Photostimulated luminescence of AgI clusters in zeolite-Y

AgI clusters in zeolite-Y (AgI/Y) were prepared by Ag+ exchange followed by reaction with NaI in solution. The formation of the clusters was determined by transmission electron microscopy and Auger electron spectroscopy. The clusters were uniform and even in size, 1.0-2.0 nm. The fluorescence spectrum of the clusters consists of two emission bands, which are attributed to AgI and Ag clusters, respectively. Photostimulated luminescence (PSL) is observed by stimulation at 675 or at 840 nm. The PSL spectrum of AgI/Y is consistent with the emission spectrum of Ag clusters and thus the PSL is considered to be caused by the charge transfer or carrier migration from the zeolite framework or from the AgI clusters to the Ag clusters. The appearance of PSL indicates that these materials may find application as a medium for erasable optical memory. (C) 1998 American Institute of Physics. [S0021-8979(98)02407-4].

Photoluminescence from Ge clusters embedded in porous silicon

Visible photoluminescence (PL) and Raman spectra of Ge clusters embedded in porous silicon (PS) have been studied. The as-prepared sample shows redshifted and enhanced room temperature PL relative to reference PS. This result can be explained by the quantum confinement effect on excitons in Ge clusters and tunnel of excitons from Si units of the PS skeleton to Ge clusters. One year storage in dry air results in a pronounced decrease in PL intensity but blue-shifted in contrast to reference PS. This phenomenon correlates to the size decrease of macerated Ce clusters and occurrence of "quantum depletion" in Ge clusters. Consequently, only excitons in Si units contribute to PL. (C) 1998 American Institute of Physics.

Preparation of I-2 clusters and their absorption spectra

Samples have been prepared at different temperatures by loading It molecules into the cages of zeolite 5A, and the measurements of the absorption spectra have been carried out for the prepared samples. It is shown that 12 molecular clusters are formed in the cages of zeolite 5A, and it is also found that molecular clusters which are bonded with intermolecular forces have an important feature, namely, the intermolecular distance in molecular clusters can be changed on different preparing conditions and the blue shift of absorption edges can not be as the criterion of forming molecular clusters.

Formation, structure and fluorescence of CdS clusters in a mesoporous zeolite

CdS clusters are formed in the pores of a mesoporous zeolite in which the size of the clusters may be adjusted. The size of the clusters increases as the CdS loading is increased. X-ray diffraction investigation shows that the lattice constants of the clusters contract upon increasing size. This contraction is attributed to an increase of the static pressure exercised by the zeolite framework as the clusters grow bigger. Both the excitonic and trapped emission bands are detected and become more intensive upon decreasing size. Three absorption bands appear in the photoluminescence excitation (PLE) spectra and they shift to the blue as cluster size decreases. Based on the effective-mass approximation, the three bands are assigned to the 1S-1S, 1S-1P and 1S-1D transitions, respectively. The size-dependence of the PLE spectra can also be explained. (C) 1997 Elsevier Science Ltd.

Packing LUT clusters with network flow programming

This paper describes a two-step packing algorithm for LUT clusters of which the LUT input multipliers are depopulated. In the first step, a greedy algorithm is used to search for BLE locations and cluster inputs. If the greedy algorithm fails, the second step with network flow programming algorithm is employed. Numerical results illustrate that our two-step packing algorithm obtains better packing density than one-step greedy packing algorithm.

Modeling and optimizing of high-concentration erbium-doped fiber amplifiers with consideration of ion-clusters

Starting from the modeling of isolated ions and ion-clusters, a closed form rate and power evolution equations for high-concentration erbium-doped fiber amplifiers are constructed. Based on the equations, the effects of the fraction of ion-clusters in total ions and the number of ions per cluster on the performance of high-concentration erbium-doped fiber amplifiers are analyzed numerically. The results show that the presence of the ion-clusters deteriorates amplifier performance, such as the signal power, signal gain, the threshold pump power for zero gain, saturated signal gain, and the maximum gain efficiency, etc. The optimum fiber length or other parameters should be modified with the ion-clusters being taken into account for the amplifiers to achieve a better performance. (c) 2007 Elsevier B.V. All rights reserved.

Quakes in solid quark stars

A starquake mechanism for pulsar glitches is developed in the solid quark star model. It is found that the general glitch natures (i.e., the glitch amplitudes and the time intervals) could be reproduced if solid quark matter, with high baryon density but low temperature, has properties of shear modulus mu(c) = 10(30-34) erg/cm(3) and critical stress sigma(c) = 10(18similar to24) erg/cm(3). The post-glitch behavior may represent a kind of damped oscillations. (C) 2004 Elsevier B.V. All rights reserved.

DISTRIBUTION FUNCTION OF A DRIFTING ELECTRON-GAS FOR GENERAL ENERGY-BAND STRUCTURES

The usual application of the Lei-Ting balance equation method for treating electron transport problems makes use of a Fermi distribution function for the electron motion relative to the center of mass. It is pointed out that this presumes the existence of a moving frame of reference that is dynamically equivalent to the rest frame of reference, and this is only true for electrons with a constant effective mass. The method is thus inapplicable to problems where electrons governed by a general energy-band dispersion E(k) are important (such as in miniband conduction). It is demonstrated that this difficulty can be overcome by introducing a distribution function for a drifting electron gas by maximizing the entropy subject to a prescribed average drift velocity. The distribution function reduces directly to the usual Fermi distribution for electron motion relative to the center of mass in the special case of E(k)=($) over bar h(2)\k\(2)/2m*. This maximum entropy treatment of a drifting electron gas provides a physically more direct as well as a more general basis for the application of the balance equation method.

Photoluminescence of ZnS clusters in zeolite-Y

Two obvious emissions are observed from the ZnS clusters encapsulated in zeolite-Y. The emission around 355 nm is sharp and weak, locating at the onset of the absorption edge. The band around 535 nm is broad, strong and Stokes-shifted. Both the two emissions shift to blue and their intensities firstly increase then decrease as the loading of ZnS in zeolite-Y or clusters size decreases. Through investigation, the former is attributed to the excitonic fluorescence, and the latter to the trapped luminescence from surface states. The cluster size-dependence of the luminescence may be explained qualitatively by considering both the carrier recombination and the nonradiative recombination rates. Four peaks appearing in the excitation spectra are assigned to the transitions of 1S-1S, 1S-1P, 1S-1D and surface state, respectively. The excitation spectra of the clusters do not coincide with their absorption spectra. The states splitted by quantum-size confinement are detected in the excitation spectra, but could not be differentiated in the optical absorption spectra due to inhomogeneous broadening. The size-dependence of the excitation spectra is similar to that of the absorption spectra. Both the excitation spectra of excitonic and of trapped emissions are similar, but change in relative intensity and shift in position are observed.