Photoconductivity and absorption in amorphous Si

The paper deals with photoconductivity and absorption in aSi specimens, prepared mainly by the decomposition of silane in a glow discharge. Substrate temperatures, T_d, between 300 K and 650 K were used during deposition. The normalised photoresponse was measured at room temperature as a function of photon energy and T_d in a spectral range from 0.5 eV to 3 eV. The absorption coefficient was determined for evaporated, sputtered and glow discharge specimens.The main features of the results are in agreement with conclusions drawn from previous electrical transport and field effect measurements and can be interpreted on the basis of the proposed model for the localised state distribution. It is confirmed that ?_c??_v is 1.5 to 1.6 eV, and that there is a local density of state maximum at about 1.2 eV below ?_c. At room temperature the steady photocurrent is carried predominantly by electrons in states above ?_c, whether excitation is from localized or extended states. Specimens prepared at T_d > 500 K are highly photosensitive, with electron recombination lifetimes, τ, of up to 10^?5s. Rise and decay times of the signal lie in the millisecond range. For T_d < 500 K there is a drastic decrease in τ, which falls to 10^?11 s at $\text{T}{}_{\mathrm{<mtext>d</mtext>}}\text{? 300}\text{K}$ and is even less for evaporated specimens. These results are discussed in some detail.     

Selective measurement of chromium(VI) by fluorescence quenching of ruthenium

The oxidative deterioration of polyunsaturated fatty acids (PUFAs) in culinary oils and fats during episodes of heating associated with normal usage (80-300 degrees C, 20-40 min) has been monitored by Fourier transform infrared spectroscopy (FTIR). The thermal oxidation of PUFAs is a free radical chain reaction, in which hydroperoxides are generally recognized as the primary major products. Hydroperoxides of PUFAs are easily decomposed into a very complex mixture of secondary products with the decrease in unsaturation. The oxidative advance of PUFAs during heating was studied by the determination of unsaturation percentage at different temperatures and heating times. Oils frequently used in food frying such as olive oil, sunflower oil, corn oil and seeds oil (sunflower, safflower and canola seed) were studied. The results show there is a decrease in unsaturation starting at 150 degrees C and becoming more pronounced at temperatures around 250 degrees C. The following variations were found in the unsaturation percentage, expressed as methyl linoleate, between the original sample and the sample heated at 300 degrees C for 40 min: olive oil (19-6%), sunflower oil (29-12%), corn oil (28-18%) and seeds oil (23-11%). This variation in unsaturation grade provides evidence of the transformation of essential PUFAs and subsequent decrease in the oils' nutritional value. The internal standard method is suitably precise when the n-valeronitrile is used as standard as shown by the 1-2% relative standard deviation (R.S.D.) found for seven replicates.     

In search of ionic liquids incorporating azolate anions

Twenty-eight novel salts with tetramethyl-, tetraethyl-, and tetrabutylammonium and 1-butyl-3-methylimidazolium cations paired with 3,5-dinitro-1,2,4-triazolate, 4-nitro-1,2,3-triazolate, 2,4-dinitroimidazolate, 4,5-dinitroimidazolate, 4,5-dicyanoimidazolate, 4-nitroimidazolate, and tetrazolate anions have been prepared and characterized by using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and single-crystal X-ray crystallography. The effects of cation and anion type and structure on the physicochemical properties of the resulting salts, including several ionic liquids, have been examined and discussed. Ionic liquids (defined as having m.p.<100 degrees C) were obtained with all combinations of the 1-butyl-3-methylimidazolium cation ([C(4)mim](+)) and the heterocyclic azolate anions studied, and with several combinations of tetraethyl or tetrabutylammonium cations and the azolate anions. The [C(4)mim](+) azolates were liquid at room temperature exhibiting large liquid ranges and forming glasses on cooling with glass-transition temperatures in the range of -53 to -82 degrees C (except for the 3,5-dinitro-1,2,4-triazolate salt with m.p. 33 degrees C). Six crystal structures of the corresponding tetraalkylammonium salts were determined and the effects of changes to the cations and anions on the packing of the structure have been investigated.     

Systematics of coulomb-nuclear interference: Implications for reliable measurement of the reorientation effect

Data are presented which show an inverse correlation between Coulomb excitation probability and the nuclear separation corresponding to onset of Coulomb-nuclear interference. They demonstrate that the safe energy for reorientation effect measurements should always be determined experimentally.     

Comments on the calculation of the extended state electron mobility in amorphous silicon

In a recent paper Silver and his collaborators concluded that the extended state electron mobility micro_c in amorphous silicon was about 500 cm²V^-1s^-1. The calculations leading to this value were based on the multi-trapping transport model, the experimental drift mobility value of μ_e = 1 cm²V^-1s^-1, the tail state distribution suggested by field effect data and an assumed thermalisation depth of the electrons of 0.29eV. In the following it will be shown from experimental results that the above thermalisation depth is fundamentally inconsistent with the observations and, when used with the measured micro_e, leads to unrealistically high calculated micro_c -values. We also show that with a thermalisation depth appropriate to the conditions under which micro_e is determined, the experimental results and the computed micro_c-values (10 - 20 cm²V^-1s^-1) form a consistent picture.     

The static quadrupole moment of the first excited state of 198Hg

The first direct evidence of oblate nuclear deformation in the even-A mercury isotopes has been obtained by measurement of the static quadrupole moment of the first excited state of ¹⁹⁸Hg using the reorientation effect in Coulomb excitation.     

Some new results on transport and density of state distribution in glow discharge microcrystalline silicon

The electronic properties of the material are discussed on the basis of the grain boundary trapping model proposed for polycrystalline Si. In spite of the difference in crystallite sizes it is shown that the model is applicable, leading to 10¹¹ cm^?2 filled states between crystallites. Gap state densities between 2 × 10¹⁸ and 4 × 10¹⁸cm^?3eV^?1 are deduced from field effect measurements.     

Data-Driven and Multiscale Modeling of DNA-Templated Dye Aggregates

Dye aggregates are of interest for excitonic applications, including biomedical imaging, organic photovoltaics, and quantum information systems. Dyes with large transition dipole moments (μ) are necessary to optimize coupling within dye aggregates. Extinction coefficients (ε) can be used to determine the μ of dyes, and so dyes with a large ε (>150,000 M⁻¹cm⁻¹) should be engineered or identified. However, dye properties leading to a large ε are not fully understood, and low-throughput methods of dye screening, such as experimental measurements or density functional theory (DFT) calculations, can be time-consuming. In order to screen large datasets of molecules for desirable properties (i.e., large ε and μ), a computational workflow was established using machine learning (ML), DFT, time-dependent (TD-) DFT, and molecular dynamics (MD). ML models were developed through training and validation on a dataset of 8802 dyes using structural features. A Classifier was developed with an accuracy of 97% and a Regressor was constructed with an R2 of above 0.9, comparing between experiment and ML prediction. Using the Regressor, the ε values of over 18,000 dyes were predicted. The top 100 dyes were further screened using DFT and TD-DFT to identify 15 dyes with a μ relative to a reference dye, pentamethine indocyanine dye Cy5. Two benchmark MD simulations were performed on Cy5 and Cy5.5 dimers, and it was found that MD could accurately capture experimental results. The results of this study exhibit that our computational workflow for identifying dyes with a large μ for excitonic applications is effective and can be used as a tool to develop new dyes for excitonic applications.