Metastable Deexcitation Spectroscopy Research Articles

Observation of a Half-Metallic Interface State for Pyridine-Adsorbed H/Fe3O4(100).

Organic molecule/half-metallic oxide interfaces play essential roles in recent molecular spintronics devices, yet factors governing the spin polarization of the hybridized interface state (HIS) formed there remain unclear because of the lack of direct spectroscopic evidence for it. Here, we present a spin-polarized metastable deexcitation spectroscopy (SPMDS) experiment, which is sensitive to the topmost surface, showing that the adsorption of pyridine on H-terminated Fe3O4(100) at 100 K produces a highly spin-polarized HIS, while no such HIS forms on a bare Fe3O4(100) surface. A density functional theory calculation has predicted the formation of half-metallic HIS, which is consistent with the SPMDS results. This can be understood on the basis of the interface chemical bonding formed by the coordination of the nitrogen end of pyridine to the surface Fe atom where half-metallic conduction electrons are distributed.

Spinterface Formation at α-Sexithiophene/Ferromagnetic Conducting Oxide

The inorganic/molecular spinterface is an ideal platform for generating extraordinary spin effects. Understanding and controlling these spin-related effects is mandatory for the exploitation of such interfaces in devices. For this purpose we have investigated the adsorption of α-sexithiophene (α-6T) on La0.7Sr0.3MnO3 (LSMO) as one of the prototypical material combinations used in organic spintronic devices. Atomic force microscopy (AFM), confocal photoluminescence, X-ray and utraviolet photoelectron spectroscopy, and metastable de-excitation spectroscopy unraveled the structure and the electronic configuration of 6T for various surface coverages. This data set allowed the determination of the characteristic features of occupied states: the band diagram and the work function. Finally, density functional theory enabled us to establish that the spin polarization in 6T molecular orbitals critically depends on the termination layer of LSMO, showing a substantial effect on the MnO-terminated one only. We believe that this research provides important hints for a comprehensive understanding of spinterface effects in general and offers key suggestions for the optimization of 6T/LSMO-based spin devices through the engineering of LSMO termination layer.

Formation of quasi-free-standing graphene on SiC(0001) through intercalation of erbium

Activation of the carbon buffer layer on 4H- and 6H-SiC substrates using elements with high magnetic moments may lead to novel graphene/SiC-based spintronic devices. In this work, we use a variety of surface analysis techniques to explore the intercalation of Er underneath the buffer layer showing evidence for the associated formation of quasi-free-standing graphene (QFSG). A combined analysis of low energy electron diffraction (LEED), atomic force microscopy (AFM), X-ray and ultraviolet photoemission spectroscopy (XPS and UPS), and metastable de-excitation spectroscopy (MDS) data reveals that annealing at temperatures up to 1073 K leads to deposited Er clustering at the surface. The data suggest that intercalation of Er occurs at 1273 K leading to the breaking of back-bonds between the carbon buffer layer and the underlying SiC substrate and the formation of QFSG. Further annealing at 1473 K does not lead to the desorption of Er atoms but does result in further graphitization of the surface.

Contact-induced spin polarization of monolayer hexagonal boron nitride on Ni(111)

Hexagonal boron nitride (h-BN) is a promising barrier material for graphene spintronics. In this Letter, spin-polarized metastable de-excitation spectroscopy (SPMDS) is employed to study the spin-dependent electronic structure of monolayer h-BN/Ni(111). The extreme surface sensitivity of SPMDS enables us to elucidate a partial filling of the in-gap states of h-BN without any superposition of Ni 3d signals. The in-gap states are shown to have a considerable spin polarization parallel to the majority spin of Ni. The positive spin polarization is attributed to the π-d hybridization and the effective spin transfer to the nitrogen atoms at the h-BN/Ni(111) interface.

Spin polarization of single-layer graphene epitaxially grown on Ni(1 1 1) thin film

The spin-resolved electronic structure of graphene on Ni(111) was investigated using spin-polarized metastable deexcitation spectroscopy (SPMDS). Graphene was grown epitaxially on a Ni(111) single-crystalline surface using the ultra high vacuum chemical vapor deposition technique with benzene vapor as a precursor. At 50L (5×10−5Torrs), a single epitaxial layer of graphene was formed, but no further growth was observed at higher exposure. The spin-summed spectrum of graphene/Ni(111) had a new peak at the Fermi level and three weak features corresponding to the molecular orbitals of graphene. Spin asymmetry analysis of the SPMDS spectra revealed that the spin polarization of the electronic states shown by the new peak was parallel to the majority spin of the Ni substrate. The appearance and spin polarization of the new electronic states are discussed in terms of the hybridization of graphene π orbitals and Ni d orbitals.

Changes in ionization potentials of MgO and CaO films upon heating in air and vacuum investigated by metastable de-excitation spectroscopy

Changes in the ionization potential of MgO and CaO film surfaces upon heating in air and vacuum, which are necessary processes in the manufacture of plasma display panels, were investigated by metastable de-excitation spectroscopy. Heating at 500°C for 1min in vacuum is effective for cleaning an MgO film surface previously heated in air, resulting in the recovery of the ionization potential of the cleaned MgO surface from the increased value upon heating in air. A similar treatment is not effective for the CaO surface. This difference in behavior can be attributed to the stronger bonding states of H2O and CO2 on CaO than those on MgO. This is consistent with the fact that CaO has not yet been practically used as a protective layer in plasma display panels. Furthermore, the firing voltages of the plasma predicted for the MgO and CaO protective layers from the ionization potentials determined by metastable de-excitation spectroscopy were found to qualitatively agree with measured firing voltages obtained for a test panel. The present results confirmed that metastable de-excitation spectroscopy is effective for evaluating protective layers used in plasma display panels.

Energy-level alignment at the Alq3/Fe3O4(001) interface

We have used the technique of metastable de-excitation spectroscopy to probe the interfacial electronic structure of the organic semiconductor (OSC) Alq3 deposited onto clean Fe3O4(001) substrates. We have measured shifts in the low-energy secondary electron cutoff and energetic onset of the highest occupied molecular orbital (HOMO) of Alq3 as the coverage increases from the sub-ML range to multilayer formation. We find that the presence of an interfacial dipole induces a uniform decrease in the valence band electronic states by 1.2 eV with respect to the vacuum level and modifies the position of the HOMO energetic onset to 1.8 eV below the substrate Fermi level. The strong intrinsic dipole moment of Alq3 is suggested as the origin for these changes in accordance with previous studies of Alq3 deposited onto various substrates.

The characterization of Electronic state from Surface to Several Nanometer Region on MgO:Si Thin Film

ABSTRACTWe report the properties of MgO:Si film as a protective cathode material on the electrical discharge, and the electronic state of the outer-most surface on MgO:Si film characterized by helium Meta-stable De-excitation Spectroscopy (MDS) and that of several nanometer region from the surface evaluated by X-ray Photoelectron Spectroscopy (XPS). Both of the spectra are discussed focusing on the dependence upon the amount of Si in the MgO film for understanding discharge phenomena. The analyses of the experimental data imply that the discharge properties are not improved due to surface degradation with the increase of Si in MgO films. However, an in-situ discharge experiment, in which MgO:Si films are not exposed for the atmosphere after its deposition, shows that the introduction of Si up to about 1 atomic% has the potential to enhance the secondary electron emission coefficient.

Metastable De-excitation Spectroscopy and Density Functional Theory Study of the Selective Oxidation of Crotyl Alcohol over Pd(111)

The extremely surface sensitive technique of metastable de-excitation spectroscopy (MDS) has been utilized to probe the bonding and reactivity of crotyl alcohol over Pd(111) and provide insight into the selective oxidation pathway to crotonaldehyde. Auger de-excitation (AD) of metastable He (23S) atoms reveals distinct features associated with the molecular orbitals of the adsorbed alcohol, corresponding to emission from the hydrocarbon skeleton, the O n nonbonding, and C═C π states. The O n and C═C π states of the alcohol are reversed when compared to those of the aldehyde. Density functional theory (DFT) calculations of the alcohol show that an adsorption mode with both C═C and O bonds aligned somewhat parallel to the surface is energetically favored at a substrate temperature below 200 K. Density of states calculations for such configurations are in excellent agreement with experimental MDS measurements. MDS revealed oxidative dehydrogenation of crotyl alcohol to crotonaldehyde between 200 and 250 K, resulting in small peak shifts to higher binding energy. Intramolecular changes lead to the opposite assignment of the first two MOs in the alcohol versus the aldehyde, in accordance with DFT and UPS studies of the free molecules. Subsequent crotonaldehyde decarbonylation and associated propylidyne formation above 260 K could also be identified by MDS and complementary theoretical calculations as the origin of deactivation and selectivity loss. Combining MDS and DFT in this way represents a novel approach to elucidating surface catalyzed reaction pathways associated with a “real-world” practical chemical transformation, namely the selective oxidation of alcohols to aldehydes.

44.3: Investigation of Effects of Heating in Air on Ionization Potential of MgO and CaO Films using Metastable De‐excitation Spectroscopy

AbstractEffects of heating in air on the ionization potentials, Eip, of MgO and CaO films were investigated using metastable de‐excitation spectroscopy (MDS). The difference in Eip between samples with and without heating in air was found to be small for MgO and significant for CaO. In addition, the difference in the ion‐induced secondary electron yield, γ, which is expected from Eip determined from the MDS spectra, between samples with and without heating in air qualitatively agrees with that expected from the measurement of the firing voltage using a test panel. The present results conformed that MDS is effective to evaluating γ of the protective layer used in plasma display panels.

Surface characterization of MgO:Al,N films with meta‐stable de‐excitation spectroscopy and x‐ray photoelectron spectroscopy

Abstract— It is shown that meta‐stable de‐excitation spectroscopy (MDS) is one of the most useful characterization methods to analyze the interaction between the discharge gas and the surface of the material and is applied to MgO:Al,N films. From the results of the measurement and analysis of helium MDS and the in‐situ discharge experiment, it is confirmed that the limited composition films of MgO:Al,N have potentially a larger secondary‐electron‐emission coefficient (γ) compared with that of MgO. The improvement in γ is caused by the electron‐occupied tailing state at around the top of the valence band which is generated by the introduction of Al,N to MgO films. Also, the O1s spectra measured by x‐ray photoelectron spectroscopy (XPS) shows that the stable surfaces are formed with the introduction of Al,N to MgO films.

Investigation of Measurement Conditions of Metastable De-excitation Spectroscopy of MgO Thin Films Used for Plasma Display Panels

K. Yoshino, Y. Morita, T. Nagatomi, M. Terauchi, T. Tsujita, T. Nakayama, Y. Yamauchi, M. Nishitani, M. Kitagawa, Y. Yamauchi, and Y. Takai Department of Material and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan 3rd Device Development Group, AVC Devices Development Center, AVC Networks Company, Panasonic Corporation, Ibaraki, Osaka 567-0026, Japan Co-operation Laboratory of Panasonic, Osaka University, Suita, Osaka 565-0871, Japan Advanced Display Technology Group, Image Devices Development Center, Panasonic Corporation, Moriguchi, Osaka 570-8501, Japan National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan yoshino.kyohei@jp.panasonic.com nagatomi@mls.eng.osaka-u.ac.jp

Spin polarization of the Si(111)-7 × 7 surface: A study with a spin-polarized metastable helium atom beam

We have observed the electron spin polarization of a clean Si(111) surface under external magnetic fields of 0–5 T using a spin-polarized metastable helium atom beam. By making comparisons with metastable deexcitation spectroscopy (MDS) measurements, the spin polarization has been assigned to surface dangling bonds. It has been found that the spin polarization is largely quenched by the adsorption of atomic hydrogen, and that its H-exposure dependence correlates well with that of the MDS spectrum and the low energy electron diffraction pattern.

A spin polarized He metastable beam investigation of the adsorption of l-cysteine on magnetic surfaces

We report first results on the deposition of l-cysteine (Cys), a fundamental amino-acid, on a magnetic substrate. Molecules have been deposited under ultra-high vacuum conditions on Fe(1 0 0) films. Electronic and magnetic properties of adsorbed molecules have been investigated by spin polarized metastable deexcitation spectroscopy (SPMDS). The spectra show a growth dynamics apparently similar to the one already reported on coinage metal surfaces with the formation of a strongly-bound thiolate species (S Fe bond) and weakly-bound second-layer molecules. SPMDS revealed the spin-polarized character of an Auger (Penning) deexcitation peak related to a Cys molecular orbital. Interestingly, these spin-polarization effects, ascribed to the interaction with the magnetized substrate, vanish before the first layer completion, suggesting a possible correlation with the SAM assembly properties.

Element-selective spin polarization analysis of surfaces by spin-polarized ion scattering spectroscopy

We discuss the validity of surface spin polarization analysis with element selectivity using spin-polarized ion scattering spectroscopy (SP-ISS). We examined the control of the incident 4 He + spins and successfully conducted magnetic hysteresis measurement on an Fe(100) surface. The spin polarization of the Fe(100) surface exposed to O 2 atmosphere measured by spin-polarized ion neutralization spectroscopy was consistent with that reported by spin-polarized metastable de-excitation spectroscopy. The element selectivity of SP-ISS is discussed in terms of ion neutralization, re-ionization, and multiple scattering.

Surface Spin Polarization of Fe Nanoclusters

The spin polarization at the surface of Fe nanoclusters has been probed using a spin-polarized metastable helium beam. The clusters, produced in a gas aggregation source, display a lognormal size distribution with a peak centered at ~11 nm. Varying coverages of both spheroid- and cuboid-shaped particles were concomitantly deposited onto clean Si(111) substrates for investigation with the extremely surface sensitive technique of metastable de-excitation spectroscopy (MDS). A nominal cluster coverage of 8 ¿ yielded a maximum asymmetry of ~10% in the ejected electron yield for He spins aligned parallel and anti-parallel to the magnetization direction of the clusters. When compared to values obtained from epitaxial Fe films on various substrates, the measured asymmetry suggests an enhancement in the surface spin polarization, as theoretically proposed. The atomic structure of the clusters and their topography on the Si(111) substrates were studied with transmission and scanning electron microscopy.

Hydrogen sorption sites in holmium silicide on silicon(1 1 1)

The hydrogen sorption sites on the surface of holmium silicide grown on Si(1 1 1) have been determined using metastable de-excitation spectroscopy, ultraviolet photoemission spectroscopy and density functional theory calculations. Comparison of calculated and measured surface density of states spectra allow us to locate the position of the second subsurface hydrogen atom in each unit cell to an interstitial site in the layer of rare earth atoms. The hydrogenation energies indicate a reaction pathway that involves concomitant site occupation.

Growth dynamics of L-cysteine SAMs on single-crystal gold surfaces: a metastabledeexcitation spectroscopy study

We report on a metastable deexcitation spectroscopy investigation of the growthof L-cysteine layers deposited under UHV conditions on well-defined Au(110)-(1 × 2) and Au(111) surfaces.The interaction of He* with molecular orbitals gave rise to well-defined UPS-like Penning spectra which providedinformation on the SAM assembly dynamics and adsorption configurations. Penningspectra have been interpreted through comparison with molecular orbital DFT calculationsof the free molecule and have been compared with XPS results of previous works.Regarding adsorption of first-layer molecules at room temperature (RT), two differentgrowth regimes were observed. On Au(110), the absence of spectral features related toorbitals associated with SH groups indicated the formation of a compact SAM of thiolatemolecules. On Au(111), the data demonstrated the simultaneous presence, since the earlystages of growth, of strongly and weakly bound molecules, the latter showing intact SHgroups. The different growth mode was tentatively assigned to the added rows of thereconstructed Au(110) surface which behave as extended defects effectively promoting theformation of the S–Au bond. The growth of the second molecular layer was insteadobserved to proceed similarly for both substrates. Second-layer molecules preferably adoptan adsorption configuration in which the SH group protrudes into the vacuum side.

Surface electronic structure of two- and three-dimensional holmium silicide on Si(111)

The surface electronic structure of two- and three-dimensional Ho silicides grown on Si(111) has been studied with the techniques of metastable de-excitation spectroscopy (MDS) and ultraviolet photoemission spectroscopy. Electronic structure differences between the two cases are examined, and the extreme sensitivity of MDS to the surface density of states (DOS) allows a direct comparison between deconvolved spectra and ab initio density-functional theory calculations. This comparison shows a good agreement between the DOS calculated for the Si(111)-1x1-Ho structure and MDS data obtained from an almost-complete layer of two-dimensional Ho silicide. The dominant role of the topmost silicon bilayer in determining the surface electronic properties is revealed. .

Scanning magneto-optical lens for surface analysis

We demonstrate the essential principles for performing a novel form of metastable-atom microscopy, namely focusing and scanning of a He 23S beam. A magneto-optical lens, in conjunction with time-varying magnetic field components, has been used to scan a beam of He 23S atoms across a sample surface in a manner analogous to other scanning probe techniques. The instrument developed is the first to apply methods in the laser manipulation of atoms specifically to the study of surfaces. With the lens in its current arrangement, a beam of He 23S atoms with an intensity of ∼6 × 1012 atoms s−1 cm−2 may be directed anywhere within a sample area of ∼64 mm2 at a focal distance of 1.4 m and with a focal spot full-width at half-maximum of <2 mm. Such control over the intensity and position of the He 23S beam greatly benefits the surface analysis technique of metastable de-excitation spectroscopy, as demonstrated with example spectra from a clean Si(1 1 1) 7 × 7 surface.