Search

Nanoparticle suspensions (nanofluids) have been subject of several investigations in the last few years for their potential applications, one of them being the improvement of heat transfer characteristics of thermal fluids. However, notwithstanding the numerous studies on thermal properties of nanofluids containing various kinds of nanoparticles, in many cases, the results are conflicting, and the models proposed to explain the experiments are controversial. In this paper, the thermal conductivity enhancement in dispersions, based on titania nanoparticles produced by laser-assisted pyrolysis, is investigated using an optical technique known as forced Rayleigh light scattering. Measurements on these systems exhibited a linear relation between the thermal conductivity enhancement and the nanoparticle volume fractions with a maximum increase of 6% with respect to the base fluid ethanol at 0.6% titania content. Comparison of the experimental data with the aggregation model developed by Prasher et al. highlighted the crucial role of the morphology of pyrolytic nanoparticle aggregates on determining the thermal conductivity enhancement. Copyright © 2014 John Wiley & Sons, Ltd.

Fundamental aspects concerning the inter and intramolecular properties of association complexes of metals and molecules of biological interest are not yet fully understood. The study of their structures and dynamics is an important research field for applications in chemistry and pharmacology. Information on their intrinsic properties can be gathered by studying isolated molecules and their complexes in a solvent-free environment, which permits a comparison with high-level theoretical calculations. Most of the biological molecules have, however, extremely low vapor pressure and are thermally labile. Many efforts have been made to find new vaporization methods. Pulsed laser ablation (PLA) technique proved to be suitable to desorbe solid amino acids and nucleic bases and their complexes with metals, in neutral and ionized states, and to perform mass spectrometry in gas-phase. Here we report mass spectra of biomolecules complexed with metals, generated by laser ablation PLA-mass spectrometry, or ablation followed by supersonic expansion (REMPI).

Impulsive stimulated Raman scattering (ISRS) is a nonlinear coherent technique for the measurement of the vibrational spectrum of samples within the bandwidth of the excitation laser pulse, which, for pulse durations of several tens of femtoseconds, translates into easy access to the low-wavenumber Raman spectrum (below few hundreds of cm−1). Recently, a degenerate ISRS Fourier-transform Michelson spectrometer for measurements in bulk, transparent liquids operating with a single, nonamplified femtosecond laser source, was demonstrated, although with low measurement speed, exploiting a position-sensitive detector to measure the time evolution of the tiny spectral asymmetries of the probe beam produced by stimulated Raman gain and loss. Building on this innovation of the detection method, in this work, we show that using a different data acquisition concept, based on the fast modulation of the delay in one of the spectrometer arms, it is possible to measure scattering and absorbing samples in a much shorter time, approaching the real-time visualization of the Raman spectrum of the sample under analysis. Furthermore, the setup described here does not suffer from creation of microbubbles, heating effects or thermal instabilities produced in the sample by increasingly high excitation fluences, and is therefore suitable to be implemented in vibrational microscopy applications with tight beam focusing. Thus, the present work is a contribution toward practical implementation of low-wavenumber vibrational imaging, which is of importance for studies of intramolecular vibrational modes of large molecules or of heavy functional groups or for the study of intermolecular force constants in biological and nanostructured materials.

Femtosecond coherent Raman techniques have significant diagnostic value for the sensitive and non-intrusive measurement of temperature, pressure, and composition of gas mixtures. Due to the low density of samples, however, such measurements make use of high-energy amplified laser sources, with unwieldy and costly experimental setups. In this paper, we demonstrate an experimental setup equipped with a low-energy and low-average-power femtosecond oscillator allowing measurement of the pure-rotational spectrum of nitrogen down to atmospheric pressure using impulsive stimulated Raman scattering. Using a simplified model to analyze the experimental data we were able to derive the gas temperature with reasonable accuracy.

Impulsive stimulated Raman scattering (ISRS) is a nonlinear pump–probe spectroscopy technique particularly suitable to study vibrational intermolecular and intramolecular modes in complex systems. For the latter, recent studies of ISRS microscopy with low-energy laser sources have attracted attention for investigation of photosensitive or biological samples. Following this stream of interest, in this paper, we report an investigation on the relationship between femtosecond ISRS data and pump–probe Z-scan measurements, showing that the latter technique is capable of capturing the Kerr nonlinearities induced by the molecular vibrational modes. To this aim, firstly, spectrally filtered and Raman-induced Kerr ISRS signals were simultaneously acquired to determine the sample nonlinear response and to establish the reference data for the Z-scan analysis. Then, by adopting a suitable experimental arrangement to avoid thermo-optical effects, we were able to unambiguously observe the Raman-induced effects in Z-scan measurements, thus obtaining a consistent picture between ISRS and Z-scan for the first time, to the best of our knowledge. Practical applications of the proposed method include calibrated measurements of the contribution of the internal (Raman) and external molecular modes to the nonlinear refractive index.

Coherent anti-Stokes Raman scattering (CARS) is a versatile technique for vibrational imaging that can be realized by a simple setup using a single femtosecond laser oscillator. For such a layout, supercontinuum generation in a photonic crystal fiber (PCF) is commonly used to produce a broadband Stokes beam in the so-called multiplex CARS spectroscopy; accordingly, characterization of the supercontinuum beam is a necessary requirement to perform controlled experiments. In this paper, we first perform simulations and characterizations of the supercontinuum spectra produced by a PCF in conditions encountered in a typical single-oscillator multiplex CARS setup. Good agreement between the experimental and simulated supercontinuum spectra is obtained in the whole emission band and for different PCF working conditions, by accurate choice of the simulation input parameters and by accounting for the input beam chirp in the numerical code. The influence of PCF working conditions on the CARS signal measured on a reference sample is also shown. To this regard, we also study the long-term stability of the Stokes beam and of the associated CARS signal, using the Allan variance to determine a limit to the acquisition time in CARS measurements. We show that Stokes and CARS long-term spectral fluctuations are on a timescale of some minutes in our setup, setting an indicative limit to the practical acquisition time for reliable CARS measurements. Based on this work, good control over the measurement of vibrational bands by CARS microscopy can be achieved in view of optimized performances in specific spectral bands.

Silicon (Si) nanowires (NWs) grown on stainless-steel substrates by Cu-catalysed Chemical Vapour Deposition (CVD) have been prepared to be used as anodes in lithium-ion batteries. The use of NWs can overcome the problems related to the Si volume changes occurring during lithium alloying by reducing stress relaxation and preventing material fragmentation. Moreover, since the SiNWs are grown directly on the substrate, which also acts as a current collector, an excellent electrical contact is generated between the two materials without the necessity to use additional binders or conducting additives. The electrochemical performance of the SiNWs was tested in cells using lithium metal as the anode. A large irreversible capacity was observed during the first cycle and, to a lesser extent, during the second cycle. All the subsequent cycles showed good reversibility even if the coulombic efficiency did not exceed 95%, suggesting the formation of an unstable SEI film and a continuous decomposition of the electrolyte on the silicon surface. The absence of a stable SEI film was assumed responsible for a linear capacity fade observed upon cycling. On the other hand, the electrochemical characterization performed at different values of the charging current showed that SiNWs possess an exceptionally high rate capability.

Photonic crystal fibers (PCFs) have gained popularity thanks to the use of their broad and efficient supercontinuum (SC) emission as Stokes source in ultrafast multiplex coherent anti-Stokes Raman scattering microscopy. Here, the infrared portion of the SC emitted by a widely used PCF is studied in a basic CARS experiment with a single laser oscillator operated at various conditions of power and chirp values. The dependence of the SC and CARS signal of reference cyclohexane sample on the operative conditions of the laser is examined and illustrated by means of statistical approaches that serve the purpose of defining and quantifying the reliability of CARS measurements in relation to the characteristic of the Stokes pulse coming from the fiber. Statistical techniques based on principal component analysis, on calculation of statistical momenta of the SC spectra, and on Allan variance studies are introduced to help to identify the best working regime. Principal component analysis was used to quantify the correlation of SC spectra with PCF input power, which is found to be stronger at high laser beam chirp values. First and second momenta of SC spectra allow identification of best operating conditions with respect to peak emission wavelength and spectral width. Finally, the behavior of Allan variance shows that the time stability of Stokes and CARS signals clearly depends on the chirp of the pump pulse, evidencing that high chirp conditions correspond to lower variance values at the expense of stability for longer measurement times.

In this work it is shown how SrTiO3 substrates interact with YBa2Cu3O7-δ thin films during film processing. The effect of this interdiffusion on morphology and transport properties is analyzed. The films were deposited by low fluorine Metal Organic Decomposition (MOD) on SrTiO3 (001) single crystals. In general, a sharp c-axis oriented growth and high critical temperature (Tc) were observed in all samples; however, the surface of the otherwise good-quality films is spoiled by the presence of inhomogeneous circular areas of seemingly different appearance and texture. In order to understand the nature of these defects, the substrates and the films were deeply investigated by optical microscopy, scanning and transmission electron microscopy (SEM, TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), contact-stylus surface profilometry (SSP), Atomic Force Microscopy (AFM) and time of flight secondary ion mass spectrometry (ToF-SIMS). The round spots are the manifestation, on the film surface, of ion interdiffusion from the substrate into the YBCO film Spot-free samples can be obtained with different pre-treatments of the SrTiO3 substrates. Although these defects are detrimental for the film local structure, Jc values are not significantly affected, although the defects' influence might become substantial while working on smaller scales.

Polycrystalline MoS 2 grown by Mo sulphurization was exposed to increasing doses of Ar + ions at 250 eV starting from 2.2 × 10 15 ions/cm 2 to 3.92 × 10 17 ions/cm 2 . Electronic structure changes were monitored by X-Ray Photoelectron Spectroscopy (XPS) and Inverse Photolectron Spectroscopy (IPES). No change in the Fermi level position was observed with Ar + dosing. Ion bombardment resulted in a new visible feature at lower binding energy in the Mo3d core level, while the S2p lineshape showed little changes. The formation of a steady state from 2.49 × 10 17 ions/cm 2 has been detected. The investigation of the occupied and unoccupied states on the steady-state surface pointed to the simultaneous presence of metallic-like Mo with amorphous MoS 2-x .