Cathodoluminescence Degradation Research Articles

Stability of Y2O3:Tm,Yb phosphor thin films during cathodoluminescence and upconversion

An Y2O3:Tm,Yb thin film was synthesized by the sol-gel method using spin coating. X-ray powder diffraction revealed the formation of a Y2O3 cubic phase. The cathodoluminescence and upconversion photoluminescence of the film were investigated, including assessment of emission stability. Auger electron spectroscopy was used to monitor its chemical state concurrently with the cathodoluminescence intensity using the same electron beam. Auger peaks associated with carbon, yttrium and oxygen were detected and their Auger peak-to-peak heights indicated that carbon was removed from the surface during the initial stage of degradation, whereas yttrium and oxygen remained almost unchanged. The cathodoluminescence degradation of the Tm emissions at 457 nm and 975 nm exhibited a continuous decline until the electron dose was about 100 C/cm2 and then stabilized. The upconversion photoluminescence intensity of the Tm emission at 488 nm remained stable for a period of 28 h and did not exhibit degradation.

Photoluminescence, cathodoluminescence degradation and surface analysis of Gd2O3:Bi pulsed laser deposition thin films

Gd2-xO3:Bix = 0.003 thin films were successfully deposited on Si (100) substrates in vacuum and an oxygen atmosphere at different substrate temperatures using the pulsed laser deposition technique. The microstructure, surface topography, chemical composition analysis and luminescent properties of the samples were studied. The influence of prolonged electron beam exposure on the surface state, chemical and cathodoluminescence (CL) stability of the samples was investigated. The background atmosphere and substrate temperature were found to significantly affect the microstructure and photoluminescence (PL). According to scanning electron microscopy cross-sections, the thicknesses for the thin films were relatively constant around 100 nm. The CL emission intensity degradations in a vacuum and an oxygen atmosphere were checked synchronously with the Auger peak-to-peak heights using the same electron beam for both measurements. The effect of the electron bombardment on the surface state of the samples was studied by using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). All major elements (gadolinium and oxygen) were located, with additional carbon and chlorine that were removed during the early stages of electron bombardment. Generally, the CL intensity of the thin films with a blue-green emission was stable under electron bombardment, indicating that this phosphor may be appropriate for field emission displays applications.

Cathodoluminescence degradation of Y2O3:Dy3+ nanophosphor for field emission displays

Cathodoluminescence (CL) degradation of Y2O3:Dy3+ nanophosphors prepared by the solution combustion method was explored for feasible applications in low voltage field emission displays (FEDs). Oxide materials are excellent candidates for FED fabrication due to their high melting points, chemical and radiation stability with long lifetimes, high color purity, and being environmentally friendly. Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) were used to monitor changes in the surface chemical composition and correlation fit with CL degradation. AES and CL spectroscopy (2 keV energy electrons and with a beam current of 15 μA) measurements were done in high vacuum (1.5 × 10−8 Torr) and oxygen pressures of 1 × 10−7 and 5 × 10−7 Torr. The Y2O3:Dy3+ nanophosphor showed strong yellow (572 nm) and relatively weaker blue (492 nm) CL emissions. These CL emissions increased as carbon (C) was depleted from the surface, and then it slightly decreased at a high electron dose in both the vacuum and oxygen atmospheres, for electron doses up to about 690 C/cm2. The C was depleted from the surface due to electron stimulated reactions. No significant change in the chemical state of Y 3d was observed with XPS high resolution spectra for the postirradiation sample. The change in the CL intensity was, therefore, attributed to the depletion of C from the surface.

Cathodoluminescence degradation of Bi doped La2O3 and La2O2S phosphor powders

Simultaneous cathodoluminescence (CL) and Auger electron spectroscopy (AES) measurements were performed on La2-xO3:Bix=0.002 and La2-xO2S:Bix=0.002 phosphor powders during long term exposure to an electron beam in order to assess the CL degradation and chemical changes on the surface. X-ray photoelectron spectroscopy (XPS) measurements were also made before and after CL degradation. After a small amount of initial CL degradation, the La2-xO3:Bix=0.002 sample remained stable under the electron beam and it may be suitable for use in field emission displays. However, the La2-xO2S:Bix=0.002 showed continuous and severe CL degradation and is not suitable for CL applications. During degradation AES measurements showed that there was a decrease in the surface concentration of S, suggesting the formation of a non-luminescent La2O3 surface layer which was responsible for degradation. However, some S remained on the surface and XPS spectra showed that a sulphate, possibly La2O2SO4, was present which may have contributed to the degradation.

Cathodoluminescence degradation study of the green luminescence of ZnO nanorods

Bright green luminescence at 500 nm was obtained from chemical bath deposited ZnO nanorods after annealing in a reducing atmosphere (H2/Ar) at 700 °C for 30 min. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and secondary electron microscopy (SEM) were used to study the effect of electron beam irradiation on the cathodoluminescence (CL) and the surface state of the sample. The CL intensity was monitored concurrently with the Auger peak-to-peak heights (APPHs) using the same electron beam for both techniques. CL intensity degradation was studied in vacuum and in an oxygen ambient. According to the AES spectra, all the principal elements (zinc and oxygen) were detected as well as carbon, which was removed at the initial stage of electron beam irradiation. No chemical change was observed during electron beam irradiation. In vacuum, the CL intensity decreased to almost half of its initial intensity after 100 C/cm2 electron dose and then stabilized. In the oxygen atmosphere, the CL intensity also decreased initially up to a dose of ~10 C/cm2 and thereafter recovered to about 90% of its original intensity and stabilized after a dose of ~100 C/cm2. No difference in the chemical state of Zn was observed with XPS for the original sample and the degraded ones. Only a small change in the defect contribution part of the O peak was observed. SEM images for the original and degraded samples showed that the electron beam irradiation induced surface changes in the morphology of the ZnO nanorods. Generally, the CL intensity was very stable under electron beam irradiation, suggesting that the material may be suitable for application in optoelectronics devices.

Luminescence properties and cathodoluminescence degradation of Bi doped SrO powder

Bi3+ doped strontium oxide (SrO:Bi) phosphor powders were synthesized by the solgel combustion method using metal nitrates as precursors and citric acid as fuel. The optimum Bi3+ doping concentration was found to be 0.2 mol. %, and the optimum annealing temperature was found to be 1100 °C (2 h). The x-ray diffraction patterns corresponded with the known face-centered-cubic structure of SrO. Williamson–Hall plots showed that the crystallite size was in the range of ∼180 nm. Diffuse reflectance measurements of the pure host material showed that it was strongly reflecting (∼100%) down to a wavelength of about 230 nm, but when doped with Bi, an absorption band at 275 nm was observed that increased with Bi concentration. Scanning electron microscopy revealed a cubic morphology, and the grain size increased with annealing temperature. Photoluminescence measurements indicated that the phosphor exhibited efficient blue emission around 445 nm under UV excitation, which also occurred for electron irradiation, but slightly shifted about 5 nm to a longer wavelength. Auger electron spectroscopy (AES) was employed to analyze the surface chemical composition of the powder after pumping to a vacuum pressure of 2.6 × 10−8 Torr and back-filling the vacuum system with O2 to a pressure of 1.0 × 10−7 Torr. By simultaneous monitoring of the cathodoluminescence (CL) and AES peak-to-peak heights over time for 22 h, the CL degradation of the phosphor was investigated. SrO was found to be stable under electron irradiation. The phosphor may have potential applications in the fields of lighting and displays due to its broad blue emission.

Surface characterization and cathodoluminescence degradation of ZnO thin films

Abstract ZnO thin films were successfully synthesized by the sol-gel method using the spin coater technique. The films were annealed at 600 °C in air for two hours and in Ar/H 2 (5%) flow for 30 and 60 min, respectively. Structural analysis, surface morphology and characterization, as well as optical analysis (photoluminescence and cathodeluminescence (CL)) were done on the samples and discussed in detail. CL degradation during prolonged electron irradiation on the films was also determined. A preferential orientation of the c -axis perpendicular to the surface was observed from X-ray diffraction data showing the peak from the (002) plane for the films annealed in both the air and in the H 2 flow. The film annealed in air exhibited a broad visible emission as well as a strong ultraviolet emission. A single-green emission peak around 511 nm was obtained from the film that was annealed in Ar/H 2 flow for 60 min. The CL study revealed that the intensity of the green emission (511 nm) was very stable during electron bombardment for electron doses of more than 160 C/cm 2 .

Surface analysis and cathodoluminescence degradation of undoped ZnO and ZnO:Zn phosphors

Commercial ZnO:Zn and undoped ZnO with a hexagonal wurtzite structure (JCPDS 80-0075) were studied. The direct optical bandgaps were 3.25 and 3.28 eV for the undoped and ZnO:Zn, respectively. Undoped ZnO exhibited strong ultraviolet band to band emission as well as weak deep level defect emission around the green, orange, and red regions. ZnO:Zn emits only green light centered at 514 nm which was attributed to oxygen vacancies. Electron irradiated degradation of the ZnO:Zn sample showed a rapid initial decrease in cathodoluminescence intensity and then a subsequent recovery and increase to up to double the initial value during prolonged electron irradiation. Deconvoluted peaks of the O1s x-ray photoelectron peaks confirmed the presence of oxygen-related defects in both samples. The ZnO:Zn sample was found to be very stable under electron bombardment, which makes it suitable for the use in field emission displays.

Luminescent properties, intensity degradation and X-ray photoelectron spectroscopy analysis of CaS:Eu2+ powder

In the present work, we report the structure, particle morphology, surface chemical states, photoluminescent properties and cathodoluminescence intensity degradation of the CaS:Eu2+ powder phosphors. X-ray diffraction, X-ray photoelectron spectroscopy, and photoluminescence (PL) spectroscopy data suggest that the powder contains secondary phases that were possibly formed during the preparation or due to unintended contamination. These phases presented an opportunity to demonstrate the dependence of the emission of the Eu2+ ion on the host lattice. An intense red PL broad band with a maximum at 650nm was observed when the powder was excited at 484nm using a monochromatized xenon lamp. When the powder was excited using a 325nm He–Cd laser an additional PL emission peak was observed at 384nm. The origin of this emission is discussed. Auger electron spectroscopy and Cathodoluminescence (CL) spectroscopy were used to monitor the changes in the surface chemical composition and CL intensity of the CaS:Eu2+ phosphors when the phosphor was irradiated with a 2keV electron beam in vacuum. The CL degradation is attributed to, among other things, the formation of a non-luminescent layer of CaSO4, surface charging, and possibly the presence of O defects. This phosphor is evaluated for applications in low voltage field emission displays and light emitting diodes.

The cathodoluminescence degradation and surface characterization of β-Ca3(PO4)2:Tb phosphor

The cathodoluminescence (CL) degradation mechanism of β-Ca3(PO4)2:Tb phosphors induced under low voltage electron beam bombardment in a vacuum chamber maintained at an O2 pressure of 1×10−6Torr and a background pressure of 1×10−8Torr was investigated. The surface chemical composition of the degraded powders analyzed by X-ray photoelectron spectroscopy, suggested that a new compound of oxide based calcium layer (i.e. CaxOy; x<1, y>1) was formed on the surface. The initial removal of C from the surface and the formation of a CaxOy over layer on the phosphor surface during electron irradiation caused a noticeably change in the CL intensity. Especially, based on the Auger electron spectroscopy combined with CL spectroscopy data, it is observed that there was an obvious correlation between the metal oxide layer build-up and the small decrease in CL brightness of the phosphors. The CaxOy, however, stabilized the phosphor preventing further degradation in the CL intensity. The CL properties and possible mechanism by which the new metal oxides were formed on the surface due to a prolonged electron beam irradiation are discussed.

Surface state of Y3(Al,Ga)5O12:Tb phosphor under electron beam bombardment

The surface state of Y3(Al,Ga)5O12:Tb phosphor was determined before and after 27h of prolonged electron beam exposure. The electron irradiation was carried out at a base pressure of 2.3×10−8Torr and an oxygen pressure of 1.0×10−6Torr. New surface layers were formed after the chemisorbed species were removed as a result of electron stimulated surface chemical reactions. The rate of the removal of the chemisorbed species from the phosphor's surface during prolonged electron irradiation was affected by the background working atmosphere as measured with Auger electron spectroscopy combined with cathodoluminescence (CL) spectroscopy. The CL intensity of the Y3(Al,Ga)5O12:Tb stabilized after removal of the chemisorbed species and stayed constant during further electron irradiation, indicating that this phosphor is good for the technology used in field emission displays. There was an increase in the Al, Y, O and Tb Auger peak intensities pointing to the formation of a complicated surface structure that was probably a combination of more than one chemical compound. X-ray photoelectron spectroscopy results suggested electron-beam induced formation of new interleave oxide layers, such as AlOx, YOx and Y(Al,Ga)O3 on the surface. These oxide layers acted as a protective layer inhibiting further CL intensity degradation during prolonged electron irradiation.

PL and CL degradation and characteristics of SrAl2O4: Eu2+,Dy3+ phosphors

Strontium aluminate phosphors are ideal for luminescent infrastructure materials. Their brightness and persistent glow time are much higher than previously used sulphide phosphors. Strontium aluminates prepared by the sol–gel and combustion methods are compared with commercially available strontium aluminate. High luminescent efficient SrAl2O4:Eu2+,Dy3+ pulsed laser deposited (PLD) thin films were also produced using the commercially available powder. Photoluminescence (PL) degradation studies showed that the phosphor intensity decreased about 20% over a period of 2 weeks under ultraviolet (UV) irradiation. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) showed that cathodoluminescence (CL) degradation is due to the formation of SrO due to electron stimulated surface reactions. The light output mechanism of the phosphor is also discussed in more detail.

Surface chemical changes of CaTiO3:Pr3+ upon electron beam irradiation

Surface chemical changes of CaTiO3:Pr3+ phosphor material and their effect on the red emission intensity of the 1D2→3H4 transition of Pr3+, upon electron beam irradiation are presented. Red emission at 613nm was obtained upon probing the surface with a 2keV electron beam. The surface chemical changes and Pr3+ red emission were monitored using an Auger Electron Spectroscopy (AES) and Cathodoluminescence (CL) spectrometer, respectively. The CL intensity decreased with a decrease in O on the surface at 1×10−8Torr base pressure and decreased with an increase in O on the surface at 1×10−6Torr O2. The X-ray Photoelectron Spectroscopy (XPS) revealed that CL degradation at 1×10−6Torr O2 is due to the formation of CaO and CaOx as well as TiO2/Ti2O3 non-luminescent species on the surface.

Luminescence response and CL degradation of combustion synthesized spherical SiO 2:Ce nanophosphor

This study was aimed to systematically investigate the luminescence response of SiO 2:Ce 3+ nanophosphors with different excitation sources. The powders were synthesized by using an urea assisted combustion method. SiO 2:Ce 1m% samples were also annealed at 1000 °C for 1 h in a charcoal environment to reduce incidental Ce 4+ to partial Ce 3+ ions. High resolution transmission electron microscopy (HRTEM) images of the as synthesized and annealed powder samples confirmed that the particles were spherical and in the size range of 3–8 nm in diameter. X-ray diffraction (XRD) and electron dispersion spectroscopy (EDS) results showed that the SiO 2 was crystalline and pure. Diffused reflectance, photoluminescence (PL) and cathodoluminescence (CL) results of the SiO 2:Ce 3+ samples were obtained and compared with each other. The CL degradation and the surface reactions on the surface of the SiO 2:Ce 3+ were studied with X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). A clear improvement in the chemical stability of the SiO 2:Ce 3+ annealed at 1000 °C were obtained.

Low voltage electron induced cathodoluminescence degradation and surface characterization of Sr3(PO4)2:Tb phosphor

Tb3+-doped Sr3(PO4)2 phosphor was prepared by a sol–gel combustion method. A trigonal structure having Sr and O atoms occupying two different lattice sites were obtained. Scanning Auger nanoprobe was used to analyze the morphology of the particles. Photoluminescence (PL) and cathodoluminescence (CL) properties of Sr3(PO4)2:Tb powder phosphors were evaluated and compared. In addition, the CL intensity degradation of Sr3(PO4)2:Tb was evaluated when the powders were irradiated with a beam of electrons in a vacuum chamber maintained at an O2 pressure of 1×10−6Torr or a background pressure of 1×10−8Torr O2. The surface chemical composition of the degraded powders, analyzed by X-ray photoelectron spectroscopy (XPS), suggests that new compounds (metal oxides) of strontium and phosphorous were formed on the surface. It is most likely that these compounds contributed to the CL intensity degradation of the Sr3(PO4)2:Tb phosphors. The CL properties and possible mechanism by which the new metal oxides were formed on the surface due to a prolonged electron beam irradiation are discussed.

Cathodoluminescence degradation of PLD thin films

The cathodoluminescence (CL) intensities of Y2SiO5:Ce3+, Gd2O2S:Tb3+ and SrAl2O4:Eu2+,Dy3+ phosphor thin films that were grown by pulsed laser deposition (PLD) were investigated for possible application in low voltage field emission displays (FEDs) and other infrastructure applications. Several process parameters (background gas, laser fluence, base pressure, substrate temperature, etc.) were changed during the deposition of the thin films. Atomic force microscopy (AFM) was used to determine the surface roughness and particle size of the different films. The layers consist of agglomerated nanoparticle structures. Samples with good light emission were selected for the electron degradation studies. Auger electron spectroscopy (AES) and CL spectroscopy were used to monitor changes in the surface chemical composition and luminous efficiency of the thin films. AES and CL spectroscopy were done with 2 keV energy electrons. Measurements were done at 1×10−6 Torr oxygen pressure. The formation of different oxide layers during electron bombardment was confirmed with X-ray photoelectron spectroscopy (XPS). New non-luminescent layers that formed during electron bombardment were responsible for the degradation in light intensity. The adventitious C was removed from the surface in all three cases as volatile gas species, which is consistent with the electron stimulated surface chemical reaction (ESSCR) model. For Y2SiO5:Ce3+ a luminescent SiO2 layer formed during the electron bombardment. Gd2O3 and SrO thin films formed on the surfaces of Gd2O2S:Tb3+ and SrAl2O4:Eu2+,Dy3+, respectively, due to ESSCRs.

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 CL degradation. The electron stimulated surface chemical reaction mechanism was used to explain the effects of S desorption and the formation of the nonluminescent CaSO4 layer on the surface.

Cathodoluminescence properties of SiO2:Pr3+and ZnO·SiO2:Pr3+ phosphor nanopowders

The successful incorporation of ZnO nanoparticles in Pr3+-doped SiO2 using a sol–gel process is reported. SiO2:Pr3+ gels, with or without ZnO nanoparticles, were dried at room temperature and annealed at 600 °C. On the basis of the X-ray Diffraction (XRD) results, the SiO2 was amorphous regardless of the incorporation of Pr3+ and nanocrystalline ZnO or annealing at 600 °C. The particles were mostly spherical and agglomerated as confirmed by Field Emission Scanning Electron Microscopy. Thermogravimetric analysis of dried gels performed in an N2 atmosphere indicated that stable phases were formed at ≥900 °C. Absorption bands ascribed to 3H4-3P(J = 0,1,2), 1I6 and 1D2 in the UV–VIS region were observed from SiO2:Pr3+ colloids. The red cathodoluminescent (CL) emission corresponding to the 3P0 → 3H6 transition of Pr3+ was observed at 614 nm from dried and annealed SiO2:Pr3+ powder samples. This emission was increased considerably when ZnO nanoparticles were incorporated. The CL intensity was measured at an accelerating voltage of 1-5 keV and a fixed beam current of 8.5 μA. The effects of accelerating voltage on the CL intensity and the CL degradation of SiO2:Pr3+ and ZnO·SiO2:Pr3+ were also investigated using Auger electron spectroscopy coupled with an Ocean Optics S2000 spectrometer.

Cathodoluminescence degradation of SiO2:Ce,Tb powder phosphors prepared by a sol-gel process

Auger electron spectroscopy (AES), cathodoluminescence (CL) spectroscopy, and x-ray photoelectron spectroscopy (XPS) were used to study the CL intensity degradation of SiO2:Cb,Tb powder phosphors prepared by a sol-gel process. The AES and the CL data were collected simultaneously when the powders were bombarded for 10h with a beam of electrons of current density of 54mA∕cm2, accelerated by 2kV in a vacuum chamber containing either 1×10−8 or 1×10−7Torr O2. A decrease of CL intensity was simultaneous with desorption of oxygen (O) from the surface, i.e., there is a correlation between the degradation of CL intensity and desorption of O. The AES and the XPS data suggest that a nonluminescent oxygen-deficient layer of SiOx (x<2) that could decrease the CL intensity was formed on the surface. Mechanisms by which oxygen desorption leads to a reduction of the CL intensity are discussed.

Cathodoluminescent properties of SrGa2S4:Eu2+ phosphor for field-emission display applications

Cathodoluminescent properties of a SrGa2S4:Eu2+ phosphor, which was synthesized by a high-temperature solid-phase reaction, have been studied. Cathodoluminescent spectra, the Commission International de l’Eclairage color coordinates, and photoluminescence spectra were obtained. The cathodoluminescent intensity was measured at an operating voltage of 0–10kV and current density of 0–160μA∕cm2. The degradation of cathodoluminescence after high-current-density bombardment was observed. X-ray photoelectron-spectroscopy analysis showed the electron-bombardment-induced chemical reaction in the SrGa2S4:Eu2+ phosphor in which SrSO4 was formed. The cathodoluminescent properties were compared with commercial green phosphor ZnS:Cu,Au,Al and it was found that the properties of SrGa2S4:Eu2+ phosphor under high current bombardment were more stable.