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ArticleApril 12, 2018

Quenching Pathways in NaYF₄:Er³⁺,Yb³⁺ Upconversion Nanocrystals
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Freddy T. Rabouw
Freddy T. Rabouw
Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands
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http://orcid.org/0000-0002-4775-0859
P. Tim Prins
P. Tim Prins
Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands
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http://orcid.org/0000-0002-8258-0074
Pedro Villanueva-Delgado
Pedro Villanueva-Delgado
Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands
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Marieke Castelijns
Marieke Castelijns
Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands
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Robin G. Geitenbeek
Robin G. Geitenbeek
Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands
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Andries Meijerink*
Andries Meijerink
Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands
*E-mail: [email protected]
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http://orcid.org/0000-0003-3573-9289

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ACS Nano

Cite this: ACS Nano 2018, 12, 5, 4812–4823

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https://doi.org/10.1021/acsnano.8b01545

Published April 12, 2018

Abstract

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Lanthanide-doped upconversion (UC) phosphors absorb low-energy infrared light and convert it into higher-energy visible light. Despite over 10 years of development, it has not been possible to synthesize nanocrystals (NCs) with UC efficiencies on a par with what can be achieved in bulk materials. To guide the design and realization of more efficient UC NCs, a better understanding is necessary of the loss pathways competing with UC. Here we study the excited-state dynamics of the workhorse UC material β-NaYF₄ co-doped with Yb³⁺ and Er³⁺. For each of the energy levels involved in infrared-to-visible UC, we measure and model the competition between spontaneous emission, energy transfer between lanthanide ions, and other decay processes. An important quenching pathway is energy transfer to high-energy vibrations of solvent and/or ligand molecules surrounding the NCs, as evidenced by the effect of energy resonances between electronic transitions of the lanthanide ions and vibrations of the solvent molecules. We present a microscopic quantitative model for the quenching dynamics in UC NCs. It takes into account cross-relaxation at high lanthanide-doping concentration as well as Förster resonance energy transfer from lanthanide excited states to vibrational modes of molecules surrounding the UC NCs. Our model thereby provides insight in the inert-shell thickness required to prevent solvent quenching in NCs. Overall, the strongest contribution to reduced UC efficiencies in core–shell NCs comes from quenching of the near-infrared energy levels (Er³⁺: ⁴I_11/2 and Yb³⁺: ²F_5/2), which is likely due to vibrational coupling to OH^– defects incorporated in the NCs during synthesis.

Subjects

Keywords

Upconversion (UC) materials are unconventional color-conversion materials that can absorb low-energy and emit higher-energy photons. Examples include polycyclic aromatic hydrocarbons, (1,2) specially designed semiconductor-quantum-dot geometries, (3,4) and crystals doped with lanthanide ions. (5−7) Of particular interest have been β-NaREF₄ nanocrystals (NCs; with RE = Y, Gd, or Lu) doped with either Er³⁺ or Tm³⁺ and often co-doped with Yb³⁺ or other lanthanides to increase absorption or tune the emission wavelength. (8−22) Such NCs exhibit narrow-line emissions, and they are photochemically and colloidally stable, making them ideal for background-free biomedical imaging. Moreover, the small size of NCs enables their use in nanoscale designs to enhance the UC luminescence through sensitization (13) or plasmon enhancement. (23−29) However, the efficiency of UC luminescence is significantly lower for nanocrystalline doped NaREF₄ than for the corresponding bulk material, because of excited-state quenching by the NC surface and the environment. (10,11,16,19−21,30) The UC efficiency is particularly low in the smallest NCs. (11,14,20)

Efforts have been made to improve the low UC luminescence efficiency of doped NaREF₄ NCs. The common strategy to achieve higher efficiencies is to grow inert nonluminescent shells of undoped NaREF₄ (with RE = Y, Gd, or Lu) around the doped upconverting core. (11,16) This serves to spatially isolate the active luminescent core from the NC surface and the environment. While this general concept has been well established, a quantitative understanding of UC luminescence quenching in NCs is still lacking. UC in lanthanide-doped crystals is a complicated process, involving (at least) two absorption events, multiple energy-migration and energy-transfer steps, and eventually emission of a high-energy photon. Meanwhile, the efficiency is limited by undesired competing processes such as cross-relaxation, back-transfer, nonradiative decay, and, in the case of NCs, quenching by energy transfer to high-energy vibrations on the surface and/or in the environment of the NCs. (10,11,16,18,22,30) To understand UC luminescence from NCs and minimize quenching requires detailed and quantitative studies that unravel the competition between the various processes at play.

Here, we investigate the quenching mechanisms of the various energy levels involved in UC in NaYF₄ NCs co-doped with Er³⁺ and Yb³⁺, a commonly studied nanocrystalline upconverter showing green and red emissions upon near-infrared excitation. (10,13,14,16,18,20,22) The dynamics of the relevant excited states of Er³⁺ and Yb³⁺ are measured upon direct excitation in NCs with and without shell, with various dopant concentrations, and dispersed in a range of organic solvents with different refractive index and chemical nature. We observe a pronounced dependence of quenching on the chemical nature of the solvent of the NCs (aliphatic versus aromatic), pointing to Förster transfer to high-energy solvent vibrations as an important quenching mechanism. (31,32) Solvent quenching affects all energy levels of Er³⁺ involved in near-infrared-to-visible UC, but most notably the red-emitting ⁴F_9/2 level. The green emission from the ²H_11/2 and ⁴S_3/2 levels is quenched by cross-relaxation and energy migration at higher Er³⁺ concentrations, while the red emission is hardly affected. The near-infrared levels ⁴I_11/2 (Er³⁺) and ²F_5/2 (Yb³⁺) are quenched by coupling to the solvent, by coupling to OH-vibrations of hydroxyl ions incorporated in the crystal structure, and by energy migration to the NC surface. The efficiencies of all quenching pathways are suppressed if the refractive index of the solvent is increased, because this enhances the rates of photon emission but not energy-transfer (i.e., quenching) rates. (33,34) We develop an analytical model for the various quenching pathways, which matches our experimental results and can predict the emission dynamics and efficiencies for NCs of different (quasi-)spherical core–shell geometries and dopant concentrations.

Results

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Solvent-Dependent Decay Dynamics

We study the popular UC material β-NaYF₄ co-doped with Er³⁺ and Yb³⁺. (5,8) Upon excitation of Yb³⁺ in the near-infrared absorption band at 980 nm, this material emits green light at 520 and 540 nm and red light at 660 nm (Figure 1a). The conversion of low-energy near-infrared photons to higher-energy visible photons is made possible by successive energy-transfer steps from two (or more) Yb³⁺ ions to Er³⁺. Both the green and the red UC luminescence can arise after two energy-transfer steps followed by partial nonradiative relaxation (indicated in Figure 1b), although additional pathways involving three energy-transfer steps have been identified leading to red luminescence. (35−37) The energy-transfer processes relevant to UC compete with undesired processes such as (multi)phonon relaxation and cross-relaxation that reduce the UC emission. The spectral characteristics of nanocrystalline β-NaYF₄ phosphors with diameters down to sub-10 nm (refs (20and50)) are similar to those of bulk β-NaYF₄, but the emission efficiencies are significantly lower. (10,11,19−21,30) In this work we systematically study the energy-loss pathways in UC NCs (Figure 1c, Figure S1 and Table S1) that lead to these lower efficiencies, as a function of the dopant concentration, for core-only and core–shell geometries and for NCs dispersed in a range of solvents.

Figure 1. (a) UC emission spectrum of β-NaYF₄ nanocrystals co-doped with 2% Er³⁺ and 18% Yb³⁺ dispersed in cyclohexane, upon laser excitation in the Yb³⁺: ²F_7/2 → ²F_5/2 transition at 980 nm. The nanocrystals emit green photons from the ²H_11/2 (520 nm) and ⁴S_3/2 (540 nm) states and red photons (660 nm) from the ⁴F_9/2 state. (b) Upon excitation of Yb³⁺ at 980 nm (dark-red wavy arrows), two consecutive energy-transfer processes (dotted lines) excite an Er³⁺ ion to a high excited state. Subsequently, after one or more nonradiative decay steps (black arrows) Er³⁺ can emit a visible photon (red or green wavy arrows). Other, more complicated, sequences of processes also contribute to the population of visible-emitting states. (35−37) (c) Transmission electron micrograph of the core-only β-NaYF₄:0.1% Er³⁺ nanocrystals used in this work. Images of other nanocrystal samples and the extracted size distributions are presented in Figure S1 and Table S1. (d) Schematic of solvent quenching, where an excited state on a lanthanide center (here, Er³⁺) doped in a nanocrystal can decay by transferring energy to high-energy vibrations in the surrounding solvent. (e) The total rate of solvent quenching by dipole–dipole interaction depends strongly on the radial position of the lanthanide center in the nanocrystal, illustrated here for a nanocrystal of 10 nm radius without (red line) and with (blue line) a 3 nm-thick nonluminescent shell. (f) The range of solvents used in our experiments below, with different chemical nature (affecting the solvent-quenching rate; eq 2) and different refractive index (affecting the radiative decay rate; eq 3).

Previous studies have identified that the environment of lanthanide-doped NCs, most notably the solvent, affects luminescence quenching. (10,11,18,22,30) It is not always realized that such quenching is a form of Förster energy transfer, by dipole–dipole coupling of an electronic transition of a lanthanide dopant to vibrations of the solvent and ligand molecules (Figure 1d). (38) The rate of energy transfer by dipole–dipole coupling, γ_ET, scales with the inverse sixth power of the separation between the energy donor (the luminescent center) and the energy acceptor (the solvent vibration):

(1)

where r₀ and r are the positions of the donor and acceptor, respectively. The parameter C is the “energy-transfer strength” prefactor, the value of which depends on the energy match of transitions in the lanthanide center to the vibrational energies of the solvent (31,32,38) as well as on the oscillator strengths of the donor and acceptor transitions involved. (43) The total solvent-quenching rate Γ_Q experienced by a luminescent center is determined by its dipole–dipole coupling to all solvent vibrations surrounding the NC, which depends on the diameter of the NC and on the location of the center inside the NC. If we assume that the NC is spherical with an outer radius a_out, we obtain a simple expression by integrating eq 1 over all solvent molecules outside the NC:

(2)

where V_out is the volume occupied by solvent, r₀ = |r₀| is the radial position of the Er³⁺ center, and ρ is the density of solvent vibrations involved in quenching. Interestingly, the amount of quenching depends only on the geometry of the NC (outer radius a_out and radial position r₀ of the dopant) and on the “quenching density” ρC—the product of the density of vibrations and the energy-transfer strength; units of volume per time—that describes the interaction with the solvent. To avoid that the quenching rate Γ_Q becomes infinite at the surface of core-only NCs (r₀ = a_out in eq 2), we assume throughout this article that the “outer radius” a_out (beyond which volume is occupied by solvent) is 0.3 nm larger than the physical radius of the NC as determined by electron microscopy. This 0.3 nm separation can be justified if one realizes that this space of approximately one atom thickness is occupied by species contributing significantly less to vibrational quenching than the solvent does, for example, fluoride anions of the host crystal or headgroups of the ligands (in our experiments, carboxylate head groups of oleate). The solvent-quenching model could be further fine-tuned, at the cost of increased complexity, by considering that the NC can be covered by long-chain organic ligands (in our experiments below, oleic acid) that occupy most of the volume up to 1–1.5 nm distance from the NC surface. Here, we approximate the combination of ligand and solvent as one continuous medium that can be described with a single quenching density ρC. In Figure 1e we plot the solvent-quenching rate normalized to the quenching density Γ_Q/ρC. We see that the solvent-quenching rate differs by orders of magnitude between luminescent centers near the surface and those near the center of a core-only NC (red line). Growing a shell of 3 nm thickness can suppress the solvent-quenching rate by a factor 2–1000, depending on the location of a center in the NC (blue line). These pronounced effects of geometry on the quenching rate are manifestations of the strong distance dependence of dipole–dipole coupling. Our model can predict the distribution of solvent quenching rates for any spherical NC geometry (core-only or core–shell) if the quenching density ρC of a particular solvent to a particular dopant’s excited state is known.

In addition to inducing nonradiative quenching, the solvent—through its refractive index n—also affects the radiative decay rate Γ_rad of dopant centers in NCs: (33,34)

(3)

where Γ_rad^bulk is the radiative decay rate in a bulk sample, and n_NC is the refractive index of the NC material [here, n_NC = 1.48 (ref (39))]. This effect is the same for all luminescent centers inside a NC independent of their exact location, if the NC is (quasi-)spherical and much smaller than the emission wavelength. We measured the values of Γ_rad^bulk for each relevant level on a bulk microcrystalline sample (Figure S2). From these reference values we can calculate the radiative decay rate in any solvent, if we know its refractive index. Throughout this article we will use tabulated refractive indices in the visible, neglecting that they are in fact slightly lower (by at most 0.02 point) in the infrared where the excited states emit (partially). This translates into an acceptable error of <1% in the calculated radiative decay rates in NCs. The series of solvents used in our experiments (Figure 1f) covers a range of refractive indices from n = 1.375 (hexane) to n = 1.627 (carbon disulfide). This corresponds to a 40% variation in radiative decay rate (eq 3). Importantly, however, the solvents are different not only in terms of refractive index but also chemically. The chemical nature of the solvent affects its vibrational spectrum and thereby the dipole–dipole coupling to various Er³⁺ transitions. We will see below that resonances of the C–H stretch vibrational energy with transitions on the Er³⁺ ion affect the strength of solvent quenching. Hence, solvent effects manifest as changes in the radiative decay of lanthanide dopants in NCs (through n in eq 3) as well as in nonradiative quenching (through ρC in eq 2).

The total decay rate Γ_tot of a luminescent center in a nanocrystal is the sum of the rates of all available decay pathways:

(4)

where we have written explicitly that the radiative decay rate Γ_rad depends on the solvent refractive index n and that the solvent-quenching rate Γ_Q depends on the radial coordinate r₀ of the center in the NC and on the quenching density ρC of the solvent. For now we assume that Γ_rad and Γ_Q are the only decay pathways for the luminescent centers. Assuming that the dopant centers are homogeneously distributed within the NC (which may in fact be not exactly true), (40) we obtain the expected decay dynamics of an ensemble of doped NCs by integrating Γ_Q over the NC volume:

(5)

where a and V are the radius and volume of the luminescent core of the NC, respectively,

(6)

is the monoexponential radiative decay component (eq 3), and

(7)

is the multiexponential solvent-quenching component to the decay (eq 2).

As we will see below, the decay dynamics of luminescent centers in NCs are accurately described by eq 5 in many cases. In other cases, at higher doping concentrations, additional decay processes become active. One of these is energy migration, that is, hopping of the excited-state energy from center to center. The effect of energy migration on the excited-state dynamics will become apparent in our experimental data. This is however difficult to take into account quantitatively with our microscopic model, because the microscopic rate distributions (e.g., for solvent quenching) are correlated for neighboring ions involved in energy migration. In this work we will therefore discuss the effects of energy migration qualitatively. Another process relevant at high dopant concentrations is cross-relaxation, in which a luminescent center transfers part of its energy to neighbors. The effect of this on the excited-state dynamics of the donor center can be taken into account analytically. (41,42) We use that the rate of cross-relaxation energy transfer by dipole–dipole interaction depends on the inverse sixth power of the distance r_da between a donor center and an acceptor center. Including this pathway adds a cross-relaxation term Γ_X = ∑_aC_X/r_da⁶ (where prefactor C_X describes the “cross-relaxation strength”, and the summation runs over all nearby acceptor centers) to the total decay rate of a donor center (eq 4). To consider all possible local electronic environments of a luminescent center, that is, its separation from nearby acceptor centers, we have to take into account the complicated crystal structure of our β-NaYF₄ NCs that has two types of cation sites for rare-earth ions (Y³⁺ or luminescent dopants), one of which is only 50% occupied. (5) If we assume a homogeneous dopant distribution and neglect finite-size effects in the NC, (33) then the local electronic environment of a luminescent center (determining Γ_X) is uncorrelated with its radial position (determining Γ_Q). Under this assumption, the total decay including solvent quenching and cross-relaxation can be written as

(8)

where the multiexponential factor X(t) in β-NaYF₄ NCs reads as

(9)

where ϕ is the doping concentration of acceptor centers, that is, the fraction of nonluminescent host cations substituted, and the products run over the (next-)nearest-neighbor shells as determined by the host-crystal structure. This long expression with four products explicitly considers the two different rare-earth cation sites in β-NaYF₄, in terms of the neighborlist r_i,n_i (refs (33,41,and42)) for like sites and r_i^*,n_i^* for unlike sites. In some of our NCs studied below, cross-relaxation takes place with both Er³⁺ and Yb³⁺ dopants as energy acceptors. The total decay function for this situation becomes

(10)

where each factor X_i(t) (i = Er, Yb) is a function as in eq 9 with ϕ = ϕ_i and C_X = C_X,i. Here we neglect correlations in the distribution of cross-relaxation rates to Er³⁺ neighbors and those to Yb³⁺ neighbors, which is justified at relatively low doping concentrations. Such correlations become important at the highest doping concentrations, because for any one excited dopant center, the cross-relaxation rates are determined by the occupation of nearby cation sites, which can each contain (apart from an inert Y³⁺ ion) an Yb³⁺ acceptor, or an Er³⁺ acceptor, but not both at the same time.

We note that all nonradiative decay processes considered here—solvent quenching, cross-relaxation, and energy migration—are manifestations of Förster resonance energy transfer by dipole–dipole coupling. For solvent quenching, an excited lanthanide ion in the NC is the energy donor and a vibrational mode in a solvent molecule is the energy acceptor, while for cross-relaxation and energy migration the donor and acceptor are both lanthanide ions. Dipole–dipole coupling has a strong distance dependence scaling with the donor–acceptor distance to the inverse sixth power. This, combined with the random distribution of luminescent dopants in a (nano)crystal, determines the multiexponential nature of the decay dynamics. The dipole–dipole coupling rate also depends on the spectral overlap between the donor transition (i.e., its emission spectrum) and the acceptor transition (i.e., its absorption spectrum), as we will discuss below. Importantly, the efficiency of dipole–dipole coupling—the rate of coupling compared to other decay processes of the donor, such as radiative decay—depends on the oscillator strength of the acceptor transition. (43) Since f–f transitions of lanthanide dopants are parity-forbidden, the dipole–dipole coupling to lanthanide acceptors is weak and therefore short-range (Å–nm). This is why cross-relaxation and energy migration are negligible except at higher doping concentrations, when the average distance between dopants is of the order of a nm or less. Many vibrational transitions of organic molecules, on the other hand, are dipole-allowed. As a result, the coupling of a luminescent dopant in a NC to a molecule of the surrounding solvent can be significant, even if the separations are as large as 1–10 nm.

Solvent Quenching of the Visible Emissions

We focus first on the dynamics of the green-emitting [⁴S_3/2; in thermal equilibrium with ²H_11/2 (ref (44)); Figure 2a] and red-emitting (⁴F_9/2; Figure 2b) levels in NaYF₄ NCs doped with 0.1% Er³⁺. In these NCs the interactions between Er³⁺ centers, for example, cross-relaxation or energy migration, are expected to be negligible because the average ion–ion separation is large at low dopant concentrations. The decay dynamics of these levels are multiexponential (Figure S3), signifying that nonradiative decay rates vary among the Er³⁺ centers as expected in the case of solvent quenching (Figure 1e). As a first simple analysis, we extract the average excited-state decay rates ⟨Γ_tot⟩ of the green-emitting (Figure 2c) and red-emitting (Figure 2d) levels from single-exponential fits, comparing core-only NCs (filled circles) and core–shell NCs (open circles) in different solvents (colors) to bulk material (black cross). The decay rates in core-only and core–shell NCs (circles) are faster than the decay rate in bulk (cross) for both levels and for all solvents, confirming the presence of nonradiative quenching pathways in NCs. Indeed, the energy gap from both the green- and red-emitting level to the next-lower-energy level is approximately 3000 cm^–1 (Figure 2a,b), which can be bridged by coupling to vibrations in organic molecules, such as the C–H stretch vibration. From the fitted decay rates ⟨Γ_tot⟩ and eq 3 we can estimate the average quantum yield η ≈ Γ_rad/⟨Γ_tot⟩ of the Er³⁺ excited states in NCs, as indicated by the gray “iso-quantum-yield” contours in Figure 2c,d. (34) The quantum yields are approximately 55% (75%) for the green-emitting levels in core-only (core–shell) NCs and 15–40% (30–60%) for the red-emitting level in core-only (core–shell) NCs. Clearly, the green emission is quenched less than the red emission. Furthermore, the growth of a nonluminescent shell suppresses quenching, as expected from Figure 1e. (11,19,20)

Figure 2. (a) Energy-level scheme of Er³⁺ with the green emission from the ⁴S_3/2 level (in thermal equilibrium with ²H_11/2; ref (44)) indicated as a green arrow and the nonradiative transition to the ⁴F_9/2 level as a black dashed arrow. (b) Same for the red emission from the ⁴F_9/2 level and the competing nonradiative transition to the ⁴I_9/2 level. (c) Average excited-state decay rates ⟨Γ_tot⟩ of the green-emitting level in NaYF₄:0.1% Er³⁺ NCs dispersed in different solvents (different colors; see Figure 1f), compared to the decay rate in bulk NaYF₄:0.1% Er³⁺ (black cross). Filled circles represent core-only NCs, open circles core–shell NCs. Gray lines are iso-quantum-yield contours plotting η ≈ Γ_rad(n)/⟨Γ_tot⟩ with Γ_rad(n) according to eq 3. (d) Same, but for the red-emitting level. (e) The emission line shapes of the ⁴S_3/2 → ⁴F_9/2 (green) and the ⁴F_9/2 → ⁴I_9/2 (red) transitions of Er³⁺, which are responsible for nonradiative decay of the green- and red-emitting levels (see panel a), estimated based on the data from ref (45). (f) Infrared absorption spectra of hexane (red), octane (orange), cyclohexane (yellow), chloroform (green), toluene (cyan), chlorobenzene (blue), and o-dichlorobenzene (purple), taken from the SDBS database of the National Institute of Advanced Industrial Science and Technology (AIST), Japan. Dashed lines indicate the approximate spectral position of the C-H stretch vibrational energy in aliphatic (∼2930 cm^–1) and aromatic (∼3070 cm^–1) molecules. (g) Photoluminescence decay curves of the red-emitting ⁴F_9/2 level upon direct excitation in bulk NaYF₄:0.1% Er³⁺ (gray), compared to core–shell NCs (dark cyan) and core-only NCs (light cyan) of the same composition dispersed in toluene. The inset shows a zoom-in of the first 0.3 ms. Solid lines are fits to the solvent-quenching model (eq 5). (h) Same, but comparing bulk to NCs dispersed in cyclohexane.

If the Er³⁺ excited states in NCs underwent some quenching mechanism independent of the surrounding solvent, we would observe decay rates Γ_tot(n) = Γ_rad(n) + Γ_nr (with a constant nonradiative component Γ_nr) that steadily increase with increasing solvent refractive index n. More precisely, the dependence would be approximately linear for refractive indices close to the bulk value (from a series expansion of eq 3):

(11)

if Γ_nr is not affected by the nature of the solvent. For the green-emitting level we observe this approximately linear increase in decay rate with refractive index (Figure 2c), but the decay rate in chloroform (green) is noticeably slower. For the red-emitting level the trend in decay rate as described by eq 11 is not apparent (Figure 2d). Instead, the decay rates in the aliphatic solvents [hexane (red), octane (orange), cyclohexane (yellow)] are nearly a factor of 2 faster than in the other solvents and even faster than in water. (18) In fact the difference between the organic solvents considered here is so large that quenching of the red emission in core–shell NCs dispersed in aliphatic solvents is similar to quenching of core-only NCs in the aromatic solvents.

We can qualitatively understand this difference in quenching rate between aliphatic and aromatic solvents by considering their vibrational energy spectra. The highest-energy vibrational modes in hydrocarbon molecules are CH-stretch modes with energies around 3000 cm^–1. This nearly matches the energies of the ⁴S_3/2 → ⁴F_9/2 and ⁴F_9/2 → ⁴I_9/2 transitions of Er³⁺ (Figure 2e), (45) which are involved in the nonradiative decay of the green- and red-emitting levels. Since we interpret solvent quenching as dipole–dipole coupling between the excited Er³⁺ ion and solvent vibrations, the coupling strength should scale with the oscillator strength of the vibrational mode, the oscillator strength of the relevant Er³⁺ transition, and its spectral overlap of the vibrational mode with the Er³⁺ transition. (43) Indeed, the oscillator strength of the ⁴F_9/2 → ⁴I_9/2 transition is approximately 5× larger than that of the ⁴S_3/2 → ⁴F_9/2 transition [2.79 × 10^–7versus 6.32 × 10^–8 (ref (45))], consistent with the 4–10× faster solvent quenching (depending on the solvent; Figure 2c,d and Figure S3). More importantly, the significantly stronger quenching of the red emission in aliphatic solvents than in aromatic solvents is explained by the energy resonance of the ⁴F_9/2 → ⁴I_9/2 transition with the CH-stretch vibrational spectra of the solvents (Figure 2f). While the aliphatic solvents (CH-stretch at ∼2930 cm^–1) have good spectral overlap with the ⁴F_9/2 → ⁴I_9/2 transition resulting in strong quenching, the aromatic solvents (CH-stretch at ∼3070 cm^–1) have less overlap resulting in weaker quenching.

Based on these clear indications of solvent-related quenching confirming results from previous studies, (10,11,18,22,30) we use the solvent-quenching model introduced above (eq 5) to quantify the quenching behavior in the different solvents. As illustrative examples, we analyze the decay dynamics of the red emission from NaYF₄:0.1% Er³⁺ NCs dispersed in aromatic toluene (Figure 2g) and in aliphatic hexane (Figure 2h). Figure S3 shows the results for other solvents and for the green emission. We compare core-only NCs (lighter colors) to core–shell NCs (darker colors) and bulk material (gray). The decay dynamics in NCs are clearly multiexponential, while those in the bulk material are not. The solvent-quenching model (solid lines) matches this multiexponential behavior very well using the quenching density ρC as the only fit parameter. More precisely, the reduced χ² parameters representing the fit quality are 1.22 ± 0.08 for green emission from core-only NCs, 1.08 ± 0.05 for green emission in core–shell NCs, 1.56 ± 0.19 for red emission from core-only NCs, and 1.26 ± 0.16 for red emission in core–shell NCs (mean ± standard deviation over eight solvents; Figure S3). The solvent-quenching model captures the effect of shell growth particularly well, (11,19,20) as evidenced by the very similar values for ρC fitted on the core-only and core–shell dynamics (Figure S3d,h). This means that, if the quenching density ρC is known for a particular dopant excited state in a particular solvent, the model can predict the solvent-quenching behavior for any (quasi-)spherical core–shell NCs, or even—with adaptations—for other shapes. (46) For example, the model predicts that a 9 nm inert shell is required to suppress solvent quenching of the green luminescence from 20 nm-diameter NCs in hexane and achieve higher than 95% luminescence quantum yield (at low doping concentration, in the limit of negligible cross-relaxation), while it requires a 20 nm shell to achieve the same for the red luminescence (Figure 5).

Ion–Ion Interactions at Higher Doping Concentrations

While NCs doped at low concentrations of 0.1% are ideal to study the fundamentals of solvent quenching, NCs must be doped at higher concentrations to achieve optimal UC efficiencies. (47) The average dopant–dopant separations are shorter, thus enhancing the rates and efficiencies of a variety of energy-transfer processes. These include the various energy-transfer processes that are necessary to achieve UC, but also undesired energy-loss pathways such as cross-relaxation and energy migration to the NC surface. The UC brightness and efficiency are determined by a subtle balance between the impact of these different energy-transfer processes.

Figure 3a shows the average decay rate (from a simple single-exponential fit to the dynamics) of the green emitting level in NCs with different doping concentrations: 0.1% Er³⁺ (left), 2% Er³⁺ (middle), and 2% Er³⁺ + 18% Yb³⁺ (right). The decay rates increase significantly going from the low-doped sample to the higher doping concentrations, both in NCs (colored bars) and in the bulk material (white line). This is mostly the result of cross-relaxation, which occurs between dopant centers at typical separations of 1 nm and closer (42) and is therefore similar in 20 nm-diameter NCs to bulk material. However, we also see that the absolute difference in decay rate between core-only (dark-colored bars) and core–shell NCs (light-colored bars) increases with increasing Er³⁺ concentration. This signifies that the surface-related contribution to the total decay becomes stronger. We ascribe this to energy migration from Er³⁺ to Er³⁺ ion within the NC, which allows the excited-state energy to reach the NC surface where it is quenched rapidly by solvent vibrations (Figure 3e). Migration becomes stronger at higher Er³⁺ concentrations (compare 0.1% Er³⁺ and 2% Er³⁺), but its effect on the decay of the green luminescence is minimized when the energy can no longer reach the surface (compare core-only and core–shell).

Figure 3. (a) The average decay rate of the green-emitting level ⁴S_3/2 for core–shell NCs (dark-colored bars) and core-only NCs (light-colors bars) with different doping concentrations (as indicated) and dispersed in different solvents (different colors), obtained from a single-exponential fit to the decay dynamics. White horizontal lines indicate the corresponding average decay rate in the bulk material. (b) Same for the red-emitting level ⁴F_9/2. (c) Possible cross-relaxation pathways from the green-emitting levels of Er³⁺ to neighboring Er³⁺ or Yb³⁺ ions in the ground state. (d) Same for the red-emitting level of Er³⁺, where all of the possible cross-relaxation pathways exhibit a large energy mismatch between the donor and acceptor transition. (e) Schematic representation of energy migration (i.e., hopping) among Er³⁺ ions to the NC surface. (f) Photoluminescence decay curves of the green Er³⁺ emission in core–shell NCs dispersed in toluene, with doping concentrations of 0.1% Er³⁺ (black), 2% Er³⁺ (blue), and 2% Er³⁺ + 18% Yb³⁺ (green). Solid lines are fits to our model including solvent quenching and cross-relaxation (eqs 8 and 10). The inset shows a zoom-in on the first 0.2 ms. (g) Same for the green emission from core-only NCs. (h) Same for the red emission from core-only NCs, doped with 0.1% Er³⁺ (black), 2% Er³⁺ (yellow), and 2% Er³⁺ + 18% Yb³⁺ (red).

Surprisingly, the decay dynamics of the red emission do not accelerate with increasing dopant concentrations, neither in NaYF₄ NCs nor in the bulk material (Figure 3b). Hence, while the green-emitting levels are quenched by cross-relaxation and energy migration, the red-emitting level is not. In fact, the quenching seems to decrease for the highest dopant concentrations, but this is an artifact due to the slightly larger sizes of our highly doped NCs (21.6 nm versus ∼19 nm diameter for the lower concentrations; Table S1), resulting in reduced solvent quenching. We can understand why cross-relaxation has a stronger effect on the green-emitting level than on the red-emitting level from the energy-level structures of Er³⁺ and Yb³⁺ (Figure 3c,d). Near-perfect energy matching is possible for cross-relaxation from the green-emitting levels of Er³⁺ to neighboring Er³⁺ ions in the ground state (blue and yellow arrows in Figure 3c), and a small mismatch exists for cross-relaxation to a neighboring ground-state Yb³⁺ ion (blue arrows). In contrast, possible cross-relaxation pathways of the red-emitting level of Er³⁺ all exhibit a larger energy mismatch (Figure 3d) and should therefore be significantly less efficient. This is consistent with previous studies on bulk NaYF₄ crystals doped with Er³⁺. (6)

To more quantitatively analyze the quenching dynamics in NaYF₄ NCs doped with high concentrations of Er³⁺ and Yb³⁺, we fit the photoluminescence decay dynamics to our full model including radiative decay, solvent quenching, and cross-relaxation (eq 8 for NCs doped with only Er³⁺ or eq 10 for NCs co-doped with Er³⁺ and Yb³⁺). Figure 3f shows the decay of the green emission in core–shell NCs dispersed in toluene and doped with 0.1% Er³⁺ (black), 2% Er³⁺ (blue), or co-doped with 2% Er³⁺ and 18% Yb³⁺ (green). In modeling the experimental data, we introduce only two new fit parameters, the cross-relaxation strengths C_Er and C_Yb, while keeping the other parameters fixed based on the solvent-quenching analysis above (see Figure 2). Our model (solid lines) matches the experimental data very well using fit parameters C_Er = 9.8 × 10⁵ Å⁶ ms^–1 and C_Yb = 3.2 × 10³ Å⁶ ms^–1 (see Figure S4 for fit results for the other solvents). This corresponds to a critical Förster radius

of 9.2 Å for Er³⁺–Er³⁺ cross-relaxation, a typical value for Förster transfer to a lanthanide acceptor. (42,51) For Er³⁺–Yb³⁺ cross-relaxation we obtain 3.5 Å, somewhat shorter because of the energy mismatch for this pathway (Figure 3c). Alternatively, we can calculate a rate of 308 ms^–1 for cross-relaxation from the green-emitting levels (²H_11/2 and ⁴S_3/2) to a nearest-neighbor Er³⁺ ion or 1.0 ms^–1 to a nearest-neighbor Yb³⁺ ion, compared to a radiative decay rate of 1.6 ms^–1 for NCs in toluene. Figure S5 confirms that quenching of the green emission at high Er³⁺ concentration is mainly due to cross-relaxation, not for example energy migration to the surface, (21) because we can perfectly reproduce the experimental decay curves from bulk NaYF₄ doped with high Er³⁺ concentration assuming only radiative decay and cross-relaxation, with all parameters fixed. This also confirms that the distribution of donor–acceptor distances, which determines the cross-relaxation rates, in our NCs is similar to bulk material. Hence, if our NCs show deviations from a statistical distribution of dopant ions as reported by Dong et al., (40) these are not strong enough to affect the interactions between dopants significantly.

The effect of concentration quenching due to energy migration becomes apparent from the green emission dynamics from core-only NCs (Figure 3g). Our model, using the fit parameters for solvent quenching and cross-relaxation we have determined before, matches the emission dynamics shortly after the laser pulse well (inset). At later times (t > 0.2 ms) the experimental decay from NCs with high Er³⁺ doping (blue and green) is clearly faster than the model predicts. This is the result of energy migration, which provides a decay pathway for the excited Er³⁺ dopants in the NC center (Figure 3e) that would otherwise decay slowly. Energy migration from Er³⁺ to Er³⁺ can occur also in core–shell NCs. However, the effect on the emission dynamics is less significant (Figure 3f), because the undoped shell prevents migration to the NC surface. This means that our model of solvent quenching plus cross-relaxation can accurately predict the dynamics and quantum yield of green Er³⁺ emission at high dopant concentrations in core–shell NCs, but will overestimate the green-emission quantum yield in core-only NCs. Modeling the effect of energy migration quantitatively is beyond the scope of this article, but can be an interesting follow-up work.

The decay dynamics of the red emission from core-only NCs at high dopant concentrations (Figure 3h) are accurately predicted by the solvent-quenching model (eq 5; quenching density ρC fixed) without introducing cross-relaxation as an additional decay pathway. A small deviation between experiment and model at late times indicates that some concentration quenching and energy migration takes place in the red (i.e., among ⁴F_9/2 levels), but weaker than in the green (i.e., among ²H_11/2 and ⁴S_3/2 levels). This modeling of the red emission dynamics is consistent with our earlier conclusion that the red emission is hardly affected by cross-relaxation or energy migration.

Quenching of the Infrared Emissions

We now turn our attention to the near-infrared energy levels of Er³⁺ (⁴I_11/2) and Yb³⁺ (²F_5/2). Figure 4a shows the average decay rate (from a simple single-exponential fit) of the near-infrared emission (∼1000 nm) from the Er³⁺⁴I_11/2 level in NCs doped with 2% Er³⁺. The decay rates, depending on the solvent, are approximately 1 ms^–1 for core–shell NCs (open circles) and 6 ms^–1 for core-only NCs (filled circles), compared to 0.13 ms^–1 in the bulk material (at 0.1% Er³⁺; Figure S2). This means that the ⁴I_11/2 level is strongly quenched in NCs, to ∼10% quantum yield for core–shell NCs and ∼2% quantum yield for core-only NCs. Quenching by cross-relaxation to neighboring Er³⁺ ions can be excluded, because the energy-level structure of Er³⁺ makes energy-conserving cross-relaxation pathways impossible (see also Figure S6). The quenching of core-only NCs is somewhat dependent on the chemical nature of the solvent, with the aliphatic solvents showing slightly faster decay rates and chloroform showing a slower decay rate. However, for core–shell NCs the linear refractive index dependence of decay rates indicates a solvent-independent nonradiative component (eq 11). Moreover, the solvent-quenching model fails to reproduce the decay dynamics of the Er³⁺ near-infrared emission in core-only and core–shell NCs with a single value for the quenching density (Figure 4b). We propose therefore that the major contributor to quenching of the near-infrared-emitting Er³⁺ level, especially in core–shell structures, is energy transfer to OH-vibrations. Indeed, the vibrational energy of approximately 3500 cm^–1 precisely matches the ⁴I_11/2–⁴I_13/2 energy gap, thus explaining why quenching is strong. OH^– ions likely substitute some native F^– sites of the NC during the synthesis, where NaOH is used as a Na⁺ source. Indeed, preventing the inclusion of oxygen in NaYF₄ crystals is notoriously difficult. (5) New synthesis procedures (20) using Na-oleate instead of NaOH as Na⁺ source can potentially prevent this and thus suppress quenching of the Er³⁺⁴I_11/2 level.

Figure 4. (a) Average decay rates ⟨Γ_tot⟩ of the near-infrared-emitting ⁴I_11/2 level of Er³⁺ in core-only NCs (filled circles) and core–shell NaYF₄ NCs (open circles) doped with 2% Er³⁺ and dispersed in different solvents, as determined from single-exponential fits. Gray lines are iso-quantum-yield contours plotting η ≈ Γ_rad(n)/⟨Γ_tot⟩, based on the bulk decay rate (black cross). (b) Photoluminescence decay curves of the near-infrared emission from core-only (blue) and core–shell (red) NaYF₄:2% Er³⁺ NCs dispersed in toluene. Inset is a zoom-in. Solid lines show an attempt to fit our solvent-quenching model (eq 5) to the experiments using one value for ρC. (c) Same as in (a), but for the ²F_5/2 level of Yb³⁺. Filled symbols represent core-only NCs, open symbols core–shell NCs. The NCs have dopant concentrations of 0.1% Yb³⁺ (squares), 18% Yb³⁺ (triangles), or 2% Er³⁺ plus 18% Yb³⁺(diamonds). The Er³⁺ and Yb³⁺ emissions cannot be spectrally distinguished for the co-doped NCs, so the recorded signal will contain contributions from both. The iso-quantum-yield contours are based on the bulk decay rate of Yb³⁺ (black cross). (d) A cartoon of rapid energy transfer and energy back-transfer between Er³⁺ and Yb³⁺ centers. (e) Photoluminescence decay curves of the near-infrared Yb³⁺ emission in NCs doped with 0.1% Yb³⁺ (red), 18% Yb³⁺ (green), or co-doped with 2% Er³⁺ and 18% Yb³⁺ (blue). The signal recorded from the co-doped NCs consists for a small part of ⁴I_11/2 emission from Er³⁺ ions. This constitutes an estimated <5% of the total emission, based on the elemental ratio 2/18 and the ratio of radiative decay rates (Figure S2).

Figure 4c shows the average decay rate of the near-infrared emission from Yb³⁺ in NCs doped with various dopant concentrations, comparing core-only to core–shell NCs. NCs doped with 0.1% Yb³⁺, with either core-only or core–shell geometry, show mainly radiative decay with a rate close to that in the bulk material. We conclude from this that solvent quenching by dipole–dipole coupling is negligible for the ²F_5/2 excited state, and we can therefore not use the solvent-quenching model. This was to be expected, because Yb³⁺ is effectively a two-level system with an energy gap of approximately 10,000 cm^–1, that is, over three times the highest vibrational energy in hydrocarbon molecules. At higher doping concentrations the Yb³⁺ decay is faster and strongly dependent on the NC geometry (core-only versus core–shell). Clearly, the Yb³⁺ luminescence is quenched by some pathway involving the NC surface that becomes more efficient at higher doping concentration. Undercoordinated Yb³⁺ centers at the NC surface could be possible quenching sites, explaining the strong concentration dependence of quenching (compare core-only NCs with 0.1% and 18% Yb³⁺). Higher Yb³⁺ concentrations would simultaneously increase the number of such defect sites per NC and enable energy migration toward them. Co-doping Er³⁺ and Yb³⁺ is likely to lead to rapid energy transfer and energy back-transfer between the nearly resonant Yb³⁺:²F_5/2 and Er³⁺:⁴I_11/2 levels (Figure 4d). As a result, the effective energy migration is enhanced, and the strong quenching pathways of Er³⁺ (likely by coupling to OH-vibrations; see above) become available to the Yb³⁺ excited state. This explains why the decay rate of near-infrared excited states in NCs co-doped with 2% Er³⁺ plus 18% Yb³⁺ is enhanced by a factor 2 compared to those doped with only 18% Yb³⁺ (Figure 4c). The luminescence decay curves of the near-infrared emissions at high dopant concentrations are close to single-exponential (Figure 4e). This confirms that fast migration averages out the inhomogeneities in decay pathways between the luminescent centers (e.g., those near the NC surface versus those in the center), resulting in one “effective” decay rate for all centers. (43)

Discussion

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In Figure 5 we summarize the quantum yields of the various emitting levels involved in UC by the Er³⁺–Yb³⁺ couple in NaYF₄ NCs, as a function of shell thickness, doping concentration, and solvent. The data points represent experimental quantum yields determined by integrating the experimental photoluminescence decay curves: (34)

(12)

where I(0) is the photoluminescence intensity at t = 0 and Γ_rad is the radiative decay rate of the emitting level, estimated from the decay rate in bulk NaYF₄ (Figure S2) and correcting for solvent refractive index (eq 3). Similarly, the lines are predictions of our model including solvent quenching and cross-relaxation, obtained by integrating the theoretical decay curves (eq 5 or 10) for UC NCs with a core diameter of 20 nm. The model reproduces the increase in quantum yield with increasing shell thickness (open versus filled symbols) and with increasing solvent refractive index [from hexane (red) to chlorobenzene (blue) to carbon disulfide (gray)]. It predicts that, because of strong solvent quenching, the quantum yield of the red luminescence can be further increased with even thicker shells than we used in the experiments presented here (requiring up to 20 nm thickness for 95% luminescence quantum yield, depending on the solvent). For the green Er³⁺ emission, our model reproduces the dependence on dopant concentration [0.1% Er³⁺ (circles) versus 2% Er³⁺ plus 18% Yb³⁺ (squares)]. The red-emission quantum yield does not show a significant dependence on doping concentration, because it is not quenched by cross-relaxation or energy migration. On the other hand, strong quenching due to cross-relaxation causes the quantum yield of the green emission in high-doping NCs to saturate at approximately 20% for shells >3 nm. Qualitatively, it is clear that the near-infrared Yb³⁺ emission becomes more efficient with increasing shell thickness, with increasing solvent refractive index, and with lower doping concentration. However, we cannot model this quantitatively because this emission is quenched by other mechanisms than dipole–dipole coupling to solvent vibrations.

Figure 5. (a–c) The quantum yields of the various emitting levels in NaYF₄:Er³⁺,Yb³⁺ UC NCs as a function of shell thickness. We consider (a) the green Er³⁺ emission from the ⁴S_3/2 level, (b) the red Er³⁺ emission from the ⁴F_9/2 level, and (c) near-infrared Yb³⁺ emission from the ²F_5/2 level. Data points are obtained by integrating experimental photoluminescence decay curves (eq 12) from core-only NCs (filled symbols) or core–shell NCs (open symbols). We compare NCs dispersed in hexane (red), chlorobenzene (blue), and carbon disulfide (gray) and NCs doped with low concentration of the active center (0.1%; circles) to NCs co-doped with 2% Er³⁺ and 18% Yb³⁺ (squares). Lines are predictions from our solvent-quenching model for the green- and red-emitting levels at low doping (solid lines) and high doping (dashed lines; only distinct for the green emission). (d) Simplified model of UC with a ground state (0), an intermediate excited state (NIR), and a highest excited state (VIS). Arrows indicate excitation, decay, and energy-transfer processes.

To get an idea of what our results imply for the efficiency of UC luminescence from NaYF₄ NCs, we consider a simplified three-level model for the UC process (Figure 5d). (48) In reality Er³⁺–Yb³⁺-based UC is of course much more complex, involving more energy levels and multiple energy-transfer pathways depending on excitation power. (49) Our simplified model consists of a ground state ‘0’, a first excited state “NIR” in the near-infrared, and a second excited state “VIS” that can emit visible photons. Excitation into the intermediate level and energy-transfer UC to the highest excited state, with rate constants Γ_exc and Γ_UC, respectively, in combination with radiative and nonradiative decay of the states NIR and VIS lead to the following set of rate equations:

(13)

(14)

where N_NIR and N_VIS are the populations in the intermediate and in the highest excited state, and Γ_i^tot denotes the total decay rate (radiative + nonradiative) of state i. The visible UC luminescence is proportional to I_UC ∝ Γ_VIS^radN_VIS. Solving the rate equations for steady-state conditions yields for the UC luminescence to second order in Γ_exc/Γ_NIR^tot, that is, in the limit of weak excitation, that

(15)

We see that the UC intensity is quadratically dependent on the quantum yield η_NIR of the intermediate near-infrared-emitting level and linearly on the quantum yield η_VIS of the highest visible-emitting level. We thus conclude that the major contribution to low UC efficiencies in NCs compared to bulk material (10,11,19,20) comes from losses in the near-infrared, in line with the interpretation of Hossan et al., (49) not only because the near-infrared-emitting levels are quenched strongly at high doping concentrations (Figure 5c) but also because losses in the infrared affect the overall UC intensity more strongly than losses in the visible. Shell growth improves the quantum yields in the near-infrared at high doping concentration by as much as a factor ∼10 in our experiments (Figure 5c), which translates into a ∼100 increase in UC intensity according to eq 15. Additionally, the shell improves the UC intensity further by a smaller factor ∼2 by increasing the quantum yield in the visible (depending on the color considered and on the solvent; Figure 5a,b). The beneficial effect of shell growth on the UC intensities thus comes mainly from the inhibition of quenching in the near-infrared. We ascribe residual quenching in the infrared in core–shell NCs with a 3 nm shell to coupling of the ⁴I_11/2 level of Er³⁺ to OH-vibrations of embedded hydroxide ions, which can potentially be prevented with improved synthesis procedures that avoid hydroxides and water. (20) The Yb³⁺²F_5/2 level is indirectly quenched by the same pathway, because at high doping concentrations the Yb³⁺²F_5/2 and Er³⁺⁴I_11/2 levels rapidly exchange energy by transfer and back-transfer. Quenching of the red-emitting ⁴F_9/2 level of Er³⁺ is due to solvent quenching and can be further suppressed with thicker shells (solid lines in Figure 5b) than the 3 nm we consider here or by excluding high-energy vibrations in the environment. Quenching of the green-emitting ⁴S_3/2 and ²H_11/2 levels at high doping concentration relevant to UC is mainly due to cross-relaxation to Er³⁺ neighbors (blue and yellow pathways in Figure 3c) and cannot be avoided by growing thicker shells. Indeed, also bulk material suffers from this loss pathway (Figure S5). (6)

Our systematic analysis of the decay dynamics of the various levels involved in Er³⁺–Yb³⁺-based UC provides important insights into the quenching pathways relevant to UC. However, many additional effects have to be considered for a full quantitative understanding of UC under all experimental conditions. For example, some studies have proposed complex population pathways for the red-emitting level, involving multiple multiphonon and/or energy-transfer processes. (35−37) Moreover, at high excitation powers the state populations can saturate, and levels higher in energy than the green- or red-emitting ones become involved in the energy-transfer and cross-relaxation pathways. (15,50) It is also important to consider that cross-relaxation and nonradiative recombination steps may not necessarily lead to irreversible energy loss, but the energy may be used in a next energy-transfer UC step. Further, the quantum yields of near-infrared and visible emission of a particular Er³⁺ center may be correlated, for example, if it is located near the NC surface. (49) This and other variations of and correlations between the rates of various UC steps can only be taken into account fully with a microscopic theoretical model that considers individual luminescent centers (e.g., Er³⁺ and Yb³⁺) explicitly, (51) as an alternative to the more common mean-field models. (35,52)

Conclusion

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In summary, we have presented a detailed study on the excited-state dynamics in NaYF₄ upconversion nanocrystals co-doped with Er³⁺ and Yb³⁺. By systematically varying important parameters such as doping concentration and the nature of the solvent, we were able to identify quenching pathways and energy-transfer processes responsible for the reduced upconversion efficiency in nanocrystals. A microscopic model was developed to provide quantitative insight in the contribution of quenching by molecular vibrations (ligands, solvent) and cross-relaxation processes. For the green and red Er³⁺ emission, the model accurately reproduces the observed decay dynamics and also demonstrates how the solvent quenching strength depends on resonances between solvent vibrational modes and electronic transitions on the Er³⁺ ion. Growing an undoped shell around the upconversion core reduces solvent quenching, and with the model the shell thickness required for a near-complete suppression of solvent quenching can be determined. For the near-infrared emission, the agreement between the observed excited-state dynamics and our model is not satisfactory. The systematic deviation indicates the presence of an additional quenching process. Based on the resonance of the O-H stretch vibrational energy and a transition on Er³⁺ for the IR emitting level, we propose that OH^– incorporated on F^– sites in NaYF₄ nanocrystals is responsible. For core–shell nanocrystals, the most significant improvement in upconversion efficiency can be achieved by suppressing quenching of the near-infrared-emitting level which may be realized by alternative synthesis strategies that prevent incorporation of OH^– in the NaYF₄ lattice.

Methods

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Nanocrystal Synthesis

The NaYF₄ core nanocrystals (NCs) doped with Er³⁺ and/or Yb³⁺ were synthesized following the procedure from ref (53) with modifications based on ref (54). A mixture of REAc₃ (4 mmol; with RE = Y, Er, Yb in the desired ratio) in oleic acid (24 mL) and 1-octadecene (68 mL) was degassed at 120 °C under vacuum for 90 min and then allowed to cool down to room temperature under nitrogen atmosphere. A solution of NaOH (10 mmol) and NH₄F (16 mmol) in methanol (40 mL) was injected into the reaction mixture under nitrogen, which was then stirred overnight at room temperature. Next, the methanol was removed by evaporation at 100 °C under vacuum for 30 min. The reaction mixture was then quickly heated to 300 °C for 110 min under nitrogen atmosphere while stirring, during which the NaYF₄ NCs form. After cooling down, the NCs were washed three times by precipitation with ethanol, centrifugation, and redispersing them in cyclohexane. In the second to last washing step, a mixture of cyclohexane (12 mL) and oleic acid (12 mL) was used to redisperse the NCs to increase ligand coverage of the surface and improve the colloidal stability. The final product was dispersed in approximately 12 mL of cyclohexane.

Core–shell NCs were synthesized by growing a nonluminescent NaYF₄ shell around (co)doped cores using the method of ref (54). A solution of YAc₃ (1.5 mmol) in oleic acid (9 mL) and 1-octadecene (25.5 mL) was degassed at 120 °C under vacuum for 60 min. The temperature was then decreased to 80 °C under nitrogen, and 5.5 nmol of core particles was added (estimated from transmission electron microscopy and the dry weight of the cyclohexane stock dispersion). Cyclohexane was removed at 100 °C under vacuum. The reaction mixture was then allowed to cool down to room temperature, a solution of NaOH (3.75 mmol) and NH₄F (6 mmol) in methanol (15 mL) was added, and the mixture was stirred overnight. Next, methanol was evaporated at 100 °C under vacuum for 10 min. The reaction mixture was then quickly heated to 300 °C under nitrogen for 120 min before cooling down to room temperature. The NCs were washed three times by precipitation with ethanol, centrifugation, and redispersing them in cyclohexane. In the second to last washing step, a mixture of cyclohexane (3 mL) and oleic acid (3 mL) was used to redisperse the NCs to increase ligand coverage of the surface and improve the colloidal stability. The final product was dispersed in approximately 1.7 mL of cyclohexane.

Microcrystalline powders of (co)doped β-NaYF₄ were prepared as described in ref (5).

Spectroscopic Experiments

Photoluminescence decay dynamics of the various Er³⁺ and Yb³⁺ emissions were recorded using an Edinburgh Instruments FLS920 fluorescence spectrometer. The samples were excited with 10 ns pulses (20 Hz) from a color-tunable optical parametric oscillator (OPO) system (Opotek HE 355 II) pumped by the third harmonic of a Nd:YAG laser. The luminescence was detected with a Hamamatsu H74220–60 photomultiplier tube for visible wavelength or with a N₂-cooled R5507-73 photomultiplier tube for the near-infrared. To measure the decay dynamics of a certain energy level, we tuned the excitation laser on the same level but a few nm blue-shifted from the detection wavelength, except for the green emission where we excited in the ²H_11/2 level and detected emission from the ⁴S_3/2 level. These levels are thermally coupled and show the same decay dynamics. (44) To minimize reabsorption of emitted light, the NCs were dispersed in an organic solvent at low concentration, while the microcrystalline powders were diluted with white BaSO₄ powder, typically to 1% w/w or lower.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b01545.

Tranmission electron microscopy of the upconversion nanocrystals, photoluminescence decay of bulk material, and a complete data set and modeling of excited-state decay dynamics of the visible-emitting levels (PDF)

nn8b01545_si_001.pdf (903.03 kb)

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Author Information

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Corresponding Author
- Andries Meijerink - Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands; http://orcid.org/0000-0003-3573-9289; Email: [email protected]
Authors
- Freddy T. Rabouw - Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands; http://orcid.org/0000-0002-4775-0859
- P. Tim Prins - Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands; http://orcid.org/0000-0002-8258-0074
- Pedro Villanueva-Delgado - Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands
- Marieke Castelijns - Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands
- Robin G. Geitenbeek - Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands
Notes
The authors declare no competing financial interest.

Acknowledgments

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The work was supported by The Netherlands Center for Multiscale Catalytic Energy Conversion (MCEC), an NWO Gravitation programme funded by the Ministry of Education, Culture and Science of the government of The Netherlands. F.T.R. acknowledges financial support from The Netherlands Organisation for Scientific Research NWO (VENI grant number 722.017.002). P.V.-D. acknowledges financial support from the Swiss National Science Foundation SNSF (grant number P2BEP2_172238). We are grateful to R. Brechbühler for critical reading of the manuscript.

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Solid-state infrared-to-visible upconversion sensitized by colloidal nanocrystals
Wu, Mengfei; Congreve, Daniel N.; Wilson, Mark W. B.; Jean, Joel; Geva, Nadav; Welborn, Matthew; Van Voorhis, Troy; Bulovic, Vladimir; Bawendi, Moungi G.; Baldo, Marc A.
Nature Photonics (2016), 10 (1), 31-34CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
Optical upconversion via sensitized triplet-triplet exciton annihilation converts incoherent low-energy photons to shorter wavelengths under modest excitation intensities. Here, we report a solid-state thin film for IR-to-visible upconversion that employs lead sulfide colloidal nanocrystals as a sensitizer. Upconversion is achieved from pump wavelengths beyond λ = 1 μm to emission at λ = 612 nm. When excited at λ = 808 nm, two excitons in the sensitizer are converted to one higher-energy state in the emitter at a yield of 1.2 ± 0.2%. Peak efficiency is attained at an absorbed intensity equiv. to less than one sun. We demonstrate that colloidal nanocrystals are an attractive alternative to existing mol. sensitizers, given their small exchange splitting, wide wavelength tunability, broadband IR absorption, and our transient observations of efficient energy transfer. This solid-state architecture for upconversion may prove useful for enhancing the capabilities of solar cells and photodetectors.
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Deutsch, Z.; Neeman, L.; Oron, D. Luminescence Upconversion in Colloidal Double Quantum Dots. Nat. Nanotechnol. 2014, 8, 649– 653, DOI: 10.1038/nnano.2013.146
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Makarov, N. S.; Lin, Q.; Pietryga, J. M.; Robel, I.; Klimov, V. I. Auger Up-Conversion of Low-Intensity Infrared Light in Engineered Quantum Dots. ACS Nano 2016, 10, 10829– 10841, DOI: 10.1021/acsnano.6b04928
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Auger Up-Conversion of Low-Intensity Infrared Light in Engineered Quantum Dots
Makarov, Nikolay S.; Lin, Qianglu; Pietryga, Jeffrey M.; Robel, Istvan; Klimov, Victor I.
ACS Nano (2016), 10 (12), 10829-10841CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
One source of efficiency losses in photovoltaic cells is their transparency toward solar photons with energies below the band gap of the absorbing layer. This loss can be reduced using a process of up-conversion whereby ≥2 sub-band-gap photons generate a single above-gap exciton. Traditional approaches to up-conversion, such as nonlinear 2-photon absorption (2PA) or triplet fusion, suffer from low efficiency at solar light intensities, a narrow absorption bandwidth, nonoptimal absorption energies, and difficulties for implementing in practical devices. These deficiencies can be alleviated using the effect of Auger-up-conversion in thick-shell PbSe/CdSe quantum dots. This process relies on Auger recombination whereby 2 low-energy core-based excitons are converted into a single higher-energy shell-based exciton. Compared to their monocomponent counterparts, the tailored PbSe/CdSe heterostructures feature enhanced absorption cross sections, a higher efficiency of the productive Auger pathway involving reexcitation of a hole, and longer lifetimes of both core- and shell-localized excitons. These features lead to effective up-conversion cross sections that are >6 orders of magnitude higher than for std. nonlinear 2PA, which allows for efficient up-conversion of continuous wave IR light at intensities as low as a few Watts per square centimeter.
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Krämer, K. W.; Biner, D.; Frei, G.; Güdel, H. U.; Hehlen, M. P.; Lüthi, S. R. Hexagonal Sodium Yttrium Fluoride Based Green and Blue Emitting Upconversion Phosphors. Chem. Mater. 2004, 16, 1244– 1251, DOI: 10.1021/cm031124o
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Suyver, J. F.; Grimm, J.; van Veen, M. K.; Biner, D.; Krämer, K. W.; Güdel, H. U. Upconversion Spectroscopy and Properties of NaYF₄ Doped with Er³⁺, Tm³⁺ and/or Yb³⁺. J. Lumin. 2006, 117, 1– 12, DOI: 10.1016/j.jlumin.2005.03.011
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Upconversion spectroscopy and properties of NaYF4 doped with Er3+, Tm3+ and/or Yb3+
Suyver, J. F.; Grimm, J.; van Veen, M. K.; Biner, D.; Kraemer, K. W.; Guedel, H. U.
Journal of Luminescence (2006), 117 (1), 1-12CODEN: JLUMA8; ISSN:0022-2313. (Elsevier B.V.)
A spectroscopic study of NaYF4 powders doped with several different concns. of Er3+, Tm3+ and/or Yb3+ is described. Rare earth-doped NaYF4 is known to be a very efficient near-IR to visible upconverter. The overview emission spectra for all samples are presented and from these the upconversion efficiency is calcd. Raman spectroscopy of undoped NaYF4 is presented here for the 1st time, demonstrating that the dominant phonon modes in NaYF4 lie 300-400 cm-1. The fact that these phonon modes are also the optically active ones is further demonstrated by temp.-dependent excitation spectroscopy. These surprisingly low-energy phonon modes explain the extraordinarily high upconversion efficiency of the rare earth-doped NaYF4 samples. Excitation spectroscopy up to ∼ 70000 cm-1 in an NaErF4 sample reveals a multitude of Er3+ 4f excitations, including the illustrious 2F(2)5/2 one that was not obsd. in excitation spectroscopy before. From the low-temp. power-dependence of the emission intensities for an Er3+, Yb3+ codoped NaYF4 sample, the dominant upconversion mechanism at low temp. is a different one than at room temp. From direct excitation, the lifetimes of the Yb3+ 2F52 → 2F7/2, Er3+ 4F9/2 → 4I15/2 and Er3+ 4S3/2 → 4I15/2 emissions are detd. as a function of temp. for all samples. At elevated temps., a significant decrease in the green lifetime is obsd., which is correlated to a simultaneous quenching in the luminescence intensity. This quenching is ascribed to cross-relaxation between two nearby Er3+ ions.
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Martín-Rodríguez, R.; Fischer, S.; Ivaturi, A.; Froehlich, B.; Krämer, K. W.; Goldschmidt, J. C.; Richards, B. S.; Meijerink, A. Highly Efficient IR to NIR Upconversion in Gd₂O₂S:Er³⁺ for Photovoltaic Applications. Chem. Mater. 2013, 25, 1912– 1921, DOI: 10.1021/cm4005745
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Highly Efficient IR to NIR Upconversion in Gd2O2S: Er3+ for Photovoltaic Applications
Martin-Rodriguez, Rosa; Fischer, Stefan; Ivaturi, Aruna; Froehlich, Benjamin; Kraemer, Karl W.; Goldschmidt, Jan C.; Richards, Bryce S.; Meijerink, Andries
Chemistry of Materials (2013), 25 (9), 1912-1921CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
Upconversion (UC) is a promising option to enhance the efficiency of solar cells by conversion of sub-bandgap IR photons to higher energy photons that can be utilized by the solar cell. The UC quantum yield is a key parameter for a successful application. Here the UC luminescence properties of Er3+-doped Gd2O2S are investigated by means of luminescence spectroscopy, quantum yield measurements, and excited state dynamics expts. Excitation into the max. of the 4I15/2 → 4I13/2 Er3+ absorption band around 1500 nm induces very efficient UC emission from different Er3+ excited states with energies above the silicon bandgap, in particular, the emission originating from the 4I11/2 state around 1000 nm. Concn. dependent studies reveal that the highest UC quantum yield is realized for a 10% Er3+-doping concn. The UC luminescence is compared to the well-known Er3+-doped β-NaYF4 UC material for which the highest UC quantum yield has been reported for 25% Er3+. The UC internal quantum yields were measured in this work for Gd2O2S: 10%Er3+ and β-NaYF4: 25%Er3+ to be 12 ± 1% and 8.9 ± 0.7%, resp., under monochromatic excitation around 1500 nm at a power of 700 W/m2. The UC quantum yield reported here for Gd2O2S: 10%Er3+ is the highest value achieved so far under monochromatic excitation into the 4I13/2 Er3+ level. Power dependence and lifetime measurements were performed to understand the mechanisms responsible for the efficient UC luminescence. We show that the main process yielding 4I11/2 UC emission is energy transfer UC.
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Heer, S.; Kömpe, K.; Güdel, H.-U.; Haase, M. Highly Efficient Multicolour Upconversion Emission in Transparent Colloids of Lanthanide-Doped NaYF₄ Nanocrystals. Adv. Mater. 2004, 16, 2102– 2105, DOI: 10.1002/adma.200400772
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Highly efficient multicolour upconversion emission in transparent colloids of lanthanide-doped NaYF4 nanocrystals
Heer, Stephan; Koempe, Karsten; Guedel, Hans-Ulrich; Haase, Markus
Advanced Materials (Weinheim, Germany) (2004), 16 (23-24), 2102-2105CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
The authors report on the successful synthesis and very intense multicolor upconversion emission of Yb3+/Er3+ and Yb3+/Tm3+ codoped NaYF4 nanocrystals transparently dispersed in soln. The upconversion efficiency of such solns. is about eight orders of magnitude higher than for the previously reported colloids of lanthanide-doped phosphate nanocrystals. This enormous improvement of the upconversion efficiency of these materials opens the door for interesting future applications in the field of biolabeling.
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Boyer, J.-C.; Vetrone, F.; Cuccia, L. A.; Capobianco, J. A. Synthesis of Colloidal Upconverting NaYF₄ Nanocrystals Doped with Er³⁺, Yb³⁺ and Tm³⁺, Yb³⁺ via Thermal Decomposition of Lanthanide Trifluoroacetate Precursors. J. Am. Chem. Soc. 2006, 128, 7444– 7445, DOI: 10.1021/ja061848b
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Synthesis of Colloidal Upconverting NaYF4 Nanocrystals Doped with Er3+, Yb3+ and Tm3+, Yb3+ via Thermal Decomposition of Lanthanide Trifluoroacetate Precursors
Boyer, John-Christopher; Vetrone, Fiorenzo; Cuccia, Louis A.; Capobianco, John A.
Journal of the American Chemical Society (2006), 128 (23), 7444-7445CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Upconverting lanthanide-doped nanocrystals were synthesized via the thermal decompn. of trifluoroacetate precursors in a mixt. of oleic acid and octadecene. This method provides highly luminescent nanoparticles through a simple 1-pot technique with only one preparatory step. The Er3+, Yb3+ and Tm3+, Yb3+ doped cubic NaYF4 nanocrystals are colloidally stable in nonpolar org. solvents and exhibit green/red and blue upconversion luminescence, resp., under 977 nm laser excitation with low power densities.
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Boyer, J.-C.; Manseau, M.-P.; Murray, J. I.; van Veggel, F. C. J. M. Surface Modification of Upconverting NaYF₄ Nanoparticles with PEG–Phosphate Ligands for NIR (800 nm) Biolabeling within the Biological Window. Langmuir 2010, 26, 1157– 1164, DOI: 10.1021/la902260j
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Surface Modification of Upconverting NaYF4 Nanoparticles with PEG-Phosphate Ligands for NIR (800 nm) Biolabeling within the Biological Window
Boyer, John-Christopher; Manseau, Marie-Pascale; Murray, Jill I.; van Veggel, Frank C. J. M.
Langmuir (2010), 26 (2), 1157-1164CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
The authors present a technique for the replacement of oleate with a PEG-phosphate ligand [PEG = poly(ethylene glycol)] as an efficient method for the generation of water-dispersible NaYF4 nanoparticles (NPs). The PEG-phosphate ligands are shown to exchange with the original oleate ligands on the surface of the NPs, resulting in water-dispersible NPs. The upconversion intensity of the NPs in aq. environments was severely quenched when compared to the original NPs in org. solvents. This is attributed to an increase in the multiphonon relaxations of the lanthanide excited state in aq. environments due to high energy vibrational modes of water mols. This problem could be overcome partially by the synthesis of core/shell NPs which demonstrated improved photophys. properties in water over the original core NPs. The PEG-phosphate coated upconverting NPs were then used to image a line of ovarian cancer cells (CaOV3) to demonstrate their promise in biol. application.
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Wang, F.; Wang, J.; Liu, X. Direct Evidence of a Surface Quenching Effect on Size-Dependent Luminescence of Upconversion Nanoparticles. Angew. Chem., Int. Ed. 2010, 49, 7456– 7460, DOI: 10.1002/anie.201003959
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Direct evidence of a surface quenching effect on size-dependent luminescence of upconversion nanoparticles
Wang, Feng; Wang, Juan; Liu, Xiaogang
Angewandte Chemie, International Edition (2010), 49 (41), 7456-7460, S7456/1-S7456/3CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
Yb/Tm co-doped NaGdF4 nanoparticles without or with a thin surface protection layer provide direct evidence of a surface quenching effect on size-dependent upconversion luminescence. The coating preserves the optical integrity of the nanoparticles (right-hand spectrum) and minimizes emission loss induced by surface quenching.
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Wang, F.; Deng, R.; Wang, J.; Wang, Q.; Han, Y.; Zhu, H.; Chen, X.; Liu, X. Tuning Upconversion through Energy Migration in Core–Shell Nanoparticles. Nat. Mater. 2011, 10, 968– 973, DOI: 10.1038/nmat3149
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Tuning upconversion through energy migration in core-shell nanoparticles
Wang, Feng; Deng, Renren; Wang, Juan; Wang, Qingxiao; Han, Yu; Zhu, Haomiao; Chen, Xueyuan; Liu, Xiaogang
Nature Materials (2011), 10 (12), 968-973CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
Photon upconversion is promising for applications such as biol. imaging, data storage or solar cells. Upconversion processes were studied in a broad range of Gd-based nanoparticles of varying compn. By rational design of a core-shell structure with a set of lanthanide ions incorporated into sepd. layers at precisely defined concns., efficient upconversion emission can be realized through Gd sublattice-mediated energy migration for a wide range of lanthanide activators without long-lived intermediary energy states. The use of the core-shell structure allows the elimination of deleterious cross-relaxation. This effect enables fine-tuning of upconversion emission through trapping of the migrating energy by the activators. The findings suggest a general approach to constructing a new class of luminescent materials with tunable upconversion emissions by controlled manipulation of energy transfer within a nanoscopic region.
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Zou, W.; Visser, C.; Maduro, J. A.; Pshenichnikov, M. S.; Hummelen, J. C. Broadband Dye-Sensitized Upconversion of Near-Infrared Light. Nat. Photonics 2012, 6, 560– 564, DOI: 10.1038/nphoton.2012.158
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Broadband dye-sensitized upconversion of near-infrared light
Zou, Wenqiang; Visser, Cindy; Maduro, Jeremio A.; Pshenichnikov, Maxim S.; Hummelen, Jan C.
Nature Photonics (2012), 6 (8), 560-564CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
Photon upconversion of near-IR photons is a promising way to overcome the Shockley-Queisser efficiency limit of 32% of a single-junction solar cell. However, the practical applicability of the most efficient known upconversion materials at moderate light intensities is limited by their extremely weak and narrowband near-IR absorption. Here, we introduce the concept of an upconversion material where an org. near-IR dye is used as an antenna for the β-NaYF4:Yb,Er nanoparticles in which the upconversion occurs. The overall upconversion by the dye-sensitized nanoparticles is dramatically enhanced (by a factor of ∼3,300) as a result of increased absorptivity and overall broadening of the absorption spectrum of the upconverter. The proposed concept can be extended to cover any part of the solar spectrum by using a set of dye mols. with overlapping absorption spectra acting as an extremely broadband antenna system, connected to suitable upconverters.
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Zhao, J.; Lu, Z.; Yin, Y.; McRae, C.; Piper, J. A.; Dawes, J. M.; Jin, D.; Goldys, E. M. Upconversion Luminescence with Tunable Lifetime in NaYF₄:Yb,Er Nanocrystals: Role of Nanocrystal Size. Nanoscale 2013, 5, 944– 952, DOI: 10.1039/C2NR32482B
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Upconversion luminescence with tunable lifetime in NaYF4:Yb,Er nanocrystals: role of nanocrystal size
Zhao, Jiangbo; Lu, Zhenda; Yin, Yadong; McRae, Christopher; Piper, James A.; Dawes, Judith M.; Jin, Dayong; Goldys, Ewa M.
Nanoscale (2013), 5 (3), 944-952CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
Despite recent achievements to reduce surface quenching in NaYF4:Yb,Er nanocrystals, a complete understanding of how the nanocrystal size affects the brightness of upconversion luminescence is still incomplete. Here we investigated upconversion luminescence of Yb,Er-doped nanocrystals in a broad range of sizes from 6 nm to 45 nm (cubic or hexagonal phases), displaying an increasing red-to-green luminescence intensity ratio and reduced luminescence lifetimes with decreasing size. By analyzing the upconversion process with a set of rate equations, we found that their asymptotic analytic solns. explain lower decay rates of red compared to green upconversion luminescence. Furthermore, we quantified the effect of the surface on luminescence lifetime in a model where nanocrystal emitters are divided between the near-surface and inside regions of each nanocrystal. We clarify the influence of the four nonradiative recombination mechanisms (intrinsic phonon modes, vibration energy of surface ligands, solvent-mediated quenching, and surface defects) on the decay rates for different-size nanocrystals, and find that the defect d. dominates decay rates for small (below 15 nm) nanocrystals. Our results indicate that a defect-redn. strategy is a key step in producing small upconversion nanocrystals with increased brightness for a variety of bioimaging and biosensing applications.
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Zhao, J.; Jin, D.; Schartner, E. P.; Lu, Y.; Liu, Y.; Zvyagin, A. V.; Zhang, L.; Dawes, J. M.; Xi, P.; Piper, J. A.; Goldys, E. M.; Monro, T. M. Single-Nanocrystal Sensitivity Achieved by Enhanced Upconversion Luminescence. Nat. Nanotechnol. 2013, 8, 729– 734, DOI: 10.1038/nnano.2013.171
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Single-nanocrystal sensitivity achieved by enhanced upconversion luminescence
Zhao, Jiangbo; Jin, Dayong; Schartner, Erik P.; Lu, Yiqing; Liu, Yujia; Zvyagin, Andrei V.; Zhang, Lixin; Dawes, Judith M.; Xi, Peng; Piper, James A.; Goldys, Ewa M.; Monro, Tanya M.
Nature Nanotechnology (2013), 8 (10), 729-734CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
High excitation irradiance can alleviate concn. quenching in upconversion luminescence when combined with higher activator concn., which can be increased from 0.5 to 8 mol% Tm3+ in NaYF4. This leads to significantly enhanced luminescence signals, by up to a factor of 70. By using such bright nanocrystals, remote tracking of a single nanocrystal with a microstructured optical-fiber dip sensor is demonstrated. This represents a sensitivity improvement of 3 orders of magnitude over benchmark nanocrystals such as quantum dots.
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Chen, D.; Huang, P. Highly Intense Upconversion Luminescence in Yb/Er:NaGdF₄ @NaYF₄ Core–Shell Nanocrystals with Complete Shell Enclosure of the Core. Dalton Trans. 2014, 43, 11299– 11304, DOI: 10.1039/c4dt01237b
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Liu, Y.; Zhao, J.; Zhang, R.; Liu, Y.; Liu, D.; Goldys, E. M.; Yang, X.; Xi, P.; Sunna, A.; Lu, J.; Shi, Y.; Leif, R. C.; Huo, Y.; Shen, J.; Piper, J. A.; Robinson, J. P.; Jin, D. Tunable Lifetime Multiplexing Using Luminescent Nanocrystals. Nat. Photonics 2014, 8, 32– 36, DOI: 10.1038/nphoton.2013.322
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Tunable lifetime multiplexing using luminescent nanocrystals
Lu, Yiqing; Zhao, Jiangbo; Zhang, Run; Liu, Yujia; Liu, Deming; Goldys, Ewa M.; Yang, Xusan; Xi, Peng; Sunna, Anwar; Lu, Jie; Shi, Yu; Leif, Robert C.; Huo, Yujing; Shen, Jian; Piper, James A.; Robinson, J. Paul; Jin, Dayong
Nature Photonics (2014), 8 (1), 32-36CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
Optical multiplexing plays an important role in applications such as optical data storage, document security, mol. probes and bead assays for personalized medicine. Conventional fluorescent color coding is limited by spectral overlap and background interference, restricting the no. of distinguishable identities. Here, we show that tunable luminescent lifetimes τ in the microsecond region can be exploited to code individual upconversion nanocrystals. In a single color band, one can generate more than ten nanocrystal populations with distinct lifetimes ranging from 25.6 μs to 662.4 μs and decode their well-sepd. lifetime identities, which are independent of both color and intensity. Such 'τ-dots' potentially suit multichannel bioimaging, high-throughput cytometry quantification, high-d. data storage, as well as security codes to combat counterfeiting. This demonstration extends the optical multiplexing capability by adding the temporal dimension of luminescent signals, opening new opportunities in the life sciences, medicine and data security.
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Arppe, R.; Hyppänen, I.; Perälä, N.; Peltomaa, R.; Kaiser, M.; Würth, C.; Christ, S.; Resch-Genger, U.; Schäferling, M.; Soukka, T. Quenching of the Upconversion Luminescence of NaYF₄:Yb³⁺,Er³⁺ and NaYF₄:Yb³⁺,Tm³⁺ Nanophosphors by Water: the Role of the Sensitizer Yb³⁺ in Non-Radiative Relaxation. Nanoscale 2015, 7, 11746– 11757, DOI: 10.1039/C5NR02100F
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Quenching of the upconversion luminescence of NaYF4:Yb3+,Er3+ and NaYF4:Yb3+,Tm3+ nanophosphors by water: the role of the sensitizer Yb3+ in non-radiative relaxation
Arppe, Riikka; Hyppanen, Iko; Perala, Niina; Peltomaa, Riikka; Kaiser, Martin; Wurth, Christian; Christ, Simon; Resch-Genger, Ute; Schaferling, Michael; Soukka, Tero
Nanoscale (2015), 7 (27), 11746-11757CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
We have studied the mechanisms of water-based quenching of the upconversion photoluminescence of upconverting nanophosphors (UCNPs) via luminescence decay measurements for a better understanding of the non-radiative deactivation pathways responsible for the relatively low upconversion luminescence efficiency in aq. solns. This included both upconversion luminescence measurements and the direct excitation of emissive energy states of Er3+ and Yb3+ dopants in NaYF4:Yb3+,Er3+ UCNPs by measuring the decays at 550 and 655 nm upon 380 nm excitation and at 980 nm upon 930 nm excitation, resp. The luminescence intensities and decays were measured from both bare and silanized NaYF4:Yb3+,Er3+ and NaYF4:Yb3+,Tm3+ UCNPs in H2O and D2O. The measurements revealed up to 99.9% quenching of the upconversion photoluminescence intensity of both Er3+ and Tm3+ doped bare nanophosphors by water. Instead of the multiphonon relaxation of excited energy levels of the activators, the main mechanism of quenching was found to be the multiphonon deactivation of the Yb3+ sensitizer ion caused by OH-vibrations on the surface of the nanophosphor. Due to the nonlinear nature of upconversion, the quenching of Yb3+ has a higher order effect on the upconversion emission intensity with the efficient Yb-Yb energy migration in the ∼35 nm nanocrystals making the whole nanophosphor vol. susceptible to surface quenching effects. The study underlines the need of efficient surface passivation for the use of UCNPs as labels in bioanal. applications performed in aq. solns.
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Fischer, S.; Johnson, N. J. J.; Pichaandi, J.; Goldschmidt, J. C.; van Veggel, F. C. J. M. Upconverting Core–Shell Nanocrystals with High Quantum Yield under Low Irradiance: On the Role of Isotropic and Thick Shells. J. Appl. Phys. 2015, 118, 193105, DOI: 10.1063/1.4936119
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Upconverting core-shell nanocrystals with high quantum yield under low irradiance: On the role of isotropic and thick shells
Fischer, Stefan; Johnson, Noah J. J.; Pichaandi, Jothirmayanantham; Goldschmidt, Jan Christoph; van Veggel, Frank C. J. M.
Journal of Applied Physics (Melville, NY, United States) (2015), 118 (19), 193105/1-193105/12CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
Colloidal upconverter nanocrystals (UCNCs) that convert near-IR photons to higher energies are promising for applications ranging from life sciences to solar energy harvesting. However, practical applications of UCNCs are hindered by their low upconversion quantum yield (UCQY) and the high irradiances necessary to produce relevant upconversion luminescence. Achieving high UCQY under practically relevant irradiance remains a major challenge. The UCQY is severely limited due to non-radiative surface quenching processes. We present a rate equation model for migration of the excitation energy to show that surface quenching does not only affect the lanthanide ions directly at the surface but also many other lanthanide ions quite far away from the surface. The av. migration path length is on the order of several nanometers and depends on the doping as well as the irradiance of the excitation. Using Er3+-doped β-NaYF4 UCNCs, we show that very isotropic and thick (∼10 nm) β-NaLuF4 inert shells dramatically reduce the surface-related quenching processes, resulting in much brighter upconversion luminescence at simultaneously considerably lower irradiances. For these UCNCs embedded in poly(Me methacrylate), we detd. an internal UCQY of 2.0% ± 0.2% using an irradiance of only 0.43 ± 0.03 W/cm2 at 1523 nm. Normalized to the irradiance, this UCQY is 120× higher than the highest values of comparable nanomaterials in the literature. Our findings demonstrate the important role of isotropic and thick shells in achieving high UCQY at low irradiances from UCNCs. Addnl., we measured the addnl. short-circuit current due to upconversion in silicon solar cell devices as a proof of concept and to support our findings detd. using optical measurements. (c) 2015 American Institute of Physics.
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Rinkel, T.; Raj, A. N.; Dühnen, S.; Haase, M. Synthesis of 10 nm β-NaYF₄:Yb,Er/NaYF4 Core/Shell Upconversion Nanocrystals with 5 nm Particle Cores. Angew. Chem., Int. Ed. 2016, 55, 1164– 1167, DOI: 10.1002/anie.201508838
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Synthesis of 10 nm β-NaYF4:Yb,Er/NaYF4 Core/Shell Upconversion Nanocrystals with 5 nm Particle Cores
Rinkel, Thorben; Raj, Athira Naduviledathu; Duehnen, Simon; Haase, Markus
Angewandte Chemie, International Edition (2016), 55 (3), 1164-1167CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
A new method is presented for prepg. gram amts. of very small core/shell upconversion nanocrystals without addnl. codoping of the particles. First, ca. 5 nm β-NaYF4:Yb,Er core particles are formed by the reaction of sodium oleate, rare-earth oleate, and ammonium fluoride, thereby making use of the fact that a high ratio of sodium to rare-earth ions promotes the nucleation of a large no. of β-phase seeds. Thereafter, a 2 nm thick NaYF4 shell is formed by using 3-4 nm particles of α-NaYF4 as a single-source precursor for the β-phase shell material. In contrast to the core particles, however, these α-phase particles are prepd. with a low ratio of sodium to rare-earth ions, which efficiently suppresses an undesired nucleation of β-NaYF4 particles during shell growth.
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Johnson, N. J. J.; He, S.; Diao, S.; Chan, E. M.; Dai, H.; Almutairi, A. Direct Evidence for Coupled Surface and Concentration Quenching Dynamics in Lanthanide-Doped Nanocrystals. J. Am. Chem. Soc. 2017, 139, 3275– 3282, DOI: 10.1021/jacs.7b00223
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Direct Evidence for Coupled Surface and Concentration Quenching Dynamics in Lanthanide-Doped Nanocrystals
Johnson, Noah J. J.; He, Sha; Diao, Shuo; Chan, Emory M.; Dai, Hongjie; Almutairi, Adah
Journal of the American Chemical Society (2017), 139 (8), 3275-3282CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Luminescence quenching at high dopant concns. generally limits the dopant concn. to <1-5 mol% in lanthanide-doped materials, and this remains a major obstacle in designing materials with enhanced efficiency/brightness. Direct evidence is provided that the major quenching process at high dopant concns. is the energy migration to the surface (i.e., surface quenching) as opposed to the common misconception of cross-relaxation between dopant ions. After an inert epitaxial shell growth, Er (Er3+) concns. ≤100 mol% in NaY(Er)F4/NaLuF4 core/shell nanocrystals enhance the emission intensity of both upconversion and downshifted luminescence across different excitation wavelengths (980, 800, and 658 nm), with negligible concn. quenching effects. The results highlight the strong coupling of concn. and surface quenching effects in colloidal lanthanide-doped nanocrystals, and that inert epitaxial shell growth can overcome concn. quenching. These fundamental insights into the photophys. processes in heavily doped nanocrystals will give rise to enhanced properties not previously thought possible with compns. optimized in bulk.
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Hyppänen, I.; Höysniemi, N.; Arppe, R.; Schäferling, M.; Soukka, T. Environmental Impact on the Excitation Path of the Red Upconversion Emission of Nanocrystalline NaYF₄:Yb³⁺,Er³⁺. J. Phys. Chem. C 2017, 121, 6924– 6929, DOI: 10.1021/acs.jpcc.7b01019
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Environmental Impact on the Excitation Path of the Red Upconversion Emission of Nanocrystalline NaYF4:Yb3+,Er3+
Hyppanen, Iko; Hoysniemi, Niina; Arppe, Riikka; Schaferling, Michael; Soukka, Tero
Journal of Physical Chemistry C (2017), 121 (12), 6924-6929CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
The mechanism for red upconversion luminescence of Yb-Er codoped materials is not generally agreed on in the literature. Both two-photon and three-photon processes have been suggested as the main path for red upconversion emission. The authors have studied β-NaYF4:Yb3+,Er3+ nanoparticles in H2O and D2O, and the authors propose that the nanoparticle environment is a major factor in the selection of the preferred red upconversion excitation pathway. In H2O, efficient multiphonon relaxation (MPR) promotes the two-photon mechanism through green emitting states, while, in D2O, MPR is less effective and the three-photon path involving back energy transfer to Yb3+ is the dominant mechanism. For the green upconversion emission, the authors' results suggest the common two-photon path through the 4F9/2 energy state in both H2O and D2O.
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Schietinger, S.; Aichele, T.; Wang, H.-Q.; Nann, T.; Benson, O. Plasmon-Enhanced Upconversion in Single NaYF₄:Yb³⁺/Er³⁺ Codoped Nanocrystals. Nano Lett. 2010, 10, 134– 138, DOI: 10.1021/nl903046r
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Plasmon-Enhanced Upconversion in Single NaYF4:Yb3+/Er3+ Codoped Nanocrystals
Schietinger, Stefan; Aichele, Thomas; Wang, Hai-Qiao; Nann, Thomas; Benson, Oliver
Nano Letters (2010), 10 (1), 134-138CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
In this Letter the authors report the plasmon-enhanced upconversion in single NaYF4 nanocrystals codoped with Yb3+/Er3+. Single nanocrystals and Au nanospheres were studied and assembled in a combined confocal and at. force microscope setup. The nanocrystals show strong upconversion emission in the green and red under excitation with a continuous wave laser in the near-IR at 973 nm. Using the at. force microscope, the authors couple single nanocrystals with Au spheres (30 and 60 nm in diam.) to obtain enhanced upconversion emission. An overall enhancement factor of 3.8 is reached. A comparison of time-resolved measurements on the bare nanocrystal and the coupled nanocrystal-Au sphere systems unveil that faster excitation as well as faster emission occurs in the nanocrystals.
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Han, S.; Deng, R.; Xie, X.; Liu, X. Enhancing Luminescence in Lanthanide-Doped Upconversion Nanoparticles. Angew. Chem., Int. Ed. 2014, 53, 11702– 11715, DOI: 10.1002/anie.201403408
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Enhancing Luminescence in Lanthanide-Doped Upconversion Nanoparticles
Han, Sanyang; Deng, Renren; Xie, Xiaoji; Liu, Xiaogang
Angewandte Chemie, International Edition (2014), 53 (44), 11702-11715CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
A review. The enthusiasm for research on lanthanide-doped upconversion nanoparticles is driven by both a fundamental interest in the optical properties of lanthanides embedded in different host lattices and their promise for broad applications ranging from biol. imaging to photodynamic therapy. Despite the considerable progress made in the past decade, the field of upconversion nanoparticles has been hindered by significant exptl. challenges assocd. with low upconversion conversion efficiencies. Recent exptl. and theor. studies on upconversion nanoparticles have, however, led to the development of several effective approaches to enhancing upconversion luminescence, which could have profound implications for a range of applications. Herein we present the underlying principles of controlling energy transfer through lanthanide doping, overview the major advances and key challenging issues in improving upconversion luminescence, and consider the likely directions of future research in the field.
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Sun, Q.-C.; Mundoor, H.; Ribot, J. C.; Singh, V.; Smalyukh, I. I.; Nagpal, P. Plasmon-Enhanced Energy Transfer for Improved Upconversion of Infrared Radiation in Doped-Lanthanide Nanocrystals. Nano Lett. 2014, 14, 101– 106, DOI: 10.1021/nl403383w
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Plasmon-Enhanced Energy Transfer for Improved Upconversion of Infrared Radiation in Doped-Lanthanide Nanocrystals
Sun, Qi-C.; Mundoor, Haridas; Ribot, Josep C.; Singh, Vivek; Smalyukh, Ivan I.; Nagpal, Prashant
Nano Letters (2014), 14 (1), 101-106CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
Upconversion of IR radiation into visible light was studied for applications in photovoltaics and biol. imaging. However, low conversion efficiency due to small absorption cross-section for IR light (Yb3+), and slow rate of energy transfer (to Er3+ states) has prevented application of upconversion photoluminescence (UPL) for diffuse sunlight or imaging tissue samples. Here, the authors use resonant surface plasmon polaritons (SPP) waves to enhance UPL in doped-lanthanide nanocrystals. The anal. indicates that SPP waves not only enhance the electromagnetic field, and hence weak Purcell effect, but also increase the rate of resonant energy transfer from Yb3+ to Er3+ ions by 6 fold. While the authors do observe strong metal mediated quenching (14-fold) of green fluorescence on flat metal surfaces, the nanostructured metal is resonant in the IR and hence enhances the nanocrystal UPL. This strong Coulombic effect on energy transfer can have important implications for other fluorescent and excitonic systems too.
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Fischer, S.; Kumar, D.; Hallermann, F.; von Plessen, G.; Goldschmidt, J. C. Enhanced Upconversion Quantum Yield near Spherical Gold Nanoparticles – a Comprehensive Simulation Based Analysis. Opt. Express 2016, 24, A460– A475, DOI: 10.1364/OE.24.00A460
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Yin, Z.; Li, H.; Xu, W.; Cui, S.; Zhou, D.; Chen, X.; Zhu, Y.; Qin, G.; Song, H. Local Field Modulation Induced Three-Order Upconversion Enhancement: Combining Surface Plasmon Effect and Photonic Crystal Effect. Adv. Mater. 2016, 28, 2518– 2525, DOI: 10.1002/adma.201502943
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Local Field Modulation Induced Three-Order Upconversion Enhancement: Combining Surface Plasmon Effect and Photonic Crystal Effect
Yin, Ze; Li, Hang; Xu, Wen; Cui, Shaobo; Zhou, Donglei; Chen, Xu; Zhu, Yongsheng; Qin, Guanshi; Song, Hongwei
Advanced Materials (Weinheim, Germany) (2016), 28 (13), 2518-2525CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
The authors present a novel device and significant modulation of gold nanorods (AuNRs)/Polymethylmethacrylate (PMMA) opal photonic crystals (OPCs) surface plasmon photonic crystal (SPPC) on upconversion luminescence (UCL) of NaYF4:Yb3+, Er3+ NPs, which has perfectly combined surface plasmon effect of AuNRs and PC effects of 3D PMMA opals. In the hybrids, the UCL of NaYF4:Yb3+, Er3+ has been enhanced more than 103 folds, which is at least an order of magnitude higher than that reported by the previous literature.
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Zhou, D.; Liu, D.; Xu, W.; Yin, Z.; Chen, X.; Zhou, P.; Cui, S.; Chen, Z.; Song, H. Observation of Considerable Upconversion Enhancement Induced by Cu_2–xS Plasmon Nanoparticles. ACS Nano 2016, 10, 5169– 5179, DOI: 10.1021/acsnano.6b00649
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Observation of Considerable Upconversion Enhancement Induced by Cu2-xS Plasmon Nanoparticles
Zhou, Donglei; Liu, Dali; Xu, Wen; Yin, Ze; Chen, Xu; Zhou, Pingwei; Cui, Shaobo; Chen, Zhanguo; Song, Hongwei
ACS Nano (2016), 10 (5), 5169-5179CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
Localized surface plasmon resonances (LSPRs) are achieved in heavily doped semiconductor nanoparticles (NPs) with appreciable free carrier concns. In this paper, we present the photonic, elec., and photoelec. properties of plasmonic Cu2-xS NPs/films and the utilization of LSPRs generated from semiconductor NPs as near-IR antennas to enhance the upconversion luminescence (UCL) of NaYF4:Yb3+,Er3+ NPs. Our results suggest that the LSPRs in Cu2-xS NPs originate from ligand-confined carriers and that a heat treatment resulted in the decompn. of ligands and oxidn. of Cu2-xS NPs; these effects led to a decrease of the Cu2+/Cu+ ratio, which in turn resulted in the broadening, decrease in intensity, and red-shift of the LSPRs. In the presence of a MoO3 spacer, the UCL intensity of NaYF4:Yb3+,Er3+ NPs was substantially improved and exhibited extraordinary power-dependent behavior because of the energy band structure of the Cu2-xS semiconductor. These findings provide insights into the nature of LSPR in semiconductors and their interaction with nearby emitters and highlight the possible application of LSPR in photonic and photoelec. devices.
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Xu, W.; Chen, X.; Song, H. Upconversion Manipulation by Local Electromagnetic Field. Nano Today 2017, 17, 54– 58, DOI: 10.1016/j.nantod.2017.10.011
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Upconversion manipulation by local electromagnetic field
Xu, Wen; Chen, Xu; Song, Hongwei
Nano Today (2017), 17 (), 54-78CODEN: NTAOCG; ISSN:1748-0132. (Elsevier Ltd.)
Rare earth doped upconversion nanocrystals (RE-UCNCs) have attracted extensive interests owing to their unique phys. properties and great potential applications in bio-application, photonic and photoelec. devices etc. Although UCNCs open doors to a wide range of new opportunities, they are confronting with some difficulties and one of the fatal problems is their low upconversion luminescent strength/efficiency. To date, various methods have been explored to solving this significant issue. Totally to say, the methods can be classified into two aspects, the traditional size, structure, surface and crystal field controls of the UCNCs, and the novel local electromagnetic field modulation surrounding the UCNCs. The local electromagnetic field modulation on UCNCs is a powerful strategy to enhance the strength/efficiency of UCNCs, reporting enhancement from several times up to four orders in short times. The timely and concise summary on the previous literatures is significant for more rapid and formulated development of this field. This review is aimed at offering a comprehensive framework for metal/semiconductor plasmon-induced and photonic crystal effect induced upconversion enhancement. Differing from the other review articles, we first introduced the generation principle of localized electromagnetic field in metal/semiconductor nanostructure/photonic crystals, and their general interaction rules with various emitters. Then, we summed up the recent published works on the local field modulation-induced upconversion enhancement, on emphasis we did our best to discover the generality of obtaining highly improved photoluminescence for any emitters and the personality of realizing highly improved upconversion enhancement. We further prospected the future development in this attractive field based on the previous theor. and exptl. results and the requirement of application. The marriage of upconversion with nanophotonic could explore a novel frontier in photonics that potentially spawn many exciting new fields.
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Stouwdam, J. W.; Hebbink, G. A.; Huskens, J.; van Veggel, F. C. J. M. Lanthanide-Doped Nanoparticles with Excellent Luminescent Properties in Organic Media. Chem. Mater. 2003, 15, 4604– 4616, DOI: 10.1021/cm034495d
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Lanthanide-Doped Nanoparticles with Excellent Luminescent Properties in Organic Media
Stouwdam, Jan W.; Hebbink, Gerald A.; Huskens, Jurriaan; Van Veggel, Frank C. J. M.
Chemistry of Materials (2003), 15 (24), 4604-4616CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
Surface-coated nanoparticles of LaF3 and LaPO4 doped with the luminescent trivalent lanthanide ions Eu3+, Nd3+, Er3+, Pr3+, Ho3+, and Yb3+ were prepd. These ions emit in the visible and in the near-IR part of the electromagnetic spectrum. The ions Nd3+, Er3+, Pr3+, and Ho3+ are the main focus in this research because they show emissions in telecommunication windows. The Yb3+ ion is of interest because it can be used as a sensitizer for Er3+. The Eu3+ ion was used as a probe for the structural environment of the luminescent ion. These lanthanide ions are incorporated in the inorg. host of the particles and the particles are dispersible in org. solvents. The luminescent lifetimes of the ions are increased by orders of magnitude compared to org. complexes, with values ranging from several microseconds for Pr3+ and Ho3+, up to ∼200 μs for Nd3+, and 1 ms for Er3+. This increase in the luminescence lifetime is indicative of an effective shielding of the lanthanide ions from nonradiative decay of the excited state by the high-energy vibrations of the solvents and the coordinated org. ligands. A model is proposed to describe the nonexponential behavior of the luminescence decay by quenching from outside the particle. It uses two fit parameters, kR, a parameter describing the luminescence decay rate in the absence of surface quenching, and C, a parameter describing the sensitivity toward quenching. Using this model, the luminescence decays can be fitted very well, and factors influencing the luminescence lifetime like concn. quenching and solvent effects can be described accurately.
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Guyot-Sionnest, P.; Wehrenberg, B.; Yu, D. Intraband Relaxation in CdSe Nanocrystals and the Strong Influence of the Surface Ligands. J. Chem. Phys. 2005, 123, 074709, DOI: 10.1063/1.2004818
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Intraband relaxation in CdSe nanocrystals and the strong influence of the surface ligands
Guyot-Sionnest, Philippe; Wehrenberg, Brian; Yu, Dong
Journal of Chemical Physics (2005), 123 (7), 074709/1-074709/7CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
The intraband relaxation between the 1Pe and 1Se state of CdSe colloidal quantum dots is studied by pump-probe time-resolved spectroscopy. IR pump-probe measurements with ∼6-ps pulses show identical relaxation whether the electron was placed in the 1Se state by above band-gap photoexcitation or by electrochem. charging. The intraband relaxation of the electrons is not affected by the photogenerated holes which were trapped. However, the surface ligands strongly affect the rate of relaxation in colloid solns. Faster relaxation (<8 ps) is obtained with phosphonic acid and oleic acid ligands. Alkylamines lead to longer relaxation times of ∼10 ps and the slowest relaxation is obsd. for dodecanethiol ligands with relaxation times ∼30 ps. In the absence of holes or when the holes are trapped, the intraband relaxation is dominated by the surface and faster relaxation correlates with larger interfacial polarity. Energy transfer to the ligand vibrations may be sufficiently effective to account for the intraband relaxation rate.
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Aharoni, A.; Oron, D.; Banin, U.; Rabani, E.; Jortner, J. Long-Range Electronic-to-Vibrational Energy Transfer from Nanocrystals to Their Surrounding Matrix Environment. Phys. Rev. Lett. 2008, 100, 057404, DOI: 10.1103/PhysRevLett.100.057404
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Long-Range Electronic-to-Vibrational Energy Transfer from Nanocrystals to Their Surrounding Matrix Environment
Aharoni, Assaf; Oron, Dan; Banin, Uri; Rabani, Eran; Jortner, Joshua
Physical Review Letters (2008), 100 (5), 057404/1-057404/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
A radiationless transition process of long-range, resonance interconversion of electronic-to-vibrational energy transfer (EVET) is discovered between the band-gap excitation of nanocrystal quantum dots to matrix vibrational overtone modes using fluorescence lifetime measurements. A theor. anal. based on long-range dipole-dipole nonadiabatic couplings, being distinct from the traditional adiabatic or static-coupling pictures, is given and is in qual. agreement with expts. EVET should be considered in matrix choices for near-IR optoelectronic applications of nanocrystals.
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Rabouw, F. T.; Den Hartog, S. A.; Senden, T.; Meijerink, A. Photonic Effects on the Förster Resonance Energy Transfer Efficiency. Nat. Commun. 2014, 5, 3610, DOI: 10.1038/ncomms4610
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Photonic effects on the Forster resonance energy transfer efficiency
Rabouw Freddy T; den Hartog Stephan A; Senden Tim; Meijerink Andries
Nature communications (2014), 5 (), 3610 ISSN:.
Forster resonance energy transfer (ET) between luminescent species is applied in bio-imaging, lighting and photovoltaics, and has an important role in photosynthesis. However, the fundamental question of whether ET rates and efficiencies can be tuned by the photonic environment remains under debate. Here we show that ET rates are independent of the photonic environment, using the model system of LaPO4 nanocrystals co-doped with Ce(3+) donors and Tb(3+) acceptors. Although the radiative emission rate of the Ce(3+) excited state increases with the refractive index of the solvent in which the nanocrystals are dispersed, the Ce(3+)-to-Tb(3+) ET rate does not. We demonstrate that, as a result, lower refractive index solvents enable higher ET efficiencies leading to higher Tb(3+) emission intensities. Furthermore, an analytical model for ET in (nano)crystalline host materials is presented, able to predict the dependence of ET efficiencies on the photonic environment and the concentration of acceptor ions.
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Senden, T.; Rabouw, F. T.; Meijerink, A. Photonic Effects on the Radiative Decay Rate and Luminescence Quantum Yield of Doped Nanocrystals. ACS Nano 2015, 9, 1801– 1808, DOI: 10.1021/nn506715t
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Photonic Effects on the Radiative Decay Rate and Luminescence Quantum Yield of Doped Nanocrystals
Senden, Tim; Rabouw, Freddy T.; Meijerink, Andries
ACS Nano (2015), 9 (2), 1801-1808CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
Nanocrystals (NCs) doped with luminescent ions form an emerging class of materials. In contrast to excitonic transitions in semiconductor NCs, the optical transitions are localized and not affected by quantum confinement. The radiative decay rates of the dopant emission in NCs are nevertheless different from their bulk analogs due to photonic effects, and also the luminescence quantum yield (QY, important for applications) is affected. In the past, different theor. models probably describe the photonic effects for dopant emission in NCs, with little exptl. validation. The photonic effects on the radiative decay rate of luminescent doped NCs were studied using 4 nm LaPO4 NCs doped with Ce3+ or Tb3+ ions in different refractive index solvents and bulk crystals. The measured influence of the refractive index on the radiative decay rate of the Ce3+ emission, having near unity QY, is in excellent agreement with the theor. nanocrystal-cavity model. The nanocrystal-cavity model can be used to quantify the nonunity QY of Tb3+-doped LaPO4 NCs and demonstrate that, as a general rule, the QY is higher in media with higher refractive index.
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Anderson, R. B.; Smith, S. J.; May, P. S.; Berry, M. T. Revisiting the NIR-to-Visible Upconversion Mechanism in β-NaYF₄:Yb³⁺,Er³⁺. J. Phys. Chem. Lett. 2014, 5, 36– 42, DOI: 10.1021/jz402366r
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Revisiting the NIR-to-Visible Upconversion Mechanism in β-NaYF4:Yb3+,Er3+
Anderson, Robert B.; Smith, Steve J.; May, P. Stanley; Berry, Mary T.
Journal of Physical Chemistry Letters (2014), 5 (1), 36-42CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
Here, we show that the long-accepted mechanism for the prodn. of red and blue emission through upconversion (UC) of 1 μm excitation in Yb3+/Er3+-doped materials does not apply in the popular β-NaYF4 host. We propose a new mechanism involving Yb3+-to-Er3+ energy-transfer UC out of the green-emitting 2H11/2,4S3/2 states that quant. accounts for all of the obsd. optical behavior. Rate consts. for the relevant radiative and nonradiative processes are reported along with a prediction of the power dependence of the pulsed and continuous-wave UC quantum efficiency.
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Berry, M. T.; May, P. S. Disputed Mechanism for NIR-to-Red Upconversion Luminescence in NaYF₄:Yb³⁺,Er³⁺. J. Phys. Chem. A 2015, 119, 9805– 9811, DOI: 10.1021/acs.jpca.5b08324
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Disputed Mechanism for NIR-to-Red Upconversion Luminescence in NaYF4:Yb3+,Er3+
Berry, Mary T.; May, P. Stanley
Journal of Physical Chemistry A (2015), 119 (38), 9805-9811CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
The most commonly proposed mechanisms for NIR-to-red upconversion in the well-studied material β-NaYF4:Er3+,Yb3+ are evaluated to resolve inconsistencies that persist in the literature. Each of 4 possible mechanisms is evaluated in terms of the direct anal. of spectroscopic data. There are no important mechanisms that involve the 1st excited state of Er3+, 4I13/2, as an intermediate state. A large body of evidence overwhelmingly supports the proposed mechanism of Anderson et al., which suggests an intimate connection between NIR-to-red and NIR-to-blue upconversion. Both red and blue upconversion are produced primarily by a 3-photon excitation process that proceeds through the green emitting state to a dense manifold of states, 4G/2K, above the blue emitting state, 2H9/2. Competing relaxation mechanisms out of 4G/2K det. the relative amts. of blue and red upconversion produced. Multiphonon relaxation from 4G/2K results in blue upconversion, whereas back energy transfer from Er3+(4G/2K) to Yb3+(2F7/2) results in red emission.
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Zhang, J.; Hao, Z.; Li, J.; Zhang, X.; Luo, Y.; Pan, G. Observation of Efficient Population of the Red-Emitting State from the Green State by Non-Multiphonon Relaxation in the Er³⁺–Yb³⁺ System. Light: Sci. Appl. 2015, 4, e239, DOI: 10.1038/lsa.2015.12
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Observation of efficient population of the red-emitting state from the green state by non-multiphonon relaxation in the Er3+-Yb3+ system
Zhang, Jiahua; Hao, Zhendong; Li, Jing; Zhang, Xia; Luo, Yongshi; Pan, Guohui
Light: Science & Applications (2015), 4 (1), e239CODEN: LSAIAZ; ISSN:2047-7538. (Nature Publishing Group)
The rare earth Er3+ and Yb3+ codoped system is the most attractive for showcasing energy transfer upconversion. This system can generate green and red emissions from Er3+ under IR excitation of the sensitizer Yb3+. It is well known that the red-emitting state can be populated from the upper green-emitting state. The contribution of multiphonon relaxation to this population is generally considered important at low excitation densities. Here, we demonstrate for the first time the importance of a previously proposed but neglected mechanism described as a cross relaxation energy transfer from Er3+ to Yb3+, followed by an energy back transfer within the same Er3+-Yb3+ pair. A luminescence spectroscopy study of cubic Y2O3:Er3+, Yb3+ indicates that this mechanism can be more efficient than multiphonon relaxation, and it can even make a major contribution to the red upconversion. The study also revealed that the energy transfers involved in this mechanism take place only in the nearest Er3+-Yb3+ pairs, and thus, it is fast and efficient at low excitation densities. Our results enable a better understanding of upconversion processes and properties in the Er3+-Yb3+ system.
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van Dijk, J. M. F.; Schuurmans, M. F. H. On the Nonradiative and Radiative Decay Rates and a Modified Exponential Energy Gap Law for 4f–4f Transitions in Rare-Earth Ions. J. Chem. Phys. 1983, 78, 5317– 5323, DOI: 10.1063/1.445485
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On the nonradiative and radiative decay rates and a modified exponential energy gap law for 4f-4f transitions in rare-earth ions
Van Dijk, J. M. F.; Schuurmans, M. F. H.
Journal of Chemical Physics (1983), 78 (9), 5317-23CODEN: JCPSA6; ISSN:0021-9606.
A modified exponential energy gap law for nonradiative decay was derived for 4f-4f transitions where only a few phonons participate in the transition. Its preexponential factor varies by only a factor of 10 for different host materials and is purely electronic in nature. The modified law is the usual law (giving a factor of 105 variation) modified by using an energy gap which is effectively 2 (max.) phonon energies smaller than the energy difference between the initial and the final electronic state. A general relation between the radiative and nonradiative decay rates was constructed. For 4f-4f transitions, this relation enables one to predict the nonradiative decay rate from a knowledge of the radiative decay rate to within 1 order of magnitude accuracy.
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Sokolov, V. I.; Zvyagin, A. V.; Igumnov, S. M.; Molchanova, S. I.; Nazarov, M. M.; Nechaev, A. V.; Savelyev, A. G.; Tyutyunov, A. A.; Khaydukov, E. V.; Panchenko, V. Ya. Determination of the Refractive Index of β-NaYF₄/Yb³⁺/Er³⁺/Tm³⁺ Nanocrystals Using Spectroscopic Refractometry. Opt. Spectrosc. 2015, 118, 609– 613, DOI: 10.1134/S0030400X15040190
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Determination of the refractive index of β-NaYF4/Yb3+/Er3+/Tm3+ nanocrystals using spectroscopic refractometry
Sokolov, V. I.; Zvyagin, A. V.; Igumnov, S. M.; Molchanova, S. I.; Nazarov, M. M.; Nechaev, A. V.; Savelyev, A. G.; Tyutyunov, A. A.; Khaydukov, E. V.; Panchenko, V. Ya.
Optics and Spectroscopy (2015), 118 (4), 609-613CODEN: OPSUA3; ISSN:0030-400X. (SP MAIK Nauka/Interperiodica)
A method for measuring refractive index n of nanosize particles in the visible and near-IR spectral ranges is proposed. The method is based on comparing refractive index ncolloid of a colloid soln. of nanoparticles in several solvents with refractive indexes nsolvent of corresponding pure solvents and has an accuracy of ±2 × 10-4. Upconversion nanosize phosphors (UCNPs) are synthesized in the form of a β-NaYF4 cryst. matrix doped with Yb3+, Er3+, and Tm3+ rare earth ions. UCNPs have a doped core with a diam. of 40 ± 5 nm and undoped shell with a thickness of 3-5 nm. Synthesized nanocrystals possess intense photoluminescence in the blue, green, and red spectral ranges upon excitation by IR radiation with a wavelength of 977 nm. Using a spectroscopic refractometer, the dispersion of the refractive index of β-NaYF4/Yb3+/Er3+/Tm3+ nanocrystals was measured for the first time in the spectral range of 450-1000 nm with an accuracy of ±2 × 10-4.
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Dong, C.; Pichaandi, J.; Regier, T.; van Veggel, F. C. J. M. Nonstatistical Dopant Distribution of Ln³⁺-Doped NaGdF₄ Nanoparticles. J. Phys. Chem. C 2011, 115, 15950– 15958, DOI: 10.1021/jp206441u
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Nonstatistical Dopant Distribution of Ln3+-Doped NaGdF4 Nanoparticles
Dong, Cunhai; Pichaandi, Jothirmayanantham; Regier, Tom; van Veggel, Frank C. J. M.
Journal of Physical Chemistry C (2011), 115 (32), 15950-15958CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
Oleate-stabilized NaGdF4 nanoparticles codoped with 20% Y3+ and 5% Tb3+ (NaGdF4:Y,Tb), with 20% Nd3+ (NaGdF4:Nd), and with 20% Tb3+ (NaGdF4:Tb) were prepd. in org. medium. The distribution of dopant ions was studied using synchrotron radiation XPS along with x-ray powder diffractometry, TEM, energy dispersive x-ray spectroscopy, and luminescence spectroscopy. These nanoparticles do not have the intended statistical dopant distribution despite the fact that different synthesis procedures and dopant ions with different ionic radii were used. NaGdF4:Y,Tb nanoparticles have a subtle gradient structure with Gd3+ more concd. toward the center and Y3+ more concd. toward the surface of the nanoparticles. NaGdF4:Nd nanoparticles have a steep gradient structure with Gd3+ more concd. toward the center and Nd3+ more concd. toward the surface of the nanoparticles. Even NaGdF4:Tb nanoparticles have a steep gradient structure with Tb3+ more concd. toward the center and Gd3+ more concd. toward the surface of the nanoparticles in spite of the very similar ionic radius of Gd3+ and Tb3+. The general assumption that Ln3+ dopant ions in lanthanide-based nanoparticles are statistically distributed in the whole nanoparticle may not be true.
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van Wijngaarden, J. T.; Scheidelaar, S.; Vlugt, T. J. H.; Reid, M. F.; Meijerink, A. Energy Transfer Mechanism for Downconversion in the (Pr³⁺, Yb³⁺) Couple. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 155112, DOI: 10.1103/PhysRevB.81.155112
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Energy transfer mechanism for downconversion in the (Pr3+, Yb3+) couple
van Wijngaarden, J. T.; Scheidelaar, S.; Vlugt, T. J. H.; Reid, M. F.; Meijerink, A.
Physical Review B: Condensed Matter and Materials Physics (2010), 81 (15), 155112/1-155112/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
Downconversion of one visible photon into two IR photons has been reported for the lanthanide ion couple (Pr3+, Yb3+) in a variety of host lattices. The mechanism responsible for downconversion is controversial and has been reported to be either a two-step energy transfer process (via two first-order transfer steps, the first being cross relaxation) or cooperative energy transfer from Pr3+ to two Yb3+ ions (a second-order process). Here the authors report expts. on downconversion for the (Pr3+, Yb3+) in LiYF4. Luminescence decay curves of the Pr3+ emission are recorded as a function of the Yb3+ concn. and analyzed using Monte Carlo simulations for both cooperative energy transfer and energy transfer through cross relaxation. A good agreement is obtained between expt. and simulations for energy transfer by cross relaxation but not for cooperative energy transfer. The observation that cross relaxation is more efficient than cooperative energy transfer is consistent with Judd-Ofelt calcns. for the transition probabilities involved in the two energy transfer processes and the lower probability for the second-order cooperative transfer.
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Yu, D. C.; Martín-Rodrïguez, R.; Zhang, Q. Y.; Meijerink, A.; Rabouw, F. T. Multi-Photon Quantum Cutting in Gd₂O₂S:Tm³⁺ to Enhance the Photo-Response of Solar Cells. Light: Sci. Appl. 2015, 4, e344, DOI: 10.1038/lsa.2015.117
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Multi-photon quantum cutting in Gd2O2S:Tm3+ to enhance the photo-response of solar cells
Yu, De-Chao; Martin-Rodriguez, Rosa; Zhang, Qin-Yuan; Meijerink, Andries; Rabouw, Freddy T.
Light: Science & Applications (2015), 4 (10), e344CODEN: LSAIAZ; ISSN:2047-7538. (Nature Publishing Group)
Conventional photoluminescence (PL) yields at most one emitted photon for each absorption event. Downconversion (or quantum cutting) materials can yield more than one photon by virtue of energy transfer processes between luminescent centers. In this work, we introduce Gd2O2S:Tm3+ as a multi-photon quantum cutter. It can convert near-IR, visible, or UV photons into two, three, or four IR photons of ∼1800 nm, resp. The cross-relaxation steps between Tm3+ ions that lead to quantum cutting are identified from (time-resolved) PL as a function of the Tm3+ concn. in the crystal. A model is presented that reproduces the way in which the Tm3+ concn. affects both the relative intensities of the various emission lines and the excited state dynamics and providing insight in the quantum cutting efficiency. Finally, we discuss the potential application of Gd2O2S:Tm3+ for spectral conversion to improve the efficiency of next-generation photovoltaics.
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Henderson, B.; Imbusch, G. F. Optical Spectroscopy of Inorganic Solids; Clarendon Press: Oxford, 1989.
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Geitenbeek, R. G.; Prins, P. T.; Albrecht, W.; van Blaaderen, A.; Weckhuysen, B. M.; Meijerink, A. NaYF₄:Er³⁺,Yb³⁺/SiO₂ Core/Shell Upconverting Nanocrystals for Luminescence Thermometry up to 900 K. J. Phys. Chem. C 2017, 121, 3503– 3510, DOI: 10.1021/acs.jpcc.6b10279
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NaYF4:Er3+,Yb3+/SiO2 Core/Shell Upconverting Nanocrystals for Luminescence Thermometry up to 900 K
Geitenbeek, Robin G.; Prins, P. Tim; Albrecht, Wiebke; van Blaaderen, Alfons; Weckhuysen, Bert M.; Meijerink, Andries
Journal of Physical Chemistry C (2017), 121 (6), 3503-3510CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
The rapid development of nanomaterials with unique size-tunable properties forms the basis for a variety of new applications, including temp. sensing. Luminescent nanoparticles (NPs) demonstrated potential as sensitive nanothermometers, esp. in biol. systems. Their small size offers the possibility of mapping temp. profiles with high spatial resoln. The temp. range is however limited, which prevents use in high-temp. applications such as, for example, nanoelectronics, thermal barrier coatings, and chem. reactors. The authors extend the temp. range for nanothermometry beyond 900 K using SiO2-coated NaYF4 nanoparticles doped with the lanthanide ions Yb3+ and Er3+. Monodisperse ∼20 nm NaYF4:Yb,Er nanocrystals were coated with a ∼ 10 nm SiO2 shell. Upon excitation with IR radiation, bright green upconversion (UC) emission is obsd. From the intensity ratio between 2H11/2 and 4S3/2 UC emission lines at 520 and 550 nm, resp., the temp. can be detd. up to at least 900 K with an accuracy of 1-5 K for SiO2-coated NPs. For bare NaYF4:Yb,Er NPs, the particles degrade >600 K. Repeated thermal cycling expts. demonstrate the high durability and reproducibility of the SiO2-coated nanocrystals as temp. probes without any loss of performance. The present results open avenues for the development of a new class of highly stable nanoprobes by applying a SiO2 coating around a wide variety of lanthanide-doped NPs.
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Villanueva-Delgado, P.; Biner, D.; Krämer, K. W. Judd–Ofelt Analysis of β-NaGdF₄: Yb³⁺, Tm³⁺ and β-NaGdF₄:Er³⁺ Single Crystals. J. Lumin. 2017, 189, 84– 90, DOI: 10.1016/j.jlumin.2016.04.023
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Judd-Ofelt analysis of β-NaGdF4: Yb3+, Tm3+ and β-NaGdF4:Er3+ single crystals
Villanueva-Delgado, P.; Biner, D.; Kramer, K. W.
Journal of Luminescence (2017), 189 (), 84-90CODEN: JLUMA8; ISSN:0022-2313. (Elsevier B.V.)
Hexagonal β-NaGdF4 single crystals were grown by the Bridgman technique, one doped with 5% Yb3+, 0.5% Tm3+ and the other with 1% Er3+. Based on polarized absorption spectra a Judd-Ofelt anal. was performed using the software RELIC. The energy barycenters of the room temp. absorption bands were used to refine the 4f wavefunctions in the intermediate coupling approxn., and obtain the Slater integrals F(2), (4), (6) and the spin-orbit coupling parameter ζ. The exptl. oscillator strengths were fitted, using calcd. matrix elements, to the three Judd-Ofelt parameters Ω2,4,6. The radiative lifetimes and branching ratios were detd. and compared to exptl. data. The oscillator strengths of selected transitions relevant to upconversion processes are discussed.
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Fischer, S.; Swabeck, J. K.; Alivisatos, A. P. Controlled Isotropic and Anisotropic Shell Growth in β-NaLnF₄ Nanocrystals Induced by Precursor Injection Rate. J. Am. Chem. Soc. 2017, 139, 12325– 12332, DOI: 10.1021/jacs.7b07496
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Controlled Isotropic and Anisotropic Shell Growth in β-NaLnF4 Nanocrystals Induced by Precursor Injection Rate
Fischer, Stefan; Swabeck, Joseph K.; Alivisatos, A. Paul
Journal of the American Chemical Society (2017), 139 (35), 12325-12332CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Precise morphol. and compn. control is vital for designing multifunctional lanthanide-doped core/shell nanocrystals. Controlled isotropic and anisotropic shell growth techniques in hexagonal Na rare earth tetrafluoride (β-NaLnF4) nanocrystals by exploiting the kinetics of the shell growth are reported. A drastic change of the shell morphol. was obsd. by changing the injection rate of the shell precursors while keeping all other reaction conditions const. The authors obtained isotropic shell growth for fast sequential injection and a preferred growth of the shell layers along the crystal's c-axis [001] for slow dropwise injection. Using this slow shell growth technique, the authors have grown rod-like shells around different almost spherical core nanocrystals. Bright and efficient upconversion was measured for both isotropic and rod-like shells around β-NaYF4 nanocrystals doped with Yb3+/Er3+ and Yb3+/Tm3+. Luminescence upconversion quantum yield and lifetime measurements reveal the high quality of the core/shell nanocrystal. Also, multishell rod-like nanostructures were prepd. with optically active cores and tips sepd. by an inert intermediate shell layer. The controlled anisotropic shell growth allows the design of new core/multishell nanostructures and enables independent studies of the chem. and physics of different nanocrystal facets.
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Wang, F.; Liu, X. Upconversion Multicolor Fine-Tuning: Visible to Near-Infrared Emission from Lanthanide-Doped NaYF₄ Nanoparticles. J. Am. Chem. Soc. 2008, 130, 5642– 5643, DOI: 10.1021/ja800868a
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Upconversion Multicolor Fine-Tuning: Visible to Near-Infrared Emission from Lanthanide-Doped NaYF4 Nanoparticles
Wang, Feng; Liu, Xiaogang
Journal of the American Chemical Society (2008), 130 (17), 5642-5643CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
A general approach to fine-tuning the upconversion emission colors, based upon a single host source of NaYF4 nanoparticles doped with Yb3+, Tm3+, and Er3+, is presented. The emission intensity balance can be precisely controlled using different host-activator systems and dopant concns. The approach allows access to a wide range of luminescence emission from visible to near-IR by single-wavelength excitation.
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Pollnau, M.; Gamelin, D. R.; Lüthi, S. R.; Güdel, H. U.; Hehlen, M. P. Power Dependence of Upconversion Luminescence in Lanthanide and Transition-Metal-Ion Systems. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61, 3337– 3346, DOI: 10.1103/PhysRevB.61.3337
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Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems
Pollnau, M.; Gamelin, D. R.; Luthi, S. R.; Gudel, H. U.; Hehlen, M. P.
Physical Review B: Condensed Matter and Materials Physics (2000), 61 (5), 3337-3346CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
The authors show theor. with the simplest possible model that the intensity of an upconversion luminescence that is excited by the sequential absorption of n photons has a dependence on absorbed pump power P, which may range from the limit of Pn down to the limit of P1 for the upper state and less than P1 for the intermediate states. The two limits are identified as the cases of infinitely small and infinitely large upconversion rates, resp. In the latter case, the dependence of luminescence intensities from intermediate excited states on pump power changes with the underlying upconversion and decay mechanisms. In certain situations, energy-transfer upconversion and excited-state absorption can be distinguished by the measured slopes. The competition between linear decay and upconversion in the individual excitation steps of sequential upconversion can be analyzed. The influence of nonuniform distributions of absorbed pump power or of a subset of ions participating in energy-transfer upconversion was studied. These results are of importance for the interpretation of excitation mechanisms of luminescent and laser materials. The authors verify the authors' theor. results by exptl. examples of multiphoton-excited luminescence in Cs3Lu2Cl9:Er3+, Ba2YCl7:Er3+, LiYF4:Nd3+, and Cs2ZrCl6:Re4+.
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Hossan, M. Y.; Hor, A.; Luu, Q.; Smith, S. J.; May, S.; Berry, M. T. Explaining the Nanoscale Effect in the Upconversion Dynamics of β-NaYF₄:Yb³⁺,Er³⁺ Core and Core–Shell Nanocrystals. J. Phys. Chem. C 2017, 121, 16592– 16606, DOI: 10.1021/acs.jpcc.7b04567
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Explaining the Nanoscale Effect in the Upconversion Dynamics of β-NaYF4:Yb3+,Er3+ Core and Core-Shell Nanocrystals
Hossan, Md Yeathad; Hor, Amy; Luu, QuocAnh; Smith, Steve J.; May, P. Stanley; Berry, Mary T.
Journal of Physical Chemistry C (2017), 121 (30), 16592-16606CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
Nanocrystals of β-NaYF4:Yb3+,Er3+ generally have lower NIR-to-visible upconversion (UC) internal quantum efficiency, IQE, compared to high-quality bulk materials, and exhibit more rapid UC dynamics, typical of quenching, when excited with a pulsed source near 980 nm. The addn. of a protective shell increases the IQE of the nanocrystals and slows the overall excited-state dynamics. An extension of a recently developed model for UC in powders of μm-sized β-NaYF4:18%Yb3+,2%Er3+ crystals correctly predicts the time-resolved luminescence curve shapes, relative intensities, and obsd. drop in IQE of the various emission lines for core and core-shell nanoparticles following pulsed excitation. The model clearly shows that the nanoscale effect on visible upconversion luminescence in these materials, with typical high-Yb3+ and low-Er3+ doping, is largely due to rapid energy migration among Yb3+(2F5/2) and Er3+(4I11/2) ions at the 1 μm energy level, such that an equil. is achieved between interior sites and rapidly relaxing surface sites. The faster kinetics obsd. in visible emission following pulsed NIR excitation is mainly a propagation of the effect of surface quenching of the 1 μm reservoir states and is not due to direct quenching of the visible emitting states themselves. For Er3+ ions contributing to UC emission, the relaxation rate consts. for the blue (2H9/2), green (2H11/2, 4S3/2), and red (4F9/2) emitting states are essentially unchanged from their bulk values, indicating that Er3+ ions close to the nanoparticle surface are nearly silent with regard to UC. The addn. of a passive β-NaYF4 shell retards the drain of the 1 μm excitation reservoir and recovers the participation of outer Er3+ sites in UC. The dependence of IQE on shell thickness is well explained in terms of a Forster-type model describing an energy donor (Er3+, Yb3+) interacting with a thin plane layer of acceptors (oleate). The UC behavior of both the core and the core-shell nanocrystals can be modeled, almost quant., solely from quenching at the 1 μm level, without sep. consideration of a near-surface Er3+ population. A 2-layer model for the core nanoparticles is revealing with regard to the modest extent to which near-surface ions do participate in UC and gives a better representation of the detailed dynamics of the NIR emitting states. A method is presented for allowing investigators to est. the IQE for any nanosample (with 18% Yb3+,2%Er3+ doping) as a function of excitation power d. (cw) or pulse-energy d. based on the low pulse energy measurement of the decay const. for the 1 μm emission.
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Gargas, D. J.; Chan, E. M.; Ostrowski, A. D.; Aloni, S.; Altoe, M. V. P.; Barnard, E. S.; Sanii, B.; Urban, J. J.; Milliron, D. J.; Cohen, B. E.; Schuck, P. J. Engineering Bright Sub-10-nm Upconverting Nanocrystals for Single-Molecule Imaging. Nat. Nanotechnol. 2014, 9, 300– 305, DOI: 10.1038/nnano.2014.29
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Engineering bright sub-10-nm upconverting nanocrystals for single-molecule imaging
Gargas, Daniel J.; Chan, Emory M.; Ostrowski, Alexis D.; Aloni, Shaul; Altoe, M. Virginia P.; Barnard, Edward S.; Sanii, Babak; Urban, Jeffrey J.; Milliron, Delia J.; Cohen, Bruce E.; Schuck, P. James
Nature Nanotechnology (2014), 9 (4), 300-305CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
Imaging at the single-mol. level reveals heterogeneities that are lost in ensemble imaging expts., but an ongoing challenge is the development of luminescent probes with the photostability, brightness and continuous emission necessary for single-mol. microscopy. Lanthanide-doped upconverting nanoparticles overcome problems of photostability and continuous emission and their upconverted emission can be excited with near-IR light at powers orders of magnitude lower than those required for conventional multiphoton probes. However, the brightness of upconverting nanoparticles was limited by open questions about energy transfer and relaxation within individual nanocrystals and unavoidable tradeoffs between brightness and size. Here, the authors develop upconverting nanoparticles under 10 nm in diam. that are over an order of magnitude brighter under single-particle imaging conditions than existing compns., allowing one to visualize single upconverting nanoparticles as small (d = 4.8 nm) as fluorescent proteins. The authors use advanced single-particle characterization and theor. modeling to find that surface effects become crit. at diams. under 20 nm and that the fluences used in single-mol. imaging change the dominant determinants of nanocrystal brightness. Factors known to increase brightness in bulk expts. lose importance at higher excitation powers and paradoxically, the brightest probes under single-mol. excitation are barely luminescent at the ensemble level.
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Villanueva-Delgado, P.; Krämer, K. W.; Valiente, R.; de Jong, M.; Meijerink, A. Modeling Blue to UV Upconversion in β-NaYF₄:Tm³⁺. Phys. Chem. Chem. Phys. 2016, 18, 27396– 27404, DOI: 10.1039/C6CP04347J
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Modeling blue to UV upconversion in β-NaYF4:Tm3+
Villanueva-Delgado, Pedro; Kramer, Karl W.; Valiente, Rafael; de Jong, Mathijs; Meijerink, Andries
Physical Chemistry Chemical Physics (2016), 18 (39), 27396-27404CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
Samples of 0.01% and 0.3% Tm3+-doped β-NaYF4 show upconverted UV luminescence at 27 660 cm-1 (361 nm) after blue excitation at 21 140 cm-1 (473 nm). Contradictory upconversion mechanisms in the literature are reviewed and two of them are investigated in detail. Their agreement with emission and two-color excitation expts. is examd. and compared. Decay curves are analyzed using the Inokuti-Hirayama model, an av. rate equation model, and a microscopic rate equation model that includes the correct extent of energy transfer. Energy migration is found to be negligible in these samples, and hence the av. rate equation model fails to correctly describe the decay curves. The microscopic rate equation model accurately fits the exptl. data and reveals the strength and multipolarity of various interactions. This microscopic model is able to det. the most likely upconversion mechanism.
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Fischer, S.; Steinkemper, H.; Löper, P.; Hermle, M.; Goldschmidt, J. C. Modeling Upconversion of Erbium Doped Microcrystals Based on Experimentally Determined Einstein Coefficients. J. Appl. Phys. 2012, 111, 013109, DOI: 10.1063/1.3674319
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Modeling upconversion of erbium doped microcrystals based on experimentally determined Einstein coefficients
Fischer, Stefan; Steinkemper, Heiko; Loeper, Philipp; Hermle, Martin; Goldschmidt, Jan Christoph
Journal of Applied Physics (2012), 111 (1), 013109/1-013109/13CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
The upconversion of IR photons is a promising possibility to enhance solar cell efficiency by producing electricity from otherwise unused sub-band-gap photons. The authors present a rate equation model and the relevant processes to describe the upconversion of near-IR photons. The model considers stimulated and spontaneous processes, multi-phonon relaxation, and energy transfer between neighboring ions. The input parameters for the model are exptl. detd. for the material system, β-NaEr0.2Y0.8F4. The detn. of the transition probabilities, also known as the Einstein coeffs., is the focus of the parameterization. The influence of multi-phonon relaxation and energy transfer on the upconversion are evaluated and discussed. Since upconversion is a nonlinear process, the irradiance dependence of the simulations is studied and compared to the exptl. data of quantum efficiency measurements. The results are very promising and indicate that upconversion is reasonably phys. described by the rate equations. Therefore, the presented model will be the basis for further simulations concerning various applications of upconversion, such as in combination with plasmon resonances in metal nanoparticles. (c) 2012 American Institute of Physics.
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Li, Z.; Zhang, Y. An Efficient and User-Friendly Method for the Synthesis of Hexagonal-Phase NaYF₄:Yb, Er/Tm Nanocrystals with Controllable Shape and Upconversion Fluorescence. Nanotechnology 2008, 19, 345606, DOI: 10.1088/0957-4484/19/34/345606
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An efficient and user-friendly method for the synthesis of hexagonal-phase NaYF4:Yb, Er/Tm nanocrystals with controllable shape and upconversion fluorescence
Li, Zhengquan; Zhang, Yong
Nanotechnology (2008), 19 (34), 345606/1-345606/5CODEN: NNOTER; ISSN:0957-4484. (Institute of Physics Publishing)
Hexagonal-phase NaYF4:Yb, Er/Tm nanocrystals are the best IR-to-visible upconverting materials to date, but user-friendly methods for making pure hexagonal-phase NaYF4:Yb, Er/Tm nanocrystals with upconversion fluorescence are still lacking. Most of the methods reported so far require excess fluoride reactants in a high-temp. reaction which are very unfriendly to users and raise safety concerns. In this work, an efficient and user-friendly method was developed for the synthesis of uniform hexagonal-phase NaYF4:Yb, Er/Tm nanocrystals with upconversion fluorescence, by forming small solid-state crystal nuclei and further growth and ripening of the nuclei. NaYF4:Yb, Er/Tm nanoplates, nanospheres and nanoellipses were also selectively produced by varying the concn. of the surfactant. All the nanocrystals showed strong upconversion fluorescence, and fluorescence from the nanoplates was obsd. even when the laser power d. was reduced to about 50 mW cm-2. These nanocrystals have great potential for use in biol. and medicine as fluorescent labels or imaging probes.
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Wang, F.; Deng, R.; Liu, X. Preparation of Core-Shell NaGdF₄ Nanoparticles Doped with Luminescent Lanthanide Ions to Be Used as Upconversion-Based Probes. Nat. Protoc. 2014, 9, 1634– 1644, DOI: 10.1038/nprot.2014.111
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Preparation of core-shell NaGdF4 nanoparticles doped with luminescent lanthanide ions to be used as upconversion-based probes
Wang, Feng; Deng, Renren; Liu, Xiaogang
Nature Protocols (2014), 9 (7), 1634-1644CODEN: NPARDW; ISSN:1750-2799. (Nature Publishing Group)
Sodium gadolinium fluoride (NaGdF4) is an ideal host material for the incorporation of luminescent lanthanide ions because of its high photochem. stability, low vibrational energy and its ability to mediate energy exchanges between the lanthanide dopants. This protocol describes the detailed exptl. procedure for synthesizing core-shell NaGdF4 nanoparticles that incorporate lanthanide ions into different layers for efficiently converting a single-wavelength, near-IR excitation into a tunable visible emission. These nanoparticles can then be used as luminescent probes in biol. samples, in 3D displays, in solar energy conversion and in photodynamic therapy. The NaGdF4 nanoparticles are grown through co-pptn. in a binary solvent mixt. of oleic acid and 1-octadecene. Doping by lanthanides with controlled compns. and concns. can be achieved concomitantly with particle growth. The lanthanide-doped NaGdF4 nanoparticles then serve as seed crystals for subsequent epitaxial growth of shell layers comprising different lanthanide dopants. The entire procedure for the prepn. and isolation of the core-shell nanoparticles comprising two epitaxial shell layers requires ∼15 h for completion.
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ACS Nano

Cite this: ACS Nano 2018, 12, 5, 4812–4823

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https://doi.org/10.1021/acsnano.8b01545

Published April 12, 2018

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Abstract
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Figure 1
Figure 1. (a) UC emission spectrum of β-NaYF₄ nanocrystals co-doped with 2% Er³⁺ and 18% Yb³⁺ dispersed in cyclohexane, upon laser excitation in the Yb³⁺: ²F_7/2 → ²F_5/2 transition at 980 nm. The nanocrystals emit green photons from the ²H_11/2 (520 nm) and ⁴S_3/2 (540 nm) states and red photons (660 nm) from the ⁴F_9/2 state. (b) Upon excitation of Yb³⁺ at 980 nm (dark-red wavy arrows), two consecutive energy-transfer processes (dotted lines) excite an Er³⁺ ion to a high excited state. Subsequently, after one or more nonradiative decay steps (black arrows) Er³⁺ can emit a visible photon (red or green wavy arrows). Other, more complicated, sequences of processes also contribute to the population of visible-emitting states. (35−37) (c) Transmission electron micrograph of the core-only β-NaYF₄:0.1% Er³⁺ nanocrystals used in this work. Images of other nanocrystal samples and the extracted size distributions are presented in Figure S1 and Table S1. (d) Schematic of solvent quenching, where an excited state on a lanthanide center (here, Er³⁺) doped in a nanocrystal can decay by transferring energy to high-energy vibrations in the surrounding solvent. (e) The total rate of solvent quenching by dipole–dipole interaction depends strongly on the radial position of the lanthanide center in the nanocrystal, illustrated here for a nanocrystal of 10 nm radius without (red line) and with (blue line) a 3 nm-thick nonluminescent shell. (f) The range of solvents used in our experiments below, with different chemical nature (affecting the solvent-quenching rate; eq 2) and different refractive index (affecting the radiative decay rate; eq 3).
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Figure 2
Figure 2. (a) Energy-level scheme of Er³⁺ with the green emission from the ⁴S_3/2 level (in thermal equilibrium with ²H_11/2; ref (44)) indicated as a green arrow and the nonradiative transition to the ⁴F_9/2 level as a black dashed arrow. (b) Same for the red emission from the ⁴F_9/2 level and the competing nonradiative transition to the ⁴I_9/2 level. (c) Average excited-state decay rates ⟨Γ_tot⟩ of the green-emitting level in NaYF₄:0.1% Er³⁺ NCs dispersed in different solvents (different colors; see Figure 1f), compared to the decay rate in bulk NaYF₄:0.1% Er³⁺ (black cross). Filled circles represent core-only NCs, open circles core–shell NCs. Gray lines are iso-quantum-yield contours plotting η ≈ Γ_rad(n)/⟨Γ_tot⟩ with Γ_rad(n) according to eq 3. (d) Same, but for the red-emitting level. (e) The emission line shapes of the ⁴S_3/2 → ⁴F_9/2 (green) and the ⁴F_9/2 → ⁴I_9/2 (red) transitions of Er³⁺, which are responsible for nonradiative decay of the green- and red-emitting levels (see panel a), estimated based on the data from ref (45). (f) Infrared absorption spectra of hexane (red), octane (orange), cyclohexane (yellow), chloroform (green), toluene (cyan), chlorobenzene (blue), and o-dichlorobenzene (purple), taken from the SDBS database of the National Institute of Advanced Industrial Science and Technology (AIST), Japan. Dashed lines indicate the approximate spectral position of the C-H stretch vibrational energy in aliphatic (∼2930 cm^–1) and aromatic (∼3070 cm^–1) molecules. (g) Photoluminescence decay curves of the red-emitting ⁴F_9/2 level upon direct excitation in bulk NaYF₄:0.1% Er³⁺ (gray), compared to core–shell NCs (dark cyan) and core-only NCs (light cyan) of the same composition dispersed in toluene. The inset shows a zoom-in of the first 0.3 ms. Solid lines are fits to the solvent-quenching model (eq 5). (h) Same, but comparing bulk to NCs dispersed in cyclohexane.
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Figure 3
Figure 3. (a) The average decay rate of the green-emitting level ⁴S_3/2 for core–shell NCs (dark-colored bars) and core-only NCs (light-colors bars) with different doping concentrations (as indicated) and dispersed in different solvents (different colors), obtained from a single-exponential fit to the decay dynamics. White horizontal lines indicate the corresponding average decay rate in the bulk material. (b) Same for the red-emitting level ⁴F_9/2. (c) Possible cross-relaxation pathways from the green-emitting levels of Er³⁺ to neighboring Er³⁺ or Yb³⁺ ions in the ground state. (d) Same for the red-emitting level of Er³⁺, where all of the possible cross-relaxation pathways exhibit a large energy mismatch between the donor and acceptor transition. (e) Schematic representation of energy migration (i.e., hopping) among Er³⁺ ions to the NC surface. (f) Photoluminescence decay curves of the green Er³⁺ emission in core–shell NCs dispersed in toluene, with doping concentrations of 0.1% Er³⁺ (black), 2% Er³⁺ (blue), and 2% Er³⁺ + 18% Yb³⁺ (green). Solid lines are fits to our model including solvent quenching and cross-relaxation (eqs 8 and 10). The inset shows a zoom-in on the first 0.2 ms. (g) Same for the green emission from core-only NCs. (h) Same for the red emission from core-only NCs, doped with 0.1% Er³⁺ (black), 2% Er³⁺ (yellow), and 2% Er³⁺ + 18% Yb³⁺ (red).
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Figure 4
Figure 4. (a) Average decay rates ⟨Γ_tot⟩ of the near-infrared-emitting ⁴I_11/2 level of Er³⁺ in core-only NCs (filled circles) and core–shell NaYF₄ NCs (open circles) doped with 2% Er³⁺ and dispersed in different solvents, as determined from single-exponential fits. Gray lines are iso-quantum-yield contours plotting η ≈ Γ_rad(n)/⟨Γ_tot⟩, based on the bulk decay rate (black cross). (b) Photoluminescence decay curves of the near-infrared emission from core-only (blue) and core–shell (red) NaYF₄:2% Er³⁺ NCs dispersed in toluene. Inset is a zoom-in. Solid lines show an attempt to fit our solvent-quenching model (eq 5) to the experiments using one value for ρC. (c) Same as in (a), but for the ²F_5/2 level of Yb³⁺. Filled symbols represent core-only NCs, open symbols core–shell NCs. The NCs have dopant concentrations of 0.1% Yb³⁺ (squares), 18% Yb³⁺ (triangles), or 2% Er³⁺ plus 18% Yb³⁺(diamonds). The Er³⁺ and Yb³⁺ emissions cannot be spectrally distinguished for the co-doped NCs, so the recorded signal will contain contributions from both. The iso-quantum-yield contours are based on the bulk decay rate of Yb³⁺ (black cross). (d) A cartoon of rapid energy transfer and energy back-transfer between Er³⁺ and Yb³⁺ centers. (e) Photoluminescence decay curves of the near-infrared Yb³⁺ emission in NCs doped with 0.1% Yb³⁺ (red), 18% Yb³⁺ (green), or co-doped with 2% Er³⁺ and 18% Yb³⁺ (blue). The signal recorded from the co-doped NCs consists for a small part of ⁴I_11/2 emission from Er³⁺ ions. This constitutes an estimated <5% of the total emission, based on the elemental ratio 2/18 and the ratio of radiative decay rates (Figure S2).
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Figure 5
Figure 5. (a–c) The quantum yields of the various emitting levels in NaYF₄:Er³⁺,Yb³⁺ UC NCs as a function of shell thickness. We consider (a) the green Er³⁺ emission from the ⁴S_3/2 level, (b) the red Er³⁺ emission from the ⁴F_9/2 level, and (c) near-infrared Yb³⁺ emission from the ²F_5/2 level. Data points are obtained by integrating experimental photoluminescence decay curves (eq 12) from core-only NCs (filled symbols) or core–shell NCs (open symbols). We compare NCs dispersed in hexane (red), chlorobenzene (blue), and carbon disulfide (gray) and NCs doped with low concentration of the active center (0.1%; circles) to NCs co-doped with 2% Er³⁺ and 18% Yb³⁺ (squares). Lines are predictions from our solvent-quenching model for the green- and red-emitting levels at low doping (solid lines) and high doping (dashed lines; only distinct for the green emission). (d) Simplified model of UC with a ground state (0), an intermediate excited state (NIR), and a highest excited state (VIS). Arrows indicate excitation, decay, and energy-transfer processes.
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  One source of efficiency losses in photovoltaic cells is their transparency toward solar photons with energies below the band gap of the absorbing layer. This loss can be reduced using a process of up-conversion whereby ≥2 sub-band-gap photons generate a single above-gap exciton. Traditional approaches to up-conversion, such as nonlinear 2-photon absorption (2PA) or triplet fusion, suffer from low efficiency at solar light intensities, a narrow absorption bandwidth, nonoptimal absorption energies, and difficulties for implementing in practical devices. These deficiencies can be alleviated using the effect of Auger-up-conversion in thick-shell PbSe/CdSe quantum dots. This process relies on Auger recombination whereby 2 low-energy core-based excitons are converted into a single higher-energy shell-based exciton. Compared to their monocomponent counterparts, the tailored PbSe/CdSe heterostructures feature enhanced absorption cross sections, a higher efficiency of the productive Auger pathway involving reexcitation of a hole, and longer lifetimes of both core- and shell-localized excitons. These features lead to effective up-conversion cross sections that are >6 orders of magnitude higher than for std. nonlinear 2PA, which allows for efficient up-conversion of continuous wave IR light at intensities as low as a few Watts per square centimeter.
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5. 5
  Krämer, K. W.; Biner, D.; Frei, G.; Güdel, H. U.; Hehlen, M. P.; Lüthi, S. R. Hexagonal Sodium Yttrium Fluoride Based Green and Blue Emitting Upconversion Phosphors. Chem. Mater. 2004, 16, 1244– 1251, DOI: 10.1021/cm031124o
  There is no corresponding record for this reference.
6. 6
  Suyver, J. F.; Grimm, J.; van Veen, M. K.; Biner, D.; Krämer, K. W.; Güdel, H. U. Upconversion Spectroscopy and Properties of NaYF₄ Doped with Er³⁺, Tm³⁺ and/or Yb³⁺. J. Lumin. 2006, 117, 1– 12, DOI: 10.1016/j.jlumin.2005.03.011
  6
  Upconversion spectroscopy and properties of NaYF4 doped with Er3+, Tm3+ and/or Yb3+
  Suyver, J. F.; Grimm, J.; van Veen, M. K.; Biner, D.; Kraemer, K. W.; Guedel, H. U.
  Journal of Luminescence (2006), 117 (1), 1-12CODEN: JLUMA8; ISSN:0022-2313. (Elsevier B.V.)
  A spectroscopic study of NaYF4 powders doped with several different concns. of Er3+, Tm3+ and/or Yb3+ is described. Rare earth-doped NaYF4 is known to be a very efficient near-IR to visible upconverter. The overview emission spectra for all samples are presented and from these the upconversion efficiency is calcd. Raman spectroscopy of undoped NaYF4 is presented here for the 1st time, demonstrating that the dominant phonon modes in NaYF4 lie 300-400 cm-1. The fact that these phonon modes are also the optically active ones is further demonstrated by temp.-dependent excitation spectroscopy. These surprisingly low-energy phonon modes explain the extraordinarily high upconversion efficiency of the rare earth-doped NaYF4 samples. Excitation spectroscopy up to ∼ 70000 cm-1 in an NaErF4 sample reveals a multitude of Er3+ 4f excitations, including the illustrious 2F(2)5/2 one that was not obsd. in excitation spectroscopy before. From the low-temp. power-dependence of the emission intensities for an Er3+, Yb3+ codoped NaYF4 sample, the dominant upconversion mechanism at low temp. is a different one than at room temp. From direct excitation, the lifetimes of the Yb3+ 2F52 → 2F7/2, Er3+ 4F9/2 → 4I15/2 and Er3+ 4S3/2 → 4I15/2 emissions are detd. as a function of temp. for all samples. At elevated temps., a significant decrease in the green lifetime is obsd., which is correlated to a simultaneous quenching in the luminescence intensity. This quenching is ascribed to cross-relaxation between two nearby Er3+ ions.
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7. 7
  Martín-Rodríguez, R.; Fischer, S.; Ivaturi, A.; Froehlich, B.; Krämer, K. W.; Goldschmidt, J. C.; Richards, B. S.; Meijerink, A. Highly Efficient IR to NIR Upconversion in Gd₂O₂S:Er³⁺ for Photovoltaic Applications. Chem. Mater. 2013, 25, 1912– 1921, DOI: 10.1021/cm4005745
  7
  Highly Efficient IR to NIR Upconversion in Gd2O2S: Er3+ for Photovoltaic Applications
  Martin-Rodriguez, Rosa; Fischer, Stefan; Ivaturi, Aruna; Froehlich, Benjamin; Kraemer, Karl W.; Goldschmidt, Jan C.; Richards, Bryce S.; Meijerink, Andries
  Chemistry of Materials (2013), 25 (9), 1912-1921CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
  Upconversion (UC) is a promising option to enhance the efficiency of solar cells by conversion of sub-bandgap IR photons to higher energy photons that can be utilized by the solar cell. The UC quantum yield is a key parameter for a successful application. Here the UC luminescence properties of Er3+-doped Gd2O2S are investigated by means of luminescence spectroscopy, quantum yield measurements, and excited state dynamics expts. Excitation into the max. of the 4I15/2 → 4I13/2 Er3+ absorption band around 1500 nm induces very efficient UC emission from different Er3+ excited states with energies above the silicon bandgap, in particular, the emission originating from the 4I11/2 state around 1000 nm. Concn. dependent studies reveal that the highest UC quantum yield is realized for a 10% Er3+-doping concn. The UC luminescence is compared to the well-known Er3+-doped β-NaYF4 UC material for which the highest UC quantum yield has been reported for 25% Er3+. The UC internal quantum yields were measured in this work for Gd2O2S: 10%Er3+ and β-NaYF4: 25%Er3+ to be 12 ± 1% and 8.9 ± 0.7%, resp., under monochromatic excitation around 1500 nm at a power of 700 W/m2. The UC quantum yield reported here for Gd2O2S: 10%Er3+ is the highest value achieved so far under monochromatic excitation into the 4I13/2 Er3+ level. Power dependence and lifetime measurements were performed to understand the mechanisms responsible for the efficient UC luminescence. We show that the main process yielding 4I11/2 UC emission is energy transfer UC.
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8. 8
  Heer, S.; Kömpe, K.; Güdel, H.-U.; Haase, M. Highly Efficient Multicolour Upconversion Emission in Transparent Colloids of Lanthanide-Doped NaYF₄ Nanocrystals. Adv. Mater. 2004, 16, 2102– 2105, DOI: 10.1002/adma.200400772
  8
  Highly efficient multicolour upconversion emission in transparent colloids of lanthanide-doped NaYF4 nanocrystals
  Heer, Stephan; Koempe, Karsten; Guedel, Hans-Ulrich; Haase, Markus
  Advanced Materials (Weinheim, Germany) (2004), 16 (23-24), 2102-2105CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
  The authors report on the successful synthesis and very intense multicolor upconversion emission of Yb3+/Er3+ and Yb3+/Tm3+ codoped NaYF4 nanocrystals transparently dispersed in soln. The upconversion efficiency of such solns. is about eight orders of magnitude higher than for the previously reported colloids of lanthanide-doped phosphate nanocrystals. This enormous improvement of the upconversion efficiency of these materials opens the door for interesting future applications in the field of biolabeling.
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9. 9
  Boyer, J.-C.; Vetrone, F.; Cuccia, L. A.; Capobianco, J. A. Synthesis of Colloidal Upconverting NaYF₄ Nanocrystals Doped with Er³⁺, Yb³⁺ and Tm³⁺, Yb³⁺ via Thermal Decomposition of Lanthanide Trifluoroacetate Precursors. J. Am. Chem. Soc. 2006, 128, 7444– 7445, DOI: 10.1021/ja061848b
  9
  Synthesis of Colloidal Upconverting NaYF4 Nanocrystals Doped with Er3+, Yb3+ and Tm3+, Yb3+ via Thermal Decomposition of Lanthanide Trifluoroacetate Precursors
  Boyer, John-Christopher; Vetrone, Fiorenzo; Cuccia, Louis A.; Capobianco, John A.
  Journal of the American Chemical Society (2006), 128 (23), 7444-7445CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Upconverting lanthanide-doped nanocrystals were synthesized via the thermal decompn. of trifluoroacetate precursors in a mixt. of oleic acid and octadecene. This method provides highly luminescent nanoparticles through a simple 1-pot technique with only one preparatory step. The Er3+, Yb3+ and Tm3+, Yb3+ doped cubic NaYF4 nanocrystals are colloidally stable in nonpolar org. solvents and exhibit green/red and blue upconversion luminescence, resp., under 977 nm laser excitation with low power densities.
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10. 10
  Boyer, J.-C.; Manseau, M.-P.; Murray, J. I.; van Veggel, F. C. J. M. Surface Modification of Upconverting NaYF₄ Nanoparticles with PEG–Phosphate Ligands for NIR (800 nm) Biolabeling within the Biological Window. Langmuir 2010, 26, 1157– 1164, DOI: 10.1021/la902260j
  10
  Surface Modification of Upconverting NaYF4 Nanoparticles with PEG-Phosphate Ligands for NIR (800 nm) Biolabeling within the Biological Window
  Boyer, John-Christopher; Manseau, Marie-Pascale; Murray, Jill I.; van Veggel, Frank C. J. M.
  Langmuir (2010), 26 (2), 1157-1164CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
  The authors present a technique for the replacement of oleate with a PEG-phosphate ligand [PEG = poly(ethylene glycol)] as an efficient method for the generation of water-dispersible NaYF4 nanoparticles (NPs). The PEG-phosphate ligands are shown to exchange with the original oleate ligands on the surface of the NPs, resulting in water-dispersible NPs. The upconversion intensity of the NPs in aq. environments was severely quenched when compared to the original NPs in org. solvents. This is attributed to an increase in the multiphonon relaxations of the lanthanide excited state in aq. environments due to high energy vibrational modes of water mols. This problem could be overcome partially by the synthesis of core/shell NPs which demonstrated improved photophys. properties in water over the original core NPs. The PEG-phosphate coated upconverting NPs were then used to image a line of ovarian cancer cells (CaOV3) to demonstrate their promise in biol. application.
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11. 11
  Wang, F.; Wang, J.; Liu, X. Direct Evidence of a Surface Quenching Effect on Size-Dependent Luminescence of Upconversion Nanoparticles. Angew. Chem., Int. Ed. 2010, 49, 7456– 7460, DOI: 10.1002/anie.201003959
  11
  Direct evidence of a surface quenching effect on size-dependent luminescence of upconversion nanoparticles
  Wang, Feng; Wang, Juan; Liu, Xiaogang
  Angewandte Chemie, International Edition (2010), 49 (41), 7456-7460, S7456/1-S7456/3CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Yb/Tm co-doped NaGdF4 nanoparticles without or with a thin surface protection layer provide direct evidence of a surface quenching effect on size-dependent upconversion luminescence. The coating preserves the optical integrity of the nanoparticles (right-hand spectrum) and minimizes emission loss induced by surface quenching.
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12. 12
  Wang, F.; Deng, R.; Wang, J.; Wang, Q.; Han, Y.; Zhu, H.; Chen, X.; Liu, X. Tuning Upconversion through Energy Migration in Core–Shell Nanoparticles. Nat. Mater. 2011, 10, 968– 973, DOI: 10.1038/nmat3149
  12
  Tuning upconversion through energy migration in core-shell nanoparticles
  Wang, Feng; Deng, Renren; Wang, Juan; Wang, Qingxiao; Han, Yu; Zhu, Haomiao; Chen, Xueyuan; Liu, Xiaogang
  Nature Materials (2011), 10 (12), 968-973CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
  Photon upconversion is promising for applications such as biol. imaging, data storage or solar cells. Upconversion processes were studied in a broad range of Gd-based nanoparticles of varying compn. By rational design of a core-shell structure with a set of lanthanide ions incorporated into sepd. layers at precisely defined concns., efficient upconversion emission can be realized through Gd sublattice-mediated energy migration for a wide range of lanthanide activators without long-lived intermediary energy states. The use of the core-shell structure allows the elimination of deleterious cross-relaxation. This effect enables fine-tuning of upconversion emission through trapping of the migrating energy by the activators. The findings suggest a general approach to constructing a new class of luminescent materials with tunable upconversion emissions by controlled manipulation of energy transfer within a nanoscopic region.
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13. 13
  Zou, W.; Visser, C.; Maduro, J. A.; Pshenichnikov, M. S.; Hummelen, J. C. Broadband Dye-Sensitized Upconversion of Near-Infrared Light. Nat. Photonics 2012, 6, 560– 564, DOI: 10.1038/nphoton.2012.158
  13
  Broadband dye-sensitized upconversion of near-infrared light
  Zou, Wenqiang; Visser, Cindy; Maduro, Jeremio A.; Pshenichnikov, Maxim S.; Hummelen, Jan C.
  Nature Photonics (2012), 6 (8), 560-564CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
  Photon upconversion of near-IR photons is a promising way to overcome the Shockley-Queisser efficiency limit of 32% of a single-junction solar cell. However, the practical applicability of the most efficient known upconversion materials at moderate light intensities is limited by their extremely weak and narrowband near-IR absorption. Here, we introduce the concept of an upconversion material where an org. near-IR dye is used as an antenna for the β-NaYF4:Yb,Er nanoparticles in which the upconversion occurs. The overall upconversion by the dye-sensitized nanoparticles is dramatically enhanced (by a factor of ∼3,300) as a result of increased absorptivity and overall broadening of the absorption spectrum of the upconverter. The proposed concept can be extended to cover any part of the solar spectrum by using a set of dye mols. with overlapping absorption spectra acting as an extremely broadband antenna system, connected to suitable upconverters.
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14. 14
  Zhao, J.; Lu, Z.; Yin, Y.; McRae, C.; Piper, J. A.; Dawes, J. M.; Jin, D.; Goldys, E. M. Upconversion Luminescence with Tunable Lifetime in NaYF₄:Yb,Er Nanocrystals: Role of Nanocrystal Size. Nanoscale 2013, 5, 944– 952, DOI: 10.1039/C2NR32482B
  14
  Upconversion luminescence with tunable lifetime in NaYF4:Yb,Er nanocrystals: role of nanocrystal size
  Zhao, Jiangbo; Lu, Zhenda; Yin, Yadong; McRae, Christopher; Piper, James A.; Dawes, Judith M.; Jin, Dayong; Goldys, Ewa M.
  Nanoscale (2013), 5 (3), 944-952CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
  Despite recent achievements to reduce surface quenching in NaYF4:Yb,Er nanocrystals, a complete understanding of how the nanocrystal size affects the brightness of upconversion luminescence is still incomplete. Here we investigated upconversion luminescence of Yb,Er-doped nanocrystals in a broad range of sizes from 6 nm to 45 nm (cubic or hexagonal phases), displaying an increasing red-to-green luminescence intensity ratio and reduced luminescence lifetimes with decreasing size. By analyzing the upconversion process with a set of rate equations, we found that their asymptotic analytic solns. explain lower decay rates of red compared to green upconversion luminescence. Furthermore, we quantified the effect of the surface on luminescence lifetime in a model where nanocrystal emitters are divided between the near-surface and inside regions of each nanocrystal. We clarify the influence of the four nonradiative recombination mechanisms (intrinsic phonon modes, vibration energy of surface ligands, solvent-mediated quenching, and surface defects) on the decay rates for different-size nanocrystals, and find that the defect d. dominates decay rates for small (below 15 nm) nanocrystals. Our results indicate that a defect-redn. strategy is a key step in producing small upconversion nanocrystals with increased brightness for a variety of bioimaging and biosensing applications.
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15. 15
  Zhao, J.; Jin, D.; Schartner, E. P.; Lu, Y.; Liu, Y.; Zvyagin, A. V.; Zhang, L.; Dawes, J. M.; Xi, P.; Piper, J. A.; Goldys, E. M.; Monro, T. M. Single-Nanocrystal Sensitivity Achieved by Enhanced Upconversion Luminescence. Nat. Nanotechnol. 2013, 8, 729– 734, DOI: 10.1038/nnano.2013.171
  15
  Single-nanocrystal sensitivity achieved by enhanced upconversion luminescence
  Zhao, Jiangbo; Jin, Dayong; Schartner, Erik P.; Lu, Yiqing; Liu, Yujia; Zvyagin, Andrei V.; Zhang, Lixin; Dawes, Judith M.; Xi, Peng; Piper, James A.; Goldys, Ewa M.; Monro, Tanya M.
  Nature Nanotechnology (2013), 8 (10), 729-734CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
  High excitation irradiance can alleviate concn. quenching in upconversion luminescence when combined with higher activator concn., which can be increased from 0.5 to 8 mol% Tm3+ in NaYF4. This leads to significantly enhanced luminescence signals, by up to a factor of 70. By using such bright nanocrystals, remote tracking of a single nanocrystal with a microstructured optical-fiber dip sensor is demonstrated. This represents a sensitivity improvement of 3 orders of magnitude over benchmark nanocrystals such as quantum dots.
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16. 16
  Chen, D.; Huang, P. Highly Intense Upconversion Luminescence in Yb/Er:NaGdF₄ @NaYF₄ Core–Shell Nanocrystals with Complete Shell Enclosure of the Core. Dalton Trans. 2014, 43, 11299– 11304, DOI: 10.1039/c4dt01237b
  There is no corresponding record for this reference.
17. 17
  Liu, Y.; Zhao, J.; Zhang, R.; Liu, Y.; Liu, D.; Goldys, E. M.; Yang, X.; Xi, P.; Sunna, A.; Lu, J.; Shi, Y.; Leif, R. C.; Huo, Y.; Shen, J.; Piper, J. A.; Robinson, J. P.; Jin, D. Tunable Lifetime Multiplexing Using Luminescent Nanocrystals. Nat. Photonics 2014, 8, 32– 36, DOI: 10.1038/nphoton.2013.322
  17
  Tunable lifetime multiplexing using luminescent nanocrystals
  Lu, Yiqing; Zhao, Jiangbo; Zhang, Run; Liu, Yujia; Liu, Deming; Goldys, Ewa M.; Yang, Xusan; Xi, Peng; Sunna, Anwar; Lu, Jie; Shi, Yu; Leif, Robert C.; Huo, Yujing; Shen, Jian; Piper, James A.; Robinson, J. Paul; Jin, Dayong
  Nature Photonics (2014), 8 (1), 32-36CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
  Optical multiplexing plays an important role in applications such as optical data storage, document security, mol. probes and bead assays for personalized medicine. Conventional fluorescent color coding is limited by spectral overlap and background interference, restricting the no. of distinguishable identities. Here, we show that tunable luminescent lifetimes τ in the microsecond region can be exploited to code individual upconversion nanocrystals. In a single color band, one can generate more than ten nanocrystal populations with distinct lifetimes ranging from 25.6 μs to 662.4 μs and decode their well-sepd. lifetime identities, which are independent of both color and intensity. Such 'τ-dots' potentially suit multichannel bioimaging, high-throughput cytometry quantification, high-d. data storage, as well as security codes to combat counterfeiting. This demonstration extends the optical multiplexing capability by adding the temporal dimension of luminescent signals, opening new opportunities in the life sciences, medicine and data security.
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18. 18
  Arppe, R.; Hyppänen, I.; Perälä, N.; Peltomaa, R.; Kaiser, M.; Würth, C.; Christ, S.; Resch-Genger, U.; Schäferling, M.; Soukka, T. Quenching of the Upconversion Luminescence of NaYF₄:Yb³⁺,Er³⁺ and NaYF₄:Yb³⁺,Tm³⁺ Nanophosphors by Water: the Role of the Sensitizer Yb³⁺ in Non-Radiative Relaxation. Nanoscale 2015, 7, 11746– 11757, DOI: 10.1039/C5NR02100F
  18
  Quenching of the upconversion luminescence of NaYF4:Yb3+,Er3+ and NaYF4:Yb3+,Tm3+ nanophosphors by water: the role of the sensitizer Yb3+ in non-radiative relaxation
  Arppe, Riikka; Hyppanen, Iko; Perala, Niina; Peltomaa, Riikka; Kaiser, Martin; Wurth, Christian; Christ, Simon; Resch-Genger, Ute; Schaferling, Michael; Soukka, Tero
  Nanoscale (2015), 7 (27), 11746-11757CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
  We have studied the mechanisms of water-based quenching of the upconversion photoluminescence of upconverting nanophosphors (UCNPs) via luminescence decay measurements for a better understanding of the non-radiative deactivation pathways responsible for the relatively low upconversion luminescence efficiency in aq. solns. This included both upconversion luminescence measurements and the direct excitation of emissive energy states of Er3+ and Yb3+ dopants in NaYF4:Yb3+,Er3+ UCNPs by measuring the decays at 550 and 655 nm upon 380 nm excitation and at 980 nm upon 930 nm excitation, resp. The luminescence intensities and decays were measured from both bare and silanized NaYF4:Yb3+,Er3+ and NaYF4:Yb3+,Tm3+ UCNPs in H2O and D2O. The measurements revealed up to 99.9% quenching of the upconversion photoluminescence intensity of both Er3+ and Tm3+ doped bare nanophosphors by water. Instead of the multiphonon relaxation of excited energy levels of the activators, the main mechanism of quenching was found to be the multiphonon deactivation of the Yb3+ sensitizer ion caused by OH-vibrations on the surface of the nanophosphor. Due to the nonlinear nature of upconversion, the quenching of Yb3+ has a higher order effect on the upconversion emission intensity with the efficient Yb-Yb energy migration in the ∼35 nm nanocrystals making the whole nanophosphor vol. susceptible to surface quenching effects. The study underlines the need of efficient surface passivation for the use of UCNPs as labels in bioanal. applications performed in aq. solns.
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19. 19
  Fischer, S.; Johnson, N. J. J.; Pichaandi, J.; Goldschmidt, J. C.; van Veggel, F. C. J. M. Upconverting Core–Shell Nanocrystals with High Quantum Yield under Low Irradiance: On the Role of Isotropic and Thick Shells. J. Appl. Phys. 2015, 118, 193105, DOI: 10.1063/1.4936119
  19
  Upconverting core-shell nanocrystals with high quantum yield under low irradiance: On the role of isotropic and thick shells
  Fischer, Stefan; Johnson, Noah J. J.; Pichaandi, Jothirmayanantham; Goldschmidt, Jan Christoph; van Veggel, Frank C. J. M.
  Journal of Applied Physics (Melville, NY, United States) (2015), 118 (19), 193105/1-193105/12CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
  Colloidal upconverter nanocrystals (UCNCs) that convert near-IR photons to higher energies are promising for applications ranging from life sciences to solar energy harvesting. However, practical applications of UCNCs are hindered by their low upconversion quantum yield (UCQY) and the high irradiances necessary to produce relevant upconversion luminescence. Achieving high UCQY under practically relevant irradiance remains a major challenge. The UCQY is severely limited due to non-radiative surface quenching processes. We present a rate equation model for migration of the excitation energy to show that surface quenching does not only affect the lanthanide ions directly at the surface but also many other lanthanide ions quite far away from the surface. The av. migration path length is on the order of several nanometers and depends on the doping as well as the irradiance of the excitation. Using Er3+-doped β-NaYF4 UCNCs, we show that very isotropic and thick (∼10 nm) β-NaLuF4 inert shells dramatically reduce the surface-related quenching processes, resulting in much brighter upconversion luminescence at simultaneously considerably lower irradiances. For these UCNCs embedded in poly(Me methacrylate), we detd. an internal UCQY of 2.0% ± 0.2% using an irradiance of only 0.43 ± 0.03 W/cm2 at 1523 nm. Normalized to the irradiance, this UCQY is 120× higher than the highest values of comparable nanomaterials in the literature. Our findings demonstrate the important role of isotropic and thick shells in achieving high UCQY at low irradiances from UCNCs. Addnl., we measured the addnl. short-circuit current due to upconversion in silicon solar cell devices as a proof of concept and to support our findings detd. using optical measurements. (c) 2015 American Institute of Physics.
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20. 20
  Rinkel, T.; Raj, A. N.; Dühnen, S.; Haase, M. Synthesis of 10 nm β-NaYF₄:Yb,Er/NaYF4 Core/Shell Upconversion Nanocrystals with 5 nm Particle Cores. Angew. Chem., Int. Ed. 2016, 55, 1164– 1167, DOI: 10.1002/anie.201508838
  20
  Synthesis of 10 nm β-NaYF4:Yb,Er/NaYF4 Core/Shell Upconversion Nanocrystals with 5 nm Particle Cores
  Rinkel, Thorben; Raj, Athira Naduviledathu; Duehnen, Simon; Haase, Markus
  Angewandte Chemie, International Edition (2016), 55 (3), 1164-1167CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A new method is presented for prepg. gram amts. of very small core/shell upconversion nanocrystals without addnl. codoping of the particles. First, ca. 5 nm β-NaYF4:Yb,Er core particles are formed by the reaction of sodium oleate, rare-earth oleate, and ammonium fluoride, thereby making use of the fact that a high ratio of sodium to rare-earth ions promotes the nucleation of a large no. of β-phase seeds. Thereafter, a 2 nm thick NaYF4 shell is formed by using 3-4 nm particles of α-NaYF4 as a single-source precursor for the β-phase shell material. In contrast to the core particles, however, these α-phase particles are prepd. with a low ratio of sodium to rare-earth ions, which efficiently suppresses an undesired nucleation of β-NaYF4 particles during shell growth.
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21. 21
  Johnson, N. J. J.; He, S.; Diao, S.; Chan, E. M.; Dai, H.; Almutairi, A. Direct Evidence for Coupled Surface and Concentration Quenching Dynamics in Lanthanide-Doped Nanocrystals. J. Am. Chem. Soc. 2017, 139, 3275– 3282, DOI: 10.1021/jacs.7b00223
  21
  Direct Evidence for Coupled Surface and Concentration Quenching Dynamics in Lanthanide-Doped Nanocrystals
  Johnson, Noah J. J.; He, Sha; Diao, Shuo; Chan, Emory M.; Dai, Hongjie; Almutairi, Adah
  Journal of the American Chemical Society (2017), 139 (8), 3275-3282CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Luminescence quenching at high dopant concns. generally limits the dopant concn. to <1-5 mol% in lanthanide-doped materials, and this remains a major obstacle in designing materials with enhanced efficiency/brightness. Direct evidence is provided that the major quenching process at high dopant concns. is the energy migration to the surface (i.e., surface quenching) as opposed to the common misconception of cross-relaxation between dopant ions. After an inert epitaxial shell growth, Er (Er3+) concns. ≤100 mol% in NaY(Er)F4/NaLuF4 core/shell nanocrystals enhance the emission intensity of both upconversion and downshifted luminescence across different excitation wavelengths (980, 800, and 658 nm), with negligible concn. quenching effects. The results highlight the strong coupling of concn. and surface quenching effects in colloidal lanthanide-doped nanocrystals, and that inert epitaxial shell growth can overcome concn. quenching. These fundamental insights into the photophys. processes in heavily doped nanocrystals will give rise to enhanced properties not previously thought possible with compns. optimized in bulk.
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22. 22
  Hyppänen, I.; Höysniemi, N.; Arppe, R.; Schäferling, M.; Soukka, T. Environmental Impact on the Excitation Path of the Red Upconversion Emission of Nanocrystalline NaYF₄:Yb³⁺,Er³⁺. J. Phys. Chem. C 2017, 121, 6924– 6929, DOI: 10.1021/acs.jpcc.7b01019
  22
  Environmental Impact on the Excitation Path of the Red Upconversion Emission of Nanocrystalline NaYF4:Yb3+,Er3+
  Hyppanen, Iko; Hoysniemi, Niina; Arppe, Riikka; Schaferling, Michael; Soukka, Tero
  Journal of Physical Chemistry C (2017), 121 (12), 6924-6929CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  The mechanism for red upconversion luminescence of Yb-Er codoped materials is not generally agreed on in the literature. Both two-photon and three-photon processes have been suggested as the main path for red upconversion emission. The authors have studied β-NaYF4:Yb3+,Er3+ nanoparticles in H2O and D2O, and the authors propose that the nanoparticle environment is a major factor in the selection of the preferred red upconversion excitation pathway. In H2O, efficient multiphonon relaxation (MPR) promotes the two-photon mechanism through green emitting states, while, in D2O, MPR is less effective and the three-photon path involving back energy transfer to Yb3+ is the dominant mechanism. For the green upconversion emission, the authors' results suggest the common two-photon path through the 4F9/2 energy state in both H2O and D2O.
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23. 23
  Schietinger, S.; Aichele, T.; Wang, H.-Q.; Nann, T.; Benson, O. Plasmon-Enhanced Upconversion in Single NaYF₄:Yb³⁺/Er³⁺ Codoped Nanocrystals. Nano Lett. 2010, 10, 134– 138, DOI: 10.1021/nl903046r
  23
  Plasmon-Enhanced Upconversion in Single NaYF4:Yb3+/Er3+ Codoped Nanocrystals
  Schietinger, Stefan; Aichele, Thomas; Wang, Hai-Qiao; Nann, Thomas; Benson, Oliver
  Nano Letters (2010), 10 (1), 134-138CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
  In this Letter the authors report the plasmon-enhanced upconversion in single NaYF4 nanocrystals codoped with Yb3+/Er3+. Single nanocrystals and Au nanospheres were studied and assembled in a combined confocal and at. force microscope setup. The nanocrystals show strong upconversion emission in the green and red under excitation with a continuous wave laser in the near-IR at 973 nm. Using the at. force microscope, the authors couple single nanocrystals with Au spheres (30 and 60 nm in diam.) to obtain enhanced upconversion emission. An overall enhancement factor of 3.8 is reached. A comparison of time-resolved measurements on the bare nanocrystal and the coupled nanocrystal-Au sphere systems unveil that faster excitation as well as faster emission occurs in the nanocrystals.
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24. 24
  Han, S.; Deng, R.; Xie, X.; Liu, X. Enhancing Luminescence in Lanthanide-Doped Upconversion Nanoparticles. Angew. Chem., Int. Ed. 2014, 53, 11702– 11715, DOI: 10.1002/anie.201403408
  24
  Enhancing Luminescence in Lanthanide-Doped Upconversion Nanoparticles
  Han, Sanyang; Deng, Renren; Xie, Xiaoji; Liu, Xiaogang
  Angewandte Chemie, International Edition (2014), 53 (44), 11702-11715CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A review. The enthusiasm for research on lanthanide-doped upconversion nanoparticles is driven by both a fundamental interest in the optical properties of lanthanides embedded in different host lattices and their promise for broad applications ranging from biol. imaging to photodynamic therapy. Despite the considerable progress made in the past decade, the field of upconversion nanoparticles has been hindered by significant exptl. challenges assocd. with low upconversion conversion efficiencies. Recent exptl. and theor. studies on upconversion nanoparticles have, however, led to the development of several effective approaches to enhancing upconversion luminescence, which could have profound implications for a range of applications. Herein we present the underlying principles of controlling energy transfer through lanthanide doping, overview the major advances and key challenging issues in improving upconversion luminescence, and consider the likely directions of future research in the field.
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25. 25
  Sun, Q.-C.; Mundoor, H.; Ribot, J. C.; Singh, V.; Smalyukh, I. I.; Nagpal, P. Plasmon-Enhanced Energy Transfer for Improved Upconversion of Infrared Radiation in Doped-Lanthanide Nanocrystals. Nano Lett. 2014, 14, 101– 106, DOI: 10.1021/nl403383w
  25
  Plasmon-Enhanced Energy Transfer for Improved Upconversion of Infrared Radiation in Doped-Lanthanide Nanocrystals
  Sun, Qi-C.; Mundoor, Haridas; Ribot, Josep C.; Singh, Vivek; Smalyukh, Ivan I.; Nagpal, Prashant
  Nano Letters (2014), 14 (1), 101-106CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
  Upconversion of IR radiation into visible light was studied for applications in photovoltaics and biol. imaging. However, low conversion efficiency due to small absorption cross-section for IR light (Yb3+), and slow rate of energy transfer (to Er3+ states) has prevented application of upconversion photoluminescence (UPL) for diffuse sunlight or imaging tissue samples. Here, the authors use resonant surface plasmon polaritons (SPP) waves to enhance UPL in doped-lanthanide nanocrystals. The anal. indicates that SPP waves not only enhance the electromagnetic field, and hence weak Purcell effect, but also increase the rate of resonant energy transfer from Yb3+ to Er3+ ions by 6 fold. While the authors do observe strong metal mediated quenching (14-fold) of green fluorescence on flat metal surfaces, the nanostructured metal is resonant in the IR and hence enhances the nanocrystal UPL. This strong Coulombic effect on energy transfer can have important implications for other fluorescent and excitonic systems too.
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26. 26
  Fischer, S.; Kumar, D.; Hallermann, F.; von Plessen, G.; Goldschmidt, J. C. Enhanced Upconversion Quantum Yield near Spherical Gold Nanoparticles – a Comprehensive Simulation Based Analysis. Opt. Express 2016, 24, A460– A475, DOI: 10.1364/OE.24.00A460
  There is no corresponding record for this reference.
27. 27
  Yin, Z.; Li, H.; Xu, W.; Cui, S.; Zhou, D.; Chen, X.; Zhu, Y.; Qin, G.; Song, H. Local Field Modulation Induced Three-Order Upconversion Enhancement: Combining Surface Plasmon Effect and Photonic Crystal Effect. Adv. Mater. 2016, 28, 2518– 2525, DOI: 10.1002/adma.201502943
  27
  Local Field Modulation Induced Three-Order Upconversion Enhancement: Combining Surface Plasmon Effect and Photonic Crystal Effect
  Yin, Ze; Li, Hang; Xu, Wen; Cui, Shaobo; Zhou, Donglei; Chen, Xu; Zhu, Yongsheng; Qin, Guanshi; Song, Hongwei
  Advanced Materials (Weinheim, Germany) (2016), 28 (13), 2518-2525CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
  The authors present a novel device and significant modulation of gold nanorods (AuNRs)/Polymethylmethacrylate (PMMA) opal photonic crystals (OPCs) surface plasmon photonic crystal (SPPC) on upconversion luminescence (UCL) of NaYF4:Yb3+, Er3+ NPs, which has perfectly combined surface plasmon effect of AuNRs and PC effects of 3D PMMA opals. In the hybrids, the UCL of NaYF4:Yb3+, Er3+ has been enhanced more than 103 folds, which is at least an order of magnitude higher than that reported by the previous literature.
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28. 28
  Zhou, D.; Liu, D.; Xu, W.; Yin, Z.; Chen, X.; Zhou, P.; Cui, S.; Chen, Z.; Song, H. Observation of Considerable Upconversion Enhancement Induced by Cu_2–xS Plasmon Nanoparticles. ACS Nano 2016, 10, 5169– 5179, DOI: 10.1021/acsnano.6b00649
  28
  Observation of Considerable Upconversion Enhancement Induced by Cu2-xS Plasmon Nanoparticles
  Zhou, Donglei; Liu, Dali; Xu, Wen; Yin, Ze; Chen, Xu; Zhou, Pingwei; Cui, Shaobo; Chen, Zhanguo; Song, Hongwei
  ACS Nano (2016), 10 (5), 5169-5179CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  Localized surface plasmon resonances (LSPRs) are achieved in heavily doped semiconductor nanoparticles (NPs) with appreciable free carrier concns. In this paper, we present the photonic, elec., and photoelec. properties of plasmonic Cu2-xS NPs/films and the utilization of LSPRs generated from semiconductor NPs as near-IR antennas to enhance the upconversion luminescence (UCL) of NaYF4:Yb3+,Er3+ NPs. Our results suggest that the LSPRs in Cu2-xS NPs originate from ligand-confined carriers and that a heat treatment resulted in the decompn. of ligands and oxidn. of Cu2-xS NPs; these effects led to a decrease of the Cu2+/Cu+ ratio, which in turn resulted in the broadening, decrease in intensity, and red-shift of the LSPRs. In the presence of a MoO3 spacer, the UCL intensity of NaYF4:Yb3+,Er3+ NPs was substantially improved and exhibited extraordinary power-dependent behavior because of the energy band structure of the Cu2-xS semiconductor. These findings provide insights into the nature of LSPR in semiconductors and their interaction with nearby emitters and highlight the possible application of LSPR in photonic and photoelec. devices.
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29. 29
  Xu, W.; Chen, X.; Song, H. Upconversion Manipulation by Local Electromagnetic Field. Nano Today 2017, 17, 54– 58, DOI: 10.1016/j.nantod.2017.10.011
  29
  Upconversion manipulation by local electromagnetic field
  Xu, Wen; Chen, Xu; Song, Hongwei
  Nano Today (2017), 17 (), 54-78CODEN: NTAOCG; ISSN:1748-0132. (Elsevier Ltd.)
  Rare earth doped upconversion nanocrystals (RE-UCNCs) have attracted extensive interests owing to their unique phys. properties and great potential applications in bio-application, photonic and photoelec. devices etc. Although UCNCs open doors to a wide range of new opportunities, they are confronting with some difficulties and one of the fatal problems is their low upconversion luminescent strength/efficiency. To date, various methods have been explored to solving this significant issue. Totally to say, the methods can be classified into two aspects, the traditional size, structure, surface and crystal field controls of the UCNCs, and the novel local electromagnetic field modulation surrounding the UCNCs. The local electromagnetic field modulation on UCNCs is a powerful strategy to enhance the strength/efficiency of UCNCs, reporting enhancement from several times up to four orders in short times. The timely and concise summary on the previous literatures is significant for more rapid and formulated development of this field. This review is aimed at offering a comprehensive framework for metal/semiconductor plasmon-induced and photonic crystal effect induced upconversion enhancement. Differing from the other review articles, we first introduced the generation principle of localized electromagnetic field in metal/semiconductor nanostructure/photonic crystals, and their general interaction rules with various emitters. Then, we summed up the recent published works on the local field modulation-induced upconversion enhancement, on emphasis we did our best to discover the generality of obtaining highly improved photoluminescence for any emitters and the personality of realizing highly improved upconversion enhancement. We further prospected the future development in this attractive field based on the previous theor. and exptl. results and the requirement of application. The marriage of upconversion with nanophotonic could explore a novel frontier in photonics that potentially spawn many exciting new fields.
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30. 30
  Stouwdam, J. W.; Hebbink, G. A.; Huskens, J.; van Veggel, F. C. J. M. Lanthanide-Doped Nanoparticles with Excellent Luminescent Properties in Organic Media. Chem. Mater. 2003, 15, 4604– 4616, DOI: 10.1021/cm034495d
  30
  Lanthanide-Doped Nanoparticles with Excellent Luminescent Properties in Organic Media
  Stouwdam, Jan W.; Hebbink, Gerald A.; Huskens, Jurriaan; Van Veggel, Frank C. J. M.
  Chemistry of Materials (2003), 15 (24), 4604-4616CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
  Surface-coated nanoparticles of LaF3 and LaPO4 doped with the luminescent trivalent lanthanide ions Eu3+, Nd3+, Er3+, Pr3+, Ho3+, and Yb3+ were prepd. These ions emit in the visible and in the near-IR part of the electromagnetic spectrum. The ions Nd3+, Er3+, Pr3+, and Ho3+ are the main focus in this research because they show emissions in telecommunication windows. The Yb3+ ion is of interest because it can be used as a sensitizer for Er3+. The Eu3+ ion was used as a probe for the structural environment of the luminescent ion. These lanthanide ions are incorporated in the inorg. host of the particles and the particles are dispersible in org. solvents. The luminescent lifetimes of the ions are increased by orders of magnitude compared to org. complexes, with values ranging from several microseconds for Pr3+ and Ho3+, up to ∼200 μs for Nd3+, and 1 ms for Er3+. This increase in the luminescence lifetime is indicative of an effective shielding of the lanthanide ions from nonradiative decay of the excited state by the high-energy vibrations of the solvents and the coordinated org. ligands. A model is proposed to describe the nonexponential behavior of the luminescence decay by quenching from outside the particle. It uses two fit parameters, kR, a parameter describing the luminescence decay rate in the absence of surface quenching, and C, a parameter describing the sensitivity toward quenching. Using this model, the luminescence decays can be fitted very well, and factors influencing the luminescence lifetime like concn. quenching and solvent effects can be described accurately.
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31. 31
  Guyot-Sionnest, P.; Wehrenberg, B.; Yu, D. Intraband Relaxation in CdSe Nanocrystals and the Strong Influence of the Surface Ligands. J. Chem. Phys. 2005, 123, 074709, DOI: 10.1063/1.2004818
  31
  Intraband relaxation in CdSe nanocrystals and the strong influence of the surface ligands
  Guyot-Sionnest, Philippe; Wehrenberg, Brian; Yu, Dong
  Journal of Chemical Physics (2005), 123 (7), 074709/1-074709/7CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
  The intraband relaxation between the 1Pe and 1Se state of CdSe colloidal quantum dots is studied by pump-probe time-resolved spectroscopy. IR pump-probe measurements with ∼6-ps pulses show identical relaxation whether the electron was placed in the 1Se state by above band-gap photoexcitation or by electrochem. charging. The intraband relaxation of the electrons is not affected by the photogenerated holes which were trapped. However, the surface ligands strongly affect the rate of relaxation in colloid solns. Faster relaxation (<8 ps) is obtained with phosphonic acid and oleic acid ligands. Alkylamines lead to longer relaxation times of ∼10 ps and the slowest relaxation is obsd. for dodecanethiol ligands with relaxation times ∼30 ps. In the absence of holes or when the holes are trapped, the intraband relaxation is dominated by the surface and faster relaxation correlates with larger interfacial polarity. Energy transfer to the ligand vibrations may be sufficiently effective to account for the intraband relaxation rate.
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32. 32
  Aharoni, A.; Oron, D.; Banin, U.; Rabani, E.; Jortner, J. Long-Range Electronic-to-Vibrational Energy Transfer from Nanocrystals to Their Surrounding Matrix Environment. Phys. Rev. Lett. 2008, 100, 057404, DOI: 10.1103/PhysRevLett.100.057404
  32
  Long-Range Electronic-to-Vibrational Energy Transfer from Nanocrystals to Their Surrounding Matrix Environment
  Aharoni, Assaf; Oron, Dan; Banin, Uri; Rabani, Eran; Jortner, Joshua
  Physical Review Letters (2008), 100 (5), 057404/1-057404/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
  A radiationless transition process of long-range, resonance interconversion of electronic-to-vibrational energy transfer (EVET) is discovered between the band-gap excitation of nanocrystal quantum dots to matrix vibrational overtone modes using fluorescence lifetime measurements. A theor. anal. based on long-range dipole-dipole nonadiabatic couplings, being distinct from the traditional adiabatic or static-coupling pictures, is given and is in qual. agreement with expts. EVET should be considered in matrix choices for near-IR optoelectronic applications of nanocrystals.
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33. 33
  Rabouw, F. T.; Den Hartog, S. A.; Senden, T.; Meijerink, A. Photonic Effects on the Förster Resonance Energy Transfer Efficiency. Nat. Commun. 2014, 5, 3610, DOI: 10.1038/ncomms4610
  33
  Photonic effects on the Forster resonance energy transfer efficiency
  Rabouw Freddy T; den Hartog Stephan A; Senden Tim; Meijerink Andries
  Nature communications (2014), 5 (), 3610 ISSN:.
  Forster resonance energy transfer (ET) between luminescent species is applied in bio-imaging, lighting and photovoltaics, and has an important role in photosynthesis. However, the fundamental question of whether ET rates and efficiencies can be tuned by the photonic environment remains under debate. Here we show that ET rates are independent of the photonic environment, using the model system of LaPO4 nanocrystals co-doped with Ce(3+) donors and Tb(3+) acceptors. Although the radiative emission rate of the Ce(3+) excited state increases with the refractive index of the solvent in which the nanocrystals are dispersed, the Ce(3+)-to-Tb(3+) ET rate does not. We demonstrate that, as a result, lower refractive index solvents enable higher ET efficiencies leading to higher Tb(3+) emission intensities. Furthermore, an analytical model for ET in (nano)crystalline host materials is presented, able to predict the dependence of ET efficiencies on the photonic environment and the concentration of acceptor ions.
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34. 34
  Senden, T.; Rabouw, F. T.; Meijerink, A. Photonic Effects on the Radiative Decay Rate and Luminescence Quantum Yield of Doped Nanocrystals. ACS Nano 2015, 9, 1801– 1808, DOI: 10.1021/nn506715t
  34
  Photonic Effects on the Radiative Decay Rate and Luminescence Quantum Yield of Doped Nanocrystals
  Senden, Tim; Rabouw, Freddy T.; Meijerink, Andries
  ACS Nano (2015), 9 (2), 1801-1808CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  Nanocrystals (NCs) doped with luminescent ions form an emerging class of materials. In contrast to excitonic transitions in semiconductor NCs, the optical transitions are localized and not affected by quantum confinement. The radiative decay rates of the dopant emission in NCs are nevertheless different from their bulk analogs due to photonic effects, and also the luminescence quantum yield (QY, important for applications) is affected. In the past, different theor. models probably describe the photonic effects for dopant emission in NCs, with little exptl. validation. The photonic effects on the radiative decay rate of luminescent doped NCs were studied using 4 nm LaPO4 NCs doped with Ce3+ or Tb3+ ions in different refractive index solvents and bulk crystals. The measured influence of the refractive index on the radiative decay rate of the Ce3+ emission, having near unity QY, is in excellent agreement with the theor. nanocrystal-cavity model. The nanocrystal-cavity model can be used to quantify the nonunity QY of Tb3+-doped LaPO4 NCs and demonstrate that, as a general rule, the QY is higher in media with higher refractive index.
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35. 35
  Anderson, R. B.; Smith, S. J.; May, P. S.; Berry, M. T. Revisiting the NIR-to-Visible Upconversion Mechanism in β-NaYF₄:Yb³⁺,Er³⁺. J. Phys. Chem. Lett. 2014, 5, 36– 42, DOI: 10.1021/jz402366r
  35
  Revisiting the NIR-to-Visible Upconversion Mechanism in β-NaYF4:Yb3+,Er3+
  Anderson, Robert B.; Smith, Steve J.; May, P. Stanley; Berry, Mary T.
  Journal of Physical Chemistry Letters (2014), 5 (1), 36-42CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  Here, we show that the long-accepted mechanism for the prodn. of red and blue emission through upconversion (UC) of 1 μm excitation in Yb3+/Er3+-doped materials does not apply in the popular β-NaYF4 host. We propose a new mechanism involving Yb3+-to-Er3+ energy-transfer UC out of the green-emitting 2H11/2,4S3/2 states that quant. accounts for all of the obsd. optical behavior. Rate consts. for the relevant radiative and nonradiative processes are reported along with a prediction of the power dependence of the pulsed and continuous-wave UC quantum efficiency.
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36. 36
  Berry, M. T.; May, P. S. Disputed Mechanism for NIR-to-Red Upconversion Luminescence in NaYF₄:Yb³⁺,Er³⁺. J. Phys. Chem. A 2015, 119, 9805– 9811, DOI: 10.1021/acs.jpca.5b08324
  36
  Disputed Mechanism for NIR-to-Red Upconversion Luminescence in NaYF4:Yb3+,Er3+
  Berry, Mary T.; May, P. Stanley
  Journal of Physical Chemistry A (2015), 119 (38), 9805-9811CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
  The most commonly proposed mechanisms for NIR-to-red upconversion in the well-studied material β-NaYF4:Er3+,Yb3+ are evaluated to resolve inconsistencies that persist in the literature. Each of 4 possible mechanisms is evaluated in terms of the direct anal. of spectroscopic data. There are no important mechanisms that involve the 1st excited state of Er3+, 4I13/2, as an intermediate state. A large body of evidence overwhelmingly supports the proposed mechanism of Anderson et al., which suggests an intimate connection between NIR-to-red and NIR-to-blue upconversion. Both red and blue upconversion are produced primarily by a 3-photon excitation process that proceeds through the green emitting state to a dense manifold of states, 4G/2K, above the blue emitting state, 2H9/2. Competing relaxation mechanisms out of 4G/2K det. the relative amts. of blue and red upconversion produced. Multiphonon relaxation from 4G/2K results in blue upconversion, whereas back energy transfer from Er3+(4G/2K) to Yb3+(2F7/2) results in red emission.
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37. 37
  Zhang, J.; Hao, Z.; Li, J.; Zhang, X.; Luo, Y.; Pan, G. Observation of Efficient Population of the Red-Emitting State from the Green State by Non-Multiphonon Relaxation in the Er³⁺–Yb³⁺ System. Light: Sci. Appl. 2015, 4, e239, DOI: 10.1038/lsa.2015.12
  37
  Observation of efficient population of the red-emitting state from the green state by non-multiphonon relaxation in the Er3+-Yb3+ system
  Zhang, Jiahua; Hao, Zhendong; Li, Jing; Zhang, Xia; Luo, Yongshi; Pan, Guohui
  Light: Science & Applications (2015), 4 (1), e239CODEN: LSAIAZ; ISSN:2047-7538. (Nature Publishing Group)
  The rare earth Er3+ and Yb3+ codoped system is the most attractive for showcasing energy transfer upconversion. This system can generate green and red emissions from Er3+ under IR excitation of the sensitizer Yb3+. It is well known that the red-emitting state can be populated from the upper green-emitting state. The contribution of multiphonon relaxation to this population is generally considered important at low excitation densities. Here, we demonstrate for the first time the importance of a previously proposed but neglected mechanism described as a cross relaxation energy transfer from Er3+ to Yb3+, followed by an energy back transfer within the same Er3+-Yb3+ pair. A luminescence spectroscopy study of cubic Y2O3:Er3+, Yb3+ indicates that this mechanism can be more efficient than multiphonon relaxation, and it can even make a major contribution to the red upconversion. The study also revealed that the energy transfers involved in this mechanism take place only in the nearest Er3+-Yb3+ pairs, and thus, it is fast and efficient at low excitation densities. Our results enable a better understanding of upconversion processes and properties in the Er3+-Yb3+ system.
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38. 38
  van Dijk, J. M. F.; Schuurmans, M. F. H. On the Nonradiative and Radiative Decay Rates and a Modified Exponential Energy Gap Law for 4f–4f Transitions in Rare-Earth Ions. J. Chem. Phys. 1983, 78, 5317– 5323, DOI: 10.1063/1.445485
  38
  On the nonradiative and radiative decay rates and a modified exponential energy gap law for 4f-4f transitions in rare-earth ions
  Van Dijk, J. M. F.; Schuurmans, M. F. H.
  Journal of Chemical Physics (1983), 78 (9), 5317-23CODEN: JCPSA6; ISSN:0021-9606.
  A modified exponential energy gap law for nonradiative decay was derived for 4f-4f transitions where only a few phonons participate in the transition. Its preexponential factor varies by only a factor of 10 for different host materials and is purely electronic in nature. The modified law is the usual law (giving a factor of 105 variation) modified by using an energy gap which is effectively 2 (max.) phonon energies smaller than the energy difference between the initial and the final electronic state. A general relation between the radiative and nonradiative decay rates was constructed. For 4f-4f transitions, this relation enables one to predict the nonradiative decay rate from a knowledge of the radiative decay rate to within 1 order of magnitude accuracy.
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39. 39
  Sokolov, V. I.; Zvyagin, A. V.; Igumnov, S. M.; Molchanova, S. I.; Nazarov, M. M.; Nechaev, A. V.; Savelyev, A. G.; Tyutyunov, A. A.; Khaydukov, E. V.; Panchenko, V. Ya. Determination of the Refractive Index of β-NaYF₄/Yb³⁺/Er³⁺/Tm³⁺ Nanocrystals Using Spectroscopic Refractometry. Opt. Spectrosc. 2015, 118, 609– 613, DOI: 10.1134/S0030400X15040190
  39
  Determination of the refractive index of β-NaYF4/Yb3+/Er3+/Tm3+ nanocrystals using spectroscopic refractometry
  Sokolov, V. I.; Zvyagin, A. V.; Igumnov, S. M.; Molchanova, S. I.; Nazarov, M. M.; Nechaev, A. V.; Savelyev, A. G.; Tyutyunov, A. A.; Khaydukov, E. V.; Panchenko, V. Ya.
  Optics and Spectroscopy (2015), 118 (4), 609-613CODEN: OPSUA3; ISSN:0030-400X. (SP MAIK Nauka/Interperiodica)
  A method for measuring refractive index n of nanosize particles in the visible and near-IR spectral ranges is proposed. The method is based on comparing refractive index ncolloid of a colloid soln. of nanoparticles in several solvents with refractive indexes nsolvent of corresponding pure solvents and has an accuracy of ±2 × 10-4. Upconversion nanosize phosphors (UCNPs) are synthesized in the form of a β-NaYF4 cryst. matrix doped with Yb3+, Er3+, and Tm3+ rare earth ions. UCNPs have a doped core with a diam. of 40 ± 5 nm and undoped shell with a thickness of 3-5 nm. Synthesized nanocrystals possess intense photoluminescence in the blue, green, and red spectral ranges upon excitation by IR radiation with a wavelength of 977 nm. Using a spectroscopic refractometer, the dispersion of the refractive index of β-NaYF4/Yb3+/Er3+/Tm3+ nanocrystals was measured for the first time in the spectral range of 450-1000 nm with an accuracy of ±2 × 10-4.
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40. 40
  Dong, C.; Pichaandi, J.; Regier, T.; van Veggel, F. C. J. M. Nonstatistical Dopant Distribution of Ln³⁺-Doped NaGdF₄ Nanoparticles. J. Phys. Chem. C 2011, 115, 15950– 15958, DOI: 10.1021/jp206441u
  40
  Nonstatistical Dopant Distribution of Ln3+-Doped NaGdF4 Nanoparticles
  Dong, Cunhai; Pichaandi, Jothirmayanantham; Regier, Tom; van Veggel, Frank C. J. M.
  Journal of Physical Chemistry C (2011), 115 (32), 15950-15958CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  Oleate-stabilized NaGdF4 nanoparticles codoped with 20% Y3+ and 5% Tb3+ (NaGdF4:Y,Tb), with 20% Nd3+ (NaGdF4:Nd), and with 20% Tb3+ (NaGdF4:Tb) were prepd. in org. medium. The distribution of dopant ions was studied using synchrotron radiation XPS along with x-ray powder diffractometry, TEM, energy dispersive x-ray spectroscopy, and luminescence spectroscopy. These nanoparticles do not have the intended statistical dopant distribution despite the fact that different synthesis procedures and dopant ions with different ionic radii were used. NaGdF4:Y,Tb nanoparticles have a subtle gradient structure with Gd3+ more concd. toward the center and Y3+ more concd. toward the surface of the nanoparticles. NaGdF4:Nd nanoparticles have a steep gradient structure with Gd3+ more concd. toward the center and Nd3+ more concd. toward the surface of the nanoparticles. Even NaGdF4:Tb nanoparticles have a steep gradient structure with Tb3+ more concd. toward the center and Gd3+ more concd. toward the surface of the nanoparticles in spite of the very similar ionic radius of Gd3+ and Tb3+. The general assumption that Ln3+ dopant ions in lanthanide-based nanoparticles are statistically distributed in the whole nanoparticle may not be true.
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41. 41
  van Wijngaarden, J. T.; Scheidelaar, S.; Vlugt, T. J. H.; Reid, M. F.; Meijerink, A. Energy Transfer Mechanism for Downconversion in the (Pr³⁺, Yb³⁺) Couple. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 155112, DOI: 10.1103/PhysRevB.81.155112
  41
  Energy transfer mechanism for downconversion in the (Pr3+, Yb3+) couple
  van Wijngaarden, J. T.; Scheidelaar, S.; Vlugt, T. J. H.; Reid, M. F.; Meijerink, A.
  Physical Review B: Condensed Matter and Materials Physics (2010), 81 (15), 155112/1-155112/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
  Downconversion of one visible photon into two IR photons has been reported for the lanthanide ion couple (Pr3+, Yb3+) in a variety of host lattices. The mechanism responsible for downconversion is controversial and has been reported to be either a two-step energy transfer process (via two first-order transfer steps, the first being cross relaxation) or cooperative energy transfer from Pr3+ to two Yb3+ ions (a second-order process). Here the authors report expts. on downconversion for the (Pr3+, Yb3+) in LiYF4. Luminescence decay curves of the Pr3+ emission are recorded as a function of the Yb3+ concn. and analyzed using Monte Carlo simulations for both cooperative energy transfer and energy transfer through cross relaxation. A good agreement is obtained between expt. and simulations for energy transfer by cross relaxation but not for cooperative energy transfer. The observation that cross relaxation is more efficient than cooperative energy transfer is consistent with Judd-Ofelt calcns. for the transition probabilities involved in the two energy transfer processes and the lower probability for the second-order cooperative transfer.
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42. 42
  Yu, D. C.; Martín-Rodrïguez, R.; Zhang, Q. Y.; Meijerink, A.; Rabouw, F. T. Multi-Photon Quantum Cutting in Gd₂O₂S:Tm³⁺ to Enhance the Photo-Response of Solar Cells. Light: Sci. Appl. 2015, 4, e344, DOI: 10.1038/lsa.2015.117
  42
  Multi-photon quantum cutting in Gd2O2S:Tm3+ to enhance the photo-response of solar cells
  Yu, De-Chao; Martin-Rodriguez, Rosa; Zhang, Qin-Yuan; Meijerink, Andries; Rabouw, Freddy T.
  Light: Science & Applications (2015), 4 (10), e344CODEN: LSAIAZ; ISSN:2047-7538. (Nature Publishing Group)
  Conventional photoluminescence (PL) yields at most one emitted photon for each absorption event. Downconversion (or quantum cutting) materials can yield more than one photon by virtue of energy transfer processes between luminescent centers. In this work, we introduce Gd2O2S:Tm3+ as a multi-photon quantum cutter. It can convert near-IR, visible, or UV photons into two, three, or four IR photons of ∼1800 nm, resp. The cross-relaxation steps between Tm3+ ions that lead to quantum cutting are identified from (time-resolved) PL as a function of the Tm3+ concn. in the crystal. A model is presented that reproduces the way in which the Tm3+ concn. affects both the relative intensities of the various emission lines and the excited state dynamics and providing insight in the quantum cutting efficiency. Finally, we discuss the potential application of Gd2O2S:Tm3+ for spectral conversion to improve the efficiency of next-generation photovoltaics.
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43. 43
  Henderson, B.; Imbusch, G. F. Optical Spectroscopy of Inorganic Solids; Clarendon Press: Oxford, 1989.
  There is no corresponding record for this reference.
44. 44
  Geitenbeek, R. G.; Prins, P. T.; Albrecht, W.; van Blaaderen, A.; Weckhuysen, B. M.; Meijerink, A. NaYF₄:Er³⁺,Yb³⁺/SiO₂ Core/Shell Upconverting Nanocrystals for Luminescence Thermometry up to 900 K. J. Phys. Chem. C 2017, 121, 3503– 3510, DOI: 10.1021/acs.jpcc.6b10279
  44
  NaYF4:Er3+,Yb3+/SiO2 Core/Shell Upconverting Nanocrystals for Luminescence Thermometry up to 900 K
  Geitenbeek, Robin G.; Prins, P. Tim; Albrecht, Wiebke; van Blaaderen, Alfons; Weckhuysen, Bert M.; Meijerink, Andries
  Journal of Physical Chemistry C (2017), 121 (6), 3503-3510CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  The rapid development of nanomaterials with unique size-tunable properties forms the basis for a variety of new applications, including temp. sensing. Luminescent nanoparticles (NPs) demonstrated potential as sensitive nanothermometers, esp. in biol. systems. Their small size offers the possibility of mapping temp. profiles with high spatial resoln. The temp. range is however limited, which prevents use in high-temp. applications such as, for example, nanoelectronics, thermal barrier coatings, and chem. reactors. The authors extend the temp. range for nanothermometry beyond 900 K using SiO2-coated NaYF4 nanoparticles doped with the lanthanide ions Yb3+ and Er3+. Monodisperse ∼20 nm NaYF4:Yb,Er nanocrystals were coated with a ∼ 10 nm SiO2 shell. Upon excitation with IR radiation, bright green upconversion (UC) emission is obsd. From the intensity ratio between 2H11/2 and 4S3/2 UC emission lines at 520 and 550 nm, resp., the temp. can be detd. up to at least 900 K with an accuracy of 1-5 K for SiO2-coated NPs. For bare NaYF4:Yb,Er NPs, the particles degrade >600 K. Repeated thermal cycling expts. demonstrate the high durability and reproducibility of the SiO2-coated nanocrystals as temp. probes without any loss of performance. The present results open avenues for the development of a new class of highly stable nanoprobes by applying a SiO2 coating around a wide variety of lanthanide-doped NPs.
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45. 45
  Villanueva-Delgado, P.; Biner, D.; Krämer, K. W. Judd–Ofelt Analysis of β-NaGdF₄: Yb³⁺, Tm³⁺ and β-NaGdF₄:Er³⁺ Single Crystals. J. Lumin. 2017, 189, 84– 90, DOI: 10.1016/j.jlumin.2016.04.023
  45
  Judd-Ofelt analysis of β-NaGdF4: Yb3+, Tm3+ and β-NaGdF4:Er3+ single crystals
  Villanueva-Delgado, P.; Biner, D.; Kramer, K. W.
  Journal of Luminescence (2017), 189 (), 84-90CODEN: JLUMA8; ISSN:0022-2313. (Elsevier B.V.)
  Hexagonal β-NaGdF4 single crystals were grown by the Bridgman technique, one doped with 5% Yb3+, 0.5% Tm3+ and the other with 1% Er3+. Based on polarized absorption spectra a Judd-Ofelt anal. was performed using the software RELIC. The energy barycenters of the room temp. absorption bands were used to refine the 4f wavefunctions in the intermediate coupling approxn., and obtain the Slater integrals F(2), (4), (6) and the spin-orbit coupling parameter ζ. The exptl. oscillator strengths were fitted, using calcd. matrix elements, to the three Judd-Ofelt parameters Ω2,4,6. The radiative lifetimes and branching ratios were detd. and compared to exptl. data. The oscillator strengths of selected transitions relevant to upconversion processes are discussed.
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46. 46
  Fischer, S.; Swabeck, J. K.; Alivisatos, A. P. Controlled Isotropic and Anisotropic Shell Growth in β-NaLnF₄ Nanocrystals Induced by Precursor Injection Rate. J. Am. Chem. Soc. 2017, 139, 12325– 12332, DOI: 10.1021/jacs.7b07496
  46
  Controlled Isotropic and Anisotropic Shell Growth in β-NaLnF4 Nanocrystals Induced by Precursor Injection Rate
  Fischer, Stefan; Swabeck, Joseph K.; Alivisatos, A. Paul
  Journal of the American Chemical Society (2017), 139 (35), 12325-12332CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Precise morphol. and compn. control is vital for designing multifunctional lanthanide-doped core/shell nanocrystals. Controlled isotropic and anisotropic shell growth techniques in hexagonal Na rare earth tetrafluoride (β-NaLnF4) nanocrystals by exploiting the kinetics of the shell growth are reported. A drastic change of the shell morphol. was obsd. by changing the injection rate of the shell precursors while keeping all other reaction conditions const. The authors obtained isotropic shell growth for fast sequential injection and a preferred growth of the shell layers along the crystal's c-axis [001] for slow dropwise injection. Using this slow shell growth technique, the authors have grown rod-like shells around different almost spherical core nanocrystals. Bright and efficient upconversion was measured for both isotropic and rod-like shells around β-NaYF4 nanocrystals doped with Yb3+/Er3+ and Yb3+/Tm3+. Luminescence upconversion quantum yield and lifetime measurements reveal the high quality of the core/shell nanocrystal. Also, multishell rod-like nanostructures were prepd. with optically active cores and tips sepd. by an inert intermediate shell layer. The controlled anisotropic shell growth allows the design of new core/multishell nanostructures and enables independent studies of the chem. and physics of different nanocrystal facets.
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47. 47
  Wang, F.; Liu, X. Upconversion Multicolor Fine-Tuning: Visible to Near-Infrared Emission from Lanthanide-Doped NaYF₄ Nanoparticles. J. Am. Chem. Soc. 2008, 130, 5642– 5643, DOI: 10.1021/ja800868a
  47
  Upconversion Multicolor Fine-Tuning: Visible to Near-Infrared Emission from Lanthanide-Doped NaYF4 Nanoparticles
  Wang, Feng; Liu, Xiaogang
  Journal of the American Chemical Society (2008), 130 (17), 5642-5643CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  A general approach to fine-tuning the upconversion emission colors, based upon a single host source of NaYF4 nanoparticles doped with Yb3+, Tm3+, and Er3+, is presented. The emission intensity balance can be precisely controlled using different host-activator systems and dopant concns. The approach allows access to a wide range of luminescence emission from visible to near-IR by single-wavelength excitation.
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48. 48
  Pollnau, M.; Gamelin, D. R.; Lüthi, S. R.; Güdel, H. U.; Hehlen, M. P. Power Dependence of Upconversion Luminescence in Lanthanide and Transition-Metal-Ion Systems. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61, 3337– 3346, DOI: 10.1103/PhysRevB.61.3337
  48
  Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems
  Pollnau, M.; Gamelin, D. R.; Luthi, S. R.; Gudel, H. U.; Hehlen, M. P.
  Physical Review B: Condensed Matter and Materials Physics (2000), 61 (5), 3337-3346CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
  The authors show theor. with the simplest possible model that the intensity of an upconversion luminescence that is excited by the sequential absorption of n photons has a dependence on absorbed pump power P, which may range from the limit of Pn down to the limit of P1 for the upper state and less than P1 for the intermediate states. The two limits are identified as the cases of infinitely small and infinitely large upconversion rates, resp. In the latter case, the dependence of luminescence intensities from intermediate excited states on pump power changes with the underlying upconversion and decay mechanisms. In certain situations, energy-transfer upconversion and excited-state absorption can be distinguished by the measured slopes. The competition between linear decay and upconversion in the individual excitation steps of sequential upconversion can be analyzed. The influence of nonuniform distributions of absorbed pump power or of a subset of ions participating in energy-transfer upconversion was studied. These results are of importance for the interpretation of excitation mechanisms of luminescent and laser materials. The authors verify the authors' theor. results by exptl. examples of multiphoton-excited luminescence in Cs3Lu2Cl9:Er3+, Ba2YCl7:Er3+, LiYF4:Nd3+, and Cs2ZrCl6:Re4+.
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49. 49
  Hossan, M. Y.; Hor, A.; Luu, Q.; Smith, S. J.; May, S.; Berry, M. T. Explaining the Nanoscale Effect in the Upconversion Dynamics of β-NaYF₄:Yb³⁺,Er³⁺ Core and Core–Shell Nanocrystals. J. Phys. Chem. C 2017, 121, 16592– 16606, DOI: 10.1021/acs.jpcc.7b04567
  49
  Explaining the Nanoscale Effect in the Upconversion Dynamics of β-NaYF4:Yb3+,Er3+ Core and Core-Shell Nanocrystals
  Hossan, Md Yeathad; Hor, Amy; Luu, QuocAnh; Smith, Steve J.; May, P. Stanley; Berry, Mary T.
  Journal of Physical Chemistry C (2017), 121 (30), 16592-16606CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  Nanocrystals of β-NaYF4:Yb3+,Er3+ generally have lower NIR-to-visible upconversion (UC) internal quantum efficiency, IQE, compared to high-quality bulk materials, and exhibit more rapid UC dynamics, typical of quenching, when excited with a pulsed source near 980 nm. The addn. of a protective shell increases the IQE of the nanocrystals and slows the overall excited-state dynamics. An extension of a recently developed model for UC in powders of μm-sized β-NaYF4:18%Yb3+,2%Er3+ crystals correctly predicts the time-resolved luminescence curve shapes, relative intensities, and obsd. drop in IQE of the various emission lines for core and core-shell nanoparticles following pulsed excitation. The model clearly shows that the nanoscale effect on visible upconversion luminescence in these materials, with typical high-Yb3+ and low-Er3+ doping, is largely due to rapid energy migration among Yb3+(2F5/2) and Er3+(4I11/2) ions at the 1 μm energy level, such that an equil. is achieved between interior sites and rapidly relaxing surface sites. The faster kinetics obsd. in visible emission following pulsed NIR excitation is mainly a propagation of the effect of surface quenching of the 1 μm reservoir states and is not due to direct quenching of the visible emitting states themselves. For Er3+ ions contributing to UC emission, the relaxation rate consts. for the blue (2H9/2), green (2H11/2, 4S3/2), and red (4F9/2) emitting states are essentially unchanged from their bulk values, indicating that Er3+ ions close to the nanoparticle surface are nearly silent with regard to UC. The addn. of a passive β-NaYF4 shell retards the drain of the 1 μm excitation reservoir and recovers the participation of outer Er3+ sites in UC. The dependence of IQE on shell thickness is well explained in terms of a Forster-type model describing an energy donor (Er3+, Yb3+) interacting with a thin plane layer of acceptors (oleate). The UC behavior of both the core and the core-shell nanocrystals can be modeled, almost quant., solely from quenching at the 1 μm level, without sep. consideration of a near-surface Er3+ population. A 2-layer model for the core nanoparticles is revealing with regard to the modest extent to which near-surface ions do participate in UC and gives a better representation of the detailed dynamics of the NIR emitting states. A method is presented for allowing investigators to est. the IQE for any nanosample (with 18% Yb3+,2%Er3+ doping) as a function of excitation power d. (cw) or pulse-energy d. based on the low pulse energy measurement of the decay const. for the 1 μm emission.
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50. 50
  Gargas, D. J.; Chan, E. M.; Ostrowski, A. D.; Aloni, S.; Altoe, M. V. P.; Barnard, E. S.; Sanii, B.; Urban, J. J.; Milliron, D. J.; Cohen, B. E.; Schuck, P. J. Engineering Bright Sub-10-nm Upconverting Nanocrystals for Single-Molecule Imaging. Nat. Nanotechnol. 2014, 9, 300– 305, DOI: 10.1038/nnano.2014.29
  50
  Engineering bright sub-10-nm upconverting nanocrystals for single-molecule imaging
  Gargas, Daniel J.; Chan, Emory M.; Ostrowski, Alexis D.; Aloni, Shaul; Altoe, M. Virginia P.; Barnard, Edward S.; Sanii, Babak; Urban, Jeffrey J.; Milliron, Delia J.; Cohen, Bruce E.; Schuck, P. James
  Nature Nanotechnology (2014), 9 (4), 300-305CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
  Imaging at the single-mol. level reveals heterogeneities that are lost in ensemble imaging expts., but an ongoing challenge is the development of luminescent probes with the photostability, brightness and continuous emission necessary for single-mol. microscopy. Lanthanide-doped upconverting nanoparticles overcome problems of photostability and continuous emission and their upconverted emission can be excited with near-IR light at powers orders of magnitude lower than those required for conventional multiphoton probes. However, the brightness of upconverting nanoparticles was limited by open questions about energy transfer and relaxation within individual nanocrystals and unavoidable tradeoffs between brightness and size. Here, the authors develop upconverting nanoparticles under 10 nm in diam. that are over an order of magnitude brighter under single-particle imaging conditions than existing compns., allowing one to visualize single upconverting nanoparticles as small (d = 4.8 nm) as fluorescent proteins. The authors use advanced single-particle characterization and theor. modeling to find that surface effects become crit. at diams. under 20 nm and that the fluences used in single-mol. imaging change the dominant determinants of nanocrystal brightness. Factors known to increase brightness in bulk expts. lose importance at higher excitation powers and paradoxically, the brightest probes under single-mol. excitation are barely luminescent at the ensemble level.
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51. 51
  Villanueva-Delgado, P.; Krämer, K. W.; Valiente, R.; de Jong, M.; Meijerink, A. Modeling Blue to UV Upconversion in β-NaYF₄:Tm³⁺. Phys. Chem. Chem. Phys. 2016, 18, 27396– 27404, DOI: 10.1039/C6CP04347J
  51
  Modeling blue to UV upconversion in β-NaYF4:Tm3+
  Villanueva-Delgado, Pedro; Kramer, Karl W.; Valiente, Rafael; de Jong, Mathijs; Meijerink, Andries
  Physical Chemistry Chemical Physics (2016), 18 (39), 27396-27404CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
  Samples of 0.01% and 0.3% Tm3+-doped β-NaYF4 show upconverted UV luminescence at 27 660 cm-1 (361 nm) after blue excitation at 21 140 cm-1 (473 nm). Contradictory upconversion mechanisms in the literature are reviewed and two of them are investigated in detail. Their agreement with emission and two-color excitation expts. is examd. and compared. Decay curves are analyzed using the Inokuti-Hirayama model, an av. rate equation model, and a microscopic rate equation model that includes the correct extent of energy transfer. Energy migration is found to be negligible in these samples, and hence the av. rate equation model fails to correctly describe the decay curves. The microscopic rate equation model accurately fits the exptl. data and reveals the strength and multipolarity of various interactions. This microscopic model is able to det. the most likely upconversion mechanism.
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52. 52
  Fischer, S.; Steinkemper, H.; Löper, P.; Hermle, M.; Goldschmidt, J. C. Modeling Upconversion of Erbium Doped Microcrystals Based on Experimentally Determined Einstein Coefficients. J. Appl. Phys. 2012, 111, 013109, DOI: 10.1063/1.3674319
  52
  Modeling upconversion of erbium doped microcrystals based on experimentally determined Einstein coefficients
  Fischer, Stefan; Steinkemper, Heiko; Loeper, Philipp; Hermle, Martin; Goldschmidt, Jan Christoph
  Journal of Applied Physics (2012), 111 (1), 013109/1-013109/13CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
  The upconversion of IR photons is a promising possibility to enhance solar cell efficiency by producing electricity from otherwise unused sub-band-gap photons. The authors present a rate equation model and the relevant processes to describe the upconversion of near-IR photons. The model considers stimulated and spontaneous processes, multi-phonon relaxation, and energy transfer between neighboring ions. The input parameters for the model are exptl. detd. for the material system, β-NaEr0.2Y0.8F4. The detn. of the transition probabilities, also known as the Einstein coeffs., is the focus of the parameterization. The influence of multi-phonon relaxation and energy transfer on the upconversion are evaluated and discussed. Since upconversion is a nonlinear process, the irradiance dependence of the simulations is studied and compared to the exptl. data of quantum efficiency measurements. The results are very promising and indicate that upconversion is reasonably phys. described by the rate equations. Therefore, the presented model will be the basis for further simulations concerning various applications of upconversion, such as in combination with plasmon resonances in metal nanoparticles. (c) 2012 American Institute of Physics.
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53. 53
  Li, Z.; Zhang, Y. An Efficient and User-Friendly Method for the Synthesis of Hexagonal-Phase NaYF₄:Yb, Er/Tm Nanocrystals with Controllable Shape and Upconversion Fluorescence. Nanotechnology 2008, 19, 345606, DOI: 10.1088/0957-4484/19/34/345606
  53
  An efficient and user-friendly method for the synthesis of hexagonal-phase NaYF4:Yb, Er/Tm nanocrystals with controllable shape and upconversion fluorescence
  Li, Zhengquan; Zhang, Yong
  Nanotechnology (2008), 19 (34), 345606/1-345606/5CODEN: NNOTER; ISSN:0957-4484. (Institute of Physics Publishing)
  Hexagonal-phase NaYF4:Yb, Er/Tm nanocrystals are the best IR-to-visible upconverting materials to date, but user-friendly methods for making pure hexagonal-phase NaYF4:Yb, Er/Tm nanocrystals with upconversion fluorescence are still lacking. Most of the methods reported so far require excess fluoride reactants in a high-temp. reaction which are very unfriendly to users and raise safety concerns. In this work, an efficient and user-friendly method was developed for the synthesis of uniform hexagonal-phase NaYF4:Yb, Er/Tm nanocrystals with upconversion fluorescence, by forming small solid-state crystal nuclei and further growth and ripening of the nuclei. NaYF4:Yb, Er/Tm nanoplates, nanospheres and nanoellipses were also selectively produced by varying the concn. of the surfactant. All the nanocrystals showed strong upconversion fluorescence, and fluorescence from the nanoplates was obsd. even when the laser power d. was reduced to about 50 mW cm-2. These nanocrystals have great potential for use in biol. and medicine as fluorescent labels or imaging probes.
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54. 54
  Wang, F.; Deng, R.; Liu, X. Preparation of Core-Shell NaGdF₄ Nanoparticles Doped with Luminescent Lanthanide Ions to Be Used as Upconversion-Based Probes. Nat. Protoc. 2014, 9, 1634– 1644, DOI: 10.1038/nprot.2014.111
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  Preparation of core-shell NaGdF4 nanoparticles doped with luminescent lanthanide ions to be used as upconversion-based probes
  Wang, Feng; Deng, Renren; Liu, Xiaogang
  Nature Protocols (2014), 9 (7), 1634-1644CODEN: NPARDW; ISSN:1750-2799. (Nature Publishing Group)
  Sodium gadolinium fluoride (NaGdF4) is an ideal host material for the incorporation of luminescent lanthanide ions because of its high photochem. stability, low vibrational energy and its ability to mediate energy exchanges between the lanthanide dopants. This protocol describes the detailed exptl. procedure for synthesizing core-shell NaGdF4 nanoparticles that incorporate lanthanide ions into different layers for efficiently converting a single-wavelength, near-IR excitation into a tunable visible emission. These nanoparticles can then be used as luminescent probes in biol. samples, in 3D displays, in solar energy conversion and in photodynamic therapy. The NaGdF4 nanoparticles are grown through co-pptn. in a binary solvent mixt. of oleic acid and 1-octadecene. Doping by lanthanides with controlled compns. and concns. can be achieved concomitantly with particle growth. The lanthanide-doped NaGdF4 nanoparticles then serve as seed crystals for subsequent epitaxial growth of shell layers comprising different lanthanide dopants. The entire procedure for the prepn. and isolation of the core-shell nanoparticles comprising two epitaxial shell layers requires ∼15 h for completion.
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Supporting Information
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b01545.
- Tranmission electron microscopy of the upconversion nanocrystals, photoluminescence decay of bulk material, and a complete data set and modeling of excited-state decay dynamics of the visible-emitting levels (PDF)
- nn8b01545_si_001.pdf (903.03 kb)
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Quenching Pathways in NaYF4:Er3+,Yb3+ Upconversion NanocrystalsClick to copy article linkArticle link copied!

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Abstract

Results

Solvent-Dependent Decay Dynamics

Figure 1

Solvent Quenching of the Visible Emissions

Figure 2

Ion–Ion Interactions at Higher Doping Concentrations

Figure 3

Quenching of the Infrared Emissions

Figure 4

Discussion

Figure 5

Conclusion

Methods

Nanocrystal Synthesis

Spectroscopic Experiments

Supporting Information

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Acknowledgments

References

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Abstract

Figure 1

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Figure 3

Figure 4

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Quenching Pathways in NaYF₄:Er³⁺,Yb³⁺ Upconversion Nanocrystals
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