The passivation of n-GaAs in different sulphur containing solutions in which sulphur chemical activity was varied, i.e. solutions of sodium and ammonium sulphides in water and alcohols, as well as a solution of sulphur monochloride in carbon tetrachloride, was studied by photoluminescence and Raman spectroscopy. The increase of the sulphur chemical activity in the passivating solution results in an increase of the photoluminescence intensity and in a decrease of the surface barrier of the passivated semiconductor.
Epitaxial BiFeO3 films pulsed laser deposited on SrTiO3, Nb:doped SrTiO3, and DyScO3 were studied using variable angle spectroscopic ellipsometry, vacuum ultraviolet ellipsometry, micro-Raman spectroscopy, and x-ray diffraction. The energy band gap of the film deposited on DyScO3 is 2.75 eV, while the one for the film deposited on Nb:doped SrTiO3 is larger by 50 meV. The blueshift in the dielectric function of the BiFeO3 films deposited on Nb:doped SrTiO3 compared to the films deposited on DyScO3, indicates a larger compressive strain in the films deposited on Nb:doped SrTiO3. This is confirmed by Raman spectroscopy and by high resolution x-ray diffraction investigations.
This work is reporting the dielectric functions of DNA base films, i.e. guanine, adenine, cytosine, and thymine in the energy range from near-infrared to ultra-violet. Spectroscopic ellipsometry using synchrotron radiation was employed in situ on DNA base films grown on hydrogen terminated Si(111) surfaces under ultra-high vacuum conditions. The optical response of adenine and guanine films is described by a uniaxial model while in the case of thymine and cytosine films an isotropic model was employed. The imaginary part of the dielectric functions ε 2 of the DNA base films is compared with the electronic transitions of single molecules calculated using time-dependent density functional theory.
We report on the recent investigations of the surface of ferroelectric crystals with scanning force microscopy. In situ investigations are demonstrated for temperatures T even above the transition temperature Tc for triglycine-sulphate (TGS) and barium-titanate (BaTiO3) (Tc,TGS=49 °C, Tc,BaTiO3=126 °C. We directly observed transition from the ferroelectric into the paraelectric phase. Our inspection of the ferroelectric crystals TGS and BaTiO3 allowed the simultaneous investigation of topographic surface features and ferroelectric domain structure in these crystals. Ferroelectric domains are clearly discernible in the noncontact mode as well as the friction force mode of imaging. The domain wall width in TGS was measured to be smaller than 8 nm. Furthermore, contact force microscopy indicated the domain wall to be atomically flat, exhibiting no surface step. While image contrast in noncontact scanning force microscopy pictures is found to be a convolution of both topographic and ferroelectric forces on TGS samples, the tip/sample interaction in BaTiO3 is dominated by van der Waals forces.
Infrared spectroscopy is applied to the characterization of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) films grown with organic molecular beam deposition (OMBD) on hydrogen-passivated Si(111). Comparing these films with powder spectra, the films show a preferential orientation of the PTCDA molecules close to coplanar with the substrate surface. For deposition of PTCDA films using laser ablation at 1064 nm, the average orientation of the molecules is more random than in the OMBD-grown films, but we still find some degree of preferential order of the molecules with respect to the substrate surface. The experimental findings are compared to density functional calculations of the single molecule and the two crystalline phases.
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