Reversible surface oxidation and efficient luminescence quenching in semiconductor single-wall carbon nanotubes

We have investigated reversible single-wall carbon nanotube (SWNT) oxidation by quantitative analysis of the oxide-induced absorption bleaching and luminescence quenching at low pH. These data, in combination with DFT structure calculations, suggest that the nanotube oxide is a 1,4-endoperoxide. At low pH, the endoperoxide protonates to create a hydroperoxide carbocation, introducing a hole in the SWNT valence band. Nanotube luminescence is extremely sensitive to quenching by hole-doping, while the absorption is relatively robust.     

Homogeneous nucleation rate measurements of 1-butanol in helium: a comparative study of a thermal diffusion cloud chamber and a laminar flow diffusion chamber

Isothermal homogeneous nucleation rates of 1-butanol were measured both in a thermal diffusion cloud chamber and in a laminar flow diffusion chamber built recently at the Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Prague, Czech Republic. The chosen system 1-butanol-helium can be studied reasonably well in both devices, in the overlapping range of temperatures. The results were compared with those found in the literature and those measured by Lihavainen in a laminar flow diffusion chamber of a similar design. The same isotherms measured with the thermal diffusion cloud chamber occur at highest saturation ratios of the three devices. Isotherms measured with the two laminar flow diffusion chambers are reasonably close together; the measurements by Lihavainen occur at lowest saturation ratios. The temperature dependences observed were similar in all three devices. The molecular content of critical clusters was calculated using the nucleation theorem and compared with the Kelvin equation. Both laminar flow diffusion chambers provided very similar sizes slightly above the Kelvin equation, whereas the thermal diffusion cloud chamber suggests critical cluster sizes significantly smaller. The results found elsewhere in the literature were in reasonable agreement with our results.     

Photoemission from diamond films illuminated by intense Nd:YAG laser harmonics

Photoemission from polycrystalline diamond films irradiated with the harmonics of a Nd:YAG laser indicates that the amorphous or quasi-amorphous zones (patches) of sp²-C on the external surface of the diamond grains make a dominant contribution to the emission yield. This is deduced from: (a) the strong reduction of the yield as a consequence of the partial removal of amorphous carbon (a-C) patches by etching with acid and (b) the correlation between the yield and the a-C content determined by Raman spectroscopy. The junction between the a-C patches and the diamond determines a configuration of the energy bands, which allows one-photon emission with an energy of 4.7 eV (4th harmonic). We have measured samples with variable a-C content and found an efficient emission, with values of quantum efficiency on the order of 10^-6. A phenomenological picture of the electron emission process leads us to locate it at the a-C–diamond–vacuum triple border. This indicates a high local efficiency and suggests that a diamond film with an appropriate distribution of a-C patches on the surface could be a very efficient material. Moreover it could work in moderate vacuum and have a high laser-radiation damage-threshold. Such a material would therefore be interesting as a robust photocathode . PACS 79.60.-i; 81.05.Uw; 81.15.Gh     

Three-dimensional single Doppler radar analysis of an occluded front

Summary Single Doppler radar analysis has been applied to an occluded front associated with the last stages of cyclonic activity over the Norrk?ping area in Central Sweden. Validation was conducted of the analysis methods, such as reflectivity contouring and wind calculation from Doppler radial velocity using velocity azimuth display (VAD). The structure of the cloud system was identified as composed of a stratocumulus cloud layer, horizontally divided into two sectors of high reflectivity (>20 dBZ), oriented parallel to the surface occluded front, each extending around 80 km W-E and 50 km S-N. Embedded in these two precipitating areas were local regions of very high reflectivity (up to 50 dBZ), indicating the presence of scattered heavy precipitation and showers. The case study represents a test of the capabilities of single Doppler radar analysis techniques to be introduced in the next future into the operative forecasting environment in Italy.

---

Riassunto Tecniche di analisi dei dati da singolo radar Doppler meteorologico vengono applicate allo studio di un fronte occluso associato alla fase terminale di decadimento di un'area ciclonica sulla verticale di Norrk?ping nel Sud della Svezia. Scopo principale dello studio è la validazione dei metodi di analisi dei dati, in particolare la mappatura dei campi di riflettività e il calcolo dei campi di vento dalla velocità radiale Doppler utilizzando il velocity azimuth display (VAD). Il sistema nuvoloso risulta costituito da stratocumuli ed è suddiviso in orizzontale in due settori di elevata riflettività (>20 dBZ), orientati parallelamente al fronte occluso e ciascuno con un'estensione intorno a 80 km in direzione W-E e 50 km in direzione S-N. I volumi ad elevata riflettività (fino a 50 dBZ) rinvenuti all'interno di queste due zone di precipitazione rivelano la presenza di intense precipitazioni isolate e di acquazzoni. Lo studio costituisce un test delle potenzialità delle tecniche di analisi dei dati da singolo radar Doppler, il quale dovrà essere quanto prima introdotto nel sistema di previsioni meteorologiche in Italia.

---

Резюме Радарный анализ с использованием эффекта Доплера применяется к поглощенному фронту, связанному с последними стадиями циклонической активности над областью Норкёпинг в центральной Швеции. Оценивается применимость методов анализа, таких как построение контура отражательной способности и вычисление ветров из радиальной скорости Доплера, исполязуя индикатор азимута скорости. Определяется структура системы облаков, которая представляет слой слоистокучевых облаков, разделённый в горизонтальном направлении на два сектора с высокой отражательной способностью (>20 dBZ), ориентированных параллельно поверхности поглощённого фронта, простирающихся на расстояния порядка 80 км W-E и 50 км S-N. Внутри этих областей имеются области с очень высокой отражательной способностью (вплоть до 50 dBZ), что указывает на присутствие рассеивающих тяжёлых осадков и ливней. Исследованный случай представляет проверку возможностей техники радарного анализа с использованием простого эффекта Доплера для применения в будущем для оперативного прогнозирования окружающей среды в йталии.

     

Heating of samples induced by fast magic-angle spinning

Intense sample heating through high-speed magic-angle spinning (MAS; up to 58 K temperature difference) is demonstrated. The role of probehead and spinner design, as well as that of the temperature of the bearing air on the heating of a rotating sample, is examined. MAS-induced heating can affect the accurate determination of the isotropic value of the chemical shift as well as the principal values, asymmetry and anisotropy parameters of the chemical shift tensor. In some cases, a very large temperature gradient (12 K) within the fast rotating sample was found, which may limit the resolution of high-speed 1H MAS nuclear magnetic resonance (NMR) spectra.     

TiO2 thin films for spintronics application: a Raman study

We present the results of a systematic study performed by micro‐Raman spectroscopy on pure anatase, pure rutile and mixed anatase–rutile TiO₂ thin films, deposited by radio frequency magnetron sputtering on quartz substrates, with different thicknesses. The crystal structures of the as‐deposited films were unambiguously determined and a good crystalline homogeneity was revealed by a systematic mapping of the samples. In the mixed‐phase films, the relative amount of the two phases was monitored by a simple analysis of the components of the multi‐Lorentzian fitting curves. For the single‐phase films, the influence of the thickness and the effect of different thermal treatments, carried out to obtain series of thin films differing only for oxygen content, are discussed. The analysis of the scattered light has provided indication about the presence of an interface layer between the substrate and the film, which can play a role in driving the interesting magnetic properties exhibited by our samples, which are of potential usefulness for spintronics application. The results obtained from other techniques are briefly reported and discussed in relation to our systematic Raman characterization. This study points out how Raman investigation can provide suggestions toward the understanding of the complex physical phenomena leading to room‐temperature ferromagnetism in TiO₂ thin films. Copyright © 2009 John Wiley & Sons, Ltd.     

Solid state NMR study of intercalate complexes of poly(ethylene oxide) and small molecules

From solid state NMR spectra, a lower shielding of poly(ethylene oxide) (PEO) protons, in contrast to higher shielding of PEO carbons, has been found for PEO/hydroxybenzene and PEO/LiCF₃SO₃ complexes in comparison with neat PEO. The same PEO chemical shifts were found both for crystalline and amorphous phase of PEO/LiCF₃SO₃ polymer electrolyte, confirming the same interaction in both phases. Measurements of 2D ¹H CRAMPS exchange NMR spectra have been used to characterize proton distances in complexes of PEO and benzene derivatives. A close contact (∼ 0.3 nm) between aromatic and PEO protons was detected in some cases. From the measurements of the cross polarization ¹H → ¹³C, using Lee-Goldburg irradiation of ¹H nuclei, the distance between LiCF₃SO₃ carbon and the nearest PEO protons in the PEO/LiCF₃SO₃ complex was determined.     

NMR and vibrational spectroscopic study of the order and mobility in polycarbonate and polycarbonate — poly(ethylene oxide) blends

Time dependence of the gel formation in toluene solutions of polycarbonate (PC) was investigated by two-dimensional Fourier-transform infrared (2D FT-IR) correlation spectroscopy. The 2D correlation approach reveals that there are at least three bands in the C=O stretching region. The intensity increase of the band at 1771 cm⁻¹ occurs later compared with the onset of the intensity changes of the bands at 1778 and 1765 cm⁻¹ corresponding to amorphous and crystalline-like domains, respectively. The band at 1771 cm⁻¹ is assigned to the chain conformations occurring in the partial-order regions accompanying crystalline-like domains. Splitting of the signals of aromatic carbons in the solid-state ¹³C CP/MAS NMR spectra of semicrystalline PC and PC-PEO blends indicates restricted mobility resulting from the fixed ordering due to partial crystallinity of PC itself and from blending of PC with PEO. The decreasing mobility of PC with the increasing content of highly mobile PEO in the blends was proved by the dipolar dephasing rates obtained in the ¹H-¹H CRAMPS (combined rotation and multi-pulse spectroscopy) NMR experiments.