Angle-resolved Secondary Electron Emission Research Articles

Angle-resolved secondary photoelectron emission from graphene interfaces

Angle-resolved secondary electron emission spectra were analyzed for studying the electron transmission properties of graphene when it is tightly bound to a metallic substrate and when it is decoupled from the latter. As respective model systems, graphene/Ni(111) and graphene/Au/Ni(111) were considered. We discovered two types of spectral structures linked with the transmission properties of graphene: bands related to scattering resonances and to umklapp reflection. These findings are interpreted based on an ab initio theory of the scattering of the outgoing electrons by the graphene overlayers. The experimental secondary electron emission maps are in good agreement with the calculations, indicating that such approach can be successfully applied for studying the electron transmission properties of a variety of two-dimensional materials.

Time-dependent first-principles study of angle-resolved secondary electron emission from atomic sheets

Angle-resolved secondary electron emission (ARSEE) spectra of atomic sheets have been considered to reflect the unoccupied energy bands of the targets. However, far less is known about the dynamics of electron excitation from the valence bands to the unoccupied bands upon electron impact, leading to emission into the vacuum. Here, the authors keep track of the electrons excited to the unoccupied bands and emitted to the vacuum in real time, position space, and k-space simultaneously by a time-dependent density-functional theory simulation.

Electronic dispersion relations of monolayer hexagonal boron nitride formed on the Ni(111) surface.

Angle-resolved ultraviolet-photoelectron spectroscopy and angle-resolved secondary-electron emission spectroscopy have been carried out for a film of single-crystalline hexagonal boron nitride (h-BN) formed on the Ni(111) surface to investigate both the valence- and conduction-band structures. The thickness of the film studied in this experiment was 1 ML. The observed electronic dispersion relations were compared with some theoretical ones reported for bulk h-BN. Among these theoretical calculations, the one by Catellani et al. [Phys. Rev. B 36, 6105 (1987)] is in the best agreement with the present results. We have discussed the strength of the interfacial bond and the influence of this bond upon the electronic states of the monolayer h-BN film on the basis of the observed band structures for the BN film and a film of monolayer graphite formed on Ni(111).

Fine structure of low-energy secondary electron emission spectra in platinum single crystal

Current spectra of angle-resolved secondary electron emission (ARSEE) of a Pt single crystal normal to the surfaces (100), (111) and (110) are calculated. Calculation results are compared to the existing experimental data. It is shown that the fine structure of ARSEE spectra is mainly due to the electron build-up of final states, from which electron emission takes place. The derived method which we have allows to directly restore specific features of band disposition in the electron dispersion law from the treatment of experimental results.

High-energy angle-resolved secondary-electron emission spectroscopy of highly oriented pyrolytic graphite

The angle-resolved secondary-electron emission (ARSEE) spectra of highly oriented pyrolytic graphite have been measured in the 7-70 eV electron kinetic energy range. A peak observed at approximately=59 eV with K/sub ///=1.35-1.65 AA-1 was tentatively explained within a semiempirical approach based on a free electron parabola with an inner potential of -16.2 eV (relative to the vacuum level) which crosses the M point at 59 eV. From the crossing of the free electron parabola with the zone centre peaks at 16 eV and 28.5 eV were independently described. These results suggest that the empty density of states for large kinetic energies are well described by a free electron parabola. In the energy range below 30 eV good agreement was obtained with previously published ARSEE experiments. XPS and AES measurements were performed to verify that the 59 eV peak did no originate from an impurity level. After argon ion bombardment the fine structure observed in the ARSEE spectra of HOPG was completely removed.

The electronic structure of Rhodium: Angle-resolved studies of photoelectron and secondary electron emission

We report electron spectroscopic studies of the Rh(111) surface, with the aim to obtain bulk band-structure information. We have measured normal photoemision using tunable synchrotron radiation in the range of photon energies ħω between 11 eV and 55 eV, and angle-dependent photoemission along the ΓLUX and ΓLKL azimuths using the He resonance lines (ħω=21.2 eV, 40.8 eV). To complement these data, we studied angleresolved secondary electron emission after excitation with electrons and photons. We derive parts of the one-electron energy dispersionE(k) along ΓL, and determine the energies of several bulk critical points (in eV):E(Γ> 7+/Γ8+)=−2.75±0.10,E(Γ> 8+=−0.85±0.10,E(Γ> 7−=16.1±0.5,E(Γ> 6−/Γ> 8−)=20.5±0.5,E(X7+)=−5.0±0.1,E(L6+)=−5.6±0.5,E(L6+/L4++5+)=−2.65±0.10,E(L6+)=9.0±0.5 eV. Our results are compared to several available band structure calculations.

Fine structure of angle-resolved secondary electron emission spectra in silicon

Experimental measurements and theoretical calculations of Si(100) and Si(110) angle-resolved secondary electron emission spectra are presented. We doubly differentiate the spectra by the energy E to extract a fine structure and show its correlation with the electron energy band boundaries. Energy broadening of the boundaries depending on electron lifetime τ (E) leads to decreasing of the fine structure amplitude as τ 2(E). This allows us to obtain from the experiment an energy dependence of the electron mean free path in the energy range 2–15 eV.

Unoccupied-electronic-band structure of graphite studied by angle-resolved secondary-electron emission and inverse photoemission.

Angle-resolved inverse photoemission spectroscopy (ARIPES) and angle-resolved secondary-electron emission spectroscopy (ARSEES) have been performed for graphite to establish experimentally the unoccupied-electronic-band structure as well as to study the difference between the two techniques. Remarkable differences have been found in the experimental two-dimensional band structures obtained by the two methods. The experimental results have been compared with the two different band calculations by R. C. Tatar and S. Rabii [Phys. Rev. B 25, 4126 (1982)] and by N. A. W. Holzwarth, S. G. Louie, and S. Rabii [Phys. Rev. B 26, 5382 (1982)] with special attention to the energy position of the three-dimensional interlayer band. The possible origin of the difference between ARIPES and ARSEES has also been discussed.

Secondary-electron emission and electron-energy-loss results on graphite single crystals.

Unoccupied states of graphite single crystals have been probed in a wide energy range. Angle-resolved secondary-electron emission excited by electrons has been used to map out the unoccupied bands above the vacuum level. Highly dispersive structures are observed in the 0--30-eV kinetic-energy range of the electron yield collected along the \ensuremath{\Gamma}--M and \ensuremath{\Gamma}--K symmetry directions of the two-dimensional Brillouin zone. The results are in good agreement with recent band-structure calculations. The angular dependence of plasmons has been investigated and clear effects related to graphite anisotropy are observed and interpreted.

Theory of the fine structure in secondary electron emission from Cu(001) c(2 × 2)− Cl

The fine structure in angle-resolved secondary electron emission spectra from the adsorbate system Cu(001) c(2 × 2)− Cl has been calculated for various off-normal emission angles by means of a layer-KKR multiple scattering theory. Close agreement with experimental data is found as regards the existence, energy position and angle dependence of features. Their physical origin is revealed by varying the adsorbate-substrate spacing and by calculating separately the scattering contributions from substrate and adsorbate and their interference. Features are thus identified as due to (a) substrate bulk bands (near critical points) (b) adsorbate resonances and (c) interference between substrate and adsorbate scattering.

Angle-resolved secondary electron investigation of graphite: A band mapping

Angle-resolved secondary electron emission excited by electrons has been used to map out the unoccupied bands of graphite single crystal. Highly dispersive structures are observed in the low kinetic energy region (0–30 eV) of the electron yield collected along the Γ-M and Γ-K lines of the two-dimensional Brillouin zone. The experimental bands are in very good agreement with recent band structure calculations. The different behaviour of the observed dispersion lines along Γ-M and Γ-K supports that effects due to single crystal of graphite are detected. Particularly, the splitting of the 2s2p xy band (∼ 3 eV kinetic energy) along the Γ-M direction has been clearly observed for the first time. The use of angular resolved spectroscopy of secondary electrons for investigating the band structures of two-dimensional systems above the vacuum level has been established as a powerful and straightforward technique.

On the analysis of secondary electron emission spectra

Angle-resolved secondary electron emission (SEE) spectra exhibit, superimposed on an intense cascade background of multiply scattered electrons, fine-structural details which contain information on both bulk and surface properties of the emitting sample. However, the interpretation of the fine structure is not at all clear at present from an experimental point of view. To gain more insight, we have studied SEE from Cu(001)c(2 × 2)-Cl which serves as a model system since both the geometrical as well as the electronic structure are well known from other investigatios. All SEE features observed may be interpreted by emission either out of excited substrate bulk bands or out of adsorbate-induced surface resonance states. However, an interpretation of the “surface resonance” features in terms of emission out of substrate bulk bands and subsequent adsorbate-induced surface umklapp processes works equally well. Some consequences for future work are considered.

Densities of states in the unoccupied bands of palladium in the region of ΓL

Features in the angle-resolved secondary electron emission from a Pd(111) surface are compared with regions of high one-dimensional densities of states in calculated band structures. Earlier measurements at normal exit are confirmed and extended over a range of angles away from ΓL and a new feature which disperses rapidly with angle is detected; very good agreement with an RAPW band structure is found.

Valence-band structure of silver alongΛfrom angle-resolved photoemission

Angle-resolved normal photoemission spectra of Ag(111) are reported for photon energies $h\ensuremath{\nu}=6\ensuremath{-}32$ eV, obtained by the use of synchrotron radiation. Very good agreement was found with the theoretical relativistic-augmented-plane-wave band structure of Christensen. Experimental dispersion relations along $\ensuremath{\Lambda}$ were derived and tabulated. The $d$-band spacings at $\ensuremath{\Gamma}$ were determined for the first time and were interpreted to yield the "ligand field" parameters $10Dq=0.865\ifmmode\pm\else\textpm\fi{}0.027$ eV, $\ensuremath{\xi}(4d)=0.232\ifmmode\pm\else\textpm\fi{}0.011$ eV. These parameters, which are meaningful only at $\ensuremath{\Gamma}$, are in excellent agreement with Christensen's band structure. The spin-orbit parameter is very close to the free-atom value, and the positive sign of $10Dq$ indicates attractive interaction of the electrons with neighbor ion cores. Two pieces of evidence for indirect transitions via thermal broadening were noted. A resonance at $h\ensuremath{\nu}=22$ eV in the band-4 (${\ensuremath{\Gamma}}_{7}$) intensity was attributed to a flat region in band $7 ({\ensuremath{\Gamma}}_{6})$. The flatness in band 7 (and pershaps band 8) also yields an effect equivalent to the angle-resolved secondary electron emission phenomenon of Willis and co-workers. In this case electrons moving in the [111] direction are scattered down into band 7 and trapped, yielding a peak at 17-eV kinetic energy.

Secondary electron emission from tungsten. Observation of the electronic structure of the semi-infinite crystal

The fine structure in the angle-resolved secondary electron emission (SEE) spectra can be interpreted fully in terms of the electronic structure of the semi-infinite crystal. A large number of elements of structure can be related to the bulk band structure. In addition, extra features are unambiguously interpreted in terms of surface-specific electronic properties. The bulk contributions to the SEE structure can be interpreted by means of a band model by assuming conservation of momentum parallel to the surface during emission. Peaks in the SEE spectra of bulk origin relate, concerning their spectral position, to high density of states regions in K-space. All such elements of structure have been identified, and the locations in K space which contribute have been determined. The observation of bulk properties as structure in the SEE spectra is in fact a consequence of the quantum-mechanical matching at the surface. In a simple mode, neglecting this matching, the density-of-states contributions fold out of the expression for the emitted current. The matching conditions, which are discussed in a muffin-tin model, also imply that a number of surface-specific effects are observed. These are referred to as 'anomalies'. Four 'types' of 'anomalies' are discussed.

Adsorbate-induced surface resonances on tungsten at energies above the vacuum level

In angle-resolved secondary electron emission normal to a W(110) surface, spectral structure, due to adsorbate-induced (i.e. “extrinsic”) surface resonances lying above the vacuum level, is resolved when the energy of the emitted electron corresponds to a gap in the energy bands of the substrate. Results are presented for ordered monolayers of O 2 and CO. The adsorbate resonances are interpreted in terms of excited two-dimensional Bloch states produced by the lattice periodicity of the adsorbate.

Angle-Resolved Secondary-Electron—Emission Spectra from Si(111)7×7 Surface States

In angle-resolved secondary-electron---emission spectra from a silicon (111)7\ifmmode\times\else\texttimes\fi{}7 surface there are two series of peaks which are not in the spectra of the unreconstructed (111)1\ifmmode\times\else\texttimes\fi{}1 surface. These peaks are interpreted by a two-step model; electrons are excited into surface-state resonances lying above the vacuum level, from which states they emerge into the vacuum. The dispersion curves of these states are presented and discussed.

Inelastic low-energy electron scattering from solid surfaces and adsorbed species. The observation of adsorbate surface resonances

Results are presented for angle-resolved secondary-electron emission normal to a W(110) surface for the special case of “band gap emission”, which arises when the energy of the emitted electron corresponds to an energy gap in the final states' bands located along a principal symmetry direction. Under such conditions, surface enhanced emission occurs, and scattering out of adsorbate-induced (i.e., “extrinsic”) surface states is resolved. Spectra are presented for the specific cases of O2 and CO adsorption on W(110) which, together with parallel LEED studies, show that such effects only appear in the presence of ordered adsorbate monolayers, and are extremely sensitive to the surface unit cell dimensions of the adsorbate–substrate superstructure. The results are interpreted in terms of adsorbate surface resonances associated with excited two-dimensional surface Bloch states produced by the periodicity of the adsorbate layers.