Enhancement Mechanism Of Photoluminescence Of Rare Earth Ions Doped Al₂O₃ Prepared By Non-Aqueous Sol-Gel Method

The Er³⁺-doped, Er³⁺-Y³⁺ and Er³⁺-Yb³⁺ codoped Al₂O₃ have been prepared by usingnon-aqueous sol-gel method instead of aqueous sol-gel method. The rare earth ions-dopedAl₂O₃ with photoluminescence (PL) properties were obtained in the near infrared regioncentered at 1.53μm. The effect of doping concentrations and sintering temperatures used inthe aqueous and non-aqueous sol-gel methods on phase structures and PL properties of therare earth ions-doped Al₂O₃ were systematically investigated by using thermogravimetry/differential thermal analysis (TG/DTA), Fourier transform infrared spectroscopy (FTIR),x-ray diffraction (XRD), PL measurment to explore the influence of aqueous and non-aqueoussol-gel method on PL properties and the enhancement mechanism for PL properties ofEr³⁺-doped Al₂O₃ induced by Y³⁺ and Yb³⁺ codoping, respectively.The Er³⁺-doped, Er³⁺-Y³⁺ and Er³⁺-Yb³⁺ codoped Al₂O₃ powders sintered at differenttemperatures were prepared by using the aluminum isopropoxide [Al(OC₃H₇)₃] as precursor,acetylacetone (AcAcH) as chelating agent, nitric acid (HNO₃) as catalyzer, and hydratednitrate of Er(NO₃)₃·5H₂O, Y(NO₃)₃·5H₂O, Yb(NO₃)₃·5H₂O as dopant under non-aqueousisopropanol environment. The phase contents diagram for the Er-doped Al-O system with thedoping concentration up to 5 molï¼…was described at the sintering temperature from 550℃to1250℃. There were the three crystalline types of Er³⁺-doped Al₂O₃ phases,γ,θandα-(Al,Er)₂O₃, and the two relative stoichiometric compounds composed of Al, Er, and O,ErAlO₃ and Al₁₀Er₆O₂₄ phases, in the Er-Al-O phase contents diagram. The Er³⁺ dopingsuppressed crystallization of theγandθphases and delayed phase transitions of theγ→θandθ→α. Compared with the undoped Al₂O₃ powders, the transition temperatures ofγ→θandθ→αwere increased about 60℃and 110℃for 5 molï¼…Er³⁺-doped Al₂O₃ powders. TheErAlO₃ and Al₁₀Er₆O₂₄ phases were obtained for Er³⁺-doped Al₂O₃ powders sintered over1150℃. The suppressed crystallization and delayedγ→θ→αphase transitions by the Er³⁺doping were more evident for the Er³⁺-doped Al₂O₃ prepared by non-aqueous method relativeto that by aqueous sol-gel method. The 0.1, 0.5, 1 molï¼…Er³⁺ with the molar ratio of 0-20between Y³⁺ and Er³⁺, and Yb³⁺ and Er³⁺ codoped Al₂O₃ powders sintered at 1000℃wereprepared by non-aqueous method. The Y³⁺, Yb³⁺ codoping further suppressed thecrystallization and enhanced the thermal stability of the Er³⁺-doped Al₂O₃ phases. With Y³⁺and Yb³⁺ content over 10 molï¼…, the (Y, Er)AlO₃ and (Yb,Er)₃Al₅O₁₂ phases precipitated for 1 molï¼…Er³⁺-doped Al₂O₃ powders, respectively.The PL properties centered at 1.535μm were obtained for the Er³⁺-doped Al₂O₃ in thebroadband extending from 1.400μm to 1.700μm. The full widths at half maximum (FWHM)from 46 nm to 85 nm were observed with a main peak at 1.535μm and a side peak at 1.556μm for the Er³⁺-doped Al₂O₃ with theγphase and the mixture ofγandθphases. With theappearance ofα-(Al,Er)₂O₃, ErAlO₃ and Al₁₀Er₆O₂₄ phases in the matrix ofγandθphases,the PL spectra were splitting and the FWHM became narrow. With increasing the sinteringtemperature from 600℃to 1100℃, the PL lifetime and intensity centered at 1.535μmincreased obviously due to the decrease in -OH content. Increasing the Er³⁺ dopingconcentration from 0.1 molï¼…to 2 molï¼…, the PL lifetime and intensity decreased due to theconcentration quenching effect of Er³⁺. The PL quenching was mainly attributed to energytransfer from Er³⁺ to -OH, and the up-conversion had only limited effect. The hydrolysiseffect of Er³⁺ was suppressed under non-aqueous sol environment, improving the PLproperties due to the increase of Er³⁺ dispersibility in the Er³⁺-doped Al₂O₃. Compared withaqueous sol-gel method, the PL intensity with broader FWHM was enhanced about an orderof magnitude for the 1 molï¼…Er³⁺-doped Al₂O₃ prepared by non-aqueous sol-gel method, andthe longer PL lifetime was detected.The PL properties centered at 1.535μm were obtained for the Er³⁺-Y³⁺ codoped Al₂O₃.The characteristic PL spectra for Er³⁺-Y³⁺ codoped Al₂O₃ with the mixture ofγandθphaseswere similar to that of Er³⁺-doped Al₂O₃. The PL spectrum was splitting with the large amountof (Y, Er)AlO₃ phase precipitating. With increasing the molar ratio between Y³⁺ and Er³⁺ from0 to 20, the PL lifetime and intensity increased for 0.1-1 molï¼…Er³⁺-doped Al₂O₃. Comparedwith the Er³⁺-doped Al₂O₃, the PL lifetime of 1 molï¼…Er³⁺-10 molï¼…Y³⁺ codoped Al₂O₃ wasincreased from 3.22 ms to 3.96 ms, and PL intensity was enhanced about an order ofmagnitude. The Y³⁺ played a slight dispersed role for the 0.1 molï¼…Er³⁺-doped Al₂O₃, andconsequently the PL lifetime and intensity increased slightly with the increase of the Y³⁺molar ratio. The dispersed role of Y³⁺ was more apparent for the 1 molï¼…Er³⁺-doped Al₂O₃,and the PL lifetime and intensity increased obviously with the increase of the Y³⁺ molar ratio.The cluster of Er³⁺ could be dispersed by the substitution of Y³⁺ for Er³⁺, which decreased thecross-relaxation rate among Er³⁺ and the multi-phonon relaxation rate between Er³⁺ and -OH.The PL properties centered at 1.535μm were also obtained for the Er³⁺-Yb³⁺ codopedAl₂O₃. The characteristic PL spectra for Er³⁺-Yb³⁺ codoped Al₂O₃ with the mixture ofγandθphases were similar to that of Er³⁺-doped Al₂O₃. The PL spectrum was splitting with the largeamount of (Yb,Er)₃Al₅O₁₂ phase precipitating. With increasing the molar ratio between Yb³⁺and Er³⁺ from 0 to 20, the PL lifetime increased for the 0.1-1 molï¼…Er³⁺-dopoed Al₂O₃correspondently. The PL intensity reached the maximum with 1:10 between Er³⁺ and Yb³⁺,then decreased with further increasing the Yb³⁺ molar ratio, while which were still larger thanthat of the Er³⁺-doped Al₂O₃. Compared with the Er³⁺-doped Al₂O₃, the PL lifetime of 1 molï¼… Er³⁺ 10 molï¼…Yb³⁺ codoped Al₂O₃ was increased from 3.22 ms to 4.17ms, and PL intensitywas enhanced about 20 times. Enhancement of PL intensity for Er³⁺-Yb³⁺ codoped Al₂O₃ wasattributed to sensitive effect between Yb³⁺ and Er³⁺ and dispersed effect of Yb³⁺ that is similarto Y³⁺. The sensitive effect of Yb³⁺ had the crucial influence on PL intensity for the 0.1 molï¼…Er³⁺-doped Al₂O₃. The more remarkable dispersed effect of Yb³⁺ on PL intensity wasobserved for the 1 molï¼…Er³⁺-doped Al₂O₃. With the molar ratio over 1:10 between Er³⁺ andYb³⁺, the decreased PL intensity was induced by the energy back transfer.The 0.1-1 molï¼…Er³⁺ with the molar ratio of 10 between Yb³⁺ and Er³⁺codoped Al₂O₃films have been prepared on the thermally oxidized SiO₂/Si(100) substrates in the dip-coatingprocess. The smooth and continuous Er³⁺-Yb³⁺ codoped Al₂O₃ films about 2μm thick wereobtained for 20 coating cycles sintered at 1000℃. The films with column grain wereobserved, which was different from the films of similar thickness with anisotropy grainprepared by aqueous sol-gel method. The doping Al₂O₃ film with column grain had a moreapparent (110) preferred orientation than that of the anisotropy grain. The PL intensity ofEr³⁺-Yb³⁺ codoped Al₂O₃ films prepared by non-aqueous sol-gel method was stronger about 2orders of magnitude than that by aqueous sol-gel method, and the correspondent FWHM was54-57 nm.