annual review of physical chemistry volume 66 issue 1 2015 [doi...

PC66CH27-Yan ARI 20 January 2015 15:28

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Upconversion of Rare EarthNanomaterialsLing-Dong Sun, Hao Dong, Pei-Zhi Zhang,and Chun-Hua YanBeijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare EarthMaterials Chemistry and Applications, PKU-HKU Joint Laboratory in Rare Earth Materialsand Bioinorganic Chemistry, College of Chemistry and Molecular Engineering, PekingUniversity, Beijing 100871, China; email: [email protected], [email protected]

Annu. Rev. Phys. Chem. 2015. 66:61942

The Annual Review of Physical Chemistry is online atphyschem.annualreviews.org

This articles doi:10.1146/annurev-physchem-040214-121344

Copyright c 2015 by Annual Reviews.All rights reserved

Keywords

anti-Stokes emission, 4f-4f transitions, nanomaterial, energy transfer,optical application

Abstract

Rare earth nanomaterials, which feature long-lived intermediate energy lev-els and intracongurational 4f-4f transitions, are promising supporters forphoton upconversion. Owing to their unique optical properties, rare earthupconversion nanomaterials have found applications in bioimaging, thera-nostics, photovoltaic devices, and photochemical reactions. Here, we reviewrecent advances in the photon upconversion processes of these nanomate-rials. We start by considering energy transfer models involved in the studyof upconversion emissions, as well as well-established synthesis strategies tocontrol the size and shapeof rare earth upconversionnanomaterials. Progressin engineering energy transfer pathways, which play a dominant role in de-termining upconversion emission outputs, is then discussed. Lastly, repre-sentative optical applications of these materials are considered. The aim ofthis review is to provide inspiration for researchers to explore novel upcon-version nanomaterials and extended optical applications.

619

Review in Advance first posted online on January 30, 2015. (Changes may still occur before final publication online and in print.)

Changes may still occur before final publication online and in print

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RE: rare earth

UC: upconversion

1. INTRODUCTION

Rare earth (RE) elements, including the lanthanide family (from lanthanum to lutetium), as wellas scandium and yttrium, are important in functional materials. Because of their similar electroncongurations ([Xe]4f n15d016s2), trivalentRE ions have similar physical and chemical properties(13). Owing to the large quantum number (n = 4, l = 3), the energy levels from the 4f electronconguration are abundant, thus allowing for many intracongurational transitions (Figure 1).However, the 4f-4f intracongurational transition is parity forbidden for free RE ions. As the REions are embedded in an inorganic lattice, the parity-forbidden rule may be partially broken dueto the mix of certain odd-parity congurations. Hence, the originally forbidden transitions arepartially allowed.

Because of their abundant energy levels and intracongurational transitions, RE ions are con-sidered promising luminescent centers (47). Moreover, the intrinsic spectroscopic character ofRE ions causes them to be less affected by their microsurroundings because of shielding from the5s25p6 subshells (5). Moreover, excellent photostability, a large anti-Stokes shift, long lumines-cence lifetime, and sharp-band emission result from the unique 4f energy levels. Three typicalenergy transfer modes have been established in RE ionactivated materials in understanding theemission behavior, namely downshifting, quantum cutting, and upconversion (UC).

The term photon UC refers to nonlinear optical processes in which the continuous absorp-tion of two or more low-energy photons leads to the emission of high-energy ones (anti-Stokesemission). In 1959, Bloembergen (8) began UC investigations with a device termed an infraredquantum counter. Since then, numerous efforts have contributed to enriching the family of UCmaterials. In contrast to simultaneous two-photon absorption and second harmonic generation,the UC process is realized via long-lived intermediate energy levels. Furthermore, a continuous-wave laser with relative low-power density (1103 W/cm) can trigger efcient UC emissions (9).

5F5

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Figure 1Energy-level diagrams of rare earth ions. Typical upconversion emissive excited states are highlighted by redbold lines.

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UCN: upconversionnanomaterial

ETU: energy transferupconversion

In recent decades, RE-based UC materials have attracted considerable attention owing totheir unique optical properties. RE-based bulk materials have been successfully employed as dis-play devices and compact solid state lasers (4, 1012). With the development of nanoscienceand nanotechnology, RE-based UC materials can now be designed on the nanoscale for morepromising prospects. Unlike semiconductor quantum dots, whose luminescence exhibits high re-liability with particle size, the emission bands of RE-based upconversion nanomaterials (UCNs)are less affected. Moreover, UCNs also exhibit high resistance to photobleaching and photoblink-ing, micro-/millisecond lifetimes, and large anti-Stokes shifts (1318). Owing to their specicnear-infrared (NIR) excitation, UCNs have excellent penetration depth in biosystems, and thereis no autouorescence from backgrounds. Because of these advantages, RE-based UCNs havefound use in a wide range of applications, from bioimaging (1921) and theranostics (2224) tophotovoltaic devices (25) and photochemical reactions (26).

Despite these features, several aspects of UCNs are of great concern to researchers. As isknown, the intrinsic energy transfers of RE ions play a dominant role in determining UC emis-sion efciency and color outputs. Energy transfer pathways must be manipulated to generate thedesired UC emissions for applications. Additionally, another challenge is the need to explore newapplication elds to efciently utilize UC emissions.

Based on these considerations, we review recent investigations in the eld of UCNs. In Sec-tion 2, we introduce the basic energy transfer mechanisms in RE-related UC emissions. Next,Section 3 presents synthesis strategies to obtain high-quality UCNs. Subsequently, we focusin Section 4 on typical approaches used for energy transfer modulations, through which UCemissions as well as excitations are selected. Section 5 discusses issues involved in typical opticalapplications.

2. ENERGY TRANSFER MECHANISM

Figure 1 depicts the energy levels of RE ions and pathways to realize UC emissions. In general,ve energy transfermechanisms are involved in the photonUCprocess (Figure 2), namely excitedstate absorption (ESA), energy transfer upconversion (ETU), photon avalanche (PA), cooperativeenergy transfer (CET) upconversion, and energy migration-mediated upconversion (EMU).

In an ESA process (Figure 2a), RE ions with multiple energy levels can undergo the successiveabsorption of two or more low-energy photons, resulting in the transition from the ground toexcited state, and further to a higher excited state. High-energy photons could be released withinsuch transitions. Although the ESA process is simple and straightforward, the requirement isrigorous. The absorption cross section of the excited ions should be adequate to absorb the secondpump photon. However, its capability of absorbing the second photon is generally rather low.

The ETU process (Figure 2b) includes two types of luminescent centers, a sensitizer and anactivator. The absorption cross section of the sensitizer is usually larger than that of the activator.Upon excitation with the pump photons, the excited sensitizer transfers energy to adjacent acti-vators resonantly. UC emission is generated from the activator when electrons drop back to theground state. In the ETU process, energy-level matching and a close spatial distance are required.RE ions with abundant energy levels provide a great advantage for ETU processes.

In 1979, Chivian and colleagues (27) discovered the PA process (Figure 2c) using Pr3+

ionbased infrared quantum counters. The energy gap between the intermediate state and theground state is in a mismatch with the energy of the pump photon. Once electrons are excitedto the intermediate state, an ESA process is likely to occur to populate the higher excited state.Subsequently, resonant cross relaxation takes place between the superexcited ion and adjacentground state ion, yielding two ions in the intermediate state. Repeating the cross-relaxation

www.annualreviews.org Upconversion of Rare Earth Nanomaterials 621


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Figure 2(ae) Basic energy transfer mechanisms of rare earthrelated upconversion (UC) emissions: (a) excited state absorption (ESA),(b) energy transfer upconversion (ETU), (c) photon avalanche (PA), (d ) cooperative energy transfer (CET) upconversion, and (e) energymigration-mediated upconversion (EMU). The gray dashed line in panel e represents the core/shell interface. ( f ) Energy-leveldiagrams and proposed UC energy transfer pathways in the Yb3+-Er3+, Yb3+-Ho3+, and Yb3+-Tm3+ pairs.

process, exponential population of the intermediate state is sure to occur, along with excitationabove the threshold. In this case, PA-induced UC emissions are readily produced as long as theconsumption of superexcited ions is less than that of ground state ions.

Similar to ETU, two types of luminescent centers are required in the CET process(Figure 2d ): a cooperative sensitizer and activator (28). Themain difference between the two pro-cesses is the absence of adequate long-lived intermediate energy levels in the activators in CET. InCET,UCemission results from simultaneous energy transfer from two sensitizers to one activator.Hence, its UC emission efciency is approximately three orders of magnitude lower than that ofETU (4). Sometimes cooperative UC emissions can be observed from cooperative dimers (29, 30).

In 2011, Liu and coworkers (31) proposed the EMU mechanism, based on energy transferwithin core/shell nanostructures. An EMU process (Figure 2e) incorporates four types ofluminescent centers with dened concentrations into separated layers: a sensitizer, accumulator,migrator, and activator. Upon excitation with low-energy photons, an ETU process occurs,

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populating the higher excited state of the accumulator. Then the energy is donated to an adjacentmigrator in the same region, followed by further energy transfer through the core/shell interfaceto a migrator in the neighboring region. Finally, the energy is trapped by an activator, giving outUC emissions as electrons drop back to the ground state. Meanwhile, UC emissions from theaccumulator ions can also occur.

As mentioned above, the UC efciency varies with different mechanisms. Because of highemission efciency, ETU-motivatedUCNs overwhelmingly dominate correlative studies. Amongthe RE ions, the Yb3+ ion is the best choice for the sensitizer. The absorption cross section of theYb3+ ion is 9.11 1021 cm2 (976 nm; 2F7/2 2F5/2), larger than most of the RE ions. Moreimportantly, the energy-level diagram of the Yb3+ ion is quite simple, with only one excited stateof 2F5/2, which matches well with those of many RE ions. As for the activators, there are manychoices. Owing to their ladder-like arrangement of energy levels and excellent level matching withthe Yb3+ ion, the Er3+, Tm3+, and Ho3+ ions are ideal activators for the ETU process (9, 32).The doping concentration of the Yb3+ ion is usually kept at 20% or higher, whereas that of theactivators is lower than 2%.

Figure 2f shows the energy-level diagrams and proposed energy transfer pathways in theYb3+-Er3+, Yb3+-Ho3+, and Yb3+-Tm3+ pairs. The number of photons involved in UC processesis obtained from log-log diagrams of the UC emission intensity versus excitation power density,so-called I-P curves (33, 34). We have comprehensively investigated the UC properties inYb3+-Er3+-codoped NaYF4 UCNs (35). Upon 980-nm excitation, Yb3+ ions absorb the pumpphoton and undergo the 2F7/2 2F5/2 transition. Subsequently, the excited Yb3+ ions donateas-absorbed energy to adjacent Er3+ ions resonantly, promoting Er3+ ions to generate the 4I15/2 4I11/2, 4I11/2 4F7/2, 4I13/2 4F9/2, and 4F9/2 2H9/2 upward transitions. After electrons havepopulated these excited states, nonradiative relaxations to the 2H11/2, 4S3/2, and 4F9/2 states occur,further yielding the 2H11/2 4I15/2 (525 nm; green), 4S3/2 4I15/2 (545 nm; green), 4F9/2 4I15/2 (655 nm; red), and 2H9/2 4I15/2 (415 nm; violet) emissions. From the monitored I-Pcurves, the green and red emissions are assigned to two-photon UC processes, and the violetemission belongs to the three-photon transitions, consistent with the proposed mechanisms.Similarly, two-photon green and red emissions can also be generated in Yb3+-Ho3+-codopedUCNs via the 5F4, 5S2 5I8 (545 nm; green) and 5F5 5I8 (650 nm; red) transitions. In addition,weak blue emission centered at 485 nm could also be observed via the 5F3 5I8 transition (36).

In contrast to the two former cases, Yb3+-Tm3+-codoped UCNs may involve more thantwo-photon UC processes (37). This may be attributed to the discretely arranged energy levels ofTm3+ ions, which reduce the possibility of nonradiative relaxations. As shown in Figure 2f, whenelectrons of Yb3+ ions relax to the ground state (2F5/2 2F7/2), the energy migrates to nearbyTm3+ ions to conduct the 3H6 3H5, 3F4 3F2, 3H4 1G4, 1G4 1D2, and 1D2 3P2upward transitions. After population on these excited states and several nonradiative relaxations(3F2 3F3, 3F2 3H4, 3P2 1I6), the following UC emissions are generated: two-photon3F3 3H6 (695 nm; red) and 3H4 3H6 (800 nm; NIR), three-photon 1G4 3F4 (645 nm;red) and 1G4 3H6 (475 nm; blue), four-photon 1D2 3F4 (450 nm; blue) and 1D2 3H6(365 nm; UV), and ve-photon 1I6 3F4 (345 nm; UV) and 1I6 3H6 (290 nm; UV).

3. SYNTHESIS STRATEGY

The ideal host matrix should possess high chemical stability and low phonon energy. Amongvarious RE compounds, RE uorides, such as REF3, REOF, and MREFn (M = Li, Na, K,or Ba; n = 4 or 5), are commonly considered as ideal host materials (9, 32). Additionally, REuorides, with low phonon energies, are especially desirable for UC studies and applications.



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The synthesis strategies of RE uorides have been well established. Thermal decomposition andhydro(solvo)thermal synthesis methods are efcient in controlling the uniformity of hydrophobicUCNs. These routes allow products to be precisely tuned in terms of size, shape, and compositionwithin the nanoscale.

3.1. Thermal Decomposition

The thermal decomposition method is an oxygen-free, organic phase, synthetic process in whichthe precursors are dissolved and decomposed in high-boiling-point organic solvents with theassistance of surfactants. Ions are then generated to combine into new nuclei at elevated tem-peratures (38, 39). By adding different kinds of precursors, one can obtain various RE uorides.Generally, the precursors are RE-based triuoroacetates and oleates. Organic solvents usuallyconsist of surfactants and coordination solvents, such as octadecene, oleic acid, and oleylamine. Itis widely accepted that coordination solvents can cap the surface of UCNs to control growth anddisperse them in organic solvents. The thermal decomposition method can also be divided intosingle-source and multisource precursor thermal decompositions according to the uorine sourceprovided by the precursors.

3.1.1. Single-source precursor thermal decomposition. In 2005, Yan and coworkers (38)reported the synthesis of LaF3 triangular nanoplates using the thermal decomposition method(Figure 3a). They used RE(CF3COO)3 as the precursor, which provided both RE and uorineions upon decomposition. This strategy has been developed into a universally applicable methodfor the synthesis of high-quality and monodispersed UCNs, including NaREF4 (39, 40), as well asREOF (41), LiREF4 (42), KREF4 (42), and BaREF5 (43) (Figure 3bf ). Single-source precursorthermal decomposition is also suitable for the preparation of UCNs with a core/shell structure(35). In addition, UCNs with a core/shell structure have been prepared to great success usinga modied hot-injection technique, in which a stock solution containing a reactive precursor oftriuoroacetates is injected into the hot solvent at a constant rate (44).

3.1.2. Multisource precursor thermal decomposition. RE and uorine precursors are re-spectively provided by two or more kinds of precursors in the multisource precursor thermaldecomposition method. Generally, RE triuoroacetates, oleates, acetates, and chlorides are em-ployed for RE ions, whereas HF, NH4F, NH4HF2, NaF, and CF3COOH are used for uorineions. Chen and coworkers (45) prepared -NaREF4 UCNs with this approach and chose NaFand RE oleates as precursors. They controlled the morphology of UCNs by simply regulating theratios of NaF-to-RE oleates or the ratio of solvents. Li & Zhang (46) developed a facile and user-friendly method for the synthesis of -NaYF4:Yb,Er/Tm UCNs in oleic acid and octadecene.The reaction occurred in an anhydrous and oxygen-free environment. In this work, a methanolsolution containingNaOH andNH4F was added to a homogeneous solution of RECl3, oleic acid,and octadecene. The most signicant part of this strategy is to consume a stoichiometric amountof uoride reagents entirely at room temperature, so as to decrease the HF gas and uorinatespecies at high temperatures.

3.2. Hydro(solvo)thermal Method

The hydro(solvo)thermalmethod is another way to yieldUCNswithwell-controlledmorphology.The strategy involves mixing RE precursors with uoride precursors in an aqueous solution,sealing and heating the solution in an autoclave (often lined with Teon). RE nitrates, chlorides,and oxides are usually chosen as REprecursors.HF,NH4F, andNH4HF2 are frequently employed

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Figure 3Transmission electron microscopy images of (a) LaF3, (b) -NaYF4, (c) -NaYF4, (d ) LaOF, (e) BaGdF5,and ( f ) LiErF4, synthesized with the thermal decomposition method, and ( g) -NaYF4 and (h) -NaYF4,prepared with the hydro(solvo)thermal method. (i ) Scanning electron microscopy image of -NaYF4prepared with the hydro(solvo)thermal method. Panel a reprinted with permission from Reference 38.Copyright 2005 American Chemical Society. Panels b and c reprinted with permission from Reference 40.Copyright 2006 American Chemical Society. Panel d reprinted with permission from Reference 41.Copyright 2008 American Chemical Society. Panel e reprinted from Reference 43 with permission of TheRoyal Society of Chemistry. Panel f reprinted from Reference 42 with permission of The Royal Society ofChemistry. Panels g and h reprinted with permission from Reference 47. Copyright 2007 AmericanChemical Society. Panel i reprinted with permission from Reference 48. Copyright 2007 Wiley-VCHVerlag GmbH & Co. KGaA.

as uoride precursors for REF3 UCNs, whereas NaF and KF are used for MREF4 (M = Na, K)UCNs. In this method, many experimental parameters, such as the reactant concentration, dosageof RE ions, temperature, reaction time, and pH value, can inuence the growth of UCNs. NearlymonodispersedNaYF4 single-crystal nanoparticles (Figure 3g), hexagonal nanorods (Figure 3h),and ower-patterned nanodisks (Figure 3i) have been synthesized with the hydro(solvo)thermalmethod (47, 48).



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4. UPCONVERSION ENERGY TRANSFER MODULATION

Due to their critical effect on optical properties and further applications, engineering UC energytransfer pathways has always been a great concern. With regard to optical properties, energytransfer modulation can be divided into three categories: multicolor tuning, the enhancement ofUC emissions, and novel UC excitations triggered by energy transfer.

4.1. Multicolor Tuning of Upconversion Emissions

In anETU-motivated system, efcient activators aremainly theEr3+, Tm3+, andHo3+ ions.Thesefew choices determine the limited emission color outputs. For example, green and red emissionscanbe simultaneously obtained inYb3+-Er3+-codopedUCNs, generating anoverall yellowoutput.Coincidently, green and red emissions with similar wavelengths are also yielded in Yb3+-Ho3+-codoped UCNs, which, to a large extent, limit the application of UCNs in multicolor encodingand multiplexed analyte detection purposes. Hence, investigations into multicolor tuning areessential. Numerous methods have been developed, which can be classied as follows: controllingthe RE doping concentration, screening the hostmatrix, introducing extraneous energy levels, andincorporating energy acceptors to undergo the luminescence resonant energy transfer (LRET)process.

4.1.1. Controlling the rare earth doping concentration. The RE doping concentration de-termines the number of luminescent centers as well as their spatial distance in the inorganic hostmatrix. Novel multiphoton cross relaxations and energy backtransfer from activators to sensitizersusually occur when their doping concentration changes.

In 2004, Capobianco and coworkers (49) demonstrated that the red to green emission ratio en-hancedmonotonicallywith an increasingdoping concentrationofYb3+ ions inY2O3:Yb,ErUCNs.They attributed this phenomenon to the multiphoton cross relaxation of 4F7/2 (Er3+) + 4I11/2(Er3+) 4F9/2 (Er3+) + 4F9/2 (Er3+), where 4F9/2 is responsible for red emissions. Inspired by theoptical advantage of -NaYF4:Yb,Er UCNs, our group systematically studied their composition-dependent UC properties. By precisely tuning the content of Yb3+ (1030%) and Er3+ (0.55%)ions, we discovered that the red to green emission ratio increased with an elevated content of bothYb3+ and Er3+ ions (35). Apart from the cross-relaxation process, the energy backtransfer alsoenhances red emissions. Wang & Liu (50) observed this phenomenon in -NaYF4:Yb,Er UCNswith an elevated Yb3+ ion content (Figure 4a). They reasoned that the energy backtransfer process4S3/2 (Er3+) + 2F7/2 (Yb3+) 4I13/2 (Er3+) + 2F5/2 (Yb3+) should contribute to the phenomenon.

As for Yb3+-Tm3+ codopedUCNs, theUC emission property also exhibits a certain regularity.Our group studied UC emission proles with various doping concentrations of Tm3+ ions (0.25%) in -NaYF4:Yb,Tm UCNs. Spectral results showed that decreasing the content of Tm3+

ions from 5% to 0.2% tended to enhance the four-photon UC emissions more than it did thethree-photon UC emissions, resulting in the transition of color output from blue to purple (37).Prasad and coworkers (51) found that the ratio of ve-photon emission to three-photon emissionincreased with the content of Yb3+ ions and reached a maximum at 90%. Han and coworkers(52) observed that an elevated doping concentration of Yb3+ ions enhanced ve- and four-photonemissions compared with three- and two-photon emissions.

4.1.2. Screening the host matrix. The host matrix provides the doping sites for RE luminescentcenters. Besides the fundamental adoption functionality, different host matrices lead to variousspatial distances, as well as different local coordination structures of luminescent centers.Moreover, the phonon energy, which signicantly affects UC energy transfer pathways, differs

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420 490 560 630 700 420 490 560 630 700

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S-0378

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Figure 4Multicolor upconversion (UC) emission modulation of Yb3+-Er3+-codoped upconversion nanomaterials (UCNs) by (a) controlling thedoping concentration of Er3+ ions and introducing extraneous energy levels of Tm3+ ions, (b) screening host matrices of -NaREF4and -NaREF4, and (c) incorporating organic dyes to generate the luminescence resonant energy transfer processes between theUCNs and the dyes. The digital photographs in panel a show the UC emission outputs of the samples listed in the upper spectra. Panela reprinted with permission from Reference 50. Copyright 2008 American Chemical Society. Panel b reprinted with permission fromReference 40. Copyright 2006 American Chemical Society. Panel c reprinted with permission from Reference 68. Copyright 2011Wiley-VCH Verlag GmbH & Co. KGaA.

in diverse host matrices. The branch ratio of UC emissions can be effectively regulated afteralternation in host matrices.

In 2005, Soukka and coworkers (53) studied the effect of the host matrix on the UC emissionsof Yb3+-Er3+-codoped systems. The results showed that the red to green emission ratio of Er3+

ions in oxides and oxychlorides was larger than that in oxysuldes, uorides, and uoride double



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salts. Huang and coworkers (54, 55) investigated Sc3+ ionbased UCNs, NaScF4 and KSc2F7.In the two kinds of UCNs, apparent red emission dominated the visible spectra, yielding a redcolor output. Recently, Capobianco and coworkers (56) studied the UC emission properties ofGdVO4:Yb,ErUCNs. Spectral results demonstrated that green emission was the dominant visibleemission.Moreover, the emission at 525 nm (2H11/2 4I15/2) wasmore intense than that at 545 nm(4S3/2 4I15/2), which was different from most cases. Recently, Liu and coworkers (57) describeda new type of UCN by adopting an orthorhombic crystallographic structure in which RE ionswere distributed in arrays of tetrad clusters. The unique arrangement enabled the preservationof excitation energy within the sublattice domain and effectively enhanced the violet emission(415 nm; 2H9/2 4I15/2), which was conrmed as a four-photon UC process.

It is worth noting that different phase structures of the same compound can also result indistinctive UC emissions. Our group investigated the phase-structure-dependent UC emissionproperties of NaYF4:Yb,Er UCNs (35). As shown in Figure 4b, there are two phase structures ofNaYF4, namely cubic () and hexagonal (). From their corresponding UC emission spectra, wecan see that the intensity of the green emission is in the same proportion of that of the red emissionof Er3+ in -NaYF4:Yb,Er UCNs, thus generating yellow emission output. However, the greenemission dominates the visible regime in -NaYF4:Yb,Er UCNs, yielding green emissionoutput.

Along with Er3+ ions, UC emissions of Tm3+ ions are also dependent on the host matrix.Typically, the four- and ve-photon emissions are small in proportion. Capobianco and coworkers(58) presented a type of LiYF4:25%Yb,0.5%TmUCN, for which spectral results exhibited intenseUV emissions with moderate Yb3+ ion content.

4.1.3. Introducing extraneous energy levels. Abundant energy levels provide RE ions withvarious energy transfer pathways, and multicolor emissions can be generated when different REactivators are appropriately codoped.Multiphoton cross relaxations occurring within the codopedactivators can also tailor the original emissions. After such energy transfers, novel emission bandsor altered branch ratios appear.

Wang & Liu (50) were able to ne-tune visible UC emissions in -NaYF4:Yb,Er,Tm UCNs(Figure 4a). Before doping with Er3+ ions, they obtained blue emission output upon 980-nmexcitation. By subtly increasing the concentration of Er3+ ions from 0.2% to 1.5%, they graduallytuned the emission output from blue to white. Wang and coworkers (59) prepared a series ofNaYbF4-based UCNs doped with Er3+-Tm3+, Tm3+-Ho3+, and Er3+-Ho3+ pairs, respectively,with the result of varying emission outputs and spectral ngerprints.

Except for multicolor emissions, Zhang and coworkers (60) successfully tailored UC emissionin Yb3+-Ce3+-Ho3+-tridoped UCNs. With an increasing doping concentration of Ce3+ ions(015%), the color emission output obviously changed from green to red. The spectral transfor-mation was ascribed to multiphoton cross relaxations: 5I6 (Ho3+) + 5F5/2 (Ce3+) 5I7 (Ho3+)+ 2F7/2 (Ce3+) and 5S2/5F4 (Ho3+) + 5F5/2 (Ce3+) 5I5 (Ho3+) + 2F7/2 (Ce3+). Unusual 5G5 5I7 and 5F2/5K8 5I8 transitions from Ho3+ ions and 5d 4f transitions from Ce3+ ionshave also been observed (61). Qin and coworkers (62) demonstrated UC emission modulation inNaYF4:Er,Tm UCNs. With the introduction of Tm3+ ions, the red to green emission ratio ofEr3+ ions increased as a result of multiphoton cross relaxation: 3F4 (Tm3+) + 4I11/2 (Er3+) 3H6(Tm3+) + 4F9/2 (Er3+). Recently, Chan and coworkers (63) employed a combinatorial methodto investigate the inuence of codoping with other RE ions on the UC emissions of Er3+ ions.They found that almost spectrally pure green emission can be generated from Er3+-Pr3+ andEr3+-Sm3+ coactivated UCNs, whereas pure red emission can be yielded from Er3+-Ho3+- and

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Er3+-Tm3+-codoped counterparts. Selective quenching of red emission was proposed to accountfor the pure green emission, whereas the pure red emission was induced by cross relaxations:4I11/2 (Er3+) + 3F4 (Tm3+) 4I13/2 (Er3+) + 3H5 (Tm3+), 3F4 (Tm3+) + 4I11/2 (Er3+) 3H6(Tm3+) + 4F9/2(Er3+), 4I11/2 (Er3+) + 5I7 (Ho3+) 4I13/2 (Er3+) + 5I6 (Ho3+), and 5I7 (Ho3+) +4I11/2 (Er3+) 5I8 (Ho3+) + 4F9/2 (Er3+), respectively.

Recently, several transition metal ions have also been incorporated as intermediate states forphoton upconverting.Mn2+ ions, which possess the unique 4T1 energy state, are employed to tailorthe UC emissions of RE ions. First, Li and coworkers (64) investigated the role of Mn2+ ions onthe UC emission of Er3+ ions in KMnF3:Yb,Er UCNs. Spectral results showed that spectrallypure red UC emissions were generated in this system. The authors reasoned that the energytransfer Yb3+ (2F5/2) Er3+ (2H11/2, 4S3/2) Mn2+ (4T1) Er3+ (4I15/2) enhanced red emissionwhile decreasing the intensity of green emission. Later, Liu and coworkers (65) conrmed thered emission output in KMnF3:Yb,Er UCNs. Moreover, they also observed spectrally pure redemission in KMnF3:Yb,Ho UCNs and spectrally pure NIR emission in KMnF3:Yb,Tm UCNs,which also mediated energy transfer via the 4T1 state of Mn2+ ions.

4.1.4. Luminescence resonance energy transfer. LRET refers to resonant energy migrationfrom UCNs to decorated energy acceptors, such as quantum dots, organic dyes, and noble metals.The key prerequisite for LRET is the spectral overlap between the emission of UCNs and theabsorption of energy acceptors. With various decorations of energy acceptors, UCN emissionscan be subtly tuned via LRET.

Li and coworkers (66) preparedNaYF4:Yb,ErUCNsdecoratedwith gold nanomaterials, whoseabsorption band heavily overlappedwith the green emission of Er3+ ions.With an elevated amountof gold nanomaterials, the green emission ofUCNsdecreased gradually. Zhang and coworkers (67)encapsulated uorescein isothiocyanate (FITC), tetramethylrhodamine isothiocyanate (TRITC),and quantum dots (QD605) into silica shells coated outside of the NaYF4:Yb,Er/Tm UCNs.The occurrence of LRET processes was conrmed by the reduction of the typical emission ofNaYF4:Yb,Er/Tm UCNs and the emergence of novel emission bands of FITC, TRITC, andQD605. Gorris and coworkers (68) employed RhB and the dye S-0378 to modulate the green andred emissions of NaYF4:Yb,Er UCNs, respectively (Figure 4c). Furthermore, they also adopteduorescein and the dyeNIR-797 to encode the blue andNIR emissions ofNaYFF4:Yb,TmUCNs,respectively.

4.2. Enhancement of Upconversion Emissions

Determined by the parity-forbidden 4f-4f intracongurational transitions, the absorption crosssection of RE ions is quite small. Hence, the UC emission efciency of RE ions is far fromsatisfactory. Many approaches exist, including forming core/shell structures, tailoring the localcrystal eld, constructing building blocks with noblemetals, and introducingNIR antenna ligands.

4.2.1. Constructing core/shell structures. UCNs prepared by wet chemical methods are facedwith surface defects, as well as the vibration of capping ligands, which are deleterious to UCemissions. Boyer & van Veggel (69) observed a sharp decrease in the quantum yield of UCNs withdecreasing particle size. The increasing surface defects in smaller particles should account for thephenomenon. Capobianco and coworkers (70) investigated the effect of the hydroxyl group on theUC emissions of Er3+ ions. After the removal of the original oleate ligands, the resulting UCNsexhibited an enhanced red to green emission ratio in the UC spectra. An efcient way to eliminatethe surface effect is the formation of core/shell structures.



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Liu and coworkers (71) investigated the surface quenching effect by comparing the branch ratioof blue to NIR emission intensity of Tm3+ ions and the overall emission intensity before and aftershell growth. van Veggel and coworkers (72) demonstrated novel epitaxial layer-by-layer growthof NaYF4:Yb,Er UCNs with Ostwald ripening dynamics. UC spectral results conrmed that theemission intensity steadily enhanced with an increasing thickness of the shell layer. Moreover, theUC emission intensity enhanced linearly with the shell thickness (73). Recently, our group demon-strated the novel generation of a heterogeneous core/shell structure,-NaYF4@CaF2 UCNs (74).The successful growth of the CaF2 layer should be ascribed to the same space group (Fm3m) and asimilar lattice constant, that is, a = 5.448 A for -NaYF4 and a = 5.451 A for CaF2.We preciselytuned the thickness of the CaF2 shell layer by controlling the molar ratio of [Ca]/[RE]. Spectralresults showed that, when the molar ratio of [Ca]/[RE] was 4, the overall integrated emissionintensity was enhanced approximately 300 times (Figure 5a). Moreover, RE ion leakage was sup-pressed after the epitaxial growth of the CaF2 layer. Very recently, Bednarkiewicz and coworkers

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Figure 5The enhancement of upconversion emissions of Yb3+-Er3+-codoped upconversion nanomaterials (UCNs) by (a) constructingcore/shell structures, (b) tailoring the local crystal eld, (c) constructing building blocks with noble metals, and (d ) introducing anear-infrared antenna ligand. Panel a reprinted with permission from Reference 74. Copyright 2012 Wiley-VCH Verlag GmbH & Co.KGaA. Panel b reprinted with permission from Reference 80. Copyright 2008 American Chemical Society. Panel c reprinted fromReference 86 with permission of The Royal Society of Chemistry. Panel d reprinted from Reference 93 by permission from MacmillanPublishers Ltd, copyright 2012.

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(75) conrmed a considerable enhancement in CET-mechanized -NaYF4:Yb,Tb@CaF2 UCNs.The maximum enhancement factor was reported to be 40.

In contrast to the optically inert shell layers mentioned above, optically active shells notonly could enhance the UC emission intensity, but also could generate novel UC emis-sions of activators embedded in the shell layer. In Capobianco and coworkers (76) workwith NaGdF4:Yb,Er@NaGdF4:Yb core/shell UCNs, the UC emission intensity of the activecore/active shell UCNs increased three times in green emission and 10 times in red emissioncompared with that of NaGdF4:Yb,Er@NaGdF4 UCNs. Very recently, Liu and coworkers (77)employed an epitaxial end-on growth technology to synthesize multicolor microrods comprisingdifferent RE activators. In this way, six kinds of multicolor-banded microrods were obtained formulticolor barcoding. Chen and coworkers (78) described the intriguing generation of UCNsby introducing energy transfer through the core/shell interface. NaGdF4:Yb,Tm@NaGdF4:Eucore/shell UCNs were prepared, and the UC emission property was investigated. Upon 980-nmexcitation, emissions from both Tm3+ and Eu3+ ions were observed. Liu and coworkers (31, 79)further promoted the application of energy transfer though core/shell interfaces. They obtainedUC emissions from a series of activators without long-lived energy levels, including Eu3+, Tb3+,Dy3+, and Sm3+ ions.

4.2.2. Tailoring the local crystal field. Because of destruction in the local symmetry, tailoringthe local crystal eld is expected to facilitate intracongurational transitions. An efcient way to doso is to compensatewith optically inert non-RE ions.Hence, several alkalimetal ions and transitionmetal ions are frequently introduced to tailor the local crystal eld of luminescent centers.

Owing to their smaller cationic radii, Li+ ions are supposed to be randomly located at thelattice site or interstices among the lattices. Zhang and coworkers (80) introduced Li+ ions toY2O3:Yb, Er UCNs. They found that the UC emission intensity of Y2O3:Li,Yb,Er UCNs showedan enhancement of two orders of magnitude compared to Y2O3:Yb,Er UCNs (Figure 5b). Theyconcluded that the prolonged lifetime of the 4I11/2 (Er3+) and 2F5/2 (Yb3+) states and the extrapopulation of the 4I13/2 (Er3+) state favored the enhancement of UC emissions. Following this,Wang & Nann (81) observed a more than 30-fold enhancement of emissions after 80% Li+

doping in NaYF4:Yb,Er UCNs. Cai and coworkers (82) investigated the Li+ iondependent UCproperties of -NaGdF4:Li,Yb,Er UCNs. They observed 47 and 23 times the enhancement forthe green and red emissions compared with the -NaGdF4:Yb,Er UCNs, respectively.

Besides Li+ ions, several transition metal ions, such as Bi3+ and Fe3+, have also been employedto tailor the local crystal eld (83, 84). Except for doping with non-RE ions, the local crystaleld could be tailored by other means as well. Recently, Hao and coworkers (85) demonstrated aninteresting method to enhance UC emissions by applying relatively low voltages to BaTiO3:Yb,Erlms. They attributed this enhancement phenomenon to the promotion in radiative probabilitiesinduced by the lower symmetry of Er3+ sites, which took place after the introduction of the externalelectric eld.

4.2.3. Constructing building blocks with noble metal nanostructures. Nobelmetal nanopar-ticles, which have localized surface plasmon resonance (LSPR) properties, have been extensivelyinvestigated in terms of their interactions withUC emissions. Themost frequently used plasmonicnoble metals are gold and silver nanostructures. Two possible mechanisms have been proposedto account for the enhancement of UC emissions caused by the LSPR property of gold and silvernanostructures. On one hand, the enhancement effect may be achieved as a result of the amplica-tion of the local incident electromagnetic eld, which arises from the coupling of excitation bandswith the surface plasmon resonance. On the other hand, the increase in the recombination rate



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by the surface plasmoncoupled emission could enhance the emission efciency, which effectivelypromotes radiative and nonradiative decay rates.

We reported the rst enhancement of the UC emission intensity assisted with LSPR(Figure 5c) (86). In our design, silver nanowires were employed to provide the LSPRs, andNaYF4:Yb,Er UCNs were chosen as the UC emission generators. Spectral results showed thatthe enhancement factor was 2.3 and 3.7 for the green and red emissions, respectively. Duan andcoworkers (87) decorated gold nanoparticles onto the surface of NaYF4:Yb,Tm UCNs. Theyfound a more than 150% increase in the emission intensity of blue emission, whereas an increaseof only 50% was observed for the red emission. Zhang and coworkers (88) investigated the inter-action between the NaYF4:Yb,Er@SiO2 UCNs and the gold shells by adjusting the LSPR peaksof the composites. Spectral results demonstrated that UC emissions were enhanced only in thecomposite whose LSPR peak was at 900 nm, nearest to the excitation wavelength of 980 nm. Suchresults indicate that the excitation ux was increased via the local eld enhancement effect.

The spatial distance between the UCNs and noble metals is crucial for the enhancementeffect. Xu and coworkers studied the separation distance-dependent UC emission prole inNaYF4:Yb,Er@SiO2@Ag core/shell composites (89). The inuence of silver nanoparticles on UCemission behavior was studied by controlling the thickness of the SiO2 layer. The maximum UCemission enhancement factor was observed at a separation distance of 10 nm. Kagan and cowork-ers (90) constructed a metal oxideUCN trilayered structure in which the thickness of the oxidespacer was tuned from 2 to 15 nm. The optimal thickness of the oxide spacer was 5 nm for gold and10 nm for silver noble metals. Additionally, ne arrays of noble metals have been demonstratedas efcient LSPR donors. May and coworkers (91) observed over three times the enhancementof UC emissions from the patterned gold surface. Compared with the smooth gold surface, theyfound an approximately twofold magnication of the excitation eld intensity. Recently, Nagpaland coworkers (92) reported at least six times the enhancement of UC emissions of Er3+ ions on agold pyramid patterned surface, while fourteen times quenching the UC emission on the at goldsurface.

4.2.4. Introducing NIR antenna ligands. Because of their large absorption cross sections, an-tenna ligands have been frequently used to enhance the downshifting emission intensity of Eu3+

and Tb3+ ions. Theoretically, appropriate NIR antenna ligands are also capable of enhancingthe intensity of UC emissions of Er3+, Tm3+, and Ho3+ ions. However, only one report has con-rmed the feasibility of the sensitization effect fromNIR antenna ligands (Figure 5d ). Hummelenand coworkers (93) prepared IR-806 dyes by grafting carboxyl-containing chains to commercialIR-780 cyanine dye molecules. The modied IR-806 molecules were capped on the surface ofNaYF4:Yb,Er UCNs by a ligand exchange strategy. Upon excitation with NIR photons (800 nm),the IR-806 dyes were excited and donated energy to the Yb3+ ions embedded in the inorganicmatrix. Finally, the energy was extracted by the Er3+ ions via the ETU process. Spectral resultsexhibited a 3,300-fold enhancement compared with the oleate-capped UCNs. Besides the signi-cant sensitization, the excitation band was shifted to 800 nm, which corresponded to the excitationof IR-806 molecules.

4.3. Energy TransferTriggered Novel Upconversion Excitation

Conventional ETU-mechanized UCNs, which are sensitized by Yb3+ ions, exhibit a narrow exci-tation band centered at 980 nm. However, the small selection of excitations limits the exibility offurther applications of UCNs. For example, the specic 980-nm excitation heavily overlaps withthe absorption of water molecules, which are the main ingredients of organisms.

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Figure 6(a) Proposed energy transfer mechanism of Nd3+ ionsensitized upconversion (UC) emissions. Differentregions are highlighted with different background colors. (b) UC emission spectra of Er3+ ions upon 980-nmand 808-nm irradiation with the same excitation power. Figure reprinted with permission from Reference94. Copyright 2013 American Chemical Society.

To solve the pending question, we developed Nd3+ ionsensitized UCNs (Figure 6) (94).After absorption of the 730-nm, 808-nm, and 865-nm NIR light, Nd3+ ions were excited, andthen the energy was transferred to adjacent Yb3+ ions through a downshifting process. Finally,activators trapped the energy and gave out UC emissions. To avoid mutual quenching, Nd3+ andEr3+ ions were incorporated into separated layers. We observed a similar excitation efciency of808 nm and 980 nm. Moreover, the heating effect induced by the irradiation of 980-nm light waslargely minimized by the illumination of 808-nm light. Along with incorporation into separatedlayers (94, 95), tridoping with Nd3+, Yb3+, and Er3+ ions in the same layer could also generatedistinguishable UC emissions (96, 97). We note that the content of Nd3+ ions should be kept ata low level to prevent considerable multiphoton cross relaxations between Er3+ and Nd3+ ions.

5. APPLICATIONS

REUCNs have been adopted in a wide range of applications, from bioimaging to photoresponsivedevices, owing to the unique NIR excitation, large anti-Stokes shifts, sharp-band emissions, andexcellent photostability. RE UCNs have miniscule biotoxicity, remarkable penetration depth, anda high signal-to-noise ratio. Therefore, they have been studied as popular luminescent materialsfor bioimaging applications (21).

Various in vitro (98101) and in vivo (102106) models have been employed to validate thebioimaging potentials of RE UCNs. For example, Li and coworkers (103) synthesized sub-10-nm-NaLuF4:Gd,Yb,Tm UCNs for sensitive in vivo imaging. Excellent detection limits of 50 and1,000 UCN-labeled cells were achieved for subcutaneous and intravenous injection, respectively.Our group reported the in vivo imaging and toxicity assessments of NaYF4:Yb,Tm UCNs withCaenorhabditis elegans (106).Theworms exhibitedNIRUC luminescence in the gut upon excitationof 980 nm. Furthermore, theUCNs can be excreted out as thewormswere fedwith onlyEscherichiacoli again, and no signicant difference in the ingesting of UCNs was observed between thehermaphrodite and the male.



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PDT: photodynamictherapy

Due to low photon energy and efcient anti-Stokes emissions, RE UCNs have also been em-ployed as energy transducers to upconvert NIR light and trigger photochemical reactions. A seriesof photoresponsive applications, such asNIR-triggered photodynamic therapy (PDT), sensing anddetection, photoswitch, photorelease, and photoisomerization, highlights the signicance of REUCNs.

In the PDT system, photosensitizers are activated by emission fromUCNs and then combinedwith adjacent oxygen atoms, producing reactive oxygen species, resulting in oxidative damage tocancer cells. Liu and coworkers (107) presented the rst UCN-based in vivo PDT. NaYF4:Yb,ErUCNs were decorated with chlorin e6 molecules to trigger the PDT process. Upon 980-nmexcitation, visible emissions from the UCNs were transferred to the photosensitizers. We con-structed triple-functional NaGdF4:Yb,Er@CaF2 core/shell UCNs loaded with hematoporphyrinand silicon phthalocyanine dihydroxide molecules, respectively (108). The composites exhibitedexcellent PDT efciency in HeLa cells upon 980-nm irradiation.

In sensing and detection studies, UCNs were decorated with specic recognition site-containing chromophoric complexes. The UC emissions could be turned on or off by the com-plexes via anLRETprocess. For example, Li and coworkers (109) demonstrated in vivo bioimagingof methylmercury (MeHg+) with cyanine-modied NaYF4:Yb,Er,Tm UCNs. The nanocompos-ites showed high specicity and sensitivity with a detection limit of 0.18 ppb. Recently, Zhang andcoworkers (110) developed a novel ligase-assisted signal-ampliable DNA detection scheme withhigh sensitivity and specicity based on UCNs via an LRET process.

The rst NIR-triggered photoswitch application was reported by Branda and coworkers (111).Photochromicmolecules, diarylthene derivatives, which are sensitive toUV and visible light with areversible ring-closing and -opening form,were stimulatedwithUCemission fromNaYF4:Yb,Tmand NaYF4:Yb,Er nanoparticles, respectively. They further demonstrated the reversible remotecontrol of diarylethene derivatives with one type of UCN (112). The strategy lies in embeddingEr3+ ions and Tm3+ ions into separated layers of nanoparticles to yield UV and visible emissionupon high and low excitation power, respectively.

Spectral match between the absorption spectra of photosensitive molecules and emission spec-tra of UCNs is the key factor for photorelease studies. Branda and coworkers (113) demonstratedthe release of carboxylic acid molecules from 3,5-dialkoxybenzoin compounds with NIR light.NaYF4:Yb,Tm UCNs were employed as photon transducers to provide UV emissions upon NIRirradiation. An apparent decrease in the absorption of reactants and a concomitant increase inthe absorption of products suggested the occurrence of photorelease. Xing and coworkers (114)demonstrated the photorelease application in vivo. D-luciferin molecules were covalently bondedto the NaYF4:Yb,Tm UCNs. Upon UV emission generated by the UCNs, these molecules werereleased and gave out bioluminescence. Both in vitro and in vivo imaging results demonstratedthe advantage of UCNs.

In the NIR-triggered photoisomerization system, azobenzene molecules are typical photosen-sitizers. They are able to conduct the trans-cis photoisomerization process upon exposure to UVand visible light fromYb3+-Tm3+-codopedUCNs.Yu and coworkers (115) observed the fast bend-ing of azotolane containing cross-linked liquid crystal polymer lms covered with NaYF4:Yb,TmUCNs upon 980-nm excitation. This occurred due to the photoisomerization of azotolane unitsand a change in the alignment of the mesogens. Li and coworkers (116) fabricated an NIR-light-responsive self-organized helical superstructure by dopingNaGdF4:Yb,Tm@NaGdF4 UCNs andchiral azobenzenes into a liquid crystal host. By tuning the excitation power density, they wereable to achieve reversible NIR-light-driven red, green, and blue reections in a single thin lm(Figure 7). Aside from the above-mentioned applications, UCNs have also been employed astemperature sensors (117) and energy transducers for solar cells (25, 118, 119).

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Figure 7Upconversion emission spectra of upconversion nanomaterials upon irradiation with a 980-nm laser at(a) 2 W mm2 and (b) 0.15 W mm2. (c) Trans-cis and (d ) cis-trans photoisomerization process ofazobenzenes in cyclohexane upon irradiation with a 980-nm laser at 2 W mm2 and 0.15 W mm2,respectively. Corresponding reection spectra of a cholesteric liquid crystal at room temperature uponirradiation with a 980-nm laser at (e) 2 W mm2 and ( f) 0.15 W mm2 for different times. Figure reprintedwith permission from Reference 116. Copyright 2014 American Chemical Society.



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6. CONCLUSIONS AND FUTURE OUTLOOK

In this review, we summarize recent developments of RE UCNs in terms of energy transferpathways, synthesis strategies, energy transfer modulations, and applications. RE-based uorides,which are excellent host matrices for UC processes, have been synthesized with high quality byvarious methods. By rationally dening the RE luminescent centers, host matrices, and energy-level-matched energy extractors, investigators can easily obtain multicolor UC emissions. Addi-tionally, efcient approaches to enhance UC emissions include constructing core/shell structures,lowering the local symmetry, and introducing noble metals and NIR antenna ligands. Further-more, we show the emerging progress of novel excitations triggered by energy transfer fromNd3+

ions. In this way, the excitation bands of UC emissions are greatly enriched. To date, various bio-logical models, such as cells, bacteria, C. elegans, mice, and rabbits, have been used to test andverify the bioimaging value of RE UCNs. The exciting results conrm that UCNs are superiorto conventionally used quantum dots and organic dyes.

Despite these accomplishments, there are still some remaining questions. For example, thefabrication of bright RE UCNs with small size (sub10 nm) is required for better bioimagingpurposes. However, the surface effects determined by the large surface-to-volume ratio are dele-terious for UC emission efciency. Presently, attention is mainly given to Er3+, Tm3+, and Ho3+

ions. Other RE activators, such as Tb3+ and Eu3+ ions, have distinctive spectral ngerprints andlonger luminescence lifetimes. Hence, skillfully mastering the UC energy transfer routes in otherRE ions is also meaningful and challenging. In addition, only one type of NIR antenna ligand hasbeen reported; nonetheless, due to the signicant enhancement effect, further investigations intoantenna-sensitized UC emissions should be urgently explored. Valuable applications of UCNsshould also be developed to make full use of UC emissions.

SUMMARY POINTS

1. RE ions are excellent supporters for photon UC emissions owing to abundant energylevels. With diverse energy transfer pathways, numerous pairs of RE ions are capable ofgenerating UC emissions.

2. RE UCNs exhibit large anti-Stokes shifts, sharp-band emissions, excellent photostabil-ity, and long luminescence lifetimes. Owing to these advantages, RE UCNs have beenadopted as more promising luminescent materials compared with quantum dots andorganic dyes.

3. Synthetic strategies for the creation of high-quality RE UCNs are well developed tosome extent. The size and morphology of UCNs could be purposefully controlled byvarious methods.

4. Multicolor RE UCNs are of signicance to multicolor imaging and multiplexed detec-tion applications. Controlling the RE doping concentration, screening the host matrix,introducing extraneous energy levels, and incorporating energy acceptors are excellentways to tune the branch ratio of UC emissions.

5. The enhancement of UC emissions is a general concern for researchers. Constructingcore/shell nanostructures, tailoring the local crystal eld, incorporating noble metals,and introducing NIR antenna ligands are methods to enhance UC emissions.

636 Sun et al.


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6. The introduction of Nd3+ ions to conventional ETU-motivated UCNs greatly enrichedUC excitations within the NIR range. Simultaneously, the heating effect induced bylong-time irradiation of 980-nm light is largely minimized by 808-nm irradiation, whichhas an excitation efciency that is similar to that of 980-nm light.

7. With their unique and fascinating optical properties, RE UCNs are used in a wide rangeof applications, from bioimaging to photoresponsive applications. UC emissions fromRE UCNs are expected to trigger more intriguing applications in the future.

DISCLOSURE STATEMENT

The authors are not aware of any afliations, memberships, funding, or nancial holdings thatmight be perceived as affecting the objectivity of this review.

ACKNOWLEDGMENTS

This work was supported by the NSFC (21371011, 21331001) and MOST of China(2014CB643800).

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