Inverse Photoemission Research Articles

Interfacial and surface magnetism in epitaxial NiCo2O4(001)/MgAl2O4 films

NiCo2O4 (NCO) films grown on MgAl2O4 (001) substrates have been studied using magnetometry and x-ray magnetic circular dichroism based on x-ray absorption spectroscopy and spin-polarized inverse photoemission spectroscopy with various thicknesses down to 1.6 nm. The magnetic behavior can be understood in terms of a layer of optimal NCO and an interfacial layer (1.2 ± 0.1 nm), with a small canting of magnetization at the surface. The thickness dependence of the optimal layer can be described by the finite-scaling theory with a critical exponent consistent with the high perpendicular magnetic anisotropy. The interfacial layer couples antiferromagnetically to the optimal layer, generating exchange-spring styled magnetic hysteresis in the thinnest films. The non-optimal and measurement-speed-dependent magnetic properties of the interfacial layer suggest substantial interfacial diffusion.

Variation of Strong Correlation Effects in A-site Ordered Perovskites CaCu3Ti4−xRuxO12: Photoemission and Inverse Photoemission Studies

We have systematically studied the strong correlation effects in A-site ordered perovskites CaCu3Ti4-xRuxO12 (x = 0, 1, 3.5, and 4) by using photoemission and inverse photoemission spectroscopies. In x = 0, 1, 3.5, the peak positions of the strongly correlated Cu 3d states around -3.8 eV and Ti 3d states around 3.6 eV little change. On the other hand, in x = 4, the Cu 3d states split into two peaks around -2.5 and -4 eV. These indicate that Ti plays an important role to retain the strong correlation effects. In addition, the multiplet structures of Cu 3d final states from -8 to -14 eV become weak as Ru increases, indicating the reduction of the localized characters of Cu 3d states. At the Fermi level, we observe the absence of spectral weight in x = 0, 1 and the development of Ru 4d in-gap states in x = 3.5, 4, which give rise to the metal-insulator transition between x = 1 and x = 3.5.

The spin polarization of palladium on magneto-electric Cr2O3

While induced spin polarization of a palladium (Pd) overlayer on antiferromagnetic and magneto-electric Cr2O3(0001) is possible because of the boundary polarization at the Cr2O3(0001), in the single domain state, the Pd thin film appears to be ferromagnetic on its own, likely as a result of strain. In the conduction band, we find the experimental evidence of ferromagnetic spin polarized in Pd thin films on a Cr2O3(0001) single crystal, especially in the thin limit, Pd thickness of around 1–4 nm. Indeed there is significant spin polarization in 10 Å thick Pd films on Cr2O3(0001) at 310 K, i.e. above the Néel temperature of bulk Cr2O3. While Cr2O3(0001) has surface moments that tend to align along the surface normal, for Pd on Cr2O3, the spin polarization contains an in-plane component. Strain in the Pd adlayer on Cr2O3(0001) appears correlated to the spin polarization measured in spin polarized inverse photoemission spectroscopy. Further evidence for magnetization of Pd on Cr2O3 is provided by measurement of the exchange bias fields in Cr2O3/Pd(buffer)/[Co/Pd] n exchange bias systems. The magnitude of the exchange bias field is, over a wide temperature range, virtually unaffected by the Pd thickness variation between 1 and 2 nm.

Chemical and Electronic Structure at the Interface between a Sputter‐Deposited Zn(O,S) Buffer and a Cu(In,Ga)(S,Se)2Solar Cell Absorber

The chemical and electronic structure of the interface between a sputter‐deposited Zn(O,S) buffer layer and an industrial Cu(In,Ga)(S,Se)2(CIGSSe) absorber for thin‐film solar cells is investigated with X‐ray and UV photoelectron spectroscopy, inverse photoemission spectroscopy, and X‐ray emission spectroscopy. We find a CIGSSe absorber surface band gap of 1.61 (±0.14) eV, which is significantly increased as compared to the minimal value derived with bulk‐sensitive methods (≈1.1 eV). We find no indication for diffusion of absorber elements into the buffer layer. Surface‐ and bulk‐sensitive measurements of the buffer layer suggest the presence of S‐Zn and S‐O bonds in the Zn(O,S) layer. We find that the naturally existing downward band bending toward the CIGSSe absorber surface is increased by the formation of the interface, likely enhancing carrier separation under illumination. We also derive a flat conduction band alignment, in line with the reported high conversion efficiencies of corresponding large‐area solar cells.

A novel high-current, high-resolution, low-kinetic-energy electron source for inverse photoemission spectroscopy

A high-current electron source for inverse photoemission spectroscopy is described. The source comprises a thermal cathode electron emission system, an electrostatic deflector-monochromator, and a lens system for variable kinetic energy (1.6–20 eV) at the target. When scaled to the energy resolution, the electron current is an order of magnitude higher than that of previously described electron sources developed in the context of electron energy loss spectroscopy. Surprisingly, the experimentally measured energy resolution turned out to be significantly better than calculated by standard programs, which include the electron–electron repulsion in the continuum approximation. The achieved currents are also significantly higher than predicted. We attribute this “inverse Boersch-effect” to a mechanism of velocity selection in the forward direction by binary electron–electron collisions.

Response to "Comment on 'Spin- and angle-resolved inverse photoemission setup with spin orientation independent from electron incidence angle'" [Rev. Sci. Instrum. 93, 093904 (2022)

We reply to the Comment by Donath et al. on our setup, which allows a total 3D control of the polarization direction of the electron beam in an inverse photoemission spectroscopy (IPES) experiment, a significant advance with respect to previous setups with partial polarization control. Donath et al. claim an incorrect operation of our setup after comparing their results, treated to enhance the spin asymmetry, with our spectra without the same treatment. They also equal spectra backgrounds instead of equaling peak intensities above the background. Thus, we compare our Cu(001) and Au(111) results with the literature. We reproduce previous results, including spin-up/spin-down spectral differences observed for Au and not observed for Cu. Also, spin-up/spin-down spectral differences appear at the expected reciprocal space regions. In the Comment, it is also stated that our tuning of the spin polarization misses the target because the spectra background changes when tuning the spin. We argue that the background change is irrelevant to IPES since the information is contained in peaks produced by primary electrons, those having conserved their energy in the inverse photoemission process. Second, our experiments agree with previous results from Donath et al. [Wissing et al., New J. Phys. 15, 105001 (2013)] and with a zero-order quantum-mechanical model of spins in vacuum. Deviations are explained by more realistic descriptions including the spin transmission through an interface. Consequently, the operation of our original setup is fully demonstrated. Our development corresponds to "the promising and rewarding angle-resolved IPES setup with the three-dimensional spin resolution," as indicated in the Comment, after our work.

Comment on "Spin- and angle-resolved inverse photoemission setup with spin orientation independent from electron incidence angle" [Rev. Sci. Instrum. 93, 093904 (2022)

The paper under discussion promises a spin- and angle-resolved inverse-photoemission (IPE) setup, where the spin-polarization direction of the electron beam used for excitation "can be tuned to any preferred direction" while "preserving the parallel beam condition." We support the idea to improve IPE setups by introducing a three-dimensional spin-polarization rotator, but we put the presented results to the test by comparing them with the literature results obtained by existing setups. Based on this comparison, we conclude that the presented proof-of-principle experiments miss the target in several aspects. Most importantly, the key experiment of tuning the spin-polarization direction under otherwise allegedly identical experimental conditions causes changes in the IPE spectra that are in conflict with existing experimental results and basic quantum-mechanical considerations. We propose experimental test measurements to identify and overcome the shortcomings.

Photoemission Study of One-Dimensional Electronic States of (6,5), (6,6), and (7,4) Single-Walled Carbon Nanotubes

We have investigated the one-dimensional electronic states of single-walled carbon nanotubes (SWCNTs) with diameters ranging from 0.8–1.4 nm by optical absorption, photoemission, and inverse photoemission measurements. In the photoemission spectra of (6,5) SWCNTs and (6,6)- and (7,4)-enriched SWCNTs, spike structures caused by Van Hove singularities (VHSs) are clearly observed. The observed VHS positions are compared with the positions calculated using the tight-binding model and density functional theory. A large curvature affects the electronic states of metallic SWCNTs with small diameters.

Micro-/macroscopic and density functional studies of the interactions between molybdenum trioxide and C60 molecule.

We have investigated the interactions between C60 and (MoO3)n using scanning tunneling microscopy with spectroscopy (STM/STS) and ex situ ultraviolet-visible-near-infrared (UV-vis-NIR) spectroscopy in combination with density functional theory (DFT) calculations. The formation of (MoO3)n chemically bound to C60 is energetically favorable due to ΔG < 0 for n = 1, 2, 4, 6, 8, and 9, and they well reproduced the histogram of the height of (MoO3)n on the C60 (111) terrace obtained by a STM height-profile. STS results demonstrated the upward energy shift of both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of C60 in the vicinity of (MoO3)n (n = 6 or 9), which is consistent with the previous results of the co-deposited C60/MoO3 film obtained using photoemission and inverse photoemission spectroscopy [Wang and Gao, Appl. Phys. Lett. 105, 111601 (2014), Yang et al., J. Phys.: Condens. Matter 28, 185502 (2016), and Li et al., J. Phys. Chem. C 118, 4869 (2014)]. Theoretical calculations of (MoO3)n (n = 1, 2, 4, 6, 8, and 9) chemically bound to C60 indicated that 0.01-0.32 holes are injected into C60 by (MoO3)n nanoclusters, and UV-vis-NIR and DFT results found that the hole doping to C60 is caused via the electron transfer from the HOMO of C60 to the LUMO of (MoO3)n. Furthermore, it is noted that the C60-(MoO3)n interactions exhibit a high heat resistance up to 250 °C by examining the UV-vis-NIR spectra of a co-deposited C60/MoO3 (6:4) film before and after thermal annealing. The present findings provide useful information for the practical use of P-type C60-based thermoelectric devices.

Metal Halide Perovskite Surfaces with Mixed A‐Site Cations: Atomic Structure and Device Stability

AbstractMixing cations in the perovskite structure has been shown to improve optoelectronic device performance and stability. In particular, CsxMA1‐xPbBr3 (MA = CH3NH3) has been used to build high‐efficiency light‐emitting diodes. Despite those advantages, little is known about the exact location of the cations in the mixed perovskite film, and how cation distribution affects device properties and stability. By using scanning tunneling microscopy , the exact atomic structure of the mixed cation CsxMA1‐xPbBr3 perovskite interface is revealed. In addition, X‐ray photoelectron spectroscopy, ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy are used to study the stability and electronic properties of the CsxMA1‐xPbBr3 perovskite film. Partial substitution of MA+ by Cs+ induces a modification of the perovskite surface structure, leading to improved device stability is shown. These results provide a better understanding of the key parameters involved in the stability of mixed cation perovskite solar cells.

Titanium dioxide surface energy levels tuning by self-assembled monolayers

Tailoring the work function of functional layers in an optoelectronic device is an important means for performance improvement, as it results in changes in charge extraction or recombination. One way to proceed is to adsorb molecules with varying dipole moment strengths and signs. In this communication, the surface of anatase TiO2 is modified using different self-assembled monolayers (4-chlorobenzoic acid, 4-nitrobenzoic acid, 4-methoxybenzoic acid, and β-alanine) and the energetics at the interface is determined using a series of photoelectron spectroscopy techniques, namely, ultraviolet photoelectron spectroscopy, x-ray photoelectron spectroscopy, and inverse photoemission spectroscopy. The observed changes in work function are correlated with the dipole moments of the respective acids, calculated by density functional theory. Finally, the relevance of this interfacial engineering for controlling the charge extraction from an optoelectronic device is illustrated.

Direct Characterization of Type‐I Band Alignment in 2D Ruddlesden–Popper Perovskites

Abstract2D Ruddlesden–Popper halide perovskites have attracted considerable attention due to their desirable optoelectronic properties, high chemical and structural tunability, and improved environmental stability. However, the understanding of their structure–properties relationships is still limited. In particular, the energy level positions and band alignments at interfaces involving these materials, which are important features to control in the context of any applications, are still under debate. Here, the electronic structure of high‐purity films of BA2MAn−1PbnI3n+1 for n = 1–5 (where BA stands for butylammonium and MA for methylammonium) is investigated, using optical absorption, ultraviolet, and inverse photoemission spectroscopies, and density functional theory calculations. This study determines the ionization energy and electron affinity of each compound and demonstrates a type‐I band alignment for the BA2MAn−1PbnI3n+1 series. This study further describes the evolution of the exciton binding energy as a function of the thickness of the inorganic layers.

Direct Observation of Increased Free Carrier Generation Owing to Reduced Exciton Binding Energies in Polymerized Small-Molecule Acceptors.

Energy loss caused by exciton binding energy (Eb) has become a key factor that restricts further advancement of organic solar cells (OSCs). Herein, we used transient mid-IR spectroscopy to study direct photogeneration of free charge carriers in small-molecule acceptors (SMAs) Y6 and IDIC as well as polymerized SMAs (PSMAs) PYFT and PZ1. We found that free carrier concentration is higher in PSMAs than in their corresponding SMAs, indicating reduced exciton Eb, which is then confirmed by ultraviolet photoelectron spectroscopy, low-energy inverse photoemission spectroscopy, and film absorption spectra measurements. The measured Eb values of PYFT and PZ1 are 0.24 and 0.37 eV, respectively, smaller than those of Y6 (0.32 eV) and IDIC (0.47 eV). This work not only provides a method to directly monitor the photogenerated free carriers in OSC materials but also demonstrates that polymerization is an effective strategy to reduce the Eb, which is crucial to decrease the energy losses in high-performance OSCs.

Polycapillary-boosted instrument performance in the extreme ultraviolet regime for inverse photoemission spectroscopy: erratum.

We correct values and figures for the resolution of the spectrometer, as proposed in [Opt. Express25, 31840 (2017)10.1364/OE.25.031840OPEXFF1094-4087]. The new results take into account previously unknown, incoherent phase fluctuations, caused by the polycapillary lens (PCL), and estimate the realistic performance of the instrument.

Oxygen contribution to the magnetic response of ultrathin Fe/Ni multilayers grown on Fe-p(1 × 1)O

We investigated the magnetic behaviour of an Fe overlayer on a Ni buffer layer (Fe/Ni multilayer system) grown on top of the Fe-p(1 × 1)O surface, with a particular focus on the modifications observed in the spin-resolved electronic structure and the role of oxygen, introduced in the system in a well-defined amount at the substrate preparation stage. The structural properties are investigated by means of low energy electron diffraction, that confirms the formation of an epitaxial system featuring a metastable surface lattice. Spin-resolved photoemission spectroscopy testifies a strong decrease of the spectral spin polarization for increasing thickness of the Ni buffer layer, reaching a minimum from a nominal thickness of 6 Ni atomic layers. Surprisingly, the growth of a single Fe overlayer is sufficient to restore most of the original polarization signal, thus creating an ultrathin bcc Fe film seemingly decoupled from the substrate. Ab initio calculations track the modifications of the magnetic moments of the surface layers that quench upon Ni deposition and restore with additional Fe growth. Spin-resolved inverse photoemission highlights a notable reduction of the density of majority states just above the Fermi level, possibly influencing the magnetic response of the system.

Spin- and angle-resolved inverse photoemission setup with spin orientation independent from electron incidence angle.

A new spin- and angle-resolved inverse photoemission setup with a low-energy electron source is presented. The spin-polarized electron source, with a compact design, can decouple the spin polarization vector from the electron beam propagation vector, allowing one to explore any spin orientation at any wavevector in angle-resolved inverse photoemission. The beam polarization can be tuned to any preferred direction with a shielded electron optical system, preserving the parallel beam condition. We demonstrate the performances of the setup by measurements on Cu(001) and Au(111). We estimate the energy resolution of the overall system at room temperature to be ∼170meV from kBTeff of a Cu(001) Fermi level, allowing a direct comparison to photoemission. The spin-resolved operation of the setup has been demonstrated by measuring the Rashba splitting of the Au(111) Shockley surface state. The effective polarization of the electron beam is P = 30% ± 3%, and the wavevector resolution is ΔkF ≲ 0.06 Å-1. Measurements on the Au(111) surface state demonstrate how the electron beam polarization direction can be tuned in the three spatial dimensions. The maximum of the spin asymmetry is reached when the electron beam polarization is aligned with the in-plane spin polarization of the Au(111) surface state.

Resonant inelastic x-ray scattering of spin-charge excitations in a Kondo system

Resonant inelastic x-ray scattering (RIXS) studies across the Ce ${M}_{5}$ edge have been carried out to investigate the electronic structure of the ferromagnetic ${\mathrm{CeAgSb}}_{2}$ Kondo system. The RIXS spectra exhibit energy loss features corresponding to final states usually observed by combining photoemission and inverse photoemission spectroscopy. At low energy loss, a clear signature of the spectral features corresponding to the spin-orbit interaction is also observed. Polarization dependence provides evidence for the ${}^{1}{S}_{0}$ symmetry singlet ground state by a total suppression of the ${f}^{0}$ final state. A simplified single-impurity Anderson model combined with full multiplet theory allows an accurate description of the spin-charge excitations. The RIXS data also reveal a strong temperature $T$ dependence of the fluorescencelike structure. We conjecture that this behavior reflects the $T$ dependence of the Kondo resonance.

Rashba-split image-potential state and unoccupied surface electronic structure of Re(0001)

The influence of spin-orbit interaction on the unoccupied electronic structure of the Re(0001) surface is investigated by spin- and angle-resolved inverse photoemission and density-functional theory calculations. In the two high-symmetry azimuths $\overline{\mathrm{\ensuremath{\Gamma}}}\phantom{\rule{0.16em}{0ex}}\overline{\text{K}}$ and $\overline{\mathrm{\ensuremath{\Gamma}}}\phantom{\rule{0.16em}{0ex}}\overline{\text{M}}$, we identify transitions into $d$-derived bulk states as well as different types of surface states. The Rashba-type spin-split hole pocket around $\overline{\mathrm{\ensuremath{\Gamma}}}$ finds continuation in empty spin-split surface states for higher ${\mathbf{k}}_{\ensuremath{\parallel}}$, thereby forming $\mathsf{W}$-shaped states whose lower parts are partially occupied. A large energy gap below and above the vacuum energy around $\overline{\mathrm{\ensuremath{\Gamma}}}$ hosts image-potential-induced surface states. The $n=1$ member of the Rydberg-like series exhibits a free-electron-like $E({\mathbf{k}}_{\ensuremath{\parallel}})$ dispersion with an effective mass of ${m}^{*}/{m}_{e}=1.2\ifmmode\pm\else\textpm\fi{}0.1$. Careful spin-resolved measurements for several angles of electron incidence allow us to detect Rashba-type spin-dependent energy splittings of this state with a Rashba parameter of ${\ensuremath{\alpha}}_{\text{R}}=105\ifmmode\pm\else\textpm\fi{}33\phantom{\rule{0.16em}{0ex}}\text{meV}\phantom{\rule{0.16em}{0ex}}\AA{}$.

Impacts of KF Post-Deposition Treatment on the Band Alignment of Epitaxial Cu(In,Ga)Se2 Heterojunctions.

In this study, we investigated band alignments at CdS/epitaxial CuInxGa1-xSe2 (epi-CIGSe) and epi-CIGSe/GaAs heterointerfaces for solar cell applications using ultraviolet, inverse, and X-ray photoemission spectroscopy (UPS, IPES, and XPS) techniques. We clarified the impacts of KF postdeposition treatment (KF-PDT) at the CdS/epi-CIGSe front heterointerfaces. We found that KF-PDT changed the conduction band alignment at the CdS/epi-CIGSe heterointerface from a cliff to flat configuration, attributed to an increase in the electron affinity (EA) and ionization potential (IP) of the epi-CIGSe surface because of a decrease in Cu and Ga contents. Herein, we discuss the correlation between the impacts of KF-PDT and the solar cell performance. Furthermore, we also investigated the band alignment at the epi-CIGSe/GaAs rear heterointerface. Electron barriers were formed at the epi-CIGSe/GaAs interface, suppressing carrier recombination as the back surface field. Contrarily, a hole accumulation layer is formed by the valence band bending, which is like Ohmic contact.

Photoemission spectral functions from the three-body Green's function

We present an original strategy for the calculation of direct and inverse photo-emission spectra from first principles. The main goal is to go beyond the standard Green's function approaches, such as the $GW$ method, in order to find a good description not only of the quasiparticles but also of the satellite structures, which are of particular importance in strongly correlated materials. To this end we use as a key quantity the three-body Green's function, or, more precisely, its hole-hole-electron and electron-electron-hole parts, and we show how the one-body Green's function, and hence the corresponding spectral function, can be retrieved from it. We show that, contrary to the one-body Green's function, information about satellites is already present in the non-interacting three-body Green's function. Therefore, simple approximations to the three-body self-energy, which is defined by the Dyson equation for the three-body Green's function and which contains many-body effects, can still yield accurate spectral functions. In particular, the self-energy can be chosen to be static which could simplify a self-consistent solution of the Dyson equation. We give a proof of principle of our strategy by applying it to the Hubbard dimer, for which the exact self-energy is available.