Low-voltage Cathodoluminescence Research Articles

Judd−Ofelt analysis, electronic and elastic properties from ab-initio study and photo-cathodoluminescence studies of Eu3+ activated CdWO4 nanophosphors

In the present work, CdWO4:Eu3+ (1–9 mol%) nanophosphors were synthesized by hydrothermal method. X-ray diffraction studies show that all compounds crystallized in monoclinic system with a wolframite structure of space group P2/c (No.13). The surface morphology studies by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed nano-rod like structures with size of the particle is approximately 40 nm. The functional group analysis were carried out by Fourier transform Infrared spectroscopy (FT-IR). The first principle quantum mechanical calculations based on density functional theory are performed using Crystal17 code to calculate the electronic and elastic properties. Luminescent characteristics, PL emission spectra show characteristics charge transfer (CT) band of CdWO4 and the emission bands corresponding to typical f-f transitions of Eu3+. The possible mechanism of concentration quenching effect is determined to be multipole-multipole interaction. The Eu3+ activated CdWO4 show red emission peak at 614 nm due to Eu3+ ion corresponding to 5D0 → 7F2 transition upon 295 nm excitation. The 5D0 → 7F1 (590 nm) magnetic dipole transition are less strong intense than 5D0 → 7F2 (614 nm) electric dipole transition. Judd–Ofelt parameters (Ω2 and Ω4), transition probabilities (AT), luminescence lifetime branching ratio are calculated first time for CdWO4 system. The value of is found greater than confirms the covalency between the metal and ligands indicates asymmetric environment around activator. Low voltage cathodoluminescent (CL) measurement were performed for Eu3+ activated CdWO4. The color chromaticity coordinates (CIE) values are estimated from the emission spectra and discussed in detail.

Highly efficient low-voltage cathodoluminescence of semiconductive nanoporous ZnMnO green phosphor films

The low voltage-operational thin-film type of the high-quality nanoporous ZnMnO (Mn: ∼18 at.%) green phosphor was fabricated by grain boundary engineering via the thermal nucleation of the well-crystallized columnar ZnMnO thin film. Vertically-aligned nanopores were observed to be formed throughout the whole area of the ZnMnO phosphor film; and the impact excitation of Mn2+ centers and the defect-mediated optical transition in nanoporous ZnMnO significantly enhanced green light emission in the nanoporous cavity. The results suggest nanoporous ZnMnO to hold promise for the next-generation color-field panels as a new type of the thin-film phosphor. The material characteristics, including the structural, electrical, and optical properties, are meticulously examined, and the formation mechanism of nanoporous ZnMnO and the origin of its low-voltage strong-green-cathodoluminescence are described.

Yb3+ sensitized Na2Y2B2O7:Er3+ phosphors in enhanced frequency upconversion, temperature sensing and field emission display

Na2Y2B2O7:Er3+/Yb3+ crystalline phosphors with single monoclinic phase have been synthesized by urea assisted solution combustion method and characterized through X-ray diffraction and Field emission scanning electron microscopy analysis. Frequency upconversion (UC) emission study has been carried out upon 980nm laser diode excitation and maximum enhancement of about ∼1433 times for the UC emission intensity of the green band in the codoped phosphor compared to that of the singly Er3+ doped phosphor has been reported. Optical temperature sensing study performed in the developed codoped phosphor reveals that the material can be used temperature sensing application within 300–613K temperature range with maximum sensitivity ∼7.9×10−3K−1 at 300K. The low-voltage cathodoluminescence measurements have been performed for Na2Y2B2O7:Er3+-Yb3+ phosphors as a function of accelerating voltage and filament current. The developed phosphors exhibit highly intense green emission with some weak emission bands in violet, blue and red regions upon electron beam excitation.

Introduction of carbon nano-tubes/graphenes cooperating conductive component into the phosphor to restrain the saturation behavior in low voltage cathodoluminescence

The saturation behavior in low voltage cathodoluminescence is originated from the low conductivity of the phosphor material. Based on the electrical percolation theory, carbon nano-tubes (CNTs) and graphenes (GPNs) were simultaneously introduced into the phosphor material to achieve a higher conductivity. Owing to the cooperating effects originated from the morphorlogical complementarities between CNTs and GPNs, the electrical percolation threshold of three-dimensional conductive network in the phosphor material was significantly decreased when the CNT/GPN mass ratio was properly set. With the introduction of CNT/GPN cooperating conductive component in the phosphor material, distinctly improved performances were achieved in low voltage cathodoluminescence.

Low-energy Cathodoluminescence for (Oxy)Nitride Phosphors.

Nitride and oxynitride (Sialon) phosphors are good candidates for the ultraviolet and visible emission applications. High performance, good stability and flexibility of their emission properties can be achieved by controlling their composition and dopants. However, a lot of work is still required to improve their properties and to reduce the production cost. A possible approach is to correlate the luminescence properties of the Sialon particles with their local structural and chemical environment in order to optimize their growth parameters and find novel phosphors. For such a purpose, the low-voltage cathodoluminescence (CL) microscopy is a powerful technique. The use of electron as an excitation source allows detecting most of the luminescence centers, revealing their luminescence distribution spatially and in depth, directly comparing CL results with the other electron-based techniques, and investigating the stability of their luminescence properties under stress. Such advantages for phosphors characterization will be highlighted through examples of investigation on several Sialon phosphors by low-energy CL.

Low-energy Cathodoluminescence for (Oxy)Nitride Phosphors

Nitride and oxynitride (Sialon) phosphors are good candidates for the ultraviolet and visible emission applications. High performance, good stability and flexibility of their emission properties can be achieved by controlling their composition and dopants. However, a lot of work is still required to improve their properties and to reduce the production cost. A possible approach is to correlate the luminescence properties of the Sialon particles with their local structural and chemical environment in order to optimize their growth parameters and find novel phosphors. For such a purpose, the low-voltage cathodoluminescence (CL) microscopy is a powerful technique. The use of electron as an excitation source allows detecting most of the luminescence centers, revealing their luminescence distribution spatially and in depth, directly comparing CL results with the other electron-based techniques, and investigating the stability of their luminescence properties under stress. Such advantages for phosphors characterization will be highlighted through examples of investigation on several Sialon phosphors by low-energy CL.

Local Field Enhancement-Induced Enriched Cathodoluminescence Behavior from CuI-RGO Nanophosphor Composite for Field-Emission Display Applications.

Field-emission displays (FEDs) constitute one of the major foci of the cutting edge materials research because of the increasingly escalating demand for high-resolution display panels. However, poor efficiencies of the concurrent low voltage cathodoluminescence (CL) phosphors have created a serious bottleneck in the commercialization of such devices. Herein we report a novel CuI-RGO composite nanophosphor that exhibits bright red emission under low voltage electron beam excitation. Quantitative assessment of CL spectra reveals that CuI-RGO nanocomposite phosphor leads to the 4-fold enhancement in the CL intensity as compared to the pristine CuI counterpart. Addition of RGO in the CuI matrix facilitates efficient triggering of luminescence centers that are activated by local electric field enhancement at the CuI-RGO contact points. In addition, conducting RGO also reduces the negative loading problem on the surface of the nanophosphor composite. The concept presented here opens up a novel generic route for enhancing CL intensity of the existing (nano)phosphors as well as validates the bright prospects of the CuI-RGO composite nanophosphor in this rapidly growing field.

Introduction of carbon nano-tubes into the phosphor to restrain the saturation behavior in low voltage cathodoluminescence.

To restrain the saturation behavior in low voltage cathodoluminescence, a small amount of carbon nano-tubes (CNT) was introduced into the phosphor to form CNT-introduced phosphor material. In the specific working conditions in low voltage cathodoluminescence, the field enhancement effect is initiated in the CNT-introduced phosphor material, and the local electric fields surrounding the top of each CNT is much stronger than the background electric field. The CNTs, with enhanced local electric fields surrounding their tops, play a key role in removing the electrons in the phosphor material, in which the CNTs act as convenient channels for electrons to be removed in cathodoluminescence. By introducing a small amount of CNTs into the phosphor, the saturation behavior in low voltage cathodoluminescence is effectively restrained, which has a similar effect as improving the conductivity of the phosphor material. The field enhancement effect in the CNT-introduced phosphor material may be activated and become more effective when the applied current density in cathodoluminescence is increased, thus the dynamic performance of the CNT-introduced phosphor material is favorable in low voltage cathodoluminescence.

Cathodoluminescence properties of gadolinium-doped CaMoO4:Eu nanoparticles

Gadolinium (Gd3+) doped CaMoO4:Eu nanoparticles were prepared via facile auto-combustion route and annealed at 900°C for ∼4h. X-ray diffraction study confirms the tetragonal scheelite type of CaMoO4 phase. Low voltage cathodoluminescent (CL) measurement were performed for Gd3+ (0 and 10at.%) co-doped CaMoO4:Eu as a function of accelerating voltage and filament current. CL studies confirm that Gd3+ co-doped CaMoO4:Eu has a visible emission peak centered at ∼615nm and a broad host emission band in the range 380–550nm. Commission Internationale de L’Eclairage chromaticity diagram co-ordinates of Gd3+ co-doped CaMoO4:Eu are found to be in reddish orange region as was observed through photoluminescence and CL spectra.

VUV–vis photoluminescence, low-voltage cathodoluminescence and electron-vibrational interaction of Mn2+ in Ba2MgSi2O7

A series of Mn2+ doped Ba2MgSi2O7 phosphors with a nominal formula Ba2Mg1−xMnxSi2O7 (x=0.01–0.13) were prepared by a high-temperature solid-state reaction technique. Their VUV–vis photoluminescence spectra, fluorescence decay curves, and low-voltage cathodoluminescence spectra were measured and discussed in detail. The results confirm that the Mn2+ ions enter into the four-coordinated Mg2+ site in Ba2MgSi2O7 host lattice. The main electron-vibrational interaction (EVI) parameters (such as Huang–Rhys factor, effective phonon energy and position of the zero-phonon line) were evaluated from the experimental spectra. Furthermore, the concentration and the temperature dependence of the emission intensity and the luminescence decays of Ba2MgSi2O7:Mn2+ were investigated, accordingly the schematic energy levels for Mn2+ in Ba2MgSi2O7 were proposed. Due to the high emission intensity under low-voltage electron beam excitation, a further optimized sample Ba2Mg0.95Mn0.05Si2O7 can be considered as a promising green phosphor for field emission displays (FEDs).

Investigation on the conductivity-dependent performance in low voltage cathodoluminescence.

With the increase of applied current density in low voltage cathodoluminescence, the exciting power tends to saturate, causing the saturation of electron-hole generation rate in the phosphor layer. Moreover, ground-state depletion could emerge for the activators owing to the increased exciting power and decreased average penetration depth of incident electrons in the phosphor layer, causing the decrease of the energy transfer probability of e-h pairs exciting ground-state activators. In addition, the radiative transition probability of excited activators could be decreased due to the increase of temperature. In view of these key factors, the efficiency decrease in cathodoluminescence is the inevitable result. To restrain the efficiency decrease so as to improve the performance in low voltage cathodoluminescence, the conductivity of the phosphor material was improved. By introducing a conductive component, which improves the conductivity of the phosphor material, the performance in low voltage cathodoluminescence was effectively improved.

Energy Transfer and Tunable Luminescence of NaLa(PO3)4:Tb3+/Eu3+ under VUV and Low-Voltage Electron Beam Excitation

Series of NaLa(PO3)(4):Tb3+/Eu3+ phosphors were prepared by a hightemperature solid-state reaction technique. Structure refinements were performed based on powder X-ray diffraction (XRD) data. VUV-UV-vis photoluminescence (PL), fluorescence decays, time-resolved emission spectra (TRES), and low-voltage cathodoluminescence (CL) spectra were utilized to investigate the luminescence and energy transfer processes. Under VUV-UV light and low-voltage electron beam excitation, NaLa(PO3)(4):Tb3+ and NaLa(PO3)(4):Eu3+ exhibit characteristic emissions of Tb3+ (D-5(4) --> F-7(J)) and Eu3+ (D-5(0) --> F-7(J)), respectively. By adjusting the doping concentration of Eu3+ ions in NaTb0.70La(0.30-x)Eux(PO3)(4), tunable emission colors are realized in a large color gamut, in which energy transfer from Tb3+ to Eu3+ was observed and discussed in detail. On the basis of the good VUV-vis PL and CL properties, NaLa(PO3)(4):Tb3+/Eu3+ phosphors might be promising for applications in plasma display panels (PDPs) and field emission displays (FEDs).

Novel orange and reddish-orange color emitting BaGd2O4:Sm3+ nanophosphors by solvothermal reaction for LED and FED applications

BaGd2O4 (BG):Sm3+ nanophosphors were synthesized by a solvothermal reaction method. The powder X-ray diffraction pattern confirmed their orthorhombic structure, and the morphological studies were carried out by taking the scanning and transmission electron microscopy images. The photoluminescence (PL) emission and PL excitation (PLE) spectra were investigated as a function of Sm3+ ion concentration. The PLE spectra revealed both Gd3+ and Sm3+ excitation bands in the shorter and longer wavelength regions, indicating that the efficient energy transfer occurred from the Gd3+ to Sm3+ ions in the BG host lattice. The PL spectra exhibited an intense orange emission due to (4G5/2→6H7/2) transition along with two other moderate intense emission peaks due to the (4G5/2→6H5/2) and (4G5/2→6H9/2) transitions. Based on the emission performance related to (4G5/2→6H7/2) transition, the Sm3+ ion concentration was optimized to be at 1mol%. The low-voltage cathodoluminescent (CL) measurements were also performed for BG:1mol% Sm3+ nanophosphors as a function of accelerating voltage and filament current. From the CL spectra, the reddish-orange emission was observed. The Commission International De I-Eclairage chromaticity coordinates of BG:Sm3+ nanophosphors were found to be very close to the chromaticity coordinates of Nichia Corporation developed amber light-emitting diodes.

Low-voltage cathodoluminescence and Eu/Tb L3-edge XANES of Na1+yCa1−x−2yEuxTbyPO4

Eu2+/Tb3+ single-doped and co-doped NaCaPO4 phosphors were synthesized by a high temperature solid-state reaction method. Their photoluminescence, low-voltage cathodoluminescence and Eu/Tb L3-edge X-ray absorption near edge structure (XANES) were studied. Under low-voltage electron beam excitation, NaCaPO4: Eu2+ exhibits a green emission and NaCaPO4: Tb3+ emits a color-tunable light resulted from cross-relaxation process. For co-doped samples NaCaPO4: Eu2+, Tb3+, dominant emission is from f–d transition of Eu2+ and the 5D4–7F5 transition of Tb3+ is overlapped to the former one. Eu/Tb L3-edge XANES spectra confirmed the occurrence of Eu3+ and inexistence of Tb4+ ions in the samples.

Homogeneous Precipitation Synthesis and Low-Voltage Cathodoluminescence of SnO2:Eu3+Phosphors for Field Emission Displays

A reddish-orange-emitting SnO2:Eu3+ phosphor for field emission displays (FEDs) was successfully synthesized via a homogeneous precipitation route using urea as a precipitant. The influences of the dopant concentration of Eu3+ and calcination temperature on optical properties were investigated. The low-voltage field emission properties of the FED device prepared using the synthesized SnO2:Eu3+ phosphors were reported. Under the UV light, SnO2:Eu3+ phosphors display the strong orange–red emission peaked at 587, 591, and 597 nm due to the 5D0→7F1 magnetic dipole transition of Eu3+. The phosphor doped with 1.0 mol% Eu3+ possesses the highest photoluminescent (PL) intensity. Under the low-voltage excitation of 300 V, the fabricated FED device exhibits the bright orange–red emission, high-voltage brightness saturation, and high color purity, which has a potential application in low-voltage full color FEDs.

Mechanisms in the saturation behavior for low voltage cathodoluminescence

The saturation behavior in exciting power Pexcite is the specific characteristic in low voltage cathodoluminescence due to the high resistivity of the phosphor, causing the saturation behavior in field emission display. With the increasing of current density j, the saturation behavior in Pexcite directly results in the saturation of electron-hole (e-h) generation rate gnum. The e-h generation region X would shrink, resulting in the drastic increase of e-h concentration generation rate gcon, causing the decrease in probability ηet of an e-h pair exciting an activator. In addition, the radiative transition probability ηrad of an excited activator would be decreased due to the shrinkage of X and increase of temperature. To restrain the saturation behavior in low voltage cathodoluminescence, the ultimate method is to lower the resistivity of the phosphor. By introducing In2O3 conductive component into Y2O3:Eu3+ phosphor, restrained saturation behavior in cathodoluminescence of Y2O3:Eu3+ phosphor was effectively achieved.

Introduction of cooperating conductive components into the phosphor to improve the low voltage cathodoluminescence

In order to improve the electric conductivity of Y2O3:Eu3+ phosphor with the least amount of conductive component so as to maximize the improvement in low voltage cathodoluminescence, In2O3 and Cu nanowires (NWs) were simultaneously introduced to form Cu NWs/In2O3-attached Y2O3:Eu3+ phosphor. In2O3 and Cu NWs play different roles in the formation of electrically conductive network, i.e., Cu NWs are suitable as conductive channels for charge transmission due to their one-dimensional morphology with large slenderness ratios, while the island-like In2O3 condensates form local conductive contacts joining the adjacent Cu NWs. Meanwhile, In2O3 forms attachment between Cu NWs and the phosphor. Owing to the cooperating effects between Cu NWs/In2O3 conductive components in the phosphor, the efficiency in low voltage cathodoluminescence was significantly improved.

Hydrothermal synthesis and improved luminescent properties of nanosheet-based rare earth orthoborates lantern-like assemblies

Hexagonal vaterite-type LuBO3:Tb3+/Eu3+ lantern-like phosphors have been successfully prepared by a simple hydrothermal process directly without sintering treatment. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, selected area electron diffraction, photoluminescence, low-voltage cathodoluminescence, and kinetic decays were used to characterize as-prepared phosphors, which present complex lantern-like 3D assemblies that are composed of nanosheets with thickness of about 23 nm and high crystallinity in spite of the moderate reaction temperature of 210 °C. The reaction mechanism and the self-assembly evolution process have been proposed. The pH, temperature, time, and ethylene glycol play critical roles in the formation of this specific hierarchical and complex 3D structure. Under UV excitation and low-voltage electron beam excitation, vaterite-type LuBO3:Tb3+ samples showed the characteristic green emission of Tb3+ corresponding to 5 D 4 → 7 F 5 transitions; vaterite-type LuBO3:Eu3+ particles exhibited a strong red emission corresponding to primary group electric dipole transition 5 D 0 → 7 F 2 with much higher R/O values (chromatically redder fluorescence, increase of 29 %) compared with samples by conventional solid-state reaction, which have potential applications in fluorescent lamps and field emission displays.

Monodisperse and core-shell structured SiO2@Lu2O3:Ln3+ (Ln=Eu, Tb, Dy, Sm, Er, Ho, and Tm) spherical particles: A facile synthesis and luminescent properties

The core-shell structured SiO2@Lu2O3:Ln3+ particles were realized by coating the Lu2O3:Ln3+ phosphors onto the surface of non-aggregated, monodisperse and spherical SiO2 particles by the Pechini sol–gel method. The as-synthesized products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), energy-dispersive X-ray (EDX) spectra, scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), photolumiminescence (PL), and low-voltage cathodoluminescence (CL). The results indicate that the 800°C annealed sample consists of crystalline Lu2O3 shells and amorphous SiO2 cores, in spherical shape with a narrow size distribution. The as-obtained particles show strong light emission with different colors corresponding to different Ln3+ ions under ultraviolet–visible light excitation and low-voltage electron beams excitation, which have potential applications in fluorescent lamps and field emission displays.

Electrospinning synthesis and luminescence properties of one-dimensional La9.33(SiO4)6O2: Ln3+ (Ln = Ce, Eu, Tb) microfibers

One-dimensional La(9.33)(SiO(4))(6)O(2): Ln(3+) (Ln = Ce, Eu, Tb) microfibers were fabricated by a simple and cost-effective electrospinning method. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL) and low voltage cathodoluminescence (CL) as well as kinetic decay were used to characterize the resulting samples. SEM and TEM results indicated that the diameter of the microfibers annealed at 1000 °C for 3 h was 200-245 nm. The microfibers were further composed of fine and closely linked nanoparticles. La(9.33)(SiO(4))(6)O(2): Ln(3+) (Ln = Ce, Eu, Tb) phosphors showed the characteristic emission of Ce(3+) (5d → 4f), Eu(3+) ((5)D(0)→(7)F(J)) and Tb(3+) ((5)D(3,4)→(7)F(J)) under ultraviolet excitation and low-voltage electron beams (3-5 kV) excitation. An energy transfer from Ce(3+) to Tb(3+) was observed in the La(9.33)(SiO(4))(6)O(2): Ce(3+), Tb(3+) phosphor under ultraviolet excitation and low-voltage electron beam excitation. Luminescence mechanisms were proposed to explain the observed phenomena. Blue, red and green emission can be realized in La(9.33)(SiO(4))(6)O(2): Ln(3+) (Ln = Ce, Eu, Tb) microfibers by changing the doping ions. So the La(9.33)(SiO(4))(6)O(2): Ln(3+) (Ln = Ce, Eu, Tb) phosphors have potential applications in full-color field emission displays.