Persistent Phosphors: From Fundamentals to Applications

By Jianrong Qiu, Yang Li and Yongchao Jia

Persistent Phosphors: From Fundamentals to Applications provides an introduction to the key synthesis methods, characterization methods, physical mechanisms, and applications of this important luminescent materials system. The book covers basic persistent phosphorescence, introducing concepts such as emission, luminescence, phosphorescence, persistent phosphorescence and the development of persistent phosphors. Then, synthesis methods are reviewed and the connections between synthesis methods and improved materials properties are discussed. Characterization methods to investigate the trapping and de-trapping mechanism are also presented. Other sections cover the theoretical framework and energy band engineering models and materials with a focus on activators, hosts, emission bands and excitation bands. Finally, the most relevant applications of persistent phosphors are included for use in displays, safety signs, bio-labels and energy.

Persistent Phosphors is an invaluable reference for materials scientists and engineers in academia and R&D. It is a key resource for chemists and physicists.

• Presents characterization techniques to reveal the photophysical and photochemical properties of defects for this important category of luminescent materials
• Discusses the structural role of defects in polycrystals and the capture-storing-migration-release progress of excited carriers
• Demonstrates the synthesis routes and potential applications for persistent phosphor materials

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 24, 2020
• ISBN: 9780128187722

Jianrong Qiu

Jianrong Qiu received his PhD from Okayama University, Japan in 1992. He worked at the Japan Science and Technology Agency as a chief researcher, Cornell University as a research associate fellow and Zhejiang University as a professor from 1995 to 2008. He was Cheung Kong chair professor of South China University of Technology from 2009 to 2014, and is now chair professor of Zhejiang University, China. He has authored more than 500 articles and has around 16 000 SCI citations to his papers. He has co-authored one book in English (Springer-Verlag) and one book in Japanese (Kagaku Dojin, Japan). He was the author of many chapters (more than 20) to various books. He has given more than 120 plenary or invited lectures in international conferences. He received the Young Scientist Award from the Rare-earth Society of Japan in 1999, Otto-Schott Research Award from the Ernst Abbe Fund, Germany in 2005, Academic Award from the Ceramics Society of Japan in 2007, and G. W. Morey Award from the American Ceramic Society in 2015. He serves as associate editor or editor of the Int. J. Appl. Glass Sci., J. Non-Cryst. Solids, J. Asian Ceram. Soc., J. Ceram. Soc. Jpn, J. Chinese Ceram. Soc., and Front. Mater.-Glass. Sci. His current research interests are photonic materials (luminescent materials and nonlinear optical materials) and femtosecond laser interaction with materials.

Book preview

Persistent Phosphors

From Fundamentals to Applications

First Edition

Jianrong Qiu

Zhejiang University, Hangzhou, China

Yang Li

Guangdong University of Technology, Guangzhou, China

Yongchao Jia

Universite Catholique de Louvain, Louvain-la-neuve, Belgium

Table of Contents

Cover image

Title page

Copyright

Contributors

Preface

1: Introduction

Abstract

1.1: Definitions of optical phenomena

1.2: Defects

1.3: Traps

1.4: Luminescent centers

1.5: Codopants

1.6: Characterization of persistent luminescence

1.7: Development history of persistent phosphors

1.8: Diverse applications of persistent phosphors

2: Synthesis methods

Abstract

2.1: Solid-state reaction

2.2: Wet-chemical approach

2.3: Physical-based routes

2.4: Molecules for surface modification

2.5: Synthesis of glassy, glass-ceramic, and composite persistent phosphors

2.6: Summary

3: Mechanisms

Abstract

3.1: Factors controlling trapping and detrapping progress

3.2: Proposed afterglow models

3.3: Essential physical processes

3.4: Discussions about metastable shallow and deep traps

4: Characterization and simulation

Abstract

4.1: Instrument measurement

4.2: Theoretical simulation

5: Persistent phosphors

Abstract

5.1: Lanthanide-activated persistent phosphors

5.2: Transition metal-activated persistent phosphors

5.3: Main group element-activated persistent phosphors

5.4: Defect-activated persistent phosphors

5.5: Influence of different hosts

6: Control of afterglow properties

Abstract

6.1: Implantation of defect

6.2: Control of emission band

6.3: Control of excitation band

6.4: Control of afterglow time

7: Applications

Abstract

7.1: Color display

7.2: Light-emitting diode

7.3: Optical information storage

7.4: Sensors

7.5: Bioapplications

7.6: Solar cells

7.7: Photocatalysis

Index

Copyright

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Contributors

Mohamed A. Ali     College of Optical Science and Engineering, Zhejiang University, Hangzhou, China

Xiaofeng Liu     School of Materials Science and Engineering, Zhejiang University, Hangzhou, China

Ibrahim Morad

Zhejiang University, Hangzhou, China

Suez University, Suez, Egypt

Preface

Jianrong Qiu, Hangzhou, China

This book is intended as an introductory level book for graduate students, professional scientists, and engineers interested in persistent phosphors. We hope that the readers have knowledge about luminescence, defect, energy transfer, etc.

Chapter 1 describes the basic concepts related with persistent phosphorescence. We pay special attention to the similarities and differences of fluorescence, phosphorescence, and persistent phosphorescence; defects and traps related with the charge carrier release and storage in persistent phosphors; the luminescent centers including rare earth ions and transition metal ions; and the effects of codoping of ions on the persistent phosphorescence. In addition, the development history of persistent phosphors is described.

In Chapter 2, we introduce various synthesis methods for the control of size, morphology, and homogeneity; the design of functionality; and the optimization of afterglow performance. We focus our discussion on the synthetic methods for persistent phosphorescence nanoparticles, which can be classified into two types: (1) wet-chemical approach, including sol–gel, combustion synthesis, hydro(solvo)thermal, coprecipitation, microwave synthesis, and template methods, and (2) physical process, such as laser ablation and electron beam irradiation processes. The methods and moieties for the functionalization of the surface of the persistent phosphors and synthetic methods for persistent luminescent glasses are also introduced.

In Chapter 3, we review the mechanisms of persistent phosphorescence. A comprehensive discussion is given on the characteristics of traps, including the trap type, trap concentration and distribution, and detrapping probability of captured charge carriers.

In Chapter 4, we introduce experimental techniques together with theoretical approaches based on the first principles and semiempirical methods. Special attention has been paid to the methodology for the determination the nature of luminescent center and trap at atomic scale and their energy levels in the bandgap of the persistent phosphors.

In Chapter 5, various persistent phosphors are introduced, which are activated with luminescent centers of rare earth (RE) ions, transition metal (TM) ions, main group elements, and native crystalline defect. The influence of different hosts on excitation band, emission band, and decay time of persistent phosphors are discussed.

In Chapter 6, we give a detailed description of the defect implanting approaches, modulating principles, and performance tunability through defect engineering.

In Chapter 7, we introduce the applications of persistent phosphors as safety indicators, road traffic signs, luminous decoration, luminous fibers, luminous watches, alternating current light-emitting diode (AC-LED), temperature sensors, photovoltaic materials, and photocatalysts.

Yang Li and Jianrong Qiu wrote Chapters 1, 2, 3, and 6; Yongchao Jia wrote Chapter 4; Mohamed Ali and Jianrong Qiu wrote Chapter 5; Ibrahim Morad and Xiaofeng Liu wrote Chapter 7; and Jianrong Qiu, Xiaofeng Liu, and Yongchao Jia revised and checked the whole manuscript.

I encountered persistent phosphor SrAl2O4:Eu² +, Dy³ + in Kyoto when I was a researcher in Hirao Active Glass Project, Japan Science and Technology Agency in 1997, and was astonished by the performance of the phosphor. I then started to study persistent phosphorescent glasses, glass ceramics, and phosphors. I also found a proverb It glows only because of defects during my research. It is really a mysterious field, and many things are still unclear and are waiting to be explored. I hope this book can give readers helpful guidance into this field.

July 1, 2020

1: Introduction

Abstract

This chapter briefly describes the basic concepts related with persistent phosphorescence. Special attentions have been devoted to the following issues: the similarities and differences of fluorescence, phosphorescence, and persistent phosphoresce; the defect types and the trap dynamics including the release and storage of charge carriers in persistent phosphors; the luminescent centers for the radiative combination; and the energy levels of rare earth ions and transition metal ions, their characteristics, and the effects of codoped ions on the duration of persistent phosphorescence. The history and research perspectives of persistent phosphors are described.

Keywords

Optical phenomena; Persistent phosphorescence; Defects; Traps; Luminescence centers

1.1: Definitions of optical phenomena

1.1.1: Fluorescence

This book deals with the persistent phosphor, which shows a unique luminescence phenomenon. Usually the luminescence is classified into fluorescence, phosphorescence, and persistent phosphorescence [1]. When the luminescent materials are irradiated by high-energy particles, such as photons, protons, or electrons, they emit photons with energies lower than that of the excitation photons, resulting in the occurrence of fluorescence. Fluorescence is the inversed process of absorption where the excited electrons return to their ground state along with a spontaneous emission of photons. Its decay lifetime can be down to 10 ns. Therefore the luminescence almost stops immediately after removal of the excitation [2].

Depending on the type of excitation source, fluorescence can be divided into several categories, such as photoluminescence excited by electromagnetic radiation (usually ultraviolet light), cathodoluminescence excited by high-energy electron beams, and electroluminescence by electric field stimulation. Notably, thermoluminescence is a reluminescence process characterized by the release of energy accumulated upon heating, regardless of the excitation source.

1.1.2: Phosphorescence

Unlike fluorescence, phosphorescence is a slow process that is associated with a forbidden process (with respect to the selection rules) in which the optical centers relax from the excited state to the ground state. In phosphorescence a typical process is that the electron that absorbs the photon (energy) undergoes an unusual intersystem crossing into an energy state of higher spin multiplicity, usually a triplet state. As a result the excited electron can be trapped in the triplet state, from which the return to the lower energy singlet state is not allowed by selection rules. These transitions, although forbidden, will still occur in quantum mechanics but are kinetically unfavored and thus progress at significantly long timescales. Therefore phosphorescent material does not immediately reemit the radiation that it absorbs, but reemits at a much lower intensity and has a longer emission lifetime in the range of about 10− 6 to 1 s. Thus the difference between fluorescence and phosphorescence is the transition nature, which leads to the different decay lifetimes. Any type of phosphorescence can show its decay process in the form of luminescence [3].

1.1.3: Persistent phosphorescence

Persistent phosphorescence is an interesting optical phenomenon, whereby a material is excited with high-energy radiation (typically ultraviolet light, x-rays and beta rays, etc.) and the resulting emission decays very slowly for an appreciable time—from seconds to many hours after the excitation source is removed. It is also called long persistent luminescence or long-lasting phosphorescence. Generally the slow liberation of trapped charge carriers by thermal excitation results in the persistent phosphorescence. Therefore it is worthy to note that the mechanisms of persistent phosphorescence and phosphorescence are totally different [4].

The intensity and lifetime of the persistent phosphorescence are the two important parameters to evaluate the persistent phosphorescence properties of materials [5]. The lifetime of persistent phosphorescence is defined as the time length between the stopping of the excitation and the dropped of the luminescence intensity to 0.32 mcd/m², which is 100 times higher than the sensitivity limit of human eyes in the dark. Commercial persistent phosphors have very high intensity of persistent phosphorescence and long persistent duration. Up to the present a myriad of long persistent materials have been developed, and the number of new materials is growing year by year. For instance, blue persistent phosphor CaAl2O4:Eu² + and Nd³ +; green persistent phosphor SrAl2O4:Eu² + and Dy³ +; red persistent phosphor Y2O2S:Eu³ +, Mg² +, Ti⁴ +; and near-infrared persistent phosphor ZnGa2O4:Cr³ + have been developed in recent years [6–9].

1.2: Defects

Defect usually means imperfection and should be avoided during material design, but defect plays indispensable roles in persistent phosphorescence. In an ideal and perfect crystal, all atoms are located strictly in regular, periodic lattice sites. However, the arrangement of atoms is not perfect in real crystals, and some atoms are not always located at their ideal sites in a three-dimensional crystal structure. The regions in the crystal structure that are different from the ideal lattice structure are considered as the structural defect of the crystal. Defects in the real crystals include thermal defect, point defect, vacancy, interstitial atom, and substitution defects [10].

1.2.1: Thermal defects

The generation and movement of vacancy and interstitial atoms are mainly dependent on the thermal fluctuation of atoms. Thermal defects, also known as intrinsic defects, are generated by thermal vibration of atoms in the crystal lattice when the temperature is higher than the absolute temperature. Some atoms, which have a high thermal energy, move away from their equilibrium position, resulting in the formation of defects. There are two types of intrinsic defects: Frenkel and Schottky defects [11, 12].

Frenkel defect: A atom jumps from the ordinary lattice site to the interstitial position due to thermal fluctuations, creating a vacancy and an interstitial atom at the same time. The features of Frenkel defect are that vacancy and interstitial atoms are generated simultaneously with the same numbers, such that the crystal volume and density remain unchanged, as shown in Fig. 1.1A [13].

Fig. 1.1 The formation process of (A) Frenkel defect and (B) Schottky defect. Reproduced with permission from K.W. Böer, Handbook of the Physics of Thin-Film Solar Cells. Springer Science & Business, 2014, Copyright Springer.

Schottky defect: An atom at its ordinary lattice site, due to large thermal vibration, squeezes out the nearest neighbor atom and occupies the nearest neighbor lattice site. In the earlier process a vacancy is formed in the original position of the atom, and this process results in the formation of Schottky defects. Similar as a row of elastic balls, the most superficial atoms eventually shift to a new position. The characteristics of Schottky defect are that new atomic layer is added to the crystal surface, and only vacancy defect is found in the bulk of the crystal; therefore the crystal expands, and the density decreases, as shown in Fig. 1.1B [13].

1.2.2: Point defects

The process of persistent phosphorescence occurs through a sequence of electronic transitions between the energy bands of the host lattice and energy states of dopants, as well as the trap in intrinsic defects. The emission wavelength of activators strongly depends on the energy levels of the emitters, as well as the influence of surrounding crystal field provided by the host crystal. The region where the crystal structure deviates from the ideal lattice structure is usually called the structural defect in the crystal [14].

Intrinsic defects, or native defects, relate only with the atoms in a perfect crystal. The simplest example of a native defect is a missing atom, or vacancy. Extrinsic defects, in contrast, refer to impurity atoms that are foreign to the crystal. Dopants are usually described as impurity atoms that are introduced intentionally. A defect complex is a combination of two or more impurities or native defects (e.g., the boron–hydrogen complex in silicon and VN center).

Three different types of native defects are of importance in most inorganic crystals. They are lattice vacancy, interstitial atom, and substitution defects [12].

Vacancy. Atomic vacancies are usually present in real crystals, as shown in Fig. 1.2A. This kind of defect is known as a vacancy. The vacancy defect is the simplest, most common defect in real crystals. This defect plays an important role in the electrical properties, optical properties, and high-temperature dynamic process of materials.

Fig. 1.2 Point defects: (A) vacancy, (B) interstitial atom, (C) small substitutional atom, and (D) large substitutional atom (all of these defects disrupt the perfect arrangement of the surrounding atoms). Reproduced with permission from K.W. Böer, Handbook of the Physics of Thin-Film Solar Cells. Springer Science & Business, 2014, Copyright 2013, Springer.

Interstice atom. Foreign impurity atoms with a size smaller than the constituent atoms of the crystals are likely to enter into the interstitial site of the lattice, resulting in the formation of interstitial impurities, as shown in Fig. 1.2B.

Substitution defect. Foreign dopant atoms replace the native atoms and occupy their lattice sites, resulting in the formation of substitutional defect in which the foreign atom may have a different size or oxidation state, as shown in Fig. 1.2C and D.

Now we take the typical III–V semiconductor GaAs as an example to illustrate these point defects. A gallium vacancy is denoted as VGa in GaAs. In the GaAs crystal, there is a new kind of defect, the antisite (e.g., a gallium atom residing on an arsenic site, GaAs, or an arsenic atom sitting on a gallium site, AsGa). Deviations from exact stoichiometry lead to the formation of large concentrations of interstitials and antisites of the excess constituent. Moreover, impurities can enter into different lattice sites depending on their size, oxidation state, and electronegativity. In GaAs, for example, silicon can substitute for gallium or arsenic sites, which are denoted as GaAs/SiGa or GaAs/SiAs, respectively. A site that is not substitutional is called interstitial and is identified by the subscript i. Interstitial copper in GaAs, for example, is denoted as Cui [15].

1.3: Traps

Generally speaking, traps, which originate from the aforementioned lattice defects, are responsible for the storage and release of charge carriers [8, 14]. It plays a dominant role in all the persistent luminescence process. Traps are characterized by their depth inside the bandgap, which is determined by the energy difference between the energy level of the trap and the conduction band (in the case of electron traps) or the valence band (in the case of hole traps). Generally, shallow traps, whose depth is lower than around 0.4 eV [16], are fully emptied at low temperatures and do not actively take part in persistent luminescence at room temperature. On the other hand, very deep traps, whose depths are around 2 eV or larger, require more energy to be emptied at room temperature. A trap depth of around 0.6–0.7 eV is often considered to be an ideal trap for persistent luminescence [17].

1.3.1: Trap depths

The replacement of atoms in a persistent phosphor with substitutional impurities, interstitial impurities, intrinsic defects, complexes between point defects, and dislocations can bring new energy states inside the bandgap of the host matrix. The creation of these energy states, also called energy levels, is the major reason why persistent phosphors have gained enormous scientific and technical importance.

In persistent phosphors, it is generally accepted that after excitation, charge carriers could be captured for a long duration by the traps, which create energy levels inside the bandgap. The captured charge carriers are stored in the traps, until the thermal excitation is strong enough to allow the slow liberation of trapped charge carriers. The released electrons can then return to the activators and produce luminescence [18].

Generally, though not universally, it is accepted that the traps are associated with the lattice defects mentioned earlier, that is, intrinsic defects, intentionally introduced defects, or both. The main trap levels are shallow traps and deep traps. The trap depth determines the release rate of the electrons captured in traps. Shallow traps are easily emptied at room temperature with a fast release rate. Deep traps with the slower release rate, on the other hand, are difficult to evacuate at room temperature in short time. A fraction of electrons are even stored in deep traps under the room temperature. A high release rate is beneficial to obtain a strong phosphorescent intensity but will quickly exhaust the stored energy. In contrast a slow release rate leads to a weaker phosphorescent intensity but with a slower decay rate.

Actually the deep trap energy states are located far from the bandgap edges. Such impurities do not generate a hydrogenic ground state because the carriers are bound strongly at a short distance from the impurity core. Due to this localization the averaging of the ground state wave function over a large volume does not occur. For these reasons, deep traps strongly depend on the particular details of the potential near the impurity core. There exists no simple model that accurately describes deep levels. Impurities that generate deep levels in the bandgap typically do not fit well into the crystal, generating significant local strains. The localized levels can trap carriers, impeding conduction. Historically, deep levels and shallow levels were always defined according to their proximity to the band edges. More recently, however, localized states have been discovered that happen to lie close to the conduction or valence band, and deep states can act as dopants. It is therefore useful to define a defect as localized or delocalized. In literature, deep generally refers to a defect wave function that is localized in real space.

1.3.2: Trap examples

The deep traps control the position of the Fermi level of semiinsulating persistent phosphors. The center of trap energy band has a number of unusual properties that are related, in part, to the change of its relative positions. This process is now known to occur for a large number of deep-level centers in various persistent phosphors.

In extreme cases the lattice relaxation can be strong enough to allow a center to bind two electrons and assume a lower total energy than binding one electron, despite the fact that two equally charged carriers repel each other. Such defects are called negative-U, where U refers to the Coulomb repulsion term in the Anderson model. One example of a negative-U center is the oxygen vacancy (VO) in ZnO, which is a deep trap. An illustration of this vacancy is shown in O bond, so they are dangling bonds. This is the neutral charge state, VO⁰. If the Fermi level is sufficiently low, then the VO defect will give up two electrons, becoming VO² +. The VO+ is not thermodynamically stable; its energy is always higher than VO or VO² + for the whole Fermi level in the bandgap region [19].

Fig. 1.3 Ionic picture of an oxygen vacancy in ZnO. The removal of an oxygen atom leaves two dangling bonds. Reproduced with permission from J. Pelleg, Defects in Materials. Diffusion in Ceramics. Springer, Cham, 2016, Copyright 2016, Springer.

In persistent phosphors, anion vacancies are generally donors, while cation vacancies are acceptors. These native defects are important as compensating centers. The formation of vacancies introduces acceptor levels into the bandgap, enabling electrons to reduce their energy by several eV. This reduction in energy encourages the formation of vacancies. The formation energy of electrically active intrinsic defects depends on the position of the Fermi level [20].

Vacancies also form complexes with hydrogen impurities. The hydrogen atoms can be usually captured by dangling bonds, which is a process known as passivation. By providing an electron to form a saturated bond, hydrogen passivation removes the energy level associated with the dangling bond from the bandgap. Hydrogen also forms complexes with cation vacancies in compound semiconductors. Nevertheless, hydrogen can also occupy anion vacancies and act as shallow donors, which has been confirmed in several persistent phosphors.

Interstitials are usually generated through electron irradiation at cryogenic temperatures. At room temperature, as in the case of vacancies, interstitials tend to diffuse rapidly until they reach another defect or the crystal surface. In addition to the high-symmetric interstitial sites, a split interstitial in which two atoms share a single lattice site is known as an interstitialcy. Intrinsic point defects can agglomerate into larger, extended defects. When interstitials condense, forming an extra section of a lattice plane, we obtain an extrinsic stacking fault, which are surrounded by dislocations. Analogously the condensation of vacancies results in the formation of intrinsic stacking faults [21].

1.3.3: Isoelectronic traps

Isoelectronic traps have the same number of valence electrons as the atoms they replace. If there is no significant lattice strain, the isoelectronic trap is electrically neutral. This kind of traps is usually found in the persistent phosphors with equivalent substitution during the doping design. Such as Cr³+ ions can substitute the octahedral lattice of Ga in Cr³+-doping ZnGa2O4 materials, generating Cr³+-Ga³+ trap cluster [4]. Actually, not so many isoelectronic traps are discovered in persistent phosphors, and now the mainly researches focus at semiconductor and metal materials. For example, in silicon, carbon is an isoelectronic impurity if it replaces Si sites. Because of its small size, it produces considerable local strain. One way to relax this strain is to have a pair of carbon atoms occupy one silicon site, similar to the case of split interstitial. Although carbon by itself does not produce any energy levels in the gap, it participates in many defect reactions that occur at high temperatures. The X center, for example, is a defect pair in silicon with a group-III acceptor on one substitutional site and a carbon on a neighboring substitutional site. The acceptor level is slightly shallower than that of the isolated group-III acceptor.

1.3.4: Doping traps

Ion doping has been a preferred strategy in the creation of traps in persistent phosphors, and it gives great freedom in the choice of the trap species and depth. In contrast to free diffusion, ion doping is a process far from equilibrium. The ions are stopped by various interactions, some of which lead to the displacement of host material atoms.

The dopant concentration profile is dependent on thermal equilibrium for a particular dopant/solid combination. Ion doping is a nonequilibrium process, the dopant atoms being driven into the solid by kinetic energy. As doped, the atoms are not necessarily on their substitutional sites. In practice, shallow implants are used to produce a well-defined Gaussian doping profile [22].

Dopants must occupy the correct substitutional lattice position to become electrically active carriers. Heating, or thermal activation, is the principal process used to move impurities into their substitutional sites. In all discussions of dopant activation, it should be remembered that the solid solubility of any particular impurity could not be exceeded for equilibrium conditions. If the doping leads to concentrations larger than the solubility limit, then precipitation and cluster formation will occur during thermal annealing [23].

1.4: Luminescent centers

1.4.1: Coordination geometry of isolated ions in solids

Luminescent centers, such as transition metal (TM) and lanthanide (Ln) ions, can emit light by d-d and f-f transitions and interband f-d transitions. Since the d electrons of TM and Ln ions are very sensitive to the surrounding environment, the energy levels of these ions can vary significantly when they are doped in different hosts. In contrast, due to the shielding effect of the outer s and p orbitals, the energy of 4f states of Ln ions are almost independent of the host material. However, the coordination environment can still control the emission of Ln ions by the constraints of the selection rules [24].

The effect of the host on the emitter’s energy levels can be described by crystal field theory (CFT). CFT is a classic model of the electronic structure commonly used to estimate ligand-stabilized transition metal complexes. In CFT a metal ion is regarded as a free ion, and a ligand is considered a point charge. It is assumed that the orbitals of the metal ion have negligible overlap with that of the ligand. Consequently the luminescence of a complex, such as a TM complex, mainly depends on the character of energy levels according to the following factors [25, 26]:

(1)the nature of transition metal ions, in particular the number of d electrons;

(2)the spatial arrangement and distance of ligands around the central metal ion;

(3)the nature of the ligand: the stronger the ligand field strength, the greater the energy difference between the high-energy state and the low energy state.

Fig. 1.4 presents spatial orientations of the five degenerate d orbitals. If TM/Ln ions are placed at the center of a spherically symmetric negative electric field, the repulsive action of the d orbital is the same; therefore the d orbital energy is elevated with respect to the energy in the free atom state but without splitting. On the other hand, in real crystals, the metal ions are placed in a nonspherical negative electric field, and the d orbitals become nondegenerate. The splitting of energy level depends on the local geometry where the metal ions are located, such as octahedron, tetrahedron, and planar square [27].

Fig. 1.4 Partial orientations of d orbitals in octahedronal coordination environment. Reproduced with permission from L.E. Orgel, Spectra of transition-metal complexes. J. Chem. Phys. 1955, 23, 1004–1014, Copyright 1955 American Institute of Physics.

Octahedron: Fig. 1.4 shows a reprehensive example about the d orbitals in the octahedral coordination environment that six ligands are distributed along the positive and negative directions of the x, y, and z axes. Due to the anisotropic spatial distribution of the electric field associated with the crystal field, the degree of energy increase is different in each direction. In this figure the coordinating ligand with the dz², dx²-y² orbital’s head meets, and the collision force is relatively close, resulting in larger increase in orbital energy compared with other orbitals. In comparison the coordination ligands with dxy, dxz, and dyz are farther from the side interaction distance; thus the orbital energy is lower [28].

Tetrahedron: In the tetrahedron coordination the five d orbits do not collide in the head-on fashion. The heads of dz² and dx²-y² orbitals both point to the face center, while the dxy, dxz, and dyz orbitals all point to the middle of each edge of the cube. As shown in Fig. 1.5, the face to the vertices is the face of the right triangle, and the point from the edge to the vertex is the right side of the right triangle. The latter is closer to the ligand than the former; therefore the former has a smaller repulsive force than the latter [29].

Fig. 1.5 Different partial orientations of d orbitals overlapping with the ligands in tetrahedron environment. Reproduced with permission from L.E. Orgel, Spectra of transition-metal complexes. J. Chem. Phys. 23 (1955), 1004–1014, Copyright 1955 American Institute of Physics.

Fig. 1.6 presents the splitting of d orbitals and their relative ligand field strength (Δ) values in octahedral and tetrahedral complexes. Quantitative calculations have been developed to determine the energy levels of a dn system in which their ligand fields strength vary from weak to strong. Based on Tanabe-Sugano diagram, the energy levels of the TM ions in different host can be calculated [30]. In this diagram the energies of d orbitals are plotted against Δ. The energy unit is the Racah parameter B. The Tanabe-Sugano diagram can be used for complexes with different B values in an octahedral coordination.

Fig. 1.6 Tanabe-Sugano energy-level diagram of a ³ d 3 system in an octahedral crystal field. Reproduced with permission from K.V. Lamonova, E.S. Zhitlukhina, R.Y. Babkin, S.M. Orel, S.G. Ovchinnikov, Y.G. Pashkevich, Intermediate-spin state of a 3d ion in the octahedral environment and generalization of the Tanabe-Sugano diagrams. J. Phys. Chem. A 115 (2011), 13596–13604, Copyright 2011 American Chemical Society.

Two parameters B and C are needed to describe the energy levels of the d electron system with known d electron numbers. In the Tanabe-Sugano diagram, each graph the ratio of C/B only affects the relative energy of the excited states with different spin multiplicities and ground states. It is worth noting that the Tanabe-Sugano diagram is calculated for the Oh symmetric ligand field. If the C/B ratio varies with the number of d electrons, the same graph can be used for the tetrahedral ligand field (d¹⁰-n equivalent to dn). The use of this energy-splitting map allows for prediction of the optical properties of TM-based coordination compounds [31].

1.4.2: Lanthanide ions

Trivalent lanthanide ions feature a Xe core electronic configuration with n 4f electrons, in which n varies from 0 (for La(III)) to 14 (for Lu(III)). Lanthanide ions are characterized by their rich 4fn levels, which endows Ln-doped materials with intriguing optical properties that are attractive for applications in diverse fields, such as lasers and phosphors. For free (gaseous) Ln³ + ions, the energy level of each electron configuration split into a series of energy levels due to