Electron Take-off Angle Research Articles

The ST component in the Si 2p photoemission spectrum from H-terminated and oxidized Si (001) surfaces

One doublet is usually employed to fit the Si0 substrate species in the Si 2p photoemission spectra from Si (001) H-terminated (after piranha treatment) and oxidized surfaces. However, there is a second substrate-top component (ST) with a binding energy of 0.3 eV higher than the bulk component; its intensity varies from ∼10% at normal emission (i.e., 90° from the surface) to ∼20% at 35°. It is present even for oxidized surfaces and does not correspond to any of the suboxide species. It corresponds to the first layers of the substrate and is responsible for the decrease in the signal dip between the two S–O branches of the Si 2p spectra for glancing electron takeoff angles. Although it is resolvable for monochromatized sources, the ST component is absent in the literature on Si 2p spectra.

Surface Propensity of Anions in a Binary Ionic-Liquid Mixture Assessed by Full-Range Angle-Resolved X-ray Photoelectron Spectroscopy and Surface-Tension Measurements.

Angle-resolved X-ray photoelectron spectroscopy and contact-angle measurements guided by a signal attenuation model are utilized to extract molar composition and anion enrichment in the vacuum interface of a binary ionic liquid mixture, having a common quaternary ammonium cation and two different anions. By using the intensity ratio of the F1s peaks belonging to the two different anions recorded at the full electron take-off angle range, from 0° to 80°, we have determined that only a fractionally covered and anion enriched surface layer can predict the AR-XPS data, which is also consistent with surface tension measurements. Moreover, the more bulky and non-spherical anion enrichment is evident even at the conventional and the so assumed bulk sensitive take-off angle of 0°. This methodology provides a surface enrichment factor of the molecular ions and clearly serves as an experimental evidence for recently debated surface layering and/or island structure in ionic liquid systems.

Non-dipolar effects in photoelectron-based normal incidence X-ray standing wave experiments

The normal incidence X-ray standing waves technique is one of the most well-established methods for investigating the geometric structure at interfaces and surfaces. It is able to measure vertical positions and distances of individual atomic species with very high precision (typically <0.02Å). These data not only yield valuable structural information, but also represent an excellent benchmark for density functional theory and ab initio calculations. Non-dipolar effects are well known to strongly affect the result, in particular when light elements are involved. A correction mechanism for these effects is established, but in its commonly-used form it is based on one essential restriction, namely the assumption of perfect normal incidence of the X-rays with respect to the relevant lattice planes of the crystal. Here, we show that small deviations from normal incidence, as they are unavoidable in typical experimental setups, lead to significant systematic errors in the NIXSW results. The magnitude of this effect depends on the specific conditions in a non-linear way and may reach up to 5%, corresponding to several tenths of an Ångström in the adsorption height. We present a straightforward way of accounting for this effect, and demonstrate that recording the photoelectron yield in an angular-resolved mode is indispensable, since the correction parameters strongly depend on the electron take-off angle.

Interlayer interaction and π-band electron occupation on top layers of freshly cleaved bulk HOPG investigated with N(E) C KVV Auger electron spectroscopy

N(E) C KVV Auger spectra (V=σsσpπ) were used for measurement of the π-band electron occupation of five outer layers on freshly cleaved bulk HOPG. The π-band electron occupation of the 1–5 graphene layers was measured relative to the electron concentration in the σp-band. In-depth π-band profiles were obtained by means of variation of the Auger electron takeoff angle within the range of 15–90°. Differences in the π-band electron occupation of the 1–5 graphene layers were determined. The π-band electron occupation varies from 0 at the top graphene layer to that of the π-band electron occupation typical for bulk HOPG at the 5th graphene layer counted from the outer surface. These results are discussed on the basis of the π-band formation under the interlayer interaction of the pz-electrons.

Study of SEM images obtained with an electron energy and take-off angle (E-θ) selective detector

Study of SEM images obtained with an electron energy and take-off angle (E-θ) selective detector

Effect of Composition Ratio on Erbium Silicide Work Function on Different Morphology of Si(100) Surface Changed by Alkaline Etching

Effect of Composition Ratio on Erbium Silicide Work Function on Different Morphology of Si(100) Surface Changed by Alkaline Etching

Valence band structure of rubrene single crystals in contact with an organic gate dielectric

Organic field effect transistors (OFETs) using crystalline organic semiconductors are of great interest because of their well-defined structural and electronic properties to study the intrinsic charge carrier transport mechanisms in π-conjugated molecular solids, as well as to unravel their potential to be applied as a novel type of electronic device. In the present study, the valence band structure of the channel region of an OFET is proposed based on photoemission results of a well-defined interface between a dielectric molecular monolayer and single crystals of 5,6,11,12-tetraphenyltetracene (rubrene) which is known to exhibit the highest field effect mobility of all organic semiconductors at room temperature. Commensurate growth of clusters of tetratetracontane (TTC; n-C44H90) on the rubrene single crystal surface and their morphological transformation into a uniform overlayer were observed by atomic force microscopy. Photoelectron spectroscopy measurements at various electron take-off angles were then conducted to derive the valance band width of the rubrene single crystal covered by the TTC overlayers. The valence band width at this hetero-interface was found to be equivalent to that of the pristine rubrene, which suggests an unchanged ‘band effective mass ħ2(d2E/dk||2)’ of accumulated holes even at the vicinity of hydrocarbon-based gate dielectrics.

Separate K-line contributions to fluorescence enhancement in electron probe microanalysis

The behaviour of the fluorescence enhancement correction factor in electron probe microanalysis, as a function of incident electron energy and take-off angle, is assessed for different binary samples in a wide range of compositions. Monte Carlo simulations are employed to validate Reed's correction algorithm [S.J.B. Reed, Characteristic fluorescence corrections in electron-probe microanalysis, Br. J. Appl. Phys. 16 (1965) 913-926], by means of estimating the primary excited radiation volume and the volume corresponding to secondary fluorescence generation. Then, Reed's expression for the fluorescence enhancement has been modified to account for Kα and Kβ line contributions separately. It is clearly shown that in certain cases the assignment of all fluorescent contribution to the Kα lines may be inadequate, particularly when trace element analysis imposes an accurate determination of elemental concentrations.

Instrument-related geometrical factors affecting the intensity in XPS and ARXPS experiments

The angular dependence of the X-ray photoelectron spectroscopy (XPS) signal is influenced not only by the electron take-off angle, but also by instrument-related geometrical factors. The XPS signal is, in fact, integrated over the overlap between the X-ray beam, the spectrometer analysis volume, and the sample surface. This overlap depends on the size and shape of the spectrometer analysis volume and X-ray beam, as well as on their relative orientation. In this paper it is described the models and protocols for the characterization of the parameters defining the geometry of an XPS instrument. The protocols include practical methods for assessing the spectrometer analysis area and the X-ray beam spot dimension. Simple systems consisting of flat and “thick” gold films on silicon wafers were employed. The parameters found with those samples are transferable to other more complex systems since they are geometrical in nature. The method allows for the prediction of the actual intensity of XPS peaks, hence removing the need of normalizing the peak areas to the area of a determined substrate peak. The associated reduction of the uncertainty in half is of special importance since the quantitative analysis of angle-resolved XPS data could be very sensitive to noise. Two rotating and one non-rotating XPS instruments are described. Some examples of the applications of the method are also provided.

On X-ray tube spectra, the dependence on the angular and electron energy of X-rays from the targets

An algorithm was developed for the theoretical intensities of both X-ray continuum and characteristic K-radiation (K α 1 , K α 2 , K α , K β 1 , K β 2 , K β 3 , K β 13 , K β 4 and K β 5 ) of different target materials (Cr, Cu, Rh, Mo, W, Ag and Au) at different electron incidence angles (0.01° − 90°), different take-off angles of the photon emission (0.01° − 90°) and different X-ray tube voltages. It was found that the intensities of continuum and characteristic X-rays increase with increasing take-off angle and the take-off angle is inversely proportional to the absorption path of X-rays in the target. At take-off angles of 20° or more, the flux of the X-ray spectra remains practically independent of the take-off angle. Furthermore, at the optimum electron incidence angle and take-off angle of emitted X-ray photons, the dependence of X-ray tube spectra on applied voltage was also investigated.

Chemical analysis of HfO2∕Si (100) film systems exposed to NH3 thermal processing

Nitrogen incorporation in HfO2∕SiO2 films utilized as high-k gate dielectric layers in advanced metal-oxide-semiconductor field effect transistors has been investigated. Thin HfO2 blanket films deposited by atomic layer deposition on either SiO2 or NH3 treated Si (100) substrates have been subjected to NH3 and N2 anneal processing. Several high resolution techniques including electron microscopy with electron energy loss spectra, grazing incidence x-ray diffraction, and synchrotron x-ray photoelectron spectroscopy have been utilized to elucidate chemical composition and crystalline structure differences between samples annealed in NH3 and N2 ambients as a function of temperature. Depth profiling of core level binding energy spectra has been obtained by using variable kinetic energy x-ray photoelectron spectroscopy with tunable photon energy. An “interface effect” characterized by a shift of the Si4+ feature to lower binding energy at the HfO2∕SiO2 interface has been detected in the Si 1s spectra; however, no corresponding chemical state change has been observed in the Hf 4f spectra acquired over a broad range of electron take-off angles and surface sensitivities. The Si 2p spectra indicate Si–N bond formation beneath the HfO2 layer in the samples exposed to NH3 anneal. The NH3 anneal ambient is shown to produce a metastable Hf–N bond component corresponding to temperature driven dissociation kinetics. These findings are consistent with elemental profiles across the HfO2∕Si(100) interface determined by electron energy loss spectroscopy measurements. X-ray diffraction measurements on similarly treated films identify the structural changes resulting from N incorporation into the HfO2 films.

XPS Characterization of Au (Core)/SiO2 (Shell) Nanoparticles

Core-shell nanoparticles with ca. 15-nm gold core and 6-nm silica shell were prepared and characterized by XPS. The Au/Si atomic ratio determined by XPS is independent of the electron takeoff angle because of the concentric spherical shape of the nanoparticles. The formula given by Wertheim and DiCenzo (Phys. Rev. B 1988, 37, 844) for spherical nanoparticles and the modified one by Yang et. al. (J. Appl. Phys. 2005, 97, 024303) for core-shell nanoparticles are used to correlate the XPS-derived composition with the geometry of the nanoparticles only after significantly modifying either the bulk density of the silica shell or the attenuation length of the photoelectrons.

Investigation of structure and chemical states of self-assembled Au nanoscale particles by angle-resolved X-ray photoelectron spectroscopy

The surface structure of the silane self-assembled monolayer (SAM) prepared by 3-mercaptopropyl-trimethyl-silane (MPTMS) on Si(111) and the chemical states of the self-assembled Au nanoparticles absorbed onto the surface of the silane SAM were studied by angle-resolved X-ray photoelectron spectroscopy. The thickness of the silane SAM was determined as the vertical distance between sulfur atoms on the surface of the silane SAM and the surface of the SiO 2 overlayer on the silicon substrate, and had a value of 4.7 Å. In addition, two types of sulfur species were detected in the self-assembled Au nanoparticles at an electron take-off angle of 5°. These species are assigned as bound thiolate and unbound thiol with S 2p binding energies of 162.6 eV and 164.6 eV respectively. The assignments indicate that the gold–thiolate bond was formed when Au nanoparticles were assembled onto the surface of the silicon substrate through MPTMS as a coupling agent.

Self-energy in surface electron spectroscopy: I. Plasmons on a free-electron-material surface

On the basis of a quantum approach, the formula for the complex self-energy, and hence also that for the energy-loss cross-section, are derived for fast electrons penetrating through the surface from the interior of a solid. Both the angular and depth dependence are included in the expression. Use of a Drude-Lindhard model bulk dielectric function for describing a bulk plasmon excitation in a free-electron-like material has yielded an analytical expression for the surface dielectric function which satisfies surface sum rules. The calculated imaginary and real parts of the electron self-energy for Si have indicated, in detail, the excitation processes of the surface plasmon and the bulk plasmon for an electron passing through the surface, with the dependency on electron energy and take-off angle. This approach provides quantitatively the electron inelastic scattering cross-section in the surface region for use in surface electron spectroscopy.

Tilted orientation of molecular chains at the surface of a series of perfluoropolyether liquids: an X-ray photoelectron spectroscopic investigation

Angle resolved X-ray photoelectron spectroscopic (XPS) studies of a series of perfluoropolyethers of general formula F[CF(CF3)CF2O] n CF2CF3, where n = 27, 65, and >70, show that there is preferential ordering of-CF3 groups at the liquid surfaces. The C 1s intensity corresponding to the -CF3 groups increases with decrease in the electron take-off angle. No measurable change in the oxygen or fluorine intensity is observed upon varying the electron take-off angle. However, experiments show that no adsorption site exists at the surfaces, indicating that the ether oxygens are not available for any of the surface processes. Relative enhancement of the C 1s intensity of the -CF2/CF3 group with decreasing take-off angle is different for different liquids, with smaller chain length liquids showing a more rapid change. This is explained as due to an increase in helicity and tilt of molecular chains from the surface normal. Computational studies have been performed at the semiempirical level to understand the molecular structure. XPS data along with the computational studies suggest that at the liquid-air interface the molecular chains are ordered nearly perpendicular to the surface with a tilt which increases with chain length. The conclusions are in agreement with recent atomic and molecular beam scattering, ion/surface scattering and theoretical modelling studies.

Model for Analysis of XPS Electron Take-off Angle Experiments in Layer-structured Samples: Determination of Attenuation Lengths in a Well-characterized Langmuir-Blodgett Film

A series of equations have been proposed to calculate take-off angle dependency of the elemental composition of layer-structured models from the experimentally obtained peak intensities of XPS. We have measured the take-off angle dependency of XPS peaks of O 1s, C 1s, Si 2p and Cd 3d from a well-characterized one-layer Langmuir–Blodgett (LB) film of polymerized cadmium 10,12-pentacosadiynoate on a silicon wafer substrate and have applied the newly proposed equations to deduce attenuation lengths of photoelectrons for every element in the layer-structured organic materials. From the best fitting between the theoretically calculated and the experimentally obtained data by the least-squares method, attenuation lengths for a monolayer film of polymerized cadmium 10,12-pentacosadiynoate have been evaluated to be 3.4, 3.7, 4.0 and 4.5 nm for the respective photoelectrons of O 1s (955 eV), Cd 3d5/2 (1082 eV), C 1s (1202 eV) and Si 2p (1388 eV) with the kinetic energy dependency ofE0.75. It is essential to use polymerized LB films and to confirm the surface structure of the film directly by suitable techniques such as atomic force microscopy, scanning electron microscopy or transmission electron microscopy.© 1997 by John Wiley & Sons, Ltd.

Model for Analysis of XPS Electron Takeoff Angle Experiments in Layerstructured Samples: Determination of Attenuation Lengths in a Wellcharacterized LangmuirBlodgett Film

A series of equations have been proposed to calculate take-off angle dependency of the elemental composition of layer-structured models from the experimentally obtained peak intensities of XPS. We have measured the take-off angle dependency of XPS peaks of O 1s, C 1s, Si 2p and Cd 3d from a well-characterized one-layer Langmuir–Blodgett (LB) film of polymerized cadmium 10,12-pentacosadiynoate on a silicon wafer substrate and have applied the newly proposed equations to deduce attenuation lengths of photoelectrons for every element in the layer-structured organic materials. From the best fitting between the theoretically calculated and the experimentally obtained data by the least-squares method, attenuation lengths for a monolayer film of polymerized cadmium 10,12-pentacosadiynoate have been evaluated to be 3.4, 3.7, 4.0 and 4.5 nm for the respective photoelectrons of O 1s (955 eV), Cd 3d5/2 (1082 eV), C 1s (1202 eV) and Si 2p (1388 eV) with the kinetic energy dependency of E0.75. It is essential to use polymerized LB films and to confirm the surface structure of the film directly by suitable techniques such as atomic force microscopy, scanning electron microscopy or transmission electron microscopy.© 1997 by John Wiley & Sons, Ltd.

XPS and SSIMS Analysis Revealing Surface Segregation and Short-Range Order in Solid Films of Block Copolymers of PEO and PLGA

The interfacial chemistry of solvent cast films of block copolymers of poly(ethylene oxide) (PEO) and poly(lactic and glycolic acid) (PLGA) have been investigated over a wide range of compositions. X-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectrometry (SSIMS) were utilized to provide both elemental and structural data. In all cases it is found that the PLGA component preferentially resides at the copolymer surface, and this is demonstrated by the use of the variable electron takeoff angle XPS. SSIMS confirms the low concentrations of PEO at the copolymer surface. Radical cation intensities within the SSIMS spectra have been correlated to PLGA block compositions. It is demonstrated that these ion intensities may be used to estimate the PLGA surface chain sequence composition and its short-range order.

Investigation of the depth of preferential surface ordering in liquids: A photoelectron spectroscopic investigation of liquid mixtures

Angle resolved x-ray photoelectron spectroscopic studies of a series of liquid mixtures of a perfluoropolyether (Krytox 16350) and a polyphenylether (Santovac-5) show that the former spreads on the latter and the film thickness could even be smaller than the photoelectron mean free path. The molecular ordering in krytox remain as that of the free liquid which is manifested in the enhancement of the relative intensity of specific features in the photoelectron spectrum as the electron take-off angle is decreased. The preferential ordering is limited to the very top and an upper limit of this thickness is estimated to be about 8 Å. This observation is in accordance with molecular dynamics simulations of long chain hydrocarbon liquids.

Characterization of PTFE on Silicon Wafer Tribological Transfer Films by XPS, Imaging XPS and AFM

Atomic force microscopy (AFM) shows that PTFE-on-silicon wafer tribological transfer films consist of narrow ribbons of PTFE, <1 μm wide and <10 nm high, aligned with the film draw direction. Within the ribbons the PTFE molecular chains are also aligned with the draw direction. Such materials are of interest as substrates for the epitaxial crystallization of polymers and small molecules. X-ray photoelectron spectroscopy demonstrates that the amount of PTFE put down increases with deposition temperature and pressure. The XPS azimuthal angle dependence at low electron take-off angle is consistent with the aligned ribbon morphology. Azimuthal and polar angle dependence and charging studies show that hydrocarbon contamination observed in the XPS spectrum is located on top of the silicon wafer substrate, in the channels between the PTFE ribbons. Imaging XPS confirms the pressure dependence of PTFE coverage, and at low deposition pressure reveals tracks of PTFE ∼100 μm wide, aligned with the film draw direction. Together with the AFM data this leads to a description of PTFE transfer films as fractal materials.