K.C. Wong

Growth and characterization of Cr2N/CrN multilayer coatings

A series of monolithic and multilayer coatings of chromium nitride with various compositions and architectures were deposited at low temperatures (<200°C) on silicon substrates using ion-assisted reactive magnetron sputtering. All coatings had a total thickness in the 1.5±0.3 μm range. The multilayer coatings were designed such that their period and CrN fraction varied in the range 30–150 nm and 0.50–0.93, respectively. Real-time in situ ellipsometry was used to monitor and control the deposition process. The deposited coatings were characterized post-deposition using X-ray diffraction (XRD), Rutherford backscattering (RBS), X-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE). The primary chromium nitride phases (Cr2N and CrN) in the films were identified using XRD. The chemical composition of selected samples was determined from RBS and XPS measurements. The phase composition of the deposited layers was deduced from the analysis of the SE data. The mechanical properties of the coatings were evaluated using a nanoindenter. The measured hardness values were in excess of 20 GPa. The results of the different characterization and testing techniques were correlated and follow-up work on this project suggested.

Some observations for effects of copper on zinc phosphate conversion coatings on aluminum surfaces

Abstract X-Ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and electrochemical measurements have been used to characterize zinc phosphate (ZPO) conversion coatings, formed on high-purity aluminum surfaces, after dipping in coating baths containing different concentrations of Cu2+ up to 50 ppm. Significant variations in the morphology, adhesion and corrosion protection afforded by the ZPO coatings are especially observed as the Cu2+ in the bath varies from 0 to 10 ppm. Comparisons are made with the effects of surface Cu segregation on ZPO coatings formed in a previous study made on Al-2024-T3 alloy.

Surface effects in chromate conversion coatings on 2024-T3 aluminum alloy

Chromate conversion coatings formed on samples of 2024-T3 aluminum alloy, which had been given different pre-treatments, were examined by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and corrosion tests. Two pre-treatments were considered, namely a simple mechanical polish, and polishing followed by an etch in a HF-H2SO4 solution. The latter treatment leads to significant Cu enrichment at the oxide-alloy interface, and this in turn can lead to a deleterious effect on the corrosion protection afforded by a subsequently applied chromate coating. Discussions are given of the behaviour of Cu in the coating formed on the sample that received an acid etch in the pre-treatment. This involves both migration through the coating and a non-uniform redeposition of Cu on to the coating surface. By contrast, the sample that initially was given just the mechanical polish in the pre-treatment does not show a Cu enrichment in the surface region, and the subsequently applied coating appeared stable after a 24 h immersion in a NaCl test solution.

Formation of zinc phosphate coatings on AA6061 aluminum alloy

Conditions for forming zinc phosphate conversion coatings on AA6061 aluminum alloy have been investigated by characterizing coatings formed for different parameters of the coating bath. Morphological and compositional information on the coatings was assessed by SEM, EDX and XPS, and simple adhesion tests were undertaken to indicate the strengths of coating attachment. The emphasis was to identify conditions that give high coverage, uniform coatings of small, strongly adhered, zinc phosphate crystals. The use of low-zinc solutions (e.g. an atomic Zn/P ratio of 0.07) and normal-zinc solutions (Zn/P ratio 0.25) were compared; coatings formed by the two solution types appear comparable at pH 2, although at pH 4 the low-zinc solution is more effective. Fluoride in the concentration range 200–400 ppm is indicated to be a useful additive for the normal-zinc coating bath and in the 600–1000 ppm range for the low-zinc process. The use of acid etching in the pre-treatment appears to yield better coatings than when mechanical polishing alone is used.

XPS study of interfaces in a two-layer light-emitting diode made from PPV and Nafion with ionically exchanged Ru(bpy)32+

Abstract X-ray photoelectron spectroscopy (XPS) was used to investigate the interfacial chemistries occurring in a prototype two-layer polymer light-emitting diode that had been fabricated from PPV and Nafion with ion-exchanged Ru(bpy) 3 2+ . Magnesium and indium tin oxide (ITO) were used as electrodes, and XPS observations were made as the different layers were added sequentially. Specifically, Mg was deposited on a layer of Nafion with incorporated Ru(bpy) 3 2+ , and this was itself deposited on PPV-coated ITO. The morphology of Nafion, which is dictated by the strongly hydrophilic and hydrophobic groups that compose it, is indicated to rearrange with the incorporation of Ru(bpy) 3 2+ ; this rearrangement appears reinforced when a thin film is deposited on PPV. The Mg deposition results in substantial changes in XPS spectra, in part due to a direct interaction with the fluorocarbon backbone of Nafion.

What determines the structures formed by oxygen at low index surfaces of copper

This article reviews structural details from tensor LEED analyses for three oxygen on copper surfaces, specifically those designated Cu(110)-(2x1)-O, Cu(110)-c(6x2)-O and Cu(100)-(2√2x√2)R45°-O. Certain common features are identified and discussed. It appears that these particular structures receive some stability from being able to fit O-Cu-O building blocks, of the sort needed to construct bulk Cu 2 O, on to the Cu substrates. Each O atom maintains a four-coordinate status, while simultaneously allowing reasonable O-Cu bond lengths. Comparisons are made with other related systems, namely O at Ni and N at Cu surfaces.

Tensor LEED analyses for the (√3 × √3)R30° and c(4 × 2) structures formed by sulphur chemisorbed on the (111) surface of rhodium

Abstract Tensor LEED analyses have been made for the Rh(111)-(√3 × √3)R30°-S and Rh(111)-c(4 × 2)-S surface structures chemisorbed at 1 3 and 1 2 monolayer coverages respectively. For the lower-coverage form, S adsorbs on the regular three-coordi sites which continue the fcc packing sequence; the S-Rh bond lengths are indicated to equal 2.23 A, and relaxations in the metallic structure are negligible. In the c(4 × 2) form, the adsorption occurs equally on both types of three-coordinate site (fcc and hcp), although some surface Rh atoms bond to two S atoms while others bond to just one, and this sets up some interesting relaxations. Specifically, the S atoms displace laterally from the centre of the three-fold sites by 0.20 to 0.29 A, and the first metal layer is buckled by about 0.23 A. The first-to-second interlayer spacing in the metal expands to 2.26 A from the bulk value of 2.20 A. The average S-Rh bond lengths equal 2.22 A, and so they are not significantly changed from that in the low-coverage form. The structural evolution for S chemisorbed on the (111) surface of rhodium with increasing coverage is compared with the corresponding evolution on the Rh(110) surface.

CHARACTERIZATION OF ALUMINUM SURFACES AFTER DIFFERENT PRETREATMENTS AND EXPOSURE TO SILANE COUPLING AGENTS

X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and scanning electron microscopy (SEM) have been used to characterize surfaces of aluminum which have been pretreated by mechanical polishing, acid etching and alkaline etching, as well as given subsequent exposures to air and water. These surfaces can differ markedly with regard to their chemical compositions and topographical structures. Characterizations of these surfaces after exposures to three organosilanes, γ-GPS, BTSE and γ-APS, indicate that the amount of silane adsorbed in each case shows a tendency to increase both with the number of OH groups detected at the oxidized aluminum and with the surface roughness. The XPS data are consistent with the adhesion of γ-APS occurring through H bonding, especially via NH3+ groups.

LEED crystallographic analysis for the Rh(111)-(√7 × √7)R19.1°P surface structure

A recent communication [11] reported a novel structure for the (√7 × √7)R19.1° reconstruction formed by P on the Rh(111) surface, and the present paper details the associated tensor LEED analysis. This reconstruction involves a packed arrangement of Rh pentagons and triangles for the topmost layer, with P atoms at 37 monolayer coverage occupying eight-coordinate sites created by the Rh pentagons. The nearly flat, densely packed RhP mixed layer (density of five Rh and three P atoms per seven Rh atoms in the substrate unit mesh) appears to provide a major stabilizing effect on this surface. The average PRh bond distance is indicated to be 2.52 A, slightly longer than that (2.38 A) in bulk Rh2P where the P atoms are also eight-coordinate. Some similarities are noted with the corresponding (rv7 × √7)R19.1° structures formed by S on the Pd(111), Cu(111) and Ag(111) surfaces.

XPS investigations of the reactivities of oxidized ZrNb interfaces formed by deposition on a gold substrate

Abstract The interaction of a film of Zr (∼ 38 A thick) deposited under ultrahigh vacuum (UHV) on to an approximately 12 A film of NbO on a gold substrate has been studied with X-ray photoelectron spectroscopy (XPS). Evidence is presented for an interfacial conversion from Zr and NbO to ZrO2 and Nb. The reactivity of this sample was studied through a series of sequential treatments. Although changes occur in the topmost layer, the Zr Nb interfacial region, as identified by a shoulder at ∼ 180 eV in Zr 3d and the 203.0 eV peak in Nb 3d, appears to be remarkably inert on heating at 300°C under UHV, as well as on subjecting to O2 and hydrogen plasma treatments.