Photo-induced Electron Emission Research Articles

Photo-induced Electron Emission of Nanostructured Carbon Thin Film Based Transmission Photocathodes at Different Electric Field

Very thin nanostructured carbon films were deposited on quartz substrate by reactive magnetron sputtering using graphite target and gas mixture of Ar and reactive gas N_2 or N_2+H_2. Film thicknesses were in the range of 20-25 nm. Rutherford backscattering spectrometry and Elastic recoil detection analytical method determined the concentration of elements in the films. Scanning electron microscopy scanned the surface morphology of carbon films. Raman spectroscopy was used for chemical structural properties determination of very thin carbon films. Raman spectra intensities were fitted with Gaussian peaks. The photo-induced (pulsed laser - 266 nm) electron emission properties of very thin nanostructured carbon films were investigated by the measurement of cathode bunch charge at different electric field and calculate quantum efficiency. The influence of different electric field on the photo-induced electron emission characteristics of prepared transmission photocathodes are discussed.

Disentangling surface atomic motions from surface field effects in ultrafast low-energy electron diffraction

Ultrafast low-energy electron diffraction holds potential to provide atomic level details to the surface dynamics controlling processes from surface chemistry to exotic collective effects. Accessing the primary timescales requires subpicosecond excitation pulses to prepare the corresponding nonequilibrium state. The needed excitation for maximum contrast above background invariably leads to photoinduced electron emission with the creation of surface fields that affect diffraction and must be quantified to recover the key structural dynamics. Using 2 keV ultrashort low-energy electron bunches, we investigate this field effect on the ensuing electron distribution in projection imaging and diffraction as a function of excitation intensity. Using a structural model, we demonstrate a quantitative separation of the surface field effect on electron diffraction, enabling isolation of the structural dynamics of interest. Particle trajectory simulations provide insight into the correlation between geometrical characteristics of the charge separated region and the corresponding intensity modulation at the detector.

Modeling of Laser-Induced Plasmon Effects in GNS-DLC-Based Material for Application in X-ray Source Array Sensors.

An important direction in the development of X-ray computed tomography sensors in systems with increased scanning speed and spatial resolution is the creation of an array of miniature current sources. In this paper, we describe a new material based on gold nanostars (GNS) embedded in nanoscale diamond-like carbon (DLC) films (thickness of 20 nm) for constructing a pixel current source with photoinduced electron emission. The effect of localized surface plasmon resonance in GNS on optical properties in the wavelength range from UV to near IR, peculiarities of localization of field and thermal sources, generation of high-energy hot electrons, and mechanisms of their transportation in vacuum are investigated. The advantages of the proposed material and the prospects for using X-ray computed tomography in the matrix source are evaluated.

The Combined Influence of Dopant Species and Surface Termination on the Electronic Properties of Diamond Surfaces

The combined effects of geometrical structure and chemical composition on the diamond surface electronic structures have been investigated in the present study by using high-level theoretical calculations. The effects of diamond surface planes [(111) vs. (100)], surface terminations (H, F, OH, Oontop, Obridge, vs. NH2), and substitutional doping (B, N vs. P), were of the largest interest to study. As a measure of different electronic structures, the bandgaps, work functions, and electron affinities have been used. In addition to the effects by the doping elements, the different diamond surface planes [(111) vs. (100)] were also observed to cause large differences in the electronic structures. With few exceptions, this was also the case for the surface termination species. For example, Oontop-termination was found to induce surface electron conductivities for all systems in the present study (except for a non-doped (100) surface). The other types of surface terminating species induced a reduction in bandgap values. The calculated bandgap ranges for the (111) surface were 3.4–5.7 (non-doping), and 0.9–5.3 (B-doping). For the (100) surface, the ranges were 0.9–5.3 (undoping) and 3.2–4.3 (B-doping). For almost all systems in the present investigation, it was found that photo-induced electron emission cannot take place. The only exception is the non-doped NH2-terminated diamond (111) surface, for which a direct photo-induced electron emission is possible.

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.

Thermally enhanced photoinduced electron emission from nitrogen-doped diamond films on silicon substrates

This work presents a spectroscopic study of the thermally enhanced photoinduced electron emission from nitrogen-doped diamond films prepared on $p$-type silicon substrates. It has been shown that photon-enhanced thermionic emission (PETE) can substantially enhance thermionic emission intensity from a $p$-type semiconductor. An $n$-type diamond/$p$-type silicon structure was illuminated with $400--450$ nm light, and the spectra of the emitted electrons showed a work function less than 2 eV and nearly an order of magnitude increase in emission intensity as the temperature was increased from ambient to $\ensuremath{\sim}400 \ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Thermionic emission was negligible in this temperature range. The results are modeled in terms of contributions from PETE and direct photoelectron emission, and the large increase is consistent with a PETE component. The results indicate possible application in combined solar/thermal energy conversion devices.

Interface and interlayer barrier effects on photo-induced electron emission from low work function diamond films

Nitrogen-doped diamond has been under investigation for its low effective work function, which is due to the negative electron affinity (NEA) produced after surface hydrogen termination. Diamond films grown by chemical vapor deposition (CVD) have been reported to exhibit visible light induced electron emission and low temperature thermionic emission. The physical mechanism and material-related properties that enable this combination of electron emission are the focus of this research. In this work the electron emission spectra of nitrogen-doped, hydrogen-terminated diamond films are measured, at elevated temperatures, with wavelength selected illumination from 340 nm to 450 nm. Through analysis of the spectroscopy results, we argue that for nitrogen-doped diamond films on metallic substrates, photo-induced electron generation at visible wavelengths involves both the ultra-nanocrystalline diamond and the interface between the diamond film and metal substrate. Moreover, the results suggest that the quality of the metal–diamond interface can substantially impact the threshold of the sub-bandgap photo-induced emission. • Spectra of photo-induced electron emission from N-doped diamond is reported. • Separate energy thresholds are observed for photo-induced and thermionic emission. • Thresholds are attributed to variance in the samples’ electronic structure. • Photoelectron generation involves both the UNCD and the metal substrate.

The effects of high purity MgO nano-powders on the electrical properties of AC-PDPs

A major goal in plasma display panel technology is to reduce power consumption and increase efficiency. A candidate method to accomplish this goal is to increase the Xe content in discharge gases. However, this method has the weak point of increasing discharge voltages. High purity MgO powders made by the self-reaction method are adopted to overcome this disadvantage. From scanning electron microscope analysis, particles of the powders were observed to have very sharp edges, and the impurity content is very low according to inductively-coupled plasma mass spectrometry analysis. Moreover, the powders show a very high photo-induced electron emission and exo-electron emission, and have a very low concentration of oxygen vacancies by cathodoluminescence. Finally, the discharge voltage of high purity MgO powder-adopted films is reduced by about 34% compared to that of conventional films.

Photo-induced electron emission from 17β-estradiol and progesterone and possible biological consequences

It was established for the first time, that the sexual hormones 17β-estradiol (17βE2) and progesterone (PRG) are able to emit electrons from their excited single state in water–ethanol mixtures. The yield of the “solvated electrons” ( e s - ) depends on the substrate concentration, the ratio of water–alcohol-mixtures and the temperature. The e s - yield obtained from 17βE2 is by two orders of magnitude higher than this of PRG. The possible relationship of the resulting hormone transients from 17βE2 leading via specific metabolites to breast cancer is discussed.

General formulation of thermal, field, and photoinduced electron emission

The canonical equations of electron emission are the Richardson-Laue-Dushman equation for thermionic emission, the Fowler-Nordheim equation for field emission, and the Fowler-Dubridge equation for photoemission. They are widely used and experimentally well vetted for the determination of current density from electron sources. While all three may be obtained from a general underlying formulation, they are treated as distinct as their domains of applicability do not overlap. Here, a tractable analytical model is given that not only devolves to the canonical equations, but also and more importantly is effective in the transition regions when the emission currents from tunneling, thermal emission, and/or photoemission become comparable but where the canonical equations are inapplicable. The resulting thermal-field-photoemission equation has application to emerging advanced electron sources as well as for emission models required by beam simulation codes.

Characterization of a Rydberg atom-based streak camera operating in synchroscan mode

A streak camera that operates in synchroscan mode has been developed with a spectral response throughout the infrared. A gas-phase sample of Rydberg atoms is used as a photocathode. This compact device possesses 5 ps time resolution and can be used with a total infrared energy of about 1 nJ, or 10 −7 of the total macropulse energy of the FELIX free electron laser. This combination of characteristics makes it not only an attractive device for use in a variety of infrared experiments, but also a powerful tool for the study of photo-induced electron emission in atomic systems. As an example, a Rydberg-atom based electron gun which produces about 20 pulses of electrons at a 70 GHz repetition frequency has been characterized using this synchroscan streak camera.

The photoemission optogalvanic effect in a Ne-Nd hollow cathode

Laser-induced photoelectric and photoemission optogalvanic effects in a Ne-Nd hollow cathode discharge have been studied using a continuous wave laser source. The potential barrier for photoinduced electron emission from the cathode decreases as the applied voltage is increased. Owing to secondary electron emission in the plasma, the photocurrent is greater than that without discharge. The multiplication of secondary electrons and the quantum efficiency are also investigated.

Laser-induced electron emission from an Au surface irradiated by single picosecond pulses at ?=2.94 ?m. The intermediate region between multiphoton and tunneling effects

The photoinduced electron emission from the surface of a solid gold target irradiated by single picosecond pulses of an erbium laser is investigated. The applied laser intensity (5–120 GW/cm2) corresponds to the intermediate interaction region between the pure multiphoton and tunnel effects, where the decisive Keldysh-parameter, γ, is in the range 1<γ<12=n0. In the light intensity region which is free of surface heating (IL<80 GW/cm2), the slope of the measured logarithmic intensity dependence of the photocurrent decreases from the n0=12 perturbative value down to n ℞ 5. Therefore the experiment shows that the Keldysh-type theories, which have recently been proved to describe correctly the ionization of atoms, are also valid to a certain extent in the case of the photoeffect in metals.