Preparation Of Rare Earth Doped Superfine Materials And Their Up-conversion Investigation

For thousands of years, people have been in persistent pursuit of some fascinatingluminescent natural ores. They call these ores "gems" and take pride in possessing andwearing them as ornaments to show off their wealth and social status. The high evaluation ofluminescent ores has helped to promote people's recognition, processing and exploration ofluminescent materials. With the establishment and the development of the theory ofluminescence, it has been possible to exploit luminescent materials with certain practicalvalues. Rare earth compounds, with their excellent properties, became one of the host spots inthe research fields of physics and material science in 1970s and 1980s. In recent years,nanonization has caused materials to form abundant new physic properties in size, structuredeformation and localization. This has led the investigation of the superfine and nanomaterials as well as the nano effect on luminescent properties and mechanisms to become newhot spots of luminescent physics, of which the investigation of rare earth doped superfine andnano materials constitutes important part. In this PHD dissertation, the up-conversion of rareearth doped nano compounds and the nano effect on spectral properties and luminescentmechanisms are investigated. Several preparation methods of rare earth doped superfineluminescent compounds which might be industrialized are explored. Some important progressis made.Er³⁺ doped Y₂O₃ nano crystal is prepared with co-precipitation method. The up-conversionluminescence properties of Er³⁺ doped Y₂O₃ nano crystal with excitation of 808 nm and 488nm are investigated, respectively. It is found that under 808 um excitation the up-convertedred emission intensity of ⁴F_9/2→⁴I_15/2 transition increases upon the excitation power morequickly than that of the green emission of ²H_11/2(⁴S_3/2)→⁴I_15/2 transition. Based on rateequation, the up-conversion mechanism is discussed. It is believed that no-resonant energytransfer of ⁴I_9/2+⁴I_11/2—⁴F_9/2+⁴I_13/2 play an important role in performing the redup-conversion emission, resulting that the red emission intensity increases upon the excitationpower more quickly than that of the green emission. Under 488 nm excitation, Er³⁺ dopedY₂O₃ nano crystal, Multiple up-converted emissions have been observed. The most intenseup-converted emission centered at 406 um, which corresponds to the ²p_3/2→⁴I_13/2 transition,is a two-photon process. Our investigation suggests that realization of violet laser operationby a four-level system might be a feasible approach by developing Er³⁺:Y₂O₃ nano-crystalline ceramics.Er³⁺ doped Y₂O₃ nano crystals with different size are prepared using combustion method.Size effect on luminescence properties of Er³⁺ doped Y₂O₃ nano crystals is investigated. Theemission spectra under 488 nm excitation indicate that the ratio of the intensity of ⁴S_3/2→⁴I_15/2transition to that of ²H_11/2→⁴I_15/2 decreases with the decrease of the particle size. It is believedthat the rate of hypersensitive transition of ²H_11/2→⁴I_15/2 increases with the decrease of theparticle size more quickly than that of transition of ⁴S_3/2→⁴I_15/2. The time dependence of the554 nm luminescence decay excited by 518 nm reveals that the rate of up-converting energytransfer occurring between two Er³⁺ ions in ⁴S_3/2 state increases with the decrease of theparticle size. The up-conversion luminescence of Er³⁺ doped Y₂O₃ nano crystals with differentsize pumped at 980 nm is investigated. As the particle size decreases, "saturation effect"becomes more serious.Transparent Er³⁺/Yb³⁺ co-doped oxyfluoride glass ceramics is prepared, which containsnano crystalline with size of 10-12 nm. The up-conversion emission in oxyfluoride glassceramics is notably stronger than that in its glass counterpart. The green luminescenceintensity of the ²H_11/2(⁴S_3/2)→⁴I_15/2 transition and the red luminescence intensity of ⁴F_9/2→⁴I_15/2 transition in the glass ceramics are 26 and 6 times stronger than those in its glasscounterpart, respectively. The ratio of the red emission intensity to the green one also changesobviously. It is believed that the low phonon energy in nano crystalline and the assemblingeffect result in the enhancement of the up-conversion emission and the change of theup-conversion properties.Er³⁺ doped and Er³⁺/Yb³⁺ co-doped superfine LaF₃ powders are prepared usingco-precipitation method. The sample shows sphere like particle with siz of about 250 nm. Thequantum efficiencies of ⁴S_3/2 and ⁴F_9/2 levels in Er³⁺ doped LaF₃ powders are 67.0% and71.9%, respectively. The introduction of Yb³⁺ largely improves the up-conversion intensity.With the enhancement of Yb³⁺ concentration the sensitizing properties change. On the otherhand, the Er³⁺-Er³⁺ interaction is reinforced as Yb³⁺ concentration increases. These lead to thevariation of the intensity distribution among the up-converted emission bands.Er³⁺/Yb³⁺ and Tm³⁺/Yb³⁺ co-doped superfine powders are prepared with a combustionsynthesis method followed with fluorization process for the first time. The fabricated particlesize is about 210 um. Under 980 nm excitation, five up-converted luminescence bandsattributed to seven transitions of Er³⁺ ion and five up-converted luminescence bands attributedto eight transitions of Tm³⁺ ion are observed. The Tm³⁺/Yb³⁺ co-doped superfine powderspresent strong blue emission. On the basis of rate equation discussion, it is believed that directcooperation sensitization mechanism from Yb³⁺ to Tm³⁺ is a main way performing the up-converted emission.