Photoelectron Emission Yield Research Articles

Photoelectron Emission Yield of Au Film: Theoretical Calculation and Measurement

Spacecraft in orbit are exposed to intense solar radiation in the space environment, causing the material surface to be charged and damaged. The incident electrons, ions, and photons on the surface of spacecraft materials produce secondary electrons, backscattered electrons, photoelectrons, and so on. The damage to the material caused by the emitted photocurrent is more serious when the incident energy is higher than the photoelectric threshold. In view of the significant threat of particle radiation to the charge protection and operational life of spacecraft materials, in this article, a theoretical model of the photoelectron emission yield (PEEY) is studied, and a test system of metal materials under ultraviolet light is developed. First, we derive a three-step model of the photoemission of metal materials and calculate the PEEY spectrum of Au. Second, the optical system is built by using a deuterium lamp, a monochromator, and an aperture. The installation structure of the photodiode is designed, and the photon number is obtained by measuring the incident luminous flux. Third, a central through-hole microchannel plate is used and a voltage division circuit module is designed to realize effective collection of photoelectrons emitted from the sample surface. This approach realizes a detectable range for an Au film from 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−7</sup> el./ph in the spectral range of 130–258 nm. The reliability of the test system is proved by comparing the experimental results with the theoretical results.

Strong-field nano-optics

The present status and development of strong-field nano-optics, an emerging field of nonlinear optics, is discussed. A nonperturbative regime of light-matter interactions is reached when the amplitude of the external electromagnetic fields that are driving a material approach or exceed the field strengths that bind the electrons inside the medium. In this strong-field regime, light-matter interactions depend on the amplitude and phase of the field, rather than its intensity, as in more conventional perturbative nonlinear optics. Traditionally such strong-field interactions have been intensely investigated in atomic and molecular systems, and this has resulted in the generation of high-harmonic radiation and laid the foundations for contemporary attosecond science. Over the past decade, however, a new field of research has emerged, the study of strong-field interactions in solid-state nanostructures. By using nanostructures, specifically those made out of metals, external electromagnetic fields can be localized on length scales of just a few nanometers, resulting in signficantly enhanced field amplitudes that can exceed those of the external field by orders of magnitude in the vicinity of the nanostructures. This leads not only to dramatic enhancements of perturbative nonlinear optical effects but also to significantly increased photoelectron yields. It resulted in a wealth of new phenomena in laser-solid interactions that have been discovered in recent years. These include the observation of above-threshold photoemission from single nanostructures, effects of the carrier-envelope phase on the photoelectron emission yield from metallic nanostructures, and strong-field acceleration of electrons in optical near fields on subcycle timescales. The current state of the art of this field is reviewed, and several scientific applications that have already emerged from the fundamental discoveries are discussed. These include, among others, the coherent control of localized electromagnetic fields at the surface of solid-state nanostructures and of free-electron wave packets by such optical near fields, resulting in the creation of attosecond electron bunches, the coherent control of photocurrents on nanometer length and femtosecond timescales by the electric field of a laser pulse, and the development of new types of ultrafast electron microscopes with unprecedented spatial, temporal, and energy resolution. The review concludes by highlighting possible future developments, discussing emerging topics in photoemission and potential strong-field nanophotonic devices, and giving perspectives for coherent ultrafast microscopy techniques. More generally, it is shown that the synergy between ultrafast science, plasmonics, and strong-field physics holds promise for pioneering scientific discoveries in the upcoming years. (Less)

Electronic control of ultrafast field emission in carbon nanotube gaps

The electronic control of an ultrafast tunneling electron emission was demonstrated in the nanogap of a single-walled nanotube (SWNT) when irradiated by a femtosecond laser pulse. The SWNT apex possesses a nanoscale morphology with a large damage threshold and thus enabled the achievement of a large emission rate. More importantly, the DC field-emission characteristics varied when the gate bias was changed. This was analyzed in terms of the change in the effective barrier height and enhancement factors. Photoinduced electron emission was observed when the gap area was illuminated with a femtosecond laser centered at a wavelength of 800 nm. As the laser power was increased, a saturated tunneling current was observed, reaching more than 10 electrons per pulse. Finally, the photoelectron emission yield was tuned with the help of gate-induced variations in the electronic band structures of the SWNTs.

Visible sub-band gap photoelectron emission from nitrogen doped and undoped polycrystalline diamond films

In this study the origin of visible sub-band gap photoelectron emission (PEE) from polycrystalline diamond films is investigated. The PEE yields as a function of temperature were studied in the wavelengths range of 360–520nm. Based on the comparison of electron emission yields from diamond films deposited on silicon and molybdenum substrates, with different thicknesses and nitrogen doping levels, we suggested that photoelectrons are generated from nitrogen related centers in diamond. Our results show that diamond film thickness and substrate material have no significant influence on the PEE yield. We found that nanocrystalline diamond films have low electron emission yields, compared to microcrystalline diamond, due to the presence of high amount of defects in the former, which trap excited electrons before escaping into the vacuum. However, the low PEE yield of nanocrystalline diamond films was found to increase with temperature. The phenomenon was explained by the trap assisted photon enhanced thermionic emission (ta-PETE) model. According to the ta-PETE model, photoelectrons are trapped by shallow traps, followed by thermal excitation at elevated temperatures and escape into the vacuum. Activation energies of trap levels were estimated for undoped nanocrystalline, undoped microcrystalline and N-doped diamond films using the Richardson-Dushman equation, which gives 0.13, 0.39 and 0.04eV, respectively. Such low activation energy of trap levels makes the ta-PETE process very effective at elevated temperatures.

Photoelectron emission yield experiments on evolution of sub-gap states in amorphous In-Ga-Zn-O thin films with post deposition hydrogen treatment

Total photoyield emission spectroscopy (TPYS) was applied to study the evolution of sub-gap states in hydrogen-treated amorphous In-Ga-Zn-O (a-IGZO) thin films. The a-IGZO thin films were subjected to hydrogen radicals and subsequently annealed in ultra-high vacuum (UHV) conditions. A clear onset of the electron emission was observed at around 4.3 eV from the hydrogen-treated a-IGZO thin films. After successive UHV annealing at 300 °C, the onset in the TPYS spectra was shifted to 4.15 eV, and the photoelectron emission from the sub-gap states was decreased as the annealing temperature was increased. In conjunction with the results of thermal desorption spectrometer, it was deduced that the hydrogen atoms incorporated in the a-IGZO thin films induced metastable sub-gap states at around 4.3 eV from vacuum level just after the hydrogenation. It was also suggested that the defect configuration was changed due to the higher temperature UHV annealing, and that the hydrogen atoms desorbed with the involvement of Zn atoms. These experiments produced direct evidence to show the formation of sub-gap states as a result of hydrogen incorporation into the a-IGZO thin films.

Direct coupling of photonic modes and surface plasmon polaritons observed in 2-photon PEEM

We report the direct microscopic observation of optical energy transfer from guided photonic modes in an indium tin oxide (ITO) thin film to surface plasmon polaritons (SPP) at the surfaces of a single crystalline gold platelet. The photonic and SPP modes appear as an interference pattern in the photoelectron emission yield across the surface of the specimen. We explore the momentum match between the photonic and SPP modes in terms of simple waveguide theory and the three-layer slab model for bound SPP modes of thin metal films. We show that because the gold is thin (30-40 nm), two SPP modes exist and that momentum of the spatially confined asymmetric field mode coincides with the dominant mode of the ITO waveguide. The results demonstrate that photoemission electron microscopy (PEEM) can be an important tool for the observation of photonic to SPP interactions in the study of integrated photonic circuits.

Work function tuning of tin-doped indium oxide electrodes with solution-processed lithium fluoride

Solution-processed lithium fluoride (sol-LiF) nanoparticles synthesized in polymeric micelle nanoreactors enabled tuning of the surface work function of tin-doped indium oxide (ITO) films. The micelle reactors provided the means for controlling surface coverage by progressively building up the interlayer through alternating deposition and plasma etch removal of the polymer. In order to determine the surface coverage and average interparticle distance, spatial point pattern analysis was applied to scanning electron microscope images of the nanoparticle dispersions. The work function of the sol-LiF modified ITO, obtained from photoelectron emission yield spectroscopy analysis, was shown to increase with surface coverage of the sol-LiF particles, suggesting a lateral depolarization effect. Analysis of the photoelectron emission energy distribution in the near threshold region revealed the contribution of surface states for surface coverage in excess of 14.1%. Optimization of the interfacial barrier was achieved through contributions from both work function modification and surface states.

Total Photoelectron Emission Yield Spectroscopy on Diamond Surfaces

Diamond has unique physical and chemical properties such as the highest thermal conductivity. Especially, the negative electron affinity (NEA), where the conduction band minimum is higher than the vacuum level, is highlighted for a long time as to be one of the best candidates for the electron emitters. We have developed growth techniques of high quality diamond films, and total photoelectron emission yield spectroscopy (TPYS) equipment to be able to investigate NEA on diamond. Now it is possible to determine NEA quantitatively, and characterize electronic properties of surface and bulk of diamond by means of NEA. According to these results, we find that free-exciton derived electron emission plays an important role in electron emission from diamond with the NEA surface, and finally realizes a NEA electron emitter with diamond p-n junction diodes. The diamond semiconductor, that makes it possible to study “excitonics” at room temperature, will be one of the most exciting materials for the next surface science.

Surface defect states analysis on diamond by photoelectron emission yield experiments

Total photoyield spectroscopy (TPYS) was applied on partially hydrogen-terminated IIa (001) diamond surfaces, to investigate the variation of the negative electron affinity (NEA) and formation of defects by hydrogen evaporation. According to the results, we find that the amplitude of negative electron affinity is not changed by partial hydrogen evaporation. The negative electron affinity of residual hydrogen-terminated areas is maintained at around − 1.0 eV. Surface states are generated by partial hydrogen desorption at around 1.6 ± 0.1 eV above the valence band maximum. Hydrogen-free surfaces exhibited positive electron affinity. These experiments demonstrate that TPYS is a promising technique for surface defect characterization.

Interaction of Evaporated Nickel Nanoparticles with Highly Oriented Pyrolytic Graphite: Back-bonding to Surface Defects, as Studied by X-ray Photoelectron Spectroscopy

Ni nanoparticle evaporation onto highly oriented pyrolytic graphite (HOPG), with low and high surface defect densities, has been studied in situ by X-ray photoelectron spectroscopic (XPS) techniques, including binding energy shifts, Auger parameters, and Wagner plots. Ni nucleates at defect sites, whether initially present or those introduced by Ar+ bombardment, with the formation of spherical nanoparticles, which adhere strongly through Ni/HOPG back-bonding. The variation of the C 1s peak intensity with Ni coverage suggests that the photoelectron emission yield may be enhanced at lower Ni coverages, due to Ni nanoparticle-induced electron localization and work function reduction at the HOPG surface, which is evidence of such back-bonding.

Photoelectron Emission Technique for the Surface Analysis of Silicon Wafer Covered with Oxide Film

AbstractA new analysis method using the photoelectron emission yield spectra near threshold has been developed to characterize solid surfaces exposed to ambient air. The application of the analysis method to the yield spectra for a Si (100) wafer surface covered with a native oxide film revealed that three emission peaks are cumulated on the basic yield spectrum line for the silicon. It was indicated that these peaks have a Gaussian-shaped density distribution and the energy levels are located at 5.37, 5.68, and 6.02 eV below the vacuum level, respectively.

Electronic Properties of Hybrid Organic/Inorganic Langmuir−Blodgett Films Containing CdS Quantum Particles

We investigated the electronic properties of hybrid inorganic/organic thin films where the inorganic components are monodisperse CdS quantum size particles (diameters 2.5 and 5 nm). For this purpose, low-energy photoelectron spectroscopy was applied in which the energy of photoelectrons, ejected from the metal substrate or from the films themselves, is measured. The electronic properties of hybrid Langmuir-Blodgett films were studied, when the films were deposited on gold or silicon surfaces and contained particles of different sizes arranged in layers at different distances from the substrate. For comparison films containing only the organic matrix were examined, so the effect of the quantum particles (QPs) on the electronic properties of the whole film could be resolved. The present study proves the importance of the organization of hybrid structure on its electronic properties. It is clearly demonstrated that the QPs cannot be considered as insolated species and that their electronic properties are affected by the structure of the film as a whole and by the substrate. It is shown that the QP size affects both the photoelectron emission yield and the scattering efficiency of electrons from the QPs.

Photoionization quantum yield for liquid squalane and squalene estimated from photoelectron emission yield

An equation relating photoelectron emission yield to initial photoionization quantum yield in the condensed phase is derived. The equation is applied to the photoelectron emission yields from liquid squalane (C 30H 62, 2,6,10,15,19,23-hexamethyltetracosane) and squalene (C 30H 50, 2,6,10,15,19,23-hexamethyl-2,6,10,14,18,22-tetracosahexaene) previously reported by the authors. On the assumption that the thermalization distance of photoelectrons is of a similar value to that of electrons produced with X-rays the photoionization quantum yields of these liquids in the energy region between 7 and 10.8 eV are estimated. The photoionization quantum yields are evidently less than unity and increase with increasing photon energy. The quantum yield for squalane increases monotonically whereas the one for squalene shows a change in its slope around the photoionization threshold of the σ electrons.

Photoelectron emission spectra of butylamine, ferrocene and benzene solutions compared with simulated spectra using gas-phase UPS

The photoelectron emission cross section as a function of exciting photon energy of the emitter molecule in solution is assumed to be the same as that of a free molecule. The photoelectron emission yield spectra for solutions can then be obtained by simulation calculation using gas-phase UPS spectra. They are compared with the experimental spectra to give the conclusion that the threshold energy determined for solution of ferrocene or benzene exhibiting a narrow first band in UPS corresponds to the vertical ionization process, while that for alkylamine exhibiting a broad band in UPS corresponds to the average value of the energies for vertical and adiabatic process. Polarization energies of the solvent, acetonitrile, determined for these molecules are well predicted by the Born model.

Spin-polarized photoelectron emission study of AlxGa1−xAs alloys grown by molecular beam epitaxy

We report on spin-polarized photoelectron emission and quantum yield measurements performed on AlxGa1−x As alloys (0.08⩽x⩽0.65) which were grown by molecular beam epitaxy and activated in situ to negative electron affinity by exposure to Cs and O2. We investigate the electronic structure of the alloys near the band-gap region. These alloys can be used as high- efficiency spin-polarized photoelectron sources at photon energies matching convenient light sources.

Photoelectron Emission from Aromatic and Metallooraganic Hydrocarbons

Abstract Photoeletron emission from various aromatic hydrocarbon crystals has been studied and some of the factors (space charge, sample preparation) which might influence the phtoelectric threshold have been investigated. The longstanding problem of determining the “true” photoelectric threshold is re-examined, and a semi-empirical expression is derived which adequately accounts for the spectral dependence of the photoelectron emission yield, and at the same time determines the threshold. New photoelectric thresholds for a number of anthracene derivatives are also reported.