Escape Depth Of Photoelectrons Research Articles

Core-ionization spectrum of liquid water.

We present state-of-the-art calculations of the core-ionization spectrum of water. Despite significant progress in procedures developed to mitigate various experimental complications and uncertainties, the experimental determination of ionization energies of solvated species involves several non-trivial steps such as assessing the effect of the surface potential, electrolytes, and finite escape depths of photoelectrons. This provides a motivation to obtain robust theoretical values of the intrinsic bulk ionization energy and the corresponding solvent-induced shift. Here we develop theoretical protocols based on coupled-cluster theory and electrostatic embedding. Our value of the intrinsic solvent-induced shift of the 1sO ionization energy of water is -1.79 eV. The computed absolute position and the width of the 1sO peak in photoelectron spectrum of water are 538.47 eV and 1.44 eV, respectively, agreeing well with the best experimental values.

Evidence for heterogeneous oxidation on the surface of a plasma SiC nanopowder based on synchrotron-excited XPS

The surface chemistry of a plasma SiC nanopowder was characterized by X-ray photoelectron spectroscopy using a tunable synchrotron X-ray source. The sample surface was representative of SiC nanopowder surfaces in interaction with ambient air, with no significant influence from carbonaceous deposits. By choosing an incident photon energy in the range 200–1200 eV, the escape depth of photoelectrons was altered from 0.5 to 3 nm based on the C 1s, Si 2p and O 1s core-level transitions. The spectral series gave evidence for surface enrichment of silica and sp2-bonded elemental carbon. Several models of surface enrichment were tested against the spectral intensities as a function of escape depth. This analysis led to sp2-carbon islands and two types of oxidation depths. While on the larger part of the surface, the silica is only a near-monolayer in thickness, there are surface regions that are oxidized to a depth of ≥3 nm. These heavily oxidized regions are attributed to defected open channel pores on the outer surface of nanoparticle agglomerates, based on the previous electron tomography results.

Experimental Band Structure of Pb(Zr,Ti)O3 : Mechanism of Ferroelectric Stabilization.

Pb(Zr,Ti)O3 (PZT) is the most common ferroelectric (FE) material widely used in solid-state technology. Despite intense studies of PZT over decades, its intrinsic band structure, electron energy depending on 3D momentum k, is still unknown. Here, Pb(Zr0.2 Ti0.8 )O3 using soft-X-ray angle-resolved photoelectron spectroscopy (ARPES) is explored. The enhanced photoelectron escape depth in this photon energy range allows sharp intrinsic definition of the out-of-plane momentum k and thereby of the full 3D band structure. Furthermore, the problem of sample charging due to the inherently insulating nature of PZT is solved by using thin-film PZT samples, where a thickness-induced self-doping results in their heavy doping. For the first time, the soft-X-ray ARPES experiments deliver the intrinsic 3D band structure of PZT as well as the FE-polarization dependent electrostatic potential profile across the PZT film deposited on SrTiO3 and Lax SrMn1- x O3 substrates. The negative charges near the surface, required to stabilize the FE state pointing away from the sample (P+), are identified as oxygen vacancies creating localized in-gap states below the Fermi energy. For the opposite polarization state (P-), the positive charges near the surface are identified as cation vacancies resulting from non-ideal stoichiometry of the PZT film as deduced from quantitative XPS measurements.

Secondary electron emission and photoemission from a negative electron affinity semiconductor with large mean escape depth of excited electrons

The formulae for parameters of a negative electron affinity semiconductor (NEAS) with large mean escape depth of secondary electrons λ (NEASLD) are deduced. The methods for obtaining parameters such as λ, B, E pom and the maximum δ and δ at 100.0 keV ≥ E po ≥ 1.0 keV of a NEASLD with the deduced formulae are presented (B is the probability that an internal secondary electron escapes into the vacuum upon reaching the emission surface of the emitter, δ is the secondary electron yield, E po is the incident energy of primary electrons and E pom is the E po corresponding to the maximum δ). The parameters obtained here are analyzed, and it can be concluded that several parameters of NEASLDs obtained by the methods presented here agree with those obtained by other authors. The relation between the secondary electron emission and photoemission from a NEAS with large mean escape depth of excited electrons is investigated, and it is concluded that the presented method of obtaining λ is more accurate than that of obtaining the corresponding parameter for a NEAS with large λ ph (λ ph being the mean escape depth of photoelectrons), and that the presented method of calculating B at E po > 10.0 keV is more widely applicable for obtaining the corresponding parameters for a NEAS with large λ ph.

A Laser‐ARPES View of the 2D Electron Systems at LaAlO3/SrTiO3 and Al/SrTiO3 Interfaces

AbstractThe electronic structure of the two‐dimensional electron system (2DES) found at the Al/SrTiO3 (Al/STO) and LaAlO3/SrTiO3 (LAO/STO) interfaces is measured by means of laser angle resolved photoemission spectroscopy, taking advantage of the large photoelectron escape depth at low photon energy to probe these buried interfaces. The possibility of tuning the electronic density in Al/STO by varying the Al layer thickness is demonstrated, and it is shown that the electronic structure evolution is well described by self‐consistent tight binding supercell calculations, but differs qualitatively from a rigid band shift model. It is shown that both 2DES are strongly coupled to longitudinal optical phonons, in agreement with previous reports of a polaronic ground state in similar STO based 2DESs. Tuning the electronic density in Al/STO to match that of LAO/STO and comparing both systems, it is estimated that the intrinsic LAO/STO 2DES has a bare band width of ≈60 meV and a carrier density of ≈6 × 1013 cm−2.

The relevance of ARPES to high-Tc superconductivity in cuprates

Angle-resolved photoemission spectroscopy, visualizing the superconducting gap in k-space, plays a pivotal role in research on cuprates and other high-Tc superconducting materials. However, there has always been an imminent doubt whether this technique truly represents the intrinsic bulk spectral function, whose response can be distorted by energy- and k-dependence of the photoexcitation matrix element, and by a small photoelectron escape depth of few surface atomic layers. Here, we address this fundamental question with soft-X-ray photoemission measurements of the superconducting gap in the paradigm high-Tc cuprate Bi2Sr2CaCu2O8. We vary the matrix element by spanning a dense k-space grid, formed by the lattice superstructure, and the probing depth by changing the emission angle. The measured gap appears independent of the matrix element effects, probing depth or photoexcitation energy. This fact proves the relevance of photoemission studies for the bulk superconductivity in Bi2Sr2CaCu2O8, and calls for similar verification experiments on other high-Tc compounds, in particular more three-dimensional ones. Bi2Sr2CaCu2O8 shows an anomalously fast decay of the coherent spectral weight with photon energy, tracing back to strong electron–phonon interaction or relaxation of the lattice coherence.

Escape Depth of Secondary and Photoelectrons from CdTe Films with a Ba Film

Escape zone depths λ' for true-secondary electrons and photoelectrons from pure CdTe and CdTe with a Ba film of thickness θ ≤ 1 monolayer have been estimated for the first time. It is shown that upon a decrease of the work function of the surface by 2 eV, the value of λ' increases by 1.2–1.3 times.

Unexpected metal-insulator transition in thick Ca1-xSrxVO3 film on SrTiO3 (100) single crystal

Epitaxial Ca1-xSrxVO3 (0 ≦ x ≦ 1) thin films were grown on (100)-oriented SrTiO3 substrates by using the pulsed laser deposition technique. In contrast to the previous report that metal-insulator transition (MIT) in Ca1-xSrxVO3 (CSVO) was achieved only for extremely thin films (several nm thick), MIT was observed at 39, 72, and 113 K for films with a thickness of 50 nm. The electronic structure was investigated by hard and soft X-ray photoemission spectroscopy (HX-PES and SX-PES). The difference between these PES results was significant due to the variation in an escape depth of photoelectrons of PES. While HX-PES showed that the V 2p3/2 spectra consisted of four peaks (V5+, V4+, V3+, and V2+/1+), SX-PES showed only three peaks (V5+, V4+, and V3+). This difference can be caused by a strain from the substrate, which leads to the chemical disorder (V5+, V4+, V3+, and V2+/1+). The thin film near the substrate is affected by the strain. The positive magnetoresistance is attributed to the effect of electron-electron interactions in the disorder system. Therefore, the emergence of MIT can be explained by the electron-electron interactions from the chemical disorder due to the strain.

Surface studies of solids using integral X-ray-induced photoemission yield.

X-ray induced photoemission yield contains structural information complementary to that provided by X-ray Fresnel reflectivity, which presents an advantage to a wide variety of surface studies if this information is made easily accessible. Photoemission in materials research is commonly acknowledged as a method with a probing depth limited by the escape depth of the photoelectrons. Here we show that the integral hard-X-ray-induced photoemission yield is modulated by the Fresnel reflectivity of a multilayer structure and carries structural information that extends well beyond the photoelectron escape depth. A simple electric self-detection of the integral photoemission yield and Fourier data analysis permit extraction of thicknesses of individual layers. The approach does not require detection of the reflected radiation and can be considered as a framework for non-invasive evaluation of buried layers with hard X-rays under grazing incidence.

Anisotropic effective mass approximation model to calculate multiple subband structures at wide-gap semiconductor surfaces: Application to accumulation layers of SrTiO3 and ZnO

We propose a simplified theoretical model that well reproduces the dispersion curves of the multiple subbands in a two-dimensional electron gas in the accumulation layer at the surfaces of wide-gap semiconductors. The electronic band structures containing multiple subbands with different orbital characteristics are derived by self-consistently solving the Poisson–Schrödinger equations with anisotropic effective mass approximations. Calculations were carried out on the two-dimensional electron gas states formed at the surfaces of SrTiO3 and ZnO for comparison. The calculated subband structures at the SrTiO3 surface were in excellent agreement with available angle-resolved photoelectron spectroscopy data, confirming the validity of the present model. The calculations further indicate the existence of a high electron density, exceeding 2×1021cm−3 and a high electric field of 20MVcm−1 at the surface. Moreover, photoelectron-escape depths are discussed quantitatively and photoelectrons from the first subband are found to dominate the total photoelectron intensity in the spectra.

(Invited) Investigation of the Si/TiO2/Electrolyte Interface Using Operando Tender X-ray Photoelectron Spectroscopy

Photoelectrochemical (PEC) cells based on semiconductor-liquid interfaces provide a method of converting solar energy to electricity or fuels 1-3. Recently, we have demonstrated operational systems that involved stabilized semiconductor-liquid junctions 4,5. We have employed operando ambient-pressure X-Ray photoelectron spectroscopy (AP-XPS) on these highly stable semiconductor-liquids junctions to directly characterize the semiconductor-liquid junction at room temperature under real-time electrochemical control 6,7. The escape depth of photoelectrons that originate from tender X-ray synchrotron radiation — 2.3 to 5.2 keV — allows for sampling of the sample simultaneously near the sample surface but through an electrolyte layer as the inelastic mean free path of such photoelectrons is on the order of 5 to 10 nanometers. Accordingly, tender X-ray AP-XPS has enabled simultaneous monitoring of the semiconductor surface, the semiconductor-electrolyte interface, and the bulk electrolyte of a PEC cell as a function of the applied potential, E. Accumulation, depletion and Fermi level pinning were observed. The observed shifts in the core level emission binding energies with respect to the applied potential directly revealed ohmic and rectifying junction behavior on metallized and semiconducting samples, respectively. 1. H. J. Lewerenz et al., Energ. Environ. Sci., 3, 748–760 (2010). 2. H. J. Lewerenz, in Photoelectrochemical Materials and Energy Conversion Processes, R. C. Alkire, D. M. Kolb, J. Lipkowski, and P. N. Ross, Editors, vol. 12, p. 61–181, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany (2010). 3. Z. Huang, C. Xiang, H. J. Lewerenz, and N. S. Lewis, Energ. Environ. Sci., 7, 1207–1211 (2014). 4. S. Hu et al., Science, 344, 1005–1009 (2014). 5. M. F. Lichterman et al., Energy Environ. Sci., 7, 3334–3337 (2014). 6. S. Axnanda et al., Meet. Abstr., MA2013-02, 921–921 (2013). 7. S. Axnanda et al., (2014), submitted.

Calculations of mean escape depths of photoelectrons in elemental solids excited by linearly polarized X-rays for high-energy photoelectron spectroscopy

We have calculated mean escape depths (MEDs, D) of photoelectrons from Si, Cu, and Au excited by linearly polarized X-rays over the 50–10,000eV energy range for high-energy photoelectron spectroscopy. These calculations were done with Monte Carlo methods using the values of electron inelastic mean free paths λ that were calculated by the relativistic full Penn algorithm and the Dirac–Hartree–Fock atomic potential for elastic scattering of electrons. In order to know the dependence of MED on the asymmetry parameter β and the emission angle θ, we calculated MEDs for β=−1 to β=2 (by 0.5 steps) and θ=0° to θ=80° (with respect to the surface normal). The resulting MED values increase as the β values decrease over the 50–10,000eV energy range. We also found that the values of D/λcosθ are approximately constant to within 10% for a given value of θ and for energies over 1000eV (except for the condition of β=−1) over a range of emission angles (typically 0–40° or 60°). We also found that the energy dependence of the ratios D/λcosθ at emission angle θ=0° can be expressed by the Jablonski–Powell predictive equation over the 50–10,000eV energy range.Finally, we have obtained a predictive MED equation that could be applied to the wide range of experimental conditions that are used in high-energy XPS with synchrotron radiation.

Beyond hard x-ray photoelectron spectroscopy: Simultaneous combination with x-ray diffraction

Hard x-ray photoelectron spectroscopy (HAXPES) is a powerful and novel emerging technique for the nondestructive determination of electronic properties and chemical composition of bulk, buried interfaces and surfaces. It benefits from the exceptionally large escape depth of high kinetic energy photoelectrons, increasing the information depth up to several tens of nanometers. Complementing HAXPES with an atomic structure sensitive technique (such as x-ray diffraction) opens a new research field with major applications for materials science. At SpLine, the Spanish CRG beamline at the European Synchrotron Radiation Facility, we have developed a novel experimental set-up that combines HAXPES and x-ray diffraction (x-ray reflectivity, surface x-ray diffraction, grazing incidence x-ray diffraction, and reciprocal space maps). Both techniques can be operated simultaneously on the same sample and using the same excitation source. The set-up includes a robust 2S + 3D diffractometer hosting a ultrahigh vacuum chamber equipped with a unique photoelectron spectrometer (few eV < electron kinetic energy < 15 keV), x-ray tube (Mg/Ti), 15 keV electron gun, and auxiliary standard surface facilities (molecular beam epitaxy evaporator, ion gun, low energy electron diffraction, sample heating/cooling system, leak valves, load-lock sample transfer, etc.). This end-station offers the unique possibility of performing simultaneous HAXPES + x-ray diffraction studies. In the present work, we describe the experimental set-up together with two experimental examples that emphasize its outstanding capabilities: (i) nondestructive characterization of the Si/Ge and HfO2/SiO2 interfaces on Ge-based CMOS devices, and (ii) strain study on La0.7Ca0.3MnO3 ultrathin films grown on SrTiO3(001) substrate.

Determination of electronic and atomic properties of surface, bulk and buried interfaces: Simultaneous combination of hard X-ray photoelectron spectroscopy and X-ray diffraction

Hard X-ray photoelectron spectroscopy (HAXPES) is a powerful novel emerging technique for bulk compositional, chemical and electronic properties determination in a non-destructive way. It benefits from the exceptionally large escape depth of high kinetic energy photoelectrons enabling the study of bulk and buried interfaces up to several tens of nanometres depth. Its advantage over conventional XPS is based on the long mean free path of high kinetic energetic photoelectrons. Using the advantage of tuneable X-ray radiation provided by synchrotron sources the photoelectron kinetic energy, i.e. the information depth can be changed and consequently electronic and compositional depth profiles can be obtained. The combination of HAXPES with an atomic structure sensitive technique, as X-ray diffraction, opens a new research field with great potential for many systems in which their electronic properties are intimately linked to their crystallographic structure. At SpLine, the Spanish CRG Beamline at the European Synchrotron Radiation Facility (ESRF) we have developed a novel and exceptional set-up that combine grazing incidence X-ray diffraction (GIXRD) and HAXPES. Both techniques can be operated simultaneously on the same sample and using the same excitation source. The set-up includes a heavy 2S+3D diffractometer and UHV chamber equipped with an electrostatic analyzer. The UHV chamber has also MBE evaporation sources, an ion gun, a LEED optic, a sample heating and cooling device, an electron gun, a UV discharge lamp, a low and medium energy X-ray tube, leak valves and a load-lock port. The photon energy ranges between 7 and 45keV. The HAXPES analyzer is an electrostatic cylinder-sector (FOCUS HV CSA), with a compact geometry and high transmission due to second order focusing. The analyzer is capable to handle kinetic energies both up to 15keV and down to a few eV with the same analyzer setup and power supply.

Photoemission yield and the electron escape depth determination in metal–oxide–semiconductor structures on N+-type and P+-type silicon substrates

This article gives a quantitative analysis of electron photoemission yield from N+-type and P+-type substrates of MOS structures. Based on this analysis, a method is presented to estimate both the scattering length, ℓ, of electrons in the image force potential well and of photoelectron escape depth, xesc, from the semiconductor substrate. This method was used to estimate the scattering length and the escape depth from the substrates of Al-SiO2-Si (N+-type and P+-type) structures. It was found that for N+-type substrate structures the scattering in the image force potential well has a dominating influence on the photoemission yield while for P+-type substrate structures both the scattering in the image force potential well and the photoemission from the subsurface regions of the photoemitter play important roles. It was found that the scattering length in the image force potential well was equal to ℓ = 6.7–6.9 nm for structures on both N+ and P+ substrates, produced in the same processing conditions. For structures on P+ substrates, the escape depth was found to be equal to xesc = 8–9 nm. The scattering length, ℓ, determined in this study is considerably larger than the one reported previously (ℓ = 3.4 nm) for similar MOS structures. The escape depth xesc determined in this study is also considerably larger than the escape depth determined previously (xesc = 1.2–2.5 nm) for the external photoemission from uncovered silicon surfaces into vacuum.

Effect of Linear Density of States on the Quasiparticle Dynamics and Small Electron-Phonon Coupling in Graphite

We obtained the spectral function of very high quality natural graphite single crystals using angle-resolved photoelectron spectroscopy. A clear separation of nonbonding and bonding bands and asymmetric lineshape are observed. The asymmetric line shapes are well accounted for by the finite photoelectron escape depth and the band structure. The extracted width of the spectral function (inverse of the photohole life time) near the K point is, beyond the maximum phonon energy, approximately proportional to the energy as expected from the linear density of states near the Fermi energy. The upper bound for the electron-phonon coupling constant is about 0.2, a much smaller value than the previously reported one.

Depth distribution of the fluorine concentration during radiative carbonization of PVDF

The XPS integral intensity of the F1s line and its satellite is measured during the long-term radiative carbonization of PVDF (polyvinylidene fluoride). A model is proposed that describes the effect of the fluorine depth distribution on the shape and intensity of the F1s spectra. A comparison of the experimental data with the model calculations provides estimates for the concentration inhomogeneity during the radiative carbonization of PVDF, for the photoelectron escape depth, and for the probability of a single energy loss by a photoelectron in its motion towards the surface. A technique determining the fluorine concentration is presented. It is based on the occurrence of chemical shifts of the C1s line towards larger bond energies for the carbon atoms chemically bonded to one or two fluorine atoms.

Probing Surface Oxidation of Reduced Uranium Dioxide Thin Film Using Synchrotron Radiation

The work presents a study of the formation of defects created by argon ion sputtering of UO{sub 2} thin film surfaces using high-resolution X-ray photoelectron spectroscopy with photon energy of 625 eV. Stable-defected surfaces of UO{sub 2-x} were monitored by the uranium core level lines found at the low-binding energy side of the U{sup +4} features. The lower end of these states gave binding energies at 377.1 and 387.7 eV attributed to U4f{sub 7/2} and U4f{sub 5/2} core levels peaks, respectively, for metallic uranium (U{sup {circ}}). In between both states (U{sup {circ}} and U{sup +4}), the clear presence of other electronic states is observed. These lines are attributed to intermediate, metastable states of uranium ions between U{sup {circ}} and U{sup +4}. Temperature-programmed X-ray photoelectron spectroscopy is performed to observe the oxidation process of these reduced states with increasing temperature. The increase in the U{sup +4} intensity is not correlated with the decrease of U ions signal with lower oxidation states indicating that the limitation is the photoelectron escape depth. In other words, sputtering has created reduced clusters or ridges on the surface of average dimensions larger than the escape depth of the 4f photoelectron with kinetic energy of 225 eV.more » The activation energy for surface oxidation to U{sup +4} is {approx}20 kJ mol{sup -1}. This value is close to other values reported for vacancy-type diffusion mechanism in the fluorite structure.« less

Electronic structure of Ti metal and TiO 2 powder studied by hard and soft (Cu Kα 1 and Al Kα1) X-ray photoelectron and Auger spectroscopy

High-energy X-ray photoelectron spectroscopy (XPS) is of particular importance for minimizing the effects of surface contamination by increasing photoelectron escape depths. In this study high-resolution high-energy Cu Kα 1 and soft Al Kα 1 XPS and Auger electron spectroscopy were used to compare the electronic structure of Ti in TiO 2 powder and Ti metal. The Ti 1s in TiO 2 XPS line is narrower and more symmetric than in Ti metal. A comparison of the relative intensities of the L 23M 23M 45 and L 23M 23M 23 Auger transitions in Ti metal and TiO 2 is consistent with the expected transfer of Ti 3d electrons away from the Ti site in the oxide. The satellites accompanying the Ti 1s XPS line excited by Cu Kα 1 X-rays occur at the same energies as the satellites accompanying the Ti 2s and 2p XPS lines excited by Al Kα 1 X-rays indicating that they do not depend on the core-level, the experimental resolution or inelastic scattering processes.

Development of hard X-ray photoelectron spectroscopy at BL29XU in SPring-8

We have realized hard X-ray (HX) photoelectron spectroscopy (PES) with high-throughput and high-energy-resolution for core level and valence band studies using high-energy and high-brilliance synchrotron radiation at BL29XU in SPring-8. Large escape depth of high-energy photoelectrons enables us to probe intrinsic bulk states almost free from surface condition. By use of a newly developed electron energy analyzer and well-focused X-rays, high-energy resolution of 75 meV ( E / Δ E = 79 , 000 ) was realized for 5.95 keV photoelectrons.