Surface Spectroscopic Techniques Research Articles

Structural defects in poly(vinylene disulfide)

We have investigated the structural defects in poly (vinylene disulfide), -(S-CH=CH-S) n -, using optical, magnetic and surface spectroscopic techniques. This has been done by comparing the analysis results obtained on pristine samples with those on protic-acid-doped samples. In the undoped sample, the main type of defect is found on carbon broken bonds which gives rise to carbon-oxygen bonding due to oxidation of the sample. In doped samples, the sulfur sites are greatly affected by doping and appear as the main protonation sites. A careful analysis of all the results has enabled the role of the defects in the electrical conduction mechanism to be clarified.

Oxygen reduction in acid media catalyzed by heat treated cobalt tetraazaannulene supported on an active charcoal: correlations between the performances after longevity tests and the active site configuration as seen by XPS and ToF-SIMS

The influence of heat treatment (HT) on CoTAA supported on SX Ultra samples before and after electrochemical tests has been investigated using the rotating disk electrode (RDE) technique and cyclic voltammetry, The heat treated sample configuration was also scrutinized by two different surface spectroscopic techniques: X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS). After a 100 h polarization period at 500 mV (RHE) (diffusion activation domain), optimal performances were found to occur with the 600 degrees C heat treated samples (HT 600). However, the most salient feature was the observation of a fair activity and stability for the 800 degrees C heat treated samples (HT 800). Similar behavior was exhibited both with H2SO4 and Nafion(R) electrolytes. Collected data could be interpreted on the basis of RDE and cyclic voltammetry theories, suggesting a fast dioxygen mass transfer within the catalytic material in comparison with interfacial process kinetics. After these electrochemical tests a complete demetallation was taking place, as seen by XPS within the HT 800 samples. A protonation process was also clearly identified on the nitrogen ions. An attempt has been made to propose a catalytic cycle in which variations of the different nitrogen oxidation states are occuring. The major obstacle encountered lay in the lack of XPS evidence of the high nitrogen oxidation states. From ToF-SIMS data the existence of bonding between C, N and O can be established with NO3- formation. Under the same treatment conditions this process occurs differently with H(2)TAA precursor. From the collected data it turned out that catalytic cycles involving nitrogen ions possess a lower turnover than those with cobalt ion participation.

In Situ Electron Spectroscopic Identification of Carbon Species Deposited by Laser Ablation

AbstractThin carbon films were grown on Si (111) substrates by ablating a graphite target utilizing an excimer pulsed laser in a UHV Riber © LDM-32 system. Two kinds of films were produced, a highly oriented pyrolytic graphite (HOPG) type and a diamond-like carbon (DLC) type. A relationship of the films microstructure with laser power density and substrate conditions was observed. The HOPG films were homogeneous but the DLC films were heterogeneous, as shown by micrographs. The thin films are monitored and analyzed in situ during the first stages of the deposition process. The monitoring was done by RHEED and the characterization by several surface spectroscopic techniques, AES, XPS and EELS. The formation of a SiC interface was observed for both films due to the reaction of the first carbon species with the substrate surface.

Investigations of zeolites by photoelectron and ion-scattering spectroscopy. Part 3.—Cation depletion at the external surface of HNa-faujasites

Zeolites containing both Na and H ions (HNa-X, HNa-Y) have been prepared by thermal decomposition of NH4+ in ammonia-exchanged precursors or by reduction of transition-metal ions (Ag+) previously introduced by ion exchange. These materials, together with other zeolites (Na-A, RuK-A, Na-MOR, Na-ZSM-5), have been studied by surface-spectroscopic techniques [X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and ion-scattering spectroscopy (ISS)]. After the thermal treatment required to obtain the H forms (calcination, reduction), the near-surface region of the zeolite crystals is depleted of Na cations, probably due to their migration into the interior of the crystallite. This is demonstrated by the relative intensities of XPS as well as of ISS signals. In UPS, the characteristic changes of the signals ascribed to the influence of the anionic excess charge on the electronic structure of the framework, which reflect the transformation of the ionic Na form into the largely covalent H form, occur at low bulk exchange degrees.

Interaction of Silane Coupling Agents with the TiO2(110) Surface

The interactions of prototypical silane coupling agents with a clean and hydroxyl-predosed TiO 2 (110) single-crystal surface have been studied using surface spectroscopic techniques. The adsorption and reactions of vinyltriethoxysilane (VTES), diethyldiethoxysilane (DEDS), and (aminopropyl)triethoxysilane (APS) were studied by means of temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Even in the absence of surface water or hydroxyls, VTES dissociates rapidly to form Si(OEt)-(CH=CH 2 ) s and two OEt s , where Et = C 2 H 5 and s refers to surface species. The DEDS similarly dissociates to produce a mixture of SiEt 2,s , EtOSiEt 2,s , and one to two OEt s . Both of these silicon moieties are bound to the surface via Si-O-Ti bonds. Any EtO- ligands, whether bound to Ti cations or to the adsorbed silanes, decompose at ∼650 K via β-hydride elimination to create ethylene gas and a surface-bound hydrogen (OH s ). This hydrogen further reacts with a second EtO- ligand to produce ethanol gas. Above 700 K, the ethyl ligands left on the adsorbed silane produced from DEDS decompose via β-hydride elimination to give ethylene gas. Similarly, the CH 2 =CH- ligands left on the adsorbed silane produced from VTES also decomposes through β-hydride elimination into acetylene gas and surface hydrogen, but they hydrogenate to give ethylene much more rapidly. When hydroxyls are predosed on the surface, they react with any Ti 4+ -bound EtO- ligands formed in the initial decomposition of VTES and DEDS to produce ethanol gas at ∼350 K. Thus, the main function of the hydroxyls is to clear Ti 4+ sites of ethoxy groups, not to act as sites for the initial attachment of the silane to the surface. Neither TPD nor XPS data indicated that APS dissociated to a measurable extent on the TiO 2 (110) surface.

Effects of deposition parameters on morphology of Langmuir-blodgett films of poly(γ-Benzyl-L-Glutamate) by scanning-probe-microscopy

Synthetic homopolypeptides, such as poly(γ-benzyl-L-glutamate)(PBLG) bear oc-helical secondary structure and exhibit liquid-crystalline behavior. These properties are useful for a range of materials applications, among them nonlinear optics (NLO). The α-helical structural motif of PBLG can be exploited by regular substitution of chemical functionalities in the sidechains along the main axis. Macromolecular order can then be established in polypeptide thin films using Langmuir-Blodgett (LB) deposition. Monolayer films of PBLG have been investigated by surface spectroscopic techniques, without having an understanding of the local morphology.Atomic force microscopy (AFM) has the potential for elucidating the microstructure of LB films, providing a mechanism for relating differing morphologies to deposition parameters. Excessive contact forces exerted by an AFM tip at small size scales, however, make the technique less viable for delicate organic monolayers. By intermittently striking a sample surface with a sharp probe, the tapping mode AFM provides a means for nondestructively imaging such soft samples.

Surface Spectroscopic Studies of the Deposition Mechanisms of TinX Films from Organometallic Precursors

ABSTRACTThe adsorption and thermal behavior of tetrakis-(dimethylamido)-titaniurn (TDMAT), Ti[NMe2]4, were investigated by surface spectroscopic techniques in the temperature range 100-1100K. A metallic Ti substrate readily dissociates TDMAT even below 300 K, producing a carbon-rich interface. When the substrate is exposed to a continuous flux of TDMAT at growth temperatures (550-700K), deposition of carbon-rich TiCxNy films readily occurs with a high precursor reactive sticking coefficient. With the addition of sufficient NH3 flux, we demonstrate the existence of a direct surface-reaction-driven deposition mechanism which involves reaction(s) between adsorbed TDMAT and NHX species on the film surface and thus leads to growth of substantially cleaner TiNx films. This growth mechanism dominates at low pressures (≤10-4Torr) where gas-phase reaction between the precursor gases becomes insignificant.

The observation and distribution of organic additives on paper surfaces using surface spectroscopic techniques

The observation and distribution of organic additives on paper surfaces using surface spectroscopic techniques

Applications of MBMS and surface spectroscopic techniques in the study of reaction mechanisms in CBE; investigations of the reactivity of tritertiarybutylgallium and triisobutylgallium as alternative precursors for epilayer growth

Current approaches to the study of reaction mechanisms in CBE and investigations of the potential of triisobutylgallium and tritertiarybutylgallium as novel CBE precursors are reviewed. Surface spectroscopic techniques indicate that adsorbed iso-butyl radicals decompose to produce gaseous butene and hydrogen at significantly lower temperatures than in the corresponding process for ethyl radicals on GaAs, resulting in lowered growth temperatures and low temperature C incorporation levels in comparison to the results obtained with triethylgallium. A β-methyl migration occurs at higher temperatures causing C to deposit irreversibly on the surface in the presence of Al. Lowered temperatures for the β-hydride elimination reaction are also observed for adsorbed tertiarybutyl radicals and the absence of β-methyl groups avoids the facile C deposition process seen for iso-butyl. These potential advantages associated with tertiarybutyl ligands cannot be realized straightforwardly in CBE using tritertiarybutylgallium however, since steric crowding effects result in the inefficient total dissociation of the adsorbing precursor molecule.

Electronic structure of epitaxial β-FeSi2 on Si(111)

We have applied a combination of surface spectroscopy techniques (XPS, Auger, electron energy loss and bremsstrahlung isochromat spectroscopy) to investigate the chemical bonding of β-FeSi2 thin films epitaxially grown on the Si(111) surface. The local structure of the films has been investigated with the extended BIS fine structure technique. The semiconducting nature of the films is evidenced by a shift of the empty d-band to higher binding energies and by the lowering of the density of the states close to EF as compared to pure Fe films. The dielectric functions extracted from the electron energy loss spectra strongly support this picture. We measured an energy gap of 1.0 ± 0.2 eV which compares favourably with density of states calculations for the β-FeSi2 semiconducting phase.

Conductance, work function and catalytic activity of SnO 2-based gas sensors

We have studied the geometric microstructure as well as the elemental composition and distribution of SnO 2-based polycrystalline sensors with X-ray diffraction and surface spectroscopic techniques. Electrical properties and responses against reducing gases are characterized by a.c. and d.c. measurements of resistances and/or conductances. The models explaining the high sensitivity of polycrystalline SnO 2 against reducing gases in terms of changes of the intergrain conductivity have been completed. The Schottky-barrier mechanism of the electron transport across the grain boundaries is valid only for SnO 2 grains larger than the Debye length of electrons. For grains smaller than the Debye length, the band bending at the surface can be neglected compared with the overall shift of the Fermi level in the grains during gas exposure. From combined measurements of conductances, work function changes and catalytic activities as functions of temperature, CO and H 2O partial pressures, we deduce that OH dipoles, which do not influence the oxidation kinetics of carbon monoxide, are formed during interaction with water at 673 K.

Interaction of Pd-overlayers with SnO2: comparative XPS, SIMS, and SNMS studies

We report about interaction processes between palladium (Pd) and tin dioxide (SnO2) studied with various surface spectroscopic techniques. Total sputter yields necessary for absolute depth calibration in SIMS are determined for SnO2. Clustering of palladium occurs at low temperatures. Small changes in the XPS relative core level intensities of Pd and Sn allow to determine cluster sizes. Oxidation of Pd in the presence of oxygen at T≥470 K is a prerequisite for diffusion of Pd2+ ions into SnO2 layers. The latter process is confirmed and described quantitatively by evaluating the SIMS and SNMS measurements.

Surface spectroscopic studies of pristine and fouled membranes part II. Method development and adsorption mechanism

The complicated problem of membrane fouling has been tackled by applying several surface spectroscopical methods. The results elucidate various chemical aspects of the interaction between polysulfone (PSf) membranes and polypropylene glycol (PPG), which combination was chosen as a model system. All analyses of the DDS GR61 PP and Dorr-Oliver S10 membranes fouled by adsorption of PPG, show that the latter is present as an additional layer on the surfaces. The atomic composition, as revealed by X-ray photoelectron spectroscopy (XPS), of the pristine membranes changes towards that of PPG in the fouled surfaces. In fast atom bombardment mass spectrometry (FAB MS) some m/z fragments occur that are not observed in the spectra of the membranes and of the foulant separately. Unfortunately, assignment of these ions, which could contribute in clarifying the chemical mechanism of adsorption, is not possible due to their complexity. On the other hand, attenuated total reflection Fourier transform infrared spectroscopy (ATR FTIR) demonstrates that the major interaction between PSf and PPG proceeds through the aryl—C (CH 3) 2—aryl structure in PSf and the CH 3 groups in PPG. Also in S10, consisting mainly of Victrex polyethersulfone with Udel as a minor surface constituent (see Part I), this interaction is clearly observed. This study shows integrated application of various surface spectroscopic techniques to be a powerful means in unravelling the mechanisms of fouling. It is especially helpful in directing research on membrane materials for special purposes or in designing cleaning agents and procedures for membranes.

Surface Chemistries and Photoelectrochemistries of Thin Film Molecular Semiconductor Materials

Thin films of vacuum deposited trivalent and tetravalent metal phthalocyanines (Pc), and derivatives of perylenes (Pe), have been explored by a combination of ultrahigh vacuum deposition, surface spectroscopic characterization techniques, and measurement of photoconductivity and photocurrent spectral responses involving a) interdigitated array microcircuits (MC), and b) photoelectrochemical cells. Of particular interest has been the way adsorbed molecules such as O 2 and NH 3 affect the photoconductivity of these thin film materials, and the effect on photoactivity of the formation of a sharp interface between a phthalocyanine and a perylene. Multiple chemisorption sites are implicated for molecules such as O 2 and NH 3 on trivalent metal Pc thin film surfaces. These sites may either increase or decrease photoconductivity, depending on the partial pressure of the adsorbant, and the coverage of molecules competing for the same chemisorption sites. Photoelectrochemical modification of the Pc surface introduces submonolayer coverages of metals like Ag, providing a chemisorption site for molecules such as NH 3 , thereby improving the performance of these photoconductivity-based chemical sensors. Photoelectrochemical techniques can be used to characterize Pc/Pe bilayers, since the electrolyte provides a noncorrosive, optically transparent electrical contact. Photocurrent yield spectra, obtained from two different illumination directions, have been used to estimate the width of the interfacial region which is most active for exciton dissociation in the Pc/Pe bilayer. Transient photocurrent yields in Pc/Pe multilayers (up to 16 total layers), show that the photocurrent increases linearly with the number of Pc/Pe interfaces. These increases occur up to the point where the width of the individual layers becomes narrower than the interface resolution that our present vacuum deposition technologies can provide

Prototype structure for systematic investigations of thin-film gas sensors

We have developed thin-film prototype structures for electronic conductivity and impedance sensors with integrated heater, temperature sensor and interdigital electrodes. These structures serve as substrates for subsequent deposition of chemically sensitive materials. The structures are based upon platinum and sapphire, they are stable even at high temperatures ( T ∼- 1073 K) under oxidizing and reducing conditions, and they have suitable dimensions for surface spectroscopic techniques to be applied as well. The last is important for optimizing sensors by characterizing their electronic and chemical structures such as stoichiometries, impurities, dopants or interdiffusion profiles of the sensitive layers. We use these structures to fabricate prototype NO 2 sensors based upon SnO 2, which show surprisingly low cross-sensitivities against CO and H 2O.

Surface Reaction Mechanisms in Chemical Beam Epitaxy

ABSTRACTSurface reaction mechanisms which underly the growth of III-V semiconductors by chemical beam epitaxy have been investigated using a combination of surface spectroscopic techniques in conjunction with modulated molecular beam scattering techniques. Emphasis is placed on understanding the complex growth rate effects observed during the growth of Ga(Al,In)As and the origin of selected area epitaxy. These effects are shown to arise from the surface sensitive nature of the decomposition of the group III alkyl source chemicals used in CBE.

Thermal and ion-beam-induced etching of InP with chlorine

Surface spectroscopic techniques have been used to investigate adsorption and thermal and ion-induced processes at the InP(100)-Cl2 interface. Two adsorption states are identified and etching reactions are interpreted in terms of surface chemical transformations and desorption processes involving these states.

Electron spectroscopic studies of the Dy/Si(111) interface

Low-energy electron diffraction (LEED), x-ray photoelectron (XPS), ultraviolet photoelectron (UPS), and Auger electron (AES) surface spectroscopic techniques were used to study the chemical interaction between Dy atoms and Si atoms at the Dy–Si interface at room temperature, as well as Dy silicides formed after annealing at 400 °C for 30 min. Experiment shows that at low coverages [≤4 monolayers (ML)] atom intermixing takes place, causing chemical shifts of Dy 4f and Dy 3d. However, no significant chemical shift of Si 2p has been observed, suggesting a lack of strong chemical bonds of the Dy–Si type. With coverage increasing, a (7×7) LEED pattern from a clean Si(111) surface turns into a (1×1), and finally becomes ambiguous. After annealing for 30 min at 400 °C, the Dy silicides formed with an atomic concentration ratio of Dy to Si of ∼1:2. The binding energies of Dy 4f (Dy Si2), Dy 3d(Dy Si2), and Si 2p(Dy Si2) are 5.5, 1295.2, and 98.65 eV, respectively.

An EXAFS study on the morphology change of Ru catalyst by CO adsorption

Great attention has recently been paid to the reconstruction of clean metal surface upon adsorption. Then a question naturally arise what happens to the small metal clusters in supported catalyst upon adsorption, which is the first step of catalytic reaction. Surface spectroscopic techniques like LEED are not available for small particles on the surface of supports, and mainly infrared absorption spectroscopy has provided information, rather indirectly, through the number of observed vibrations and the frequency shifts of adsorbed species such as CO. Recently Van't Blik et al. reported in a series of papers direct evidence of a structural change of Rh clusters supported in Al/sub 2/O/sub 3/ upon CO adsorption. They observed a drastic change in EXAFS (extended X-ray absorption fine structure) by the admission of CO. It was concluded that this change was caused by the disruption of the metal-metal bond, resulting in formation of Rh(CO)/sub 2/ species. As far as the authors know, this is the only example of straightforward evidence of morphology changes caused by adsorption, and it is not certain whether or not this phenomenon is general. In this communication they present another example of the morphology change upon adsorption for Ru supported on ..gamma..-Al/sub 2/O/submore » 3/ and show this phenomenon is not limited to Rh.« less

An extended x-ray absorption fine structure study on the morphology change of ruthenium catalyst by carbon monoxide adsorption

Great attention has recently been paid to the reconstruction of clean metal surface upon adsorption. Then a question naturally arise what happens to the small metal clusters in supported catalyst upon adsorption, which is the first step of catalytic reaction. Surface spectroscopic techniques like LEED are not available for small particles on the surface of supports, and mainly infrared absorption spectroscopy has provided information, rather indirectly, through the number of observed vibrations and the frequency shifts of adsorbed species such as CO. Recently Van't Blik et al. reported in a series of papers direct evidence of a structural change of Rh clusters supported in Al/sub 2/O/sub 3/ upon CO adsorption. They observed a drastic change in EXAFS (extended X-ray absorption fine structure) by the admission of CO. It was concluded that this change was caused by the disruption of the metal-metal bond, resulting in formation of Rh(CO)/sub 2/ species. As far as the authors know, this is the only example of straightforward evidence of morphology changes caused by adsorption, and it is not certain whether or not this phenomenon is general. In this communication they present another example of the morphology change upon adsorption for Ru supported on ..gamma..-Al/sub 2/O/submore » 3/ and show this phenomenon is not limited to Rh.« less