Hendrik C. Swart

Degradation of zinc sulfide phosphors under electron bombardment

ZnS phosphor powders have been subjected to electron bombardment (2 keV, 2 mA/cm2) at a residual gas pressure of 1.2×10−8 Torr and oxygen pressures of 10−6 Torr. Auger electron spectroscopy and cathodoluminescence (CL), both excited by the same electron beam were used to monitor changes in surface state and luminous efficiency during electron bombardment. A direct correlation between the surface reactions and the degradation of CL brightness was observed. Both C and S were depleted from the surface during electron bombardment. The postulated mechanism for the electron stimulated reactions on the phosphor surface is electron beam dissociation of molecular species to atomic species which subsequently react with C to form volatile compounds (COx, CH4, etc.) and with ZnS to form a nonluminescence layer of either ZnO or ZnSO4. The growth in thickness of the nonluminescence surface layer is directly responsible for the degradation in CL brightness.

Doped zinc oxide window layers for dye sensitized solar cells

The present paper reports the fabrication of dye sensitized solar cell (DSSC), where boron doped ZnO (BZO) and aluminum-boron co-doped ZnO (AZB) thin films were used as front window electrodes. The highly crystalline zinc oxide (ZnO) nanoparticles (NPs) synthesized by the sol-gel route were used as host material for the dye. The efficiencies of the DSSCs formed using the BZO and AZB as window layers were obtained to be 1.56 and 1.84%, respectively. The enhanced efficiency in the case of an AZB window layer based DSSC is attributed to the increase in conductivity induced by co-doping of Al and B and an increase in the number of conducting pathways between the window layer and NPs provided by the nanorods. This facilitates a new approach in the window layer (doped ZnO) for DSSC application.

DEGRADATION OF ZNS FIELD-EMISSION DISPLAY PHOSPHORS DURING ELECTRON-BEAM BOMBARDMENT

Phosphor thin films of ZnS:Mn on Si(100) have been subject to electron bombardment (0.6–4 keV) over a range of pressures from 1×10−6 to 5×10−8 Torr. Various gases including hydrogen, oxygen, and water vapor were introduced into the ambient during bombardment, to assess their effects on the phosphor surface. Auger electron spectroscopy data indicate that electron bombardment in the presence of O2 and H2O caused depletion of sulfur and accumulation of oxygen on the surface. Hydrogen also caused depletion of sulfur. Removal of sulfur was shown to be consistent with electron-beam dissociation of molecular species to atomic hydrogen and/or oxygen, followed by a surface reaction to form high vapor pressure sulfur compounds (e.g., SOx and H2S). In the case of oxygen and water vapor, ZnS was converted to ZnO or ZnSO4. These changes in surface chemistry reduced the intensity of cathodoluminescence from ZnS-based phosphors. The mechanisms leading to loss of CL intensity are discussed.

Cathodoluminescent properties and surface characterization of bluish-white LiAl5O8:Tb phosphor

Cathodoluminescence (CL) characteristics and electron-beam induced surface chemical changes in nanocrystalline Tb3+ doped LiAl5O8 powder phosphors are presented. Bluish-white CL with a maximum at ∼543 nm was observed when the powders were irradiated with a 2 keV electron beam. The emissions in the green and the blue regions arise from the magnetic dipole D54-F7J (J=6–0) and D53-F7J transitions of the Tb3+ ion. The appearance of the line emissions in the blue region are discussed in terms of Tb oxidation states and their corresponding interconversion. Auger electron spectroscopy and x-ray photoelectron spectroscopy (XPS) were used to probe the chemical changes on the surface of the LiAl5O8 phosphor under electron bombardment. The XPS data suggest that the Tb ions exist both in trivalent and tetravalent oxidation states which could be the reason for the observed green as well as blue CL emissions. A thermodynamically stable Al2O3 layer formed on the surface as a result of the electron stimulated surface che...

Synthesis and properties of poly(acrylamide-aniline)-grafted gum ghatti based nanospikes

In this work, we have synthesized poly(acrylamide-aniline)-grafted gum ghatti based crosslinked conducting hydrogel via a two-step synthesis method. The first step involved the microwave assisted synthesis of a semi-interpenetrating polymer network (semi-IPN) based on acrylamide and gum ghatti using N,N′-methylene-bis-acrylamide and ammonium persulfate as a crosslinker-initiator system. The semi-IPN has been observed to exhibit as much as 2183% swelling in aqueous solution. The effect of several variables such as time, solvent, pH, microwave power, crosslinker amount, aniline concentration, initiator concentration and monomer concentration on the swelling capacity was explored. In the final step, polyaniline was entrapped within a semi-IPN (optimized reaction condition) followed by doping with hydrochloric acid, which leads to the formation of conducting IPN. The synthesized hydrogels, as monitored by the swelling behaviour were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy and thermogravimetric analysis. Finally, the synthesized crosslinked networks have been used in malachite green (MG) adsorption. The result indicates that IPN of poly(acrylamide-aniline)-grafted gum ghatti are potential candidates for dye removal from water.

Deep level defect correlated emission and Si diffusion in ZnO:Tb(3+) thin films prepared by pulsed laser deposition.

Terbium (Tb(3+)) doped zinc oxide (ZnO) or (ZnO:Tb(3+)) thin films were grown on silicon substrates by the pulsed laser deposition technique at different growth temperatures that were varied from room temperature (RT) to 400°C. The effects of substrate temperature on the structural and optical properties of the ZnO:Tb(3+) films were investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and RT photoluminescence spectroscopy. The band to band and deep level defect emissions were observed for all substrate temperatures. The silicon that has diffused from the substrate has occupied the position of the Zn vacancies in the ZnO:Tb(3+) thin films at the higher substrate temperatures (400°C). A blue emission was observed for all the ZnO:Tb(3+) thin films deposited at the different substrate temperatures.

Synthesis and characterization of white light emitting Ca x Sr 1-x Al 2 O 4 :Tb 3+ ,Eu 3+ phosphor for solid state lighting

A white light emitting CaxSr1-xAl2O4:Tb3+;Eu3+ phosphor was synthesized by a combustion method using metal nitrates as precursors and urea as a fuel. The X-ray diffraction patterns from the samples showed phases associated with monoclinic structures of CaAl2O4 and SrAl2O4. White photoluminescence with the CIE coordinates (x = 0.343, y = 0.325) was observed when the phosphor was optically-excited at 227 nm using a monochromatized xenon lamp. The white photoluminescence was a result of the combination of blue and green line emissions from Tb3+, and red line emission from Eu3+. The structure and photoluminescence properties of this phosphor are reported.

Surface chemical reactions during electron beam irradiation of nanocrystalline CaS:Ce3+ phosphor

The effects of accelerating voltage (0.5–5 keV) on the green cathodoluminescence (CL) of CaS:Ce3+ nanocrystalline powder phosphors is reported. An increase in the CL intensity was observed from the powders when the accelerating voltage was varied from 0.5 to 5 keV, which is a relevant property for a phosphor to be used in field emission displays (FEDs). The CL degradation induced by prolonged electron beam irradiation was analyzed using CL spectroscopy, x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The AES data showed the decrease in the S peak intensity and an increase in the O peak intensity during electron bombardment. The CL intensity was found to decrease to 30% of its original intensity after about 50 C/cm2. XPS was used to study the chemical composition of the CaS:Ce3+ nanophosphor before and after degradation. The XPS data confirms that a nonluminescent CaSO4 layer has formed on the surface during the degradation process, which may partially be responsible for the C...

Correlating the magnetism and gas sensing properties of Mn-doped ZnO films enhanced by UV irradiation

In this study, we report on the correlation between the magnetism and gas sensing properties of Mn-doped ZnO films grown via aerosol spray pyrolysis. The evolution of the structure, morphology, optical properties, and chemical state of ZnO with the Mn concentration was also investigated. ZnO doped with Mn (0.1 at%) demonstrated room-temperature ferromagnetism (RTFM) due to the uncompensated surface spins primarily originating from structural defects and oxygen vacancies (VO) on the surface, which act as active sites for the adsorption of oxygen species. The undoped ZnO structure revealed both FM and paramagnetism (PM) at the near surface of the film. Increased Mn doping destroyed the RTFM ordering due to improved PM features induced by Mn clusters on the ZnO surface and reduced amount of VO on the surface. However, ZnO films doped with Mn (0.1 at%) exhibited an improved sensing response to oxidizing gases compared to their counterparts, showing that films with RTFM with no PM contribution exhibit improved sensing properties. These analyses revealed that the nature of the film surface plays a substantial role in both their magnetic and sensing behaviors.

Enhanced green emission from UV down-converting Ce3+–Tb3+ co-activated ZnAl2O4 phosphor

Ce3+–Tb3+ co-activated ZnAl2O4 powder phosphors were prepared by a solution combustion method using urea as a fuel. X-ray diffraction characterization showed that all the powders crystallized in the well known cubic spinel phase of ZnAl2O4. An enhanced down-converted green emission associated with the 5D4→7F5 transitions of Tb3+ ions was observed at 543 nm from the ZnAl2O4:Ce3+, Tb3+ powders with different concentrations of Ce3+ and Tb3+. It was inferred from the fluorescence decay data that the enhancement was due to energy transfer from Ce3+ to Tb3+. Further, cathodoluminescence intensity degradation of the ZnAl2O4:Ce3+, Tb3+ powder phosphors was investigated when the powders were irradiated with 2 keV electrons. X-ray photoelectron spectroscopy was used to analyze the chemical and electronic states of individual elements before and after electron irradiation. The ZnAl2O4:Ce3+, Tb3+ phosphor was evaluated to be used as a UV down-converting layer in conventional silicon photovoltaic cells or as a source ...