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A special issue of Crystals (ISSN 2073-4352). This special issue belongs to the section "Inorganic Crystalline Materials".

Deadline for manuscript submissions: closed (15 June 2022) | Viewed by 35675

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Prof. Dr. Alessandra Toncelli

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Department of Physics, University of Pisa, 56126 Pisa, PI, Italy
Interests: photonics; rare-earth luminescence; optical spectroscopy; THz spectroscopy; crystal growth
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Dr. Željka Antić

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Vinča Institute of Nuclear Sciences, National Institute of the Republic of Serbia, University of Belgrade, Belgrade, Serbia
Interests: luminescence; luminescence thermometry; phosphors; advanced optical materials; sensors
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Special Issue Information

Dear Colleagues,

Photonics applications based on rare-earth (RE) doped crystals are developing in many different fields, such as photovoltaic, laser technology, optical data storage, sensing, bioimaging, diagnosis, and therapy. RE-doped inorganic bulk materials are long been known to have luminescence emissions that are spectrally distributed throughout the whole optical range from the ultraviolet (UV) to the mid-infrared region with unique features which have made these materials very important especially for laser applications. Moreover, when grown in nanometric size, these materials exhibit peculiar behaviours for their efficiency, lifetimes, energy transfer processes, interaction with the environment, etc., which have stimulated new research devoted, on the one hand, to the physical understanding of these phenomena and on the other hand, to the development of many new applications. This Special Issue is devoted both to the study of RE-doped bulk crystal and nanocrystal materials from a scientific point of view and to the presentation of their possible applications in any field.

We invite researchers to contribute to the Special Issue “Optical and Spectroscopic Properties of Rare-Earth Doped Crystals”, which is intended to serve as a unique multidisciplinary forum covering all aspects of science, technology, and applications of RE-doped crystals starting from the growth techniques with specific attention to the emission features of these materials and their applications.

Potential topics include, but are not limited to:

synthesis and growth of RE-doped crystals
new host materials for efficient emission
upconversion efficiency
characterisation of RE-doped crystals by spectroscopic, microscopic, and other advanced techniques
analysis of the energy transfer processes
biomedical and imaging applications
integrated optics applications
photovoltaic applications
sensing applications

Prof. Dr. Alessandra Toncelli
Dr. Željka Antić
Guest Editors

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Published Papers (13 papers)

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Research

12 pages, 3195 KiB

Open AccessEditor’s ChoiceArticle

Saturation Spectroscopic Studies on Yb³⁺ and Er³⁺ Ions in Li₆Y(BO₃)₃ Single Crystals

by Gábor Mandula, Zsolt Kis, Krisztián Lengyel, László Kovács and Éva Tichy-Rács

Crystals 2022, 12(8), 1151; https://doi.org/10.3390/cryst12081151 - 16 Aug 2022

Viewed by 1820

Abstract

The results of a series of pump–probe spectral hole-burning experiments are presented on Yb³⁺- or Er³⁺-doped Li₆Y(BO₃)₃ (LYB) single crystals in the temperature range of 2–14 K and 9–28 K, respectively. The spectral hole has a complex structure for Yb³⁺ with superposed narrow and broad bands, while a single absorption hole has been observed for Er³⁺. Population relaxation times (T₁) at about 850 ± 60 μs and 1010 ± 50 μs and dipole relaxation times (T₂) with values of 1100 ± 120 ns and 14.2 ± 0.3 ns have been obtained for the two components measured for the Yb³⁺:²F_7/2—²F_5/2 transition. T₁ = 402 ± 8 μs and T₂ = 11.9 ± 0.2 ns values have been found for the Er³⁺:⁴I_15/2—⁴I_11/2 excitation. The spectral diffusion rate at about 1 and 5 MHz/ms has been determined for the narrow and broad spectral line in Yb³⁺-doped crystal, respectively. The temperature dependence of the spectral hole halfwidth has also been investigated. Full article

(This article belongs to the Special Issue Optical and Spectroscopic Properties of Rare-Earth-Doped Crystals)

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14 pages, 10198 KiB

Open AccessArticle

Optical Properties of Yttria-Stabilized Zirconia Single-Crystals Doped with Terbium Oxide

by Yazhao Wang, Zhonghua Zhu, Shengdi Ta, Zeyu Cheng, Peng Zhang, Ninghan Zeng, Bernard Albert Goodman, Shoulei Xu and Wen Deng

Crystals 2022, 12(8), 1081; https://doi.org/10.3390/cryst12081081 - 1 Aug 2022

Cited by 9 | Viewed by 2800

Abstract

A series of yttria-stabilized zirconia single-crystals doped with 0.000–0.250 mol% Tb₄O₇ was prepared by the optical floating-zone method. As shown by XRD and Raman spectroscopy, all of the crystals had a cubic-phase structure. These were initially orange–yellow in color, which is indicative of the presence of Tb⁴⁺ ions, but they then became colorless after being annealed in a H₂/Ar atmosphere as a result of the reduction of Tb⁴⁺ to Tb³⁺. The absorption spectra of the unannealed samples show both the 4f ⁸→4f ⁷5d¹ transition of Tb³⁺ ions and the Tb⁴⁺ charge-transfer band. In addition, the transmittance of the crystals was increased by annealing. Under irradiation with 300 nm of light, all of the single-crystal samples showed seven emission peaks in the visible region, corresponding to the decay from the ⁵D_3,4 excited state of Tb³⁺ to the ⁷F_J (J = 6–0) states. The most intense emission was at 544 nm, which corresponds to the typical strong green emission from the ⁵D₄→⁷F₅ transition in Tb³⁺ ions. Full article

(This article belongs to the Special Issue Optical and Spectroscopic Properties of Rare-Earth-Doped Crystals)

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12 pages, 2295 KiB

Open AccessArticle

Infrared Photoluminescence of Nd-Doped Sesquioxide and Fluoride Nanocrystals: A Comparative Study

by Fulvia Gennari, Milica Sekulić, Tanja Barudžija, Željka Antić, Miroslav D. Dramićanin and Alessandra Toncelli

Crystals 2022, 12(8), 1071; https://doi.org/10.3390/cryst12081071 - 31 Jul 2022

Cited by 3 | Viewed by 1774

Abstract

Lanthanide ions possess various emission channels in the near-infrared region that are well known in bulk crystals but are far less studied in samples with nanometric size. In this work, we present the infrared spectroscopic characterization of various Nd-doped fluoride and sesquioxide nanocrystals, namely Nd:Y₂O₃, Nd:Lu₂O₃, Nd:Sc₂O₃, Nd:YF₃, and Nd:LuF₃. Emissions from the three main emission bands in the near-infrared region have been observed and the emission cross-sections have been calculated. Moreover, another decay channel at around 2 μm has been observed and ascribed to the ⁴F_3/2→⁴I_15/2 transition. The lifetime of the ⁴F_3/2 level has been measured under LED pumping. Emission cross-sections for the various compounds are calculated in the 1 μm, 900 nm, and 1.3 μm regions and are of the order of 10⁻²⁰ cm² in agreement with the literature results. Those in the 2 μm region are of the order of 10⁻²¹ cm². Full article

(This article belongs to the Special Issue Optical and Spectroscopic Properties of Rare-Earth-Doped Crystals)

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9 pages, 3866 KiB

Open AccessArticle

Rapid Aqueous-Phase Synthesis and Photoluminescence Properties of K_0.3Bi_0.7F_2.4:Ln³⁺ (Ln = Eu, Tb, Pr, Nd, Sm, Dy) Nanocrystalline Particles

by Weili Wang, Shihai Miao, Dongxun Chen and Yanjie Liang

Crystals 2022, 12(7), 963; https://doi.org/10.3390/cryst12070963 - 10 Jul 2022

Cited by 2 | Viewed by 1844

Abstract

Trivalent lanthanides (Ln³⁺) doped bismuth-based inorganic compounds have attracted considerable interest as promising candidates for next-generation inorganic luminescent materials. Here, a series of K_0.3Bi_0.7F_2.4 (KBF) nanocrystalline particles with controlled morphology have been synthesized through a low-temperature aqueous-phase precipitation method. Using KBF as the host matrix, Eu³⁺, Tb³⁺, Pr³⁺, Nd³⁺, Sm^3+, and Dy³⁺ ions are introduced to obtain K_0.3Bi_0.7F_2.4:Ln³⁺ (KBF:Ln) nanophosphors. The as-prepared KBF:Ln nanophosphors exhibit commendable photoluminescence properties, in which multicolor emissions in a single host lattice can be obtained by doping different Ln³⁺ ions when excited by ultraviolet light. Moreover, the morphology and photoluminescence performance of these nanophosphors remain unchanged under different soaking times in water, showing good stability in a humid environment. The proposed simple and rapid synthesis route, low-cost and nontoxic bismuth-based host matrix, and tunable luminescent colors will lead the way to access these KBF:Ln nanophosphors for appealing applications such as white LEDs and optical thermometry. Full article

(This article belongs to the Special Issue Optical and Spectroscopic Properties of Rare-Earth-Doped Crystals)

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10 pages, 3743 KiB

Open AccessArticle

Room Temperature Synthesis of Various Color Emission Rare-Earth Doped Strontium Tungstate Phosphors Applicable to Fingerprint Identification

by Soung-Soo Yi and Jae-Yong Jung

Crystals 2022, 12(7), 915; https://doi.org/10.3390/cryst12070915 - 27 Jun 2022

Cited by 5 | Viewed by 1818

Abstract

Crystalline SrWO₄ was synthesized at room temperature using a co-precipitation method. To use the SrWO₄ as a phosphor, green and red phosphors were synthesized by doping with Tb³⁺ and Eu³⁺ rare earth ions. The synthesized samples had a tetragonal structure, and the main peak (112) phase was clearly observed. When the sample was excited using the absorption peak observed in the ultraviolet region, SrWO₄:Tb³⁺ showed an emission spectrum of 544 nm, and SrWO₄:Eu³⁺ showed an emission spectrum of 614 nm. When Tb³⁺ and Eu³⁺ ions were co-doped to realize various colors, a yellow-emitting phosphor was realized as the doping concentration of Eu³⁺ ions increased. When the synthesized phosphor was scattered on a glass substrate with fingerprints, as used in the field of fingerprint recognition, the fingerprint was revealed by green, red, and yellow emissions in response to a UV lamp. Full article

(This article belongs to the Special Issue Optical and Spectroscopic Properties of Rare-Earth-Doped Crystals)

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11 pages, 4253 KiB

Open AccessArticle

Preparation of Cerium Oxide via Microwave Heating: Research on Effect of Temperature Field on Particles

by Chao Lv, Hong-Xin Yin, Yan-Long Liu, Xu-Xin Chen, Ming-He Sun and Hong-Liang Zhao

Crystals 2022, 12(6), 843; https://doi.org/10.3390/cryst12060843 - 15 Jun 2022

Cited by 5 | Viewed by 1979

Abstract

Micro–nano cerium oxide particles with regular morphology and good dispersity have been widely used in polishing and other industrial fields. Microwave heating is an effective, controllable and green heating technology. The Venturi reactor used for microwave heating developed by our team was the core equipment utilized to study the effects of pyrolysis conditions on the purity and microstructure of cerium oxide particles. The experiments were carried out and the products were characterized using XRD, SEM and EDS. Microwave heating, fluid flow and chemical reaction were coupled using numerical simulation, the effects of microwave power, reactor location and waveguide arrangement on temperature fields were investigated. The results showed that with the microwave power increasing, the degree of crystallinity and purity of cerium oxide improved. The morphology gradually became sphere-like. Varied reactor locations and waveguide arrangements changed the gradient and dispersity of temperature fields. Bulk particles and agglomeration could be avoided, and cerium oxide particles with average size of 80 nm were produced when the reactor was located in the center of the cavity. Vertical arrangement of waveguides had the advantages of higher temperature value, gentle gradient and better dispersity. Full article

(This article belongs to the Special Issue Optical and Spectroscopic Properties of Rare-Earth-Doped Crystals)

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11 pages, 2666 KiB

Open AccessArticle

Photoluminescence of the Eu³⁺-Activated Y_xLu_1−xNbO₄ (x = 0, 0.25, 0.5, 0.75, 1) Solid-Solution Phosphors

by Milica Sekulić, Tatjana Dramićanin, Aleksandar Ćirić, Ljubica Đačanin Far, Miroslav D. Dramićanin and Vesna Đorđević

Crystals 2022, 12(3), 427; https://doi.org/10.3390/cryst12030427 - 19 Mar 2022

Cited by 7 | Viewed by 2418

Abstract

Eu³⁺-doped Y_xLu_1−xNbO₄ (x = 0, 0.25, 0.5, 0.75, 1) were prepared by the solid-state reaction method. YNbO₄:Eu³⁺ and LuNbO₄:Eu³⁺ crystallize as beta-Fergusonite (SG no. 15) in 1–10 μm diameter particles. Photoluminescence emission spectra show a slight linear variation of emission energies and intensities with the solid-solution composition in terms of Y/Lu content. The energy difference between Stark sublevels of ⁵D₀→⁷F₁ emission increases, while the asymmetry ratio decreases with the composition. From the dispersion relations of pure YNbO₄ and LuNbO₄, the refractive index values for each concentration and emission wavelength are estimated. The Ω₂ Judd–Ofelt parameter shows a linear increase from 6.75 to 7.48 × 10⁻²⁰ cm² from x = 0 to 1, respectively, and Ω₄ from 2.69 to 2.95 × 10⁻²⁰ cm². The lowest non-radiative deexcitation rate was observed with x = 1, and thus LuNbO₄:Eu³⁺ is more efficient phosphor than YNbO₄:Eu³⁺. Full article

(This article belongs to the Special Issue Optical and Spectroscopic Properties of Rare-Earth-Doped Crystals)

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14 pages, 5846 KiB

Open AccessArticle

Low-Temperature Magnetic and Magnetocaloric Properties of Manganese-Substituted Gd_0.5Er_0.5CrO₃ Orthochromites

by Neeraj Panwar, Kuldeep Singh, Komal Kanwar, Yugandhar Bitla, Surendra Kumar and Venkata Sreenivas Puli

Crystals 2022, 12(2), 263; https://doi.org/10.3390/cryst12020263 - 15 Feb 2022

Cited by 8 | Viewed by 2911

Abstract

Rare-earth chromites have been envisioned to replace gas-based refrigeration technology because of their promising magnetocaloric properties at low temperatures, especially in the liquid helium temperature range. Here, we report the low-temperature magnetic and magnetocaloric properties of Gd_0.5Er_0.5Cr_1−xMn_xO₃ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) rare-earth orthochromites. The Néel transition temperature (T_N) was suppressed from 144 K for Gd_0.5Er_0.5CrO₃ to 66 K for the Gd_0.5Er_0.5Cr_0.5Mn_0.5O₃ compound. Furthermore, magnetization reversal was observed in the magnetization versus temperature behavior of the Gd_0.5Er_0.5Cr_0.6Mn_0.4O₃ and Gd_0.5Er_0.5Cr_0.5Mn_0.5O₃ compounds at 100 Oe applied magnetic field. The magnetic entropy change (−∆S) value varied from 16.74 J/kg-K to 7.46 J/kg-K, whereas the relative cooling power (RCP) ranged from 375.94 J/kg to 220.22 J/kg with a Mn ion concentration at 5 T field and around 7.5 K temperature. The experimental results were substantiated by a theoretical model. The present values of the magnetocaloric effect are higher than those of many undoped chromites, manganites and molecular magnets in the liquid helium temperature range. Full article

(This article belongs to the Special Issue Optical and Spectroscopic Properties of Rare-Earth-Doped Crystals)

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19 pages, 10588 KiB

Open AccessArticle

Thermal Stability and Radiation Tolerance of Lanthanide-Doped Cerium Oxide Nanocubes

by Kory Burns, Paris C. Reuel, Fernando Guerrero, Eric Lang, Ping Lu, Assel Aitkaliyeva, Khalid Hattar and Timothy J. Boyle

Crystals 2021, 11(11), 1369; https://doi.org/10.3390/cryst11111369 - 11 Nov 2021

Cited by 5 | Viewed by 3366

Abstract

The thermal and radiation stability of free-standing ceramic nanoparticles that are under consideration as potential fillers for the improved thermal and radiation stability of polymeric matrices were investigated by a set of transmission electron microscopy (TEM) studies. A series of lanthanide-doped ceria (Ln:CeO_x; Ln = Nd, Er, Eu, Lu) nanocubes/nanoparticles was characterized as synthesized prior to inclusion into the polymers. The Ln:CeO_x were synthesized from different solution precipitation (oleylamine (ON), hexamethylenetetramine (HMTA) and solvothermal (t-butylamine (TBA)) routes. The dopants were selected to explore the impact that the cation has on the final properties of the resultant nanoparticles. The baseline CeO_x and the subsequent Ln:CeO_x particles were isolated as: (i) ON-Ce (not applicable), Nd (34.2 nm), Er (27.8 nm), Eu (42.4 nm), and Lu (287.4 nm); (ii) HMTA-Ce (5.8 nm), Nd (6.6 nm), Er (370.0 nm), Eu (340.6 nm), and Lu (287.4 nm); and (iii) TBA-Ce (4.1 nm), Nd (5.0 nm), Er (3.8 nm), Eu (7.3 nm), and Lu (3.8 nm). The resulting Ln:CeO_x nanomaterials were characterized using a variety of analytical tools, including: X-ray fluorescence (XRF), powder X-ray diffraction (pXRD), TEM with selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDS) for nanoscale elemental mapping. From these samples, the Eu:CeO_x (ON, HMTA, and TBA) series were selected for stability studies due to the uniformity of the nanocubes. Through the focus on the nanoparticle properties, the thermal and radiation stability of these nanocubes were determined through in situ TEM heating and ex situ TEM irradiation. These results were coupled with data analysis to calculate the changes in size and aerial density. The particles were generally found to exhibit strong thermal stability but underwent amorphization as a result of heavy ion irradiation at high fluences. Full article

(This article belongs to the Special Issue Optical and Spectroscopic Properties of Rare-Earth-Doped Crystals)

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9 pages, 3949 KiB

Open AccessArticle

Calcium Tungstate Doped with Rare Earth Ions Synthesized at Low Temperatures for Photoactive Composite and Anti-Counterfeiting Applications

by Soung-Soo Yi and Jae-Yong Jung

Crystals 2021, 11(10), 1214; https://doi.org/10.3390/cryst11101214 - 8 Oct 2021

Cited by 12 | Viewed by 3260

Abstract

A precursor was prepared using a co-precipitation method to synthesize crystalline calcium tungstate. The prepared precursor was dried in an oven at 80 °C for 18 h. The dried powders, prepared without a heat treatment process, were observed in XRD analysis to be a crystalline CaWO₄ phase, confirming that the synthesis of crystalline CaWO₄ is possible even at low temperature. To use this crystalline CaWO₄ as a light emitting material, rare earth ions were added when preparing the precursor. The CaWO₄ powders doped with terbium (Tb³⁺) and europium (Eu³⁺) ions, respectively, were also observed to be crystalline in XRD analysis. The luminescence of the undoped CaWO₄ sample exhibited a wide range of 300~600 nm and blue emission with a central peak of 420 nm. The Tb³⁺-doped sample showed green light emission at 488, 545, 585, and 620 nm, and the Eu³⁺-doped sample showed red light emission at 592, 614, 651, and 699 nm. Blue, green, and red CaWO₄ powders with various luminescence properties were mixed with glass powder and heat-treated at 600 °C to fabricate a blue luminescent PiG disk. In addition, a flexible green and red light-emitting composite was prepared by mixing it with a silicone-based polymer. An anti-counterfeiting application was prepared by using the phosphor in an ink, which could not be identified with the naked eye but can be identified under UV light. Full article

(This article belongs to the Special Issue Optical and Spectroscopic Properties of Rare-Earth-Doped Crystals)

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9 pages, 2634 KiB

Open AccessArticle

Charge Compensation in Europium-Doped Hafnia Nanoparticles: Solvothermal Synthesis and Colloidal Dispersion

by Xavier H. Guichard, Francesco Bernasconi and Alessandro Lauria

Crystals 2021, 11(9), 1042; https://doi.org/10.3390/cryst11091042 - 30 Aug 2021

Cited by 2 | Viewed by 2518

Abstract

Effective charge compensation of europium in hafnium oxide nanoparticles was achieved at low temperature, allowing high doping incorporation (up to 6 at.%) and enhanced luminescence. The efficiency of the incorporation and charge compensation was confirmed by scanning electron microscope energy dispersive X-ray spectroscopy and powder X-ray diffraction measurements. Despite the known polymorphism of hafnium oxide, when doped to a concentration above 3 at.%, only the pure monoclinic phase was observed up to 6 at.% of europium. Furthermore, the low-temperature solvothermal route allowed the direct formation of stable dispersions of the synthesized material over a wide range of concentrations in aqueous media. The dispersions were studied by diffuse light scattering (DLS) to evaluate their quality and by photoluminescence to investigate the incorporation of the dopants into the lattice. Full article

(This article belongs to the Special Issue Optical and Spectroscopic Properties of Rare-Earth-Doped Crystals)

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8 pages, 2131 KiB

Open AccessArticle

Heat-Induced Transformation of Luminescent, Size Tuneable, Anisotropic Eu:Lu(OH)₂Cl Microparticles to Micro-Structurally Controlled Eu:Lu₂O₃ Microplatelets

by Madeleine Fellner, Alberto Soppelsa and Alessandro Lauria

Crystals 2021, 11(8), 992; https://doi.org/10.3390/cryst11080992 - 20 Aug 2021

Cited by 1 | Viewed by 2006

Abstract

Synthetic procedures to obtain size and shape-controlled microparticles hold great promise to achieve structural control on the microscale of macroscopic ceramic- or composite-materials. Lutetium oxide is a material relevant for scintillation due to its high density and the possibility to dope with rare earth emitter ions. However, rare earth sesquioxides are challenging to synthesise using bottom-up methods. Therefore, calcination represents an interesting approach to transform lutetium-based particles to corresponding sesquioxides. Here, the controlled solvothermal synthesis of size-tuneable europium doped Lu(OH)₂Cl microplatelets and their heat-induced transformation to Eu:Lu₂O₃ above 800 °C are described. The particles obtained in microwave solvothermal conditions, and their thermal evolution were studied using powder X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), optical microscopy, thermogravimetric analysis (TGA), luminescence spectroscopy (PL/PLE) and infrared spectroscopy (ATR-IR). The successful transformation of Eu:Lu(OH)₂Cl particles into polycrystalline Eu:Lu₂O₃ microparticles is reported, together with the detailed analysis of their initial and final morphology. Full article

(This article belongs to the Special Issue Optical and Spectroscopic Properties of Rare-Earth-Doped Crystals)

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19 pages, 1729 KiB

Open AccessArticle

Luminescence Intensity Ratio Thermometry with Er³⁺: Performance Overview

by Aleksandar Ćirić, Tamara Gavrilović and Miroslav D. Dramićanin

Crystals 2021, 11(2), 189; https://doi.org/10.3390/cryst11020189 - 14 Feb 2021

Cited by 41 | Viewed by 5265

Abstract

The figures of merit of luminescence intensity ratio (LIR) thermometry for Er³⁺ in 40 different crystals and glasses have been calculated and compared. For calculations, the relevant data has been collected from the literature while the missing data were derived from available absorption and emission spectra. The calculated parameters include Judd–Ofelt parameters, refractive indexes, Slater integrals, spin–orbit coupling parameters, reduced matrix elements (RMEs), energy differences between emitting levels used for LIR, absolute, and relative sensitivities. We found a slight variation of RMEs between hosts because of variations in values of Slater integrals and spin–orbit coupling parameters, and we calculated their average values over 40 hosts. The calculations showed that crystals perform better than glasses in Er³⁺-based thermometry, and we identified hosts that have large values of both absolute and relative sensitivity. Full article

(This article belongs to the Special Issue Optical and Spectroscopic Properties of Rare-Earth-Doped Crystals)

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