Differential expression of adenylate kinase isozymes in human tissues

Abstract

The discovery of strong visible photoluminescence (PL) in porous silicon (Si) has kindled a broad range of experimental and theoretical investigations into the luminescence mechanism. In spite of the vast research efforts, however, there yet remain unresolved problems in porous silicon. In this work, using a series of porous Si samples, we have characterized their optical and electrical properties by measuring PL, Raman, dark and photoconductivity, and photoinduced current transient spectroscopy (PICTS). It is found that visible PL peaks shift to higher energies and confined phonon spectra appear with increasing porosity of these materials, as supported by the quantum confinement of carriers and phonons in nano-sized porous Si crystallites. To be more specific about the influence of porosity, we have also investigated the transport mechanism of charge carriers by the combined measurements of dark and photoconductivity of free-standing porous Si. In a sample of porosity ~40%, prepared on p-type c-Si, we have found a persistent photoconductivity (PPC) which was thermally annealed at about 290 K. The relaxation of PPC shows a temperature dependence where the activation energies of the relaxation are found to be 70 ~ 100 meV. Further, it is of interesting to note, at least in this work, that the PPC was not noticeable in porous Si which was prepared on n-type c-Si. Because a metastable defect is expected to cause the PPC, we have carried out PICTS measurement and found a direct evidence of deep level defects near 0.5 eV below the conduction band only in the sample prepared on p-type c-Si. Photovoltage measurements show that these defects are electron trapping centers. By using an electron paramagnetic resonance (EPR) spectroscopy, we have identified the presence of $P_b$-like Si dangling bonds on the surface of as-prepared porous Si cystallites. In order to examine if these Si dangling bonds are involved in the creation of PPC, we have performed photo-EPR experim...