Er3+-Doped Anatase TiO2 Nanocrystals: Crystal-Field Levels, Excited-State Dynamics, Upconversion, and Defect Luminescence

✍ Scribed by Wenqin Luo; Chengyu Fu; Renfu Li; Yongsheng Liu; Haomiao Zhu; Xueyuan Chen

• Publisher: John Wiley and Sons
• Year: 2011
• Tongue: English
• Weight: 1017 KB
• Volume: 7
• Category: Article
• ISSN: 1613-6810
• DOI: 10.1002/smll.201100838

✦ Synopsis

Abstract

A comprehensive survey of electronic structure and optical properties of rare‐earth ions embedded in semiconductor nanocrystals (NCs) is of vital importance for their potential applications in areas as diverse as luminescent bioprobes, lighting, and displays. Er^3+^‐doped anatase TiO~2~ NCs, synthesized via a facile sol–gel solvothermal method, exhibit intense and well‐resolved intra‐4f emissions of Er^3+^. Crystal‐field (CF) spectra of Er^3+^ in TiO~2~ NCs are systematically studied by means of high‐resolution emission and excitation spectra at 10–300 K. The CF analysis of Er^3+^ assuming a site symmetry of C~2__v__~ yields a small root‐mean‐square deviation of 25.1 cm^−1^ and reveals the relatively large CF strength (549 cm^−1^) of Er^3+^, thus verifying the rationality of the C~2__v__~ symmetry assignment of Er^3+^ in anatase TiO~2~ NCs. Based on a simplified thermalization model for the temperature‐dependent photoluminescence (PL) dynamics from ^4^S~3/2~, the intrinsic radiative luminescence lifetimes of ^4^S~3/2~ and ^2^H~11/2~ are experimentally determined to be 3.70 and 1.73 μs, respectively. Green and red upconversion (UC) luminescence of Er^3+^ can be achieved upon laser excitation at 974.5 nm. The UC intensity of Er^3+^ in Yb/Er‐codoped NCs is found to be about five times higher than that of Er‐singly‐doped counterparts as a result of efficient Yb^3+^ sensitization and energy transfer upconversion (ETU) evidenced by its distinct UC luminescence dynamics. Furthermore, the origin of defect luminescence is revealed based on the temperature‐dependent PL spectra upon excitation above the TiO~2~ bandgap at 325 nm.