Concept: Auger electron spectroscopy
A novel two-step approach for preparing carbon nanotube (CNT) systems, exhibiting an extraordinary combination of functional properties, is presented. It is based upon nanocomposite films consisting of metal (Me = Ni, Fe, Mo, Sn) nanoparticles embedded into diamond-like carbon (DLC). The main concept behind this approach is that DLC inhibits the growth of Me, resulting in the formation of small nanospheres instead of layers or extended grains. In the second step, DLC:Me substrates were used as catalyst templates for the growth of CNTs by the thermal chemical vapor deposition (T-CVD) process. X-ray photoelectron spectroscopy (XPS) has shown that at the T-CVD temperature of 700 °C DLC is completely graphitized and NiC is formed, making DLC:Ni a very effective catalyst for CNT growth. The catalyst layers and the CNT systems have been characterized with a wide range of analytical techniques such as Auger electron spectroscopy and X-ray photoelectron spectroscopy (AES/XPS), X-ray diffraction, reflectivity and scattering, Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and optical and electrical testing. The produced CNTs are of excellent quality, without needing any further purification, durable, firmly attached to the substrate, and of varying morphology depending on the density of catalyst nanoparticles. The produced CNTs exhibit exceptional properties, such as super-hydrophobic surfaces (contact angle up to 165°) and exceptionally low optical reflection (reflectivity <10(-4)) in the entirety of the visible range. The combination of the functional properties makes these CNT systems promising candidates for solar thermal harvesting, as it is demonstrated by solar simulation experiments.
By use of synchrotron X-ray fluorescence and Rutherford backscattering spectrometry, we show the SU-8 soft lithographic process contaminates PDMS. Residues of the antimony containing photoinitiator are transferred from the master mold to the surface of PDMS, uncontrollably intensifying the surface potential, leading to electroosmotic flow variability in PDMS microfluidic devices.
In this work, a novel evaluation strategy for the X-ray photoelectron spectroscopy (XPS) chemical assessment is proposed to identify the corrosion products formed on the surface of hot-dip galvanized ZnMgAl coatings after exposure to standardized salt spray tests. The experiments demonstrate that the investigated system exhibits a problematic differential charging behavior between the different compounds, an effect which cannot be fully compensated for by the flood gun of the XPS system, making a reliable evaluation of the individual spectra impossible by using standard evaluation and fitting methods. For that reason, a new effective approach-taking the different charge shifts into account-was implemented and its reliability experimentally verified on model mixtures of assumed corrosion products with known composition. With this new approach, the chemical surface composition of an industrial sample of a corroded ZnMgAl coating was revealed and discussed in order to clearly demonstrate the potential of the proposed method for future corrosion studies.
Iron pyrite (cubic FeS2) is a promising candidate absorber material for earth-abundant thin-film solar cells. Here we report on phase-pure, large-grain, and uniform polycrystalline pyrite films that are fabricated by solution-phase deposition of an iron(III) acetylacetonate molecular ink followed by sequential annealing in air, H2S, and sulfur gas at temperatures up to 550°C. Phase and elemental composition of the films is characterized by conventional and synchrotron X-ray diffraction (XRD), Raman spectroscopy, Auger electron spectroscopy, secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectroscopy (XPS). These solution-deposited films have more oxygen and alkalis, less carbon and hydrogen, and smaller optical band gaps (Eg = 0.87 ± 0.05 eV) than similar films made by chemical vapor deposition. XPS is used to assess the chemical composition of the film surface before and after exposure to air and immersion in water to remove surface contaminants. Optical measurements of films rich in marcasite (orthorhombic FeS2) show that marcasite has a band gap at least as large as pyrite and that the two polymorphs share similar absorptivity spectra, in excellent agreement with density functional theory (DFT) models. Regardless of the marcasite and elemental impurity content, all films show p-type, weakly-activated transport with curved Arrhenius plots, a room-temperature resistivity of ~1 Ω cm, and a hole mobility that is too small to measure by Hall effect. This universal electrical behavior strongly suggests that a common defect or a hole-rich surface layer governs the electrical properties of most FeS2 thin films.
Cellular adhesion and proliferation inside three-dimensional (3D) synthetic scaffolds represent a major challenge in Tissue Engineering (TE). Besides the surface chemistry of the polymers, it is well recognized that scaffold internal architecture, namely pore size/shape and interconnectivity, has a strong effect on the biological response of cells. This study reports for the first time how polycaprolactone (PCL) scaffolds with controlled micro-architecture can be effectively produced via bioextrusion and used to enhance the penetration of plasma deposited species. Low pressure nitrogen-based coatings were employed to augment cell adhesion and proliferation without altering the mechanical properties of the structures. X-ray photoelectron spectroscopy (XPS) carried out on different sections of the scaffolds indicates a uniform distribution of nitrogen-containing groups throughout the entire porous structure. In vitro biological assays confirm that plasma deposition sensitively promotes the activity of Saos-2 osteoblast cells, leading to a homogeneous colonization of the PCL scaffolds.
Structural and compositional characterization of ultrathin titanium oxide films grown on Pt(3)Ti(111)
- Journal of physics. Condensed matter : an Institute of Physics journal
- Published over 6 years ago
We have investigated the growth of ultrathin titanium oxide (TiO(x)) films on a Pt(3)Ti(111) single crystal surface as a function of oxidation temperature (300-1000 K) and oxygen exposure (up to 4500 l) by means of Auger electron spectroscopy, low-energy electron diffraction, ultraviolet photoelectron spectroscopy and high-resolution electron energy loss spectroscopy (HREELS). Both the surface composition and the surface structure of the resulting TiO(x) films exhibit a strong dependence on the preparation conditions. Loss of the chemical order and Ti segregation are observed at the Pt(3)Ti(111) surface upon oxygen exposures of more than 135 l at 1000 K. Increasing oxygen exposure enhances Ti segregation and oxide growth. At a threshold of ≈220 l (at 1000 K) a transition in the oxide structure occurs, namely from a (6 × 3√3) rectangular structure (a = 16.6 Å, b = 14.4 Å) below 220 l to a (7 × 7)R21.8° hexagonal structure (a = b = 19.3 Å) above 220 l. Two additional incommensurate rectangular metastable structures are observed for the highest oxygen exposures (above 900 l) at intermediate oxidation temperatures (800-900 K). In all cases the changes in the valence band spectra and the work function with respect to the clean Pt(3)Ti(111) surface are independent of the chosen oxidation parameters. Based on their HREELS spectra we identify the (6 × 3√3) and (7 × 7)R21.8° structures grown at 1000 K with a stoichiometric TiO phase, while the other and less stable oxide phases grown at 800-900 K exhibit more complex phonon structures that could not simply be associated with any of the stoichiometric phases TiO, Ti(2)O(3) or TiO(2). Our results are rather similar to those found by Granozzi et al for the deposition of Ti onto a Pt(111) surface in an oxygen atmosphere, except a few interesting deviations as a consequence of the different preparation conditions.
Controlling the structure of colloidal nanocrystals (NCs) is key to the generation of their complex functionality. This requires an understanding of the NC surface at the atomic level. The structure of colloidal PbS-NC passivated with oleic acid has been studied theoretically and experimentally. We show the existence of surface OH- groups, which play a key role in stabilizing the PbS(111) facets, consistent with x-ray photoelectron spectroscopy as well as other spectroscopic and chemical experiments. The role of water in the synthesis process is also revealed. Our model, along with the existing observations of NC surface termination and passivation by ligands, helps to explain and predict the properties of NCs and their assemblies.
The hybrid structure of Ag nanowires (AgNWs) covered with graphene (Gr) shows synergetic effects on the performance of transparent conducting electrodes (TCEs). However, these effects have been mainly observed via large-scale characterization, and precise analysis at the nanoscale level remains inadequate. Here, we present the nanoscale verification and visualization of the improved chemical and electrical stabilities of Gr-covered AgNW networks using conductive atomic force microscopy (C-AFM), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS) combined with the gas cluster ion beam (GCIB) sputtering technique. Specifically by transferring island Gr on top of the AgNW network, we were able to create samples in which both covered and uncovered AgNWs are simultaneously accessible to various surface-characterization techniques. Furthermore, our ab initio molecular dynamics (AIMD) simulation elucidated the specific mechanistic pathway and a strong propensity for AgNW sulfidation, even in the presence of ambient oxidant gases.
Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons. But a detailed understanding of such non-equilibrium excited-state dynamics and their interplay with structural changes is challenging: a multitude of excited states and possible transitions result in phenomena too complex to unravel when faced with the indirect sensitivity of optical spectroscopy to spin dynamics and the flux limitations of ultrafast X-ray sources. Such a situation exists for archetypal polypyridyl iron complexes, such as [Fe(2,2'-bipyridine)3](2+), where the excited-state charge and spin dynamics involved in the transition from a low- to a high-spin state (spin crossover) have long been a source of interest and controversy. Here we demonstrate that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2'-bipyridine)3](2+) on photoinduced metal-to-ligand charge transfer excitation. We are able to track the charge and spin dynamics, and establish the critical role of intermediate spin states in the crossover mechanism. We anticipate that these capabilities will make our method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered molecular phenomena involving 3d transition metal complexes.
Chemical waves that arise in the H2 + O2 reaction on a bimetallic Rh(111)/Ni surface have been studied in the 10-6 and 10-5 mbar range at T = 773 K with photoelectron emission microscopy (PEEM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). Nickel coverages of 0.3, 0.6, and 1.0 monolayers were investigated. Coadsorbed with some oxygen, Ni starts to penetrate the Rh bulk region substantially only beyond 900 K. In the 10-5 mbar range, chemical waves are characterized by three distinct gray levels in PEEM. This number reduces to only two levels in the 10-6 mbar range. In situ LEED showed the periodic appearance of a (n × 1) (n = 8, 10) pattern during chemical waves which was assigned to a 2D-Ni oxide. With in situ AES, one observes that the bright phase in PEEM correlates with a high Ni coverage and the dark phase with a low Ni coverage.