Low-frequency surface-enhanced Raman scattering spectroscopy at metal electrode surfaces

Low-frequency surface-enhanced Raman scattering (SERS) spectroscopy is a versatile tool for studying surface phenomena under electrochemical conditions. This spectroscopy enables us to obtain rich information on extramolecular vibrations between substrate and adsorbates, which are sensitive to atomistic surface features of the substrate. Owing to recent advancements in optical filter technology, low-frequency SERS signals are now becoming easily detectable using conventional Raman systems equipped with holographic notch filters. In addition, SERS background signals, which have been simply ignored, can provide electronic information on the metal substrate. This allows us to observe both sides of electrode–electrolyte interfaces in situ and simultaneously, which is never expected in far-infrared or terahertz absorption spectroscopy. This advanced SERS spectroscopy can help our understanding of electrochemical and electrocatalytic reactions at the molecular scale.     

Investigation of Biopag films adsorbed on metal and ceramic surfaces by X-ray photoelectron spectroscopy

Using X-ray photoelectron spectroscopy, it was found that the treatment of aluminum, nickel, and ceramic surfaces with a 1% Biopag aqueous solution (Biopag, poly(hexamethylene guanidine chloride)), was synthesized at the Institute of Ecological and Technological Problems in accordance with TU (Specifications) 9392-020-41547288-02) results in the formation of thin polymer films on the surfaces. The chemical composition of the films was determined. It was shown that the chemical composition of the films is changed on long standing in air. Microbiological tests were carried out, and the minimum suppressing concentration toward Pseudomonas aeruginosa was determined. It was shown that the Biopag films aged for 6 months retain up to 84% of their initial activity.     

Characterization of metal oxide surfaces and thin semiconductor films by inelastic electron tunneling spectroscopy.

Inelastic electron tunneling spectroscopy (IETS) is a unique surface and interface analytical technique using electron tunneling through a metal/insulator/metal tunneling junction at cryogenic temperatures. It gives the vibrational spectrum of a very thin (nm) insulator film and the adsorbed species on it. The high sensitivity, good resolution, and wide spectral range inherent in IETS enable us to analyze the surface and interface of the insulator in detail. The tunneling junction is a good model system for oxide catalysts, electronic devises, and solid state sensors. Information about the surfaces of alumina and magnesia, the adsorption states and chemical reactions of adsorbed species occurring on these oxides can be obtained through an analysis of the tunneling spectra. The structures and properties of evaporated thin semiconductor films can also be studied. In this review, the surface characterization of alumina and magnesia, the adsorption and surface reactions of organic acids, esters, amides, and nitryls on these oxides, and the characterization of thin evaporated films of Si, Ge, and the oxides are summarized.     

Thin metal films on quasicrystalline surfaces

Thin metal films with a thickness of one or over one monolayer formed on quasicrystalline surfaces were studied using reflection high-energy electron diffraction, X-ray photoelectron spectroscopy, X-ray photoelectron diffraction and scanning tunneling microscopy. The substrates were the 10f surface of d-Al–Ni–Co and the 5f surface of i-Al–Pd–Mn. The metals deposited were Au, Pt, Ag and In. None of these metals forms any ordered layer by deposition onto clean quasicrystalline surfaces. However, if a submonolayer of In is present atop the 10f surface, an epitaxial layer of multiply-twinned AuAl₂ crystals is formed by Au deposition and subsequent annealing. This is also the case for Pt deposition, but not for Ag deposition. Although the surfactant effect of In is also observed in the case of Au deposition on the 5f surface of i-Al–Pd–Mn, the ordered layer formed is a film of Au–Al alloy with icosahedral symmetry. No ordered films are formed by Pt or Ag deposition onto the 5f surface, regardless of the presence of an In-precovered layer. A Sn film monolayer induced by surface diffusion was also studied.     

Surface bonding on silicon surfaces as probed by tip-enhanced Raman spectroscopy

Tip-enhanced Raman spectroscopy (TERS) has been used to obtain the Raman signal of surface species on silicon single crystal surfaces without the necessity for surface enhancement by addition of Ag nanoparticles. By illuminating the hydrogen terminated silicon surface covered with a droplet of 4-vinylpyridine with UV light, a 4-ethylpyridine modified silicon surface can be easily obtained. By bringing a scanning tunneling microscope (STM) Au tip with a nanoscale tip apex to a distance of ca. 1 nm from the m...     

Depth profiling of polymer films by confocal Raman spectroscopy

Confocal Raman microspectroscopy has many potential applications in the study of polymer-solvent interactions, including the determination of solvent and polymer-solvent complex depth profiles. This contribution focuses on preventing the formation of polymer-solvent complexes, using surface chemical modification of PVC films. While the surface-specific nature of the film modification is easily demonstrated,^[1] confocal Raman measurements clearly show the effects of film refractive index: the modifier depth profile shows a lack of symmetry and the film thickness is underestimated. A spectral normalisation method is described, and this is shown to result in a modifier depth profile which is in good agreement with data obtained by Raman microspectroscopy following physical cross-sectioning of a sample. Alternative techniques for Raman depth profiling are also discussed.     

Optical tomography of non-crystalline films by interference enhanced Raman spectroscopy

The depth dependence of Raman spectra of a-GeS₂-type films having a different optical thickness (/4 and /2) and their refractive index profile have been investigated. The model of a layered-inhomogeneous structure of films has been proposed. There have been distinguished three regions: near-surface region (up to 50 Å), central part and transition film-substrate region (up to 300 Å).     

Optical tomography of non-crystalline films by interference enhanced Raman spectroscopy

The depth dependence of Raman spectra of a-GeS(2)-type films having a different optical thickness (lambda/4 and lambda/2) and their refractive index profile have been investigated. The model of a layered-inhomogeneous structure of films has been proposed. There have been distinguished three regions: near-surface region (up to 50 A), central part and transition film-substrate region (up to 300 A).     

Characterization of thin metastable vanadium oxide films by Raman spectroscopy

Thin films are potentiodynamically generated on vanadium in Ba²⁺/acetate electrolyte systems at high voltages. The influence of the anodic potential up to 400 V on the composition and structure of the about 500 nm thin anodic conversion films are investigated. Raman spectroscopy indicates that different film types depend on the electrochemical process parameters. The relationship between the Raman laser excitation power and the amorphous or microcrystalline film structure is also discussed. Beside metastable disordered structures the films contain crystalline phases of V₂O₅, V₄O₉ and barium vanadate, respectively. Received: 15 July 1997 / Revised: 16 February 1998 / Accepted: 21 February 1998     

Characterization of thin metastable vanadium oxide films by Raman spectroscopy

Thin films are potentiodynamically generated on vanadium in Ba²⁺/acetate electrolyte systems at high voltages. The influence of the anodic potential up to 400 V on the composition and structure of the about 500 nm thin anodic conversion films are investigated. Raman spectroscopy indicates that different film types depend on the electrochemical process parameters. The relationship between the Raman laser excitation power and the amorphous or microcrystalline film structure is also discussed. Beside metastable disordered structures the films contain crystalline phases of V₂O₅, V₄O₉ and barium vanadate, respectively.     

Study on functionalization of metal surfaces. III. Photopolymerization of monomer films on metal surfaces

The three kinds of monomer films on metal surfaces were deposited by adsorption from a solution of 6-polymerizable substituents-1,3,5-triazine-2,4-dithiol monosodium salts (RTDN); the polymerizable substituents such as cis-9-octadecenylamino, di(cis-9-octadecenyl)amino, and p-vinylbenzyl(cis-9-octadecenyl)amino groups were selected in view of the polymerization activity of unsaturated groups in the substituents and the packing degree of monomer molecules. The monomer films were estimated to consist of mainly 6-substituents-1,3,5,-triazine-2,4-dithione (3H, 5H) and to be multimolecular layers that are considerably cross-packed and ordered. The monomer films on metal surfaces were polymerizable under a UV light irradiation in air atmosphere to give polymer films. In the photopolymerization, azobis(isobutyronitrile) (AIBN) was very effective for increasing the monomer conversion and the polymerization rate. The optimum concentration of AIBN in monomer films was very small, about 0.025 mol %. The monomer conversion was influenced by the kind of monomers, namely, the polymerization activity and the packing degree. The effect of the packing degree was especially remarkable. The monomer conversion decreased with an increase in the thickness of monomer films. This is because the polymerization was initiated by oxygen and AIBN, which were diffused into the inner of monomer films. The possibility of polymerization of the unsaturated groups and the thione groups in monomer molecules under UV light irradiation is discussed.     

In situ Raman spectroscopy of surfaces modified by ion soft landing

The design and characterization of a system for in situ Raman analysis of surfaces prepared by ion soft landing (SL) is described. The performance of the new high vacuum compatible, low cost, surface analysis capability is demonstrated with surface enhanced Raman spectroscopy (SERS) of surfaces prepared by soft landing of ions of crystal violet, Rhodamine 6G, methyl orange and copper phthalocyanine. Complementary in situ mass spectrometric information is recorded for the same surfaces using a previously implemented secondary ion mass spectrometer (SIMS). Imaging of the modified surfaces is achieved using 2D Raman imaging as demonstrated for the case of Rhodamine 6G soft landing. The combination of the powerful molecular characterization tools of SERS and SIMS in a single instrument fitted with in-vacuum sample transport capabilities, facilitates in situ analysis of surfaces prepared by ion SL. In particular, information is provided on the charge state of the soft landed species. In the case of crystal violet the SERS data suggest that the positively charged ions being landed retain their charge state on the surface under vacuum. By contrast, in the case of methyl orange which is landed as an anion, the SERS spectra suggest that the SL species has been neutralized.     

Analysis of thin solid films and surfaces by infrared spectroscopy

Thin solid films and surfaces are characterized by IR spectroscopy, based on reflectance and transmittance measurements, in particular with polarized light at oblique incidence. Thus two independent data sets fors- andp-polarization are available. Atp-polarization additional absorption lines at the zeros of the dielectric function are observed (Berreman-mode). The interpretation of the measured spectra is carried out by a fit procedure to simulate the observed spectra. As a result the specimens are characterized in terms of vibronic resonances and their oscillator strengths or concentration, thicknesses of various films in a stack of layers, profiles of depth depending chemical composition, or concentration and mobility of conduction electrons.All examples are relevant for application in technology, as microelectronics, thin film technology, catalysis, e.g. The results of the IR analysis are compared with those of other analytical methods as SIMS, RBS, and AES. The agreement is very good. One important advantage of the IR analysis, however, is the fact that it is a non-destructive method.     

Characterisation of salt films on dissolving metal surfaces in artificial corrosion pits via in situ synchrotron X-ray diffraction

The salt films formed on metal surfaces dissolving inside artificial corrosion pits formed in 1 M HCl have been probed with synchrotron X-ray diffraction. NiCl₂ · 6H₂O is the main phase in the salt film on nickel, whereas salt films on both iron and 316 L stainless steel are predominantly FeCl₂ · 4H₂O. However, the salt film on iron has a very fine homogeneous crystallite size whereas that on stainless steel is much coarser. The potential-dependence of the film formed on iron has been determined.     

Raman spectroscopy of selected copper minerals of significance in corrosion

The Raman spectroscopy of selected minerals of the corrosion products has been measured including nantokite, eriochalcite, claringbullite, atacamite, paratacamite, clinoatacamite and brochantite and related minerals. The free energy of formation shows that each mineral is stable relative to copper metal. The mineral, which is formed in copper corrosion, depends on the kinetics and conditions of the reaction. Raman spectroscopy clearly identifies each mineral by its characteristic Raman spectrum. The Raman spectrum is related to the mineral structure and bands are assigned to CuCl stretching and bending modes and to SO stretching modes. Clinoatacamite is identified as the polymorph of atacamite and not paratacamite. Paratacamite is a separate mineral with a similar but different structure to that of atacamite.     

Microanalysis of solid surfaces by Secondary Neutral Mass Spectrometry

Summary The application of Secondary Neutral Mass Spectrometry (SNMS) for high resolution microanalysis of solid surfaces is a rapidly evolving field. Significant progress which has recently been made, as the different SNMS techniques are further developed, includes two important aspects. First the outstanding capability of SNMS for depth profiling with high depth resolution, which in the past was essentially restricted to electrically conducting samples, has been extended towards the analysis of insulating materials with dielectric constants approaching unity. Second, great effort has been made to develop a secondary neutral microprobe which allows SNMS surface and in-depth analysis with high lateral resolution. Here two promising approaches exist, which are based on different methods for post-ionization of sputtered neutral particles. The present state-of-the-art of both techniques is shown and a comparison in respect to detection limits and image acquisition times is attempted.