Photoelectron Lifetime Research Articles

Nanostructures with Quantum Dot Clusters: Long Photocarrier Lifetime

Major restrictions of semiconductor optoelectronic devices operating at room temperatures are caused by short photoelectron lifetime, which strongly reduces the photoresponse and puts strong limitations on manipulations with photoelectrons. Here we present results of modeling novel optoelectronic materials, based on structures with correlated dot clusters. The main distinctive characteristic of these quantum-dot structures are collective potential barriers around dot clusters. The barriers provide an effective control of photoelectron capture due to separation of highly mobile electron states transferring the photocurrent from the localized electron states in quantum dots. The novel nanostructured materials combine manageable photoelectron lifetime, high mobility, and quantum tuning of localized and conducting states. Thus, these structures have strong potential to overcome the limitations of traditional quantum dot and quantum-well structures. Besides manageable photoelectron kinetics, the advanced quantum-dot structures will also provide high coupling to radiation, low generation-recombination noise, and high scalability.

Preparation, characterization and luminescent properties of spherical CaTiO 3:Pr 3+ phosphors by spray pyrolysis

In this paper, spherical Pr 3+-doped CaTiO 3 phosphor particles were fabricated through a two-step spray pyrolysis process, using citric acid and polyethylene glycol (PEG) as additives. X-ray diffraction (XRD), scanning electron microscopy (SEM), High-resolution transmission electron microscopy (HRTEM), thermogravimetric and differential thermal analysis (TG–DTA), X-ray photoelectron spectra (XPS), photoluminescence (PL), cathodoluminescence (CL) spectroscopy, and lifetime measurements were employed to characterize these samples. The results reveal that the as-prepared CaTiO 3:Pr 3+ phosphors are spherical with submicron particle size. The particles show a strong red emission corresponding to 1D 2– 3H 4 (612 nm) of Pr 3+ under the ultraviolet excitation (325 nm) and low voltage electron beams (1–5 kV). Furthermore, the morphology, PL and CL intensities of the CaTiO 3:Pr 3+ phosphors can be tuned by altering the concentration of PEG, annealing temperature, and acceleration voltage. These phosphors show potential applications in the field of field emission displays (FEDs).

Synthesis and photoluminescent properties of α-NaYF 4:Nd/ α-NaYF 4 core/shell nanostructure with enhanced near infrared (NIR) emission

A simple and efficient method has been established for the synthesis of α-NaYF 4:Nd/ α-NaYF 4 core/shell nanoparticles under hydrothermal conditions. The as-prepared products were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectra and lifetimes. TEM images show that the as-prepared samples have sphere-like shape. The lifetime values of the 4F 3/2 energy level of the α-NaYF 4:Nd/ α-NaYF 4 core/shell nanoparticles increase significantly in comparison with those of α-NaYF 4:Nd core nanoparticles, with a quantum efficiency of as high as 52%.

The local electronic structure of α-Li3N

New theoretical and experimental investigations of the occupied and unoccupied local electronic densities of states (DOS) are reported for α-Li3N. Band-structure and density-functional theory calculations confirm the absence of covalent bonding character. However, real-space full-multiple-scattering (RSFMS) calculations of the occupied local DOS find less extreme nominal valences than have previously been proposed. Nonresonant inelastic x-ray scattering, RSFMS calculations, and calculations based on the Bethe–Salpeter equation are used to characterize the unoccupied electronic final states local to both the Li and N sites. There is a good agreement between experiment and theory. Throughout the Li 1s near-edge region, both experiment and theory find strong similarities in the s-and p-type components of the unoccupied local final DOS projected onto an orbital angular momentum basis (l-DOS). An unexpected, significant correspondence exists between the near-edge spectra for the Li 1s and N 1s initial states. We argue that both spectra are sampling essentially the same final DOS due to the combination of long core-hole lifetimes, long photoelectron lifetimes, and the fact that orbital angular momentum is the same for all relevant initial states. Such considerations may be generally applicable for low atomic number compounds.

Facile sonochemical synthesis ofCePO4:Tb/LaPO4 core/shell nanorods with highly improved photoluminescent properties

A simple, efficient and quick method has been established for the synthesis ofCePO4:Tb nanorodsand CePO4:Tb/LaPO4 core/shell nanorods via ultrasound irradiation of inorganic salt aqueous solution underambient conditions for 2 h. The as-prepared products were characterized by meansof powder x-ray diffraction (PXRD), transmission electron microscopy (TEM),high-resolution transmission electron microscopy (HRTEM), selected area electrondiffraction (SAED), x-ray photoelectron spectroscopy (XPS), photoluminescence (PL)spectra and lifetimes. TEM micrographs show that all of the as-prepared ceriumphosphate products have rod-like shape, and have a relatively high degree ofcrystallinity and uniformity. HRTEM micrographs and SAED results prove that thesenanorods are single crystalline in nature. The emission intensity and lifetime of theCePO4:Tb/LaPO4 core/shell nanorods increased significantly with respect to those ofCePO4:Tb core nanorods under the same conditions. A substantial reduction in reaction time as wellas reaction temperature is observed compared with the hydrothermal process.

DECAY PROCESS OF PHOTOELECTRONS RELATED TO SHALLOW ELECTRON TRAP IN SILVER HALIDE MICROCRYSTAL

Combining the microwave absorption dielectric spectrum experiment with computer simulation, the decay process of free photoelectrons related to shallow electron trap in silver halide microcystal is investigated. It is found that when shallow electron traps exist in silver halide, the free photoelectron decay will be sped up at the beginning, and then slowed down in the subsequent process. Shallow electron traps like a reservoir can store up photoelectrons temporarily, and then release them tardily? which results in lengthening free photoelectron lifetime or decay time and enhancing the efficiency of latent image formation.

Investigation of photoelectron temporal characteristics in silver halide microcrystals using the microwave absorption technique

The free photoelectron lifetime reflects to a large extent the latent image formation efficiency and sensitivity of silver halide material. The microwave absorption dielectric-spectrum technique enables measurement of the photoelectron decay process of silver halide emulsion exposed to 35ps laser pulse. For T-grain AgBr emulsion, the relationship between exciting energy and photoelectron action has been obtained and the influence of iodide dopants on photoelectron lifetime was measured and analysed. The photoelectron lifetime of dye-sensitized AgBr emulsion with tabular grains is shorter than that with cubic grains and the latent image formation efficiency of the former is higher than the latter.

Investigation of the photoelectron decay of silver halide microcrystal

The microwave absorption dielectric spectrum can be used to study the decay process of free photoelectrons and shallow-trapped electrons in semiconductor crystals. The decay curve of free photoelectrons and shallow-trapped electrons of silver halide grains is measured using this technique. The influence of iodide and K4Fe(CN)6 shallow electron trap dopants on the photoelectron lifetime of silver halide grains is studied. For the unsensitized cubic AgCl crystals, when the free photoelectron lifetime (FLT) reaches a maximum, the photographic efficiency is optimal. From our analysis, we conclude that FLT is the longest for the cubic AgCl crystals doped with 0.5% iodide at 80% doping position and 1×10−6 mol K4Fe(CN)6/molAg, whereas, for the highly photosensitized cubic AgBrCl crystals doped with K4Fe(CN)6, the photographic efficiency is optimal when the FLT reaches its minimum. The free photoelectron lifetime reaches minimum and the sensitivity of AgBrCl emulsion reaches maximum when the doping position is 30%Ag at K4Fe(CN)6 content of 10−6mol/molAg.

Origin of photoemission final-state effects inBi2Sr2CaCu2O8by very-low-energy electron diffraction

Very-low-energy electron diffraction with a support of full-potential band calculations is used to achieve the energy positions, dispersions, lifetimes, and Fourier compositions of the photoemission final states in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ at excitation energies below 45 eV. We find (1) highly structured final states that explain the dramatic photon energy dependences in photoemission; (2) a photoelectron lifetime decrease above 30 eV, resulting in increased damping of these dependences; (3) an intense $c(2\ifmmode\times\else\texttimes\fi{}2)$ diffraction below 25 eV, which reveals a significant extrinsic contribution to the shadow Fermi surface due to final-state diffraction. In a perspective, the experimental final states can be utilized to tune the photoemission experiment on specific valence states or Fermi surface replicas.

Li 2 CsSb : A highly-efficient photocathode material

The electronic band structure of the alkali antimonide Li2CsSb has been calculated. Based on this calculation, estimates are given for the absorption coefficient and the radiative lifetime of the photoelectron. The optical generation of photoelectrons in this material proceeds via a direct optical transition, but after relaxation of the photoelectron it is prohibited to recombine with a valence band hole due to the indirect band gap. The photoelectron lifetime and the diffusion lengths are compared with other photocathode materials. The results suggest that for Li2CsSb the photocathode can be made significantly thicker and therefore more efficient.

The kinetics simulation of trap depths and capture cross sections of SETs in AgC l microcrystals doped with ［Fe(CN)6］4- complex

To describe the photoelectron rise and decay process of AgCl microcrystals doped with ［Fe(CN)6］4-, a kinetics model composed of three in trinsic centres and a shallow electron traps(SETs)is set up, and further more, a set of differential equations is deduced. By solving the differential equations, photoelectron decay curve and photoelectron lifetime that is in accord with the transient microwave photoconductivity experimental data are obtained. Adjusting related simulation parameters, the SET depths and capture cross sections at room temperature for ［Fe(CN)6］4- are obtained and they are 0.115eV and 2 .136×10-17cm2, respectively.

Substituent Effects in Molecular Electronic Relaxation Dynamics via Time-Resolved Photoelectron Spectroscopy: ππ* States in Benzenes

We study the applicability of femtosecond time-resolved photoelectron spectroscopy to the study of substituent effects in molecular electronic relaxation dynamics using a series of monosubstituted benzenes as model compounds. Three basic types of electronic substituents were used: CC (styrene), CO (benzaldehyde), and C⋮C (phenylacetylene). In addition, the effects of the rigidity and vibrational density of states of the substituent were investigated via both methyl (α-methylstyrene, acetophenone) and alkyl ring (indene) substitution. Femtosecond excitation to the second ππ* state leads, upon time-delayed ionization, to two distinct photoelectron bands having different decay constants. Variation of the ionization laser frequency had no effect on the photoelectron band shapes or lifetimes, indicating that autoionization from super-excited states played no discernible role. From assignment of the energy-resolved photoelectron spectra, a fast decaying component was attributed to electronic relaxation of the ...

Electronic properties and Fermi surface of Ag(111) films deposited onto H-passivated Si(111)-(1×1) surfaces

Silver films were deposited at room temperature onto H-passivated Si(111) surfaces. Their electronic properties have been analyzed by angle-resolved photoelectron spectroscopy. Submonolayer films were semiconducting and the onset of metallization was found at a Ag coverage of $\sim$0.6 monolayers. Two surface states were observed at $\bar{\Gamma}$-point in the metallic films, with binding energies of 0.1 and 0.35 eV. By measurements of photoelectron angular distribution at the Fermi level in these films, a cross-sectional cut of the Fermi surface was obtained. The Fermi vector determined along different symmetry directions and the photoelectron lifetime of states at the Fermi level are quite close to those expected for Ag single crystal. In spite of this concordance, the Fermi surface reflects a sixfold symmetry rather than the threefold symmetry of Ag single crystal. This behavior was attributed to the fact that these Ag films are composed by two domains rotated 60$^o$.

Photoelectron lifetime determination of Ag(1 1 1) films at the Fermi surface

The electronic properties of 10 monolayers Ag(111) films deposited onto Si(111)-7×7 substrates at room temperature have been studied by scanning the photoelectron intensity at the Fermi level in different symmetry directions. The main features observed in these profiles correspond to Lorentzian-like peaks produced by the pass of the sp band through the Fermi level. A simple model has been developed, which connects the photoemission peak linewidth with the lifetime of photoelectrons excited from the Fermi level. The obtained inverse photoelectron lifetime values have been found to be in excellent agreement with the typical values of the Ag single crystals. These results support the fact that the transport properties of well ordered Ag films grown onto silicon substrates are quite close to those expected in bulk-like silver crystals.

Electronic Band Structure of Gd: A Consistent Description

The dispersion of the Gd (5d6s)-valence bands has been investigated by means of spin- and angle-resolved photoemission. The spin analysis of various spectral features shows that their weak dispersion and unusual broadening is due to the photoelectron lifetime rather than to correlation induced band narrowing as previously proposed. These results resolve a long-standing discrepancy between theoretical and experimental descriptions of the rare earth band structure.

Properties of ultrafine silver iodobromide emulsions

Three ultrafine silver iodobromide emulsions with the same iodide contents (0.3 mol%) were prepared. Their sizes were determined by transmission electron microscopy and X-ray diffraction, which proved that their sizes were about 30, 50 and 100 nm respectively. The emulsions showed the ‘quantum-size effect’, in that their absorption peaks shifted to short wavelengths with decreasing grain size and their sensitivities were far lower than those of normal emulsions. However, the sensitivities of these emulsions were increased significantly when they were reduction sensitized with (NH2)2CSO2, which was more effective than sulphur, gold and sulphur-plus-gold sensitization. The smaller the grain sizes, the better was the reduction-sensitized effect for these ultraline grain emulsions. It was also observed that the ionic conductivities were directly proportional to the surface area volume ratio of the grains; i.e. for cubic grains the ionic conductivity was inversely proportional to the edge length of the grain, d. Similarly the photoelectron lifetimes were directly proportional to the cubic grain edge length d. The result of low-temperature (77 K) fluorescence also demonstrated that recombination of positive holes and photoelcctrons increased as the grain size decreased.

Phase Sensitive Microwave Absorption Studies on AgBr<sub>1-x</sub>I<sub>x</sub>Emulsion Crystals

The influence of phase boundaries on the photoelectron lifetime of T-grain emulsion crystals is examined with the microwave absorption method. For the measurements, AgBr1−xIxT grains with a core-shell and a so-called “gradient” structure were used. The core-shell ratio, the grain volume, and the iodide content of the core were varied. The microwave absorption measurements indicated that a maximum photoelectron lifetime can be reached for a special combination of these parameters. In the case of core-shell T grains, maximum values were found for a core-shell ratio of 1:1 and iodide contents x > 0.08. For iodide values x < 0.02 “gradient” grains lead to the highest photoelectron lifetimes. A model is proposed to explain the behavior of the structured grains influenced by the interplay of the geometrical grain structure and the iodide content.

The Effect of Three Different 4-hydroxy-1,3,3a,7-tetraazaindenes on the Melt pAg, Ionic Activation Energies and Photocarrier Responses of an AgBr Emulsion

Melts and coatings of a 0.3-μm octahedral AgBr emulsion were prepared with four concentrations of each of three different 4-hydroxy-1,3,3a,7-tetraazaindenes (TAIs). pAg measurements were made on the melt samples and ionic thermocurrent and radiofrequency photoconductivity measurements were made on the coated samples. The melt pAg (i.e., a measure of the Br− desorbed from the grain surface) increased as the pKsp and the concentration of the TAI added to the sample increased. The concentration of desorbed Br− measured as a function of TAI concentration was consistent with the proposal that TAI anions compete with Br− for adsorption to the surface of the grain. The ionic activation energy for formation of interstitial silver ions and the photoelectron lifetime increased as the pKsp and the concentration of TAI− increased. A linear correlation between desorbed Br− and ionic activation energy was observed. The log of the rate constant for electron trapping decreased linearly as the ionic activation energy increased. The presence of TAI did not affect the photohole response. For TAIs with pKa<5.0, these effects were independent of pH for pH values between 4.5 and 7.0, whereas when pKa > 6, these effects decreased with decreasing pH.

The Effect of TAI Addition on the Ionic Conductivity and Photoelectron Lifetime of Silver Halide Emulsion Microcrystals

The effect of 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindenee (TAI) addition on the electric properties (photoelectric lifetime and ion conductivity) of tabular silver halide microcrystals and their sensito-metric properly has been studied. The result shows that the addition of TAI decreases the ionic conductivity of the tabular silver halide microcrystals and prolongs their photoelectron lifetimes, thus to some extent promoting the sensitivity enhancement of the emulsion.

Mechanism of Sensitization by Hydroxy-tetraazaindene

Effects of 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene (TAI) on the electronic properties of unsensitized and sulphur sensitized cubic AgCl emulsion grains were investigated by means of the Dember effect, and microwave photoconductivity. The results indicated that, in the presence of TAI, Dember photovoltage and its decay time and the lifetime of photoelectrons were increased, resulting from the decrease of the concentration of interstitial silver ions produced by TAI. The mechanism of sensitization by TAI suggested urns as follows: decrease of interstitial silver ions in silver halide grains by TAI modified the formation and the growth of primitive latent image centres, as well as the specks concentration process.