Department of Physics
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Item Influence of substrate domain boundaries on surface reconstructions of Ga/Si(1 1 1)(Elsevier, 2004-03) Gangopadhyay, SubhashisThe temperature induced phase transition of the Ga/Si(1 1 1) surface for submonolayer Ga coverages has been monitored by variable temperature scanning tunneling microscopy. After room temperature deposition of about 1/3 monolayer of Ga on Si(1 1 1), the Si(1 1 1) surface is mostly covered with Ga-induced magic clusters in a 7 × 7 like arrangement whereas at the domain boundary regions of the former Si(1 1 1)-7 × 7 reconstruction an increased density of excess Ga islands is found. The magic clusters are stable against annealing up to 350 °C. At this temperature, however, the Ga-islands coalesce and a mixture of and 6.3 × 6.3 structures is formed at the domain boundary regions. At an annealing temperature of 400 °C a phase transformation of the structure to 6.3 × 6.3 is found at the domain boundary region opposite to the usual thermal phase sequence. This can be explained in terms of an interplay of surface kinetics and surface stress.Item Surface oxidation of GaN(0001): Nitrogen plasma-assisted cleaning for ultrahigh vacuum applications(AIP, 2014) Gangopadhyay, SubhashisThe cleaning of metal-organic vapor-phase epitaxial GaN(0001) template layers grown on sapphire has been investigated. Different procedures, performed under ultrahigh vacuum conditions, including degassing and exposure to active nitrogen from a radio frequency nitrogen plasma source have been compared. For this purpose, x-ray photoelectron spectroscopy, reflection high-energy electron diffraction, and scanning tunneling microscopy have been employed in order to assess chemical as well as structural and morphological surface properties. Initial degassing at 600 °C under ultrahigh vacuum conditions only partially eliminates the surface contaminants. In contrast to plasma assisted nitrogen cleaning at temperatures as low as 300 °C, active-nitrogen exposure at temperatures as high as 700 °C removes the majority of oxide species from the surface. However, extended high-temperature active-nitrogen cleaning leads to severe surface roughening. Optimum results regarding both the removal of surface oxides as well as the surface structural and morphological quality have been achieved for a combination of initial low-temperature plasma-assisted cleaning, followed by a rapid nitrogen plasma-assisted cleaning at high temperature.Item 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 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 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.Item Scanning tunneling spectroscopy of thin films of carbon nanotubes(AIP, 2013) Gupta, Raj Kumar; Manjuladevi, V.The thin films of single walled carbon nanotubes (CNT) were deposited on indium tin oxide (ITO) glass substrates. The films were characterized using scanning tunneling microscopy and spectroscopy. The topographic images reveal a high degree of orientational order in the arrangement of bundles of CNT onto the ITO substrates. We found a significant narrowing of the band gap of the CNT films on ITO as compared to that of ITO film on glass substrate.