Search results for " Spectra"
showing 10 items of 709 documents
Non-degenerate 1,2-silyl shift in silyl substituted alkyltrimethylcyclopentadienes
2005
Abstract The five new silanes C5Me3RSiMenCl3 − n (n = 3, R = i-Pr (5); n = 2, R = i-Pr (6); n = 2, R = s-Bu (7); n = 2, R = cyclohexyl (8); and n = 3, R = t-Bu (9)) were synthesized by reaction of 1-alkyl-2,3,4-trimethylcyclopentadienyl lithium salts with appropriate chlorosilane and characterized by NMR, MS, and IR spectra. At elevated temperatures (250–360 K), all the silanes undergo a non-degenerate sigmatropic silyl rearrangement, which generates non-equivalent structures a and b. The presence of minor structure c was observed in compounds 5 and 7 only. The Diels–Alder cycloaddition of 5 with strong dienophiles tetracyanoethylene (TCNE), and dimethylacetylenedicarboxylate (DMAD) provide…
Defect-related visible luminescence of silica nanoparticles
2013
The high photon emissivity in the visible spectral range is one of the most relevant phenomena emerging from the reduction of silica down to nanoscale; hence it is promising for the development of optical nanotechnologies (down converter, probes, display). It is well accepted that the origin of this luminescence is related to the high specific surface (~100 m2/g) that favors the formation of optically active defects at the nanosilica surface. With the aim to clarify the role of specific luminescent defects, here we report a detailed study of spectral and decay features by time-resolved photoluminescence spectra under a visible-UV tunable laser excitation. Our study is carried out on differe…
Vibrational properties of the surface-nonbridging oxygen in silica nanoparticles
2008
By studying the site-selective luminescence spectra of oxidized silica nanoparticles we identify the electronic and the vibrational lines associated with the surface nonbridging oxygen, $\ensuremath{\equiv}{\text{Si-O}}^{\ifmmode\bullet\else\textbullet\fi{}}$. This defect emits a zero-phonon line inhomogeneously distributed around 2.0 eV with full width at half maximum of 0.04 eV, weakly coupled with the local ${\text{Si-O}}^{\ifmmode\bullet\else\textbullet\fi{}}$ stretching mode whose frequency is measured to be $920\text{ }{\text{cm}}^{\ensuremath{-}1}$. These findings are different from those of the well-characterized defect in the bulk silica thus evidencing structural peculiarities of …
Nitrogen interstitial defects in silicon. A quantum mechanical investigation of the structural, electronic and vibrational properties
2019
The vibrational features of eight interstitial nitrogen related defects in silicon have been investigated at the first principles quantum mechanical level by using a periodic supercell approach, a hybrid functionals, an all electron Gaussian type basis set and the Crystal code. The list includes defects that will be indicated as Ni (one N atom forming a bridge between two Si atoms), Ni-Ns (one interstitial and one substitutional N atom linked to the same Si atom), Ni-Ni (two Ni defects linked to the same couple of silicon atoms) and Ni-Sii-Ni (two Ni defects linked to the same interstitial silicon atom). Four 〈0 0 1〉 split interstitial (dumbbell) defects have also been considered, in which …
High accuracy Raman measurements using the Stokes and anti-Stokes lines
1997
We show that by measuring the separation between the Stokes and anti-Stokes peaks excited by two different laser lines we obtain a very precise determination of absolute phonon energies. The method is useful for measuring small changes of these energies with strain, temperature, laser power, etc. It doubles the changes and avoids the necessity of using the reference lines in the Raman spectra. The method can be applied for the determination of phonon deformation potentials, for the characterization of strained heteroepitaxial layers, and for micro-Raman analysis of strain in silicon integrated circuits. We give examples of phonon shifts in Si, Ge, GaAs, InAs, and GaP as a function of applie…
Site effects assessment through historical and instrumental data integration
2006
ABSTRACT This work illustrates the historical data contribution to the analysis of the microtremor survey results carried out in May 2003 in the city of Palermo by the Istituto Nazionale di Geofisica e Vulcanologia and the Dipartimento di Geologia e Geodesia – University of Palermo. The microtremor survey has been carried out within the project SESAME (Site EffectS assessment using AMbient Excitations) funded by the European Commission, devoted to test the effectiveness of the seismic noise, and in particular of Nakamura technique, in the site effects assessment. The Nakamura technique singles out zones characterized by a near-surface geology suitable to produce site effects, by exploiting …
Optical determination of pH on surfaces using immobilized Fluorescent dyes.
1993
The attachment of pH-sensitive Fluorescent dyes on to hydroxylapatite for measuring the pH of small volumes is described. Fluorescein and acridine were used, both activated with reactive functions, such as isothiocyanate or succinimidyl ester groups, to enhance the possibility of a covalent linkage to the surface. First investigations were carried out on synthetic hydroxylapatite as the surface material. After the preparation of a particular surface with the fluorescent dye, steady-state and time-resolved fluorescence spectroscopies were employed for estimating the pH value of a solution applied to the surface. In this paper we present the results of our investigations done with both method…
Optimization criteria in sample selection step of local regression for quantitative analysis of large soil NIRS database
2012
International audience; Large soil spectral libraries compiling thousands of NIR (Near Infrared) reflectance spectra have been created encompassing a wide diversity and heterogeneity of spectra. Among the many chemometric approaches to the calibration of chemical and physical properties from these large libraries, local calibrations have the advantage of being able to select the most similar spectra to the spectrum of a target sample. This is particularly relevant when dealing with highly heterogeneous media such as soils, where the mineral matrix has a strong influence on spectral features. A crucial step in the implementation of local calibration procedures is the construction of local ne…
The origin of slow electron recombination processes in dye-sensitized solar cells with alumina barrier coatings
2004
We investigate the effect of a thin alumina coating of nanocrystalline TiO2 films on recombination dynamics of dye-sensitized solar cells. Both coated and uncoated cells were measured by a combination of techniques: transient absorption spectroscopy, electrochemical impedance spectroscopy, and open-circuit voltage decay. It is found that the alumina barrier reduces the recombination of photoinjected electrons to both dye cations and the oxidized redox couple. It is proposed that this observed retardation can be attributed primarily to two effects: almost complete passivation of surface trap states in TiO2 that are able to inject electrons to acceptor species, and slowing down by a factor of…
Bis(pyridylvinyl)diaminobenzenes: synthesis, acidochromism and solvatochromism of the fluorescence
2005
Abstract C2-symmetrical 1,4-distyrylbenzenes with 4-dimethylaminobenzene or pyridine as terminal rings, and propyloxy or dipropylamino groups in the central 2,5-positions were prepared. Solvatochromism of the absorption is small, but more pronounced in the fluorescence spectra. Some compounds exhibit huge Stokes shifts. Acid strongly alters the fluorescence: red shifts and decreasing quantum yields is the general result, but depending on the position and character of the basic sites, a strong recovery of fluorescence efficiency combined with large hypsochromic shifts may result in highly acidic solutions.