Piezoelectric scattering limited mobility characteristics of a degenerate surface layer in compound semiconductors at low lattice temperatures

The theory is developed for piezoelectric scattering rate of carriers in a degenerate surface layer under the condition of low temperature when the approximations of the well-known traditional theory are not valid. The scattering rates thus obtained are then used to estimate the zero-field mobility characteristics for the surface layers under similar condition of low temperature. The results for the surface layers in GaAs and ZnO show that when one takes into account either the degeneracy of the carrier ensemble or the finite energy of the phonons or both, the energy dependence of the scattering rates changes significantly from what follows for a non-degenerate ensemble or from the traditional theory, where one makes use of the high-temperature approximation and thus assumes equipartition law for the phonon distribution, and neglects the phonon energy in the energy balance equation of the electron–phonon system. It is observed that the zero-field mobility characteristics that follow from these scattering rates are interesting in that they are quite different from what turns out either for a non-degenerate ensemble or in the high-temperature approximations.     

Approximation of the Posterior Distribution in a Change-Point Problem

We consider a family of models that arise in connection with sharp change in hazard rate corresponding to high initial hazard rate dropping to a more stable or slowly changing rate at an unknown change-point . Although the Bayes estimates are well behaved and are asymptotically efficient, it is difficult to compute them as the posterior distributions are generally very complicated. We obtain a simple first order asymptotic approximation to the posterior distribution of . The accuracy of the approximation is judged through simulation. The approximation performs quite well. Our method is also applied to analyze a real data set.     

Metal-free inorganic ligands for colloidal nanocrystals: S2-, HS-, Se2-, HSe-, Te2-, HTe-, TeS3(2-), OH-, and NH2- as surface ligands

All-inorganic colloidal nanocrystals were synthesized by replacing organic capping ligands on chemically synthesized nanocrystals with metal-free inorganic ions such as S(2-), HS(-), Se(2-), HSe(-), Te(2-), HTe(-), TeS(3)(2-), OH(-) and NH(2)(-). These simple ligands adhered to the NC surface and provided colloidal stability in polar solvents. The versatility of such ligand exchange has been demonstrated for various semiconductor and metal nanocrystals of different size and shape. We showed that the key aspects of Pearson's hard and soft acids and bases (HSAB) principle, originally developed for metal coordination compounds, can be applied to the bonding of molecular species to the nanocrystal surface. The use of small inorganic ligands instead of traditional ligands with long hydrocarbon tails facilitated the charge transport between individual nanocrystals and opened up interesting opportunities for device integration of colloidal nanostructures.     

Application of scanning calorimetry to estimate soil organic matter loss after fires

The severe heating of soil during wildfires and prescribed burns may result in adverse effects on soil fertility due to organic matter loss. No rapid and reliable procedure exists to evaluate soil organic matter (SOM) losses due to heating. Enthalpy of SOM combustion correlates with organic matter content. Quartz is a ubiquitous mineral in soils and has a remarkably constant composition and reversible α–β phase transition at 575 °C. We suggest that SOM content in heated and unheated soils can be compared using the ratio of SOM combustion enthalpy on heating to the β–α quartz transition enthalpy measured on cooling of the same sample. This eliminates the need to dry and weigh the samples, making possible field applications of the proposed method. The feasibility of using the (ΔH _{comb SOM})/(ΔH _β–α Qz) ratio was established with experiments on soil samples heated in the laboratory and the method was then used for evaluation of SOM loss on two pile burn sites at UC Berkeley’s Blodgett Forest Research Station in Georgetown, California.     

A Micellar Complex of a Conjugated Polyelectrolyte for Efficient FRET to Dye‐Labeled DNA

A polymer–surfactant micellar complex has been studied as a fluorescence resonance energy transfer (FRET) donor to fluorescein‐labeled DNA (ssDNA‐Fl). In water, the molar absorptivity and fluorescence quantum efficiency of cationic poly(fluorene‐co‐phenylene) (c‐PFP) are substantially increased in the presence of non‐ionic surfactants. A TEM microscopic study shows the formation of a nanowire micellar complex of c‐PFP and the surfactants. About a 400% enhancement of the FRET signal is measured in c‐PFP/ssDNA‐Fl with Brij 30, relative to that without surfactants. The signal amplification is successfully modulated using different types of non‐ionic surfactants which perturb the complexation, fine‐structure of the complex (i.e., donor‐acceptor separation), and the resulting energy transfer process.

     

MWCNTs-ZrO2 as a reusable heterogeneous catalyst for the synthesis of N-heterocyclic scaffolds under green reaction medium

A simple, efficient, and facile heterogeneous multi-walled carbon nanotubes-zirconia nanocomposite (MWCNTs-ZrO₂) has been synthesized using natural feedstock coconut juice (água-de-coco do Ceará). The synthesized catalyst was characterized by Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy, and X-ray photoelectron spectroscopy analysis. The heterogeneous nanocomposite has been used for one-pot synthesis of various N-heterocyclic compounds like pyrazoles, 1,2-disubstituted benzimidazoles, 2-arylbenzazoles, and 2,3-dihydroquinazolin-4(1H)-ones under green reaction medium at room temperature. This novel method has several advantages, such as short reaction time, simple work-up, excellent yield, and green reaction conditions. The catalyst was recycled up to four times without significant loss in catalytic activity.     

Synthesis,Crystal Structure and Thermal Behavior of Gadolinium Dicyanamide Dihydrate Gd[N(CN)2]3 · 2 H2O

Gadolinium dicyanamide dihydrate Gd[N(CN)₂]₃ · 2 H₂O was prepared by ion exchange in aqueous solution followed by evaporation of the solvent at room temperature. Gd[N(CN)₂]₃ · 2 H₂O was characterized by single‐crystal structure analysis, FTIR spectroscopy and DSC analysis. In the crystal there are three crystallographically independent [N(CN)₂]^? ions and Gd³⁺ which are coordinated by six N atoms from six different [N(CN)₂]^? ions and two O atoms from two water molecules forming an irregular quadratic antiprism. Four H bonds have been identified in the structure of Gd[N(CN)₂]₃ · 2 H₂O, two of them running to terminal N atoms and two to the bridging N atoms of dicyanamide ions (Gd[N(CN)₂]₃ · 2 H₂O: P2₁/n (no. 14), a = 7.4845(15) Å, b = 11.529(2) Å, c = 13.941(3) Å, β = 93.98(3)°, Z = 4, 1948 reflections, 175 parameters, R1 = 0.0493). The DSC analysis indicates that Gd[N(CN)₂]₃ · 2 H₂O looses the crystal water at temperatures around 130 – 140 °C forming anhydrous Gd[N(CN)₂]₃, the structure of which has been refined by the Rietveld method based on X‐ray powder diffraction data. Gd[N(CN)₂]₃ was found to be isotypic with Ln[N(CN)₂]₃ (Ln = La, Ce, Pr, Nd, Sm and Eu) which previously have been described in the literature.     

Modulation of the lowest metal-to-ligand charge-transfer state in [Ru(bpy)2(N-N)]2+ systems by changing the N-N from hydrazone to azine: photophysical consequences

Two Ru(II) complexes, [Ru(bpy)2L](ClO4)2 (1) and [Ru(bpy)2L'](BF4)2 (2), where bpy is 2,2'-bipyridine, L is diacetyl dihydrazone, and L' 1:2 is the condensate of L and acetone, are synthesized. From X-ray crystal structures, both are found to contain distorted octahedral RuN(6)(2+) cores. NMR spectra show that the cations in 1 and 2 possess a C2 axis in solution. They display the expected metal-to-ligand charge transfer (1MLCT) band in the 400-500 nm region. Complex 1 is nonemissive at room temperature in solution as well as at 80 K. In contrast, complex 2 gives rise to an appreciable emission upon excitation at 440 nm. The room-temperature emission is centered at 730 nm (lambda(em)(max)) with a quantum yield (Phi(em)) of 0.002 and a lifetime (tau(em)) of 42 ns in an air-equilibrated methanol-ethanol solution. At 80 K, Phi(em) = 0.007 and tau(em) = 178 ns, with a lambda(em)(max) of 690 nm, which is close to the 0-0 transition, indicating an 3MLCT excited-state energy of 1.80 eV. The radiative rate constant (5 x 10(4) s(-1)) at room temperature and 80 K is almost temperature independent. From spectroelectrochemistry, it is found that bpy is easiest to reduce in 2 and that L is easiest in 1. The implications of this are that in 2 the lowest (3)MLCT state is localized on a bpy ligand and in 1 it is localized on L. Transient absorption results also support these assignments. As a consequence, even though 2 shows a fairly strong and long-lived emission from a Ru(II) --> bpy CT state, the Ru(II) --> L CT state in 1 shows no detectable emission even at 80 K.     

Metallo-organic domino reactions: C-H versus C-C bond breaking

HL and MeL are prepared by condensing benzil dihydrazone with 2-formylpyridine and 2-acetylpyridine, respectively, in 1:2 molar proportions. While in a reaction with [Ru(C(6)H(6))Cl(2)]2, HL yields the cation [Ru(C(6)H(6)){5,6-diphenyl-3-(pyridin-2-yl)-1,2,4-triazine}Cl]+, MeL gives the cation [Ru(C(6)H(6))(MeL)Cl]+. Both the cations are isolated as their hexafluorophosphate salts and characterised by X-ray crystallography. In the case of HL, double domino electrocyclic/elimination reactions are found to occur. The electrocyclic reaction occurs in a C=N-N=C-C=N fragment of HL and the elimination reaction involves breaking of a C-H bond of HL. Density functional calculations on model complexes indicate that the identified electrocyclic reaction is thermochemically as well as kinetically feasible for both HL and MeL in the gas phase. For a double domino reaction, similar to that operative in HL, to occur for MeL, breaking of a C-C bond would be required in the elimination step. Our model calculations show the energy barrier for this elimination step to be much higher (329.1 kJ mol(-1)) for MeL than that for HL (96.3 kJ mol(-1)). Thus, the domino reaction takes place for HL and not for MeL. This accounts for the observed stability of [Ru(C(6)H(6))(MeL)Cl]+ under the reaction conditions employed.     

Accumulation of radiocesium by <Emphasis Type="Italic">Pleurotus citrinopileatus</Emphasis> species of edible mushroom

Pleurotus citrinopileatus, a species of edible mushrooms, is widely accepted food component, especially in Indian subcontinent. The accumulating susceptibility of this edible mushroom species towards long-lived radioisotopes of cesium was studied in controlled laboratory condition using the ¹³⁴Cs (T _1/2 = 2.06 y) radioisotope. It was observed that the experimental mushroom species accumulated ¹³⁴Cs and maximum accumulation took place in the cap portion. The pileus (cap)/stipes (stem) ratio of each ¹³⁴Cs accumulated mushroom sample was determined and found 2.22±0.74. The protein and fat fractions of the experimental mushroom species were extracted separately after accumulation of radiocesium and it was found that most of the radiocesium accumulation occurred in the protein fraction of the mushroom. The mushroom Pleurotus citrinopileatus which is white in color, turned completely black after radiocesium accumulation. The black mushroom so obtained was produced upto fourth generation by tissue culture method without using any radiocesium further. All the successors were found to be black indicating a permanent mutation of the mushroom species.