Markus Kuhn

Growth, structure and thermal properties of chromium oxide films on Pt(111)

Abstract We have prepared chromium oxide films on Pt(111) with thicknesses ranging from less than a monolayer to more than eight monolayers. The films were grown by Cr vapor deposition under an oxygen background pressure of 2 × 10 −6 mbar and a sample temperature of 600 K. The growth, structure and thermal properties of the resulting overlayers have been studied using a combination of ISS, XPS, LEED and STM. During the initial stage of growth, the film nucleates at Pt step edges, and grows in a step-flow mode. For the first two monolayers, a well-ordered p(2 × 2) structure is observed and attributed to the epitaxial growth of metastable Cr 3 O 4 . At higher coverage, a (√3 × √3)R30° structure appears, due to the formation of the stable Cr 2 O 3 phase. After heating in vacuum to about 700 K, the p(2 × 2) film begins to decompose and agglomerate, and finally becomes a (√3 × √3)R30° overlayer. The latter is stable up to the highest temperature used in these studies (1000 K), although at this temperature it also shows dewetting and further clustering.

H2S adsorption on chromium, chromia, and gold/chromia surfaces: Photoemission studies

The reaction of H2S with chromium, chromia, and Au/chromia films grown on a Pt(111) crystal has been investigated using synchrotron-based high-resolution photoemission spectroscopy. At 300 K, H2S completely decomposes on polycrystalline chromium producing a chemisorbed layer of S that attenuates the Cr 3d valence features. No evidence was found for the formation of CrSx species. The dissociation of H2S on Cr3O4 and Cr2O3 films at room temperature produces a decrease of 0.3–0.8 eV in the work function of the surface and significant binding-energy shifts (0.2–0.6 eV) in the Cr 3p core levels and Cr 3d features in the valence region. The rate of dissociation of H2S increases following the sequence: Cr2O3<Cr3O4<Cr. For chromium, the density of states near the Fermi level is large, and these states offer a better match in energy for electron acceptor or donor interactions with the frontier orbitals of H2S than the valence and conduction bands of the chromium oxides. This leads to a large dissociation probabili...

Adsorption of sulfur on bimetallic surfaces: Formation of copper sulfides on Pt(111) and Ru(001)

The interaction of sulfur and copper on Pt(111) and Ru(001) has been examined using x‐ray photoelectron spectroscopy and thermal desorption mass spectroscopy. Cu/Pt(111) and Cu/Ru(001) surfaces were exposed to S2 gas at 300 K. It was found that for both substrates stable copper–sulfide films were formed, of which the Cu–S/Ru(001) system was the more stable. The decomposition of the Cu2S films on Pt(111) occurred at 600–850 K and on Ru(001) at 900–1000 K. Breakdown of the films led to evolution into the gas phase of sulfur (mainly as S2) without desorption of copper or the sulfur chemisorbed on the substrate. This chemisorbed sulfur desorbed in a broad feature from 1000 to 1500 K. For the Ru(001) substrate the Cu atoms remained on the surface until they desorbed at 1080 K, while on Pt(111) the Cu atoms migrated into the Pt(111) surface to form a subsurface Cu–Pt alloy; no Cu desorption features were seen at temperatures as high as 1300 K. On Pt(111), copper sulfide promoted the formation of a bulklike plat...

Growth and structure of ultrathin Cr films on Pt(111)

The growth and structure of ultrathin chromium films on Pt(111) have been studied using low-energy He+ ion scattering spectroscopy, X-ray photoelectron spectroscopy, low energy electron diffraction and scanning tunneling microscopy. Upon initial deposition at room temperature, the Cr atoms migrate along the Pt terraces and populate the step edges. Further growth results in two-dimensional islands of chromium developing from the step edges and across the Pt terraces. The film is pseudomorphic to the Pt(111) substrate. At coverages above 3 ML, chromium forms three-dimensional elongated islands oriented along the three closepacked directions of the Pt(111) surface. The chromium overlayer adopts a bcc (110) surface structure with the close-packed overlayer directions, 〈111〉, aligned with the close-packed Pt(111) directions, 〈110〉. The presence of the Cr overlayer was found to electronically perturb the Pt surface atoms leading to a redistribution of charge between Pt and Cr.

Epitaxial growth of ultrathin films of chromium and its oxides on Pt(111)

The growth of vapor-deposited Cr and chromium oxide on Pt(111) has been studied by ion scattering spectroscopy, x-ray photoelectron spectroscopy, low energy electron diffraction, and scanning tunneling microscopy. Both metal Cr film and its oxide nucleate at Pt(111) step edges, and grow in a step-flow mode. The metal Cr film is pseudomorphic to the Pt(111) substrate within the first two monolayers. At higher coverages, the chromium overlayer adopts a bcc(110) surface structure and forms three-dimensional elongated islands oriented along the three close-packed directions of the Pt(111) surface. For the chromium oxide, a well ordered p(2×2) structure is observed within the first two monolayers and attributed to the epitaxial growth of metastable Cr3O4. At higher coverages, a (√3×√3)R30° structure appears, due to the formation of the stable Cr2O3 phase.

Epitaxially Grown Fe3O4 Thin Films: An XPS Study

Magnetite is a technologically important material with interesting magnetic and electronic properties. The applications of this oxide include, but are not limited to, magnetic storage media, as a catalyst in the plastics industry, electronic circuit applications, and may have important future applications in the micro-electronics industry as well. Since the transition between different iron oxide phases can be easily achieved, characterization of well-defined materials with XPS is quite important. We report XPS measurements of a 1 μm thick Fe3O4 thin film on a MgO(001) substrate. The film was grown using plasma-assisted MBE, and was characterized in situ with RHEED which indicated a well ordered surface. A companion sample was characterized ex situ with XRD and SQUID which showed it to be single-crystalline, stoichiometric Fe3O3. The sample was transported under argon atmosphere and introduced into the XPS chamber under flowing nitrogen. The sample was then cleaned by annealing in a 2 × 10−6 oxygen at 620...

Surface Structure and Morphology of Mg-Segregated, Epitaxial Fe 3 O 4 Thin Films on Mgo(001)

We have investigated the structural and compositional changes that are induced by the segregation of substrate Mg to the surface of 1μm-thick Fe 3 O 4 films on MgO(001). The thin films have been grown with plasma-assisted MBE, and characterization with RHEED (reflection high-energy electron diffraction), x-ray diffraction (XRD), and Superconducting Quantum Interference Device (SQUID) magnetometry show slightly strained, single-crystalline Fe 3 O 4 films. For the surface studies, we have combined Low-Energy Electron Diffraction (LEED) and Scanning Tunneling Microscopy (STM). Initial and final surface characterization employed X-ray Photoelectron Spectroscopy (XPS) and Ion Scattering Spec-troscopy (ISS) respectively. The surfaces of the MBE-grown samples are flat and show a (√2 × √2)R45° reconstruction with respect to the Fe 3 O 4 surface unit cell. We observe the onset of Mg segregation to the surface at around 700 K, with long, narrow extensions of terraces being observed growing along the [110] and [1 10 ] directions. Upon prolonged heating at 800 K, massive Mg segregation to the surface is observed. Heating in an oxygen atmosphere induces a 1×4 surface reconstruction, and results in extremely long (≈ 1000 A ), wide terraces.

Magnesium outdiffusion through magnetite films grown on magnesium oxide (001) (abstract)

Scanning tunneling microscopy (STM) studies of 1 μm thick films of single crystalline Fe3O4 grown on MgO(001) indicate that repeated annealing of the sample in UHV causes Mg diffusion through the Fe3O4 film. The onset of this effect was clearly seen by STM at room temperature for samples raised above 400–430 °C. It appears that the annealing process causes the migration of Mg from the substrate entirely through the Fe3O4 lattice, and that the migration tends to fill the surface layer first, with lower layers filling as anneal time is increased. Upon detection of this effect, several complementary sample analysis techniques were employed to determine the extent of the changes observed. X-ray diffraction studies indicate shifts in the lattice constant from the cubic constant of magnetite, Fe3O4, (8.396 A), which is already strained in thin-film growth on a substrate, further toward the cubic lattice constant of magnesioferrite, MgFe2O4, (8.375 A) in order to accommodate the Mg that has migrated to the surfa...