Resonant Photoemission Spectroscopy Research Articles

Heterostructured organic interfaces probed by resonant photoemission

The application of the resonant photoemission spectroscopy (RPES) to various organic molecule based systems is reviewed. The chemical specificity and the possibility to conduct experiments in the energy domain that provides a time scale for charge dynamics, make the RPES a powerful tool to study organic heterojunctions and in particular to probe the charge transfer processes at organic interfaces. We briefly discuss the models used in RPES data analysis to extract the time scale of the excited charge delocalisation and the spatial correlation of core, valence occupied and unoccupied molecular states. As an example we report on 3,4,9,10-perylene tetracarboxylic acid dianhydride (PTCDA) on (1 × 2) Au(1 1 0) surface where organic layer metallicity is directly evidenced in RPES experiments. A particular attention is dedicated to bio-mimetic model molecules whose electronic structure at interfaces is the fundamental key for the design of real devices. In the last section we consider recent experiments that could open the way to new fields of applications regarding biological molecules and single molecule systems where RPES could elucidate the link between the quantum and the meso-scopic properties of such systems.

Site-specific electronic structure of bacterial surface protein layers

We applied resonant photoemission and X-ray absorption spectroscopy for a detailed characterization of the valence electronic structure of the regular two-dimensional bacterial surface protein layer of Bacillus sphaericus NCTC 9602. Using this approach, we detected valence electron emission from specific chemical sites. In particular, it was found that electrons from the π clouds of aromatic systems make large contributions to the highest occupied molecular orbitals.

Photoemission, resonant photoemission, and x-ray absorption of a Ru(II) complex adsorbed on rutile TiO2(110) prepared by in situ electrospray deposition

An experimental study of the bonding geometry and electronic coupling of cis-bis(isothiocyanato)bis(2,2(')-bipyridyl-4,4(')-dicarboxylato)-ruthenium(II) (N3) adsorbed on rutile TiO(2)(110) is presented, along with supporting theoretical calculations of the bonding geometry. Samples were prepared in situ using ultrahigh vacuum electrospray deposition. Core-level photoemission spectroscopy was used to characterize the system and to deduce the nature of the molecule-surface bonding. Valence band photoemission and N 1s x-ray absorption spectra were aligned in a common binding energy scale to enable a quantitative analysis of the bandgap region. A consideration of the energetics in relation to optical absorption is used to identify the photoexcitation channel between the highest occupied and lowest unoccupied molecular orbitals in this system, and also to quantify the relative binding energies of core and valence excitons. The core-hole clock implementation of resonant photoemission spectroscopy is used to reveal that electron delocalization from N3 occurs within 16 fs.

Femtosecond Charge Transfer in Assemblies of Discotic Liquid Crystals

The electronic coupling strength within columns of discotic liquid crystals is investigated using core-level resonant photoemission spectroscopy. Coexisting well-ordered and disordered regions are identified in thin films of tetra-alkoxy-substituted phthalocyanines with the aid of near edge X-ray absorption fine structure and photoelectron spectroscopies. These different regions are used to derive a lower limit for the intermolecular charge transfer bandwidth within the framework of the core-hole clock principle. We find average charge transfer times on the order of a few femtoseconds, that is, significantly faster than the C(1s) core-hole lifetime, which indicates a surprisingly strong electronic coupling between the phthalocyanine units as compared to what is expected from the charge transport characteristics of this material.

Soft X-ray Absorption and Photoemission Studies of Ferromagnetic Mn-Implanted 3C-SiC

We have performed X-ray photoemission spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and resonant photoemission spectroscopy (RPES) measurements of Mn-implanted 3C-SiC (3C-SiC:Mn) and carbon-incorporated Mn5Si2 (Mn5Si2:C). The Mn 2p core-level XPS and XAS spectra of 3C-SiC:Mn and Mn5Si2:C were similar to each other and showed “intermediate” behaviors between the localized and itinerant Mn 3d states. The intensity at the Fermi level was found to be suppressed in 3C-SiC:Mn compared with Mn5Si2:C. These observations are consistent with the formation of Mn5Si2:C clusters in the 3C-SiC host, as observed in a recent transmission electron microscopy study.

Photoemission evidence for a Mott-Hubbard metal-insulator transition inVO2

The temperature $(T)$-dependent metal-insulator transition (MIT) in ${\text{VO}}_{2}$ is investigated using bulk sensitive hard-x-ray $(\ensuremath{\sim}8\text{ }\text{keV})$ valence-band, core-level, and $\text{V}\text{ }2p\ensuremath{-}3d$ resonant photoemission spectroscopies (PESs). The valence-band and core-level spectra are compared with full-multiplet cluster model calculations including a coherent screening channel. Across the MIT, $\text{V}\text{ }3d$ spectral weight transfer from the coherent ($3{d}^{1}\underset{̱}{C}$ final) states at Fermi level to the incoherent ($3{d}^{0}+3{d}^{1}\underset{̱}{L}$ final) states, corresponding to the lower Hubbard band, leads to gap formation. The spectral shape changes in $\text{V}\text{ }1s$ and $\text{V}\text{ }2p$ core levels as well as the valence band are nicely reproduced from cluster model calculations, providing electronic structure parameters. Resonant PES finds that the $3{d}^{1}\underset{̱}{L}$ states resonate across the $\text{V}\text{ }2p\ensuremath{-}3d$ threshold in addition to the $3{d}^{0}$ and $3{d}^{1}\underset{̱}{C}$ states. The results support a Mott-Hubbard transition picture for the first-order MIT in ${\text{VO}}_{2}$.

Electronic structure of Fe (0–5 at.%) dopedMoO2 thin films studied by resonant photoemission spectroscopy

The electronic structure of pulsed laser-depositedMo1−xFexO2 (x = 0,0.02 and 0.05) thin films has been investigated using resonant photoemission spectroscopynear the Mo 4p absorption edge. In all the samples a broad Fano-like resonance peak at∼46 eV is observed in the whole area of the valence band, which indicates the contribution ofthe Mo 4d states in the entire valence band region. The doping of Fe in these films leads toa decrease in Mo 4d states contributing to electronic states at lower binding energy region.In addition to this, we also observe a shoulder at 4.9 eV in the valence band spectra ofdoped samples. It is proposed that the origin of this shoulder is due to the Fe hybridizedstates.

Electronic structure of the TiO thin film on Ag(1 0 0): Angle-resolved photoemission study

Angle-resolved photoemission spectroscopy (ARPES) has been used to study the electronic structure of the TiO film formed on a Ag(1 0 0) surface. The film was prepared by Ti deposition on Ag(1 0 0) at room temperature in an O 2 atmosphere at 3.0 × 10 −9 Torr and subsequent annealing at 600 °C, and the film with the composition of TiO 0.92 was obtained by this procedure. The conduction band and the valence band of the film are observed at 0–2 eV and 4–8 eV, respectively, and thus these bands are separated by a gap of about 2 eV. Resonant photoemission spectroscopy measurements show that the hν-dependence of the conduction band intensity shows a resonant behavior characteristic of the Ti 3d states. The two-dimensional electronic structure of the states in the conduction band along the ( Γ ¯ X ¯ ) direction of Ag(1 0 0) are measured with ARPES, and it was found that the states show dispersions consistent with the (1 × 1) periodicity.

ULTRAFAST CHARGE TRANSFER ACROSS MOLECULE/METAL INTERFACES BY RESONANT PHOTOEMISSION SPECTROSCOPY

The authors review recent studies of charge transfer dynamics across molecule/metal interfaces by resonant photoemission spectroscopy done at the Surface, Interface and Nanostructure Science (SINS) beamline, Singapore Light Source Synchrotron. The interfacial charge transfer takes place in the femtosecond scale due to the strong coupling between molecules and metal substrate, indicating that the transport properties of the molecule/metal junction is dominated not only by the chemical nature of molecules but also the interfacial properties of molecule/metal. Resonant photoemission spectroscopy is demonstrated to be a powerful tool for the study of ultrafast charge transfer dynamics across molecule/metal interfaces.

Direct Observation of Dispersive Kondo Resonance Peaks in a Heavy-Fermion System

Ce 4d-4f resonant angle-resolved photoemission spectroscopy was carried out to study the electronic structure of strongly correlated Ce 4f electrons in a quasi-two-dimensional nonmagnetic heavy-fermion system CeCoGe1.2Si0.8. For the first time, dispersive coherent peaks of an f state crossing the Fermi level, the so-called Kondo resonance, are directly observed together with the hybridized conduction band. Moreover, the experimental band dispersion is quantitatively in good agreement with a simple hybridization-band picture based on the periodic Anderson model. The obtained physical quantities, i.e., coherent temperature, Kondo temperature, and mass enhancement, are comparable to the results of thermodynamic measurements. These results manifest an itinerant nature of Ce 4f electrons in heavy-fermion systems and clarify their microscopic hybridization mechanism.

Hybridization effects inCeCoIn5observed by angle-resolved photoemission

We have investigated the low-energy electronic structure of the heavy fermion superconductor $\mathrm{Ce}\mathrm{Co}{\mathrm{In}}_{5}$ by angle-resolved photoemission. We focus on the dispersion and the peak width of the prominent quasi-two-dimensional Fermi surface sheet at the corner of the Brillouin zone as a function of temperature along certain $k$ directions with a photon energy of $h\ensuremath{\nu}=100\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. We find slight changes of the Fermi vector and an anomalous broadening of the peak width when the Fermi energy is approached. Additionally, we performed resonant angle-resolved photoemission spectroscopy experiments with $h\ensuremath{\nu}=121\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. A flat $f$-derived band is observed with a distinct temperature dependence and a $k$-dependent spectral weight. These results, including both off- and on-resonant measurements, fit qualitatively to a two-level mixing model derived from the periodic Anderson model.

Effects of SrTiO3 capping in La2∕3Ca1∕3MnO3 electrodes of different orientations

We report on the study of the structural, magnetic, and electronic properties of SrTiO3 capped La2∕3Ca1∕3MnO3 electrodes grown on (001) and (110) SrTiO3 substrates. Magnetic properties of the (001) and (110) capped electrodes evolve differently when the capping layer thickness increases, revealing a reduction of the saturation magnetization for the (001) ones. Electronic properties are studied combining Mn55 nuclear magnetic resonance (NMR) and x-ray photoemission spectroscopy (XPS). NMR experiments highlight that electronic phase separation in the (001) electrodes is enhanced by the presence of the SrTiO3 capping layer and XPS measurements show that the electronic state of interfacial Mn ions from (001) electrode is more sensitive to the capping layer.

Carrier Compensation Mechanism of Highly Conductive Anatase Ti0.94Nb0.06O2 Epitaxial Thin Films

ABSTRACTWe investigated electrical conduction of anatase Ti0.94Nb0.06O2 (TNO) epitaxial thin films in relation to oxygen defects generated by post-annealing. Annealing of TNO in oxygen was found to cause dramatic decreases in ne. Resonant photoemission spectroscopy measurements revealed that a deep acceptor state just above the top of valence band evolves, synchronized with the decrease of ne. We proposed that the acceptor state originates from interstitial oxygen atoms combined with Nb dopants and compensates electron carriers.

Band structure and Fermi surface of heavy Fermion compounds Ce 2TIn 8[formula omitted] studied by angle-resolved photoemission spectroscopy

We report a systematic study on the electronic structure of Ce-based heavy Fermion materials Ce 2TIn 8 ( T = Rh , Co, Ir) by high-resolution angle-resolved (ARPES) and 4d–4f resonant (RPES) photoemission spectroscopies. We have experimentally determined the band structure and Fermi surface of these three compounds, and found that the Fermi surfaces of three compounds are very similar to each other, having quasi two-dimensional cylindrical Fermi surfaces centered at the M(A) point in the Brillouin zone. RPES results have confirmed the localized picture and, in addition, revealed that Ce 4f electrons in Ce 2RhIn 8 have a relatively stronger localized nature than those in Ce 2CoIn 8. This difference in the localized character explains well the difference in the magnetic property of these compounds.

Angle-resolved and resonant photoemission study of the ZrO-like film on ZrC(1 0 0)

Angle-resolved photoemission spectroscopy (ARPES) and resonant photoemission spectroscopy (RPES) have been used to study the electronic structure of the ZrO-like film formed on a ZrC(1 0 0) surface. It is found that, in addition to the O 2p band observed at 6–8 eV, states exist at 0.15 and 0.75 eV around the M ¯ point. ARPES measurements show that the states at 0.15 and 0.75 eV disperse towards the lower binding energy side and cross the Fermi level along the M ¯ → Γ ¯ direction. RPES measurements show that the former state shows a resonant behavior characteristic of the intra-atomic Zr 4d resonance, suggesting that the state includes substantial contribution of Zr 4d orbitals. On the other hand, the latter state shows a resonant behavior similar to that of the O 2p state in ZrO. The resonance is thought to be caused through the inter-atomic deexcitation mechanism involving the emissions of O 2p electrons, and thus the latter state is ascribed to that mostly composed of the O 2p component.

Sensitivity to hole doping of CuL3resonant spectroscopies: Inelastic x-ray scattering and photoemission ofLa2−xSrxCuO4

We present Cu ${L}_{3}$ edge resonant inelastic x-ray scattering (RIXS) and resonant x-ray photoemission spectroscopy (RXPS) of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ ($x=0.0$, 0.13, and 0.27). RIXS is found to be much more sensitive to the hole doping level than resonant photoemission. RIXS allows us to determine the average energy of $dd$ excitations of both undoped and hole doped sites. The trade-off between the Raman and non-Raman spectral features depends on the doping level strongly in RIXS but weakly in RXPS. These experimental findings are consistently interpreted within a cluster model: The role of nonlocal core hole screening in the intermediate state is shown to be crucial. Nonlocal screening has an impact also on the interpretation of the ${L}_{3}$ x-ray absorption spectra in cuprates.

Photoemission Study of Mn 3d Electrons in the Valence Band of Mn/GeMnTe

We present the results of the electronic band structure study of the Ge0.9Mn0.1Te epilayers, clean and modified in situ by deposition of manganese atoms under ultra high vacuum conditions. Ge1-xMnxTe is a ferromagnet with a relatively high Curie temperature (up to 140 K) strongly depending on Mn concentration [1]. Thus, investigation of its properties, including the band structure, attracts considerable interest, in view of possible applications of IV-VI-based systems in fabrication of spintronic devices. The Ge0.9Mn0.1Te layers were grown on BaF2 (111) substrates by an MBE method with use of effusion cells as GeTe, Te2 and Mn solid sources. The substrate temperature was 400-450 o C. The content of Mn in Ge0.9Mn0.1Te was checked by energy dispersive X-ray fluorescence analysis. The sample surface was protected by a layer of amorphous tellurium. Prior the photoemission experiments, the tellurium layer was removed by annealing. The Mn overlayers were deposited in situ at room temperature. The valence band and shallow core levels of Ge0.9Mn0.1Te and ultrathin Mn overlayers have been investigated by means of resonant photoemission spectroscopy (RPES). In this technique the radiation energy is tuned to the intra ion transition (like Mn 3p-3d) in order to enhance emission from partly occupied shells of transition metal atoms. Sets of resonant photoemission spectra were measured for the photon energy range covering the energy of the Mn 3p→3d transition (40<hν<60 eV). They were acquired for the clean surface of the Ge0.9Mn0.1Te sample, after evaporation of 0.5 ML Mn and after annealing at 200 o C for 2 hours. The annealing was carried out in order to induce the diffusion of Mn atoms into the Ge0.9Mn0.1Te crystal. The spectra (photoelectron energy distribution curves) covered the range of electron binding energy starting from the valence band edge down to the Mn 3p level. The Mn 3d states contribution occurred in the upper part of the valence band, with a maximum at the binding energy of about 4 eV. Its shape corresponded to that expected for Mn 2+ ions surrounded by six Te ions in the octahedral coordination [2]. Deposition of Mn and annealing the system led to an increase of Mn 3d feature intensity but did not induce any change in the shape of the spectrum. This proved that diffusing Mn ions occupied the same sites in the lattice as those introduced during the layer growth. The results acquired for the Ge0.9Mn0.1Te epilayers are compared with those recently obtained for bulk Ge1-xMnxTe polycrystals in which Mn possibly forms a mixed valence system [3]. The photoemission results are also related to perceptible differences in magnetic properties of theses two systems.

X-ray Absorption and Inverse Photoemission Study of Y1-xCaxTiO3

We have performed the O 1 s x-ray absorption (XAS) and resonant inverse photoemission spectroscopy (RIPES) in Mott insulator YTiO 3 , band insulator CaTiO 3 and Y 0.61 Ca 0.39 TiO 3 with a metal–in...

Resonant photoemission spectroscopy of thick Mn films on Si(111) at the2pedge: Detection of the two-hole valence-band satellite of Mn

We have measured valence-band spectra of a thick Mn metal film deposited on Si(111) by means of resonant photoemission across the $\mathrm{Mn}\phantom{\rule{0.2em}{0ex}}2p\ensuremath{\rightarrow}3d$ absorption edge. A resonant Raman-Auger signal is detected for excitation energies down to $3.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the Mn ${L}_{3}$ absorption threshold. This feature exhibits a crossover from the Raman to the normal Auger behavior at a photon energy close to the $\mathrm{Mn}\phantom{\rule{0.2em}{0ex}}2p$ photoemission ionization threshold. The resulting radiationless resonance Raman behavior gives rise to a signal equivalent to the $6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ satellite in Ni, with energy at $3.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below ${E}_{F}$. Moreover, the presence of many-electron effects is detected for the Mn spectral weight in the valence band, with a clear contribution of Mn-derived states at the Fermi edge.

MnAsdots grown onGaN(0001¯)−(1×1)surface

MnAs has been grown by means of MBE on the GaN(0001)-(1x1) surface. Two options of initiating the crystal growth were applied: (a) a regular MBE procedure (manganese and arsenic were delivered simultaneously) and (b) subsequent deposition of manganese and arsenic layers. It was shown that spontaneous formation of MnAs dots with the surface density of 1$\cdot 10^{11}$ cm$^{-2}$ and $2.5\cdot 10^{11}$ cm$^{-2}$, respectively (as observed by AFM), occurred for the layer thickness higher than 5 ML. Electronic structure of the MnAs/GaN systems was studied by resonant photoemission spectroscopy. That led to determination of the Mn 3d - related contribution to the total density of states (DOS) distribution of MnAs. It has been proven that the electronic structures of the MnAs dots grown by the two procedures differ markedly. One corresponds to metallic, ferromagnetic NiAs-type MnAs, the other is similar to that reported for half-metallic zinc-blende MnAs. Both system behave superparamagnetically (as revealed by magnetization measurements), but with both the blocking temperatures and the intra-dot Curie temperatures substantially different. The intra-dot Curie temperature is about 260 K for the former system while markedly higher than room temperature for the latter one. Relations between growth process, electronic structure and other properties of the studied systems are discussed. Possible mechanisms of half-metallic MnAs formation on GaN are considered.