensitivity. Micro-PL measurements performed on the growth surface and on the cross-section of the samples have shown higher incorporation of Si impurities in films having lower detection performance. Optical microscopy on the nucleation side of such films has evidenced a lower nucleation density, leading to diamond grains of greater size, but also to higher incorporation of Si atoms. Thus, a greater grain size does not necessarily guarantee a higher film quality, if the nucleation density is low. On the basis of these results, the nucleation density is identified as the 'hidden' variable in the growth process of our CVD diamond films, determining, ultimately, their detection sensitivity.

This work demonstrates that upon anodic polarization in an aqueous fluoride-containing electrolyte, TiO2 nanotube array films can be formed with a well-defined crystalline phase, rather than an amorphous one. The crystalline phase was obtained avoiding any high temperature annealing. We studied the formation of nanotubes in an HF/H2O medium and the development of crystalline grains on the nanotube wall, and we found a facile way to achieve crystalline TiO2 nanotube arrays through a one-step anodization. The crystallinity of the film was influenced by the synthesis parameters, and the optimization of the electrolyte composition and anodization conditions (applied voltage and time) were carried out. For comparison purposes, crystalline anatase TiO2 nanotubes were also prepared by thermal treatment of amorphous nanotubes grown in an organic bath (ethylene glycol/NH4F/H2O). The morphology and the crystallinity of the nanotubes were studied by field emission gun-scanning electron microscopy (FEG-SEM) and Raman spectroscopy, whereas the electrochemical and semiconducting properties were analyzed by means of linear sweep voltammetry, impedance spectroscopy, and Mott-Schottky plots. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) allowed us to determine the surface composition and the electronic structure of the samples and to correlate them with the electrochemical data. The optimal conditions to achieve a crystalline phase with high donor concentration are defined. © 2018 IOP Publishing Ltd.

The work herein presented investigates the sensing properties of a graphene-based Schottky diode, operating in real environmental conditions, that is at room temperature and in presence of humidity. The role of water in the field of gas sensors for air quality monitoring has always been a crucial point: the water is always present in the environment and in most cases acts as interfering molecule, by altering or masking the effects of the other analytes. In this work, reporting device employed for NH3 detection, we show that the adsorption kinetics of water takes place on completely different time scale so that the ammonia contribution can be distinguished. The hetero-junction consists of graphene sheets obtained by Liquid Phase Exfoliation and deposited onto a Si cathode by drop casting. Cross interference effect between water vapour and NH3 was studied by exposing the diode at several Relative Humidity values ranging from 20% up to 70%. All tests were performed in air at 500sccm total flow at room temperature. A current drift due to the water is mainly visible whereas the effect of the pollutant is revealed as an abrupt decrease of the diode current

In this work, we present the behavior of a graphene/silicon Schottky diode exposed to NH3 flow of few tens of parts-per-million (ppm), at standard temperature and humidity conditions. Graphene was synthesized by Liquid Phase Exfoliation and transferred onto the Silicon substrate by drop casting. The Schottky barrier characterization towards NH3 was performed at a reverse bias of -3V in the range 10 ppm-200 ppm. Results show the effect on the device electric current of ammonia concentrations as low as 10 ppm, with a good repeatability of the voltamperometric response. The variations NH3, of the Schottky barrier, are reported as a function of the gas concentration. A spontaneous restoring is finally observed for the device Keywords— graphene; Schottky diode; gas sensing; environmental monitoring

High conductive graphene films can be grown on metal foils by chemical vapor deposition (CVD). We here analyzed the use of ethanol, an economic precursor, which results also safer than commonly-used methane. A comprehensive range of process parameters were explored in order to obtain graphene films with optimal characteristics in view of their use in optoelectronics and photovoltaics. Commercially-available and electro-polished copper foils were used as substrates. By finely tuning the CVD conditions, we obtained few-layer (2-4) graphene films with good conductivity (∼500 Ohm/sq) and optical transmittance around 92-94% at 550 nm on unpolished copper foils. The growth on electro-polished copper provides instead predominantly mono-layer films with lower conductivity (≥1000 Ohm/sq) and with a transmittance of 97.4% at 550 nm. As for the device properties, graphene with optimal properties as transparent conductive film were produced by CVD on standard copper with specific process conditions.

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 growth of graphene on copper from ethanol, focusing on processes of short duration (up to one min). We investigated the produced films 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. (C) 2014 Elsevier B.V. All rights reserved.

Some crucial aspects of the functionalisation of carbon nanotubes by nitric acid vapors are investigated, namely selectivity of the process and thermal stability in air of the functionalised materials. Results obtained by varying duration (0.5–5.0 h) of the conventional treatment with vapors at azeotropic HNO3 concentration are compared with those obtained, for fixed duration (2.0 h), by using sub-azeotropic HNO3+H2O+Mg(NO3)2 solution to generate acid vapors with acid concentration in the range 25–93 wt%, as recently proposed by us. A thorough picture is drawn based on evidences of high-resolution transmission electron microscopy, micro Raman spectroscopy, thermo-gravimetry, derivative thermo-gravimetry, and temperature-programmed desorption systematic analyses.

Taguchi method is for the first time applied to optimize the synthesis of graphene films by copper-catalyzed decomposition of ethanol. In order to find the most appropriate experimental conditions for the realization of thin high-grade films, six experiments suitably designed and performed. The influence of temperature (1000-1070 C) and synthesis duration (1-30 min) and hydrogen flow (0-100 sccm) on the number of graphene layers and defect density in the graphitic lattice was ranked by monitoring the intensity of the 2D- and D-bands relative to the G-band in the Raman spectra. After critical examination and adjusting of the conditions predicted to give optimal results, a continuous film consisting of 2-4 nearly defect-free graphene layers was obtained. © 2013 Elsevier B.V.