Lanthanide-doped semiconductor nanocrystals: electronic structures and optical properties

Abstract

solid state lightings [11,12]. Nevertheless, due to the parity forbidden transition nature, the absorption cross-section of f-f transition is small and usually high power light sources such as laser are needed to excite the Ln 3+ ions. The ability to excite Ln 3+ ions efficiently in a broad spectral range is strongly desired for realizing their full potentials in signaling and lighting applications. To improve the excitation efficiency of Ln 3+ , sensitization is an efficient way to avoid the direct excitation of the Ln 3+ . Charge transfer, electronics transfer from ligand ground states (e.g., O 1s) to the Ln 3+ excited states, has proved to be an efficient means to achieve intense Ln 3+ emissions, due to its large band absorption cross-section. By employing this strategy, commercial phosphor Y 2 O 3 :Eu 3+ has been demonstrated to be an excellent red phosphor, which can be effectively excited at Eu-O charge transfer band at 255 nm. Another strategy for efficient sensitization of Ln 3+ is via the energy transfer (ET) from semiconductor nanocrystals (SNCs), which generally possess large absorption cross-section for Ln 3+ excited states. Moreover, it is known that the exciton Bohr radius of semiconductors is much larger than that of insulators [13], which could result in pronounced quantum confinement effect for small nanocrystals (NCs) (e.g., 2−10 nm for In 2 O 3 , ZnO and TiO 2 ). As a result, the optical properties of Ln 3+ incorporated in SNCs could be tailored via size control or bandgap engineering, which is very attractive in fabricating a nano-device for technological applications. It is anticipated that the luminescence of Ln 3+ ions can be efficiently sensitized via the energy transfer from the excited host to Ln 3+ , which thereby overcomes the inefficient direct absorptions of the parity forbidden 4f-4f transitions of Ln 3+ ions (Fig. 1). To realize the efficient energy transfer and intense Ln 3+ PL, the successful incorporation of Ln 3+ into the lattices of SNCs is of utmost importance, which still remains a great challenge via conventional wet-chemical methods especially for some widely used wide bandgap Trivalent lanthanide (Ln 3+ ) ions doped semiconductor nanomaterials have recently attracted considerable attention owing to their distinct optical properties and their important applications in diverse fields such as optoelectronic devices, flat plane displays and luminescent biolabels. This review provides a comprehensive survey of the latest advances in the synthesis, electronic structures and optical spectra of Ln 3+ ions in wide band-gap semiconductor nanocrystals (SNCs). In particular, we highlight the general wet-chemical strategies to introduce Ln 3+ ions into host lattices, the local environments as well as the sensitization mechanism of Ln 3+ in SNCs. The energy levels and crystal-field parameters of Ln 3+ in various SNCs determined from energy-level-fitting are summarized, which is of vital importance to understanding the optical properties of Ln 3+ ions in SNCs. Finally, some future pros...

White-light upconversion emission of lanthanide double-doped oxide nanoparticles via defect state luminescence of ZnO

“…A significant improvement in ET from the semiconductor nanocrystals (SNC) has led to an efficiently sensitized UCL emission. Meanwhile, the inefficient absorptions of the forbidden 4f-4f transitions of Ln ions have been overcome [27]. With the doped (Eu 3+ )Y 2 O 3 nanocrystals, it was reported that the UCL intensity of SNC was enhanced via the ET from the excited host to the Eu 3+ ions [28].…”

Graphene‐Oxide‐Modified Lanthanide Nanoprobes for Tumor‐Targeted Visible/NIR‐II Luminescence Imaging

“…[1] Thep ast decade has witnessed the tremendous development of luminescent nanoprobes based on trivalent lanthanide doped nanocrystals (Ln 3+ -NCs) due to the unique electronic structures and optical properties of Ln 3+ emitters. [2] Through the sequential absorption of two or more nearinfrared (NIR) photons,L n 3+ -NCs can emit visible upconversion luminescence (UCL) and down-shifting luminescence (DSL) in the second NIR window (NIR-II, 1000-1700 nm) to markedly eliminate autofluorescence signals and light scattering from biological molecules/tissues, [3] resulting in greatly improved light penetration depth and signal-to-noise ratio (SNR) in bioimaging. [4] Moreover,L n 3+ -NCs have tunable luminescence wavelengths and lifetimes by properly doping Ln 3+ or designing core-shell structures,w hich opens new avenues for multiplex imaging through color and lifetime encoding.…”

Graphene‐Oxide‐Modified Lanthanide Nanoprobes for Tumor‐Targeted Visible/NIR‐II Luminescence Imaging

“…[1] Thep ast decade has witnessed the tremendous development of luminescent nanoprobes based on trivalent lanthanide doped nanocrystals (Ln 3+ -NCs) due to the unique electronic structures and optical properties of Ln 3+ emitters. [2] Through the sequential absorption of two or more nearinfrared (NIR) photons,L n 3+ -NCs can emit visible upconversion luminescence (UCL) and down-shifting luminescence (DSL) in the second NIR window (NIR-II, 1000-1700 nm) to markedly eliminate autofluorescence signals and light scattering from biological molecules/tissues, [3] resulting in greatly improved light penetration depth and signal-to-noise ratio (SNR) in bioimaging. [4] Moreover,L n 3+ -NCs have tunable luminescence wavelengths and lifetimes by properly doping Ln 3+ or designing core-shell structures,w hich opens new avenues for multiplex imaging through color and lifetime encoding.…”