Photoluminescence Intensity Of Eu3 Research Articles

АЗИД-ИОН КАК ВОССТАНОВИТЕЛЬ Eu 3+ ДО Eu 2+

The behavior of the Eu3+ /N3- system in dimethyl sulfoxide was studied using the spectral methods. It was found that a redox interaction occurs between the components of the system, leading to the formation of divalent europium ion. The reaction proceeds spontaneously, quickly, almost during the time of mixing the solutions. A wide band in the region of 320–400 nm with a maximum at 350 nm was detected in the absorption spectrum of the reaction products; while a wide structureless 350–450 nm emission band with a maximum at 380 nm, characteristic of the Eu 2+ ion, was observed in the photoluminescence (PL) spectrum. An increase in the optical density and PL intensity in these regions is accompanied by a decrease in the absorption and PL intensity of Eu3+ ion in the longer wavelength region of the spectrum. A possible scheme of chemical processes in the Eu3+ /N3- – DMSO system has been proposed, including: 1) replacement of the Cl- ion by an azide ion in the Eu3+ coordination sphere; 2) electron transfer from N3- to Eu 3+, leading to the reduction of the latter to the divalent state. The prerequisite for the occurrence of this stage of the reaction is the low electron affinity (–2.68 eV) of the azide radical intermediate compared to the relatively high redox potential of the Eu3+ /Eu2+ pair (–0.36 eV); 3) complexation of the resulting Eu 2+ ion with N3- ligand, which is accompanied by an increase in the optical density of the solution at a maximum of 350 nm. The possibility of this stage of the reaction has been confirmed experimentally: an in crease in the concentration of the chloride ion in the reaction mixture, as a competing ligand with N3- , leads to a decrease in the optical density of the solution. It has been proposed to use the reduction reaction of Eu3+ with azide ion as a simple method for the preparation of divalent lanthanides, since salts of azide acid are readily available compounds.

Multimode fluorescence intensity ratio thermometer based on synergistic luminescence from Eu3+ to Mn4+ of SrTiO3:Eu3+-ZnTiO3:Mn4+ nanocomposites

SrTiO3:Eu3+-ZnTiO3:Mn4+ nanocomposites were successfully synthesized to obtain highly sensitive optical thermometers. The synthesized SrTiO3:Eu3+ was cubic perovskite structure, and the synthesized ZnTiO3:Mn4+ was cubic perovskite structure accompanying with hexagonal ilmenite structure. The results of X-ray diffraction and Fourier transform infrared spectra suggested that the mixture of SrTiO3:Eu3+-ZnTiO3:Mn4+ nanophosphors had no obvious change in the structure after heat treatment at 800 °C. The photoluminescence intensity of Eu3+ in SrTiO3 phosphors revealed a stable temperature dependence, while that of Mn4+ in ZnTiO3 phosphors revealed a sharp temperature dependence. The emission band ranging from 650 nm to 800 nm were separated into two bands centered at 690 nm and 714 nm, which were assigned to 4T2g→4A2g and 2Eg→4A2g transitions of Mn4+ ions in cubic perovskite and hexagonal ilmenite ZnTiO3 matrix, respectively. The maximum relative sensitivity revealed a dependence on the molar ratio between SrTiO3 and ZnTiO3. The maximum value of 3.3 %K−1 was achieved for the composites with the molar ratio of 1:3 between SrTiO3:Eu3+ and ZnTiO3:Mn4+ nanophosphors, which was higher than that of the composites before heat treatment, 3.1 %K−1. The results suggested the potential application in optical temperature probes for the composites.

A Hydrophilic Mixed Lanthanide Metal-Organic Framework Monitoring H2O in D2O.

Lanthanide metal-organic framework (Ln-MOF) luminescent sensors monitoring the H2O content in D2O are still rare. We designed and built a hydrophilic mixed Ln-MOF (Eu0.4Tb0.6-MOF) monitoring the H2O content in D2O. By designing a ligand and controlling the synthesis method, we achieved a balance between the structural stability and sensing capacity. When the H2O content ranges from 0 to 100%, the photoluminescence color of Eu0.4Tb0.6-MOF can change from yellow to green, which can be observed by the naked eye. The mechanism is that the photoluminescence intensity of Eu3+ decreases faster than that of Tb3+ when the H2O content increases. The sensing mechanism was studied further by transient fluorescence spectrometry.

Energy transfer dynamics in thermally stable Sm3+/ Eu3+ co-doped AEAlBS glasses for near UV triggered photonic device applications

Transparent, Sm3+/Eu3+ co-doped alkaline earth alumino borosilicate (AEAlBS) glasses have been synthesized by employing melt quenching process and explored their down-shifting luminescent properties to understand their utility in visible red photonic devices applications. The amorphous (non-crystalline) nature of the as-prepared glasses was analyzed with help of an X-ray powder diffraction (XRD) pattern, containing a broad peak. Photoluminescence (PL) properties demonstrate that the glasses can be proficiently excited by near-UV with a dominant peak centered at 402 nm. The PL emission spectra exhibit four emission peaks with an intense peak placed at 599 nm under 402 nm excitation. The optimum emission intensity was obtained for 0.5 mol% Sm3+ ions doped in AEAlBS glasses. In the present work, Sm3+ ion is acting as an effective sensitizer for Eu3+ activator in AEAlBS glasses, and part of energy gets transferred from sensitizer (Sm3+) to activator (Eu3+) ions. The PL intensity of Eu3+ ions is increasing at the expense of a decrease in the intensity of Eu3+ ion peaks in the co-doping AEAlBS glasses at λex = 402 nm. The efficient ET from sensitizer to activator ions proved to be dipole-dipole in nature via employing Dexter's formula with Reisfeld's approximation. The experimental lifetime values calculated from the PL decay profiles are decreasing with the surge in Eu3+ ion concentration in the as-prepared glasses. Inokuti Hirayama (I-H) model applied to the PL decay profiles confirms the ET process responsible for a decrease in experimental lifetimes as dipole-dipole in nature. The outcome of the I-H model is in consonance with the result given by Dexter theory. The CIE coordinates for single Sm3+ doped glasses are falling in the orange region and gradually surge into the red region by co-doping with Eu3+ ions in AEAlBS glasses. The temperature-dependent emission analysis reveals that the PL intensity at 150 °C and 200 °C perseveres up to 94.34 and 91.30% of the PL intensity at ambient temperature, respectively. All the obtained results contemplate the superiority of the multifunctional Sm3+/Eu3+ co-doped AEAlBS glasses for near UV triggered photonic device applications.

Improved photoluminescence in Eu3+ doped LaVO4 phosphor via co-doping of Li+/Ca2+ ions

This paper investigates the structural and optical properties of Eu3+ doped LaVO4 phosphor via co-doping of Li+ and Ca2+ ions. The unit cell volume and particles size vary with Li+ and Ca2+ ions. The Eu3+ doped phosphor emits intense red color at 613 nm due to Eu3+ ion on excitation with 266, 355 and 394 nm. The photoluminescence intensity of Eu3+ doped phosphor is enhanced up to 2.1 and 4.5 times via co-doping of Li+ and Ca2+ ions, respectively. Therefore, the Eu3+, Li+, Ca2+ co-doped LaVO4 phosphor may be suitable candidate for the development of display devices and white light emitting diodes.

Surface functionalized GdVO4:Eu3+ nanocrystals as turn-off luminescent probe for selective sensing of Cu2+ ions: role of pH

Poly(ethylene glycol) bis(carboxymethyl) ether (PEG-diacid) functionalized GdVO4:Eu3+ (5 at%) nanocrystals have been synthesized by polyol method. X-ray diffraction and transmission electron microscopy were used for the structural and morphology characterization of GdVO4:Eu3+ samples. Fourier transform infrared spectroscopy confirms the functionalization of nanocrystal surface by PEG-diacid. The resonance energy transfer from the vanadate (VO43−) to Eu3+ results in enhanced red emission. The GdVO4:Eu3+ as luminescent probe can selectively detect the presence of Cu2+ ions out of various metal ions investigated. Upon addition of Cu2+ ions, photoluminescence intensity of Eu3+ reduces instantaneously more than 90%, which is related to energy transfer from Eu3+ to Cu2+ ions. This shows the fast response during sensing. Surface functionalized particles shows a significant sensitivity as compared to the non-functionalized particles. These probes show more favourable in the alkaline aqueous medium for sensing. The possibility of reabsorption is ruled out. The limit of detection is about 9 nM of Cu2+ with the first response time and this method will be useful in selective sensing of Cu2+ ions in aqueous medium.

Effect of gadolinium on europium luminescence and its in-situ valence conversion in SrBPO5 host

Gadolinium/europium-activated strontium borophosphate (SrBPO5) phosphors were synthesized through solid-state reaction route in air atmosphere. Morphology and particle size were analyzed using scanning electron microscopy (SEM). Both europium and gadolinium were observed to emit strongly in this host. Interestingly, photoluminescence (PL) of both Eu2+ and Eu3+ was also observed. In the co-activated SrBPO5: Eu, Gd phosphors, the PL intensity of Eu3+ was enhanced significantly with simultaneous quenching of Eu2+ luminescence. The excitation and emission profiles of europium indicated the valence conversion (VC) of Eu2+ to Eu3+ in the presence of gadolinium. In addition to the VC, the luminescence of Eu3+ was also enhanced due to the energy transfer (ET) from Gd3+ to Eu3+. The ET was maximum for SrBPO5: Eu0.01, Gd0.075 phosphor at a critical distance of 2.76 Å. The co-activated phosphors exhibited a high color rendering index for Eu3+, suggesting it to be an efficient orange-red phosphor. PL lifetime decay studies suggested a single site occupancy for Eu2+ though the number of sites for Gd3+ and Eu3+ varied depending on their concentrations. The time-resolved emission spectroscopy (TRES) revealed different emission profiles for Eu3+ in varying time domains corroborating its different site occupancies. Strong thermoluminescence (TSL) was observed from these phosphors, though Eu2+ and Eu3+ PL were quenched following γ-irradiation. The calculated trap parameters and dose-response of γ-irradiated Eu, Gd-coactivated phosphor, suggested it to be a good thermoluminescent phosphor.

Impact of thermal annealing time on luminescence properties of Eu3+ ions in silver nanoparticles embedded lanthanum sodium borate glasses

Eu3+ ions doped Ag NPs embedded glasses have been prepared in a glass composition (80-x-y)B2O3-15Na2O-5La2O3-xEu2O3-yAgCl (where × = 0.0, 0.5 mol% and y = 0.1 mol%) by a well-known conventional melt quenching method. Tuning of the surface plasmon resonance band and the nucleation of silver nanoparticles (Ag NPs) was controlled by a thermal annealing schedule. Annealed samples displayed a broad surface plasmon resonance band positioned at 426 nm and its intensity gradually increases with increasing annealing time. HR-TEM micrographs reveal the formation of spherical Ag NPs inside the glass system with an average particle size around 6 nm (for annealed samples). The structural characterizations FESEM and EDX measurements has been also carried out. Photoluminescence (PL) excitation and emission spectra were recorded for studied glasses. The optimum PL enhancement of the Ag NPs embedded glass system was obtained at 10 h of heat treatment and subsequently PL intensity decreased beyond 10 h annealing time (PL quenching was observed). Photoluminescence results reveal that the improvement in the PL intensity of Eu3+ ions is predominantly due to the local field effect attributed by the silver NPs and the PL intensity quenching of Eu3+ ions attributed to the resonance energy transfer from Eu3+ ions to silver nanoparticles. Judd-Ofelt theory has been used to calculate the radiative parameters like intensity parameters, radiation transition probabilities, branching ratios and radiative lifetimes of Eu3+ ions. The calculated radiative parameters and luminescence studies suggested that the investigated 10 h annealed glass system was practically appropriate material for the development of LEDs and solid state lasers.

Luminescence behavior of Eu3+ in silica glass containing GdVO4: Eu nanocrystals

Transparent nanocrystalline glass (NCG) with GdVO4:Eu nanocrystals embedded in sintered nanoporous silica glass (NPSG) is synthesized, and intense photoluminescence and energy transfer are observed. X-ray diffraction (XRD), micro-Raman spectroscopy, transmission electron microscopy (TEM), and high-resolution TEM (HR-TEM) analyses confirm that GdVO4:Eu nanocrystals are dispersed in silica glass. The photoluminescence intensity of Eu3+ is significantly enhanced by the microcrystallization process. After microcrystallization, the photoluminescence intensity increases by 41 and 14 times for glass that has Eu3+ singly doped and glass codoped with Gd3+, VO43−, and Eu3+, respectively. When Gd3+ and Eu3+, Gd3+ and V5+, and V5+ and Eu3+ are respectively double-doped, the efficiencies of Gd3+ → Eu3+, VO43- → Eu3+, and Gd3+ → VO43− energy transfers are assayed to be 22.3%, 24.7%, and 70.1%, respectively. In Eu3+, Gd3+, and V5+ triple-doped samples, in addition to the above energy transfer processes, there is a Gd3+ → VO43- → Eu3+ energy transfer. Nevertheless, VO43- → Eu3+ is still the main process. Especially after microcrystallization, the transfer efficiency of VO43- → Eu3+ is significantly increased to 90.9% because the lattice site of Gd3+ is replaced by Eu3+ in GdVO4 crystallites.

Globular and Optically Transparent Photonic Crystals Based on 3D-opal Matrix and REE

By repeatedly filling the octahedral and tetrahedral pores of 3D-silica opal matrices with silica sol doped with rare-earth elements with subsequent heat treatment globular photonic crystals filled with mesoporous glass and optically transparent photonic crystals (quantytes) containing 10-30ppm REE were produced, depending on the annealing temperature. Voids of fcc lattice formed by amorphous spherical globules of SiO2 in globular photonic crystals are filled (up to 70%) by mesoporous glass doped with rare earth elements. Pores in the transparent photonic crystals disappear during sintering of globules of silica and mesoporous glass, but the periodic arrangement of REE-enriched silica areas (quantum dots) is retained. The reflection and luminescence spectra of photonic crystals filled with sols doped with europium Eu3+ and terbium Tb3+ were experimentally studied. A significant increase in the photoluminescence intensity of Eu3+ ions at the approach of the spectral position of the transition 5D0 → 7F2 to the edge of the bandgaps of the photonic crystal was determined. The authors come to the conclusion that a lowering of the threshold for lasing transitions in ions of rare elements is possible.

Synthesis and Characterization of SiO2@Y2MoO6:Eu3+ Core-Shell Structured Spherical Phosphors by Sol-Gel Process.

SiO2@Y2MoO6:Eu3+ core-shell phosphors were prepared by the sol-gel process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectra (EDS), transmission electron microscopy (TEM), photoluminescence (PL) spectra as well as kinetic decays were used to characterize the resulting SiO2@Y2MoO6:Eu3+ core-shell phosphors. The XRD results demonstrated that the Y2MoO6:Eu3+ layers on the SiO2 spheres crystallized after being annealed at 700 °C and the crystallinity increased with raising the annealing temperature. The obtained core-shell phosphors have spherical shape with narrow size distribution (average size ca. 640 nm), non-agglomeration, and smooth surface. The thickness of the Y2MoO6:Eu3+ shells on the SiO2 cores could be easily tailored by varying the number of deposition cycles (70 nm for four deposition cycles). The Eul+ shows a strong PL emission (dominated by 5D0-7F2 red emission at 614 nm) under the excitation of 347 nm UV light. The PL intensity of Eu3+ increases with increasing the annealing temperature and the number of coating cycles.

Influence of high magnetic field on the luminescence of Eu3+-doped glass ceramics

Rare earth (RE) doped materials have been widely exploited as the intriguing electronic configuration of RE ions offers diverse functionalities from optics to magnetism. However, the coupling of magnetism with photoluminescence (PL) in such materials has been rarely reported in spite of its fundamental significance. In the present paper, the effect of high pulsed magnetic field on the photoluminescence intensity of Eu3+-doped nano-glass-ceramics has been investigated. In our experiment, Eu-doped oxyfluoride glass and glass ceramic were prepared by the conventional melt-quenching process and controlled heat treatment. The results demonstrate that the integrated PL intensity of Eu3+ decreases with the enhancement of magnetic field, which can be interpreted in terms of cooperation effect of Zeeman splitting and magnetic field induced change in site symmetry. Furthermore, as a result of Zeeman splitting, both blue and red shift in the emission peaks of Eu3+ can be observed, and this effect becomes more prominent with the increase of magnetic field. Possible mechanisms associated with the observed magneto-optical behaviors are suggested. The results of the present paper may open a new gate for modulation of luminescence by magnetic field and remote optical detection of magnetic field.

Enhanced Eu3+ emission of Eu2+/Eu3+:(Y0.9La0.1)2O3 transparent ceramics synthesized in H2 atmosphere for modern lighting and display

Sintering in reducing atmosphere of H2 was employed to obtain Eu2+/Eu3+ co-existed (Y0.9La0.1)2O3 transparent ceramics. The co-existence of Eu2+ and Eu3+ was observed in radioluminescence spectrum. Minor content of Eu2+ is ascribed to the capture of oxygen vacancies for some Eu3+ ions in reducing atmosphere. Eu2+ was found to be stimulatory to the emission intensity of Eu3+. With the presence of Eu2+, the photoluminescence intensity of Eu3+ was greatly enhanced due to the energy transfer from Eu2+ to Eu3+. The results showed that Eu2+/Eu3+:(Y0.9La0.1)2O3 ceramics with low content of Eu3+ and minor Eu2+ have enormous potential to surpass the conventional Eu3+:Y2O3 red phosphors with high content of Eu3+ and no Eu2+ in the application of modern lighting and display.

Preparation and photoluminescence of Eu-doped oxyfluoride borosilicate glass ceramics

Eu-doped transparent oxyfluoride borosilicate glass ceramics containing Ba2GdF7 nanocrystals were prepared by controlling crystallization of melt-quenched glass fabricated under a reductive atmosphere. In the oxyfluoride borosilicate glass ceramics, the mean crystal size of Ba2GdF7 nanocrystals was about 30 nm, which could be observed by X-ray diffraction (XRD) and transmission electron microscopy analysis. The photoluminescence spectra of the samples excited at 392 nm showed that, besides the characteristic sharp emissions of Eu3+ ions, a very intense broadband emission of Eu2+ ions centered at 450 nm appeared. The photoluminescence intensity of Eu3+ and Eu2+ ions in the glass ceramics was much stronger than that in the as-made precursor. The long decay lifetimes of Eu3+ and Eu2+ ions evidenced the partitions of Eu3+ and Eu2+ ions into the Ba2GdF7 nanocrystals. The energy transfer from Gd3+ ions to Eu3+ and Eu2+ ions was confirmed by the excitation and emission spectra.

处理温度及CdS量子点对Eu3+掺杂SiO2干凝胶光致发光性质的影响

Silica xerogels doped with Eu3+ ions and co-doped with Eu3+/CdS nanoparticles were prepared through a two-step hydrolysis process.The effect of annealing temperature on photoluminescence properties of Eu3+-doped silica xerogel was investigated.The results showed that the photoluminescence intensity of Eu3+-doped silica xerogels was significantly dependent on the annealing temperature,which was closely related to the amount of the defect in xerogel network.The emission intensity increased with the annealing temperature from 40 ℃ to 300 ℃.But when the temperature exceeded 300 ℃,the intensity decreased with the annealing temperature.The study of the photoluminescence of co-doped xerogels showed that the intrinsic emissions due to 5D0→7FJ transitions of Eu3+ were observed,and the intensity was sensitive to the surface states of CdS particles and of the porous silica gel.

General Route to the Fabrication of ZnS and M‐Doped (M = Cd2+, Mn2+, Co2+, Ni2+, and Eu3+) ZnS Nanoclews and a Study of Their Properties

AbstractIn a mixed solvent composed of water and ethylenediamine, pure and ion‐doped (Cd2+, Mn2+, Co2+, Ni2+, Eu3+, etc.) clew‐like ZnS nanostructures, self‐assembled by curled ZnS nanowires, were synthesized by means of a hydrothermal route. These nanoclews possess a high specific surface area and can be separated easily from solution by natural settlement due to the self‐assembled structure; thus, they can be used in the field of photocatalysis. Photocatalytic experiments indicated that the pure ZnS nanoclews presented higher photocatalytic activity under UV irradiation, and the photocatalytic activity of the recycled ZnS nanoclews does not change noticeably after five reaction cycles. Furthermore, the CdxZn1–xS nanoclews showed high photocatalytic activity driven by visible light when the value of x was in the range from 0.3 to 0.7. The photoluminescence intensity of Eu3+‐doped ZnS nanoclews increased significantly compared with pure ZnS nanoclews. The formation mechanism of the ZnS nanoclews is also discussed.

Photoluminescence Characteristic of Ce3+-Eu3+ Co-doped Y3Al5O12 Phosphor Prepared by Combustion Method

For this study, Yttrium aluminum garnet (YAG) particles co-doped with Eu3+ and Ce3+ were prepared by combustion process using the 1:1 ratio of metal ions as reagents. The characteristics of the synthesized nano powder were investigated by means of X-ray diffraction (XRD), Scanning Electron Microscope (SEM), and photoluminescence (PL). The various YAG peaks, with the (420) main peak, appeared at all Ce concentration in XRD patterns. The YAG phase crystallized with results that are in good agreement with the JCPDS diffraction file 33-0040. The SEM image showed that the resulting YAG:Ce,Eu powders had uniform sizes and good homogeneity. The grain size was about 50 nm. The photoluminescence spectra of the YAG:Ce,Eu nanoparticles were investigated to determine the energy level of electron transition related to luminescence processes. It was composed of a broad band of Ce3+ activator, inclusive of the line peaks of Eu3+. The PL intensity of Ce3+ has the wavelengths of 480–650 nm and the PL intensity of Eu3+ has main peak at 590 nm.

Evidence of Trivalent Europium Incorporated in Anatase TiO2 Nanocrystals with Multiple Sites

Trivalent europium ions have been successfully incorporated in anatase TiO2 nanocrystals via a sol−gel solvothermal synthesis, in spite of a large mismatch in ionic radius between Eu3+ and Ti4+. The photoluminescence intensity of Eu3+:TiO2 nanocrystals has been significantly improved, which is comparable to that of common red phosphors. Multiple sites of Eu3+ in TiO2 nanocrystals have been identified by the technique of site-selective spectroscopy. Eu3+ ions at two lattice sites exhibit sharp emission and excitation peaks with site symmetries descending from D2d to approximate C2v and D2 as a result of the lattice distortion, whereas Eu3+ ions at disordered site near the surface are analogous to Eu3+ ions located in a glasslike environment. Extended X-ray absorption fine structure is utilized for identifying the local structure and Eu3+ coordination. The luminescence dynamics and crystal-field levels of Eu3+ at different sites have been analyzed. A growth mechanism for the incorporation of Eu3+ in the anatase lattice is also suggested.

Enhanced photoluminescence of Ca2Al2SiO7:Eu3+ by charge compensation method

The effect of compensator on optical properties of Ca2Al2SiO7:Eu3+ is systematically investigated by the X-ray powder diffraction, photo-luminescence (PL) properties and lifetime. It is obviously observed that the PL intensity of Eu3+ under 394 nm excitation increases in the order of Ca1.86Eu0.14Al2SiO7 (CAS), Ca1.72Na0.14Eu0.14Al2SiO7 (CASNa) and Ca1.86Eu0.14Al2.14Si0.86O7 (CASAl), the intensity of Eu3+ are 100%, 134%, 184%, and the lifetime of Eu3+ are 0.75 ms, 1.28 ms and 1.39 ms, respectively. A charge compensation model is proposed to explain the changes in the emission intensity and lifetime of Eu3+ in Ca2Al2SiO7 with different compensation methods.

Photoluminescence Spectral Change in Layered Titanate Oxide Intercalated with Hydrated Eu3+

A number of interesting photoluminescence properties of titanate layered oxide intercalated with hydrated Eu3+ have been demonstrated. Photoluminescence intensity of Eu3+ decreased rapidly with time during irradiation by UV light having energy higher than the band gap energy of the host TiO (Ti(1.81)O4) layer. This is presumably due to the decrease in energy transfer from the host TiO layer to Eu3+ as a result of the change in the hydration state of water molecules surrounding Eu3+, which is caused by the hole produced in the TiO valence band. When irradiation was discontinued, the emission intensity gradually recovered. The recovery time increased when the water in the interlayer is removed by heat treatment. This indicates that the state of interlayer water changes during irradiation and returns to its initial state after discontinuation of irradiation. The excitation spectra changed drastically at any given wavelength upon irradiation with UV light. A comparison of the excitation spectra before and after irradiation reveals that only the excitation peak at around the irradiation wavelength decreased upon irradiation, as in the case of spectral hole burning. The hydration state of water molecules surrounding Eu3+ presumably changes depending on the irradiation wavelength, leading to the above spectral change because the Eu/TiO film has a superlattice structure producing holes with different energies.