Synchrotron Radiation Photoemission Research Articles

In Situ Investigations of Al/Perovskite Interfacial Structures.

Interfacial properties of perovskite layers and metal electrodes play a crucial role in device performance and long-term stability of perovskite solar cells. In this work, we performed a comprehensive study of the interfacial structures and ion migration at the interface of a CH3NH3PbI3 perovskite layer and an Al electrode using in situ synchrotron radiation photoemission spectroscopy measurements. It was found that the Al electrode can react with the perovskite layers, leading to the formation of aluminum iodide species and the bonding between Al and N, as well as the reduction of Pb2+ ions to metallic Pb species at the interface. Moreover, during the Al deposition, iodide ions can migrate from the CH3NH3PbI3 subsurface to the Al electrode, while the reduced Pb remains at the subsurface. The depth profile photoemission measurements, made by varying the photon energies of incident synchrotron radiation X-rays, demonstrate that the reaction occurs at the Al/CH3NH3PbI3 interface at least with a thickness of ∼3.5 nm below the perovskite surface. This study provides an atomic-level fundamental understanding of the Al/CH3NH3PbI3 interfacial structures and insight into the degradation mechanisms of perovskite solar cells when using Al metal as the electrode.

Photoemission Spectroscopy Study on Hydrogen Termination Effect on SiO2/Si Structure Fabricated Using H+-Implanted Si Substrate

Using proton (H+)-implanted silicon (Si) substrates, we clarified the effect of dangling bond termination by hydrogen on the interfacial strain in the silicon dioxide (SiO2)/Si system. The variations of the SiO2/Si interface structure caused by H+ implantation into a SiO2/Si sample and by hydrogen out-diffusion heat treatment were analyzed by high-resolution synchrotron radiation photoemission spectroscopy. We found that H+ implantation into the SiO2/Si sample [intentional generation of the interfacial dangling bonds] can increase the intensity of the strained-Si peaks in the Si 2p photoemission spectrum. In addition, our study revealed that the strained Si atom amount and dangling bond density are reduced by hydrogen out-diffusion heat treatment. These findings suggest that the increase/decrease in the dangling bond density by H atoms results in the increase/decrease in local strain field around a dangling bond, thereby changing the length of the Si–Si bonds beneath the SiO2/Si interface. Out-diffused hydrogen seems to play roles to not only reduce the dangling bond density but also relax the local strain at the SiO2/Si interface. The hydrogen termination effect is expected to have an advantage in structural stability in the SiO2/Si system as compared with the pure thermal termination effect.

Synchrotron radiation photoemission study of Ba adsorption on the vicinal SiC(111)-8° surface

The electronic structure of the Ba/SiC(111)-8° interface has been studied in detail in situ in an ultrahigh vacuum using synchrotron radiation photoelectron spectroscopy. The SiC(111)-8° samples were grown by an original method of epitaxy of low-defect unstressed nanoscaled silicon carbide films on silicon substrates. The Si 2p, C 1s core level spectra have been investigated as a function of Ba submonolayer coverage with photon energies in the range of 100–450 eV. Upon Ba adsorption, a drastic change in the C 1s spectrum was revealed. It was found that Ba adsorption leads to the formation of a new, previously unknown carbon-based structure of C nanoclusters on the surface. Experimental data show that the nanoclusters can be created exclusively on the vicinal SiC(111)-8° surfaces in the presence of stabilizing adsorbed metal atoms, specifically, Ba atoms. It is shown that the surface nanoclusters are formed due to the local interaction between silicon vacancies, carbon atoms and Ba adatoms. It is established that the carbon clusters are composed of carbon rings and their chemical bonds are similar in nature to the bonds in aromatic-like compounds.

Amorphous Gadolinium Aluminate as a Dielectric and Sulfur for Indium Phosphide Passivation

The passivation of n-type InP (100) using sulfur in combination with a gadolinium aluminate (GAO) dielectric layer has been studied. Photoluminescence, minority-carrier lifetime, and capacitance–voltage measurements indicate that a (NH4)2S vapor passivation step prior to atomic layer deposition of the oxide effectively lowers the interface state density. Surface and interface chemistry were studied by synchrotron radiation photoemission spectroscopy (SRPES). The effect of ex situ surface passivation after native oxide removal in HCl solution was examined. It was observed that surface reoxidation occurred during (NH4)2S vapor exposure, leading to the formation of Inx(HPO4)y. S was present on the surface as a sulfide in both surface and subsurface sites. After atomic layer deposition of GAO, sulfates were detected in addition to Inx(HPO4)y, which was confirmed by near-edge X-ray absorption fine structure analysis. The S in the stack was quantified using reference-free grazing incidence X-ray fluorescence an...

Supramolecular Tessellations at Surfaces by Vertex Design

Assembly and tessellation of organic species at surfaces are important for the design of advanced materials, particularly for the development of spontaneous self-assemblies of supramolecular systems of increasing complexity. However, there are few examples where the ability to steer the system between supramolecular tessellations has been achieved. Here, we demonstrate a series of steps to reduce and then restore molecular symmetry; those variations impact vertex symmetry and thus generate a series of tessellations that reflect the molecular symmetry. We deposit 4,4'-dihydroxybiphenyl on the Ag(111) surface, then anneal at specific temperatures to achieve stepwise dehydrogenation of the terminal hydroxyls. The symmetry of tessellation vertices in the self-assembled structure also changes, as characterized by scanning tunneling microscopy and synchrotron radiation photoemission spectroscopy. This control over vertex geometry and spontaneous tessellation structure extends our understanding of supramolecular design control and advances architectural complexity for the development of functional surfaces.

The photoemission study of InSb/HfO2 stacks upon N2 rapid thermal annealing

Antimonide based III-V materials are widely used in quantum-well transistors and long wavelength optoelectronic devices benefit from their narrow bandgaps and high carrier mobilities. Interface chemistry has proven to be important in establishing reliable devices. The InSb/HfO2 stacks have been systematically studied upon atomic layer deposition (ALD) and rapid thermal annealing (RTA) at 325 °C and 400 °C, utilizing X-ray photoelectron spectroscopy (Al Kα1) and synchrotron radiation photoemission spectroscopy. No “clean up” effect was observed after the ALD process. The interface oxidization, elemental diffusion and substrate oxide desorption have been observed upon the RTA process. This work highlights the importance of substrate passivation prior to ALD process to obtain a thermally stable InSb/HfO2 interface for InSb based devices.

Magnetic and electronic properties of B -site-ordered double-perovskite oxide La2CrMnO6 thin films

We report on magnetic and electronic properties of the $B$-site-ordered double-perovskite $\mathrm{L}{\mathrm{a}}_{2}\mathrm{CrMn}{\mathrm{O}}_{6}$ films grown by pulsed-laser deposition. The magnetic-field dependence of magnetization curves showed hysteresis at low temperatures regardless of the degree of Cr/Mn order and the saturation magnetization became smaller for the higher-Cr/Mn-ordered film. The x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements suggested antiferromagnetic coupling between $\mathrm{C}{\mathrm{r}}^{3+}$ and $\mathrm{M}{\mathrm{n}}^{3+}$ ions, resulting in ferrimagnetism of the $B$-site-ordered double-perovskite $\mathrm{L}{\mathrm{a}}_{2}\mathrm{CrMn}{\mathrm{O}}_{6}$. A band structure was established by combining these results together with the synchrotron-radiation photoemission and optical spectra. We discuss the magnetic states among the $B$-site-ordered $\mathrm{L}{\mathrm{a}}_{2}{B}^{\ensuremath{'}}\mathrm{Mn}{\mathrm{O}}_{6}$ (${B}^{\ensuremath{'}}:\phantom{\rule{4pt}{0ex}}3d$ transition-metal elements from V to Ni) being basically consistent with the Kanamori-Goodenough rule.

The band structure change of Hf0.5Zr0.5O2/Ge system upon post deposition annealing

Abstract Hafnium zirconium oxide films have been utilized in negative capacitance (NC) field-effect transistors (FETs). The band alignment of semiconductor and HfZrOx film is critical to obtain high device performance. The band alignment of Hf0.5Zr0.5O2/SiOx/Ge system before and after post deposition annealing at 500 °C is studied via angle resolved X-ray photoelectron spectroscopy, synchrotron radiation photoemission spectroscopy and UV–Visible spectroscopy. The band gap of Hf0.5Zr0.5O2 is seen narrowed 0.27 ± 0.05 eV, and the valence band offset between Hf0.5Zr0.5O2 and Ge decreases 0.25 eV ± 0.05 eV after PDA at 500 °C. Therefore, the conduction band offset is nearly unchanged. This work gives insights into the interface physics about Hf0.5Zr0.5O2/SiOx and is valuable for Ge-based NC pFETs.

Microscopic Views of Atomic and Molecular Oxygen Bonding with epi Ge(001)-2 × 1 Studied by High-Resolution Synchrotron Radiation Photoemission.

In this paper, we investigate the embryonic stage of oxidation of an epi Ge(001)-2 × 1 by atomic oxygen and molecular O2 via synchrotron radiation photoemission. The topmost buckled surface with the up- and down-dimer atoms, and the first subsurface layer behaves distinctly from the bulk by exhibiting surface core-level shifts in the Ge 3d core-level spectrum. The O2 molecules become dissociated upon reaching the epi Ge(001)-2 × 1 surface. One of the O atoms removes the up-dimer atom and the other bonds with the underneath Ge atom in the subsurface layer. Atomic oxygen preferentially adsorbed on the epi Ge(001)-2 ×1 in between the up-dimer atoms and the underneath subsurface atoms, without affecting the down-dimer atoms. The electronic environment of the O-affiliated Ge up-dimer atoms becomes similar to that of the down-dimer atoms. They both exhibit an enrichment in charge, where the subsurface of the Ge layer is maintained in a charge-deficient state. The dipole moment that was originally generated in the buckled reconstruction no longer exists, thereby resulting in a decrease in the ionization potential. The down-dimer Ge atoms and the back-bonded subsurface atoms remain inert to atomic O and molecular O2, which might account for the low reliability in the Ge-related metal-oxide-semiconductor (MOS) devices.

Molecular beam epitaxy, atomic layer deposition, and multiple functions connected via ultra-high vacuum

Molecular beam epitaxy (MBE) invented for the growth of compound semiconductors in the 70’s has been successfully extended to the advanced growth of metals, oxides, oxide/semiconductor interfaces and emergent topological materials by our endeavor in the past three and half decades. In the 80’s, Kwo et al. have invented metal MBE and oxide MBE methods in pioneering spintronics and high-temperature superconducting oxide films. In driving compound semiconductors for optoelectronics, Hong et al. have produced distributed Bragg reflectors with a continuously graded composition between each constituent without shutter operation, and greatly reduced the electrical resistance; this simple method has made easy manufacture of vertical-cavity surface-emitting lasers. In the 90’s, combining (In)GaAs and oxide MBE chambers via ultra-high vacuum, Hong et al. were the first to unpin the Fermi level in oxide/GaAs, which led to the first demonstration of inversion-channel (In)GaAs metal-oxide-semiconductor (MOS) field-effect transistors (MOSFETs). Integrating MBE, atomic layer deposition (ALD), and many other functions in ultra-high vacuum, advances have been made in pushing ultimate complementary MOS (CMOS) of record-high device performances and beyond in growing emergent topological materials for spintronics. Our novel method in preserving as-grown (In)GaAs surfaces and interfaces with high-κ oxides and metals enables employing in-situ synchrotron radiation photoemission to study electronic structures in an atomic scale.

Surface composition of MAPb(IxBr1−x)3 (0 ≤ x ≤ 1) organic-inorganic mixed-halide perovskites

Organic-inorganic halide perovskites have been used as light absorbers in photovoltaic cells. The energy band gap of these perovskites can be tuned by mixing various halides at different concentrations. In this work, mixed iodide-bromide perovskite films at various concentrations were deposited on fluorine-doped tin oxide glass substrates via a spin coating technique. The films were characterized using X-ray diffraction (XRD), ultraviolet-visible spectroscopy and synchrotron-radiation photoemission spectroscopy. The XRD results suggest that the lattice spacing decreased with increasing bromide concentration, which also relates to an increase in the energy band gap. The mixed halide films showed less crystallinity than that of pure halide films. The photoemission results showed that the iodide concentration at the surface was higher than expected. This could have been due to the higher solubility of iodide perovskite in the dimethylformamide solvent than that of bromide perovskite. These results suggest that the energy levels at the surface of the films can be different from those of the bulk material.

Multiple Reaction Paths for CO Oxidation on a 2D SnOx Nano‐Oxide on the Pt(110) Surface: Intrinsic Reactivity and Spillover

AbstractAn interface stabilized SnOx/Pt(110) nano‐oxide characterized by a c(2 × 4) surface reconstruction is prepared and characterized by low‐energy electron diffraction (LEED), synchrotron radiation photoemission spectroscopy (SRPES), and scanning tunneling microscopy (STM). Based on the experimental data, atomic models for the nano‐oxide are proposed and then validated by comparing the experimental results with the outcome of first‐principle calculations. The reactivity of the nano‐oxide toward CO is investigated, obtaining that the c(2 × 4) reconstruction efficiently oxidizes CO to CO2. The SnOx nano‐oxide on the Pt(110) surface can act as a reservoir for oxygen that can diffuse on the adjacent Pt areas where it oxidizes CO. This spillover effect endows the SnOx/Pt(110) system with enhanced tolerance to CO poisoning.

Germanene Epitaxial Growth by Segregation through Ag(111) Thin Films on Ge(111).

Large-scale two-dimensional sheets of graphene-like germanium, namely, germanene, have been epitaxially prepared on Ag(111) thin films grown on Ge(111), using a segregation method, differing from molecular beam epitaxy used in previous reports. From the scanning tunneling microscopy (STM) images, the surface is completely covered with an atom-thin layer showing a highly ordered long-range superstructure in wide scale. Two types of protrusions, named hexagon and line, form a (7√7 × 7√7) R19.1° supercell with respect to Ag(111), with a very large periodicity of 5.35 nm. Auger electron spectroscopy and high-resolution synchrotron radiation photoemission spectroscopy demonstrate that Ge atoms are segregated on the Ag(111) surface as an overlayer. Low-energy electron diffraction clearly shows incommensurate "(1.35 × 1.35)" R30° spots, corresponding to a lattice constant of 0.39 nm, in perfect accord with close-up STM images, which clearly reveal an internal honeycomb arrangement with corresponding parameter and low buckling within 0.01 nm. As this 0.39 nm value is in good agreement with the theoretical lattice constant of free-standing germanene, conclusively, the segregated Ge atoms with trivalent bonding in honeycomb configuration form a characteristic two-dimensional germanene-like structure.

Probing surface electronic properties of a patterned conductive STO by reactive ion etching

SrTiO3 (STO) is a highly attractive oxide material due to its flexible tunability of electrical properties. It can be designed to exhibit a high mobility with a tunable carrier concentration by creating oxygen vacancies, or by doping with Nb or La, which substitute the Ti and Sr sites, respectively. Here we show a micro-patterned surface doping of STO by using reactive ion etching (RIE). The creation and pattering of a conductive STO surface were achieved by sequential treatments with Ar and O2 plasma. The patterned conductive surface edge was well defined as confirmed by an electrostatic force microscopy. The electronic characteristics of the RIE treated STO surface were probed by a synchrotron radiation photoemission spectroscopy, which shows the emergence of Ti3+, Ti2+, Ti1+ states and metallic states near the Fermi level. The electrical mobility of the conductive STO surface can be increased up to 12000 cm/V s with a typical sheet carrier concentration around 1013–1014 cm−2. Increasing Ar plasma time elongate the depth of the conductive surface, which reflects the change of magnetoresistance behavior at low temperature. The demonstrated control of the STO surface conductivity along with a large area and high precision patterning method can be widely used for a variety of oxide electronic and spintronic devices.

Atom-to-atom interaction of O2 with epi Ge(001)-2 × 1 in elucidating GeOx formation

A microscopic view of O2 adsorption on an epi Ge(001)-2 × 1 surface at room temperature, as studied by synchrotron radiation photoemission, is presented. One of the dissociated O atoms etches off the up-dimer atom to form the GeO species. The other dissociated O atom fills the vacancy and bonds with the underneath Ge atom in the subsurface layer. The down-dimer atom and its backbond with the subsurface atom remain inert to O2. The present investigation reports the notorious effect of O2 on the Ge surface, which should not be overlooked in Ge metal–oxide–semiconductor (MOS) device development.

Mesoporous Pd@Ru Core-Shell Nanorods for Hydrogen Evolution Reaction in Alkaline Solution.

The activity and stability of bimetallic nanocatalysts strongly depend on their structures, compositions, and interfaces. Here, we report the synthesis of mesoporous Pd@Ru core-shell bimetallic nanorods composed of face-centered cubic Pd and hexagonal close-packed Ru. The nanorods have two types of cavities with diameters of 3.0 ± 0.9 and 20.3 ± 8.1 nm. The mutual diffusion process between Ru and Pd is characterized by the high-angle annular dark-field scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy mapping, and the synchrotron radiation photoemission spectroscopy measurements. The mesoporous Pd@Ru nanorods exhibit superior catalytic performance and stability for hydrogen evolution reactions (overpotentials of 30 mV at 10 mA·cm-2 in 1.0 M KOH solution and 37 mV at 10 mA·cm-2 in 0.5 M H2SO4 solution).

Direct Formation of C-C Double-Bonded Structural Motifs by On-Surface Dehalogenative Homocoupling of gem-Dibromomethyl Molecules.

Conductive polymers are of great importance in a variety of chemistry-related disciplines and applications. The recently developed bottom-up on-surface synthesis strategy provides us with opportunities for the fabrication of various nanostructures in a flexible and facile manner, which could be investigated by high-resolution microscopic techniques in real space. Herein, we designed and synthesized molecular precursors functionalized with benzal gem-dibromomethyl groups. A combination of scanning tunneling microscopy, noncontact atomic force microscopy, high-resolution synchrotron radiation photoemission spectroscopy, and density functional theory calculations demonstrated that it is feasible to achieve the direct formation of C-C double-bonded structural motifs via on-surface dehalogenative homocoupling reactions on the Au(111) surface. Correspondingly, we convert the sp3-hybridized state to an sp2-hybridized state of carbon atoms, i. e., from an alkyl group to an alkenyl one. Moreover, by such a bottom-up strategy, we have successfully fabricated poly(phenylenevinylene) chains on the surface, which is anticipated to inspire further studies toward understanding the nature of conductive polymers at the atomic scale.

Synchrotron radiation study of metallic titanium deposited on dielectric substrates

In this work, the authors present a detailed synchrotron radiation photoemission study of the interactions of metallic titanium with dielectric substrates. In particular, the authors focus on the formation of titanium silicide upon deposition when titanium interacts with SiO2 based substrates. Both TiSi and TiSi2 are present, but TiSi2 grows preferentially upon annealing. When deposited on carbon-containing low-k materials, titanium interacts with the surface to form titanium carbide and titanium silicide. At elevated temperature, carbon in the low-k dielectric assists in the conversion of titanium silicide to titanium silicate.

Ni-doped ZnCo2O4 atomic layers to boost the selectivity in solar-driven reduction of CO2

Regulating the selectivity of CO2 photoreduction is particularly challenging. Herein, we propose ideal models of atomic layers with/without element doping to investigate the effect of doping engineering to tune the selectivity of CO2 photoreduction. Prototypical ZnCo2O4 atomic layers with/without Ni-doping were first synthesized. Density functional theory calculations reveal that introducing Ni atoms creates several new energy levels and increases the density-of-states at the conduction band minimum. Synchrotron radiation photoemission spectroscopy demonstrates that the band structures are suitable for CO2 photoreduction, while the surface photovoltage spectra demonstrate that Ni doping increases the carrier separation efficiency. In situ diffuse reflectance Fourier transform infrared spectra disclose that the CO2·− radical is the main intermediate, while temperature-programed desorption curves reveal that the ZnCo2O4 atomic layers with/without Ni doping favor the respective CO and CH4 desorption. The Ni-doped ZnCo2O4 atomic layers exhibit a 3.5-time higher CO selectivity than the ZnCo2O4 atomic layers. This work establishes a clear correlation between elemental doping and selectivity regulation for CO2 photoreduction, opening new possibilities for tailoring solar-driven photocatalytic behaviors.

Direct solution deposition of device quality Sb2S3-xSex films for high efficiency solar cells

Sb2S3-xSex (0 ≤x ≤ 3) is promising for practically applicable solar cell since it possesses excellent stability, suitable band gap, high extinction coefficient and abundant elemental storage. This study demonstrates a novel methodology towards simultaneous dissolution of Se and Sb2O3 for direct deposition of Sb2S3-xSex films. Mechanistic study shows that the formation of selenium-nitrogen free radical in sulfur-containing complex is responsible for the dissolution of selenium. The energy levels of the as-synthesized Sb2S3-xSex were examined by synchrotron radiation photoemission spectroscopy, revealing that more Se in the Sb2S3-xSex leads to the valence band shifting considerably upward while the conduction band remaining nearly unchanged. Moreover, for the first time we present that the solution processed Sb2S3-xSex can generate photovoltaic performance with an efficiency of 5.8%. This research provides a straightforward strategy for the fabrication of Sb2S3-xSex solar cells and offers an effective method for the synthesis of metal selenosulfide films.