Metals In Ultrahigh Vacuum Research Articles

Nanoscale architecture of ceria-based model catalysts: Pt–Co nanostructures on well-ordered CeO2(111) thin films

We have prepared and characterized atomically well-defined model systems for ceria-supported Pt–Co core–shell catalysts. Pt@Co and Co@Pt core–shell nanostructures were grown on well-ordered CeO2(111) films on Cu(111) by physical vapour deposition of Pt and Co metals in ultrahigh vacuum and investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy. The deposition of Co onto CeO2(111) yields Co–CeO2(111) solid solution at low Co coverage (0.5 ML), followed by the growth of metallic Co nanoparticles at higher Co coverages. Both Pt@Co and Co@Pt model structures are stable against sintering in the temperature range between 300 and 500 K. After annealing at 500 K, the Pt@Co nanostructure contains nearly pure Co-shell while the Pt-shell in the Co@Pt is partially covered by metallic Co. Above 550 K, the re-ordering in the near surface regions yields a subsurface Pt–Co alloy and Pt-rich shells in both Pt@Co and Co@Pt nanostructures. In the case of Co@Pt nanoparticles, the chemical ordering in the near surface region depends on the initial thickness of the deposited Pt-shell. Annealing of the Co@Pt nanostructures in the presence of O2 triggers the decomposition of Pt–Co alloy along with the oxidation of Co, regardless of the thickness of the initial Pt-shell. Progressive oxidation of Co coupled with adsorbate-induced Co segregation leads to the formation of thick CoO layers on the surfaces of the supported Co@Pt nanostructures. This process is accompanied by the disintegration of the CeO2(111) film and encapsulation of oxidized Co@Pt nanostructures by CeO2 upon annealing in O2 above 550 K. Notably, during oxidation and reduction cycles with O2 and H2 at different temperatures, the changes in the structure and chemical composition of supported Co@Pt nanostructures were driven mainly by oxidation while reduction treatments had little effect regardless of the initial thickness of the Pt-shell.

Reactions of Adsorbed Porphyrins on Oxides and Metals in Ultrahigh Vacuum and Liquids

Adsorption of porphyrins on metal surfaces has been studied extensively in ultrahigh vacuum, but, despite the technological importance of oxides, much less work exists on the adsorption of porphyrins on oxide surfaces. Using X-ray photoelectron spectroscopy (XPS) the adsorption and reactions of 5,10,15,20-tetraphenylporphyrin on MgO(100) and TiO2(110) surfaces in ultrahigh vacuum have been studied and will be compared with reactions on metal surfaces both in ultrahigh vacuum and immersed in aqueous solution. Focus will be on metalation of free-base molecules and metal center exchange of metalloporphyrins where the porphyrin molecules extract metal ions from the oxide surfaces. The behavior on oxide surfaces will be shown to have a lot in common with reactions in the liquid phase, such as ion exchange and protonation.

Correction to Improved Contacts to MoS2 Transistors by Ultra-High Vacuum Metal Deposition.

Correction to Improved Contacts to MoS2 Transistors by Ultra-High Vacuum Metal Deposition.

Improved Contacts to MoS2 Transistors by Ultra-High Vacuum Metal Deposition.

The scaling of transistors to sub-10 nm dimensions is strongly limited by their contact resistance (RC). Here we present a systematic study of scaling MoS2 devices and contacts with varying electrode metals and controlled deposition conditions, over a wide range of temperatures (80 to 500 K), carrier densities (10(12) to 10(13) cm(-2)), and contact dimensions (20 to 500 nm). We uncover that Au deposited in ultra-high vacuum (∼10(-9) Torr) yields three times lower RC than under normal conditions, reaching 740 Ω·μm and specific contact resistivity 3 × 10(-7) Ω·cm(2), stable for over four months. Modeling reveals separate RC contributions from the Schottky barrier and the series access resistance, providing key insights on how to further improve scaling of MoS2 contacts and transistor dimensions. The contact transfer length is ∼35 nm at 300 K, which is verified experimentally using devices with 20 nm contacts and 70 nm contact pitch (CP), equivalent to the "14 nm" technology node.

Model of the effect of local frictional heating on the tribodesorbed gases from metals in ultra-high vacuum

In this paper, a model of thermal desorption of gases from the friction zone of metals in ultra-high vacuum is presented. Comparison of the model with the experimental data of tribodesorbed gases from AISI 304 steel shows that for mild friction the amount of desorbed gases owing to the local temperature increase is much lower than the experimentally obtained values. This finding supports the hypothesis that structural modification of material is the main driving process for tribodesorption. The study also shows that a significant volume of material around the friction zone contributes to the tribodesorption process.

Growth and magnetism of self‐organized Co nanoplatelets on Si(111) surface

AbstractMagnetic nanodots have great potential for applications in ultrahigh‐density information storage, quantum computing and many other fields. In this work, Co nanoplatelets with uniform size, height and shape were fabricated by ultrahigh vacuum metal evaporation on the Si(111)‐(7 × 7) surface, using a two‐dimensional identical Al cluster array as the template and spacer. The Al clusters reduce Si dangling bonds significantly and prevent the reaction between Si and the ferromagnetic metal Co. Almost all Co nanoplatelets appear as equilateral triangles with a side length of 5.4 nm and a magic thickness of 2 ML, pointing in the [211] direction. In situ scanning tunneling microscopy (STM) was used to determine the atomic structure and atomistic formation process of the Co nanoplatelets. Magnetic properties of the Co platelets were measured by SQUID at 5 K. Such quantum dots exhibit unusually high perpendicular magnetic anisotropy. The present study demonstrates a promising pathway to directly integrate magnetic nanostructures with Si‐based electronic devices. Copyright © 2006 John Wiley & Sons, Ltd.

Hybrid interfaces of conjugate polymers: Band edge alignment studied by ultraviolet photoelectron spectroscopy

The control of hybrid interfaces in polymer-based electronic devices may be enabling in many applications. The engineering of hybrid interface involves (requires) an understanding of the electronic structure of materials—one organic and one inorganic—that form the two halves of hybrid interfaces, as well as the electronic and chemical consequences of the coupling of the two. Although much literature exists describing the interfaces between vapor-deposited organic molecules and model molecules for polymers on the surfaces of clean metals in ultrahigh vacuum, few studies have been reported on spin-coated, semiconducting polymer films on realistic substrates. Spin coating in an inert atmosphere (or even air) is a central part of the process of the fabrication of polymer-based light-emitting devices and other modern polymer-based electronic components. Here, work on the electronic structure of semiconducting (conjugated) polymer films spin-coated onto selected inorganic substrates, carried out using ultraviolet photoelectron spectroscopy, is reviewed and summarized to generate a generalized picture of the hybrid interfaces formed under realistic device fabrication conditions.

MOMBE growth studies of GaN using metalorganic sources and nitrogen

Abstract Using modulated beam mass spectrometry we have investigated the species in the gas phase within the reaction chamber of an ultra-high vacuum metal organic vapour phase epitaxy system, when growing gallium nitride (GaN). We use high purity 14 N and 15 N to identify the species containing nitrogen within the post-reaction products and have identified a number of parasitic reactions which consume the active nitrogen and thus limit the growth rate at a level below the expected value.

Hydrogen and metals for ultrahigh vacuum construction materials-What limits and reduces hydrogen outgassing rate from construction metals in ultrahigh vacuum ?-

Hydrogen and metals for ultrahigh vacuum construction materials-What limits and reduces hydrogen outgassing rate from construction metals in ultrahigh vacuum ?-

Surface-Enhanced Raman Scattering as a Ubiquitous Vibrational Probe of Transition-Metal Interfaces: Benzene and Related Chemisorbates on Palladium and Rhodium in Aqueous Solution

A recently developed strategy for utilizing surface-enhanced Raman scattering (SERS) to obtain uniquely detailed vibrational information for a myriad of organic (and other) adsorbates on transition metals in electrochemical and other ambient environments is illustrated for benzene and a pair of monosubstituted benzenes, toluene and benzonitrile, on palladium and rhodium films in aqueous solution. The transition-metal layers, formed by constant-current deposition onto SERS-active gold substrates, can be sufficiently thin (3−5 monolayers, ML) so to yield near-optimal Raman scattering intensities, yet are essentially devoid of exposed “pinhole” sites, thereby eliminating spectral interferences from adsorption onto gold. Benzene was selected in view of the detailed vibrational information also available for this archetypical organic chemisorbate on transition metals in ultrahigh vacuum (UHV) by means of electron energy loss spectroscopy (EELS). Comparison of the spectral information obtained by SERS and EELS ...

Study of iron–chromium alloys and mixed oxides using focused ion beam optical spectroscopy

Focused ion beam optical spectroscopy (FIBOS) is a material analysis technique in which information is derived from optical emission generated when a finely focused energetic ion beam hits a sample. It is capable of providing information on the atomic composition and the chemical state of the sample. Chemical mapping and depth profiling of samples are also possible. Here FIBOS has been used to study a series of Fe–Cr alloys and mixed oxides of the same transition metals in ultra-high vacuum under Ar+ and Ga+ ion bombardment, with and without an oxygen bleed. Linear fits of the signal ratios versus composition ratios were observed.

超高真空中での液体金属の濡れ性測定と評価

This paper presents a new evaluation method of wettability between solid and liquid metals in ultra-high vacuum. An argon ion sputter cleaning process of the specimen and a highly magnified observation of the contact angles by scanning electron microscope were adopted to obtain the most reliable experimental values of the contact angles. It was actually found that the wettability between silver and tungsten, that had been thought to be poor, was rather good, when both of the surfaces were perfectly cleaned in ultra-high vacuum. The contact angles for various liquid metals on tungsten were also determined using this experimental method. The empirical tight binding calculations were performed and the local density of states was used to evaluate the wetting interaction energy. The calculated local values were qualitatively proportional to the experimental values of the interaction energies. In order to control wettability, it is necessary to control the electronic states of the substrate surface.

Using C60 molecules for deep carbonization of rhenium in ultrahigh vacuum

The adsorption of C60 molecules on the surface of a rhenium ibbon heated to 1800 K is investigated quantitatively. It is found that the C60 molecules are adsorbed without reflection and then dissociate, and the carbon formed in the process is dissolved in the bulk of the rhenium to the limit of solubility. Then a graphite monolayer forms on the rhenium surface, and the dissociation of the C60 molecules stops. It is shown that the C60 molecules can be used as an effective carbonizing object on metals in ultrahigh vacuum.

Spectroellipsometric study of phase transitions in d and f metals in ultrahigh vacuum

The temperature behaviour of the optical properties of iron and nickel single crystals and of polycrystalline cerium has been studied in the spectral range from 0.2 to 5.2 eV by an ellipsometric method in vacuum of p≈10−9−10−11 Torr.The influence of ferromagnetic (Fe, Ni) and isostructural (Ce) phase transitions on the optical conductivity σ are discussed in detail. From the analysis of experimental results a conclusion is made about the character of change of the electronic structure during phase transitions.

Argon isotope analysis by a newly developed mass spectrometric system for K-Ar dating.

A new mass spectrometer and the associated analytical systems, called HIRU, was designed and constructed for the argon isotope analysis of minerals from young volcanic rocks as well as metamorphics and granitoids. HIRU is composed of a sample holder, an extraction oven, purification lines, standard gas lines, a mass spectrometer, and an ultra high vacuum pumping system. All the parts, except for the sample holder, were made of stainless steel and connected with ICF flanges using Cu gaskets or ultra high vacuum metal valves. The mass spectrometer is a 15cm sector type with an oblique incidence-single focusing system using an electron bombard ion source and three collectors which contain 8 (for 36Ar), 6 (38Ar) and 4 (40Ar) stage secondary electron multipliers respectively.Argon isotope analysis by HIRU is summarized and the precision and reliability of the new mass spectrometric system are discussed in this paper. A series of analysis for argon isotopes, such as taking a set of spectrum, the calculation of isotopic ratios, argon content, and ages is carried out with a computer-controlled system. HIRU has mde it possible to date geological materials with high sensitivity (eg. tens mg of mica of Cretaceous age), high precision (ca. 1% of error for argon content), reliability (possible for rocks older than 0.1 Ma), and convenience (eg. 5–6 samples can be dated per day).

New Preparation Method For Liquid Alloy Surfaces and Their Characterization by Electron Spectroscopy

ABSTRACTSurface investigations of liquid metals in ultra-high vacuum are scarce as compared with solid state studies, mainly because of experimental difficulties of preparing atomically clean liquid surfaces. A new technique has been developed in order to prepare clean liquid metal surfaces. According to the characterization with electron spectroscopy it proved to be an effective way to obtain high purity liquid surfaces and hence offers the possibility to study a variety of metals and alloys in the liquid state.Information about the surface composition, segregation properties and its purity in the liquid state and the valence electron structure has been obtained by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS, respectively). Exciting new insights in the electronic structure of various liquid alloys such as Au-Si, Cu-Sn, Ga-Sn and TI-Bi have been obtained from these measurements.

Nucleation of AuAg binary alloys from the vapour phase on KBr single crystals

The nucleation, growth and composition of particulate AuAg alloy deposits on a KBr(100) crystal cleavage surface was investigated by coevaporation of the metals in ultrahigh vacuum. Series of specimens were prepared by increasing the deposition time at constant fluxes of the vapour beams. Analysis of the nucleation rates by transmission electron microscopy revealed that they are mainly determined by the nucleation of gold. Correspondingly, the preferred condensation of gold was found in the early stages of particle growth, as measured by quantitative X-ray fluorescence analysis. During further deposition the composition approached that of the vapour beam, while the substrate coverage increased. The results were interpreted by means of an extended kinetic nucleation model in which it is assumed that the nucleation and growth of alloy particles is mainly governed by the differences in the adatom adsorption energies E a, in the activation energies E d for surface diffusion and in the pre-exponential constants of the components. Evaluation of the experimental data resulted in E a, Au − E a, Ag = 0.03 eV and E d, Au − E d, Ag = -0.02 eV, in close agreement with data for AuAg on NaCl reported in earlier work. As small variations of these values were found to result in large deviations in the condensation behaviour, particularly in the composition in the nucleation phase, the error limits were only about ±0.02 eV. The ratio of the pre-exponential constants was found to be about unity.

The surface composition of Pd-Cu alloys: A comparative investigation of photoelectric work function measurements, Auger electron spectroscopy and calculations based on a broken bond approximation

The surface composition of Pd-Cu alloys has been studied by Auger electron spectroscopy, photoelectric work function measurements and by calculations based on a broken bond model of surface segregation. The measurements were performed on thin films with a bulk composition varying from 0% to 100% Pd. The films were prepared by sublimation of both metals in ultrahigh vacuum onto a Pyrex glass support. For the work function measurements both the absolute value of the work function and the change in work function due to hydrogen chemisorption were used for an estimate of the surface composition. The Auger spectra were evaluated in the way proposed by Gallon to calculate the surface composition of the alloys. The data derived from the experimental results are in good agreement with the calculated values. The results show that the surfaces of clean and equilibrated Pd-Cu alloys are only slightly enriched in copper.

Preparation of metals in ultra high vacuum by electromagnetic levitation

The conditions for dynamic instabilities of electromagnetically levitated metal specimens have been investigated. Methods which eliminate the instabilities have been developed and were used to decarburize and degas a levitated niobium sphere in ultra-high vacuum for a period of 16 hours.

175. Measurement of exoelectron emission on elongation of metals in ultra-high vacuum: V S Kortov and V G Teplov, Rep Ural Polytech Inst, 215, 1973, 5–57 ( in Russian)

175. Measurement of exoelectron emission on elongation of metals in ultra-high vacuum: V S Kortov and V G Teplov, Rep Ural Polytech Inst, 215, 1973, 5–57 ( in Russian)