Down-Shifting in the YAM: Ce3+ + Yb3+ System for Solar Cells

In this work, we investigate Ce³⁺ to Yb³⁺ energy transfer in Y₄Al₂O₉ (YAM) for potential application in solar spectrum down-converting layers for photovoltaic devices. Photoluminescence properties set, of 10 samples, of the YAM host activated with Ce³⁺ and Yb³⁺ with varying concentrations are presented, and the Ce³⁺ to Yb³⁺ energy transfer is proven. Measurement of highly non-exponential luminescence decays of Ce³⁺ 5d band allowed for the calculation of maximal theoretical quantum efficiency, of the expected down-conversion process, equal to 123%. Measurements of Yb³⁺ emission intensity, in the function of excitation power, confirmed the predominantly single-photon downshifting character of Ce³⁺ to Yb³⁺ energy transfer. Favorable location of the Ce³⁺ 5d bands in YAM makes this system a great candidate for down-converting, and down-shifting, luminescent layers for photovoltaics.     

Role of Eu2+ and Dy3+ Concentration in the Persistent Luminescence of Sr2MgSi2O7 Glass-Ceramics

In this study, glass-ceramics based on Sr₂MgSi₂O₇ phosphor co-doped with Eu/Dy were obtained from the sintering and crystallisation of glass powders. The glasses were melted in a gas furnace to simulate an industrial process, and the dopant concentration was varied to optimise the luminescence persistence times. The doped parent glasses showed red emission under UV light excitation due to the doping of Eu³⁺ ions, while the corresponding glass-ceramics showed persistent blue emission corresponding to the presence of Eu²⁺ in the crystalline environment. The dopant concentration had a strong impact on the sintering/crystallisation kinetics affecting the final glass-ceramic microstructure. The microstructures and morphology of the crystals responsible for the blue emission were observed by scanning electron microscopy–cathodoluminescence. The composition of the crystallised phases and the distribution of rare-earth (RE) ions in the crystals and in the residual glassy phase were determined by X-ray diffraction and energy dispersive X-ray analysis. The emission and persistence of phosphorescence were studied by photoluminescence.     

Computer Modelling of Energy Structure of Yb3+ and Lu3+ Doped LaF3 Crystals

The energy band structure, as well as partial and total densities of states have been calculated for LaF₃:Yb and LaF₃:Lu crystals within density functional theory using the projector augmented wave method and Hubbard corrections (DFT + U). The influence of geometric optimization on the results of energy band calculations of LaF₃:Ln crystals (Ln = Yb, Lu) was analysed and the absence of relaxation procedure is confirmed to negatively influence the energy position of states, and the variability between obtained results of different optimization algorithms are within the calculation accuracy. The top of the valence band of LaF₃ is confirmed to be formed by the 2pF^--states and the bottom of the conduction band is formed by the 5d-states of La³⁺. The positions of the 4f-states and 5d-states of activator ions in LaF₃ were studied. It is shown that the 4f-states of Yb³⁺ are slightly above the top of the valence band and the 4f-states of Lu³⁺ to be 3.5 eV below the top of the valence band. The energy levels of the 5d states of the impurities are energetically close to the bottom of the LaF₃ conduction band. The calculated band gap of 9.6 eV for LaF₃ is in a good agreement with the experimental result and is not affected by impurity ions.     

Energy Transfer from Pr3+ to Gd3+ and Upconversion Photoluminescence Properties of Y7O6F9:Pr3+, Gd3+

Upconversion materials have numerous potential applications in light energy utilization due to their unique optical properties. The use of visible light excitation to obtain ultraviolet emission is a promising technology with broad application prospects, while relevant research is absent. A series of Pr³⁺, Gd³⁺ doped Y₇O₆F₉ phosphors were synthesized by traditional solid–state reaction. X-ray diffraction, scanning electronic microscopy, steady-state photoluminescence spectra, a decay dynamic, and upconversion emission spectra of the samples were studied. Under the excitation of 238 nm, the energy transfer from Pr³⁺ to Gd³⁺ was realized and a strong ultraviolet B emission due to the ⁶P_7/2→⁸S_7/2 transition of the Gd³⁺ ions was achieved. Under the excitation of a 450 nm blue laser, Pr³⁺ absorbed two blue photons to realize the upconversion process and then transferred the energy to Gd³⁺ to obtain the ultraviolet B emission.     

The pH Value Control of Morphology and Luminescence Properties of Gd2O2S: Tb3+ Phosphors

Developing rare-earth doped oxysulfide phosphors with diverse morphologies has significant value in many research fields such as in displays, medical diagnosis, and information storage. All of the time, phosphors with spherical morphology have been developed in most of the related literatures. Herein, by simply adjusting the pH values of the reaction solution, Gd₂O₂S:Tb³⁺ phosphors with various morphologies (sphere-like, sheet-like, cuboid-like, flat square-like, rod-like) were synthesized. The XRD patterns showed that phosphors with all morphologies are pure hexagonal phase of Gd₂O₂S. The atomic resolution structural analysis by transmission electron microscopy revealed the crystal growth model of the phosphors with different morphology. With the morphological change, the band gap energy of Gd₂O₂S:Tb³⁺ crystal changed from 3.76 eV to 4.28 eV, followed by different luminescence performance. The samples with sphere-like and cuboid-like microstructures exhibit stronger cathodoluminescence intensity than commercial product by comparison. Moreover, luminescence of Gd₂O₂S:Tb³⁺ phosphors have different emission performance excited by UV light radiation and an electron beam, which when excited by UV light is biased towards yellow, and while excited by an electron beam is biased towards cyan. This finding provides a simple but effective method to achieve rare-earth doped oxysulfide phosphors with diversified and tunable luminescence properties through morphology control.     

Effect of H2O Molecule Adsorption on the Electronic Structure and Optical Properties of the CsI(Na) Crystal

We investigated H₂O molecule adsorption that had an effect on the luminescence properties of the CsI(Na) crystal using experiments and first-principle calculations. We measured the emission spectra of the CsI(Na) crystal at different exposure times under gamma ray excitation. The experimental results showed that the energy resolution of the CsI(Na) crystal was worse when the crystal surface adsorbed more H₂O molecules, and the crystal surface deliquescence decreased the luminescence efficiency of the CsI(Na) crystal. We studied the band structure, density of states, and optical properties changes caused by H₂O molecule adsorption on the CsI(Na) (010) surface. The generalized gradient approximation (GGA) was used to describe the exchange and correlation potential between the electrons. Our calculation results showed that the band gap width of the CsI(Na) (010) surface decreased after adsorbing H₂O molecules, while three new peaks appeared in the valence band, and the absorption coefficient decreased from 90,000 cm⁻¹ to 65,000 cm⁻¹, and the reflection coefficient decreased from 0.195 to 0.105. Further, the absorption coefficient was reduced by at least 25% because of H₂O molecule adsorption, which led to the luminescence degradation of the CsI(Na) crystal.     

Ce3+, Pr3+ Co-Doped Lu3Al5O12 Single Crystals and Ceramics: A Comparative Study

Ce³⁺, Pr³⁺ co-doped Lu₃Al₅O₁₂ (Ce, Pr:LuAG) single crystals and ceramics were prepared using the optical floating zone (OFZ) and reactive vacuum sintering methods, respectively. The microstructure, photo- (λ_ex = 450 nm), and radio-luminescence (under X-ray excitation) performance, as well as scintillation light yield (LY, under γ-ray, ¹³⁷Cs source) of both materials, were investigated and compared. Ce, Pr:LuAG ceramics had an in-line transmittance of approximately 20% in the visible light range, while the analogous crystals were more transparent (~65%). The X-ray excited luminescent (XEL) spectra showed the characteristic Ce ³⁺ and Pr³⁺ emissions located at 310 nm, 380 nm, and 510 nm. The highest LY of the Ce, Pr:LuAG ceramics reached 34,112 pho/MeV at 2 μs time gate, which is higher than that of a single crystal. The ratio of LY values (LY2/LY0.75) between shaping times of 0.75 μs and 2 μs indicated a faster scintillation decay of ceramics regarding single crystals. It was ascribed to the lower effective concentration of luminescent activators in single crystals because of the coefficient segregation effect.     

The Effect of Fluorides (BaF2, MgF2, AlF3) on Structural and Luminescent Properties of Er3+-Doped Gallo-Germanate Glass

The effect of BaF₂, MgF₂, and AlF₃ on the structural and luminescent properties of gallo-germanate glass (BGG) doped with erbium ions was investigated. A detailed analysis of infrared and Raman spectra shows that the local environment of erbium ions in the glass was influenced mainly by [GeO]₄ and [GeO]₆ units. Moreover, the highest number of non-bridging oxygens was found in the network of the BGG glass modified by MgF₂. The ²⁷Al MAS NMR spectrum of BGG glass with AlF₃ suggests the presence of aluminum in tetra-, penta-, and octahedral coordination geometry. Therefore, the probability of the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ions increases in the BGG + MgF₂ glass system. On the other hand, the luminescence spectra showed that the fluoride modifiers lead to an enhancement in the emission of each analyzed transition when different excitation sources are employed (808 nm and 980 nm). The analysis of energy transfer mechanisms shows that the fluoride compounds promote the emission intensity in different channels. These results represent a strong base for designing glasses with unique luminescent properties.     

Energy Transfer Studies in Tb3+-Yb3+ Co-Doped Phosphate Glasses

Detailed optical properties of Tb³⁺-Yb³⁺ co-doped phosphate glasses were performed based on their emission spectra and decay measurements. Under blue excitation of Tb³⁺ at 488 nm, the intensity of Yb³⁺ emissions gradually enhanced upon increasing the Yb³⁺ content until 1 mol% indicated an energy transfer from Tb³⁺ to Yb³⁺. Otherwise, under near infrared excitation of Yb³⁺ at 980 nm, these glasses exhibit intense green luminescence, which led to cooperative sensitization of the ⁵D₄ level of Tb³⁺ ions. A cooperative energy transfer mechanism was proposed on the basis of the study on the influence of Yb³⁺ concentration on up-conversion emission intensity, as well as the dependence of this up-conversion intensity on near infrared excitation power. Moreover, the temporal evolution of the up-conversion emissions have been studied, which was in positive agreement with a theoretical model of cooperative up-conversion luminescence that showed a temporal emission curve with rise and decay times of the involved levels.     

Phonon Sideband Analysis and Near-Infrared Emission in Heavy Metal Oxide Glasses

In this work, spectroscopic properties of europium and erbium ions in heavy metal oxide glasses have been studied. The phonon energy of the glass host was determined based on Eu³⁺ excitation spectra measurements. Near-IR emission spectra at 1550 nm related to ⁴I_13/2 → ⁴I_15/2 transition of erbium in heavy metal glasses were examined with special regards to luminescence bandwidth and measured lifetime. In particular, correlation between phonon energy and the measured lifetime ⁴I_13/2 (Er³⁺) was proposed. The luminescence lifetime for the ⁴I_13/2 upper laser state of erbium decreases with increasing phonon energy in glass matrices. Completely different results were obtained glass samples with europium ions, where the ⁵D₀ lifetime increases with increasing phonon energy. Our investigations suggest that the values of measured ⁵D₀ lifetime equal to radiative lifetimes for all heavy metal oxide glasses.     

Luminescence Properties and Judd–Ofelt Analysis of Various ErF3 Concentration-Doped BaF2 Crystals

The influence of erbium ion concentration on the optical properties of BaF₂:ErF₃ crystals was investigated. Four ErF₃ concentration (0.05, 0.08, 0.15 and 0.5 mol% ErF₃)-doped BaF₂ crystals were obtained using the Bridgman technique. Room temperature optical absorption in the 250–850 nm spectral range was measured, and the photoluminescence (PL) and decay times were also investigated. The Judd–Ofelt (JO) approximation was used, taking into account four absorption peaks (at 377, 519, 653 and 802 nm). The JO intensity parameters, Ω_t (t = 2, 4, 6), were calculated. The influence of the ErF₃ concentration on the JO parameters, branching ratio, radiative transition probability and radiative lifetime were studied. The obtained results were compared with measured values and with those reported in the literature. Under excitation at 380 nm, the well-known green (539 nm) and red (668 nm) emissions were obtained. The calculated and experimental radiative lifetimes were in millisecond range for green and red emissions. The intensity of the PL spectra varied with the Er³⁺ ion concentration. The emission intensity increased linearly or exponentially, depending on the ErF₃ concentration. Under excitation at 290 nm, separate to the green and red emissions, a new UV emission band (at 321 nm) was obtained. Other research has not reported the UV emission or the influence of ErF₃ concentration on emission behavior.     

Tunable Luminescence and Energy Transfer of Sr3B2O6:Ce3+, Sm3+ Phosphors with Potential Anti-Counterfeiting Applications

Sm³⁺ and Ce³⁺ singly doped and Sm³⁺ and Ce³⁺ co-doped Sr₃B₂O₆ phosphors are prepared via a high-temperature solid-state reaction method. The crystal structure and phase purity are characterized by X-ray diffraction (XRD) analyses. The Sm³⁺-doped sample displays an emission in the orange-red region, with the strongest emission line at about 648 nm and possessing a good luminescence thermal stability between 78 and 500 K. With the increase in the Sm³⁺ content, the concentration quenching is observed due to the cross-relaxation (CR) processes among the Sm³⁺ ions. Upon 340 nm excitation, the Ce³⁺-doped phosphor presents a broad emission band in the blue region with a maximum at about 420 nm, which overlaps well with the ⁶H_5/2 → ⁶P_3/2 excitation line of Sm³⁺ and implies the possible energy transfer from Ce³⁺ to Sm³⁺. The spectral and decay measurements of the Ce³⁺ and Sm³⁺ co-doped samples are conducted and the Inokuti–Hirayama (I-H) model is adopted to analyze the luminescence decay dynamics of the donor Ce³⁺. Owing to the evident sensitization of the Sm³⁺ by the Ce³⁺ ions, the co-doped samples exhibit color variation under different wavelength excitations, endowing them with potential applications in optical anti-counterfeiting.     

Structure and Photoluminescence Properties of Dy3+ Doped Phosphor with Whitlockite Structure

Whitlockite has the advantages of a low sintering temperature, high stability, and a low fabrication cost, and it is widely used as the host for luminescent material. In this study, Ca_1.8Li_0.6La_0.6−x(PO₄)₂:xDy³⁺ phosphor was prepared by the high-temperature solid-state method, and its structure, composition, and luminescence properties were systematically studied. The results showed that a new whitlockite type matrix was prepared by replacing Ca²⁺ in whitlockite with monovalent and trivalent cations. The prepared phosphors belonged to a hexagonal crystal system with a particle size in the range of 5–20 μm. Under the excitation of 350 nm UV light, the samples emitted white light, and there were mainly two stronger emission peaks at 481 nm in the blue band and 573 nm in the yellow band, which correspond to the electron transitions at ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 of Dy³⁺, respectively. The optimal doping concentration of Dy³⁺ in Ca_1.8Li_0.6La_0.6(PO₄)₂ matrix was 0.03 (mol%). The main mechanism of concentration quenching in the sample was dipole–dipole energy transfer. When the temperature was 130 °C, the luminescence intensity of the samples was 78.7% of that at 30 °C, and their thermal quenching activation energy was 0.25 eV. The CIE coordinates of the sample at 30 °C were (0.2750, 0.3006), and their luminescent colors do not change with temperature. All the results indicate that Ca_1.8Li_0.6La_0.6−x(PO₄)₂:xDy³⁺ phosphor is a luminescent material with good luminescence performance and thermal stability, which shows a promising application in the field of LED display.     

Structural and Photoluminescence Investigations of Tb3+/Eu3+ Co-Doped Silicate Sol-Gel Glass-Ceramics Containing CaF2 Nanocrystals

In this work, the series of Tb³⁺/Eu³⁺ co-doped xerogels and derivative glass-ceramics containing CaF₂ nanocrystals were prepared and characterized. The in situ formation of fluoride crystals was verified by an X-ray diffraction technique (XRD) and transmission electron microscopy (TEM). The studies of the Tb³⁺/Eu³⁺ energy transfer (ET) process were performed based on excitation and emission spectra along with luminescence decay analysis. According to emission spectra recorded under near-ultraviolet (NUV) excitation (351 nm, ⁷F₆ → ⁵L₉ transition of Tb³⁺), the mutual coexistence of the ⁵D₄ → ⁷F_J (J = 6–3) (Tb³⁺) and the ⁵D₀ → ⁷F_J (J = 0–4) (Eu³⁺) luminescence bands was clearly observed. The co-doping also resulted in gradual shortening of a lifetime from the ⁵D₄ state of Tb³⁺ ions, and the ET efficiencies were varied from η_ET = 11.9% (Tb³⁺:Eu³⁺ = 1:0.5) to η_ET = 22.9% (Tb³⁺:Eu³⁺ = 1:2) for xerogels, and from η_ET = 25.7% (Tb³⁺:Eu³⁺ = 1:0.5) up to η_ET = 67.4% (Tb³⁺:Eu³⁺ = 1:2) for glass-ceramics. Performed decay analysis from the ⁵D₀ (Eu³⁺) and the ⁵D₄ (Tb³⁺) state revealed a correlation with the change in Tb³⁺–Eu³⁺ and Eu³⁺–Eu³⁺ interionic distances resulting from both the variable Tb³⁺:Eu³⁺ molar ratio and their partial segregation in CaF₂ nanophase.     

Primary electron transfer reactions in modified reaction centers from Rhodopseudomonas sphaeroides

Absorption spectra were measured by means of an optical multichannel analyzer in Rhodopseudomonas sphaeroides R-26 reaction centers (RCs) modified by treatment with NaBH₄ at various times (≥1 ps) after the onset of a short excitation flash at 880 nm. Most of these RCs (75-95%) have only one “monomeric” bacteriochlorophyll-800 (B₁) molecule and are as active as the original RCs. The duration of the excitation and measuring pulses was ≈33 ps. If the center of the excitation pulse preceded the center of the measuring pulse by 36-40 ps, the formation of a state P^E (early state), which is converted to the state P^F (P⁺ bacteriopheophytin^-) in 4 ± 1 ps (1/e time), was observed. Also the kinetics and the spectrum of the stimulated emission (reflecting the kinetics and the emission spectrum of the excited state P^*) were determined. The difference spectrum of the state P^E approximately equals the sum of the spectra of the states P^* (≈65%) and ¹[P⁺B₁^-] (≈35%). This indicates that B₁^- is an intermediate in the electron transfer from P^* to bacteriopheophytin, H₁, transferring this electron with a rate constant of (4 × 0.35 ps)^-1 = 7 × 10¹¹ s^-1.     

Effects of Ionic Liquid, 1-Ethyl-3-methylimidazolium Chloride ([EMIM]Cl), on the Material and Electrical Characteristics of Asphaltene Thin Films

Asphaltene is a component of crude oil that has remained relatively unexplored for organic electronic applications. In this study, we report on its extraction technique from crude oil tank bottom sludge (COTBS) and its thin-film characteristics when 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) ionic liquid (IL) was introduced as dopants. The extraction technique yielded asphaltene with more than 80% carbon content. The IL resulted in asphaltene thin films with a typical root-mean-square surface roughness of 4 nm, suitable for organic electronic applications. The thin films each showed an optical band gap of 3.8 eV and a sheet resistance as low as 10⁵ Ω/□. When the film was used as a conductive layer in organic field-effect transistors (OFET), it exhibited hole and electron conduction with hole (µ_h) and electron (µ_e) mobilities in the order of 10⁻⁸ and 10⁻⁶ cm²/Vs, respectively. These characteristics are just preliminary in nature. With the right IL, asphaltene thin films may become a good alternative for a transport layer in organic electronic applications.     

The Upconversion Luminescence of Er3+/Yb3+/Nd3+ Triply-Doped β-NaYF4 Nanocrystals under 808-nm Excitation

In this paper, Nd³⁺–Yb³⁺–Er³⁺-doped β-NaYF₄ nanocrystals with different Nd³⁺ concentrations are synthesized, and the luminescence properties of the upconversion nanoparticles (UCNPs) have been studied under 808-nm excitation for sensitive biological applications. The upconversion luminescence spectra of NaYF₄ nanoparticles with different dopants under 808-nm excitation proves that the Nd³⁺ ion can absorb the photons effectively, and the Yb³⁺ ion can play the role of an energy-transfer bridging ion between the Nd³⁺ ion and Er³⁺ ion. To investigate the effect of the Nd³⁺ ion, the decay curves of the ⁴S_3/2 → ⁴I_15/2 transition at 540 nm are measured and analyzed. The NaYF₄: 20% Yb³⁺, 2% Er³⁺, 0.5% Nd³⁺ nanocrystals have the highest emission intensity among all samples under 808-nm excitation. The UC (upconversion) mechanism under 808-nm excitation is discussed in terms of the experimental results.     

Evaluation of Effective Mass in InGaAsN/GaAs Quantum Wells Using Transient Spectroscopy

Transient spectroscopies are sensitive to charge carriers released from trapping centres in semiconducting devices. Even though these spectroscopies are mostly applied to reveal defects causing states that are localised in the energy gap, these methods also sense-charge from quantum wells in heterostructures. However, proper evaluation of material response to external stimuli requires knowledge of material properties such as electron effective mass in complex structures. Here we propose a method for precise evaluation of effective mass in quantum well heterostructures. The infinite well model is successfully applied to the InGaAsN/GaAs quantum well structure and used to evaluate electron effective mass in the conduction and valence bands. The effective mass m/m₀ of charges from the conduction band was 0.093 ± 0.006, while the charges from the conduction band exhibited an effective mass of 0.122 ± 0.018.     

Transition Metals (Cr3+) and Lanthanides (Eu3+) in Inorganic Glasses with Extremely Different Glass-Formers B2O3 and GeO2

Glasses containing two different network-forming components and doped with optically active ions exhibit interesting properties. In this work, glass systems based on germanium dioxide and boron trioxide singly doped with lanthanides (Eu³⁺) and transition metals (Cr³⁺) ions are research subjects. Optical spectroscopy was the major research tool used to record excitation and emission spectra in a wide spectral range for studied systems. The emitted radiation of glasses doped with Cr³⁺ ions is dominated by broadband luminescence centered at 770 nm and 1050 nm (⁴T₂ → ⁴A₂). Interestingly, the increase of concentration of one of the oxides contributed to the detectable changes of the R-line (²E → ⁴A₂) of Cr³⁺ ions. Moreover, EPR spectroscopy confirmed the paramagnetic properties of the obtained glasses. The influence of molar ratio GeO₂:B₂O₃ on spectroscopic properties for Eu³⁺ ions is discussed. The intensity of luminescence bands due to transitions of trivalent europium ions as well as the ratio R/O decrease with the increase of B₂O₃. On the other hand, the increase in concentration B₂O₃ influences the increasing tendency of luminescence lifetimes for the ⁵D₀ state of Eu³⁺ ions. The results will contribute to a better understanding of the role of the glass host and thus the prospects for new optical materials.     

Determination of the primary charge separation rate in isolated photosystem II reaction centers with 500-fs time resolution

We have measured directly the rate of formation of the oxidized chlorophyll a electron donor (P680⁺) and the reduced electron acceptor pheophytin a^- (Pheoa^-) following excitation of isolated spinach photosystem II reaction centers at 4°C. The reaction-center complex consists of D₁, D₂, and cytochrome b-559 proteins and was prepared by a procedure that stabilizes the protein complex. Transient absorption difference spectra were measured from 440 to 850 nm as a function of time with 500-fs resolution following 610-nm laser excitation. The formation of P680⁺-Pheoa^- is indicated by the appearance of a band due to P680⁺ at 820 nm and corresponding absorbance changes at 505 and 540 nm due to formation of Pheoa^-. The appearance of the 820-nm band is monoexponential with τ = 3.0 ± 0.6 ps. The time constant for decay of ^1*P680, the lowest excited singlet state of P680, monitored at 650 nm, is τ = 2.6 ± 0.6 ps and agrees with that of the appearance of P680⁺ within experimental error. Treatment of the photosystem II reaction centers with sodium dithionite and methyl viologen followed by exposure to laser excitation, conditions known to result in accumulation of Pheoa^-, results in formation of a transient absorption spectrum due to ^1*P680. We find no evidence for an electron acceptor that precedes the formation of Pheoa^-.