Europium-Doped NaYF4 Nanocrystals as Probes for the Electric and

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Europium-Doped NaYF4 Nanocrystals as Probes for the Electric and Magnetic Local Density of Optical States throughout the Visible Spectral Range Freddy T. Rabouw, P. Tim Prins, and David J. Norris Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b03730 • Publication Date (Web): 24 Oct 2016 Downloaded from http://pubs.acs.org on October 24, 2016

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Europium-Doped NaYF4 Nanocrystals as Probes for the Electric and Magnetic Local Density of Optical States throughout the Visible Spectral Range Freddy T. Rabouw, P. Tim Prins, and David J. Norris∗ Optical Materials Engineering Laboratory, ETH Zurich, 8092 Zurich, Switzerland E-mail: [email protected]

1 ACS Paragon Plus Environment

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Abstract Absorption and emission in the ultraviolet, visible, and infrared spectral range are usually mediated by the electric-field component of light. Only some electronic transitions have significant ‘magnetic-dipole’ character, meaning that they couple to the magnetic field of light. Nanophotonic control over magnetic-dipole emission has recently been demonstrated, and magnetic-dipole transitions have been used to probe the magnetic-field profiles of photonic structures. However, the library of available magnetic-dipole emitters is currently limited to red or infrared emitters, and mostly doped solids. Here, we show that NaYF4 nanocrystals doped with Eu3+ have various electric- and magnetic-dipole emission lines throughout the visible spectral range from multiple excited states. At the same time, the colloidal nature of the nanocrystals allows easy handling. We demonstrate the use of these nanocrystals as probes for the radiative electric and magnetic local density of optical states in a planar mirror geometry. A single emission spectrum can reveal enhancement or suppression of the density of optical states at multiple frequencies simultaneously. Such nanocrystals may find application in the characterisation of nanophotonic structures, or as model emitters for studies into magnetic light–matter interaction at optical frequencies. Keywords: Magnetic dipole, local density of optical states, lanthanide luminescence, nanocrystals


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A major topic in traditional photonics research has been the control and enhancement of spontaneous emission from electric-dipole (ED) emitters, i.e. those with electronic transitions coupling to the electric-field component of light. Other types of electronic transitions, such as higher-multipole transitions that couple to electric-field gradients of light, have recently gained interest. 1,2 Moreover, it has been realized that some technologically relevant emitters, such as infrared-emitting Er3+ in telecommunication 3,4 and red-emitting Eu3+ in lamp phosphors, 5,6 have electronic transitions with significant magnetic-dipole (MD) character, meaning that they are mediated by the magnetic component of light. In the past few years, several theoretical studies have been performed on emission control of MD emitters in various photonic environments. 7,8 For example, possible geometries for plasmonic or dielectric antennas have been explored with magnetic hotspots where emission and excitation of MD emitters can be enhanced. 4,9–15 Clearly, control over the interaction of light with MD transitions in matter is of fundamental and technological interest. In addition, MD emitters can be used to map the optical magnetic fields of laser beams, 16 plasmonic structures, 17 antennas, and metamaterials, as an alternative to near-field microscopy tips. 14,18,19 If an emitter exhibits ED and MD transitions from the same excited state, the ratio of ED to MD emission intensity is proportional to the ratio of radiative electric LDOS to magnetic LDOS. 6,17 The characterization of optical antennas using simultaneous ED and MD emitters 17 can thus complement lifetime-based experiments, 20,21 which measure the total LDOS including quenching pathways that contribute negatively to the brightness of emitters. 22 Typically, the coupling of electronic transitions to the magnetic component of light is much weaker than to the electric component. Magnetic interactions are important only for materials where electronic transitions are ED forbidden to first order, such as intraconfigurational transitions in transition metals (electronic configuration mdn ) or lanthanides (4fn ). Most experiments until now have studied the competition between the MD and the ED emission from the 5 D0 state of Eu3+ , at free-space wavelengths of approximately 600 nm. 5,6,23–28 In addition, the partial MD contribution has been characterized for the emission lines of several other lanthanides 3,4,29 and transition-metal ions, 30,31 embedded in a solid host material and emitting in the near infrared. The


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development of suitable MD emitters in the visible is desirable for further research in this new field, especially colloidal emitters that can easily be coupled to any photonic or plasmonic structures. Many candidate MD emitters have been identified from quantum-mechanical calculations on the lanthanide ions, which have multiple transitions with strong MD character and with emission wavelengths throughout the near-infrared, visible, and ultraviolet (UV) spectral range. 32,33 However, most of these transitions involve highly excited states, which typically decay rapidly through nonradiative pathways such as multi-phonon relaxation. 34 To make MD emission lines from such states in a lanthanide ion experimentally accessible, the lanthanide must be doped into a host material with a low phonon energy, where multi-phonon relaxation processes are relatively slow. NaYF4 is useful for this purpose, 35,36 as shown in the context of upconversion luminescence which requires emission from highly excited states upon excitation at low energy. 37–39 Here we study various ED and MD emission lines from Eu3+ doped in NaYF4 nanocrystals (NCs). This low-phonon host material allows the observation of ED as well as MD emission lines from three excited states of Eu3+ , covering the visible spectral range from the blue (464 nm) to the deep red (690 nm). We place the emitters at a controlled separation from a flat silver mirror to systematically vary the electric and magnetic LDOS, 6,23 and examine how the decay pathways in NaYF4 :Eu3+ NCs are affected. Depending on the electric and magnetic LDOS, excited Eu3+ ions relax primarily via ED or MD transitions. This competition between radiative decay pathways (ED versus MD) determines the relative emission intensities of transitions from the same excited state, while competition from nonradiative loss pathways affects the emission intensities from different excited states. 40 To highlight the potential application of nanocrystalline MD emitters as probes for the radiative electric and magnetic LDOS, the spectral characteristics of NaYF4 :Eu3+ NCs in a photonic environment (here, on a mirror) are compared to NCs dispersed in a solvent or to bulk NaYF4 :Eu3+ . Synthesis procedures for NaYF4 NCs are well developed, mostly because they are a popular host material for upconversion. 35,39,41,42 Since Y3+ ions are chemically similar to lanthanide ions, NaYF4 NCs can be doped with different types and combinations of lanthanide ions by simply


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a

b

c

Energy (eV)

3 NaYF4 Eu3+

2

ED: 464 nm 488 nm MD: 510 nm

395 nm

1

ED: 534 nm 584 nm MD: 525 nm 554 nm

0

5

D0

10–1 5

10

D1

–2

f

1.0 1*

* = MD

2 5

0.5

5

D1

7

FJ

D0

7

FJ

4

3 1 2*

Intensity (norm.)

e

100

Intensity (norm.)

d

5 10 Delay time (ms)

ED: 615 nm 690 nm MD: 593 nm

2 1 0

L6

5

DJ

J=6 45 23 01

7

FJ

0.04 5

D2

7

0.02 0

FJ 0* 3* 2

3 0

0.00

0.0 0

5

J=3

Nonradiative relaxation

100 nm

Intensity (norm.)

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500 600 700 Emission wavelength (nm)

500 600 700 Emission wavelength (nm)

Figure 1: (a) Transmission electron micrograph of the NaYF4 :Eu3+ NCs used in this study. (b) Cartoon of a NaYF4 NC doped with Eu3+ . For NCs with 15 nm diameter, a nominal 5% doping concentration corresponds to approximately 1300 Eu3+ ions per NC. (c) The energy-level diagram of Eu3+ up to 3.2 eV above the ground state, resulting from spin–orbit and Coulomb interactions experienced by the seven 4f-electrons of Eu3+ . Following excitation in the 5 L6 level (blue arrow; Supporting Information, Figure S2), relaxation can occur through multiple pathways. These involve nonradiative relaxation down the ladder of 5 DJ energy levels (black arrows) as well as radiative transitions to the 7 FJ ground-state multiplet (green, yellow, and red arrows). Some radiative transitions have magnetic-dipole (MD) character, others electric-dipole (ED). (d) The decay dynamics of the 5 D0 level (red) and the higher-energy 5 D1 level (yellow), following excitation of NCs dispersed in tetradecane at 395 nm. The 5 D1 level decays faster because of the possibility of rapid nonradiative relaxation to the 5 D0 level, in addition to the various radiative transitions. (e) The emission spectrum of NaYF4 :Eu3+ NCs dispersed in tetradecane upon excitation at 395 nm, with (f) a zoom-in on the weaker emission lines. Transitions from the different 5 DJ excited states are indicated with a green (J = 2), yellow (J = 1), or red (J = 0) shading. Transitions to different 7 FJ ground states are labeled with the corresponding J-value. Transitions with a dominant MD character are marked with an asterisk.


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using a mixture of Y3+ precursors and dopant precursors during their synthesis. 43 Our synthesis method for NaYF4 NCs doped with 5% Eu3+ is based on existing procedures, 41,44 and described in the Supporting Information (see also Figure S1). Transmission electron microscopy (Figure 1a) shows that the NCs are approximately 15 nm in diameter. A nominal 5% doping concentration then corresponds to 1300 Eu3+ ions per NC, that each act as an individual color center (Figure 1b). The localized electronic states of lanthanide dopant ions are not influenced by the size or shape of the (nano)crystal host, in contrast to the delocalized conduction- and valence-band states of semiconductor nanocrystals. Organic ligands on the NC surface provide colloidal stability, so that the NCs make clear dispersions in many common organic solvents, such as hexane, toluene, or chloroform. In principle, this allows the NCs to be placed on or near a (nano)photonic structure by spin-coating, lithographic positioning, 45,46 or electro-hydrodynamic printing. 47 Independent of how the NCs were processed, the crystalline host material ensures that the electronic structure of the Eu3+ dopants is protected and constant.

Figure 1c shows the energy-level diagram of Eu3+ , and the excited-state decay pathways relevant to our experiments. The strongest excitation line is at 395 nm (Supporting Information, Figure S2), corresponding to the transition from the 7 F0 ground state to the 5 L6 excited state (blue arrow in Figure 1c). Following excitation, a Eu3+ ion can decay down the ladder of energy levels by nonradiative relaxation pathways. The rate of nonradiative multi-phonon relaxation depends exponentially on the phonon energy of the host material. 34 NaYF4 has a relatively low phonon energy (300–400 cm−1 ), 36 and hence relatively low losses from multi-phonon relaxation. 37 As a result, radiative decay can compete with nonradiative channels in NaYF4 :Eu3+ not only from the lowest 5 D0 (red arrows in Figure 1c) excited state, but also from the higher 5 D1 (yellow arrows) and 5 D2 (green arrows) excited states, 48 that are quenched in other host materials. 5,6 Nevertheless, nonradiative decay pathways make the photoluminescence decay rate of the higher excited state 5

D1 considerably faster (yellow data points in Figure 1d) than that of the lowest excited state 5 D0

(red data points). From any of the excited states, the ion can decay radiatively to one of the seven


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roughness values of less than 1 nm. 50 Al2 O3 spacer layers of different thicknesses were deposited on the Ag by reactive sputtering of Al in an O2 /Ar plasma. The NCs were then spin-coated from a dispersion in hexane, resulting in a sub-monolayer coverage (Figure 2b). See Supporting Information for fabrication details. This geometry ensures that the Eu3+ ions in a sample are located at the same distance to the Ag mirror to within 15 nm (the average diameter of the NCs; Figures 1a, 2a), so that each emitter experiences the same LDOS. Analytical expressions can be derived for the rate of radiative decay of MD and ED emitters in a planar geometry. 6,51 We consider a three-layer geometry (Figure 2a) with a semi-infinite layer of air on top (dielectric constant ǫ0 = 1), a layer of Al2 O3 (ǫ1 = 2.82) with thickness d in the center, and a semi-infinite layer of Ag on the bottom (with frequency-dependent ǫ2 ; Ref. 53 ). The Eu3+ ions are modeled as isotropic point dipoles 52 at h = 7.5 nm (half a NC diameter) above the Al2 O3 layer, while the effect of the refractive-index contrast between NC and air is taken into account using a local-field factor χ. 54 We explicitly distinguish the radiative decay rate γ det into photon modes that can be collected by the objective of our microscope with numerical aperture NA = 0.9, and the total radiative decay rate γ tot including energy transfer to the metal, plasmon emission, and emission of photons under large angles from the surface normal. See the Supporting Information for details of the calculations. There, we also show the theoretical difference between the parameters γ det and γ tot for the various geometries considered in this work (Figure S4). For a particular excited state in Eu3+ , the total rate of decay is given by the sum over the total radiative decay rates for each of the transitions (colored arrows in Figure 1c) plus intrinsic nonradiative pathways (black arrows). For example, for the 5 D1 excited state the total decay rate is tot + γ tot + γ tot + γ tot + γ Γ1 = γ1→0 1→1 1→2 1→3 nonrad ,

(1)

where the subscripts 1 → J denote the radiative transitions from excited state 5 D1 to ground state 7

FJ , and γnonrad is the nonradiative decay rate. The branching ratios of the Eu3+ emission lines

as measured in an emission spectrum, depend only on the part of the radiative decay that enters the


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microscope objective. For example, for the branching ratio of the 5 D1 → 7 F0 emission line is I1→0 γ det β1→0 = P = P 1→0 , det J I1→J J γ1→J

(2)

where IJ→J ′ denotes the emission intensity of the transition 5 DJ → 7 F′J . The photoluminescence lifetime of an emitter is the inverse of the total decay rate (Equation 1): τ = 1/Γ. Hence, lifetimebased characterization of plasmonic nanostructures or antennas using a probe emitter 20,21,55 is sensitive to the total decay rate, including intrinsic nonradiative decay γnonrad and possible metalinduced quenching contained in γ tot . Characterization based on the relative intensities of ED and MD emission lines from the same excited state of a probe emitter can provide useful complementary information. It probes plasmonic enhancement of the detectable part of the radiative decay, and therefore, by reciprocity, about enhancement of absorption. Figure 2c,d shows the theoretical radiative LDOS of our planar Ag mirror structure, i.e. the enhancement of the (detectable) radiative decay rate γ det , for an ED emitter (panel c) or a MD emitter (panel d). The radiative LDOS depends on the separation between emitter and mirror, with a first maximum at a spacer thickness of 40–80 nm (depending on emission wavelength) for an ED emitter, and at 120–180 nm for a MD emitter. These regions of high electric LDOS (Figure 2c) and high magnetic LDOS (Figure 2d) are also directly evident in the series of experimental emission spectra of Figure 2e, from the intensities of ED and MD emission lines. Strong ED emission is observed at a spacer thickness of 40–80 nm (highlighted in blue), while strong MD emission is observed for spacers of 120–180 nm. In this way, interesting locations of high radiative electric or magnetic LDOS for wavelengths throughout the visible spectral range are readily identified using Eu3+ -doped NaYF4 NCs. For a more quantitative analysis of the competition between ED and MD emission, we focus on the branching ratios of the various emission lines originating from the same excited state. As discussed above (Equation 2), the branching ratios are determined by the rate γ det of radiative decay into photon modes collected by the spectrometer. It is important to realise that they do not


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depend on anything else. Any intrinsic nonradiative decay pathways or quenching induced by a plasmonic structure would deplete the excited state. However, since this reduces the recorded intensity of all excited states by the same factor, the branching ratios would be unaffected. We can therefore analyse the branching ratios of the different excited states using Equation 2 (Figure 3), despite significant nonradiative decay of the higher excited states 5 D1 and 5 D2 (see Figure 1d). Further, note that as different emission lines come from the same excited state, the branching ratios provide information about the LDOS ratio at different frequencies, not the absolute LDOS. In other words, if both electric and magnetic LDOS in the visible are enhanced by the same factor,

1.0

b Branching ratio

a

3* 0.5

5

D2

7

FJ 2 0

0.0 0 100 200 ’ Spacer thickness (nm)

1.0

5

7

D1

FJ 2*

3 0.5

1 0*

0.0 0 100 200 ’ Spacer thickness (nm)

c

1.0

Branching ratio

the branching ratios in the Eu3+ emission spectrum remain the same.

Branching ratio

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0.5

5

7

D0 2

FJ 1*

4 0.0 0 100 200 ’ Spacer thickness (nm)

Figure 3: Branching ratios of the most important Eu3+ emission lines from (a) the 5 D2 highenergy excited state, (b) the 5 D1 high-energy excited state, and (c) the 5 D0 lowest excited state, as a function of the spacer thickness that separates the emitter from the Ag mirror. The data points are the experimental data upon excitation at 395 nm. The lines are fits to Equation 2. MD emission lines are those with a maximum in branching ratio around 150 nm spacer thickness, and labeled with as asterisk. The rightmost data point in each plot represents the branching ratios of NCs in a homogeneous environment (dispersed in tetradecane), with the theoretical branching ratios depicted as a horizontal line, and calculated from the fit results on the mirror data. The data points in Figure 3 are the experimental branching ratios (from the emission spectra in Figure 2e) for emission lines from the 5 D2 high-energy excited state (Figure 3a), from the 5 D1 highenergy excited state (Figure 3b), and 5 D0 lowest excited state (Figure 3c). The different transitions are labeled with the J value of the ground state 7 FJ . For each excited state, the experimental bulk and γ bulk of data are fitted to Equation 2, with as fit parameters the radiative decay rates γED MD the different transitions in homogeneous bulk NaYF4 (see Supporting Information). If we allow for transitions with mixed ED and MD character in the fits, 29–31,49 we obtain ratios of ED/MD 10 ACS Paragon Plus Environment

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character (or vice versa) of 90% or greater. To reduce the number of fit parameters, we therefore approximate that each transition has pure ED or pure MD character. The match between data and fit is good for the 5 D1 (Figure 3b) and the 5 D0 emission lines (Figure 3c). The model performs less well for the 5 D2 state (Figure 3a) where integration of the emission lines is complicated by relatively strong background emission, presumably from the Al2 O3 spacer itself. The transitions with MD character are labeled with an asterisk in Figure 3. They clearly stand out as those with a maximum in branching ratio around 150 nm spacer thickness. Table 1 summarizes the estimated values of γ bulk for each of the transitions, obtained from the fits to the branching ratios (Figure 3) and the theoretical MD transition rates. 33 The values add up to a total radiative bulk +γ bulk +γ bulk = 161 s−1 . decay rate of the lowest 5 D0 excited state in bulk NaYF4 of Γ0 = γ0→1 0→2 0→4 This would translate into a radiative lifetime of 5.7 ms for NCs dispersed in an organic solvent (e.g. tetradecane, see below and Supporting Information), matching well with the experimental decay time of 6.3 ms for the 5 D0 luminescence (Figure 1d). We should however be careful not to make a quantitative comparison, because the experimental ‘apparent’ lifetime for the 5 D0 state is affected by the slow relaxation from higher excited states and potentially by nonradiative quenching. The last data points in each of the panels of Figure 3 are the measured branching ratios for NCs dispersed in a homogeneous photonic environment, tetradecane. (This nonvolatile solvent was chosen because a droplet containing the NCs could be placed on the inverted microscope without evaporating during the measurement.) The horizontal lines in Figure 3 are the theoretical values based on the fit results. The branching ratios on top of a mirror oscillate between values that are higher and lower than those in tetradecane, both in experiment and in theory. This shows that characterizing a batch of Eu3+ -doped NCs in a homogeneous environment provides a useful reference measurement. Subsequently, it is immediately clear from the emission spectrum of the NCs on a photonic structure (here, a mirror) whether the ratio of radiative electric over magnetic LDOS (via Equation 2) is enhanced or reduced by the structure. Finally, in Figure 4 we compare the emission intensities from the different emissive excited states of Eu3+ . Figure 4a shows the fraction of the total emission that comes from the 5 D1 state


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Table 1: Overview of the dominant emission lines from the three lowest excited states in NaYF4 :Eu3+ . The decay rates γ bulk are determined from fits of the experimental branching ratios as a function of distance from a Ag mirror (Figure 3a). Fitting of the branching ratios yields only the relative transition rates, which are converted to absolute rates using the theoretical MD rates labeled with a dagger (†), taken from Ref. 33 and corrected for the refractive index of NaYF4 of n = 1.48 (Ref. 59 ). We estimate the uncertainty in the relative rates to be approximately 20%, determined by uncertainties in the calibration of our experimental setup, variations in background fluorescence from the Al2 O3 spacer, approximations made in the model (see Supporting Information), and errors from the integration of the emission lines. initial state 5

D2

final state 7

F0 F2 7 F3 7 F0 7 F1 7 F2 7 F3 7 F1 7 F2 7 F4 7

5

D1

5

D0

emission wavelength (nm) dipole character transition rate γ bulk in bulk NaYF4 (s−1 ) 464 ED 2.9 488 ED 5.4 510 MD 37.6† 525 MD 4.4 534 ED 15.7 554 MD 39.8† 584 ED 37.9 593 MD 46.6† 615 ED 65.2 690 ED 49.2


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(yellow) and from the 5 D2 state (green), as a function of separation from the Ag mirror. For both excited states, the intensity has a minimum at a spacer thickness of 100 nm, and a maximum at a spacer of 160 nm. While the relative intensities of the emission lines from the same excited states can be modeled relatively easily considering competition between radiative decay pathways (Figure 3; Equation 2), understanding the relative emission intensities from different excited states is more complex, because many more decay processes can play a role. However, with some simple assumptions, we can extract an estimate for the relative intensities of different excited states in terms of rate constants for the individual transitions. First, most of the emission (∼ 90%; see Figure 1e) originates from the lowest excited state 5 D0 , so the total measured emission intensity can be approximated as Itot ≈

P

J′

I0→J ′ , where

P

J′

denotes summation over ground states 7 FJ ′ . Second,

we neglect that the population of the 5 D0 state is slightly affected by the relaxation pathway of the highly excited states 5 D1,2 , which, if nonradiative, can feed into the 5 D0 state. Finally, we assume that nonradiative relaxation from the 5 D0 state is negligible, because the gap to the closest ground state 7 F6 is many phonon energies. 54 The 5 D0 intensity is then simply proportional to the collection efficiency:

P

J′

I0→J ′ ∝

P

J′

det ′ / γ0→J

P

J′

tot ′ . The intensity from the higher excited states is γ0→J

determined by competition between ‘detectable’ radiative decay and all other decay pathways: P

J′

IJ→J ′ ∝

P

J′

det ′ /Γ . The relative intensity from a high-energy excited state (5 D or 5 D ) γJ→J J 2 1

compared to the total emission intensity can then be written as P

J′

IJ→J ′

Itot

P

IJ→J ′ ≈ ≈ P J ′ I0→J ′ J′

P

J′

P

det ′ tot ′ γJ→J ′ γ × PJ 0→J det ΓJ J ′ γ0→J ′

(3)

where J labels the excited state 5 D1 or 5 D2 . Figure 4b shows how the different factors of Equation 3 depend on the spacer thickness in our experimental mirror geometry. The factor

P

J′

tot ′ / γ0→J

P

J′

det ′ is the inverse collection efficiency γ0→J

for emission from the lowest-energy excited state 5 D0 , plotted in red. The total radiative decay rate into collectable photon modes

P

J′

det ′ is plotted in yellow for the 5 D state, and in green γJ→J 1

for the 5 D2 state. We see that the dependence of these three factors on the spacer thickness is


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relatively weak (±50%) and the maxima do not match the maximum of the high-energy excitedstate intensity in the experiment (Figure 4a). These three factors alone can therefore not explain the experimental observations. We argue that the dependence of excited-state intensity on spacer thickness arises mostly from variations in the total decay rate ΓJ of the excited states. The spacer thickness affects this rate indirectly, not via changes in emission rates, but excitation rates. The situation is schematically depicted in Figure 4c. At low excitation rates (left) an important mechanism of nonradiative decay from high-energy excited states of the Eu3+ ion to lower-energy states is cross-relaxation, 48 i.e. the transfer of part of the excited-state energy to a neighboring ion in the ground state. At higher excitation rates (right) the ground-state population decreases, so that cross-relaxation should become less likely. 56 This results in less nonradiative relaxation and more emission from high-energy states (see Supporting Information, Figure S5). Indeed, our experimental conditions correspond to an estimated excitation rate of approximately 0.1 ms−1 (see Supporting Information), sufficiently high to lead to depletion of the Eu3+ ground-state population. The blue line in Figure 4b is the excitation rate of the Eu3+ ions, which by reciprocity is equal to γ det for the excitation transition 7

F0 → 5 L6 (an ED transition at 395 nm). This factor has a strong dependence on spacer thickness,

with a minimum at 100 nm and a maximum at 160 nm coinciding with the experimental extremes in high-energy excited-state intensity (Figure 4a). This indicates that the photonic effect on ΓJ via the excitation rate strongly affects the relative intensities from different excited states. Figure 4d,e summarizes how strongly the excited-state decay pathways of Eu3+ in NaYF4 NCs are affected simply by the presence of a mirror, by considering emitter–mirror separations of 100 nm (Figure 4d) and 160 nm (Figure 4e). At 100 nm separation, 24% of the emission comes through MD transitions (red) and 76% through ED transitions (blue), while at 160 nm separation these numbers are 65% and 35%, respectively. These differences comes mainly from competition between different radiative pathways from the same excited state (Figure 3; Equation 2), and provide information about the radiative electric and magnetic LDOS. In addition, the different spacer thicknesses show different relative amounts of high-energy excited-state emission (dark shading):


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6% at 100 nm spacer versus 13% at 160 nm spacer. As discussed above, multiple radiative and nonradiative processes determine the relative emission intensities from different excited states (Figure 4b), where under our experimental conditions the most prominent effect is due to the photonic enhancement of the excitation rate. As we showed in Figures 2f and 3, the various ED and MD emission lines from Eu3+ -doped NaYF4 NCs can be used to probe the radiative electric and magnetic LDOS near a photonic or plasmonic structure. Many more MD transitions in the visible and infrared spectral range from lanthanide-doped colloidal NC emitters are accessible experimentally, 33 if one uses (A) the right host material of low phonon energy to avoid multi-phonon relaxation, 36 and (B) low doping concentration 38,48 or high excitation power 38,56 to avoid cross-relaxation. As a few examples, the 5 D3 excited state of Tb3+ , 48 the 2 H11/2 excited state of Er3+ , 35,36 the 4 G1 excited state of Tm3+ , 38 and the 2 F5/2 excited state of Yb3+ have emission lines with (partial) MD character. 33 Some of these may be useful as probes for the electric and magnetic LDOS of plasmonic antennas and metamaterials. More generally, our work highlights how strongly even a relatively simple photonic environment affects the competition between ED and MD transition rates (Figure 4d,e). This is of importance for the photonic or plasmonic enhancement of lanthanide and transition metal luminescence, for example for optical communication 4 or upconversion. 57,58

Associated Content Supporting Information Supporting Information Available: Experimental methods, additional sample characterization, and further details related to our calculations. This material is available free of charge via the Internet at http://pubs.acs.org.


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Author information Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgements F.T.R. acknowledges support from the Netherlands Organisation for Scientific Research (NWO; Rubicon grant nr. 680-50-1509). This work used equipment and resources funded by the European Research Council under the European Union’s Seventh Framework Program (FP/2007-2013) / ERC Grant Agreement Nr. 339905 (QuaDoPS Advanced Grant).


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Europium-Doped NaYF4 Nanocrystals as Probes for the Electric and

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