We have studied photoluminescence (PL) from porous Si anodized laterally along the length of the Si wafer. Broad PL peaks were observed with peak intensities at ∼640 to 720 nm. Strong PL intensity could be observed from 550 to 860 nm. Room-temperature peak intensities were within an order of magnitude of peak intensities of AlGaAs/GaAs multi-quantum wells taken at 4.2 K, and total intensities were comparable. A blue shift of peak intensities from ∼680 to 620 nm could be observed after thermal anneal at 500 °C in O2 and subsequent HF dip.
We demonstrate the applicability of thermal oxidation to control the photoluminescence (PL) from quantum-sized structures in porous silicon. Uniform photoluminescence samples with intense visible light observed under ultraviolet light at room temperature were quickly obtained without a long time hydrofluoric acid (HF) immersion. Applying different oxidation times or temperatures provides a very practical technique to control the luminescence color. By this way, we have observed a shift in the luminescence peak from 7600 to 6200 Å and a reduction in the spectral width from ∼1600 to ∼950 Å.
In this study, single-electron transistors and memory cells with Au colloidal islands linked by C60 derivatives have been fabricated by hybridization of top–down advanced electron-beam lithography and bottom–up nanophased-material synthesis techniques. Low-temperature transport measurements exhibit clear Coulomb-blockade-type current–voltage characteristics and hysteretic-type gate-modulated current. The hysteresis is attributed to the presence of electrically isolated charge–storage islands. With the guidance provided by Monte Carlo simulation, we propose a circuit model and give an estimate of the sample parameters.
The cobaltic trifluoroacetate oxidation of mesitylene in a rapid mixing flow system resulted in the formation of the mesitylene dimer radical cation. The hyperfine splitting constants obtained from the ESR spectrum indicated the removal of orbital degeneracy in the dimer radical cation by a cooperative intermolecular vibronic effect.
Electron spin resonance spectra were obtained for the radical cations of the nonalternant hydrocarbons, azulene and 4,6,8-trimethylazulene. Coupling constants were determined and compared with those predicted by valence bond and various molecular orbital calculations. The experimental results were clearly more consistent with the molecular orbital approaches which satisfactorily predicted the correct symmetries and relative spin densities, whereas the valence bond method did not. Splitting constants calculated directly by the INDO method agreed quite well with the observed values.
We have developed a new, minimal damage approach for examination of luminescent porous Si (PS) layers by transmission electron microscopy (TEM). In this approach, chemically etched (CE) PS layers are fabricated after conventional plan-view TEM sample preparation. Our TEM studies show that crystalline, polycrystalline, and amorphous phases exist in the same CE sample. The microstructure is believed to gradually change from crystalline to amorphous during chemical etching in a HF-HNO3-H2O solution. The microcrystallites in the polycrystalline region are estimated to be 15–100 Å, while the pore size is on the order of 400 Å.
We have studied the photoluminescence (PL), structure, and composition of laterally anodized porous Si. Broad PL peaks were observed centered between ∼620–720 nm with strong intensities measured from 500 to 860 nm. Macroscopic variations in PL intensities and peak positions are explained in terms of the structure and anodization process. Structural studies suggest that the PL appears to originate from a multilayered porous Si structure in which the top two layers are amorphous. X‐ray diffraction spectra also suggest the presence of a significant amorphous phase. In addition to high concentrations of B and N, we have measured extremely high concentrations (≫1020 cm−3) of H, C, O, and F. Our results indicate that laterally anodized porous Si does not fit the crystalline Si quantum wire model prevalent in the literature, suggesting that some other structure is responsible for the observed luminescence.
A numerical investigation was conducted to determine the effect of bleed on oblique shock wave/turbulent boundary layer interactions. The numerical solution to the compressible Navier-Stokes equations reveal the flow details throughout the interaction zone and inside the normal bleed slot. Results are presented for an incident oblique shock of sufficient strength to cause boundary layer separation in the absence of bleed. Bleed is applied across the shock impingement location over a range of bleed mass flow rates corresponding to different values of plenum pressures. The results indicate a complex flow structure with large variations in both normal and tangential flow velocities across the bleed slot. The flow entrainment into the slot is accompanied by an expansion-compression wave system with a bow shock originating inside the bleed slot. Increasing the bleed mass flow by decreasing the plenum pressure caused an initial decrease then a later increase in the boundary layer momentum and displacement thickness downstream of the interaction.
The electron spin resonance spectra of several previously unobserved alkyl- and cycloalkyl-aromatic radical cations were obtained in a flow system, and the ratio of the methylene hyperfine splittings to the corresponding methyl hydrogen splittings was measured. These ratios, which ranged from 0.46 to 1.79, were rationalized on the basis of the preferred geometrical conformations of these molecules.
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