Enhancement of infrared phosphorescence properties in lithium spinal materials with chromium doped

Luminescence is briefly defined as the irradiation of materials without heat source. The luminescence phenomenon was first used by the German physicist and science historian Eilhardt Wiedemann in 1888 as a luminescence, and he described it as "a phenomen of not merely dependent on temperature increase." At the same time, Wiedemann classified luminescence into six distinct categories according to the excitation method. These are photoluminescence, thermoluminescence, electroluminescence, crystalloluminescence, triboluminescence, chemiluminescence. Photoluminescence is the phenomenon of light emission after the absorption of photon in any form of matter. The absorbed photon is usually shorter in wavelength than ultraviolet or violet, while the emitted photons are blue, green, and red with longer wavelengths. The photoluminescence phenomenon is caused by the electronic conduction of electrons at energy levels. Absorbed photons are analyzed under two different headings as photoluminescence fluorescence and phosphorescence according to the passage duration of energy levels. The luminescence is a phenomenon observed by human beings that is more than 2000 years . The first scientific studies began in the 17th century. Long lasting luminescent materials have attracted great interest in the past 30 years and have been extensively researched and used both in the scientific community and in industrial areas. A lot of study has been done on long-term phosphorescence materials that emit blue and green colors. The major ones are CaAl2O4: Eu2 +, Nd3 + and SrAl2O4: Eu2+, Dy3+. On the other hand, the number of studies on phosphorescent materials emitting in the red and infrared regions is not as high as those of blue and green emitting phosphors. Singlet-triplet state transitions in electronic transitions are less likely than singlet-singlet state transitions, which is often called forbidden transitions. If electrons are stimulated in triplet state, the time of arrival of the electrons to the fundamental state, that is the average life span, can be extended to a longer period than one second. In this case, the duration of the emitted light during the transition is longer, this is called phosphorescence. In recent years, many potential applications of inorganic phosphors have been achieved, including luminescence in the infrared region, as well as in night vision systems and in the field of in-vivo tissue imaging. The infrared light wavelength is an electromagnetic light with a visible light (400-700 nm) greater than 700 nm and a red limit up to 1 mm. If this wavelength interval is examined in the frequency band, it covers an approximate range of 430 to 300 GHz. Infrared light can be divided into three subgroups as Near Infrared (NIR), Middle Infrared and Far Infrared. The NIR region defines between 700nm and 2500nm (120-430 THz) in the electromagnetic spectrum. The NIR provides additional benefits due to the sensitivity and detection of light in the NIR region, being invisible and less interference more penetrating for living tissue in addition to that absorption and auto- fluorescence is minimum. The choice of doping in phosphorescent materials that emit to the near infrared region is rather limited. Although many rare earth elements are used to obtain emission in phosphorescence materials, the doping materials used in phosphorescence emitting in the near infrared region are limited due to the high density of the 5d band, or due to emission in the extrinsic visible region. Transition metals are frequently used as additives in the production of phosphorescent materials that act as Cr3+ emission centers and emit to the near infrared region. In this study, it is desired to improve the phosphorescence properties obtained by adding trivalent chromium ion (Cr3+) to the lithium aluminate and lithium gallate matrix. For this purpose, the chromium concentration to be added to the LiGa5O8 structure and sinter temperature were determined and then the rare earth elements such as Nd, Pr, Eu, Gd, Dy, Ce, Yb and transition metals as Bi were doped at different concentrations. The effects of these second additives on the phosphorescence emission intensity and the emission decay time were investigated. The next step was to investigate the changes in the phosphorescence properties of the LiGa5O8 structure with the displacement of gallium - aluminum. Experimental studies were carried out to determine the chromium concentration to be added to the host matrix of LiGa5O8 and the temperature of sintering of the matrix. When 1.5% - 1.5% - 2% chromium doped LiGa5O8 samples were compared, the 1.5% chromium concentration was higher than the other additive concentrations and the experiment work was continued with 1.5% chromium addition. In order to determine the sinter temperature, LiGa5O8: 1.5% Cr3+ was sintered at 1250 ° C - 1300 ° C - 1350 ° C. The sinter temperature was selected at 1300 ° C and the samples were sintered at 1300 ° C in the subsequent runs. Solid state reaction method and Sol-Gel method were used in the experiments. Samples of Cr 3+ transition metal at 1%, 1.5% and 2% concentrations added to the LiGa5O8 matrix were also obtained by solid state reaction method in the stoichiometric exchange of gallium and aluminum. In the co-dopand studies, samples were obtained using the Sol-Gel method. In order to avoid concentration quenching, chromium concentration is selected %1 for codopand experiments. Pr, Bi, Nd, Dy, Gd were added at a concentration of 0.5% and as a result of spectrophotometric measurements; Dy and Gd doping causes a decrease in the intensity of emmission; Pr, Bi and Nd caused an increase in emmission intensity. Codopand elements Pr, Bi, Nd and additionally Yb, Eu, Ce co-dopands contributing to the increase in the intensity of emmission were doped 1% and the phosphorescence properties were compared. Bi and Nd increase the emmision intensity 4 times, and Pr, Yb and Eu increase the emmission intensity 2 times. Ce decreased the intensity of emmision. Also, the decay time of the sample doped with Bi is longer when compared to other samples. Finally, the concentration of Bi and Nd additive elements, which provide a significant increase in the intensity of emmission, are increased by raising the concentration density up until 1%. However, when the additive concentration is increased above 1%, the emmission intensity is decreased due to the concentration excess. Although the luminescence intensity of the material formed by the chromium contribution to the main matrix of LiAl5O8 is higher, the persistence of the radiance during the irradiation is very low because the energy band of the main matrix is higher. The energy level of the conductivity band of the new matrix formed by the aluminum atoms entering the structure is higher than LiGa5O8, so the wavelength is as low as 711 nm. Thus, the LiAl5O8 matrix is subjected to a higher energy-dependent transition, resulting in more energy release. According to the measured results, it is observed that LiGa5O8 and LiAl5O8 crystal structures are formed. When aluminum is added stoichiometrically by subtracting the matrix from the gallium, the LiAl5O8 peaks begin to dominate. This change in the main matrix structure is observed to be a change in the emmission intensity and the decay time. The compositions of the LiGa5O8 and LiAl5O8 matrices are higher than the other compositions. The LiGa5O8 structure is the host matrix structure with the longest duration of emission. In the host matrix structures in which gallium and aluminum are combined in the structure, the luminous intensities and durations are lower. The reason for this is the impurities formed by atoms that do not enter the structure, just as with concentration quenching, excess activators in the molecular crystal structure reduce the intensity of emmision and decay times. In cases where the Ga-Al-O structure consists entirely of aluminum or gallium (LiGa5O8 and LiAl5O8), the emmission intensities are maximum. However, the luminous intensity of the matrix structures in which gallium and aluminum are formed together is at a lower level. As a result, Bi and Nd increased emission intensity 4 times, Pr, Yb and Eu increased emission intensity 2 times, and Ce reduced emission intensity. Also, the concentration quenching of the sample doped with Bi is minimum when compared to other samples.