Luminescent Materials
Luminescence is an energy state transition that results from chemical changes, electrical energy, subatomic motions, reactions in crystals, or stimulation of an atomic system.
Luminescent materials are an important class of devices for display and lighting applications. However, they are susceptible to thermal degradation. Increasing their quantum efficiency (QE) and thermal stability is essential to improve these materials for display and lighting applications.
Chemiluminescence
Chemiluminescence is the emission of light resulting from the excitation of electrons that are in excited quantum states. It is a common property of many materials, but it has been demonstrated in some rare and unique instances.
Luminescent materials can be used in various types of lighting and devices. They are used in glow sticks, kites, emergency lights and more. They are also used for medical testing and in the pharmaceutical industry, where they are often used to detect drug contamination.
In addition, some chemiluminescent materials can be converted to other luminescent substances by a process known as indirect chemiluminescence. This allows the detection of ultra-low concentrations (femtomoles) of a target molecule. This method of detection is more efficient than direct chemiluminescence because it does not involve the cleavage of label molecules prior to emission, but instead uses a label that reacts with the sample and delivers a single photon.
This is a common way to detect certain chemicals, such as drugs and hormones in body fluids. The reaction can also be used to detect trace amounts of gas such as nitric oxide and sulfur compounds.
Some chemiluminescent reactions are also useful for detection in capillary electrophoresis. Examples include the luminol reaction and the tris(2,2′-bipyridyl)ruthenium(II) reaction. Usually, a trigger solution is added to the reaction compartment before the measurement takes place.
These reactions are often a dull green or blue in color and the intensity of the glow can vary significantly, depending on the chemistry involved. The chemistry is usually simple, but it can be challenging to interpret the results.
Another chemiluminescent technique is enhanced chemiluminescence, which luminescent materials involves the use of a horseradish peroxidase enzyme and an antibody to sensitize a reagent near the target molecule of interest. This reagent then undergoes a series of oxidative changes to produce an excited triplet carbonyl that emits light when it decays into a singlet carbonyl.
Chemiluminescence is an essential tool for a variety of applications. It is useful in liquid-phase detection for HPLC, flow analysis, DNA probes, immunoassay labels, and enzyme reactions. It is also a valuable method for protein blotting, and a number of other types of analysis.
Photoluminescence
Photoluminescence is a process that involves a combination of absorption and emission of light. This occurs via excitations and relaxation processes that occur when light is absorbed by a substance.
It is a contactless, nondestructive technique that is used to probe the electronic structure of materials. In the ultraviolet-visible spectral region, absorbance spectroscopy or reflectance spectroscopy can be used to measure transitions from the ground state to an excited state.
Luminescent materials are important in applications such as lighting, displays, and systems that detect X-rays or g-rays. They can be characterized in terms of their luminescence spectrum, and their optical properties can be optimized to improve their efficiency.
The development of luminescent materials has mainly relied on simple design rules, systematic materials synthesis, and serendipitous discovery. Today, researchers are turning to data-driven methods that can accelerate the exploration of new luminescent materials.
This approach uses artificial intelligence (AI) to mine and analyze large amounts of data that contain information about a material’s physics, surface engineering, and device encapsulation. The resulting feature sets can then be used to identify promising materials that can be synthesized and optimized using high-throughput methods.
AI has the potential to help accelerate the discovery of promising luminescent materials by identifying patterns and relationships in data. It can also help to accelerate the synthesis and optimization of these materials.
However, despite the enormous amount of research in this field, there is still a gap between the available data and what is most needed for successful application of luminescent materials. The main issue is that current feature sets are based largely on property-related compositions and average crystal structures, which are not specific to luminescent centers.
To reduce this gap, it is vital to develop features more specifically for luminescent centers. This can be achieved by developing models that better represent the luminescent center’s local environment. This will be essential in predicting the orbital interactions, crystal field splitting, and optical properties of luminescent centers.
Moreover, because the luminescent center’s local environment is distorted during synthesis and device encapsulation, it is necessary to know how these distortions affect the phosphor’s ability to emit radiation. This knowledge is essential for determining the luminescence spectrum, and it luminescent materials can be obtained through a wide range of spectroscopic techniques, including photoluminescence.
Scintillation
Luminescent materials exhibit scintillation when they absorb energy from ionizing radiation and then re-emit it in the form of light. The process occurs in a chain of steps that involves the formation of excited electronic states and the transfer of their excitation to luminescent centers. The rate of this process depends on the luminescent material and can be characterized by different time constants.
In fluorides and fluorite, the first step in the conversion of radiation to light is the formation of primary electron-hole pairs with characteristic time constants. This occurs within less than a picosecond and is one of the most important and widely studied processes in scintillation crystals.
The second step is the thermalization of these ionized pairs, which occurs within less than a few tens of nanoseconds. This is a critical stage because it leads to a significant loss of scintillation efficiency due to non-radiative recombination, trapping and other charge transport barriers.
This phase can be further divided into two sub-phases: the charge transport phase and the conversion of thermalized electrons and holes to luminescent centers. The latter is a more complicated process that is influenced by atomic vacancies, defects and grain boundaries in the crystal structure of the scintillator.
Another critical aspect of the conversion process is that if the luminescent center is a molecular system or defect, it must contain an energy band that is forbidden to the electrons and holes in the medium. This is called the forbidden energy band and it prevents photo-ionization or reabsorption of the emitted light from the excited state of the center.
Several classes of scintillators are known, including organic, metal and inorganic. Among them, organic scintillators, such as benzene rings, contain luminescent centers in the delocalized molecular orbitals (p-orbitals). These are excited to vibrational states belonging to higher energy levels on interaction with radiation.
A common method of measuring the luminescence of these organic scintillators is to use spectral measurements, using a laser or an ionizing radiation source. However, this requires an extremely sensitive instrument that can detect and measure the luminescence of the entire spectrum of a material.
Persistent luminescence
Persistent luminescence, also known as afterglow luminescence, is a type of luminescent material that shows a long-lasting light emission even after the excitation source is no longer present. The research of this phenomenon has a lot of interest and has led to the development of new types of materials with many different applications.
The main mechanism behind this phenomenon is the creation of energy traps in the material that are filled upon excitation and release during detrapping. This can happen by means of electron trapping or hole trapping. The charge carriers released from these defects can recombine with their counterparts at the luminescence centers, producing luminescence.
Luminescent materials can be used for many different purposes, including lighting, display technology, and optical sensors. Some of these materials may also be used for biomedical imaging, as they can detect cancer cells or other diseases in the body.
However, despite their wide application, it is difficult to synthesize and characterize these types of materials. In particular, their synthesis often involves harsh chemical procedures that involve rare-earth metals and are prone to contamination. In order to overcome these problems, a variety of organic and nanoscale phosphors are being developed.
These materials can be used for a variety of purposes, including lighting, display technology, and solar cells. They can also be used for biomedical imaging, since they can detect cancer cells or other diseases in an animal’s body.
One of the most common phosphors used in these applications is Strontium Aluminate (SAl2O4). This material exhibits strong, UV-emitting luminescence, which is beneficial for a wide range of applications.
This material is not only highly effective for X-ray detection but also has strong emission at the visible and near-infrared wavelengths, which makes it an excellent candidate for IR-based microscopy and spectroscopy. Moreover, it can be used as an illuminator for a variety of applications, including imaging, light-emitting diodes (LEDs), and microfluorescence.
A number of luminescent materials have been developed for a wide range of applications, including LEDs and phosphors. Some of the most interesting ones include SrAl2O4 phosphor, which emits strong, blue-green light and is useful for a variety of applications; YbAl2O4 phosphor, whose strong luminescence allows it to be used as a sensor for X-ray detection; and NaYF4:Yb,Er UCNPs, which can be used as a fluorescent marker for detecting cancer cells.
