The progress of single-band upconversion nanomaterials

Abstract

Upconversion (UC) nanomaterials (NMs) with single-band emission properties provoke widespread interest in the bio-medical field due to their advantages over those with multi-band emissions such as for high resolution in situ multiplexed molecular mapping. Several methods have been applied in recent years to realize single-band emission with different colors. In this feature article, for the first time, we provide an overview of the recent progresses in realizing single-band UC emission through different methods and the related mechanisms. Three types of strategies, including choosing an appropriate matrix, a doping ion with suitable energy level and a coating organic dye with a certain absorption wavelength, are discussed in detail. Finally, the challenges and future perspectives for these novel NMs with single-band UC emission are stated.

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1 Introduction

UCNMs have been widely researched in recent years due to their unique properties that enable the conversion of low-energy photons (infrared or near infrared photons (NIR)) into high-energy photons (visible (VIS) to ultraviolet (UV) photons) via multi-photon processes.^1–15 Benefitting from this feature, lanthanide (Ln) doped UCNMs possess low background auto-fluorescence, deep light penetration depth, and minimal photo-damage to bio-tissue.^16–20 In addition, these UCNMs offer sharp emission peaks, large anti-Stokes shifts, long-lived excited electronic states, and high resistance to photo-bleaching.^21–27 Considering all of these excellent characteristics, Ln³⁺-doped UCNMs are expected to be an alternative to organic fluorescent dyes or quantum dots for various bio-medical applications. For some special fields, single-band optical properties are required, e.g., in in situ multiplexed molecular mapping, accurate molecular profiling can only be achieved with single-band emission nanoprobes by excluding crosstalk between different labelling signals.²⁸

Ln³⁺ ions have a 4fⁿ5s²5p⁶ electron configuration with n = 0–14. The partly-filled 4f shell is responsible for the unique optical and magnetic properties. For n electrons in 14 available orbitals, there are 14 over n possible configurations and all configurations can have different energies. This gives rise to a rich energy level structure in the NIR, VIS and UV spectral range (Table 1). Owing to the long lifetime of the excited states, an excited Ln³⁺ ion can sequentially absorb a second photon of suitable energy and reach an ever-higher excited state,^29,30 and then various emission bands can be obtained when those electrons in different excited states relax to the ground state. The emission wavelengths are independent of host materials due to the transitions between different 4fⁿ states being parity forbidden (no change in dipole moment); however, the ratio of different emission bands can be tuned. Actually, several methods, such as changing the possibilities of cross-relaxation (CR) processes between different energy levels or using an organic dye with a high molar absorption coefficient to absorb the unwanted emission bands, have been used to realize single-band UC luminescence.^28,31,32 To the best of our knowledge, there is no review reported about single-band UC emission to date.

Table 1 Typical lanthanide activators, the corresponding major emissions and energy transitions²⁹

Activators	Major emissions (nm)	Energy transitions
Er^3+	411, 523, 542, 656	^2H9/2 → ^4I15/2, ^2H11/2 → ^4I15/2, ^4S3/2 → ^4I15/2, ^4F9/2 → ^4I15/2
Ho^3+	542, 645, 658	^5S2/^5F4 → ^5I8, ^5F5 → ^5I8
Tm^3+	294, 345, 368, 450, 475, 650, 700, 800	^1I6 → ^3H6, ^1I6 → ^3F4, ^1D2 → ^3H6, ^1D2 → ^3F4, ^1G4 → ^3H6, ^1G4 → ^3F4, ^3F3 → ^3H6, ^3H4 → ^3H6
Tb^3+	490, 540, 580, 615	^5D4 → ^7F6, ^5D4 → ^7F5, ^5D4 → ^7F4, ^5D4 → ^7F3
Eu^3+	590, 615, 690	^5D0 → ^7F1, ^5D0 → ^7F2, ^5D0 → ^7F4
Sm^3+	555, 590	^4G5/2 → ^6H5/2, ^4G5/2 → ^6H7/2

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In this review, we systematically summarize the progress of different methods to actualize single-band UC emission in various systems and the related mechanisms, which is useful for understanding single-band optical properties and promoting their further development. For better comparison, some basic knowledge about UC with multi-band emission is first discussed in Section 2. Finally, the challenges and future outlook of this novel single-band UC emission property are pointed out.

2 UC with multi-band emission and the related mechanism

Ln³⁺-doped UCNMs generally comprise an inorganic host matrix co-doped with sensitizer and activator. The host lattice determines the relative spatial distribution of the dopant ions, the coordination numbers and the type of anions surrounding the dopant. The properties of the host lattice and its interaction with the dopant ions have a strong influence on UC processes. Moreover, the host lattice with low phonon energies can minimize nonradiative losses and maximize radiative emission. For example, fluorides, which are widely chosen as matrixes due to the relatively low phonon energies (ca. 350 cm⁻¹) of their crystal lattices, generally have higher luminescent efficiency than that of oxides (ca. 600 cm⁻¹).^33–35 Although UC can be actualized in principle from most lanthanide-doped crystalline host materials, efficient UC only occurs when appropriate dopants (including activators and sensitizers) are selected. To actualize multicolor emissions, different Ln³⁺ ions (such as Er³⁺, Ho³⁺ and Tm³⁺) with ladder-like arranged energy levels are widely chosen as activators and doped into different matrixes. However, most Ln³⁺ activators exhibit low absorption cross-section, leading to low pump efficiency and resulting relatively low UC efficiency for the singly-doped NMs. To improve the UC emission intensity, the sensitizer, which have large absorption cross-section in the excitation wavelength region, is used to absorb the photon energy and then transfer to the activators, e.g., Yb³⁺ ions possesses large absorption cross-section (1.2 × 10⁻²⁰ cm²) at around 980 nm due to the ²F_7/2 → ²F_5/2 transition; Nd³⁺ ions have a larger absorption cross-section (1.2 × 10⁻¹⁹ cm²) at around 800 nm due to the ⁴F_5/2/²H_9/2 → ⁴I_9/2 transition.^36–39

For example, Yb/Er and Yb/Tm co-doped NaYF₄ nanoparticles show strong yellow and blue emissions attributed to ²H_11/2/⁴S_3/2 → ⁴I_15/2 (∼539 nm), ⁴F_9/2 → ⁴I_15/2 (∼650 nm) transitions of Er³⁺ and ¹D₂ → ³F₄/¹G₄ → ³H₆ (∼470 nm) transitions of Tm³⁺, respectively (Fig. 1a and b).^40–46 The yellow color of Er³⁺ ions results from two major constituent emissions, which can be separately observed with the aid of green and red color filters. Nd³⁺ is also used as a sensitizer to actualize green UC emission in Yb/Er co-doped NMs under excitation with 980 nm laser.^38,39 Yb/Ho:NaGdF₄ is another luminescent material that exhibits multi-band emission with yellow light.^47,48 Actually, several other Ln ions, including Tb³⁺, Eu³⁺, Dy³⁺ and Sm³⁺, without long-lived intermediary energy states, also serve as the activators to generate multicolor UC emission by controlling gadolinium sublattice-mediated energy migration through a well defined core/shell structure (Fig. 1c and d).^49,50 The main mechanism in these systems is energy transfer upconversion (ETU). In an ETU process, taking Yb/Er:NaYF₄ as an example, an initial energy transfer from an Yb³⁺ ion in the ²F_5/2 state to an Er³⁺ ion populates the ⁴I_11/2 level. A second 980 nm photon or energy transfer from an Yb³⁺ ion can then populate the ⁴F_7/2 level of the Er³⁺ ion. The Er³⁺ ion can then relax nonradiatively (without emission of photons) to the ²H_11/2 and ⁴S_3/2 levels, and the green ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 emissions occur. Alternatively, the ion can further relax and populate the ⁴F_9/2 level leading to the red ⁴F_9/2 → ⁴I_15/2 emission. The ⁴F_9/2 level may also be populated from the ⁴I_13/2 level of the Er³⁺ ion by absorption of a 980 nm photon or energy transfer from an Yb³⁺ ion with the ⁴I_13/2 state being initially populated via the nonradiative ⁴I_11/2 → ⁴I_13/2 relaxation.

Fig. 1 (a) Proposed energy transfer mechanisms exhibiting the upconversion processes in Er³⁺, Tm³⁺, and Yb³⁺ doped crystals under 980 nm excitation. (b) Luminescence emission spectra of 1 wt% colloidal solutions of nanocrystals in toluene excited with a 980 nm laser diode (power density = 100 W cm²): NaYF₄:2% Er³⁺,20% Yb³⁺ and NaYF₄:2% Tm³⁺,20% Yb³⁺. Inset: total upconversion luminescence of NaYF₄:2% Er³⁺,20% Yb³⁺ solution and isolation of green and red luminescence using appropriate filters; total upconversion luminescence of NaYF₄:2% Tm³⁺,20% Yb³⁺ solution. (c) Proposed energy transfer mechanisms in the core–shell–shell nanoparticles. Note that only partial energy levels of Tm³⁺, Gd³⁺, and A³⁺ (A = Dy, Sm, Tb, and Eu) are shown for clarity. The optical emissions from higher-lying energy levels of Tb³⁺ and Eu³⁺ are highlighted with colored arrows. (d) Luminescence images of representative samples in cyclohexane solution (2 mg mL⁻¹) under irradiation of a 980 nm laser.^40,50

3 UC with single-band emission and the related mechanism

Ln³⁺ activators generally have abundant meta-stable excited states enabling them to emit photons covering from the ultraviolet to infrared spectral regions in appropriate hosts, and the emission profiles are various with different electron transition or relaxation pathways. For example, the ⁴F_7/2 and ⁴F_9/2 levels of Er³⁺ are the origin of the green and red emissions, respectively;⁵¹ if the population of ⁴F_7/2 level is suppressed or the electrons in the ⁴F_7/2 state relax nonradiatively to the ⁴F_9/2 state, the green emission will disappear and the single red one will be achieved.

Compared with multi-band UC emission, the single-band emission has much higher chromatic purity, resulting in better sensitivity and spatial resolution in bio-imaging applications, e.g., KMnF₃:Yb/Er with strong pure red emission can afford deeper tissue penetration than NaYF₄:Yb/Er with both green and red emission, and the red emission intensity of the KMnF₃:Yb/Er NCs is found to be substantially higher than that of the NaYF₄:Yb/Er nanocrystals of similar particle size irrespective of the pump power.⁵² In addition, for in situ multiplexed molecular mapping, nanoprobes with single-band emission, which can exclude crosstalk between different labeling signals, is the prerequisite for accurate molecular profiling.²⁸ Recently, a variety of novel techniques, including choosing the appropriate matrix, a doping ion with suitable energy level and coating with an organic dye with a certain absorption wavelength, are applied to realize single-band UC emission.

3.1 Choosing an appropriate matrix

The selection of an appropriate matrix is essential for obtaining Ln³⁺-doped materials with favorable optical properties and controllable emission profiles. Normally, the matrix requires similar lattice parameters with dopant ions for high solubility and low lattice phonon energy to minimize nonradiative loss and maximize the radiative emission. To actualize single-band UC emission, the matrix needs to benefit a certain CR or the depopulation of one metastable energy level so as to suppress the other emissions.

In fluoride materials, long lifetimes of the excited states are commonly observed because of the low phonon energies (ca. 350 cm⁻¹) of the crystal lattice, whereas conventional oxygen-based systems often exhibit large phonon energies above 500 cm⁻¹, so fluoride materials are better for obtaining high UC efficiency in comparison with oxides. However, the normally adopted fluorides, such as CaF₂ and NaYF₄, only exhibit cubic and hexagonal phases, whereas lanthanide oxyfluorides present more than three crystal phases, which is better for tuning the emission wavelengths.

3.1.1 Fluoride. Fluoride with low phonon energy has been widely investigated as a UC host and researchers in the bio-medical field benefit from its low toxicity to bio-tissues.^53–55 For most of the fluorides, such as lanthanide or alkaline fluorides, multi-band emissions of the doped Ln³⁺ activators were observed, whereas single-band emission can be actualized through changing the component or structure of fluorides.

Wang et al. reported the single red (∼660 nm) and NIR emission (∼800 nm) observed, respectively, in Yb/Er and Yb/Tm co-doped Na₃MF₇ (M = Zr, Hf) nanocrystals (NCs) with tetragonal phase, and the red upconversion intensity of the Yb/Er:Na₃ZrF₇ nanocrystals is about 5 times as high as that of the hexagonal Yb/Er:NaYF₄ ones with a similar crystal size.³² The authors propose that the Ln³⁺ clusters, which can shorten the distances between Ln³⁺ and then assist or enhance the CR (⁴F_7/2 + ⁴I_11/2 → 2 ⁴F_9/2) of Er³⁺/(¹G₄ + ³F₄ → ³H₄ + ³F₂) of Tm³⁺ ions, play a key role to induce this intrinsic single-band emission (Fig. 2). Yan et al. reported that the single-band red UC luminescence was achieved in Yb/Er co-doped K₃MF₇ NCs.⁵⁶ Their study suggest that the UC luminescence feature of Yb/Er:K₃MF₇ NCs is independent of the doping levels of Yb³⁺/Er³⁺ and the pump power of incident light, and the high-energy vibrational oscillators on the NCs surfaces are likely to result in a striking increase in the population of the ⁴F_9/2 level of Er³⁺, accounting for the spectrally pure emission. A similar result was reported in perovskite Yb/Er:KMgF₃ NCs as well by Zhang et al.⁵⁷ Actually, the reasons for the single-band emission in those matrixes given in the related papers need more strong evidence, such as the ion distribution in the crystal structure and the real-time electron population in different energy levels, which are significant for realizing this novel optical property in other hosts.

Fig. 2 (a) TEM micrograph and (b) SAED pattern of Yb/Er (20/0.5 mol%):Na₃ZrF₇ NCs; the inset of (a) shows a histogram of the particle size distribution, (c) HRTEM image of an individual NC and its FFT pattern, and (d) EDS spectrum of the product, showing the existence of Na, F, Zr and Yb signals (Cu signals come from the copper grid, and an Er signal is not detected owing to the low doping content), (e) and (f) are the UC emission spectra of Yb/Er(Tm) (20/0.5 mol%):Na₃ZrF₇ NCs under 980 nm laser excitation, (g) energy level diagrams of Er³⁺, Yb³⁺ and Tm³⁺ ions, showing the possible energy transfer mechanisms for the single-band red (NIR) UC emission of Er³⁺(Tm³⁺) activators in Na₃ZrF₇ host, under 980 nm laser excitation.³²

In addition, although the Mn²⁺-containing matrix, such as KMnF₃,⁵² can also give rise to single-band UC emission of Er³⁺/Ho³⁺/Tm³⁺, the mechanisms are quite different, which will be discussed in Section 3.2, because the optical properties and mechanism are similar to the Mn²⁺-doped situation.

Actually, for the widely investigated NaYF₄ matrix, a few methods were applied by several groups to tailor the emission profile of the doped Ln³⁺ activators approximate to single-band. Yan et al. reported that the red emission of Er³⁺ was greatly enhanced through engineering the local structure of Ln³⁺ activators in Na_xYF_3+x.⁵⁸ The local structure engineering was achieved through precisely tuning the composition of NCs, with different [Na]/[RE] ([F]/[RE]) ratios, and a significant difference in the red to green emission ratio, which varied from 1.9 to 71 and 1.6 to 116, was observed for Yb/Er:Na_xYF_3+x and Yb/Er:Na_xGdF_3+x NCs, respectively (Fig. 3). It can also be realized through doping Gd³⁺ ions at Sc³⁺ sites and attaching gold NCs at the surface in NaSc_0.8Er_0.02Yb_0.18F₄ NCs, which was reported by Rai et al.³¹ The intensity of NIR emission (∼802 nm) in Yb/Tm:NaYF₄ NCs with ultrasmall size (7–10 nm) was demonstrated to increase by up to 43 times and approach to single-band, along with an increase in the relative content of Yb³⁺ ions from 20% to 100% and a corresponding decrease in the Y³⁺ content from 80% to 0%, which was reported by Prasas et al.⁵⁹

Fig. 3 Schematic of Na_xYF_3+x and the corresponding UC emission spectra upon 980 nm excitation.⁵⁸

3.1.2 Lanthanide oxyfluoride. In comparison with fluorides, lanthanide oxyfluorides (LnOF), such as Lu₆O₅F₈, have a much higher molar density of Ln³⁺ ions in the matrix, which indicate that a larger amount of Ln luminescent micelles can be transformed from a single oxyfluoride NC. Moreover, the Ln–O bond in LnOF can be more easily loosened than the Ln–F bond via the protonation reaction with H⁺, which may facilitate Ln³⁺ ion extraction from the NC surface into micelles. In addition, they possess abundant structure types (i.e., rhombohedral SmSI-type R3̄m, tetragonal PbFCl-type P4/nmm and cubic fluorite-like Fm3̄m) and ever-present low symmetry of Ln³⁺ due to the distortion in their crystal lattices.⁶⁰ Hence, excellent and novel UC performance of activators was observed in LnOF matrixes such as Er:YbOF,⁶⁰ Yb/Er:Lu₅O₄F₇,^61,62 Yb/Er(Tm):ScOF⁶³ and Yb/Er:YOF@YOF⁶⁴ NMs.

Wang et al. reported that in Er³⁺ (2–12 mol%)-doped Vernier phase ytterbium oxyfluoride (V-YbOF) host, single-band red emission centered at 660 nm (⁴F_9/2 → ⁴I_15/2) was observed and the maximum UC output was achieved when the Er³⁺ doping concentration is about 4 mol% (Fig. 4).⁶⁰ The high concentration of Yb³⁺ in the host lattice greatly facilitated the energy-back-transfer (EBT) process from Er³⁺ to Yb³⁺ [⁴F_7/2 (Er³⁺) + ²F_7/2 (Yb³⁺) → ⁴I_11/2 (Er³⁺) + ²F_5/2 (Yb³⁺)] and the photon excitation to higher ²H_9/2, ²H_11/2 and ⁴S_3/2 levels of Er³⁺ was suppressed, which results in the absence of blue (²H_9/2 → ⁴I_15/2) and green (²H_11/2, ⁴S_3/2 → ⁴I_15/2) emissions. Sun et al. and Chen et al. reported that the UC spectrum of Lu₅O₄F₇:Er³⁺/Yb³⁺ nanoparticles can also be tailored into red single-band due to a large amount of nonradiative (⁴I_11/2 → ⁴I_13/2), which benefits red emission.^61,62 Gao et al. observed strong red single-band emission in Er³⁺/Yb³⁺:YOF@YOF NCs with a core@shell structure. Their study suggests that the YOF shell enhanced the red emission at ∼669 nm (∼18 times) and suppressed the green emission of Er³⁺ at ∼530 nm.⁶⁴

Fig. 4 Schematic of the orthorhombic Vernier phase structure of Yb₆O₅F₈ and the possible sites for Ln³⁺ doping: (a) the a × nb × c unit cell with n = 6 along the b-axis and (b) the assorted array of O²⁻ and F⁻ ions offers four different types of Yb³⁺ cation sites. Note that the O²⁻ and F⁻ anions located at 8e sites shown as green balls are of disordered distribution; (c) TEM image of Ln³⁺ doped V-YbOF nanoparticles. Inset shows the high resolution TEM image of an individual V-YbOF particle; (d) UC emission spectra of Er³⁺ doped V-YbOF samples under 980 nm excitation (∼3 W cm⁻²) and the emission intensity variations of Er³⁺-doped V-YbOF samples with the increase of Er³⁺ doping concentration (inset).⁶⁰

3.1.3 Oxide. Oxide not only serves as an important semiconductor but also a significant host for Ln³⁺ ions due to its high chemical stability. Recently, Zhang et al. reported that the single red emission was achieved in Yb/Er:Bi₂O₃ nanospheres; the intensity of the red emission achieves a maximum when the Yb³⁺ concentration reached 20%, whereas the green emission increases slowly but weaker than that without Yb³⁺ ions.⁶⁵ The authors propose two reasons for these UC properties: (1) the high Yb³⁺ ion concentration decreases the interatomic distance of Yb³⁺ and Er³⁺, which can efficiently accelerate the EBT process from Er³⁺ to Yb³⁺ ions, and this EBT process can suppress the population of Er³⁺ ions in levels ²H_11/2 and ⁴S_3/2 by depopulating the excited state ⁴F_7/2 (Er³⁺), which results in the decrease of green emission; (2) the intrinsic lifetime of the ⁴I_13/2 level being much longer than that of ⁴I_11/2, which makes the energy transfer of ²F_5/2 (Yb³⁺) + ⁴I_13/2 (Er³⁺) → ²F_7/2 (Yb³⁺) + ⁴F_9/2 (Er³⁺) more favorable than the ²F_5/2 (Yb³⁺) + ⁴I_11/2 (Er³⁺) → ²F_7/2 (Yb³⁺) + ⁴F_7/2 (Er³⁺), leading to the red emission (⁴F_9/2 → ⁴I_15/2) improving significantly. Similar results are also observed in Yb/Er:ZrO₂ (ref. 66) and Yb/Er:ZnO⁶⁷ NCs, which was reported by Song et al. and Xu et al., respectively. Although the UC emission was realized in part in oxides, there still exist several challenges, such as improving the synthetic procedure to prepare uniform NCs and controlling the structure (size, morphology and crystal structure) and enhancing the luminescent efficiency.

3.1.4 Other matrixes. Tian et al. reported that the single-band red and blue emissions were observed, respectively, in Yb³⁺/Ho³⁺- and Yb³⁺/Tm³⁺-doped P₂O₅–MgO₂–Sb₂O₃–MnO₂–AgO glasses.⁶⁸ For the Yb³⁺/Ho³⁺ situation, the multi-phonon relaxation probability from ⁵I₆ to ⁵I₇ in phosphate glass is very large (1.39 × 10⁶ s⁻¹), indicating that the ions in the ⁵I₆ state are rapidly relaxed to the ⁵I₇ state. The electron in ⁵I₇ state will transit to ⁵F₅ state after absorbing another 980 nm photon, and then give rise to single-band red emission. For the Yb³⁺/Tm³⁺ situation, the multi-phonon relaxation probability of ³H₅ and ³F_2,3 is also very large, which results in a great population in ³F₄ and ³H₄ states and then the ions transfer to ¹G₄ state through (²F_5/2 (Yb³⁺) + ³H₄ (Tm³⁺) → ²F_7/2 (Yb³⁺) + ¹G₄ (Tm³⁺)), from which the single-band blue emission was obtained.

The UC emissions of Er³⁺ ions normally contain green (²H_11/2, ⁴S_3/2 → ⁴I_15/2) and red (⁴F_9/2 → ⁴I_15/2) wavelengths (Fig. 1b); the ratio of green to red depends on the competition between ⁴F_7/2 → ²H_11/2/⁴S_3/2 and ⁴S_3/2 → ⁴F_9/2, ⁴I_13/2 → ⁴I_11/2 or ⁴F_7/2 + ⁴I_11/2 → 2 ⁴F_9/2. It is relatively easier to transfer most of the high energy photons to the lower ones at a certain condition and the single red emission can be realized; however, the green one originates from two different excited-state levels (²H_11/2, ⁴S_3/2), and it is unlikely to suppress all the CR and depopulation for the red emission under strong green luminescence, so the single-band green emission is difficult to be realized. However, pure green light containing ∼525 nm and ∼545 nm was achieved in Yb/Er:LaVO₄ (ref. 69) and Yb/Er(Ho):NaCe(MoO₄)₂ (ref. 70) phosphors. The CR ⁴F_7/2 + ⁴I_11/2 → 2 ⁴F_9/2 of Er³⁺ and ⁵F₃ + ⁵I₈ → ⁵F₅ + ⁵I₇ of Ho³⁺, which contribute to the red population, were suppressed due to the boundary effect of nanorods in Yb/Er:LaVO₄ and the different band edge absorption by adding EDTA into the Yb/Er(Ho):NaCe(MoO₄)₂ reaction system.

3.2 Doping ion with suitable energy level

It is well known that ion doping is an important route for imparting new and useful properties for numerous functional NMs. For luminescent materials, doping ions with suitable energy level can be used to realize single-band emission through changing the energy transfer processes or enhancing one certain CR possibility.

Mn²⁺ ions can be used as an exchange-energy transfer medium in Yb/Er-, Yb/Ho- and Yb/Tm-containing systems and it then actualizes single-band UC emission.^71–74 Zhao et al. reported that Yb/Er:NaYF₄ phase changed from hexagonal nanorods to small nanocubes with cubic phase after doping 30% Mn²⁺ ions.⁷¹ The emission profile altered from multi-band to single band and the emission color changed from green to red, which was ascribed to nonradiative energy transfer from the ²H_9/2 and ⁴S_3/2 levels of Er³⁺ to the ⁴T₁ level of Mn²⁺, followed by energy-back-transfer to the ⁴F_9/2 level of Er³⁺ (Fig. 5). Several Mn²⁺-containing matrixes, such as Yb/Er:MnF₂,⁷⁵ Yb/Er(Ho,Tm):KMnF₃ (ref. 52 and 76) and Yb/Er:NaMnF₃,^77,78 were also applied to realize single-band UC emission.

Fig. 5 (a) XRD patterns of NaYF₄ doped with 0, 3, 5 and 30 mol% Mn²⁺ ions. TEM micrographs of NaYF₄ NCs (b) without Mn²⁺ dopants and (c) doped with 30 mol% Mn²⁺ ions. (d) UC emission spectra of Yb/Er (18/2 mol%):NaYF₄ NCs doped with 0, 5 and 30 mol% Mn²⁺ ions; insets are luminescent images of the corresponding samples. (e) Schematic energy level diagram showing the possible UC mechanism of Mn²⁺-doped Yb/Er:NaYF₄ NCs. (f) Luminescence time traces of the 30 mol% Mn²⁺-doped Yb/Er:NaYF₄ NCs acquired with 200 ms time bins under continuous 980 nm laser illumination for more than 3 h, suggesting the durable photostability of the UC NCs. (g) Luminescent image of 30 mol% Mn²⁺-doped UC NCs dispersed in hexane.⁷¹

Ce³⁺ ions have appropriate energy levels to facilitate CR with Ho³⁺ ions, which was used to modify the emission profile of Ho³⁺ ions. Chen et al. reported the single-band red luminescence of Ho³⁺ upon excitation of 808 nm based on the elaborate combination of Nd³⁺ sensitization, Ce³⁺-assisted CR and the active-core@active-shell structure design.⁷⁹ They found that increasing Ce³⁺ dopants results in monotonic enhancement of red to green UC intensity ratio of Ho³⁺ and the CR processes involving Ce³⁺ and Ho³⁺, i.e., CR1: Ho³⁺: ⁵I₆ + Ce³⁺: ²F_5/2 → Ho³⁺: ⁵I₇ + Ce³⁺: ²F_7/2, CR2: Ho³⁺: ⁵S₂/⁵F₄ + Ce³⁺: ²F_5/2 → Ho³⁺: ⁵F₅ + Ce³⁺: ²F_7/2 and CR3: Ho³⁺: ⁵F₅ + Ce³⁺: ²F_5/2 → Ho³⁺: ⁵I₄ + Ce³⁺: ²F_7/2 were the main reason for the observed optical phenomena (Fig. 6). Hao et al. reported that a tunable UC multicolor output from green/yellow to red was achieved in a fixed Yb³⁺/Ho³⁺ composition in BaGdF₅ by doping Ce³⁺ under the excitation of 980 nm, and the emission spectrum approached single-band with Ce³⁺ concentration increased to 30%.⁸⁰

Fig. 6 (a) TEM and (b) HAADF-STEM micrographs of (a) 20Yb/1Ho/30Ce:NaGdF₄@20Yb/20Nd:NaYF₄ core–shell NCs. Insets of (a) show the SAED pattern of these core–shell NCs (bottom) and the enlarged core–shell NCs (top); (c) Ce³⁺ content-dependent UC emission spectra for Yb/Ho/Ce:NaGdF₄ core NCs; (d) the integrated UC intensity as well as the ratio of red to green emission versus Ce³⁺ content; (e) energy level diagrams of Ce³⁺, Ho³⁺, Yb³⁺, and Nd³⁺ as well as the proposed mechanisms for the achievement of pure red UC luminescence under 980 or 808 nm laser excitation in Yb/Ho/Ce:NaGdF₄@Yb/Nd:NaYF₄ core–shell NCs.⁷⁹

3.3 Coating organic dye with a certain absorption wavelength

Coating material with high reabsorption ability at a certain wavelength outside UCNMs to eliminate unwanted emission bands is also an efficient method to actualize single-band UC emission. Zhang et al. achieved single-band UC emission with different colors by coating the UCNCs with a screen layer containing an organic dye with a high molar absorption coefficient as a nanofilter to filter the unwanted emission bands.²⁸ To obtain green and blue single-band emission, nickel(II)phthalocyanine-tetrasulfonic acid tetrasodium salt (NPTAT), organic dyes with a maximum absorption wavelength of 657 nm, were added with tetraethyl orthosilicate (TEOS) to form NPTAT-doped silica nanofilters on the 20Yb/2Er:β-NaGdF₄@NaGdF₄@SiO₂ and 20Yb/0.2Tm:β-NaGdF₄@NaGdF₄@SiO₂ NCs to filter red emission bands, respectively (Fig. 7a–h) and to obtain the red single-band emission, coating pure SiO₂ layers and rhodamine B isothiocyanate outside 10Er:α-NaYbF₄@NaYF₄ NCs with strong red to green UC emission to filter the green emission (Fig. 7i–l). We believe that amounts of other absorbents will be applied in various Ln³⁺-doped systems to actualize single-band emission with different colors in the future.

Fig. 7 TEM images and size distributions of (a) 20Yb/2Er:β-NaGdF₄@NaGdF₄ NCs, (b) green emission sb-UCNPs (20Yb/2Er:β-NaGdF₄@NaGdF₄@SiO₂@NPTAT-doped SiO₂), (e) 20Yb/0.2Tm:β-NaGdF₄@NaGdF₄ NCs, (f) blue emission sb-UCNPs (20Yb/0.2Tm:β-NaGdF₄@NaGdF₄@SiO₂@NPTAT-doped SiO₂), (i) a-NaYbF₄:10% Er@NaYF₄ NCs and (j) red emission sb-UCNPs (NaYbF₄:10% Er@NaYF₄@SiO₂@rhodamine B isothiocyanate-doped SiO₂). UC photoluminescence spectra of the green, blue and red sb-UCNPs (d, h, and l) and original UCNCs measured in water and cyclohexane, respectively (c, g, and k) (insets: corresponding photoluminescent images of the colloidal solutions under continuous wavelength 980 nm laser excitation). Scale bars, 100 mm.²⁸

4 Conclusions and outlook

We summarized the research progress of single-band UC emission with different colors, including various routes and related mechanisms. To obtain spectra with single-band, several strategies have been investigated, such as choosing an appropriate matrix, a doping ion with suitable energy level and coating an organic dye with a fixed absorption wavelength. In general, the matrix accelerates a certain CR, the ion with suitable energy level alters the energy transfer processes and the organic dye absorbs unwanted emission bands. These materials with single-band emission have been widely employed as a platform for bio-imaging and therapeutic applications. Among the abovementioned three approaches, we believe choosing the appropriate matrix exhibits the highest UC efficiency, because a doping ion with suitable energy level will introduce extra energy transfer paths and coating with an organic dye with a fixed absorption wavelength will waste part of the absorbed energy. Although tremendous achievements have been made, there still exist many fundamental and practical issues to be solved in this field, which will provide great opportunities and challenges for related applications in the future.

First, more different single-band emission wavelengths with various output colors should be investigated in the future. Different wavelengths have different advantages in various applications. To date, most of the single-band emissions are obtained from Er³⁺ (∼540 nm, ∼650 nm), Ho³⁺ (∼650 nm) or Tm³⁺ (∼479 nm, ∼800 nm). The activators may be extended to other Ln³⁺ ions (such as Tb³⁺, Eu³⁺, Dy³⁺ and Sm³⁺) or transition metal ions (such as Mn²⁺ and Cr³⁺). Second, the single-band UC emission efficiency is very low for practical application. The existing methods for actualizing single-band emission are mainly through CR, different energy transfer processes or reabsorption to suppress the unwanted emission, which will result in an amount of energy loss. It is important to develop some superior methods for realizing single-band UC emission. Third, the mechanisms for single-band UC emission are still unclear. Although several single-band emissions are observed in different systems, the related mechanisms lack strong evidence, particularly concerning the energy transfer processes. More experiments and characterizations need to be carried out systematically.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (No. 51472225 and 51372235) and the Zhejiang Provincial Natural Science Foundation of China (LR14E020003).

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