The open-circuit photovoltage Vx of a photoelectrochemical cell equipped with a platinum-coated n-Si semiconductor electrode remarkably increased when the Pt layer was made microscopically discontinuous. A strikingly high Vx of 0.685 V has been obtained for a Pt-coated and alkali-etched n-Si electrode in which Pt existed in the form of islands 5-20 nm wide. The Vx is much higher than those for normal p-n-junction Si solid solar cells (about 0.59 V). This is an encouraging result, which possibly makes a novel approach to highly efficient solar cells. Theoretical considerations on the effect of the discontinuity of the metal overlayers in metal-coated electrodes have shown that high photovoltages can be generated in cases where the metal exists in the form of extremely small, sparsely scattered islands, say about 5 nm wide, separated by about 20 nm from each other, even though no increase in the intrinsic barrier height at the metal-island/semiconductor interface is assumed to occur. The effective barrier height at the semiconductor-liquid junction can increase to the equivalent of the band gap by choosing appropriate redox couples due to the rapid decay of the potential modulation near the metal-island/semiconductor interface. In silicon electrodes covered with minute metal islands, the naked parts of the silicon surface are covered with insulating thin oxide layers, and the metal-covered parts serve as gates for the carrier transport. Accordingly, the saturation current can become quite low because most of the silicon surface is covered with the oxide layer, producing only low densities of intragap states, and the surface recombination at these parts becomes negligibly slow.
To investigate the frequency of heterogeneity among the multiple 16S rRNA genes within a single microorganism, we determined directly the 120-bp nucleotide sequences containing the hypervariable α region of the 16S rRNA gene from 475 Streptomyces strains. Display of the direct sequencing patterns revealed the existence of 136 heterogeneous loci among a total of 33 strains. The heterogeneous loci were detected only in the stem region designated helix 10. All of the substitutions conserved the relevant secondary structure. The 33 strains were divided into two groups: one group, including 22 strains, had less than two heterogeneous bases; the other group, including 11 strains, had five or more heterogeneous bases. The two groups were different in their combinations of heterogeneous bases. The former mainly contained transitional substitutions, and the latter was mainly composed of transversional substitutions, suggesting that at least two mechanisms, possibly misincorporation during DNA replication and horizontal gene transfer, cause rRNA heterogeneity.
The morphological effects of micafungin, a member of a new class of antifungal agents candins, on growing hyphae of Aspergillus fumigatus were studied by the use of Nomarski microscopy, scanning electron microscopy and thin-section electron microscopy. Micafungin at concentrations of 0.001-0.1 microg ml(-1) strongly inhibited the in vitro growth of this fungus and induced striking changes in the hyphal morphology, depending on the drug concentration and the length of the incubation period. The changes observed included increased formation of branches on the lateral walls, disruption of the tips of both hyphal cells and branches, and crushing and collapse of whole hyphae. In addition, micafungin was also effective in damaging membranous structures, including disruption of the cell membrane, partial loss of nuclear membranes and expansion of endoplasmic reticula. From these results, it was concluded that micafungin primarily affected the normal formation of cell walls and septa of growing hyphae. The inhibition of apical growth of hyphae accompanied by excessive lateral branching that was followed by disruption of both hyphal tips and branch tips ultimately led to the destruction of whole hyphae. This cytological effect of micafungin on the hyphal growth of A. fumigatus can explain its potent anti-Aspergillus activity.
The open-circuit photovoltage (Vx) generated at an n-Si electrode coated with a microscopically discontinuous platinum layer in a redox solution is increased by changing the redox potential of the redox couple toward the positive, until it reaches about 0.5 V, much higher than the Vx (ca. 0.2 V) for an n-Si electrode coated with a continuous Pt layer. These results verify a theory proposed by us on the discontinuous metal coated semiconductor electrodes and open up a new approach to efficient photoelectrochemical solar cells.
We simultaneously measured the force and tunneling current in three-dimensional (3D) space on the Si(111)-(7 × 7) surface using scanning force/tunneling microscopy at room temperature. The observables, the frequency shift and the time-averaged tunneling current were converted to the physical quantities of interest, i.e. the interaction force and the instantaneous tunneling current. Using the same tip, the local density of states (LDOS) was mapped on the same surface area at constant height by measuring the time-averaged tunneling current as a function of the bias voltage at every lateral position. LDOS images at negative sample voltages indicate that the tip apex is covered with Si atoms, which is consistent with the Si-Si covalent bonding mechanism for AFM imaging. A measurement technique for 3D force/current mapping and LDOS imaging on the equivalent surface area using the same tip was thus demonstrated.
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