Intervalence Charge Transfer State Research Articles

3P0 → 1D2 non-radiative relaxation control via IVCT state in Pr3+-doped Na2Ln2Ti3O10 (Ln=La, Gd) micro-crystals with triple-layered perovskite structure

A comparative study on luminescence properties and their nature in relation to different substitutions at the rare-earth site of Pr3+-doped layered perovskite Na2Ln2Ti3O10 (Ln = La, Gd) micro-crystals has been reported. At room temperature, the sample for Ln = La exhibits intense greenish-blue (3P0 → 3H4) and red (1D2 → 3H4) emissions under bandgap excitation, whereas for Ln = Gd the red emission can be only observed and all 3P0-related emissions were completely quenched. It turns out that 3P0→ 1D2 nonradiative relaxation in Na2Ln2Ti3O10 (Ln = La, Gd) critically depends on the energy location of Pr3+-Ti4+ intervalence charge transfer state and thus on the location of Pr3+ ground state 3H4 with respect to the top of the valence band. Temperature-dependent photoluminescence spectra in the 4.5–300 K range reveals a significant increase of Pr3+ luminescence, which is ascribed to an efficient thermally-activated energy transfer process from host to Pr3+ ions. Lower energy of self-trapped exciton state relative to the bottom of conduction band for Ln = Gd is responsible for thermoluminescence observed at higher temperatures than that for Ln = La sample. In the case of Ruddlesden-Popper type layered perovskite oxide compounds our results show a possibility of controlling the 3P0 → 1D2 nonradiative relaxation of Pr3+ ions and the energy distance of the relatively shallow traps in relation to the bottom of conduction band, giving a way for specific band-gap engineering in these materials.

Hole Trapping Process and Highly Sensitive Ratiometric Thermometry over a Wide Temperature Range in Pr3+-Doped Na2La2Ti3O10 Layered Perovskite Microcrystals.

We demonstrate a potential optical thermometric material, Pr3+-doped triple-layered perovskite Na2La2Ti3O10 microcrystals, which promises a remarkable performance in temperature sensing over a wide temperature range (125-533 K), with a maximum relative sensitivity of 2.43% K-1 at 423 K. Both temperature and high-pressure dependent photoluminescence measurements were performed for this compound. It turns out that the Pr3+-Ti4+ intervalence charge transfer state is the primary cause for the very efficient thermometric characteristics in the 296-533 K range. In the 125-300 K range, 3P1 and 3P0 levels of Pr3+ can be exploited as thermally coupled energy levels for temperature sensing with high sensitivity at and below room temperature. A significant enhancement of the Pr3+ ions' luminescence observed in the 4.5-300 K range is ascribed to an efficient, thermally activated energy transfer process from the host to Pr3+ ions. Carrier recombination on Pr3+ related hole traps was proposed in the studied system. The thermoluminescence properties are investigated, and possible mechanisms for the interpretation of the experimental results are discussed as well. This work may provide a perspective approach to design a high-performance, self-calibrated optical thermometer operating over a wide temperature range.

Direct Evidence of Intervalence Charge-Transfer States of Eu-Doped Luminescent Materials

Direct evidence is given for the existence of intervalence charge transfer (IVCT) states of Eu2+/Eu3+ pairs in Eu-doped CaF2, SrF2, and BaF2. They are detected in diffuse reflectance spectra. In doped materials, IVCT states, in which an electron transfer occurs between two metal sites differing only in oxidation state, are rather difficult to observe because the absorption bands are extremely broad and flat, their intensity is low, and no emission follows the IVCT absorptions. Their assignment as IVCT states is provided by state-of-the-art multiconfigurational ab initio calculations. Although IVCT states of lanthanide-doped materials have largely been overlooked so far, they can cause luminescence quenching and even complete luminescence excitation loss. Their direct observation and independent assignment in classical dopant (Eu) and hosts (CaF2, SrF2, BaF2) are very significant: They suggest that the occurrence of IVCT states in other lanthanide-activated materials is very likely overlooked and their impact is ignored.

Enhanced red emissions and higher quenching temperature based on the intervalence charge transfer in Pr3+ doped LiNbO3 with Mg2+ incorporation

For investigating the effect of exceeding Mg2+ doping on optical properties and luminescent mechanism, detailed spectroscopic characterizations of Pr3+ (1 mol%) and Mg2+ (0 and 5mol%) doped congruent LiNbO3 (CLN) single crystals were studied. The exceeding Mg2+ doping of 5mol% was found to enhance the dominant red emissions (1D2→3H4) under 360nm excitation with a larger population of 1D2 via the Pr3+-Nb5+ intervalence charge transfer (IVCT). Quenching of blue-green emissions in Pr3+ doped CLNs was ascribed to the depopulation of 3P0 via IVCT state. The lifetimes related to 1D2→3H4 were significantly increased with Mg2+ incorporation at a temperature range from 20K to 430K. As the temperature rises, the progressively thermal quenching of the red emission and lifetime was observed, which was interpreted with the thermally activated energy transfer from the IVCT state to ground level 3H4. More importantly, exceeding Mg2+ doping led to a displacement of the IVCT state in the configuration coordinate diagram, resulting in larger quenching activation energy and higher quenching temperature (310K→370K) in Pr:Mg:CLN. The emission enhancement and quenching temperature increase at exceeding Mg2+ concentration were considered to be of practical value in luminescent and laser applications.

Tuning photoluminescence in layered Pr doped Bi4Ti3O12 nanosheets via molten salt process

Novel single-phased and emission-tunable layered Bi4Ti3O12:Pr3+ nanosheets have been successfully synthesized via molten salt method. The as-prepared samples were characterized by X-ray diffraction and scanning electron microscopy to investigate their crystalline phases and morphologies. Furthermore, we explored the photoluminescence properties of the Bi4Ti3O12:Pr3+ phosphors, while the cross-relaxation processes between 1D2, 3P0 emitting states and the Pr3+–Ti4+ intervalence charge transfer state, color tuning and thermally stability were discussed in detail. Besides, the emission hue of Bi4Ti3O12:Pr3+ could be adjusted from white-light (0.3059, 0.2731) to yellow (0.5167, 0.4792) by tuning the excitation wavelength, in which the ratio of 3P0→3H4 and 1D2→3H4 was changed. In addition, the increases in temperatures led to the cooperative effects of thermal-quenching behavior and the cross-relaxation processes, thus resulting in the significant decrease of 3P0→3H4 emission in comparison to the 1D2→3H4 emission. The results suggested that the Bi4Ti3O12:Pr3+ is an effective color-tuning phosphor.

Intervalence charge transfer state interfered Pr3+ luminescence: A novel strategy for high sensitive optical thermometry

In this study, the Ruddlesden-Popper type perovskite Na2La1.96Pr0.04Ti3O10 micro-crystals are synthesized and demonstrated to be one of the most promising optical temperature sensing materials. The measured maximal absolute temperature sensitivity of this phosphor reaches as high as 0.40K−1, remarkably superior to those of the inorganic optical thermometric materials reported previously; while the relative sensitivity is 1.96%K−1, ranking among the highest ones for the inorganic optical thermometric materials. Analysis of the configurational coordinate diagram indicates that thermo-induced relaxation between the Pr3+ 3Px and 1D2 levels through Pr3+-Ti4+ intervalence charge transfer state is the primary cause for the temperature sensing characteristics. Based on these results, a novel temperature sensing strategy utilizing the intervalence charge transfer state interfered Pr3+ luminescence to perform optical thermometry is proposed, which is further demonstrated to be universally valid for the Pr3+ doped oxides containing d0 configured transition metal ions, as long as energy of the intervalence charge transfer state is moderate. This work may provide useful inspiration for developing high sensitive optical thermometric materials.

Excitation Wavelength Dependent Luminescence of LuNbO4:Pr3+—Influences of Intervalence Charge Transfer and Host Sensitization

A series of LuNbO4:Pr3+ phosphors was prepared by a solid-state reaction method at high-temperature. Rietveld refinements were performed based on powder X-ray diffraction (XRD) data. Diffuse reflectance spectra (DRS), UV–vis photoluminescence (PL), time-resolved emission spectra (TRES), and fluorescence decays were utilized to study the luminescence and host sensitization processes of Pr3+ in LuNbO4. Excitation wavelength dependent luminescence of LuNbO4:Pr3+ was investigated and explained in consideration of the processes of nonradiation relaxation via cross-relaxation, multiphonon relaxation, and crossover to the intervalence charge transfer (IVCT) state. Furthermore, the host sensitization of Pr3+ emission in LuNbO4 was confirmed and the energy transfer efficiency from host to Pr3+ increased with increasing Pr3+ doping concentration/temperature. Because the change of emission intensities for both blue from the host and red from 1D2 is sensitive to temperature, a large variation of emission color is obs...

A Novel Optical Thermometry Strategy Based on Diverse Thermal Response from Two Intervalence Charge Transfer States

In this work, a novel thermometry strategy based on the diversity in thermal quenching behavior of two intervalence charge transfer (IVCT) states in oxide crystals is proposed, which provides a promising route to design self‐referencing optical temperature sensing material with superior temperature sensitivity and signal discriminability. Following this strategy, uniform Tb3+/Pr3+:NaGd(MoO4)2 micro‐octahedrons are directionally synthesized. Originated from the diverse thermal responses between Tb3+‐Mo6+ and Pr3+‐Mo6+ IVCT states, fluorescence intensity ratio of Pr3+ to Tb3+ in this material displays excellent temperature sensing property in a temperature range from 303 to 483 K. The maximum absolute and relative sensitivity reaches as high as 0.097 K−1 and 2.05% K−1, respectively, being much higher than those of the previously reported optical thermometric materials. Excellent temperature sensing features are also demonstrated in the other Tb3+/Pr3+ codoped oxide crystals having d0 electron configured transition metal ions (Ti4+, V5+, Mo6+, or W6+), such as scheelite NaLu(MoO4)2 and NaLu(WO4)2, and monazite LaVO4 and perovskite La2Ti3O9, evidencing the universal validity of the proposed strategy. This work exploits an effective pathway for developing new optical temperature sensing materials with high performance.

Ultrahigh-sensitive optical temperature sensing based on ferroelectric Pr3+-doped (K0.5Na0.5)NbO3

Optical temperature sensing based on the variation of the fluorescence intensity ratio of rare-earth materials has become appealing due to the multiple superiorities over the electrical temperature sensing. However, confined by the largest energy separation of two thermally linked levels of rare-earth ions, the highest sensitivity of such temperature sensing is essentially smaller than 2878/T2, as reported previously from diverse systems. In this work, we demonstrate that ultrahigh-sensitive temperature sensing can be achieved from Pr3+-doped (K0.5Na0.5)NbO3 based on the intensity ratio of the 1D2-3H4 emission to the 3P0-3H4 emission. The ratio can be increased as high as 18-fold when temperature rises from room temperature to 456 K, nicely fitting a thermally linked-levels-like equation and showing an ultrahigh sensitivity of 7997/T2. The striking change of the ratio is attributed to the interaction between the two emission levels and the intervalence charge transfer state. This work may have provided a distinct route in the field of optical temperature sensing utilizing rare-earth-doped materials. In addition, the resultant product also possesses excellent photoluminescence and ferroelectric properties, showing promising potentials in multifunctional devices for practical applications.

Photoluminescent properties of Pr3+ activated Y2WO6 for light emitting diodes

A series of Pr3+ activated Y2WO6 phosphors are prepared by solid state reaction method. The crystalline phase and luminescent properties of samples are discussed by X-ray diffraction spectra and photoluminescence spectra, respectively. The photoluminescence excitation spectrum indicates that the sample can absorb both UV and blue light. And the samples emit a blue emission of WO66− and the characteristic red emission of Pr3+. The partial quenching of the other prominent blue luminescence from 3P0 state is ascribed to a radiationless relaxation pathway involving a low-lying Pr4+–W5+ intervalence charge transfer (IVCT) state. Through adjusting the excitation wavelength, Y2WO6: Pr3+ can not only emit warm white light with CIE coordinates of (0.33, 0.37) but also show bright red emission with CIE coordinates of (0.66, 0.33). In addition, the thermal properties of the samples are also investigated in detail. The results show that the application as a white and red component for white light emitting diodes is proposed.

Configuration coordinate energy level diagrams of intervalence and metal-to-metal charge transfer states of dopant pairs in solids.

Configuration coordinate diagrams, which are normally used in a qualitative manner for the energy levels of active centers in phosphors, are quantitatively obtained here for intervalence charge transfer (IVCT) states of mixed valence pairs and metal-to-metal charge transfer (MMCT) states of heteronuclear pairs, in solid hosts. The procedure relies on vibrational frequencies and excitation energies of single-ion active centers, and on differences between ion-ligand distances of the donor and the acceptor, which are attainable empirically or through ab initio calculations. The configuration coordinate diagrams of the Yb(2+)/Yb(3+) mixed-valence pair in Yb-doped YAG and the Ce(3+)/Yb(3+) heteronuclear pair in Ce,Yb-codoped YAG, are obtained and described. They are drawn from empirical data of the single-ions and their usefulness is discussed. The first diagram suggests that IVCT states of Yb(2+)/Yb(3+) pairs may play an important role in the quenching of the Yb(3+) emission and it provides the details of the quenching mechanism. The second diagram supports the interpretation recently given for the energy transfer from Ce(3+) to Yb(3+) in Ce,Yb-codoped YAG via a MMCT Ce(4+)-Yb(2+) state and it provides the details. The analyses of the two diagrams suggest the formation of Yb(2+)/Yb(3+) pairs after the Ce(3+)-to-Yb(3+) MMCT, which is responsible for the temperature quenching of the Yb(3+) emission excited via Ce(3+) (4f → 5d) absorption in Ce,Yb-codoped YAG.

Photocatalyzed Conversion of CO2to CH4: An Excited-State Acid–Base Mechanism

Photoinduced reduction of CO2 by H2O occurs in nanoporous Vycor glass doped with tungsten oxides derived from 312 nm photolysis of physisorbed W(CO)6. In polished forms of Vycor, a hydrated precursor to monoclinic WO3 is formed, whereas in the unpolished glass, the WO3 precursor and mixed valence tungsten oxides that exhibit lower energy intervalence charge transfer (IVCT) transitions are formed. The relative amounts of the different tungsten oxide are attributed to structural differences between the polished and unpolished forms of PVG. Chemisorption converts CO2 to a formic acid-like species, and population of the conduction band of the WO3 precursor with 312 nm light or population of the IVCT state of the mixed valence oxide with ≥437 nm light photocatalyzes the reduction of chemisorbed by coadsorbed water yielding CH4 and O2. Regardless of the excitation wavelength, electronic spectra and the dependence of methane yield on absorbed energy and the energetics of the conversions implies that neither metal oxide acts as a source of reducing equivalents. Instead, excitation of the metal oxide induces a charge polarization that creates local acidic and basic regions about the oxide in which the reduction of chemisorbed CO2 and oxidation of chemisorbed H2O occur exergonically. The dependence of methane yield on surface pH, and the pH dependencies of the respective half reactions show that the pH gradient needed for the reactions to occur spontaneously fall within the range of known photoinduced changes in acid-base properties. Within this excited-state acid-base mechanism, the metal oxide is not a source of electrons, but a conduit of electrons and protons between two exergonic processes.

Cathodoluminescent stability of rare earth tantalate phosphors

Surface chemical changes were probed on the surface of YTaO4:Pr3+ (0.5mol%) and LaTaO4:Pr3+ (0.5mol%) phosphors that were prepared by solid state reaction at 1200°C, after they were exposed to a 900Ccm−2 electron dose. X-ray diffraction confirmed the successful formation of these phosphors. The structures of YTaO4:Pr3+ (0.5mol%) and LaTaO4:Pr3+ (0.5mol%) were found to be M′ monoclinic phases. The blue cathodoluminescence (CL) emission coming from the 3P0→3H4 transition of Pr3+ was found to be relatively more intense than the red emission from the 1D2→3H4 transition under electron beam irradiation of LaTaO4:Pr3+ (0.5mol%) and the red emission was dominant over the blue emission in YTaO4:Pr3+ (0.5mol%). This was attributed to the position of the intervalence charge transfer state as well as a possible cross-relaxation mechanism between Pr3+ ion pairs in the case of YTaO4:Pr3+ (0.5mol%). The CL and chemical changes of the surfaces of these phosphors were probed by a 2keV electron beam at a 1×10−6Torr O2 pressure using an Auger electron spectroscopy system in combination with a CL spectrometer. X-ray Photoelectron spectroscopy was used to analyze the chemical changes that took place on the surface as a result of the electron beam impingement, and it revealed the formation of new species on the surface of YTaO4 that were called YxOz, with 0<x<2, 0<z<3 and an increment contribution from Ta2O5 and Y2O3. No chemical changes were found for LaTaO4:Pr3+ during electron bombardment. The stability of the CL intensity of both phosphors opens the possibility of investigating the phosphors to be used as a possible field emission display phosphor.

Strong red emission in lead-free ferroelectric Pr3+-doped Na0.5Bi0.5TiO3 thin films without the need of charge compensation

We report on the strong red emission properties of lead-free perovskite ferroelectric Pr3+-doped Na0.5Bi0.5TiO3 (NBT: Pr3+) thin films without the need of charge compensation. The thin films were prepared by a chemical solution deposition method combined with a rapid thermal annealing process at 700 °C. The strong red emission assigned to prominent 1D2→3H4 transitions of the Pr3+ ions at 611 nm was observed together with a very weak blue-green emission, indicating the commercial use potential of the lead-free perovskite NBT: Pr3+ ferroelectric thin film materials. The energy transfer from the host lattice to the Pr3+ ions should be responsible for the strong red emission, while the weak blue-green emission might be due to the quenching of 3P0 induced by a low-lying Pr3+/Ti4+↔Pr4+/Ti3+ intervalence charge transfer state. The lead-free NBT: Pr3+ thin films also exhibit good ferroelectric and dielectric properties.

Sensitization mechanisms of 1 µm luminescence in Tb3+–Yb3+ co‐doped borate glasses

AbstractTb3+–Yb3+ co‐doped borate glasses were prepared and the ultraviolet/visible sensitization of infrared luminescence of Yb3+ has been evaluated by the photoluminescence (PL), the PL excitation (PLE) spectra, and the lifetime measurement. The Tb3+: 5D4–7FJ luminescence in visible range and the Yb3+: 2F5/2–2F7/2 luminescence at 1000 nm were observed upon the excitation of Tb3+: 4f levels by 378 and 488 nm. On the basis of the difference in behavior of the Yb3+ concentration dependence of the integrated PL intensity and lifetimes of the Yb3+:2F7/2 level by UV and blue excitations, the energy transfer (ET) paths by the UV, and blue excitation are different. The intervalence charge transfer (IVCT) band between the Tb3+–Yb3+ pair were observed in the PLE spectra monitoring the Yb3+ emission. We conclude that the ET is caused through the IVCT state by the UV excitation and the ET upon the 5D4 level excitation of Tb3+ by 488 nm is caused by phonon assisted ET or cooperative downconversion.

Emission properties of (SrTiO 3–TiO 2):Pr 3+ eutectic with self-organized fractal microstructure

An investigation of spectroscopic properties of (SrTiO 3–TiO 2):Pr 3+ eutectic and, for comparison, of bulk SrTiO 3:Pr 3+ and TiO 2:Pr 3+crystals is presented. Luminescence spectra have been measured under both 450 nm and 350 nm excitation wavelength. For UV excitation they are characterized by a dominant red luminescence corresponding to transition from the 1D 2 level of Pr 3+ ions. The mechanism responsible for quenching of blue (from 3P 0 state) and intensification of red luminescence is proposed to be thermally-induced radiationless relaxation involving a low-lying Pr 3+–Ti 4+ intervalence charge transfer state. Measured decay constants of 1D 2 excited state of Pr 3+ are compared with values obtained for other praseodymium doped titanate hosts.

The excited state dynamics of KLa(MoO 4) 2:Pr 3+: From a case study to the determination of the energy levels of rare earth impurities relative to the bandgap in oxidising host lattices

The luminescence properties of KLa(MoO 4) 2 (KLM) single crystals doped with Pr 3+ have been measured in the 10–600 K temperature range in order to investigate the mechanisms involved in the radiationless processes. At variance with previously studied scheelite-like molybdates activated with Pr 3+, no effects attributed to the formation of intervalence charge transfer states have been observed. The model proposed in order to account for this behaviour allows the determination of the energy of the Pr 3+ levels relative to the valence and conduction bands of the host. This model has firstly been confirmed for Tb 3+-doped KLM, for which suitable experimental data are available, and then extended to the other rare earth ions on the basis of the systematic nature of the lanthanide energy levels properties. The obtained conclusions are finally supported in the light of the comparison with some other representative cases.

Emission quenching induced by intervalence charge transfer inPr3+- orTb3+-dopedYNbO4 and CaNb2O6

The excitation and emission properties ofPr3+ andTb3+ ions dopedinto YNbO4 (hereafterYNB) and CaNb2O6 (hereafter CNB) have been studied as a function of the temperature inthe 10–600 K range. The observed quenching of the luminescence from the3P0 (Pr3+)and 5D3 (Tb3+) states has been related to an electron transfer process from the trivalent rare earth to theNb5+ ion inducing the formation of an intervalence charge transfer (IVCT) state. On this basis adetailed picture of the excited-state dynamics of the investigated compounds is presented,taking into account the characteristics of the host lattices and of the involved opticallyactive ions.

Quenching of Lanthanide Emission by Intervalence Charge Transfer in Crystals Containing Closed Shell Transition Metal Ions

The quenching of the luminescence originating from the excited states 3P0 and 1D2 of Pr3+ and 5D3 and 5D4 of Tb3+ has been studied in oxide crystals containing closed shell transition metal ions, such as titanates, vanadates, niobates, and tantalates. It has been shown that the emission from these excited states can be quenched by an intervalence charge transfer mechanism. The temperature dependence of the emission intensities has allowed estimating indicative activation energies for the crossover to the intervalence charge transfer state. In the case of Tb3+, the quenching gives rise to relatively short decay times for the 5D4 state.

Red luminescence induced by intervalence charge transfer in Pr 3+-doped compounds

The excited state dynamics of the Pr 3+ in oxide lattices is revisited in the light of the ‘virtual recharge’ model. The connection between the quenching of the blue-green emission from the 3P 0 level and the formation of a Pr 3+ to metal intervalence charge-transfer state (IVCTS) is discussed on the basis of new experimental evidences. Updated information is presented, concerning the energy position of the IVCTS in various materials and its relation with the host properties. Finally, the limits of the proposed model are outlined and the perspectives for future work are pointed out.