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Publication Title | Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals Feng Wang and Xiaogang Liu

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imaging. As high quality UC nanocrystals can now be routinely prepared, the study of these UC processes has evolved into a highly interdisciplinary field that has rapidly expanded at the frontiers of photochemistry, biophysics, solid state physics, and materials science. In particular, UC emission from lanthanide (Ln)-doped nanocrystals offers an attractive optical labeling technique in biological studies without many of the constraints associated with organic fluorophores and quantum dots.2,3 The UC technique utilizes near infrared (NIR) excitation rather than ultraviolet (UV) excitation, thereby significantly minimizing background autofluorescence, photobleaching, and photodamage to biological specimens. The technique also allows in vivo observation with substantially high spatial resolution and offers remarkable sample penetration depths that are much higher than those obtained by UV excitation.4 In addition, UC processes can be induced by a low power (1–103 W cm 2) continuous wave laser, as opposed to a costly high-intensity (106–109 W cm 2) pulse laser source for the generation of a simultaneous two-photon process.

UC processes are mainly divided into three broad classes: excited state absorption (ESA), energy transfer upconversion (ETU), and photon avalanche (PA). All these processes involve the sequential absorption of two or more photons (Fig. 1). Thus, UC processes are different from the multi- photon process where the absorption of photons occurs simultaneously.

In the case of ESA, excitation takes the form of successive absorption of pump photons by a single ion. The general energy diagram of the ESA process is shown in Fig. 1a for a simple three-level system. If excitation energy is resonant with the transition from ground level G to excited metastable level E1, phonon absorption occurs and populates E1 from G in a process known as ground state absorption (GSA). A second pump photon that promotes the ion from E1 to higher-lying state E2 results in UC emission, corresponding to the E2 - G optical transition.

ETU is similar to ESA in that both processes utilize sequential absorption of two photons to populate the metastable level. The essential difference between ETU and ESA is that the excitation in ETU is realized through energy transfer between two neighboring ions. In an ETU process,

Fig. 1 Principal UC processes for lanthanide-doped crystals: (a) excited state absorption, (b) energy transfer upconversion, (c) photon avalanche. The dashed/dotted, dashed, and full arrows represent photon excitation, energy transfer, and emission processes, respectively.

each of two neighboring ions can absorb a pump phonon of the same energy, thereby populating the metastable level E1 (Fig. 1b). A non-radiative energy transfer process promotes one of the ions to upper emitting state E2 while the other ion relaxes back to ground state G. The dopant concentration that determines the average distance between the neighboring dopant ions has a strong influence on the UC efficiency of an ETU process.

The phenomenon of PA was first discovered by Chivian and co-workers5 in Pr3+-based infrared quantum counters. PA-induced UC features an unusual pump mechanism that requires a pump intensity above a certain threshold value. The PA process starts with population of level E1 by non-resonant weak GSA, followed by resonant ESA to populate upper visible-emitting level E2 (Fig. 1c). After the metastable level population is established, cross-relaxation energy transfer (or ion pair relaxation) occurs between the excited ion and a neighboring ground state ion, resulting in both ions occupying the intermediate level E1. The two ions readily populate level E2 to further initiate cross-relaxation and exponentially increase level E2 population by ESA, producing strong UC emission as an avalanche process.

The UC luminescent efficiency in these three processes varies considerably. ESA is the least efficient UC process. Efficient UC is possible in PA with metastable, intermediate levels that can act as a storage reservoir for pump energy. However, the PA process suffers from a number of drawbacks, including pump power dependence and slow response to excitation (up to several seconds) due to numerous looping cycles of ESA and cross-relaxation processes. In contrast, ETU is instant and pump power independent, and thus has been widely used to offer highly efficient UC (B two orders of magnitude higher than ESA)1 over the past decade.

Nanoscale manipulation of lanthanide-doped UC nano- crystals leads to important modification of their optical properties in excited-state dynamics, emission profiles and UC efficiency. For example, the reduction in particle size provides the ability to modify the lifetime of intermediate states.6 The control of spatial confinement of dopant ions within a nanoscopic region can lead to marked enhancement of a particular wavelength emission as well as generation of new types of emissions.

This tutorial review focuses primarily on advances devel- oped within the past five years in the rational design and synthesis of lanthanide-doped UC nanocrystals. In section 2, an effort has been made to present an overview of the dopant/ host design principle for efficient UC in nanocrystals. In section 3, we discuss a variety of synthetic approaches that offer control over particle size, shape, dispersion, and emission properties. We also provide examples of various systems. In section 4, we attempt to provide general strategies for surface modification of UC nanocrystals, leading to high resistance to quenching induced by solvents or surface defects such as absorbed contaminants. The surface modification also imparts improved particle dispersibility in the solvent and provides active surfaces for further biological functionalization. In section 5, we provide a general picture of chemical tuning of emission colors, as it emerges from a consideration of the interplay of different parameters that control the emission

This journal is c The Royal Society of Chemistry 2009

Chem. Soc. Rev., 2009, 38, 976–989 | 977

Published on 12 February 2009. Downloaded by National University of Singapore on 11/09/2014 11:52:48.

Image | Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals Feng Wang and Xiaogang Liu

article online quality nanocrystals routinely prepared processes evolved highly interdisciplinary
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