P. Pianetta

Elemental compositions of comet 81P/Wild 2 samples collected by Stardust

We measured the elemental compositions of material from 23 particles in aerogel and from residue in seven craters in aluminum foil that was collected during passage of the Stardust spacecraft through the coma of comet 81P/Wild 2. These particles are chemically heterogeneous at the largest size scale analyzed (∼180 ng). The mean elemental composition of this Wild 2 material is consistent with the CI meteorite composition, which is thought to represent the bulk composition of the solar system, for the elements Mg, Si, Mn, Fe, and Ni to 35%, and for Ca and Ti to 60%. The elements Cu, Zn, and Ga appear enriched in this Wild 2 material, which suggests that the CI meteorites may not represent the solar system composition for these moderately volatile minor elements.

UPS studies of the bonding of H2, O2, CO, C2H4 and C2H2 on Fe and Cu☆

UPS (ultraviolet photoemission spectroscopy) data of H2, O2, CO, C2H4 and C2H2 chemisorbed on polycrystalline Fe and Cu are presented. Together with data already available on Ni, this represents a comprehensive study of the adsorption of simple molecules on three representative metals of the first row transition metals. Adsorption of O2, or H2 on Fe gives rise to a single resonance level at 6 eV below EF. CO dissociatively adsorbs on Fe at room temperature but remains in molecular form at T = 110 K. O2 adsorbed on Cu produces two resonance levels at −5.5 eV and −1.5 eV respectively. In analyzing the chemicorbed CO, C2H4 and C2H2spectra of the three metals, we have considered the fact that the heat of adsorption of these gases is much higher on Fe or Ni than on Cu. Thus, even though the heat of adsorption of CO on Ni is a factor of two higher than on Cu, the chemisorbed CO spectra on these two metals are extremely similar. For the chemisorbed hydrocarbons, we have measured the π level bonding shifts and found that there is no correlation between the level shifts and the heats of adsorption. Application of Grimleys chemisorption model to calculate the chemisorption energy (using the π level shifts as input parameters) results in slightly better agreement between calculated and measured heats of adsorption. Such behavior suggests that the substrate contribution to the bonding is crucial in determining the heats of adsorption. From an examination of the difference spectra of the valence band of Fe and Cu, it is concluded that UPS should be able to supply information as to which group of the substrate orbitals participate in the bonding. However, much more experimental and theoretical work is required before such information can be made quantitative.

Ge-Interface Engineering With Ozone Oxidation for Low Interface-State Density

Passivation of Ge has been a critical issue for Ge MOS applications in future technology nodes. In this letter, we introduce ozone oxidation to engineer Ge/insulator interface. Density of interface states (D_it) across the bandgap and close to the conduction band edge was extracted using conductance technique at low temperatures. D_it dependence on growth conditions was studied. Minimum D_it of 3 times 10¹¹ cm^-2V^-1 was demonstrated. Physical quality of the interface was investigated through Ge 3d spectra measurements. We found that the interface and D_it are strongly affected by the distribution of oxidation states and the quality of the suboxide.

Three-Dimensional Imaging of Chemical Phase Transformations at the Nanoscale with Full-Field Transmission X-Ray Microscopy

The ability to probe morphology and phase distribution in complex systems at multiple length scales unravels the interplay of nano- and micrometer-scale factors at the origin of macroscopic behavior. While different electron- and X-ray-based imaging techniques can be combined with spectroscopy at high resolutions, owing to experimental time limitations the resulting fields of view are too small to be representative of a composite sample. Here a new X-ray imaging set-up is proposed, combining full-field transmission X-ray microscopy (TXM) with X-ray absorption near-edge structure (XANES) spectroscopy to follow two-dimensional and three-dimensional morphological and chemical changes in large volumes at high resolution (tens of nanometers). TXM XANES imaging offers chemical speciation at the nanoscale in thick samples (>20 µm) with minimal preparation requirements. Further, its high throughput allows the analysis of large areas (up to millimeters) in minutes to a few hours. Proof of concept is provided using battery electrodes, although its versatility will lead to impact in a number of diverse research fields.

Determination of the bonding of alkyl monolayers to the Si(111) surface using chemical-shift, scanned-energy photoelectron diffraction

The bonding of alkyl monolayers to Si(111) surfaces has been studied by conventional x-ray photoelectron spectroscopy (XPS) and chemical-shift, scanned-energy photoelectron diffraction (PED) using synchrotron radiation. Two very different wet-chemical methods have been used to prepare the alkyl monolayers: (i) olefin insertion into the H–Si bond on the H–Si(111) surface, and (ii) replacement of Cl on the Cl–Si(111) surface by an alkyl group from an alkyllithium reagent. In both cases, XPS has revealed a C 1s signal chemically shifted to lower binding energy, which we have assigned to carbon bonded to silicon. PED has shown that both preparative methods result in carbon bonded in an atop site with the expected C–Si bond length of 1.85±0.05 A. Chemical-shift, scanned-energy photoelectron diffraction is a particularly valuable probe of local structure at surfaces that contain the same element in multiple, chemically distinct environments.

Near‐edge x‐ray absorption of carbon materials for determining bond hybridization in mixed sp2/sp3 bonded materials

Near‐edge x‐ray absorption fine structure (NEXAFS) measurements were performed on a variety of carbon materials, covering a range of hybrid bonding character from pure sp3 type to pure sp2 type. Diamond, chemical vapor deposited (CVD) diamond films of varying quality. Diamond‐like carbon (DLC) films, and graphite were examined with this technique and these measurements were compared with Raman spectroscopy results and scanning electron microscopy images of carbon film morphology. For the mixed sp2 and sp3 bonded DLC materials, NEXAFS does not suffer from the large Raman cross‐section difference between sp2 and sp3 type bonds, thus allowing unambiguous characterization of carbon thin films with a broader range of sp2/sp3 bonding ratios than possible with Raman spectroscopy alone. This capability was used to determine the transition point where the sequential‐CVD carbon film growth technique produces predominately sp3 or sp2 bonded material.

TXM-Wizard: a program for advanced data collection and evaluation in full-field transmission X-ray microscopy

A suite of GUI programs written in MATLAB for advanced data collection and analysis of full-field transmission X-ray microscopy data including mosaic imaging, tomography and XANES imaging is presented.

The effect of rapid thermal N2O nitridation on the oxide/Si(100) interface structure

High‐resolution x‐ray photoelectron spectroscopy (XPS) was used to study the chemical nature and physical distribution of N in oxynitride films formed by rapid thermal N2O processes (RTPs). High‐resolution synchrotron Si 2p core level photoemission spectroscopy (PES) was used to study the oxide/Si(100) interface suboxide structures with and without the presence of N. XPS N 1s studies indicated that there are two types of N in the RTP oxynitride films. The chemical bond configuration of the first type of N is similar to that N in Si3N4 and is mainly distributed within the first 1 nm from the interface. The second type of N is distributed mainly outside of the first 1 nm region, and the N is likely bonded to two Si and one oxygen atom. PES studies showed that Si formed suboxides with oxygen at the interface for all oxynitride films. It is found that there is no change in the Si+1 structure while there is a dramatic intensity decrease in the Si+2 and Si+3 peaks with the inclusion of N in the oxide. Both the ...

Synchrotron radiation studies of electronic structure and surface chemistry of GaAs, GaSb, and InP

The oxidation properties of GaAs(110), GaSb(110), and InP(110) have been studied with the photoemission technique using synchrotron radiation. One part of the work consisted of an investigation of the chemical shifts in the core levels upon adsorption of oxygen in submonolayer quantities. For GaAs and InP, oxygen removed electrons preferentially from the surface column V elements (As and P), leaving the column III elements (Ga and In) unaffected; whereas, for GaSb, both constitutents were involved in the oxidation suggesting a breaking of bonds between the surface atoms and the rest of the crystal. The second part of the study consisted of a careful investigation of the Fermi level (Ef) pinning in the GaAs(110) surface. Three different samples and many cleaves were studied. For one sample, Ef was always at the bulk position. On the other two crystals, both pinned and unpinned cases were found on various cleaves. The unpinned samples had sharp electron distribution curves (EDC’s), while the pinned samples ...

Alkyl-terminated Si(111) surfaces: A high-resolution, core level photoelectron spectroscopy study

The bonding of alkyl monolayers to Si(111) surfaces has been studied with high-resolution core level photoelectron spectroscopy (PES). Two very different wet-chemical methods have been used to prepare the alkyl monolayers: (i) Olefin insertion into the H–Si bond of the H–Si(111) surface, and (ii) replacement of Cl on the Cl–Si(111) surface by an alkyl group from an alkyllithium reagent. In both cases, PES has revealed a C 1s component shifted to lower binding energy and a Si 2p component shifted to higher binding energy. Both components are attributed to the presence of a C–Si bond at the interface. Along with photoelectron diffraction data [Appl. Phys. Lett. 71, 1056, (1997)], these data are used to show that these two synthetic methods can be used to functionalize the Si(111) surface.