G. Faggio

Optical trapping of porous silicon nanoparticles

Silicon nanoparticles obtained by ball-milling of a 50% porosity silicon layer have been optically trapped when dispersed in a water-surfactant environment. We measured the optical force constants using linearly and radially polarized trapping beams finding a reshaping of the optical potential and an enhanced axial spring constant for the latter. These measurements open perspectives for the control and handling of silicon nanoparticles as labeling agents in biological analysis and fluorescence imaging techniques.

Nitrogen-doped graphene films from chemical vapor deposition of pyridine: Influence of process parameters on the electrical and optical properties

Summary Graphene films were produced by chemical vapor deposition (CVD) of pyridine on copper substrates. Pyridine-CVD is expected to lead to doped graphene by the insertion of nitrogen atoms in the growing sp2 carbon lattice, possibly improving the properties of graphene as a transparent conductive film. We here report on the influence that the CVD parameters (i.e., temperature and gas flow) have on the morphology, transmittance, and electrical conductivity of the graphene films grown with pyridine. A temperature range between 930 and 1070 °C was explored and the results were compared to those of pristine graphene grown by ethanol-CVD under the same process conditions. The films were characterized by atomic force microscopy, Raman and X-ray photoemission spectroscopy. The optical transmittance and electrical conductivity of the films were measured to evaluate their performance as transparent conductive electrodes. Graphene films grown by pyridine reached an electrical conductivity of 14.3 × 105 S/m. Such a high conductivity seems to be associated with the electronic doping induced by substitutional nitrogen atoms. In particular, at 930 °C the nitrogen/carbon ratio of pyridine-grown graphene reaches 3%, and its electrical conductivity is 40% higher than that of pristine graphene grown from ethanol-CVD.

Exciton condensation in homoepitaxial chemical vapor deposition diamond

In this work, the characteristics of the edge emission of a homoepitaxial diamond sample grown by chemical vapor deposition (CVD) are reported. Photoluminescence has been excited at 220 nm by using a tunable optical parametric oscillator laser, giving ∼5 ns wide laser pulses. The temperature of the sample has been decreased from room temperature down to 30 K. Free exciton emission and its phonon replicas have been observed at all the temperatures explored. Excitonic lifetime shows a nonmonotonic dependence on the sample temperature. Luminescence at low temperatures from electron-hole drops at approximately 5.18 eV has been observed for the first time in CVD diamond.

A joint macro-/micro- Raman investigation of the diamond lineshape in CVD films: the influence of texturing and stress

Abstract A systematic Raman analysis has been carried out on diamond films prepared by microwave plasma enhanced chemical vapour deposition, using a CH 4 –CO 2 gas mixture at methane concentrations varying between 47 and 52%, at 750 and 850°C substrate temperatures, in order to assess the influence of the growth conditions on the film crystalline quality, as measured by the linewidth of the diamond peak. The trends observed by changing substrate temperature and CH 4 concentration are understood on the basis of the results of a complementary film characterisation by means of micro-Raman spectroscopy, scanning electron microscopy and X-ray diffraction, aimed at investigating the local stress distribution, the surface morphology and the preferential orientation, respectively. The existence of a strong correlation is evidenced between the texturing achieved by varying the gas-mixture composition and the lineshape of the diamond peak in the micro-Raman spectra. The competition between different growth sectors within the film promotes the occurrence of large anisotropic stresses that split and inhomogeneously broaden the diamond peak. As a result, quite large macro-Raman diamond peaks are correspondingly detected, suggesting the possible unreliability of linewidth as defect density indicator in the presence of large anisotropic stresses. Finally, a quantitative estimation of the stress level in the investigated films is preliminarily presented.

Spectral response of large area CVD diamond photoconductors for space applications in the vacuum UV

Abstract This work reports on the development and characterization of large area (1 cm2) vacuum UV CVD diamond photodetectors to address the requirements of space missions where pixel and 2D arrays are used. The quality of the CVD diamond was characterized by photoluminescence and Raman spectroscopy. The performance of these devices in the dark and under illumination was investigated and the results compared to those from small area detectors based on similar material. Planar and transverse electrode configurations were used in order to evaluate the possibility of producing imaging detectors. The spectral analysis of the photocurrent was measured as a function of several functional parameters and experimental conditions.

Generalized mean-spherical-approximation description of highly asymmetric hard-sphere mixtures

We use thermodynamically self-consistent integral equation theories to determine the structure of binary hard-sphere mixtures in a regime of moderate to high size asymmetry, and for low concentration of the species with bigger particle size. Calculations are performed by applying the generalized mean-spherical approximation (GMSA) and the Rogers-Young (RY) approximation. The thermodynamic consistency of the GMSA is implemented in terms of adjustable parameters which are used in order to reproduce the Mansoori-Carnahan-Starling-Leland equation of state, and to impose the equality of the osmotic isothermal compressibilities estimated through the virial and fluctuation routes. The structural results obtained for a moderate size asymmetry of the particle species compare rather satisfactorily with the available Monte Carlo (MC) data and their parametrizations, and with previously reported modified hypernetted-chain results. The relative performances of the GMSA and of the RY approximations are also examined for strongly asymmetric mixtures. A regime of semi-dilute concentration for which no simulation data are available is investigated first and a very close agreement emerges between the RY and GMSA radial distribution functions. The case of very high dilution of the component with bigger particle size, for which RY and MC results already exist, is then considered, but it appears impossible to achieve a thermodynamically consistent solution for the GMSA according to the consistency prescriptions adopted. Other possible implementations of the thermodynamic consistency of the GMSA for HSMs and other multicomponent fluids are discussed.

Highly Versatile and Efficient Process for CNT Oxidation in Vapor Phase by Means of Mg(NO3)2‒HNO3‒H2O Ternary Mixture

Nitric acid vapor phase oxidation of carbon nanotubes using Mg(NO3)2‒HNO3‒H2O ternary mixture is proposed as a further development and significant improvement of the method employing the sole azeotropic HNO3 solution. Based on the so-called “salt effect,” the addition of Mg(NO3)2 to HNO3‒H2O solution allows varying the acid vapor concentration within a wide range, even beyond the azeotropic one (>68wt%). Ternary mixture containing 20wt% Mg(NO3)2 and variable amount of liquid HNO3 (30–58wt%) is used for the functionalization process, achieving nitric acid vapor concentrations in the range 25–93wt%. The increase of acid concentration in vapor phase produces a significant augment of the functionalization degree and a linear increase of the concentration of carboxylic acids groups, which can thus be simply tuned.

Chemical Vapor Deposited Graphene-Based Derivative As High-Performance Hole Transport Material for Organic Photovoltaics

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:[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.

Fast growth of polycrystalline graphene by chemical vapor deposition of ethanol on copper

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.

On the CVD Growth of C Nanotubes Over Fe-Loaded Montmorillonite Catalysts

The synthesis of carbon nanotubes (CNTs) by chemical vapor deposition (CVD) of isobutane (i-C4H10) over sodium-exchanged K10-montmorillonite based iron-catalysts is investigated. By studying the influence of iron-addition (5-25 wt%) on the catalyst performances, at 700 °C, an empirical relationship is derived relating the mass of CNTs synthesized with the exposed surface of loaded iron, as resulting from simultaneous change of number, size and dispersion of Fe-nanoparticles available for the growth.