Low Energy Photoelectron Spectroscopy Research Articles

Polarity and surface structural evolution of iron oxide films

Various ordered Fe oxide films were prepared in an ultrahigh vacuum system, and their surface and electronic structure were studied in situ by low-energy electron diffraction and photoelectron spectroscopy. We obtain Fe2O3(0001) films with oxygen termination (O–Fe2O3(0001)), a polar surface, via FeO(111) films. The O–Fe2O3(0001) films can be transformed to a non-polar surface of iron-terminated Fe2O3(0001) (Fe–Fe2O3(0001)) films. Fe3O4(111) films are formed after annealing O–Fe2O3(0001) films. A co-existed phase of Fe–Fe2O3(0001) and Fe3O4(111) is observed via annealing Fe–Fe2O3(0001) films. These Fe oxide films with different surface structures and properties can be designedly used as model plates for further studies in surface physics and chemistry.

Surface Explosion Chemistry of Malic Acid on Cu(110)

The thermal decomposition chemistry of malic acid adsorbed on a copper(110) surface was studied with thermal desorption spectroscopy, reflection–absorption infrared spectroscopy, low-energy electron diffraction and X-ray photoelectron spectroscopy in ultrahigh vacuum. In contrast to tartaric acid on Cu(110), no differences between racemate and pure enantiomers have been identified.

Investigation of Carbon Supported Pt and PtCo Electrocatalysts by Low-Energy Ion Scattering and X-ray Photoelectron Spectroscopy: Influence of the Surface Characteristics on Performance and Degradation

An investigation of the behaviour of carbon-supported Pt and PtCo cathode electro-catalysts was carried out to evaluate their performance and resistance to degradation. Nanosized Pt and PtCo catalysts with similar crystallite size (2.7-2.9 nm) were prepared by using a colloidal route. The surface properties were investigated by X-ray photoelectron spectroscopy (XPS) and low-energy ion scattering spectroscopy (LE-ISS). The formation of a Pt skin layer on the surface of the alloy electro-catalyst was obtained by using a pre-leaching procedure. As result, a significant surface Pt enrichment was observed for the leached Pt-Co electrocatalyst. This characteristic appeared to influence catalysts' performance and degradation. Accelerated tests (electrochemical cycling) at 130 °C in a pressurised PEMFC showed a better stability for the PtCo alloy as compared to Pt. Furthermore, better performance was obtained at high temperatures for the pre-leached PtCo/C as compared to the Pt/C cathode catalyst.

Polymer Material as a Gate Dielectric for Graphene Field-Effect-Transistor Applications

Epitaxial-graphene field-effect transistor (EG-FET) with a polymer gate dielectric was fabricated and their electrical characteristics were investigated. The epitaxial graphene layer was formed on a semi-insulating 6H-SiC substrate by a high-temperature annealing in ultrahigh vacuum. The formation of graphene was confirmed by low-energy electron diffraction (LEED), Raman-scattering spectroscopy and X-ray photoelectron spectroscopy (XPS). The polymer gate dielectric (ZEP520a) layer was formed by spin coating, which exhibits good dielectric properties without noticeable structural degradation of the graphene layer. The EG-FETs with this polymer gate dielectric shows an n-type characteristic, with the field-effect mobility of 580 cm2 V-1 s-1.

Low-Energy-Electron-Diffraction and X-ray-Phototelectron-Spectroscopy Studies of Graphitization of 3C-SiC(111) Thin Film on Si(111) Substrate

Epitaxial graphene can be formed on silicon substrates by annealing a 3C-SiC film formed on a silicon substrate in ultrahigh vacuum (G/3C-SiC/Si). In this work, we explore the graphitization process on the 3C-SiC(111)/Si(111) surface by using low-energy electron diffraction and X-ray photoelectron spectroscopy (XPS) and compare them with that on 6H-SiC(0001). Upon annealing at T≥1150 °C, the 3C-SiC(111)/Si(111) surface follows the sequence of (√3×√3)R30°, (6√3×6√3)R30°, and (1×1)graphene in the surface structures. The C 1s core level according to XPS indicates that a buffer layer, identical with that in G/6H-SiC(0001), exists at the G/3C-SiC(111) buffer. These observations strongly suggest that graphitization on the surface of the 3C-SiC(111) face proceeds in a similar manner to that on the Si-terminated hexagonal bulk SiC crystals.

Energetics of CdSe Quantum Dots Adsorbed on TiO2

Understanding how quantum dot (QD)-sensitized solar cells operate requires accurate determination of the offset between the lowest-unoccupied molecular orbital (LUMO) of the sensitizer quantum dot and the conduction band of the metal oxide electrode. We present detailed optical spectroscopy, low-energy photoelectron spectroscopy, and two-photon photoemission studies of the energetics of size-selected CdSe colloidal QDs deposited on TiO2 electrodes. Our experimental findings show that in contrast to the prediction of simplified models based on bulk band offsets and effective mass considerations, band alignment in this system is strongly modified by the interaction between the QDs and the electrode. In particular, we find relatively small conduction band-LUMO offsets, and near “pinning” of the QD LUMO relative to the conduction band of the TiO2 electrode, which is explained by the strong QD-electrode interaction. That interaction is the origin for the highly efficient QD to electrode charge transfer, and it also bears on the possibility of hot carrier injection in these types of cells.

Ultrathin (1×2)-Sn layer on GaAs(100) and InAs(100) substrates: A catalyst for removal of amorphous surface oxides

Amorphous surface oxides of III–V semiconductors are harmful in many contexts of device development. Using low-energy electron diffraction and photoelectron spectroscopy, we demonstrate that surface oxides formed at Sn-capped GaAs(100) and InAs(100) surfaces in air are effectively removed by heating. This Sn-mediated oxide desorption procedure results in the initial well-defined Sn-stabilized (1×2) surface even for samples exposed to air for a prolonged time. Based on ab initio calculations we propose that the phenomenon is due to indirect and direct effects of Sn. The Sn-induced surface composition weakens oxygen adsorption.

Doping type and thickness dependence of band offsets at the amorphous/crystalline silicon heterojunction

We conduct a systematic investigation of the valence band offset ΔEv for amorphous/crystalline silicon heterojunctions (a-Si:H/c-Si) using low-energy photoelectron spectroscopy in the constant final state mode. The dependence of ΔEv on a-Si:H thickness as well as on the possible combinations of c-Si substrate and a-Si:H film doping types are explored. ΔEv is found to be independent of both substrate and film doping and amounts to ΔEv¯=0.458(6) eV, averaged over all doping combinations and thicknesses, with a systematic error of 50–60 meV. A slight but statistically significant dependency of ΔEv on the a-Si:H film thickness may be explained by a changing interface dipole due to variations in dangling bond saturation during a-Si:H growth.

Investigation of Carbon Supported Pt and PtCo Electrocatalysts by Low-Energy Ion Scattering and X-ray Photoelectron Spectroscopy: Influence of the Surface Characteristics on Performance and Degradation

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Band alignment at amorphous/crystalline silicon hetero-interfaces

ABSTRACTWe present an investigation of the band offsets in amorphous/crystalline silicon heterojunctions (a-Si:H/c-Si) using low energy photoelectron spectroscopy, ellipsometry and surface photovoltage data. For a variation of deposition conditions that lead to changes in hydrogen content and the thereby the a-Si:H band gap by ∼180 meV, we find that mainly the conduction band offset ΔEV varies, while ΔEC stays constant within experimental error. This result can be understood in the framework of charge neutrality (CNL) band lineup theory.

Layer-by-Layer Transfer of Multiple, Large Area Sheets of Graphene Grown in Multilayer Stacks on a Single SiC Wafer

Here we report a technique for transferring graphene layers, one by one, from a multilayer deposit formed by epitaxial growth on the Si-terminated face of a 6H-SiC substrate. The procedure uses a bilayer film of palladium/polyimide deposited onto the graphene coated SiC, which is then mechanically peeled away and placed on a target substrate. Orthogonal etching of the palladium and polyimide leaves isolated sheets of graphene with sizes of square centimeters. Repeating these steps transfers additional sheets from the same SiC substrate. Raman spectroscopy, scanning tunneling spectroscopy, low-energy electron diffraction and X-ray photoelectron spectroscopy, together with scanning tunneling, atomic force, optical, and scanning electron microscopy reveal key properties of the materials. The sheet resistances determined from measurements of four point probe devices were found to be ∼2 kΩ/square, close to expectation. Graphene crossbar structures fabricated in stacked configurations demonstrate the versatility of the procedures.

Adsorption Properties of the Cu(115) Surface: Basic Interfaces

The interfaces: K/Cu(115) and CO/Cu(115) have been characterized using surface sensitive techniques, including low energy electron diffraction and photoelectron spectroscopy. K adatoms show tendency to occupy the sites close to the step edges. At low temperature (near 125 K), after completion of two layers, potassium grows in 3D islands (the Stranski-Krastanov mode). At higher temperature, e.g. at room temperature, potassium introduces reconstruction of the substrate even at low coverages. Calibration of the alkali coverage, up to completion of the first layer, using the work function changes curve has been confirmed as a very convenient and precise procedure. The adsorbed state of CO at 130 K has been identified by registration of core levels obtained by the use synchrotron radiation photoelectron spectroscopy. The characteristics of the main is and satellite peaks have been analyzed in context of substrate geometry and compared with the ones of other copper planes. There are no indications of dissociative adsorption of CO, only residual carbon and oxygen were found after adsorbate desorption around 220 K. CO molecules show a strong tendency to on top adsorption in sites far from the step edges of the Cu(115) surface.

Photoelectron Spectroscopy of LiV2O4with Photons from 8.4 to 8100 eV: Bulk Sensitivity, Hybridization, and Recoil Effects

Hard- and soft-X-ray photoelectron spectroscopies (HAXPES and SXPES) are very powerful for studying the bulk electronic structures of strongly correlated electron systems. The presence of an intrinsic surface layer on clean fractured surfaces is demonstrated for LiV 2 O 4 . In this material, single-nucleus recoil effects are very prominent not only for all core levels but also for valence states at 20 K. However, such recoil effects are negligible in VO 2 even at 350 K in spite of the fact that VO 2 has a similar V–O 6 octahedron structure. The marked intensity increase in the high-binding-energy part of the so-called O 2p band relative to the V 3d band in HAXPES is interpreted to be due to the V 4s state contribution. Very high resolution extremely low energy photoelectron spectroscopy (ELEPES) is performed with Kr and Xe lamps at 10.1 and 8.4 eV, respectively, demonstrating its bulk sensitivity for this material with heavy-Fermion-like behavior.

Layer Growth, Thermal Stability, and Desorption Behavior of Hexaaza-triphenylene-hexacarbonitrile on Ag(111)

The layer growth, thermal stability, and desorption kinetics of ultrahigh vacuum (UHV)-grown organic discoid molecules (hexaaza-triphenylene-hexacarbonitrile (HATCN)) on Ag(111) have been studied by atomic force microscopy (AFM), thermal desorption spectroscopy (TDS), low-energy electron diffraction (LEED), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). In the initial growth stage (≤0.24 nm mean film thickness) a strongly bonded monolayer of face-on oriented molecules is formed which does not desorb upon heating to 900 K, probably due to the formation of paracyanogen. With increasing film thickness the monolayer of face-on oriented molecules transforms at room temperature into a monolayer built up of edge-on oriented molecules, which saturates at a mean film thickness of around 0.8 nm. This layer partially decomposes during heating, and C2N2 fragments desorb between 650 and 900 K. On the monolayer of standing molecules a second, metastable, layer forms, which saturates at 1.36 nm. Wi...

Probing Electronic Properties of Molecular Engineered Zinc Oxide Nanowires with Photoelectron Spectroscopy

ZnO nanowires (NWs) are emerging as key elements for new lasing, photovoltaic and sensing applications but elucidation of their fundamental electronic properties has been hampered by a dearth of characterization tools capable of probing single nanowires. Herein, ZnO NWs were synthesized in solution and integrated into a low energy photoelectron spectroscopy system, where quantitative optical measurements of the NW work function and Fermi level location within the band gap were collected. Next, the NWs were decorated with several dipolar self-assembled monolayers (SAMs) and control over the electronic properties is demonstrated, yielding a completely tunable hybrid electronic material. Using this new metrology approach, a host of other extraordinary interfacial phenomena could be explored on nanowires such as spatial dopant profiling or heterostructures.

Retention mechanisms and binding states of deuterium implanted into beryllium

The retention of 1 keV D+ ions implanted into clean and oxidized single crystalline Be at room and elevated temperatures is investigated by a combination of in situ analytical techniques including temperature programmed desorption (TPD), nuclear reaction analysis, low-energy ion spectroscopy (LEIS) and x-ray photoelectron spectroscopy. For the first time, the whole temperature regime for deuterium release and the influence of thin oxide films on the release processes are clarified. The cleaned and annealed Be sample has residual oxygen concentration equivalent to 0.2 monolayer (ML) BeO in the near-surface region as the only contamination. LEIS shows that Be from the volume covers thin BeO surface layers above an annealing temperature of 1000 K by segregation, forming a pure Be-terminated surface, which is stable at lower temperatures until again oxidized by residual gas. No deuterium is retained in the sample above 950 K. By analyzing TPD spectra, active retention mechanisms and six energetically different binding states are identified. Activation energies (EA) for the release of D from binding states in Be are obtained by modelling the experimental data. Two ion-induced trap sites with release temperatures between 770 and 840 K (EA= 1.88 and 2.05 eV, respectively) and two trap sites (release between 440 and 470 K) due to supersaturation of the bulk above the steady state fluence of 2×1017 cm-2 are identified. None of the release steps shows a surface recombination limit. A thin BeO surface layer introduces an additional binding state with a release temperature of 680 K. Implantation at elevated temperatures (up to 530 K) changes the retention mechanism above the saturation limit and populates a binding state with a release temperature of 570 K.

Atomic structure of a regular Si(2 2 3) triple step staircase

An atomically accurate regular triple step array with a period of 4.8 nm has been fabricated on the vicinal Si(5 5 7) surface. Its atomic structure was studied on different length scales by scanning tunneling microscopy, low energy electron diffraction and photoelectron spectroscopy. These complementary methods allowed to identify the average orientation of the regular triple step staircase as Si(2 2 3) and to give a deeper insight into the atomic arrangement of this structure.

Investigation of epitaxial Nd1.85Ce0.15CuO4 − y film surface by low energy electron diffractometry

The surface of epitaxial Nd1.85Ce0.15CuO4 − y (001) (NCCO) film has been studied by low energy electron diffractometry (LEED) and photoelectron spectroscopy. Ar+ ion etching of a surface with subsequent annealing in oxygen at atmosphere pressure has been found to lead to the ordered structure restoration of surface layers with the symmetry type and lattice parameters corresponding to the NCCO phase. Annealing in vacuum at temperatures close to the boundary of thermodynamic phase stability results in the formation of epitaxial Ce0.5Nd0.5O1.75 phase on a surface that is indicated in the LEED pattern as additional spots corresponding to the surface lattice (√2 × √2)R45°.

Interface formation in CdTe solar cells: Nucleation of CdTe on CdS(0001) and (10$ \bar 1 $0)

AbstractThe growth of CdTe on (0001) and (10$ \bar 1 $0) surfaces of CdS was studied using low energy electron diffraction and photoelectron spectroscopy. The results indicate oriented growth of the CdTe films with a faster nucleation of CdTe on CdS(0001), explaining the preferred (111) orientation of CdTe films on polycrystalline substrates. The faster nucleation on the (0001) surface is attributed to the formation of a stable CdS(0001):Te surface termination, which is identified from surface sensitive Te 4d spectra and a 2√3 × 2√3 surface reconstruction. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrahigh-Resolution Vacuum Ultraviolet Light Source System for Extremely Low Energy Photoelectron Spectroscopy

We propose an efficient monochromatization method for the new ultrahigh-resolution Ar, Kr, and Xe light sources to be used for extremely low energy photoelectron spectroscopy (ELEPES). By fully utilizing the short-wavelength transmittance limits known as the Urbach tail of LiF, CaF2, and sapphire, we can extract only the longest wavelength lines of the resonance lamp with Ar, Kr, and Xe gases by using LiF heated up to ∼50 °C as well as CaF2 and sapphire at room temperature. The full width at half maximum of the longest wavelength Xe I resonance line is found to be narrower than 500 µeV. Using these light sources, we can perform ELEPES in laboratories at three photon energies with ultrahigh energy resolution and high efficiency.