Hemispherical Energy Analyzer Research Articles

Ru(0001) and SiO2/Ru(0001): XPS study

X-ray photoelectron spectroscopy (XPS) is used to analyze the chemistry of the Ru(0001) film surface and the Ru/SiO2 interfacial region at different annealing conditions. The XPS spectra are collected under ultrahigh vacuum (base pressure of ∼5 × 10−10 Torr) condition using a SPECS electron spectrometer with a PHOIBOS 100 hemispherical energy analyzer and an XR 50 Al Kα x-ray source (1486.67 eV). High-resolution spectra of O 1s, Ru 3d/C 1s, and Si 2p together with survey scans are presented. The presence of 1 × 1 low energy diffraction pattern, collected from a 950 °C Ar/H2 step-annealed Ru(0001) sample, confirms the hexagonal periodicity of Ru(0001) surfaces.

Compact setup for spin-, time-, and angle-resolved photoemission spectroscopy.

We present a compact setup for spin-, time-, and angle-resolved photoemission spectroscopy. A 10 kHz titanium sapphire laser system delivers pulses of 20 fs duration, which drive a high harmonic generation-based source for ultraviolet photons at 21 eV for photoemission. The same laser also excites the sample for pump-probe experiments. Emitted electrons pass through a hemispherical energy analyzer and a spin-filtering element. The latter is based on spin-polarized low-energy electron diffraction on an Au-passivated iridium crystal. The performance of the measurement system is discussed in terms of the resolution and efficiency of the spin filter, which are higher than those for Mott-based techniques.

Electron emission spectra by charging analysis

The energy spectra of electrons emitted due to electron irradiation are essential in relevant scientific and technological fields. While the energy spectra of conductor materials are generally acquired using hemispherical electron energy analyzers, technical difficulties remain in the acquisition of these spectra when dielectric materials are involved, as these materials charge due to electron irradiation. In this work, the energy spectra were obtained taking advantage of the unavoidable effect of charging on the energy of the secondary electrons emitted from non-conductive surfaces. A novel time-resolved low-dose irradiation method was applied for the first time to obtain the energy spectra of copper, silver, and gold samples, with the results compared with the spectra measured by the hemispherical energy analyzer. This new method could be promising for obtaining the energy spectra of dielectric materials.

An investigation of photo-double ionization in benzene

SynopsisA photo-double ionization (PDI) coincidence technique has been used to study electron correlation as a function of photon impact energy on Benzene (C6H6). The simultaneous emission of two photoelectrons from the benzene π orbital has been measured using two sets of hemispherical electron energy analysers. Angle resolved single and coincidence photoelectron spectra are reported, providing new data on the PDI mechanism.

Imaging properties of hemispherical electrostatic energy analyzers for high resolution momentum microscopy

Hemispherical deflection analyzers are the most widely used energy filters for state-of-the-art electron spectroscopy. Due to the high spherical symmetry, they are also well suited as imaging energy filters for electron microscopy. Here, we review the imaging properties of hemispherical deflection analyzers with emphasis on the application for cathode lens microscopy. In particular, it turns out that aberrations, in general limiting the image resolution, cancel out at the entrance and exit of the analyzer. This finding allows more compact imaging energy filters for momentum microscopy or photoelectron emission microscopy. For instance, high resolution imaging is possible, using only a single hemisphere. Conversely, a double pass hemispherical analyzer can double the energy dispersion, which means it can double the energy resolution at certain transmission, or can multiply the transmission at certain energy resolution.

OPTIMAL VOLTAGE PROFILE FOR HEMISPHERICAL DEFLECTOR ENERGY ANALYZERS USING GENETIC ALGORITHM

In this study, the genetic algorithm (GA) method is used to optimize the hemispherical deflector analyzer, which can analyze according to the energies of charged particles. The GA inspired by evolutionary codes consists of the population of individuals. At each step, the GA, selects the population individuals as parents and uses them to produce children for the next generation. Thus throughout generations, the population evolves to an optimal solution. The purpose of this study is to achieve the best voltage profile for the 180° hemispherical energy analyzers through the voltage equations of the energy analyzer. The best voltage values ​​for the 180˚ hemispherical deflector analyzer, which is the most used analyzer in experimental studies, are found with high accuracy in this study. Optimization of a complex electron optical system, such as an analyzer system with a GA, has been achieved using voltage equations and has been found to work very well given the difficulty of the problem. In contrast to other techniques in the literature, the best voltage profile is obtained in a short time by means of the proposed GA method.

Influence of Charge Transport Layers on Open-Circuit Voltage and Hysteresis in Perovskite Solar Cells

Summary Perovskite materials have experienced an impressive improvement in photovoltaic performance due to their unique combination of optoelectronic properties. Their remarkable progression, facilitated by the use of different device architectures, compositional engineering, and processing methodologies, contrasts with the lack of understanding of the materials properties and interface phenomena. Here we directly target the interplay between the charge-transporting layers (CTLs) and open-circuit potential ( V OC ) in the operation mechanism of the state-of-the-art CH 3 NH 3 PbI 3 solar cells. Our results suggest that the V OC is controlled by the splitting of quasi-Fermi levels and recombination inside the perovskite, rather than being governed by any internal electric field established by the difference in the CTL work functions. In addition, we provide novel insights into the hysteretic origin in perovskite solar cells, identifying the nature of the contacts as a critical factor in defining the charge accumulation at its interface, leading to either ionic, electronic, or mixed ionic-electronic accumulation.

A Full Electrostatic High-Brightness Slow Positron Beam Apparatus Using a Remoderator in Conjunction with a Hemispherical Energy Analyzer

The development of a full electrostatic slow positron beam apparatus at the Tokyo University of Science is described. The performance of different types of material as the primary moderator and the energy resolution of the beam are detailed.

Laser Multiphoton Ionization of Tetrakis(dimethylamino)ethylene

The tetrakis(dimethylamino)ethylene (TDAE) molecule possesses the lowest known molecular ionization potential (<5.4 eV) and exhibits an intense Rydberg series between the first and second ionization limit (∼14 eV). The ionization of TDAE using multiphoton ionization photoelectron spectroscopy was carried out using laser light at a variety of wavelengths with a hemispherical energy analyzer. Interestingly, photoelectron signal due to direct two-photon ionization was not seen, rather ionization from a fluorescent charge-transfer state located ∼2.5 eV below the ionization limit was evident and in general agreement with a previous study. In addition, a second intense peak exists corresponding to thermal energy electrons. Measurements of the angular distribution for the electrons due to photoionization from the intermediate state are peaked along the electric field vector of the laser and the thermal electrons direction is independent of this angle. From this, we propose that the thermal peak is most likely due to thermionic emission initiated through excitation of a known long-lived Rydberg state at ∼6.5 eV. Alternately, we speculate that excitation leading to thermionic emission could result from a "collective" excitation mechanism.

Photonically excited electron emission from modified graphitic nanopetal arrays

Efficient electron emission for energy conversion requires a low work function and a stable emitter material. The work function of graphene-based carbon materials can decrease significantly by intercalation with alkali metals, thus increasing their emission current. In this work, electron emission from potassium-intercalated carbon nanosheet extensions grown on electrode graphite is investigated. These petal-like structures, composed of 5–25 layers of graphene, are synthesized using microwave plasma chemical vapor deposition. Samples are intercalated with potassium, and a hemispherical energy analyzer is used to measure the emission intensity caused by both thermal and photonic excitation. The emission from the potassium-intercalated structures is found to consistently decrease the work function by 2.4 to 2.8 eV relative to non-intercalated samples. High emission intensity induced by photonic excitation from a solar simulator, with a narrow electron energy distribution relative to established theory, suggests that electron scattering decreases emitted electron energy as compared to surface photoemission. A modified photoemission theory is applied to account for electron scattering, and the sample work function and mean number of scattering events are used as parameters to fit theory to experimental data. The thermal stability of the intercalated nanopetals is investigated, and after an initial heating and cooling cycle, the samples are stable at low temperatures.

A hemispherical retarding field energy analyzer to characterize spatially and angularly extended electron beams

We have designed and built a hemispherical retarding field energy analyzer in order to facilitate characterization of large area electron emitters (typically field emitter arrays with active areas up to 1 cm2) with large angular aperture. A complete numerical model of the analyzer has been built, including perturbations due to secondary particles, in order to determine the analyzer performances. The analyzer energy resolution is better than 100 meV for an energy range up to 120 eV. The analyzer has a global field of view of 112° and allows measurements of the energy distribution of the beam as a function of the emission angle, as well as measurements of the beam intensity profile along any section of the beam. We have successfully used the analyzer to characterize the electron beam emitted by 1 cm2 Mo microtips-based field emitter arrays.

The influence of surface conditioning on the thermionic electron emission from polycrystalline diamond films

AbstractThermionic electron emission (TEE) properties of as deposited, bare surface, in situ hydrogen‐terminated and in situ oxygen‐terminated polycrystalline diamond (Poly‐Di) films surfaces are reported. Continuous Poly‐Di films of a thickness of ∼200 nm grown on B‐doped Si substrates by hot‐filament chemical vapor deposition (HF CVD) were used. The TEE measurements were carried out in the 500–650 °C temperature range by recording the energy spectra of the electrons emitted using a hemispherical energy analyzer under ultra‐high vacuum (UHV) conditions. As‐deposited Poly‐Di surfaces may exhibit variations in their TEE properties depending on history and surface conditioning, thus indicating the need to control the diamond surface chemistry to obtain reproduceable TEE properties. Annealing to 1000 °C results in significant decrease of the TEE intensities, related to thermal desorption of hydrogen and creation of a bare surface. in situ annealing to 1000 °C followed by in situ hydrogenation of the Poly‐Di film surface results in significant enhancement of the TEE. Several cycles of the last procedure show that the TEE of hydrogen‐terminated Poly‐Di surface can be restored, whereas minor decrease in TEE intensity can be associated with slow surface degradation of the Poly‐Di surface following this conditioning. It is suggested that the TEE from hydrogen‐terminated Poly‐Di film may involve two mechanisms. Oxygenation of the bare Poly‐Di film results in low TEE intensities, very similar to those obtained from the bare Poly‐Di film surface, indicating that the TEE process from Poly‐Di film is somehow insensitive to the difference in the surface potential induced by oxygenation and bare surface conditionings.

Nanoscale 3-D $(E, k_{x}, k_{y})$ band structure imaging on graphene and intercalated graphene

An x-ray photoemission electron microscope (X-PEEM) equipped with a hemispherical energy analyzer is capable of fast acquisition of momentum-resolved photoelectron angular distribution patterns in a complete cone. We have applied this technique to observe the 3-D (E, kx, ky) electronic band structure of zero-, one-, and two-monolayer (ML) graphene grown ex situ on 6H-SiC(0001) substrates where a carbon buffer layer (zero ML) forms underneath the graphene layer(s). We demonstrate that the interfacial buffer layer can be converted into quasi-free-standing graphene upon intercalation of Li atoms at the interface and that such a graphene is structurally and electronically decoupled from the SiC substrate. High energy and momentum resolution of the X-PEEM, along with short data acquisition times from submicrometer areas on the surface demonstrates the uniqueness and the versatility of the technique and broadens its impact and applicability within surface science and nanotechnology.

He I Ultraviolet Photoelectron Spectroscopy of Benzene and Pyridine in Supersonic Molecular Beams Using Photoelectron Imaging

We performed He I ultraviolet photoelectron spectroscopy (UPS) of jet-cooled aromatic molecules using a newly developed photoelectron imaging (PEI) spectrometer. The PEI spectrometer can measure photoelectron spectra and photoelectron angular distributions at a considerably higher efficiency than a conventional spectrometer that uses a hemispherical energy analyzer. One technical problem with PEI is its relatively high susceptibility to background electrons generated by scattered He I radiation. To reduce this problem, we designed a new electrostatic lens that intercepts background photoelectrons emitted from the repeller plate toward the imaging detector. An energy resolution (ΔE/E) of 0.735% at E = 5.461 eV is demonstrated with He I radiation. The energy resolution is limited by the size of the ionization region. Trajectory calculations indicate that the system is capable of achieving an energy resolution of 0.04% with a laser if the imaging resolution is not limited. Experimental results are presented for jet-cooled benzene and pyridine, and they are compared with results in the literature.

Energy band dispersion in photoemission spectra of argon clusters

Using photoemission we have investigated free argon clusters from a supersonic nozzle expansion in the photon energy range from threshold up to 28 eV. Measurements were performed both at high resolution with a hemispherical electrostatic energy analyser and at lower resolution with a magnetic bottle device. The latter experiments were performed for various mean cluster sizes. In addition to the ≈1.5 eV broad 3p-derived valence band seen in previous work, there is a sharper feature at ≈15 eV binding energy. Surprisingly for non-oriented clusters, this peak shifts smoothly in binding energy over the narrow photon energy range 15.5–17.7 eV, indicating energy band dispersion. The onset of this bulk band-like behaviour could be determined from the cluster size dependence.

Physical characterization of molybdenum microtips field emitter arrays

We used a dedicated hemispherical energy analyzer to measure energetic and angular distributions of electrons emitted from molybdenum microtips integrated in a 1 cm2 field emitter array designed by the CEA/LETI laboratory. Such cathodes typically deliver about 25 mA at an extraction voltage of 100 V, and are studied in order to replace heated wires as electron sources for space applications. We find that the energy distribution of the beam strongly depends on the extraction voltage, and is therefore expected to vary across the emission lifetime of the device, at a rate depending both on the alteration of the resistive structure with time and on the fate of adsorbed contaminants at the tip surface. A semi-empirical model of the emitters is proposed and used to determine parameters of energetic and angular distributions. The energy dispersion of the beam is found to increase from 2 eV ± 20% up to 9 eV ± 20% eV, for extraction voltages varying from 40 to 100 V. The mean angular dispersion of the beam is found to be 42° ± 20% when a null electric field is set at the grid extraction surface.

An electrostatic slow positron beam apparatus for positron scattering experiments

An electrostatic slow positron beam system has been developed for positron scattering studies. The apparatus uses a remoderator in reflection geometry in conjunction with an electrostatic hemispherical energy analyzer for the brightness enhancement. It is possible to determine absolute total scattering cross section values under magnetic field-free conditions down to a few mG. Measurement of the energy distribution of the positrons that are reemitted from the remoderator and total cross sections for positron-He scattering are presented.

Time- and energy resolved photoemission electron microscopy-imaging of photoelectron time-of-flight analysis by means of pulsed excitations

The present work enlightens the developments in time- and energy resolved photoemission electron microscopy over the past few years. We describe basic principles of the technique and demonstrate different applications. An energy- and time-filtering photoemission electron microscopy (PEEM) for real-time spectroscopic imaging can be realized either by a retarding field or hemispherical energy analyzer or by using time-of-flight optics with a delay line detector. The latter method has the advantage of no data loss at all as all randomly incoming particles are measured not only by position but also by time. This is of particular interest for pump–probe experiments in the femtosecond and attosecond time scale where space charge processes drastically limit the maximum number of photoemitted electrons per laser pulse. This work focuses particularly on time-of-flight analysis using a novel delay line detector. Time and energy resolved PEEM instruments with delay line detectors enable 4D imaging ( x, y, Δ t, E Kin) on a true counting basis. This allows a broad range of applications from real-time observation of dynamic phenomena at surfaces to fs time-of-flight spectro-microscopy and even aberration correction. By now, these time-of-flight analysis instruments achieve intrinsic time resolutions of 108 ps absolute and 13.5 ps relative. Very high permanent measurement speeds of more than 4 million events per second in random detection regimes have been realized using a standard USB2.0 interface. By means of this performance, the time-resolved PEEM technique enables to display evolutions of spatially resolved (<25 nm) and temporal sliced images life on any modern computer. The method allows dynamics investigations of variable electrical, magnetic, and optical near fields at surfaces and great prospects in dynamical adaptive photoelectron optics. For dynamical processes in the ps time scale such as magnetic domain wall movements, the time resolution of the delay line detectors even allows to be used directly for real-time experiments as well.

Correlated positron-electron emission from LiF (100)

Using two hemispherical energy analyzers with transfer lenses we have observed positron-electron pair emission from a surface upon low energy positron impact. An 85 eV positron beam from the NEPOMUC positron source was incident upon a LiF(100) surface and emitted electrons were detected in coincidence with the positrons. Energy analysis of the positron-electron pair confirms that electrons are emitted from the valence band upon positron impact. Electron-electron pairs emitted upon electron and positron impact are also discussed. The information obtainable from these experiments has the potential to provide important insights into electron-electron correlation in solids.

A novel time‐of‐flight spectrometer for PAES

AbstractPositron annihilation induced Auger electron spectroscopy (PAES) uses the annihilation of a positron with an electron of the inner shell of atoms whereas X‐rays or high‐energy electrons initiate the ionisation in conventional AES. Advantages of PAES are the extremely high surface sensitivity and the higher signal to noise ratio. In order to benefit from PAES one has to use a low‐energy positron beam of high intensity, which is available at NEPOMUC at the new Munich research reactor FRM‐II. The energy of the Auger electrons is mostly analysed by hemispherical energy analysers. Due to their small solid angle (typically ≥0.1%) and the necessity to scan over certain energy ranges in ≥1eV steps one still needs several hours using an intense positron source like NEPOMUC (at the FRM‐II) or even weeks at the lab‐beam based on a beta‐emitter to obtain an Auger spectrum. For this reason, a novel time‐of‐flight spectrometer (TOF‐PAES) was developed, which efficiently collects the Auger electrons emitted in a solid angle of 2π around the sample, and hence reduces the measurement time to less than one hour. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)