Exoelectron Research Articles

Surface plasmon coupled chemiluminescence during adsorption of oxygen on magnesium surfaces.

The dissociative adsorption of oxygen molecules on magnesium surfaces represents a non-adiabatic reaction exhibiting exoelectron emission, chemicurrent generation, and weak chemiluminescence. Using thin film Mg/Ag/p-Si(111) Schottky diodes with 1 nm Mg on a 10-60 nm thick Ag layer as 2π-photodetectors, the chemiluminescence is internally detected with a much larger efficiency than external methods. The chemically induced photoyield shows a maximum for a Ag film thickness of 45 nm. The enhancement is explained by surface plasmon coupled chemiluminescence, i.e., surface plasmon polaritons are effectively excited in the Ag layer by the oxidation reaction and decay radiatively leading to the observed photocurrent. Model calculations of the maximum absorption in attenuated total reflection geometry support the interpretation. The study demonstrates the extreme sensitivity and the practical usage of internal detection schemes for investigating surface chemiluminescence.

Defect creation via dissociative recombination of ionic centers in solid Ne matrices

Recombination of the intrinsic ionic centers Ne2+ (self-trapped holes) with the detrapped electrons in solid Ne matrices and relaxation channels have been studied. The experiments were performed employing combination of the cathodoluminescence (CL) with current and optical activation spectroscopy techniques. CL spectra were recorded simultaneously in the VUV and visible range. Yields of spectrally resolved thermally and photon-stimulated luminescence (TSL, PSL) and thermally and photon-stimulated exoelectron emission (TSEE, PSEE) were measured in the time-correlated manner. It was found that the recombination reaction proceeds with irreversible dissociation of the transient Ne2∗∗ centers and the dissociative recombination (DR) products exit the matrix cage. Products of the DR reaction are found to be in 3s and 3p states. The detection of “defect” components in the TSL and PSL points to the defect formation via DR in Ne matrices. The temperature range of the electron traps stability is elucidated. A long-lasting “afteremission” of electrons and afterglow of VUV photons observed on switching off the irradiation suggest the accumulation of the uncompensated negative charge.

METAL SURFACE LAYER ADHESION DESTRUCTION AT TOOL-METAL TORSION CONTACT

Adhesive interaction of metal products with a tool inherent in the process of metal forming. Defects on the products and tools out of the order are the consequences of this. The problem of the choice of material for the instrument has been considered. On the example of mandrel tube drawing mechanism of fracture of adhesive fracture surface layer of metal has been described. Based on an analysis of the solids adhesion hypothesized that the adhesive activity is associated with the electron work function of the material tools. By method of photo-stimulated exoelectron emission electron work function for a number of ceramic materials has been determined. The relationship between the electron work function of the material and surface roughness of workpiece was detected. Criterion for selecting the tool material is formulated and a method of tube drawing fundamentally new quality has been developed.

Estimation of tension of the surface layer of parts after hardening by different methods of surface plastic deformation

A method of control of shot impact methods of surface plastic deformation is presented in the paper. The possibility of controlling the hardening process of parts made of titanic alloys in terms of deflection of plates made of carbon steels is shown. Because of differences in the resilient and plastic properties of materials and as they show different tendencies to hardening their deformation will occur with different speed and intensity over time. In the case of the greatest deflection of the testplate it may not be up to the required degree of hardening of the part made of another material. Cases of insufficient hardening or rehardening of the part’s surface layer may occur in controlling the hardening on plates made of U8A steel. One of the indices of the surface layer is its energy level, therefore the intensity of exoelectronic emission of the surface of the part treated can be used as a criterion of estimation of the stress-strain state of the surface layer. Non-destructive testing of hardening and finishing of the surface layer can be accomplished comparing the part tested with the reference standard.

Charged defects and defect‐induced processes in nitrogen films

AbstractRadiation effects in solid nitrogen irradiated with an electron beam were studied with emphasis on the role of charged centers in radiation‐induced phenomena. The experiments were performed employing luminescence method and activation spectroscopy techniques – spectrally resolved thermally stimulated luminescence TSL and thermally stimulated exoelectron emission. Samples were probed in depth by varying electron energy, thus discriminating radiation‐induced processes in the bulk and at the surface. Spectroscopic evidence of the excited N2* (C3Πu) molecule desorption was obtained for the first time and mechanism of the phenomenon based on recombination of electron with intrinsic charged center N4+ was proposed. The key role of N3+ center dissociative recombination in generation of N radicals is suggested. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Radiation Effects in Solid Nitrogen and Nitrogen-Containing Matrices: Fingerprints of N4+ Species

The radiation effects and relaxation processes in solid N2 and N2-doped Ne matrices, preirradiated by an electron beam, have been studied in the temperature range of 5-40 and 5-15 K, respectively. The study was performed using luminescence methods: cathodoluminescence CL and developed by our group nonstationary luminescence NsL, as well as optical and current activation spectroscopy methods: spectrally resolved thermally stimulated luminescence TSL and exoelectron emission TSEE. An appreciable accumulation of N radicals, N(+), N2(+) ions, and trapped electrons is found in nitrogen-containing Ne matrices. Neutralization reactions were shown to dominate relaxation scenario in the low-temperature range, while at higher temperatures diffusion-controlled reactions of neutral species contribute. It was conceived that in α-phase of solid N2, the dimerization reaction (N2(+) + N2 → N4(+)) proceeds: "hole self-trapping". Tetranitrogen cation N4(+) manifests itself by the dissociative recombination reaction with electron: N4(+) + e(-) → N2*(a'(1)Σ(u)(-)) + N2 → N2 + N2 + hν. In line with this assumption, we observed a growth of the a'(1)Σ(u)(-) → X(1)Σ(g)(+) transition intensity with an exposure time in CL spectra and the emergence of this emission in the course of electron detrapping on sample heating in the TSL and NsL experiments.

Non-adiabatic processes in the charge transfer reaction of O2 molecules with potassium surfaces without dissociation

Thin potassium films grown on Si(001) substrates are used to measure internal chemicurrents and the external emission of exoelectrons simultaneously during adsorption of molecular oxygen on K surfaces at 120 K. The experiments clarify the dynamics of electronic excitations at a simple metal with a narrow valence band. X-ray photoemission reveals that for exposures below 5 L almost exclusively peroxide K2O2 is formed, i.e., no dissociation of the molecule occurs during interaction. Still a significant chemicurrent and a delayed exoelectron emission are detected due to a rapid injection of unoccupied molecular levels below the Fermi level. Since the valence band width of potassium is approximately equal to the potassium work function (2.4 eV) the underlying mechanism of exoemission is an Auger relaxation whereas chemicurrents are detected after resonant charge transfer from the metal valence band into the injected level. The change of the chemicurrent and exoemission efficiencies with oxygen coverage can be deduced from the kinetics of the reaction and the recorded internal and external emission currents traces. It is shown that the non-adiabaticity of the reaction increases with coverage due to a reduction of the electronic density of states at the surface while the work function does not vary significantly. Therefore, the peroxide formation is one of the first reaction systems which exhibits varying non-adiabaticity and efficiencies during the reaction. Non-adiabatic calculations based on model Hamiltonians and density functional theory support the picture of chemicurrent generation and explain the rapid injection of hot hole states by an intramolecular motion, i.e., the expansion of the oxygen molecule on the timescale of a quarter of a vibrational period.

Weak electron emission current for characterization of nanomaterials, gas and radiation sensing towards medical applications

Rapid development of nanomaterials opens a wide horizon for their applications, medicine being one of them. Nanomaterials and nanodevices are in use both in the human body and as medical nanosensors. Reliable employment of materials requires trustworthy detection of their properties. Characterization of both the nanomaterials and nanosensors should be supplied at the nanoscaled dimension. To avoid disturbing gentle nanoobjects, measurements of them with contactless techniques are preferable. Low energy electron has a mean free path in a solid that is of the order of nanoscale. Therefore, a prethreshold (energy of the emitting electron is close to the electron work function) electron emission contactless spectroscopy could become an efficient instrument both for characterization of nanostructured materials and nanosensing. Weak emission (~ 10 -16 …10 -15 Q/cm 2 ) of electrons from a solid does not give a significant feedback to measurements in the sense of the negligible inducted electrical charge at the material surface (the density of the surface electrons in the solid is around 10 14 cm -2 ). The paper reviews photo-, dual-, and exo-electron emission fundamentals and their applications for the characterization of nanoobjects (concentration of pointlike imperfections, their annealing, migration, surface charge of nanoparticles, energy gap, electron density of states, thickness of thin films and interfaces between them and the substrate) as well as gas and ionizing radiation. The ways for medical applications are indicated.

Exoemission Properties of PDP Protective Layers

The priming properties of four different types of protective layers inside plasma display panels (PDPs) are compared, using undoped MgO, Si-doped MgO, Sc-doped MgO, and MgCaO (15%) plasma protective layers. In a two-electrode type of PDP, the statistical spread in discharge formation time is measured, as a function of the waiting time from 5 μs to 50 ms, after excitation by a different number of sustain cycles (1-1024) at 200 and 10 kHz. The results are interpreted in terms of the delayed emission of exoelectrons from these oxide cathodes. The kinetics of the emission process is compared with that of the existing recombination emission models. An exocurrent contribution is present in all of the materials that decays with the square root of the waiting time. It is proportional to the electron density itself, and can qualitatively be explained by a three-particle Auger electron loss mechanism.

Study on the Addressing Characteristics of an ac PDP With Sc-Doped MgO-Protecting Layer

In this paper, the addressing characteristics of an ac plasma display panel (PDP) with Sc-doped MgO-protecting layer are investigated. The characteristics of addressing voltage and temporal distribution of addressing discharge event in Sc-doped MgO panel are different from those in a conventional MgO panel. In particular, the characteristics depending on the operation temperature are changed drastically in Sc-doped MgO panel. The formative discharge time lag increases as the addressing pulse application time and operation temperature increase regardless of protecting layers. The statistical time lag shows the contrary trend compared to the formative discharge time lag except for the MgO when operated in room temperature. The variation of addressing voltage and temporal distribution of addressing discharge event in different protecting layers can be understood through the survey of previous research result on the exoelectron emission characteristics from protecting layers in an ac PDP. It is assumed that the emission of electrons from a protecting layer results in the decrease of wall voltage but increase of priming effect for the addressing discharge. Also, the possibility for the control of addressing characteristics is suggested by changing scan voltage level during address period.

SU-E-T-103: Radiochromic Film Measurement of Nanoparticle Radiation Dose Enhancement

Purpose: Bombardment of high-Z material with x-ray radiation leads to release of fluorescent x-rays, photoelectrons, and Auger electrons. The electrons deposit their energy in tissue within micrometers distance of the high-Z material surface when in contact with tissue. Gold nanoparticles have been studied as a high-Z vehicle for taking advantage of that phenomenon to enhance radiotherapy dose at the cellular level. Our goal is to use unlaminated radiochromic film to measure the gold nanoparticle dose enhancement factor (DEF) associated with 40 and 50 kVp tungsten bremsstrahlung spectrums. Methods: A 10 nm gold foil was placed between an x-ray source and radiochromic film and exposed to lightly filtered 40 kVp and 50 kVp tungsten bremsstrahlung x-rays. A Monte Carlo simulation was performed to estimate the energy deposition with depth in the film active layer, and energy deposition with depth in water. The film was calibrated at x-ray energies ranging from 4.5 keV to 50 kVp to account for energy-dependent response. Optical densities of film exposed with and without gold nanoparticles were used to derive a dose enhancement factor, defined as the ratio of dose attributed to Auger electrons to dose attributed to bremsstrahlung x-rays. Results: Dose enhancement factors within the mean pathlength of Auger electrons ranged from 34.6 +/− 8.6 to 45.0 +/− 11.0 for the 40 kVp spectrum, and 30.7 +/− 11.0 to 44.2 +/− 7.7 for the 50 kVp spectrum. Conclusion: These results compare favorably with other studies that have found varying gold nanoparticle DEF values for 50 kVp bremsstrahlung by Monte Carlo calculation ranging from 1.5 to 2 within a tumor volume to 400 within 10 urn of the gold nanoparticle/tissue interface. Measurements by thermally stimulated exoelectron emission beryllium oxide dosimetry* found a DEF of 55.7 in a single layer of cells on thick gold foil. Wayne State University Fund for Medical Research and Education

Anomalous low-temperature “post-desorption” from solid nitrogen

Anomalous low-temperature post-desorption (ALTpD) from the surface of nominally pure solid nitrogen preliminary irradiated by an electron beam was detected for the first time. The study was performed using a combination of activation spectroscopy methods—thermally stimulated exoelectron emission (TSEE) and spectrally resolved thermally stimulated luminescence (TSL)—with detection of the ALTpD yield. Charge recombination reactions are considered to be the stimulating factor for the desorption from pre-irradiated α-phase solid nitrogen.

The kinetics of phase transitions in vitreous chalcogenide semiconductors As10.2Se89.8 and As9Se90Bi in early stage of physical ageing process

The kinetics of glass transition in selenide glasses As10.2Se89.8 and As9Se90Bi in early stage of physical ageing process has been investigated by parallel differential scanning calorimetry (DSC) and exoelectron emission (EEE). It has been found that the glass transition process occurring in investigated glasses is evidenced by peaks on EEE intensity and DSC curves. Admixture of bismuth causes a distinct lowering of the temperature of glass transitions process both in the surface layer and in the volume. The addition of Bi causes a decrease in the value of the activation energy for glass transition process in both the volume and in the surface layer, thus reducing the thermal stability of investigated glasses. Physical ageing in Se-rich chalcogenide glasses leads to a significant increase of endothermic peak area A, temperature of glass transition T g and decrease of the activation energy value E. All these effects are strongly dependent on glass composition.

Emission of weak electrons: Dosimetry of nanovolumes

Modern radiation therapy (RT) technologies and the needs of molecular and micro radiobiology have necessitated the detection of radiation absorbed by micro/nano volumes. The possibility of developing a nano/micro-scale dosimeter is demonstrated via the use of prethreshold photoelectron and exoelectron emission from solid-state nanolayers (Si, bone) and nanofilms (ZrO2:PbS). The experiments indicate that the absorbed dose threshold depends on the detector origin. • Molecular radiobiology requires radiation detection in micro/nano volumes. • The escape length of weak electrons (several eV) of solids is at the nanometer scale. • Therefore, a solid-state dosimeter at the nanoscale could be developed. • Photo and exoelectron emissions (P-EE) have demonstrated dose detection in nanovolumes. • Nanolayers (Si, bone) and nanofilms (ZrO2:PbS) can be used for P-EE-based dosimeters.

The Long Last Sustain Method for Improving High-Temperature Misdischarge in an AC-PDP

This paper proposes a long last sustain waveform to improve the high-temperature misdischarge in an AC-PDP. We have confirmed that, when applying the interval time during which the X electrode and the Y electrode are grounded before the reset period in the next subfield after the sustain period at high temperature, the VA minimum voltage is reduced. The interval time reduces the probability of exoelectron emission at high temperatures. Therefore, the interval time can improve the high-temperature misdischarge. However, due to the nature of the ADS driving method, there are limitations when increasing the driving time. Thus, we proposed the long last sustain waveform, in which either the X electrode or the Y electrode is applied to VS voltage, instead of the interval time. The VA minimum voltage is significantly decreased, when the long last sustain waveform was applied within the same interval time, rather than grounding the X and Y electrodes. In particular, the effect of the X long last sustain waveform showed the best results. We also proposed the ascending X long last sustain waveform to optimize the length of the long last sustain waveform according to each subfield. The ascending X long last sustain waveform further reduced the VA minimum voltage. In this paper, the VA minimum voltage can be decreased to approximately 12 V, by applying the proposed ascending X long last sustain waveform, in comparison with the existing drive method, at higher temperatures.

Polarization of poly(vinylidene fluoride) and poly(vinylidene fluoride-trifluoroethylene) thin films revealed by emission spectroscopy with computational simulation during phase transition

The electronic structure and self-polarization of P(VDF-TrFE) Langmuir-Blodgett nanofilms were analyzed under temperature-driven phase transitions, according to their thickness, composition, and structural conformation. Both thermo-stimulated exoelectron emission (TSEE) spectroscopy and computational simulation, including quantum-chemical calculations from first principles, were carried out. PVDF and composite P(VDF-TrFE) (70:30) molecular chains as Trans and Gauche conformers, as well as crystal cells, were modeled for these TSEE analyses. The quantum-chemical calculations and the computational simulation were based on the density functional theory (DFT) as well as semi-empirical (PM3) methods. It was demonstrated that the energy of electron states, as well as the total energies of the studied P(VDF-TrFE) molecular clusters during phase transformation, is influenced by electron work function and electron affinity. Analysis was performed by combining TSEE experimental data with the computational data of the molecular models, demonstrating the effectiveness of this joint approach. For the first time, TSEE was used for contactless measurements of nanofilm polarization, and characterization of the phase transition. The proposed new method can be widely applied in nanobiomedicine, particularly in development of new bone bio-implants, including built-in sensors (new smart nanotechnology).

AC-PDP 특성 개선을 위한 MgCaO 보호층과 단결정 MgO 파우더 코팅에 대한 연구

In this study, we attempted to reduce firing voltage of AC-PDP by alloying MgO protective Layer with CaO. Also, spray coating of single crystal MgO powder has been used to improving Exo-electron emission characteristics and reducing the statistical discharge delay in plasma displays. The properties of discharge depending upon the single crystal MgO powder are investigated. Plasma display having powder coated MgCaO Protective Layer shows lower driving voltage and higher efficacy than of uncoated, conventional panel.

Charging effect and relaxation processes in electron bombarded cryogenic solids

Charging effect and relaxation processes in cryogenic solids pre-irradiated by an electron beam are discussed with the example of solid Xe. The experiments were performed employing combination of the cathodoluminescence method with activation spectroscopy techniques: thermally stimulated luminescence (TSL) and thermally stimulated exoelectron emission (TSEE). Relaxation emissions of photons and electrons were detected in the time correlated manner. A long-lasting “afteremission” of electrons and afterglow of VUV photons were observed on completing irradiation. The experiments performed with a variable voltage applied to the Faraday plate enabled us to demonstrate appreciable accumulation of electrons and ascertain the spatial distribution of the charge centers. The uncompensated negative charge is found to be localized preferentially near the surface. Xe layers close to the sample-substrate interface are shown to be positively charged. The part played by pre-existing and radiation-induced defects as well as dopants is considered and the temperature range of the electron traps’ stability is elucidated.

ChemInform Abstract: Quantum Mechanochemistry Understanding of Tribochemical Reactions

AbstractReview: 80 refs.

A New Reset Waveform for Stable Discharge Under Variable Panel Temperatures in AC-Plasma Display Panel

The discharge characteristics, especially the reset discharge characteristics, were examined relative to the panel temperature from -5 to 65°C in an ac-plasma display panel. As the panel temperature decreased, the reset discharge became unstable, due to a decrease in the exo-electron emission from the MgO surface by the thermal activation, thereby inducing the unstable address discharge at a low panel temperature. To stabilize the reset discharge, the effects of the ramp-slope rates during the reset period on the reset discharge were examined under variable panel temperatures. Based on the experimental observation, the new temperature-adaptive reset waveform is proposed to produce a stable reset discharge irrespective of variable panel temperatures.