About Long-afterglow materials

Long-lasting glow materials referred to long afterglow materials, also known as the luminescent material, luminous material, which is essentially a photoluminescent material.    

Brief introduction   

Long-lasting glow materials referred to afterglow materials, also known as the luminescent material, which is essentially a photoluminescent material, he is a kind of absorbed energy as visible light, ultraviolet, X-ray, etc., and The substance can continue to emit light after excitation stop, he can be able to trap the energy stored in the material, is a kind of promising.  

History  

Long afterglow is one of the earliest materials research and application, many natural ore itself has long afterglow emission characteristics, and for the production of items such as "Moonwalker", "Night Pearl", etc. (Figure 1). Real written records might be in our Taizong during the Song Dynasty (AD 976--997 years) described (FIG. 2) with a "long-lasting paint" draw "cow painting", painting the cow into the night'll see that the reason is that this painting cow is made of photoluminescent pigment painted with oysters, Western earliest record of such light-emitting material in 1603 when an Italian cobbler local ore roasting alchemists, got some in the dark the red light material, after analysis showed that, within the ore containing barium sulfate, after reduction roasting became a part of barium sulfide long-lasting material. Since then, the 1764 British and sulfur mixed with oyster fired a blue-white light-emitting materials, namely calcium sulfide long afterglow.      

Various materials   

Aluminate base  

Since 1993, the co-doped synthetic Matsuzawa Dy of SrAl2O4: Eu study found that the afterglow decay time of up to 2000min. Subsequently, it has successively developed a series of rare earth activated aluminate long afterglow materials, such as blue CaA12O4: Eu, Nd and cyan Sr4Al14O25: Eu, Dy, its long-lasting materials and afterglow performance parameters in Table 1. Aluminate long-lasting material, which is mainly the activator Eu, afterglow luminescent color mainly in blue-green wavelength range. Today, although the water was not very good aluminate, aluminate long afterglow material SrA12O4: Eu, Dy and Sr4Al14O25: Eu, Dy still get a great commercial application is the main stage of long-lasting material Follow materials research and applications.

Silicate-based  

Silicate matrix using long-lasting materials, due to the silicate has excellent chemical stability and thermal stability, while raw material SiO2 cheap, easy to get, in recent years more and more people's attention, and this silicates material widely used in lighting and display areas. Since 1975 Japan first developed a long-lasting material silicate Zn2SiO4: Mn, As, its afterglow time was 30min.  

Since then, a variety of silicate long-lasting materials have also been developed, such as Sr2MgSi2O7: Eu, Dy, Ca2MgSi2O7: Eu, Dy, MgSiO3: Mn, Eu, Dy, material and performance parameters in Table 1. Silicate matrix long-lasting material for the main activator Eu2 +, its luminous colors still concentrated in blue-green, although there are reports of red silicate long-lasting material. Afterglow better performance is co-doped Eu and Dy Sr2MgSi2O7 and Ca2MgSi2O7, its afterglow duration longer than 20h. In addition, observed in Mn, Eu, Dy co-doped MgSiO3 three elements into a red long-lasting phenomenon. Silicate aluminate long-lasting material has incomparable advantages in terms of water resistance, but its poor performance than aluminate.

Other  

In addition to the above-mentioned categories of long-lasting materials, research materials as well as more long-lasting sulfide ZnS: Cu, Co and CaS: Eu, Tm represented, which is characterized by the color variety, but the maximum initial afterglow luminance only 40mcd / m or so, and this type of material in the first few minutes, the afterglow of a sharp decline in effective afterglow time is very short. There Pr doped titanate CaTiO3: Pr, Al. Up to now, the long-lasting luminescence in the oxidation system has been extensively studied, it is worth noting that chlorine oxide Ca8Zn (SiO4) 4Cl2: Eu, the nitride-containing Ca2Si5N8: Eu is also a long-lasting phenomenon.

In addition, Professor Pan Zhengwei research group has developed a long near-infrared long-lasting material Zn3Ga2Ge2O10: Cr, its afterglow time up to 360h    

Some of the common long-lasting materials and twilight time  

Long afterglow emission peak time color

SrAl2O4: Eu, Dy 520nm> 60h green

CaAl2O4: Eu, Nd 450nm> 20h Blu

Sr4Al14O25: Eu, Dy 486nm> 30h blue

Sr2MgSi2O7: Eu, Dy 466nm> 20h blue

Ca2MgSi2O7: Eu, Dy 545nm> 20h green

MgSiO3: Mn, Eu, Dy 644nm ~ 4h red

Cd2Ge7O16: Mn 585nm> 3h orange light

MgSnO4: Mn 499nm> 5h green

Zn3Ga2Ge2O10: Cr 700nm> 360h red

 Emitting mechanism

1. hole transport model
For these materials, the first model consists in SrAl2O4 Matsuzawa et: Eu, Dy hole transport model proposed system. Based on this model, Matsuzawa think, in the long-lasting material SrAl2O4: Eu, Dy in, Eu electron trapping centers, Dy is a hole-trapping centers. When the material is excited by UV, Eu can become trapped electrons Eu, the resulting hole valence band is generated by capturing Dy Dy, after stoppage of the excitation, because the relationship between the movement of the heat, the occurrence of the escape hole, and the opposite through the above process The characteristics of the process and lead to Eu emitting a schematic diagram shown in Figure 3. The model in various Eu and Dy co-doped mechanism to explain the long-lasting materials are widely as a reference, become a common interpretation Eu and Dy co-doped mechanism of long-lasting material.


2. shifting coordinates model
Coordinate displacement model was first and Su Qiang Qiu Jianrong et al. Figure 3 is a schematic diagram of the displacement coordinate model. A is the ground level Eu2 +, B for the excited state level, C level of defect levels. C may be incorporated into the impurity ions, the matrix may be a defect by the defect of some levels. Su Qiang and others that C can play a role in the capture of electrons. Under the influence of an external light source, the electron is excited from the ground state to an excited state (1), part of the electronic transition back to a lower energy state luminescence (2). Another part of the relaxation process of electrons through a defect can be stored and C in (3). When the defect level electronic absorb energy and re-excited by return to excited states, the transition to the ground state and emit light. The length of time and number of electrons stored in the defect levels, and absorb energy (heat) about twilight, the more the number of electrons in the defect level, the longer the afterglow time, more energy absorption, resulting in sustained light.

Preparation of material


A high temperature solid
Prepared by high temperature solid long-lasting material is the more traditional method of reaction, this method is used widely. In general, the general operation of the solid phase reaction is based on the solid powder as the raw material. We will meet the requirements of purity of the raw material weighed by a certain percentage, and add a certain amount of flux mill fully mixed evenly, then under certain conditions (temperature, atmosphere, time, etc.) for burning. Stoichiometric ratio of the luminescent material according to precise recipes, high temperature resistance furnace placed in a protective gas atmosphere or reducing atmosphere 900 ℃ ~ 1450 ℃ burning 2h-5h serve. Burning process, types and distribution ratio of the co-solvent and dopant ions and so on structure and optical properties of long afterglow luminescent material have a significant influence.
Since the reaction conditions of high temperature solid control, a reducing agent, flux the selection and preparation of raw materials are increasingly optimized, mature production technology, they have been widely used. For example: Red Afterglow sulfide system material is alkaline earth metal carbonate, sulfur powder, and select the appropriate mix of rare earth oxides and fluxes obtained by firing, there are direct use of alkaline earth metal sulfates and rare earth oxides, flux Mix baking process.

Sol - gel method
For high temperature solid ignition temperature is high, the preparation of sub-luminescent material particles coarser, the milled material is vulnerable to the shortcomings crystal damaged, it has developed many other ways. Wherein the sol-gel method as a wet chemical methods in materials science community has attracted wide attention. This method originated in the 18th century, the application of this method has been very extensive.
Sol-gel method is the use of a specific precursor material under conditions hydrolyzed to form a sol, and then the solvent was evaporated by a heat treatment, the sol into a gel network structure, and then into the post-treatment process appropriately formed nanomaterials A method for preparing the basic processes of nanomaterials used as follows:
Materials -> dispersible System -> Sol -> Gel -> Nanomaterials
Prepared by the sol-gel technique is the use of luminescent materials are mainly metal alkoxide method, i.e., a metal alkoxide as a starting material into the perhydrolysis reaction, the polymerization reaction and sol gel. Zhang Dong and co-workers were prepared by sol-gel ZnAl2O4: Mn material, the sintering temperature than the traditional method lower by 100 至 200 ℃. [3]
In recent years, inorganic complexes prepared by sol-gel gradually by the people's attention, which is mainly preparation Pechin method is used, the use of citric acid and ethylene glycol esterification reaction of the sol, this method is fast and simple experiment cost method lower than the metal alkoxide.

Combustion
Combustion method refers to a method of combustion synthesis precursor material. When the reaction was exothermic reaction reaches ignition temperature, somehow ignited the subsequent reaction that is exothermic heat to maintain, material combustion product is intended to be produced. The main principle of this method is that the reaction raw material corresponding nitrate, urea is added as a fuel (reducing agent), at a certain temperature heat a few minutes, after intense oxidation-reduction reaction, spilling large amounts of gas, and then burning a few After ten seconds to get fluffy foam-like material, not Jietuan friable. This method has considerable applicability, the gas produced by the combustion process may also be used as protective gas. General operation is as follows, in order to synthesize SrAl2O4: Eu, Dy, for example, will SrCO3, Al (NO3) 3, and Dy2O3 Eu2O3 nitrate solution by chemical proportions, adding the appropriate urea and boric acid, dissolved quickly into already warming to about 600 ℃ muffle furnace, with the evaporation of water, a few minutes later, as the oxidant nitrate and urea as a reducing agent reaction occurs, and then burn, spilling gas, this process is very short, only ten seconds , to give the product after combustion. The product was removed and cooled to obtain abrasive product. When using this method can greatly reduce the oven temperature, it is an energy efficient way.
In addition to the several methods for preparing the afterglow material, there is a hydrothermal synthesis method, microwave-assisted synthesis, chemical precipitation method. By using these new synthetic technology adoption, research results show that the performance improvement achieved on the light emitting material of a breakthrough, it is possible to obtain a light emitting material can not be prepared by conventional techniques, resulting in a new kind of light emitting material, to further broaden long-lasting material research applications.


Biological applications
Long-lasting light-emitting excitation light irradiation after stopping substance can still continue to light the phenomenon. Long-lasting light-emitting material does not contain toxic heavy metals, can be in front of detection and imaging excitation, realize biological sensing and imaging in the "free excitation" conditions, thus effectively avoiding the excitation background interference generated in situ. Despite the long-lasting light-emitting materials have an advantage, but until 2007 only the long-lasting material used in biological sensing and imaging reports. Chermont use of sol-gel method and other high temperature synthesis having a near-infrared fluorescence long afterglow silicate nano-materials, and applied vivo imaging (Figure 3). The method successfully avoided the potential harm to organisms traditional fluorescence excitation light source method, the first time in vivo "Free excite" fluorescence imaging.

Figure 5Ca0.2Zn0.9Mg0.9Si2O6: Eu, Dy, Mn Afterglow nanoparticles for in vivo imaging results (A) prior to injection, the nanoparticles placed first 6 W UV light excitation at 2 cm 5 min. (B) respectively, then three different positions on the back of mice injected with different concentrations (100, 10 and 1μg / mL) of the nanoparticle suspension (20μl). Use 2 min acquisition time, it is easy to monitor the fluorescence of which two larger doses (2μg and 200 ng) issued. Nanoparticles (C) of mice after intramuscular injection of 90 s dispersal
Li Zhan-jun and his collaborators reported the use of MCM-41 molecular sieve as a template for preparing long-lasting materials and for imaging biological cells (Fig. 6), the clever use of MCM as a template, the use of MCM-41 can get clever morphology preferably spherical nanoparticles, although the material at high temperature sintering longer prone to collapse, but the size of the sintered material obtained in the nm scale and better particle dispersion.
Figure 6 PEG-coated nanoparticles for long-lasting in vivo imaging

2012 Maldiney, who used PLNP PEG coating material and the surface-modified biotin, between biotin and avidin strongly about each other, their Kw dissociation constant of 10 or less. They group using such material detection surface is highly expressed avidin glioma cells, the effect is very sensitive and can be used for imaging of glioma cells (Figure 7).
Figure 7 biotin and PEG-coated nanoparticles for long-lasting avidin detection and BT4C sensing cells.


Development trend
Experienced a long period of development, long-lasting materials are self-contained, with its own unique "charm" to emerge and show broad application prospects. Although research in this area is very active. However, in its research and applications, there are still many problems to be solved. Long-lasting luminescence mechanism of research is not very good, there are still many issues that need explanation; matrix material and less active ions of choice and simple; how to use better synthetic methods such as sol-gel method instead of high-temperature solid-phase synthesis method is a serious problem ...... long-lasting material because of its excitation in vitro in biological applications to avoid the influence of in vivo autofluorescence and very bio-imaging could be used in the reservoir. We believe that through control of material composition, structure, improving the preparation process, long-lasting materials will be in more and more widely used.