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Nanometer-sized particles of lead iodide layered semiconductors were embedded in Si02 films. X-ray diffraction and photoluminescence (PL) spectroscopy showed the preservation of the bulk layered structure and symmetry.The P L and PL excitation (PLE) spectroscopy exhibited a blue-shift due to quantum size effect. The bulk lead iodide showed exciton transition with an exceptionally small Bohr radius ( a g = 19 A) and a large Rydberg constant ( R = 70.8 meV). In addition, the lead iodide represents a special case in which me >> mh. The prepared samples contained particles with mean radii, a, in the range UB < a < 3 a~. In this limit (with me >> mh) the experimental results are in agreement with a model in which the electron is localized nearly at the center of the particle, enabling the hole to move around it. According to this model, the localization of the electron is further enhanced due to the confinement of the hole motion by the particle boundaries. Thus, the size confinement permits the creation of an acceptor-like exciton. The simulated blue-shift of this acceptor-like exciton has been determined, utilizing a variational method. The P L spectrum revealed additional states, associated with stoichiometric and structural defects. These defects were created during the growth process.

The photoluminescence (PL) spectrum of nanometer-sized particles of PbI2, at 77 K, is dominated by an acceptor-like exciton emission. When the specimens were irradiated with visible light at temperature of 70 "C, the exciton energy was blue-shifted and its intensity decreased. This blue shift is a manifestation of a quantum size effect, and it indicates the reduction of the particle's size. The changes in the PL spectra showed strong dependence on the radiation conditions. These results suggest that the irradiation produces chemical decomposition of Pb-I bonds. Moreover, retreatment of the particles with iodine vapors nearly regenerated the original PbIz particle's size

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Fluorescence detected magnetic resonance (FDMR) has been applied to approximately 25-nm-thick porphyrin films, containing ordered domains of zinc tetra-(p-octylphenyl)-porphyrin (ZnTOPP) spin-coated onto quartz slides. Illuminating the films at 1.4 K with 457.9-nm light from a continuous wave Ar(+) laser produces at least two different, Jahn-Teller-distorted, ZnTOPP triplet species, labeled i and ii. Microwave-induced magnetic resonance of i and ii in the absence or presence of an externally applied magnetic field affects the fluorescence intensity of ZnTOPP, thus allowing FDMR. For triplet species i, formed in films spin-coated from toluene solution, the zero-field splitting (ZFS) parameters were determined as |D| = (316.9 +/- 0.1) x 10(-4) cm(-1) and |E| = (32.0 +/- 0.5) x 10(-4) cm(-1). By exposure of the spin-coated films to chloroform vapor at room temperature, triplet i is converted into species ii, with |D| = (295 +/- 3) x 10(-4) cm(-1) and |E| = (121 +/- 3) x 10(-4) cm(-1). For the excited triplet state of ZnTOPP in a toluene glass, ZFS parameters with values of |D| = (295 +/- 1) x 10(-4) cm(-1) and |E| = (91 +/- 1) x 10(-4) cm(-1) are found. From a combined study of the FDMR- and microwave-induced fluorescence spectra, i and ii are identified as unligated and ligated ZnTOPP triplet species, respectively. From the asymmetrically shaped zero-field FDMR signals of i, we conclude that the local crystal field perturbations of the stacked molecules are anisotropic. The FDMR results of the ZnTOPP films are compared with those for a film of zinc tetraphenylporphyrin (ZnTPP), which lacks the octyl substituents, and therefore is nonordered. Upon illumination, the ZnTPP films contain only a single, ligated, triplet species with ZFS parameters very similar to those of ligated ZnTOPP. At approximately 5 K, the lifetime of triplet i is considerably shortened compared to that of ZnTOPP in a glass at the same temperature.