Electron Emission Spectra Research Articles

Resonant emission of UO2, U3O8, and UO2+x valence electrons under SR excitation near the O4,5(U) absorption edge

The structure of the resonant electron emission (REE) spectra of UO2 (REE appears under the excitation with synchrotron radiation near the O4,5(U) absorption edge at ∼100 eV and ∼110 eV) is studied with regard to the X-ray O4,5(U) absorption spectrum of UO2 and a quantitative scheme of molecular orbitals based on the X-ray electron spectroscopy data and the results of a relativistic calculation of the electronic structure of UO2. The structure of the REE spectra of U3O8 and UO2+x is studied for comparison, and the effect of the uranium chemical environment in oxides on it is found. The appearance of such a structure reflects the processes of excitation and decay involving the U5d and electrons of the outer valence MOs (OVMOs, from 0 to ∼13 eV) and inner valence MOs (IVMOs, from ∼13 eV to ∼35 eV) of the studied oxides. It is noted that REE spectra show the partial density of states of U6p and U5f electrons. Based on the structure of REE spectra, it is revealed that U5f electrons directly participate (without losing the f nature) in the chemical bonding of uranium oxides and are delocalized within CMOs (in the middle of the band), which results in the enhancement of the intensity of the REE spectra of CMO electrons during resonances. The U6d electrons are found to be localized near the bottom of the outer valence band and are observed in the REE spectra of the studied oxides as a characteristic maximum at 10.8 eV. It is confirmed that U6p electrons are effectively involved in the formation of IVMOs, which leads to the appearance of the structure in the region of IVMO electron energies during resonances. This structure depends on the chemical environment of uranium in the considered oxides.

Phase effects in resonant multiphoton emission

The dependence of resonant multiphoton emission of electrons on the relative and carrier-envelope phases of bichromatic laser fields is studied theoretically by numerically solving the time-dependent Schrodinger equation. The laser field is described by a space- and time-dependent electric field. In order to solve numerically the Schrodinger equation, we modify the transfer-matrix algorithm such that it becomes numerically stable for a very large number of matching points. This modification enables one to investigate space effects related to a finite size of a laser focus. Numerical analysis shows the importance of finite penetration depth of laser fields in solids and the ponderomotive energy related to the quiver motion of electrons in the laser focus. We demonstrate that the energy spectrum of emitted electrons and the total photocurrent depend on the relative and carrier-envelope phases, indicating that multiphoton dynamics of electrons emitted from a solid surface can be efficiently controlled by varying these phases.

Photoelectron emission from LiF surfaces by ultrashort electromagnetic pulses

Energy- and angle-resolved electron emission spectra produced by incidence of ultrashort electromagnetic pulses on a LiF(001) surface are studied by employing a distorted-wave method named the crystal surface-Volkov (CSV) approximation. The theory makes use of the Volkov phase to describe the action of the external electric field on the emitted electron, while the electron-surface interaction is represented within the tight-binding model. The CSV approach is applied to investigate the effects introduced by the crystal lattice when the electric field is oriented parallel to the surface plane. These effects are essentially governed by the vector potential of the external field, while the influence of the crystal orientation was found to be negligible.

Spectral studies on Cr 3+ ions doped in sodium–lead borophosphate glasses

Electron paramagnetic resonance (EPR), optical absorption and emission spectra of Cr 3+ ions doped in (30− x) (NaPO 3) 6+30PbO+40B 2O 3+ xCr 2O 3 ( x=0.5, 2.0, 3.0, 4.0 and 5.0 mol%) glasses have been studied. The EPR spectra exhibit resonance signals with effective g values at g≈4.55 and g≈1.97. The EPR spectra of x=3.0 mol% of Cr 2O 3 in sodium–lead borophosphate glass sample were studied at various temperatures (295–123 K). The intensity of the resonance signals increases with decrease in temperature. The optical absorption spectrum exhibits four bands characteristic of Cr 3+ ions in octahedral symmetry. From the analysis of the bands, the crystal-field parameter Dq and the Racah interelectronic repulsion parameters B and C have been evaluated. The emission spectrum exhibit one broad band characteristic of Cr 3+ ions in octahedral symmetry. This band has been assigned to the transition 4T 2g (F)→ 4A 2g (F). Correlating EPR and optical data, the molecular bonding coefficient ( α) has been evaluated.

FORMATION OF ONE-DIMENSIONAL Ga STRUCTURES ON Si(5 5 12)-2 × 1 TRENCHED TEMPLATES

In the advent of GaN photonic devices, there is renewed interest in looking at the growth of nanostructures of III-Nitrides that can yield novel properties. To grow a high density of self-assembled GaN nanostructures on a large scale, there is a need to use templates with well-defined nanometer scale features. The Si (5 5 12)-2 × 1 surface is a naturally faceted stable surface that has several nanotrenches of (225) and (337) with a large unit cell of 5.35 × 0.77 nm2. In this work we present here, the first report on the X-ray photoelectron spectroscopy study of the adsorption kinetics of Ga on Si (5 5 12)-2 × 1 reconstructed surface. The uptake curve which is the plot of the ratio of Ga (2 p ) to Si (2 p ) intensity with deposition time, shows that Ga grows in a Stranski–Krastnov growth mode on Si (5 5 12) surface. The core level spectra of Si (2 p ) and Ga (2 p ) have been deconvoluted to see the evolution of the interface during the adsorption process of Ga . The valence-band spectra show the modification of the electronic structure and the secondary electron emission spectra yield the change in work function during Ga adsorption. Correlating the electronic and structural aspects of the interface formation results the study demonstrates the possibility of integrating Si and group III-nitride technologies while exploiting the novel properties due to reduced dimensions.

Energy transfer and luminescence properties of Eu 3+-doped NaTb(WO 4) 2 phosphor prepared by a facile hydrothermal method

NaTb (1− x )Eu x (WO 4) 2 ( x = 0–100%) phosphors have been synthesized via a mild hydrothermal process directly without further sintering treatment. X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence excitation and emission spectra and decay curve were used to characterize the samples. Moreover, the luminescence colors of NaTb (1− x )Eu x (WO 4) 2 (NTbW:Eu) samples can be tuned from green, green–yellow, and yellow to red by simply adjusting the relative Eu 3+-doping concentrations under a single wavelength excitation, which might find potential applications in the light displays systems.

Effect of Oxygen Plasma Treatment on Crystal Growth Mode at Pentacene/Ni Interface in Organic Thin-Film Transistors

We report how treatment of nickel (Ni) with O(2) plasma affects the polarity of Ni surface, crystallinity of pentacene film on the Ni, and electrical properties of pentacene organic thin-film transistors (OTFTs) that use Ni as source-drain electrodes. The polar component of surface energy in Ni surface increased from 8.1 to 43.3 mJ/m(2) after O(2)-plasma treatment for 10 s. From X-ray photoelectron spectra and secondary electron emission spectra, we found that NiO(x) was formed on the O(2)-plasma-treated Ni surface and the work function of O(2)-plasma-treated Ni was 0.85 eV higher than that of untreated Ni. X-ray diffraction and atomic force microscopy measurements showed that pentacene molecules are well aligned as a thin-film and grains grow much larger on O(2)-plasma-treated Ni than on untreated Ni. This change in the growth mode is attributed to the reduction of interaction energy between pentacene and Ni due to formation of oxide at the Ni/pentacene interface. Thus, O(2)-plasma treatment promoted the growth of well-ordered pentacene film and lowered both the hole injection barrier and the contact resistance between Ni and pentacene by forming NiO(x), enhancing the electrical property of bottom-contact OTFTs.

Two-Center Resonant Photoionization

Photoionization of an atom A, in the presence of a neighboring atom B, can proceed both directly and via resonant excitation of B with subsequent energy transfer to A through two-center electron-electron correlation. We show that in such a case the photoionization process can be very strongly enhanced and acquire interesting characteristic features, both in its time development and the electron spectrum.

Potassium-benzene interactions on Pt(111) studied by metastable atom electron spectroscopy

Electron emission spectra obtained by thermal collisions of He(∗)(2(3)S) metastable atoms with C(6)H(6)/Pt(111), C(6)H(6)/K/Pt(111), and K/C(6)H(6)/Pt(111) were measured in the temperature range of 50-200 K to elucidate the adsorption/aggregation states, thermal stabilities of pure and binary films, and local electronic properties at the organic-metal interface. For C(6)H(6)/Pt(111), the He(∗)(2(3)S) atoms de-excite on the chemisorbed overlayer predominantly via resonance ionization followed by Auger neutralization and partly via Penning ionization (PI) yielding weak emission just below the Fermi level (E(F)). We assigned this emission to the C(6)H(6) π-derived states delocalized over the Pt 5d bands on the basis of recent density functional calculations. During the layer-by-layer growth, the C(6)H(6)-derived bands via PI reveal a characteristic shift caused by the final-state effect (hole response at the topmost layer). C(6)H(6) molecules chemisorb weakly on the bimetallic Pt(111) (θ(K)=0.1) and physisorb on the K multilayer. In both cases, the sum rule was found to be valid between the K 4s and C(6)H(6)-derived bands. The band intensity versus exposure plot indicates that the C(6)H(6) film grows on the K multilayer by the Volmer-Weber mechanism (island growth), reflecting the weak K-C(6)H(6) interactions. In case of K/C(6)H(6)/Pt(111), the K atoms are trapped on the topmost C(6)H(6) layer at 65 K, forming particlelike clusters. The surface plasmon satellite was identified for the first time and the loss energy increases with increasing cluster size. The K clusters are unstable above ∼100 K due to thermal migration into the C(6)H(6) film. When the cluster coverage is low, the K 4s band extends below and above E(F) of the Pt substrate and the anomaly is discussed in terms of vacuum level bending around the cluster.

Luminescence properties of monodispersed spherical BaWO4:Eu3+ microphosphors for white light-emitting diodes

Monodispersed spheres (1–4 μm in diameter) of BaWO4:Eu3+ (hereafter BWO:Eu) red-phosphor exhibiting intense emission at 615 nm were synthesized via a mild hydrothermal method. X-ray diffraction, scanning electron microscope, photoluminescence excitation and emission spectra, and decay curve were used to characterize the properties of BWO:Eu phosphors. An intense red emission was obtained by exciting either into the 5L6 state with 394 nm or the 5D2 state with 465 nm, that correspond to two popular emission lines from near-UV and blue LED chips, respectively. The values of Ω 2,4 experimental intensity parameters (13.8 × 10−20 and 8.2 × 10−20 cm2) are determined. The high-emission quantum efficiency of the BWO:Eu phosphor suggests this material could be promising red phosphors for generating white light in phosphor-converted white light-emitting diodes.

Hole Injection Layer of Thermally Evaporated Copper Oxide for Top Emitting Organic Light Emitting Diodes

We report the enhancement of the electroluminescent property of top emitting organic light emitting diodes (TEOLEDs) using thermally evaporated copper oxide as a hole injection layer (HIL) between UV ozone-treated silver (Ag) anodes and -bis[-(1-naphthyl)--phenylamino]biphenyl . The operation voltage of TEOLEDs at decreased from 6.2 to 5.0 V as a 2 nm thick layer was used as the HIL. layers were separately in situ deposited on both Ag and thermally evaporated -coated Ag and their interface dipole energies were quantitatively determined using synchrotron radiation photoemission spectroscopy. Secondary electron emission spectra revealed that the work function of is higher by 0.53 eV than that of Ag. The corresponding interface dipole energies were for Ag and for . As a result, plays a role in reducing the hole injection barrier from 1.63 to 0.96 eV, resulting in a decrease in the turn-on voltage of TEOLEDs.

Influence of aluminium and lead on activation of magnesium as anode

Mg-6%Al, Mg-5%Pb and Mg-6%Al-5%Pb (mass fraction) alloys were prepared by induction melting with the protection of argon atmosphere. Their electrochemical activations in different electrolyte solutions were investigated by galvanostatic test. The microstructures of these alloys and their corroded surfaces were studied by scanning electron microscopy, X-ray diffractometry and emission spectrum analysis. The results show that the activation of magnesium is not prominent when only aluminum or lead exists in the magnesium matrix, but the coexistence of the two elements can increase the activation. The activation mechanism of Mg-Al-Pb alloy is dissolving-reprecipitating and there is a synergistic effect between aluminium and lead: the precipitated lead oxides on the surface of the alloy can facilitate the precipitation of Al(OH) 3, which can peel the Mg(OH) 2 film in the form of 2Mg(OH) 2·Al(OH) 3 and activate the magnesium matrix.

Luminescent properties of Eu3+-activated nanophosphors Ca3Y0.8Gd0.2(VO4)2.4(PO4)0.6: Eu3+ for light-emitting diodes

Eu3+-doped Ca3Y0.8Gd0.2(VO4)2.4(PO4)0.6 nanophosphors have been prepared by modified solid-state reaction. X-ray powder diffraction, transmission electron microscopy (TEM), photoluminescence excitation and emission spectra were used to characterize the resulting samples. X-ray powder diffraction (XRD) analysis confirmed the formation of YVO4. Photoluminescence (PL) results showed that the phosphor could be efficiently excited by UV–visible light from 350 to 550nm, exhibiting bright orange-red emission(excited by 397) and red emission(excited by 467), which has potential application as a phosphor for UV and blue GaN-based light-emitting diodes (LEDs). TEM images show that the grain size of Ca3Y0.45Eu0.35Gd0.2(VO4)2.4(PO4)0.6 is about 39nm, which is in full agreement with the theoretical calculation data from the XRD patterns.

Secondary electron emission spectra from clean and cesiated Al surfaces: the role ofplasmon decay and data analysis for applications

We report measurements of energy spectra of secondary electrons emitted from clean andcesiated aluminum surfaces under the impact of 130 eV electrons. Measurements show thatthe decay of bulk and surface plasmons dominates the electron emission. In contrast withtheoretical calculations, our experiments indicate that the electron collision cascade insidethe solid produced by electrons excited by plasmon decay do not contribute significantly toelectron emission.A simple analysis of electron energy distributions measured as a function of Cs surfacecoverage allows separation of rediffused incident electrons from the continuumbackground of true secondary electrons. The result shows that yields of rediffusedelectrons used in several applications may have been significantly overestimated.

Efficient Top-Emitting Organic Light Emitting Diode with Surface Modified Silver Anode

The enhancement of quantum efficiency using a suface modified Ag anode in top-emitting organic light emitting diodes (TEOLEDs) is reported. The operation voltage at the current density of 1 mA/cm 2 of TEOLEDs decreased from 9.3 V to 4.3 V as the surface of anode coated with CuOx layer. The work function of these structures were quantitatively determined using synchrotron radiation photoemission spectroscopy. Secondary electron emission spectra revealed that the work function of the Ag/CuOx structure is higher by 0.6 eV than that of Ag. Thus, the CuOx structure acts as a role in reducing the hole injection barrier by about 0.6 eV, resulting in a decrease of the turn-on voltage of top-emitting light emitting diodes.

Photoluminescence properties of La 2− xEu x(WO 4) 3 red phosphor prepared by hydrothermal method

La 2− x Eu x (WO 4) 3 red phosphors have been synthesized by a hydrothermal process with further sintering treatment. X-ray diffraction (XRD), thermogravimetry (TG), the field emission scanning electron microscope (FE-SEM), photoluminescence excitation and emission spectra and decay curve were used to characterize La 2− x Eu x (WO 4) 3 phosphors. The results showed that La 2− x Eu x (WO 4) 3 phosphor exhibits intensive red emission under 395 nm excitation. The strongest line (395 nm) in excitation spectra of these phosphors matches with the output wavelength of UV InGaN-based light-emitting diodes (LEDs) chip. Hence, it is considered to be a new promising phosphor for generating white light devices.

Ionization of the hydrogen atom by short half-cycle pulses: dependence on the pulse duration

A theoretical study of the ionization of hydrogen atoms by short external half-cycle pulses (HCPs) as a function of the pulse duration, using different quantum and classical approaches, is presented. Total ionization probability and energy distributions of ejected electrons are calculated in the framework of the singly-distorted Coulomb-Volkov (SDCV) and the doubly-distorted Coulomb-Volkov (DDCV) approximations. We also performed quasiclassical calculations based on a classical trajectory Monte Carlo method which includes the possibility of tunneling (CTMC-T). Quantum and classical results are compared to the numerical solution of the time-dependent Schrodinger equation (TDSE). We find that for high momentum transfers the DDCV shows an improvement compared to the SDCV, especially in the low-energy region of the electron emission spectra, where SDCV fails. In addition, DDCV reproduces successfully the TDSE electron energy distributions at weak momentum transfers. CTMC-T results reveal the importance of tunneling in the ionization process for relative long pulses and strong momentum transfers but fails to overcome the well-known classical suppression observed for weak electric fields.

Electron emission features of the derivatives of radiation carbonization of poly(vinylidene fluoride)

It has been studied how photoelectron and CKVV spectra of partially crystalline poly(vinylidene fluoride) (PVDF) are modified during a long-term degradation of its surface under soft X-rays (AlKα), which is accompanied by a flow of secondary electrons having different energies, and upon exposure to a unfocused beam of 600 eV Ar+ ions. In both cases, the surface layer of the sample is enriched with carbon owing to defluorination. The shape of the electron emission spectra of the carbonized layer depends on an external effect; that is, whether soft X-ray photons or ions are used for defluorination. In the case of bombardment with Ar+, there is clear evidence for the dominance of the sp2 bonds between carbon atoms, as can be seen from the specific shape of the C KVV band and the C1s spectrum. The most surprising result of this study is that both photons and ions produce the same depth gradient of residual fluorine at an equal fluorine concentration in the carbonized surface layer. The reason for this is not clear and needs further investigation.

Kinetic of poly(vinylidene fluoride) defluorination under X-ray irradiation

Modification of the photoelectron and C KVV spectra during the long-term surface degradation of partially crystalline PVDF under simultaneous soft X-ray and electron irradiation are reported. Deep radiative carbonization brings about the formation of carbynoid structures (chain-like carbon) in the surface; as a result the shape of the electron emission spectra of carbon in the carbonized sample essentially differs from that of graphite and PVDF. Analysis of carbon core-level electron spectra via decomposition onto spectral components shows presence of partially (CH, CF) and fully (=C=, -C≡) carbonized units of polymeric chain thus pointing on two-step mechanism of polyme ric chain transformation from the initial to a carbynoid. Results of the mathematical modeling of the first step of the chain transformation show it to be a second order process.

Chemisorption-induced gap state at organic–metal interface: Benzenethiol on Pt(111)

Electron emission spectra obtained by thermal collisions of He*(2(3)S) metastable atoms with benzenethiol (C(6)H(5)SH) on Pt(111) were measured to characterize the chemisorption-induced gap state (CIGS) formed at the organic-metal interface. First-principles calculations using density functional theory were also performed for an ordered thiolate (C(6)H(5)S) monolayer on Pt(111). Our data exhibit that the CIGS due to the S 3p-Pt 5d mixings appears just below the Fermi level (E(F)) of the substrate, where the local density of states decreases drastically from the S terminal to the benzene ring. Furthermore, strong benzene pi(1e(1g))-S 3p couplings are apparently lifted upon the formation of thiolate. These features indicate that thiolate is not a good mediator of metal wave functions at E(F), which is closely related to tunneling probability (and eventually electric conductance) in the relevant metal-organic-metal junctions at zero bias.