Views:368 Author:Site Editor Publish Time: 2020-06-25 Origin:Site
Long afterglow luminescent materials are referred to as long afterglow materials, also known as luminous materials. It is a kind of photoluminescence material that emits visible light under the excitation of a light source and stores part of the obtained light energy. After the excitation stops, the energy is slowly released in the form of light. Therefore, it is also called "green light source material. Luminescent material is also called luminous body. It is a functional material that can convert various forms of energy absorbed from the outside into unbalanced light radiation. Because it can use sunlight or light to store light and then It emits light at night or in the dark, so it is widely used in night emergency indication, optoelectronic devices or components, instrument display, low-level lighting, home decoration and defense military (such as night travel map) and so on. It is more expected to be applied in information processing and new energy, life science and advanced science and technology fields, affecting the development of future science and technology. There are various types of light-emitting materials, and the main types are: photoluminescence, cathode-ray emission, electroluminescence, pyroluminescence, radiation emission, etc.
Organic long afterglow materials are a class of luminescent materials developed in recent years. However, most organic molecules can exhibit room-temperature long afterglow luminescence only in an aggregated state (such as a crystal) or need to be doped in a special host. The main reason is that the molecules can achieve electronic coupling between molecules in the aggregated state. Intersystem crossing (ISC) sensitizes triplet excited states. In order to achieve long afterglow emission of organic molecules in an amorphous state, a team collaborated to build a small organic molecule (CzDPS) with intramolecular electron coupling. The carbazole (donor) and diphenylsulfone (acceptor) units in the molecule are close in space. This close spatial action of the receptor can effectively mediate the passage between lines and achieve long persistent luminescence in non-clustered states. The results show that CzDPS not only has room temperature phosphorescence in the crystal state, but also has long afterglow luminescence in the photocurable adhesive with doping concentration of only 1wt %, indicating that this kind of material has a good potential application in the fields of plastics and photocurable 3D printing.
On this basis, the team also explored the application of the organic long afterglow material in time-resolved imaging. The luminescence lifetime of traditional organic materials is generally on the order of nanoseconds to milliseconds, and complex and sophisticated imaging equipment is often required to distinguish the luminescence of materials from background scattered light. The long afterglow material has a luminous lifetime of seconds, and a long-life luminescence with a millisecond delay can be detected by a general CMOS camera. Using the long afterglow luminescence property of CzDPS, the team only realized the time-resolved luminescence imaging of fingerprints by illuminating the sample attached to the fingerprint under the UV LED light, using the mobile phone to take pictures after turning off the UV LED light, and effectively eliminating the scattered light and the interference of substrate autofluorescence on fingerprint recognition. It provides a new and simple method for fingerprint identification. The findings are published in the journal Materials Horizons.
Long afterglow luminescent materials have been studied a lot, and a variety of inorganic materials have emerged, ranging from simple matrix luminescence to current rare earth ion-doped special matrix solid materials. The luminous performance is also continuously improved. The research status of some materials will be introduced below.
(1) Sulfide system
Sulfide series long afterglow luminescent materials have a long history of development. In 1866, France's Sidot first produced long-glow afterglow materials, and industrialized production in the early 20th century. Since then, a variety of sulfide system long afterglow materials have been developed, such as blue-violet light CaS: Bi, yellow-emitting ZnCd: Cu. However, long afterglow materials of sulfide systems have low luminous brightness, short afterglow time, poor chemical stability, and are prone to deliquescent. Although these shortcomings can be overcome by adding radioactive elements, material coating, etc., the addition of radioactive elements causes harm to human health and the environment. Therefore, it is greatly restricted in practical use.
(2) Aluminate matrix system
In the 1960s, Japanese researchers discovered the long afterglow phenomenon of SrAl204: Eu2 +, and developed extensive research interest in it. Further in-depth research on this material has shown that afterglow properties are significantly improved over sulfide matrix materials. Luminous brightness, afterglow time and chemical stability of aluminate systems are incomparable with the long afterglow materials of the first generation sulfide systems. However, alkaline earth aluminate materials have a single luminous color, high synthesis temperature, and are prone to deliquescence in contact with water.
(3) Silicate system
In fact, people have been attaching importance to the research and development of silicate phosphors for a long time. Because the silicate-based luminescent materials in the silicate system have good chemical and thermal stability, and high-purity silica raw materials are cheap and easily available, the sintering temperature is lower than 100 ° C compared to the aluminate system. The characteristics of silicate long afterglow luminescent materials are as follows: the external irradiance is stable, which expands the material's luminous color range, especially blue materials in the field of fire emergency safety. Due to its characteristics of light absorption and luminescence, no need for power, convenient installation, low cost, maintenance-free and high safety factor, special materials not only have excellent application characteristics, but also have high afterglow brightness and long time, adding new varieties for light-emitting materials. In addition, because the application characteristics of silicate system materials are in the case of emergency power failure, it does not need artificial energy to achieve rapid lighting, so the application in certain fields (such as the ceramic industry) is in great demand. It is currently better than aluminate systems in fire emergency systems and safety indication systems. The latest high-tech products such as passages, safety exit signs, fire extinguishing equipment signs, warning signs, etc. have been widely used in the fire protection industry at home and abroad.
(4) Titanate system
Rare earth ion-activated alkaline earth metal titanates are another type of long red afterglow luminescent material with stable chemical properties, high luminous intensity and good color purity. The luminescent matrix of rare earth activated titanates is mainly alkaline earth carbonate. Early research has Sm3 + activated BaTiO3 and Cr3 + activated CaTiO3. Currently reported more are Pr3 + activated SrTiO3 and CaTiO3. Its production process uses Ti0. And alkaline earth carbonate as raw materials, adds a certain amount of activating ions, etc., and mixes ZnO, MgO, Al203 into the matrix through high-temperature solid-phase reaction to make Zn, Mg, AlI and other parts Replace the alkaline earth element in the matrix of the matrix, so as to achieve good luminous brightness and long afterglow time. The alkaline earth titanate red long afterglow luminescent material represented by CaTiO3: Pr3 + is not only stable, good in performance, but also pure in color. Studying its luminescence mechanism and finding effective ways of high afterglow properties are of great significance for the study of new red long afterglow materials. However, the biggest shortcoming of this system is that the luminous brightness is not enough, and the afterglow time cannot meet the requirements of practical applications, and the excitation intensity in the visible light region needs to be further improved.
(5) Sulfur oxide system
The sulfur oxide series long afterglow luminescent material is a new type of long afterglow luminescent material. Its matrix system is mainly Eu2 + and Sm3 + activated red-emitting Y202S rare earth sulfur oxide material. Sulfur oxide series long afterglow luminescent materials are currently the best type of long afterglow luminescent materials among red long afterglow luminescent materials. However, the matrix material of the luminescent powder is composed of rare earth sulfur oxide, the cost is too high, and the luminous brightness and afterglow duration are not as good as the blue-green alkaline earth aluminate and silicate series luminescent materials.
So far, long afterglow luminescent materials are still used in noctilucent materials. Based on the characteristics of these materials, we believe that these materials may have applications in the fields of environmental catalysis and biomedicine and energy. We have conducted preliminary explorations in environmental catalysis and biomarkers, and have achieved certain results. For example, by compounding with photocatalytic materials, it is found that such composite materials still have photocatalytic effect in the absence of light or darkness, and are expected to become a class of multifunctional materials. As mentioned earlier, spherical CaWO; Eu * red long afterglow luminescent materials were prepared by using the long afterglow luminescence characteristics of the tungstate system we found (the afterglow time just meets the requirements of biomarkers). After that, we carried out research as a new type of bioluminescent marker, which may open up new directions for the application of luminescent materials in the biomedical field. In addition, such materials are also likely to be applied in solar energy, becoming a new type of photoelectric conversion materials and energy materials.