Browsing by Author "Kulju, Sampo"

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  • Atomic Structure of a Spinel-Like Transition Al2O3(100) Surface
    (2014) Jensen, Thomas N.; Meinander, Kristoffer; Helveg, Stig; Foster, Adam S.; Kulju, Sampo; Musso, Tiziana; Lauritsen, Jeppe V.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We study a crystalline epitaxial alumina thin film with the characteristics of a spinel-type transition Al2O3(100)surface by using atom-resolved noncontact atomic force microscopy and density functional theory. It is shown that the films are terminated by an Al-O layer rich in Al vacancies, exhibiting a strong preference for surface hydroxyl group formation in two configurations. The transition alumina films are crystalline and perfectly stable in ambient atmospheres, a quality which is expected to open the door to new fundamental studies of the surfaces of transition aluminas.
  • Real-space Wigner-Seitz Cells Imaging of Potassium on Graphite via Elastic Atomic Manipulation
    (2015-02-05) Yin, Feng; Koskinen, Pekka; Kulju, Sampo; Akola, Jaakko; Palmer, Richard E.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Atomic manipulation in the scanning tunnelling microscopy, conventionally a tool to build nanostructures one atom at a time, is here employed to enable the atomic-scale imaging of a model low-dimensional system. Specifically, we use low-temperature STM to investigate an ultra thin film (4 atomic layers) of potassium created by epitaxial growth on a graphite substrate. The STM images display an unexpected honeycomb feature, which corresponds to a real-space visualization of the Wigner-Seitz cells of the close-packed surface K atoms. Density functional simulations indicate that this behaviour arises from the elastic, tip-induced vertical manipulation of potassium atoms during imaging, i.e. elastic atomic manipulation, and reflects the ultrasoft properties of the surface under strain. The method may be generally applicable to other soft e.g. molecular or biomolecular systems.
  • Simple metal under tensile stress: Layer-dependent herringbone reconstruction of thin potassium films on graphite
    (2015-05-11) Yin, Feng; Kulju, Sampo; Koskinen, Pekka; Akola, Jaakko; Palmer, Richard E.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    While understanding the properties of materials under stress is fundamentally important, designing experiments to probe the effects of large tensile stress is difficult. Here tensile stress is created in thin films of potassium (up to 4 atomic layers) by epitaxial growth on a rigid support, graphite. We find that this "simple" metal shows a long-range, periodic "herringbone" reconstruction, observed in 2- and 3-(but not 1- and 4-) layer films by low-temperature scanning tunneling microscopy (STM). Such a pattern has never been observed in a simple metal. Density functional theory (DFT)simulations indicate that the reconstruction consists of self-aligned stripes of enhanced atom density formed to relieve the tensile strain. At the same time marked layer-dependent charging effects lead to substantial variation in the apparent STM layer heights.