Oxidized Al Surfaces Research Articles

Morphology and Tribological Behaviour of Amorphous and Crystalline Aluminum Oxide Layers

Anodization of aluminum (Al) is a well-known process for the production of oxidized Al surfaces. Within the scope of the present work, three different Al-oxide layers, produced with different electrochemical methods, were investigated in view of their layer morphology and tribological properties. For this purpose, a newly developed PCO layer (Innovent e.V.), and two commercially available layers, Ematal and a CERANOD®, were compared. Al-oxide layers produced on flat samples were tested against cylindrical DIN 100Cr6 rings under dry sliding conditions. It was observed that under the selected conditions, the frictional behavior of the Al-oxide layers can be influenced by their structural composition and their surface topography, i.e. is different when their structure is amorphous or crystalline. The newly developed Al-oxide layer, which possesses the lowest surface roughness and a very uniform porosity, provided the longest lasting low friction period. Since γ-Al2O3 was also observed in this layer, it is believed that the structural composition of the Al-oxide layer has additionally influenced the frictional response of these samples.

Experimental example of isotropic surface second-harmonic generation: dc-sputtered air-exposed aluminum thin films

We have determined the amplitudes and relative phases of the allowed components of the effective surface second-harmonic susceptibility, Χ ( 2 ) , for a set of air-exposed dc-sputtered Al thin films. The second-harmonic intensity and polarization state was measured as a function of linear input polarization state and azimuthal orientation of the sample using a Ti:sapphire laser at 810 nm. The films are found to be optically isotropic with respect to azimuthal rotation angle, consistent with an isotropic Χ ( 2 ) with seven elements in three families. Using measured linear optical properties, the elements of Χ ( 2 ) were determined from the data. In terms of the dimensionless parameters a, b, and d [J. Rudnick and E. A. Stern, Phys. Rev. B 4, 4274 (1971)], we find ‖a‖=2.30′0.72, in rough agreement with previous measurements of the surface current perpendicular to the surface on oxidized Al surface, ‖b‖=0.013′0.004 for the surface current parallel to the surface, and ‖d‖ =0.009′0.002 for the bulk contribution. The relative phases of a and b with respect to d are found to be (-0.4′6)° and (63′12)°, respectively. The measurement indicates a large discrepancy between measured values and the universal theoretical expectation that b=-1 and d= 1.

Density functional study on the reactivity of oxidized aluminum surfaces: effects of adsorbed metallic atoms (Au, Cu, Ti, V)

Theoretical studies on the possibility of restoring the reactivity of an oxidized Al surface by the adsorption of a metallic atom, such as Au, Cu, Ti, V or Al, have been carried out. We studied: (i) the population distributions; (ii) the energy levels of the highest occupied molecular orbital (HOMO); (iii) the softness and the local softness; and (iv) the adsorption energy and structure of AlH 3. The latter is a model for dimethylaluminum hydride, which is used as a source gas for chemical vapor deposition (CVD). The indicators of reactivity (i–iv) were calculated for three systems: (a) a clean Al surface, (b) an oxidized Al surface, and (c) an oxidized Al surface adsorbing a metallic atom, using density functional theory (B3P86/Lanl2DZ) with a cluster model for the Al surface. It was found that the adsorption of these metallic atoms can restore the reactivity of the oxidized surface.

スピンバルブライクトンネル接合における下地電極層の磁気抵抗への影響

The relation between the under-electrode layers of spin-valve-like tunneling junctions and the tunneling magnetoresistive effect was investigated. Al, Cu, and Pt, which all have low resistivity, were selected for the electrode layer. The roughness of the oxidized Al surface with an Al electrode layer of more than 10 nm thickness increased drastically. In all junctions with Al layers thicker than 30 nm, the tunneling magnetoresistive effect was not observed. On the other hand, junctions with a Cu electrode exhibited an MR ratio of over 10%, even for Cu thicknesses of up to 80 nm. The MR ratio decreased with increasing roughness of the oxidized Al surface, irrespective of the electrode materials.

Visual detection of protein adsorption onto electrochemically oxidized aluminum surfaces

Adsorption of proteins onto electrochemically oxidized aluminum surfaces by a simple visual observation was investigated. For this purpose, Ta and then Al were sputtered onto glass slides (Al/Ta/glass slides). Al/Ta/glass slides were electrochemically oxidized in 0·4 M H 3PO 4 under the potentiostatic conditions. After the application of aqueous solutions of bovine, rabbit or human immunoglobulin onto the solid Al 2O 3 surfaces, a change in colour was monitored visually. It was found that all investigated proteins could successfully be adsorbed onto oxidized Al surfaces. This was manifested by a change in colour of the surface from tan to a purple or blue, depending on the concentration of proteins, coating time and degree of oxidation of the Al layer. Most importantly, when an aqueous solution of human IgG was applied on an antihuman IgG coated surface, a change in colour was also observed indicating that the adsorption process did not denature the molecular recognition sites. This type of antibody-antigen reaction was confirmed on the example of antihuman prothrombin — human prothrombin. It is believed that this technology may be useful in developing immunosensors for a variety of applications.

Ultraviolet laser excited adsorbate–surface interactions: NH3, C2H4, and CH2I2 on Al and oxidized Al surfaces

The chemical interactions and the 308-nm laser radiation effects on NH3, C2H4, and CH2I2 adsorbed on Al and oxidized Al surfaces have been studied by x-ray photoemission, thermal desorption, and time-of-flight mass spectrometry. The mass and the translational energy distributions are determined as a function of surface coverage and laser fluence. For NH3 and C2H4, the major radiation effect appears to be laser-induced thermal desorption. For CH2I2 on Al, the adsorbate electronic excitation effect is evident at both submonolayer and multilayer coverages. At a submonolayer coverage, CH2 radicals and C2H4 molecules are produced and can be desorbed by laser pulses. Part of the products may be trapped on the surface. At a multilayer coverage on Al and any coverage on the oxidized Al, photodissociation leads to CH2I and I formation. At very high coverages, adsorbate-mediated ‘‘explosive’’ desorption can take place on any substrate. The results yield the characteristic electronic, thermal, and explosive desorption patterns depending on the gaseous exposure, the substrate, and the laser irradiation conditions.

Force sensing in scanning tunnelling microscopy: observation of adhesion forces on clean metal surfaces

SUMMARYTip sample interaction forces were investigated during normal tunnelling operation of the STM using an Ir tip and a polycrystalline Ir sample. Metallic adhesion interaction was observed for tunnel conductivity ranging from 10−6 to 19−9 Ω‐1 implying that the actual gap width was of the order of 1–4 Å. Similar experiments performed on a polycrystalline Al sample exposed to 1 Langmuir O2 showed that tip sample interaction changed from attractive to repulsive on well‐defined areas extending over ∼100 Å2 which we identified with the oxidized Al surface.

Surface extended x-ray absorption fine structure in the soft-x-ray region: Study of an oxidized Al surface

The extended x-ray absorption fine structure (EXAFS) above the oxygen K edge (h..nu.. approx. = 535 eV) has been investigated for an oxidized Al surface. The spectra were obtained using monochromatized synchrotron radiation in the 500--800-eV range and detecting the secondary electron yield from the sample. Pronounced EXAFS was observed extending at least 300 eV beyond the edge. Experiments of the present kind appear to have great applicability for the study of the oxidation and catalytic activity of surfaces.

Reactivity of clean and oxidized aluminum surfaces towards sulphur compounds

The interaction of H2S, CS2 and SO2 with clean and oxidized Al films has been investigated in the temperature range –78 to 200°C. Dissociative adsorption of H2S on Al films at –78°C was accompanied by H2 evolution, while CS2 uptake at this temperature was mainly molecular and reversible. With SO2, extensive adsorption and incorporation was possible at –78°C. The completion of the surface layer with each gas at temperatures –78°C was followed by a tarnishing reaction for which both the energy of activation and the pre-exponential factor were determined.The metal film of Al had a large and stable area after sintering at 70°C for 20 min; the roughness factor was usually 7.0. Oxygen adsorption on the film at 30°C caused some further reduction in area, but the roughness factor did not fall below 5.0 even when the film was saturated with oxygen at 30°C under an oxygen pressure of 5 N m–2. The presence of an oxide layer on the Al film decreased its tendency for subsequent adsorption of the sulphur compounds at all temperatures in the range –78 to 200°C. The uptake of CS2 by oxidized Al films above 50°C was extremely slow but accompanied by CO evolution.The pre-exponential factor for the reaction of each gas increased as the energy of activation increased and a compensation effect was assumed to operate in the tarnishing reactions by sulphur compounds on both clean and oxidized Al surfaces.