Persistently Luminescent Materials E-Book

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Beschreibung

A broad view of the booming field in optical materials based on their applications and potentials.

Persistent luminescence refers to a property of materials in which they retain luminescence for hours or even days without a source of excitation once being charged. Persistently luminescent materials have found a huge range of technological applications, from optical displays to chemical sensing to information storage and security technologies. This growing field of research and development has never been such an important area of materials science.

Persistently Luminescent Materials offers a comprehensive overview of these emerging optical materials and their applications. It presents all major research trends, new information, and novel properties of the major and developing persistently luminescent materials. With detailed information on how to design new materials and create new functionalities, it’s a must for researchers and technologists in virtually every major area of industry.

Persistently Luminescent Materials readers will also find:

• Practical applications in phototherapy, biological imaging, displays, and more
• Detailed discussion of topics including sustainability techniques, temperature sensing, sterilization, and optical information storage
• Analysis of how to use persistently luminescent materials to create new theranostic functionalities

Persistently Luminescent Materials is ideal for specialists and professionals in the field, but also for a broader readership seeking a perspective of practical needs related to the applications of optical and perticularly persistently luminescent materials.

Leseprobe

Persistently Luminescent Materials

From Development to Applications

Edited by Yuanbing Mao

Editor

Prof. Yuanbing MaoIllinois Institute of Technology3101 South Dearborn StreetChicagoIL 60616USA

Cover Image: © Denis Larkin/Shutterstock

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Print ISBN: 978‐3‐527‐35313‐2ePDF ISBN: 978‐3‐527‐84474‐6ePub ISBN: 978‐3‐527‐84475‐3oBook ISBN: 978‐3‐527‐84476‐0

About the Editor

Dr. Yuanbing Mao is the Department Chair and a Professor of Chemistry at the Illinois Institute of Technology. He completed his BSc at Xiangtan University, MSc at the Institute of Chemistry, Chinese Academy of Sciences, and Ph.D. from the State University of New York at Stony Brook. He has earned several awards, including the Young Investigator Award from the U.S. Department of Defense and the Outstanding Mentorship Award from the Council on Undergraduate Research, and is a recipient of the DOE Visiting Faculty Program. His research interests include nanomaterials, solid‐state science, and nanoscience, with expertise in optoelectronics, energy storage and conversion, and environmental remediation. To date, he has published more than 185 peer‐reviewed journal articles, as well as several book chapters and patents.

Preface to “Persistently Luminescent Materials: From Development to Applications”

Persistent luminescence is a phenomenon in which luminescence is maintained for minutes to hours or even days without an excitation source after initial excitation. Owing to their unique optical properties, the past couple of decades have witnessed the emerging and rapid development of persistently luminescent materials, including lanthanide and transition metal‐ion‐activated inorganic materials and molecular materials, providing persistent luminescence in a wide spectral range from ultraviolet (UV, 200–400 nm) and visible (Vis, 400–700 nm) to infrared (IR, 700–1400 nm). Their development has become one of the most dynamic areas in the field of functional materials along with multiple realized and potential applications in self‐sustaining optical displays, phototherapy, anti‐counterfeiting, in vivo biological imaging, chemical sensing, optical data storage, security technologies, etc. In other words, they have found diverse applications in industry, military, medicine, firefighting, daily life, etc. These applications have been greatly dependent on their characteristic performance of persistently luminescent materials, particularly in their persistent luminescence emission intensity and duration by tuning trap depth, distribution and density.

Although the development and applications of persistently luminescent materials have been highly successful, there are still many missing puzzles and explorations to be made. Moreover, there are no recent collections of such diverse work on persistently luminescent materials, such as books, connecting their fundamental development and application perspectives in a concise fashion to give a broad view of the status of this emerging area. Therefore, we invited authors to contribute comprehensive review articles from the perspective of practical needs related to the applications of persistently luminescent materials, which is expected to attract a broader readership, including not just specialists and professionals in the field, but also extending to beginners to this diverse field along with any interested parties to this fascinating field. The wide variety of applications of persistently luminescent materials covered by the eight review articles published here is proof of the growing attention that their development and applications have received in recent years. Specifically, this book entitled Persistently Luminescent Materials: From Development to Applications covers not only the basic science and engineering of both inorganic and organic persistently luminescent materials but also their applications in color display and lighting along with the state‐of‐the‐art aspects of sustainability techniques, temperature sensing, sterilization, and optical information storage. It also provides insight into their use to create new theranostic functionalities when interfaced with biological molecules or species.

Featuring contributions from leading experts in the field, Persistently Luminescent Materials: From Development to Applications offers a broad coverage from display, lighting, and sensing to bioimaging, phototherapy, data‐storage and security technologies. It looks at development perspectives on and trends in persistently luminescent materials in biological, display and data security technologies, and discusses the influence of newer persistent luminescence use on niche application fields of sustainability, temperature sensing, and sterilization. The book also covers such topics as molecular persistently luminescent materials and their hybrids along with their broad application potentials, and much more.

Presents the most comprehensive trends in persistently luminescent materials relating to their development and applications

Systematically presents new information about persistently luminescent materials, expanding applications and exploring novel properties and their features on their application potentials

Describes how to design persistently luminescent materials to meet application needs and create new functionalities when interfacing with various practical fields

Offers a range of practical applications, including optical displays, lighting, phototherapy, biological imaging, chemical sensing, data storage, and security technologies

Delineates essential requirements for applicable persistently luminescent materials

Discusses experimental results obtained in relevant applications

Persistently Luminescent Materials: From Development to Applications is an excellent resource for materials scientists and engineers, optoelectronic scientists, and device engineers. It will also benefit biological and data storage scientists and engineers who work with luminescent materials scientists. To give readers a broader view of this booming field, this edited book covers both inorganic and organic as well as their hybrids. To attract readers with a wide range of backgrounds, this edited book is organized based on the applications and potentials of persistently luminescent materials.

Persistently Luminescent Materials: From Development to Applications presents just the tip of the iceberg of the broad, dynamic, and active fundamental research and applications in the developing field of persistently luminescent materials by collecting a few examples of the latest advancements. We hope that readers will enjoy reading these articles and find them useful for their research.

I appreciate all the contributors of this book for their wisdom and solid scientific work. I also thank Program Manager of Books and References Dr. Shaoyu (Alice) Qian and the rest of the Wiley team for their tireless support.

Yuanbing MaoEditor

1Introduction to Persistently Luminescent Materials

Nimai Pathak1,2 and Yuanbing Mao1

1Illinois Institute of Technology, Department of Chemistry, 3101 S Dearborn Street, Chicago, IL 60616, USA

2Bhabha Atomic Research Centre, Radiochemistry Division, Central Avenue Road, Trombay, Mumbai, Maharashtra, 400085, India

1.1 Introduction

The term “persistent luminescence” (hereinafter PersL) generally refers to the glow‐in‐the‐dark or the “afterglow” property of a light emitting material, which continuously emits light lasting for a relatively long time, starting from seconds to even days after ceasing the excitation sources like ultraviolet (UV) or visible light (visible light in rare cases), electron beams, or high energy radiation such as X‐, α‐, β‐, or γ‐rays. It has a rich history which can be traced back as long as 1000 years ago when a Chinese artist Z. Xu painted a cow resting inside a barn with a special “night‐vision” ink imported from Japan. This inexplicable magic was later described by Y. Wen in a Chinese miscellaneous note called “Xiāng Shán Yě Lù” in the ancient Song dynasty (960–1279 CE) [1–3]. There was no scientific evidence about the raw material used for the preparation of this special “night‐vision” ink, but it was possibly sulfide compounds made of calcium from pearl shells and sulfur from volcanic activities [4]. Although most of the literature on persistently luminescent materials (PLMs) were available since the beginning of twenty‐first century (Figure 1.1a and c), a well‐organized report on any such night‐vision PLM substance was first written on Bologna stone by O. Montalbani in the book “De Illuminabili Lapide Bononiensi Epistola” in 1634 and then by F. Licetus in the book “Litheosphorus Sive de Lapide Bononiensi” in 1640 as shown in Figure 1.1b [2, 4]. An orange and reddish afterglow was observed from this stone in the dark after prior illumination by either sunlight or flame and described as “golden light of the Sun.” The stone was first synthesized by the Italian alchemist V. Cascariolo by calcining the mineral barite (BaSO4), found in Bologna, Italy. This marked the beginning of modern luminescence materials. The word “luminescence” was firstly used by E. Wiedemann, a German physicist in 1888, originating from the Latin word lumen with the meaning of light [6].

Figure 1.1 (a) Number of publications on PLMs in the Web of Science. (b) The book “Litheosphorus Sive de Lapide Bononiensi” about the afterglow phenomenon of the Bologna stone written by Fortunius Licetus in 1640 (Bologna, Italy).

Source: Adapted from Yuan [5].

(c) History of the development of PLMs.

Source: Yuan [5]/IOP Publishing.

Limited research was carried out to develop new afterglow materials until the end of the twentieth century. It took ∼400 years to prove that, instead of any “magic” power, the 3d94s1 → 3d10 transition of Cu+ impurities present in the Bologna stone is responsible for reddish‐orange afterglow peaking at ∼610 nm in the year 2012 [6]. For about 130 years, zinc sulfide (ZnS) doped with copper (Cu+) and codoped with cobalt (Co2+) emitting green emission (∼530 nm) was dominating commercial afterglow materials. It was particularly used in the military during World War I and World War II as well as for various civilian purposes such as luminous paints, watch dials, and “glow‐in‐the‐dark” toys, among others [2, 3, 7, 8]. Later on, trace amount of radioactive elements such as radium (Ra), promethium (Pm), or tritium (3H) were introduced into ZnS:Cu+,Co2+ system in order to improve its weak brightness and short persistency of luminescence. Although continuous irradiation by α‐ or β‐rays due to radioactive decay of those radioisotopes helped to improve the light output [4], the use of radioactive materials became a huge public concern in late 1990 owing to their unavoidable health problems and environmental pollution [4, 9]. This resulted in a sharp decrease in the annual sales of watches made of radioactive LPPs. Later, considering the huge potential market of watches based on PLMs, Japanese company Nemoto & Co., Ltd. gave a specific focus on developing radioactive‐free afterglow phosphors. After much trial and error, they finally succeeded in 1996 in developing the new generation PLM of SrAl2O4:Eu2+,Dy3+ with a bright green emission peaking at ∼520 nm and lasted for over 30 hours [10–12]. Since the development of SrAl2O4:Eu2+,Dy3+ PLM, the searching for various other novel PLMs began to speed up around the world.

Most of the PLMs developed before and after the discovery of SrAl2O4:Eu2+,Dy3+ are inorganic in nature as discussed in Section 1.3.2. Molecular organic solid‐state materials with persistent luminescence, originated from room temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF), are also becoming increasingly popular. Molecular PLMs (MPLMs) have several advantages over their inorganic counterparts containing transition metals and/or rare earth metals, such as potentially facile tuning of structure, simple processing, and easy fabrication of soft and flexible optical devices. They have shown potential applications in biological imaging, optical recording devices, chemical sensing, and security systems [13–15]. However, many scientific and technical issues still remain to be resolved before their widespread implementations. For instance, their optical properties are highly dependent on their crystalline state and need to be used in the solid state because their solution or amorphous forms show weak persistent luminescence. PersL based on molecular hybrid structures can overcome these limitations because intermolecular interactions such as hydrogen bonding enhance the rigidity of molecular conformations and restrict molecular motions and vibration. This helps to minimize the nonradiative decay of triplet excitons and improve their photoemission lifetimes and quantum efficiencies (QEs) [14]. As studies on MPLMs have been advanced tremendously in recent years, inorganic PLMs (IPLMs) have been investigated more in both research and development. Hence, we have mostly focused discussion on IPLMs with limited examples of MPLMs in this chapter as well as this book.

So far applications of IPLMs such as LumiNova® products are mostly in the areas of decoration, toys, safety signage, watch dials, and displays (Figure 1.2a–f). Continuous efforts have been put into exploring their new applications, such as glow‐in‐the‐dark road markings, bio‐imaging, photocatalysis, AC‐driven light‐emitting diodes (LEDs) with reduced flickering, or pressure sensors to visualize ultrasound beams [18–21]. Research work on these new applications has progressed well while many challenges are still facing as shown in Figure 1.2e. In other words, after the successful development of the bright green emitting SrAl2O4:Eu2+,Dy3+ PLM, research interest and effort on both finding new and efficient PLMs and exploring their potential applications on various advanced fields have been significantly increased as suggested by hundreds of new publications reported yearly on various PLMs as shown in Figure 1.1a. Hence, many review papers, book chapters, and sometimes entire books have been published on PLMs to cover the whole field, focused on a specific class of materials including Eu2+, Cr3+, and Mn2+‐doped PLMs, or targeted specific applications like near‐infrared (NIR) bio‐imaging by researchers worldwide [5, 12, 16, 17, 22–27]. Most of these publications introduce key synthesis methods, characterization methods, physical mechanisms, and applications of this important luminescent materials system. They cover mostly the basics of PersL followed with a concise description of the most relevant applications of related PLMs. To advance PersL and PLMs to the next stage, we dedicate this book to the perspective of practical needs related to the applications of PLMs, which is expected to attract a broader readership, including not just specialists and professionals in the field, but also extending to beginners to this diverse field along with any interested parties to this fascinating field.

Figure 1.2 (a) Examples of LumiNova® products and their practical usages for (b) emergency signage, (c) “night‐vision” luminous paints, and (d) watch dials.

Source: [16]/Nemoto & Co., Ltd.

(e) A summary of various established, in progress, and challenging applications of PLMs.

Source: Poelman et al. [17]/AIP Publishing.

(f) Applications of PLMs based on optically stimulated luminescence.

Source: Adapted from Yuan [5].

With this intention in mind, in this introductory (Chapter 1), we provide a brief description of various PLMs, the basic mechanism involved in PersL in both inorganic and organic PLMs, various synthesis methods for making PLMs, main characterization techniques for PersL properties, and trap manipulation for improving PersL of PLMs. This chapter is helpful for readers to brush up their background knowledge of PersL and PLMs. It provides some fundamental guidelines for future development of efficient PLMs before introducing other chapters in this book which are focused on various advanced applications of PLMs, specifically, advances in the applications of long persistent phosphors in display technology field of long persistent phosphor (Chapter 2), persistent luminescence‐based LEDs (Chapter 3), persistently luminescent nanoparticles for bioimaging and biosensors (Chapter 4), persistently luminescent nanoparticles for cancer therapy (Chapter 5), persistently luminescent inorganic materials for anticounterfeiting (Chapter 6), recent advances of the deep trap persistent phosphor for optical data storage (Chapter 7), afterglow thermometer using persistent phosphors (Chapter 8), and persistent luminescence and applications of molecular materials and their hybrids (Chapter 9).

1.2 PersL Processes and Emission Mechanisms

It is now widely accepted and well supported by mounting evidence that various defects or impurity‐related traps inside the band gap of material play the crucial role of trapping excited electrons or holes followed by a delay in the electron–hole recombination and are responsible for luminescence lasting from several seconds to hours. As the simple pictorial representation is given in Figure 1.3a, during a PersL process, electrons are first excited from a ground state to an excited state by a charging source such as UV or visible light, or high‐energy ionizing radiation like X‐rays. The excited electrons are then further photo‐ionized into the conduction band (CB) of host materials with the help of continued excitation energy. Since the electrons are free to move in the CB, they can be easily trapped by either shallow (close to CB) or deep (a bit far from CB) traps. Since these electron traps form localized electronic states at energy levels which are different from carrier transport bands, they will get trapped energetically whenever photo‐generated free carriers fall into these localized states (Figure 1.3b, top). Since these trap states have different equilibrium nuclear configurations, there is an additional energy barrier between the delocalized carrier transport energy band (CB or valence band [VB]) and localized state (in a trap). The mobility of the charge carriers (electrons and holes) in the crystal system therefore restricted energetically (Figure 1.3b, bottom). Such energy barriers occur because the empty and occupied defect energy states have different equilibrium nuclear configurations [28]. An additional external activation energy with an energy of kBT ∼ 26 meV wherein kB = Boltzmann constant and T = temperature at room temperature is required by these trapped carriers in order to escape the localized state and get back into the CB or free carrier transport level. Moreover, only those defect‐related traps whose energy levels are positioned between CB and VB are suitable to trap charge carriers [28]. The trap depth is defined by the energy difference between transport states and defect levels (ΔE). Any defects whose energy level exists above CB or below VB edges cannot act as trap states. As shown in Figure 1.3c, traps having small energy difference to CB and VB edges (ΔE ≤ kBT) are called as shallow trap states while those energetically positioned toward the middle of band gap (ΔE > kBT) are called deep trap states. The small energy difference of the shallow traps makes them involved in both trapping and detrapping processes of the free charges and restricts their efficient movement. In contrast, due to higher energy difference, a deep trap sometime hinders detrapping process and facilitates the nonradiative recombination pathways (Figure 1.3c). Therefore, an additional form of energy by means of optical, thermal, or mechanical source is required to release these trapped charge carriers. The room temperature thermal energy may release the shallowly trapped carriers slowly and help to get back into the CB followed by emission due to electron–hole recombination resulting persistent luminescence (Figure 1.3d).

Figure 1.3 (a) Basic steps of PersL process: (1) excitation of electrons, (2) photoionization of electrons into CB, (3) trapping of the excited electrons and holes into shallow and deep traps, (4) detrapping of the electrons and holes, and (5) recombination of electron–hole pairs as light emission. (b) The top part represents how nonperiodic disorder and defects in a semiconductor lattice create localization of the charge carriers inside the trap's energy well and the bottom part shows the newly introduced energetic levels and how traps slow down the transport of charge carriers through trapping and detrapping events.

Source: Adapted from Jin et al. [28].

(c) The state density in a disordered semiconductor and the radiative (rad.) and nonradiative (nonrad.) recombination processes.

Source: Reproduced with permission from Jin et al. [28]; © 2020, Royal Society of Chemistry.

(d) Summary of various possible excitation paths using various pumping sources from low energy ultraviolet (UV), visible (Vi), and NIR light, to ionizing irradiation like X‐rays, trapping detrapping mechanisms of charge carriers involving various shallow and deep electron/hole traps, quantum tunneling of charge carriers, up‐conversion persistent luminescence, and energy transfer process involved in persistent luminescence.

Source: Liang et al. [29]/Springer Nature.

Recently Liang et al. provided an excellent summary of various complex mechanisms (steps 1–19) involve in the PersL process of PLMs, starting from possible excitation paths, trapping–detrapping mechanisms involve in various shallow and deep traps (Figure 1.3d) [29]. The electrons from the activator can be pumped to a higher energy level (step 1) by using a variety of excitation sources. Although both UV and visible light sources are widely used as pumping sources for PLMs, UV excitation source with photon energy higher than the band gap of host materials generates strong PersL than visible light (step 12) in most of the cases. However, when PLMs are considered for application in biological field, especially for bioimaging, it is necessary to have good charging ability with high tissue penetrating deep‐red or NIR lights sources in order to facilitate the required in‐situ recharging. Lanthanide‐based photon up‐conversion process can be used to design such NIR excitable PLMs with anti‐Stokes emission. This can be achieved through the energy transfer (ET) processes between the sensitizers and activators. Two or more incident low‐energy NIR photons are absorbed by the sensitizers followed by sequential populations of the higher excited states of the activator via an energy transfer as shown in steps 15 and 16. The activators then emit a high‐energy photon in the UV–Vis region to charge PLMs (step 18). On the other hand, high‐energy X‐rays will produce abundant electron–hole pairs through the Auger effect and Compton scattering (step 19). The negligible scattering and higher penetration depth of X‐rays make them an alternative excitation source for PLMs. Both electrons and holes coexist as the primary charge carriers in PLM host matrices. The photo excitation of an electron may create a hole either in the activator or in valance band, which can be subsequently captured and then released by hole traps through migration across the VB (steps 7, 8, and 14). All these charge carriers created by various sources of excitation energies are then subsequently captured by the shallow and deep traps (steps 2, 7, 13, and 14), followed by their gradual released back to the activators (steps 3 and 8) to trigger PersL (step 4). In many times, an ET process between two different types of emitters is an effective way to expand the spectrum of PLMs (steps 9–11). As mentioned before, shallowly trapped electrons can be transferred back to the activators easily via CB at room temperature to generate PersL; however, deeply trapped electrons find such escape difficult and need external stimuli like heating, light irradiation, and mechanical force (step 5). Because the shallow‐trap‐based discharge is fast and generates only short PersL while the deep‐trap‐based discharge is difficult and needs external stimuli, a quantum tunneling (QT) model has been proposed for the deeply trapped electrons, which showed the ultralong (tens of hours) PersL (step 6). Therefore, the depth of the traps plays the most crucial role in determining the performance of PLMs. Shallow trap states result in high intensity and short PersL due to quick electron–hole recombination, while deep traps result in low intense long PersL. Many times, deep traps hinder detrapping process due to higher energy difference and facilitates nonradiative recombination pathways, which causes less light yield. Hence, defect‐related trap depth engineering plays a crucial role in designing efficient PLMs. Different types of crystal imperfections, such as vacancies, interstitial ions, impurities, and dopants, can act as electron traps with a broad energy distribution in depth.

After a brief discussion of PersL processes, we will now explain the PersL mechanisms by considering various proposed models, including the CB‐VB model, the energy band model through CB and VB engineering, the self‐electron/hole trapping–detrapping model, the QT model, the cation vacancy model, and the anion vacancy model, as discussed below.

1.2.1 CB‐VB Model

The CB‐VB model is based on hole and electron trapping–detrapping processes through VB and CB, respectively. This requires the energetic position of traps near VB or CB because the localization or trapping of the free charge carriers (holes and electrons) in the traps happens via VB and CB only. As discussed earlier, the detrapping process of shallow trap carriers is easier than that of the deep trap carriers; we shall discuss the hole and electron trapping–detrapping processes below by giving some specific examples.

Hole trapping–detrapping processes can be found in the case of SrAl2O4:Eu2+,Dy3+ as proposed by Yamamoto and Matsuzawa (Figure 1.4a) [34]. They considered holes as the main charge carriers as supported by their photoconductivity measurements. When an Eu2+ ion is excited by a photon, a hole is possibly escaped from the ground state of Eu2+ ion to the VB and leaving behind an Eu+ ion. In the VB, the hole is free to move and subsequently captured by the Dy3+ ions, which has formed a shallow hole‐trap near the VB and resulting in a Dy4+ ion. The room temperature thermal energy then releases the hole from Dy4+ ion into the VB and followed by recombination with the electron of Eu+ ion and generating Eu2+ again by releasing a photon.

Figure 1.4 Various proposed PersL mechanisms: (a) Hole trapping–detrapping model through VB and CB. (b) Electron trapping–detrapping model through VB and CB.

Source: Li et al. [18]/Royal Society of Chemistry.

(c) Energy level scheme of SrAl2O4 host.

Source: Dorenbos [30]/IOP Publishing.

(d) Stacked VRBE diagrams of Y3Al5−xGaxO12:Ce3+,Cr3+ (x = 0, 1, 2, 2.5, 3, 3.5, 4, and 5),

Source: Ueda et al. [31]/Royal Society of Chemistry.

(e) Stacked VRBE diagram of REPO4 (RE = La, Gd, Y, and Lu) with the binding energy in the ground states of Eu2+, Pr3+, and Tb3+.

Source: Lyu and Dorenbos [32]/Royal Society of Chemistry.

(f) Stacked VRBE diagram of Gd1−xLaxAlO3 (x = 0, 0.25, 0.5, and 1).

Source: Luo et al. [33]/American Chemical Society.

Later, Tanabe et al. disagreed with this hole trapping–detrapping model because the reduction of Eu2+ to Eu+ and the oxidation of Dy3+ to Dy4+ requires huge amount of energy and possible only under ambient conditions. They propose a new model in which Eu3+ is created after leaving an electron to CB while the Dy3+ ion captures the escaped electron through CB and forms Dy2+ [35, 36]. This is known as electron trapping–detrapping model (Figure 1.4b), which is exactly a mirror image of the hole trapping–detrapping model. This mechanism was strongly supported by the existence of Eu3+ species under excitation through the X‐ray Absorption Near Edge Spectroscopy (XANES) results [37], but the reduction of Dy valence state from 3+ to 2+ is still a debate and has not been confirmed experimentally by Electron Paramagnetic Resonance (EPR), X‐ray photoelectron spectroscopy (XPS) or XANES techniques. Later, Holsa et al. also agreed that Eu+ species are unstable in aluminate. They proposed that codoping Re3+ ion like Dy3+ may create new defect‐modulated trap centers due to the charge imbalance [38]. Moreover, they also observed a higher duration of PersL from SrAl2O4:Eu,Sm than SrAl2O4:Eu when Sm3+ ion is added. The argument that co‐dopant ions like Dy3+ and Sm3+ are only modulating trap centers due to charge imbalance was supported by the fact that lower PersL duration was observed when Sm3+ ions are easily reduced to Sm2+ in the N2–H2 atmosphere. Recently, Wang et al. have demonstrated super‐long PersL from SrGa2O4:Cu2+[39]. Through a combined experimental and theoretical studies, they proposed that O vacancies and the −1 charged Ga vacancies create hole traps close to the VB. Herein, the excitation occurs through charge transfer transition from O− to Cu2+ and leaves behind a hole in VB. The hole is then trapped by −1 charged Ga vacancies followed bydetrapping and recombination to give red PersL from SrGa2O4:Cu2+.

1.2.2 Energy Band Model via CB and VB Engineering

It is always easier and in a more convincing way to explain electron and hole trapping–detrapping mechanisms if the absolute energy of VB, CB, excited and ground states of luminescence centers as well as trap states can be determined and presented in one energy level diagram. While there has been an ongoing debate for about 10 years on PersL mechanism behind SrAl2O4:Eu2+,Dy3+, P. Dorenbos first constructed the famous energy level scheme of SrAl2O4 host with two zigzag curves of divalent and trivalent lanthanide ions in 2005 (Figure 1.3c) [30]. From the respective energy level position of various divalent lanthanides, it was proposed that trivalent lanthanide ions such as Nd3+, Dy3+, Ho3+, and Er3+ can act as potential electron trapping centers in SrAl2O4:Eu2+,Ln3+ since the ground states of the Ln2+ species are located below the bottom of CB (Figure 1.4c). On the other hand, since the trap depths of Eu2+ from VB is very high, Eu2+ can act as a stable hole trapping center and thereby act as emitting or electron–hole recombination center.

Thus, the electron trapping–detrapping mechanism involving the Eu2+/Dy3+ and Eu3+/Dy2+ redox couple, that is, excitation of 5d electron of the Eu2+ ion to the CB followed by trapping at Dy3+ trap, can be explained. Since Nd2+ ions have similar depth like Dy2+, the same diagram can also explain the sensitization of Nd3+ when codoped with Eu2+ as reported in Matsuzawa's paper [10]. Therefore, with the help of such diagram shown in Figure 1.4c, it is now possible to predict a few critical things as follows: (i) the nature of the charging process, that is, electron and/or hole transfer, (ii) differentiation of the emission canters and trap centers of PersL and the role of co‐dopants. From Figure 1.4c, it can be said that Eu2+ and Ce3+ usually act as emission centers because they prefer to be stable as hole traps due to large energy gap between the ground state and the top of VB. On the other hand, due to moderate or small trap depth with respect to CB, Nd3+, Dy3+, Ho3+, and Er3+ can act as efficient electron trapping centers [4].

Such energy level scheme can also provide good guidance to control and alter PersL properties by controlling the trap depth or by altering the energy level of CB or VB edge in a controllable manner, known as CB engineering and VB engineering. For example, the PersL mechanism due to Ce3+ in Y3Al5−xGaxO12:Ce3+,Cr3+ (x = 0–5) can be tuned by altering the trap depth by changing the energy gap between CB edge and the ground state of Cr2+. As shown in Figure 1.4d, it is possible to construct the vacuum referred binding energy (VRBE) diagrams of Y3Al5O12‐Y3Ga5O12 solid solutions wherein the energy level of 4f and 5d1,2 states of Ce3+ as well as 3d ground state of Cr2+ are shown, and by substituting Ga3+ at the tetrahedral and/or octahedral sites, a systematic variation of trap depth can be realized as strongly supported by the thermoluminescence (TL) study [31].

Similarly, when La, Gd, Y, and Lu orthophosphates were doped with different lanthanide ions, their VRBE diagrams can be constructed as shown in Figure 1.4e [32]. From the TL 2D mapping of Eu3+ and Tb3+ codoped YPO4 after ceasing excitation, it was observed that only TL emission due to 5D0 → 7FJ (J = 1, 2, 3, and 4) transitions of Eu3+ were present while 4f‐4f transitions of Tb3+ were totally absent. This confirmed that Eu3+ act as electron trapping and recombination centers while Tb3+ act as hole trapping centers. This experimental result can be clearly explained by considering the trap depths from the VRBE diagram. It was also explained that when Y3+ ions were substituted by isovalent La3+, Gd3+, or Lu3+ ions, the top of VB was altered in a controllable way. This is known as VB engineering to tune the trap depths of hole trapping centers. Likewise, the VRBE diagram of Gd1−x LaxAlO3 (x = 0, 0.25, 0.5, and 1) in Figure 1.4f showed how the trap depth can be altered by doping La at Gd position [33].

1.2.3 Self‐Electron/Hole Trapping–Detrapping Model

The examples described above indicate that it is feasible to tailor the trap depth (ΔE) of electron or hole traps by CB or VB engineering and subsequently controlling the recombination process and PersL characteristics of PLMs with prior knowledge of various energy levels of lanthanide ions within the bandgaps of certain matrices. However, these CB‐VB engineering models are based on the energy level position of dopants and co‐dopants, which act as trap centers inside the band gaps of PLM hosts. Also, it is difficult to explain the intense PersL in some single activator‐doped PLMs by Cr3+ or Mn2+[40]. Although there are many explanations that intrinsic and lattice defects may act as electron or hole traps, in many cases, it is possible that the activation center itself is serving as both emission and trap centers, which leads to the self‐electron/hole trapping–detrapping model. This happens only when activators have multi‐valence states with a self‐redox nature in particular host matrices. For example, the VRBE diagrams of Cr‐doped LaAlO3 can be constructed by putting absolute energy levels of Cr3+ and Cr2+ (Figure 1.5a) [41]. The VRBE diagram indicates that Cr3+ ion may act as a deep hole trapping or a recombination center due to the large trap depth (∼1.5 eV) from the top of the VB, while it may also act as an intermediate or shallow electron trapping canter due to relatively lower trap depth (∼1.3 eV) of Cr2+ from the bottom of the CB. A similar explanation can also be given in other matrices such as GdAlO3, YAlO3, and LuAlO3 due to similar ground state locations of Cr2+ and Cr3+ in their VRBE diagrams.

Figure 1.5 (a) The VRBE diagram with self‐electron and hole trapping–detrapping model in chromium‐doped LaAlO3.

Source: Adapted from Katayama et al. [41].

(b) QT model of NIR PersL in Cr3+‐doped LiGa5O8.

Source: Adapted from Liu et al. [42].

(c) Oxygen vacancy as electron traps for the PersL mechanism of YAGG:Ce3+,Yb3+.

Source: Yang et al. [43]/American Chemical Society.

(d) Oxygen and Ga vacancy as hole trap for the PersL mechanism of SrGa2O4:Cu2+.

Source: Wang et al. [39]/American Chemical Society.

1.2.4 QT Model

While most of the above‐mentioned CB‐VB models explained the charge trapping in shallow trap states, QT model explains super‐long PersL involving deep traps with higher trap‐depth energy. The words of quantum tunneling describe a quantum mechanical phenomenon when particles tunnel through a barrier, which is not possible classically. In a few cases of deep traps, it is possible that room temperature is not sufficient to release charge carriers from traps, but tunneling of electrons or holes might occur between deep trap states and near energy state levels of activators. For example, Cr3+‐doped zinc gallogermanate showed super‐long PersL in 650–1000 nm region and lasted for 15 days (360 hours) [44]. Similarly, Cr3+‐doped LiGa5O8 showed NIR PersL, which lasted more than 1000 hours [42]. When charged by UV light, these PLMs showed a strong afterglow suggesting that the approximate energy required for ionizing Cr3+ activator in Zn3Ga2Ge2O10 or LiGa5O8 host is near to the CB. However, these same PLMs were also found to give weak but super long PersL when charged by low‐energy visible light (400–630 nm). This interesting phenomenon can be explained based on QT process as shown in Figure 1.5b. When charged by visible light, electrons from the ground state of Cr3+ ions may be promoted to higher energy levels, which are close to deep traps below ionization threshold. Subsequently, the deep traps get filled through the tunneling process from the closest energy levels of Cr3+ ions. This explains the charging of deep traps by low energy light while the reverse tunneling recombination procedure explain the super long afterglow as it is difficult to release the carriers into CB or VB directly due to high energy barrier.

1.2.5 Cation and Anion Vacancy Model

Most of the examples given above involve various lanthanide or transition metal ions as traps, either when being used as co‐dopants or acting as self‐trap centers in some cases. However, there are also many examples in which intrinsic defects, such as cationic or anionic vacancies, act as hole or electron trap centers. Especially, in oxide‐based PLMs, oxygen vacancy is often found to act as electron traps. For instance, Clabau et al. suggested that oxygen vacancies can act as efficient electron traps in SrAl2O4:Eu2+,Dy3+[45]. Therefore, even if codopant ions such as Dy3+ act as efficient traps for Eu2+ doped PLM matrices as explained by the CB‐VB energy diagram, it is very much feasible that oxygen vacancies also form different types of shallow or deep traps. Furthermore, these models could be constructed based on the identification and characterization of vacancies by many sophisticated techniques, such as XPS, EPR, and positron annihilation lifetime spectroscopy (PALS). For example, oxygen vacancies were found to be responsible for enhancing the PersL of Y3Al2Ga3O12:Ce3+,Yb3+ significantly when this PLM was heated in 10% H2/90% Ar atmospheres [43]. In addition to Yb3+, oxygen vacancies also act as efficient electron trap canters as shown in Figure 1.5c. In this case, the existence of oxygen vacancies in the Y3Al2Ga3O12:Ce3+,Yb3+ PLM was characterized by XPS and EPR measurements. Similarly, both neutral oxygen vacancy and negatively charged Ga vacancies were found to act as hole‐trap in SrGa2O4:Cu2+ and generate super‐long red PersL (Figure 1.5d) [39]. Therefore, depending on its charge, oxygen vacancies can act both as electron and hole trap centers while the negatively charged cationic vacancies act as hole trap centers.

1.3 Types of PLMs

1.3.1 Inorganic PLMs

Figure 1.6a represents a typical PersL material comprises three components: host (H), luminescent emitter (E), and trapping centers (T). IPLMs can be categorized into ultraviolet (UV), visible, and near‐infrared (NIR) light‐emitting PLMs according to the emitter's specific emission range. Figure 1.6b,c shows various UV and visible light emitters, respectively. In many cases, some emitters have shown PersL both in the UV and visible wavelength regions.

Figure 1.6 (a) Schematic representation of a PersL material comprising a host (H), an emitter (E), and traps (T). The PersL material emits possibly in UV, Vis, or IR range upon UV irradiation.

Source: Adapted from Wang and Mao [46].

(b) Examples of various UV PersL emitters, such as Pr3+, Bi3+, Pb2+, Gd3+, Ce3+, Tb3+, and wide‐band compounds, and their emitting wavelengths distributed in UVA, UVB, and UVC regions.

Source: Adapted from Wang and Mao [46].

(c) Example of various visible light PersL emitters, such as Eu2+/3+, Ce3+, Dy3+, Sm3+, Tm3+, Tb3+, Pr3+, Mn2+, Mn4+, Cr3+, Bi3+, Pb2+, and wide‐band compounds, and their emitting wavelengths distributed in UVA, blue, green, yellow, red and NIR regions.

Source: Adapted from Kang et al. [47].

In Figure 1.7, we present a few examples of PersL spectra along with the decay profiles of these UV, visible, and NIR PLMs. The applications of PLMs are mostly dependent on their emission wavelengths; hence, some PLMs are more suitable for specific applications than others. For instance, PLMs having visible PersL are useful as luminous signage and lighting sources when it is desired to have high visibility in the dark. For biomedical applications, NIR PLMs are the most desirable candidates due to the high tissue penetrability of NIR light [17, 18]. On the other hand, UV PLMs find applications in persistent disinfection, “solar‐blind” tagging, persistent photocatalysis, and “device‐free” phototherapy [46]. Below we have summarized many representative UV, visible, and NIR PLMs.

Figure 1.7 Representative PLMs of Pr3+, Bi3+, Gd3+, Ce3+, Mn2+, and Cr3+‐doped compounds. (a) PersL decay curves of Ca2Al2SiO7:Pr3+ at 268 nm in different light environments, including in the dark, under room light, in direct sunlight, and in the shade after charged by 254 nm UV light along with its emission spectrum as shown in the inset.

Source: Wang et al. [48]/Springer Nature/CC BY 4.0.

(b) PersL emission spectra of YPO4:Bi3+ recorded at different delayed times including 4, 5, 7, 10, 20, 30, 60, and 120 minutes after ceasing X‐ray irradiation.

Source: Liu et al. [49]/John Wiley & Sons.

(c) PersL decay curves of Sr3Gd2Si6O18:Pr3+ peaking and Sr3Gd2Si6O18 at 311 nm upon 254 nm charging along with the PersL spectrum of Sr3Gd2Si6O18:Pr3+ shown in the inset.

Source: Wang et al. [50]/Royal Society of Chemistry.

(d) PersL intensity of Zn3Ga2Ge2O10:0.5%Cr3+ monitored at 713 nm as a function of time after charged by UV light at 254 nm. The right‐hand side of the picture shows the photograph of NIR PersL of the same compounds at various time interval after ceasing the excitation source.

Source: Pan et al. [44]/Springer Nature.

(e) PersL emission spectra of Li2ZnGeO4:tMn2+ (t = 0, 0.0025, 0.005, 0.01, 0.015, and 0.02) visible PLMs after removing excitation source at 254 nm. Insets: the photos of Li2ZnGeO4 and Li2ZnGeO4:Mn2+ samples under sunlight, under the irradiation by 254 nm of excitation wavelength and at different time intervals after ceasing the excitation source.

Source: Adapted from Jin et al. [51].

(f) Photographs of PL (top; under UV light illumination) and PersL (bottom; 10 seconds after excitation ceased) from YSiO2N:Ce3+ and YSiO2N:Ce3+,Ln3+ (Ln = Sm, Tm) samples.

Source: Adapted from Kitagawa et al. [52].

1.3.1.1 UV PLMs

While most efforts have been devoted to design and develop efficient visible and NIR PLMs along with critically understanding of their PersL mechanism in the last three decades, UV PLMs have started to receive attention from the research community only in the last decade. Except for a few Pb2+ or Ce3+ doped UVA PLMs reported in 2002 and 2003, major interest in developing UV PLMs was aroused only after 2015 [48, 53–55]. The challenges in finding emitters with high emissive energy levels and suitable host matrices with large band gaps, along with having efficient charge traps located at high energy positions, are the main reasons for the limited number of reported UV PLMs. However, since the demand for UV light emitting devices for disinfection technology has grown significantly during the COVID‐19 pandemic [56], researchers are giving significant effort to design and develop various UV PLMs as reflected by their increasing number of reports [46]. Depending on the emission wavelengths, UV PLMs can be further classified into UVC (200–280 nm), UVB (280–315 nm), and UVA (315–400 nm) PLMs. Due to numerous unique applications of functional materials within each range of these UV wavelengths, UV PLMs show unparalleled potentials over visible and NIR PLMs. So far, UV PLMs can be developed by doping activators such as Pr3+ (4f2), Gd3+ (4f7), Ce3+ (4f1), Bi3+ (6s2), or Pb2+ (6s2) into suitable host matrices. Among them, Pr3+ and Bi3+ activators can generate PersL in the UVA, UVB, and UVC regions, Pb2+ can yield UVA and UVB PersL, Gd3+ may produce UVB PersL, and Ce3+