Phosphor Thin Films Research Articles

Luminescence properties of Li-codoped Lu2SiO5:Ce thin-film phosphors prepared by sol–gel processing

Lu2SiO5:Ce (LSO:Ce) is one of the promising candidates for applications in X-ray micro-imaging and flat panel display devices. In order to improve its luminescence performances, the effect of Li+ on Lu2SiO5:Ce (LSO:Ce) thin-film phosphors was investigated. The LSO: x mol% Ce, y mol% Li (x=0.6–5, y=0–8) thin-film phosphors were fabricated by sol-gel processing and characterized by measuring their structure and luminescence. The results indicated that an optimal Ce-doping concentration for photoluminescence (PL) emission intensity was achieved at x=3 (equivalent 0.375at%). After Li-codoping (y=2), the PL intensity of the LSO:Ce thin-film phosphor was increased by a factor 1.9 with respect to the Li-free thin-film phosphor. The enhanced luminescence was dominated by the improvement of the crystallinity, and the decreased luminescence at y>2 could be attributed to the creation of excessive defects, which reduced the crystallinity of thin-film phosphors.

Enhanced light extraction from Y2O3: Eu3 + phosphor films via vacuum nano-imprint lithography using spin-on dielectric materials

This paper reports a simple process to enhance the extraction efficiency of photoluminescence from Eu-doped yttrium oxide (Y2O3: Eu3+) thin-film phosphor. The two-dimensional (2D) SiO2 photonic crystal layer was fabricated by vacuum nano-imprint method using a conventional spin-on dielectric solution as a nano-imprint resin and a 2D patterned Si stamp. The applied pressure and temperature of the patterning and annealing process facilitates the conversion of the spin-on dielectric solution into a SiO2 hemisphere structure resulting in high definition transfer of the 2D reverse pattern of the Si mold. The light extraction efficiency of the Y2O3: Eu3+ thin-film phosphor assisted by 2D SiO2 photonic crystal layer was approximately 1.72 times higher than that of the conventional Y2O3: Eu3+ thin-film phosphor.

Probing and Controlling Photothermal Heat Generation in Plasmonic Nanostructures

In the emerging field of thermoplasmonics, Joule heating associated with optically resonant plasmonic structures is exploited to generate nanoscale thermal hotspots. In the present study, new methods for designing and thermally probing thermoplasmonic structures are reported. A general design rationale, based on Babinet's principle, is developed for understanding how the complementary version of ideal electromagnetic antennae can yield efficient nanoscale heat sources with maximized current density. Using this methodology, we show that the diabolo antenna is more suitable for heat generation compared with its more well-known complementary structure, the bow-tie antenna. We also demonstrate that highly localized and enhanced thermal hot spots can be realized by incorporating the diabolo antenna into a plasmonic lens. Using a newly developed thermal microscopy method based on the temperature-dependent photoluminescence lifetime of thin-film thermographic phosphors, we experimentally characterize the thermal response of various antenna and superstructure designs. Data from FDTD simulations and the experimental temperature measurements confirm the validity of the design rationale. The thermal microscopy technique, with its robust sensing method, could overcome some of the drawbacks of current micro/nanoscale temperature measurement schemes.

Metal-ion doped luminescent thin films for optoelectronic applications

One type of commonly used luminescent material is so-called metal-ion doped phosphor. In such materials, optical and luminescent characteristics may be controlled by the host and metal-ion dopants (lanthanide and transition metal) of the phosphor. Many metal-ion doped luminescent materials can be made in the form of thin films. This feature article briefly reviews the development in the basic design rules of such materials, synthetic approaches (physical vapour deposition and chemical methods), and fabrication processes of metal-ion doped thin-film phosphors. We provide an overview of various techniques used for the structural characterization of metal-ion doped thin-film phosphors. Metal-ion doped luminescent thin films are attracting widespread interest in the optoelectronics field, ranging from flat-panel displays, solid-state lighting, optical waveguides, solar cells, and multifunctional applications. A critical issue for these applications is enhancing luminescence efficiency by considering the specific configuration of thin-film structure differing from the corresponding bulk material. Hence, the factors affecting luminescent properties of thin-film phosphors are identified. Some strategies in improving the performances of thin-film luminescent devices are discussed. Finally, the state of the art in some emerging fields is reviewed and future directions are indicated.

Luminescent properties and structural characteristics of sputter-deposited ZnGa2O4:Mn phosphor thin films

The correlation between the luminescent properties and structural characteristics of ZnGa2O4:Mn phosphor thin films has been investigated. The ZnGa2O4:Mn films have been prepared by radio frequency planar magnetron sputter deposition from a 2 mol. % Mn-doped ZnGa2O4 target in an Ar-O2 gas mixture whose pressure ranged between 2 and 20 mTorr. Films deposited at gas pressures above 10 mTorr showed a random crystallographic orientation, while the films deposited ≤10 mTorr exhibited a preferential orientation of the [110] direction perpendicular to the film surface. Internal stress in the ZnGa2O4:Mn films became increasingly compressive upon decreasing the gas pressure, reaching a value of ∼1.7 × 1010 dyne/cm2 at 2 mTorr. The observed change in internal stress correlated with the changes in the crystalline structure of the films. After a post-deposition anneal at 800 °C, the crystallinity of the films was improved. The rms surface roughness of the annealed ZnGa2O4:Mn films did not systematically depend on the gas pressure. Both photoluminescence (PL) and cathodoluminescence (CL) resulted in green light, and the emission peaked at ∼505 nm. This was attributed to the Mn2+ 3d-3d intrashell 4T1 → 6A1 transition. The intensities of both PL and CL emission increased as the gas pressure decreased, presumably due to the better crystalline quality with a highly [110]-textured structure and a more densely-packed microstructure in the ZnGa2O4:Mn films deposited at lower pressures.

Luminescence of Cr3 +-doped ZnGa2O4 thin films deposited by pulsed laser ablation

Chromium-doped zinc gallate powder is synthesized via a solid-state reaction and subsequently deposited as a thin film on quartz substrates by using a pulsed laser deposition technique under two different deposition conditions. The films are characterized with X-ray diffraction, scanning electron microscopy, UV–vis spectrophotometry and luminescent measurements. As the oxygen pressure is changed from 0 to 1Pa, we find that the grain size gets smaller, the crystallinity improves, the band-gap energy increases, the excitation peaks of the charge transfer band exhibit a remarkable blue-shift from 263 to 247nm and the intensity of the red emission (694nm) is enhanced. The results suggest that the structural and luminescent properties of ZnGa2O4:Cr3+ thin film phosphors are improved by deposition at an oxygen pressure of 1Pa.

Electroluminescence from Cr3+ in New Perovskite Thin-Film Phosphors using LaAlO3 and LaGaO3 as the Host

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Crystal growth and photoluminescence characteristics of Ca2MgSi2O7:Eu3+ thin films grown by pulsed laser deposition

Ca2MgSi2O7:Eu3+ films were deposited on Al2O3 (0001) substrates by pulsed laser deposition. The films were grown at various oxygen pressures ranging from 100 to 400mTorr. The crystallinity and surface morphology of the films were examined by X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. XRD and AFM respectively showed that the Ca2MgSi2O7:Eu3+ films had a zircon structure and consisted of homogeneous grains ranging from 100 to 400nm depending on the deposition conditions. The radiation emitted was dominated by a red emission peak at 620nm. The maximum PL intensity of the Ca2MgSi2O7:Eu3+ films grown at 300mTorr was increased by a factor of 1.3 compared to that of Ca2MgSi2O7:Eu3+ films grown at 100mTorr. The crystallinity, surface roughness and photoluminescence of the thin-film phosphors were strongly dependent on the deposition conditions, in particular, the oxygen partial pressure.

Multiple excitation process in deep-ultraviolet emission from AlGdN thin films pumped by an electron beam

We studied the deep-ultraviolet emission properties of Al0.999Gd0.001 N thin films pumped by an electron beam. The Al0.999Gd0.001 N thin films were grown on fused silica substrates using an ultra-pure reactive sputtering technique. The intra-orbital electron transition of the Gd3+ ions in Al0.999Gd0.001 N showed an extremely narrow luminescence line at 318 nm. We fabricated field-emission devices using an Al0.999Gd0.001 N phosphor thin film and analyzed the dependence of the device characteristics on the injected current and acceleration voltage. The maximum output power was 1.0 mW/cm2. The excitation cross section was of the order of 10−13 cm2 and was found to depend on the acceleration voltage. These results indicate that injected high-energy electrons multiply excite Gd3+ ion.

Study of crystallization process of Eu doped SrGa2S4 thin film phosphors by two electron beams evaporation and 355 nm Nd:YAG laser-annealing

In this study, the detailed crystallization mechanism of SrGa2S4: Eu2+ thin film phosphors annealed by Nd:YAG laser (355nm) was examined. Using the spectral transmittance measurements of SrS and Ga2S3 thin films, the influence of temperature on SrGa2S4 thin film formation by laser irradiation was investigated using the thermal conducting equation. From the calculation, it is thought that Ga2S3 absorbed greater laser energy than SrS and generated the heat. By diffusing heat throughout the thin film, crystallization was promoted.

ゾルゲル法によるCa0.6Sr0.4TiO3:Pr蛍光体薄膜の作製とその光学特性および電気特性

ゾルゲル法によるCa0.6Sr0.4TiO3:Pr蛍光体薄膜の作製とその光学特性および電気特性

Red photoluminescence of pulsed laser deposited Eu:NaY(MoO4)2 thin film phosphors on sapphire substrates

We have successfully fabricated Eu3+ ion doped NaY(MoO4)2 thin film phosphors using a pulsed laser deposition (PLD) method and studied their photoluminescence properties. The NaY(MoO4)2 thin films were deposited on single crystal sapphire substrates (0001) using deposition temperatures of 300 and 600 °C and O2 partial pressures of 100 and 300 mTorr. The properties of the NaY(MoO4)2:Eu3+ thin films were characterized by means of X-ray photoelectron spectroscopy (XPS), analyzing photoluminescence (PL) spectra, X-ray diffraction (XRD), atomic force microscopy (AFM) and field-emission scanning electron microscopy (FE-SEM). The degree of crystallinity increased with an increase in oxygen pressure and the thickness was around 220 nm for the films coated on the Al2O3 substrate. The photoluminescent property of the thin films was enhanced by increasing substrate temperature and the film deposited at 300 mTorr exhibited the highest luminance. Intense and efficient PL emission was observed from the thin films. The broad absorption bands covering the UV region from 215 to 350 nm are attributable to the O → Mo and O → Eu charge transfer transitions. The thin film phosphors show a strong red emission at 617 nm due to the electric dipole transitions of 5D0 → 7F2 of Eu3+ ions, which clearly indicates that the Eu3+ occupies a non centro-symmetic site.

Synthesis of Green Emitting and Transparent Zn2SiO4:Mn2+ Thin Film Phosphors on 2D Photonic Crystal Patterned Quartz Substrates

Zn2SiO4:Mn2+ thin film phosphors (TFPs) have been synthesized by RF magnetron sputtering, using a single multicomponent stoichiometric target. And 2D photonic crystal patterns were introduced on a quartz substrate to enhance the light extraction efficiency. In order to introduce 2D photonic crystal patterns on a quartz substrate, nanosphere lithography was used. Polystyrene spheres, with diameter of 330 nm, were transferred on the quartz substrate and subsequently were served as an etch mask. Quartz substrates were patterned by CF4 gas-based reactive ion etching. Zn2SiO4:Mn2+ were deposited on that 2D photonic crystal patterned quartz substrate and the effect of height of photonic crystal layers were investigated. The light extraction efficiency of Zn2SiO4:Mn2+ thin film phosphors deposited on the photonic crystal patterned quartz substrate was enhanced three times to compared with that of flat Zn2SiO4:Mn2+ thin film phosphors due to the Bragg diffraction and leaky mode caused by PCLs. Transmittance of Zn2SiO4:Mn2+ TFPs deposited on the photonic crystal patterned substrate was high enough, above 70% in the visible light region with respect to that of quartz substrate.

Low Temperature Growth of CaTiO3: Pr Phosphor Thin Film on Flexible Substrate by Photo- induced Chemical Solution Process

ABSTRACTPreparation of the CaTiO3:Pr (CTO:Pr) phosphor thin film on PET substrate was investigated by using the excimer laser-assisted metal organic decomposition(ELAMOD) and photo reaction of nano-particles (PRNP) process. The effects of the substrate material, starting materials, and UV sources on photoluminescence (PL) were investigated. By using the BaTiO3(BTO) nano-particles buffer layer and the CTO: Pr nano-particles as a starting material, CTO: Pr thin film on the PET substrate was successfully obtained by using the KrF laser and excimer lamp irradiation at 25°C. It was found that excimer lamp irradiation is effective for improving the PL of the films.

Selecting Morphology of Y3Al5O12:Ce3+ Phosphors for Minimizing Scattering Loss in the pc-LED Package

This paper reports the conversion efficiency (CE) and packaging efficiency (PE) of white phosphor-converted light-emitting diodes (pc-LED) with four morphologically different phosphors, such as nanophosphors (NPs), thin-film phosphors (TFPs), ceramic plate phosphors (CPPs), or micro-size powder phosphors (MPPs) to search for additional room for minimizing the scattering loss of conventional MPPs used in a conventional white pc-LED. The newly developed film-type phosphors with optical structures employ a two dimensional photonic crystal layer (2D PCL) to improve the extraction efficiency as well as optical short-wave pass filters (SWPFs) for recycling of backward emission loss. A careful evaluation of the CE, PE and internal quantum efficiency (IQE) shows that the 2D PCL, SWPF-assisted NP thick films characteristics and 2D PCL-assisted CPP characteristics, such as an improved PE and reduced scattering loss, satisfy the requirements for current white pc-LED applications. Therefore, a 2D PCL-assisted CPP-capped pc-LED shows a CE of ∼56% and a PE of ∼70%, surpassing the values (∼55, ∼63%) observed in the typical MPP-based white pc-LED at 350 mA. In addition, a 2D PCL, SWPF-assisted NP thick film-capped LED shows a CE of ∼31% and PE of ∼82%, indicating that there is a large room for further improvement in the IQE of as-prepared NPs.

Influence of local atomic configuration in AlGdN phosphor thin films on deep ultra-violet luminescence intensity

We investigated the narrowband ultraviolet emission properties of Al0.94Gd0.06N phosphor thin films pumped by an electron beam. An extremely narrow luminescence line, which was less than 1 nm from the intra-orbital f-f transition in Gd3+ ions, was confirmed at 318 nm. The corresponding emission efficiency was improved by decreasing the growth temperature. The extended X-ray absorption fine structure analysis of the local atomic structure revealed that a low-temperature growth led to the formation of a uniform atomic configuration around Gd, which was found to play a key role in improving the luminescence intensity of the films.

Photoluminescence of Titanium-Doped Zinc Orthosilicate Phosphor Gel Films

Titanium-doped zinc orthosilicate (α-Zn2SiO4:Ti) phosphor thin films were deposited on silicon wafer substrates by the sol–gel process. The crystallization processes and photoluminescence properties of the films were investigated. X-ray diffraction revealed that the dried films were amorphous and converted into single-phase willemite structure following annealed at 600 °C and above. Upon thermal annealing at 800 ° –1000 °C, Ti-doped willemite thin films had the average crystallite sizes of 17∼28 nm. The luminescence properties of phosphor films were characterized by excitation and emission spectra. Photoluminescence spectra of Ti-doped films exhibited prominent blue emission bands centered at 402 nm under an excitation wavelength of 225 nm. The emission intensity was dependent on the level of titanium doping, annealing temperature, and film thickness.

X-ray photoelectron spectroscopy analysis for undegraded and degraded Gd2O2S:Tb3+ phosphor thin films

This paper presents the X-ray Photoelectron Spectroscopy (XPS) analysis for the undegraded and degraded Gd2O2S:Tb3+ thin film phosphor. The thin films were grown with the pulsed laser deposition (PLD) technique. XPS measurements were done on Gd2O2S:Tb3+ phosphor thin films before and after electron degradation. The XPS technique has proven the presence of Gd2O3 on the degraded and undegraded thin film spots. The presence of the SO2 bonding was also detected after degradation. This clearly indicates that surface reactions did occur during prolonged electron bombardment in an oxygen atmosphere.

Patterning of Gd 2(WO 4) 3:Ln 3+ (Ln = Eu, Tb) luminescent films by microcontact printing route

Gd 2(WO 4) 3 doped with Eu 3+ or Tb 3+ thin phosphor films with dot patterns have been prepared by a combinational method of sol–gel process and microcontact printing. This process utilizes a PDMS elastomeric mold as the stamp to create heterogeneous pattern on quartz substrates firstly and then combined with a Pechini-type sol–gel process to selectively deposit the luminescent phosphors on hydrophilic regions, in which a Gd 2(WO 4) 3:Ln 3+ (Ln = Eu, Tb) precursor solutions were employed as ink. X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectra, as well as low voltage cathodoluminescence (CL) spectra were carried out to characterize the obtained samples. Under ultraviolet excitation and low-voltage electron beams excitation, the Gd 2(WO 4) 3:Eu 3+ samples exhibit a strong red emission arising from Eu 3+ 5D 0,1,2– 7F 1,2 transitions, while the Gd 2(WO 4) 3:Tb 3+ samples show the green emission coming from the characteristic emission of Tb 3+ corresponding to 5D 4– 7F 6,5,4,3 transitions. The results show that the patterning of rare earth-doped phosphors through combining microcontact printing with a Pechini-type sol–gel route has potential for field emission displays (FEDs) applications.

Enhancement of Photoluminescence by Photonic Crystal Scattering and Nanocrystalline Surface Scattering From Y2O3:Tb3+ Film Phosphors

Thin film phosphors (TFPs) are generally less efficient than powder phosphors because a large fraction of the light generated within the film is lost by the wave-guiding effect. This study examined the effects of the grain size of sol-gel derived nanocrystalline Y2O3:Tb3+ TFPs on the extraction efficiency of emission light and the absorption of UV excitation light caused by Bragg scattering from an over-coated two-dimensional (2D) polystyrene (PS) photonic crystal layer (PCL). The total scattering enhancement of the integrated photoluminescence (PL) efficiency from the 20 mol % Li-doped TFP modified with the 580 nm 2D PS was 9.3 times higher than that of pure flat Y2O3:Tb3+. This was attributed to a combination of increased emission in the anomalous surface scattering enhancement of nanocrystalline films, and both emission and excitation in the Bragg scattering enhancement of 2D PCLs.