Sensitizing Up-conversion Luminescence And Mechanism Of Er³⁺ Under Multi-band Excitations

Up-conversion is a special luminescence phenomenon that can convert the long wavelength emission into short wavelength emission and it has widely potential applications in laser, multicolor display, anti-counterfeit, bio-probes, solar cell and so on. Among which, the application of up-conversion in anti-counterfeit techniques have some advantages, for example, strong concealment, long availability, low cost and convenient for encryption. However, some disadvantages are also existent in up-converted anti-counterfeit techniques, such as not enough emission colors, singly excitation wavelength and so on. Based on these, some work are investigated and presented as follows:In up-converted anti-counterfeit techniques application, the excitation wavelength at 980nm was applied widely, due to the special sensitizing property of Yb³⁺ ions. Although the strong up-converted red and green could be obtained under the 980nm excitation, the enhancements of blue and violet emission were rather hard. On the other hand, it was single to only adopt the 980nm excitation and to explore the new excitation bands was needed, which can enhance the anti-counterfeiting function and improve the degree of security. The semiconductor laser technique in excitation wavelength of 812nm reached maturity and the energy level of Er³⁺ can match this excitation wavelength well. Hence, it is significant to carry out the up-converted strong emissions of Er³⁺ under the 812nm excitation for broadening the up-converted anti-counterfeit techniques.A novel stoichiometric material named Ba₂ErF₇ was synthesized by co-precipitation method. Under the 812nm excitation, the sample presented the bright red and green up-conversion emission bands of Er³⁺. In order to compare with luminescence property of Er³⁺, the property of sample under the 980nm excitation was also studied. It was found that abundant up-conversion emission bands of Er³⁺ from ultraviolet to visible region were observed. Moreover, based on the special sensitizing property of Yb³⁺ ions, the effects of Yb³⁺ on the luminescence property of Er³⁺ in Ba₂ErF₇ sample under the two excitations were investigated. The Yb³⁺ ions played a role of quenching ion under the 812nm excitation, resulting in the decreases of green and red emission intensities. However, with the introduction of Yb³⁺ ions, the blue and violet emission intensities were efficiently enhanced under the 980nm excitation. When the doping concentration exceeded 5mol%, all the emission intensities of Er³⁺ increased, due to the sensitization from Yb³⁺ to Er³⁺.In order to further enhance the up-conversion emission intensity under 812nm excitation, based on the strong absorption of Nd³⁺ in the region of 812nm, the effects of Nd³⁺ on the up-conversion property of Er³⁺ and mechanism were investigated in Er³⁺/Nd³⁺ co-doped and Er³⁺/Nd³⁺/Yb³⁺ triple doped Ba2YF7, which adopted the directed and indirectly sensitizing energy transfer of Nd³⁺. It was found that in codoped system, the instruction of Nd³⁺ make a part of blue emissions quenched and the green emission decreased while the red one increased in a certain concentration. However, in triple doped system, due to the instruction of Yb³⁺, all the emission intensities of Er³⁺ were stronger than that in singly doped and co-doped systems and an ultraviolet emission centered at 380nm was observed. Moreover, with increasing the doping concentration of Nd³⁺, all the emission intensities of Er³⁺ increased firstly and then decreased and the optimal doping concentration obtained at 2.5mol%. It was indicated that the Yb³⁺ played an important role of bridging ion on the energy transfer from Nd³⁺ to Er³⁺, resulting in the efficient emission of Er³⁺.Recently, the significant development of visible semiconductor laser technique provided the new pumping source for up-converted anti-counterfeit techniques. Based on these, we also studied the up-conversion property of Er³⁺ singly doped NaYF4 material under red excitation source of 671nm. Under the 671nm excitation, the sample presented the ultraviolet (382nm), violet (400nm and 408nm), blue (468nm and 492nm) and green (525/540nm) of Er³⁺. While the doping concentration of Er³⁺ increased from 1mol% to 3mol%, all the emission intensities of Er³⁺ increased. When the doping concentration of Er³⁺ exceeds 3mol%, the emissions centered at 400nm, 468nm and 540nm became to decrease. Based on the wide absorption of Tm3+ in the region of 671nm, the energy transfer between Tm3+ and Er³⁺ was investigated. It was found that, the instruction of Tm3+ could enhance the emission intensity of Er³⁺ in a certain concentration and the optimal doping concentration obtained at 0.7mol%.