Emission of positronium in a nanometric PMMA film

Positron beam experiments have been performed for the first time on a self-supporting polymethyl metacrylate (PMMA) film of 310 nm-thick made by spin coating. The positronium (Ps) emission from the PMMA surface is studied as a function of the positron implantation energy by using Doppler profile spectroscopy and Compton-to-peak ratio analysis. When the sample and the Ge-detector are perpendicular to the positron beam, the emission of para-positronium (p-Ps) is detected as a narrow central peak. By rotating the sample 45° with respect to the beam, the emission of p-Ps is detected as a blue-shifted fly-away peak. The bulk Ps fraction, the efficiency for the emission of Ps by picking up an electron from the surface, and the diffusion lengths of positrons (thermal and or epithermal), p-Ps and ortho-positronium (o-Ps) are obtained.     

Formation of defect structures in Au-polysilane interfaces probed by low energy positron beams

Formation and accumulation of defect structures at interfaces between polysilanes and vacuum-evaporated gold (Au) electrodes are discussed quantitatively by low energy positron annihilation spectroscopy. The size distribution of the defects at the interface is analyzed based on the values of ortho-positronium (o-Ps) lifetime (τ₃), and no effect of the evaporation process is observed in the polymer films. The intensity of o-Ps (I₃) indicates no considerable change before and after Au evaporation on dialkyl-substituted polysilanes, however, the values of I₃ is increased ∼20% in phenyl-substituted polysilane (PMPS) by the evaporation. The I₃ dependence on incident positron energy suggests the formation of the defects not only at an Au-PMPS interface but also in PMPS bulk phase as deep as 400 nm from the interface. Phenyl ring dissociation from the polymer backbone will play a significant role in the selective formation of the defects in PMPS. This is the first report on the direct measurement of defect structures at conjugated polymer-metal interface with non-destructive way, implying that electrode fabrication by vacuum evaporation affects the solid state structure of polymers.     

Some aspects of positronium physics

Some aspects of both theoretical and experimental study of the positronium system to probe physics beyond the Standard Model are reviewed. In particular, new experiments to search for the invisible decay of orthopositronium (o-Ps) with the sensitivity in the branching ratio Br(o-Ps → invisible) ≃ 10⁻⁸–10⁻⁷ are discussed. The experimental technique involves a specially designed high-efficiency pulsed slow positron beam, which is also applicable for other experiments with o-Ps in vacuum. Details of the beam design, as well as the first measurements results are presented. Possible applications of the slow-pulsed positron beam for materials research are discussed. The text was submitted by the authors in English.     

Quantum beats in the 3γ annihilation decay of Positronium observed by perturbed angular distribution

We have applied conventional Time Differential Perturbed Angular Correlation (TDPAC) method to observe the anisotropy oscillations in the 3γ annihilation decay of polarized Positronium in a weak magnetic field. The effect, as predicted theoretically and experimentally demonstrated by Barishevsky et al. [V.G. Barishevsky, O.N. Metelitsa, V.V. Tikhomirov, Oscillations of the positronium decay γ-quantum angular distribution in a magnetic field, J. Phys. B: At. Mol. Opt. Phys.22 (1989) 2835], is induced by the coherent admixture of the m = 0 states of ortho-Positronium (o-Ps) and para-Positronium (p-Ps) in interaction with the magnetic field.The following experimental characteristics are to be considered:

(i)

the oscillation frequency corresponds to the difference in energy of the Ps atom levels in magnetic field and is proportional with H²;

(ii)

in a fixed geometry the modulation depth (oscillations amplitude) depends on the mean positron polarization;

(iii)

privileged angles of the polarization vector, magnetic field and detectors are required for optimizing the observed oscillations amplitude.

The normalized difference spectrum function (R(t)) obtained from time spectra measured in vacuum and in different gaseous atmospheres (Ar, H₂, N₂) have the oscillations amplitude constant and we conclude that the Ps atoms are not fully thermalized over a time interval of about 400 ns.The R(t) functions obtained for o-Ps annihilation decays, in dry air or Ar-O mixture, have the oscillations amplitude time dependent due, probably, to the paramagnetism of the Oxygen molecules.     

Dependence of positron lifetimes on the dielectric constants of some organic media

An attempt has been made to correlate the orthopositronium (o-Ps) pickoff annihilation lifetime with the dielectric constant for some organic liquids. A semi-empirical free volume model, developed by Thosar et al., is used.     

Porosity depth profiling of spin-coated silica thin films produced by different precursors sols

The structural evolution of nanoporous silica thin films was studied by Doppler broadening spectroscopy (DBS), 2-3 gamma ratio of positronium (3γ-PAS) and Fourier transform infrared spectroscopy (FT-IR). Four series of silica films with thickness in the 300-600 nm range were deposited by spin coating on Si substrate changing the content of sacrificial porogen in the sol precursors. The effect on the porosity of different amount of porogen and of the thermal treatments in the 400-900 °C temperature range have been highlighted. The evolution of the porosity is discussed considering the removal of porogen and of the silanol Si-OH groups by thermal treatments as pointed out by FT-IR. Pores with size from less than 1 nm up to sizes larger than 2.0 nm have been detected. In samples with maximum porogen load oPs escaping was observed indicating onset of connected porosity. At temperatures higher than 700 °C a decrease of the porosity due to a progressive pore collapsing was evidenced. A strong correlation was found between the shift of the Si-O-Si transversal optical (TO₃) mode in the FT-IR spectra and the pore size in the porous silica films as revealed by DBS and 3γ-PAS.     

Determination of the positron diffusion length in Kapton by analysing the positronium emission

Doppler profile spectroscopy and Compton-to-peak ratio analysis have been used to study the positronium (Ps) emission from the Kapton surface as a function of the positron implantation energy E.Two different positions for the sample have been performed in the experiment.In the first case the sample and the Ge-detector are perpendicular to the positron beam. With this geometry the emission of para-positronium (p-Ps) is detected as a narrow central peak.In the second case, by rotating the sample 45° with respect to the beam axis, the emission of p-Ps is detected as a blue-shifted fly away peak. The implantation of the positrons is described by the Makhov profile, where we used the modified median implantation for polymers as given by Algers et al. [J. Algers, P. Sperr, W. Egger, G. Kögel, F.H.J. Maurer, Phys. Rev. B 67 (2003) 125404].Thermalised positrons can diffuse to the surface and may pick up an electron to be emitted as Ps. We found a thermal and or epithermal positron diffusion length L₊ = 5.43 ± 0.71 nm and L₊ = 5.51 ± 0.28 nm correspondingly for both cases, which is much more than the one found by Brusa et al. [R.S. Brusa, A. Dupasquier, E. Galvanetto, A. Zecca, Appl. Phys. A 54 (1992) 233]. The respective efficiency for the emission of Ps by picking up an electron from the surface is found to be f_pu = 0.247 ± 0.012 and f_pu = 0.156 ± 0.003.     

Microstructure characterization of porous silicon as studied by positron annihilation measurements at low temperatures and high vacuum

Atomic scale properties of thin porous silicon (PSi) layers, characterized by the formation of positronium, are investigated using positron annihilation lifetime spectroscopy in the temperature range 20-300 K under 10⁻⁷ Torr vacuum. The longest orthopositronium as well as the shortest parapositronium components are found to have quite low intensities in the thin layer at room temperature. It is also found that at temperatures ≤240 K, these two components do not show up in the spectrum. The reason for this absence of the longest lifetime component is suggested.     

A comparative study of field emission properties of carbon nanotube films prepared by vacuum filtration and screen-printing

A comprehensive comparative study of electron field emission properties of carbon nanotube (CNT) films prepared by vacuum filtration and screen-printing was carried out. Field emission performance of vacuum filtered CNT films with different filtered CNT suspension volumes was systematically studied, and the optimum electron emission was obtained with a low turn on field of ∼0.93 V/μm (at 1 μA/cm²) and a high field enhancement factor β of ∼9720. Comparing with screen-printed CNT films, vacuum filtered CNT films showed better electron emission performance, longer lifetime, and greater adhesive strength to substrates. This work reveals a potential use of vacuum filtered CNT films as field emission cathodes.     

PL and EL characterizations of ZnO:Eu, Li films derived by sol-gel process

ZnO:Eu³⁺, Li⁺ films prepared by the dip-coating method were characterized by photoluminescence (PL) and electroluminescence (EL). When the ZnO:Eu³⁺, Li⁺ films were excited using UV light with energy corresponding to the band-to-band excitation of the host matrix, the PL spectra showed emissions from both ZnO and Eu³⁺ ions, while their EL spectra showed emissions only from Eu³⁺ ions, and no emission from ZnO could be detected. It is found that the EL emission intensity B is dependent on the applied voltage, B=B_o exp(−bV^−1/2). With increasing frequency, the EL intensity dramatically increases at lower frequencies (<1000 Hz), and then increases gradually at higher frequencies (>1000 Hz).     

Optical and electrical properties of thermally oxidized bismuth thin films

Bismuth trioxide (Bi₂O₃) thin films were prepared by dry thermal oxidation of metallic bismuth films deposited by vacuum evaporation. The oxidation process of Bi films consists of a heating from the room temperature to an oxidation temperature (T_o = 673 K), with a temperature rate of 8 K/min; an annealing for 1 h at oxidation temperature and, finally, a cooling to room temperature. The optical transmission and reflection spectra of the films were studied in spectral domains ranged between 300 nm and 1700 nm, for the transmission coefficient, and between 380 nm and 1050 nm for the reflection coefficient, respectively. The thin-film surface structures of the metal/oxide/metal type were used for the study of the static current-voltage (I-U) characteristics. The temperature of the substrate during bismuth deposition strongly influences both the optical and the electrical properties of the oxidized films. For lower values of intensity of electric field (ξ < 5 × 10⁴V/cm), I-U characteristics are ohmic.     

Surface characterization and luminescent properties of SrAl2O4:Eu,Dy nano thin films

SrAl₂O₄:Eu²⁺, Dy³⁺ thin films were grown on Si (1 0 0) substrates in different atmospheres using the pulsed laser deposition (PLD) technique. The effects of vacuum, oxygen (O₂) and argon (Ar) deposition atmospheres on the structural, morphological and photoluminescence (PL) properties of the films were investigated. The films were ablated using a 248 nm KrF excimer laser. Improved PL intensities were obtained from the unannealed films prepared in Ar and O₂ atmospheres compared to those prepared in vacuum. A stable green emission peak at 520 nm, attributed to 4f⁶5d¹→4f⁷ Eu²⁺ transitions was obtained. After annealing the films prepared in vacuum at 800 °C for 2 h, the intensity of the green emission (520 nm) of the thin film increased considerably. The amorphous thin film was crystalline after the annealing process. The diffusion of adventitious C into the nanostructured layers deposited in the Ar and O₂ atmospheres was most probably responsible for the quenching of the PL intensity after annealing.     

Temperature dependence of positron annihilation characteristics on the surfaces of graphite powders

Positron lifetime measurements have been made on graphite powders, grafoils, and pyrolytic graphite crystals with different surface areas in the temperature range between 25° and 600 °C. Three positron lifetimes were found in these systems: a short-lived component (0.2 ns) due to positrons in the bulk; a component (0.45 ns) due to surface-trapped positrons; and a long-lived component (2 ns) ofo-Ps in the voids or the interfacial spaces of powders. Both bulk and surface positron lifetimes increase as a function of temperature. Correlations between the intensity of surface-trapped positrons and the surface area and between Ps formation and the surface area of graphite are found. The Ps formation probability increases as a function of temperature. A thermal desorption model interprets the emission process of Ps atoms from the surface of graphite to the vacuum and gives an activation energy of 0.23±0.02 eV.Preliminary results of this paper were presented at the March Meeting of American Physical Society, Los Angeles. Bull. Am. Phys. Soc.28, 347 (1983)     

The effect of annealing on the photoluminescent and optical properties of porous anodic alumina films formed in sulfamic acid

Photoluminescent and optical properties of porous oxide films formed by two-step aluminum anodization at a constant potential of 30 V in sulfamic acid have been investigated after their annealing, ranging from room temperature up to 600 °C. X-ray diffraction reveals the amorphous nature of porous oxide films. Infrared and energy dispersive spectroscopy indicates the presence of sulfuric species incorporated in oxide films during the anodization. Photoluminescence (PL) measurements show PL bands in the range from 320 to 600 nm. There are two peaks in emission and excitation spectra. One emission peak is at constant wavelength centered at 460 nm and the other shifts from 390 to 475 nm, depending on excitation wavelength. For excitation spectra, one spectral peak is at constant wavelength at 270 nm and the other also shifts to longer wavelengths while increasing emission wavelength. Upon annealing of the as-prepared oxide films PL increases reaching maximum value at about 300 °C and then decreases. The results indicate the existence of two PL centers, one placed at surface of the pore wall, while the other positioned inside the oxide films.     

Characteristics of filtered vacuum arc deposited ZnO-SnO2 thin films on room temperature substrates

ZnO, SnO₂ and zinc stannate thin films were deposited using filtered vacuum arc deposition (FVAD) system on commercial microscope glass and UV fused silica substrates (UVFS) at room temperature (RT). The structural and morphological analyses were performed using X-ray diffraction (XRD) and Atomic Force Microscopy (AFM), respectively. XRD patterns of ZnO films deposited at RT had strongly c-axis orientation, whereas SnO₂ and zinc stannate films had amorphous structure as they did not have any defined patterns. Average crystalline size and surface grain size of ZnO films were ∼16 nm, as determined from diffraction line broadening and AFM images, respectively. Optical constants in the 250-1100 nm wavelength range were determined by variable angle spectroscopic ellipsometry and transmission measurements. The transmission of the deposited films in the VIS was 80-90%, affected by interference. The refractive indices and the extinction coefficients of deposited ZnO, SnO₂ and zinc stannate films were in the range 1.87-2.15 and 0.02-0.04, depending on wavelengths and deposition parameters. The optical band gap (E_g) was determined by the dependence of the absorption coefficient on the photon energy at short wavelengths. Its values for ZnO, SnO₂ and zinc stannate were in the range 3.25-3.30 eV, 3.60-3.98 eV and 3.43-3.52 eV, respectively, depending on the deposition pressure.     

Oxidation and thermal reduction of the Cu(1 0 0) surface as studied using positron annihilation induced Auger electron spectroscopy (PAES)

Changes in the surface of an oxidized Cu(1 0 0) single crystal resulting from vacuum annealing have been investigated using positron annihilation induced Auger electron spectroscopy (PAES). PAES measurements show a large increase in the intensity of the annihilation induced Cu M_2,3VV Auger peak as the sample is subjected to a series of isochronal anneals in vacuum up to annealing temperature 300 °C. The intensity then decreases monotonically as the annealing temperature is increased to ∼600 °C. Experimental probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons are estimated from the measured intensities of the positron annihilation induced Cu M_2,3VV and O KLL Auger transitions. Experimental PAES results are analyzed by performing calculations of positron surface states and annihilation probabilities of surface-trapped positrons with relevant core electrons taking into account the charge redistribution at the surface, surface reconstructions, and electron-positron correlations effects. The effects of oxygen adsorption on localization of positron surface state wave function and annihilation characteristics are also analyzed. Possible explanation is proposed for the observed behavior of the intensity of positron annihilation induced Cu M_2,3VV and O KLL Auger peaks and probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons with changes of the annealing temperature.     

Fabrication and application of highly ordered mesoporous Co3O4, NiO, and their metals

Highly ordered mesoporous Co₃O₄, NiO, and their metals were synthesized by nanocasting method using there corresponding mesoporous SBA-15 silica as a template. The obtained porous metal oxides have high surface areas, large pore volume, and a narrow pore size distribution. The N₂-adsorption data for mesoporous metal oxides have provided the BET area of 257.7 m² g⁻¹ and the total pore volume of 0.46 cm³ g⁻¹. The mesoporous metals were employed as a catalyst in the synthesis of (S)-3-pyrrolidinol from chiral (S)-4-chloro-3-hydroxybutyronitrile, and a high yield to (S)-3-pyrrolidinol-salt was obtained on the mesoporous Co metal catalyst.     

Influence of porous morphology on optical dispersion properties of template free mesoporous titanium dioxide (TiO2) films

This paper focuses the influence of porous morphology on the microstructure and optical properties of TiO₂ films prepared by different sol concentration and calcination temperatures. Mesoporous TiO₂ thin films were prepared on the glass substrates by sol-gel dip coating technique using titanium (IV) isopropoxide. Porous morphology of the films can be regulated by chemical kinetics and is studied by scanning electron microscopy. The optical dispersion parameters such as refractive index (n), oscillator energy (E_d), and particle co-ordination number (N_c) of the mesoporous TiO₂ films were studied using Swanepoel and Wemple-DiDomenico single oscillator models. The higher precursor concentration (0.06 M), films exhibit high porosity and refractive index, which are modified under calcination treatment. Calcinated films of low metal precursor concentration (0.03 M) possess higher particle co-ordination number (N_c = 5.05) than that of 0.06 M films (N_c = 4.90) due to calcination at 400 °C. The lattice dielectric constant (E_∞) of mesoporous TiO₂ films was determined by using Spintzer model. Urbach energy of the mesoporous films has been estimated for both concentration and the analysis revealed the strong dependence of Urbach energy on porous morphology. The influence of porous morphology on the optical dispersion properties also has been explained briefly in this paper.     

Properties of boron and phosphorous incorporated tetrahedral amorphous carbon films grown using filtered cathodic vacuum arc process

This paper reports the electrical, mechanical, structural and field emission properties of as grown and also boron and phosphorous incorporated tetrahedral amorphous carbon (ta-C) films, deposited using a filtered cathodic vacuum arc process. The effect of varying boron and phosphorous content (up to 2.0 at.% in to ta-C) on the conductivity (σ_D), activation energy (ΔE₁), hardness, microstructure, emission threshold (E_turn-ON) and emission current density (J) at 12.5 V/μm of ta-C: B and ta-C: P films deposited at a high negative substrate bias of −300 V are reported. It is observed that both boron and phosphorous incorporation leads to a nearly an order increase in σ_D and corresponding decrease in ΔE₁ and a slight increase in hardness as compared to as grown ta-C films. In the case of field assisted electron emission, it is observed that E_turn-ON increases and J decreases. The changes are attributed to the changes in the sp³/sp² ratio of the films due to boron and phosphorous incorporation. The effect of boron on ta-C is to give a p-type effect whereas the effect of phosphorous gives n-type doping effect.     

Properties of Zn3N2-doped ZnO films deposited by pulsed laser deposition

The optical properties of N-doped ZnO films grown by pulsed laser deposition are examined for which zinc nitride is used as the source of nitrogen. The motivation for this study is to determine if nitrogen-related acceptor state formation can be achieved in ZnO films using Zn₃N₂ doping in the ablation target. The films were deposited in oxygen or nitrogen on c-plane sapphire. Photoluminescence measurements at 20 K reveal a 3.31 eV acceptor-bound exciton emission due to nitrogen substitution on the oxygen site, donor-acceptor pair emission at 3.23 ± 1 eV and free electron-acceptor at 3.27 eV. The binding energy of the N-related acceptor is estimated to be in the range of 170-15 meV. While the as-deposited films were n-type, thermal annealing in oxygen yielded insulating behavior, consistent with compensating acceptor states.