Enhanced Luminescence of Eu-Doped TiO2Nanodots
© to the authors 2009
Received: 19 February 2009
Accepted: 6 April 2009
Published: 25 April 2009
Monodisperse and spherical Eu-doped TiO2nanodots were prepared on substrate by phase-separation-induced self-assembly. The average diameters of the nanodots can be 50 and 70 nm by changing the preparation condition. The calcined nanodots consist of an amorphous TiO2matrix with Eu3+ions highly dispersed in it. The Eu-doped TiO2nanodots exhibit intense luminescence due to effective energy transfer from amorphous TiO2matrix to Eu3+ions. The luminescence intensity is about 12.5 times of that of Eu-doped TiO2film and the luminescence lifetime can be as long as 960 μs.
It is well-known that rare earth (RE) ions can exhibit rich spectral properties [1–4]. The direct excitation of the RE ions is inefficient because of parity-forbidden f–f transitions. Therefore, host materials are required to excite the RE ions efficiently in a wide spectral range for realizing their full potential in optoelectronic devices and flat panel displays [5, 6]. For these applications, inorganic oxide materials exhibit superior advantages in terms of their good chemical, thermal, and mechanical properties [7–9]. For example, Y2O3:Eu is one red emitting phosphor compound commonly used [7, 10–12]. However, high costs prevent its further developments. As one of the recently developed alternative oxide host materials, titanium oxide (TiO2) is demonstrated to be a good sensitizer to absorb light and transfer energy to Eu3+ ions [4, 8, 13–16]. It also has advantages in practical applications because of its low cost, chemical and thermal stability, and good mechanical properties . However, the Eu–Eu interaction in TiO2 matrix may greatly decrease the luminescence intensity. It has been demonstrated that TiO2 amorphous region is an ideal framework for Eu3+ ions by significantly decreasing the non-desired Eu–Eu interaction [8, 14, 16]. Moreover, monodisperse spherical and small phosphor particles prepared on a substrate are greatly demanded not only for improvement of luminescence performance and screen resolution, but also for technological applications, such as light emitting devices and flat panel displays.
In our previous work, we have developed a novel method, i.e., phase-separation-induced self-assembly, to synthesize monodisperse polycrystalline TiO2nanodots on substrate (unpublished). The TiO2nanodot was found to be composed of many small nanocrystallites embedded in amorphous surrounding, which could be an ideal host matrix for Eu3+ions. In the present study, monodisperse and spherical Eu-doped TiO2nanodots were successfully prepared on substrate via the facile approach. The size of the Eu-doped TiO2nanodots can be controlled by varying the preparation condition. The Eu-doped TiO2nanodots exhibited intense sharp luminescence under ultraviolet excitation. The luminescence intensity could be 12.5 times as strong as Eu-doped TiO2film with the luminescence lifetime to be 960 μs.
Preparation conditions and morphology features of different samples
Density (×1010 cm−2)
Results and Discussion
The formation mechanism of the Eu-doped TiO2 nanodots by the phase-separation-induced self-assembly is based on Marangoni effect [18–20]. After the precursor sol is spin-coated on the substrate, the ethanol in the liquid film is evaporated but the evaporation rate is gradient in thickness direction. The Marangoni effect can lead to convective flows in the liquid film with a large temperature gradient during the spin-coating process. The requirement of minimizing the extra surface free energy induces the formations of the TBOT/Eu(TMHD)3 droplets and the PVP phase. After hydrolysis, the TBOT/Eu(TMHD)3 droplets become gel nanodots, which form Eu-doped TiO2 nanodots after calcination.
In this work, monodisperse Eu-doped TiO2nanodots with spherical shape were successfully synthesized on substrate by utilizing the phase-separation-induced self-assembly during the spin-coating process. The size of the Eu-doped TiO2nanodots can be controlled by changing the preparation condition. The average diameter of nanodots reduces from 70 to 50 nm if the TBOT concentration in the precursor sol decreases from 0.1 to 0.08 M. After calcining at 500 °C, the Eu-doped TiO2nanodots remain amorphous with the Eu3+ions well-dispersed in the amorphous TiO2matrix. The amorphous TiO2framework acts as an effective sensitizer to absorb light and transfer energy to Eu3+ions, resulting in strong luminescence from Eu3+ions. The PL intensity of Eu-doped TiO2nanodots (nanodot-1) can be 12.5 times as strong as film, and the PL lifetime is determined to be as long as 960 μs. It is believed that the good luminescence properties endow the Eu-doped TiO2nanodots with potentials in many fields, such as light emitting devices, flat panel displays, etc.
This work was supported by the Nature Science Foundation of China (Grant no. 50572093, 30870627).
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