KCl Matrix Research Articles

Characterization of CdS nanocrystals embedded in KCl single crystal matrix grown by Czochralski method

Abstract The growth of CdS nanocrystals (NCs) embedded in bulk KCl single crystal matrix is performed using the Czochralski method. The X-ray diffraction reveals the incorporation of the CdS NCs with a cubic structure inside the KCl matrix. The optical density measurements of the CdS NCs embedded in KCl single crystal show a shift of the absorption edge towards higher energies. The optical band-gap is estimated to be about of 2.60 eV. The photoluminescence (PL) spectrum of the studied samples presents four emission bands in the range of 2.20–2.56 eV.

Influence of thermal and field stress on trapped electrons in single and doped alkalihalides

Injection experiment has been performed to extract injection and photo-currents at constant field stress under different temperature ambient between 795 and 995 K in KCl, KBr and KBr doped KCl single crystals. The variation of the conductivity of injecting carriers with the color centers yielded activation energy value, which is found to have a threshold limit of doping with KBr in KCl matrix.

Silver halidemicro- and nanocrystals embedded in an alkali halide matrix:suppression of the Jahn-Teller effect in nanoparticles

Self-organized microcrystals and nanocrystals (quantumdots) of AgCl and AgBr embedded in KCl and KBr crystallinematrices and retaining the orientation of the host latticewere studied by optically detected magnetic resonance. It wasunambiguously shown that self-organized microcrystalline silverhalides can be grown inside alkali halide crystals with theproperties of bulk crystals since the optically detectedmagnetic resonance spectra of the embedded microcrystals werepractically the same or close to those in bulk AgCl and AgBrand could be used as a `fingerprint' for AgCl and AgBr. ForAgCl nanocrystals in a KCl matrix the anisotropy of theg-factor both for isolated self-trapped holes and forself-trapped holes forming self-trapped excitons was found tobe substantially reduced compared with those of bulk AgClcrystals. This implies a considerable suppression of theJahn-Teller (JT) effect in nanoparticles. A rather general mechanismof the suppression of the JT effect in nanocrystalsis proposed, taking into account the additional deformationfield appearing because of the strong vibronic interaction atthe interface. It was concluded that the distribution ofexchange interactions for electron-hole pairs and triplet excitons in the KBr:AgBr system is due to a distribution of AgBrcrystal sizes. The holes seem to be self-trapped in the AgBrbecause of the dynamical JT effect. The exchange splittingincreases for distant electron-hole pairs with a decrease ofAgBr size. The spectra with exchange splitting larger thanthat in bulk AgBr (1.9 cm-1) seem to belong to AgBr nanocrystals. In contrast to AgCl the wavelength of theluminescence in AgBr micro- and nanocrystals embedded in a KBrmatrix decreases with the decrease of AgBr crystal size.

Cathodoluminescence in Europium doped KCI crystals

Spectral cathodoluminescence of KCI:Eu2+ (0.2% wt) crystals has beenmeasured at different temperatures. In the 80—300K temperature range the luminescence consists of three main broad emission bands with maxima around 418, 452, and 619 nm. Based on photoluminescence studies the 418 nm could be associated with the 4f65d (t2g)-4f7 (8S7/2) radiative transition of the Eu2+ ions in the KCl matrix. The band with maximum at 452 nm could be associated with the α-centere emission as in pure KCl. The 619 nm band has been considered as a part of the luminescence modified by the absorption of the F centers formed during irradiation. The intensity of all these bands versus temperature shows a minimum around 150K, an indication of a low defect mobility produced with the electron-beam irradiation.

Orientedsilver chloride microcrystals and nanocrystals embedded in acrystalline KCl matrix, as studied by means of electron paramagneticresonance and optically detected magnetic resonance

Self-trapped holes (STH), self-trapped excitons (STE) and shallowelectron centres in small AgCl crystals embedded in a KClcrystalline matrix have been observed by means of optically detectedmagnetic resonance (ODMR). The existence of the impurity clustersin heavily doped KCl:AgCl single crystals, ranging from single andpaired Ag ions to AgCl nanometre- and micrometre-size crystals(nanocrystals and microcrystals) retaining the orientation of thematrix, was confirmed. ODMR spectra were used as a fingerprint ofthe embedded AgCl microcrystals and a signature of theircrystallinity. For AgCl nanocrystals the anisotropy of theg-factor both for isolated STH and for STH forming STE was foundto be substantially reduced compared with those of bulk AgClcrystals and AgCl microcrystals embedded in KCl. This implies aconsiderable suppression of the Jahn-Teller effect innanoparticles. A rather general mechanism of the suppression of theJahn-Teller effect in nanocrystals is developed, taking intoaccount the additional deformation field appearing because of thestrong vibronic interaction at the interface. It allows evaluationof the critical size of the embedded AgCl nanocrystals, at which thesuppression of the Jahn-Teller effect has its onset; the valueobtained is ≈10 nm, in agreement with experiment.

Photoluminescence spectra of ZnO particles embedded in thin alkali halide crystals

Sample specimens of ZnO particles embedded in thin alkali halide crystals (KI, KBr and KCl) have been prepared from their melts and photoluminescence spectra have been measured at low temperature. In cases of KI and KBr matrices, a bound exciton and a free exciton emission line appear clearly, and in KCl matrix, the emission lines are very sharp in comparison with the other cases. The sharpest lines in KCl matrix are only obtained during the time of a few minutes. The particles are kept in the melt above the melting temperature. These facts suggest that the use of the alkali halide melt and rapid solidification of the melt result in a proper etching of the particle surface.

Exciton self-trapping in AgCl nanocrystals

Self-trapping of excitons is reported in AgCl nanocrystals embedded in a crystalline KCl matrix. The particles, observed by atomic force microscopy, have radii of several nanometers. Due to the spatial confinement only recombination of the self-trapped exciton (STE) is observed. $\mathrm{STE}({\mathrm{Br}}^{\ensuremath{-}})$ and donor-acceptor pair recombination are absent. The time and temperature behavior of the emission is found to be significantly different from that in bulk AgCl. It is concluded that this is due to different self-trapped exciton configurations: The diffuse electron is spatially confined and centered in the nanocrystal whereas the compact hole, self-trapped at different lattice sites, causes variations in wave-function overlap in the different STEs. Optically detected magnetic resonance measurements reveal changes in g-values and a decrease by a factor of two in the magnitude of the zero-field splitting of the triplet state. They also demonstrate that the AgCl nanocrystal lattice axes are oriented along the axes of the KCl matrix.

Silver nanocrystals obtained by the ionization of Ag −-ions in KCl

This paper discusses the formation of “corals” arranged silver nanocrystals in a KCl matrix starting from Ag − ions (∼10 17 ions/cm 3) by subsequent thermal annealing at temperatures between 400–700°C and 250°C. In the last case only, by illumination with total radiation of a 100W Xe-lamp, spherical separated silver clusters were obtained. The stepwise annealing processes were followed by the recording of the optical absorption spectra at 20°C. The sizes of silver clusters were determined through transmission electron microscopy.

Octanol-, chloroform-, and propylene glycol dipelargonat-water partitioning of morphine-6-glucuronide and other related opiates.

The pKa and log P values of morphine-6-β-d-glucuronide (M6G) and morphine-3-β-d-glucuronide (M3G) and a range of structurally-related opiates (morphine, normorphine, codeine, norcodeine, 6-acetylmorphine, diacetylmorphine, and buprenorphine) were accurately measured using a potentiometric approach. The measured lipophilicity profiles (pH 2−11, 0.15 M KCl matrix) of M3G and M6G were compared using a proton donor solvent (chloroform) and a proton acceptor solvent (propylene glycol dipelargonate, PGDP), in addition to octanol. The log P values and lipophilicity profiles of M6G and M3G determined in octanol−water have confirmed the unexpectedly high lipophilicity of the two glucuronides. These results show the importance of measuring the effect of pH on lipophilicity, since log D (pH 7.4) values gave a notably different order of lipophilicity for the opiates compared with log P. M6G, but not M3G, showed significant differences in log P between different types of partitioning solvents. The observed order of li...

The determination of thallium with the stabilized temperature platform furnace and Zeeman background correction

Palladium, about 5–10 μg as Pd (NO 3) 2, is confirmed as an effective matrix modifier for Tl. A pyrolysis temperature of 1000°C can be used, which is a much higher temperature than could be used with the previously proposed matrix modifier, H 2SO 4. The recovery of Tl was quite good in the presence of large amounts of NaCl but was inadequate in the presence of KCl. Abandoning the pyrolysis step and the matrix modifier proved to be convenient for determining Tl in both an NaCl and a KCl matrix. Thallium was fully recovered in these matrices in the presence of about 30μg of either salt, or about 0.15%, assuming a 20μl sample.

Spectroscopic behaviour of Cr(CN) 3−6 ion isolated in KCl host

Vibrational, electron spin resonance (ESR), absorption and luminescence spectra of the Cr(CN) 3− 6 ion in a KCl matrix are reported. The results indicate the complex ion is trapped in three non-equivalent symmetries. However, the metal ion in the CrC 6 polyhedron possesses practically undistorted octahedral symmetry. Following the vibrational analysis the vibronic assignment of the phosphorescence spectrum is proposed.

Electromagnetic propagation in close-packed disordered suspensions.

Multiple-scattering theory is used to evaluate the effective dielectric function \ensuremath{\epsilon}(\ensuremath{\omega}) for disordered systems in which spherical inclusions are densely packed in a host medium. We compare the well-known quasicrystalline approximation (QCA) with Roth's effective-medium approximation (EMA). Both approaches are studied in the long-wavelength limit and, also, in the regime where the wavelength and particle diameter are comparable. In the long-wavelength case, the QCA reduces to the elementary Maxwell-Garnett formula. By contrast, the EMA is shown to include the effects of local-field fluctuations. The QCA and EMA are applied to two situations of experimental interest: (1) a strong-scattering system comprised of metal spheres embedded in a KCl matrix, and (2) a weak-scattering system made of pressed ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ particles. In both cases there are significant differences between the two approximation schemes and the EMA is generally to be preferred.

Low‐Temperature Photoluminescence of the Aggregate Centres in Eu2+‐Doped KCl Crystals

AbstractThe emission spectra and fluorescence lifetimes of different aggregation products of Eu2+ dipoles in KCl matrix are examined in the range 15 to 300 K. Two aggregation types are formed in dependence on the annealing temperature. The optical properties of the aggregates (i) and (ii) formed at RT and 125 °C are very similar to those characteristic of isolated dipoles. Above 125 °C both aggregates undergo decomposition. A third aggregation product is formed between 200 and 250 °C. Differently from (i) and (ii) the pertinent absorption and emission spectra do not show any vibronic structure. The effect of the above aggregation products on the fluorescence lifetime of isolated dipoles present in crystals is ascertained.

Manifestation of Spin–Rotation Coupling in Intersystem Crossing in a Rotating Impurity Molecule

AbstractThe dependence of the probability of the singlet‐triplet S1–T1 radiationless transition on the total angular momentum J is observed for an NO impurity molecule in the KCl matrix. This dependence disappears in the presence of a magnetic field (H ≧ 0.2 T), which breaks the spin‐rotation coupling.

Far-infrared absorption of small-palladium-particle composites

We use the recently derived size-distribution-dependent expression for the effective dielectric constant of a composite medium. Because of observed clustering of Pd particles we assume a broad size distribution for the small Pd particles embedded in KCl matrix. We obtain very good agreement between our theoretical calculation and experimental results. Values given by earlier theoretical calculations were by a factor of 10 below those supported by the measured data.

Analysis of Pb dopant distributions in KCl monocrystals by energy dispersive X‐ray spectrometry

AbstractThe energy dispersive X‐ray microanalysis in combination with the scanning electron microscopy was used for investigations of the microdistribution of Pb dopants in solution‐grown KCl crystals. In applying an absorption filter technique (using an about 30 μm thick aluminium foil arranged into the X‐ray path between the specimen and the detector) the resulting detectability of the Pb minor constituents is improved by a factor of about 3. The minimum detectable content of lead within the KCl matrix could be estimated to have the order of magnitude of 0.1 mole per cent Pb.

Circular Dichroism of [PtCl2(R)-pn] and [PtCl2(S)-pn] in Solution and in KCl Matrix.

Circular Dichroism of [PtCl2(R)-pn] and [PtCl2(S)-pn] in Solution and in KCl Matrix.

Critical Behavior of the Dielectric Constant of a Random Composite near the Percolation Threshold

Measurements are reported of a singularity in the dielectric constant of a random metal-insulator composite below the percolation threshold. The composite sample consists of small silver particles randomly dispersed throughout a KCl matrix. The dielectric constant is found to obey a scaling relation with a critical exponent of $s=0.73\ifmmode\pm\else\textpm\fi{}0.07$.