High-resolution Photoemission Spectroscopy Research Articles

Coverage Dependent Variation of the Adsorption Structure of 2-Thiophenecarboxaldehyde on the Ge(100)-2 × 1 Reconstructed Surface

High-resolution photoemission spectroscopy (HRPES) measurements were collected and density functional theory (DFT) calculations were performed to track the exposure-dependent variation of the adsorption structure of 2-thiophenecarboxaldehyde (C4H3SCHO: TPCA) on the Ge(100) 2 × 1 reconstructed surface at room temperature. In an effort to identify the most probable adsorption structures on the Ge(100)-2 × 1 reconstructed surface, we deposited TPCA molecules at low exposure and at high exposure and compared the differences between the electronic features measured using HRPES. The HRPES data suggested three possible adsorption structures of TPCA on the Ge(100)-2 × 1 reconstructed surface, and DFT calculations were used to determine the plausibility of these structures. HRPES analysis corroborated by DFT calculations, indicated that an S-dative bonded structure is the most probable adsorption structure at relatively low exposure levels, the [4 + 2] cycloadduct structure is the second most probable structure, and the [2 + 2]-C=O cycloadduct structure is the least probable structure on the Ge(100)-2 × 1 reconstructed surface at relatively high exposure levels.

Efficiency enhancement caused by using LiF to change electronic structures in polymer photovoltaics

The enhancement of device performance achieved by inserting a LiF (lithium fluoride) interfacial layer between the active layer and aluminum (Al) cathode was investigated. Energy band diagrams and interfacial chemical reactions were examined using high-resolution synchrotron-radiation photoemission spectroscopy. If only a thin layer of Al is deposited on the P3HT:PCBM/LiF, the highest occupied molecular orbital level of the active layer does not change. This indicates that shifts in the energy band are only strongly influenced by the insertion of LiF, not the subsequent deposition of an Al layer. We found that the molecular energy band shifts occurred at the interface because of the presence of LiF increases the open-circuit voltage. Furthermore, the thin layer of LiF reduces the work function and improves contact with the cathode, thereby reducing the series resistance and improving the short circuit current density. Thus, without the influence of the Al layer, inserting LiF improves the device performance. The level of performance differs substantially from that observed at the organic layer/LiF/Al interfaces in organic light-emitting devices.

Soft x-ray spectroscopic investigation of Zn doped CuCl produced by pulsed dc magnetron sputtering

We report on a systematic investigation of the electronic properties of UV-light emitting Zn doped CuCl thin films implemented using near edge x-ray absorption fine structures (NEXAFS) and high-resolution x-ray photoemission spectroscopy. A clear shift of the valence band maximum towards higher binding energy by 0.2 ± 0.1 eV was observed in Zn doped CuCl as compared to undoped CuCl. This shift is in correlation with the increase in conductivity measured by the Hall effect measurements. A decrease in the optical band gap of CuCl film is also observed as a function of Zn doping. The profound changes in the full width at half maximum and the gradual disappearance of satellite features of Cu 2p core level photoemission as a function of Zn dopant are attributed to the reduced presence of the surface layer of Cu2+ species with d9 configuration in the doped films. These investigations help us to understand the doping mechanisms and underlying physics. The reduced presence of the Cu2+ related surface layer as a function of Zn doping is also verified using NEXAFS.

Electronic structure of EuFe2As2

Employing high resolution photoemission spectroscopy, we studied the temperature evolution of the electronic structure of EuFe2As2, a unique pnictide, where antiferromagnetism of the Eu layer survives within the superconducting phase due to ‘FeAs’ layers, achieved via substitution and/or pressure. High energy and angle resolution helped to reveal the signature of peak–dip features, having significant p orbital character and spin density wave transition induced band folding in the electronic structure. A significant spectral weight redistribution is observed below 20 K manifesting the influence of antiferromagnetic order on the conduction electrons.

Band alignment of ZnTe/GaAs heterointerface investigated by synchrotron radiation photoemission spectroscopy

ZnTe films have been grown on GaAs substrate by metalorganic vapor phase epitaxy. High-resolution synchrotron radiation photoemission spectroscopy measurements are used to determine the valence band offset of ZnTe/GaAs heterojunctions. Based on the binding energies of Zn 3d and Ga 3d core levels and valence band maximum values, the valence band offset has been determined to be 0.19 eV for ZnTe/GaAs heterojunction. The heterojunction shows type I band configuration with a conduction band offset of 0.65 eV. The accurate determination of the band alignment of ZnTe/GaAs heterojunction facilitates the design of optical and electronic devices based on ZnTe/GaAs structure.

Formation of one-dimensional self-assembled silicon nanoribbons on Au(110)-(2 × 1)

We report results on the self-assembly of silicon nanoribbons (NRs) on the (2 × 1) reconstructed Au(110) surface under ultra-high vacuum conditions. Upon adsorption of 0.2 monolayer (ML) of silicon, the (2 × 1) reconstruction of Au(110) is replaced by an ordered surface alloy. Above this coverage, a new superstructure is revealed by low energy electron diffraction (LEED), which becomes sharper at 0.3 Si ML. This superstructure corresponds to Si nanoribbons all oriented along the [1¯10] direction as revealed by LEED and scanning tunneling microscopy (STM). STM and high-resolution photoemission spectroscopy indicate that the nanoribbons are flat and predominantly 1.6 nm wide. In addition, the silicon atoms show signatures of two chemical environments corresponding to the edge and center of the ribbons.

Accommodation of Lattice Mismatch in a Thiol Self-Assembled Monolayer

The structure of self-assembled monolayers (SAMs) of 3-(4′-(methylthio)-[1,1′-biphenyl]-4-yl)propane-1-thiol (CH3S(C6H4)2(CH2)3SH) formed on Au(111)/mica has been investigated by scanning tunneling microscopy (STM), high-resolution X-ray photoemission spectroscopy (HRXPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT) calculations. A highly crystalline monolayer terminated by thioether moieties is formed, which adopts a structure that differs from the previously studied CH3 terminated analogue with its 2√3 × √3 unit cell. An oblique 2√3 × √61 chiral unit cell containing 8 molecules is proposed. Accommodation of the substantial mismatch between the lattices of the SAM and substrate is explained by molecular design. Serving as a buffer layer, the alkane spacer decouples the SAM lattice, defined by the functionalized aromatic moieties, from the substrate.

Adlayers of Alkanedithiols on Au(111): Effect of Disulfide Reducing Agent

High-resolution photoemission spectroscopy is used to characterize adlayers of ethane-, hexane-, and nonanedithiol molecules grown on Au(111) surfaces by the immersion method. The effect of using a reducing agent during and after the immersion to inhibit or eliminate S-S bonds is investigated. Our results demonstrate that immersion 24 h in millimolar dithiol ethanolic solutions gives rise to the formation of multilayers; this effect is more pronounced in the case of ethanedithiol, the shortest molecule. A post-treatment with a disulfide reducing agent is effective to produce monolayers of standing-up molecules; this effect is again more pronounced in the case of ethanedithiol. Finally, the immersion 24 h in a solution containing dithiol and the reducing agent gives an unexpected result: most molecules remain adsorbed in the lying-down configuration; in this case, the almost complete suppression of the standing-up phase occurs equally with the three types of molecules, which suggests that the formation of S-S bonds must be important for the lifting of the molecules.

ChemInform Abstract: Kondo Resonance in Magnetic and Non‐Magnetic Ce‐Intermetallics

AbstractReview: 26 refs.

Does scandium resemble transition or rare earth metals when it is grown on silicon surfaces?

Scandium (Sc) has long been considered a rare earth (RE) element rather than a transition metal (TM) when grown on silicon (Si) surfaces, although little experimental evidence supports it. When scandium nitride (ScN) grown on Si is a promising buffer material for combining gallium nitride (GaN) and Si, a thin Sc silicide layer may be formed at the ScN/Si interface and, therefore, the electronic properties of Sc on Si are becoming an important issue to address. In this study, the interfacial atomic and electronic properties of Sc on n-type Si (111)-7 × 7 reconstructed surfaces have been studied by low energy electron diffraction and synchrotron radiation based high-resolution X-ray photoemission spectroscopy. By varying the Sc overlayer thickness, different silicide phases were observed and the Schottky barrier (SB) evolution was obtained. The SB evolution of Sc on Si is closer to TM than RE. This barrier may interfere with carrier transportation if GaN/ScN/Si tri-layered heterostructures are grown.

MPc/Au(111) interface electronic structure investigated by high-resolution photoemission spectroscopy

MPc/Au(111) interface electronic structure investigated by high-resolution photoemission spectroscopy

A tunable low-energy photon source for high-resolution angle-resolved photoemission spectroscopy

We describe a tunable low-energy photon source consisting of a laser-driven xenon plasma lamp coupled to a Czerny-Turner monochromator. The combined tunability, brightness, and narrow spectral bandwidth make this light source useful in laboratory-based high-resolution photoemission spectroscopy experiments. The source supplies photons with energies up to ~7 eV, delivering under typical conditions >10(12) ph/s within a 10 meV spectral bandwidth, which is comparable to helium plasma lamps and many synchrotron beamlines. We first describe the lamp and monochromator system and then characterize its output, with attention to those parameters which are of interest for photoemission experiments. Finally, we present angle-resolved photoemission spectroscopy data using the light source and compare its performance to a conventional helium plasma lamp.

Te concentration dependent photoemission and inverse-photoemission study of FeSe1−xTex

We have characterized the electronic structure of FeSe1−xTex for various x values using soft x-ray photoemission spectroscopy (SXPES), high-resolution photoemission spectroscopy (HRPES) and inverse photoemission spectroscopy (IPES). The SXPES valence band spectral shape shows that the 2 eV feature in FeSe, which was ascribed to the lower Hubbard band in previous theoretical studies, becomes less prominent with increasing x. HRPES exhibits systematic x dependence of the structure near the Fermi level (EF): its splitting near EF and filling of the pseudogap in FeSe. IPES shows two features, near EF and approximately 6 eV above EF; the former may be related to the Fe 3d states hybridized with chalcogenide p states, while the latter may consist of plane-wave-like and Se d components. In the incident electron energy dependence of IPES, the density of states near EF for FeSe and FeTe has the Fano lineshape characteristic of resonant behavior. These compounds exhibit different resonance profiles, which may reflect the differences in their electronic structures. By combining the PES and IPES data the on-site Coulomb energy was estimated at 3.5 eV for FeSe.

Schottky barrier height of Au on the transparent semiconducting oxide β-Ga2O3

The Schottky barrier height of Au deposited on (100) surfaces of n-type β-Ga2O3 single crystals was determined by current-voltage characteristics and high-resolution photoemission spectroscopy resulting in a common effective value of 1.04 ± 0.08 eV. Furthermore, the electron affinity of β-Ga2O3 and the work function of Au were determined to be 4.00 ± 0.05 eV and 5.23 ± 0.05 eV, respectively, yielding a barrier height of 1.23 eV according to the Schottky-Mott rule. The reduction of the Schottky-Mott barrier to the effective value was ascribed to the image-force effect and the action of metal-induced gap states, whereas extrinsic influences could be avoided.

Electronic Structure of SrTi0.99Sc0.01O3 Thin Film Studied by High-Resolution Soft-X-ray Spectroscopy

The electronic structure of SrTi 0.99 Sc 0.01 O 3 thin film has been studied by X-ray absorption spectroscopy (XAS) and high-resolution photoemission spectroscopy (HRPES). The evidence of Sc 3+ substitution into Ti 4+ sites is obtained from the Ti 2 p and Sc 2 p XAS spectra. The HRPES spectra in the valence band and the core level indicate that the valence band is mainly composed of O 2 p state hybridized with Ti 3 d state. The Fermi level ( E F ) locates at ∼1.2 eV from the top of the valence band. The energy separation between the top of the valence band and E F is in good agreement with the activation energy ( E a ), which is estimated from the Arrhenius plot of electrical conductivity. These findings may indicate that the SrTi 0.99 Sc 0.01 O 3 thin film is a p -type oxide semiconductor with a large E a .

Electrochemical, High-Resolution Photoemission Spectroscopy and vdW-DFT Study of the Thermal Stability of Benzenethiol and Benzeneselenol Monolayers on Au(111)

The preparation and thermal stability of benzenethiol and benzeneselenol self-assembled monolayers (SAMs) grown on Au(111) have been investigated by electrochemical experiments and high-resolution photoemission spectroscopy. Both techniques confirm the formation of monolayers with high packing densities (θ = 0.27-0.29 ML) and good degrees of order in both cases. Despite many similarities between the two SAMs, the thermal desorption is distinctly different: whereas the benzenethiol SAM desorbs in a single steplike process, the desorption of the benzeneselenol SAM occurs with a much lower activation energy and involves the cleavage of some Se-C bonds and a change in molecular configuration from standing up to lying down. This behavior is explained by considering the different nature of the bonding of the headgroup with the metal surface and with the phenyl ring. Density functional theory calculations show that the breakage of the Se-C bond has a lower activation energy barrier than the breakage of the S-C bond.

Electronic structure near the quantum critical point in V-doped Cr —A high-resolution photoemission study

We studied the evolution of the electronic structure across the quantum critical point in V-doped Cr employing high-resolution photoemission spectroscopy. Experimental results exhibit signatures of pseudogap and orbital Kondo resonance peak at low temperatures for all the compositions studied. The pseudogap and the Kondo peak reduce with V doping but remain finite across the quantum critical point indicating their relevance in widely discussed quantum phases in correlated electron systems. The spectral lineshape near the Fermi level exhibit (ϵF − ϵ)0.25 dependence evidencing deviation from Fermi-liquid behavior.

Comparative photoemission studies on the superconducting gap of the filled skutterudite superconductors LaPt4Ge12and PrPt4Ge12

We performed a comparative study of the superconducting gap in the new filled skutterudite superconductors LaPt4Ge12 and PrPt4Ge12 using high-resolution photoemission spectroscopy. We succeeded in observing spectral changes across Tc that reflect the opening of the superconducting gap in both compounds and also in observing a noticeable difference in their respective superconducting spectral shapes near the Fermi level, pointing toward a more complex superconducting gap structure in PrPt4Ge12. In addition, we found that the two-gap model is more suitable for describing the superconducting-state spectrum of PrPt4Ge12 than the single-isotropic-gap and single-anisotropic-gap models, which suggests an explanation that multiband effects may possibly induce the anomalous superconducting properties of PrPt4Ge12.

Low-energy scale excitations in the spectral function of organic monolayer systems

Using high-resolution photoemission spectroscopy we demonstrate that the electronic structure of several organic monolayer systems, in particular 1,4,5,8-naphthalene tetracarboxylic dianhydride and Copper-phtalocyanine on Ag(111), is characterized by a peculiar excitation feature right at the Fermi level. This feature displays a strong temperature dependence and is immediatly connected to the binding energy of the molecular states, determined by the coupling between the molecule and the substrate. At low temperatures, the line-width of this feature, appearing on top of the partly occupied lowest unoccupied molecular orbital of the free molecule, amounts to only $\approx 25$ meV, representing an unusually small energy scale for electronic excitations in these systems. We discuss possible origins, related e.g. to many-body excitations in the organic-metal adsorbate system, in particular a generalized Kondo scenario based on the single impurity Anderson model.

Thermal stability of the Co/β-Si3N4/Si(111) interface: A photoemission study

The thermal stability of the Co/β-Si3N4/Si(111) interface has been studied by high-resolution photoemission spectroscopy in a temperature range extending from room temperature to 650°C. It is demonstrated the ability of a very thin crystalline buffer layer of silicon nitride to prevent the interfacial reaction between cobalt and silicon at room temperature. The behaviour of the interface at higher temperature shows the formation of cobalt silicides already at 300°C. Moreover, the presence of new components in the decomposition of the photoemission spectra is discussed in the light of the existing literature.