H.C. Swart

Defect correlated fluorescent quenching and electron phonon coupling in the spectral transition of Eu3+ in CaTiO3 for red emission in display application

This paper reports on the defect correlated self-quenching and spectroscopic investigation of calcium titanate (CaTiO3) phosphors. A series of CaTiO3 phosphors doped with trivalent europium (Eu3+) and codoped with potassium (K+) ions were prepared by the solid state reaction method. The X-ray diffraction results revealed that the obtained powder phosphors consisted out of a single-phase orthorhombic structure and it also indicated that the incorporation of the dopants/co-dopants did not affect the crystal structure. The scanning electron microscopy images revealed the irregular morphology of the prepared phosphors consisting out of μm sized diameter particles. The Eu3+ doped phosphors illuminated with ultraviolet light showed the characteristic red luminescence corresponding to the 5D0→7FJ transitions of Eu3+. As a charge compensator, K+ ions were incorporated into the CaTiO3:Eu3+ phosphors, which enhanced the photoluminescence (PL) intensities depending on the doping concentration of K+. The concentratio...

Zns:Cu,Al,Au phosphor degradation under electron excitation

Abstract ZnS:Cu,Al,Au phosphor powder has been subjected to 2 keV electron bombardment at an oxygen pressure of 1×10−6 Torr. Auger electron spectroscopy (AES) and cathodoluminescence (CL), both excited by the same electron beam were used to monitor changes in the surface composition and luminous efficiency during electron bombardment. It was found that the phosphor surface degrades during prolonged electron bombardment. The chemical state of the surface before and after degradation was determined by X-ray photoelectron spectroscopy (XPS). A residual gas analyser was used to detect the volatile compounds coming from the surface during electron bombardment. It is shown that the electron beam stimulates the molecular oxygen into atomic species, which react with the ZnS to form ZnO and volatile SO2 with the consequent loss of CL intensity. The growth of a non-luminescent ZnO surface layer is one reason leading to the degradation in CL intensity.

Effect of annealing on the structural, morphological and photoluminescence properties of ZnO thin films prepared by spin coating

Zinc oxide (ZnO) thin films were deposited on silicon substrates by a sol-gel method using the spin coating technique. The ZnO films were annealed at 700°C in an oxygen environment using different annealing times ranging from 1 to 4 h. It was observed that all the annealed films exhibited a hexagonal wurtzite structure. The particle size increased from 65 to 160 nm with the increase in annealing time, while the roughness of the films increased from 2.3 to 10.6 nm with the increase in the annealing time. Si diffusion from the substrate into the ZnO layer occurred during the annealing process. It is likely that the Si and O2 influenced the emission of the ZnO by reducing the amount of Zn defects and the creation of new oxygen related defects during annealing in the O2 atmosphere. The emission intensity was found to be dependent on the reflectance of the thin films.

The oxidation of industrial FeCrMo steel

Abstract Fe–9Cr–1Mo steel is used as a corrosion resistant alloy in pressurised water reactors. The high steam temperature and pressure inside the reactors result in erosion corrosion. The steel’s hardness and resistance to various forms of corrosion are determined by the composition of the oxide layer formed on the alloy at elevated temperatures. Samples were taken from pieces of pipe removed after failure and have an operation history in excess of 20 years in a local power generation facility. The samples were oxidised in UHV for 1000 L between 157°C and 758°C. AES (Auger electron spectroscopy) and XPS (X-ray photo electron spectroscopy) depth profiles of the oxide layer were obtained at room temperature. At high oxidation temperatures the layer forms a duplex structure consisting of Fe oxide at the gas/oxide interface and Cr oxide at the oxide/metal interface. The ratio between these oxides is temperature dependent. Below 400°C the oxide layer consists mainly of Fe 2 O 3 and a small amount of Cr 2 O 3 at the metal/oxide interface. Between 400°C and 600°C the oxide layer consists of a mixture of FeO, Fe 2 O 3 and Cr 2 O 3 and above 600°C the layer consists mainly of Cr 2 O 3 . No influence of Mo or Ni on the oxidation process was measurable with the techniques used.

The role of surface and deep-level defects on the emission of tin oxide quantum dots

This paper reports on the role of surface and deep-level defects on the blue emission of tin oxide quantum dots (SnO₂ QDs) synthesized by the solution-combustion method at different combustion temperatures. X-ray diffraction studies showed the formation of a single rutile SnO₂ phase with a tetragonal lattice structure. High resolution transmission electron microscopy studies revealed an increase in the average dot size from 2.2 to 3.6 nm with an increase of the combustion temperature from 350 to 550 °C. A decrease in the band gap value from 3.37 to 2.76 eV was observed with the increase in dot size due to the quantum confinement effect. The photoluminescence emission was measured for excitation at 325 nm and it showed a broad blue emission band for all the combustion temperatures studied. This was due to the creation of various oxygen and tin vacancies/defects as confirmed by x-ray photoelectron spectroscopy data. The origin of the blue emission in the SnO₂ QDs is discussed with the help of an energy band diagram.

The difference in degradation behaviour of ZnS:Cu,Al,Au and ZnS:Ag,Cl phosphor powders

Abstract Auger electron spectroscopy (AES) and cathodoluminescence (CL) measurements showed that surface reactions with the consequent loss of CL intensity occur for both the ZnS:Cu,Al,Au and ZnS:Ag,Cl phosphor powders during prolonged electron bombardment. Although ZnO layers formed on both the ZnS phosphor surfaces there is a difference between the degradation behaviour of these two phosphors. Surface charging occurs during electron bombardment of the aged ZnS:Ag,Cl phosphor due to the formation of a non-conductive ZnO surface layer. Charging of the ZnS:Ag,Cl phosphor resulted in a delay in the surface reactions and to an increase in the CL degradation rate due to band-bending. A much slower degradation in CL intensity was measured for ZnS:Cu,Al,Au, where no charging occurred.

Degradation behaviour of ZnS phosphor powders under different experimental conditions

The phosphor powder ZnS:Cu,Al,Au has been investigated extensively as a possible green phosphor for field emission displays (FEDs). Phosphor powders of ZnS:Cu,Al,Au have been subjected to electron bombardment (1–5 keV) at oxygen pressures ranging from 1×10−7 to 2×10−6 Torr and different electron current densities (45–58 mA cm−2). Auger electron spectroscopy (AES) and cathodoluminescence (CL), both excited by the same electron beam, were used to monitor changes in surface composition and luminous efficiency during electron bombardment. It was shown that the degradation rate of the CL intensity is a linear function of oxygen pressure. The interactive constant (a measure of the probability of surface reactions) and the burning parameter (a measure of the probability of CL degradation per incident electron) plotted as a function of electron beam energy follow the same trend as the electron impact ionization cross-section for an atom.

Influence of ultrasonication times on the tunable colour emission of ZnO nanophosphors for lighting applications.

This paper reports on the sonochemical synthesis of zinc oxide (ZnO) nanophosphors (NPr) at different ultrasonication times (5 min, 30 min, 1h, 5h, 10h and 15 h) for near white light emission applications. X-ray photoelectron spectroscopy indicated that the O1s peak consists of two components. These were O1 (ZnO) and O2 (deficient oxygen; OH groups) centred at 529.7±0.3 eV and 531.1±0.3 eV, respectively. All samples showed UV and defect level emission (DLE). The DLE enhancement was due to the increase in oxygen related defects such as oxygen vacancies/interstitials. Due to the combination of near UV and DLE near white light emission in ZnO NPr was obtained. The emission could be tuned with different ultrasonic times. It was found that the ultrasonication time influenced the growth mechanism and luminescence properties of the ZnO NPr.

A promising orange-red emitting nanocrystalline NaCaBO3:Sm3+ phosphor for solid state lightning

Orange-red emitting NaCaBO3:Sm 3+ nanophosphors were prepared by combustion method using metal nitrates and urea as initial materials. The phase of the product was identified by x-ray diffraction technique (XRD) and was found to be orthorhombic with a space group Pmmn and lattice parameters a=16.12 A, b=10.25 A, c=3.52 A and V=581.60 A 3 calculated with the POWD program. The average particle size of the nanophosphor calculated from the XRD data and cross verified by TEM was 34 nm. The spectral properties of the NaCaBO3: Sm 3+ nanophosphors were studied under near UV and electron excitation, which shows characteristic emission lines assigned to the transition ( 4 G5/2 to the 6 Hj, j=5/2, 7/2, 9/2, 11/2) of the Sm 3+ ion. The optimal Sm 3+ ion concentration and its critical energy distance were determined as x=2 mol% and 59.97 A, respectively and the dipole–dipole interactions were responsible for concentration quenching in the present phosphor. The band gap of the phosphor was calculated to be 5.82 eV. The phosphorescence decay curve was also measured.

Potential of Sr4Al14O25: Eu2+,Dy3+ inorganic oxide-based nanophosphor in Latent fingermark detection

In this paper, the potential of the Sr4Al14O25: Eu2+,Dy3+ fluorescent nanophosphor is explored to visualize the latent fingermarks. The nanophosphor has been synthesized using self-propagating high temperature synthesis and shows intense luminescence and strong long after glow properties. These nano particles have been characterized for their particle size, crystalline phase, and photoluminescent properties. The calculated PL CIE co-ordinates (0.138, 0.359) correspond to the blue–green shade. Different porous, non-porous, as well as slightly non-smooth surfaces are taken and successfully tested for latent fingerprints development with the prepared Sr4Al14O25: Eu2+,Dy3+ fluorescent nanophosphor powder. Present nano powder is advantageous to use on luminescent or multicolored surfaces as it gives greater contrast and also eliminates the problem of background interferences with the friction ridges due to its strong long after glow property. The results are very good when latent fingerprints were developed on the currency notes with optical variable ink and highly fluorescent pattern areas.