Department of Physics
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Item Surface Structure Of Ale-grown Polycrystalline Calcium Sulfide Thin Films Studied By Afm(Materials Research Society of Korea, 1998) Dey, SrijataItem Laser-Assisted Scanning Tunneling Microscopy Studies of Thin Ordered Molecular Layers(Springer, 1995) Dey, SrijataThe irradiation of the junction of an STM with laser light may give access to the study of resonant effects. Since high laser intensities are required, an active compensation setup, designed to suppress the dominating nonresonant effects is used. Mixed films of the dye perylene-tetracarboxylic-dianhydride (PTCDA) and the liquid crystal octylcyanobiphenyl (8CB) were prepared on graphite and MoS2. STM measurements show small isolated domains of PTCDA embedded in 8CB. The islands could be imaged with high resolution even under the illumination with intense laser light. First experiments investigating the laser-induced contrast observed on the dye islands under active compensation of the nonresonant effects are discussed.Item STM studies on dye molecules embedded in ordered liquid crystal structures and an approach for laser-assisted scanning tunneling microscopy(SPIE, 1993-06) Dey, SrijataScanning tunneling microscopy, although capable of yielding very high resolution on periodic structures, very often provides only moderate resolution on singular features. Our work aims at the use of laser light to improve the identification of individual molecules. We report on scanning tunneling microscopy measurements performed on dye molecules dissolved in a liquid crystal and adsorbed onto highly oriented pyrolytic graphite. Either localized perturbations of the liquid crystal structure with the size of single molecules or more or less extended ordered domains of well resolved dye molecules were reproducibly imaged for several dyes. To study light-induced resonant effects the influence of non-resonant absorption leading to thermal expansion of tip and sample has to be suppressed. Therefore, an electro- optical system was realized using an ArPLU- and a dye laser of different wavelengths power-modulated with a relative phase shift of 180 degree(s). Preliminary results obtained with this setup are presented documenting the efficiency of the compensation.Item An ultrahigh‐vacuum system for STM studies(AIP, 1994) Dey, SrijataAn ultrahigh‐vacuum (UHV) system for STM studies is described where a home‐built STM is combined with a novel facility for in situ exchange of samples and tips. Technical details concerning the design of the sample and tip holders and the manipulation and storage equipment are given. For the preparation of organic films a compact evaporation source comprising two ovens with an excellent thermal insulation was constructed. First STM results of a coronene film on graphite and of tungsten diselenide are presented.Item Scanning tunneling microscopy studies of diazo dye monolayers on HOPG(Elsevier, 1995-05) Dey, SrijataWe report on scanning tunneling microscopy (STM) studies of monolayers of the diazo dye 4-[4-(N,N-dimethylamino)phenylazo]azobenzene (D2, summation formula C20H19N5) on the basal plane of highly oriented pyrolytic graphite (HOPG). Monolayers of the dye were prepared by vapour deposition or by dissolving the molecules in the liquid crystal octylcyanobiphenyl (8CB). The STM images show a double-row structure exhibiting different types of lattice defects and various domains. High-resolution images allow the identification of individual molecules and the observation of intramolecular contrast. The different orientations of the rows can be explained by a commensurate registry of the molecules with the substrate. A model for the unit cell is proposed.Item Contrast mechanisms in photothermal scanning tunneling microscopy(Springer, 1994-08) Dey, SrijataBy irradiation of the tunneling junction of a scanning tunneling microscope with intensity-modulated laser light a gap-width modulation due to thermal expansion of tip and sample was produced. Photothermal images were obtained by spatial mapping of the resulting modulation of the tunneling current or its logarithm. The various mechanisms responsible for the observed contrast are discussed quantitatively. In case of a highly corrugated gold film on mica the contrast arises mainly from either the current variations caused by the non-zero reaction time of the current control loop or from a geometry factor. In both cases the images reflect certain properties of the sample topography. On the other hand, for a liquid-crystal film adsorbed on graphite a contrast on a molecular scale was found which is attributed to variations of the effective barrier height.Item Zinc sulfide and terbium-doped zinc sulfide films grown by traveling wave reactor atomic layer epitaxy(IAEA, 1998) Dey, SrijataZinc sulfide (ZnS) and terbium-doped ZnS (ZnS:Tb) thin films were grown by traveling wave reactor atomic layer epitaxy (ALE). In the present work, ZnCl2, H2S, and tris (2,2,6,6-tetramethyl-3,5-heptandionato) terbium (Tb(tmhd)3) were used as the precursors. The dependence of crystallinity and Cl content of ZnS films was investigated on the growth temperature. ZnS and ZnS:Tb films grown at temperatures ranging from 400 to 500 .deg. C showed a hexagonal-2H crystalline structure. The crystallinity of ZnS film was greatly enhanced as the temperature increased. At growth temperatures higher than 450.deg.C, the films showed preferred orientation with mainly (002) diffraction peak. The Cl content decreased from approximately 9 to 1 at.% with the increase in growth temperature from 400 to 500 .deg. C. The segregation of Cl near the surface region and the incorporation of O from Tb(tmhd)3 during ALE process were also observed using Auger electron spectroscopy. The ALE-grown ZnS and ZnS:Tb films revealed very uniform thickness and smooth surface morphology in the observation using atomic force microscopy and transmission electron microscopyItem AFM studies of polycrystalline calcium sulfide thin films grown by atomic layer deposition(Elsevier, 1999-04) Dey, SrijataPolycrystalline CaS thin films were grown on Al2O3 films deposited on Si-wafer using the atomic layer deposition (ALD) technique. The surface structure of these films was studied by AFM and compared with respective SEM images. The polycrystalline film surfaces comprise regular shaped crystallites. First report of a possible growth mechanism is presented, on studying the variation of morphological features (i.e., roughness and size of crystallites) with thickness and growth rate.Item Scanning tunneling microscopy of liquid crystals, perylene‐tetracarboxylic‐dianhydride, and phthalocyanine(AIP, 1994-05) Dey, SrijataScanning tunneling microscopy measurements conducted on pure liquid crystals and liquid crystals in combination with the dyes perylene‐tetracarboxylic‐dianhydride (PTCDA) and phthalocyanine (Pc), all adsorbed on highly oriented pyrolytic graphite (HOPG), are being reported on. The liquid crystal octylcyanobiphenyl (8CB) was used as a solvent for the dyes, allowing the preparation of ordered dye layers in a very simple way. A combination of vapor deposition of PTCDA and liquid deposition of 8CB provided films, where 8CB and PTCDA are embedded in each other on a small length scale. The samples were prepared in two different ways yielding either 8CB islands within a PTCDA monolayer or PTCDA islands surrounded by 8CB. The Pc films on HOPG exhibit a hexagonal molecular lattice that could be related to the lattice of the graphite substrate and for which a structural model is proposed. For dodekylcyanobiphenyl (12CB) on HOPG the ordering in unit cells containing 10 rather than 8 molecules is reported for the first time.