R. Franchy

Formation of a well-ordered aluminium oxide overlayer by oxidation of NiAl(110)

We have investigated the electronic and geometric structure of a thin oxide film grown by oxidation on NiAl(110) using electron spectroscopic techniques, i.e., LEED, EELS, XPS and ARUPS. This film is inert to adsorption of, respectively reaction with many molecules up to temperatures of about 800 K. It is well ordered as deduced from the LEED pattern and covers the whole surface. We find that the oxide film is about 5 A thick, consisting of aluminium oxide as shown by EELS, XPS and ARUPS. It is most likely formed of two aluminium layers and two quasihexagonal oxygen layers with oxygen surface termination. Since the oxide film is rather thin it only shows a two-dimensional band structure which has been investigated using ARUPS. For the electronic levels of the oxide strong periodic dispersions are observed with bandwidths compatible to dispersion bandwidths calculated for the ΓX direction of α-Al2O3.

Investigations on phase transitions within thin Al2O3 layers on NiAl(001) — HREELS on aluminum oxide films

Abstract We have investigated the phase transitions and the structure of thin aluminum oxide layers on NiAl(001) using high-resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The oxide films were prepared either by oxygen adsorption at 300 K and annealing to different temperatures or by oxidation at different temperatures up to 1500 K. We have demonstrated that in connection with LEED and AES data the HREEL spectra allow the determination of the structure of the alumina layers. Different phases of the Al2O3 layers have been observed depending on annealing or oxidation temperature. The observed energy losses are high-frequency branches ω+ of Fuchs-Kliewer (FK) surface phonons. Calculated spectra based on dielectric theory reproduce the measured spectra very well. The thickness of the well-ordered θ-Al2O3 layer is estimated to be about 10 A.

Oxygen on Cu(100) – a case of an adsorbate induced reconstruction

We show that at and above room temperature only a single long range ordered superstructure exists on Cu(100). This is the (2 × 22)R45° structure, which corresponds to a saturation coverage of θo = 0.48 ± 0.05. A well-ordered c(2 × 2) superstructure is not observed in the investigated range of adsorption temperatures (Tads = 300–600 K). Above a critical oxygen coverage θc = 0.34 ±0.02 long range order starts to develop. The disordered adsorption region below θc = 0.34 is characterized by two different adsorption sites. One of these sites already is saturated at relatively low coverages (θo ≅ 0.15). This site appears to corresponds to a local reconstruction with the neighbouring atoms of the adsorbate atom engaging in an intralayer contraction. Diffraction studies show a first order phase transition at θc from the disordered phase to a phase with long range order. The long range ordered domains are larger than the coherence length of the EELS diffraction experiment already slightly above θc. For the “Perfect” (2 ×22)R45° structure only a single fourfold coordinated adsorption site is occupied. The large number of total symmetric vibrations as well as the observed LEED pattern show that the substrate surface is reconstructed. An assignment of the observed vibrations allows us to determine the reconstruction pattern which is characterized by a local symmetry reduction. This lowering of symmetry might be the driving force of the reconstruction pointing towards a Peierls instability or a cooperative Jahn-Teller effect.

The oxidation of the NiAl(111) surface

Abstract The oxidation of NiAl(111) was investigated in the temperature region between 300 and 1500 K by HREELS (high resolution electron energy loss spectroscopy), AES (Auger electron spectroscopy) and LEED (low energy electron diffraction). The adsorption of oxygen up to saturation at room temperature leads to the formation of a thin amorphous Al-oxide film. After annealing the oxygen-covered sample to temperatures between 900 and 1100 K a well-ordered Al-oxide with hexagonal symmetry is found. The HREEL spectrum exhibits losses at 53, 79 and 110 meV. This suggests the formation of thin ordered γ′-Al2O3 films on the surface. After annealing the sample above 1100 K the HREEL spectrum exhibits only two losses at 78 and 113 meV which are attributed to thin α-like Al2O3 layers. These Al-oxide overlayers contain less oxygen and the surface is facetted. The surface consists in (111) terraces and (110) facets which form triangular pyramids.

A combined scanning tunneling microscopy and electron energy loss spectroscopy study on the formation of thin, well-ordered β-Ga2O3 films on CoGa(001)

The formation of thin, well-ordered β-Ga2O3 films on CoGa(001) was studied by means of high resolution electron energy loss spectroscopy (EELS), scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy. The crystalline β-Ga2O3 films on CoGa(001) are prepared upon adsorption of O2 at 300 K and subsequent annealing at 700 K or by oxidation of the sample directly at 700 K, respectively. EEL spectra of these films exhibit Fuchs–Kliewer modes at 305, 455, 645, and 785 cm−1 in good agreement with calculated spectra using the IR parameters of Ga2O3. The band gap was determined to be 4.5±0.2 eV. In addition, a gap state at 3.3 eV was found. The observed LEED pattern of β-Ga2O3/CoGa(001) can be explained by a (2×1) structure in two perpendicularly oriented domains. STM images exhibit atomically flat and large oxide terraces (up to 2500×700 A2) mainly of rectangular shape. STM pictures with atomic resolution confirm the (2×1) structure. In addition, a square subst...

The interaction of oxygen with Nb(110) at 300, 80 and 20 K

The adsorption of O2 on Nb(110) at 300, 80 and 20 K has been studied using high-resolution electron energy loss spectroscopy (HREELS), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). At 300 and 80 K O2 is dissociatively chemisorbed and thin Nb-oxide (NbO + NbO2) layers are formed. At saturation, signature of Nb2O5 islands on top of NbO and NbO2 layers have been found. At 20 K and low exposures (< 2 L) oxygen is dissociatively adsorbed and Nb oxide layers grow on the surface. The oxidation features of Nb(110) at 20 K are very similar to those found at 80 and 300 K. But, after forming protective oxide layers on the surface, the O2 molecules are physisorbed. The frequency of the OO stretching vibration of physisorbed O2 is 193 meV, which is close to the Raman frequency of O2 molecules in the gas phase.

The Rayleigh phonon dispersion on Cu(100): A stress induced frequency shift?

AbstractThe dispersion of the Rayleigh wave on Cu(100) has been measured by high resolution electron energy loss spectroscopy (EELS) throughout the two-dimensional Brillouin zone in the

Structural models for the Cu(100)(2 × 22)R45°-O phase

\overline {\Gamma M} {\text{ - direction}}