Monocrystalline Surfaces Research Articles

Surface characterization of Cu-Ti systems: an IR study

A comparative study of the surface properties of Cu-Ti samples prepared by impregnation of TiO 2 with a Cu(NO 3) 2 solution or from amorphous CuTi alloy precursors diluted with TiO 2 has been made. The samples were submitted to vacuum and H 2 treatments at increasing temperatures, from 493 up to 773 K; at the end of the pretreatments the specimens were contacted with CO, from RT up to 573 K. The surface species produced were monitored by FTIR spectroscopy. On all the samples a band of CO adsorbed on Cu surface sites, significantly blue-shifted in comparison with that observed on Cu monocrystalline surfaces and on Cu particles supported on other n-type oxidic semiconductors, was observed. The blue-shift, that is maximum on the most strongly reduced samples, was interpreted as due to an electronic interaction of copper with reduced titania and to the fact that, on the more strongly reduced samples, copper particles are almost completely covered by reduced titania and copper sites are almost isolated. The NO interaction on CO precovered samples produces a strong intensity and a red-shift of the CO absorption band, indicating that in oxidizing conditions and in the presence of CO the exposed Cu grows up.

Recoil implantation effects in ion impact desorption of monolayer adsorbates on solid surfaces

Ion impact desorption of monolayer adsorbates on solid surfaces has been generally explained by three main terms: (I) reflection of direct knock-on adsorbate recoils, (II) knock-off by reflected projectiles and (III) knock-off by cascade collisions. In the case I, some fraction of the knock-on adsorbate recoils, not reflected, are implanted into the substrate. For evaluation of recoil-implantation effects on the adsorbate decay profile, mass balance equations for adsorbates at the top-surface and recoil-implanted in the sub-surface layers are proposed taking into account the desorption, recoil-implantation and collisional mixing with anisotropic effect. It is shown that the experimental decay profiles obtained by surface non-sensitive analysis techniques such as RBS are non-(bi-) exponential due to the recoil-implantation effect and that the analysis of the decay profiles provides the cross-sections for desorption and recoil-implantation. The recoil implantation cross-sections are compared with the theoretical recoil-production cross-sections. Finally, it is demonstrated that the recoil-implantation cross-section is used to characterize the well-defined ordered-adsorbates on monocrystalline surfaces, namely Si(111)√3 × √3-Au and -Ag surfaces.

Dynamic scaling in polycrystalline growth.

The dynamic scaling properties of a polycrystalline growth model proposed by Van der Drift are discussed. We present an analytic derivation of the dynamic exponent, describing the growth of monocrystalline surface domains, yielding p=1/2 and 1/4 for two and three dimensions, respectively. For specific, highly nonuniform, initial conditions in the two-dimensional model we find that initially the exponent p is equal to 1, but that after some time, crossover takes place to p=1/2. The results are confirmed by numerical simulations for the two-dimensional case. We investigate the relation between our model and the Huygens model for amorphous growth, formulated by Tang, Alexander, and Bruinsma and examine both models in the context of a differential equation for interface growth, analyzed by Kardar, Parisi, and Zhang.

Arrhenius parameters for rate processes on solid surfaces

During the last decade, many attempts have been made to measure accurately the apparent Arrhenius parameters (i.e., the activation energy and pre-exponential factor) for various rate processes on monocrystalline surfaces. The main attention was directed to studying desorption of the simplest molecules such as CO, H 2 and NO. Interesting results have also been derived for CO oxidation on different metals and for surface diffusion. The present review contains a detailed catalogue of the experimental data obtained in this field of surface science. The data collected indicate that the coverage dependence of the apparent Arrhenius parameters is, as a rule, rather strong. With increasing coverage, the change in the activation energy usually exceeds 10 kcal/mol and the pre-exponential factor often varies over three or more orders of magnitude. The direction of variation, as a rule, satisfies the compensation effect. The coverage dependence of the activation energy is customarily explained by lateral interactions between adsorbed particles, by adsorbate-induced changes in the surface, or by different types of adsorption sites. Various models collected predict also the coverage dependence of the pre-exponential factor for different rate processes in adsorbed overlayers. However, the predicted coverage dependences of the pre-exponential factor are usually not so strong as that measured in real systems.

Formation and dissipation of adlayer domains during electrooxidation of carbon monoxide on ordered monocrystalline and disordered platinum and rhodium surfaces as probed by FTIR spectroscopy

Abstract The stability of adlayer domains (“islands”) of carbon monoxide formed during electrooxidation on ordered and disordered monocrystalline platinum and rhodium electrodes in aqueous 0.1M HClO 4 has been characterized by means of real-time surface FTIR spectroscopy. The presence of such domains is discerned in part from the retention of high-frequency CO stretching ( v CO ) bands for both terminal and twofold bridging CO, characteristic of close-packed adlayer structures, during electrooxidative CO removal down to low average coverages, θ. Further evidence for the presence of stable adlayer domains on ordered Pt(111) and Pt(100) was obtained by sequential dosing and partial electrooxidation of 13 CO followed by 12 CO dosing. The dipole-dipole coupling properties of such adlayers indicate partial segregation of the isotopic forms. Subsequent rapid electrooxidation of these mixed isotope layers yields initially preferential formation of 12 CO 2 , pointing to the presence of reactive domain regions on the ordered surfaces. While the adlayer domains are stable for extended periods (ca. 1 h) on most ordered monocrystalline surfaces, they undergo relaxation on Pt(100) as discerned from decreases in the v CO frequency with time at a fixed θ as well as with decreasing coverage. The dissipation of adlayer structures on disordered monocrystalline Pt and Rh was also observed during slow potentiostatic oxidation from θ-induced v CO frequency decreases, which become progressively greater for longer oxidation timescales. Similar findings were also obtained for CO on both mechanically polished and annealed polycrystalline platinum; domain dissipation is especially facile on the former surface. Such electrooxidation-induced CO domain dissipation is attributed to the nucleation of the reaction front at surface defect sites. The stability of adlayer domains during electrooxidation on most ordered surfaces, and for short reaction timescales on disordered surfaces, is ascribed to a lower density of nucleation sites operative under these conditions.

Formation and dissipation of adlayer domains during electrooxidation of carbon monoxide on ordered monocrystalline and disordered platinum and rhodium surfaces as probed by FTIR spectroscopy

The stability of adlayer domains (“islands”) of carbon monoxide formed during electrooxidation on ordered and disordered monocrystalline platinum and rhodium electrodes in aqueous 0.1M HClO 4 has been characterized by means of real-time surface FTIR spectroscopy. The presence of such domains is discerned in part from the retention of high-frequency CO stretching ( v CO) bands for both terminal and twofold bridging CO, characteristic of close-packed adlayer structures, during electrooxidative CO removal down to low average coverages, θ. Further evidence for the presence of stable adlayer domains on ordered Pt(111) and Pt(100) was obtained by sequential dosing and partial electrooxidation of 13CO followed by 12CO dosing. The dipole-dipole coupling properties of such adlayers indicate partial segregation of the isotopic forms. Subsequent rapid electrooxidation of these mixed isotope layers yields initially preferential formation of 12CO 2, pointing to the presence of reactive domain regions on the ordered surfaces. While the adlayer domains are stable for extended periods (ca. 1 h) on most ordered monocrystalline surfaces, they undergo relaxation on Pt(100) as discerned from decreases in the v CO frequency with time at a fixed θ as well as with decreasing coverage. The dissipation of adlayer structures on disordered monocrystalline Pt and Rh was also observed during slow potentiostatic oxidation from θ-induced v CO frequency decreases, which become progressively greater for longer oxidation timescales. Similar findings were also obtained for CO on both mechanically polished and annealed polycrystalline platinum; domain dissipation is especially facile on the former surface. Such electrooxidation-induced CO domain dissipation is attributed to the nucleation of the reaction front at surface defect sites. The stability of adlayer domains during electrooxidation on most ordered surfaces, and for short reaction timescales on disordered surfaces, is ascribed to a lower density of nucleation sites operative under these conditions.

Composition profile at Pt xNi 100−x alloy surfaces by ideas (incidence dependent excitation for Auger spectroscopy)

The IDEAS method is applied to measure the composition profile of very different PtNi monocrystalline surfaces: Pt 50Ni 50 (110) and (100), and Pt 10Ni 90 (110). The basic ingredients of the method are carefully discussed and an optimized procedure is presented. The results are in very good agreement with previous results obtained mainly from LEED but also from ISS and RBS under channeling and blocking conditions. The accuracy is typically of 5 to 10 at% Pt and 10 to 20 at% Pt on the determination of C 1 and C 2 respectively: this is well suited to segregation studies for comparison with model predictions or in conjunction with catalysis work. Being a spectroscopic instead of a scattering method, it is possible to apply it to a wide range of situations: less ordered surfaces, high temperature, compounds made of chemical elements with close atomic numbers. Although no geometrical information is accessible, IDEAS seems a valuable complementary tool to scattering methods in a complete surface crystallographic study.

Argon-induced Auger emission from amorphous and crystalline silicon: A comparison between experiment and simulation

Measurements of absolute Si L23 Auger emission yields are compared with computer simulations in the case of 40 keV argon irradiation of amorphous and monocrystalline silicon. A model parameter adjustment was found which allows a fair comparison between the incidence-angle dependence of the Auger yield obtained by simulation and experimentally. As in the case of aluminium, studied previously, the neutralization of sputtered ions had to be taken into account. A Hagstrum process is assumed with an energy dependence of the characteristic velocity. Lattice effects on both atomic- and bandlike emission are observed and calculated, which are not significantly affected by thermal atomic displacements. The enhancement of the Auger yield observed experimentally with the transition from the amorphous to the single-crystal phases is well reproduced by computer simulation, except for normal incidence. In this latter case, the calculated Auger yields for both amorphous and monocrystalline surfaces do not differ strongly.

Hydrous phase in the crystalline vanadium oxide spheres

The presence of a hydrous phase was investigated in fine crystalline V 2O 5 spheres which contain a cavity inside. A high-resolution electron micrograph shows the formation of hydrous V 2O 5 in the grain boundaries between V 2O 5 crystallites. The experiments using sorption of gases, thermal desorption spectroscopy (TDS), and IR spectroscopy have suggested that the hydrous phase is composed mainly of a layered compound, probably V 2O 5 · 1.6H 2O. The physisorption of water was almost completely reversible up to a relative pressure of 0.3. The interlayer spacings seems to work as sorption sites. When H 2O is sorbed as a monolayer, determined by applying the BET equation to the sorption isotherm, the hydrous phase is identified as V 2O 5 · 2.6H 2O. Most of the H 2O molecules in the hydrous phase can be removed below 200°C; removal of oxygen commences at 250°C in vacuum. Various states of H 2O phase were demonstrated TDS with peaks at 100, 170, 260, and 420°C. The heating of V 2O 5 spheres in O 2 at 300–400°C produces a mosaic particle with monocrystalline (010) surfaces.

Mechanism of the reaction NO + NH 3ON V 2O 5 catalysts

XPS analysis of monocrystalline surfaces of V 2O 5 revealed that the (010) crystal plane contains only V +5 and O 2− ions, whereas the (100) plane is covered by OH groups. In the case of powdered samples of different grain shapes, varying between thin plates and bars, a correlation was found between the intensity of the IR band of ammonia adsorbed in the form of NH + 3 ions, and the value of the X-ray morphological factor f = I(101)/ I(010), indicating that also in the case of dispersed samples the OH groups of Brönsted acid character are located on the crystal planes perpendicular to (010), e.g. on the (100), (001) and (101) planes. An identical correlation was obtained between the value of ‘ f’ and the yield of N 2 from the reaction NO+ NH 3, which indicates that this reaction proceeds with participation of ammonia adsorbed on Brönsted acid sites on the crystal planes perpendicular to (010). Thus, the amount of N 2 formed in this reaction cannot be taken as a measure of the concentration of VO groups at the surface of V 2O 5 catalysts, as postulated in the literature.

Information on adsorbate positions from low-energy recoil scattering: Adsorption of hydrogen on Pt.

Low-energy (10-keV) recoil scattering is proposed as a tool for measurement of adsorbate positions on monocrystalline surfaces. This technique, which makes use of high-energy-recoil adsorbate atoms (energy \ensuremath{\ge} 1 keV), supplies real-space structure information. Its potentialities are illustrated in a study of the adsorption system H on (111) sites of Pt. By exploiting surface channeling of incident 10-keV Ne ions on vicinal Pt surfaces we show unambiguously that H occupies fcc-type adsorption sites on Pt(111) terraces.

Adsorption of O 2 and CO on thin films of palladium

The adsorption of O 2 and CO on polycrystalline and (111) monocrystalline thin palladium films formed by vapor deposition has been studied by means of Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and mass spectrometry. The palladium film thickness ranged from 15 to approximately 1500 Å. The experiments were carried out at 298 K. Surface topographical effects were studied in various ways, one of which was a method consisting of repeated alternating vapor deposition of palladium and gaseous exposure. In all cases studied, no LEED pattern other than the normal Pd(1x1) pattern was observed after gaseous exposures up to 6000 langmuir of O 2 or 7200 langmuir of CO. However, AES showed clearly that some O 2 and CO adsorbed on polycrystalline and monocrystalline surfaces that were either contaminated or rough. Thick (1500 Å) smooth Pd(111) films grown on mica showed no evidence of significant O 2 or CO adsorption, and in both cases the AES intensity ratio of the O(512 eV) line to the Pd(329 eV) line was near zero.

Stability and reactivity of (001) and (111) NiO: a RHEED-AES investigation of Si surface segregation and Ni formation by gas reduction

Abstract In this contribution are reported recent results regarding the preparation and the reactivity of different nickel oxide (NiO) monocrystalline surfaces. At first, it concerns a structural investigation of metal formation by hydrogen reduction of (001) NiO. In most cases, Ni nuclei form three-dimensional islands as observed by RHEED and corroborated by AES. At low temperatures, metal stacking, previously described as pure hcp stacking from (1120) planes, is shown to develop, in reality, with stacking faults from which fcc stacking is often formed. At high temperatures, metal stacking is retained as within the oxide and, from a comparative transmission electron microscopy investigation, a model for the topotactic nucleus formation is put forward. Secondly, the preparation and the stability of (111) NiO are investigated. Apparent (111) NiO stability is found to be due to the thermal surface segregation of silicon. Even in very low amount, this very mobile impurity segregates at the (111) NiO surface leading to a two-dimensional O-Si-Ni compound.

Stability and reactivity of (001) and (111) NiO: A RHEED-AES investigation of Si surface segregation and Ni formation by gas reduction

In this contribution are reported recent results regarding the preparation and the reactivity of different nickel oxide (NiO) monocrystalline surfaces. At first, it concerns a structural investigation of metal formation by hydrogen reduction of (001) NiO. In most cases, Ni nuclei form three-dimensional islands as observed by RHEED and corroborated by AES. At low temperatures, metal stacking, previously described as pure hcp stacking from (112̄0) planes, is shown to develop, in reality, with stacking faults from which fcc stacking is often formed. At high temperatures, metal stacking is retained as within the oxide and, from a comparative transmission electron microscopy investigation, a model for the topotactic nucleus formation is put forward. Secondly, the preparation and the stability of (111) NiO are investigated. Apparent (111) NiO stability is found to be due to the thermal surface segregation of silicon. Even in very low amount, this very mobile impurity segregates at the (111) NiO surface leading to a two-dimensional O-Si-Ni compound.

Optical Radiation from He Particles Scattered at a Monocrystalline Surface at Grazing Incidence

In detailed investigations, the scattering and the interaction mechanisms of 70 to 200 keV He+ ions at grazing incidence on a monocrystalline (110) NiFe surface are studied both by experiments and by computer simulations. The light intensity I and the Stokes parameters S/I, C/I, and M/I of the He+–468.6 nm and of the He0–587.6 nm radiation emitted by the scattered particles were observed as function of the angles of incidence relative to surface lattice directions and of the cleanness of the surface. The results exhibit a strong dependence of I and S/I on the surface crystallography and coverage. An interpretation of the results is proposed on the basis of two excitation mechanisms, (i) excitation by electron capture from the metal surface, (ii) excitation by collisions with adsorbed atoms, and (iii) an Auger deexcitation or resonance ionization mechanism which quenches the light emission of particles with small scattering angles.

Infrared investigations of adsorbed species on films and monocrystalline surfaces : A literature search

Publisher Summary This chapter presents several tabulations that are used for various infrared techniques. First table presented in the chapter provides the meaning of the symbols used for the various infrared techniques. Another table presented contains the identification of the investigated systems. When an oxide layer on a metallic film results from the oxidation of this film, the references are introduced in the corresponding metal section. The chapter also deals with significant emission, ellipsometric, transmission, and reflection methods. The main theoretical references listed in the chapter discusses frequency shifts upon adsorption as well as electrodynamical and dipole coupling interactions.

Study of buried silicon nitride layers synthesized by ion implantation

The formation of buried layers of silicon nitride by nitrogen-ion implantation in single-crystal silicon is studied. He+ backscattering, x-ray diffraction, scanning and transmission electron microscopies, and infrared absorption measurements were used for the physico-chemical characterization; sheet resistivity determination, spreading-resistance profile, and current-voltage characteristics for the electrical characterization. It is shown that, for 180-keV nitrogen ions, a fluence about 1018 N/cm2 must be implanted in order to obtain a continuous layer of silicon nitride and that annealing must be performed at 1200 °C to make it homogeneous and electrically insulating. The Si3N4 layer obtained crystallizes in the α phase and presents properties nearly similar to those of deposited layers. It is demonstrated that the conditions of implantation (energy, substrate temperature, beam intensity) play a fundamental role in the structure of the superficial silicon layer: the substrate must not be amorphized up to the surface during the implantation to obtain, after annealing, a monocrystalline surface layer suitable, for instance, for further epitaxy.

X‐ray investigations of friction stressed α‐Fe single crystals

AbstractA translation of a α‐Fe single crystal on a glass plate causes elastic and plastic deformations in regions near the surface. The dislocation density caused by the deformations and its dependence on the friction way were determined from Rocking curves. After having reached a certain friction way the monocrystalline surface layer changes into a polycrystalline layer. At the same time a friction texture develops within the polycrystalline layer.

Computer simulation of low energy hydrogen and helium ion reflection from monocrystalline surfaces

physica status solidi (a)Volume 55, Issue 1 p. K53-K58 Short Note Computer simulation of low energy hydrogen and helium ion reflection from monocrystalline surfaces M. A. Chubisov, M. A. Chubisov Institute of High Energy Physics, Academy of Sciences of the Kazakh SSR, Alma-Ata Search for more papers by this authorA. F. Akkerman, A. F. Akkerman Institute of High Energy Physics, Academy of Sciences of the Kazakh SSR, Alma-Ata Search for more papers by this author M. A. Chubisov, M. A. Chubisov Institute of High Energy Physics, Academy of Sciences of the Kazakh SSR, Alma-Ata Search for more papers by this authorA. F. Akkerman, A. F. Akkerman Institute of High Energy Physics, Academy of Sciences of the Kazakh SSR, Alma-Ata Search for more papers by this author First published: 16 September 1979 https://doi.org/10.1002/pssa.2210550152Citations: 3 480082 Alma-Ata, USSR. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume55, Issue116 September 1979Pages K53-K58 RelatedInformation

Optical excitation of he projectiles during reflection at a monocrystalline surface under channeling conditions

The emission of visible light from 195 keV He projectile ions excited by grazing scattering at a (101)Ni surface is used to perform a detailed investigation of “surface channeling”. The experiments exhibit focusing in semichannels and critical conditions for axial and planar channeling. The dependence of the light emission of excited singly charged He + and neutral He 0 on incidence direction towards the (101) surface is different at small angles. An interpretation of this behaviour is proposed which is based on a correlation between energy loss and charge state of axially surface channeled projectiles.