Very Low Energy Electron Diffraction Research Articles

低速電子回折型電子スピン検出器を用いた高分解能スピン・角度分解光電子分光(実験技術)

低速電子回折型電子スピン検出器を用いた高分解能スピン・角度分解光電子分光(実験技術)

Electronic structure of the conduction band of the interface region of ultrathin films of substituted perylenedicarboximides and the germanium oxide surface

The results of the investigation of the electronic structure of the conduction band and the interfacial potential barrier during the formation of interfaces of dioctyl-substituted perylenedicarboximide (PTCDI-C8) and diphenyl-substituted perylenedicarboximide (PTCDI-Ph) ultrathin films with the oxidized germanium surface have been presented. The experimental results have been obtained using the very low energy electron diffraction (VLEED) technique in the total current spectroscopy (TCS) mode at energies in the range from 5 to 20 eV above the Fermi level EF. The positions of the maxima of the fine structure of total current spectra (FSTCS) of the PTCDI-C8 and PTCDI-Ph films differ significantly in the energy range from 9 to 20 eV above the Fermi level EF, which can be associated with the difference between the substituents of the chosen molecules, dioctyl- and diphenyl-, respectively. At the same time, the positions of the lowenergy maxima in the FSTCS spectra at an energy 6–7 eV above the Fermi level EF for the PTCDI-C8 and PTCDI-Ph films almost coincide with each other. It has been suggested that these maxima are attributed to the electronic states of the perylene core of the molecules under investigation. The process of the formation of interfacial potential barriers of the PTCDI-C8 and PTCDI-Ph films with the oxidized germanium surface has been analyzed. It has been found that the work functions of the surface, Evac–EF, differ little from 4.6 ± 0.1 eV over the entire range of organic coating thicknesses from 0 to 6 nm.

Electronic structure of the conduction band upon the formation of ultrathin fullerene films on the germanium oxide surface

The results of the investigation of the electronic structure of the conduction band in the energy range 5–25 eV above the Fermi level EF and the interfacial potential barrier upon deposition of aziridinylphenylpyrrolofullerene (APP-C60) and fullerene (C60) films on the surface of the real germanium oxide ((GeO2)Ge) have been presented. The content of the oxide on the (GeO2)Ge surface has been determined using X-ray photoelectron spectroscopy. The electronic properties have been measured using the very low energy electron diffraction (VLEED) technique in the total current spectroscopy (TCS) mode. The regularities of the change in the fine structure of total current spectra (FSTCS) with an increase in the thickness of the APP-C60 and C60 coatings to 7 nm have been investigated. A comparison of the structures of the FSTCS maxima for the C60 and APP-C60 films has made it possible to reveal the energy range (6–10 eV above the Fermi level EF) in which the energy states are determined by both the π* and σ* states and the FSTCS spectra have different structures of the maxima for the APP-C60 and unsubstituted C60 films. The formation of the interfacial potential barrier upon deposition of APP-C60 and C60 on the (GeO2)Ge surface is accompanied by an increase in the work function of the surface Evac–EF by the value of 0.2–0.3 eV, which corresponds to the transfer of the electron density from the substrate to the organic films under investigation. The largest changes occur with an increase in the coating thickness to 3 nm, and with further deposition of APP-C60 and C60, the work function of the surface changes only slightly.

Density of the unoccupied electronic states of the ultrathin films of the aziridinylphenylpyrrol substituted fullerene

The ultrathin films of aziridinylphenylpyrrol-C60 (APP-C60) and of the unsubstituted C60 thermally deposited in UHV on an oxidized silicon substrate (SiO2)n-Si were studied experimentally using the very low energy electron diffraction (VLEED) method and the total current spectroscopy (TCS) measurement scheme. The density of the unoccupied electronic states (DOUS) located 2-20eV above the Fermi level (EF) of the films under study was determined using the TCS results and using the results of the density functional theory (DFT) calculations of the vacant electronic orbitals of the APP-C60 and C60 molecules. The DOUS peak structure obtained on the basis of the calculation results corresponds well to the structure of the major DOUS bands obtained from the TCS experiment. The comparison of the DOUS spectra of the APP-C60 and C60 films under study showed that the introduction of the APP substituent to the C60 molecule was accompanied by the pronounced changes of the π* DOUS bands in the energy range from 2eV to 5eV above EF and of the DOUS band composed from both π* and σ* - type orbitals in the energy range from 5eV to 9eV above EF. The formation of the low-lying σ* DOUS band in the APP-C60 film in the energy range from 8eV to 13eV above EF can be explained by the superposition of the relevant DOUS maxima from the C60 film and from the APP fragment.

Formation of the conduction band electronic structure during deposition of ultrathin dicarboximide-substituted perylene films on the oxidized silicon surface

The results of the investigation of the conduction band electronic structure and the interfacial potential barrier during deposition of ultrathin dicarboximide-substituted perylene films (PTCBI-C8) on the oxidized silicon surface have been presented. The measurements have been performed using the very low energy electron diffraction (VLEED) technique implemented in the total current spectroscopy (TCS) mode with a variation in the incident electron energy from 0 to 25 eV. Changes in the intensities of the maxima from the deposited PTCBI-C8 film and from the substrate with an increase in the organic coating thickness to 7 nm have been analyzed using TCS measurements. A comparison of the structure of the maxima of PTCBI-C8 and perylene-tetracarboxylic-dianhydride (PTCDA) films has made it possible to distinguish the energy range (8–13 eV above EF) in which distinct differences in the structures of maxima for PTCDA and PTCBI-C8 films are observed. This energy range corresponds to low-lying σ*-states of the conduction band of the films studied. The formation of the interfacial region of the PTCBI-C8 film and (SiO2)n-Si substrate is accompanied by an increase in the surface work function by 0.6 eV, which corresponds to the electron density charge transfer from the (SiO2)n-Si substrate to the PTCBI-C8 film.

Design of a multi-channel spin polarimeter**Supported by National Natural Science Foundation of China (10979021, 11027401, 11174054, 11304338, 11227902), the Ministry of Science and Technology of China (2011CB921800) and the “Strategic Priority Research Program (B)” of the Chinese Academy of Sciences (XDB04010100)

All commercial electron spin polarimeters work in single channel mode, which is the bottleneck of researches by spin-resolved photoelectron spectroscopy. By adopting the time inversion antisymmetry of the magnetic field, we developed a multichannel spin polarimeter based on normal incident very low energy electron diffraction (VLEED). The key point to achieve the multi-channel measurements is the spatial resolution of the electron optics. The test of the electron optics shows that the designed spatial resolution can be achieved and an image type spin polarimeter with 100 times 100, totally ten thousand channels is possible to be realized.

Transmission of low-energy electrons through ultrathin layers of tin(IV) phthalocyanine oxide

The results of the investigation of the transmission of low-energy electrons through 8-nm-thick films of tin(IV) phthalocyanine oxide (SnOPc) on the surface of the oxidized silicon substrate are presented. The procedure of detecting the reflection of testing low-energy electron beam from the surface was implemented using the very low-energy electron diffraction (VLEED) technique in the total current spectroscopy (TCS) mode with a change in the incident electron beam energy from 0 to 25 eV. The structure of maxima in the total current spectra of SnOPc films was established and the variation in intensities of the maxima of total current spectra emitted from the deposited SnOPc film and (SiO2)n-Si substrate during an increase in the thickness of the organic coating to 8 nm was analyzed. With such an increase in the thickness of the organic coating, the work function of the surface increases by 0.7 eV, which corresponds to the transfer of the electron density from the (SiO2)n-Si substrate to the SnOPc film. Optical absorption spectra of SnOPc films were measured. The absorption spectra were compared with the spectra measured by the TCS method for the SnOPc films and the films of molecules of oxygen-free metal phthalocyanine.

Unoccupied Electronic States at the Interface of Oligo(phenylene-vinylene) Films with Oxidized Silicon

Ultrathin films of trioligo(phenylene-vinylene) and of dinitro-substituted trioligo(phenylene-vinylene) end terminated by dibutyl-thiole were thermally deposited in an ultrahigh vacuum (UHV) on an oxidized silicon substrate (SiO2)n-Si. The surface work function and the density of the unoccupied electron states (DOUS) located 5–20 eV above the Fermi level (EF) were studied during the film deposition using the very low energy electron diffraction (VLEED) method and the total current spectroscopy (TCS) measurement scheme. The electronic structure typical for the organic films studied was formed, while the organic deposit thickness increased up to 5–6 nm. The DOUS peak structure of the organic films studied obtained from the TCS results showed good correspondence to the DOUS peaks obtained from the density functional theory (DFT) calculations. The calculations were made for the model oligo(phenylene-vinylene) films where the neighboring molecules had relatively weak interaction, which is also characteristic of the disordered films studied experimentally. The comparison of the DOUS spectra of the unsubstituted film and of the dinitro-substituted film showed that the substitution had a minor effect on the DOUS for the energies higher than 10 eV above EF, while in the energy range from 5 to 10 eV above EF a significant difference between DOUS of the two films studied was observed. The 0.3 and 0.7 eV increase of the surface work function values was observed during the formation of the unsubstituted trioligo(phenylene-vinylene) and of the dinitro-substituted trioligo(phenylene-vinylene) deposits on the substrate surface, respectively. The interfacial charge transfer was characterized by the formation of the 2 nm thick polarization layer in the organic films and by the negative charge transfer to the organic layer from the (SiO2)n-Si substrate.

Spin-dependent surface barrier from very-low-energy electron diffraction fine structures: A feasibility study

The surface barrier is a key issue for understanding and describing the results of surface-sensitive electron spectroscopies, such as photoemission and inverse photoemission. We present a feasibility study showing that the shape of the surface barrier can be derived from very-low-energy electron diffraction (VLEED) fine-structure measurements. In particular, we focus on the spin dependence of the barrier probed by a spin-polarized experiment. We show model calculations of spin-dependent fine structures for specific ferromagnetic surfaces, where spin-dependent barrier models were successfully employed to describe spectroscopic results. We come to the conclusion that spin-polarized VLEED experiments with state-of-the-art energy and angular resolution are able to reveal details of the spin-dependent shape of the surface barrier.

Electronic properties of the polycrystalline tin dioxide interface with conjugated organic layers

The results on the electronic structure of the unoccupied electronic states of the polycrystalline SnO 2 in the energy range from 5 eV to 25 eV above the Fermi level are presented. The modification of the electronic structure and of the surface potential upon deposition of the ultrathin films of copper phthalocyanine (CuPc) and of perylene tetracarboxylic acid dianhydride (PTCDA) film onto the SnO 2 surface were studied using the very low energy electron diffraction (VLEED) method and the total current spectroscopy (TCS) measurement scheme. A substantial attenuation of the TCS signal coming from the SnO 2 surface was observed upon formation of a 1.5–2 nm thick organic deposit layer while no new spectral features from the deposit were distinguishable. It was observed that the electronic structure typical for the organic films was formed within the organic deposit thickness range from 2 nm to 7 nm. The interfacial charge transfer was characterized by the formation of the polarization layer up to 5 nm thick in the organic films. The PTCDA deposition on SnO 2 was accompanied by the negative charge transfer onto the organic layer and to the 0.65 eV increase the surface work function. At the CuPc/SnO 2 interface, the negative charge was transferred to the SnO 2 surface and the overall surface work function decreased by 0.15 eV.

Electron transition probabilities in elementary electron-phonon scattering processes using electron-energy-loss spectroscopy: Application to graphite

We have investigated coupling constants in elementary electron-phonon scattering processes on a graphite surface by the combined use of high-resolution electron-energy-loss spectroscopy (HREELS) and very low-energy electron diffraction (VLEED). HREELS is used to measure the modulations of electron transition probabilities from incoming electrons in vacuum to outgoing electrons in vacuum where the transition includes one-phonon scattering processes inside a solid. Determining the electronic band structures of graphite with VLEED, we defined electronic states of the solid surface that electrons entered before and after scattering off phonons. Thus, we observed that the measured electron transition probabilities significantly depended on whether the electrons were in a bulk Bloch state or an evanescent state before scattering off the phonons. This result clearly indicates that the measured electron transition probabilities reflect the strength of the coupling constants in the solid.

Surface electron band structure and very low-energy electron diffraction reflectivity for Al(111)

The two-dimensional 2D layer Green's function scattering method is used to calculate the energy of surface states and resonances at $\overline{\ensuremath{\Gamma}}$ for Al(111) for both below and above the vacuum level. The surface-barrier potential is represented by an empirical form. The above vacuum-level surface electron band structure for this surface has not been calculated before and it is important in understanding many surface phenomena. The geometric structure of the Al(111) surface is known from intensity analysis in low-energy electron diffraction at energies 60--450 eV. The details of the surface barrier for Al(111) were obtained from a match with the below vacuum-level experimental energy position of the first Rydberg surface resonance and the Shockley surface state at ${\mathbit{k}}_{\ensuremath{\parallel}}=0(\overline{\ensuremath{\Gamma}})$. The calculation was then extended to the above vacuum-level case for 0--27 eV with the inclusion of inelastic electron interactions. Tamm-type resonances at 6.9 eV and possibly also at 8.3 eV, a Shockley-type resonance at $14.0\ifmmode\pm\else\textpm\fi{}0.5\text{ }\text{eV}$ and a series of Rydberg (image) resonances near 24 eV all above vacuum level are found at ${\mathbit{k}}_{\ensuremath{\parallel}}=0$. The same 2D layer Green's function scattering method using the same input data was then used to calculate the intensity of the 00 beam for ${\mathbit{k}}_{\ensuremath{\parallel}}=0$ (normal incidence) in very low-energy electron diffraction (VLEED) from this surface in the energy range 0--65 eV. Features in the VLEED intensities are found due to the Shockley and Rydberg resonances. Experimental data from over 26 years ago found surface features near the energies found in this work. Beam intensities from low-energy electron microscope measurements at normal incidence and new data from other surface spectroscopies could provide experimental confirmation of the resonances predicted in this work.

High efficiency and high energy-resolution spin-polarized photoemission spectrometer

We have developed a new spin-polarized photoemission spectrometer combined a commercial hemispherical electron energy analyzer with an electron spin-polarimeter employing the very low energy electron diffraction (VLEED) at a Fe(001)p(1×1)-O surface. The new polarimeter has realized the effective Sherman function of 0.40 and the figure of merit of 1.9×10-2, which is approximately two orders of magnitude higher than that expected with a compact Mott scattering spin-polarimeter. The new system enabled us to measure spin- and angle-resolved photoemission spectra with high energy and momentum resolutions.

A new spin-polarized photoemission spectrometer with very high efficiency and energy resolution

A new spin- and angle-resolved photoemission spectrometer was developed adopting the very-low-energy-electron-diffraction (VLEED)-type spin polarimeter. The Fe(001)p(1x1)-O film grown on MgO(001) crystal for the VLEED target yields significantly high spin-resolving power, the effective Sherman function of 0.40+/-0.02, with long lifetime and stability compared to the conventional Fe(001) target. Under the favor of high resolving power, approximately 100 times higher efficiency than that of conventional Mott-type spin polarimeter, the figure of merit of 1.9+/-0.2x10(-2) was achieved. Owing to this high efficiency, high-energy resolution can be realized with this new spin-polarized photoemission spectrometer. The simplified ways of target preparation and revitalization make the VLEED spin polarimeter much more convenient and feasible for the spin-polarized photoemission spectroscopy.

Elastic scattering effects in the electron mean free path in a graphite overlayer studied by PES and LEED

We measure variations due to a an epitaxial graphite overlayer on SiC in the Si 2p core level signal using synchrotron radiation photoelectron spectroscopy at low kinetic energy. The intensity minima in the Si 2p signal correspond to the final-state band gaps predicted from Very Low Energy Electron Diffraction (VLEED), whereby the experimental values of the energy dependent V i are derived from the widths of the I( V) spectral structures, and calculations of the complex band structure of the final states are performed with the Bloch wave damping constants giving the mean free path. In graphite there are significant diffraction minima in the final-state band gaps.

Excited Surface Band Structures and Surface Barrier Features in VLEED

We have tested a surface barrier model for Cu(001) for consistency with below vacuum level data from inverse photoemission and above vacuum level data from very low energy electron diffraction (VLEED). The model has an exponential-type saturation which extends to the center of the first row of atoms in the crystal. The barrier model can account for the limited data available for the surface barrier band structure below the vacuum level but not for the strong surface barrier features which occur in VLEED reflectivity spectra for θ = 80° and ϕ = 45°. This is the case even when allowance is made for dynamical variation of the key barrier structural features. It is concluded that this form of the barrier saturation is not correct for real crystals.

Band- andk-dependent self-energy effects in the unoccupied and occupied quasiparticle band structure of Cu

Excited-state self-energy effects in the electronic structure of Cu, a prototype weakly correlated system containing states with different degrees of localization, are investigated with emphasis on the unoccupied states up to 40 eV above the Fermi level. The analysis employs the experimental quasiparticle states mapped under full control of the 3-dimensional wavevector k using very-low energy electron diffraction for the unoccupied states and photoemission for the occupied states. The self-energy corrections to the density-functional theory show a distinct band- and k-dependence. This is supported by quasiparticle GW calculations performed within the framework of linearized muffin-tin orbitals. Our results suggest however that the GW approximation may be less accurate in the localized d-bands of Cu with their short-range charge fluctuations. We identify a connection of the self-energy behavior with the spatial localization of the one-electron wavefunctions in the unit cell and with their behavior in the core region. Mechanisms of this connection are discussed based on the local-density picture and on the non-local exchange interaction with the valence states.

THREE-DIMENSIONAL BAND MAPPING BY COMBINED VERY-LOW-ENERGY ELECTRON DIFFRACTION AND PHOTOEMISSION

Resolving the 3D wave vector k in photoemission mapping of the band structure E( k ) requires knowledge of the unoccupied final states. Both dispersions and lifetimes of these states can be achieved by very-low-energy electron diffraction (VLEED). By incorporating the non-free-electron and excited-state effects in the final states, combining VLEED with photoemission provides accurate mapping of the valence E( k ) resolved in the 3D wave vector and under control of the intrinsic accuracy. We here concentrate on the most accurate combined method which uses angle-dependent VLEED and photoemission measurements. It provides access to many Brillouin-zone lines using one crystal surface, and benefits from an intensity gain and a better intrinsic accuracy near the Fermi level.

Momentum dependence of the surface potential barrier for above-barrier electrons

Fine structure features in very low-energy electron diffraction (VLEED) near beam thresholds are sensitive to the form of the surface barrier potential. The VLEED features give information about the barrier for an electron incident on the barrier from the crystal side almost parallel to the surface and with total energy above the barrier height. Theoretically, the barrier form is known to depend on the kinetic energy and angle of approach, that is the momentum, of the incoming electron, but the various theoretical methods of calculation presently provide information which is quantitatively different. The calculation of the fine structure features from models which do not take into account the energy dependence of the barrier height and saturation for the electrons involved in VLEED have not provided consistent and definitive information about the barrier form. It is shown that this information can be extracted from a large data set over an extended energy range 8–19eV on Cu(001). To obtain the required information, it is necessary to have overlapping energy ranges which cover all of the vacuum emergence energies for at least one specific VLEED electron beam.

Very low-energy electron diffraction as a method of band structure investigations: Applications in photoelectron spectroscopy

This review presents the theoretical and experimental foundations of very low-energy electron diffraction (VLEED) spectroscopy, which is the most direct method of studying the band structure E(k) of unoccupied higher-lying electronic states. The experimental results presented here indicate that, contrary to the generally accepted point of view, these states may differ significantly from free-electron dispersion and may experience considerable many-electron effects. It is shown that the higher-lying states observed in the experiment can be used directly in photoelectron spectroscopy, which is the basic method for studying valence states. This makes it possible to solve the basic problem of resolving the band structure in the three-dimensional quasi-wave vector.