Low Phonon Energy Research Articles

High color‐purity broad orange‐red emission of Mn2+‐doped transparent phosphate glass–ceramics embedded NaYP2O7 crystals

AbstractIn this work, novel Mn2+‐doped transparent NaYP2O7 glass–ceramics were prepared through melt‐quenching and subsequent thermal treatment. The surface crystallization method was confirmed based on the differential scanning calorimeter, X‐ray diffraction, and scanning electron microscope characterization. As the increase and prolong of the heat‐treatment temperature and time, the crystallinity of the NaYP2O7 glass–ceramics increases, while the transmittance gradually decreases. The optimal heat‐treatment regime is identified to be crystallized over 4 h at 580°C based on the crystallinity and transmittance. Under the excitation of 408 nm, both the Mn2+‐doped precursor glass and glass–ceramic crystallized at 580°C for 4 h generate a broad emission at 620 nm (4T2(g)→6A1(s)) with a full width at half‐maximum (FWHM) of ∼107 nm. The glass–ceramics exhibit higher emission intensity and longer decay lifetime than that of precursor glass with the aid of the low phonon energy NaYP2O7 crystals. The Mn2+‐doped glass–ceramic exhibits stable orange‐red luminescence with the chromaticity coordinates of (0.568, 0.428), low correlation color temperature of 1 800 K, high color purity of 98.88%, and low thermal quenching (70% at 373 K of the initial intensity at 298 K). The above results indicate that Mn2+‐doped transparent NaYP2O7 glass–ceramics have promising potential for orange‐red color display devices.

Effect of concentration of Er3+ on up-conversion luminescence in NaYF4 embedded oxy-fluoride glass-ceramics under 1550 nm excitation

Oxy-fluoride glass-ceramics (GC) has advantages such as low phonon energy of fluorides and the excellent physical and chemical properties of oxides. Here we report the synthesis of a series of transparent oxy-fluoride GCs with various doping contents of Er3+ (0.2 mol%-10 mol%) synthesized under different temperatures, and their UCL properties under the excitation of 1550 nm laser. We found that Er3+-doped glass matrix with NaYF4 fluoride nanocrystals demonstrated greatly enhanced UCL compared with as-prepared glass. The samples showed red light emission with 3 mol% Er3+ as the optimal doping content. The wavelength of UCL could be fine-tuned by the temperature of heat treatment, and 893 K seems to be the optimal temperature. We also studied the photoelectric conversion characteristics of the optimal Er3+ doped oxy-fluoride GCs with a polycrystalline silicon solar cell under the excitation of a 1550 nm laser as a possible strategy to effectively improve the photo-electric conversion efficiency of solar cells.

Less is more: An oxyfluoride garnet broadband cyan-green phosphor towards highly efficient full-spectrum WLEDs

Full-spectrum lighting has become the development direction for human-centric illumination. However, the reabsorption effect caused by cyan, green and yellow multiple-phosphor packaging leads to low luminous efficiency. In this work, a broadband cyan-green phosphor is proposed to replace the cyan and yellow phosphors for highly efficient and thermally robust full-spectrum WLEDs. A new family of oxyfluoride garnet phosphors Lu2QMg2Al3O9F3: Ce3+ (Q = Ca, Sr, Ba) has been developed by incorporating F- into the classic LuAG: Ce3+ green phosphor. The excess charge was compensated by substituting trivalent Lu3+ and Al3+ with divalent alkaline earth element. The low covalency and phonon energy of fluorine-containing system endow these serial phosphors with efficient and thermally stable blue-shifted emission. The representative Lu2SrMg2Al3O9F3: Ce3+ exhibits a broadband cyan-green emission peaked at 513 nm with a 105 nm FWHM. Furthermore, outstanding quantum efficiency (88.4%) and thermal stability (94.3% @ 423 K) were achieved. This oxyfluoride phosphor was packaged with red phosphor without cyan phosphor on a blue chip. The obtained WLEDs show an excellent color rendering index (Ra = 96.8) with luminous efficiency (LE = 106 lm/W). This research might pave a promising avenue for the development of next-generation full-spectrum lighting.

Fabrication, microstructure and upconversion luminescence properties of Er:Sr5(PO4)3F transparent nanostructured ceramics

Fluoride-upconversion transparent ceramic materials doped with Er ions have attracted extensive interest due to their higher stability, lower phonon energy, and higher upconversion efficiency. xEr:Sr5(PO4)3F (S-FAP) (x=0.5–5 at%) transparent ceramics have been synthesized by hot pressing sintering in this work, and the maximum optical transmittance of the sample reached 74.52 and 82.45% at 500 and 1000 nm, respectively. The XRD results of the powder and ceramic samples were consistent with the diffraction characteristics of the hexagonal S-FAP crystal structure. The SEM and TEM images of powder revealed that the morphology of the sample was composed of mostly rice grains with a diameter of around 20 nm. The XPS and EDS results confirmed the presence of Er3+ and the uniform distribution of Er elements. The thermally etched surface and cross section of ceramics revealed residual pores as the most important scattering source, and the SEM images of the ceramics show that Er doping can obviously promote the growth of the average grain size of S-FAP transparent ceramics. Furthermore, the upconversion luminescence characteristics and fluorescence lifespan of S-FAP transparent ceramics with varied Er doping concentrations were examined, as well as the upconversion luminescence process. The findings in this work indicate that Er:S-FAP transparent ceramics can be employed as a potential upconversion fluorescent material in display technology, biomedicine, temperature sensing, and other applications.

Role of Gd2O3 on tuning structural and optical properties of low phonon energy MgF2 borate glass

Role of Gd2O3 on tuning structural and optical properties of low phonon energy MgF2 borate glass

Efficient energy transfer between Er and Dy achieved in perfluoride glass ceramics by tailoring crystallization characteristics

Perfluoride glass ceramics (FGCs) with SrF2/CaF2 mixed crystals were prepared using a concurrent rapid-quenching-crystallization process. The FGCs contained dense, homogeneous, round crystals of sizes less than 1 μm with high transmittance (exceeding 80 % at 3–8 μm) and low phonon energy (508 cm−1). The fluorescence properties of the FGCs doped with Er and Dy were investigated. The energy transfer efficiency between Er and Dy reached 93.81 % and 98.13 %, corresponding to Er: 4I11/2 to Dy: 6H5/2 and Er: 4I13/2 to Dy: 6H11/2, respectively. This enhancement is attributed to the enrichment of rare-earth ions in the crystals induced by phase-separation and low phonon energy of the host material, FGC. Moreover, the average lifetime of Er: 4I11/2 in FGC reached 10.13 ms, which is the highest reported value for glass ceramics to our knowledge. This study enriches the research theory of FGC and provides guidance for expanding the properties of materials for mid-infrared photonics.

Intense O + E + S-band emission from Pr3+-doped ZnF2-based glasses

Pr3+-doped ZnF2-based glasses were prepared by using a melt-quenching method in dry N2 atmosphere. Under the excitation of a 588 nm light emitting diode (LED), ultrabroadband emissions ranging from 1245 to 1640 nm were obtained from the Pr3+-doped ZnF2-based glasses, which originate from the transitions 1D2→1G4 (producing E + S-band emission) and 1G4→3H5 (producing O-band emission) of Pr3+. The shape of the emission spectra could be tailored by varying the concentration of Pr3+. Emission spectra with the maximum full width at half maximum (FWHM) of 215 nm (1289 nm-1504 nm, covering the O + E + S-band) was obtained in the ZnF2-based glass at a doping concentration of 5000 ppm. The effects of the phonon energy of the matrix on O + E + S-band emission were also investigated. Our results showed that Pr3+-doped ZnF2-based glasses with low phonon energy might be used for constructing O + E + S-band lasers and optical amplifiers.

Superradiant Thomson scattering from graphite in the extreme ultraviolet

We study the Thomson scattering from highly oriented pyrolitic graphite excited by the extreme ultraviolet, coherent pulses of FERMI free electron laser (FEL). An apparent nonlinear behavior is observed and fully described in terms of the coherent nature of both exciting FEL beam and scattered radiation, producing an intensity-dependent enhancement of the Thomson scattering cross-section. The process resembles Dicke's superradiant phenomenon and is thus interpreted as the observation of superradiant Thomson scattering. The process also triggers the creation of coherent, low-q ([Formula: see text] 0.3 Å[Formula: see text]), low energy phonons. The experimental data and analysis provide quantitative information on the sample characteristics, absorption, scattering factor, and coherent phonon energies and populations and open the route for the investigation of the deep nature of complex materials.

Enabling Broadband Solar-Blind UV Photodetection by a Rare-Earth Doped Oxyfluoride Transparent Glass-Ceramic.

Oxyfluoride transparent glass-ceramics (GC) are widely used as the matrix for rare-earth (RE) ions due to their unique properties such as low phonon energy, high transmittance, and high solubility for RE ions. Tb3+ doped oxyfluoride glasses exhibit a large absorption cross section for ultraviolet (UV) excitation, high stability, high photoluminescence quantum efficiency, and sensitive spectral conversion characteristics, making them promising candidate materials for use as the spectral converter in UV photodetectors. Herein, a Tb3+ doped oxyfluoride GC isdeveloped by using the melt-quenching method, and the microstructure and optical properties of the GC sample are carefully investigated. By combining with a Si-based photo-resistor,a solar-blind UV detector is fabricated, which exhibits a significant photoelectric response with a broad detection range from 188 to 400nm. The results indicate that the designed UV photodetector is of great significance for the development of solar-blind UV detectors.

Bulk-like phonon transport in multilayer graphene nanostructures with consecutive twist angles

We investigate phonon transport in consecutively twisted multilayer graphene (CTMG) via atomistic simulations, assuming the same twist angle between any adjacent layers. Intriguingly, the calculated temperature drop between each two layers along the cross-plane direction in CTMG appears nearly identical, indicating an ill-defined Kapitza length. Moreover, the bulk-like thermal resistance increases with rising temperature. Both phenomena suggest that heat conduction in CTMG more closely resembles that in bulk materials as compared to layered materials with an individual mismatched interface, such as twisted multilayer graphene (TMG). This peculiar behavior originates from the unique nanostructure of CTMG. First, the common twist angle creates a consistent charge distribution at each interface, leading to a higher binding energy and reduced mismatch of the phonon density of states across the interface, and therefore a more uniform temperature drop. Further, the continuum of interfaces induces strong scattering of low-energy acoustic phonons generated by emission processes at the interfaces, which results in a temperature dependence opposite to that of TMG. Our findings enable a better understanding of phonon transport in twisted multilayer graphene, and may also facilitate the exploration of other interface-engineered van der Waals nanostructures.

976 nm continuous-wave laser damage of Er:CaF2 crystals

Abstract Er:CaF2 crystals are crucial gain media for producing 3 μm mid-infrared (MIR) lasers pumped by 976 nm continuous-wave (CW) lasers owing to their low phonon energy and high conversion efficiency. This study investigated the damage characteristics and mechanism of Er:CaF2 crystals irradiated with a 976 nm CW laser. The laser-induced damage threshold of Er:CaF2 crystals with different Er3+ doping levels was tested; the damage morphology consists of a series of regular 70° cracks related to the angle of the crystal slip system on the surface. A finite-element model was used to calculate the temperature and stress fields of the crystals. The results indicated that the damage can be attributed to surface tensile stresses caused by the temperature gradient, and crystals with higher doping concentrations were more susceptible to damage owing to stronger light absorption. These findings provide valuable insights into the development of high-power MIR lasers.

Exploring the potential of lanthanide-doped oxyfluoride materials for bright green upconversion and their promising applications towards temperature sensing and drug delivery.

The most efficient upconversion (UC) materials reported to date are based on fluoride hosts with low phonon energies, which reduce the amount of nonradiative transitions. In particular, NaYF4 doped with Yb3+ and Er3+ at appropriate ratios is known as one of the most efficient UC phosphors. However, its low thermal stability limits its use for certain applications. On the other hand, oxide hosts exhibit better thermal stability, yet they have higher phonon energies and are thus prone to lower UC efficiencies. As a result, developing host nanomaterials that combine the robustness of oxides with the high upconversion efficiencies of fluorides remains an intriguing prospect. Herein, we demonstrate the formation of ytrrium doped oxyfluoride (YOF:Yb3+,Er3+) particles, which are prepared by growing a NaYF4:Yb3+,Er3+ layer around SiO2 spherical particles and consecutively applying a high-temperature annealing step followed by the removal of SiO2 template. Our interest lies in employing these materials as Boltzmann type physiological range luminescence thermometers, but their weak green emission is a drawback. To overcome this issue, and engineer materials suitable for Boltzmann type thermometry, we have studied the effect of introducing different metal ion co-dopants (Gd3+, Li+ or Mn2+) into the YOF:Yb3+,Er3+ particles, focusing on the overall emission intensity, as well as the green to red ratio, upon 975 nm laser excitation. These materials are explored for their use as ratiometric thermometers, and further also as drug carriers, including their simultaneous use for these two applications. The investigation also includes examining their level of toxicity towards specific human cells - normal human dermal fibroblasts (NHDFs) - to evaluate their potential use for biological applications.

The impact of low-energy phonon lifetimes on the magnetic relaxation in a dysprosocenium single-molecule magnet.

Developing molecular spin technologies requires microscopic knowledge of their spin-dynamics. Calculation of phonon modes, phonon scattering and spin-phonon coupling for a dysprosocenium single-molecule magnet (SMM) give simulations of spin-dynamics that agree with experiment. They show that low-energy phonon scattering is a significant contribution to the high-performance of dysprosocenium SMMs.

Color tunable luminescence in ThO2:Er3+,Yb3+ nanocrystals: a promising new platform for upconversion.

Lanthanide-doped luminescent nanoparticles are an appealing system for many applications in the area of biomedical, solar cell, thermometry, anti-counterfeiting, etc. due to their sensitivity, reliability, high photochemical stability, and high optical transparency in the visible-NIR range. A color-tunable upconversion-luminescence (UCL) in a new low phonon energy ThO2 host based on modulating sensitizer concentration has been realized in this work and it may work as a potential candidate to replace corrosive and toxic fluoride based hosts in the future. Er3+-Yb3+ co-doped thoria nanoparticles were prepared using a gel combustion route and their structural and luminescence properties were determined as a function of the Yb3+ concentration. Phonon dispersion measurements have established the dynamic structural stability of the thoria nanoparticles. Density functional theory (DFT) was used to calculate the defect formation energy, highlighting the feasibility of dual ion (Er3+ and Yb3+) doping in thoria. The morphology and average size of the doped thoria was studied using high resolution transmission electron microscopy (HRTEM), and any defects evolving as a result of aliovalent doping were probed using positron annihilation lifetime spectroscopy (PALS). With 980 nm laser excitation, the nanothoria emits green and near-red light. A significant enhancement of the red-to-green intensity ratio of Er3+ ions in nanothoria was observed with an increase in Yb3+ concentration which resulted in beautiful color tunability from green to yellow light in going from lower (up to ∼5 mol%) to higher (10 and 15 mol%) Yb3+ concentration. The power dependence and the dynamics of the upconverted emission confirm the existence of two-photon upconversion processes for the green and red emissions.

Coexistence of topological node surface and Dirac fermions in phonon-mediated superconductor YB2C2.

The interaction between nontrivial topology and superconductivity in condensed matter physics has attracted tremendous research interest as it could give rise to exotic phenomena. Herein, based on first-principles calculations, we investigate the electronic structures, mechanical properties, topological properties, dynamic stability, electron-phonon coupling (EPC), and superconducting properties of the synthesized real material YB2C2. It is a tetragonal structure with P4/mbm symmetry and exhibits excellent stability. The calculated electronic band structures reveal that a zero-dimension (0D) Dirac point and two-dimensional (2D) nodal surface coexist near the Fermi level. A spin-orbit coupling (SOC) Dirac point with the topological Fermi arc is observed on the (001) surface. These nodal surfaces are protected by a two-fold screw axis and time-reversal symmetry. Based on the Bardeen-Cooper-Schrieffer theory, the superconducting transition temperature (Tc) in the range 1.25-4.45 K with different Coulomb repulsion constant μ* for YB2C2 is estimated to be consistent with previous experimental results. In addition, the EPC is mainly from the coupling between the dx2-y2 and dz2 orbitals of the Y atom and low-energy phonon modes. The presence of superconductivity and nontrivial topological surface state in YB2C2 suggests that it may be a candidate material for topological superconductors.

Constructing Efficient and Thermostable Red‐NIR Emitter via Cross Relaxation and Crystal‐Field Engineering of Holmium‐Based Perovskite‐Type Half Metal

AbstractLanthanide ions, as dopants, have evoked widespread research interest owing to the rich optical, magnetic, and electrical properties, but their luminescent intensity is always limited by typical concentration quenching. Herein, a holmium‐based double perovskite of Cs2NaHoCl6 is reported, which surprisingly breaks through this barrier and achieves efficient red‐NIR emission by virtue of the large unit cell, low phonon energy, high content of activators, and cross relaxation phenomenon between Ho3+. The heavy Ho3+ also endows the intriguing half‐metallic nature with a down‐spin conducting band and an up‐spin insulating band. After performing ion doping on crystallographic sites of Na+ and Ho3+, the photoluminescence quantum yield of such red‐NIR emitter under 450 nm excitation is dramatically promoted to 82.3%, benefiting from the improved crystal field environment that alleviates the parity forbidden rule and suppresses non‐radiative recombination loss. Furthermore, the heat‐favorable phonon‐assisted population processes enable the robust photostability against thermal quenching. By combining a 450 nm chip, the red‐NIR light‐emitting diodes are fabricated, in which the wide‐coverage NIR emissions are ideally suited for medical light source, night vision, nondestructive examination, and transmission imaging. It is believed that this work will open an avenue for enhancing the fluorescence of lanthanide ions and developing advanced spintronic materials.

Efficient O-band emission from Pr3+ doped low phonon energy ZnF2-based glasses

Pr3+ doped ZnF2-based (ZnF2-BaF2-SrF2-YF3) glasses with a low phonon energy of ∼420 cm−1 are prepared by using a conventional melt-quenching method in dry N2 atmosphere. Under the excitation of a 980 nm laser diode, intense emission at ∼1310 nm is observed from the Pr3+ doped ZnF2-based glasses. The full spectral width at half maximum is 107 nm (1275–1382 nm), which is ∼7 nm wider than that of Pr3+ doped ZrF4-based glass (∼100 nm, 1279–1379 nm). The measured lifetime (∼183 μs) of the excited energy level 1G4 for Pr3+ ions in the ZnF2-based glass is also much longer than that of Pr3+ doped in ZrF4-based glass (∼110 μs). According to the theoretically calculated results, a positive gain of 31.7 dB at 1304 nm can be obtained from O-band optical amplifiers based on the Pr3+ doped ZnF2-based glass, which is 5 dB higher than that obtained from amplifiers based on Pr3+ doped ZrF4-based glass with a same pump condition. The corresponding bandwidth for a net gain of >20 dB is ∼51 nm (1277–1328 nm), and the noise figure is calculated to be < 5.8 dB in above spectral range. Our results indicate that Pr3+ doped ZnF2-based glasses have the potential for constructing efficient O-band optical amplifiers.

ХАЛЬКОГЕНІДНІ СТЕКЛА: СТРУКТУРНІ І ОПТИЧНІ ВЛАСТИВОСТІ (ОГЛЯД)

Structural properties of chalcogenide glasses mainly on the example of binary As-S(Se) and Ge-S(Se) systems and ternary Ge-As-S(Se) systems, structural models, parameters of short range order of glasses obtained using diffraction methods, EXAFS and Raman spectroscopy are considered. Raman spectra of binary As-S(Se) and Ge-S(Se) systems and ternary Ge-As-S(Se) systems, structural models that are used for interpretation of Raman spectroscopy results are considered. Optical properties of chalcogenide glasses and optical absorption edge in binary and multicomponent systems are discussed. The refractive index and its wavelength dependence, other optical properties are among important parameters that determine the suitability of materials as optical media. Refractive and absorption indexes, optical band gap of chalcogenide glasses can be changed by doping of different elements. The results suggest a combined effect of chemical ordering and topological in such glasses (parameters dependence on average coordination number, composition, nanophase separation, etc.). Importance of study of interrelation of structural and physico- chemical properties is stated. As frequently pointed out by various researchers, chalcogenide glasses are promising materials for various applications because they are transparent over a wide range of wavelengths in the infrared region, they possess high linear and non-linear refractive indices, number of photoinduced effects, low phonon energies and are easy to fabricate. Applications of chalcogenide glasses cover wide range, among them: IR optics, recording and storage of information, xerography, thermoplastic and holographic media, inorganic resists, optical filters, diffraction optical elements, non-linear elements, fiber and integrated optics, etc. Composition-structure-properties correlations are convenient to tailor the physical, optical and other properties of chalcogenide glasses and provide an important reference for the further development of new chalcogenide glasses taking into account their possible applications.

Pressure modified upconversion luminescence of Yb3+ and Er3+-doped Bi3TeBO9 ceramics

Yb3+ and Er3+-doped Bi3TeBO9 ceramics exhibit efficient upconversion luminescence in the visible spectral range upon laser diode excitation at 975 nm. Bi3TeBO9:Yb3+/Er3+ powder was prepared by means of the modified Pechini method. A series of Bi3TeBO9:Yb3+/Er3+ ceramic samples was fabricated using the high-pressure low-temperature technique, by sintering Bi3TeBO9:Yb3+/Er3+ powder under different pressures (2, 4, 6 or 8 GPa). The structure and morphology of the above-mentioned samples were analyzed using XRD, SEM and EDX measurements. The phonon energy of Bi3TeBO9 matrix associated with the vibrations of TeO6 and BO3 molecular groups was revealed using μ-Raman spectroscopy. The low phonon energy of Bi3TeBO9 matrix allows the effective energy transfer between Yb3+ and Er3+ ions as well as the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions from the excited to ground states of Er3+ ions, resulting in efficient upconversion emission at 523, 540 and 653 nm, respectively. The decay times determined for all analyzed emissions were longer for Bi3TeBO9:Yb3+/Er3+ ceramics than for the powder sample. The highest values of decay times were detected for the ceramics obtained under a pressure of 8 GPa and were equal to 174, 172 and 175 μs for the emissions at 523, 540 and 653 nm, respectively. The obtained results suggest that Bi3TeBO9:Yb3+/Er3+ ceramics can be proposed as efficient spectral converters in the new type solar cells enhancing the efficiency of the photovoltaic effect.

ErF3 microcrystals controllably deposited in perfluoride glass for upconversion red emission

To the best of our knowledge, this paper first reports ErF3 microcrystals controllably deposited in perfluoride glass using phase-separation engineering techniques. The sample exhibited strong upconversion red-light emission owing to the small distance between Er3+ ions and low phonon energy (585 cm−1). The sample has a high red/green ratio of up to 18.6, which, to our knowledge, is the highest reported value in Er3+-doped fluoride glass ceramics. Furthermore, the sample has a long fluorescence lifetime (3.18 ms @660 nm), good color saturation (0.6255,0.3707), and good thermal stability (ΔE=0.31eV). Therefore, this sample has the potential for application across multiple fields, such as color display, visible laser, and lighting.