| 1. |
- Åhlund, John, et al.
(författare)
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The adsorption of iron phthalocyanine on graphite : A scanning tunnelling microscopy study
- 2007
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 601:17, s. 3661-3667
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Tidskriftsartikel (refereegranskat)abstract
- Different adsorption phases of iron phthalocyanine (FePc) on highly oriented pyrolitic graphite (HOPG) have been characterized by scanning tunnelling microscopy (STM). Evaporation of FePc onto the graphite (0 0 0 1) surface, kept at room temperature, results in the formation of three-dimensional molecular islands.After annealing to 400 °C different two-dimensional features are identified, depending on the initial coverage. At low doses, domains with well defined boundaries have been observed, within which molecules tend to organise in chains. At higher coverage, islands exhibiting well-ordered densely-packed square or hexagonal molecular arrangement have been resolved. For the adsorption structures corresponding to one monolayer islands our results show that the molecules adsorb with the molecular plane parallel to the surface. The high resolution STM images allow us to resolve the orientation of single molecules and subsequently we suggest that the molecular monolayer is stabilized by van der Waals interactions. The characterization of the observed Moiré contrast and a comparison with other similar systems underlines the importance of the central metal in the molecule–molecule and molecule–substrate interactions, which govern the molecular adsorption geometry.
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| 2. |
- Ahlund, John, et al.
(författare)
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Molecular growth determined by surface domain patterns
- 2008
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 112:17, s. 6887-6890
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Tidskriftsartikel (refereegranskat)abstract
- The growth of iron phthalocyanine (FePc) on InSb(001) c(8 x 2) at submonolayer coverage has been investigated with scanning tunneling microscopy (STM). FePc adsorbs flat centered on the In rows both at 70 K and at room temperature (RT). However, the shapes of the two-dimensional molecular islands are fundamentally different; while the RT growth results in chainlike structures along the [I 10] direction, as already observed for other Pc's adsorbed on the same surface, the islands are prolonged along [110], i.e., perpendicular to the substrate rows, at 70 K. These observations are explained on the basis of a recently observed new surface phase at low temperature, resulting in structural domains on the surface. The molecular growth front follows the propagating domain boundary that freezes at low temperature.
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| 3. |
- Castleton, Christopher, 1969-, et al.
(författare)
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Managing the supercell approximation for charged defects in semiconductors : Finite-size scaling, charge correction factors, the band-gap problem, and the ab initio dielectric constant
- 2006
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 73:035215, s. 11-
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Tidskriftsartikel (refereegranskat)abstract
- The errors arising in ab initio density functional theory studies of semiconductor point defects using the supercell approximation are analyzed. It is demonstrated that (a) the leading finite size errors are inverse linear and inverse cubic in the supercell size and (b) finite size scaling over a series of supercells gives reliable isolated charged defect formation energies to around +-0.05 eV. The scaled results are used to test three correction methods. The Makov-Payne method is insufficient, but combined with the scaling parameters yields an ab initio dielectric constant of 11.6+-4.1 for InP. Gamma point corrections for defect level dispersion are completely incorrect, even for shallow levels, but realigning the total potential in real-space between defect and bulk cells actually corrects the electrostatic defect-defect interaction errors as well. Isolated defect energies to +-0.1 eV are then obtained using a 64 atom supercell, though this does not improve for larger cells. Finally, finite size scaling of known dopant levels shows how to treat the band gap problem: in < or = 200 atom supercells with no corrections, continuing to consider levels into the theoretical conductin band (extended gap) comes closest to experiment. However, for larger cells or when supercell approximation errors are removed, a scissors scheme stretching the theoretical band gap onto the experimental one is in fact correct.
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| 4. |
- Höglund, Andreas, et al.
(författare)
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Relative concentration and structure of native defects in GaP
- 2005
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Ingår i: Physical Review B. ; 72:19, s. 195213-
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Tidskriftsartikel (refereegranskat)abstract
- The native defects in the compound semiconductor GaP have been studied using a pseudopotential Density Functional Theory method in order todetermine their relative concentrations and the most stable charge states. The electronic and atomic structures are presented and the defect concentrations are estimated using calculated formation energies. Relaxation effects are taken into account fully and produce negative-Ucharge transfer levels for V\sS{P}{} and P\sS{Ga}{}. The concentration of V\sS{Ga}{} is in good agreement with the results of positron annihilation experiments. The charge transfer levels presented compare qualitatively well with experiments where available. The effect of stoichiometry on the defect concentrations is also described and is shown to be considerable.The lowest formation energies are found for P\sS{Ga}{+2} in p-type and V\sS{Ga}{-3} in n-type GaP under P-rich conditions, and for Ga\sS{P}{-2} in n-type GaP under Ga-rich conditions.Finally, the finite size errors arising from the use of supercells with periodic boundary conditions are examined.
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| 5. |
- Karlsson, P. G., et al.
(författare)
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Initial stages of ZrO2 chemical vapor deposition on Si(100)-(2x1) from zirconium tetra-tert-butoxide
- 2008
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 602:10, s. 1803-1809
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Tidskriftsartikel (refereegranskat)abstract
- The initial stages of chemical vapor deposition of ZrO2 from zirconium tetra-tert-butoxide (ZTB) on Si(100)-(2 x 1) have been studied by scanning tunnelling microscopy (STM) and synchrotron radiation excited photoelectron spectroscopy (PES). The STM images and core level (PES) spectra indicate that the predominant surface modifications induced by ZTB are due to silicon carbonization and formation of zirconium dioxide. The carbonization reaction leads to formation of subsurface carbon and two types of reconstructions are discussed: dimer vacancies and dimer vacancies in conjunction with a rotated surface Si-dimer. Indications for the formation of small amounts of zirconium silicide are also found. No evidence for silicon oxidation can be observed with PES, in contrast to the interface properties previously found after larger exposures to ZTB.
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| 6. |
- Nilson, Katharina, et al.
(författare)
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STM and XPS characterization of Zinc Phthalocyanine on InSb(001)
- 2008
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 602:2, s. 452-459
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Tidskriftsartikel (refereegranskat)abstract
- Zinc phthalocyanine (ZnPc) adsorbed on the InSb(0 0 1)-c(8 x 2) surface has been studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Coverages from sub-monolayer to monolayer (ML) have been investigated. The molecules form ordered structures on the reconstructed rows of the surface with the molecular plane parallel to the surface. A change in the electronic structure between the sub-ML and higher coverages has been observed. Moreover, in order to study the influence of annealing on the electronic and geometric structures, the samples have been heated to elevated temperatures (about 640 K). In addition, multi-layer ZnPc films have been characterized by XPS measurements.
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| 7. |
- Ekström, Mikael, et al.
(författare)
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Antiferromagnetism in Zn-doped La2CuO4 as observed by muon spin resonance spectroscopy
- 2001
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 0163-1829. ; 64:18, s. 1845221-1845226
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Tidskriftsartikel (refereegranskat)abstract
- The local fields seen by positive muons implanted in Zn-doped La2CuO4 show a distribution with a main peak attributed to muon sites far from the Zn ions and a satellite structure corresponding to muons residing closer to the Zn. The temperature dependence indicates a strong loss of magnetic order for Cu moments near the Zn atoms. The data can be understood in terms of a model where a Zn ion not only introduces a vacancy in the magnetic Cu lattice but also creates a RKKY-type disturbance. The electron spin polarization around the Zn ions induces a change of the magnetic moments on surrounding Cu ions. The AF lattice is found to be strongly perturbed within a radius of 10 Angstrom around each Zn ion. Possible consequences for the superconductivity of the corresponding Sr-doped materials are discussed.
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| 8. |
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| 9. |
- Prytz, Kjell, 1961-
(författare)
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Meissner Effect in Classical Physics
- 2018
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Ingår i: Progress In Electromagnetics Research M. - : The Electromagnetics Academy. - 1937-8726. ; 64, s. 1-7
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Tidskriftsartikel (refereegranskat)abstract
- The Meissner effect is explored based on the acceleration-dependent component of the Weber force. According to the Maxwell theory, a steady circulating current does not produce any dynamics on external resting charges; however, according to the Weber theory, the charges of the circulating current exhibit a centripetal acceleration, which affects the external charges at rest. It is demonstrated that the current generated in this manner can explain the Meissner effect in classical physics.
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| 10. |
- Wolf, Matthew J., et al.
(författare)
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STM Images of Anionic Defects at CeO2(111)-A Theoretical Perspective
- 2019
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Ingår i: Frontiers in Chemistry. - : FRONTIERS MEDIA SA. - 2296-2646. ; 7
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Tidskriftsartikel (refereegranskat)abstract
- We present a theoretically oriented analysis of the appearance and properties of plausible candidates for the anionic defects observed in scanning tunneling microscopy (STM) experiments on CeO2(111). The simulations are based on density functional theory (DFT) and cover oxygen vacancies, fluorine impurities and hydroxyl groups in the surface and sub-surface layers. In the surface layer, all three appear as missing spots in the oxygen sublattice in filled state simulated STM images, but they are distinguishable in empty state images, where surface oxygen vacancies and hydroxyls appear as, respectively, diffuse and sharp bright features at oxygen sites, while fluorine defects appear as triangles of darkened Ce ions. In the sub-surface layer, all three defects present more complex patterns, with different combinations of brightened oxygen ion triangles and/or darkened Ce ion triangles, so we provide image maps to support experimental identification. We also discuss other properties that could be used to distinguish the defects, namely their diffusion rates and distributions.
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