Color‐Tunable Up‐Conversion Emission and Infrared Photoluminescence and Dielectric Relaxation of Er3+/Yb3+ Co‐Doped Bi2Ti2O7 Pyrochlore Thin Films

The color‐tunable up‐conversion (UC) emission and infrared photoluminescence and dielectric relaxation of Er³⁺/Yb³⁺ co‐doped Bi₂Ti₂O₇ pyrochlore thin films prepared by a chemical solution deposition method have been investigated. The pyrochlore phase structure of Bi₂Ti₂O₇ can be stabilized by Er³⁺/Yb³⁺ co‐doping. Intense color‐tunable UC emission and infrared photoluminescence can be detected on the thin films excited by a 980 nm diode laser. Two UC emission bands centered at 548 and 660 nm in the spectra can be assigned to ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ions, respectively. A Stokes infrared emission centered at 1530 nm is due to ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions. The dependence of UC emission intensity on pumping power indicates that the UC emission of the thin films is a two‐photon process. The thin films also exhibit a relatively high dielectric constant and a low dissipation factor as well as a good bias voltage stability. Temperature‐ and frequency‐dependent dielectric relaxation has been confirmed. This study suggests that Er³⁺/Yb³⁺ co‐doped Bi₂Ti₂O₇ thin films can be applied to new multifunctional photoluminescence dielectric thin‐film devices.     

Preparation,Structure, and Up‐Conversion Luminescence of Yb3+/Er3+ Codoped SrIn2O4 Phosphors

SrIn₂O₄, which shows lower phonon energy than CaIn₂O₄, is not only a good photocatalyst but also can be an excellent up‐conversion (UC) host to exhibits UC luminescence. In this work, Yb³⁺ and/or Er³⁺ doped SrIn₂O₄ phosphors were synthesized, and their UC luminescence properties were studied and compared with those in the CaIn₂O₄ host. The structure of SrIn₂O₄: 0.01Er³⁺ and SrIn₂O₄: 0.1Yb³⁺/0.01Er³⁺ samples were refined by the Rietveld method and found to that SrIn₂O₄: 0.1Yb³⁺/0.01Er³⁺ showed increasing unit cell parameters and cell volume, indicating In³⁺ sites were substituted successfully by Yb³⁺ and/or Er³⁺ ions. From the UC luminescence spectra and diffuse reflection spectra, Er³⁺‐doped SrIn₂O₄ showed very weak luminescence due to ground state absorption of Er³⁺; Yb³⁺/Er³⁺ codoped SrIn₂O₄ presented strong green (550 nm) and red (663 nm) UC emissions which were assigned to energy transfer from Yb³⁺ transition ²F_7/2→²F_5/2 to the Er³⁺ transition ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2. Comparing with CaIn₂O₄, Yb³⁺/Er³⁺ codoped SrIn₂O₄ showed obvious advantages with higher UC luminescent intensity. The pumping powers study showed that UC emissions in Yb³⁺/Er³⁺ codoped SrIn₂O₄ were attributed to energy transfer of Yb³⁺→Er³⁺ with a two‐photon process. The possible UC luminescent mechanism of Yb³⁺/Er³⁺‐doped SrIn₂O₄ was discussed.     

Dielectric Properties and Relaxation of Bi2Ti2O7

The dielectric properties of Bi₂Ti₂O₇ were explored as a function of temperature and frequency. A comparison between the dielectric response of the well‐known Bi_1.5Zn_0.92Nb_1.5O_6.92 (BZN) pyrochlore and the recently available Bi₂Ti₂O₇ sintered ceramic revealed considerable differences, which indicate that chemical disorder, and not atomic displacement on its own, is chiefly responsible for the dielectric relaxation in bismuth pyrochlores. A low‐frequency (<10 kHz) and relatively high‐temperature (~125 K) dielectric relaxation was observed in Bi₂Ti₂O₇. An Arrhenius function was used to model the relaxation behavior and yielded an activation energy of 0.162 eV and an attempt jump frequency of ~1 MHz. This response is consistent with space charge polarization and not the result of dipolar or ionic disorder.     

Infrared to Visible Up‐Conversion Emission in Er3+/Yb3+ Codoped Fluoro–Phosphate Glass–Ceramics

Erbium Er³⁺ and ytterbium Yb³⁺ codoped fluoro‐phosphate glasses belonging to the system NaPO₃–YF₃–BaF₂–CaF₂ have been prepared by the classical melt‐quenching technique. Glasses containing up to 10 wt% of erbium and ytterbium fluorides have been obtained and characterized using differential scanning calorimetry (DSC) and UV–visible and near‐infrared spectroscopy. Transparent and homogeneous glass–ceramics have been then reproducibly synthetized by appropriate heat treatment above glass transition temperature of a selected parent glass. Structural investigations of the crystallization performed through X‐ray diffractometry (XRD) and scanning electron microscopy (SEM) have evidenced the formation of fluorite‐type cubic crystals based during the devitrification process. Finally, infrared to visible up‐conversion emission upon excitation at 975 nm has been studied on the Er³⁺ and Yb³⁺ codoped glass–ceramics as a function of thermal treatment time. A large enhancement of intensity of the up‐conversion emissions–about 150 times‐ has been observed in the glass–ceramics if compared to the parent glass one, suggesting an incorporation of the rare‐earth ions (REI) into the crystalline phase.     

Tailorable Multicolor Up‐conversion Emissions in Tm3+/Ho3+/Yb3+ Co‐Doped LiLa(MoO4)2

Using a modified sol–gel method, LiLa(MoO₄)₂: Tm³⁺/Ho³⁺/Yb³⁺ phosphors with tailorable up‐conversion (UC) emission colors were prepared. Under the excitation of a 980 nm laser diode, up‐conversion red and green emissions in Ho³⁺/Yb³⁺ co‐doped and blue emission in Tm³⁺/Yb³⁺ co‐doped LiLa(MoO₄)₂ were observed, respectively. The intensities of the RGB (red, green, and blue) emissions could be controlled by varying concentrations of Tm³⁺ or Ho³⁺, and the optimal composition was also determined. In Tm³⁺/Ho³⁺/Yb³⁺ co‐doped LiLa(MoO₄)₂, the UC emission colors could be tuned from blue through white to yellow by adjusting the concentrations of Tm³⁺ or Ho³⁺. The UC excitation mechanisms were also investigated based on the power dependence of UC luminescence intensity.     

Fabrication and Upconversion Luminescent Properties of Er3+‐Doped and Er3+/Yb3+ Codoped La2O2CN2 Nanofibers

La₂O₂CN₂:Er³⁺and La₂O₂CN₂:Er³⁺/Yb³⁺ upconversion (UC) luminescence nanofibers were successfully fabricated via cyanamidation of the respective relevant La₂O₃:Er³⁺ and La₂O₃:Er³⁺/Yb³⁺ nanofibers which were obtained by calcining the electrospun composite nanofibers. The morphologies, structures, and properties of the nanofibers are investigated. The mean diameters of La₂O₂CN₂:Er³⁺ and La₂O₂CN₂:Er³⁺/Yb³⁺ nanofibers are 179.46 ± 12.58 nm and 198.85 ± 17.07 nm, respectively. It is found that intense green and weak red emissions around 524, 542, and 658 nm corresponding to the ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_l5/2 energy levels transitions of Er³⁺ ions are observed for La₂O₂CN₂:Er³⁺ and La₂O₂CN₂:Er³⁺/Yb³⁺ nanofibers under the excitation of a 980‐nm diode laser. Moreover, the emitting colors of La₂O₂CN₂:Er³⁺ and La₂O₂CN₂:Er³⁺/Yb³⁺ nanofibers are all located in the green region. The upconversion luminescent mechanism and formation mechanism of the nanofibers are also proposed.     

REVO4‐Based Nanomaterials (RE = Y,La, Gd,and Lu) as Hosts for Yb3+/Ho3+, Yb3+/Er3+, and Yb3+/Tm3+ Ions: Structural and Up‐Conversion Luminescence Studies

Rare‐earth vanadates of the form REVO₄ (RE = Y, La, Gd, and Lu) doped by Yb³⁺/Ho³⁺, Yb³⁺/Er³⁺_, or Yb³⁺/Tm³⁺ lanthanide ions were successfully synthesized using the sol–gel method and annealing at 600°C in an air atmosphere. The structure and morphology of the prepared nanocrystals were investigated by X‐ray diffraction, thermogravimetric analysis, transmission electron microscopy, and energy‐dispersive X‐ray spectroscopy. All prepared materials were homogenous and had nanosized dimensions. Their elemental compositions were confirmed by optical emission spectrometry. Spectroscopic analysis of the materials was carried out by measuring excitation and emission spectra, luminescence decays, and dependence between the intensity of the luminescence and the laser energy. Following effective excitation by NIR radiation, Ln³⁺ co‐doped vanadate matrices exhibited a strong up‐conversion (UC) luminescence. Differences in spectroscopic properties between monoclinic LaVO₄ and tetragonal YVO₄, GdVO₄, or LuVO₄ doped by Ln³⁺ ions were observed, indicating the influence of the crystal structure on the UC emission. Drawing conclusions from these spectroscopic investigations, the UC mechanisms were proposed, including energy‐transfer processes between Yb³⁺ ions and emitting ions.     

Near‐Infrared Quantum Cutting via Downconversion Energy Transfers in Ho3+/Yb3+ Codoped Tellurite Glass Ceramics

The different concentration of Ho³⁺/Yb³⁺ codoped tellurite glasses were prepared by high‐temperature melting‐quenching method. On excitation of Ho³⁺ ions with blue photon at 449 nm as well as ultraviolet (UV) photon at 360 nm, the near infrared emission at 977 nm from Yb³⁺ and 981, 1020 nm from Ho³⁺, which could be absorbed by silicon and enhance the efficiency of the silicon‐based solar cell, were observed. The energy‐transfer process of Ho³⁺ and Yb³⁺ ions and involved mechanism have been investigated and discussed. The first‐order energy transfer (ET) through cross relaxation and a back ET from Yb³⁺ to Ho³⁺ occurred in the near‐infrared quantum cutting (NIR QC) system are proposed and verified. The NIR quantum efficiency achieved 166% when Yb³⁺ doping concentration is 20 mol%.     

Intense upconversion luminescence and energy‐transfer mechanism of Ho3+/Yb3+ co‐doped SrLu2O4 phosphor

A series of novel SrLu₂O₄: x Ho³⁺, y Yb³⁺ phosphors (x=0.005‐0.05, y=0.1‐0.6) were synthesized by a simple solid‐state reaction method. The phase purity, morphology, and upconversion luminescence were measured by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL) spectroscopy. The doping concentrations and sintering temperature were optimized to be x=0.01, y=0.5 and T=1400°C to obtain the strongest emission intensity. Under 980 nm laser diode excitation, the SrLu₂O₄:Ho³⁺, Yb³⁺ phosphors exhibit intense green upconversion (UC) emission band centered at 541 nm (⁵F₄,⁵S₂→⁵I₈) and weak red emission peaked at 673 nm (⁵F₅→⁵I₈). Under different pump‐power excitation, the UC luminescence can be finely tuned from yellow‐green to green light region to some extent. Based on energy level diagram, the energy‐transfer mechanisms are investigated in detail according to the analysis of pump‐power dependence and luminescence decay curves. The energy‐transfer mechanisms for green and red UC emissions can be determined to be two‐photon absorption processes. Compared with commercial NaYF₄:Er³⁺, Yb³⁺ and common Y₂O₃:Ho³⁺, Yb³⁺ phosphors, the SrLu_1.49Ho_0.01Yb_0.5O₄ sample shows good color monochromaticity and relatively high UC luminescence intensity. The results imply that SrLu₂O₄:Ho³⁺, Yb³⁺ can be a good candidate for green UC material in display fields.     

Highly Sensitive Optical Thermometry of Yb3+‐Er3+ Codoped AgLa(MoO4)2 Green Upconversion Phosphor

Er³⁺–Yb³⁺ codoped AgLa(MoO₄)₂ phosphors with intense green emission from ²H_11/2/⁴S_3/2 → ⁴I_15/2 transitions and negligible red emission from ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺ were synthesized by sol–gel process. Its temperature sensing performance was evaluated based on the temperature dependence of fluorescence intensity ratio (FIR) of two green emission bands in the range 300–510 K. The maximum sensitivity of AgLa(MoO₄)₂: 0.02Er³⁺/0.4Yb³⁺ is approximately 0.018 K^?1 at 480 K, which is much higher than those of reported samples based on green emissions of Er³⁺. Result suggests that AgLa(MoO₄)₂: Er³⁺/Yb³⁺ has a great potential application in optical temperature sensors.     

Ion‐Beam Etching on Nanostructured La2Ti2O7 Piezoelectric Thin Films

The impact of generated Ga⁺ ion beams on the functional properties of crystallized lanthanum dititanate oxide (La₂Ti₂O₇) ferroelectric thin films has been investigated at the nanometer scale by means of the scanning probe microscopy. Both the surface and the electrical response are shown to undergo sensitive modifications for areas exposed to ion irradiation. These are revealed through dynamic force mode, Kelvin force, and piezoresponse force (PFM) microscopies. Despite the nanometer film‐dimensions, local piezoactivity is still detected, thus confirming the high resistance of the material to ion‐beam exposures. In addition, crystallized La₂Ti₂O₇ film, with a Pt electrode on top, is successfully patterned by focused ion beam (FIB) process. This allows the achieving of localized islands with lateral sizes ranging from 500 to 300 nm. Nanoscale electromechanical response is measured inside these nanostructures with the help of the PFM technique. In this case, the local piezosignal exhibits a level similar to the one obtained for the unetched film. The measures evidence no obvious sidewall effect in spite of not using the usual postannealing treatment that leads to functional properties recovery. This study demonstrates that the use of La₂Ti₂O₇ materials combined with the FIB technique constitute a suitable and promising test‐bench to pattern low‐dimensional nanostructures. These investigations bear significant implications in view of the development of lead‐free piezoelectric thin films for nanoelectromechanical systems applications.     

Spectral Conversion From Ultraviolet to Near Infrared in Yb3+‐Doped Pyrovanadate Zn2V2O7 Particles

Yb³⁺‐doped Zn₂V₂O₇ particles were synthesized via the Pechini method. The crystal structure and morphology of the polycrystalline samples were investigated by X‐ray powder diffraction and scanning electron microscopy measurements, respectively. The reflectance spectrum, photoluminescence excitation, emission spectra, and the absolute quantum efficiency of the IR emission (900–1100) were measured. The intense near‐IR emission around 1000 nm attributed to the ²F_5/2 → ²F_7/2 transition of Yb³⁺ was observed under the excitation of ultraviolet light in the Yb³⁺‐doped pyrovanadate. The efficient energy transfer from VO₄ groups into Yb³⁺ ions was confirmed by the optical spectra and fluorescent lifetime measurements. These results demonstrate that the Yb³⁺‐doped pyrovanadate particles are promising materials for spectral conversion from visible sunlight to near‐infrared emission and it may have potential application for spectral convertor to enhance the photoelectric conversion efficiency of c‐Si solar cells.     

Electro‐Optic Property of Mg2+/Er3+‐Codoped LiNbO3 Crystal: Mg2+ Concentration Threshold Effect

A series of Mg²⁺/Er³⁺‐codoped congruent LiNbO₃ crystals were grown by Czochralski method from the growth melts containing 0.5 mol% Er₂O₃ while varied MgO content from 0.0 to 7.0 mol%. The unclamped electro‐optic coefficients γ₁₃ and γ₃₃ of these crystals were measured by Mach–Zehnder interferometry. Two different voltage‐applying schemes were adopted: one is the DC voltage applied to the crystal via Al films coated onto crystal surfaces and another is via a pair of external Cu slab electrodes. The coefficients measured by the two schemes show similar strong dependence on Mg²⁺ concentration. The dependence is non‐monotonous, dramatic, and unusual, and reveals the features of two Mg²⁺ concentration thresholds of optical damage: one in the Mg²⁺ concentration range of 1.2–2.0 mol% (in crystal) and another in 4.5–5.0 mol%. Around the threshold the electro‐optic coefficient decreases abruptly at first and then recovers quickly, and the coefficient drops by >20% (12%) at the first (second) threshold, which exceeds the error 3% considerably. The dramatic behavior is qualitatively explained on the basis of the EO coefficient model of LiNbO₃ and the defect structure model for Mg²⁺‐doped LiNbO₃.     

Optical Thermometry Based on Up‐Conversion Luminescence Behavior of Er3+ ‐Doped Transparent Sr2YbF7 Glass‐Ceramics

Transparent novel glass‐ceramics containing Sr₂YbF₇:Er³⁺ nanocrystals were successfully fabricated by melt‐quenching technique. Their structural and up‐conversion luminescent properties were systemically investigated by XRD, HRTEM, and a series of spectroscopy methods. The temperature‐dependent up‐conversion spectra prove that ²H_11/2 and ⁴S_3/2 levels of Er³⁺ are thermally coupled energy levels (TCEL). Consequently, the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 emissions of Er³⁺ in Sr₂YbF₇:Er³⁺ glass‐ceramics can be used as optical thermometry based on fluorescence intensity ratio (FIR) technique. Combined with low phonon energy and high thermal stability, Er³⁺ ions in Sr₂YbF₇ glass‐ceramics present broad operating temperature range (300–500 K), large energy gap of TCEL (786 cm^?1) and high theoretical maximum value of relative sensitivity (62.14 × 10^?4 K^?1 at 560 K), which suggests that Sr₂YbF₇:Er³⁺ glass‐ceramics may be excellent candidates for optical temperature sensors.     

Microstructures and Microwave Dielectric Properties of Bi2O3‐Deficient Bi12SiO20 Ceramics

The Microstructure and microwave dielectric properties of Bi₂O₃‐deficient Bi₁₂SiO₂₀ ceramics were investigated. A small amount of unreacted Bi₂O₃ phase melted during sintering at 825°C and assisted with densification and grain growth in all samples. The melted Bi₂O₃ reacted with remnant SiO₂ during cooling to form a Bi₄Si₃O₁₂ secondary phase. The nominal composition of Bi_11.8SiO_19.7 ceramics sintered at 825°C for 4 h had a high relative density of 97% of the theoretical density, and good microwave dielectric properties: ε_r = 39, Q × f = 74 000 GHz, and τ_f = ?14.1 ppm/°C. Moreover, this ceramic did not react with Ag at 825°C.     

Tunable upconversion luminescence in new Ho3+/Yb3+-doped SrBi4Ti4O15 photochromic ceramics for switching application

Upconversion (UC) luminescence modulation is quite important in controlling and processing light for active components of light sources, photoswitches, optical memories, and optical sensing devices. In this work, we reported one kind of novel phosphor, Ho³⁺/Yb³⁺-doped SrBi₄Ti₄O₁₅ ceramics, which displayed both strong UC luminescence and obvious photochromic (PC) reaction. The UC luminescence, PC effect, and the modulation of UC performance based on PC behavior were investigated in detail. By alternating visible light irradiation and thermal stimulus, the UC luminescence could be reversibly regulated. Meanwhile, the modulation was unveiled to tightly rely on the irradiation time and thermal treatment processes. Excellent reproducibility was also achieved. In addition, as an alternative method to thermal treatment, the manipulation of luminescence by electric field was also explored. Finally, the mechanism related to the UC luminescence manipulation was illustrated. The results indicate that these samples could be potentially utilized in optical data storage and anti-counterfeiting security fields.     

Synthesis and Up‐conversion Luminescence of Yb3+/Ln3+ (Ln = Er,Tm, Ho) Co‐Doped Strontium Cerate by Pechini Method

Novel up‐conversion (UC) luminescent nanopowders, Sr₂CeO₄:Yb³⁺,Ln³⁺ (Ln = Er, Tm, Ho) were prepared with Pechini method. The Sr₂CeO₄:Yb³⁺,Ln³⁺ (Ln = Er, Tm, Ho) nanopowders had an orthorhombic crystal structure, and showed olive‐like morphology with the length of about 260 nm and width of about 130 nm. Under 980 nm lazer excitation, the Sr₂CeO₄:Yb³⁺/Er³⁺, Sr₂CeO₄:Yb³⁺/Tm³⁺, and Sr₂CeO₄:Yb³⁺/Ho³⁺ nanophosphors exhibit strong green, blue, and green UC luminescence, respectively. The luminescence mechanisms for the doped lanthanide ions were thoroughly analyzed.     

高介电材料Y2O3 薄膜的微结构及界面特性的研究

采用射频反应磁控溅射金属钇耙的方法,在硅衬底成功制备了高介电Y2O3薄膜.并深入研究了Y2O3薄膜的微结构和Y2O3/Si 体系的界面结构在高温退火过程中的变化规律.研究结果表明:在400～500 ℃之间,Y2O3 薄膜有一个从单斜相向立方相的物相结构转变过程.在高温退火过程中,Y2O3/Si有界面层SiO2生成,并且随着退火温度增加,界面层SiO2 的厚度也在逐渐增加.     

Enhanced Sunlight Excited 1‐μm Emission in Cr3+–Yb3+ Codoped Transparent Glass‐Ceramics Containing Y3Al5O12 Nanocrystals

Cr³⁺–Yb³⁺ codoped transparent glass‐ceramics containing Y₃Al₅O₁₂ nanocrystals were prepared by heat treatment of as‐prepared glass sample and characterized by X‐ray diffraction and transmission electron microscopy. The efficient energy transfer from Cr³⁺ to Yb³⁺ ions through multi‐phonon‐assisted process was confirmed by the luminescence spectrum and fluorescent lifetime measurements. When excited by the lights from a solar simulator in the wavelength region of 400–800 nm, greatly enhanced near‐infrared emission around 1 μm was achieved from Cr³⁺–Yb³⁺ codoped glass ceramic compared with that from as‐prepared glass and Ce³⁺–Yb³⁺ codoped glass ceramic. These results demonstrate that the Cr³⁺–Yb³⁺ codoped glass ceramic is a promising material for enhancement of the efficiency of solar energy utilization.