Synchrotron Photoemission Research Articles

Thermal- and air- stability of the compositional variants of van der Waals Pt-Telluride thin films probed by high resolution photoemission spectroscopy

The Pt-Te compositional phase diagram consists of at least three different compositional line phases (PtTe2, Pt3Te4, and Pt2Te2) that can be described as layered van der Waals materials. This presents challenges in controlling the composition of ultrathin Pt-telluride 2D materials by physical vapor deposition methods. Here we show by temperature programmed synchrotron photoemission spectroscopy that the different phases have varying thermal stability in vacuum. This enables the synthesis of these materials by preparation of PtTe2 films at low growth temperatures and subsequent vacuum annealing to ∼ 350 °C for Pt3Te4, or ∼ 400 °C for Pt2Te2. Such prepared phases are characterized by high resolution core level spectroscopy to provide reference spectra for these materials. Moreover, the chemical stability of these materials was tested by exposure to oxygen and air. Even after prolonged air exposure only the surface Te layer was modified by oxygen chemical bonds that caused a 3-eV shift to higher binding energy of the Te-3d core levels. However, these oxygen species could be desorbed by vacuum annealing at 280 °C and pristine Pt-telluride samples can be re-established. This shows the excellent chemical stability of these materials, important for practical applications.

Sn-induced [formula omitted] reconstruction on the C-incorporated Si(001)-4[formula omitted] off surface

The reconstruction induced by Sn atoms on the C-incorporated Si(001)-4∘ off surface has been studied using scanning tunneling microscopy and synchrotron photoemission spectroscopy. When 0.6-ML Sn atoms were deposited on the Si(001)-4∘ off surface with subsurface C atoms at 630 °C, the whole (001) terraces were covered with a single c(4×4) reconstruction, where each Sn addimer bridges two Si dimers, reducing the density of dangling bonds to 0.5 ML. Such a reconstruction is different from the Sn-induced c(8×4) reconstruction formed on the clean Si(001) terraces without subsurface C atoms, where Sn dimers replace Si dimers and every fourth dimer is vacant. In both cases, the adjacent dimers buckle out of phase and the DB steps are debunched into single-layer steps. The reconstruction change from c(8×4) to c(4×4) is explained by the surface stress exerted by the subsurface C atoms much smaller than Si atoms.

Observation of topological superconductivity in a stoichiometric transition metal dichalcogenide 2M-WS2

Topological superconductors (TSCs) are unconventional superconductors with bulk superconducting gap and in-gap Majorana states on the boundary that may be used as topological qubits for quantum computation. Despite their importance in both fundamental research and applications, natural TSCs are very rare. Here, combining state of the art synchrotron and laser-based angle-resolved photoemission spectroscopy, we investigated a stoichiometric transition metal dichalcogenide (TMD), 2M-WS2 with a superconducting transition temperature of 8.8 K (the highest among all TMDs in the natural form up to date) and observed distinctive topological surface states (TSSs). Furthermore, in the superconducting state, we found that the TSSs acquired a nodeless superconducting gap with similar magnitude as that of the bulk states. These discoveries not only evidence 2M-WS2 as an intrinsic TSC without the need of sensitive composition tuning or sophisticated heterostructures fabrication, but also provide an ideal platform for device applications thanks to its van der Waals layered structure.

Increased Stability of Subsurface C Induced by Ca on the C-Incorporated Si(001)-4°-off Substrate

The effects of Ca atoms adsorbed on the C-incorporated Si(001)-4°-off substrate have been investigated by using scanning tunneling microscopy (STM) and synchrotron photoemission spectroscopy (PES). The C atoms incorporated into the subsurface by C2H2 exposure and postannealing at 630 °C induce the c(4 × 4) structure and debunch the double-layer DB steps. The Ca atoms additionally adsorbed on this C-incorporated substrate induce terraces with a width twice that of the clean surface. Until postannealing up to 680 °C, no SiC trace is detected. The SiC islands start to be detected after postannealing at 780 °C, which is about 100 °C higher than that for the identical surface without Ca atoms. After the adsorbed Ca atoms are mostly desorbed by 880 °C postannealing, the SiC islands only remain on the surface. Such a result implies that the adsorbed Ca atoms act as a stabilizer to the subsurface C atoms and increase the onset temperature of SiC formation by 100 °C.

Band-dependent superconducting gap in SrFe2(As0.65P0.35)2 studied by angle-resolved photoemission spectroscopy

The isovalent-substituted iron pnictide compound SrFe2(As1−xPx)2 exhibits multiple evidence for nodal superconductivity via various experimental probes, such as the penetration depth, nuclear magnetic resonance and specific heat measurements. The direct identification of the nodal superconducting (SC) gap structure is challenging, partly because the presence of nodes is not protected by symmetry but instead caused by an accidental sign change of the order parameter, and also because of the three-dimensionality of the electronic structure. We have studied the SC gaps of SrFe2(As0.65P0.35)2 in three-dimensional momentum space by synchrotron and laser-based angle-resolved photoemission spectroscopy. The three hole Fermi surfaces (FSs) at the zone center have SC gaps with different magnitudes, whereas the SC gaps of the electron FSs at the zone corner are almost isotropic and kz-independent. As a possible nodal SC gap structure, we propose that the SC gap of the outer hole FS changes sign around the Z-X [(0, 0, 2π) − (π, π, 2π)] direction.

Attosecond Dynamics of sp-Band Photoexcitation.

We report measurements of the temporal dynamics of the valence band photoemission from the magnesium (0001) surface across the resonance of the Γ[over ¯] surface state at 134eV and link them to observations of high-resolution synchrotron photoemission and numerical calculations of the time-dependent Schrödinger equation using an effective single-electron model potential. We observe a decrease in the time delay between photoemission from delocalized valence states and the localized core orbitals on resonance. Our approach to rigorously link excitation energy-resolved conventional steady-state photoemission with attosecond streaking spectroscopy reveals the connection between energy-space properties of bound electronic states and the temporal dynamics of the fundamental electronic excitations underlying the photoelectric effect.

Fabrication of Amorphous Carbon Thin Film from CH<sub>4</sub> Using PEVCD

Thin films of amorphous carbon have successfully been fabricated by radio frequency-plasma enhanced chemical vapor deposition (RF-PECVD). Carbon film fabrication with a methane gas (CH4) source has been fabricated with deposition parameters: 15 Watt of RF power, 13.5 MHz of frequency, 100 °C of substrate temperature, 450 m Torr of pressure and 120 minutes of deposition time. Methane gas flows were 40, 60, 80 sccm, respectively. Thin film can be in the form of DLC, Graphite or Amorphous Carbon depending on the ratio of hybridized orbitals of sp3, sp2 and sp. The X-ray diffractrometry (XRD) show the amorphous phase of the thin film has been formed. Further analysis using Fourier transform infrared (FTIR) combined with synchrotron photoemission spectroscopy (PES) showed that the sp3 hybridization was more dominant than sp2. This gives an indication that as fabricated thin films are tetrahedrally amorphous carbon (ta-C). Keywords: Amorphous carbon, methane gas, thin film.

Sn-induced 1D nanostructure formed on Si(5 5 12)-2 × 1: Faceting followed by preferential adsorption

The structural evolution of Si(5 5 12)-2 × 1 induced by submonolayer Sn atoms has been investigated using scanning tunneling microscopy and synchrotron photoemission spectroscopy. At a Sn coverage of 0.12 ML, Sn atoms induce a well-ordered nanostructure composed of (112) and (113) facets and they are adsorbed selectively on the (112) facet to form Sn/Si(112)-c(2 × 2). Along with the compensating Si(113)-3 × 2 facet, they form the one-dimensional (1D) sawtooth-like nanostructure having a high aspect-ratio and a well-defined cross-section of an average width of 14 nm and an average height of 0.7 nm. After the Sn/Si(112)-c(2 × 2) facet is saturated with Sn atoms, Sn atoms start to be adsorbed on the (113) facets. However, due to the instability of the Sn-adsorbed (113) facet, the ordered nanofacets start to be collapsed. At a coverage of 0.96 ML, the surface loses such regular 1D nanofacets and undergoes a transition from semiconductor to metal.

Initial reactions of ultrathin HfO2 films by in situ atomic layer deposition: An in situ synchrotron photoemission spectroscopy study

An in-depth exploration of the initial reaction and interfacial characteristics of ultrathin HfO2 films was implemented using in situ atomic layer deposition (ALD) and synchrotron radiation photoemission spectroscopy (SRPES). A newly developed Hf(mp)4 [mp = 3-methyl-3-pentoxide, OC(CH3)(C2H5)2] precursor, and H2O were adopted for the ALD of ultrathin HfO2. During the ALD process, the chemical composition and energy bandgap below the Fermi level of the HfO2 thin film were investigated at each cycle of the precursors by in situ measurements of SRPES. The Hf 4f, O 1s, and Si 2p core level and valence band spectra suggested that the initial ALD reactions yielded a Si suboxide and Hf silicate over three ALD cycles. An observation of the abnormal phenomena originating from the interfacial layer between HfO2 and Si revealed that the uniform HfO2 thin films exhibited a sufficient valence band offset as an insulating layer, which ensured that the valance band offset between Si 3p and O 2p reached to 2.78 eV over three ALD cycles, which coincided with that of the typical HfO2/Si structures.

Surface reconstruction switching induced by tensile stress of [formula omitted] steps: From Ba/Si(0 0 1)-[formula omitted] to Ba/Si(0 0 1)-4° off-[formula omitted

The alkaline-earth metal adsorption on Si(001) has attracted much interest for finding a proper template in the growth of high-κ and crystalline films. Up to now on the flat Si(001) surface with double domains and single-layer steps, the adsorbed Ba atoms are known to induce the 2×3 structure through removing two Si dimers and adding a Ba atom per unit cell in each domain. In the present investigation, the Si(001)-4° off surface with DB steps and single domains has been employed as a substrate and the reconstruction at the initial stage of Ba adsorption has been investigated by scanning tunneling microscopy and synchrotron photoemission spectroscopy. On this vicinal and single domain terrace, a novel 3×2 structure rotated by 90° from the 2×3 structure has been found. Such a 3×2 structure turns out to be formed by adding a Ba atom and a Si dimer per unit cell. This results from the fact that the adsorbed Ba2+ ions with a larger ionic radius relieve tensile stress on the original Si dimers exerted by the rebonded atoms at the DB step.

Unexpected Rotamerism at the Origin of a Chessboard Supramolecular Assembly of Ruthenium Phthalocyanine.

We have investigated the formation and the properties of ultrathin films of ruthenium phthalocyanine (RuPc)2 vacuum deposited on graphite by scanning tunneling microscopy and synchrotron photoemission spectroscopy measurements, interpreted in close conjunction with ab initio simulations. Thanks to its unique dimeric structure connected by a direct Ru-Ru bond, (RuPc)2 can be found in two stable rotameric forms separated by a low-energy barrier. Such isomerism leads to a peculiar organization of the molecules in flat, horizontal layers on the graphite surface, characterized by a chessboard-like alternation of the two rotamers. Moreover, the molecules are vertically connected to form π-stacked columnar pillars of akin rotamers, compatible with the high conductivity measured in (RuPc)2 powders. Such features yield an unprecedented supramolecular assembly of phthalocyanine films, which could open interesting perspectives toward the realization of new architectures of organic electronic devices.

On-Surface Synthesis of a Pure and Long-Range-Ordered Titanium(IV)-Porphyrin Contact Layer on Titanium Dioxide

We show the possibility of tailoring the molecular arrangement, as well as the chemical and structural modifications, of porphyrins at the monolayer saturation coverage on TiO2(110) by Synchrotron photoemission, electron diffraction, STM topography and DFT calculations. Free-base tetra-phenyl-porphyrins (2H-TPP) adsorb on the oxygen rows, where they can spontaneously capture two additional hydrogen atoms at their iminic nitrogens (4H-TPP). Both 2H-TPP and 4H-TPP molecules aggregate into a commensurate phase at the saturation coverage of one monolayer. Upon sample heating, a self-metalation reaction sets in at 100C, yielding full metalation of the saturated monolayer at ~200C. The Ti atoms are extracted from the substrate and, by simultaneous dehydrogenation of the pyrrolic nitrogen atoms, incorporated into the porphyrin macrocycle, where they remain coordinated to two oxygen atoms underneath. Neither the adsorption geometry (on-bridge, atop the oxygen rows) nor the molecular arrangement change across the self-metalation transition up to 300C. On one side, the robustness of this saturation phase makes it a promising system for its implementation into applications for photocatalysis and photovoltaic devices. On the other side, the possible manifestation of metal exchange with the very reactive Ti atoms must be taken into account when designing porphyrin-sensitized solar cells since the critical temperature for the onset of self-metalation is very close to the normal operating temperature of photovoltaic devices.

Critical Role of Oxygen in Silver-Catalyzed Glaser–Hay Coupling on Ag(100) under Vacuum and in Solution on Ag Particles

The essential role of oxygen in enabling heterogeneously catalyzed Glaser-Hay coupling of phenylacetylene on the Ag(100) was elucidated by STM, laboratory and synchrotron photoemission and DFT calculations. In the absence of co-adsorbed oxygen, phenylacetylene formed well-ordered dense overlayers which, with increasing temperature, desorbed without reaction. In striking contrast, even at 120 K, the presence of oxygen led to immediate and complete disruption of the organic layer due to abstraction of acetylenic hydrogen with formation of a disordered mixed layer containing immobile adsorbed phenylacetylide. At higher temperatures phenylacetylide underwent Glaser-Hay coupling to form highly ordered domains of diphenyldiacetylene that eventually desorbed without decomposition leaving the bare metal surface. DFT calculations showed that while acetylenic H abstraction was otherwise an endothermic process, oxygen adatoms triggered a reaction-initiating exothermic pathway leading to OH(a) + phenylacetylide, consistent with the experimental observations. Moreover, it was found that with a solution of phenylacetylene in nonane and in the presence of O2, Ag particles catalyzed Glaser-Hay coupling with high selectivity. Rigorous exclusion of oxygen from the reactor strongly suppressed the catalytic reaction. Interestingly, too much oxygen lowers the selectivity towards diphenyldiacetylene. Thus vacuum studies and theoretical calculations revealed the key role of oxygen in the reaction mechanism, subsequently borne out by catalytic studies with Ag particles that confirmed the presence of oxygen as a necessary and sufficient condition for the coupling reaction to occur. The direct relevance of model studies to mechanistic understanding of coupling reactions under conditions of practical catalysis was reaffirmed.

Correction to “Interfacial Electronic Structure of Methylammonium Lead Iodide Grown on a Mesoporous TiO2 Layer on F-Doped Tin Oxide Substrate”

Interfacial electronic structure at the perovskite CH3NH3PbI3/TiO2 or MAPbI3/TiO2 interface has been investigated using a high-resolution synchrotron photoemission spectroscopy. The core levels and valence band spectra of our solution-processed MAPbI3 layers spin-coated on a mesoporous TiO2 substrate are consistent with a pure iodide perovskite (MAPbI3) with residual methylammonium (CH3NH3+) trapped within the perovskite crystal lattice, which is in contact with TiO2. In addition, we show spectroscopic evidence for the presence of PbI2 and methylamine (CH3NH2) at the MAPbI3/TiO2 interface. Interestingly, the Fermi level (EF) aligned to the conduction band minimum (CBM) of both layers is found to be lifted via a band bending which may be attributed to characteristic chemical bonding structures at the interface. The band bending may act as an electron transfer barrier between MAPbI3 and TiO2. Thus, our findings in interfacial electronic structure of MAPbI3 layers on TiO2 will be crucial in understanding its...

First in-flight synchrotron X-ray absorption and photoemission study of carbon soot nanoparticles.

Many studies have been conducted on the environmental impacts of combustion generated aerosols. Due to their complex composition and morphology, their chemical reactivity is not well understood and new developments of analysis methods are needed. We report the first demonstration of in-flight X-ray based characterizations of freshly emitted soot particles, which is of paramount importance for understanding the role of one of the main anthropogenic particulate contributors to global climate change. Soot particles, produced by a burner for several air-to-fuel ratios, were injected through an aerodynamic lens, focusing them to a region where they interacted with synchrotron radiation. X-ray photoelectron spectroscopy and carbon K-edge near-edge X-ray absorption spectroscopy were performed and compared to those obtained for supported samples. A good agreement is found between these samples, although slight oxidation is observed for supported samples. Our experiments demonstrate that NEXAFS characterization of supported samples provides relevant information on soot composition, with limited effects of contamination or ageing under ambient storage conditions. The highly surface sensitive XPS experiments of airborne soot indicate that the oxidation is different at the surface as compared to the bulk probed by NEXAFS. We also report changes in soot’s work function obtained at different combustion conditions.

Selenium, Benzeneselenol, and Selenophene Interaction with Cu(100)

A high-resolution synchrotron photoemission study of selenium, selenophene (Seph), and benzeneselenol (BSe) adsorption on a Cu(100) surface was performed, to investigate the film structure, chemical adsorption properties, and molecular orientation. Complementary measurements for selenophene adsorption on Au(111) were also performed. The investigation is based on determination of core level binding energies (CLBEs) by X-ray photoelectron spectroscopy (XPS) and near-edge adsorption fine structure (NEXAFS). Se chemisorption was studied as a complement to the work on molecules in order to obtain data on the CLBEs for the atomic case. Se adsorption was performed from a Na2Se solution and thereafter annealing in vacuum. Annealing leads to appearance of ordered structures in LEED. Se 3d CLBEs were determined for the different cases. Fits of Se 3d XPS spectra show existence of several components attributable to different adsorption configurations. Also coadsorption with S was studied, which leads to changes in ad...

Custom sample environments at the ALBA XPEEM

A variety of custom-built sample holders offer users a wide range of non-standard measurements at the ALBA synchrotron PhotoEmission Electron Microscope (PEEM) experimental station. Some of the salient features are: an ultrahigh vacuum (UHV) suitcase compatible with many offline deposition and characterization systems, built-in electromagnets for uni- or biaxial in-plane (IP) and out-of-plane (OOP) fields, as well as the combination of magnetic fields with electric fields or current injection. Electronics providing a synchronized sinusoidal signal for sample excitation enable time-resolved measurements at the 500MHz storage ring RF frequency.

Origin of ambipolar graphene doping induced by the ordered Ge film intercalated on SiC(0001)

Transfer doping mechanism of the quasi-free-standing monolayer graphene (QFMLG), depending on the thickness as well as the atomic structures of the intercalated Ge film, has been revealed by combined investigations of scanning tunneling microscopy and synchrotron photoemission spectroscopy. The ordered Ge film has been formed between QFMLG and the n-type 6H-SiC(0001) substrate by depositing Ge on the (63×63)R30° zero layer and postannealing. At postannealing temperatures lower than 900 °C, the interfacial structures inducing p-type QFMLG, like 2×1.5n3 and 4×33, are formed. On the other hand, at those higher than 900 °C, the interfacial structures inducing n-type QFMLG, like 7×43 [expressed as a (5−376) matrix], 8×33 and 21×21 [expressed as a (5115) matrix], are formed as a result of partial deintercalation of Ge atoms. Such a transition from p-type to n-type doping turns out to be associated with an increase in the density of states of an upper Hubbard band originating from strong correlation of the electrons at the Ge atoms on the first and the second Ge interfacial layers.

Initial CaF2 reactions on Si(1 1 4)-2 × 1: Isolated silicides, faceting and partial CaF adsorption

When CaF2 molecules are deposited on Si(114)-2×1 held at 500°C, two kinds of isolated and symmetric Ca-silicide units are initially formed. With increasing CaF2 deposition to 0.4ML, instead of the terrace being filled with them, a trench composed of (113) and (117) facets appears on the surface as a result of substrate etching induced by dissociated F atoms. Selectively on this (113) facet, a 2×2 CaF overlayer is formed uniformly. In the present studies, using scanning tunneling microscopy and synchrotron photoemission spectroscopy, the origins of such isolation of Ca-silicide units on the (114) terrace as well as selective adsorption of CaF on the (113) facet have been disclosed.

Bifunctional effects of the ordered Si atoms intercalated between quasi-free-standing epitaxial graphene and SiC(0001): graphene doping and substrate band bending

Bifunctional effects of the Si atoms intercalated between the n-type 6H–SiC(0001) substrate and the zero layer have been disclosed by scanning tunneling microscopy, low-energy electron diffraction, high-resolution synchrotron photoemission spectroscopy and angle-resolved photoemission spectroscopy. As a result of Si intercalation, an ordered Si interfacial layer composed of a Si adlayer and Si adatoms with dangling bonds has been formed under quasi-free-standing epitaxial graphene (EG). It turns out that the SiC(0001) band bending is determined by the Fermi level located close to the lowest states of the upper Hubbard band. The Hubbard bands originate from strong correlation effects of the electrons in the dangling bonds of the Si adatoms ordered on the Si adlayer. The doping level of the decoupled graphene is determined by the amount of charge transferred from the Si adatoms ordered on the Si adlayer to the quasi-free-standing EG.