P. N. Henriksen

Adsorption of Phosphorus Acids on Alumina.

Abstract Vibrational spectra of several phosphorus acids adsorbed on oxidized aluminum surfaces have been measured using inelastic electron tunneling spectroscopy. From analysis of the spectra, it is concluded that these acids chemisorb on alumina by a condensation reaction in which resonance stabilized structures are formed involving deprotonated hydroxilic and phosphoryl oxygen atoms. These structures are symmetric PO 2 groups in the case of phosphinic acid, whereas for phosphonic acids, they are symmetric PO 3 groups. In the spectrum of the hydration inhibitor N[CH 2 P(O)(OH) 2 ] 3 adsorbed on alumina, unexpected bands attributable to P-H motions appeared. It is therefore inferred that this material dissociates upon adsorption. The various phosphonic acids produced in this decomposition subsequently adsorb as symmetrical tridentate species.

Inelastic electron tunneling spectroscopy of silane coupling agents adsorbed on alumina

Inelastic electron tunneling spectroscopy (IETS) has been used to obtain vibrational spectra of alkoxysilanes adsorbed on aluminum oxide. The spectra reveal that: (1) monoalkoxysilanes are not adsorbed whereas trialkoxysilanes are readily adsorbed, (2) the reaction mechanism of triethoxysilanes and trimethoxysilanes is the same, and (3) the reaction mechanism under anhydrous conditions utilizes surface OH or OD groups present on alumina. These results support the hypothesis that the alkoxy groups exchange with OH groups on the surface to form siloxane bonds, and that these bonds are stabilized with trialkoxysilanes. For monoalkoxysilanes, either the bonds do not form so readily, or they are not stabilized by further condensation of the adsorbed species. Instead, the monoalkoxysilanes appear to catalyze further oxidation of the alumina surface.Inelastic electron tunneling spectroscopy (IETS) has been used to obtain vibrational spectra of alkoxysilanes adsorbed on aluminum oxide. The spectra reveal that: (1) monoalkoxysilanes are not adsorbed whereas trialkoxysilanes are readily adsorbed, (2) the reaction mechanism of triethoxysilanes and trimethoxysilanes is the same, and (3) the reaction mechanism under anhydrous conditions utilizes surface OH or OD groups present on alumina. These results support the hypothesis that the alkoxy groups exchange with OH groups on the surface to form siloxane bonds, and that these bonds are stabilized with trialkoxysilanes. For monoalkoxysilanes, either the bonds do not form so readily, or they are not stabilized by further condensation of the adsorbed species. Instead, the monoalkoxysilanes appear to catalyze further oxidation of the alumina surface.

A vibrational spectroscopic comparison of vinyltriethoxysilane and vinylphosphonic acid adsorbed on oxidized aluminum

Results from inelastic electron tunneling spectroscopy (IETS), IR transmission, and multiple reflection-absorption infrared spectroscopy are presented in a comparison of the adsorption of monolayers of vinylphosphonic acid (VPA) and vinyltriethoxysilane (VTES) on oxidized aluminum. Spectra obtained from VPA suggest that adsorption occurs via a condensation reaction that forms a resonance structure involving deprotonated hydroxyl oxygen atoms and the phosphoryl oxygen. A symmetric tridendate structure is proposed and discussed for the adsorbed state of VPA. For adsorbed VTES, these results support a previously proposed reaction involving condensation and formation of siloxanes at the oxide surface. The relative intensities of the bands associated with the vinyl group are similar for both compounds, suggesting that the organofunctional groups are oriented on the surface in the same way and, in the case of IETS, that the Pb top electrode has little effect on band position.

An atomic force microscopy study of polyester surfaces

The surface properties of amorphous and crystalline polyester films, well below their glass transition temperature, have been studied with an atomic force microscope. For amorphous films a corrugated pattern develops on the surface as a result of scanning and the corrugations are always perpendicular to the scan direction. When scanning is stopped the pattern shows a slight relaxation; however, the surface is plastically deformed. When crystalline films are scanned, similar patterns are seen which are less pronounced and require a much longer scan time. These results suggest that the physical properties of a glassy polyester surface may be different from the bulk, and the freedom of macromolecules is reduced upon crystallization, thus suppressing molecular motion at the surface.

Easily realized inelastic electron tunneling spectrometer

An easily realized inelastic electron tunneling spectrometer (IETS) controlled by computer through an IEEE‐488 interface bus is described. Components and circuits of the system are described in detail in order to help newcomers to IETS build a research quality spectrometer on a relatively low budget. The system design is much simpler and easier to implement than others reported in the literature, and experimental results indicate that the spectrometer has comparable resolution and signal‐to‐noise ratio. Additionally, in‐house software routines offer the system considerable flexibility in spectral data manipulation, for example, background correction, numerical differentiation, and subtraction of one spectrum from another may be performed.

Comparison of the vibrational spectra of pyruvic acid on alumina using inelastic electron tunneling spectroscopy (IETS) and Fourier transform multiple reflection absorption infrared spectroscopy (MRAIRS)

Abstract Vibrational spectra obtained using IETS and MRAIRS are presented for monolayers of pyruvic acid adsorbed on alumina, incorporated in thin film Al/alumina/pyruvic acid/Pb samples. Spectra obtained using both techniques do not exhibit a carbonyl peak, however MRAIR spectra for similar samples without a Pb cover film do. This behaviour may be attributed to interactions between the adsorbed molecules and the cover film. Possible reasons for this, and the disappearance of certain peaks in the IET spectra when Sn and Al are employed as cover films instead of Pb, are discussed.

Inelastic electron tunneling spectroscopic studies of alkoxysilanes adsorbed on alumina

Inelastic electron tunneling spectroscopy is used to investigate the adsorption of dimethyldimethoxysilane, dimethyldiethoxysilane, and dimethylvinylethoxy silane on alumina at the monolayer level. Data obtained indicate that different adsorbed layers are produced when the silanes are introduced onto the oxide surface from solution or as a vapor. Silanes introduced in the same way onto different types of oxides suggest that alumina morphology also affects the adsorbed configuration.

Rearrangement of Au(111) surface as a result of scanning with scanning tunneling/atomic force microscopes

Rearrangements of monolayer islands on the (111) surface of Au films, grown epitaxially on mica substrates, are observed as a result of continuous scanning with either a scanning tunneling microscope (STM) or an atomic force microscope (AFM) under ambient conditions. With the STM, a contiguous monolayer as large as 150×150 nm2 can be moved from atop to the edges of underlying terrace in about an hour of continuous scanning at a rate of 2.0 μm/s. In general, islands coalesce, vacancies fill, and terrace edges become straighter as a result of the tip scanning over the Au surface. Crystal defects such as screw dislocations, however, are not modified appreciably with scanning, although rearrangements are seen in regions near the defects. Similar effects are observed with an AFM scanning, but to a lesser extent, and only on freshly grown films.

Epitaxial growth of ultrathin films of bismuth: an atomic force microscopy study

Abstract Ultrathin films of bismuth have been grown epitaxially onto mica substrates by vapor deposition and atomic force microscopy has been used to image the growth surface to determine optimum conditions for fabricating films having the least variations in thickness and largest possible grain size. The films always grow from three-dimensional islands which coalesce to form crystallites with their trigonal axis perpendicular to the mica surface. Crystallites as large micrometers squared with monatomic stepped topology and an rms surface roughness on the order of 1.0 nm have been observed. For mica substrates, it has been found that cleavage conditions, substrate preheating, substrate temperature during deposition. Bi deposition rate, and annealing temperature all affect nucleation and growth of Bi films.

An investigation of the temporal coherence length of light

An interference filter is constructed from a pair of front-surface two-way mirrors. These partially reflecting mirrors are clamped at both ends with a small spacer placed at one end, thus forming a thin wedge of air between their face-to-face mirrored surfaces. A series of thin film interference spectra are recorded by moving this air wedge apparatus in incremental steps across a beam of white light. The mirror separation as a function of position along the wedge is obtained from analysis of the transmitted spectra. As the separation increases, temporal coherence is lost for successively longer wavelengths until interference effects are no longer observed. We define coherence length measured in this way as twice the wedge thickness where the interference fringe pattern is reduced to the noise level of the transmitted spectrum. Using this definition and technique, a value of 224 ± 20 µm is obtained using a white light source. This value is much larger than one would expect for the coherence length of white light measured by more standard techniques, e.g. Michelson interferometry. The implication of our experiment is that coherence length is a property of the light reaching the detector, which in our case is the light transmitted by the interference filter.