Luminescent Materials: Synthesis, Characterization and Applications

Luminescent materials are capable of transforming certain types of energy into electromagnetic radiation, which means that in response to a specific stimulus, these materials emit light typically within the ultraviolet (UV) to the infrared (IR) spectral region [1,2].

Although they have been identified for over 1000 years from natural materials found in Chinese and Japanese paintings, one of the first documented luminescent materials is the Bologna Stone, found in 1603 in a village near Bologna by a shoemaker named Vincenzo Cascariolo [3,4]. After heating the mineral in a charcoal furnace, he observed that exposure to sunlight led to a strong orange-red emission that could persist for hours—this is the first scientifically documented persistent luminescence (PersL) material [2,3]. Later, the material in question was identified as barite (BaSO₄), which converted into barium sulfide (BaS) upon calcination [3,4]. Still, it took more than four centuries for scientists to understand the wonders involved in this curious light emission. This phenomenon has long held the interest of scientists and remains a hot topic to this day. From the development of the cathode-ray tube using phosphors based on zinc sulfide (ZnS) to the semiconductor-based solid-state lighting that dominates our screens and displays, or even the illumination solutions governing our daily lives, luminescent materials have become essential to our modern society. Indeed, the luminescence field has seen tremendous progress through advances in material synthesis/growth procedures, new spectroscopic techniques to assess their properties, as well as the need to fulfil the requirements of novel technologies [2,5]. Although this field is still being dominated by lighting and display applications, technologies such as bioimaging, biosensing, cell tracking, optical thermometry, photonics, information storage, disinfection and even water remediation have found new opportunities in luminescent materials [2,6,7,8,9,10,11]. Hence, such applications became trendy topics that have driven increasing research with the commendable purpose of addressing pressing societal demands. To achieve such goals, the selection of the materials that best fit a specific application must begin with a fundamental understanding of the physical processes involved.

One example of materials with intriguing luminescent phenomena is PersL materials, which have aroused much interest from researchers [6,12,13]. These materials have the ability to store energy in their defect (trap) centers and release it gradually by emitting light after the removal of the excitation source [6,12,14]. This phenomenon is commonly designated as afterglow. The trap centers frequently originate from impurities, lattice defects, or dopants intentionally introduced for this purpose. Despite not emitting radiation, they store energy while the excitation is on, and then, after the illumination stops, gradually release it to the emitting centers due to thermal (room temperature and above) or other physical activation, leading to delayed luminescence that can prevail for hours or even days [6,12,14,15]. Due to this remarkable property, PersL materials have been considered for a wide range of applications, such as safety signals, information storage, bioimaging, anti-counterfeiting, photodynamic therapy, and even for photocatalysis, among others [8,12,16]. PersL materials can be seen as energy-saving materials, whose afterglow time will be strongly dependent on the concentration and depths of the traps involved in the process. On the other hand, the emission energy is dictated by the nature of the emitting centers [14]. Therefore, the design of PersL materials must involve a careful choice of a suitable emitter and a host that can provide adequate trap centers to feed the long-lasting persistent luminescence process, which is often a major challenge [6,12,17].

Likewise, up-conversion is another appealing effect in luminescent materials, enabling near-IR light to be converted into visible radiation, which is of considerable significance for many research areas, especially in bioapplications, since it minimizes photo-damage to the biological tissues, as well as their autofluorescence background [18,19,20]. This is a nonlinear anti-Stokes process in which the emitted photons have higher energy than the absorbed ones, i.e., two or more low-energy photons are absorbed sequentially by long-lived intermediate energy states, resulting in spontaneous or stimulated emission at energies significantly higher than those of the excitation [18,19,21]. Up-conversion has been established as an efficient process, particularly when involving RE ions which have ladder-like energy level schemes, enabling the absorption of photons in equal increments [18,21]. Analogously to the PersL, up-conversion materials are mainly composed of the inorganic host, which radically influences the radiative and non-radiative processes, and the dopant ions, which can act either as activators or sensitizers, being responsible for the energy of the emitted photons and for absorbing light and efficiently transferring energy to the activator, respectively [19]. The wide possibilities of selecting the host lattice and the dopant(s) can lead to a myriad of up-conversion properties, allowing plenty of room for creativity and imagination [19].

The variety of luminescence materials is almost infinite, from organic polymers, organic dyes, transition metal complexes (iridium, copper, platinum, gold), metal–organic materials, to inorganic phosphors, etc. [22]. In this Special Issue, we welcome research on all luminescence materials, with particular emphasis on defect-engineered inorganic materials, such as transition metal (TM) and rare-earth (RE) doped semiconductors and isolators, to tailor their emission and promote fascinating phenomena like PersL or up-conversion.

Therefore, although research on luminescent materials has come a long way since the reports on the Bologna Stone, with many breakthroughs and a great maturing of the optical spectroscopy techniques used to assess them, there is still a vast path to be taken to respond to all the demands of the emerging technologies. In addition, many pieces of the puzzle are yet to be fitted to master the underlying physical phenomena, leaving researchers with an immense playground for improvement and novel discoveries.